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PROGRESS IN INORGANIC CHEMISTRY

Edited by

STEPHEN J. LIPPARD

DEPARTMENT OF CHEMISTRY MASSACHIJSFITS INSTITUTE OF TECI INOLOGY

CAMBRIDGE, MASSACHUSEITS

VOLUME 36

AN INTERSCIENCE@ PUBLICATION JOHN WILEY & SONS New York Chichester Bnsbane - Toronto Singapore

Progress in Inorganic Chemistry Volume 36

Advisory Board

THEODORE L. BROWN UNIVERSITY OF ILLINOIS, URBANA, ILLINOIS

JAMES P. COLLMAN STANFORD UNIVERSITY, STANFORD, CALIFORNIA

F. ALBERT COTTON TEXAS A & M UNIVERSITY. COLLEGE STATION. TEXAS

RONALD J. GILLESPIE McMASTER UNIVERSITY, HAMILTON, ONTARIO, CANADA

RICHARD H. HOLM HARVARD UNIVERSITY, CAMBRIDGE. MASSACHUSETTS

GEOFFREY WILKINSON IMPERIAL COLLEGE OF SCIENCE AND TECHNOLOGY, LONDON. ENGLAND

PROGRESS IN INORGANIC CHEMISTRY

Edited by

STEPHEN J. LIPPARD

DEPARTMENT OF CHEMISTRY MASSACHIJSFITS INSTITUTE OF TECI INOLOGY

CAMBRIDGE, MASSACHUSEITS

VOLUME 36

AN INTERSCIENCE@ PUBLICATION JOHN WILEY & SONS New York Chichester Bnsbane - Toronto Singapore

An Interscience@ Puhlication

Copyright" 1988 b>- John Wiley & Sons, Inc.

All rights reserved. Published simultaneously in Canada.

Reproduction or translation of any part of this work beyond that permitted by Section 107 or 108 of the 1976 United States Copyright Act without the permission of the copyright owner is unlawful. Requests for permission or further information should be addressed to the Permissions Department. John Wiley & Sons, Inc.

Library o f Congress Catalog Card Number: 59-13035 ISBN 0-471-61144-1

Printed in the United States of America

10 9 8 7 6 5 4 3 2 1

Contents

Carbon-Hydrogen-Transition Metal Bonds . . . . . . . . . . By MAURICE BROOKHART Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina and MALCOM L. H. GREEN and LUET-LOK WON(; Inorganic Chemistry Laboratory, University of Oxford, Oxford, United Kingdom

Mechanistic Aspects of Organometallic Radical Reactions . . . . 125 By DAVID R. TYLER Department of Chemistry, University of Oregon, Eugene, Oregon

Chemical and Physical Properties of Triangular Bridged Metal Complexes. . . . . . . . . . . . . . . . . . . . . . . . . 195

By RODERICK D. CANNON and Ross P. WHITE School of Chemical Sciences, University of East Anglia, Norwich, United Kingdom

Cyclic and Heterocyclic Thiazenes . . . . . . . . . By RICHARD T. OAKLEY Department of Chemistry and Biochemistry, University of Guelph, Guelph, Onturio, Canada

. 299

Ligand Additivity in the Vibrational Spectroscopy, Electrochemistry, and Photoelectron Spectroscopy of Metal Carbonyl Derivatives . . . . . . . . . . . . . . . . . . . . 393

By BRUCE E. BURSTEN and MICHAEL R. GREEN Department of Chemistry, The Ohio State University, Columbus, Ohio

Subject Index . . . . . . . . . . . . . . . . . . . . . . . 487

Cumulative Index, Volumes 1-36 . . . . . . . . . . . . . . . 505

Progress in Inorganic Chemistry Volume 36

Carbon-Hydrogen-Transition Metal Bonds

MAURICE BROOKHARI'

Depurtmerit of Chemistry, University of North Curolinu, Chapel Hill, Xorth Curolina

arid

MALCOLM L. H. GREEN and LUET-LOK WONG

Inorganic Chemistry Luborator?, University of Oxford, Oxford, United Kingdom

CONTENTS

I. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . 2

I1 EARLY OBSERVATIONS OF INTERACIIONS BETWEEN C-€1 BONDS AND TRANSIIION MEI'AL CENTERS . . . . . . . . . 3

111. CLASSIFICATION OF COMPOUNDS W I I H AGOSIIC BONDS . . . 8

IV. GENERAL PIIYSICAI. AND CHEMICAL PROPERTIES . . . . . . 9

A. Structural Determinations Using X-Ray, Neutron. and Electron Diffraction Techniques . . . . . . . . . . . . . . . . . . . 9

B. Nuclear Magnetic Resonance Studies . . . . . . . . . . . . . . 10 C. Infrared Spectroscopy . . . . . . . . . . . . . . . . . . . 17 D. General Rcactivity Patterns . . . . . . . . . . . . . . . . . 17

V TIIE NrVI'URE OF AGOSTIC BONDING . . . . . . . . . . 18

A. General Considerations Concerning Three-Center, Two-Electron Bonds. . . . . . . . . . . . . . . . . . . . 20

21 B. The Representation o f Agostic Bonds: The IIalf-Arrow Convention . . . C. Theoretical Discussions of Molecules Containing Distortions at the

a-Carbon Atom: Distorted Methyl Groups and Distorted Alkylidene Ligands . . . . . . . . . . . . . . . . . . . . 22

25 E. Intermolecular M- --II-C Bonds. . . . . . . . . . . . . . . . 27 D. General Observations on the Occurrence of Agostic Bonds . . . . . .

