Advances in Organometallic Chemistry, Volume 14

Advances in

ORGANOMETALLIC CHEMISTRY VOLUME 14

CONTRIBUTORS TO THIS VOLUME

V. G. Albano Kenneth P. Callahan

P. Chini Ernst Otto Fischer

M. Frederick Hawthorne James A. lbers Steven D. lttel

M. F. Lappert P. W. Lednor G. Longoni Yoshio Matsumura Akira Nakamura Rokuro Okawara Sei Otsuka

V. S. Petrosyan

0. A. Reutov Hubert Schmidbaur Dietmar Seyferth

N. S. Yashina

Advances in Organometallic Chemistr y EDITED BY

ROBERT W E S T

F. G. A. STONE

DEPARTMENT OF CHEMISTRY UNIVERSITY OF WISCONSIN MADISON, WISCONSIN

DEPARTMENT OF INORGANIC CHEMISTRY THE UNIVERSITY BRISTOL, ENGLAND

VOLUME 14

@ 1976

ACADEMIC PRESS

New York

*

S a n Francisco

A Subsidiary of Harcourt Brace Jovanovich, Publishers

*

London

COPYRIGHT 0 1976, BY ACADEMIC PRESS,INC. ALL RIGHTS RESERVED. N O PART O F 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.

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United Kingdom Edition published by ACADEMIC PRESS, INC. (LONDON) LTD. 24/28 Oval Road. Lo n d o n NWI

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Contents LIST OF CONTRIBUTORS . PREFACE

.

ix xi

. . .

1 3 6

On the Way to Carbene and Carbyne Complexes

ERNST OTTO FISCHER Introduction . Transition Metal-Carbene Complexes . . Other Syntheses of Cnrbene Complexes . Reaction Possibilities of Carbene Complexes . Transition Metal-Carbyne Complexes Reaction of Other Peptacarbonylcarbene Complexes with Boron Trihalides . VII. Reaction of Pentacarbonylcarbene Complexes with Halides of Aluminum and Gallium . VIII. Reaction of Lithium Benzoylpentacarbonyltungstate with Triphenyldibromophosphorane . . IX. Reactivity of the Carbyne Ligarid . References

I. 11. 111. IV. V. VI .

.

.

8

.

21

.

24

.

27

. . .

27 28 29

.

33 35 37 59 60

Coordination of Unsaturated Molecules to Transition Metals

STEVEN D. ITTEL and JAMES A. IBERS I. 11. 111. IV.

Introduction . Theoretical Models . Structural Results . Summary . References .

.

.

. .

.

Methyltin Halides and Their Molecular Complexes

V. S. PETROSYAN, N. S. YASHINA, and 0. A. REUTOV I. 11. 111. IV. V.

Introduction . Methods of Study . Structures of Methyltin Halides . Molecular Complexes of Methyltin Halides . Conclusion . References . V

. . .

. . .

.

63 64 68 76 91 92

CONTENTS

vi

Chemistry of Carbon-Functional Alkylidynetricobalt Nonacarbonyl Cluster Complexes

DIETMAR SEYFERTH I. Introduction : General Properties of Alkylidynetricobalt Nonacarbonyl Complexes . 11. Synthesis of Alkylidynetricobalt Nonacarbonyl Complexes . 111. Chemistry of the Trirobaltcarbon Decacnrbonyl Cation . Iv. Highly Stable Nonacarbonyl Tricobaltcarbon-Substituted Carbonium Ions v. Decomposition Reactions and Derived Synthetic Applications of Alkylidynetricobalt Konacarbonyl Complexes . VI. Concluding Remarks References .

.

98 100 . 110

.

. 119

.

135

. 138

.

141

Ten Years of Metallocarboranes

KENNETH P. CALLAHAN and M. FREDERICK HAWTHORNE I. Introduction 11. 111. IV. V. VI. VII. VIII. IX. X.

. . .

145 150 155 167 . 171 . 171 . 175 . 178

.

