Group 13 Chemistry I: Fundamental New Developments (Structure and Bonding, Volume 103)

103 Structure and Bonding Managing Editor: D.M.P. Mingos Editorial Board: A.J. Bard. I.G. Dance. P. Day. J.A. Ibers • ...

Author: Herbert W. Roesky | David A. Atwood

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103 Structure and Bonding Managing Editor: D.M.P. Mingos

Editorial Board: A.J. Bard. I.G. Dance. P. Day. J.A. Ibers • T. Kunitake T.J. Meyer. H.W. Roesky-J.-P. Sauvage A. Simon. F. Wudl

Springer

Berlin Heidelberg New York Barcelona Hong Kong London Milan Paris Tokyo

Group 13 Chemistry I Fundamental New Developments Volume Editors: H.W. Roesky and D.A. Atwood

With contributions by H. H6pfl, L. Mahalakshmi, P.P. Power, S. Schulz, D. Stalke

~ Springer

The series Structure and Bonding publishes critical reviews on topics of research concerned with chemical structure and bonding. The scope of the series spans the entire Periodic Table. It focuses attention on new and developing areas of modern structural and theoretical chemistry such as nanostructures, molecular electronics, designed molecular solids, surfaces, metal dusters and supramolecular structures. Physical and spectroscopic techniques used to determine, examine and model structures fall within the purview of Structure and Bonding to the extent that the focus is on the scientific results obtained and not on specialist information concerning the techniques themselves. Issues associated with the development of bonding models and generalizations that illuminate the reactivity pathways and rates of chemical processes are also relevant. As a rule, contributions are specially commissioned. The editors and publishers will, however, always be pleased to receive suggestions and supplementary information. Papers are accepted for Structure and Bonding in English. In references Structure and Bonding is abbreviated Struct Bond and is cited as a journal. Springer WWW home page: http://www.springer.de Visit the SB home page at http://link.springer.de/series/sb/or http://link.springer-ny.com/series/sb/

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Managing Editor

Volume Editors

Professor D. M i c h a e l P. M i n g o s

P r o f e s s o r H e r b e r t W. R o e s k y

Principal St. Edmund Hall Oxford OX1 4AR, UK

Institut Fir Anorganische Chemie der Universit~it G6ttingen Tammannstr. 4 37077 G6ttingen, Germany

E-mail: [email protected]

E-mail: hroeskyOgwdg.de

P r o f e s s o r Dr. D a v i d A. A t w o o d Department of Chemistry University of Kentucky Lexington, Kentucky, 40506 0055, USA

E-mail: Datwood@pop. uky.edu

Editorial Board Prof. Allen J. Bard Department of Chemistry and Biochemistry University of Texas 24th Street and Speedway Austin, Texas 78712, USA

Prof. Ian G. Dance Department of Inorganic and Nuclear Chemistry School of Chemistry University of New South Wales Sydney, NSW 2052, Australia

Prof. Peter Day, FRS Director and Fullerian Professor of Chemistry The Royal Institution of Great Britain 21 Albemarle Street London WIX 4BS, UK

Prof. James A. Ibers Department of Chemistry North Western University 2145 Sheridan Road Evanston, Illinois 60208-3113, USA

E-mail: [email protected]

E-mail: [email protected]

E-mail:[email protected]

E-mail: [email protected]

Prof. Toyohi Kunitake Faculty of Engineering: Department of Organic Synthesis Kyushu University Hakozaki 6-10-1, Higashi-ku Fukuoka 812, Japan

Prof. Thomas J. Meyer Associate Laboratory Director for Strategic and Supporting Research Los Alamos National Laboratory PO Box 1663 Mail Stop A 127 Los Alamos, NM 87545, USA

E-mail: [email protected] Prof. Jean-Pierre Sauvage Facult6 de Chimie Laboratoires de Chimie Organo-Min6rale Universit~ Louis Pasteur 4, rue Blaise Pascal 67070 Strasbourg Cedex, France

E-mail: [email protected] Prof. Fred Wudl Department of Chemistry University of California LosAngeles, CA 90024-1569, USA

E-mail: [email protected]

E-mail: [email protected]

Prof. Herbert W. Roesky Institut fiir Anorganische Chemie der Universit~it GiSttingen Tammannstra£e 4 D-37077 G6ttingen, Germany

E-mail: [email protected]

Prof. Arndt Simon Max-Planck-Institut f/Jr Festk6rperforschung Heisenbergstraige 1 70569 Stuttgart, Germany

E-mail: [email protected]

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Preface

Over the last decade our view of chemistry has evolved substantially. Whereas individual researchers previously focused on specific areas of chemistry, such as inorganic, organic, etc. we now take a more holistic approach. Effective and efficient research projects now incorporate whatever aspects of the chemistry subdisciplines that are needed to complete the intended work. The main group elements have always been used in this manner. Depending on the use of the elements, the resulting work can be described under any heading of chemistry. The group 13 elements have been special in this regard due to the very unique characters of the constituent elements. Thus, there is a dramatic change in the properties of the elements when proceeding through the series, B, A1, Ga, In, T1. This difference is one of the main reasons why these elements have seen, and continue to see, such widespread usage in such disparate applications as organic synthesis, electronic and structural materials, and catalysis, to name but a few. The widespread use and applicability, however, can be directly attributed to the deep understanding of the fundamental properties associated with these elements and their attendant compounds. The fundamental research conducted with these elements forms the foundation upon which all applications and utility are built. The present Structure and Bonding volume is part of a threepart series focused on fundamental, biological, and applied aspects of the group 13 elements. It will showcase four new areas of fundamental work that will either grow in importance in the coming years, or provide new insight into potentially new areas. The first chapter, "Structure and Bonding in Boron-Containing Macrocycles and Cages - Comparison to Related Structures with Other Elements Including Organic Molecules" is authored by Herbert H6pfl. It details the rich structural chemistry that can be orchestrated using boron as a "linker" between wellchosen organic units. An impressive range of three-dimensional compounds results from these systems. This chapter will be the seminal work in this area for many years to come. In the second chapter, Phil Power continues his remarkable and continuously successful search for new main group metal multiple bonding. The chapter is entitled: Multiple Bonding Between Heavier Group 13 Elements, and covers the astounding new developments that have occurred in this area in

VIII

Preface

only the last few years. Much of this work has resulted from Power's own activities. The vast majority of group 13 compounds are neutral. In contrast, charged compounds, and in particular systematic studies of compounds incorporating these elements are rare. Due to their relevance in catalysis some studies of anionic compounds are known. Studies of cationic compounds, however, are essentially nonexistent. This area will be addressed by L. Mahalakshmi and D. Stalke in chapter 3 entitled: The R2M+ Group 13 OrganometaUic Fragment Chelated by P-Centered Ligands. This may be, perhaps, the first review focused exclusively on cationic group 13 compounds. While the chapter itself is fundamental, the implications of the work in Lewis acid-based reactions is very clear. This chapter is surely a signal for others to begin exploring this unique and interesting area. Group 13-15 element compounds are now fairly commonplace semiconducting materials, usually incorporating phosphorus or arsenic. A great deal, however, remains to be discovered in group 13 combinations with the heavier group 15 congeners, Sb and Bi. Stephan Schulz is at the leading edge of this new, very difficult, area of endeavor. His chapter, the fourth in this book, "Synthesis, Structure and Reactivity of Group 13/15 Compounds Containing the Heavier Elements of Group 15, Sb and Bi" clearly outlines the unique nature of this chemistry, and provides a "road-map" for any other researchers wishing to get into this area. Although containing only four chapters, this book really represents the "tip of the iceberg" for fundamental research in these areas. Utilizing the information clearly disseminated in these pages, many other researchers can build and improve upon their own fundamental and applied group 13 science. lune 2002

H.W. Roesky David A. At-wood

Contents

Structure and Bonding in Boron-Containing Macrocycles and Cages - Comparison to Related Structures with Other Elements Including Organic Molecules H. H6pfl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Multiple Bonding Between Heavier Group 13 Elements P. P. Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

57

The R2M+ Group 13 Organometallic Fragment Chelated by P-Centered Ligands L. Mahalakshmi, D. Stalke . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

85

Synthesis, Structure and Reactivity of Group 13/15 Compounds Containing the Heavier Elements of Group 15, Sb and Bi S. Schulz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

117

Author Index Volumes 101-103 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

167

Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

169

Contents of Volume 104 Group 13 Chemistry II Biological Aspects of Aluminum

Volume Editors: H.W. Roesky, D.A. Atwood Acute Aluminum Intoxication K. Berend, G.B. van der Voet, F.A. de Wolff A New Effect of Aluminum on Iron Metabolism in Mammalian Cells S. Oshiro The Complexity of Aluminum-DNA Interactions: Relevance to Alzheimer's and Other Neurological Diseases S. Anitha, K.S.J. Rao Aluminum: Interaction with Nudeotides and Nudeotidases and Analytical Aspects of its Determination M.R.C. Schetinger, V.M. Morsch, D. Bohrer Aluminofluoride Complexes in the Etiology of Alzheimer's Disease A. Strunecka, J. Patocka Fluoroaluminate Chemistry B. Conley, D.A. Atwood

Contents of Volume 105 (preliminary)

Group 13 Chemistry III Industrial and Applied V o l u m e Editors: H.W. R o e s k y , D.A. A t w o o d Borates in Industrial Use D.M. Schubert Aluminum and Gallium Hydrazides W. Uhl The Synthesis and Structural Properties of Aluminum Oxide, Hydroxide and Organooxide Compounds D.J. Linton, A.E.H. Wheatley Insertion and I]-hydrogen Transfer at Aluminum P.H.M. Budzelaar Higher Coordinate Group 13 Compounds D.A. Atwood

Structure and Bonding in Boron-Containing Macrocycles and Cages ± Comparison to Related Structures with Other Elements Including Organic Molecules Herbert HoÈp¯ Centro de Investigaciones QuõÂmicas, Universidad AutoÂnoma del Estado de Morelos, Av. Universidad 1001, C. P. 62210 Cuernavaca, Mexico e-mail: hhop¯@buzon.uaem.mx

The present revision describes the generation of macrocyclic, cage-like and supramolecular structures incorporating three- or four-coordinate boron atoms. Preparative strategies are discussed and include, aside from traditional preparative methods known from organic chemistry, one-step syntheses by self-assembly and template syntheses. Emphasis is placed on chemical and structural analogies to well-known related structures from organic, coordination and organometallic chemistry. Applications of boron-containing hosts in ionic and molecular recognition, including chiral recognition, as well as selective molecular transport through lipophilic membranes are discussed. Keywords: Boron, Macrocycle, Cage, Supramolecular chemistry, Molecular recognition

1

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

2

2

Boron Compounds with Macrocyclic Structures . . . . . . . . . . . .

4

2.1 2.1.1 2.1.2 2.1.3

Calixarene-Type Macrocycles . . . . . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Borazine Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-(Diethylboryl)pyridines and (3-Aminophenyl)boronic Acid Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Porphyrinogen and Porphyrin-Type Macrocycles . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Triazolyl- and Imidazolylboranes . . . . . . . . . . . . . . . . . . . . . Boron-Bridged Tetrathiaporphyrinogens . . . . . . . . . . . . . . . . Porphyrin-Type Macrocycles . . . . . . . . . . . . . . . . . . . . . . . . Crown Ether-Type Macrocycles . . . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (Acyloxy)boranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (Acylamino)boranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (Phosphinoyloxy)-, (Phosphorylamino)-, (Sulfonyloxy)and (Seleninoyloxy)boranes . . . . . . . . . . . . . . . . . . . . . . . . . Cyclophane-Type Macrocycles . . . . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Borates and Boronates from Bidentate Ligands . . . . . . . . . . . Borates and Boronates from Tridentate Ligands . . . . . . . . . .

.. .. ..

4 4 5

. . . . . . . . . .

. . . . . . . . . .

6 8 8 8 11 11 12 12 13 15

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. . . . .

17 17 17 17 18

2.2 2.2.1 2.2.2 2.2.3 2.2.4 2.3 2.3.1 2.3.2 2.3.3 2.3.4 2.4 2.4.1 2.4.2 2.4.3

Structure and Bonding, Vol. 103 Ó Springer-Verlag Berlin Heidelberg 2002

2

H. HoÈp¯

3

Boron Compounds with Cage-Like Structures . . . . . . . . . . . . .

23

3.1 3.2 3.3 3.4

Borate Cages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Borasiloxane Cages . . . . . . . . . . . . . . . . . . . . . . . . . Borophosphonate Cages . . . . . . . . . . . . . . . . . . . . . . Vanadium Borate and Borophosphate Cluster Anions

. . . .

23 24 27 29

4

Application of Boron-Containing Hosts in Ionic and Molecular Recognition . . . . . . . . . . . . . . . . . . . . . . . . . . .

31

4.1 4.1.1 4.1.2 4.1.2.1 4.1.2.2

35 37 38

4.1.3.1 4.1.3.2 4.2 4.2.1 4.2.2

Metal Complexes with Boron-Containing Hosts . . . . . . . . . . . . Boron-Containing Pseudocryptates . . . . . . . . . . . . . . . . . . . . . . Borylated Bis(dioxime)metal Complexes and Related Compounds Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Complexes of the M(dioxime-BR2)2 and M(dioxime-BR2)2L Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Complexes of the RCo(dioxime-BR2)2L and Fe(dioxime-BR2)2LL¢ Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . Complexes of the M(LBR2)X Type . . . . . . . . . . . . . . . . . . . . . . Other Related Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . Borylated Tris(dioxime)metal Complexes and Related Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Complexes of the M[RB(dioxime)3BR] Type . . . . . . . . . . . . . . . Other Related Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . Molecular Recognition by Boronic Acid Derivatives . . . . . . . . . Molecular Recognition by Monoboronic Acid Derivatives . . . . . Molecular Recognition by Diboronic Acid Derivatives . . . . . . . .

5

Conclusions and Perspectives . . . . . . . . . . . . . . . . . . . . . . . . .

47

6

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

47

4.1.2.3 4.1.2.4 4.1.2.5 4.1.3

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

31 31 33 33 33

39 39 40 41 41 43

1 Introduction At present, supramolecular chemistry and the related construction of macrocyclic and cage-like assemblies are among the most studied scienti®c topics and interest in these systems lie in the ®elds of chemistry, biology, physics, and material science [1]. Four principal synthetic strategies can be applied for the preparation of macrocyclic or cage-like assemblies: (i) (ii) (iii) (iv)

traditional organic synthesis with formation of covalent bonds [2], organic synthesis with templates [2, 3], metal coordination of bi-, tri-, or oligodentated ligands [4], and hydrogen-bonding between suitable donor-acceptor fragments [5].

Organic synthetic methods have one big advantage over the other two preparative strategies and that is the formation of strong covalent bonds that

Structure and Bonding in Boron-Containing Macrocycles and Cages

3

normally give thermodynamically stable products. However, the synthesis of large, discrete supermolecules becomes increasingly dif®cult, as the scale and complexity of the target molecules increase. Thus, the ®nal yields of the desired purely organic macrocyclic or cage-like products can be quite low. Coordinative and hydrogen bonds are much weaker than covalent bonds, but the advantage of metal coordination and hydrogen-bonding directed assembly is frequently the kinetic instability of intermediate products that permits the occurrence of reorganizing processes in order to conduct the whole reaction to one unique product, which is the thermodynamically most favored one [4n,4r]. The disadvantage of having thermodynamically weaker coordinative or hydrogen bonds in these assemblies can be overcome by the formation of more than one of these bonds at a time. This strategy has opened a completely new ®eld in chemistry and month for month new, exciting supramolecular assemblies are reported, which often are obtained quantitatively and with high thermodynamic stability. Boron being a non-metal forms boron-oxygen and boron-nitrogen bonds of mainly covalent character with bonding energies of approximately 536 (BAOR) and 446 kJ/mol (B3N3H3Cl3), respectively. The corresponding carbon-oxygen and carbon-nitrogen bonds are weaker [6]. In spite of the strong bonding energies, three-coordinate boron complexes can be quite reactive with nucleophilic reagents because of the `vacant' p-orbital on boron. Thus, as long as a boron-complex is three-coordinate, reversible reactions and transformations into other three-coordinate derivatives can occur with relatively low activation energy barriers. In four-coordinate boron complexes the reactivity is drastically reduced like in the corresponding carbon analogues. The fourth bond in four-coordinate boron complexes can be a covalent or a coordinative bond with a basic donor atom, e.g., nitrogen, oxygen, sulfur, or phosphorus [7]. The strengths of coordinative D ® B bonds are generally much weaker than those of the corresponding covalent bonds, e.g., with nitrogen donors the dative bond energy has values between 50 and 150 kJ/mol [8]. Nevertheless, Lewis bases frequently allow for a stabilization of initially three-coordinate boron complexes, especially when they form part of a chelate ring, and this stabilization is adjustable by the D ® B bond strength. From these considerations, an interesting strategy for the synthesis of stable macrocyclic or cage-like assemblies can be developed. That is the initial use of three-coordinate boron compounds for the construction of supramolecular assemblies with all the advantages outlined above for metal coordinative assemblies, i.e., formation of labile bonds in intermediate products, possibility of structural reorganization, etc. Afterwards, if necessary, the so-formed macrocyclic or cage-like assembly can be stabilized by a coordinative D ® B bond with all advantages of a frame formed uniquely by covalent bonds, in this case BAX bonds (X = O, N, S, P, etc.), i.e., high thermodynamic stability, molecular ¯exibility, etc. So far, little attention has been paid to these advantageous aspects of boron chemistry and, therefore, only a limited number of macrocycles or cages

4

H. HoÈp¯

incorporating boron is known. Moreover, many of these compounds have been obtained accidentally or have been prepared without a useful strategy for the design of new, interesting chemical compounds and materials. With these observations in mind, the aim of the following review is three-fold: (i) to classify and organize all chemistry related to this ®eld, (ii) to describe similarities with organic, coordination and organometallic compounds, and (iii) to present already known applications in ionic and molecular recognition as well as selective molecular transport through lipophilic membranes in order to vision future developments.

2 Boron Compounds with Macrocyclic Structures 2.1 Calixarene-Type Macrocycles

2.1.1 Introduction Calixarenes are cyclophane-type molecules, in which at least four aromatic units are joined via methylene or related groups [9]. It is known that calix[4]arenes with hydroxy groups in the 2-position normally prefer a conelike conformation, however, depending on the substituents a more or less rapid dynamic interconversion with three possible partial cone conformations is possible (Fig. 1) [9, 10]. The preferred conformation of the unsubstituted calixarene shown in Fig. 1 (X = CH2) is 1,3-alternating, whereby two opposite rings are perpendicular to the plane formed by the four methylene groups and the other two are parallel to the same plane [11]. In the case of calix[6]arenes the conformational mobility is enhanced and 1,3,5-alternating or 1,4-alternating `winged' conformations have been identi®ed as the preferred ones [9]. Very few heteratom-

Fig. 1. Known calixarenes 1 and possible conformations for calix[4]arenes

Structure and Bonding in Boron-Containing Macrocycles and Cages

5

bridged calixarene derivatives have been structurally characterized [12±14], and part of this chemistry has been reviewed recently [15]. 2.1.2 Borazine Derivatives One of the ®rst reports related to the preparation of macrocyclic ring systems with boron atoms deals with the transformation of m-difunctionalized borazines into cyclic di-, tetra-, or hexaborazines [16±19]. The cyclic dimeric borazine derivatives 2 and 3 can be prepared by reaction of 2,4-dichloro-1,3,5,6-tetramethylborazine with aromatic 1,3-dihydroxy and 1-amino-3-hydroxy compounds (Fig. 2). The products are obtained in yields

Fig. 2. Di-, tetra-, and hexameric borazine macrocycles

6

H. HoÈp¯

of 33±48% after puri®cation by sublimation. Attempts to get pure samples of analogous compounds with aromatic 1,3-diamino, 1,3-dithiol, 1-amino-3-thiol or 1,3-xylylene derivatives failed. The macrocycle formation is also inhibited when 2-methylresorcinol is used as bridging ligand instead of the unsubstituted resorcinol [16]. By reaction of 2-alkyl-4,6-dichloro-1,3,5-trimethylborazines (alkyl = methyl, ethyl, i-propyl) with bis(trimethylsilyl)amine the tetrameric borazine ring systems 4±6 are produced (Fig. 2); they can be puri®ed by several successive vacuum sublimations (yields: 4±60%). If the borazines carry n-propyl and tertbutyl groups in the 2-position or if methylbis(trimethylsilyl)amine is used to bridge the borazine molecules, the macrocyclic ring formation is inhibited [17, 18]. In a similar way, reaction of 2,4-dichloro-1,3,5,6-tetramethylborazine with N,N-dimethylformamide and dimethylamine gives a 36-membered macrocyclic oligoborazine 7 with six borazine rings linked by oxygen bridges (Fig. 2). Again, puri®cation through several vacuum sublimations is necessary and yields are quite low [19]. Considering the relationship between borazine and benzene, the abovementioned systems could be considered as cyclophane or calix[n]arene derivatives (n = 4, 6). 1HNMR spectra of all macrocyclic borazines described above show only one set of signals for the 2, 4, and 6 borazine moieties, respectively, which may be indicative of a highly symmetric ring conformation or a rapid interconversion between other possible conformers [16, 17, 19]. Unfortunately, low temperature NMR has not been realized in order to clarify the conformational preference of these ring systems. For the hexameric borazine ring system a lowering of the local symmetry can be recognized from the IR spectra and a crown-like ring conformation with a 3-fold symmetry axis is proposed by molecular modeling [19]. 2.1.3 3-(Diethylboryl)pyridines and (3-Aminophenyl)boronic Acid Derivatives Although 3-(diethylboryl)pyridine has been known since 1983 [20], it was not until 1994 that it was discovered to have a tetrameric structure both in solution (CHCl3, THF) and in the solid state (Fig. 3) [21, 22]. The tetrameric macrocycle 8 is formed via intermolecular coordinative N ® B bonds that are strong enough to keep the heteromacrocyclic ring system intact in solution, even at higher temperatures and in the presence of relatively strong amines like piperidine, quinuclidine or N,N-dimethylaminopyridine. The tetramer is also stable in a mixture of THF and water (up to 33%). The solid-state structure of this compound can be considered as a heteroatomic calixarene derivative with C2-symmetry and an approximate cone conformation, in which an opposite pair of the four pyridine rings is perpendicular to the molecular plane, while the other one has a more parallel orientation to this plane. The carbon-carbon distances between the parallelÊ and the shortest carbonorientated pyridine rings range from 4.23 to 5.12 A Ê carbon distance between the other pair is 5.80 A [21]. In contrast to compound

Structure and Bonding in Boron-Containing Macrocycles and Cages

7

Fig. 3. Dimeric and tetrameric macrocyclic structures obtained from (diethylboryl)pyridines

8, the solid-state structure of compound 9 [22] has a conformation that is related to the structure of the unsubstituted calix[4]arene shown in Figure 1 [11]. Attempts to prepare macrocyclic structures from a mixture of compounds 10 and 11 failed and only tetrameric (8) and dimeric (12) structures with a sixmembered C2N2B2 heterocycle can be isolated (Fig. 3) [23]. A structurally related tetrameric macroheterocycle is compound 13 that is prepared in a one-pot synthesis (yield 64%) from salicylaldehyde and (3-aminophenyl)boronic acid in methanol (Fig. 4). Due to its insolubility it has been characterized only by mass spectrometry. If a substitutent is introduced at the imine function (R = Me, Ph), trimeric structures (14 and 15) are

Fig. 4. Tetrameric 13 and trimeric 14, 15 macrocyclic structures obtained from

(3-aminophenyl)boronic acid derivatives

8

H. HoÈp¯

produced, possibly because of a repulsive steric interaction between this substituent and a neighboring B-phenyl group. The triboronate 14 is slightly soluble and has been studied by 1H-, 13C-, and 11B-NMR spectroscopy as well as X-ray crystallography. Both studies con®rm a completely asymmetric, 15membered ring system with a hydrophobic cavity. In the crystal lattice the trinuclear complexes are stacked along one of the crystallographic axes producing in®nite channels, whereby the cavity is occupied by one of the three B-methoxy groups [24]. 2.2 Porphyrinogen and Porphyrin-Type Macrocycles

2.2.1 Introduction The scienti®c interest in porphyrin ligands (Fig. 5) derives in part from their ability to accommodate a large series of different elements, often in various oxidation states. On the other hand porphyrins are planar molecules with a delocalized 18 p-electron system and a diatropic ring current [25], which makes them interesting for the design of new materials with applications in photochemistry [25±27]. In contrast, porphyrinogens 16 (Fig. 5) possess only 16 p-electrons and as a consequence the delocalization over the whole macrocycle is absent. The conformations of porphyrinogens are not planar any more and can approximate to the conformations known for calix[4]arenes (vide supra). Compounds 16 (Fig. 5) may, therefore, be considered as heteroatomic calixarene derivatives, of which some have additional heteroatoms in the bridging positions [15, 28±30]. 2.2.2 Triazolyl- and Imidazolylboranes Triazolylboranes form intermolecular complexes by the preferential engagement of non-adjacent nitrogen atoms as the linking sites to boron. Both

Fig. 5. Porphyrin and porphyrinogens 16

Structure and Bonding in Boron-Containing Macrocycles and Cages

9

Fig. 6. The triazolylboranes 17 and 18 can be considered as porphyrinogen analogues

dimethyl(1,2,3-triazol-1-yl)borane 17 and dimethyl(benzotriazol-1-yl)borane 18 exist as tetrameric macrocycles in the solid state (Fig. 6), however, with different conformations [31, 32]. In both cases there is no evidence for localization of the formally single and double NAN bonds, and the same is true for the formally covalent and coordinative NAB bonds. Compound 17 adopts a conformation with approximate D2d-symmetry, in which the ®ve-membered rings are 1,2-alternately bent up and down from the plane of the bridging boron atoms. Theoretical calculations (ASED-MO) have shown that this conformation probably corresponds to the global minimum of the potential-energy surface and that cone-like conformations are energetically much less favored [32]. Compound 18 adopts a 1,3-alternating conformation with Ci-symmetry, in which one pair of triazole rings is nearly parallel to the plane formed by the boron atoms, while the other one is oriented almost perpendicular to the same plane. The N¼N distances across the macrocyclic Ê for 17 and 4.89/5.14 A Ê for 18, respectively, however, cavities are 4.92 A theoretical considerations predict that a coordination of the four apical nitrogen atoms with a metal atom would be unlikely in a square-planar geometry [32]. As in the case of the tetrameric macrocycles discussed above, compounds 17 and 18 can be considered as porphyrinogen analogues or as heteroatombridged heteroaromatic calix[4]arene derivatives. The two different conformations observed for 17 and 18 have analogues in compounds 16 (Fig. 5) [15, 28±30]. Recently, the chemistry of imidazolylboranes has been revisited with some very interesting observations [33]. One of the factors that induces the formation of macrocyclic ring systems instead of non-cyclic oligomers or polymers is the substitution on the boron atom, e.g., 1-imidazolyldiethyl- and ~ diphenylborane give chain-like arrangements [34, 35], while 1-imidazolyldihydroborane and ~ dimethylborane give macrocyclic tetrameric, pentameric or even higher-membered (R = R¢ = Me) systems (Fig. 7) in high yields (85% for 19±23).

10

H. HoÈp¯

Fig. 7. Tetrameric and pentameric imidazolylboranes 19±23

The tetrameric macrocycle of 21 has Ci-symmetry and its conformation is similar to that of 18 with one opposite pair of imidazole rings in the plane of the molecule, and the other pair perpendicular to it. In 22 all ®ve imidazole rings are almost perpendicular to the molecular plane, so that a CH2Cl2 molecule can be included in the cavity of the pentamer. Unfortunately, compounds 19±23 are unstable under in¯uence of air and moisture [33]. From dihydrobis(1-imidazolyl)borate or tetrakis(1-imidazolyl)borate and silver salts the interesting mixed tetrametallic macrocycles 24 and 25 (Fig. 8) are prepared in high yields (40±60%) [36]. Thereby, the presence of tertiary monophosphines is critical, because both polymeric (26) and cyclic tetrameric (24 and 25) complexes can be obtained depending on the phosphine added to the reaction mixture. Furthermore, with some of the phosphines rapid decomposition and deposition of metallic silver occurs. In the macrocyclic molecules 24 and 25 a pair of silver atoms is bridged by two bis(1-imidazolyl)borate moieties, with a transannular Ag¼Ag Ê for 24 and 8.89 A Ê for 25. The conformations of 24 and 25 are distance of 8.61 A

Fig. 8. Tetrameric 24 and 25 and polymeric 26 complexes between silver(I) salts and bis (1-imidazolyl)borates

Structure and Bonding in Boron-Containing Macrocycles and Cages

11

centrosymmetric and analogous to that of 21. In solution partial dissociation through breaking of both AgAN and AgAP bonds takes place, while apparently the BAN bonds stay intact. This is an important observation that indicates a higher kinetic stability of BAN bonds (in tetracoordinated BN4 complexes) in comparison to AgAN bonds, which are widely used for the construction of supramolecular arrays [37, 38]. In the Cambridge Structural Database [39] only two macrocyclic molecules with transition metals, in which the metal ions are joined only by imidazolyl units, have been reported. One structure is trimetallic and contains platinum(II) [40a] and the second one is tetrametallic with copper(II) ions [40b]. 2.2.3 Boron-Bridged Tetrathiaporphyrinogens The boron-bridged tetrathiaporphyrinogen 28 can be prepared in good yields (62%) from the dilithiated di-2-thienylborane amine 27 (Fig. 9). In the solid-state the macrocycle has a cone-shaped conformation with S4-symmetry, however, the four boron atoms are not in the same plane. This conformation is different to the ones discussed above for the triazolyl- and Ê. imidazolylboranes. The distance between opposite sulfur atoms is 5.01 A Compound 28 is an interesting species, because the boron atoms are only tricoordinated. Therefore, their empty pz-orbitals may allow for p-electron conjugation and two-fold reduction might generate a delocalized 18 p-electron species with a diatropic ring current [25]. Unfortunately, both X-ray crystallography (non-planarity) and 11B-NMR spectroscopy (d = 40 ppm) of 28 exclude the possibility of any delocalization of the heteroaromatic ring current through the boron atoms [41]. 2.2.4 Porphyrin-Type Macrocycles So far, only one completely planar porphyrin-type macrocycle with boron atoms 29 has been reported (Fig. 9) [42]. This molecule has D4h-symmetry and is composed of four ®ve-membered B2S3 rings that are linked through the

Fig. 9. Reaction of Cl2BNiPr with the dilithiated di-2-thienylborane amine 27 gives the

tetrathiaporphyrinogen 28. Compound 29 is a porphyrin-type macrocycle

12

H. HoÈp¯

Ê . The boron atoms by sulfur bridges. The transannular S¼S distance is 4.67 A BAS bond lengths to the inner sulfur atoms are shorter than the ones to the Ê ) is still outer sulfur atoms, but the average BAS bond length (1.811 A indicative of strong pp-pp interactions [43]. In spite of the structural relationship with porphyrin, the two systems are not isoelectronic, B8S16 (29) having 32 p-electrons while porphyrin has only 26. Theoretical studies (extended HuÈckel calculations) have demonstrated that the extra electrons in B8S16 occupy molecular orbitals that are more antibonding than bonding, resulting in a smaller p-delocalization energy. Furthermore, the HOMO-LUMO gap is much larger in agreement with the white color of this compound [44]. Nevertheless, it has also been predicted that complexes with copper(II) might be about as stable as the corresponding porphyrin complex [45]. 2.3 Crown Ether-Type Macrocycles

2.3.1 Introduction The functional groups outlined in Fig. 10 are all bifunctional ligands that can, at least from a theoretical point of view, form monomeric, dimeric, or oligomeric complexes with metal ions or a diorganoboryl group. In all cases a delocalization of the p-electron density through the central carbon, phosphorus, sulfur, or selenium atom can take place and in the case of a delocalized system the bond energies between the donor atoms of the ligand (X = O, N) and the metal ion or the boryl group will be intermediate between the strengths of corresponding covalent and coordinative bonds. In other words, in dimeric or oligomeric ring systems formed with the bidentate ligands outlined in Fig. 10, the thermodynamic and kinetic stability of the cyclic structure should be enhanced in those cases in which the p-electron density is delocalized in the bidentate ligand (Fig. 11).

Fig. 10. Functional groups that may form monomeric, dimeric or oligomeric complexes with metal ions or a diorganoboryl group

Structure and Bonding in Boron-Containing Macrocycles and Cages

13

Fig. 11. The EAX bond energy should be enhanced in ligands which are delocalized

So far, only for the ®rst two of the six ligand types outlined in Fig. 10 have oligomeric structures with boron been reported. This may be in part due to the relatively small number of publications related to this ®eld and the fact that a systematic attempt to prepare boron macrocycles with these ligands has not been realized so far. The following structures will show that macrocyclic rings between metal ions or BR2 units and acyloxy or acylamino groups have some relationship with rigid coronands [1c]. 2.3.2 (Acyloxy)boranes Acyloxy derivatives of boron, R2C(O)-OBR¢2 (R = alkyl, aryl; R¢ = alkyl, aryl, F, OAc), can be considered as anhydrides of borinic R2B(OH), boronic RB(OH)2 and boric acid derivatives, respectively, with carboxylic acids. Most of them have been obtained in high yields (»90%) from reaction of the corresponding carboxylic acid with triethylborane or 9-BBN [46, 47]. Interestingly, no reduction of the carboxylic group occurs in the latter reaction. The ®rst published data available on this class of compounds suggest the presence of a monomeric structure I or II in the gas phase [48] and a dimeric structure III with an eight-membered B2O4C2 ring in solution and the solid-state (Fig. 12) [49]. More recent 11B-NMR studies at room temperature have shown that (acyloxy)diethylboranes are monomeric also in non-polar solvents like chloroform [d(11B) » 60 ppm] [46]. Under the same conditions the corresponding 9-BBN derivatives are monomeric (both types I and II) or dimeric, depending on the electron-donating or -withdrawing effect of the substituent on the carboxyl group, the temperature, and the concentration of the

Fig. 12. Possible monomeric (I and II) and dimeric (III) structures for (acyloxy)boranes in

gas phase, solution and solid-state

14

H. HoÈp¯

compound in solution. However, for the dimeric structures the association is only loose [d(11B) = 28 ppm]. These results have been con®rmed by IR studies in solution [46, 49]. X-ray crystallographic studies reveal that the central eight-membered ring in the dimeric (acyloxy)borane obtained from 9-BBN and benzoic acid has a chair conformation with Ci-symmetry, in which the six atoms of the two carboxyl groups form a perfect plane [46]. In contrast, the corresponding derivative of trimethylacetic acid has a twisted boat conformation with C2symmetry that originates from the steric demand of the tert-butyl groups [47]. Interestingly, in the ®rst case all four BAO bond lengths are identical Ê ] thus indicating complete delocalization of the carboxylates, while [1.555(3) A in the second case there is a signi®cant difference between two pairs of BAO Ê ]. bond lengths [1.546(7) and 1.572(7) A The 2:1 reaction of 9-BBN with a series of dicarboxylic acids, namely oxalic acid, malonic acid, 2,2-dimethylmalonic acid, and succinic acid, in dimethoxyethane gives in some cases dimeric and in other cases macrocyclic (acyloxy)diorganoboranes. This has been proved by IR spectroscopy (all C = O groups are bidentate), 11B-NMR (d = 10 ppm) and X-ray crystallography [47]. With oxalic acid two structures are possible (IV and V), of which the ®rst with a ®ve-membered boron heterocycle instead of a four-membered one is the more probable formulation (Fig. 13).

Fig. 13. Possible structures for (acyloxy)boranes formed from the reaction of 9-BBN with oxalic acid and solid state structure of the complex with 2,2-dimethylmalonic acid 30

Structure and Bonding in Boron-Containing Macrocycles and Cages

15

Bis(9-borabicyclo[3.3.1]nonyl)-2,2-dimethylmalonate 30 is a tetrameric 32-membered macrocyclic ring system with crystallographic C2- and approximate D2-symmetry, in which two different coordination modes of the 9-borabicyclo[3.3.1]nonyl units are apparent (Fig. 13). Half of the 9-borabicyclo[3.3.1]nonyl moieties are each complexed to two of the four oxygen atoms from the malonic acid to form a six-membered 1,3,2-dioxaborinane ring, while the other half act as bridging units to join these BO2C3 heterocycles between each other through the remaining two oxygen atoms of the dicarboxylic acid. Unfortunately, the structural data are of low quality in this case, so that the BAO bond lengths cannot be compared in order to determine if there is delocalization in the carboxylate groups. Similar structures may form with malonic and succinic acid [47]. That carboxylates act as bridging ligands between two metal ions is widely known and metal analogues to the eight-membered B2O4C2 rings discussed above are uncountable. The union of four metal ions by four carboxyl bridging moieties to a macrocyclic ring system is less frequent, but examples have been reported repeatedly during the past two decades. Such complexes are known with monocarboxylic acids, e.g., with platinum(II) [50], rhenium(I) [51], zinc(II) [52], ytterbium(II) [53], tin(IV) [54], bismuth(III) [55], copper(I) [56], and silver(I)/platinum(II) [57], as well as dicarboxylic acids like oxalic acid, malonic acid or maleic acid, e.g., with ytterbium(III) [58], copper(II) [59], chromium(III)/lanthanum(III) [60], chromium(III)/cerium(III) [60], manganese(II) [61], and manganese(II)/ iron(III) [62]. Macrocyclic structures with three or six central metal atoms are still more rare and have been observed only for palladium(II) [63] and iron(II)/ manganese(II) [64]. 2.3.3 (Acylamino)boranes Amides are derivatives of carboxylic acids, so that their coordination behavior to boranes might be similar to that of their parent compounds. 11B-NMR spectroscopic studies have shown that compounds 31 and 32 are monomeric species in solution, while compounds 33 and 34 with the more Lewis acidic 9-borabicyclo[3.3.1]nonyl unit form aggregates that may be dimeric, oligomeric, or polymeric. The grade of association could not be determined by mass spectrometric analyses, because in all cases only the monomer is liberated into the gas phase [65].

16

H. HoÈp¯

The solid state structure of 33 reveals a dimeric molecule with an eightmembered B2C2N2O2 heterocyclic ring in a chair conformation. The NAB and Ê , respectively, as well as the CAN BAO bond lengths of 1.585(2) and 1.541(2) A Ê , respectively, indicate and CAO bond lengths of 1.294(3) and 1.285(3) A delocalization of the p-electron density in the acylamino moiety [65]. During the study of the reaction products between six different x-lactams and triethylborane, monomeric (for 36, 39), dimeric (for 38), tetrameric (for 40) and oligomeric (for 35) species have been detected in solution by cryoscopic methods. For 37 [X = -CH2)5-] only lateral reaction products could be observed (Fig. 14) [66]. Apparently, the ring sizes of the x-lactams, which must result in different ring conformations, play a key role in the formation of higher aggregates. Only compound 38 could be characterized by X-ray crystallography showing the presence of a symmetric eight-membered B2C2N2O2 ring in a chair conformation [65] similar to the structure formed with benzoic acid (vide supra). In the light of the CAN and CAO bond lengths it can be demonstrated that there is some delocalization of the p-electron density, however, with a dominance of a C@N instead of a C@O bond. Therefore, dimer 38 must be considered as cislactim derivative (Fig. 14). For the tetrameric (40) and oligomeric (35) (acylamino)diethylboranes trans-lactim con®gurations (Fig. 14) have been proposed [66].

Fig. 14. Dimeric 38 is a cis-lactim derivative, while tetrameric 40 and oligomeric 35 are trans-lactim derivatives with a partially delocalized NACAO central moiety. 36 and 39 are monomeric complexes

Structure and Bonding in Boron-Containing Macrocycles and Cages

17

Some similar bimetallic acylamino complexes are also known with transition metal ions, e.g., with vanadium(II) [67], palladium(II) [68], and especially platinum(II) [69]. In the Cambridge Structural Database [39] only one trimetallic structure is found in which three iron(II) ions are bridged by a total number of six acylamino ligands [70]. 2.3.4 (Phosphinoyloxy)-, (Phosphorylamino)-, (Sulfonyloxy)- and (Seleninoyloxy)boranes A series of (phoshinoyloxy)- [71], (phosphorylamino)- [72], (sulfonyloxy)[73±78] and (seleninoyloxy)boranes [79] are also known, however, only monomeric or dimeric molecules with similar structures and conformations to I±III (Fig. 12) have been reported so far. 2.4 Cyclophane-Type Macrocycles

2.4.1 Introduction Cyclophanes or p-spherands have played a central role in the development of supramolecular chemistry forming an important class of organic host molecules for the inclusion of metal ions or organic molecules via p-p interactions. Particular examples are provided by their applications in synthesis [80], in the development of molecular sensors [81], and the development of cavities adequate for molecular reactions with possible applications in catalysis [82]. The classical organic synthesis of cyclophanes can be quite complex [83], so that the preparation of structurally related molecules via coordination or organometallic chemistry might be an interesting alternative. 2.4.2 Borates and Boronates from Bidentate Ligands While 1,2- and 1,3-alkanediols form only monomeric 1,3,2-dioxaborolanes and 1,3,2-dioxaborinanes with triethylborane, 1,4- and 1,5-alkanediols give mainly dimeric macrocyclic boronates 41 and 42, which are obtained in yields of 70±75% after puri®cation by vacuum distillation. As lateral products oligoand polymeric molecules are formed. The main products have a 14- and 16-membered heterocyclic ring, respectively (Fig. 15) [84]. In solution a complex mixture of monomeric, dimeric, and oligomeric boronates can be detected that are in equilibrium between each other, probably via ligand exchange reactions. The association grade determined by cryoscopic measurements has an average value of 2±3 depending on the concentration of the compounds in solution. During vacuum distillation the equilibrium is apparently displaced in the direction of the smaller aggregates due to their higher volatility [85].

18

H. HoÈp¯

Fig. 15. Macrocyclic diboronates of alkanediols (41 and 42) and a bisphenol derivative (43

and 44)

Dimeric diboronates 43 and 44 are obtained if the dithioboronic ester RB(SBu)2 (R = nBu, Ph) is heated with bisphenol HOC6H4CMe2C6H4OH (Fig. 15) [86]. 2.4.3 Borates and Boronates from Tridentate Ligands Salicylideneamino alcohols are easily prepared ligands that react readily with arylboronic acids to form the corresponding esters, which may be monomeric 45±49 or dimeric 50±59 (Fig. 16) [87±94].

