Activation of Small Inorganic Molecules PDF
Preview Activation of Small Inorganic Molecules
Homogeneous Catalysis by Metal Complexes VOLUME I Activation of Small Inorganic Molecules M. M. TAQUI KHAN ARTHUR E. MARTELL Department of Chemistry Department of Chemistry Nizam College Texas A & M University Osmania University College Station, Texas Hyderabad, India ACADEMIC PRESS New York and London 1974 A Subsidiary of Harcourt Brace Jovanovich, Publishers COPYRIGHT © 1974, BY ACADEMIC PRESS, INC. ALL RIGHTS RESERVED. NO PART OF THIS PUBLICATION MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM OR BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCLUDING PHOTOCOPY, RECORDING, OR ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, WITHOUT PERMISSION IN WRITING FROM THE PUBLISHER. ACADEMIC PRESS, INC. Ill Fifth Avenue, New York, New York 10003 United Kingdom Edition published by ACADEMIC PRESS, INC. (LONDON) LTD. 24/28 Oval Road. London NW1 Library of Congress Cataloging in Publication Data Taqui Khan, M M Homogeneous catalysis by metal complexes. Includes bibliographical references. CONTENTS: v. 1. Activation of small inorganic molecules.-v. 2. Activation of alkenes and alkynes. 1. Catalysis. 2. Metal catalysts. 3. Complex compounds. I. Martell, Arthur Earl, Date joint author. II. Title. QD505T36 546 72-9982 ISBN 0-12-406101-X (v. 1) PRINTED IN THE UNITED STATES OF AMERICA Preface During the past twenty-five years the development of the field of coordina- tion chemistry has gone through several general yet discernible phases. After a classic beginning based largely on descriptive and stereochemical studies, a more physical approach developed in which quantitative equilibrium studies led to the understanding of the thermodynamics of complex formation in solution. Subsequently the development of the ligand field theory and bonding concepts made possible successful correlations between electronic and mag- netic spectra and the constitution and properties of coordination compounds. Presently, improvements in X-ray crystallographic techniques are providing a large body of structure-property correlations and producing a new level of understanding of coordination chemistry. Also, synthesis of new types of metal complexes of small molecules, many of which contain metal-carbon sigma and pi bonds, has opened new vistas in coordination chemistry, since these compounds are frequently catalysts or intermediates in the synthesis of new organic and organometallic compounds. In addition to their general applications to homogeneous catalysis, the complexes of nitrogen and oxygen in particular are of interest as models for biological oxidation and nitrogen fixation. The purpose of this two-volume work is to review and systematize the chemistry of reaction of metal ions with small molecules, and to include chapters on hydrogen, oxygen, nitrogen, carbon monoxide, nitric oxide, and the alkenes and alkynes. Because many coordination chemists are primarily interested in the inorganic complexes, the decision was made to publish the metal ion activation of the small diatomic molecules as Volume I. Metal ion activation of alkenes and alkynes comprises Volume II. While the subject of metal ion activation of alkenes and alkynes is of vital importance to the field of coordination chemistry, it is broader in scope and would be of interest to organic and organometallic chemists as well as to coordination chemists. As with many monographs, this work is the result of an earlier attempt to review and systematize the subject of interest for our own purposes. This effort began about seven years ago with a review of metal ion activation of molecular oxygen, a general subject closely related to previous research work vii viii PREFACE on which we had collaborated. Completion of this review led over the next few years to the development of similar reviews on the activation of hydrogen, carbon monoxide, and unsaturated hydrocarbons. In the initial phase, the subject of nitrogen activation was not given sufficient weight to justify a full chapter, however the rapid growth of that field over the past few years led to the development of a nitrogen chapter that is now the most extensive in the work. Similar recent developments in nitrosation reactions have led to a chapter which is greatly expanded over its