Carbon Dioxide as Chemical Feedstock

Edited by Michele Aresta

Carbon Dioxide as Chemical Feedstock

Edited by Michele Aresta Carbon Dioxide as Chemical Feedstock

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Edited by Michele Aresta

Carbon Dioxide as Chemical Feedstock

The Editor Prof. Dr. Michele Aresta University of Bari Department of Chemistry and CIRCC Via Celso Ulpiani 27 70126 Bari Italy

All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate. Library of Congress Card No.: applied for British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at . © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Composition Toppan Best-set Premedia Limited, Hong Kong Printing and Bookbinding betz-druck GmbH, Darmstadt Cover Design Schulz Grafik-Design, Fußgönheim Printed in the Federal Republic of Germany Printed on acid-free paper ISBN: 978-3-527-32475-0

V

To Federica, Mattia and Nicolò

Carbon Dioxide as Chemical Feedstock. Edited by Michele Aresta Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32475-0

VII

Contents Preface XV List of Contributors 1

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

2

2.1 2.2 2.3 2.4 2.5 2.5.1 2.5.2 2.6

XVII

Carbon Dioxide: Utilization Options to Reduce its Accumulation in the Atmosphere 1 Michele Aresta Carbon Dioxide Emission 1 The Accumulation of CO2 in the Atmosphere, and the Effects that We Fear 1 Technologies to Reduce CO2 Accumulation in the Atmosphere 4 The Utilization of CO2 6 Conditions for Using CO2 8 CO2: Sources and Prices 8 The Potential for CO2 Utilization, and the Content of This Book 9 The Need for Research to Speed an Exploitation of the Utilization Option 11 References 13 Utilization of Dense Carbon Dioxide as an Inert Solvent for Chemical Syntheses 15 Alessandro Galia and Giuseppe Filardo Introduction 15 Dense Carbon Dioxide as Solvent Medium for Chemical Processes 15 Enzymatic Catalysis in Dense Carbon Dioxide 18 Other Reactions in Dense Carbon Dioxide 19 Polymer Synthesis in Supercritical Carbon Dioxide 20 Chain Polymerizations: Synthesis of Fluoropolymers 22 Step Polymerizations: Synthesis of Biodegradable Polymers Conclusions 27 Acknowledgments 27 References 28

Carbon Dioxide as Chemical Feedstock. Edited by Michele Aresta Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32475-0

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Contents

3

3.1 3.2 3.2.1 3.2.2 3.2.3 3.2.4 3.2.5 3.2.6 3.3 3.4 3.5

4

4.1 4.2 4.3 4.3.1 4.3.2 4.3.2.1 4.3.2.2 4.3.2.3 4.3.2.4 4.4 4.4.1 4.4.2 4.4.3 4.5 4.5.1 4.5.2 4.5.3 4.5.4 4.6

Autotrophic Carbon Fixation in Biology: Pathways, Rules, and Speculations 33 Ivan A. Berg, Daniel Kockelkorn, W. Hugo Ramos-Vera, Rafael Say, Jan Zarzycki, and Georg Fuchs Introduction 33 The Mechanisms of CO2 Fixation 34 The Calvin–Benson–Bassham (CBB) Cycle 34 The Reductive Citric Acid Cycle (Arnon–Buchanan Cycle) 37 The Reductive Acetyl-CoA Pathway (Wood–Ljungdahl Pathway) 39 The 3-Hydroxypropionate/Malyl-CoA Cycle 40 The 3-Hydroxypropionate/4-Hydroxybutyrate Cycle 42 The Dicarboxylate/4-Hydroxybutyrate Cycle 44 Rules to Explain the Diversity 46 Evolutionary Aspects 49 Chemical Aspects of CO2 Fixation 50 Acknowledgments 51 References 51 Carbon Dioxide Coordination Chemistry and Reactivity of Coordinated CO2 55 Joëlle Mascetti Introduction 55 Carbon Dioxide Bonding to Metals 56 Synthesis and Structure of CO2 Complexes 59 Low-Temperature Matrix Isolation and Theoretical Studies 59 Synthesis of Stable Complexes 64 End-On Complexes 65 Side-On Complexes 67 Bridged Complexes 67 Bridged Complexes Obtained by In-situ Synthesis 67 Reactivity of CO2 Complexes 69 C–O Bond Cleavage and O Transfer 70 Reactions with Electrophiles 72 Reactions with Nucleophiles 73 CO2 Complexes as Reaction Intermediates in CO2 Utilization Processes 75 Oxidative Coupling Reactions 76 Reduction Reactions 79 Catalytic Processes 81 Bioinspired Reactions 82 Conclusions 84 Acknowledgments 85 References 85

Contents

5

5.1 5.2 5.2.1 5.2.2 5.2.3 5.2.4 5.3 5.3.1 5.3.2 5.3.2.1 5.3.2.2 5.3.2.3 5.4

6

6.1 6.2 6.3 6.3.1 6.3.2 6.3.3 6.3.4 6.3.5 6.3.6 6.4 6.5 6.6

7 7.1 7.2

Main Group Element- and Transition Metal-Promoted Carboxylation of Organic Substrates (Alkanes, Alkenes, Alkynes, Aromatics, and Others) 89 Thomas Zevaco and Eckhard Dinjus Introduction 89 Formation of Aromatic Carboxylic Acids: The Kolbe–Schmitt Synthesis 90 Kolbe–Schmitt Synthesis: Generalities 90 Reaction Parameters and Mechanistic Studies of the Kolbe–Schmitt Synthesis 91 Recent Applications of the Kolbe–Schmitt Carboxylation: Synthesis of 1,3-Dialkylimidazolium-2-Carboxylates 97 Carboxylation of C–H-Acidic Compounds 99 Reactive Organometallic Derivatives in the Synthesis of Carboxylic Acids 102 Generalities 102 Bimetallic Catalytic Systems 104 Pd/Sn Systems 104 Rh/B and Cu/B 107 Ni/Zn 108 Palladium (0)-Catalyzed Telomerization of Butadiene with CO2: Synthesis of δ-Lactone 112 References 116 The Chemistry of N–CO2 Bonds: Synthesis of Carbamic Acids and Their Derivatives, Isocyanates, and Ureas 121 Eugenio Quaranta and Michele Aresta Introduction 121 Synthesis of Carbamic Acids and Alkylammonium Carbamates 122 Synthesis of Carbamate Esters 125 Transfer of Carbamate Group to Alkyl Halides 126 Transfer of Carbamate Group to Acylating Agents 131 Transfer of Carbamate Group to Alcohols 134 Transfer of Carbamate Group to Epoxides 138 Transfer of Carbamate Group to C–C Double Bonds 142 Transfer of Carbamate Group to C–C Triple Bonds 145 Synthesis of Isocyanates 148 Synthesis of Ureas 154 Conclusions 159 References 159 Synthesis of Linear and Cyclic Carbonates 169 Danielle Ballivet-Tkatchenko and Angela Dibenedetto Introduction 169 Acyclic Organic Carbonates 170

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X

Contents

7.2.1 7.2.2 7.2.3 7.2.4 7.2.5 7.3 7.3.1 7.3.1.1 7.3.1.2 7.3.1.3 7.3.1.4 7.3.2 7.3.2.1 7.3.2.2 7.3.3 7.3.3.1 7.3.3.2 7.3.3.3 7.3.4 7.3.5 7.3.6 7.3.7 7.3.8 7.4 7.4.1 7.4.2

8

8.1 8.2 8.3 8.4 8.4.1 8.4.2 8.5 8.5.1 8.5.2 8.5.3 8.6

Market and Production 170 Current Trends with CO2 as Feedstock 172 Alcoholysis of Urea 174 Direct Route 175 The Future of CO2-Based Routes to Acyclic Carbonates 180 Synthesis of Organic Cyclic Carbonates 181 Carboxylation of Epoxides 182 Use of Conventional Solvents 182 Use of Ionic Liquids 184 Use of Supercritical CO2 185 Combined Reaction Media: sc-CO2 and Ionic Liquids 185 Oxidative Carboxylation of Olefins 186 Use of Oxygen as Oxidant 186 Use of Other Oxidants 188 Other Synthetic Routes to Cyclic Carbonates 189 From Halohydrins 189 From Halogenated Carbonates 190 Reaction of Cyclic Ketals with Carbon Dioxide 191 Synthesis of Cyclic Carbonates from Propargylic Alcohols 191 Reaction Between Carbon Dioxide and Diols 192 Reaction of Urea and Diols 194 Reaction of Carbon Dioxide or Urea with Glycerol 195 Reactivity of Cyclic Alkylene Carbonates 198 Transesterification Reactions 200 Synthesis of Acyclic Carbonates 200 Synthesis of Cyclic Carbonates 204 References 205 Polymers from Carbon Dioxide: Polycarbonates, Polythiocarbonates, and Polyurethanes 213 Donald J. Darensbourg, Jeremy R. Andreatta, and Adriana I. Moncada Introduction 213 Historical Perspective 215 Metal Catalysts for the Copolymerization of Epoxides and CO2 215 Metal Catalysts for the Copolymerization of Oxetanes and CO2 228 Early Studies 228 Recent Studies using Transition Metal Catalysts 229 Physical Methods for the Characterization of Copolymers Produced from Epoxides or Oxetane and Carbon Dioxide 235 Cyclohexene Oxide Monomer 235 Propylene Oxide Monomer 237 Oxetane Monomer 239 Copolymer Isolation and Catalyst Recycling 241

Contents

8.7 8.8 8.9

9

9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9

10

10.1 10.2 10.2.1 10.2.2 10.2.3 10.3 10.3.1 10.3.2 10.3.3 10.3.4 10.3.5 10.4 10.4.1 10.4.2

Copolymerization of Carbon Disulfide and Epoxides and Episulfides 244 Copolymers from Aziridines and Carbon Dioxide 245 Concluding Remarks 245 Acknowledgments 246 References 246 In-Situ Study of Carbon Deposition during CO2 Reforming of Methane for Synthesis Gas Production, Using the Tapered Element Oscillation Microbalance 249 Wie Pan and Chunshan Song Introduction 249 Thermodynamic Analysis of Carbon Formation from CH4 or CO 252 Thermodynamic Analysis of Carbon Formation in CO2 Reforming of Methane 254 TEOM Measurement of Carbon Formation in CO2 Reforming of Methane 256 TPO Analysis of Carbon Formation in CO2 Reforming 257 TEM Analysis on Carbon Formed on Catalysts After CO2 Reforming of Methane 259 Kinetic Study of Carbon Formation on PCH4 and PCO in CO2 Reforming 260 H2O Effect on Carbon Formation in CO2 Reforming 262 Conclusions 263 Acknowledgments 263 References 263 Appendix A9.1 264 Utilization of Carbon Dioxide through Nonthermal Plasma Approaches 267 Ji-Jun Zou and Chang-Jun Liu Introduction 267 Nonthermal Plasma Phenomena 268 Electron/Molecular Reactions 270 Atom/Ion/Molecule Reactions 270 Heterogeneous Reactions 271 CO and/or H2 Production from CO2 272 CO2 Dissociation 272 Reforming of CH4 with CO2 273 Reforming of Aliphatic Hydrocarbons with CO2 275 Other Reforming Reactions with CO2 276 Reduction of CO2 276 Hydrocarbons Synthesis from CO2 277 Oxidative Coupling of CH4 with CO2 277 Hydrogenation of CO2/CO to C2 Hydrocarbons 278

XI

XII

Contents

10.4.3 10.5 10.5.1 10.5.2 10.5.3 10.5.4 10.5.5 10.6 10.6.1 10.6.2 10.7

Higher Hydrocarbons from CH4 and CO2 279 Oxygenates Synthesis from CO2 280 Methanol from CO2 Hydrogenation 280 Methanol from CO2 and CH4 281 Aldehydes from CH4 and CO2 282 Acetic Acid from CH4 and CO2 282 Oxygenates from H2O and CO2 283 Combination of Plasma with Catalyst 284 Catalysts in Plasma Utilization of CO2 284 Interaction Between Plasma and Catalyst 285 Summary 286 Acknowledgments 287 References 287

11

Photochemical, Electrochemical, and Photoelectrochemical Reduction of Carbon Dioxide 291 Emily Barton Cole and Andrew B. Bocarsly Introduction 291 Homogeneous Photochemical Reduction 292 General Considerations 292 Transition Metal Complexes 294 Ruthenium Complexes 294 Rhenium Complexes 295 Macrocyclic Complexes 296 Electrochemical Reduction 296 Reduction in Aqueous Solutions at Metal Electrodes 297 Reduction to CO and HCOOH 298 Reduction to CH3OH and Alcohols 299 Reduction to CH4 and Hydrocarbons 300 Reduction in Nonaqueous Solutions at Metal Electrodes 302 Reduction Mediated by Metal Complexes 303 Transition Metal Complexes 303 Macrocyclic Complexes 304 Metal-Containing Enzyme-Mediating Complexes 305 Semiconductor Systems for Reduction 305 Photoelectrochemical Semiconductor Electrode Systems 305 Unmodified Semiconductor Electrode Interfaces 306 Modified Semiconductor Electrodes 307 Homogenous Solution Catalysts at Semiconductor Electrodes 308 Heterogeneous Photochemical Semiconductor Systems 309 Unmodified Semiconductor Colloids and Powders 309 Metal-Coated Semiconductor Colloids and Powders 310 Concluding Remarks and Future Directions 311 References 312

11.1 11.2 11.2.1 11.2.2 11.2.2.1 11.2.2.2 11.2.3 11.3 11.3.1 11.3.1.1 11.3.1.2 11.3.1.3 11.3.2 11.3.3 11.3.3.1 11.3.3.2 11.3.3.3 11.4 11.4.1 11.4.1.1 11.4.1.2 11.4.1.3 11.4.2 11.4.2.1 11.4.2.2 11.5

Contents

12

12.1 12.2 12.2.1 12.2.2 12.2.3 12.3

13

13.1 13.2 13.3 13.3.1 13.3.2 13.4 13.5 13.5.1 13.5.2 13.6 13.7

14

14.1 14.2 14.3 14.3.1 14.3.2 14.3.3 14.3.3.1 14.3.3.2 14.3.3.3 14.3.3.4 14.4 14.4.1 14.4.2

Recent Scientific and Technological Developments in Electrochemical Carboxylation Based on Carbon Dioxide 317 Giuseppe Silvestri and Onofrio Scialdone Introduction 317 Electrocarboxylation 318 Electrocarboxylation of Organic Halides 319 Electrocarboxylation of Aromatic Ketones 324 Electrocarboxylation of Other Substrates 326 The Electroreduction of Carbon Dioxide in Protic Media (Water and Alcohols) 327 Acknowledgments 330 References 330 Indirect Utilization of Carbon Dioxide: Utilization of Terrestrial and Aquatic Biomass 335 Michele Aresta and Angela Dibenedetto Introduction 335 The Natural Carbon Cycle 336 The Utilization of Terrestrial Biomass 337 Residual Biomass 338 Cultivated Biomass 339 The First-Generation Biofuels 339 The New Generations of Biofuels 339 Second-Generation Biofuels 340 Third-Generation Biofuels 341 Implementation of the Biorefinery Concept 347 Concluding Remarks 349 References 349 Fixation of Carbon Dioxide into Inorganic Carbonates: The Natural and Artificial “Weathering of Silicates” 353 Ron Zevenhoven and Johan Fagerlund Introduction: Inorganic Carbonate Uses and Natural Resources 353 Natural Fixation of CO2 in Carbonates 355 Process Routes to Valuable Carbonate Products 357 Material Resources 357 Direct (Single-Step) Process Routes 358 Indirect (Single-Step) Process Routes 359 General Aspects of Calcium Carbonate Production 359 Acetic Acid Route 361 Two-Step Aqueous Carbonation of Solid Residues 362 The pH-Swing Process 363 Mineral Carbonation for Carbon Capture and Storage (CCS) 364 Material Resources 366 Direct (Single-Step) Process Routes 367

XIII

XIV

Contents

14.4.2.1 14.4.2.2 14.4.3 14.4.3.1 14.4.3.2 14.5 14.5.1 14.5.2

Gas–Solid Processes 367 Aqueous Solution Processes 369 Indirect (Multistep) Process Routes 369 Gas–Solid Processes 369 Aqueous Solution Processes 372 Other Carbonate Production Processes and Applications Carbonation of Brines 374 Straightforward Carbonation 375 Acknowledgments 375 References 376 Index 381

374

XV

Preface

In Nature, the carbon cycle has the ability to recycle some 203 gigatons (Gt) of carbon dioxide each year (see Figure 13.1). Although anthropogenic (“man-made”) CO2 which, at 7 Gt per year, represents approximately 3.4% of the total CO2 converted in the natural cycle – an apparently small amount – the natural carbon cycle, despite having become extremely proficient over millions of years, is unable to recycle this excess CO2. As a result, CO2 becomes accumulated in the atmosphere, leading in turn to an expansion of the “greenhouse effect” with, potentially, a major impact on climate change. Hence, it is vital that strategies are developed to limit CO2 accumulation in the atmosphere. The use of CO2 – as either a technological fluid or a raw material in chemical processes and in biotechnological applications, such as enhanced biological fixation – provides the potential to reduce CO2 emissions. Indeed, CO2 utilization may become an efficient tool if it can be merged with the development of innovative, “sustainable” technologies with less-intensive energy and materials requirements than those currently on stream. For this purpose, CO2 may be used either as a technological fluid, as a building block for complex molecules, or as a carbon source for fuels. It is important to remember that, whichever compound is prepared from CO2, during the use of that compound the CO2 will be returned. The competition is, therefore, between the rate of formation of CO2 and the rate of its fixation. Most CO2 is produced in combustion processes which, in general, are very fast – much faster than any reaction that may convert CO2 into chemicals. Even natural processes that fix CO2 into biomasses function much more slowly than combustion. In fact, the rate of depletion of natural fossil sources is higher than the rate of formation of fossil carbon, and consequently any competition based on the rapidity of these opposing processes will be lost in favor of CO2 production. Thus, it is unrealistic to imagine that the conversion of CO2 might solve the problem of its accumulation in the atmosphere, as kinetic – and often also thermodynamic – factors are braced against such possibility. Hopefully, it might in time be possible to convert a limited fraction – perhaps up to 7% – of the anthropogenic CO2 back into useful chemicals or fuels. Although this will not solve the problem of CO2 accumulation in the atmosphere, when added to other CO2 utilization technologies it Carbon Dioxide as Chemical Feedstock. Edited by Michele Aresta Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32475-0

XVI

Preface

may provide an effective level. In fact, no technology is available today that is capable of performing much better under the same conditions of energy consumption and economics. The use of CO2 represents one option that might provide a serious contribution to reducing CO2 accumulation in the atmosphere, speaking not in terms of its storage but rather but in terms of its efficiency. An exception might be to use CO2 as a co-monomer so as to afford polycarbonates and polyurethanes that may persist for decades and, therefore, represent an optional means of CO2 storage. Carbon dioxide is considered to be an inert molecule since, with water, it is the end product of any combustion process, including biological cellular oxidation reactions. Although it is produced by all living organisms, whether animal or vegetable (for example, an adult man emits about 0.9 kg CO2 per day), by far the main source of CO2 is the combustion of fossil carbon (coal, oil, gas) used for the production of energy. This book contains highlights of the research carried out during the past 20 to 30 years or so, with emphasis placed not only on fundamental concepts but also on applications to synthetic chemistry and the development of new processes based on metal-catalyzed conversion reactions. The collection of review material gathered here provides a cross-disciplinary insight into a very wide-ranging field that bridges the chemical and biotechnological aspects of CO2 conversion with its technological use. The most recent discoveries and creative synthetic strategies are discussed in detail, while the barriers that must be overcome in order to fully exploit the utilization option are highlighted. This book represents an updated tool for those teachers, research workers and students who wish to become acquainted with the most advanced knowledge of CO2 chemistry, and strategies for its conversion and utilization. Consequently, these chapters offer a diversity of options in such a complex area, and illustrate the links between chemistry and biology, showing how hybrid technologies may contribute to identify operative and effective solutions to these problems. The challenges within the different areas are launched through open questions, and it is hoped that the readers may indeed use their own initiative to discover new and more efficient solutions. In this way, a practical application may be found that will transform a dream into reality. I wish to express my most sincere thanks to my colleague Professor Angela Dibenedetto for her precious collaboration, and her excellent help provided during the preparation of this book. University of Bari Department of Chemistry and Interuniversity Consortium on Chemical Reactivity and Catalysis-CIRCC Campus Universitario 70126 Bari, Italy June 2009

Michele Aresta

XVII

List of Contributors Jeremy R. Andreatta Texas A&M University Department of Chemistry College Station, TX 77843 USA

Andrew B. Bocarsly Princeton University Frick Laboratory Princeton, NJ 08544 USA

Michele Aresta University of Bari Department of Chemistry and CIRCC 70126 Bari Italy

Donald J. Darensbourg Texas A&M University Department of Chemistry College Station, TX 77843 USA

Danielle Ballivet-Tkatchenko Université de Bourgogne and CNRS, UMR 5260 Institut de Chimie Moléculaire 9, avenue A. Savary 21000 Dijon France

Angela Dibenedetto University of Bari Department of Chemistry and CIRCC Campus Universitario 70126 Bari Italy

Emily Barton Cole Princeton University Frick Laboratory Princeton, NJ 08544 USA Ivan A. Berg Universität Freiburg Fakultät Biologie Schänzlestr. 1 79104 Freiburg Germany

Eckhard Dinjus Forschungszentrum Karlsruhe GmbH Institute of Technical Chemistry Division of Chemical–Physical Processing Hermann-von-Helmholtz-Platz 1 76344 Eggenstein-Leopoldshafen Germany Johan Fagerlund Abo Akademi University 20500 Abo/Turku Finland

Carbon Dioxide as Chemical Feedstock. Edited by Michele Aresta Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32475-0

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List of Contributors

Giuseppe Filardo Università di Palermo Dipartimento di Ingegneria Chimica dei Processi e dei Materiali Viale delle Scienze Ed. 6 90128 Palermo Italy Georg Fuchs Universität Freiburg Fakultät Biologie Schänzlestr. 1 79104 Freiburg Germany Alessandro Galia Università di Palermo Dipartimento di Ingegneria Chimica dei Processi e dei Materiali Viale delle Scienze Ed. 6 90128 Palermo Italy Daniel Kockelkorn Universität Freiburg Fakultät Biologie Schänzlestr. 1 79104 Freiburg Germany Chang-Jun Liu Tianjin University Key Laboratory for Green Chemical Technology of Ministry of Education School of Chemical Engineering and Technology Tianjin, 300072 China Joëlle Mascetti Université Bordeaux 1 Institut des Sciences Moléculaires (UMR 5255 CNRS) 351 cours de la Libération 33405 Talence Cedex France

Adriana I. Moncada Texas A&M University Department of Chemistry College Station, TX 77843 USA Wie Pan Pennsylvania State University Clean Fuels and Catalysis Program EMS Energy Institute, and Department of Energy & Mineral Engineering 209 Academic Projects Building University Park, PA 16802 USA Eugenio Quaranta University of Bari Department of Chemistry and CIRCC 70126 Bari Italy W. Hugo Ramos-Vera Universität Freiburg Fakultät Biologie Schänzlestr. 1 79104 Freiburg Germany Rafael Say Universität Freiburg Fakultät Biologie Schänzlestr. 1 79104 Freiburg Germany Onofrio Scialdone University of Palermo Dipartimento di Ingegneria Chimica dei Processi e dei Materiali Viale delle Scienze 90128 Palermo Italy

List of Contributors

Giuseppe Silvestri University of Palermo Dipartimento di Ingegneria Chimica dei Processi e dei Materiali Viale delle Scienze 90128 Palermo Italy Chunshan Song Pennsylvania State University Clean Fuels and Catalysis Program EMS Energy Institute, and Department of Energy & Mineral Engineering 209 Academic Projects Building University Park, PA 16802 USA Jan Zarzycki Universität Freiburg Fakultät Biologie Schänzlestr. 1 79104 Freiburg Germany

Thomas Zevaco Forschungszentrum Karlsruhe GmbH Institute of Technical Chemistry Division of Chemical–Physical Processing Hermann-von-Helmholtz-Platz 1 76344 Eggenstein-Leopoldshafen Germany Ron Zevenhoven Abo Akademi University 20500 Abo/Turku Finland Ji-Jun Zou Tianjin University Key Laboratory for Green Chemical Technology of Ministry of Education School of Chemical Engineering and Technology Tianjin, 300072 China

XIX

1

1 Carbon Dioxide: Utilization Options to Reduce its Accumulation in the Atmosphere Michele Aresta

1.1 Carbon Dioxide Emission

Carbon dioxide (CO2) is considered to be the major cause of climate change, because of its greenhouse properties and continuous accumulation in the atmosphere. Indeed, the atmospheric concentration of CO2 has risen from 278 ppm during the preindustrial era to a current level of 387 ppm (Figure 1.1) [1]. The origin of this massive emission of CO2, and of its steady accumulation in the atmosphere, has been the use of carbon-based fossil fuels in human activities (Figure 1.2). Moreover, as carbon-based fossil fuels currently represent 80–85% of the world’s energy sources, and will continue to play such role at least in the short to medium term, a major expansion is visualized in the emission of CO2 (Figure 1.3). Today, the robust growth of emerging economies such as China and India is driving worldwide energy demand and usage such that is increasing at a rate never before experienced; in fact, a further expansion of 50–100% is envisaged by the year 2030 [2]. The novel point here is that, compared to the recent past, these emerging economies will drive the expansion [2] of energy use, with the energy requirements of the OECD (Organisation for Economic Co-operation and Development) countries envisaged to grow at a rate of 0.7% per year, while that of nonOECD countries will grow at 2.5% per year. For example, China and India increased their combined share of world energy consumption from 8% in 1980, to 18% in 2005. Moreover, it is estimated that, in 2009 the energy consumption of China will become equal to that of the USA, and that by the year 2010 China will become the world leader in terms of CO2 emission.

1.2 The Accumulation of CO2 in the Atmosphere, and the Effects that We Fear

Today, as CO2 continues to accumulate in the atmosphere, serious concerns are being raised regarding its influence on climate change. The greenhouse effect that is associated with energy production and use – whether by the direct release into Carbon Dioxide as Chemical Feedstock. Edited by Michele Aresta Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32475-0

1 Carbon Dioxide: Utilization Options to Reduce its Accumulation in the Atmosphere

Figure 1.1 The steady increase in CO2 concentrations in the atmosphere in recent years. Solid and open symbols indicate average and seasonal trends, respectively.

8000 7000 6000

Total CO2 emissions from fossil-fuels (million metric tons of C) CO2 emissions from gas fuel consumption CO2 emissions from liquid fuel consumption CO2 emissions from solid fuel consumption CO2 emissions from cement production CO2 emissions from gas flaring

Million metric tons of carbon

2

5000 4000 3000 2000 1000 0 1751

Figure 1.2

1801

1851

1901

1951

2001

The origin of the emission of CO2.

the atmosphere of flue gas-associated heat produced by the energy, industrial, and transport sectors, or by the emission of greenhouse gases – is slowly causing the temperature of the Earth to rise, as revealed in a report by the Intergovernmental Panel on Climate Change (IPCC) (Figure 1.4) [3]. Inevitably, these changes in the thermal structure of the atmosphere, and of the Earth’s solid surface, are causing the temperatures of the planet’s waters to rise and their volume to increase, to a point where the rising water levels on the Earth’s surface are seen as a major problem, especially in coastal areas.

1.2 The Accumulation of CO2 in the Atmosphere, and the Effects that We Fear 40.000 35.000 30.000

3

mongabay.com United States Mexico South Korea Russia India Africa OECD Europe

Canada Japan Australia/New Zealand China Middle East Brazil source data: EIA 2007

OECD Europe Brazil Africa Middle East India

25.000 20.000

China

15.000

Russia Australia/NZ South Korea Japan Mexico Canada United States

10.000 5.000 0 1990

2002

2003

2010

2015

2020

2025

Figure 1.3 Scenario of the emission of CO2 until 2030. In 2010, China will become the leading CO2 emitter worldwide.

Figure 1.4 Variation of the temperature of the Earth during the last millennium. The upper panel shows the trend during the past 140 years (IPCC data).

2030

4

1 Carbon Dioxide: Utilization Options to Reduce its Accumulation in the Atmosphere

Figure 1.5 Correlation among the emission of CO2 into the atmosphere, the time of stabilization of its concentration, the temperature variation, and the rise in sea-level.

Unfortunately, the time required to stabilize the effects of increased greenhouse gas concentrations in the atmosphere will be very long. Any response by the Earth to measures that we might take today will not be effective tomorrow, nor the day after tomorrow – it will take centuries (Figure 1.5). So, with such a long time-scale, the ability to forecast scenarios will be affected by any major errors that might emerge as a result of any, often unpredictable, changes that might occur. During the past twenty years, there have been major changes in the social, political, and economical structures of a large part of society that, in 1985, were not considered realistic when forecast scenarios were first constructed. The subsequent development of a market economy in Eastern Europe and Russia has led to large demands for energy, whilst the current changes in China and India are creating much uncertainty with regards to future energy needs. Today, the economies of large areas of the Earth, including Eastern Europe and Russia, Southern America, other Asian countries in addition to China and India, and almost the whole Africa, are undergoing continuous evolution to a point where their real rate of growth is not exactly known. Clearly, under such conditions the variability and uncertainty of any foreseen scenario is very large, and it will inevitably become essential that responsible measures are taken in order to avoid reaching a ‘point of no return.’