Fig. 16. Salicylideneamino alcohols react with arylboronic acids to form monomeric (45±49)

or dimeric complexes (50±59)

Structure and Bonding in Boron-Containing Macrocycles and Cages

19

All compounds of this type are isolated in high yields by one-step syntheses (normally more than 70%) and are air-stable due to the presence of N ® B coordinative bonds. A comparative X-ray crystallographic study has shown that in the case of compounds 50±56 the dimeric structure is produced preferably, in order to avoid the annular ring strain in the monomeric species that would be related to the junction of a planar heterocyclic BNOC3 ring to a non-planar ®ve-membered BNOC2 ring through a boron atom with tetrahedral geometry [87, 88]. In the case of compound 47 a dimeric structure cannot be obtained for steric reasons [89]. The transannular strain is released in compound 46 and only the existence of a monomeric complex can be proved. If the ring size is enhanced further to a seven-membered ring, then an equilibrium between the monomeric and the dimeric complex 57 exists, whereby the dimeric species seems to be the kinetic product [94]. Dimeric species are also produced with ligands that have ®ve (58) or six (59) methylene groups between the imino and the hydroxy groups [88]. The formation of dimeric products is unique for the case of boron, because analogous complexes with other elements are all monomeric [95]. This can be attributed to the small covalent radius of the boron atom and its tetrahedral geometry in four-coordinate boron complexes. Molecular modeling shows that bipyramidal-trigonal and octahedral coordination geometries are more favorable for the formation of monomeric complexes with these ligands. A total number of eight dimeric structures with a central ten-membered heterocycle, but different substituents in the ligand, has been characterized by X-ray crystallography [87, 88, 91] and interestingly all of them have crystallographic Ci-symmetry. The central ten-membered heterocycle B2N2O2C4 has a chair-boat-chair conformation and in all cases there are intramolecular CAH¼O transannular interactions with distances between 2.4 Ê . For comparison, the sum of the van der Waals radii between oxygen and 2.6 A Ê [96]. The N ® B bond lengths have values between and hydrogen is 2.70 A Ê , which are in the upper range of bond lengths found for 1.609(8) and 1.64(1) A diphenylboron chelates with salicyclaldehyde azomethines [1.572(2) to Ê ] [97]. 1.634(5) A Based on the same synthetic strategy compounds 60±67 can be prepared (Fig. 17). Yields are generally high (69±98%) and the products are air-stable, but have only low solubility. Molecules of both structural types have been analyzed by X-ray crystallography (60 and 66). Both examples have Ci-symmetry, so that the con®gurations of the two chiral boron atoms are RS or SR, respectively. The central macrocyclic units consist of a 14-membered B2N2O2C8 and an 18-membered B2N2O2C12 heterocyclic ring for 60 and 66, respectively. The nearest B¼B, Ê, O¼O, and C¼C distances in compound 60 are 6.35, 4.29, and 3.84 A respectively, in compound 66 the corresponding distances are 8.26, 7.02, and Ê . Strong intramolecular CAH¼O interactions in the cavity of 60 are 3.59 A responsible for the extreme low-®eld shift of the aromatic hydrogen atoms directed to the center of the macrocycle. The mean planes of the two central aromatic rings of complex 66 have a parallel orientation, but are displaced between each other. The distance between the centroids of the aromatic rings

20

H. HoÈp¯

Fig. 17. Compounds 60±63 are [4.4]metacyclophane and 64±67 [5.5]paracyclophane-type

molecules

Ê . Both compounds can be considered as cyclophane derivatives, is 4.16 A whereby 60±63 would be [4.4]metacyclophane and 64±67 [5.5]paracyclophane derivatives [98]. Cyclophane-like binuclear metallomacrocycles can be constructed also in one-step syntheses and in high yields if metal complexes with free coordination sites in cis-orientation are brought to reaction with diamines [99], dialcohols [100], or dithiols [101]. Similar results are obtained with bis (b-diketone)- [102], bis(b-keto enamine)- [103] and diphosphine-ligands [104]. Thereby, it is advantageous that metal vertices can introduce additional functional properties such as Lewis acidity [103], luminescence [105], magnetism [106], or redox activity [107]. Trinuclear [100c, 100d, 101, 107, 108] and tetranuclear [100d, 101] species are also known. In distinction to these metallocyclophanetype molecules, in the boron analogues 60±67 the macrocyclic ring consists only of covalent bonds, even if the N ® B coordination is omitted. This covalent macrocyclic skeleton should have more kinetic and thermodynamic stability than many of the metallocyclophanes cited above. A further example of a macrocyclic ring system with boron that is related to the structures of the metallocyclophanes mentioned before is compound 68 that can be prepared via condensation between two bis(borazaphenanthrene) molecules. The phenyl ether oxygen atoms allow the two borazaphenanthrene rings to pivot with respect to each other, therefore this dimeric boronic acid anhydride can potentially exist in two isomeric forms, either face-to-face or helical (Fig. 18). In the face-to-face form the boron atoms of the bis(borazaphenanthrene) moieties have syn-orientation, while they have approximate antiorientation in the helical form. Compound 68 has been characterized by X-ray crystallography in the helical form [109]. The dimensions of the cavity can be described by the transannular C¼C contacts between the carbon atoms in Ê. 2-position of the phenyl ether units, which have values of 5.12 and 6.21 A A different approach to the preparation of boron macrocycles can be made by using diboronates 69±71 for the construction of the macrocyclic derivatives

Structure and Bonding in Boron-Containing Macrocycles and Cages

21

Fig. 18. The dimeric boronic acid anhydride 68 can potentially exist in a face-to-face or a helical form. X-ray crystallography proves the helical conformation

outlined in Fig. 19 via condensation with triethylene glycol, but all attempts to transform 69±71 have failed so far [110]. In contrast, reaction of ligand 72 with 4,4¢-biphenyldiboronic acid has been successful and diboronate 73 is obtained in yields of 33%. This complex acts as a receptor for the paraquat dication forming a 1:1 complex with an association constant of 320 M)1 in acetone. The intermolecular forces responsible for the complexation are ion-dipole stabilization between the dative N ® B dipoles and the two cationic centers in paraquat, attractive p-p interactions between

Fig. 19. Transformation of diboronates 69±71 to macrocyclic derivatives was not possible,

however, with 4,4¢-biphenyldiboronic acid 72 compound 73 is formed

22

H. HoÈp¯

the electron-rich and electron-poor biphenyl groups, and CAH¼O hydrogen bonding between the paraquat methyl groups and the boronate oxygen atoms [111]. If 2,6-pyridinedimethanol is condensed with arylboronic acids in non-polar solvents, the tetrameric boron complexes 74 and 75 are formed rapidly (within 15±30 min) in yields of 80 and 93% (Fig. 20). In both cases only the RSRS/ SRSR enantiomeric pair with approximate S4-symmetry is obtained, so that the reaction is diastereoselective. The unsymmetrical coordination mode of 2,6-pyridinedimethanol can be proved by 1H- and 13C-NMR spectroscopy that allow one to differentiate between the OCH2 groups involved in a ®ve-membered heterocycle and the ones which are bridging. The X-ray structures of 74 and 75 show that the tetrahedral geometry of the boron atom and the planarity of the ligand, whose primary bonds are also collinear, dictate the tetrameric macrocyclic structure formation. The four boronate moieties are almost perpendicular to each other. Ê and The average distance between two neighboring boron atoms is 5.4 A molecular modeling has shown that there is a cavity with a diameter of about Ê in the center of the molecule which is, however, accessible only by a 1.4 A Ê [87, 91]. narrow channel formed by the B-aryl groups with a diameter of 0.6 A At ®rst sight the isolation of 74 and 75 may seem surprising, since 2,6pyridinedimethanol normally acts as a tridentate monochelating ligand as it has been shown for a series of complexes with metal ions and organometallic compounds [112]. As is the case of the dimeric complexes 50±59 (Fig. 16), this may be explained with the small covalent radius of boron and its tetrahedral geometry, which is not as adequate for the coordination to this ligand as it would be a trigonal-bipyramidal or an octahedral polyhedron. This statement is con®rmed by the molecular structures of compounds 76±78, in which the central elements silicon [113] and sulfur [114] both have tetrahedral geometry, too (Fig. 20).

Fig. 20. Reaction of 2,6-pyridinedimethanol with arylboronic acids gives the tetrameric

macrocycles 74 and 75. Dimeric compounds are known with silicon 76 and 77 and sulfur 78

Structure and Bonding in Boron-Containing Macrocycles and Cages

23

3 Boron Compounds with Cage-Like Structures 3.1 Borate Cages

A crystalline sample of diborate 79 that is bridged via three 1,4-alkanedioxy moieties has been obtained accidentally during the reduction of trans-1,2cyclopropanedicarboxylic acid with H3B á THF in a yield of 22% (Fig. 21). The central 20-membered B(OC4O)3B cage consists of two eclipsed BO3 groups that are bridged by three -CH2C3H4CH2- units and has a C3-symmetry Ê and axis that passes through the two boron atoms. The B¼B distance is 3.9 A Ê the intramolecular distance between the nearest H-atoms is 4.1 A. The BAO bond lengths are shortened due to p-back bonding to the boron atom that stabilizes the compound against hydrolysis with dilute acid or base. Because of the trans-substitution of the cyclopropane rings the molecule is chiral with a de®nite helicity, in this case in the sense of a left-handed screw [115]. Diborate 80 is produced from a 3:2 mixture of 1,1¢-bi-2-naphthol and boric acid in re¯uxing benzene as a racemic mixture of RRR and SSS combinations [116]. Alternatively, the molecule can be prepared in almost quantitative yield starting from (R,S)-binaphthol by treatment with H2BBr á SMe2 [117]. The diborate possesses again a central B(OC4O)3B cage that is similar to the one described above, with the difference that here the two eclipsed BO3 groups are bridged by three chiral 1,1¢-binaphthyl units that can be considered as ÔbladesÕ Ê and therefore 0.5 A Ê in this propeller compound. The B¼B distance is 3.4 A shorter than in 79 [117]. Because of its chirality diborate 80 has been applied as a catalyst for an asymmetrically induced Diels-Alder reaction (cyclopentadiene + methacrolein), where it showed high exo- and enantioselectivity [117]. With tripodal tris(diphenylphosphinito)¯uoroborate [FB(OPPh2)3]) ligands and gold(I) complexes two structurally related, completely inorganic cages 81 and 82 are known (Fig. 22) [118, 119]. The complex cation 81 contains an asymmetrical triangle of gold atoms capped above and below by the tripodal [FB(OPPh2)3]) anions. The tetrahedra of the ¯uoroborate groups are staggered due to a disrotatory twist of the two triangles of phosphorus atoms against the triangle of gold atoms, that have

Fig. 21. Diborates 79 and 80 are propeller compounds with a cage-like cavity

24

H. HoÈp¯

Fig. 22. Compounds 81 and 82 are prepared by treatment of [H(Ph2PO)2Au]2 with an excess of F3B¼OEt2

Ê [118]. In contrast, the auriphilic Au¼Au contacts of 3.04, 3.16, and 3.42 A cage-like cation 82, in which the three-fold axis passes through the boron atoms, has C3h-symmetry. The central part of this molecule is a hexanuclear gold cluster with the geometry of a trigonal prism and three chlorine atoms bridging the vertical edges. The cluster is stabilized by Au¼Au auriphilic Ê . The 1H-, 13C-, 11B-, and 31P-NMR data con®rm a highly contacts of 3.17 A symmetric structure in solution for both cases [118, 119]. 3.2 Borasiloxane Cages

Borasiloxanes are derivatives of the well-studied class of siloxanes (R2SiO)n, in which part of the four-coordinate silicon atoms have been substituted by three-coordinate boron atoms. They are therefore characterized by the presence of Si-O-B units and can have one-dimensional oligomeric [120] or polymeric [121], two-dimensional cyclic [122±126], or three-dimensional cagelike [127±131] structures 83±92 as outlined in Figs. 23 and 24. The Si-O-B fragment can be formed from different starting materials that generally include chlorosilanes or silanols and boron halides or boric acid derivatives. For the preparation of the cyclic six-, eight-, ten- and twelvemembered borasiloxanes 83±89 four synthetic approaches are known so far, starting from either (i) an arylboronic acid ArB(OH)2 and an organosilanediol R2Si(OH)2 or a disiloxanediol (R2SiOH)2O [122, 123], (ii) phenylboronic acid PhB(OH)2 and a diethoxysilane R2Si(OEt)2 or 1,3diethoxydisiloxane (R2SiOEt)2O [132, 133],

Structure and Bonding in Boron-Containing Macrocycles and Cages

25

Fig. 23. One-dimensional and two-dimensional structures of borasiloxanes

(iii) phenylboronic acid PhB(OH)2 and a 1,3-, 1,5- or 1,7-dichlorosiloxane (R2SiCl)2O, [(R2SiCl)O]2SiR2 or [R2SiCl)OSiR2]2O in the presence of triethylamine [124], (iv) phenyldichloroborane PhBCl2 and a diorganosilanediol R2Si(OH)2 [125], and (v) boron tri¯uoride, BF3, and a trisiloxanediolate [(R2SiOLi)O]2SiR2 [126]. Generally, the products are obtained in moderate to good yields. They are inde®nitely stable in the pure state and unaffected by atmosphere and moisture; however, when dissolved in moist solvents they slowly hydrolyze. Compound types 85 and 86 are cyclo-boratrisiloxane and cyclodiboratetrasiloxane derivatives, respectively, with 11B-NMR displacements at 27 2 ppm that are typical for boronates with a trigonal CBO2-environment [122, 123]. Analogous to trialkyl- and triarylboroxines (RBO)3 [134], the eightmembered borasiloxanes 86 can from 1:1 adducts with amines, however this does not occur with the six-membered rings 85 [122]. Boracyclotetra-, boracyclopenta-, and diboracyclohexasiloxanes 87±89 are obtained according to method (iii) and have been identi®ed by 1H-, 13C-, 11B-, and 29Si-NMR spectroscopy as well as mass spectrometry [124]. X-ray structures are known for the three smaller borasiloxane ring systems 85±87 and in all cases similar BAO and SiAO bond lengths are observed. The BAO bond lengths in these and related structures range from 1.313(2) to Ê , and the SiAO bond lengths from 1.615(3) to 1.639(1) A Ê [122±126]. 1.375(6) A Ê, The sums of the covalent radii for B/O and Si/O are 1.54 and 1.91 A respectively [135], thus indicating that there are p-bonding interactions in

26

H. HoÈp¯

Fig. 24. Cage-like borasiloxanes 90±92

both cases. The OABAO and OASiAO bond angles are relatively constant and vary from 120.7(4) to 122.8(3)° for the trigonal boron and from 106.6(8) to 112.0(2)° for the tetrahedral silicon atom. In contrast, the BAOASi bond angle is more ¯exible and values from 128.9(1) to 159.8(2)° are reported [122±126]. In most of the cases the central heterocyclic ring is essentially planar, only in two examples does a slightly non-planar conformation occur [124, 126]. The phenyl rings attached to boron also approach co-planarity with the ring, but it has not been possible so far to prove any appreciable p-delocalization from the aromatic ring onto boron and hence the other heteroatoms. Other heterocyclosiloxanes in which one or more of the skeletal silicon atoms have been replaced by an atom of another main group element or transition metal are well-known and have been extensively studied [136]. Some of the cyclic borasiloxane motifs described above participate as central parts in the three-dimensional cage-like borasiloxanes 90±92 (Fig. 24) [127±131]. Compound 90 can be considered as bicyclic derivative of the diboradisiloxanes 86, in which an additional RBO2 fragment bridges the two silicon atoms [128]. This eleven-membered Si(OBO)3Si cage is formed in 91% yield by reaction of tert-butylsilanetriol with 4-bromophenylboronic acid in a 2:3 molar ratio. The solid-state structure has approximate D3h-symmetry with the fourcoordinate silicon atoms located on the C3-axis and the three-coordinate boron atoms in the rh-plane. When viewed along the axis passing through the two silicon atoms, the tert-butyl groups are in an almost eclipsed arrangement. All relevant structural parameters like the BAO and SiAO bond lengths as well as the BAOAB, SiAOASi and SiAOAB bond angles are within the limits observed for the corresponding monocyclic borasiloxanes. The intramolecular Ê and the average B¼B distance is 3.77 A Ê [128]. Si¼Si distance is 3.49 A In compound 91 the diborahexasiloxane ring system 89 can be identi®ed, which has been expanded to a three-dimensional structure by an additional (R2SiO)2O fragment bridging now, in contrast to compound 90, two boron atoms [127]. This cage can be obtained in yields of 45% from tetraphenyldisiloxanediol and boric acid when reacted in a 6:1 stoichiometry. The molecule contains a

Structure and Bonding in Boron-Containing Macrocycles and Cages

27

17-membered B2Si6O9-cage, in which two BO3 units are bridged by three -SiR2-O-SiR2- groups. When viewed along the B¼B axis, the BO3 units are in a staggered con®guration, but the structure has a C2-symmetry axis which is perpendicular to the B¼B axis. Also in this case, the BAO and SiAO bond lengths as well as the OABAO, OASiAO and BAOASi bond angles approach the expected values. The BAOASi bond angles of 156.7(1) and 167.2(2)° are larger than in compounds 85±87, but they are still within the range of values found in Ê. other related ring systems [136a]. The B¼B bond distance is 3.48 A Interestingly, some other compounds with the same empirical formula as 91, but with Me2Si, MeVySi (Vy = vinyl), Et2Si or MePhSi units instead of Ph2Si are reported to be non-crystalline polymers [121]. Borasiloxane 92 with boron and silicon atoms at alternate corners and the general formula B4Si4O10 for the central cage is composed of two diboratetrasiloxane rings, that are joined by two Si-O-Si bridges [129]. Unlike the related and extensively studied silasesquioxanes 93 (Fig. 25), in which the silicon atoms can be substituted by various combinations of group 13 and 15 elements [137], this molecule contains alternately three-coordinate boron and four-coordinate silicon corner sites. Compound 92 can be synthesized in 19% yield from tert-butyltrichlorosilane and 4-vinylphenylboronic acid in the presence of aniline and water. The incomplete cube structure of 92 possesses an inversion center. In each B2Si2O4 ring one silicon, both boron, and all oxygen atoms are essentially coplanar, while the other silicon atom clearly lies out of this plane. All bond lengths and bond angles are within the expected ranges [129]. Due to the structural relationship with silasesquioxanes 93, these cage-like borasiloxanes may be interesting building blocks for the construction of more complex materials like zeolites or catalyst supports. One of such more complex building blocks has been already prepared in yields of 43%. The product is an interesting boroncontaining silasesquioxane dimer with two planar BO3 moieties that is stable also in solution [130]. The somewhat different bis(diboratetrasiloxane) derivatives 94 and 95 have been prepared from bimetallic salen{B(OEt)2}2 derivatives and diphenylsilanediol [131, 138]. In this case the four boron atoms of two diboratetrasiloxane rings 86 are chelated by salen-type ligands in order to produce cages of cylindrical geometry (Fig. 25). Ê , respectively, thus The intramolecular B¼B distances are 4.0 and 9.8 A Ê 3 that could be large enough to accommodate forming a cavity of about 157 A small molecules or cations. There is a strong relationship between the structure of compound 92 and compounds 94 and 95. In 94 and 95 the cycloborasiloxane rings are joined by two B(NCH2CH2XCH2CH2N)B (X = CH2, NH) bridges, while 92 has two SiAOASi bridges instead. 3.3 Borophosphonate Cages

The borophosphonate cages described in what follows are also analogues of silasesquioxanes 93. In contrast to the cage-like borasiloxanes discussed above,

28

H. HoÈp¯

Fig. 25. General formula for silasesquioxanes 93. Compounds 94 and 95 are cage-like

bis(diboratetrasiloxane) derivatives, in which two eight-membered diborasiloxane rings are joined by two salen ligands

they have a higher degree of structural relationship, since P-O-B units are isoelectronic to Si-O-Si units. So far, two different methods have been elaborated for the preparation of borophosphonates 96±104 (Fig. 26), starting from either (i) the tert-butylphosphonic bis(trimethylsilyl) ester t-BuP(O) (OSiMe3)2 and dichlorophenylborane PhBCl2 (yield 14%) [139] or (ii) a phosphonic acid R¢P(O)(OH)2 and a trialkyl- or triarylborane R3B [140, 141]. All borophosphonates obtained so far are air and thermally stable (m.p. >210 °C). They are soluble in all common organic solvents if they carry bulky groups either on phosphorus or on boron. Borophosphonates that contain only methyl or phenyl groups show poor solubility [140]. 1H-, 11B-, and 31 P-NMR spectra are in agreement with the highly symmetric silasesquioxanetype structure in solution [140, 141].

Fig. 26. Borophosphonates 96±104 are derivatives of silasesquioxanes, since P-O-B units are

isoelectronic to Si-O-Si units

Structure and Bonding in Boron-Containing Macrocycles and Cages

29

Compounds 96, 99, and 103 have been characterized by X-ray crystallography that con®rms the 20-membered B4P4O12 cubanoid framework surrounded by hydrophobic organic groups. Phosphorus and boron atoms, which both have tetrahedral geometry and are bridged by oxygen atoms, alternately occupy the corners of the cube. The BAO bond lengths with values from Ê lie in the same range as for borate ions with 1.450(5) to 1.493(10) A tetracoordinate boron atoms[142], but the PAO bond lengths with values that Ê are typical for partial multiple bonds [142]. range from 1.486(3) to 1.516(3) A In all three molecules the six eight-membered B2P2O4 rings adopt a pseudo-C4 crown conformation (saddle conformation). The OABAO, OAPAO, and BAOAP bond angles range from 107.4(6) to 111.5(7)°, 107.0(3) to 113.1(4)°, and 146.2(5) to 148.9(5)°, respectively. The average B¼P edge length of the Ê [139±141]. cube is 2.85 A 3.4 Vanadium Borate and Borophosphate Cluster Anions

The cage-like borasiloxanes and borophosphonates discussed above display features of both inorganic and organic chemistry. They have completely inorganic cores of the borosilicate or borophosphate type that carry organic substituents only on its outer sphere providing them some of the advantageous properties of organic molecules like solubility in non-polar solvents or occurrence of monomeric cage-like species in solution. Considering that inorganic borosilicates and borophosphates often form complex two- or threedimensional frameworks of low solubility, the design of other related structure types may be interesting with respect to the development of materials with new physical and chemical properties. Another approach to the synthesis of cage-like structures with borates consists in utilizing polar organic molecules to direct the crystallization of inorganic frameworks, e.g., through hydrogen bonding. This strategy has been applied together with hydrothermal synthesis techniques in the last few years for the preparation of a number of vanadium borate and vanadium borophosphate cluster anions [143±147]. Vanadium borates and vanadium borophosphates form part of the extensively studied compound class of polyoxoanions, whose structural skeletons are mainly derived from vanadium [148] and molybdenum oxide clusters [149] that frequently have included additional heteroatoms like phosphorus, arsenic, boron, silicon, etc. [150]. The general procedure for the preparation of vanadium borates consists in heating a concentrated H2O solution of boric acid and vanadium oxide in an autoclave at 170 °C for several days [143]. Two different vanadium borate clusters 105 and 106 are obtained, one with two polyborate chains coordinated to a contorted vanadium oxide ring (105) and another one with a macrocyclic B18O36(OH)6 ring (106). The latter ring is composed of six B3O6(OH)4) units and has a chair-like conformation (Fig. 27) [143]. The macrocyclic borate ring is sandwiched by two smaller (VO)6O12 rings of triangular form via six axial B-(l3-O)-V bonds, of which three are directed to one side of the macrocycle and three to the other one. On average, ten of the

30

H. HoÈp¯

Fig. 27. In vanadium borate 106 the macrocyclic borate ring (left) is sandwiched by two smaller (VO)6O12 rings (right) of triangular form via six axial B-(l3-O)-V bonds to form a cage-like cluster anion

twelve vanadium ions per cluster are vanadium(IV), and two are vanadium(V). The anionic charge of the cluster is balanced by ethylene-dimmonium ions [143]. In a related structure six Zn(en)22+ complexes are coordinated to the macrocyclic borate through the oxygen atoms that bridge the B3O3 rings thus providing a type of organic environment to the cluster [144]. Vanadium borophosphates 107±109 that contain cluster anions with different ring sizes can be synthesized by hydrothermal reactions of vanadium(III) oxide, boric acid and an appropriate phosphate salt [145, 146]. The ring size of the cluster anion with the composition {[(VO)2BP2O10]12)}4)6 is determined by the size of the cation that is, however, included only in the interior of the cages with n = 5 and 6 (Fig. 28). In all three cluster anions two PO4 tetrahedra and one BO4 tetrahedron are connected through common corners to form BP2O10 trimers, in which the external phosphorus-bound oxygen atoms are unshared. The BP2O10 trimers are connected to (VO)2O6 dimers by sharing oxygen atoms thus building (VO)2BP2O10 units. One boron-bound oxygen atom adopts a l3-O arrangement and is bound to two vanadium atoms. The cluster ion charge is balanced by piperazinium, ammonium, or alkaline earth metal ions (K+, Rb+, Cs+). With piperazinium ions a tetrameric [145], with Na+ a pentameric [146], and with NH4+, K+, Rb+ or Cs+ a hexameric [145, 147] cluster is formed. The cluster ions must have some conformational ¯exibility in order to accommodate cations over a variable size range. The ring diameters of the cluster anions do indeed Ê , while vary, e.g., the average O¼O distance in the NH+4 centered cage is 5.61 A + Ê the corresponding distance in the K centered ring is 5.36 A [147]. Although the centered cations should act as templates in the formation of 108 and 109, templating is perhaps not an indispensable requisite, because the cluster anion

Structure and Bonding in Boron-Containing Macrocycles and Cages

31

Fig 28. Vanadium borophosphates with the composition {[(VO)2BP2O10]12)}n form cage-like cluster anions, which may be empty (107, n = 4) or centered by a cation (109, n = 6). For clarity only half of the vanadium and phosphorus atoms of the cluster anions are shown

107 with a 4-ring is formed in the absence of any small cation and is not centered. It should be noticed that the formation of these compounds is somewhat fortuitous, since a series of other vanadium borophosphates of similar composition, but with polymeric chain-like structures, are known [151, 152].

4 Application of Boron-Containing Hosts in Ionic and Molecular Recognition 4.1 Metal Complexes with Boron-Containing Hosts

4.1.1 Boron Containing Pseudocryptates The supramolecular structures of the four closely related natural products boromycin [153], aplasmomycin [154], borophycin [155], and tartrolon [156] are characterized by the presence of a central anionic BO4 fragment that rigidi®es the organic macrocyclic frames through ester links with diol fragments to generate a cleft suitable for the inclusion of a sodium cation. The sodium cation that is housed in this cleft is coordinated by six oxygen atoms, including two oxygens of the BO4 unit, in an irregular arrangement. The overall complex is charge neutral. These ionophores with antibiotic [153±155] and cytotoxic [156] properties are unique, since very few metabolic products are known containing the element boron. The structural organization of these boron-containing natural products has inspired the synthesis of pseudocryptand 110 [157], which combines the

32

H. HoÈp¯

Fig. 29. The pseudo-cryptand 110 can be transformed with boric acid and a metal hydroxide to the corresponding charge neutral pseudocryptates 111±116. 117 serves as precursor ligand for Li+

18-membered [2.2]macrocyclic core of the [2.2.2]cryptand with two bidentate catechol units that can be transformed with boric acid and a metal hydroxide MOH (M = Li, Na, K, Rb, Cs, NH4) to the corresponding charge neutral pseudocryptates 111±116 (Fig. 29) [157±159]. The binding ability towards alkali metal cations was studied in solution by NMR spectroscopic techniques with the result that the potassium complex is the most stable one. The binding constant is estimated to be about 1012.5 mol L)1 and the selectivity factors for K+ over Na+ and Cs+ cations are more than 103 and 102, respectively [157]. NH+4 is bound less strongly [158]. Responsible for the relatively large binding constants are the additional attractive charge-charge and charge-dipole interactions between the BO4 unit and the cation. X-ray structure analyses have shown that chiral R and S isomers are formed due to the tetrahedral environment of the spiroborate ester function. The lone pairs of the two nitrogen atoms are directed towards the metal ion, the average Ê . Further coordination occurs with six of the eight N¼N distance being 6.27 A oxygen atoms present in the host, namely the four oxygen atoms of the ether junctions and two of the four borate oxygen atoms. The polyhedra are irregular [157, 158]. Based on these observations a selective receptor for lithium 117 has been synthesized from a smaller diazadioxa macrocycle [160]. Ligands 110 and 117 may be regarded also as binucleating ligands capable of binding two cations at a time. Another related host-guest complex has been constructed from citric acid, boric acid, and a strontium salt in a 2:1:1 stoichiometry. In this complex the strontium cation is surrounded by four water molecules, two monodentate carboxyl groups and one oxygen atom of the BO4 unit [161]. A cage-like borate host for the simultaneous inclusion of two metal cations is also known. It can be prepared in high yields from a N,N¢,N¢¢,N¢¢¢-tetrakis(2-hydroxyethyl)cyclene derivative by reaction with sodium borohydride in DMSO at 120 °C. Only three of the four hydroxyethyl functions of each ligand bind the boron atom, so that the resulting complex 118 is a centrosymmetric diborate with two BO3(OH) units (Fig. 30). The sodium cations are coordinated each by four nitrogen atoms and three oxygen atoms. The host-guest

Structure and Bonding in Boron-Containing Macrocycles and Cages

33

Fig. 30. Compound 118 is a cage-like host-guest complex with the simultaneous inclusion of two metal cations

complex is further stabilized by two intramolecular OAH¼O hydrogen bonds between the BAOH group and an oxygen atom of the opposite BO4 group [162]. 4.1.2 Borylated Bis(dioxime)metal Complexes and Related Compounds 4.1.2.1 Introduction Dioximes are widely used bidentate ligands for the complexation of transition metal ions with the characteristic property to form complexes with a pseudomacrocyclic structure if two or more of these ligands are chelating the same metal ion (119, Fig. 31) [163]. The low solubility of many of these complexes can be enhanced replacing the intramolecular OAH¼O hydrogen bridges by diorganoboryl groups [163] forming complexes with the general formula M(dioxime-BR2)2 120±122 [164±167], M(dioxime-BR2)2L (L = neutral ligand) 123±126 [168±172], RM(dioxime-BR2)L 127 [173±178], and M(dioxime-BR2)2LL¢ 128 [179, 180] (Figs. 31, 33). The complexes are obtained by reacting the corresponding metal dioxime complexes with an adequate boron reagent, e.g., BF3 or diphenylborinic acid anhydride (Ph2B)2O. The resulting products are frequently neutral and can contain either one or two boron bridges [181]. 4.1.2.2 Complexes of the M(dioxime-BR2)2 and M(dioxime-BR2)2L Type The M(dioxime-BR2)2 class of complexes 120±122 with four-coordinate metal ions in a square-planar environment has attracted attention in view of possible columnar M¼M interactions that may result in interesting semiconducting properties in the solid state [182]. Therefore, a series of nickel(II) complexes

34

H. HoÈp¯

Fig. 31. Complexes of type 120±126 are derivatives of M(dioxime)2 complexes 119, in which the hydrogen bridges have been replaced by diorganoboryl groups

with different boryl groups and dioxime ligands have been structurally characterized (Fig. 31), but only some have a M¼M interaction, however without the expected columnar structure. In Ni(dmg-BF2)2 (dmg = dimethylglyoxime) 120, the glyoxime residues and the nickel atom in each complex are coplanar, but the BF2 groups are displaced out of the plane with one of the ¯uorine atoms in each BF2 group in axial orientation adopting a ciscon®guration (conformation type A in Fig. 32). Due to the steric requirement of these substituents the formation of a columnar structure with in®nite M¼M interactions is inhibited, and only the Ê association of pairs of molecular units is allowed. The Ni¼Ni distance is 3.21 A [164]. If the same compound is crystallized in the presence of benzimidazole, the [Ni(dmg-BF2)2]2 dimer units are sandwiched between sheets of benzimidazole molecules due to p-p interactions resulting in an increased Ni¼Ni Ê [165]. With anthracene the p-p interactions seem to be separation of 3.358 A stronger, because in this case the parent dimer molecule is cleaved. Each monomer now has a conformation of type B (Fig. 32) and is sandwiched by anthracene molecules [166]. Compound 121 has the same con®guration [163d]. The dimeric structure is retained in a series of ®ve-coordinate 1:1 adducts of Ni(dmg-BF2)2 123 with nitrogen bases like aniline [168], benzylamine [169], and 4,4¢-bipyridine [170]. In all cases the Lewis base coordinates to the nickel atom in an axial position forming a square-based pyramidal arrangement. In Ê comparison to Ni(dmg-BF2)2 the Ni¼Ni separation increases by 0.45 to 0.70 A and each nickel atom is displaced from its N4-plane towards the apical nitrogen atom. The BF2 groups have cis-con®guration with the axial ¯uorine atoms being oriented in the same direction as the nitrogen base (conformation

Structure and Bonding in Boron-Containing Macrocycles and Cages

35

Fig. 32. Possible conformations for complexes of the M(dioxime-BR2)2 and M(dioximeBR2)2L type

type A, Fig. 32). Interestingly, in the anion [Co(dmg-BF2)2py]) 124, which contains a cobalt(I) center that is isoelectronic to nickel(II), only a monomeric structure is observed, in which the axial substituents of the cis-con®gured BF2 groups are oriented in the opposite direction to the nitrogen base (conformation C in Fig. 32) [171]. The same conformation is displayed by compounds 125. A monomeric complex with copper(I) 126 is also known, [Cu(dmgBF2)2CO]), however, in this case the axial ¯uorine atoms have the same orientation as the monodentate CO ligand (conformation D in Fig. 32) [172]. If the BF2 groups in Ni(dmg-BF2)2 are substituted by BPh2 units (122), the complex also adopts the saddle-shaped conformation of type D (Fig. 32), in which the two dimethylglyoxime fragments are bent down from the N4 plane with a dihedral angle of about 27° between the two least-squares planes of the dioxime units. The coordination geometry around the nickel ion is distorted square-pyramidal, but there are no intermolecular Ni¼Ni interactions [167]. 4.1.2.3 Complexes of the RCo(dioxime-BR2)2L and Fe(dioxime-BR2)2LL¢ Type Complexes of the composition RCoIII(dioximeH)2L (R = alkyl, L = neutral ligand) and their parent complexes with BR2 bridges RCo(dioxime-BR2)2L 127 (Fig. 33) are known as organocobaloximes [173±178] and have received attention being models for vitamin B12 (cobalamines) [183]. A series of related complexes of the composition FeII(dioxime-BR2)LL¢ 128 (Fig. 33) without the metal-carbon bond is also known [179, 180].

36

H. HoÈp¯

Fig. 33. Cobalt(III) complexes 127 are known as organocobaloximes and have received attention being models for vitamin B12. Iron(II) complexes 128 are analogues without the metal-carbon bond

The replacement of the OAH¼O bridges with BF2 of BPh2 may affect both the complex geometry [178] and the electron density at the central metal ion [184], providing the opportunity of adjusting the CoAC bond strength towards homolytic cleavage, which is currently accepted to be the ®rst step of the reactions catalyzed by the vitamin B12 coenzyme [185]. Similar to the four- and ®ve-coordinate complexes 120±126, for RCo (dioxime-BR2)2L 127 and Fe(dioxime-BR2)LL¢ 128 different conformations are possible in solution and in the solid state, in which the substituents of the boron atoms may adopt cis- or trans-con®gurations and in which the alkyl group R may have a parallel or an antiparallel orientation with respect to the BR2 substituents [173±180]. Some related examples with BR2 monocapped ligands are also known [181] and one of these molecules, 129, assembles to an interesting supramolecular structure, in which the cobalt(III) ions in a methylcobaloxime unit are coordinated to the pyridine residues of the bridging B(py)(OMe) group (Fig. 34). The dinuclear complex forms a large rectangular cage that is limited by the two pyridine residues and the cobaloxime moieties. The two pyridine

Fig. 34. Compound 129 assembles to a supramolecular structure through coordinative bonds between the cobalt(III) ions and the pyridine residues of the bridging B(py)(OMe) group

Structure and Bonding in Boron-Containing Macrocycles and Cages

37

Ê [186]. In a rings are nearly coplanar with a shortest C¼C approach of 3.85 A similar way a zinc dicatechol porphyrin has been assembled into dimers using 3-pyridylboronic acid as template [187]. However, in this case the boron atom remains three-coordinate, whereas in the cobaloxime derivative it assumes tetrahedral coordination. 4.1.2.4 Complexes of the M(LBR2)X Type The macrocyclic bis(oximato)borates 130±131 and its tetramethylene analogue 132 are derivatives of the bisborylated dioxime complexes discussed above, in which one of the OBR2O groups has been substituted by a trimethylene or tetramethylene unit, respectively (Fig. 35). A series of monomeric copper(II) complexes Cu(LBF2)X (X = H2O, I), py, CN), NCO)) 130 has been synthesized [188±192] and proposed as model systems for redox proteins [193, 194]. The capacity of coordination to soft ligands, e.g., CN), in these complexes puts ligand LBF2 in the relatively small class of stabilizing ligands for copper(II), which have been shown to be capable of preventing reduction to copper(I) [195]. Several characteristic structural changes occur in the order H2O, I), py, CN), NCO) upon variation of ligand X. Ê and are The average CuAN bond lengths increase from 1.940(8) to 2.001(4) A accompanied by parallel changes in the apical displacement of the copper(II) Ê ) [188±192]. In all these cases ion from the basal N4-plane (Dd = 0.32 to 0.58 A the degree of apical displacement is much larger that it would be expected in a ``normal'' square pyramidal copper(II) coordination environment (0.1 to Ê ) [196]. The different apical displacements of the copper(II) ions in this 0.2 A series of complexes are also accompanied by changes in the overall conformation of the macrocyclic ligand. As the metal ion is progressively moving farther out of the basal plane, an increasing `¯exing' of the macrocycle is induced, so that maximum overlap between the nitrogen lone pairs and the

Fig. 35. Compounds 130±132 are derivatives of the bisborylated dioxime complexes, in which

one of the OBR2O groups has been substituted by a trimethylene or tetramethylene unit, respectively

38

H. HoÈp¯

d orbitals of the metal ion is guaranteed. The propylene portion and the BF2 section of the macrocycle can be folded in two distinct ways, giving a chair or a boat conformation, however, the energy barrier seems to be relatively low, since both conformations have been observed for one of the compounds in the same crystal structure [188, 194]. Electrochemical reduction of [CuLBF2](ClO4) yields selectively a neutral copper(I) complex [CuLBF2] with a four-coordinate tetrahedrally distorted square-planar geometry [197]. This complex reacts rapidly with carbon monoxide to the ®ve-coordinate copper(I) complex [Cu(LBF2)CO] with a square-pyramidal geometry similar to that of the copper(II) analogues discussed above (131, Fig. 35) [194]. Related compounds with other transition metals have been studied only sparsely, e.g., with nickel(II) [198], cobalt(III) [174], and rhodium(III) [199, 200]. A series of dimeric copper(II) complexes {[Cu(LÕBF2)S}[X] is also known and exhibits interesting magnetic effects associated with electron spin exchange between the copper(II) ions [201]. As already mentioned, the macrocyclic ring can be further enhanced if the propylene moiety is substituted by a butylene group. Visible spectral data of a series of related compounds [Cu(L¢BF2)D]X 132 and the structural study of the derivative with D = H2O show that the seven-membered chelate ring in¯uences the structure and other properties like electronic absorption of the copper(II) complex [193]. 4.1.2.5 Other Related Compounds From 2,6-diacetylpyridine dioxime, ferric chloride hydrate, and phenylboronic acid as starting materials the macrocyclic dinuclear iron(II) complexes 133 can be prepared (Fig. 36).

Fig. 36. Complexes 133 and 134 are dinuclear bis(dioxime) metal complexes

Structure and Bonding in Boron-Containing Macrocycles and Cages

39

Compounds 133 contain two seven-coordinate iron(II) centers, which are bridged by two oxygen atoms and an additional bridging group X (X = OMe, OH, Cl) in a pentagonal bipyramidal environment. The pentagonal equatorial plane is made up by the three nitrogen atoms from one of the dioximes and two (boron)-oxygen atoms. The chlorine atoms and the bridging group X serve as axial ligands for the metal. This arrangement causes the macrocycle to take on a `butter¯y' shape with the axial bridging site above the `wings' which are formed by the diacetylpyridine dioxime units. The dihedral angles between the planes containing the equatorial ligands range from 102.2(1) to 105.0(1)°. One of the bridging oxygen atoms is protonated and therefore both iron ions are in the iron(II) oxidation state [202]. Other closely related boron-containing dinuclear macrocyclic complexes with copper(II) and nickel(II) ions are compounds 134 (Fig. 36). The central part of these complexes can be described as an N2MO2MN2 center, in which the two metal ions are bridged by the phenolate moieties and further coordinated by two oxime nitrogen atoms. Each metal ion has a square-planar coordination environment, however, the overall molecular geometry is saddleshaped due to bending of the macrocyclic ring system. The torsion angle between the planes of the phenyl rings is 68.4° [203]. A a very interesting macrocyclic ring system of the composition Pt(LBF2)2 with L = ethylenediphosphine oxide, Ph(O)HP(CH2)2PH(O)Ph, was also reported a few years ago [204]. 4.1.3 Borylated Tris(dioxime)metal Complexes and Related Compounds 4.1.3.1 Complexes of the M[RB(dioxime)3BR] Type Complexes containing encapsulated metal ions (clathrochelates ) with the formula [M(dioxime)3(BR)2] are known with iron(II) 135, cobalt(II) 136, cobalt(III) 137, and ruthenium(II) 138 (Fig. 37) [205±220]. Generally, these macrobicyclic complexes are prepared by template synthesis from a mixture of

Fig. 37. [M(dioxime)3(BR)2] complexes 135±138 contain encapsulated metal ions and are clathrochelates. The coordination geometry of the metal ion is described by the distortion angle /

40

H. HoÈp¯

the dioxime, the metal ion and the boron reagent. Clathrochelate complexes afford extremely high thermodynamic stability and kinetic inertness to the encapsulated metal ion [221], and can therefore allow for the study of usually labile and reactive metal centers [222] as well as electron transfer reactions [223]. Several studies have been undertaken with the aim to functionalize the chelating fragments of clathrochelates, expecting steric and electronic effects on the polyhedron geometry and the central metal ion properties [219, 220]. If donor atoms are introduced in the external part of the macrobicyclic trisdioximate complex, further metal ions can be coordinated in the peripheral fragments thus generating polynuclear complexes. Oligometallic complexes are being investigated extensively as (i) models for metalloproteins and metalloenzymes (biomimetics) [224], (ii) ef®cient catalysts for chemical reactions [225] and (iii) promising materials for molecular electronics [1e, 226]. So far, hexasubstituted derivatives 135 have been prepared with aryl and alkyl thiolates, phenolates as well as oxo- and thioether precursors [220]. From a structural point of view the most important variation observed in clathrochelates 135±138 is related to the coordination geometry of the central metal ion. The two possible extreme coordination geometries are trigonalprismatic or regular octahedral and intermediate coordination geometries can be described by the distortion angle / with values between 0±60° (Fig. 37). For all complexes studied so far by X-ray crystallography /-values between 5.4 and 29.3° have been reported indicating that the coordination polyhedron is more similar to a trigonal prism than to an octahedron, although normally octahedral geometry is preferred for six-coordinate metal ions. The MAN bond distances are identical for a series of iron(II) complexes 135, so that apparently the macrobicyclic ligand cavity is `matched' to the iron(II) size by a rotationaltranslational change of the trigonal prism about the axis of three-fold symmetry. This observation is con®rmed by a decrease of distance between the Ê with increasing distortion angle [220]. prism bases from 2.39 to 2.30 A Theoretical calculations give evidence that forces other than ligand ®eld stabilization energy, e.g., metal ion size, intra-ligand steric repulsion, and p-backbonding are primarily responsible for the unusual geometry of the central metal ions in borylated tris-dioximated complexes [217, 218]. Tris(dioxime) complexes that are capped with a boryl group BR at only one end of the molecule are also known, e.g., with technetium(III) [227, 228] and rhenium(III) [229]. 4.1.3.2 Other Related Compounds Two interesting monoborylated clathrochelates 139 and 140 are known (Fig. 38), in which three oxime functions are joined together by a BR group, while the other half of the ligand is capped by a carbon or a phosphorus atom, respectively.

