High Pressure Chemistry

Synthetic, Mechanistic, and Su percritical Applications

Edited by Rudi van Eldik and Frank-Cerrit Klarner

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High Pressure Chemistry Synthetic, Mechanistic,

and Supercritical

Applications

Edited by Rudi van Eldik and Frank-Gerrit Klamer

Further Titles of Interest

B. Comils, W. A. Herrmann, R. Schlogl, C.-H. Wong (Eds.)

Catalysis from A to Z A Concise Encyclopadie

1 :S”rN 3-527-29588-X

F. Diederich, P. J. Stang (Eds.)

Metal-catalyzed Cross-coupling Reactions

1999

ISBN 3-527-29421-X

P. Braunstein, L. A. Oro, P. R. Raithby (Eds.)

Metal Clusters in Chemistry Three Volumes

1999 ISBN 3-527-29549-6

D. E. De Vos, I. F. J. Vankelecom, P. A. Jacobs (Eds.)

Chiral Catalysts Immobilization and Recycling

2000

ISBN 3-527-29952-1

FIZ CHEMIE Berlin (Ed.)

Dictionary o f Common Names

Second, Extensively Enlarged Edition (5 Vol.) 2001 ISBN 3-527-30288-3

High Pressure Chemistry

Synthetic, Mechanistic, and Su percritical Applications

Edited by Rudi van Eldik and Frank-Cerrit Klarner

Prof: Dr. Rudi yon Eldik University of Erlangen-Numberg Institute of Inorganic Chemistry EgerlandstraBe 1 91058 Erlangen Germanv

Pro$ Dr. Frank-Cerrit Klarner University of Essen Institute of Organic Chemistry UniversitatsstraXe 5 45141 Essen Germany

Cover Dr. Frank Wurche University of Essen

This book was carefully produced. Never- theless, editors, authors and publisher do not warrant the information contained therein 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 A catalogue record for this book is available from the British Library. Die Deutsche Bibliothek - CIP Cataloguing-in-Publication-Data A catalogue record for this publication is available from Die Deutsche Bibliothek

0 WILEY-VCH Verlag GmbH, 69469 Weinheim (Federal Republic of Germany). 2002 All rights reserved (including those of translation in 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 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.

Printed in the Federal Republic of Germany. Printed on acid-free paper.

Typesetting Asco Typesetters, Hong Kong Printing betz-dmck gmbH, Darmstadt Bookbinding J. Schaffer GmbH & Co. KG, Griinstadt

ISBN 3-527-30404-5

Contents

I Basic Principles 1

l v

1

1.1 1.2 1.3 1.3.1 1.3.1.1 1.3.1.2 1.3.2 1.3.2.1 1.3.2.2 1.3.3 1.3.4 1.4 1.5

2

2.1 2.2 2.2.1 2.2.2 2.2.3 2.2.4 2.2.5 2.2.6 2.3 2.3.1

Effect of Pressure on Inorganic Reactions: Introduction and Mechanistic Applications 3 Rudi van Eldik and Colin D. Hubbard Introduction 3 Determination of Volumes of Activation Thermal-Induced Reactions 12 Ligand Substitution Reactions 12

Octahedral Complexes 13

Square-Planar Complexes 19

Electron Transfer Reactions 21

Self-Exchange Reactions 22

Non-Symmetrical Reactions 22 Actvation of Small Molecules 25 Addition and Elimination Reactions 30

Photo- and Radiation-Induced Reactions 32

Concluding Remarks 35 Acknowledgments 36

References 37

6

The Effect of Pressure on Organic Reactions: Basic Principles and Mechanistic Applications 41

Frank Wurche and Frank-Cerrit Kliirner Introduction 41

Cycloadditions 45 Diels-Alder Reactions, Mechanistic Aspects 45 Complex Reactions and Synthetic Applications 1,3-Dipolar [3 + 21 Cycloadditions [2 + 21 Cycloadditions 66

[6 + 41 and [8 + 21 Cycloadditions Cheletropic Reactions 70

Pericydic Rearrangements 71 Sigmatropic [ 3.31 Shifts: Cope and Claisen Rearrangement

56 64

68

71

vi I Contents

2.3.2 2.3.3 2.3.4 2.3.5

2.4 2.4.1 2.4.2 2.4.3 2.5 2.6

3

3.1 3.2 3.3 3.4 3.5 3.6 3.7

II

4

4.1 4.2 4.3 4.3.1 4.3.2 4.3.3 4.3.4 4.4 4.5

5

5.1 5.2

Electrocyclic Rearrangements 74

Intramolecular Diels-Alder Reactions 76 Ene Reactions 77 The Relationship Between Activation or Reaction Volume and Ring Size 81 Free-Radical Reactions 84