Metallocarboranes: Synthetic Methods , Twelve-Vertex Metallocarboranes . Thirteen-Vertex Metallocarboranes . Fourteen-Vertex Metallocarboranes . Eleven-Vertex hletallocarboranes . Ten-Vertex Metallocarboranes . Nine-Vertex Metallocarboranes . . Oxidative Addition to B-H Bonds . Metallocarboranes in Homogeneous Catalysis References .

.

. .

.

180

182

. 183

Recent Advances in Organoantimony Chemistry

ROKURO OKAWARA and YOSHIO MATSUMURA I. 11. 111. IV.

Introduction . Hexacoordinate Mono- and Diorganoantimony Compounds . Triorganostibine Sulfide . Tertiary Stibines . References .

.

. . . . .

187 188 192 197 202

.

205 207

Pentaalkyls and Alkylidene Trialkyls of the Group V. Elements

HUBERT SCHMIDBAUR I. Introduction . 11. Simple Nitrogen Ylides

.

CONTENTS 111. IV. V. VI. VII.

Phosphorus Ylides and Pentaalkylphosphoranes . Arsenic Ylides and Pentaalkylarsoranes . Antimony Ylides and Pentaalkylstiboranes . . Bismuth Compounds . Related Compounds of Vanadium, Niobium, and Tantalum . References .

vii

.

209 224 23 1 236 236 240

Acetylene and Allene Complexes: Their Implication in Homogeneous Catalysis

SEI OTSUKA and AKIRA NAKAMURA I. Introduction . 11. Acetylene Complexes. 111. Allene Complexes . References .

245 246 265 279

.

High Nuclearity Metal Carbonyl Clusters

P. CHINI, I. 11. 111. IV. V. VI . VII. VIII. IX. X. XI. XII. XIII. XIV.

xv.

G.LONGONI,

Introduction . Structural Data in the Solid State Structural Data in Solution . Syntheses. . Methods of Separation . Reactivity . Iron Derivatives . Ruthenium Derivatives , Osmium Derivatives . . Cobalt Derivatives , Rhodium Derivatives . Iridium Derivatives . . Nickel Derivatives . Platinum Derivatives . Bonding Theories . References .

and V. G. ALBANO

.

285 286 306 311 316 317 323 324 325 325 327 332 333 334 336 34 1

Free Radicals in Organometallic Chemistry

M. F. LAPPERT and P. W. LEDNOR I. 11. 111. IV.

Introduction . Metal-Centered Organometallic Radicals . Other Organometallic Radicals . . Bimolecular Homolytic Substitution ( S H ~at ) the Metal Center of an Organometallic Substrate .

345 349 367 370

viii

CONTENTS

V. Addition or Elimination Radical Reactions VI. Appendix . References

.

,

.

. 381 . 390

.

392

SUBJECT INDEX .

. 401

CUMULATIVE LISTOF CONTRIBUTORS .

. 410

CUMULATIVE LISTOF TITLES

. 412

,

fist of Contributors Numbers in parentheses indicate the pages on which the authors’ contributions begin.

V. G. ALBANO(ass),Istiticto de Chtmicu Generale dell’ Universitd, Milano, Italy KENNETH 1’. CALLAHAN (143), Metcalj Research Laboratory, Department of Chemistry, Brown University, Providence, Rhode Island P. CHINI(as,;), Istituto de Chirnica Generale dell’ Universitd, Milano, Italy

ERNST OTTO FISCHER (1), Inorganic Chemistry Laboratory, Technical University, Munich, West Germany M. FREDERICK HAWTHORNE (143), Department of Chemistry, University of California, Los Angeles, Caltjornia

JAMES A. IBERS (33), Depnrtntent oj Chemistry, Northwestern University, Evanston, Illinois STEVEND. ITTEL (33), Central Research and Development Department, E. I . d u Pont de Nemours and (’onipany, Wilmington, Delaware