original concept; however in this case the field does not seem to have developed sufficiently to justify a treatment comparable to the other subjects. Although the work began as a review for our own use only, we were pre- vailed upon by friends to prepare the work for publication. In a treatment of this nature, it is impossible to be expert in all phases of the field. Thus first- hand knowledge of metal ion activation of oxygen and nitrogen does not provide much insight into the fine points of reactions such as catalytic hydro- génation and nitrosation. In areas outside our range of expertise we have tried to present reactions and interpretations in a manner that seems reason- able to us. If our points of view do not coincide with those who are more experienced with the reactions under consideration, we hope for a charitable judgment of our treatment. We hope we have managed to present interpreta- tions that are sufficiently original to make positive contributions to the subject at hand. We have used our own preferred conventions for representation of covalent and coordinate bonds of organometallic compounds and metal complexes. We have related this to a consistent distribution of formal charges in the formulas used to represent these compounds. Details of the method employed have been presented in Appendix I. It is our hope that the readers will consider this approach to be both reasonable and satisfying. In any case one of its strong points is the designation of all formal charges for metal ions and ligand atoms, and where reasonably possible, their locations. While the formal charges assigned to the metal ions in many complexes seem to deviate widely from the accepted oxidation states, it is believed that the formalism employed corresponds more closely to (or reflects more satisfactorily variations in) the true charge distribution. In any case it is felt that our methods allow consistent and complete electron bookkeeping for the formulas under consideration. We express our thanks to all those who provided valuable assistance in the preparation of the manuscript. Reviews of various chapters by the following professional friends and associates were particularly helpful: Professor Minoru Tsutsui (nitrogen), Professor Gordon Hamilton (oxygen), Dr. Elmer Wymore (hydrogen), and Dr. J. L. Herz (oxygen). Thanks and appreciation are also extended to Dr. R. Motekaitis and Dr. Badar Taqui Khan for assis- PREFACE ix tance in proofreading the typescript and the galleys, to Robert M. Smith for literature searches, and to Mary Martell for editorial assistance and for typing the several versions of the manuscript that were produced over the long period of time during which this work developed. M. M. TAQUI KHAN ARTHUR E. MARTELL Contents of Volume II Introduction Migration of π Bonds The Oxo Reaction Hydrosilation of Alkenes and Alkynes Oxidation of Alkenes and Alkynes Multiple Insertion Reaction of Alkenes and Alkynes Bibliography Appendix I: Formulas, Bonding, and Formal Charges Appendix II : Glossary of Terms and Abbreviations Author Index—Subject Index xi 1 Activation of Molecular Hydrogen I. Introduction The activation of molecular hydrogen in homogeneous systems by metal complexes has received considerable attention because of possible commercial and synthetic applications, as well as interest in the catalytic processes and reaction intermediates in themselves as examples of new types of inorganic chemical reactions. Since many of the reactions of molecular hydrogen that are catalyzed by metal ions are carried out in common solvents and even in aqueous solution, and since the reactions frequently occur under mild condi- tions, there has been considerable success in interpreting the reaction mechanisms in terms of the nature of reactive intermediates and the properties of the metal catalysts. In the extensive investigations of the activation of molecular hydrogen in aqueous solution by Halpern and co-workers [1-4], the metal ions copper(II), mercury(II), mercury(I), copper(I), and silver(I) were reported to be effective catalysts. It has been noted that metals that are good heterogeneous cat- alysts—ruthenium, cobalt, nickel, palladium, and platinum—have the same number of electrons in their valence shell as the catalytically active metal ions—palladium(II), copper(II), copper(I), silver(I), and mercury(ll), re- spectively. Thus homogeneous and heterogeneous catalysis of the activation of molecular hydrogen seems to arise from similar electron characteristics, the most important of which are the d electron configurations and electron affinities of the catalytic species. From the results published thus far it may be concluded that only transition metal ions that possess electron configurations in the d5-d10 range are catalysts in the activation of molecular hydrogen. 