1.3 Technologies to Reduce CO2 Accumulation in the Atmosphere

It has been predicted that carbon-based fossil fuels will continue to provide 80– 85% of the world energy consumption at least until 2030. Consequently, it is clear that, despite uncertainties regarding the role of atmospheric CO2 on climate

1.3 Technologies to Reduce CO2 Accumulation in the Atmosphere

change and on the occurrence of catastrophic events, as well as links between population growth and energy consumption, the predicted large rise in CO2 emissions is raising serious concern in relation to the rise in the Earth’s temperature. Today, the need to control CO2 production and emissions is the center of attention of both scientific and industrial worlds and, as a consequence, several methods have been developed to achieve this goal (Table 1.1). None of the technologies listed in Table 1.1 can alone provide a short- to medium-term solution to reduce atmospheric CO2 emissions at a level necessary to stabilize current concentrations. Rather, the correct blend of technologies

Table 1.1

Technologies for controlling the emission of CO2 into the atmosphere.

Technology

Examples of application

Comments

Efficiency

Production of electric energy

The efficiency of the technologies of conversion of chemical-into-electric energy may rise from 32% to over 50%

Use of any form of energy

Saving energy through a responsible use

Fuel shift

Substitution of coal with oil or gas

Emission value expressed as kg CO2 per kWh electric energy produced: 1 for coal, 0.75 for oil, 0.5 for gas.

Advanced technologies for electric energy production

IGCC

Concentration of the production of CO2 via decarbonization of fossil fuels. CO2 can be either disposed of or used

Non-carbon-based fuels

Nuclear energy

Use of nuclear fuels for the production of electric energy; used by sectors that require a high intensity

Perennial energies

Solar, wind, hydro, geothermal

Their exploitation depends on the geographic position of a country

Renewables

Biomass utilization

Residual and cultivated terrestrial and aquatic biomass can produce liquid or gaseous fuels suitable for the transportation sectors.

Carbon dioxide capture and storage (CCS)

Capture and disposal of CO2

This technology has a large disposal potential, but sites are not available in all counties. It is energy-intensive and will expand the use of fossil-C well beyond the current limits

CO2 utilization

CO2 is recycled, mimicking Nature

Chemical, technological, and enhanced-biological uses are the only ones that generate a profit, adding value to a waste

5

1 Carbon Dioxide: Utilization Options to Reduce its Accumulation in the Atmosphere 38

36

36

36

34

34 32

30

30 Volume Energy Density/GJ/m3

6

28 26 24 22 20

18 17

18 16

13

14 12

9

10

8

8 6 4

2

2 0 diesel

Figure 1.6

bio-diesel from algae

gasoline

carbon coke

brown coal

methanol

bio-oil from algae

H2 (l)

methane (g)

H2 (g) 20,0 MPa

Energy density of several different liquid and gaseous vectors.

must be identified to achieve the most effective reduction in CO2 emissions, while paying the lowest social (by boosting development) and economic (affordable technologies for all societies) costs. In this way, these methods may be developed over time such that they become effective for application on the large scale. Likewise, the optimal use of resources may be maximized by applying intelligent solutions. Based on the different intensities of energy required, industrial and collective electrified transport uses may be separated from personal transportation and municipal uses of energy. For example, gasoline, diesel and methane might represent the best options for the individual and mass road-transportation sector, as they have the correct energy density (Figure 1.6), whereas ‘renewables’ might represent the ideal candidate to partially substitute fossil fuels in the transportation sector and in the chemical industry. Perennial sources could be used to best advantage in the short term by municipalities or small industries, fossil carbon could be used in the production of certain necessary chemicals or fuels, while nuclear power would be better used for industrial other applications that demand a highly intensive use of energy.

1.4 The Utilization of CO2

In 2008, increases in the price of oil and the recognition of a need to reduce the impact of the chemical and energy industries on climate change, notably by reducing the emission of CO2, directed interest towards the use of renewable sources of energy and alternative feedstocks for the chemical industry. This strategy

1.4 The Utilization of CO2 Coal, 23% Natural gas, 24% Biomass, 47% Renewable energy, 6% Nuclear, 8%

Hydroelectric, 45% Geothermal, 5% Wind, 2% Solar, 1%

Petroleum, 39% Figure 1.7

Potential of biomass in the USA (from EIA-Paris, 2004).

included the industrial utilization of CO2 and the enhanced fixation of CO2 in aquatic biomass, with both applications having huge potential for the recycling of carbon and reductions in CO2 emissions. The enhanced, direct fixation of CO2 into fast-growing biomasses might contribute towards reducing its accumulation in the atmosphere, under non-natural conditions. Such an approach could be used for the production of chemicals and energy (e.g., conversion into gaseous and liquid fuels, rather than direct combustion of the solid biomass), with beneficial effects on reducing CO2 emissions and accumulation in the atmosphere. The potential of biomass as a possible substitute for fossil fuels in the USA is shown in Figure 1.7. The implementation of an artificial photosynthetic cycle which could recycle CO2 [4, 5], thus complementing the natural cycle, could also make an effective contribution to reducing CO2 emissions. The utilization of CO2 through technological, chemical, or enhanced biological methods may lead to reductions in CO2 emissions, with these innovative processes being substituted for older technologies and products, and imparting direct and indirect benefits on the impact of climate change. In this case, a direct effect might be considered the reduction of both CO2 emissions and fossil fuels extractions, whereas an indirect effect would be a substitution, with CO2, of chemicals such as chlorofluorocarbons and congeners having a climate change power (CCP) many thousand-fold that of CO2 [6]. It should be remembered that, whilst the use of CO2 in this way will not solve the problem of atmospheric CO2 accumulation, it might contribute to such an issue by reducing the volume of CO2 produced. It is also worth noting that the fraction of CO2 produced via the use of chemicals is approximately 10% of the total, the remainder being derived from energy products. It follows, therefore, that if all chemicals were to be produced from CO2, then 10% of the emitted CO2 could be recycled so as to achieve an almost carbon-neutral use of chemicals. Unfortunately, this situation would be impossible to achieve for several reasons, among which are the energy costs. A realistic estimate of the total amount of CO2 to be avoided [7] has been set at 7%, this being the contribution of CO2 use in the short to medium term. However, if efficient technologies capable of converting CO2 into energy-rich products (fuels) were to be developed, then a much greater amount of

7

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1 Carbon Dioxide: Utilization Options to Reduce its Accumulation in the Atmosphere

CO2 could be converted into usable products. This would result in a much more significant step in the direction of chemicals and energy production, with a close to zero carbon-emission level.

1.5 Conditions for Using CO2

The utilization of CO2, whether technological, biological, or chemical, to cause an effective reduction in its emission into the atmosphere, must comply with certain key rules, namely:

• • • •

the new process must reduce the overall CO2 emissions; it must be less energy- and material-intensive with respect to the on-stream processes that it aims to replace; it must employ using safer and more eco-friendly working conditions; and it must be economically viable.

The reduction in overall CO2 emission for a given application of CO2 is not easily quantified. Both, the energy and material consumption must be minimized, through the control of process parameters, such as the conversion yield and selectivity, the temperature, the pressure, and the post-reaction operations such as isolation and purification. Mass control requires more direct (fewer steps), effective (high-yield) and selective (product entropy control) processes, with waste (gas, liquid, solid) minimization at source, and with a better carbon-atom utilization. As CO2 is not a toxic substance, under controlled conditions of utilization it does not cause any serious worries. In particular, its fire-extinguishing properties ensure that the risk of combustion is close to zero when a high pressure is used [30–40 MPa when supercritical CO2 (sc-CO2) is used as solvent and/or reagent]. Hence, CO2 can be considered to be safe reagent or solvent, especially when sc-CO2 is used as a process fluid (see Chapter 2). The economics of CO2 utilization depend on its quality, with the price reaching up to US$ 400 per ton, according to the purity required and the purification technologies involved [8].

1.6 CO2: Sources and Prices

As noted above, CO2 can be obtained from several sources, with a variety of prices. Although natural wells are an important and cheap source (€15–20 per ton) [9] of pure (>99%) CO2, this source should be discontinued and the use of recovered CO2 (from power-generation plants or industrial processes) encouraged as an alternative. The captured CO2 is characterized by different degrees of purity, according to its origin, and may require extensive purification in specific applications, such as in the food industry. These purification steps will, of course, affect

1.7 The Potential for CO2 Utilization, and the Content of This Book

the price of CO2, and this will in turn invite the discovery and use of new sources, an example being fermentation reactors which may provide high-purity gas. One possible drawback of such a source might be its seasonality (e.g., sugar cane harvests), although this could easily be circumvented by the year-round storage of these raw materials. In the case of several applications in the chemical industry, the presence of contaminants such as O2, SOx, or NOy, which may be present in the flue gases of power plants, might be deleterious by causing negative (poisoning) effects on the catalysts.

1.7 The Potential for CO2 Utilization, and the Content of This Book

The large-scale separation of CO2 from power and industrial plant flue gases, or its recovery via new technological applications (e.g., integrated gasification combined cycle; IGCC) which produce concentrated flows of CO2, will make available huge volumes of CO2. The subsequent fate of the recovered CO2 would be either disposal in natural fields (spent gas or oil wells, unmined coal seams, aquifers), or its utilization. Whilst, in principle, the former option can be used to eliminate large volumes of CO2, in practice the disposal sites are few in number and the procedure would involve high energy and economic costs. Eventually, however, the implementation of this technology will surely lead to an expansion of the extraction of fossil carbon-based fuels. Conversely, the utilization of CO2 has already been implemented at the level of over 130 Mt per years in the energy and chemical industries [6], and is a technology that is known to add value to the waste CO2. Today, new technologies using CO2 are under development, the exploitation of which will greatly expand the amount of CO2 that can be either used or recycled. The large-scale utilization of CO2 can be integrated very well with its large-scale recovery, yet, whilst the utilization process will produce economic benefits, the disposal process will incur economic costs. It is essential that these new applications of CO2 are energetically more convenient than the existing processes, and in this respect the biological [10] and technological [6, 11] applications of CO2 utilization have been extensively reviewed. Indeed, the aim of this book is to describe in detail the chemistry of the utilization of CO2, presenting an up-date in each specific field of CO2 use. The technological use of CO2 contributes to a reduction in its atmospheric accumulation, through CO2 being substituted into species with a much higher CCP [6]. Some examples where the use of CO2 lowers the impact on climate change (even if the CO2 is ultimately vented to the atmosphere) are listed in Table 1.2. Among the applications listed in Table 1.2, the CO2 substitutes either species with a higher CCP or an organic solvent which, on completion of the process, may be partly recovered using energy-expensive technologies, or burned so as to

9

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1 Carbon Dioxide: Utilization Options to Reduce its Accumulation in the Atmosphere Table 1.2

Some examples of technological applications of CO2.

Example of application

Product substituted

Example of application

Product substituted

Example of application

Product substituted

Dry cleaning

Chlorinated solvents

Fire extinguisher

Flame retardants

Extraction of caffeine

Hexane

Water treatment

Sulfuric acid

Cleaner in electronics

Fluorinated solvents

Solvent in reactions

Various organics

Air conditioning

Fluorinated compounds

Mechanical industry

Fluids with higher CCP

Production of nanomaterials

Organic solvents

Antibacterial

Complex pharmaceuticals

Extraction of fragrances

Hexane

Solvent in polymerization

Organic solvents

produce large amounts of CO2 [11]. Specific aspects of the use of CO2 as a technological fluid are described in Chapter 2. Enhanced biological fixation [10] has great potential, especially if an aquatic biomass can be created that is capable of producing chemicals and fuels; this topic is discussed in Chapter 13. The chemical utilization of CO2 is also considered in detail, with the pros and cons of each application highlighted to provide a detailed picture of the state of the art, and of the use of CO2. Areas in which further research in order to fully implement this utilization strategy are noted as key points. The potential of such a technology is especially interesting with regards to reducing CO2 emissions. In fact, often the most important aspect in the chemical utilization of CO2 is not the amount of CO2 used (if a chemical made from CO2 is used, the CO2 will be re-formed within a short time, except in the case of polymers that may last for decades!); rather, it is the introduction of innovative technologies that may lead to a reduction in the use of materials and energy. The potential uses of CO2 in chemical applications are shown in Figure 1.8, where some of the products (carboxylates, carbonates, and carbamates in routes A and B) are obtained by incorporation of the entire CO2 molecule. The reactions bearing such products will have a low energy content and may occur at room temperature, or lower (see Chapters 5 to 8). Processes in which CO2 is reduced to other C1 or Cn molecules (Figure 1.8, routes C and D) require an input of energy. In order to be consistent with the information provided in Section 1.5, such energy cannot be provided by fossil fuels, and alternative sources must be found, with solar energy the best candidate. In Chapters 9 to 12, details are provided of the potential for reducing CO2 to fuels or chemicals. Most processes involving CO2 conversion can be compared to the natural processes described in Chapter 3 (biological reactions) and Chapter 14 (geological inorganic reactions). The fundamentals of CO2 chemistry and reactivity are described in Chapter 4.

1.8 The Need for Research to Speed an Exploitation of the Utilization Option

11

O NH2

C

H2N

ONa/K

NH2

HO H3COH

COONa/K

A HCOOH

O

O

O

O

+

O O

O

O

O

n

O

D O

C

RO

HCONHR CnH2n+2

H2

CnH2n

CO CO 22

O ROH

RNH2

OR

CO COOH COOH

O

O2

H2C=CH2

O

B

O

HOOC

COOR

R

RC ≡ CR

Br

R

C

O

e-, H+

O

COOH

HOOC

O RNH2 + R’X

COOH

COOH

+

RNHC OR’ O

N H

Figure 1.8

O

C

H N N H

O

O OH

3

O

n

The possible applications of CO2 in chemical syntheses.

1.8 The Need for Research to Speed an Exploitation of the Utilization Option

The first intensive investigations into the chemistry of CO2 followed the initial description of the transition-metal complex, Ni(CO2)(PCy3)2 [12]. Subsequently, during the late 1970s and 1980s, much emphasis was placed on investigating CO2 conversions and the associated reaction mechanisms [13, 14]. Unfortunately, however, this early enthusiasm in academia was not supported by industrial investment (at the time, there was no compulsion to reduce CO2 emissions and waste production!), and consequently during the following decades the research effort was decreased and the benefits were lost. Yet, today, interest in CO2 utilization has been totally re-vamped, based on the above-mentioned aspects. Clearly, whilst current research must be funded along selected axes so as to identify the most

12

1 Carbon Dioxide: Utilization Options to Reduce its Accumulation in the Atmosphere

useful results in terms of exploitable technologies, it is vital that the fundamental research should also be continued. The use of CO2 in the synthesis of carboxylates (including acrylates), esters and lactones, carbonates, carbamates, and polymers may help to solve substantial problems encountered in the chemical industry, and hopefully advance towards sustainability. In all such cases, the new processes based on CO2 would replace old energy- and material-intensive technologies that employed either the use of toxic compounds (e.g., phosgene) or were characterized by a high E-factor [6]. The use of sc-CO2 as a solvent and reagent represents an application that reduce not only the amount of waste organic solvents, but also the emission of CO2 derived from the combustion of spent solvents. The dry-reforming of methane is a technology that may pressurize the gas-toliquid (GTL) approach by converting methane and CO2 into liquid fuels at the liquefied natural gas (LNG) extraction well. The photoassisted electrochemical reduction of CO2 in water represents a very interesting technology that may allow the efficient use of residual or intermittent energies [6], with the concomitant conversion of large volumes of CO2 into chemicals or fuels. Unfortunately, the technological use of CO2 has several hidden benefits [6], the most prominent of which is that it should be implemented on a large scale so to replace toxic species and compounds which have a high CCP. The use of sc-CO2 has already been exploited in several areas (e.g., as a solvent in drywashing, or in extractions or chemical reactions); however, its use will inevitably be expanded into the chemical industry, as well as in the process of enhanced oil recovery. By mimicking Nature and combining biotechnology and chemistry, it may be possible to bring about the discovery of new technologies which, together with a greater use of biomass, may support the substitution of fossil-carbon with quasizero-emission technologies. One major exploitation of the concept that “Nature makes and Chemists re-shape” may bring about important benefits that have not yet been fully identified. The “inorganication” of CO2 is a technology that may, at least potentially, be used to store large volumes of CO2 over the long term, in the form of “safe” chemicals. Of course, such an approach would be especially welcome in situations where residual inorganic oxides and sludge from industrial processes could be used for the CO2 fixation. Whilst, in future, it is essential that industry, international and national funding organizations all support research investigations in the above-mentioned areas, it is equally vital that these studies are conducted in complementary fashion. In this way, it will be possible to investigate a wider variety of applications, taking into account specific factors applicable to different countries, and consequently producing the greatest benefit to Society in general.

References

References 1 Mauna Loa Laboratory data, http://www. esrl.noaa.gov/gmd/ccgg/trends/. 2 Department of Energy (2008) International Energy Agency Report, 0484. 3 Solomon, S., Qin, D., Manning, M., Marquis, M., Averyt, K., Tignor, M.M.B., LeRoy Miller Jr, H., and Chen, Z. (2007) Climate Change 2007; The Fourth Assessment Report (AR4) of the United Nations Intergovernmental Panel on Climate Change (IPCC). 4 Aresta, M. (1990) Enzymatic and Model Carboxylation and Reduction Reactions for Carbon Dioxide Utilization (eds M. Aresta and J.V. Schloss), Kluwer Academic Publishers, Dordrecht, The Netherlands, p. 1. 5 Aresta, M., Quaranta, E., and Tommasi, I. (1991) Photochemical Conversion of Solar Energy (eds E. Pelizzetti and M. Schiavello), Kluwer Academic Publishers, Dordrecht, The Netherlands, p. 517. 6 Aresta, M. (2009) Carbon Dioxide Capture and Storage (ed. M. MarotoValer), Woodhead Publishing Limited, Abington Hall, Granta Park, Cambridge, CB21 6AH, UK. 7 The Conference Participants (1990) IEA Executive Conference on “Solar Photoconversion Processes for Recycling

8 9

10

11

12

13

14

Carbon Dioxide from the Atmosphere”, Colorado Spring. http://ceh.sric.sri.com/Public/Reports/ 743.2000. Graziano, P. (1990) Enzymatic and Model Carboxylation and Reduction Reactions for Carbon Dioxide Utilization (eds M. Aresta and J.V. Schloss), Kluwer Academic Publishers, Dordrecht, The Netherlands, p. 19. Aresta, M. (2010) Energy from aquatic biomass, in Handbook of Combustion (eds M. Lackner, F. Winter, and M. Agarwal), vol. 5, ch. 13, Wiley-VCH, Weinheim. Aresta, M., and Dibenedetto, A. (2002) CO2 Conversion and Utilisation (eds C. Song, A.M. Gaffney, and K. Fujimoto), ACS Series Book 809, American Chemical Society, p. 54. Aresta, M., Nobile, C.F., Albano, V.G., Forni, E., and Manassero, M. (1975) J. Chem. Soc. Chem. Commun., 15, 636. Aresta, M., and Forti, G. (eds) (1987) Carbon Dioxide as a Source of Carbon, Kluwer, Dordrecht, The Netherlands. Aresta, M., and Schloss, J.V. (1990) Enzymatic and Model Carboxylation and Reduction Reactions for Carbon Dioxide Utilization, Kluwer Academic Publishers, Dordrecht, The Netherlands.

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2 Utilization of Dense Carbon Dioxide as an Inert Solvent for Chemical Syntheses Alessandro Galia and Giuseppe Filardo

2.1 Introduction

Studies of the use of liquid and supercritical carbon dioxide (scCO2) for chemical processes represent a vast field which covers many interdisciplinary areas, including industrial chemistry, transport phenomena, catalysis, phase and chemical equilibria, and the design of high-pressure experimental and analytical apparatuses. In addition, CO2 can be used simply as a solvent or also as a reactant, thus further complicating the tassonomic approach to the problem. Another consideration that makes difficult an original approach to analyzing the use of dense CO2 as a solvent for chemical synthesis is that overviews of the subject have been regularly provided in excellent reviews and books describing many different aspects of interest, including homogeneous and heterogeneous catalytic processes [1–12], polymerizations [13–23], green syntheses [24–27], and biocatalysis [28–31]. Consequently, the aim of this chapter is to highlight the main features of dense CO2 that permit the syntheses of both low-molecular-weight and macromolecular compounds. No attempt has been made to provide a comprehensive analysis of the literature in the different areas that, as noted previously, has been very effectively and regularly reviewed by others.

2.2 Dense Carbon Dioxide as Solvent Medium for Chemical Processes

A gaseous pure component can be defined as supercritical when its state is determined by values of temperature T and pressure P that are above its critical parameters (Tc and Pc). In the proximity of its critical point, a pure supercritical fluid (or a dense gas as it is alternatively known) has a very high isothermal compressibility, and this makes possible to change significantly the density of the fluid with relatively limited modifications of T and P. On the other hand, it has been shown that the thermodynamic and transport properties of supercritical fluids can be tuned simply by changing the density of the medium. This is particularly interesting for Carbon Dioxide as Chemical Feedstock. Edited by Michele Aresta Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32475-0

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2 Utilization of Dense Carbon Dioxide as an Inert Solvent for Chemical Syntheses

the chemical engineer, as density becomes an additional parameter that can be used to improve the performances of a process. Supercritical carbon dioxide was initially mainly investigated as a processing medium for extractions and fractionations in the field of natural product processing. Subsequently, the past fifteen years has witnessed a rapidly growing interest in the use of this unconventional solvent medium to conduct chemical reactions, mainly because it can serve as a substitute for liquid organic solvents [32]. Such interest in supercritical fluids has been shown to depend not only on their attractive chemical– physical properties, such as density-adjustable solvent strength, low surface tension and viscosity, but also on specific technical–economical properties, such as low cost, high availability, high stability, good biocompatibility, low toxicity and nonflammability, and readily accessible critical parameters (Tc = 304.12 K, Pc = 7.37 MPa). In particular, when gaseous reagents are involved, scCO2 can allow the operator to perform the reaction under single-phase conditions, thus greatly increasing the local concentration of the dissolved gases and markedly accelerating the mass transfer kinetics both for the elimination of the gas–liquid interphase and for the better transport properties of the supercritical medium in comparison with liquid solvents. These considerations may explain why the most extensively investigated reactions using scCO2 are hydroformylation, hydrogenation, and oxidation. The hydroformylation reaction (Scheme 2.1), originally discovered by Otto Roelen in 1938 [33], is an industrial process of strategic importance for the manufacture of aldehydes from olefins and syngas. Most research on the hydroformylation of olefins in scCO2 has been carried out using homogeneous catalytic systems. From an applications point of view, it is interesting to note that good results have been obtained with high-molecularweight olefins which, owing to their poor water-solubility, cannot be hydroformylated in the aqueous liquid/liquid/gas system based on the use of a water-soluble rhodium (Rh) catalyst (Ruhrchemie/Rhône-Poulenc). Rh has frequently been adopted as the metal center in scCO2 and one crucial point that must be resolved for transferring the synthetic route from the laboratory to the industrial plant, is the availability of low-cost CO2-philic ligands to render the organometallic catalyst soluble in the reaction medium at a sufficiently high concentration. This problem is particularly difficult to overcome because the solvent power of CO2 will become progressively reduced, as the result of antisolvent effects, when the molar fractions of low-critical-temperature gaseous reagents are increased [34–36]. To date, the most effective approach to this problem has been to functionalize the aryl ring of the triphenylphosphine with fluorinated alkyl groups [37–41];

COH R Scheme 2.1

+ CO+H2

catalyst

COH R

+

R

Schematic representation of the hydroformylation of terminal alkenes.

2.2 Dense Carbon Dioxide as Solvent Medium for Chemical Processes

however, this solution proved to be too expensive for industrial applications unless a quantitative recovery of the catalytic system could be achieved. More recently, Sarbu et al. showed that the incorporation of carbonyl groups could significantly increase the solubility of macromolecular materials in scCO2, provided that the enthalpic and entropic contributions were properly balanced [42, 43]. This result was subsequently extended to the design of carbonylated phosphines that were tested as ligands in the Rh-catalyzed hydroformylation of 1-decene in scCO2 [44]. Phosphite, phosphonite and phosphinite ligands with nine-carbon branched alkyl substituents have also been synthesized, and proved to be active in the rhodiumcatalyzed hydroformylation of 1-octene in scCO2, although the catalytic system was insoluble in the dense gas [45]. Heterogeneously catalyzed processes have also been the object of much investigation. In this case the problem of catalyst recycling was considerably reduced and the excellent transport properties of the supercritical medium still granted rapid mass transfer rates. As an example, interesting results were obtained by the group of Poliakoff, when they studied the hydroformylation of 1-octene using a Rh complex with aryl phosphine ligands immobilized on silica [46]. The reasons for studying hydrogenation reactions in scCO2 are similar to those described previously for oxo-syntheses, where the complete miscibility of molecular hydrogen with scCO2 was invoked as an important precondition to achieve process intensification. In this context, it should be noted that the hydrogenation of organic substrates using continuous fixed-bed reactors is one of the few processes that have been developed effectively from the laboratory scale to the industrial application. This was achieved on the basis of an excellent collaboration between the Clean Technology Group at the University of Nottingham and the fine chemicals manufacturer Thomas Swan & Co. Ltd [47]. Despite the above-mentioned information relating to gaseous reactant solubility seeming to push towards achieving a one-phase reaction system, it has been shown [48] that faster reaction rates can occasionally be obtained under biphasic conditions. Notably, this occurs when CO2 dissolved in the organic reactants permits a significant increase in the solubility of permanent gases in the expanded liquid phase. This, in turn, allows the problems associated with low reactant concentrations, which are often observed in single-phase processes and which nullify the high local concentration of hydrogen, to be overcome. It is important to note that the formation of CO from the reverse water gas shift reaction (CO2 + H2 = CO + H2O) has been detected at low temperature with the commonly used Pt group metals [49, 50]. Under the operational conditions used to carry out these reactions, the CO coverage is relatively low and concentrated on specific sites such as the step and kink. This has been considered a possible explanation for the fact the catalytic performances are generally unaffected unless demanding reactions that require the catalytic action of specific sites on the catalyst surface are considered, as is the case with enantioselective hydrogenation reactions [49]. Another important class of reactions that has been the object of intense investigation is the selective oxidation of organic substrates in dense CO2. Clearly, the first consideration supporting the use of CO2 as solvent for such reactions is that,

17

18

2 Utilization of Dense Carbon Dioxide as an Inert Solvent for Chemical Syntheses HO

O2 catalyst Scheme 2.2

O

+

+ H2O

Oxidation of cyclohexane to a mixture cyclohexanol/cyclohexanone.

in contrast to most adopted organic solvents, this compound cannot be further oxidized, thus preventing the formation of byproducts and waste materials. Many of the above considerations relating to hydroformylation and hydrogenation reactions in CO2 can be extended to oxidations, particularly when O2 is used as the oxidant agent. Indeed, O2 is the most widely investigated oxidizing compound, due to its low cost and high atom-efficiency. In these processes, significant process advantages have been observed arising from the reduction of transport resistance through elimination of the gas–liquid interface. This is usually coupled to a higher local concentration of the oxidizing agent, accompanied by a better safety profile of the process due to the solvent acting as inert diluent. In this context, it is of interest to note dense CO2 is one of the few reaction media that can be used for the direct synthesis of hydrogen peroxide from H2 and O2 [51]. The majority of studies on oxidation reactions in scCO2 have involved catalyzed processes promoted by molecular oxygen, in which the role of the catalyst is to generate free radicals that will react with the chemical oxidant, leading to a product distribution that is typical of an unselective chain process. Among these can be mentioned the oxidation of cyclohexane to cyclohexanol and cyclohexanone (Scheme 2.2) as an intermediate step in the production of the adipic acid that is a key component in the production of Nylon 6,6 polyamide [52–54]. Olefin epoxidation is another synthetic route of great interest that has been the object of several investigations, not only in multiphase reaction systems but also with oxidants other than molecular oxygen, such as inorganic and organic peroxides [55–57]. The partial oxidation of alcohols, to afford carbonyl or carboxylic compounds, is another synthetic route of high industrial interest. For this, scCO2 was investigated as a reaction medium for the aerobic oxidation of aliphatic, unsaturated, aromatic and benzylic acids with different catalytic systems, mainly based on the use of noble metals, both in batch [58–64] and in continuous fixed-bed reactors [65–70]. In this context, very promising results have been obtained when studying the catalytic activity of supported palladium and gold nanoparticles in the oxidation of benzyl alcohol to benzaldehyde; these allowed conversions and selectivities in excess of 90% to be achieved [71–73].