Homolytic Bond Dissociations 84

Quinone Oxidations (Hydrogen Transfer Reactions) Free Radical Cyclizations 87

Ionic Reactions 88 Concluding Remarks 90 Acknowledgments 91

References 91

85

High-pressure Kinetics and Highly Viscous Media Tsutomu Asano High Pressure and Dynamic Solvent Effects Selection of Reaction Systems 101 Z / E Isomerization of N-Benzylideneanilines 103 Z / E Isomerization of Push-Pull Substituted Azobenzenes Z/E Isomerization of Carbocyanine Cations 124

Z / E Isomerization of DNAB in a Polymeric Medium Concluding Remarks 125 Acknowledgments 126 References 126

97

97

109

122

Mechanistic and Synthetic Applications o f High Pressure 129

Water Exchange on Metal ions: The Effect o f Pressure Lothar Helm and Andrh E. Merbach Introduction 13 1

Concepts of Solvent Exchange Reactions Water Exchange from the First Coordination Shell First Row Transition Metal Ions Second- and Third-Row Transition Metal Ions Lanthanides 144

Main Group Elements 151 Water Exchange from the Second Coordination Shell Conclusions 157 Acknowledgments 157 References 158

231

133 136

136 142

255

insights into Solution Chemistry from High Pressure Electrochemistry Thomas W. Swaddle Introduction 161

Pressure Effects on the Kinetics of Self-Exchange Reactions

261

162

5.2.1 5.2.2 5.2.3 5.2.4 5.3 5.4 5.4.1 5.4.2 5.5

6

6.1 6.1.1 6.1.2 6.2 6.2.1 6.2.2 6.3 6.3.1 6.3.2 6.3.3 6.4 6.5

7

7.1 7.1.1 7.2 7.2.1 7.2.2 7.2.3 7.3 7.4 7.5

8

8.1 8.2

Principles 162

Experimental Observations with Aqueous Media Experimental Observations with Non-aqueous Media Approaches Involving Electrochemistry 168 Electrochemical Measurements at High Pressure 170 Homogeneous Versus Heterogeneous Electron Transfer Aqueous Solutions 171

Non-aqueous Media 174

Conclusions 180

Acknowledgments 181

References 18 1

164

167

172

Pressure Effects on the Photoreactions o f Transition Metal Complexes

Peter C. Ford and Leroy E. Laverman Introduction 284

Pressure Effects on Excited State Energies Pressure Effects on Excited State Kinetics Unimolecular Excited State Reactions 196 Ligand Photosubstitution Reactions 196 Unimolecular Photoredox Decompositions 205 Bimolecular Pathways For Excited State Deactivation Excited State Electron and Energy Transfer Reactions Brarnsted Base Quenching 211 Reactions with Lewis Bases to Give an Excited State Complex Photochemically Generated Reactive Intermediates 213

Summary 218 Acknowledgments 22 9 References 22 9

184

186 188

206 207

212

Application of High Pressure in Transition Metal-Catalyzed Reactions

Oliver Reiser Introduction 223 General Principles 224

Lewis Acid Catalyzed Reactions 224

Cycloadditions 224

Nucleophilic Substitution 229 Addition of Nucleophiles to Carbonyl Compounds Palladium Catalyzed Reactions 230 Rhodium-Catalyzed Hydroboration 236

Conclusion 237 References 237

223

229

High Pressure in Organic Synthesis: Influence on Selectivity Lutz F. Tietze and Peter L. Steck Introduction 239

Influence of High Pressure on Selectivity

239

243

viii I Contents

8.2.1 8.2.2 8.2.3 8.2.3.1 8.2.3.2 8.2.4 8.3

9

9.1 9.2 9.3 9.3.1

9.3.1.1 9.3.1.2 9.3.2 9.3.2.1 9.3.3

9.4

9.5

10

10.1 10.2 10.2.1 10.2.2 10.2.3 10.2.4 10.3 10.3.1 10.3.2 10.3.2.1 10.3.2.2 10.3.2.3 10.3.3

General Remarks 243 Chemo- and Regioselective Transformations 245 Diastereoselective Transformations 253 Reactions with Simple Diastereoslectivity 253 Reactions with Induced Diastereoselectivity 267 Enantioselective Transformations 275 Conclusion 280

Acknowledgements 281 References 281

High-pressure Promoted Cycloadditions for Application in Combinatorial Chemistry 284 George J. T. Kuster and Hans W. Scheeren Introduction 284 High-pressure Diels-Alder Reactions on the Solid Phase High-pressure Multicomponent Domino Cycloaddition Reactions High Pressure-Promoted One-Pot Three-Component [4 + 2]/[3 + 21 Cycloadditions: Scope and Limitations 287 Mono Substituted Dipolarophiles 291 Di-Substituted and Cyclic Dipolarophiles 292 Novel 5,5-Membered Bi- and Tricyclic Nitroso Acetals Pressure and Solvent Dependency 296 High Pressure-Promoted Domino [4 + 2]/[4 + 2]/[3 + 21 Cycloaddition of 2-Methoxy- 1,3-Butadiene and p-Nitrostyrene 298