M. F. LAPPERT(345), School of Molecular Sciences, University of Sussex, Brighton, England

P. W. LEDNOR* (345), School of Molecular Sciences, University of Sussex, Brighton, England G. LONGONI(ass), Istituto de Chiniica Generale dell’ Universitd, Milano, Italy

YOSHIOMATbUMURAt (187), Department of Applied Chemistry, Osaka University, Yamadakartzi, Suita, Osaka, J a p a n AKIRA N A K A l i U R A (24,5), Department of Chemistry, Faculty of Engineering Science, Osaka Universit!y, Toyonaka, Osaka, J a p a n ROKURO OKAWARA (187), Department of Applied Chemistry, Osaka University, Yamadakami, Suita, Osaka, J a p a n * Present address: Institut fur Anorganische Chemie der Universitat Munchen, 8 Miinchen 2, Meiserstrasse 1, Germany. t Present address: Japan Synthetic Rubber Co., Ltd., Research Laboratory, 7569 Ikuta, Tama, Kawasaki, Japan. ix

X

LIST

OF CONTRIBUTORS

SEI OTSUKA(245), Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka, J a p a n V. S. PETROSYAN (63), Chemistry Department, M . 8. Lomonosov Moscow State University, MOSCOW, USSR

0. A. REUTOV(63), Cheniistry Department, M . V . Lomonosov Moscow State University, MOSCOW, USSR HUBERTS c m m B A m (205), Anorganisch-chemisches Laboratorium, Technische Universitat Munchen, Munich, West Germany

DIETIIARS E Y F E w r n (97), Department of Chemistry, Massachusetts Institute oj Technology, Cambridge, Massachusetts

N. S.YAsnIxA (63), Chemistry Department, M . V . Lomonosov Moscow State University, Moscow , U S S R

Preface The first volume of Advances in Organometallic Chemistry was published early in 1964, and twelve other volumes have appeared since that date. The Editors have sought to produce a series of books containing specialist articles on all aspects of this field. The success of the series, as judged by the reviews of the books published in the journal literature, is due in large measure to the cooperation and help we have received from some one hundred and ten contributors. However, the demand for authoritative surveys of topics in organometallic chemistry derives mainly from the continued resilience of this area of endeavor, one measure of which is the annual appearance of over 2000 primary journal articles. After a little over a decade of publication it seemed to the Editors that we should arrange for the appearance of a commemorative Volume containing articles by distinguished chemists which would emphasize both the wide scope of organometallic chemistry and its international character. The number of contributors was necessarily limited by the need to keep the book to a reasonable length. This presented a problem in relation to selection of authors. Our choice is, therefore, a personal one guided to some degree by geographical distribution and the desire to balance transition metal chemistry versus main group metal chemistry.

F. G. A. STONE

It. WEST

xi

This Page Intentionally Left Blank

Advances in

ORGANOMETALLIC CHEMISTRY VOLUME 14

This Page Intentionally Left Blank

On the Way to Carbene and Carbyne Complexes* ERNST OTTO FISCHER fnorganic Chemistry l a b a r d o r y Technical University Munich, W e s t Germoqy

I. Introduction . 11. Transition Metal-Carbene Complexes .

.

A. Preparation of the First Carbene Complexes . . B. Bonding Concepts and Spectroscopic Findings . . 111. Other Syntheses of Carbene Complexes . IV. Reaction Possibilities of Carbene Complexes . A. Addition and CO Substitution B. Transition Metal-Carbene Complex Residues aa Amino-Protective Groups for Amino Acids and Peptides . . C. Addition-Rearrangement Reactions. . . D. Substitution of Hydrogen a t \:- r-Carbon Atom E. Liberation of the Carbene Ligand . . V. Transition Metal-Carbyne Complexes . A. Preparation of the First Carbyne Complexes . B. X-Ray Structural Analyses . VI. Reaction of Other Pentacarbonylcarbene Complexes with Boron Trihalides . . VII. Reaction of Pentacarbonylcarbene Complexes with Halides of Aluminum and Gallium . VIII. Reaction of Lithium Benzoylpentacarbonyltungstate with Triphenyldibromophosphorane . . . IX. Reactivity of the Carbyne Ligand References .

