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Organometallic Bonding and Reactivity (Topics in Organometallic Chemistry Volume 4) PDF

205 Pages·1999·3.95 MB·English
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4 Topics in Organometallic Chemistry Edi tor ia l Boa rd : J.M. B r o w n � P. D i x n e u f - A. Ff i r s tner � L.S. H egedus P. H o f m a n n � P. K n o c h e l � G. v a n Koten � T.J. Marks S. Mura i � M. Reetz Springer Berlin Heidelberg New York Barcelona Hong Kong London Milan Paris Singapore Tokyo Organometallic Bonding and Reactivity Fundamental Studies Volume Editors: J.M. Brown and P. Hofmann With contributions by P.B. Armentrout, D. Braga, A. Dedieu, P. Gisdakis, A. G6rling, E Grepioni, E Maseras, N. R6sch, S.B. Trickey ~ Springer The series Topics in Organometallic Chemistryp resents critical overviews of research results in organometal- lic chemist ry, where new developments are having a significant influence on such diverse areas as organic syn- thesis, pharmaceutical research, biology, polymer research and materials science. Thus the scope of coverage includes a broad range of topics of pure and applied organometallic chemistry. Coverage is designed for a broad academic and industrial scientific readership starting at the graduate level, who want to be informed about new developments of progress and trends in this increasingly interdisciplinary rid& Where appropri- ate, theoretical and mechanistic aspects are included in order to help the reader understand the underlying principles involved. The individual volumes are thematic and the contributions are invited by the volumes editors. In references Topics in Organometallic Chemistry is abbreviated Top. Organomet. Chem. and is cited as a jour- nal. Springer WWW home page: http://www.springer.de ISSN 1436-6002 ISBN 3-540-64253-6 Springer-Verlag Berlin Heidelberg New York Library of Congress Cataloging-in-Publication Data Organometallic bonding and reactivity : fundamental studies / volume editors, J. M. Brown and P. Hofmann ; with contributions by P. B. Armentrout ... [et al.]. p. cm.- - (Topics in organometaUic chemistry; 4) Includes bibliographical references. ISBN 3-540-64253-6 (hardcover : alk. paper) 1. OrganometaUic compounds. 2. Chemical bonds. 3. Reactivity (Chemistry) I. Brown, John M. II. Hof- mann, E (Peter), 1947- III. Series. QD411.5.072 1999 547’.05--dc21 99-33293 CIP This work is subject to copyright. All rights are reserved, whether the whole part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9,1965, in its current ver- sion, and permission for use must always be obtained from Springer-Verlag.Violations are liable for prosecu- tion under the German Copyright Law. ' Springer-Verlag Berlin Heidelberg 1999 Printed in Germany The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Cover: Friedhelm Steinen-Broo, Pau/Spain; MEDIO, Berlin Typesetting: Data conversion by MEDIO, Berlin SPIN: 10543767 66/3020 - 5 4 3 2 1 0 - Printed on acid-free paper. Compiled by Matt Pretender Volume Editors Dr. John M. Brown Prof. Peter Hofmann Dyson Perrins Laboratory Organisch-Chemisches Institut South Parks Road Universit~it Heidelberg Oxford OX1 3QY, Im Neuenheimer Feld 270 E-mail: Preface General The making and breaking of carbon-metal bonds is fundamental to all the pro- cesses of organometallic chemistry and moreover plays a significant role in ho- mogeneous as well as heterogeneous catalysis. This rather blunt statement em- phasises the extent to which a proper understanding of the structure, energetics and reactivity of C-M bonds is at the core of the discipline. In order to accept it, a proper definition of the terms involved is required. Quite simply we define the metal-carbon bond in its broadest sense to embrace carbon linked to transition- metals, lanthanides and actinides, and main group metals. We do not distin- guish between formally covalent single or multiple bonding on the one hand and q-bonding on the other. In the studies to be described in the following chapters, the emphasis will be on transition metal complexes and insofar as the funda- mentals come under scrutiny, simple metal alkyls or related species (metal alke- nyl, alkynyl, aryl, or allyl) will play an emphatic part. The central role of metal alkyls and their congeners and especially the role of their metal carbon linkage in homogeneous catalysis may be appreciated by considering some key reaction steps leading to their formation or breakdown. There follows a few prominent examples of transition metal mediated stoichiometric or catalytic processes: - In homogeneous hydrogenation of double