High-pressure Domino [4 + 2]/[3 + 21 Cycloaddition Reactions on the Solid Phase 299 Conclusions and Outlook 302 References 303

285 286

295

Catalytic and Solvophobic Promotion o f High Pressure Addition Reactions 305 GerardJenner Introduction 305 Chemical Activation 306 Catalysis by Lithium Salts 309 Catalysis by Lanthanides and Related Periodic Elements Catalysis with Other Lewis Acids Catalysis by Phosphines 320 Solvophobic Activation 321 Water and Water-like Solvents 322 Kinetic Studies in Water and Water-like Solvents Michael Reactions 325 Diels-Alder Reactions 327 Effect of Addition of Micelles and Cyclodextrines in Aqueous Media Svnthetic Aovlications 337

312 319

324

336

Contents I ix

10.3.3.1 Diels-Alder Reactions 338 10.3.3.2 Michael Reactions 342

10.3.2.3 Baylis-Hillman Reactions 343

10.3.3.4 Triactivation 343 10.4

11

11.1 11.2 11.3 11.3.1

11.3.2

11.3.3

11.3.4

Conclusions 344

References 345

Future Perspectives: Applications o f High Pressure in Supramolecular Chemistry 348 Robert Rulofl Christophe Saudan, Andri E. Merbach and Frank-Cerrit Klarner Introduction 348

Biomolecules under Extreme Conditions 349

The Effect of Pressure on the Formation of Host-Guest Complexes Inclusion of Helium Atoms or Acetonitsile Molecules in C ~ O or a Hemicarcerand as Molecular Containers 351 Complexation of Cations and Cbo as Guests with Crown Ethers, Cryptands, and Calixarenes as Hosts Molecular Tweezers as Synthetic Receptors: Focussing on Volume and Entropy of Association 354 Formation of Host-Guest Complexes of a-Cyclodextrins with Azo Dyes: Determination of Activation and Reaction Volumes

352

353

357

11.3.5

11.4

Ill

12

12.1 12.1.1 12.1.2 12.1.3 12.2 12.2.1 12.2.2 12.3

12.3.1 12.3.2

11.3.4.1 Inclusion of Short Guests into the a-Cyclodextrin Cavity 11.3.4.2 Sequential Threading of a-Cydodextrin onto a Long Guest

357

Self-Assembled Multinudear Coordination Species with Chiral 362

Bipyridine Ligand 363 Conclusion and Outlook 367 Acknowledgments 368 References 368

Chemical Reactions in Supercritical Fluids 369

Catalytic Reactions in Supercritical Fluids Jason Hyde, Walter Leitner and Martyn Poliakoff Introduction to Catalytic Reactions in Supercritical Fluids Solvent Properties of Supercritical Fluids Temperature and Pressure Relations 372

Decaffeination of Coffee via scCOz Extraction of Caffeine Practical Aspects of Catalpc Reactions in scC02 Heterogeneously Catalyzed Reactions 375 Homogeneously Catalyzed Reactions 377

Acid-Catalyzed Continuous Flow Processes in Supercritical Fluids 379 Heterogeneously Catalyzed Alkylation Reactions 379 Heterogeneously Catalyzed Etherification and De-symmeterization Reactions 380 ~

372

372 372

374

375

x I Contents

12.4 12.5 12.5.1 12.5.2 12.6 12.6.1 12.6.2 12.7

13

13.1 13.2 13.2.1 13.2.2 13.3 13.3.1 13.3.2 13.3.3 13.3.4 13.3.5 13.4

14

14.1 14.2

14.3 14.3.1

Homogeneously Catalyzed C-C Coupling Reactions Hydrogenation Reactions 383 Heterogeneously Catalyzed Hydrogenation Reactions 383 Homogeneously Catalyzed Hydrogenation Reactions 386

Hydroformylation Reactions 389 Homogeneously Catalysed Hydroformylation Reactions 389

Heterogeneously Catalyzed Hydroformylation Reactions 393

Closing Remarks 394 References 395

381

Application of Supercritical Fluids in the Fine Chemical Industry Werner Eonrath and Reinhard Karge Introduction 398 Supercritical Fluids in Separation/Purification 400

Supercritical Fluid Extraction (SFE) 400

Supercritical Fluid Chromatography (SFC) 403 Catalytic Reactions in Supercritical Fluids 405 Hydrogenation 406 Methylation 42 1

Friedel-Crafts Alkylation Reactions 412

Oxidation 415 Other Reaction Types 41 7

Concluding Remarks 419

Acknowledgments 41 9 References 41 9

398

Applications of Supercritical Water 422 Eckhard Dinjus and Andrea Kruse Introduction 422