11 13 13 14 21 21 22

24 27 27 28 29

I INTRODUCTION In 1960 I had the honor to lccturt: a t this University about our investigations in the area of sandwich complexes. Today I do not wish to return to the results of these earlier studies, but instcad I would like to report on an area of work that has occupied us intensively for several years, namely that of carbene and, more recently, carbyne complexes.

* A Nobel lecture translated by P. Legzdins and G. 0. Wiedersatz, Technical University, Munich. Copyright @ The Nobel Foundation, 1974. 1

2

ERNST OTTO FISCHER

If one formally replaces one of the hydrogen atoms in an alkane hydrocarbon, such as ethane, by a metal atom (which may also have other ligands attached to i t ) , one obtains an organometallic compound in which the organic entity is bonded to the metal by a r-bond (Fig. l a ) . Such compounds were first prepared more than 100 years ago by Bunsen who obtained cacodyl (tetramethyldiarsine) (1) , as well as by Frankland who prepared various dialkylzincs ( 2 ) . Later, Grignard succeeded in synthesizing alkylmagnesiurn halides by the treatrncnt of magnesium with alkyl halides ( 3 ), and for this awompliahment he was awarded thc Nobel Prize in 1912. I n addition, thc organoaluminum cornpounds ( 4 ) of Ziegler, which made possible the low-prcssurc polymerization of olefins such as ethylene, should be remembered. Professor Zieglcr, together with Natta, were honored with the Nobel Prize in 1963. If one next considers a system with 2 carbon atoms bonded to each other by a double bond, i.e., an alkene molecule, one recognizes a number of separate paths leading to organometallic derivatives. One path involves the replacement of a substituent by a metal atom in a manner that we have just seen and leads to u compounds that are exemplified by vinyllithium derivatives. Alternatively, only the a-electrons of the double bond need be employed to bind the organic molecule t o the metal. One, thus obtains a-complexes (5, 6) (Fig. l b ) , the first example of which, Zeise’s salt, Ii[PtC13(C2H4)], was prepared as early as 1827 ( 7 ) . Such metal r-complexes involving olefins are preferentially formed by transition metals ; the main group elements, on the other hand, are less able to form such bonds. In this class of compound one can also include sandwich complexes (8, 9) in which the bond between the metal and the ligand is no longer formed by just 2 *-electrons but by a delocalized, cyclic ?r-electron system. A particular example is dibenzenechromiuin(0) (1 0 ) in which the chromium atom lies between two parallel and eclipsed benzene rings. One attains the third variant when one formally cleaves the double bond and attaches one of the resulting halves of the olefin molecule to a

(a)

(b)

(C)

FIG.1. Production of organometallic compounds from hydrocarbon derivatives, M = a metal or a metal-complex fragment.

Carbene and Carbyne Complexes

3

transition metal component (Fig. l b ) . This concept is realized in the transition metal-carbene coniplcxes in which carbenes ( :CRIX’ that are short-lived in the free stat(, arc‘ stabilized by bonding to the metal. The first part of my account will bt. dcvoted to complexes of this kind. Finally, if one considers molccules with a carbon-carbon triple bond of the type that exists in alkyncs, OIW realizes that there also are three possible paths t o metal-cootairiing derivatives (Fig. l c ) . One may construct a-compounds as discussed in the previous cases or one may use both a-bonds to synthesize. complexes (11) in which the two metal-ligand bonds exist a t a right angles to each other. Let us finally consider a cleaved triple bond in which one-half is replaced by a metal complex fragment; thus we come to the carbytic mmplcxes about which I shall report in the second part of this review.