2 BROOKIIARI'. GREEN . AND WONG

VI . COMPOUNDS CONTAINING AGOSTIC M-H-C GKOUPS . . . . . 28

A . Agostic Alkyls . . . . . . . . . . . . . . . . . . . 1 . a-Agostic M-. 1i.C Bonds . . . . . . . . . . . . . 2 . P-Agostic Alkyl Complexes . . . . . . . . . . . . .

B . Agostic Alkylidene Compounds . . . . . . . . . . . . . C . Agostic Compounds Involving Unsaturated Hydrocarbon Ligands .

1 . q'-Ene-agostic-yl Compounds . . . . . . . . . . . . 2 . q3-Enyl-agostic-yl Compounds . . . . . . . . . . . . 3 . Cyclic and Acyclic q-Diene-agostic-yl Compounds . . . . .

. . . 28

. . . 29

. . . 33

. . . 46

. . . 58

. . . 59

. . . 60

. . . 79 4 . qS-Dienyl-agostic-yl Compounds . . . . . . . . . . . . . . 83

D . Remote M-H-C Bonds . . . . . . . . . . . . . . . . . . 83 E . Agostic Interactions in Polynuclear Systems . . . . . . . . . . . 95

VII . COMPOUNDS CONTAINING METAL-HYDROGEN-M' BRIDGES WHERE M' 1 B. N . Si, CI. F . . . . . . . . . . . . . . . . 104

A . Metal-I Iydrogen-Boron Bridges . . . . . . . . . . . . . . . 10-1 B . .Three.Center, Two-Electron Bonding in the M-H-Si System . . . . . 106 C . Metal-IIydrogen-Nitrogen Bridges . . . . . . . . . . . . . . 108 D . Carbon-Hydrogen-Boron Bridges . . . . . . . . . . . . . . . 109 E . Carbon-Hydrogen-<:arbon Bonds . . . . . . . . . . . . . . . 109

VIII . MISCELLANEOUS AGOSTIC SYSTEMS . . . . . . . . . . . . 110

IX . IMPLICATIONS OF AGOSTIC INTERACTIONS IN TRANSITION METAL CATALYZED REACTIONS OF IIYDROCARBONS . . . . . 111

A . The Relationship between Agostic Interactions and Carbon-Carbon Bond Forming Reactions via Alkyl Migration Reactions: Implications in Olefin Polymerizations . . . . . . . . . . . . . . . . . . 111

B . General Observations . . . . . . . . . . . . . . . . . . . 114

X . CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . 115

ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . 1 IS

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . 115

I . INTRODUCTION

Carbon-hydrogen bonds. especially those of saturated ($) carbon ccn- ters. arc normally considered to be chemically inert . There is a rapidly increasing body of evidence. however. which shows that C-H bonds can act as ligands to transition metal centers by formation of three.center . two- electron bonds (3c-2e) and that the extent of the interactions is such as to have a marked effect on the molecular and electronic structure and hence the reactivity of the molecule .

CAKIWN HYDKOCrtN-TII,~USITIC)N ME IAI BONDS 3

The term agosticf (1) will be used to discuss the various manifestations of covalent interactions between carbon-hydrogen groups and transition metal centers in organometallic compounds, in which a hydrogen atom is covalently bonded simultaneously to both a carbon atom and to a transition metal atom.

11. EARLY OBSERVATIONS OF INTERACTIONS BETWEEN C-H BONDS AND TRANSITION METAL CENTERS

In 1965 Mason ( 3 ) , Ibers (4), and their co-workers observed close approach of the ortho-hydrogen atoms of aryl-phosphine ligands to the metal center in the compounds [truns-Pd12(PMe-Ph),] (1) and [ R u C I ~ ( P P ~ ~ ) ~ ] (2), Fig. 1. The relevance o f this observation to catalysis and metal catalyzed €€-transfer reactions was noted ( 3 ) . The crystal structure o f [Rh(€I)Cl(SiCI,)(PPh,)i] (3) shows there to be a Kh-€I,,,h, interaction with an estimated Rh-H~,prhcl distance of 2.79 A ( 5 ) . Maitlis and co-workers reported the crystal structure of rrans-[Pd(CMeCMeCMeCMeH,)Br( PPh,),] (4) and showed there to be a close approach o f the €1, to the palladium

1 2

3 4

Figure 1 tarly examples of close approache5 of hydrogen atoms to metal center5

:-The term agostic i s derived from the Circck word & ~ o v T ~ ; (2). which may he translated as to clasp, to draw towards, to hold to oneself. M.L.H.G. wishes to acknowledge the advice o f Mr. J . Griffin. Ralliol College, Oxford, who suggested the word.