Structure and Bonding in Boron-Containing Macrocycles and Cages

41

Fig. 38. Compounds 139 and 140 are monoborylated clathrochelates

Compounds 139 are tris(oximehydrazone) derivatives with an iron(II) ion in the center of the cavity [230]. Compound 140 (Fig. 38) has been known for 30 years [231, 232] and was prepared from a tris(2-aldoximo-6-pyridyl)phosphine that is capped by a BF unit to encapsulate cobalt(II), zinc(II), nickel(II), and iron(II). All four macrocyclic complexes were characterized later by a comparative X-ray crystallographic study [233±236]. 4.2 Molecular Recognition by Boronic Acid Derivatives

4.2.1 Molecular Recognition by Monoboronic Acid Derivatives A new class of host molecules for the selective complexation of salts [237], alcohols [238], amines [239], and catecholamines [240] has been designed by combining crown ethers of different sizes with a boronic acid or boronate (Figs. 39 and 40). In this way hosts 141±145 with both a r-bonded Lewis acidic boron atom for complexation of anions and a conventional multidentate ligand for cations are generated. Complexation experiments of the 21-membered crown[6] boronate 142 with different potassium salts KX (X = F, Cl, Br, I, SCN, CN, OMe) indicate that there is a high speci®city for the incorporation of KF, whereby F) is bound covalently to the boron atom and K+ is complexed by the crown ether (146, Fig. 39). An X-ray study has shown that the complexation of KF is heterotopic, i.e., both ions are complexed inside the same host. Some of the salts can only be bound in a monotopic way (KI and KSCN) [237]. Alcohols can be selectively bound to the same host type if they are combined with an amine and vice versa, considering that a cation and an anion will be formed through a proton transfer. The so-formed alkoxide anion will bind to the boron atom, while the ammonium ion will be complexed by the crown ether (147, Fig. 39). Competition experiments involving benzylamine have shown enhanced selectivity for the complexation of alcohols with

42

H. HoÈp¯

Fig. 39. Molecules 141±145 and can function as host molecules for the selective complexation

of salts, alcohols, and amines (146 and 147)

sterically less demanding substituents. X-ray structure analyses of the complexes with benzylamine/methanol (ethanol) reveal that the ammonium ion is completely embedded in the crown ether moiety, while the methoxy or ethoxy group, respectively, is coordinated to the boronate [238]. Amines can bind both to the boron atom and the crown ether, so that high association constants may be expected for their complexation. Competitive complexation experiments between hosts 141±145 and four different primary amines RNH2 (R = PhCH2, Ph(CH3)CH, nPr, PhCH2CH2) give association constants between 1.3 and 795 depending on the steric strain of the corresponding amine. The binding of secondary amines is less favored [239]. The slightly modi®ed host 148 is able to bind catecholamines with high selectivity (Fig. 40).

Fig. 40. Host 148 is capable to bind catecholamines with high selectivity

Structure and Bonding in Boron-Containing Macrocycles and Cages

43

Considering that some catecholamine derivatives like dopamine, norepinephrine, epinephrine, and tyramine are important drugs or biosynthetic compounds, these hosts can be possibly applied as synthetic carriers with the ability to selectively transport catecholamines through a lipophilic membrane. Host 148 functions as a ditopic carrier, in which the catechol moiety of the corresponding catecholamine derivative is complexed by the boron atom and the ammonium group is hydrogen-bonded to the crown-ether (149±151). Furthermore, the overall resulting supramolecular structure is charge neutral, since the accompanying anion of the dopamine derivative is complexed also by the boron atom. Liquid membrane transport experiments have shown that the transportation rate of dopamine and other derivatives is enhanced up to 160 times in the presence of host 148. The structure of the proposed host-guest complex has been veri®ed by mass spectrometry [240]. 4.2.2 Molecular Recognition by Diboronic Acid Derivatives Bidentate binding of two Lewis acidic boron centers to one methoxide anion was ®rst reported in 1967 [241]. Further examples did not appear until 1985 [242]. Today, other bis(boronates) like 152 and 154±158 (Fig. 41) are known that can be applied to the selective complexation of amines and diamines [243±247]. The bidentate Lewis acidic complex 152 has an s-indacene framework with two cis-oriented boronate groups that can bind amines forming either concavetype 153a or convex-type 153b 1:1 complexes (Fig. 41). From variable temperature NMR experiments with 4-dimethylaminopyridine (DMAP) or its analogues it has been concluded that the concave-type to convex-type complex ratio is about 3:1 in CD2Cl2 at 170 K. With excess of diamine a 1:2 complex is formed as the only product [243]. The molecular structure of this latter hostguest complex has been analyzed by X-ray crystallography. Each boron atom is coordinated to a different DMAP ligand via the pyridine nitrogen atoms, one in the concave-type and the other one in the convex-type coordination mode [244]. The chiral diboronic esters 154±156 are easily prepared from o-, m-, and p-phenyldiboronic acid and (R,R)-1,2-diphenyl-1,2-ethanediol. 1H-NMR titration experiments of 154 with benzylamine reveal that a 1:2 complex is formed, suggesting that each boron center accepts one amine molecule [245, 246]. A 1:1 complex cannot be detected, even when 154 and benzylamine are mixed exactly in a 1:1 ratio. Structure 157 is proposed as a possible model for this complex (Fig. 41), in which the nitrogen atom of the amine is coordinated to a boron atom, while one of the NH protons interacts with one of the two oxygen atoms of the other dioxaborolane ring to form a hydrogen bond. An allosteric effect is thereby predicted, because the coordination of the ®rst amine molecule ®xes the two dioxaborolane rings by two-point binding thus providing a preferable binding site for the second amine molecule. Additionally, the electron density on the second boron atom is reduced by hydrogen bonding resulting from the ®rst amine binding. This allosteric binding cannot be realized with the meta- and para-diboronates 155 and 156 [246]. The

44

H. HoÈp¯

Fig. 41. Bis(boronates) 152 and 154±158 can selectively bind amines or diamines

association constants for the complexes between 154 and other chiral and nonchiral amines con®rm in all cases an allosteric effect, because in all runs the second association constants K2 are larger by at least two orders of magnitude than the ®rst ones K1. Secondary amines form stronger complexes than primary amines due to their stronger basicity [245]. Another chiral diboronic ester 158 is derived from phenylboronic acid and L -tartaric acid. This bimetallic complex has been designed to evaluate its interaction with diamines of different chain lengths H2N-(CH2)n-NH2 (n = 2±6). Two binding modes between host and guest have been identi®ed by 1H NMR spectroscopy and theoretical considerations (AM1). Diamines with n £ 4 are only bound via formation of two coordinative N ® B bonds, while diamines n ³ 5 additionally have two hydrogen bonds between two of the four amino protons and the two carbonyl oxygens of the host. This results in a higher association constant for the larger diamines. The chiral diboronic ester 158 is also capable to recognize the chirality of the (R,R)- and (S,S)-isomers of 1,2diamino-1,2-diphenylethane. The (R,R)-isomer can bind to both boron atoms, while the (S,S)-isomer can form only one N ® B bond and one hydrogen bond [247]. Finally, a huge number of different boronic and diboronic acids has been designed with the aim to use them as molecular receptors for saccharide

Structure and Bonding in Boron-Containing Macrocycles and Cages

45

sensing in aqueous media. Since this important and very interesting topic was already reviewed a few years ago [248], only some of the most striking features will be described here. Boronic and diboronic acids readily form covalent interactions with different saccharides, even in aqueous media, whereby the application of diboronic acids may be advantageous, if a selective recognition of one speci®c polyol like D -fructose, D -galactose, D -mannose or D -glucose is desired. Especially the selective recognition of D -glucose is of interest due to its medicinal and industrial applications. Therefore, a series of boronic acids 159±166 with chromophoric groups attached has been synthesized and evaluated with respect to the capacity to selectively bind D -glucose or other saccharides (Fig. 42).

Fig. 42. A series of boronic acids 159±166 with chromophoric groups has been evaluated

with respect to its capacity to selectively bind D -glucose or other saccharides

In 159 and 163±166 the tertiary amine function is coordinated to the boron atom and transmits the electronic change due to the ester formation to the chromophore. In 160±162 the boron atom is directly connected to the chromophore. After the complexation of the saccharide, the change of the charge transfer, e.g., for 159 [249±251], or the ¯uorescence bands, e.g., for 160±166 [252±255], can be measured and interpreted. The most selective binding of D -glucose has been achieved with host 164 that forms a 1:1 complex with a macrocyclic structure (Scheme 1).

46

H. HoÈp¯

Scheme 1. Formation of a macrocyclic complex between a diboronic acid and a saccharide

Competitive binding studies have shown that 164 and 166 are suitable for the detection of glucose at physiological levels [253±255]. Diboronate 165 is capable of chiral recognition of monosaccharides and gave the best chiral recognition for fructose when tested with a series of different saccharides [255]. Compounds 167±171 outlined in Fig. 43 form another series of diboronic acids that form complexes with mono- and disaccharides. In these cases the asymmetrical immobilization of chromophoric functional groups, e.g., aromatic rings in 167±170 or Fe2+-complexation with the related boronate 171, can be analyzed by circular dichroism measurements [256±262].

Fig. 43. Compounds 167±171 form a series of diboronic acids for the complexation of

saccharides with functional groups that permit analysis by circular dichroism measurements

Further interest is related to the design of adjustable hosts that permit to mimic cooperative interactions present in many biological functions, e.g., for the transport of sugars across cell membranes. In this respect, crown ethers have been functionalized with boronic acid units, e.g., compound 172, so that the

Structure and Bonding in Boron-Containing Macrocycles and Cages

47

binding interactions with the saccharide molecules can be modi®ed by another species like a metal ion through a cooperative (allosteric) interaction [263]. In this particular case the binding of an alkaline or alkaline earth metal ion to the crown ether decreases the stability of the host with D -allose (negative allosterism), since the conformation of the crown-ether changes due to the metal ion complexation [264±268].

5 Conclusions and Perspectives This review has shown that there exist already a signi®cant number of macrocyclic and cage-like compositions incorporating boron atoms in the skeleton, part of which can be related to already known organic and inorganic macromolecular and supramolecular assemblies. Some of these boron compounds are thermodynamically and kinetically quite stable in solution and the solid state, even under hydrolytic conditions, a fact that has made them interesting for applications in host-guest chemistry and molecular recognition. The author of this review is sure that the full potential of boron in this ®eld of chemistry is still to be reached and important future developments will follow. Acknowledgement. The author thanks CONACyT for ®nancial support.

6 References 1. See for example: (a) Lehn JM (1988) Angew Chem Int Ed Engl 27: 89; (b) Atwood JL (ed) 1990 Inclusion phenomena and molecular recognition, Plenum, New York; (c) VoÈgtle F (1991) Supramolecular Chemistry, Wiley, Chichester; (d) Schneider HJ, DuÈrr H (1991) Frontiers in supramolecular chemistry and photochemistry, VCH, New York; (e) Lehn JM (1995) Supramolecular chemistry, VCH, New York; (f) Lehn JM (ed) (1996) Comprehensive supramolecular chemistry, Pergamon, New York; (g) Lent CS (2000) Science 288: 1597 2. For comprehensive literature see for example: (a) Parker D (1996) (ed) Macrocycle synthesis. Oxford University Press, New York; (b) Schmidtchen, Berger M (1997) Chem Rev 97: 1609 3. See for example: Hoss R, VoÈgtle F (1994) Angew Chem Int Ed Engl 33: 375 and references cited therein 4. For recent reviews see: (a) Lawrence DS, Jiang T, Levett M (1995) Chem Rev 95: 2229; (b) Hunter CA (1995) Angew Chem Int Ed Engl 34: 1079; (c) Philp D, Stoddart JF (1996) Angew Chem Int Ed Engl 35: 1154; (d) Piguet C, Bernardinelli G, Hopfgartner G (1997) Chem Rev 97: 2005; (e) Linton B, Hamilton AD (1997) Chem Rev 97: 1669; (f) Slone RV, Benkstein KD, BeÂlanger S, Hupp JT, Guzei IA, Rheingold AL (1998) Coord Chem Rev 171: 221; (g) Batten SR, Robson R (1998) Angew Chem Int Ed Engl 37: 1460; (h) Stang PJ (1998) Chem Eur J 4:19; (i) Jones CJ (1998) Chem Soc Rev 27: 289; (j) Albrecht M (1998) Chem Soc Rev 27: 281; (k) Caulder DL, Raymond KN (1999) Acc Chem Res 32: 975; (l) Langley PJ, Hulliger J (1999) Chem Soc Rev 28: 279; (m) Navarro JAR, Lippert B (1999) Coord Chem Rev 185±186: 653; (n) Swiegers GF, Malefetse TJ (2000) Chem Rev

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6. 7.

8. 9. 10. 11. 12.

13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

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100: 3483; (o) Saalfrank RW, Uller E, Demleitner B, Bernt I (2000) Synergistic effect of serendipity and rational design in supramolecular chemistry. In: Structure and Bonding, Vol 96. Springer, Berlin Heidelberg New York, p 150; (p) Fujita M (2000) Molecular panelling through metal-directed self-assembly. In: Structure and Bonding, Vol 96. Springer, Berlin Heidelberg New York, p 178; (q) Leininger S, Olenyuk B, Stang PJ (2000) Chem Rev 100: 853; (r) Holliday BJ, Mirkin CA (2001) Angew Chem Int Ed 40: 2022 For recent reviews see: (a) Etter MC (1990) Acc Chem Res 23: 120; (b) Lehn JM (1990) Angew Chem Int Ed Engl 29: 1304; (c) Whitesides GM, Mathias JP, Seto CT (1991) Science 254: 1312; (d) Etter MC (1991) J Phys Chem 95: 4601; (e) Lindsey (1991) New J Chem 15: 153; (f) Aakeroy CB, Seddon KR (1993) Chem Soc Rev: 397; (g) Subramanian S, Zaworotko MJ (1994) Coord Chem Rev 137: 357; (h) Whitesides GM, Simanek EE, Mathias JP, Seto CT, Chin DN, Mammen M, Gordon DM (1995) Acc Chem Res 28:37; (i) Desiraju GR (1995) Angew Chem Int Ed Engl 34: 2311; (j) Conn MM, Rebek J Jr (1997) Chem Rev 97: 1647; (k) Fyfe MCT, Stoddart JF (1997) Acc Chem Res 30: 393; (l) Jeffrey GA (1997) An introduction to hydrogen bonding. Oxford University Press, New York; (m) Melendez RE, Hamilton AD (1998) Top Curr Chem 198: 97; (n) Krische MJ, Lehn JM (2000) The utilization of persistent H-bonding motifs in the self-assembly of supramolecular architectures. In: Structure and Bonding, Vol 96, Springer, Berlin Heidelberg, p 3; (o) Braga D, Grepioni F (2000) Acc Chem Res 33: 601; (p) Prins LJ, Reinhoudt DN, Timmerman P (2001) Angew Chem Int Ed 40: 2382 (a) Charnley T, Skinner HA, Smith NB (1952) J Chem Soc: 2288; (b) Cottrell TL (1958) The strengths of chemical bonds. 2nd edn, Butterworths, London Housecraft CE (1995) Compounds with three- or four-coordinate boron, emphasizing cyclic systems. In: Abel EW, Stone FGA, Wilkinson G (eds), Comprehensive Organometallic Chemistry II, A Review of the Literature 1982±1994, Vol 1. Pergamon Press, Oxford HoÈp¯, H. (1999) J Organomet Chem 581: 129 and references cited therein See for example: (a) Gutsche CD (1989) Calixarenes. In: Stoddart JF (ed) Monographs in supramolecular chemistry. Royal Society of Chemistry, Cambridge (a) KaÈmmerer H, Happel G, Caesar F (1972) Makromol Chem 162: 179; (b) Happel G, Mathiasch B, KaÈmmerer H (1975) Makromol Chem 176: 3317 McMurry JE, Phelan JC (1991) Tetrahedron Lett 32: 5655 With sulfur as heteroatom: (a) Zamaev IA, Shklover VE, Ovchinnikov YE, Struchkov YT, Astankov AV, Nedel'kin VI, Sergeyev VA (1989) Acta Cryst, Sect C 45: 1531; (b) Zamaev IA, Shklover VE, Ovchinnikov YE, Struchkov YT, Astankov AV, Nedel'kin VI, Sergeyev VA (1990) Acta Cryst, Sect C46: 643 With nitrogen as heteroatom: Ito A, Ono Y, Tanaka K (1998) New J Chem 22: 779 With silicon as heteroatom: Yoshida M, Goto N, Nakanishi F (1999) Organometallics 18: 1465 KoÈnig B, Fonseca MH (2000) Eur J Inorg Chem 2303 Meller A, FuÈllgrabe HJ, Habben CD (1979) Chem Ber 112: 1252 Meller A, FuÈllgrabe HJ (1975) Angew Chem Int Ed 14: 359 Meller A, FuÈllgrabe HJ (1978) Z Naturforsch 33b: 156 Meller A, FuÈllgrabe HJ (1978) Chem Ber 111: 819 (a) Ishikura M, Kamada M, Terashima M (1984) Heterocycles 22: 265; (b) Ishikura M, Kamada M, Terashima M (1984) Synthesis 936; (c) Ishikura M, Kamada M, Ohta T, Terashima M (1984) Heterocycles 22: 2475 Sugihara Y, Miyatake R, Takakura K, Yano S (1994) J Chem Soc Chem Commun 1925 Sugihara Y, Takakura K, Murafuji T, Miyatake R, Nakasuji K, Kato M, Yano S (1996) J Org Chem 61: 6829 Murafuji T, Mouri R, Sugihara Y (1996) Tetrahedron 52: 13933 Barba V, Gallegos E, Santillan R, FarfaÂn N (2001) J Organomet Chem 622: 259 Smith KM (1984). In: Katritzky AR, Rees CW (eds) Comprehensive Heterocyclic Chemistry. Vol 4, Pergamon Press, Oxford, p 378

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26. Pohl M, Schmickler H, Lex J, Vogel E (1991) Angew Chem Int Ed Engl 30: 1693 27. Vogel E, Hass W, Knipp B, Lex J, Schmickler H (1988) Angew Chem Int Ed Engl 27: 406 28. For structurally characterized tetraoxaporhyrinogens, see: (a) Haas W, Knipp B, Sicken M, Lex J, Vogel E (1988) Angew Chem Int Ed Engl 27: 409; (b) Hazell A (1989) Acta Cryst Sect C45: 137; (c) Jones PG, Bubenitschek P, Hopf H (1994) Z Kristallogr 209: 777 29. For structurally characterized tetraazaporhyrinogens, see: (a) Meng JB, Fu DC, Wang YM, Jia WL, Yiao XK, Wang HG (1992) Chinese J Struct Chem 11: 376; (b) Gale PA, Sessler JL, Kral V, Lynch V (1996) J Am Chem Soc 118: 5140; (c) Allen WE, Gale PA, Brown CT, Lynch VM, Sessler JL (1996) J Am Chem Soc 118: 12471; (d) Turner B, Botosohansky M, Eichen Y (1998) 37: 2475 30. For a structurally characterized tetrathiaporhyrinogen, see: Vogel E, Rohrig P, Sicken N, Knipp B, Herrmann A, Pohl M, Schmickler H, Lex J (1989) Angew Chem Int Ed Engl 28: 1651 31. Niedenzu K, Woodrum KR (1989) Inorg Chem 28: 4022 32. Brock CP, Comanion AL, Kock LD, Niedenzu K (1991) Inorg Chem 30: 784 33. Weiss A, Pritzkow H, Siebert W (2000) Angew Chem Int Ed 39: 547 34. MuÈller KD, Komorowski L, Niedenzu K (1978) Synth React Inorg Chem: 149 35. Boenig IA, Conway WR, Niedenzu K (1975) Synth React Inorg Chem 5: 1 36. Effendy, Lobbia GG, Pellei M, Pettinari C, Santini C, Skelton BW, White AH (2001) J Chem Soc, Dalton Trans 528 37. See for example: Garret TM, Koert U, Lehn JM, Rigault A, Meyer D, Fischer J (1990) J Chem Soc 557 38. Baxter PNW, Lehn JM, Fischer J, Youinou MT (1994) Angew Chem Int Ed Engl 33: 2284 39. Cambridge Structural Database System (2000), v5.21. Cambridge Crystallographic Data Centre, Cambridge 40. (a) Chaudhuri P, Karpenstein I, Winter M, Lengen M, Butzlaff C, Bill E, Trautwein AX, Florke U, Haupt HJ (1993) Inorg Chem 32: 888; (b) Hoskins BF, Robson R, Slizys DA (1997) J Am Chem Soc 119: 2952; (c) Lai SW, Chan MCW, Peng SM, Che CM (1999) Angew Chem Int Ed Engl 38: 669 41. CarreÂ FH, Corriu RJP, Deforth T, Douglas WE, Siebert W, Weinmann W (1998) Angew Chem Int Ed 37: 652 42. Krebs B, HuÈrter HU (1980) Angew Chem Int Ed Engl 19: 481 43. Diercks H, Krebs B (1977) Angew Chem Int Ed Engl 16: 313 44. Gimarc BM, Trinajstic (1982) Inorg Chem 21: 21 45. Gimarc BM, Zhu JK (1983) Inorg Chem 22: 479 46. Yalpani M, Boese R, Seevogel K, KoÈster R (1993) J. Chem Soc Dalton Trans 47 47. Lang A, NoÈth H, Schmidt M, (1995) Chem Ber 128: 751 48. Kramer GW, Brown HC (1977) J Organomet Chem 132: 9 49. Shorygin PP, Lopatin BV, Boldyreva OG, Bogdanov VS (1971) J Gen Chem USSR (Engl Transl) 41: 2731 50. De Meester P, Skapski AC (1973) J Chem Soc Dalton Trans 1194 51. Cotton FA, Dikarev EV, Petrukhina MA (1999) Inorg Chim Acta 284: 304 52. (a) Hursthouse MB, Malik MA, Motevalli M, O'Brien P (1991) Chem Commun 1690; (b) Abrahams I, Malik MA, Motevalli M, O'Brien P (1995) J Chem Soc Dalton Trans 1043 53. Rad'kov YF, Bochkarev MN, Zakharov LN, Saf 'yanov YN, Khorshev SY, Struchkov YT (1991) Metallorg Khim (Organomet Chem in USSR) 4: 920 54. (a) Ng SW, Das VGK, Pelizzi G, Vitali F (1990) Heteroatom Chem 1: 433; (b) Gielen M, Khlou® AE, Biesemans M, Kayser F, Willem R, Mahieu B, Maes D, Lisgarten JN, Wyns L, Moreira A, Chattopadhay TK, Palmer RA (1994) Organometallics 13: 2849 55. Troyanov SI, Pisarevsky AP (1993) Chem Commun 335 56. (a) Rodesiler PF, Amma EL (1974) Chem Commun 599; (b) Drew MGB, Edwards DA, Richards R (1977) J Chem Soc Dalton Trans 299 57. Kozitsyna NY, Ellern AM, Struchkov YT, Moiseev IY (1992) Mendeleev Commun 100

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202. Vasilevsky I, Rose NJ, Stenkamp RE (1992) Acta Cryst B48: 444 203. Nanda KK, Addison AW, Paterson N, Sinn E, Thompson LK, Sakaguchi U (1998) Inorg Chem 37: 1028 204. Powell J, Horvath MJ, Lough A (1995) J Chem Soc Dalton Trans 2975 205. Boston DR, Rose NJ (1968) J Am Chem Soc 90: 6859 206. Zakrzewski GA, Ghilardi CA, Lingafelter EC (1971) J Am Chem Soc 93: 4411 207. Boston DR, Rose NJ (1972) J Am Chem Soc 96: 4163 208. Jackals S, Rose NJ (1973) Inorg Chem 12: 1232 209. Robbins MK, Naser DW, Heiland JL, Grzybowski JJ (1985) Inorg Chem 24: 3381 210. Muller JG, Grzybowski JJ, Takeuchi KJ (1986) Inorg Chem 25: 2665 211. Blandamer MJ, Burgess J, Fawcett J, Radulovic S, Russel DR (1988) Transition Met Chem 13: 120 212. Voloshin YZ, Kostromina NA, Nazarenko AY (1990) Inorg Chim Acta 170: 181 213. Lindeman SV, Voloshin YZ, Struchkov YT (1990) Koord Khim 16: 1367 214. Lindeman SV, Struchkov YT, Voloshin YZ (1991) Inorg Chim Acta 184: 107 215. Lindeman SV, Struchkov YT, Voloshin YZ (1993) Pol J Chem 67: 1575 216. Zavodnik VE, Belsky VK, Voloshin YZ, Varzatskii OA (1993) J Coord Chem 28: 97 217. Kubow SA, Takeuchi KJ, Grzybowski JJ, Jircitano AJ, Goedken VL (1996) Inorg Chim Acta 241: 21 218. See RF, Churchill MR, Lance KA, Mersman DP, Williams KR (1997) Inorg Chim Acta 257: 285 219. Voloshin YZ, Varzatskii OA, Palchik AV, Stash AI, Belsky VK (1999) New J Chem 23: 355 220. Voloshin YZ, Varzatskii OA, Kron TE, Belsky VK, Zavodnik VE, Strizhakova NG, Palchik AV (2000) Inorg Chem 39: 1907 221. Kirchner RM, Mealli C, Bailey M, Howe N, Torre LP, Wilson LJ, Andrews LC, Rose NJ, Lingafelter EC (1987) Coord Chem Rev 77: 89 and references cited therein 222. Comba P, Sargeson AM (1986) Phosphorus Sulfur 28: 137 223. (a) Gribble J, Wherland S (1989) Inorg Chem 28: 2859; (b) Borchardt D, Wherland S (1986) Inorg Chem 25: 901 224. Lippard SJ, Feig AL (1994) Chem Rev 94: 759 225. Cotton FA, Adams R (eds) (1997) Catalysis by di- and polynuclear complexes. VCH, Weinheim 226. (a) Kahn O (1993) Molecular Magnetism. VCH, Weinheim; (b) Kahn O (1995) Adv Inorg Chem 73: 49; (c) Balzani V, Juris A, Ventury M, Campagha S, Serroni S (1996) Chem Rev 96: 759 227. Treher EN, Francesconi LC, Malley MF, Gougoutas JZ, Nunn AD (1989) Inorg Chem 28: 3411 228. Linder KE, Malley MF, Gougoutas JZ, Unger SE, Nunn AD (1990) Inorg Chem 29: 2428 229. Jurisson S, Francesconi L, Linder KE, Treher KE, Malley MF, Gougoutas JZ, Nunn AD (1991) Inorg Chem 30: 1820 230. Voloshin YZ, Stask AI, Varzatskii OA, Belsky VK, Maletin YA, Strizhakova NG (1999) Inorg Chim Acta 284: 180 231. Parks JE, Wagner BE, Holm RH (1970) J Am Chem Soc 92: 3500 232. Parks JE, Wagner BE, Holm RH (1971) Inorg Chem 10: 2472 233. Churchill MR, Reis AH (1973) J Chem Soc Dalton Trans 1570 234. Churchill MR, Reis AH Jr (1972) Inorg Chem 12: 2280 235. Churchill MR, Reis AH (1972) Inorg Chem 11: 1811 236. Churchill MR, Reis AH (1972) Inorg Chem 11: 2299 237. Reetz MT, Niemeyer CM, Harms K (1991) Angew Chem Int Ed Engl 30: 1472 238. Reetz MT, Niemeyer CM, Harms K (1991) Angew Chem Int Ed Engl 30: 1474 239. Reetz MT, Niemeyer CM, Hermes M, Goddard R (1992) Angew Chem Int Ed Engl 31: 1017 240. Paugam MF, Valencia LS, Boggess B, Smith BD (1994) J Am Chem Soc 116: 11203 241. Shriver DF, Biallas MJ (1967) J Am Chem Soc 89: 1078

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242. (a) Katz HE (1985) J Am Chem Soc 107: 1420; (b) Katz HE (1985) J Org Chem 50: 5027; (c) Katz HE (1986) J Am Chem Soc 108: 7640; (d) Katz HE (1987) Organometallics 6: 1134; (e) Katz HE (1989) J Org Chem 54: 2179 243. Narasaka K, Sakurai H, Kato T, Iwasawa N (1990) Chem Lett 1271 244. Sakurai H, Iwasawa N, Narasaka K (1996) Bull Chem Soc Jpn 69: 2585 245. Nozaki K, Yoshida M, Takaya H (1996) Bull Chem Soc Jpn 69: 2043 246. Nozaki K, Yoshida M, Takaya H (1994) Angew Chem Int Ed Engl 33: 2452 247. Nozaki K, Tsutsumi T, Takaya H (1995) J Org Chem 60: 6668 248. James TD, Sandanayake KRAS, Shinkai S (1996) Angew Chem Int Ed Engl 35: 1910 249. Sandanayake KRAS, Shinkai S (1994) J Chem Soc Chem Commun 1083 250. Koumoto K, Shinkai S (2000) Chem Lett 856 251. Mizuno T, Takeuchi M, Shinkai S (1999) Tetrahedron 55: 9455 252. James TD, Sandanayake KRAS, Shinkai S (1994) J Chem Soc Chem Commun 477 253. James TD, Sandanayake KRAS, Shinkai S (1994) Angew Chem Int Ed Engl 33: 2207 254. Sandanayake KRAS, James TD, Shinkai S (1995) Chem Lett 503 255. James TD, Sandanayake KRAS, Shinkai S (1995) Nature 374: 345 256. Tsukagoshi K, Shinkai S (1991) J Org Chem 56: 4089 257. Shiomi Y, Saisho M, Tsukagoshi K, Shinkai S (1993) J Chem Soc Perkin Trans 1 2111 258. Shiomi Y, Kondo K, Saisho M, Harada T, Tsukagoshi K, Shinkai S (1993) Sup Mol Chem 2: 11 259. Kondo K, Shiomi Y, Saisho M, Harada T, Shinkai S (1992) Tetrahedron 48: 8239 260. Nakashima K, Shinkai S (1994) Chem Lett 1267 261. Sandanayake KRAS, Nakashima K, Shinkai S (1994) J Chem Soc Chem Commun 1621 262. Takeuchi M, Imada T, Shinkai S (1996) J Am Chem Soc 118: 10658 263. Shinkai S, Ikeda M, Sugasaki A, Takeuchi M (2001) Acc Chem Res 34: 494 264. Deng G, James TD, Shinkai S (1994) J Am Chem Soc 116: 4567 265. Imada T, Kijima K, Takeuchi M, Shinkai S (1995) Tetrahedron Lett 36: 2093 266. James TD, Shinkai S (1995) J Chem Soc Chem Commun 1483 267. Sugasaki A, Ikeda M, Takeuchi M, Shinkai S (2000) Angew Chem Int Ed 39: 3839 268. Sugasaki A, Ikeda M, Takeuchi M, Koumoto K, Shinkai S (2000) Tetrahedron 56: 4717

Multiple Bonding Between Heavier Group 13 Elements Philip P. Power Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA e-mail: [email protected]

Experimental and theoretical data for stable compounds that have the potential for multiple bonding between heavier group 13 metals have been surveyed. In spite of the fact that only about a dozen such compounds have been isolated under ambient conditions, the area has generated widespread debate owing to widely differing opinions on the multiplicity of the bonding in some compounds, especially the `gallyne' Na2Ga2(C6H3-2,6-Trip2)2 (Trip = C6H2-2,4,6-i-Pr3). The experimental and theoretical data show that group 13A13 heavier element multiple bonding is weak. Only two neutral multiply bonded compounds, i.e., the radical species (t-Bu)3SiMM{Si(t-Bu)3}2 (M = Al or Ga), are known to be stable at room temperature. The other compounds, the majority of which are gallium derivatives, are either mono- or dianionic. All the currently known dianionic species depend on the presence of bridging alkali metal counter cations for stability. Recent computational data also indicate the presence of some alkali metal-group 13 metal bonding in the dianion complexes. Possible future directions and experimental routes to resolving bonding arguments are outlined. Keywords: Multiple bonding, Clusters, Group 13 elements, Radicals, Terphenyls, Silyl ligands, Bulky groups

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

58

2 Bonding in the Neutral Dimers RMMR (M = Al, Ga, In, Tl, R = H, alkyl or aryl) and the Neutral Radicals RMMR2´ [M = Al or Ga, R = Si(t-Bu)3] . . . . . . . . . . . . . . . . . . . . . .

59

3 Multiple MAM Bonds via Reduction of R2MMR2 Species [M = Al, Ga; R = C6H2-2,4,6-i-Pr3 (Trip) or ±CH(SiMe3)2] and R2MMR´ [M = Al, Ga; R = Si(t-Bu)3] to Monoanions . . . . . . . .

64

4 Heavier Group 13 MAM Multiple Bonding in Reduced Metal Cluster Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

70

5 Bonding in the Compound Na2Ga2(C6H3-2,6-Trip2)2 . . . . . . . . . . . .

75

6 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

80

7 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

82

Structure and Bonding, Vol. 103 Ó Springer-Verlag Berlin Heidelberg 2002

58

P. P. Power

List of Abbreviations Mes THF TMEDA Trip

C6H2-2,4,6-Me3 (mesityl) tetrahydrofuran 1,1,2,2-tetramethylethylenediamine C6H2-2,4,6-i-Pr3

1 Introduction The investigation of multiple bonding between heavier main group elements of periods 13 through 16 has been a major focus of inorganic and organometallic research since the mid 1970s [1, 2]. However, this work has been concentrated mainly on the multiple bonding of elements of groups 14, 15, or 16, and the ®rst authentic reports of multiple bonding between heavier group 13 elements were not published until 1993 [3±5]. There were several reasons for this belated development. The major one was the undoubted weakness of bonding between these elements. Although there are no thermochemical data currently available for heavier group 13 element-element bonds in molecular species, spectroscopic and computational data on a range of compounds point to bond energies that are considerably lower than those of analogous bonds between group 14 or 15 elements of the same period [6±17]. The low bond strengths have their origin in the fact that these elements are relatively large and electropositive [18]. Consequently, their valence electrons lie at a relatively high energy and are less strongly bound. These atomic properties are perpetuated in their homonuclear covalent bonding. The weakness of the homonuclear bonding of these elements is in sharp contrast to the high strength of the bonds they form to more electronegative atoms [19] such as C, N, O, S, P, or the halogens. There is a large ionic component in these semipolar bonds which greatly increases their strength. An additional important factor affecting the bonding of the heavier group 13 elements is the limited number of valence electrons available for bond formation. In neutral molecules the use of the three valence electrons to form three electron pair bonds necessarily leaves a valence orbital unoccupied. This usually results in association or, in cases where one of the bonds involves another group 13 metal, a disproportionation reaction such as that shown in Eq. (1). 1

Thus, attempts to isolate and characterize stable, singly bonded R2MAMR2 species with common organic substituent groups such as Me, Et, i-Bu, or Ph were generally unsuccessful. However, in 1988 Uhl and coworkers showed that

Multiple Bonding Between Heavier Group 13 Elements

59

a stable aluminum compound of this type could be isolated with use of a bulky substituent as in R2AlAAlR2 [R = CH(SiMe3)2] [20]. This compound was the ®rst structurally characterized molecule incorporating an AlAAl single bond. Compounds containing single GaAGa [21] or InAIn bonds [22] had been known from earlier work although these were not organometallic species. At present ca. 120 compounds with MAM (M = Al, Ga, In, Tl) bonds have been structurally characterized and these have been the subject of a number of reviews [2d, 23±25]. Like single bonds, the stabilization of multiple bonds between group 13 metals also requires the use of bulky substituents. This requirement is unsurprising since, with rare exceptions, bulky groups are essential for stability in multiply bonded compounds of the neighboring heavier group 14 or 15 elements. However, the low numbers of valence electrons in the group 13 derivatives introduce added dif®culties, since there are usually insuf®cient numbers of electrons available for the formation of multiple bonds. For this reason multiple bonding is usually observed only when electrons are added to the group 13A13 bonded system. With the exception of the radical species RMMR2´ {M = Al [26] or Ga [27, 28]; R = ASi(t-Bu3)}, there are no stable, neutral compounds having heavier main group 13A13 multiple bonding. The currently known, multiply bonded species are all anionic in character, with either solvent separated or contact alkali metal counter cations. As will be seen, the incorporation of contact alkali metal ions into a multiply bonded group 13A13 structure can have signi®cant effects on the stability of the group 13A13 bond, especially in dianionic species. This occurs through interaction of the alkali metal ion with the group 13A13 bonding molecular orbitals and with the organic ligand substituents. The latter type become particularly important when alkali metal cation-aryl ring interactions are present. The importance of such interactions can be gauged from the fact that no group 13A13 bond has been shown to exist in dianionic species without supporting (i.e., bridging) metal cations. The ambiguity introduced by such bridging is one of the reasons why the extent of group 13A13 metal multiple bonding in such compounds has resulted in such lively controversy. In this review the currently available data, both theoretical and experimental, are summarized. Although aspects of multiple bonding between heavier group 13 elements have been covered in a number of reviews [1a, 11c, 2, 19], there has been no treatment that has covered all the known classes of compounds.

2 Bonding in the Neutral Dimers RMMR (M = Al, Ga, In, Tl, R = H, alkyl or aryl) and the Neutral Radicals RMMR2´ [M = Al or Ga; R = Si(t-Bu)3] The simplest compounds that may contain multiple bonds between the heavier group 13 elements are the dimers of formula RMMR (M = Al, Ga, In or Tl, R = H, alkyl or aryl group). Simpler M2 diatomic molecules of the elements, which can be generated in the gas phase, are essentially, singly bonded, having triplet 3P ground states and dissociation energies [6, 14, 27±32] that range from

60

P. P. Power

Fig. 1. Drawing of the structure of [Tl{g5-C5(CH2Ph)5}]2 [35] without H atoms showing the long TlATl interaction

ca. 36 kcal mol)1 for Al2 to 14 kcal mol)1 for Tl2 (cf. single bond dissociation energies of ca. 50±23 kcal mol)1 in the neighboring elements Si to Pb [33]). The only structurally characterized group 13 compounds of the nominal dimeric formula RMMR are the indium and thallium cyclopentadienide derivatives [M{g5-C5(CH2Ph)5}]2 (M = In [34], Tl [35]) (Fig. 1), the thallium slippedicosahedral compound [TlB9H9C2Me2]2 [36], and the pentafulvalene dithallium species (g5:g5-t-Bu4C10H4)Tl2 [37]. In the cyclopentadienide compounds the Ê . In the slippedInAIn and TlATl distances are 3.631(2) and 3.632(1) A Ê icosahedral species the TlATl separation is 3.67 A, and in the pentafulvalene Ê . These distances are far derivative the shortest TlATl separation is 3.760 A Ê longer than the ca. 2.8 and 3.0 A expected for InAIn or TlATl single bonds on the basis of the sum of their radii [18], and indicate that the metalAmetal bonding is extremely weak and consists of feeble intermolecular interactions possibly involving lone pairs and metal p orbitals. It is interesting to note that no lower valent derivatives of either aluminum or gallium with organic or related ligands have been observed to associate as dimers. Instead, tetrameric, hexameric or octameric clusters are most commonly encountered [2d, 38], and many of these dissociate in solution or the vapor phase to afford monomers [2d, 38, 39]. There have a number of computational studies of hypothetical RMMR species [10±13, 40, 41]. The simplest compounds are the hydrides HMMH. Some calculated structural parameters and energies of the linear and transbent metal-metal bonded forms of the hydrides are given in Table 1. It can be seen that in each case the trans-bent structure is lower in energy than the linear con®guration. However, these structures represent stationary points on the potential energy surface, and are not the most stable forms. There also exist mono-bridged, vinylidene or doubly bridged isomers as shown in Fig. 2 The doubly hydrogen bridged isomer [11, 12] is the most stable in all cases except boron [13]. The energies cited in Table 1 are relative to the zero value for the doubly bridged form. For the linear geometry, the ground state is a triplet (3R)g) owing to the degeneracy of valence p-orbitals. For the trans-bent

61

Multiple Bonding Between Heavier Group 13 Elements

Table 1. Calculated geometries for linear and trans-bent HMMH (M = Al, Ga, In, or Tl)

species

Structure

Ê) MAM (A

MAMAH/C (°) Rel. Energy (kcal mol)1)a

Ref.

HAlAlH

Linear Trans-bent Linear Trans-bent

2.298 2.613 2.295 2.737

180 120 180 120.1

28.8 13.5 29.1 13.7

[12] [12] [13] [13]

HGaGaH

Linear Trans-bent Linear Trans-bent Linear Trans-bent

2.218 2.951 ± 2.656

180 116.6 180 120.4

36.9 20.2 ± 12.4

[13] [13] [11] [11]

2.608

102.8

±

[41]

HInInH

Linear Trans-bent

2.545 3.329

180 116.7

44.3 24.0

[13]

HTlTlH

Trans-bent

3.28

115.1

±

[10]

a

These energies are relative to the most stable doubly hydrogen bridged form.