Physico-Chemical Properties of Water at High Temperature and Pressure and their Relation to Applications Supercritical Water in Chemical Synthesis Organic Reactions 425

423 425

14.3.1.1 Hydrolysis Reactions 425 14.3.1.2 Condensations 427

14.3.1.3 Diels-Alder Reactions 427 14.3.1.4 Rearrangements 428 14.3.1.5 Friedel-Crafts Reactions 429 14.3.1.6 Partial Oxidations 430 14.3.1.7 Reduction Reactions 431 14.3.2 Organometak Reactions 43 I

14.3.2.1 Heck Coupling 431

14.3.2.2 Cyclotrimerization of Alkynes 432 14.3.2.3 Hydroformylation 433

14.3.3 Inorganic Reactions 434

14.4 Supercritical Water in Decomposition Reactions 434

14.4.1 Oxidation in Supercritical Water 434

14.4.2 Gasification in Supercritical Water 437

14.5 Conclusions 440

14.6 Outlook 441

References 442

Contents I xi

Index 447

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I xiii

High pressure chemistry is an area that has developed a vigorous activity over the past decades. Although most of the earlier work was mainly performed in the area of organic chemistry, a major contribution from inorganic chemists over the past two to three decades resulted in the development of sophisticated instrumentation that enables the study of fast chemical reactions under high pressure. More re- cently, the application of supercritical fluids has received much attention especially in chemical industry. Numerous reviews have reported on the progress made in these areas over the past years.

The monograph consists of fourteen contributions bascd on oral presentations at the European High Pressure Research Group Meeting held at Kloster Banz, Germany, in September 2000. The theme of the meeting was High Pressure Chemistry. It covers contributions from high pressure inorganic and organic chemistry, as well as the application of supercritical fluids in chemical synthesis and processes. The monograph is subdivided into three sections. The first three chapters are devoted to basic principles involved in the application of high pressure techniques in inorganic and organic chemistry. The subsequent eight chapters are devoted to mechanistic and synthetic applications of high pressure in inorganic, organometallic, organic, and supramolecular chemistry. The final three chapters are devoted to chemical reactions in supercritical fluids and cover catalytx re- actions, applications in the fine chemical industry and the application of super critical water. All in all, the individual chapters reveal the present status of high pressure chemistry and its application in a variety of areas.

The editors appreciate the co-operative support they received from the individual authors of the chapters, as well as the effective interaction with Wiley-VCH. The efforts of numerous scientific coworkers and the financial support from many funding agencies have all contributed to bringing high pressure chemistry to where it stands at present. May we all in future benefit from these developments and stimulate further activities of the next generation in this area of chemistry.

Rudi van Eldik University of Erlangen-Niirnberg

Frank-Gerrit Klarner University of Essen

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I xv

List of Authors

Tsutomu Asano Department of Applied Chemistry Faculty of Engineering Oita University 700 Dannoharu Oita 870-1192

Japan

Werner Bonrath Vitamins and Fine Chemicals Division Chemical Process Technology F. Hoffmann-LaRoche Ltd. GrenzacherstraRe 124 CH-4070 Basel Switzerland

Eckhard Dinjus Institut fur Technische Chemie Chernisch Physikalische Verfahren Forschungszentrum Karlsruhe Hermann-von-Helmholt-Platz 1 76344 Eggenstein-Leopolshafen Germany

Rudi van Eldik Institute for Inorganic Chemistry University of Erlangen-Nurnlierg EgerlandstraRe 1 91058 Erlangen Germany

Peter C. Ford Department of Chemistry and Biochemishy University of California Santa Barbara CA 93106 U.S.A.

Lothar Helm Institut de Chimie Minerale et Analytique Universite de Lausanne BCH

CH-1015 Lausanne Switzerland

Colin D. Hubbard Institute for Inorganic Chemistry University of Erlangen-Niirnberg Egerlandstrage 1 91058 Erlangen Germany

Jason Hyde School of Chemistry University of Nottingham llniversity Park Nottingham NG7 2RD U.K.

Gerard Jenner Laboratoire de Piezochimie Organique Universite Louis Pasteur 1: Rue Blaise Pascal 67008 Strasbourg Ckdex France

Reinhard Karge Vitamins and Fine Chemicals Division Chemical Process Technology F. Hoffmann-LaRoche Ltd. Grenzacherstrage 124 4070 Basel Switzerland

Frank-Gemt Klarner Institut fur Organische Chemie Universitiit Essen UniversitatsstraBe 5 45141 Essen Germany

Andrea h s e Institut fur Technische Chemie, Chemisch Physikalische Verfahren

xvi I List ofAuthors

Forschungszentrum Karlsruhe Hermann-von-Helmholtz-Platz 1 76344 Eggenstein-Leopolshafen Germany

George J . T. Kuster Department of Organic Chemistry NSR Center for Molecular Structure, Design

University of Nijmegen Toernooiveld 1 6525 ED Nijmegen The Netherlands

Leroy E. Laverman Department of Chemistry and Biochemistry University of California Santa Barbara CA 93106 U.S.A.