4 BROOKHART, GREEN, AND WONG

[estimated r(Pd-H) 2.23 A]. Also. 31P-H, coupling was observed in the ‘H NMR spectrum. Some interaction of a hydrogen-bonding nature with the metal orbitals was proposed (6).

Important developments arose in the studies of polypyrazolyl metal complexes by Trofimenko (7-10) and Cotton and co-workers (1 1-14). Trofimenko observed unusual chemical shifts and low u(C-H) stretching frequencies in a number of metal compounds containing polypyrazolyl ligands (8-10). It was found that the compound [Ni{EtzB(pz),},] (S), was unexpectedly inert to substitution at the metal and this result, together with the observation of a low field shift of the methylene hydrogens, led to the proposal of some screening of the nickel center by the ethyl groups (10). The crystal structure of 5 showed that one hydrogen of each of the methylene groups lies in an apical position above or below the nickel atom. The Ni-H distances were 2.459 A and the possibility of there being a C-H bonding interaction to the metal was raised (15). The crystal structure of bis[dihydrobis-(1-pyrazolyl)borato]nickel, [Ni{H,B(p~)~},](pz =

CIN,Hj) (6), however, showed the dimensions of the pyrazolyl ligand to be very similar to those of the diethyl analogue 5, and it was concluded that no significant interaction occurs between the methylene hydrogens and the nickel (16). More recently. the absence of significant Ni-H interaction in 5 has been supported by calculations (17).

The polypyrazolyl derivatives [Mo{R2B(3,S-RR’pz)2}(-q-CH,CH-CH2) (CO),] (7), R = H, R’ = Me; or R = Et, R’ = H (S), were prepared and 8 showed unusual high-field chemical shifts assignable to the methylene hydrogens (1V.B .) and exceptionally low frequency bands assignable to u(C-H) stretching modes, at 2701 and 2664 cm ’. It was suggested that the C-H groups were intruding into empty metal orbitals (8) giving rise to some Mo-H-C interaction (9). The crystal structure of [Mo{Et,B(pz),}(q- CH2CPhCH,)(CO),] (9), gave a Mo-C distance to the methylene carbon of the ethyl group of 3.06 A, from which a Mo-H distance of 2.27-2.15 A was estimated. A 3 ~ 2 e Mo-H-C bond was proposed by analogy to the B-H-Mo bond in [Mo{H~B(~z)~}(-~’-C,H~)(CO),~ (14) (see Section VII). It was noted that such an interaction could serve as a model for the activation of C-H bonds by metals (12, 13). Furthermore. the ‘H NMR spectrum of 9 showed it t o be fluxional and the variable temperature spectra revealed exchange between the C-H-Mo and the free C-H of the mcth- ylene group. A bond energy of 17-20 kcal mol-’ was estimated from the data (13). The relevance of these Mo-H-C interactions to C-H activ, A t’ ion was emphasized.

The crystal structure of the compound [MO{E~,B(~~)~}(~-C,H,)(CO)~] (10) showed a short Mo-C(methy1ene) distance of 2.92 A with estimated

5

Et I

H \ H.

l.lO/ ' > , l,G/ ' \ \

' -2158. ,c-,, ' \ w193ii

. \ . \

,/.-., \

..%

B . \ /

Mo 292 0 3-055

' Mo

9 10

Figure 7. pounds 9 and 10.

The molecular structures and the dimensions of the Mo-11-C moiety of com-

r(Mo-H) and r(C-H) distances of 1.93 and 1.1 A, respectively. A strong Mo-H-C (3c-2e) bond was proposed and it was argued that the observed Mo(q'-C,H,) structure was preferred to that of the possible isomer with no M-11-C bond but with a Mo(q'-C,H,) moiety as a consequence of the strength of the Mo-H-C bond (14). The molecular structures of 9 and 10 are shown in Fig, 2.

Further early evidence for M-H-C interactions came primarily from the observation of short M---€1-C distances in a number of compounds. The compounds discussed in Section VI are examples of these interactions. Also, the correct characterization of cations formed by protonation of iron- qJ-diene derivatives, [Fe(q4-diene)L3], contributed significantly to the appreciation of the diversity of the occurrence of agostic M-H-C bonds. This early history is outlined in Section VI.C.2. The presence of Fe-11-C interactions was eventually proposed on the basis of detailed NMK studies and confirmation came from a neutron diffraction study of [Fe(q-C,H,,){P(OMe),),] ' (11) (see Table IV). This neutron diffraction study was the first on a compound with an agostic M-H-C group and showed distances that were interpreted in terms of a very strong Fe-H-C interaction. This bonding was described as aprewrsor C-H.-.Fe bond rather than a fully delocalized 3c-2e bond (18).

