Advances in Catalysis and Related Subjects, Volume 25

ADVANCES IN CATALYSIS VOLUME 25 Advisory Board G. K. BORESKOV Novosibirsk, V.S.S.R. P. H. EMMETT Baltimore, Marylan...

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ADVANCES IN CATALYSIS VOLUME 25

Advisory Board

G. K. BORESKOV Novosibirsk, V.S.S.R.

P. H. EMMETT Baltimore, Maryland

G. NATTA Milan, Italy

M. BOUDART Stanford, California

M. CALVIN Berkeley, California

W. JOST

J. HORIUTI Sapporo, Japan

Gatringen, Germany

P. W. SELWOOD Santa Barbara, California

ADVANCES IN CATALYSIS VOLUME 25

Edited by D. D. ELEY The University Nottingham, England

HERMAN PINES Northwestern University Evanston, Illinois

PAULB. WEISZ Mobil Research and Development Corporation Princeton, New Jersey

1976

ACADEMIC PRESS

NEW YORK

SAN FRANCISCO

LONDON

A Subsidiary of Harcourt Brace Jovanovich, Publishers

COPYRIGHT 0 1976, BY ACADEMIC PRESS, INC. ALL RIGHTS RESERVED. N O PART O F THIS PUBLICATION MAY BE REPRODUCED OR TRANSMITTED I N ANY FORM OR BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCLUDING PHOTOCOPY, RECORDING, OR ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, WITHOUT PERMISSION IN WRITING FROM THE PUBLISHER.

ACADEMIC PRESS, INC.

111 Fifth Avenue, New York. New York 10003

United Kingdom Edition published by ACADEMIC PRESS, INC. (LONDON) LTD. 24/28 Oval Road, London N W l

LIBRARY OF CONGRESS CATALOG

CARD

NUMBER: 49-7755

ISBN 0-12-007825-2 PRINTED I N THE UNITED STATES O F AMERICA

Contents .............................................................. ...................................................................

CONTRIBUTORS PREFACE

vii ix

Application of Molecular Orbital Theory to Catalysis ROGERC. BAETZOLD I. 11. 111. IV. V.

Introduction ................................................... Calculational Procedures. .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . MetalClusters.. Chemisorption .................................................. Conclusions .................................................... References ....................................................

. .. . ...

. . . .. . .

... .

. . . ..

.. . .. .

'..

1

3 . 16 34 51 53

The Stereochemistry of Hydrogenation of a$-Unsaturated Ketones ROBERT L. AUGUSTINE I. 11. 111. IV. V.

Introduction ................................................... Mechanistic Proposals . . . .. . . . . . . . , , . , . . . . .. . . . ... Effect of Variables on the Hydrogenation of p-Octalone and Related Compounds Hydrogenation of Other Ring Systems . . . . . . . . . . . . . . . . . . . ... Conclusions .................................................... References ....................................................

. .. ..

.. . .. ..

. . . . . . .. . ... .

56 59 63 75 78 79

Asymmetric Homogeneous Hydrogenation J. D. MORRISON, W.F. MASLER, AND

I. 11. 111. IV. V. VI.

M.K. NEUBERG

Introduction ................................................... Homogeneous Rhodium-Chiral Phosphine Catalyst Systems . . . . . . . . . . Chiral Amide-Rhodium Complexes as Catalysts . . . . . . . . . . . . . . .... . Chiral Cobalt Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chiral Ruthenium Catalysts . . . . . . . . . . . . . . -. . .. . .. .. .. . . . . Concluding Remarks. . . . . . , . . . . . . . .. .. .. . .. Note Added in Proof. . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . .: . . . . . . . . . References ....................................................

. . .. . . . . . . .. . .. .. .. V

. . .. .... . .. . .. . .. . ... . .. .. . . . .. . .

81

85 115 118 120 121 122 122

vi

CONTENTS

Stereochemical Approaches to Mechanisms of Hydrocarbon Reactions on Metal Catalysts J . K. A. CLARKE AND J . J. ROONEY I . Introduction .................................................. -125 I1. The Horiuti-Polanyi Mechanism .................................... 127 111. Reactions of Olefins ............................................ 136 IV . Skeletal Rearrangement of Alkanes in Platinum and Other Noble Metals .... 141 V. Recent Experimental Approaches to Skeletal Rearrangements ............ 158 VI Influence of Carbonaceous Deposits ................................ 176 VII . Conclusions ................................................... 180 References ................................................... 180

.

Specific Poisoning and Characterization of Catalytically Active Oxide Surfaces HELMUT KNBZINGER

. Introduction ..................................................

I I1. I11. IV .

General Scope and Definitions ..................................... Experimental Methods ........................................... Interaction of Specific Poisons with Oxide Surfaces .................... V . Specific Poisoning on Alumina Surfaces ............................. VI . Conclusions ................................................... References ....................................................

184 187 195 203 249 258 260

Metal-Catalyzed Oxidations of Organic Compounds in the Liquid Phase: A Mechanistic Approach ROGERA . SHELDON AND JAY K. KOCHI I . Introduction .................................................. I1 Homolytic Mechanisms .......................................... I11 Heterolytic Mechanisms .......................................... IV. Heterogeneous Catalysis of Liquid Phase Oxidations .................... V. Biochemical Oxidations .......................................... VI . Summary-Directions for Future Development ........................ References ....................................................

. .

............................................................ ............................................................

AUTHORINDEX SUBJECTINDEX CONTENTS OF PREVIOUS VOLUMES .............................................

274 275 339 377 381 390 391 415 443 452

Contributors Numbers in parentheses indicate the pages on which the authors’ contributions begin.

ROBERTL. AUGUSTINE, Department of Chemistry, Seton Hall University, South Orange, New Jersey (56) ROGERC. BAETZOLD, Research Laboratories, Eastman Kodak Company, Rochester, New York (1) J. K. A. CLARKE,Chemistry Department, University College, Bepeld, Dublin, Ireland (125) HELMUTKN~ZINGER, Physikalisch-Chemisches Institut, Universitdt Miinchen, Miinchen, West Germany (1 84) JAYK. KOCHI,Department of Chemistry, Indiana University, Bloomington, Indiana (272) W. F. MASLER, Department of Chemistry, University of New Hampshire, Durham, New Hampshire (81) J . D. MORRISON, Department of Chemistry, University of New Hampshire, Durham, New Hampshire (81) M. K. NEUBERG, Department of Chemistry, Stanford University, Stanford, California (81) J . J . ROONEY, Department of Chemistry, The Queen’s University, Belfast, Northern Ireland (125) ROGERA. SHELDON, Koninklijke/Shell-Luboratorium,Amsterdam, The Netherlands (272)

vii

This Page Intentionally Left Blank

CATALYSISAFTER TWENTY-FIVEADVANCES We had a telephone call the other day to ask if we would write a preface, and also to remind us that this was the twenty-fifth volume (25th!) of Advances in Catalysis, and therefore something of a special occasion. The number 25 is magical only because of our base-ten system of counting, of course, but it does appear to present a long enough history of reviewing catalytic science that we may ask ourselves what has been accomplished. As Editors, we may ask how well we have done our job as suppliers of reviews. As scientists, we may ask what basic advance the science of catalysis has made. We are reminded of a friend who said: “I’ve seen so many things going on that I ask myself, ‘Just what is going on?”’ In some ways, this defines our task: to invite, find, and print summary views on “just what is going on.” Figuratively speaking, each individual piece of research digs a hole, more or less deep, somewhere in a vast field of science. Reviews should provide a view of the most relevant findings in a somewhat integrated province of nearby holes. The trouble is that the field of catalytic science is so broad that even the wellreviewed provinces (e.g., even if they cover summary topics like stereochemistry of hydrogenation, the polymerization of olefins, the state of oxide catalysts, etc.) can still be far apart, and basic and common truths of catalysis may not emerge from these views. Reviews (reviewers) that tackle the next broader level of integration of “just what is going on” are difficult to find. It takes not only wisdom but also courage to attempt such basic analyses and reviews. We believe in a need for our evolving human society to practice the skills of the viewing of landscapes, in addition to the study of trees and groves. Speaking of desirable goals, some years ago we resolved to help bring together the basically related knowledge of the catalytic and of the enzymatic researchers-but, alas, with little success. Common phenomena, laws, concepts, and mechanisms do exist, but are busily pursued by linguistically separated groups (like those who do Langmuir-Hinshelwood and those who do Michaelis-Menten kinetics, or those for which “substrate” not only has different but opposite connotations, etc.). These two areas of endeavor remain perfect examples of the ivory towers of Babel which often characterize scientific pursuits. We will continue to try to bridge a regrettable gap, but it will take editors and authorsauthors fluent in both languages. We are looking for the rare talent. . . ix

X

AN EDITORIAL PREFACE

Where has catalytic science itself gone? We have turned to a colleague who has no editor’s bias, but one who manages somehow to keep up with a vast cross section of literature as well as being professionally versed in theories as well as current practice of catalysis. We asked him for his views on “where does the understanding of the active center or the catalytic site stand today?” The following reply is that of Dr. Werner 0. Haag, a former student of one (HP) and current colleague of another (PBW) of the Editors:

* * * Fifty years ago, H. S . Taylor wrote his classic paper [Proc. Roy. Soc., Ser. A 108,105 (1925)] in which he developed the concept of the active sites. Some twenty-five years ago, attention focused heavily on the electronic properties of metal and semiconductor catalysts as the key to an understanding of catalytic activity. Was it a fad? Are there new and different fads? It seems fair to say that there is no one particular aspect now that dominates attention. Perhaps this reflects the growing and proper realization that there is no single or universal “secret” to explain the great variety of catalytic activities any more than the accepted diversity of molecular reactivities among the many molecules of general chemistry. The initial collective electronic theory of the fifties, in its simplest form, implied that the electrons and holes-controlled by the bulk structure-of the catalytic solid are available for reactants anywhere on the surface. It largely ignored the “geometric factor” inherent in Taylor’s active site concept or in Balandin’s Multiplet Theory. In 1958, Advances in Catalysis carried an article by Gwathmey and Cunningham whose pioneering experiments simply and visually, but dramatically, forecast the great importance of the structural surface details of the crystal planes of solids for adsorption and reactivity. In recent years we have seen the development of several new and sophisticated tools such as lowenergy electron diffraction (LEED) and Auger electron spectroscopy (AES). These have made it possible to focus on the technicalities of detailed surface topography and the unique chemical behavior of surface sites. Adsorption and reactions are sometimes described as occurring on corners, edges, dislocations, surface steps, and kinks. Also, crystal surfaces have been found to reorganize as a resplt of adsorption. This current ability to study the solid surface on an atomic scale represents one remarkable step of progress. The surface topography is not only found to be important for metals, but also for elemental and compound semiconductors; adsorption often occurs preferentially on the incompletely coordinated surface atoms of disordered surfaces, while ordered surfaces are relatively inert. Notable progress is also being made on the theoretical side. While earlier solid

AN EDITORIAL PREFACE

xi

state approaches described solids as infinitely large crystals, efforts are now directed toward computing charge densities at surface irregularities, especially sites of low coordination number such as corner atoms. Thus, new evidence for the importance of topographically distinct parts of the surface has emerged. Yet, this must not be taken as a simplistic decision in favor of an “active sites theory” as a matter of distinction from, or versus, the “electronic factor” approach in catalysis. On the contrary, the two viewpoints have become complementary. Electronic surface states due to topographically distinct surface sites become necessary ingredients of the collective electronic theory. A new concept has emerged that distinguishes between structure-sensitive and structure-insensitive reactions; the specific rate of the latter is independent of particle size. This is a useful concept with many implications. A complete independence from crystal size, if applicable to the transition from “solid” to “atom” would, however, seem incongruous in the light of any electronic theory-physical or chemical. We must realize that the metal particles, even at the low end of the range investigated, are still relatively large: a 20-A metal crystallite contains 300 to 400 atoms. The difficulty of preparing and characterizing smaller particles has remained great. But there is now a possibility of forming clusters of a few atoms of precisely predetermined number. For example, (Rh), and (Rh)6 clusters have apparently been made [J.Amer. Chem. Soc. 94, 1789 (1972)l on a phosphinated polystyrene polymer. (w)6 catalyzed the hydrogenation of aromatics at 25°C and 1 atm H2 pressure, while (Rh)4 is apparently inactive! The chemistry of metal organic complexes has already done much to bring homogeneous and heterogeneous catalysis closer to being a unifiable catalytic science in other ways. Most reactions such as hydrogenation or C -H bond activation, once thought t o be typically metal catalyzed, can now be effected with mononuclear metal complexes. Activated adsorption and reaction on incompletely coordinated transition metal complexes is the analog of the preferential adsorption on topographically distinct surface sites of low coordination number on solids mentioned above. Attention is increasingly being given to using synthetic organic methods to create well-defined heterogeneous catalysts. It is obvious that these brief thoughts on progress can only provide selected highlights rather than providing a full review.

* * * On the scene of industrial chemistry, too, many sizable advances have occurred. Among them are processes for production of vinyl acetate from ethylene/OJacetic acid over a heterogeneous Pd-catalyst; the manufacture of acetic acid by carbonylation of methanol using a transition metal complex homoge-

xii

AN EDITORIAL PREFACE

neous catalyst; acrylonitrile production by ammonoxidation using a bismuthmolybdate based solid; the widespread adoption of crystalline alumino-silicate (zeolite) based catalysts in the petroleum industry; the use of bimetallic catalyst combinations in petroleum naphtha reforming; the first commercial uses of matrix-supported enzymes (for example, a supported penicillin amidase in the production of semisynthetic penicillins). It would seem that in these past years, many interdisciplinary approaches and concepts, involving many parts of chemistry and physics, have clearly merged and promise to develop further insights and applications in the future; and that is a joyous and satisfying observation. The present volume continues our effort to provide diverse exposure. We include two articles devoted to stereochemical aspects of catalytic reactions (J. K. A. Clarke and J. J. Rooney; R. L. Augustine), and one (J. D. Morrison, W. F. Masler, and M. K. Neuberg) devoted to the control of a yet more subtle level of chemical structure: asymmetry (or optical activity); a comprehensive review of liquid phase organic oxidation catalysis (R. A. Sheldon and J. K. Kochi); a review of specific adsorption and poisoning action as a means to learn more about active sites (H. Knozinger); and some of the latest considerations to catalysis of molecular orbital theory (R. C. Baetzold).

P. B. WEISZ

Application of Molecular Orbital Theory to Catalysis ROGER C . BAETZOLD Research Laboratories Eastman Kodak Company Rochester. New York

.

I Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A QuestionsExamined . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B Applications of Approximate Molecular Orbital Theory . . . . . . . . . I1 Calculational Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. General Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Extended Hlickel Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . C Complete Neglect of Differential Overlap . . . . . . . . . . . . . . . . . D. Properties Calculable by Approximate Molecular Orbital Theory E Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F Comparisons with Experiment . . . . . . . . . . . . . . . . . . . . . . . . 111. Metalclusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A . Silver Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B PalladiumClusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Cadmium Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D. Nickelclusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E . Low Atomic Number Clusters . . . . . . . . . . . . . . . . . . . . . . . . F. Silver-Palladium Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . IV. Chemisorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B Carbonsubstrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C Silver Clusters o n Silver Bromide . . . . . . . . . . . . . . . . . . . . . . D Metal Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E Palladium on Various Substrates . . . . . . . . . . . . . . . . . . . . . . . V . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A . Specific Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B Future . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

. .

. . .

....

.

. . . .

.

1 2 2 3 3 3 9 11 13 13 16 17 27 29 31 32 33 34 34 36 41 47 49 51 52 53 53

1 . Introduction Molecular orbital (h40)calculations treating the electronic properties of isolated and chemisorbed clusters of atoms will be examined in this article. Small clusters of metal atoms serve as catalytic centers for many reactions. yet their electronic properties are poorly understood . Chemisorbed species are likewise 1

2

ROGER C. BAETZOLD

of importance since this phenomenon frequently takes place at catalytic centers. The description of the properties of possible catalytic centers in terms of energy levels offers a problem where MO theory is particularly applicable. In this article we examine approximate MO theory t o see whether it is useful for the study of catalysis. The theory is applied t o diverse systems in order t o make predictions o r explain experimental data. It is still too early t o judge the eventual impact of this method, but its success for molecular problems suggests its possible importance. A. QUESTIONSEXAMINED Through the application of MO theory to several systems, some questions of fundamental importance emerge. We are particularly interested in the following concepts. 1. At what size does a cluster of metal atoms possess metalliie properties? 2. Are the interactions between atoms in a cluster pairwise additive? 3. What makes a particular metal cluster effective as a catalyst? 4. What effect does substrate have on electronic properties of adsorbed clusters of atoms? 5. What size substrate representation is required t o treat chemisorption?

B. APPLICATIONS O F APPROXIMATE MOLECULAR ORBITALTHEORY In recent years a new line of attack has been made o n the catalysis problem using MO theory. The theory itself is approximate and, therefore, its application is limited to a semiquantitative explanation of phenomena. More elaborate calculations that are applied to small molecules are not useful for these problems because they are too complex for the computer facilities generally available. Instead of probing quantitative details, approximate MO theory is useful to test concepts. The widespread application of MO theory to systems containing u bonds was sparked in large part by the development of extended Hiickel (EH) theory b y Hoffmann (I) in 1963. At that time, nMO theory was practiced widely by chemists, but only a few treatments of u bonding had been undertaken. Hoffmann’s theory changed this because of its conceptual simplicity and ease of applicability to almost any system. It has been criticized on various theoretical grounds but remains in widespread use today. A second approximate MO theory with which we are concerned was developed by Pople and co-workers (2) in 1965 who simplified the exact Hartree-Fock equations for a molecule. It has a variety of names, such as complete neglect of differential overlap (CNDO)or intermediate neglect of differential overlap (INDO). This theory is also widely used today.

MOLECULAR ORBITAL THEORY

3

Recently, approximate MO theories have been applied to a wide range of solid-state phenomena in addition to thwe reviewed in this paper. A short review of some of these problems indicates its versatility. Messmer and Watkins (3) have used EH to predict the position of N impurity levels in diamond using a 35-atom C lattice. Their calculations indicated the presence of a JahnTeller effect in accordance with electron paramagnetic resonance (EPR) experiments. The calculation was successful in explaining the deepening of the N donor level as due to Jahn-Teller distortion. The MO-type calculation has been employed by Bramanti et al. (4) to explain the absorption spectrum of T1' in KCl. The calculated positions of the T1' energy levels in KCl explained changes in the spectrum on going from free ions to the solid state. Similarly, in the case of hydrogen impurity in LiF, Hayns (5) has shown that the excitation energy predicted by calculation is in accord with experimental results. These results have inspired confidence in the semiempirical method as a means for providing qualitative explanations for several electronic phenomena.

11. Calculational Procedures A. GENERALAPPROACH Sigma MO theory is directly applicable to determination of electronic properties of atom clusters or other finite-size arrays. Such systems, varying in size from 1 atom to nearly infinite, are not amenable to the well-known calculational procedures of solid-state physics which employ periodic boundary conditions. The MO procedures have been applied to a variety of inorganic systems including Mn04, C T O ~ ~and - , C104-, which were first treated in 1952 by Wolfsberg and Helmholtz ( 6 ) . The procedure employed in this early work formed a basis for later methods such as the one employed by Ballhausen and Gray (7) for vanadyl ion calculations and eventually the EH theory. These calculational methods are briefly outlined here so that we can use them with understanding in this chapter.

B. EXTENDED HUCKELTHEORY 1. Equations

Extended Hiickel theory is useful for calculating the properties of molecules containing u or IT bonds. The molecular orbitals ( $ i ) are taken as a linear combination of the valence atomic orbitals (xi):

4

ROGER C. BAETZOLD

Two assumptions are made in this choice. Core orbitals are deemed to have negligible influence on bonding, and the shape of atomic orbitals is used to describe the molecular orbitals. More complete ab initio calculations often allow for orbital variation so the latter assumption is a possible source of error. The neglect of core orbitals is justified by their localized nature, which excludes significant participation in bond formation. A recent pseudopotential formulation by Cusachs (8), in which core orbitals were included, has shown that the form of the equations used in MO theory is unchanged although the input parameters may require some modification. Thus, most workers do not consider core orbital effects significant. A secular equation of the form n

C (Hij - ESij)Cij = O

j = 1 , 2 , ..

. ,n

i=l

is obtained by variation of the Cii coefficient to achieve an energy minimum. In a system consisting of n orbitals, the matrix elements appearing in the secular equation have the form

(Hamiltonian matrix element) and

A

(overlap matrix element), where H is taken as an effective Hamiltonian operator of the system. This introduces a degree of arbitrariness in choosing the Hamiltonian matrix elements and leads to a nonexplicit accounting for electron repulsion. In EH theory, the atomic orbitals are usually taken as Slater orbitals,

(2.)" Xi =

lJ2

&x!

rN-l e-" y,, (0, $1,

where N = principal quantum number, r = distance from nucleus, a = orbital exponent, and Y,,,, (0, $) = spherical harmonic for I, m quantum numbers. Although the single Slater function is nodeless, linear combinations of Slater functions are often used to achieve nodes in the radial function of a single orbital. The choice of orbital exponent (a) to use. for a particular atomic Slater orbital has been the subject of several investigations. Originally, Slater ( 9 ) proposed a set of empirical rules for choosing exponents; however, these are not used frequently in modern calculations. Hartree-Fock self-consistent-field (SCF)

MOLECULAR ORBITAL THEORY

5

calculations using single Slater orbitals for atoms up to atomic number 86 have been performed by Clementi et al. ( 1 0 , I I ) in order to determine orbital exponents. These functions are frequently used in EH calculations. The orbitals determined by Clementi do not yield the same overlap integrals as calculated by atomic Hartree-Fock wave functions, and, therefore, they have some deficiencies. Cusachs and Corrington (12) attempted to remedy this difficulty by fitting orbitals to give the proper overlap. These orbitals, called matching overlap orbitals, have reduced quantum numbers and have been determined for elements through atomic number 86. Linear combinations of two Slater orbitals have been determined for atomic orbitals in transition elements by Richardson et al. (13) and Basch and Gray (14). These functions are determined by fit to SCF atomic wave functions and describe the overlap integrals more accurately than do single Slater functions. Their use, however, increases computer time requirements, a situation that must be weighed in determining what kind of orbitals to use. Once a choice of atomic functions is made, the overlap integrals appearing in Eq. (2) are all calculated. This calculation can be performed accurately, since formulas have been derived by Mulliken et al. (15) giving closed-form expressions for overlap integrals between Slater orbitals. The remaining choice of parameters for an EH calculation lies in the Hamiltonian matrix elements (Hij). The diagonal elements are taken as Hii = - IP,

(6)

the negative ionization potential of the orbital in question. Experimental atomic ionization potentials are available and employ spectroscopic data tabulated by Moore (16). The off-diagonal elements are calculated by the Wolfsberg-Helmholtz formula

where K is an empirical constant usually taken as 1.75. It may be assigned other values if one attempts to fit calculated and experimental data. An alternative procedure for calculating off-diagonal Hamiltonian elements is provided by Hij =

3 Sjj(2 - ISjjI) (Hji + Hjj),

(8)

the formula of Cusachs and Cusachs (17). Here there is no undetermined constant. The secular equation (2) is solved by applying standard matrix diagonalization procedures programmed for most computers. The eigenvalues obtained are the energy levels of the system ( E ~ ) , and the eigenvectors are the coefficients (Cjj) used in EQ. (1). Using this result, the total energy in an EH procedure is calculable from

E=

CgiEi, i

(9)

6

ROGER C. BAETZOLD

in which g i is the number of electrons in molecular orbital i, and lowest molecular orbitals are filled first. Other procedures for calculating energy have been employed since, in order for Eq. (9) to be derivable from Hartree-Fock theory, electron-electron and nuclear-nuclear repulsion must cancel. Although this condition is not always met, Goodisman (18) has justified the use of Eq. (9) to calculate binding energies based on the isoelectronic principle often used in chemistry. The electron distribution in a system is calculable using the Cii coefficients. The Mulliken procedure (19), based on the fact that the integral of $*$ represents an electron density, defines the charge on atom r as

where m and n refer to atomic orbitals on atoms r and s, respectively, and gi is the number of electrons in molecular orbital i. Another definition of charge has been proposed by Lowdin (20) for use when the atomic wave functions, i.e., xi in Eq. (9,are orthogonal. This corresponds to the situation of Smn = 0, m # n in Eq. (10). The relative merits of the two approaches have been discussed by Cusachs and Politzer (21), but since nonorthogonal basis functions are typically used, the Mulliken procedure is more often used.

2. Approximations The approximations used in EH theory have included (1) neglect of core electrons, (2) use of atomic basis functions, (3) use of effective Hamiltonian resulting in arbitrariness in choice of matrix elements, (4) lack of explicit accounting for electron-electron and nuclear-nuclear repulsion, and (5) approximate energy calculation procedure. Although these approximations seems severe in light of Hartree-Fock treatments of MO theory, the important feature to remember is that EH theory has proved useful in several systems. The virtue of EH theory is its simplicity and ease of application to a wide variety of atoms in various geometries. It enables calculation by a defined procedure using input data chosen by defined rules, and it is therefore useful to make comparisons of similar systems. Since interactions between all orbitals in a system are included through the use of all overlap integrals in Eq. (2), no assumptions about the distance of interactions are arbitrarily introduced. 3 . Ionic Modifications

The previous description of EH theory applies when the effect of electron distribution is not taken explicitly into account. When a calculation is performed for an ionic molecule or solid, some form of interaction due t o charge

MOLECULAR ORBITAL THEORY

7

transfer may be included. This effect is usuallyincluded by modifying the diago. nal Hamiltonian elements (Hii), which, in turn, influence the off-diagonal elements (Hii). A procedure is to employ the formula

-Hii = IPi + AQ ,,

(1 1)

where Q , is the charge of the atom on which orbital i is located, and A is a constant on the order of 1 to 2 eV. This choice is based on the experimental information that changes by 1 electron of the charge on an atom may change the atom’s ionization potential by as much as 10 eV. However, opposite charges on surrounding atoms act to reduce this effect to 1-2 eV. Nevertheless, this choice of value A is empirical. When charge-dependent Hamiltonian matrix elements, as in Eq. (ll), are employed, an iterative calculation is required. An initial guess of the charges Qi in the system is made and the secular equation is set up and solved. The resulting charges calculated by Eq. (10) are used as input for the next cycle. Convergence of output and input charges to within a selected tolerance determines the final charge distribution. An alternative procedure has been employed by Corrington and Cusachs (22) to introduce charge dependence for diatomic ionic molecules. They take

-Hii = IPi + BiQi where the Madelung-type terms form the Bi are given by

+ C

V(RAB),

(12)

B#A

V ( R A B ) have

been added to Eq. (1 1). In this

Bi = IPi - EAi, where EAi is the electron affinity of orbital i. Thus, Bi has a value on the order of 10 eV and has been determined (22) for several orbitals. The V ( R A B ) term is calculable as a quantum mechanical expression which can be easily programmed. 4. Infinite System Modifications

The EH-type calculation may be extended to infinite periodic systems in addition to the finite systems. Messmer et al. (23,24) have described the calculation for infinite graphite sheets. A Bloch function is formed from the molecular orbitals (JIJ,

where the sum is over unit cells; 6 denotes a position of the appropriate atom in the Eth cell, and is the wave number. Using these Bloch functions, a secular

8

ROGER C. BAETZOLD

equation such as Eq. (2) is to be solved. The matrix elements are given by

p = -(N-1)

where R p is the displacement of the p-th unit cell from the origin and H i ; f is the Hamiltonian matrix element between orbitals p and v in cells o and p , respectively. Equations (6) and (7) are employed to determine these matrix elements. In practice, the summation in Eq. (14) is truncated to a finite number of unit cells in order to conserve computer time while taking into account all significant interactions. A separate calculation is performed for each value of E. 5 . Other Modifications Several modifications of EH theory for transition elements have been proposed, including those of Cotton et al. (25), Fenske et al. (26), and Canadine and Hiller (27). The explanation of the use of several different versions of EH theory lies in the use of an effective Hamiltonian and the attempt to identify it with the Hamiltonian used by Roothaan (28) in Hartree-Fock molecular theory. The SCF Hamiltonian is written A

H = - & V 2 -+

c V,,,, m

-3

where V2 is the kinetic-energy operator, and Vm is a contribution to the potential-energy operator caused by atom m. The diagonal Hamiltonian matrix element can be written

with orbital p on atom a. The first two terms in the integral are atomic or ionic and identifiable with the experimental ionization potential,

c

~ b M = - ~ . - + j x ; m #a

Vrn xpd7,

(1 7)

but the next term involves the crystal-field interactions with the surrounding atoms and ions. The crystal-field interaction may be ignored as in the standard version of EH, or treated as a point charge-type interaction,

MOLECULAR ORBITAL THEORY

9

where Zm is an effective nuclear charge, and rim is the electron-nuclear distance, or it may be calculated by the method of Corrington and Cusachs (22), which we have described. The choice of procedure may depend on the atoms present but it should be used consistently. In addition to the explicit dependence of diagonal matrix element on charge and surrounding ions, an explicit dependence may be derived for the off-diagonal elements. Of course, the dependence of Hit on charge effects is also transferred to the off-diagonal elements whenever Eq. (7) or Eq. (8) is used. The additional charge dependence has been found important by Canadine and Hiller (27,29) in their calculations for MnO, and transition metal carbonyls. They find the charge and orbital occupancy of the metal are influenced strongly by the use of this type of procedure.

c. COMPLETE NEGLECT OF DIFFERENTIAL OVERLAP 1. Equations

The complete neglect of differential overlap (CNDO) procedure was developed by Pople and co-workers (2) and has been widely used. It suffers from some of the same limitations as EH but makes different approximations. It is a simplification of the exact Hartree-Fock equations for a molecule. In this procedure mathematical approximations leading to neglect of “small” terms are employed rather than the intuitive approximations employed in EH. In addition, electrons having different spin are treated in this procedure. In the Hartree-Fock procedure, the wave function of the system is taken as a Slater determinant

where $ i ( j ) is the molecular spin orbital i, in which electron j is located. The electronic Hamiltonian of the system is explicitly

where 2, is the charge on atom A, riA is the distance of atom A to electron i, rij is the distance between electrons i and j , and - h Z / 2 m i 0; is the kineticenergy operator.

10

ROGER C. BAETZOLD

A solution of the Schrodinger equation is achieved by variation of the coefficients Cii to determine an energy minimum. A secular equation of the form of Eq.(2), as used in EH theory, is arrived at:

c (Fw - EiSpu)Cui=

i = 1,2, . . . ,n.

0

(22)

U

The explicit form of the matrix elements is complex and will not be detailed here. Since complex integrals are involved in these terms, a complete solution is usually not attempted except for small molecules. In the ChDO approximation the terms x i x i appearing in integrals are multiplied by the Kronecker delta, greatly simplifying the integral calculation. In addition, an empirical constant for each kind of atom (&) is introduced in the off-diagonal elements, and the similarity of molecular terms to atomic terms in the diagonal Hamiltonian matrix elements is used to introduce experimental atomic information. Although the physics of these approximations is interesting, we shall not repeat them. Instead, the final formulas derived for closed-shell systems in which the d orbitals may have variable degree of occupancy as derived by Baetzold (30) are presented: Fpp= -t

a (IPt EA)d t

1

c [(P# -

rd41"-t

B

MB)

AA rdd

(1 - ppp)

(e

- NB)

r 2I .

(23)

For s orbitals the interchange of s with d and MB with N B provides the proper formula for diagonal element. The off-diagonal elements are

F,, =

3 (p", -t &)S,

-

a

ppv

7;:.

(24)

In the preceding equations, the symbols are identified as follows: IP =atomic orbital ionization potential EA = atomic orbital electron affinity ra4fB = electron repulsion integral between d orbital on atom A and s orbital on atom B = electron repulsion integral, where p and v denote s or d nature of these orbitals MB = number of d-valence electrons on atom B NB = number of s-valence electrons on atom B

ybB

c

occ

Pw =

i=l

giC&,i

(occ refers to occupied orbitals)

MOLECULAR ORBITAL THEORY

Pf

=

11

c P,,,,

sum over d orbitals on atom B

c1

lf

=

,, P,,,,sum over s,p orbitals on atom B

The energy of the system is calculated by an expression that explicitly takes into account electron-electron and nuclear-nuclear repulsion unlike Eq. (9). The form of this equation is

1

E = i ZP,,,, Hcw+FPu t W

[

c

ZAZBe’IRAB,

(25)

B’*

where H,,,, corresponds to a resonance integral and the other terms have been identified previously. Note that the P,,,, terms require eigenvectors (C,,,,) that are obtained by diagonalization. Since these coefficients contain information on the charge distribution in the cluster, the dependence of energy on charge is built into the procedure directly.

2 . Application More input information is required to perform a CNDO calculation than an EH calculation. The same requirements for choice of atomic orbitals and ionization potentials, described before for EH, must be made. In addition, electron affinity data for each orbital must be employed and usually this is known with least accuracy. Tables of data for some orbitals have been compiled by Zollweg (31); however, in some cases these data must be estimated. The resonance parameter 05 must be chosen by some procedure for each kind of atom. Pople et al. (2) have recommended values for low atomic number elements, and the fitting of calculated to experimental diatomic molecule data has been used (30) as a criterion for 0: choice in other work. Table I lists input data that we have used for previous MO calculations.

D. PROPERTIES CALCULABLE BY APPROXIMATE MOLECULAR ORBITALTHEORY The physical properties of a cluster of bonded atoms are determined from equilibrium conditions. The potential-energy curve is constructed and the bond length minimizing this energy is taken as the equilibrium internuclear distance (Req). Other equilibrium properties are calculated as follows. Bond energy (BE): the difference in energy between the isolated atoms and the bonded atoms is taken as the bond energy. Equation (9) for EH calculation or Eq. (25) for CNDO calculation determines the energy.

12

ROGER C. BAETZOLD TABLE I

Input Data for Molecular Orbital Calculations Element

Po

Orbital

Ag

-1

cu

-1

AU

-1

Pd

-8

Na

-1

Cd

-3

4d 5s 5P 3d 4s 4P 5d 6s 6P 4d 5s 5P 3s 3P 4d

3.691 1.351 1.351 4.400 1.461 1.461 4.025 1.823 1.823 3.404 1.568 1.568 0.836 1.486 3.969 1.638 1.638 4.18 1.43 1.43

5s Ni

-7

Orbital exponent

5P 3d 4s 4P

IP

$UP + EA)

11.58 7.56 3.83 9.23 7.72 3.94 11.09 9.22 4.37 8.33 7.32 2.00 5.14 3.04 17.66 8.96 4.19

8.28 4.26 2.39 6.46 4.45 2.56 8.10 5.11 2.19 5.17 4.16 1.00 2.57 1.52 11.33 4.98 2.10 4.97 4.70 1.92

-

Ionization potential (IP): the ionization potential is taken equal to the highest occupied molecular orbital (HOMO) in accordance with Koopmans’ theorem (32). Alternatively, in CNDO the energy difference, Ecation - Eneutral,

(26)

is a measure of IP. Electron affinity (EA): the electron affinity is frequently taken as the lowest unoccupied molecular orbital (LUMO) or the energy difference,

Eneutral- Eanion

9

(27)

in CNDO calculations. Excitation energy (AE): the excitation energy required to promote an electron from ground t o excited state is

HOMO - LUMO. Alternatively, in CNDO it may be calculated as the energy difference, Eexcited - Eground. state

state

(28)

MOLECULAR ORBITAL THEORY

13

Vibrational frequency (We):the vibrational frequency is determined by fitting a harmonic or other empirical potential function to the calculated potential-energy curves. Atomic charge (Q): the atom charge is calculated according to Eq. (lo), employing coefficients Cij determined by calculation. Overlap population (QAB): the reduced overlap population is given as QAB=

C

C

PpvSpv,

pon von atom A atom B

where the symbols have the meanings given earlier. It is a measure of the covalent bonding between atoms A and B. E. PERSPECTIVE Computer programs for performing EH and CNDO calculations are generally available from such organizations as the Quantum Chemistry Program Exchange (33). The choice of which program to use for a particular problem is arbitrary since few hard and fast rules can be made concerning the relative merits of the two procedures. Each investigator may have a preference for a particular version of the calculation and each may employ somewhat different input parameters. The problem of attempting to apply semiempirical calculations to catalytic and surface phenomena should not be minimized. The calculation is performed for a well-defined model which is a representation of an ill-defined experimental situation. The experimental system in the case of catalysis is seldom specified in detail such as surface structure, surface composition, site of reaction, ratedetermining step, or a multitude of other factors. This lack of definition is an experimental and theoretical limitation. The objective of these theoretical investigations in the field of catalysis is to gain a general understanding of the phenomena, It is apparent that this must be the objective at this point because the calculational approximations and model approximations discussed above provide definite boundaries to one’s expectations. On the other hand, a good qualitative description of catalysis is useful for many types of reactions.

F. COMPARISONSWITH EXPERIMENT 1. Diatomic Molecules

The reliability of MO calculations for metal atoms can be judged by application to homonuclear diatomic molecules. Experimental electronic properties have been measured using mass spectrometers for many such molecules. Dimers

14

ROGER C. BAETZOLD

and, in some cases, larger clusters exist in the gas phase above melts of many metals. Thus, the calculation is tested or calibrated with data for these molecules. A recent EH-type calculation by Hare et al. (34) and Cooper et al. (35) has been applied t o diatomic transition metal molecules. Input data were chosen from previously explained procedures to determine which input data sets give the best fit t o experimental data. The offdiagonal Hamiltonian elements were calculated using Eq. (8). A comparison of calculated and experimental data for transition element diatomics is shown in Table 11. Although some discrepancies are apparent, the procedure seems qualitatively correct for these molecules. A comparison of EH and CNDO with experimental data has been made by Baetzold (30) for other metal homonuclear diatomic molecules. This work has employed the orbital exponents of Clementi et al. (10, 11) and experimental atomic data for ionization potentials. Table I11 lists representative data for transition metal molecules calculated by CNDO and EH. No one procedure is universally superior t o another. A comparison of data calculated b y EH and CNDO with experimental data for metal homonuclear diatomic molecules has been made by Baetzold (30). Employing the input data of Table I leads t o the data compiled in Table 111. Calculated binding energies, excitation energies, and ionization potentials generally agree better with experiment than calculated bond lengths or vibration frequency. The observation of lower ionization potential for Ag2 than for Ag (also Cu2, Au2) is predicted by CNDO but not by EH.

TABLE I1 Properties of Diatomic Molecules

Molecule

Method

Dissociation energy (eV)

sc2

Calc EXP Calc EXP Calc EXP Calc EXP Calc EXP

1.25 1.13 2.6 1.9 2.45 2.37 0.36 0.29 5.0 1.0

Cr2 Ni2 Zn2 Fez

Bond length (A)

Ionization potential (eV)

Vibrational frequency (cm-')

2.20 2.50 1.90 2.22 2.2 1 2.30 2.60 2.50 1.25 2.22

5.5 5.7 7.9 5.8 8.9 6.6 7.1 8.4 8.6 5.9

250 230 300 400 370 3 25 100 -

365

15

MOLECULAR ORBITAL THEORY TABLE 111 Calculated vs. Experimental Data for Transition Metal Diatomic Molecules

~

Quantity

Eq. (7) K = 1.30

Eq. (7) K = 1.75

Eq. (8)

CNDOO po = -1

Experimental

2.1 1.74 8.5 1 3.35 410

Ag2 molecule 2.1 3.2 4.80 2.60 9.86 9.86 4.58 2.98 735 313

3.0 2.60 7.23 3.80 500

2.5 1.63 <7.6 2.7 192

1.75 1.60 8.50 2.60 450

Cu2 molecule 2.1 4.14 9.21 0.24 390

3.0 2.17 7.21 3.07 510

2.22 1.98 <6.7 2.70 26 5

Au2 molecule 2.75 3.24 10.6 8 3.65 328

3.25 2.46 7.67 2.87 5 00

2.47 2.24 <9.2 2.4 142

Pd2 moleculeb 2.00 1.25 7.94 0.37 264

2.50 1.40 9.22 0.10 410

2.57 1.10 < 8.3 344

-

2.1 2.0 10.26 3.57 464

-

1.75 1.30 7.92 0.21 346

-

"CNDO = complete neglect of differential overlap. bFor Eq. (7), K = 1.44 and for CNDO, po = -8.

2. (CuCl), Clusters Recent experiments (36,37)using the mass spectrometer have provided values for the electronic properties of (CuCI), clusters. The CNDO procedure has been applied to these clusters by Baetzold and Mack (unpublished data) to determine whether trends in a series of similar clusters would be reproduced. Input parameters were chosen by fit to experimental data for Cu2 and Clz molecules. Geometries are predicted with general accuracy although fine detail may be missed. For example, a regular hexagon is predicted as the low-energy

16

ROGER C. BAETZOLD

form of (CuC1)3, whereas experimentally a distorted hexagon is the structure observed. The binding energy per CuCl unit within the (CuCl), series is predicted within 10-20% accuracy up to n = 5 . The calculated ionization potential is near constant at 10 eV for the series and is within 1 eV of the experimental values. The experimental trends are reproduced by CNDO, thus justifying its usefulness for such systems.

I 11. Metal Clusters A study of the electronic nature of metal clusters is a difficult theoretical and experimental problem. Information on small isolated clusters has been obtained with the field-ion microscope, mass spectrometer, or electron microscope, but other techniques are needed for such studies. The semiempirical MO procedure is applicable to this problem and can provide a great deal of detailed information to compare with experiment concerning the electronic structure of the metal cluster. The transition in properties from single atom to bulk metal is a region of interest. The question of the transition point at which the properties of clusters begin to resemble bulk properties is of interest, and one may speculate whether all properties become bulklike at the same size. This question, of course, is sig nificant for catalysis since an optimization of catalytic activity with dispersion or particle size is desired to achieve the most efficient catalyst. It is also important for phenomena involving nucleation since the bond energy and surface energy increase with increasing nucleus size. These opposing energies change until the critical nucleus size is reached, at which point the free energy change for further growth becomes negative. Calculations have been performed on atom clusters by Hoare and Pal (38) and Burton (39) to determine their geometrical and thermodynamic properties using empirical potential functions. In this technique the pairwise potentials V(Rij) between atoms, a distance Rij apart, are assumed additive to give a potential energy V(Rij).

E= i> j

Potential functions that have been used include the Morse type or the LennardJones type. The potential function is generally minimized by the greatest number of nearest-neighbor bonds. A relative free energy for clusters of a given size is obtained by employing the potential function and calculating the entropy (S) of the cluster. To compare various geometries, only the entropy component from vibration needs to be cal-

MOLECULAR ORBITAL THEORY

17

culated. The latter is found from the vibrational partition function for a cluster

(a,

where = l/kT, Eo = zero-point vibration energy, and ui = vibrational frequency. The vibrational frequencies are determined by normal coordinate analysis leading to a free energy (C), G = E - TS.

(33)

Burton (39) has calculated properties of Ar clusters containing up to 87 atoms. He finds that the viirational entropy per atom becomes constant for about 25 atoms. The entropy per atom for spherical face-centered cubic structures exceeds that of an infinite crystal and reaches a maximum between 19 and 43 atoms. An expression for the free energy ofthe cluster as a function of size was derived and shows that the excess free energy per atom increases with cluster size up to the largest clusters calculated. Although this approach yields valuable thermodynamic information on small clusters, it does not give electronic information. A. SILVERCLUSTERS 1. Potential-Eneey Curves Electronic properties of small silver clusters have been calculated by Baetzold (40) using the EH and CNDO MO methods. The Ag valence orbitals are occupied according to 4d"5s15p in the atom. It has been found that 5 p orbitals are required in the calculation even though they are not occupied in the atom, in order to prevent the formationof severely destabilized 5s antisymmetric molecular orbitals. The 5s orbitals account for most of the bonding in the silver cluster, but they do not provide a repulsive force in the EH-type calculation for small Ag clusters: the 4d orbitals achieve this effect. In Ag clusters containing 5 or more atoms, a repulsive force is induced through the antisymmetric 5s orbitals. Calculations reported here employ only the 5s and 5p orbitals for the large Ag clusters. Potential-energy curves for Ag2 are shown in Fig. 1 as calculated by MO techniques. Data in Table 111 are derived from the properties calculated at the minima of these curves. All orbitals including 4d, Ss, and 5 p are included in this calculation. As the data in Table I11 indicate, EH is better than CNDO in predicting R, and BE, and the CNDO procedure predicts a longer-range interaction. For IP, CNDO is more accurate. The IP of Ag2 is experimentally less

18

ROGER C. BAETZOLD

1

I

I

I

I

I

2

4

6

8

10

Internuclear distance (A")

FIG. 1. Potential-energy curves calculated for Agz : curve A-extended Hiickel (EH) calculations, K = 1.3; curve B-EH, Eq. (8); curve C-complete neglect of differential overlap.

than the ZP of Ag atom, and only the CNDO calculation is in accord with this trend.

2. Geometry In calculations, a choice must be made of the geometry to be employed for the cluster. This was accomplished by determining potential-energy curves for the common conformations of atoms for each cluster size. It has been found that linear chains are more stable than other conformations of small neutral silver atom clusters. The stability of straight linear Ag chains is explained by examining the contributions of 5s orbitals to the molecular orbitals of clusters. Antibonding levels are occupied in the clusters since some molecular orbitals less stable than the atomic level are filled. In these states, the parity of orbitals on adjacent centers is opposite so that the proximity of atoms destabilizes the level. The greater proximity of atoms in symmetric aggregates than in linear aggregates leads to greater destabilization of the antibonding levels. Figure 2 illustrates this effect for Ag3 : each antibonding HOMO is half-occupied, and the destabilization for the triangle offsets the added stability of the lowest level. A comparison of the bonding energy per atom (BE/n) for silver clusters of one- and three-dimensional structure is made in Fig. 3 . The stability increases in the order 3d < 2 d < Id in this size range. The geometries treated were straight-chain linear, square planar, and symmetric three-dimensional structures.

19

MOLECULAR ORBITAL THEORY A9 3 Triangle Energy Levels

Ag 3 Linear

Molecular Orbitals

Energy Levels

Molecular Orbitals

FIG. 2. Occupied 5s molecular orbitals for Ag3, linear and triangle. The atom positions are denoted by black dots, and the ‘‘plus’’ and “minus” refer to the sign of the atomic orbital in the molecular orbital.

I

I

Ag Clusters

I

to

I

I

I

40

60

Number of atoms

FIG. 3. Bonding energy per atom for Ag clusters vs. size.

The last large structures are built on the face-centered cubic (fcc) geometry illustrated in Fig. 4. Shells of neighboring atoms equally spaced from the central atom are added resulting in 13-, 19-, 43-, and 55-atom clusters. Bond energies per atom for other three-dimensional geometries are not increased over the linear geometry. The trend in Fig. 3 shows BE/n t o be constant at -0.94 eV for straight chains above about 10 atoms. At smaller sizes, the zigzag behavior indicates even-size clusters to be more stable than odd. The increase in BE/n with n for the fcc structures suggests that this geometry will become more stable than the straight chains at sufficiently high n. The two-dimensional square planar clusters have intermediate values of bond energy.

20

ROGER C. BAETZOLD

FIG. 4. Thirteen-atom cluster model for facecentered cubic geometry.

3. Size Effects The increased bond energy of linear over three-dimensional silver clusters contradicts the proposition that more bonds in a particular size cluster give greater stability. The interaction energy calculated by molecular orbital theory is not additive pairwise. For example, Ag, has a total bond energy of 1.74 eV, whereas that of linear Ag3 is 1.89 eV. In Ag4, the total bond energy 3.76 eV also does not reflect the 3 :1 ratio of bonds with Ag,. The greater stability of small linear Ag clusters compared to three-dimensional ones is due partly to the shorter bond length in the former. For example, the potential-energy curves of Fig. 5 show the bond length in linear Ag, is 0.5 A less than in cubic Age. This trend also holds true for the other cluster sizes. In each conformation, the equilibrium bond length increases with size. For linear clusters, it increases I

I

I

I

1

-3 -

-

2- - 5 -

-

c

0

w

-

I

W

-7

-

t I

1 2

3

Internuclear distonce

6)

4

FIG. 5. Potentialenergy curves for Ag, in cubic and in planar geometry.

21

MOLECULAR ORBITAL THEORY TABLE IV Calculated Properties of Age Clusters Linear Parameter

Re,

(a)

BE (eV) IP (eV) EA (eV)

Re,

(a)

BE (eV) IP (eV) EA (eV)

K = 1.30 2.25 3.54 7.93 5.85

2.75 2.65 2.85 2.85

Tetrahedral CNDO" Ag4 3.0 4.96 6.85 -

K = 1.30 3.25 0.27 6.30 6.30

CNDO~ 3.50 4.00 5.94

-

Ag4 5.5 0.45 6.95 6.95

A&+ 2.25 3.85 8.50 8.50

2.50 2.55 7.0 7.0

uCNDO = complete neglect of differential overlap.

from 2.1 A for Ag, t o 2.65 A for Ag,, ,where it becomes constant with further size increase. It approaches the value 3.25 A for the large three-dimensional structures, compared to the nearest-neighbor distance in bulk Ag of 2.88 A. The electronic properties of Ag4 as well as its ionized forms have been examined in detail by CNDO and EH, as shown in Table IV. Both procedures predict the linear form to be the stable neutral cluster, but as the cluster loses electrons the tetrahedral geometry becomes more stable. This is because the symmetric molecular orbitals are lower in energy for the tetrahedral than linear geometry and only these would be occupied as the cluster loses electrons. These effects are in accord with the electron spin resonance (ESR) experiments of Eachus and Symons (41) on the cationic forms of Ag4 clusters in frozen glasses. A bond elongation occurs when electrons are added t o the neutral cluster. This occurs particularly for the tetrahedral geometry but also for the linear geometry. The monoanions of Ag are calculated to be stable with the exception of Agz- (i.e., the reaction 3 Ag +. Agz- t A 6 requires energy). It is calculated that Ag,- is unstable by 0.5 eV, but Ag; is stable by 1.33 eV. The electronic properties of linear Ag particles may be deduced as a function of size in Fig. 6. The HOMO, which indicates the ionization potential, moves

22

ROGER C. BAETZOLD

-5

x

P W

-9

FIG. 6. Dependence of energy bands on number of atoms in linear Ag cluster. LUMOlowest unoccupied molecular orbital.

toward the vacuum level as size increases. The zigzag behavior indicates higher ZP values for even- rather than odd-size clusters. The EA value or LUMO shown in Fig. 6 increases with size in a zigzag fashion. Odd-size clusters have the largest EA. Since the EA of isolated Ag atoms is measured as 1.3 eV by Hotop and Bennett (42), the clusters have better electron-accepting abilities than the single atom. The electronic properties of silver aggregates change from a semiconductor type at a few atoms toward a metal type at several atoms. In a metal the ZP and EA are equal and become the work function (i.e., spacing of a few kT between energy levels). Figure 6 shows the energy gap between HOMO and LUMO as a function of aggregate size for linear aggregates. Apparently the ZP and EA are converging t o a value =7 eV. These results indicate a longwavelength shift in light absorption as particle size increases. This effect is consistent with experiments by Comes (43) on the absorption of Ag particles as size changes. The d states have little interaction with the s states since they deviate little from the atomic energy level - 11.58 eV. The dependence of the electronic structure of three-dimensional Ag clusters on size is shown in Fig. 7. Several trends are comparable t o the trends observed for linear Ag particles. The HOMO and LUMO converge t o -6 eV at the size range 30-50 atoms with a level spacing of 0.1 to 0.2 eV. The lowest 5s MO drops in energy with increasing size, unlike the behavior observed for linear Ag clusters. The occupied band width at 55 atoms is -5 eV. This lowest state (- 11 ev) has dropped to an energy that should overlap d-orbital states since the latter are spread to almost the required degree in Fig. 6. This property would not be found in the linear clusters.

MOLECULAR ORBITAL THEORY

1\

23

$=-

-12'

LUMO

I

10

I

30

50

Number of otoms

FIG. 7. Dependence of energy bands of three-dimensional Ag clusters on size. LUMOlowest unoccupied molecular orbital; HOMO-highest occupied molecular orbital.

4. Density of States

The density of states for large linear and three-dimensional Ag clusters is quite different. Figure 8 compares the states found in hexagonal fcc Ag,, and linear Ag3,,. Note the greater spread of the occupied states for the fcc structures. The state density decreases with energy separation from the lowest level for the one-dimensional structure and tends to increase slightly with energy separation for the fcc structure. This behavior is typical of free elec-

Energy

(ev)

FIG. 8. Number of energy levels in 0.5eV interval vs. energy for Ag27 hexagonal facecentered cubic geometry (solid curve) and Ag30 linear geometry (dotted curve).

24

ROGER C. BAETZOLD

tron models such as those discussed by Kittel (44). In the one-dimensional case the state density is proportional to E-'I2 ;whereas for the three-dimensional case it is proportional to E l l 2 . Apparently, the electrons in these clusters behave very much as free electrons do. This is not surprising since the free electron model works well for bulk Ag. 5 . Wave Functions Wave functions calculated for the lowestenergy levels of linear Aglg are shown in Fig. 9. The coefficient of the 5s orbital for each atom is plotted versus atom number along the chain. It is apparent that the electrons in the lowest-energy level are spread almost uniformly throughout the cluster. As energy increases, the uniformity decreases since nodes appear in the wave function. The uniform spreading of electron density would be predicted by the free electron model.

6 . CNDO Properties The predictions of EH described previously for Ag clusters are confirmed b y the CNDO-type calculation. The linear geometry is more stable than other geometries when 4 d , 5s, and 5p orbitals are included in the calculation. The data calculated by CNDO in Table V for linear Ag particles confirm the behavior of ZP, EA, BE/n, and AE with size found earlier b y EH calculation. The binding energy per atom is greater for even- than for odd-size particles. The band gap r

I

1

I

I

FIG. 9. Coefficient of 5 s orbital in lowest s molecular orbital (MO)($ 11, next highest MO ($2), and next highest MO ($3) vs. atom in Ag19 chain.

25

MOLECULAR ORBITAL THEORY TABLE V Properties of Linear Silver Aggregates Calculated by CNDO" Aggregate size

BE/atom (eV)

IP (eV)

A E l (ev)

1.30 0.78 1.24 0.97 1.23 1.01 1.13

7.23 4.20 6.85 4.29 6.94 4.24 6.04

3.80 3.39 3.14 2.71 2.91 2.4 1 2.28

aCNDO = complete neglect of differential overlap. Bond lengths = 3.0 A.

becomes smaller with increasing particle size, but it has a value larger than that calculated by EH for comparable-size clusters. The ionization potential decreases with size and approaches the bulk value much faster than comparable quantities calculated by EH.

I . Infinite A g Clusters The EH calculation was applied t o infinite Ag clusters using the procedure described in Section II.B.4. Cubic and linear Ag8 unit cells were employed to calculate the energy bands shown in Fig. 10. The number of degenerate

-12

Linear Ag

w

0

k

1 7r/a

0

Cubic Ag

k

FIG. 10. Band structure of linear and cubic Aga.

(Ill) 7r/o

26

ROGER C. BAETZOLD

levels are labeled along each energy band in the figure. The input parameters chosen were the same as those used for the 5s and 5 p orbitals of the largest silver clusters. The cubic structure has a BE/n = 1.5 eV averaged over the (1 11) direction in k space compared to the value 0.9 eV obtained for linear Ag. These values confirm the extrapolation of the trends in Fig. 3, suggesting that a critical size exists for the transition from linear to three-dimensional as the most stable form of Ag. The parabolic dependence of energy band on k expected by the free electron model is not observed in Fig. 10. Different choices of orbital exponent giving less diffuse atomic orbitals promote such a behavior, but more work is needed t o determine the criteria for choosing parameters in the infinite-model EH theory. Other choices of orbital exponents also predict cubic geometry t o be more stable than linear.

8. Other Group IB Metal Clusters The properties and trends calculated for silver aggregates are similar to those obtained for Cu and Au clusters. The data in Table VI show that the number of s states below d states is in accord with the energy difference between d and s atomic orbitals which decreases in the order Ag > Au > Cu. A comparison of the ionization potential and electron affinity of the 19-atom fcc cluster shows that IP increases in the order Ag < Cu < Au. Electrons are more easily transferred to clusters with high EA and are removed with greater difficulty from clusters of high IP. The gap between unoccupied and occupied states shows that Cu has greater metallic properties at this size range. The binding energies per atom calculated using K = 1.75 indicate values about half the cohesive energy of the bulk metal.

TABLE VI Extended Huckel Calculation of Properties of M19 Face-Centered Cubic Clustersa

Metal ~~

~~~~

Ag

cu

Au

BE/n (eV)

HOMOb (eV)

LUMOC(eV)

A E (eV)

No. of s states below d states

1.68 1.54 1.44

6.93 7.48 8.59

6.32 7.19 8.17

0.6 1 0.29 0.42

1 4 4

~

~~

aK = 1.75. bHOMO = highest occupied molecular orbital. =LUMO = lowest unoccupied molecular orbital.

MOLECULAR ORBITAL THEORY

27

B. PALLADIUM CLUSTERS 1. Geometry The valence orbitals of the Pd atom have the electronic configuration 4dl05s5p. All of these orbitals have been included in the calculations, and it is found that because of the greater diffuseness of the 5s orbitals, practically all of the bonding is due to them. The 4d orbitals, being less diffuse, form a narrow band, as opposed to the 5s orbitals, which form a band several electron volts wide. Several geometries of Pd clusters have been examined, and three-dimensional symmetric has been determined as the most stable form of small cluster. The data in Table VII, obtained by using EH and K = 1.44, are calculated for symmetric geometries and large fcc clusters. Parameter BE/n increases with size of Pd cluster and indicates a trend to aggregation, although the value is much less than the bulk cohesive energy, 3.9 eV. This trend is similar to data for Ag clusters as well as the lengthening of equilibrium bond with size.

2 . Size Effects The ionization potential and electron affinity of Pd clusters converge to a common value at small size, as shown in Table VII. The HOMO (I.) and LUMO (EA) are composed of antisymmetric d molecular orbitals in both cases and, TABLE VII Electronic Properties of Palladium ClusterP' _ _ _ _ ~ ~

~~

Size

Structureb

Req (A)

dHoles peratom

BE/n (eV)

HOMOC (eV)

LUMOd (eV 1

2 3 4 5 6 8 9 13 19

Linear Triangle Pyramid Bipyramid Square pyramid Cube bcc Cube fcc fcc

1.75 2.30 2.10 2.30 2.30 2.40 2.75 2.50 2.75

1 .oo 0.67 0.52 0.4 1 0.35 0.35 0.40 0.62 0.42

0.60 0.80 0.74 0.67 0.55 0.56 0.47 1.19 1.29

8.20 8.03 8.01 8.11 8.08 8.14 8.25 8.19 8.27

7.25 7.85 8.02 8.1 1 8.04 8.14 8.25 8.19 8.26

aExtended Hiickel method; K = 1.44. bbcc = Body-centered cubic; fcc = face-centered cubic. CHOMO= highest occupied molecular orbital. dLUMO = lowest unoccupied molecular orbital.

28

ROGER C. BAETZOLD

owing t o the narrow spread of these states, are very closely spaced. In silver aggregates, a similar close spacing is present only when *55 atoms are present. 3. d Holes

The behavior of energy bands in Pd aggregates is shown as a function of size in Fig. 11. Several states are located within the occupied band. The lowest state in this band is almost exclusively 5s in character even though these orbitals are higher in energy than 4d-energy levels in the atom. The position of the nexthighest s state drops in energy as size increases until, at 8 atoms, it becomes filled with 2 electrons. At the same size, a level with 4d character becomes unoccupied, increasing the number of holes in the d band. For example, in cubic Pd8 the HOMO and LUMO d o not contain any contributions from s orbitals. There are, however, several unoccupied states above LUMO that do contain s orbitals. Electron transfer t o these unoccupied d states is more likely than t o s states because of the greater directional property of the d orbital enabling greater overlap. Paramagnetism observed in bulk Pd is attributed t o the presence of holes in the Pd d band. The EH and CNDO calculations are in agreement with this finding. The calculated values of d-band holes are close t o bulk experimental values: 0.36 d-band hole/atom determined by Kimura et al. (45) or 0.60 dband holelatom determined by Wohlfarth (46). In the property of orbital occupancy, small Pd aggregates are very similar t o bulk metal.

0

-4

Isolated

-8

1--;, ''; ,:

I,

C

W

,

I

Bonded

Isolated

F,iIIed,stat;

" " " " " ' " ' ' . ~ . ...... ~ .,.~ ,,,~ ,,,,~ ,,,, ~ ,, ,,~ ,,,~ ,,,,~ ,,,, ~,,,

3

5

7

Number of Pd atoms

FIG. 11. Energy levels for Pd clusters vs. size (lowest filled state is s type).

29

MOLECULAR ORBITAL THEORY Y

0

z = 0 , 2 a , 4 a ...

0 z = a,3a,5a

...

El ,

FIG. 12. Model of Pd15.

* x (0)

Pd atoms at 2 = 0 , 2 a ; (0)Pd atoms at 2 = a (u = 2.35 A).

4. Electron Distribution

The electronic structure of Pd has been further examined by employing the 15-atom model shown in Fig. 12. In this model a distribution of atoms with different numbers of nearest neighbors is examined because it offers the opportunity to investigate surface vs. bulk phenomena. Atom number 9 has 10 nearest neighbors and will be classified as bulk, whereas atom number 4 has 3 nearest neighbors and will be classified as surface. In this cluster we find that the bulk atom becomes positively charged relative to the surface atom and has a greater number of vacant d states. The density of electrons at a particular energy for surface and bulk atoms can be calculated according to the Mulliken procedure outlined in Section 1I.B. 1. The data in Fig. 13 show a different electron distribution on surface and bulk atoms. The bulk electron density is split-the maximum in surface electron density occurs at the minimum in bulk density. This behavior indicates that the bulk electron distribution is different from the surface electron distribution. C. CADMIUM CLUSTERS Cadmium clusters have been treated by Baetzold (47)using EH and CNDO calculations. With atomic valence electron configuration 4d1'5s25p, the clusters are calculated to be weakly stable. Linear geometry is more stable than symmetric three-dimensional geometries or even the bulk crystal structure for small Cd clusters. Poor stability is a consequence of the closed atomic 5s shell in Cd. Unstable antisymmetric 5s molecular orbitals are filled in the small clusters, but the amount of bonding by 5 p orbitals increases with size. This leads to the trend of increasing stability with size as observed in Table VIII. Compari-

30

ROGER C. BAETZOLD

3 ’ Bulk atom

.....,,.

t

Surface atom

0.4

0.0 -12.0 0

Energy levels

FIG. 13. Density of electrons on surface and bulk Pd atoms vs. energy. TABLE VIII Extended Hiickel Calculationsfor Cadmium Clustersa Cluster

Geometry

BElatom (eV)

Req (A)

IP (eV)

EA (eV)

2 3 3. 4 4 5 5 15 15 28 28

Linear Linear Triangle Linear Pyramid Linear Bipyramid Linear Hexagonal close-pack Linear Hexagonal close-pack

0.29 0.43 0.40 0.64 0.4 1 0.14 0.02 0.96 0.19 1.01 0.23

2.0 2.0 2.25 2.25 2.50 2.25 2.15 2.50 2.50 2.50 2.75

6.30 5.12 7.18 5.68 5.62 5.15 5.52 5.68 6.11 5.69 5.82

5.38 5.12 5.81 5.68 5.42 5.15 5.30 5.68 5.11 5.69 5.17

2 Bulk

Linear Hexagonal close-pack

4.1

4.1

Experimental data 0.09 2.46 1.16 2.91

-

“K = 1.90. son of calculated data with experimental data shows that the cohesive energy of large linear Cd clusters is close t o that of bulk Cd, although the hexagonal closepacked (HCP) structure is calculated t o be much less stable. The ionization potential has decreased from the Cd atom value of 8 9 6 eV t o within l .5 eV of the bulk Cd work function and the gap between HOMO and LUMO is approaching kT.

31

MOLECULAR ORBITAL THEORY

The electronic configuration of the large 28-atom HCP structure is calculated as p 0 .5 5 , indicating that p orbitals have participated significantly in the bonding.

4d105s1.455

D. NICKELCLUSTERS Nickel clusters behave similarly to Pd clusters. The Ni atomic configuration (3d84s2) indicates the LUMO is expected to be of d character. Symmetric

three-dimensional geometries are the most stable geometry, and data in Table IX indicate that trends in BElatom, IP, and EA calculated by EH are similar to those described before for Pd and other metal clusters. The set of d orbitals of Ni clusters have smaller fractional occupancy than the set of d orbitals of Pd clusters, although LUMO in both cases hasd character. The CNDO calculations for Ni clusters shown in Table IX offer some interesting comparisons with EH data. The CNDO data predict much larger bond enerTABLE IX Electronic Properties of Nickel Clusters No. of atoms

Geometry

2 3 4 5 6 8

Linear Equilateral triangle Pyramid Bipyramid Square bipyramid Cube

2 3 4 5 6

Linear Equilateral triangle Pyramid Bipyramid Square bipyramid

2 Bulk

Linear ( R = 2.30 A) fcce (R = 2.49 A)

BElatom (ev)

HOMO" (eV)

LUMOb (eV)

d Holeslatom

EHC-R = 2.5 A 1.31 1.63 1.34 7.63 1.24 1.63 1.11 7.63 1.03 7.63 1.01 7.63

1.63 7.63 1.63 7.63 1.63 1.63

1.0 0.67 0.50 0.40 0.36 0.70

CNDOd-R = 2.15 A 0.98 1.61 3.9 1.29 3.1 1.96 6.2 6.93 6.1 6.20

2.32 3.09 3.18 4.08 4.43

1.0 2.0 2.0 2.3 2.5

-

-

Experimental 1.2 6.4 4.44 4.5-5.2

aHOMO = highest occupied molecular orbital. bLUMO = lowest unoccupied molecular orbital. CEH = extended Hiickel (calculations). dCNDO = complete neglect of differential overlap. efcc = face-centered cubic.

-

32

ROGER C. BAETZOLD

gies and d holes per atom than the EH calculations. In addition, the HOMO and LUMO are more sensitive t o size as calculated by CNDO. These conflicting predictions are an indication of the difficulty in choosing parameters to fit nickel correctly. Recently, Wan and Tong (48) have performed an EH-type calculation for the d band of Ni using periodic boundary conditions. Using two Slater orbitals for each d orbital, they found good agreement with previous ab initio calculations for bulk Ni. The d band was calculated to be 2.48 eV wide, and the ionization potential was 6.38 eV. Despite the fact that no s orbitals were included in the calculations, the results suggest that EH is as reliable a procedure as more expensive ones for the calculation of bulk Ni properties.

E. Low ATOMICNUMBERCLUSTERS 1. Lithium

Lithium clusters have been a popular model for the calculation of metal properties because of their low atomic number. Lasarov and Markov (49) used a Huckel procedure to determine the properties of a 48-atom Li crystal. They found a transition t o metal properties with the binding energy per atom a p proaching 1.8 eV at 30 atoms. The ionization potential approached the bulk value since some electrons occupy antibonding molecular orbitals, as observed for Ag clusters. The calculated properties of the largest cluster were not those of a bulk metal. Stoll and Preuss (50) have examined Li clusters using a SCF-MO calculational procedure. Although they found convergence problems for the large clusters, chains were more stable than layer structures or three-dimensional crystal structures for the smaller clusters, and BE/n is an increasing function of size. The width of the occupied part of the conduction band was in the order 3d > 2d > 1d, as described for silver clusters. The lowest state in the conduction band also drops in energy with increasing size. The calculated work function is within 10% of the experimental bulk work function. 2. Carbon Rings and chains of C atoms up to 18 atoms have been examined b y Hoffmann (51). He found chains to be more stable than rings up t o about 10 atoms, and then on rings have better stability. There is an odd-even alternation in the BE/n observed for C clusters. The gap between HOMO and LUMO was found t o diminish with increasing size of the cluster. These properties are similar t o the ones described earlier for Ag clusters.

MOLECULAR ORBITAL THEORY

33

3. Sulfur Meyer and Spitzer (52) have calculated the electronic properties of rings and chains of sulfur atoms. An EH procedure was employed with parameters chosen t o fit the energy levels of Sg. For S4 it was found that the tetrahedron was only weakly stable, with branched chains and zigzag chains more stable. The calculations predict that above S5 the gas-phase species are cyclic. In addition t o these geometry effects, a long-wavelength shift was observed for light absorption b y the chains.

F. SILVER-PALLADIUM CLUSTERS Alloys are employed as catalysts because there is the opportunity for continuous variation of their electronic properties with composition. A small number of metal atoms having various compositions have been employed t o simulate alloys using the MO-type calculations. Whether such a model can be an adequate representation for bulk alloy particles is questionable in light of the differences between properties of Ag or Pd clusters and those of bulk metals noted earlier in this paper. Therefore, these data are best taken t o represent small mixed-metal cluster particles. A mixed-metal cluster representing Ag-Pd has been investigated using the 15atom model shown in Fig. 12 as the basic structure. Properties of Ag-Pd alloys are well known, and a recent review by Allison and Bond (53)has summarized them well. It is indicated that according to the rigid band model, 1 electron is transferred from each Ag atom t o Pd, resulting in the filling of d-band holes that exist at a level of about 0.6 per atom of Pd. Thus, at 60% Ag composition, the abrupt changes observed in magnetic and catalytic properties of the alloy could be associated with the filling of d holes. The EH MO method is not in complete agreement with the rigid band picture of Ag-Pd alloys. It predicts that 0.3-0.4 electron per Ag atom is transferred from Ag t o Pd when Ag atoms are added to random lattice positions of the 15atom model. The number of d holes on bulk and surface Pd atoms in the cluster are shown as a function of composition in Fig. 14. Here bulk Pd atoms have more holes than do surface Pd atoms. The number of Pd d holes decreases with added Ag but does not equal zero at 60% Ag. The qualitative predictions of EH show that the number of bulk d holes per Pd atom changes sharply at 40%Ag. The composition where this sharp decrease takes place may vary with particle size and with the position of Ag atoms. The surface-to-volume ratio of Pd particles determines the average number of d holes per atom. It is expected that similar trends will be found in other Group IBVIII alloy systems.

34

ROGER C. BAETZOLD

U

a

-

1.0-

-

0 0

1

20

I

I

40

60

80

100

IV. Chemisor ption A.

SCOPE

The MO procedure has recently been applied t o adsorbates o n a wide variety of substrates. We shall be concerned primarily with the effect of substrate o n electronic properties of adsorbed atoms. Clusters of Ag and Pd on nonionic (carbon) substrate and ionic (AgBr) substrate have been examined for this effect. In addition, properties of small atomic number adatoms on graphite were calculated in an extensive study, which is reviewed here. Metal substrates have also been treated by MO procedure. 1. Early Work

Chemisorption is a phenomenon of importance in catalysis which may be treated by MO theory. Experimental studies have been carried out for a variety of systems, but theoretical descriptions of the electronic features of chemisorption beyond simple considerations are in a primitive stage. There are several factors responsible for this state of affairs. One is, of course, the complexity of the substrate system t o be modeled, which has forced theorists to work with a small-size representation for the surface, as implied by the surface molecule concept of localized interactions. Although some early work has been done b y

MOLECULAR ORBITAL THEORY

35

Sherman and Eyring (54, 55), it is only recently that quantum mechanical techniques have become available for the treatment of chemisorption. In addition, lack of precise experimental definition of surfaces has made it difficult t o construct proper surface models. It is expected that theoretical models of surfaces will become increasingly useful in the future owing t o the active work being performed o n these problems. Early quantum mechanical calculations treating chemisorption employed a valence bond-type method and a few atoms to simulate the surface. Sherman and Eyring (54) used a 4-atom model t o simulate Hz dissociation on charcoal. The calculations were unable to duplicate the low activation energy observed experimentally for this reaction. Later calculations by Sherman and Eyring (55) showed that Hz dissociation on Niz has a low activation energy, suggesting that quantum mechanics is a useful tool for such studies.

2 . Surface Molecule Concept The surface molecule concept used by Grimley (56) is often employed in quantum mechanical theories of chemisorption as a necessary simplification. The concept implies that adsorbed atoms interact stronglywith a limited number of atoms on the surface. Van der Avoird (57) has discussed some of the experimental evidence indicating that adsorption on transition metals is localized. He has also performed calculations treating Hz dissociation o n 2 atoms of transition metal including Ni, Pd, and Pt. These calculations indicate that Hz dissociative chemisorption requires no activation energy and, thus, lend support for the use of the surface molecule concept. The concept has also been employed by Thorpe (58) for transition-metal chemisorption on W surfaces. Here it is concluded on the basis of trends in adsorbed atom-binding energies that the surface molecule concept is useful to describe the system. 3 . General Features A general quantum mechanical description of chemisorption has been made for semifinite models containing atoms with s-type orbitals by Blyholder and Coulson (59). The Huckel procedure and a more refined SCF-LCAO-MO procedure were employed and both gave essentially the same type of results. The appearance of localized states on a substrate was investigated because such sites may form a potential site for adsorption. These states were observed on linear chains using the Huckel procedure where only nearest-neighbor interactions are included. A sufficient condition for localized states to appear on linear chains is for the Coulomb integral of an end atom [analogous to Hii of Eq. (6)] t o differ in magnitude from the Coulomb integrals of other atoms by more than one resonance integral between adjacent atoms [analogous to Hii of Eq. (7)]. In the

36

ROGER C. BAETZOLD

case of square planar substrates, a difference in magnitude equal t o three resonance integrals is required for localized states. Such a condition would be expected for some but not all impurity atoms. Adsorption of an atom to these models gave greatest bonding energies at sites of greatest free valence or number of missing bonds. However, contributions to the bond strength from nonlocalized states were important and not negligible compared to localized state contributions. This suggests that, although surface states are important in determining sites of adsorption, interactions with the other states of the substrate cannot be neglected. The conclusions reached by Coulson and Blyholder in this work form a frame of reference for MO calculations t o be examined. B. CARBONSUBSTRATES 1. Ag Clusters on Carbon The electronic properties of Ag cluster adsorbed on a C model have been examined by Baetzold (60). In this CNDO calculation, properties of interest included bond energies, ionization potential, and charge transfer with the substrate. A 10-atom C model consisting of fused hexagons was employed as the substrate. The 2 s and 2 p orbitals of C are included in this calculation as well as the 4d, Ss, and 5 p orbitals of Ag. a. Potential-Energy Curves. Carbon surface possesses sites of differing bond energy for the Ag atom. Only sites of high symmetry were examined in the calculations because of computer time limitations. The potential-energy curves of Fig. 15 indicate that the Ag atom is adsorbed most strongly over a tricoordi-

Binding energy

(ev)

12'

1.5

2.0

1

I

2.5

30

Distance Ag to nearest C

FIG. 15. Potentialenergy curves for Ag on carbon. (a) over hexagon center.

(8)

(0)Over

carbon atom; (0)over bond;

MOLECULAR ORBITAL THEORY

37

nate C atom in preference to other sites with greater numbers of nearest neighbors. A calculation of the overlap population between adsorbed Ag atoms and C atoms shows it is 4 times larger for nearest C atoms than next-nearest C atoms. The interaction falls off rapidly with distance. b. Binding Energy. Trends in the binding energy per adsorbed Ag atom have been examined by placing several atoms on the substrate. When Ag atoms are placed over C atoms, the bond energy per atom follows the trend shown in Fig. 16. This behavior indicates that when all Ag atoms are in direct contact with the lattice, aggregation is unfavored. Experiments by Hamilton and Logel ( 6 I ) , studying gas-phase nucleation of Ag on amorphous C are in accord with this effect. Greater stabilization results when three-dimensional Ag clusters are placed on the C substrate. When 5 Ag atoms are added so that only 3 are in contact with the substrate, the total bonding energy is more than 5 times the adsorption energy of a single Ag atom. Thus, the aggregate would be stable. Similar effects are found for Pd and Ni. Thus, growth of metal clusters on the plane carbon surface from gas-phase condensation is favored energetically if the incoming metal atoms form o n top of another metal atom bound t o the surface. Experimental measurements of the temperature coefficient of nucleation of Ag on carbon by Lewis (62) have given bonding energies of adsorbed Ag clusters. Values for the monomer, dimer, and trimer are 0.95, 2.8, and 4.95 eV, respectively. The calculated values, after use of the appropriate scaling factor, are 2.7, 2.4, and 4.2 eV for Ag atoms placed directly over C atoms. Although a major discrepancy is present for the monomer, the other values agree reasonably well. The experimental work gave 0.4 eV as the diffusion energy of Ag o n car-

BEfatom (eV)

0 1

2

3

4

5

6

Number of Ag atoms

FIG. 16. Average BElatom (dashed curve) and charge (solid curve) of Ag clusters adsorbed on carbon.

ROGER C. BAETZOLD m-

::

5 4 -

3 -

d states

\

4

In4I 0

5

2 -

+ Ag2

Carbon

I I I

Number of

I

J

slates

Carbon

3 0

E,

2 -

I 1 -

II

I , Ill

1

bon compared to 0.58 eV calculated for diffusion over C atoms t o hexagon tenters. Apparently diffusion across bonds on the carbon surface is a much higherenergy path. c. Charge. Silver atoms transfer charge to the carbon model. Density of state is shown versus snergy for Ag, interacting with the carbon model in Fig. 17. The HOMO level of isolated Ag is - 4.25 eV, which is in the gap between HOMO and LUMO of the carbon model. When Ag is allowed to interact with carbon, orbitals mix so that 5s character of Ag is added to HOMO and states just below HOMO. In addition, HOMO is lowered whereas LUMO does not change much. The LUMO does not contain Ag orbitals, so that additional electrons added t o the system would not go to the Ag cluster. The average charge of a cluster atom versus cluster size is shown in Fig. 16. The metal nuclei have a partial positive charge, but addition of 1 electron to the system does not completely neutralize this charge. There is an oscillation of HOMO and LUMO with size of silver cluster on carbon, as was observed previously for isolated silver clusters. Maxima in HOMO and minima in LUMO occur at odd-size particles.

2. Pd Clusters on Carbon a. Energy Levels. Palladium atoms interact with the carbon model largely by their 5s orbitals. Previous calculations have shown that isolated clusters of Pd atoms containing 2 or more atoms have d-band holes caused by occupancy of the 5s symmetric molecular orbital that lies lower in energy than the

39

MOLECULAR ORBITAL THEORY

Energy level (eV)

,

,

I

3

I

5

Number of Pd atoms

FIG. 18. Energy levels of Pd clusters adsorbed on carbon model.

4d molecular orbitals. When Pd atoms are in direct contact with carbon atoms, no d-band holes are found for as many as 6 clustered Pd atoms. An illustration of this effect (Fig. 18) showing positions of d states and s states results from

EH-type calculation. The CNDO calculations agree with the prediction. Note that in Fig. 18 s states are destabilized relative to atomic levels so that the electrons remain in lower-lying d levels. We observe a partial electron transfer to the Pd atom from the carbon lattice in CNDO calculations. The filled atomic d levels of Pd lie below HOMO of carbon and the unfilled atomic s levels lie above LUMO. b. Potential-Energy Curves. Calculations of potential-energy curves (Fig. 19) for Pd o n carbon show that the most stable position is over a carbon atom. It

Distance Pd to nearest C

(A)

FIG. 19. Potentialenergy curves for Pd adsorbed on carbon. bond; (A) over hexagon center.

Over carbon; (0) over

(0)

40

ROGER C. BAETZOLD TABLE X

Palladium Atoms on Carbona No. of atoms

HOMOb (eV)

LUMOC(eV)

BE/atom (eV)

d Holes/atom

Charge/atom

1 2 3 4

10.22 9.83 8.98 9.46

2.74 3.21 4.85 4.91

3.24 1.34 0.27 0.52

0.12 0.33 0.27 0.35

-0.27 -0.27 -0.26 -0.23

Tomplete neglect of differential overlap. bHOMO = highest occupied molecular orbital. CLUMO= lowest unoccupied molecular orbital.

is t o be noted that these curves differ from Ag potential-energy curves in that center positions have a larger R,. Calculations indicate that BElatom decreases with the number of Pd atoms, as shown in Table X. This behavior is similar t o the behavior described for Ag and indicates an inability to aggregate when Pd atoms touch carbon atoms. The data in this table indicate partial electron transfer from carbon to palladium. The trend in LUMO indicates an increasing electron affinity with size. 3 . Graphite Substrate for H , C, F, 0,and N

a. Model. The graphite surface involved in chemisorption has been examined recently in an extensive study b y Bennett et al. (63,64, 65) which has laid the groundwork for much of chemisorption theory. Both EH and CNDO procedures were employed for this problem, although these workers found CNDO t o be more reliable for adsorption of electrophilic adsorbates. The model employed was an 18-atom planar C lattice with so-called periodic connectivity boundary conditions. This model enables the simulation of an infinite two-dimensional array using only a finite-model calculation. The comparison of the properties of the infinite model with a similar finite-size model showed that the position of maximum adsorbate stability depends strongly o n the substrate model chosen. Thus, Bennett et al. conclude that models with periodic connectivity conditions are important for adsorption calculations 9 n graphite. b. Position of Adsorption. The atomic adsorbates H, C, F, 0, and N were studied on the graphite model. Adsorption positions of high stability were chosen, and it was found that H, F, and 0 adsorb over the center of a C-C bond and N and C adsorb over the center of a C hexagon. The CNDO procedure gave potential-energy curves with the deepest minima at these sites. Charge was transferred between the adsorbed atom and substrate with electrons generally flowing to the lattice at shorter bond lengths. At the potential

MOLECULAR ORBITAL THEORY

41

minima, H and N have positive charge, whereas C, 0, and F have negative charge. This prediction suggests a work-function decrease on adsorption of the former and a work-function increase on adsorption of the latter. These predictions are not in accord with simple electronegativity considerations of charge transfer on adsorption. Some general considerations learned from these studies include the fact that adatoms interact with more than one surface C atom at its position of maximum stability. The magnitude of overlap population correlates directly with the strength of bonding. In addition, the free valence of the adatom varies systematically with atomic number. c. Symmetry Principles. A study of the symmetry principles embodied in the Woodward-Hoffmann rules has led to rules for chemisorption and catalysis that have proven useful for the graphite surface (65). The principle essentially states that for chemisorption to take place with low activation energy, it is necessary that the wave functions of the reactants and surface have positive overlap. The Messmer and Bennett procedure requires the following: 1. The surface wave function is determined from the bulk band structure of the solid. 2. The wave functions so determined are expressed as a linear combination of Bloch functions. 3. Only those Bloch functions with eigenvalues close to the Fermi level are considered. 4. The symmetry properties of the reactant molecular orbitals are determined. 5. Simple chemisorption or dissociation will only occur if the wave functions involved in the electron transfer have positive overlaps. These rules have been applied to H2,N2,02, and F2 on the graphite surface.

c. SILVER CLUSTERS O N SILVER BROMIDE The electronic properties of small silver clusters chemisorbed on AgBr have been calculated by Baetzold (66) using MO theory. This problem deals with catalysis since, as Hamilton and Urbach (67) have described, the silver centers are catalysts for the chemical reduction of AgBr grains. By using various experimental techniques, they indicate that a minimum size cluster of 4 Ag atoms is required for the catalysis. This suggests that some properties of 4 bonded silver atoms are different from atomic and perhaps like bulk properties, which could account for the catalysis.

1 . Model CNDO A rectangular 12-ion model (2 X 3 X 2) was employed as the AgBr substrate for the CNDO calculations. Only the s and p valence orbitals on each ion were

42

ROGER C. BAETZOLD

,

I

0-

A g on ( A g B r & CNDO Over B r

Distance to lattice

(i)

FIG. 20. Potential-energy curves for adsorption of Ag to (AgBr)6. CNDO-complete neglect of differential overlap.

included in the calculation since these are the ones primarily involved in electron transfer. The potential-energy curves for adsorption of 1 Ag atom t o this lattice are shown in Fig. 20. Here it is seen that different sites have different adsorption energies. The bond energy to these sites increases in the order: over Br ion < over Ag ion < between 4 ions.

Electronic Properties. When the silver aggregate grows, its properties become more like those of bulk silver. These calculations for Ag atoms added at lattice positions show that a single atom on the crystal has a charge of +0.52, which is comparable t o the average AgBr lattice cation charge of t0.55 calculated by

on CAgBr)6 CNDO Lattice Ag Charge = t.55

Ag,

A

e

d

Electron trapped Number of Ag atoms

FIG. 21. Average charge on Ag atoms adsorbed to (AgBr)B. CNDO-complete neglect of differential overlap.

43

MOLECULAR ORBITAL THEORY

Mulliken analysis. Figure 21 shows the average charge o n a silver aggregate in contact with AgBr as a function of size. The average charge decreases as size increases such that for Age the total positive charge is t0.65,which corresponds roughly to Ag;. The probabilities of electron or hole capture b y the silver aggregate are also indicated by this figure and were obtained by performing the Mulliken analysis o n the anionic and cationic clusters. The small difference in charge between systems with different net charge indicates that a hole or electron is only partially localized on the added silver cluster. The electronic properties of neutral and charged silver clusters adsorbed o n the AgBr model are listed in Table XI. Here the bonding energy per atom is generally an increasing function of size for neutral and cationic clusters, indicating that aggregation of isolated atoms and cations is energetically favored. This is untrue when the cluster has a net negative charge. Comparison of bondTABLE XI

Silver Clusters on (AgBr) 6'

No. of Ag atoms

BE/atom (ev)

HOMO

1.97 3.50 3.29 3.81 3.35 3.32

9.88 5.83 8.08 5.66 7.83 5.51 6.03

-

2.11 4.29 1.86 4.15 2.30

(ev)

LUMO~ (eV)

Neutral clusters 3.85 5.83 3.42 5.66 3.89 5.51 4.95

IP (eV)

EA (eV)

Isolated cluster BE/atom (eV1

9.54 6.53 9.55 6.47 8.51 6.09

3.14 4.85 2.58 4.76 3.05 -

0 2.96 1.73 2.39 3.54 3.7 1

-

-

Anionic clusters

6.82 4.80 4.87 4.37 1.53 1.78 3.16 3.00 3.35

Cationic clusters 11.94 12.36 10.36 12.06 9.42 11.36

'Complete neglect of differential overlap. bLUMO = lowest unoccupied molecular orbital.

2.65 2.78 2.59

1.48 1.97 0.38 2.30

44

ROGER C. BAETZOLD

ing energies per atom for adsorbed and isolated Ag clusters shows that the AgBr lattice stabilizes the Ag cluster in most cases. This effect is due t o the electron donation to the lattice. The electron affinities of adsorbed silver clusters, calculated using Eq. (27), are shown in Table XI. These values indicate that electron-trapping ability of Ag and Ag3 is greater than that of AgBr with the reverse true for Ag, and Ag4. With increasing Ag cluster size, EA increases, as expected for isolated Ag clusters and shown in Fig. 7, so that at sufficient size, EA is greater for the cluster than the AgBr model. When this is true, electrons added to the crystal become localized at the Ag cluster where Ag' reduction takes place. 2. ModelEH Charge iterative EH calculations were performed for Ag atoms adsorbed to a %-ion model of AgBr containing a ledge. Using this type of calculation, a larger AgBr model is treated than in CNDO, and adsorbed Ag atoms have the opportunity t o interact with more nearest-neighbor ions. Most of the effects observed with this model were similar to those observed with the CNDO model. Silver atoms were placed on the AgBr lattice at virtual sites because no minima were found when potential-energy curves were calculated. Electronic Properties. There is a stabilization of Ag o n the lattice relative to the isolated silver particle. Stabilization results from electron transfer t o the AgBr model. The binding energy per atom is near constant and independent of size. This behavior is unlike the increasing trend in BElatom observed for isolated silver aggregates. The average charge o n the silver aggregate is plotted versus size in Fig. 22 for the charge-dependent and noniterative calculations. It is t o be noted that the results of the charge-dependent calculation agree in form with CNDO results, although the silver particle has a lower charge in EH calculation. The noniterative calculation shows that silver is strongly ionized even at a size of a few atoms. This behavior is unrealistic based o n stable aggregates of silver comprising 4 atoms. The charges on the silver aggregate after adding a hole or an electron t o the system indicate that neither of these particles is localized completely on the silver center. It is apparently the ionic and Madelung effects that result in localization of charge carriers o n the silver aggregate. The AgBr substrate controls the most stable geometry of silver aggregates through the Madelung effect. A right triangle of 3 silver atoms is found to be more stable than a triatomic straight chain. For 4 atoms, a very stable geometric configuration results for a square array with one side of the square touching the crystal and the other side above it. This is more stable b y 1.29 eV than the square array touching the crystal at four sites. These results violate the intuitive idea that the site of maximum reactivity can form the most bonds. The effect of AgBr model size on EH and CNDO calculations is minor. Calcu-

45

MOLECULAR ORBITAL THEORY

-

W

F

c

+I

-

D I

1

2

I

I

1

4

Number of Ag

I

I

I

6

8

atoms

FIG. 22. Average charge on Ag atoms adsorbed to (AgBr)14 (extended Hiickel calculation). (A = neutral, noniterative calculation; B = hole trapped; C = neutral; D = electron trapped). TABLE XI1 Calculationsfor Agz on AgBr

Ag2 chargesa

Charge on nearest Br of AgBr

Charge on nearest Ag of AgBr

9.49 8.87

EH +0.355, +0.236 +0.361, +0.242

-0.487 -0.485

+0.378 +0.373

7.11 8.01

CNDO +0.332, +0.246 +0.278, +0.166

-0.681 -0.6 11

+0.387 +0.389

Geometry of AgBr

Total BE (eV)

(AgBr)i4 (AgBrh (AgBr)6 (AgBr)s "EH-extended

Hiickel (calculations); CNDO-complete neglect of differential overlap.

lations for Ag, adsorbed on two different-size models are shown for each calculation in Table XII. The charge on Ag, and nearest-lattice ions shows little dependence on model size although the total bonding energy does show a small effect.

3 . Photolysis Mechanism The calculated data for aggregate energies can be applied t o describe a mechanism of photolysis of AgBr. A small silver center produced by photol-

46

ROGER C. BAETZOLD

ysis grows by attracting photoelectrons and mobile interstitial silver ions. The ability to attract electrons is determined by the EA of the center relative t o the EA of the rest of the crystal, AEe

= EA(AgBr)AgN - EA(AgBr).

(34)

The energy released by adding a silver ion is given by

Table XI11 lists these values for small-size aggregates. Clearly, Ag, and Ag4 centers in CNDO calculation cannot accept a photoelectron from the conduction band. As size increases, however, the larger even-sized neutral aggregates can accept electrons, as the trend in LUMO shows in Table XI. The path of growth for the CNDO calculation that releases the most energy for each step is shown in Fig. 23. Kinetic factors are not included in this mechanism; it must be remembered that these may exert a controlling influence on the reaction path. The EH calculations agree with the CNDO results if a planar nondefect geometry isused. When the model containing defects serves as the lattice, electroncapture processes are favored at the expense of A$ capture at the Ag center, as shown in Table XIII. This leads t o the alternative pathway shown in Fig. 24, and would explain a dependence of photochemistry on surface-defect structure. The MO calculations explain a possible route for silver cluster formation and catalysis in silver bromide. The electron affinity of the silver center determines TABLE XI11 Energy Released at Silver Center No. of Ag atoms

AEe (eV)

A E k + (eV)

~~

1 2 3 4

CNDV 1.71 -0.56 1.62 -0.09

0.05 0.90 0.6 1 0.78

1 2 3 4

EHb defect model -0.96 1.84 2.38 1.75

1.10 0 0 0

OCNDO = complete neglect of differential overlap. &EH= extended Huckel (calculations).

MOLECULAR ORBITAL THEORY

47

J

FIG. 23

FIG. 24

FIG. 23. Thermodynamic path for Ag cluster growth on plane surface (complete neglect of differential overlap). FIG. 24. Thermodynamic path for Ag cluster growth on defect surface (extended Hiickel calculation).

the fate of electrons transferred to the crystal from a developer in solution. The electron affinity of a small AgBr grain containing silver clusters is greater than in grains without the silver clusters. This energy difference makes it possible for certain developers to transfer electrons to and reduce the AgBr grains containing Ag clusters.

D. METAL SUBSTRATES 1. H o n N i

The adsorption of H on Ni has been the subject of a recent EH-type calculation by Fassaert et al. (68). A finite-size representation consisting of up to 13 atoms was employed for the (1 1l), (loo), and (1 10) nickel surfaces. In this calculation, the d orbitals of nickel were taken as a linear combination of two Slater orbitals in order to improve the fit with more exact SCF atomic calculations, as described in Section II.B.l. In addition, the diagonal Hamiltonian matrix elements were modified to depend on charge, similar to Eq. (1 l), in order to check the charge separation predicted by a noniterative calculation. The 13-atom crystal model has been used to calculate properties qualitatively similar to those of bulk nickel. The calculated d-band width is 1.81 eV versus 2.44 eV found experimentally. The calculated value of d-band holes per atom is 0.68 compared to an experimental value of 0.6,and the cohesive energy is about

48

ROGER C. BAETZOLD

10% less than the experimental value of 4.4 eV. These comparisons suggest that the finite-size model can be used as a substrate for adsorption calculations. The bond energy of H to Ni surface calculated according to this procedure does not show a minimum as a function of H-Ni distance. This is a weakness of the noniterative EH procedure and requires calculated bond energies to be taken at a Ni-H distance equal to the s u m of the atomic radii. The covalent bond energy calculated in this manner is -2 eV for most surfaces and compares well with the experimental value of 2.9 eV. The adsorption of H to Ni surface results in the transfer of electrons to H. Strongest bonding takes place when there is a single nickel nearest neighbor to the H and the adsorption energy increases in the order (1 11) < (100) < (1 10). The.bond energy decreases when the surface is more closely packed. The orbitals important in this bonding are the 1 s of H and the 3dzl and 4s of Ni. This finding is in conflict with recent models discussed by Bond (69) who contends that eg orbitals (i.e., d x l - y l and d Z 1 )are nonbonding in the metal and, therefore, particularly important for adsorption bonding. It is significant that the 4s orbital of Ni, being more diffuse than the d orbitals, interacts strongly with the H orbital forming a bond. It was concluded that the structure of the surface molecule is sufficient for determining which d orbitals of the metal interact with H. Fassaert ef al. (68) simulated H adsorption on a Cu surface by adding an additional electron per metal atom to the system. This approximation relies on the fact that atomic wave functions and energy levels are not too different for Ni and Cu and that their principal difference lies in the number of valence electrons. In the case of adsorption to Cu substrate, which has no unfilled d orbitals, the metal d orbitals do not participate in the bonding to H. All bonding takes place using the metal 4s orbitals. The calculated covalent bond energy is comparable on the Ni and Cu substrate models, so that from the results a distinction between the catalytic properties of the two metals cannot be made. 2. C O o n N i A recent paper by Robertson and Wilmsen (70) has treated the adsorption of CO by EH on an 8-atom planar nickel surface. Results for larger nickel substrates showed that the size of the surface plays a limited role in determining relative energies of chemisorption. It was found that the most favorable position for adsorption is bridge bonding of 2 Ni to C. This calculated binding energy is 2.53 eV versus an experimental value of 1.98 eV. Bonding to a single Ni atom through C is only 0.14 eV less stable than the above configuration, so that there is a possibility of both kmds of CO existing on the surface of Ni. When the oxygen end of the molecule was adsorbed to Ni, the bond energy decreased significantly. In addition to single CO molecule adsorption, the adsorption of several CO

MOLECULAR ORBITAL THEORY

49

molecules was treated in order to calculate the surface coverage. Calculations for several bridge-bonded arrangements of adsorbed CO molecules gave the l o w est energy for a random distribution of nontouching CO molecules with a CO/Ni ratio of 0.5. The latter results agrees with experiment, as does the prediction of bonding of CO through C rather than 0 t o the surface.

3 . CO on NiO The changes in infrared vibration frequency of CO upon adsorption t o NiO have been examined by Politzer and Kasten (71) using EH theory. These chargedependent iterative calculations were used to determine energies of adsorption, and the wave functions were analyzed t o determine overlap populations, which were found t o be a measure of vibrational frequency. The substrate model consisted of a single Ni or 0 ion with charge appropriate to the system under investigation. The most stable configuration of CO on NiO was found t o be OC-Ni. This geometry was of lower energy than CO-Ni, including bonding t o t h e oxide ion for all charges on Ni. Thus, the IR bands are attributed to bonding between CO and metal ions on the NiO surface. The CO overlap population is increased over the free molecule value on adsorption to Ni(I), Ni(II), and Ni(III), increasing in this order. Adsorption to Ni(0) decreases the overlap population. Thus, an increase in vibration frequency of CO is expected as the charge of nickel ion becomes more positive because overlap population is a measure of bond stiffness. This effect has been observed experimentally and illustrates a potentially valuable application of MO theory.

4. Organic Species on Pb Chemisorption calculations by the EH method for organic species o n lead substrate have been performed by Robertson and Wilmsen (72). Substrate models ranging in size from 5 t o 13 atoms proved useful for this work. It was found that electron-rich adsorbates such as methylamine do not adsorb o n the surface, whereas electron-poor species such as ethyl radical do. Potential-energy curves show that bond energy is positive only for the electron-poor species. This behavior is in accord with the known reactivity of lead surfaces. In addition, the degree of surface coverage was predicted t o be 0.5 based on the adsorption energies for models with different degrees of coverage.

E. PALLADIUM ON VARIOUS SUBSTRATES The interaction of adsorbed metal atoms with substrate may be treated b y EH calculation for situations of low charge transfer. The general effects are similar

50

ROGER C. BAETZOLD

-

x a

\ \

\

Is0 la ted

orbital

‘-”

Interacting orbitals

I

Is0 la ted

orbi ta I

FIG. 25. Orbital interaction scheme.

to what happens when two differentenergy orbitals interact as depicted in Fig. 25. The lower-energy orbital is stabilized and the upper-energy orbital is destabilized by the interaction. A general rule (73) regarding the direction of orbital shifting can be learned from Fig. 25. Orbitals of adsorbed atoms are raised or lowered in energy depending on whether the ionization potentials of substrate orbitals are smaller or greater, respectively, than the ionization potentials of orbitals on adsorbed atoms. The extent of orbital shifting is dependent on the magnitude of overlap between interacting orbitals. When significant electron transfer occurs, Madelung effects must be included in the considerations; however, we restrict our attention to the nonionic case. Extended Huckel calculations illustrate this principle for Pdz interacting with various substrates. The substrate models for B, Li, Cu, and A1 consisted of 9 atoms in a 3 X 3 plane with the bulk nearest-neighbor distance for bond lengths. The Pdz molecule is placed above the plane so that each Pd atom is 2 A away from a nearest substrate atom. Vacant d states will exist on Pd clusters if a molecular orbital composed primarily of 5s orbitals is lower in energy than the highest molecular orbital composed of d orbitals. Figure 26 shows the 5s energy level and the band of 4d energy levels of Pdz adsorbed on various substrates. Substrates composed of atoms with IP < 7.33 (atomic 5s level for Pd) would have vacant 4d states since electrons would flow from the high-energy 4d states to the lower-energy 5s states. Figure 26 shows that each energy level is shifted by interaction with the substrate, but shifts in 5s levels are much greater. The condition necessary for vacant d states is that the molecular orbitals composed primarily of 5s Pd orbitals lie below HOMO composed of 4d Pd orbitals. Furthermore, the band of d energy levels and the lowest s energy level for Pdz on various substrates are shown in Fig. 26. When the atom of the substrate has a smaller ZP than 7.33 eV (atomic 5s level for Pd), the s state is below a d state, resulting

MOLECULAR ORBITAL THEORY I

I

51

,

Ionization potential of substrate atom , e V

FIG. 26. Energy levels of Pd2 adsorbed on various substrate models vs. substrate atom ionization potential.

in flow of electrons from d to s states. This diagram shows that each energy level is shifted by interaction with substrate, but shifts in 5s levels are much greater than shifts in 4d levels because 5s orbitals are more diffuse. This analysis holds for nonionic situations, but impurities that may exist on the substrate surface could alter the interpretation. This description of the interaction of Pd2 with various substrates serves to illustrate principles rather than make specific predictions for these substrates. It is interesting that the orbital-shifting effect does not occur over distances greater than two bond lengths. This simple calculation does not take into account charge-transfer effects, and it is expected that these would become large as the ionization potential difference increases in Fig. 26. Calculations for Pd2 on the oxide substrate MgO have been performed using EH. The planar substrate model showed the greatest Pd2-binding energy when both Pd atoms were bonded to Mg ions rather than to 0 ions or between lattice sites. The number of vacant d states on Pd is dependent on the site of adsorption. One d hole per Pd atom is observed for Pd2 near Mg ions or interstitial sites, but only 0.46 per atom is observed for Pd2 near oxide sites. These results would be in accord with the data shown in Fig. 26 since oxygen has a much higher ZP than Mg. The adsorbed Pd atom has its level ordering and binding energy determined by the type of surface ion that is adjacent to it.

V. Conclusions We have investigated the application of approximate MO theory to a number of problems that are important in catalysis. One of the striking features of

52

ROGER C. BAETZOLD

this theory is its versatility and interpretation of complex phenomena in simple terms. It is a model that fills a gap in the field of catalysis and, therefore, should be of interest t o many people. Although some calculation is required to examine most systems, certain generalizations can often be made. The issue of reliability of approximate MO theory is complex. The work presented here tends to support its validity as a means of explaining and predicting phenomena. For example, the calculated electronic properties of diatomic molecules and CuCl clusters in Section II.F.2 agree well with experiment. The properties of metal clusters are reasonable in light of the small amount of experimental data available on them. The chemisorption calculations in Section IV all support the available experimental evidence. More critical comparisons of experiment and theory are necessary to establish the degree of usefulness of a p proximate MO theory. A. SPECIFIC QUESTIONS A number of specific questions were posed early in this paper and we shall now attempt to collect information and answer them. The approximate MO theory leads to the following conclusions that deserve further experimental examination. 1. Clusters of Ag atoms have metal-like properties at 55 atoms, but they are not metals. The gap between HOMO and LUMO being -0.2 eV is still 10 times larger than kT expected for a bulk metal. The work function is calculated to be 1 eV larger than that of the bulk, and the cohesive energy is only about onethird that expected for bulk. The electrons behave like free electrons. The trend in each of these quantities with size is in the direction from single atom properties toward bulk metal properties. Clusters of Pd have characteristics that differ from bulk metal properties in much the same way as do Ag clusters. One exception is the calculated number of unoccupied d states per atom of small clusters, which is very close to bulk values of 0.6. 2. The MO theory does not give bond energies in accord with a pairwise interaction scheme. The trends in Fig. 3 indicate bond energy is not proportional to average bond number and would seem to discourage the use of continuum thermodynamic approaches at this size range. 3. The theory suggests orbital symmetry to be an important property in catalysis, A catalyst with a set of partially occupied d orbitals having energy near that of the energy level of interacting species can interact strongly with reactants. The symmetry of the d orbitals on the surface of the catalyst must match the symmetry of reactant species to produce positive overlap in order to give low activation barriers. 4. Substrate effects are important in determining the electronic properties of adsorbed catalyst. The data for Pdz on substrate show that atoms in the

MOLECULAR ORBITAL THEORY

53

first adsorbed layer can have their energy levels reordered. This interaction is short range, not extending further to the second layer. Charge can be exchanged with the substrate, and it thus determines whether or not the metal atoms are stable when aggregated. In addition there are particular sites, even on a plane surface, where adsorbed atoms tend to bond. These effects suggest that the substrate should play a role in determining the catalytic activity of supported catalysts of small size. As the size becomes larger, this effect would diminish. 5 . Most of the calculations support the use of the surface molecule concept. The features predicted for metals such as Ni and Pb and nonmetals such as C are in accord with what is expected from experimental measurements. One exception is the graphite surface where different adsorption properties were found, depending on whether a finite or infinite surface model was employed.

B. FUTURE A continuing need exists in the field of catalysis for a calculational procedure such as MO theory. The degree of usefulness of this theory will be known when sufficient direct experimental and theoretical comparisons are made and this will be an active area in the near future. The examples treated here indicate the versatility of MO theory, but other areas, particularly mechanistic studies in catalysis, could be treated. The testing of reaction mechanisms or intermediate species will probably be a profitable area for further application. ACKNOWLEDGMENTS I am grateful to my colleagues in the Kodak Research Laboratories whose questions and interest provided the basis for this program. In particular, appreciation is given to Dr. Joseph Yudelson whose advice and encouragement have been instrumental in sustaining this work. REFERENCES 1. Hoffmann, R.,J. Chem. Phys. 39,1397 (1963). 2. Pople, J. A., Santry, D. P., and Segal, G. A., J. Chem Phys. 43, Part 2, 129 (1965); Pople, J. A., and Segal, G. A., J. Chem Phys. 43, Part 2, 136 (1965); 44, 3289 (1966); Santry, D. P., and Segal, G. A.,J. Chem. Phys. 47, 158 (1967); Pople, J. A.,

3. 4. 5. 6.

Z 8. 9.

and Beveridge, D. L., “Approximate Molecular Orbital Theory.” McGraw-Hill, New York, 1970. Messmer, R. P., and Watkins, G. D., Phys Rev. Lett. 25,656 (1970). Bramanti, D., Mancini, M., and Ranfagni, A.,Phys Rev. B 3,3670 (1971). Hayns, M. R., Phys. Rev.B 5,697 (1972). Wolfsberg, M., and Helmholz, L.,J. Chem. Phys. 20,837 (1952). Ballluusen, C. J., and Gray, H. B., Inorg. Chem. 1,111 (1962). Cusachs, L. C., Spectrosc. Lett. 3,7 (1970). Slater, J. C.,Phys. Rev. 36,57 (1930).

54

ROGER C. BAETZOLD

10. Clementi, E., and Raimondi, D. L., J. Chem. Phys. 38,2686 (1963). 11. Clementi, E., Raimondi, D. L., and Reinhardt, W. P., J. Chem Phys. 47,1300 (1967). 12. Cusachs, L. C., and Corrington, J. H., in “Sigma M.O. Theory” (0. Sinanoglu and K. Widberg, eds.), p. 256. Yale Univ. Press, New Haven, Connecticut, 1970. 13. Richardson, J. W., Nieuwpoort, W. C., Powell, R. R., and Edgell, W. F., J. Chem Phys. 36,1057 (1962). 14. Basch, H., and Gray, H. B., Theor. Chim. Acta 4,367 (1966). 15. Mulliken, R. S . , Rieke, C. A., Orloff, D., and Orloff, H., J. Chem Phys. 17, 1248 (1949). 16. Moore, C. E., Nut. Bur. Stand. (US.),Circ. 467,Vols. 1-3 (1949). 17. Cusachs, L. C., and Cusachs, B. B.,J. Phys Chem. 71,1060 (1967). 18. Goodisman, J., J. Amer. Chem. SOC.91,6552 (1969). 19. Mulliken, R. S . , J . Chem. Phys. 23,1833 (1955). 20. Lowdin, P. O., J. Chem. Phys. 18,365 (1950). 21. Cusachs, L.C., and Politzer, P., Chem. Phys. Lett. 1,529 (1968). 22. Corrington, J. H., and Cusachs, L. C., Int. J. Quantum Chem., Symp. 3,207 (1969). 23. Messmer, R. P., McCarroll, B., and Singal, C. M., J. Vac. Sci Technol. 9, 891 (1972). 24. Messmer, R. P., Chem. Phys Lett. 11,589 (1971). 25. Cotton, F. A., Harris, C. B., and Wise, J. J., Inorg. Chem. 6,909 (1967). 26. Fenske, R. F., Caulton, K. G., Radtke, D. D., and Sweeney, C. C., Inorg. Chem. 5, 951 (1966). 27. Canadine, R. M., and Hiller, I. H., J. Chem. Phys. 50,2984 (1969). 28. Roothaan, C. C. J., Rev. Mod. Phys. 23,69 (1951). 29. Hiller, I. H., J. Chem. Phys. 52, 1948 (1970). 30. Baetzold, R. C., J. Chem. Phys. 55,4355 (1971). 31. Zollweg, R. J., J. Chem. Phys. 50,4251 (1969). 32. Koopmans, T. A., Physica (Utrecht) 1,104 (1933). 33. Quantum Chemistry Program Exchange, Chem. Dep., Indiana Univ., Bloomington. 34. Hare, C. R., Sleight, T. P., Cooper, W., and Clarke, G. A., Inorg. Chem. 7, 669 (1968). 35. Cooper, W. F., Clarke, G. A., and Hare, C. R.,J. Phys Chem. 76,2268 (1972). 36. Guido, M. Balducci, G., Gigli, G., and Spoliti, M., J. Chem. Phys. 55,4566 (1971). 37. Guido, M. Gigli, G., and Balducci, G.,J. Chem Phys. 57, 3731 (1972). 38. Hoare, M. R., and Pal, P.,J. Cryst. Growth, 17, 77 (1972). 39. Burton, J. J., J. Chem. Phys. 52, 345 (1970);Burton, J. J., Chem Phys Lett. 17, 199 (1972);Burton, J. J., J. Chem. SOC.,Faraday Trans II 69,540 (1973). 40. Baetzold, R. C., J. Chem. Phys. 55,4363 (1971). 41. Eachus, R. S . , and Symons, M. C. R., J. Chem. Soc. A p. 1329 (1970). 42. Hotop, H., and Bennett, R. A., J. Chem. Phys. 58,2373 (1973). 43. Gomes, W.,Trans Faradoy SOC.59,1648 (1963). 44. Kittel, C., “Introduction to Solid State Physics,” 3rd Ed. Wiley, New York, 1968. 45. Kimura, H.,Katsuki, A,, and Shimizu, M.,J. Phys SOC.Jap. 21,307 (1966). 46. Wohlfarth, E. P.,J. Phys Chem. Solids 1,35 (1956). 47. Baetzold, R. C., J. Catal. 29,129 (1973). 48. Wan, C. C., and Tong, S . Y.,Surface Sci. 34,739 (1973). 49. Lasarov, D., and Markov, P., Surface Sci. 14,320 (1969). 50. Stoll, H.,and Preuss, H.,Phys. StatusSolidi(B) 53,519 (1972). 51. Hoffmann, R., Tetrahedron 22,521 (1966). 52. Meyer, B., and Spitzer, K.,J. Phys. Chem. 76,2274 (1972). 53. Allison, E. G., and Bond, G. C., Catal Rev. 7,233 (1972). 54. Sherman, A., and Eyring, H.,J. Amer. Chem. SOC.54,2661 (1932).

MOLECULAR ORBITAL THEORY

55

55. Sherman, A., and Eyring, H., J. Chem. Phys. 3,49 (1935). 56. Grimley, T. B., in “Molecular Processes on Solid Surfaces” (E. Drauglis, R. D. Gretz, and R. J. Jaffee, eds.), p. 299. McGraw-Hill, New York, 1969. 5 % Van der Avoird, A., Surface Sci. 18,159 (1969). 58. Thorpe, B. J., Surface Sci. 33,306 (1972). 59. Blyholder, G., and Coulson, C. A., Discuss. Faraday Soc. 63,1782 (1967). 60. Baetzold, R. C., Surface Sci. 36,123 (1973). 61. Hamilton, J. F., and Logel, P. C., Thin Solid Films 16,49 (1973). 62. Lewis, B., Surface Sci. 21,273 (1970). 63. Bennett, A. J., McCarroll, B., and Messmer, R. P., Surface Sci. 24, 191 (1971). 64. Bennett, A. J., McCarroll, B., and Messmer, R. P., Phys Rev. B 3, 1397 (1971). 65. Messmer, R. P.,and Bennett, A. J., Phys. Rev. B 6,633 (1972). 66. Baetzold, R. C., J. Solid State Chem. 6, 352 (1973). 67. Hamilton, J. F., and Urbach, F., in “The Theory of the Photographic Process” (C. E. K. Mees and T. H. James, eds.), 3rd ed., Ch. 5 . Macmillan, New York, 1966. 68. Fassaert, D. J. M., Verbeek, H., and Van der Avoird, A., Surface Sci. 29,501 (1972). 69. Bond, G. C., Phtinum Metals Rev. 10,87 (1966). 70. Robertson, J. C., and Wilmsen, C. W., J. Vac. Sci Technol. 9,901 (1972). 71. Politzer, P., and Kasten, S. D., Surface Sci. 36,186 (1973). 72. Robertson, J. C., and Wilmsen,C. W., J. Vac. Sci Technol. 8,53 (1971). 73. Hoffmann, R., Accounts Chem. Res. 4 , l (1971).

The Stereochemistry of Hydrogenation of c~,P-UnsaturatedKetones ROBERT L. AUGUSTINE Department of Chemistry Seton Hall University South Orange, New Jersey I. Introduction.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Mechanistic Proposals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111. Effect of Variables on the Hydrogenation of pOctalone and Related Compounds.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Solvents.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Catalysts.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Substituents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV. Hydrogenation of Other Ring Systems . . . . . . . . . . . . . . . . . . . . . A. Heterocycles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Hydrindenones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

56 59 63 63 72 73 75 75 76 77 78 79

I. Introduction Catalytic hydrogenation has been widely used by the synthetic organic chemist in the preparation of a variety of compounds. In some cases, particularly in the synthesis of the more complex molecules such as those broadly categorized as natural products, the utility of this reaction is limited because it is difficult to predict the stereochemistry of the product that would be formed on hydrogenation of an olefinic or a ketonic intermediate. This control of product stereochemistry is often of considerable importance, particularly in the preparation of pharmacologically active molecules. Although examination of a molecular model of the compound to be hydrogenated will often show with a reasonable degree of certainty which side of the molecule offers less hindrance to adsorption on the catalyst surface, factors other than this “steric approach control” must also be considered. The stereochemistry of the hydrogenation of isolated olefins is dependent not only on the steric bulk of the substituents on the double bond but also on the nature of the catalyst used in the reaction and the hydrogen availability to the catalyst surface ( I ) . The effect of these variables 56

HYDROGENATION OF UNSATURATED KETONES

57

on product stereochemistry can best be understood using the classic HoriutiPolanyi (2) mechanism shown in Scheme 1. Examination of this mechanistic se-

-

”*

I

-C-C-

1

! L I

(1)

2~ I

-

H I

1

1

-c-c-

(3)

a

I

-c-c-

A 1

H

1

*

-

I

I

-c-c-

I 1

(4)

H H

Scheme 1. Classic HoriutiPolanyi mechanism for olefin hydrogenation (2).

quence shows that under conditions of low hydrogen availability t o the catalyst (for instance, low hydrogen pressure or slow stirring), the stereochemistry of the product is determined primarily by the relative stabilities of the various “half-hydrogenated states” that are available to the system [Eq. (3) in Scheme I]. Under high hydrogen availability conditions (high pressure or very rapid stirring), product stereochemistry is fixed by the mode of the initial adsorption of the olefin on the catalyst surface [Eq. (2)]. It has also been proposed that over palladium the various steps in this reaction sequence are more readily reversible than they are over platinum (1). When the double bond is polarized by being in conjugation with an electronwithdrawing group as in an a$-unsaturated ketone, other factors enter into product stereochemistry considerations. In the hydrogenation of these compounds the stereochemistry of the product can be influenced by the nature of the solvent used in the reaction as well as by the hydrogen availability and the nature of the catalyst. There have been only a few reasonably general studies of the effect of solvent on the hydrogenation of unsaturated ketones and most of these have been on the A43-ketosteroids. Although the general trends observed in the solvent polarity-product stereochemistry relationship are usually consistent, care must be taken in extrapolating absolute stereochemical data from one study to another as shown by the data listed in Table I. In this table is given the percent A-B cis ring-fused product (111) (the 50 isomer) obtained on hydrogenation of cholestenone (I) and testosterone (11). These data show that the absolute value of the amount of 33 product, 111, obtained in specific solvents is not reproducible. The fact that all of these reactions were run using different types of palladium catalysts and analyzed in different ways is undoubtedly the reason for this lack of similarity between the different sets of results. What is impor-

58

ROBERT L. AUGUSTINE TABLE I Solvent Effect on Sp-Product Formation in the Hydrogenation of Cholestenone and Testosterone Percent 5p isomer in product Solvent

N-Methyl pyrrolidone Acetonitrile Acetone Dioxane Tetrahydrofuran Cyclohexane Hexane Methanol Ethanol i-Propanol t-Butanol Acetic acid N-Methyl pyrrolidine Methanol-KOH Ethanol-NaOH i-PropanoliPrOMethanokHz SO4 i-Propanol-H2S04 Ethanol-HCI i-Propanol-HCI Acetic acid-HCl iPropanol-HBr Acetic acid-HBr

From cholestenone (I) Ref. 3 Ref. 4 Ref. 5 Ref.6

From testosterone (11) Ref. 3 Ref.4 Ref. 5 Ref. 6

HYDROGENATION OF UNSATURATED KETONES

59

tant, however, is the fact that a specific change in solvent usually causes the same general change in the amount of 50 isomer formed. Most of the predictions concerned with the stereochemical outcome of the hydrogenation of a,p-unsaturated ketones have been based on empirical conclusions derived from studies on model compounds. This is particularly true in the steroid field [e.g., Slomp et al. (7)]. One result of this type of empirical approach is the development of the classic method for preparing 50 steroids by hydrogenating the corresponding A4 -3-keto species in the presence of hydroxide ions (7, 8). A more complete discussion of other generalizations which have been made concerning the hydrogenation of steroids is beyond the scope of this review but such information is available elsewhere (9,10).

I I. Mechanistic Proposals As already mentioned, the stereochemistry of simple olefin hydrogenation can usually be understood by utilizing the classic Horiuti-Polanyi mechanism (1,2). A number of different mechanistic rationales have been put forth, however, to account for the stereochemicaldata obtained on hydrogenation of a,P-unsaturated ketones in different media. Actually, no single explanation can be used to account for all of the stereochemical observations, but it is possible to blend the various proposals to give a mechanistic framework from which it is possible by extrapolation to obtain the desired stereochemical information. One of the earliest of these mechanistic proposals was suggested by Weidlich (11) and is outlined in Scheme 2. He found that the hydrogenation of 2, 3diphenylindenone (IV) gave the cis-indanone (V) in acid and the trans isomer (VI) in a basic medium. To account for these results he proposed that in the hydrogenation of the unsaturated ketone in acid the initial step is the protonation of the carbonyl oxygen to form a carbonium ion (VII), which would then polarize the double bond making it easier for the cis addition of hydrogen to occur. Despite this polarization and the fact that a resonance form (VIII) that placed the positive charge on the @-carbonwas recognized by Weidlich, he still stated that 1,2-addition of hydrogen to the double bond occurred under acidic conditions except, possibly, in rare instances. In bases he proposed that the reaction occurred by way of a 1,4-addition of hydrogen. This would give the enol (IX)which on ketonization would lead to the trans-disubstituted species (VO. Somewhat later, Brewster (12) proposed that in an acidic medium a metal surface would interact with an a, 0-unsaturated carbonyl system to form intermediates such as compounds X and XI in which the metal was complexed either with the carbonyl carbon (X) or the /3-carbon (XI). Such complexation would most probably take place after protonation of the carbonyl oxygen. Hydride

60

ROBERT L. AUGUSTINE

Ph

on

+

on

on (VIII)

(VII)

Ph

Ph

0

0

Pd

"2

on

0

(IX)

(VI)

Scheme 2. Proposed mechanism for the hydrogenation of a,p-unsaturated ketones in acidic media [Weidlich ( 1 1 ) ] .

ion transfer from the metal surface gives the saturated product. Since unsaturated alcohols are not formed in the hydrogenation of unsaturated ketones, it was reasoned that the &complex (XI) was preferred. An extension to this proposal was made later to account for the fact that hydrogenation of A1~g-octalone-2 (XII) in acid gave about 90% of cis-&decalone (XIII), whereas in neutral medium only about 50% of this isomer was formed. It was reasoned that protonation of the octalone would give the fl-carbonium ion which could then be adsorbed on

n

n

HYDROGENATION OF UNSATURATED KETONES

cis

61

trans

FIG. 1. Adsorbed protonated p-octalone in cis and trans configurations [Augustine et a1. (24)I .

the catalyst surface in either the cis or the trans configuration as shown in Fig. 1 (13). If it is assumed that the adsorbed carbonium ion has a tetrahedral configuration, it is evident that the cis-adsorbed species is sterically more favorable than the trans species. Thus, hydride ion transfer to the adsorbed charged species from the catalyst would lead to predominant cis-product formation. In contrast to the Weidlich proposal, these mechanisms call for a 1,4-addition of hydrogen in acidic media. The concept of a hydride ion transfer from the catalyst is not unique to these mechanistic proposals. Hydride ions have been proposed to take part in the catalytic hydrogenolysis of certain substituted cyclopropanes (14). More recently, attack by “electrophilic hydrogen” on a catalyst-stabilized carbonium ion has been proposed to occur in the hydrogenolysis of C-Br bonds (15) as well as in the cleavage and rearrangement of certain acetyl epoxides (16). In yet another mechanistic proposal, McQuillin et al. (3)suggested that in the unsaturated ketonic system the oxygen must be considered adsorbed on the catalyst surface along with the double bond, but adsorption of the carbonyl group was likely to be influenced by the solvent medium. The polarized carbony1 group would act as a surface ligand, and the catalyst, as the electron acceptor, should be regarded as a Lewis acid. The influence of acid in determining product stereochemistry was explained as permitting easier polarization of the carbonyl group in the same way that polar solvents do. It was further indicated that bases do not fundamentally alter the mode of adsorption or the process of hydrogen transfer in these reactions. It should be mentioned, however, that, although in a neutral medium an isolated olefin is usually hydrogenated more readily than is an cu,P-unsaturated carbonyl compound, the reverse is true in either acid (1 7) or base (18). This indicates that in acidic or neutral medium the unsaturated carbonyl moiety interacts in some way with the solvent to form an intermediate species that is more strongly adsorbed on the catalyst than is the neutral molecule. Both the carbonium ion formed in acid and the enolate anion formed in base would be expected to have strong adsorption characteristics. About the only mechanistic rationale put forth t o account for product stereochemistry differences between a neutral and a basic hydrogenation medium was that proposed by Wilds et al. (19) (Scheme 3) to explain the fact that the equilinone derivative (XV) was hydrogenated in a neutral solvent to the trans C/D ring fused system (XVI), whereas in base only the cis isomer (XVII) was

62

ROBERT L. AUGUSTINE

HI Pd P EtOH

cI

\

/

I

boHbO” (XIX)

Scheme 3. Rationale for the influence of base on the hydrogenation of a,@-unsaturated ketones [Wilds et al. (I 9)] .

formed. He suggested that, in the basic reaction, initial formation of the enolate anion (XVIII) occurred, and, on hydrogenation, this enolate gave a resonancestabilized anion (XIX) which, when protonated at the 0 position from the solvent and ketonized gave product XVII. Since equilinone is planar, adsorption on the catalyst is regulated by the angular methyl group. Thus, in neutral medium, adsorption and hydrogen transfer occurs trans to this group and the trans product is formed. In base, again adsorption takes place trans to the angular methyl but, since the product stereochemistry is determined by protonation of the adsorbed anion from the solution on the side of the molecule away from the catalyst, cis-isomer formation will occur. None of these mechanistic proposals is sufficiently general to use to rationalize all of the stereochemical data observed on the hydrogenation of a,b-unsaturated ketones. By a judicious combination of segments of each of these proposals along with the Horiuti-Polanyi mechanism (2),it is possible, however, to develop a uniform mechanistic rationale that can be useful in determining the effect of solvent on product stereochemistry . In addition, the influence of hydrogen availability, the type and quantity of catalyst, and the nature of other substituents on the reacting molecule on the product isomer distribution can also be more readily understood. As mentioned previously, it is difficult to extrapolate data from one reaction system to another and come up with a reasonable mechanistic formula. Because

HYDROGENATION OF UNSATURATED KETONES

63

of this difficulty most of the discussions in the following sections are based on the systematic studies made on the Hydrogenation of 0-octalone (XII) which were run to determine the effect of the various reaction parameters on the ratio of cis-(XIII) and trans -0-decalones(XN) formed (13,20-24).

I I I. Effect of Variables on the Hydrogenation of P-Octaloneand Related Compounds A. SOLVENTS 1. Neutral Media The hydrogenation of 0-octalone (XII) in a number of neutral solvents over a palladium catalyst gave mixtures of the cis- and trans-0-decalones. Initial examination of these product ratios showed no correlation between the dielectric constant of the solvent and the amount of cis-0-decalone (XIIII) obtained. When the solvents were separated into protic and aprotic categories, however, a direct relationship between product stereochemistry and solvent polarity in each series became apparent, as shown by the data given in Tables I1 and 111 (24). In aprotic solvents, an increase in solvent polarity resulted in an increase in the amount of cis-0-decalone formed. Similar results were also obtained in the hydrogenation of cholestenone and testosterone (see Table I). If, as suggested by McQuillin et al. (3), a more polar aprotic solvent will facilitate complexation of the carbonyl oxygen of an a,&unsaturated ketonic system in the same way that it increases its polarization (25), it can be assumed that what is occurring in these polar solvents is a 1,4-addition of hydrogen to the conjugated system. TABLE I1

Product Stereochemistry Obtained on Hydrogenation of p-Octalone in Neutral Aprotic Solventsa Solvent

Dielectric constant

Percent cisp-decalone

N,N-Dimethylformamide Acetone Ethyl acetate Diethyl ether Dioxane Cyclohexane n-Hexane

38.0 21.0 6.0 4.34 2.2 2.07 1.89

79 63 51 58 52 51 48

aData from Augustine et al. (24).

64

ROBERT L. AUGUSTINE TABLE I11 Product Stereochemistry Obtained on Hydrogenation of p-Octalone in Neutral Hydroxylic Solventsa

Solvent

Dielectric constant

Percent cisp-decalone

Methanol i-pro pano Ethanol n-Propanol s-Butanol n-Butanol t-Butanol

33.6 26.0 25.1 21.8 18.7 17.8 10.9

41 49 55 68 70 71 91

~~

~

~~~~

‘Data from Augustine et al. (24).