Fig. 2. Schematic drawing of some of the isomers of M2H2 (M = BATl)

isomers, the ground state is the singlet 1Ag. In this respect, the trans-bent dimer may be regarded as an associated pair of 1R+ molecules, as illustrated in Fig. 3 in which the nr lone pair of one MH unit is delocalized into one of the pp orbitals of its partner. The bond dissociation energies of the dimers relative to 2 MH (1R+) units were calculated to be very small ± just 10.3 kcal mol)1 for

Fig. 3. Schematic drawing of the polarized donor acceptor (paw-paw) bonds of trans-bent

HMMH

62

P. P. Power

Al2H2 and only ca. 3 kcal mol)1 for Ga2H2, In2H2, and Tl2H2 [13]. The low values for the heavier Ga, In, and Tl derivatives suggest that they are best regarded as weak intermolecular complexes rather than doubly bonded molecules. The trans-bending in these molecules may also be accounted for through a molecular orbital model involving mixing of MAM r* and p (as well as r and p*) levels to afford orbitals of increasing non-bonding character [40, 42] which, in the limit, would result in purely lone pair orbitals of the :MAH fragments. Trinquier and Malrieu have shown that related derivatives in geometries in heavier group 14 compounds could be correlated with the singlet-triplet energy separations in the molecular fragments [43]. This technique was applied to the group 13 M2H2 species by Treboux and Barthelat [13] who showed that a trans-bent distortion occurs when the singlet triplet separation (DES)T) of the MAH fragment is greater than half the bond energy in the linear singlet 1Dg state. If (DES)T) is greater than this bond energy, monomers should result. The singlet-triplet 1R+)3P separations in MAH species were calculated to be 42.7(Al), 46.6 (Ga), 47 (In) and 51.9 (Tl) kcal mol)1 (Table 2) whereas the MAM (r+p) bond dissociation energies (DETBE) for the linear (1dg) form of HMMH were calculated to be 75 (Al), 68 (Ga), 67 (In) and 50 (Tl) kcal mol)1 (Table 2). Thus for all the heavier group 13 element HMMH species the sum of DES)T values is calculated to be always greater than DETBE and a trans-bent structure is therefore predicted to be more stable than a linear structure except for Tl where a dissociated structure is predicted. It is interesting to compare the singlet tripletseparations with those of the neighboring group 14 element :MH2 (M = Si, Ge, Sn or Pb) species where values of ca. 20 (Si), 22 (Ge), 23 (Sn) and 41 (Pb) kcal mol)1 were calculated [43]. These energies are signi®cantly less than those in the group 13 elements whereas the calculated r + p bond energies of the planar group 14 elements dimers H2MMH2, 104 (Si), 96 (Ge), 82 (Sn) are higher than those of the group 13 dimers. Thus, the MAM bonding in the heavier group 13 HMMH species is much weaker than it is in the corresponding heavier group 14 H2MMH2 compounds. In fact, for the silicon and Table 2. Singlet-triplet energy separations (DEST) in M-H monomers, MAM bond dissociation energies (BDEs), SCF calculated bond lengths for the hypothetical, linear dimers HMMH (M = B, Al, Ga, In, or Tl)c

BAH AlAH GaAH InAH TlAH a

DEST a

MAM (BDEs)b

MAM (trans-bent)

27.7 42.7 46.6 47.0 51.9

138 75 68 67 50

± 2.737 2.951 3.329 ±

Ê A Ê A Ê A

All energies in kcal mol)1. Obtained from CI calculations using a linear HMMH species with 1Dg ground state. c Ref. [13]. b

Multiple Bonding Between Heavier Group 13 Elements

63

germanium species, the SDES)T value is less than half DETBE, so that planar or almost planar structures are predicted. Calculations have also been carried out on the hypothetical species Ga2Me2 which is predicted to have a trans-bent structure, a calculated GaAGa bond Ê , and a GaAGaAC angle of 123°. In terms of the canonical length of 2.654 A molecular orbitals, it was described as having a weak single bond rather than a double one [40]. The highest occupied molecular orbital (HOMO) (15bu) was characterized as mainly lone pair in character with the HOMO-1 (15 ag) being a weak r-bond. Removal of an electron from the HOMO (15bu) gave Ê (cf. 2.654 A Ê [MeGaGaMe]+ which has a shorter GaAGa bond length of 2.476 A in the neutral species) suggesting that the 15bu orbital of the neutral molecule exerts some antibonding effect which lengthens the GaAGa bond. These results are in basic agreement with the calculations on the Ga2H2 species indicating that the GaAGa bonding is quite weak, having a bond order that much is less than one. Very recent calculations [41] on a series of RGaGaR compounds with Ê, various substituents afford GaAGa distances and GaAGaAR angles of 2.608 A Ê Ê Ê 120.8° (R = H); 2.662 A, 123.7° (R = Me); 2.575 A, 123.3° (R = SiH3); 2.724 A, Ê , 115.9° (R = C6H3-2,6-Ph2). The results for the H and 121.2° (R = Ph); 2.716 A Me species resemble those in the early reports [10±13, 40]. Interestingly, the shortest GaAGa interaction is calculated for the SiH3 derivative which is in agreement with earlier calculations on group 14 triply bonded species [44]. It Ê ) were is also worth noting that the longest GaAGa interactions (ca. 2.72 A calculated for the aryl (i.e., Ph or C6H3-2,6-Ph2) substituted compounds. To summarize, the computational data indicate that the MAM bonding in the dimers RMMR (M = Al, Ga, In, Tl; R = H, Me, etc.) is quite weak, with a maximum bond dissociation energy of ca. 10 cal mol)1 being calculated for the aluminum derivative. As yet, no stable aluminum or gallium compounds of this formula have been experimentally characterized, and, in view of the expected weakness of the MAM bonding, their isolation and characterization is a formidable synthetic challenge. The few indium and thallium dimers discussed above [34±37] have very feeble metal-metal interactions with MAM Ê longer than single bonds. distances 0.6±0.8 A The only stable, neutral species with multiple bonding between group 13 metals are the odd-electron radical, compounds RMMR2´ {M=Al [26] or Ga [27,28]; R=Si(t-Bu)3}, which were synthesized by the reaction of excess NaSi(tBu)3 with MCl3. Only the gallium compound has been characterized by single crystal X-ray diffraction, and it is illustrated in Fig. 4. It features a GaAGa Ê (cf. the GaAGa single bond distances in Table 3 bond length of 2.423(1) A below) with a wide [170.34(6)°] Si-Ga(1)AGa(2) angle at the two-coordinate Ga(1). A simple bonding picture of this compound and its aluminum analogue involves overlap of sp(M(1)) and sp2(M(2)) hybridized metals to form an MAM r-bond. The bonding p-molecular orbital, which is formed by the overlap of pz orbitals of the two metal atoms, is occupied by the odd electron to afford a formal MAM bond order of 1.5. EPR spectroscopy showed that the unpaired electron displayed hyper®ne coupling to the two distinct aluminums or galliums with values of 18.9 and 21.8 G for aluminum and in the range

64

P. P. Power

Fig. 4. Schematic drawing of the structure of the odd-electron species t-Bu3SiGaGa{Si(t-Bu)3} [27] without H atoms

32±64 G for gallium. These values (see also in the next section) are consistent with the unpaired electron residing in a p-orbital. The stability of these compounds was attributed to the large size and electronic properties of the Si(t-Bu)3 substituents [26±28]. Computational data for the aluminum comÊ and a wide AlAAl-Si angle of pound indicate an AlAAl distance of 2.537 A 174.90° [26]. The longer distance for the aluminum species is a result of the larger covalent radius for this metal [18].

3 Multiple MAM Bonds via Reduction of R2MMR2 Species [M = Al, Ga; R = C6H2-2,4,6-i-Pr3 (Trip) or ±CH(SiMe3)2] and R2MMR´ [M = Al, Ga; R = Si(t-Bu)3] to Monoanions Compounds of the formula R2MMR2 (M = Al, Ga, In, Tl; R = alkyl or aryl) [19, 20, 23, 25] possess a single MAM bond. Since the metals are three coordinate, they also have an empty p-orbital at each metal that lies perpendicular to the metal coordination plane. It is therefore possible, at Table 3. Metal-metal bond lengths and torsion angles in some tetraorganodialanes

and -digallanes and their reduced monoanion derivatives

R2AlAlR2 (R = CH(SiMe3)2) R02 AlAlR02 (R¢ = C6H2-2,4,6-i-Pr3) [R2AlAlR2]) [R2AlAlR02 ]) R2GaGaR2 (R = CH(SiMe3)2) R02 GaGaR02 (R¢ = C6H2-2,4,6-i-Pr3) [R2GaGaR2]) [R02 GaGaR02 ]) a

Ê) MAM (A

Torsion Anglea

MAC

2.660 (1) 2.647 (3) 2.53 (1) 2.470 (2) 2.541 (1) 2.515 (3) 2.401 (1) 2.343 (2)

0 44.8 0 1.4 0 43.8 0 15.5

1.982 1.996 2.040 2.021 1.995 2.008 2.064 2.038

Angle between metal coordination planes.

Ref. (3) (3) (5) (1) (5) (7) (5) (10)

[20] [54] [3] [54] [53] [4] [55] [4]

Multiple Bonding Between Heavier Group 13 Elements

65

least in principle, to add either one or two electrons to these orbitals to form a p-bond (of formal order 0.5 or 1.0) between the metals. Products of this type were reported for aluminum and gallium by three different groups in 1993 [3±5]. Prior to this work it had been shown that singly reduced [R2BBR2]) radical anions could be generated in solution [44±46], and that doubly reduced contact ion pair salts of the formula [LnLi2R2BBR2] (L = donor ligand) could be isolated and structurally characterized [47±50]. The structures of solvent separated, singly reduced salts of the radical anions also were determined subsequently [51, 52]. Stirring the neutral precursors R2MMR2 {M = Al or Ga; R = ACH(SiMe3)2 [20,53] or AC6H2-2,4,6-i-Pr3 (Trip) [4, 54]} with alkali metal afforded singly reduced, radical anions [R2MMR2]´) which could be crystallized as solvent

Fig. 5. Schematic drawing of the structure of the salt [Li(TMEDA)2][R2AlAlR2] [R = CH(SiMe3)2] [3] without H atoms

Fig. 6. Schematic drawing of the structure of the salt [Li(12-crown-4)2] [Trip2GaGaTrip2] [4] without H atoms

66

P. P. Power

separated ion pairs with the addition of alkali metal complexing agents such bidentate alkyl amines, ethers or crown ethers (Figs. 5 and 6) [3±5, 54, 55]. Some important structural parameters for these anions and their neutral precursors are presented in Table 3. The addition of one electron causes an Ê (ca. 7±8%) and the torsion MAM bond shortening in the range 0.13±0.18 A angle between the metal planes decreases in the case of the aryl derivatives. The torsion angles in both the alkyl precursors and their anions are either at or near 0°, but this may be a fortuitous effect of packing in these cases. EPR spectroscopy of the anions shows that the unpaired electron displays equal hyper®ne coupling to each metal nucleus which shows that the unpaired electron occupies an orbital that is shared equally between the metals. The EPR spectrum of [Trip2AlAlTrip2]´) is presented in Fig. 7 and shows an 11-line pattern owing to coupling to two 27Al (I = 5/2, 100%) nuclei. The relatively low values of the hyper®ne coupling constants (ca. 10±12 G for 27Al and 35±40 G for 69/71Ga) indicate that this orbital is p in character [54]. In summary, the structural and spectroscopic data are consistent with the generation of a p-bond of formal order 0.5 between the metals. Furthermore, there are no complicating factors due either to bridging of the group 13 metal-metal bonds by counter cations or the complexation of counter cations in close proximity to the metals. In other words, these multiple MAM bonds are unsupported by ancillary effects. The attempted addition of a second electron to afford the dianion [R2MMR2]2) (M = Al or Ga) has not been successful to date. The various attempts [3±5] to synthesize these doubly bonded aluminum and gallium ethene analogues stand in contrast to the situation for the corresponding

Fig. 7. EPR spectrum of [Li(TMEDA)2] [Trip2AlAlTrip2] [54] showing the hyper®ne coupling to two equivalent 27Al (I =5/2) nuclei

Multiple Bonding Between Heavier Group 13 Elements

67

boron dianions which can be isolated as stable species [47±50]. Electrochemical studies on the gallium dimer Trip2GaGaTrip2 (Trip = C6H2-2,4,6-i-Pr3) showed that it underwent two reductions [56]. The ®rst was quasi-reversible. In contrast, the second reduction produced an irreversible pattern in the cyclic voltammogram at a very negative voltage. This raised the question of whether the failure to isolate the dianionic salt was due to its reactivity toward the ether solvent or to an inherent instability of the dianion owing to coulombic repulsions. It was found that if the less reactive solvent NEt3 were employed in the reduction of Trip2GaGaTrip2 instead of an ether, the previously unobserved species Na2{Ga(GaTrip2)3} (Fig. 8) was isolated in preference to the expected product containing the dianion [Trip2GaGaTrip2]2) [57]. The structure illustrated in Fig. 8 features a central gallium bound to three GaTrip2 moieties which yields the dianion [Ga(GaTrip2)3]2). This forms a contact ion triple with two bridging Na+ ions. The four galliums provide an approximately trigonal planar array of four p-orbitals which can be combined to afford a1 (bonding), e (non-bonding) and a1 (anti-bonding) molecular orbitals by D3 symmetry. The 2) charge is thus accommodated in the bonding a1 molecular orbital which renders multiple character (formal bond order Ê . The Na+ 1.33) to the GaAGa bonds ± average GaAGa length = 2.389(17) A ions are complexed by the aryl rings of the Trip substituents and the closest Ê . The crowded nature of Na+-Ga approach involves Ga(2)-Na(2) at 3.144(2) A the molecule is indicated by torsion angles of 17.7, 32.2 and 28.5° (instead of the ideal 0°) between the planes at Ga(2), Ga(3) and Ga(4) and the coordination plane at central Ga(1). The presence of multiple GaAGa bonding delocalized across the Ga4 array is supported by the structural changes observed upon two electron oxidation of the neutral molecule Ga(GaTrip2)3. Ê becomes signi®cantly longer than that The average GaAGa distance 2.476(7) A in the reduced species and the torsion angles between the planes at the

Fig. 8. Schematic drawing of the structure of the salt Na2Ga(GaTrip2)3 [57]. H atoms are not shown

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P. P. Power

peripheral galliums and the central gallium increase to an average value of 77°. In effect, the two-electron oxidation destroys the multiple character of the GaAGa bonding by removing two electrons from the delocalized a1 p-bond. The decomposition of the Trip2MMTrip2 upon double reduction is therefore not a consequence of a reaction with solvent in this case, but is due to the inherent instability of the dianion [Trip2MMTrip2]2). Nonetheless, Uhl and coworkers have clearly shown that such anions can also react with solvent through further reduction of the related radical anion species [R2AlAlR2]) [R = CH(SiMe3)2] in 1,2-dimethoxyethene, which leads to the formation of ether cleavage products that have no AlAAl bonds [24, 58, 59]. Overall, the experimental ®ndings show that the putative [R2MMR2]2) species are both highly reducing and unstable. These properties are probably due to the electropositive character of the group 13 metals and the large Coulombic repulsion arising from the double negative charge localized for the most part on the M2 moiety. The experimental results have received support from recent computational data on various [15, 60] H2MMH2, [H2MMH2]), [H2MMH2]2) and Li2{H2MMH2} (M = B)Tl) species (Table 4) [15]. The calculations show that the addition of one electron to form the monoanion [M2H4]) is exothermic in all cases. Furthermore, the planar (D2h) form of the anion is more stable than the twisted (by 90°) D2d form by 22.7 (B), 14.8 (Al), 16.5 (Ga), 13.4 (In) kcal mol)1 suggesting that p-bonds of these approximate strengths have formed. These calculated numbers are in reasonable agreement with values inferred from p-p* transitions in the anions [Trip2MMTrip2]) from which single p-bond energies of ca. 19 (Al) and 17 (Ga) kcal mol)1 were estimated [19]. The bond order in the monoanions is formally 1.5 in each case, although the calculated bond orders are less than this value. In any event, the extent of the p-bonding is suf®ciently great for the planar form (in which the p-overlap is maximized) to be favored. Ê are in The calculated bond distances 1.63 (B), 2.48 (Al), 2.33 (Ga) and 2.72 (In) A good agreement with the known experimental values (cf. Table 3). The most interesting results of the computational data, however, concern the addition of the second electron which is strongly endothermic in all cases. In other words, these dianions are predicted to dissociate an electron in the gas phase spontaneously. In solution or the solid state, it is possible that the ions could be stabilized by the effects of counter cations. This stabilization has been accomplished for the boron dianions which crystallized as contact ion pairs [47±50]. However, as indicated above, no success has been achieved so far in Ê ) for group 13 (M = B, Al, Ga, In) [M2H4]0,)1,)2 systemsa Table 4. Optimized bond lengths (A M

H2MAMH2

[H2M@MH2])

[H2M@MH2]2)

B Al Ga In

1.74 2.61 2.45 2.83

1.63 2.48 2.33 2.72

1.61 2.46 2.46 2.71

a

Data from Ref. [15].

Multiple Bonding Between Heavier Group 13 Elements

69

the isolation of the heavier analogues. Apparently, simple interactions between the cations and the anionic centers are insuf®cient to enable their isolation although calculations show that the stabilization of the p-orbital by the alkali metal cation complexation is signi®cant. The cation complexation also results in decreased MAM bond lengths in all cases which further underlines the key importance of the presence of alkali metal cations for stability [15]. It seems probable that the employment of ligands which can also complex alkali metal cations in close proximity to the group 13 metal-metal moiety may stabilize it against decomposition. These results, as well as earlier calculations on [Ph2MMPh2]n) (M = B or Al; n = 0, 1 or 2) clearly show that the added electrons occupy a p-orbital [61]. It had earlier been suggested [19] that the second electron might have entered a r*- instead of p-level in order to account for the instability of the dianions. This is clearly not the case, and, as already discussed, the instability is due to Coulombic repulsions. Since both electrons occupy the p-level, the MAM bonds in the dianions are predicted to be double ones although the calculated bond orders are all less than 2. The calculated MAM distances in the [H2MMH2]2) are Ê [15] and in [Ph2MMPh2]2) they are 1.61 (B), 2.46 (Al), 2.46 (Ga), 2.71 (In) A Ê 1.622 (B) and 2.443 (Al) A [61] which are marginally shorter than those in the singly reduced species. The relatively modest decreases in MAM bond length upon addition of the second electron have been rationalized on the basis of a build up of electronic charge which causes the overall electron density to expand and the effective radius of the atom to increase. This offsets the expected bond length decrease as a result of the bond order increase. Since the dianions [H2MMH2]2) (M = B)Tl) are isoelectronic to the corresponding neutral H2MMH2 (M = C)Pb) species, an increasing tendency toward pyramidalization of the group 13 metal geometry analogous to that observed in the group 14 elements might be expected. This bending has been accounted for in terms of mixing of the MAM r* and p orbitals which have the same symmetry (bu) in the trans-bent (C2h) structure. By the same token, Bridgeman has shown that p*-r (both ag symmetry) mixing, which had hitherto been essentially neglected, becomes increasingly important as the group is descended. Calculations on [H2MMH2]2), [Me2MMMe2]2) (M = Al, Ga or In) do indeed predict trans-bending [15]. However, the energy differences between the planar and trans-bent forms are relatively small (maximum calculated value = 7.6 kcal mol)1 for [Me2InInMe2]2)) which indicates that the geometry is ¯oppy and can be easily affected by weak forces such as those encountered in crystal packing. In very recent work, Wiberg and coworkers have shown that the neutral RGaGaR2 [R = Si(t-Bu)3] radical may also be reduced by either Na or NaR to afford (THF)3NaRGaGaR2 which exists as a contact ion pair [Na-Ga(1) = Ê ] as shown in Fig. 9 [28]. The coordination at Ga(2) is planar and 3.205(2) A Ê which is signi®cantly shorter than the the GaAGa distance is 2.3797(6) A Ê observed in the neutral radical precursor RGaGaR2. The Si(1)2.423(1) A Ga(1)-Ga(2) angle closes to 142.41(4)°, owing to increased steric congestion caused by complexation of the Na(THF)3+ moiety to Ga(1). In spite of the very crowded nature of the molecule, the shortening of the GaAGa bond is

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Fig. 9. Schematic drawing of the structure of (THF)3NaRGaGaR2 [R = Si(t-Bu)3] [28]. H atoms are not shown

consistent with the addition of the ``extra'' electron to the p-orbital and the formation of Ga@Ga double bond.

4 Heavier Group 13 MAM Multiple Bonding in Reduced Metal Cluster Species Currently, the only stable examples of heavier group 13 element molecules in this class concern gallium derivatives. Reduction of the arylgallium dihalide Cl2GaC6H3-2,6-Mes2 (Mes = AC6H2-2,4,6-Me3) with either sodium [62] or potassium [63] by Robinson and coworkers afforded the product M2(GaC6H3 -2,6-Mes2)3 (M = Na or K). These were described as [(GaC6H3-2,6-Mes2)3]2) dianions complexed to alkali metal counter cations as shown in Fig. 10. The structures of the anions feature essentially equilateral triangles of galliums Ê (Na+ salt) [62] and with very similar average GaAGa distances of 2.411(1) A + Ê 2.425(5) A (K salt) [63]. The putative anionic rings contain two electrons in a delocalized (a1) p-orbital and are thus in agreement with the HuÈckel 4n+2 rule for aromaticity (n = 0 in this case). The formal bond order in the ring is 1.33 and the relatively short GaAGa bond lengths are in reasonable agreement with the multiple character of the bonds. Calculations also indicate the presence of a ring current consistent with aromaticity [64]. There are NaAGa and KAGa Ê and short Na±C and KAC contacts to the contacts near 3.23(1) and 3.56(2) A ortho-mesityl rings which probably play a key roÃle in the stabilization of the complex. An important feature of both structures is that the two GaAGaAC angles at each gallium vertex are unequal. Thus, the (dotted) lines drawn through the centroid are not coincident with the GaAC bond. In other words, the GaAC vector if extended through the gallium into the ring does not bisect the angle at the gallium vertex. This deviation may be illustrated by Fig. 11 which affords C3h rather than D3h local symmetry for the ring. This angular deviation averages ca. 14° in the potassium salt but is 32.6° for the sodium salt. The structural distortions are also seen in computational work [64]. The result

Multiple Bonding Between Heavier Group 13 Elements

71

Fig. 10. Schematic drawing of the Ga3 ring species K2(GaC6H3-2,6-Mes2)3 [63]. H atoms are not shown

of these are that the r-bonds assume the character of polar dative bonds (see illustration) and are weakened as a result. Indeed, the hypothetical, neutral three-membered structure (GaH)3 was calculated to be unstable. However, the presence of the 2p-electron cloud, formed by overlap of three gallium p-orbitals, and the two bridging alkali metal cations (which are also complexed by the mesityl rings), stabilize the molecule. Although the average GaAGa Ê is consistent with multiple character, it is not as bond length of ca. 2.42 A Ê short as the ca. 2.39 A in [Na2Ga(GaTrip2)2] (Fig. 6) and this ®nding is

Fig. 11. Illustrations of the distortions in the geometry of the [(GaC6H3-2,6-Mes2)3]2) ion as a

result of lone pair character

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P. P. Power

consistent with the weakening of the r-bond as suggested by the structural data. It would be interesting to know the values for the energies of electron dissociation of the [M3R3]2) (M = Al, Ga, In; R = H or Me) ions in order to assess the inherent stability of these dianions. There have also been computational studies of several three-membered rings related to the above doubly reduced cyclotrigallenes [65]. These concern species of formula o-{(GaH)(CH)2}, M[c-{GaH)2(CH)}], [c-{(GaH)2(CH)}]), c-{(GaH)(SiH)2}, M[c-{(GaH)(SiH)2}] (M = Li, Na or K) and [c-{(GaH) (SiH)2}]). It was concluded that all these hypothetical compounds were also aromatic with a 2p-electron orbital delocalized over the three-membered ring. Reduction of Cl2GaC6H3-2,6-Trip2 (Trip = C6H2-2,4,6-i-Pr3) with potassium affords the unusual K2Ga4 cluster species of formula K2Ga4(C6H3-2, 6-Trip2)2 whose structure is illustrated in Fig. 12 [66]. This ®nding is in contrast with results already obtained for the corresponding reduction using sodium which afforded the controversial species Na2Ga2(C6H3-2,6-Trip2)2 (see next section). The structure of K2Ga4(C6H3-2,6-Trip2)2 is centrosymmetric and features an Ê . Two almost perfectly square Ga4 core with GaAGa distances of ca. 2.47 A galliums [Ga(1) and Ga(1A)] carry a terphenyl substituent whose central ring is oriented almost perpendicularly to the Ga4 core. Ga(1) and Ga(1A) are planar coordinated and the external GaAGaAC angles at these galliums differ by about 6°. Two potassiums are located above and below the gallium plane. They do not lie directly above the centers of the Ga4 array but are displaced so Ê ) are longer than the that the K(1)AGa(1) and K(1)AGa(2) distances (ca. 3.82 A Ê K(1)AGa(1A) and K(1)AGa(2A) distances (ca. 3.53 A). The potassiums ions

Fig. 12. Schematic drawing of the K2Ga4 cluster species K2Ga4(C6H3-2,6-Trip2)2 [66]. H atoms are not shown

Multiple Bonding Between Heavier Group 13 Elements

73

Ê also interact with the Trip rings, and the closest K ¼ C distances are ca. 3.29 A Ê [C(0), C(11)] and 3.30 A [C(16A), C(17A)]. The GaAGa bonding was described as single on the basis that Ga(1) and Ga(1A) provide two electrons each for core bonding while Ga(2) and Ga(2A) (which carry a lone pair instead of an organic substituent) provide one electron each. Two further electrons are supplied by potassiums to afford a total of eight electrons for four GaAGa bonds and hence an average formal GaAGa bond order of one. This bond order is consistent with the GaAGa distance which is slightly shorter than the GaAGa single bonds in the tetraorganogallanes (Table 3). This shortening can be attributed in part to the unsubstituted, two-coordinated environment at two of the four galliums comprising the square which eases steric congestion. A more detailed picture of the bonding in this species is available from computational data [67]. A simple MO treatment of the Ga4 ring shows that only four GaAGa bonding molecular orbitals are allowed by symmetry. The remaining molecular orbitals concern two GaAC (bonding), six Ga4 (non-bonding) and six Ga4C2 (antibonding) levels. However, four of the Ga4 non-bonding MOs of eu symmetry can be combined to give two weakly transannular bonding and two corresponding antibonding MOs to afford a total of six bonding ring orbitals. The computational data for the hypothetical species [Ga4H2]2) and Na2{Ga4H2} allow evaluation of the energies and electron density surfaces for these MOs. Several are illustrated in Fig. 13. The uncomplexed Ê [Ga4H2]2) ion is predicted to be planar (C2h) with a Ga±Ga bond length of 2.504 A

Fig. 13. The energies and electron density surfaces of the orbitals relevant to GaAGa bonding in [Ga4H2]2) [67]

74

P. P. Power

which is slightly longer than the experimental ®nding but consistent with single bonding. The Ga4 arrangement is almost square with internal angles of 86.3° (GaH) and 93.7° (Ga). The HOMO is a r-orbital with components on four gallium gallium bonds. The HOMO-1 is a p-orbital delocalized over the four galliums which results from overlap of four gallium 4pz orbitals. The HOMO-2 is mainly a gallium lone pair orbital with some density located on the hydrogens, as well as some GaAGa transannular bonding involving the galliums of the GaAH units. The HOMO-3 also possesses some transannular GaAGa bonding character, but it is also localized to some extent on two of the gallium-hydrogen bonds. In addition, it appears to have GaAH r* antibonding character. The HOMO-4 is a gallium lone pair orbital. HOMO-5 has a large GaAH bonding localization as well as a minor transannular GaAGa component involving the lone pair galliums. The HOMO-6 primarily represents GaAH bonding, while HOMO-7 is formed by a positive overlap of orbitals from the six (i.e., Ga4H2) atoms, and is of uniform sign throughout. In sum, the HOMO, HOMO-1 and HOMO-7 are the levels that are most strongly associated with bonding between adjacent galliums of the ring. There are also varying transannular GaAGa bonding components in the lone pair orbitals in the HOMO-3 and HOMO-5 as well as some in the HOMO-2. Overall, however, it seems probable that the net result of these bonding orbitals is that the formal GaAGa bond order in the gallium square is close to, or slightly greater than one, which is in good agreement with that predicted by simple electron counting. Interestingly, since the HOMO-1 is a p orbital, the Ga4 ring is a delocalized 2p electron system and quali®es the [Ga4H2]2) species as an aromatic ring ± at least according to the HuÈckel rule ± although the bond order of the GaAGa bonds is ca. 1. In addition, the possible aromatic character of the free ion [Ga4H2]2) should not be allowed to obscure the fact that it is unstable toward electron dissociation as in Eq. (2). 2

which has been calculated to be exoergic to the tune of 62.5 kcal mol)1. Dianions of the type [Ga4R2]2) are only likely to be stable when they are complexed to counter cations. The importance of the alkali metals is readily apparent from calculations on Ê the hypothetical cluster Na2Ga4H2. The GaAGa bond is lengthened to 2.587 A although the Ga4 array remains almost square. The major effect of Na+ complexation on orbital energies is that GaAGa p-orbital (HOMO-1) in [Ga4H2]2) becomes stabilized by ca. 80 kcal mol)1 in Na2Ga4H2 and becomes HOMO-2. The r-bonding HOMO in [Ga4H2]2) becomes the HOMO in Na2Ga4H2 although it is almost identical in energy with the HOMO-1. The HOMO-1 in [Ga4H2]2) is also stabilized although to a much lesser extent (ca. 26 kcal mol)1) than the p-orbital. The HOMO-2 is also destabilized slightly. The difference between the calculated and experimental GaAGa bond lengths may be due to the complexation of the alkali metal ions by the aryl rings of the ligand. There is a relationship between these ®ndings for these M2Ga4H2 systems and those recently published for the gas phase ions [LiAl4]) and [NaAl4])

Multiple Bonding Between Heavier Group 13 Elements

75

which were shown to have square pyramidal structures on the basis of spectroscopic and computational data [68]. These structures can be regarded as being composed of an alkali metal ion and a square planar Al42) unit. A molecular orbital analysis of the latter shows that it bears a striking resemblance to that discussed above for the hydride derivatives [Ga4H2]2) and Na2Ga4H2. The HOMO is a p-orbital and, as this is delocalized over the four metals and contains two electrons, it conforms to the HuÈckel rule. The HOMO-1 is primarily lone pair in character with some transannular GaAGa bonding. The HOMO-2 is GaAGa r-bonding although HOMO-3 has antibonding and lone pair character; while the degenerate HOMO-4 levels are lone pair orbitals. The HOMO-5 has a strong resemblance to the HOMO-7 of [Ga4H2]2) and Na2Ga4H2. Thus, the maximum number of bonding molecular orbitals in this ring is four which affords an approximate bond order of one for the AlAAl bonds. The calculated AlAAl distance in the [NaAl4]) cluster is Ê which is consistent with AlAAl single bonding. The bond distance in 2.60 A Ê and the shorter distance in the the [Al4]2) dianion was calculated to be 2.58 A uncomplexed species parallels the behavior of Na2Ga4H2 and [Ga4H2]2). It seems likely that the longer AlAAl and GaAGa distances in the complexed species are the easier is the formation of alkali metal-Al or Ga bonds which lowers the electron density in the AlAAl or GaAGa bonding orbitals. In spite of the aromatic character of the Al42) ring, computational data indicate that it is unstable and spontaneously loses an electron. Thus, the presence of a counter cation is required for stability. The instability of the Al42) ring is probably due to the Coulombic repulsion from the double negative charge and the weakness of the AlAAl bonding which leads to molecular orbitals that are insuf®ciently low in energy to prevent the spontaneous dissociation of an electron. The initial results on the [NaAl4]) clusters have been extended to the heterocyclic ring species [XAl3]) (X = Si, Ge, Sn, Pb) bearing a single charge and as a result are not prone to dissociation of an electron like the [Al4]2) or [Ga4H2]2) anions [69]. A planar four-membered ring structure is in all cases the Ê. most stable structural isomer with AlAAl distances in the range 2.57±2.61 A

5 Bonding in the Compound Na2Ga2(C6H3-2,6-Trip2)2 The synthesis of this compound (by the reduction of Cl2GaC6H3-2,6-Trip2 with sodium) were published by Robinson and coworkers in mid 1997 [70]. It was described as a ``gallyne'' which involved a GaAGa triple bond. Without a doubt this description has resulted in more controversy and debate than has been associated with any other main group compound in recent years. Before attempting to discuss the arguments regarding the bonding, it is worthwhile to summarize the experimentally determined properties of this species. The structure is illustrated in Fig. 14. The C(ipso)Ga(1)Ga(2)C(37) array has a Ê , which planar, trans-bent structure with a Ga(1)AGa(2) distance of 2.319(2) A is the shortest GaAGa distance currently known, and an average GaAC bond Ê . The Ga-Na distances are in the range 3.056(6)-3.106(6) A Ê length of 2.04(2) A

76

P. P. Power

Fig. 14. Schematic drawing of the `gallyne' Na2(GaC6H3-2,6-Trip2)2 [70]. H atoms are not

shown

Ê . In addition, there are relatively close with an average value of 3.08(1) A contacts between the sodiums and the aromatic ring carbons that range from Ê . The GaAGaAC angles are 128.5(4) and 133.5(4)°. These ca. 2.85 to 3.15 A structural parameters have been independently con®rmed by another data set Ê, which afforded the very similar structural parameters: GaAGa = 2.324(1) A Ê Ga-C(av) = 2.041(5) A, GaAGaAC = 125.9(2); 134.0(2)° [66]. The compound Na2Ga2(C6H3-2,6-Trip2)2 was also characterized by 1H-NMR, 13C-NMR, and IR spectroscopy, but these data do not provide direct information on the GaAGa bonding. There have been no further spectroscopic studies that might shed light on the nature of the GaAGa bonding in this compound. In fact, all bonding arguments since the original report have been based exclusively on the original structure and the computational data. Several points of view have emerged on the basis of these calculations. Although there is some agreement between all these opinions, they can be conveniently grouped into the following categories: a) The GaAGa consists of two weak donor-acceptor bonds (similar to those proposed earlier for the RMMR species) plus a p-bond which can be illustrated schematically by Fig. 15. On this basis the bond is a triple one although it differs from the conventional r + 2p triple bond seen in acetylene. The bonding picture is derived from the use of localized rather than canonical MOs, and consists of two polar-dative bonds and a p-bond [2a, 2c, 70±72]. b) An alternative model, based on canonical molecular orbitals, views the GaAGa bond as consisting of a r-bond, a p-bond, and a weak [42], or

Multiple Bonding Between Heavier Group 13 Elements

77

Fig. 15. Schematic illustration of one view [(a) see text] of the Ga-Ga bonding in the dianion [RGaGaR]2)

``slipped'' [73], p-bond. More recently, this model was also justi®ed on the basis of ELF calculations [1d]. As a result, these authors view the GaAGa bond as a triple one although the bond order is less than three since one of the p-bonds is very weak. c) A third model views the GaAGa bond as double [74] or even less than double [40] in character. This view is based on DFT and SCF/RHF calculations that use canonical molecular orbitals. The bonding picture resembles (b) in the sense that the main controversy concerns the description of the HOMO 15bu orbital as shown in Figs. 16 and 17. These authors maintain that it is a non-bonding lone-pair orbital although it more or less corresponds to the slipped p-orbital described in (b). The non-bonding character of this orbital can also be justi®ed on the basis that the maximum in the electron density of this orbital occurs at a 95° GaAGa-lone pair angle [41] which, being outside the perpendicular to the GaAGa bond line at gallium, quali®es it as a lone pair (LP) rather than a bonding orbital. Moreover, computational data showed that this orbital is actually anti-bonding in its effect [40] (con®rmed also by later calculations) [41]. By this criterion the bond order is less than two. The DFT calculations [74] were also of further interest in that they showed that the interactions between the Na+ ions and the ortho-aryl rings played a roÃle in shortening the GaAGa distance and this was con®rmed by later calculations with larger basis sets [72, 41].

Fig. 16. Contour diagram of the 15bu lone pair orbital in trans-Li2[MeGaGaMe] obtained with a 6±31G* basis set [40]

78

P. P. Power

Fig. 17. Molecular orbital energy and correlation diagram for Li2[MeGaGaMe] with energies in eV [40]

d) The most recently published high level calculations by Nagase and coworkers are unique in that they deal not with a model species but with the whole molecule including the i-Pr groups on the ortho-aryl rings [41]. These calculations have shown that the short GaAGa distance is the result of several factors that include Na-terphenyl interactions, i-Pr-i-Pr interactions, GaANaAGa bridge bonding, as well as adjustments in CAGaAGa angles due to the steric requirements of the i-Pr groups. Apart from the important Na-terphenyl interactions [74] the latter three factors had not been considered in much depth in earlier work. The majority of the earlier Ê or greater calculations had led to estimates of GaAGa distances of ca. 2.4 A Ê Ê which were ca. 0.08 A longer than the ca. 2.32 A experimental distance. The latest calculations, which include these other factors, resulted in a GaAGa Ê and thus has provided the best agreement bond length of around 2.367 A currently available between theoretical and experimental data. FurtherÊ of the GaAGa bond more, the calculations indicated that ca. 0.1 A Ê shortening occurs as a result of sodium-aryl ring bonding, and ca. 0.04 A shortening is due to the presence of i-Pr substituents. In the absence of Ê was calculated for the these effects a GaAGa distance as long as 2.535 A

Multiple Bonding Between Heavier Group 13 Elements

79

uncomplexed dianionic species [Ga2(C6H3-2,6-Ph2)2]2). An orbital analysis [41] of the simpler model species Na2[MeGaGaMe] showed the presence of an ag GaAGa r-bonding orbital, an au orbital of p symmetry which had maxima in the electron density along the Na-Ga bonds and not midway between the galliums, a bu orbital (i.e., the slipped p-bond or lone pair orbital) whose maximum electron density lies outside a 90° line with respect to the GaAGa bond. It was noted that a small amount of p-bonding exists in this orbital but it is antibonding in its effect at the center of this orbital lying between the galliums [41] in agreement with previous results [40]. There is also a correlating ag lone pair orbital that has some GaAGa bonding character which causes the GaAGa bond to shorten. The authors' main conclusion was that it may not be reasonable to discuss whether the experimental compound contains a GaAGa triple bond since the contributions of the sodiums to the short GaAGa bond is considerable. In essence, the heart of the molecule is the Na2Ga2 cluster rather than a simple GaAGa bond. Furthermore, it was predicted that the use of lithium instead of sodium in such clusters could lead to much shorter GaAGa bond distances owing to the smaller size of the Li+ cation which would more strongly attract the aryl rings. Since the bonding picture discussed in (d) is based upon the application of some of the most sophisticated calculations to the actual compound itself, rather than to a model species, it seems that the limits of the currently available theoretical sophistication as applied to this problem are being approached. These theoretical data have shown that in the absence of sodium ions and the i-Pr groups, the GaAGa bond distance would probably be as long Ê which is consistent with a much weaker GaAGa bond than that seen as 2.5 A experimentally in the contact ion triple Na2Ga2(C6H3-2,6-Trip2)2 [75]. The weakness of the GaAGa bond in these uncomplexed dianions has been con®rmed by force constant calculations [16, 17] which, although they do not provide details of the orbital interactions, show that the GaAGa bond strengths in the model species lie between the values calculated for single and double bonds. In essence, there is little doubt that GaAGa multiple bonding, if it exists in the real molecule, is very weak and most authors are agreed on relative weakness of the bonding. It is probable that theoretical data may provide a reliable estimate of the actual strength of the GaAGa interactions in a free dianion [Ga2R2]2). Finally, there is the question of the stability of these ions toward the dissociation of an electron. Will they behave like the [R2MMR2]2) (M = Al, Ga, In, Tl) species discussed earlier and spontaneously self ionize to afford a monoanion and a free electron? It seems likely that this will be the case and will show that the [RMMR]2) anions are inherently unstable. With regard to the different points of view outlined in (a), (b) and (c), it should be pointed out that these differences arise mainly from the use of localized (a, LMO), or canonical (CMO, b, and c) molecular orbitals. In principle LMOs and CMOs are equivalent and are related by a unitary transformation. This can be illustrated by the CBC bonding in acetylene,

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which consists of a r and 2 p MOs in the case of the CMO model and three banana bonds for the LMO model to give a bond order of 3 in each case. For the [RGaGaR]2) dianion and related isoelectronic species such as RGeGeR, the CMO model affords a r, a p and a lone pair (n) orbital [in the case of (c)]; the lone pair arising from mixing a r* and one of the p-orbitals. In contrast, the LMO gives two polarized donor-acceptor (paw-paw) bonds and a p-bond. Thus, three banana bonds of the LMOs of a light element compound are transformed to two polar-dative and one p-bond for heavier element derivatives. In other words, the CMO model gives a formal bond-order of 2, whereas the LMO model gives a formal bond order of 3. One advantage of the CMO picture is that the same orbital descriptions r, p and lone pair apply to bonding in light and heavier elements whereas it is necessary to change from unpolarized banana bonds (light element) to polarized dative (or paw-paw) and p-bonds (heavier element) for the LMOs. Another advantage of the CMO picture is that it results in a lower bond order which is consistent with a weaker bonding, whereas the LMO method produces a formal bond order of three. Thus, the CMO approach is consistent with a closer relationship between bond order and molecular properties such as bond length.

6 Future Work It bears repeating that the characteristic feature of the literature debate on the Na2Ga2(C6H3-2,6-Trip2)2 compound has been its almost complete reliance on theoretical arguments. Apart from the original report describing the structure of Na2Ga2(C6H3-2,6-Trip2)2 [70] there has been no further experimental information on the GaAGa bond in this molecule. Several lines of inquiry seem feasible. One such direction could be based on the decomposition of these bonds into their component parts. For example, if it is assumed that the Na2Ga2(C6H3-2,6-Trip2)2 species is triply bonded, it follows that the GaAGa bond will involve three electron pairs in three bonding orbitals. Removal of one of these pairs (i.e., from the HOMO which corresponds to the p bonding CMO or LMO) should result in an RGaGaR doubly bonded species as shown Eq.(3). 3

This experiment was proposed in 1998 [1c] but, as already noted earlier, there have been no reports of such a compound. It is noteworthy, however, that the congeneric neutral indium [75] and thallium compounds [76] M(C6H3-2,6-Trip2) are not dimeric in their crystal structures but exist as monomers. This immediately raises the question of the possible structure of their aluminum or gallium analogues. Such compounds should be capable of existence, since a number of Al(I) and Ga(I) alkyls and aryls have already been characterized [21, 23±26, 38, 39] and these are monomers in the vapor or solution phase although they are weakly associated as tetramers or hexamers in the solid. Would the species {M(C6H3-2,6-Trip2)}n (M = Al or Ga; n = 1

Multiple Bonding Between Heavier Group 13 Elements

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or 2) exist as monomers or dimers in the crystalline state? The synthesis of these compounds could represent an important step in the resolution of the bonding arguments. If their structures are dimeric in solution and the solid state, it would lend support to the concept of triple bonding between these elements. If, however, they prove to be monomeric, the existence of triple bond in the (RMMR)2) (R = C6H3-2,6-Trip2) or Na2RMMR dianion would be dif®cult to sustain. Either of these possibilities should lead to a resolution of the bonding arguments. However, it is possible that their solid state structure could exhibit long, and relatively weak, M-M interactions in a dimeric species, in which case the question becomes how long should a double bond be? The bond energies discussed earlier in Sect. 2 for RMMR suggest that the galliumAgallium bond is a weak intermolecular interaction species (similar in strength to that of indium ca. 3 kcal mol)1 [13]) rather than a double bond. This is consistent with calculations by Nagase and coworkers which indicate a Ê for the Ga2(C6H3-2,6-Ph2)2 molecule [41]. This GaAGa bond length of 2.716 A Ê longer than a normal single bond between galliums distance is ca. 0.2 A substituted by organic substituents and as a result the bond energy should be quite low. If such a species can exist, and if it is double bonded, oxidation of a neutral RGaGaR species to the atom [RGaGaR]+ should lead to a lengthening of the GaAGa bond. However, calculations on the hypothetical species MeGaGaMe show that this is not the case and the GaAGa bond is predicted to shorten in [MeGaGaMe]+ [40]. At present, therefore, it seems likely that a neutral species of formula Ga(C6H3-2,6-Trip2) could only, at best, be weakly dimerized in the solid, and would almost certainly be dissociated in solution. Such dimers, if indeed they are formed in the crystal phase, would probably be best regarded as weak molecular complexes rather than ``digallenes''. This view is in line with the low calculated bond strengths mentioned above [13] and the monomeric structure of the indium congener In(C6H3-2,6-Trip2) [75]. If these predictions are experimentally proven, the GaAGa bond in Na2Ga2(C6H3-2,6-Trip2)2 would be best described as an essentially single one. A second line of investigation could focus on the role of the alkali metals in the structure. At present the type of structure seen for Na2Ga2(C6H3-2, 6-Trip2)2 appears to be unique to that molecule. This view arises from the fact that changing the alkali metal from sodium to potassium does not afford the same structure but the K2Ga4 cluster species K2Ga4(C6H3-2,6-Trip2)2 as discussed earlier [66]. The alkali metal roÃle is thus more complicated than that of simple reductant or an innocent counter cation for the stabilization of the dimeric structure. It seems that size and electronic characteristics are also crucial for the stability of each structure. Calculations [41] indicate that the Li+ salt should give GaAGa distances that are shorter than those of the Na+ salt. It is unlikely that that free dianion [Ga2(C6H3-2,6-Trip2)2]2) will be stable in the absence of counter cations. However, it may be possible to obtain a singly reduced anion of formula [Ga2(C6H3-2,6-Trip2)2]) either as a contact or solvent separated ion pair. The formal bond order in this species is less than that in the dianion. However, the Coulombic repulsion should be reduced. If such effects are in approximate balance, a similar GaAGa distance would be measured for the monoanion.