2.3 Enzymatic Catalysis in Dense Carbon Dioxide

Enzymes are natural biomacromolecules that belong to the class of proteins. In an aqueous environment, they assume conformations that impart them with high

2.4 Other Reactions in Dense Carbon Dioxide

activities and selectivities under mild conditions. Although, in the natural world, enzymes have adapted to function in aqueous biological media, they have also been shown capable of functioning in organic solvents, provided that a small number of water molecules are bound to the biocatalyst; this allows the enzyme to exist in its active conformation [74]. As enzymes are insoluble in organic media, they represent a special class of heterogeneous catalysts. The use of enzymatic catalysts in scCO2 would seem the perfect union between a highly selective and active green catalytic system and an environmental friendly solvent with excellent transport properties. Such a union could achieve a marked reduction in the mass transfer resistance that is, most likely, the slowest step in a heterogeneously catalyzed process carried out with a chemically active catalytic system. For these reasons, many groups have investigated CO2 enzymatic catalysts that have been either freely suspended or stabilized by using different approaches [31]. Interestingly, the hydrophobic ion pairing of cytochrome c with fluorinated anionic surfactant proved to be an effective strategy for preparing enzyme–surfactant complexes that were soluble in dense CO2, without any modification of the secondary structure of the protein [75]. Despite, in theory, this approach having a high potential for this reaction, the practical use of biocatalysts in scCO2 was frequently – but not always – prevented by solvent–catalyst interactions that led to the formation of carbamates. Such a reaction changes the enzyme’s conformation, and in turn affect its catalytic performances. As a result, the industrial use of these catalysts presents several problems, the solutions of which are partially dependent on the ability to identify enzymes that could be obtained from organisms adapted to living in a CO2-rich environment.

2.4 Other Reactions in Dense Carbon Dioxide

Whilst a major proportion of studies have been targeted at using scCO2 as a green solvent in organic syntheses, and have focused on hydrogenation, hydroformylation and oxidation reactions, other classes of organic reactions have been studied in dense carbon dioxide. Some of these were mainly used to assess density inhomogeneity effects that, in the proximity of the critical point, can markedly affect the kinetics of reactions carried out in a supercritical fluid (SCF), as in the case of Diels–Alder reactions [76–83]. Other reactions were investigated within the frame of cooperation between academia and industry, and led to patents that were the object of industrial development, as was the case of acid-catalyzed Friedel–Craft reactions [84, 85]. Apart these cases, many other reactions have been investigated, including carbonylations, Heck reactions, vinyl substitutions, and hydrosylation. In general, based on available data, it is difficult to establish whether these studies could extend beyond the laboratory scale. However, as Eric Beckman has stressed, when reviewing the use of supercritical or near-critical carbon dioxide in “green” chemical syntheses [26], the role of these studies in demonstrating the extreme versatility of

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dense CO2 as a solvent for chemical syntheses, thus stimulating new approaches that could become relevant for industrial applications, cannot be neglected.

2.5 Polymer Synthesis in Supercritical Carbon Dioxide

The primary consideration is that polymer synthesis in SCFs are generally carried out at high pressure; consequently, it is important to analyze the influence of this parameter on the polymerization process. The increase of pressure can affect a polymerization process by:

• • •

increasing the concentration of gaseous monomers; changing the kinetic constants for the individual steps involved in the process; modifying the equilibrium constant of the process.

Apart from pressure effects, which are not unique to SCFs, some other peculiar aspects of polymerization in SCFs are related to the physico-chemical properties of compounds under supercritical conditions. In traditional liquid solvents, the polymerization reaction rates are often limited by the local increase in viscosity during the process, as this lowers the mass transfer rate of the monomer to the reaction site. A lower viscosity and a higher diffusion coefficient in SCFs each contribute to overcome this limitation, however, allowing the polymerization rate to be significant up to high value of monomer conversion. The initiation step could also be positively affected by the above-mentioned transport properties, as the efficiency factor f assumes higher values with respect to conventional liquid solvents due to the diminished solvent cage effect. One further advantage is constituted by the tunability of the compressibilitydependent properties such as density, dielectric constant, heat capacity, and viscosity, all of which offer additional possibilities to modify the performances of the polymerization process. This aspect could be particularly relevant in the case of copolymerization reactions, where the reactivity ratios of the two monomers, and ultimately the final composition of the copolymer, could be controlled by modifying the pressure of the reaction system. Apart from polymerization processes with gaseous monomers above their critical points – for example, the synthesis of low-density poly(ethylene) – several SCFs have been tested as inert reaction media, such as ethane, propane, butane, and CO2. Among these, scCO2 is by far the most widely investigated, because it links positive fluid effects on the polymers with environmental advantages; this makes scCO2 the main candidate as an alternative to traditional solvents used in polymer syntheses. Compared to water – another green solvent which is frequently adopted as a dispersing medium in industrial heterogeneous polymerizations – scCO2 eliminates the need for energy-intensive drying processes, as the polymer product can

2.5 Polymer Synthesis in Supercritical Carbon Dioxide

be isolated completely dry upon venting. As the critical temperature of CO2 is very close to room temperature, this solvent can be used in applications that involve polymers and heat-sensitive materials such as enzymes, flavors, pharmaceuticals, and highly reactive monomers. With regards to the physico-chemical properties, the solvency of scCO2 as a medium and its plasticizing effects on macromolecular materials are of central importance. The solubility of reagents and polymer product in the continuous phase of a polymerization medium is the first parameter to consider in the selection of the polymerization technique (whether homogeneous or heterogeneous). CO2 has a low dielectric constant, and by varying T and P, its value can be changed from 1.01 to 1.45 for the gas phase and to 1.60–1.67 for the liquid phase. It has been noted that CO2 behaves very much like a hydrocarbon solvent with regards to its ability to dissolve small molecules; consequently, many monomers exhibit a high solubility within CO2. On the other hand, most high-molar-mass polymers are scarcely soluble in CO2, and the only polymers that show good solubility under relatively mild conditions (T < 373 K, P < 35 MPa) are amorphous fluoropolymers, silicones, and polyether polycarbonate copolymers. To date, the parameters that govern the solubility of polymers in CO2 are not fully understood, despite several investigations having been conducted to clarify the nature of specific solute–solvent interactions between various polymers and CO2. In the case of silicones, a specific interaction between CO2 and the silicone portion of the polymer backbone determines the high solubility of these types of macromolecular compound. Lewis acid–base-type interactions have been detected by using Fourier transform infrared (FTIR) spectroscopy between CO2 and the electron-donating functional groups of polymer chains [e.g., the carbonyl group of poly(methyl methacrylate)] and high-pressure 19F nuclear magnetic resonance (NMR) studies have indicated specific interactions between the CO2 and fluorocarbons. Quite recently, Sarbu et al. [42, 43] proposed a notable thermodynamic approach for the rational design of macromolecules with high solubility in scCO2. The basic concept of such a soluble macromolecule was to select its constituent repeat units so as to induce favorable enthalpic and entropic interactions with the compressible fluid to ensure a high gain in the free energy change of mixing: ΔGmix = ΔH mix − TΔSmix . This conceptual design was successfully used as a guide to identify nonfluorous, nonsilicon polymers that were soluble in CO2 [86, 87]. Regardless of the exact nature of the polymer–solvent interactions, it is clear that the poor solubility of most polymers in CO2 compels their syntheses to be performed via heterogeneous polymerization techniques to operate at acceptable values of pressure and temperature. Owing to the good solubility of many monomers and initiators in CO2, dispersion polymerization is today the most investigated heterogeneous method used for the synthesis of high-molecular-weight CO2-insoluble polymers. One central role for the performance of a successful dispersion polymerization is the surfactant.

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Since traditional surfactants (which have been designed for use in aqueous or organic continuous phases) are scarcely soluble in CO2, specialized stabilizer compounds have been developed that are capable of stabilizing the polymer colloid. All of these molecules are characterized by a CO2-philic portion which shows high solubility in the solvent, and a CO2-phobic portion which has a high affinity to the polymer phase. The latter part, which constitutes the anchoring portion of the surfactant, attaches to the surface of the polymer particle by either physical adsorption or chemical grafting, depending on its nature. The CO2-philic portion extends into the continuous phase and prevents the onset of flocculation of the particles by imparting long-range repulsion among them. However, these effects must be strong enough to compensate the long-range van der Waals attractions. Beside the consideration of solubility, another property of scCO2 which is relevant in polymer synthesis is its capability to highly plasticize polymers, thereby lowering their glass transition temperature (Tg). This plasticization effect is especially important in heterogeneous processes where a polymer coagulum with a high local viscosity is formed, as this effect may lead to an increase in the free volume of the polymer chains that induce an enhancement in the value of the diffusion coefficient of species inside the coagulated particles. Under these circumstances, the polymerization process can proceed up to high value of monomer conversion. The translational and segmental diffusivity of the growing polymer chains are also enhanced by the plasticizing effect, and this may lead to a shift in the onset of the gel effect at a higher value of the conversion or, occasionally, to a practical suppression of such an effect. Nonetheless, such behavior can simplify the thermal control of the process. 2.5.1 Chain Polymerizations: Synthesis of Fluoropolymers

Several monomers have been polymerized in scCO2 by particle-forming chain free-radical polymerizations, the most investigated of these being methylmethacrylate, styrene, N-vinylpyrrolidone, acrylic acid, vinylidene fluoride, and tetrafluoroethylene [13–23]. Among these monomers, those of major interest in terms of the industrial use of the synthetic route are fluorinated vinyl monomers, as these allow the preparation of high-performance polymers that exhibit a unique combination of excellent chemical resistance, a high thermal stability, a low dielectric constant and dissipation factor, unusual surface properties, low water absorptivities, excellent weatherability, and low flammabilities. These positive properties are related to the presence of fluorine substituents in the chain, although this may lead to a more difficult control of the polymerization process when compared to their hydrocarbon homologues. In fact, the high electrophilicity of the fluorinated macroradical species that propagate the chain represents the origin of a strong kinetic competition between chain propagation and chain transfer reactions involving H-atom donor species [88]. Fluoropolymers are usually obtained through heterogeneous (emulsion or suspension) polymerization techniques in aqueous systems. As a consequence of the

2.5 Polymer Synthesis in Supercritical Carbon Dioxide

adopted aqueous initiators, thermally unstable end groups (carboxylic acid and acid fluoride) are generated that must be removed before the polymer melt can be processed. In order to reduce the amount of thermally unstable end groups, the polymerization could be performed in aprotic solvents such as chlorofluorocarbons (CFCs) or perfluorocarbons, perfluoroalkyl sulfide and perfluorinated cyclic amines; however, these solvents are not only prohibitively expensive but also nonenvironment friendly. In this context, dense CO2 represents a sustainable alternative to such volatile organic compounds (VOCs), as demonstrated implicitly by the huge investments made over the past few years, both in the USA (i.e., the cooperation of DuPont with University of North Carolina) and in the European Union (European Research Grants Superpol and Ecopol), in investigating the use of scCO2 for the production of fluorinated polymers. High-molecular-mass polytetrafluoroethylene (PTFE) was synthesized in a heterogeneous CO2/aqueous medium at 5–12 MPa, using a water-soluble persulfate initiator, with and without a sodium or ammonium salt of perfluoro-octanoic acid (C7F15COO−Na + NH+4 ) as surfactant (to stabilize the water-rich micelles) [89]. In this process, by correctly selecting a water-soluble perfluorinated initiator, the compartmentalization of monomer, polymer and initiator was achieved, thus obtaining rapid polymerization kinetics and high values of both yields (up to 90%) and molecular weights (Mn = 900 kg mol−1) [89]. Moreover, the monomer/CO2 system behaved like a pseudoazeotropic mixture and could be safely handled. In fact, as TFE is known to form explosive mixtures with air, it must be kept scrupulously free from oxygen so as to prevent the formation of any shock-sensitive polymeric peroxides. Moreover, in the absence of air, it can disproportionate violently to form CF4 and elemental carbon [90]: (CF2=CF2 → C + CF4). More recently, the polymerization of TFE was carried out in a “water-free” scCO2-based medium, at 323 K and 12.1–13.3 MPa, with tertiary-amyl-per-pivalate as a free radical initiator, using a 5 liter batch reactor [91]. The experiments were performed both in the absence and presence of perfluoropolyether carboxylate, employed as surfactant, under the free acid form and as sodium and calcium salts (Figure 2.1). An expanded fibrillated PTFE morphology, which probably arose from the plasticizing effect of scCO2 on the amorphous domains of the polymer, was obtained in all experiments. Such a morphology could be particularly interesting for applications in the manufacture of solvent-free hydrophobic microporous membranes. In the case of surfactant-free polymerizations, the product was mainly constituted by irregular macroparticles having sizes in the range of 200 to 500 μm. Interestingly, when the free acid was used a clear acceleration of the polymerization rate was observed, and the polymer was obtained in the form of smaller particles with a more regular quasi-spherical shape. Poly(vinylidene fluoride) (PVDF) is the second most important thermoplastic within the fluoropolymer family after PTFE. Although, both the thermal and chemical stability of PVDF are somewhat lower compared to PTFE, the hydrogenated polymer can be more easily processed with conventional equipment, and it offers an advantageous compromise between quality and price. When the

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FLUOROLINK® C stabilizers CF3 CF2COO-NH4+

Cl CF2CFO

7004A n=4-6, Mw=937

n CF3 CF2COO- Ca2+

Cl CF2CFO

n

7004Ca n=4-6, Mw=1878

2

CF3 Cl CF2CFO

CF2COOH

7004H n=4-6, Mw=920

n Figure 2.1 Perfluoropolyether (PFPE) carboxylate compounds used as stabilizers in the polymerization of tetrafluoroethylene in scCO2.

synthesis of PVDF in scCO2, using a continuous stirred tank reactor, was recently investigated [92–95] the synthesized polymer showed properties similar to those of commercial samples prepared by the conventional route. The precipitation polymerization of VDF in dense carbon dioxide, initiated by γ-radiation, was studied as part of the aforementioned Superpol EU research program [96]. The absence of any chemical initiators, the fragments of which would ultimately be incorporated into the polymer chains, together with the easy removal of volatile carbon dioxide and monomer, resulted in a high-purity product obtained under mild process conditions (T ≤ 313 K, P ≤ 25 MPa). A further improvement in the process could be achieved performing the synthesis of fluorinated monomers in scCO2 in the presence of suitable stabilizers, so as to modify the polymerization technique from precipitation to dispersion or emulsion. When a research group at the University of Nottingham investigated the polymerization of VDF in the presence of a reactive polydimethylsiloxane monomethacrylate (PDMS-mMA) [97], even when an increased polymer molecular weight was obtained, a high level of aggregation of the primary particles was observed. The same group achieved a more effective stabilization by using a home-synthesized fluorinated graft maleic anhydride copolymer stabilizer [98]. In this case, the formation of spherical microparticles was clearly observed, but a monomer conversion less than 15% was reported, which corresponded to a maximum solid content of polymer in the autoclave of approximately 90 g l−1. Other groups [99, 100] investigated the possibility of carrying out a surfactantassisted polymerization of VDF in scCO2, using diethyl peroxydicarbonate (DEPDC) as a free radical initiator, with the aim of testing several perfluoropolyether compounds synthesized by Solvay Solexis as stabilizers. When ammonium carboxylate derivatives were used, an effective stabilization of the polymer particles was obtained, and the polymer was collected from the reactor with yields of up to 63%. This corresponded to a solid content in the reactor of 220 g l−1, in the form

2.5 Polymer Synthesis in Supercritical Carbon Dioxide

of a powder constituted by spherical particles, the diameters of which decreased as a function of the concentration of the stabilizer. Both, TFE and VDF may be copolymerized with suitable comonomers in order to obtain macromolecular materials with improved end-use properties. One fluorinated monomer that was copolymerized under mild conditions (308 K and 9–11 MPa) was perfluoropropyl-vinylether (PPVE) (CF3CF2CF2OCF=CF2). The copolymer was obtained in the form of a free-flowing powder, with almost quantitative yields; moreover, its composition could be altered by changing the molar ratio of the comonomers in the feed. Interestingly, chains were synthesized with molecular weights greater than that of the commercial product with a concentration of carboxylic acid or acid fluoride end groups similar to those detected in polymers prepared in CFCs [101]. Macromolecular matrixes suitable for the preparation of perfluorinated ionexchange membranes can be obtained through the copolymerization of TFE and functionalized perfluoro-vinyl-ether monomers, such as sulfonyl-vinyl fluoride CF2=CFOCF2CF(CF3)O(CF2)2SO2F (PSEVPE). These copolymers should couple to provide a good mechanical strength with a high ionic conductivity. In order to achieve this goal, the polymer must have a high molecular weight and a high concentration of ionic groups grafted onto the backbone. The main drawback in the synthesis of such a polymer with these joined properties is constituted by the β-scission of vinyl ether-terminated macroradicals [101]. Poly(TFE-co-PSEVPE) can be synthesized in scCO2 with a concentration of ionic groups similar to those obtained in conventional routes, but virtually free of acid end groups as a result of a substantial suppression of the β-scission reactions. Copolymers of TFE and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole (PDD) were synthesized in CO2 at low temperatures using bis(perfluoro-2-N-propoxypropionyl) peroxide as initiator [102, 103]. In this case, the copolymers were prepared with different compositions and having a broad range of Tg-values, from 340 to 607 K. No difference could be found between a fluorinated commercial sample (Teflon AF 1601) and a copolymer synthesized in CO2 having a similar composition, by using 19F NMR spectroscopy, IR spectroscopy, and differential scanning calorimetry (DSC). TFE may be copolymerized in dense CO2 also with hydrocarbon vinyl monomers such as vinyl acetate (VAc). In this case, it is possible to prepare an organic solventsoluble fluoropolymer that might be functionalized in a further step, for example through the hydrolysis of VAc groups to vinyl alcohol groups [104, 105]. Poly(TFEco-VAc) can be synthesized in an aqueous medium (e.g., by emulsion), although a branched structure is obtained via this route due to hydrogen abstraction from the VAc groups. In contrast, copolymers produced in scCO2 (318 K, 23–26 MPa initial pressure) are predominantly linear, most likely due to the swelling properties of CO2 towards fluoropolymer matrices; this facilitates the diffusion of monomers into the polymer phase, thus increasing the rate of propagation (bimolecular process) relative to the intramolecular chain transfer (unimolecular process). Among VDF copolymers, those that include hexafluoropropylene (HFP) have an important commercial value. The properties of the copolymer depend on the

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HFP content; when it is less than 19–20 mol% a thermoplastic and semicrystalline macromolecule is obtained, whereas for higher HFP contents the copolymers are amorphous and elastomeric. Most commercial poly(VDF-co-HFP) elastomers contain approximately 22 mol% HFP, a composition that represents the best compromise between a low Tg and a fully amorphous polymer. These materials may also be produced commercially via suspension and emulsion polymerization in water. Both processes generate large quantities of wastewater, and have high energy costs for drying of the polymer. Moreover, ammonium perfluoro-octanoate surfactants, which are environmentally suspect, are frequently adopted. In the case of copolymers, consideration must be taken of the effect of chain composition on the solubility of the macromolecules in scCO2. For poly(VDF-coHFP), it has been proposed that the copolymer is insoluble in the polymerization medium when the HFP content is low, but it becomes soluble at a high HFP content [106]. Interestingly, the composition of the copolymer can be controlled by altering the monomer feed composition. For example, when VDF and HFP are copolymerized, two types of propagating macroradicals are possible – one with VDF at the propagating end, and the other with HFP. The reactivity ratios were estimated by performing batch polymerizations in scCO2 at low values of monomer conversion, and analyzing the copolymer composition with 19F NMR. The HFP was found to undergo cross-propagation with VDF, while the latter self-propagated randomly and also cross-propagated with HFP [107, 108]. It was also noted that the copolymer was always less rich in HFP with respect to the feed composition. From an industrial viewpoint, it is interesting to note that poly(VDF-co-HFP) can be synthesized via a surfactant-free copolymerization in scCO2, and would have properties similar to those of the copolymers obtained via an aqueous emulsion copolymerization. Advantageously, however, this approach would avoid wastewater streams, reduce the energy costs (no drying step is required), and also improve the chemical stability of the polymers due to the absence of reactive acid fluoride end groups. 2.5.2 Step Polymerizations: Synthesis of Biodegradable Polymers

Although the main body research into polymerization in scCO2 has been focused on chain free radical polymerization processes, step growth polymerizations have also been object of much investigation. In these processes, low-molecular-weight molecules are often coproduced that must be removed in order to shift the equilibrium towards the synthesis of high-molar-mass polymers. However, the increase in viscosity that accompanies the growth of the chains also lowers the diffusivity of such small molecules, which makes the process mass transfer-controlled. But, scCO2 can be used to overcome this drawback since, by plasticizing the polymer phase, it induces an acceleration of the kinetics of diffusion and of the mobility of the polymer chain in the melts. Within the frame of these processes, it is interesting to note that scCO2 appears to serve as an effective dispersion medium for the synthesis of biodegradable poly-

Acknowledgments

mers such as poly(D,L-lactide-co-glycolide) and poly(ε-caprolactone). These macromolecular materials can be used in biomedical applications, an example being the preparation of a “scaffold” for tissue growth. However, given their end-use, it is crucial that these materials are free from any residues of the catalyst, monomer, and solvent. In this regard, scCO2 may represent a valuable solution, as it can be used not only as a nontoxic dispersion medium for the heterogeneous ringopening polymerization (ROP) of lactide, glycolide and ε-caprolactone, but also as an extraction solvent to recover any unreacted monomers. Some interesting results have been reported for these polymerization processes, both in the absence [109, 110] and presence of a suitable surfactant [111–114]. Apart from potentially toxic Lewis acid catalysts, enzymes have also been used successfully to perform the ROP of ε-caprolactone [115].

2.6 Conclusions

The benefits that scCO2 offers as a reaction medium have been the object of intense research activity, and it is foreseeable that this field of study will continue to improve. This suggestion is justified by the fact that scCO2 demonstrates both favorable physico-chemical properties (tunability of the chemico-physical properties, complete miscibility with permanent gases, plasticizing effect of polymers, inertness to free chain reactions) and positive technical and environmental qualities (naturally occurring and abundant, inexpensive, nontoxic, nonflammable). In many cases, scCO2 may offer not only an easier separation of the product but also an improved safety profile for a process; examples of this are the oxidation reactions or polymerization of TFE. At present, the majority of these research studies are still confined to the laboratory scale, with transfers to the industrial scale – such as the DuPont process for the synthesis of fluorinated polymers in scCO2, or the Thomas Swan plant for continuous reactions in dense CO2 – still quite rare. Whilst this diversity is partly due to the natural diffidence that envelops a new technology, it is also due to the fact that, in many cases, the environmental benefits imparted by the use of CO2 are not accompanied by clear technical– economical advantages, which are the main driving forces of industry. Yet, it is clear that a more integrated interdisciplinary cooperation among chemists, engineers, and biologists could play an important role in the development and promotion of more promising processes utilizing scCO2.

Acknowledgments

Part of the research reported in this chapter was supported by the Ministero dell’Istruzione, dell’Università e della Ricerca (MIUR) and by the Università di Palermo.

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3 Autotrophic Carbon Fixation in Biology: Pathways, Rules, and Speculations Ivan A. Berg, Daniel Kockelkorn, W. Hugo Ramos-Vera, Rafael Say, Jan Zarzycki, and Georg Fuchs

3.1 Introduction

Animals, fungi, and many microbes, that require organic compounds as a carbon source are referred to as heterotrophs. They oxidize such compounds to carbon dioxide. Autotrophs (= self-nourishing organisms), in contrast, are able to assimilate inorganic carbon to build all cell constituents, and are responsible for the primary production on Earth. Autotrophic carbon fixation, therefore, is essential for life, representing the most important synthetic process in biology (2 × 1011 tons of CO2 are fixed annually). This capability is a characteristic feature of prokaryotes, and only by endosymbiosis was it conferred on eukaryotic cells [1]. Chloroplasts, as well as many autotrophic bacteria, use the Calvin–Benson– Bassham (CBB) cycle for CO2 fixation [2]. The later discovery of a second autotrophic pathway, the reductive citric acid cycle [3], was unexpected and for a long time this was not accepted by the scientific community as it seemingly contradicted a biological dogma of that time, namely the biochemical unity of life. Yet, today, four other fundamentally different autotrophic processes have been identified in prokaryotes that allow cellular building blocks to be synthesized from CO2. In general terms, the endergonic assimilation of CO2 into cellular building blocks requires reducing equivalents and an input of energy. It can be formulated as: CO2 + 4 [H] + nATP  (CH2O) + H2O + nADP + nPi

(3.1)

where (CH2O) represents cell carbon at an average oxidation state zero, as in carbohydrates. Reducing power is provided by inorganic compounds such as water, hydrogen gas, reduced sulfur compounds, or ammonia. Likewise, energy is provided by photosynthesis or by respiration – that is, by the exergonic reduction of oxidized inorganic compounds such as oxygen, nitrate, or sulfate that function as “terminal electron acceptors” in respiration. Therefore, autotrophic CO2 fixation occurs both in the light and in the dark, and in both aerobic and anaerobic environments. Carbon Dioxide as Chemical Feedstock. Edited by Michele Aresta Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32475-0

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3 Autotrophic Carbon Fixation in Biology: Pathways, Rules, and Speculations

The usage of energy produced through the oxidation of inorganic compounds is termed chemosynthesis; this is in contrast to photosynthesis, where light energy is used. Although, nowadays, phototrophic green plants are responsible for the main primary production, they were preceded by chemotrophs in evolution, and the niches where chemoautotrophs are responsible for primary CO2 production are relics today. An example of such an ecosystem is the deep-sea vents on the dark seafloor, where the energy and carbon sources, mainly H2, H2S, CO and CO2, are supplied by magma degassing and from high-temperature reactions between seawater and rocks [4]. Such ecosystems are ultimately driven by the energy of sunlight, because the energy-producing respiration processes of the organisms depend on oxidized inorganic compounds (O2 or sulfate). These electron acceptors of respiration are the products of oxygenic photosynthesis (O2), or of the oxidation of reduced sulfur compounds with photosynthetically produced oxygen. The question is therefore, what are the principal requirements of an autotrophic carbon-fixation mechanism? An organic molecule serves as a CO2 acceptor molecule, which becomes carboxylated by a carboxylase enzyme. This CO2 acceptor molecule needs to be regenerated in a reductive autocatalytic cycle. The product that can be drained off from such a metabolic cycle should be a central cellular metabolite, from which all cellular building blocks for polymers can be derived; examples of such central metabolites are acetyl-CoA, pyruvate, oxaloacetate, 2-oxoglutarate, phosphoenolpyruvate, and 3-phosphoglycerate. Importantly, the intermediates should not be toxic to the cell. The irreversible steps of the pathway are driven by ATP hydrolysis, while the reduction steps are driven by low-potential reduced coenzymes. This chapter summarizes the current knowledge of CO2 fixation pathways, and emphasizes the characteristic features that determine their distribution in the modern biosphere.

3.2 The Mechanisms of CO2 Fixation

Presently, six CO2 fixation pathways are known; these are listed and described in the order of their discovery. 3.2.1 The Calvin–Benson–Bassham (CBB) Cycle

This cycle represents the quantitatively most important CO2 fixation pathway in Nature. It is found in most aerobic autotrophic organisms, ranging from diverse photosynthetic and chemolithoautotrophic bacteria to chloroplasts of eukaryotic algae and higher plants [5]. It is centered around carbohydrates, with ribulose 1,5-bisphosphate being the CO2 acceptor (Figure 3.1). CO2 (and oxygen; see below) adds to C2 of this molecule in its enediol form. Basically, only two key enzymes are required in addition to the enzymatic outfit of most cells, namely the carboxylase ribulose 1,5-bisphosphate carboxylase/oxy-

3.2 The Mechanisms of CO2 Fixation

CH2OH C O 5 x Glyceraldehyde 3-phosphate

CHOH

Glyceraldehyde 3-phosphate

CHOH CH2O P 3 x Ribulose-5-phosphate 3 ATP

6 ADP + 6 Pi

1 6 NADPH

3 ADP

COOH

CH2O P

CHOH

C O

CH2O P

CHOH

6 x 3-Phosphoglycerate

CHOH CH2O P

6 ATP

3 CO2

2

3 x Ribulose 1,5-bisphosphate 3 H2O

Figure 3.1 The Calvin–Benson–Bassham (CBB) cycle. 1,5-bisphosphate carboxylase/oxygenase (RubisCO).

, phosphoribulokinase;

, ribulose

genase (RubisCO), and phosphoribulokinase that is required for regeneration of the CO2 acceptor. Ribulose 1,5-bisphosphate carboxylation and hydrolysis of the resulting C6 intermediate leads to two molecules of 3-phosphoglycerate, which are further reduced with NAD(P)H to glyceraldehyde 3-phosphate, driven by ATP hydrolysis. The regeneration part of the cycle consists of the interconversion of triosephosphates via various sugar phosphates to ribulose 5-phosphate, which is phosphorylated by phosphoribulokinase to ribulose 1,5-bisphosphate. Three rounds of the cycle generate one molecule of triosephosphate. The energy demand of the CBB cycle is high, since nine ATP are required to form one triosephosphate molecule from three molecules of CO2 (see Table 3.1 for a comparison with other cycles). The key CBB cycle enzyme, RubisCO, is the most abundant protein in the world [8], as it can comprise up to 50% of the total soluble protein in the chloroplasts or in bacteria using this cycle. This fact is a consequence of the notorious catalytic inefficiency of RubisCO, that is, a low affinity for CO2, a slow catalytic turnover rate, and a wasteful oxygenase side reaction responsible for photorespiration, resulting in a futile cleavage of the substrate to form phosphoglycolate as a side product. However, the CBB cycle enzymes are oxygen-insensitive and can easily be controlled, because the whole pathway is separated from

35

Comparison of autotrophic CO2-fixation pathways.