6 BROOKIIART, GREEN, AND WONG

11

The next class of agostic compounds of note were the highly distorted tantalum-alkylidene systems described in Section V1.B. The compound [Ta(q-CSMe5)(PMe3)(q-C211,)(CHCMe3)] (12), characterized by neutron diffraction, shows r(C-lI), r(Ta-H) distances of 1.135 and 2.042 A. rc- spectively. As discussed in Section V.C. however. initially there was some doubt as to the extent of M-H-C, bonding.

The observation of a Ti-H,-C, bond in the ethyl compound [Ti($- Et)Cli(dmpe)] (13), provided a strikingly simple example of agostic bonding (19, 20).

Me3 Cp-

CI & R5

I

1-

12 13 14

It seemed clear that the dose approach of the Ha hydrogen reflected the requirement of the otherwise 12-electron titanium center to gain more electrons. This report was rapidly followed by the correct formulation of the 18-electron cation [Co(q-C,Me5)(q'-Et)(q-C2H4)] + (14), which showed for the first time that the agostic q'-alkyl ( b ) could be the stable ground state rather than the olefin-hydride (a) , or the q'-alkyl ( c ) , shown in Fig. 3. The identification of the q2-alkyl group in these otherwise simple compounds strongly suggested that such q'-alkyl systems might be much more common than previously suspected as has since been abundantly confirmed. Furthermore, at this point, the implication that agostic interactions would have a general involvement with many aspects of organo-transition metal chemistry became inescapable.

CARBON-~IYC)KO~iEN-'I 'KANSII'I~~N METAL. BONDS 7

A --__ .

. N , . , . --: , , .

. - 1 % B _ - * *

-. Et Y - !I -~

18e

16

I 0 I '8e I b i 14

16e i c i

15

M 2- CO(~-C'JI . ) I ,

Figure 3. Alternative isomeric structures for a metal -ethyl compound. ( a ) the previously assumed olefin-hydride (or "classical") structure 16; ( h ) the B-agostic $-ethyl (14): (c) the q'-(or a-)ethyl (15). Pathway A is that norrnallv proposed for the olefin insertion reactions. Pathway H shows the agostic structure as the grourtd ,state rather than the normally supposed transition state.

The first example of an agostic interaction to be characterixd in a polynuclear system is that in [OS,II(CO),~( k*.?-CH,)J (17) (21,22). Neutron diffraction studies showed, however, that only the p,-CH2 isomer (18) exists in the solid state (23). 111 solution there is an equilibrium mixture of 17 and 18. The elegant technique of partial deuteration was developed by Shapley to characterize the agostic interaction hy NMR spectroscopy (see Section 1V.B).

17 18

The first neutron diffraction characterization of an agostic bond in a cluster system was carried out by Muetterties and co-workers for the

8 BROOKHART, GREEN, AND WONG

tetranuclear compound [Fe4( p-H)( p4-CH)(C0)12] (19) (24-26). The agostic interaction was characterized by the r(C-HJ, r(Fe-Ha) distances of 1.191 and 1.927 A, respectively, and by the low value of J(C-Ha) 103 Hz in the I3C NMR spectrum.

[Fe: = Fe(C0I3

19

111. CLASSIFICATION OF COMPOUNDS WITH AGOSTIC BONDS

It is convenient and of some chemical significance to classify the dis- cussion of the chemistry of compounds with agostic bonds under the head- ings given at the beginning of this chapter. The a-agostic alkyls discussed in Section VI.A.l are the first members of the series shown in Fig. 4a. The alkylidene complexes with an agostic hydrogen on the alkylidene car-

Compounds a B Y Remote Section VI.A.1 VI.A.2 V1.D V1.D

Figure 4a. The series of agostic alkyl complexes.

C l l > H PH M I VH /6L, 5 M

M - H M

M T H

Compounds ylidene- yl ene-yl enyl-yl diene-yl dicnyl-yl

Section V1.R VI.C.1 VI.C.2 VI.C.3 VI.C.4

Figure 4h. The series unsaturated-hydrocarbons-agostic-yl complexes

CARBON-HYDROGEN 3RANSITION METAI. BONDS 9

bon (Section V1.B) can be considered as the first members of the series of complexes shown in Fig. 4b, which have unsaturated hydrocarbon frag- ments attached to the carbon atom of the agostic C-H bond. In Section VII those compounds with related M--H-X interactions, where X 7L c', and where M # metal. are described. In Section VIII M-X-C interactions with X # H are discussed.

IV. GENERAL PHYSICAL AND CHEMICAL PROPERTIES

A. Structural Determinations Using X-Ray, Neutron, and Electron Diffraction Techniques

The most interesting structural feature of M-H-C bonds is thc location of the hydrogen atom and the C-H and M-€4 bond distances. In several X-ray structure determinations, particularly the early ones, evidence for interaction o f the C-€4 group with metal was inferred from a close M-C distance. Many X-ray structures locatc and refine hydrogen atom positions, but such data give only approximate M-H and C-H distances.