1 2 1. ACTIVATION OF MOLECULAR HYDROGEN The electronic configuration, however, is not the only factor required for catalytic activity of a metal ion. The transition metal ions manganese(II), cobalt(II), nickel(II), and iron(III) are all inactive in spite of the fact that they have electrons in the d5-d10 range. It has also been observed that metal ions that possess catalytic hydrogénation activity seem to be those that form labile hydrido complexes in solution [4]. It seems, therefore, that hydrido complexes are probably essential intermediates in metal-catalyzed homogeneous hydro- génation reactions, and in metal-catalyzed reactions in which hydrogen acts as a reducing agent. Since metal hydrides apparently have an important role in the mechanism of activation of molecular hydrogen, it is useful at this stage to consider the nature of metal-hydrogen bonds and the factors influencing lability and stability in metal hydride complexes. Π. Structure and Reactivity in Metal Hydride Complexes Considerable physical data are now available on the structures of stable hydrido complexes in the solid state [5]. The nature of the metal-hydro- gen bond in stable hydrido complexes, such as ReH ~, HCo(CN) , and 4 5 HPtBr(PEt), has been shown to be covalent by NMR (nuclear magnetic 3 2 resonance) studies of Wilkinson [6] and of Griffith and Wilkinson [7], In these complexes the hydride ion may be considered as an anionic donor that occupies one of the normal coordination positions of the metal ion. Further examples of stable metal hydride complexes for which X-ray data show the hydride ion to occupy one of the coordination positions in the metal complex are the following: i/ww-PtHBr(PEt ) [8], HMn(CO) [9], OsHBr(CO)(PPh) 3 2 4 3 2 [10], RhH(CO)(PPh ) [11], and K (ReH ) [12]. The metal-hydrogen distance 3 3 2 9 estimated for all these complexes is that expected for a normal covalent bond. The presence of a hydride ion in the coordination sphere of a metal ion is also indicated by the presence of PMR (proton magnetic resonance) resonances at high field (~ 20-30 τ) [13]. Spin-spin coupling with the central metal ion has been studied for the hydrido derivatives of 103Rh, 195Pt, and 183W complexes. The coupling constant / _ increases with increasing strength of the metal- M H hydrogen bond and thus lends strong support for the concept of highly covalent bonds in these complexes. The infrared spectra of the hydrido complexes show bands due to the metal-hydrogen stretching vibrations, ranging from 1726 to 2242 cm -1. The presence in the trans position of ligands, such as cyanide and carbon monoxide that usually have large trans effects, reduces the strength of the metal- hydrogen bond, and the observed metal-hydrogen stretching frequencies are accordingly reduced. In the complexes Ir(H) Cl(CO)(PhP) (1) and 2 3 2 II. STRUCTURE AND REACTIVITY IN METAL HYDRIDE COMPLEXES 3 Os(H)(CO)(PPh ) (2) the stretching frequencies of the metal-hydrogen bonds 2 3 3 trans to the carbonyl group are 2100 and 1882 cm"1, respectively. The hydro- gen trans to Cl ~ in 1 has a metal-hydrogen stretching frequency of 2196 cm ~1, reflecting a change of 96 cm"1 reulting from the trans effect of the carbonyl group in this iridium complex [14]. The hydrogen trans to triphenylphosphine in 2 absorbs at 2051 cm-1, indicating a large difference, 169 cm-1, in the trans effects of CO and PPh in this osmium complex [14]. Although not 3 indicated by these data, the trans effect of a tertiary phosphine substituent is usually much greater than that of the chloride ligand. On deuteration all the metal-hydrogen bands shift to lower frequencies as expected. The infrared spectrum of the dihydrido complex cation IriH^iPr^PCHsCHaPPha^+ (3) shows metal-hydrogen absorption bands at 2091 and 2080 cm"1 [15]. In this complex the two hydrogens probably occupy eis positions, as in the case of Ir(H)Cl(CO)(Ph P) (1). 