Walter Leitner Max-Planck-Institut fur Kohlenforschung Kaiser-Wilhelm-Platz 1 45470 Miilheim an der Ruhr Lehrstuhl fur Technische Chemie und Petrolchemie RWTH Aachen Woringer Weg 1 52056 Aachen Germany

Andre E. Merbach Institut de Chimie Minkale et Analytique Universiti. de husanne BCH 1015 Lausanne Switzerland

Martyn Poliakoff School of Chemistry University of Nottingham University Park Nottingham NG7 2RD U.K.

Oliver Reiser Institut fur Organische Chemie Universitat Regensburg UniversitatsstraBe 31 93053 Regensburg Germany

and Synthesis

Robert Ruloff lnstitut de Chimie Mmerale et Analytique Universite dc Lausanne BCH 1015 Lausanne Switzerland

Christophe Saudan Institut de Chimie Minerale et Analytique Universitk de Lausanne BCH 1015 Lausanne Switzerland

Hans W. Scheeren Department of Organic Chemistry NSR Center for Molecular Structure. Design

university of Nijmegen Toernooiveld 1 6525 ED Nijmegen The Netherlands

and Synthesis

Peter L. Steck Institut fur Organische Chemie Georg-August-Universitat Tammannstrasse 2 37077 Gottingen Germany

Thomas W. Swaddle Department of Chemistry University of Calgary Calgary Alberta T2N 1N4 Canada

Lutz F. Tietze Institut fur Organische Chemie Georg-August-Universitat Tammannstrasse 2 37077 Gottingen Germany

Frank Wurche Institut fur Organische Chemie Universitat Essen UniversitatsstraRe 5 45141 Essen Germany

I'

I

Basic Principles

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1 3

1

Effect of Pressure on Inorganic Reactions:

Introduction and Mechanistic Applications

Rudi van €Idib: and Colin D. Hubbard

I. I

Introduction

Chemistry literature is to a large extent concerned with preparative work and the structural and spectroscopic characterization of reaction products. The velocity of the reactions and efficiency of product formation as manifested in the reaction yield, are also of importance in synthetic studies, particularly when the products are of direct use or are intermediates in commercially relevant activities. The ki- netics of reactions can be very informative in combination with other information for revealing the details of the reaction mechanism. Once a chemical reaction mechanism is fully understood, the insight gained can be used to tune the chemi- cal process in any desired direction. The evidence for a particular mechanism is often circumstantial, and therefore kineticists try to employ the widest set of ex- perimental variables available in an effort to interpret the resulting kinetic data in the least equivocal manner possible.

The value of the mechanistic information that emerges from kinetics measure- ments over a series of elevated pressures for solution reactions in inorganic and organic chemistry has been realized for some time [ l - 3 ) . However, many inor- ganic reactions are too fast to follow using conventional instrumentation. Hence the momentum regarding investigations at high pressures vis-a-vis organic re- actions was delayed somewhat until adaptation of rapid reaction techniques for operation at high pressures had been achieved, mostly in the period from 1975 to 1985. This fertile period has been recorded in reviews, in conference proceedings, and in monographs, and readers may obtain a thorough background and sense of historical development by consulting this literature [4-111. Even until quite re- cently, suitable instrumentation was not widely available.

The purpose of this chapter is to familiarize the reader with the current status of activities in the application of hydrostatic pressure to mechanistic studies in the areas of inorganic and organometallic chemistry, as well as in the blossoming field of bioinorganic chemistry. Although the basic principles involved in high pressure kinetics for reactions in general have been the subject of many reports [12-141, some essential aspects and the most frequently used methods will be presented

4 I 7 Effect of Pressure on Inorganic Reactions: Introduction and Mechanistic Applications

here to form a basis for the subsequent chapters dealing with the effect of pressure on particular types of reactions in inorganic and organic chemistry.

The parameter that is derived from high pressure kinetic experiments in solution is the difference in partial molar volume between the activated complex of transi- tion state theory and the reactant state, and is known as the volume of activation. AVf . If the particular reaction is reversible and the system experimentally acces- sible, AVf for the reverse reaction can also be obtained and the difference between these two quantities results in the reaction volume, AVO. The latter quantity may also be determined by measuring the equilibrium constant ( K ) for the reaction as a function of pressure, or from the partial molar volumes of the reactants and prod- ucts, derived from solution density measurements. The volume of activation itself is determined from measurements of the reaction rate constant k at different hydrostatic pressures p at a given absolute temperature T, since (? In k/?p), =