TRANSITION METAL-CARBENE COMPLEXES

A. Preparation of the First Carbene Complexes In a short communication in 1964, Maasbol and I (12) reported for the first time stable carbene complexes. We had treated hexacarbonyltungsten with phenyl- or methyllithium in ether with the intention of adding the carbanion t o a CO ligand a t the carbon which, relative to the oxygen, is the more positively charged ligand site. Indeed, we did obtain the lithium acylpentacarbonyltungstates, which were converted to pentacarbonyl[hydroxy (organo) carbene]tungstc~i(0) complexes by subsequent acidification in aqueous solution (Scheme I ) . We quickly established that) these complexes are not particularly stable. They tend t o split off tho carbene ligand with a simultaneous hydrogen shift thereby liberating aldchydcs, a fact that Japanese investigators also discovered (13). Very recent13 , we learned how to prepare these hydroxycarbene complexes analytically pure (14). Previously, these complexes, without isolation, had been successfully converted to the substantially more stable methoxycarbene (.ompounds by treatment with diazomethane (12). We soon found a more elegant route to the latter complexes involving the direct alkylation ( 2 5 ) of the lithium acylcarbonylmetalates with trialkyloxonium tetrafluoroborates ( 1 6 ) (Scheme 1) which can be prepared

4

ERNST OTTO FISCHER

co

co oc~T=co co OC,I

according t o the method of ilIecrwein et al. This preparative route to the methoxycarbene compounds possesses the advantage of being simple and straightforward and of leading to the desired conipounds in very high yields. There arose, therefore, the possibility of synthesizing a broad spectrum of carbene romplexes. Instead of phcnyllithium many other organolithium reagents (17-23) can be employed. Moreover, hexacarbonylchromium (17 ) , hexacarbonylmolybdcnum (17 ), the bimetallic decacarbonyls of manganese (24,2 5 ) , technetium ( 2 5 ) and rhenium ( 2 5 ) , and pentacarbonyliron ( 2 8 ) as well as tetracarbonylnickel (27) may be used instead of hexacarbonyltungsten, but the resulting carbene complexes become increasingly more labile in thc indicated order. Finally, substituted metal carbonyls (27-30) can also be subjected t o the carbanion addition and subsequent alkylation. The carbene complexes are generally quite stable, diamagnetic, soluble in organic solvents, and sublimable. Before we consider their reactions in more detail, I want t o discuss briefly carbene ligand-metal bonding.

B. Bonding Concepts and Spectroscopic Findings The first X-ray crystal structure determination, carried out by Mills in cooperation with us ( 3 1 ) on pentacarbonyl[niethoxy (phenyl)carbenelchromium (0), confirmed our originally postulated bonding concept. According to this concept, the carbene carbon atom is sp2 hybridized. It should therefore possess an empty p-orbital and be electron-deficient. Substantial compensation for this strong electron deficiency is provided by a pr-pa bond between one of the free elcctron pairs on the oxygen atom of the methoxy group and the unused p-orbital of the carbene

5

Carbene and Carbyne Complexes

carbon. To a lesser but no less certain extent, there is also a &-pn backbond from a filled central metal orbital of suitable symmetry t o this empty p-orbital of the carbenc carbon. These bonding features affect the distances of the carbene-carbon atom from the oxygen atom on the one side, and from the central chromium atom on the other side: the Ccarbeno-O distance, for which a value of 1.33 A was found, lies between the values for a single (1.43 in diethyl ether) and a double (1.23 A in acetone) bond. Although the average Cr-Cco distance in the carbene complex amounted to 1.87 8, the Cr-Ccarbene distance was 2.04 A. However, according to the considerations of Cotton ( 3 2 ), a distance of 2.21 d would have been expected for a pure chromium-carbon a-bond. Consequently, the bond order of the Cr-Ccarbene bond is distinctly less than that of the Cr-Cco bonds in thc same complex, but it is greater than that of a single bond. That the phenyl group, a t least in the crystal lattice, does not engage in pr-pn bonding with the carbene carbon can be seen by the significant twisting of the plane containing the Cr, C, and 0 atoms relative to the one formed by the phenyl ring. At the same time, i t should be recognized that the double-bond character of the Ccarbene-O bond is so substantial that cis and trans isomers can easily exist relative to this bond (Fig. 2 ) . In the case of pentacarbonyl[methoxy (phenyl)carhenelchromium(O), only molecules of the trans type are found in the lattice, but a t low temperatures lH NMR spectroscopy reveals in solution the coexistence of the cis isomers (33, 34). Further important insights into the bonding relationships of the carbene complexes are made possible by a consideration of the vco bands of vibrational spectra (20, 35-37). As we know, the carbonyl ligands in metalcarbonyl complexes may be considered as very weak donor systems. They donate electron density from the carbon's free electron pair to unused orbitals on the metal atom, a process that formally leads to a negative charge on the metal. This is reduced primarily by a back donation