2 3 2 Although considerable data have accumulated on the structures of solid hydrido complexes, the structures of labile hydrido complexes in solution are not known with certainty. According to Halpern [4] the nature of the metal- hydrogen bond in solution is probably the same as that of crystalline hydrido complexes. The reactivities of labile hydrides as intermediates in the catalytic reactions of molecular hydrogen seem to depend on their stabilities. In order that a particular transition metal ion may act as a catalyst in the activation of molecular hydrogen, its hydrido complex should have sufficient thermo- dynamic stability to be formed readily in solution, but should be sufficiently labile to react rapidly with the substrate [4]. If the hydrido complex is thermo- dynamically too unstable it is not formed at all, as may be the case for manganese(II), cobalt(II), nickel(II), and iron(III). The oxidation potential of the metal ion or complex seems to offer an important criterion for pre- dicting the formation of thermodynamically stable hydrido intermediates. 4 1. ACTIVATION OF MOLECULAR HYDROGEN Although the cobalt(II) ion by itself is inactive as a hydrogénation catalyst, pentacyanocobaltate(II) is a very active catalyst for the activation of molecular hydrogen. For each of the metal ions that acts as a catalyst there seems to be a rough correlation between catalytic activities and the stabilities (or oxidation potentials) of its complexes. Thus for silver(I) complexes, the catalytic activities decrease in the order: Ag(C H 0 )2~ > Ag(NH ) + » 2 3 2 3 2 Ag(CN) '. This is also the inverse order of stability. The oxidation potentials 2 of these complexes vary in the order: —0.643 v, —0.373 v, and +0.31 v, respectively. Although superficially the lack of catalytic activity of the most stable silver(I) complex, Ag(CN) ", and the high catalytic activity of pentacyano- 2 cobaltate(II), Co(CN) 3-, seem mutually contradictory, the catalytic effects 5 may be readily explained on the basis of stability. Silver(I) complexes function by heterolytic fission of hydrogen to form mixed complexes of silver(I) in which the hydride ion is one of the ligands. Thus the less stable silver(I) com- plexes will have the greatest affinity for the hydride ligand, whereas the cyanide ions are so strongly bound that they are not displaced by hydride ion. On the other hand, a pentacoordinated cobalt(II) complex functions through homolytic fission of hydrogen and simultaneous oxidation of the cobalt(II) to the very stable cobalt(III) species. Thus the cyanide ion will assist the electron transfer to hydrogen by forming the most stable monohydrido- cobalt(III) complex, HCo(CN) 3-. 5 The hydrido complex of a catalytic species must also be labile, in order that hydrogen (or hydride ion) be transferred to a substrate and regenerate the original catalytic complex. The lability of a hydrido complex depends on the total ligand-field stabilization energies of the ligands in the complexes of both the original catalyst and the hydrido complex formed after hydrogen fission [16]. According to Chatt and Shaw [17,18], the energy separation between the highest occupied bonding or nonbonding levels and the lowest vacant anti- bonding levels (Δ) must be greater than some critical value in order to confer stability on the hydrido complex. In the case of octahedral aqueous transition metal ions, especially those of the first transition series, the separation between the t and e(o*) levels (Fig. 1) is not enough to achieve stability of 2g g the hydride. This results in the formation of labile hydrido complexes in solution. In such cases, bonding electrons from the metal-hydrogen a bond are easily promoted through the t level into the vacant antibonding e(c*) 2g g level, with the effective dissociation of the metal-hydrogen bond and the consequent separation of a hydride ion or hydrogen atom that reacts with substrate or solvent and is thus destroyed. In the absence of a substrate the metal ion itself is reduced, in some cases with the liberation of a proton in solution. In the presence of ΤΤ-bonding ligands such as tertiary phosphines and carbon monoxide, the backbonding interaction of metal t orbitals with the 2g
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