For more reliable r(C-H) and r(M-H) distances, neutron or possibly electron diffraction data are required and these have been reported for several compounds. These data will be discussed in more detail later, however, it is clear that all agostic bonds are bent. Furthermore, the agostic C-H distance is in the range 1.13-1.19 A and is elongated 5-10% relative to a nonbridging C-H bond. The M-H distances in M-H-C bonds are also substantially longer (10-20%) than expected for a normal terminal M-H bond. These effects can clearly be ascribed to the presence of a three- center, two-electron M-H-C bond (see Section V) with the consequent reduction of the C-H and M-H bond orders. Some typical examples are:

r (C-H)( A) 1.16

r(M- 11 ) (A) 1.X7

1.19

1.84

1.19

1 .!I3

10 BROOKHARI'. GREEN. AND WONG

It is not always the case that a M-H--C interaction gives rise to an elongated C-€1 bond as shown for the unsymmetrical distorted [Ti(dmpe)(CL),(-q'-CI I?)] (21) (20, 28). The recent neutron structure (20) established that one Ti-C-H angle closes down to 93.5" but that all three C- H bonds do not deviate significantly from 1.10 A:

r(Ti-1 l;) 2.447 A

1.090 A r(C-14,) 1.09s 'A

1.082 A i (1 i -C - l I > ) 93.5'

i (?'i-C-HI) 118.4"

i(Ti-C--HJ 112.9"

21

B. Nuclear Magnetic Resonance Studies

The most useful spectroscopic technique for detecting the presence of M-H-C systems in compounds is NMK spectroscopy. Where spectra of static agostic systems can be obtained. the ' I t and I3C chemical shifts, and in particular J(C-H) values can be used with confidence to assign agostic structures. Many agostic compounds are, however, highly fluxional and undergo rapid exchange of the agostic hydrogen with other hydrogens, normally those attached to the same carbon atom. These fluxional compounds give averaged spectra at 25°C. Harriers to these hydrogen exchange reactions are frequently of such a magnitude (>8 kcal mol-') that static (slow exchange) spectra can be obtained at low temperatures (e.g., -80 to -10OOC). In cases where even at the lowest attainable temperatures static spectra cannot be observed, it is often difficult to distinguish between the agostic formulation and classical structures. Partial deuteration experiments. coupled with careful analysis of the chemical shift and J(C-H) values, are useful in these cases and examples are discussed below.

Static Systems. The most characteristic feature of a M-H-C agostic interaction (see general structure 22) is the low value of J(C,-H,) due to the reduced C,-H, bond order in the (3c-2e) system and the resultant elongated C--H bond. Typical values for J(C',-Ha) are in the range of 60- 90 Hz. as can be seen by examining Tables I to VII. These values are significantly lower than those expected for normal C(sp3)-H bonds (120-

CARBON- I~YI>KO<;EN-'I'RANSI'TION METAL I3ONI>S 11

130 € 1 ~ ) in, for example, the coordinatively unsaturated structure 23. Con- versely, they are much higher than expected for a classical C-M-€1 alkyl hydride system o f general structure 24 [J(C'-H) < 10 Hz] and are thus a very reliable indicator of an agostic interaction.

23 Unsaturated

22 Agostic

24

Terminal hydride

It is probable that, depending on the compound, there can be substantial variations in the bond strengths of the M-H and C-H components in agostic M-H-C bonds. The most common situation is where the C-H component is stronger than the M-€€ component, but the converse can also be true. For example, some values of J(C-H) are above 90 Iiz (especially for agostic alkylidene compounds. see Table III ) , and some are substantially lower than 65 Hr.

Most agostic systems in which tertiary phosphine or tertiary phosphite ligands are present show very small or nonobservable two-bond J(P-H,J coupling constants, whereas if the compound contained a terminal hydride ligand 24, then substantially larger values for J(P-H) would be expected. The low values for l(C-€i,) may be associated with longer M-H, boncls, compared to terminal M-H bonds. Some examples are:

g'+ !?

Mn -.- H

-I+

12 HROOKIIAKI, GREEN, AND WONG

An interesting interplay between J(C-Ha) and J(P-H,) values is illustrated in the following two compounds (32):

The unsubstituted agostic ethyl compound exhibits a "normal" J(C-H) value of 61 Hz and a nonobservable J(P-H,) < 4 Hz. Upon substitution of a methyl group at C,, the J(C-HA) decreases dramatically to 38 Hz and the J(P-Ha) increases to 32 Hz. These changes suggest that there is a delicate balance in energy between the agostic and ethylene-hydridc structures in this system and that methyl substitution can perturb the stable geometry such that the C-H, is elongated [decreasing J(C-H,)] and M- H, is shortened (J(P-H) is increased]. This observation supports the idea that, depending on the compound, a range of stable structures for M-€I- C bonds are observed.

