Activation of Small Inorganic Molecules

425 Pages·1974·5.745 MB·English

by  M.M. Taqui Khan (Auth.)

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.
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United Kingdom Edition published by
ACADEMIC PRESS, INC. (LONDON) LTD.
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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