I

0

0

trans

CiS

FIG.2. The structure of pentacarbonyl [methoxy(phenyl)carbene]chromium(0) ; bond lengths in angstroms.

6

ERNST OTTO FISCHER

of charge density from the metal to the carbon monoxide via a d r - p backbond. Therefore, carbon monoxide has simultaneously an acceptor function as well as a donor function. The a-donor/*-acceptor ratio of the CO ligand in a complex is a very sensitive probe of the electronic character of the other ligands bonded to the metal. It can be qualitatively estimated by determination of the CO-stretching frequency. Let us coniparc carbon monoxide with methoxy(phcny1)carbene as a ligand in complexcs of the type (CO) &rIl (whcre L = CO or [C (OCH3)CsHs], respectively) by considering the vco absorptions. While the totally symmetric Itaman-active vco-stretching frequency in Cr 6 appears at 2108 cm-I ( A , g ) (3 8 ), we find the absorption of the CO group that is situated trans to the carbcne ligand to be shifted drastically t o lower wave numbers and to occur a t 1953 cn1-l ( A , ) ( 1 7 ) .This means t h a t the carbene ligand possesses a substantially larger a-donor/a-acceptor ratio than CO. In other words, the entire rarbene ligand is positively polarized with the @r(CO)6 part bcing negative. Thus, the dipole moments of the complcxrs (ca. 5 D) are also relatively large. I n the remainder of this account I shall not consider further purely spectroscopic studies. Hoivever, special at tention should be briefly given t o I3C N M R measuremcnts as they represent extraordinarily valuable aids for the devdopment of this area of chemistry. I n the first study of this kind, Kreiter (39) succceded in showing that in pentacarbonyl[methoxy(phenyl)carbenc]chroniiuni (0) the carbene-carbon atom is very positively charged. The observed chcmical shift of 351.42 ppni lies well within the range of shifts exhibited by the carbo-cations of organic chemistry. Thus, this modern investigative method confirmed once again our original concept. With its intensely positively charged character, the carbene carbon behaves as an elcctrophilic center, a feature of paramount importance in the reactivity of such compounds. Wc shall return t o this point.

(co)

111 OTHER SYNTHESES OF CARBENE COMPLEXES Since the time t h a t our first paper about metal carbene complexes was published in 1964, this area of research has expanded quickly. Today there are available several major review articles (40-43) dealing with the chemistry of carbene coniplexes. Therefore I want now only t o single out particularly interwting syntheses. In our laboratory in 1968, Ofele (44) treatcd l,l-dichloro-2,3-diphenyl2-cyclopropene with disodium pentatcarbonylchromate and, with the concomitant elimination of sodium chloride, he obtained pentacarbonyl(2,3diphenylcyclopropenyliderie) chromium (0). This compound is stable up

7

Carbene and Carbyne Complexes

to 2OOOC and is notable for thci fact that the carbene ligand does not contain any heteroatom. The elrctronic requirements of the cwbene carbon here are satisfied by the three-nicinbtrcd cyclic a-system :