Figure 2 shows the 1,2- and 1,4-diadsorbed octalone, each in the cis and trans configuration. It can be seen that the cis 1,4-diadsorbed species is less hindered than is the trans species, and, thus, &&decalone formation should be favored by a 1,4-addition sequence. In the 1,2-diadsorbed species, little difference between cis and trans adsorption can be detected, and, therefore, nearly equal mixtures of the two isomeric decalones should be formed from a 1,2-addition process. Thus, the product distribution data given in Table I1 can be understood if it is considered that the reaction is a combination of 1,2- and 1,4-addition processes. The more polar the solvent, the more the 1,4-addition occurs and, as 1,2-Adsor pti on

1,4-Ad s o r p t i o n

FIG. 2. 1,2-Diadsorbed and 1 ,rl-diadsorbed octalone under low hydrogen availability conditions [Augustine et al. (24)l.

HYDROGENATION OF UNSATURATED KETONES

65

shown above, the more cis-o-decalone is formed. Further support for this concept of a 1,4-addition process to unsaturated ketones can be found in the isolation of enolic species from the hydrogenation of several qfl-unsaturated carbony1 compounds (26). If these 1,2- and 1,4-addition processes are considered as merely extensions of the HoriutiPolanyi reaction sequence (see Scheme l), it should be possible to change the product stereochemistry by modifying the hydrogen availability to the catalyst surface, as is observed in the hydrogenation of isolated olefins (I). Since the product data shown in the Tables I1 and 111 were obtained using relatively slow magnetic stirring (2I),it could be assumed that these reactions were run under conditions of low hydrogen availability to the catalyst. This was verified by the fact that the apparent activation energies for these reactions were only about one-tenth (24) of what would be expected if they were not diffusioncontrolled (27). Thus, under these conditions the adsorbed species would have a near-tetrahedral configuration ( I ) and the product stereochemistry would be determined by the relative stability of the cis- and trans-adsorbed species, as depicted in Fig. 2. If hydrogen pressure is increased in these reactions the product stereochemistry should be determined by the mode of initial adsorption ( I ) of the conjugated system with the carbon atoms maintaining a trigonal configuration. Drawings of the 1,2- and 1,4-diadsorbed species having trigonal carbon atoms are shown in Fig. 3. Under these conditions both the 1,2- and 1,4-diadsorbed species show less steric hindrance to trans adsorption when compared to the drawings in Fig. 2. Thus, it would be expected that under high hydrogen availability conditions more of the trans-0-decalone should be formed than is observed in diffusion1,2-A d s o rp t i on

H

1,4-Ad sor p t i o n

H

FIG. 3. 1,2-Diadsorbed and 1,4diadsorbed octalone under high hydrogen availability conditions [Augustine e l al. ( 2 4 ) ] .

66

ROBERT L.AUGUSTINE

90

-

7,,

60

- w--\ H n 70 0 W

-

Dimethylformomide

a_---

-- - --

I

SL .*

‘

60-

$

50 4G

--

-

n - Hexone

-

0

2280

304C

n

I 760

1520

controlled reactions. The product distribution dependency on hydrogen pressure in both hexane and dimethylformanide is shown in Fig. 4. In both instances, lowering the hydrogen availability t o the catalyst surface resulted in an increase in the amount of cis-0-decalone obtained. It should be noted that, at least in the hydrogenation of octalone, the point at which a difference between high and low hydrogen availability is first observed is near atmospheric pressure. Therefore, the product stereochemistry obtained is markedly dependent on factors such as the rate of agitation, the quantity of catalyst, and the surface area of the solution exposed to the gas phase. Results similar to those shown in Fig. 4 were obtained by modifying the rate of agitation of the reaction mixture and changing the amount of catalyst used (21). Slowing the rate of stirring gave more of the cis product, whereas using very rapid stirring gave nondiffusion-controlled conditions and a marked decrease in cis-isomer production. With large quantities of catalyst the limited amount of hydrogen available under diffusion-controlled conditions is more diffused and there is less hydrogen available per “active site” with a resultant increase in the amount of cis product formed. Decreasing the quantity of catalyst resulted in an increase in the availability of hydrogen per active site and, thus, a decrease in cis-isomer production.

67

HYDROGENATION OF UNSATURATED KETONES

In hydroxylic media the effect of solvent polarity on product stereochemistry is the reverse of that observed in aprotic solvents (see Table 111). In these protic solvents, it has been proposed that participation by the hydroxy group via solvation or interaction with the carbonyl group can modify the participation of the carbonyl group in adsorption on the catalyst with the double bond (3). Since this interaction is regulated by the steric bulk of the alcohol, the smaller the alcohol, the more interaction with the carbonyl group and the more 1,2-addition occurs. In t-butyl alcohol, 1,4-addition predominates, whereas in methanol hydrogenation occurs primarily by way of a 1,2-addition process (24). Modifying the hydrogen availability t o the catalyst by changing the hydrogen pressure, the rate of agitation, or the quantity of catalyst affects the product sterochemistry in the same way as it does in aprotic solvents (21). Figure 5 shows the effect of changing hydrogen pressure on product stereochemistry obtained in the hydrogenation of 0-octalone in t-butyl alcohol and methanol. Comparison of these data with those shown in Fig. 4 shows the similarity between t-butyl alcohol and dimethylformamide (1 ,4-addition) and between methanol and hexane (1,2-addition). The major difference between protic and aprotic solvents is

90

-

a-. 99 9.

a,

c -0 80

8

-. --.*

0 0 - - 1 ,

0,

n

$1 70 I

-

d

2280

3040

t- But$ Alcohol

\ \ \

!?

c,

60-

50

-

90% Ethanol

760

I520 Hydrogen pressure (rnrn)

FIG. 5. Effect of hydrogen pressure on product stereochemistry in neutral hydroxylic solvents [Migliorini (21) and Augustine et al. ( 2 4 ) ] .

68

ROBERT L. AUGUSTINE

that in protic media the solvent interaction with the carbonyl group must be taken into consideration.

2. Acidic Media The acid-promoted hydrogenation mechanism proposed by Brewster (12) is essentially a special type of the 1,4-addition process. In this mechanism, a proton comes from the solvent and a hydride ion comes from the catalyst, but the two hydrogens are added t o the ends of the conjugated system. In almost every hydrogenation of a substituted 0-octalone the presence of acid changes the amount of the cis ringfused product that is formed (3-6,13,24). It is apparent from the data in Table I that the strength of the acid used is very important in determining the extent of cis-product formation from the hydrogenation of A4-3-ketosteroids. The use of alcoholic hydrochloric or sulfuric acid gives only a slight change in 50-product formation, but in alcoholic HBr a marked increase in cis-product formation is observed. The use of an acetic acid-HBr solvent is considered a good way of obtaining excellent yields of a 50-steroid in the hydrogenation of the corresponding A4-3-keto compound (5, 6). On the other hand, a large amount of cis-0-decalone was formed on hydrogenation of 0-octalone in ethanolic HCl(13). To examine more fully this effect of acid strength on product stereochemistry, p-octalone was hydrogenated in a variety of ethanolic HC1 solutions (21, 24). The product distribution curve shown in Fig. 6 is comprised of three straight lines. If it is assumed, first, that protonation of the octalone (XII) gives a 0-carbonium ion that is adsorbed more readily in the cis configuration (13) (see Fig. 1) and, second, that this carbonium ion is more readily adsorbed on the catalyst surface than is the neutral species (17), these data can be readily understood. In very weak acid, protonation of the unsaturated moiety has not yet begun and the increase in the amount of cis product obtained is a result of the increase in solvent polarity, and, thus, of the increase in the amount of 1,4-addition occurring. The area of sharp rise in percent cis-0-decalone formation is caused by the competitive hydrogenation of the unprotonated species and the small amount of readily adsorbed carbonium ions formed by the reaction of the weak acid with the octalone. Since the carbonium ion gives predominantly the cis product, the more protonation, the more cis product is formed. At the acid strength of the second breakpoint, sufficient carbonium ion formation has occurred so that, in conjunction with the facile adsorption of this species, only carbonium ion hydrogenation takes place with the product stereochemistry remaining essentially constant with any further increase in acidity. In support of these conclusions it was found that the position of the first breakpoint was dependent on the amount of catalyst present, whereas the position of the second breakpoint depended o n the carbonium ion-octalone ratio (24).

HYDROGENATION OF UNSATURATED KETONES

I I I I I I I 16' 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

69

I

[H'I FIG. 6. Effect of [H+]on product distribution in the hydrogenation of octalone [ Migliorini (21) and Augustine ef al. (24)j

.

The differences noted in Table I in the hydrogenation of cholestenone and testosterone are probably due to differences in solvent acidity used in each case. This is difficult to check since the acid concentrations used were not always reported. It is further possible that with substrates such as A4-3-ketosteroids the acid concentrations corresponding t o the two product distribution breakpoints shown in Fig. 6 are different from those found for octalone hydrogenation. Thus, the change in acidity between alcoholic HCl and alcoholic HBr could correspond to a change analogous t o that found between the two breakpoints in Fig. 6. As was observed in neutral solvents, increasing the hydrogen availability to the catalyst resulted in a decrease in the amount of cis product formed, as shown in Fig. 6 . Under high hydrogen availability conditions, equilibration of the adsorbed carbonium ion does not occur and, hence, more of the trans-saturated product is obtained.

3. Basic Media The presence of base in the hydrogenation of A4-3-ketosteroids has long been known t o lead to the almost exclusive formation of the 50 product ( 9 , l o ) , but, when base is added to the reaction medium in the hydrogenation of fl-octalone

70

ROBERT L. AUGUSTINE

(XU), there is only a slight increase in the amount of cis product formed (13). The effect of varying base concentrations on product stereochemistry in the hydrogenation of octalone is shown in Fig. 7. As in the acid-promoted reaction (Fig. 6 ) , the product distribution in base is composed of three straight line segments: one in very dilute base, a second at intermediate concentrations, and a third in more concentrated basic media. The position of the first breakpoint is dependent on the quantity of catalyst used, whereas the position of the second breakpoint depends on the amount of substrate present (20,24). In contrast to what is observed in both neutral and acidic solutions, in basic medium there is no change in product distribution when the hydrogen availability is varied. This, and the fact that a,&unsaturated ketones are hydrogenated in preference to olefins in base but not in neutral media (18) indicate that a strongly adsorbed species, such as an enolate anion, is present in basic solutions of unsaturated ketones. If it is assumed that the enolate anion is involved in the base-promoted hydrogenation of unsaturated ketones and also that the initial adsorption of this species is the product-determining step in the reaction, a reasonable mechanistic hypothesis, based on that initially proposed by Wilds et al. (19 ) (see Scheme 3), can be put forth for this reaction. Such a process involves a hydride ion transfer from the catalyst to an adsorbed enolate

I X lo'* 5 X lo-'

I x lo-'

' 0.I

0.3

0.5

0.7

0.9

[OH-]

FIG. 7 . Effect of [OH-] on product distribution in the hydrogenation of octalone [Foscante (20)and Augustine et al. ( 2 4 ) ] .

HYDROGENATION OF UNSATURATED KETONES

71

H

FIG. 8. Modes of adsorption for the enolates of octalone [Augustine ef al. (24)l.

followed by protonation of the resultant species at the P-carbon from the solution in the product-determining step. The different enolate anions that may be obtained from octalone (XII) are shown in Fig. 8. It has been shown that the homoannular enolate corresponding to compound XX is the initial product formed on reaction of A4-3-ketosteroids with a strong base and the only product formed on reaction with a weak base (28). This enolate isomerizes to the heteroannular species, such as compound XXI, on further treatment with strong base. Similar results have also been observed on reaction of octalone (XII) with strong base (29). This later work as well as a study of enamine formation (29) from XI1 indicated that the other enolate (XXII) is not present to any large extent. The homoannular enolate (XX) can be adsorbed in the cis and trans arrangement (Fig. 8), but the cis-adsorbed species is the less hindered. The heteroannular enolate (XXI) is almost planar with cis and trans adsorption occurring with almost equal facility. Figure 9 shows a proposed mechanism (24) to account for the product distribution data given in Fig. 7. In very dilute base, the homoannular enolate (XX) is formed first and, as it forms, is preferentially adsorbed on the catalyst in the cis configuration. Hydrogenation of this species with proton transfer from the solution will give predominantly the trans product. In these very dilute basic solutions, competitive hydrogenation of the neutral molecule also occurs. As the base concentration increases more of the homoannular enolate is formed, adsorbed, and hydrogenated. At the base concentration corresponding to the first breakpoint in Fig. 7, the number of enolate ions formed equals the number of active sites on the catalyst and, hence, there is a dependency on the quantity of catalyst at this point. In more concentrated base solutions, more homoan-

72

ROBERT L. AUGUSTINE

+

some

trans-adsor b e d

c is-adsor b e d

' XI11

'

XIV

FIG. 9. Mechanism for the hydrogenation of octalone in basic media [Augustine ef al. (2411.

nular enolate is formed than can be adsorbed so that the excess remains in solution and is converted to the heteroannular species. Between the two breakpoints there is a competition between the hydrogenation of the homoannular and heteroannular enolates and, in more concentrated base, essentially only the heteroannular species is present. This mechanistic rationale can also be used t o explain the product stereochemistry data from the base-promoted hydrogenation of other a,@unsaturated ketones as well. For example, if an angular methyl group were present in the heteroannular enolate depicted in Fig. 8, trans adsorption would be greatly favored. Thus, on hydride ion transfer from the catalyst and protonation from the solution, cis-product formation would predominate, as observed (9,10).

B. CATALYSTS All of the data discussed in the foregoing were obtained using palladium catalysts. Using platinum oxide for the hydrogenation of octalone gives a product distribution similar to that obtained over palladium under moderately high hydrogen availability conditions (23). It would appear, then, that as with simple olefins ( I ) , in the hydrogenation of a,punsaturated ketones over platinum in neutral and acidic media, less equilibration of the adsorbed species occurs than is observed with palladium. In basic medium, because of the irreversible adsorption of the enolate anion, the same product stereochemistry is

HYDROGENATION OF UNSATURATED KETONES

73

obtained with both platinum and palladium (19, 23). Several homogeneous catalysts have been used to hydrogenate a,@nsaturated ketones (30-32), but no general conclusions can be made concerning any catalyst type-product stereochemistry correlation.

C. SUBSTITUENTS The presence of substituents on an unsaturated ketonic species can modify product stereochemistry in a variety of ways. When an angular methyl group is present in an octalone ring (e.g., compound XXIII) (33),the formation of the cis product usually predominates.

(XXIII)

(XXIV)

The presence of an angular carbalkoxy group (e.g., compound XXIV) (34) leads to the primary formation of the trans product. An attempt has been made to ascertain the reasons for these specific directional effects but it has not been successful (35). In the deuteration or tritiation of A' -3-ketosteroids (XXV), attack occurs from the a (or back) side of the molecule (36) as would be expected with the steric hindrance to adsorption exhibited by the P-oriented angular methyl group. On the other hand, deuteration or tritiation of the doubly unsaturated A'' 4-3-ketosteroid (XXVI) still takes place selectively at the A' -double bond,

but it occurs from the side of the molecule (37). The introduction of the second double bond makes the A-ring of the steroid essentially planar, thus bending the angular methyl group back so that it does not interfere with adsorption. It is interesting to note, however, that the use of the homogeneous catalyst, (PhJP)3RhC1, for the deuteration of either the A1-3-keto- or the A'14-3-ketosteroids gives the a-deuterated products in both instances (30,31).

74

ROBERT L. AUGUSTINE

The presence of a 4-substituent on a A4 -3-ketosteroid (XXVII) increases the amount of 5a products obtained on hydrogenation (38),even though it would not be expected that a substituent on a trigonal carbon atom would have any appreciable influence on the direction in which the molecule is adsorbed on the catalyst surface. In every hydrogenation of these 4-substituted steroids, as well as the analogous hydrindenones (see Section IV.B), the 4-substituent in the product (XXVIII) has the equatorial configuration. Thus, the product appears to be formed from a trans addition of hydrogen t o the double bond. These reactions were, however, run in solvents that favor a lP-addition process. 1,4-Addition would give the intermediate enol (XXIX) which would

(XXVII)

(XXVIII)

(XXIX)

then ketonize t o give the product having the more favorable equatorial substituent. This 1,4-addition sequence also appears to be responsible for the increase in Sa-product formation from these compounds. Examination of molecular models of the 1,4-diadsorbed species, as depicted in Fig. 2, shows that in the cis-adsorbed entity there is an appreciable peri-interaction between the 4substituent and the 6P-hydrogen. Thus, trans adsorption is favored. One of the more subtle effects that a substituent can have on product stereo-

HYDROGENATION OF UNSATURATED KETONES

75

chemistry is that exerted by a hydroxy gfoup. It has been established that the hydrogenolysis of benzyl alcohols over palladium occurs with inversion of configuration at the benzyl carbon (39). It would appear, then, that on this catalyst an interaction with the carbinol carbon assumes some importance. The hydrogenation of double bonds in rigid molecules containing a hydroxy group, preferably one in an axial configuration, usually occurs from the side of the molecule trans t o the hydroxy group, again indicating that a catalyst-carbinol carbon interaction is occurring (9). This “palladium-hydroxy. group effect” (9) has been shown to be quite important in determining the product stereochemistry in the hydrogenation of hydroxy-substituted A4 -3-ketosteroids. The presence of either a 170- (3,4,5)or 1 10-hydroxy (40) group leads to the formation of more of the 5 0 product than is observed in the hydrogenation of the 17P-acetate or the deoxy compounds. Comparison of the testosterone and cholestenone hydrogenation data (see Table I) shows the extent to which this effect can be operational. M e n the steroid contains an 1la-hydroxy group more of the 50 product is obtained (41). How extensive this hydroxy group effect is remains to be evaluated, but it appears to be reasonably effective in determining the direction of hydrogenation of hydroxy-substituted molecules over a palladium catalyst (9). This effect is not operative over platinum or rhodium catalysts (42).

IV. Hydrogenation of Other Ring Systems A. HETEROCYCLES Replacement of one of the carbon atoms of the octalone ring system, as in compounds XXX, XXXI, and XXXII, by a nitrogen results in different hydrogenation stereochemistry, as shown in Table IV. It is possible that in the TABLE IV Percent cis Product Obtained on Hydrogenation of Heterocycles XXX, XXXI, and XXXII in Neutral and Acidic Mediaa % cis product

Medium

XXXb

XXXIb

XXXIIC

Neutral Acid

35 86

60 90

30 85

aStructures XXX-XXXII are shown on p. 76. bData from Durand-Henchoz and Morrau (43). CDatafrom Augustine (13).

76

ROBERT L. AUGUSTINE 0

Ph-C-N

"

0

(XXX)

(XXXI)

mo (XXXII)

N-methyl compounds (XXX and XXXI) the unshared electron pair on the nitrogen is facilitating adsorption on the catalyst in a specific way, but this is unlikely with theN-benzoyl species (XXXII). It is also possible that in the N-methyl compounds the reaction is analogous t o the hydrogenation of octalone in very weak base. With the N-benzoyl material, the introduction of the trigonal nitrogen causes conformational changes in the molecule that change the preferred mode of adsorption. In acid, the protonation, hydride ion transfer process, discussed in the foregoing, is probably occurring.

B. HYDRINDENONES The hydrogenation of hydrindenones with the unsaturated ketonic system in either the five- or six-membered ring (XXXIII and XXXIV) gives almost exclusive cis-product formation, regardless of the nature of the solvent used or the type of angular substituent present (22, 44-46). The presence of a substituent at the 4-position in 9-methyl-hydrind-4-ene-3-ones (XXXV) leads to preferential trans-isomer formation (45-48). The fact that the 4-substituent in the saturated

(XXXIII)

(

x x x Iv )

product (XXXVI) is invariably in the equatorial configuration strongly suggests a 1,4-addition process as described previously for the hydrogenation of the 4-substituted- A4 -3-ketosteroids. In the hydrindenone series, however, molecular models do not show the presence of a peri-interaction in either the cis or the trans 1,4-diadsorbed species so that the formation of the trans product must be caused by some other factor. Utilization of a boat-type intermediate has been proposed (48) t o explain these observations with the equatorial conformation acquired by equilibration of the 4-substituent in the reaction medium. Such

(XXXV)

(XXXVI)

77

HYDROGENATION OF UNSATURATED KETONES

facile equilibration of a 2-substituted ketone in a neutral ethanol solution is rather unlikely, however, and the 1,4-addition process seems more reasonable. Further examination of the molecular models of the cis and trans 1,4-diadsorbed hydrindenones shows that the presence of an angular methyl group causes a tilting of the cis-diadsorbed moiety toward the double bond side of the molecule. In the absence of a 4-substituent, cis adsorption is still favored. However, when a 4-substituent is present the tilting of the molecule in its direction causes a severe interaction between this substituent and the catalyst surface, thus causing the trans-adsorbed species to be favored. In the absence of an angular methyl group this effect should not be seen, but no data are available to verify this hypothesis. C. MISCELLANEOUS

The hydrogenation of unsaturated ketonic moieties in ring systems other than those already discussed has not been reported very frequently. The size of the ring containing the double bond is very important. When the unsaturated carbonyl grouping has the double bond in a five-membered ring, hydrogenation appears to give only the cis product. A similar relationship also appears to be present when the double bond is exocyclic to rings of different size as, for example, in the hydrogenation of the various dibenzylidene cyclohexanones 9h

(XXXVII)

e

Yh

PF

0

9h

(XXXIX)

(XXXVIII)

(XXXVII) (49). With the cyclopentanone (n = 0), only the cis-dibenzyl product (XXXVIII) is obtained; from cyclohexanone (n = 1) the product is primarily the cis isomer. A mixture of the cis and trans products is obtained from cycloheptanone (n = 2), whereas from the cyclooctanone (n = 3) the product is primarily the trans isomer (XXXIX). Similar effects are also observed in the hydrogenation of the bicyclic systems (XL) in acidic media, as shown in Table V (SO). The hydrogenation of 3,s-disubstituted cyclohexenones (XLI) gives exclusively cisdisubstituted cyclohexanones regardless of reaction conditions (22). 0

0

78

ROBERT L. AUGUSTINE TABLE V Product Stereochemistry Obtained on Hydrogenation of Bicyclic Unsaturated Ketones in Acidic Mediuma I

+

n

%

1

99 93 89 85

2 3 4

I

I

I

%

1

I 11 15

aData from Granger er al. (50).

V. Conclusions The product stereochemistry obtained on hydrogenation of a,P-unsaturated ketones is dependent on a number of reaction variables. The effect of solvent on this hydrogenation of ketones can be summarized rather simply. In neutral media the reaction occurs essentially by way of the classic Horiuti-Polanyi (2) process which has been modified to incorporate both 1,2- and lY4-addition of hydrogen to the conjugated system. The relative amounts of 1,2- and 1,Caddition that take place are dependent on the nature of the solvent. In polar aprotic solvents, 1,4-addition predominates, whereas in nonpolar aprotic solvents the hydrogenation occurs primarily by a 1,2-addition sequence. In hydroxylic media the smaller polar solvents, because of an interaction with the carbonyl group, promote 1,2-additionYwhereas in the more bulky solvents, 1,4-addition takes place. In acid there is an initial protonation of the conjugated moiety followed by adsorption of the P-carbonium ion on the catalyst surface and hydride ion transfer from the catalyst to this charged species. Since product stereochemistry is regulated by the hydrogen availability to the catalyst, adsorption of the carbonium ion must be a reversible process. In base, however, the formation of an enolate anion occurs first. This species is then irreversibly adsorbed on the catalyst. Hydride ion transfer from the catalyst followed by protonation at the P-carbon from the solution regulates the product stereochemistry. With these mechanistic rationales and a knowledge of how the nature of the catalyst affects the reversibility of the individual steps in the hydrogenation process as well as how steric factors can be involved in olefin adsorption, it

HYDROGENATION OF UNSATURATED KETONES

79

should be possible to understand more thoroughly the effect of different reaction parameters on the outcome of the hydrogenation. REFERENCES 1. Siegel, S . , Advan. Catal. 16,123 (1966). 2. Horiuti, I., and Polanyi, M., Trans. Faraday SOC. 30,1164 (1934). 3. McQuillin, F. J., Ord, W. O., and Simpson, P. L., J. Chem. SOC.,London p. 5996 (1963). 4. Combe, M. G., Henbest, H. B., and Jackson, W. R., J. Chem. SOC.C p. 2467 (1967). 5. Nishimura, S., Shimahara, M., and Shiota, M., J. Org. Chem. 31,2394 (1966). 6. Nishimura, S., Shimahara, M., and Shiota, M., Chem. Ind. (London) p. 1796 (1966). 7. Slomp, G., Jr., Shealy, Y. F., Johnson, J. L., Donia, R. A., Johnson, B. A., Holysz, R. P., Pederson, R. L., Jenson, A. D., and Ott, A. C.,J. Amer. Chem. SOC.77,1216 (1955). 8. Djerassi, C., Yashkin, R., and Rosenkranz, G., J. Amer. Chem. SOC.74, 422 (1952); Gabbard, R. B., and Segaloff, A., J. Org. Chem. 27,655 (1962). 9. Augustine, R. L., in “Organic Reactions in Steroid Chemistry” (J. Fried and J. Edwards, eds.), p. 111. Van Nostrand-Reinhold, New York, 1972. 10. Loewenthal, H. J. E., Tetrahedron 6,269 (1959). 11. Weidlich, H. A., and Meyer-DeLius, M., Ber. Deut. Chem. Ges. 74, 1195 (1941); Weidlich, H. A., Chemie 58,30 (1945). 12. Brewster, J. H., J. Amer. Chem. SOC. 76,6361 (1954). 13. Augustine, R. L., J. Org. Chem. 23,1853 (1958). 14. Ullman, E. F., J. Amer. Chem. SOC.81,5386 (1959). 15. Read,G., and Ruiz, V. M., J. Chem. SOC.(PerkinI) p. 235 (1973). 16. Read, G. and Ruiz, V. M., J. Chem SOC.(PerkinI) p. 368 (1973). 17. Markman, A. L., Zh. Obshch. Khim. 24,2184 (1954); [Chem. Abstr. 50,5426 (195611. 18. Howe, R., and McQuillin, F. J., J. Chem. SOC.,London p. 1194 (1958). 19. Wilds, A. L., Johnson, J. A., Jr., and Sutton, R. E., J. Amer. Chem. SOC.72,5524 (1950).

20. 21. 22. 23. 24.

Foscante, R. E., Ph.D. Thesis, Seton Hall Univ., South Orange, New Jersey, 1966. Migliorini, D. C., Ph.D. Thesis, Seton Hall Univ., South Orange, New Jersey, 1966. Augustine, R. L., and Broom, A. D., J. Org. Chem. 25,802 (1960). Augustine, R. L., J. Org. Chem. 28,152 (1963). Augustine, R. L., Migliorini, D. C., Foscante, R. E., Sodano, C. S., and Sisbarro, M. J., J . Org. Chem. 34, 1075 (1969); Augustine, R. L., Ann. N . Y. Amd. Sci. 145, Art. 1, 19 (1967).

25. Kosower, E. M., J. Amer. Chem. SOC.80, 3253 (1958); Kosower, E. M., and Remy, D. C., Tetrahedron 5,281 (1959). 26. Fuson, R. C., Course, J., and McKeever, C. H., J. Amer. Chem. SOC.62,3250 (1940); Fuson, R. C., Byers, D. J., and Rachlin, A. I., J. Amer. Chem. SOC.64, 2891 (1942); Attenburrow, J,, Connett, J, E., Graham, W.,Oaghton, J. F., Ritchie, A. C., and Wilkinson, P. A., J. Chem SOC. London p. 4547 (1961); Heusler, K., Wieland, P., and Wettstein, A., Helv. Chim Acta 42,1586 (1959). 27. Yao, H. C., and Emmett, P. H., J. Amer. Chem SOC.81,4125 (1959). 28. Malhotra, S. K., and Ringold, H. J., J. Amer. Chem. SOC.86, 1997 (1964). 29. House, H. O., Trost, B. M., Magin, R. W.,Carlson, R. G . , Franck, R. W., and Rasmusson, G. H., J. Org. Chem. 30,2513 (1965). 30. Djerassi, C., and Gutzwiller, J., J. Amer. Chem SOC.88,4537 (1966). 31. Voelter, W., and Djerassi, C., Chem. Ber. 101,58 (1968). 32. Jardine, I., and McQuillin, F. J., J. Chem. Soc D p. 503 (1969).

80

ROBERT L. AUGUSTINE

33. Dauben, W. G., Rogan, I. B., and Blanz, E. J., Jr., J. Amer. Chem. SOC.75, 6384 (1954); Sondheimer, F., and Rosenthal, D., J. Amer. Chem. SOC.80, 3995 (1958); Futaki, R., J. Org. Chem. 23,451 (1958). 34. Dreiding, A. S., and Tomasewski, A. J., J. Amer. Chem. SOC.77,168 (1955);Dauben, W. G., Tweit, R. C., and MacLean, R. L., J. Amer. Chem. SOC.77,48 (1955);Turner, R. B., Lee, R. E., Jr., and Hildenbrand, E. G., J. Org. Chem. 26,4800 (1961). 35. Thompson, H. W., Ann. MY. Acnd. Sci. 214, 195 (1973); Thompson, H. W., and Naipawer, R. E., J. Amer. Chem. SOC.95,6379 (1973). 36. Brodie, H. J., Hayano, M., and Gut, M., J. Amer. Chem. SOC.84,3766 (1962);Schmitz, F. J., and Johnson, W. S., Tetrahedron Len. p. 647 (1962); Ringold, H. J., Gut, M., Hayano, M., and Turner, A., Tefrahedron Left. p. 835 (1962); Ringold, H. J., M. Hayano, and Stefanovic, V., J. Biol. Chem. 238,1960 (1963). 37. Brodie, H. J., and Waig, P. A., Tefruhedron 23, 535 (1967); Brodie, H. J., Raab, K., Possanza, G., Seto, N., and Gut, M.,J. Org. Chem. 34,2697 (1969). 38. Rosenthal, D., Niedermeyer, A. O., and Fried, J., J. Org. Chem. 30, 510 (1965); Henbest, H.B., Jackson, W. R., and Malunowicz, I., J. Chem. SOC.C p. 2469 (1967). 39. Mitsui, S., and Imaizumi, S., Bull. Chem. SOC. Jap. 34, 774 (1961); Mitsui, S., and Kudo, Y.,Chem. Ind. (London) p. 381 (1965);Mitsui, S., Senda, Y.,and Konno, K., Chem. Ind. (London) p. 1354 (1963). 40. Palaki, J., Rosenkranz, G., and Djerassi, C., J. Biol. Chem. 195,751 (1952). 41. Mancera, O.,Ringold, H. J., Djerassi,C., Rosenkranz, G., and Sondheimer, F.,J. Amer. Chem. SOC. 75, 1286 (1953); Suvorov, N. N., and Yaroslavtseva, Zh. Obshch. Khim. 31,1372 (1961); [Chem. Absfr. 55,23593 (1961)l. 42. Caspi, E., J. Org. Chem. 24,669 (1959). 43. DurandNenchoz, S., and Monau, R. C., Bull. SOC.Chim. Fr. p. 3416 (1966). 44. Boyce, C. B. C., and Whitehurst, I. S., J. Chem. Soc., London p. 4547 (1960);Dauben, W. G., McFarland, J. W., and Rogan, J. B., J. Org. Chem. 26,297 (1961);Chaykovsky, M., and Ireland, R. E., J. Org. Chem. 28,748 (1963). 45. Baggaley, K. H., Brooks, S. G., Green, J., and Redman, B. T., J. Chem. SOC.C p. 2671 (1971). 46. Hajoz, Z.G., and Parrish, D. R., J. Org. Chem. 38,3239 (1973). 47. Hajoz, Z. G., Panish, D. R., and Oliveto, E. P., Tetrahedron Lett. p. 6495 (1966); Hajoz, Z.G., Micheli, R. A., Parrish, D. R., and Oliveto, E. P., J. Org. Chem. 32,3008 (1967);Nomine, G., Amiard, G., and Torelli, V., Bull. SOC.Chim. Fr. p. 3664 (1968); Gardner, J. N., Anderson, B. A., and Oliveto, E. P., J. Org. Chem. 34,107 (1969). 48. McKenzie,T. C . , J. Org. Chem. 39,629 (1974). 49. Irvine, J. L., Hall, I. H., Carlson, G. L., and Piantadosi, C., J. Org. Chem. 37, 2033 (1972). 50. Granger, R., Chapat, J.-P., and Crassous, J., C. R. Acnd. Sci, Ser. C 265, 529 (1967).

Asymmetric Homogeneous Hydrogenation J. D. MORRISON

AND

W. F. MASLER

Department of Chemistry University of New Hampshire Durham, New Hampshire AND

M. K. NEUBERG Department o f Chemistry Stanford University Stanford, California I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Asymmetric Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Chiral Heterogeneous Catalysts . . . . . . . . . . . . . . . . . . . . . . . C. Archiral Rhodium-Phosphine Catalysts. . . . . . . . . . . . . . . . . . . 11. Homogeneous RhodiumChiral Phosphine Catalyst Systems . . . . . . . . A. An Overview of Germinal Experiments . . . . . . . . . . . . . . . . . . B. Synthesis of Chiral Phosphine Ligands . . . . . . . . . . . . . . . . . . . C. Asymmetric Reduction of Ketones and Imines . . . . . . . . . . . . . . D. Reaction Conditions and Tabulated Data for Olefiic Substrates 111. Chiral Amide-Rhodium Complexes as Catalysts . . . . . . . . . . . . . . . IV. Chiral Cobalt Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V. Chi~alRuthenium Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . VI. Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Note Added in Proof . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

....

81 82 83 83 85 85

93 103 106 115 118 120 121 122 122

I. Introduction In recent years there has been much interest in homogeneous hydrogenations catalyzed by transition metal complexes (I). One facet of research in this area is the search for chiral catalysts (catalysts that are dissymmetric, i.e., optically active) that can be used to produce chiral compounds via asymmetric reactions. In this review, we survey asymmetric homogeneoug hydrogenation reactions, that is reactions that create asymmetric carbon atoms by the addition of hydrogen across multiple bonds under the influence of soluble chiral catalysts. Before launching into this subject, we briefly review the concept of an asymmetric synthesis, say a few words about some asymmetric heterogeneous hydrogenations, and outline basic information about soluble achiral rhodium81

82

J. D. MORRISON, W. F. MASLER, AND M. K. NEUBERG

phosphine complexes that have stimulated much of the chiral catalyst research to date.

A. ASYMMETRIC SYNTHESIS The basic principles of asymmetric synthesis have been discussed in a number of recent reviews ( 2 , 3 ) . The heart of an asymmetric synthesis is a reaction in which an achiral unit in an ensemble of substrate molecules is converted into a chiral unit in such a manner that the stereoisomeric products are obtained in unequal amounts. For almost all cases this is equivalent to the statement that in an asymmetric reaction a prochiral unit is converted into a chiral unit. There are two common ways to accomplish an asymmetric reaction. Either a second chiral center is created in a molecule under the influence of an existing chiral center in that molecule or a chiral reagent acts on a prochiral substrate to create a new chiral center. The conversion of chiral a-keto esters to di,astereomeric a-hydroxy esters is an example of the first type of asymmetric reaction, and the asymmetric hydroboration of alkenes with chiral boranes is an example of the second type (Fig. 1). OH

0

(a)

II Ph-C-C-O-;dR' I1 0

I.MeMel 2. H'

I*

+ Ph- C-C

1

bR,,

II

-0-6RR'R"

Me 0

HOS03NHl

H20,. OH-

(7% HO-C*-H

I

CHlCH3

A4kCH3

1 - u-cI--Nn, I

1. Pd-Silk fibroin.hydropen

(')

2. hydrolysis

HC Ph

CH,Ph

FIG. 1. Examples of asymmetric synthesis. (a) Addition of a Grignard reagent to a chiral a-keto ester. (b) Chiral hydroboration with di-3-pinanylborane. (c) Chiral heterogeneous hydrogenation.

ASYMMETRIC HOMOGENEOUS HYDROGENATION

83

Asymmetric hydroboration and conceptually similar reactions involving chiral reagents have been used with great success. Their principal shortcoming is that stoichiometric quantities of chiral compounds must be invested and only rarely can these compounds be recycled. An asymmetric reaction involving a catalyst that is chiral would be a superior way to accomplish an asymmetric synthesis since, with only a small amount of chiral material, large quantities of optically active product could, in principle, be obtained. The idea of chiral catalysis has fascinated chemists for some time, in part because enzymes are chiral catalysts. The greatest amount of work has been done in the area of chiral hydrogenations.

B. CHIRALHETEROGENEOUSCATALYSTS Heterogeneous catalysts modified by the addition of chiral substances have been used to hydrogenate olefins asymmetrically, but only a few effective chiral heterogeneous catalyst systems have been found. Palladium deposited on silk fibroin was used to hydrogenate 4-benzylidene-2-methyl-5-oxazolone asymmetrically to give, after hydrolysis, optically active phenylalanine (Fig. lc). The optical purity' of the product was found to be dependent on the origin of the fibroin and its chemical pretreatment (4-6). Raney nickel has been modified with amino acids and other chiral reagents to give catalysts that have been used to effect asymmetric reductions (7). However, these catalysts suffer from some of the same kinds of vagaries that have been observed for the palladium on silk fibroin catalysts. For example, the optical purities of the products were found to be very dependent on pH and the method of catalyst preparation. The generally lowpercent asymmetric synthesis in asymmetric heterogeneous hydrogenations may be due, in part, to a nonuniform distribution of chiral modifying agents over the catalytic surfaces. In the case of silk fibroin, metal clumping on the chiral support or dissociation of the metal from the fibroin may allow some reduction to occur in an achiral local environment.

C. ACHIRALRHODIUM-PHOSPHINE CATALYSTS Most of the asymmetric homogeneous hydrogenations reported to date have used rhodium-phosphine complexes as catalysts. In a majority of these cases, it has been assumed that the active catalysts are similar to the well-studied 'We use the terms optical purity and percent enantiomeric excess (V&e) to express the extent to which one enantiomer is produced in excess over the other in an asymmetric reaction; i.e., optical purity or %ee= %R - V& (or vice versa). Normally, %eeis determined by measuring the rotation of the product and expressing it as a percentage of the maximum rotation for that product ([OL]obs/[Q]max X loo), although increasingly the percent enantiomeric excess is being determined by absolute NMR methods involving diastereomeric derivativesor chiral shift reagents.

84

J. D. MORRISON, W. F. MASLER, AND M. K. NEUBERG

Wilkinson catalyst, tris (triphenylphosphine) chlororhodium (I). With this in mind, it will be helpful to review briefly a few facts about [(C6H5)3P]&C1 before discussing the asymmetric rhodium catalysts in greater detail. Since the first report in 1965, on the catalytic activity of solutions of [(C6H&P] ,RhCl, extensive mechanistic studies have been carried out on this system (I,8 , 9 ) . However, the mechanistic picture is still somewhat clouded. The various pathways which have been suggested to be operating are shown in Fig. 2. Of the four catalytic routes illustrated, two (A+ B-+ C-+ G-+ H+ I+ products and A+ E+ F-+ G-+ H-+ I-+ products) consider Wilkinson's catalyst

saturated product

I

saturated product

+

A

L = Ph,P ligand * = ligand dissociation step (S)= coordinated solvent

-

= steps in

-

t D

the cycle that considers

A to be the active catalyst.

---c =steps in the cycle that considers

D to be the active catalyst. A and D cycles.

= steps common to

FIG. 2. Some catalytic schemes proposed for Wilkinson's catalyst.

ASYMMETRIC HOMOGENEOUS HYDROGENATION

85

(A) to be the so-called active species, i.e., the species directly involved in the catalytic cycle that first coordinates one of the reactants, hydrogen or olefin. The other two routes (A+ D+ C+ G+ H+ products and A+ D+ F+ G+ H+ products) consider A to be only the “catalyst precursor,” with species D being the actual active catalyst. The experimental evidence is in some respects equivocal, but on balance appears to favor the A+ B+ C+ G pathway over the other alternatives ( I ) . Without presenting any of the voluminous and sometimes contradictory experimental data that have been accumulated, three points on which there is general agreement, will be emphasized: 1. At some point in the catalytic cycle one phosphine ligand is dissociated, leaving an empty coordination site on the rhodium. 2. An intermediate (or at least an activated complex) exists in which phosphine, hydrogen, and olefin are all coordinated to the metal (species G in Fig. 2). 3. Hydrogen transfer is stereospecifically cis and occurs in a stepwise manner,’ passing through a metal alkyl complex (species H in Fig. 2), the so-called half-hydrogenated state.

I I. Homogeneous Rhodium-Chiral Phosphine Catalyst Systems A. AN OVERVIEWOF GERMINALEXPERIMENTS At about the time of Wilkinson’s discovery, new schemes were developed by others for the preparation and configurational correlation of chiral phosphines (I&-e). The combination of advances in homogeneous catalysis and chiral phosphine technology prompted research on chiral phosphine complexes. Horner et al. ( 1 1 ) were the fiist to hypothesize in print that rhodium complexes containing optically active tertiary phosphine ligands should effect the asymmetric hydrogenation of unsymmetrically substituted olefins. The first examples of asymmetric hydrogenation based on this principle were reported by Knowles and co-workers (the Monsanto group) in 1968 (12). Rhodium complexes of the type RhL3c13 (where L was a chiral phosphine) were used in the hydrogenation of a-phenylacrylic acid (atropic acid) and itaconic acid under the conditions indicated in Fig. 3. When L was (I?)-(-)methylphenyl-n-propylphosphine; 15% optically pure (S)-(+)-cu-phenyl-propionic acid and 3% optically pure methylsuccinic- acid (configuration unreported) were obtained. ’However, the second hydrogen transfer may follow the fiist so closely in many instances that the distinction between a “two-step” and “simultaneous” process is fragile. 3The phosphine ligand (PPhMePr”) had an optical purity of about 70%. In the original publication (12) there is a misprint that shows the chiral phosphine as PPhMePr’ rather than PPhMeR”.

86

J. D. MORRISON, W. F. MASLER, AND M. K. NEUBERG

.

PI1 ..

\

LjRhC13

C=CHI /

nooc

Ph-CH-CH,

20 alm. HI. 6 0 T bcnrene-ClOH-Cl~N

15% ee(S)

atropic acid

H,C=C

I

COOH

/CHzCooH

LfRhC13

20 a m . H2, 60°C benrcne-EIOH-Et,N

'COOH

-

cn,-cn-cn,coon I

COOH 3% ee

itaconic acid

Piiii CH,

L* = CH,CH,H,C'

'Ph

FIG. 3. Early examples of homogeneous asymmetric hydrogenation (12).

Although the structure of the active catalyst obtained in solution was uncertain, the Monsanto group suggested at the time that Wilkinson-type Rh(1) complexes might be involved. They did not speculate on how the reduction of Rh(II1) to Rh(1) might be accomplished, but one possibility is Rh(III)L3C13 + Hz HRh(III)L3CIz

HRh(III)CIzL3 + HCI,

(1)

Rh(I)L3CI + HCl

(2)

Presumably, added base (such as triethylamine) promotes metal reduction by removing acid. Almost simultaneously with Knowles and Sabacky, Horner and co-workers (13) tested their own hypothesis using a catalyst prepared in situ from p-dichlorobis (1,5-hexadiene)dirhodium(I) and (S)-(+)-methylphenyl-n-propylphosphine (Fig. 4). In this case the structure of the catalyst was more certain [Rh(l.S-HD)CII,

PMePh-n-Pr H2(1 a m ) , bcnzenc. 25°C

b * "(n-PrPhMeP),RhCI"

in siru catalyst

R = Et. 7 -a%>e e ( S ) R=OMe,3-4%ee(R)

** (S)-Methylpropylphenylphosphine

FIG. 4. Early examples of asymmetric homogeneous hydrogenation with a Wilkinsontype catalyst. 1,s-HD = 1,s-hexadiene.

ASYMMETRIC HOMOGENEOUS HYDROGENATION

87

prb:*4 I Ph

CI

FIG. 5. Presumed intermediate in asymmetric homogeneous hydrogenation, analogous to G in Fig. 2.

since related reactions of [Rh(olefin),Cl] compounds and tertiary phosphines were known to give neutral, square planar Rh(1) complexes of the type RhL3C1. Therefore, for their hydrogenations of a-substituted styrenes, Horner and coworkers assumed a mechanism that paralleled the one proposed for Wilkinson’s catalyst and involved the intermediate shown in Fig. 5 . The authors suggested that the preferential formation of (S)-(t)-2-phenylbutane (7-8% optical purity) from a-ethylstyrene and of (R)-(t)-l -methoxy-1-phenylethane (3-4% optical purity) from a-methoxystryrene could be rationalized by assuming a skewed arrangement of methyl and n-propyl groups of both phosphines and a preferred low-energy orientation for olefin binding. Subsequent to these experiments, Horner and Siege1 investigated the reduction of a number of other a-substituted styrenes, C6H5C(R)=CH2 (R = Et, n-Pr, bPr, OEt, a-naphthyl, CH2C6H5,or Br), using a number of chiral phosphines, MefiPhR’ (R’=n-Pr, i-Pr, n-Bu, or t-Bu). The optical purities of the hydrogenation products varied between 2-19% (14). The Monsanto group also extended their studies to other substrates (particularly a$-unsaturated acids) and other phosphines. In these later experiments, they generated the catalyst in situ, as the Horner group had done. A methanol solution containing an a-substituted acrylic acid and a trace amount of triethylamine was then added, and hydrogenations were carried out at 30 atm H2 and 6OoC. For various combinations of olefin and chiral ligand, optical purities ranged from 3 to 21% (Fig. 6). ‘R

R

\

IRhll,5-HD)CI] *, L‘

TI,N. bmrene-MeOH,

HOOC’

60’C. H2. 10 alm

=

CH,-

I

CU-COOU 3-21% ee

L* = PMePhR’ (chiral) R’ = n-Pr, i-Pr, cyclohexyl IS-HD = I S-hexddienc

FIG. 6. Asymmetric homogeneous hydrogenation of a-substituted acrylic acids with a Wilkinson-type catalyst under “extended” conditions (higher temperature and pressure than usual).

88

J. D. MORRISON, W. F. MASLER, AND M. K. NEUBERG

During the course of their work on the atropic acid-MePPhPr" system, the Monsanto group discovered some unusual effects on the hydrogenation rate and optical yield when the ligand/Rh ratio was varied. In the absence of any added base, both the rate and the optical purity increased to a maximum as L/Rh was increased from 2 to 8. This is in contrast to the general observation that excess ligand lowers the activity of a Wilkinson-type homogeneous hydrogenation catalyst by competing with substrate for vacant coordination sites on the metal. A number of experiments indicated that the observed effects were related to the formation of a phosphobetaine by the reaction of atropic acid with any ligand in excess of 2 equivalents per equivalent of rhodium. In the presence of the phosphobetaine, atropic acid was hydrogenated rapidly to chiral a-phenylpropionic acid. It was concluded that the phosphobetaine influenced the rate and optical yield only because it converted the substrate to the carboxylate anion (Fig. 7). This conclusion was supported by an experiment using L/Rh = 2 and the triethylamine salt of atropic acid in which a thirty-fold rate increase, compared to the rate at L/Rh = 2 in the absence of triethylamine, and also increased optical purity (28%ee) were obtained (15). The Monsanto group has considered various ways in which the carboxylate anion could promote such dramatic effects. First, coordination through the carboxylate group might permit more rapid binding of the substrate to the catalyst than occurs with the neutral acid, which coordinates only through the olefinic bond. Second, such carboxylate coordination would place the double bond in a position accessible to easy and rapid intramolecular hydrogen transfer, regardless of whether or not the olefin actually occupied a coordination site of the metal. Third, should the olefmic bond of the substrate indeed also bind, then the chelating ring formed would be expected to increase steric control. As is seen in later discussions, these ideas have been supported by the results of other workers. As described above, the rhodium-phosphine catalysts used extensively by the Horner and the Monsanto groups contained phosphines that were asymmetric

R,P:

+

H,C=C

/Ph 'COOH

R,&-CH,CHCOOH I

-

+ R,P - CH,CHCOO-

H,C=C

Ph

jh phosphobetaine

/Ph

1-

1 'roo-

'coo-

phosphobetaine salt

FIG. 7. Reaction of a tertiary phosphine with atropic acid to produce a phosphobetaine salt.

ASYMMETRIC HOMOGENEOUS HYDROGENATION

89

at phosphorus. A negligible optical yield was obtained in the one reported reaction involving an optically active phosphine ligand with a chiral carbon atom somewhat remote from the metal [ P ~ I P ( C H , & M ~ E ~ )However, ~]. in 1971, Morrison and co-workers (the New Hampshire group) showed that a successful asymmetric hydrogenation could be carried out with at least one phosphine not asymmetric at phosphorus, namely, neomenthyldiphenylphosphine (NMDPP). In fact, by using a catalyst believed to be Rh(NMDPP),Cl, these workers achieved what was, at the time, the highest degree of asymmetric induction ever accomplished with a chiral hydrogenation catalyst, either homogeneous or heterogeneous (16). A 61% ee of (S)-(+)-3-phenylbutanoic acid was obtained by hydrogenation of (E)-P-methylcinnamic acid in the presence of the Rh(NMDPP),Cl catalyst-(in situ preparation) and triethylamine (0.17 mole of EtJN per mole substrate) (Fig. 8). The New Hampshire group noted that in their work a,P-unsaturated carboxylates uniformly yielded products of higher optical purity than simple olefms. For example, the product enantiomeric excesses for (E)-P-methylcinnamic acid (61% ee), Q-a-methylcinnamic acid (52% ee), and a-phenylacrylic acid (28% ee) were in the range of 4 to 9 times greater than that for a-ethylstyrene (7% ee). In agreement with the Monsanto group, it was suggested that these observations might indicate a mechanism involving bifunctional substrate coordination through both the carboxylate anion and olefmic bond. The relatively high optical purities obtained with the Rh-NMDPP system are particularly interesting from a practical viewpoint since the NMDPP ligand is prepared from an inexpensive, commercially available, chiral precursor, (-)menthol (I 7). Tertiary phosphines chiral at phosphorus, on the other hand, are much less accessible and require a classic resolution step (see later discussion for details; Section 1I.B). Almost coincident with the New Hampshire group's report of dramatic success with NMDPP, Kagan and co-workers (the Paris group) reported that optical purities of 60-70% could be obtained with a new chiral diphosphine-rhodium(1) Ph IRh(CHz=CH2)2CIl *, NMDPP

/'="\

Ph

>

I

H,C-CH-CH,COOH

E13N, knrcne-EIOH. 60'C.

COOH

Hz(20 a l m )

61% e e ( S )

FIG. 8. Asymmetric homogeneous hydrogenation with a neomenthyldiphenylphosphine

(NMDPP)catalyst.

90

J. D. MORRISON, W. F. MASLER, AND M. K. NEUBERG IRh(cyclooctene),CI]

t

I

2D10P

benzene-EtOH IlPO".

,.1.

> 2 [Kh(DIOP)CI(S)] in siruDlOP catalyst

(S) = solvent

R' \

/R

H

'COOH

'

C=C

R I

(-1-DIOP catalyst

H2(1 aim). r. t . 1:2 benzene-Et0H

El,N,

2

R'CH,CHCOOH

A: R' = Ph; R = NHCOCH,

A: 12% e e ( R )

B: R' = H; R = NHCOCH,Ph

B : 68%e e ( R )

C:R'=H;R=Ph

C : 63%ee(S)

FIG. 9. Asymmetric homogeneous hydrogenations with the 2,34-isopropylidene-2,3dihydroxy-1,4-bis(diphenylphosphino)butae (DIOP) catalyst. Hydrogenation with a (+)-DIOP catalyst would, of course, give enantiomericproducts. (% ee = percent enantiomeric excess.)

catalyst system (18). Like NMDPP, the new phosphine ligand (-)-2,3-O-isopropylidene-2,3-dihydroxy-l,4-bis(diphenylphosphino)butane [(- )-DIOP] could be prepared from a readily available chiral compound, L(+)-tartaric acid. The DIOP catalyst, often represented as [Rh(DIOP)Cl(solvent)] was generated in situ as shown in Fig. 9. The substrates used in initial experiments and hydrogenated to products having up to 72% ee were two a-acylaminoacrylic acids and a-phenylacrylic acid. The high stereoselectivities observed with the DIOP catalyst have been attributed to the appreciable conformational rigidity due to the trans-fused dioxolane ring and also to the presence of the metal-containing chelate ring. Stereochemical control through participation of the carboxylic acid function of the substrate also seemed to be indicated since, in contrast to the result shown in Fig. 9 for free a-phenylacrylic acid, hydrogenation of methyl a-phenylacrylate gave methyl-2-phenylpropanoateof the R configuration, and only 7% ee. Later experiments, however, showed that although the carboxyl group was important in the case of a-phenylacrylic acid, it was not crucial for a successful asymmetric hydrogenation when the substrate also contained the enamide function (19). For example, compound I was hydrogenated with high asymmetric bias (78% ee). This result and others were taken as evidence that coordination through the enamide group may influence the stereochemical course of the reaction.

ASYMMETRIC HOMOGENEOUS HYDROGENATION

91

,NHCOCH, CH,HC‘=C,

Ph (1)

The favorable effect of the enamide function on asymmetric induction is indicated not only by the result with compound I, but also by later results summarized in Table I, where optical purities in the range of 70 to 80% were generally obtained for various derivatives of alanine, phenylalainine, tyrosine, and 3,4-dihydroxyphenylalanine(DOPA). The Paris group found that the Rh-(-)DIOP catalyst yielded the “unnatural” R or amino acid derivatives, whereas L-amino acid derivatives could be obtained with a (+)-DIOP catalyst. Since the optical purity of the N-acylamino acids can often be considerably increased by a single recrystallization (fractionation of pure enantiomer from racemate) and the N-acetyl group can be removed by acid hydrolysis, this scheme provides an excellent asymmetric synthesis route to several amino acids. An even more efficient asymmetric synthesis of a-amino acid derivatives has been described by the Monsanto group (2Ua-e). They have found that chiral o-anisylcyclohexylmethylphosphine (ACMP) (11), like DIOE’, exerts an extraTABLE I

Asymmetric Hydrogenations of c+Acylaminoac?yh?. Acids with the Soluble DIOP Catalysta R’HC=C

/

NHCOR

+R’CHzCHNHCOR

‘COOH

I COOH

R’ H

Ph p-OH-phenyl

p-OH-phenyl

R

Yield (%)

%) eeb

CH3 CH3 CH3

96 95 92

13

95

62

72 80

‘Reaction conditions as in Fig. 9, but without NEt3. DIOP = 2,3-O-isopropylidene-2,3dihydroxy-l,4-bis(diphenylphosphino) butane. b% ee = percent enantiomeric excess. (-)-DIOP gives D amino acid derivatives;(+)-DIOP gives L .

92

J. D. MORRISON, W. F. MASLEK, AND M. K. NEUBERG

(t)- ( R ) -ACfVIP

(11)

ordinarily effective chiral influence in the reduction of a-acylaminoacrylic acid substrates. Catalysts prepared from (+)-ACMP give L-amino acid derivatives and those containing the (-)-phosphine give derivatives of the D series. Many instances of 85-90% ee have been observed (Table 11). The ACMP ligand was deliberately designed to create the opportunity for secondary bonding between the substrate and the ligand. It was felt that a-acylaminoacrylic acid substrates might possibly act as “tridentate ligands” toward the catalyst: the olefmic and carboxylate groups interacting with the rhodium and the acylamino groups and hydrogen bonded to the methoxy groups of the ACMP ligands. It should be pointed out that asymmetric reactions other than hydrogenation have been carried out with chiral phosphine complexes of rhodium (and a few other metals). For example, asymmetric hydrosilylations (addition of Si-H across C=C, C=O, and C=N bonds) have been catalyzed by such complexes TABLE I1 Asymmetric Homogeneous Hydrogenations of a-Acylaminaacrylic Acids with the Monsanto Group [ (+)-(R)-ACMP] Catalysta COOH

I R-CH=C

R’CONH-C-H

\

I

NHCOR’

CH2 R

R

R’

Product (%ee)b

3-OMe, 4-OH-phenyl 3-OMe, 4-OAc-phenyl Phenyl Phenyl pC1-phenyl 3-(l-Acetylindolyl)

Ph Me Me Ph Me Me Me

90

H

88 85 85

I1 80 60

’(+)-(R)-ACMP = (+)-(R)+-anisylcyclohexyhnethylphosphme. bWith (+>(R>ACMP the products all have the S (or L) configuration. % ee = percent enantiomeric excess.

ASYMMETRIC HOMOGENEOUS HYDROGENATION

93

TABLE 111

Homogeneous Hydroformylation of Olefins in the Presence of ChiraI Rhodium(1)-Phosphine Catalysts Substrate

Chiral phosphinea

Product

PhCECH2 PhCH=CH2 PhC(Et)=CH2 PhOCH=CH2 PhCH=CH2 PhC(Me)=CH2 PhCH=CH2 PhCH=CHCH3 PhCH=CH2 cisCH 3CH= CHCH3

(+)-R-(PhCHz)MePhP (+)-NMDPP (+)-NMDPP (+)-NMDPP (-)-DIOP (-)-Drop (-)-DIOP (-)-DIOP (+)-DIOP (+)-DIOP

(9-PhCHMeCHO (9-PhCHMeCHO (R)-PhCHEtCH2CHO (R)-PhOCHMeCHO (R)-PhCHMeCHO (R)-PhCMeCH2CHO (R)CHMeCHO (R)-PhCH2CHMeCHO (S)-PhCHMeCHO QCH CH2CHMeCHO

Productb (% eel

17.5 Low Low LOW

3.8 1.7 25.2 15.5 16 27

aNMDPP = neomenthyldiphenylphosphine; DIOP = 2,34-isopropylidene-2,3dihydroxy1,4-bis(diphenylphosphino)butane. *% ee = Percent enantiomeric excess; “LOW” indicates that the optical purity was less than 2% ee.

(213-e). When a ketone or imine is hydrosilylated the intermediate silyloxy or silylamino compound can be hydrolyzed to an alcohol or amine. Thus the overall result is equivalent to direct hydrogenation:

H

H

Asymmetric hydroformylations (22a-c) and a variety of other chiral reactions (23a-d) have also been observed with metal complexes made from chiral phosphines (Table 111).

B. SYNTHESIS

OF

CHIRAL WOSPHINE LIGANDS

Two kinds of chiral tertiary phosphine ligands have been used in asymmetric hydrogenation experiments involving rhodium complexes: the Horner and Monsanto groups have concentrated on ligands whose chirality is centered at an asymmetric phosphorus atom, and the New Hampshire and Paris groups have focused their attention mainly on phosphiries that carry chiral carbon moieties. 1. Phosphine Ligands Chiral at Phosphorus

The earliest method of preparation of an optically active phosphorus compound was by resolution of a phosphine oxide: Meisenheimer resolved ethyl-

94

J. D. MORRISON, W. F. MASLER, AND M. K. NEUBERG

R

S

FIG. 10. Synthesis of chiral oxides by reaction of Grignard reagents with diastereomerically pure menthyl phosphinates. Deoxygenation of chiral phosphine oxides gives chiral phosphines.

methylphenylphosphine oxide as the d-bromocamphorsulfonate salt (24). Optically active phosphine oxides have also been prepared from resolved quaternary phosphonium salts (25) by reaction with sodium hydroxide (26) or by a Wittig sequence (27). Optically active phosphines can be obtained by cathodic reduction (28) of resolved quaternary phosphonium salts or by various silane reductions of resolved phosphine oxides (29a, b). The synthesis of chiral phosphines from resolved phosphonium salts or phos. phine oxides is an intrinsically limited approach. The groups attached to phosphorus must be present prior to resolution and, furthermore, the preparation of phosphine oxides and phosphines from phosphonium salts by chemical or electrochemical cleavage reactions requires that one of the groups bonded to phosphorus be substantially easier to cleave than the other three. A newer synthetic approach that overcomes some of the limitations inherent in the earlier methods described above has been developed by Mislow and co-workers (I0a-c). When unsymmetrically substituted phosphinyl halides are esterified with (-)-menthol, the resulting diastereomeric phosphinates can be separated by fractional crystallization (Fig. lo)." Displacement of the menthylAlternative methods of preparation of chiral phosphinates have also been reported (IOd, e).

95

ASYMMETRIC HOMOGENEOUS HYDROGENATION

oxy group by an appropriate Grignard reagent gives chiral tertiary phosphine oxides. The chiral tertiary phosphine oxides can be reduced to chiral tertiary phosphines by one of several methods: trichlorosilane (retention of configuration), trichlorosilane and a weakly basic amine (retention), trichlorosilane and a strongly basic amine (inversion), hexachlorodisilane (inversion), or phenylsilane (retention) (29). Although it does not circumvent a classic resolution step, the Mislow approach does introduce greater flexibility since a number of chiral phosphines can be obtained from a single resolved precursor. Unfortunately, the multistep synthesis of the diastereometically pure methyl phosphinate is tedious and normally gives rather low overall yields. ACMP and Related Ligands. The Monsanto group has applied the Mislow synthetic sequence to the synthesis of chiral ACMP, which is an especially effective ligand in asymmetric hydrogenation systems that produce optically active amino acids. Figure 11 shows the reaction sequence starting with the (R)p menthyl ester. The Grignard reaction gave a 70-90% yield of phosphine oxide. The selective reduction of the phenyl group in the chiral phosphine oxide was accomplished in about 60% yield. Deoxygenation of the ACMP oxide was carried out with Si2C16 or HSiC13-Et3N (inversion of configuration at phosphorus) in about 50% yield. The ACMP ligand can be used in situ with a soluble Rh(1)-alkene complex to produce a catalytically active system, but normally it is converted to a stable crystalline complex of the type [(ACMP),Rh (diene)] +X-,where the diene is, for example, 1,5-cyclooctadieneand X-is BF4-, PF6-, or B(Ph)4-. The Monsato application of the Mislow scheme has also produced other ligands of the ACMP type, for example with i-propyl, i-butyl, or benzyl groups in place of cyclohexyl and i-propyl, ethyl, or benzyl in place of methyl in the ether function. Ligands with these structural variations gave catalysts that were less effective than ACMP in terms of the optical purities of hydrogenation products (30). 0 0 II PhwP-0-Menthyl

I

II

PhmP1Me t

Me

HI. R h K

FIG. 11. Synthesis of (S)-o-anisylcyclohexylmethylphosphine (ACMP). The (R)-ACMP ligand is prepared from the phosphinate that is epimeric at phosphorus (see Fig. 10).

96

-

J. D. MORRISON, W. F. MASLER, AND M. K. NEUBERG

.

CH,CH,CH(Ph)COOH

a-methylbmzylamme

LiAIH4

*I SorIl. pyridinc

Ph

Hl

FIG. 12. Synthesis of (,S)-2-phenylbutyldiphenylphosphine. Catalysts prepared from this ligand, and structurally related ligands, typically give products of low optical purity (31).

2. Phosphine Ligands Chiral at Carbon a. NMDPP. MDPP, and CAMPHOS Ligands. The New Hampshire group has prepared a number of chiral phosphine ligands from chiral alkyl halides (Fig. 12) and tosylates via displacements with diphenylphosphide anion. For example, in some early experiments lithium diphenylphosphide was used to prepare (+)S2-methylbutyldiphenylphosphine, (+)S-2-phenylbutyl-diphenylphosphine,(-)R-3-phenylbutyldiphenylphosphine, (t)-R-2-octyldiphenylphosphine(configuration presumed but not rigorously proved), and (+)-neomenthyldiphenylphosphine (NMDPP) from the appropriate chloride or bromide (31). The (+)-NMDPP ligand proved to be especially effective in hydrogenation experiments (16) but was also found to be unexpectedly difficult to synthesize. Several complications were encountered. First, displacement of halogen from (-)-menthy1 chloride by “lithium diphenylphosphide” (prepared from chlorodiphenylphosphine and lithium in tetrahydrofuran), which proceeded readily at room temperature with some other primary and secondary halides, was very slow, and prolonged reaction times and elevated temperatures were required to effect complete reaction. Second, the yield of tertiary phosphine product was lowered due to a competing elimination reaction in which the phosphide anion functions as a base rather than a nucleophile. Third, the product was contaminated with two tenacious impurities, 4-hydroxybutyldiphenylphosphine(from the ring opening of the tetrahydrofuran solvent by lithium diphenylphosphide) and NMDPP oxide arising, most likely, from air oxidation of (+)-NMDPP during work-up. Chlorodiphenylphosphine has been shown to react with alkali metals and magnesium in tetrahydrofuran solution to give 4-hydroxybutyldiphenylphosphine so this by-product was not unexpected. The ring-opening reaction is specific for tetrahydrofuran; dioxane and aliphatic ethers are not affected. It was found, however, that sodium diphenylphosphide prepared from diphenylphosphine and sodium metal in either tetrahydrofuran or liquid ammonia gave no detectable ring opening of the tetrahydrofuran solvent, and the use of sodium diphenylphosphide prepared in this way became the preferred method of gen-

ASYMMETRIC HOMOGENEOUS HYDROGENATION

97

Ph2PNs TH I

NMDPP

FIG. 13. Synthesis of neomenthyldiphenylphosphine(NMDPP).

erating diphenylphosphide anion for displacement on menthyl chloride (I 7, 32) (Fig. 13). The reaction of (-)-menthy1 chloride with sodium diphenylphosphide in tetrahydrofuran requires 48-54 hr at reflux temperature for completion. The elimination side reaction is still observed. However, by-products (isomeric menthenes and diphenylphosphine) arising from the elimination reaction are easily removed by distillation. The overall conversion of (-)-menthy1 chloride to (+)-NMDPP is about 34%, not counting the (+)-NMDPP oxide produced during a typical work-up. The (t)-NMDPP ligand is rather sensitive to air oxidation in solution and (+)-NMDPP oxide can be a very tenacious impurity, but careful crystallization of the phosphine from deoxygenated ethanol gives (+)-NMDPP in 95%(or higher) purity. The reaction of sodium diphenylphosphide with (t)-neornenthyl chloride (Fig. 14) gives (-)-menthyldiphenylphosphine (MDPP). The overall conversion of (+)-neomenthyl chloride to (-)-MDPP in a typical experiment is 25-30%.' The yield of (-)-MDPP was lower than the yield of (t)-NMDPP because elimination is a more serious competitive process for (+)-neomenthyl chloride6 than for (-)-menthy1 chloride. The MDPP ligand is easily purified by crystallization from ethanol, and a purity of 98%(2%oxide) is attainable with one crystallization. 'The diphenylphosphine elimination by-product from both the (+)-NMDPP and (-)-MDPP syntheses can be recovered so that the syntheses are more economical than they may appear to be. 6The transdiaxial relationship between the halogen and the hydrogens at C-2 and C-4 accounts for the relatively greater ease with which (+)-neomenthyl chloride undergoes E2 elimination.

98

J. D. MORRISON, W. F. MASLER, AND M. K. NEUBERG

PhzPNa THF

MDPP

FIG. 14. Synthesis of menthyldiphenylphosphine (MDPP).

In conjunction with the syntheses of (+)-NMDPP the relative effectiveness of lithium, sodium, and potassium diphenylphosphides was determined. Under a standard set of conditions the reaction of (-)-menthy1 chloride with sodium diphenylphosphide gave the highest yield of (+)-NMDPP. The ratios of the yields of (+)-NMDPP were 1.O : 1.55 : 1.16 for lithium, sodium, and potassium diphenylphosphide, respectively ( I 7,32). The c h i d diphosphine ligand, (+)(lR, 3S)-1,2,2-trimethyl-l,3-bis(diphenylphosphhomethyl)cyclopentane, commonly called (+)-CAMPHOS (111), has also

(+) -CAMPHOS

been prepared by the New Hampshire group (32). The synthesis of this ligand posed special challenges and ultimately resulted in some new synthetic approaches that may be useful in other ligand syntheses. The starting compound in the CAMF'HOS synthesis, commercially available (+)-camphoric acid, was reduced to 1,2,Ztrimethyl-l,3-bis(hydroxymethyl)cyclopentane with lithium aluminum hydride in ether (Fig. 15). In the initial trials to synthesize CAMPHOS, many procedures were used in an attempt to prepare dihalide from the diol. None of a great many standard methods met

ASYMMETRIC HOMOGENEOUS HYDROGENATION

JCooH

99

coon A LIAIH

FIG. 15. Attempted synthesis of dihalide precursor of 1,2,2-trimethyl-l,3-bis(diphenylphosphino)cyclopentane.

with any success.' Reaction mixtures that could not be adequately characterized by IR and NMR were obtained. Reaction of the diol with p-toluenesulfonyl chloride in pyridine, however, pro~ by chloduced the ditosylate in nearly quantitative yield. S N displacements ride on neopentyl tosylate, which bears certain structural similarities to the ditosylate precursor of CAMPHOS, have been shown to give good yields of neopentyl chloride. However, when 1,2,2-trimethyl-l,3-bis(hydroxymethyl)cyclopentane ditosylate was allowed to react with sodium chloride in hexamethylphosphoramide, in an attempt to form the dichloride, only N, N-dimethylp-toluenesulfonamide was isolated. Reaction of the ditosylate with lithium chloride in ethoxyethanol was exothermic and HCl was evolved but the dichloride was not isolated. The isolated product contained at least one olefinic bond. Similarly, in N, N-dimethylformamide, lithium chloride and the ditosylate gave a product that decomposed on distillation. Faced with such repeated failures, a dihalide route to CAMPHOS was abandoned in favor of a more direct approach: reaction of the ditosylate with diphenylphosphide anion. The synthesis of CAMPHOS by displacement on its ditosylate precursor with the diphenylphosphide anion appeared promising on paper, but initially was a dismal failure in practice. The reaction of lithium diphenylphosphide (from PhzPCl and Li) gave no CAMPHOS. However, when potassium diphenylphosphide (from PhzPH and K) in tetrahydrofuran was used, in place of the lithium reagent, (+)CAMPHOS was formed (Fig. 16). The reaction of the ditosylate with potassium diphenylphosphide is initially exothermic. However the reaction does not go to completion under its own power-heat must be applied. It is likely that the less hindered a-tosylate group is displaced or eliminated rather readily at room temperature, but the neopentyl-like P-tosylate group apparently requires more strenuous conditions to effect its displacement. 'Among the procedures tried were thionyl chloride and pyridine. phosphorus pentachloride, triphenylphosphine dibromide in N, Ndimethylformamide, triphenylphosphine and carbon tetrachloride, tris(dimethylamino)phosphine and bromine, o-phenylenephosphorochloridite and bromine, tris(dimethylamino)phosphine and carbon tetrachloride, and trin-octylphosphine and carbon tetrachloride.

100

J. D. MORRISON, W. F. MASLER, AND M. K. NEUBERG

FIG.16. Synthesis of (+)-1,2,2-trimethyl-1,3-bis(diphenylphosphino)cyc~opentane by tosylate displacement.

(t)-CAMPHOS is a viscous oil that cannot be purified by distillation or crystallization. After distilling the reaction mixture to remove low boiling by-products such as diphenylphosphine, the pot residue, chiefly (t)-CAMPHOS and (t)CAMPHOS dioxide, is subjected to column chromatography on silica gel or alumina, eluting the purified (t) -CAMPHOS with benzene. An alternative and more circuitous route to (t)-CAMPHOS from (+)-camphor has been developed but the much more direct phosphide route is preferable (32). The observations made by the New Hampshire group concerning the variable reactivity of metal phosphides with alkyl halides and tosylates should be kept in mind when planning ligand syntheses by these routes. It appears that, for any particular halide or tosylate substrate, the best metal phosphide for displacement can be determined only by experiment. b. DIOP and Related Ligands. The Paris group has achieved much success with diphosphine ligands derived from chiral tartaric acid, both enantiomers of which are commercially available. The “parent ligand” in the Paris arsenal is DIOP which is prepared as shown in Fig. 17. Compounds IV and V, which are similar tojDIOP, have also been synthesized (33). Chiral, insolubilized, catalytically active, transition metal complexes that incorporate DIOP moieties have also been developed by the Paris group (34). Insolubilized complexes exhibit some features of both homogeneous and heterogeneous catalysts. The catalyst can be more easily recovered during product work-up, and greater air stability is observed. In addition, although solvent

ASYMMETRIC HOMOGENEOUS HYDROGENATION

B

Y

HO/

c

‘COOH

HO,=,COOEt EtOH,H*-

7

-

Me,C(OMe),

,,y,COOEt

101

c

Hi, benzene

COOEt

pO/$.OOEt

(+)-tartaric acid

(-)-DIOP

FIG. 17. Synthesis of (-)-2,3~-isopropylidene-2,3dihydroxy-l,4-bis(diphenylphosphino)butane (DIOP).

channels in the polymer support allow many soluble substances to enter and leave the reaction site, the pores of the polymer are capable of excluding certain olefins on the basis of molecular size (35-37). Also important is the fact that polymer-supported homogeneous catalysts lend themselves to continuous flow processes and are not limited to more inefficient batch processes as are their soluble counterparts. In early studies on insolubilized systems, Grubbs and Kroll(35) found that when chloromethylated polystyrene beads, Merrifield resins (38), were treated

102

J. D. MORRISON, W. F. MASLER, AND M. K. NEUBERG

Merrifield resin

aldehyde polymer

insolubilized DIOP

FIG. 18. Synthesis of insolubilized 2,3.O-isopropylidene-2,3dihydroxy-l,4-bis(diphenylphosphin0)butane (DIOP).

with lithium diphenylphosphide, 80% of the chlorine atoms were replaced to give a polymer containing tertiary phosphine groups. This polymer was then equilibrated with tris(triphenylphosphine)rhodium(I) chloride to give an insolubilized catalyst which was used to hydrogenate a variety of olefins. The hydrogenation rate was found to be dependent on the molecular size of the olefm. A decreased relative rate for large olefins was attributed to their exclusion from the catalytically active sites due to restrictions in the size of the solvent channels caused by cross-links in the polymer. This observation supports the view that the major portion of the reduction takes place inside the polymer beads. The insolubilized catalyst could be recovered by filtration and used again many times. The Paris group has used a modification of the Grubbs and Kroll system to insolubilize a rhodium derivative of DIOP (Fig. 18). A Merrifield resin was allowed to react with dimethylsulfoxide to convert the chloromethyl groups to aldehyde groups. The aldehyde resin was then allowed to react with (t)-l,4ditosylthreitol to give an acetal resin. Displacement of the tosyl groups by sodium diphenylphosphide gave a phosphinated resin. Reaction of the phosphinated resin with pdichlorotetraethylenedirhodium(1) gave an active chiral catalyst (34).

ASYMMETRIC HOMOGENEOUS HYDROGENATION

103

c. ASYMMETRIC REDUCTIONOF KETONES AND IMINES Homogeneous rhodium(1)-chiral tertiary phosphine catalysts have been used to hydrogenate ketones directly and t o hydrosilylate ketones and imines thus accomplishing, after hydrolysis, indirect hydrogenation. Bonvicini and co-workers (39) were the first to report a direct asymmetric homogeneous ketone hydrogenation with a chiral rhodium-phosphine catalyst. Hydrogenation of acetophenone and 2-butanone at 1 atm H2 and 25" in the presence of [Rh(nbd)L2 ]+C104- (L = (t)@)-benzylmethylphenylphosphine; nbd = norbornadiene) gave alcohols having the R configuration, 8.6 and 1.9% ee, respectively. Tanaka and co-workers (40) observed very low (<1% ee) optical purities when they reduced ketones with a similar catalyst incorporating (-)@)-methylethylphenylphosphine as the chiral ligand (40). The low optical purities in this case are not too surprising in view of the steric bulk similarity of the methyl and ethyl groups in the ligand and the lack of secondary bonding opportunities. Optical yields as high as 56% (but more typically 10-2W) have been recorded by Solodar (41) in the direct asymmetric hydrogenation of ketones with [Rh(COD) (ACMP),]' BF4-.8 Catalyst turnover ratios of over 1000 were observed. It was found that the stereochemistry was quite dependent on the choice of solvent and its water content. For example, the hydrogenation of 2-octanone in ethanol gave the (t)S-carbinol with 1.6% ee; in N , N-dimethylformamide (DMF) the R-carbinol was observed in 5.1% ee, and in acetic acid the R-carbinol was observed in 12.0% ee. In varying the water content of the isobutyric acid solvent in the hydrogenation of 2-octanone from 0.1 t o 8%, the optical yield dropped from 13.9 t o 5.3%. A synthetic application of the asymmetric homogeneous hydrogenation of ketones has been demonstrated by Sih and co-workers (42). The homogeneous, selective reduction of 2(6-carbomethoxyhexyl) cyclopentane-l,3,4-trionein the presence of 1,5-cyclooctadienebis(ACMP)rhodium(I) tetrafluoroborate and trie thylamine in methanol gave 246-carbomethyoxyhexyl)-4-R-hydroxycyclopentane-l,3-dione (68% ee). This compound was converted to prostaglandin El via a series of steps (Fig. 19). The 68% ee reported is not a valid representation of the asymmetric bias of the reduction. The product was isolated by crystallization and the chemical yield was less than 50%. The mother liquors from the crystallization were inactive, indicating optical enrichment due to fractionation of the enantiomer from the racemate. A cationic catalyst prepared from [Rh(R{PhCH2)MePhP),(nbd)]+C1O4- has been used to hydrosilylate several alkylphenylketones (43). Optical yields ranging from 5 t o 62% were reported, the most impressive being those for acetophenone (31.6% ee), ethyl phenyl ketone (43.1% ee), and t-butyl phenyl ketone 8Solodar (41) has also observed the asymmetric hydrogenation of ketones with related iridium catalysts but much more slowly than with rhodium catalysts.

104

J. D. MORRISON, W.

F. MASLER, AND M. K. NEUBERG

..(CH,),COOH HO."

on WE,

FIG. 19. Asymmetric homogeneous hydrogenation of a ketone in the synthesis of prostaglandin El. ACMP = o-anisylcyclohexylmethylphosphine.

(61.8% ee) when the hydrosilylations were accomplished with phenyldimethylsilane. Trimethylsilane gave lower optical yields. Asymmetric hydrosilylation of imines followed by hydrolysis of the N-silylamine intermediate yields chiral amines. The Kagan group hydrosilylated a series of prochiral imines with both polymethylhydrogensiloxane and diphenylTABLE IV

Asymmetric Hydrosilylations of Imines with the (+) -DIOP Catalysp CH3,

C=NR'

--+ CH3CH(R)-N(R')Si-

R'

Substrate PhC(CH3)= NCHz F'h PhC(CH3)= NCHz F'h PhC(CHs)=NPh PhC(CH3)=NPh PhCHzC(CH3)=NCHzPh PhCHzC(CH+ NCHz Ph

I

n20

+CH3CH(R)-NR'H

I

Amine product configuration

Silane HzSiPhz [-SiH(CH3)O-] HzSiPhz [-SiH(CH3)0-] HzSiF'h2 [-SiH(CH3)0-]

n

n

Amine product optical purity (% eel

S S S S S S

'DIOP = 2,3-Oisopropylidene-2,3dihydroxy-l,4-bis(diphenylphosphino)butane. b%ee = Percent enantiomeric excess.

50

3 40 47 12 14

ASYMMETRIC HOMOGENEOUS HYDROGENATION

105

silane over the DIOP catalyst (Table IV). Product optical purities varied with the temperature, and low temperatures were found to give the highest optical yields (21e). The insolubilized DIOP catalyst (34) was found to be rather ineffective for the asymmetric hydrogenation of olefinic substrates; the hydrogenation of a-ethylstyrene proceeded readily but gave (-)-R-2-phenylbutane with an optical purity of only 1.5%. Methyl atropate was hydrogenated to (+)a-methylhydratropate (2.5% ee). The soluble DIOP catalyst gave 15 and 17% ee, respectively, for the same reductions. The optical purity of the products was lower when recovered insolubilized catalyst was used. There was no reduction of a-acetamidocinnamic acid in ethanol-benzene with the insolubilized catalyst, presumably due to the hydrophobic nature of the polymer support causing it to shrink in hydroxylic solvents. More impressive results were obtained for the asymmetric hydrosilylation of ketones over the insolubilized catalyst (34). Acetophenone, for example, was hydrosilylated with phenylnaphthylsilane and the intermediate was hydrolyzed with HCI to give (-)Sphenylmethylcarbinol(58% ee). Similar results were obtained with the homogeneous DIOP catalyst. Using diary1 or arylalkyl silanes, yields from 52 to 100% and optical purities from 7 to 59% were realized (Table V). TABLE V Asymmetric Synthesis of Alcohols by Hydrosilylation of Ketones: A Comparison of an Insolubilized (+) -DIOP Catalyst vs. a Solution (+)-DIOP Catalyst" RIRZC=O

+RlRZCH-0-4-

I I

Ketone Acetophenone

Phenylisopropyl ketone

n+ + RIRzCHOH Product insolubilized catalyst (%ee)b

Product soluble catalyst

Silane

KetonelRh

Time (hr)

Hz SiPhCH3 HzSiPhz HZSiPh(C 10H7) HzSiPh(C10 H,)

25 35 33 20

42 24 45 24

12 29 59 55

13 28 58 58

HZSiPhCHJ Hz SiPhz

25 25

48 48

I 28

20 35

(%eelb

% all cases, the hydrosilylated product was hydrolyzed to the alcohol; (-)S-carbinol was obtained in every case. DIOP = 2,30-isopropylidene-2,3dihydroxy-l,4-bis(diphenylphosphino)butane. b% ee = Percent enantiomeric excess.

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D. REACTIONCONDITIONS AND TABULATED DATA FOR OLEFINICSUBSTRATES 1. The DIOP Catalyst: Reaction Conditions for Asymmetric Hydrogenation of Enamides Typically, the catalyst is prepared in situ from the reaction of 2 moles of DIOP with 1 mole of [RhCl(COD)] or a similar alkene complex. Excess phosphine greatly reduces or destroys entirely the catalyst activity. Hydrogenations have most often been carried out in a 1 : 2 benzene-ethanol solution, 2-5 mM in catalyst, at room temperature, and 1 atm hydrogen pressure. At the conclusion of the hydrogenation the reaction solution is evaporated to dryness and one of several work-up procedures is used. (a) For N-acetylalanine and N-acetyltyrosine water was added to the residue to dissolve the organic products. The catalyst residue is insoluble and can be filtered. (b) For other acylamino acids the residue was treated with 0.5N NaOH and filtered; the filtrate was acidified with dilute HC1, extracted with ether, and washed with a small amount of water. The ether solution was dried (Naz SO4) and evaporated to give the product. (c) For N-acetylphenylalaninamide and N-acetylamino-lphenyl-1-propane the organic products were isolated by thin-layer chromatography on silica gel. Table VI gives data for some hydrogenations with the DIOP catalyst.

TABLE VI Asymmetric Hydrogenation of or-Acylaminoacrylic Acids with the Soluble DIOP Catalyst” Substrate PhCH=C(NHCOCH3)COzH PhCH=C(NHCOPh)COZ H PhCH=C(NHCOCH3)COzCH3 PhCH=C(NHCOCH3)CONHz CHz=C(NHCOCH3)COz H

~-(OH)C~H~CH=C(NHCOCHJ)COZH 3,4-(CHzOz )C~HJCH=C(NHCOPH)CO~H ~-(OH)C~H~CH=C(NHCOP~)COZ H (CH3)zCHCH=C(NHCOPh)COz H

Substrate/Rh ratio 540

200 150 I5 150 100 50 80 15

Synthetic yield (%)

95 96 90 12 96 92 91 95 98

Product (% ee)

b

12 64

55 I1 13 80 19 62 22

“DIOP = 2,3-O-isopropylidene-2,3dihydroxy-l,4-bis(diphenylphosphino)butane. bResults thus far show that (-)-DIOP gives R- (or D)-amino acid derivatives and (+)-DIOP gives S (or L ) products. % ee = Percent enantiomeric excess.

ASYMMETRIC HOMOGENEOUS HYDROGENATION

107

2. Monsanto ACMP Catalyst: Reaction Conditions for Asymmetric Hydrogenations of a-Acylaminoacrylic Acids Either the free acid or its triethylamine salt may be hydrogenated. The free acid substrates give the best percent enantiomeric excess results at low pressures and this parameter is not especially sensitive to temperature. The anion substrate form is best reduced at lower temperatures (typically O'C), and percent enantiomeric excess values do not show much pressure sensitivity. Almost any alcohol solvent may be used and small amounts of water are tolerable. Hydrogenations are usually carried out in methanol solution (or, with some substrates, a 20-25% slurry is used) being careful t o purge oxygen which is a serious poison. At 40 psig, SO", and about 0.01%metal levels, the reduction of most acylaminoacrylic substrates is complete in about 4 hr (often less). The amino acid product may be crystallized from methanol, often with an increase in optical purity due to fractional crystallization of the pure enantiomer from the racemate. Rhodium may be recovered, but the ligand is oxidized during work-up. Table VII lists data for the ACMP and related catalysts. 3. The NMDPP, MDPP, and CAMPHOS Catalysts: Reaction Conditions for Asymmetric Hydrogenations of a,&Unsaturated Carboxylic Acids The New Hampshire group has compared the effectiveness of various catalysts toward a,&unsaturated carboxylic acid substrates. The typical set of reaction conditions used was as follows. Reductions were carried out in a 1 : 1 ethanol-benzene solvent, at 300 psi H z , for 24 hr at 60°C. The solvent was deoxygenated immediately prior to use. The catalyst was formed in 100 ml of solvent from 34 pmole of [Rh(COD)Cl] and 0.5 mmole of phosphine, was prereduced for 0.5 hr at 3.5 atm Hz, and then the substrate (most often 25 mmole; a substrate-rhodium ratiog of -375) was added in 100 ml of solvent containing 4 mmole of triethylamine. Table VIII shows the structure of the substrates that have been used in comparative studies by the New Hampshire group. Some of these substrates are quite hindered, but, with the NMDPP, CAMPHOS, DIOP, and ACMP catalysts, quantitative yields are usually obtained under the "extended" reaction conditions described above." The MDPP catalyst does not give a good yield in most instances even under extended conditions. At the conclusion of a hydrogenation reaction, solutions were evaporated to dryness and the residue was partitioned between 10% NaOH solution (50 ml) gSubstrate-rhodium ratios between 185 and 435 have been used in comparative studies. ''The "extended" reaction conditions involve higher pressure and tempmature than are normally used for Wilkinson-type catalysts and unhindered substrates.

108

J. D. MORRISON, W. F. MASLER, AND M. K. NEUBERG

TABLE VII Reduction of Some or-AcylaminoacrylicAcids (Mainly DOPA Precursors) with Rhodium(I)-Tertiary Phosphine Catalysts (200) Chiral Phosphine: R1RZR3P

Substrate: R~CH=C(NHCORS)COzH

R1

Rz

R3

o-Anisyl Me Me m-Anisyl o-Anisyl 0-Anisyl C6Hll 0-Anisyl o-Anisyl o-Anisyl o-Anisyl o-Anisyl o-Anisyl o-Anisyl o-Anisyl o-Anisyl

Me Ph Ph Me C6Hll

Ph n-Pr i-Pr Ph Me Me Ph i-Pr Ph n-Pr Me Me Me Me Me Me

Me Ph Me Me C6Hll C6Hll C6Hll C6Hll C6H112 C6Hll

Phosphine (%eePIb

95 90 90 80 95 95 15

80 95 95 90 95 95 95

95 95

R4 3-OMe-4-OH-phenyl 3-OMe- 4- OH-phenyl 3-OMe-4-OH-phenyl 3-OMe-4-OH-phenyl 3-OMe-4-OH-phenyl 3-OMe-4-OH-phenyl 3-OMe-4-OH-phenyl 3-OMe-4 -OH-phenyl 3-OMe-4-OAc-phenyl 3-OMe-4-OAc-phenyl 3-OMe-4-OAc-phenyl 3-OMe-4-OAc-phenyl 3-OMe-4-OAc-phenyl Ph Ph H

R5

Ph Ph

Ph Ph Ph Ph Ph Ph Me Me Me Me Me Me Ph Me

Product optical purity (% eela 5 8‘ 2gd 2gd

Id 81e 9of 32d Id 5‘ 2Od

I 18 8Se

88f 8Se

6 Oe

“% ee = Percent enantiomeric excess. %robably minimum values in most cases. ‘These reductions were run in a stirred autoclave in methanol at 55 psi (absolute) of H2, SO”, with 1 equivalent of NaOH. The catalyst-to-substrate mole ratio was 1:3000. %hese reductions were run in a Parr shaker in methanol at 55 psi (absolute) at 25°C. eThese reductions were run in a Parr shaker in 95% ethanol at 10 psi (absolute). fThese reductions were run in a Pan shaker in isopropanol. gThis reduction was run as described in footnote c, but with triethylamine (0.05%) added instead of NaOH.

and methylene chloride (50 ml). The aqueous layer was separated, washed with ether, and then was acidified with hydrochloric acid. The organic acid product was extracted with ether. The ether solution was dried (MgS04) and concentrated t o give a “crude” product. Itaconic, citraconic, mesaconic, and (E) and (Z)a-phenylcinnamic acids give solid products; the other cinnamic acid derivatives and atropic acid give liquids. “Crude” solid products were analyzed by NMR, and optical rotations were taken on the unrecrystallized crude material. Crude liquid products were analyzed by NMR, then distilled, and optical rotations were taken o n distilled material.

ASYMMETRIC HOMOGENEOUS HYDROGENATION

109

TABLE VIII or,pUnsaturated Carboxylic Acid Substrates Used in Comparative Studies of Chiral Rhodium-Phosphate Catalysts Ph

nzc=c< co,n Atropic acid

( E ) - a -Methylcinnamic acid

( Z )- a -Methylcinnamic

( E ) - a -Phenylcinnamic acid

acid

( Z ) - a -Phenylcinnamic acid

( E ) - p -Methylcinnamic acid

,Cn,CO,H n,c=c, co,n ( Z ) -0 -Methylcinnamic acid

noc n, >c=c n,c ‘co,n Mesaconic acid

ltaconic acid

H,C\ /n ,c=c no,c \co,n Citraconic acid

Tables IX-XI11 show the stereochemical results obtained with each catalyst system and give some additional experimental detail. Table XIV is a composite table that pulls together the stereochemical data for all of the substrates and catalysts used.

4. Stereochemical Relationships It is possible to perceive a number of interesting relationships in the data of Table XIV. It is clear that the phosphoms-chiral ACMP catalyst, which is so

110

J. D. MORRISON, W. F. MASLER, AND M. K. NEUBERG

TABLE IX Asymmetric Homogeneous Hydrogenation of or,@ Unsaturated Carboxylic Acids with the Rhodium(I)-(+)-NMDPP Catalyst"

Substrateb

Substrate-Rh mole ratio

Reduction yield (%)'

Synthetic yield (%)d

Product (% ee)e

Product confiiuration

435

100

49.2

29.6

S

375

100

90.3

60.0

R

375

29

25.2

R

185

100

88.5

34.4

S

185

100

92.0

9.1

S

375

100

72.3

61.8

S

375 375 375 375

100 100 100 10

80.5 85.O 67.0

31.2 8.1 5.9

R R R

h

-

-

Atropic acidf (E)-a-Methylcinnamic acid (Z)-a-Methylcinnamic acid (E)-ar-Phenylcinnamic acid (Z)-ar-Phenylcinnamic acid (E)-0-Methylcinnamic acid (Z)-0-Methylcinnamic acid Itaconic acid Mesaconic acid Citraconic acid ~~

~

"NMDPP = neomenthyldiphenylphosphine. All reactions were carried out in a mediumpressure Parr apparatus for 24 hr at 300 psi hydrogen at 60°C in 200 ml 1 : 1 (v/v)deoxygenated ethanol-benzene with a substrate-to-triethylamine mole ratio of 6.25,unless otherwise noted. See Table VIII. 'The reduction yield was determined on the crude reduced acids by NMR spectroscopy. dThe synthetic yield data are for distilled products where the product was a liquid and for crude products where the product was a solid. % ' ee = Percent enantiomeric excess. fThe stubstrate-to-triethylamine mole ratio was 7.35. g(Z)-arMethylcinnamic acid (25 mmole) gave a mixture of saturated acid (29%) and starting material (71%), 3.85 g. The mixture was purified to 76.8% saturated product for the determination of the optical rotation. 'Citraconic acid (25 mmole) gave a mixture of saturated product (10%) and starting material (90%), 2.8 g. A rotation was not taken.

outstandingly effective for the asymmetric reduction of a-acylaminoacrylic acids, does not compete favorably with carbon-chiral ligands in terms of the percent enantiomeric excess values obtained with aJ-unsaturated carboxylic acids. This is not to say that other phosphorus-chiral ligands will also be less effective. An important point, however, is that the match-up of ligand and substrate is a critical, specific, and unpredictable feature of such reactions. A good ligand for one kind of substrate is not necessarily best for another kind.

111

ASYMMETRIC HOMOGENEOUS HYDROGENATION TABLE X

Reduction of a,0-Unsaturated Carboxylic Acids with the R hodium(I)- (-) -MDPP Catalyst a

Substrateb

Substrate-Rh mole ratio

Reduction yield (%)c

Atropic a c i d Q-a-Me thyicinnamic acid (Z)-a-Methylcinnamic acid (8)aPhenylcinnamic acid (Z)a-Phenylcinnamic acid Q-p-Methylcinnamic acid (Z)-fl-Methylcinnamic acid I taconic acid Mesaconic acid Citraconic acid

435 375 375 185 185 375 375 375 375 375

100 67 16 25 23 38 77 100 50 27

Synthetic Product yield (%)d (% ee)e

61.5 g i

i k 1 "'

91 0

P

0.0 16.8h 0.Oh.i

27. 2iJ 3.2Ck 1.2'9' 30.6iJ" 18.1" 7.2i*r -

Product configuration

S R S S S R S -

aMDPP = menthyldiphenylphosphine. All reactions were carried out in a medium-pressure Parr apparatus for 24 hr at 300 psi hydrogen at 60°C in 200 ml 1 : 1 (v/v) deoxygenated ethanol-benzene with a substrate-to-triethylamine mole ratio of 6.25. bSee Table VIII. reduction yield was determined by NMR analysis of the crude products. dThe synthetic yield was determined on the distilled product for liquid products and on the crude product for solids. e% ee = Percent enantiomeric excess. fThe substrate-to-triethylamine mole ratio was 7.35. BReduction of 25 mmole of (E)-a-methylcinnamic acid gave a mixture of saturated acid (67%) and starting material (33%), 3.9 g. The mixture was purified to 94.4% saturated acid for the determination of the rotation. hThe optical rotation measurement assumes no contribution by the starting material other than a dilution effect. 'Reduction of 25 mmole of (Z)-a-methylcinnamic acid gave a mixture of saturated acid (16%) and starting material (84%), 3.9 g. The rotation wasdetermined on thecrude product. iReduction of 12.5 mmole of (E)-a-phenylcinnamic acid gave a mixture of saturated acid (25%) and starting material (75%), 2.6 g. The rotation was determined on the crude product. kReduction of 12.5 mmole of (Z)-a-phenylcinnamic acid gave a mixture of saturated acid (23%) and starting material (77%), 2.6 g. The rotation was determined on the crude product. lReduction of 25 mmole of (E)-p-methylcinnamic acid gave a mixture of saturated acid (37.5%) and starting material (62.5%), 3.5 g. The optical rotation was taken on the crude product. "'Reduction of 25 mmole of (Z)-0-methylcinnamic acid gave a mixture of saturated acid (77%) and starting material (23%). The optical rotation was measured on a sample of 85% purity (15% starting material). "The optical rotation was measured on the crude product. OReduction of 25 mmole of mesaconic acid gave a mixture (2.8 g) of saturated acid (50%) and starting material (50%). The optical rotation was measured on the crude product. PReduction of 25 mmole of citraconic acid gave a mixture of saturated acid (27%) and starting material. The optical rotation was not taken.

112

J. D. MORRISON, W. F. MASLER, AND M. K. NEUBERG TABLE XI

Asymmetric Homogeneous Hydrogenation of a,&Unsaturated Carboxylic Acids with the Rhodium(I)-(+)-CAhfPHOS Catalyst" ~

Substrateb

Substrate-Rh mole ratio

Reduction yield (%)'

Synthetic yield (%)d

Product (% ee)e

Atropic acidf (,!?)-a-Methylcinnamic acid (Z)-a-Methylcinnamic acid (ma-Phenylcinnamic acid (Z)-a-Phenylcinnamic acid (E)-p-Methylcinnamic acid (Z)-p-Methylcinnamic acid Itaconic acid Mesaconic acid Citraconic acid

435 375 375 185 185 375 375 375 375 375

100 100 100 100 100 100 100 100 100 14

69 93 88 88 88 78 90 74 79 B

6.05 15.2 11.0 11.8 13.9 9.7 11.4 10.7 1.8 -

Product configuration S

R S S S S

R R R -

%AMPHOS = 1,2,2-trimethyl-l,3-bis(diphenylphosphinomethyl)cyclopentane. All reactions were carried out in a medium-pressure Parr apparatus for 24 hr at 300 psi hydrogen at 60°C in 200 ml 1: 1 (v/v) deoxygenated ethanol-benzene with a substrate-to-triethylamine m. le ratio of 6.25. 'See Table VIII. 'The reduction yield was determined on the crude reduced acids by NMR spectroscopy. dThe synthetic yield data were based on distilled products when the product was a liquid and on crude product when the product was a solid. e% ee = Percent enantiomeric excess. fThe substrate-to-triethylamine mole ratio was 7.35. gA crude solid (2.1 g) was isolated and was shown by NMR spectroscopy to be 14% 2-methylsuccinic acid; the balance was starting material. The optical rotation of the product was not determined.

Tables IX and X also reveal some dramatic differences between NMPP and MDPP. These Iigands are diastereomers; more precisely, they are epimers since they differ only in configuration at C-3. It is quite reasonable that these ligands should behave differently, since diastereomers have different chemical and physical properties, although sometimes only slightly different. However, NMDPP and MDPP generate considerably disparate behavior both in terms of the activity and the chiral influence of the catalysts derived from them. Toward every substrate examined thus far the MDPP catalyst has had a very low activity, much lower activity than the NMDPP catalyst. Also, the MDPP catalyst generally gave much lower asymmetric bias than the NMDPP catalyst, and was the only chiral catalyst to give an archiral product" (two examples). In principle, all chiral catalysts should give chiral products. However, the energy difference between diastereomeric transition states can be so slight that the product does not have an observable rotation.

113

ASYMMETRIC HOMOGENEOUS HYDROGENATION TABLE XI1

Asymmetric Homogeneous Hydrogenation of a,p-Unsaturated Carboxylic Acids with the Rhodium(I)-(-)-DIOP Catalyst‘?

SubstrateC

Substrate-Rh mole ratio

Yield (%)d

Atropic acidf (E)-or-Methylcinnamicacid (Z)-or-Methylcinnamicacid @)-a-Phenylcinnamic acid (Z)-or-Phenylcinnamicacid (0-p-Methylcinnamic acid (Z)-0-Methylcinnamic acid

435 315 315 185 185 315 315

81 14 89 68 85 81

I0

Product (% ee)e

Product configuration

43.9 24.6 33.0 14.9 1.0 13.5 28.0

S S

R R S R S

‘DIOP =2,3-O-isopropylidene-2,3dihydroxy-l,4-bis(diphenylphosphino)butane. All reactions were carried out in a medium-pressure Parr apparatus for 24 hr at 300 psi hydrogen at 60°C in 200 ml 1 : 1 (v/v) deoxygenated benzene-ethanol with asubstrate-to-triethylamine mole ratio of 6.25. In all cases the mole ratio of (-)-DIOP t o rhodium was 1.5. bThe authors wish to express their gratitude to Ms. Susan J. Hathaway who collected the data in this table. %ee Table VIII. all cases, reduction of the substrate was quantitative (determined by NMR). Yield refers to isolated yield-distilled in the case of liquid products, crude in the case of solids. e% ee = Percent enantiomeric excess. fThe substrate-to-triethylamine mole ratio was 7.35.

If one inspects molecular models, it is possible to envisage a possible rationalization for the lower activity of the MDPP catalyst compared to the NMDPP catalyst. It appears that the MDPP ligand is less hindered around phosphorus than is the NMDPP ligand. It may be that MDPP more effectively competes for unsaturated coordination sites on the metal (especially under the high ligand loading conditions used by the New Hampshire group). This is equivalent to the proposition that a MDPP ligand is less easily dissociated from a (MDPP)3RhCl species and consequently catalysis is retarded. Of course, other explanations are also possible, one being that the MDPP more effectively hinders the coordination sites of the metal complex and in this way reduces its catalytic effectiveness. There appears to be no general stereocorrelation model that can be perceived for the NMDPP and MDPP ligands. As has been pointed out, these ligands are epimeric, being “locally enantiomeric” at C-3. One might be tempted to presume that catalysts prepared from them would produce enantiomeric products since the C-3 chiral carbons are closest to the metal. However, such an intuitively comfortable presumption is as dangerous as the equally satisfying premise that the better ligands will always be those that are chiral at phosphorus rather than at some more remote carbon atom. It is clear from the data in the tables that NMDPP and MDPP do sometimes induce the production of enantiomeric

114

J. D. MORRISON, W. F. MASLER, AND M. K. NEUBERG

TABLE XI11 Asymmetric Homogeneous Hydrogenation of a,p-Unsaturated Carboxylic Acids with [Rh (COD) (ACMP)2/+ BF4-a,b

Substrate' @)a-Methylcinnamic acid (2)a-Methylcinnamic acid (E)a-Phenylcinnamic acid (Z)-or-Phenylcinnamic acid (0-P-Methylcinnamic acid (Z)-0-Methylcinnamic acid

Yield (%)d

Product (% eele

Product configurationf

88 88 85 93 85 81

12.1 23.5 24.4 1.5 37.1 13.2

R R S

R S R

aACMP = o-anisylcyclohexylmethylphosphine. All reductions were carried out in a medium-pressure Parr apparatus for 24 hr at 300 psi hydrogen at 60°C in 200 ml deoxygenated 1:1 (v/v) benzeneethanol with a substrate-to-rhodium ratio of 362. The substrateto-triethylamine mole ratio was 6.25. % h e authors wish to express their gratitude to Ms. Susan J. Hathaway who collected the data for the last four entries in this table. %ee Table VIII. all cases reduction of the substrate was quantitative (determined by NMR). Yield refers to isolated yield-distilled in the case of liquid products, crude in the case of solids. e% ee = Percent enantiomeric excess. f(+)-(R)-ACMP was used as the phosphine ligand in every case.

products from the same substrate; but just as often they give products with the same chiralities. There appears to be no general relationship on the basis of comparative data collected thus far. The data in Table XIV can also be used to provide insight on another point. The first six substrates listed in Table XIV comprise a set of three diastereomeric12 (geometrically isomeric) pairs. The question is, With the same catalyst, d o E and Z isomers give enantiomeric products? The answer is that from the data in Table XIV there is no generality that covers this situation when all catalysts are considered. With DIOP,enantiomers are obtained from diastereomeric substrates in each instance, but with the other catalysts there is no regularity. There is almost an equal number of examples of each of the two possible patterns. This is not too surprising if one remembers that diastereomeric substrates, like diastereomeric ligands, can be thought of as simply different compounds. There is no reason to presume that diastereomers must display enantiomeric patterns of behavior but neither is there any stereochemical prin12The olefinic substrates that are cis-trans isomers are by modern stereochemical nomenclature more generally termed diastereomers. That is, they are stereoisomers that are not enantiomers. The fact that they contain no asymmetric carbons is irrelevant to this classification.

115

ASYMMETRIC HOMOGENEOUS HYDROGENATION TABLE XIV

Asymmetric Hydrogenation of a,p- Unsaturated Carboxylic Acids: A Comparison of Product Percent Enantiomeric Excess Values for Several Ligandsa-c Substrates

ACMP

DIOP

NMDPP

MDPP

CAMPHOS

@)-or-Methylcinnamicacid (Z)*-Me thylcinnamic acid @)a-Phenylcinnamic acid (Z)*-Phenylcinnamic 'acid (E)-p-Methylcinnamic acid (Z)-p-Methylcinnamic acid Atropic acid I taconic acid Mesaconic acid Citraconic acid ' condensation of Tables IX-XI11 in which additional details are given. A 'Abbreviations: ACMP = o-anisylcyclohexylmethylphospine;DIOP =2,34Xsopropylidene2,3-dihydroxyl-l,4-bis(diphenylphosphino)butane; NMDPP = neomenthyldiphenylphosphine; MDPP = menthyldiphenylphosphine; CAMPHOS = 1,2,2-trimethyl-l,3-bis(diphenylphosphinomethy1)cyclopentane. 'Data from unpublished research of the New Hampshire Group.

ciple that prevents them from doing so. In another chiral hydrogenation system (see discussion in Section HI), the fact that diastereomeric olefinic substrates gave products of the same configuration and almost the same optical purity with the same chiral catalyst has been taken as a possible indication that hydrogen transfer had occurred after a loss of diastereomeric identity. It is important t o recognize that, whereas this is a sufficient explanation, it is not a necessary one,

I I I. Chiral Amide-Rhodium Complexes as Catalysts Abley and McQuillin (44) have reported asymmetric homogeneous hydrogenations catalyzed by rhodium complexes of chiral amides. In initial experiments the catalyst was generated in situ by treating trichlorotripyridylrhodium(II1) with sodium borohydride in an optically active amide solvent (Fig. 20). In later work a 5% solution of the amide in diethylene glycol monoethyl ether was used and products with the same optical purities were obtained. This evidence indi-

FIG. 20. Synthesis of rhodium catalysts containing chiral amide ligands.

116

J. D. MORRISON, W. F. MASLER, AND M. K. NEUBERG

cates that the induced asymmetry was not due to asymmetric solvation but to the formation of a specific rhodium-amide-substrate complex. Spectral and conductivity measurements on the amide complex indicated that a possible formulation for it was [pyz(amide)RhC1(BH4)]+C1-. It was proposed that the borohydride group is coordinated through hydrogen as a bidentate ligand and the amide is bound through the carbonyl oxygen. Geometrically isomeric methyl$-methylcinnamates were hydrogenated with homogeneous catalysts prepared from several amides (Fig. 21), and product optical purities ranging from 14 to 58% were obtained (Table XV). It was observed that with two catalysts the products from (Z)- and (E)-methyl-0-methylcinnamate had the same sign and almost the same magnitude of rotation. This could suggest, according to the authors, that “at the decisive stage the molecule has lost the olefm geometry.” Although very little is known about the mechanism of such hydrogenations, Abley and McQuillin assumed that hydrogen transfer occurred in a stepwise fashion. It was claimed that the configuration of the methyl 3-phenylbutanoate

CH,OH I

R CH,CHCNH, I OH

(e)

HO

(f)

FIG. 21. Chiral amide ligands used in homogeneous hydrogenation reactions.