-AV#/RT (R is the ideal gas constant), an equation was developed within transi- tion state theory based upon the analogous equilibrium constant relationship, ( 3 In K/Sp), = -AVo/RT. The former equation, upon integration. can be em- ployed to determine AVz from a plot of In k versus p . Providing the pressure is no higher than 200 MPa, in the vast majority of cases AV# is pressure independent and the plot is linear. A nonlinear behaviour is usually encountered when dealing with a compressible solvent where both the reaction and activation volume become pressure sensitive. For such cases often encountered in organic systems (see Chapter 2), where it is necessary to consider the pressure dependence of AV#, i.e. to extrapolate the data to ambient pressure, there are various treatments available for processing the primary data [5, 151. In this introductory chapter the focus will be on reactions in which there is a negligible or absence of pressure dependence of the volume of activation. In general, volume of activation data quoted in this report will refer to ambient conditions, i.e. close to room temperature, and readers are advised to consult the cited literature for more detailed information on the exact experimental conditions employed.

Equilibrium and kinetic parameters obtained as a function of temperature per- mit the drawing of diagrams illustrating the Gibbs free energy (C), enthalpy (H) and entropy (S) changes in proceeding in the sequence reactant state/transition state/product state, and including intermediates when they are formed. Corre- spondingly, a volume diagram or volume profile can chart the respective volume changes along the reaction coordinate, and when appropriate actual partial molar volumes are known, on an absolute rather than a relative basis, something that cannot realized for G, H or S. Hence if reactants A and B form a product AB and no intermediates are formed, i.e. there is a single step reaction, a volume profile in which the reaction volume is, for example, negative and the volume of activation is such that the transition state is almost halfway between reactant and product states, is depicted in Fig. 1.1.

As shown in Fig. 1.1, other forms of the volume profile are possible depending on the particular character of the system. Thus, in principle, a volume profile rep- resents a simple and lucid way of describing a reaction and diagnosing the mech- anism, but with the caveat that mechanistic diagnosis is uncomplicated when only intrinsic changes (changes in bond lengths, bond angles for example) occur. In

1 5 7 . 1 Introduction

I Reactants , Transitionstate , Products

Reaction Coordinate Fig. 1.1. Volume profile for the overall reaction A + B -+ AB. The activated complex is [A----B)#

many actual reactions, when charged species are produced or neutralized during the reaction, or increases or decreases in polarity occur, then there is also a change in the volume occupied by the solvent molecules surrounding the system by virtue of an increase or decrease in (at least) the first solvation layer. Volume reduction of solvent from this source is known as electrostriction. Thus the facile interpretation of measured values of AVO or AV# can be compromised by the existence of the two contributions which are difficult to quantify. The intrinsic and solvational con- tributions to AVf can schematically be visualized as shown in Fig. 1.2.

A B (A---B)* A- B

o+o-m-m Forward reaction: A + B - A B AViZtr = - Reverse reaction: A B - A + B AVittr = +

o+ 0-00 -m Forward reaction: A' + B- - A 8 AV:,,, = Reverse reaction: AB - A+ + B- AV&,, = -

Overall volume effect: AV = AVintr + AVsolv * * *

Fig. 1.2. Intrinsic and solvational contributions to the volume of activation.

6 7 Effect of Pressure on Inorganic Reactions: Introduction and Mechanistic Applications

Clearly a reaction accelerated by pressure has a negative volume of activation and one retarded by pressure. a positive volume of activation. Most inorganic re- actions that have been studied yield AVc values within the range of ' 30 to -30 cm' mol-'. which corresponds to retardation and acceleration respectively of a factor of about 4 at 100 MPa (1 kbar) compared to 0.1 MPa (atmospheric pressure;. In the absence of solvational contributions. positive AVf values are indicative of the commencement of bond breakage. whereas negative AVc values are indicative of reactions in which a bond is beginning to be established upon reaching the transition state. Further classification of reaction types will be presented later.

A brief account of experimental methods follows with illustrations in some cases. Thereafter thermal reactions grouped by reaction type from inorganic, organo- metallic and bioinorganic chemistry will be described from the perspective of the mechanistic insight gained from the application of high pressure techniques. A section on photo- and radiation-induced chemical reactions is also included. At this stage readers are referred to more detailed reports on water cxchange processes in Chapter 4, application of electrochemical techniques in Chapter 5. and photo- chemical processes in Chapter 6.