X-Ray structural analysis (45) showed that the three C-C distances of the ligand are not identical: the distance between the 2 carbon atoms bearing the phenyl substituents is shorter than the other two distances. The carbene carbon-chromium distance of 2.05 A lies within the range of values found for our carbene complcxcs, thereby indicating that in this case as well a n authentic clarbcne complex exists. A vcry interesting synthetic mcthod was published in 1969 by Richards and co-workers ( 4 6 ) . They found that, in the reaction of alcohols with certain isocyanide complexes [such as those of platinum( 11) an addition of the alkoxy group t o thc (.arbon atom as well as of the hydrogen t o the nitrogen atom of the isocyanidc ligarid occurs, and one thus obtains the corresponding carbene cornplcxcs :

I,

c1

P(C,H5)3

Cl’

CeN\

’t.:

+

C,H50H

pt/P ( cZ

~

Cl’ C6H5

H5) 3

1C-N-C,H, ,7

(2)

II OCZH5

This procedure has led subsequently to many compounds of this kind. The relationship between the complcx chemistry of isocyanides and of carbon monoxide comes to mind a t this point. In 1971 we succeeded, again for the first time, in transferring a carbene ligand from one metal atom t o another (26, 47). For example, if one irradiates a solution of cycloprntadic~nyl(carbonyl)[methoxy (phcnyl)carbcnelnitrosylmolybdenum (0) in the prcscricc of a n cxccss of pentacarboiiyliron, one obtains tctracarbonyl[mothoxy (phcnyl)carbencliron(0) with the simultaneous formation of c yclopcntadienyl (dicarbonyl) nitrosylmolybdenum :

a

ERNST OTTO FISCHER

Finally, an additional synthesis of recent times (1971) comes from Lappert and his group (48).They treated an electron-rich olefinic system, such as 1,1‘ ,3,3’-tetraphenyl-2, 2‘-biimidazolidinylidene, with a suitable complex compound. In this manner, they attained cleavage of the C=C double bond and attachment of the carbene fragment to the metal: (C,H5),P\ /C1\ /C1 Pt Pt / \ / \ C1 C1 P(C,H,),

C H lB

CH l6

N I

N I

C6H5

C6H5

xylene 140°C

(4) CH l6

c1

I

C85

This brief sketch summarizes some of the other independently discovered methods for synthesis of carbene complexes.

IV R E A C T I O N POSSIBILITIES OF CARBENE COMPLEXES I now wish to confine my attention to carbene complexes of “our type” and to show with recent examples the kinds of reactions we were able t o produce. We have already established that the carbene carbon is an electrophilic center and, hence, i t should be very easily attacked by nucleophiles. I n most reactions we believe that the first reaction step probably involves attachment of a nucleophile to the carbene carbon. In some cases, for instance with several phosphines ( 4 9 ) and tertiary amines ( 5 0 ) , such addition products are isolable analytically pure under certain conditions (1 in Fig. 3 ) . For the second step there exists the possibility that the nucleophilic agent may substitute a carbon monoxide in the complex with preservation of the carbenc ligand ( 2 in Fig. 3 ) . One can also very formally think of the carbene complex as an ester type of system [X=C(R’)OR with X = bf(C0) 6 instead of X = 01, because the oxygen atom as well as the metal atom in the A3 (0) 6 residue are each missing 2 electrons for attainment of an inert gas configuration. So, i t is not surprising that the

Carbene and Carbyne Complexes

9

0

carbene

Liberation of the carbene ligand

FIG.3. Reaction possibilities of alkoxycarbene complexes.