ASYMMETRIC HOMOGENEOUS HYDROGENATION

117

TABLE XV Hydrogenation of (E)- and (Z)-Methyl-p-methylcinnamate with Rhodium(III)-Borohydride Complexes of Chiral Amides

PH

\

/

COOCH3

Ph Ipy~(amide)RhCI(BH,)]*

or

CH3

(E)

Substrate configuration

I *

'

CH3-CH-CH2COOCH3

COOCH3

\

Ph

H2

/

/

c=c

\

H

P-midea

Optical purity of amide (% ee)b

SolventC

Optical purity of product (% ee)b

~~~~

100 96 100

? 92 99 96 96 100 100

A A B C A C B B B B

'The letters in parentheses refer to structures in Fig. 21. b% ee = Percent enantiomeric excess. 'Solvents: (A) With the amide as solvent; (B) with the amide as a 5 % solution in diethylene glycol monoethylether; (C) with the amide as a 5% solution in diethylene glycol monoethylether-water (10: 1).

obtained using the various chiral amides could be predicted by the use of stereocorrelation models in which steric repulsions between the amide substituents and the butanoate group are minimized. A correlation between the degree of induced chirality and the size of the large and medium-sized groups of the amide was also perceived. However, the data supporting the stereochemical correlation rationale were generated using only two substrates, (E)- and (Z)methyl-P-methylcinnamate,and the (2) ester was reduced in the presence of only two different ligands. It would be dangerous to infer that the correlation scheme will be valid for other olefins and ligands.

118

J. D. MORRISON, W. F. MASLER, AND M. K. NEUBERG

IV. Chiral Cobalt Catalysts From an economic viewpoint it would be desirable to develop efficient chiral homogeneous catalyst systems based on metals other than those from the expensive noble group. Ohgo and co-workers (45u-c) have made some progress with chiral cobalt catalysts, but much remains to be done in this area. In early studies, a catalyst solution believed to contain a cyanocobalt(I1)chiral amine complex was prepared (Fig. 22). The chiral amines (-){R)-1,2propanediamine (Pn) or (+)-(S)-N,N'-dimethyl-l,2-propanediamine(diMPn) were used. It was suggested that the catalytically active species might resemble a previously characterized compound, pethylenediaminebis [tetracyanocobaltate(II)] (compound VI). Whatever the precise structure of the active species, the catalyst solution did effect the asymmetric reduction of atropic acid, but with low asymmetric induction (Fig. 23). More successful asymmetric reductions have been based on amine (particularly alkaloid) complexes of bis(dimethy1glyoximato) cobalt(II), also known as cobaloxime(I1) and represented Co(dmg), (compound VII). Cobaloxime-chiral amine complexes have been used to catalyze the hydrogenation of both olefinic and ketonic substrates (Fig. 24). It has been determined that hydroxyamine modifiers, for example, alkaloids such as quinine, quinidine, and cinchonidine, are most effective. The highest optical purity obtained thus far has been 71%, observed for reduction of benzil in benzene solution at 10" using quinine as the CoCI,.OH,O

t

4KCN

L*, H20.25°C underN2

--

"catalyst solution"

L* = ( R ) - l , 2-propanediamine(Pn) (S)-N. N-dimethyl- I.?-propanediamine(diMPn)

I(CN).,Co'") -NH2CH2CH2NH2-

Co"lXCN),]

4-

(V1)

FIG. 22. Preparation of a cyanocobalt(I1)-chiral diamine catalyst solution. The cobalt species in solution may resemble the ethylenediamine complex (VI).

Pn: I % ee(S) diMPn: 7%,ee(S)

FIG. 23. Asymmetric hydrogenation with cyanocobalt(I1)lchiraI diamine solutions. % ee = percent Pn = 1,2-propanediamine; diMPn = N,N-dimethyl-l,2-propanediamine; enantiomeric excess.

ASYMMETRIC HOMOGENEOUS HYDROGENATION

119

R’

A: R = OMe, R’= Ph B: R = OMe, R’ = NHCOCH, C: R = OMe, R‘ = NHCOCH,Ph D: R = Ph, R’ = Ph

0

II Ph-C-C-Ph

II

Co(dmp),-quinine H2(1 a m ) . benzene, 25”:

0

A: 7%ee(S) B: 19%ee(S) C: 7%ee(S) D: 49%ee(S)

PH Ph-cH-c-ph II 0 61 %ee(S)

HCZCH,

J3

HC(0H) N

quinine

FIG. 24. Asymmetric hydrogenations with a quinine complex of cobaloxime(I1) [Co (dmgI2]. % ee = Percent enantiomeric excess.

chiral amine modifier. The use of more polar solvents and higher temperatures gave low optical purities. Various ratios of Co(dmg), to quinine have been used, but a 1 : 1 mole ratio is sufficient.

Co(drng),

(VII)

It appears probable that the effectiveness of the chiral cobalt systems studied thus far is a function, in part at least, of secondary bonding between the quinine ligand and the substrate, possibly via hydrogen bonding between carbonyl and hydroxyl groups.

120

J. D. MORRISON, W. F. MASLER, AND M. K. NEUBERG

V. Chiral Ruthenium Catalysts Apparently the fnst asymmetric hydrogenation with a chiral ruthenium catalyst was that reported by Hirai and Furuta (46a,b) using a ruthenium(II1) complex of poly-L-methylethylenimine(PLMI) (VIII). The complex was not isolated, but a catalyst solution was prepared in situ by mixing RuC13 * 3 H 2 0 * * +NH-CH-CH,-NHCH-CH,+ I

I

CH3

CH3

PLMI

(VIII)

and the polymer in acetate buffer for a specified period of time at 25°C. Methylacetoacetate was added to the resulting catalyst solution and hydrogenation was carried out at 80°C and an initial hydrogen pressure of 80 atm: CH3COCHzCOOCH3

catalyst solution

CH3&I(OH)CH2COOCH3

(4)

054.3% ee

The optical yield of the methyl (-)-3-hydroxybutyrate thus obtained was found to be dependent on several factors. When the molar ratio of ligand (calculated as the monomer) to Ru(II1) was increased in the range 2.5-10.0, the optical purity increased. The yield could be further improved by lengthening the standing time of the catalyst solution before use and by adjusting the pH of this solution to an optimum value of 5.5. The highest optical purity reported, 5.3%, was obtained with a ligand monomer-to-ruthenium ratio of 10, a standing time of 6 days, and at pH 5.5. It has been proposed that the optically active polymer coordinates to ruthenium as a bidentate ligand. The effect of the solution standing time on asymmetric induction was interpreted in terms of the time required for this multicoordination to Ru(1II) to occur. Bidentate coordination of the substrate to the catalyst through carbonyl and ester groups was also suggested. According to the authors, the catalytically active species is not the initially formed Ru(II1) complex but a Ru(I1) complex, presumably formed by hydrogen reduction. The Ru(1II)PLMI catalyst was later shown to catalyze the asymmetric hydrogenation of mesityl oxide [(CH3)2C=CHC(0)CH3], Because this substrate was only partially soluble in the aqueous reaction medium, the hydrogenation proceeded in an emulsion state rather than in a truly homogeneous solution state. Both the olefmic bond and the carbonyl group were reduced (47). By determining the composition of the reaction mixture at different times, it was established that two pathways to completely saturated product were operative. The dominant route was A -* B + C (Fig. 25), as indicated by the rapid

ASYMMETRIC HOMOGENEOUS HYDROGENATION

R

(CH,),C=CHCCH,

-

D

!

(CH,),CHCH,CCH,

B

A

(CHJ,C=CHCHCH,

121

-

(CH,),CHCH,~HCH, C

FIG. 25. Hydrogenation of mesityl oxide with a Ru(II1)-poly-L-methylethylemine catalyst. The dominant route is A + B +C.

formation of methyl isobutyl ketone, followed by its slow disappearance and the gradual appearance of 4-methyl-2-pentanol. The presence of a small amount (ca. 0.02 mole%) of the unsaturated alcohol D was evidence for a slight contribution from the path A + D + C. Asymmetric induction leading to optically active 4-methyl-2-pentanol(O.5% ee at L/Ru = 5.0, standing time = 3 days, pH = 5.5) was shown to occur only during hydrogenation by this minor route, since reduction of authentic methyl isobutyl ketone yielded optically inactive product. It was concluded that the bidentate coordination possible in mesityl oxide but not methyl isobutyl ketone was essential for stereoregulation of reduction.

VI. Concluding Remarks Chiral catalysis is in its infancy. The results described in this review represent only crude pylons marking the entrance to what will probably prove to be an extraordinarily productive and useful arena for future research. There are a great many catalytically active achiral systems which can, in principle, be modified by the incorporation of chiral ligands to produce catalysts for asymmetric hydrogenation and other chiral reactions. Only a few chiral ligands have been synthesized; there are almost limitless possibilities in this area for the synthetic chemist. In the short-term future, we hope for many new developments and, in the long-term, perhaps even for some totally new concepts and major theoretical breakthroughs that will make it possible to perceive structure-efficiency relationships for chiral catalysis. The possibilities for valuable contributions in this area are vast. We have every confidence that great progress will be made. For in the words of E. J. Corey: “The synthetic chemist is more than a logician and strategist; he is an explorer strongly influenced to speculate, to imagine, and even to create.” (48)

122

J. D. MORRISON, W. F. MASLER, AND M. K. NEUBERG

NOTEADDEDI N PROOF The Monsanto Group has recently reported enantiomeric excesses of 95-96% for the hydrogenation of a-acylaminoacrylic acids using a chiral diphosphine [ 1,2di-(o-anisylphenylphosphino) ethane] as a ligand (49). The chiral phosphine was prepared by oxidative coupling of chiral o-anisylmethylphenylphosphine oxide (50), followed by deoxygenation with trichlorosilane and tri-n-butylamine in acetonitrile. The Paris Group has reported studies of various chiral diphosphines related to DIOP (51 1. Enantiomeric excesses as high as 90% were obtained. Structural analogs in which the acetonide ring was replaced by a carbon ring were shown to be capable of high asymmetric induction, as high as that obtained with DIOP. The asymmetric reduction of enamides to produce chiral amine derivatives has also been examined by the Paris Group (52). Subsequent unpublished studies (53)have shown that the degree of asymmetric synthesis is much higher in benzene than it is in ethanol for such systems; up to 92%enantiomeric excess was achieved in one case. A stereocorrelation model for DIOP hydrogenations has been proposed (54). Further results on asymmetric hydrogenations of activated carbonyl compounds catalyzed by bis(dimethylg1yoximato) cobalt (II)-chiral amine complexes have been reported (55,561. Some chiral reductive dimerizations were observed (55).

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Stereochemical Approaches to Mechanisms of Hydrocarbon Reactions on Metal Catalysts J. K. A. CLARKE Chemistry Department University College Belfield, Dublin, Ireland AND

J. J. ROONEY Department of Chemistry The Queen’s University Belfast, Northern Ireland

............................................... ............................... A. General Character of the Olp Process. .......................... B. Problem A: Mechanism of Two-Set Exchange . . . . . . . . . . . . . . . . . . . C. Problem B: Nature of the ap-Diadsorbed Species and Rollover . . . . . . 111. Reactions of Olefins ......................................... A. Competitive Hydrogenation of Cycloalkenes .................... B. Deuteration of Olefins ..................................... C. Homogeneous Complexes. .................................. IV. Skeletal Rearrangement of Alkanes on Platinum and Other Noble Metals . A. The Bond-Shift Mechanism ................................. B. The Dehydrocyclization-Hydrogenolysis(or “Cyclic”) Mechanism ... V. Recent Experimental Approaches to Skeletal Rearrangements . . . . . . . . . A. Surface-Structure Sensitivity ................................ B. I3C-Labeling Studies ...................................... C. Studies with Alloy Catalysts. ................................ VI. Influence of Carbonaceous Deposits ............................. VII. Conclusions ................................................ References ................................................. I. Introduction

11. The Horiuti-Polanyi Mechanism..

125 127 127 129 134 136 136 140 141 141 142 150 158 158 166 173 176 180 180

1. Introduction Transition metals catalyze a very wide variety of hydrocarbon reactions ranging from hydrogenation of olefins and exchange of paraffins with deuterium at lower temperatures to skeletal rearrangement, cyclization, hydrogenolysis, cracking, and carbiding under more severe conditions. Because of this flexibility in 125

126

J . K. A. CLARKE AND J. J. ROONEY

catalytic behavior and multitude of adsorbed species of different types, several of which may be present simultaneously in any given system, progress in developing detailed mechanisms is understandably slow, in spite of a vast amount of work. The major purpose of this review is t o show that even though the problems are formidable, isotopic tracers and stereochemistry are particularly useful in gaining mechanistic insight. In order to choose the correct mechanism from among several possibilities, suitable model compounds can be designed, synthesized, and reacted. This often leads to compounds of increasingly complex structure but, paradoxically, t o simpler means of obtaining definitive mechanistic information. We have emphasized throughout the review the value of this approach in spite of the synthetic difficulties encountered. The exchange of model compounds with deuterium has been increasingly employed during the last decade, and key results pertaining to the detailed understanding of the classic Horiuti-Polanyi mechanism of hydrogenation of olefins are summarized. Studies of competitive reactions have also been valuable in this area and are briefly described for hydrogenation of cycloalkenes. Interest is, however, now focused more and more on the mechanisms of the higher-temperature reactions in which deuterium reacts far too rapidly to be of much value as a tracer. In this area carbon isotopes have become increasingly important in mechanistic studies as amply demonstrated by Gault and his school at Strasbourg. They have shown that the labor involved in synthesizing model compounds labeled by carbon isotopes in special positions, and difficulties in analyzing products, are abundantly rewarded by their contribution t o an understanding of skeletal isomerization, cyclization, hydrogenolysis, and cracking. This is now a rapidly developing area, and we have attempted an assessment of the current state of knowledge of the many mechanistic possibilities emerging from a variety of studies. Finally, matrix isolation of active transition metals either as single atoms or discrete ensembles by alloying with Group IB and main group metals can also reasonably be regarded as another aspect of the stereochemical approach, even though geometric and electronic factors are never separate variables. Moreover, the technological superiority of many alloy catalysts has given a new impetus in recent years to this approach, so that we feel justified in including some work on alloys pertaining to mechanisms. Obviously, the field covered is very large so the choice of material throughout the review is selective. This has the inherent danger that certain mechanisms and areas have been given undue emphasis. The reader should bear this in mind so that we do not leave the impression that all the mechanisms described are definitely established. At best, many can still only be regarded as useful postulates that may serve as a guide to further experiments.

HYDROCARBON REACTIONS ON METAL CATALYSTS

127

11. The Horiuti-PolanyiMechanism A. GENERALCHARACTER OF

THE

a@ PROCESS

The classic Horiuti-Polanyi mechanism proposed in 1934 for hydrogenation of ethylene on Ni is shown in Scheme 1. Since then isotopic tracer studies,

* *

qcn, I

t

n I

-

cnp,

t

2*

(d)

Scheme I

especially reactions with deuterium, and stereochemistry have been extensively employed to characterize this mechanism in detail. In a recent review Burwell (1) has given an excellent account of the philosophy behind this approach and the theory involved in interpreting exchange data. The following basic facts have been established from exchange with deuterium of numerous alkanes and polycycloalkanes especially on Pd catalysts. 1. Initial distributions of products from reactions of ethane and higher alkanes in excess D2 show that step (c) in Scheme 1 is reversible. Thus, interconversion of monoadsorbed and a@-diadsorbedspecies can be very rapid especially on Pd and Rh before desorption of alkane. This interconversion is now referred to as the a@ process. 2. The a0 process propagates readily along a chain of carbon atoms, and, in acyclic paraffins with rapid rotation about C-C bonds, every H atom is readily replaced, as evidenced by the very large quantities of the perdeutero isomer observed by Gault and Kemball (2) in initial products from exchange of n-hexane on Pd films. 3. Propagation of the exchange reaction is blocked if the chain of carbon atoms contains a quaternary center or a bridgehead such as that in bicyclo[2,2,1] heptane.

128

J. K. A. CLARKE AND J. J. ROONEY

FIG. 1. Typical examples of compounds possessing isolated pairs of vicinal hydrogen atorns'(4,5).

4. The a0 process is limited to cis addition and elimination of H atoms. Evidence for this is also found in studies of olefin hydrogenation (3),where both H atoms add to the side of the double bond facing the metal surface. Exchange of a variety of cyclic paraffins provides clear confirmation of this description. Two examples, reactions of bicyclo [2,2,1] heptane (4) and 1,1,3,3-tetramethyIcyclopentane (5) on Pd, will serve as illustrations (Fig. 1). In both cases propagation of the a0 process is blocked, by bridgeheads and quaternary groups, respectively, and, since there is no rotation about ring C-C bonds, only one isolated set of 2 H atoms is initially replaced in each molecule (Fig. 2).

FIG. 2. The ap process for a pair of isolated vicinal hydrogen atoms.

5 . However, the exchange data also afforded details that gave rise to two major problems:

Problem A . When a cycloalkane contains an isolated unit of 3 or more consecutive carbon atoms, none of which is a blocking atom, initial exchange on both faces of the ring is observed. Thus, the patterns for cyclopentane reacted on Pd (1) show not only a large maximum in the d 5 isomer, but substantial amounts of the d6-dI0isomers as well with small and large maxima, respectively, in the d B and dlo isomers (1) (Fig. 3). The a0 process predicts initial replacement of only 5H atoms on one face of the C5 ring so that some additional process is important. Problem B . The discovery in the 1960s of many transition metal complexes and homogeneous catalysts in which hydride, alkyl, olefin, and ally1 ligands, etc., are present and reactive focused attention on the possibility that bonds between the intermediates and metal surfaces (Scheme 1) may be very similar to those of their homogeneous analogs. The logical and drastic conclusion from this line of thought, in contrast to earlier theories, is that individual metal atoms in the surfaces rather than aggregates are the essential loci of reactions in heterogeneous catalysis. Thus, monoadsorbed alkyl is a o-bonded species, but the questions remain: Is the 43-diadsorbed species a n-bonded olefin and do these species interconvert as ligands of the same metal atom? The alternative view is that the as-

HYDROCARBON REACTIONS ON METAL CATALYSTS

129

FIG. 3. Effect of temperature("C) on the isotopic distribution patterns ( d l - d l o ) resulting from exchange of cyclopentane on a Pd/A1203 catalyst. The paterns are normalized to 1.0 at the dlo isomer ( I ) . Reprinted with permission from Accounts Chem. Res. 2, 289 (1969). Copyright by the American Chemical Society.

terisk (Scheme 1) is a site rather vaguely described as a multicentered molecular orbital, capable of u bonding to a carbon atom and involving several contiguous metal atoms in a regular array of a crystal face. The a(l-diadsorbed species is then regarded as a di-o-bonded alkane. A considerable amount of work on exchange of hydrocarbons with Dz on metals during the last decade has been done with the purpose of providing solutions t o these two problems. The remainder of this section is devoted t o a review of key results which ultimately yielded unequivocal answers.

B. PROBLEMA: MECHANISM OF TWO-SETEXCHANGE Rooney ( 6 ) favored a mechanism involving interconversion of intermediates as ligands of the same metal atom. Moreover, he suggested that n-bonded olefin further interconverts on certain metal atoms, especially on Pd surfaces, with n-ally1 complexes and that the latter process may occur with trans elimination and addition of hydrogen atoms. This was suggested to explain initial exchange on both faces of a ring as in cyclopentane. The idea fitted well with the known propensity of Pd t o form n-ally1 complexes and evidence was obtained that such

130

J . K. A. CLARKE AND J. J. ROONEY TABLE I

Initial Distributions for Exchange of 1 , I ,3,3-TetramethylcycIohexane on Pd Films"

TC C)

dl

d2

d3

d4

d5

d6

42 110

39.5 28.7

3.0 3.9

12.6 8.0

8.1 14.1

36.0 39.2

0.8 6.1

"Data from Rooney ( 6) .

species (n-1-methylbutenyl) are especially important in hydrogenation of buta-l,3-diene on this metal (7, 8). Besides, allylic species of some sort must be involved in dehydrogenating cyclohexane to benzene and in disproportionating cyclohexene to cyclohexane and benzene (9). An investigation of reactions of 1,l,3,3-tetramethylcyclohexane with D2 on Pd films was carried out as a test of Rooney's theory (6), since the latter predicted that only 5 (and not 6) of the hydrogen atoms of the isolated trimethylene unit should be easily replaced. The results were exactly as predicted (Table I) by the mechanism (Fig. 4). The detailed reaction steps shown in Fig. 4 demonstrate that an H atom on the central carbon atom of the isolated trimethylene unit cannot be replaced initially by the proposed mechanism. A significant point in these results was that, even at 196"C, the d5/d6 isomer ratio was still greater than unity thus demonstrating that oar-bonded species are not readily formed on Pd in accordance with Kemball's (10) earlier finding of almost exclusively simple exchange of methane at elevated temperatures on this metal. Burwell (I) disagreed with Rooney's solutions to both problems. First, he maintained that the a@-diadsorbedspecies is eclipsed vicinal diadsorbed alkane. Part of his evidence for this view was that only molecules containing 2 or more vicinal hydrogen atoms that are already eclipsed or may easily move into eclipsed positions undergo the a0 process. For example, bicyclo[2,2,1] heptane (Fig. 1) initially exchanges only one set of 2H atoms, those that are eclipsed on C2 and

FIG. 4. The n-ally1 mechanism postulated to explain initial exchange of 5 hydrogen atoms in an isolated trimethylene unit on Pd (6).

HYDROCARBON REACTIONS ON METAL CATALYSTS

131

FIG. 5. Roll-over mechanisms of 1,2diadsorbed species (1, 11-13). Reprinted with permission from J. Amer. Chem. SOC. 88,4555 (1966). Copyright by the American Chemical Society.

C3 or, equivalent, on C5 and C6. The absence of propagation through the bridgehead was explained by the impossibility of obtaining an eclipsed pair of hydrogens at C1 and Cz , and equivalent positions. Adamantane with no possibility of possessing a pair of eclipsed hydrogens only gives simple exchange. However, Rooney’s mechanism suggests the alternative argument that bicyclo [2,2,1] hept1-ene and adamantene are too strained even as A-complexed olefins. Burwell also provided an interesting alternative mechanism to explain the d5 maximum in exchange of 1,1,3,3-tetramethylcyclohexaneon Pd. This involved rollover of his ab-diadsorbed species while still attached to the surface (11-13) as shown in Fig. 5. This mechanism not only explains the absence of easy replacement of a central H atom in an isolated trimethylene unit but also has the advantage of accounting for the small maximum in the d s isomer in exchange of cyclopentane. Thus, one rollover of C5HSD3 with repeat of the a0 process on certain sites can only give C5Hz D8. Easy multiple rollover and a rapid a0 process on other sites explain the large maximum in the d l o isomer. Burwell (1) further tested the rollover mechanism by studying the exchange of bicyclo[3,3,1] nonane (I) and bicyclo[3,3,0] octane (11) on Pd (Fig. 6; Table 11). Compound I contains isolated trimethylene units and exhibits maxima in the d s , d l o , and d12isomers. The a0 process now rapidly propagates through the bridgehead (eclipsing is possible in the chair-boat form) giving the ds maximum. Maxima dlo and d12 are due to replacement of 2H atoms each (h sets) in the isolated trimethylene units. Isolation is due to the impossibility of rollover

TABLE I1 Distributions for Exchange of Bicyclo[3,3, I J nonane (I) and Bicyclo[3,3,0] octane (II) on PdIA1203 cOtalystsa Compound

T("C)

do

d,

d2

d3

d4

d5

d6

d7

ds

I I1

50

97.77 93.44

0.43 0.18

0.05 0.36

0.12 0.27

0.08 0.12

0.11 0.15

0.09 0.30

0.06 0.54

0.38 1.45

68

d9

dlo

d,,

d12

d13

d14

0.09 0.33

0.45 0.42

0.05 0.12

0.33 0.30

0.00 0.54

0.00 1.45

7.5 5.0 0.7

6.6 6.5 1.2

0.0 0.7

0.0

=Data from Burwell ( I ) .

TABLE rrI Distibutions for Exchange of I-Methylbicyclo/3,3, OJ ocane on Pd CataZysts'

(a) (b)

(4

30 90 120

-

77.3 92.1

'Data from Quinn et al. (14). b(a) Initial distribution for a f

4.0 1.9 0.7

2.6 1.1 0.5

5.9 0.7

0.3

6.7 0.9 0.4

~(b);fdm; ( c ) 2 wt % P d / A I 2 0 3 .

7.2 1.2 0.5

13.7 1.6 0.6

32.2 1.7 0.6

6.9 2.2

0.7

7.0 4.0 0.7

0.3

0.5 0.1

HYDROCARBON REACTIONS ON METAL CATALYSTS

133

I II m FIG. 6. Model compounds designed to distinguish n-ally1 and roll-over mechanisms of two-set exchange on Pd (I, 11, 14). (I) Adapted from Burwell (I). Reprinted with permission from Accounts Chem. Res. 2, 289 (1969). Copyright by the American Chemical Society. (11) Adapted from Roth et al. (IZ). By permission of the Journal of the Research Institute for Catalysis, Hokkaido University.

of the 1,2-diadsorbed bicyclononane. However, the same would be true if n-ally1 complexes (C, -C2-Cg and other equivalent positions) could not form. Compound I1 is particularly interesting since epimerization at one tertiary C atom would generate the trans isomer, a molecule too strained to be significant. Rollover of 1,2-diadsorbed cis-bicyclo[3,3 ,O] octane is, therefore, excluded and edge-on rollover of the 1,5-diadsorbed octane (A or B in Fig. 5) is sterically impossible. However, compound I1 yields about equal amounts of d14(perdeutero isomer) and d8 isomers (one-set exchange) clearly indicating that the trimethylene units are not isolated in this compound. Burwell suggested that end-on rollover (C in Fig. 5) would be necessary, but special sites are required (Fig. 7).

FIG. 7. Bonding of 1,2-diadsorbed species postulated for end-on rollover ( I ) . Reprinted with permission from Accounts Chem. Res. 2, 289 (1969). Copyright by the American Chemical Society.

The n-ally1 mechanism also accounts for the results if one accepts the rather strained n-ally1 (C1-C2-C3)as readily participating. So far none of the model compounds seemed to distinguish clearly the two mechanisms. However, Roth et al. (11) suggested that l-methylbicyclo[3,3,0] octane (111) should show this distinction since edge-on rollover is impossible in this case but the n-ally1 mechanism should still be feasible. Quinn et al. (14) synthesized this compound and found initial exchange of only 11 hydrogens (and not 13) with a maximum in the d, isomer at lower temperatures (Table 111) on Pd catalysts. These results

134

J. K. A. CLARKE AND J. J. ROONEY

FIG. 8. Hydrogen atoms of interest in initial exchange of endo-trimethylenenorbornane and possible olefinic derivatives for rollover (14,15).

obviously ruled out the n-ally1 mechanism. Quinn er al. (14, 15) noted that several other compounds reacted in a fashion that also clearly supported Burwell's view. endo-Trimethylenenorbornane (Fig. 8) exchanges only 5 hydrogens initially, whereas 7 hydrogens should have been readily accessible by the n-ally1 mechanism. A variation of the latter (16) is suprafacial 1,3 shift of a hydrogen atom via a transient n-ally1 complex, a mechanism that has been discussed theoretically by Mango (17). But this mechanism should also have allowed exchange of 7 hydrogen atoms (5H and 2h) of the C5 ring. An examination of the corresponding olefins (Fig. 8) shows that rollover and, therefore, exchange of the h and h' atoms is sterically very hindered. The n-ally1 mechanism also prenonane may initially exchange all 15 dicts that endo-3-methylbicyclo[3,3,1] hydrogen atoms shown but can never invert the tertiary center in an isolated iso-C4 unit. However, under conditions where initial exchange of all of those hydrogens was observed, the reaction was accompanied by endo-exo isomerization ( 1 9 ,as expected from the roll-over mechanism. In retrospect, it might have been realized earlier that trans addition and/or elimination involving olefmln-ally1 interconversions are impossible. There are now good theoretical reasons for believing that this interconversion does not occur in one step but proceeds via intermediate a-bonded allyls.

c. PROBLEM B: NATUREOF THE QP-DIADSORBED SPECIES AND ROLLOVER The roll-over mechanism is now clearly established and this brings us back to Problem B, the nature of the afl-diadsorbed species. Apart from the argument concerning the necessity for eclipsed pairs of vicinal hydrogens for the afl process to operate, Burwell (I) stressed that in his view conversion of certain cyclic

HYDROCARBON REACTIONS ON METAL CATALYSTS

135

compounds to n-bonded olefins is too endothermic to be acceptable but that these compounds could form the corresponding eclipsed a0-diadsorbed species without additional strain. Bicyclo[3,3,1] nonane is a good example because exchange propagates very readily through the bridgeheads (Table 11). The n-bonded olefin model requires easy formation of n-complexed bicyclo [3,3,1] non-1-ene, a rather strained olefin, but eclipsed 1,2-di-u-bonded bicyclo[3,3,1] nonane could readily be obtained in the chair-boat conformation. Quinn et al. (15) stressed their view that arguments based on strain energy of free olefins are not strictly valid. When olefins are n-complexed to zero-valent metal, there is considerable bond lengthening and deviation from planarity about the unsaturated carbon atoms, resulting in considerable relief of strain (18). Conformations of n-bonded olefins are, therefore, not directly related to conformations of free alkenes or alkanes. A corollary is that eclipsing of vicinal pairs is not in itself the essential criterion for the a0 process but that a reasonable possibility of forming an olefin complex is. Since bicyclo[3,3,1] non-1-ene is a moderately stable olefin in the free state (19,20),it could well meet this criterion. Conformation and conformational flexibility can then be considered as secondary factors in determining the ease of alkyl/n-bonded olefin interconversion. The only way of using exchange with deuterium to solve Problem B is to study compounds that have eclipsed vicinal pairs of hydrogens but with only the remotest chance of forming n-bonded olefinic complexes. Caged compounds are necessary and a very suitable choice is the heptacyclotetradecane shown in Fig. 9

(1)

(na)

( n b)

FIG. 9. Examples of the type of compound required to distinguish n-bonded alkene and eclipsed 1,Zdiadsorbed alkane by exchange on Pd (21).

whose structure (I) consists of two bicyclo [2,2,1] heptane units orthogonally fused together. If Burwell’s formulation is correct, the tetradecane should behave in a fashion very similar to that of bicyclo [2,2,1] heptane whichundergoes initial multiple exchange of 2 hydrogens. However, McKervey et al. (21) found that the tetradecane gives only simple exchange on Pd with the observed distributions of deutero isomers closely agreeing with those calculated on the basis of 16 exchangeable hydrogens using binomial theory (Table IV). On the other hand, the tricyclodecane (IIa in Fig. 9) initially exchanges all 10 hydrogen atoms shown on the same catalyst indicating that the corresponding olefin is readily formed as a n complex. The exchange results are, therefore, a sensitive diagnostic test of the degree of strain in olefins, and the as yet unknown tricyclodecene (IIb in Fig. 9) with a

136

J . K. A. CLARKE AND J. J. ROONEY

TABLE IV Observed (X) and Calculated ( Y ) Distributions for Exchange of Heptacyclotetradecane on 5 wt % Pd/Pumicea

X Y X Y

80 80 90 90

14.9 14.3 49.5 41.3

21.1 22.3 33.0 36.3

3.1 3.1 13.4 13.0

0.3 0.3 3.3 2.9

0.6 0.5

0.2 0.1

‘Data from McKervey et al. (21).

new type of strain is predicted to be moderately stable as a free entity. In accordance with the above conclusion, recent work shows that bicyclo[2,2,1] hept1-ene and adamantene, which are not formed as A complexes on Pd surfaces, are so unstable that they immediately dimerize in the free state (22,23). Now that the ao-diadsorbed species is known to be n-complexed olefin, the simplest interpretation of rollover is that the metal-olefin bond breaks; the free olefin has then a transient existence in the gas phase and can migrate from one type of site to another. That this occurs t o an appreciable extent even at ambient temperatures starting with alkane in excess Dz may seem surprising but is powerful support for the olefin migration step postulated in hydrocracking and hydroreforming o n dual-functional catalysts. If rollover can only occur with some residual bonding t o the surface, then a variety of species, edge-bonded and side-bonded, and, thus, special sites have to be postulated. A logical conclusion is that two-side, as opposed to one-side, initial exchange of compounds such as cyclopentane is a “demanding” reaction. Plunkett and Clarke (24) searched for surface structure sensitivity in the epimerization of cis-l,4-dimethylcyclohexaneon a series of well-characterized supported Pd catalysts with average crystallite diameters ranging from 240 to 45 A. The specific rates varied very little so they concluded that special sites are not required for rollover. Another group (25), using a simple, effective comparative method, has also reported no difference in activity for cis-l,2-dimethylcyclohexane epimerization between a dispersed and sintered Pt catalyst.

I 11. Reactions of Olefins A. COMPETITIVE HYDROGENATION OF CYCLOALKENES A valuable indirect method of probing the Horiuti-Polanyi mechanism is the study and comparison of competitive rates of hydrogenation of olefins using both homogeneous and heterogeneous catalysts. Comparisons of individual rates

HYDROCARBON REACTIONS ON METAL CATALYSTS

137

and interpretations, on the other hand, suffer from the disadvantage that with heterogeneous catalysts the rates are markedly affected by trace impurities or minor variations in the history of the catalyst (26). When 2 olefins or 2 aromatics (27-29), in a binary mixture in solution, are simultaneously hydrogenated invariably a plot of log CA versus log CBis linear, where CA and CB are the respective substrate concentrations throughout the course of hydrogenation, and the slope provides a competition ratio, R . Moreover, Maurel and Tellier (29) have shown that even if the overall rates of hydrogenation of olefins are subject to diffusion control by hydrogen, the values of R are unaffected. Particular attention has been paid t o the cycloalkenes, and values of R relative to unity for cyclopentene, obtained by Graham et al. (30) from hydrogenation of pairs in the C5-C9 series on several supported metals in ethanol at a constant hydrogen pressure of 1 atm, are given in Fig. 10. Data from various groups for both competitive and individual rates of hydrogenation of cycloalkenes are summarized in Table V. The noteworthy feature of Table V is that, although the competitive sequences agree well for both a homogeneous catalyst and a variety of heterogeneous catalysts, the competitive and individual rate sequences do not parallel each other. The major difficulty in interpreting such data is deciding which step of the Horiuti-Polanyi mechanism (Scheme 1) is rate-controlling. The relative importance of different steps may A

1.0

A

A

A

A

0 0

0

0

0

0 X

lo

X 0.

B

x

-

6

Ir

Pt

X.

I

Ru

Rh

Pd

FIG. 10. Competitive hydrogenation of cycloalkenes on metal catalysts. Competition ratios are normalized to 1.0 for C5. A,C5;0,C7; 0 , C6; X , C8; 0 , C9 (30).

138

J. K. A. CLARKE AND J. J. ROONEY TABLE V

Rate Sequences for Hydrogenation of Cycloalkenes Catalyst (Ref.)

Solvent

Competitive

Individual

vary with catalyst, olefin, and solvent. Invariably linear relationships between log C , and log CB are obtained in competitive hydrogenation, and Hussey et al. (27) point out that this is predicted even if the rate of olefin chemisorption is rate-controlling. However, this can hardly be true since reaction is normally zero order in olefin, reflecting the fast rate of step (b) (Scheme I), and first order in hydrogen. Whether the reverse of step (b) is sufficiently fast to allow equilibrium t o be set up is a matter of dispute (27). This is an important point since one of the major objectives of the work is to find the relative strengths of adsorption of different olefins, i.e., ratios of Langmuir adsorption coefficients. Any analysis of competitive rates that isolates ratios of these coefficients is based on the assumption that the following equilibrium obtains: Asoln

+

Bads

*

Bsoln + Aads

Jardine and McQuillin (31) believed that hydrogen transfer [steps (c) and (d) of Scheme 11 was rate-controlling in their work, whereas Hussey et al. (27) emphasize that hydrogen diffusion through the solution can be rate-controlling as found by Maurel and Tellier (29). Although various groups (27-29, 33, 34) have described kinetic analyses for competitive reactions on heterogeneous catalysts, the following suffices t o d e m onstrate the complexities of the problem of hydrogenation of olefins. Applying steady state analysis t o the system,

gives for the rate of hydrogenation

HYDROCARBON REACTIONS ON METAL CATALYSTS

139

where Bo and f?H are the fractions of surface covered by olefin and hydrogen atoms, respectively. When 2 olefins are in competition, the rate ratio (rA/rB) is given by -=-.- k4(A)

k3(A)

. BA

'

IB

k3(B)

oB

'

k4(B)

. k-3(B)

oh

+ k4(B) *

k-3(A) -I-k4(A)

eH

(2) '

*

If adsorption of olefin is competitive and equilibrium is set up between solution and surface oA/eB =b A

.C A / b B

'

CB?

(3)

where b A and b B are the respective Langmuir adsorption coefficients. Substitution of Eq. (2) by Eq. (3) gives

-

.

.

.

.

rA - k4(A) - k3(A) - bA C A k-3(B) IB

k4(B)

k3(B)

bB

CB

k-3(A)'

'

k4(B)

'

OH

(4)

k4(A) ' e H

If alkyl reversal [reverse of step (c) in Scheme 11 is fast compared to step (d), then k-, >> k40H and Eq. (4) approximates t o -= rA rB

[::; -.

k3(A) 'k-3(B) k3(B)

'

k-3(A)

. b_A ]

cA

bB

CB

(5)

It can easily be shown that integration of Eq. (5) affords a linear relationship between log CA and log CB with slope, or competition ratio, R , as the multiple of constants within the square brackets. The ratio of Langmuir coefficients is obviously coupled with ratios of rate constants and alkyl reversal equilibrium constants. On the other hand, if alkyl reversal is very slow such that k-3 << k4 OH, then Eq. (4)reduces t o

-

which is very similar to that developed by Wauquier and Jungers (33),and used by Rader and Smith (28). The latter authors studied competitive hydrogenation of polymethylbenzenes and assessed the ratios kJ(A)/kJ(B) from individual rate studies. They then assumed that the same ratios apply in competitive hydrogenation and simply calculated the ratios of strengths of adsorption from the experimental values of R . However, it is obvious from this treatment that in dealing with olefins the degree of alkyl reversal is of key importance in interpreting values of R . Alkyl reversal can be separately assessed from studies of hydrogenation of olefins by deuterium.

140

J . K. A. CLARKE AND J. J. ROONEY

B. DEUTERATION OF OLEFINS Deuteration of olefins often results in initial alkane products that do not have deuterium incorporated (35) because of dissociative adsorption of alkene (36). The initial surface pool contains a large proportion of hydrogen atoms and, hence, the initial alkane products contain little deuterium. To make a proper study of deuteration, a means of enriching the surface pool of hydrogen atoms has to be found. Phillipson and Burwell (37) achieved this by using solvents such as CH3COOD and CH30D in the deuteration of cycloalkenes on a Pt/A1203 catalyst. They found that incorporation of deuterium is a function of the olefin, the order of increasing deuterium content of cycloalkane product (exchangeaddition) is C5 > C7 > C8 > c6, and for cycloalkene the order of degree of exchange, C8 > C5, C7 > c6. For both these sequences, reversibility of step (c) (Scheme 1) is important, and, for the latter sequence, reversibility of step (b) must operate. These results also clearly indicate that Eq. ( 6 ) is not a good approximation for competitive hydrogenation of alkenes. However, the exact duplication of the exchangeaddition order of the competitive hydrogenation sequence (Table V) for Pt catalysts strongly suggests that the factor controlling competition ratios also controls the deuterium content of the cycloalkane products. If step (d) is rate-controlling in competition [Eq. ( 5 ) ] , then a low deuterium content in cycloalkane would indicate a high position in the competition sequence. Clearly this is not so, and k4(A)/k4(B) cannot be the major factor in determining the R values. A similar situation also holds if alkyl reversal is the most important factor in competition [(kJ(A)/k-3(A))/(kJ(B)/k-J(B)) in Eq. ( 6 ) ] . Slow alkyl reversal also implies a low deuterium content in cycloalkane but again a high position in the competition sequence. The competition rate order can only parallel the exchange-addition order if the proportion of olefin coverage (ratio of Langmuir adsorption coefficients) is the major control in competitive reactions. Thus the metal-olefin bond strength is in the order c5> C7 > c61CS ,C 9 . The relative strengths of adsorption of c6 and C8 are difficult to assess from competition ratios. There are several reasons for believing that the value of R for this mixture cannot be simply equated with the ratio of the Langmuir adsorption coefficients and the Eq. ( 5 ) may not be a valid approximation. The individual rates for hydrogenation of C8 are very low (Table V), and C8 seems t o have a pronounced inhibitory effect on the hydrogenation of c6 (27,31) even though c6 is preferentially hydrogenated on Pd. Maurel and Tellier (29) found that for a wide range of olefins the results were self-consistent in that the competition ratio between any two could be predicted by appropriate division or multiplication of their respective competition ratios with a third (R(A,B) X R(B/c) = R(A/c)). Graham et ~ l(30) . found the same consistency for their data (Fig. 10) with the exception of the c6 and C8 mixture on Pt/Si02, and Hussey ef d. (27) also report the same feature for this mixture. A probable clue to an

HYDROCARBON REACTIONS ON METAL CATALYSTS

141

understanding of this behavior is that in the C5-Cs series of cycloalkanes c6 exhibits the lowest degree of multiple exchange with deuterium on Pd catalysts and CB the highest (4, 38). Thus, alkyl reversal is of least significance for c6 and most important in this series for c8, in line with their position in the sequence for exchange of cycloalkenes observed by Phillipson and Burwell (37). A very low steady-state concentration of cyclooctyl is, therefore, indicated with a concomitant diminution in the steady-state concentration of adsorbed hydrogen. Thus CB interferes with the rate of c6 hydrogenation and degree of cyclohexyl reversal, because c8 occupies approximately the same number of sites as (26 and, therefore, exerts a major influence on the surface concentration of hydrogen. C. HOMOGENEOUS COMPLEXES Apart from the above rate studies, there is additional evidence for the suggested sequence of metal-cycloalkene bond strengths. In a series of experiments, Quinn et al. (39) formed [a-cycloalkenyl] Pd2Br4 compounds competitively from binary mixtures of 3-bromocycloalkenes with Pd(I1) in solution. The competition order was C, > C6 > c 8 , the same as for hydrogenation of cycloalkenes on Pd, in a reaction in which complexing the double bond in the bromocycloalkene to the metal is the critical step. Hartley (40) also reports that the stabilities of Ag(1) complexes of cycloalkenes decrease in the order C5 > C7 > c6 > CB. There is, therefore, powerful evidence from both homogeneous and heterogeneous systems that during hydrogenation alkenes a-bond to individual metal atoms in surfaces, in complete agreement with the conclusion arrived at from exchange of polycycloalkanes with deuterium on Pd. The original Horiuti-Polanyi mechanism is, therefore, now much better understood and the parallel between homogeneous and heterogeneous catalysis clearly established in this area. However, one caveat concerning the restriction to ciselimination and addition of hydrogen atoms in the a0 process may be necessary. For example, Pecque and Maurel (41) reported evidence for direct trans addition of hydrogen to 2,3-dimethylbicyclo [2,2,2] oct-2-ene. However, they used ethanol as solvent, and, as seen from the work of Phillipson and Burwell ( 3 3 , polar hydrogen-bonding solvents are not inert but are intimately involved in hydrogenation.

IV. Skeletal Rearrangementof Alkanes on Platinum and Other Noble Metals The action of monofunctional platinum catalysts in effecting hydrocarbon skeletal rearrangement at temperatures as low as 250°C was noted as long ago as 1936 by Kazanskii and his school (42-44). Overshadowed by the technically

142

J. K. A. CLARKE AND J. J. ROONEY

rewarding developments in bifunctional reforming catalysts since the 1940% active study of this area did not begin until circa 1960-at least in Western countries. As is apparent in the account that follows, platinum catalysts have been the most studied, being (it seems) more active than other metals.

A. THE BOND-SHIFTMECHANISM Anderson and his co-workers examined the reactions of small alkanes mainly on platinum and palladium (45-48). Isobutane was isomerized to n-butane on platinum and on palladium, neopentane isomerized to isopentane on platinum, whereas other metals (including palladium) caused hydrogenolysis predominantly or exclusively. It was proposed that the slow step in the isomerization was the formation of a bridged intermediate (C) from an wry-triadsorbed species (A, B) (Fig. ll).' Huckel MO calculations based on this proposal suggested,

(A)

(B)

(C)

FIG. 11. The Anderson-Avery mechanism for bond-shift isomerization on Pt (47).

they believed, that partial electron transfer from hydrocarbon to metal encourages the rearrangement. Also, the influence of methyl substituents could be rationalized (48) on the basis of hyperconjugative interaction. Thus, energies liberated in formation of the bridged intermediate were in the order n-butane < isobutane < neopentane which was the order of relative reactivities found over platinum. This mechanism was termed a bond shift. Palladium acts differently in key respects from platinum; for example, as noted, neopentane is not isomerized. Muller and Gault (50) have suggested an alternative type of intermediate for isomerization on palladium because they have also noted that 1,1,3trimethylcyclopentane is largely converted to para- and rneta-xylenes by ring expansion at the quaternary center on Pt, but ring expansion at the tertiary center is preferred on palladium giving an adsorbed 1,l-dimethylcyclohexane that demethylates and is finally desorbed as toluene. On the basis of the above Deuterolysis results for 1,l -dimethylcyclopropane on Pt and a comparison with Pd, Co, and Fe have encouraged Muller and Gault to agree that this triadsorbed reactant is more probable on the former metal (49).

HYDROCARBON REACTIONS ON METAL CATALYSTS

CjHB

143

+ C Hq

FIG. 12. Possible mechanism of skeletal isomerization and hydrogenolysis of isobutane on Pd (50).

facts and the known propensity of palladium to form allylic complexes, they suggest the mechanism illustrated for isobutane in Fig. 12. To provide adequate background for the work to be described next, some further findings by Anderson and Avery may be mentioned. The selectivity for isomerization versus hydrogenolysis (Si = rr/rH) of isobutane on evaporated films of platinum claimed to expose (1 11) faces predominantly was found to be enhanced by a factor of 5 relative to unoriented films; this enhancement was not observed for n-butane (Table VI). Anderson and Avery (47) proposed that a symmetrical triadsorbed species (Diagram 1) is the preferred reaction intermediate for isobutane, such an intermediate not being possible for n-butane. This intermediate fits the triplets of metal atoms on the (111) plane of platinum, suggesting, they believed, a basis for the “enhanced efficiency” of the (1 11) plane for the isomerization of isobutane. We note that inspection of rates of isomerization given in the paper of Anderson and Avery shows a factor of only TABLE VI Relative Proportions of Isomerization and Hydrogenolysis with Butanes on Platinuma

Reactant hydrocarbon Isobutane nButane

Catalystb

Isomerization rate Hydrogenolysis rate

(111)Pt (100) Pt Unoriented Pt (111)Pt (loo) Pt Unoriented Pt

10.4 2.95 2.08 0.23 0.39 0.23

OData from Anderson and Avery (47). bTemperature range: 256’-320” C.

144

J . K. A. CLARKE AND J. J . ROONEY

f\

*

C

C Ir

about 2 in favor of isobutane over n-butane on (1 11) Pt at 320°C [there is, indeed, a similar factor at 300°C in favor of isobutane over n-butane on (1 00) Pt for which there is not a natural “fit” for a triadsorbed intermediate]. Boudart and his group (51) have subsequently found that the specific activity for isomerization of neopentane to isopentane of a series of supported platinum catalysts of differing dispersion varied by a factor of perhaps 15 whereas the specific activity for hydrogenolysis changed over 300-fold. In other words, both reactions were facilitated by high metal dispersion but this was most distinct for hydrogenolysis. It must be stressed that this suppression of hydrogenolysis, rather than enhancement of isomerization, on (1 11) facesis the main cause of increased selectivity on (111) Pt; (111) faces probably have poor hydrogenolysis characteristics (52). Boudart and his co-workers have found that high-temperature prior heat treatment (900°C) leads t o highest selectivity. Such severely fired catalysts are expected to contain metal crystallites exposing predominantly (1 11) faces (53). Anderson and Aveiy proposed that the same intermediate for isomerization was also responsible for hydrogenolysis of isobutane (47), but very recently Hagen and Somorjai have studied reactions of isobutane and propane on Pt and Ir catalysts in which Au was incorporated in increasing amounts. They concluded from the results that the sites responsible for isomerization are distinct from those causing hydrogenolysis (53a). Boudart and Ptak (54) have reported that, among all the metals of Group VIII plus Cu and Au, only Pt, Ir, and Au isomerize neopentane t o isopentane. If rate data are extrapolated to a common reaction temperature, their results suggest the activity of gold to be 104-10s less, and iridium lo2 more, than that of platinum. These differences were contained solely in the Arrhenius frequency factor. Anderson and Avery (47) failed t o find bond-shift isomerization activity with iridium films. Boudart and Ptak (54) point out that isopentane could be missed among the products because the hydrogenolysis activity of iridium is so large-about two orders of magnitude greater than for platinum. No matter how this problem may be resolved, an interesting idea has been put forward by Boudart and Ptak t o explain the activity of Pt and Au (and, perhaps, Ir). Two requirements are considered necessary: (a) the surface atoms must be suffi-

HYDROCARBON REACTIONS ON METAL CATALYSTS

145

ciently electronegative (copper is inactive) and (b) the surface valencies of the metal must be able to shift readily from one value to another in the rearrangement in which they schematically depict the transition state (Diagram 2)2 (54).

Diagram 2

According to Boudart and Ptak, the shifting of surface valency hinges on the ease of the electron promotion step 5dl06s' + 5d96s2 for Au (1.1 eV only) and the corresponding process for Ag which is catalytically inactive (2.7 eV). They foresee that the requirement b applies also in the mechanism proposed by Muller and Gault (49) which is slightly different from the Anderson-Avery conception. In the formulation of the latter, the Australianworkers supposed that the reverse reaction (isopentane + neopentane) could not happen. Muller and Gault recall (49)from some of their earlier work (55)that nonnegligible amounts of neohexane were formed from 3-methylpentane on platinum films. For this reason they preferred the mechanism shown in Diagram 3 for the case of ring enlarge-

Diagram 3

ment. This mechanism may act reversibly and can explain the formation of a quaternary carbon atomY3as depicted in the case of neopentane isomerization (Diagram 4). Gault (56) favors this adsorbed-cyclopropane scheme on the fur2The present authors believe that species C should more correctly be depicted as CH,-

CH=C(CH,),

or CH -CH-C(CH,),

T I

I ! since the adsorbed product must also be triply bonded to two sites as in the transition state. Although Anderson has never described in detail the exact nature of the bonding of the adsorbed product to the surface, it is highly unlikely that the orbitals depicted in the transition state (Fig.11) transform into those that bind the product, CH2-CH-C(CH3)2, to 2 Pt atoms. The Anderson-Avery mechanism is in fact incomplete. 3We believe that concern over feasibility of the reverse reaction is unnecessary in the arguments. By the principle of microscopic reversibility, the forward path can be reversed by the same elementary steps. The incomplete nature of the Anderson-Avery mechanism (Fig. 11)has apparently caused some confusion.

146

J. K. A. CLARKE AND J. J. ROONEY H3C

CH,

HC

CH,

\/

y43

II

M

I

M

2

HLC ' M2

I

H3C-C-CH~CH

I

H

M M Diagram 4

II

M

ther grounds that 2-meth~l-2.'~ C-butane isomerizes largely to 2-" C-n-pentane and to a smaller extent only to 3-13C-n-pentane as the latter is the expected predominant product from the Anderson-Avery mechanism. He interprets t h i s observation4 on the basis that the intermediate

is preferred over

r

1

Recently, Rooney and co-workers (23,58,59) have questioned the view that triadsorption by loss of 3 hydrogen atoms from the alkane is the minimum requirement for bond-shift reactions. They studied the isomerization of a series of caged hydrocarbons in excess hydrogen on palladium and platinum catalysts. The compounds were chosen in order to render difficult or totally exclude a mechanism involving army-triadsorbed species. Thus, 1,7,7-trimethyl[2,2,1] heptane interconverts with its endo- and exo-2,3,3-trimethyl isomers, bicyclo[3,2,2] octane changes to bicyclo [3,3,1]nonane, and protoadamantane to

-

4This experimental result is, in fact, readily explained by the bond-shift mechanism described in the following paragraphs. A vinyl shift (Diagram 5) is predicted to be much easier

&gram

5

than a methyl or ethyl shift from molecular orbital theory (57), so that a partially dehydrogenated species would exhibit the observed bondshift direction. Unsaturation in the shifting group would result in a larger negative pressure dependence index than in the case of a saturated shifting group. For example, in the reaction of isopentane, a vinyl shift would lead to a pressure dependence index of -1.5 for hydrogen.

HYDROCARBON REACTIONS ON METAL CATALYSTS

147

adamantane on Pd and Pt catalysts in the range 150°-3500C. The last example afforded activation energies of 24.1 and 10.1 kcal mole-' for rearrangement on palladium and platinum, respectively, and also illustrates the difficulty of invoking the Anderson-Avery mechanism.

I

dehydroadamantane

*/ Diagram 6

Examination of models (Diagram 6) shows that, apart from a very skewed initial acq species, it seems impossible to have the triple Ogy attachment of the product to the surface. Even the 09 process cannot operate in the exchange of adamantane (see Section 11). Rooney and his co-workers argue that loss of only 1H atom generating a surface alkyl may be sufficient to allow bond shift. This view is vindicated by the observation (59) that the very strained adamantene dimer (Diagram 7) isomerizes at rates comparable to those of exchange with

Diagram 7

deuterium on a Pd/pumice catalyst, with both products and reactants having simple distributions only of deuteroisomers. Besides, the activation energy for isomerization of protoadamantane on platinum is less than the additional strain energy of dehydroadamantane, thus ruling out the possibility of isomerization via loss of 2 hydrogen atoms to give an uy-diadsorbed species with subsequent formation and cleavage of a C3 ring. The isomerization of adsorbed alkyls using neopentane as an example is explained as in Diagram 8. The half-reaction state is normally energy forbidden for

148

J . K. A. CLARKE AND J. J. ROONEY ,' s 3 /CH3 +CH

CH-C

*

CH

-MDiagram 8

3

(\CH3 -M-

a free radical because the second molecular orbital has net antibonding character and would have to contain one electron. However, bonding to metal overcomes this problem because the organometallic complex has molecular orbitals very similar to those in olefin-metal complexes. Clearly, the antibonding orbital as a result of pn-dn interaction (Diagram 9) is sufficiently stable to be occupied.

f" -M-

1 U

Diagram 9

Since this mechanism is so similar to that of bond shift in carbonium ions, it is not surprising that the ease of rearrangement on metals parallels the ease of isomerization via acid catalysis. The order of rearrangement of neopentane > isobutane > n-butane on platinum is also that expected from consideration of the energetics of neopentyl, 2-methylprop-l-yl, and n-but-2-yl ions converting to 2-methylbut-2-yl, n-but-2-yl, and 2-methylprop-1-yl ions, respectively. The mechanism also explains why platinum, which forms much stronger metalolefin bonds than palladium (60), is much the better catalyst for bond shift. In fact, the very large difference in activation energies for isomerization of protoadamantane (a strained compound) agrees well with the finding that bond shift involving gem-dimethyl groups is so difficult on palladium. The probability that the mechanism of Rooney and co-workers could be important for simple paraffins as well has received strong support from work on rearrangement of n-pentane and n-hexane on very dilute Pt-in-Au alloys (see Section V). We may in the light of the foregoing discussion conclude with some firmness that bond shift is more selective on (1 11) faces of platinum, not because of suitable triangular arrays of atoms, as suggested for a so-called demanding reaction, but that this face has not the capacity for the extensive bonding to the surface required for hydrogenolysis. In fact, if the terminology has any merit the latter could well be regarded as the demanding and the bond shift as a facile reaction, i.e., the latter is important when there are a significant number

149

HYDROCARBON REACTIONS ON METAL CATALYSTS

of sites with very low bonding capacity. It is of interest in this connection that in a recent study Brunelle and co-workers (60a) conclude that bond-shift isomerization (of n-pentane) on Pt is not dependent on surface structure. Very recent work (60b) has confirmed that Ir films do not isomerize neopentane; most of the transition metals as well as palladium (60c) rearrange isobutane to n-butane but are also inactive for the former conversion. This clearly indicates that isomerization of neopentane on Pt is mechanistically rather special and, in view of the known propensity of Pt to promote ay exchange with deuterium of paraffins (5,49), refocuses attention on the a y species diadsorbed on one metal atom as the precursor for bond shift in simple alkanes. The following mechanism for neopentane isomerization on Pt is feasible, where the shifting

methyl group in the half-reaction state bridges the C, and C2 atoms of a transient n-ally1 system. A simple molecular orbital treatment of this system predicts that this migration should be easier than, and is indeed a simple extension of, the mechanism discussed (Diagrams 8 and 9). Furthermore, there is a clear analogy to such metallocyclobutane rearrangements in homogeneous catalysis. For example, Ag+ ions isomerize certain C3-ring compounds and the mechanism involves insertion of the Ag' ion into one of the bonds of the C3-ring with methyl migration ( 6 0 4 . Because transition metals other than Pt (as far as they have been examined) have not such a propensity to form a metallocyclobutane directly from a gemdimethyl group the way to ready isomerization of neopentane is barred. However, an alkane such as isobutane may have an indirect route to the aydiadsorbed species provided that lY2-migrationof hydrogen is relatively easy ( 6 0 4 ,as follows.

H

I

CHz=C-

CH2- CH,

The attractive feature of this mechanism is that it utilizes the same type of 13diadsorbed intermediate and bridging transition state for rearrangement of these simple alkanes. Furthermore, this mechanism is also a simple extension of the mechanism of Diagrams 8 and 9.

150

J. K. A. CLARKE AND J. J. ROONEY

Reference will be made again to bond-shift mechanisms in the following section in considering ring enlargement.

B. THEDEHYDROCYCLIZATION-HY DROGENOLYSIS (OR “CYCLIC”)MECHANISM The isomerization of larger alkane molecules at or about 250°C on platinum has been found to proceed substantially, or in some cases entirely, through a cyclic intermediate (55, 61). Suggestively, methylcyclopentane from dehydrocyclization always accompanies the rearrangement of methylpentanes and of n-hexane. Initial product distributions were found to be identical in the isomerization of methylpentanes and n-hexane and in the hydrogenolysis of methylcyclopentane (62) in the case of a highly dispersed supported platinum (0.2% Pt-A1203) (Table VII). A common intermediate for the three reactions, isomerization, dehydrocyclization, and methylcyclopentane hydrogenolysis, has been inferred; the reactions may then be represented by the unified formal scheme shown in Fig. 13. TABLE VII

Initial Ratios of Hexane Isomers from Reactions of Methylcyclopentane, n-Hexane (I),2-Methylpentane (ll),and 3-Methylpentane (Ill) on Platinum-Alumina at 300°C‘

~~~

Hydrogenolysis Isomerization of I Isomerization of I1 lsomerization of I11 Equilibrium ~~~

0.9

-

2.15 2.2 -

0.55 0.55

0.9 0.55

1.65

1.1

-

-

~

‘Data from Maire et al. (62). See also Fig. 14.

FIG. 13. Common intermediate for dehydrocyclization and isomerization of n-hexane and hydrogenolysis of methylcyclopentane (61).

HYDROCARBON REACTIONS ON METAL CATALYSTS

151

C

FIG. 14. Nonselective ring opening of methylcyclopentane in interconversionof n-hexane, 2-methylpentane, and 3-methylpentane (62).

In comparative experiments, isomerization of 2,3-dimethylbutane was found to be slow at 277"-350°C on 0.2% Pt-A1203 and also (see later) on a much less dispersed 10% Pt-Al,O,. Barron el al. (61) argued from these results that the intermediate (C) in Fig. 13 was an adsorbed entity having a methylcyclopentane structure. Dehydrocyclization followed by ring opening was accordingly the inferred isomerization route for hexanes (Fig. 14). Product distributions depend on the metal dispersion. Preferences for certain modes of ring scission depending on metal loading were reported as far back as 1957 by Gault (63). Thus, with 6% or more of platinum on alumina, and with platinum films at the lower temperatures, a selective hydrogenolysis of the disecondary C-C bonds took place ("mechanism B"); on a catalyst of low metal content, i.e. 0.6% and lower, and with platinum and palladium films at the higher temperatures, the five bonds in the ring were opened with almost equal probability ("mechanism A") (62, 63). The latter process appears to be pertinent to the dehydrocyclization process itself, now to be discussed, and there is considerable interest in the dependence on metal dispersion of the individual steps in Fig. 13 (see Section V). A number of mechanisms of ring closure have been suggested. In connection with a study of the interconversion of n-propylbenzene and a-ethyltoluene on platinum, Shephard and Rooney (64) proposed the pathway in Diagram 10, by

=q* Q -n

M

Diagram 10

M

M

152

J . K. A. CLARKE AND J. J . ROONEY

analogy with organometallic reaction mechanisms. As noted in the preceding paragraph, Barron et al. (61) related nonselective hydrogenolysis of the several types of C-C bonds in methylcyclopentane with edge sites occurring most particularly on the highly dispersed type of platinum catalyst. They inferred also a type of intermediate, which they termed ad,?-triadsorbed. In their wellknown follow-up paper ( 5 9 , they suggested that the ring closure process, which appeared t o be effected by the same type of sites, might also proceed by the same kind of intermediate. These authors favored a modified form of the Rooney-Shephard intermediate which they represented as shown in Diagram 11 for the case of 2-methylpentane reactant.

Diagram 11

Muller and Gault (50) inferred subsequently from comparative rates of dehydrocyclization of 2,2,4-trimethyl(I), 2,2,3-trimethyl(II), and 2,2,4,4-tetramethylpentanes(II1) on Pd and Pt films the need for supposing a dehydrocyclization mechanism other than alkene/alkyl insertion for platinum. Thus, dehydrocyclization rates on palladium at 300°C were 0.18,2.6, and < 0.1 units of activity per unit weight, respectively, but, on platinum, the rates were closely similar for reactants I, 11, and 111. An aaw-triadsorbed intermediate is clearly suggested by this insensitivity of reaction rates t o (even gern-dimethyl) substituents at the penultimate carbon atoms. The intermediate suggested for platinum (Diagram 12) can, by a simple cis-ligand insertion, yield an adsorbed

Diagram 12

cyclopentyl radical. Examination of orbital dispositions suggests, Muller and Gault argue, an alternative and possibly more energetically favorable route, namely the transient formation of an intermediate in which the two p-orbitals of carbon atoms 1 and 5 are coupled together with a metal d orbital, resulting in a filled bonding and two empty nonbonding and antibonding molecular orbitals (Diagram 13).

153

HYDROCARBON REACTIONS ON METAL CATALYSTS

C

/c\

I C

C

C

/c\c

C

I

/c\c

\

/

rc

\/

M Dingram I3

Because of the marked differences in reactivity of 'I, 11, and 111 on palladium, Gault and co-workers prefer the metal-olefin/metal-alkyl insertion mechanism outlined above for this metal in contrast to platinum. It may be noted, incidentally, that the results described for reaction of I, 11, and 111 on platinum would be consistent with the simplest possible mechanism of ring closure, namely through aw-diadsorption: CJ\$

I - \ c\M'c

C

c-c

I

'M

This intermediate certainly cannot be ruled out with the information available. It is attractive in at least one respect; i.e., it is the simplest species for nonselective ring opening. Direct insertion of a metal atom into a C-C bond of a C3 ring is known, and Whitesides (65) has found that the ligand, 1,Ctetramethylene, may eliminate as cyclobutane from certain Pt compounds. Until circa 1960, it was generally accepted that C 6 cyclization was direct when the alkane structure permitted. On platinum at moderate temperatures, it now appears that 1,s-cyclization is preponderant over 1,6- or 1,7-~yclization,the extent of the preponderance being difficult to explain on mechanistic grounds (66a,b). There is a question whether production of c6 ring compounds from a reactant alkane having a 6-carbon chain takes place by direct closure to the C6 ring or by 1,s-cyclization followed by ring enlargement before desorption. Gault has favored rather the latter view and points out (66b) that simultaneous initial production of c6 rings at 300°C tends to depend on the presence of larger metal particles which are believed, somewhat circumstantially, to promote bond-shift-type ring enlargement. On the other hand, it has been argued that 1,s- and 1,6-cyclization are parallel processes for alkane reactants having a 6-carbon chain. Dautzenberg and Platteeuw ( 6 7 ) report that benzene is produced from a successive reaction step in isomerization of 2-methylpentane on supported platinum; methylcyclopentane is an initial product. Both benzene and methylcyclopentane were initial products in isomerization of n-hexane on the same catalyst. Their conclusion was, therefore, that the intermediate Cs-ring structure was not involved in the conversion of n-hexane to benzene. Accordingly, 1,s- and 1,6-cyclization must be parallel reactions. Davis and Venuto (68)

154

J. K. A. CLARKE AND J. J. ROONEY TABLE VlIl

Aromatization of n-Octane, 2-Methylheptane, and 3-Methylheptane over P t / ( ~ - A 1 ~ 0 3 ~ ’ Composition of Cs-aromatic fraction in mole % Hydrocarbon

Ethylbenzene

o-Xylene

m-Xylene

p-Xylene

n-Octane 2-Methylheptane 3-Methylheptane

39.7 1.8 17.1

55.9 2.1 25 .O

2.5 93.6 1.9

1.9 2.5 56.1

’

‘Data from Fogelberget al. (70). bExperimental conditions: temperature 525°C; hydrogen pressure 1.5 atm; 1 g catalyst. The a-A1203 was prepared by heating aluminum hydroxide to 1200°C; specific area 40 m2/g.

rn-xylene

m-xylene

.1 toluene+ methane or o-xylene

FIG. 15. Possible closures to six-membered rings with n-octane (A), 2-methylheptane (B), and 3-methylheptane (C). The dots represent carbon atoms bonded to the catalyst (70).

HYDROCARBON REACTIONS ON METAL CATALYSTS

155

report that the major aromatic products obtained from ten different C8C9 paraffins (including some unsaturateds) at 482°C were only those predicted by a direct six-membered ring closure. Confirmation was obtained by Davis (69) who reported similar aromatics distribution at 500°C from alkanes containing a quaternary carbon and from corresponding naphthenes. Fogelberg et al. (70) reported that dehydrocyclization of n-octane and of methylheptanes on Pt/a-A12O3 at 380"-525"C matched closely expectations based on direct 1,6-ring closure (Table VIII; Fig. 15). For n-pentane having methyl substituents, 1,5-cyclization may be followed by ring expansion at the metal sites to yield a c6 cycle before desorption (55, 71, 72) or, alternatively, by ring opening to a straight 6-carbon chain hydrocarbon that undergoes 1,Qcyclization as argued forcibly by Dautzenberg and Platteeuw (67). The latter authors reason that if aromatization were to proceed through a five-membered ring hydrocarbon, one would expect the same product composition of the C8 aromatics as formed from 2,5-dimethylhexane, 2,2,44rimethylpentane, and 1,1,3-trimethylcyclopentane(Diagram 14). In fact, whereas 2,2,4-

2.5-dinwthylh.xon

I,t,3- trimelhylcybpmlow

-

2,2,4 lrimelhylpentans

Diagram 14

trimethylpentane and 1,1,3-trimethylcyclopentane yield very similar distributions of xylene (425" and 48OoC), they point out that this does not hold for 2,5-dimethylhexane (Table IX). In the latter case, p-xylene is the predominant product pointing to a direct 1,6-ring closure. Other similar results (e.g., reaction of 1,l-dimethylcyclohexane compared to isopropylcyclopentane) were given by Dautzenberg and Platteeuw in support of this conclusion. We may note here that at temperatures above about 350"C, conversion of a c6 ring to "benzene" is thermodynamically so favorable that the only carbocyclic pathway then to isomerized alkanes is by a Cs-ring intermediate. Gault (66b) has noted that on platinum films with large crystallites at 300"C, 2,3-dimethylpentane and 3-methylhexane react to give 1,2-dimethylcyclopentane as the main initial product. Findings that 3-methylhexane gives 1,5-ring closure, whereas 2,sdimethylhexane gives mainly 1,6-closure (Table IX) seem to conflict. However, the latter study (67) and that of Davis and Venuto (68) were carried out at -480°C so this apparent contradiction is resolved if it is accepted that such a drastic alteration in conditions causes a change in the preferred mechanism from metal-carbenelmetal-alkylinsertion (1,5closure) to metal-olefin/metal-alkyl insertion ( I ,6-closure of 2,5-dimethylhex-len-6-~1 being the only possibility

156

J. K. A. CLARKE AND J. J. ROONEY TABLE IX

Product Compositions fiom Reactions on Platinum Supported on Nonacidic Aluminas" Reactant hydrocarbon

Products

c1-c7

2,2,4-TMP 1,1,3-TMCP ClH8 Ethylbenzene P-CsH10 m-CBHlo OC8HlO

2, 2,4-TMPb 425OC

1,1,3-TMCP' 425°C

2, 2,4-TMPb 480°C

1,1,3-TMCf 480°C

21.0 37.4 32.4 1.3 0.0 3.2 4.7 0.0

17.0 14.6 59.8 0.0 0.0 3.9 4.7 0.0

52.3 14.9 2.4 12.9 0.0 7.0 10.5 0.0

41.2 8.2 24.3 7.8 0.0 7.1 11.0 0.4

2,5-DMHd 48OoC N.R.~ N.R.~ N.R~ 0.0 0.6 87.0 10.8 1.6

"Products given in weight percent; columns 2-5 are expressed as percent of total hydrocarbon, and column 6 is expressed as percent of total C8 aromatics. bTMP = trimethylpentane. Data from Lester (72). 'TMCP = trimethylcyclopentane. Data from Lester (72). dDMH = dimethylhexane. Data from Dautzenberg and Platteeuw (67). 'N.R., not reported. Conversion into C8 aromatics: 60 mole %.

on steric grounds for 2,5-dimethylhexane). The influence of carbiding at higher temperatures on dehydrocyclization mechanism is considered in Section VI. A further possible route to benzene is by cyclization of hexatrienes (67, 73, 74) arising from dehydrogenation at higher temperatures and lower hydrogen partial pressures. Hexatriene-1,cis3,5 has been shown to undergo C6-ring closure by a thermal reaction at temperatures in the range 117°-1610C (7.9, and order-of-magnitude extrapolation of Lewis and Steiner's kinetic results suggests the quite appreciable first-order rate constants of 10-20 sec-' at 350°C and 1-2 X lo3 sec-' at 500°C. Guczi and TBtBnyi (75a) report formation of a hexatriene-l,3,5 in reaction of n-hexane at 350°C on platinum with simultaneous benzene formation. Publications from two laboratories (55, 72) have argued that aromatization to xylenes of 1,1,3-trimethylcyclopentane on platinum films at 300"-330°C can be explained only by a ring enlargement at the quaternary carbon atom:

Thus, an opening of the cyclopentane ring followed by a 1,6-ring closure would lead only to 1,l-dimethylcyclohexane and toluene. A carbonium ion mechanism (see below) would lead to all xylenes, ortho as well as para and meta. Barron

HYDROCARBON REACTIONS ON METAL CATALYSTS

157

er al. (55) liken this ring enlargement to the bond-shift isomerization with neo-

pentane which is also restricted to platinum catalysis.' Significantly, they point out the extent of aromatization of hexanes is larger on Pt films, where also 2,3-dimethylbutane is formed in large amounts by bond shift, supporting the relationship between bond shift and ring enlargement. Their more recent version of this mechanism involving an adsorbed cyclopropane species has been referred to above. Lester (72) believes that the close similarity of aromatics distribution from 2,2,Ctrimethylpentane and of 1,1,3-trimethylcyclopentaneat 425" and 480°C over Pt/inert-A1203 (see Table IX and earlier remarks) means that the cyclopentane is an important intermediate over this catalyst. Further, he points out, the rate of aromatization of the two reactant hydrocarbons is about the same, implying that the ring formation is probably not the rate-determining step. Since aromatization of cyclohexanes is known to be rapid under these catalyst conditions, it is inferred that the slow step in the aromatization of the two reactants studied is the ring expansion of the cyclopentane. Lester argues, by analogy with the ready action of acidic catalysts in effecting ring expansion, that platinum acts as an electron sink (weak Lewis acid) for absorbed cyclopentenes, creating electron-deficient species that can rearrange in a manner analogous to carbonium ions (Scheme 2). His mechanism involves formation of

Scheme 2

an electron-deficient cyclopentyl species by electron withdrawal from a halfhydrogenated cyclopentene to surface platinum. This is followed either by a 1,4-hydrogen shift or by a series of 1,2-hydrogen shifts and ring insertion of a methyl carbon via a bicyclic intermediate (or transition state) to yield a cycloMetals Fe, Rh,and Pd (76) give relatively much less xylenes and a greater proportion of benzene and other products not necessitating the type of bond shift suggested for Pt.

158

J. K. A. CLARKE AND J. J. ROONEY

hexyl surface species, with rapid dehydrogenation to an aromatic. Step 1 is one way of representing the formation of the electron-deficient structure. We may note that Lester’s scheme was devised in the light of an earlier mechanism put forward by Barron et al. (55) involving an cwcryy intermediate, and it was developed along lines to avoid the necessity of removing 4 hydrogen atoms from two methyl groups in the way proposed. Lester’s scheme is rather similar to the latter mechanism for bond shift of McKervey et al. (59);the only distinction is that a free carbonium ion is postulated in the former, whereas the latter provides an explanation of rearrangement of a covalently bonded alkyl or cycloalkyl. The reaction scheme of McKervey et al. has, therefore, the twofold advantage of not having to postulate change separation and at the same time meeting Lester’s criticism about the number of hydrogens removed before bond shift is possible in the earlier Barron-Gault mechanism.

V. Recent Experimental Approaches to Skeletal Rearrangements Three lines of enquiry have been pursued actively in recent years to uncover more fully details of isomerizations and dehydrocyclization. The use of 13Clabeled hydrocarbons, pioneered by Beeck and his group in the 1940s for acidic isomerization of alkanes, is now proving the most informative of these approaches. The technique has been applied most particularly in connection with the study of metal-dispersion dependence of isomerization activity. Pertinent alloy work is of quite recent origin. Although it tends to be more inferential in nature, this approach offers a further basis on which reaction mechanisms may be tested and holds promise of giving further commercially useful forms of catalyst for processes including skeletal rearrangements. We shall begin by examining results of studies on the dependence of isomerization activity on metal dispersion. A. SURFACE-STRUCTURE SENSITIVITY

As discussed in Section IV, Barron et al. (55,61) found the “cyclic mechanism of isomerization to be predominant, perhaps the sole route, on a highly dispersed platinum-alumina (0.2%w/w Pt). The cyclic mechanism was shown to be important also over platinum films and supported platinum of moderate dispersion (> 100 A). Here, although the product distributions were very different from that found over the dispersed catalyst, the initial product distributions at 300°C were practically identical in the isomerization and in methylcyclopentane hydrogenolysis. At lower temperatures they were somewhat different as they also were at all temperatures on platinum films. It was suggested that, especially on platinum films, a bond-shift isomerization could accompany the cyclic

HYDROCARBON REACTIONS ON METAL CATALYSTS

159

mechanism (55). Further, the differences between the dilute and concentrated Pt/A1203 catalysts were taken to be due to a different relative number of two types of site appropriate to the different reaction routes (62). There has been some dispute and counterargument as to the validity of this conclusion. The nature of the exchanges serve as a useful reminder of several sources of complexity that accompany tests of surface-structure sensitivity of catalytic reactions by use of a series of catalysts having differing metal-loading or differing average metal-crystallite size. Dautzenberg and Platteeuw (67), working with platinum catalysts, concluded that isomerization of n-hexane (predominantly cyclic mechanism) was independent of particle size. The findings of Barron et al. were attributed to adventitious chlorine in the alumina support leading to bifunctional catalyst action. Maire et al. (77) have responded by reporting product ratios for hydrogenolysis of methylcyclopentane and isomerization of 2-methylpentane at 200"-340°C on a variety of platinum catalysts prepared from different platinum compounds and having different supports. The n-hexane/3-methylpentaneratios lay in two well-separated regions corresponding to the 0.2%Pt and the 10% Pt catalysts, respectively. Even deliberate acidification of a support did not affect the product distributions appreciably at the temperatures in question. Results from "C experiments on their high and low dispersion catalysts were becoming available (78) allowing an even more definite statement as to the broad mechanism of isomerization to be made. Two types of site were believed to be involved: (i) on dispersed catalysts, where purely cyclic isomerization took place as well as nonselective hydrogenolysis, the sites concerned involve probably only 1 metal atom; and (i) on concentrated catalysts, sites comprising several contiguous surface atoms are thought to determine the selective type of hydrogenolysis (preferential cleavage of -CH2 -CH2-) and allow both bond-shift and cyclic mechanism. In another exchange, the Dutch workers (79, 80) report further results, including a product ratio for a 10% Pt/A1203 catalyst that did not differ significantly from that found for loadings of metal as low as 1%. Gault et al. argue that, whereas the 10% Pt/A1203 catalyst of Barron et al. (55) was of average metal-crystallite size, ca. 200 A, the 10% Pt catalyst of Dautzenberg and Platteeuw (67) was only ca. 80 A, both estimates being derived from X-ray diffraction line broadening. Although Gault et al. in this communication do draw attention to the effect on mean particle size of several catalyst preparation conditions, there remains the very clear problem that real metal crystallites may have other than ideal shapes; that is, regular cubo-octahedra or other idealized shapes are not to be expected in all catalyst preparations [cf. 0 Cinneide and Clarke (81)l.Higher metal loadings may lead to agglomerates of smaller units which bring with them a surface topography rather similar to crystallites produced under conditions of small metal loadings. Further, Gault (82) reports that when a highly dispersed 10% Pt/A1203 catalyst is sintered in hydrogen at 500°C

160

J . K. A. CLARKE AND J. J. ROONEY

the size of the crystallites increases, the activity decreases, but the selectivity for methylcyclopentane hydrogenolysis does not change. Thus, on sintering the number of sites decreased but the relative numbers of those for selective and nonselective ring opening remain unchanged. Gault takes the view that the sites consist of defects of different kinds the relative numbers of which are determined in the early stage of reduction in the catalyst preparation. The implication here is that temperature of catalyst pretreatment is also a determining factor for producing a variation in proportion of the different possible types of surface site (83). Anderson el al. (84) propose that UHV “ultrathin” (0.3-1 .O pg/cm’) films can form a useful model system for studying particle size effects on catalyst selectivity without complication due to possible surface contamination. The particle sizes in their ultrathin films were, on average, about 20 W with particles apparent down to the limit of their electron-microscope resolution (ca. 8 A) and possibly, they thought, with a proportion of monodispersed platinum atoms. They report that the selectivity in n-hexane reaction for formation of Cb products versus hydrogenolysis products at 273°C was much higher on such films than on “thick” platinum films (Table X). These workers suggest that their results are consistent with Gault’s view that low coordination metal atoms, which would occur in greater proportion in their ultrathin films, favor carbocyclic reaction intermediates, whereas, as they previously argued (see Section IV), bond-shift isomerization and hydrogenolysis may be favored by having 2 or 3 adjacent atoms on a crystal face. In their interpretations, they attach most significance to the larger proportion of cyclic products on ultrathin as compared to thick films. Anderson (83) also reports that there is less distinction between ultrathin and thick films in cyclization plus isomerization versus hydrogenolysis ratio in the reaction of 2-methylpentane. Gault has argued (85) that only l3C-1abelingexperiments can tell whether this is an adequate measure of the proportion of isomerization proceeding by the “cyclic” mechanism in any particular case. There is clearly a source of complexity in attempts at comparing isomerization with hydrogenolysis if hydrogenolysis can proceed by different mechanisms. Thus, a n-olefm or n-ally1 route (reverse of the ringclosure type of process discussed in Section 1V.B) may be expected to be favored on the same kind of sites as the dehydrocyclization reaction. This has been clearly enunciated in the papers of Gault. Similarly, the reverse of the carbene/alkyl insertion route may occur on single metal atoms. By contrast, ao9p precursors of selective ring opening of methylcyclopentane to 3-methylpentane may require regions of lower index. Bond-shift isomerization is an even more open question. Anderson believes that its intermediate is identical to that of the 1,3-&adsorbed hydrogenolysis intermediate, whereas McKervey er al. differ (Section IV). C-Labeling experiments (see below) confirm that the bond-shift route becomes relatively more

’’

TABLE X Reaction of n-Hexane over Pktinum Film Catalysts:

c6

Reaction Product?’

c6 Reaction products (mole %)

Run No! 1 2 3 4 5 6 7 8 9 14-22

Film type

2-MP

3-MP

2,3-DMB, neo-H, CHe

Ultrathin Ultrathin Ultrathin Ultrathin Thick polycrystalline Thick polycrystalline Thick polycrystalline Thick polycrystalline, sintered Thick polycrystalline, sintered Thick, free from thin film “fringe,” sintered

10.5 12.5 13.9 17.6 25.5 23.9 32.4 39.0 34.0 43-20

4.5 5.6 5.4 6.4 11.7 10.6 14.7 19.0 17.0 22-11

0 0 0 0 0 0 0 0 0 0

MCP

CH + B

Selectivity

2-MP/3-MP

73.7 72.4 7 2.9 69.6 47.6 52.0 41.0 27.0 34.0 10-50

11.3 9.5 8.0 6.4 15.2 13.5 11.9 14.0 15.0 16-19

>10 >10 17.1 11.6 <2 <2 <2 1.8 1.0 Not available

2.3 2.2 2.6 2.8 2.2 2.3 2.2 2.1 2.0 ?2

~

‘Data from Anderson et al. (84). ’Key to abbreviations: 2-MP = 2-methylpentane; 3-MP = 3-methylpentane; 2,3-DMB = 2,3dimethylbutane; neo-H = neohexane; CHe = cyclohexene; MCP = methylcyclopentane; CH = cyclohexane; B = benzene. ‘Conversions were generally 1-2%. Reaction temperature: 273OC.

TABLE XI

Reaction of Methylcyclopentane on PIatinum Film Catalysts at 273"Ca ~~~~

Film Type Ultrathin Ultrathin Ultrathin Ultrathin Ultrathin Thick

bg 0.01 0.08 0.60 1.o 2.5

-

Average crystallite diam. (A)

Initial hydrogenolysis rate (10'~molec. sec-I cm-2)

<15 20 40 43 58 -

-

-

26 14 14 8.4

6.1 1.7 2.4 2.2

OData from Anderson and Shimoyama (86).

bMP = methylpentane; CH = cyclohexane.

Steady hydrogenolysis rate (10'~molec. se~-'cm-~)

~

~~

Productsb (mole %)

2-MP

3-MP

nC6H14

CH

c1-C~

45.8 41.2 51.6 57.6 51.6 61.3

18.8 17.4 21.2 23.0 20.4 32.2

23.5 28.9 18.0 18.4 20.2 1.8

6.9 2.9 0.4 3.4 -

2.4 3.0 -0.3 -0.6 2.3 2.5

163

HYDROCARBON REACTIONS ON METAL CATALYSTS

important for metal of lesser dispersion, but no precise relationship is yet in sight. Anderson and Shimoyama (86) have attempted to assess qualitatively and in a relative way the crystallite-size dependence of these processes using ultrathin platinum films of increasing crystallite size, in the range < 15-40 A, and also normal “thick” films. Hydrogenolysis of methylcyclopentane, n-hexane, 3-methylpentane, and 2-methylpentane at 273°C showed a steady decrease in the specific rate, measured in their static reactor either as the fast initial reaction or as the “steady” rate (Tables XI and XII). Methylcyclopentane undergoes hydrogenolysis, it is argued, solely by the n-olefin/n-ally1 route because 1,3diadsorption is not possible for the C5 ring. Both of these mechanisms can operate for the remaining three hydrocarbons. Faute de rnieux, these authors assume that the 1,3-hydrogenolysis pathway is independent of particle size in constructing an interim representation of particle size influence (Fig. 16). Two experimental facts are also of use: (i) carbocyclic isomerization, involving a n-adsorbed entity for ring closure, is dominant on the smallest crystallitesfollowing Gault-and is never trivial, so that a similar relative importance might TABLE XI1 Hydrogenolysis of Hexanes on Platinum Film Catalysts at 273’C” ~~

~

Film Reactant n-Hexane

3-Methylpentane

2-Methylpentane

Type

b g cm”)

Ultrathin Ultrathin Ultrathin Ultrathin Ultrathin Ultrathin Ultrathin Thick Ultrathin Ultrathin Ultrathin Thick Ultrathin Ultrathin Ultrathin Ultrathin Thick

0.060 0.080 0.125 0.25 0.50 1.0 2.5 0.125 0.60 2.5 0.08 0.5 1.0 2.5 -

‘Data from Anderson and Shimoyama (86).

~

Average crystallite diam. (A) 15 20 20 36 38 43 58

-

20 40 58

-

20 38 43 58 -

Initial hydrogenolysis rate ( 1 0 ’ ~molec. sec-’ cm-2)

Steady hydrogenolysis rate (10’~ molec. sec-’ cm-2)

4.0 2.2 1.45 0.55 0.53 0.35 0.35 0.37 15 13 8.8 6.1 29 14 13 8.5 5.0

0.48 0.22 0.18 0.073 0.060 0.037 0.045 0.039 2.2 1.7 1.3 0.84 5.2 1.9 1.7 1.3 0.5

164

J. K. A. CLARKE AND J. J . ROONEY

17-adsorbed mechanlsrns

1,3-adsorbed

l13-adsorbed

rnec ha nisms

rnec hanisms

2

1

I

2

1

I

T 15A 40&

thick f i l m l l j i

thick fi&

CRYSTAL SIZE-

FIG. 16. Schematic representation of the variation of individual reaction rates with Pt crystaliite size: a (left) corresponds to the type of behavior of 2-methylpentane; b (right) to n-hexane (86).

be expected for hydrogenolysis for the two paths; (ii) judging from neopentane behavior ( 4 9 , a 1,3-diadsorbed radical may be estimated to give -20% hydrogenolysis and -80% isomerization. These considerations yield the schematic representation shown in Fig. 16. The conclusion Anderson and Shimoyama draw with some firmness from this scrutiny of experimental data is that both hydrogenolysis and isomerization via n-adsorbed species decrease rapidly with increasing platinum particle size. This behavior could most readily be ascribed to the action of step sites, as argued by Gault for carbocyclic isomerization. Gault (85) has responded by stating his belief that two, and probably three, different mechanisms contribute to methycyclopentane hydrogenolysis, and

HYDROCARBON REACTIONS ON METAL CATALYSTS

165

a0 adsorption may lead to one of these. One route is the selective breaking of disecondary bonds on large metal crystallites (> 1SO A) for which the results of Anderson and Shimoyama (Table XI) provide a clue in the shape of trace n-hexane only in the hydrogenolysis of methylcyclopentane on the thick film. The second is the direct rupture of an cup-diadsorbed species demonstrated in the case of the ring opening of the cis- and trans-disubstituted cyclobutanes (87), which can take place on small crystallites and corresponds to equal scission of any endocyclic bond. Later work, discussed below, demonstrates that methylcyclopentane hydrogenolysis on highly dispersed platinum corresponds to an equal chance of breaking any endocyclic bond and the distribution does not vary with temperature: the ring opening seems to be very simple in nature. This work emphasizes, therefore, superposition of particle size effects for the contributing processes in hydrogenolysis and skeletal isomerization and exposes, again, absence of firm information on bond-shift isomerization sites. The existence of at least three mechanisms of hydrogenolysis with different surface-structure sensitivity is important to bear in mind in considering d o y catalyst action. The association of the dehydrocyclization step (at least) of the carbocyclic mechanism of isomerization with edge sites has been strongly vindicated by Joyner et al. (88) using a combination of LEED, Auger, and mass spectrometry. These workers found that dehydrocyclization of n-heptane in the range 100"400°C took place most readily on a stepped platinum surface with (1 11) orientation terraces (Fig. 17). Such surfaces, consisting of low-index planes linked by steps often of monatomic height, result from cutting a single crystal along a high-index plane and have a high thermal stability for kinetic reasons (89). Other surfaces studied by Joyner et al. (88),besides having reduced initial activity for the dehydrocyclization, showed a steady decline in activity with time due to the formation of disordered carbon-containing deposit. By contrast, the stepped surface with (1 11) orientation developed a carbonaceous layer that appears to play an important role in maintaining the catalytic activity. Apart,

FIG. 17. Stepped sites designated by LEED as Pt(s)- [6(111) X (lOO)] (88).

166

J. K. A. CLARKE AND J. J. ROONEY

then, from a greater reactivity of such steps in dissociative chemisorption, as found by this group (90) for dissociating diatomic molecules such as hydrogen, the inevitable carbonaceous deposit at the surface with hydrocarbon reactants is induced to form an ordered structure at the step which conserves catalytic action. There is a further consideration. Steric demands and the need for, perhaps, three (or more) free coordination positions at the dehydrocyclization surface site appear to necessitate a step location or kink site. No particular choice between possible step sites seems possible at present on organometallic grounds, but the rather general relevance of the stepped surfaces with (1 11) orientation arises from (i) nucleation of carbon deposition (as above) and (ii) the likelihood of the strong persistence of such regions on the surface of practical catalysts (52,91).

B. ”c-LABELING

STUDIES

The ”C-labeling technique has been exploited in recent years to determine the relative importance of the bond-shift and carbocyclic isomerization mechanisms and their intercombination, as the size of the metal particles on a supported catalyst is varied. Corolleur er al. (92) give conclusive evidence that the carbocyclic mechanism predominates in isomerization of 2-methyl- and 3-methylpentanes on their 0.2-wt % Pt/A1203 catalyst (crystallites < 50 A) at 265O-275”C. Thus, the positions of the labeled carbon atoms in most of the isomeric initial reaction products were consistent with a simple dehydrocyclization to a methylcyclopentane intermediate followed by the opening of its ring. However, 10-20% of the isomers produced required in addition the intervention of a methyl or ethyl shift, or a more extended “mixed” sequence of rearrangement steps. The latter finding, they believed, was indicative of reaction on a small proportion of larger crystallites as will be discussed presently. To illustrate this method of approach, results of three of the experiments by Corolleur er al. are shown in Tables XI11 and XIV. In the examples selected here, a I3C-labeled 3-methylpentane and a labeled 2-methylpentane are reacted, in turn, and distribution of 13C is shown for the methylpentane products in Table XI11 and for the n-hexane products in Table XIV. Distributions expected for a pure cyclic mechanism (C) and for a methyl shift (T) are indicated. Detailed discussion by the authors of “abnormal” products (i.e., those not predicted by the single-stage purely carbocyclic mechanism) led to the conclusion that these are formed on a second, less numerous, type of surface site by the action of which a succession of several rearrangements, according to a cyclic or a bond-shift mechanism, takes place. In Table XIV it can be seen that assumption of a simple skeletal rearrangement of the type

N--

HYDROCARBON REACTIONS ON METAL CATALYSTS

167

following cyclic isomerization is adequate to explain n-hexane distributions; the sums of columns 4 and 5 , noted in column 7, are in good agreement with an observed n-hexane distribution. Formally, the authors state that on the principal sites the bond-shift probability (t) is approximately zero, whereas the cyclization probability (c) is much less than the probability of desorption (d). A single cyclic isomerization transformation is the result, For the minority sites, d becomes = c, and a follow-up study, described below, has confirmed the association of these sites with larger crystallites of metal in the catalyst. Results of similar experiments by Gault and his co-workers (93) with a 10-wt % F’t/Alz03 catalyst (mean crystallite size 150-2OOA) required the assumption that several successive rearrangements took place in the adsorbed phase before desorption. A model was developed in which either a dehydrocyclization-hydrogenolysis event or a methyl or ethyl shift involving a tertiary atom competed with desorption. By assuming that the isomeric hexanes had the same desorption probability (d) and the different bond-shift processes proceeded with the same chance (t), it was found possible to reproduce the observed initial product distributions with these two independent parameters. In general, values of d = 0.5 and t = 0.10-0.20 fitted the results best. As an additional refmement, the ratio of the Cz-C3 and C3-C4 bond scission probabilities for methylcyclopentane (0)was taken to be 3.3, rather than the statistical value of 2, to improve further the fit. An attempt was made by these authors to fit all the experimental initial product distributions for the isomerizations of 2-methylpentane-2-’3C, 3-methylpentane-3-13C, and the hydrogenolysis of methyl (’3C) cyclopentane; these reactions were performed under identical experimental conditions with 10% Pt/ Alz 0 3 . Simplifying assumptions were made that excluded all possible bondshift isomerizations other than two types of methyl shift and one ethyl shift. Product distributions for the three reactions were calculated as a function of d and t using three different values of /3 (2, 3.3, and 5). For each value of /3 the calculated distributions giving best fit with the experimental data are reproduced in Table XV. Agreement in each case between calculated and observed distributions is extremely good. On the basis of the

ratio obtained in the isomerization of

An

TABLE XI11 Reactions on 4 0.2% PtIA1203 Cutulyst: '3CDistribution for Methylpent~nes~

I

L

Q\

00

Reacting hydrocarbonb

^r

Expt. No. Calcd. Mech. C Calcd. Mech. T

-

Obsd.

1 2

Calcd. Mech. C Calcd. Mech. T Obsd.

Isomerization conversion (%)

9.8 10.3

6

'Data from Corolleur et 4Z. (92). bMech. C = cyclic mechanism;Mech. T = methyl shift.

4.4 3.8

X

Y

z+r

0 0

100 0 77.6 f 1.6 68.7 f 2.0 50 50 50.0 f 0.15

0 100 17.6 f 0.5 27.1 f 0.1

f f

1.0 1.4

50 0 10.2

42.7

f

0.1

0 50 7.4 f 0.15

TABLE XN Reactions on a 0.2% PtIA1203 Catalyst: I3C Distribution for n-Hexanes'

w c

Isomerization conversion Expt.No. (%)

Reacting hydrocarbonb

X

Y

z

100 a 89.4 f 1.8 93.2 f 1.6 0 50 7.2 f 2.4

0 100-a -1.1 f 3.4 -1.0 f 3.4 50 50 49.1 f 1.9

x +Y

m W

0 0

Calcd. Mech. C Calcd. Mech. T Obsd. Calcd. Mech. C Calcd. Mech. T Obsd.

1 2

9.8 10.3

11.7 2 2.6 7.9 f 2.4 50

6

10.2

43.8

0

'Data from Corolleur et al. (92). bMech. C = cyclic mechanism;Mech. T = methyl shift.

f

2.9

100 a 100.1 101.1 50 50 51

2

s

d

9 m

N

o! 3 m

t

-4

:

\o

v!

9

t

z

c4-

0

v!

170

v,

-a

3

W

m

W

9

v,

s

171

?

m

2

0

2 m

2 m

p'

m

172

J. K. A. CLARKE AND J. J. ROONEY

they were enabled to set for the intermediate value of 3.3. This value also corresponds to that obtained in “pure” hydrogenolysis without rearrangement of methylcyclopentane. It appears then that the fourteen observed data were fittedbyfl=3.3,t=0.17+-O.Ol,andd=0.52. Some consequences of their analysis are pointed out by Corolleur et al. (93). A desorption probability of 0.5 means that four steps per surface sojourn are necessary to account for 90% of the reaction products and seven to account for 99%. The condition for a similarity of the hexane distributions in hydrogenolysis of methylcyclopentane and in isomerization was developed. The requirement is that quantity [t/(l - t ) ] [d/(l - d)] be as small as possible, that is, either t or d must be small. On the dispersed sites of 0.2% Pt/AlzO3, t is very small so that, they argue, identity of both distributions was observed with this catalyst (61). However, on their 10% Pt/A1203, t is significant so that concurrence depends on the value of d. At higher temperatures (300°C) the distributions are identical (55), indicating probably a rather low value ford, but results at lower temperatures (down to 230°C) (93)become quite dissimilar, d not being small. They point out that, in general, d will depend on temperature,pH,, and pdbne, consistent with observed dependences of product distribution. In a further study, Muller and Gault (94) reported that isomerization of 2,3dimethylbutane on thick platinum films yielded, as well as the expected bondshift initial products (2-methylpentane and 2,2dimethylbutane), substantial amounts of 3-methylpentane, n-hexane, and methylcyclopentane even at 273°C. Clearly, this is another example of a multistep mechanism. On the same basis, isomerization of 2,2dimethylbutane should give only 3-methylpentane, 2,3dimethylbutane, and 2-methylpentane as initial products; in fact, Muller et al. report that n-hexane, methylcyclopentane, and benzene represented 15% of their initial products at 275°C. Somewhat in contrast to the situation for Pt/A1203, the number of surface reactions before desorption appeared to be no greater than two or three. It turns out that in the formation of 3-methylpentane the distribution was best explained by the succession of a bond shift and cyclic mechanism. This is quite distinct from the formation of n-hexane where two consecutive bond shifts occur. Perhaps in consequence of this difference, they conclude, a marked variation with temperature of the product distributions is observed. In view of the proven multistep catalytic process, transport of the reacting species from one type of site to another before desorption as an alkane, cycloalkane, or benzene seems necessary. Since desorbed olefin plays a significant role in exchange of cycloalkanes with deuterium on Pd films even at ambient temperatures, olefins and even dienes could be responsible in the transport steps. It is useful to recall that this is, in fact, the basis of the classic theory of dualfunctional catalysis.

-

HYDROCARBON REACTIONS ON METAL CATALYSTS

173

C. STUDIESWITH ALLOYCATALYSTS Particularly since the time of the experimentation of Sinfelt and his group .(9.5, 96) on ethane hydrogenolysis with nickel and rhodium catalysts of different metal dispersion, the belief has become strong that alkane hydrogenolysis is a “demanding” process. Multiply bonded intermediates have been proposed for this reaction from time to time [see in particular Maire et al. (62)]. When these participate-in the form of radicals diadsorbed on adjacent metal atoms, as in the “selective” scission of a disecondary CH2-CH2 link discussed in the papers of Gault and his group-they are likely to make stringent demands, geometric (distances separating contiguous sites in an ensemble) and possibly electronic (distinctions between crystal faces, perhaps also within a face), of surface sites if the radicals are to be effective intermediates. Arguments have been adduced to show that three (possibly four) distinguishable reaction intermediates, namely, 1,3diadsorbed (as for neopentane), 1,2diadsorbed (as for ethane), n-alkenelallyl (suggested for cyclopentane), are required to account for hydrogenolysis findings [Anderson and Shimoyama (86)] ; a further possibility following the work of Muller and Gault is the reversal of metal-carbene/metal-alkyl insertion (cycloalkanes, alkanes larger than ethane). Three groups of workers have reported that small additions of copper as an alloying component to nickel leads to a sharp fall in activity for alkane hydrogenolysis, in examples that involve each of these types of intermediates, namely, for propane ( 9 9 , ethane (98, 99), and cyclopentane (100). Because the hydrogenolysis studies with propane and with ethane have been, at least in significant part,6 carried out at temperatures above 320°C the main finding may be taken to refer to the single-phased alloy state. The balance of opinion inclines toward the interpretation, advanced most forcibly by Ponec and Sachtler (1OZ), that at least two contiguous nickel atoms are necessary in this reaction, with 6The cyclopentane hydrogenolysis experiments of Ponec and Sachtler (100)were carried out below 300°C. It would be of interest to have measurements of this reaction in the region above 320°C because, as discussed earlier, low coordination sites-perhaps single metal atoms-may be effective in n-alkene-type hydrogenolysis and, incidentally, in the carbene route. If the latter is the case, a less severe influence of copper addition might be found in the single-phased Ni-Cu temperature range. Some indication that this may be the case appears from an examination of cyclohexane hydrogenolysis on 0s--cU and Ru-Cu (101). Two further points are worth noting about cyclopentane (and cyclohexane) hydrogenolysis. First, Gault’s work has suggested strongly that at least rhree mechanisms of hydrogenolysis are operative, their proportion depending on metal dispersion. Infuence of surface-structure sensitivity effects per se in alloy studies on this reaction cannot therefore be ignored. Secondly, there is a strong tendency for methane to become the dominant reaction product from these cycloalkanes in the alloy studies. Initial ring scission (slow) is followed, then, by (faster) successive degradation involving in the final stages a 1,2diadsorbed intermediate. The concern in the present discussion is in the slow step.

174

J. K. A. CLARKE AND J. J. ROONEY

most probably a restriction on their geometry. In principle, alloy experiments of suitable design offer a way of testing relative site-number requirements of parallel reaction routes (103). Experiments on the conversion of n-hexane on Ni-Cu alloys reported by Ponec and Sachtler (102) are of special interest. Hydrogenolysis may be expected in this case to be composite of a 1,3-diadsorbed intermediate, a a-alkene process,, and a metal-carbene route. Isomerization was found to proceed in preference to hydrogenolysis with large copper “additions,” consistent with the idea put forward that a lesser number of contiguous site atoms are needed for isomerization than for hydrogenolysis. We note that even the population of pair sites is reduced to a negligible amount at less than about 7% Ni in a square array of nickel and copper atoms. Specifically, Ponec and Schtler (102) obtained the following results (Fig. 18).

1. Between 0 and 23% C u , A z (=log r,, where r, is the specific rate of the overall reaction per square centimeter metal) decreased sharply and t h i s was accompanied by an increase in the selectivity (S) for production of c6 hydrocarbons to values comparable with those found by Anderson et al. (84) for their ultrathin nickel films. 2. For 40-73% C u , although the overall activation energy changed only marginally, a much more pronounced change in S than in the first result was ob-

act

40 €02x-

-0-

-m(r-=

\t-+\

b S

40

20

I

*I--‘, \ A1 11- 14-

I

I

1 - 1

+

A i

I

20

40

60

80

100%

cu

FIG. 18. Reaction parameters for n-hexane conversion by Ni and NiCu alloys at 330°C; A1 = log rw (rate per gram of catalyst); A2 = log rs (rate per square centimeter of total surface); Eact is activation energy of the overall process; S is the selectivity for producing c6 products; M is a fission parameter whose value inversely reflects the degree of multiple fragmentation to methane (102).

175

HYDROCARBON REACTIONS ON METAL CATALYSTS

served, reaching values common for platinum. The A parameters changed only slightly in this region. 3. The selectivity showed only a small temperature dependence. Theoretical calculations by Burton et al. ( 1 0 2 ~show ) in fact that even small incorporations of Cu into Ni can furnish a surface largely composed of Cu and, correspondingly, having a high proportion of isolated Ni atoms. In view of the foregoing discussion about site requirement for bond-shift reaction, this situation would be expected to favor bond-shift rearrangement relative to either the carbocyclic mechanism or hydrogenolysis. van Schaik et al. (104) have recently reported results for reactions of n-pentane and of n-hexane on supported Pt and Pt-Au alloys within the single-phaseregion of composition (alloys of 1 to 12.5% Pt). Extremely diluted Pt in Au (1 and 2.5% Pt) catalyzed isomerization almost exclusively in the case of n-pentane (Table XVI), which clearly must occur by the bond-shift route; further, the 2.5% Pt alloy yielded only benzene and cyclohexane from methylcyclopentane at 359°C. Hydrogenolysis was not found at temperatures below 400°C with n-pentane on the 1% Pt alloy. The first finding appears to give additional support to the one-metal site bond-shift isomerization mechanism discussed in Section IV. In contrast to absence of dehydrocyclization of n-pentane on the 1% Pt alloy, van Schaik et al. found production of methylcyclopentane and benzene TABLE XVI Product Distribution in n-Pentane Reactions on Pt and Pt-Au Alloys" Pt content

(%I

T("c)

Conv. (%)

1

318 343 371 398 350 370 400 425 320 348 389 417 298 312 326 346

0.06 0.11 0.3 0.6 0.35 0.8 2.6 5.8 0.11 0.9 2.3 7.0 3.9 7.9 12.7 11.3

2.5

12.5

100

~1

CZ

~3

-

-

-

-

1 5 4 10 21 5 5 6 6

"Data from van Schaik e f al. (104). b c ~ =s cyclopentane.

-

1 2 7 8 19 27 15 17 20 20

n ~ 4 i ~ s

CCS-~

-

100 100 100 100 100 2 2 9 6 2 2 95 94 2 1 26 6 3 7 5 22 17 14 8 3 17 13 54 12 2 52 15 3 46 18 3 18 4 43

-

-

1 51 53 30 18 11 6 5

7

Sis

s,,

1.0 1.0 1.0 1.0 1.0 0.98 0.97 0.96 0.30 0.26 0.23 0.05 0.68 0.67 0.62 0.59

0 0 0 0 0 0 0 0.01 0.59 0.61 0.41 0.29 0.12 0.08 0.07 0.09

176

J. K. A. CLARKE AND J. J. ROONEY

from n-hexane at 388°-4300C on this catalyst; these then represented 14-22% of total products. On both a 2.5 and 3.7% Pt alloy, however, little or no cyclic products were observed up to about 400°C. Finally, on 8 and 12.5% Pt alloys, the two cyclic hydrocarbons grew in amount to nearly 50% of total products. Product patterns (hydrogenolysis, isomerized alkane) for n-hexane were otherwise broadly comparable to those found for n-pentane and the study provides another alloy system in which selectivity for nondestructive reactions increases with (at least a large) content of Group IB metal. Some further findings on hydrogenolysis/isomerization ratios have been made by Sinfelt and his group. It was briefly reported (105) that, rather generally, Group IB metals inhibit markedly the hydrogenolysis activities of Group VIII metals. The Esso group investigated the use of supported alloys of such high dispersion that otherwise unavailable ranges of surface composition [see Hofmann (106)] were made possible. Ru-Cu and 0s-Cu alloys, examined in this way, showed depression of hydrogenolysis of cyclohexane at 316°C compared to isomerization (101), the hydrogenolysis rate falling by nearly 3 orders of magnitude over the range of Group VIIII metal content for ethane reaction but, interestingly, only about 1 order of magnitude for cyclohexane hydrogenolysis. Dehydrogenation rates of cyclohexane varied insignificantly over the same composition range. Atomic ratios Cu/Ru or Cu/Os from 0 to 1.O at the surface were determined by H2 and CO chemisorption. Karpinski and Clarke found that small incorporations of tin in platinum films led to a decrease of hydrogenolysis selectivity of about one order of magnitude ( 1 0 6 ~ ) . Incorporation of copper in rhodium (106b) and of gold in palladium ( 1 0 6 ~gave ) a similar behavior.

VI. Influence of Carbonaceous Deposits Brief reference has been made to the influence of surface carbon on dehydrocyclization (88),but the general implications of carbonaceous residues for hydrocarbon reactions at elevated temperatures have to date been somewhat neglected. Carbon deposits are always present to a greater or lesser degree either as interstitial atoms or graphitic-like material. Thus, self-poisoning is frequently noted and particularly suppression of severe fragmentation, especially to methane in the very early stage of reaction (64,104,107). Moreover, interstitial compounds, such as borides and carbides, tend to have much lower hydrogenolysis activity relative to the parent metals, but retain activity for hydrogenation, bond-shift isomerization, and ring enlargement (108-110). Both the 1,2and 1,3-adsorptions which lead to C-C bond fission must involve (YOI attachments (carbenes) at some stage in the fragmentation process. Thus, the interstitial surface compounds seem to have reduced bonding capacity for

HYDROCARBON REACTIONS ON METAL CATALYSTS

177

carbene intermediates relative to the parent metals. Consequently, the carbene/ alkyl insertion mechanism for dehydrocyclization and its reverse for hydrogenolysis on a carbided surface may be less important than alkene/alkyl insertion, which derives from the easy alkyl reversal type of surface dehydrogenation. Considerations of relative strain indicate that 1,5- is more likely than 1,6cyclization by the carbene/alkyl mechanism, but inspection of models gives no grounds for such a preference via the alkene/alkyl insertion route. However, Beckwith and Moad (111) recently found that homogeneous cyclization of n-hex-1-en-6-yl radicals to methylenecyclopentyl radicals is the major reaction at ambient temperatures, although extrapolation of their data shows that, at -600"K, 1,6cyclization should be preferred because of the activation entropy term. There are several clues that alkene/alkyl insertion is the major mechanism of ring closure on heavily carbided surfaces, and, under such conditions, chain length permitting, 1,6cyclization is preferred. The studies of Pad and TBtdnyi (73, 74,112-115a) on reactions of hydrocarbons on Pt,both in hydrogen and helium carrier gases, and on deliberately deactivated catalysts are significant in this respect. When the surface coverage by hydrogen is low and carbiding undoubtedly extensive, n-hexane is converted mainly to benzene, but, under conditions of less carbiding, 1,5cyclization and isomerization are the major reactions. An increase in temperature, other conditions being the same, also increases the ratio of benzene to other c6 products. This is also a feature of the results of van Schaik et al. (104) for n-hexane conversion on a 12.5%Pt/Au alloy. Thus there may be a genuine shift from 1 3 - to 1,6-Mg closure with increasing temperature due to a change to the alkene/alkyl insertion mechanism on increasingly carbided surfaces. Higher yields of benzenes are not, therefore, simply a consequence of the relative thermodynamic stability of the aromatic ring under more severe conditions. Independent evidence that this general conclusion is correct may lie in recent work of the British Petroleum group (11%) who have used small additions of sodium ions to form a (1 11) overlayer on the platinum component in a reforming catalyst. This treatment resulted in up to a fourfold increase in the ratio of benzene to methylcyclopentane from n-hexane at 500°C at the same levels of conversion. Apparently, (1 11) faces have low activity for hydrogenolysis (see Section IV.A) indicating a reluctance to form adsorbed carbenes. The logical inference then is that the alkene/alkyl mechanism and, as a result, 1,6-ring closure become preferred. The results of Plunkett and Clarke (107) for n-hexane conversion on Ir and Ir/Au alloys provide good evidence for the above conclusion. Benzene is the only c6 product even at as low a temperature as 325"C, but at the same time cracking is much more pronounced and severe than on t'F or R/Au alloys, with methane the major fragment, indicating that the Ir and Ir/Au catalysts are extensively carbided. One of the most interesting findings of PaAl and TBtBnyi is that on fresh Pt

178

J. K. A. CLARKE AND J. J. ROONEY

catalysts or in hydrogen, the olefmic products are mainly those with the most substituted double bonds, whereas terminal olefms become dominant with catalyst deactivation (1152). They conclude that the primary act of adsorption is preferentially at tertiary centers on Pt with little carbiding and at terminal carbon atoms on deactivated surfaces. Thus, the initial attack on carbided surfaces seems to be more carbanionic in character, in agreement with the view that the carbene carbon atom is a distinctly positive center (116) and buildup of negative charge on metal a characteristic of carbides (11 7). The position of the initially formed double bond may, therefore, be decisive in controlling the relative importance of 1,5- and lY6-ringclosure in a 6-carbon chain by the alkene/alkyl insertion mechanism. These considerations neatly explain certain puzzling results and help to resolve the controversy (Section 1V.B) about direct versus indirect modes of formation of benzenes. Thus, van Schaik et al. (104) found that n-pentane did not cyclize,whereas n-hexane gave significant quantities of methylcyclopentane on a 1%Pt/Au alloy. There is obviously very little if any carbiding on such a dilute alloy, so that preferential internal olefin formation is inferred. Clearly n-pent-2-ene would not cyclize but n-hex-2-ene gives methylcyclopentane by alkene/alkyl insertion:

The observation by Gault (66b) that 3-methylhexane gives 1,2-dirnethylcyclopentane as the main initial product at 300°C on Pt is dso interesting since two other products, 1,3dimethyl and ethylcyclopentane, are possible through 1, k i n g closure by either carbene/ or alkene/alkyl insertion. However, this result is also explained on the assumption that the most substituted olefin is again preferred on surfaces with little carbiding. There are two possible tertiary olefins and only one can undergo 1,5cyclization giving the product mentioned.

On the other hand, the reaction of 2,Sdimethylhexane on Pt was carried out at a much higher temperature (671, i.e., on a carbided surface, and as we have seen (Section IV.B) the terminal olefin can only yield para-xylene.

HYDROCARBON REACTIONS ON METAL CATALYSTS

179

Alternatively, the possibility that an alkyl/carbene mechanism of ring closure is more important than the alkene/alkyl insertion mechanism under all conditions on platinum cannot be ruled out, provided that carbene formation at internal positions relative to terminal is similarly preferred here. Such a preference is reasonable if the carbene is electron-donating toward the metal. With such a constraint on position of attachment, this mechanism goes quite far toward explaining the above three results. For example, an internal carbene in the case of n-pentane cannot give rise to C5-ring formation whereas the analogous internal carbene arising from n-hexane clearly can. Finally, the characteristics of exchange of methane with deuterium on several metals (Table XVII) provide some additional evidence for the validity of these ideas about the role of surface carbide. The virtual lack of multiple exchange on Pd indicates that this metal is too inert to give the aa-bonded species required for ring closure via carbene/alkyl insertion. However, not only does multiple exchange proceed readily on Pt at temperatures used by Anderson and Gault and their co-workers in most of their detailed studies, but the a&-bonded species are readily hydrogenated off the surface, as indicated by the low d 4 / d 2isomer ratio. Thus, Pt should be relatively free of interstitial carbon atoms at lower temperatures. The other metals mentioned in Table XVII apparently carbide readily; the low temperatures and high d 4 / d z isomer ratios imply a pronounced tendency for the surface reactions, CH,(,d,) + CH(,d,) + C(ads). Such metals (Ir, Re, W, Ta) seem to give exclusive 1,6cyclization with 6-carbon chains even at 300°C or lower (107, 107a). However, the major limitation of the methane exchange data in attempting such correlations is that they give no indication of the influence of the other type of carbonaceous residues, namely, condensed aromatic or graphitic-type deposits.

TABLE XVII Initial Distributions of Products for the Exchange of Methane on Metal Filmsa Metal (Ref.)

!}I0 Pd W Ru (est.) I18

T("C)

dl

d2

d3

d4

d4bz

162 237

21 12 36 95 76 37

4.5 3 12 0.6 1 8

29 24 25 2 9 15

45 61 27 2 14 41

10 20 2.3 3.0 14 5

259 254 151 157

'Ratio PD, /PCH, in reaction mixture was 1.0 except with nickel (0.75) and ruthenium (2.0).

J. K. A. CLARKE AND J. J. ROONEY

180

VI I. Conclusions Although main reaction pathways are now fairly well established, the position regarding detailed mechanisms and site requirements is still far from being satisfactory. There is no doubt that certain metal atoms in highly dispersed Pt catalysts have bonding capacity that is well suited to the carbocyclic mechanism of isomerization, but conclusions about bond-shift isomerization may have to be reexamined. Not only are the mechanisms of individual reactions, cyclization, isomerization, hydrogenolysis, etc., difficult to elucidate, but there is the further possible complication emerging from C-labeling experiments of adsorbed species undergoing more than one type of reaction, perhaps several times, with possible intersite transfer via transient dehydrogenated entities before desorption of detectable products. A further source of difficulty is carbiding, the degree of which varies with conditions and from metal to metal including alloys. In fact the distinctions in behavior of different metals, even for the Horiuti-Polanyi mechanism, still remain to be adequately explained. Perhaps at this stage a tendency to have too firm a view about site requirements for different mechanisms and a rather uncritical usage of the terms “demanding” and “facile” should be avoided. REFERENCES

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60a. Brunelle, JP., Sugier, A., and Le Page, J-F., J. Catal. in press (see also ref. 53a). 60b. Karpihski, Z., and Clarke, J. K. A.,J. Chem. SOC.,Faraday Z71, 2310 (1975). 60c. Taylor, J. F., and Clarke, J. K. A., in preparation. 60d. Paquette, L. A., and Leichter, L. M., J. Amer. Chem. SOC.94,3653 (1972). 61. Barron, Y., Maire, G., Cornet, D., and Gault, F. G.,J. Catal. 2, 152 (1963). 62. Maire, G . , Plouidy, G., Prudhomme, J. C., and Gault, F. G.,J. Catal. 4,556 (1965). 63. Gault, F. G., C. R. Acad. Sci. 245,1620 (1957). 64. Shephard, F. E., and Rooney, J. J., J. Catal. 3,129 (1964). 65. Whitesides, G . M., Symp. Organometal. Chem., Leeds, 1974. 66a. Callender, W. L., Gault, F. G., and Pines, H., Proc. Znr. Congr. Catal., 5th, 1972 2,1265 (1973). 66b. Callender, W. L., Gault, F. G., and Pines, H., Proc. Int. Congr. Catal., A h , 1972 2,1275 (1973). 67. Dautzenberg, F. M., and Platteeuw, J. C.,J. Catal. 19,41 (1970). 68. Davis, B. H., and Venuto, P. B., J. Catal. 15,363 (1969). 69. Davis, B. H.,J. Caral. 23, 340 (1971). 70. Fogelberg, L. G., Gore, R., and Ranby, B., Acta Chem. Scand. 21,2041,2050 (1967). 71. Kazanskii, B. A., Fadeev, V. S., and Gostunskaya, I. V., Izv. Akad. Nauk SSSR, Ser. Khim. p. 677 (1971). 72. Lester, G. R . , J C a d . 13,187 (1969). 73. Pail, Z.,and Titihyi, P., Kem. Kozlem. 31,129 (1972). 74. Pail, Z.,and TBtBnyi, P., J. Catal. 30, 350 (1973). 75. Lewis, K. E., and Steiner, H.,J. Chem. SOC.,London p. 3080 (1964). 75a. Guczi, L., and TBtBnyi, P., Ann. N. Y. Acad. Sci. 213, 173 (1973). 76. Muller, J. M., and Gault, F. G., Bull SOC.Chim. Fr. No. 2,416 (1970). 77. Maire,G., Corolleur, C., Juttard, D., and Gault, F. G., J. Catal. 21,250 (1971). 78. Tomanova, D., Corolleur, C., and Gault, F. G., C. R. Acad. Sci., Ser. C 269, 1605 (1969). 79. Dautzenberg, F.M., and Platteeuw, J. C., J. Catal. 24,364 (1972). 80. Corolleur, C., Gault, F. G., Juttard, D., Maire, G., and Muller, J. M., J. Catal. 27,466 (1 972). 81. 0 Cinneide, A. D., and Clarke, J. K. A., Catal. Rev. 7,213 (1973). 82. Gault, F. G . ,Proc. Znt. Congr. Catal., 5th, 1972 1,753 (1973). 83. Anderson, J. R., Advan. Catal. 23, 1 (1973). 84. Anderson, J. R., Macdonald, R. J., and Shimoyama, Y.,J. Catal. 20, 147 (1971). 85. Gault, F. G., Proc. Znt. Congr. Catal., A h , 1972 1,707 (1973). 86. Anderson, J. R., and Shimoyama, Y., Proc. Znt. Congr. Catal.,5th, 1972 1,695 (1973). 87. Maire, G., and Gault, F. G., Bull. SOC.Chim. Fr. p. 894 (1967). 88. Joyner, R.W., Lang, B., and Somorjai, G. A., J. Catal. 27,405 (1972). 89. Somorjai, G. A., Catal. Rev. 7,87 (1973). 90. Lang, B., Joyner, R. W., and Somorjai, G. A., Surface Sci. 30,454 (1972). 91. Somorjai, G. A., J. Catal. 27,453 (1972). 92. Corolleur, C., Corolleur, S., and Gault, F. G., J. Catal. 24, 385 (1972). 93. Corolleur, C., Tomanova, D., and Gault, F. G., J. Catal. 24,401 (1972). 94. Muller, J. M., and Gault, F. G., Proc. Int. Congr. Catal., Sth, 1972 1,743 (1973). 95. Sinfelt, J. H., J. Amer. Chem. SOC.86, 2996 (1964). 96. Yates, D. J. C., and Sinfelt, J. H.,J. Catal. 8, 348 (1967). 97. Beelen, J. M., Ponec, V., and Sachtler, W. M. H., J. Catal. 28,376 (1973). 98. Sinfelt, J. H., Carter, J. L., and Yates, D. J. C.,J. Catal. 24, 283 (1972). 99. Plunkett, T. J., and Clarke, J. K. A., J. Chem. SOC.,Faraday 168,600 (1972). 100. Ponec, V., and Sachtler, W. M. H.,J. Catal. 24,250 (1972).

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101, Sinfelt, J. H., J. Catal. 29,308 (1973). 102. Ponec, V., and Sachtler, W. M. H.,Proc. Int. Congr. Catal.,5th, 1972 1,645 (1973). 102U. Burton, J. J., Hyman, E., and Fedak, D. G., J. Catal. 37, 106, 114 (1975). 103. Clarke, J. K. A.,Chem. Rev. 75,291 (1975). 104. van Schaik, J. R. H.,Dessing, R. P., and Ponec, V.,J. Catal. 38,273 (1975). 105. Sinfelt, J. H.,Proc. Int. Contr. Catal., 5th, I972 1,653 (1973). 106. Hofmann, D. W., J. Catal. 27,374 (1972). 106u. Karpihski, Z., and Clarke, J. K. A., J. Chem. SOC.,Faraday Z 71, 893 (1975). 106b. Peter, A., and Clarke, 3. K. A., J. Chem. SOC.,Faraday Z in press. I06c. Visser, C., Zuidwijk, J. G. P., and Ponec, V., J. Cutal. 37,407 (1974). 107. Plunkett, T. J., and Clarke, J. K. A., J. Catal. 35,330 (1974). 107a. Clarke, J. K. A., and Taylor, J. F., J. Chem. SOC.,Faraday 171, 2063 (1975). 108. Levy, R. B., and Boudart, M.,Science 181,547 (1973). 109. Free], J., and Galwey, A. K.,J. Catal. 10,277 (1968). 110. Brown, C. A., J. Org. Chem. 35, 1900 (1970). I l l . Beckwith, A. L. J., and Moad, G., J. Chem. SOC.Chem. Commun. p. 472 (1974). 112. Pad, Z., and Tbtbnyi, P., Dokl. Akad. Nauk SSSR, 201, 868 (1971);ibid., 201, 1119 (1973). 113. Pail, Z., and TBtBnyi, P., J. Catal., 29, 176 (1973). 114. Pail, Z., and TBtbnyi, P., Acta Chim. (Budapest), 72,277 (1972). 115a. Pad, Z., and Thomson, S . J., J. Catul., 30,96 (1973). 115b. Clark, J. T. K., Edmonds, T., and McCarroll, J. J. (to British Petroleum Co. Ltd.) British Patent 1,397,293 (1975). 11.5~.Pail, Z., and Tkthyi, P.,unpublished observations. 116. Farrell, L. F., Randall, E. W., and Rosenberg, E., J. Chem. Soc. Chem. Commun., 1078 (1971). I 1 7. Frad, W . A., Advan. Znorg. Rudiochem., 11,153 (1968). 118. McKee, D. W., and Norton, F. J., J. Phys. Chem., 68,481 (1964).

Specific Poisoning and Characterization of Catalytically Active Oxide Surfaces HELMUT KNOZINGER Physikalischthemisches Institut Universitat Miinchen Miinchen. West Germany I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I1 . General Scope and Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . A. Surface Sites. Adsorption Sites. Active Sites . . . . . . . . . . . . . . . . B . Selective and Specific Poisoning . . . . . . . . . . . . . . . . . . . . . . . . C. Suitable Poisons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I11 . Experimental Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Methods for Characterization of Catalyst Surfaces by Specific Poisons B . Methods of Specific Poisoning . . . . . . . . . . . . . . . . . . . . . . . . IV. Interaction of Specific Poisons with Oxide Surfaces. . . . . . . . . . . . . . A. Interaction of Water with Oxide Surfaces-Surface Models. . . . . . . . B Adsorption of Ammonia . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Adsorption of Amines (Other than Ammonia and Pyridine) . . . . . . D Adsorption of Pyridine . . . . . . . . . . . . . . . . . . . . . . . . . . . . E Adsorption of Nitrogen Dioxide . . . . . . . . . . . . . . . . . . . . . . . F. Adsorption of Ketones and Nitriles . . . . . . . . . . . . . . . . . . . . . G . Adsorption of Carbon Dioxide . . . . . . . . . . . . . . . . . . . . . . . . H . Adsorption of Acids. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I . Adsorption of Electron-Donor and Electron-Acceptor Molecules . . . . V . Specific Poisoning on Alumina Surfaces . . . . . . . . . . . . . . . . . . . . A. Dehydration of Alcohols on Alumina . . . . . . . . . . . . . . . . . . . . B Isomerization and Exchange Reactions on Alumina . . . . . . . . . . . . VI Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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184 187 187 189 189 195 195 202 203 203 217 221 222 230 232 234 243 245 249 249 254 258 260

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1 Introduction In 1925. Taylor (I. 2) introduced the concept of geometric and energetic heterogeneity of solid catalyst surfaces. Since that time the importance of heterogeneity in chemisorption and catalytic processes is undisputed and the existence of active sites on catalytically active solid surfaces is no longer a matter of controversy Our knowledge about the chemical nature of these sites. however. is still very poor and there are only few cases in which their concentration (or number per unit surface area) could be determined satisfactorily .

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Modem trends favor models in which catalytic activity is attributed to the surface coordinative unsaturation rather than to the collective properties of the solid, but this problem is still far from being resolved. If the surface coordinative unsaturation is accepted as the source of catalytic activity, the question arises whether all surface atoms-e.g., on a metal surface-are equally active or whether there is a dependence of the specific catalytic activity per site on the coordination number of the respective atom in the surface. As these coordination numbers for surface atoms may vary over wide ranges if the occurrence of different crystallographic faces, of edges of intersecting planes, steps, point defects, and dislocations are taken into consideration, one may favor a model with the surface atoms of lowest coordination number and thus highest valence unsaturation to be the sites of highest activity. A particle size effect will be the consequence. In fact, some chemisorption processes (3) and a few catalytic reactions on metals, namely isomerization and hydrogenolysis of hydrocarbons on platinum (4, 5), have been found to show a particle size effect. This kind of reactions was classified by Boudart (6) as structure-sensitive or demanding. Most reactions, however, do not show very large differences on different crystal faces and particle size effects are usually small; for example, Ertl and Koch (7)observed identical catalytic activities for the CO oxidation on different Pd singlecrystal planes and on a polycrystalline Pd wire. Many other reactions on metals also fall into the group of reactions that are insensitive to orientation of the exposed crystal planes and to particle size (6). These reactions are, therefore, classified as structure-insensitive or facile (6), and most of the surface atoms irrespective of their valence unsaturation are expected to be reactive in these cases with approximately the same activity per surface atom. The low sensitivity versus the valence unsaturation of the active surface atoms of these facile reactions may be due to internal compensation effects (8),or to surface reconstructions during the course of the reaction (Sa). The problem, however, appears still not to be well understood (9). A further important point is the question whether only single surface atoms or ensembles of a number of surface atoms are needed for a given catalytic process. Finally, the effects of interactions between neighboring atoms or between these atoms and the matrix in which they are embedded are of fundamental interest. The study of the influence on selectivities of alloying a catalytically active metal with an inert partner has proved rewarding in this connection (10-13). Analogous information on the nature of active sites and their interaction with their environments may be obtained for active ions in studies of the catalytic properties of solid solutions (14-16). In this particular case, catalytically active ions, e.g., Cr3+, are embedded in an inactive oxide matrix, such as a-AlzO3 (1 7,18). For facile reactions on metals the entire catalyst surface or all surface atoms may be presumed to be active. Multicenter sites of a proper configuration (composed of identical atoms) may thus be accessible for reactant molecules to form

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di- or triadsorbed intermediates. Multisite processes will, therefore, occur quite frequently on metals for structure-insensitive reactions. On oxides, however, the packing of equivalent sites is most probably less dense so that multisite processes on equivalent surface ions or groups must be less probable. Homolytic bond cleavage is, therefore, certainly a very scarce dissociative adsorption step on oxides, contrary to the situation on metals. Oxides, on the other hand, expose cations, oxygen ions, and hydroxyl groups [see, e.g., Boehm ( 1 9 ) ] ,often in unusual coordination numbers. Thus, again surface coordinative unsaturation may be considered as the cause for the activity or reactivity of the various surface sites (21). The surface properties of many oxides can also conveniently be described in terms of surface acidic and basic sites (20) and even reducing and oxidizing sites may occur. Acidic and basic sites may exist in many different configurations which will lead to a distribution of their strength. Probably only very few sites in a given distribution can be considered as catalytically active sites and the coordination of these active sites may be assumed to be extremely unusual. Acid-base pair sites may play a substantial role and they will be involved in heterolytic bond cleavage or can at least lead to very strong polarizations. Heterolytic bond cleavage on such pair sites has been shown to lead to important intermediates in hydrocarbon reactions on chromia by Burwell and co-workers (21) and by Kokes (9,22-24). Finally, there are some oxidespreferrentially mixed oxides such as silica-alumina-that possess an appreciable number of acidic protons. These provide the active sites for typically acidcatalyzed reactions. It may be seen from t h i s superficial summary that there is a wide variety of potentially active single or multicenter sites on oxide catalysts. There will be sites that are distinct with respect to their chemical nature, and for chemically equivalent sites there is a certain energy distribution. Naturally, a vital interest in heterogeneous catalysis is the chemical nature of surface sites in general and information regarding catalytically active sites, in particular of their energy distributions and their absolute numbers per unit surface area. Independent knowledge of the chemical nature of an active site will provide important complementary information in additon to the more easily accessible data on the behavior of the reactant itself. A reaction mechanism that may be proposed from the latter information could then be ascertained by the former, and qualitative comparisons with analogous reactions in homogeneous and enzymatic systems would be facilitated. For quantitative comparisons of activities of different heterogeneous catalysts or of the activity of a heterogeneous catalyst with those of homogeneous catalysts or enzymes, turnover numbers (i.e., numbers of molecular conversions per site per second) must be available. For their calculation, a knowledge of the absolute number of active sites is indispensable. There are various approaches for obtaining this information on the chemical

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nature and number of active sites. The most direct method, which was most successfully applied by Kokes (24), involves using the reactant itself as a probe molecule. From the surface compounds formed, one may then draw conclusions as to the chemical nature of the respective surface sites. If one is able to show that the observed surface compounds are intermediates of the catalytic reaction, and not only their precursors, one is allowed to identify these thus characterized surface sites with the active sites for the process in consideration. The number of the intermediates formed per unit surface area gives directly the active site density. Another rather direct approach involves detailed analysis of absolute rates for the estimation of the active site density from the preexponential factors. This “kinetics method” was discussed very recently by Maatman (25). Finally, there is a more indirect method, which may be called specific poisoning of a catalyst surface. A probe molecule other than the reactant is usually preadsorbed as a poison and its effect on the catalytic activity studied. From the nature of interaction of t h i s probe molecule with the surface sites one may obtain information regarding the chemical nature of these sites, and from the number of poisoning molecules necessary to bring the activity to zero one may estimate an upper limit of the active site density. Beside the obvious difficulty to discriminate between surface sites that purely absorb the probe molecule and those that are really involved in the catalytic process as active sites, there is a number of other limitations making it difficult to obtain clear-cut information on the nature of active sites. The present contribution will, therefore, preferentially describe the proposals that are indispensable for a proper application of t h i s method and the precautions that have to be kept in mind while drawing conclusions from specific poisoning experiments. An entire compilation of studies in which specific poisoning had been applied has been renounced, and only two examples will be discussed in some detail in the last section of this article, namely the dehydration of alcohols and the isomerization of olefins on alumina.