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As previously mentioned, there have been no detailed spectroscopic investigations of Na2Ga2(C6H3-2,6-Trip2)2. It is possible that studies as basic as UV-visible spectrum could provide information. Raman spectroscopy could also allow an estimate of the approximate strength of the GaAGa bond. In addition, it should be possible to extend the range of compounds to include aluminum and indium. In principle, there are no compelling reasons why these analogues of the currently known gallium species should not exist. The aluminum compounds would be of particular interest as calculations on the neutral trans-bent HAlAlH model species indicate a bond strength of ca. 10 kcal mol)1 which is over three times higher than that calculated (ca. 3 kcal mol)1) for the gallium congener. It is more likely that in this case a dimeric RAlAlR could be isolated. Acknowledgements. We are grateful to the National Science Foundation and the Alexander von Humboldt Stiftung for ®nancial support.

7 References 1. There is a vast literature associated with this general area which has regularly been reviewed. For a general overview of many of the classes of compounds involved see: (a) Power PP (1999) Chem Rev 99: 3463. In addition, various models for the bonding in such compounds which differs markedly from those in their lighter congeners, have been discussed in (b) Driess M, GruÈtzmacher H (1996) Angew Chem Int Ed Engl 35: 828; (c) Power PP (1998) Dalton Trans 2939; (d) GruÈtzmacher H, FaÈssler TF (2000) Chem Eur J 6: 2317 2. In addition to Refs. [1a±d] there have been several recent reviews that have dealt with various aspects of multiple bonding to the group 13 elements: (a) Robinson GH (1999) Acc Chem Res 32: 773; (b) Downs AJ (1999) Coord Chem Revs 189: 59; (c) Robinson GH (2000) Chem Commun 2175. Low-valent heavier group 13 species and their bonding to transition metal moieties have also been reviewed recently; (d) Linti G, SchnoÈckel H (2000) Coord Chem Rev 206±207: 285 3. Pluta C, PoÈrschke K-R, Kruger C, Hildenbrand K (1993) Angew Chem Int Ed Engl 32: 388 4. He X, Bartlett RA, Olmstead MM, Ruhlandt-Senge K, Sturgeon BE, Power PP (1993) Angew Chem Int Ed Engl 32: 717 5. Uhl W, Vester A, Kaim W, Poppe J (1993) J Organomet Chem 454: 9 6. Balasubramanian K (1986) J Phys Chem 90: 6786; (1989); 93: 8388 7. Janiak C, Hoffmann R (1989) Angew Chem Int Ed Engl 28: 1688; (1990) J Am Chem Soc 112: 5924 8. Budzelaar PHM, Boersma J (1990) Recl Trav Chem Pays-Bas 109: 187 9. Meier U, Peyerimhoff SD, Green F (1990) Z Phys D 17: 209 10. Schwerdtfeger P (1991) Inorg Chem 30: 1660 11. Palagyi Z, Schaefer HF (1993) Chem Phys Lett 203: 195 12. Palagyi Z, Grev RS, Schaefer HF (1993) J Am Chem Soc 115: 1936 13. Treboux G, Barthelat J-C (1993) J Am Chem Soc 115: 4870 14. Balducci G, Gigli G, Melone G (1998) J Chem Phys 109: 4384 15. Bridgeman AJ, Nielsen NA (2000) Inorg Chem Acta 303: 107 16. Grunenberg J, Goldberg N (2000) J Am Chem Soc 122: 6045 17. KoÈppe R, SchnoÈckel H (2000) Z Anorg Allg Chem 626: 1095

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18. For a discussion of the atomic properties of the group 13 metals see: Downs AJ (1993) In: Downs AJ (eds) Chemistry of aluminum gallium, indium and thallium. Blackie, London, Chapter 1 19. Brothers PJ, Power PP (1996) Adv Organomet Chem 39: 1 20. Uhl W (1988) Z Naturforsch B, Chem Sci 43B: 1113 21. Beamish JC, Small RWH, Worrall IJ (1979) Inorg Chem 18: 220 and references cited therein 22. Tuck DJ (1993) Chem Soc Rev 269 23. Uhl W (1993) Angew Chem Int Ed Engl 32: 1386 24. Uhl W (1997) Coord Chem Rev 163: 1±32 25. Uhl W (2000) Chem Soc Rev 29: 259 26. Wiberg N, Blank T, Kaim W, Schwerderski B, Linti G (2000) Eur J Inorg Chem 1475 27. Wiberg N, Amelunxen K, NoÈth H, Schwenk H, Kaim W, Klein A, Scheiring T (1997) Angew Chem Int Ed Engl 36: 1213 28. Wiberg N, Blank T, Amelunxen K, NoÈth H, Knizek J, Habereder T, Kaim W, Wanner M (2001) Eur J Inorg Chem 1719 29. Huber KP, Herzberg G (1979) Molecular spectra and molecular structure IV. Constants of diatomic molecules. Van Nostrand, New York 30. Balasubramanian K (1990) Chem Rev 90: 93 31. Miedema AR, Gingerich KA (1979) J Phys B 12: 2081 32. Brewer L, Gywynn JS (1980) Faraday Symp Chem Soc 14: 13b 33. Martinho SimoÄes JA, Liebman JF, Slayden SW (1995) In: Chemistry of Organic Germanium Tin and Lead Compounds. Wiley, Chichester, p 245 34. Schumann H, Janiak C, GoÈrlitz F, Loebel J, Dietrich A (1989) J Organomet Chem 363: 243 35. Schumann H, Pickhardt J, BoÈruer U (1987) Angew Chem Int Ed Engl 26: 790 36. Jutzi P, Wegener D, Hursthouse MB (1991) Chem Ber 124: 295 37. Jutzi P, Schnittger J, Hursthouse MB (1991) Chem Ber 124: 1693 38. Dohmeier C, Loos D, SchnoÈckel H (1996) Angew Chem Int Ed Engl 35: 129 39. Haaland A, Martinsen K-G, Volden, HV, Kaim W, WaldhoÈr E, Uhl W, Schutz U (1996) Organometallics 15: 1146 40. Allen TL, Fink WH, Power PP (2000) Dalton Trans 407 41. Takagi N, Schmidt MW, Nagase S (2001) Organometallics 20: 1646 42. Byetheway I, Lin Z (1998) J Am Chem Soc 120: 12133 43. Malrieu JP, Triniquier G (1989) J Am Chem Soc 111: 5916 44. Klusik H, Berndt A (1981) Angew Chem Int Ed Engl 20: 870 45. Klusik H, Berndt A (1981) J Organomet Chem 222: c25 46. Lef¯er JE, Watts GB, Tanigaki T, Dolan E, Miller DS (1970) J Am Chem Soc 92: 6825 47. Power PP (1992) Inorg Chem Acta 200: 443 48. Moezzi A, Olmstead MM, Power PP (1992) J Am Chem Soc 114: 217 49. Moezzi A, Bartlett RA, Power PP (1992) Angew Chem Int Ed Engl 31: 1082 50. NoÈth H, Knizek J, Ponikwar W (1999) Eur J Inorg Chem 1931 51. Grigsby WJ, Power PP (1996) Chem Commun 2235 52. Grigsby WJ, Power PP (1997) Chem Eur J 3: 368 53. Uhl W, Layh M, Hildenbrand T (1989) J Organomet Chem 364: 289 54. Wehmschulte RJ, Ruhlandt-Senge K, Olmstead MM, Hope H, Sturgeon BE, Power PP (1993) Inorg Chem 32: 2983 55. Uhl W, Schutz U, Kaim W, WaldhoÈr E (1995) J Organomet Chem 501: 79 56. Wehmschulte RJ, He X, Power PP. Unpublished results 57. Wehmschulte RJ, Power PP (1998) Angew Chem Int Ed Engl 37: 3152 58. Uhl W, Vester A, Fenske D, Baum G (1994) J Organomet Chem 464: 23 59. Uhl W, Gerding R, Vester A (1996) J Organomet Chem 513: 613 60. Kaufmann E, Schleyer PvR (1988) Inorg Chem 27: 3987 61. Hamilton EL, Prius JG, DeKock RL, Jalkanen KJ (1998) Main Gp Met Chem 21: 219 62. Li, X-W, Pennington WT, Robinson GH (1995) J Am Chem Soc 117: 7578

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63. Li X-W, Xie Y, Schreiner PR,Gripper KD, Crittendon RC, Campana CF, Schaefer HF, Robinson GH (1996) Organometallics 15: 3798 64. Xie Y, Schreiner PR, Schaefer HF, Li X-W, Robinson GH (1996) J Am Chem Soc 118: 10635 65. Xie Y, Schreiner PR, Schaefer HF, Li X-W, Robinson GH (1998) Organometallics 17: 114 66. Twamley B, Power PP (2000) Angew Chem Int Ed Engl 39: 3500 67. Phillips AD, Power PP. Unpublished work 68. Li X, Kuznetsov AE, Zhang H-F, Boldyrev AI, Wang L-S (2001) Science 291: 859 69. Li X, Zhang H-F, Wang L-S, Kuznetsov AE, Cannon NA, Boldyrev AI (2001) Angew Chem Int Ed Engl 40: 1867 70. Su J, Li X-W, Crittendon C, Robinson GH (1997) J Am Chem Soc 119: 5471 71. Xie Y, Grev RS, Gu J, Schaefer HF, Schleyer PvR, Su J, Li X-W, Robinson GH (1998) J Am Chem Soc 120: 3773 72. Xie Y, Schaefer HF, Robinson GH (2000) Chem Phys Lett 317: 174 73. Klinkhammer KW (1997) Angew Chem Int Ed Engl 36: 2320 74. Cotton FA, Cowley AH, Feng X (1998) J Am Chem Soc 120: 1795 Ê is consistent with a GaAGa bond order near one; cf. GaAGa 75. A GaAGa distance of 2.5 A distances in Table 3 76. Haubrich ST, Power PP (1998) J Am Chem Soc 120: 2202 77. Niemeyer M, Power PP (1998) Angew Chem Int Ed Engl 37: 1277

The R2M+ Group 13 Organometallic Fragment Chelated by P-Centered Ligands Lepakshaiah Mahalakshmi, Dietmar Stalke Institut fuÈr Anorganische Chemie der UniversitaÈt WuÈrzburg, Am Hubland, 97074 WuÈrzburg, Germany e-mail: [email protected]

The chapter discusses the synergistic combination of two important concepts in ligand design to furnish catalytically improved moieties in Group 13 organometallics: heteroatomic chelation in coordination site selectivity and heteroaromatic coordination known from porphyrins. This new approach might raise the Group 13 organometallics from their current important but subordinate role as co-catalysts to outstanding catalysts in their own right. Keywords: Group 13 metals (aluminum, gallium, indium, thallium), Ambidentate ligands,

Phosphorus-nitrogen bidentate ligands, Pyridyl phosphanes, Aminoiminophosphoranes, Lewis acid catalysis

1

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

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1.1 1.2

Scope of the Chapter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalysis and the Ligand Effect . . . . . . . . . . . . . . . . . . . . . . . .

86 86

2

Organometallic Derivatives of Group 13 Elements Al, Ga, In or Tl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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2.1 2.1.1 2.1.2 2.2 2.2.1 2.2.2 2.2.3 2.2.3.1 2.2.3.2 2.2.4

With Phosphanes . . . . . . . . . . . . . . . . . . . . Primary, Secondary and Tertiary Phosphanes Diphosphanes . . . . . . . . . . . . . . . . . . . . . . . With Phosphorous-Nitrogen Ligands . . . . . . Pyridyl Phosphanes . . . . . . . . . . . . . . . . . . . Imino and Amino Phosphanes . . . . . . . . . . . Cyclic Phosphazenes and Phosphazanes . . . . Cyclotriphosphazenes . . . . . . . . . . . . . . . . . Cyclodiphosphazanes . . . . . . . . . . . . . . . . . Acyclic Phosphazenes and Phosphazanes . . .

3

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

4

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

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List of Abbreviations Ad Bipy Bu Cp dppa dppe dppm Et Et2O Me N NMR P Ph Pr Py t THF Tol

1-adamantyl 2,2¢-bipyridine butyl cyclopentadienyl diphenylphosphino amine 1,2-di(diphenylphosphino)ethane 1,1-di(diphenylphosphino)methane ethyl diethyl ether methyl nitrogen nuclear magnetic resonance phosphorus phenyl propyl pyridyl tertiary tetrahydrofuran tolyl

1 Introduction 1.1 Scope of the Chapter

One of the motivating objectives for the study of organometallic chemistry is the commercial importance of chemicals produced by reactions that are catalyzed by organometallic compounds. Organometallic derivatives of Group 13 elements are utilized extensively in industrial processes. In particular, they are of interest as polymerization co-catalysts, ceramic precursors, as volatile organometallic precursors for semiconductor materials in the electronic industry (e.g. GaAs, GaN, InN) and as selective reagents in organic synthesis [1, 2]. The scope of this chapter is to provide an overview of the trends in Group 13 organometallic derivatives with phosphorus based ligands and their emerging signi®cance in catalysis. 1.2 Catalysis and the Ligand Effect

One of the landmark discoveries in catalytic chemistry was the Ziegler-Natta catalyst. The award of the Nobel prize in chemistry in 1963 to Ziegler and

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Natta further crowned this important discovery. The Ziegler-Natta catalyst, which is heterogeneous, is composed of a transition metal catalyst and a co-catalyst based on group 13 organometallics normally containing aluminum. Classical examples of such catalyst/co-catalyst pairs are TiCl3/ Al(C2H5)2Cl or TiCl4/Al(C2H5)3. These catalysts polymerize alkenes at atmospheric pressure and ambient temperature. Furthermore, methyl alumoxane (MAO) has become a key component in single site catalytic processes [3]. Predominantly, these developments have emphasized the signi®cant role of aluminum alkyls in catalysis and led to a vast number of papers and patents which portray the several possible modi®cations of the organoaluminum reagents. The present data substantiate the important role of cationic group 13 complexes in catalytic chemistry and fuel the resurgence of interest in this ®eld [4, 5, 6]. Neutral aluminum alkyls have also been used as reagents or catalysts for Lewis acid mediated reactions and were found to catalyze the polymerization of ethylene to a-ole®ns at elevated temperatures and ethylene pressure [7]. Although alkyl aluminum compounds were mainly relegated to the role of co-catalysts in the past, recent trends in research show that they are now being looked at as new catalysts devoid of transition metal ingredients. In particular cationic alkyl aluminum complexes are seen as transition metal free ole®n polymerization catalysts. Cationic aluminum compounds can be generated by the reaction of the corresponding neutral species with a Lewis acid, e.g., B(C6F5)3, or other suitable alkyl anion abstractors (e.g., Ph3 C BAr4 ). Bochmann et al. showed for the ®rst time that the aluminocenium per¯uorophenylborate 1 exhibits a high initiator activity in polymerization of isobutene and in copolymerization of isobutene, Eq. (1) [8]. These cationic species presumably function in a manner similar to group 4 catalysts of the general formula [Cp2MR]+ [9, 10]. 1

Furthermore, gallium compounds can serve as model systems for aluminum congeners. Cationic gallium alkyls are of interest in synthesis and catalytic applications involving polar substituents because of the relative stability of the GaAR bond toward hydrolysis and electrophilic cleavage compared to the otherwise superior Al±R species [11]. Research in catalysis chemistry reveals the overall trend that the ef®ciency of a phosphane based catalyst is much higher than that of a phosphane free catalyst. Thus, the signi®cance of phosphane ligands in catalysis needs no explanation. One of the classical examples in this category is the Wilkinson catalyst (Ph3P)3RhCl which was found to catalyze alkene hydrogenation effectively. A second basic principle seems to be that heteroaromatic architectures promote catalytic abilities considerably. Hence, heteroarene containing ligands are one of the prospective candidates in the search for a most effective and ef®cient catalyst [12]. The design of heterotopic ligands bearing phosphorus and nitrogen or oxygen donor

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atoms is a ®eld of constantly ongoing research owing to the often unique properties that such ligands confer to their metal complexes in stochiometric or catalytic reactions [13±16]. Associated with such a ligand system is the further incentive of several possible modes of tuning the stereoelectronic factors to an extent unmarked by any other class of ligands. Literature on transition metal chemistry of phosphorus-nitrogen systems is ubiquitous [17]. However, there are no detailed reports of systematic studies of phosphorus nitrogen systems with group 13 elements. Thus, the several new promising industrial applications of organometallic derivatives of group 13 elements give impetus to the driving force behind this chemistry. The need for the development of new materials has led to the recent search for new ligand systems. Considering the importance of phosphorus based ligands in catalysis, this article attempts to give an overview of studies conducted on phosphorus based ligands with group 13 elements (Al, Ga, In, Tl) based on literature available up to mid 2001. In this section, a brief discussion of the importance of ligand frameworks in catalytic chemistry is given. The nature and structure of ligands used plays an important role in designing and tailoring a new catalyst. Various studies show that qualities required for the most ef®cient and effective catalyst have been achieved by employing bulky and/or chelating ligands. Use of ligands with extremely bulky substituents led to the successful isolation of monomeric metal complexes of group 13 elements and lanthanides essential for studying the factors inducing the catalytic abilities. On the other hand the signi®cance of chelating ligands is well documented in nature by the porphyrin catalysts and related compounds [18]. In this context, complexes of chelating bidentate ligands are well-established in catalytic applications of organometallic derivatives. Furthermore, such bulky or chelating ligands stabilize metal centers in unusual coordination geometries or in a low valence state [19, 20]. A recent impressive example was provided by Roesky et al. They demonstrated the monomeric aluminum(I) derivative a to be stabilized by employing such a chelating ligand [21], (Chart 1). Metallocenes are well established as single-site catalysts in polymerization reactions [22]. The search for new ancillary ligands to replace the classical Cp ligand has given us several non-Cp based ligands with diverse structural motifs. The nitrogen donor chelating ligands such as amidinates [23, 24] and guanidinates [25, 26] have been found to be suitable alternatives to Cp systems. In spite of the importance of phosphane ligands in catalysis, the design of a most effective phosphane ligand still remains a great challenge. In this context, the advantageous combination of a N donor and a P donor center in a single system is well studied as exempli®ed by several reviews and papers in this ®eld [12±16]. The different electronic and steric characteristics of the donor groups often control the reactivity at the metal site. Nitrogen being a hard base can stabilize metals in high oxidation states, while phosphorus, as a soft base, is suitable for stabilizing metals in low oxidation states. Stalke et al. have designed several such ambidentate P/N ligands as shown in Scheme 1 [12]. Monoanionic ligands were prepared by introducing, for instance, p-block elements as bridging atoms with a formal negative charge, (Scheme 1).

The R2M+ Group 13 Organometallic Fragment Chelated by P-Centered Ligands

Chart 1.

Scheme 1.

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Bidentate ligands result when group 15 elements are applied to bridge two heteroaromatic rings. These ligands can coordinate a metal by r coordination of lone pairs localized either at the ring nitrogen atoms and/or at the bridging element [27]. In addition, they could exhibit p-interactions to the metals through the aromatic rings [28] or the E±C±N allylic moiety. Lithiated ligands afford hydrocarbons soluble molecular precursors to synthesize a variety of complexes in transmetalation reactions [29]. Unlike rigid bidentate ligands, pyridyl based ligands can also adopt different types of chelating and bridging modes. Thus, incorporation of pyridyl substituents at the phosphorus center instead of the frequently employed but restricted phenyl groups, alters and augments the coordination capability of the ligand system and leads to the design of multi-dentate Janus head ligands. They take advantage of the synergism of P/N coordination, chelation effect, and heteroaromatic coordination . Although being ambidentate, they prevent a high degree of association and cause less aggregated oligomers to be generated. The bridging group 15 element in oxidation state III remains divalent. When considering parameters for ligand design in catalysis in addition to the relative donor±acceptor strength, among the steric factors the bite angle (h) is most important for chelating ligands. With bidentate ligands the bite angle is de®ned as the angle enclosed by the two donor atoms of the ligand and the metal. The bite angle is an indirect parameter to quantify the number of steric interactions a ligand imposes in an organometallic complex. Obviously, it is also a function of the M±P distance in phosphorus based bidentate ligands [30].

2 Organometallic Derivatives of Group 13 Elements Al, Ga, In or Tl 2.1 With Phosphanes

2.1.1 Primary, Secondary and Tertiary Phosphanes Coates et al. ®rst reported that the trialkyl derivatives of group 13 elements form adducts with donor molecules such as amines and phosphanes. They observed that the reaction of trimethyl derivatives of Al, Ga or In with diphenyl phosphane gives dimeric products of the composition (Me2M á PPh2)2 (M = Al, Ga, In), on heating by elimination of methane [31], whereas the similar reaction with dimethyl phosphane leads to polymeric products of the type (Me2Ga±PMe2)x [32]. Similarly, reactions of primary phosphanes with silylalanes [33] gave adducts, which, on heating, undergo an elimination condensation reaction to give dimeric species, (Scheme 2). It was observed that four membered rings like 2 were favored over six membered rings like 3 when the substituents are large, keeping to the proposed idea that main group ring size depends in part on the steric demand of the substituent. However, it was also observed that coupled byproducts were obtained instead

91

The R2M+ Group 13 Organometallic Fragment Chelated by P-Centered Ligands

of phosphino alane ring compounds in reactions of chloro phosphanes with silylalanes.

Scheme 2.

Barron et al. report the synthesis of several adducts of tertiary phosphanes with trimethyl aluminum, Eq. (2) [34]. 2

A systematic NMR spectroscopic study of these adducts suggests that the steric repulsion between the trimethyl aluminum Lewis acid and the phosphane Lewis base rather than the electronic factors account for the detected changes in the 31 P-NMR spectroscopic chemical shifts (Table 1). The change in the chemical shift (D) of the phosphanes on coordination to AlMe3 has been correlated to the Table 1.

4a 4b 4c 4d 4e 4f 4g 4h 4i 4j 4k 4l 4m 4n a

31

P-NMR value of trimethylaluminum±phosphane adducts Compound

d (ppm)

D (ppm)

Me3AlPMe3 Me3AlPMe2Ph Me3AlPEt3 Me3AlP(CH2CH2CN)3 Me3AlPMePh2 Me3AlP(C6H4Me-p)3 Me3AlPPh3 Me3AlP(C6H4F-p)3 Me3AlPPh2(C6H11) Me3AlPPh(C6H11)2 Me3AlP(CH2Ph)3 Me3AlP(C6H11)3 Me3AlPBut3 Me3AlP(C6H4Me-o)3

)47.5 )36.9 )17.0 )19.5 )24.2 )9.5 )7.3 )10.2 )6.6 )12.0 )15.5 )3.7 41.4 )22.5

+12.5 +10.1 +4.0 +3.5 +3.9 )1.5 )1.4 )1.2 )2.2 )9.5 )3.5 )13.1 )20.5 +7.7

These energies are relative to the most stable doubly hydrogen bridged form.

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steric demand of the phosphane. It is an indication of the difference in the R±P±R angles between free and coordinated phosphane. These observations were substantiated by X-ray structural data of 4g and 4n [35]. In recent years, Driess et al. have shown that aluminum phosphanide compounds reminiscent to 3 can be utilized as precursors toward cluster formation [36]. On heating 3¢ was found to undergo cyclocondensation and dimerizes to give a (AlP)6 cluster, whereas treating with nBuLi and Me2AlCl gave a solvent separated ion pair 5, (Scheme 3), which consists of an anionic Al4P3 cage. On the other hand, double deprotonation with R2PLi and further treatment with Me3Al gave rise to a donor solvent free cluster consisting of a Al4Li4P6 framework.

Scheme 3.

2.1.2 Diphosphanes With diphosphanes recently Stephan et al. reported an intriguing Al and P based macrocyclic structure [37]. A zirconium based catalyst precursor ®rst was employed in the catalytic dehydrocoupling of the primary bidentate phosphane to give the tetraphosphane 6, (Scheme 4). The function of 6 as a molecular building block has been demonstrated by its reaction with MMe3(M = Al, Ga). Although, the gallium derivative 7 has not been

Scheme 4.

The R2M+ Group 13 Organometallic Fragment Chelated by P-Centered Ligands

93

characterized well due to its instability the aluminum derivative 8 has been structurally characterized. It shows four P4 chains linking eight aluminum centers to form a macrocycle composed of Al2P3 rings. A variety of complexes with numerous novel structural features have also been unearthed in the study of phosphorus ligands with group 13 halides which will not be described in this review [38, 39]. 2.2 With Phosphorous-Nitrogen Ligands

2.2.1 Pyridyl Phosphanes N-Heteroaryl ring systems are well-known as bridging functions between transition metal centers [12, 40, 41]. The pyridyl substituent is one of the representative examples of such systems that has been a widely used ligand in transition metal coordination chemistry [42]. However, the interest in group 13 metal chemistry of these ligands is of recent interest. Stalke et al. [43] have reported the synthesis and structure of the aluminum adduct Me3Al(l±Py)PPy2 10. Equation (3) shows that when tri(2-pyridyl)phosphane 9 [44] is treated directly with trimethyl aluminum in diethyl ether the adduct complex Me3Al(l-Py)PPy2 10 is obtained which has structurally been characterised, (Fig. 1). 3

Fig. 1. The solid state structure of Me3Al(l-Py)PPy2 10

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It has to be emphasized that in contrast to the analogous reactions of tertiary phosphanes with aluminum derivatives [35], where a phosphorus atom is coordinated to the aluminum atom, in 10 it was observed that only one of the three pyridyl ring nitrogen atoms coordinates to the aluminum atom. The `hard' aluminum atom prefers the `hard' nitrogen donor site rather than the `soft' phosphane function. Three methyl groups and the single N-donor atom leave the central aluminum atom tetrahedrally coordinated. There is no signi®cant difference in the PAC-bond lengths of the single ring coordinated to the aluminum atom and the two non-coordinated pyridyl substituents, respectively. It is interesting to note that, unlike the reaction of 9 with organo lithium compounds, the reaction stops with the adduct species rather than undergoing ligand coupling to the corresponding aluminum phosphide. Reaction of tri(2-pyridyl)phosphane [44] with lithium metal followed by hydrolysis gives the di(2-pyridyl)phosphane via cleavage of one P±aryl bond [27]. The principal feature of this ligand is the coordination ¯exibility towards various metal centers. The N-donor groups in this di(2-pyridyl)phosphane would inhibit polymerization and facilitate the formation of monomeric complexes on reaction with alkali metals. The reaction with group 13 organometallics resulted in low molecular aggregates which are of interest as precursors for III/V semiconducting ®lms [45]. The resulting materials are volatile as they are monomers and the hypothetical leaving groups (pyridine or picoline) in the MOCVD (metal organic chemical vapor deposition) process are both volatile and thermally stable. [(thf)2LiPy2P] 11 is the starting material in various transmetalation reactions and 11 is either synthesized in the reaction of Py3P 9 with elemental lithium or in the lithiation of Py2PH 12 with n-butyllithium. It is one of the sources for [Me2AlPy2P] 13a, (Scheme 5). The X-ray structure determination proves 11 does not exhibit a single Li±P contact although being formally a lithium phosphide. The phosphorus(III) center remains divalent (Fig. 2) [27]. The alkali metal coordination in the substituent periphery of the ligand opens the door to unique reactivity in these complexes, e.g., the reduction of the P(V) center in the iminophosphorane Py3P = NSiMe3 to chiral phosphane amines RPyPN(H)SiMe3 via lithium organics. This

Scheme 5.

The R2M+ Group 13 Organometallic Fragment Chelated by P-Centered Ligands

95

Fig. 2. The solid state structure of [(thf)2LiPy2P] 11

involves the unprecedented cleavage of two PAC(Py) bonds in a single step [46]. With the phenyl substituents in the analogue iminophosphorane Ph3P = NSiMe3 only a single PAC(Ph) bond cleavage is achieved with elemental sodium [47]. [(thf)2LiPy2P] 11 is a lead structure as primary and secondary alkali metal phosphides MPR2 and MPHR are key transfer reagents in organo phosphorus synthesis [48]. Lithiated compounds in general are also good precursors to obtain organometallic compounds by transmetalation reactions [49±50]. In this context even more important is that they are target materials in tailoring catalytically active transition metal fragments containing phosphide ligands. Compound 13a can been obtained via two different routes: ®rstly in the reaction of 11 with dimethyl aluminum chloride where LiCl is eliminated and secondly by the reaction of di(pyridyl) phosphane 12 (Py2PH) with trimethyl aluminum where methane is formed, (Scheme 5). The X-ray structure determination of [Me2AlPy2P] 13a, (Fig. 3) elucidates the aluminum atom to be coordinated by the two nitrogen atoms of the pyridyl rings in addition to the two remaining methyl groups leaving the aluminum four

Fig. 3. The solid state structure of [Me2AlPy2P] 13

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and the phosphorus atom two coordinated. In contrast to other organo aluminum phosphides no Al±P contact is established. The formation of monomeric structures is also a favorable point for catalysis. Quantum mechanical simulations on ethylene insertion in an aluminum catalyst [AlMe{MeC[NMe]2}]+ i, (Chart 1), has shown that the calculated barrier for insertion into a methyl-bridged dinuclear aluminum species j, (Chart 1), is over 10 kcal mol)1 higher than that in the mononuclear species suggesting that the monomeric species is the active catalyst [51]. The structure of 13a reveals that, in order to accommodate the aluminum atom, the pyridyl ligands are twisted along the PAC bonds and the metal atom is displaced from the ligand plane. This distortion of the P-ligand gives rise to a butter¯y arrangement in 13a. The solid state structures of the di(pyridyl)-phosphide [27] 13a, -arsenide [43], -amide [52], and -methanide [53], all containing the Me2Al+ cationic fragment have been established. Although the AlAN distance is almost invariant in all structures the more acute C±E±C angle (As < P < N < CH) forces the complex in a more pronounced butter¯y arrangement. It would be worth noting that unlike the pyridyl phosphanes the pyridyl amide exhibit different reactivity and structural features depending on the nature of the metal [52]. The Me2Al+- and Me2Ga+-containing complexes of di(2-pyridyl)amide are monomeric as in 13 with the metal centers coordinated exclusively through the ring nitrogen atoms, while in the Me2In+- and Me2Tl+containing congeners a different coordination mode is observed. As shown in Scheme 6, the dipyridyl amides can exhibit three different coordination modes.

Scheme 6.

Consequently, due to preferred cis-cis orientation a dimeric structure is observed for the indium complex and an unprecedented cis-trans arrangement in the thallium structure leads to a polymeric aggregate. Further 15N-NMR spectroscopic studies show that the aluminum and gallium complexes are stable contact ion pairs even in solution whereas the indium and thallium compounds are solvent-separated ion pairs in THF solution. In accordance with the electropositive nature of the bridgehead atoms, all di(pyridyl) substituted anions behave like amides with the electron density accumulated at the ring nitrogen atoms rather than carbanions, phosphides or arsenides. The divalent bridging atoms (N, P, As) in the related complexes should in principle be able to coordinate either one or even two further Lewis acidic metals to form heterobimetallic derivatives. According to the mesomeric structures, (Scheme 7), it can act as a 2e- or even a 4edonor. However, theoretical calculations, supported by experiments, have shown that while in the amides (E = N) the amido nitrogen does function as

The R2M+ Group 13 Organometallic Fragment Chelated by P-Centered Ligands

97

Scheme 7.

a typical Lewis base [52], the situation in the corresponding phosphides (E = P) is different [54], (Fig. 4). In the latter nearly all the charge density couples into the pyridyl rings leaving the central phosphorus atom only attractive for soft metals in the form of a p-acid type of coordinating center. The Lewis basicity of the central bridging nitrogen atom in di(pyridyl) amide is still high enough to coordinate a second equivalent of AlEt3. This further suggests that due to the higher electronegativity of the central nitrogen atom compared to the bridging divalent phosphorus atom the di(pyridyl)amide is the harder Lewis base. To test the coordination abilities of the divalent phosphorus atom in [Me2Al(Py2P)], 13a was reacted with [CpFe(CO)3][BF4] in an attempt to prepare heterobimetallic compounds where in general a carbonyl group can easily be replaced by a phosphane [55]. The vacant phosphorus site was expected to coordinate with Fe(II) while retaining the AlAN bonds. However, the AlAN bonds in the complex were cleaved presumably by the formation of thermodynamically favorable AlF3 accompanied by the alkylation of the tetra¯uoroborate anion. This leads to the formation of [{Cp(CO)2Fe}2 {(l-P)Py2}][BMe4] 14, (Scheme 8). Although not containing

Ê )3 level (bottom) Fig. 4. The calculated total electron density at the 2.0 (top) and 1.7 e-A shows considerable Lewis base abilities at the central nitrogen atom in [Me2AlPy2N] (left) while at the central phosphorus atom of [Me2AlPy2P] 13 hardly any electron density is left (right)

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any group 13 organometallic fragment the structure of 14 con®rms the anticipated ability of the phosphorus center in the Py2P) anion to coordinate soft metals [28]. In 14 two soft Lewis acidic metal fragments [CpFe(CO)2]+ are bridged by the phosphorous atom of a single di(2-pyridyl)phosphide ligand to give the [{Cp(CO)2Fe}2{(l-P)Py2}]+ cation, (Fig. 5).

Scheme 8.

Fig. 5. The solid state structure of the cation in [{Cp(CO)2Fe}2{(lP)Py2}] [BMe4] 14

In general it is well established that bulky groups and heavier alkali metals prompt M±aryl p interactions in addition to M±P r bonds in the case of primary and secondary phosphides [56] or terphenyl substituted phosphides [57, 58]. Rather than varying the bulk of the aryl substituent incorporation of donor centers in group 14 and 15 element bonded rings such as di(2-pyridyl)amides, -phosphides and -arsenides Py2E) (E = N, P, As) to modulate the coordination behavior gives rise to an interesting chemistry. The adaptability of these pyridyl substituted ligands is exempli®ed by the ¯exible

99

The R2M+ Group 13 Organometallic Fragment Chelated by P-Centered Ligands

Scheme 9.

coordination behavior towards various metal centers. They either coordinate exclusively by both pyridyl nitrogen atoms leaving the bridging E atom two coordinated without any metal contact (A in Scheme 9). The coordination ¯exibility of such an ambidentate Py2P) Janus head ligand has been found to span the wide range from r-all-nitrogen chelation to hard organometallic moieties, r-phosphorous l2-bridging in dinuclear iron complexes to p-N, C, P heteroallyl coordination to the soft cesium atom [12, 28, 59±62]. The mixed bridging-N/ring-N coordination is observed in some di(2-pyridyl)amides (B in Table 2.

31

P-NMR spectroscopic data for pyridyl phosphanes and their complexes

Compound

d (ppm)

Py3P 9 Me3Al (l-Py)PPy2 10 [(thf)2Li(Py2P)] 11 Py2PH 12 [Me2Al(Py2P)] 13 [Et2Al(Py2P)] [{Cp(CO)2Fe}2{(l-P)Py2}][BMe4] 14

0.26 )0.4 13.0 )34.1 25.7 23.7 39.1

Fig. 6. The

31

X-ray

Ref. 44 43 27 27 27 54 28

P-NMR Chemical shift scale for pyridyl phosphane based systems

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Fig. 7. The solid state structures of the di(pyridyl)methanide [53], -amide [52], -phosphide[27] 13 and ±arsenide [43] all containing the Me2Al+ cationic fragment. Although the Al±N distance is almost invariant in all structures the more acute C±E±C angle (As < P < N < CH) forces the complex in a more pronounced butter¯y arrangement

Scheme 9) [63±65]. Table 2 and Fig. 6 show the trends in phosphorus NMR data for pyridyl phosphanes on complexation. Figure 7 gives an excellent view along the E M axis of di(pyridyl) systems illustrating the deviation from planarity on coordination. In any case, a monomeric compound similar to the corresponding phosphorus analogue 13 is formed. Of the Py2E-ligand only the pyridyl nitrogen atoms coordinate the metal center leaving the bridging group 15 atom separated from the cation. Geometric differences occur with respect to ligand planarity when comparing the respective group 13 complexes. While the C(H) analogous ligand system remains coplanar without exception the corresponding group 15 derivatives of aluminum reveal a distinct deviation of

101

The R2M+ Group 13 Organometallic Fragment Chelated by P-Centered Ligands

the anion from planarity. Figure 7 illustrates these deviations in contrast to the lithium complex 11. In [Me2Al(Py2P)] 13 (E = P, M = Al) and the isotype [Me2Al(Py2As)] (E = As, M = Al) complex, the pyridyl ring planes intersect at an angle of 155°. The bridging angle C±E±C becomes more acute: 110.4(2)° (E = P, M = Li), 106.6(1)° (E = P, M = Al) and, as a consequence of the increased p-character in the C±As bonds, 103.0(3)° (E = As, M = Al). In addition, the intramolecular N¼N¢ distance (the ``bite'') of the ligand differs in both phosphorus compounds (306.4 pm when E = P, M = Li and 292.2 pm when E = P, M = Al). On the other hand, the two EAC bond lengths are equal within the standard deviations in all anions with a bond order between a single and a double bond (av. P±C = 179 pm vs 185 pm for a single and 161 to 171 pm for a double bond in phosphaalkenes [66]. The same is valid for the av. AsAC bond length of 190 pm (198 pm in diphenylarsenides [67] and 182 pm in arsaalkenes [68]). Moreover, the pyridyl rings exhibit alternating bond lengths indicating partial double bond localization in the 3 and 5 position as well as accumulation of negative charge at the ring nitrogen atoms. Although, an X-ray structure analysis of the gallium complex Py2AsGaMe2 could not be obtained a geometry very similar to that of Py2AsAlMe2 can be deduced from the very similar NMR spectroscopic properties. Compared to the starting material Py3As, the highest energetic pyridyl ring deformation vibration in the IR spectrum of Py2AsAlMe2 and Py2AsGaMe2 is shifted to higher wave numbers by coordination to the metal centers t 1570 AsPy3 , [12] 1600 (Py2AsAlMe2), 1595 (Py2AsGaMe2) cm)1). The metal coordination also causes an up ®eld shift of the 6-H signal in the 1H-NMR spectrum of more than 1 ppm (d 8.67 (Py3As), 7.61 (Py2AsAlMe2), 7.49 (Py2AsGaMe2)). Hence, the monoanionic ligands of the heavier group 15 elements show a certain coordination ¯exibility toward different metal centers while not giving up the full conjugation. The bent conformation of the anion, however, is not static as veri®ed by the 1H- and 13C-NMR spectroscopic data. Despite the nonequivalence of both methyl groups at the group 13 element in the solid state only a single signal is detected in solution even at low temperatures ()80 °C). Table 3. N-M-N Bite angle in di(pyridyl) ligand systems

Di(pyridyl) ligands

Bite angle (°) Py(N)±M±(N)Py

M

Ref.

Py2N

93.52 91.57 92.00 92.42 98.98 100.03 101.16 97.10 72.51 76.00

AlMe2 AlEt2 GaMe2 InMe2 AlMe2 AlEt2 AlMe2 AlMe2 TlMe2 InMe2(ONO2)(H2O)

52 52 52 52 27 54 43 53 53 53

Py2P Py2As Py2CH

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The bite angle concept is systematically studied with phosphorus based bidentate ligands and has been successfully used to explain the transition metal complexes of diphosphane ligands of type c in Chart 1. On the other hand, similar systematic studies with nitrogen based bidentate ligands (d, e in Chart 1) are still not available. Stalke et al. have studied the main group element bridged di(pyridyl) systems with different transition and main group elements. Due to the ready availability of the bite angle h value from the crystallographic data, Table 3 lists the changes in the h values observed for the various di(pyridyl) systems. The subtle changes associated with the group 14 or group 15 bridged di(pyridyl) systems on coordination to group 13 is represented in terms of the bite angle h. Closer analysis shows that in the case of di(pyridyl) amide, either the change in the nature of the group 13 element or substituent does not show signi®cant difference. Nevertheless, the change of bridging atom in the di(pyridyl) system does seem to have an effect on the bite angle. In the case of numerous phosphorus based bidentate ligands, it is observed that the larger the bite angle, the more the catalytic ef®ciency is expected to rise. The steric effect induced by ligands with large bite angle prevents any further coordination which in turn reduces the unnecessary energy expenditure required for breaking or making bonds to achieve the active catalyst. A bidentate nitrogen ligand such as trans-4, 5-di(2-pyridyl) norborane e in Chart 1 is found to potentially have a bite angle as large as 115.5° as deduced from molecular modeling calculations [69]. However, the Pd complex of this ligand has not been found to show any appreciable rate in a CO insertion reaction compared to the complex of a ligand with a smaller bite angle. As the di(pyridyl) phosphanes are monomers anyway, the bite angle h might not have that impact as found in diphosphane complexes. However, further studies are clearly required to explain the different trends observed for bidentate nitrogen ligands in comparison to the bidentate phosphorus ligands during catalytic reactions. 2.2.2 Imino and Amino Phosphanes In an excellent review by Roesky et al. in 1994 [70a] a vast number of examples for coordination complexes of cyclic phosphazanes and phosphazenes and other related systems have already been compiled. In the following section, an attempt is made to cover the latest features of group 13 systems along with some earlier examples with phosphorus-nitrogen based systems other than pyridyl phosphanes. As early as in the 1960s, Schmidbaur et al. [71±73] showed that imino phosphoranes react with group 13 (Al, Ga, In) alkyls to give ionic structures, (Scheme 10). However, the reaction of triphenyliminophosphorane with triphenyl aluminum or gallium (M = Al, Ga) ®rst affords the complex where the group 13 metal is coordinated through the imino nitrogen atom. On heating one P-bound phenyl substituent is ortho-deprotonated and coordi-

The R2M+ Group 13 Organometallic Fragment Chelated by P-Centered Ligands

103

nated to the PhM+ cation. Together with the maintained N±M contact this gives a MNPC four membered metallacycle 16 in Scheme 11a.