I

1.2 Determination of Volumes of Activation

The scope of activity in the overall field in question may be gauged by the number of pertinent papers published or the number of volume parameters reported. Up to 1978 about 170 of the latter values had been published. while in the subsequent two decades approximately 1000 and 1600 values of the activation volume. respec- tively. have been reported [8, 16, 171. The most frequently used method of rnon- itoring a rcaction in coordination chemistry is by following changes in the UV:Vis spectrum either with a conventional spcctrophotometer or with a stopped-flow in- strument. For conventional time range reactions (reaction times longer than a few minutes) using Uy/Vis spectroscopy, a two-window cell (Fig. 1.3) and a pressuriz- able cuvette (pill-box) (Fig. 1.4) may be used for high pressure measurements 1181. The advantage of the pill-box cuvette is that pressure can be transmitted through the compression of the movable, closcly fitting cylindrical parts: it can be easily filled using a syringe needle techniquc. after which the two cylindrical parts are turned 180" to seal the cuvette. When the cell is pressurized. the two cylindrical parts move closer together as a result of the compression of the solvent used in the sample solution within the cell. and therefore the pressure from the pressurizing medium is transmitted to the sample solution. The cell is pressurized with a pres- sure generating system which typically consists of the components shown in Fig. 1.5. An hydraulic pump is used to gcnerate an oil pressure. which is then trans- mitted by the separator to the pressurizing medium (for instance water) used within the optical cell. Compression and expansion can be controlled with the series of mechanical valves and monitored with a pressure gauge.

The type of high pressure cell in Fig. 1.3 can also be used to construct a three-

1 7 1.2 Determination of Volumes of Activation

I

Fig. 1.3. Schematic view o f a two-window high-pressure cell: 1 - pressure plug; 2 - O-ring; 3 - reaction compartment; 4 - A- and O-ring; 5 - sapphire window; 6 - pressure connection.

or four-window cell which may be used for flash photolysis and pulse radiolysis applications. Technical details of these cells and methods of use may be found in recent literature [ 18-20].

The development of high pressure stopped-flow instruments opened up the possibility to study reactions in the millisecond and second time range as a func- tion of pressure [21-271. A stopped-flow instrument is designed to enable the rapid

Hole

\

Slot

. -.

Fig. 1.4. Schematic presentation of a "pill-box'' optical cell for measurements in a high pressure optical cell. The slot and hole allow the pill-box cell to be filled and extra liquid to be released on closing the cell.

8 7 €fled of Pressure on Inorganic Reactions: Introduction and Mechanistic Applications I 7 1

oil reservoir

reservoir filled with I high-pressure I cell compartment with optical windows and

n-heptane

valve

&-I pill-box cell 1

teflon membrane in a steel cylinder, used for pressure transmission and for separatlon of oil and n-heptane

o vacuum pump

Fig. 1.5. Typical system to generate high pressure.

mixing of two solutions containing the reactants, followed by the monitoring of the reaction progress when mixing is completed. Two high pressure versions of stopped-flow instruments are shown in Figs 1.6 and 1.7, with the difference that in the first case (Fig. 1.6) the activation of the syringes occurs by means of a motor inside the high pressure cell, whereas in the second case (Fig. 1.7) the syringes are activated from outside the cell. Activation of the sample syringes causes a flow of the two reagent solutions through a mixing jet and optical path into a receiver syringe, which is followed by the activation of the optical detection system that then monitors the reaction progress of the rapidly mixed reagents occurring in the optical path. The deadtime of the mixing process is between 2 and 10 ms.

A second important method for determining AV# is by application of N M R spectroscopy. Progress in NMR instrumentation from electromagnets to super- conducting magnets and higher field strengths has largely been matched by de- velopments in construction of suitable high pressure probes for newer instruments in individual laboratories [29-38). Investigations of solvent exchange (see Chapter 4) and electron self-exchange reactions have been the principal beneficiaries of progress in high pressure NMR techniques. A typical example of an NMR high pressure probe developed in our laboratories, is shown schematically in Fig. 1.8. The operation principle of both these high pressure probes is that the NMR sam- ple tube is placed within a high pressure cell and is pressurized with a suitable fluid by a movable stopper that transmits the pressure from the pressurizing fluid to the sample solution by moving down the NMR tube, which is controlled by the compressibility of the solvent used in the sample solution. With the aid of these high pressure probes practically all possible NMR measurements can be per- formed as a function of pressure up to 200-300 MPa (i.e. 2 to 3 kbar) at a fixed

1 9 7.2 Determination of Volumes of Activation

~. ~ _ _ _ ~

Fig. 1.6. Schematic representation of a high pressure stopped-flow unit: 1 - lid to overall unit; 2 - outer vessel; 3 - window holder; 4 - quartz windows; 5 - electric motor; 6 - motor actuator; 7 - stopped-flow unit positioning

rod; 8 - syringe-driving plate; 9 - drive syringe (inner); 10 - drive syringe (outer); 11 - block holding windows, mixer and syringe attachment points; 12 - mixing jet; 13 - stop syringe (outer); 14 - stop syringe (inner).

temperature. The only restriction is that the sample tube within the high pressure probe cannot be spun.