bonds, the stepwise reaction of an q2-coordinated alkene with dihydrogen gives first an alkyl metal hydride, and then the decoordinated alkane by elimination. - In the heterogeneous catalysis of hydrogenation, surface-bound metal alkyls play a pivotal role in the reaction cycle. - Homogeneous or heterogeneous dehydrogenation reactions of hydrocarbons involve transition metal alkyl hydrides, which may undergo fl-elimination and decoordination of H 2 and an alkene. - In hydroformylation, a metal alkyl is formed in similar manner but intercept- ed by cis-ligand migration to coordinated CO; the reductive elimination then involves an acyl metal hydride. - Hydrosilylation and many more related addition reactions of X-H or X-Y units to unsaturated organic substrates proceed via metal alkyl (or alkenyl, aryl) in- termediates, which are produced by insertion steps into M-H or M-X,Y bonds. Hydrocyanation of alkenes and dienes figures prominently in this context. VIII ].M. Brown, P. Hofmann - For transition metal catalysed alkene amination, a process of great industrial potential, the most promising catalytic cycles are based upon the intermedi- acy of alkyl metal complexes, formed either by amine addition to a metal- coordinated olefin or by olefin insertion into M-H and M-N bonds, respec- tively. Metal catalysed alkyne amination and hydration reactions are related cases. - For the simplest mechanism of alkene polymerisation the alkyl chain grows through an alkyl migration to coordinated alkene; the same process is re- sponsible for C-C bond formation in alkene dimerisations and oligomerisa- tions. - The copolymerisation ofalkenes and CO to 1,4-polyketone polymers involves successively a palladium alkyl and acyl, the sequence being continued by mi- gration of the acyl to q2-coordinated alkene, and further cis-ligand migration to coordinated CO. - In metathesis and ROMP polymerisation, the key steps are a template cy- cloaddition between metal alkylidene and alkene, leading to metal alkyl bonds in a metallacyclobutane structure, and the reverse process with oppo- site regioselectivity. - Palladium and nickel-catalysed cross-couplings involve successive addition of a carbon electrophile and a carbon nucleophile to the metal and then an elimination of cis-adjacent alkyl groups; for the related Heck reaction the key step is the cis-ligand migration of a palladium alkyl of electrophilic origin to a coordinated alkene. - The catalytic amination or carboxyalkylation of halogenated arenes as well as the catalytic arylation of carbonyl compounds using palladium catalysts cre- ate aryl metal intermediates en route to the C-N bond forming elimination step. - Intermediates of olefin oxidation reactions of the Wacker-type are hydroxy- substituted metal alkyls of e.g. palladium. - Metal q3-allyls, often in equilibrium with their ql-allyl isomers, have a broad base of catalytic involvement best appreciated through the exometallic reac- tion of cationic palladium allyls with nucleophiles or the intermediacy of allyl nickel complexes in hydrocyanation ofbutadiene. The chemistry of q3-benzyl systems is related. - Migration of an unsaturated alkyl group from iron to carbon is the basis of the most convincing explanation for Fischer-Tropsch telomerisation. - C-H activation of alkanes, a fundamental step for C-H functionalization re- actions in both chemical and biological systems gives a metal alkyl as the first formed intermediate. C-H functionalization reactions of alkenes and arenes, e.g. hydrovinylation or the Murai reaction and related processes, involve met- al aryls or alkenyls en route to functionalized hydrocarbons. - Last but not least, numerous stoichiometric reactions of reactants where the transition metal acts as a template, permit the chemo- and stereoselective synthesis of complex organic molecules through intermediates with M-C bonds. Preface IX Naturally this constitutes an incomplete list. Given the enormously broad scope of known or potential transformations of organic substrates involving M-C bonds, how then may the experimentalist or theoretician contribute to basic un- derstanding? Here it is convenient to separate the contributions of these two communities, although in practice there is considerable convergence of effort. Experimental Studies On the structural side there is an accumulating body of results from X-ray, neu- tron and electron diffraction invaluable for developing a systematic corpus of data on bond lengths and bond angles, and defining the trends with respect to variation of metal and co-ligands. Fast, highly efficient X-ray instrumentation in the