Scheme 10.

Scheme 11.

Stalke et al. have recently shown that ortho-lithiation of the imino phospharane Ph3P = NSiMe3 affords a new type of side-arm donating ligand [Li(o-C6H4PPh2NSiMe3)]2 á Et2O 17 [49, 74]. The reaction of this lithiated iminophosphorane with indium chloride proceeds smoothly by the complete replacement of chlorine atoms, with the elimination of lithium chloride and the formation of three M-C r bonds to give the indium complex [In(o-C6H4PPh2NSiMe3)3] 18, (Scheme 11b). Structural elucidation of this complex shows it to be monomeric with indium ®ve-coordinated, (Fig. 8), [29]. The coordination environment around indium composed of three aryl substituents and two imino nitrogen which function as side-arm donors. The third imino nitrogen atom is not involved in coordination to the indium atom. The signi®cant feature deduced from the structure of 18 is the participation of the side-arm imino nitrogen in coordination to the metal leading to the formation of ®ve-membered metallacycle structure. This is further con®rmed by the lengthening of the P@N bond on coordination. Recently, Stephan et al. [75a] show that iminophosphoranes react with Lewis acids such as AlCl3, AlMeCl2, and AlMe3 to give the corresponding donor acceptor adducts 19 wherein only the iminophosphorane nitrogen atom coordinates to aluminum which has been con®rmed by a structure determination. These compounds on treatment with B(C6F5)3 lead to the formation of the ionic species 20, one of these compounds undergoes further reaction with

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Fig. 8. The solid state structure of [In(o-C6H4PPh2NSiMe3)3] 18

PMe3 to give the complex salts 21 as shown in Scheme 12. Compounds of type 20 and 21 have successfully been tested for their catalytic ef®ciency. However, it was observed that tBu3PNSiMe3 failed to show similar adduct formation and this failure was attributed to steric crowding.

Scheme 12.

On the other hand, with the NH substituted triorgano iminophosphorane R3PNH, adduct formation occurs readily which is attributed to the decrease in steric crowding by the replacement of the trimethylsilyl substituent. However, on heating the adducts gave dimeric species such as 22, (Scheme 13). In contrast to the reactions reported by Schmidbaur et al. [73, 76±78], here the NH function rather than the organic periphery supplies the hydrogen atom to form methane. The structure of 22 con®rms the formation of a dimer with two iminophosphorane N-termini l2-bridging two aluminum centers [75a]. Dehnicke et al. report a similar structure with halide substituents at the aluminum atom [79]. The dimeric species shows similar reactivity as 19 to give the ionic product analogous

The R2M+ Group 13 Organometallic Fragment Chelated by P-Centered Ligands

105

to 20 although, even in the presence of excess of the borane reagent the ionization is observed only at one of the aluminum centers.

Scheme 13.

Similarly, Niecke et al. [80] show that Lewis acid-base adducts of type 23 were only obtained when di(dialkylamino)phosphanes were treated with aluminum trialkyls, (Scheme 14). In contrast, the reaction with dialkyl aluminum hydride gave dimeric phosphiniminoalanes 24 due to the dehydrogenation reaction. The structure of the products is similar to that reported by Stephan et al. Keeping to the HSAB principle, only the Al±N coordination mode was observed and not the AlAP bonding mode. Due to the strong Lewis acidic character of the Al center only the iminophosphorane form (PH) was observed and not the phosphanylamido form (NH), (Scheme 14). Metal complexes of di(dialkylamino) phosphanes have been found to exist in an equilibrium between the kinetically favored (NH)±phosphanylamido A and the thermodynamically stable tautomeric (PH) iminophosphorane B. The equilibrium position corresponds to the Lewis acidity of the metal center. A stronger acidic character of the metal center stabilizes the PH form as seen in 23. These experimental observations has been substantiated by computational studies [81]. The formation of the (PH) iminophosphorane is also con®rmed in solution by a doublet observed in 31P-NMR spectroscopic spectra with a typical 1JHP coupling constant in the range of 540±580 Hz.

Scheme 14.

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2.2.3 Cyclic Phosphazenes and Phosphazanes 2.2.3.1 Cyclotriphosphazenes In addition to the review by Roesky et al. [70], several other reviews also deal with speci®c interest and developments in the coordination chemistry of cyclophosphazenes [82, 83]. However, the chemistry of cylophosphazenes with group 13 organometallics is relatively underdeveloped in comparison to transition metal chemistry in this area. One recent interesting example reported by Steiner et al. demonstrates the ability of substituted cyclophosphazenes to function as multiprotic acids on suitable modi®cation [84]. The amino substituted cyclophosphazene 25 is able, on treatment with trimethyl aluminum, to facilitate multinuclear metal arrangement by undergoing full deprotonation at the amino nitrogen atoms. Structural elucidation of the resulting pentanuclear organo aluminum complex 26 shows that the complex exhibits all three different possible coordination modes of this ligand in a single molecule, (Scheme 15). All ®ve aluminum atoms have tetrahedral environments and are accommodated in bidentate chelating coordination sites at the centrally arranged phosphazenate core. Although similar coordination patterns have been observed in other aluminum complexes containing P-N ligands before but only with one or two metal centers unlike in 26 [85±89].

Scheme 15.

2.2.3.2 Cyclodiphosphazanes Cyclodiphosphazanes(III) 27 shown in Scheme 16 undergo oxidation reactions to give the cyclodiphosphazanes(V) of type 28. These are prospective ligands in catalysis since these ligands due to lack of phosphorus lone-pairs are less susceptible to the destructive cycloreversion of the ligands. Hence they could prevent catalyst deactivation in the process. When treated with trimethyl aluminum the cyclodiphosphazanes form symmetrically substituted bimetallic species of type 29 [90]. Characterization by single-crystal X-ray studies show

The R2M+ Group 13 Organometallic Fragment Chelated by P-Centered Ligands

107

Scheme 16.

that these are trispirocyclic complexes in which the ligands coordinate both dimethyl aluminum moieties in an (N, E) g2-fashion (E = O, S, Se, NR). The important point here is that the phosphorus ligand on coordination with transition or main group metals were not found to coordinate as di(amido) ligands but side-on as aminophosphoranates. Amino phosphoranes, especially aminoiminophosphoranes, are structural equivalents to amidinates and guanidinates. They are documented as already being prospective molecules as ancillary ligands for alkyl aluminum based catalysts. It has been shown that aminoiminophosphoranates of the type [Me3Si±N±P(R2)±N±Me3Si]) derived formally by the deprotonation of R2P(NSiMe3)(NHSiMe3) are versatile chelating ligands [91±93]. 2.2.4 Acyclic Phosphazenes and Phosphazanes The review by Roesky et al. [70] covered some of the earlier work related to the group 13 complexes of acyclic phosphazanes and phosphazenes. In the search for pentacoordinate aluminum complexes, instead of the more ubiquitous tetra coordinate aluminum complexes, Clemens et al., as early as 1966, showed that alkyl aluminum compounds react with di(diphenylphosphino) amines and several other phosphane ligands to give the 1:1 addition products [94]. The pentacoordinate nature of aluminum in these compounds has been established by 31P-NMR spectroscopic data. Schmidbaur et al. have reported the different reactivity of group 13 alkyls with tetraphenyl diphosphazane (dppa) [95]. With gallium alkyls, a zwitterionic eight membered ring 30 is obtained by alkane elimination from GaR3, (Scheme 17), whereas aluminum trialkyl and gallium trimethyl starting materials gives zwitterionic sixmembered rings 31 with gallium methyl forming both kind of isomers. The structures have been proved by 1H- and 31P-NMR spectroscopy and an X-ray structure determination of 31a (M = Al, R = Me). It shows an unsymmetrical

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L. Mahalakshmi á D. Stalke

Scheme 17.

PN coordination towards aluminum and the bond lengths favors the depicted charge separation over coordinative bonds and a P@N double bond. Furthermore, 31a has been shown to undergo selective oxidation of one of the phosphorus centers to give the (N,N ¢)-chelated aluminum complex 32 [88]. However, an attempted synthesis of 33, the monoimidic analogue of dppa, led to the formation of the salt 34 due to the rapid deprotonation of 33. On demetalation and further reaction with trimethyl silyl chloride 32 gives the 1,3 disilylated P(III)±N±P(V) chain molecule 34. The structure was determined by single crystal X-ray crystallography [88]. This was interesting in the sense that monooxidation of dppa (HN(PPh2)2) with either PhN3 or Me3SiN3 in a controlled stoichiometry could not be achieved since the reaction of 31a with PhN3 gave only the amidodiphosphanimide aluminium chelate complex 36 reported earlier by the direct reaction of AlMe3 with diphosphazide of dppa [89, 96]. On the other hand, a similar reaction with dppm, (Scheme 18), showed that no deprotonation of the bridging methylene group was observed [97, 98] and resulted in the mono adduct formation 37. Triethyl aluminum adds on a second equivalent to give the diadduct 38 not observed in the reaction with gallium alkyl derivatives. In contrast, the di(iminophosphorano) methane derivative undergoes single deprotonation to give aluminum chelate complex 39 [99]. An alternative approach to 39 has been the reaction of lithiated di(iminophosphorano)methane with group 13 elements (Al, Ga, In)

The R2M+ Group 13 Organometallic Fragment Chelated by P-Centered Ligands

109

Scheme 18.

halides followed by reaction with Me2AlCl or the appropriate Grignard reagents [75a]. In the oxidized products of dppm the protons are acidic enough to undergo deprotonation by organometallic bases. However, addition of a second equivalent of AlMe3 leads to the formation of the bimetallic bridged carbene complex 40 due to double deprotonation at the methylene carbon atom [99]. Double deprotonation of the methylene backbone has also been observed in the aluminum complexes of H2C(Ph2P = X)2 (X = O, S) to give bridging trimetallic (X = O) [100, 101] and tetrametallic (X = S) [102] complexes. Similarly, cyclic silylated diphosphazanes react quantitatively to give six membered ring compounds by methane evolution 41. The cyclohexane-like framework in the various complexes shows different conformations. 31P-NMR spectroscopy shows decreasing Lewis acidity of the metal fragment along AlMe2 ³ GaMe2 > InMe2 (see Table 4). The hapticity in these 1,5-g2 bound zwitterionic chelates with a cationic N3P2 backbone has been con®rmed by the structurally characterized analogues [89]. The phosphinimide phosphanes 42 [75b], which are similar to the acyclic diphosphazanes comprising of one nitrogen and phosphorus atom, when treated with AlMe3, (Scheme 19), were found to coordinate to the P(III) center to yield the alane adducts 43 which were characterized by the 31P-NMR spectroscopy and structure elucidation of 43a. Again, the utility of lithiated starting materials in the formation of new organometallic reagents is re¯ected in another related ligand system, the amido diphosphane anion N(SiMe2CH2PPr2i )2 44 [103]. It is prepared by reacting the lithiated derivative with AlCl3. Metathesis reaction of lithium or magnesium alkyl gives the di(hydrocarbyl) aluminum derivatives 45 and 46. The 31P-NMR spectroscopic data of 44±46 indicate weak phosphane

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Table 4. Group 13 complexes of phosphazane ligands

Compound

d (ppm)

Ref.

MeN[P(Ph)2]2.AlEt3 EtN[P(Ph)2]2.AlEt3 30 R = Me 30 R = Et HN[P(Ph)2]2.AlMe3 31a R = Me 31a R = Et 32 R = Me

)64 )59 41.8 66 36.3 42.9, 52.8 (P(III)-Al) 43.0, 49.0 (PIII-Al) 38.9 PIII, 35.7 PV (100 Hz)a 39.7 PIII, 35.7 PV (98 Hz)a 24.6 PV )19.7 29.5* 27.6* 23.4* 23.1 22.0 23.4 17.6* 20.3 42.4 Pv, 38.9 PIII (80 Hz)a 47.9, 40.8 (62 Hz)a 39.7, 26.3* 45.0, 28.1 (26 Hz)a )10.5 )4.9 )4.2 )3.6 R.T. )5.4 L.T.. )6.8L.T. 268.4* 359.2* 126.6 121.2*

94 94 95 95 88 88, 95 88, 95 88

32 R = Et 36 38 39 M = Al, R = Me 40 41 a R = Ph 41 a R = NMe2 41 b R = Ph 41 b R = NMe2 41 c R = Ph 41 c R = NMe2 42a 42b 43a 43b 44 45a 45b 46 47 48 49 50 a 2

J(P-P) 2J(P-P); * X-ray data available;

R.T.

Room temperature;

88 96 97 99 99 89 89 89 89 89 89 75b 75b 75b 75b 103 103 103 103 116 117 106 120 L.T.

Low temperature.

coordination to the aluminum atom which has been con®rmed in the structures of 44 and 46. The structure of 44 shows the aluminum center to be ®ve coordinated whereas in 46 it is only four coordinated via only one phosphane side arm of the ligand, Scheme 20. The difference in the structural features has been attributed to steric factors. The solid state structure of 46 shows the frozen ¯uxional behavior of these complexes in solution state which has been veri®ed by low temperature spectroscopic data.

The R2M+ Group 13 Organometallic Fragment Chelated by P-Centered Ligands

111

Scheme 19.

Scheme 20.

The diazaphosphane or aminoiminophosphane ligands with a NPN framework are another subclass of cyclophosphazenes. These compounds with both phosphorus in oxidation state (III) [104±110] and (V) [111±112] have been employed in the synthesis of four membered heterocycles and coordination chemistry with group 13 derivatives. Several complexes of trivalent phosphorus derivatives with both aluminum halide and alkyls are known as illustrated for 48 in Scheme 21 [113±119]. The structure determination of 48 con®rms the formation of a four membered metallacycle [116, 117].

Scheme 21.

A similar series of zwitterionic compounds have also been isolated in the case of PV derivatives but will not be discussed in detail [70, 78, 85, 86, 88, 111, 113]. The ®rst example of a neutral aluminum complex of diazaphosphane, the 1,3,2,4-diazaphosphaluminetidine 50, Eq. (4), has been synthesized by the dehydrogenation reaction between Lewis acid-base adduct H3Al ¬ NMe3 and tBuP{N(H)tBu}2 49. The product tBuP(NtBu)2(H)Al ¬ NMe3 50 was structurally and spectroscopically characterized [120]. 4

112

L. Mahalakshmi á D. Stalke

3 Conclusions The reaction of tri(2-pyridyl)phosphane and tri(2-pyridyl)arsane, respectively, with lithium metal leads to (THF)2Li(-Py)2E via cleavage of one E-aryl bond and ligand coupling yielding bipyridine. Transmetallation reactions yield complexes of the composition Me2M(-Py)2E (E = N, P, As; M = Al, Ga). The hard Group 13 metal centers are exclusively chelated by the pyridyl-N-atoms leaving the E-atom two-coordinated. The negative charge is largely delocalized throughout the whole [Py2E]) anion. Nevertheless, this delocalization permits coordinational ¯exibility and deviation from planarity. In the [Py2P]) anion the central phosphorus atom seems basic enough to coordinate soft d-block metal centers. This might show the way to hard-soft bimetallic reagents due to coordination site selective behavior. Substitution of the organic alkyl or aryl groups in the classical NP(R)2N) chelating anionic ligand by two pyridyl groups converts it into a Janus face NP(Py)2N) tripodal ligand. At least one pyridyl ring nitrogen in addition to only one imido nitrogen atom is used in metal coordination. The active ligand periphery opens up several avenues: (a) coordination site selectivity NPN) versus PyPPy, (b) adaptability in ligation composition depending on the geometric constraints and coordination capability of the metals and (c) the possibility of forming heterobimetallic complexes where the dipyridyl aminoiminophosphoranes are employed as ¯exible metal linkers and not only as bulky protectants. Undoubtedly, the combination of N/P coordination in ambidentate chelating ligands will further fuel the design of new target molecules of a tailored pro®le in catalysis and material science. Acknowledgements. The authors thank the Deutsche Forschungsgemeinschaft (especially the SFB 347 `Selective reactions of metal-activated molecules' and the Graduiertenkolleg `Electron density') and the Fonds der Chemischen Industrie for generous ®nancial support. D S wants to thank the students involved in phosphorus chemistry over the years (A Steiner, H Gornitzka, S Wingerter, M Pfeiffer, A Murso, T Stey, F Baier). Without their tremendous contributions, this chapter would have been impossible.

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Synthesis, Structure and Reactivity of Group 13/15 Compounds Containing the Heavier Elements of Group 15, Sb and Bi Stephan Schulz Institut fuÈr Anorganische Chemie der UniversitaÈt Bonn, Gerhard-Domagk-Strasse 1, 53121 Bonn, Germany e-mail: [email protected]

Triorganostibines and -bismuthines ER03 as well as tetraorganodistibines and -dibismuthines E2R¢4 (E = Sb, Bi) react with trialkylalanes, -gallanes and ±indanes R3M (M = Al, Ga, In) under formation of simple Lewis acid-base adducts of the type R3MAER¢3 and bisadducts of the type [R3M]2[E2R¢4]. Their structures and stabilities were investigated by single crystal Xray diffraction, NMR spectroscopy and theoretical calculations. In addition, general pathways for the synthesis of heterocycles [R2MSbR¢2]x will be presented. Stibinogallanes and -indanes can generally be prepared by dehalosilylation reactions, while stibinoalanes are formed by dehydrosilylation reactions. This particular pathway is also applicable for the synthesis of [Me2AlBi(Tms)2]3. In addition, MSb heterocycles (M = Al, Ga, In) can be synthesized by reaction of tetraorganodistibines and trialkylalanes, -gallanes and -indanes. Monomeric compounds R2MER¢2 and RMER¢ (M = Al, Ga; E = Sb, Bi) have not been reported to date, but Lewis base-stabilized monomers of the type baseÐM(R2)ER¢2 (M = Al, Ga; E = P, As, Sb, Bi) are formed by reaction of the corresponding heterocycle with 4(dimethylamino)pyridine (dmap). So prepared monomers react with transition metal complexes to give bimetallic complexes of the type baseÐM(R2)ER¢2AM(CO)n. Keywords: Aluminum, Gallium, Antimony, Bismuth, Structures

1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

2 2.1 2.1.1 2.1.2 2.1.3 2.1.4 2.2 2.2.1

Lewis Acid-Base Adducts . . . . . . . . . . . . . . . . . . . . . . . . . Adducts of the Type R3MAER¢3 ± General Trends . . . . . . . . Synthesis of Group 13-Stibine and -Bismuthine Adducts . . . Temperature-Dependent NMR Investigations . . . . . . . . . . . Single Crystal X-Ray Structure Determinations . . . . . . . . . . Computational Calculations . . . . . . . . . . . . . . . . . . . . . . . . Bisadducts of the Type [R3M]2[E2R¢4] . . . . . . . . . . . . . . . . . Syntheses and Solid State Structures of Group 13-Distibine and -Dibismuthine Bisadducts . . . . . . . . . . . . . . . . . . . . . .

3 3.1 3.2 3.3 3.3.1

Heterocycles of the Type [R2MER¢2]x . . . . . . . . . . . . General Synthetic Pathways . . . . . . . . . . . . . . . . . . . Single Crystal X-Ray Structure Analyses . . . . . . . . . . Reactivity of Heterocycles . . . . . . . . . . . . . . . . . . . . Synthesis of Lewis Base-Stabilized Monomers of the Type BaseAM(R2)ER¢2 . . . . . . . . . . . . . . . . . . . . . . . 3.3.2 Solid State Structures ± General Trends . . . . . . . . . .

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3.3.3 Transition Metal Complexes of Lewis Base-Stabilized Monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 4

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160

5

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161

List of Abbreviations Me Et Pr i-Pr Bu i-Bu t-Bu Ph Cp* Tmp Dipp Trip Mes Mes* Ada dmap Tms

methyl ethyl propyl iso-propyl butyl iso-butyl tert-butyl phenyl pentamethylcyclopentadienyl 2,2,6,6-tetramethylpiperidine 2,6-di(iso-propyl)phenyl 2,4,6-tri(iso-propyl)phenyl 2,4,6-trimethylphenyl 2,4,6-tri(tert-butyl)phenyl adamantyl 4-(dimethylamino)pyridine trimethylsilyl (SiMe3)

1 Introduction Group 13/15 compounds belong to the most intensely studied class of inorganic main group element compounds. In particular, Lewis acid-base adducts of the type R3MAER¢3 (M = group 13 element, E = group 15 element) have been investigated for more than 2 centuries. Early reports dealing with this speci®c class of compounds date back to 1809, when Gay-Lussac successfully synthesized F3BANH3 [1], the historical prototype of a Lewis acidbase adduct. Since this initial study, numerous group 13/15 adducts have been synthesized and structurally characterized, in particular those of boranes and alanes with amines and phosphines. Besides ``simple'' Lewis acid-base adducts, numerous compounds of the type [R2MER¢2]x and [RMER¢]x (xP2), containing ``real'' r-bonds between the group 13 and 15 elements, have been prepared. Epoch-making studies were performed by famous chemists such as Stock and Poland, who synthesized the

Synthesis, Structure and Reactivity of Group 13/15 Compounds

119

so-called ``inorganic benzene'', B3N3H6, Wiberg, who investigated the reaction of AlH3 with NH3 in detail, and others [2].

Scheme 1. Reactions of Stock and Poland and of Wiberg et al.

Not only the preparation of so far unknown compounds, but also the development of new synthesis techniques such as the Schlenk technique and vacuum-line technique which allow the handling and manipulation of air- and moisture-sensitive, pyrophoric compounds, are the most striking breakthroughs of their fascinating work. The majority of so-prepared compounds [R2MER¢2]x adopt four- and six-membered heterocyclic structures, which are usually described as head-to tail adducts, but higher oligomeric structures have also been found (xP4). However, monomeric compounds of the type R2MER¢2 (M = Al, Ga, In; E = N, P, As), which are of great interest in respect to their bonding properties, have been obtained to a far lesser extent. This is in sharp contrast to well-known boron compounds of the type R2B@ER2, in particular aminoboranes, containing r- and p-bonding parts [3]. The question whether the MAE bond in compounds of this speci®c type containing the heavier elements of group 13 does have (partial) double bond character was elucidated by computational calculations, resulting in a much deeper understanding of the nature of the MAE bond [4]. Monomeric compounds of the type RMER¢ have been completely unknown. They usually tend to oligomerize, leading to the formation of the corresponding dimeric, trimeric and tetrameric forms [RMER¢]x (x = 2)4; M = Al, Ga, E = N, P, As), but higher oligomers also have been obtained [5]. However, only the heterocycles (x = 2, 3) have been studied in detail, since they are of theoretical interest in respect to their number of p-electrons (quasi-aromaticity) [6]. A major breakthrough on monomeric compounds of this speci®c type was reported only recently, when Power and Roesky et al. succeeded in the synthesis and structural characterization of the ®rst iminogallane RGaNR¢ [7]. This particular compound features the shortest GaAN bond length ever observed. In addition to such fundamental studies essentially based on academic interest, investigations related to material science undoubtedly had the biggest impact on group 13/15 chemistry. The potential of R3MAER¢3 adducts and [R2MER¢2]x heterocycles to serve as precursors for the synthesis of the corresponding binary semiconducting materials ME (in particular M = Al, Ga, In; E = N, P, As), render such compounds very interesting for chemists, physicists and material scientists [8]. In a pioneering work, Manasevit introduced the MOCVD (metal organic chemical vapor deposition) process using two metal organic starting compounds (Et3Ga and AsH3) for the

120

S. Schulz

synthesis of GaAs layers in 1968 (Scheme 2) [9]. In the following years several research groups have demonstrated this technology to be generally useful for the synthesis of thin ®lms of various metals, semiconducting, superconducting or insulating materials [10].

Scheme 2. Manasevit's original reaction for the synthesis of GaAs

Later on, this concept was extended to precursors containing both elements of the desired material already connected by a chemical bond in a single molecule. Such precursors are mainly referred to as single source precursors. Their potential application for the deposition of thin ®lms of the corresponding binary materials by MOCVD processes could be demonstrated. In particular Lewis acid-base adducts R3MAER¢3 and four- and six-membered heterocycles [R2MER¢2]x (Fig. 1) have been in the focus of research groups both in industry and university. Consequently, the development of powerful synthetic pathways for the preparation of such precursors has been forced.

Fig. 1. Potential single source precursors for CVD reactions

However, the majority of such studies is related to organometallic precursors containing the lighter elements of group 15 (E = N, P, As) [11]. In contrast, the corresponding group 13-stibines and -bismuthines have been investigated to a far lesser extent. Until four years ago, only a handful of group 13-stibines have been known, while group 13-bismuthines have not been prepared at all. (Cp*Al)3Sb2, containing two ``naked'' Sb-atoms bridged by three Cp*Al units, was synthesized by Roesky et al. by reaction of [Cp*Al]4 and [t-BuSb]4 [12]. Cowley et al. [13] prepared four- and sixmembered heterocycles 1±4 by salt elimination and dehalosilylation reactions (Scheme 3). Their solid state structures were determined by single crystal Xray diffraction. In 1996, we started more detailed investigations on group 13/15 compounds containing the heavier elements of group 15, Sb and Bi, focussing on the synthesis of aluminum and gallium stibines and bismuthines. At the same time, Wells et al. also began to prepare MASb adducts and heterocycles (M = B, Ga, In). These studies, which are the object of this review, resulted in

Synthesis, Structure and Reactivity of Group 13/15 Compounds

121

Scheme 3. Synthesis of the ®rst MASb compounds

the synthesis of a large amount of new compounds, which can be divided into two general classes: a) Lewis acid-base adducts of the type R3MAER¢3 and [R3M]2[E2R¢4] containing dative MAE bonds and b) compounds with MAE r-bonds, mainly in form of four- and six-membered heterocycles [R2MER¢2]x. A major goal was to investigate the solid state structures of such compounds by single crystal X-ray diffraction. It was found that Lewis acid-base adducts R3MAER¢3 show general structural trends, which allow estimations on the relative stability of the adducts. The experimental results were con®rmed by computational calculations, giving even deeper insights into the structural parameters and the thermodynamic stability of simple Lewis acid-base adducts. In addition, their thermodynamic stability in solution was investigated by temperature-dependent NMR spectroscopy. The heterocycles were also investigated in respect to their chemical reactivity. They can be generally converted into monomeric, Lewis basestabilized compounds of the type dmapAM(R2)ER¢2. These monomers are powerful reactants in further complexation reactions leading to transition metal complexes of the type dmapAM(R2)ER¢2AM¢(CO)n. Such complexes containing a main group metal and a transition metal bridged by a group 15 element seem to be generally accessible by this particular reaction pathway.

2 Lewis Acid-Base Adducts 2.1 Adducts of the Type R3MAER¢3 ± General Trends

The reaction between a Lewis acid R3M and a Lewis base ER¢3, usually resulting in the formation of a Lewis acid-base adduct R3MAER¢3, is of fundamental interest in main group chemistry. Numerous experiments, in particular reactions of alane and gallane MH3 with amines and phosphines ER03 , have been performed [14]. Several general coordination modes, as summarized in Fig. 2, have been identi®ed by X-ray diffraction.

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S. Schulz

Fig. 2. Possible coordination modes of MH3 (M = Al, Ga)

The stability of such adducts, usually expressed by their dissociation enthalpies DHDiss, has been intensely investigated in the past, both in solution and in the gas phase (Table 1). Additional computational studies [16] provided detailed information about the nature of the dative bond, the strength of the acid-base interaction and the Table 1. Dissociation enthalpies DHDiss of adducts R3AlAER3 [15]

Donor

Acceptor

DHo [kJ mol)1]

State of aggregation

H3N Me3N Et3N Et3P Ph3P H3N Me3N H3N Et3N Bu3P Ph3P

AlMe3 AlMe3 AlMe3 AlMe3 AlMe3 AlCl3 AlCl3 AlBr3 AlBr3 AlBr3 AlBr3

27.6 0.3 30.0 0.2 26.5 0.2 22.1 0.3 17.6 0.2 41 48 2 41.0 44.8 47.5 34.9

Solution Solution Solution Solution Solution Gas Solution Gas Solution Solution Solution

123

Synthesis, Structure and Reactivity of Group 13/15 Compounds

in¯uence of the central elements M and E on the adduct stability. The role of the substituents R and R¢ exhibiting different steric and electronic properties was also studied in detail. However, comparable Lewis acid-base adducts containing the heavier elements of group 15, Sb [17] and Bi, have been investigated to a far lesser extent and there are only a few reports on their thermodynamic stability. To the best of our knowledge, only for one compound, Br3AlASbBr3 5, the enthalpy of formation has been reported (4.3 0.6 kJ mol)1 [18]). Compound 5 is a molecular adduct only in the gas phase as was shown by electron diffraction [17d], whereas it is ionic in the solid state ([SbBr2][AlBr4] [17e]). In the early 1950s, Coates [19] investigated the reaction between Me3Ga and EMe3 (E = N, P, As, Sb, Bi). While the reactions with Me3N, Me3P, Me3As and Me3Sb led to the formation of the expected Lewis acid-base adducts Me3GaAEMe3, Me3Bi did not react. The stability of so-formed adducts decreases from Me3GaANMe3 (21.0 kcal/mol) and Me3GaAPMe3 (18 kcal/ mol) to Me3GaAAsMe3 (10 kcal/mol) [15]. Me3GaASbMe3 was to unstable to allow the determination of its gas phase dissociation enthalpy. Comparable results were obtained by Mills et al. [20], who investigated in detail the reaction of trimethylpnictines EMe3 (E = P, As, Sb) with boron trihalides BX3, diborane B2H6 and trimethylborane BMe3. The results clearly proved the decreasing stability of so-formed adducts with increasing atomic number of the group 15 element. Tables 2 and 3 display thermodynamic data of several group 13-trimethylpnictine adducts as obtained from calorimetric gas phase measurements. Table 2. Adduct dissociation enthalpiesa (kcal ¤ mol) [15]

NMe3 PMe3 AsMe3 a b c

BF3

BCl3

BBr3

BH3

BMe3

AlMe3

GaMe3

InMe3

26.6 18.9

30.5

c

c

c

31.5

c

c

17.6 16.5

30.0 21.0

21.0 18 10

19.9 17.1

c

c

c

b

c

c

Data for SbMe3 and BiMe3 reactions are not available. No reaction. No data available.

Table 3. Reaction enthalpiesa obtained by Mills et al. (kJ ¤ mol) [20]

PMe3 AsMe3 SbMe3 a

BF3

BCl3

BBr3

BH3

BMe3

)45.5 )20.4 )4.2c

)68.6 )46.2 )26.8

)122.3 )81.2 )19.8

)79.9 )49.6 )6.6c

)41.0 b

b

For the reaction acid(g) + base(g) ® adduct (sum of gas-phase acid-base reaction and the heat of sublimation!). b No reaction observed. c No simple adduct formation.

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S. Schulz

Table 4. Bond angles (°) found in group 15-triorganyls ER3

EH3 EPh3 EMe3

E=N

E=P

E = As

E = Sb

E = Bi

106.8 [22] 119.6 [24] 110.6 [29]

93.5 [22] 102.8 [22] 98.9 [30]

92.0 [22] 99.7 [25] 96.1 [31]

91.5; 91.3 [23] 96.6 [26], 96.3 [27] 94.2 [32]

± 93.9 [28] 96.7 [33]

The observed trend, which is now generally accepted, corresponds to the increasing s-character of the lone pair, indicated by the increasing degree of pyramidalization (decreasing bond angular sum) of analogously substituted (triorganopnictines) ER¢3. This was found both in experimental studies (see Table 4) and by computational calculations [21]. In contrast, the Lewis acidity of group 13 compounds does not follow a steady trend but takes the order: R3Al > R3Ga > R3In » R3B > R3Tl. However, both the Lewis acidity and Lewis basicity do not only depend on speci®c electronic properties of the central group 13/15 elements, but also on substituent effects. Electron-withdrawing substituents lead to stronger Lewis acids and weaker Lewis bases, while electron-donating substituents increase the Lewis basicity and decrease the Lewis acidity. Following trends for the strengths of Lewis acids MR3 (M = Al, Ga, In) are valid: MF3 > MCl3 > MBr3 > MI3 > MH3 > MMe3 > MEt3 > M(t-Bu)3. Boranes BR3, however, exhibit a slightly different tendency. The acidity of the trihalides BX3 follows exactly the reverse sequence [34]: BF3 < BCl3 < BBr3 < BI3. As expected, the Lewis basicity of ER03 (E = NABi) takes the opposite trend: EF3 < ECl3 < EBr3 < EI3 < EH3 < EMe3 < EEt3 < E(t-Bu)3. These trends correspond to the increasing +I effect of the substituents. In addition, repulsive steric interactions between bulky substituents such as t-Bu, leading to larger CAEAC bond angles, may also affect the Lewis basicity by reducing the s-character and increasing the p-character of the lone pair. However, the strength of the Lewis acid-base interaction does not necessarily correspond with the Lewis acidity and basicity of the fragments, since steric (repulsive) interactions between the substituents may be a limiting factor. For instance, NMe3 is expected to be a stronger Lewis base than N(H)Me2, but H3BAN(H)Me2 (DHDiss = 36.41.0 kcal mol)1) and Me3BAN(H)Me2 (DHDiss = 19.30.2 kcal mol)1) are stronger Lewis acid-base adducts than H3BANMe3 (DHDiss = 34.80.5 kcal mol)1) and Me3BANMe3 (DHDiss = 17.6 0.2 kcal mol)1) [15]. 2.1.1 Synthesis of Group 13-Stibine and -Bismuthine Adducts Wells et al. characterized group 13-stibine adducts by single crystal X-ray structure analyses ®rst in 1997 [35]. The solid state structures of three boranestibine adducts of the type X3BASb(Tms)3 (X = Cl 6, Br 7, I 8), obtained by reaction of boron trihalides BX3 and Sb(Tms)3 in n-pentane, were determined.

Synthesis, Structure and Reactivity of Group 13/15 Compounds

125

These initial studies were expanded in our group and by Wells et al. to analogous reactions of trialkylalanes [36], -gallanes [37] and -indanes [38] R3M as well as dialkylaluminum chlorides R2AlCl [36a]. In addition, completely alkyl-substituted adducts were obtained from reactions of trialkylalanes [39] and -gallanes [40] with trialkylstibines (Scheme 4). Very recently, we synthesized stable group 13-bismuthine adducts by reaction of trialkylalanes and -gallanes with triorganobismuthines BiR¢3 (R¢ = Tms, i-Pr) [41]. These compounds represent the ®rst structurally characterized R3MABiR¢3 adducts.

Scheme 4. Synthesis of Lewis acid-base adducts R3MÐER¢3 (E = Sb, Bi) since 1996

The acid-base interaction in group 13-stibine and -bismuthine adducts seems to be very weak as is indicated by mass spectroscopic studies, which never showed the molecular ion peak but only the respective Lewis acid and Lewis base fragments. The extreme lability in the gas phase may also account for the fact that there are only very few reports on thermodynamic data of group 13-stibine or bismuthine adducts in the literature. Therefore, multinuclear NMR spectroscopy and single crystal X-ray diffraction are the most important analytical tools for the characterization of such adducts. 1 H-NMR spectra of stibine adducts R3MASbR¢3 typically exhibit resonances due to the organic ligands bound to the group 13 element shifted to lower ®eld and those of the ligands bound to Sb shifted to higher ®eld compared to the pure trialkyl species. Analogues results were reported for several group 13amine and -phosphine adducts such as Me3AlAPR03 [42], R3GaAPR¢3 (R = Me, Et) [43] and Me3InANR03 [44]. The relative stability of R3MAER¢3 adducts containing a given Lewis acid MR3 and different Lewis bases ER¢3 can be estimated from the difference Dd of the chemical shifts of the MAR group in the adduct R3MAER¢3 and in the pure trialkyl compound MR3. It was found that Dd is strongly affected by the organic substituents bound to both central atoms. Lewis acids containing small ligands such as MMe3 or MEt3 show the biggest down®eld shift of the proton resonance with the strongest Lewis bases such as Sb(t-Bu)3 or Sb(Tms)3, containing large and electropositive substituents [45]. In contrast, adducts of Lewis acids with sterically more demanding substituents such as t-Bu3Al and t-Bu3Ga show the biggest down®eld shift when combined with sterically less hindered Lewis bases such as SbEt3 and Sb(n-Pr)3, while adducts with Sb(t-Bu)3 and Sb(Tms)3 show Dd values near zero, indicating excessive dissociation in solution.

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S. Schulz

In contrast to these trends observed for the stibine adducts, the 1H-NMR spectra of the bismuthine adducts R3MABiR¢3 without exception show almost the same chemical shifts due to the organic groups as the starting trialkylalanes, -gallanes and -bismuthines, again indicating very weak acidbase interactions in solution. 2.1.2 Temperature-Dependent NMR Investigations Reliable information on the thermodynamic stability of group 13/15 adducts is usually obtained by gas phase measurements. However, due to the lability of stibine and bismuthine adducts in the gas phase toward dissociation, temperature-dependent 1H-NMR studies are also useful for the determination of their dissociation enthalpies in solution [41b]. We focussed on analogously substituted adducts t-Bu3AlAE(i-Pr)3 (E = P 9, As 10, Sb 11, Bi 12) since they have been fully characterized by single crystal X-ray diffraction, allowing comparisons of their thermodynamic stability in solution with structural trends as found in their solid state structures. The observed dissociation enthalpies of t-Bu3AlAE(i-Pr)3 adducts (12.2 kcal/mol 9, 9.9 kcal/mol 10, 7.8 kcal/mol 11 and 6.9 kcal/mol 12) steadily decrease with increasing atomic number of the pnictine, as was expected (Fig. 3). Since steric interactions within analogously substituted adducts should become less effective with increasing atomic radius of the central group 15 element, the observed trend obviously results from the decreased Lewis basicity of the heavier pnictines. Additional studies on R3AlABi(Tms)3 (R = Me 13, Et 14) showed the extreme lability of alane-bismuthine adducts toward dissociation in solution. Their 1H-NMR spectra at ambient temperature only show one resonance due to the AlAR groups, while at )70 °C two resonances of the AlAR groups in a

Fig. 3. Dissociation enthalpies of adducts t-Bu3Al±E(i-Pr)3 as obtained from temperaturedependent NMR Studies

127

Synthesis, Structure and Reactivity of Group 13/15 Compounds

relative intensity of 1:2 are observed, caused by the presence of (R3Al)2 dimers (terminal and bridging substituents) in solution. The two resonances coalesce between )30 and )50 °C (Al-Me) and )25 and )40 °C, respectively, as was known for (Me3Al)2 and (Et3Al)2. Interestingly, Et3AlABi(Tms)3 14 undoubtedly is a stable adduct in the absence of any solvent, as was shown by single crystal X-ray diffraction. These experimental ®ndings strongly underline the very weak Lewis acid-base interaction within group 13-bismuthine adducts. 2.1.3 Single Crystal X-Ray Structure Determinations Up to now, ®fteen group 13-stibine R3AlASbR¢3 and four group 13-bismuthine adducts R3AlABiR¢3 have been structurally characterized by single crystal Xray diffraction studies. Their central structural parameters are summarized in Table 5. Structures 1±4 show the solid state structures of four representative adducts. Table 5. Selected bond lengths (pm) and angles (°) of adducts R3MAER¢3 (M = B, Al, Ga, In; E = Sb, Bi) as obtained from single crystal X-ray diffraction

Adduct

MAE

R3MASb(Tms)3 M = B; R = Cl 6 M = B; R = Br 7 M = B; R = I 8 M = Al; R = Et 15 M = Al; R = i-Bu 16 [46] M = Ga; R = Et 17 M = Ga; R = t-Bu 18 M = In; R = Me3SiCH2 19

225.9 226.8 225.7 284.1 284.8 284.6 302.7 300.8

MAR (ave.) S(XAEAX) S(RAMAR) (21) (17) (8) (1) (1) (5) (2) (1)

185.5 201.8 224.2 198.4 199.5 200.7 201.3 220.8

327.7 327.2 325.6 310.8 312.2 308.8 302.0 312.8

328.5 329.9 ± 347.3 350.5 348.3 349.9 353.1

282.1 (1); 279.8 (1)

199.1; 199.4

312.6; 309.1

339.6; 341.5

R3MASbR¢3 M = Al; R = Me; R¢ = t-Bu 21 M = Al; R = Et; R¢ = t-Bu 22 M = Al; R = t-Bu; R¢ = i-Pr 11 M = Al; R = t-Bu; R¢ = Et 23 M = Ga; R = t-Bu3; R¢ = Et 24 M = Ga; R = t-Bu3; R¢ = i-Pr 25

283.4 287.3 292.7 284.5 284.8 296.2

(1) (1) (1) (1) (1) (1)

196.7 198.1 203.0 202.7 204.4 204.2

319.1 317.8 294.1 301.5 292.8 300.5

347.2 343.7 348.7 346.9 349.3 347.6

R3MABi(Tms)3 M = Al; R = Et 14 M = Ga; R = Et 26

292.1 (2) 296.6 (1)

197.8 199.0

305.7 303.5

350.8 353.9

R3MABiR¢3 M = Al; R = t-Bu; R¢ = i-Pr 12 M = Ga; R = t-Bu3; R¢ = i-Pr 27

308.8 (1) 313.5 (1)

201.8 203.0

286.5 286.1

350.4 352.1

R2MClASb(Tms)3 M = Al; R = t-Bu 20a

a

Two molecules within the asymmetric unit.