Pathway

Amount of ATP for synthesis of one triose phosphate

CO2-fixing enzymes

Active “CO2” species

CO2 fixation products which may be used for biosynthesis

Key enzymes 36

Reductants for synthesis of one triose phosphate

Calvin–Benson–Bassham cycle (reductive pentose phosphate cycle)

9

6 NAD(P)H

RubisCO

CO2

3-Phosphoglycerate

RubisCO; Phosphoribulokinase

Reductive citric acid cycle

5

3 NAD(P)H, 1 unknown donora, 2 ferredoxin

2-Oxoglutarate synthase; Isocitrate dehydrogenaseb; Pyruvate synthase; PEP carboxylase

CO2; CO2; CO2; HCO3−

Acetyl-CoA, pyruvate, PEP, oxaloacetate, succinyl-CoA, 2-oxoglutarate

2-Oxoglutarate synthase, ATP-citrate lyase

Reductive acetyl-CoA pathway

4–5

3 NAD(P)H, 2–3 ferredoxin, 1 H2 (in methanogens)

Acetyl-CoA synthase/ CO dehydrogenase, formate dehydrogenase, pyruvate synthase

CO2

Acetyl-CoA, pyruvate

Acetyl-CoA synthase/CO dehydrogenase, enzymes reducing CO2 to methyltetrahydropterin

7 NAD(P)H, but 1 FAD is reduced in the cycle

Acetyl-CoA/propionylCoA carboxylase

HCO3−

Acetyl-CoA, pyruvate, succinyl-CoA

Malonyl-CoA reductase, propionyl-CoA synthase, malyl-CoA lyase

3-Hydroxypropionate/ malyl-CoA cycle

10

3-Hydroxypropionate/ 4-hydroxybutyrate cycle

9

6 NAD(P)H

Acetyl-CoA/propionylCoA carboxylase

HCO3−

Acetyl-CoA, succinyl-CoA

Acetyl-CoA/propionyl-CoA carboxylasec, enzymes reducing malonyl-CoA to propionyl-CoA, methylmalonyl-CoA mutasec, 4-hydroxybutyryl-CoA dehydratase

Dicarboxylate/ 4-hydroxybutyrate cycled

8

2 NAD(P)H, 3 ferredoxin, 1 unknown donor

Pyruvate synthase, PEP carboxylase

CO2; HCO3−

Acetyl-CoA, pyruvate, PEP, oxaloacetate, succinyl-CoA

4-Hydroxybutyryl-CoA dehydratase

a) b) c) d)

NADH in Hydrogenobacter thermophilus [6]. Biotin-dependent 2-oxoglutarate carboxylase in Hydrogenobacter thermophilus [7]. The presence of these enzymes is usual for Bacteria, but not Archaea, where this pathway was discovered. As studied in Ignicoccus hospitalis.

3 Autotrophic Carbon Fixation in Biology: Pathways, Rules, and Speculations

Table 3.1

3.2 The Mechanisms of CO2 Fixation

most of the central metabolism of the cell, and sugar phosphates are common metabolites. Up to now, four forms of RubisCO have been recognized, with forms I–III being bona fide RubisCO, and form IV referred to as RubisCO-like protein (RLP). First identified in the phototrophic green sulfur bacterium Chlorobium tepidum [9], RLPs are found in a wide variety of prokaryotes. Although RLPs are structurally related to the bona fide RubisCO, they do not function as RubisCO enzymes but catalyze a reaction in sulfur metabolism [5]. Form III RubisCO is confined to Archaea, and its role in their metabolism is at issue [5, 10–12]. Interestingly, RubisCO was found in many Archaea that were not reported to be able to grow autotrophically. On the contrary, RubisCO has also been found in Archaea that grow autotrophically but are considered to use the reductive acetyl-CoA pathway for CO2 fixation. Hence, they contain at the same time RubisCO activity and the key enzyme activities of the reductive acetyl-CoA pathway. Notably, phosphoribulokinase, the second key enzyme of the CBB cycle, was not detected in Archaea, and the cycle cannot be closed yet. Although, the possibility to synthesize the substrate of RubisCO, ribulose 1,5-bisphosphate, was shown in some Archaea containing RubisCO form III [10, 11], such a pathway cannot be closed in a cycle, and the understanding of the function of this enzyme is still elusive; however, its participation in AMP metabolism has been proposed recently [11]. The finding that genomes of three methanogens contain a gene for phosphoribulokinase in addition to a form III RubisCO gene [12] reinforces the possibility that the CBB cycle may function in some Archaea in addition to the reductive acetyl-CoA pathway. A recent phylogenetic analysis showed a possible archaeal origin of both RubisCO and RLP, with form III proteins from Methanomicrobia being the likely precursors of all modern RubisCO and RLP lineages [5]. This is in agreement with the proposed late appearance of the CBB cycle in evolution [13–15]. Its immediate success is not obvious, when considering the adverse characteristics of its key enzyme, RubisCO. However, the oxygen-insensitivity of this carboxylase may have been decisive, despite its oxygenase side activity. The CBB cycle likely evolved before the oxygen concentration in the atmosphere increased to modern high values. 3.2.2 The Reductive Citric Acid Cycle (Arnon–Buchanan Cycle)

This cyclic pathway was first proposed for Chlorobium sp. that use an anoxygenic photosynthesis for energy supply [3]. It reverses the reactions of the oxidative citric acid cycle (Krebs cycle) and forms acetyl-CoA from two CO2 (Figure 3.2). Three modifications of the conventional oxidative citric acid cycle are needed, which substitute irreversible enzyme steps. Succinate dehydrogenase is replaced by fumarate reductase, 2-oxoglutarate dehydrogenase by ferredoxin-dependent 2-oxoglutarate oxidoreductase (2-oxoglutarate synthase), and citrate synthase by ATP-citrate lyase [3, 16]; it should be noted that the carboxylases of the cycle catalyze the reductive carboxylation reactions. There are variants of the ATP-driven cleavage of citrate as well as of isocitrate formation [7]. The reductive citric acid

37

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3 Autotrophic Carbon Fixation in Biology: Pathways, Rules, and Speculations

CO2

COOH C

O 2 Fdred

CH3

4 COSCoA CH3 Acetyl-CoA CoA

Pyruvate

COOH COOH

ATP

ADP+Pi

C O

1

CH2 HO

C COOH

CH2

CH2

COOH

COOH

Oxaloacetate

Citrate

COOH CHOH

COSCoA

HC COOH

CH2 CH2 COOH Succinyl-CoA

COOH

CO2

2

C O

CO2

CH2 2 Fdred CoA

3

CH2 COOH Isocitrate

NAD(P)H

CH2 COOH 2-Oxoglutarate

Figure 3.2 Reductive citric acid cycle. , ATP-citrate lyase; , 2-oxoglutarate : ferredoxin oxidoreductase (2-oxoglutarate synthase); , isocitrate dehydrogenase; , pyruvate : ferredoxin oxidoreductase (pyruvate synthase). Fdred = reduced ferredoxin.

cycle (in Chlorobium) requires five ATP to form triosephosphates from three CO2, and depends on oxygen-sensitive carboxylases that use reduced ferredoxin as the electron donor for the reductive carboxylation of the CoA thioesters of acetate and succinate (see Table 3.1). This small iron–sulfur protein is also oxygen-sensitive. The pathway may be used even in the reversed oxidative direction for the oxidation of acetyl-CoA in one and the same organisms (Desulfobacter sp.) [17]. The use of ATP citrate lyase reaction (Equation 3.2), instead of the irreversible citrate synthase reaction, allows the generation of a second molecule of ATP in the oxidative cycle, in addition to ATP synthesis via succinate thiokinase. It is not known, which mechanism determines the direction of the cycle in Desulfobacter – that is, whether a compound is preferentially oxidized or assimilated. Acetyl-CoA + Oxaloacetate + ADP + Pi → Citrate + CoA + ATP

(3.2)

This pathway involves oxygen-sensitive enzymes and electron carriers, and therefore is found in anaerobic or microaerobic bacteria, such as anaerobic sulfate reducers

3.2 The Mechanisms of CO2 Fixation

(e.g., Desulfobacter sp., Deltaproteobacteria), microaerobic sulfur oxidizers (Epsilonproteobacteria), anaerobic phototrophic green sulfur bacteria (Chlorobium), and hydrogen-oxidizing microaerobic Aquificales (Aquifex, Hydrogenobacter) [3, 7, 18, 19]. The functioning of this cycle in anaerobic Crenarchaeota (e.g., Thermoproteus sp.) [20] is now questioned, as all enzymes of the dicarboxylate/4-hydroxybutyrate cycle have been found there (see below). The low ATP requirement, as well as the moderate oxygen sensitivity of some of its enzymes, makes the reductive citric acid cycle a suitable pathway mainly for anaerobes, but also for some microaerobes. 3.2.3 The Reductive Acetyl-CoA Pathway (Wood–Ljungdahl Pathway)

This is a noncyclic pathway that also results in the fixation of two molecules of CO2 to form acetyl-CoA. It was elucidated by Wood, Ljungdahl, Thauer and others as a pathway which is used by acetogenic bacteria to synthesize acetate from CO2 in their energy metabolism [21]. The acetyl-CoA pathway resembles the Monsanto process of acetate synthesis from CO and methanol, with one molecule of CO2 being reduced to the level of methyltetrahydropterin, while another CO2 molecule is reduced to the level of carbon monoxide in the reaction catalyzed by the nickeldependent carbon monoxide dehydrogenase (Figure 3.3). This enzyme also acts as acetyl-CoA synthase, by accepting the methyl group from a methylated corrinoid protein (the methyl group being derived from methyltetrahydropterin), combining it with the carbon monoxide group to form an enzyme-bound acetyl group, and then releasing this group thiolytically with coenzyme A to form acetyl-CoA. This key enzyme CO dehydrogenase/acetyl-CoA synthase has probably common roots in all prokaryotes, in contrast to those enzymes involved in the formation of methyltetrahydropterin from CO2. There are many variants of the pathway which differ in the use of coenzymes and electron carriers; the pathway can also be reversed and used for the oxidation of acetyl-CoA, instead of the citric acid cycle [22]. The reductive acetyl-CoA pathway is unique in several aspects. For example, the pathway makes extensive use of coenzymes (tetrahydropterin, cobalamin) and of

6 [H]

CO2

[CH3 ] 2 H2O 2 [H]

CO2

1

1

CH3 COSCoA

CoASH

[CO ] H2O

Acetyl-CoA

CO2

2 Fdred

2

CH3 CO COOH Pyruvate

Figure 3.3 Reductive acetyl-CoA pathway. , pyruvate : ferredoxin oxidoreductase.

, CO dehydrogenase/acetyl-CoA-synthase;

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3 Autotrophic Carbon Fixation in Biology: Pathways, Rules, and Speculations

metals (Mo or W, Co, Ni, Fe–S centers). It also enables CO, formaldehyde, methanol, methylamine, or methylmercaptane to be assimilated. Many of the onecarbon units react spontaneously with the cofactors and prosthetic groups of this pathway. This pathway depends on strict anoxic conditions, as some of its enzymes – notably CO dehydrogenase/acetyl-CoA synthase – are highly oxygensensitive. It also requires the above-mentioned metals, which are water-soluble preferentially in the reduced oxidation state – that is, under anoxic conditions. The process can be simulated in the laboratory to produce not only acetylthioesters but also derived products by simply incubating CO, H2 and H2S or methylmercaptane with Ni and Fe salts; these inorganic metals form mixed Ni–Fesulfides that act as catalysts [23, 24]. Among the CO2 fixation pathways, this pathway has the lowest energetic costs, requiring only four ATPs per triose phosphate (see Table 3.1). However, its requirements for metals, cofactors, anaerobiosis are among the highest, which causes it to be restricted to a limited number of ecological niches. The reductive acetyl-CoA pathway is the favored autotrophic pathway in strict anaerobes. Notably, it is found in those bacteria that encounter traces of the abovementioned gases and one-carbon compounds in their environment. Acetyl-CoA is assimilated into cell material as described for the reductive citric acid cycle, and can simultaneously serve as an energy source for ATP synthesis during its conversion to acetate. This pathway is found in acetogenic bacteria and methanogenic Archaea, in sulfate-reducing bacteria (Desulfobacterium sp. (Deltaproteobacteria), Desulfotomaculum sp. (relatives of Clostridia), Archaeoglobus (Euryarchaeota)), as well as in anaerobic ammonia-oxidizing bacteria [13, 25, 26]. 3.2.4 The 3-Hydroxypropionate/Malyl-CoA Cycle

This pathway results in the fixation of three molecules of bicarbonate, and forms pyruvate as the central carbon precursor molecule. The main CO2-fixing enzyme is acetyl-CoA/propionyl-CoA carboxylase. The pathway can be divided into two metabolic cycles (Figure 3.4). In the first cycle, acetyl-CoA is carboxylated to malonyl-CoA, which is subsequently reduced and converted into propionyl-CoA via 3-hydroxypropionate as a free intermediate. Propionyl-CoA is carboxylated to methylmalonyl-CoA, which is subsequently converted to succinyl-CoA; the latter is then used to activate L-malate by succinylCoA:L-malate coenzyme A transferase, which forms L-malyl-CoA and succinate. Succinate is oxidized to L-malate via conventional steps. L-Malyl-CoA, the second characteristic intermediate of this cycle, is cleaved by L-malyl-CoA/β-methylmalylCoA lyase, thus regenerating the starting molecule acetyl-CoA and releasing glyoxylate as a first carbon-fixation product [27]. Glyoxylate is an unconventional cell carbon precursor that requires special enzymes to be used in its biosynthesis; a second cycle serves as the glyoxylate assimilation pathway [28]. Glyoxylate is combined with propionyl-CoA to β-

3.2 The Mechanisms of CO2 Fixation Glyoxylate

41

O

H

(S)-Malyl-CoA CoA-S

Pyruvate

CoA-S

COOH O

COOH

H3C

COOH O CH3

O

2 Acetyl-CoA O

OH

2 HCO3¯ CoA-S

OH

(S)-Malate H2O

CoA-S

Succinyl-CoA 2 Propionyl-CoA (S)-Methylmalonyl-CoA

CH3

1st Cycle

HCO3¯

S-CoA O

H2O CH3

HOOC

S-CoA

ADP + Pi ATP

COOH

CH3

Mesaconyl-C1-CoA

S-CoA OH

O

O

CH3

Mesaconyl-C4-CoA

2 CoA 2 NADPH + 2 H+ + 2 ATP 2 NADP+ + 2 AMP + 2 PPi

S-CoA

HOOC

O

HOOC

2 3-Hydroxypropionate

CH3

H2O

COOH

HO COOH

OH

CoA-S

CoA-S

4 NADPH + 4 H+ 4 NADP+ 2 CoA

2 [H] O

O

COOH

2 Malonyl-CoAO

CoA

OH

2 ATP 2 ADP + 2 Pi

COOH

HOOC

(S)-Citramalyl-CoA H3C

O

β-Methylmalyl-CoA

H HOOC O

Glyoxylate

2nd Cycle

Figure 3.4 3-Hydroxypropionate/malyl-CoA cycle, as studied and proposed in Chloroflexus aurantiacus.

methylmalyl-CoA, which in turn is converted via mesaconyl-CoA to citramalylCoA. The latter is cleaved into pyruvate and acetyl-CoA. Acetyl-CoA conversion to propionyl-CoA occurs as described above, closing the second, glyoxylate assimilation cycle. The distribution of this pathway is quite limited; to date, it has only been observed in the phototrophic bacterium Chloroflexus aurantiacus, a member of the green nonsulfur bacteria [27]. Although the genomes of closely related bacteria, Chloroflexus aggregans and Roseiflexus spp., contain the genes for this pathway [29], they appear to be unable to grow autotrophically. Interestingly, another green nonsulfur bacterium, Oscillochloris sp., uses the CBB cycle for autotrophy [30]. It seems reasonable to suggest that the 3-hydroxypropionate/malyl-CoA cycle evolved in the Chloroflexaceae, and has close connections with heterotrophic metabolism. The cycle allows mixotrophic growth by assimilating fermentation products such as acetate, propionate, or succinate, which makes it suitable for bacteria living in habitats where such fermentation products may occur. Indeed, the preferred growth mode of Chloroflexus sp. is photoheterotrophy.

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3 Autotrophic Carbon Fixation in Biology: Pathways, Rules, and Speculations

An interesting feature of this cycle is the presence of a number of bi-/multifunctional enzymes: bifuctional malonyl-CoA reductase, which catalyzes two consecutive reductase steps leading to 3-hydroxypropionate; trifunctional propionyl-CoA synthase, a fusion protein converting 3-hydroxypropionate to propionyl-CoA in three steps; and trifunctional malyl-CoA/β-methylmalyl-CoA/citramalyl-CoA lyase, which not only cleaves L-malyl-CoA into acetyl-CoA and glyoxylate (last reaction of the first cycle, Figure 3.4), but also synthesizes β-methylmalyl-CoA from glyoxylate and propionyl-CoA and splits L-citramalyl-CoA into pyruvate and acetylCoA, thus opening and closing the glyoxylate assimilation cycle [31–33]. The energy costs of the 3-hydroxypropionate/malyl-CoA cycle are high, with ten ATP required per triose phosphate (see Table 3.1). However, bicarbonate rather than CO2 is the actual inorganic carbon species used by acetyl-CoA/propionyl-CoA carboxylase (this is discussed in Chapter 4). Moreover, as this enzyme is virtually irreversible and has a high affinity for bicarbonate, this cycle is expensive although kinetically effective. 3.2.5 The 3-Hydroxypropionate/4-Hydroxybutyrate Cycle

In this cycle, one molecule of acetyl-CoA is formed from two molecules of bicarbonate (Figure 3.5). The key carboxylating enzyme is the bifunctional biotindependent acetyl-CoA/propionyl-CoA carboxylase. In Bacteria and Eukarya, acetyl-CoA carboxylase catalyzes the first step of fatty acid biosynthesis. However, Archaea do not contain fatty acids in their lipids, and acetyl-CoA carboxylase cannot serve as the key enzyme of fatty acid synthesis; rather, it is responsible for autotrophy. The product of acetyl-CoA carboxylase reaction, malonyl-CoA, is reduced via malonate semialdehyde to 3-hydroxypropionate, which is further reductively converted to propionyl-CoA. Propionyl-CoA is carboxylated to (S)-methylmalonyl-CoA by the same carboxylase. (S)-Methylmalonyl-CoA is isomerized to (R)-methylmalonyl-CoA, followed by carbon rearrangement to succinyl-CoA by coenzyme B12dependent methylmalonyl-CoA mutase. Succinyl-CoA is further reduced to succinate semialdehyde and then to 4-hydroxybutyrate. The latter compound is converted into two acetyl-CoA molecules via 4-hydroxybutyryl-CoA dehydratase, a key enzyme of the pathway. 4-Hydroxybutyryl-CoA dehydratase is a [4Fe-4S] cluster and FAD-containing enzyme that catalyzes the elimination of water from 4-hydroxybutyryl-CoA by a ketyl radical mechanism to yield crotonyl-CoA [34]. Conversion of the latter into two molecules of acetyl-CoA proceeds via normal βoxidation steps. Hence, the 3-hydroxypropionate/4-hydroxybutyrate cycle (as illustrated in Figure 3.5) can be divided into two parts. In the first part, acetyl-CoA and two bicarbonate molecules are transformed to succinyl-CoA, while in the second part succinyl-CoA is converted to two acetyl-CoA molecules. The 3-hydroxypropionate/4-hydroxybutyrate cycle functions in autotrophic Sulfolobales (Crenarchaeota) [35–37]. These are extreme thermoacidophiles from volcanic areas which grow best at a pH of about 2 and temperatures of 60 to 90 °C.

3.2 The Mechanisms of CO2 Fixation

43

CoAS

acetyl-CoA

O

HCO3

ADP+Pi

CoAS acetoacetyl-CoA

CoAS NADH+H NAD

acetyl-CoA CoASH

+

+

O

malonyl-CoA

O ATP

CoAS

O

O O

CoAS

NADPH+H+

OH

NADP++CoASH

malonate semialdehyde

O

O

OH

(S)-3-hydroxybutyryl-CoA

NADPH+H+

O

H2O

OH NADP

CoAS

3-hydroxypropionate

+

O

O

crotonyl-CoA

OH OH ATP+CoASH

H2 O

OH

AMP+PPi

CoAS O

3-hydroxypropionyl-CoA

4-hydroxybutyryl-CoA

O

OH

AMP+PPi ATP+CoASH

acryloyl-CoA

HO 4-hydroxybutyrate NADP

SCoA

+

NADPH+H+

H 2O

O

O

NADPH+H+

O HO O

NADP+

propionyl-CoA

succinic semialdehyde

NADP

O

ATP +

NADPH+H+

O HO

SCoA O succinyl-CoA

O

SCoA

OH O

HCO3

-

ADP+Pi

SCoA

(S)-methylmalonyl-CoA

Figure 3.5

SCoA

OH

3-Hydroxypropionate/4-hydroxybutyrate cycle, as studied in Metallosphaera sedula.

Most Sulfolobales can grow chemoautotrophically by oxidizing sulfur, pyrite, or hydrogen under microaerobic conditions. This cycle was also proposed for the mesophilic ammonia-oxidizing symbiotic Cenarchaeum symbiosum, a member of the “marine group”-1 Crenarchaeota which are abundant in the sea [37]. The enzymes of the 3-hydroxypropionate/4-hydroxybutyrate cycle are oxygen-tolerant. Although 4-hydroxybutyryl-CoA dehydratase is inactivated by oxygen, it may be sufficiently stable at low oxygen tensions to maintain activity, especially in the protected environment of the cell. Therefore, the 3-hydroxypropionate/4-hydroxybutyrate cycle fits well to the lifestyle of (micro)aerobic Crenarchaeota. The active “CO2” species in the 3-hydroxypropionate/4-hydroxybutyrate cycle is bicarbonate (see Table 3.1). The use of bicarbonate as a substrate may be advantageous for organisms using this cycle in comparison with, for example, the CBB

44

3 Autotrophic Carbon Fixation in Biology: Pathways, Rules, and Speculations

cycle. Metallosphaera sedula acetyl-CoA/propionyl-CoA carboxylase has a turnover number at 65 °C of 28 s−1 [38]. The Km value of RubisCO for dissolved CO2 (average 0.05 mM, but it may be as high as 0.34 mM in some marine cyanobacteria [39]) is lower than or even comparable to the apparent Km value of archaeal acetyl-CoA/ propionyl-CoA carboxylase for bicarbonate (0.3 mM) [38]. However, it should be noted that the CO2 concentration in water equilibrated with air (1 atm pressure, 20 °C, 370 ppm CO2) is 0.012 mM at pH 7.4 (assumed intracellular pH), and that of bicarbonate is 0.26 mM. The concentration of bicarbonate will be even higher at pH 7.8–8.2 of sea water (pKapp. CO2 HCO3− = 6.3, 20 °C). Thus, the acetyl-CoA carboxylase affinity for the active “CO2” species appears not to be inferior to that of RubisCO. In addition, in volcanic gases and in animal hosts, the CO2 concentration is much higher than in air. It should also be noted that carboxyphosphate, which is used in these biotin-dependent carboxylase reactions, is an attractive model for carbon fixation during chemoevolution. Although the 3-hydroxypropionate part of this cycle resembles the first part of the 3-hydroxypropionate/malyl-CoA cycle functioning in Chloroflexus, the enzymes used to synthesize propionyl-CoA from malonyl-CoA are not homologous, although the intermediates are the same [31, 32, 37]. Therefore, this pathway appears to have evolved independently in Sulfolobales and in the Chloroflexaceae, representing an interesting example of convergent evolution. The energy requirement of the 3-hydroxypropionate/4-hydroxybutyrate cycle is nine ATPs for one triosephosphate synthesized (see Table 3.1). 3.2.6 The Dicarboxylate/4-Hydroxybutyrate Cycle

This cycle resembles the 3-hydroxypropionate/4-hydroxybutyrate cycle, but with pyruvate : ferredoxin oxidoreductase (pyruvate synthase) and phosphoenolpyruvate (PEP) carboxylase as the carboxylating enzymes (Figure 3.6). The dicarboxylate/4-hydroxybutyrate cycle starts from acetyl-CoA, which is reductively carboxylated to pyruvate. Pyruvate is converted to PEP and then carboxylated to oxaloacetate. The latter is reduced to succinyl-CoA by the reactions of an incomplete reductive citric acid cycle. Succinyl-CoA is reduced to 4-hydroxybutyrate, the subsequent conversion of which into two acetyl-CoA molecules proceeds in the same way as in the 3-hydroxypropionate/4-hydroxybutyrate cycle. The cycle can be divided into part 1 transforming acetyl-CoA, one CO2 and one bicarbonate to succinyl-CoA via pyruvate, PEP, and oxaloacetate, and part 2 converting succinyl-CoA via 4-hydroxybutyrate into two molecules of acetyl-CoA. This cycle was shown to function in Ignicoccus hospitalis, an anaerobic autotrophic hyperthermophilic Archaeum (Desulfurococcales) [40]. Moreover, this pathway functions in Thermoproteus neutrophilus (Thermoproteales), where the reductive citric acid cycle was earlier assumed to operate, but was later disproved (W.H. Ramos-Vera et al., unpublished results). The active “CO2” species in the dicarboxylate/4-hydroxybutyrate cycle are CO2 as cosubstrate for pyruvate synthase, and bicarbonate as cosubstrate for PEP

3.2 The Mechanisms of CO2 Fixation

45

CoAS

acetyl-CoA

O

CO2 CoAS

2 Fdox+CoASH

O

acetyl-CoA

CoAS

2 Fdred

CoASH

NADH+H+

O NAD

HO

O

O

O

pyruvate

ATP+H2O

acetoacetyl-CoA

+

Pi+AMP

HO

CoAS O

O

O

OH

(S)-3-hydroxybutyryl-CoA

P

PEP

-

HCO3 Pi

O

H2 O

O

HO

CoAS crotonyl-CoA

OH

O

O

oxaloacetate NADH+H+

H2 O NAD

OH

OH O

CoAS O

HO

4-hydroxybutyryl-CoA

H2 O

O

OH HO

HO

fumarate

4-hydroxybutyrate

O

O NAD(P)+

O

O

NAD(P)H+H+

HO

HO O

succinic semialdehyde

HO

2 MVox

OH

O succinate SCoA

O succinyl-CoA

OH

2 MVred

O 2 MVox+CoASH 2 MVred

Figure 3.6

OH

(S)-malate

O

AMP+PPi ATP+CoASH

+

ATP+CoASH ADP+Pi

Dicarboxylate/4-hydroxybutyrate cycle, as studied in Ignicoccus hospitalis.

carboxylase (see Table 3.1). However, as the organisms bearing this cycle live in volcanic areas with a high ambient CO2 partial pressure, the affinity of the carboxylases for CO2 or bicarbonate may be less critical than in the case of the CBB cycle. Triose phosphate formation in this cycle requires eight ATP equivalents (see Table 3.1). A comparison to the crenarchaeal 3-hydroxypropionate/ 4-hydroxybutyrate cycle reveals that the dicarboxylate/4-hydroxybutyrate cycle preferentially uses reduced ferredoxin instead of NAD(P)H as electron donor in the reduction steps. The oxygen sensitivity of some of its enzymes and electron carriers restricts this cycle to anaerobic organisms. Indeed, Thermoproteus and Ignicoccus species grow as strict anaerobes by reducing elemental sulfur with hydrogen gas.