The chemical shifts of agostic hydrogens in C-Ha-M systems for d", n > 0, metal centers normally occur at high fields (6(Ha) < 0 ppm and up to -16 ppm] and occur in the range typical for normal terminal metal hydrides. For this reason. care must be exercised in interpreting these values; there are several examples in the early literature where such high- field shifts led to incorrect assignments of agostic structure as classical (normally olefin-hydride) structures (see later). For d" systems, resonances due to the agostic hydrogens normally do not occur at higher fields than 0 ppm. For example, in d" alkylidene system the chemical shifts fall in the range 2-6.5 ppm; however, these shifts, in general, do occur at higher fields than those of analogous Welectron d" alkylidene systems that show no M-H, interaction (Section V1.B).

The values for the "C chemical shifts of C, in the agostic structure 22 occur over a wide range depending on the system and substituents R, and R2, and are not particularly useful in identifying agostic structures. The exceptions are systems in which C, in the hydride form (24) is part of a rr system (see Sections VI.C.2-VI.C.4). In these compounds 6(C,,) is often near or above 0 ppm and at higher fields than one would expect for either

CARBON-IIYDKOC;EN-'I'KANSII'ION MEI'AL, BONDS 13

the hydride 24 or the coordinatively unsaturated structure 23. Two examples follow:

&(C,) -4.5 ppm &(C,) --5 to - 6 ppm

(33) (32, 34-36)

Fluxional Systems. When a spectrum of the static species cannot be obtained (i.e., when the barriers to interconversion are <7-8 kcai mol '1 then only averaged values of chemical shifts and J(C-H) can be measured. The problem that then arises is to distinguish between the three possible structures, namely, the unsaturated 23, the agostic 22, and the terminal hydride 24 structures. When the fluxional process does not involve scrambling of H, with other hydrogen atoms in the system, then the averaged values of J(C-H,) normally give a clear indication of agostic structures. For example, protonation of [ C O ( ~ - C , M ~ ~ ) ( ~ - C H ~ ) ~ ] was originally thought to give a product with the structure 16. The 13C NMR spectrum at - 90°C is consistent with either the terminal hydride 16, or the rapidly equilibrating bridged structures 14 and 14', (e.g., C,eC:, C,eCb) (37):

16 14 14'

'The observed value for averaged J(C,,,,-H) of 33.5 Hz clearly indicates the rapidly equilibrating agostic structure 14, where the observed value of 33.5 Hz arises from the average of J(C,--H) = 67 Hz and J(C,r-H) -0 Hz. For the static structure 16 a J(C,-H) of -0 Hz would be predicted

The most common problem arising in fluxional systems is to distinguish (37).

14 BRC)OKHAKI', GREEN, AND WON<;

among systems 25-27 in which the agostic interaction occurs in an unsym- metrically bridged methyl group:

25 26 21

The averaged value for J(C-H) expected for 25 is the normal J(C,,a- H) value of 120-130 Ht. For 27 a value of J(C-€1) may lie in the range of 80-100 Hz. I(H-M-C) is estimated to be 0-10 Hz and J(C,-H,) as 120-150 Hz (the high value would occur when C, is part of a 7~ system and ha\ S J J ~ hybridization). For 26 the values of J(C-H),, would be -105- 125 HL if ranges for J(C,-H,,) are taken as 60-90 Hz and for J(C,-H,) as 120-145 HL. These overlapping ranges indicate that .\(C,-H)d5 values alone can seldom be used Hith certainty to assign agostic structures.

Compounds for which the metal has d", IZ > 0 configurations, the averaged '€1 chemical shift for the fluxional agostic methyl group of 26 is normally above 0 ppm and at higher fields than the noninteracting methyl group of 25. If the unsaturated isomer 27 were highly fluxional then the averaged chemical shift of the methyl group in this system would also be at fields higher than 0 ppm (see later). Since the chemical shifts of terminal and agostic hydrogens are quite variable and fall in similar ranges, the averaged chemical shift does not permit a distinction to be made between these two systems. In d" systems, where the structures 25 and 26 are favored, the averaged chemical shift in each case will be at fields below 0 ppm and thus cannot be used to distinguish 25 from 26.

An important NMR method for probing agostic interactions in fluxional systems is that involving partial deuteration developed by Cahert and Shapley (21) on the trinuclear osmium system [Os,(CO),,,(CH3)(H)] (17). They observed that the average 'H chemical shifts and J(C-H) values are quite sensitive to the extent of deuteration of the methyl group and fall in the order (lowest field) 6(CH1) > G(CH2D) > 6(CHD2) (highest field) and J(C-H)(CH;) > J(C-H)(CH,I>) > J(C-H)(CHDZ). Furthermore, both the chemical shifts and J(C-H) values of the partially deuterated species CH3D and CHDZ (but not CH3) are strongly temperature dependent. These effects arise because there is a thermodynamic preference for the deuterium atom to occupy the terminal positions and hydrogen atoms to occupy the bridging, agostic site. The reason for this preference is illustrated in Fig. 5 . The bridging M-11-C and M-D-C bonds are longer and weaker than