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S. Schulz

Structure 1. Ortep diagram (50% thermal ellipsoids) showing the solid state structure for 19

Structure 2. Ortep diagram (50% thermal ellipsoids) showing the solid state structure for 22

Structure 3. Ortep diagram (50% thermal ellipsoids) showing the solid state structure for 12

Synthesis, Structure and Reactivity of Group 13/15 Compounds

129

Structure 4. Ortep diagram (50% thermal ellipsoids) showing the solid state structure for 26

Stibine adducts. All R3MASbR¢3 adducts display distorted tetrahedral coordination geometries around the central atoms. The substituents R and R¢ are arranged in a staggered conformation in relation to one another. The MASb bond lengths are signi®cantly elongated compared to the sum of the covalent radii [47] [Srcov(AlSb): 266; Srcov(GaSb): 267; Srcov(InSb): 285 pm] except for the borane adducts X3BASb(Tms)3, which show distances between 225.7 8 and 226.8 pm 7 [Srcov(BSb): 223 pm]. This is likely a result of their increased Lewis acidity due to the halogen substituents (strong )I effect). Interestingly, the BASb distances do not re¯ect the different Lewis acidities of the boron trihalides, since they differ only by about 1 pm. Experimental studies and computational calculations on Me3N-adducts with BF3, BCl3, BBr3 and BI3 con®rm these results [48]. In contrast, the MASb bond distances (M = Al, Ga) observed in the solid state structures differ signi®cantly depending on the steric demand of the substituents. This is somewhat surprising since repulsive steric interactions should generally decrease with increasing atomic radii of the central atoms. The AlASb distances range from 279.8(1) 20 to 292.7(1) pm 11 and the GaASb distances vary between 284.6(5) 17 and 302.7(2) pm 18. The longest MASb bond distances in both groups were observed for the sterically extremely crowded adducts t-Bu3MASbR¢3 (M = Al, Ga; R¢ = i-Pr, Tms), while the shortest AlASb distance was found in the partially chloro-substituted adduct tBu2AlClASb(Tms)3 20, likely due to the electron-withdrawing effect of the Cl atom. Br3AlASbBr3 5 exhibits a much shorter AlASb bond length (252.2 pm), as was determined by electron diffraction [17d]. This bond distance is even shorter than the sum of the covalent radii, which is rather unusual for adducts. Unfortunately, to the best of our knowledge, no other electron diffraction studies of group 13-stibine adducts have been reported to date. The only structurally characterized InASb adduct is (Me3SiCH2)3 InASb(Tms)3 19 [38], featuring an InASb bond distance of 300.8(1) pm. Due to the lack of other structurally characterized InASb adducts, no structural comparisons can be made. The InASb bond length found in 19 is supposed to be at the lower end of the InASb dative bond range since the covalent radius of In (rcov: 143 pm) is about 17 pm larger than those of the lighter elements Al and Ga. Therefore, InASb dative bonds are expected to

130

S. Schulz

vary from 300 to 320 pm. Further investigations are necessary to gain deeper insights into the nature of dative InASb bonds. Considering the almost equal covalent radii of Al and Ga, analogously substituted AlASb and GaASb adducts are very attractive compounds for comparative studies. Thus, qualitative estimations on the in¯uence of the particular Lewis acid on the structural parameters of the adducts should be possible. Differences between identically substituted alane and gallane adducts in such cases are only based on different electronic properties of the Lewis acid, whereas steric effects can be neglected. Since gallanes are expected to be weaker Lewis acids than the corresponding alanes, the GaASb distances are expected to be elongated compared to the AlASb distances. However, experimental ®ndings meet this expectation only in a limited way. The MASb bond lengths of sterically crowded adducts such as t-Bu3MASb(i-Pr)3 differ by 4 pm (M = Al: 292.7(1) 11, Ga: 296.2(1) pm 25), but adducts of the type t-Bu3MASbEt3 (M = Al: 284.5(1) 23; Ga: 284.8(5) pm 24) and Et3MASb(Tms)3 (M = Al: 284.1(1) 15; Ga: 284.6(5) pm 17) bearing less bulky substituents show almost the same MASb distances. Bismuthine adducts. As was observed for group 13-stibine adducts, the central atoms in group 13-bismuthine adducts R3MABiR¢3 (M = Al [41b], Ga [41a]) also reside in distorted tetrahedral environments. The MABi bond distances [292.1(2) 14; 296.6(1) 26; 308.8(1) pm 12; 313.5(1) pm 27] are signi®cantly elongated compared to the sum of the covalent radii {Srcov(AlBi): 275; Srcov(GaBi): 276 pm [47]}, in particular those of 12 and 27. The AlABi bond lengths are shorter than the respective GaABi distances found in analogously substituted gallane-bismuthine adducts. When compared to the MASb bond lengths, the MABi bond lengths are signi®cantly elongated, in particular those of t-Bu3MABi(i-Pr)3 [M = Al: 308.8(1) pm 12; M = Ga: 313.5(1) pm 27]. The increase by 16 (M = Al) and 17 pm (M = Ga), respectively, clearly exceeds the difference of the covalent radii of 9 pm (Sb: 141 pm, Bi: 150 pm). Estimation of the adduct strength in the solid state. Haaland [49] and Frenking et al. [48] demonstrated for several borane and alane adducts that the adduct formation process between a group 13 trialkyl and an amine or a phosphine is accompanied by a decrease of the CAMAC bond angles (from 120° toward tetrahedral) and an increase of the MAC bond distances. According to this model, the bigger the decrease of the CAMAC bond angular sum and the bigger the increase of the MAC bond length, the stronger is the acid-base interaction (Fig. 4). Single crystal X-ray structure analyses of analogously substituted adducts con®rmed the applicability of this model. Adducts of the type Et3AlAE(Tms)3 and t-Bu3AlAE(i-Pr)3 (E = P, As, Sb, Bi) were structurally characterized [50], allowing detailed comparisons of their solid state structural parameters. The trends observed for the average AlAC bond lengths and the CAAlAC bond angular sums are summarized in Figs. 5 and 6. Both ®gures clearly show the steadily increasing CAAlAC bond angular sum and the decreasing (average) AlAC bond length from the phosphine adducts toward the bismuthine adducts, as was expected by the Haaland model. The slightly smaller value of the CAAlAC bond angular sum for t-Bu3AlABi(i-Pr)3

Synthesis, Structure and Reactivity of Group 13/15 Compounds

131

Fig. 4. Changes of the structural parameters resulting from adduct formation

12 compared to Et3AlABi(Tms)3 14, which indicates a slightly increased stability of 12, agrees with experimental ®ndings observed in temperaturedependent NMR experiments. These have proven the existence of tBu3AlABi(i-Pr)3 12 in solution (DHDiss = 6.9 kcal/mol), while Et3AlABi(Tms)3 14 is fully dissociated. In spite of the signi®cantly shorter AlABi bond distance found for 14, 12 seems to be the more stable adduct, at least in solution. However, the Haaland model can only give a qualitative trend of the stability. It should not be overestimated, in particular when comparing adducts containing different substitution patterns.

Fig. 5. MAC bond distances of analogously substituted Lewis acid-base adducts

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S. Schulz

Fig. 6. CAAlAC bond angular sums of analogously substituted Lewis acid-base adducts

2.1.4 Computational Calculations Detailed computational analyses were performed to assess the coordination properties of various triorganopnictines toward AlH3 and AlMe3, respectively [50]. As was observed for the solid state structures, the adducts typically display C3v symmetry with the substituents adopting a staggered orientation in relation to one another. The calculated dissociation energies within a homologous series of adducts decrease with increasing atomic number of the pnictine (Table 6). The dissociation energies of Me3AlAtrialkylpnictine adducts typically range from 13 (AlAP) to 4 kcal mol)1 (AlABi), while AlH3 adducts are signi®cantly more stable [22 (AlAP) to 10 (AlABi) kcal mol)1]. According to the decreased Lewis basicity of EH3 compared to trialkylstibines ER03 (R¢ = Et, i-Pr), EH3Aadducts constantly show the lowest dissociation energies within each adduct group (Fig. 7). As was expected, the dissociation energies of adducts of a given Lewis acid AlR3 and different Lewis bases ER¢3 increase with increasing basicity of the pnictine ER¢3 (R¢ = H < Et < i-Pr) due to different electronic and steric substituent effects. However, the in¯uence of substituents on the adduct stability is less pronounced than the in¯uence of the group 15 element itself. The decreasing Lewis acidity of the alanes (H3Al > Me3Al) is also re¯ected by the decreasing dissociation energies. According to the Haaland model, an increase of the Lewis acid-base interaction is accompanied by a decrease of both the RAAlAR bond angles and the AlAR bond distances. However, comparisons are possible only for adducts containing the same alane to exclude any steric or electronic effects of

133

Synthesis, Structure and Reactivity of Group 13/15 Compounds

Table 6. B3LYP/SDD optimized structural parameters (pm, °) and dissociation energies (kcal/mol) of H3AlAER¢3 and Me3AlAER¢3 adducts; De = dissociation energy at 0 K

Adduct

AlAE

AlAR (ave.) RAAlAR

EAR¢

R¢AEAR¢

De

H3AlAEH3 E=P E = As E = Sb E = Bi

265.4 276.5 304.9 317.4

159.5 159.3 159.2 159.0

118.7 119.0 119.3 119.7

142.5 151.8 169.8 179.9

98.8 96.4 94.9 93.4

13.5 9.5 6.4 2.9

H3AlAEEt3 E=P E = As E = Sb E = Bi

256.9 266.2 290.5 300.6

160.2 160.0 159.8 159.5

117.4 117.8 118.3 118.8

189.6 199.0 217.5 227.7

103.3 101.5 99.3 97.5

21.3 16.6 12.6 9.2

H3AlAE(i-Pr)3 E=P E = As E = Sb E = Bi

257.5 266.4 289.8 299.3

160.3 160.1 159.8 159.7

117.1 117.5 118.1 118.5

193.2 202.1 220.1 229.9

104.3 103.2 100.9 99.4

22.3 17.7 13.6 10.3

Me3AlAEH3 E=P E = As E = Sb E = Bi

279.1 298.5 345.3 382.2

198.8 198.3 196.1 197.7

118.5 119.0 119.4 120.0

142.9 152.5 170.7 180.9

97.6 94.8 93.0 91.5

7.4 4.3 2.0 0.4

Me3AlAEEt3 E=P E = As E = Sb E = Bi

266.1 279.3 311.6 328.4

199.6 199.3 198.9 198.4

116.9 117.4 118.3 119.0

189.9 199.4 218.4 228.3

102.5 100.3 97.6 95.8

12.6 8.8 5.7 3.6

Me3AlAE(i-Pr)3 E=P E = As E = Sb E = Bi

269.4 283.8 310.0 324.8

199.7 199.3 199.0 198.6

116.2 116.9 117.9 118.8

193.1 202.8 220.7 230.5

100.7 103.0 99.4 98.2

13.6 9.9 6.7 4.4

the substituents bound to Al. In addition, the calculations clearly demonstrate that there is no correlation between the AlAE bond distances and the thermodynamic stabilities of the adducts. Thermodynamically more stable adducts do not necessarily feature shorter AlAE distances. Comparison of calculated and experimental data. Since the homologous series of adducts of the type Et3AlAE(Tms)3 (E = P 28, As 29, Sb 15, Bi 14) has been structurally characterized by single crystal X-ray diffraction, their structures and thermodynamic stabilities were calculated to allow a comparison between experimental and theoretical data (Table 7). The dissociation energies constantly decrease from Et3AlAP(Tms)3 28 (De = 13.8 kcal mol)1) to Et3AlABi(Tms)3 14 (De = 2.2 kcal mol)1). Consequently, the increase of the AlAC bond lengths and the decrease of the CAAlAC bond angles in respect to pure Et3Al become less intense with

134

S. Schulz

Fig. 7. Calculated dissociation energies of alane-pnictine adducts

increasing atomic number of the group 15 element. Bond distances and bond angles obtained from the calculations are only slightly bigger than those experimentally observed. In contrast, the calculated and experimentally observed MAE bond distances differ signi®cantly. The AlAE bond lengths obtained from single crystal structure analyses (255.5 28, 265.4 29, 284.1 15, 292.1 pm 14) show an increase of 37 pm in agreement to the increase of the covalent radii of the group 15 elements (P 110; Bi 150 pm), whereas the calculated data show an elongation by almost 50 pm (AlAP: 269.7 pm; AlABi: 321.4 pm). In particular, the calculated and experimental data of the stibine and bismuthine adducts differ by almost 8 and 10%, respectively (Fig. 8). 2.2 Bisadducts of the Type [R3M]2[E2R¢4]

The synthesis of tetraalkyldipnictines of the type R04 E2 (E = P, As, Sb, Bi), containing a central EAE bond, goes back to Cadet¢s initial discovery of the Table 7. B3LYP/SDD optimized and experimental structural parameters (in parentheses) for

Et3AlAE(Tms)3 adducts (pm, °)

Et3AlAE (Tms)3

AlAE

E E E E

269.7 276.9 306.2 321.4

= = = =

P As Sb Bi

(255.5) (265.4) (284.1) (292.1)

AlAC (ave.)

CAAlAC

EAX (ave.)

XAEAX

199.6 200.9 200.5 200.1

116.2 116.7 117.6 118.4

234.2 244.7 264.7 271.3

108.1 106.1 103.4 100.2

(198.9) (198.9) (198.4) (197.8)

(113.6) (114.1) (115.8) (116.9)

(227.3) (236.5) (256.0) (263.2)

De

(106.6) 13.8 (105.0) 8.8 (104.1) 5.1 (101.9) 2.2

Synthesis, Structure and Reactivity of Group 13/15 Compounds

135

Fig. 8. AlAE bond distances [calculated, experimental and Srcov(AlE)] of Et3AlAETms3

adducts

``fuming liquid'' in 1757. The major components of the fuming liquid are tetramethyldiarsine Me4As2 and [Me2As]2O, which are also known as cacodyl and cacodyl oxide, as was shown later by Bunsen [51]. Since these early investigations, numerous compounds of this speci®c type, in particular diphosphines and diarsines, have been synthesized. Their reactivity toward transition metal complexes was studied in detail, leading to the formation of monodentate (types A and B) and bidentate complexes (types C and D) as well as heterocycles (types E and F) (Fig. 9) [52]. In contrast, distibines and dibismuthines have been investigated to a far lesser extent. This is mainly due to their lability toward disproportionation into the element E and R03 E [53]. In addition, their lone pairs exhibit a much higher s-character, as was also found for ER¢3 compounds, severely limiting their potential for further complexation reactions. Therefore, only a handful monoand bimetallic complexes [LnM¢]x[R04 Sb2] (M¢ = transition metal; x = 1, 2) and heterocycles (R02 SbM¢Ln)2 have been synthesized [54], while analogous dibismuthine complexes are entirely unknown. The same is true for group 13 metal complexes of distibines and dibismuthines. However, the lack of any data is not surprising since the number of reports on group 13-dipnictine complexes in general is very limited. Prior to our studies, only two diphosphine-borane

Fig. 9. Transition metal complexes of dipnictines E2R04

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S. Schulz

bisadducts of type C, [H3B]2[Me4P2] 30 [55] and [H2(Br)B]2[Me4P2] 31 [56], had been synthesized and structurally characterized. 2.2.1 Syntheses and Solid State Structures of Group 13-Distibine and -Dibismuthine Bisadducts Our investigations concerning the reactions of trialkylalanes and -gallanes R3M with tetraalkyldistibines and -dibismuthines R¢4E2 led to the synthesis of several bisadducts [R3M]2[E2R04 ] (M = Al, Ga; E = Sb [57], Bi [58]) of type C (Scheme 5).

Scheme 5. Synthesis of tetraalkyldistibine and-dibismuthine bisadducts [R3M2][E2R4¢]

The so-prepared compounds are stable in the pure form, whereas they easily undergo consecutive reactions in solution under cleavage of the EAE bond (see Sect. 3.1). Four tetraalkyldistibine (32±35) and two -dibismuthine bisadducts (36 and 37) have been structurally characterized (Table 8). Table 8. Selected bond lengths (pm) and angles (°) of tetraorgaonodipnictines and their main group metal and transition metal complexes

Compound

EAE

MAE

MAR (ave.) SYAEAXa SRAMAR

P2Me4 [59] [H3B]2[P2Me4] 30 [H2(Br)B]2[P2Me4] 31 Sb2Me4

221.2 202.6b 218.9 (5) 286.2 [60]; 283.0 (1), 283.8 (1) [61] 286.7 (1) 2.866 (1) 283.7 (1) 283.0 (1) 286.6 (1) 286.1 (1) 282.6 (4) 281.1 (1) 281.4 (1) 283.8 (1) 283.9 (1) 303.5 (3) 299.0 298.3 (1) 298.4 (1)

± 196.9 197.1 (1) ±

± 117.1 ± ±

265.6 ± 313.4 328.6 318.9 ± 285.4 [60]; 289.4 [61]

± ± ± ± 262.6 274.9 272.6 291.9 291.9 300.1 302.8 ± ± 308.4 309.9 311.4

± ± ± ± ± ± ± 202.0 203.0 202.4 203.8 ± ± 201.6 203.6; 204.1

287.6 278.1 284.7 286.3 298.4 299.2 310.5 295.1 292.8 292.9 291.1 282.1 280.8 287.7 285.9; 291.6

Sb2(Tms)4 [62] Sb2(Me3Sn)4 [63] Sb2Ph4 [64] [Sb(H)CHTms2]2 [65] [(OC)5Cr]2[Sb2Ph4] [64] [(OC)5W]2[Sb2Ph4] [64] [(OC)5Re]2[Sb2Ph4] [64] [t-Bu3Al]2[Sb2Me4] 32 [t-Bu3Ga]2[Sb2Me4] 33 [t-Bu3Al]2[Sb2Et4] 34 [t-Bu3Ga]2[Sb2Et4] 35 Bi2(Tms)4 [66] Bi2Ph4 [67] [t-Bu3Al]2[Bi2Et4] 36 [t-Bu3Ga]2[Bi2Et4] 37 a b

(1) (1) (3) (1) (1) (1) (1) (2) (2); (2)

RYAEAX E-E-X1;2 X1 -E-X2 (degree of pyramidalization). Structural data of the trans-isomers.

± ± ± ± ± ± 351.1 352.2 350.2 351.1 ± ± 352.7 354.3; 353.4

Synthesis, Structure and Reactivity of Group 13/15 Compounds

137

Structures 5 and 6 display the solid state structures of two representative distibine and dibismuthine adducts. The ligands bound to the central Sb and Bi atoms adopt a staggered conformation in relation to one another, with the bulky M(t-Bu)3 groups arranged in a trans-position. This is likely due to repulsive steric interactions. The central SbASb [281.1(1) 32; 283.9(1) pm 35] and BiABi bond distances [298.3(1) 36 and 298.4(1) pm 37] are almost unchanged compared to the uncomplexed distibines and dibismuthines, as can be seen

Structure 5. Ortep diagram (50% thermal ellipsoids) showing the solid state structure

for 34

Structure 6. Ortep diagram (50% thermal ellipsoids) showing the solid state structure

for 37

138

S. Schulz

from Table 8. So no indication for an EAE bond weakening in the solid state due to the increased steric pressure resulting from the adduct formation is observed. Comparable SbASb bond distances were found in transition metal complexes such as {[I2Cd]2[Sb2Et4]}n [278.4(2) pm], [(CO)5W]2[Sb2Ph4] [286.1(1) pm] and [(CO)5Cr]2[Sb2Ph4] [286.6(1) pm]. The AlASb and GaASb bond lengths of the [t-Bu3M]2[Sb2Me4] bisadducts [291.9(1) 32, 291.9(1) pm 33] are signi®cantly shorter compared to those of the [t-Bu3M]2[Sb2Et4] bisadducts [300.1(1) 34, 302.2(2) pm 35], which is likely due to minor steric hinderance. The MASb bond distances found for 34 and 35 and the MABi bond distances displayed by 36 and 37 belong to the longest MAE bond lengths ever observed. The AlABi [308.4(1) pm 36] and GaABi bond lengths [309.9(2), 311.4(2) pm 37] are comparable to those found for (t-Bu)3MABi(iPr)3 [M = Al: 308.8(1) pm 12, Ga: 313.5(1) pm 27]. Both the MASb and MABi bond lengths are signi®cantly (up to 35 pm) elongated compared to the sum of the covalent radii [Srcov(AlSb): 266 pm; Srcov(AlBi): 275 pm; Srcov(GaSb): 267 pm; Srcov(GaBi): 276 pm]. As was found for analogously substituted stibine and bismuthine adducts of the type R3MAER¢3, the GaAE (E = Sb, Bi) and GaAC distances of the gallane bisadducts [t-Bu3Ga]2[E2R04 ] are slightly longer and the CAGaAC bond angular sum is slightly bigger than the respective bonding parameters of the alane bisadducts [t-Bu3Al]2[E2R04 ]. These ®ndings agree to the reduced Lewis acidity of t-Bu3Ga compared to t-Bu3Al. The average SbAC distances (214.6 pm 32; 214.4 pm 33; 216.7 pm 34; 216.8 pm 35) are comparable to those observed in Me4Sb2 (216.2 pm). In contrast, the sums of the CASbAC and CASbASb bond angles (295.1° 32; 292.9° 33; 292.9° 34 and 291.1° 35) are signi®cantly increased compared to those of Me4Sb2 (average 283.1°). The same trend, which points to a partial rehybridization of the Sb centers, was found in simple R3MAER¢3 Lewis acid base adducts (M = Al, Ga). The former lone pair of the distibine, which had a high scharacter, gets more p-character upon coordination to the Lewis acid. Simultaneously, the s-character of the former SbAC and SbASb bonding electron pairs increases, resulting in a widening of the CASbAC and CASbASb bond angles. The tetraalkyldibismuthine adducts 36 and 37 exhibit comparable structural parameters. The MAC [average values: 201.6 pm 36; 203.6 pm (Ga1), 204.1 pm (Ga2) 37] and BiAC distances [average: 226.7 pm 36; 227.9 pm (Bi1), 228.6 pm (Bi2) 37] as well as the sum of the CABiAC and CABiABi bond angles [287.7° 36; 285.9° (Bi1), 291.6 (Bi2) 37] are almost identical. However, they are slightly bigger than those of the analogously substituted distibine bisadducts [t-Bu3M]2[Sb2Et4] (M = Al 292.9° 34; Ga 291.1° 35). This points to a higher s-character of the dative BiAM bonding electron pairs and a higher p-character of the BiAC and BiABi bonding electron pairs compared to the distibine bisadducts. Consequently, the sum of the CAMAC bond angles of the dibismuthine bisadducts (352.8° 36; 354.3°, 353.4° 37) are slightly bigger than those of the distibine derivatives (M = Al, 350.2° 34; Ga, 351.1° 35), indicating tetraalkyldibismuthines to be weaker Lewis bases than tetraalkyldistibines.

Synthesis, Structure and Reactivity of Group 13/15 Compounds

139

3 Heterocycles of the Type [R2MER¢2]x 3.1 General Synthetic Pathways

Group 13/15 heterocycles of the type [R2MER¢2]x and [RMER¢]x containing the lighter elements of group 15, N and P, are usually synthesized by alkane, hydrogen or salt elimination reactions, as can be seen in Scheme 6 [8b, 11].

Scheme 6. General synthetic pathways for the synthesis of group 13/15 heterocycles

Unfortunately, these standard reaction types almost completely failed for the synthesis of the corresponding stibides and bismuthides. Hydrogen and alkane elimination reactions are inappropriate since the EAH group is less acidic than the NAH and PAH group due to the reduced electronegativity of Sb and Bi. Salt elimination reactions very often occur under reduction of the stibide and bismuthide, respectively, leading to the formation of elemental Sb and Bi. Only two successful salt elimination reactions have been reported so far. Reactions of Me2MCl (M = Ga, In) and t-Bu2SbLi yield six-membered heterocycles [Me2MSb(t-Bu)2]3 (M = Ga 1, In 2) [13b]. Therefore, alternative synthetic pathways had to be developed to obtain a deeper understanding of this class of compounds. To date, three different reaction types have been found to give the desired heterocycles. Dehalosilylation reaction. The dehalosilylation reaction was introduced by Wells et al. in 1986, who obtained arsinogallanes by reaction of R2AsTms with chlorogallanes under elimination of TmsCl [68] (Scheme 7).

Scheme 7. First synthesis of GaAAs compounds by dehalosilylation reaction

The most important improvements resulting from this powerful reaction type, which is now well established for the synthesis of arsinogallanes, are the easy work-up of the reaction products as well as the possibility to obtain bis-

140

S. Schulz

and tris(arsino)gallanes of the type RGa(AsR2)2 and Ga(AsR2)3 [69]. In 1989, Cowley et al. ®rstly used this speci®c reaction type for the preparation of a heterocyclic stibinogallane {[Cl2GaSb(t-Bu)2]3 3} and an -indane {[t-Bu2Sb (Cl)In-l-Sb(t-Bu)2]2 4}. Compounds 3 and 4 were obtained by reaction of MCl3 and t-Bu2SbTms [13a,c]. The products 3 and 4 were the only structurally characterized group 13/Sb heterocycles, until we [37c] and Wells et al. [37a,b, 38, 70] started to investigate the synthesis of this particular class of compounds in more detail in 1996. Since then, several GaASb and InASb heterocycles, generally obtained as four- and six-membered rings, have been synthesized and structurally characterized (Scheme 8).

Scheme 8. Dehalosilylation pathway for the synthesis of GaASb and InASb heterocycles

Dehydrosilylation reaction. While the dehalosilylation reaction is very powerful for the synthesis of GaASb and InASb heterocycles, this reaction type completely failed for the synthesis of the corresponding stibinoalanes. Only the reaction of Me2AlCl with Sb(Tms)3, leading to the formation of [Me(Cl)AlSb(Tms)2]3 38, yielded an AlASb heterocycle [36a]. However, 38 obviously was not formed by dehalosilylation but by tetramethylsilane elimination reaction (Scheme 9).

Scheme 9. Synthesis of [Me(Cl)AlSb(Tms)2]3

Chloroalanes obviously display a different reactivity compared to chlorogallanes and -indanes, which is mainly based on two speci®c characteristics: 1) The AlACl bond energy is much higher compared to a GaACl and an InACl bond, resulting in a higher thermodynamic stability; 2) Chloroalanes exhibit a much higher Lewis acidity compared to analogously substituted chlorogallanes and -indanes. These particular properties of chloroalanes favor the formation of simple Lewis acid-base adducts, as was observed for the reaction of R2AlCl with Sb(Tms)3 (R = Et, t-Bu). In contrast, reactions of the analogous gallanes and indanes yielded the desired heterocycles. The same tendencies were observed in reactions of R2MCl (M = Al, Ga, In; R = Et, i-Bu) with P(Tms)3 and As(Tms)3. The gallane and indane react under formation of the expected MAE heterocycles [71], while the corresponding alanes yield the simple adducts

Synthesis, Structure and Reactivity of Group 13/15 Compounds

141

[72]. However, in contrast to the chloroalane-stibine adduct, the phosphine and arsine adducts could be converted into the corresponding heterocycles by thermal activation (Scheme 10) [73].

Scheme 10. Synthesis of Lewis acid-base adducts R2AlClAE(Tms)3

These speci®c problems were overcome by substituting the chlorine atom in R2AlCl by a hydrogen atom, leading to dialkylalanes R2AlH. They exhibit a signi®cantly reduced Lewis acidity and the hydrogen atom is much more weakly bound than the chlorine atom, hence depressing the tendency to form an adduct but to react with silyl-substituted group 15 compounds under elimination of Me3SiH. This reaction type, which is known as dehydrosilylation reaction, was introduced by NoÈth et al. [74] in 1961, who reacted diborane B2H6 and R2PTms, resulting in the formation of [H2BPR2]3. Later on, Fritz et al. [75] reported on the reaction of Et2PTms and HAlCl2 and H2AlCl, which proceeds under elimination of TmsH rather than TmsCl, proving the dehydrosilylation reaction to be the favored reaction pathway. In addition, in the reaction of Me2AlH and HP(Tms)2 the elimination of TmsH is preferred over H2, as was demonstrated by Krannich et al. [76] (Scheme 11).

Scheme 11. Initial dehydrosilylation reactions as performed by NoÈth et al., Fritz et al. and Krannich et al.

Our investigations started on reactions of R2AlH with Sb(Tms)3 and R2SbTms, giving the desired AlASb heterocycles of the type [R2AlSbR¢2]x (R = Me, Et, i-Bu; R¢ = t-Bu, Tms; x = 2, 3) [36a, 77] 39±42 in excellent yields. In addition, the reaction of Me2AlH with Bi(Tms)3 was found to give [Me2AlBi(Tms)2]3 43 [78], the ®rst (and so far the only) example of a heterocyclic group 13-Bi compound. Analogous reactions of Me2AlH with P(Tms)3 and As(Tms)3 also resulted in the formation of the corresponding heterocycles [Me2AlP(Tms)2]2 and [Me2AlAs(Tms)2]2, demonstrating the dehydrosilylation reaction to be generally applicable for the synthesis of heterocyclic group 15-alanes [79]. Its major advantages are based on the mild

142

S. Schulz

reaction conditions (no solvents; easy work-up; quantitative yields; low reaction temperatures), which are in particular necessary for the formation of the AlABi ring (Scheme 12).

Scheme 12. Synthesis of AlAE heterocycles by dehalosilylation reactions

Distibine cleavage reaction. As mentioned previously (see Sect. 2.2), tetraalkyldistibines Sb2R¢4 show a remarkable lability toward SbASb bond cleavage in reactions with electrophilic reagents [53]. While reactions of this speci®c type with transition metal complexes are well established, Breunig et al. ®rst described the reaction of a tetraalkyldistibine with group 13-trialkyls in 1998 [80]. Me4Sb2 was reacted with (TmsCH2)3In, leading to the formation of the six-membered heterocycle [(TmsCH2)2InSbMe2]3 44. The formation of a distibine adduct of type A or C (Fig. 9) was not observed. In contrast, our investigations on reactions of distibines with trialkylalanes and gallanes led to the formation of several distibine bisadducts [R3M]2[Sb2R¢4] 32±35 (see Sect. 2.2) [57], clearly proving the adduct formation to be the ®rst step in this reaction. While 32±35 are stable in the solid state below 0 °C under an inert gas atmosphere, their stability in solution is signi®cantly reduced. In agreement with Breunig¢s observations, consecutive reactions under SbASb bond cleavage were observed, yielding MASb heterocycles of the type [R2MSbR¢2]x, as was demonstrated for the gallane bisadducts 33 and 35 [57]. The resulting heterocycles [t-Bu2GaSbMe2]3 45 and [t-Bu2GaSbEt2]2 46 were isolated and structurally characterized. The most striking property of this particular reaction pathway is related to the simple generation of completely alkyl-substituted MASb heterocycles. These might be useful precursors for the synthesis of thin ®lms of the corresponding semiconducting MSb-materials by MOCVD processes (metal organic chemical vapor deposition) [81]. Prior to our studies, MASb heterocycles were only available by dehalosilylation (GaASb) and dehydrosilylation reactions (AlASb), respectively. Both reaction pathways suffer from the lack of stable silylstibines of the type R2SbTms as starting compounds. In addition, the resulting heterocycles are often partially contaminated with silyl groups, as was shown in the reaction of t-Bu2SbTms and Me2AlH [77b]. Under non-optimal reaction conditions, the partially silyl-substituted heterocycle (Me2Al)3(t-Bu2Sb)2Sb(Tms)2 47 was obtained (Fig. 10). The reaction of tetraalkyldistibines with group 13-trialkyls now offers an alternative synthetic pathway for the synthesis of completely alkyl-substituted heterocycles. The ®rst step of the reaction consists of a formation of a 1:1 or

143

Synthesis, Structure and Reactivity of Group 13/15 Compounds

Fig. 10. Reactions between Me2AlH and t-Bu2SbTms

1:2 adduct, followed by simultaneous MAC and SbASb bond cleavage. According to this reaction type, ``mixed-substituted'' stibines of the type R02 SbR are formed as byproducts, as was experimentally veri®ed by multinuclear NMR studies (1H, 13C). The formation of 1:1 adduct intermediates in solution starting from the bisadducts seems to be possible due to the weak MASb bonds and the extensive degree of dissociation of 32±35 in solution, as was con®rmed by NMR studies [82] (Fig. 11).

Fig. 11. Proposed reaction mechanism for the Sb2R¢4 bond cleavage reaction

3.2 Single Crystal X-Ray Structure Analyses

In general, MASb heterocycles either adopt four- or six-membered ring geometries of the type [R2MSbR¢2]x (x = 2, 3). The only structurally characterized AlABi heterocycle, [Me2AlBi(Tms)2]3 43, also forms a six-membered ring. AlAE heterocycles. The central structural parameters of AlASb heterocycles are summarized in Table 9. Table 9. Selected AlASb bond lengths (pm) and endocyclic AlASbAAl and SbAAlASb bond angles (°) of AlASb heterocycles

Heterocycle

MASb

AlASbAAl

SbAAlASb

[Me2AlSb(Tms)2]3 39 [36a] [Et2AlSb(Tms)2]2 40 [77a] [i-Bu2AlSb(Tms)2]2 41 (Me2Al)3(Sbt-Bu2)2Sb(Tms)2 47 [77b] [Me2AlSb(t-Bu)2]3 42 [77b]

270.3 272.3 274.3 271.9

118.5 (1)±128.2(1) 91.7 (1) 93.7 (1) 115.4 (1)±128.4 (1)

103.5 (1)±106.5 (1) 88.3 (1) 86.3 (1) 103.1 (1)±106.9 (1)

(1)±273.6 (1) (1), 272.9 (1) (1), 274.6 (1) (2)±278.0 (2)

271.9 (1)±278.4 (1) 115.3 (1)±128.9 (1) 102.8 (1)±108.2 (1)

144

S. Schulz

Structure 7. Ortep diagram (50% thermal ellipsoids) showing the solid state structure for 42

[Me2AlSb(Tms)2]3 39 and [Me2AlSb(t-Bu)2]3 42 are isostructural and adopt non-planar, distorted twist-boat conformations. Both the Al and Sb atoms of the central six-membered rings are arranged in distorted tetrahedral environments. The AlASb bond lengths found in 39 are almost equal [270.3(1)±273.8(1) pm], while those of 42 differ by about 6 pm [271.9(1)±278.4(1) pm]. The average bond distances of 272 (39) and 275 pm (42) are elongated compared to the sum of the covalent radii [Srcov (AlSb):266 pm], but signi®cantly shortened in respect to the AlASb bond lengths found for the Lewis acid-base adducts R3Al±SbR¢3 and [t-Bu3Al]2[Sb2R04 ] (280±300 pm). In spite of the increased steric demand of Tms compared to t-Bu groups (effective steric parameters: Tms: 1.40; t-Bu: 1.24) [83], the central ring in 39 seems to be less strained due to the replacement of a tertiary C atom by a larger Si atom (CMe3 versus Tms). This is expressed by the smaller variation of the endocyclic bond angles (3° 39; 6° 42) as well as the smaller exocyclic bond angles found in 39 [SiASbASi:100.7(1)±102.3(1)°] compared to those of 42 [CASbAC:104.9(2)±106.0(2)°]. The endocyclic SbAAlASb bond angles are smaller [39: 103.5(1)±106.5(1)°; 42: 102.8(1)±108.2(1)°] and the AlASbAAl bond angles wider [39: 118.5(1)±128.2(1)°; 42: 115.3(1)±128.9(1)°] than tetrahedral. Comparable structural parameters were reported for other six-membered Al-pnictine rings such as [Me2AlAs(CH2Tms)Ph]3 [AlAAsAAl: 118.2(2)±122.2(2)°, AsAAlAAs: 102.6(2)±104.8(2)°] [84], [Me2AlAsPh2]3 á (C7H8)2 [AlAAsAAl: 118.1(1)±122.7(1)°, AsAAlAAs: 99.1(1)±101.1(1)°] [84], [Me2AlN(CH2)2]3 [AlANAAl: 119.9(5)°, NAAlAN: 102.0(5)°] [85].

145

Synthesis, Structure and Reactivity of Group 13/15 Compounds

Structure 8. Ortep diagram (50% thermal ellipsoids) showing the solid state structure for 40

The AlASb bond distances of the planar, four-membered AlASb heterocycles [Et2AlSb(Tms)2]2 40 and [i-Bu2AlSb(Tms)2]2 41 are comparable [272.3(1)±272.9(1) pm 40, 274.3(1)±274.6(1) pm 41] to those observed in the six-membered rings. Due to the somewhat larger i-Bu substituents, 41 exhibits slightly longer AlASb bond distances than 40. The endocyclic bond angles (AlASbAAl: 91.7(1)° 40; 93.7(1) 41; SbAAlASb: 88.3(1)° 40; 86.3(1) 41) also re¯ect the different steric demand of the substituents. More bulky substituents bound to Al lead to an increase of the CAAlAC and AlASbAAl bond angles, while the SbAAlASb bond angles decrease. Analogous results were found in the corresponding ring systems [R2AlE(Tms)2]2 (R = Me, Et, i-Bu; E = P, As), as shown in Table 10. The AlABi heterocycle [Me2AlBi(Tms)2]3 43 [78] exhibits almost the same structural parameters as were found in other six-membered heterocycles of the type [R2AlE(Tms)2]3 (E = P, As, Sb). Table 10. Selected bond lengths (pm) and angles (°) of four-membered heterocycles

[R2AlE(Tms)2]2 (E = P, As, Sb)

Heterocycle

AlAE

AlAEAAl EAAlAE

RAAlAR

SiAEASi

[Me2AlP(Tms)2]2 [76, 86] [Et2AlP(Tms)2]2 [72a] [i-Bu2AlP(Tms)2]2 [76b] [Me2AlAs(Tms)2]2 [72b] [Et2AlAs(Tms)2]2 [73] [i-Bu2AlAs(Tms)2]2 [76] [Et2AlSb(Tms)2]2 [77a] [i-Bu2AlSb(Tms)2]2 [77a]

245.7 245.7 247.6 253.6 253.5 255.0 272.6 274.4

90.6 90.2 91.0 91.7 91.0 92.2 91.7 93.7

113.4 114.2 117.1 115.0 115.0 118.8 117.3 121.2

108.4 108.0 106.3 108.1 107.6 105.6 107.3 102.7

89.4 89.8 89.0 88.3 89.0 87.8 88.3 86.3

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S. Schulz

Structure 9. Ortep diagram (50% thermal ellipsoids) showing the solid state structure for 43

However, the endocyclic AlABiAAl bond angles [121.7(1)±130.5(1)°] are slightly increased and the BiAAlABi bond angles [101.0(1)±104.1(1)°] decreased compared to heterocycles containing the lighter group 15 elements. The AlABi bond distances range from 275.5(3)±279.3(3) pm. This is only marginally longer than the sum of the covalent radii of 275 pm, but signi®cantly shorter than the distances found in the AlABi adducts (292± 308 pm). Unfortunately, no other group 13-Bi heterocycle has been reported to date, allowing no further structural comparisons. Ga-Sb heterocycles. Structurally characterized GaASb heterocycles are summarized in Table 11. The six-membered heterocycles [Me2GaSb(t-Bu)2]3 1, [Cl2GaSb(t-Bu)2]3 3, [Me2GaSb(Tms)2]3 48 and [t-Bu2GaSb(Me)2]3 49 adopt distorted twist-boat-type conformations. The shortest GaASb bond distances were found in [Cl2GaSb (t-Bu)2]3 3 (average Ga-Sb: 266.1 pm), obviously resulting from the stronger Lewis-acidic character of the Ga atoms due to the electron-withdrawing effect of Table 11. Selected GaASb bond lengths (pm) and endocyclic GaASbAGa and SbAGaASb bond angles (°) of GaASb heterocycles

Heterocycle

GaASb

GaASbAGa

SbAGaASb

[Cl2GaSb(t-Bu)2]3 3 [13a] [Me2GaSb(Tms)2]3 48 [37c] [t-Bu2GaSb(Me)2]3 49 [57] [Et2GaSb(Tms)2]2 50 [70a] [t-Bu2GaSb(Tms)2]2 51 [37b] (t-Bu2Ga)2ClSb(Tms)2 52 [37b] [t-Bu2GaSb(Et)2]2 53 [57]

265.9 267.7 271.3 271.8 276.5 273.4 273.1

109.9 (average) 118.3 (1)±127.6 (1) 127.3 (1)±133.3 (1) 92.7 (1) 94.4 (1), 94.5 (1) 85.7 (1) 96.6 (1)

108.4 (average) 103.6 (1)±107.3 (1) 96.7 (1)±102.8 (1) 87.3 (1) 85.5 (1) ± 83.4 (1)

(2)±266.2 (2) (1)±271.4 (1) (1)±275.1 (1) (1), 272.9 (1) (1), 276.8 (1) (2) (1), 273.5 (1)

Synthesis, Structure and Reactivity of Group 13/15 Compounds

147

the Cl substituents. The distortion of the six-membered rings is re¯ected both by the wide range of the GaASb bond lengths and the endocyclic GaASbAGa and SbAGaASb bond angles. Again, the bond angles at the Ga centers are bigger and those at the Sb atoms smaller than tetrahedral.

Structure 10. Ortep diagram (50% thermal ellipsoids) showing the solid state structure for 49

The central structural parameters of the four-membered GaASb heterocycles [Et2GaSb(Tms)2]2 50, [t-Bu2GaSb(Tms)2]2 51, (t-Bu2Ga)2(Cl)Sb(Tms)2 52 and [t-Bu2GaSbEt2]2 53 are similar to those found in four-membered AlASb rings.

Structure 11. Ortep diagram (50% thermal ellipsoids) showing the solid state structure for 53

The observed GaASb bond lengths of 272.3 pm 50, 276.7 pm 51, 273.4 52 and 273.3 53 strongly depend on the steric demand of their substituents. The longest GaASb bond length was observed in 51 (276.5 pm), obviously due to increased steric hinderance between the very bulky substituents (t-Bu and Tms). These

148

S. Schulz

distances are both elongated compared to the sum of the covalent radii (267 pm) and to the GaASb bond distances found in the six-membered rings, re¯ecting the increased ring strain. (t-Bu2Ga)2Sb(Tms)2Cl 52, obtained by the reaction of t-Bu2GaCl and Sb(Tms)3 in 2:1 stoichiometry, show slightly shorter GaASb bond lengths (273.4 pm) compared to [t-Bu2GaSb(Tms)2]2 51 (276.7 pm), likely due to the electron-withdrawing in¯uence of the Cl atom.

Structure 12. Ortep diagram showing the solid state structure for 52

InASb heterocycles. InASb heterocycles display structural parameters comparable to the AlASb and GaASb heterocycles, as shown in Table 12. The InASb bond distances of six-membered rings are slightly shorter than those of four-membered rings, but somewhat longer than the sum of the covalent radii (283 pm) [47]. [t-Bu2Sb(Cl)In-l-Sb(t-Bu)2]2 4 exhibits two different InASb bond distances due to the endocyclic and the exocyclic InASb bonds. As expected, the InASb bond distance of the terminal, threecoordinated t-Bu2Sb fragment (279.7 pm) is shorter than those of the bridging t-Bu2Sb moiety (286.5 pm). However, NMR spectra recorded in solution at Table 12. Selected InASb bond lengths (pm) and endocyclic InASbAIn and SbAInASb bond angles (°) of InASb heterocycles

Heterocycle

InASb

282.2 [Me2InSb(t-Bu)2]3 2 [13a] [Me2SbIn(CH2Tms)2]3 44 [80] 285.2 284.4 [Me2InSb(Tms)2]3 54 [87] 282.4 [Et2InSb(Tms)2]3 55 [70b] ([t-Bu2Sb(Cl)In-l-Sb(t-Bu)2]2 286.5 4 [13c] [t-Bu2InSb(Tms)2]2 56 [38] 292.7 288 [(TmsCH2)2InSb(Tms)2]2 57 [37a]a a

(1)±288.9 (1) (1)±286.9 (1) (1)±287.0 (1) (2)±291.1 (2) (1), 287.0 (1)

InASbAIn 115.8 (1)±127.8 129.2 (1)±137.7 119.8 (1)±127.0 119.1 (1)±129.7 94.9 (1)

SbAInASb (1) (1) (1) (1)

(1), 293.4 (1) 94.8 (1), 95.1 (1) 95.2

103.7 (1)±109.4 (1) 92.4 (1)±98.6 (1) 102.7 (1)±106.8 (1) 102.4 (1)±106.2 (1) 85.1 (1) 85.0 (1) 84.8

The poor quality of the crystals did not facilitate a complete data set collection.

Synthesis, Structure and Reactivity of Group 13/15 Compounds

149

ambient temperature only show one signal due to the t-Bu groups, indicating a rapid interchange between the terminal and bridging moiety. The longest InASb bond lengths were found for the sterically most crowded heterocycle [t-Bu2InSb(Tms)2]2 56.