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3 Autotrophic Carbon Fixation in Biology: Pathways, Rules, and Speculations

3.3 Rules to Explain the Diversity

The six autotrophic pathways differ in various aspects such as: the use of CO2 or bicarbonate; the affinity of the key carboxylase(s) for CO2 or bicarbonate; the redox potential of the reduction steps; the coenzymes involved and the natural inorganic substrates used for the reduction of the electron carriers; the ATP requirement; the requirements for metals (Fe, Co, Ni, Mo); the oxygen sensitivity of the enzymes; the interconnection of the carbon metabolism and the energy metabolism; the regulation mechanisms; the capability of concomitant assimilation of common low-molecular-mass compounds, such as fermentation products; the possibility to use the reverse reactions of the pathways for the complete oxidation of organic compounds; the evolutionary origin of the machinery; and the distance to the presumed chemoevolutionary scenario. These criteria determine the distribution of the pathways in autotrophic organisms in different habitats, in addition to their phylogeny. The pathways also differ in 12C/13C isotope discrimination, a fact that has long been neglected and which has an impact on the interpretation of carbon isotope fractionation data and geological records. The distribution of the CO2 fixation pathways in the different lineages of Prokaryotes depends on the individual characteristic features of the pathways and enzymes, which determine the adaptation of their bearer to its natural environment. For instance, highly oxygensensitive enzymes are useless for aerobic organisms, while pathways with high energy demands are not appropriate for strict anaerobes that live at the expense of an energy metabolism that is close to the thermodynamic equilibrium. Furthermore, organisms belonging to different phylogenetic groups have inherited a complex enzyme outfit that cannot be changed suddenly at will. Different ecological strategies may also be applied in the different niches and by the different prokaryotic groups [41]. For instance, some autotrophic microorganisms settle in environments in which the fermentation products of other microorganisms frequently occur and can be used as an additional carbon source. The efficiency of different CO2 fixation pathways is normally judged by comparing their energy demands. However, such a simple comparison is not always justified. Autotrophic CO2 fixation pathways differ in their products and intermediates, and organisms belonging to different phylogenetic groups and living in different environments have different biosynthetic requirements. Traditionally, the costs of autotrophic carbon-fixation mechanisms are judged by a comparison of how many ATP molecules are required to synthesize one molecule of triosephosphate from three molecules of inorganic carbon. However, the biosynthetic demands for triosephosphates and derived sugar phosphates differ dramatically. Plants using the CBB cycle synthesize huge amounts of sugars, whereas carbohydrates comprise just a minor fraction of archaeal cells. The direct energetic comparison of different cycles is sometimes questionable, as the reducing power of various electron donors is not the same. For example, the reduction potential of reduced ferredoxin is stronger than that of the reduced pyridine nucleotides. In growing cells, ferredoxin is often reduced by a hydroge-

3.3 Rules to Explain the Diversity

nase, but under a low hydrogen partial pressure the reduction of ferredoxin may be forced by energy-driven reverse electron transport from NAD(P)H, the costs of which are difficult to evaluate [42]. This questions the possibility of comparing the energetic efficiency of, for example, the dicarboxylate/4-hydroxybutyrate cycle and the 3-hydroxypropionate/4-hydroxybutyrate cycle (see Table 3.1). The intrinsic properties of the CO2-fixation-related enzymes may be important in terms of energetic costs of the whole pathway. A poor catalytic efficiency of RubisCO not only leads to a loss of energy, associated with photorespiration, but also pushes the organisms to synthesize special carbon-concentrating mechanisms (even C4 carbon fixation and Crassulacean acid metabolism may be regarded as carbon-concentrating mechanisms). Moreover, carboxylases with a low catalytic efficiency need to be synthesized in large amounts; in the case of RubisCO, this may represent up to one-half of the soluble cellular protein. In other words, the synthesis of the carboxylase catalyst itself may devour a huge amount of energy. Not only the synthesis of the amino acids is costly; to build in one amino acid into protein, an additional four ATP are required. The amino acid demand for the synthesis of the additional enzymes of an autotrophic carbon-fixation pathway may actually determine its energy costs. In terms of energy costs, the CBB cycle is one of the most expensive, as it requires nine ATP for the synthesis of one triosephosphate (see Table 3.1). However, most species that use this cycle are phototrophs (green plants, algae, cyanobacteria, purple bacteria); photosynthesis results in ATP production and, therefore, the bottleneck in the growth of phototrophs is normally not ATP supply, but rather the supply of phosphorus, nitrogen, or iron. Interestingly, the group of anoxygenic phototrophic bacteria adapted to low light intensities – the green sulfur bacteria [43, 44] – uses the reductive citric acid cycle for autotrophic CO2 fixation. Indeed, the energy yield of their metabolism is much lower than that of the purple sulfur bacteria that use the CBB cycle. As both groups are successful in their environments, this simply reflects different ecological strategies applied by these organisms. Recent results reported by Markert et al. [45] are in agreement with this consideration; although, normally, an organism has only one pathway for autotrophic CO2 fixation, an uncultured endosymbiont of a deep-sea tube worm uses both the reductive citric acid cycle and the CBB cycle, depending on the energy supply. In a high-energy situation the symbiont uses the CBB cycle, whereas under low-energy conditions it switches to the energetically more favorable reductive citric acid cycle. An important feature of the CBB cycle that may have ensured its success is its connection with carbohydrate metabolism. Carbohydrates are the most abundant biomolecules in plants, playing numerous roles as storage and structural compounds. The product of the CBB cycle, glyceraldehyde 3-phosphate, is a substrate of the gluconeogenetic pathway that directly connects CO2 fixation with sugar synthesis; hence, this pathway may be advantageous for those organisms requiring a plentiful supply of carbohydrates. In four autotrophic CO2-fixation cycles, succinyl-CoA plays a central role (Figure 3.7):

47

48

3 Autotrophic Carbon Fixation in Biology: Pathways, Rules, and Speculations

Acetyl-CoA

Glyoxylate

Oxaloacetate

Acetyl-CoA

HCO3CO2 HCO33-OH-Propionate

4-OH-Butyrate

Oxaloacetate 1

2

3

Citryl-CoA

5

4

CO2

HCO3Succinyl-CoA 1+4 1+3 2+5 2+4

Malyl-CoA

CO2

3-hydroxypropionate/malyl-CoA cycle 3-hydroxypropionate/4-hydroxybutyrate cycle reductive citric acid cycle dicarboxylate/4-hydroxybutyrate cycle

Figure 3.7 General scheme representing the strategy used by four different carbon dioxide-fixation pathways [40]. These pathways have in common the formation of succinylCoA from acetyl-CoA and two inorganic carbons. The vertical arrows point to the CO2-fixation products released from these

metabolic cycles. The combination of the metabolic modules 1 and 4 results in the 3-hydroxypropionate/malyl-CoA cycle; 1 and 3 = the 3-hydroxypropionate/4-hydroxybutyrate cycle; 2 and 5 = the reductive citric acid cycle; 2 and 3 = the dicarboxylate/4-hydroxybutyrate cycle.

i) In the 3-hydroxypropionate/malyl-CoA cycle, where acetyl-CoA/propionylCoA carboxylase(s) is (are) used as carboxylating enzyme(s) to form succinylCoA (route 1), from which acetyl-CoA is regenerated via malyl-CoA cleavage (route 4) and glyoxylate is the carbon-fixation product (see Figure 3.4). ii) In the 3-hydroxypropionate/4-hydroxybutyrate cycle, where route 1 is used for succinyl-CoA formation, as in (i), but acetyl-CoA regeneration proceeds via 4-hydroxybutyrate (route 3), and acetyl-CoA is the product (see Figure 3.5). iii) In the dicarboxylate/4-hydroxybutyrate cycle, where acetyl-CoA is converted to succinyl-CoA using pyruvate synthase and PEP carboxylase (route 2); acetyl-CoA regeneration via 4-hydroxybutyrate is similar to the route in Sulfolobales (route 3) (see Figure 3.6). iv) In the reductive citric acid cycle, where succinyl-CoA is formed via route 2; however, succinyl-CoA is further reductively carboxylated to 2-oxoglutarate and isocitrate and converted to citrate, which is cleaved into acetyl-CoA and oxaloacetate (route 5) (see Figure 3.2). In principle, other combinations of the five partial routes indicated in Figure 3.7 are conceivable. The individual partial routes differ not only with respect to ATP

3.4 Evolutionary Aspects

requirement, but also with respect to oxygen sensitivity of the enzymes, and the use of reduced ferredoxin instead of NAD(P)H as reductant. Routes 2 and 5 represent typical “anaerobic” pathways that are unlikely to occur in strict aerobes, whereas routes 1, 2, and 4 are (micro)aerobic pathways. For example, the autotrophic Crenarchaea studied so far use one of two pathways, either the 3-hydroxypropionate/ 4-hydroxybutyrate (routes 1 + 3) or dicarboxylate/4-hydroxybutyrate (routes 2 + 3) cycle, depending on an aerobic or anaerobic mode of growth of the corresponding species. Although the reductive citric acid cycle (routes 2 + 5) functions in some bacteria requiring oxygen for growth (Aquificales), they require not only a reduced O2 content in the gas mixture, but also a high temperature (≥70 °C), further decreasing the oxygen concentration in the medium. Whether combinations others than those discussed exist remains unknown; however, interestingly, the combination of routes 2 and 4 (dicarboxylate/malyl-CoA cycle) was formerly proposed as an autotrophic CO2 fixation pathway in Chloroflexus aurantiacus [46]. Autotrophic organisms preferentially incorporate 12CO2 (relative to 13CO2) into their biomass. This carbon isotopic fractionation cannot be achieved abiotically, and the occurrence of this effect in a specimen is an indication of its biological origin. Different CO2 fixation pathways lead to the different isotopic depletions; for example, an isotopic signature of −20 to −30‰ is typical for the CBB cycle, of −2 to −13‰ – for the reductive citric acid cycle, the depletion greater than −30‰ is a characteristic of the reductive acetyl-CoA pathway, and values of −12.5 and −13.7‰ were reported for Chloroflexus aurantiacus in which the 3-hydroxypropionate/malyl-CoA cycle is operating [30, 47–50]. The 3-hydroxypropionate/4-hydroxybutyrate cycle and the dicarboxylate/4-hydroxybutyrate cycle both exhibited similar and very low fractionations, with reported values of between −0.2 and −3.6‰ [49]. Therefore, differences in the stable isotopic content may be used to suggest a specific mechanism of CO2 fixation, and the pathways leave an isotopic signature in the geologic record of 13C-depleted sedimentary organic matter. Such results should be interpreted with great caution, however, because – as seen above – different pathways may have similar discrimination factors, and the discrimination may differ even among organisms using the same pathway. Furthermore, the contribution of different pathways leads to a mixed situation. For example, C4 and Crassulacean acid metabolism plants both have the CBB cycle, but their isotopic signatures are in the range of those for the reductive citric acid cycle. Moreover, a single amino acid substitution in the RubisCO active site can alter the discrimination significantly, from −22 to −11‰ [50], thus questioning the possibility of identifying a particular pathway in the geological records by a fractionation value.

3.4 Evolutionary Aspects

The common ancestor of life was probably a chemoautotrophic hyperthermophilic anaerobe. This “metabolism first” theory assumes that life started with catalytic metal sulfide surface/compartments in a hydrothermal-vent setting in the Hadean

49

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3 Autotrophic Carbon Fixation in Biology: Pathways, Rules, and Speculations

ocean [24, 51, 52]. According to this theory, inorganic carbon fixation proceeded on minerals and was based on transition metal sulfide catalysts. Given the structural (and catalytic) similarity between the minerals themselves and the catalytic centers of the enzymes in the acetyl-CoA pathway, an attractive idea is that minerals catalyzed a primitive acetyl-CoA pathway [15]. In fact, experimental evidence supports this idea. For example, both the thioester acetyl methylsulfide and its hydrolysis product, acetate, can be produced from CO and CH3SH by using only Fe- and Ni-sulfides as catalysts [23]. The following characters of the reductive acetyl-CoA pathway may indicate its antiquity [13]: the enzymes and electron carriers of this pathway are extremely oxygen-sensitive and can only function in strict anaerobes; the pathway occurs in both Bacteria and Archaea, and also in hyperthermophiles; it has minimal energy requirements; it allows the concomitant utilization of volatile reduced one-carbon compounds (e.g. CO, CH2O); metal sulfides (in the form of different Fe–S centers) are important for the catalysis; it requires different ubiquitous, yet varied coenzymes, which probably preceded the more complex proteins as catalysts; and – most importantly- it combines energy metabolism and carbon assimilation mechanisms. The reductive acetyl-CoA pathway shares some of these features with the reductive citric acid cycle, which may also be regarded as primordial [14]. The dicarboxylate/4-hydroxybutyrate cycle also requires various iron–sulfur proteins such as ferredoxin, as well as thioesters to facilitate chemical reactions, and fits well into a simple primordial carbonfixation scheme in an “iron–sulfur world” [24, 51]. At the same time, both phylogenetic analyses and general considerations denote the CBB cycle as a later innovation, the appearance of which might reflect the adaptation to oxygen [5, 13].

3.5 Chemical Aspects of CO2 Fixation

There are various aspects of autotrophic carbon fixation linked to chemistry: 1) The need to reduce the accumulation of CO2 into the atmosphere requires new technologies capable of reducing the CO2 emission. One possibility would be to use CO2 as a raw material for syntheses, as Nature does on an incredible large scale using different mechanisms, as shown here. On the one hand, CO2 is available in virtually unlimited amounts, yet on the other hand it is very little reactive and its use often requires the input of energy. Hence, the use of CO2 in chemical syntheses requires dream reactions and molecular-defined catalysts that are not yet available. This represents a great challenge for chemistry. 2) The use of CO2 in chemistry normally requires its interaction with metal centers of catalysts; one such example is the Kolbe–Schmitt carboxylation of phenol to produce salicylic acid. The potential of CO2 as a raw material in the synthesis of carboxylates, carbonates, or carbamates is rather limited. A future aim is the economically attractive synthesis of carboxylic acids, or optically

References

active β-hydroxycarbonic acids from ketones with CO2 and H2, under environment-friendly conditions. The development of catalysts for the synthesis of urea, formic acid, methanol, and cyclic carbonates are further future goals [53–55]. Unfortunately, the biological catalysts for similar reactions provide no direct models for chemical catalysts. 3) For various reasons, the carboxylases and reductive carboxylases of autotrophic CO2-fixation pathways have not been applied in synthetic processes. Their high substrate specificity and target substrates make these biocatalysts unattractive, and therefore an alternative approach might be to develop existing autotrophic bacteria by metabolic engineering to produce useful chemical building blocks, and then to use their natural potential to synthesize an overwhelming variety of different compounds. The knowledge of new autotrophic pathways that involve completely different intermediates compared to the Calvin cycle opens new perspectives and potentials [7, 56]. 4) Another scientific “chemical problem of the century” is to mimic those reactions that, in chemoevolution, have resulted in the autocatalytic formation of various organic compounds from inorganic volcanic gases [53]. The existing autotrophic pathways may serve as guidelines showing in which direction to search for primordial processes.

Acknowledgments

These studies were supported by Deutsche Forschungsgemeinschaft and Fonds der Chemischen Industrie.

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3 Autotrophic Carbon Fixation in Biology: Pathways, Rules, and Speculations 15 Russell, M.J. and Martin, W. (2004) Trends Biochem. Sci., 29, 358–363. 16 Ivanovsky, R.N., Sintsov, N.V., and Kondratieva, E.N. (1980) Arch. Microbiol., 128, 239–241. 17 Schauder, R., Widdel, F., and Fuchs, G. (1987) Arch. Microbiol., 148, 218–225. 18 Hügler, M., Wisen, C.O., Fuchs, G., Taylor, C.D., and Sievert, S.M. (2005) J. Bacteriol., 187, 3020–3027. 19 Hügler, M., Huber, H., Molyneaux, S.J., Vetriani, C., and Sievert, S.M. (2007) Environ. Microbiol., 9, 81–92. 20 Strauss, G., Eisenreich, W., Bacher, A., and Fuchs, G. (1992) Eur. J. Biochem., 205, 853–866. 21 Ragsdale, S.W. and Wood, H.G. (1991) Crit. Rev. Biochem. Mol. Biol., 26, 261–300. 22 Thauer, R.K., Möller-Zinkhan, D., and Spormann, A.M. (1989) Annu. Rev. Microbiol., 43, 43–67. 23 Huber, C. and Wächtershäuser, G. (1997) Science, 276, 245–247. 24 Wächtershäuser, G. (2007) Chem. Biodivers., 4, 584–602. 25 Vorholt, J.A., Kunow, J., Stetter, K.O., and Thauer, R.K. (1995) Arch. Microbiol., 163, 112–118. 26 Schouten, S., Strous, M., Kuypers, M.M., Rijpstra, W.I., Baas, M., Schubert, C.J., Jetten, M.S., and Sinninghe Damsté, J.S., (2004) Appl. Environ. Microbiol., 70, 3785–3788. 27 Strauss, G. and Fuchs, G. (1993) Eur. J. Biochem., 215, 633–643. 28 Herter, S., Fuchs, G., Bacher, A., and Eisenreich, W. (2002) J. Biol. Chem., 277, 20277–20283. 29 Klatt, C.G., Bryant, D.A., and Ward, D.M. (2007) Environ. Microbiol., 9, 2067–2078. 30 Ivanovsky, R.N., Fal, Y.I., Berg, I.A., Ugolkova, N.V., Krasilnikova, E.N., Keppen, O.I., Zakharchuc, L.M., and Zyakun, A.M. (1999) Microbiology, 145, 1743–1748. 31 Alber, B.E. and Fuchs, G. (2002) J. Biol. Chem., 277, 12137–12143. 32 Hügler, M., Menendez, C., Schägger, H., and Fuchs, G. (2002) J. Bacteriol., 184, 2404–2410. 33 Friedmann, S., Alber, B.E., and Fuchs, G. (2007) J. Bacteriol., 189, 2906–2914.

34 Martins, B.M., Dobbek, H., Cinkaya, I., Buckel, W., and Messerschmidt, A. (2004) Proc. Natl Acad. Sci. USA, 101, 15645–15649. 35 Ishii, M., Miyake, T., Satoh, T., Sugiyama, H., Oshima, Y., Kodama, T., and Igarashi, Y. (1997) Arch. Microbiol., 166, 368–371. 36 Menendez, C., Bauer, Z., Huber, H., Gad’on, N., Stetter, K.O., and Fuchs, G. (1999) J. Bacteriol., 181, 1088–1098. 37 Berg, I.A., Kockelkorn, D., Buckel, W., and Fuchs, G. (2007) Science, 318, 1782–1786. 38 Hügler, M., Krieger, R.S., Jahn, M., and Fuchs, G. (2003) Eur. J. Biochem., 270, 736–744. 39 Tcherkez, G.G., Farquhar, G.D., and Andrews, T.J. (2006) Proc. Natl Acad. Sci. USA, 103, 7246–7251. 40 Huber, H., Gallenberger, M., Jahn, U., Eylert, E., Berg, I.A., Kockelkorn, D., Eisenreich, W., and Fuchs, G. (2008) Proc. Natl Acad. Sci. USA, 105, 7851–7856. 41 Valentine, D.L. (2007) Nat. Rev. Microbiol., 5, 316–323. 42 Hedderich, R. (2004) J. Bioenerg. Biomembr., 36, 65–75. 43 van Gemerden, H., and Mas, J. (1995) Anoxygenic Photosynthetic Bacteria (eds R.E. Blankenship, M.T. Madigan, and C.E. Bauer), Kluwer Academic Publishers, Dordrecht, The Netherlands. 44 Beatty, J.T., Overmann, J., Lince, M.T., Manske, A.K., Lang, A.S., Blankenship, R.E., Van Dover, C.L., Martinson, T.A., and Plumley, F.G. (2005) Proc. Natl Acad. Sci. USA, 102, 9306–9310. 45 Markert, S., Arndt, C., Felbeck, H., Becher, D., Siever, S.M., Hügler, M., Albrecht, D., Robidart, J., Bench, S., Feldman, R.A., Hecker, M., and Schweder, T. (2007) Science, 315, 247–250. 46 Ivanovsky, R.N., Krasilnikova, E.N., and Fal, Y.I. (1993) Arch. Microbiol., 159, 257–264. 47 Quandt, L., Gottschalk, G., Ziegler, H., and Stichler, W. (1977) FEMS Microbiol. Lett., 1, 125–128. 48 Holo, H. and Sirevåg, R. (1986) Arch. Microbiol., 145, 173–180. 49 House, C.H., Schopf, J.W., and Stetter, K.O. (2003) Org. Geochem., 34, 345–356.

References 50 McNevin, D.B., Badger, M.R., Whitney, S.M., von Caemmerer, S., Tcherkez, G.G.B., and Farquhar, G.D. (2007) J. Biol. Chem., 282, 36068–36076. 51 Wächtershäuser, G. (1988) Microbiol. Rev., 52, 452–484. 52 Martin, W., Baross, J., Kelley, D., and Russell, M.J. (2008) Nat. Microbiol. Rev., 6, 805–814.

53 Aresta, M. and Dibenedetto, A. (2002) Rev. Mol. Biotechnol., 90, 113–128. 54 Aresta, M. and Dibenedetto, A. (2007) Dalton Trans., 2975–2992. 55 Sakakura, T., Choi, J.C., and Yasuda, H. (2007) Chem. Rev., 107, 2365–2387. 56 Ishii, M., Chuakrut, S., Arai, H., and Igarashi, Y. (2004) Appl. Microbiol. Biotechnol., 64, 605–610.

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4 Carbon Dioxide Coordination Chemistry and Reactivity of Coordinated CO2 Joëlle Mascetti

4.1 Introduction

Despite the fact that carbon dioxide (CO2) is used in a great number of industrial applications, it remains a molecule of low reactivity, and methods have still to be identified for its activation. Both thermodynamic and kinetic problems are connected with the reactivity of CO2, and few reactions are thermodynamically feasible. A very promising approach to activation is offered by its coordination to transition metal complexes, as both stoichiometric reactions of C–C bond formation and catalytic reactions of CO2 are promoted by transition metal systems. Efforts to enhance the yield of hydrogen in water gas-shift (WGS) reactions have also been centered on CO2 interactions with transition metal catalysts. The coordination on metal centers lowers the activation energy required in further reactions with suitable reactants involving CO2, making it possible to convert this “inert” molecule into useful products. This chapter considers this important aspect of CO2 organometallic chemistry. As the subject has been reviewed previously, in 1995, 1996 and 1999 [1–5], attention has been focused here on the period between 1999 and early 2009. The first section describes the general concepts of CO2 bonding to metals, and this is followed by details of electron-deficient CO2–metal complexes characterized spectroscopically in low-temperature matrices, and stable complexes obtained at room temperature, in order to demonstrate the different coordination modes identified, trends along the Periodic Table, and theoretical contributions to the understanding of bonding in these systems. The subsequent sections involve the reactivity of coordinated CO2, focusing initially on reactions proceeding via O transfer or electrophilic or nucleophilic attacks on a bent coordinated OCO moiety with increased C–O distance (i.e., “true” reactions of coordinated CO2). Examples are then provided of oxidative coupling, reduction reactions, catalytic processes, and bioinspired reactions where the formation of an intermediate metal CO2 complex is suspected in the reaction mechanism.

Carbon Dioxide as Chemical Feedstock. Edited by Michele Aresta Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32475-0

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4 Carbon Dioxide Coordination Chemistry and Reactivity of Coordinated CO2

4.2 Carbon Dioxide Bonding to Metals

Carbon dioxide is a 16e− molecule that belongs to the D∞h symmetry group and is linear in its ground state. It is a nonpolar molecule containing two polar C=O bonds, with two sets of orthogonal π orbitals. Initially thought to be a poor ligand, it is now evident that CO2 exhibits several coordination sites and a great variety of coordination modes in its complexes. The carbon atom (LUMO orbitals) has a Lewis acid character and can be described as an electrophilic center, whereas the oxygens (HOMO orbitals) are weak Lewis bases and described as nucleophilic centers. It is worth noting that most of CO2 catalytic reactions require a simultaneous acid–base activation, with the carbon atom and one of the oxygen atoms involved in the interaction with the metal. The two double C=O bonds contain π electrons that can interact with d electrons of transition metals in a Dewar–Chatt– Duncanson bonding scheme. When the LUMO orbitals of CO2 are occupied (via electron transfer), the lowest energy state corresponds to a bent geometry. As an example, the radical anion CO2−• is a bent molecule with an equilibrium angle of 134 °. So, any interaction of carbon dioxide with a metal will induce a loss of linearity. There are four basic modes of CO2 coordination, as described in Figure 4.1. For the η1-C coordination mode, there is a strong charge transfer between a d2z metal orbital and the anti-bonding π* orbital of CO2. This bonding mode is preferred with electron-rich metals, and may be facilitated by an additional weak interaction between one or two oxygen atoms of CO2 with a Lewis acid center located in the coordination sphere of the metal. In the η2(C,O) bonding mode, there is a double bonding scheme with a σ bond from the π orbital of CO2 to an empty d2z metal orbital, together with a “back-bonding” from a filled dxy metal orbital to the empty π* CO2 orbital. The η1(O) end-on coordination mode is preferred with electronpoor metals, and the CO2 molecule can remain linear or be weakly bent. The η2(O,O) coordination mode can be described as a metal carboxylate with an ionic

O O

C

O

O

O

C

Lewis acid (at C)

C

O

M

η1O

η1C M

LUMO

+

M

M

O

O C

O

C

O

Lewis base (at O)

O C

η2CO HOMO

Figure 4.1

O

M

CO2 coordination to a single metal center.

η2O,O

4.2 Carbon Dioxide Bonding to Metals

bond M+ CO2− and is often encountered with alkali or alkaline-earth metals or, in the case of CO2 adsorption, with metal surfaces. The interaction of CO2 with transition metal complexes has been the subject of extensive studies [1–6], both experimental and theoretical. It is remarkable that only a few authenticated examples of metal complexes of CO2 have been reported in the literature. The first side-on complex to be structurally characterized was reported by Aresta and colleagues [7, 8] with a Ni atom coordinated to one CO2 and two phosphine ligands in an almost planar environment: (PCy3)2Ni(CO2). Coordinated CO2 has two nonequivalent CO bonds (1.17 and 1.22 Å) and an OCO angle of 133 °. Later on, an iron complex with a trigonal bipyramidal geometry and CO2 lying in the equatorial plane was synthesized by Karsch [9]: (PMe3)4Fe(CO2). In this complex, CO2 has two almost equivalent relatively long CO bonds (1.25 and 1.28 Å) and a small OCO angle of 124 °, very close to that of a CO2− moiety. The first Ccoordinated complex to be structurally characterized was reported by Herskovitz et al. [10] for Rh: Rh(diars)2Cl(CO2), with CO bonds lengths of 1.20 and 1.25 Å and an OCO angle of 126 °. Other CO2 complexes of Nb [11], Mo [12, 13], and Co [14] were structurally characterized during the 1980s and, more recently, a CO2 complex of Ru [15] has been described. The CO2-bridged polymetallic complexes involve coordination of the carboxyl carbon to one metal and bonding of one or two carboxyl oxygens to a second (or third) metal center, leading to compounds of the μm-ηn type, as described in Figure 4.2. The first bridged CO2 complexes to be structurally characterized were Fe–Re, Ir–Zr, and Rh–Os bimetallic complexes, or polymeric Co, Os, and Ru clusters (for a review, see Ref. [4] and references therein for details). When X-radiography studies are not possible, theoretical calculations and spectroscopic studies can help in the structural characterization of CO2 complexes. The most largely used as a diagnostic tool is infrared (IR) spectroscopy, both for the quantitative determination of CO2 and for structural analysis. Because CO2 has a center of symmetry, the asymmetric stretching (2340 cm−1) and the degenerate bending modes (667 cm−1) are IR-active and Raman-inactive. The C=O symmetric stretching mode is IR-inactive, whereas a Fermi resonance is observed as a doublet located at 1285 and 1388 cm−1 in the Raman spectrum of CO2. When the LUMO orbitals are occupied (as may occur in the radical anion CO2−•, in CO2 adducts, or in electronically excited CO2) it induces the bending of the molecule and an increase in the CO bond length, and, consequently, large modifications in the IR spectrum of the CO2 moiety. The anti-symmetric νa(C=O) stretching mode is lowered in the region 1500–2000 cm−1, the symmetric νs(CO) stretching mode becomes IR-active and observed in the region 1400–1100 cm−1, the bending mode δ(OCO) is shifted from 667 cm−1, and additional modes (metal– C and/or metal–O stretching modes, C=O out-of-plane bending mode) may appear in the low-frequency region, between 300 and 800 cm−1. The positions and intensities of these modes can provide some information on the bonding mode of CO2

57

4 Carbon Dioxide Coordination Chemistry and Reactivity of Coordinated CO2

58

binuclear complexes

M O

O O

M

C

O

O C

M

O C

M

μ2−η2C,O

M

O

O M

μ3−η3

Figure 4.2

O,O'

O

M O

O M

C

M

2

tetranuclear complexes M

C

M

μ2−η

M

trinuclear complexes

O

O

μ2−η4

M

μ2−η3

M

C

M

M

O

M

μ3−η4

M

C

μ4−η5

Coordination modes in polynuclear metal–CO2 complexes.

in the complex, together with an estimation of the OCO angle value. Together with the use of isotope-labeled CO2, Fourier transform infrared (FTIR) spectroscopy can then be very precise and helpful in the characterization of CO2–metal complexes. Jegat et al. [16] have shown that, if ΣΔν represents the sum of the frequency shifts observed on the two ν(CO) stretching modes in isotope-labeled 13C and 18O complexes, then the following relationships are to be expected:

• • •

side-on coordination: ΣΔν(13C) > ΣΔν(18O) and ΣΔν(18O) < 60 cm−1 η1-C coordination: ΣΔν(13C) > ΣΔν(18O) with 60 < ΣΔν(18O) < 70 cm−1 end-on coordination: ΣΔν(13C) < ΣΔν(18O) with ΣΔν(18O) > 70 cm−1

In C-coordinated complexes, the frequency splitting between the two ν(CO) stretching modes is less than 400 cm−1, and in end-on complexes, the out-of-plane bending mode γ(C=O), located in the 500–650 cm−1 region, exhibits a preponderant 18 O effect, in contrast with other coordination modes, where a larger 13C effect is observed (namely Δγ(13C) = 10 to 20 cm−1, versus Δγ(18O) = 5 cm−1). Nuclear magnetic resonance (NMR) spectroscopy is also largely used to characterize CO2 complexes. The 13C NMR spectrum of CO2 dissolved in a nonpolar solvent shows a resonance at 124 ppm, which is shifted when CO2 is bonded to a metal center. Depending on the mode of bonding, the shift may be up or down field, and may vary from a few ppm up to several hundreds of ppm. A few examples are given below for different types of bonding.