CAKHON-HYDROGEN-7'RANSI'IION MEIAL. UOKI)S 1s

E

C-H

c- 0 A 5

E

Figurc 5 . Potential energy curves for C-14 and C-I> bonds illustrating the origin of the Shapley effect.

the corresponding terminal G I 1 or C-1) bonds and hence have a shallower potential well. Therefore there is a smaller zero-point energy difference between them, relative to the stronger terrnirzal C-H, versus C-D, bonds. In consequence, the deuterium prefers to occupy the terminal site and drives the hydrogen to occupy the bridging site. The temperature depend- ence of the equilibrium, as measured by the change in average chemical shifts, allows calculation of the AH" values for the thermodynamic equilibrium isotope effect. Two examples are:

(CO!, 0 s

H D

Partial deuterium labeling experiments must be interpreted critically since there are two specific situations from which incorrect conclusions could be drawn. First, as Faller and co-workers noted (39) and as com- mented on in our earlier review (1). if the classical hydridc 27 were the

16 BROOKHART. GREEN, AND WONCi

stable structure but rapid hydrogen scrambling were occurring via unsaturated 25, then a “Shapley effect” would still be observed, since the M- HJM-D, zero-point energy difference would be significantly less than for C-HJC-D,. In practice, we are not aware of any case where a classical olefin-hydride has been established crystallographically, but which cannot be “frozen out” by low-temperature NMR spectroscopy. If 27 is the ground state, the barrier to formation of 26 will most likely exceed 7-8 kcal mol ’, the minimum value required for low-temperature observation of the static structure (39).

Second, it is more difficult to distinguish between structures 26 and 25 in d” metal alkyl complexes, as illustrated by the agostic complex [Ti(dmpe)Cl,($-Me)] (21) (20. 28). The methyl hydrogens are equivalent down to -100°C. and no Shapley effect for [Ti(dmpe)C1,CH2D] is observed. The X-ray and neutron structures show a distorted methyl group, however, Section V1.A. 1. Interestingly, the neutron structure shows that all three C-H distances of the methyl group are essentially equal and thus zero-point energy differences between C-H and C-D will also be nearly equal for all sites. Therefore, no Shapley effect is to be expected. In such do systems crystal or molecular structure determination may be required to establish the presence of distortions arising from agostic interactions.

+ lo [

-25

90 100 110 120 LH-C-M l o

Figure 6. M = H (O), ?‘iCl1 (j), ZnMe (+), or Mn(CO), (A).

Variation of 2.T(H,H)La,c with the H-C-M bond angle of MCH, molecules where

CARRON-IIYDKOGEN-TRANSITION METAL BONDS 17

Green and Payne (40, 41) suggested that the magnitude and sign of 'J(H-H) may give useful information regarding the degree of distortion of methyl groups in L,MCH, complexes. The concept should be especially useful for unsaturated high valent do complexes, as illustrated for [TiMeCl,] (28). An electron diffraction study (41) of this molecule shows there to be a symmetrical flattening of the methyl group with Ti-C-H angles of -101.1(2.2)" and unusually long C-H bonds, 1.16 A. Infrared studies provide further support for this structure (42) (Section 1V.C). The geminal 'J(H-H) value was found to be + 11.3 €12, the first example of a methyl group with a positive geminal coupling constant. Calculations by Green and Payne (40) show the magnitude of ?J(H-H) to be quite sensitive to the degree of flattening of the methyl group. A linear relationship between 'J(H-H) and the M-C-H angle was calculated for ML, = TiCl,, Mn(C0)5, and ZnCH, , with 2J(H-H) becoming increasingly positive with decreasing M-C-H bond angle, Fig. 6.

C. infrared Spectroscopy

The stretching frequencies of agostic M-H-C bonds have been reported for relatively few of the large number of agostic compounds described in the following sections. Consequently u(C-H) data have not often been used as a probe of agostic interactions. In all cases, however, bands assignable to u(C-H) are found at lower frequencies than for normal sp3- C-€1 bonds and occur in the range 2250-2800 cm-'; data are given in the tables. Uercaw and co-workers assigned a p-agostic structure to [Sc(q- C,Me,),CH,CH,] (29), on the basis of low frequency u(C-H) bands observed in the range 2440-2593 cm ' (43). The propyl analogue [Sc(q- C,Mej),CH2Cf12CH7] has no corresponding low frequency hands and is thought not to be agostic (see Section VI.A.2).

McKean et al. (44) reviewed the vibrational analysis of ML,,C€& compounds and reported vibrational spectra of several deuterium labeled complexes ML,CD3 and ML,CHD2. The latter species allow evaluation of an isolated C-H stretching mode. Analysis of the vibrational spectra of [TiCH,C13] 28, and [TiCH2DCl,] led them to conclude, in agreement with electron diffraction data (31), that the H-C-H angle opens to -115" and the Ti-C,-H angle is correspondingly reduced.