form of area detector, CCD, rotating anode and synchrotron technology has brought about a revolution in speed for the determination of molecular struc- tures of even the largest organometallic systems in the solid state. Dunitz, Biirgi and others made seminal contributions to our knowledge of solid state struc- ture/reactivity relationships. Now a large body of X-ray and neutron diffraction data is quickly and easily retrievable from structural databases, and can be wide- ly used to "map out" parts of energy surfaces or of specific reaction pathways or to derive subtle variations of molecular structure from large series of related compounds. The accuracy of X-ray data permits answers to questions about the nature of C-M bonding versus Van der Waals contacts. Taken together, informa- tion from diffraction experiments form the basis of efforts to tailor the structure of organometallic compounds ("ligand design") for specific functions in orga- nometallic chemistry and catalysis. Solid state structure determination provides the theme for the Chapter by Braga and Grepioni "Static and Dynamic Struc- tures of Organometallic Molecules and Crystals". Despite the high level of precision of contemporary solid state structural studies, more detailed information on energetics and reactivity patterns need to be collected from other experimental sources. Two areas of current endeavour provide significant results. Mass spectrometric techniques, which are elaborated in Armentrout’s Chap- ter"Gas Phase Organometallic Chemistry", possess the power to provide direct information on the energetics of transient species generated in the gas-phase. Recent reports have shown, that gas phase investigations of reaction pathways and energetics are feasible even for "real" catalytically active complexes, as for C-H activating [Cp~Ir(PR3)] 14-electron intermediates, for Grubbs type (PR3)zClzRu(carbene) olefin metathesis and Cp2Zr(R) + olefin polymerisation catalysts. Armentrout’s Chapter is largely concerned with guided ion beam tan- dem MS, and other workers have applied FT Ion Cyclotron Resonance [FTICR]. By analysis of the kinetic energy release distribution, experimental bond ener- gies may be derived, and compared with the predictions of increasingly sophis- ticated calculations. Much of the mass spectrometric work involves bare metal cations (or metal oxide cations MO +) and permits direct comparisons of chem- oselectivity, regioselectivity and reactivity. For example, the reaction of light X J.M. Brown, E Hofmann metal cations with hydrocarbons can result in some C-C cleavage in competition with C-H activation. Heavier transition metal cations lead to dehydrogenation via C-H activation. MS experiments may be extended to ligated metal ions; a sig- nificant reaction between ScMe2+ and cycloalkanes is a sigma-bond metathesis occurring by a four-centre transition state, in competition with dehydrogena- tion so that a range of R2Sc+ species is observed. Interesting proposals of a"two- state-reactivity" have been employed to explain the gas phase reactivity of MO + fragments with organic substrates. Important questions concerning the trans- ferability of gas phase reaction patterns to solution chemistry remain to be an- swered. In catalysis it is a familiar truism that many of the most interesting species are highly elusive; their short lifetime under normal reaction conditions precludes detection. Time-resolved IR spectroscopy holds considerable promise for the definition of species in this category. Given a spectrometer with 200 femtosec- ond time resolution, intermediates of very short lifetime may be detected pro- vided that their transient concentration is sufficient. Laser photolysis of the Tp*Rh(CO) 2 complex at 295 nm occurs with a high quantum yield (0.3) for CO dissociation and C-H activation from hydrocarbon solvent. This lies in contrast to the quantum yield of 0.01 for the corresponding Cp * complex. In the period of 500 ns after CO dissociation, several intermediates are observed. First a mo- lecular alkane complex ensues, which dissociates one of the pyrazole units over 200 ps. The dissociated species undergoes first C-H insertion and then rechela- tion of the pyrazole, both on a 200 ns timescale, to give the stable C-H activation product. The energy barrier for the critical C-H insertion is around 35 kJmo1-1. The fast IR approach is made more powerful when coupled to classical mecha- nistic probes. In a related instance where Cp*Ir(PMe 3) is the coordinatively un- saturated fragment, the existence of an alkane complex en route to the C-H ac- tivation/insertion product