II. General Scope and Definitions A. SURFACE SITES,ADSORPTIONSITES,ACTIVESITES The terms surface site, adsorption site, and active site have already been used throughout the Introduction, and in part they may be self-explanatory. It is, however, desirable to use only well-defined terms in the following and, therefore, a brief discussion follows on how these terms are used throughout this paper. Depending on the state of hydroxylation, any oxide surface will be made up by incompletely coordinated cations, oxygen ions, and hydroxyl groups (and

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often by more complex surface groups which may be described as surface impurities). All these groups and different species of these, which may be distinct by their neighboring groups and configurations, are denoted sutjiuce sites in general. Some of these surface sites may be able to interact specifically with an adsorbate molecule (either a reactant, a poison, or even an inert molecule), and, then, the surface site is an adsorption site. A particular chemisorption process for an adsorbate under consideration may require sites of characteristic geometry or configuration (multicenter sites, see below) which may be rather scarce. The number of the respective adsorption sites will then be very low. Various examples are given in Section IVYwhere the site density for chemisorption turned out to be of the order of magnitude of 10’2-10’3 sites/cm2. This corresponds to only approximately 1-10% of a densely packed oxygen layer [e.g., (100) plane in r-AlZO,] if one oxygen ion is assumed to cover 8 A’. Among the adsorption sites for a particular reactant there may be a certain number of sites that possess the ability to activate the adsorbed molecule and to form an intermediate. This may be a short-living surface compound, sufficiently loosely bound by labile chemical bonds to readily form the reaction products on desorption. Such sites are denoted active sites. However, initial activities are often observed to be very high and to fall to lower values of fmal stable activity. Catalyst surfaces may expose active sites that are “too active,” so that fragmentation of the reactant may occur or surface reactions may lead to strongly held species which then lead to a blocking of these extremely active sites (selfpoisoning; see Section 1I.B). The corresponding active sites will, therefore, not contribute to the catalytic conversion under conditions of stable activity. Usually an active site has to fulfill particular geometric, configurational, and energetic requirements, so that the site density may be expected to be low. The site densities, as estimated by the kinetics method by Maatman (25), cover the range from 3 X lo3 siteslcm’ (t-butylbenzene cracking on a SiOz-Alz03 catalyst) to approximately 1014 siteslcm’ (isopropanol dehydrogenation on ZnO). The nearest and next-nearest neighbors of a central surface ion or group most probably play. an important role in active sites, since they determine the site configurations and, thus, the geometry and energetics of a particular site. The role of the neighboring centers may merely be to assess the proper configuration, but in other cases neighboring centers may actively interfere in a catalytic elementary process. Such active sites will be called multicenter sites. Examples are acid-base pair sites [e.g., on ZnO (231 or the complex active sites required for mechanisms similar to the hydrogen shift mechanism as proposed by Turkevich and Smith (26). There are other processes, such as the dehydration of alcohols (27, 28), that also require acidic and basic sites. In these cases, however, the two functions are most probably spacially more separated and should, therefore, be distinguished from multicenter sites.

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B. SELECTIVE AND SPECIFIC POISONING Poisoning of a catalyst leads to a decrease in activity for a given catalytic process. The phenomenon may be brought about by the reacting system itself or

by some additives, which will be denoted poisons. If no additional poison is present, poisoning (or inhibition) may be due to a competitive adsorption of reactant and reaction products or by a strong adsorption on the active sites of the reactant itself. During the latter process which is usually referred to as sevpoisoning (or coking), chemical transformations of the reactants lead to stable surface species that cannot be desorbed or displaced by the reactant. The effect of additional poisons may be reversible or irreversible, depending on the nature and strength of interaction between poison and active site. According to Kemball (29), selective poisoning refers to the case in which a poison causes a catalyst to operate selectively or to improve its selectivity. Assume, for example, a catalyst that is active for two parallel or consecutive reactions, where two different types of active sites are required for each reaction path. Then, if a poison can be found that specifically interacts with only one type of these active sites, the corresponding reaction path may be suppressed and the catalyst selectivity improved. Typical examples of selective poisoning have been described by Pines and Haag (30) and by Berlnek et al. (31)who were able to suppress the secondary isomerization of olefins during the dehydration of alcohols on alumina by selectively operating poisons. Rosynek et al. (32,33)reported on the selective operation of carbon dioxide on the D2 exchange reaction with 1-butene on 7-alumina while the parallel double-bond isomerization remained unaffected. If poisoning is carried out with the aim to improve the catalyst selectivity toward a desired product, then the only requirement is that the active sites responsible for the undesired reaction path be among the adsorption sites for the poisoning additive. If the aim of the poisoning experiment, however, is the analysis of the nature and an estimate of the number of the active sites, then a necessary proposal is that the poison specifically and solely interacts with the active sites. Although this requirement can probably never be met completely, this type of experiment will be denoted specific poisoning. Naturally, specific poisoning, if possible and applied to a complex reaction system, may lead to selective poisoning.

C. SUITABLE POISONS 1. General Requirements

As we deal with oxide surfaces, the existence of protonic (Brq5nsted) acid sites, of Lewis acidic and Lewis basic sites, and of reducing and oxidizing sites is to be

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expected.' A whole set of probe molecules of various properties is, therefore, needed for probing a given surface with respect to the various types of sites. These molecules must allow for the proper intermolecular interactions between the probe molecule and a particular adsorption site. If the corresponding interactions between probe molecule and adsorption site, which may be an active site for a certain catalytic process, is sufficiently strong as compared to the interaction between a reactant molecule and the site in consideration, then the probe molecule may be used as an effective specific poison. Such specific poisons have to fulfill additional requirements, most of which are intimately related to the fact that the poison has to be used under catalytic conditions, that is, at elevated temperatures in the presence of reactants and product molecules. Furthermore, the system should be kept as simple as possible so that the results can be interpreted unequivocally. The following criteria should be taken into consideration for the selection of suitable poisons. a. Specific Interactions. The poison should interact specifically only with the active site under consideration. It should preferentially form only one surface species that is characteristic for the nature of the active site. Usually, there are hardly any poisons that meet these requirements, rather a poison molecule may interact with adsorption sites, only part of which are active sites. In this case some qualitative information on the chemical nature of the active sites may be obtained, but only an upper limit of the number of active sites can be determined from the number of poisoning molecules per unit surface area necessary just to eliminate catalytic activity. b. Detectability of Surface Species. The adsorbed species formed on interaction of a poison with an adsorption or active site must be detectable, that is, concentration and lifetime of the adsorbed species must be sufficiently high for the sensitivity and time scale of the experimental technique applied. Surface concentrations may well be critical due to the low active site density, if a specific poison can be found. The lifetime of the adsorbed species may be low for interactions of probe molecules with protonic sites due to a high proton mobility in the adsorbent-adsorbate system. c. Molecular Size of Poison. The molecular size of the poison should be as low as possible for various reasons. First, the active sites may be located in narrow pores of the catalyst that might be inaccessible for large poison molecules. Large molecules may also close the pore mouths of narrow pores and, thus, 'As pointed out by Flockhart and Pink (34),a clear distinction should be drawn between Lewis acid-Lewis base reactions, in which a partial two-electron transfer is involved, and redox processes, in which only a single electron is transferred. The surface sites initiating these interactions may be the same for both processes or different in nature. This point is still uncertain.

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prevent reactant molecules from diffusing into these pores, or at least increase the activation energy of diffusion appreciably. Second, the active site, for example, a Lewis acid site, may be exposed in an oxygen vacancy and it may be sterically shielded by the neighboring oxide ions. Steric hindrance will then strongly influenced the intensity of the molecular interaction of the poison with the active site. As an example the adsorption of pyridine on alumina is discussed in Section IV. On the other hand, the steric hindrance in Lewis acid-Lewis base interactions on adsorption of substituted pyridines has been utilized by Benesi (35)and by Jacobs and Heylen (36) for the specific detection and poisoning of Brqjnsted sites in the presence of Lewis acid sites. Third, a large molecule may adsorb on a particular adsorption site and may shield an active site located nearby. A poisoning will be the consequence which, however, is due to the molecular size of the poison and is not caused by specific poisoning. d. Strength of Interaction. The poison should be strongly held to the active site as compared to the reactant or product molecules. This is particularly necessary if the poison is “irreversibly” preadsorbed (see Section 1II.B). Too low relative strength of interaction of the poison may lead to a displacement by reactant or products. e. Thermal Stability. The poison should be thermally stable even on the catalyst surface up to temperatures at which the catalytic reaction proceeds. f. Reactivity of the Poison. The poison should be nonreactive under catalytic conditions, that is, the poison should not undergo chemical reactions with the surface other than the acid-base or redox processes characteristic for the direct interaction with the active site. Particularly, many oxide surfaces possess oxidative or hydrolyzing properties that may lead to additional surface compounds on reaction with the poison (see, e.g., Sections IV. F and G). Furthermore, the adsorbed poison should be unreactive versus any component of the catalyzed reaction.

g. Surface Reconstruction. If the poison is used for probing the active sites of a given catalyst surface, then one should, make sure that the adsorption of the poison does not lead to any surface reconstruction. Even locally very restricted changes of the surface geometry may strongly influence the adsorption and catalytic behavior of the respective surface. h. Promotional Effects of Poisons. Flockhart and his co-workers reported a reinforcement of the adsorption of an electron-acceptor molecule in the presence of an electrondonor molecule and vice versa on an alumina surface (37) and on zeolites (38). This phenomenon was explained by the assumption of an inductive interaction between the respective donor and acceptor sites via the lattice. Similar effects may occur during the specific poisoning of catalyst surfaces due to an interaction of sites, which may possibly cause an enhancement

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of the specific activity of certain sites. Since such effects may also promote initially inactive sites, it could become difficult to count the actual number of active sites present on the surface of the unpoisoned catalyst. This may be particularly troublesome if the promotional effect does not show up, since the poisoning effect remains dominating. Too high numbers of active sites will then be obtained by the method of specific poisoning. In some cases the enhancement in activity may overcome the poisoning effect. Thus, Minachev and Isakov (39) observed a remarkable C 0 2-promotion effect on toluene disproportionation and benzene alkylation over X and Y zeolites. The C 0 2-promotion effect seems to be complex in nature but, most probably, is related to some interactions of C 0 2 with residual water resulting in the formation of new Bransted acid sites (40). Water itself is well known to increase the catalyst activity of acid-catalyzed processes on, for example, silica-alumina, where a conversion of Lewis acid sites into Brgnsted acid sites occurs on adsorption of water (41). 2 . Selection of Suitable Poisons For the characterization of the nature of surface sites, probe molecules are needed. In the following, however, we confine ourselves to those probe molecules that can principally be used as poisons under catalytic conditions. Thus, for example, the indicator molecules usually used for the titration of surface acidity and basicity will not be treated. The conditions under which these measurements are carried out differ greatly from those applied during actual catalysis, and the molecular size of the probe molecules is unfortunately usually very large? These methods have been reviewed very recently by Tanabe (20) and by Fomi (42). Suitable poisons should be selected according to the criteria compiled in the foregoing. Some of these can hardly be fulfilled and others cannot easily be proved, e.g., criteria g and h. Thus, a compromise has usually to be made to fmd the optimum poisoning compound for a given purpose. Burwell (43) has proposed the application of the principle of hard and soft acids and bases (HSAB) (44, 45) as a basis for a qualitative discussion of adsorption. Burwell et a2. (46) have applied this principle to considerations regarding adsorption on chromia. In fact, the HSAB concept seems to be a very useful tool for the prediction of suitable poisons, and its application will particularly meet criteria a and d. In general, “hard” indicates low polarizability and small size, and “soft” indicates high polarizability and large size. Hardness will increase, and softness correspondingly decrease, with increasing positive oxidation state, with decreasing degree of unsaturation, and with decreasing size. Thus, the most typical members of the group of hard acids and hard bases are the proton 2The size of the indicator molecules most probably affects also the observed acid strength distributions.

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and the hydroxide ion, respectively. The HSAB concept can be formulated as follows: hard acids prefer to associate with hard bases and soft acids prefer to bind to soft bases. This rule can be successfully utilized for the prediction of the compounds that may act as effective poisons in a given process and it becomes particularly fruitful when the strength of interaction of the poison with an active site is to be considered in relation to the strength of interaction of this site with the reactants and products. In the most general form, the adsorption equilibrium of a poisoning compound N on an adsorption site S may be written N + S e N - S

(1)

where the nature of N-S may be a coordination surface compound, a surface complex ion, or a charge transfer complex. Equation (1) can, therefore, be used as a general description of both, simple acid-base interactions (N being either the acidic adsorbate and S a basic site, or N being the basic adsorbate and S an acid site) and of redox processes (with N being either the electron-donor or -acceptor molecule and S, respectively, either the reducing or oxidizing site). The constituents of most catalytically active oxide surfaces, that is, incompletely coordinated metal ions, oxygen and hydroxide ions, and protons, have to be classified as hard. Consequently, hard acids and bases will be preferentially held by basic and acidic sites on oxide surfaces. Whether a given compound is an effective poison for a catalytic process under consideration, however, must strongly depend on the relative hardness of the poison as compared to that of reactants and products. Thus, a hard poison may be effective if reactants and products are soft. A still harder poison, however, has to be used if a reactant or product belongs to the group of hard compounds; for example, pyridine, a fairly hard base, is an effective poison for the olefin isomerization on alumina, but a partial displacement of pyridine has been observed during the dehydration of alcohols on alumina (47). The reason for these observations is simply the fact that olefins are rather soft bases, whereas alcohols and water are hard. Beside the acid-base equilibrium described by Eq. (l), competition or displacement reactions, N+X-S e N - S + X

(2)

may also play a role in some adsorption equilibria. Here N again denotes an acidic or basic poison and X-Sa surface acid-base complex; e.g. X may be a surface proton and S an oxygen ion, or X is a hydroxide ion coordinated to a metal ion S. Finally, acid-base pair sites can be blocked by compounds that can dissociate heterolytically on these sites. Table I summarizes compounds that can be expected to be poisons on oxide surfaces according to the HSAB concept. Column 1 denotes the surface site to be detected or poisoned by a compound given in column 2. Column 3 specifies the most probable adsorption processes for the various systems according to

194

HELMUT KNOZINGER TABLE I Potential Specific Poisons Selected According to the HSAB Concepta

Type of site

Type of poison

Possible type of adsorption process

Section of text

H20+~[Men']s~[HzO+Me"'], [H3N +Me"'], NH3 + [Me"'], Py+ [Me"'],*[Py~Me"'], N z 0 4 + [Me"'], [N03Me"+] + NO RCN + [Me"'] [ RCN +Me"'] R2CO+ [Me"'],= [R2CO-+Me"'ls

IV. A IV. B + C IV. D IV. E IV. F IV. F

~

Lewisacid

HzO Amines N-heterocycles NO2 Nitriles Ketones

* *

~-

Br$nsted acid

Amines N-heterocycles

Lewis base

C02 BF3

~

= [NH:], * [PyH'I,

NH3 + [H'], Py + [H'I,

* BF3 + [MeOH] ,* [MeOH: BF31s HAc + [OH-] * [Ac-I , H2O +

PhOH+ [OH-]s*[PhO-],+

HzO

~

IV.G IV. H 1V. H IV. H

H 2 0 + [Me-01, [MeOH], + [OH], [OH], NH3 + [Me-O],*[MeNH2],+ See Eqs. (21) t o (27)

Iv. A 1V.B IV. G

,* [TCNE- . .

D'],

IV. I

- A -1 ,

IV. I

~~~~

Electrondonor sites [D],

Tetracyanoethylene [TCNE] Trinitrobenzene [TNBI

TCNE + [D]

Electronacceptor sites [A]

Polynuclear aromatic hydrocarbons Pheno thiazine WThI

PhTh + [A]

,

IV.B+C IV. D

=

Hz 0 NH3 COZ

~~

,

c02 + [ 0 2 - 1 ~ ~ [ c 0 3 2 - 1 s COz + [OH-] s [ HCOiI s

Carboxylic acids Phenol Acid-base pairs

,

,=

,

[PhTh'

.

aHSAB = hard and soft acids and bases.

Eq. (1) or (2). Details of these processes are discussed in Section IV; the corresponding subsections for the respective systems are mentioned in column 4. It is seen from Table I that the number of possible specific poisons is very restricted, and, in practice, it may become even more restricted, since some of the listed compounds will not fulfill satisfactorily the criteria for specific poisons. Thus, the applicability of soft bases such as ketones or nitriles will certainly be very scarce. The electron-acceptor molecules that tend to adsorb on electron-

SPECIFIC POISONING OF OXIDE SURFACES

195

donor sites are also rather soft compounds, with, however, a high electron affinity. Despite their softness they are well known to form radical anions on many oxide surfaces, although this occurs at very low concentrations, typically of the order of magnitude of 10” to 10’’ spinslcm’. Surface anions that most probably are the electrondonor sites have to be classified as hard; however, their hardness must depend on their environments, that is, on the configuration of the particular surface site. One may, thus, expect a distribution of the hardness of these surface sites, and there may be some weakly coordinated surface anions sufficiently soft to undergo charge transfer interactions with the soft acceptor molecules. This explanation is in agreement with the conclusions of Che et al. (48) who consider weakly coordinated 02ions as the donor sites on magnesium oxide and titanium dioxide, and it would also correlate with the low site densities.

I I I. Experimental Methods A. METHODS

FOR

CHARACTERIZATION OF CATALYST SURFACES BY SPECIFIC POISONS

A catalyst surface may be assumed to be characterized by specific poisoning if the number of adsorption sites, the strength (or the strength distribution) of the adsorbate-catalyst interaction, and the nature of this interaction as well as the chemical nature of the adsorbed species can be determined. All three properties are equally important to characterize fully, i. e., qualitatively and quantitatively, a catalyst surface. The number of adsorption sites may be determined from the adsorbed amount of poison as measured by conventional techniques, whereas thermoanalytical methods have to be applied for a quantitative characterization of the adsorption bond strength. Spectroscopic methods will be most suitable for studies of the chemical nature of the adsorbed species and the nature of the adsorbate-surface interaction. 1. Determination of Adsorbed Amounts The number of molecules of a poison that are retained on a catalyst surface after equilibration at low temperature and desorption at a predetermined higher temperature into a vacuum or an inert gas stream, is usually said to be adsorbed irreversibly. Here, irreversibly describes the fact that at the desorption temperature the activation energy for desorption cannot be overcome for a certain number of molecules. This number of irreversibly adsorbed molecules is generally a function of the desorption temperature, indicating a distribution of the activation energy for desorption. As an example, Fig. 1 shows this dependence

196

HELMUT KNOZINGER

O'

100

a

300

Desorption temperature

500 (OC)

FIG. 1. Irreversibly adsorbed amounts of pyridine as a function of the desorption temperature of vAl2O3, pretreated at 500" and 6OO0C, respectively.

for the system pyridine/q-Alz 0 3 . If the nature of the chemisorption process is known, e.g., from spectroscopic information, the number of adsorption sites can be calculated from the number of irreversibly retained molecules at any desorption temperature. Adsorbed amounts can easily be measured with high accuracy by means of the conventional volumetric (49, 50) and gravimetric (50) techniques. For static measurements the desorption temperature is increased by certain increments and the desorption carried out at constant temperature. Thermogravimetric analysis (TGA) is a dynamic method by which the weight loss is detected by a balance while heating the catalyst continuously. The same type of information can be obtained as with the static methods, provided the heating rate is sufficiently low to attain the characteristic irreversibly adsorbed amount at any temperature. Gas chromatographic (GC) methods also provide possibilities to detect the irreversibly held amounts of a poison (51), although these techniques are less accurate than the gravimetric methods. The number of irreversibly adsorbed molecules can be calculated from the material balance for successively injected pulses of the poison and the eluted amounts. Alternatively, adsorption equilibrium can be attained at low temperature, the adsorbed amount being determined by frontal analysis(51). Desorption may then be carried out at the same temperature and the irreversibly held amount can be calculated either from the difference between adsorbed and desorbed amounts of a first cycle or from the difference of the adsorbed amounts of a first and a second adsorption (52). Desorption temperatures can then be raised stepwise after the first desorption and the dependence of the irreversibly adsorbed amounts on desorption temperature determined from the corresponding desorbed amounts. The accuracy of these GC measurements is limited because of the usually very pronounced tailing of the desorption trace for the systems of interest.

SPECIFIC POISONING OF OXIDE SURFACES

197

2 . Determination of Strength of Interaction between Adsorbate and Surface The techniques described in the foregoing do not easily provide information about the chemisorption bond strength. The Clausius-Clapeyron equation is not applicable in the range of irreversible adsorption. Only by measurements of the desorption rates during the thermal desorption processes at two slightly different temperatures can the activation energy of desorption be estimated. This method has been used by Kubokawa (53). Desorption rates can be calculated from the evolution curves obtained during isothermal desorption as shown, for example, by Czanderna (54). Thermoanalytical methods recommend themselves for direct measurements of enthalpy changes on adsorption and desorption. Among these, the calorimetric techniques are the most sensitive and precise ones. However, as pointed out by Gravelle (55), they have not been utilized in the past as much as would have been desirable according to their very promising potentialities. Heat-flow calorimetry-applying preferentially Calvet microcalorimeters-has been discussed very recently by Gravelle (55,56). E.: has shown that this technique permits the precise determination of the energy spectrum of an adsorbent surface with respect to a given probe molecule. Measurements can be extended over wide temperature ranges from room temperature up to approximately 5OO0C, so that practically the whole temperature range in which catalytic surface processes occur can be covered. Temperature-programmed desorption (TPD) is another powerful technique that is useful for the quantitative characterization of catalyst surfaces by probe molecules. Temperature-programmed desorption is the removal of chemisorbed species on continuously and programmed heating the sample into a vacuum or an inert carrier gas stream. During the increase of the adsorbent temperature, chemisorbed species are desorbed at different temperatures according to their binding energies. The TPD method provides information on (a) the number of chemisorption states or chemisorbed species, ( b ) the population of each state, (c) the activation energy, and ( d ) the frequency factor and, thus, the entropy term of the desorption processes of each chemisorption state. Various different techniques have been developed in the past, since Cvetanovic and Amenomiya (57, 58) first described their GC-approach. In this case, desorption occurs into an inert carrier gas and the desorbed material is detected by a GC detector of high sensitivity, such as a thermal conductivity or flame ionization detector. For the chemical analysis of the desorption products, these can be collected in traps for subsequent analysis (59). However, since changes in the product distribution during the desorption process as a function of the temperature should provide valuable information, Schubart and Knozinger (60) have applied an on-line mass spectrometer (MS) analysis system. This consists of a GC-MS coupling system by which the desorption vessel and a quadrupole mass spectrometer are coupled,

198

HELMUT KNOZINGER

so that rapid scans of the mass, spectra of the desorbed material in short time intervals during the desorption experiment are possible. Stakebake et al. (61) used a time-of-flight mass spectrometer as the detector system to follow desorption processes under vacuum. A direct partial-pressure analysis can be made in this device. Czanderna (54, 62) and Czanderna et al. (63) have described the application of the microbalance in TPD experiments. This technique permits observation of the evolution curves in the gas phase and simultaneously of the weight loss of the adsorbent-adsorbate system. The mathematical analysis of the observed desorption data is by no means simple (64). Thus, particularly in the case of heterogeneous surfaces, analysis by modeling the desorption process by means of analog computers has been shown to be extremely valuable (65, 66). Enthalpy changes on adsorption and desorption of probe molecules on catalyst surfaces may also be followed by differential thermal analysis (DTA) (67) although this method has been used only sporadically in the past. The experimental techniques have been described by Landau and Molyneux (67) very recently. As an example, Bremer and Steinberg (68) observed three endothermic peaks during the desorption of pyridine from a MgO-Si02 catalyst; these peaks were assigned as three different chemisorption states of pyridine. The assignment of desorption peaks in TPD and DTA experiments as indicating different adsorption states formed on adsorption of a probe molecule at the adsorption temperature may be ambiguous, since a transformation of lowtemperature adsorption states into other states may occur on steadily increasing the desorption temperature above the adsorption temperature. Still more detailed information on the chemisorption bond is obtained from photoelectron spectroscopy studies, because these methods permit the determination of binding energies and electron distributions in the chemisorbed species. The application of photoelectron spectroscopy as a surface chemistry technique has been confined mostly to metal surfaces (69) since charging effects may be expected on nonconducting materials such as oxides. It has been shown, however, very recently (70) that these problems can be overcome and some studies on surfaces of nonconducting materials have already been published (69, 71, 71a-c). Thus, photoelectron spectroscopy may become an experimental tool that could provide us with the most direct information on the adsorbatesurface interaction and thus on the very nature of the adsorption sites. By using W radiation, the outer molecular orbital energies, and their bonding characteristics, in the adsorbed species may be determined. Soft X-rays as the ionizing radiation [electron spectroscopy for chemical analysis (ESCA)] will eject electrons from inner shells; therefore, ESCA will primarily be characteristic for the individual atoms in a chemisorbed molecule or a surface atom or ion. Chemical shifts of ESCA peaks may additionally be a measure of the environments of the considered atom in a chemisorption complex, and they may be related to the oxidation states and coordination numbers, e.g., of surface ions

SPECIFIC POISONING OF OXIDE SURFACES

199

that are part of a chemisorption site or complex in the surface. Radial electron distribution studies (72) may be combined with such ESCA work.

3 . Characterization of Chemical Nature of Specifically Adsorbed Probe Molecules a. Infrared Spectroscopy. Among the spectroscopic methods used for the characterization of the chemical nature of chemisorbed species, infrared spectroscopy has certainly found the most frequent application due to the highly instructive information that can be gained if compared to the relative experimental simplicity of many of its variants. The various techniques by which infrared spectroscopy has been adapted to surface chemical problems have been reviewed in the past (73-78). The transmission method is a comparatively simple technique which is easily applicable in studies on most oxide surfaces. Usually, frequency shifts of internal vibrations of molecularly adsorbed species or the occurrence of new vibrations due the formation of new surface compounds are detected. This information permits a characterization of the chemical nature of the surface species by comparison of their infrared spectra with those of known compounds or by normal coordinate analysis. An indirect conclusion may then be drawn regarding the type of adsorbate-surface interaction and the nature of the adsorption site. Unfortunately, the low-frequency range (below approx. 1000 cm-') is hidden by the absorption of the adsorbent, so that the most interesting and important bands of the stretching mode of the chemisorption bond cannot be obtained by the transmission method. In fact, far infrared spectra of systems of interest for specific poisoning have still not been published. Fourier transform spectroscopy, which has now become available commercially, should be valuable at least in fortunate cases, if the adsorbent absorption is sufficiently low in the low-frequency ranges of interest. Alternatively, the diffuse reflectance method (79, 80) may help to overcome these problems, although no diffuse reflectance spectra in the low-frequency range have yet been published. This frequency range also becomes accessible by means of Raman spectroscopy, since oxides are usually very bad Raman scatterers so that the adsorbents do not contribute to the Raman bands. Therefore, the application of Raman spectroscopy in connection with signal-averaging techniques for improvement of signal-to-noise ratios should have high potentialities for the study of the lowfrequency region. Till now, however, only preliminary results have been obtained. Nevertheless, Raman bands of adsorbed molecules have already been observed by Winde (81),Hendra and co-workers (82,83), Knozinger and Jeziorow ski (84, and Egerton and co-workers (85) in frequency regions in which the adsorbents are completely opaque in the infrared transmission technique. In addition, Raman spectra of chemisorbed species may be obtained fairly easily at elevated temperatures. Two review articles have appeared very recently (SSa, b). Infrared emission spectra (86,87)are usually obtained at elevated temperatures,

200

HELMUT KNOZINGER

but they can also be scanned near room temperature if a highly sensitive Fourier transform spectrometer is used (87). Internal reflectance spectroscopy (IRS) (88) may find application in the low-frequency range, although the in situ treatment of the adsorbents seems to be difficult, and, depending on the adsorbent particle size, still bigger difficulties may arise from a poor optical contact between IRS element and adsorbent. b. Ultraviolet-Visible Spectroscopy. Whereas infrared spectroscopy is preferentially used to study the nature of adsorbed species produced on adsorption of fairly small molecules, electronic spectra provide us with valuable additional information on the state of more complex adsorbed molecules. Electronic spectra of adsorbed species are particularly informative if they are complemented by other spectral data such as infrared or electron spin resonance (ESR) spectra. The wavelength of the radiation in the ultraviolet and visible region is usually small as compared to the particle size of adsorbents or catalysts. Due to the correspondingly high scattering losses, the transmission technique can be applied only very sparingly. Therefore, diffuse reflectance is the most powerful technique for the detection of electronic spectra of adsorbed species. Band positions can be obtained directly from the diffuse reflectance spectra, whereas the Kubelka-Munk function has t o be applied for quantitative measurements. Theory and experimental details have been described by Wendlandt and Hecht (89),Kortiim (90),and Klier (91) and reviews on the subject have been published by Terenin (92) and by Leftin and Hobson (93). The potentialities of high-temperature reflectance spectroscopy (HTRS) and of dynamic reflectance spectroscopy (DRS), as described by Wendlandt (94), should be emphasized in connection with surface chemical problems. In the HTRS technique, reflectance spectra are recorded at constant elevated sample temperature, whereas in the DRS technique, the temperature is uniformly changed (usually increased) and the reflectance recorded at a fixed wave number. The results of DRS experiments if carried out under vacuum or with an inert carrier gas may, therefore, valuably,complement thermogravimetric or TPD experiments. The following observations may be obtained from electronic spectra of adsorbed species, depending on the nature of the adsorbate-surface interaction: spectral shifts, changes in absorption coefficients, alterations in vibrational frequencies, and the occurrence of new bands. The last phenomenon is the most important one regarding the characterization of active sites and specific poisoning, since the appearance of new bands indicates the formation of a new surface species due to specific interactions. The assignment of these bands is usually made by comparison with the spectra of known chemical compounds. A characterization of adsorbent surface properties is then possible b y taking into consideration the nature of the newly formed surface species. As the spectral region below about 200 nm is usually not accessible (commonly used quartz or fused silica optics cut off there), electronic spectra of adsorbed species on catalyst

SPECIFIC POISONING OF OXIDE SURFACES

20 1

surfaces have so far involved mostly molecules containing electrons, the electronic transitions of which are observed at wavelength larger than 200 nm. The observation of fluorescence and phosphorescence spectra of surface complexes may also be useful (95, 95a). c. Electron Spin Resonance. Electron spin resonance is limited to the detection of unpaired electrons. Thus, systems that either possess intrinsic paramagnetism or in which paramagnetism is induced, e.g., by adsorption of a suitable probe molecule, can be studied by ESR. The energy-level separation of the states between which ESR transitions take place is only of the order of magnitude of 1 cm-' . Accordingly, ESR spectra are extremely sensitive to environmental changes near the paramagnetic species. The adsorption of paramagnetic molecules such as 0;and NO as studied by ESR is, therefore, an excellent way of probing the environments of adsorption sites. With regard to specific poisoning, the ESR technique is a most powerful method for the detection of radicals and radical ions which may be generated on adsorption of some specific poisons. The presence of reducing and oxidizing sites on catalyst surfaces can, thus, be tested and the number of these sites estimated from the spin concentration. The extremely high sensitivity of the ESR technique is of particular interest in this connection, since it permits the study of very low concentrations of adsorption and active sites. The essential features of the ESR method as applied to surface studies have been discussed by Kokes (96), Adrian (97), Aston (98), and very recently by Lunsford (99). Electron spin resonance spectra permit conclusions on the nature of the adsorbed paramagnetic species, their surface binding characteristics, and the geometrical configuration in the adsorbed state. Thus, very detailed information on the nature and properties of the adsorption sites may be obtained if an unambiguous interpretation of the ESR spectra is possible. This, however, can sometimes be difficult, since analogy conclusions may be dangerous due to the high sensitivity toward environmental changes. Furthermore, most organic molecules that have so far been used as probe molecules or specific poisons are unfortunately large, so that they may interact simultaneously with various sites (possibly other than the radical-generating site) and they may sterically shield these neighboring sites. An unambiguous description of a seemingly detected catalytically active site may be questionable under such conditions. d. Nuclear Magnetic Resonance (NMR). The essential features of the various NMR techniques as applied to adsorption studies have been dealt with in the publications by Aston (98), Resing (ZOO), and Pfeifer (ZOZ), and an extensive literature review has been given recently by Derouane et al. (102). As in the present context, we are primarily interested in the chemical nature and the chemisorption bond strength of localized (high residence times) and specifically

202

HELMUT KNOZINCER

adsorbed poisons, NMR relaxation studies will usually not provide us with the necessary information. However, Resing (100) has shown how relaxation studies may in favorable cases yield information about the energy distribution of adsorption sites. High-resolution proton NMR has been applied to identify adsorbed species. This approach, however, seems to be restricted because the intrinsic chemical shift of adsorbed species is strongly influenced by various external factors (102). Furthermore, proton chemical shifts are of the order of magnitude of only 10 ppm, so that the resolution of neighboring resonance lines may become poor due to line broadening in the adsorbed system and lines may even vanish. The hyperfine splitting can never be resolved for adsorbed species. A recent very promising development is the application of the l 3 C Fourier transform NMR (13C-FT-NMR) (103). The "C chemical shifts are larger than proton chemical shifts (order of magnitude: 300 ppm), and the line widths are smaller due to reduced dipolar interactions. Resolution is, therefore, much better than for the proton NMR. The FT-NMR permits the rapid accumulation of spectra so that 13C high-resolution spectra can be obtained at natural abundance (1.108%) within a few minutes. Utilization of the Overhauser effect and decoupling of the indirect l 3 C-' H spin-spin interactions yields an additional gain in signal intensity (103). Although "C-FT-NMR has been applied to only a few systems (104-107)which are of little interest in the present context, the study of specific poisons adsorbed on catalytically active surfaces seems very promising and is strongly recommended. The technique should be particularly valuable for the detection of adsorbed molecules or poisons, such as C 0 2 , that do not contain protons. Carbon dioxide, however, has to date only been studied on silica surfaces (106). The various forms of double resonance may also be adapted to their applicability for surface studies (102). The four-pulse technique (1074b) may find application to surface chemical problems.

B. METHODS O F SPECIFIC POISONING After the type and strength of interaction of a potential poison with the catalyst surface has been studied and the number of adsorption sites estimated, its effect on the rate of a given catalytic reaction can be studied. Any kind of catalytic reactor may in principle be used for these studies, that is, static as well as dynamic methods are suitable, and the various forms of pulse techniques are applicable. The real distinction between the two types of poisoning experiments that have been performed lies in the fact that the poison is either fed together with the reactant and is present in the gas phase throughout the run or the surface is poisoned by irreversibly preadsorbing the poison while the gas phase is kept free of it. If the poison is present in the gas phase, a larger number of modes of interaction with different surface sites may be possible than for the

SPECIFIC POISONING OF OXIDE SURFACES

203

irreversibly held portion alone; for example, strongly adsorbed pyridine is coordinately bound to Lewis acid sites o n alumina, whereas H bonding takes place additionally at finite pyridine partial pressures. A clear distinction between active sites and adsorption sites for the poison may, therefore, become impossible. Thus, the preadsorption of a strongly held portion of the poison should always be the preferred experimental method. In any case, one has to be sure that the poison is not displaced from active sites by the reactants or products (see Section 11. C. 1 ; criterion d). Preadsorption may conveniently be carried out as follows: after thermal pretreatment, the catalyst is saturated with the poison at low temperatures, either under vacuum system or in an inert carrier gas. Misono et al. (108) used a GC column that was packed with the catalyst and determined the retained amounts by frontal analysis (see Section II.A.1). Gati and Knozinger (109), using the microcatalytic pulse technique, connected the reactor directly with the detector (the GC column was bypassed) and saturated the catalyst surface by successive pulses of the poison. The first pulses are usually completely retained and saturation is achieved as soon as the eluted amounts remain constant and correspond t o the injected pulse size. Various irreversibly absorbed amounts can be attained after saturation by stepwise desorption at increasingly higher temperatures. The number of retained molecules at any given desorption temperature can be determined directly in the catalytic reactor, according to the methods mentioned in Section III.A.1, or they may be taken from independently determined adsorption isobars. Naturally, the highest temperature at which the reaction is subsequently carried out should never exceed the highest desorption temperature.

IV. Interaction of Specific Poisons with Oxide Surfaces The adsorption of potential poisons listed in Table I according to the HSAB concept is discussed in this section. From the information accumulated regarding the modes of interaction of any one of these adsorbates with oxide surfaces, it will be concluded whether a particular molecule may be suitable as an effective specific poison or as a probe molecule for the characterization of certain surface properties.

A. INTERACTION O F WATER WITH OXIDE SURFACES-SURFACE MODELS It is well known that the presence of water on an oxide catalyst surface effectively reduces the catalytic activity versus a wide variety of reactions. On the other hand, promotional effects have also been observed, so that criterion h (Section II.C.1) is invalidated. Water can be held by incompletely coordinated

204

HELMUT KNOZINGER

surface cations according t o HzO + [Me"'],

[HzO--+ Me"'],

(3)

or it may be adsorbed dissociatively on metal-oxygen pair sites,

Additionally, there are various types of H-bonded species possible, which have recently been discussed by Knozinger (110). Thus, a wide variety of adsorbed species held by sites of distinct nature must be envisaged. Criterion a will, therefore, most probably lose its validity or the assignment of a certain adsorption site for water as an active site is at least not unambiguous in most cases. Water is, therefore, not considered t o be a suitable specific poison. However, the elements of water, hydrogen and hydroxyl group, are known to be constituents of all oxide surface layers (19), and the dehydroxylation of hydroxyl-containing surfaces is an important process during which surface sites of different nature are being created. Studies of the interaction of water with oxide surfaces have, thus, cast some light on the structure of surface layers, and surface structure models have been developed on the basis of such studies. Burwell et al.'s (21) concept of the coordinative unsaturation of surface atoms and ions is particularly valuable in the discussion of surface structures. Surface structure models have been described for alumina, titania, chromium oxide, zinc oxide, magnesium oxide, and silica-alumina.

1. Alumina Alumina exists in a wide variety of crystallographic modifications ( I l l ) , among which the transition phases Q- and yA1203 are of particular interest in adsorption and catalysis since they develop fairly high specific surface areas (typically between 100 and 200 m'/g). Both modifications are made up by a defect lattice of the spinel type, and they may be distinguished by their defect character and the distribution of the cations in tetrahedral and octahedral sites (111, 112). It is not clear how structural differences of the bulk reflect themselves in the surface properties of these two modifications. According to Leonard et al. (112), one has t o assume that the less organized the starting material is the more defective and the more energetic will the surface be. The 6 modification has also frequently been used in adsorption studies. The 6 phase may be regarded as a superstructure of three Alz03-spinel cells containing an integer number of aluminum ions (111). Alumina surfaces previously exposed t o water vapor (or moist air) at temperatures above 100°C are terminated by a monolayer of hydroxyl groups. The presence of hydroxyl groups in the surface has been shown by deuterium exchange and infrared spectroscopy (113-116)and by chemical methods (19).

*.

SPECIFIC POISONING OF OXIDE SURFACES

205

The rehydroxylation of dehydroxylated alumina surfaces on exposure to water vapor is accompanied by a strong heat evolution, which amounts t o as much as 100 kcal/mole at the lowest coverages (52, 117-119, 119u), indicating the strong chemical interaction between surface and water molecules. Dissociative chemisorption and coordinative interaction of water molecules occur simultaneously during rehydration of q-A12O3 (1194. Regarding 7-A1203,Peri and Hannan (113) observed three resolved OHstretching bands at 3698, 3737, and 3795 cm-’ after degassing the surface at 650°C; these bands are assigned as “isolated” noninteracting OH groups. With higher resolution, two additional bands at 3733 and 3780 cm-’ were observed by Peri (114). At degassing temperatures below approximately 500°C, broad unresolved bands, due to mutual interactions of hydroxyl groups are obtained in the OH-stretching region. Isolated surface OH groups are progressively removed from the surface at increasingly higher degassing temperatures but with different rates according to their nature. Even after heat treatment in vacuo at 8OO”C, about 2% of the total hydroxyl content is still retained on the surface (114). The removal of hydroxyl groups from the surface was postulated to generate “strained oxide linkages” (117),a reaction that may be depicted as

The condensation of two neighboring hydroxyl groups leads to the formation of a water molecule that is then expelled from the surface. It is now agreed that this process leaves an oxide ion in the outermost surface layer and an exposed incompletely coordinated aluminum ion in the next-lower layer; this exposed cation is located in a “hole” that is electron-deficient and acts thus as a Lewis acid site. Relying on his weight loss, rehydration, and infrared studies, Peri (120) developed a model of the 7-A1203surface. The (100) plane was assumed to be exposed predominantly, with an aluminum ion in an octahedral site lying immediately below each surface hydroxyl group of a fully hydroxylated surface. The dehydroxylation process was simulated by a statistical (Monte Carlo) method. Assuming a random removal of hydroxyl pairs without creation of “defects” (adjacent oxide ions or holes), a regular surface lattice can be maintained up to the stage where about 67% of a hydroxyl monolayer has been removed. This stage corresponded to a degassing temperature of approximately 500°C. Further adjacent hydroxyls can only be condensed to eliminate water with creation of defects, which comprise adjacent aluminum and oxide ions. This process can be continued until 90.4% of the surface hydroxyl content has been eliminated, which can be achieved by degassing at approximately 670°C.

206

HELMUT KNOZINCER

At this stage no hydroxyls exist on adjacent sites and further dehydroxylation is possible only by migration of surface ions. At least protons will possess a high mobility at these temperatures. The slow formation of high-temperature forms of alumina, however, indicate that oxide and aluminum ion migration must also occur accompanied by surface sintering effects. These phenomena explain the fact that the number of the most energetic defects (triplet defects in Peri’s model) decreases as the degassing temperature is raised beyond approximately 650°C (121). The five observed OH-stretching bands were attributed by Peri (120) to hydroxyl groups in distinct surface environments. Thus, the high wave-number band at 3800 cm-’ (type A) was assumed to be characteristic for an OH group with four oxide nearest neighbors in the surface, whereas the low wave-number band at 3700 cm-’ should point to an OH group that contains no oxide ions as nearest neighbors, but four aluminum ions (type C). The other three hydroxyl groups that give rise to the intermediate wave numbers at 3733,3744, and 3780 cm-’ (type E, B, and D) are located at one, two, and three nearest-neighbor oxides, respectively. These environmental differences were assumed to lead to different acidic properties of the various OH-groups. Peri (120) suggested that the C-type hydroxyls were the most acidic, and the A-type the least acidic ones. However, the exchange with Dz of the A-type hydroxyls is appreciably higher than that of the other hydroxyls (115, 116). Dunken and Fink (116) therefore, proposed the reverse acidity sequence where the A-type hydroxyls (3800 cm-’) are the most acidic and should have four nearest-neighbor oxide “holes.” The C-type hydroxyls (3700 cm-’), on the other hand, are assumed to be immediately adjacent to four oxide ions with the 0-H force constant and the proton reactivity (acidity) being lowered due to H-bond interactions with the neighboring oxide ions. Thus, the assignment of the observed OH-stretching bands seems still to be ambiguous. This situation unfortunately renders a discussion of the nature of surface sites in general rather difficult. A totally different interpretation of multiple frequencies in the OH-stretching region has been suggested by Hallam (122), who points to the possibility that multiple frequencies in the OH-stretching region may arise from combination sum and difference modes of the fundamental stretch and low-frequency vibrations. The number of OH groups present in the surface would then be lower than the number of bands observed in the infrared spectra. In this case, the central band and the corresponding sum and difference modes belonging to a single OH group should respond to exchange or adsorption processes in the same manner. Inspection of spectra taken during Dz exchange (115, 116), however, seem not to show a behavior consistent with Hallam’s suggestion, at least for the aluminas used in these studies. Any observed infrared band in the OH-stretching region must, therefore, be assigned to an individual surface OH group.

SPECIFIC POISONING OF OXIDE SURFACES

207

Zecchina (123) mentioned the appearance of two complex groups of OHstretching bands at 3800 to 3780 cm-’ and 3740 to 3690 cm-’ , respectively, on surfaces of 7)- and 7-Al2 0 3 . The first group of bands was assigned to hydroxyls belonging to the coordination sphere of tetrahedral A13+ ions, and the second group to hydroxyls shared by an octahedral and a tetrahedral aluminum ion. This interpretation of two groups of surface OH-stretching bands points to the probably weakest point in Peri’s surface model, i.e., the assumption of the (100) face forming the surface layer of y-A1203 predominantly. In fact, the occurrence of various exposed crystal planes, which would offer different local environments for surface OH groups, appears to be likely for polydisperse materials such as the transitional aluminas. According to Lippens (124, 7-Al2O3 predominantly terminates in (1 10) faces, and 7)-A12O3 in (1 11) faces. Some evidence for the (1 l l ) face being exposed on q-A1203 was reported by Borello et al. (125). These authors described two types of electron-deficient sites, I and 11, on 9-A1203 after degassing at 500’C and higher temperatures, which were responsible for CO adsorption:

(1)

(11)

Here the square represents an anion vacancy. Thus, type I sites correspond to incompletely coordinated Al ions in tetrahedral sites, whereas type I1 sites are generated by the removal of an anion bridging an octahedral and a tetrahedral cation. Hydroxyl groups in the respective positions will exhibit distinct properties and give rise to different OH-stretching bands as pointed out by Zecchina (123). Certainly, Peri (120) was aware of this situation when he wrote: “The surface may be very different from that depicted and possibly much more complex. Faces other than the (100)-face may be exposed to a major extent. The model should prove applicable to other faces which can be approximated by a square lattice, but could not be readily applied, for example, to a (1 11) face.” Surface models are usually deduced by extrapolating the bulk structure up to the surface. It seems, however, possible that in surface layers a structure that is distinct from the bulk structure is stabilized. Although not strictly comparable with the situation for transitional aluminas, some evidence for such phenomena has been obtained by French and Somorjai (126)for the (0001) face of c u - A l 2 0 3 by means of LEED. They showed that the surface structure was distinct from that of the bulk and that the surface layer was oxygen-deficient and the aluminum ions were in a reduced valence state. A chemical composition corresponding to A120(or AlO) was proposed for the surface layer. Weller and Montagna (127) have shown that also 9- and 7-A1203 may exist in a nonstoichiometric composition (in the surface in particular), and they also believe in the existence of surface aluminum ions in a reduced valence state. It seems interesting in this

208

HELMUT KNOZINGER TABLE I1

Comparison of the OH (OD)-Stretching Fundamental Wave Numbers for y-, v ,and S-Al203'

-

WH)

.. WD) ~

~

~~~~~~~~~

3700 3733 3744 3780 3800

3700 -

3710

-

3785

3670 3685 3727 3715 3790

3745 3760 3785

3740

3795

2733 2759 2803

2725 2760 2790

2730 2755 2790

-

2715 2155 2800

-

~

3680 -

3730 -

~

=Reference numbers are cited in parentheses in the column headings.

connection to state that very similar OH-stretching bands have been observed for different alumina modifications, as shown in Table 11. Since equal numbers of OH-stretching bands and nicely comparable positions are obtained, one may conclude that the respective surface OH groups reside in comparable local environments on the surfaces of the three modifications of alumina. The stabilization of comparable surface layers irrespective of the bulk structure may be the reason for this. These surface layers may be disordered, the degree of disorder being determined by the state of organization of the starting material (112). The disorder of the surface layer may be responsible for the creation of particular sites that cannot be readily predicted by idealized models. Impurities (such as iron, fluorine, chlorine, and sulfate) of low concentrations are also known to play an important role in surface properties (111). These impurities probably can influence the structure of surface layers. Tung and McIninch (129) describe still another rehydroxylation process of alumina that leads to the formation of passive protons. It is assumed that a water molecule initially coordinated to a cation in an anion vacancy, dissociates, the OH group being held in the vacancy while the proton is trapped in a cation vacancy:

From catalytic studies, it was concluded that the depth of the trap corresponded to 0.06 eV, so that these passive protons could be promoted at temperatures

SPECIFIC POISONING OF OXIDE SURFACES

209

above 40OoC. This picture could, therefore, account for the high-temperature Brensted acidity of aluminas. Any surface model that is based on the assumption of a single exposed crystal plane will fail in the case of the frequently used 6-A12O3 type P1 lOCl (supplied by Degussa, Germany). This material consists of microspheres of 50 to 300 A diameter. Parkyns (130) has, therefore, described the surface of this particular alumina by approaching the microspherical particle form by a rhombooctahedron of twenty-six sides made up by low index planes of the alumina spinel structure. These planes were the (loo), (110), and the (111) planes. The surface of this material should then contain 4.7 octahedral and 2.8 tetrahedral cations/l00 A’, whereas a maximum number of only octahedral cations of 6.25/ 100 A’ is possible on a (100) face according to Peri’s model (120). In conclusion, all the available surface models for high-area aluminas are moreor-less idealized, and they describe, therefore, only roughly the geometrical and energetic situation that a molecule will “see” when it approaches the surface from the gas phase. The description of adsorption and active sites on the basis of such idealized models are, therefore, equally idealized.

2. Titanium Dioxides Titanium dioxide Ti02 occurs in three crystalline modifications, i.e., anatase, rutile, and brookite. Anatase and rutile are important as adsorbents, white pigments, and catalysts. In both modifications, the Ti4+ ions are octahedrally coordinated by oxygen 02-, and each 0’-has three Ti4+neighbors. The structural unit in both forms is the Ti06 octahedron with similar Ti-0 bond length in each case. The difference between the two structures is due to the different packing of these octahedra (131). As in the case of alumina, surface structures have been characterized mainly by the hydroxylation/dehydroxylationbehavior of the two modifications. The first infrared spectra of surface OH groups were reported by Yates (132),by Lewis and Parfitt ( 1 3 4 , and later by Criado et al. (134). In the case of anatase, the (001) face is the most atomically dense plane and is, therefore, usually assumed to terminate the crystallites predominantly. In the clean (001) cleavage plane of anatase, five-coordinate Ti4+ions are exposed with the 0’-ions bridging two cations. According to Boehm el al. (19, 135a-c), viater molecules will complete the coordination sphere of the surface cations and subsequently dissociate to form two types of surface OH groups-one monodentate terminal and another bidentate bridged hydroxyl. These two types of surface OH groups also exhibit different chemical properties, as shown by the same authors, and they should give rise to different OH-stretching frequencies. Parkyns (136) observed five bands at 3728, 3707, 3672,3654, and 3636 cm-’ , their intensities depending on the heat treatment of the samples, which can be

210

HELMUT KNOZINCER

interpreted in two ways. Both interpretations were entirely based on Boehm's model, which allows for the occurrence of five distinct OH groups by assuming either different electrostatic environments for the groups on a single-crystal plane (compare Peri's statistical model of the yA1203 surface) or the contribution of planes other than the (001). Primet ef al. (137, 138), observed only two OH-stretching bands at 3715 and 3665 cm-' on an anatase surface, which they explain by a simple model based again on the (001) face. It is assumed that only the oxygen ions just above this plane can be substituted by hydroxyl groups and that these may occur in two configurations, one in which they are on adjacent sites and mutually interact via weak H bonds (band at 3665 cm-') and one in which they are isolated from each other (band at 3715 cm-'). On dehydroxylation, the two types of OH groups will lead to different types of surface sites. The H-bonded OH groups will be removed according to

'/

OH. * * * *OH I/

TTi-0-Ti-

I

'I

-

H,O

+

(7) (111)

whereas the isolated OH groups can only be removed at higher temperatures when protons start migrating over the surface,

(IV)

As a consequence, incompletely coordinated titanium ions will be exposed. Type 111 sites would be able to re-form the original OH groups through chemisorption of water, whereas type IV would only coordinate molecular water. Recently, Munuera er al. (139) reported on TCA, TPD, and infrared studies of dehydration and rehydration of an anatase surface. They could best explain the adsorption capacity for water of the anatase surface by assuming the (1 11) face to be exposed. This face contains 1.9 four-coordinate Ti4+ions/100 A2 and the same number of highly exposed supraplanar oxide ions. Additionally, there are 3.8 five-coordinate Ti4' ions/100 A'. To complete the coordination spheres of these cations, a total of 7.6 H 2 0 molecules/100 A' is needed, which agreed with the experimental value. This water is coordinately bonded to the cations and gave rise to infrared absorptions at 3480 cm-' (Cl t C3) and at 1600 cm-' (&). Further bands at 3730, 3680, 3645, and 3620 cm-' were assigned as OHstretching bands of OH groups probably located at the positions of different oxide ions. Some support for this surface model was obtained from the adsorption of C1-C4 primary alcohols (140). Surface models of rutile are based on the (110) face. The selection of this crystallographic plane is justified by the fact that electron micrographs of

SPECIFIC POISONING OF OXIDE SL'RFACES

21 1

crystalline rutile powders show a morphological resemblance to massive rutile crystals, in which the external crystal surface is composed predominantly (60%) by the (1 10) face; the (101) and (100) faces contribute equally, roughly 20% each (141,142). Primet et al. (137) reported infrared bands of an isolated surface OH group at 3685 cm-' and of H-bonded groups located on adjacent unit cells at 3655 and 3410 cm-' . These assignments were based on a model essentially similar to that proposed by these authors for the anatase surface, the differences in frequencies for the H-bonded groups being due to the different o . . . .0 distances in the two modifications (138). Jackson and Parfitt (141) observed four OH-stretching bands at 3700, 3670,3690, and 3420 cm-' . They assign the two high-frequency bands as isolated OH groups, which are formed on dissociative chemisorption of a water molecule. One OH group formed on an anion vacancy has monodentate attachment (3670 cm-' ), whereas the second formed by addition of a proton onto a surface oxygen is bidentate or bridged (3700 cm-'). The two low-frequency bands were assigned as H-bonded species of these two types of surface OH groups. Munuera and Stone (142) successfully applied the model of a (1 10) plane to their adsorption studies of water, isopropanol, and acetone. The "dry" (1 10) face exposes 5.1 five-coordinate Ti4+ ions. Half of these were able to chemisorb water dissociatively, leading to the formation of two types of surface OH groups. The activation energy for desorption of this water was 107 kJ mole-'. At this state of hydroxylation the rutile surface consists of isolated five-coordinated Ti4+ ions, which on further addition of water coordinate water molecuions, and of pairs of OH groups, one being monodentate larly, of isolated 02and the other bidentate with respect to the cations. Jones and Hockey (143) took into consideration the occurrence on the surface of rutile of all three low-index planes (1 lo), (101), and (100). They suggest that dissociative chemisorption can only occur on the (1 10) face producing a monodentate OH group with infrared absorption at 3650 cm-' and a bridging OH group absorbing at 3410 cm-' . The "dry" (101) and (100) faces expose five-coordinate Ti4+ ions which were assumed to complete their coordination sphere by molecular water. The corresponding sets of infrared bands were 3680,3610,1610 cm-' and 3550, 1610 cm-', respectively. Although the surface models for anatase and rutile, as proposed by different authors, are idealized and differ from each other in details, it can certainly be ions, and OH groups concluded that coordinatively unsaturated Ti4+cations,02in widely varying configurations should be exposed on partially hydrated and/or hydroxylated surfaces. Depending on the local environments of these sites, a wide spectrum of possible intermolecular interactions should be the consequence which may render specific adsorption processes possible. Finally, the ease of the surface reduction of titanium dioxides due to hydrocarbon contamination (19) leads to the formation of new types of surface sites and to drastic changes of the surface properties.

212

HELMUT KNOZINGER

3 . &Chromium Oxide The crystal structure of a-Cr203 is made up by a hexagonal close-packed lattice of oxide ions (sequence ABAB -). Two-thirds of the octahedral sites are occupied by Cr3+ ions. Possible idealized surface structures, based on the (OOl), (loo), and (101) planes and the creation of surface sites in the form of coordinatively unsaturated cations and anions on dehydroxylation of the surface, have been discussed by Burwell et al. (21) and by Stone (144). The (001) face is the most likely crystal plane to predominate in the external surface of wellcrystallized a-Cr2O3 (145). A possible surface model that maintains the overall as well as the local electrical neutrality, as proposed by Zecchina et al. (145) for the dehydroxylated (001) face, is shown in Fig. 2a. It can clearly be seen that equal numbers of four- and five-coordinate Cr3+ ions are to be expected on this idealized surface. Dissociative chemisorption of water would lead to the formation of surface OH groups, as shown in Fig. 2b, for a partially hydroxylated model surface. In fact, on adsorption of DzO, Zecchina et al. (145) observed OD-stretching fundamental bands at 2700 and 2675 cm-' , which were narrow and isolated. As evidenced by the appearance of a H 2 0 bending band at 1590 e

-

0

b

FIG. 2. (a) Possible model for a completely dehydroxylated (001) face of a-chromia. 0, 02-ions of the underlying layer; 0, Cr3+ions; 8,02-ions of the upper layer. (b) Possible model of a partially hydrated (001) face of a-chromia. @, surface hydroxyl. [Reproduced with permission from Zecchina ef nl. (145).]

SPECIFIC POISONING OF OXIDE SURFACES

I{\[

213

cm-' at increasing coverages, molecular adsorption also takes place. This band seems t o be due t o coordinatively adsorbed water molecules:

cr3+

(9)

which are released from the surface only at temperatures above 150"-200"C. A complete hydroxylation of the cr-CrzO 3 does, therefore, apparently not occur. Assuming a random removal of OH groups from a hydroxylated cu-Cr203 surface similar to that proposed by Peri (120) for yA1203, one would expect the generation of coordinatively unsaturated Cr3+ ions and 02-ions of widely differing local configurations, so that a heterogeneous surface would result. Depending on the degree of hydroxylation, five-coordinate and four-coordinate Cr3+ ions may be exposed. Three of the ligands of these coordinatively unsaturated cations are lattice oxygen ions; coordination numbers 5 and 4 are adopted by surface oxygen ions or OH groups as shown in Scheme 1 (a square denotes an

0'-

0'-

Scheme I

anion vacancy). Surface heterogeneity is, therefore, caused by two factorsdifferent degrees of coordinative unsaturation and varying distribution of nearest neighbors. Because of this distinction, Zecchina et al. (146) have introduced the terms coordinative heterogeneity and ligand heterogeneity. 4. Zincoxide

The chemisorption of water on ZnO has been investigated by Nagao and Morimoto (147). Dent and Kokes (148) explained H2 chemisorption and ethylene hydrogenation on the basis of a model in which the (OOOI), (OOOi), and non-close-packed faces such as (1010) planes of the wurtzite crystal structure are assumed to form the external crystal surface. Chemisorption of Hz was suggested to occur on Zn-0 pair sites, since the Zn-H and 0-H stretching

214

HELMUT KNOZINGER

fundamentals appeared simultaneously. These pair sites should be located on (0001) faces and reconstructed (OOOi) faces, where coordinatively unsaturated ZnZ+ions can be expected t o occupy trigonal holes. These sites as well as sites on other planes, such as lOiO), can also chemisorb water dissociatively. By using a very similar picture of the ZnO surface, Atherton et al. (149) have recently assigned the observed OH-stretching fundamental bands of surface OH groups. The narrow bands at 3670 and 3620 cm-' were attributed to isolated OH groups located at (0001) and (OOO'i) planes, respectively, whereas the broader bands at 3440 and 3550 cm-' were suggested to arise from mutually H-bonded OH groups on the less densely packed (1010) and (1010) or (1071) planes, respectively. Mattmann et al. (150) observed isolated OH-stretching fundamental bands for a variety of ZnO samples at 3660 to 3685 cm-' and at 3610 to 3630 cm-' , which they assigned, in agreement with Atherton et al. (149), t o free OH groups on the (0001) and (OOOi) planes, respectively. They disagree with Atherton et al. in that bands below 3400 cm-' were attributed to OH groups o n (1010) faces, whereas bands around 3450 and 3550 cm-' were assigned as the asymmetric and symmetric OH-stretching fundamentals of molecularly adsorbed water. 5 . Magnesium Oxide Magnesium oxide crystallizes in the NaCl structure. According to the surface hydration characteristics of high surface area MgO (prepared by decomposition of the hydroxide), Anderson et al. (151) suggested the (100) plane to be exposed at the external surface of the crystallites predominantly. The (100) face of MgO is the crystal plane of lowest energy and presents a cubic array of Mg2+ and 02-ions at a separation of 2.107 A. The simplest model for the chemisorption of H z O on this plane consists of an OH group coordinated on a surface cation (A type) and the remaining proton forming a second type of OH group with an adjacent 0'- ion (B type). This may lead to an ideally hydroxylated MgO surface as shown in Fig. 3. A sharp infrared band at 3752 cm-' was, consequently, assigned as the OH-stretching fundamental of the A-type hyE-type

A-tVDe

6 2.107A

FIG. 3. Schematic representation of section through ideal hydrated (100)face of MgO. [Reproduced with permission from Anderson el a). (151).]

SPECIFIC POISONING OF OXIDE SURFACES

215

droxyls, whereas a broad band at 3610 cm-’ was attributed to B-type hydroxyls that can interact with their environments by H-bonding. The NMR second moments of the surface OH groups were consistent with this simple model as shown by Webster et al. (152). These NMR studies furthermore showed that the dehydroxylation occurs by growth of hydroxyl free areas, leaving clusters of OH groups on the surface. Water chemisorption on MgO was reinvestigated very recently (152a). Beside exposed cations, 0’- ions, and OH groups, a number of paramagnetic centers have been detected by ESR techniques on the surface of MgO. The relevant literature has recently been reviewed by Derouane and Vedrine (153). Such studies have revealed the fact that (1 11) orientations must also be exposed to some degree on highly activated MgO, which is an effective catalyst for H2-Dz equilibration reaction at low temperature. Thus, Boudart et al. (154) have proposed a model for the active site for this reaction. The site consists of an OH group that is located next t o a so-called Vl site, perhaps at a crystal corner or some other surface site characterized by a high binding energy of the OH group on MgO. The VI paramagnetic center is pictured as a B3 site on a (1 11) face, where the three Oz-ions have been replaced by three 0- ions. The maximum concentration of these active sites was found t o be of the order of magnitude of lo9/cm2. 6 . Silica-Alumina Much work has been done t o characterize surface and catalytic properties of SiOz-Alz03 and other mixed-oxide systems. It is well known from these studies that SiOz-Alz03 possesses Lewis as well as Brqhted acid sites that are interconvertible. Different models for the site structures have been proposed. Tanabe (20) has recently reviewed these attempts to characterize the surfaces of silica-alumina mixed oxides as well as those of crystalline silica-aluminas. Until recently, when Peri (155) reported on a model of the silica-alumina surface, there were no detailed models for the surfaces of mixed oxides available. Beside the presence of Brdnsted and Lewis acid sites, Peri (156) had proposed the existence of “a sites” on the Si02-Alz03 surface, which he described as acid-base pair sites rather than simple Lewis acid sites. Various molecules, such as acetylene, butene, and HCl, are adsorbed very selectively on these a sites, whereas NK3 and H z O are also held by many other sites (157). To rationalize the formation of these sites, Peri (155) developed a semiquantitative surface model for certain silica-aluminas, which were prepared by reaction of AICIJ with the surface silanol groups of silica and subsequent hydrolysis and dehydration. The model is entirely based on a surface model of silica, which suggests an external surface resembling a (100) face of the cristobalite structure (158). It should be mentioned in this connection that Peri’s surface model of silica may

216

HELMUT KNOZINCER

be oversimplified in that it takes only one type of exposed crystallographic plane into consideration (159). The generation of a sites is described as being due t o Al-0-Al links that form on geminal silanol groups, whereas those Al-0-Al links that form on vicinal silanol groups are assumed t o exhibit lower acidity. These latter sites are identified as ‘‘p sites.” The estimated numbers of a and 0 sites in the model surface are 6.6 X 10’2/cm2 and 5.1 X 10’3/cm2, respectively. Certain other sites are visualized by the surface model that explain the persistence of SiOH rather than AlOH groups. The ‘ti sites” are described as Si-OAI=O

/OAI=O

Si

and ‘OH

Si-OH

and the “y sites” are assumed to be formed by double bridges in which two oxide ions are held between A1 atoms. This model had been developed for a special preparation of silica-alumina, which may be designated as an “aluminum-on-silica” catalyst according t o Bourne et al. (160). Peri (15.9, however, argues that similar sites should also be formed on typical silica-aluminas that are usually prepared in aqueous media. Bourne et al. ( I 6 0 ) , on the other hand, state on the basis of pyridine adsorption studies onto “aluminum-in-silica’’ and “aluminum-on-silica” that both preparations behave differently on dehydration. It seems, thus, that there is still no generally accepted surface model for silica-aluminas available, although Peri’s approach seems to be a good basis for further developments that should permit a deeper insight into the nature of surface sites on such mixed oxides. It might be worth mentioning that Tanabe et al. (161) have very recently proposed a concept t o explain the generation of acidity in binary mixed oxides that allows the prediction whether or not a novel mixed oxide will develop surface acidity.

7. General Considerations on Surface Models The surface structure models discussed in the preceding sections were all based on an extrapolation of the bulk structure to the surface layers. In many cases, models differing in details are competing for the same oxide, and the assignment of infrared absorptions of surface OH groups is still a matter of controversy. The influence of the preparative conditions and of impurities that may concentrate in the surface layers could be the reason for this. Furthermore, the question whether surface layers really correspond to the structure of the bulk or whether the structure of the surface deviates more or less from that of the underlying bulk is still not clarified. Surface structures may be strongly disordered or even amorphous, which should permit the generation of sites of unusual configurations.

SPECIFIC POISONING OF OXIDE SURFACES

217

Nevertheless, these idealized surface structure models have proved valuable in many cases, and the simulation of adsorption/desorption processes by statistical methods is certainly an extremely rewarding approach that will be developed further and lead t o more realistic pictures of the nature and structure of oxide surfaces and of sites on them. It is ascertained at present that coordinatively unsaturated atoms or ions constitute the surface layer of oxides. The capability of these surface atoms or ions to bind adsorbate molecules depends on the metal-oxygen bond character and their local environment, i.e., coordination number and nature and distribution of nearest neighbors. Sites of particular nature will usually show energetic heterogeneity that may be a coordinative or ligand heterogeneity or both. The particular properties of surface sites can be tested by suitable probe molecules, and in the following sections the interaction of the probe molecules, selected according t o HSAB concept and listed in Table I, with the surfaces of oxides are discussed. This discussion is confined t o those oxides of which the surface models have been described, and spectroscopic data, infrared in particular, are considered preferentially because of their high information content. B. ADSORPTIONOF AMMONIA Ammonia is a strong Lewis base and it is small in size. The various forms of ammonia expected t o occur on surfaces due to specific interactions (see Table I) can be detected by infrared spectroscopy. Interactions should be quite strong since ammonia is a fairly hard base. Thus, criteria a-d (see Section II.C.l) could hopefully be fulfilled. 1. Alumina

The infrared bands that have been obtained on adsorption of NH3 on transitional aluminas are summarized in Table 111. Four surface species have mainly been identified beside weakly held ammonia, which was simply H-bonded. The dominant species is coordinately held ammonia that adsorbs on incompletely coordinated A13+ ions: [Al3'I s + NH3

E:],

(10)

A limiting form of this surface species may be the NH3+form. At high pretreatment temperatures (>500°C) and low hydroxyl densities, NH2 groups are formed, probably due to dissociative chemisorption on acid-base pair sites, according to [Al-O],+NH3~[Al-NH2],

+ [OH],

(1 1)

TABLE 111 Infrared Spectra of Ammonia Adsorbed on Alumina

Infrared absorption bands (cm-')

Ref.

Type of Alz 0 3 and pretreatment

162, I 6 3

-

3200- 3115- 1550- 11703412 3330 1655 1369

164

-

-

3030-3130

165

-

-

-

I66

T-412 0 3 , 4OO0-80WC

167-169

?-A12 0 3

Coordinately held NH3

3400

3355

1620

<500°C

3415

3370

1625

>5OO"C

3415

3370

1625

12601285 12601285

3265

1615

1270

3280

1620 1622

1230 1255

1610

1230

7

~~

~

I70

6-&03, 150"-800"c

3350

1 71

AI2O3,55O0C Al203,HF-treated

3400 3400

172

7-Al203,

< 4OOOC

-NH2

NH: -

14851550

NH;

-

-

-

-

-

3100-3332

1390-1484

3386 3335 1510

3100

-

-

3150 3050 1680 1410

3180 1630 15001510

3520 3440 1560

-

-

-

-

~

3100 3100

1580 1580

-

1498 1455

-

3210 1430 1450-1470

SPECIFIC POISONING OF OXIDE SURFACES

-

219

Protonated ammonia, NH;, was observed on surfaces containing high OH concentrations (pretreatment < 400°C): [H+l s + NH3

"HA

s

(12)

Thus, adsorption of NH3 on alumina resembles that of water in many respects. Both molecules are adsorbed molecularly at low temperatures but are chemisorbed dissociatively at higher temperatures. Ammonia is held strongly on A 1 2 0 3 surfaces and cannot be removed completely even on desorption at 500°C. Various species occur simultaneously, their relative importance being determined by the OH content of the surface. Furthermore, displacement adsorptions may take place. Thus, NH; ions readily replaced chloride ions on surfaces of chlorided aluminas (166). One has, therefore, to conclude that ammonia retention on aluminas cannot be an acceptable measure of surface acidity and can hardly be related to catalytic activity. Ammonia adsorption on aluminas as studied by infrared spectroscopy, perhaps combined with TPD experiments ( I 73), gives ample information on surface properties; but ammonia cannot be used as a specific poison on alumina. 2 . Titanium Dioxides Infrared studies of the adsorption interaction of ammonia with rutile (174176) and anatase (1352, 136,174,176) have been reported. Bands similar to those obtained for the system ammonia-alumina have been found. Although some contradiction regarding detailed assignments of the infrared bands still exists, it is generally agreed that at least three chemically different surface species are formed on both crystallographic modifications. Ammonia is H-bonded onto surface OH groups. This form is readily desorbed on evacuation. The more strongly bonded species are mainly ammonia molecules held by coordinatively unsaturated surface cations. Infrared spectra of the coordinated ammonia suggest the simultaneous formation of two such species held by two types of Lewis sites (175, 176), which correspond to Ti4+ ions in different stereochemical environments (e.g., type 111 and IV sites; see Section IV.A.2). In addition, ammonia is adsorbed dissociatively with the production of surface NH; groups (136, 175). An NH; species was only observed by Boehm and Hermann (1357) on anatase. This species may be due to an increase of the protonic acidity brought about by some chlorine impurities. The NHC was also formed on addition of HC1 to the hH3-Ti02 system (175); but addition of water vapor to this system does not produce the NH; species (175). A reduction of the surface OH content on adsorption of ammonia has been reported for titanium dioxides (176) and also for alumina (177) and for a Cr203-W03 catalyst (178). This phenomenon points to the fact that the original surface properties may also be modified by displacement adsorption.

220

HELMUT KNOZINGER

3 . Chromium Oxide Filimonov et al. (1 74) first reported infrared spectra of ammonia adsorbed on chromium oxide. Recently, Eley el al. (179) have shown that ammonia is H bonded by surface OH groups and coordinately held by two distinct types of Lewis acid sites. Oxidation of adsorbed ammonia molecules at 100°C with the production of water that was chemisorbed on the surface was also observed ( I 79).

4.Magnesium Oxide Magnesium oxide is considered to exhibit basic properties (20). It is thus not unexpected that neither Bransted nor Lewis acid sites could be detected by ammonia adsorption (180, 181). Hydrogen-bondingis the only type of interaction that ammonia probably undergoes with surface oxide ions on dehydroxylated surfaces (180) and with surface OH groups on hydroxylated surfaces (181).

5 . Silica-Alumina and Zeolites Pure oxides develop Lewis acid sites preferentially. Mixed oxides, in general, and silica-aluminas, in particular, are known to develop protonic as well as Lewis acidity (20). The distribution of Bransted and Lewis acid sites depends on the degree of hydroxylation and on the distribution of aluminum cations in octahedral and tetrahedral sites (182). On adsorption of ammonia onto silicaalumina, the simultaneous formation of NHC species and of coordinately held NH3 is thus observed by infrared spectroscopy (157, 170, 182-184). Beside these species, Peri (157) detected NH2 groups. At higher temperatures many types of adsorption sites for ammonia were recognized (157). The a! sites (see Section IV.A.6) that seem to be active for various hydrocarbon reactions were among these sites, but ammonia was not held specifically by this type of site (157). Similarly, zeolites possess intrinsic Brgnsted and Lewis acid sites that on ammonia adsorption, lead to the simultaneous formation of NHC species and coordinately adsorbed NH3 (185-187). Besides, strong complex formation with the exchangeable cations is observed (188-190).

6 . General Conclusions on Ammonia Adsorption The adsorption of ammonia leads to a variety of chemically distinct species in most cases. Different types of sites are responsible for the formation of these surface species. Any correlation between rates of catalytic reactions and quantities of adsorbed ammonia may, therefore, be misleading and the characterization of active sites becomes ambiguous. Furthermore, ammonia is a very strong

SPECIFIC POISONING OF OXIDE SURFACES

22 1

base and will, therefore, interact even with weak sites with an appreciable heat evolution. Isosteric (191) and calorimetric (192) heats of chemisorption ranging from 45 kcal/mole at low coverages to approx. 10 kcal/mole at higher coverages have been reported for alumina, silica-alumina, and zeolites. Ammonia will consequently be adsorbed by a large number of sites, only a few of which are active sites in a given reaction; for example, 2 X 1013 sites/cm2 have been counted by trimethylamine adsorption on dumina, whereas the number of active sites for cyclohexene isomerization was considered to be only 8 X 10"/cm2 (30). This means that only the fraction of ammonia adsorption sites with heats of chemisorption above 20 kcal/mole are active sites in this reaction (192). Ammonia seems to be too strong a base if specific adsorption is required. A characterization of the chemical nature and a determination of the number of catalytically active sites by means of poisoning experiments with ammonia will, therefore, not readily be possible. Ammonia can thus not be recommended as a simply acting specific poison. Conclusive results may, however, be obtained by stepwise poisoning, adding successive small quantities of ammonia, provided that the modes of interaction with the catalyst of this ammonia are controlled by spectroscopic techniques under the reaction conditions.

C. ADSORPTION OF AMINES(OTHERTHAN AMMONIA AND PYRIDINE) Although a variety of amines, particularly trimethylamine and n-butylamine have widely been used as poisons in catalytic reactions and for surface acidity determinations (20), comparably few spectroscopic data of adsorbed amines are available. As with ammonia, coordinatively adsorbed amines held by coordinatively unsaturated cations have preferentially been found on pure oxides (176, 193-196), whereas the protonated species were additionally observed on the surfaces of silica-aluminas and zeolites (196-199). However, protonated species have also been detected on n-butylamine adsorption on alumina (196) and trimethylamine adsorption on anatase (176) due to the high basicity of these aliphatic amines. In addition, there is some evidence for dissociative adsorption of n-butylamine (196) and trimethylamine (221) on silica-alumina. Some amines undergo chemical transformations at higher temperatures (195, 200) and aromatic amines, such as diphenylamine, have been shown to produce cation radicals on silica-alumina (201, 2 0 1 ~ ) . In conclusion, the use of amines for the characterization of adsorption and active sites raises the same problems as ammonia. The interaction of the amines, saturated aliphatic amines in particular, with oxide surfaces is certainly still less specific than that of ammonia because of their higher basicity and their larger molecular size. The influence of steric effects on amine adsorption has been discussed (172, 201b). Thus, Medema et al. (172) came to the conclusion that the adsorbed amount of an amine on y-A1203 primarily depends on its molec-

222

HELMUT KNOZINGER

ular cross-sectional area and not on its basicity. Furthermore, chemical transformations of chemisorbed amines such as disproportionation and deamination may occur at elevated temperatures (201b, c). It seems, thus, difficult to draw unambiguous conclusions on the chemical nature and number of active sites from poisoning experiments with amines.

D. ADSORPTION OF PYRIDINE Pyridine, although less basic than ammonia, is still a fairly hard base and may be expected to be an effective specific poison. Its molecular size, however, may bring about some difficulties. Due to the nitrogen lone electron pair, pyridine should interact with acidic oxides in a specific way to form coordinated species PyL on Lewis acid sites and the pyridinium ion PyH' on protonic sites. The infrared spectra of pyridine coordination compounds (162,202)are clearly distinct from those of PyH' (203) and of H-bonded pyridine, so that the corresponding surface species can quite easily be distinguished. The ring vibration modes 19b and 8a-according to the assignments of Kline and Turkevich (204)are the most sensitive vibrations with regard to the nature of intermolecular interactions via the nitrogen lone pair electrons. These two modes are observed at 1440 to 1447 and at 1580 to 1600 cm-' ,respectively, for H-bonded pyridine, at 1535 to 1550 and around 1640 cm-' for PyH', and at 1447 to 1464 and 1600 to 1634 cm-' for coordination compounds. Electronic spectra, on the other hand, are not very sensitive versus the type of surface bond (204u,210). 1. Alumina Since the time that Parry (205) first published his paper on pyridine adsorption on an r)-Al2O3, the surface acid sites of a number of crystallographically different alumina samples and the influence of pretreatment conditions on the pyridine adsorption have been studied. Relevant results are summarized in Table IV. Hydrogen-bonded pyridine is observed in all cases, provided pyridine is present in the gas phase. Besides, depending on the pretreatment and desorption temperatures and on the type of alumina, varying numbers of PyL species have been detected (Table IV), which are identified by their characteristic 19b and 8a vibrational modes. Increasing wave number of these modes indicates increasing coordination bond strength. These assignments were confirmed recently by Raman spectroscopy (82,83,211), which permits the corresponding bands between 990 and 1050 cm-' to be observed. This spectral range is barely accessible by the infrared transmission technique (see Section III.A.3a). Kirina e t ul. (209) report on a transformation of Lewis into Brgnsted sites by addition of water vapor-the water is assumed to be coordinated to the Lewis site and to provide acidic protons. Other authors (121,196,208) could not confirm this

TABLE IV

Infrared Bands of Pyridine Adsorbed on Alumina

Sample V-&%

~ 4 1 023

Q-Ab03

Pretreatment temperature ("C)

450

500 650

Desorption temperature (" C )

19b Mode (an-') 1450

RT 150-230 325-565

1453 1457 1458

150 500 RT 100-200 300

8a Mode (cm-I)

1583 1600 1621 1622 1632 1632

205

1618 1625 1625

208

1453 1457 1445

1597 1610 1450 1455 1455

Ref.

0 cd

9

z 5 z

U

1620 1617 1623 1623

210

1615

I94

0

208

2 w

Q

0

3 Y-A203

7-Ab03

650

500

RT 250

150 500

y-A203

500

100

6-A1203

150-800

RT 125

350 S-Al2O3

300-800

-

RT-200 250 300-400

1620 1453

1623 1623

1456 1457

1615

209

1589 1606 1606 1620 1620

206

1449 1449 1449 1449

1614 1617 1617

1444

1452

E

h

!2

121,207 h)

1624

h)

w

224

HELMUT KNOZINGER

kind of acid site transformation. Although some weak protonic sites have been detected on aluminas by ammonia adsorption (see Section IV,B.l), no such intrinsic Brgnsted sites could be found by most authors by pyridine adsorption, since the PyH+ species is not formed at a detectable level due to the lower basity of pyridine. Even when the spectra were recorded at temperatures up to 300°C the PyH+ species could not be detected (212), indicating that the protonic acidity of aluminas is not appreciably increased in this temperature range. Only Bremer et al. (213) claim to detect the PyH+ species on p A l 2 O 3 . It was suggested that this species should be formed on intrinsically present acidic protons. Knozinger and Stolz (121) reported some evidence for sterical hindrance between the pyridine ring and surface anions in the formation of the coordination surface bond. Two “outer complexes” with characteristic 8a modes at 1614 and 1617 cm-’ were observed at desorption temperatures below roughly 200°C (207) . At increasingly higher temperatures, a thermally more stable “inner complex” with an 8a mode at 1624 cm-’ was detected. This activated chemisorption was explained by the assumption of a steric hindrance in the approach between pyridine nitrogen and aluminum ion (probably located in a triplet anion vacancy). The importance of steric effects governing the chemisorption bond strength could also be envisaged in the adsorption of substituted pyridines (214). Thus, 4-methylpyridine, which exhibits the same steric requirements as pyridine but is a stronger base, is more strongly held than pyridine. 2,4,6-Trimethylpyridine-a still stronger base-is much more weakly held than pyridine because of the steric hindrance of the methyl groups in 2- and 6-positions. These conclusions are in good agreement with the results of Medema et al. (172) on the adsorption capacities of y-AlzO3 for a series of amines of different molecular size (see Section 1V.C). Benesi (35)has tried specifically to detect Brtdnsted sites in the presence of Lewis sites by using 2,6-dimethylpyridine as a probe molecule. He suggests that the interaction of this base with Lewis acid sites should be completely suppressed. However, spectroscopic data (214) show that this is not true and that the adsorption of 2,6-dimethyl-substitutedpyridines is unfortunately not as specific as assumed by Benesi. The main chemisorption of pyridine on alumina surfaces at temperatures roughly below 350°C is, thus, by coordination on to coordinatively unsaturated A13 ions: +

The bond strength of this surface coordination complex should depend on the Lewis acidity of the particular site (coordination number and ligand distribu-

225

SPECIFIC POISONING OF OXIDE SURFACES

tion) and on the steric situation. At temperatures higher than roughly 35OoC, an additional surface reaction between pyridine and surface OH groups has been observed, leading to a surface pyridone species on 6- and 9-Al2O3 (210). This reaction may tentatively be described by

The pyridone surface species has a C=O stretching band at 1634 cm-' .3 Hydrogen gas has been detected by mass spectrometry (210), and the formation of this surface compound has been established by chemical methods by Boehm (215). This surface reaction points to the existence of strongly basic OH- ions held to certain sites on alumina surfaces, their number being of the order of magnitude of 10'3/cm2 (121). Additional evidence for the existence of these reactive and strongly basic OH- ions on aluminas comes from surface reactions observed on adsorption of nitriles and ketones (see Section IV.F) and of carbon dioxide (see Section IV.G). These reactions may, thus, be valuable for the detection of the corresponding sites that most probably have to be considered as acid-base pair sites. 2. Titanium Dioxides Infrared spectroscopic studies regarding the adsorption of pyridine on both anatase and rutile have been reported (136, 176, 194, 216,217). Hydrogenbonded pyridine is readily desorbed on pumping at room temperature, whereas pyridine held by coordinatively unsaturated Ti4+ ions is thermally stable up to approximately 400°C. As ammonia, pyridine forms two distinct coordinately held species ( I 76,217) indicating the existence of two types of Lewis acid sites, which should correspond to Ti4 ions in different stereochemical environments. According to Primet et al. ( I 76), the more stable species is chemisorbed on type 111 sites (see Section IV.A.2) which are assumed to be more acidic than type IV sites. A protonated pyridine has never been observed, and the Lewis acid sites on titanium oxides cannot be converted into Brfinsted sites by water vapor adsorption (21 7). Although Jones and Hockey (216) suggest that the chemistry of surface hydroxyl on rutile corresponds more closely to that of the OH- ion rather than that of the hydroxyl group, no surface reactions similar to that observed with alumina [Eq. (14)] have since been reported. +

3This interpretation of the band at 1634 cm-' replaces the one given in Knozinger and Stolz (121), where this band had been assigned as being due to a hypothetical Py species. +

226

HELMUT KNOZINGER

3. cYChromium Oxide According to the surface models of a-Cr203, as described in Section IV.A.3, OH groups and coordinately unsaturated Cr3+ ions may be considered to act as adsorption sites for pyridine. In fact, the infrared spectra of the pyridineaCr203 system as reported by Zecchina et al. (218) clearly show the reversible formation of strong H bonds between pyridine and surface OH groups. Species b H + were not formed at a detectable level, but the occurrence of the 19b and 8a modes of pyridine at 1449 and 1610 cm-’ , respectively, indicate the formation of a PyL species on both a partially dehydroxylated and a hydrated a-CrzO3 after pyridine adsorption at room temperature. This species was still observed after preadsorption of 02. It was suggested (218) that pyridine completes the coordination sphere of coordinatively unsaturated surface Cr3 ions, but it also forms the PyL species by displacement chemisorption: +

In their surface model, Zecchina et al. (145, 146) propose the presence of five distinct types of Cr3 ions which differ in the coordination number (4 or 5 ) and in the nature of their ligands. Nevertheless, only one set of infrared bands for the PyL species could be observed. This indicates that the differences in the acid strength of the different Cr3+ ions are small and/or that the vibrational modes of the coordinated pyridine do not respond sensitively enough to the intrinsic acid strength distribution. +

4. Zinc Oxide

Acidic properties of ZnO have been established by various methods (20). However, only very little information on the spectroscopic behavior of pyridine adsorbed on ZnO is available. Tanabe et al. (20,220) report on the observation of the PyL species, whereas PyH+ could not be detected. 5 . Magnesium Oxide The 8a mode of pyridine adsorbed on MgO was reported to occur at 1583 cm-’ (219), and it was concluded from the absence of a shift of this band to higher values that MgO does not contain aprotonic acid sites. However, the same authors (219) found a positive wave-number shift of approximately 30 cm-’ of the C Z N stretching vibration of adsorbed benzonitrile. This indicates the coordination of the nitrile onto weak aprotonic sites despite the lower

SPECIFIC POISONING OF OXIDE SURFACES

227

basicity of benzonitrile as compared to pyridine. The absence of any wavenumber shifts in the spectra of adsorbed pyridine might, therefore, be explained by only very weak interaction due to a steric hindrance rather than by the absence of any aprotonic sites. Anyhow, strong coordinative interactions of pyridine with the MgO surface could probably not be expected, since basic properties of the MgO surface generally predominate in adsorption processes (20).

6 . Silica-Alumina and Zeolites As expected for silica-alumina as a mixed oxide (see also Section IV.B.5), the PyH' and PyL species are observed simultaneously (160,205,206,221-223). Two distinct types of Lewis acid sites could be detected (19b mode at 1456 and 1462 cm-' , respectively) on a specially prepared aluminum-on-silica catalyst (160). On water addition, the Lewis sites can be converted into Br$nsted sites (160, 205, 221). The effect of Na' ions on the acidity of silica-aluminas has been studied by Parry (205) and by Bourne et al. (160). It can be concluded from Parry's results that Na' ions affect both types of acid sites, so that alkali poisoning does not seem to eliminate the Brgnsted sites selectively. For quantitative determination of the surface density of Lewis and Br$nsted acid sites by pyridine chemisorption, one requires the knowledge of at least the ratio of the extinction coefficients for characteristic infrared absorption bands of the PyH' and PyL species. Attempts have been made to evaluate this ratio for the 19b mode, which occurs near 1450 cm-' for the PyL species and near 1545 cm-' for the WH+ species (160,198,206,221,224,225). The most reliable value as calculated from the data given by Hughes and White (198) seems to be

The data reported by Basila et al. (221,224) lead to a value of 1.6 +- 0.3. Using the apparent integrated absorption intensities as given by Hughes and White (198), Ward and Hansford (226) estimated the limits of detection of Brgnnsted acidity to be of the order of magnitude of lo-' meq g-' for silica-aluminas with Brunauer-Emmett-Teller (BET) surface areas between 350 and 500 m2/g. Since silica-alumina contains Brgnsted as well as Lewis acid sites, a clear correlation between rates of a heterogeneously catalyzed reaction and surface acidity as measured by pyridine adsorption is only possible if a distinction between PyH' and PyL is made. This is possible by infrared spectroscopy as shown in this section. Thus, Ward and Hansford (226) found a good linear correlation between the percent conversion of o-xylene and the Bransted acidity of synthetic silica-alumina catalysts. This correlation is shown in Fig. 4, where the Brgnsted acidity is expressed as peak height of the band at 1545 cm-' per unity of catalyst weight.

228

HELMUT KNOZINGER / 5OOOC

Brdnsted acidity

FIG. 4 . Conversion of o-xylene as a function of Brgnsted acidity (expressed as peak height of 1545cm-' band per sample mass) for a silica-alumina catalyst. [Reproduced with permission from Ward and Hansford (226).]

The PyH' and PyL species are also formed on various types of decationated (198,227-233) and cation-exchanged (189,234-239) zeolites. The PyH' usually predominates on decationated zeolites after heat treatments below 550°C (227,228). Pyridine seems to form the protonated species selectively on interaction with the OH groups, giving rise to a band around 3630 to 3660 cm-' (198,227-229),whereas ammonia (187)and piperidine (198) are protonated by this OH group and by another type that absorbs at 3540 to 3605 cm-' as well. This improved selectivity of pyridine is due to its lower basicity. In addition to the PyL species that is formed by coordination of pyridine onto Lewis acid sites (tricoordinated aluminum ions), a coordination bond is formed with the cations in cationexchanged zeolites (189,235,236,238). Ward (235,236) has shown that the strength of this type of interaction with alkali and alkaline earth cations increases as the cation size decreases and the electrostatic field strength increases. The Br#nsted acidity of X- and Y-type zeolites strongly depends on the type of the cation (235,236). Furthermore, cations can be displaced on interaction with pyridine (240). 2,6Dimethylpyridine has been shown to interact selectively with the Brgnsted sites in Y zeolites (36),whereas pyridine interacted with both types of acid sites. The substituted pyridine has, thus, been used as a specific poison to count the number of active sites for the cracking of cumene. The value of 1.8 X lo2' sites per gram of catalyst was much lower than that obtained by pyridine poisoning. Analogous poisoning experiments have recently been carried out during n-butene . results show that the coordinaisomerization over NH4Y zeolites ( 2 4 0 ~ ) These tion on to Lewis acid sites of the 2,6-disubstituted pyridine is strongly restricted due to steric hindrance. Furthermore, Yashima and Hara (241) recently pointed

SPECIFIC POISONING OF OXIDE SURFACES

229

out, that, due to its molecular size, pyridine adsorption occurs much more specifically than that of ammonia with respect t o active sites for the disproportionation of alkylbenzenes. The molecular size of pyridine and of alkylbenzenes is comparable, whereas the much smaller ammonia finds access t o sites in narrow channels that remain inaccessible for the reactants. As pyridine forms the PyH' and PyL species simultaneously on most zeolites, meaningful correlations between rates and surface acidity can again only be obtained from discrimination of these species. Thus, Karge (230) could show by a similar method as applied by Ward and Hansford (see above) that Br$nsted acidic OH groups of an H mordenite are the most probable active sites in benzene alkylation. 7. General Considerations on Pyridine Adsorption

It is concluded from the foregoing considerations that pyridine may successfully be applied as a specific poison, provided the possible pitfalls are carefully kept in mind. The lower basicity of pyridine as compared t o ammonia renders its chemisorption more selective. However, its basicity is in most cases still much higher than that of the commonly used reactants, so that one is usually able t o determine an upper limit for the number of active sites by pyridine poisoning (239). On the other hand, the hardness of reactants or reaction products may be comparable with that of pyridine [e.g., dehydration of alcohols (491 ; t h e poison will then be partially displaced. The molecular size of pyridine may bring about difficulties, since it restricts the accessibility of pyridine to narrow pores or even the approach to an adsorption site (214). In favorable cases, however, steric effects may be utilized t o improve the specificity of poisoning (35,36,241). The high thermal stability of pyridine and of PyH' and PyL species recommends these compounds as a poison at higher temperatures, although roughly above 350°C undesired surface reactions or chemisorption processes can occur as, e.g., o n alumina surfaces (210). The distinction between PyL species of different coordination bond strengths by infrared spectroscopy may be difficult due t o the low sensitivity of the ring vibrational modes toward slight changes in the electron densities at the ring nitrogen (218). Considering the function of Lewis acid sites, special care has t o be taken in the case of cation-exchanged zeolites, since pyridine will then form coordination compounds with Lewis sites (tricoordinated A13' ions of the lattice) and simultaneously with the exchangeable cations. Pyridine chemisorption may also transfer these cations to other equilibrium positions and, thus, change indirectly the environments of surface sites near the original or new cation positions. Although pyridine seems to be an optimal specific poison, independent control of the modes of interaction in the particular system should always be carried out. This procedure is indispensable as soon as more than one surface species is formed, e.g., PyH' and

230

HELMUT KNOZINGER

PyL. Infrared spectroscopy in combination with poisoning experiments is a recommendable technique and has already been proved to permit meaningful conclusions t o be drawn on the nature of active sites and reaction intermediates (47,226,230)(see also Section V).

E. ADSORPTIONOF NITROGENDIOXIDE Nitrogen dioxide, NOz, is a fairly small molecule with an unpaired electron and may be expected t o be a selective molecule for electron-deficient or Lewis acid sites. Nevertheless, only very little spectroscopic information on the nature of surface species formed on adsorption of NOz is available. Naccache and Ben Taarit (242) have shown by infrared spectroscopy and ESR that NOz forms Cr+NOz+ and Ni+NOz+ complexes on chromium and nickel zeolites. Thus, NOz behaves as an electron donor and reducing agent in these zeolites. Boehm (243) has studied the adsorption of NOz on anatase and on 7)-Alz03, which were pretreated at very low temperatures of only l0Oo-l5O0C. At 1380 cm-' , a band which is t o be attributed t o a free nitrate ion, was observed. Boehm (243) has explained the formation of the nitrate ion by the disproportionation by basic OH- ions: [OH-] + 3NOZ -+ [NO;]

+ HNO3 + NO

(16)

Infrared spectra of absorbed NOz on different types of AZO3and silicaalumina were reported by Parkyns (130). Similar experiments with A12 0 3 , MgO, NiO, and ZrOz were carried out by Pozdnyakov and Filimonov (244). The samples in these studies were pretreated at elevated temperatures. Infrared bands of the surface species and their assignments for the adsorption on Alz O3 and MgO are shown in Table V. The most intense bands are those near 1600 and 1230 cm-' , w h c h are attributed t o the nitrate ion on sites in different environments. Parkyns (130) found three poorly resolved bands in the 1600-cm-' region, whereqs only two are reported by Pozdnyakov and Filimonov (244) on their alumina. Unidentate, bidentate, and bridging NO3- ions can be assumed t o be responsible for these adsorptions. Contrary to Pozdnyakov and Filimonov, Parkyns attributed the band at 1960 t o 1977 cm-' t o an NO' species that is only partly ionized and associated with NO3- by some partly covalent bond. The formation of these surface species can be explained consistently by the following reaction scheme which was put forward by Parkyns (130):

TABLE V Infrared Bands and Assignments of NO2 Adsorbed on A1203 and MgO

Sample

Ref.

A1203 A1203

130 243 243

Mgo

Unidentate NO;

1570,1290

Bidentate NO;

1597,1225-1250 1570,1480 1330,1310

-

Bridging NO;

1615,1225-1250 1620,1600,1245 -

NO+

MetNO

1960-1977 2260 -

1985 1935

NO:(?)

2000 -

232

HELMUT KNOZINGER

Nitric oxide is rapidly desorbed on evacuation. The number of adsorption sites for this reaction was found t o increase from 1.5/100A2 after pretreatment at 25°C to only 2.3/100 Az after pretreatment at 800°C for a 6-A1203 (Degussa). These values are about an order of magnitude higher than those obtained by pyridine adsorption on the same type of alumina (121) and by NH3 adsorption on a y-Alz03 (168). The number of a sites as determined by COz adsorption by Peri (157)is still lower. The chemisorption of NO2, therefore, seems t o be less specific than that of pyridine, NH3, and C 0 2 . The weak absorptions at wave numbers above 2000 cm-' were tentatively assigned by Parkyns t o NO?' species. These may be comparable t o the species found in zeolites by Naccache and Ben Taarit (242). Nitrogen dioxide adsorbed on silica-alumina behaves similarly as on alumina (130),and uni and bidentate NO: ions appear on MgO (244). The NO3- species are fairly strongly held and can only be completely removed from aluminas o n evacuation at 500°C. Thus, NOz is very specifically held by Lewis acid sites o n surfaces that are evacuated at sufficiently high temperatures. Although the specificity of the NOz chemisorption seems to be less pronounced, for example, than that of pyridine, it might be a useful poison for certain catalytic studies. Unfortunately, no attempts of this kind have yet been undertaken.

F. ADSORPTION OF KETONESAND NITRILES Ketones and nitriles are rather soft bases; their coordination onto electrondeficient sites o n oxides is, therefore, relatively weak. One may, however, expect an improved specificity of chemisorption due to their softness. Unfortunately, however, these substances very easily undergo chemical transformations at oxide surfaces. Thus, carboxylate structures are formed on adsorption of acetone on alumina (194, 24.5-243, titanium dioxide ( 1 9 4 , and magnesium oxide (219, 248, 249). Besides, acetone is also coordinated onto Lewis acid sites. A surface enolate species has been suggested as an intermediate of the carboxylate formation (248, 249). However, hexafluoroacetone also leads to the formation of trifluoroacetate ions (219). The attack of a basic surface OH- ion may, therefore, be envisaged as an alternative or competing reaction path: (CH3)zCO + [OH-],

---+

[CH3C00-], + CH4

(18)

The formation of methane has been proved by Fink (245, 246) and Deo et d. (247) by mass spectrometry to occur On acetone adsorption on alumina. This surface reaction thus lends some support to the assumption of basic OH- ions on the surface of alumina and titanium dioxide (see Sections IV.D.l and 2). Hair and Chapman (250) have proposed the use of hexachloroacetone as a probe molecule for the detection of electrondeficient sites. The infrared bands

SPECIFIC POISONING OF OXIDE SURFACES

233

observed between 1550 and 1700 cm-' had been assigned as carbonyl-stretching vibrations of coordination compounds of hexachloroacetone with Lewis sites of different strengths on alumina and silica-alumina surfaces. Tretyakov and Filimonov (219) later speculated that the trichloroacetate ion might have been formed by analogy with the adsorption of hexafluoroacetone. In fact, bands at 1630 and 1430 cm-' have very recently been detected on adsorption of hexachloroacetone on alumina (251). These bands must be assigned as the asymmetric and symmetric stretching vibrations of the trichloroacetate ion. The simultaneous formation of chloroform was checked by gas chromatography. The attack of a basic OH- ion according t o (C13C)2CO + [OH-],

[C~JCCOO-],+ HCCl3

(19)

is, indeed, highly probable in this case, since this type of reaction is well established in organic chemistry for the detection of CXJC

I1

0

groups and for the formation of chloroform. Thus, hexachloroacetone certainly cannot be utilized as a probe molecule or specific poison. Infrared spectra of nitriles adsorbed o n zeolites have been reported by Angel1 and Howell (252), by Karge (238), by Ratov et al. (239, and by Butler and Poles (253). Some Raman spectra are also available (254, 255). Besides H-bonding, the nitriles appear t o interact primarily with the exchangeable cations. Acetonitrile, CH3CN, has been used frequently. However, assignment of the u2 mode (C=N stretch) in coordination compounds is quite confused due to the occurrence of Fermi resonance between the v2 mode and the (u3 + v4) combination mode. This problem has recently been dealt with by Knozinger and Krietenbrink (255). Tretyakov and Filimonov (219) describe a coordinative interaction between benzonitrile and aprotic sites on magnesium oxide, and Zecchina et al. (256) came to the same conclusion for the adsorption of propionitrile, benzonitrile, and acrylonitrile on a chromia-silica catalyst. Chapman and Hair (257) observed an additional chemical transformation of benzonitrile on alumina-containing surfaces, which they describe as an oxidation. Knozinger and Krietenbrink (255) have shown that acetonitrile is hydrolyzed on alumina by basic OH- ions, even at temperatures below 100°C. This reaction may be described as shown in Scheme 2. The surface acetamide (V) is subsequently transformed into a surface acetate at higher temperatures. Additional reactions o n alumina are a dissociative adsorption and polymerizations (255) analogous to those observed for hydrogen cyanide by Low and Ramamurthy (258), and a dissociative adsorption. Thus, acetonitrile must certainly be refused as a probe molecule and specific poison.

234

HELMUT KNOZINGER

(V)

Scheme 2

However, these surface reactions can be suppressed by proper substitution of the methyl group by stronger electron-releasing groups. Thus, t-butylnitrile was shown to be adsorbed on alumina only through coordination bonds onto Lewis acid sites (beside weak H-bonding), and n o chemical transformations occurred at temperatures below 200°C (255). One can show that similarly suitable substitutions of ketones prevent the surface reactions observed with acetone (251). These compounds, t-butylnitrile and some alkyl-substituted ketones, are more weakly held on surface sites than pyridine and should, therefore, show an improved chemisorption specificity. They might be very useful complementary specific poisons for certain classes of reactions in which only soft reactants and products occur. Research in this field is presently in progress in the author's laboratories. G. ADSORPTION OF CARBON DIOXIDE

Carbon dioxide, C 0 2 , is a fairly small molecule with acidic properties, which has frequently been used as a probe molecule for basic surface sites and as a poison in catalytic reactions. As shown in the following, C 0 2 adsorption onto oxide surfaces leads t o a variety of surface species such as bicarbonates and carbonates that coordinate to surface metal ions in various ways. The type of the coordination influences the symmetry of these ligands so that different surface species held by distinct surface sites can be distinguished by means of their infrared absorptions (162). The characteristic infrared (and Raman) bands of COz and possible surface species are summarized in Table VI. The wave-number range below 1000 cm-' was usually not accessible in studies on adsorbed C 0 2 because of the strong absorption of the oxides at lower wave numbers. 1. Alumina

Infrared spectra have been reported for C 0 2 adsorbed o n y-A1203 by Little and Amberg (259),Peri (157,260,263),Fink (246,262), Yakerson er al. (264),

TABLE VI Infrared (and Raman) Bands and Assignments of COz and Carbonate Species

Carbonate ion (an )

Vibrational mode

53 t14

55

"6

1337 667 2349

1020-1090 860-880 1420-1470 680-750

Unidentate carbonate (cm )

Bidentate carbonate (cm -l)

''OrgaIliC" (bridging) carbonate (cm-l)

1300-1370 1040-1080 670-690 1470-1530 750-820 850-880

1590-1630 1020-1 030 660-680 1260-1270 740-760 830-840

1750-1870 1150-1280 -

Bicarbonate (cm-')

1290-1410 990-1050

0

E

1620-1660 695-705 830-840

h)

w

wl

236

HELMUT KNOZINCER

and Parkyns (261,263, on K - AO3 ~ by ~ G r e g and Ramsay (265), on x-Al2O3 by Parkyns (267), and on mixtures of various transition phases by Parkyns (266, 267). The spectra obtained by various authors differed considerably which was due to differences in the state of hydroxylation as shown by Parkyns (267). The structure of the solid phase, on the contrary, does not influence the spectra significantly since y-, x-, and a mixture of 6 - , x-, 9-,and K - A ~ all ~ Obehave ~ in much the same way (267). At least seven species of adsorbed C 0 2 have been detected, their formation being determined mainly by the heat pretreatment. As some of these species exist simultaneously on the Alz O3 surface and the infrared bands may be broad and ill-defined, their detection and the assignment of the respective bands may be ambiguous in some cases. After heat treatment at roughly below SOO'C, the alumina surface is still strongly hydroxylated and COz adsorption leads t o the formation of a surface bicarbonate ion predominantly. This species absorbs at 3605, 1640, 1480, and 1233 cm-' (262,264,266,267). On formation of this species, COz selectively reacts with the highest-frequency (3800-cm-') OH groups of the alumina surface, and no bicarbonate was formed when the respective OH groups were eliminated from the surface (266). It is assumed that the bicarbonate ion forms on an A1-OH pair site, which was called "X-site" by Fink (246,262). These sites allow for the formation of an intermediate species of COz held by the cation (266): o on on yo2 NC/

, Al\O/A1\

'

y

' 0 2

2

,AI,O,AI,

I

__t

0

I

(2 1)

,*l\o,A1,

The number of these sites, as measured by Fink (246,262),vary between 1.2 and 1.8 X 1013/cm2. This value nicely coincides with the number of sites that convert pyridine t o the pyridone species (see Section IV.D.l). Thus, the X-sites certainly contain reactive and strongly basic OH groups and they may be identical with the sites responsible for the pyridone formation and the hydrolysis of ketones and nitriles. The highest-frequency OH group vanishes preferentially in all these surface reactions. Because the X-sites are Al-OHpair sites created by the formation of oxide vacancies in the immediate vicinity of the reactive OH groups, the above result lends some support t o the interpretation of Dunken and Fink (116) that the reactive OH groups (3800 cm-') are surrounded by four oxide vacancies rather than Peri's (120) assumption that they are surrounded by four 02-ions (see Section IV.A.1). Rosynek ( 2 6 7 ~thinks ) that a free carbonate ion also exists on the surface and contributes t o the band at 1480 cm-' . On more extensively dehydroxylated alumina surfaces, pressure-dependent bands appear at 2340 t o 2370 cm-' (frequency increases as compared t o gaseous C02) and in the range 1780-1850 cm-' . Peri (157) assigned the band at 2370

SPECIFIC POISONING OF OXIDE SURFACES

237

cm-’ as the asymmetric C02-stretching mode of an undissociated COz molelinkage similar t o those cule, which is held by an a site (a strained Al-0-Al postulated for Si02-A120,) by ion-quadrupole interactions. There are 5 X 10l2 a sites/cmZ on a 7-Al2O3 surface after pretreatment at 800°C. Peri (157) assumed that this surface species could possibly be related through an equilibrium with an “organic” bridging carbonate type structure,

for which Parkyns (266) suggests a band pair to be responsible at 1850 and 1180 cm-’ . Two other bands at 1820 and 1780 cm-’ seem t o be related t o structures such as

&0 0 ’

‘0’

II

I

A1

0

and +

Al

respectively (266). If these structures are realistic, then they clearly indicate that COz does by no means interact selectively with basic surface sites. The chemisorption of C 0 2 on strongly dehydroxylated surfaces leads t o the formation of bidentate (1660 and 1230-1 270 cm-’ ) and unidentate (1 530 and 1370 cm-’) carbonate groups (259, 262,264,265, 267). Reactive oxide ions must be involved in this chemisorption step; the particular structure of the surface species formed is determined by the local environment of the coordinatively unsaturated oxide ion. Parkyns (267) argues that the carbonate formation might be most likely to occur, for example, with exposed oxide ions located in “steps” in crystal faces. According to the data of Gregg and Ramsay (265),only 1 in 10 oxide ions of a K - A ~ surface ~ O ~ after heat treatment at 1000°C is reactive. Although this number might be higher for more disordered surfaces after treatments at lower temperatures, Gregg and Ramsay’s results seem t o indicate that only a small percentage of the surface oxide ions are located in suitable environments for carbonate formation. Most probably oxide vacancies in the vicinity of the exposed anion are indispensable for the properties of the respective sites; this is quite evident in the case of the bidentate carbonate group. In fact, Schubart and Knozinger (60) have shown that pyridine preadsorption reduces the chemisorption of C 0 2 , particularly the bicarbonate formation and the formation of species that typically absorb in the 1800-1900 cm-’ region. Peri (267b) has tested the reactivity of the reactive 0’- ions by isotope exchange with C1802. The existence of 0 2 - ions of varying reactivity on a y-A1203 is clearly demonstrated by these experiments and different chemisorbed species of C 0 2 seem to be responsible for the 180-exchange in different temperature

238

HELMUT KNOZINGER

ranges. Again, the participation of exposed metal ions seems to be necessary in these exchange reactions, a conclusion that is in accord with the above mentioned reduction by pyridine presorption of the formation of the species that give bands i.n the 1800-1900 cm-’ range. It thus follows from the foregoing discussion that C 0 2 adsorption on alumina is a key compound for the study of the chemical nature of a variety of distinct surface sites. However, it is also apparent that unambiguous conclusions can hardly be drawn from COz -poisoning experiments due to manifold, simultaneously existing surface species. 2. Titanium Dioxides Carbon dioxide adsorption by highly dehydroxylated titanium dioxides gives rise to bands that are best ascribed t o a bidentate carbonate species. The corresponding band pairs were reported t o appear at 1580 and 1320 cm-’ by Yates (132) and O’Neill and Yates (268) and at 1584 and 1375 cm-’ by Primet et al. (176, 269) on anatase and at 1485 and 1325 cm-’ on rutile (132). These bidentate carbonate species are stable during pumping. Bicarbonate species were formed on OH-bearing surfaces on both anatase (176) and rutile (176,270). These species on Ti02 surfaces, however, behave differently from those on A1203 surfaces in that they are very labile and are destroyed even by mere pumping at 25°C. As shown by Jackson and Parfitt (270), the reactive OH groups that form the bicarbonate species on rutile selectively, are those that give rise t o the 0-H stretching band at 3700 cm-’ . This indicates the chemical identity of this OH group and of the sites t o which it belongs. One may probably assume that a mechanism similar to that on A l z 0 3 [Eq. (21)] is responsible for the bicarbonate formation on T i 0 2 , thus indicating again the importance of acid-base pair sites.

3. Supported Chromia and Unsupported a-Chromia Little and Amberg (259) reported bands at 1430 and 1750 cm-’ for C 0 2 adsorbed by a chromia-alumina catalyst, which they ascribed to a carbonate ion and an “organic” bridging carbonate species. A variety of bands were observed on a reduced chromia-silica catalyst by Zecchina et al. (271). These authors ascribed the bands in the 1350-1700 cm-’ region to carboxylate rather than carbonate species. They distinguished a strongly bonded C02- species (bands at 1600 and 1415 cm-’), a less strongly bonded C02’- (bands at 1630 and 1350 cm-’) that is formed without complete charge transfer, and very weakly bonded, quasi-neutral C0:’- (band at 1700 cm-’). Very complex spectra are obtained on C 0 2 adsorption by partially dehydroxylated a-chromia surfaces (272). Seven surface species have been identified by

SPECIFIC POISONING OF OXIDE SURFACES

239

Zecchina et al. (272), the most stable and important of which are a bicarbonate species, two types of bidentate bicarbonate groups in different environments, and a monodentate carbonate group. aChromia does not possess any adsorption capacity for C 0 2 if the surface is fully hydroxylated. This is a first indication that a coordinatively unsaturated cation must be involved in the bicarbonate formation beside a reactive OH group. The bicarbonate gives rise to infrared bands at 1620, 1430, and 1225 cm-' and is stable at room temperature but removed at 200°C. A mechanism similar t o that proposed for the corresponding surface reaction on alumina [Eq. (21)] was adopted by Zecchina et al. (272). The two bidentate carbonate species are assumed t o be formed on coordinatively unsaturated Cr3+ ions that do not bear any OH groups but only 02-ions in their coordination sphere (see Section IV.A.3). The coordinative heterogeneity is then responsible for the formation of carbonate ligands of different stability. The participation of the coordinatively unsaturated cations in this surface reaction is confirmed by the inhibiting action of O2 and pyridine in blocking the cations. A band pair at 1560 and 1340 cm-' is thus ascribed to the more strongly bonded species which is assumed t o be held by a site with two anion vacancies:

The less strongly held species that absorbs at 1580 and 1200-1325 cm-' , on the contrary, is assumed to be bonded according t o

The lower stability is due to the lower crystal field stabilization on passing from coordination 5 t o 6 than from 4 to 5. Finally, the monodentate carbonate species (bands at 1490 and 1365 cm-') may be formed on sites with the Cr3+ ion having coordination number 5 (only one anion vacancy):

aChromia seems to behave similarly t o alumina. Carbon dioxide chemisorption allows for the identification of distinct adsorption sites, but its applicability as a specific poison is at least questionable.

240

HELMUT KNOZINGER

4. Zincoxide The species formed on COz adsorption by ZnO unfortunately are not as welldefined as for the previously discussed oxides. Taylor and Amberg (273) reported the appearance of bands at 1618, 1431, and 1230 cm-' which most probably have to be ascribed t o surface bicarbonate species. No such bands, however, were observed by Matsushita and Nakata (274) and Atherton el al. (149). It may well be that the bicarbonate species on ZnO is very labile as observed for TiOz and that they were already pumped off before recording the spectra. This possibility was stressed by Borello (275). Morimoto and Morishige (276), however, have shown that the adsorption capacity of ZnO for COz increases linearly with decreasing number of surface OH groups. Very recent additional data of the same authors (276a) seem t o clearly rule out the formation of bicarbonate species on ZnO. This result suggests that most of the COz is not combined with OH groups on the ZnO surface. Atherton el al. (149) found the formation of carbonate ions which they claim t o result from the reaction of COz with a coordinatively unsaturated surface oxygen. Bands at 1640 and 1430 cm-' were observed by Matsushita and Nakata (274). The results of both research groups are consistent with the assumption of a bidentate carbonate ion as pointed out by Borello (275). It seems, therefore, likely that a Zn-0 cation-anion pair site is responsible for the carbonate formation, and these sites may be compared with the pair sites proposed by Dent and Kokes (148) for the Hz chemisorption on ZnO (see Section IV.A.4).

5 . Magnesium Oxide Magnesium oxide is usually considered as an oxide with predominantly basic surface properties (20). One would, therefore, expect COz t o interact specifically and strongly with basic surface sites. Infrared data are available for the interaction of COz with strongly dehydroxylated MgO (heat treatment at 850°C). A set of bands at 1665, 1005, 1325, and 850 cm-' were assigned the vl, v z , v4, and v 6 modes of a bidentate surface carbonate species by Evans and Whateley (277). Gregg and Ramsay (278) described two band pairs at 1670 and 1320 cm-' and at 1650 and 1280 cm-' ,the latter being observed by Evans and Whateley (277) at 1625 and 1275 cm-' as being caused by two energetically distinct bidentate carbonate species that have slightly different surface environments. The bidentate species is transformed into a unidentate carbonate (bands at 15 10 and 1390 cm-') when water vapor is adsorbed. A bicarbonate species is, however, formed when water is adsorbed first (277). It absorbs at 1655, 1405, and 1220 cm-' and is reversibly adsorbed at room temperature. The bicarbonate species is also observed on adsorption of COz onto hydroxylated MgO surfaces (279).

SPECIFIC POISONING OF OXIDE SURFACES

24 1

Assuming a (100) face to be exposed (see Section IV.A.5 and Fig. 3), one carbonate group is formed for 2.2Mg2+ ions (278). Thus, only every other coordinatively unsaturated surface cation bears a carbonate group. This result can be attributed to steric reasons and to the electrostatic repulsion of these anions. Slow formation of C 0 32- ions in a fairly symmetrical environment has also been observed a t higher pressures (278). The infrared bands of this species are similar t o those of bulk MgC03, the formation of which is probably restricted to the surface layers.

6 . Silica-Alumina and Zeolites Carbon dioxide adsorption on silica-alumina (after heat treatment above 600°C) produced an infrared band at 2375 cm-I (157). This band was assigned by Peri (157) t o a linear COz molecule being strongly held by an (Y site by ionquadrupole interaction. Strong COz adsorption was blocked by chemisorption of HCl and of 1-butene. Titration of the C 0 2 chemisorption sites (a sites) with HC1 and 1-butene gave site concentrations of 2 to 9 X 10” siteslcm’, depending on the heat pretreatment. Ammonia adsorption led to site concentrations that were 2-4 times as high as these values, indicating the lower specificity of NH3 adsorption on silica-alumina. Other surface species were not detected on COz chemisorption on silica-alumina. The adsorption of COz on X and Y zeolites also leads t o the formation of a species which gives rise to an increased v3 frequency and activation of the v2 mode. The position of the v3 mode is cation-dependent (280-282). Angel1 (283) was able to correlate the frequency of this vibration linearly with electrostatic field strength of the exchangeable cations. A linear configuration of COz was suggested for the respective species, the adsorption bond being due to ion-dipole interactions (284). Carbonate formation is also observed on zeolites (280-282, 285, 286), which strongly depends on the zeolite framework structure, on the type of exchanged cation, and the degree of exchange, and on the adsorption temperature. Two types of carbonate structures were reported t o exist on NaX zeolites. These absorb at 1700 and 1365 cm-’ and at 1485 and 1425 cm-’ , respectively, and were described by Jacobs et al. (282) b y structures VI and VII. In structure VI an O1 lattice oxygen is involved as well as a Na ion

(VI)

(VII)

located at a site III’, whereas the carbonate ion in structure VII is coordinated to a Na,,,g ion and seems to be located in a more symmetrical environment.

24 2

HELMUT KNOZINCER

Structure VI is transformed into the more stable compound VII. This transformation is restricted t o NaX zeolites. Although LiX and KX give rise to the formation of species VI at room temperature, carbonate structures were completely absent on CaX, SrX, and BaX at room temperature (280) but were formed at elevated temperatures (286). This behavior of various zeolites, which in detail depends on the degree of exchange, is explained by the location of the exchangeable cations on different sites in the zeolite framework (282). Unidentate carbonate species are formed on heating bivalent cation-exchanged Y zeolites in COz (286). The CaZ+ions are involved in the formation of this surface carbonate (281) in CaY zeolites. Jacobs et al. (281) have shown that lattice oxygen must be incorporated in COz t o form the carbonate on a dehydrated zeolite. Provided there are some residual water molecules retained in the zeolite, carbonate formation is explained by the following reactions: X zeolite, +

t

2H'

t

Y zeolite,

Here the third oxygen in the carbonate species comes from a water molecule (281, 282). Partially hydrated Ca ions o n sites I1 move to sites 111' in the case of the Y zeolite [Eq. (27)] before they are involved in the carbonate formation. The protons formed after dissociation of the residual water molecules lead t o an increase in intensity of the OH-stretching band at 3650 cm-' . The corresponding OH group is the most acidic one and gives rise t o formation of the PyH' species on pyridine adsorption (see Section IV.D.6). Reactions (26) and (27) may, therefore, account for the promotion effect of COz in various catalytic reactions (39; 40, 287). The existence o f reactive 0'- ions on amorphous silica-alumina and on zeolites containing different exchangeable cations has recently been demonstrated by isotope exchange with Cl8OZ by Pen (267b).