Structure 13. Ortep diagram (50% thermal ellipsoids) showing the solid state structure for 54

Structure 14. Ortep diagram showing the solid state structure for 4

Ring size studies. The ring size (degree of oligomerization x) of analogously substituted heterocycles [R2MER¢2]x in the solid state strongly depends on the atomic radius of the group 15 element, as was rationalized by comparison of single crystal X-ray structures of a homologous series of heterocycles

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S. Schulz

Table 13. Selected structural parameters of heterocycles [Me2ME(Tms)2]x

M

E

x

MAE (ave.)

EAMAE

MAEAM

CAMAC

SiAEASi

Al

N [88]a P [76,86] As [72b] Sb [36a] Bi [78]

2 2 2 3 3

199.6 245.7 253.6 271.8 277.4

90.1 89.4 88.3 104.9 102.3

89.9 90.6 91.7 124.0 126.8

109.7 113.4 115.0 117.9 119.2

117.5 108.3 108.1 101.7 100.5

Ga

N [89] P [90] As [91] Sb [37c]

2 2 2 3

208.2 245.0 253.0 269.1

89.8 88.2 87.0 105.2

90.3 91.8 93.0 123.6

113.8 114.4 116.8 118.1

118.7 108.0 107.7 101.6

In

N [92] P [93] As [87] Sb [87]

2 2 2 3

230.4 263.0 270.1 285.3

89.7 86.7 85.5 104.1

90.3 93.3 94.5 124.3

108.8 116.9 118.8 120.5

111.4 109.8 109.4 103.0

a

[Me2AlN(SiHMe2)2]2.

[Me2ME(Tms)2]2 (M = Al, Ga, In; E = N, P, As, Sb, Bi) (Table 13) [87]. N, P- and As-containing derivatives adopt four-membered ring structures in the solid state [Me2ME(Tms)2]2, while the somewhat larger Sb and Bi atoms favor the formation of six-membered rings [Me2ME(Tms)2]3. The group 13 element seems to have no signi®cant in¯uence on the preferred ring size within this particular family of heterocycles. The formation of the preferred ring size depends on both ring strain effects, generally favoring the less strained six-membered heterocycles, and entropy effects, generally favoring the formation of the less-aggregated four-membered rings. If both energies have comparable values, as was found in initial computational calculations [94], steric interactions between the substituents may control the degree of cyclization. Substituents in four-membered heterocycles generally have more space available due to the (idealized) 90° endocyclic bond angle of the central group 13/15 ring atoms, while sixmembered rings display endocyclic bond angles of 109.5° (ideal chair conformation) or 120° (ideal planar ring), respectively. Additional repulsive interactions between the substituents in the 1,3-position may also affect the ring geometry. Consequently, small ring atoms such as N, P, As favor the formation of four-membered rings [Me2ME(Tms)2]2 in order to reduce steric interactions between the substituents, while larger central ring atoms (Sb, Bi) yield six-membered heterocycles. In contrast, somewhat bigger Et-groups bound to group 13 elements in most cases lead to the formation of four-membered heterocycles [Et2ME(Tms)2]2 (M = Al, Ga, (In); E = P, As, Sb), resulting from increased steric interactions (Table 14). Only [Et2InSb(Tms)2]3 55, containing the largest group 13 element In, adopts a six-membered ring structure in the solid state. Only for 55 the central ring atoms are big enough to suf®ciently decrease steric interactions between the ligands, leading to the formation of the less-strained six-membered ring

151

Synthesis, Structure and Reactivity of Group 13/15 Compounds Table 14. Selected structural parameters of heterocycles [Et2ME(Tms)2]x

M

E

x

MAE (ave.)

MAEAM

Al

P [72a] As [73] Sb [77a]

2 2 2

245.7 256.5 272.6

90.2 89.6 91.7

Ga

P [71a] As [71a,g] Sb [70a]

2 2 2

245.8 254.4 272.3

In

P [95] As [95] Sb [70b]

2 2 3

264.6 271.2 287.3

EAMAE

CAMAC

SiAEASi

89.8 90.4 88.3

114.6 115.1 114.5

108.0 109.3 107.3

91.4 92.2 92.7

88.6 87.8 87.3

113.9 114.2 114.2

107.8 107.5 106.9

92.5 93.6 125.1

87.5 86.4 104.4

114.2 114.6 116.8

109.1 108.5 101.2

geometry. In this context, the question whether the analogous Bi-containing heterocycles [Et2MBi(Tms)2]3 (M = Al, Ga, In), which are currently unknown, will either form four- or six-membered rings seems to be interesting. 3.3 Reactivity of Heterocycles

So far, only heterocyclic MASb and MABi compounds have been obtained, whereas the synthesis of monomeric compounds R2MER¢2 failed. This is not surprising since monomeric group 13/15 compounds of this type generally have been realized only by use of sterically extremely demanding ligands on both M and E such as Mes*, Dipp or Ada. Their increased steric interactions inhibit the formation of heterocyclic or higher oligomeric structures [R2MER¢2]x (xP2) and favor the formation of kinetically stabilized, monomeric compounds [96]. However, heterocycles may serve as starting compounds for the generation of their monomeric forms. According to their formulation as ``headto-tail adducts'' (Fig. 12), strong Lewis bases are able to cleave the heterocycles by coordinating to the group 13 element, leading to the formation of electronically stabilized monomers of the type baseÐM(R2)ER¢2 (Scheme 13). Compounds of this type have been reported by different groups. Me3NAAl(H2)NR2 and Me3NAAl(H2)As(Tms)2 were synthesized by reaction of H3AlANMe3 with sterically demanding secondary amines R2NH [97] and As(Tms)3 [98], respectively. Reactions of H2AlClANMe3 and LiPMes2 led to the formation of Me3NAAl(H2)PMes2 [99]. In each

Fig. 12. Head-to-tail adducts

152

S. Schulz

Scheme 13. Synthesis of MAE monomers from MH3ANMe3 (M = Al, Ga)

compound, the group 13 element is coordinatively saturated by interaction with Me3N. The lone pair of the group 15 atom is potentially active for further coordination chemistry. Unfortunately, no general pathway for the synthesis of this speci®c class of compounds was known. The reactions have been limited to Me3N-stabilized AlH3 and H2AlCl as well as amines, phosphines and arsines as starting compounds. In addition, no prediction whether monomeric or heterocyclic structures will be formed was possible, as can be conducted from the reactions of H3AlANMe3 and H3GaANMe3 [100] with E(Tms)3 (E = P, As). Besides such intermolecular stabilized compounds, intramolecular stabilized compounds have also been reported. Rettig et al. described the synthesis of phosphine-stabilized AlAN monomers by using a tripodal ligand [101]. More recently, Raston et al. reported on the synthesis of compounds of the type [H2AlE(H)R]2, containing only weak AlAH bridges [102]. 3.3.1 Synthesis of Lewis Base-Stabilized Monomers of the Type BaseAM(R2)ER¢2 Intermolecular stabilized compounds of the desired type were generally formed either by a ring cleavage reaction of the MAE heterocycle (M = Al, Ga) and dmap or by reaction of R2AlH with E(Tms)3 in solution in the presence of dmap (Scheme 14)[41a, 103, 104]. The second reaction type is very useful if the corresponding heterocycle is unknown. For example, dmapAAl(Et2)Bi(Tms)2 64 can only be synthesized by this reaction pathway since [Et2AlBi(Tms)2]x is unknown, so far. The alanes dmapAAl(R2)E(Tms)2 (E = P, As, Sb, Bi; R = Me, Et) 58±64 were obtained in almost quantitative yield. In contrast, the analogous Ga monomers 65±67 are temperature labile, decomposing signi®cantly at room temperature both in solution and in the solid state. In-containing compounds have not been reported to date. The so-prepared compounds are very sensitive toward air and moisture, in particular in solution. 1H- and 13C-NMR spectra prove the formation of 1:1 adducts of the type dmapAM(R2)E(Tms)2. The proton resonances of the dmap molecule are shifted to higher ®eld, as was observed for similar borane adducts

153

Synthesis, Structure and Reactivity of Group 13/15 Compounds

Scheme 14. General synthetic pathways for the synthesis of monomeric, Lewis base-stabilized compounds dmapAM(R2)E(Tms2)

[105]. These ®ndings are in agreement with a partial rearrangement of the charge distribution within the aromatic ring as is shown in Scheme 15.

Scheme 15. Rearrangement of the charge distribution within the dmap molecule by coordination to a Lewis acid

3.3.2 Solid State Structures ± General Trends Single crystal X-ray analyses of 58±67 reveal the formation of monomeric R2ME(Tms)2 molecules, to which one dmap molecule is coordinated. AlAE monomers. Table 15 summarizes the central structural parameters as determined by single crystal X-ray diffraction. Table 15. Selected MAE bond distances (pm) and bond angles (°) of base-stabilized

monomeric group 15-alanes dmapAAl(R2)E(Tms)2

Compound

MAE

dmapAAl(Me2)P(Tms)2 58 [103b] dmapAAl(Me2)As(Tms)2 59 [103b] dmapAAl(Et2)As(Tms)2 60 [103b] dmapAAl(Me2)Sb(Tms)2 61 [103a] dmapAAl(Et2)Sb(Tms)2 62 [103a] dmapAAl(Me2)Bi(Tms)2 63 [41a] dmapAAl(Et2)Bi(Tms)2 64 [41a]

237.9 247.2 247.3 269.1 268.0 275.5 275.0

MAN (1) (2) (1) (1) (1) (2) (2)

198.4 197.5 198.8 197.8 198.0 197.2 197.8

(2) (4) (3) (2) (2) (4) (5)

ave. MAR

S(XAEAX)

197.5 196.8 197.7 197.0 198.0 197.2 198.8

309.1 304.1 306.6 302.4 298.9 296.8 293.4

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S. Schulz

Structure 15. Ortep diagram (50% thermal ellipsoids) showing the solid state structure for 61

Structure 16. Ortep diagram (50% thermal ellipsoids) showing the solid state structure for 64

The substituents bound to Al and the group 15 element E generally adopt a staggered conformation relative to one another. The AlAN bond distances, which range from 197.2(4) 63 to 198.8(3) pm 60, are almost equal, indicating no signi®cant electronic in¯uence of the speci®c group 15 element on the

Synthesis, Structure and Reactivity of Group 13/15 Compounds

155

acceptor property of the Al-fragment. Me3N-stabilized monomers such as Me3NAAl(H2)N(tmp)2 [205.8(2) pm], Me3NAAl(H)(Cl)N(Tms)2 [201.6(5) pm], Me3NAAl(H2)As(Tms)2 [199.8(7) pm] and Me3NAAl(H2)PMes2 [200.9 (8) pm] exhibit longer AlAN distances. AlAN adducts containing tricoordinated N centers show comparable AlAN distances, while the AlAN bond length found for 2-Me-pyridineAAlCl3 [194.2(2) pm] is shortened due to the stronger Lewis acidic character of the AlCl3-fragment [106]. The average AlAC and EASi bond distances and the CAAlAC bond angles are comparable to the starting compounds. In contrast, the AlAE bond lengths of 58±64 are relatively short. Thus, dmapAAl(Me2)P(Tms)2 58 shows an AlAP bond distance of 237.9(1) pm. Comparable AlAP bond lengths were found for the monomeric phosphinoalanes Trip2AlP(Ada)(SiPh3) [234.2(2) pm] [107] and Tmp2AlPPh2 [237.7(1) pm] [96a], while Me3NAAl(H2)PMes2 shows a slightly elongated AlAP bond distance [240.9(3) pm]. The AlAAs bond distances [247.2(2) 59, 247.3(1) 60] are comparable to those observed for Tmp2AlAsPh2 [248.5(2) pm)]. Me3NAAl(H2)As(Tms)2 [243.8 (2) pm], which is sterically less hindered and more Lewis-acidic due to the weaker +I effect of an H- compared to a Me-substituent, and [Me3NA(H)AlAsSi(iPr)3]2 [108] show slightly elongated AlAAs distances. The shortest AlAAs distances were found in the borazine analogue [Mes*AlAsPh]3áEt2O [109] [241.8(3) ± 243.5(3) pm], containing only tricoordinated Al and As centers. The AlASb [269.1(1) 61, 268.0(1) pm 62] and AlABi bond lengths [275.5(2) 63, 275.0(2) pm 64] are the shortest AlAE bond distances reported to date. Due to the lack of any structurally characterized monomeric aluminum stibides and bismuthides, they can only compared to AlAE heterocycles (R2AlER¢2)x. These display AlAE distances ranging from 273 to 278 pm (AlASb) and 275 to 279 pm {[Me2AlBi(Tms)2]3 43}, respectively. The sum of the AlAE covalent radii are only slightly shorter [235 (AlP); 246 (AlAs), 266 (AlSb), 275 pm (AlABi)]. Interestingly, the degree of the AlAE bond length shortening between Lewis base-stabilized compounds and their corresponding heterocycles strongly depends on the group 15 element. AlAP and AlAAs bond distances of the base-stabilized monomers are shortened by 8 pm (AlAP) and 6 pm (AlAAs), while this effect is signi®cantly reduced for the monomeric stibides (3±5 pm) and bismuthide (2 pm), respectively. This is likely due to reduced intramolecular repulsive interactions between the ligands, which become less important for heterocycles containing bigger group 15 elements. Analogously substituted alanes dmapAAl(Me2)E(Tms)2 (E = P 58, As 59, Sb 61 and Bi 63) also show a remarkable difference in respect of the degree of pyramidalization of the group 15 element, which steadily decreases from the phosphide 58 (309.1°) toward the higher homologues (As: 304.1° 59, Sb: 302.4° 61, Bi: 296.8° 63). Comparable trends are found for the Et-substituted Lewis base-stabilized monomers dmapAAl(Et2)E(Tms)2 (E = As: 306.6° 60, Sb: 298.9° 62, Bi: 293.4° 64), as can clearly be seen in Fig. 13. These ®ndings agree with the trends observed previously for group 15 triorganyls such as EH3, EPh3 and EMe3 (see Table 4, Sect. 2.1.). The decreasing bond angular sums result from decreased steric interactions between the Tms

156

S. Schulz

Fig. 13. Degree of pyramidalization of monomeric base-stabilized alanes

groups and the alane fragment and from the increased s-character of the lone pair and the increased p-character of the bonding electron pairs due to relativistic effects and the lanthanoid-contraction (inert-pair effect) [21]. GaAE monomers. The central structural parameters of Ga-containing monomers dmapAGa(R2)E(Tms)2 (E = P 65, As 66, Sb 67), which are summarized in Table 16, are comparable to those of analogously substituted AlAE monomers.

Structure 17. Ortep diagram (50% thermal ellipsoids) showing the solid state structure for 67

157

Synthesis, Structure and Reactivity of Group 13/15 Compounds

Table 16. Selected MAE bond distances (pm) and bond angles (°) of base-stabilized

monomeric group 15-gallanes dmapAGa(R2)E(Tms)2

Compound

MAE

MAN

ave. MAR

S(XAEAX)

dmapAGa(Me2)P(Tms)2 65 [104] dmapAGa(Me2)As(Tms)2 66 [104] dmapAGa(Et2)Sb(Tms)2 67 [103b]

237.2 (1) 245.5 (1) 264.8 (1)

208.0 (2) 208.2 (2) 206.6 (2)

198.5 198.2 199.4

305.3 300.2 298.0

Their structural parameters are very similar to those of the Al-derivatives, but show one major difference. According to the reduced Lewis acidity of the R2Gacompared to the R2Al-fragment, the GaAN distances [206.6(2) in 67 to 208.2(2) pm in 66] are signi®cantly elongated. Almost the same GaAN distances were found for 2-(methylamino)pyridineA(Me2)GaCl [110] [206.6(3) pm], Me3NAGaH3 [111] [(208.1(4) pm], 4-(methyl)pyridineAGa(Mes)2SeMes [112] [209.5(3) pm], and for quinuclidine-stabilized amido- and azidogallanes [113] (206±210 pm). The GaAE bond distances [237.2(1) 65, 245.5(1) 66, 264.8(1) 67 pm], belong to the shortest GaAE bond lengths ever observed. As was observed for the corresponding MAE heterocycles (M = Al, Ga), they are slightly shorter than AlAE bond distances of analogously substituted Lewis base-stabilized AlAE monomers. The CAGaAC and SiAEASi bond angles as well as the GaAC and EASi bond lengths of 65±67 are almost the same. 3.3.3 Transition Metal Complexes of Lewis Base-Stabilized Monomers Lewis base-stabilized compounds dmapAM(R2)ER¢2 are attractive reactants for further reactivity studies. They contain a lone pair, which is potentially active for further complexation reactions. In particular, reactions with transition metal complexes seem to be very interesting. In contrast to the steadily growing number of complexes containing a direct bond between a transition metal and a group 13 metal [114], complexes containing a group 13 metal fragment (RxM) and a transition metal fragment (M¢Ln) bridged by a group 15 element are almost unknown [115]. Me3NAAl(CH2Tms)2PPh2ACr(CO)5 68, obtained by a ring cleavage reaction between [(TmsH2C)2AlPPh2]2 and (Me3N)Cr(CO)5 [116], was the only structurally characterized example of this class of compounds containing a MAEAM¢ backbone chain [117]. However, this reaction pathway only succeeded in this particular case, but it is not generally applicable for the synthesis of the desired class of compounds. Beachley suggested the reactivity of the group 13/15 heterocycle toward (Me3N)Cr(CO)5 to depend on both the degree of association and the Lewis acidity of the heterocycle in solution [118]. Lewis base-stabilized compounds have the major advantage to be monomeric in solution, containing an active lone pair as was shown in complexation reactions with different transition metal carbonyls such as Fe2(CO)9, (Me3N)Cr(CO)5 and Ni(CO)4. Complexes of the type dmapAM(Me2)E(Tms)2AM¢(CO)n (M = Al, Ga; E = P, As, Sb; M¢ = Ni 69±74, Cr 75, 76, Fe 77) were synthesized (Scheme 16) and studied by single crystal X-ray diffraction [104, 119].

158

S. Schulz

Scheme 16. Synthesis of bimetallic complexes of the type dmapAM(R2)E(Tms2)AM¢(CO)n

The formation of the complexes 69±77 is clearly revealed by the presence of the carbonyl resonances in the 13C-NMR spectra and the carbonyl stretching vibrations in the infrared spectra. Comparative studies between the Ni-complexes 69±71 reveal the in¯uence of the group 15 element on the properties of the complex. The increasing down®eld shift of the carbonyl resonances and the decreasing wave numbers of the A1 vibration in respect to pure Ni(CO)4, point to a decrease of the CBO and an increase of the NiAC bond order. According to the synergistic r-donor/p-acceptor bonding concept, these ®ndings agree to a slight increase in r-donor/p-acceptor ratio with increasing atomic number of the group 15 element. Comparable trends were observed by Bodner et al. for more than 100 transition metal complexes (R¢3E)M¢Ln (E = group 15 element) [120]. Single crystal X-ray diffraction studies con®rmed the results as obtained from the NMR and IR studies, showing an increase of the NiAC bond order [NiAC: 179.6 69 [121], 179.1 70, 177.9 pm 71] and a decrease of the CAO bond order (CAO: 113.9 69, 113.9 70, 114.6 pm 71) in respect to Ni(CO)4 (Ni-C: 181.6, CAO: 112.7 pm) (Table 17) [122].

Structure 18. Ortep diagram (50% thermal ellipsoids) showing the solid state structure for 71

159

Synthesis, Structure and Reactivity of Group 13/15 Compounds

Table 17. Bond distances [pm], 13C-NMR chemical shifts [ppm] and A1 vibration numbers [cm)1] of pure Ni(CO)4 and selected complexes

Compound

NiAC

CAO

d13C (CO)

IR m (CO, A1)

Ni(CO)4 dmapAAl(Me2)P(Tms)2ANi(CO)3 69 dmapAAl(Me2)As(Tms)2ANi(CO)3 70 dmapAAl(Me2)Sb(Tms)2ANi(CO)3 71

181.6 179.6 179.1 177.9

112.7 113.9 113.9 114.6

191.9 199.5 199.8 200.9

2057 2048 2046 2042

Structure 19. Ortep diagram (50% thermal ellipsoids) showing the solid state structure for 75

The structures of 69±77 are very similar. The ligands bound to the group 13 and group 15 elements adopt a staggered conformation in relation to one another. The MAC, MAE, MAN and EASi bond lengths are comparable to those found in the starting monomeric compounds dmapAM(Me2)E(Tms)2. The coordination of a transition metal fragment seems to have almost no in¯uence on these particular structural parameters, as can be seen from Table 18. Interestingly, the AlAP distance in 75 [242.8(1) pm] is signi®cantly shorter than those of Beachley¢s complex 68 [248.5(1) pm]. The SiAEASi and AlAEASi bond angular sum of the bimetallic complexes are signi®cantly increased compared to the Lewis base-stabilized monomers. The SiAEAX (X = Si, Al) bond angular sums of the AlAEANi complexes 69±71 are widened by about 17° (309.1 58 to 326° 69), 13° (304.1° 59 to 317.7° 70) and 12° (302.4° 61 to 314.3° 71), respectively. The corresponding bond angles of analogous Ga-containing complexes 72 and 73 show the same structural trend. The larger steric demand of a lone pair in dmapAM(Me2)E(Tms)2 (M = Al, Ga) compared to an E-M¢ bonding electron pair, which was expected from the VSEPR concept, is clearly revealed. It also indicates a partial rehybridization of the EASi bonding electron pairs, becoming more s-character, and the former lone pair, getting more p-character. The EANi bond lengths [PANi: 231.9(2) pm 69; AsANi: 241.9(1) 241.9(1) 70, 241.9(1) pm 72; SbANi:

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Table 18. Selected MAE bond distances (pm) and bond angles (°) of base-stabilized

monomers dmapAM(R2)E(Tms)2 and their bimetallic complexes dmapAM(R2)E(Tms)2AM¢(CO)n Compound

MAE

Me3NAAl(CH2Tms)2PPh2ACr(CO)5 68 [116]248.5 dmapAAl(Me)2P(Tms)2ACr(CO)5 75 [104] 242.8 dmapAAl(Me)2P(Tms)2AFe(CO)4 77 [104] 243.2 dmapAAl(Me)2P(Tms)2ANi(CO)3 69 [104] 240.0 237.9 dmapAAl(Me)2P(Tms)2 58 [103b] dmapAAl(Me)2As(Tms)2ANi(CO)3 70 [119] 247.9 dmapAAl(Me)2As(Tms)2ACr(CO)5 76 [104] 251.2 247.2 dmapAAl(Me)2As(Tms)2 59 [103b] dmapAAl(Me)2Sb(Tms)2ANi(CO)3 71 [119] 268.0 269.1 dmapAAl(Me)2Sb(Tms)2 61 [103a] dmapAGa(Me)2As(Tms)2ANi(CO)3 72 [104]246.5 245.5 dmapAGa(Me)2As(Tms)2 66 [104] dmapAGa(Me)2Sb(Tms)2ANi(CO)3 73 [104]264.7 264.8 dmapAGa(Et2)Sb(Tms)2 67 [103b] a

EAM¢ (1) (1) (1) (2) (1) (1) (1) (2) (2) (1) (1) (1) (1) (1)

248.2 252.8 237.7 231.9 ± 241.9 260.0 ± 255.6 ± 241.9 ± 255.4 ±

S(SiAEAX)a

MAN (1) (1) (1) (2) (1) (1) (1) (1) (1)

204.9 196.3 196.1 196.1 198.4 196.6 195.5 198.9 196.5 197.8 204.5 208.2 204.6 206.6

(3) (2) (2) (5) (2) (2) (2) (4) (4) (2) (2) (2) (2) (2)

308.3 313.5 318.9 326.0 309.1 317.7 313.0 304.1 314.3 302.4 316.3 300.2 312.8 298.0

S(SiAEAX) = Si1AEASi2 + Si1,2AEAM (degree of pyramidalization).

255.6(1) 71, 255.4(1) pm 73] increase according to the increase of the atomic radii of the group 15 elements. The SiAEAX bond angular sum is strongly affected by the speci®c transition metal carbonyl complex, as can be seen by comparison of the Ni(CO)3- 69, Fe(CO)4- 77 and Cr(CO)5-complexes 75 of dmapAAl(Me)2P(Tms)2 58. According to the number of CO substituents and the geometry of the transition metal carbonyl, the bond angular sum around P steadily decreases from 326.0° 69 (tetrahedral environment around Ni) and 318.9° 77 to 313.5° 75 (octahedral coordination around Cr). In contrast, the arsine complexes 70 and 76 show a much smaller decrease (317.7° 70 to 313.0° 76). This is likely due to the increased atomic radius of As compared to P, reducing steric interactions between the Cr(CO)5- and the R03 As-fragments. Further experiments have to give a more detailed insight into the bonding situation of such complexes.

4 Conclusions Lewis acid-base adducts R3MAER¢3 (M = Al, Ga, In; E = Sb, Bi) have been synthesized and structurally characterized. Comparative NMR studies and computational calculations for Et3AlAE(Tms)3 and t-Bu3AlAE(i-Pr)3 clearly demonstrated their weak (attractive) acid-base interactions. According to these studies, their dissociation energies range from 2±8 kcal mol)1. The AlAC bond lengths and CAAlAC bond angles as obtained from single crystal X-ray diffraction studies qualitatively re¯ect the relative stability of analogously substituted adducts. In contrast, the central MAE bond length is mainly a function of steric repulsion between the ligands, giving no reliable information about the thermodynamic stability of the particular adduct.

Synthesis, Structure and Reactivity of Group 13/15 Compounds

161

Reactions of tetraalkyldistibines and -dibismuthines with trialkylalanes and gallanes yield bisadducts of the type [R3M]2[E2R¢4]. These compounds are stable in the solid state, while they easily undergo consecutive reactions in solution. Completely alkyl-substituted heterocycles [R2MER¢2]x are formed under cleavage of the SbASb bond, as was demonstrated for two GaASb heterocycles. Additional synthetic pathways for the synthesis of AlA, GaA and InASb heterocycles are dehydrosilylation reactions between dialkylaluminum hydrides and silyl-substituted stibines R02 SbTms (R¢ = alkyl, Tms) as well as dehalosilylation reactions between dialkylgallium- and -indium chlorides and R02 SbTms. The synthesis of [Me2AlBi(Tms)2]3 clearly demonstrates the general potential of the dehydrosilylation reaction for the synthesis of group 13/15 compounds. The so-formed heterocycles usually adopt four- or six-membered ring structures. The preferred ring geometry is mainly in¯uenced by the central group 15 element as well as by the substituents. Small ring atoms such as N, P, As and sterically demanding substituents lead to the formation of four-membered heterocycles, while bigger group 15 elements and smaller ligands also allow the formation of six-membered rings, as was found in [Me2ME(Tms)2]3 (M = Al, Ga, In; E = Sb, Bi). The heterocycles can be cleaved by reaction with 4-(dimethylamino)pyridine, yielding Lewis base-stabilized monomeric compounds of the type dmapAM(R2)E(Tms)2 (M = Al, Ga; E = P, As, Sb, Bi). This general reaction now offers the possibility to synthesize electronically rather than kinetically stabilized monomeric group 13/15 compounds. These can be used for further complexation reactions with transition metal complexes, leading to bimetallic complexes of the type dmapAM(Me2)E(Tms)2AM¢(CO)n (M = Al, Ga; E = P, As, Sb; M¢ = Ni, Cr, Fe). Acknowledgements. I am very grateful to my co-workers Andreas Kuczkowski and Florian Thomas, who have spent tremendous time and enthusiasm in the preparation of the compounds reported herein. I am also very grateful to Dr. M. Nieger for the competent crystallographic analyses and Prof. Peter Schreiner, University of Athens, for detailed computational calculations concerning the R3MAER¢3 Lewis acid-base adducts. Generous ®nancial support by the DFG, the Fonds der Chemischen Industrie and the Bundesministerium fuÈr Bildung, Wissenschaft, Forschung und Technologie (BMBF) as well as by Prof. E. Niecke, UniversitaÈt Bonn, is also gratefully acknowledged.

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76. (a) Krannich LK, Watkins CL, Schauer SJ (1995) Organometallics 14: 3094; (b) Krannich LK, Watkins CL, Schauer SJ, Lake CH (1996) Organometallics 15: 3980 77. (a) Schulz S, Nieger M (1998) Organometallics 17: 3398; (b) Schulz S, Kuczkowski A, Nieger M (2000) Organometallics 19: 699 78. Schulz S, Nieger M (1999) Angew Chem 111: 1020; Angew Chem Int Ed Engl 38: 967 79. See, for example: Thomas F, Schulz S, Nieger M (2001) Eur J Inorg Chem 161 80. Breunig HJ, Stanciu M, RoÈsler R, Lork E (1998) Z Anorg Allg Chem 624: 1965 81. (a) Cowley AH, Jones RA, Nunn CM, Westmoreland DL (1990) Chem Mater 2: 221; (b)Park HS, Schulz S, Wessel H, Roesky HW (1999) Chem Vap Deposition 5: 179 82. NMR spectra of the bisadducts almost show identical chemical shifts compared to the pure starting compounds, indicating 32±35 to be extensively dissociated in solution 83. Charton M (1982) Top Current Chem 114: 57 84. Laske Cooke, JA, Purdy AP, Wells RL, White PS (1996) Organometallics 15: 84 85. Atwood JL, Stucky GD (1970) J Am Chem Soc 92: 285 86. Hey-Hawkins F, Lappert MF, Atwood JL, Bott SG (1991) J Chem Soc Dalton Trans 939 87. Thomas F, Schulz S, Nieger M (2002) Z Anorg Allg Chem 628: 235 88. (a) Byers JJ, Pennington WT, Robinson GH, Hrncir DC (1990) Polyhedron 9: 2205; (b) Anwander R, Runte O, Eppinger J, Gerstberger G, Herdtweck E, Spiegler M (1998) J Chem Soc Dalton Trans 847 89. Hill JB, Pennington WT, Robinson GH (1994) J Chem Cryst 24: 61 90. (a) Dillingham MDB, Burns JA, Byers-Hill J, Gripper KD, Pennington WT, Robinson GH (1994) Inorg Chim Acta 216: 267; (b) Wiedmann D, Hausen HD, Weidlein J (1995) Z Anorg Allg Chem 621: 1351 91. Schaller A, Hausen HD, Schwarz W, Heckmann G, Weidlein J (2000) Z Anorg Allg Chem 626: 1047 92. Aitchison KA, Backer-Dirks JDJ, Bradley DC, Faktor MM, Frigo DM, Hursthouse MB, Hussain B, Short RL (1989) J Organomet Chem 366: 11 93. Stuczynski SM, Opila RL, Marsh P, Brennan JG, Steigerwald ML (1991) Chem Mater 3: 379 94. Schulz S, Gudat D, unpublished results 95. von HaÈnisch C (2001) Z Anorg Allg Chem 627: 68 96. (a) Knabel K, Krossing I, NoÈth H, Schwenk-Kirchner H, Schmidt-Amelunxen M, Seifert T (1998) Eur J Inorg Chem 1095; (b) Leung WP, Chan CMY, Wu BM, Mak TCW (1996) Organometallics 15: 5179; (c) Prust J, MuÈllerP, Rennekamp C, Roesky HW, Uson I (1999) J Chem Soc Dalton Trans 2265; (d) Brothers PJ, Power PP (1996) Adv Organomet Chem 39: 1; (e) Wehmschulte RJ, Ruhlandt-Senge K, Power PP (1994) Inorg Chem 33: 3205; (f) Byrne EK, Parkanyi L, Theopold KH (1988) Science 241: 332 97. (a) Atwood JL, Koutsantonis GA, Lee FC, Raston CL (1994) J Chem Soc Chem Commun 91; (b) Gardiner MG, Koutsantonis GA, Lawrence SM, Lee F-C, Raston CL (1996) Chem Ber 129: 545; (c) Janik JF, Duesler EN, Paine RT (1997) J Organomet Chem 539: 19 98. Janik JF, Wells RL, White PS (1998) Inorg Chem 37: 3561 99. Atwood DA, Contreras L, Cowley AH, Jones RA, Mardones MA (1993) Organometallics 12: 17 100. Janik JF, Wells RL, Young Jr. VG, Rheingold AL, Guzei IA (1998) J Am Chem Soc 120: 532 101. Fryzuk MD, Giesbrecht GR, Olovsson G, Rettig SJ (1996) Organometallics 15: 4832 102. Andrews PC, Raston CL, Roberts BA (2000) J Chem Soc Chem Commun 1961 103. (a) Schulz S, Nieger M (2000) Organometallics 19: 2640; (b) Thomas F, Schulz S, Nieger M (2001) Eur J Inorg Chem 161 104. Thomas F, Schulz S, Nieger M (2002) Chem Eur J 8: 1915 105. Lesley GMJ, Woodward A, Taylor NJ, Marder TB (1998) Chem Mater 10: 1355 106. Engelhardt LM, Junk PC, Raston CL, Skelton BW, White AH (1996) J Chem Soc Dalton Trans 3297 107. Wehmschulte RJ, Ruhlandt-Senge K, Power PP (1994) Inorg Chem 33: 3205 108. Driess M, Kunz S, Merz K, Pritzkow H (1998) Chem Eur J 4: 1628

166

S. Schulz

109. Wehmschulte RJ, Power PP (1996) J Am Chem Soc 118: 791 110. Koide Y, Francis JA, Bott SG, Barron AR (1998) Polyhedron 17: 983 111. Brain PT, Brown HE, Downs AJ, Greene TM, Johnson E, Parsons S, Rankin DWH, Smart BA, Tang CY (1998) J Chem Soc Dalton Trans 3685 112. Rahbarnoohi H, Wells RL, Liable-Sands LM, Yap GPA, Rheingold AL (1997) Organometallics 16: 3959 113. Luo B, Young Jr. VC, Gladfelter WL (2000) Inorg Chem 39: 1705 114. For a recent review, see: Fischer RA, Weiû J (1999) Angew Chem 111:3002; Angew Chem Int Ed Engl 38: 2830 115. Only some Lewis acid-base adducts of group 13 trialkyls R3M or trihalides Cl3M and transition metal complexes of the type LnFeAE@CR2, (E = P, As): Weber L, Scheffer MH, Stammler HG, Stammler A (1999) Eur J Inorg Chem 1607; (LnWºP): Scheer M, MuÈller J, Baum G, HaÈser M (1998) J Chem Soc Chem Commun 1051, have been synthesized and structurally characterized 116. Tessier-Youngs C, Bueno C, Beachley Jr. OT, Churchill MR (1983) Inorg Chem 22: 1054 117. A comparable compound, (Me2N)2BPPh2-Cr(CO)5, was synthesized by NoÈth et al. but not structurally characterized by single crystal X-ray diffraction: NoÈth H, Sze SN (1978) Z Naturforsch B: Anorg Chem Org Chem 33: 1313 118. Beachley Jr. OT, Banks MA, Kopasz JP, Rogers RD (1996) Organometallics 15: 5170 119. Thomas F, Schulz S, Nieger M (2001) Organometallics 20: 2405 120. Bodner GM, May MP, McKinney LE (1980) Inorg Chem 19: 1951 121. Compound 69 contains 6 independent molecules in the unit cell and the NiAC distances vary between 177.5 and 180.5 pm. Therefore, this value has to be seen with care 122. Braga D, Grepioni F, Orpen G (1993) Organometallics 12: 1481

Author Index Volumes 101-103

Aldinger F, see Seifert HJ (2002) 101:1-58 Friihauf S, see Roewer G (2002) 101:59-136 Haubner R, WiLhelm M, Weissenbacher R, Lux B (2002) Boron Nitrides - Properties, Synthesis and Applications. 102:1-46 Herrmann M, see Petzow G (2002) 102:47-166 Herzog U, see Roewer G (2002) 101:59-136 H6pfl H (2002) Structure and Bonding in Boron Containing Macrocycles and Cages. 103:1-56 Jansen M, Jfischke B, J~schke T (2002) Amorphous MultinaryCeramics in the Si-B-N-C System. 101:137-192 Jfischke B, see Jansen M(2002) 101:137-192 J~schke T, see Jansen M(2002) 101:137-192 Lux B, see Haubner R (2002) I02:1-46 Mahalakshmi L, Stahlke D (2002) The R2M+Group 13 Organometallic Fragment Chelated by P-centered Ligands. 103:85-116 Miiller E, see Roewer G (2002) 101:59-136 Petzow G, Hermann M (2002) Silicon Nitride Ceramics. 102:47-166 Power P (2002) Multiple Bonding Between Heavier Group 13 Elements. 103:57-84 Roewer G, Herzog U, Trommer K, Mfiller E, Friihauf S (2002) Silicon Carbide - A Survey of Synthetic Approaches, Properties and Applications. 101:59-136 Schulz S (2002) Synthesis, Structure and Reactivity of Group 13/15 Compounds Containing the Heavier Elements of Group 15, Sb and Bi 103:117-166 Seifert HI, Aldinger F (2002) Phase Equilibria in the Si-B-C-N System. 101:1-58 Stahlke D, see Mahalakshmi L (2002) 103:85-116 Trommer K, see Roewer G (2002) 101:59-136 Weissenbacher R, see Haubner R (2002) I02:1-46 Wilhelm M, see Haubner R (2002) I02:1-46

Subject Index

(Acylamino)boranes 15-17 (Acyloxy)boranes 13-15 Allosteric effect 43, 47 Aminoiminophosphorane, ancillary ligands 107 -, reduction to phosphanamines 94 3-(Aminophenyl)boronic acid 7-8 Base-stabilizedmonomer 119,121 - -,A1-E monomer 153, 155 - -, Ga-E monomer 156 Bimetallic complex 159 Bis(dioxime)metal complexes, conformation 34-36 -, definition 33 -, derivatives with boron 33-39 Bismuthides 139 Bismuthine adduct 125,130 Bond order, in group 13 molecules 66, 68, 70, 73, 74, 77, 80 Borasiloxanes, cage-like structures 26-27 -, cyclic structures 25-26 -, preparative methods 24 Borates, cage-like structures 23-24, 31-33 -, dinuclear structures 17-22 Borazines 5-6 Boronates, dinuclear structures 17-22, 43-47 -,mononuclear structures 18,41-43 -, tetranuclear structures 22 Borophosphonates 27-29 Bridging, alkali metals 59, 67, 71 -

-

Calixarenes, conformation 4 -, definition 4 -, derivatives with boron 6-7 -, heteroaromatic calixarenes 4-5, 8-9 Catalysis, ancillary ligands 107 -,bite angle 90,101-102 -, deactivation 106

-, metallocenes 88 -, methyl alumoxane (MAO) 87 -, Ziegler-Natta 87 Chiral recognition 23, 43-44, 46 Clathrochelates 39-41 Cobaloximes 35-37 Coulombic repulsion, group 13 element compounds 67, 69, 75 Crownethers, derivatives with boron 12-17, 31-33, 41-43, 46-47 Cyclophanes, derivatives with boron 17-23 -, general information 17 Dehalosilylation 120, 139 Dehydrosilylation 140, 141 Dibismuthine bisadduct 136, 138, 142 3- (Diethylboryl)pyridines 6-7 Digallene 81 Dissociation enthalpy 123, 126, 133 Distibine bisadduct 136, 137,142 Distibine cleavage reaction 142 EPR spectroscopy Gallyne

63, 66

75

Haaland model 130-132 Heterocycles 120, 139 -,AI-Bi 141,143,145 -, AI-Sb 141, 143 -, four-membered 119, 140, 145,147, 150 -,Ga-Sb 140,146 -, In-Sb 140, 148 -, six-membered 119, 140, 144-150 Imidazolylboranes 9-11 Iminophosphorane, donor-acceptor adducts 103

170

Subject Index

Iminophosphorane

- , o r t h o - d e p r o t o n a t e d triphenyl iminophos-

phorane

102

- , o r t h o - l i t h i a t e d triphenyl iminophos-

phorane 103 -, reduction to phosphanamines 94 Inclusion compounds, with alcohols 41-42 -, with amines 41-44 -, with catecholamines 42-43 - -, with metal ions 30-42 Intermolecular complexes, group 13 molecules 62 -

-

Janus head ligand, pyridylphosphides 93-102 , containing phosphorus/nitrogen Lewis acid-base adduct Lewis acidity 124, 129 Lewis basicity 124

90

118,121,144

Metallomacrocycles, dinuclear 15,17, 20, 22 -, hexanuclear 15 -,trinuclear 15, 17,23-24 MOCVD process 119, 142 Molecular recognition, by boronic acid derivatives 41-43 - -, by diboronic acids 43-47 Mono-/bimetallic complexes 135 M o n o m e r i c c o m p o u n d 151,153 -, electronic stabilization 151 -, kinetic stabilization t51 Multiple bonding, in group 13 elements 59, 63, 64, 69, 70, 75 -

-

Oligomeric compounds

119,151

Phosphane, amino and iminophosphanes 102-106 -, bite 101 -, catalysis 87 -, chiral phosphanamines 94 -, diazaphosphane 111 -, diphosphane 92 -, donor molecules 90, 91 -, NMR data ofpyridylphosphanes 99 -, phosphanamine 105 -, phosphanimide 108 -, pyridylphosphane 93, 99 -, tetraphosphane 92

Phosphazane 106-112 -, acyclic phosphazanes 102,107 -, cyclotriphosphazanes 106 -, diphosphazanes 109 (Phosphinoyloxy)boranes 17 (Phosphorylamino)boranes 17 Porphyrinogens, definition 8 -, derivatives with boron 9-11 Porphyrins, definition 8 -, derivatives with boron 11-12 Pyridylphosphane 93-102 -, coordination 93 -, reactivity 93 Pyridylphosphide 93-102 -, coordination 96 -, reactivity 94 Radicals 59, 63, 65, 66 Recognition, chiral 23, 43-44, 46 -, selective 31-33,41-43,45-47 Reduction, double 65, 68 -, group 13 molecules 58,64 Ring size 149 - -, entropy effect 150 - -, ring strain effect 150 Saccharide sensing 44-47 Salt elimination 120,139 (Seleninoyloxy)boranes 17 Semiconducting materials 119 , -donor/-acceptor concept 158 Single source precursor 120, 142 - - - , a d d u c t 120 - - -, heterocycle 120 Solid state structures 127-129, 137, 144-149, 154-159 Steric interaction 129 Stibides 139 Stibine adduct 124,129 (Sulfonyloxy)boranes 17 Supramolecular chemistry, cage-like assemblies 2-3 -, kinetic stability 3 - -, macrocyclic assemblies 2-3 -, synthetic strategies 2 - -, thermodynamic stability 3 -

-

Thermodynamic stability 123, 133 -, computational calculation 122,124,132 - -, electronic effect 132 - - , N M R s t u d y 125, 126 ,steric effect 132

SubjectIndex Transition metal bisadduct 138 Transition metal complexes 121,157,158 Transport through cell membranes 43, 46 Triazolylboranes 8-9 Triple bonding, group 13 element compounds 75, 76

171 Triplet state, group 13 element dimers 60 Tris(dioxime)metal complexes 39-40 Vanadium borates 29-31 Vanadium borophosphates 29-31