was proved by the photolysis of alkylhydride iso- topomers and satisfactory correlation of the results with a kinetic model requir- ing an alkane complex. Photoelectron spectroscopy is another important experimental tool which has provided deeper insight into bonding patterns and electronic structures of organometallic compounds and into M-C interactions. Here - in contrast to simple organic molecules - one observes the breakdown of Koopmans’ theorem. This inevitably necessitates either the spectroscopic comparison of series of re- lated and specifically modified model compounds, or the use of appropriate computational procedures in order to identify the nature of observed ionisation events. These can then be related to a qualitative or quantitative bonding de- scription of the species in question. A large body of PE spectroscopic informa- tion on organometallics has been collected in the past, but surprisingly its direct influence and use as a guideline for synthesis and its impact for expanding mechanistic knowledge and devising novel structures or reaction pathways has been somewhat limited. Certainly further effort will be very important here. Modern spectroscopic techniques also provide intimate details of the struc- ture of surface bound groups. For example, high-resolution electron energy loss Preface XI spectroscopy (HREELS) provides an equivalent IR spectrum of adsorbent which can be compared with theoretical calculation (DF calculations). Solution thermochemistry should be mentioned as a further area of funda- mental studies, which are of great importance, because they can provide refer- ence data for estimating reaction enthalpies or for establishing useful additive and incremental schemes for energy calculations of single step organometallic reactions or catalytic cycles. Unfortunately, solid and reliable thermochemistry data for organometallic reactions in condensed phase are rather scarce, and only a few groups are operating seriously in this field. Their results form an important link to the results of theoretical calculations and may serve as a credibility nexus between theory and experiment. Theoretical Studies All types of fundamental experimental studies of organometallic structures, structural dynamics, energetics and reactivity in the solid state, in solution or in the gas phase are intimately connected to theoretical chemistry with its large body of modern computational tools. It is certainly adequate to state, that dur- ing the last 10 to 15 years we have witnessed a dramatic change of the role that is played by theoretical chemistry for organometallic chemistry and catalysis re- search. The rapid development of computers and of programming technology and the concomitant commercial availability or free accessibility of often easy- to-handle, graphics- and screen-oriented program packages have caused a rev- olutionary change in attitudes towards theory among organometallic chemists. The 1998 Nobel Prize in Chemistry was awarded to two of the pioneers of theo- retical and computational chemistry, John A. Pople and Walter Kohn and nicely testifies to this statement. The experimental chemist has access to most levels of theory, ranging from molecular mechanics approaches and semiempirical quantum chemistry to highly sophisticated, correlated density functional and ab initio (molecular orbital, valence bond) calculations. For this reason most organometallic and catalysis research laboratories have come to use quantum chemical calculations on a routine basis during the past 10 years. It is interesting - and to some extent surprising - to realise that the em- ployment of theoretical methods either for analysing experimental results or to plan organometallic molecular structure and function is an even more routinely established tool in industrial R&D labs engaged in organometaUic or catalysis research, than in academic laboratories. Contemporary quantum chemistry al- lows one to perform calculations not only for small model systems, from which basic electronic structure patterns and unifying concepts can be derived, but also allows modelling of real systems. Models of bonding and electronic struc- ture, based upon more qualitative or semi-quantitative concepts and methods like ligand and crystal field theory, the angular overlap model, PMO theory and orbital interaction rules, all variants of Extended Hiickel-type calculations and their descriptive one-electron MO theory tools for molecular or extended sys- tems are useful tools for analysing and understanding many features of electron-

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