7. General Considerationson Carbon Dioxide Adsorption Carbon dioxide fulfills some of the relevant criteria and contradicts others. Evidently, although COz exhibits acidic properties, the adsorbed amounts cannot be taken as a measure of surface basicity; strong chemisorption of COz occurs through interaction with acid-base pair sites preferentially. Thus, specific poisoning of basic sites by COz chemisorption is not possible. Furthermore, a

243

SPECIFIC POISONING OF OXIDE SURFACES

sometimes rather large number of distinct surface species exist simultaneously on different sites. The identification of a catalytically active site by specific poisoning seems t o be hopeless in such a situation. The promotional effect of C 0 2 observed on zeolites hurts criterion h (see Section II.C.1). In conclusion, poisoning experiments that only report adsorbed amounts of COz and do not deal with the nature of the chemisorbed species must be looked at with reservation and their interpretation is usually ambiguous. Nevertheless, C 0 2 is an extremely valuable probe molecule because the infrared spectra of the chemisorbed species respond very sensitively t o their environments, Thus, the frequency separation of the typical band pairs of the carbonate structures may be taken as a measure of the local asymmetry at the chemisorption site. The application of I3C-FT-NMR should be extremely valuable for a still more extensive study of the nature of sites by COz adsorption. Due t o the very detailed information on the structure of sites on oxide surfaces that can be obtained by C 0 2 chemisorption studies, this compound should in some cases also be applicable as a specific poison. A very careful study of the type of interaction with the surface, however, has t o be undertaken for each particular system before any conclusive interpretation of poisoning experiments becomes meaningful.

H. ADSORPTIONO F ACIDS As COz has been shown t o be unsuitable for studying the basic properties of an oxide surface selectively, the further search for sufficiently unreactive probe molecules and specific poisons of small molecular size has still not been very encouraging. Schwab and Kral (288) have used the Lewis acid BF3 t o detect basic sites on the surface of pure and doped aluminas. The use of BF3 as a specific poison of basic active sites seems t o be unfavorable, since the promotion of catalytic activity and selectivity in acidcatalyzed reactions on silica-alumina and alumina by BF3 treatment is well known (289,290). Matsuura et al. (291) have in fact detected the creation of new strong acid sites on alumina after BF3 treatment. Infrared studies of the chemisorption of BF3 on alumina and silicaalumina surfaces have clearly shown that BF3 does not simply coordinate t o basic sites such as OH groups or oxygen ions or atoms in surface M-0-M' linkages (M and M' are Si or Al), instead surface chemical reactions occur between the acid and the surface sites. The infrared data were interpreted most satisfactorily by Rhee and Basila (292) suggesting a scheme of seven reactions: H BF3 + [MOHl,--+[MO:BF3],~[M-G-BF21,

HF+ [ M O H ] , ~ [ M F ] , + H 2 0

+ HF

(28a) (28b)

244

HELMUT KNOZINGER

L ' I F

[MOBF2],+ [M'OH],-

,+ H F

-0-B-0-M'

(28d)

F [MOBF2], + [MOM'],+

r e 1 M

[M202BF],+ [M'OH],

_ I )

[M202BF], + [MOM'],

__*

[a] M-0-B-0-M

s + [M'F],

(28g)

Water and HF could be detected in the gas phase and the infrared spectra were consistent with the formation of -0-BF2and

-o\

BF

' 0 -

surface compounds. The compounds BC13 (292) and BH3 (293) behave completely analogously. At higher BF3 pressures than used in the infrared studies of Rhee and Basila (292) and at elevated temperatures, additional reactions occur with the formation of volatile SiF4 and (BOF)3 on silica and kaolin, as proposed by Baumgarten and Bruns (294). In view of the complexity of these systems and because the boron trihalides completely alter the surface properties of oxide catalysts, these compounds cannot be used as specific poisons. The adsorption of formic acid and acetic acid leads to the formation of carboxylate groups on aluminas (194, 295-299), titanium dioxides, (134, 1 3 3 , 176, 194, 300, 301), chromium oxide (134, 302, 303), zinc oxide (298, 304306), and magnesium oxide (299, 304, 306). The corresponding dissociative chemisorption step most probably takes place on acid-base pair sites of the type

M

/O\

M

In the case of ZnO and MgO, formic acid is even absorbed in the bulk forming a bulk phase of metal formate on top of the underlying oxide (304). Furthermore, OH groups newly formed by the dissociative chemisorption step may be

SPECIFIC POISONING OF OXIDE SURFACES

245

strongly acidic and provide acidic protons as active sites for acid-catalyzed reactions. In fact, Tamaru and co-workers (296-298) have conclusively shown that formic acid decomposition at low temperatures on Alz03is catalyzed by such acidic protons, which result from dissociative adsorption of HCOOH in an induction period. Similar results have been reported for the HCOOH decomposition at low temperatures on rutile by Munuera (300) and Trillo er al. (307). Thus, the adsorption of carboxylic acids may in some cases alter the surface properties of an oxide dramatically since the acid delivers acidic protons rather than block a basic site. It should be remembered in this connection that neither A1203 nor Ti02 provide acidic protons as active sites in the absence of carboxylic acids. These compounds are, therefore, not generally suitable as specific poisons of basic sites. Primet er al. (176) have shown that phenol also adsorbs dissociatively on dehydroxylated T i 0 2 , but it does not react with OH groups. Thus, again there might be some danger of the newly formed OH groups providing acidic protons. Phenol will also detect acid-base pair sites that are able t o dissociate the molecule but will less probably interact with purely basic sites. Furthermore, phenol as well as carboxylic acids may undergo catalytic transformations with the reactants or products under investigation. Examples are given in Section V.A. I. ADSORPTION OF ELECTRON-DONOR AND ELECTRON-ACCEPTOR MOLECULES It is now well established that a variety of organic molecules such as polynuclear aromatic hydrocarbons with low ionization energies act as electron donors with the formation of radical cations when adsorbed on oxide surfaces. Conversely, electron-acceptor molecules with high electron affinity interact with donor sites on oxide surfaces and are converted to anion radicals. These surface species can either be detected by their electronic spectra (90-93, 308-310) or by ESR. The ESR results have recently been reviewed by Flockhart (311). Radical cation-producing substances have only scarcely been applied as poisons in catalytic reactions. Conclusions on the nature of catalytically active sites have preferentially been drawn by qualitative comparison of the surface spin concentration and the catalytic activity as a function of, for example, the pretreatment temperature of the catalyst. Only phenothiazine has been used as a specific poison for the butene-1 isomerization on alumina [Ghorbel e l al. (312)]. Tetracyaonoethylene, on the contrary, has found wide application as a poison during catalytic reactions for the detection of active sites with basic or electron-donor character. This is probably due to the lack of other suitable acidic probe or poison molecules. There exists still some controversy as to the nature of the electron-acceptor sites on oxide surfaces that lead t o the formation of radical cations. Various

246

HELMUT KNOZINCER

authors have described this process as being due to an electron transfer to a Lewis acid site (194, 313-316). Fog0 (317) and Hirschler and Hudson (318), on the contrary, assume that the oxidation occurs through molecular oxygen catalyzed by a Brdnsted site or that the electron is accepted by a surface proton, this process being catalyzed by molecular oxygen (319). Molecular oxygen seems in fact to play an important role in the radical cation formation in many cases, as shown by Hall and Dollish (320) and by Porter and Hall (310). On alumina, in particular, molecular oxygen seems to be required for the radical cation formation (321), whereas on silica-alumina two types of oxidizing sites may exist, one that involves molecular oxygen and another that does not (321). Flockhart et al. (321) suggested that the aromatic hydrocarbon molecule is adsorbed by a Lewis site (abnormally coordinated aluminum ion) of the alumina surface. The electron affinity is too low to abstract an electron from the hydrocarbon, but the energy levels of the donor molecule may be significantly altered. In consequence, an electron transfer may occur in the presence of molecular oxygen t o this acceptor molecule whose energy levels may also be suitably altered. It has been shown by Knozinger and Miiller (322) that the spin concentration of the perylene cation on alumina surfaces can be reduced significantly by pyridine adsorption. This result also suggests the participation of Lewis sites in the radical cation formation. Tricoordinated aluminum ions have been considered as the oxidizing sites on silica-aluminas (321) and on zeolites (323325), although multivalent exchangeable cations also act as electron-acceptor sites in zeolites (325-327). Muha (3270) concluded from his ESR results that the perylene cation on a silica-alumina surface is relatively unrestricted in its motion. Consequently, the counterion may also be mobile on the surface. This conclusion is consistent with the assumption of molecular oxygen being the acceptor site and lends some support to the existence of one type of site on the silica-alumina surface that involves molecular oxygen. Hoang-Van and co-workers (328) studied radical cation formation on an amorphous alumina using phenothiazine as the donor molecule. The spin concentration (Fig. 5) shows two maxima at about 470" and 600°C and a third increase at around 800°C as a function of the heat treatment of the alumina. This behavior correlated qualitatively with the isomerizing activity of the catalyst (329) and was taken as evidence for the creation of different types of acceptor sites on the surface. Incompletely coordinated A13+ ions were assumed t o be responsible for the first maximum. It should be remembered in this connection that a maximum can only be formed if the electron affinity of the acceptor sites is sensitive toward the ligand heterogeneity of the respective sites. The ligand type of the incompletely coordinated A13+must change from mainly OH t o mainly 02-in the temperature range between 350 and 550°C. The second maximum at 600°C was attributed to the appearance and healing of surface defects with more than one A13+in the immediate vicinity.

SPECIFIC POISONING OF OXIDE SURFACES

24 7

40'7 2 E

em a kl

1'

.-C a v)

O'

Lbo

5do

600

760

oc'

Activation temperature

FIG. 5 . Spin concentration of phenothiazine cations formed on amorphous A1203 as a function of the activation temperature. [Reproduced with permission from Hoang-Van et aJ. (328).]

Tetracyanoethylene (TCNE), tetracyanoquinodimethane (TCNQ), and various mono-, di-, and trinitroaromatic compounds are the preferred electron-acceptor molecules for the detection of donor sites on oxide surfaces. Mostly TCNE has been used as a poison in catalytic research. Electronic and ESR spectra of the adsorbed acceptor molecules are characteristic of the surface anion radicals which are assumed to be formed according to TCNE+[D], * [ T C N E - . . . * D + l s

(29)

The nature of the donor site D depends on the type of oxide and its pretreatment temperature for pure oxides and, additionally, on the composition in the case of mixed oxides. The radical anion formation from TCNE (electron affinity 2.89 eV) on aluminas occurs on extraordinarily coordinated hydroxide ions on hydroxyl-rich surfaces, whereas exceptionally coordinated 0'- ions play the role of the donor sites on more strongly dehydroxylated surfaces (328, 330). Accordingly, as the chemical nature of the donor site changes with the degree of surface hydroxylation, the spin concentration of the anion radical passes through two maxima: the first is located between 400" and 500°C (OH- donor sites), and the second (brought about by the 0'- ions) is between 600" and 700°C (328, 331). Trinitrobenzene (TNB) (electron affinity 1.O eV) is a weaker electron acceptor than TCNE and interacts only with the 0'- sites (332), thus acting more selectively than TCNE. On titanium dioxide and magnesium oxide, the spin concentration analogously passes through two maxima as a function of the dehydroxylation temperature, and weakly coordinated OH- and 0'- ions are considered as the donor sites (48,

24 8

HELMUT KNOZINGER

333). On zinc oxide, on the other hand, the donor sites have been associated with Zn' ions or oxygen ion vacancies with trapped electrons (334). Silica-aluminas also develop reducing activity; the reducing power of the corresponding donor sites increases with increasing Al content (332). The OHions are responsible for the reducing activity of HY zeolites after activation at around 250°C (33.9, whereas electronegative and electropositive sites are created at higher activation temperatures of around 650°C. In conclusion, TCNE and probably also TNB are suitable probe molecules for the detection of electron-donor sites, and they may be used as specific poisons in catalytic reactions. The molecular size of these molecules may be a disadvantage. However, the concentration of the donor sites is extremely low and amounts t o a typical order of magnitude of less than 10'6-10'7/m2. The reducing sites are, thus, most probably sufficiently separated from each other so that the number of spins per unit surface area is in fact a good measure of the number of sites. In poisoning experiments, there is still some danger that sites located near the reducing sites are shielded for steric reasons. The sites detected by the TCNE and TNB are certainly to be considered as basic beside their reducing properties. There may, however, exist still other purely basic sites that are not detected by electron-acceptor molecules. The nitroaromatic compounds appear not to undergo any other chemical transformation than the electron transfer reaction (332). Tetracyanoethyne also seems t o be comparably stable. Ghorbel and co-workers (312) used TCNE as a poison on alumina at fairly high temperatures and claim that the compound and the respective radical anion is stable even at 450°C. However, there is some danger of chemical transformations of TCNE on strongly basic surfaces, on which a hydrolysis may occur to form tricyanoethenol which may then undergo secondary reactions with additional TCNE (336). Furthermore, one should keep in mind that the interactions of electronacceptor molecules are most probably more complex than usually assumed. As pointed out by Kern (336), all generally applied acceptor molecules bear electron-rich functional groups at the periphery. Due to their molecular size, these molecules can, therefore, interact simultaneously with the electron-donor site and electron-deficient sites. These interactions may mutually influence each other and determine the strength of interaction. The inhibiting effect of ammonia (312) and pyridine (322) on the radical anion formation from TCNE may be an indication of such complex interactions. The existence of interdependent electron-donor and -acceptor sites on various surfaces has been demonstrated by Flockhart and co-workers (37,38,332,335). These authors have shown that the spin concentrations of perylene are increased on surfaces that are precovered with TNB, and vice versa. Up t o tenfold enhancement of the reducing activity of a zeolite sample was observed when electron-donor molecules are preadsorbed o n the surface (38). These results

SPECIFIC POISONING OF OXIDE SURFACES

249

show that on surfaces of oxides and zeolites, electron-donor and -acceptor sites exist in close proximity and that they are interdependent.

V. Specific Poisoning on Alumina Surfaces Two reactions for which specific poisoning experiments have contributed to the elucidation of the reaction mechanisms and permit evaluation of the possibilities and pitfalls of the technique are discussed as examples in this section. The first example is the dehydration of alcohols on alumina catalysts, and the second, the isomerization of olefins on the same type of catalyst. A. DEHYDRATION OF

ALCOHOLS ON

ALUMINA

Various reviews have appeared in the past dealing with the dehydration reaction of alcohols (27, 28, 337-339). The elimination of water from aliphatic alcohols on alumina is known to proceed through two possible routes, namely, monomolecular olefin formation,

and bimolecular ether formation, 2ROH

ROR + HzO

Reaction temperature and alcohol structure are the main factors determining the reaction route. Thus, methanol and ethanol are the most prominent examples for ether-forming alcohols, whereas increase in chain length and, much more pronounced, chain branching and increasing reaction temperature act in favor of the monomolecular elimination reaction (28, 340). Typical examples of olefin-forming alcohols are t-butanol and isobutanol. Poisoning experiments have been carried out with the aim to determine the chemical nature of the active sites and of the reaction intermediates. The two reaction routes will be treated separately in the following. 1. Olefin Formation

Early poisoning experiments using nitrogen bases such as ammonia, pyridine, and piperdine have shown that the secondary isomerization of the primary olefinic products can be completely suppressed, whereas the dehydration activity of the alumina catalyst was only slightly influenced by these poisons (30,31,341344). This is a typical example of selective poisoning, where a consecutive reac-

250

HELMUT KNOZINGER

tion step is suppressed. The general conclusion from these results was that two different types of active sites were responsible for the olefin isomerization and the alcohol dehydration. It was argued that the dehydration reaction occurs on only weakly acidic or even nonacidic sites. However, a detailed description of the active sites was still not possible from these experiments, the more so as the poison was fed over the catalyst together with the reactants. Similar experiments were carried out by Jain and Pillai (345) who tried to test the participation of basic sites by using phenol and acetic acid as poisons. The addition of small amounts of phenol (5%) t o the reaction feed led t o a spectacular increase in the rate of olefin formation. Particularly in the case of t-butanol the rate increased by a factor of about 3. Jain and Pillai explained this effect as being due t o an increase in acidity of the surface. Strong intrinsic acid sites have already been excluded as active sites by pyridine poisoning, and the presence of intrinsic Brgnsted sites on alumina is disregarded by the infrared spectra of adsorbed pyridine (see Section IV.D.l). The promoting effect of phenol is, therefore, most likely to be due to newly formed acidic protons that result from a dissociative adsorption of the phenol, as described in Section 1V.H. The active participation of these protons must consequently influence not only the catalyst activity but also the reaction mechanism. The rate of olefin formation is retarded with further increasing phenol concentration, and this effect is due to the competitive formation of alkyl phenyl ethers. Similar phenomena were observed when acetic acid was used as a poison. Alkyl acetates were formed-a competition reaction that usually led to a strong decrease of the rate of olefin formation. These examples nicely demonstrate that phenol and acetic acid cannot be used as suitable specific poisons according to the criteria put forward in Section II.C.l. Poisoning experiments with varying amounts of preadsorbed pyridine have recently been carried out by Knozinger and Stolz (47). Pyridine is solely held by Lewis acid sites under the experimental conditions as shown by infrared spectroscopy. The rate of isobutylene formation from t-butanol was essentially independent of the degree of poisoning, and the true activation energy of the reaction remained constant at 25 kcal/mole, when the number of preadsorbed pyridine molecules varied between 3 and 9 X 10"/mZ. It thus, appears that Lewis sites which retain pyridine at temperatures between 550' and 15OoC, respectively, do not interfere in this reaction. In the case of isobutanol dehydration, a promotional effect is observed (47). Isobutanol forms a surface carboxylate under reaction conditions (344, and this surface species gives rise to a typical symmetric COO--stretching vibration at 1567 cm-' . The CH-stretching vibration of the methylene group of isobutanol at 2870 cm-' disappears on formation of the oxidized species. Consequently, the intensity of the 1567-cm-' band can be taken as a measure of the surface concentration of the carboxylate species, whereas the intensity of the 2870-cm-' band represents the surface concentration of molecular alcohol. The concentra-

SPECIFIC POISONING OF OXIDE SURFACES

Pyridine coverage (mg/g)

25 1

Integrated band area

FIG. 6. FIG. 7 . FIG. 6. Adsorption of isobutanol on pyridine-poisoned b - A l 2 0 3 . Integrated band areas (I.b.a., arbitrary units) of bands at 2870 and 1567 cm-l as a function of pyridine coverage. [Reproduced with permission from Knozinger and Stolz (43.1 FIG. 7. Rate of olefin formation from isobutanol at 220°C on 6-Alz03 [expressed as olefin pressure (mm Hg) formed at constant contact time] versus integrated band area (arbitrary units) of band at 2870 cm-'. [Reproduced with permission from Knozinger and Stolz (43.1

tions of these surface species can be altered by the number of preadsorbed pyridine molecules. As shown in Fig. 6 , the number of molecularly adsorbed isobutanol molecules increases as the surface concentration of the carboxylate species is reduced by pyridine preadsorption. The rate of isobutylene formation increases roughly linearly with the surface concentration of the molecular form of adsorbed isobutanol as shown in Fig. 7. Thus, again the Lewis sites can be excluded as active sites in the olefin formation. These sites are blocked either by the carboxylate species or by the preadsorbed pyridine. However, the carboxylate formation binds a surface oxygen additionally, which remains accessible as an adsorption site for the alcohol when carboxylate formation is suppressed by preadsorbed pyridine. This leads t o the increased surface concentration of the molecular form of adsorbed alcohol and to the increased rate of olefin formation. The same surface oxygen may also act as a basic site for the 0-hydrogen abstraction from the carbon skeleton of the alcohol. The participation of basic sites as active sites in the reaction has in fact been tested by Figueras Roca and co-workers (346) by specific poisoning of alumina catalysts with TCNE. The olefin formation from iso-propanol was strongly reduced by preadsorbed TCNE. It could furthermore be shown that OH groups also participate in the dehydration reaction; oxygen ions and OH groups in suitable arrangements and configurations, therefore, appear to form the active sites in olefin formation. The molecular form of adsorbed alcohol in which the reaction is initiated should be a H-bonded molecule (347). This assumption is in agreement with the effect of pyridine on its surface concentration, and infrared spectroscopy has shown (348) that the preferred H-bonded structure of alcohols on alumina is a species in which an 0'- ion of the surface is the acceptor.

252

HELMUT KNOZINGER

Relying on this adsorption structure, a model mechanism has been put forward by Knozinger and co-workers (349, 350). The activation is assumed to be initiated by proton fluctuations between adsorbed alcohol molecule and surface which may result in polarization of the molecule. The alcohol molecule itself is suggested t o possess some vibrational or rotational freedom relative t o the surface so that the 0 proton may approach a basic 02ion while the alcohol is in the necessary antiperiplanar conformation. Specific poisoning studies with pyridine and TCNE have led to this picture that excludes the participation of Lewis acid sites in the reaction. It should, however, be mentioned that this interpretation of the reaction mechanism is not completely undisputed. Thus, Soma and co-workers (351) conclude from their dynamic treatment that a surface alkoxide species is the intermediate in olefin formation. Since the alkoxide is formed by dissociative adsorption on Al-0 pair sites, one would expect a strong poisoning effect by pyridine. Bremer and co-workers (352,353) propose a mechanism in which a coordinately held alcohol molecule , H

0

/R

I

0-AI-0

I

is assumed as being the adsorbed species responsible for olefin formation. These authors based their conclusions particularly on Na+ poisoning experiments. However, there seems to be some danger of a serious surface reconstruction during the poisoning procedure so that a direct comparison of Na+-poisoned and unpoisoned catalysts may be difficult. More detailed studies seem to be necessary t o solve these discrepancies. 2. Ether Formation It has been shown that only those alcohols that form detectable surface alcoholate species on alumina undergo bimolecular dehydration with ether and water as reaction products (340). Thus, ether formation is the dominant reaction direction of methanol and ethanol at low temperatures, and the tendency toward ether formation is reduced as the chain length increases and chain branching occurs (28, 340). The same trends are observed for the stability and surface concentrations of the surface alcoholate species (27, 28, 47, 340). Alcoholate formation is due t o a dissociative chemisorption step of the alcohol that occurs on 4-0 pair sites (47,340,354-358). One is, thus, led to the conclusion that ions are both important sites in the incompletely coordinated A13+ ions and 02ether formation from alcohols and that their participation should be detectable by specific poisoning. Jain and Pillai (345) have shown that the ether formation from methanol, n-propanol, and isopropanol is inhibited when phenol and acetic acid were

SPECIFIC POISONING OF OXIDE SURFACES

25 3

added to the feed. Alkyl phenols and alkyl phenyl ethers in the presence of phenol and alkyl acetates in the presence of acetic acid were observed in fairly high yields, indicating again that the poison undergoes chemical transformations and actually interferes as a main reactant in the system. Phenol and acetic acid are, therefore, not suitable as specific poisons according to the definitions and criteria put forward in Section II.C.l. However, Jain and Pillai (345) were led to a probably very important conclusion from their results insofar as they assume that two chemically distinct surface species-one an alcoholate, the other an H-bonded species-are reacting with each other t o form an ether. This reaction was visualized as a nucleophilic displacement reaction, and a condensation reaction between two chemically identical alcoholate groups was thus ruled out. Parera and his co-workers (359-362) have studied the poisoning effect of amines, pyridine, phenol, and acetic acid. A reduced rate of ether formation from methanol at the standard temperature of 230°C was observed when the poisons were present in the feed. In most cases the original activity was recovered, although rather slowly. Most probably the poisons were either displaced by alcohol and/or water or removed from the surface by chemical transformations. Figueras Roca and co-workers (346) have used preadsorbed TCNE t o poison the basic sites specifically. The rate of ether formation from methanol and ethanol responded very sensitively to the poisoning with TCNE, so that the participation of basic sites in the bimolecular alcohol dehydration seems to be proved. The active participation of coordinatively unsaturated A13+ ions could be demonstrated by poisoning experiments with preadsorbed pyridine (4 7). Pyridine was held by coordination bonds under the experimental conditions and influenced the surface concentration of a surface ethanolate as shown by infrared spectroscopy. The integrated band intensity of a typical band at 11 14 cm-* could be taken as a measure of the surface concentration of the alcoholate groups. The rate of diethyl ether formation of these pyridine-poisoned alumina catalysts was directly proportional to the number of alcoholate groups in the surface. The straight line that was obtained does not pass through the origin, indicating that a certain fraction of the alcoholate groups were not reactive. The others appear to behave energetically uniformly. The proportionality of the rate of ether formation and the surface density of alcoholate groups is in favor of the participation of only one alcoholate group per reaction step, the second reaction partner being assumed t o be a H-bonded alcohol molecule. This picture agrees with the mechanism proposed by Jain and Pillai (345) and with the relative reactivity of a series of substituted alcohols (363). Furthermore, from pyridine poisoning studies, one estimates an alcoholate density of an order of magnitude of 10"/m2. This corresponds to one alcoholate group per 1000 A*. These groups seem, therefore, to be so strongly separated from each other that a condensation reaction appears to be highly improbable. Poisoning experiments have thus shed some light on the chemical nature of the

254

HELMUT KNOZINCER

active sites in the ether formation. The importance of AI-0 pair sites seem t o be undisputed today, although some authors still prefer a condensation mechanism for the ether formation (351). B. ISOMERIZATION AND EXCHANGE REACTIONSON ALUMINA Partially dehydroxylated alumina surfaces are able to activate C-H bonds in saturated and unsaturated hydrocarbons. Aluminas are, therefore, active catalysts for double bond and cis-trans isomerization reactions and also for exchange reactions such as D2 exchange with hydrocarbons or deuterium scrambling (e.g., C6H6/C6D6 or CH4/CD4). The behavior of aluminas in these reactions turned out t o be extremely complex, and a number of chemically distinct sites have been postulated. Although the problem of the nature and concentration of active sites on aluminas for the different reactions is still far from being resolved, specific poisoning experiments have shed some light on the nature of the peculiar surface sites of aluminas. In most cases the technique proposed by Larson and Hall (364) that counts the number of posion molecules by stepwise desorption of the preadsorbed poison at increasingly higher temperatures has been applied (see also Section II1.B). This is certainly the most adequate technique, although some authors (32, 33) have undertaken poisoning experiments by introducing certain small amounts of poison into the reaction mixture or by measuring the conversion in a pulse reactor after injection of successive small doses of the poison (365). This last technique is certainly inadequate since a homogeneous poisoning cannot be attained, rather the catalyst is poisoned layer by layer for successive pulses. Carbon dioxide poisoning has been applied for the identification of active sites of exchange reactions, such as CH4 t Dz and CH4 t CD4 (364), ortho-para Hz conversion and H2-D2 exchange (366), exchange of olefins with D2 (32,33, 367), and exchange of benzene with D2 (368). The standard activation temperature of the ?)- and y-A1203 mixtures used in these studies was 53OoC. The problems that arise from the use of C 0 2 as a poison are clearly seen if one is concerned with the interpretation of data and the description of the nature of the active sites. The exchange reactions were all strongly influenced by the COz chemisorption, and Hightower and co-workers (32, 33, 368) suggested those sites as active sites that give rise t o the infrared band at 1780 cm-' on adsorption of C 0 2 . This band was ascribed to a linear form of chemisorbed C 0 2 held by especially exposed A13+ cations (see Section IV.G.l). Later the same authors (369) and Rosynek (267a) preferred a band at about 1480 cm-' as indicating the COz chemisorbed species that blocked the exchange sites. This band, however, probably arises from an uncoordinated carbonate ion, so that it is hardly significant for any particular surface site other than a reactive 02-ion. The numbers of sites active in the exchange reactions as determined by C 0 2

SPECIFIC POISONING OF OXIDE SURFACES

255

poisoning vary between 2-4 X 10'2/cm2 for the exchange of methane with D2 (364) and about 1.4 X 10'3/cm2 for the exchange of olefins with Dz (367). According to Hall and co-workers (364, 366, 367), the low site density and the strength of interaction of the COz chemisorption suggest that a low probability surface configuration is required of these sites, e.g., a multiple vacancy, a cluster of oxide ions, or a combination of both. Since COz still chemisorbs to a large extent as a surface bicarbonate species after activation of alumina at 530"C, the active site for the exchange reaction was assumed additionally to involve an adjacent hydroxyl group (364, 366), although the bicarbonate appears to be very labile and to decompose on evacuation already at temperatures below 100°C (60, 2 6 7 ~ ) . This type of active site has been schematically pictured (364) as H

o,A,fo

I

A similar type of site has been postulated by Knozinger and co-workers (370373) for the double-bond isomerization of olefins on an r)-Alz03that was activated at 300°C. These two types of sites, however, cannot be identical, since Hightower and Hall (367, 374) have shown that under their conditions Dz exchange and the intramolecular double-bond isomerization of olefins are independent processes, whereas under the conditions used by Knozinger and coworkers (371-373) D2 exchange only occurred through the intermolecular isomerization step. In particular, Hightower and co-workers (32,33)have shown that the sites that catalyze the Dz exchange with olefins are different from those that are active in the double-bond shift. The exchange sites are blocked by C 0 2 , but the isomerization sites remain unaffected by COz chemisorption on aluminas that were activated at 530°C. Peri ( 1 5 7 , 3 7 9 , on the contrary, has shown that C 0 2 adsorbs onto 5-8 X 10l2 OL sites/cm2 on his y-Alz03 after activation at temperatures above 600°C. This COz species gave rise t o a band at 2 3 7 0 cm-' , was readily desorbed by evacuation at lOO"C, and was displaced by butene. Peri has associated these sites with the isomerization of olefins. Rosynek er al. (32) reported that neither ammonia nor pyridine had a significant poisoning effect on the double-bond shift or cis-trans isomerization of butenes. This result appears to be quite questionable, since the retarding effects on the double-bond shift of ammonia (312, 3 76), triethylamine (365), and pyridine (377, 378) have clearly been demonstrated. Ghorbel and co-workers (329) have shown that the dependence of the rate of 1-butene isomerization at 260°C on the activation temperature of an amorphous alumina shows two maxima near 470" and 650OC. This behavior nicely parallels the surface concentration of the cation radicals formed on PhTh adsorption (see Fig. 5 ) as well as that of the anion radicals formed on TCNE adsorption (see

256

HELMUT KNOZINGER

Section IV.1). Both compounds, when used as poisons for the 1-butene isomerization at 260°C on aluminas activated at 470' and 650"C, effectively reduced the catalyst activity (312). Ghorbel and co-workers, therefore, suggested that the corresponding oxidizing and reducing sites are active sites in the double-bond shift. A certain fraction of chemisorbed ammonia and chemisorbed acetic acid was also shown to block the active sites involved in the double-bond shift (312). It was, therefore, assumed that the oxidizing sites should simultaneously exhibit Lewis acid character and that the reducing sites should act as basic sites. The active sites for olefin isomerization, as proposed by Ghorbel and co-workers (312), are, thus, complex multicenter sites in which acid sites with oxidizing character and basic sites with reducing character are simultaneously involved. This model seems t o be plausible on a qualitative basis. Some doubts, however, may arise when one compares the numbers of chemisorbed species as given for the various poisons at a temperature of 260°C on alumina activated at 470°C (312). About 2 X 10" PhTh' cations, 4 X 10" TCNE- anions, and 2.6 X 1014 chemisorbed acetic acid molecules were detected per square centimeter. These numbers differ by orders of magnitude, and, regarding the size of these poison molecules, it cannot be possible that more than one radical ion is formed per multicenter site active in olefin isomerization. The picture of these multicenter sites, as proposed by Ghorbel and co-workers (312), therefore, only holds if one considers the lowest number of sites detected (i.e., 2 X 10" oxidizing siteslcm') as an upper limit of the total number of active sites; the other poisons must then be adsorbed on these active multicenter sites and, additionally, on many other sites nonselectively. It is interesting t o note that the cis-trans ratios obtained by Ghorbel and coworkers (312) on 1-butene isomerization were mostly unaffected by the chemisorption of the different poisons and corresponded to the thermodynamically determined ratio. The same result was obtained by Knozinger and Aounallah (377), when 1-butene was isomerized in a recirculating reactor at 120°C on r)A 1 2 0 3 (activated at 500°C) that was partially poisoned by pyridine. In microcatalytic pulse experiments, on the other hand, one observes kinetically determined primary product distributions with cis-trans ratios of about 2 (370). Different types of active sites appear, therefore, t o be involved in the doublebond shift and in the cis-trans isomerization , the latter remaining unaffected by pyridine chemisorption. The existence of various types of active sites all involved in the isomerization of olefins-their creation depending on the activation temperature-has already been postulated (365, 373, 379). Recent poisoning experiments with pyridine on r ) - A 1 2 0 3 (activated at 500" and 600°C) seem t o give some evidence for the existence of at least two chemically distinct active sites. Their true nature is still obscure (378), but that they are blocked by pyridine indicates that Lewis sites should be involved. It had been shown that the r ) - A 1 2 0 3 was deactivated

SPECIFIC POISONING OF OXIDE SURFACES

0

1

2 3 L Pyridine molecules

5

6

257

7

xIOl7 per m2

FIG. 8. (a) Deactivation of q-AI203 (activated at 500°C) at 80°C for successive pulses of 2,3-dimethyl-l-butene. (b) Relative conversion (with respect to unpoisoned catalyst) of 2,3-dimethyl-l-butene at 8OoC for Fist pulse ( 0 ) and final activity ( 0 ) as a function of pyridine coverage.

during successive pulses in microcatalytic runs, when the double-bond shift of 2.3-dimethyl-1-butene was studied at 80°C (373). This deactivation process (Fig. 8a) can be interpreted by assuming two types of active sites. Type A sites are predominating on the fresh catalyst, but they are blocked by self-poisoning during successive pulses; type B sites remain active and are responsible for a final, nearly stable activity. The chemisorption of pyridine appears t o affect the A and B sites in a different manner, as shown in Fig. 8b. On adsorption of 3 X 10'' pyridine molecules/m2, the conversion in the first pulse is reduced by only lo%, whereas the final activity drops to 50% of the value on the untreated catalyst. The final conversion amounts t o only 10% of the initial value on adsorption of 4 X 1017 pyridine molecules/m2 whereas the conversion in the first pulse is still as high as 50% of that obtained for the unpoisoned catalyst. Apparently, the B sites are blocked preferentially by pyridine and they should, therefore, involve the most acidic Lewis sites. The lethal dose of pyridine is approx. 6 X lo" molecules/mZ. A value of 8 X l O I 7 pyridinelm' was obtained for the isomerization of 1-butene at 120°C on the same type of catalyst (377). In summary, various results have been obtained by poisoning experiments, which, however, still d o not give a clear picture of the active sites. This may be

258

HELMUT KNOZINGER

due t o the particularly complex behavior of alumina as catalyst for exchange and isomerization reactions as well as to the complex interactions of the applied poisons with the surface of aluminas. This second point is particularly true for C 0 2 . Nevertheless, it is felt that the results already obtained are encouraging, although much remains t o be done. The spectroscopic study of the adsorption of olefins on poisoned surfaces in connection with TPD and tracer experiments seem to be promising if they are carefully carried out on aluminas at different states of hydroxylation. Such studies should be even more informative if more than one poison is used.

VI. Conclusions The aim of specific poisoning is the determination of the chemical nature of catalytically active sites and of their number. The application of the HSAB concept together with eight criteria that a suitable poison should fulfill have been recommended in the present context. On this basis, the chemisorptive behavior of a series of hard poisoning compounds on oxide surfaces has been discussed. Molecules that are usually classified as soft have not been dealt with since hard species should be bound more strongly on oxide surfaces. This selection is due t o the very nature of the HSAB concept that allows only qualitative conclusions t o be drawn, and it is by n o means implied that compounds that have not been considered here may not be used successfully as specific poisons in certain cases. Thus, CO (145, 380-384), NO (242, 381, 385-392, 398), and sulfur-containing molecules (393-398) have been used as probe molecules and as specific poisons in reactions involving only soft reactants and products (32,364,368). The HSAB concept is recommended as a convenient guideline for the selection of potential poisons and the validity of the criteria has to be tested in each particular case. These criteria are probably rather restrictive and can hardly ever be fulfilled simultaneously by a single poisoning compound. They have been quoted to facilitate the choice of optimum poisons and one has to judge in each particular case whether or not one or the other of the criteria may be weakened. Some comments are possibly still necessary regarding criterion d, which demands a high strength of interaction of the poison. High strength of interaction may in many cases lead to a fairly nonspecific chemisorption of the poison, and one will thus count active site densities higher than actually present. Low strength of interaction, on the other hand, may lead to a displacement of the poison by reactants or products. It is, therefore, recommended that specific poisoning experiments should be carried out with a series of poisons, the strength of interaction of which slowly approaches that of the reactant itself (provided products are not held more strongly than reactants). In any case,

SPECIFIC POISONING OF OXIDE SURFACES

259

however, the particular system must be studied carefully regarding the interaction of the poison with the catalyst surface in the presence of the reaction components. Criterion b demands detectability of the chemisorbed species of the poison. This point is particularly important in the case of proton acids, since the lifetime of protonated species may be very low due to the high mobility of surface protons. Thus, the pyridinium ion cannot be detected on silica surfaces, although some protonated species must have been formed (399), as can be shown from a continuous absorption in the infrared spectra. Protons that can hardly be detected directly by protonated probe molecules may well initiate catalytic reactions due to their polarizing action during their fluctuations (349,350). There is still a lack of acidic poisons and the search for suitable and unreactive acidic compounds is strongly needed. Furthermore, the study of the chemisorptive behavior of bifunctional molecules, such as diketones, diamines, and cyclic compounds such as diazines (400), with two heteroatoms in varying relative orientations seems to be promising, since such compounds may shed some light on the configurations of exposed cations and on their geometric arrangements in the exposed crystal faces. Much of what has been said in the preceding sections may sound quite pesaimistic. This standpoint is taken because the possible pitfalls and the ambiguities that may influence the interpretation of data have t o be emphasized. In fact, there are certainly only very few poisoning experiments known today that allow a clear and convincing picture of the chemical nature of active sites to be drawn, and the numbers of active sites as counted by specific poisoning are certainly always upper limits. Nevertheless, specific poisoning is a valuable catalytic technique and will hopefully be further developed in the future. The problems that were involved in experiments in the past are mainly owing to the fact that the complex multicomponent systems of a reacting mixture and the poison on a catalyst surface were not sufficiently understood and conclusions were drawn without this fundamental knowledge. It is, therefore, expected that progress in understanding and interpretation of specific poisoning experiments will come mainly from a deeper knowledge of the chemisorption behavior of poisons in the presence of reactants and products, which should be obtained from the application of modern spectroscopic techniques. For the determination of the active site densities, measurements with a series of poisons that progressively approach the basicity or acidity of the reactants should be carried out to improve stepwise the specificity of the chemisorption of the poison. A control of possible displacement adsorptions is indispensable in these studies, and it becomes all the more important the more the basicity or acidity of the poison approaches that of the reactant. Studies of the simultaneous or successive chemisorption of poisoning molecules and reactants are, therefore, one of the most important prerequisites for further progress. Spectroscopic techniques will

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Metal-Catalyzed Oxidations of Organic Compounds in the Liquid Phase: A Mechanistic Approach ROGER A . SHELDON Konin klQkelShell-Laboratorium Amsterdam. The Netherlands AND

JAY K . KOCHI Department of Chemistry Indiana University Bloomington. Indiana

.

I Introduction

....................................

I1. Homolytic Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Liquid Phase Autoxidations in the Absence of Accelerators or Inhibitors

1. Initiation Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Propagation Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Termination Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . 4 . Autoxidation of Aldehydes . . . . . . . . . . . . . . . . . . . . . . . 5 . Autoxidation of Olefms- Addition Mechanisms . . . . . . . . . . . . 6 . Co-oxidations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B . Mechanisms of Redox Catalysis by Transition Metal ComplexesElectron and Ligand Transfer Processes . . . . . . . . . . . . . . . . . . 1. Reactions of Metal Complexes with Peroxides . . . . . . . . . . . . a . Hydrogen Peroxide . . . . . . . . . . . . . . . . . . . . . . . . . . b . Alkyl Hydroperoxides . . . . . . . . . . . . . . . . . . . . . . . . c. Peracids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . d . Kinetics of Autoxidation Involving Redox Initiation with Alkyl Hydroperoxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 . Activation of Molecular Oxygen by Metal Complexes . . . . . . . . 3. Reactions of Metal Complexes Directly with Substrate and Autoxidation Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a. Alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . b . AromaticHydrocarbons . . . . . . . . . . . . . . . . . . . . . . . i . Effect of Halide Ions . . . . . . . . . . . . . . . . . . . . . . . ii. Effect of Strong Acids . . . . . . . . . . . . . . . . . . . . . . . c. Alkanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . d . Comparison between Chemical and Electro-oxidation of Hydrocarbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

212

274 215 215 276 218 280 281 281 282 283 285 285 281 295 295 296 303 305 308 316 320 322 326

METAL-CATALYZED OXIDATIONS

. Aldehydes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

e f. g. h.

Carboxylic Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . Glycols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phenols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i. Thiols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . j . Effect of the Direct Reaction on Kinetics of Autoxidation . . . 4 . Reaction of Metal Catalysts with Free Radicals-Catalyst-Inhibitor Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 . Factors Affecting the Activity of Metal Catalysts . . . . . . . . . . . a. Influence of the Particular Metal Complex . . . . . . . . . . . . . b . Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . c . Solvent Effects-Physicochemical Properties of Metal Catalysts in Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . d . Catalyst Deactivation-Macroscopic Stages in Metal-Catalyzed Autoxidations of Hydrocarbons . . . . . . . . . . . . . . . . . . . . . e. Effects of Products of Oxidation-Co-oxidations . . . . . . . . . f Ligand Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . g. Mixed-Metal Catalysts-Synergism and Antagonism . . . . . . . . I11. Heterolytic Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A Fundamental Roles of Metal Catalysts in Heterolytic Oxidations . . . . B. Heterolytic Reactions of Metal-Hydroperoxide Complexes . . . . . . . 1. Hydrogen Peroxide-Metal Catalyst Systems . . . . . . . . . . . . . . 2. Alkyl Hydroperoxide-Metal Catalyst Systems . . . . . . . . . . . . . a. Metal-Catalyzed Epoxidations . . . . . . . . . . . . . . . . . . . . b Thecatalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . c. Generation of Hydroperoxide in Situ . . . . . . . . . . . . . . . . d . Oxidations of Other Substrates . . . . . . . . . . . . . . . . . . . C . Oxygen Activation-Direct Oxygen Transfer from Metal-Dioxygen Complexes to Organic Substrates . . . . . . . . . . . . . . . . . . . . . . . . . D. Activation of Substrate by Coordination to Metals . . . . . . . . . . . . 1 Palladium-Catalyzed Oxidations of Olefins . . . . . . . . . . . . . . a . Mechanisms of Palladium-Catalyzed Oxidations of Olefiis . . . . b . n Complexes As Intermediates . . . . . . . . . . . . . . . . . . . . c . Decomposition of Pd(I1)-Olefin n Complexesin Aqueous Solution d . Reactions of Palladium-Olefin Complexes in Nonaqueous Solvents e . Formation of Glycol Esters and Related Reactions . . . . . . . . f . Oxidative Carbonylation of Olefins . . . . . . . . . . . . . . . . . g. Oxidative Coupling of Olefins . . . . . . . . . . . . . . . . . . . . 2. Oxidation of Aromatic Hydrocarbons by Pd(I1) Complexes . . . . . a . Oxidative Coupling Reactions . . . . . . . . . . . . . . . . . . . . b . Oxidative Nuclear Substitution of Arenes . . . . . . . . . . . . . c. Oxidative Substitution of Aromatic Side Chains . . . . . . . . . d . Oxidative Carbonylation of Arenes . . . . . . . . . . . . . . . . . 3 . Activation of Saturated Hydrocarbons by Metal Complexes . . . . . IV. Heterogeneous Catalysis of Liquid Phase Oxidations . . . . . . . . . . . . . V. Biochemical Oxidations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Monooxygenases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Dioxygenases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Mechanisms of Enzymatic Oxidations . . . . . . . . . . . . . . . . . . . D. Chemical Models for Oxygenases . . . . . . . . . . . . . . . . . . . . . .

.

.

.

.

273 326 330 331 331 334 334 334 336 336 336 336 331 331 338 338 339 342 342 344 344 346 352 353 354 360 361 361 362 362 36 3 365 361 361 361 361 310 312 314 314 311 381 382 383 384 381

274

ROGER A. SHELDON AND JAY K. KOCHI

VI. Summary-Directions for Future Development . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

390 391

I. Introduction The study of the oxidation of organic compounds by molecular oxygen has a long history.' Indeed, Priestley's discovery of oxygen in 1774 and Lavoisier's subsequent explanation of the process of combustion marked the beginning of the modern era of chemistry. Observations made in the nineteenth century linked the deterioration of many organic materials, such as rubber and natural oils, to the absorption of oxygen. Early studies were mainly concerned with finding ways of inhibiting such processes. Around the turn of the century it was recognized that the formation of organic peroxides was involved in these processes. Subsequent studies of the interaction of simple hydrocarbons with molecular oxygen, carried out in the 1940's,' provided the basic concepts for the development of the free radical chain theory of autoxidation. Control of autoxidation is desirable from the point of view of inhibiting reactions such as the rancidification of fats and the oxidative deterioration of plastics, gasoline, lubricating oils, and rubber, and of promoting a variety of desirable reactions including the drying of paints and the synthesis of industrial organic chemicals by selective oxidation of petroleum hydrocarbons. Catalysis of the latter reactions, particularly by metal complexes, is of considerable technological interest and plays a vital role in many important chemical and biochemical oxidations. During the last two decades there has been renewed interest in the field of homogeneous catalysis, brought on partly by the renascence of inorganic chemistry, in which attention has been focused on the preparation and properties of coordination complexes of transition metals. Much effort has also been directed toward the elucidation of the fundamental roles of transition metal complexes in homogeneous liquid phase oxidations. Liquid phase oxidation of hydrocarbons by molecular oxygen forms the basis for a wide variety of petrochemical p r o c e s ~ e s , ~including -~~ the manufacture of phenol and acetone from cumene, adipic acid from cyclohexane, terephthalic acid from p-xylene, acetaldehyde and vinyl acetate from ethylene, propylene oxide from propylene, and many others. The majority of these processes employ catalysis by transition metal complexes to attain maximum selectivity and efficiency. The purpose of this article is to review the subject of metal-catalyzed oxidations of organic compounds in the liquid phase, largely within a mechanistic framework.* A better understanding of the catalytic action of metal complexes *The literature has been covered selectively through 1974 in this review.

METAL-CATALYZED OXIDATIONS

27 5

is essential from the point of view of increasing selectivity and efficiency. In the present climate of spiraling prices for petrochemical feedstocks, improving the performance of catalysts has become of ever increasing importance. Emphasis is placed here on homogeneous rather than heterogeneous catalysis, primarily owing to the greater number of mechanistic studies carried out in homogeneous systems. Metal-catalyzed oxidations may be conveniently divided into two types, which we arbitrarily designate as homolytic and heterolytic. The first type of catalysis usually involves soluble transition metal salts (homogeneous), such as the acetates or naphthenates of Co, Mn, and Cu, or the metal oxides (heterogeneous). Furthermore, homolytic catalysis necessitates the recycling of the metal species between several oxidation states by one-equivalent changes. Free radicals are formed as intermediates from the organic substrate. Heterolytic catalysis involves reactions of organic substrates coordinated t o transition metals. It is characterized by the metal complex acting as a Lewis acid or formally undergoing two-equivalent changes. Free radicals are not intermediates. These two types of catalytic processes will be treated separately in the ensuing discussion, although the distinction is not always clear since there are transition metal complexes that are capable of participating in both types of catalysis. Homolytic and heterolytic catalysis also fall into the categories that have been described as “hard” and “soft” processes, respectively. l6 Historically the homolytic type of catalysis has been known and studied for a long time. The heterolytic catalysts represent a relatively recent innovation but, nevertheless, include important developments such as the Wacker process for the oxidation of olefins. Regardless of the mechanism involved, the most important characteristics of metal catalysts for effecting oxidation are the accessibility of several oxidation states as well as the accommodation of various coordination numbers, both of which are properties of transition metal complexes.

11. Homolytic Mechanisms A. LIQUIDPHASEAUTOXIDATIONS I N THE ABSENCEOF ACCELERATORS OR INHIBITORS Many liquid phase oxidations, hereafter known as autoxidations, occur virtually spontaneously under relatively mild conditions of temperature and oxygen pressure. They are frequently subject t o autocatalysis by products (i.e., hydroperoxides, peracids, etc.). The liquid phase autoxidation of hydrocarbons has been studied extensively and is the subject of several monographs and rev i e w ~ . ’ ’ - ~ ~With few exceptions, the majority of liquid phase autoxidations

276

ROGER A. SHELDON AND JAY K. KOCHI

proceed via a free radical chain mechanism, which may be described by the following general scheme. Initiation:

Propagation: R * + 0 2 +RO2’ RO2- + RH

-% RO2H + R.

(3) (4)

Termination:

+RO2R -!+ RO4R +nonradical products + 0 2 R. + ROz.

2RO2.

2k

(5)

(6)

Alkylperoxy radicals play vital roles in both propagation and termination processes. Hydroperoxides, R 0 2 H , are usually the primary products of liquid phase autoxidations [reaction (4)] and may be isolated in high yields in many cases. Much of the present knowledge of autoxidation mechanisms has resulted from ~ the parent hydrostudies of the reactions of alkylperoxy r a d i ~ a l s j ’ - ~and peroxide^,'^^-^ independently of autoxidation. Thus, the various modes of reaction of organic peroxides are now well-~haracterized.~’ - 39 At partial pressures of oxygen greater than approximately 100 Torr, chain termination occurs exclusively via the mutual destruction of two alkylperoxy radicals [reaction (6)]. The cross-termination reaction (5) may be neglected. The predicted rate expression, under steady-state conditions, is then given by

which is usually observed in practice. The susceptibility of any particular hydrocarbon to autoxidation is determined by the ratio k,/(2kr)1’2. 1. Initiation Reactions Chain initiation is readily accomplished by deliberately adding initiators, that is, compounds yielding free radicals on thermal decomposition. In practice, initiators should have substantial rates of decomposition in the temperature range 5Oo-15O0C. The rate of chain initiation, Rj, is given by

R i = 2eki[In2],

(8)

277

METAL-CATALYZED OXIDATIONS

where e is the efficiency of radical production (i.e., the fraction that escapes from the solvent cage), and ki is the unimolecular rate constant for decomposition of the initiator In2. Typical initiators are aliphatic azo compounds, dialkyl peroxides, diacyl peroxides, and peresters. Table I gives the bond energies of some common initiators. Kinetic studies have been greatly simplified by the deliberate use of initiators. This technique circumvents the long and generally irreproducible induction periods that often marred earlier kinetic studies. Initiation by direct reaction of the organic substrate with molecular oxygen, namely, RH+02

+R . + H O 2 .

(9)

is thermodynamically and kinetically unfavorable for most hydrocarbons, although it has been observed in a few c a ~ e s . ~ ' ~Chain - ~ initiation in the absence of added initiators is usually attributable t o radicals formed by decomposition of adventitious impurities present in the substrate. Direct attack can be favorable when it involves compounds in which hydrogen is bonded to elements other than carbon. Such processes are illustrated by the facile air oxidation of thiols, phosphines, and a variety of organometallic compound^.^^ TABLE I

Some Common Initiators for Autoxidation Activation energy (kcal/mole)

Structure

Name

"C for 1 hr half-life -

Hydrogen peroxide tert-Butyl hydroperoxide tert-Butyl peroxide

HO -OH t-BuO-OH t-BuO-OBu-t 0

48 42 31

150

tert-Butyl . perbenzoate -

t-BuO- OCPh 0 0

34

125

Benzoyl peroxide

PhCO -0CPh 0 0

30

95

Acetyl peroxide

CH~CO-OCCHJ CN CN

30-32

85

Azoisobutyronitrile tert-Butyl hyponitrite

(CH&C-N=N-C(CH& t-BuO-N=N-OBU-t 00

30 28

60

tert-Butyl peroxalate

t-BuO-OCCO-OBu-t

25.5

40

II

II

II

II

II

I

I

II II

85

278

ROGER A. SHELDON AND JAY K. KOCHI

A third mechanism for initiation is the reaction of carbanions with molecular ~ x y g e n ~ :~ ' - ~ R-+02

+R * + 0 2 :

(10)

However, except for highly acidic hydrocarbons, this pathway is not a very common one. Thermal decomposition of alkyl hydroperoxides represents a major source of free radicals in autoxidation reactions. Unless hydrocarbons are rigorously purified before use, the trace amounts of hydroperoxides present can lead to erroneous results in kinetic studies, especially when there are no added initiators. If initiation involves simple unimolecular homolysis of the alkyl hydroperoxide, kd

ROOH

+RO. + *OH

(11)

and the autoxidation is carried out at sufficiently high temperatures so that it does not accumulate, the limiting rate is given b y 4 j

Only one-third of the RH is consumed by alkylperoxy radicals under these conditions. Thus, the chain lengths are short, and substantial amounts of RH are attacked by alkoxy and hydroxy radicals generated from the thermolysis of the hydroperoxide.

2. Propagation Reactions The addition of the radical (R.) to oxygen is extremely rapid, being diffusioncontrolled in most cases (k, > lo9 liters mole-' sec-'). At partial pressures above 100 Torr, the rate-controlling step in autoxidations is hydrogen transfer from substrate t o the alkylperoxy radical, i.e., reaction (4). The rate constants for hydrogen transfer from similar compounds can be roughly correlated with the exothermicity of reaction (4). Oxidations are likely t o be rapid if the bond that is formed (ROO-H) is at least as strong as that which is broken(R-H). Some pertinent bond dissociation energies are listed in Table 11. The ROO-H bond has been estimated44 t o be about 90 kcal mole-', which is larger than that for a benzylic or allylic C-H bond (-85 kcal mole-') or aldehydic C-H bond (86 kcal mole-'). It is comparable to a tertiary C-H bond in a saturated hydrocarbon. The relatively weak 0-H, S-H, N-H, and P-H bonds of phenols, thiols, aromatic amines, and phosphines, respectively, also provide readily abstractable hydrogens. Alkylperoxy radicals, being relatively stable and unreactive, are quite selective and preferentially abstract the most weakly bound hydrogen. The selectivity of

279

METAL-CATALYZED OXIDATIONS

Compound CH3-H n-Cs H7-H i-C3 H7- H I-C4 Hg-H CHz=CH-H C6Hs-H CH2=CH-CH2-H

Energy (kcal/mole)

Compound

Energy (kcal/mole)

103 99 94 90 105 103 85

PhCH2-H RCO-H CHjS-H CH3PH-H PhO-H PhNH-H ROO-H

85 86 88 85 88 80 90

alkylperoxy radicals is similar to that of bromine atoms [D(H-Br) = 87 kcal mole-’]. The relative rates of attack on the primary, secondary and tertiary C-H bonds of 2-methylpentane are roughly in the order: 1 :30: 300.33 Propagation rate constants have been found to depend not only on the substrate but also on the nature of the attacking alkylperoxy radical. Thus, in order to obtain a meaningful correlation of propagation rate constants withC-H bond energies, the rate constants should be compared for the reactions of a series substrates RH with the same alkylperoxy radical. These rate constants can be measured experimentally by carrying out the autoxidations of the various substrates RH in the presence of moderate concentrations of an alkyl hydroperoxide R‘OzH. Under these conditions all of the alkylperoxy radicals derived from RH undergo chain transfer with the added hydroperoxide, ROz. + R’OzH

+RO2H + R’O2.

(13)

and the rate-controlling propagation and termination steps are represented by R’Oz. + RH 2R’02.

kb + R’02H + R. 2k’

nonradical products

(14) (15)

The overall rate of oxidation is given by

and determination of the absolute rate constants gives the “crossed” propagation rate constant, kb. In Table I11 the rate constants, k;, for reaction of several substrates with ferfbutylperoxy radicals are compared with the rate constants, kp,for reaction with their own peroxy radicals.

280

ROGER A. SHELDON AND JAY K. KOCHI TABLE 111 Rate Constants per Labile Hydrogen for Reaction of Substrates with Their Own Peroxy Radicals, (k,) and with tert-Butylperoxy Radicals (kb)at 30°C"

Substrate

k,(M-'

Octene-1 Cyclohexene Cy clopentene 2,3-Dimethylbutene-2 Toluene Ethylbenzene Cumene Tetralin Benzyl ether Benzyl alcohol Benzyl acetate Benzyl chloride Benzyl bromide Benzyl cyanide Benzaldehyde

kb(M-' sec-')

sec-')

0.084 0.80 0.85 0.14 0.012 0.10 0.22 0.5 0.3 0.065 0.0075 0.008 0.006 0.01 0.85

0.5 1.5 1.7 0.14 0.08 0.65 0.18 1.6 7.5 2.4 2.3 1.50 0.6 1.56 33,000

kplkb 6.0 1.9 2.0 1.o 6.7 6.5 0.9 3.2 25.0 37.0 307 190 100 156 -40,000

"See Howard (Refs. 26 and 32).

Examination of Table 111 reveals that reactivities of peroxy radicals are strongly dependent on their structure. Reactivities are influenced by both steric and polar e f f e ~ t ~ , 2 and, ~ * in ~ ~general, - ~ ~ increase as the electron-withdrawing capacity of the a! substituent increases. Acylperoxy radicals, which possess a strong electron-withdrawing substituent, are considerably more reactive than other alkylperoxy radicals. For example, the benzoylperoxy radical is 4 X lo4 times more reactive than the terr-butylperoxy radical.

3 . Termination Reactions Under normal autoxidation conditions, the termination step occurs exclusively by the self-reaction of two alkylperoxy radicals, which combine to form unstable tetroxides: 2R02.

RO4R

(16)

The modes of decomposition of tetroxides are dependent on the structure of the alkyl group." - 33*45ai The overall rate of autoxidation of a substrate is determined not only by the propagation rate constant, k,, but also by the termination rate constant, k t , as given in Eq. (7). Table IV lists approximate rate constants for various peroxy radical terminations. Examination of Table IV reveals that the lower rates of autoxidation for primary and secondary hydrocarbons compared to tertiary

METAL-CATALYZED OXIDATIONS

28 1

TABLE IV Approximate Rate Constants for Various Alkylperoxy Radical Terminations at 30"Ca

Xt

(M-' sec-')

Alkylperoxy radical

~

~~

1.6 x 105 107

H02 ' Primary, RCHz02 * Secondary, R2CH02Tertiary, R3CO2

106

-

103

'Data from Howard (Ref. 32).

hydrocarbons is not only due to the lower reactivity of the C-H bonds in the former but also to the significantly higher rates of termination of primary and secondary alkylperoxy radicals. 4. Autoxidation of Aldehydes

Autoxidation of aldehydes is analogous to that of hydrocarbons. Acylperoxy radicals are involved as principal chain carriers and peracids are the primary products in the following manner:

+R e 0 R e 0 + 0 2 +RC03* RCHO

RC03* + RCHO +RC03H + R e 0

5 . Autoxidation of Olefins-Addition Mechanisms There are additional possibilities for chain propagation in the autoxidation of olefins. These reactions involve the addition of the alkylperoxy radical to the double bond RO2'+

\

/

C=C,+

/

I / R02C-C. I .'

(20)

followed by reactions that lead to epoxides

or p oly pe roxide s I / RO?C-C;+

I

O2

I I +R02C-CO2. I

I

etc.

(22)

282

ROGER A. SHELDON AND JAY K. KOCHI

Much of the present knowledge of the addition mechanism of olefin autoxidation has resulted from the studies of Mayo and c ~ - w o r k e r s . ~The ~ ~abstrac~~~-~ tion of hydrogen from the olefin by alkylperoxy radicals occurs exclusively at the reactive allylic position. Abstraction and addition are competing processes in olefin autoxidations. The ratio of addition t o abstraction products is strongly dependent on the structure of the ~ l e f i n . ~ ’

6 . Co-oxidations In recent years much emphasis has been placed on studies of co-oxidations, since they can provide quantitative data about fundamental processes (such as the relative reactivities of peroxy radicals toward various hydrocarbon^^^ which are difficult to obtain by other methods. Co-oxidations are also quite important from a practical viewpoint since it is possible t o utilize the alkylperoxy intermediates for additional oxidation processes instead of wasting this “active oxygen.” That the addition of a second substrate to an autoxidation reaction can produce dramatic effects is illustrated by Russell’s observation5’ that the presence of 3 mole % of tetralin reduced the rate of cumene oxidation by twothirds, despite the fact that tetralin itself is oxidized 10 times faster than cumene. The retardation is due to the higher rate of termination of the secondary tetralylperoxy radicals compared t o the tertiary cumylperoxy radicals (see above). The kinetics of autoxidation of mixtures of substrates have been discussed by Walling.43 He finds that large increases in the rate of oxidation of the unreactive component are possible in the presence of small amounts of a substrate readily attacked by alkylperoxy radicals, if the rate of termination remains more or less constant. An example of the utilization of active oxygen is illustrated by the cooxidation of aldehydes and ole fin^,^^ in which both the acylperoxy radical and the peracid are used for epoxidation of the olefin: RCHO ----+ R e 0

(23)

RCO + o2 4 R C O ~ . R C 0 3 - + RCHO \

RCO3. +, C=C

/

\

+RCO3H + RCO I / ----+ RCO3C-C; I

RCO?. + RCHO +RCOzH + R e 0

(26)

(28)

METAL - CATALY ZED OXIDATIONS

283

This reaction affords much higher yields of epoxide than those obtained from the autoxidation of the olefin alone since acylperoxy radicals are more selective than alkylperoxy radicals in favoring addition relative to abstraction.

B. MECHANISMS OF REDOXCATALYSIS BY TRANSITION METAL COMPLEXES-ELECTRON AND LIGANDTRANSFERPROCESSES In recent years, there has been a great deal of interest in the mechanisms of electron transfer p r o c e s ~ e s . ~ It~ -is~now ~ recognized that oxidation-reduction reactions involving metal ions and their complexes are mainly of two types: inner-sphere (ligand transfer) and outer-sphere (electron transfer) reactions. Prototypes of these two processes are represented by the following reactions. Electron transfer (outer-sphere): (k =

liter mole-' sec-'1

Ligand transfer (inner-sphere): Co(NH&CIZ+ + Cr2+ -3 CrC12++ CO(NH~)~'+etc.

(31)

(k = 6 X lo5 liters mole-' sec-')

During electron transfer reactions, the coordination spheres of the metal ions remain intact. By contrast, ligand transfer reactions proceed via a bridged activated complex in which the two metal ions are connected by a common bridging ligand. In the examples above, replacement by chloride of only one of the six ammonia ligands bound to cobalt accelerates the rate by a factor of over lo9. The concepts of electron and ligand transfer can be applied t o the oxidation and reduction of organic substrates by metal c ~ m p l e x e s , ~ ' since - ~ ~ oneequivalent changes in the oxidation states of metals in inorganic redox reactions also have analogies in organic chemistry. Thus, the interconversion of the series of species carbonium ion (R'), free radical (R.), and carbanion (R-) results from one-equivalent changes, namely,

Redox reactions of organic substrates with metal species involving a oneequivalent change in the oxidation state of the metal will generate free radical intermediate^.^^' 6 5 Whether the subsequent reaction between a free radical and a metal complex occurs via electron transfer or ligand transfer is determined largely by the nature of the ligand. A unified theory of the mechanisms of oxidation of alkyl radicals by copper(I1) complexes has been proposed,63a9 which is based on the hard and soft acid-base (HSAB) classification delineated by Pearson and others.66ai When the metal is bonded to hard ligands, such as acetate ion, reaction preferentially occurs at the metal atom (i.e., electron

284

ROGER A. SHELDON AND JAY K. KOCHI

transfer). When the metal is bonded to soft ligands, such as bromide or iodide, reaction occurs primarily on the ligand, and an atom transfer (inner-sphere) mechanism usually prevails. Attack on ligand and attachment to the metal are competitive in the case of chloride, which lies in the borderline region. Alkyl radicals can be similarly placed on a HSAB scale. A multitude of apparently different types of redox reactions may be classified within the general scheme contained in Eq. (32). The following reactions (R = alkyl) represent a few examples of such processes.* Each reaction can also be represented by a microscopic reverse process. Electron transfer processes: A.

X- + Mn+ RC02-

+ Mn+

_+

X. + M(n-I)+

(33)

RC02. (or R . + C 0 2 ) + M(" -

(34)

[M"' = Pb(IV), Ce(IV), Mn(III), Co(III), Tl(III)] R02-

+ M"+ +R 0 2 . + M("-

')+

(35)

[M"+= Mn(III), Co(III), Pb(IV), Ce(IV)] B.

Ligand transfer processes:

+R3SnC1+ R. RBI + Cr(I1) +R. + Cr(1II)Br

RCl+ R3Sn.

(41) (42)

Since these reactions are influenced by changes in the redox potential of the metal complex, it is possible to change from one process to the microscopic reverse process by changing the ligands attached to the metal. For example, with acetate ligands cobalt(I1) is stable with respect to cobalt(III), and, in the presence of bromide ions, cobalt(II1) is reduced by alkyl radicals in a ligand transfer oxidation: Co(0Ac)zBr + R-

+Co(0Ac)z + RBI

(43)

*Coordination around the metal will be included hereafter only if required for the discussion.

METAL-CATALYZED OXIDATIONS

285

With cyanide ligands, however, Co(II1) is stable with respect to Co(1I) and the microscopic reverse process obtains, [CO(CN)~] 3- + RBr

+[Co(CN)sBr] 3- + R -

(44)

These concepts are important for an understanding of the roles played by metal ions and their complexes in the catalysis of oxidation reactions via homolytic mechanisms. Thus, metal complexes may function as catalysts by interfering with any of the various initiation, propagation, and termination steps out.lined earlier. The participation by metal catalysts in autoxidations may be divided into four main groups: (a) reaction with peroxides; (b) reaction with substrate; (c) reaction with oxygen; ( d ) reaction with alkoxy and alkylperoxy radicals. The latter ( d ) leads to inhibition rather than to catalysis. Each of these types of participation will be discussed in the following sections. 1. Reactions of Metal Complexes with Peroxides

There is an extensive literature dealing with metal-catalyzed decompositions of peroxides.67968 For the purposes of this article we will concentrate primarily on the reactions of metal complexes with hydrogen peroxide, alkyl hydroperoxides, and peracids, since these are the usual peroxidic intermediates in autoxidations.

a. Hydrogen Peroxide. The mild oxidizing action of hydrogen peroxide is considerably enhanced in the presence of certain metal catalyst^.^'^-^ The best known of these reagents is Fenton’s reagent, which consists of Fe(I1) and H2O2. Iron(I1)-catalyzed decomposition of hydrogen peroxide proceeds via a free radical chain process involving hydroxyl radicals as transient intermediates7’ : Fe(I1) + HzOz Fe(II1) + HzOz

+Fe(III)+ HO. + HO-

(45)

+Fe(I1) + HOz- + H+

Fe(I1) + HO- +Fe(II1) + HOFe(II1) + HOz

- +Fe(I1) + O2 + H+

HO. + H202

+H2O + HO2.

Reaction (48) would be expected on energetic grounds to be more rapid than reaction (46). A catalytic cycle is possible via reactions (49,(49), and (48) without including reaction (46). In the reaction of other metal ions with alkyl hydroperoxides (to be discussed later), the reaction analogous to Eq. (48) is energetically unfavorable. In the presence of organic substrates the hydroxyl radicals can react with the For substrate leading to a number of interesting reacti0ns.6~”b, 71a, b,

286

ROGER A. SHELDON AND JAY K. KOCHI

example, Fenton’s reagent is used for the hydroxylation of aromatic hydrocarbons to the corresponding phenols,”*

’’*’‘

Biphenyls, formed via dimerization of the hydroxycyclohexadienyl radical intermediates are by-products in these reactions. Competition between these reactions is dependent on the Fe(II1) concentration, since phenols are formed via electron transfer oxidation of the intermediate by Fe(II1) [reaction (51)]. It should be emphasized, however, that the yields of phenols in these reactions are generally not impressive. The difficulty can mainly be attributed to further oxidation of the phenol product under reaction conditions. Reactivities of a variety of alcohols, ethers, and amides toward hydroxy radicals derived from Fenton’s reagent have been compared with those obtained from radiation chemistry in the absence of iron species.72a9 Reactivities of different C-H bonds indicate that hydroxyl radical is a strongly electrophilic radical so that electron supply is more important than C-H bond strength in determining its reactivity. Fenton’s reagent thus serves as a useful means for studying the one-electron oxidation and reduction of the resulting carboncentered radicals with iron(I1,III) specie^."^^ Furthermore, in these systems the addition of copper(I1) complexes that can intercept free radicals effectively often leads to enhanced yields of oxidation products.n* Comparisons of the type described above suggest that the reactivity of the hydroxyl radical is not strongly affected by the presence of various iron species in the Fenton’s reagent. However, a careful study of the oxidation of cyclohexanol has recently revealed a pronounced regioselectivity and stereoselectivity for hydroxylation to cyclohexanediols.74 Thus, hydrogen at C-3 of cyclohexano1 was found to be more reactive than that at either C-2 or C-4. Moreover, deuterium-labeling studies showed that the hydrogen at C-3 which is cis to the hydroxyl group is preferentiaIly (8%) removed, and the resulting radical is converted almost completely to cis-l,3-cyclohexanediol.Such a series of stereoselective processes represent an overall net retention in the oxidative conversion from cyclohexanol. It was concluded from these results that an iron-bound oxidant subject to strong substitutent-derived steric effects, and not a free hydroxyl radical, was involved.

’’

METAL-CATALYZED OXIDATIONS

287

Another interesting use of Fenton's reagent is the conversion of hydrocarbons t o carboxylic acids in the presence of carbon monoxide": RH + CO + HzO2

Fe(11)

+RCO2H + HzO

(5 2)

Hydroxyl radicals are also intermediates in the reaction between Ti(II1) and H2O2 and are capable of hydroxylating aromatic^.'^-^'^ A number of other metal complexes can decompose hydrogen peroxide via reactions analogous to Eqs. (45) and/or (46), including those of cerium81** copper,82ai cobalt,83a* r n a n g a n e ~ e ,and ~ ~ silver.85 Many of these electron transfer reactions are thought to proceed via inner-sphere complexes of metalhydrogen peroxides (M-OOH).84i 86 b. Alkyl Hydroperoxides. The most common pathway for catalysis of liquid phase autoxidations undoubtedly involves the metal-catalyzed decomposition of alkyl hydroperoxides. Much information concerning the roles of metal complexes in oxidations has been gained from studies under nonautoxidizing conditions, that is, by examining the decomposition of alkyl hydroperoxides alone in an inert atmosphere and an inert s ~ l v e n t . ~ 'The rapid decomposition of alkyl hydroperoxides in hydrocarbon solutions in the presence of trace amounts of iron, manganese, cobalt, and copper naphthenates is well known.18a-C*219 The two reactions for hydroperoxides, R 0 2 H + M("-')+

+RO. + M"+ + HO-

(53)

are analogous to reactions (45) and (46), respectively, previously described for hydrogen peroxide. If a particular metal ion is capable of effecting only one of these reactions, a stoichiometric but not a catalytic decomposition of alkyl hydroperoxide would be expected. However, a catalytic reaction is feasible if there is a regenerative pathway for the metal complex capable of reacting again with the alkyl hydroperoxide. Moreover, metal complexes can initiate the radical-induced chain decomposition of the hydroperoxide. Thus, a metal ion could produce radicals via reactions (53) or (54) which then cause the radical chain decomposition to occur:

+2RO. + 0 2 RO. + RO2H +RO2* + ROH 2RO2.

(55)

(56)

The metal ion in the foregoing example is acting as an initiator rather than a catalyst. Much confusion exists in the literature because of the indiscriminate use of the term catalyst for what is really an initiator. The relative rates of reactions (53) and (54) are roughly correlated with the couple. Table V lists the redox redox potential of the particular M"+/M(" -

288

ROGER A. SHELDON AND JAY K. KOCHI TABLE V

Redox Potentials (Aqueous Solution)

1.98 1.82 1.61 1.51 0.77 0.15 -0.2a -0.03 -0.20 -0.37 -0.41

Ag(I1) + e Co(II1) + e Ce(1V) + e Mn(II1) + e Fe(II1) + e Cu(I1) + e Mo(V1) + e W(V1) + e V(II1) + e Ti(IV) + e Cr(II1) + e