4.3 Synthesis and Structure of CO2 Complexes

4.3 Synthesis and Structure of CO2 Complexes

Here, the different identified coordination modes of CO2 with transition and nontransition metals are described, together with trends along the Periodic Table, and theoretical contributions to the understanding of bonding in these systems through three types of study: (i) low-temperature matrix isolation spectroscopy of electrondeficient metal/CO2 moieties; (ii) theoretical studies of reactions of metals with CO2; and (iii) the synthesis of stable complexes. No information is provided on gas-phase studies, nor on the surface chemistry of CO2 (such details are provided in Refs [4, 17]). 4.3.1 Low-Temperature Matrix Isolation and Theoretical Studies

Matrix isolation is a technique whereby an unstable or reactive chemical species can be immobilized by cocondensing it at very low temperature with copious quantities of an inert substance, such as a noble gas [18]. A large number of matrix isolation studies have focused on the reactions of atoms, which can be thermally produced without any excess kinetic energy. Due to the combination of the matrix “cage effect” and low temperature, dynamic processes are then often hindered, and even a small barrier to reaction appears as a mountain at 10 K when kT = 0.08 kJ mol−1. However, this provides access to loosely bound complexes, which may act as precursors to insertion or other reactions. The technique of laser ablation is now widely used in many of these studies to generate metal atoms, and also to initiate atomic reactions in cryogenic matrices. In this case, it must be borne in mind that atoms have an excess kinetic energy that may allow them to overpass energy barriers and prevent the observation of reaction intermediates, such as coordination complexes. Photochemical effects may also occur, due to the light being emitted during the ablation process. Infrared spectroscopy remains, together with theoretical [mainly density functional theory (DFT)] calculations, the workhorse of these studies, allowing reasonably certain identification. Therefore, a convenient route to the preparation of metal–CO2 complexes and their identification by means of spectroscopy might be achieved through the condensation reaction of metal atoms with CO2 at cryogenic temperatures. As this particular aspect of metal–CO2 complex formation has not been reviewed specifically, the following section will include a more important period than 1999–2009, though not exhaustively. The first matrix isolation spectroscopic study conducted by M.E. Jacox et al. [19] on alkali atoms and CO2 co-deposited at 14 K in Ar matrices, showed IR absorptions near 1600 cm−1 that were assigned to the anti-symmetric stretching mode ν3(OCO) of an M+ …CO2− ion pair, with an OCO valence angle near 130 ° and a significant contribution of molecular aggregates to the observed spectrum. However, the first attempts to prepare isolated metal–CO2 complexes in a cryogenic matrix appeared during the early 1980s. An example was that of Kafafi et al. [20], who condensed alkali atoms (Na, K, Cs) with pure CO2, CO2/Ar or CO2/N2

59

60

4 Carbon Dioxide Coordination Chemistry and Reactivity of Coordinated CO2

mixtures and formed M+ CO2− (with a η2-O,O coordination and C2v symmetry) and 2− M2+ 2 CO2 (with C2v and Cs structures). Shortly before this, Hauge et al. had claimed that CO2 was reductively coupled by Li to give LiC2O4 by condensing Li atoms in CO2/Ar mixtures [21]. Later on, Manceron studied very carefully the reactions of Li atoms with CO2 in Kr [22], and observed the formation of two isomers of LiCO2 with η1-O coordinated bent structures, namely LiC2O4 and Li2C2O4. The η1-O coordinated Li–OCO complex has long been a unique example of end-on coordination, and the question arose of whether a true oxalate (with a C–C bond formation) was formed, or not. The result of these studies was that a quite rich reactivity was observed (except for Na), with the formation of M–CO2 complexes with various structures (C2v, Cs), leading to the formation of oxalates MC2O4 by a concentration effect or annealing, and to the decomposition in carbonates M2CO3 and CO by annealing above 200 K. Surprisingly, sodium atoms appeared almost nonreactive, except in neat CO2 matrices. The decreasing order of reactivity of alkaline metals with CO2 was Li > Cs > K > Na. Other metals, such as alkaline-earth Be [23], Mg [24–26], Ca [27], or Group 13 metals such as B [28, 29] and Al [30], have been studied, both experimentally and/ or theoretically. The reaction M + CO2 → MO + CO is calculated for all laser ablated alkaline-earth atoms as endothermic by 26, 66, and 35 kcal mol−1 for Be, Mg, and Ca, respectively [23, 25, 27]. The reactions proceed through the formation of cyclic (η2-O,O) MOCO structures via barriers of 23, 20, and 14 kcal mol−1, respectively. A previous experimental study of Mg + CO2, conducted with thermal Mg atoms [24], led to the formation of a MgOCO C2v cyclic structure with a calculated OCO angle of 128 °. The IR bands of the complex were observed at 1580, 1385, and 866 cm−1, although it should be noted that, in line with calculations and as previously observed for Na atoms, only a small amount of the Mg atoms was reactive, even in neat CO2. Interestingly, the condensation of Mg atoms with CO2/C2H4/Ar mixtures exhibited IR bands at 1768, 1284, 1256 cm−1, that were assigned to the formation of a five-membered cycle MgC2H4CO2 through the formation of a C–C bond between ethylene and CO2 (binding energy: 18 kcal mol−1). Similar attempts to obtain a reductive coupling between ethylene and CO2 with Ni atoms in cryogenic matrices were unsuccessful. The reforming of CO2 to CO is significantly enhanced in the presence of B [28, 29], as the reaction B + CO2 → BO + CO becomes exothermic by 64 kcal mol−1. The recombination of BO and CO, leading to OBCO, is predicted to be barrier-free [29], and the OBCO (νCO = 1863 cm−1) product detected in matrix isolation experiments [28] was probably a secondary product of the BO + CO recombination. The codeposition of Al atoms with CO2 in Ar matrices [30] leads to the formation of AlCO2 molecules, which interconvert into two isomers: the low-temperature isomer presents a Cs symmetry, with two nonequivalent CO bonds, whereas the highertemperature (25 K) isomer has a ring structure in which the metal interacts with both oxygen atoms. For Al clusters, a reductive elimination is observed upon warming up above 30 K, yielding Al2O and CO. When investigating transition metals, Ozin et al. made the first attempts with Au and Ag atoms [31], and observed very labile MCO2 π complexes by means of

4.3 Synthesis and Structure of CO2 Complexes

Raman and UV-visible spectroscopy, but no precise characterization was made. A report was made in 1981 where the IR spectrum of Cu atoms deposited with CO2 at 80 K was interpreted in terms of the formation of a π-coordinated complex between CO2 and zerovalent copper [32]. Almond et al. [33] prepared a (CO2) M(CO)5 molecule (M = Cr, W), that led to the formation of CO and oxometal carbonyl under UV irradiation. The first complete study of the reactivity of CO2 with the first row of transition metals was made by Mascetti et al. [34, 35]. Here, it was shown that the late transition metal atoms (Fe, Co, Ni, and Cu) formed one-to-one M(CO2) complexes, where CO2 was bonded in a side-on (Ni), end-on (Cu), or C-coordinated (Fe, Co) manner, while the earlier metal atoms (Ti, V, and Cr) spontaneously inserted into a CO bond to yield oxocarbonyl species OM(CO) or OM(CO)(CO2). Normal coordinate analysis showed that the force constants of CO bonds were significantly decreased by 50%, compared to free CO2, and that the OCO angle was bent between 120 and 150 °. Since then, most studies have associated experimental matrix isolation studies with DFT calculations: Sc [36–38], Ti [36, 39–41], V [40, 42], Cr [43], Mn [43], Fe [43, 44], Co [43, 45], Ni [36, 43, 46–48], and Cu [36, 43, 49] for the first-row transition metals. This has allowed for the prediction of reaction mechanisms, a better description of the bonding modes, and a more reliable identification of the species observed in experiments. Examples of this include titanium, nickel, and copper. The interaction of s2d2 and s1d3 Ti atoms with CO2 has been studied by Papai et al. [39] using DFT calculations. It has been shown that the ground-state Ti atom could insert with no energy barrier into a CO bond, and this would result in an OTiCO insertion product (see Figure 4.3). The comparison of the calculated data with those from previous matrix isolation experiments [35, 40, 50] revealed that the insertion product formed in a low-temperature argon matrix with thermal- [35] or laser-evaporated [40] Ti atoms corresponded to the singlet state OTiCO. Ti(CO2) complexes, in various coordination modes, were located on the triplet and quintet potential surfaces, from which the triplet state (O,O) coordination mode was shown to be the most stable, but this lay above the OTiCO molecule by about 30 kcal mol−1. Later, Mebel et al. [41] conducted a very detailed study of the same reactions, and reached the conclusion that the most energetically favorable reaction mechanism was an insertion of the Ti atom into a CO bond via a η2(C,O) -coordinated Ti(CO2) complex, to produce the triplet OTi(CO) molecule. A comparison of the reaction mechanisms for alkaline earth and early transition metal atoms indicated that, although all of them could enhance CO2 reforming into CO, the early transition metal atoms (Sc, Ti, V) were the best for this purpose. The case of nickel is completely different. Although laser-ablated atoms exhibit insertion reactions [43], thermal atoms are almost nonreactive [35] unless working in a neat CO2 matrix [35, 48] or using a coreactant such as N2 [51]. Andrews et al. [43] have shown that laser-ablated late transition metals (from Cr to Ni), react with CO2 to give the insertion products OMCO, as observed in solid argon matrices. Mebel et al. [47] showed, via DFT studies, that this reaction occurred preferentially in the triplet electronic state, through the formation of a cyclic four-membered

61

62

4 Carbon Dioxide Coordination Chemistry and Reactivity of Coordinated CO2

Figure 4.3 Potential energy curve for the Ti + CO2 → OTiCO insertion reaction based on DFT calculations.

ring NiOOC intermediate, with an energy barrier of about 23 kcal mol−1, although the basis sets used did not allow accurate results to be obtained. Later on, Hannachi et al. [48] revisited the theoretical study of this system by using combined B3LYP/CCSD(T) calculations, and showed that the insertion–elimination route Ni + CO2 → NiO (3Σ−) + CO (1Σ+) was the most favorable mechanism (Figure 4.4). In this case, the metal insertion reaction is initiated by an electron transfer from the Ni atom to the CO2 molecule, after which the insertion and oxygen abstraction steps take place in a concerted fashion, along with the charge-transfer processes. So, it is clear that for nickel, the insertion reaction is not preceded by the formation of a η2-CO2 complex, as with Sc, Ti, or V. Likewise, in contrast to the Sc, Ti and V reactions, the insertion of Ni into CO2 is an endothermic reaction (by 15.4 kcal mol−1) with respect to Ni (s1d9) + CO2 and, as a consequence, was not observed for thermally evaporated nickel atoms, which have insufficient kinetic energy to overcome the barrier of reaction. Similar to nickel, thermal copper atoms are not especially reactive towards CO2: the formation of a weak 1 : 1 end-on complex has been observed in neat CO2 matrix by Mascetti et al. [35], characterized by an important charge transfer from copper to CO2 [52]. Later, theoretical investigations were conducted by Hannachi et al. [49] which showed that the insertion of Cu into the CO bond of CO2 was direct. with an activation energy of 61 kcal mol−1 and also endothermic, by 30.6 kcal mol−1. Although no Cu(CO2) complex was found, a weak η1O -Cu2 (OCO) was calculated, which suggested that the species observed experimentally in the matrices was in

4.3 Synthesis and Structure of CO2 Complexes 60.0

Ni

59.9

3A" surface

2.111

Relative energy (kcal/mol)

3A' surface

NiO(3Π)+CO

1A' surface

51.7

45.0 40.0

20.0

O

14.3 = == Ni+CO2 0.0

?

h2oo (3B2)

O

1.846

1.700

Ni 179.9

h2co (1A')

1.129 O

1.838

1.131 C 180.0

O

ONiCO (3A') (3A")

Ni 1.864

-7.8

C 170.7

ONiCO (3A")

15.4 ONiCO

h2oo(3B2)

1.2287

145.0

(3A')

17.4

17.2

Ni

1.665

37.4 ONiCO

O

C

NiO(3Σ-)+CO

34.6

132.1

O

h2co (1A')

O

1.249

1.904 C 147.6

1.180 O

Figure 4.4 Energy diagram of the Ni + CO2 → NiO + CO reaction based on DFT calculations.

fact a Cu2 end-on complex [53]. The formation of OCuCO− anions was observed with laser-ablated Cu atoms by Andrews et al. [43], produced by the addition of Cu to CO2− , formed by electron capture during the experiments. Some metals from the second and third rows of the Periodic Table, that are largely used in catalytic systems and reduction processes, have also been studied; these include Zr [54], Ru [55], Rh [45], Pd [56], Nb [57], Ta [58], Re [59], and Os [55]. For laser-ablated Zr, Nb, and Ta atoms, Qin et al. have observed the formation of OM(CO) and O2M(CO)2 insertion products. Subsequent calculations proved that the insertion process was barrierless for Zr, whereas Nb and Ta atoms insertions proceeded via electron transfer from the metal to CO2 and the formation of adduct complexes as intermediates. Laser-ablated Co and Rh atoms, when studied by Xu et al., showed similar reactions with the formation of the following neutral species: OM(CO), O2M(CO), and OCo2(CO). For laser-ablated Ru and Os atoms cocondensed with CO2 in Ar and Ne matrices, Andrews et al. have identified several neutral insertion species, including OM(CO), O2M(CO), and O2Os(CO)2, formed by the insertion of OOs(CO) into a second CO2 molecule, and OCRu(O2)(CO) by the addition of a CO2 molecule to ORu(CO). Although, osmium has been shown to be more reactive than ruthenium, both metals have the potential to be good catalysts for CO2 reduction. Similar results have been obtained for Re by the same group. A theoretical study performed by Liu et al. [56], of CO2 coordination in a series of phosphine-substituted Pd complexes, showed that in all cases the η2C,O side-on bonding mode was the lowest in energy, independently of the basicity

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4 Carbon Dioxide Coordination Chemistry and Reactivity of Coordinated CO2

or the bulkiness of the ligand phosphine; however, the relative stabilities were different. Several studies have also been performed on heavy metals such as lanthanides or actinides. In the case of La atoms [60], the formation of OLa(CO) as the primary product was observed, whilst UV-visible rearrangements have been seen to lead to the formation of La(η2-CO)O and OLa(η2-CO). Finally, a comparison of the decreasing CO stretching frequency of OMCO in the series Sc, Y, La, indicated an increase in the metal d orbital → CO π* back-donation, going from Sc to La. In the case of uranium [61], Andrews et al. showed that pulsed laser-evaporated U atoms were sufficiently energetic to insert into CO2 upon condensation in argon matrices, forming OUCO (1799, 804 cm−1), and with a larger charge transfer from OU to CO than for OBCO [29]. Further reaction with CO molecules isolated in the matrix led to the formation of OCU(O)CO species. It appears, then, that the most-often observed reactions of a metal atom with CO2 in a low-temperature matrix are the formation of a metal complex, and/or the insertion into one CO bond of CO2. These studies, which were conducted at low temperature on “naked” metal atoms, could not reproduce the reactions obtained with “real” metal complexes containing ligands, which can in turn influence further reactions with CO2 (some examples are provided in Sections 4.3 and 4.4). However, with the assistance of theoretical calculations, the studies have allowed the identification of general trends in the Periodic Table, as well as a description of the different CO2 bonding modes through the vibrational analysis of isolated M(CO2) moieties. 4.3.2 Synthesis of Stable Complexes

The synthesis of stable CO2 complexes usually proceeds via four different methods: (i) the substitution of labile ligands (as for Ni(PCy3)2(CO2) [7]); (ii) addition to d16 complexes (as for Rh(diars)2Cl(CO2) [10]); (iii) insertion into binuclear complexes (as for Co(Pr-salen)K(CO2) [14]) and the reduction of complexes (as for Nb(CO2) (C5H4Me)(CH2SiMe3) [11]); or (iv) by the in situ synthesis of CO2 as for ((CO)5Re2 (CO2)2Re(CO)4)2 [62]). The most important of these reactions, however, are substitutions and additions, as these also play a decisive role in the catalytic and stoichiometric reactions of CO2. At this point, only those compounds that have been clearly identified as having CO2 bound to metal centers through one at least of its atoms will be considered. Some relatively recent reviews by Gibson [2] and Leitner [3] in 1996, and by Gibson [4] and Yin [5] in 1999, described the most recent developments of the organometallic chemistry of CO2 up until 1998; hence, these will not be discussed here in detail, other than to mention any relevant aspects of CO2 activation. Rather, attention will be focused on data produced between 1999 and 2009. Perhaps the most important report made during the past decade has been the first synthesis of stable end-on CO2 coordinated complexes. The first such complex was a linear η1-O-coordinated CO2 bound to uranium, obtained by

4.3 Synthesis and Structure of CO2 Complexes

Castro-Rodriguez et al. in 2004 [63]. Subsequently, a linear μ(O, O′)-coordinated CO2 to magnesium was synthesized by Chang et al. in 2005 [64] whilst, very recently, in 2008, Gao et al. reported a μ-η1,η1-OCO linear coordination mode of CO2 in inorganic polyoxoanions in water [65]. 4.3.2.1 End-On Complexes The electron-rich six-coordinate (AdArO)3tacnUIII complex (where (AdArOH)3tacn = 1,4,7-tris(3-adamantyl-5-tert-butyl-2-hydroxybenzyl)1,4,7-triazacyclononane) was shown to react with CO2 to yield (AdArO)3tacnUIV(CO2), a colorless complex in which CO2 was linearly coordinated to U through one oxygen [63]. When this complex was characterized with X-ray diffraction (XRD), the OCO angle was close to linear (178 °), the U–OCO group had a U–O bond length of 2.351 Å, a short C–O bond length of 1.122 Å, and a longer terminal CO bond length of 1.277 Å. A bonding model UIV=O=C•–O− ↔ UIV-O≡C–O−, where the uranium center is oxidized to UIV and the CO2 ligand reduced by one electron, was supported by electronic and vibrational spectroscopic studies. Both, visible and near-infrared (NIR) band intensities and positions were characteristic of the UIV f2 ion, while the IR spectra exhibited a ν(CO) band at 2188 cm−1, which was shifted by 60 cm−1 when using 13CO2. In 2005, Chang et al. [64] reported a one-pot synthetic pathway to novel aluminum–magnesium complexes containing CO2 in the linear μ(O,O′) bonding mode. For this, one equivalent of AlR3 (R=Me, Et) was added to a solution of Mg(N(SiMe3)2)2 in tetrahydrofuran (THF) at room temperature, and an excess of CO2 was bubbled through the stirred, ice-cooled mixture to yield compounds 1 (R = Me) and 2 (R = Et): (R2Al(μ-NSiMe3)(μ-OSiMe3)Mg(thf)2(μ-O2C))3. The proposed reaction pathway suggested a first reaction between CO2 and Mg(N(SiMe3)2)2 to give an oxo-transfer product Mg(N(SiMe3)2)(OSiMe3), which was assumed to form a bridged Al–Mg intermediate with AlR3; this then lost a ligand from the Mg center and was attacked by a second CO2 molecule, with the oxygen atom as a weak electron donor. Finally, the CO2 acted as a bridging ligand to form a trimer (Figure 4.5). The structures of 1 and 2 were confirmed by XRD, with a C3 symmetry axis, and three almost linear CO2 bridges that formed a 12-membered ring. The CO bond lengths were very close to that of free CO2 (between 1.149 and 1.233 Å), which suggested the presence of double C=O bonds. The OCO angles (between 169.6 and 175.3 °) were close to 180 ° and suggested sp-hybridized C atoms. The Mg←O=C=O→Mg moieties showed some disorder, with a shape like an hourglass. The 13C NMR spectra of 1 and 2 showed peaks that were slightly shifted (ca. 121 ppm) relative to free CO2 (124 ppm), whereas the side-on transition metal complexes usually showed 13C resonances at around 200 ppm. The IR spectra exhibited strong absorptions at 2267 and 2275 cm−1, respectively, these being slightly red-shifted by 73 and 65 cm−1 relative to free CO2, whereas the side-on complexes usually exhibited shifts greater than 300 cm−1. Only a small amount of electron density was transferred from the coordinated CO2 to the more electropositive Mg atoms in molecules 1 and 2 (and less than that observed in the uranium complex described above).

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4 Carbon Dioxide Coordination Chemistry and Reactivity of Coordinated CO2

Figure 4.5

Schematic representation of Al–Mg complex with linearly bridging CO2 [64].

By bubbling CO2 through an aqueous solution containing (H2PMo11CoO40)5−, Gao et al. were able to obtain the CO2 adduct compound (C3H5N2)3(C3H4N2) (PMo11CoO38(CO2))·4 H2O, where single crystal XRD showed that the CO2 was μη1,η1-coordinated [65]. The skeleton was composed of (PMo11CoO38)n polymeric chains, bridged by the CO2 ligand in an axial direction. The CO2 groups showed slightly bent arrangements, with OCO angles of 158.7 ° and a CO bond length of 1.287 Å. The two O atoms of CO2 linked MoVI and CoII from two neighboring polyoxoanions, respectively. The IR spectrum exhibited an absorption band at 2169 cm−1, shifted to 2108 cm−1 by 13C labeling. In the NMR 13C spectrum, the chemical shift for the C atom of CO2 was 113.86 ppm, appearing in an up-field shift relative to free CO2. Similar results have been obtained for the analogue compound (C3H5N2)4(SiMo11CoO38(CO2))·4 H2O. Beyond the fact that it was one of the few examples of stable end-on CO2 complexes, this synthesis provided a promising route to capture CO2 by simple inorganic polyoxoanions in an aqueous medium. Interestingly, in 1998 Kozik et al. [66] had described quite different results on the interaction of CO2 with various tungsten heteropolyanions. Reactions with CO2 occurred for germano- and silicotungstates when substituted with Co(II), Ni(II) and Mn(II). Although no structural data were available, multiple spectroscopic evidence indicated that reversible CO2 complexes had been formed, with a direct η1-C metal– carbon bond (the so-called C-coordination), and with the O atoms of CO2 being involved in H bonds with water or alcohol included in the structure. The IR spectrum of the tetraheptylammonium salt of α-(SiW11O39Co(CO2))6− showed strong bands located around 1675 and 1350 cm−1, respectively shifted to 1633 and 1340 cm−1 with 13CO2, and to 1657 and 1331 cm−1 with C18O2. The 13C NMR study was the first to report the existence of octahedral paramagnetic Co(II) complexes, with two considerably downfield shifted signals observed at 792 and 596 ppm, due to two different types of H bonding, and with sensitivity to the temperature.

4.3 Synthesis and Structure of CO2 Complexes

4.3.2.2 Side-On Complexes A new bis(carbon dioxide) side-on adduct of molybdenum: transMo(CO2)2HN(CH2CH2PMe2)2(PMe3) has been synthesized from transMo(CO2)2(PMe3)4 [12] and structurally characterized by Carmona et al. [67]. Similar to the parent complex, this new compound had a distorted octahedral geometry with two staggered CO2 ligands that were strongly bonded to Mo through one π-CO bond, in a side-on manner. The CO2 groups had CO bond lengths ranging from 1.199 to 1.284 Å, and OCO angles equal to 131 °, whereas the Mo–C distance was 2.08 Å. The IR data showed red-shifted bands at 1660, 1155, and 1100 cm−1. The 13C NMR spectra revealed a fluxionality that consisted of a synchronous motion of the two CO2 ligands, rotating in the same direction, with two signals centered at 213 and 217 ppm. One of the reactions of coordinated CO2 is its reduction to CO by means of oxygen atom transfer to another substrate (an oxophilic metal or a readily oxidized ligand). However, CO2 can also act as its own oxygen sink, giving rise to CO + CO23 − . The carbonyl carbonate compound trans-Mo(CO)(CO3)HN(CH2CH2PMe2)2(PMe3) has also been prepared from the tetrakis phosphine carbonyl carbonate parent compound. In addition, the reactivity of Mo(CO3)(CO)(PMe3)4 towards CO has been studied: a dicarbonyl complex Mo(CO3)(CO)2(PMe3)3 was formed at room temperature. Interestingly, this latter compound could undergo a conproportionation reaction and liberate CO2 upon heating under a carbon monoxide atmosphere; this was seen as a rare example of the reversibility of the disproportionation process. 4.3.2.3 Bridged Complexes Sadighi et al. reported in 2007 [68] the reduction of CO2 by a (N-heterocyclic carbene)Ni(0) complex that gave rise to a new μ-η2, η2-CO2 coordination geometry at a dinickel core: ((IPr)Ni)2(μ-CO)(μ-η2,η2-CO2), with IPr = 1,3-bis(2,6diisopropylphenyl)imidazol-2-ylidene. The reduction of CO2 was evident from the presence of the bridging CO in the complex, which showed two NMR 13C signals at 172.6 ppm (CO2) and 246.4 ppm (CO). This complex also exhibited strong IR absorptions at 1773, 1630, and 1205 cm−1, shifted to 1731, 1586, and 1183 cm−1 upon 13C isotopic substitution. The most remarkable feature of the crystal structure was one CO2 ligand bridging two nickel atoms via its two π-CO bonds in a side-on manner (Figure 4.6). The bridged CO2 was bent at 133.4 ° and the CO bond lengths were 1.255 and 1.257 Å, which were much longer than for free CO2 (1.16 Å) and for the bridged CO ligand (1.184 Å). Other Ni(0) complexes, such as biscarbene (IMe)2Ni(0) formed analogous complexes (IMe = 1,3-dimesitylimidazol-2-ylidene). As with the previous example, a carbonyl carbonate Ni(II) isomer was formed by a disproportionation reaction during the synthesis. 4.3.2.4 Bridged Complexes Obtained by In-situ Synthesis A few stable CO2 complexes resulting from a formal oxidation of metal carbonyl complex have been described. Although not strictly true CO2 complexes, these are important intermediates in the photocatalytic and electrocatalytic reductions of CO2 to formate and CO, and also in biologic systems. A few examples of such species are described below.

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4 Carbon Dioxide Coordination Chemistry and Reactivity of Coordinated CO2

O(1)

C(6)

C(3) Ni(1)

C(1)

Ni(2)

C(2) O(3)

Figure 4.6

O(2)

The molecular structure of ((IPr)Ni)2(μ-CO)(μ-η2,η2-CO2) [68].

In 1998, Gibson et al. reported [69] the synthesis of ruthenium μ2-η2-type CO2bridged complexes from the reaction of cis-Ru(phen)2(CO)(CHO)(PF6) with O2 in dimethyl ether (DME)/H2O: (Ru(phen)2(CO)(CO2)Ru(phen)2(CO))(PF6)2, and of the related complex (Ru(phen)2(CO)(CO2)Ru(bpy)2(CO))(PF6)2. The latter complex was structurally characterized as having the following values: the OCO angle was 122.3 °; the CO bonds were 1.235 and 1.319 Å; and the IR bands for the carboxyl bridge were located at 1499 and 1183 cm−1. Another example of a multisite bridging CO2 ligand, starting from a carbonyl complex, was reported in 2001 by Chi et al., with the preparation and characterization of two CO2-bridged osmium cluster complexes [70]. These authors reported the synthesis of Os4(μ-H)(μ-CO2)(thd)(CO)13 (tdh = 2,2,6,6-tetramethyl-3,5-heptandionate) by the direct treatment of Os3(CO)12 with a β-diketone molecule (thd)H. The XRD study showed how a unique CO2 ligand could bridge a triosmium metal fragment, Os3(μ-H)(CO)10 through the C and O atoms, and a monometallic osmium moiety, Os(CO)3(thd) through one O atom. This was a different pattern from that of cluster complexes with a bridged carboxylate ligand, where the CO2 was linked to the nearby Os–Os edge via two direct Os–O σ-bonds. The OCO angle was 118 °, which was close to the value encountered in CO2− moieties, while the average CO bond length was 1.27 Å. In 2004, Wang et al. showed that air-stable aminorhenium–CO2 complexes could be easily obtained in good yields from the corresponding carbonyl complexes η5:η1-C5H4CH2CH2NR(CH3)Re(CO)2, by oxidation with peroxy-acids [71]. The binding mode of the CO2 ligand to the metal was the “side-on” type, with a OCO angle of 133 °, and CO bond lengths of 1.19 and 1.23 Å, similar to other η2-CO2 complexes reported elsewhere (Figure 4.7). On this basis, a reaction mechanism

4.4 Reactivity of CO2 Complexes

Figure 4.7

ORTEP view of η5:η1-C5H4CH2CH2NR(CH3)Re(CO)(η2-CO2) complex [71].

was proposed whereby a transient Re(CO)2(O−) intermediate underwent intramolecular cyclization to produce the CO2 complex. The final example is related to the redox transformations of CO2 in biological cycles. Carbon monoxide dehydrogenases (CODHases) are the biological catalysts for the reversible oxidation of CO to CO2, with water as the source of oxygen: CO + H2O → CO2 + 2e− + 2H+. In 2007, Dobbek et al. [72] reported crystal structures of anaerobic CODHases containing Ni-, Fe-, S- cluster in three different states. In the intermediate structure, CO2 was seen to act as a bridging ligand between Ni, completing the square-planar coordination of Ni2+, and the asymmetrically coordinated Fe, replacing a water hydroxo-ligand bound to Fe2+ in the two other states. In the cluster–CO2 complex, Ni acted as the Lewis base, and Fe was the Lewis acid that stabilized the negative charge on oxygen. CO2 acted as a η1-OCO end-on ligand at Fe, and as a η1-C ligand at Ni, which resulted in a μ2-η2 binding mode of CO2 bridging the Ni–Fe site. The oxygen atoms were also H-bonded to two protein fragments (Lys and His), while the OCO angle was equal to 133 °.