The temperature-jump technique [39, 401 is frequently used to study the kinetics of rapidly equilibrating processes in solution on a microsecond time scale. This technique can only be applied to equilibria that are sensitive to temperature, such that a rapid temperature jump of a few degrees will result in a relaxation of the system to the new temperature, a process that can be followed on a micro- or milli- second time scale.

10 I 7 Efect of Pressure on Inorganic Reactions: introduction and Mechanistic Applications

-~~ b-Thrust rod 7

sapphire window

window mount

G P

hsc

Anvil

Bomb body Plunger

Reagent Outer sleeve syringes

W a n t ami t

Wndowmnt

Mixer

Wndow mount for the entwing light

cell Observatim for the PM

Lower plate Hydraulic pressure connection Port of the Coolant

temperature Sensor I I

'

Fig. 1.7. Schematic representation of the commercially available Hi-Tech HPSF-56 high pressure stopped-flow unit [28]

Electrochemical methods have also been adopted for application of high pressure [41-431 (see Chapter 5). Correlations emerging from these investigations have val- uable application in the interpretation of partial molar volume changes associated with electron transfer reactions (see Sect. 1.3.4). A potential future interest is in reactions carried out at elevated pressures in a supercritical fluid medium; in view of this a special optical cell has been developed for studying organometallic re- actions initiated by flash photolysis in supercritical fluids [20] (see Chapters 12 to 14).

The principles and instrumentation outlined above have been applied to numer- ous types of reactions in inorganic chemistry. A systematic treatment of the differ- ent reaction types and specific examples to illustrate the role of high pressure measurements in such studies, now follows.

TI A16 V4 - vessel a polyvinylchlorid IPVC)

0 sample soiut on

upper plug

aluminum

probe jacket

lower plug

titanium ring

lower pressure screw

eroded titanium tube

Q rnacor

... PTFE

I ” 7.2 Determination of Volumes of Activation

15

16

17

1 8

1 9

2 0

21

2 2

23

24

Fig. 1.8. 13 - capacitor holder; 14 - aluminum tube; high-pressure NMR (400 MHz) measurements: 15 - upper plug; 16 - sample tube; 17 - 1 - O-ring; 2 - probe jacket; 3 - thermal saddle coil; 18 - Macor; 19 - TiA16V4 vessel; insulation; 4 - polyvinyl chloride; 5 - O-ring; 20 - lower plug; 21 - lower pressure screw; 6 - O-ring; 7 - semi-rigid coaxial cable; 22 - capacitor; 23 - coaxial cable; 24 - 8 - connection to thermostat; 9 - titanium capacitor holder. tube; 10 - lid; 11 - screw; 12 - capacitor;

Design features of a probe head for

12 I 7 Effect of Pressure on lnorganic Reactions: lntroduction and Mechanistic Applications

1.3 Thermal-Induced Reactions

In this section of our presentation we will focus on different types of reactions in inorganic chemistry that occur thermally. In Sect. 1.4 we will present an account of radiation-induced inorganic reactions. Photo-induced inorganic reactions are dealt with in Chapter 6.

1.3.1 Ligand Substitution Reactions

Ligand substitution reactions of metal complexes have been the topic of many mechanistic studies in coordination chemistry because of the fundamental role of such reactions in many chemical, biological and catalytic processes. For a general ligand substitution reaction as shown in Eq. (1.1),

where X is the leaving group, Y the entering ligand, and L,, the spectator ligand(s) (charges are omitted for clarity), there are basically three simple pathways: (i) the dissociative (D) process with an intermediate of lower coordination number; (ii) the associative (A) process with an intermediate of higher coordination number; (iii) the interchange (1) process, in which no intermediate of lower or higher coor- dination number is involved. The interchange of the ligands X and Y can be more dissociative ( I d ) or more associative (Ia) in nature, depending on whether bond breakage or bond formation is more important, respectively. These mechanisms are outlined schematically in Fig. 1.9,

Such ligand substitution reactions should exhibit very characteristic AV + values depending on the degree of bond breakage or bond formation in the transition state. The most simple type of ligand substitution reaction involves the symmetri- cal exchange of coordinated solvent or ligand with bulk solvent or ligand mole- cules, respectively.

[M(S),]"+ + S' --f [M(S),-,(S')]"' + S

Exchange of a unidentate solvent molecule (S) between the first coordination sphere of a solvated metal ion (M"+) and the bulk solvent (Eq. (1.2)) has been studied for cations of many elements of the Periodic Table. The incoming solvent molecule S" is denoted with an Q to distinguish it from the initially coordinated molecule with which it exchanges. Such reactions are very important and a knowl- edge of the kinetic and associated activation parameters represents important background to the understanding and tuning of substitution of a solvent by other ligands 144). The focus has frequently been; but by no means exclusively, on water as solvent. There is no reaction volume and the solvent exchange process is as- sumed to have zero solvational change. Thus A V f should be a direct measure of the intrinsic volume changes that occur, such that a continuous spectrum of tran-