1.69 1.25 0.92

Pb(IV) + 2e Tl(II1) + 2e 2Hg(II) + 2e HOz*+ e + H+ HzOz + 2e + 2H+

1.68b 1.77‘ -0.32b 0.68c -0.45b

Oz + e + H + Oz + 2e + 2H+ Oz + e ~~

~~~~~~~

~

~

aWilliams, R. J. P., Advan. Chem. Coord. Compounds p. 279 (1961). bSee George, P., “Oxidases,” Vol. I (Ref. 641). ‘See Jones (Ref. 380, p. 99).

potentials of some systems that are known to react with hydroperoxides. It should be emphasized, however, that these values pertab only to aqueous solutions. Redox potentials are influenced by the nature of the ligands and the solvent. Unfortunately, redox potentials of metal complexes in organic solvents are not generally known. It is possible to divide the metals that react with alkyl hydroperoxides into four groups: (i) metals that effect reaction (53), (ii) metals that participate in reaction (54), (iii) metals that are involved in both, and (iv) metals that effect heterolytic reactions of the hydroperoxide (see Section 1II.B). Other routes that do not involve changes in the oxidation state of the catalyst are also possible for the homolytic decomposition of alkyl hydroperoxides. The following scheme, for example, is presented speculatively:

METAL-CATALYZED OXIDATIONS R02H + MX2

289

+ROOMX + HX

ROOMX +RO- + -0MX *OMX+ RO2H

HOMX + R 0 2 .

HOMX + HX +MX2 + H2O

(60)

Catalysis in such a mechanism can be attributed to weakening of the peroxidic linkage by formation of an inner-sphere complex with the metal. It could be especially applicable to compounds of the main group elements. Such processes are probably involved in the catalytic decomposition of hydroperoxides by sulfonium compounds (see later), boron esters, or S e 0 2 , e.g., R02H + Se02

+ROOSe(0)OH +RO. + .OSe(O)OH

(61)

+ H 2 0 + SeOz etc.

(62)

.OSe(O)OH + R02H +R 0 2 .

i. When the metal complex is a strong oxidant, reaction (54)predominates. For example, Pb(1V) reacts stoichiometrically with 2 moles of alkyl hydroperoxide to afford alkylperoxy radical^^*-^'^: Pb(OAc)4 + 2RO2H

+Pb(0Ac)z + 2 R 0 2 . + 2HOAc

(63)

Cerium(1V) resembles Pb(1V) in its reactions with alkyl hydroperoxides, i.e., R O ~ H+ Ce(IV)

+R O ~ +. Ce(II1) + H +

(64)

These reactions have been used for producing high concentrations of alkylperoxy radicals for ESR studies." Other strong oxidants that might be expected to behave similarly are TIU' or Ag". ii. When the metal ion is a strong reducing agent, reaction (53) predominates. Chromous ion, Cr(II), reduces alkyl hydroperoxides to the corresponding alcohols61993:

The decomposition cannot be catalytic since there is no convenient route available for the regeneration of Cr(I1). The reaction has been utilized for the preparation of benzylchromium in high yield as follows: PhCH2C(CH3)202H + Cr(I1) +PhCHzC(CH&O* + Cr(1II)OH PhCH2C(CH3)20.

-

PhCH2 * + (CH&CO

PhCHz + Cr(I1) +PhCH?Cr(III)

(67) (68) (69)

Copper(1) similarly reduces hydroperoxides to the corresponding alcohols. 94 Indeed, the reaction of the Cu(I)/Cu(II) couple with peroxides has been thorto the reaction with Cr(II), alkyl hydroperoughly s t ~ d i e d . ~ ~ -In ' ~ contrast ' oxides can react by a catalytic process with Cu(I), since there are several routes

290

ROGER A. SHELDON AND JAY K. KOCHI

available for regenerating Cu(1). Thus, in the copper-catalyzed reduction of teertamyl hydr~peroxide,'~the Cu(1) is regenerated via electron transfer oxidation9s-100 of the ethyl radicals formed by the rapid fragmentation of tertamyloxy radicals: C H ~ C H Z C ( C H ~ ) ~+OCU(I) ~ H -+ Cu(I1)OH + C H ~ C H Z C ( C H ~ ) Z O . (70) CH~CH~C(CHJ)ZO*J+(CH&CO + CzHs * CzHS. + Cu(I1) -+ CzH4 + H + + Cu(1) etc.

(71)

(72)

Copper(1) can also be regenerated by electron transfer oxidation of radicals produced by hydrogen transfer from solvent by alkoxy radicals."' Thus, reactions carried out in hydrocarbon solvents will produce alkyl radicals that are oxidized by Cu(I1) at rates approaching diffusion c o n t r 0 1 . ~ ~ - ' ~ ~ In view of the low redox potential of the Cu(II)/Cu(I) couple, regeneration of Cu(1) by reaction of Cu(I1) with the hydroperoxide, Cu(I1) + ROzH -+Cu(1) + ROz' + H+

(73)

appears unlikely. Indeed, Hiatt has r e p ~ r t e d ~ that ~ ~alkyl - ~ hydroperoxides at room temperature are inert to cupric acetate alone. Ferrous complexes also reduce alkyl hydroperoxides (cf. Fenton's reagent). In the presence of butadiene the following sequence of reactions was postulated t o account for the observed products"'* l o 3 : RO,H

. RO.

t Fe'"'

+

Fe""

LR O .

RO-

t Fe'""0H

+

Fe'"')

(74)

(77)

An interesting series of reactions has been developed based on the reaction between Fe(I1) and hydrogen peroxide adducts of ketones. The cyclohexanonehydrogen peroxide a d d ~ c t " ~ ~reacts ' with ferrous sulfate in acidic solutions t o produce the 5-carboxypentyl radical.lo5 In the absence of reactive substrates, dodecanedioic acid is formed by dimerization of these radicals:

29 1

METAL -CATALYZED OXIDATIONS

In the presence of butadiene, the 5-carboxypentyl radical generates an allylic adduct that dimerizes t o a mixture of C2t3 dicarboxylic acids'" :

(81)

C,, diacid

H02C(CH,),* t Cu""

-

HO,C(CH,),CH=CH,

-t

(83)

Cu'" + H i

A further modificationla6 is achieved by intercepting the allylic radicals with Cu(I1) shown in Eq. (82). The 5-carboxypentyl radical may also be directly intercepted by Cu(I1) t o afford w-hexenoic acid [Eq. (83)]. Also, the ligand transfer oxidation of alkyl radicals by cupric halides may be employed to produce the corresponding w-halo acids, e.g., HOzC(CH2)s. + CuC12

(84)

4 H02C(CH2)5Cl+ CuCl

Thus, by variation of the ketone, olefin, and catalyst these reactions can be utilized for the synthesis of a wide variety of polyfunctional long-chain rn01ecules.~~~~* A method for introducing remote double bonds by the ferrous sulfate-cupric acetate-promoted decomposition of certain alkyl hydroperoxides has recently been reported,lo8 e.g., +

+O,H

Fe""

="'//'O.

(85)

Fe(""

t

*o. H O*

(86) A

e

O

H

t

&

OH

(87)

It is noteworthy that reactions such as the foregoing should also be possible in the presence of a Cu(I)/Cu(II) couple since Cu(1) is also able to effect reaction (85).95

The metal ion-promoted decomposition of alkyl hydroperoxides can be employed as a method for introducing the alkylperoxy group into various substrates67&b, 103, 1 0 9 : 2RO2H + R'H

CU(I)/CU(II) ___)

R02R' + H2O + ROH

(88)

Copper salts are usually the superior reagents for this reaction. Reaction (88) is analogous to the peroxyester reaction,67a9 9 7 t ' O 0 * '01* "09 in which a variety of organic substrates are selectively oxidized by tert-butyl peresters in the presence of catalytic amounts of transition metal salts, particularly copper complexes: bi

'"

CUX"

RH + t-BuO2Ac

--3 ROAC + t-BuOH

(89)

292

ROGER A. SHELDON AND JAY K. KOCHI

It has been showngsy97 that the relevant oxidation steps in these reactions are t-BuOzAc + CU(I) +t-BuO* + Cu(1I)OAc t-BuO2H + CU(I) t-BuO. + RH

(91)

+t-BuOH + R .

(92)

<== r-BuOOH

R* + &(I11

(90)

+t-BuO, + Cu(1I)OH R O ~ B U '+ H+ + &(I)

(93)

ROAc + H+ + Cu(1)

(94)

Metals Ti(III), V(II), and V(I11) are also known to reduce alkyl hydroperoxides in a manner analogous to Cr(II), Fe(II), and Cu(1) although they have not been studied as extensively.61' 93 iii. When the metal has two oxidation states of comparable stability, reactions (53) and (54) occur concurrently. Cobalt and manganese compounds are among the most effective catalysts for autoxidation since they are able to induce the efficient catalytic (as opposed to stoichiometric) decomposition of alkyl hy droperoxides. Aqueous solution:

+RO. + Co(1II)OH RO2H + Co(1II) +ROz* + Co(II)+ H+ ROzH + Co(I1)

(95)

(96)

Nonpolar solvents: RO2H + Co(1I) +RO. + Co(1II)OH

(97)

or ROzCo(I1)

+RO. + CO(III)O

RO~CO(III)

RO2. + Co(I1)

(98) (99)

In aqueous solution outer-sphere electron transfer between metal ions and alkyl hydroperoxides [reactions (95) and (96)] is expected to be favorable. In nonpolar solvents, electron transfer probably proceeds via the formation of inner-sphere, covalently bonded complexes. The overall reaction constitutes a catalytic decomposition of the hydroperoxide into alkoxy and alkylperoxy radicals: 2RO2H

+ROz. + RO. + HzO

(100)

Cobalt compounds are generally the more effective catalysts and consequently have received the most attention.34i112-131c Much information has been gained from studies of cobalt-catalyzed decompositions of alkyl hydroperoxides under nonautoxidizing conditions. One important point to be borne in mind in these studies is that radical-induced chain decomposition of the hydroperoxide, via reactions (55) and (56), is always in competition with decomposition via the foregoing cycle.

METAL-CATALYZED OXIDATIONS

293

The electron spin resonance (ESR) spectrum of alkylperoxy radicals has been observed in hydrocarbon solutions during the catalytic decomposition of alkyl hydroperoxides with cobalt acetylacet~nates.’~~ The spectrum of alkoxy radicals under the same conditions is too broad to observe. The ESR signal of alkylperoxy radicals generated in this manner decayed much more slowly than those generated photochemically. To account for these “long-lived” peroxy radicals, it was suggested that they were stabilized by complex formation with cobalt ~pecies.”~ Subsequent kinetic and ESR analysis, however, showed that the earlier work was an artifact of high peroxide concentration^.'^^ At the present there is no unambiguous evidence for the importance of peroxy complexes with cobalt in these systems. Hiatt et al.34a-dstudied the decomposition of solutions of tert-butyl hydroperoxide in chlorobenzene at 25OC in the presence of catalytic amounts of cobalt, iron, cerium, vanadium, and lead complexes. The time required for complete decomposition of the hydroperoxide varied from a few minutes for cobalt carboxylates to several days for lead naphthenate. The products consisted of approximately 86% tert-butyl alcohol, 12% di-tert-butyl peroxide, and 93% oxygen, and were independent of the catalysts. A radical-induced chain decomposition of the usual type,’35 initiated by a redox decomposition of the hydroperoxide, was postulated to explain these results. When reactions were carried out in alkane solvents (RH), shorter kinetic chain lengths and lower yields of oxygen and di-tert-butyl peroxide were observed due to competing hydrogen transfer of tert-butoxy radicals with the solvent. t-BuO.

+ t-Bu02H ___) t-BuOH + t-BuOz *

t-BuO.

+ RH d

R

.

+ t-BuOH

(101)

(103)

Competition between metal ion-induced and radical-induced decompositions of alkyl hydroperoxides is affected by several factors. First, the competition is influenced by the relative concentrations of the metal complex and the hydroperoxide. At high concentrations of the hydroperoxide relative to the metal complex, alkoxy radicals will compete effectively with the metal complex for the hydroperoxide. Competition is also influenced by the nature of the solvent (see above). Contribution from the metal-induced reaction is expected to predominate at low hydroperoxide concentrations and in reactive solvents. The contribution from the metal-induced decomposition to the overall reaction is readily determined by carrying out the reaction in the presence of free radical inhibitors, such as phenols, that trap the alkoxy radicals and, hence, prevent radical-induced d e c o m p o ~ i t i o n . ’130a ~ ~ ~ Thus, Kamiya e l a1.”’ showed that the initial rate of the cobalt-catalyzed decomposition of tetralin hydroperoxide, when corrected for the contribution from radical-induced decomposition by the

294

ROGER A. SHELDON AND JAY K. KOCHI

addition of an inhibitor, was (within experimental error) equal t o the limiting rate of cobalt-catalyzed autoxidation of tetralin under the same conditions. This result demonstrates that, under the autoxidizing conditions, chain initiation occurs exclusively via cobalt-catalyzed decomposition of the hydroperoxide. The relative rates of reactions (95) and (96) are markedly dependent on the solvent. Shchennikova and c o - ~ o r k e r s ' ~ ~31a-c ~ ~ have studied the influence of solvent on the relative rates of these reactions. In polar solvents, such as water, carboxylic acids, or mixtures of carboxylic acids and chlorobenzene, reaction (95) is the slow, rate-determining step and the cobalt catalyst is present almost completely in the divalent state during reaction. In nonpolar solvents, such as benzene or chlorobenzene, reaction (96) is the slower step and the catalyst is present mainly in the trivalent state. The reasons for this dramatic solvent effect have not been discussed at any length. It has been suggested'24 that in aqueous solution, reaction (96) takes place only between the ionic species (Co3+ and R02-) and is very rapid. In a hydrocarbon medium, in which the alkyl hydroperoxide does not dissociate appreciably, this reaction is expected to be very slow. Reaction ( 9 9 , on the other hand, may proceed via an inner-sphere, covalently bonded complex and should be facilitated in nonpolar, poorly coordinating solvents in which complex formation is more favorable. Exchange of ligands in Co(I1) complexes, which are generally substitution labile, is rapid compared to Co(II1) complexes, which are inert to substitution. A further point t o be considered is that reaction (99) is reversible, whereas (97) is irreversible. The equilibrium constant of reaction (99) is probably markedly dependent on the solvent. Other w ~ r k e r s ~124 ~ ~ have - ~ . reported that commencement of the cobaltcatalyzed autoxidation of pure hydrocarbons, i.e., nonpolar solvent, is accompanied by oxidation of Co(I1) t o Co(II1). The transformation is easily observed by the change in color from pale violet or pink [Co(II)] t o intense green [Co(III)] . Similarly, manganese-catalyzed autoxidations were observed to start when Mn(I1) was converted to Mn(I1I). The concentration of Co(II1) reached a maximum during the course of autoxidation and then decreased. This maximum coincided with the appearance of aldehydes in the reaction mixtures. The authors also showed by c a l ~ u l a t i o n 'that ~ ~ reduction of Co(II1) by a secondary product accounted for the observed kinetics much better than reduction by hydroperoxide. Hence, the decrease in concentration of Co(II1) after it had reached a maximum was attributed to reduction by aldehydes (see Section II.B.3.e), which was much more facile than reduction by alkyl hydroperoxide [reaction (96)]. The Mn(II)/Mn(III) system has not been studied in as much detail as cobalt, but the principles discussed above are also probably applicable t o the manganesecatalyzed reactions: b9

Mn(II)+ R02H Mn(III)+ R 0 2 H

'

+Mn(II1)OH + RO.

(104)

+Mn(II)+ R 0 2 . + H +

(105)

METAL-CATALYZED OXIDATIONS

295

As mentioned above, autoxidation ~ t u d i e s " ~indicate that reaction (104) is much faster than reaction (105) in hydrocarbon media. The kinetics of the reaction between manganous stearate and n-decyl hydroperoxide have been studied, and both metal-induced and radical-induced decomposition of hydroperoxide were observed. Rhodium and iridium, which are in the same group as cobalt in the Periodic Table, are also expected to effect reactions analogous to Eqs. (95) and (96); this view is supported by recent studies of autoxidations catalyzed by Rh and Ir complexes136-140 (see Section II.B.2). Complexes of these metals rapidly decompose hydroperoxides in a catalytic r e a ~ t i 0 n . l ~ ' c. Peracids. Metal ions of variable valence catalyze the decomposition of peracids via redox reactions analogous to those with alkyl hydroperoxides130a, b, 131a-c. 0 II RCOOH + M(n-l)+

*M"+OH + R C O ~ .

(106)

*Mn+02CR + HO.

(107)

+M("- ')+ + RC03 - + H+

(108)

0

II RCOOH + M(n-l)+

0 II RCOOH + M"+

Oxidation of Co(I1) or Mn(I1) complexes by peracids [reaction (106) or (107)] is a facile r e a ~ t i 0 n . I ~Reaction ~ (108) proceeds much more slowly, if

at all. Thus, in contrast to the reaction with alkyl hydroperoxides (see preceding section), during the decomposition of peracetic acid the cobalt catalyst is present largely as Co(1II) in both chlorobenzene and acetic acid solutions. Peracids are intermediates in the autoxidation of aldehydes, but the direct oxidation of aldehydes is a more favorable pathway for the regeneration of the reduced form of the catalyst (see Section II.B.3.e). The predominance of Co(II1) supports the reaction (109) as the slower step in both solvents. RCHO + Mn+

+R e 0 + M(n-l) + H+

(109)

d. Kinetics of Autoxidation Involving Redox Initiation with Alkyl Hydroperoxides. The kinetics of metal-catalyzed autoxidation were initially formulated by Woodward and Mesrobian.112ai If it is assumed that initiation occurs only by reactions (95) and (96) [which are equivalent t o the combined form represented in Eq. (loo)] , the limiting rate of oxidation is given by

Walling43 has shown that the expression given in Eq. (1 10) is part of a general expression for any system involving low steady-state concentrations of alkyl hy-

29 6

ROGER A. SHELDON AND JAY K. KOCHI

droperoxide that produce an average of n initiating radicals. Iff is the fraction of substrate consumed through attack by alkylperoxy radicals, the limiting rate expression is given by

The limiting rates observed in practice are generally in good agreement with these predictions. 2. Activation of Moleculur Oxygen by Metal Complexes The most common pathway for catalysis of autoxidations by transition metal complexes involves the decomposition of alkyl hydroperoxides. Another route that may be possible for chain initiation involves direct oxygen activation, whereby the complexation of molecular oxygen by a transition metal would lower the energy of activation for direct reaction with the substrate [reaction ( 9 ) ] For example, oxygen coordinated to a metal might be expected to possess properties similar to alkylperoxy radicals and undergo hydrogen transfer with a hydrocarbon:

.

M-0-0.

+ RH +M-0-OH

+ Re

(112)

Many authors'2s* 142-146 have proposed reaction (1 12), or a variant of it, in an attempt to explain kinetic data. For example, Uri14' proposed the following mechanism for the initiation of cobaltous stearate-catalyzed autoxidation of methyl linoleate in benzene:

eCO(II)02 C0(11)02 + Co(1I)XH *Co(I1) + Co(1II)X + HO2. Co(I1) + 0 2

(113)

(1 14)

A number of transition metals are now k n o ~ n ' ~ ' - to ' ~form ~ stable dioxygen complexes, and many of these reactions are reversible. In the case of cobalt, numerous complexes have been shown to combine oxygen reversibly.'57* Since cobalt compounds are also the most common catalysts for autoxidations, cobalt-oxygen complexes have often been implicated in chain initiation of liquid phase autoxidations. However, there is no unequivocal evidence for chain initiation of autoxidations via an oxygen activation mechanism. Theories are based on kinetic evidence alone, and many authors have failed to appreciate that conventional procedures for purifying substrate do not remove the last traces of alkyl hydroperoxides from many hydrocarbons. It is usually these trace amounts of alkyl hydroperoxide that are responsible for chain initiation during catalytic reaction with metal complexes. In the last few years, numerous stable, diamagnetic oxygen complexes of other Group VIII elements (Ir, Rh, Ru, Os, Ni, Pd, Pt) have been prepared.'47-'5'

297

METAL-CATALYZED OXIDATIONS

A few examples are shown here [(I)-(VII)] Ph,P,. 0, I ,O CI,if. I PPh,

.

Many of these complexes have

Ph,P,M/O

co

(M

(1)

t-BuNC\ '0 I

Ph,P' = Ni,

t-BuNCYM

/O

\b

(M = Ni, Pd)

Pd. Pt)

(11)

(111)

NO

CNCH,

P

(VI)

(VII)

0-0 bond lengths that correspond closely to those of the peroxide (OZ2-) or superoxide (0,:)anions and, hence, may be regarded as inorganic peroxides'59 (Table VI). Some of these complexes form hydrogen peroxide on protonation, in agreement with this formulation.'60-'62 TABLE VI

0-0 Bond Lengths in Metal-Dioxygen Complexes and Metal Peroxides Compound

0-0 bond length (A)

Reference

1.21 1.28 1.47 1.30 1.51 1.63, 1.52 1.2 1.26 1.42 1.47 1.31 1.40-1.47 1.36-1.55 1.50 1.3 1-1.45 1.40-1.48

159,163 163 156,159 164 165a 165b, c 166a, b 167 168 169 170a, b 171 171 171 171 171

298

ROGER A. SHELDON AND JAY K. KOCHI

For the oxygen to form a n complex with the metal, an empty orbital of the correct energy and symmetry must be available on the metal to form a u bond to oxygen and a filled d orbital to back-bond to the antibonding orbitals of 0 ~ y g e n . l ~ The ' degree of back-bonding will depend on the relative energies and the amount of overlap between the orbitals and may contribute to the two extreme types of bonding in oxygen adducts with metal complexes represented by compounds I and VII. u-Peroxy complexes such as VI will be formed by metals in high oxidation states in which the d orbitals are less available for back-bonding. Metals in low oxidation states, on the other hand, will have relatively expanded d orbitals and the n-bonded arrangement in compound I will be favored. Finally, a series of complexes in which dioxygen is weakly bound to the metal have been used as models for biological oxygen transport.'66aib The mode of bonding of the O2 group in compound VII is bent, with a Co-0-0 bond angle of 153". The complex may be formulated as a low-spin octahedral Co(1II) complex of the superoxide ion, 02;.Electron spin resonance studies of other Schiff base and porphyrin complexes also support superoxide-type structures. '66b Due to the possibility of chain initiation by direct reaction of a metal-dioxygen complex with substrate, many of these complexes have been examined as autoxidation catalysts, particularly for the oxidation of olefins.' 369 39-1419 72-1 7 9 Thus, Collman et reported that dioxygen complexes of Ir(I), Rh(I), and Pt(0) catalyzed the autoxidation of cyclohexene at 25" to 60°C in benzene or methylene chloride. Cyclohexene-3-one is the major product (together with water) and cyclohexene oxide a minor product:

'

'

n

Other workers reported that the Rh(1) complex, RhCl(Ph3P)3, catalyzed the 82 liquid phase autoxidation of alkylbenzenes.' More recent investigations have shown that these reactions involve metalcatalyzed decomposition of hydroperoxides via the usual redox cycles. Thus, inhibition, polymerization and product studies in the FU~Cl(Ph~P)~-catalyzed autoxidation of cyclohexene,' 36 ethylbenzene,' 36 and diphenylmethane' were compatible only with metal-catalyzed decomposition of the alkyl hydroperoxide and not a direct reaction of the metal-dioxygen complex with substrate. Complexes Rh(II1) ( a c a ~ ) ~Rh(II1) , (2eth~lhexanoate)~, and Co(I1) (2ethylhe~anoate)~, gave results that were almost the same as those obtained with RhCl (Ph3P)3. The redox cycle may involve Rh(I1) and Rh(II1):

METAL-CATALYZED OXIDATIONS

299

+Rh(II1) OH + RO.

(116)

+Rh(I1) + RO2.

(117)

Rh(I1) + ROzH Rh(III)+ RO2H

+ H+

Alternatively, a similar cycle can be written with Rh(1) and Rh(I1). Analogous reactions presumably occur in Ir(1)-catalyzed autoxidations since Ir(1) complexes, such as compound I, rapidly decompose alkyl hydroperoxides under anaerobic conditions.' 8391 84 Similarly, a recent study14' of the homogeneous oxidation of cyclohexene by various low-valent phosphine complexes of Group VIII transition metals yielded no definite proof for initiation by oxygen activation. Results were consistent with reactions involving chain initiation via the usual redox reactions of the metal complexes with traces of hydroperoxides. Long induction periods were observed with peroxide-free hydrocarbons. Further evidence against initiation by direct oxygen activation in the oxidation of olefms is provided by the following two observation^.'^^ First, no reaction was observed between olefins (e.g., cyclohexene, 1-octene, and styrene) and metal-dioxygen complexes, such as I, 11, and V, when they were heated in an inert atmosphere (nitrogen). Second, no catalysis was observed with these metal complexes in the autoxidation of olefms, such as styrene, that cannot form hydroperoxides. Direct reaction between oxygen and a substrate would be expected to be more favorable when both molecules are coordinated to a metal. Indeed, hydrogen abstraction from a coordinated olefin by coordinated dioxygen was observed in the reaction of a Rh(1)-cyclooctene complex with molecular oxygen.' 8683b The following mechanism was proposed:

-

CI

'\

'

&

CI

H' \

..o,..

CI /

Rh I

\

3, (119)

+

0,

-

300

ROGER A. SHELDON AND JAY K. KOCHI

It should be borne in mind, however, that the foregoing reaction is a stoichiometric one. In a catalytic autoxidation such a process would be readily masked by one involving metal-catalyzed decomposition of the hydroperoxide. An oxygen activation mechanism has also been po~tulated"~for the autoxidation of cumene at 35°C in the presence of the Pd(0) complex, (Ph3P)4Pd: (Ph3P)d Pd + 2 0 2

+ (Ph3P)z PdOz + 2PhjPO

(Ph3P)zPdOz + RH + (Ph3P)zPdOzH + R.

R. + 0 2 ROz' + RH

(121)

(122)

ROz*

(123)

+ROzH + R.

(124)

4

However, a recent kinetic study"' has shown unequivocally that chain initiation proceeds via the usual metal-catalyzed decomposition of the hydroperoxide. Thus, the rate of initiation of the autoxidation of cumene was, within experimental error, equal to the rate of production of radicals in the (Ph3P)4Pdcatalyzed decomposition of tert-butyl hydroperoxide in chlorobenzene at the same temperature and catalyst concentration. Moreover, long induction periods were observed (in the absence of added tert-butyl hydroperoxide), when the cumene was purified by passing it down a column of basic alumina immediately prior to use. Autoxidation of cumene purified by conventional procedures showed only short induction periods. These results further demonstrate the necessity of using highly purified substrates in kinetic studies. Another class of compounds that readily combine reversibly with molecular oxygen are the metal phthal~cyanines."'*~'~ A number of oxidative enzymes contain the metal-porphyrin structure as the prosthetic group. In view of the

mctal phtlialocyanines

metal porphyrin

formal structural similarities between porphyrins and phthalocyanines one might expect parallels between the catalytic properties of metal phthalocyanines and those of enzymes involved in biological oxidations. Thus,many metal phthalocyanines are known'66b91909191 to be reversible oxygen carriers in solution anal-

METAL-CATALYZED OXIDATIONS

301

ogous to the oxygencarrying heme enzymes. A number of metal phthalocyanines have been shown to be active catalysts for the decomposition of hydrogen and alkylarop e r o ~ i d e ~ ~ ' and - ' ~ ~the autoxidation of 01efins'~'~1969 matic hydrocarbon^.'^^^ 92p198-'07b Kropf and co-workers200-'07bcarried out detailed investigations of the autoxidation of alkyl aromatic hydrocarbons, such as cumene, catalyzed by metal phthalocyanines. They concluded that in the initial stages of reaction, in which the concentration of alkyl hydroperoxides is quite low, initiation occurred by an oxygen activation mechanism, e.g., CUPC + 0 '

e CUPC

(125)

0 2

CuPc 0 ' + RH +CuPc + R. + HOz- etc.

(126)

When the concentration of the hydroperoxide builds up, the copper phthalocyanine-dioxygen complex reacts with alkyl hydroperoxide, CUPC0 2 + [R02H, RH]

+CUPC+ ROz.

+ R. + H 2 0 2

(127)

The proposal for initiation by reactions (1 25) and (1 26) at temperatures below 95°C was based on the almost quantitative yield of alkyl hydroperoxide obtained. However, this result is not an adequate argument by itself for an oxygen activation mechanism since the yield of alkyl hydroperoxide is dependent on the chain length of the process. For example, a reaction with a chain length of 50 will afford a 98% yield of alkyl hydroperoxide irrespective of the mode of initiation. Since copper phthalocyanine is an efficient catalyst for the decomposition of cumene hydroperoxide, other workers have rejected the Kropf mechanism in favor of initiation via the usual redox decomposition of hydroperoxide.'98~'99~'08Moreover, a recent kinetic studyzo9 of the autoxidation of cumene in the presence of a variety of homogeneous and heterogeneous metal phthalocyanine catalysts (Cu, V, Ni, Co, Zn, Mg) has convincingly demonstrated that initiation is due to the metal-catalyzed decomposition of trace amounts of hydroperoxide. Iron(II1) meso-tetraphenylporphyrin chloride [Fe (TPP)Cl] will induce the autoxidation of cyclohexene at atmospheric pressure and room temperature via a free radical chain process.'" The iron-bridged dimer [Fe(TPP)] 2 0 is apparently the catalytic species since it is formed rapidly from Fe(TPP)Cl after the 2-3 hr induction period. In a separate study, cyclohexene hydroperoxide was found to be catalytically decomposed by Fe (TPP)Cl to cyclohexanol, cyclohexanone, and cyclohexene oxide in yields comparable to those obtained in the direct autoxidation of cyclohexene. However, [Fe(TPP)] '0 is not formed in the hydroperoxide reaction. Furthermore, the catalytic decomposition of the hydroperoxide by Fe (TPP)Cl did not initiate the autoxidation of cyclohexene since the autoxidation still had a 2-3 hr induction period. Inhibitors such as 4-tert-butylcatechol quenched the autoxidation but had no effect on the decom-

3 02

ROGER A. SHELDON AND JAY K. KOCHI

position of the hydroperoxide by Fe(TPP)Cl. It was concluded that if the hydroperoxide was an intermediate in the Fe (TPP)Cl-induced autoxidation of cyclohexene, its decomposition did not produce chain-carrying species. The results suggest that autoxidation of cyclohexene and decomposition of hydroperoxide are catalyzed by different iron species, namely, [Fe(TPP)J 2 O and Fe (TPP)Cl, respectively. Unfortunately, the crucial experiment, that is, to determine the role of [Fe(TPP)J 2 Oin the decomposition of the hydroperoxide, was apparently not carried out. An oxygen activation mechanism is favored by Ochiai for the autoxidation of cyclohexene catalyzed by copper p h t h a l ~ c y a n i n e . ’ ~ ’Kamiya ~ ~ ~ also proposed that the CuPc-0, complex initiates the autoxidation of cu-methylstyrene by an addition mechanism’ 96 ; CUPC 0 2 +

)=<-

I I

cuPc-o~-c-c*

(128)

However, no convincing evidence has been presented to support such mechanisms. The contrary notwithstanding, these reactions almost certainly involve initiation via the usual redox decomposition of trace hydroperoxides. An interesting example of oxygen activation by “onium” salts of nontransition elements has been claimed. Sulfonium salts were found to be especially effective?” and it was proposed” 2-2 14b that dioxygen complexes of sulfonium compounds can initiate autoxidations via hydrogen transfer with hydrocarbons, e.g., R3SX + 0 2

X . . . R3S

[X

. . . R3S. .

*

021

. . . 0 2 + R’H +R’* + HOz’ + R3SX

(129) (130)

The increase in the reactivity of the oxygen-oxygen bond in the complex was attributed to interaction between the partially occupied d orbitals on the sulfur with the Ing orbitals of oxygen. A more recent i n v e s t i g a t i ~ nhas ~ ~ shown, ~ however, that initiation of cumene autoxidation by sulfonium compounds involves the production of radicals by reaction of the’ sulfonium compound with trace amounts of cumene hydroperoxide. The following catalytic mechanism accounted for the observed results [cf. reactions (57)-(6O)J : R3SX + PhCMez02H R3SOOCMe2Ph

R3SOOCMezPh + HX

+R3SO- + PhMezCO.

R3S0. + PhMe2CH ----* R3SOH + PhMezC. R3SOH + HX

-+RjSX + HzO

(131) (1 32) (1 33)

(1 34)

In agreement with the proposed mechanism, the catalytic activity of sulfonium salts was found to be related to the basicity of the anion X. The activity could be enhanced by the addition of bases, such as calcium carbonate (or better by

METAL-CATALYZED OXIDATIONS

303

the use of hydroxide as the counterion X), which displace the equilibrium (131) to the right. It may be concluded from the preceding discussion that at this juncture there is no bona fide evidence for the initiation of autoxidations by direct hydrogen transfer between metal-dioxygen complexes and hydrocarbon substrates. Although such a process may eventually prove feasible, in catalytic systems it will often be readily masked by the facile reaction of the metal complex with hydroperoxide. The choice of cumene as substrate by many investigators is somewhat unfortunate for several reasons. Cumene readily undergoes free radical chain autoxidation under mild conditions and its hydroperoxide readily decomposes by both homolytic and heterolytic processes. The existence of numerous oxygenases that catalyze the “direct” oxygenation of organic substrates continues to stimulate the search for atom transfer oxidations of hydrocarbons by simple metal-dioxygen complexes. (For a further discussion of reactions of metal-dioxygen complexes with organic substrates via heterolytic pathways, see Section 1II.C).

3 . Reactions of Metal Complexes Directly with Substrate and Autoxidation Products The classic method for carrying out catalytic oxidations of hydrocarbon substrates has involved the use of the hydrocarbon itself as solvent and trace amounts of hydrocarbonsoluble metal complexes, such as metal stearates, naphthenates, or acetylacetonates, as catalysts. These reactions were generally carried out to low conversions to avoid excessive by-product formation. Catalysis involved redox reactions of the metal catalyst with intermediate hydroperoxides. In recent years increasing use has been made of an alternative procedure involving the oxidation of hydrocarbon substrates in polar solvents, usually acetic acid, in the presence of relatively large amounts of metal catalysts, usually the metal acetate. These reactions are characterized by high rates of oxidation, high conversions, and more complete oxidation of the substrate. For example, the classic autoxidation of cyclohexane is carried out to rather low conversions and affords mainly cyclohexyl hydroperoxide, cyclohexanol, and cyclohexanone . Autoxidation of cyclohexane in acetic acid, in the presence of substantial amounts of cobalt acetate catalyst, results in the selective formation of adipic acid at high conversions (see Section II.B.3.c). In addition to the usual reactions of the catalyst with intermediate hydroperoxides, the second type of reaction undoubtedly involves direct reaction of the metal catalyst with the hydrocarbon substrate and/or with secondary autoxidation products. Two possible pathways can be visualized for the production of radicals via direct interaction of metal oxidants with hydrocarbon substrates, namely, electrophilic substitution and electron transfer. Both processes are depicted below for the reaction of a metal triacetate with a hydrocarbon.

304

ROGER A. SHELDON AND JAY K. KOCHI

Electron transfer : RH + M(OAc)3

e [RH]:

+ M(0Ac)z + AcO-

(135)

[RHIt 4 R * + H +

(136)

Electrophilic substitution:

e RM(0Ac)z + HOAC

RH + M(OAC)~ RM(0Ac)z

4 R'

(137)

+ M(0Ac)z

(138)

The overall result is a I-electron reduction of the metal oxidant with concomitant formation of the substrate radical (R.) and is the same in both processes. The ease of electron transfer oxidation of hydrocarbons by a particular oxidant is related to their ionization potentials (Table VII). However, the ease of elecTABLE VII Ionization Potentials of Organic Substratesa Substrate

I.P. (eV)

Alkanes: Methane Ethane Propane n-Butane Isobutane n-Pentane n-Hexane Cyclohexane

12.8 11.5 11.1 10.7 10.6 10.5 10.4 9.9

Alkenes: Ethylene Propylene 1-Butene 2-Bu tene Isobutene 1-Hexene Cyclohexene Butadiene

10.45 9.75 9.6 9.15 9.2 9.45 8.95 9.1

Substrate

I.P. (eV)

Aromatics: Nitrobenzene Benzene Fluorobenzene Chlorobenzene Bromobenzene Toluene Ethylbenzene Cumene o-Xylene Mesitylene Phenol Anisole Aniline Naphthalene Biphenyl Anthracene Methylnaphthalene Methylanisole Thioanisole Acetophenone Benzyl chloride Pyridme Methylpyridine tertButylbenzene Benzaldehyde

10.0 9.25 9.2 9.1 9 .o 8.8 8.75 8.7 8.55 8.4 8.5 8.2 7 .? 8.1 8.3 I .3 7.9 79 8.9 9A 9.2 9.25 9 .o 9.35 9.5

aIonization Potentials, Appearance Potentials and Heats of Formation of Gaseous Positive Ions, National Standard Reference Data System, National Bureau of Standards, 1969.

METAL -CATALYZED OXIDATIONS

3 05

trophilic substitution of a substrate by a metal complex is expected to parallel that of electron transfer. A distinction between the two processes on structurereactivity relationships alone is difficult. A clear delineation between the two processes would be provided by the observation of the intermediate species, i.e., the radical cation [RH] or the organometal [RM(OA&]. These two processes are used as a basis for the discussion of various examples of redox reactions between metal complexes and hydrocarbon substrates, or secondary autoxidation products, in the following. a. Alkenes. Bawn and Sharp216 studied the reaction between various olefms and Co(II1) salts in aqueous acid. The rate-determining step was considered to be the formation of the radical cation by 1-electron transfer. In aqueous medium, the radical cation formed, + . RCH=CHz + Co(II1) +RCH-CH2 + Co(I1) (139) reacts further with water to afford a complex mixture of aldehydes, carboxylic acids, ketones, and dienes. The powerful 1electron oxidant, cobalt(II1) trifluoroacetate in trifluoroacetic acid, readily oxidizes ethylene at ambient temperatures to afford ethylene glycol The oxidizing properties of cobalt (111) trifluoroacetate in ditrifluoroacetate? trifluoroacetic acid are probably due to the formation via dissociation of cationic species, such as C O + ( O ~ C C F ~which ) ~ , are very strong oxidants (electrophiles). The following electron transfer mechanism is suggested for the oxidation of ethylene:

''

CHz=CH2 + Co(02CCF3):

+ . CH2-CH2

+ CF3C02H

CF~CO~CHZCH + ~Co(02CCF3)3 '

-

C?H2-bH2

+ Co(02CCF3)2

+CF3C02CH2CH2' + H+ +[CF3C02CH2] 2 + Co(02CCF3)2

(1404 (140b) (141)

The oxidation of cyclohexene under the same conditions afforded a variety of products including cyclohexenyl and cyclohexyl trifluoroacetates, 1,2-cyclohexanediol ditrifluoroacetate, and a number of unidentified components. The complex mixture of products is probably formed via rearrangement of various cationic intermediates subsequent to the initial electron transfer process. As a preparative method, the oxidation of alkenes under these conditions also suffers from the competitive addition of trifluoroacetic acid to the olefip: \C=C

/

/

\

+ CF3COzH

I I +HC-C-OzCCF3 I I

(142)

In support of the electron transfer mechanism [Eqs. (139)-(141)], the ESR spectra of various radical cations have been observed during reaction of alkenes with Co(II1) in trifluoroacetic acid mixtures?" However, a very different situation may obtain in the cobalt-catalyzed autoxidation of olefms in neutral non-

306

ROGER A. SHELDON AND JAY K. KOCHI

polar media in which the electron transfer step [Eq. (139)] is much less facile and is in competition with the reaction of Co(II1) with hydroperoxide. reThe oxidation of olefins by manganese(II1) acetate in acetic sulted in the formation of ylactones,

and is analogous to the formation of lactones during the lead tetraacetate oxidation of olefins.222 The proposed mechanism?' involves the addition of carboxymethyl radicals, formed by the oxidation of acetic acid by Mn(III), followed by oxidation of the resulting radical: HOAc

+ *CH2CO?H

Mn(OAc)s-Mn(OAc)z 'CH2C02H + R C H = C H R 4 RGH-CHR

(144) (145)

I

CH2CO2H RCH-CHR

I

+ Mn(III)+

CHzCOzH RFH-CHR

I

RCH-CHR

+

I

+ Mn(I1)

(146)

CHzCOzH

9 , RCH-CHR

CHzCOzH

I

I

+H+

II

0

A further variant is the oxidation of olefins by Mn(II1) acetate in the presence of halide ions. Thus, oxidation of cyclohexene by Mn(II1) acetate in acetic acid at 70°C is slow, but addition of potassium bromide leads to a rapid reaction. Cyclohexenyl acetate was formed in 83% yield.223 In contrast to what would be expected for an electron transfer mechanism, norbornene (ionization potential 9 .O eV) was unreactive at 70"C, whereas cyclohexene (ionization potential 9.1 eV) and bicyclo[3,2,1] oct-2-ene reacted rapidly. The low reactivity of norbornene can be explained, if oxidation involves attack at the allylic position

bicyclo (3, 2, 1)octene-2

norborne ne

of the olefin. Hydrogen abstraction from the allylic position of bicyclo[3,2,1] oct-2-ene leads to a stabilized radical, but with norbornene this process is less

307

METAL-CATALYZED OXIDATIONS

favorable. The authors did not suggest, however, which radical is responsible for the hydrogen transfer. The following mechanism involving hydrogen abstraction by bromine atoms can be visualized: Mn(II1) + Br-

Bra + RCHzCH=CHz

RqH

CH2 + Mn(OAc)3

. )

Mn(I1) + Br'

(148)

+HBr + RCHICH\Hz +RCHCH=CHz I

+ RCH=CHCH20Ac + Mn(0Ac)z

I

OAc (150)

However, if reaction does involve bromine atoms as intermediates, one would expect to find products (including acetolysis) derived from addition of these radicals to the double bond, especially with olefins, such as norbornene, that possess unreactive allylic positions. Obviously the detailed mechanism of this reaction remains to be resolved. Recently, a new process for the conversion of ethylene to ethylene glycol has been developed?24a Oxidation of ethylene by molecular oxygen in acetic acid, in the presence of a manganese acetate-potassium iodide catalyst, gives ethylene glycol diacetate in 98%selectivity:

No mechanism has been proposed for this reaction but it could involve steps such as Mn(III)+ I-

+Mn(II)+ I *

(152)

I * + CHzrCHz

ICH2CHz.

(153)

ICH2CH2* + Mn(0Ac)s +ICH2CHzOAc + Mn(0Ac)z ICHzCH2 OAC+ AcO - +AcOCH~CHz OAC+ I

-

(154) (155)

Peroxy intermediates are also probably involved in the catalytic cycle, since Mn(II1) species are not readily regenerated from Mn(I1) with oxygen. Oxidation of alkenes to glycol mono- and diacetates has also been reported to proceed with oxygen in acetic acid in the presence of catalytic amounts of cerium acetate?Z4b It is reasonable to assume that reoxidation of Ce(II1) to Ce(1V) is a vital step in the catalytic cycle after electron transfer [cf. Eqs. (14011. Ketones, esters, and aldehydes (see Section II.B.3.e) are oxidized by Mn(I1I) and Ce(1V) acetates to give radicals that can add to olefins to form a variety of

308

ROGER A. SHELDON AND JAY K. KOCHI

interesting products. For example, oxidation of acetone with Mn(II1) or Ce(1v) acetate in the presence of olefins led to the following reactionszz5: 0

0

I1

CH3CCH3 + Mn(II1) [Ce(IV)]

II +CH3CCHz' + Mn(II)[Ce(III)] + H+

0

(156)

0

I1

II

CH3CCHz' + RCH=CHz

4

RtHCHzCHzCCH3

0

II

R6HCHzCHzCCH3 + Mn(0Ac)s or Ce(OAc)4 +

0

0

It

II

RCH=CHCHzCCH3 + RCHCHzCH2CCH3 (158)

I

OAc

Similarly, reaction of diethylmalonate with olefins in the presence of a mixture of Cu(I1) acetate and Mn(1II) or Co(II1) acetate provides a convenient route to y, &unsaturated acids or to y-lactonesz26a*b : CHZ(C0zEt)z + M(OAc)3 *CH(COzEt)z + RCHzCH=CHz

-

.CH(COZEt)z + M(0Ac)z + HOAC

(159)

RCH~CHCH~CH(C0~Et)~

(160)

b. Aromatic Hydrocarbons. The metal-catalyzed oxidation of alkylated aromatic hydrocarbons forms the basis for a number of important industrial processes such as the manufacture of terephthalic acid by liquid phase oxidation of p - ~ y l e n e ? ' ~ ~Oxidations *~ are usually carried out in acetic acid in the presence of high concentrations of cobalt and/or manganese acetate catalysts together with various promotors, such as metal bromides, aldehydes, or ketones. These reactions doubtlessly involve direct reaction between the aromatic hydrocarbon and the metal oxidant. Because of the commercial importance of these processes, extensive studies have been carried out on the reactions of cobalt and manganese acetates with aromatic hydrocarbons under autoxidizing conditions as well as in an inert atmosphere~27a~b*2Z8a*b Dewar and co-workerszz9-z31studied the stoichiometric oxidation of a number of aromatic ethers and amines in acetic acid. The inverse dependence of the

METAL-CATALYZED OXIDATIONS

309

rate on Mn(I1) concentration was interpreted as a reversible electron transfer oxidation by Mn(II1) to form the aromatic radical cation followed by ratedetermining loss of a proton. The resultant benzylic radical is oxidized further by Mn(III), and the overall stoichiometry for p-methoxytoluene is given by

The catalytic process is represented as ArCH3 + Mn (111)

[ ArCH3]? + Mn (11)

%ArCH; + Mn (11) + HOAC +ArCHzOAc + Ht

ArCHz* + Mn(II1) ArCH;

In this scheme, the rate of oxidation is dependent on the ionization potential of the aromatic substrate, in agreement with an electron transfer mechanism. Kooyman e l al.232-234showed that no direct reaction between the arene and Mn(II1) occurred in the manganic acetate oxidation of benzene, chlorobenzene, and toluene in acetic acid. Products were satisfactorily explained by the participation of carboxymethyl radicals as intermediates analogous to those formed in the manganic acetate oxidation of olefins [see Section II.B.3.al. Substituted benzyl acetates are produced as follows: Mn(0Ac)s

AIH+ .CHzCOzH ArCHzCOzH + Mn(II1)

-

ArCH2 + Mn (111) ArCH: + HOAc

The apparent duality of mechanism has been resolved by more recent studies?35*z36 in which the oxidation of aromatic hydrocarbons by manganic acetate has been shown to proceed by two competing pathways: (i) a free radical mechanism resulting from interaction of the arene with *CH2C02H radicals generated by thermolysis of manganic acetate, and (ii) and electron transfer mechanism that is of importanceZ35in the oxidation of aromatic hydrocarbons having ionization potentials less than ca. 8 eV. Products of the reactions were consistent with the following scheme^.?^' i. Hydrocarbons, such as toluene, that have ionization potentials greater than 8 eV are shown in Scheme 1.

310

ROGER A. SHELDON AND JAY K. KOCHI HOAc

Mn(OAc), --CH,CO,H PKH/

+ Mn(OAc), PIC.+

H3caC H

J J

MnW)

MnW)

t CH,CO,H H

.OAC

C C H z O A c

CH,CO,H

Scheme I

The mixture of arylacetic acids is subsequently decarboxylatedZ3' by Mn(0Ac) to a mixture of xylyl acetates:

ii. Hydrocarbons, such as 2-methylnaphthalene, that have ionization potentials less than 8 eV are shown in Scheme 2 on page 3 1 1. The route involving electron transfer can be suppressed by potassium acetate or manganous acetate or by carrying out the reaction under completely anhydrous conditions. Similar processes are also involved in oxidations with Ce(1V) acetate Catalysis of these reactions by potassidm bromide has been Aralkyl bromides are formed as intermediates that undergo acetolysis to the corresponding acetates under the reaction conditions. ResultsZJ6were consistent with a free radical mechanism, even with reactive toluenes, in contrast to the electron transfer mechanism observed in the absence of potassium bromide. Relative r e a c t i ~ i t i e scorresponded ~~~ closely to those observed in photochemical brominations, suggesting that bromine atoms, formed by electron transfer oxida.238a9b

METAL-CATALYZED OXIDATIONS

31 1

Scheme 2

tion of bromide ion by Mn(III), undergo hydrogen transfer with the substrate, i.e.,

+Bra + Mn(I1) ArCH3 + Br- +ArCHz * + HBr ArcHz * + Mn(II1)Br +ArCHz Br + Mn(I1) ArCHzBr + AcO- +ArCHzOAc + BrBr- + Mn(II1)

HBr + AcO- +HOAc + Br-

(1 73) (1 74) (1 75) (176)

(1 77)

In contrast t o oxidations with Mn(II1) acetate, the oxidation of alkylbenzenes by the stronger oxidant, Co(II1) acetate, appears to involve only electron transfer. No competition from classical free radical pathways is apparent. Waters and c o - w ~ r k e r s studied , ~ ~ ~ the ~ ~ oxidation ~~ of a series of alkylbenzenes by Co(II1) perchlorate in aqueous acetonitrile. They observed a correlation between the reactivity of the arene and the ionization potential of the hydrocarbon which was compatible with the formation of radical cations in an electron transfer process.

312

ROGER A. SHELDON AND JAY K. KOCHI

Other a ~ t h o r s observed ~ ~ ~ - that ~ ~ the ~ relative rates of oxidation of alkylbenzenes by Co(II1) acetate in acetic acid are the reverse of what would be expected for a classical free radical mechanism. For example, toluene is oxidized approxiSimilarly, in the stoichiometric mately 10 times as fast as cumene.z41~z42~z45 oxidation of p-cymene by Co(II1) acetate in acetic product formation occurred virtually exclusively via reaction at the methyl group t o give p-isopropylbenzyl acetate and p-isopropylbenzaldehyde:

$

CH,OAc

-L?E!L+ HOAc, 65’C

p

%

p%) CHO

)

t

(178)

By contrast, oxidation of p-cymene by Mn(II1) acetatez46 afforded products derived from reactions of intermediate p-cymyl and carboxymethyl radicals, indicating that reaction did not proceed via electron transfer. Similarly, the cobalt acetate-catalyzed autoxidation of p-cymene afforded mainly p-isopropylbenzoic a ~ i d , 2 derived ~ ~ *from ~ selective ~ ~ ~ oxidation ~ ~ ~ of the methyl group. Manganese acetate, on the other hand, was involved in a catalyzed autoxidation, CO,H

CO,H

I

I

$ 4Q 2 CH3

COlH (68%)

’

,c30

( 16%)

(180)

+

( 16%)

which gave mainly p-toluic acid and its precursor, p-methylacetophenone. p-Ethyltoluene , sec-butyltoluene and 1,l -di(p-toly1)ethane similarly afforded products mainly derived from oxidation of the methyl :

(181)

_L

68%

25%

METAL-CATALYZED OXIDATIONS

313

67%

These results were reconciled with a mechanism involving reversible formation of a radical cation by electron transfer oxidation of the hydrocarbon by C O ( I I I ) :~ ~ ~ ArCH3 + Co(II1)

[ArCH3]: +Co(II)

(184)

In the next step benzyl radicals are formed from the radical cation by loss of a proton: [A~CHJ]

+ArCHz- + H+

(185)

Products are then derived from subsequent reactions of the benzyl radical with Co(II1) acetate,

-

A ~ C H+~~ ~ ( I I I+ ) A ~ C H +~ c0(11) + %A ~ C H ~ O A ~

(1 86)

or, under autoxidizing conditions, with oxygen, ArCHz' + 0

2

-+ ArCHz02* +products

(187)

The authors suggestedz44 that proton loss [Eq. (lSS)] is controlled by stereoelectronic considerations and not by the thermodynamic stability of the product. In the preferred conformation, the tertiary hydrogen on the isopropyl group of the p-cymene radical cation is located in the nodal plane of the benzene ring where its interaction with the n system in the transition state is minimized. The methyl group, on the other hand, rotates rather freely, and the loss of any one of the hydrogens is, therefore, not conformationally restricted. The same electron transfer mechanism was proposed by Heiba et ~ 1 . and was supported by the observation by ESR of the radical cations of several arenes when they were treated with Co(II1) acetate in trifluoracetic acid.248 Cobalt(II1) is a stronger oxidant in trifluoracetic acid than in acetic (see later). In some cases (with electron-rich aromatics), radical cations were observed in acetic acid.242 Further evidence for the radical cation mechanism was obtained in the oxidation of p-methoxybenzyl phenyl sulfide.z42 The pro-

~

~

314

ROGER A. SHELDON AND JAY K. KOCHI

ducts were consistent with the following mechanism and could not be explained by a classical free radical mechanism:

0%

0%

OCH,

(3%

Similarly, the oxidation of thiophenol ethers by Mn(II1) acetate also proceeds via electron transfer”’ : H

C

e SCH,

Mn(0Ac)3

L

H

3

C

e SCH,OAc

(189)

Despite the overwhelming evidence in support of an electron transfer mechanism for the oxidation of aromatic hydrocarbons by Co(III), there is still much disagreement about the detailed mechanism, especially with the less reactive alkyl aromatics. Several detailed kinetic studies of these reactions have been carried out both in the a b s e n ~ e ~ ~ ’and - ~ ’p~r e ~ e n c e ~ ’ ~ - ~of’ *oxygen. The kinetics of these reactions are complicated by the fact that cobaltic acetate in acetic acid is a dimeric p-hydroxo species, which may be formulated as (ACO)~CO(OH)~CO(OA~)~ .259 Moreover, the rate-determining step in reactions (184)-(186) may vary for different hydrocarbons, depending on the stability of the radical cation that is formed. Most authors consider only reaction (184) t o be reversible. However, reactions (1 85) and (1 86) may also be reversible. Assuming that reaction (186) is rate-determining, the rate of disappearance of Co(III), in the absence of oxygen, should be second order in Co(II1) concentration and inversely dependent on Co(I1) concentration. Such kinetics have been observed in p r a ~ t i c e . ~ ~ ’ > ~ ’ ~ Indeed, these reactions are strongly retarded by Co(I1). In the stoichiometric oxidation of ethylbenzene by cobalt(II1) acetate in acetic acid, no reaction was observed253 when [Co(III)] = [Co(II)], This result was attributed t o the formaSimilarly, cobalttion of mixed Co(III)-Co(II) dimers that are i n a ~ t i v e . 1-253 ~’ (111) acetate oxidation of toluene is strongly retarded by other metal acetates, such as Mn(II), Mn(III), Ce(IV), and Cu(II), due to the formation of inactive polynuclear complexes (mixed valence d i m e r ~ ) . ~ ” Although the observed kinetics have generally been reconciled with an electron transfer mechanism via reactions (184)-(186), some authors253 prefer a mechanism involving direct, reversible formation of a benzyl radical without the intermediacy of a radical cation. Thus, in the stoichiometric oxidation of ethylben-

315

METAL-CATALYZED OXIDATIONS

zene, the rate of disappearance of Co(II1) was given by

d[Co(III)-Co(III)] 2kl k z [Co(III)-Co(III)] [RH] kz [CO(III)-CO(III)] + k1 [CO(III)-CO(II)] dt

.

(190)

The authors interpreted the kinetics to include a reversible formation of the benzylic radical by reaction of a cobalt(II1) dimer with the substrate: PhCHzCH3 + Co(III)Co(III)

k & PhtHCH3 + Co(III)-Co(II) + H+ k-I

PhdHCH3 + Co(III)Co(III) &products

(191) (192)

We suggest that the importance of a radical cation as a discrete intermediate will depend on its reactivity. With very reactive radical cations, e.g., monoalkylbenzenes, rapid proton transfer [Eq. (18S)l may occur predominantly within the solvent cage and free radical cations as such are never formed, e.g., r

0

+

+

HOAc

In the presence of oxygen the reduction of Co(II1) is not a good measure of the reaction rate, since the oxidant can be regenerated by reaction of Co(I1) with peroxy radicals or hydroperoxides. These reactions lead to the formation of aromatic aldehydes, which are the primary products of oxidation of methylbenzenes, e.g.,254

+ArCHO+ Co(II1)OH ArCHzOzH + Co(I1) +ArCH20' + Co(1II)OH ArCH202. + Co(I1)

bArCHO

(193)

(194)

The aldehyde is subsequently oxidized to the corresponding benzoic acid via the peroxy acid. In the presence of oxygen, reaction (186) is replaced by reaction (187). The 0 t her rate of reaction is first order in Co(II1) under these workers241*254-2s6have observed, however, a second-order dependence on Co(II1) concentration, which is more difficult to explain. A rate law containing both second-order and half-order terms in Co(II1) has also been reported"' for the cobalt-catalyzed autoxidation of toluene in acetic acid. The mixed kinetic expression was explained by the participation of reactions of both a Co(II1) monomer and a Co(II1) dimer with the substrate. The question arises as to whether inner-sphere complexes of the aromatic hydrocarbon with cobalt(II1) are involved in electron transfer. An investigation260 was carried out of the oxidation of alkylaromatic hydrocarbons by the heteropoly compound KS[Co(III)04W12036] *HzO. Electron exchange between the Co(II1) complex, which contains tetrahedral cobalt, and the corre-

316

ROGER A. SHELDON AND JAY K. KOCHI

sponding Co(I1) form proceeds via an outer-sphere mechanism. The oxidation products derived from xylenes and this oxidant in heterogeneous (hydrocarbon) and homogeneous (aqueous acetic acid) systems were also consistent with an aromatic radical cation as an intermediate. It is likely that radical ions were formed by outer-sphere electron transfer. Inner-sphere electron transfer is eliminated, considering the stability and inherent resistance to destruction of the polytungstate framework which totally screens the Co(II1) from direct interaction with the aromatic IT system. i. Effect of halide ions. The rates of oxidation of aromatic hydrocarbons are enhanced, often dramatically, by the presence of halide ions. Thus, bromide ions have a pronounced synergistic effect on cobalt- and manganese-catalyzed autoxidations of alkyl aromatic hydrocarbon^.?^^-^^' The discovery of this effect provided an important breakthrough in the manufacture of terephthalic acid.261 The normal cobalt-catalyzed autoxidation of p-xylene affords p-toluic acid, and further oxidation is very slow. In the presence of sodium bromide, further rapid oxidation takes place and terephthalic acid is formed in near quantitative yields. The addition of an equimolar amount of hydrogen bromide t o cobalt(I1) acetate in acetic acid produces cobalt acetate bromide, CO(OAC)~ + HBr

+CO(OAC)BI+ HOAC

(195)

which is claimed t o be an active catalyst for the oxidation of hydrocarbons.262 Thus, p-xylene is readily oxidized to terephthalic acid at atmospheric pressure and at temperatures as low as 60"-100°C. Tetralin is oxidized rapidly at room temperature to a-tetralone. The active catalyst is also formed from cobalt(I1) acetate and metal bromides via the equilibrium, Co(0Ac)z + NaBr

+Co(0Ac)Br + NaOAc

(1 96)

High concentrations of catalyst (approximately 0.1 M) are required for optimum reaction rates. Other metal ions, such as cerium and manganese, showed the same effect, but to a more limited extent. None of the other halogens approach bromide in activity. Hydrogen bromide enhances the rate of autoxidation of cumene.z62 The effect can be explained by the following scheme, in which a bromine atom replaces an alkylperoxy radical in the usual propagation sequence272: RO2' + HBr

+R02H + BI'

Br*+RH+R'+HBr R.+02

+RO2'

Such a scheme by itself is insufficient to explain the accelerating effect of hydrogen bromide on cobalt-catalyzed autoxidations since optimum rates are achieved only in the presence of both hydrogen bromide and cobalt. One of the functions of the Co(I1) is to maintain the concentration of hydrogen bro-

METAL-CATALY ZED OXIDATIONS

317

mide at a sufficiently low level by reaction (195), in order to prevent the acidcatalyzed rearrangement of aralkyl hydroperoxides to phenols. A second function of the cobalt is to reconvert benzylic bromides, formed during reaction, to bromide ion in order t o maintain the catalyst. Thus, addition of cobalt(I1) acetate to a warm solution of benzyl bromide in acetic acid immediately generates the intense blue color of cobalt acetate bromidez6' : Co(0Ac)z

+ PhCHzBr

+Co(0Ac)Br + PhCHzOAc

(200)

The rate-enhancing effect of bromide ion is explained by a scheme involving the formation of bromine atoms via electron transfer oxidation of bromide ion by Co(II1):

+Co(11) + Br. AICH3 + BI* +AICHz' + HBr

Co(1II) + Br-

ArCHz ' + 02 4 AICHzOz' AICHzOz* + Co(I1)

(203)

+AICHO + Co(1II)OH

bCo(I1I)Br

(204)

Reaction (201) occurs instantaneously on mixing cobalt(II1) acetate with lithium bromide in acetic acid.269 A trace of tert-butyl hydroperoxide is usually required to initiate these reactions. Oxidation of Co(I1) by hydroperoxide provides the Co(II1) necessary for reaction (201). In the presence of bromide ion there is apparently no direct reaction of Co(II1) with the hydrocarbon substrate, in contrast to cobalt-catalyzed autoxidations carried out in the absence of bromide. That different mechanisms are operating is illustrated by the relative rates of oxidation of alkylbenzenes catalyzed by cobalt acetate alone compared to those obtained in the presence of added bromide ion (Table VIII). In the presence of bromide ion, the relative reactivities are consistent with a mechanism involving attack by bromine atoms but not one involving electron transfer. Individual discrepancies in selectivities between bromine atom and the species active in the Co(OAc)z-NaBr system (Table VIII) were attributed to a bromine complex, Co(1II)Br

Co(II)(Br *)

of intermediate rea~tivity.'~' However, the differences in selectivity between the two series are not large, and the discrepancy can probably be attributed to the behavior of bromine atoms to the different conditions (solvent, temperature) under which these selectivities were measured. The rates of cobalt(II1) oxidations are also enhanced by chloride ions. Thus, the oxidation of toluene by Co(1II) acetate in acetic acid required more than a week for reaction at 65"C,but reacted in less than 2 hr at room temperature in the presence of a tenfold excess of lithium chloride. The products and relative reactivities of various alkylbenzenes were consistent with an electron transfer mechanism. The dramatic enhancement in rate was attributed to the formation

318

ROGER A. SHELDON AND JAY K. KOCHI TABLE VIII Relative Reactivities of Hydrocarbons toward Cobalt Oxidation Relative reactivity (per active hydrogen)

Hydrocarbon Toluene Ethylbenzene Cumene p-Methoxytoluene Durene p-Xylene

CO(OAC)~ (65°C)u 1.o 1.3 0.3 71e 275f 10.3

Co(OAc)2--NaBr (60" C) 1.o 8.3 16.8 3.4 3.8 1.5

-

RO2' (3 0" C)'

Br (40"Qd

1.0 9.3 15.9 -

1.o 17 37 -

-

-

-

-

'Heiba et al. (Ref. 242). %rniya (Ref. 265). CCumylperoxy radical [Howard, J. A., and Ingold, K. U., Can. J. Chem. 46, 1017 (1968)l. dRussell, G. A,, and DeBoer, C., J. Amer. Chem. SOC.85, 3136 (1963). eAt 105°C [Onopchenko et al. (Ref. 244)]. fCo(II1) + LiCl. [Heiba et al. (Ref. 242)].

of a Co(II1) complex of higher oxidation Holtz2703271compared the oxidation of alkylaromatics with two different catalysis, Co(OAc), HCl and Co(OAc),-NaBr. Oxidations were carried out at high temperatures (1 82OC) which are comparable to conditions of commercial processes. Oxidation of p-tert-butyltoluene and 2,2-bis(p-tolyl)propane in acetic acid, in the presence of Co(OAc), -NaBr afforded the corresponding carboxylic acids in high yield:

-

CO,H

I

319

METAL-CATALYZED OXIDATIONS

By contrast, oxidations catalyzed by CO(OAC)~ -HC1 were much less selective, and tert-butyltoluene, for example, gave considerable amounts of products resulting from C-C bond cleavage: I

oy

I

cIt13

C02H

CO,H

I

I

CO,H

t

CH3

I

t

CO2H

CHO I

t CO,H

I

A

0

These results are most readily explained by a radical mechanism involving bromine and chlorine atoms as the chain transfer agents, respectively, since it is known that chlorine atoms are much less selective than bromine atoms. Chlorine atoms will attack tert-butyltoluene at methyl groups in the benzylic and 0 positions (i.e., tert-butyl). Bromine atoms react selectively at only the benzylic methyl group. Attack at the tert-butyl methyl group could afford carboxylic acid by the following sequence of steps: CH3

I Ar-C-CH2' I CH3

CH3

CH3

CH 3

I I

I + 0 2 -Ar-C-CH202*-Ar-CC-CO2H I

CH3

I Ar-C-CO2 I

CH3

0

CH3 H

Co(II1)

Ar-C

CH3

II I -% AI-C-CH3 I

&Arc02

H

CH3

In summary, the oxidation of aromatic hydrocarbons carried out with high concentrations of cobalt catalysts involve two competing processes, namely, electron transfer oxidation of the hydrocarbon to the radical cation and electron transfer oxidation of the ligand to the corresponding radical: Co(II1)X

s

Co(II)+ [ArH]?+X-

(208)

Co(I1) + x-

(209)

The relative rates of these processes are dependent on several factors: (i) the ionization potential of the hydrocarbon, (ii) the oxidation potential of the anion in which the relative ease of oxidation is in the order Br- > C1- >> AcO-, and (iii) the temperature. The different results discussed in the foregoing for the

320

ROGER A. SHELDON AND JAY K. KOCHI

oxidation of alkylbenzenes in the presence of CO(III)-Cl- at various temperatures suggest that electron transfer to hydrocarbon is favored at lower temperatures. The electron transfer oxidation of acetate ion is rapid only at relatively high temperatures.274 ii. Effect of strong acids. The rates of oxidation of aromatic hydrocarbons by metal acetate oxidants are also dramatically enhanced in the presence of strong acids. It has recently been reported275 that Mn(II1) and CO(II1) acetates in the presence of strong acid activators, such as trichloroacetic or trifluoroacetic acid (TFA), rapidly and selectively oxidize aromatic side chains at 25OC. iVArCH2OAc

L

ArCH3 + CO(OAC)~

0 2

HOAc-H2S04

ArCH3 +Mn(OAc)j

o2

ArCO2H

(211)

>ArCHO

(212)

Enhancement by strong acids such as TFA is a general feature of oxidations with metal acetates. Metal trifluoroacetates in TFA are much more powerful oxidants (electrophiles) than the corresponding acetates in acetic acid. Activation of the metal oxidant in TFA has been observed with cobalt(III)2 7 i 2 4 9 ~ 2 75 76 manganese(II1): 3 7 i 2 75 le ad( IV) ,' 7-28 thallium( I I I ) ; ~ ~ -~~e~r ~i u m ( I V ) ? ~and ~ *~~o ~p p~e r ( I I ) . ~ ~Similarly, ' the electrophilic properties of ~ o p p e r ( 1 ) ~and ~ ' m e r ~ u r y ( I 1 )acetates ~ ~ ~ are strongly enhanced by replacement of acetate by trifluoroacetate. It has been p r o p ~ s e d that ~ ' ~ ~ ~ ~ ~ the potent oxidizing properties of Co(II1) trifluoroacetate are due to ionization t o the cationic Co(II1) species,

'

"12

'

l2

Co(02CCF3)j

C O + ( O ~ C C F ~+) ?CF3CO;

(213)

which would be a very reactive electrophile (oxidant). Coordinative unsaturation in such species would be optimized in highly acidic (poorly nucleophilic) media such as TFA. In accordance with these expectations, even electron-poor arenes, such as benzene, chlorobenzene, and bromobenzene are readily oxidized by Co(II1) in TFA at room temperature, t o give the corresponding aryl trifluoroacetates in high yield.276 By contrast, these arenes are completely inert to cobalt(II1) acetate in acetic acid even at higher temperatures. Formation of aryl trifluoroacetates involves two successive lelectron transfers. Such a reaction may proceed via radical cations

-0

+ CO(O,CCF~)~

+

Co(0,CCFJ;

32 1

METAL-CATALYZED OXIDATIONS

or arylcobalt(II1) species as intermediates, +

Co(O,CCF,),

+

Co(O,CCF,),

+

-

Co(O,CCF,),

+

CF,CO,H

2 Co(O,CCF,),

(217)

(218)

O,CCF,

Results of kinetic and ESR studies are consistent with an electron transfer mechanism [reactions (214)-(216)] . The electron transfer mechanism of oxidative substitution of arenes by Co(II1) in TFA contrasts with the analogous oxidation of the same arenes with Pb(IV) trifluoroacetate in TFA, AIH + 2Pb(OzCCF3)4

+ArOzCCF3 + CFjCOzH + 2Pb(OzCCF3)2

(219)

in which the detection and isolation of aryllead(1V) intermediates support an electrophilic substitution mechanism278~279a~b :

+ArPb(OzCCF3)3 + CF3COzH ArPb(OzCCF3)3 +ArOzCCF3 + Pb(OzCCF3)2

AIH + Pb(OzCCF3)4

(220) (221)

Similarly, unactivated arenes readily react with thallium(II1) trifluoroacetate in TFA to give the corresponding arylthallium trifluoroacetates, ArT1(0zCCF3)z, which are stable and do not readily decompose to aryl trifluoroacetates and T1(I).z82-z86 The rate of aromatic mercuration is increased by a factor of 7 X lo5 in TFA relative t o acetic acid as solvent.z92 The electron transfer mechanism [Eqs. (2 14)-(2 16)] cannot be distinguished a priori from the electrophilic substitution path [Eqs. (217), (218) and (220), (221)] . Both mechanisms depend on nelectron availability. Ionization potentials and Hammett parameters are indirectly relatedz9’ in these benzenoid systems insofar as the orbital from which the electron is removed by charge transfer has the same symmetry as the orbital that participates in electrophilic attack. In other words, mechanistic distinctions between rate-limiting electron transfer [Eq. (214)] and electrophilic substitution [Eq. (217)] cannot easily be made on the basis of substituent effects. Electron-releasing substituents facilitate and electron-attracting substituents hinder both processes. The difference between the Co(II1) and Pb(IV) oxidations is well illustrated by the reaction with toluene. Lead(1V) oxidation gives a mixture of tolyl trifluoroacetates in high yield.

322

ROGER A. SHELDON AND JAY K. KOCHI

By contrast, Co(II1) oxidation gives oligomeric products that result from further reaction of the toluene radical cation with toluene: H3C-(TJ

+

I

L

C

G

--+H3 , etc.

(222)

The decreased yields of aryl trifluoroacetates generally observed at high arene :Co(II1) ratios can be attributed to competition between reactions (222) and (215), (216). With reactive arenes, such as toluene and anisole, reaction (222) predominates even at low arene: Co(II1) ratios. We suggest that electron transfer and electrophilic substitutions are, in general, competing processes in arene oxidations. Whether the product is formed from the radical cation (electron transfer) or from the aryl-metal species (electrophilic substitution) is dependent on the nature of both the metal oxidant and the aromatic substrate. With “hard” metal ions, such as Co(III), Mn(III), and Ce(IV):89 reaction via electron transfer is preferred because of the low stability of the arylmetal bond. With “soft” metal ions, such as Pb(IV) and Tl(III), and Pd(I1) (see later), reaction via an arylmetal intermediate is predominant (more stable arylmetal bond). For the latter group of oxidants, electron transfer becomes important only with electron-rich arenes that form radical cations more readily. In accordance with this postulate, the oxidation of several electron-rich arenes by lead(IV)’* 1 * 2 8 9 and thalli~m(II1)’~’ in TFA involve radical cation formation via electron transfer. Indeed, electrophilic aromatic substitutions, in general, may involve initial charge transfer, and the role of radical cations as discrete intermediates may depend on how fast any subsequent steps involving bond formation takes place.

Finally, it should be mentioned that all the oxidative substitution reactions of aromatics discussed above are stoichiometric processes. Rather expensive reagents are employed, and the processes would not generally be suitable for syntheses on the industrial scale. They do, however, provide simple, attractive routes for bench-scale syntheses for &wide variety of substituted a r e n e ~284 ~~~, that are difficult to prepare by other methods. Moreover, electrochemical regeneration of the oxidant could provide for the use of catalytic amounts of expensive metal oxidants. c. Alkanes. Classical metal-catalyzed autoxidations of saturated hydrocarbons via the usual free radical chain mechanism tend t o produce complex mixtures of products. This difficulty can be largely attributed to the high temperatures generally required for the oxidation of alkanes because of their low reactivity. Extensive thermolysis of the labile products often result in smaller fragments.

METAL-CATALYZED OXIDATIONS

323

Moreover, the slight differences in reactivity of C-H bonds in alkanes to free radicals lead to indiscriminate attack of the hydrocarbon chains. Improvements in the efficiency and selectivity in the conversion of saturated hydrocarbons under relatively mild conditions is a desirable goal. This objective may be achieved by the selective activation of C-H bonds in alkanes by the use of suitable metal catalysts. The latter condition obtains in the selective microbiological hydroxylation of saturated hydrocarbons (see Section V.A), in which the enzyme probably interacts with the hydrocarbon via metal-catalyzed redox reactions. Onopchenko and Schulz have recently reported294a9 that simple alkanes can be selectively oxidized by molecular oxygen in the presence of relatively high concentrations of cobalt acetate in acetic acid as solvent (cf., arene oxidations). Thus, the usual autoxidation of n-butane employing small amounts of metal catalysts requires temperatures up to 170°C and higher. Acetic acid is formed with roughly 40% selectivity as the main component in a complex mixture of oxygenated products. By contrast, the oxidation of n-butane in the presence of high concentrations of cobalt(I1) acetate in acetic acid, together with methyl ethyl ketone as a promoter, proceeded readily at temperatures in the range 100°-1250C. Under these conditions acetic acid was formed in 83% selectivity (at approximately 80% conversion).294a’ Conversion of Co(I1) into Co(II1) preceded the attainment of the maximum rate. Moreover, the induction period was shortened and finally eliminated by increasing the Co(II1) concentration. This trend is consistent with a route for the direct interaction of substrate with Co(II1). Manganese acetate was ineffective as a catalyst under these conditions. Similarly, cyclohexane is readily oxidized by cobalt(II1) acetate in acetic acid at moderate temperature^.^^"-^ In the absence of oxygen at 80°C the main products were 2-acetoxycyclohexanone and cyclohexyl acetate. Cyclohexane was about half as reactive as toluene under these conditions. Oxidation with Co(II1) acetate in the presence of oxygen gave adipic acid as the main product. This reaction has been developed into a process for the single-stage oxidation of cyclohexane to adipic acid.296p2 9 7 Selectivities of approximately 75% have been claimed at roughly 80%cyclohexane conversion. Surprisingly, alkanes containing tertiary C-H bonds showed poor reactivity in these bi 295a-d Thus, isobutane was less reactive than n-butane, and methylcyclohexane less reactive than cyclohexane (cf., lower reactivity of cumene to toluene). In the series of normal alkanes, n-butane reacted faster than n-pentane. n-Undecane was unreactive. These results are inconsistent with a normal free radical autoxidation. The authors used the analogy with arene oxidations to postulate that formation of radical cations by electron transfer from the alkane to Co(II1) was a critical factor: RH+Co(III)

+

[ R H ] t + Co(1I)

k-1

324

ROGER A. SHELDON AND JAY K. KOCHI [RH]

R.

+ H+

(225)

In contrast to alkylaromatic oxidations,z42 no kinetic isotope effect was observed with the alkanes. This result was r e ~ o n c i l e d ~ ~ "with - ~ a mechanism in or kz,depending which the rate-controlling step is governed by K,,, i.e., (kl/kz) on the stability of the radical cation. For aromatic substrates K,, is significantly larger than that for the alkanes. Thus, electron transfer is rate-determining for alkanes. However, it is difficult to reconcile the observed relative reactivities of hydrocarbons with a mechanism involving electron transfer as the rate-determining process. For example, n-butane is more reactive than isobutane despite its higher ionization potential (see Table VII). Similarly, cyclohexane undergoes facile oxidation by Co(1II) acetate under conditions in which benzene, which has a significantly lower ionization potential (Table VII), is completely inert. Perhaps the answer to these apparent anomalies is to be found in the reversibility of the electron transfer step. Thus, k-l may be much larger than kz for substrates, such as benzene, that cannot form a stable radical by proton loss from the radical cation [Eqs. (224) and (225)]. With alkanes and alkyl-substituted arenes, on the other hand, proton loss in Eq. (225) is expected to be fast. Tanakaz96 found the relative rates of oxidation of cycloalkanes by Co(II1) acetate in acetic acid at 90°C to decrease in the order: C5 >C6 >C7-C12. He concluded that the rate-controlling step did not involve C-H bond rupture but, instead, formation of a complex between the alkane and Co(II1). The relative reactivities were attributed to steric hindrance in the formation of the complex, the structural features of which were not elaborated further. The rate of oxidation of cyclohexane by Co(II1) acetate in acetic acid is enhanced in the presence of bromide ions.265 By analogy with alkylaromatic oxidations (see Section II.B.3.b), these reactions probably involve chain transfer by bromine atoms [cf. Eqs. (201)-(204)]. In the presence of strong acid activators, such as TFA, cobalt(II1) acetate is capable of the selective oxidation of alkanes under mild conditions to alkyl acetates, ketones, or alkyl chlorides, depending on the reagents used.298 For example, the oxidation of n-heptane carried out at 25"C, is illustrated in the following examples: OAc

" .

n-C,H,,

+

Co(OAc),

&TFA-HOAc

\

C13CC02H HOAc, N 2

=

(81% selectivity)

(83% selectivity)

2

(80%selectivity)

325

METAL-CATALYZED OXIDATIONS

In the last example, trichloroacetic acid not only acts as a strong acid but also as a source of chlorine atoms, R. + Cl-C-

I

4 RCl

I

+

4

(226)

/

A combination of TFA and carbon tetrachloride, as chlorine atom source, gave similar results. An even more remarkable example of the unusual selectivity of this oxidant was observed in the oxidation of 2 - r n e t h y l ~ e n t a n e ~ ~ ~ :

r

i37 1

CH~CH(CH~)CH~CH~+ C CO(OAC)~ HJ C13CC02H)RCl HOAc

selectivity (%)

c-c-c-c-c 2 74

(227)

13

These unusual selectivities, which are analogous to those observed in the absence of strong acid activators (see the foregoing), is not easily explained by a mechanism simply involving hydrogen abstraction by a free radical. It was con~ l u d e d ' that ~ ~ these reactions involve reversible formation of alkyl radicals by direct reaction of the alkane with Co(II1): k

RH + Co(III)&

k-I

R- + Co(II1)

R * + Co(I1) + H+

(228a)

Co(I1) + R+. +products

(228b)

The actual mode of interaction between Co(II1) and the alkane was not elucidated. It could involve electron transfer as described above or it may be an example of a general class of electrophilic substitutions at saturated carbon centers in which attack at a u bond occurs via a trigonal (three-center) transition state,'Ooa e.g., -C-HI

I

+ COX:

-[-i

-3H

]

cox2

-H*\ -d-Co& I

+-C*

I + Cox2 I

I

,(229)

The situation is also highly reminiscent of hydrogen abstraction with rather reactive alkylammonium cation radicals.300b Reactions of alkanes with other large electrophiles, such as PCls, also exhibit unusually high (secondary C-H:tertiary C-H) reactivity ratios.300a It is expected that steric effects would become magnified with large electrophiles. Lead(IV)277 and ~ilver(II1)~~' trifluoroacetates in TFA also oxidize alkanes at room temperature to give alkyl trifluoroacetates [see also Section III.D.3 for reactions of alkanes with Pd(I1) trifluoroacetate] . The stoichiometric oxidation of cyclohexane to a mixture of cyclohexanol, cyclohexanone, and adipic acid by cobalt(II1) perchlorate in aqueous acetonitrile has also been reported. 240

32 6

ROGER A. SHELDON AND JAY K. KOCHI

Finally, it should be noted that the rate of oxidation of cyclohexane by Co(II1) trifluoroacetate in TFA was less than 10% that of benzene.'85 This comparison contrasts sharply with the much faster rate of reaction of cyclohexane with Co(II1) acetate in acetic acid (see p. 324). Obviously more work is required to explain such apparent anomalies and to elucidate the mode of interaction of Co(II1) with saturated hydrocarbons. d. Comparison between Chemical and Electro-oxidation of Hydrocarbons. The similarity between the chemical oxidation of alkenes, arenes, and alkanes by electron transfer oxidants and electrochemical o ~ i d a t i o n s ~ of ~ ~these -~~'~ substrates is noteworthy. The anodic processes are known to involve two 1-electron transfers and radical cations are intermediates. A variety of alkylbenzenes undergo anodic acetoxylation, in which the loss of an (Y proton and solvation of the radical cation intermediate form the basis of side-chain and nuclear acetoxylation, respectively.305a9 The nucleophilicity of the solvent can be diminished by replacing acetic acid with TFA. The attendant increase in the lifetimes of aromatic radical cations has been illustrated in anodic oxidations.308 Radical cations also appear to be intermediates in the electrochemical oxidation of alkanes and a l k e n e ~ . ~ ~ ~ ~ - ~ One aspect of electrochemistry that does not seem to have been studied very extensively is the oxidation of organic substrates with electrochemically generated This technique would allow for the oxidation of organic substrates by electricity (instead of oxygen) as the reagent and metal complexes as catalysts. One example of this type of reaction is the reported3" oxidation of substituted toluenes to the corresponding benzaldehydes by electrogenerated Mn(II1) salts. This approach is certainly worthy of further investigation and could prove to be a useful method for the facile and relatively inexpensive synthesis of a variety of compounds. e. AMehydes. The autoxidation of aldehydes is important industrially since it provides a simple route for converting linear aldehydes, obtained by hydroformylation of terminal olefins, to linear carboxylic acids. Traces of iron, copper, cobalt, and manganese salts catalyze the oxidation of aldehydes by air. Initiation by direct interaction between the metal catalyst and the aldehyde was proposed as long ago as 1931 by Haber and W i l l ~ t a t e r . ~ ' ~ ~ RCHO + Fe(II1) - - - + R e 0 + Fe(I1) + H+

(230)

Bawn and co-workers carried out detailed investigations of metal-catalyzed autoxidations of acetaldehyde3l3. 314 and b e n ~ a l d e h y d e . ~'16*',~ ~ The rate of chain initiation in the autoxidation of benzaldehyde catalyzed by cobalt acetate in acetic acid was equal to the rate of reaction of Co(II1) with benzaldehyde in acetic acid in the absence of oxygen. Moreover, the onset of oxygen absorption coincided with the conversion of Co(I1) to Co(II1). The catalyst was maintained

327

METAL-CATALYZED OXIDATIONS

largely in the Co(II1) state throughout the reaction. Hence, the rate-determining step is attributed to the reaction of benzaldehyde with Co(II1) to form chaininitiating benzoyl radicals Co(II1) + PhCHO +Co(I1) + PheO + H+

(231)

which are converted to perbenzoic acid PhdO + O2 PhCOj. + PhCHO

+FW03*

(232)

+PhC03H + PhcO

(233)

Cobalt(II1) is regenerated via oxidation of cobalt(I1) by perbenzoic acid: Co(I1) + PhC03H

*Co(II1) + PhCO2* + HO(01PhCO2-

(234)

+ HO')

Co(II1) + PhC03H 4 Co(I1) + PhCOj' + H+

(235)

Reaction (234) was shown in separate experiments to be very rapid. The reaction of Co(II1) with perbenzoic acid plays no fundamental role in the autoxidation, since the rate of reaction (235) is much slower than reaction (23 1). '. The oxidation of benzaldehyde is also catalyzed by nickel(I1) Reaction (237) is faster than reaction (236). Hence, oxidation of nickel(I1) is Ni(I1) + PhCO3H

Ni(II1) + PhCHO

+Ni(II1) + PhCOz + HO+Ni(I1) + PhcO + H+

(236) (237)

the rate-limiting step in the nickel-catalyzed autoxidation of benzaldehyde, in contrast t o the situation with cobalt- and manganese-catalyzed reactions. Consequently, most of the nickel species remains mainly in the lower oxidation state throughout the reaction. The oxidation of acetaldehyde31313149 318 differs from that of benzaldehyde in that acetaldehyde reacts with peracetic acid to form acetaldehyde monoperacetate3lg: cn,cno

t

cH,co,n

-

H

lo

c-cn H3

0 II

c

(238)

'0-0' 'CH,

In the cobalt- and manganese-catalyzed autoxidation of acetaldehyde, direct reaction of the latter with the catalyst in its higher oxidation state constitutes the rate-determining step. Co(II1) + CH3CHO +Co(I1) + CHJCO + H+

(239)

Regeneration of Co(II1) [or Mn(III)] occurs by Co(I1) + CH3CO3H

+Co(II1) + CHjC02' + HO(01CHjCOz-

+ HO')

(240)

ROGER A. SHELDON AND JAY K. KOCHI

328 and/or

0

II

Co(I1) + CH3CH(OH)02CCH3

+Co(II1) + CHJCHO. + CH3C02I

(241)

OH

Oxidation of cobalt(I1) acetate by peracetic acid is rapid and forms the basis of a method for the preparation of cobalt(II1) acetate.320 Depending on the conditions, metal-catalyzed autoxidation of acetaldehyde can be utilized for the manufacture of either acetic acid or peracetic acid.321 In addition, autoxidation of acetaldehyde in the presence of both copper and cobalt acetates as catalysts produpes acetic anhydride in high yield.3228* The key step in anhydride formation is the electron transfer oxidation of acetyl radicals by Cu(II), which competes with reaction of these radicals with oxygen:

C H ~ C O+ C ~ ( I I )+CH~EO+ c ~ ( I ) CH360 + CH3C02H +(CHBCO)~O+ H+

(242) (243)

Copper(I1) is known61-63b, t o be more effective than other metal oxidants for the electron transfer oxidation of radicals t o the corresponding cations. Oxidation of p i ~ a l a l d e h y d e ~by ~ ' Mn(II1) acetate affords carbon monoxide, isobutane, and isobutylene, presumably by the following steps: (CH&CCHO + Mn(II1)

+(CH3)3CCO + Mn(I1)

(244)

(CH3)3C' + CO

(245)

(CH3)3CCO

+(CH3)jCH + (CH3)jCkO

(CH3)3CS + (CH&CCHO

(246)

(CH3)sC- + Mn(II1) +Mn(I1) + (CH3)3C+ -+ ( C H B ) ~ C = C H +~ H+ (247) Decarbonylation of the pivaloyl radical, t o afford the stable tert-butyl radical is rapid. Nikishin and co-workers have carried out extensive studies of the reactions of aliphatic aldehydes with Mn(II1) and Co(II1) acetates in acetic acid in the presence of olefins. Depending o n the reaction conditions, a variety of interesting products are formed. In the presence of catalytic amounts of cobalt(I1) acetate and a limited oxygen supply, ketones are formed via the cobalt-initiated addition of acyl radicals to the olefin,324-326be.g.,

RCO

(248)

R C H O ~

0 RCO + R'CH=CH20

I1

II

R'CHCH2CR

(249)

0

II

+ RCHOR'CH2CH2CR + R e 0 (250) Reaction of aliphatic aldehydes with stoichiometric amounts of Mn(II1) acetate in acetic acid, by contrast, produces cy-formylalkyl r a d i ~ a l s ~ ~ ' - ~ ~ l : R'eHCH2CR

METAL-CATALYZED OXIDATIONS RCHzCHO + Mn(II1)

+RCHCHO + Mn(I1) + H+

329 (251)

A kinetic isotope effect was observed327 only for cm-substituted aldehydes. Acyl radicals, thus, are formed only as a result of the secondary reactions,

RCHCHO + RCH~CHO-+

RCH~CHO+ RCH~CO

(25 2)

In the presence of olefins, the a-formylalkyl radicals add to the 0lefin,3~'-~~'

+R'kHCH2CH(R)CHO

ReHCHO + R'CH=CHz

(253)

The resulting alkyl radical undergoes hydrogen transfer with the aldehyde R'CHCHzCH(R)CHO + RCHzCHO +R'CH2CH2CH(R)CHO + RCHzeO

(254)

or electron transfer with Mn(III), R'CHCHzCH(R)CHO + M n ( 0 A c ) ~+R'CH(OAc)CHzCH(R)CHO

(255)

In the presence of a catalytic amount of copper(I1) acetate, unsaturated aldehydes were formed331 by electron transfer oxidation of the intermediate alkyl radical by Cu(II), e.g., R'?HCH~CH(R)CHO+ CU(II) +R'CH=CHCH(R)CHO

(256)

With branched olefins, oxidative elimination was the main reaction even in the absence of Cu(II), e.g., with isobutene,

)=+

RCHCH~-)--CH~CH(R)CHO

(257)

These reactions provide yet another example of the generally observed trend (see Section II.B.3.c) that oxidations in the presence of high concentrations of metal catalysts proceed by different pathways than those in the presence of catalytic amounts. In the former case, direct reaction of the metal oxidant with the substrate is often implicated. At lower concentrations, the metal species produce chain-initiating radicals by reaction with peroxides. The precise mode of interaction between metal oxidants, such as Co(II1) and Mn(III), has not generally been discussed. It i s tempting to speculate, by analogy with the oxidation of hydrocarbons (see earlier), that oxidation involves direct electron transfer, e.g., H (25%

3 30

ROGER A. SHELDON AND JAY K. KOCHI

or electron transfer may proceed via the enolate in an outer-sphere process, RCHzCHO Mn(II1) + RCHCHO-

& RCHCHO+RCHCHO. + Mn(I1)

(260a)

or it may involve homolytic decomposition of an enolate salt, e.g.,

+RCH=CH-0-

RCH=CH-OMn(II1)

+ Mn(I1)

(260b)

Rate-limiting electron transfer has been suggested as the first step in the Co(II1) oxidation of ketones. However, the oxidation of aldehydes was thought to proceed by initial enoli~ation.~~' Aromatic aldehydes cannot, of course, react via the enol. f. Carboxylic Acids Direct reactions of metal catalysts with carboxylic acids are important for two reasons: (i) metal catalysts are usually present in the form of carboxylic acid salts (e.g., acetate or stearate), and (ii) carboxylic acids are secondary products of hydrocarbon autoxidation. Moreover, many autoxidations are carried out in acetic acid as solvent. The mechanism of the decarboxylation of carboxylic acids by lead(IV),333 ~nanganese(III),~~~ ~ o b a l t ( I I I ) , ~and ~ cerium(IV)288 has been well studied. Although there are some mechanistic differences, the formation of alkyl radicals by the reaction, RC02H -6 M"+

+M("-

')+ + R' + COZ + H+

(26 1)

represents an important pathway. In an inert atmosphere, alkyl radicals are converted to alkanes by hydrogen transfer with solvent. Radicals can also undergo electron transfer oxidation by the metal oxidant and afford products (alkene, ester, etc.) ascribable to carbonium ion intermediate^,'^^' z49* 2 8 8 9 333 namely, R* + SH R* + M"+

+RH + S -

(262)

+[R'] + .("-I)+

(263)

In the presence of oxygen, these alkyl radicals may initiate chain reactions leading to autoxidation. An alternative route for the oxidation of carboxylic acids not involving decarboxylation has been demonstrated for the reaction of manganese(III)z19-22'9 z32-234 and c e r i ~ m ( I V ) . ~ ~ ~ Carboxymethyl ~. radicals are formed in the reaction. M"+ + CH3COzH

-+

M("- ')+ + *CH2COZH+ H+

(264)

These pathways represent competing processes, and their relative contributions in the reactions of metal complexes with carboxylic acids are influenced by several factors. The structure of the alkyl group is important in oxidations with Mn(II1) and Co(III), since reaction (261) is a concerted process with these oxi-

METAL-CATALYZED OXIDATIONS

331

d a n t ~ . ~249 ’ ~ ? On the other hand, the photochemically induced decarboxylation with Ce(1V) proceeds stepwise via acyloxy radicals and the influence of the alkyl group on the rate is minimal.288 Availability of a-hydrogens is a factor in the nondecarboxylative pathway (264). Thus, it is most important in the reactions of acetic acid, in which the alkyl radical (methyl) is also of low stability. The Ce(IV) oxidation of carboxylic acids proceeds by reaction (261) when induced photochemically288 but mainly by reaction (264) when carried out thermally.238a~b The rates of these reactions are markedly enhanced by strong acids such as perchloric, sulfuric, TFA, and boron trifl~oride.”~’ 2 4 9 * 2889 ’34 g. Glycols. Oxidative cleavage of 1,2-glycols to carbonyl compounds is an important reaction in organic synthesis. It is usually achieved by the use of stoichiometric quantities of expensive reagents, such as heavy metal carboxylates or periodic deVries and S ~ h o r s ” have ~ reported that 1,2-glycols can be selectively cleaved by molecular oxygen at 100°C in aprotic polar solvents in the presence of catalytic amounts of cobalt(I1) salts. Depending on the reaction time, aldehydes or carboxylic acids can be isolated in high yields. No mechanism was suggested for the reaction. It presumably involves homolytic cleavage of the diol by Co(II1) and recycling of the Co(I1) by oxidation with intermediate peracids formed by further oxidation of the aldehydes. The mechanistic distinction between 1-electron and 2-electron oxidation of glycols by heavy metal acetates has been made.33sb

OH OH

I

I

0- OH

+ Co(1II)

RCH-CH’

I I +RCH-CHR’

0. OH

I

(265)

OH

I

RCH-CHR

+ Co(I1) + H+

I

.--)

RCHO + RGH +R’CHO etc.

(266)

h. Phenols. One of the characteristic chemical properties of phenols is their facile oxidation, which can be accomplished with almost any These reactions are not only of synthetic importance but they are also implicated in many biogenetic pathways.338 In 1967, van Dort and Geursen3” examined the oxidation of phenols by molecular oxygen in chloroform and methanol solutions at room temperature, in the presence of bis(salicylidene)ethylenediiminocobalt(II) (salcomine) as catalyst.

(=pJ<J= A

salcomine

332

ROGER A. SHELDON AND JAY K. KOCHI

The corresponding p-benzoquinones were formed in moderate yields, e.g.,

ll

0

This work was extended by Vogt and c o - w o r k e r ~ ~who ~ ' showed that the salcomine-catalyzed autoxidations of 2,6-disubstituted phenols can give high yields of p-benzoquinones [reaction (267)] or diphenoquinones,

@.- OH

(268)

, l C 0 -r 0

R

R

R

depending on the conditions. The formation of benzoquinones was favored by high catalyst concentrations and at low temperatures. Diphenoquinones were obtained in good yields at low catalyst concentrations and high temperatures. Higher rates and improved selectivities to p-benzoquinones were also obtained in dimethylformamide as solvent.341 The mechanism of this reaction is rather obscure. It has been known for more than 25 years that salcomine can combine reversibly with molecular oxygen via a two-step sequence [L = bis(salicy1idene)ethylenedimine] 342a-d : LCO(I1) + 0 2 Lco(III)-o-o'

+ LCo(I1)

e Lco(III)-o-o' Lco(III)-o-o-co(III)L

(269)

(270)

The paramagnetic 1 : 1 adduct is probably the active catalyst in these reactions. The initial step may involve hydrogen transfer or electron transfer to give aryloxy radicals that react further immediately or diffuse out of the solvent cage and react with another molecule of catalyst or with themselves (Scheme 3). More work 'is necessary to resolve the mechanism of this interesting and synthetically useful reaction. The stoichiometric coupling of phenols to biphenols by oxidation with manganic tris(acety1acetonate) has also been reported,343e.g.,

METAL-CATALYZED OXIDATIONS

333

R

t

R\ r "

R

Scheme 3

When phenols are oxidized by molecular oxygen in the presence of coppermine complexes as catalysts, oxidative polymerization to polyphenylene ethers r e s ~ l t s , 3 e.g.9 ~~-~~~

334

ROGER A. SHELDON AND JAY K. KOCHI

R

The latter have a wide variety of applications. The mechanism has been discussed, but the precise. role of the catalyst in these reactions still remains somewhat o b s c ~ r e . ~ ~ ~ ~ - ~ i. Thiols. The oxidation of thiols is an important process in the petroleum industry since it occurs during the "sweetening" of oil products to remove obnoxious thiols. Metal catalysts are usually employed to enhance the rate of oxidation. A number of studies of the direct reactions of metal complexes of variable valency with thiols have been carried out.351-355 The facile oxidation of thiols by M n ( a ~ a c )in ~ the absence of oxygen was explainedJ5' by the following reaction RSH + Mn(II1) + RS' + Mn(I1) + H+ (273) RS* + Mn(II1) + RS+ + Mn(I1) (274) RS' + RSH

+ RSSR + H+

(275)

The inefficient trapping of thiyl radicals by dodecene-1 was attributed to the effective interception of the radicals by Mn(III), resulting in electron transfer oxidation to the thioxonium ion. By contrast, thiyl radicals formed in the oxidation of thiols by the weaker oxidant, ferric octanoate, were scavenged by dodecene-1. Disulfide was formed by dimerization of thiyl radical^.^" Thus, the mechanism for disulfide formation is dependent on the nature of the metal oxidant. j. Effect of Direct Reaction on Kinetics o f Autoxidation. If the reduction of the metal complex in the higher oxidation state occurs by direct attack on the substrate, 02

M"+

+ RH + M("-')+ + H+ + R- + ROz*

(276)

the kinetics of autoxidation still retain43 the form given in Eq. (1 11). Specifically, n = 2 and f = 4 provided that the alkyl hydroperoxide is involved as an oxidant in a reaction such as (53). Under these conditions the rate expression has the same kinetic form as that obtained by thermal initiation, as given in Eq. (12). Thus, replacing the alkyl hydroperoxide with the substrate RH to carry out the reduction of M"' does not lead to a basic change in kinetic form. 4. Reaction of Metal Catalysts with Free Radicals-Catalyst-Inhibitor Converxion

The reactions of metal catalysts with alkylperoxy radicals must be considered in liquid phase autoxidations, since peroxy radicals are the most abundant species in solution. The reduction of alkoxy radicals to the corresponding

METAL-CATALYZED OXIDATIONS

335

alcohols is well-known (see earlier), but the reaction, RO-

+ I&-')+

-+

RO-

+ M"+

is relatively unimportant in autoxidations since alkylperoxy radicals are present in much higher concentrations. Inhibition of autoxidations by transition metals in low oxidation states, such as Co(I1) or Mn(II), has often been Transition metal complexes often behave as catalysts at low concentrations but as inhibitors at high c o n ~ e n t r a t i o n s . ' ~ ~ There ' ' ~ ~ has been some question as to the cause of this phenomenon. Since alkylperoxy radicals are relatively strong oxidants, they can react with the reduced form of metal catalysts: ROz. + M("-')+ A RO~M"+ (278)

-

In the early stages of autoxidations, hydroperoxide concentrations are low and chain initiation is inefficient. Under these conditions, Mn(I1) and Co(I1) can act as inhibitors by scavenging alkylperoxy radicals [reaction (278)] . Competition in the termination step between the usual bimolecular termination of peroxy radicals and their reaction with metal complexes can affect the chain length of the autoxidation. The expression for the chain length in a process involving bimolecular termination of peroxy radicals is chain length = k , [RH]/(2k&)'".

(279)

If termination occurs with the metal complex, it is chain length = k, [RH] / k ; [M"].

(280)

The phenomenon of catalyst-inhibitor conversion'42i143, 3 5 6 may be understood and critical concentration of metal can be deduced by reference to Eq. (280). If decomposition of the hydroperoxide is the source of initiation, it must be formed as rapidly as it is consumed to maintain a steady rate. If termination by metal complex predominates, a steady state occurs when the right-hand side of Eq. (280) equals unity. No oxidation will occur when this quantity is less than unity. Hence, catalyst-inhibitor conversion is observed as the metal concentration is increased to the point that the chain length becomes less than unity. If termination occurs by the bimolecular reaction of peroxy radicals, a chain length of less than unity will result in the depletion of the hydroperoxide until the rate of initiation has decreased to the point where the chain length is unity again. No inhibition is expected or observed. A topic related to the foregoing discussion concerns metal complexes, such as many sulfur-containing metal c h e l a t e ~ , 3 ~ 'that - ~ ~are ~ capable only of inhibiting autoxidations. The detailed mechanisms of the inhibiting action of these metal complexes are not very well understood. Recent results362 suggest that zinc dialkyldithiophosphates react with alkylperoxy radicals at the metal center, which could involve electron transfer or an S H reaction: ~

336

ROGER A. SHELDON AND JAY K. KOCHI

5 . Factors Affecting the Activity of Metal Catalysts In the preceding sections, we discussed the various interactions that may be implicated in metal-catalyzed autoxidations. In a particular system, several reactions may be occurring simultaneously. The overall influence due to environmental factors affecting each reaction is difficult to predict. Some general points concerning the influence of the factors o n metal-catalyzed autoxidations will be treated separately, although they are all interrelated. a. Influence of the ParticularMetal Complex. The reactions described heretofore are redox processes and are only affected by metals species with variable oxidation states. Hence, the redox potential of the metal couple (see Table V) is a factor to be considered. Generally, the maximum rate of oxidation increases with the redox potential of the metal. Thus, cobalt, manganese, and cerium usually induce the highest rates. Copper and iron give somewhat lower rates. It should be emphasized, however, that there is no a priori reason for expecting a correlation between a thermodynamic quantity such as a redox potential and a kinetic phenomenon associated with catalytic activity. If such a correlation exists it is highly qualitative at best. b. Temperature. At relatively low temperatures, the rates of the catalyzed oxidation are quite different from the uncatalyzed rates. However, as the temperature is raised, the difference in these oxidation rates decreases, since the chain process can develop rapidly at sufficiently high temperatures.

c. Solvent Effects-Physicochemical Properties of Metal Catalysts in Solution. Metal catalysts are usually added in the form of carboxylic acid salts (stearates, acetates, naphthenates). In a polar solvent the salts dissociate into ions, but, even in acetic acid, very few salts are dissociated beyond ion pairs.36s Conductivity measurements show that no dissociation takes place in hydrocarbon s01vents.j~~ Increasing the concentration of carboxylic acid salts in nonpolar solvents leads to micelle f ~ r m a t i o n . ~ ~ ~ - ~ ~ ' Many transition metal carboxylates exist as dimers or higher aggregates in solution. Such an association utilizing bridging ligands is chemical in nature, in contrast to micelle formation which is largely a physical process. For example, cobalt (111) has a pronounced tendency to form multicenter complexes with bridging hydroxyl groups. Detailed studies of the properties of solutions of cobalt (111) carboxylates have been reported.217 , 2 4 9 Cobalt(II1) acetate was shown to possess the dimeric structure, (AcO)~CO(OH)~CO (OAc), .249 Other authors370 have concluded that Co (111) acetate has the trimeric structure,

METAL -CATALYZED OXIDATIONS

337

CO~O(OAC)~(HOAC)~. Similarly, copper(I1) acetate371 exists as a dimer and palladium(I1) acetate372 as a trimer in acetic acid. The solvent may also influence the rates of the various steps in the autoxidation to differing degrees. For example, in the autoxidation of cyclohexane in a variety of solvents,373aibthe dielectric constant of the medium had no effect on the rate constant for propagation. The medium, however, strongly influences the rate constant for termination (ROz * t ROz *), which involves an interaction of two dipoles. d. Catalyst Deactivation-Macroscopic Stages in Metal-Catalyzed Autoxidations of Hydrocarbons. A phenomenon commonly observed in metal-catalyzed autoxidations of hydrocarbons is the buildup of the rate to a maximum value followed by a subsequent decrease, possibly even to zero in some cases. The effect is often due to catalyst deactivation and may be caused by a number of factors.18a-c In the autoxidation of neat hydrocarbons, catalyst deactivation is often due to the formation of insoluble salts of the catalyst with certain carboxylic acids that are formed as secondary products. For example, in the cobalt stearatecatalyzed oxidation of cyclohexane, an insoluble precipitate of cobalt adipate is formed. Separation of the rates of oxidation into macroscopic stages is not usually observed in acetic acid, which is a better solvent for metal cornplexes. Furthermore, carboxylate ligands may be destroyed by oxidative decarboxylation or by reaction with alkyl hydroperoxides. The result is often a precipitation of the catalyst as insoluble hydroxides or oxides. The latter are neutralized by acetic acid and the reactions remain homogeneous. In some cases (e.g., gasoline), autoxidation of hydrocarbons is undesirable, and trace amounts of metal catalysts may often be deactivated by the addition of suitable chelating agents. The latter affect the catalytic activity of metal complexes by hindering or preventing the formation of catalyst-hydroperoxide or catalyst-substrate complexes by blocking sites of attack or by altering the redox potential of the metal ion. e. Effects of Products of Oxidation-Co-oxidations. The products of autoxidations can have a marked effect on the rates of these reactions. The dramatic effect caused by aldehydes formed in the metal-catalyzed autoxidations of hydrocarbons is a pertinent example. Thus, in the oxidation of n-decane, the Mn(II1) concentrations passes through a maximum that coincides with the appearance of aldehydes in the reaction products.18a-c In general, liquid phase autoxidations on hydrocarbons after the initial stages take place, may be considered as co-oxidations with aldehydes, alcohols, ketones, carboxylic acids, etc. Often aldehydes or ketones are deliberately added to hydrocarbon autoxidations in order to promote the reaction. For example, in the cobalt-catalyzed oxidations of alkylaromatics (see Section II.B.3.b), aldehydes, or methyl ethyl ketone are usually added in commercial processes in order to attain high rates and eliminate induction periods.

338

ROGER A. SHELDON AND JAY K. KOCHI

f. Ligand Effects. The ligands coordinated to the central metal atom can affect its activity in several ways: (i) the ligand can simply influence the solubility of the catalyst in the reacting medium; (ii) the ligand may affect the redox potential of the metal ion, or (iii) ligands may affect complex formation between catalyst and substrate. The wide use of metal catalysts in the form of salts of carboxylic acids, particularly those of long-chain fatty acids or naphthenic acids, is due to their increased solubility in hydrocarbons and to their ready availability. In acetic acid as solvent, metal catalysts are usually added as their acetates. Acetylacetonate complexes are also readily available. They are usually soluble in nonpolar solvents and are often used as autoxidation catalysts.374aib It is doubtful, however, whether the acetylacetonate ligands survive these conditions since they readily undergo destructive o ~ i d a t i o n . ~ ~ ’ - ~ ~ ~ Metal phthalocyanine complexes are also frequently used as autoxidation catalysts (see Section II.B.2). They have generally been found to be more active than the corresponding stearates or acetylacetonates. Thus, Uri14’ compared the catalytic activity of a series of transition metal stearates with the corresponding metal phthalocyanines in the autoxidation of methyl linoleate. The phthalocyanine complexes afforded faster rates of oxidation. In addition, the phthalocyanine ligand is stable and is not easily destroyed under autoxidizing conditions. Interest in metal phthalocyanine catalysts has also been stimulated by their resemblance to the metal-porphyrin structures contained in many oxidative enzymes (see Sections II.B.2 and V). In recent years, many new processes, such as hydrogenation, hydroformylation, isomerization, oligomerization, and polymerization, have utilized organometallic complexes of the platinum group metals as catalysts. These complexes are “soft” catalysts,16 and catalysis generally involves activation of some small molecule378 such as hydrogen or carbon monoxide by coordination. Many of these complexes have been tested as autoxidation catalysts (see Section II.B.2). However, it is extremely doubtful whether such complexes, which usually contain readily oxidizable ligands (e.g., triphenylphosphine), are stable under oxidizing conditions. They generally give the same results as the corresponding metal carboxylates or acetylacetonates. Their activities can be explained adequately via the usual redox cycles involving metal-catalyzed decomposition of alkyl hydroperoxides. There is, however, still a great deal of interest in this type of catalyst (see Section 111).

g . Mixed-Metal Catalysts-Synergism and Antagonism. When the combined effect of several catalysts (or inhibitors) on the rate of reaction is greater than that expected by simple addition of the effect due to each catalyst, the action is referred to as synergism. If the result is less than that expected from combination of the separate effects, it is described as antagonism. These two effects are often observed in metal-catalyzed autoxidations. The causes are often not very well understood, especially in view of the complications encountered with only

3 39

METAL-CATALYZED OXIDATIONS

one metal. The myriad of possible reactions that may be affected when two catalysts are present makes it very difficult to assign any quantitative interpretations to these effects except in a few cases. These effects, however, are very important from a technological point of view since many catalysts used in industrial processes are mixtures of more than one metal. Whether antagonism or synergism is observed is often dependent on slight changes in the structure of the substrate. Thus, Kamiya263a9bobserved that the activity of cobalt was very much lowered by mixing it with manganese during the metal-catalyzed oxidation of tetralin in acetic acid in the presence or absence of sodium bromide. The antagonism was attributed to more efficient termination resulting from the interaction between peroxy radicals and manganese(I1). By contrast, when approximately 20% of the cobalt was replaced by manganese in the oxidation of p-xylene in acetic acid solutions, a fivefold increase in the rate of oxidation was ~ b s e r v e d . ~Similar ~ ~ ~ - synergistic ~ effects were observed in the oxidation of e t h y l b e n ~ e n e , ? ~cumene ~ ~ - ~ ,264a-c and p-toluic acid.262 Synergism was a s ~ r i b e d to ~ ~increased ~ ~ - ~ chain propagation by reactions. M(I1) Br + RO2 M(II1) Br

j

M(II1) Br + ROY

(283)

M(II) + Br*

(284)

As the relative percentage of manganese was increased beyond 70%, a pronounced drop in the rate occurred and chain termination was predominant. Other workers have suggested different explanations for these results (see discussion by de Radzitzky in Ref. 264a). The use of mixed-metal catalysts can also dramatically affect the products of autoxidations. An example mentioned earlier is the selective oxidation of acetaldehyde to acetic anhydride in the presence of a mixture of cobalt and copper acetates. Another example is the co-oxidation of alkanes and olefins in the presence of both an autoxidation and an epoxidation catalyst (see Section 1II.B): autoxidation catalyst

RH + 0 2

(285)

RO2H

0 \

/

/

\

C=C

epoxidation catalyst

+RO2HF -

\

/ \

C-C

/

/ \

+ROH

(286)

A further variant on this theme utilizes the olefin itself as the hydroperoxide source.

I I I. Heterolytic Mechanisms In contrast to the homolytic autoxidations discussed in the preceding section, there is no heterolytic process known for the oxidation of organic substrates

340

ROGER A. SHELDON AND JAY K. KOCHI

by molecular oxygen in the absence of metal catalysts. (Oxygen in its first excited 'A, state is not considered in this ont text.)^" Although reactions of organic substrates with molecular oxygen are generally favorable thermodynamically, there are no low-energy pathways for reaction since molecular mechanisms for these interactions are both spin and symmetry forbidden. In principle, one may expect the heterolytic oxidation of organic substrates by molecular oxygen to be promoted by transition metal catalysts. This process may be achieved by activation of either the substrate or oxygen. Homolytic autoxidations of hydrocarbons often give complex mixtures of products-the autoxidation of olefins is a prime example. There is a great incentive, therefore, to search for catalysts that can promote the selective oxidation of olefins by essentially nonradical mechanisms. For example, there is no method available for carrying out the selective epoxidation or oxidative cleavage of olefins (see Section 1II.C) by molecular oxygen. In order to be successful, any heterolytic pathway for the metal-catalyzed oxidation of a substrate must, of course, be considerably faster than the ubiquitous homolytic processes for autoxidation. Thus, the metal catalysts discussed in the following sections, in addition to being able to promote heterolytic oxidations, are also able to catalyze homolytic processes. Similar to homolytic mechanisms, the heterolytic reactions can be divided into three groups: (i) reactions with hydroperoxides, (ii) activation of molecular oxygen, and (iii) direct reaction of metal complexes with substrate.

A. FUNDAMENTAL ROLESOF METAL CATALYSTS IN HETEROLYTIC OXIDATIONS We have seen in the first section how the concepts of electron and ligand transfer via 1-electron changes provides a basis for the understanding of homolytic oxidation mechanisms. Similarly, the concepts of substrate activation by ~l-~~~ coordination380 to metal complexes and by oxidative a d d i t i ~ n ~provide a basis for discussing heterolytic mechanisms. Examples of the former are the activation of hydroperoxides (Section 111.B.2) and olefins (Section II1.D) to nucleophilic attack by coordination to metal centers. Although the addition of free radicals to metal centers, leading to oneequivalent oxidation of the metal [see Eq. (288)J is an oxidative addition, we use the term here to describe those additions of substrates to metal centers that involve overall twoequivalent change^.^^'-^^^ The reactions of alkyl halides with cobalt (11) or with iridium(1) provide examples of one-equivalent and twoequivalent oxidations of the metal center, respectively: Co(I1) + RX Co(I1) + R *

Ir(1) + RX

----j

Co(II1) X + R'

+Co(II1) R + RIr(1II)X

(287) (288)

(289)

341

METAL-CATALYZED OXIDATIONS

Activation of small covalent molecules such as hydrogen or oxygen, or of organic substrates by oxidative addition plays a vital role in many homogeneous, transition metal-catalyzed p r o c e ~ s e s . ~ ' ~ -The ~ ~ 'detailed mechanisms of many of these processes are not particularly well understood. For example, reactions of alkyl halides involving two-equivalent changes have been postulated to proceed via an s N 2 process, a concerted three-center addition, or a free radical chain process to conform to the observation of or racemization3g0a'b*391a1b at the alkyl center, respectively. The relative reactivities of alkyl halides, e.g., methyl > ethyl > isopropyl> tert-butyl, which are often observed are consistent with nucleophilic attack by the metal at carbon in an s N 2 ~ ~ o c ~ However, s s . the~ facile ~ ~addition ~ ~ of~aryl~ and vinyl halides to nickel(O), palladium(O), and platinum(0) complexes is not consistent with a ~ presented ~ S ~ simple SN 2 process.394-401 Osborn and C O - W O ~ a*b~have evidence for a free radical chain process in the addition of alkyl bromides to iridium(1). The propagation sequence involves a ligand transfer process: R*+ h(I) Rh(I1)

+ RBI

+ RIr(I1) 4

(290)

RIr(II1) Br + R' etc.

(291)

Recently, a nonchain radical mechanism has been proposed402 for the oxidative addition of methyl iodide, ethyl iodide, and benzyl bromide to tris(tripheny1phosphine)platinum (0): PtL3

e PtLz + L slow

PtLz +RX R*+PtXLz

(292)

PtXLz + R'

(293)

RPtXLz

(294)

(L = PPh3)

This mechanism was based on the ESR observation of nitroxide adducts, f-Bu(R)NO*, using tert-nitrosobutane as a spin trap. However, interpretation of the results should be treated with caution since spin trapping by itself is an unreliable criterion, and alkyl radicals may be formed by homolytic decomposition of the product(s). Thus, the analogous oxidative adduct PhCHzPd(Ph3P)zC1, formed from (Ph3P)4Pd and benzyl chloride, afforded the nitroxide radical, f-Bu(PhCH,)NO*, on mixing with fert-nitrosobutane in b e n ~ e n e . 4 " ~ HO ~ ~Wever, it was shown that addition of optically active ( ~ - ~ - b e n zchloride yl to PdL4 proceeded with inversion of configuration, consistent with an s N 2 process: PdLz + RX RPdL:+X-

* RPdL?++X* RPdXLz

(295) (296)

One-electron and 2-electron (sN2) transfers are probably competing processes in the reactions of many low-valent metal complexes with organic halides.3g1a,b9404The mechanism dominant in a particular situation will no doubt be dependent on the nature of the substrate and the metal complex. It is

~

~

~

~

342

ROGER A. SHELDON AND JAY K. KOCHI

dangerous to draw conclusions from relative reactivities alone, since steric effects can play an important role in reactions of many of these complexes, particularly those containing bulky ligands (e.g., triarylphosphines). The activation of C-H bonds in hydrocarbons by oxidative addition to lowvalent platinum group metal complexes is also feasible. This problem is discussed in more detail in Section III.D.3).

B. HETEROLYTIC REACTIONSOF METAL-HYDROPEROXIDE COMPLEXES Metal-catalyzed reactions of hydrogen peroxide and organic hydroperoxides can be divided into two groups. The first group, which consists of metals such as Co, Mn, Fe, and Cu, facilitate homolysis of the 0-0 bond and have already been discussed. The second group includes metals such as V, Mo, W, and Ti, which in their high oxidation states can facilitate heterolysis of the 0-0 bond in hydrogen peroxide and organic hydroperoxides. 1. Hydrogen Peroxide-Metal Catalyst Systems

Many acidic metal oxides, such as Os04, W 0 3 , MooB,CrO, ,VzO,, Tid2, and SeOz, catalyze the reactions of hydrogen peroxide via the formation of inorganic peracids. These reagents, generally known as Milas' reagent^:^^^*^ closely resemble organic peracids and readily undergo reactions with nucleophiles at the 0-0 bond. Thus, many of these metal oxides (or salts) catalyze the oxidation of iodide ion by hydrogen peroxide. The mechanism involves nucleophilic displacement by iodide ion on a peroxidic oxygen, in which the conjugate base of an inorganic acid provides a good leaving group, namely, 0

II

07

0

H O - M ~ 'OH

(1

,)

___t

I1 HO-Mo-0-

t

II 0

0

IOH

These reagents were first used for the bishydroxylation of ~ l e f i n s . " ~ ~It- ~ ~ * was later found that many of these reactions proceed via epoxides that undergo subsequent hydrolysis under the acidic conditions employed. Under basic or neutral conditions, these reagents can be used for the epoxidation of olefhs409-414. OH OH RCHTCHR' + H2Oz

catalyst

/O\ RCH-CHR'

H20

I

I

7 RCH-CHR'

(298)

It is significant that all of these catalysts are known'" to form stable inorganic peracids. Similar mechanisms are probably applicable to the epoxidation of olefins by both organic and inorganic peracids as follows:

343

METAL-CATALYZED OXIDATIONS

n Low selectivities in metal-catalyzed epoxidations with H202 are generally caused by further facile reactions of HzOz with the epoxide. Perhydrolysis [Eq. (300)] is also catalyzed by the metal complex, e.g.,415

0

OH

0 / \ +(CH&C-CH2 MoWI)

(CH~)~C--CH~OZH + (CH&C=CHz

I

+ (CH3)2C-CH20H

OH

I

OH

(301)

In general, the metal catalyst-hydrogen peroxide reagent is inferior to the corresponding metal catalyst-alkyl hydroperoxide systems for the epoxidation of olefins (see Section III.B.2). Tungsten and vanadium compounds also catalyze the oxidation of mines4 l6 and sulfides41 by hydrogen peroxide in a manner analogous to oxidations with organic peracids. Cyclohexanone oxime is produced by the reaction of cyclohexanone with ammonia and hydrogen peroxide in the presence of tungstic acid as catalyst!18 The key step in the reaction is probably a W(V1)-catalyzed epoxidation of cyclohexanone imine, that is,

’

Mimoun and c o - w o r k e r ~ ~prepared ~’ a series of stable covalent Mo(V1) and W(V1) peroxides of structure (VIII), by reaction of the corresponding peracids with organic bases, such as pyridine, hexamethylphosphorous triamide (HMPA), 0

M

O,IO

A~P,4 L,

L2

(VIII)

= Mo or W L = DMF, DMAC, HMPA, Py, etc.

344

ROGER A. SHELDON AND JAY K. KOCHI

dimethyl formamide (DMF), or dimethyl acetamide (DMAC). These complexes stoichiometrically and selectively epoxidize olefins under mild conditions in organic solvents420:

The authors suggested the following mechanism420:

However, more recent mechanistic suggest that epoxidation involves oxygen transfer via a three-membered transition state. The proposed mechanism423 resembles that for the enzymatic epoxidation of olefins by ironbased mixed-function oxygenases discussed in Section V.

to The Mo(V1)-peroxide complex, MoO,(Py)(HMPA), has been hydroxylate selectively enolizable esters, lactones, and ketones, presumably via epoxidation of the enolate: 0

II

RCCHzR'