4.4 Reactivity of CO2 Complexes

It is clear that CO2 is in an “activated state” when the bonding involves the central carbon. The activation is evident from structural data such as the bent OCO moiety

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4 Carbon Dioxide Coordination Chemistry and Reactivity of Coordinated CO2

and the increased CO distances, and from spectroscopic data such as lower frequency vibration modes in IR spectra and low field shifts in 13C NMR spectroscopy. It can be expected that this “activation” would also be reflected in the reactivity of coordinated CO2. However, the relationships between these model complexes and their reactivity are not yet understood, nor the structural and electronic properties of metal–CO2 moieties that are required for catalytic activity. Very few examples of reactions of “true” CO2 complexes are known. Among them, the cleavage of a CO bond is often observed and yields metal oxides that can react further with CO2, leading to a net conversion of two CO2 molecules to CO and CO2− 3 . The same conversion can occur via the intermediate coupling of two CO2 molecules at a metal center, as described previously for Carmona’s Mo complex [12, 67]. Coordinated CO2 exhibits an enhanced reactivity at oxygen towards electrophiles, leading to metallacarboxylic acids and their derivatives. Electrophilic attack at oxygen is also the prevalent reactivity of η1-CO2 complexes and CO2-bridged complexes with alkali cations. Many stoichiometric and most catalytic reactions involving CO2 activation proceed via the formal insertion of CO2 into highly reactive M–L bonds, with the formation of new C–L bonds (where L = ligand). These reactions might not necessarily require strong coordination of CO2, as in the stable complexes described in Section 4.3.2, but are generally initiated by a nucleophilic attack of L at the Lewis acidic carbon atom of CO2. A weak interaction between the metal and the lone pairs of one oxygen atom or the π-C=O bond of CO2 may also play a role in supporting the insertion process. Three types of possible reaction of coordinated CO2, illustrated with recent examples of the past decade, are described in the following sections. These include CO bond cleavage and oxygen transfer, reactions with electrophiles, and reactions with nucleophiles. 4.4.1 C–O Bond Cleavage and O Transfer

A reaction that may be characteristic of η2- and η1-complexes formed from transition metals is thermolysis with cleavage of one CO bond resulting in the loss of CO and formation of an oxo compound. The conversion of CO2 into the coordinated CO and phosphine oxide was observed by Ohnishi et al. [73] in the reaction of the Mo(0) complex Mo(P4)(dppe) with CO2 to give a M(0) carbonyl complex fac-Mo(CO)(η3-P4=O)(dppe), where the O abstraction from CO2 by one terminal P atom in P4 (meso-o-C6H4(PPhCH2CH2 PPh2)2) takes place to give the dangling P(=O)Ph2 moiety together with the coordinated CO. The proposed mechanism involves the initial displacement of one Mo–P bond to give a η2–CO2 complex Mo(η2-CO2)(η3-P4)(dppe) that undergoes O atom abstraction by the dangling P atom of the η3-P4 ligand. Later, Srivastava et al. [74] reported the catalytic activities of several Cu and Mn aza complexes for cyclic carbonate synthesis from CO2 and epoxides. These authors showed that the reaction of CO2 with the catalyst system generated two new IR bands

4.4 Reactivity of CO2 Complexes O–

O C +

R

O

Cu

CO2 Cu

O R O O

DMAP

Cu

DMAP

O R O Cu

O

– O O

Cu

+ C

O R

R

Cu

DMAP

O

Cu R

DMAP

O Cu

O

C

Cu

DMAP

O Figure 4.8 Possible reaction mechanism for cyclic carbonate synthesis from CO2 and epoxides on Cu aza complexes [74].

at 1716 and 1225 cm−1, due to the formation of an activated Cu complex in a pentacoordinated structure with CO2 bound to the metal in a η1-C mode. The activated CO2 complex then attacked the activated epoxide at the less-hindered carbon, leading to the formation of a dimer intermediate species which yielded the cyclic carbonate (Figure 4.8). The nature of the ligands, and of their substitutions, influenced the lability of the metal–CO2 bonding, and the differences in the catalytic activities were correlated with the mode and strength of CO2 binding. The metal–CO2/epoxide bonding should neither be too strong nor too weak since, by a suitable fine-tuning of the ligand system, it would then possible to develop a highly active catalyst. One of the most interesting recent examples of oxygen transfer reaction has been the studies conducted by Meier and colleagues [75–77] of the activation of CO2 by uranium complexes. Certain ligands form multiple bonds to uranium, and small inert molecules such as CO2 become reactive when coordinated to the metal. The ability of U to use its outermost f electrons for binding ligands enables it to catalyze reactions that should be impossible with “conventional” transition metal catalysts. The oxygen transfer to a metal center, with formation of the O=M(CO) moiety, may be driven by the ancillary ligands, as shown recently in the case of the U(III) complex (see Refs [63] and [75]). A new rather stable complex

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4 Carbon Dioxide Coordination Chemistry and Reactivity of Coordinated CO2

LU + CO2 1

LU=

O

N O UIII N N O

LU–(CO2)–UL 3

CO + LU–O–UL 2 LU + CO 1

LU–CO–UL 4

Figure 4.9 Molecular representation of ((AdArO)3tacn)U(CO2) with core structure and geometric parameters in Å and degrees, and possible mechanism of CO2 reduction by a less-bulky UIII complex [75].

((AdArO)3tacnUIII)=O=C•–O−, where CO2 is coordinated to U in a η1-O mode and reduced by one electron to a radical anion, has been described in Section 4.3.2.1. However, depending on the steric hindrance of the ligand around the U center, a further reaction can occur. By replacing the adamentane ligand (Ad) with a lesslarge tert-butyl ligand (t-Bu), an initial formation of a CO2 adduct will occur, following which a second equivalent of (t-BuArO)3tacnUIII enters to complete the reduction process, to form a bridging oxide complex ((t-BuArO)3(tacn)U-O-U(tacn) (t-BuArO)3) and releasing CO (Figure 4.9). 4.4.2 Reactions with Electrophiles

Although the reports of chemical reactivity of coordinated CO2 ligand in wellcharacterized complexes are still very limited, several examples have been described showing that the oxygen of the η2- or η1-bonded coordinated CO2 can undergo an electrophilic attack by protons or other similar reagents. For example, the CO2 complex of iron(0), Fe(CO2)(depe)2, which has a trigonal bipyramidal geometry with a side-on bonded CO2, reacts with electrophiles such

4.4 Reactivity of CO2 Complexes

as MeI and MeOTf to give Me2O and the corresponding cationic iron(II) carbonyl complexes FeX(CO)(depe)2)+X−, with X = I or OTf [78]. The reaction of Fe(CO2) (depe)2 with R3SnCl in diethyl ether at 195 K gives the iron carboxylate complexes FeCl(CO2SnR3)(depe)2 with R = Me or Ph, where the CO2 moiety bridges the Fe and Sn atoms in a μ-η1(C):η2(O,O)–CO2 fashion. The formation of the iron carboxylate and the cationic carbonyl complexes can be interpreted by the following mechanism. First, the electrophile interacts directly with one of the oxygen atoms of the CO2 ligand to give a metal carboxylate FeC(O)OR. Then, with organotin chlorides, a stable heterodinuclear carboxylate FeCO2Sn is formed. With stronger electrophiles such as RX, decarboxylation takes place to give an alkoxocarbonyl iron Fe+(CO)(OR)X− that further reacts with another electrophile RX to form the corresponding ether R2O and a halogenocarbonyliron(II) halide XFe+(CO)X−. Another interesting example is the reaction of hydrosilylation of CO2. A recent report by Deglmann et al. [79] has described, both experimentally (by using in situ IR spectroscopy) and theoretically (by running DFT calculations with a Me3SiH model), the catalytic hydrosilylation of CO2 by Me2PhSiH with ruthenium nitrile complexes mer-(RuX3(MeCN)3) and cis/trans-(RuX2(MeCN)4), with X = Br, Cl. The key steps of the reaction were shown to be the transfer of the Me3Si moiety to a coordinated halide ligand, resulting in a LnRuH–(XSiMe3) intermediate, CO2 coordination to Ru in a side-on fashion, Me3Si transfer to CO2, and reductive elimination of the formoxysilane product Me3SiOCHO (Figure 4.10). The general orders of activity for CO2 hydrosilylation were found to be RuClmLn > RuBrmLn and RuIII > RuII. The highest point on the energy pathway was found to be the silyl transfer from the R3SiCl ligand to the oxygen atom of the η2-bound CO2. In 2001, Matsubara et al. [80] performed a theoretical study of the mechanism of the hydrido migration to CO2 of Fe, Ru, and Os complexes containing a protonated amine arm by DFT calculations, asking the question: which pathway did the reaction take? This might be an abstraction of the hydrido ligand by an incoming CO2 without direct coordination of CO2 to the metal atom (pathway a, where the amine arm promotes the electrophilic attack of CO2 and stabilizes the product), or the insertion of CO2 into the M–H bond with the previous η2-CO coordination of CO2 to the metal atom (pathway b). Matsubara and coworkers concluded that the reactivity was reduced in the order Fe > Ru > Os and that, in the case of Fe, pathway b was preferred, whereas pathway a was more favorable for Ru and Os, by a careful analysis of geometries, charges and energy barriers along the reaction paths. This showed how charge distributions and bond populations were important clues for understanding the mechanisms of reactions initiated by electrophilic or nucleophilic attacks. 4.4.3 Reactions with Nucleophiles

Very few reactions of CO2 complexes with nucleophiles have been reported. One of the rare examples of nucleophilic attack at CO2 has been provided by Aresta et al. [81], in a study of the coordination chemistry of phenoxide ligands to MnII

73

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4 Carbon Dioxide Coordination Chemistry and Reactivity of Coordinated CO2 L L L RuI CI CI A L -L

L L L RuIII CI H CI O C O +L SiMe3 I

L L RuI CI L CI -Product

H SiMe3 L L RuII CI L CI C

B

H L C O L RuII CI CI O H SiMe3

‡ SiMe3 L H L RuII CI L CI TS1

L H CI L Ru O CI C O SiMe3

‡ L H SiMe3 L RuII CI L CI D

TS3

-L H SiMe3 L I CI Ru L CI E

L H L RuIV CI O CI C ‡

O G

SiMe3

L H L Ru CI O CI C SiMe3 O TS2

+CO2 L H SiMe3 L RuII CI C O CI O F

Figure 4.10 Calculated mechanism of the hydrosilylation of CO2 involving a neutral RuII nitrile complex as catalyst [79].

because of the possible implication of MnII–phenoxide complexes as intermediates in the phenylphosphate carboxylase enzyme, a protein which catalyzes the selective carboxylation of phenylphosphate to 4-OH-benzoic acid using CO2. Aresta and colleagues reported the synthesis and the characterization of mixed-sandwich complexes with Cp and phenate as π-ligands CpMn(η5-ArO), with ArO = 2,6substituted phenoxide being π coordinated to the CpMn+ moiety. When these complexes were exposed to CO2, the formation of 4-OH-3,5-substituted benzoic

4.5 CO2 Complexes as Reaction Intermediates in CO2 Utilization Processes

acid was observed, because the π coordination forced electron dislocation in the 4-position to facilitate the nucleophilic attack of the ring carbon of the phenoxide on CO2. This produced the ring carboxylation, whereas the μ-O-bonded phenols (CpMn(μ-OAr)(THF))2 underwent CO2 insertion into the Mn–O bond. Another example involving Aresta’s complex (PCy3)2Ni(η2-CO2) is the Wittig reaction, observed for this compound by Gong et al. [82]. The so-called Wittig reaction is a (2+2) cycloaddition mechanism between strong nucleophiles such as trialkyl phosphorous ylides R3P=CH2 and organic ketones or aldehydes to form alkenes. Actually, when CO2 is bubbled in THF with Me3P=CH2, free ketene CH2=C=O is formed, but this immediately dimerizes to form methylene-βpropiolactone. Ketenes can be stabilized by coordination on transition metal complexes in a η2-C,O (for early transition metals) or η2-C,C (for late transition metals) manner. In this study, Aresta’s complex was reacted with Me3P=CH2 in toluene at 253 K for 2 h, and a unique nickel ketene complex (Cy3P)2Ni(η2-C,O)–CH2=CO was isolated and characterized (1611, 1570 cm−1). The reaction mechanism could be a nucleophilic attack of coordinated CO2 by the ylide; alternatively, it is also possible that the CO2 had dissociated from Ni, reacted with the ylide in the solvent, and that the ketene further coordinated to the nickel. Nevertheless, the unusual η2-C,O ketene bonding mode observed on Ni was more in favor of a nucleophilic attack of coordinated CO2. In contrast, by using kinetic measurements, Konno et al. [83] showed that the nucleophilic attack of the hydride ligand of Ru(tpy)(4,4′-X2bpy)H+ (X = H, MeO) to the carbon atom of CO2 was the rate-determining step for the formation of the formate complex Ru(tpy)(4,4′-X2bpy)(OCHO)+, while the η1-O or η2-C,O coordination of CO2 on Ru should be less important, if at all, in the transition state. Another recent example of the question of the formation of intermediate metal– CO2 complexes in these reactions was the theoretical study by Ohnishi et al. [84] of the hydrogenation of CO2 to formic acid by RuII catalysts. In the presence of water, there was no direct metal coordination of CO2, but formation of adducts in which the C and O atoms of CO2 interacted with the H (hydride) ligand and the H atom of H2O: cis-Ru(H)2(PMe3)3(H2O)(CO2). In the absence of water molecules, CO2 directly coordinated to the Ru center to afford Ru(H)2(η2-CO2)(PMe3)3. The Ru-(η1-formate) intermediate was then produced via CO2 insertion, instead of a nucleophilic attack of the H ligand to CO2 in the presence of water. As a result, the activation barrier was higher, the exothermicity lower, and a back reaction could occur. Hence, this is an example where the formation of a metal–CO2 complex is not favorable to the further reaction.

4.5 CO2 Complexes as Reaction Intermediates in CO2 Utilization Processes

From the point of view of synthesis, the most interesting reactions of CO2 result in the formation of new bonds between the carbon atom and a second group, L. Many reactions of this type are known to proceed in the presence of stoichiometric

75

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4 Carbon Dioxide Coordination Chemistry and Reactivity of Coordinated CO2

or catalytic amounts of transition metal complexes. In principle, these conversions can occur via one of the three following pathways:

• • •

Attack of the reactant at the coordinated CO2: LnM(CO2) + L → LnM-O-C(O)L (see Section 4.3) Reaction of free CO2 with a metal complex: LnM(L) + CO2 → LnM-O-C(O)L Simultaneous coordination of CO2 and L at the metal: LnM(CO2)(L) → LnM-OC(O)L

The thermal lability of CO2 complexes influences their further chemical reactions. The η1 and η2 complexes are very prone to dissociate the coordinated CO2. For example [85], when Aresta et al. studied the mechanism of formation of the peroxocarbonate complex (PCy3)2Ni(CO4) from solid (PCy3)2Ni(CO2) and dioxygen, it was proved – via 13C and 31P NMR spectra at variable temperatures, together with a FTIR study using 18O2-, 13CO2-, and C18O2-labeled compounds – that the reaction involved CO2 de-coordination, O2 coordination, and reinsertion of CO2 into the O–O bond of the reactive newly formed oxygen complex (PCy3)2Ni(O2). When CO2 is bound between two metal centers, the ones that lose CO2 most readily are the μ2-η2 and μ2-η3 complexes, in which one or two of the carboxylate oxygen atoms is (are) bound to a main group atom. These complexes are often intermediates in photochemical and electrochemical reduction reactions of CO2 to CO. Oxidative coupling reactions, reduction reactions and catalytic processes are described in other chapters of this book and in recent reviews [86–88]. In this section, attention is focused only on some recent examples of synthetically useful processes where new C–L bonds are formed, and where the formation of a metal– CO2 complex as an intermediate has been suspected or demonstrated by spectroscopic evidence and/or calculations. The peculiar case of a few bioinspired reactions is also cited. 4.5.1 Oxidative Coupling Reactions

Most olefins undergo oxidative coupling reactions with CO2 in the presence of transition metal centers such as Ni, Fe, Mo, or W, to produce lactones, carboxylic acids, and acrylate derivatives. The reaction of alkenes or alkynes with CO2, mediated by nickel complexes, was reported by Hoberg et al. during the 1980s (see Refs [89, 90]), and supplemented by theoretical studies (see Refs [91, 92]). Since then, studies of oxidative coupling reactions of CO2 with unsaturated compounds at a transition metal center have received much attention, though attempts to overcome the noncatalytic nature of the coupling reaction have proven unsuccessful. In this section, attention will be focused only on recent theoretical studies, the aim being to understand what these reaction mechanisms are, and why so far it has been difficult to obtain catalytic cycles. Detailed theoretical studies have been conducted by Papai et al. on acrylate formation mechanism via Mo- [93] and Ni-assisted [94] C–C coupling between CO2

4.5 CO2 Complexes as Reaction Intermediates in CO2 Utilization Processes

77

and C2H4 from DFT calculations. In the reaction of trans-Mo(C2H4)2(PMe3)4 with CO2, one of the key intermediates was shown to be a complex involving both coordinated CO2 and C2H4 ligands. The coupling took place in a coplanar arrangement via a five-center transition state, and resulted in a near-planar metalla-lactone species. Thus, the coordination of CO2 to the metal center was seen to be a necessary step to achieve the coupling, and was made possible by one phosphine dissociation. The electronic effects of the phosphine ligands were also seen to be important, as the C–C coupling was facilitated by σ-donation, because the metal atom acted as a basic center in the coupling process via metal → CO2 and metal → C2H4 electron donation mechanisms. The overall reaction was found to be exothermic (by 27 kcal mol−1), with all intermediates lying below the reactants level and separated by reasonably low energy barriers; the highest barrier (11 kcal mol−1) corresponded to the C–C coupling step. In contrast, in the reaction Ni (cdt ) + L2 + CO2 + C2H4 → L2Ni (CH2CH2C (O) O) + cdt cdt = cyclododecatriene L2 = bipyridine ( bpy ) or bis (dicyclohexylphosphino ) ethane ( dcpe) the same authors [94] showed that the C–C bond formation occurred in a single step by the reaction of a nickel–ethylene complex with an incoming CO2 molecule, and that the lowest energy pathway did not involve CO2 coordination, as suggested previously [91] (Figure 4.11). Although the L2Ni(CO2)(C2H4) complex has been identified as a minimum on the potential energy surface of the reaction, the barrier to dissociation is extremely low, and so its presence in the reaction mixture would be rather unlikely. The nature of the ancillary ligands was also shown to be important, as it affected the relative stability of the minima associated with the C–C bond formation, and, therefore, the related activation barriers. As noted previously, this was related to the σ-donating ability of the chelating ligand, and, for example, for L2 = dcpe, the C–C bond formation was reversible. This reaction was recently reinvestigated [95] in order to propose a calculated route for the catalytic production of acrylic acid. Similar to previous studies [94],

N

N +

Ni

N

N

N

+ C2H4

Ni

N

N

Ni + CO2 N

N

N

N C O

O

Ni C O O Figure 4.11

N Ni

Ni

Mechanistic details of Ni(0)-assisted oxidative coupling of CO2 with C2H4 [94].

O O

78

4 Carbon Dioxide Coordination Chemistry and Reactivity of Coordinated CO2

Figure 4.12 [95].

Proposed catalytic cycle for the coupling of CO2 and C2H4 to produce acrylic acid

the authors concluded that the formation of the intermediate nickelacycle occurred in a single step via the reaction of nickel-coordinated ethylene with an incoming CO2 molecule (Figure 4.12). The whole calculated catalytic cycle was shown to contain three main barriers: nickelacycle formation (122 kJ mol−1); β-H elimination (147 kJ mol−1); and the reductive elimination of acrylic acid (104 kJ mol−1), that were not expected to hinder the progress of the reaction. Likewise, the catalyst deactivation reaction should not be the cause of noncatalytic activity. It was suggested that the lack of catalytic activity was more likely due to the free energy of the reaction CO2 + C2H4 → CH2=CHCO2H, calculated to be unfavorable by 43 kJ mol−1. Although, the overall thermodynamics created an unfavorable equilibrium, it was possible that the catalytic formation of acrylic acid would occur at the start of the reaction that led to undetectable amounts. Possible solutions may include preventing the reaction from reaching equilibrium through removal of the product. Recently, Aresta et al. [96] also reported a new synthetic approach to acrylates, starting from a preformed Pd-COOMe moiety, and the first formation of ethyl acrylate from ethene and CO2 on a palladium complex (L-L)PdH+. Although, at

4.5 CO2 Complexes as Reaction Intermediates in CO2 Utilization Processes

present the mechanism involved remains unknown, theoretical calculations are currently in progress in an effort to elucidate its details. Factors controlling the regio- and stereo-selectivity of metal-centered CO2 reactions are also often not understood. Theoretical studies performed using DFT calculations on coupling reactions of CO2 with terminal and internal alkynes mediated by Ni(0) complexes were also recently reported [97]. Here, the authors showed that the coupling reaction mainly proceeded through an associative mechanism in which a direct electrophilic attack of CO2 at the π⊥ bond of the η2-coordinated alkyne occurred. Coupling reactions with terminal alkynes provide the same regioselectivity, whatever the alkyne electronic properties are, while electronic factors affect only the reaction barriers. For substituted internal alkynes, both steric and electronic factors favor coupling between CO2 and the substituted carbon. Again, the nature of the ancillary ligand is important, and a more σ-donating ligand, such as dbu (diazabicycloundecene), has been found to be a better ligand than bpy – that is, it leads to lower energy coupling barriers. 4.5.2 Reduction Reactions

By employing enzymes, Nature easily uses CO2 as a one-carbon building block for the synthesis of organic molecules. Yet, the reduction of CO2 to CO requires a large amount of energy, and even in the presence of a strong reducing agent, overcoming the O=CO bond enthalpy (532 kJ mol−1) often presents kinetic difficulties. Some metal complexes (such as the uranium complexes described above [75–77]) abstract oxygen from CO2, but the metal–oxygen bonds formed are strong, and catalytic cycles are difficult to achieve. Recently, Sadighi et al. [98] reported a copper–boryl complex (NHC)Cu(Bpin) (NHC = N-heterocyclic carbene; pin = pinacolato) that abstracts oxygen from CO2 and readily undergoes subsequent turnover: L-Cu-Bpin + CO2 → L-Cu−O-Bpin + CO L-Cu−O-Bpin + ( pinB)2 → ( pinB)2 O + L-Cu-Bpin Shortly afterwards, Zhao et al. proposed a mechanism based on DFT calculations for this reaction [99]. They showed the first step of the reaction to be a η2-CO2 coordination to the copper center, that activates significantly the C=O bond, with an electron transfer of 0.53e from the metal fragment to the CO2 moiety, suggesting a significant back-bonding in the complex. The Cu–B σ-bond was also involved in a bonding interaction with an empty π* orbital on the η2-CO2 ligand. The coordinated CO2 then inserted into the Cu–B bond with a small barrier of 3.8 kcal mol−1, giving a Cu–O–C–B linkage and boryl migration from C to O, followed by a σ-bond metathesis between pinB–Bpin and (NHC)Cu(OBpin) (Figure 4.13). The boryl migration from C to O, which releases CO, was shown to be the rate-determining step of the reaction. The electron richness of the Cu–B bond, due to the low electronegativity of B, gave rise to a small CO2 insertion barrier because the

79

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4 Carbon Dioxide Coordination Chemistry and Reactivity of Coordinated CO2

Figure 4.13

Mechanism of reduction of CO2 to CO catalyzed by Cu(I) boryl complex [99].

back-bonding interaction between the Cu–B bond and the CO2 carbon was recognized as being important in the insertion process. A catalytic reduction of CO2 to CH4 was described in 2006 by Matsuo et al. [100], using hydrosilanes: CO2 + 4 Si–H → CH4 + 4 Si–O, catalyzed by zirconium–borane complexes LZr(CH2Ph)(B(C6F5)4). These authors showed that the combination of both zirconium and boron provided the catalytic activity. In the absence of Zr, the reduction of CO2 was not observed, which suggested that the coordination of CO2 to the Zr center took place prior to the actual reduction. This coordinated CO2 would then be more reactive towards hydrosilylation so as to yield the initial product CH2(OSiR3)2, although the nature of the counterion was also shown to be critical during the course of the reaction. The reductive disproportionation of CO2 to give CO and CO2− 3 , promoted by electron-rich transition metals [9, 101] or to produce C2O2− using lanthanides such 4 as Sm [102, 103], has been observed in a number of systems. For example, Allen et al. have recently isolated an iron carbonyl complex with a bicarbonate counterion (Fe(dmpe)2(CO)H)(HCO3) with dmpe = 1,2 bis(dimethylphosphino)ethane, resulting from the iron(II)-mediated reductive disproportionation of CO2 [104], although the reaction mechanism is still speculative. In a related system, Karsh et al. [9] had observed, during the reaction of CO2 with Fe(PMe3)4, the formation of a side-on complex Fe(CO2)(PMe3)4 and of a carbonyl carbonate species Fe(PMe3)3(CO)(CO3). With regards to actinides, Summerscales et al. [105] have also reported the reductive disproportionation of CO2 to carbonate and squarate products, using COTR/ CpR′ mixed-sandwich U(III) complexes (COT = cyclooctatriene, Cp = cyclopentadienyl, R = SiiPr3, R′ = H, Me). U(III) systems have been shown previously to reduce CO2 to a linear η1-O-coordinated mono-anion [63], and to doubly reduce to give O2− salts plus CO [75, 76]. In this case, the reactions can be considered

4.5 CO2 Complexes as Reaction Intermediates in CO2 Utilization Processes R

R

10

U

+

O

8 CO2

2 R = SiiPr3

d8-toluene -30

25°C

R

R

R 4

U

R

O O

C O

U

1

O

O

U O

R

5b

+

R

O

4

U

R R

Figure 4.14 Reductive disproportionation of CO2 to carbonate and squarate products using a mixed-sandwich U(III) complex [105].

as successive 2e- reductions, although the reaction mechanism has not been described: 8 CO2 + 8 e− → 4 CO23 − + 4 CO + 2 e− → 4 CO23 − + C4O24− The bridging carbonate has an unusual μ-η1:η2-mode of coordination to U atoms with a fluxional structure, whereas the squarate species has a μ-η2:η2 structure (Figure 4.14). This is the first example of the formation of a carbocycle derived purely from a CO2 carbon source. 4.5.3 Catalytic Processes

Recent studies of three different catalytic reactions involving CO2 complexes as reaction intermediates are listed below. In the first reaction, Choi et al. [106] studied the oxidative addition of benzene to RhCl(PMe3)3 for the photochemical formation of mer-Rh(C6H5)(H)Cl(PMe3)3. Interestingly, the presence of an excess of CO2 in the solution promoted the

81

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4 Carbon Dioxide Coordination Chemistry and Reactivity of Coordinated CO2

Figure 4.15 Possible mechanisms for disubstituted urea synthesis (I) and cyclization reactions of epoxides (II) [107].

reaction. The authors suggested possible explanations that included: (i) the formation of a small amount of CO and O=PMe3, followed by the preferential photoreaction of RhCl(CO)(PMe3)2; or (ii) the reversible coordination of CO2 to the Rh center, resulting in the stabilization of productive intermediates such as RhCl(C6H5)(H) (CO2)(PMe3)2. No insertion of CO2 into the Rh–H or Rh–C bond was observed, but the prolonged photoreaction produced a small amount of a phenyl(carbonato) rhodium complex, due to the disproportionation of CO2 to CO2− 3 and CO. The second example dealt with the reactions of amines or epoxides with CO2 on supported nanogold catalysts [107]. These authors showed that polymer-immobilized nanogold catalysts had a very high catalytic activity for the activation of carbon dioxide, with a turnover frequency (TOF) in excess of 50 000 mol mol−1 h−1 for the synthesis of cyclic carbonate, and a turnover frequency of product (TOFP) of 3000 mol mol−1 h−1 for the synthesis of disubstituted ureas. Whereas, the reaction mechanism is not yet clear, it was suggested that the coordination onto the gold particles might represent the key step, and that the activation of CO2 over polymersupported nanogold clusters could be universal (Figure 4.15). The third reaction was related to the hydrocarboxylation of allenes with CO2, catalyzed by a tridendate silyl pincer-type palladium complex [108]. In this reaction, a σ-allyl palladium species (via hydropallation of allene) was formed, and its trigonal bipyramidal geometry allowed the facile coordination of CO2 (presumably in a side-on fashion) and a following nucleophilic addition to realize the carboxylation of allene (Figure 4.16). This reaction proved to be very attractive not only as a CO2 fixation reaction, but also as a general method for the synthesis of βγ-unsaturated carboxylic acids. 4.5.4 Bioinspired Reactions

Among the enzymes that occur in biological systems and which utilize CO2 as a source of carbon, some contain metal atoms as the active center, where CO2 is converted (as in RuBisCO or biotin-dependent carboxylases). However, some of these enzymes do not require metal, or, if so, it does not interact directly with CO2.

4.5 CO2 Complexes as Reaction Intermediates in CO2 Utilization Processes

Figure 4.16 Utilization of silyl pincer-type Pd complex for the hydrocarboxylation of allenes with CO2 [108].