D. General Reactivity Patterns

Two general features dominate the reactivity patterns of agostic systems. (a) The agostic C-H group can be considered as a weak ligand and as such can often be displaced by more strongly donating ligands (Eq. 1). Since the agostic C-H interactions are intramolecular, steric factors can favor

18 HKOOKHART. GREIiN, AND WONG

1-

CH,OTf I ' m C H 3 H 7 c-

H- Mn M n r H Mn CH3 (CO), (CO),

Figure 7. Some reactions of 30.

the agostic system and there are now several examples where the equilibrium shown in Eq. 1 favors the M-I-I-C system. (b) In the M-H-C system the C-H group is interacting with an essentially electrophilic metal center, which results in the agostic hydrogen becoming acidic; for example, deprotonation reactions can occur (Eq. 1). The pK, values of agostic C- €1 hydrogens vary over a wide range, and in some systems strong bases are required. When anionic. nucleophilic species are generated, reactions with other electrophiles can result in new C,-electrophile bonds and a net overall electrophilic substitution of I I,.

A system that illustrates these reactivity patterns is the complex [Mn(q- butenyl)(CO),] (30) (33) , Fig. 7.

V. THE NATURE OF AGOSTIC BONDING

The structural and spectroscopic data given in the Tables I to VII and reviewed in Section VI are clearly consistent with the description of the

large majority of M-11-C systems in terms of three-centered, two-electron (3c-2e) bonding.

Ah irzitio LCAO-SCF-MC) calculations on an analogue of [Ti- EtCl,(dmpe)], namely, [TiEt(PH3);(CI)J I] (31), provided the first theoretical evidence for an agostic M-H-C interaction (45) and calculated values of 89" for the T-Ca-Cp angle. short Ti-11, (2.23 A) and long C,- H, (1.11 A) distances were found in the fully optimized geometry (35). Analysis of the ligand effects suggested that there was a donative interaction from the Cp-H, bond to a vacant Ti-d orbital.

Ah initio calculations on the compound [I'd(C3H5)(PH3)(H)] (32) led to a structure with an agostic interaction. In optimized geometry the Pd-C,- C, angle is 88" and there is a short Pd-I3 distance (2.13 A). The agostic C-II bond length was calculated to be 1.13 A, that is. 0.05 A longer than the other C-H distances in the same molecule (46, 47).

Crabtree et al. compared available structural parameters for a range of compounds containing M--I1-C bonds (48). The data are represented in Fig. 8, which shows a trajectory, whereby as the C-H bond approached the surface of the metal atom M the C-II vector is pointing towards the metal with a M-€1-C angle of 130". Furthermore, as it approaches the metal the C-H bond rotates and lengthens. The lengthening may be associated with greater metal to C-H bonding. A similar trajectory has been proposed for the approach of the H2 molecule to a metal center (40).

,

Figure 8. A reaction trajectory for the reaction

<:-I$ t M - ML-H-C -* H-M-C (SO)

20 HKOOKHART, GREEN, AND WONG

A. General Considerations Concerning Three-Center, Two-Electron Bonds

The molecular ion 11; has two bonding electrons, the most stable structure has been calculated to be triangular. The molecular ion €I, has four electrons and is linear. The MO diagrams for the two ions are shown in Fig. 9, in which the three-center MO’s are illustrated.

The I I ; ion is bent because there is constructive overlap between the terminal hydrogen Is orbitals. In the 4-electron H3 ion the nonbonding orbital is filled and there is no net overlap between the terminal hydrogens. Electron repulsion drives the molecule to adopt the linear structure. Anal- ogous heteronuclear systems will adopt bent or linear structures according to whether they have two or four bonding electrons, respectively. Similarly, in all three-centered, two-electron bridging systems such as M-H-X (where M and X = main group metals, transition metals, boron, carbon, or silicon) the M-€I-X fragment is bent (see Section VII). Well-known examples include B-H-€3 bridges in diborane, the borohydride-transition metal compounds M-p-H2BHI, the bridging hydride compounds [ (CO),M-H- M(CO),]-, M = Cr, Mo, and W, (Fig. 10) (1) and, of course, the agostic bonds M-H-C. Some specific examples of M-€1-X bonds, where X # C, are discussed in Section VII.

It should be noted that hydrogen is unique as a bridging ligand since it has only one valence orbital in contrast to all other atoms that can act as bridging systems. Distinction should be drawn between 3c-2e(or 4e) bonds that have only three valence orbitals, that is, those discussed prcviously. and those 3c-2e(or 4e) bonds with more than three valence orbitals. A familiar example of a 3c-4e bond that employs four orbitals is that in bent bridging halogens M-X-M {e.g., in [CI,AI(p-CI),AlCl,]}. The 3c-4e bonds

“1 A + *- 09 +I-

B e n t H 3 + L inea r H3-

Figure 9. The simple MO diagrams of the H i and 11; ions.

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