HO

\

C=CCHR' / R

HO

\ /"\

MoOSL,L~

C-CHR'

0 OH

II I + RC-CHR'

/

R

It should be emphasized, however, that the reactions of Mo(V1)-peroxide complexes with the organic substrates just described, require stoichiometric amounts of reagents. Hence, they are not as useful as the metal-catalyzed reactions of organic substrates with alkyl hydroperoxides described in the following section. The latter reagents tend to give the same reactions as the Mo(V1)-peroxides. 2. Alkyl Hydroperoxide-Metal Catalyst Systems a. Metal-Catalyzed Epoxidations. Indictor and Bri11425 studied the effect of small quantities of a number of metal acetylacetonates on the epoxidation of 2,4,4-trimethyl-l -pentene with tert-butyl hydroperoxide at 25OC. Predictably, autoxidation catalysts (Co, Mn, Fe, Cu) gave poor yields of epoxides owing to the rapid catalytic decomposition of the hydroperoxide into free radicals. The acetylacetonates of Cr(III), V(III), VO(IV) and Mo02(VI), by contrast, gave

345

METAL-CATALYZED OXIDATIONS

quantitative yields of epoxide. No reaction occurred in the absence of catalysts, and the reaction was found to be applicable to other olefins. The epoxidation was stereospecific since frans-4-methyl-2-pentene gave the trans-epoxide and cis-4-methyl-2-pentene gave the cisepoxide. The authors suggested a molecular mechanism in which a metal-hydroperoxide complex was the active epoxidizing agent. The reaction may be described by the general equation, 0

\

I

1/ \ / C=C +ROzH C-C +ROH / \ / 1 Industrial interest in this reaction was stimulated by the discovery that it constitutes a commercially attractive route to propylene oxide!26aib Thus, the metal-catalyzed epoxidation of propylene with ethylbenzene hydroperoxide ~ o c for ~the coproduction s s ~ of pro~ ~ forms the basis of the Halcon ~ pylene oxide and styrene from propylene, ethylbenzene, and oxygen via the following sequence: catalyst

+ PhCH(CH3)02H

F'hCHzCH3 + 0 2

catalyst

PhCH(CH3)OzH + CH3CH=CH2

(304)

P\

PhCH(CH3)OH + CH3CH-CH2

PhCH(CH3)OH 4 PhCH=CH2 + HzO

(305) (306)

Sheng and c o - w o r k e r ~ ~carried ~ ~ - ~out ~ ~extensive studies of the epoxidation of olefins with alkyl hydroperoxides in the presence of a wide variety of metal catalysts. Soluble molybdenum complexes, such as Mo(CO)~,were shown to be the most effective catalysts. Vanadium, tungsten, and titanium complexes were also active epoxidation catalysts, whereas compounds of Mn, Fe, Co, Rh, Ni, Pt, and Cu gave negligible yields of epoxides. Optimum rates and selectivities were obtained at temperatures in the range 100'-120°C in hydrocarbon solvents. The method is suitable for the epoxidation of a wide range of substituted olefins in high yields!29i430 The high yields of epoxides and the stereospecificity of the reaction are only consistent with a heterolytic mechanism. Substituent effects indicate that the active epoxidizing agent is an electrophilic species. The authors proposed428 that epoxidation involved transfer of oxygen from a molybdenumhydroperoxide complex in which the electrophilic character of the peroxidic oxygens is enhanced by coordination to the metal catalyst namely, RO~H +M

O ~ + + MO"+

RO2H + Mo"+

\

+[Mo"'

ROzH]

\ lo\ /

/

[Mo"+R02H] + C=C / \

(active catalyst)

-3

C-C

/

\

+ROH+Mo"+

(307) (308)

(309)

~

346

ROGER A. SHELDON AND JAY K. KOCHI

In recent years this important reaction has been the subject of intensive st~dy?~*"~ b. The Catalyst. Recent s t ~ d i e s ~ ~have ~ - demonstrated ~~' that the metalcatalyzed epoxidation of the olefin and metal-catalyzed homolytic decomposition of the hydroperoxide are competing processes in these systems. Complexes of metals in low oxidation states [e.g., Mo(CO)6, W(CO)6 J are rapidly oxidized by the hydroperoxide to their high oxidation states. The active epoxidizing agents in these reactions are complexes of the hydroperoxide with the catalyst in its oxidation states such as M o o ,W(VI), V O ,and Ti(IV).

\ [M"+RO?HJ + C=C / \

0 \"/ -3 C-C k,

/

\

+ROH+M"'

(311)

[M"* R02HJ -% M("-l)' + R 0 2 * + H+

(312)

+ RO?H %M"+ + RO. + HO-

(313)

M("-')+

The selectivity to epoxide is determined by the realtive rates of reaction of the catalyst-hydroperoxide complex with the olefin [Eq. (3 1l)] in competition with its homolytic decomposition [Eq. (312)l. By-products are formed in subsequent reactions of the tot-alkoxy and tertalkylperoxy radicals with the hydroperoxide, solvent or olefin. For example, in the metal-catalyzed epoxidation of cyclohexene with tert-butyl hydroperoxide in benzene, the main by-product was 3-tert-butylperoxy-1cyclohexene, formed via the sequence433shown in Eq. (314) [cf. reactions (89)-(94)] :

If one assumes that there is no radical-induced chain decomposition of the hydroperoxide and small amounts of epoxide formed via a radical pathway are neglected, then the selectivity is given by epoxide selectivity =

k,[olefm] kd t k, [olefm]

x

100%.

Two factors are important in determining the relative values of kd and k,-the oxidation potential of the catalyst and its Lewis acidity. In general, the ease with which transition metal complexes catalyze the decomposition of hydroperoxides is related to their redox potentials (see Table V). Hydroperoxides are strong oxidants but weak reducing agents. Hence, reaction (312) is the slower,

METAL-CATALYZED OXIDATIONS

347

rate-determining step in hydroperoxide decomposition and is facile only with strong oxidants such as Co(III), Mn(III), and Ce(1V). Significantly, all of the active epoxidation catalysts, with the exception of V(V), are very weak oxidants in their high oxidation states. This trend explains why vanadium catalysts generally give lower epoxide selectivities compared to molybdenum, tungsten, and titanium catalysts? In the epoxidation step [reaction (31 l)] ,the principal function of the catalyst is to withdraw electrons from the peroxidic oxygens, making them more susceptible to attack by nucleophiles such as olefms. In so doing the catalyst acts as a Lewis acid. The Lewis acidity of metal complexes generally increases with increasing oxidation state of the metal. Active epoxidation catalysts should, therefore, be found among compounds of metals in high oxidation states. The Lewis acidity of transition metal oxides decreases in the order: CrOJ, Moo3 >> W 0 3 > TiOz, V 2 0 5 , U 0 3 . Thus, the high activity of Mo(VI) as an epoxidation catalyst is in accord with this trend. On the basis of its Lewis acidity, Cr(V1) is also expected to be a good catalyst. However, Cr(V1) is also a strong oxidant and readily participates in the homolytic decomposition of hydroperoxides. Since the epoxidation step involves no formal change in the oxidation state of the metal catalyst, there is no reason why catalytic activity should be restricted to transition metal complexes. Compounds of nontransition elements which are Lewis acids should also be capable of catalyzing epoxidations. In fact, Se02, which is roughly as acidic as Moo3, catalyzes these reacti0ns.4~~It is, however, significantly less active than molybdenum, tungsten, and titanium catalysts. Similarly, boron compounds catalyze these reactions but they are much less effective than molybdenum c a t a l ~ s t s . 4 ~The ' ~ ~low ~ ~ activity of other metal catalysts, such as Th(1V) and Zr(1V) (which are weak oxidants) is attributable to their weak Lewis acidity. The Lewis acidity of the catalyst is also influenced by the nature of the coordinating ligands. In practice, however, a ligand effect may be observable only in the initial stages of reaction due to rapid destruction of the original ligands during the reaction. Thus, the rates of the molybdenumcatalyzed epoxidation of olefms varied (with the structure of the catalyst charged) only in the initial phases of the reaction. Thereafter the rates became independent of the molybdenum complex!35 This observation suggests that all the additives were eventually modified to the same catalytic species. The conclusion was confirmed by isolation of the catalysts at the end of the reaction as Mo(VI)-1,2diol complexes (IX) in all the cases studied>36a3bIndependent experiments showed that ti

H

348

ROGER A. SHELDON AND JAY K. KOCHI

they were formed in situ during molybdenum-catalyzed epoxidations via reaction of the catalyst with the epoxide in the presence of the hydroperoxide. The structure of the catalyst is, therefore, determined by the structure of the olefin being epoxidized. It should be emphasized, however, that Mo(VI)-l,2diol complexes are not the only active Mo(V1) compounds; nor are they necessarily more active than other Mo(V1) compounds. Thus, M ~ O ~ ( a c a cgenerally )~ gave a higher rate of epoxidation initially, but the rate decreased with time due to the formation of the less active 1,2-diol complex?35 studied the effect of different ligands on molybdeOther num-catalyzed epoxidations. They generally concluded that complexes with very strongly bound ligands show low activity, presumably due to hindrance of complex formation between the catalyst and the hydroperoxide. Catalysts with very loosely bound ligands, such as Mo02(acac), , were active but less selective than those with ligands of intermediate stability, such as MoO2(oxine),. It was proposed that the latter formed a complex with the hydroperoxide by opening only one of the bonds of the chelating ligand to molybdenum. In order to be active and selective, catalysts should contain molybdenum-ligand bonds of intermediate strength. Two possible mechanisms for the transfer of oxygen from the catalysthydroperoxide complex to the olefm can be The first involves a cyclic transition state in which an M=O group in the catalyst functions in a manner similar to the carbonyl group in organic peracids. The M=O group may be part of a soluble metal complex or it may be present on the surface of a heterogeneous catalyst (see below). This mechanism is preferred by those complexes that contain an M=O group (molybdenyl, vanadyl, titanyl, etc.). Mechanism 1 :

R

Apart from the M=O moiety, an M-OX group could also act as a proton acceptor as illustrated in the second mechanism, which pertains to catalysts, such as boron compound^^^'^ 4 3 8 with no M=O group. Mechanism 2:

These reactions are also catalyzed by insoluble compounds of Mo,W,Ti, V, etc. For example, molybdenum t r i ~ x i d e ? 442 ~ ~ ’molybdates?” and molybdeny1 p h t h a l ~ c y a n i n e(Moo2 ~ ~ ~ Pc) are active catalysts. However, these reactions are not truly heterogeneous in many cases, since the catalyst dissolves

METAL -CATALYZED OXIDATIONS

349

during the 439*440a9b3 443 probably via the formation of soluble Mo(VI)-l,2-diol complexes435 (see the preceding). A series of catalysts consisting of metal oxides on a silica carrier have been These catalysts remain heterogeneous during the reaction433 and they are very active.433, 443,444 1ndeed, they are considerably more active, in general, than the simple metal oxides. For example, TiOz alone is a poor catalyst, but TiOz-onSiOz gives selectivities as high as soluble molybdenum c0mplexes.4~~The enhanced activity of TiOz-on-SiOz can be ascribed to its much stronger Lewis acidity compared to TiOz or homogeneous Ti(1V) complexes.433’445 A wide variety of solvents has been used for epoxidations, but hydrocarbons Recently, it has been that the are generally the solvent of choice!28 highest rates and selectivities obtain in polar, noncoordinating solvents, such as polychlorinated hydrocarbons. Rates and selectivities were slightly lower in hydrocarbons and very poor in coordinating solvents, such as alcohols and ethers. The latter readily form complexes with the catalyst and hinder both the formation of the catalyst-hydroperoxide complex and its subsequent reaction with the olefin. The retarding effect of alcohols on the rate of epoxidation manifests itself in the observed autoretardation by the alcohol coproduct!’83434’4469 447 The extent of autoretardation is related to the ratio of the equilibrium constants for the formation of catalyst-hydroperoxide and catalyst-alcohol complexes. This ratio will vary with the metal. In metal-catalyzed epoxidations with tert-butyl hydroperoxide, autoretardation by ferf-butyl alcohol increased in the order: W < Mo < Ti < V; the rates of Mo- and W-catalyzed epoxidations were only slightly affected. Severe autoretardation by the alcohol coproduct was also observed in vanadium-catalyzed epoxidations!’89 434* 4469 447 The formation of strong catalyst-alcohol complexes explains the better catalytic properties of vanadium compared to molybdenum for the epoxidation of allylic alcohols!29. 430* 45’ On the other hand, molybdenum-catalyzed epoxidations of simple olefins proceed approximately 10’ times faster than those catalyzed by vanadium!34y 4 4 7 Thus, the facile vanadium-catalyzed epoxidation of allyl alcohol with tert-butyl hydroperoxide may involve transfer of an oxygen from coordinated hydroperoxide to the double bond of allyl alcohol which is coordinated to the same metal atom,430 namely,

The rates of metal-catalyzed epoxidations are also influenced by the structure of the olefin and the structure of the hydroperoxide. The relative rates of epoxidation of a series of olefins using a mixture of t-BuOzH and Mo(CO), paralleled quite closely those for epoxidations with organic per acid^.^"

350

ROGER A. SHELDON AND JAY K. KOCHI

Electron-attracting substituents in the hydroperoxide increase the rate of e p o ~ i d a t i o n ~434 ' ~ ' by increasing the electrophilic character of the peroxidic oxygens. With alkylaromatic hydroperoxides, a competing metal-catalyzed heterolytic decomposition of the hydroperoxide can take place. The problem becomes especially important in epoxidations of unreactive ole fins such as ally1 For example, cumene hydroperoxide affords phenol and acetone, Lewis acid

P h C ( C H 3 ) 2 0 2 H F P h O H + (CH&C=O

(316)

This reaction is catalyzed by Lewis acids such as acidic metal Electron-attracting substituents in the aromatic ring, in addition to enhancing the rate of epoxidation, decrease the rate of heterolytic decomposition of the hydr~peroxide.~~~ Several groups have carried out detailed kinetic studies of metal-catalyzed epoxidations with alkyl hydro peroxide^.^^" 44694479 4 4 9 - 4 5 6 The reactions have generally been found to be first order in catalyst and olefin, but the dependency on hydroperoxide is complicated by autoretardation due to the alcohol coproduct (see p. 349). Thus, Could et uZ?46i447studied the kinetics of the vanadium-catalyzed epoxidations with tert-butyl hydroperoxide. They found a first-order dependence on catalyst and olefin but a Michaelis-like dependence on hydroperoxide, due to strong autoretardation by tert-butyl alcohol. Molybdenum-catalyzed epoxidations, on the other hand, were generally 449 to be first-order in catalyst, olefin, and hydroperoxide. The addition of fairly large quantities of tert-butyl alcohol did cause a significant decrease in the rate."28' 433 More detailed investigations4319450- 4 s 3 revealed that these reactions exhibit apparent first-order dependence on hydroperoxide. Thus, the molybdenum-catalyzed epoxidation may be described by the following scheme431*4 5 0 : ki

ROzH + MoWI) ROH + Mo(V1)

[ROzH Mo(VI)]

(317)

[ ROH Mo(VI)]

(318)

k-1 ki k-2

0

\

/

C=C + [ROzH Mo(VI)] / \

k3

\/ \/

+ C-C I

\

+ ROH + M O W )

(319)

The general rate equation is given by431i4 5 0 d[R02H] - d[epoxide] - k 3 [olefin] [R02H] [Mo] 0 dt dt K1 + (Kl/K2) P O H I + W 2 H l ' where K , = k-2/k2 and k , K , = k-, can be rewritten

(3 20)

+ k , [olefin]. When k-, >> k , , Eq. (320)

351

METAL-CATALYZED OXIDATIONS

d[ROZHl dt

-

k3 [olefin] [Mole KiI[ROzHl

i(Ki/Kz)

[RO2HIo/[ROzHl + (1 - KiIK2)'

(321)

where [ROH] = [ROzH]o-[R02H]. When 1 - Kl/Kz is small (i.e., K 1 K z relative to the other terms in the denominator, Eq. (321) becomes d[R02H] - k3 [olefin] [Mo], [R02H] dt

K1 + (KJK2) [RO*HIo

(322) *

The rate given by Eq. (322) explains the apparent first-order dependence on hydroperoxide. In other words, when the dissociation constants for the catalysthydroperoxide complex and the catalyst-alcohol complex are approximately the same, apparent first order dependence in hydroperoxide obtains. This kinetic result is observed in molybdenum-catalyzed epoxidation~.~" Metal-catalyzed epoxidations with alkyl hydroperoxides have mainly been used for the epoxidation of simple olefins and polymers.458a* Recently, however, there have been several reports of the use of these reagents for the synthesis of complex molecules. Tolstikov and c o - w ~ r k e r s ~ have ~ ~used - ~ ~tert-amyl ~ hydroperoxide in the presence of catalytic amounts of MoC15 or Mo(CO)~ for the synthesis of a variety of steroidal epoxides. The same group has also reported the selective epoxidation of enol esters with these reagents.464a9 For example, I-acetoxycyclohexene with tert-my1 hydroperoxide, in the presence in benzene at 80°C, gave the corresponding of MoCl, , Mo(CO), , or VO(a~ac)~ epoxide in quantitative yield:

Steroidal enol acetates were similarly epoxidized. This method has undoubted advantages over the tradional epoxidation by peracids. Metal-catalyzed epoxidation of 1-acetoxycyclohexene is a key step in a novel synthesis of catechol from cyclohexanone via the sequence465

p>

6

rzw;-pJ

0

- (y

H O & ;

The exceptionally facile epoxidation of allylic alcohols by tert-butyl hydroperoxide in the presence of vanadium catalysts, discussed earlier, has been used4663467for the synthesis of complex molecules. Thus, geraniol (X) and linalool (XI) are selectively epoxidized to the previously unknown mono466 : epoxides with t-BuO, H-Vo(aca~)~

352

ROGER A. SHELDON AND JAY K. KOCHI

p -F OH

(XI)

Similarly, the selective epoxidation of the bisallylic alcohol (XII) to the bisepoxyalcohol (XIII), with t-B~O,H-V0(acac)~,is the crucial step in a synthesis of juvenile hormone from farnes01~~':

HO

fi

:I:::

'

(326)

' L&OH

(XI11

(XIII)

Such remarkable regioselectivities are not obtainable with any other reagent. Metal catalyst-hydroperoxide systems also exhibit extremely high stereoselectivities. For example, in contrast to its reaction with peracids, the homoallylic alcohol (XIV) afforded only the syn-epoxy alcohol with t-BuO, H-MO(CO)~466 : O

O

H

Z

r-BUO,H o \

a

-

O

H

(XIV)

The use of these reagents for stereo- and regioselective syntheses of complex molecules is clearly worthy of further attention. c. Generation of Hydroperoxide in Situ. In metal-catalyzed epoxidations with hydroperoxides, the hydroperoxide is usually prepared in a separate step by autoxidation of the corresponding alkane (isobutane, ethylbenzene, etc.). However, by carrying out the co-oxidation of the alkane and the olefin in the presence of an epoxidation catalyst, it is possible to dispense with the first step. For example, the preparation of propylene oxide and cyclohexanol (together with some cyclohexanone) by co-oxidation of cyclohexane and propylene in the presence of molybdenum catalysts has been reported468:

For industrial-scale syntheses of simple epoxides, however, these reactions all suffer from the drawback that they produce a coproduct (tert-butyl alcohol, styrene, cyclohexanol, etc.). The ultimate goal of industrial research on epoxi-

METAL-CATALYZED OXIDATIONS

353

dations is still the direct selective epoxidation of ole fins with molecular oxygen (see following section). d. Oxidations of Other Substrates In addition to olefins, other nucleophilic reagents undergo oxygen transfer reactions with these metal catalysthydroperoxide systems. Thus, VO(a~ac)~ has been used to catalyze the oxida: tion of tertiary amines with tert-butyl hydr~peroxide~~' VO(acac)z

R3N + t-BuOzH)-.

R3NO + t-BuOH

(330)

Tolstikov and c o - w o r k e ~ s 472 ~ ~ ~used - ferf-amyl hydroperoxide (TAHP) in the presence of molybdenum or vanadium catalysts for the oxidation of nitrogen heterocycles,

QrR*QfR 1

0

nitrosamines, R' \

/

N-NO----+

R

TAHP

MO(C0)6

R' \ /

N-NO2

Mc%l,

and Schiff bases, . R' \ C=N-R" / R

R'

0 \ / \ C-N-R" MO(CO)~ / M& R TAHP

(333)

Aniline is oxidized to nitrobenzene by tert-butyl hydroperoxide in the presence of molybdenum or vanadium catalyst^:^^ (334)

In the presence of titanium catalysts, on the other hand, the corresponding azoxy compounds are formed4%:

Titanium-catalyzed oxidation of primary aliphatic amines with organic hydroperoxides'givesthe corresponding 0 x i m e s , 4 ~476 ~ ~e.g.,

The molybdenum- and vanadium-catalyzed oxidation of sulfides to sulfoxides has also been described.417*4 7 7 - 4 8 0 In the presence of excess hydroperoxide, further oxidation to the sulfone O C C U I S , ~ ~480 ~ ' e.g.,

354

ROGER A. SHELDON AND JAY K. KOCHI f-BuOiH

I-BuO~H

Bu2S "0(acac),) Bu2SO

vo(acac)l)Bu2SO2

(337)

Sulfides are generally oxidized much faster than olefins. For example, with t-B~O~H-V0(acac)~ in ethanol at 25"C, the relative rates decreased in the order: Bu!:S(lOO) > PhSBu"(58) > Bu"S0 (1.7) > cyclohexene (0.2)."80 Unsaturated sulfides are selectively oxidized at the sulfur atom as shown in the following example477: t-BuO2H

C H ~ ( C H Z ) ~ S C H ~ C H = CMool(acac)2) H~ CH3(CH2)3SO2CH2CH=CH2

(338)

Similarly, molybdenum and vanadium complexes catalyze the oxidation of triphenylphosphine by tert-butyl hydroperoxide."81 All of the reactions just described closely parallel the reactions of the same substrates with organic peracids. They probably involve rate-determining oxygen transfer from a metal-hydroperoxide complex to the substrate via a cyclic transition state, described earlier for the epoxidation of olefins with these 435 1eagents.4~~3

C. OXYGEN ACTIVATION-DIRECT OXYGEN TRANSFERFROM METAL-DIOXYGEN COMPLEXESTO ORGANIC SUBSTRATES We have already mentioned that a wide variety of stable diamagnetic complexes of dioxygen with transition metals is known. The ability to oxygenate substrates under mild conditions is an important chemical property of these complexes.'47-' Reactions between singlet molecules and free (triplet) dioxygen usually experience high activation energies because of the problem of spin conservation.482 In principle, this barrier may be overcome by forming singlet complexes between transition metals and dioxygen. Both catalytic and stoichiometric oxidations of substrates by metal-dioxygen complexes are k n ~ w n . ' ~ ~ For - ' ~ example, ~ stoichiometric oxidations of a number of nonmetal oxides occurs readily3''? 381avb :

\

\

'0-NO 2N0,

=

/o--Noa (Ph3P),Pt '0-NO,

METAL -CATALYZED OXIDATIONS

355

The platinum complex (XV; M = P t ) also undergoes facile addition to the carbonyl group of carbon dioxide, aldehydes, and ketones483- 4 8 6 :

For a catalytic reaction to be feasible, the product should be readily released from the metal complex in order that the cycle may continue. In other words, the substrate should coordinate more strongly than the product to the metal catalyst. A few catalytic oxidations are known. Thus, autoxidation of triphenylphosphine and fert-butyl isocyanide is catalyzed by several Group VIII metal-dioxygen c o m p l e x e ~8,7~- 49 e4.9

2-t-BuNC + 0

-

(Bu‘NC)nNiOz 2

2-t-BuNCO

(343)

The main interest in these complexes, however, stems from the possibility of effecting selective nonradical oxidations of hydrocarbons under mild conditions. There is considerable industrial interest in the direct epoxidation or oxidative cleavage of olefins with molecular oxygen by the following overall transformations: 0

/ \ RCH=CHR’ + 1 0 2 + RCH-CHR’ RCH=CHR’ + 02

RCHO + R’CHO

(344a) (344b)

Ethylene oxide is prepared industrially by the vapor phase oxidation of ethylene over a supported silver catalyst at elevated t e r n p e r a t ~ r e s . ~ ” ~ -Application of this reaction to higher olefins results in complete oxidation of the olefin to carbon dioxide and water. In general, autoxidations of olefins are notoriously unselective because of the many competing reactions of the intermediate peroxy radicals in these systems. Rouchaud and c o - w o r k e r ~ ~- ’494 ~ studied the liquid phase oxidation of propylene in the presence of insoluble silver, molybdenum, tungsten, and vanadium catalysts. Moderate yields of propylene oxide were obtained in the presence of molybdenum catalysts. These reactions almost certainly proceed via the initial formation of alkyl hydroperoxides, followed by epoxidation of the propylene by a Mo(V1)-hydroperoxide complex (see preceding section).

356

ROGER A. SHELDON AND JAY K. KOCHI

It has recently been reported495 that the complex CsH5V(C0)4 (CsH5 = cyclopentadienyl) is an efficient catalyst for the stereoselective oxidation of cycloin good yield (65% at 10%conversion). hexene to cis-l,2-epoxycyclohexane-3-01 This high stereoselectivity is reminiscent of the highly selective vanadiumcatalyzed epoxidations of allylic alcohols with alkyl hydroperoxides discussed earlier. The mechanism of reaction, OH

was not discussed, but is probably involves the catalytic sequence: cyclohexene, cyclohexenyl hydroperoxide, cyclohexenol to epoxide, etc. We have mentioned in Section II.B.2 studies of the oxidation of olefins by molecular oxygen in the presence of low-valent Group VIII metal complexes, with the expectation of effecting homogeneous, nonradical oxidation processes. However, these reactions were shown to involve the usual free radical chain autoxidation, and no direct transfer of oxygen from a metal-dioxygen complex to an olefin was demonstrated. Two research 497 have recently studied the autoxidation of cyclohexene at 60" to 65°C in the presence of a mixture of a low-valent Group VIII metal complex, e.g., RhC1(Ph3P), or (Ph,P),PtO,, and an epoxidation catalyst (molybdenum complexes). Cyclohexen-1-01and cyclohexene oxide are formed in roughly equimolar amounts. The results could be explained by a scheme involving two successive catalytic processes:

The first reaction (346) consists of hydroperoxide formation by a typical autoxidation process, and the second represents selective epoxidation by the hydroperoxide. In the absence of the autoxidation catalyst, no reaction is observed under these conditions due to efficient removal of chain-initiating hydroperoxide molecules by reaction (347). Optimum selectivities obtain when the autoxidation catalyst is of low activity, which implies a low total activity of the catalytic system. The molybdenum complexes related to Mooz(oxine), are among the most effective catalysts for e p o ~ i d a t i o n . 4Although ~~ the autoxidation catalysts were limited to two types (phosphine complexes of noble metals and transition metal acetylacetonates), there is no reason, a priori, why other complexes such as naphthenates should not produce similar results.

METAL-CATALYZED OXIDATIONS

357

Direct oxygen transfer from a metal-dioxygen complex to molybdenum may represent an alternative explanation. The resultant molybdenum(v1)-peroxide complex would be responsible for epoxidation according to 02

MA

*

(348)

This mechanism seems unlikely, in view of the large amounts of alcohol and ketone formed. (In some cases more epoxide was formed than alcohol plus ketone, suggesting that perhaps both mechanisms are operating simultaneously.) A more serious obstacle is encountered in reaction (349), in which MA undergoes a two-equivalent oxidation to M i . For a catalytic cycle, however, there is no obvious method of reducing M i back to MA under these oxidative conditions. On the other hand, it may be possible for MA and MB to be both converted to metal-dioxygen complexes. In such an event, both oxygen moieties in dioxygen must be formally utilized as oxygen atoms in the overall transformation (i.e., O2 + 0 t 0), in contrast to the disproportionation of peroxide (i.e., OZ2- + 02-t 0) represented in reaction (350). The distinction between a metal-dioxygen complex and a metal-peroxide complex lies in the observation that the former is generated from molecular oxygen, whereas the latter is derived from hydrogen peroxide. These two types of complexes may have similar structures in some cases. The possibility of oxygen atom transfer from metal-dioxygen complexes as well as the possibility of forming metal peroxides via oxygen transfer from metal-dioxygen complexes are worthy of further attention. A requirement for high reactivity of a peroxidic species toward typical olefins rests on the presence of an electrophilic oxygen center. An explanation for the low reactivity of coordinated dioxygen in d6 and d8 metal-dioxygen complexes may be found by considering the nature of the peroxidic species in metaldioxygen complexes [in addition to Mo(V1) and related do transition metal peroxides]. The ease with which lithium n-butoxide is formed by reaction of n-butyllithium with a complex may be taken as a measure of the electrophilicity of the peroxidic oxygen^.^'^ Typical high-valent metal peroxides, such as Mo02*HMPA or CrO,.py, form lithium butoxide readily at -78°C. Metaldioxygen complexes, such as (Ph3P),Pt02 or (Ph3P),Ir(CO)(O2)C1, resemble sodium peroxide (Na202)in that they do not afford lithium butoxide. Reaction of n-butyllithium with (Ph3P)2Pt02 produced (Ph3P),PtBu2 by nucleophilic

358

ROGER A. SHELDON AND JAY K. KOCHI

attack on platinum.498 Thus, the chemical reactivity of the peroxide moiety in d6 and d8 metal-dioxygen complexes apparently resembles that of nucleophilic peroxide anions more than that of electrophilic peracids. For oxygen transfer to typical olefins to be feasible, it may be concluded that a peroxide moiety should be coordinated to metals in high oxidation states. Transfer of negative charge from the peroxide moiety to the metal atom under these circumstances enables the peroxidic oxygens to be more electrophilic. Unfortunately, direct combination of metal complexes with molecular oxygen has only been observed with metals in low oxidation states. The facile addition of (Ph3P),Pt02 to the carbonyl group of aldehydes and ketones [see Eqs. (339)-(341)] is in agreement with the nucleophilic character of the coordinated dioxygen in this complex. Thus, it is expected that metaldioxygen complexes would react with olefins susceptible to nucleophilic addition. Indeed, the dioxygen complexes (Ph3P)2MO2 (where M = Pd, Pt) readily add to electrophilic olefins such as 1,l-dicyanoolefins or 1-nitroolefins, at room temperature, to give cyclic peroxy adducts in essentially quantitative yield?" e.g., H,C\ /C=C,cN-

+

(Ph,P),MO,

FN

/O-O \ (Ph,P),M\ C/C(CH,),

H3C

NC' ( M =Pd,Pt

(351)

'CN )

Simple olefins, such as cyclohexene, styrene, or tetramethylene, were unreactive even at 60°C. For facile reaction, the olefin must be substituted with powerful electron-attracting substituents capable of stabilizing a negative charge. A schematic mechanism showing the stepwise nucleophilic addition of (Ph3P), M 0 2 to the olefin may be represented as follows: t o

(Ph,P)2M' NC,? NC

+P-"\

'0'CH,

,c=c<

(Ph,P),M

,C(CH,),

NC Yc 'CN

CH,

,,)

( P W 2 M , /"-"

),

This process is analogous to the nucleophilic addition of alkylperoxy anions to electrophilic olefins,5" e.g., H3C, /C" /c=C

H,C

\

CN

H3C, t

ROO-

--t

HC/I

fc?,

CN

H3C\

1\cN

H,C

C-4

'TOR

,CN

0

CN

+

RO-

(352)

359

METAL-CATALYZED OXIDATIONS

Thermal decomposition of the cyclic peroxy adducts in reaction (351) can lead to selective cleavage of the double bond of the original olefin. For example, the adduct with 1,l -dicyanoisobutene produces acetone in quantitative yield. This oxidative cleavage appears to be a general reaction since Rh(1)- and Ir(1)dioxygen complexes [being less reactive than the Pt(0) and Pd(0)-complexes] reacted with 1,l-dicyanoisobutene at 6OoC to give acetone, presumably via an intermediate cyclic peroxy adduct. The lower reactivity of Rh(1) and Ir(1) complexes is consistent with their lower nucleophilicity compared to Pt(0) and Pd(0) complexes. This reaction is interesting because it constitutes the first clear-cut example of selective, transition metal-promoted cleavage of an olefinic double bond by molecular oxygen. Unfortunately, only 1 mole of olefin is converted per mole of metal complex since the original Pd(0) or Pt(0) complex is not regenerated. The other half of the molecule equivalent to (NC)2C0 remains bonded to the metal. /O-O (Ph,P),M

‘C(CH,),

---+ (CH,),C=O

+

(Ph,P),M<S)

i2cG

A ( P h 1M ’ 3 2

NC/‘\CN

COCN ‘CN

An analogous stoichiometric reaction between (Ph3P)?Pd02 and ketoximes has recently been described.501 A mechanism involving cyclo addition followed by cleavage of the cyclic peroxy adduct was suggested: R;

,C=NOH

+

(Ph,P),PdO,---

R

0-0,

(Ph,P),Pd’ L N / C \ ,

I OH

/R’

-

RC,

R/c=o

An interesting stoichiometric oxidation of terminal olefins, such as 1-octene, to the corresponding methyl ketones occurs with molecular oxygen at ambient temperatures in the presence of RhH(CO)(Ph,P), or € U I C ~ ( P ~ ~.502 P )a*b ~ It was suggested that these reactions involve co-oxygenation of coordinated Ph3P and olefin at the metal center, that is,

R

This reaction constitutes an Rh(1)-catalyzed co-oxidation of triphenylphosphine with an olefin. Displacement of coordinated Ph3P0 by fresh Ph3P provides for a catalytic cycle. Unfortunately, the use of stoichiometric amounts of triphenylphosphine is rather unattractive in practice. However, this reaction does indicate the important principle that oxygen transfer to typical olefins may be possible when accompanied by the simultaneous transfer of the second oxygen atom to an

360

ROGER A. SHELDON AND JAY K. KOCHl

oxygen acceptor. The role of the oxygen acceptor (Phj P) in this scheme resembles that of the cofactor in enzymatic oxygenations described in Section V.C. Sire6 de Roch and co-workersSo3have demonstrated an alternative means of activating molecular oxygen in the liquid phase. The principle depends on the lability of an M=O bond in 0x0-molybdenum complexes. Thus, oxo-dialkylthiocarbamate complexes of molybdenum catalyze the oxidation of triphenylphosphine via the sequence: M o ( V I ) ~ ~ ( S ~ C N+RPhjP ~ ) ~ --3 M o ( V I ) O ( S ~ C N R ~+)PhjPO ~ ~ M o ( V I ) O ( S ~ C N R+~0)2~ ----) ~ M O ( V I ) O ~ ( S ~ C N R ~ ) ~

(353) (354)

Strictly speaking, this reaction does not constitute an example of “oxygen activation.” The function of the molecular oxygen is merely to reoxidize the reduced form of the catalyst. As such, the reaction is no different from the palladium-catalyzed oxidations of olefins (see Section 1II.D. 1) which are not usually considered to be examples of oxygen activation. Reaction (353) involves oxygen transfer from a labile Mo=O bond and is related to many heterogeneous gas phase oxidations of hydrocarbons over molybdenum trioxide or molybdates at high temperatures. In principle one might expect oxygen transfer from the Mo(V1) 0x0-dithiocarbonate to an olefm to be feasible. However, these and similar complexes are completely unreactive to a wide range of olefins.’*’ The study of oxygen activation and oxygen transfer reactions of metal-dioxygen complexes and related species is of continuing theoretical and practical interest.

D. ACTIVATION OF SUBSTRATE BY COORDINATIONTO METALS This section is concerned with the activation of hydrocarbon molecules by coordination to noble metals, particularly p a l l a d i ~ m . ~ ~An ~ - ~important ~ landmark in the development of homogeneous oxidative catalysis by noble metal complexes was the discovery in 1959 of the Wacker process for the conversion of ethylene to acetaldehyde (see below). The success of the Wacker process provided a great stimulus for further studies of the reactions of noble metal complexes, which were found to be extremely versatile in their ability to catalyze homogeneous liquid phase reaction. The following reactions of olefins, for example, are catalyzed by noble metals: hydrogenation, hydroformylation, oligomerization and polymerization, hydration, and oxidation. Palladium complexes are generally superior catalysts for oxidation reactions, whereas other noble metals are more active for other reactions, e.g., rhodium for hydroformylation. All of these reactions seemingly involve activation of the olefm substrate by n-complex formation with the noble metal cataly~t.”~ The oxidation reactions discussed in the following generally depend on nucleophilic attack on the coordinated olefins (or other hydrocarbons) to effect oxidation of the substrate.

METAL-CATALYZED OXIDATIONS

361

1. Palladium-Catalyzed Oxidations of Olefins

The palladium-catalyzed oxidation of ethylene to acetaldehyde (the Wacker process) was discovered by Smidt and c o - ~ o r k e r s ’ ~in~ -1959. ~ ~ ~ This process combines the stoichiometric reduction of Pd(I1) with reoxidation of metal in situ by molecular oxygen in the presence of copper salts. The overall reaction constitutes a palladium-catalyzed oxidation of ethylene to acetaldehyde by molecular oxygen:

+ PdCl2 + H2O +CH3CHO + W + 2HQ (355) Pd + 2CuCl2 ----) PdCl2 + 2CuQ (356) 2CuQ+ 2HCl+ to2 +2CuC12 + H2O (357) Oxidation of olefms other than ethylene occurs more slowly and often in l9 and normal lower yields. Propylene is oxidized selectively to acetone,’ butenes give methyl ethyl ketone.’19 Higher olefins generally produce mixtures as a result of olefin isomerization.’20 Aqueous solutions of other Group VIII metal salts, such as Pt(II), Ir(III), Ru(III), and Rh(II1) oxidize olefms in an analogous manner to Pd(II), although at significantly lower rates.512 C2H4

a. Mechanisms of Palladium-Catalyzed Oxidations of Olefins. The kinetics and mechanism of the palladium-catalyzed oxidations of olefins have been studied in detail in aqueous and nonaqueous solvent^.'^^^'^^-'^^ Unlike the metal-catalyzed oxidations described in the preceding sections, the Pd(I1)catalyzed oxidations of olefms proceed by heterolytic mechanisms. Free radicals as such do not appear to be intermediates. Although Pd(II)-catalyzed oxidations bear a formal resemblance to lead(IV)5278~b and t h a l l i ~ m ( I I 1 ) ’ ~ ~ ~ ~ ~ ~ oxidations, which also involve organometallic intermediates, the mechanisms of the reactions are d i f f e ~ e n t . ’ ~The ~ rates of reaction of Pb(IV) and Tl(II1) increase with increasing alkyl substitution of the double bond with alkyl groups in agreement with a mechanism involving electrophilic attack on the 0lefix-1.’~~The rates of the former, in contrast, decrease with increasing substitution of the double bond, consistent with a reaction occurring via nucleophilic attack on the coordinated olefin.’ 1 9 * 5 2 3 Reactions in both aqueous and nonaqueous solvents may be rationalized in terms of the following steps: (i) n-complex formation; (ii) nucleophilic addition of a coordinated ligand (OH, OR, OAc, etc.) to the bound olefin with simultaneous rearrangement to a a-bonded complex, and (iii) intramolecular hydride ion transfer with concomitant decomposition of the o-bonded complex into products. The generally accepted mechanism in aqueous solution, in the presence of excess chloride, is as follows”2~’21-52si’29: PdC14’-+ C2H4 [PdC13C2b]-+ H2O

[PdCl~C2Hql-+C1-

(358)

[PdC12(HzO)C2&] + Cl-

(359)

362

ROGER A. SHELDON AND JAY K. KOCHl [PdClz(HzO)CzHq] + HzO

[PdCl2(OH)CzHq]-+ H30’

+HOCHzCHzPdCl+ UHOCHzCHzPdCl --+ CH3CHO + Pd + HQ

[PdClz(OH)C2Hq]-

(360) (361) (362)

b. n Complexes As Intermediates. The first step is the reversible formation of a n-olefm complex with Pd(I1). Complexation reduces the electron density of the double bond, rendering it more susceptible to attack by nucleophiles. Palladium(I1) salts generally catalyze nucleophilic attack on olefms more readily than Pt(I1) salts,’06 for the following reasons: (i) Pd(I1)-olefin complexes are formed more rapidly than their Pt(I1) analogs; (ii) the back-donation of charge from metal to olefm is less for Pd than for Pt and results in the electron density around the olefin being less when it is coordinated to Pd(II) relative to Pt(I1); (iii) the Pd-olefin bond is weaker than the Pt-olefin bond so that the activation energy of any rearrangement process that is necessary during the course of reaction will be lower for Pd than for Pt; and (iv) Pd(I1) can readily expand its coordination sphere to accept a fifth or a sixth ligand and allow the incoming nucleophile to coordinate to Pd(I1) before attacking the olefm, thus lowering the activation energy for nucleophilic attack. The product of the Pd(l1)-catalyzed oxidation of olefms depends on the nature of the nucleophile involved in the addition to the coordinated olefin. The nucleophile is usually the solvent (e.g., water, alcohol, or acetic acid), but others have also been studied. c. Decomposition of Pd(II)-Olefin n Complexes in Aqueous Solution. The relevent experimental data can be summarized as follows: i. The reaction is inhibited by chloride consistent with the presence of equilibria (358) and (359) ii. The reaction is inhibited by protons~22~’26i530 in accord with equilibrium (360) iii. The kinetic isotope effect (kH/kD = 4.05) observed in DzOS3l is also consistent with Eq. (360) iv. Ethyl alcohol is not an intermediate since it r e a ~ t ~ ’ ~ ~more ~ ’ ~slowly ’~’~~ than ethylene with PdClz and cannot be detected in reaction mixtures v. The reaction is first-order’14~’199’24in olefm and palladium(I1) vi. No deuterium is present in the acetaldehyde formed in DzO, indicating that an intramolecular hydrogen shift occurred during reaction’ l 4 vii. In the oxidation of CzD4 in water, only a very small isotope ( k ~ / k= ~ 1.07) effect was observed, suggesting that the carbon-hydrogen bond is either highly stretched or not broken at all in the rate-limiting step The mechanistic details of steps (361) and (362) have still not been clarified. Reaction (362), which probably represents the rate-determining step, involves nucleophilic addition of coordinated hydroxide ion to the olefin with con-

363

METAL-CATALY ZED OXIDATIONS

comitant formation of a Pd-C u bond. Reaction (362) involves a hydride shift assisted by the palladium, that is, H F'dfC1 "\>-$C-H

I I' I

t

H H

-

OHC-CH,

+

Pd

+

HCI

(363)

Alternatively, it may occur by 1,2-elimination of HPdCl followed by reverse readdition and reductive elimination. Moiseev and VargaftikS3j prepared the a-bonded complex independently by reaction of PdClz with P-chloromercuriethanol and showed that it afforded acetaldehyde on decomposition: PdClz + Q H ~ C H Z C H ~ O H * C ~ P ~ C H ~ C H ~ O H ClPdCH2CHzOH-Pd

+ CHjCHO + HQ

(364) (365)

d. Reactions of Palladium-Olefin Complexes in Nonaqueous Solvents. Formation of vinyl acetate from ethylene by reaction with PdClz in acetic acid solution containing sodium acetate was first reported by Moiseev et aLSMa The reaction was developed into a catalytic process by using copper salts to reoxidize the palladium: C H ~ = C H+~ HOAC + ~ O ~ ~ C H ~ = C H+O H A~ O C

(366)

Ethylene is readily absorbed by solutions of PdClz in acetic acid,s14 but the presence of acetate ions (as NaOAc or LiOAc) is essential for Van Helden et aI.'jsa found that ethylene at 1 atm reacts with Pd(OAc)z at 70°C to form palladium metal and vinyl acetate (40-SO%), together with acetaldehyde, ethylidene diacetate, and acetic acid. The rate of reaction increases considerably in the presence of sodium acetate and the selectivity to vinyl acetate is much higher (80-90%). The reaction probably follows a course similar to that taken in aqueous solution. In acetic acid, nucleophilic attack of coordinated acetate occurs with simultaneous rearrangement to a a-bonded complex which subsequently decomposes to products, presumably via a 0-hydrogen elimination* :

+[Pd(OAc)jCzHq]+AcOCHzCHzPdOAc + AcO- OAc +AcOCH =CHz + Pd + HOAc

[Pd(OAc)j]-+ CzH4

(367)

[Pd(OAc)jCzH4]-

(368)

AcOCH

CHz I*Pd

I 2

(369)

H

or [HPdOAc]

+HOAc + Pd

*p-Hydrogen elimination is a common mode of decomposition for transition metal a ~ c y ~ s . ~ ~ ~ ~ - ~

364

ROGER A. SHELDON AND JAY K. KOCHI

Some acetaldehyde formation is observed even under rigorously anhydrous ~ o n d i t i o n s . ~ ~ ~Indeed, ~ ~ ~ ' separate - ~ ~ ~ experiments show that Pd(1I) salts catalyze the reaction of vinyl acetate with acetic acid to produce acetaldehyde and acetic anhydride, that is,

+ H O A c s C H 3 C H O + (AcO)20 (370) The oxidation of higher olefins by Pd(I1) in acetic acid has also been studied:40-s51 but these reactions are complex.s12 Thus, terminal olefins afford the enol acetate as the major product. Internal olefins, on the other hand, give mainly allylic acetates.543 For example, propylene produces isopropenyl acetate whereas cis- and trans-2-butenes are oxidized to the secondary allylic acetate (97%). These results are rationalized as follows: CH2=CHOAc

4+

-\"

PdOAc

Pd(OAc),

___t

OAc

OAc

+

+

Pd

HOAc

(371)

I

+

pAtnAn\ ",",.'I,

-

L

?+

PdOAc

,

OAc

Pd

OAc

+

HOAc . . /9.l-+\

(J111

Reactions with higher olefins are further complicated by olefin isomerization. The formation of n-ally1 complexes of Pd(II), which occurs readily at higher temperature^:^^ retards the rate of reaction. The formation of vinyl acetate from vinyl chloride, although not formally an oxidation process, is pertinent to the oxidation of ethylene by palladium acetate complexes. The inertness of vinyl chloride to nucleophilic substitution is modified considerably by coordination to Pd(1I). Thus, chloride is readily displaced553-556from vinyl chloride by acetate ion in the presence of catalytic amounts of Pd(I1): PdCI, CH,= CHCI

+

t

PdCI,

CH,= CHCI

AcO,

Ad-

C1;CH-CH,PdCI Ad)

AcO, C H - G P d c L

CHZCH,

t

PdCI,

C 9

A similar mechanism is applicable to the catalytic conversion of ally1 chloride to ally1 acetate5": PdCI, CH,=CHCH,CI

+

PdCI,

t

CH1=CHCH2C1

PdCl PdCI,

+

AcOCH,CH= CH,

t-

'\

AcOCH,CH- C H , - .

11

365

METAL-CATALYZED OXIDATIONS

The transvinylation reaction between carboxylic acids and vinyl acetate presumably follows a course similar to the reaction of vinyl chloride and acetate just described.558 C H =~C H O A ~ + R C O ~ H ~ =CC H HO ~ C R + H O A ~

(373)

A number of other nucleophiles are capable of nucleophilic attack on coordinated olefins. For example, when ethylene is oxidized by PdC12 in alcoholic solvents, the corresponding vinyl ethers,

c 2 b+ ROH%CH?

=CHOR

(374)

and acetals, C2H4 + 2ROH=CH3CH(OR)2

(375)

~ ether ~ ~ is *not ~an * intermediate ~ ~ ~ in~acetal ~ ~ for-~ ~ ~ ~ are f ~ r m e d . ~Vinyl mation since no deuterium is incorporated in the acetal product when the reaction is carried out in CH30D. Primary amines and amides afford secondary amines and N-alkenylamides, respectively, with ethylene and Pd(I1) salts.555 Cyanide under similar conditions produces a c r y l ~ n i t r i l e . ~ ~ ~ All of the foregoing reactions involve a formal nucleophilic substitution of a group (e.g., H and Cl) attached to the double bond of a coordinated olefm. Under different conditions, Pd(I1)-catalyzed oxidations of olefins can give products (e.g., glycol esters) resulting from formal addition to the double bond of coordinated olefins.S61a e. Formation of Glycol Esters and Related Reactions. Ethylene or terminal olefins react with palladium nitrate in acetic acid as solvent to produce glycol ~ e~reaction ~ ~ ~ can ~ ~ be ' - made ~ ~ ~catalytic by remono- and d i a ~ e t a t e s . ~ ~Th acting an a-olefin and oxygen in the presence of a Pd2+/Cu2+/Li+/AcO-/C1-/ NO3- catalyst combination, using acetic acid as solvent.535ai This reaction has been developed into a process for the manufacture of ethylene glycol from ethylene via ethylene glycol m o n o a ~ e t a t :e ~ ~ ~ W(II)/CU(II) + HOAC ~ 4 + $0, No;,a+ AcOCH~CH~OH+HOCH2CH2OH (376) In the presence of high concentrations of Cu(II), 1,2-disubstituted ethanes are formed even in the absence of These reactions are described by the general equation (X = AcO, C1, etc.):

~

2

CH2=CH2

+ 2C-2

ma

+ AcO-&X-CH~CH~-OAC

+ 2CuX + X-

(377)

Thus, in the absence of nitrate, it is possible to obtain a chloroalkyl acetate as the main The most reasonable rationalization for these results is to include the formation of vinyl acetate and saturated ester as proceeding through a common Pacetoxyalkylpalladium(I1) intermediate. Depending on the conditions, the latter can decompose via P-hydrogen elimination to vinyl acetate or via nucleophilic displacement at the a-carbon giving 1,2-disubstituted ethanes:

366

ROGER A. SHELDON AND JAY K. KOCHI AcOCH =CH2 + Pd + HCI

(369)

AcOCHzCHzNu + Pd + C1-

(378)

AcOCHZCH2PdCl

(Nu = Ad), C1, HO, etc.)

The role of nitrate ion or Cu(1I) salts in promoting these nucleophilic displacements is not clear. Nitrate would be coordinated less strongly than chloride or acetate to Pd(I1). Ionization of the Pd(I1) intermediate would be more complete and enable the Pd(1I) to be more electrophilic and a better leaving group (AcOCH2CH2Pd+ compared to AcOCHzCHzPdCl). However, this ionization should also enhance &hydrogen elimination. It has been s ~ g g e s t e d ' ~ ~that '>~ Cu(I1) increases the activity of an organoPd(I1) species by withdrawing electrons from the environment of Pd(1I) by forming bridged binuclear complexes. The oxypalladium adduct may be captured directly by the oxidant [compare T ~ ( O A C )P~~,( O A C )KzCr207, ~, and AuC13 in addition to nitrate and copper(I1) complexes]. The oxidant may facilitate the heterolytic reaction (378) to avoid the necessity of forming the metastable Pd(0) by (i) directly transferring the alkyl group to oxidant, (ii) oxidative transfer of the alkyl group, or (iii) oxidation of the alkyl-Pd(I1) to a higher oxidation state, all followed by decomposit i ~ n . ' ~ ~Recently, ' PtC1, was found to be more reactive than PdClz in the oxidation of cyclohexene to saturated esters.564b The product distributions from PtC12 were different from those obtained with PdC12 with regard to positional isomerism, geometric isomerism, and ratios of chloro- and diacetates. Contrary to the author's claim, these results do not serve to distinguish among the various mechanistic possibilities presented in the foregoing. The competing &hydrogen elimination and oxidative substitution of the acetoxyalkylpalladium(I1) intermediate bear many similarities to the competing oxidative elimination and oxidative substitution mechanisms observed in elecAn altertron transfer reactions of alkyl radicals with Cu(I1) native explanation for the competing pathways in the decomposition of the acetoxyalkylpalladium( 11) intermediate can be represented by oxidative elimination versus 1electron transfer followed by a subsequent electron or ligand transfer, that is, AcOCH'CH~ + [HWX] AcOCHzCHzPdX

-€ \t/;R/

cuxz AcOCHzCH2- + PdX'

*AcOCH2CH;

+ PdX

AcOCH~CH~X

Oxidation of ethylene to ethylene glycol and its mono- and diacetates has also been catalyzed by tellurium and thallium complexes in the presence of halides

367

METAL-CATALYZED OXIDATIONS

and acetic acid.561b 9 c Both processes probably proceed via an organometallic intermediate similar t o that described in Eq.(378) for palladium. Oxygen would then be involved in the oxidation of the organometal or reoxidation of the metal after reductive elimination. f. Oxidative Carbonylatwn of Olefins. A process for the conversion of ethylene to acrylic acid by the Pd(IQcata1yzed oxidative carbonylation has been described?08 * 5 6 5 The overall stoichiometry is described by C H =~ C H ~ + co + ~

O

~ =CH ~ C O ~CH

H

~

(379)

Copper salts are used to reoxidize the palladium and the following mechanism was suggested: 0 PdCldp + 2CO + H 2 0

--+

II

[Cl2(CO)Pd-COH]

-

+ HCl + Cl-

(380)

0

II

[Clz(CzHq)Pd -COH]

-

%[C12(CO)Pd-CH2CH2C02H]-

[ C ~ Z ( C O ) P ~ - - C H ~ C H ~ C O ~+CHz=CHC02H H]-

(382)

+ Pd + HCl+ C1-+ CO

I C ~ Z ( C O ) P ~ - C H ~ C H ~ C O ~% H ] - A c O C H ~ C H ~ C O+~ Pd H + 2Cl- + CO

(383) (384)

The by-product, 0-acetoxypropionic acid, may be thermally decomposed to yield more acrylic acid: AcOCHzCHzCOzH

CH2=CHCOzH + AcOH

(385)

g. Oxidative Coupling of Olefins. Under certain conditions, oxidative coupling of olefins occurs in the presence of Pd(1I) salts.535apb3566a9b Dimerization of ethylene to butene competes with ethylene oxidation at high molar of chloride: Pd(I1). It was also found566athat vinyl acetate undergoes oxidative coupling to 1,4-diacetoxy-1,3-butadienein the presence of Pd(OAC)Z 9

2CHz =CHOAc

+ Pd(0Ac)z

+Pd + AcOCH=CHCH=CHOAc

+ ZHOAC

(386)

When PdClz-LiOAc was used instead of Pd(OAc)2, vinyl acetate was converted to a mixture of acetaldehyde and acetic anhydride [see Eq. (370)l. No oxidative coupling took place under these conditions.566a

2 . Oxidation of Aromatic Hydrocarbons by Pd(II) Complexes a. Oxidative Coupling Reactions, Van Helden and Verberg567 found that palladium metal and biphenyl are formed when benzene is heated with PdClz

368

ROGER A. SHELDON AND JAY K. KOCHI

and NaOAc at 90°C in acetic acid:

+ 2NaOAc 4 CsH5-CbH5 + Pd + 2NaCl+ 2HOAc (387) No reaction took place in the absence of NaOAc. They suggested a ratedetermining formation of a a-bonded organo-Pd(I1) complex, followed by a fast reaction with acetate t o yield biphenyl: C6H6 +PdClz

C,H,

+

PdCI,

0" +

CI-

PdCl

With monosubstituted benzenes a mixture of isomeric biphenyls is obtained. The substitution pattern corresponds t o that observed in electrophilic aromatic substitution, consistent with a mechanism involving electrophilic attack by PdClz shown in Eq. (388). Davidson and c o - ~ o r k e r s ~ ~found ~ - ~ that ' ~ benzene reacts with Pd(0Ac)z in acetic acid at 100°C t o give roughly equal amounts of biphenyl and phenyl acetate. The reaction time was reduced from 16 hr to 5 min at 100°C in the presence of perchloric and the yield of biphenyl was higher. Formation of phenyl acetate was also inhibited in the presence of ~ x y g e n . " ~ These authors concluded that oxidative coupling proceeds via an unstable o-bonded arylpalladium complex, which rapidly decomposes t o biaryls and Pd(I), that is, C6H6 + PdZ++CsHsPd' + H ' 2C6HsPd' +(C6H5)z + 2Pd'

(391) (392)

Perchloric acid increased the electrophilicity of Pd(I1). A strong parallel between palladation and mercuration of aromatic hydrocarbons was drawn.568 (The effect of strong acids in increasing the electrophilicity of metal acetates has been discussed earlier.) Arylmercury complexes are relatively stable and do not afford biaryl readily. Analogous arylthallium(II1) complexes only afford biaryls on photolysis in the presence of arenes via the following sequence: ArTl(02CR)~

At* + T(OzCR)2

~ r * + ~ t ~ 1 0 1A'~ - - A ~ + H +

(393) (394)

3 69

METAL-CATALYZED OXIDATIONS

Such a homolytic mechanism cannot be entirely ruled out in the analogous reactions with Pd(I1). Reactions of arenes with Pd(I1) are also very fast in TFA solution.572 Biaryls were formed. The following mechanism was suggested572: R

R

R

PdX

t IHPdXl

+

R

W

R

-

R

eR XPd

It is also likely that radical cation formation occurs in reactions of very reactive arenes with Pd(1I) (see Section II.B.3.b), which would also lead to biaryl formation. That the reactions of arenes with Pd(I1) compounds are far from simple is illustrated by the work of Arzoumanidis and R a u ~ h . ’ ’ ~ ~In’ ~the ~ reaction of Pd(02CCF3)2 with benzene or naphthalene in TFA, a variety of polynuclear complexes, containing both Pd(1) and Pd(I1) and arenes, were isolated574 in addition to the usual biaryls. Arylmercury(I1) compounds have been shown’ 7 5 to undergo substitution by Pd(1I) salts with subsequent biaryl formation. The following mechanism was suggested: ArHgX + PdX2 +ArPdX + HgX2 (395) ArPdX+ArH+Ar-Ar+Pd+HX

(396)

or ArPdX + & H a

+Ar-Ar

+ W + HgX2

(397)

In the presence of oxygen, the formation of biaryls by Pd(I1) oxidation of arenes can be made to be catalytic in palladium.576 For example, toluene with Pd(OAc)2 and O2 at 150°C for 16 hr afforded bitolyls in 20,600% yield based on palladium. It was concluded that biaryl formation in these systems occurs via free aryl radicals.576 The role of homolytic processes in these reactions is not clear, and further clarification of the mechanism is desirable. The arylation of olefins in the presence of Pd(I1) salts is similar to the coupling of arenes discussed above. For example, styrene and benzene afford transstilbene in the presence of P ~ ( O A Cor ) ~PdC12’77 : PhCH=CH2

+ PhH

Pd(OAc)2

HOAc.

,phC.f=Cm

(398)

370

ROGER A. SHELDON AND JAY K. KOCHI

It is generally thought that these reactions proceed via addition of a a-arylpalladium(I1) complex to the olefm, followed by palladium hydride elimina~ion,57'-5'5 ArPdX + HzC=CHz

+Ar-CH-CH2-PdX 7

+ArCH=CHz

I 2

+ [HPdX]

H

Palladium salts can be utilized catalytically in the presence of silver or copper acetate and o ~ y g e n . ~This ~ reaction ' ~ ~ offers ~ ~ ~an ~attractive ~ ~ route for the direct vinylation of benzene to ~ t y r e n e ~: ' ~ ? ~ ~ ~ PhH CZH4 +

W(OAc)z

02

c

~

() PhCH=CHz ~ ~ ~ )+ HzO ~

(399)

HOAc

b. Oxidative Nuclear Substitution of Arenes. We have already mentioned that phenyl acetate is generally formed as a by-product in the oxidation of benzene with Pd(OAc)z.56' This reaction is of practical interest since direct hydroxylations or acetoxylations of arenes do not occur Thus, a process for the direct catalytic oxidation of benzene to phenyl esters would be of commercial interest. Many workers have, therefore, addressed themselves to the problem of increasing the yield of substitution products in the oxidation of arenes with Pd(1I) compounds. In principle, nucleophilic displacement at the a-carbon of a-arylpalladium(I1) complexes should be possible, by analogy with the corresponding a-alkylpalladium(I1) complexes [cf. Eq. (378)] : /* Ar -Pd-X +ArNu + Pd + X(400)

5 Nu

(Nu = AcO, etc.)

The following oxidative substitution of arenes has been reported by Henrys6': ArH + X- + oxidant %ArX

(401)

Substitution products were obtained for X = OAc, N3, C1, NO2, CN, and SCN and for K2Crz07,P ~ ( O A C )KMn04, ~, NaC103, NaN03, and NaNOz as oxidants. Two possible mechanisms were envisaged: ArH + PdX2

+ArPdX

or PdXz + oxidant

%PdX4

>ArX + PdXz

*

ArX + WX2 + X-

(402)

(403

Other workers have reported the palladium-catalyzednitration of ben~ene,5~' PhH + NaNOz (or NzO4)

L~OAC.ma2 HOAc ) PhNOz

(+ PhOAc)

(404)

Eberson and co-workers590-594 have made detailed studies of the Pd(I1)catalyzed nuclear acetoxylation of arenes. The conditions for optimum yields of nuclear acetate were established using p-xylene as a model compound. It was

37 1

METAL-CATALYZED OXIDATIONS

shown that nuclear acetoxylation, giving 2,5-dimethylphenyl acetate, becomes the preferred reaction if excess of acetate ion is avoided and the reaction is performed under oxygen. Addition of alkali metal acetates (LiOAc, NaOAc, etc.) favors the formation of p-methylbenzyl acetate, the product of side-chain acetoxylation (see Section III.D.2.c). It was also foundsg1 that biaryl formation is favored by the addition of a Lewis acid (cf. effect of perchloric and trifluoracetic acids). The same ~ ~ r k also e showed r ~ that,~ in ~ the Pd(I1)-catalyzed ~ ~ ~ ~ acetoxyla~ tion of substituted arenes, a complete reversal of the usual pattern of isomer distribution for electrophilic aromatic substitution or anodic oxidation of aromatics is observed. To explain these results it was suggested that acetoxylation by P ~ ( O A Ctakes ) ~ place via the following addition-elimination sequence: X I

X I

X I

\

,/

ddOAc

+

[HPdOAcl

(X = t-Bu, MeO,CI,Br, e t c . )

Although the first step represents an electrophilic attack by Pd(I1) at the ortho and para positions, subsequent elimination leaves the acetate group in the meta position. The precise role of acetate ion and oxygen in determining the ratio of nuclear to side-chain acetoxylation has yet to be clarified. These reactions are limited by being generally too slow to be of any practical value. A continuation of the foregoing studies, using chlorobenzene as sub~trate,”~ was aimed at speeding up the reaction while retaining the meta selectivity. The latter feature is of interest from a synthetic point of view since it provides a one-step route to phenol derivatives that are otherwise accessible only by multistep procedures. It was foundsg3 that the addition of co-oxidants, such as nitrate or dichromate, increases the rate by a factor of about 50, but decreases the meta selectivity somewhat. With nitrate as the co-oxidant, the selectivity was improved by the addition of amines, such as bipyridine, as complexing agents. The palladium-catalyzed acetoxylation of arenes, using persulfate as the oxidant, in acetic acid has also been r e p ~ r t e d . ” ~ Arpe and HornigSgs carried out a thorough investigation of palladiumcatalyzed acetoxylation of benzene in acetic acid. Selectivities as high as 78% were observed with a heterogeneous Pd/Au-on-Si02 catalyst at 155°C.

372

ROGER A. SHELDON AND JAY K. KOCHI

c. Oxidative Substihrtwn of Aromatic Side Chains. Oxidation of alkylbenzenes with P ~ ( O A C )in~ acetic acid can also lead to acetoxylation of the side chain. For example, toluene produces benzyl acetate in 40 to 50% yield, together with bitolyls.568*569a-dA mechanism involving radical cations as intermediates and both Pd(I1) and Pd(1) as oxidants was proposed568: Pd(OAc)++ PhCH3 [PhCH3Ii PhCHz' + Pd(0Ac)z

+[PhCHs]?+ PdOAc

(405)

+PhCHz- + H+

(406)

----)

PhCHzOAc + PdOAC

(407)

These reactions are similar to those described earlier with Co(II1) and Mn(II1). Bryant et al.596 found that the relative amounts of oxidative coupling and side-chain acetoxylation of toluene are determined by the molar ratio of PdClz and acetate present. For molar ratios of NaOAc:PdC12 equal to 5 and 20, the relative yields of bitolyl: benzyl acetate were 64: 2 and 1 :68, respectively. Similarly, in the oxidation of toluene with P~(OAC)~-KOAC in acetic acid, increasing the molar ratio of KOAc to Pd(OAc), from 5 to 20 improved the yield of benzyl acetate from 53 to 93%. A catalytic process was de~eloped,'~'using a Pd(OAc)z-Sn(OAc)z catalyst and molecular oxygen in acetic acid at 100°C:

It represents a convenient method for the synthesis of benzyl esters from methylbenzenes. Small amounts of benzylidene diacetates are formed at high conversions. For a series of methylbenzenes, the rates decreased in the order: toluene > xylenes > mesitylene > durene > hexamethylbenzene. This order of reactivity is the reverse of that expected for a mechanism involving electrophilic substitution or electron transfer. However, B ~ s h w e l l e r ~found ~ * that electron-releasing groups facilitate the benzylic oxidation of substituted toluenes by Pd(OAc), in acetic acid. p-Methoxytoluene gave a 96% yield of p-methoxybenzyl acetate, and p-nitrotoluene gave only 2%p-nitrobenzyl acetate, in agreement with either an electrophilic substitution or electron transfer mechanism. More mechanistic studies are necessary to clear up these anomalies. Steric effects may play an important role in these reactions. Stern has suggested5" a mechanism involving the rearrangement of a o-arylpalladium(I1) species (the presumed intermediate in aromatic coupling) to a o-benzylpalladium(I1) species. The latter undergoes subsequent reductive elimination,

373

METAL-CATALYZED OXIDATIONS AcOPd PhCH,

t

Pd(OAc),

@CH,

AcOPd

t

HOAc

(409)

0

@

CH,

CH,PdOAc

Support for this scheme comes from the observation that the benzylpalladium(I1) complex formed from (Ph3P)4Pd and benzyl chloride reacts with acetate to produce benzyl acetate and benzylidene acetate599:

(PhjP)4Pd

+ PhCHzCl +

a

Ph3P \

/

/ Pd

PhCH2

\

AcO-

+PhCHzOAc

(411)

100°C

PPh3

If Stern’s and Eberson’s mechanisms are combined, the following tentative scheme can be formulated for competing nuclear acetoxylation, side-chain acetoxylation, and oxidative coupling in Pd(I1) oxidations of alkylbenzenes (initial attack is shown only for the para position; a similar scheme can be written with initial attack at the ortho position):

8

Pd(OAc),+

H$ f

+

AcO-

AcOPd

PdOAc

I

t bitolyls

It is difficult, however, to explain the inhibiting effect of oxygens9’* 591 and the promoting effect of acetate596on side-chain acetoxylation by this scheme.

374

ROGER A. SHELDON AND JAY K. KOCHI

Moreover, it is difficult to visualize a simple mechanism for the proposed rearrangement of a u-aryl to u-benzyl species. More studies are clearly needed before any definite conclusions can be drawn about the mechanisms of Pd(I1) oxidations of arenes. d. Oxidative Carbonylation of Arenes. By analogy with the Pd(I1)-catalyzed oxidative carbonylation of ole fins described earlier, one would expect oxidative carbonylation of arenes to be feasible according t o the reaction ArH + CO + 3 0 2

a Pd(I1)

ArCOz H

However, this reaction does not appear to have been reported. The stoichiometric carbonylation of phenylpalladium chloride, prepared in situ from phenylmercuric chloride, has been described6" : PhHgCl + PdClz + CO

CH3CN

PhCOCl + Pd + HC1+ HgCl2

(413)

3. Activation of Saturated Hydrocarbons by Metal Complexes The interaction of saturated C-H and C-C bonds with heterogeneous metal catalysts forms the basis of widely applied reactions such as isomerization, cracking, and re-forming of alkanes. In recent years, much attention has been devoted to the selective activation of C-H bonds by transition metal complexes in homogeneous solution under mild conditions.601 - '04 In principle, an alkane can undergo oxidative addition to a noble metal complex according t o

or electrophilic substitution according to

I I

-c-H

I

+ M"+ +-c-M"+ I

+ H+

Subsequent reactions of the adduct could provide methods for the mild, selective functionalization of alkanes, e.g., by dehydrogenation, carbonylation, and oxidation. Reaction (414) is formally analogous to the oxidative addition of alkyl halides to noble metal complexes described earlier, and both homolytic and heterolytic processes can be envisaged. Heterolytic cleavage of C-H bonds represented in Eq. (415) is analogous to the interaction of the powerful oxidant Co3+ with alkanes in TFA in reaction (229). Since the C-H bonds in alkanes are generally stronger than the corresponding C-M bonds, equilibrium (415) is likely to lie far t o the left. However, the existence of such an equilibrium should lead to hydrogen-deuterium exchange under

375

METAL-CATALYZED OXIDATIONS

suitable conditions. Indeed, there are reports of the rapid deuteration of alkanes in D20-CH3C02D mixtures in the presence of chloroplatinum(I1) complexes under comparatively mild conditions (1O0°C).605 Indirect evidence for alkylplatinum(I1) intermediates was obtained from reactions of Pt(I1) salts with alkylmercury compounds in D20-CH3C02D at room temperature, which led t o the formation of deuterated alkanes.606a The following scheme was proposed t o explain the results: X PtX2 + RHgX

HgX

'' /

X RPtX + DX

Pt

\

*HgX,+RPtX

(416)

R RD + PtXz

(417)

Deuterium exchange has also been reported for the linear alkanes from methane through hexane in a mixture containing 0.02 M Na31rC16 in 25 mole % DOAc/D2 0 at 150" to 170°C.607 Orientation of the deuterium by mass spectral analysis during the initial exchange appeared t o favor the exchange of hydrogens attached to the methyl groups. Unfortunately these studies lack definitiveness. Iridium catalysis is slower than that for platinum, but has the advantage that exchanges can be carried out at higher temperatures. The interaction of saturated C-H bonds with metal ions is facilitated when the alkyl group foims part of a coordinated ligand (proximity effect). Several intramolecular reactions of metal ions with saturated C-H bonds have been reported. For example,608a. Br, PtCI,[P(t-Bu)(ir-Pr),l

-2

Br-

,P(t-Bu)(n-Pr),

(4 18)

(I-Bu)(N -Pr)P, /Pt, f'H2 H,C - cn,

Reaction (418) is analogous to the many known examples609aof intramolecular electrophilic substitution reactions of metal ions with aromatic rings of ligands, e.g.3

2H'

t

6cI-

(419)

Homogeneous catalytic activation of C-H bonds has also been recently summarized.609b Thus, R u H C ~ ( P F % ~catalyzes )~ the exchange of ortho hydrogens on the phenyl groups of triphenylphosphine with deuterium gas. Under similar conditions, TaH, ( d m ~ e ) ~[where dmpe = (CH3),PCH2 CH2 P(CH3)2] catalyzes the deuterium substitution in the meta and para positions.609c Ex-

ROGER A. SHELDON AND JAY K. KOCHI

37 6

change with the Ru catalyst proceeds by the conventional ortho-metallation :

The Ta catalyst, however, can apparently effect exchange without coordination. Interestingly, RuHCl(PPh& is also a catalyst for deuterium exchange in the saturated C-H bonds of PhzP(CH2),CH3. For example, a given methyl hydrogen in diphenyl-n-propylphosphine (n = 2) exchanges at the same rate as an ortho-phenyl hydrogen over a temperature range of 140". The activation energies for both exchanges are approximately 6 kcal mole-'. Intramolecular oxidative addition to a coordinatively unsaturated Ru(I1) species is favored for C-H activation by a transition state such as,

in which ring size is an important consideration.609aMasters6loa> reported that complexes of the type PtzC14Lz [where L = ( ~ z - P ~ or ) ~ (P~ - B U ) ~ Preadily ] undergo H-D exchange in D20-CH3C0,D containing perchloric acid. The resultant complexes contain deuterium incorporated into the phosphine ligand specifically at C-3 position of the alkyl group. The following mechanism, involving oxidative addition of saturated C-H to Pt(II), was suggested:

A similar mechanism was proposed610ai to account for the observed regiospecific deuteration of olefins coordinated to Pt(I1). However, an electrophilic substitution mechanism without invoking a Pt(1V) adduct also seems reasonable. Although the direct interaction of metal complexes with alkanes under mild conditions has been demonstrated by the foregoing results, selective and catalytic functionalization of alkanes remains to be found. Success is probably more likely to be achieved by way of mild dehydrogenation (e.g., transfer of hydrogen to a suitable or carbonylation rather than by oxidation. By analogy with Co(1II) oxidations of alkanes, these reactions should be facilitated by increasing the electrophilicity of the metal ion, e.g., by using the metal tri-

METAL -CATALYZED OXIDATIONS

377

fluoroacetate in TFA. Indeed, mild stoichiometric dehydrogenation of cyclohexane by Pd(02CCF3)2 in TFA has recently been described611c: 3Pd(02CCF& + C6H12 + 3Pd + C6H6 + 6CF3COzH (420) Transient formation of alkyl-Pd(I1) species seems likely. The absence of deuterium exchange in saturated hydrocarbons with Pd(02CCF3)2 as catalyst was attributed to the lower stability of Pd alkyls compared t o Pt alkyls.6''C Thus, alkylation of Pd(02CFF3)2 with CH3HgBr and CH3CH2HgBr under conditions for deuterium exchange gave undeuterated ethane and ethylene, respectively. Rapid decomposition of the intermediate Pd alkyls by dimerization or /3-hydrogen elimination was indicated.

IV. Heterogeneous Catalysis of Liquid Phase Oxidations Mechanistic studies in homogeneous systems have the advantages over heterogeneous studies in relating kinetic observables directly t o chemical steps. One of the major problems encountered with heterogeneous catalysts is that they are not composed of a single active material that can be identified and purified as the active component. The simplicity and reproducibility of homogeneous systems as well as the ease of detecting intermediates represent other beneficial aspects. Consequently, the mechanisms of homogeneous processes are generally much better understood. Homogeneous catalysts are usually more active than their heterogeneous counterparts, in which the reaction is restricted to the surface of the catalyst. Moreover, the reactivity and selectivity of a homogeneous catalyst can, in principle, be controlled by varying the steric and electronic environment around the metal nucleus. The activity of heterogeneous catalysts can often be changed by varying the support, but the factors influencing activities are generally less well understood than those in homogeneous systems. From a practical point of view, however, heterogeneous catalysis has the important advantage in allowing for easy separation of products from the catalyst and facilitating a continuous process. Many results obtained from homogenous processes also appear to apply t o heterogeneous catalysts. A more direct relationship has begun t o emerge in the study of new types of systems. These combine the best features of homogeneous and heterogeneous systems in which known metal complexes are chemically bonded t o specific groups on a catalyst support. The principle of hard and soft catalysts developed for homogeneous catalysts may also be extended to heterogeneous catalysts. Soft surfaces are typified by transition metals and their alloys as well as transition metal oxides having metal-type conduction bands. Hard surfaces are typified by most transition metal oxides. In the first case, chemisorption can best be compared t o

378

ROGER A. SHELDON AND JAY K. KOCHI

coordination to metal complexes in low oxidation states, whereas coordination to high oxidation states pertains t o hard catalysts. Transition metal oxides (often supported on oxide carriers such as A1203or S i 0 2 ) have generally been used as heterogeneous oxidation catalysts in both the liquid and gas phases.6'2-621 Despite the enormous importance of metal oxides as catalysts, there are considerable differences of opinion concerning the mechanisms of these processes. According to one view,4429614a9 metal ions in solution have much in common with the same metal ion in an oxide lattice. The catalytic action of the metal ion should significantly resemble that of the oxide. Similarities in behavior tend t o be overlooked, however, since heterogeneous catalysts are generally employed under quite different conditions (at higher temperatures, often in the gas phase) from their homogeneous counterparts. In recent years, several authors have expressed the view that, in general, there is much overlap between homogeneous and heterogeneous catalysis. 622 - 6 2 8 The distinction between activation of molecules by coordination to metal complexes in solution and chemisorption at metal centers on surfaces is becoming less clear as more is learned about adsorption. Many homogeneous catalysts can be converted into heterogeneous ones, by anchoring coordination complexes by covalent bonding to insoluble supports.624- 628 High activity and selectivity, characteristic of homogeneous catalysis, are retained together with the ready recovery of the catalyst. Immobilization of homogeneous catalysts has generally been applied heretofore to hydrogenation and carbonylation reactions (soft catalysts). In principle, this technique should be applicable to oxidation and other reactions. The same mechanistic pathways are available for heterogeneous liquid phase oxidations as those described in preceding sections for homogeneous reactions. Thus, homolytic and heterolytic mechanisms do involve interactions of metal centers with hydrocarbon substrates, with molecular oxygen, and with intermediate hydroperoxides. Moreover, the mechanistic concepts of electron and ligand transfer, as well as activation by coordination and oxidative addition, provide a mechanistic basis for discussing heterogeneous oxidations. The local molecular structure of the active site is more pertinent than a superficial examination of the macroscopic feature of the reacting system (such as its physical state). The following examples of liquid phase oxidations have been chosen t o illustrate some of the general principles discussed above. Thus, Could and Rado442 compared the catalytic properties of transition metal oxides with the corresponding metal acetylacetonates in the autoxidation of cyclohexene. A striking parallelism was noted between the product distributions obtained in heterogeneous and homogeneous oxidations carried out with the same metal.

METAL-CATALYZED OXIDATIONS

379

Typical autoxidation catalysts, such as Co, Mn, Cu and Fe, afforded mainly cyclohexen-1-01 and cyclohexen- 1-one. Other epoxidation catalysts, such as Mo or V, afforded mainly cyclohexene oxide and cyclohexen-1-01 (see Section III.B.2). It was concluded that the atomic number and the oxidation state of the transition metal are more important than the detailed catalyst structure in determining the course of the reaction. Valendo and N ~ r i k o v ~studied ~’ heterogeneous catalysis by metal oxides in the decomposition of cumene hydroperoxide in cumene solutions. Transition metal oxides were divided into two classes of catalysts. The first group (class I) catalyzed the homolytic decomposition of alkyl hydroperoxides to ketones and alcohols and consisted of the amphoteric oxides Crz 0 3 ,MnOz, Fez 0 3 ,FeO, CoZO3, Niz03, NiO, CuO, Ag,O, and CeO,. Acidic oxides, such as TiO,, Vz 05,Moo3, and W 0 3 , are class I1 catalysts and they are characterized by their ability to catalyze the heterolytic decomposition of aralkyl hydroperoxides to phenols and ketones. The underlying reasons for the divergent behavior of these two groups of catalysts in their reactions with organic hydroperoxides have been discussed in Sections II.B.1 and III.B.2. The important conclusion to be drawn from these results is the parallel reactivity and mode of reaction observed with transition metal oxides and the corresponding metal complexes in solution. The formation of phenolic inhibitors during heterolytic decomposition of the hydroperoxides by class I1 metal oxides accounts for the inhibition often observed in the autoxidation of alkylaromatic hydrocarbons. We mentioned a series of epoxidation catalysts comprising class I1 metal oxides on silica supports, whose activities are considerably enhanced compared to the metal oxides a l 0 n e . 4 ~ ~This phenomenon is commonly observed in heterogeneous catalysis. When transition metal oxides are combined with oxide carriers, the carrier affects not only the mechanical and physical properties of the catalyst, but it also affects catalytic properties, such as activity and selectivity. These fundamental changes in catalytic properties generally result ,from compound formation between the catalyst and the carrier. Such an interaction can be compared to ligand effects in homogeneous catalysis. The increased activity of TiOz-on-SiOz is consistent433 with the much stronger Lewis acidity of Ti02on-SiOz compared to TiO, .445 In other words, the electron-withdrawing effect of the silica “ligands” renders the Ti(IV) more electrophilic and enhances its activity as an epoxidation catalyst.433 The chemisorption of oxygen at metal centers on the surface of heterogeneous catalysts can be compared to the formation of dioxygen complexes with transib1615e-c tion metals in solution (see Section II.B.2). It is genefally thought614a9 that chemisorption of oxygen on transition metal oxides plays an important role in the overall behavior of these catalysts in oxidation reactions. It is possible for molecular oxygen to be adsorbed in the following ways:

380

ROGER A. SHELDON AND JAY K. KOCHI

O’u

I

0-M-0 (a)

0-0

0-0

.1

\/ 0-M-0

0-M-0

(b)

0-0

0-M

(C)

I

I M-0

(d)

Free radical character would be expected for chemisorption on oxides of Co(II), Mn(II), etc., that form paramagnetic complexes in solution as depicted in structure a. Similarly, by analogy with homogeneous complexes, ionic structures b and c are expected for metal centers (e.g., noble metals) that can readily undergo oxidative addition via a two-equivalent change. Oxidation of organic substrates by oxygen transfer from chemisorbed oxygen can be compared t o the examples of metal dioxygen complexes discussed earlier. The gas phase oxidation of ethylene to ethylene oxide over silver catalysts has been studied e x t e n s i ~ e l y . ~It~ has ~ ~ -been ~ suggested that epoxide formation involves transfer of oxygen from a silver-oxygen complex t o the olefin on the catalyst ~urface.4’~ a Silver-on-silica also catalyzes the liquid phase oxidation of cumene to cumene hydroperoxide. A mechanism that involved insertion of coordinated oxygen into a C-H bond was proposed6”: [Agl

+

02

[ A g ] 0 2 + RH

[&I02

(421)

[Ag] + ROzH

(422)

However, this interpretation has been questioned by V r e ~ g d e n h i l . ~He ~~ showed in a careful kinetic study that the catalytic activity of Ag-on-Si02 is mainly, if not entirely, due t o its capacity t o decompose hydroperoxides into chain-initiating radicals. (Approximately one-third of the ethylene is burned to carbon dioxide during the silver-catalyzed epoxidation.) Thus, mechanistic studies of autoxidations in heterogeneous systems suffer from ambiguities similar to their homogeneous counterparts. There is n o unequivocal evidence for the direct reaction of chemisorbed oxygen with hydrocarbon substrates under mild conditions. Catalysis of autoxidations proceeding by reaction with intermediate hydroperoxides is a more likely explanation. In the oxidation of ethylene (which only occurs at elevated temperatures) there is, of course, no possibility of forming hydroperoxides by the usual free radical process involved in autoxidation. In oxidations with metal oxides, the initial step could involve reaction of the organic substrate with the lattice (oxide) oxygen. This process could then be followed by a second step involving reoxidation of the reduced form of the catalyst by molecular oxygen. In the gas phase oxidations of olefins over metal oxide catalysts, such as Bi203-Moo3, it is generally accepted that an adsorbed allylic intermediate is formed by hydrogen transfer to lattice oxygen.632a The following initial steps would seem reasonable [cf. reactions with MoOz(SzCNRz) described in Section IILC] :

METAL-CATALYZED OXIDATIONS

38 1

Such reactions are only known to occur at elevated temperatures in the gas phase. Activation of olefins to nucleophilic attack, by n-complex formation at soft metal centers (Section IILD), can also occur with heterogeneous catalysts. Thus, the oxidation of ethylene to acetaldehyde or vinyl acetate, as described earlier for homogeneous Pd(I1) catalysts, can also be carried out heterogeneously in either the liquid or gas phase.512 Despite the enormous importance of zeolites (molecular sieves) as catalysts in the petrochemical industry, few studies have been made of the use of zeolites exchanged with transition metal ions in oxidation r e a ~ t i o n s . ~ ’ 634a-f ~~-~~ van Sickle and Prest6” observed large increases in the rates of oxidation of butenes and cyclopentene in the liquid phase at 70°C catalyzed by cobaltexchanged zeolites. However, the reactions were rather nonselective and led to substantial amounts of nonvolatile and sieve-bound products. Nevertheless, the use of transition metal-exchanged zeolites in oxidation reactions warrants further investigation.

V. Biochemical Oxidations A constant supply of oxygen is crucial t o the existence of most living things. Numerous important biochemical processes636. in living systems involve the participation of oxidative enzymes that can be broadly classified into three groups. The enzymes that catalyze the dehydrogenation of primary substrates are designated dehydrogenuses. When oxygen serves as the intermediate electron acceptor, to form water or hydrogen peroxide, the enzymes are called oxiduses. The third group of enzymes participate in a diverse group of reactions that involve the direct incorporation of molecular oxygen into organic substrates.

‘’’

382

ROGER A. SHELDON AND JAY K. KOCHI

They were discovered independently by Mason638 and Hayaishi et ~ 1 . in 6 ~ ~ 1955 and are called o x y g e n a ~ e s . ~ ~ ~ - ~ ~ ' Oxygenases are found widely distributed in animals, plants, and microorganisms. They play important roles in the metabolism of aromatic, alicyclic, and aliphatic hydrocarbons. Oxygenases can be conveniently divided into two types: monooxygenases, that catalyze the incorporation of a single oxygen atom into the substrate and dioxygenases that incorporate both oxygen atoms of an oxygen molecule into the substrate.

A. MONOOXYGENASES Most enzymatic oxygenations catalyzed by monooxygenases require the presence of both molecular oxygen and an electron donor, usually referred t o as the cofactor. These oxygenations are described by the general equation, S+02+XHz+

SO+X+HzO

(426)

in which S = substrate, XH2 = reduced electron donor, SO = oxygenated substrate, and X = oxidized electron donor. Thus, one of the oxygen atoms is used t o oxidize the substrate and the other serves as an electron acceptor, as is the case of oxidases. The origin of the term mixed-function oxidase, first proposed by describes this type of enzyme. The cofactor is dehydrogenated during the course of the reaction. In these reactions, the cofactor utilized varies with the enzyme. Reduced pyridine nucleotides, tetrahydropteridines, and I ascorbate have all been shown to function as cofactors with various enzymes. For example, with phenylalanine hydroxylase a tetrahydropteridine is used, and this enzyme will not function with ascorbate or pyridine nucleotides. The reason for this cofactor specificity is not known. In certain cases, the role of cofactor is fulfilled by the substrate itself, which is dehydrogenated and oxidized at the same time, namely, SH2+02+

SO+HzO

(427)

The name of infernal monooxygenase was proposed by Hayaishi and N ~ z a k i ~ ~ ~ for this type of enzyme; those that require a cofactor are called external monooxygenases. Most of the reactions catalyzed by monooxygenases result in the formation of hydroxyl groups, and, hence, the name hydroxylases. A wide variety of enzymatic hydroxylations of aliphatic and aromatic substrates involve monooxygenases. In addition, monooxygenases catalyze a seemingly diverse group of reactions, including epoxidation, lactonization, N- or S-oxide formation, and demethylation, depending on the nature of the substrate (Table IX). For example, the monooxygenase squalene e p ~ x i d a s econverts ~ ~ ~ the polyolefin squalene into squalene-2,3-epoxide, in one of the steps in the biosynthesis of cholesterol.

383

METAL-CATALYZED OXIDATIONS TABLE IX Reactions Effected by Monooxygenases ~

Type of reaction

~~

Substrate Rl

Product R1

\

Saturated C -H hydroxylation

\ R2-C-H

Aromatic hydroxylation

R3 ArH

R3 ArOH

Epoxidation of a double bond

RICH =CHR2 0

RICH-CHRZ 0

Lactonization

RlCR2

RlCOR2

Hydroxylation of a nitrogen atom

R1,NH R2’ R1

R 1 >N

/ II

Formation of an amine oxide

Formation of a sulfoxide

\

R2/N R3 R1, R2 /s

Rz-C-OH

/

/”\

II

R2

-OH

R1\ R2-N-0

/

R3 R1, RZ/s=o

B. DIOXYGENASES The following oxidative cleavage reactions of catechol are typical examples of reactions catalyzed by dioxygenases:

OH

rnetaeatechnle

Both of the oxygen atoms incorporated into catechol have been shown63g by labeling studies to be derived from molecular oxygen. Pyrocatechase and other related oxygenases are known to require either copper or iron for maximum activity. An efficient, nonenzymatic oxidation of catechol to cis,&-muconate with molecular oxygen activated by cuprous chloride has been recently presented as a model reaction for pyrocate~hase.6~~ Oxidation catalyzed by another dioxygenase, lipoxygenase, is worthy of mention because of the close similarity to classic free radial autoxidations. This

3 84

ROGER A. SHELDON AND JAY K. KOCHI

enzyme catalyzes the oxidation of lipids containing a cis,cis-l,4-pentadiene unit directly to an allylic hydroperoxide according to RICH=CHCH~CH=CHR~ + 0 2

----)

(428)

RlCH=CHCH=CHCHR2

I

O2H

The biosynthesis of prostaglandins from unsaturated fatty acids involves the successive participation of both a dioxygenase and a m o n o o x y g e n a ~ e . 6 ~ ~ ~ ~ ~ ~ ~ The mechanism shown in Scheme 4 has been suggested6493650a9b

PGE,

+

(

PG = prostaglandin

)

HO

PGFp

Scheme 4

C. MECHANISMS OF ENZYMATIC OXIDATIONS Enzymatic oxidations are generally thought6" to involve the following sequence of reactions: (i) binding of the substrate, (ii) reduction of the enzyme by electron transfer, (iii) binding of molecular oxygen to form a ternary reduced enzyme-substrate-O2 complex; and, finally (iv), a second electron transfer resulting in the liberation of the products. Although metal-free oxygenases are known, the majority are metalloenzymes. Prosthetic groups containing iron or copper are particularly prevalent among these enzymes. The metal ion apparently performs several functions652 in the various redox reactions, such as (i) binding substrates to the enzyme, (ii) activating molecular oxygen by forming a metal-dioxygen complex, and (iii) transferring electrons from the electron donor (cofactor) to the substrate via coordinated dioxygen. Since the metal ions [Cu(II), Fe(II), Co(II), Mn(II), etc.] contained in oxidative enzymes all readily undergo 1-electron redox reactions, one would expect

385

METAL-CATALYZED OXIDATIONS

1electron transfers and transient free radicals to be important in enzymatic oxidation (no doubt considerably modified by the rigid steric requirements of enzymes). In enzymatic reactions, highly selective reactions can result from preferential attack at specific sites of a bound substrate. Consequently, the least reactive site in a molecule, e.g., the terminal methyl group in a hydrocarbon chain, may undergo reaction preferentially. By contrast, the less selective autoxidations generally involve attack at the most reactive C-H group (for proximity effects in model systems, see Ref. 653). Radicals formed by 1electron transfers within the enzyme-substrate-02 complex could undergo a subsequent rapid lelectron transfer to afford the product, without actually proceeding through a “free” radical. Homolytic mechanisms have generally been rejected for enzymatic oxidations on the grounds that they are not consistent with the observed retention of configuration for oxidation at an asymmetric C-H bond.654atb However, it has been pointed out that retention of configuration is possible if the substrates are so orientated on an enzyme that C-H bond fission and C-0 bond formation must occur on the same side of the carbon atom. Retention of configuration in the cage reactions of free radical pairs has been It should be pointed out that no X-ray structure of a metalloenzyme capable of catalyzing a redox reaction has been reported. Thus, the detailed ennronment of the metal ion in most redox enzymes is largely unknown. The porphyrin ring system is known to be present in many metalloenzymes, including certain oxygenases. These ligands are probably intimately involved in catalysis carried out by these enzymes. Much of the current understanding of the mechanisms of enzymatic oxidations mediated by oxygenases has resulted from extensive of the monooxygenases that catalyze the formation of phenols from a variety of aromatic substrates. Udenfriend and c o - ~ o r k e r s ~ showed, ~ ‘ ~ ’ ~ in studies with deuteriumand tritium-labeled hydrocarbons, that aromatic hydroxylations with monooxygenases involve an intramolecular migration of the group displaced by hydroxyl to an adjacent position on the aromatic ring. This phenomenon has become known as the “NIH shift.” The ubiquitous nature of the NIH shift among monooxygenases has resulted in such migrations being a criterion for this class of enzymes. To account for the NIH shift, arene oxides were proposed as intermediates in enzymatic hydroxylations. One major aspect of the mechanism of the NIH shift is as follows:

H & x + X

+&+;++ X

( X = D,Cl,

0

alkyl, etc

)

HO

X

386

ROGER A. SHELDON AND JAY K. KOCHI

The intermediacy of arene oxides in these systems has been firmly established by subsequent mechanistic ~ t u d i e s . ~ Nonenzymatic rearrangement of 3,4-toluene-4-’H oxide gave 4-hydroxytoluene with the same amount of deuterium retention (in the 3-position) as that observed in the enzymatic hydroxylation of toluene-4-’H by liver micro some^.^^' 1,2-Naphthalene oxide could actually be isolated as the initial product of the enzymatic hydroxylation of na~hthalene.~” Isomerization of arene oxides to phenols proceeds by complex and often multiple pathways660-668that show a marked dependence on pH. Arene oxides as key intermediates provide the basis for explaining the ortho/meta/para isomer ratios observed in enzymatic h y d r o ~ y l a t i o n s . 6 Fo ~ ~r ~example, ~~ the absence of m-cresol in the metabolites of toluene can be attributed to the fact that none of the three possible isomers of toluene oxide rearranges to this product. Other products formed during the metabolism of aromatic substrates have been shown to be derived from intermediate arene 0 x i d e s . 6 ’ ~ ~ For ~ ~example, the formation of both phenol and catechol from benzene is rationalized as follows:

”-“’

0

-0 isomerase

dehydrogenare

OH

Similarly, epoxides have been shown669 to be obligatory intermediates in the metabolism of olefins to glycols. The demonstration of the intermediacy of arene oxides and epoxides in the enzymatic oxidation of arenes and olefins provides considerable insight into the mechanism of action of monooxygenases. The active oxidant in these reactions must be an electrophilic species. These reactions formally involve oxygen atom transfer reactions that resemble the heterolytic oxidations of organic substrates by electrophilic metal-peroxide complexes discussed in Section 111.B. The insertion of an oxygen atom into saturated C-H bonds and the addition of oxygen atoms to double bonds are formally reminiscent of the reactions of electrophilic carbenes. This analogy to carbenes led to the p o s t ~ l a t i o n ~of’ ~the so-called oxenoid mechanism for monooxygenases (see Scheme 5). Any mechanism for monooxygenases must include the activation of molecular oxygen to form some type of electrophilic oxygen species. One approach to

387

METAL-CATALYZED OXIDATIONS

understanding these complex systems is to devise simple chemical models for the monooxygenases and to determine the nature of the active oxidant in these model systems.

D. CHEMICAL MODELS FOR OXYGENASES Interest in devising chemical models for oxygenases is twofold: first, to provide a basis for understanding enzymatic oxidations and, second, to develop simple catalytic systems that can emulate the high selectivity under mild conditions characteristic of enzymatic oxidations. Much of the work on model systems was stimulated by the observation of Udenfriend and co-workers in 1954654aybthat a mixture of Fe(II), EDTA, ascorbic acid, and molecular oxygen could hydroxylate arenes to phenols under mild conditions. Udenfriend’s reagent also hydroxylates alkanes to alcohols and epoxidizes ole fin^.^^'-^ 74 The EDTA in Udenfriend’s reagent probably reduces the redox potential of the Fe(II)/Fe(III) couple. The ascorbic acid functions as an electron donor, analogous to the cofactor in monooxygenases, and can be replaced by other e n e d i o l ~ . ~ ~ ~ The nature of the active oxidant in this system has been the subject of controversy. In one rationale, molecular oxygen is initially reduced to hydrogen peroxide. Hydroxylation then proceeds by hydroxyl radicals formed by reaction of Fe(I1) with hydrogen peroxide (Fenton’s reagent). However, Fenton’s reagent and Udenfriend’s reagent give different isomer distributions in the hydroxylation of arenes. Hamilton et aL ,672-674on the basis of product studies, proposed that the reaction does not involve hydrogen peroxide or hydroxyl radicals. These authors6 72 pointed out the structural similarities between ascorbic acid (XV) and tetrahydroptenidine (XVI), which has been identified as the cofactor of phenylalanine hydroxylase.6 75a-c 2,4,5 -Triamino-6-hydroxypyrimidine (XVII), which is a model for the tetrahydropteridine system, can replace the ascorbic acid in Udenfriend’s system and gives similar results in the hydroxylation of anisole.672 0

H

HzNxy

“X;qYNHz

R

I CH,OH

(XV)

H

\

N

HZN

\

N

OH

(XVI)

(XVII)

Hamilton674 proposed the “oxenoid” mechanism shown in Scheme 5 for both the model and enzymatic systems. (The iron-dioxygen complex is assumed to react as an oxenoid species and transfer an oxygen atom to the substrates S . )

ROGER A. SHELDON AND JAY K. KOCHI

388

xoH t

Fez'-

-*"+

x o > F e

P

0

OH

t

Fe"

i

-2 "0-

Scheme 5

However, since Udenfriend's reagent does not cause the NIH hydroxylation by it does not proceed by the same mechanism as that involved in the enzymatic oxidation. A radical process seems to be more likely. Norman and Lindsay Smith7'- have suggested a mechanism without directly implicating the hydroxyl radical:

0 (active oxidant)

Furthermore, variations in the reaction conditions cause changes in the distributions of phenol isomers. Thus, several mechanisms may be operative, depending on the reaction conditions. Despite numerous investigations, the mechanisrn(s) of Udenfriend's system remains obscure.656 Other workers677 showed that a model system consisting of Fe(II), EDTA, molecular oxygen and a tetrahydropteridine can hydroxylate phenylalanine to tyrosine. The following mechanism was suggested:

%p

0

.O-O-ArH

HN

) '1

H

HN

x?:)

+

ArOH

t

FeO

n

Several other systems, such as Fe(II1)-0, -N-benzyl-l,4-dihydronicotinamide678 and Fe(III)-H202 -catech01,6~' also hydroxylate arenes. However, none of these systems produce the NIH shift, suggesting that they probably

METAL-CATALYZED OXIDATIONS

389

proceed via radical mechanisms.6s6 The hydroxylation of arenes by molecular oxygen also occurs678 in the presence of metal ions, such as Cu', Ti3+,SnZ+,and Fez++ EDTA, all having a redox potential of around 0.15 eV. Despite several investigations, the mechanisms of these hydroxylations remain unclear. However, further study of these systems is warranted since the Sn(I1)-0, system has been shownds6 to produce the NIH shift, probably via arene oxides. The Sn(II)-02 system has also been shown6" to oxidize alkanes to the corresponding alcohols at ambient temperatures. It was suggested6" that the active oxidant is an Sn(II1)-O2 species, which can be regarded as an inorganic analog of alkylperoxy radicals. Several chemical oxidants produce the NIH shift in the hydroxylation of 4-ani~ole-'H.6~~Significantly, the system consisting of ~ - B U O ~ H - M O (inC~)~ duced deuterium migration the most. This reagent is known to effect many other reactions characteristic of monooxygenases. The yields obtained in these hydroxylations were not reported. Sharpless and Flood"' observed reactions characteristic of monooxygenases, such as stereospecific epoxidation of olefins and arene hydroxylation, with oxotransition metal (M=O) oxidants. For example, Cr02X2(X=Cl, OAc) oxidized tritium labeled naphthalene to naphthoquinone accompanied by tritium migration. Rearrangement is suggestive of arene oxide transients. Metelitsa er al. 683 recently reported the hydroxylation of naphthalene to &-naphthol with MO(CO)~-O~ at 75" to 95°C in acetonitrile solutions. Oxygen activation by the (C10H6)Mo(CO)3complex was suggested:

Cl0",

W-M~,

oc'

co

+

02-

+H; W--0-0OC/&Cd

-

O+O

0 II

two-&

Since the reaction involves irreversible oxidation of molybdenum, it would not be expected to be catalytic. The various chemical models discussed in the preceding may be divided into two groups. The first group resembles enzymes in that the models consist of Fe(II), 0 2 ,and a biological reducing agent. However, they do not produce arene oxides as primary products, since no NIH shift is observed. The second group bears little resemblance to enzymes but these models are able to produce the NIH shift. The proposed mechanisms for both of these groups, as shown in the foregoing, are highly speculative. It may be concluded that our understanding of the nature of the active oxidants in oxygenases is far from complete. Moreover, the model systems are of dubious synthetic value since they generally afford low yields of products and they often utilize stoichiometric quantities of expensive reagents. Nonetheless, studies of model systems have demonstrated the need for a better understanding of the interaction of molecular oxygen with metal ions, in par-

390

ROGER A. SHELDON AND JAY K. KOCHI

ticular Fe(I1) and Cu(I), which are prevalent in oxygenases. The reaction of molecular oxygen with simple Fe(I1) complexes is irreversible. Initial coordination of oxygen t o Fe(I1) is followed by a rapid bimolecular redox process, which leads irreversibly t o Fe(II1) species: Fe(I1) + O2

Fe(11)

Fe(II)02

+ 2Fe(III)

(429)

Reversible oxygenation of Fe(I1) is possible in biological systems, such as the oxygen-transporting proteins, hemoglobin and myoglobin. The steric bulk of the porphyrin ligands surrounding the Fe(I1) prevents the approach of a second Fe(I1). Recently, three research g r o ~ p s ha ~ ve~ succeeded ~ - ~ ~ in~ synthesizing models of the active site of hemoglobin and myoglobin. Thus, the Fe(I1) “picket fence” p ~ r p h y r i n s and ~ ~ ~related ~ - ~ complexes6853686 reversibly bind molecular oxygen at ambient temperatures. These results sustain the expectation that more light may be shed in the future o n the nature of the activation of oxygen by the active sites of oxygenases, several of which are also thought to contain metal porphyrin structures. Finally, an alternative to devising chemical models for enzymes is to make the enzymes themselves more attractive for large-scale synthesis by immobilization on insoluble supports.687 Immobilization of an enzyme would make a continuous process more practical and probably improve its stability, at the same time retaining the high selectivity and stereospecificity under mild conditions.

VI. Summary-Directions for Future Development We have attempted in this review to present the various aspects of metalcatalyzed oxidations largely within a mechanistic framework. The development of this field of research has depended strongly on the interaction and crossfertilization of ideas from many branches of chemistry. To understand the complex and diverse art requires a knowledge of free radical chemistry, organometallic chemistry, biochemistry, catalysis, and many more. We have seen that the recurring themes of electron and ligand transfer reactions, activation by coordination, etc., are fundamental to a unified presentation of catalysis in oxidations, whether it occurs in homogeneous, heterogeneous, or enzymatic processes. Catalysis of oxidation reactions will continue to be of enormous importance in the future. Areas that continue to be of active interest are the development of efficient methods for the direct epoxidation of olefins, hydroxylation and substitution of aromatics as well as the selective oxidation of alkanes. The application of methods developed for industrial chemicals to the synthesis of more complex molecules is worthy of more attention. A few examples have been discussed in the text. On the whole, however, synthetic chemists have not exploited these methods. Further work o n chemical models for enzymatic oxidations is desirable, as is the investigation of the direct use of enzymes by immobilization. A pertinent

METAL-CATALYZED OXIDATIONS

39 1

example is the use of immobilized enzymes for the oxidation of saturated hydrocarbons. In the words of the eminent chemist, N. N. Semenov,688 “By applying the ideas of biochemistry, chemical science may solve the energy crisis, make industrial production infinitely more efficient, and provide mankind with wings.” REFERENCES 1. For historical background, see Moureu, C., and Dufraisse, C., Chem. Rev. 7, 113 (1926); Milas, N. A., Chem. Rev. 10,295 (1932). 2. Bolland, J. L., Quart. Rev., Chem. SOC. 3, 1 (1949); Bateman, L., Quart. Rev., Chem. SOC.8,147 (1954). 3. Sittig, M., “Combine Hydrocarbons and Oxygen for Profit,” Chem. Process Rev. No. 11. Noyes Develop. Co., Park Ridge, New Jersey, 1968. 4. Sittig, M., “Catalysts and Catalytic Processes,” Chem. Process Rev. No. 7. Noyes Develop. Co., Park Ridge, New Jersey, 1967. 5. Sittig, M., “Organic Chemical Processes.” Noyes Press, New York, 1962; Sittig, M., “Organic Chemical Process Encyclopedia,” 2nd Ed. Noyes Develop. Co., Park Ridge, New Jersey, 1969. 6. Dumas, T., and Bulani, W., “Oxidation of Petrochemicals: Chemistry and Technology.” Appl. Sci. F’ubl., London, 1974. 7. Thomas, C. L., “Catalytic Processes and Proven Catalysts.” Academic Press, New York, 1970. 8. Szonyi, G., Advan. Chem. Ser. 70,53 (1968). 9. Hatch, L. F., Hydrocarbon Process. 49,101 (1970). 10. Prengle, H. W., and Barona, N., Hydrocarbon Process. 49,106 (1970). 11. Lloyd, W. G., Chemtech. pp. 176,371,687 (1971); p. 182 (1972). 12. Sheldon, R. A., and Kochi, J. K., Oxid. Combust. Rev. 5,135 (1973). 13. Candlin, J. P., Taylor, K. A., and Thompson, D. T.,Znd. Chim. Belge 35,1085 (1970). 14. Twigg, G. H., Chem. Ind. (London) p. 4 (1962). 15. Discuss. Faraday soc. 46, (1968). 16. Ugo, R., Chim. Ind. (Milan) 51,1319 (1969). 17. Reich, L., and Stivala, S. S., “Autoxidation of Hydrocarbons and Polyolefins.” Dekker, New York, 1969. 18a. Emanuel, N. M., Denisov, E. T., and Maizus, Z. K., “Liquid Phase Oxidation of Hydrocarbons” (B. J. Hazzard, transl.). Plenum, New York, 1967. 18b. Denisov, E. T., and Emanuel, N. M., Russ. Chem. Rev. 29,645 (1960). 18c. Maslov, S. A., Blyumberg, E. A., Norikov, Y. D., and Emanuel, N. M.,Dokl. Akad. Nauk SSR 210,131 (1973). 19. Lundberg, W. O., ed., “Autoxidation and Antioxidants,” Vols. I and 11. Wiley, New York, 1962. 20. Scott, G., “Atmospheric Oxidation and Antioxidants.” Elsevier, Amsterdam, 1965. 21. Emanuel, N. M., ed., “Oxidation of Hydrocarbons in the Liquid Phase.” Pergamon, Oxford, 1965. 22. Berezin, I. V., Denisov, E. T., and Emanuel, N. M., “The Oxidation of Cyclohexane” (K. A. Allen., transl. ). Pergamon, Oxford, 1966. 23. Mayo, F. R., ed., Oxidation of Organic Compounds. Advan. Chem. Ser. 75, 76, and 77 (1968). 24. Mayo, F. R.,Accounts Chem. Res. 1,193 (1968). 25. Russell, G . A., J. Chem. Educ., 36,111 (1959).

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ROGER A. SHELDON AND JAY K. KOCHI

26. Howard, J. A., in “Free Radicals” (J. K. Kochi, ed.), Vol. 11, p. 3. Wiley, New York, 1973. 27. Huyser, E. S., “Free Radical Chain Reactions,” p. 39. Wiley (Interscience),New York, 1970. 28a. Betts, J., Quart. Rev. Chem. SOC. 25,265 (1971). 28b. Lloyd, W. G.,Methods Free Radical Chem. 4 , l (1973). 29. Sire6 de Roch, I.,lnd. Chim. Belge 33,994 (1968). 30. Ingold, K. U., Accounts Chem. Res. 2 , l (1969). 31. Ingold, K. U.,Pure Appl. Chem. 15,49 (1967). 32. Howard, J. A., Advan. Free Radical Chem. 4 5 5 (1972). 33. Bennett, J. E., Brown, D. M., and Mile, B., Trans. Faraday Soc. 66,386,397 (1970). 34a. Hiatt, R., Mill, T., and Mayo, F. R.,J. Org. Chem. 33,1416 (1968). 34b. Hiatt, R., Mill, T., Irwin, K. C., and Castleman, J. K.,J. Org. Chem 33,1421,1428 (1968). 34c. Hiatt, R., Irwin, K. C., and Gould, C . W.,J. Org, Chern. 33, 1430 (1968). 34d. Hiatt, R., and Irwin, K. C., J. Org. Chem. 33,1436 (1968). 35. Swern, D., ed., “Organic Peroxides,” Vol. I. Wiley, New York, 1970; Vol. 11. 1971; Vol. 111. 1973. 36a. Hawkins, E. G. E., “Organic Peroxides,” Van Nostrand, New York, 1961. 36b. Martin, L. F., “Organic Peroxide Technology.” Noyes Data Corp., Park Ridge, New Jersey, 1973. 37. Davies, A. G., “Organic Peroxides.” Butterworth, London, 1961. 38. Tobolsky, A. V., and Mesrobian, R. B., “Organic Peroxides.” Wiley (Interscience), New York, 1954. 39. Edwards, J. O., ed., “Peroxide Reaction Mechanisms.” Wiley (Interscience), New York, 1962. 40a. Russell, G. A.,J. Amer. Chem SOC.78,1035,1041 (1956). 40b. Carlson, D. J. and Robb, J. C., Trans. Faruduy SOC. 62,3403 (1966). 40c. Dulog, L., Mukromol. Chem. 76,119 (1964). 40d. Bromberg, A., and Muzzket, K. A.,J. Amer. Chem. SOC. 91,2860 (1969). 41. Brilkina, T.G., and Shushunov, V. A., “Reactions of Organometalk Compounds with Oxygen and Peroxides” (Engl. transl., A. G. Davies, ed.). Iliffe, London, 1969. 42a. Russell, G. A., Bemis, A. G., Janzen, E. G., Geels, E. J., and Moye, A. J., Advan. Chem. Ser. 75,174 (1968). 42b. Russell, G. A., Bemis, A. G., Janzen, E. G., Geels, E. J., Moye, A. J., Mak, A. J., and Strom, E. T.,Aduan. Chem. Ser. 51,112 (1965). 42c. Sosnovsky, G., in “Organic Peroxides” (D. Swern, ed.), Vol. 111, p. 354. Wiley, New York, 1971. 42d. Ingold, K. U., and Roberts, B. P., “Free Radical Substitution Reactions.” Wiley (Interscience). New York. 1971. 42e. Garst, J. F., “Free Radicals” (J. K. Kochi, ed.), Vol. I, p. 503. Wiley, New York, 1973. 43. Walling, C.,J. Amer. Chem SOC. 91,7590 (1969). 44. Benson, S . W., J. Amer. Chem. SOC. 87,972 (1965). 45a. Russell, G. A.,J. Amer. Chem SOC. 79,3871 (1957). 45b. Bartlett, P. D., and Traylor, T. G., J. Amer. Chem. SOC. 85,2407 (1963). 46a. van Sickle, D. E., Mayo, F. R., and Arbuck, R. M., J. Amer. Chem. SOC. 87,4824 (1965). 46b. van Sickle, D. E., Mayo, F. R., Could, E. S., and Arbuck, R. M.,J. Amer. Chem. Soc. 89,977 (1967).

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46c. van Sickle, D. E., Mayo, F. R., Arbuck, R. M., and Syz, M. G., J. Amer. Chem. SOC. 89,967 (1967). 47. Koelewijn, N., Rec. Trav. Chim. Pays-Bas 91,759 (1972). 48. Niki, E., Kamiya, Y., and Ohta, N., Bull. Chem. SOC.Jap. 42,512 (1969). 49. Mayo, F. R., Syz, M. G., Mill, T., and Castleman, J. K., Advan. Chem. Ser. 75, 38 (1968). 50. Hendry, D. G., Advan. Chem. Ser. 75,24 (1968). 51. Tsuchiya, F., and Ikawa, T., Can. J. Chem. 47,3191 (1969). 52a. Taube, H., Advan. Inorg. Chem. Radiochem. 1, 372 (1957); Chem. Rev. 35,432 (1945). 52b. Taube, H., and Gould, E. S . , Accounts Chem. Res. 2,321 (1969). 53. Reynolds, W., and Lumry, R., “Mechanisms of Electron Transfer.” Ronald Press, New York, 1966. 54a. Sutin, N., Accounts Chem. Res. 1,225 (1968). 54b. Sutin, N., in “Oxidases and Related Redox Systems” (T. E. King, H. S. Mason, and M.Morrison, eds.), Vol. I, p. 37. Wiley, New York, 1965. 55. Sykes, A. G., Advan. Inorg. Chem. Radiochem. 10,153 (1967). 56. Denisov, E. T.,Russ. Chem. Rev. 40,24 (1971). 57. Brown, S. B., Jones, P., and Suggett, A.,&ogr. Inorg. Chem. 13,334 (1970). 58. Edwards, J. O., ed., “Inorganic Reaction Mechanisms.” Wiley (Interscience), New York, 1970. 59. Littler, J. S . , Chem. SOC.Spec. Publ. 24,383 (1970). 60. Linck, R. G., in “Transition Metals in Homogeneous Catalysis” (G. N. Schrauzer, ed.), p. 297, Dekker, New York, 1971. 61. Kochi, J. K.,Rec. Chem. Progr. 27,207 (1966). 62. Kochi, J. K., Science 155,415 (1967). 63a. Kochi, J. K.,Proc.Int. Congr. Pure Appl. Chem., 23rd, Boston,Mass. 4,377 (1971). 63b. Waters, W. A., Proc. Int. Congr. Pure Appl. Chem., 23rd, Boston, Mass. 4, 307 (1971). 64. Kochi, J. K., in “Free Radicals” (J. K. Kochi, ed.), Vol. I, p. 591. Wiley, New York, 1973. 65. Kochi, J. K., Accounts Chem. Res. 7,351 (1974). 66a. Pearson, R. G., Science 151,172 (1966). 66b. Pearson, R. G., ed., “Hard and Soft Acids and Bases.” Dowden, Hutchison & Ross, Stroudsburg, Pennsylvania, 1973. 67a. Sosnovsky, G., and Rawlinson, D. J., in “Organic Peroxides” (D. Swern, ed.),.Vol. I, Chs. X and XI. Wiley, New York, 1970; Vol. 11, Chs. I1 and 111. 1971. 67b. Rawlinson, D. J., and Sosnovsky, G., Synthesis p. 1 (1972). 68. Doumaux, A. R., in “Oxidation” (R. Augustine, ed.), p. 141. Dekker, New York, 1971. 69a. Baxendale, J. H., Advan. Gztal. 4,31 (1952). 69b. Uri, N., Chem Rev. 50,343 (1952). 69c. Weiss, J., Advan. Catal. 4,343 (1952). 70. Norman, R. 0. C., and Lindsay Smith, J. R., in “Oxidases and Related Redox Systems” (T. E. King, H. S. Mason, and M. Morrison, eds.), p. 131. Wiley, New York, 1965. 71a. For a review concerning the synthetic utility of redoxcatalized decomposition of hydrogen peroxide and organic peroxides, see Boguslavskaya, L., Russ. Chem. Rev. 34,503 (1965). 71b. Cf. also Berdnikov, V. M.,Russ J. Phys. Chem. 47,1547 (1973).

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410

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584. Danno, S.,Moritani,I.,and Fujiwara, Y.,Chem. Commun. p. 610 (1970); Tetrahedron 25,4819 (1969). 585. Heck, R. F.,J. Amer. Chem. SOC. 90,5518 (1968); 91,6707 (1969). 586. Shue, R. S., Chem. Commun. p. 1510 (1971);J. Amer. Chem. Soc. 93,7116 (1971). 587. Shue, R. S.,J. Catal. 26,112 (1972). 588. Henry, P. M.,J. Org. Chem. 36,1886 (1971). 589. Tissue, T., and Downs, W. J., Chem. Commun. p. 410 (1969). 590. Eberson, L., and GomezGonzales, L., Chem. Commun. p. 263 (1971). 591. Eberson, L., and GomezGonzales, L.,Acta Chem. Scand. 27,1162 (1973). 592. Eberson, L., and GomezGonzales, L., Acra Chem. Scand. 27, 1249, 1255 (1973). 593. Eberson, L., and Jonsson, E., Acra Chem. Scand., B 28,771 (1974). 594. Eberson, L., and Jonsson, L., Chem. Commun. p. 885 (1974). 595. Arpe, H. J., and Hornig, L., Erdoel Kohle, Erdgas, Petrochem. 23,79 (1970). 596. Bryant, D. R., McKeon, J. E., and Ream, B. C., Tetrahedron Lett. p. 3371 (1968). 597. Bryant, D. R., McKeon, J. E., and Ream, B. C., J. Org. Chem. 33, 4123 (1968). 598. Bushweller, C. H., Tetrahedron Lett. p. 6123 (1968). 599. Fitton, P., McKeon, J. E., and Ream, B. C., Chem. Commun. p. 370 (1969). 600. Henry, P. M., Tetrahedron Lett. p. 2285 (1968). 601. Parshall, G. W., et al., in “Catalysis, Progress in Research” (F. Basolo and R. L. Burwell, eds.), p. 165. Plenum, London, 1973. 602. Shilov, A. E., and Shteinman, A. A., Kinet. Catal. (USSR) 14,117 (1973). 603. Shilov, A. E., in “Some Theoretical Problems of Catalysis” (T. Kwan, G. K. Boreskov, and K. Tamaru, eds.), pp. 231-240. Tokyo Univ. Press, Tokyo, 1973. 604. Parshall, G. W., Chemtech. p. 445 (1974). 605. Gol’dshleger, N. F., Tyabin, M. B., Shilov, A. E., and Shteinman, A. A,, Russ. J. Phys. Chem. 43,1222 (1969). 606a. Gol’dshleger, N. F., Moiseev, I. I., Khidekel, M. L., and Shteinman, A. A., Dokl. Chem. 206,694 (1972). 606b. Hodges, R. J., Webster, D. E., and Wells, P. B., J. Chem. SOC.A p. 3230 (1971). 606c. Davis, K., Garnett, J. L., Hon, K.,Kenyon, R. S., and Long, M. A.,Proc. Int. Congr. Catal., Sth, Miami Beach, Flu. p. 491 (1972). 607. Garnett, J. L., Long, M. A., and Peterson, K. B., Aust. J. Chem. 27, 1823 (1974). 608a. Cheney, A. J., Mann, B. E., Shaw, B. L., and Slade, R. M., J. Chem. SOC.A p. 3833 (1971). 608b. Cotton, F. A., and Day, V. W., Chem. Commun. p. 415 (1974). 609a. Parshall, G. W., Accounts Chem. Res. 3,139 (1970). 609b. Parshall, G. W., Accounts Chem. Res. 8,113 (1975). 609c. Klabunde, U., unpublished results, cited in Ref. 609b. 610a. Masters, C., Chem. Commun. p. 192 (1973). 610b. Masters, C., Chem. Commun. p. 1258 (1972). 611a. Cf. Brieger, G., and Nestrick, T. J., Chem. Rev. 74,567 (1974). 611b. Nishiguchi, T., Kurooka, A., and Fukuzumi, K., J. Org. Chem. 39, 2403 (1974). 611c. Gol’dshleger, N. F., Khidekel, M. L., Shilov, A. E., and Shteinman, A. A., Kiner. Catal. (USSR) 15,235 (1974). 612a. Schuit, G. C. A., Chim. Ind. (Milan) 51,1307 (1969). 612b. Scurell, M. S., Ann. Sep. Chem. Soc.,A 70,87 (1973). 612c. Morrison, S . R.,J. Catal. 34,462 (1974). 613. Giordano, N., Chim. Ind. (Milan) 51,1189 (1969). 614a. Carra, S., Ugo, R., and Zanderighi, L., Inorg. Chim. Acta Rev. 2,55 (1969). 614b. Carra, S . , Chim. Ind. (Milan) 53,366 (1971).

METAL-CATALYZED OXIDATIONS

41 1

615a. Isaev, 0. V., Margolis, L. Y., and Kushnerev, M. Y., Russ. J. Phys. Chem. 47,1198 (1973). 615b. Haber, J., Z Chem. (Cracow) 13,241 (1973). 615c. Andreev, A., and Neshev, N., React. Kinet. Catal. Lett. 1,297 (1974). 616. Hucknall, D. J., “Selective Oxidation of Hydrocarbons.” Academic Press, New York, 1974. 617. Margolis, L. Y., Advan. Catal. 14,429 (1963). 618. Sampson, R. J.,and Shooter, D.,Oxid. Combust. Rev. 1,225 (1965). 619a. Voge, H. H., and Adams, C. R., Advan. Catal. 17,151 (1967). 619b. Voge, H. H., Advan. Chem. Ser. 76,242 (1968). 620a. Sachtler, W. M. H., Catal. Rev. 4,27 (1970). 620b. Adams, C. R., Ind. Eng. Chem. 61(6), 30 (1969). 621. Dowden, D. A., Chem. Eng. Progr.,Symp. Ser. 63,73 (1967). 622a. Garnett, J. L., Catal. Rev. 5,229 (1971). 622b. Manassen, J., Chim. Ind. (Milan) 51,1058 (1969). 623. Ballard, D. G. H., Chem. Brit. p. 20 (1973). 624. Heinemann, H.,Chemtech. 1,286 (1971). 625a. Pittman, C. U., and Evans, G. O., Chemtech. 1,416 (1971). 625b. Evans, G. O., Pittman, C. U., McMillan, R., Beach, R. T., and Jones, R., J. Organometal. Chem. 67,295 (1974). 626. Rony, P. R., J. Catal. 14,142 (1969). 627. Bailar, J. C., Catal. Rev. 10,17 (1974). 628. Michalska, Z. M.,and Webster, D. E., Platinum Metals Rev. 18,65 (1974). 629. Valendo, A. Y., and Norikov, Y. D., Petrol. Chem. USSR 8,234 (1969). 630. van Ham, N. H. A., Nieuwenhuys, B. E., and Sachtler, W. M. H., J. Catal. 20, 408 (1971). 631. Vreugdenhil, A. D., J. Catal. 28,493 (1973). 632a. Simons, T. G. J., Verheijen, E. J. M., Batist, P. A., and Schuit, G. C. A., Advan Chem. Ser. 76,261 (1968). 632b. Cf. Buiten, J., J. Catal. 27,232 (1972). 632c. Gelbshtein, A. I., Mishchenko, Y.A., and Goldstein, N. D., Dokl. Akad. Nauk SSSR 20,374 (1972). 632d. Nakagawa, K., Konaka, R., and Sugita, J., Shionogi Kenkyusho Nempo 19, 141 (1969); Yakugaku Zasshi 94,1180 (1974). 632e. Belew, J. S., Garza, C.,and Mathieson, J. W., Chem. Commun. p. 634 (1970). 632f. Balachandran, K. S., and George, M. V.,Indian J. Chem. 11,1267 (1973). 633a. Breck, D. W., “Zeolite Molecular Sieves: Structure, Chemistry and Use.” Wiley, New York, 1974. 633b. Molecular Sieve Zeolites, Advan. Chem. Ser. 101,102 (1971). 633c. Molecular Sieves, Advan. Chem. Ser. 124 (1974). 634a. Rudham, R., and Sanders, M. K., J. Catal. 27,287 (1972). 634b. fitterski, L. N., Zakharova, V. I., Artemova, N. C., and Kamneva, A. I., Tr. Mosk. Khim.-Tekhnol. Inst., 74,14 (1974). 634c. Malashevich, L. N., Borisevich, A. D., and Ermolenko, N. F., Dokl. Akad. Nauk Beloruss. SSR 16,723 (1972). 634d. lone, K. G., Bobrov, N. N., Boreskov, K. G.,andVostrikova,L. A.,Dokl. Akad. Nauk SSSR 210,102 (1973). 634e. Leach, H. F., Ann. Rep. Chem. Soc.,A 68,195 (1971). 634f. Wolf, F., Bergk, K. H., and Mueller, B., East Ger. Patent 106,273 (1974). 635. van Sickle, D. E., and Prest, M. L.,J. Catal. 19,209 (1970).

412

ROGER A. SHELDON AND JAY K. KOCHI

636. Vandecasteele, J. P., Kinet. Catal. (USSR) 14,95 (1973). 637. Bennett, J. E., Prop. Inorg. Chem. 18,p. 1 (1973). 638. Mason, H. S . , Amer. Chem. SOC. Meet., 130th, Atlantic City, N.J. Abstr. p. 55c (1956);Science 125,1185 (1957). 639. Hayaishi, O., Rothberg, S., and Mehler, A. H., Amer. Chem. SOC.Meet., 130th, Atlantic City, N.J. Abstr. p. 53c (1956); Hayaishi, O., Katagari, M., and Rothberg, S . , J. Amer. Chem SOC. 77,5450 (1955). 640a. Hayaishi, O., ed., “Oxygenases.” Academic Press, New York, 1962. 640b. Hayaishi, O., ed., “Molecular Mechanisms of Oxygen Activation.” Academic Press, New York, 1974. 641. King, T. E., Mason, H. S., and Morrison, M., eds., “Oxidases and Related Redox Systems,” Vols. I and 11. Wiley, New York, 1965. 642. Bloch, K., and Hayaishi, O., eds., “Biological and Chemical Aspects of Oxygenases.” Maruzen, Tokyo, 1966. 643. Hayaishi, O., and Nozaki, M., Science 164,389 (1969). 644a. Hayaishi, O., Annu. Rev. Biochem. 38,21 (1969). 644b. Cf. May, S . W., et al., Biochem. Biophys. Res. Commun. 48,1230 (1972);54,1540 (1973); J. Biol. Chem. 248,1725 (1973). 645. Mason, H. S., Annu. Rev. Biocbem. 34,595 (1965). 646. Mason, H. S., Advan. Enzymol. 19,79 (1957). 647. Advan. Chem. Ser. 77,170-307 (1968). 648. Yamamoto, S., and Bloch, K.,J. Biol. Chem. 245,1670 (1970). 649. Tsuji, J., and Takayanagi, H., J. Amer. Chem. SOC. 96,7350 (1974). 650a. Bergstrum, S . , Science 158,382 (1967). 650b. Samuelson, B.,J. Amer. Cbem. SOC. 87,3011 (1965). 651. Urich, V. U., and Diehl, H., Euro. J. Biochem. 20,509 (1971). 652. Hamilton, G. A., in “Catalysis, Progress in Research” (F. Basolo and R. I.. Burwell, eds.), p. 47. Plenum, New York, 1973; Hamilton, G. A., Advan. Enzymol. 32, 55 (1969). 653. Breslow, R., Chem. SOC.Rev. 1,553 (1972), and related papers. 654a. Jones, D. F., and Howe, R., J. Chem. SOC. C pp. 2801, 2809, 2816, 2821, 2827 (1968). 654b. Hamberg, M., and Bjorkhem, I., J. Biol. Chem. 246,7411,7417 (1971). 655a. Engstrom, J. P., and Greene, F. D., J. Org. Cbem. 37,968 (1972). 655b. Koenig, T., and Owens, J. M., J. Amer. Chem SOC. 96, 4054 (1974); 95, 8485 (197 3). 656. Jerina, D. M.,Cbemtech. p. 120 (1973). 657a. Guroff, G., Daly, J. W., Jerina, D., Renson, J., Witkop, B., and Udenfried, S., Science 157,1524 (1967). 657b. Daly, J., Guroff, G., Jerima, D., Udenfriend, S., and Witkop, B., Advan. Chem. Ser. 77,270 (1968). 658. Jerina, D. M., Daly, J. W., and Witkop, B., J. Amer. Cbem. SOC. 90, 6523 (1968). 659. Jerina, D. M., Daly, J. W., and Witkop, B., J. Amer. Chem. Soc. 90, 6525 (1968). 660. Jerina, D. M., Kaubisch, N., and Daly, J. W., Proc. Nut. Acad. Sci. US. 68, 2545 (1971); Biochemistry 11,3080 (1972). 661. Boyd, D. R., Daly, J. W., and Jerina, D. M.,Biochemistry 11,1961 (1972). 662. Jerina, D. M.,Daly, J. W., Witkop, B., Zaltzman-Nirenberg, P.,and Udenfriend, S., Biochemistry 9,147 (1970). 663a. Kasparek, G. J., and Bruice, T. C., J. Amer. Chem. SOC. 94,198 (1972). 663b. Whelan, D. L., and Ross, A. M., J. Amer. Chem. Soc. 96,3678 (1974).