O O

H L

O Zn

L

H + CO2 L

1

L

O Zn

L

H O C O L

L

O

Zn

L L 3 (TS)

2

O H

O Zn

O L

bond rotation vs. proton transfer

L

L 4

H

O

O L

O Zn L

L

5

+ H2O - H+/HCO3Figure 4.17

The mechanism of CO2 fixation by carbonic anhydrase [110].

In natural processes, metal ions are often in high oxidation states (2 or 3), whereas in chemical systems the metals are in low oxidation states (0 or 1). This fact inverts the role of the metal center, such that it acts as a one-electron sink in a natural system, but as a nucleophile in an artificial ones (see other chapters of this book and the review by Aresta et al. [109]). Nevertheless, important biochemical processes such as the reversible enzymatic hydration of CO2, or the formation of metal carbamates, may serve as natural models for many synthetic purposes. Starting from the properties of carbonic anhydrase (a zinc metalloenzyme that performs the activation of CO2), Schenk et al. proposed a review [110] of perspectives to build biomimetic chemical catalysts by means of high-level DFT or ab initio calculations for both the gas phase and in the condensed state. The fixation of CO2 by Zn(II) complexes to undergo the hydration of CO2 (Figure 4.17); the use of Cr, Co, or Zn complexes as catalysts for the coordination–insertion reaction of CO2 with epoxides; and the theoretical aspects of carbamate synthesis, especially for the formation of Mg2+ and Li+ carbamates, are discussed in the review of Schenk

83

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4 Carbon Dioxide Coordination Chemistry and Reactivity of Coordinated CO2

et al. The aim was to suggest novel synthetic pathways that might result in modified metal complexes with catalytic activities comparable to those of natural precursors. The mechanism of the formation of peroxocarbonates LnM(CO4)Xm, and their reactivity as oxygen-transfer agents, is important owing to their potential applications as oxidants of phosphines, olefins, methylene groups, and also as compounds mimicking monooxygenases. Aresta et al. have shown [111], via extended FTIR experiments with labeled 13CO2 and C18O2, that the formation of rhodium peroxocarbonate from CO2 and RhCl(η2-O2)(P)3, with P = PEt2Ph or PEtPh2, proceeds through O–O bond cleavage and CO2 insertion into the O–O bond, rather than the insertion of CO2 into a Rh–O bond, allowing the synthesis of asymmetric 16 O–18O moieties. The further O-transfer to ancillary phosphine ligand to give R3P=O selectively involves the Rh-linked O atom of the peroxo group of RhCl(CO4) (P)3. In this case, no evidence has been found for a direct coordination of CO2 on to the metal center during the reaction.

4.6 Conclusions

Knowledge of CO2 binding and activation has increased dramatically since the first stable CO2 complex Ni(CO2)(PCy3)2 was structurally characterized more than three decades ago [7, 8]. Different binding modes of CO2 in mononuclear and polynuclear metal complexes have been characterized. In 2004, the first stable end-on coordinated CO2 ligand was characterized in a uranium complex [63], followed in 2005 by a novel Al–Mg complex containing CO2 in a linear μ(O,O′) bonding mode [64]. The major improvement that have been made in theoretical calculations and in spectroscopic methods now allow the identification and characterization of stable or unstable complexes, together with the elucidation of reaction mechanisms. Less attention has been paid, however, to CO2 organometallic chemistry during the past decade. Whilst many reduction or coupling reactions are known to proceed in the presence of stoichiometric or catalytic amounts of transition metal complexes, very few examples remain where the formation of a metal–CO2 complex has led to an effective, catalytic reduction reaction of CO2. Carbon dioxide complex photoactivation also represents an attractive route to CO bond cleavage, coupled with O-atom transfer. However, progress in the area of CO2 utilization requires a better understanding of the reaction mechanisms, of the thermodynamics of reaction intermediates, and of structure–reactivity relationships. Although today, more is understood about CO2 coordination, knowledge of the coordination site requirements in various CO2 reactions remains poor, as does that of cooperative interactions with co-ligands. Consequently, systematic studies of this important mechanistic aspect are required, using physico-chemical techniques and computer modeling. It is important to elucidate reaction mechanisms at the molecular level, and topics such as acrylic synthesis from ethylene and CO2

References

or the carboxylation of CH bonds are of major interest. The effective activation of CO2 by metal complexes is a goal that may be difficult to reach, but which represents an exciting research area in organometallic chemistry.

Acknowledgments

J.M. thanks Dr Imre Papai, of the Chemical Research Centre, Budapest, for helpful discussions, and for providing Figures 4.1 to 4.3.

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4 Carbon Dioxide Coordination Chemistry and Reactivity of Coordinated CO2 105 Summerscales, O.T., Frey, A.S.P., Geoffrey, F., Clocke, N., and Hitchcock, P.B. (2009) Chem. Commun., 198–200. 106 Choi, J.C. and Sakakura, T. (2003) J. Am. Chem. Soc., 125, 7762–7763. 107 Shi, F., Zhang, Q.H., Ma, Y.B., He, Y.D., and Deng, Y.Q. (2005) J. Am. Chem. Soc., 127, 4182–4183. 108 Takaya, J. and Iwasawa, N. (2008) J. Am. Chem. Soc., 130, 15254–15255.

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5 Main Group Element- and Transition Metal-Promoted Carboxylation of Organic Substrates (Alkanes, Alkenes, Alkynes, Aromatics, and Others) Thomas Zevaco and Eckhard Dinjus

5.1 Introduction

Carbon dioxide (CO2) is considered today as one of the “bad guys of the chemical scene,” due mainly to its greenhouse gas character and the connected global warming. As outlined in the course of this book, many ways have been proposed from different communities to control the volume of CO2 in the atmosphere; some of these might have concrete applications, whereas the leftovers fall into the “wishful thinking” category. It is therefore a remarkable incentive for “synthetic” chemists to have a share in solving this global problem and to find new promising synthesis routes involving CO2, even though it is clear that chemical ways will still remain a small part of a more comprehensive solution. Carbon dioxide is, from many points of view, an ideal C1-synthon for organic synthesis because of its benignity, low cost, and sheer endless availability as a renewable resource. Therefore, the development of new catalytic methods to activate the otherwise thermodynamically stable CO2 and the resultant formation of new C–C bonds represents a challenge for both academia and industry. From an “atom efficiency” point of view, the C–C coupling of CO2 to an organic substrate to form in a few steps valuable products containing the intact COO moiety (carboxylic acids and carbonates) constitutes a good example for a sustainable “green” chemistry. In this chapter attention will be focused on the chemical transformations of CO2 to yield carboxylic acids and related molecules (e.g., cyclic esters), while “natural” carboxylations reactions such as those dealing with RuBisCO (ribulose-1,5 bis(phosphate) carboxylase/oxidase) and its genetic engineering or the carboxylation of phenol via a phenol phosphate intermediate by the phenol-carboxylase enzyme, will be left aside.

Carbon Dioxide as Chemical Feedstock. Edited by Michele Aresta Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32475-0

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5 Main Group Element- and Transition Metal-Promoted Carboxylation

5.2 Formation of Aromatic Carboxylic Acids: The Kolbe–Schmitt Synthesis 5.2.1 Kolbe–Schmitt Synthesis: Generalities

One of the most reported examples of an industrial synthesis using CO2 as a reagent is the synthesis of aromatic hydroxy-carboxylic acids from the parent alkali metal aryloxides, amino-phenoxides and, to a lesser extent, alkali salts of hydroxypyridines [1]. The products obtained (e.g., 2-hydroxy-benzoic acid, 4-hydroxybenzoic acid, 3-hydroxy-2-naphthoic acid 6-hydroxy-2-naphthoic acid) and the metal salts thereof can be used as starting compounds for pharmaceuticals (aspirin), antibacterial agents and color-developing agents for pressure-sensitive recording sheets and heat-sensitive recording paper [2]. Furthermore, some of these molecules find an application as additives for lubricating oils, as building blocks in polyester derivatives, and as specialty liquid crystals [3]. This synthesis, which is also called the Kolbe–Schmitt synthesis, after its inventors, is possibly the oldest organic synthesis using CO2. Kolbe reported, in 1860, the successful synthesis of 2-hydroxy-benzoic acid (also called salicylic acid due to its initial extraction as methyl salicylate from the bark of willow trees; genus: Salix linnaeus) by heating a mixture of phenol and sodium under an atmosphere of CO2 [4]. This solid-state reaction is usually carried out by contacting dry CO2 with alkali metal aryloxides, or by rapidly passing CO2 through the melted aryloxides. The thus-obtained sodium salicylate is then dissolved in water, the carboxylic acid precipitating upon acidification with sulfuric acid. The reaction is quite selective with sodium and lithium phenoxides, in general generating small amounts of 4-hydroxy benzoic acid as well as traces of 2-hydroxy-isophthalic acid and 4-hydroxyisophthalic acid as side products. Using the same general procedure, Kolbe also produced p-cresotic acid and o-thymotic acids from p-cresol and o-thymol, respectively [5]. The carboxylation occurs more easily when the aromatic compounds are activated by electron-donating substituents, as in the case of aminophenols or diand trihydroxyaromatic compounds. In order to optimize the general yield of this solid-state reaction (less than 50%, due partly o the formation of disodium salicylate and phenol in equivalent amounts), Schmitt investigated the use of a higher CO2-pressure and reported, in 1885, on greatly improved yields (above 80%) [6]. A typical reaction was performed in an autoclave at 403 K with dry sodium phenoxide “stirred” for several hours under a pressure of CO2 ranging from 8 to 13 MPa, to eventually yield (almost quantitatively) the pure sodium salicylate. One of the shortcomings of this method was that the reaction mixture actually is a waxy solid or a highly viscous liquid with a limited contact surface between alkali metal phenoxide and CO2. The Kolbe– Schmitt reaction has also been reported to take place in solution or suspension, which allows a better stirring of the reaction mixture and thus a better contact between the alkali phenoxide and CO2 [7]. Hence, many solvents have been investigated in this role: polar media such as phenol [8], ketones [9], dimethylsulfoxide

5.2 Formation of Aromatic Carboxylic Acids: The Kolbe–Schmitt Synthesis

(DMSO) [10], N-methylcaprolactam, tetrahydrofuran (THF), and tetramethylurea [11] each provided fair yields of the desired acids. Using an apolar medium such as gasoline and xylene affords suspensions, with the medium acting then more as a dispersant [12]; the yields are in those cases slightly better than in the polar media. Although this variant of the Kolbe–Schmitt synthesis can be practically performed as a continuous process, it was found that solvating the alkali metal aryloxides did not provide any significant improvement in the yields, and that the solvent-free Kolbe–Schmitt procedure was still unbeaten in term of high yields. This behavior was understood some years later following a computer-aided molecular design [13], which showed that the solvents, although capable of dissolving the alkali phenoxide and improving the reaction’s rate constants, would also allow the reverse reaction (decarboxylation) to take place because the final product was retained in the reactive phase. However, this is not the case in the classical solventfree Kolbe–Schmitt reaction, as the final step of the reaction mechanism is irreversible. A quite elegant optimization of the Kolbe–Schmitt synthesis was patented by Marassé in 1893 [14]. His modification consisted of reacting, at high temperature, a mixture of free phenol and anhydrous potassium, rubidium, or cesium carbonate under CO2-pressure to give as the main product the related salt of the salicylic acid. Interestingly, running the reaction with the cheaper sodium, magnesium and calcium carbonates afforded disappointingly low yields. Interestingly, this reaction has been newly rediscovered by Yamagushi et al. [15], who reported on the use of supercritical carbon dioxide (scCO2) and various alkali and alkaline earth metal carbonates as promoters of the reaction. K2CO3 and KHCO3 displayed the best catalytic activity for salicylic acid synthesis (36% and 17%, respectively), thus confirming the earlier findings of Marassé. Yamagushi et al. also investigated Lewis acids such as SiO2 and activated ZrO2 (SO2− 4 ), as well as basic oxides (CaO, MgO), and showed these compounds to be inadequate for such reactions in scCO2. In the industrial process used today to manufacture salicylic acid, dry sodium phenoxide obtained from phenol and soda is contacted with CO2 under 0.5 MPa and at temperatures around 373 K. After absorption of approximately one molar equivalent of CO2, the temperature is raised and held at 423–433 K for several hours to fulfill the reaction [16]. The final “technical-grade” salicylic acid is obtained after successive purification steps and, eventually, upon acidification with sulfuric or hydrochloric acid (Figure 5.1). 5.2.2 Reaction Parameters and Mechanistic Studies of the Kolbe–Schmitt Synthesis

Speaking more generally of the selectivity of the Kolbe-Schmitt reaction, the applicability of the synthetic method is narrowed by the presence in aryloxides of numerous potential sites for C–C coupling. The reaction of an alkali phenoxide with one molecule of CO2 can, in theory, generate three isomers: 2-hydroxybenzoic; 3-hydroxy-benzoic acid, the formation of which is not expected due to the activating effect of the hydroxyl group (ortho- and para-directing group [17]); and

91

92

5 Main Group Element- and Transition Metal-Promoted Carboxylation Pure phenol

50% NaOH

Water CO2 Phenol Water 1

Mixer

2

Decolorizing agent

3 Filtration

Treatment

Mill autoclave 1. Phenoxide preparation 2. Carboxylation 3. Dilution

60%H2SO4

Precipitation

Figure 5.1

+

OM

Separation

Drier

Technical grade salicylic acid

Typical industrial synthesis of salicylic acid [7].

CO2 0.5-1 MPa 393-413 K

OH -

+ HX +

OH CO2H

CO2 M

- MX

M = Li, Na Scheme 5.1

Formation of salicylic acid from alkali phenoxides and CO2 under pressure.

4-hydroxy-benzoic acid, which is actually also isolated and finds applications mostly in the polymer industry. In order to understand the general mechanism of the Kolbe–Schmitt reaction and to fine-tune the reaction parameters, many experimental and spectroscopic investigations have been undertaken. In general, these have been tedious, due mainly to the moisture-sensitivity of the starting products and the easy contamination by side products such as inorganic alkali carbonates and hydrogen-carbonates. However, a careful study of the experimental data has led to some general trends. For instance, the nature of the final products depends heavily on the alkali cations used in the starting compounds; sodium and lithium phenoxides reacting under similar experimental conditions yield the related salicylates as major products [18] (Scheme 5.1), whereas potassium, rubidium, and cesium phenoxides yield mixtures of 2-hydroxy-benzoic acid and 4-hydroxy-benzoic acid [1] (Scheme 5.2). As a rule of thumb, the yield of p-hydroxybenzoic acid generally increases with the increasing ionic radius of the alkali metal. Both, temperature and CO2pressure were also reported to be paramount in the selectivity of the carboxylation;

5.2 Formation of Aromatic Carboxylic Acids: The Kolbe–Schmitt Synthesis O M+

CO2 0.5-1 MPa

OH

-

+

CO2 M

CO2H

+ HX +

+ > 473 K -

CO2 M+

OH

OH

OH

93

- MX CO2H

M = K, Rb Cs

Scheme 5.2 Formation of hydroxy benzoic acids from alkali (K, Rb, Cs) phenoxides and CO2.

operating at lower temperatures produces 2-hydroxy-benzoic acids, whereas 4-hydroxy-benzoic acids need a higher temperature for their formation. Hence, running the carboxylation reaction at a higher temperature will yield 4-hydroxybenzoic acid, even from sodium phenoxide. The same trend was identified when reacting potassium phenoxide under one atmosphere of CO2, namely that a higher yield of the para product was delivered with increasing temperature. Interestingly, the same reaction when performed under pressure afforded another pattern, with the yield of 4-hydroxy-benzoic acid decreasing with increasing temperature. The same outcome was observed for the carboxylation of potassium and cesium phenoxides at temperatures in the range of 423 to 533 K, at constant pressure [19]. However, this situation was explained by the impossibility in a closed system to distil the phenol formed in situ, so as to displace the equilibrium towards the alkali salts of the hydroxy-carboxylic acids. Among the numerous reaction mechanisms proposed previously [20–22], the most plausible have involved the formation of an alkali metal phenoxide–CO2 complex at an early stage of the reaction. Sodium and potassium phenoxide–CO2 compounds can be easily formed at room temperature after some hours under 0.1 MPa CO2 [11]. At a higher temperature (423 K) and high CO2-pressure, the sodium phenoxide–CO2 complex intermediate undergoes an intramolecular rearrangement to produce the sodium salt of salicylic acid. In comparison, heating such intermediates under normal pressure mostly results in a decarboxylation of the compound and the liberation of phenol. Despite numerous spectroscopic and theoretical studies, the exact structure of the active species (the alkali metal phenoxide–CO2 complex) formed in the first stage of the Kolbe–Schmitt reaction remains the subject of much debate. Some of the structures proposed, which range from pure alkali phenyl carbonates to “CO2-solvated alkali phenoxides”, are shown in Figure 5.2 for the case of potassium phenoxide. Early studies postulated the straightforward formation of alkali phenyl carbonate 1 (PhOCO2M, with M = K or Na) as the reactive intermediate [4, 5]. Major discrepancies in the in situmeasured infrared (IR) spectra, such as no characteristic carbonate νas(C=O) absorption band around 1750 cm−1, were the main reasons for the search for more suitable structures. On the basis of further IR studies and differential scanning calorimetry (DSC) analyses [11], it appeared that the alkali metal phenoxide–CO2 complex, when reacted under Kolbe–Schmitt conditions, underwent an irreversible phase transition and evolved to produce at least two further intermediates. In

94

5 Main Group Element- and Transition Metal-Promoted Carboxylation O

K O

O C

O

C

M

O

O

OCO2K 2

101.236

1.407 1.410

1

K

O

O

3

2 1.413

4

1.597

1 5

6

8 1.397

2.594

2.624

K

C O 3 Figure 5.2

13 1.185 12

7* 1.270 9

16 15 14 1.186

1.201

11

2.775

2.772

1.198

Proposed structures for the PhOK–CO2 complex in the Kolbe–Schmitt synthesis.

complementing other studies, this investigation rules out a direct carboxylation at the aromatic ring. Based on the results of these studies, the alkali metal phenoxide–CO2 complex definitely plays a key role in the formation of the hydroxy benzoic acids, and an understanding of its particular structure would elucidate the ortho/para selectivity of the reaction. Structure 2 is a promising candidate able to explain the para and ortho reaction pathways. This intermediate exhibits weak interactions between π-electrons of the aromatic ring and the carbon atom of CO2, whereas CO2 is activated by the cation and the oxygen-alkali ion bond is weakened. This structure was proposed on the basis of IR and nuclear magnetic resonance (NMR) data [22], and was assumed to further react via C-alkylation at the ortho position and proton transfer to the phenolic oxygen. The existence of structure 2 was later reinforced by an X-ray diffraction (XRD) structural study of the solvent-free sodium phenoxide [11]. This crystal structure displayed as its main feature polymeric chains of Na2O2 four-membered rings generated by the stacking of PhO−Na+ pairs. The structure showed significant short distances between the cation of one unit and the aromatic ring of the next unit (average 3.05(8) Å), which spoke in favor of potential weak interactions between an electron-rich aromatic ring and the partly positively charged carbon atom of CO2. Such interaction with the bulky phenyl group led to a remarkable low-coordinated sodium ion suitable to coordinate and activate CO2, but this postulated PhONa–CO2 π-complex could not be confirmed experimentally. Structure 3, as proposed by Kosugi et al., displayed a complex transient carbonate species formed via concurrent interactions between the phenoxide ion, CO2, and the potassium cation [23]. The structure of intermediate 3, as well as the proposed mechanism, were rather speculative, however. Swimming against the mainstream, Kosugi proposed on the basis of NMR experiments with 13C-labeled CO2 and MOPAC/PM3 calculations, that the reaction proceeded by direct carboxylation of

5.2 Formation of Aromatic Carboxylic Acids: The Kolbe–Schmitt Synthesis

the aromatic ring. Consequently, the PhONa*CO2 complex was deemed not to be the key intermediate in this process; rather, its formation should be considered more as a side reaction competing with the formation of either salicylic acid or p-hydroxybenzoic acid. Whilst the studies focused mainly on the carboxylation of potassium phenoxide, an exact mechanism confirming their hypothesis was not presented. Considering the experimental difficulties and the relative inconsistency of some experimental data, computer-aided molecular design (CAMD) and molecular modeling have attracted ever-growing interest to evaluate the structures of these intermediates and to better understand their activation mechanism(s) in the Kolbe–Schmitt synthesis [24, 25]. The comprehensive studies of Markovic et al. have shed new light on the structure of the intermediate(s), and may actually clarify many experimental findings. The research group first investigated the role and structure of the sodium phenoxide−CO2 complex 4 (Figure 5.2) in the formation of salicylic acid [24a] and, subsequently, also the influence of the alkali ion on the ortho/para selectivity of the Kolbe–Schmitt synthesis [24b]. Their calculations confirmed that the reactions would most likely proceed in the ortho and para positions, with the noticeable exception of lithium phenoxide which yields only salicylic acid. In fact, the yield of the para-substituted product was found to increase as the ionic radius of the alkali metal used was increased, and that the high ortho selectivity of the lithium and sodium phenoxides was a direct consequence of their short ionic radii. The group of Markovic eventually examined the structure–reactivity relationship of the potassium phenoxide−CO2 complex [24c], with special attention being paid to the last reaction step of the para route of the Kolbe−Schmitt reaction [24d] (Scheme 5.3). These studies, which employed density functional theory (DFT) methods (B3LYP/LANL2DZ/Gaussian 98) proposed that the reactions of all alkali metal phenoxides with CO2 followed a similar ground mechanism that comprised three intermediates and three transition states. In step 1, CO2 must first be activated by an alkali metal phenoxide. In the case of the sodium phenoxide [24a], CO2 can only attack at the polarized O–Na bond to form a PhONa/CO2 complex as the first intermediate (structure 4). The calculation definitely rules out a direct C–C bond formation at the aromatic ring. PhONa + CO2 → B → TS1 (o- and p-) → C → TS2 → D → TS3 → Sodium salicylate

(5.1)

Scheme 5.3 Structure of the first ortho- and para-intermediates formed in the Kolbe–Schmitt synthesis, as proposed by Markovic et al. [24c, d].

95

5 Main Group Element- and Transition Metal-Promoted Carboxylation 50 40 30

1.446

3

1.382

2

4

1.435

1.453

1

5

4

1.313

1.386

O

1.441 1.308 O 1.462 1

6

3

1.448

2 2.134

2.259

20

2.191

7 O 1.1281.246O

Na

10

2.126

Na 2.187

pTS2-K

PhO-K+CO2 0

pTS1-K

–10

oTS1-K

pC-K oC-K

O

C

O C O M O oC-M O M O O C

M

O

H

pD-K oD-K

oTS2-K

H C O

H

–20 B-K

–30 –40

oTS3-K pTS3-K

1.384

2.120

7 O 1.222 1.246 O

E (kcal mol–1)

96

O

O M O

O M C O H

pC-M

oE-K

oD-M

O

pE-K

pD-M

–50 Reaction coordinate Figure 5.3 Calculated energy diagram for the formation of ortho- and para-hydroxyl benzoic acid from potassium phenoxide (also shown are the transition states p-TS-Na and o-TS-Na for the sodium derivatives) [24c, d].

In step 2 of the reaction, the electrophilic carbon atom of CO2 attacks the ring at the ortho position. The reaction proceeds via the transition states oTS1 and TS2 (Figure 5.3), with formation of the intermediates C and D. The final product, sodium salicylate, is obtained from intermediate D through a proton shift from the ring to the phenoxide ion via the transition state TS3. It can be shown from the figure that intermediates C and D can easily undergo the reverse reactions, thus confirming the role of the CO2-pressure to shift the equilibrium towards the final products of the reaction. Interestingly, studies dealing with the potassium phenoxide−CO2 complex [24d, e] and the nature of the intermediates responsible for the ortho and para routes could partly explain the result obtained by Kosugi and coworkers in their experiment with 13C-labeled CO2 [23]. Markovic et al. completed their Kolbe−Schmitt models with a microsolvated model, and distinguished two starting geometries for the potassium phenoxide−CO2 complex: (i) a structure with no supplementary coordinated CO2 molecules (atmospheric pressure); and (ii) a structure with one to two CO2 molecules coordinated at the potassium atom (maximum of two due to computing limits) (Figure 5.4) [24d]. The latter microsolvated structure represents the potassium phenoxide− CO2 complex under Kolbe−Schmitt experimental conditions (high CO2-pressures). Carbon dioxide can then be seen as a solvent stabilizing the KOPh–CO2 complex.

5.2 Formation of Aromatic Carboxylic Acids: The Kolbe–Schmitt Synthesis 10 10 1.406 1.410

3

2

4

1.412 1.635

1 5

6

1.233 7* 1.266

8

1.396

9

1.410

3

2

4

1.413 1.597

1 5

2.535

1.407

1.397

1.236 1.270

7*

K

12

8

6

2.594 2.624

K 2.564

13

9

16 15 14

1.185

1.201

11

2.775

2.772

1.186 1.198

Figure 5.4 Calculated structures of a potassium phenoxide–CO2 “nonsolvated” and “solvated” with two CO2 molecules [24d].

It appears that, under the conditions of the Kolbe–Schmitt reaction (high pressure and temperature), the labeled potassium phenoxide–*CO2 complex of Kosugi undergoes a scrambling of labeled and unlabeled CO2 molecules: KOPh-13CO2 ⋅2 ( 12 CO2 ) → TS1⋅2 ( 12 CO2 ) → TS1⋅( 13 CO2 )⋅( 12 CO2 ) → KOPh12CO2K ⋅( 13 CO2 ) ⋅ ( 12 CO2 )

(5.2)

As the ortho and para intermediates formed within the KS synthesis are also CO2solvated, a constant exchange of labeled and unlabeled CO2 molecules will occur such that the amounts of 13C-labeled products of the Kolbe–Schmitt reaction are constantly leveled. The results obtained by Markovic et al. were confirmed, using quantum mechanical calculations, by Stanescu et al. [13, 25], with another software package (Jaguar 4.2.) and a slightly different data set B3LYP/CEP-31+G(d). The energy profiles calculated for the reaction path followed the same pattern. Stanescu took this model as basis to understand the influence of different “solvents” on the course of the Kolbe–Schmitt reaction, and was able to confirm the experimental trends. Markovic and coworkers extended these DFT calculations (B3LYP/LANL2DZ) so as to understand the mechanism of the carboxylation of 2-naphthoxide which yields, at high temperatures, mixtures of 3-hydroxy-1naphthoic acid [26a] as a main product, as well as 2-hydroxy- and 6-hydroxy-2naphthoic acids [26b]. 5.2.3 Recent Applications of the Kolbe–Schmitt Carboxylation: Synthesis of 1,3-Dialkylimidazolium-2-Carboxylates

Tommasi et al. recently reported the carboxylation of 1,3-dialkylimidazolium-2carboxylates, which can be seen as a remarkable variant of the Kolbe–Schmitt synthesis [27]. In this case, it was shown that 1,3-dialkylimidazolium-2carboxylates could be synthesized from 1,3-dialkylimidazolium chlorides and CO2 via a Kolbe–Schmitt-type reaction (actually, this was more of a Marassé variant, run in solution). The starting compounds were carboxylated in anhydrous dimethylformamide (DMF) under approximately 5 MPa CO2, at temperatures ranging from 353 to 408 K and with Na2CO3/CO2 as a “catalyst,” according to Scheme 5.4.

97

98

5 Main Group Element- and Transition Metal-Promoted Carboxylation

P(CO2)=5 MPa 2

1

R

R

Cl 1

+ Na2CO3

2

1

Δ DMF

R

R

+ NaHCO3

C O

O

2

R =R = CH3 1

2

R = CH3, R = n-Bu Scheme 5.4 Formation of 2-dialkyl-imidazolium-carboxylates following the Kolbe–Schmitt synthetic procedure [27].

The yields of the reaction were very high, as was the selectivity of the C–C coupling at carbon 2 (although traces of 4- and 5-carboxylate isomers were identified at higher temperatures). The synthesis of such 2-imidazolium carboxylates is a topic of growing interest, owing to their use as potential ionic liquids (ILs) with tunable characteristics, and their ability to carboxylate C–H acidic compounds [28]. The results showed that a direct carboxylation could be carried out at relatively moderate temperatures (353–383 K), in good yield, and without the formation of significant amounts of side products (Scheme 5.4). For instance, running the reaction at 383 K for 36 h delivered 1,3-dimethylimidazolium-2-carboxylate in high yield (92%) and selectivity (91%), with the 4-carboxylate isomer as a minor side product (9%). The presence of other substituents caused the formation of other isomers, as shown by the case of the 1-butyl-3-methylimidazolium salt which, under the same conditions, produced a mixture of 1-butyl,3-methylimidazolium-2carboxylate (82%), 1-butyl-3-methylimidazolium-4-carboxylate (10%), and 1-butyl-3methylimidazolium-5-carboxylate (8%) [27]. Incidentally, another synthesis route of 1,3-dimethylimidazolium-2-carboxylate was reported by Holbrey et al., using 1-methyl-imidazole as substrate and dimethyl carbonate as a “carboxylating” agent [29a]. This synthesis has been extensively investigated, using DFT calculations (B3PW91), by Voutchkova et al. [29b]. Both, the Kolbe–Schmitt reaction and the dialkyl carbonate route may be beneficial, notably when taking into account the former syntheses used to produce these carboxylato derivatives. Prior to the study of Tommasi et al., the formation of imidazolium carboxylates via highly reactive N-heterocyclic carbenes (NHC) had