METAL-CATALYZED OXIDATIONS

413

664. Kasparek, G. J., Bruice, T. C., Yagi, H., Kaubisch, N., and Jerina, D. M., J. Amer. Chem. SOC. 94,7876 (1972). 665. Kasparek, G . J., Bruice, T. C., Yagi, H., and Jerina, D. M., Chem. Commun. p. 784 (1 972). 666. Yagi. H., Jerina, D. M., Kasparek, G. J., and Bruice, T. C., Proc. Nat. Acad. Sci. U.S. 69,1985 (1972). 667a. Kasparek, G. J., Bruice, P. Y., Bruice, T. C., Yagi, H., and Jerina, D. M., J. Amer. Chem. SOC. 95,6041 (1973). 667b. Bruice, P. Y.,Bruice, T. C., Selander, H. G., Yagi, H., and Jerina, D. M., J. Amer. Chem. SOC. 96,6814 (1974). 668. Fehnel, E. A., J. Amer. Chem. SOC. 94,3961 (1972). 669. Maynert, E. W.,Forman, R. L., and Watanabe, T., J. Biol. Chem. 245, 5234 (1970). 670. Udenfriend, S.,Clark, C. T., Axelrod, J., and Brodie, B. B., J. Biol. Chem. 208,731 (1954). 671. Norman, R. 0. C., and Radda, G. K., Proc. Chem. SOC.,London p. 138 (1962). 672. Hamilton, G. A., Workman, R. J., and Woo, L., J. Amer. Chem SOC. 86,3390 (1964). 673. Hamilton, G. A., and Friedman, J. P.,J. Amer. Chem. SOC.85,1008 (1963). 674. Hamilton, G. A., J. Amer. Chem. SOC.86,3391 (1964). 675a. Kaufman, S., in “The Enzymes” (P.D. Boyer, H. Lardy, and K. Myrback, eds.), Vol. 8,p. 373. Academic Press, New York, 1963. 675b. Kaufman, S.,Proc. Nat. Acad. Sci. US. 50,1085 (1963). 675c. Kaufman, S.,in “Oxygenases” (0.Hayaishi, ed.), p. 129. Academic Ress, New York, 1962. 676. Jerina, D. M., Daly, J. W., Landis, W., Witkop, B., and Udenfriend, S., J. Amer. Chem. SOC.89,3347 (1967). 677. Bobst, A., and Viscontini, M., Helv. Chim. Acta 49, 884 (1966);Viscontini, M., Leidner, H., Mattern, G., and Okada, T., Helv. Chim. Acfa 49,1911 (1966). 678. Dearden, M. B., Jeffcoate, C. R. E., and Lindsay Smith, J. R., Advan. Chem. Ser. 77, 260 (1968). 679. Jerina, D. M., Daly, J. W., and Witkop, B., Biochemistry 10,366 (1971). 680. Muradov, N. Z.,Shilov, A. E., and Shteinman, A. A., Kinet. Catal. (USSR) 13,1219 (1972). 681. Jerina, D. M., Boyd, D. R.,and Daly, J. W., Tetrahedron Lett. p. 457 (1970). 682. Sharpless, K. B., and Flood, T. C., J. Amer. Chem. SOC. 93,2316 (1971). 683. Akhrem, A. A., Abeliovich, M. L., and Metelitsa, D. I., Dokl. Chem. 213,909 (1973). 684a. Collman, J. P.,and Reed, C. A., J. Amer. Chem. SOC. 95,2048 (1973). 684b. Collman, J. P., Gagne, R. R., Halbert, T. R., Marchon, J. C., and Reed, C. A., J. Amer. Chem. SOC. 95,7868 (1973). 684c. Collman, J. P.,Gagne, R. R., and Reed, C. A., J. Amer. Chem. SOC. 96,2629 (1974). 684d. Collman, J. P.,Gagne, R. R., Reed, C. A., Robinson, W.T., and Rodley, G . A., Proc. Nat. Acad. Sci. US. 71,1326 (1974). 684e. Collman, J. P.,Gagne, R. R., Gray, H. B., and Hare, J. W., J. Amer. Chem. SOC. 96, 6722 (1974). 684f. Collman, J. P., Gagne, R. R., Kouba, J., and Ljusberg-Wahren, H., J. Amer. Chem. SOC. 96,6800 (1974). 685. Baldwin, J. E., and Huff, J., J. Amer. Chem. SOC. 95,5757 (1973). 686. Cheng, C. K.,and Traylor, T. G.,F?oc. Nat. Acad. Sci. US. 70,2647 (1973);J. Amer. Chem. SOC. 95,5810,8415,8477(1973). 687. Vieth, W. R.,and Venkatasubramanian, K., Chemtech. p. 677 (1973). 688. Semenov, N. N., Chem. Brit. p.471 (1974).

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Author Index Numbers in parentheses are reference numbers and indicate that an author’s work is referred to although his name is not cited in the text. Numbers in italics show the page on which the complete reference is listed. Anderson, C. B., 364(544), 408 A Abakumov, G. A., 292(130a, 131b), 293 Anderson, J. M., 310(237), 320(237), 330 (237), 331(237), 398 (130a), 294(130a, 131b), 295(130a, Anderson, J. R., 141(38), 142(45, 46, 47, 131b),395 48), 143, 144, 160, 161, 162, 163, 164 Abdennur, L., 308(228b), 314(252), 398, (47, 86), 173, 174, 181, 182, 185(5), 399 260 Abel, E. W., 297(166b), 298(166b), 300 Anderson, P. J., 214, 215(152),264 (166b), 396 Abeliovich, M. L., 318(569d), 370(569d), Andreev, A., 378(615c), 379(615c), 411 Andrku, P., 249(339), 269 372(569d), 389(683), 409,413 Andrulis, P. J., 308(229, 331), 398 Abley, P., 115,124 Angell, C. L., 233(254), 241(286), 242 Abraham, S. C., 297(163), 396 (286), 267,268 Abramov, V. N., 220(185), 265 Antipina, T. V., 218(171), 222(209), 223 Achrem, A. A., 344(422), 404 (209), 243(289), 265,266,268 Adams, C. R., 378(619a, 620b),411 Aomura, K., 222(204a), 266 Adams, J. Q., 335(359), 402 Aounallah, B., 255(377), 256(377), 257 Adderley, C. J., 366(563b), 409 (377),270 Addison, C. C., 336(365), 403 Arai, H., 252(357), 270 Adrian, F. J., 201,263 Aratani, T., 308(230), 398 Agoh, B., 351(458b), 405 Arbuck, R. M., 282(46a, 46b, 46c), 392, Aguiar, A. M.,89(16), 96(16), 123 393 Aguilo, A., 327(328), 361(521), 401,407 Arimoto, F. S., 290(102), 394 Akabori, H., 334(351), 402 Akabori, S., 83(4,5,6), 122 Armistead, C. G., 216(159), 264 Akhrem, A. A., 368(569c), 370(569d), Arnett, E. M.,338(375, 376,377),403 372(569d), 389(683), 409,413 Arpe, H. J., 371,410 Aldag, A. W.,144(51), 181, 185(4),260 Artemova, N. C., 381(634b), 411 Aleksandrov, V. N., 316(264b), 339(264b), Arzoumanian, H., 295(139), 298(139,173), 399 356(497), 395,397,407 Aleksanyan, V. T., 141(43), 181 Arzoumanidis, G. G., 369,409 Allan, G. G., 342(412,413), 404 Asano, R., 370(582), 409 Allen, A. D., 218(163), 265 Aston, J. G., 201,263 Allen,G. C., 327(318),402 Atherton, K., 214,240,264 Allison, E. G., 33,54 Amberg, C. H., 199(75), 234, 237(259), Attenburrow, I.,65(26), 79 238,240,258(397), 262,267,271 Augustine, R. L., 59(9), 61(13), 62, 63(13, Amenomiya,Y., 197,219(173),261,265 22, 23), 64, 65, 66, 67, 68(13, 24), Amiard, G., 76(47), 80 69(9), 70(13), 71, 72(9, 23), 73(23), Anderson, B. A., 76(47), 80 75(9), 76(22), 77(22), 79 415

416

AUTHOR INDEX

Avery, N. R., 142(47, 48), 143, 144, 164 (47), 181 Axelrod, J., 387(670), 413 Azuma, H., 298(177), 397

B Baer, S., 287(81a), 394 Baetzold, R. C., 10, 14, 17, 29, 36, 41,54, 55 Baggaley, K. H., 76(45), 80 Bailar, J. C., 378(627), 411 Baud, W. C., 364(546), 408 Baizer, M. M., 326(303), 400 Baker, B. G., 142(45,46), 181 Baker, R. H., 137(26, 27), 138(27), 140 (27), 181 Baker, T. N., 345(429), 349(429), 404 Balachandran, K. S., 411 Balandin, A. A., 301(194), 397 Balducci, G., 15(36, 37), 54 Baldwin, J. E., 390(685),413 Baldwin, R. H., 308(227a), 398 Ballard, D. G . H., 378(623), 411 Ballhausen, C. J., 3 , 5 3 Ballivet, D., 227(223), 228(239), 229(239), 266,267 Balyer, W.D., 94(29a), 95(29a), I 2 3 Bandiera, J., 258(380), 270 Barachevsky, V., 221(201), 245(308), 265, 268 Barezin, B. D., 301(195), 397 Barnes, R. K., 347(438), 348(438), 405 Baron, K., 144(52), 166(52), 181 Barona, N., 274(10), 391 Barral, R., 360(503), 407 Barron, Y.,145(55), 150(55, 61), 151,152 (551, lSS(SS), 156, 157(55), 158, 159, 172(55,61), 181,182 Barthomeuf, D., 227(223), 228(232, 239), 229(239), 266,267 Bartlett, P. D., 280(45b), 289(91a, 91b), 392,394 Basch, H., 5,54 Basila, M. R., 199(76), 220(184), 221(221), 227,243,244,262,265,266,268 Basolo, F., 297(166b), 298(166b), 300 (166b), 396 Bateman, L., 274(2), 391 Batist, P. A., 380(632a), 411 Battersby, A. R., 331(338),402

Baumgarten, P., 244,268 Bawn, C. E. H., 305, 316(269), 317(269), 326,327(313,314,315) 398,399,401 Baxendale, J. H., 285(69a), 287(83a), 393, 394 Baiant, V., 189(31), 249(31, 341), 261, 269 Beach, R. T., 378(625b), 411 Beaton, S., 292(129), 293(129), 395 Beaumont, R., 228(232), 266 Beck, P.,94(26), 1 2 3 Becker, J. Y., 298(181), 326(309c), 397, 401 Beckwith, A. L. J., 177,183 Beelen, J. M.,173(97), 182 Belew, J. S., 411 Bell, H. C., 320(279b), 321(279b),400 Bellamy, L. J., 218(164), 265 Bemis, A. G., 278(42a, 42b), 289(94, 96a, 96b), 290(94,96a, 96b), 392,394 Benesi, H. A., 191,224,229(35),261 Bennett, A. J., 40,44(65), 55 Bennett, J. E., 276(33), 279(33), 280(33), 381(637), 392,412 Bennett, R. A., 22,54 Benson, D., 289(90), 394 Benson, J. E., 144(51), 181, 185(4),260 Benson, S. W., 278(44), 279,392 Ben Taarit, Y., 228(240), 230, 246(324, 325, 326), 258(242, 387), 267,269,270 Berhek, K., 189,249(31), 261 Beranek, L., 249(341), 269 Berdnikov, V. M.,285(71b), 393 Berezin, I. V., 275(22), 391 Bergk, K. H., 381(6340,411 Bergmann, E. D., 298(180, 181), 397 Bergstrum, S., 384(650a),412 Berlin, A. A., 301(193), 397 Bernath, T., 320(276), 400 Berther, C., 343(416), 404 Bertoluzza, A., 227(222), 266 Bertsch, L., 241(285), 268 Betts, A. T., 296(142,143), 335(142, 143), 396 Betts, J., 275(28a), 392 Beveridge, D. L., 2(2), 9(2), 11(2), 53 Biale, G., 367(565), 409 Bickel, A. F., 292(126), 395 Biegen, J. R., 198(63), 262 Bielanski, A., 227(225), 266 Bielsky, B. H. J., 287(81b), 394

AUTHOR INDEX

Bigot, J. A., 292(116), 395 Bingham, A. G., 364(548), 365(548), 408 Birch, A. J., 298(182),397 Bird, C. W.,360(507a), 364(551), 407,408 Bischof, B., 292(118), 395 Bishop, C. E., 290(104b), 394 Bjorlchem, I., 385(654b), 387(654b), 412 Blackburn, D. M., 221(191),265 Blake, D. M., 355(483,484), 406 Blakely, D. W.,144(52), 166(52), 181 Blanc, A., 295(139), 298(139, 173), 356 (497), 395,397,407 Blanchard, H. S., 316(262), 317(262), 333 (344,348), 339(262), 399,402 Blanz, E. J., Jr., 73(33),80 Bloch, K., 382(642,648),412 Blum, J., 298(180, 181),397 Blyholder, G., 35,55,199(77), 262 Blyumberg, E. A., 275(18c), 287(18c), 335 (18c), 337(18c),391 Bobrov, N. N., 381(634d),411 Bobst, A., 388(677), 413 Bobylev, B. N., 349(452), 450(452), 405 Bocard, C., 296(152, 1531, 360(503), 396, 40 7 Boehm, H. P., 186, 204(19), 209, 211(19), 219(135a), 225, 230, 231(243), 244 (135b),260,264,266,267 Boga, E., 326(316b), 327(317a),401 Bogdanovic, B., 93(23c), 123 Boguslavskaya, L., 285(71a), 393 Bolland, J. L., 274(2), 391 Bond, G. C., 33,48,54,55, 130(8), 180 Bondarenko, A. V., 350(450, 451, 453), 405

Bonvicini, P., 103, I24 Booth, B. L., 299(183), 397 BoreIIo, E., 205(119a), 207, 213(146), 226 (146), 233(256), 240, 258(383, 389), 263,264,267,268,270 Boreskov, K. G., 381(634d), 411 Borisevich, A. D., 381(634c), 411 Boss, C. R., 335(361),402 Botteghi, C., 93(22c), 123 Boudart, M., 144(51), 145(54), 176(108), I81,183,185(4,8a), 215,260,264 Bourne, K. H.,216(160), 227(160), 264 Boyce, C. B. C., 76(44), 80 Boyd, D. R., 386(661), 389(681),412,413 Bozon-Verduraz, F., 244(295), 268 Bradley, J. S., 341(390a, 391a),403

417

Brady, D. A., 364(557), 408 Bramanti, D., 3 , 5 3 Brandon, R. W., 293(132), 395 Braterman, R. S., 363(536c), 408 Braus, R. J., 341(388), 403 Breck, D. W.,381(633a),411 Bremer, H., 198, 224, 252,262,266,269, 2 70 Breslow, R., 385(653), 412 Brewster, J. H., 59,68, 79 Brieger, G., 376(61 la), 410 Brilkina, T. G., 277(41), 392 Brill, W.F., 308(227b), 344,398,404 Brinigar, W. S., 297(166b), 298(166b), 300 (166b), 396 Brodie, B. B., 387(670),413 Brodie, H. J., 73(36, 37), 80 Brodskii, M. S.,292(127b), 395 Broidy, J. M., 320(286), 321(286), 400 Bromberg, A., 277(40d), 392 Brooks, S. G., 76(45), 80 Brouwer, D. M., 246(314), 269 Brown, C. A., 176(1lo), 183 Brown, D. J., 84(9), 122 Brown, D. M., 276(33), 279(33), 280(33), 392 Brown, H. C., 320(292), 321(292), 400 Brown, L. D., 297(166a), 298(166a), 396 Brown, R. G., 314(547), 365(547), 368 (570), 408,409 Brown, S.B., 283(57), 393 Bruice, P. Y.,286(667a, 667b),413 Bruice, T. C., 386(663a, 664, 665, 666, 667a, 667b), 412,413 Brunelle, J-P., 149(60a), 182 Bruns, W.,244,268 Bryant, D. R., 364(535b, 5421, 365(535b), 367(535b), 372(597), 373(596), 408, 410 BUC,S. R., 343(417), 353(417), 404 Biihl, H., 249(340), 250(340), 252(340, 349), 253(363), 259(349), 269,270 Buiten, J., 411 Bulani, W.,274(6), 391 Bulanova, J. F., 141(43), 181 Bulgakov,O. V., 218(171),265 Bulgakova, G. M., 292(127a), 395 Bunton, C. A., 331(335a), 402 Burn, A. J., 335(358),402 Burnett, R. E., 89(16), 96(16,31), 123 Burton, J. J., 16, 17,54,175,183

41 8

AUTHOR INDEX

Burwell, R. L., Jr., 127, 128(1, 4), 129(1), 130(7), 131(11, 12, 13), 132, 133(1, 11), 134, 140(35), 141(4), 180, 181, 186(21), 192,204,212,260,261 Busch, P.,290(104b), 394 Busetto, C., 332(342d), 402 Bush, J. B., 306(220), 330(220), 398 Bushweller, C. H., 372,410 Buthe, H., 85(11), 86(13),122 Butler, J. D., 233,267 Byers, D. J., 65(26), 79

C Calderazzo, F., 332(342b, 342c), 363(536b), 402,408 Callender, W. L., 153(66a, 66b), 155(66b), 178(66b), 182 Calligaris, M., 297(166b), 298(166b), 300 (166b), 396 Calvin, M., 332(342a),402 Camerman, P.,324(298), 325(298), 400 Campbell, J. R., 320(278),400 Campbell, P. G. C., 364(550), 365(550), 408

Canadine, R. M.,8 , 9 , 5 4 Candlin, J. P., 138(32), 181,274(13), 391 Cannings, F. R . , 216(160), 227(160), 264 Capka, M.,101(36), 124 Capparella, G., 298(179); 397 Carle, K. R., 292(121), 395 Carlson, D. J., 277(40b), 392 Carlson, G. L., 77(49), 80 Carlson, R. G., 71(29), 79 Cam, L. J., 323(295c), 324(295c), 400 Can, B., 334(350b), 402 Carra, S., 340(384), 341(384), 378(614a, 614b), 379(614a, 614b),403,410 Carrizosa, I., 210(140), 264 Carter, J. L., 173(98), 182, 185(12), 204 (113, 206(115), 208(115), 260,263 Carter, M. J., 297(166b), 298(166b), 300 (166b), 396 Caspi, E., 75(42), 80 Castleman, J. K., 276(34b), 282(49), 290 (34b), 292(34b), 293(34b), 294(34b), 392,393 Catone, D. L., 297(161),396 Caulton, K. G., 8,54 Cecere, M.,291(107a), 394 Cecil, R., 335(358), 402

eekovic, Z., 291(108), 331(337a), 395,401 Cenini, S., 295(141), 298(141, 179), 299 (141), 341(393), 355(485), 396, 397, 403,406 Cerny, M.,101(36), 124 Cerutti, L., 213(146), 226(146, 218), 229 (218), 258(389), 264,266 Chafetz, H., 351(465), 406 Chalk, A. J., 292(114), 395 Chambers, C. A., 198(61), 261 Chang, C. C., 186(22), 260 Chang, C. K., 297(166b), 298(166b), 300 (166b), 396 Chang, J. J., 202(107), 263 Chapat, J.-P.,71(50), 78(50), 80 Chapman, J . D., 232,233,267 Chaykovsky, M., 76(44), 80 Che, M., 195, 247(48,331), 258(386),261, 269,270 Cheney, A . J., 375(608a),410 Cheng, C. K., 390(686), 413 Cheng, P., 297(167), 396 Chenier, J. H. B., 335(362), 402 Cherkashina, L. G., 301(193), 397 Chernov, V. A., 243(289), 268 Cherrier, J. H. B., 219(173),265 Chervinskii, K. A., 316(267d), 399 Chester, A. W., 314(257), 315(257, 260), 318(273), 399 Chickos, J., 85(10b), 94(10b), 122 Chien, J. C. W., 335(361),402 Chien-Chung Chao, 258(391), 270 Chiu-Sheng, N., 326(312b), 401 Chock, P. B., 341(392),403 Choy, V. J., 296(147b), 354(147b),396 Christner, L. G., 192(40), 220(187), 228 (187),242(40),261,265 Christoph, G. G., 297(170b), 397 Chuang, T. T., 232(247), 258(395), 267, 2 71 Chuev, I. I., 292(130a, 130b), 293(130a, 136),294(130a, 130b), 295(130a, 130b), 395 Chukin, G. D., 222(209), 223(209), 266 Cimino, A., 185(14, 15), 198(71a), 207 (125),260,262,263 Clark, A., 221(191), 255(376), 265,270 Clark, C. T., 387(670), 413 Clark, D., 364(535b, 537a), 365(535b), 366(563a), 367(535b), 408,409 Clark, F. R. S., 369(572), 409

419

AUTHOR INDEX

Clark, J. T. K., 177(115b), 183 Clarke, G. A., 14(35), 54 Clarke, J. K. A., 136, 149(60b, ~ O C ) , 159, 173(99), 174(103), 176(106a, 106b, 107), 177, 179(107, 107a), 181, 182, 183 Clement, W. H., 361(520), 364(538), 407, 408 Clementi, E., 5, 14,54 Clifford, A. A., 311(239), 331(334), 398, 402 Clipsham, J., 293(135), 395 Closs, G. L., 301(210), 398 Coenen, J. W.E., 185(8), 260 Coffman, D. D., 287(77), 394 Coleman, I., 199(87), 200(87), 262 Collman, J. P., 101(37), 124, 297(166b), 298(166b), 300(166b), 340(381a, 383), 341(381a, 383), 354(381a), 390(684a, 684b, 684c, 684d,684e, 684f,396,397, 403,413 Coluccia, S., 212(145, 146), 213(146), 226(145,146,218), 229(218), 233(256), 238(272), 239(272), 258(145), 264, 266,267 Combe, M. G., 58(4), 68(4), 79 Comer, W.C., 186(22), 260 Connett, J. E., 65(26), 79 Connor, D. E., 341(390a), 403 Connor, J. A., 297(159), 342(159),396 Consiglio, G., 93(22c), 123 Conti, F., 355(485), 406 Cook, A. H., 307(192), 397 Cook, C. D., 297(167), 396 Cook, D., 222(203), 266 Cooke, M. P., 101(37), 124 Cooney, R. P., 199(85a), 262 Cooper, T. A,, 311(239, 240), 325(240), 398,399 Cooper, W. F., 14,54 Copping, C., 335(357), 402 Corado, A., 255(371, 372, 373), 256(373), 257(373), 270 Corey, E. J., 121,124 Corneil, P., 204(115), 206(115), 208(115), 263 Cornelius, E. B., 205(117), 263 Cornet, D., 150(61), 151(61), 152(61), 158(61), 172(61), 182 Corolleur, C., 159(77, 78,80), 166,167(93), 168,169,171,172,182

Corolleur, S., 166(92), 168(92), 169(92), 182 Corrington, J. H., 5, 7, 9,54 Corriu, R. J. P.,93(23a), 1 2 3 Costa Novella, E., 348(440b), 349(440b), 405 Cotton, F. A., 8,54, 375(608b),410 Coulson, C. A,, 35,55 Course, J., 65(26), 79 Cram, D. J., 85(10e), 94(10e), 122 Crassous, J., 77(50), 78(50), 80 Criado, J. M., 209, 244(134), 245(307), 264,268 Criegee, R., 289(88, 89), 394 Crivello, J. V., 325(301), 400 Cross, R. J., 363(536c), 408 Cullis, C. F., 353,402 Curci, R., 353(480), 354(480), 406 Curthoys, G., 199(85a), 262 Cusachs, L. C., 4 , 5 , 6 , 7, 9,53,54 Cusumano, J. A., 185(12), 260 Cutting, J. D., 351(467), 352(467), 406 Cvetanovic, R. J., 197, 219(173), 261,265 Czanderna, A. W., 197,198,261,262 Czapski, G., 287(80b), 394

D Dadashev, T. B., 350(454), 405 Dalin, M. A,, 350(454), 405 Dalla Lana, J. G., 232(247), 258(395), 267, 2 71 Daly, J. W., 385(657a, 657b), 386(658, 659, 660, 661, 662), 388(676, 679), 389(681), 412,413 Dang, T. P., 90(18, 19), 93(21e), 100(34), 102(34), 105(21e, 34), 122(51, 52), 123,124 Danno, S., 370(580, 581, 584), 409,410 Danyushevskii, V. Y., 234(264), 236(264), 237(264), 267 Datka, J., 227(225), 266 Dauben, W.G., 73(33,34), 76(44),80 Dautzenberg, D., 252, 255(378), 256(378), 259(350), 269,270 Dautzenberg, F. M.,153, 155, 156, 159(79), 178(67), 182 Davidson, J. M., 364(547), 365(547), 368, 370(568), 372(568), 408,409 Davies, A. G., 276(37), 392 Davis, B. H., 153,155,182

420

AUTHOR INDEX

Davis, K., 41 0 Davydova, E. M., 244(302), 268 Day, V. W., 375(608b), 410 Dean, M. H., 292(113), 395 Deane, A. M., 215(152a), 264 DeAngelis, B. A., 198(71a), 262 Dearden, M. B., 388(678), 389(678), 413 DeBoer, C., 318 Declerek, L. J., 228 (240a), 267 Deeming, A. J., 340(386), 341(386), 403 Degtyarova, T, G . , 296(146), 396 Deininger, D., 202(103), 263 de Korte, R. W.,309(232), 310(232), 330 (23 2), 398 De La Cuesta, P. J. M., 348(440b), 349 (440b), 405 Delafosse, D., 220(189), 228(189), 265 Delbouille, A,, 215(154), 264 Delfs, H., 199(79), 262 Delgass, W . N., 198(69), 262 Della Gatta, G., 205(119a), 263 Delmon, B., 185(18), 260 de Mourgues, L., 251(346), 253(346), 269 Dempsey, E., 241(284), 268 den Hertog, H. J., 309(233), 330(233), 398 Denisov, E. T., 275(18a, 18b, 22), 283(56), 287(18a, 18b, 871, 292(124), 294(124), 295(124), 296(146), 335(18a, 18b), 337(18a, 18b),391,393,394,395,396 Denisova, L. N., 296(146), 396 Deno, N . C . , 325(300b),400 Denson, D. D., 290(104b), 394 Dent, A. L., 186(23), 188(23), 213, 240, 260,264 Deo, A. V., 232,267 DePauw, M., 355(493b), 406 de Radzitzky, P., 320(275), 324(298), 325 (298, 299), 399,400 Derouane, E. G., 201, 202(102), 215(154), 263,264 de Ruiter, E., 352(468), 406 Descotes, G., 350(456), 405 Dessau, R. M.,305(218), 306(219a, 219b, 2221, 308(225), 309(235), 310(238a), 312(242), 313(242, 247, 248), 318(242, 273), 324(242), 330(219a, 219b, 238a), 331(238a), 398,399 Dessing, R. P., 175(104), 176(104), 177 (1041, 178(104), 183 de Vries, G . , 331,402 Dewar, M. J. S., 308, 332(343), 398,402

Dewing, J., 199(86), 262 Dianis, W., 198(71b), 262 Diehl, H., 384(651), 412 Diemente, D. L., 297(166b), 298(166b), 300(166b), 396 Dietsche, T. J., 93(23d), 123 Dietz, R., 308(229), 326(310a), 398,401 Di Furia, F., 353(480), 354(480), 406 Digurov, N. G . , 316(266a, 266b), 399 Djerassi, C., 59(8), 73(30, 31), 75(40, 41), 79,80 Dittmann, W.,351(458a), 405 Dixon, W. T., 287(78), 289(92), 394 Dobroselskaya, N. P., 301(194), 397 Doedens, R. J., 297(165a), 396 Dokukina, E. S . , 301(193), 397 Dolcetti, G., 81(1), 84(1), 85(1), 101(37), 122,124 Dollish, F. R., 246,269 Dolphin, D., 341(388, 380a), 403 Dominguez, I., 209(134), 244(134), 264 Donia, R. A., 59(7), 79 Dori, Z.,297(166b), 298(166b), 300(166b), 396 Doumaux, A. R., 285(68), 393 Dowden, D. A., 185(13), 260,378(621), 411 Downs, W. J., 370(589), 410 Doyle, J. R., 365(55a), 409 Dozono, T., 361(526), 362(526), 407 Dreiding, A. S., 73(34), 80 Dudley, C. W.,359(502a, 502b), 407 Dufaux, M., 247(331), 269 Dufraisse, C., 274(1), 391 Dulog, L., 277(40c), 392 Dumas, T., 274(6), 391 Dumont, W., 100(34), 102(34), 105(34), 124 Dunken, H., 204(116), 206, 208(116), 218 (167, 168, 1691, 232(168), 236, 263, 265 DurandRenchoz, S., 75,80 Dyall, L. K., 364(548), 365(548), 408 Dyer, E., 292(121), 395 Dzhemilev, U. M.,351(460,461,462,463), 353(471,412,478,479), 405,406

E Eachus, R. S., 21,54 Eberson, L., 326(305a, 305b), 370, 371 (590, 591, 593, 594), 373(590, 591), 400,410

AUTHOR INDEX Ebsworth, E. A. V., 297(159), 342(159), 396 Edgell, W. F., 5(13), 54 Edmonds, T., 177(115b), 183 Edwards, J. O., 276(39), 283(58), 328(320), 392,393,401 Egerton, T. A., 199(85b), 262 Eiben, K., 287(79), 394 Eichens, R. P., 220(183), 265 Eley, D. D., 220, 258(382), 265,270 Ellett, J. D., Jr., 202(107b), 263 Elliot, C. S., 293(132), 395 Ellis, J., 297(166b), 298(166b), 300(166b), 396 Elson, I. H., 320(287), 322(287), 400 El-Taliawi, G. M., 285(72b), 286(72b, 73a), 394 Emanuel, N. M., 275(18a, 18b, 18c, 21, 22), 287(18a, 18b, 18c, 21, 87), 292(124), 294(124), 295(124), 335(18a, 18b, 18c), 337(18a, 18b, 18c), 391,394,395 Emmett, P. H., 65(27), 79 Emmick, T. L., 85(lOd), 94(10d), 122 Endres, G. F., 333(344, 346, 348, 349), 402 Engelhardt, L. M., 297(166b), 298(166b), 300(166b), 396 Engstrom, J. P., 385(655a), 386(655a), 412 Erbse, H., 201(95,95a), 263 Erkelens, J., 130(9), 180 Ermolenko, N. F., 381(634c), 411 Ertl, G., 185,260 Eustance, J. W., 333(344,349),402 Evans, G. O., 378(625a, 625b), 411 Evans, J. V., 240,268 Evmenko, N. P., 316(267b), 399 Eyring, H., 35,54

F Fabbri, G., 227(222), 266 Fadeev, V. S., 155(71), 182 Fadley, C. S., 198(69), 262 Faigenbaum, H. M., 296(155), 396 Falk, C. D., 314(259), 320(259), 399 Fallab, S., 296(154), 396 Farberov, M. I., 349(452), 350(450, 451, 452), 353(476), 405,406 Far&, G., 227(222), 266 Farnham, W. B., 85(10c), 94(10c), 122 Farrell, L. F., 178(116), 183 Fassaert, D. J. M., 47(68), 48(68), 55

42 1

Faubl, H., 135(20), 180 Fedak, D. G., 175(102a), 183 Fedorova, T. M., 308(226a), 398 Fehnel, E. A., 386(668), 413 Fenske, R. F., 8,54 Fenton, D. M., 360(508), 367(508, 565), 407,409 Fields, E. K., 316(268), 399 Fiermans, L., 198(71c), 262 Fieser, L. F., 342(405b), 404 Fieser, M.,342(405b), 404 Figoli, N. S., 253(361, 362), 270 Figueras Roca, F., 251, 253,269 Filby, W.G., 287(79), 394 Filimonov, V. N., 219(174), 220,226(219), 230,232(219, 244), 233, 258(392), 265, 266,267,270 Finch, J. N., 255(376), 270 Fine, L. W., 295(137), 298(137), 395 Fink, P.,204(116), 206, 208(116), 218(167, 168, 169), 232(168), 234, 236(116), 237(262), 263,265,267 Finkbeiner, H. L., 306(220), 330(220), 332 (340), 333(348), 398,402 Fioshin, M. Y.,326(302), 400 Fitton, P., 341(394), 373(599), 403,410 Flaig-Baumann, R., 209(135b), 244( 135b), 264 Fleischmann, M., 326(309a), 401 Fliegel, P., 361(517), 362(517),407 Flier], C., 287(82a), 394 Flockhart, B. D., 190, 191(37, 38), 221 (201a), 245, 246(316), 247(330, 332), 248(37,38, 332, 335), 261,266,269 Flood, T. C., 389,413 Floriani, C., 332(342b, 342c), 402 Flygare, H.,289(89), 394 Fogelberg, L. G., 154, 155,182 Fogo, J. K., 246,269 Fono, A., 290(103), 291(103, 109), 292 (118), 394,395 Foote, C. S., 340(379), 341(379), 403 Forman, R. L., 386(669), 413 Forni, L., 192,261 Forzatti, P.,348(439,44Oa), 349(439,44Oa, 444), 405 Foscante, R. E., 61(24), 62(24), 63(20, 24), 64(24), 65(24), 66(24), 67(24), 68(24), 69(24), 70(24), 71(24), 72(24), 79 Founty, R. A., 369(575), 409 Fowler, J. S., 320(282), 321(282), 400

422

AUTHOR INDEX

Gailyunas, I. A., 351(459,460,464a, 464b), 405,406 Gal, D., 345(426b),404 Gallezot, P., 228(240), 267 Galli, R., 291(107a), 394 Galwey, A. K., 130(9), 176(109), 180, 183 Gardner, J. N., 76(47), 80 Garnett, J. L., 375(607), 378(622a), 410, 411 Garst, J. F., 278(42e), 392 Garza, C., 411 Giti, G., 203, 255(370, 372), 256(370), 263,2 70 Gault, F. G., 127,128(5), 142,143(50), 145 ( 5 5 ) , 149(5, 49), 150(55, 61, 621, 151 (61, 62), 152(55, 61), 153(66a, 66b), lSS(SS), 156(55), 157(55, 76), 158(55, 61), 159(55, 62, 77, 78, 80), 160, 164, 165(87), 166(92), 167, 168(92), 169 (92), 171(93), 172(85,61,93), 173(62), 178(66b), 180,181,182 Gay, I. D., 202(106), 263 Gebert, W., 301(206), 398 Gedra, A., 345(426b), 404 Geels, E. J., 278(42a, 42b), 392 Geibel, J., 297(166b), 298(166b), 300 (166b), 396 Gelbshtein, A. I., 411 Geller, B., 131(11), 133(11), 180 George, M. V., 41 I George, P., 288 Gerberich, H. R., 192(41), 261 Gerchikova, F. G., 353(472), 406 Gerlach, D. H., 341(400), 404 Gernet, 0. D., 292(131c), 294(131c), 295 (131c),395 Gershanov, F. B., 351(461), 353(470), 405, 490 Gervits, M., 292(127b), 395 Geschke, D., 202(104), 263 Geursen, H. J., 331,402 Ghiotti, G., 207(125), 212(145), 226(145), 238(271, 272), 239(272), 258(145, 383), 263,267,270 Ghorbel, A., 245,246(328), 247(328), 248, G 255(312), 256,269 Gibby, M. G., 202(107b), 263 Gabbard, R. B., 59(8), 79 Gigli, G., 15(36,37), 54 Gachechladze, G. G., 328(326b), 401 Giles, D., 220( 180), 265 Gaevskii, V. F., 316(267b), 399 Gagne, R. R., 390(684b, 684c, 684d, 684e, Giletii, Y.V.,316(267a), 399 Gilliom. R. D.. 289(98). 290(98). 394

Fox, F., 295(141), 298(141), 299(141), 396 Frad, W. A., 178(117), 183 Fraissard, J., 201(102), 202(102), 263 France, G., 219(178), 265 Franck, J. P., 296(152), 396 Franck, R. W., 71(29), 79 Franke, K., 301(206), 398 Freel, J., 176(109), 183 Freiesleben, W., 360(512), 361(512), 364 (512), 381(512), 407 Freiser, H., 338(375, 376), 403 French, T. M., 207,263 Freyermuth, H. B., 343(417), 353(417), 404 Fried, J., 74(38), 80 Friedman, J. P., 387(673), 413 Friedrich, W., 361(517), 362(517),407 Fries, R. W., 341(403b), 404 Frillette, V. J., 242(287), 268 Fripiat, J. J., 201(102), 202(102, 107), 204(112), 208(112), 220(182), 221 (197), 244(293), 263, 265, 268, 355 (493a), 406 Fritch, J. R., 297(170b), 397 Fry, A. J., 326(304), 400 Fubini, B., 205(119a), 263 Fuchs, H., 94(28), 123 Fueki, K., 221(193), 265 Fuhrhop, J. H., 300(191), 397 Fujii, Y.,83(4,5,6), 122 Fujiwara, Y.,298(174, 175, 176, 177), 369 (577), 370(578, 579, 580, 581, 582, 583,584), 397,409,410 Fukuda, K., 244(296, 297, 298, 304), 245 (296,297,298), 268 Fukui, K., 302(21 l), 398 Fukuzumi, K., 376(611b),410 Furman, M. S., 323(295d), 324(295d),400 Furuta, T., 120(47), 124 Fusi, A,, 295(141), 298(141, 179), 299 (141), 341(393), 356(496), 396, 397, 403,407 Fuson, R. C., 65(26), 79 Futaki, R., 73(33), 80

AUTHOR INDEX

423

Gilmore, J. R., 306(223), 309(236), 310 Guglielminotti, E., 212(145), 226 (145, (236), 314(250), 398,399 2181, 229(218), 233(256), 238 (272), Giordano, N., 378(613), 410 239(272), 258(145, 389), 264, 266, Gitis, S . S., 316(264b), 339(264b), 399 267, 270 Glaser, R., 122(54), 124 Guido, M., 15(36,37), 54 Glietsch, J., 252(353), 270 Guilleux, M. F., 220(189), 228(189), 265 Goldina, L. A., 292(127b), 395 Gunther, K., 287(79),394 Gol’dshleger, N. F., 375(605, 606a), 377 Gunther, P., 289(91a), 394 (61 lc), 410 Gupta, S. K., 84(9), 122 Goldstein, N. D., 411 Guroff, G., 385(657a, 657b),412 Golovina, 0. A., 301(193), 397 Guryanova, V. A., 292(131a), 294(131a), Goluber, G. S., 316(264b), 339(264b), 399 295(131a),395 Gomes, W., 22,54 Gut, M., 73(36,37), 80 Gomez-Gonzales, L., 370(590, 591, 592), Gutzwiller, J., 73(30), 79 371(590,591), 373(590,591), 410 Gonzalez, F., 209(134), 210(139), 244(134, 301),264,268 Goodisman, J., 6,54 H Gore, R., 154(70), 155(70), 182 Haag, W. O., 189,221(30), 249(30), 261 Gorzynski, J. D. 341(395), 403 Haas, L. A., 258(393), 271 Gostunskaya, I. V.,155(71), 182 Gould, C. W., 276(34c), 290(34c), 292 Haber,F.,326,401 Haber, J., 198(70), 262, 378(615b), 379 (34c), 293(34c), 294(34c), 392 (615b), 411 Gould, E. S., 282(46b), 283(52b), 348 (442), 349(446, 447), 350, 378, 392, Habgood, H. W., 241(280, 285), 242(280), 268 393,405 Hackerman, N., 205(118),263 Graham,B. W., 355(490), 406 Graham, J. H., 132(14), 133(14), 134(14, Haeberlen, U., 202(107a, 107b), 263 15), 135(15), 137, 138(30), 140, 146 Hafner, W., 361(514, 515, 517), 362(514, 515,517), 363(514), 407 (58),180,181 Hagen, D. I., 144(53a), 181 Graham,W., 65(26), 79 Hair,M. L., 199(74), 232,233,262,267 Granger, R., 77(50), 78,80 Hajoz, Z. G., 76(46,47), 80 Grant, D., 136(23), 146(23), 181 Halbert, T. R., 390(684b), 413 Gravelle, P. C., 197,261 Hall, I. H., 77(49), 80 Gray, H. B., 3,5,53,54,390(684e),413 Hall, W. K., 192(40, 41), 220(187), 228 Grayson, M., 295(137), 298(137), 395 (187, 228), 242(40), 245(310), 246 Green, J., 76(45), 80 (315), 254(366, 367), 255(364, 366), Green, M. M., 291(108), 297(166b), 298 258(364), 261,265,266, 268,269,270 166b), 300(166b), 395,396 Hallam, H. E., 206,263 Green, N., 364(545), 408 Haller, G. L., 186, 192(46), 204(21), 212 Greene, F. D., 385(655a), 386(655a), 412 Greenler, R. G., 252(354), 270 (21), 260,261 Gregg, S . J., 196(50), 236, 237, 240, 241 Halpern, J., 340(382), 341(382,392), 403 Hamberg, M., 385(654b), 387(654b), 412 (278), 261,267,268 Hambleton, F. H., 216(159), 264 Griesser, R., 287(82a), 394 Griffiths, D. L., 215(152a), 264 Hamilton, G. A., 286(73b), 331(337b), 344 Grimley,T. B.,35,55 (423), 368(569c), 370(569c), 372(569c), Gross, D. E., 364(541),408 384(652), 386(652), 387, 394, 402, Groves, J. T., 286(74), 394 404,409,412,413 Hamilton,J.F.,37,41,55 Grubbs, R. H., 101,124 Gurualdi,G., 289(91b), 394 Hammerich, O., 326(308), 401 Guczi, L., 156,182 Handel, K. D., 355(491c), 380(491c), 406

424

AUTHOR INDEX

Hannan, R. B., 204(113), 205, 208(113), 263 Hanotier, J., 314(253), 315(253), 320 (275), 324(298), 325(298, 2991, 399, 400 Hanotier-Bridoux, H., 314(253), 315(253), 320(275), 324(298), 325(298), 399,400 Hansford, R. C., 227,228, 230(226), 266 Hanzlik, R. P., 297(166b), 298(166b), 300 (166b), 396 Hara,N., 228,229(241), 267 Hara, T., 301(207b), 398 Hardin,A. H., 199(85,85b),262 Hare, C. R., 14(35),54 Hare, J. W., 390(684e),413 Harmon, R. E., 84(9), 122 Harris,C. B., 8,54 Harrod, J. F., 334(350b), 402 Hartig, U., 356(497), 407 Hartley, F. R., 135(18), 141, 180, 181, 360(505,506), 362(506),407 Hartog, F., 185(3), 260 Hasegawa, M., 244(299), 268 Haszeldine, R. N., 299(183), 397, 364 (545), 408 Hashimoto, T., 302(212b), 398 Hatch, L. F., 274(9), 391 Hattori, H., 220(220), 266 Hawkins, E. G. E., 276(36a), 392 Hay, A. S., 316(262), 317(262), 333 (344, 345, 347, 348, 349), 339(262), 399,402 Hayaishi, O., 382(640a, 640b, 642, 644a), 383(639),412 Hayano, M., 73(36), 80 Hayashi, T., 93(21a, 21d), 103(43), 123, 124 Hayden, P., 364(535b, 537a), 365(535b), 366(563a), 367(535b),408,409 Hayns, M. R., 3 , 5 3 Hayward,P. J., 355(483,484,486),406 Heather, J. B., 103(42), 124 Hecht, H. G., 200,262 Heck, R. F., 370(585), 410 Hegedus, L. S., 101(37), 124 Heiba, E. I., 306(219a, 219b, 222), 308 (225), 309(235), 310(238a), 312(242), 313,313(248), 318,318(273), 324(242), 330(219a, 219b, 238a), 331(238a),398, 399

Heimbach, P., 297(162), 355(489), 396, 406 Heinemann, H., 378(624), 411 Hell, P., 136(25), 181 Helmholz, L., 3,53 Helquist, P. M., 341(395,396), 403 Henbest, H. B., 58(4), 68(4), 74(38), 79,

80 Henc, B., 93(23c), 123 Hendra, P. J., 199, 222(82, 83, 211), 262, 266 Hendriksen, E., 205(119), 263 Hendry, D. G., 282(50), 335(356), 393,402 Henrici-Oliv6, G., 296(148), 354(396), 396 Henry, P. M., 337(372), 361(519, 522,523 529), 362(519, 522), 364(539, 549), 365(523, 529, 549), 366(564b), 370, 374(600), 403,407,408,409,410 Hensley, A. L., 215(158), 264 Herberhold, M.,360(513),407 Herrmann, M., 209(135a, 135b, 135c), 219(135a), 244(135b), 264 Hetflejus, J., 101(36,37), 124 Heusler, K., 65(26), 79 Heylen, C. F., 191, 228(36), 229(36), 261 Hiatt, R., 276(34a, 34b, 34c, 34d), 290, 292(34a, 34b, 34c, 34d), 293(135), 294(34a, 34b, 34c, 34d), 349(446), 350 (446), 353(473), 354(481), 392, 395, 405,406 Hierstetter, H., 255(371,372), 270 Hightower, J. W., 189(32, 33), 254(32, 33, 367, 368, 369), 255,258(32,368),261, 2 70 Hildenbrand, E. G., 73(34), 80 Hillar, S. A., 253(361,362), 270 Hiller, L. H., 8,9,54 Hirai, H., 120(47), 124 Hirota, K., 221(193), 244(305), 265,268 Hirschauer, A., 93(22c), 123 Hirschler, A. E., 219(177), 246(319), 265, 269 Hoang-Van, C., 245(312), 246(329), 247, 248(312), 255(312, 329), 256(312), 269 Home, M.R., 16,54 Hobert, H., 198(65,66), 262 Hobin, T. P., 326(314), 327(314), 401

425

AUTHOR INDEX Hobson, M. C., 200; 245(93), 262 Hock, H., 301(200,201), 397 Hockey, J . A., 211, 214(149), 216(159), 225,240(149), 264,266 Hodges, R. J., 410 Hoffman, B. M., 297(166b), 298(166b), 300(166b), 396 Hoffman,N. W.,81(1), 84(1), 85(1),122 Hofmann, D. W., 176,183 Hoffmann, R., 2,32,50(73),53,54,55 Hogan, P., 222(201c), 266 Holdsworth, J. D., 335(364), 402 Holm, V. C. F., 221(191), 265 Holtz, H. D., 316(270,271), 318,399 Holysz, R. P., 59(7), 79 Hon, K., 410 Hoogerwijs, R., 198(71c), 262 Hopfe, V., 196(51), 261 Hopton, J. D., 334(355), 402 Horder, J. R., 222(211), 266 Horiuti, I., 57,59,62, 79 Horlock, R. F., 214(151), 264 Homer, L., 85, 86, 87(14), 94(26, 27, 28, 29a), 95(29a), 122, 123 Hornig, L., 371, 410 Hosking, J. W., 298(172), 397 Hosokawa, T., 359(501), 407 Hotop, H., 22,54 Hotta, N., 368(569b), 310(569b), 372 (569b), 409 House, H. O., 71(29), 79 Houtman, J. P. W., 254(365), 255(365), 256(365), 270 Howard, J. A., 275(26), 276(32), 280(26), 281,318,335(362), 392,402 Howe, G. R., 350(449), 353(473), 405,406 Howe, R., 61(18), 70(18), 79, 385(654a), 387(654a), 412 Howell, M. V., 233, 241(286), 242(286), 26 7,268 Huber, L. M., 202(107a, 107b), 263 Hucknall, D. J., 378(616), 411 Hudson, J. O., 246(318), 269 Huff, J., 390(685), 413 Hughes, T. R., 198(69), 221(197), 227, 228(197), 262,265 Hunger, M., 249(339), 269 Hunt, J. D., 320(282), 321(282), 400 Hunt, R. L., 308(229), 398 Husain, M., 287(83b), 394

Hussey, A. S., 137(26,27), 138, 140, 181 Hiittel, R., 360(507b), 407 Huyser, E. S., 275(27), 392 Hyman, E., 175(102a), 183

1 Ibers, J. A., 297(164, 165a, 165b, 166b, 168), 298(166b), 300(166b), 341(401), 396,404 Ichikawa, J., 220(220), 266 Ichikawa, K., 320(285), 321(285), 400 Ignatenko, A. V., 328(330), 329(330), 401 Ikawa, T., 282(51), 393 Ikeda, Y.,93(22b), 123 Il’ina, G. P., 328(329, 330, 3311, 329(329, 330,331), 401 Imai, J., 221(196), 222(196), 265 Imaizumi, S., 75(39), 80 Imanaka, T., 298(174,175,177), 397 Imelik, B., 195(48), 218(170), 220(170), 223(206), 227(206), 228(240), 246 (324), 247(48), 258(386, 3871, 261, 265,266,267,269,270 Imoto, E., 326(326b),401 Imoto, M., 334(351), 402 Imparto, L., 335(360), 402 Indictor, N., 344,404 Indovina, V., 215(154), 264 Ingersoll, H. G., 335(359), 402 Ingold, K. U., 276(30, 311, 278(42d), 280 (30, 311, 292(129), 293(129,134), 318, 335(362), 392,395,402 Inoue, H., 326(326b), 401 Ione, K. G., 381(634d),411 Ireland, R. E., 76(44), 80 Irvine, J. L., 77(49), 80 Irwin, K. C., 276(34b, 34c, 34d), 290(34b, 34c, 34d), 293(34b, 34c, 34d), 294(34b, 34c, 34d), 349(446), 350(446, 449), 392, 405 Isaev, 0. V., 378(615a), 379(615a), 411 Isaeva, Z. G., 351(464b),406 Isakov, Y.J., 192,242(39), 261 Ishiya, C., 220(220), 266 Itakura, J., 342(414), 404 Itatani, H., 369(576), 409 Ito, H., 342(414), 404 Itoh, M., 243(290,291), 268 Itoh, T., 298(176), 397

426

AUTHOR INDEX

Ivanov, S. K., 335(363), 337(373b), 402, 403 Ivanov, V. P., 321(295d), 400 lwane, H., 103(40), 124 Iyengar, R. D., 248(334), 269 Izumi, Y.,83(4,5,6,7), 122

J Jackson,P., 211,238(270),264,267 Jackson, W. R., 58(4), 68(4), 74(38), 79, 80, 141(39), 181 Jacobs, P. A., 191, 228(36,24Oa), 229(36), 241(281), 242(282), 261,267,268 Jain, J. R., 250,252,253,269 James,B. R., 84(8), 122, 299(186a, 186b), 39 7 Janzen, E. G., 278(42a, 42b), 392 Jardine, I., 73(32), 79, 138, 140(31), 181 Jeffcoate, C. R. E., 388(678), 389(678), 413 Jemilev, U. M., 353(470),406 Jenkins, C. L., 289(96a), 290(96a), 320 (2901,394,400 Jenner, E. L., 287(77), 394 Jensen, A. D., 59(7), 79 Jerina, D. M., 385(656, 657a, 657b), 386 (658, 659, 660, 661, 662, 664, 665, 666, 667a, 667b), 388(656, 676, 679), 389(656,681), 412,413 Jerkins, G. I., 140(36), 181 Jesson, J. P., 341(400),404 Jeziorowski, H., 199,25 1(348), 262,269 Jira, R., 360(512), 361(512, 514, 515, 517, 518), 362(514, 515, 517), 363 (514), 364(512), 365(558), 381(512), 407,409 Jir8, P., 240(279), 268 John, J. A,, 328(321), 401 Johnson, B. A., 59(7), 79 Johnson, J. A., Jr., 61(19), 70(19), 73(19), 79 Johnson, J. L., 59(7), 79 Johnson, M. P., 341(394), 403 Johnson, R. A., 285(72b), 286(72b), 394 Johnson, W. S., 73(36), 80 Jolley, J. E., 326(315), 401 Jones, D. F., 385(654a), 387(654a), 412 Jones, L. D., 341(396),403

Jones, M. M., 288, 340(380), 341(380), 354(380), 403 Jones, P., 211, 225,264,266,283(57), 393 Jones, R., 378(625b), 411 Jones, T. L., 215(152), 264 Jonkhoff, T., 363(535a), 364(535a), 365 535a), 367(535a), 408 Jonsson, E., 370(593), 371(593),410 Jonsson, L., 370(594), 371(594), 410 Joy, V. J. R., 362(532), 408 Joyner, R. W., 165, 166(90), 176(88), 182 Jungers, J. C., 138(33, 341, 139,181 Jiintgen, H., 198(64), 262 Jurjev, V. P., 353(470), 406 Juttard, D., 159(77,80), 182

K Kablaoui, M. S., 351(465), 406 Kaerlein, C. P., 224(212), 266 Kagan, H. B., 90(18, 191, 93(21e), lOO(33, 34), 102(34), 105(21e, 34), 122(51,52, 53), 123,124 Kagel, K. O., 252(358), 270 Kageyama, Y.,222(204a), 266 Kahr, K., 343(416), 404 Kalechits, I. V., 298(178), 397 Kalinin, V. P., 228(237), 233(237), 267 Kalman, J. R., 320(278, 279a, 279b), 321 (278,279a, 279b),400 Kaloustian, J., 351(457), 405 Kaluza, U., 209( 135c), 264 Kaman, A. J., 342(409), 404 Kamiya, Y., 282(48), 292(129), 293, 301 (196, 198), 302(196, 198), 312(241), 314(241, 251, 254, 255,256), 315(241, 254, 255, 2561, 316(263a, 264a, 264c, 2651, 317(265), 318, 324(265), 338 (374a, 374b), 339(264a, 26413, 393, 395,397,399,403 Kamneva, A. I., 381(634b), 411 Kanaeda, H., 302(213), 398 Kane, A. R., 341(400), 404 Kaneda, K., 298(176), 397 Kantner, T. R., 220(184), 221(221), 227 (224), 265, 266 Kantyukova, R. G., 353(478), 406 Karge, H., 228(230, 233, 238), 229, 230 (230), 233,266,267 Karpihski, Z., 149(60b), 176(106a), 182, 183

AUTHOR INDEX

Karshalykov, C., 337(373b), 403 Kashima, M., 314(254, 255,256), 315(254, 255,256), 316(264c), 339(264c), 399 Kasparek, G. J., 386(663a, 664, 665, 666, 667a),412,413 Kasten, S. D., 49,55 Katagari, M.,382(639), 383(639),412 Kateva, I., 335(363), 402 Kato, H., 334(350c),402 Kato, S., 285(72a), 286(72a), 348(441), 394,405 Katsuki, A., 28(45), 51 Kaubisch, N., 386(660,664),412,413 Kaufman, S., 387(675a, 675b, 675c),413 Kawai, R., 314(251), 338(374b), 399,403 Kazanskii,B. A., 141, 155(71), 181,182 Kazanskii, V. B., 258(394), 271 Keese, R., 136(22), 181 Keier, N. P.,301(208), 398 Kemball, C., 127, 128(5), 130(9), 141(38), 149(5), 179(10), 180,181,189,261 Kenyon, R. S., 41 0 Kereselidze, R. V., 327(319), 328(324,325, 326a, 328), 329(328),401 Kern, H., 248,269 Kerr, J. A., 279 Keulks, G. W.,137(26, 27), 138(27), 140 (27), 181 Khallafalla, S. E., 258(393), 271 Kharasch, M. S., 290(102, 103, 104a, 105), 291(103,105, 107), 292(117,118), 394, 395 Khidekel, M. L., 298(178), 375(606a), 377 (6 1lc), 397,410 Kilty, P. A., 355(491a),406 Kimura, H., 28,54 Kimura, Y., 326(326b), 401 King, T. E., 382(641), 412 Kirchhof, J., 198(65,66), 262 Kirk, T. M., 350(450,453), 405 Kirina, 0. F., 222,223(209), 266 Kiselev, A. V., 220(185, 186, 190), 221 (194), 223(194), 225(194), 228(229), 232(194), 244(194), 246(194), 265,266 Kishi, K., 244(305), 268 Kiso, Y., 93(23b), 123 Kiss, A., 255(371, 372, 3731, 256(373), 257(373), 270 Kitagawa, J., 216(161), 264,349(445), 379 (4451,405 Kitching, W.,361(527b), 364(543), 408

427

Kittel, C., 24, 54 Kiviat, F. E., 222(208), 223(208), 266 Kiyoura, T., 216(161), 264, 349(445), 368 (569a), 370(569a), 372(569a), 379(445), 405,409 Klabunde, U., 375(609c), 376(609c), 410 Klevan, L., 296(151), 396 Klier, K., 200,245(91), 262 Kline, C. H., 222,266 Klose, K., 228(233), 266 Knaak, K., 301(207a), 398 Knabjohann, W.,301(205), 398 Knappe, B., 198(65), 262 Knehr, H.,258(388), 270 Knor, Z., 196(49), 261 Knowles, W. S., 85,86(12), 88(15), 91(20a, 20b, 20c, 20d, 20e), 95(30), 122(49), 122,123,124 Knozinger, H., 188(27, 281, 193(47), 196 (52), 197, 199(78), 203, 204, 205(52), 206(121), 208(121), 222(121,210), 223 (121, 206, 210), 224(206, 212, 214), 225(210), 229(47, 210, 214), 230(47), 232(121), 233(251), 234(251, 255), 231, 246, 248(322), 249(27, 28, 340), 250(340), 251(347, 348), 252(27, 28, 47, 340), 253(47, 363), 254(60), 255 (377, 378), 256(370, 373, 377, 378), 257(373,377), 259(349,350,399), 261, 262,263,266,267,269,270,271 Koch, J., 185,260 Koch, P.,346(436b), 347(436b), 405 Koch, V. R., 326(309b), 407 Kochi, J. K., 274(12), 283(61, 62, 63a, 64, (65), 289(61, 93, 94, 95, 96a, 96b, 97, 98, 99, loo), 290(94, 95, 96a, 96b, 97, 98,99, loo), 291(95,97,100, l06), 292 (61, 93, 95, 97), 305(217), 310(237), 313(217, 249), 314(259), 320(217, 237, 249,259,274,276,287,288, 290, 291), 322(281), 328(61, 62, 63a, 65), 330 (237, 249, 288, 333), 331(237, 249, 288), 336(217,249), 337(371), 366(63a, 64), 391, 393,394,398,399,400,401, 403 Kochloefl, K., 189(31), 249(31, 341), 252 (349), 259(349), 261,269 Koehl, W. J., 306(219a, 222), 309(235), 312(242), 313(242,247), 318(242,273), 324(242), 330(219a), 398,399 Koelewijn, N:, 282(47), 393

428

AUTHOR INDEX

Koenig, T., 385(655b), 386(655b),412 Kohll, C. F., 363(535a), 364(535a, 553), 365(535a), 367(535a, 566a), 408,409 Kojer, H., 361(514), 362(514), 363(514), 407 Kokes, R. J., 185(9), 186, 187(24), 201, 213,240,260,263,264 Kolbel, H., 240(279), 268 Kolesnikov, I. M., 350(455), 405 Kollar, J., 307(224a), 345(426a), 353(475), 367(561b), 398,404,406,409 Kollen, W., 198(63), 262 Kolmogorov, V., 221(201), 245(308), 265, 268 Konaka, R., 411 Kondo, T., 244(305), 268 Kondratov, V. K., 316(267c), 399 Konno, K., 75(39), 80 Konoval’chukov, A. G., 350(454), 405 Koopmans, T. A., 12,54 Kooyman, E. C., 292(126), 309, 310(232), 330(232,233,234), 395,398 Korpium, O., 85(10a, lob), 94(10a, lob), 122 Kortum, G., 199(79), 200, 245(309), 258 (388), 262,268,270 Koshel, G. N., 353(476), 406 Kosower, E. M., 63(25), 79 Kosswig, K., 353(474), 406 Kostyukov, I. N., 328(328), 329(328), 401 Kotov, E., 221(201), 245(308), 265,268 Kotov, E. J., 221(199), 265 Kouba, J., 390(6846,413 Koubek, E. O., 328(320), 401 Koubek, J., 221(201b), 222(201b), 266 Kozirovski, Y.,199(85), 262 Kral, H., 243,268 Kramer, A. V., 341(390b, 391a, 391b),403 Kraus, M., 189(31), 249(31,341), 261,269 Krebs, E. P., 136(22), 181 Krebs, H., 209(131), 264 Krietenbrink, H.,233,234(255), 267 Kroll, L. H., 101,124 Kropf, H., 292(119), 301,395,397,398 Kruchinin, V. A., 316(263b), 339(263b), 399 Krushinskaya, G. A., 353(476), 406 Kubasov, A. A., 228(237), 233(237),267 Kubokawa, Y., 197, 232(248, 249), 261, 26 7 Kubota, M.,298(172), 397

Kudo, Y.,75(39), 80 Kuhnen, L., 353(477), 354(477), 406 Kuiper, A. E. T., 218(172), 221(172), 224 (1 72), 265 Kumada, M.,93(21a, 21b, 21c, 21d, 23b), 103(43), 123,124 Kumli, K. F., 94(25), I 2 3 Kunath, D., 199(80), 262 Kurata, T., 308(226b), 398 Kurkov, V. P., 295( 136), 298(136), 395 Kurooka, A., 376(611b),410 Kushnerev, M. Y., 378(615a), 379(615a), 411 Kuzmenko, N. M., 220(190), 265 Kuz’mina, E. A., 292(131b, 131c), 294 (131b, 131c), 295(131b, 131c),395 Kwiatek, J., 333(346), 402

L Labinger, J. A., 341(388, 390a, 390b, 391a), 403 Lafer, L. I., 234(264), 236(264), 237(264), 267 Lafortune, A., 292(129), 293(129), 395 Laing, K. R., 355(490), 406 Laing, M.,297(165c), 396 Landau, M., 198,262 Landau, R., 345(427), 404 Lande, S . S., 313(249), 314(259), 320(249, 259, 2741, 330(249), 331(249), 336 (249), 399 Lander, J. L., 365(336),403 Landis, W., 388(676), 413 Lang, B., 165(88), 166(90), 176(88), 182 Langlois, N., 93(21e), 105(21e), 122(52), 123,124 LaPlaca, S. J., 297(164) Lappert, M. F., 341(402), 404 Lapshina, T. V., 141(44), 181 Larson, J. G., 254,255(364), 258(364), 270 Lasarov, D., 32,54 Lau, K. S. Y.,341(403a, 403b),404 Lavigne, J. B., 295(136), 298(136), 395 Lawesson, S. O., 291(111), 321(293), 395, 400 Leach, H. F., 381(634e),411 Lebedev, M. N., 316(266a, 266b), 328 (322b), 399,401 Leclercq, C., 136(25), 181 Leclercq, L., 136(25), 181

429

AUTHOR INDEX Lednor, P. W., 341(402), 404 Lee, B., 290(104b), 394 Lee, L. F. H., 103(42), I24 Lee, R. E., Jr., 73(80),80 Lefranpois, M., 228(231), 266 Leffin, H. P., 200, 245(93), 262,364(555), 365(555), 408 Legrand, P., 350(456), 405 Leichter, L. M., 149(60d), I 8 2 Leidner, H., 388(677), 413 Leith, I. R., 191(37), 247(330, 332), 248 (37,332), 261,269 Lena, L., 351(457),405 LBonard, A., 220(182), 221(197), 265 Leonard, A. J., 199(72), 204, 208(112), 262,263 Le Page, J.-F., 149(60a), 182 Lester, G. R., 155(72), 156, 157,182 Lester, J. E., 198(71b),262 Lethbridge, A., 361(528), 408 Letsinger, R. L., 85(10d), 94(10d), I 2 2 Letterer, R., 255(371, 372),270 Levanda, 0. G., 361(525),407 Levanevskii, 0. E., 292(128), 296(128), 395 Lever, A. B. P., 300(189),397 Levy,A., 103(39), 124 Levy, R. B., 176(108), 183 Lewis, B., 37,55 Lewis, I. A., 215(152a), 264 Lewis, K. E., 156(75), 182,209,264 Lewis, R. A., 85(10b), 94(10b), 122 Liberman, A. L., 141(43,44),181 Lichenstadt, L., 94(24), I23 Liengme, B. V., 192(40), 228(228), 242 (401,261,266 Linck, R. G., 283(60), 393 Lindley, J., 364(545), 408 Lindsay Smith, J. R., 285(70), 286(70), 388(678), 389(678), 393,413 Lippens, B. C., 204(111), 207, 208(111), 263 Lipscomb, R. D., 287(77), 394 List, V. F., 353(477), 354(477), 406 Liston, T. V., 335(359), 402 Litovka, A . P., 328(322b), 401 Little, L. H., 199(73), 234, 237(259), 238, 262,267 Littler, J. S., 283(59), 330(332), 393,401 Liu, C. L., 258(395), 271 Ljusberg-Wahren,H., 390(684f), 413

Lloyd, W.G., 274(1 l ) , 275(28b), 391,392 Loader, E. J., 199(83), 222(83, 211), 262, 266 Loewenthal, H. J. E., 59(10), 69(10), 72 (10),79 Logel, P. C., 37,55 Lo Jacono, M., 207(125), 263 Long, M. A., 375(607), 410 Lopatin, Y.N., 219(174), 220(174),265 Loser, R.W., 198(61), 261 Loshchilova, A. V., 301(195), 397 Low, M. J. D., 199(87), 200(87), 233, 244 (299), 262,267,268 Lowdin, P. O., 6,54 Lucchesi, P. J., 204(115), 206(115), 208 (115), 263 Lumry, R., 283(53), 393 Lund, H., 326(310a), 401 Lundberg, W.O., 275(19), 391 Lunsford, J. H., 201, 220(188), 258(390, 391,396,398),263,265,270,271 Lusky, H., 252(353), 270 Lutinski, F. E., 192(41),261 Lutz, E. F., 307(224b), 398 Lygin, V. J., 220(185, 186, 190), 228(229), 265,266 Lyon, H. B., 144(53), 181 Lyons, J. E., 295(138a, 138b, 140), 298 (140),356(495), 395,407

M Maatman, R. W., 187, 188,261 McCarroll, B., 7,40(63,64), 5 4 , 5 5 McCarroll, J. J., 177(115b), 183 McColeman, C., 354(481), 406 McDonald, P. D., 331(337b), 402 Macdonald, R. J., 160(84), 161(84), 174 (84), 182 McEwen, W. E., 94(25), 123 McFarland, J. W., 76(44), 80 McGillivray, G., 320(282), 321(282), 400 McGinnety, J. A., 297(165a, 165b, 168), 396 Mclninch, E., 208,264 McKee,D. W., 179(118),183 McKeever, C. H., 65(26), 79 McKenzie, T. C., 76(48), 80 McKeon, J. E., 341(394), 364(535b, 542). 365(535b), 367(535b), 372(596, 597), 373(596,599), 403,408,410

430

AUTHOR INDEX

McKervey, M. A., 132(14), 133(14), 134 (14, 15), 135(15), 136(23), 146(23, 58, 59), 147(59), 158,180,181 Mckillop, A., 320(282, 283, 284), 321(282, 283,284), 322(283, 284), 400 MacLean, R. L., 73(34), 80 McLoughlin, L., 248(335), 269 McMillan, R.,378(625b),411 McQuillin, F. J., 58(3), 61(18), 63, 67(3), 68(3), 70(18), 73(32), 79, 138(31), 140(3 l), 181, 115,124 Madan, S. K., 296(151), 396 Maeda, K., 359(501), 407 Magin, R. W., 71(29), 79 Mains, G. J., 345(431), 350(431),404 Mains, H. E., 289(97), 290(97), 291(97), 292(97), 394 Mairanovsky, S . G., 326(302),400 Maire, G., 145(55), lSO(S5, 61), 151(61, 62), 152(55, 61), lSS(SS), 156(55), 157 (55), 158(55, 61), 159(55, 62, 80), 165 (87), 172(55,61), 173(62), 181,182 Maitlis, P. M., 360(504), 407 Maizus, Z. K., 275(18a), 287(18a), 292 (127a, 127b), 298(178), 327(317b), 335 (18a), 337(18a, 373a), 391, 395, 397, 401,403 Mak, A. J., 278(42b), 392 Makishima, S . , 120(46b), 124 Malashevich, L. N., 381(634c), 411 Malbois, G., 228(231), 266 Malhotra, S. K., 71(28), 79 Malunowicz, I., 74(38), 80 Manakov, M. N., 328(322b), 401 Manassen, J., 249(337), 269, 326(310b), 378(622b), 401,411 Mancera, O., 75(41), 80 Mancini, M., 3(4), 53 Mango, F. D., 134,180 Mann, B. E., 375(608a), 410 Manokov, M. N. 316(263b), 339(263b), 399 Manzerall, D., 300(191), 397 Mapes, J. E., 220(183), 265 Marchon, J. C., 390(684b), 413 Marcilly, C., 185(18), 260 Margolis, L. Y.,378(615a, 617),379(615a), 411 Mark, H. B., 200(88), 262 Markham, A. L., 61(17), 68(17), 79 Markov, P., 32,54

Marquardt, D. N., 101(37), 124 Marsh, R. E., 297(170a),396 Marshall, J. A., 135(20), 180 Marsi, K. L., 94(29b), 95(29b), 1 2 3 Marta, F., 326(316b), 327(317a), 401 Martell, A. E., 296(150), 332(342a), 338 (378), 341(378), 354(150), 396, 402, 403 Martin, E. P., 336(367), 403 Martin, L. F., 276(36b), 392 Martinez, E. R., 348(440b), 349(440b), 405 Marx, G., 196(51), 261 Maryanoff, B. E., 122(SO), 124 Maryanoff, C. A., 122(50), 124 Mashio, F., 348(441), 405 Mashkina, A. V., 244(302), 268 Masler, W. F., 89(17), 97(17, 32), 98(17, 32), 100(32), 123 Maslov, S. A., 275(18c), 287(18c), 335 (18c), 337(18c),391 Mason, H. S., 382(641,645,646),412 Mason, R., 355(485), 406 Masters, C., 376,410 Mathieson, J. W.,411 Mathieu, M.-V., 210(137, 138), 211(137, 138), 218(170), 219(176, 178), 220 (170), 221(176), 222(206), 223(206), 225(176), 238(176, 269), 244(176), 245 (176), 246(326), 258(380, 386), 264, 266,267,269,270 Matienko, L. I., 327(317b), 401 Matkovskii, K. I., 316(267b), 399 Matok, M., 327(317a), 401 Matotov, Y.,308(228a), 398 Matsubara, F., 308(226b), 398 Matsuda, M., 370(578), 409 Matsushita, S., 240,267 Matsurra, K., 243(290), 268 Matsuzaki, J., 220(220), 266 Mattern, G., 388(677), 413 Mattmann, G., 214,264 Matucci, A. M., 343(415), 404 Maurel, R., 136(25), 137,138, 140, 141,181 Mawaka, J., 355(494),407 May, S. W., 382(644b), 412 Maynert, E. W., 386(669), 413 Mayo, F. R., 275(23,24), 276(34a), 282(49), 290(34a), 292(34a), 293(34a), 294(34a), 391,392,393

AUTHOR INDEX

43 1

Mays, D., 287(84,85), 394 Mill, T., 276(34a, 34b), 282(49), 290(34a, Medema, D., 363(535a), 364(535a), 365 34b), 292(34a, 34b), 293(34a, 34b), (535a), 367(535a, 566b), 408,409 294(34a, 34b), 392,393 Medema, J., 218(172), 221, 224,254(365), Miller, L. L., 326(309b, 309c), 401 Miller, S. A., 363(534b), 365(534b), 408 255(365), 256(365, 379), 265,270 Milliken, T. H., 205(117), 263 Mee, A., 323(295b), 324(295b), 400 Mills, D. J., 31 1(239), 398 Megarry, M. C., 191(38), 248(38), 261 Mills, G. A., 205(117),263 Mehler, A. H., 382(639), 383(639), 412 Mimoun, H., 343,344(420), 404 Mehring, M., 202(107b), 263 Minachev, K. M., 192,242(39), 261 Meisenheimer, J., 94(24), 123 Mingiedi, F., 355(492b), 406 Meister, B., 93(23c), 123 Melamud, E., 297(166b), 298(166b), 300 Minisci, F., 291(107a, 107b), 394,395 Minkov, A. I., 301(208), 398 (166b), 396 Mellor, J. M., 306(223), 309(236), 310 Mishchenko, Y.A., 411 Mislow, K., 85(10a, lob, lOc), 94(10a, lob, (236), 314(250), 398,399 lOc), 122(50),122,124 Mel’nik, L. V., 349(452), 350(452), 405 Misono, M., 203,263 Mendelsohn, M., 338(375,376,377), 403 Mitauer, L. E., 323(295d), 324(295d), 400 Mentrup, A., 94(26), 123 Mitchell, S. A., 216(159), 264 Mercier, J., 351(458b), 405 Mitsudo, T., 93(22a), 103(40), 123,124 Merrifield, R. B., 101,124 Mesrobian, R. B., 276(38), 292(112b), 295, Mitsui, S., 75(39), 8 0 Mitsutani, A,, 365(562), 409 392,395 Messmer, R. P., 3,7,40(63,64,65),44(65), Miyake, N., 93(23b), 123 Miyata, H., 232(248,249), 267 53,54,55 Metelev, A. K., 292(130b, 131a), 294 Moad, G., 177,183 Mocadlo, P., 289(93), 293(93), 394 (130b, 131a), 295(130b, 131a), 395 Metelitsa, D. I., 286(76), 344(422), 346 Modena, G., 103(39), 124, 353(480), 354 (480), 406 (432b), 368(569d), 370(569d), 372 Modone, E., 258(389), 270 (569d), 389,394,404,409,413 Moiseev, I. I., 361(524, 525), 362(524, Metz, D. J., 292(112b), 295(112b), 395 Metzger, J., 295(139), 298(139, 1731, 351 530, 531), 363, 365(534a), 375(606a), 407,408,410 (457), 356(497), 395,397,405,407 Mollan, P. A. F., 221(201a), 266 Meye, W., 208(128), 251(348), 264,269 Molyneux, A., 198,262 Meyer, B., 33,54 Moore, C. E., 5,54 Meyer, E. F., 130(7), 180 Moore, K. A., 320(286), 321(286), 400 Meyer-DeLius,M., 59(1 l ) , 60(11), 79 Moreau, J. J. E., 93(23a), I 2 3 Meyerson, S., 316(268), 399 Michaelson, R. C., 351(466,467), 352(466, Moreno, F., 210(139), 244(301), 264,268 Morgan, R. A., 365(559), 409 467), 406 Moriarty, R. M., 361(527a), 407 Michalska, Z. M., 378(628), 411 Morimoto, T., 213, 221(196), 222(196), Michel, D., 202(103, 1051,263 240,264,265,268,314(258), 315(258), Micheli, R.A., 76(47), 80 399 Migliorini, D. C., 61(24), 62(24), 63(21, 24), 64(24), 65(21, 24),66(24), 67(24), Morishige, K., 240,268 68(21, 24), 69(24), 70(21, 24), 71(24), Moritani, I., 359(501), 369(577), 370(578, 579, 580), 581, 582, 583, 584), 407, 72(24), 79 409, 410 MihailoviE, M. L., 331(337a), 402 Morrau, R. C., 75,80 Miki, M., 355(487), 406 Morrison, J. D.. 82(2), 89(17), 96(16, 31), Milas, N. A., 321c,404 97(17), 98(17), 122,123 Mile, B., 276(33), 279(33), 280(33),392

432

AUTHOR INDEX

Morrison, M., 382(641), 412 Morrison, S. R., 378(612c),410 Morrow, C. J., 89(16), 96(16), 123 Morterra, C., 205(119a), 238(271), 258 (3831,263,267,270 Moser, F. H., 300(188), 397 Mosher, H. S., 82(2), 122 Moureu, C., 274(1), 391 Moye, A. J., 278(42a, 42b), 392 Mueller, B., 381(634f), 41 1 Muetterties, E. L., 341(400), 404 Mugden, M., 342(407),404 Muha, G. M., 246,269 Muir, W. R.,341(389),403 Muller, H. D., 222(210), 223(210), 225 (210), 229(210), 246, 248(322), 255 (371, 372, 3731, 256(373), 257(373), 266,269,270 Muller, J. M., 142, 143(50), 145(55), 150 ( 5 3 , 152(55), ISS(S5), 156(55), 157 (55, 76), 158(55), 159(55, 80), 172(55), 181.182 Mulliken, R. S., 5 , 6 , 5 4 Munns, G. W., Jr., 242(287), 268 Munuera, G., 209(134), 210(140), 211, 244(134, 300, 301), 245(307), 264,268 Muradov, N. Z., 389(680), 413 Murahashi, S. I., 359(501),407 Muratov, V. M., 316(267a), 399 Murry, R. K., Jr., 85(10c), 94(10c), 122 Muzzket, K. A., 277(40d), 392

N Naccache, C., 195(48), 230, 246(324, 325, 326), 247(48, 331), 258(242, 380, 386, 387), 261,267,269,270 Nagao, M., 213, 221(196), 222(196), 264, 265 Naipawer, R. E., 73(35), 80 Nakagawa, K., 41 1 Nakamoto, K., 218(162), 224(162), 334 (1621,265 Nakamura, A,, 355(487,488), 406 Nakano, T., 370(285), 321(285), 400 Nakata, T., 240,267 Nakaya, T., 332(343), 334(351), 402 Nardin, G., 297(166b), 298(166b), 300 (166b), 396 Natori, Y.,118(45c), 122(53,56), 124 Neikam, W.C., 246(327), 269 Nelson, R. L., 248(333), 269

Nelson, S. M., 336(366,369),403 Nemeth, A., 345(426b), 404 Neogi, A. N., 342(412,413),404 Neri, C., 332(342d), 402 Neshev, N., 378(615c), 379(615c),411 Nestrick, T. J., 376(611a),410 Neuss, G. R. H., 299(183), 397 Newbold, G., 214(149), 240(149), 264 Newby, W.J., 326(307a), 400 Ng, F. T. T., 299(186a, 186b),397 Nguyen The Tam, 199(85a), 262 Niedermeyer, A. O., 74(38), 8 0 Nieuwenhuys, B. E., 380(630),411 Nieuwpoort, W. C., 5(13), 54 Niki, E., 282(48), 393 Nikishin, G. I., 308(226a), 327(319), 328 (323, 327, 328, 329, 330, 3311, 329 (327, 328, 329, 330,331), 398,401 Ninomiya, T., 302(213), 398 Nishiguchi, T., 376(611b), 410 Nishimura, S., 58(5,6), 68(5,6), 79 Nizova, S. A., 350(455), 405 Nohl, A., 251(346), 253(346), 269 Noller, H., 249(339), 269 Nolte, M. J., 297(165c), 396 Nomine, G., 76(47), 80 Norikov, Y. D., 275(18c), 287(18c), 335 (18~1,337(18c), 350(448), 379, 391, 405,411 Norman, J. G., Jr., 296(149), 298(149), 354(149), 396 Norman, R. 0. C., 285(70), 286(70), 287 (78,80a), 289(92), 310(238b), 320(280, 281, 2891, 322(281, 2891, 330(238b), 331(238b), 361(528), 364(548), 365 (548), 369(572), 387(671), 393, 394, 398,400,408,409,413 Norton, F. J., 179(118), 183 Norton, J. R., 101(37), 124 Norton, R. V., 301(199), 397 Notari, B., 249(338), 269 Noto, Y., 244(296, 297, 298, 304), 245 (296,297,298), 268 Novitskaya, N. N., 353(478,479),406 Nowack, G. P., 137(26), 181 Nozaki, H., 351(467), 352(467), 406 Nozaki, M., 382,412 Nsumba, P., 355(492a), 406 Nudelman, A., 85(10e), 94(10e), 122 Nudenberg, W., 290(102, 105), 291(105), 292(117,118),394,395

433

AUTHOR INDEX

Nyberg, K., 326(305b), 400 Nyburg, S. C., 297(167), 396 Nyman, C. J., 355(483,484,486),406

0 Oaghton, I. F., 65(26), 79 Oberemko, A. V., 323(295a), 324(295a), 400 Oblad, A. G., 205(117), 263 Ochiai, E., 299(186a, 186b), 301(197a), 302(197a), 334(350a), 397,402 0 Cinneide, A. D., 159(81), 182 O’Connor, C. J., 296(147b), 354(147b), 355(490), 396,406 Odaira, Y., 365(560), 409 Oellcrug, D., 201(95,95a), 263 Ogasawara, S., 221(195,200), 265 Ogata, I., 93(22b), 123 Ogata, Y., 314(258), 315(258), 399 Ogawa, T., 244(305), 268 Ogilne, J. L., 198(71), 262 Ohgo, Y., 118,122(55,56), 124 Ohkatsu, Y., 301(207b), 335(362), 398, 402 Ohkubo, K., 302(211, 212a, 212b, 213, 214a, 214b), 398 Ohta, N., 282(48), 312(241), 314(241), 315(241),393,399 Oishi, T., 365(560), 409 Okada, T., 388(677), 413 Okano, M., 306(221), 330(221), 398 Olah, G. A., 325(300a),400 Oldham, A. R., 138(32), 181 Oliv6, S., 296(148), 354(148), 396 Oliver, J. F., 214(151), 264 Oliveto, E. P., 76(47), 80 Olivier, K. L., 360(508), 367(508, 565), 407, 409 Omelka, L., 293(133), 395 O’Neill, C. E., 238, 267 Onishi, T., 244(296, 297, 298, 304, 3061, 245(296,297,298), 252(351), 254(351), 268,269 Onopchenko, A., 312(243, 244, 245, 246), 313(244), 318, 323(294a, 294b), 399, 400 Orchin, M., 362(532), 408 Ord, W. O., 58(3), 61(3), 63(3), 67(3), 68(3), 79 Orloff, D., 5(15), 54

Orloff, H., 5(15), 54 Osa, T., 301(207b), 398 Osbom, J. A., 341(388),403 Oswald, H. R., 214(150),264 Otsuka, S., 355(487,488), 406 Ott, A. C., 59(7), 79 Owens, J. M.,385(655b), 386(655b), 412

P Pail, Z., 156(73, 74), 177(112, 113, 114, 115a),182,183 Pal, P., 16,54 Palaki, J., 75(40),80 Palladino, N., 332(342d), 402 Pandey, R. N., 337(372), 403 Pannetier, G., 244(295), 268 Paquette, L. A., 149(60d), 182 Parera, J. M., 253,270 Parfitt, G. D., 209, 211, 219(175), 225 (217), 238(270), 264,265,266,267 Parker, V. D., 326(308), 401 Parkyns, N. D., 209, 219(136), 230, 231 (130), 232(130), 236, 237, 258(384), 264,26 7 , 2 70 Parrish, D. R., 70(46,47),80 Parry, E. P., 222,223(205), 227,266 Parshall, G. W., 341(398), 374(601, 6041, 375(609a, 609b), 376(609a, 609b), 404, 410 Partch, R. E., 320(277), 325(277), 400 Pabk, J., 221(201b), 222(201b, 201c), 266 Pasini, A., 295(141), 298(141), 299(141), 341(393), 396,403 Pasky, J. Z.,295(136), 298(136), 395 Pasquon, I., 348(439), 349(439,443), 405 Pathy, M.S. V., 326(311),401 Pauling, H., 93(23c), 123 Paulson, D. R.,301(210), 398 Paushkin, Y. M., 350(455), 405 Pauson, P., 292(117), 395 Payne, G. B., 342(411), 358(500), 404,407 Payne, N. C., 297(168), 396 Pearce, D. R., 205(119), 263 Pearson, R. G., 192(44,45), 261, 283, 341 (389,401), 393,403,404 Pedersen, C. J., 300(190), 397 Pedersen, E. B., 321(293), 400 Pederson, R. L., 59(7), 79 Pecque, M., 141,181 Peone, J., 296(151), 396

434

AUTHOR INDEX

Pepe, F., 185(14, 15,17),260 Perchenko, A. A., 323(295a), 324(295a), 400

Peri, J. B., 204(113, 114), 205, 207, 208 (113, 114), 209, 213, 215(157, 1581, 216, 218(166), 219(166), 220, 232, 234, 236,237,241,242,255,258(381), 263,264,265,267,270 Perrotti, E., 332(342d), 343(415), 402,404 Peruzotti, G. P., 103(42), 124 Peter, A., 176(106b), 183 Petersen, T. E., 321(293), 400 Peterson, K. B., 375(607), 410 Petrakis, L., 222(208), 223(208), 266 Pfeifer, H., 201,263 Pfeifer, J., 255(378), 256(378), 270 Phillips, C., 89(16), 96(16), 123 Phillipson, J. J., 140, 141,181 Piantadosi, C., 77(49), 80 Pichat, P., 210(137, 138), 211(137, 138), 218(170), 219(176), 220(170), 221 (176), 223(206), 225(176), 227(206, 223), 228(232, 239), 229(239), 238 (176, 269), 244(176), 245(176), 259 (400), 264,265,266,267,271 Pickert, P. E., 241(284), 268 Pillai, C. N., 249(342, 343, 344), 250,252, 253,269 Pilz,E., 218(168), 232(168),265 Pilz, H., 289(89), 394 Pines, A., 202(107,107b), 263 Pines, H., 153(66a, 66b), 155(66b), 178 (66b1, 182, 189, 221(30), 249(30, 337,342,343,344), 261,269 Pinhey, J. T., 320(278, 279a, 279b), 321 (278,279a, 279b), 400 Pink, R. C., 191(37, 381, 221(201a), 246 (313, 321), 247(330, 332), 248(37, 38, 332, 335), 261,266,269,336(366,367, 368, 369), 403 Pino, P., 93(22c), 123 Pitkerthly, R. C., 216(160), 227(160), 264 Pitterski, L. N., 381(634b), 411 Pittman, C. U., 378(625a, 625b), 411 Plate, A. F., 141(42), 181 Platteeuw, J. C., 153, 155, 156, 159(79), 178(67), 182 Plauschinat, M.,201(95a), 263 Pletcher, D., 326(309a), 401 Plouidy, G., 150(62), 151(62), 159(62), 173(62), 182

Plunkett, T. J., 136, 173(99), 176(107), 179,181,182,183 Podgornova, V. A., 349(452), 350(452), 4 05 Pohl, D. G., 325(300b), 400 Polanyi, M., 57,59,62,79 Poles, T. C., 233,267 Politzer, P., 49,55 Ponec, V., 173(97), 174, 175(104), 176 (104, 106c), 177(104), 178(104), 182, 183, 185(10),260 Pople, J.A.,2,9, 11,53 Porter, R. P., 245(310), 246,268 Possanza, G., 73(37), 80 Poulin, J. C., 100(34), 102(34), 105(34), 122(5 l), 124 Powell, R. R., 5(13), 54 Pozdeeva, A. A., 353(472), 406 Pozdnyakov, D. V., 230, 232(244), 258 (392), 267,270 Pratt, J. M., 297(166b), 298(166b), 300 (166b), 396 Preite, S., 348(439), 349(439), 405 Prengle, H. W., 274(10), 391 Prest, M. L., 381,411 Prettre, M.,238(269), 267 Preuss, H., 32,54 Price, P., 103(42), 124 Prieto, J . A., 244(301),268 Prigge, H., 365(558), 409 Primet, M.,210, 211(138), 219(176), 221 (176), 225, 238(176, 269), 244(176), 245,258(380,386),264,265,267,270 Privalora, L. G., 298(178), 397 Pruchnik, F., 336(370), 403 Prudhomme, J. C., 150(62), 151(62), 159 (62), 173(62), 182 Przhevalskaya, L. K., 258(394), 271 Ptak, L. D., 144(51), 145(54), 181, 185(4), 260 Pudel, M. E., 298(178),397

Q Quinn, H. A., 132, 133, 134(14, 15), 135 (151, 137(30), 138(30), 140(30), 141 (39), 146(58), 180,181

R Raab, K., 73(37),80 Rabo, J. A., 241(284), 268 Rachlin, A. I., 65(26), 79

AUTHOR INDEX Raciszewski, Z., 342(410), 404 Radda, G. K., 387(671), 413 Rader, C. P.,137(28), 138(28), 139,181 Radjaipour, M.,201(95), 263 Rado, M., 348(442), 378,405 Radtke, D. D., 8,54 Rafii, E., 351(457),405 Rafikov, S. R., 351(459, 461, 463), 353 (470), 405,406 Ragaini, V., 301(197b), 397 Raimondi, D. L., 5(10, l l ) , 14(10,11),54 Rajaran, J., 341(401), 404 Ralek, M.,240(279), 268 Ramamurthy, P.,233,267 Ramaswamy, R., 326(311), 401 Ramsay, J. D. F., 236, 237,240,241(278), 267,268 Ramsbotham, J., 219(175), 225(217), 265, 266 Ranby, B., 154(70), 155(70), 182 Randaccio, L., 297(166b), 298(166b), 300 (166b), 396 Randall, E. N., 200(88), 262 Randall, E. W.,178(116),183 Ranfagni, A., 3(4), 53 Raphael, L., 326(314), 327(314),401 Rapp, A., 94(26), 123 Rappoport, Z., 364(543), 408 Rasmusson, G. H., 71(29), 79 Ratnasamy, P.,199(72), 262 Ratov, A. N., 228(237), 233,267 Rauch, F. C., 369,409 Ravens, D. A. S., 316(261),399 Rawlinson, D. J., 285(67a, 67b), 291(67a, 67b), 393 Raymond, K. N., 297(166a), 298(166a),396 Razin, V. L., 316(263b), 339(263b), 399 Read, G., 61(15, 16), 79, 359(502a,502b), 40 7 Read, J. F., 186(21), 192(46), 204(21), 212(21), 260,261 Ream, B. C., 372(596,597), 373(596,599), 410 Redman, B. T., 76(45), 80 Reed, C. A., 390(684a, 684b, 684c, 684d), 413 Reed, J. W. 349(447), 350(447),405 Regen, S. L., 357(498), 358(498),407 Reich, L., 275(17), 391 Reinhardt, W. P., 5(11), 14(11),54 Reitman, G. A., 350(454), 405

435

Reklat, A., 199(80), 262 Remy, D. C., 63(25), 79 Renson, J., 385(657a), 412 Resing, H. A., 201,202(107),263 Ress, E., 249(340), 250(340), 252(340), 269 Revenko, L. V., 298(178), 397 Reyerson, L. H., 258(385), 270 Reynolds, W.,283(53), 393 Rhee, K. H., 221(221), 227(221), 243,244, 266,268 Richardson, J. W.,5,54 Richardson, W. H., 292(115a, 115b), 395 Rideal, E. K., 140(36), 181 Rieke, C. A., 5(15), 54 Rijnen, H. T., 185(8), 260 Rillema, D. P., 297(166b), 298(166b), 300 (166b), 396 Ringold, H. J., 71(28, 36), 73(36), 75(41), 1 79,80 Ritchie, A. C., 65(26), 79 Robb, J. C., 277(40b), 392 Roberts, B. P., 278(42d), 392 Roberts, H. L., 287(86), 394 Robertson, A., 292(123), 395 Robertson, G. B., 355(485), 406 Robertson, J. C., 48,49,55 Robinson, W. T., 298(166a), 390(684d), 396,413 Rocek, J., 326(312b),401 Rochester, C. H., 219(175), 220(179), 225 (217), 258(382), 265,266,270 Rodewald, P. G., 306(219a), 330(219a), 398 Rodley, G. A., 298(166a), 390(684d), 396, 413 Rogan, J. B., 73(33), 76(44), 80 Rogerson, T. D., 341(397),403 Roginskii, S. Z., 301(193), 3 9 7 Rony, P.R., 364(537b), 378(626),408,411 Rooney, J. J., 128(5), 129, 130, 132(14), 133(14), 134(14, 15), 135(15, 211, 136 (21), 137(30), 138(30), 140(30), 141 (39), 146, 147, 148(60), 149(5), 151, 158(59), 176(64), 180, 181, 182, 246 (313), 269 Roothaan, C. C., 8,54 Roper, W. R., 340(383), 341(383), 355 (490), 403,406 Rosenberg, E., 178(116), 183 Rosenkranz, G., 59(8), 75(40,41), 79,80 Rosenman, H., 298(180, 181), 397

436

AUTHOR INDEX

Rosenthal, D., 73(33), 74(38), 80 Rosolovskaya, E. N., 228(237), 233(237), 26 7 Rosri, E., 335(360), 402 Ross, A. M.,386(663b), 412 Rosynek, M. P.,189(33), 236,254,254(32, 33), 255(33), 258(32, 398), 261, 267, .Z 71 Roth, J. A., 131(11), 133,180 Roth, J. F., 198(71), 262 Rothberg, S., 382(639), 383(639), 412 Rouchaud, J., 346(432c), 355, 404, 406, 407 Rowlinson, H. C., 128(4), 141(4), 180 Rozanov, V. V., 197(59), 261 Rubinshtein, A. M., 197(59), 234(264), 236(264), 237(264), 261,267 Rudenko, A. P., 301(194),397 Rudham, R., 205(119),263,381(634a),411 Ruiz, V. M., 61(15, 16), 79 Russell, G. A., 275(25), 277(40a), 278(42a, 42b), 280(45a), 318,391,392 Russell, J. L., 353(475),406 Rust, F. F., 291(106), 316(272), 394, 399 Rusyanova, N. D., 3 16(267c), 399 Ruttinger, R., 361(514), 362(514), 363 (5 14), 407 Rylander, P. N., 360(509),407 Ryono, L., 341(399),404

Sakurai, S., 83(4, 5), 122 Salomon, C., 93(22c), 123 Salomon, R. G., 320(291),400 Saltzmann, J. J., 332(342b), 402 Samman, N. G., 135(21), 136(21, 23), 146 (23,59), 147(59), 158(59), 181 Sampson, R. J., 378(618),411 Samuelson, B., 384(650b),412 Sanders, M. K., 381(634a), 411 Sangster, D. F., 286(75), 394 Santambrogio, A., 343(415), 404 Santry, D. P., 2(2), 9(2), 11(2), 53 Sapunov, V. N., 308(228b), 314(252), 316 (266b), 398,399 Saravalle, R., 301(197b), 397 Sasaki, K., 326(307a), 400 Saunders, P. C., 254(368), 258(368), 270 Savinova, L. P., 292(131c), 294(131c), 295 (13 lc), 395 Schafer, H., 326(305a), 400 Schaeffer, W. P., 297(169, 170a, 170b), 396,397 Schiavello, M., 185(15), 207(125), 260,263 Schlichenmaier, V., 245(309), 268 Schmitz, F. J., 73(36), 80 Schomaker, V., 241(284), 268 Schoonheydt, R. A., 258(396), 271 Schors, A., 331,402 Schott, H., 297(162), 355(489), 396,406 Schrage, K., 131(12, 13), 180 S Schrauzer, G. N., 341(387), 403 Sabacky, M. J., 85(12), 86(12), 88(15), Schubart, W., 197,237,254(60), 261 91(20a, 20b, 20c, 20d, 20e), 122(49), Schuetzle, D., 335(356), 402 Schuit, G. C. A., 378(612a), 380(632a), 122,123, I24 410,411 Sabel, A., 361(515), 362(515), 365(558), Schultz, H. S., 343(417), 353(417), 404 407,409 Sachtler, W. M. H., 173(97), 174,182,183, Schultz, R. G., 364(537b, 541),408 185(10, l l ) , 260, 355(491a), 378(620a), Schulz, J. G. D., 312(243, 244, 245, 246), 313(244), 318(244), 323(294a, 294b), 380(491a, 630), 406,411 399, 400 Saegebarth, K. A., 342(406), 404 Schulz, W.,233(251), 234(251), 267 Saftich, S. J., 355(486), 406 Schwab, G. M., 243,268 Saito, E., 287(81b), 394 Saito, Y., 203(108), 252(357), 263, 270, Schweizer, F., 214(150), 264 Scorrano, G., 103(39), 124 296(156), 297(156), 396 Sajus,L.,93(22c), 123,296(152), 343(419), Scott, E. J. Y., 292(120), 314(257), 315 (257), 395,399 344(420), 360(503), 396,404,407 Scott, G., 275(20), 335(364), 391,402 Sakai, T., 221(193), 265 Scott, J. A. N., 246(321),269 Sakamoto, H., 302(214a), 398 Scott, J . W.,82(3), 122 Sakharov, M. M.,301(193), 397 Sakota, K., 312(241), 314(241), 315(241), Scurrell, M. S., 220(179), 258(382), 265, 270, 378(612b),410 399

AUTHOR INDEX

Seckircher, R., 312(243, 244, 245), 313 (244), 318(244), 399 Sedlmeier, J., 361(514,515, 517, 518), 362 (514, 515,517), 363(514),407 Sedlyarov, V. A., 316(266a), 399 Segal, G. A., 2(2), 9(2), 11(2),53 Segaloff, A., 59(8), 79 Selander, H. G., 386(667b), 413 Selva, A., 291(107a), 394 Selwitz, C. M., 361(520), 364(538), 407, 408 Selyutina, E. F., 316(266b), 399 Semenov, N. N., 391,413 Semmelhack, M. F., 341(395, 396, 391, 399), 403,404 Senda, Y., 75(39),80 Senoff, C. V., 218(163),265 Serebryakov, B. R., 350(454), 405 SBre6 de Roch, I., 275(29), 296(152, 1531, 343(419), 344(420), 360, 392,396,404, 40 7 Servais, A., 221(197), 265 Seto, N., 73(37), 80 Sharp, J. A., 292(125), 305,395,398 Sharpless, K. B., 344(421), 351(466,467), 352(466,467), 389,404,406,413 Shaw, B. L.,375(608a),410 Shaw, R., 279 Shchekochikhin, Y.M.,244(302), 268 Shchennikova, M. K.,'292(130a, 130b, 131a, 131b, 131c), 293(130a), 294,295(130a, 130b, 131a, 131b, 131~1,395 Shealy, Y. F., 59(7), 79 Sheldon, R. A., 274(12), 299(185), 320 (288), 326(185), 330(288, 333), 331 (288), 346(433, 434, 435, 436a, 437), 349(433, 434), 350(433,434,435), 354 (433, 435), 358(499), 360(185), 379 (433), 391, 397, 400, 401, 404, 405, 40 7 Sheng, M.N., 345,348(428), 349(428,429, 430), 350(428, 431), 353(469), 404, 406 Shephard, F. E., 151,176(64),182 Sheppard, N., 199(85), 262 Sherbanenko, B. T., 350(455), 405 Sherman, A., 35,54,55 Shestakova, A. D., 323(295d), 324(295d), 400 Shiba, T., 361(526), 362(526),407

437

Shibata, K., 216(161), 264, 349(445), 379 (443,405 Shih, S . , 313(248), 399 Shilov, A. E., 374(602,603), 375(605), 377 (611c), 389(602,680),410,413 Shim, B. K. C., 128(4), 141(4),180 Shimada, K., 368(569b), 370(569b), 372 (569b), 409 Shimahara, M.,58(5,6), 68(5,6), 79 Shimiza, M., 28(45), 54 Shimoyama, Y., 160(84), 161(84), 162, 163, 164(86), 173, 174(84), 182, 185 (3% 260 Shiota, M., 58(5,6), 68(5,6), 79 Shooter, D., 378(618), 41 1 Shopov, D., 335(363), 402 Shteinman, A . A., 375(605, 606a), 377 (611c), 389(680), 410,413 Shue, R. S., 370(586,587), 410 Shushunov,V. A., 277(41), 292(130a, 131a, 131b), 293(130a), 294( 130a, 13la, 131b), 295(130a, 131a, 131b), 392,395 Shvets, V. A., 258(394), 271 Sieber, R., 361(514), 362(514), 363(514), 40 7 Siegel, H., 85(11), 86(13), 87(14), 122 Siegel, S., 56(1), 57(1), 59(1), 65(1), 72(1), 79, 128(3), 180 Siegel, T. M., 326(309c), 401 Sigel, H., 287(82a), 394 Sih, C. J., 103,124 Silver, B. L., 297(166b), 298(166b), 300 (166b), 396 Simon, A., 252(356), 270 Simons, T. G. J., 380(632a), 411 Simpson, P. L., 58(3), 61(3), 63(3), 67(3), 68(3), 79 Sinfelt, J. H., 173(98, 101), 176(101),182, 183, 185(12), 204(115), 206(115), 208 (115), 260,263 Sing, K. S. W., 196(50), 261 Singal, C. M., 7,54 Singleton, E., 297(165c), 396 Sisbarro, M. J., 61(24), 62(24), 63(24), 64(24), 65(24), 66(24), 67(24), 68(24), 69(24), 70(24), 71(24), 72(24), 79 Sittig, M., 274(3,4,5), 391 Skibida, I. P., 292(127a, 127b), 346(436b), 347(436b), 395,405 Skirrow,G., 292(113),395 Slade, R. M.,375(608a), 410

438

AUTHOR INDEX

Slager, T. L., 258(397), 271 Slater, J. C., 4,53 Sleight, T. P., 14,54 Sloane, R. B., 301(210), 398 Slomp, G., Jr., 59, 79 Smidt, J., 361, 362(514, 515, 517), 363 (514), 365(558), 407,409 Smirnov, S. V., 316(266a), 399 Smirnov, V. A., 326(302), 400 Smith, D. F., 297(166b), 298(166b), 300 (166b), 396 Smith,G. V., 134(16), 140(35), 180,181 Smith,H.A., 137(28), 138(28), 139,181 Smith, J. F., 292(114), 395 Smith, R. D., 314(535b), 365(535b), 366 (563a), 367(535b), 408,409 Smith, R. K. 188(26), 261 Smith, W. D., 189(32), 254(32), 255(32), 258(32), 261 Snook, M.E., 286(73b), 394 Sodano, C. S., 61(24), 62(24), 63(24), 64 (24), 65(24), 66(24), 67(24), 68(24), 69(24), 70(24), 71(24), 72(24), 79 Solbakken, A., 258(385), 270 Solodar, J., 103,124 Soloveva, S., 323(295d), 324(295d), 400 Soma, Y.,252,254(351),269 Somarjai, G. A., 207,263 Somorjai, G. A., 144(52, 53, 53a), 165(88, 89), 166(52, 90, 91), 176(88), 181,182 Sondheimer, F., 73(33), 75(41),80 Sonoda, A., 342(408), 370(583), 404,409 Sood, R., 103(42), I24 Sosnovsky, G., 278(42c), 285(67a, 67b), 290(104a), 291(67a, 67b, 110, l l l ) , 392,393,394,395 Spandau, H., 336(365), 403 Spannheimer, H., 196(52), 205(52), 196 Spath, H. T., 355(491c), 380(491c), 406 Spector, M. L., 364(555, 556), 365(555, 556), 408 Spitsyn, V. I., 301(194), 397 Spitzer, K., 33,54 Spoliti, M., 15(36), 54 Stacey, M.,199(83), 222(83), 262 Stakebake, J. L., 198,261 Stamires, D. N., 246(323), 269 Starcher, P. S., 364(542), 408 Stauffer, R. D., 341(397), 403 Stefanovic, V., 73(36), 80

Steggerda, J. J., 204(11 l),208(11 l), 263 Stein, G., 287(81a), 394 Steinberg, K. H., 224(213),266 Steinberg, K.-H., 198, 252(352, 353), 262, 269,270 Steiner, H., 156(75), 182 Step, G., 136(23), 146(23), 181 Sterin, E. K., 141(43), 181 Stern, E. W., 299(184), 300(187), 360(510, 511), 364(540, 555, 556), 3654555, 556), 372,397,407,408 Stern, R., 93(22c), 123 Sternhell, S., 320(279, 279a, 279b), 321 (278,279a, 279b), 400 Stevens, H. C., 342(409), 404 Stille, J. K., 341(403a, 403b), 365(559), 404,409 Stivala, S. S., 275(17), 391 Stoll, H., 32,54 Stolz, H., 193(47), 206(121), 208(121), 222(121), 223(121, 207), 224(207, 214), 225, 229(47, 214), 230(47), 232 (121), 250, 251, 252(47), 253(47), 261,263,266 Stomberg, R., 297(171), 397 Stone, F. S., 185(16, 17), 211, 212, 221 (192), 260,264,265 Stone, W.E. E., 201(102), 202(102),263 Story, P. R., 290(104b), 394 Stozhkova, G. A., 350(450,451,453), 405 Strom, E. T., 278(42b), 392 Stynes, H. C., 297(166b), 298(166b), 300 (166b), 396 Su, C. C., 349(447), 350(447), 405 Subba Rao, G. S., 298(182), 397 Subba Rao, V. V., 248(334), 269 Subramanian, R. V., 337(371), 403 Suchkov, V. V., 316(266a), 399 Suggett, A., 283(57), 393 Suggs, V. H., 295(137), 298(137), 395 Sugier, A., 149(60a), 182 Sugita, J., 411 Sukhopar, P. A., 316(267d), 399 Sukhov, D. A., 219(174), 220(174), 265 Sumiyoshi, T., 216(161), 264, 349(445), 379(445), 405 Sumegi, L., 345(426b), 404 Sussman, S., 342(405a), 404 Sutcliffe, L. H., 289(90), 394 Sutin, N., 283(54a, 54b), 393

AUTHOR INDEX Sutton, R. E.; 61(19), 70(19), 73(19), 79 Suzuki, A., 243(290,291), 268 Suzuki, N., 368(569b), 570(569b), 572 (569b), 409 Svensson, I. B., 297(171), 397 Swan, C. J., 334(354, 3 5 9 , 4 0 2 Sweeney, C. C., 8,54 Swern, D., 276(35), 392 Swoap, J. R., 134(16),180 Syeher, A. Y., 287(82b), 394 Sykes, A. G., 283(55), 296(157), 393,396 Symes, W.R., 287(86), 394 Symons, M. C. R., 21,54 Syrkin, Y., 362(530, 531), 363,363(534a), 365(534a), 408 Syz, M. G., 282(46c, 49), 393 Szonyi, G., 274(8), 391 Szymanska-Buzar, T., 336(370), 403

T Takagawa, M.,221(200), 265 Takahashi, K., 221(200), 265 Takahashi, T., 368(569a), 370(569a), 372 (569a), 409 Takao, K., 298(174,175, 177), 397 Takayanagi, H., 383(649), 384(649), 412 Takegami, Y., 93(22a), 103(40), 123,124 Takeuchi, S., 118(45a, 45b, 45c), 122(55, 56), 124 Takezawa, N., 244(303), 268 Tamao, K., 93(21b, 23b), 1 2 3 Tamaru, K., 244(296, 297, 298, 304, 306), 245(296,297,298),252(351), 254(351), 268,269 Tamura, M.,365(561a), 409 Tanabe, K., 186(20), 192, 215, 216, 220 (20), 221(20), 226, 227(20), 240(20), 260,264,266,349(445), 379(444), 405 Tanaka, H., 342(414), 404 Tanaka, K.,323(296,297), 324,367(561c), 400,409 Tanaka, M., 93(22a), 103, 123, 124, 221 (195), 265 Tanaka, S., 351(467), 352(467), 406 Tang, R. T., 122(50), 124, 305(217), 313 (217), 320(217, 276), 336(217), 398, 400 Taqui Khan, M. M., 296(150), 338(378), 341(378), 354(150), 396,403

439

Tatsuno, Y., 355(487,488), 406 Taube, H., 283(52a, 52b), 354(482), 393, 406 Taylor, E. C., 320(282,283, 284), 321(282, 283,284),322(283,284), 400 Taylor, H. S., 184,260 Taylor, J. F., 149(60c), 179(107a), 182, 183 Taylor, J. H., 240,267 Taylor, K. A., 274(13), 391 Taylor, K. C., 186(21), 192(46), 204(21), 212(21), 260,261 Taylor, W.I., 331(338), 402 Teichner, S. J., 245(312), 246(328, 329), 247(328), 248(312), 255(312,329), 256 (312), 269 Tellier, J., 137, 138, 140, I81 Tempere, J . F., 220(189), 228(189), 265 Tench, A. J., 220(180, 181), 248(333), 265,269 Terakawa, A., 368(569b), 370(569b), 372 (569b), 409 Teranishi, S., 298(174, 175, 176, 177), 370 (579,580,581,582), 397,409 Terenin, A., 200, 221(201), 245(92, 308), 262,265,268 Testi, R., 353(480), 354(480), 406 Tdtdnyi, P., 156(73, 74), 177(112, 113, 114), 182,183 Thomas, A. L., 300(188),397 Thomas, C. B., 310(238b), 320(280, 281, 289), 322(281, 289), 330(238b), 331 (238b), 361(528), 364(548), 365(548), 369(572), 398,400,408,409 Thomas, C. L., 274(7), 391 Tomasewski, A. J., 73(34),80 Thompson, D. T., 274(13), 391 Thompson, H. W.,73(35), 80 Thomson, S. J., 177(115a), 183 Thorpe, B. J., 35,55 Timoschtschuk, T. A., 344(422), 404 Tissue, T., 370(589),420 Titova, T. J., 220(186), 228(229), 265,266 Tkac, A., 293(133), 395 Tkho, Bui Ngok, 287(82b), 394 Tobolsky, A. V., 276(38), 292(112b), 295 (112b), 392,395 Toda, Y., 232(248), 267 Tolstikov, G. A., 351(464b), 353(478,479), 405,406

440

AUTHOR INDEX

Tomanova, D., 159(78), 167(93), 171(93), 172(93), 182 Tomisha, H., 292(122), 395 Tomilov, A. P., 326(302), 400 Tong, S. Y.,32,54 Topchieva, K. V., 228(237), 233(237), 267 Torelli, V., 76(47), 8 0 Torssell, K., 321(293),400 Towle, P. H., 308(227a), 398 Townsend, J. M., 344(421), 404 Trahanovsky, W. S., 331(335b), 402 Trambouze, Y., 228(232), 251(346), 253 (346), 266,269 Trauzher, G., 297(166b), 298(166b), 300 (166b), 396 Traylor, T. G., 280(45b), 297(166b), 298 166b), 300(166b), 390(686), 392, 396, 413 Treibmann, D., 252(356), 270 Trench, A. J., 215(152a), 264 Tretyakov, N. E., 226(219), 232(219), 233, 266 Trifiro, F., 348(439,44Oa), 349(439,44Oa, 443), 405 Triggs, C., 368(568, 570, 571), 370(568), 372(568), 409 Trillo, J. M., 245,268 Trimm, D. L., 334(353,354,355),402 Trommett, A., 361(517), 362(517), 407 Trost, B. M., 71(29), 79,93(23d), 123 Tsuchiya, F., 282(51), 393 Tsuda, S., 343(418), 404 Tsuji, J., 383(649), 384(649), 412 Tsuruya, S., 334(350c), 402 Tsutsumi, S., 342(408), 365(560), 404,409 Tsyskovskii, V. K., 350(454), 405 Tulupov, V. A., 308(228a), 398 Tung, S. E., 208,264 Turkevich, J., 188,222,246(323), 261,266, 269 Turner, A., 73(36), 80 Turner, I. D. M.,199(83), 222(83), 262 Turner, J. O., 295(138b, 140), 298(140), 395 Turner, R. B., 73(34), 80 Tweit, R. C., 73(34), 80 Twigg, G., 274(14), 328(322a), 391,401 Tyabin, M.B., 375(605), 410 Tyler, A. J., 216(159), 264

U Udenfriend, S . , 385(657b), 386(662), 387, 388(676), 412,413 Udupa, H. V. K., 326(311), 401 Uegaki, E., 320(285), 321(285), 400 Uemura, S., 320(285), 321(285), 400 Ueno, A., 244(306), 268 Ugo, R., 274(16), 275(16), 295(141), 298 (141), 299(141), 338(16), 340(384, 3851, 341(384, 385, 393), 355(485), 356(496), 378(614a), 379(614a), 391, 396,403,406,407,410 Ullman, E. F., 61(14), 79 Ullman, R., 301(210), 398 Unger, M. O., 369(575), 409 Uramoto, Y.,93(21c), I 2 3 Urbach, F., 41,55 Uri, N., 285(69b), 296(142, 143, 144), 335 (142,143,357), 338,393,396,402 Urich, V. U., 384(651), 412 Utley, J. H. P., 326(307b), 401 Uvarov, A. V., 218(171), 221(194), 223 (1941, 225(194), 244(194), 246(194), 252(355), 265,270 Uytterhoeven, J. B., 220(182, 187), 228 (187, 240a), 241(281), 242(281), 265, 26 7,268

V Vaerman, J., 325(299), 400 Valendo, A. Y.,350(448), 379,405,411 Valentine, D., Jr., 82(3), 122 Valentine, J. S., 296(147a), 354(147a), 396 van Bokhoven, J. G. M., 218(172), 221 (172), 224(172), 265 van Cauwelaert, F. H., 204(112), 208(112), 241(281, 282),242(281,282), 254(366), 255(366), 263,268,270 Vandamme, L. J., 228(240a), 267 Vandecasteele, J. P.,381(636), 412 Van der Avoird, A., 35,47(68), 48(68), 55 van der Ploeg, R. E., 390(232), 310(232), 330(232), 398 Van Der Puy, M., 286(74), 394 Vanderwerf, C. A., 94(25), 1 2 3 van Doorn, J. A., 299(185), 326(185), 346 (433, 434, 437), 347(433, 437), 348

441

AUTHOR INDEX (433, 437), 349(433, 434), 350(433, 434), 354(433), 358(499), 360(185), 379(433), 397,404,405,407 van Dort, H. M., 331,402 van Ham, N. H. A., 380(630),411 van Hardeveld, R., 185(3), 260 van Heeck, K. H., 198(64), 262 van Helden, R., 309(234), 330(234), 363, 364(535a, 5531, 365(535a), 367(535a, 566a, 566b), 398,408,409 van Meerter, R. E. C., 185(8), 260 Vansant, E. F., 220(188), 241(281, 282), 242(281,282), 265,268 van Schaik, J. R. H., 175, 176(104), 177, 178,183 van Sickle, D. E., 282(46a, 46b, 46c), 381, 392,393,411 van Tilborg, W. J. M.,301(209), 302(215), 398 van Tongelen, M., 244(293), 268 Vargaftik, M. N., 362(530,531), 363(534a), 365(534a), 408 Vasilev, 316(267d), 399 Vaska, L., 297(160, 161), 340(381b), 341 (381b), 354(381b), 396,403 Vaughan, W. E., 316(272), 399 Vedejs, E., 344(424), 404 Vedrine, J. C., 215,264 Venable, R. L., 205(118), 263 Venkatasubramanian, K., 390(687), 413 Vennik, J., 198(71c),262 Venturello, G., 205(119a), 263 Venuto, P. B., 153, 155,182 Verbeek, H., 47(68), 48(68), 55 Verberg, G., 363(535a), 364(535a), 365 (535a), 367, 367(535a), 408,409 Verenchikov, S. P.,328(323,327,328), 329 (327,328), 401 Verheijen, E. J. M., 380(632a),411 Vesely, K., 293(133), 395 Vickerman, J. C., 185(16), 260 Vieth, W. R., 390(687), 413 Vilenskii, L. M., 350(455), 405 Vincent, J. E., 295(139), 298(139, 173), 395,397 Vineyard, B. D., 88(15), 91(20a, 20b, ~ O C , 20e), 122(49), 122,123,124 Vinogradov, M. G., 308(226a), 327(319), 328(323,324,325,326a, 327,328,329,

330, 331), 329(328, 329, 330, 331), 398,401 Viscontini, M., 388(677), 413 Visser, C., 176(106c), 183 Visser, T., 293(135), 395 Voelter, W., 73(31), 79 Voge, H. H., 378(619a, 619b),411 Vogt, L. H., 296(155), 332.396,402 Volger, H. C., 364(552,554), 408 Vostrikova, L. A., 381(634d), 411 Vreugdenhil, A. D., 301(209), 380,398,411

W Waddington, T. C., 218(165), 265 Wade, W. H., 205(118),263 Waig, P.A., 73(37), 80 Wakamiya, M., 232(249), 267 Walker, P. J. C., 359(502a), 407 Wallace, T. J., 334(352),402 Walling, C., 278(43), 282, 285(72a, 72b), 286(72a, 72b, 73a), 289(101), 290 (101), 291(101), 295,334(43), 392,394 Waiters, A. B., 215(154), 264 Wan, C. C., 32,54 Ward, G. A., 364(549), 365(549), 408 Ward, J. W., 227, 228(234), 230(226), 241 (280), 242(280), 266,268 Ward, P. J., 310(238b), 320(289), 322 (289), 330(238b), 331(238b), 398,400 Watanabe, I., 367(56 lc), 386(669), 409,413 Watanabe, M., 370(583), 409 Watanabe, T., 243(290), 268 Watanabe, Y.,103(40), 124 Waters, W. A., 283(63b), 292(123), 311, 325(240), 328(63b), 330(332), 331 (334), 393,395,398,399,401,402 Watkins, G. D., 3,53 Waugh, J. S., 202(107a, 107b), 263 Wauquier, J. P.,138(33, 34), 139,181 Wayaku, M., 298(174), 397 Webb, G., 130(8), 148(60), 180,181 Webster, D. E., 378(628),410,411 Webster, R. K., 215,264 Weidlich, H. A., 59, 60(11), 79 Weil, J. A., 296(157), 396 Weiman, D. E., 292(121), 395 Weinburg, H. R., 326(306), 400 Weinberg, N. L., 326(306), 400 Weinkauff, D. J., 122(49), 124

442

AUTHOR INDEX

Weiss, J., 285(69c), 393 Weller, S. W., 207,264 Wells, C. F., 287(83a, 83b, 84, 85), 394 Wells, P. B., 130(8), 180,410 Wendlandt, K.-D., 224(213), 252(352, 353), 266,269,270 Wendlandt, W. W., 200,262 Werner, U., 252(353), 270 Wettstein, A., 65(26), 79 Weymouth, F. J., 328(321), 401 Whalley, L., 221(192),265 Whateley, T. L., 240,268 Whelan, D. L., 386(663b), 412 Whelan, R., 297(166b), 298(166b), 300 (166b), 396 White, H. M.,221(198), 227,228(198), 265 Whitehurst, D. D., 365(559), 409 Whitehurst, J . S., 76(44), 80 Whitesides, G. M., 153, 182, 357(498), 358 (498),407 Wiberly, S. E., 296(155), 396 Wichterlova, B., 355(491b), 380(491b), 406 Wieland, P., 65(26), 79 Wilds,A. L.,61,70,73(19), 79 Wilke, G., 93(23c), 123, 297(162), 355 (489), 396,406 Wilkins, R. G., 296(158), 396 Wilkinson, G., 355(483, 484), 363(536a), 406,408 Wilkinson, P. A., 65(26), 79 Wilkinson, P. J., 297(166b), 298(166b), 300 (166b), 396 Williams, D. R., 344(421), 404 Williams, D., 335(364), 402 Williams, P. H. 342(41 l), 342 Williams, R. J . P.,288 Williamson, J. B., 326(313), 327(313),401 Willstater, K., 326,401 Wilmsen, C. W., 48,55 Wilson, J. S., 320(280,281), 322(281), 369 (5721,400,409 Winde, H., 199,262 Winkler, H.,94(26, 27, 28), 123 Winstein, S., 364(543,544), 408 Winter, J. A., 215(152a),264 Winterbottom, J. M.,130(8), 180 Wirkkala, R. A., 320(292), 321(292), 400 Wirth, J. G., 332(340),402 Wise, J. J., 8,54 Wiseman, J. R., 135(19), 180

Witkop, B., 385(657a, 657b), 386(658,659, 662), 388(676,679), 412,413 Wohlforth, E. P., 28,54 Wolberg, A., 198(71), 262 Wolf, F., 347(438), 348(438), 381(634f), 405,411 Wolfsberg, M., 3,53 Wolfe, S., 364(550), 365(550), 408 Wong, P. K., 341(403b), 404 Woo, L., 368(569c), 370(569c), 372 (569c), 387(672), 409,413 Woodward, A. E., 292(112a), 295,395 Workman, R. J., 368(569c), 370(569c), 372 (569c), 387(672), 409,413 Wright, T. K., 316(269), 377(269), 399

Y Yablonskii, 0. P.,327(319), 401 Yagi, H., 386(664, 665, 666, 667a, 667b), 413 Yakerson, V. I., 197(59), 234, 236, 237 (264), 261,267 Yambe, T., 302(21 l), 398 Yamamoto, H., 351(467), 352(467), 406 Yamamoto, K., 93(21a, 21b, 21c, 21d,22a, 23b), 103(43),123,124 Yamamoto, S., 382(648), 412 Yamamura, M., 370(583), 409 Yao, H. C., 65(27), 79 Yarwood, J., 222(202),266 Yashima, T., 228,229(241), 267 Yashkin, R., 59(8), 79 Yasui, T., 365(561a), 409 Yates, D. J. C., 173(96,98),182, 204(115), 206(115), 208(115), 209, 238, 263, 264,267 Yoneda, Y., 203(108), 252(357), 263,270 Yonezawa, T., 334(350c), 402 Yoshimoto, H., 369(576), 409 Yoshimura, J., 118(45a, 45b, 4 5 ~ 1 122(55, , 56), 124 Yotsuyanagi, T., 222(204a), 266 Young, D. P., 342(407), 404 Young, V. O., 335(358), 402 Young, W. G., 364(543), 408 Yukawa, T., 365(560), 409 Yur'ev, V. P., 351(459,460,461,462,463, 464a, 464b), 353(471, 472, 478, 479), 405,406

AUTHOR INDEX

Z Zaikov, G. E., 337(373a), 403 Zajacek, J. G., 323(295c), 324(295c), 345 (428, 429, 430, 4311, 348(428), 349 (428, 429, 4301, 350(428, 431), 353 (469), 400,404,406 Zakharov, I. V., 316(267a), 399 Zakharova, V. I., 381(634b), 411 Zalygin, L. L., 353(476), 406 Zaltzman-Nirenberg, P., 386(662), 412 Zanderighi, G . M., 356(496), 407 Zanderighi, L., 378(614a), 379(614a), 410 Zavitsas, A. A., 289(101), 290(101), 291 (101), 394

443

Zecchina, A., 207(125), 212, 226, 229 (218), 233, 238(272), 239(272), 258 (145, 146,383), 263,266,267,270 Zelesko, M. J., 320(282), 321(282), 400 Zettlemoyer, A. C., 248(334), 269 Zhdanov, S. P . , 221(199), 228(229), 265, 266 Zhumadylov, T., 292(128), 296(128), 395 Zimmerman, H. E., 146(57), 181 Zingery, L. M.,258(398), 271 Ziolkowski, J. J., 336(370), 403 Zollweg, R. J., 11,54 Zolotareva, L. K.,316(266b), 399 Zuidwijk, J. G. P., 176(106c), 183

Subject Index A Acetaldehyde, oxidation of, 327, 328 Acetic acid, adsorption of, 244,245 Acetophenone hydrogenation of, 103 hydrosilylation of, 105 Acids, see also specific compounds adsorption of, on oxide surfaces, 243-245 effect on oxidation, 320-322 a$-unsaturated, 87 Acrylic acids, a-substituted, 87 a-Acylaminoacrylic acid asymmetric hydrogenation with Rh-AMCP catalyst, 104, 108 with Rh-DIOP catalyst, 106 hydrogenation of, 91,92 substrates of, 92 Adamantene, 147 Adsorption equilibrium in poisoning, 193 Adsorption sites, 187, 188 Aldehydes autoxidation of, 28 1 reaction with metal complexes, 326-330 Alkanes reaction with metal complexes, 322-326 skeletal rearrangement on platinum, 141158 Alkenes, reaction with metal complexes, 305-308 Alkyl hydroperoxide(s), reaction of metal complexes with, 287 Alkyl hydroperoxide-metal catalyst systems, 344-354 Alkylphenylketones, hydrosilylation of, 103,104 Alloy catalysts, 173-176 n-Ally1 complexes, 129-134 Alumina adsorption of acids on, 243-245 of amines on, 221,222 of ammonia on, 217-219 infrared spectra, 218 444

of carbon dioxide on, 234, 236-238 of ketones on, 232-234 of nitriles on, 233, 234 of nitrogen dioxide on, 230-232 of pyndine on, 222-225,256,257 infrared spectra, 223 dehydration of alcohols on, 249-254 ether formation, 252-254 interaction of water with, 204-209 isomerization and exchange reactions on, 254-258 OH stretching frequencies, 208 olefin formation, 249-252 Amines, adsorption of, on oxide surfaces, 221,222 Amino acids, synthesis of, 91 Ammonia, adsorption of, on oxide surfaces, 217-221 Anatase, see Titanium dioxide Anderson-Avery mechanism, 142-147 o -Anisylcyclo hexylmethylphosp hine (ACMP) as catalyst, 91,92 synthesis of, 95 Antagonism, 338, 339 Arene(s) oxidative carbonylation, 374 oxidative nuclear substitution of, 370,371 Arene oxides, 386 Asymmetric synthesis, 82,83 examples of, 82 Atropic acid, hydrogenation of, 88 Autocatalysis, 275 Autoxidation, see also Oxidation without accelerators or inhibitors, 275283 of aldehydes, 281 catalyst deactivation, 337 effect of ligands on, 337,338 of mixed-metal catalysts on, 338, 339 of oxidation products on, 337 of solvent on, 336,337 of specific metal complex on, 336 of temperature on, 336

SUBJECT INDEX homolytic mechanisms, 275-283 initiation reactions, 276-278 initiators for, 277 kinetics of, 334 metal-catalyzed, 28 3-3 39 kinetics of, 295,296 of olefins, 281,282, 305-308 propagation reactions, 278-280 termination reactions, 280, 281

B Benzaldehyde, autoxidation of, 326, 327 Bicyclo[ 3.3.11 nonane, deuterium exchange, 131,132,134,135 Bicyclo[ 3.3.01 octane, deuterium exchange, 131,132 Bloch functions, 7,8 Bond dissociation energies, 278, 279 Bond-shift mechanism, 142-150 Bromide ions, effect on oxidation, 316, 317 Brookite, see Titanium dioxide Butanone, hydrogenation of, 103

C Cadmium clusters, 29-31 extended Hiickel calculations, 30 Carbon, chemisorption on, 36-41 Carbon clusters, 32 Carbon-1 3, labeling in isomerization, 166172 Carbon dioxide adsorption of, on oxide surfaces, 234-243 infrared spectra, 235 Carboxylic acids reaction with metal complexes, 330, 331 a,@-unsaturated,109 hydrogenation of, 107-115 Catalyst(s1, see also specific substances chiral, 81, 82 heterogeneous, 83 Catalyst-inhibitor conversion, 334, 335 Catalyst surfaces, see Surfaces Cerium complexes, reaction with peroxides, 289 Chemisorption bond strength, 197, 198 Hiickel procedure, 35 MO theory and, 34-5 1

445

olefin, 138, 139 SCF-LCAO-MO procedure, 35, 36 spectroscopy, 198-202 surface molecule concept, 35 Chloride ions, effect on oxidation, 317-320 Cholestenone, hydrogenation, 57,58 Cholesterol, biosynthesis of, 382 aChromia, adsorption of carbon dioxide on, 238,239 Chromia-alumina, adsorption of carbon dioxide on, 238 Chromia-silica adsorption of carbon dioxide on, 238 of nitriles on, 233 Chromium complexes, reaction with peroxides, 289 Chromium oxide adsorption of acid on, 244 of ammonia on, 220 aChromium oxide adsorption of pyridine on, 226 interaction of water with, 212,213 Cluster compounds, see also specific types atomic charge, 13 bond energy, 11 electron affinity, 12 excitation energy, 12 ionization potential, 12 overlap population, 1 3 vibrational frequency, 13 CNDO theory, 2,9-11 applications, 11 computer programs for calculations, 13 equations, 9-1 1 Cobaloxime, hydrogenation of, 118, 119 Cobalt acetate, 323, 324 Cobalt catalysts, chiral, 118, 119 Cobalt complexes autoxidation of olefins, 305, 306 reactions with aromatic hydrocarbons, 3 11-322 with peroxides, 292 Cobalt-oxygen complexes, 296,297 Co-oxidation, 282,283, 337 Copper catalysts, 144 Copper clusters, 26 extended Huckel calculations, 26 Copper complexes autoxidation of cumene, 300 of cyclohexene, 302 reaction with peroxides, 289-292

446

SUBJECT INDEX

Crystal-field interactions, 8, 9 (CuCI), clusters, 15, 16 Cumene, autoxidation of, 300, 302, 303 1,5-Cyclization, 153, 155 1,6-Cycliiation, 153, 155 Cy cloalkenes competitive hydrogenation of, 136-139 homogeneous complexes, 141 rate sequences for hydrogenation, 138 Cy clohexane autoxidation of, 303 oxidation of, 323, 324 Cyclohexene, autoxidation of, 300,301 Cyclopentane, deuterium exchange, 128, 129

D @-Decalone,formation of, 63-66, 68 Dehydrocyclization-hydrogenolysis mechanism, 150-158 Dehydrogenases, 381 Desorption, 196,197 temperature-programmed (TPD), 197, 198 Deuteration of olefins, 140, 141 of or,@-unsaturatedketones, 73 Diatomic molecules molecular orbital calculations for, 13-15 properties of, 14 Dibenzylidene cyclohexanones, hydrogenation of, 77 Differential thermal analysis (DTA) for adsorption and desorption, 198 Dimethylcyclohexane, epimerization of, 136 Dioxygenases, 383, 384 2,3-Diphenylindenone, hydrogenation, 59, 60

E Electron spin resonance of alkylperoxy radicals, 293 of chemisorbed species, 201 Electron transfer processes, 283, 284, see also Redox catalysis Electronic spectra of chemisorbed species, 200,201 Enamides, hydrogenation of, 106 Epoxidation, metal-catalyzed, 344-354 Equilinone, hydrogenation, 6 1 , 6 2

Ethylene hydrogenation, 127 oxidation of, 361 Ethylene oxide, preparation of, 355

F Fenton’s reagent, 285-287, 387 Formic acid, adsorption of, 244,245

..

b

Gas chromatography, for desorption, 196 Glycol(s), reaction with metal complexes, 331 Glycol esters, formation of, 365, 366 Gold catalyst, 144 Gold clusters, 26 extended Hiickel calculations, 26 Graphite, chemisorption of H, C, F, 0, and N on, 40,41 Group VIII-oxygen complexes, 297

H Halides, effect on oxidation, 3 16-320 Hamiltonian matrix elements, 4, 5, 7, 8, 10 Hartree-Fock equations for CNDO, 9 , l O for MO theory, 2,4-6,8 Hemoglobin, 390 Heptacyclotetradecane, deuterium exchange, 135, 136 Heterocycles, hydrogenation of, 75,76 Hexane(s), isomerization of, 170, 171 n-Hexane 3C-labeled, 169 dehydrocyclization of, 150 isomerization of, 150 reaction of, over Pt films, 161, 163, 164 Hexatrienes, ring closure, 156 Homolytic catalysis, 275-339 Horiuti-Polanyi mechanism, 57, 62, 78, 127-138,141 deuterium tracers studies, 127, 128 orpdiadsorbed species and rollover, 134136 or@ process, 127-129 two-set exchange, 129-134 Hiickel theory, extended (EH), 2 , 3 approximations, 6

447

SUBJECT INDEX computer programs for calculations, 13 equations, 3-6 infinite system modifications, 7,8 ionic modifications, 6,7 Hydrindenones, hydrogenation of, 76, 77 Hydroboration, chiral, 82,83 Hydrocarbon(s) activation of, by metal coordination,

360-377 aromatic oxidation of, 367-374 oxidative coupling of, 367-370 oxidative side-chain substitution, 372-

314 rate of oxidation effect of halide ions, 316-320 of strong acids, 320-322 reaction with metal complexes, 308-

322 autoxidation, 303 '3C-labeled, 158,166-172 caged, isomerization of, 146 effect of carbonaceous deposits on reactions of, 176-179 ionization potentials, 304 oxidation of, 274 chemical vs. electrochemical, 326 reactions on metal catalysts, 125-180 saturated, activation of, by metal complexes, 374-377 Hydroformylation of olefins, 93 Hydrogen peroxide, reaction of metal complexes with, 285-287 Hydrogen peroxide-metal catalyst systems,

of imines, 104,105 of ketones, 103,104

I Imines asymmetric homogeneous hydrogenation Of, 103-105 hydrosilylation of, 104, 105 INDO theory, 2 Infrared spectroscopy of chemisorbed species, 199,200 Iridium catalysts, 144, 149,177 Iridium-gold alloy catalysts, 177 Iron complexes autoxidation of cyclohexene, 300, 301 reaction with peroxides, 290 Isobutane, isomerization of, 142-144 Isomerization(s), 150-158 on alloy catalysts, 173-176 on alumina, 254-258 13C-labeling studies, 166-172 carbocyclic, 164-166 surface-structure sensitivity, 158-166

2,3-O-Isopropylidene-2,3-dihydroxy-l,4-bis(diphenylphosphino) butane (DIOP) as catalyst, 90,91 hydrogenation of or-acylaminoacrylic acid,

91 synthesis of, 100-102

J Jahn-Teller effect, 3

342-344 Hydrogenation asymmetric homogeneous, 81-122 chiral heterogeneous, 82 of diabenzylidene cyclohexanones, 77 of heterocycles, 75,76 of hydrindenones, 76,77 of imines, 103-105 of ketones, 103-105 of olefins, 57,83,106-115,136-139 stereochemistry , 56-79 of or,@-unsaturatedketones, 56-79 Hydroperoxide, generation of in situ,

352-354 Hydrosilylation asymmetric, 92,93

K cu-Keto esters, chiral, addition to, 82 Ketones adsorption of, on oxide surfaces, 232-234 asymmetric homogeneous hydrogenation Of, 103-105 or,@-unsaturated deuteration, 73 effect of substituents on stereochemistry,

73-75 hydrogenation of, 56-79 in acidic media, 59,60 effect of base, 61,62 tritiation, 73 Koopmans' theorem, 12

448

SUBJECT INDEX

L Langmuir adsorption coefficients, 138, 140 Lead, adsorption of organic compounds on, 49 Lead complexes, reaction with peroxides, 289 Ligand transfer processes, 283, 284, see also Redox catalysis Lipoxygenase, 383 Lithium clusters, 32

M Magnesium oxide adsorption of acid on, 244 of ammonia on, 220 of carbon dioxide on, 240, 241, 254, 255 of ketones on, 232 of nitriles on, 233 of nitrogen dioxide on, 230, 231 of pyridine on, 226, 227 interaction of water with, 214, 215 Manganese complexes autoxidation of olefins, 306-308 reaction with aromatic hydrocarbons, 309-312 with peroxides, 294, 295 Mechanisms, see specific types Menthyldiphenylphosphine (MNPP), synthesis of, 97, 98 Mesityl oxide, hydrogenation of, 121 Metal(s), see also specific elements chemisorption on, 47-49 Metal catalysts, see also specific catalysts competitive hydrogenation of cycloalkenes on, 137 hydrocarbon reactions on, 125-180 Metal clusters, 16-34, see also specific types Metal complexes, see also specific element activation of molecular oxygen by, 296303 of saturated hydrocarbons by, 374-377 activity of, 336-339 heterolytic oxidations, 340-377 physicochemical properties in solution, 336,337 reaction with aldehydes, 326-330 with alkanes, 322-326 with alkenes, 305-308

with aromatic hydrocarbons, 308-322 with carboxylic acids, 330, 331 with free radicals, 334, 335 with glycols, 331 with peracids, 295 with peroxides, 285-296 with phenols, 331-334 with substrate and autoxidation products, 303-334 Metal-dioxygen complexes, 296-298, 303 bond lengths in, 298 oxygen activation, 354-360 Metal-hydroperoxide complexes, heteroly tic reactions of, 342-354 Metal phthalocyanines, 300-302 Metal porphyrins, 300-302 Metallocyclobutane, 149 Metalloenzymes, 385 Methylcinnamic acid, hydrogenation of, 89 Meth ylcyclopentane "C-labeled, 166 hydrogenolysis of, 150, 170, 171 reaction of, over Pt films, 162 ring opening of, 15 1 Methylheptane, ring closure, 154 Methylhexane, ring closure, 155 Methyl-fl-methylcinnamates, hydrogenation of, 116, 117 Methylpentane(s) 13C-labeled, 166-168 hydrogenolysis of, over Pt films, 163, 164 isomerization of, 150 oxidation of, 325 ring closure, 151, 152, 155 Methyl a-phenylacrylate, hydrogenation of, 90 Molecular orbital (MO) theory application of, to catalysis, 1-53 approximate, 2, 3 , 5 1-54 properties calculable by, 11-13 diatomic molecules, 13-15 orbital data, 12 Molybdenum catalyst, 350, 351, 354, 355 Monooxygenases, 382, 383 reactions effected by, 383 Myoglobin, 390

N Neomenthyldiphenylphosphine (NMDPP) as catalyst, 89, 90 synthesis of, 96-98

449

SUBJECT INDEX Neopentane, isomerization of, 142, 144, 145,147 Nickel catalyst, 127, 174 adsorption of CO on, 48,49 of H on, 47,48 Nickel clusters, 31, 32 electronic properties of, 31 Nickel-copper alloy catalysts, 173-175 Nickel oxide, adsorption of CO on, 49 Nitriles, adsorption of, on oxide surfaces, 232-234 Nitrogen dioxide, adsorption of, on oxide surfaces, 230-232 Nuclear magnetic resonance of chemisorbed species, 201, 202

0 p-Octalone 1,2- and 1,4diadsorbed, 64,65 effect of H + on product distribution, 69 of hydrogen on stereochemistry, 64-68 of OH- on product distribution, 70 of substituents, 73-75 enolates of, 7 1 hydrogenation of, 60,61, 63-75 in acidic media, 68,69 in basic media, 69-72 in neutral media, 63-68 nactane, ring closure, 154 Octanone, hydrogenation of, 103 Olefins, see also specific compounds autoxidation of, 281, 282, 305-308 bishydroxylation of, 342 degree of strain, 135 deuteration of, 140, 141 homogeneous hydroformylation of, 93 hydrogenation of, 83,106-115 competitive, 136-1 39 rate of, 138, 139 oxidation of, 361-367 oxidative carbonylation of, 367 oxidative coupling of, 367 n-bonded, 134-1 36 Osmium-copper alloy catalysts, 173, 176 Oxidases, 3 81 Oxidation, see also Autoxidation biochemical, 381 enzymatic, 384-387 heterolytic mechanisms, 339-377 homolytic mechanisms, 275-339

in liquid phase, 272-391 heterogeneous catalysis of, 377-381 me tal-catalyzed, 272-39 1 of olefins, 361-367 Oxide surfaces, see also specific oxides adsorption of acids on, 243-245 of amines on, 221,222 of ammonia on, 217-221 of carbon dioxide on, 234-243 of electrondonor and electron-acceptor molecules, 245-249 of ketones and nitriles on, 232-234 of nitrogen dioxide on, 230-232 of pyridine on, 222-230 catalytically active, 184-260 interaction of water with, 203-217 Oxygenases, 303,382 chemical models for, 387-390

P Palladium adsorption on various substrates, 49-51 oxidation of aromatic hydrocarbons, 367-374 of olefins, 361-367 Palladium-alumina catalyst, 129, 132 Palladium catalysts, 127-130, 132, 133, 136,141-143,146-150,152 for hydrogenation, 63-72 on silk fibroin, 83 Palladium clusters, 27-29 on carbon, 38-40 energy levels, 38, 39 potentialenergy curves, 39,40 d holes, 28 electron distribution, 29 electronic properties, 27 geometry, 27 sue effects, 27, 28 Peracids, reaction of metal complexes with, 295 Peresters as initiators, 277 Peroxides, see also specific compounds as initiators, 277 rate constants, 279,280 reaction of metal complexes with, 285296 Petrochemicals, see also Hydrocarbons oxidation of, 274 Phenols, reaction with metal complexes, 33 1-3 34

450

SUBJECT INDEX

Phenothiazine as poissn, 245-247 Phosphine(s) as catalysts, 83-1 15 Phosphine ligands chiral at carbon, 96-102 at phosphorus, 93-95 synthesis of chiral, 93-102 Phosphine oxides, synthesis of, 94 Phosphobetaine, 88 Photoelectron spectroscopy for adsorption, 198 Platinum-alumina catalyst, 138, 150, 154157,159,168,169 Platinum catalysts, 142-150, 152, 153, 156-158,175 skeletal rearrangement of alkanes on, 141158 surface studies, 158-166 ultrathin films, 160-163 Platinum-gold alloy catalysts, 175-1 78 Platinum oxide as catalyst for hydrogenation, 72,73 Poisoning of catalytic surface, 184-260, see also Poisons adsorption equilibria, 193 on alumina, see Alumina methods of, 202,203 selective, 189 specific, 189 Poisons, 189 ff., see also specific substances characterization of surfaces by, 195-202 chemical nature of adsorbed probe molecules, 199-202 detection of surface species, 190 determination of adsorbed amounts, 195, 196 HSAB concepts, 192-194 interactions between adsorbate and surface, 197-199 with oxide surfaces, 203-249 specific, 190 strength of, 191 molecular size of, 190, 191 promotional effects of, 191, 192 reactivity of, 191 selection of, 192-195 surface reconstruction, 191 thermal stability, 191 Polycycloalkanes, deuterium exchange, 127, 128 Polymethylbenzenes, hydrogenation of, 139 Prostaglandin(s), biosynthesis of, 384 Prostaglandin E l , synthesis of, 103, 104 Protoadamantene, isomerhation of, 146,147

Pyridine adsorption on aluminum oxide, 196 on oxide surfaces, 222-230

R Raman spectroscopy of chemisorbed species, 199,200 Raney nickel, 83 Reactions, see specific types Redox catalysis, electron and ligand transfer processes, 283-339 Redox potentials of metal complexes, 288 Rhenium complexes, reaction with hydroperoxides, 298,299 Rhodium-ACMP catalyst, 91, 92 asymmetric hydrogenation of a-acylaminoacrylic acid, 104, 108 of a,p-unsaturated carboxylic acids, 114 synthesis of prostaglandin E l , 103, 104 RhodiumCAMPHOS catalyst, asymmetric hydrogenation of @-unsaturated carboxylic acids, 107-109,112 Rhodium catalyst, 127 Rhodium-chkial amide catalysts, 115-1 17 Rhodium-chiral phosphine catalysts asymmetric reduction of ketones and imines, 103-105 homogeneous, 85-1 15 Rhodium-DIOP catalyst, 90,91 asymmetric hydrogenation of a, p-unsaturated carboxylic acids, 113 of enamides, 106 hydrosilylation of irnines, 104, 105 Rhodium-MDPP catalyst, asymmetric hydrogenation of a,p-unsaturated carboxylic acids, 107-109, 111 Rhodium-NMDPP catalyst, 89,90 asymmetric hydrogenation of a,p-unsaturated carboxylic acids, 107-1 10 Rhodium-phosphine catalysts achiral, 83-85 chiral, see Rhodium-chiral phosphine catalysts Rollover mechanism, 131-136 Ruthenium cataly,sts, chiral, 120, 121 Ruthenium-copper alloy catalysts, 173,176 Rutile, see Titanium dioxide

S Silica-alumina adsorption of acids on, 243,244 of amines on. 221 of ammonia on, 220

SUBJECT INDEX of carbon dioxide on, 241,242 of ketones on, 233 of nitrogen dioxide on, 230,232 of pyridine on, 227-229 interaction of water with, 215,216 Silver catalyst, 145,149 Silver clusters, 17-26 bonding energy, 18,19 on carbon, 36-38 binding energy, 37,38 charge, 38 potentialenergy curves, 36,37 CNDO properties, 24,25 density of states, 23,24 electronic properties, 21,22 extended Huckel calculations, 26 geometry, 18-20 infinite, 25,26 molecular orbitals, 19 potentialenergy curves, 17,18,20 on silver bromide, 41-47 CNDO, 41-44 extended Huckel, 44,45 photolysis, 45-47 size effects, 20-23 wave functions, 24 Silver-palladium clusters, 33,34 Skeletal rearrangements, 158-176 Slater determinant, 9 Slater orbitals, 4,5 Sulfonium compounds, 302 Sulfur clusters, 33 Surfdce(s), 195-203,see also Adsorption, Chemisorption characterization of, by poisons, 195-202 interaction between adsorbate and, 197-

199 structure models, 216,217 Surface sites, 187,188 active, 187,188 multicenter, 188 Surface-structure sensitivity, 158-1 66 Synergism, 338,339

45 1

Titanium dioxide adsorption of acid on, 244,245 of ammonia on, 219 of carbon dioxide on, 238 of ketones on, 232 of nitrogen dioxide on, 230 of pyridine on, 225 interaction of water with, 209-211 Transition metal diatomic molecules, 15 Tricyclodecane, deuterium exchange, 135 1,2,2-Trimethyl-l, 3-bis(diphenylphosphinomethy1)cyclopentane (CAMPHOS), synthesis of, 98-100 endo-Trimethylenenorbornane, deuterium exchange, 133,134 1,1,3-TrimethyIcyclopentane,isomerization of, 142 Trimethylpentane(s), dehydrocyclization,

152 Tritiation of cY,&unsaturated ketones, 73

U Udenfriend’s reagent, 387 Ultraviolet radiation, for adsorption, 198 Ultraviolet spectroscopy of chemisorbed species, 200,201

W Water, interaction of, with oxide surfaces,

203-217 Wilkinson’s catalyst, 84,86-88 Wolfsberg-Helmholtz formula, 5 Woodward-Hoffmann rules for symmetry,

41

X X-ray studies for chemisorption, 198,199

Z

Zeolites, 192,381 adsorption of amines on, 221 of ammonia on, 220 of carbon dioxide on, 241,242 T of nitriles on, 233 of nitrogen dioxide on, 230 Testosterone, hydrogenation, 57,58 of pyridine on, 227-229 Tetracyanoethylene as poison, 245, 247, Zinc oxide 248 adsorption of acid on, 244 1,1,3,3-Tetramethylcyclohexane, deuterium of carbon dioxide on, 240 exchange, 130 Thermogravimetric analysis (TGA) for adof pyridine on, 226 sorption, 196 interaction of water with, 213,214

Contents of Previous Volumes Volume 1 The Heterogeneity of Catalyst Surfaces for Chemisorption HUGHS. TAYLOR Alkylation of Isoparaffins V. N. IPATIEFF AND LOUISSCHMERLING Surface Area Measurements. A New Tool for Studying Contact Catalysts P. H. EMMETT The Geometrical Factor in Catalysis R. H. GRIFFITH The Fischer-Tropsch and Related Processes for Synthesis of Hydrocarbons by Hydrogenation of Carbon Monoxide H.H.STORCH The Catalytic Activation of Hydrogen D. D. ELEY Isomerization of Alkanes HERMAN PINES The Application of X-Ray Diffraction to the Study of Solid Catalysts AND I. FANKUCHEN M. H. JELLINEK Volume 2

About the Mechanism of Contact Catalysis GEORGE-MARIA SCHWAB Volume 3 Balandin’s Contribution to Heterogeneous Catalysis B. M.W. TRAPNELL Magnetism and the Structure of Catalytically Active Solids P. W.SELWOOR Catalytic Oxidation of Acetylene in Air for Oxygen Manufacture J. HENRYRUSHTON AND K. A. KRIEGHR The Poisoning of Metallic Catalysts E. B. MAXTED Catalytic Cracking of Pure Hydrocarbons VLADIMIR HAENSEL Chemical Characteristics and Structure of Cracking Catalysts A. G. OBLAD,T. H. MILLIKEN, JR., AND G. A. MILLS Reaction Rates and Selectivity in Catalyst Pores AHLBORN WHEELER Nickel Sulfide Catalysts WILLIAMJ. KIRKPATRICK

The Fundamental Principles of Catalytic Activity FREDERICK SEITZ The Mechanism of the Polymerization of Volume 4 Alkenes Chemical Concepts of Catalytic Cracking AND V. N. IPATIEFF LOUISSCHMERLING R. C. HANSPORD Early Studies of Multicomponent Catalysts Decomposition of Hydrogen Peroxide by ALWINMITTASCH Catalysts in Homogeneous Aqueous SoluCatalytic Phenomena Related to Phototion graphic Development J. H. BAXENDALE T. H. JAMES Catalysis and the Adsorption of Hydrogen on Structure and Sintering Properties of Cracking Catalysts and Related Materials Metal Catalysts HERMAN E. RIES,JR. OTTOBEECK Acid-base Catalysis and Molecular Structure Hydrogen Fluoride Catalysis R. P. BELL J. H. SIMONS Theory of Physical Adsorption Entropy of Adsorption TERRELL L. HILL CHARLES KEMBALL 452

CONTENTS OF PREVIOUS VOLUMES The Role of Surface Heterogeneity in Adsorption GEORGE D. HALSEY Twenty-Five Years of Synthesis of Gasoline by Catalytic Conversion of Carbon Monoxide and Hydrogen HELMUT PICHLER The Free Radical Mechanism in the Reactions of Hydrogen Peroxide JOSEPH WEIS The Specific Reactions of Iron in Some Hemoproteins PHILIPGEORGE

453

Noble Metal-Synthetic Polymer Catalysts and Studies on the Mechanism of Their Action WILLIAMP. DUNWORTH A N D F. F.NORD Interpretation of Measurements in Experimental Catalysis P. B. WEISZAND C. D. PRATER Commercial Isomerization B. L. EVERING Acidic and Basic Catalysis MARTINKILPATRICK Industrial Catalytic Cracking RODNEY V. SHANKLAND Volume 7

Volume 5

Latest Developments in Ammonia Synthesis ANDERS NIELSEN Surface Studies with the Vacuum Microbalance: Instrumentation and LowTemperature Applications T. N. RHODIN,JR. Surface Studies with the Vacuum Microbalance: High-Temperature Reactions EARLA. GULBRANSEN The Heterogeneous Oxidation of Carbon Monoxide MORRISKATZ Contributions of Russian Scientists to Catalysis J. G. TOLPIN,G. S . JOHN, AND E. FIELD The Elucidation of Reaction Mechanisms by the Method of Intermediates in QuasiStationary Concentrations J. A. CHRISTIANSEN Iron Nitrides as Fischer-Tropsch Catalysts ROBERT B. ANDERSON Hydrogenation of Organic Compounds with Synthesis Gas MILTONORCHIN The Uses of Raney Nickel EUGENE LIEBERAND FREDL. MORRITZ Volume 6

Catalysis and Reaction Kinetics at Liquid Interfaces J. T. DAVIES Some General Aspects of Chemisorption and Catalysis TAKAO KWAN

The Electronic Factor in Heterogeneous Catalysis M. McD. BAKERAND G. I. JENKINS Chemisorption and Catalysis on Oxide Semiconductors G.PARRAVANO AND M.BOUDART The Compensation Effect in Heterogeneous Catalysis E. CREMER Field Emission Microscopy and Some Applications to Catalysis and Chemisorption ROBERT GOMER Adsorption on Metal Surfaces and Its Bearing on Catalysis JOSEPH A. BECKER The Application of the Theory of Semiconductors to Problems of Heterogeneous Catalysis K. HAUFFE Surface Barrier Effects in Adsorption, Illustrated by Zinc Oxide S. ROYMORRISON Electronic Interaction between Metallic Catalysts and Chemisorbed Molecules R. SUHRMANN Volume 8

Current Problems of Heterogeneous Catalysis J. ARVIDHEDVALL Adsorption Phenomena J. H. DE BOER Activation of Molecular Hydrogen by Homogeneous Catalysts S. W. WELLER AND G. A. MILLS Catalytic Syntheses of Ketones V. I. KOMAREWSKY AND J. R. COLEY

454

CONTENTS OF PREVIOUS VOLUMES

Polymerization of Olefins from Cracked Gases EDWINK. JONES Coal-Hydrogenation Vapor-Phase Catalysts E. E. DONATH The Kinetics of the Cracking of Cumene by Silica-Alumina Catalysts CHARLES. D. PRATERAND RUDOLPHM. LAGO Volume 9 Proceedings of the International Congress on Catalysis, Philadelphia, Pennsylvania, 1956 Volume 10 The Infrared Spectra of Adsorbed Molecules R. P. EISCHENS AND W. A. PLISKIN The Influence of Crystal Face in Catalysis ALLANT. GWATHMEY AND ROBERTE. CUNNINGHAM The Nature of Active Centres and the Kinetics of Catalytic Dehydrogenation A. A. BALANDIN The Structure of the Active Surface of Cholinesterases and the Mechanism of Their Catalytic Action in Ester Hydrolysis F. BERGMANN Commercial Alkylation of Paraffins and Aromatics EDWINK. JONES The Reactivity of Oxide Surfaces E. R. S. WINTER The Structure and Activity of Metal-on-Silica Catalysts G. C. A. SCHUITAND L. L. VAN REIJEN Volume 11 The Kinetics of the Stereospecific Polymerization of a-Olefins G. NATTAAND I. PASQUON Surface Potentials and Adsorption Process on Metals R. v. CULVER AND F. c. TOMPKINS Gas Reactions of Carbon P. L. WALKER, JR.,FRANK RUSINKO,JR., AND L. G. AUSTIN The Catalytic Exchange of Hydrocarbons with Deuterium C. KEMBALL

[mmersional Heats and the Nature of Solid Surfaces J. J. CHESSICK AND A. C. ZETTLEMOYER The Catalytic Activation of Hydrogen in Homogeneous, Heterogeneous, and Biological Systems J. HALPERN Volume 12 The Wave Mechanics of the Surface Bond in Chemisorption T. B. GRIMLEY Magnetic Resonance Techniques in Catalytic Research D. E. O’REILLY Bare-Catalyzed Reactions of Hydrocarbons PINES AND LUKEA. SCHAAP HERMAN The Use of X-Ray and K-Absorption Edges in the Study of Catalytically Active Solids ROBERT A. VAN NORDSTRAND The Electron Theory of Catalysis on Semiconductors TH. WOLKENSTEIN Molecular Specificity in Physical Adsorption D. J. C. YATES Volume 13 Chemisorption and Catalysis on Metallic Oxides F. S.STONE Radiation Catalysis R. COEKELBERGS, A. CRUCQ,AND A. FRENNET Polyfunctional Heterogeneous Catalysis PAULB. WEISZ A New Electron Diffraction Technique, Potentially Applicable to Research in Catalysis L. H. GERMER The Structure and Analysis of Complex Reaction Systems D. PRATER JAMES WEI AND CHARLES Catalytic Effect in Isocyanate Reactions AND G. A. MILLS A. FARKAS Volume 14 Quantum Conversion in Chloroplasts MELVINCALVIN The Catalytic Decomposition of Formic Acid P. MARS, J. J. F. SCHOLLEN,AND P. Zw IETERING

CONTENTS OF PREVIOUS VOLUMES Application of Spectrophotometry to the Study of Catalytic Systems H. P. LEFTINAND M. C. HOBSON, JR. Hydrogenation of Pyridines and Quinolines MORRISFREIFELDER Modern Methods in Surface Kinetics: Flash, Desorption, Field Emission Microscopy, and Ultrahigh Vacuum Techniques GERTEHRLICH Catalytic Oxidation of Hydrocarbons L. YA. MARGOLIS Volume 15 The Atomization of Diatomic Molecules by Metals D. BRENNAN The Clean Single-Crystal-Surface Approach to Surface Reactions N. E. FARNSWORTH Adsorption Measurements during Surface Catalysis KENZITAMARU The Mechanism of the Hydrogenation of Unsaturated Hydrocarbons on Transition Metal Catalysts G. C. BONDAND P. B. WELLS Electronic Spectroscopy of Adsorbed Gas Molecules A. TERENIN The Catalysis of Isotopic Exchange in Molecular Oxygen G. K. BORESKOV Volume 16

455

Chemical Identification of Surface Groups H. P. BOEHM Volume 17 On the Theory of Heterogeneous Catalysis JURO HORIUTIAND TAKASHI NAKAMURA Linear Correlations of Substrate Reactivity in Heterogeneous Catalytic Reactions M. KRAUS Application of a Temperature-Programmed Desorption Technique to Catalyst Studies R. J. CVETANOVIC AND Y. AMENOMIYA Catalytic Oxidation of Olefins HERVEY H. VOCE AND CHARLES R. ADAMS The Physical-Chemical Properties of Chromia-Alumina Catalysts CHARLES P. POOLE,JR. AND D. S. MACIVER Catalytic Activity and Acidic Property of Solid Metal Sulfates Kozo TANABE AND TSUNEICHI TAKESHITA Electrocatalysis S . SRINIVASEN, H.WROBLOWA, AND J. O'M. BOCKRIS Volume 18 Stereochemistry and Mechanism of Hydrogenation of Napthalenes in Transition Metal Catalysts and Conformational Analysis of the Products A. W. WEITKAMP The Effects of Ionizing Radiation on Solid Catalysts ELLISON H. TAYLOR Organic Catalysis over Crystalline Alumiosilicates P. B. VENUTOAND P. S. LANDIS On the Transition Metal-Catalyzed Reactions of Norbornadiene and the Concept of B Complex Multicenter Processes G. N. SCHRAUZER

The Homogeneous Catalytic Isomerization of Olefins by Transition Metal Complexes MILTONORCHIN The Mechanism of Dehydration of Alcohols over Alumina Catalysts HERMAN PINES AND JOOST MANASSEN B Complex Adsorption in Hydrogen ExVolume 19 change on Group VIII Transition Metal Modern State of the Multiplet Theory of Catalysts Heterogeneous Catalysis J. L. GARNETTAND W. A. SOLLICHA. A. BALANDIN BAUMGARTNER Stereochemistry and the Mechanism of The Polymerization of Olefins by Ziegler Catalysts Hydrogenation of Unsaturated HydroM. N. BERGER,G. BOOCOCK,AND R. N. carbons HAWARR SAMUEL SIBCEL

45 6

CONTENTS OF PREVIOUS VOLUMES

Dynamic Methods for Characterization of Adsorptive Properties of Solid Catalysts L. POLINSKI AND L. NAPHTALI Enhanced Reactivity at Dislocations in Solids J. M. THOMAS Volume 20 Chemisorptive and Catalytic Behavior of Chromia ROBERT L. BURWELL, JR.,GARYL. HALLER, KATHLEEN c. TAYLOR,AND JOHNF.READ Correlation among Methods of Preparation of Solid Catalysts, Their Structures, and Catalytic Activity KIYOSHIMORIKAWA, TAKAYASU SHIRASAKI, AND MASAHIDE OKADA Catalytic Research on Zeolites J. TURKEVICH AND Y. ON0 Catalysis by Supported Metals M. BOUDART Carbon Monoxide Oxidation and Related Reactions on a Highly Divided Nickel Oxide P. C. GRAVELLE AND S. J. TEICHNER Acid-Catalyzed Isomerization of Bicyclic Olefins JEANEUGENEGERMAINAND MICHEL BLANCHARD Molecular Orbital Symmetry Conservation in Transition Metal Catalysis FRANK D. MANGO Catalysis by Electron Donor-Acceptor Complexes KENZITAMARU Catalysis and Inhibition in Solutions of Synthetic Polymers and in Micellar Solutions H. MORAWETZ Catalytic Activities of Thermal Polyanhydroa-Amino Acids DUANEL. ROHLFING AND SIDNEY w.F O X Volume 21 Kinetics of Adsorption and Desorption and the Elovich Equation C. AHARONI A N D F. C. TOMPKINS Carbon Monoxide Adsorption on the Transition Metals R. R. FORD

Discovery of Surface Phases by Cow Energy Electron Diffraction (LEED) JOHN W. MAY Sorption, Diffusion, and Catalytic Reaction in Zeolites L. RIEKERT Adsorbed Atomic Species as Intermediates in Heterogeneous Catalysis CARLWAGNER Volume 22 Hydrogenation and Isomerization over Zinc Oxide R. J. KOKESAND A. L. DENT Chemisorption Complexes and Their Role in Catalytic Reactions on Transition Metals 2. KNOR Influence of Metal Particle Size in Nickel-onAerosil Catalysts on Surface Site Distribution, Catalytic Activity, and Selectivity R. VANHARDEVELD AND F. HARTOG Adsorption and Catalysis on Evaporated Alloy Films R. L. Moss AND L. WHALLEY Heat-Flow Microcalorimetry and Its Application to Heterogeneous Catalysis P. C. GRAVELLE Electron Spin Resonance in Catalysis JACK H. LUNSFORD Volume 23 Metal Catalyzed Skeletal Reactions of Hydrocarbons J. R. ANDERSON Specificity in Catalytic Hydrogenolysis by Metals J. H. SINFELT The Chemisorption of Benzene R. B. Moves AND P. B. WELLS The Electronic Theory of Photocatalytic Reactions on Semiconductors TH. WOLKENSTEIN Cycloamyloses as Catalysts DAVID W. GRIFFITHSAND MYRONL. BENDER Pi and Sigma Transition Metal Carbon Compounds as Catalysts for the Polymerization of Vinyl Monomers and Olefins D. G. H. BALLARD

CONTENTS OF PREVIOUS VOLUMES

Volume 24 Kinetics of Coupled Heterogeneous Catalytic Reactions L. BERXNEK Catalysis for Motor Vehicle Emissions JAMES WEI The Metathesis of Unsaturated Hydrocarbons Catalyzed by Transition Metal Compounds J. C. MOLAND J. A. MOULIJN One-Component Catalysts for Polymerization of Olefins Y U . YERMAKOV AND v. ZAKHAROV

A 6 6 C D E F 6 H 1 J

7 8 9 O 1

2 3 4

5

457

The Economics of Catalytic Processes J. DEWINGAND D. S. DAVIES Catalytic Reactivity of Hydrogen on Palladium and Nickel Hydride Phases W. PALCZEWSKA Laser Raman Spectroscopy and Its Application to the Study of Adsorbed Species R. P. COONEY, G. C U R ~ O YAND S , NGUYEN THETAM Analysis of Thermal Desorption Data for Adsorption Studies AND MILOS SMUTEK, SLAVOJ CERN;, FRANTISEK BUZEK