OR group can be replaced by amino, thio, or seleno groups ( 3 in Fig. 3 ) . I n this way, the amino- (36, 51-64), thio- (51, 5 5 ) , and seleno(organ0)carbene complexes (56) are accessible, but the synthesis of the two latter species requires a special experinirntal skill. We can also observe reactions that lead t o a more stable arrangement of the whole system very probably via primary addition and subsequent rearrangement (4in Fig. 3 ) . In addition, i t can be established that because of the electron withdrawal of the l f ( C 0 ) S moiety, hydrogen atoms in a-alkyl positions to the carbcnc carbon develop such an acidic character that their acidity corresponds to that of nitromethane (5 in Fig. 3 ) . Finally, by cleavage of the carbene ligand from the metal complex, pathways in synthetic organic chemistry are opened (6 in Fig. 3 ) .

A. Addition and C O Substitution If one treats trialkylphosphinm with pentacarbonyl[alkoxy (organo) carbelie] complexes of chroiniuni (0) arid tungsten (0) , typically in hexane a t temperatures below - 30"C, the corresponding phosphorylide complexes (addition compounds) can be isolated analytically pure and studied ( 4 9 ) . The formerly carbene carbon is now sp3 hybridized and exhibits only a a-bond to the central metal. In the case of triaryl- and mixed alkylarylphosphines, the addition-dissociation cquilibriurn (5'7) (Fig. 4) lies largely on the side of the starting materials, and so the ylide complexes can merely be detected spectroscopically. Figure 4 shows the reaction scheme for pentacarbonylCmethoxy (methyl)carbcne]chromium(O) and tertiary phosphines. Upon irradiation of solutions of these ylide complexes in hexane-toluene mixtures a t -15"C, a CO ligaiid is eliminated from the cis position and thereby the cis-tetracarbonylCalkoxy (organo) carbenelphosphine complexes are obtained (58). The phosphine that was initially added t o the carbene carbon of the starting material thus takes the place of a CO

10

ERNST OTTO FISCHER d'" olrfin cwmplexes, the results arr applicable t o d8 coniplcxrs also. Thus both stwic and electronic effects again predict the sanic conformation. Several five-coordinat e coniplr,xc.s involving the cyclic diolefin, 1 ,5-cyclooctadienc (COD), have brcn invcstigatcd ( X X I X and XXX, Table I ) . The olefiri coordinates in an (quatorial-axial manner, thus allowing the equatorial double bond to lit. ill thc, trigonal plane. If the olefin had coordinated in an cquatorial~cciuatorialfashion, the t\\ o double bonds would necessarily be pcrpcndicular to tho trigonal plane. This prcferred orientation may also be influencrd by the approximately 90" bite of COD. I n kwping with other five-coordinatr complexrs, the axial hl-C distances are greater than the correspoiiding equatorial distances. The stronger equatorial interaction also rcwilts in a longer C=C distance.

C. Complexes of Nonolefinic Unsaturated Molecules 1. Acetylene Coniplexes

A modest numbvr of acrtylcw complexes has bccri invrstigated structurally. Marly of the features of olcfin complexes are also observed in acetylenc complexes, the major difference being the change in geometry of the coordinated acetylenc. The acctylene molecule approaches the geometry of a cis-olefin with the C=C-R angle deviating frorii lS0" by the angle a. From Table I one finds that, for a variety of complexes, a ranges from 12" t o 40". There

56

STEVEN D. ITTEL AND JAMES A. IBERS

again seems to be no correlation between C=C distances and the bending back of substituent groups, but the number of structures is limited. It should be noted that metal-carbon distances are about 0.07 8 shorter in acetylene complexes than in related olcfin Complexes. Much of this difference could be attributed to the 0.04 8 change in the carbon single-bond radius on going from sp2 to s p hybridization. Yet the carbon-carbon distance between the bridge and ring carbon atoms of coordinated diphenylacetylene and trans-stilbene are not appreciably different. Thus, if one equates bond length with bond strength, then one concludes that acetylme's intckract mom strongly with metal complexes than do similarly substituted olefins. This conclusion is in accord u ith theoretical predictions (56). When the usual description of metal-olefin bonding is extended t o metal-acetylene systems, the same orbitals are used in the model. But acetylenes possess an additional set of T- and a*-orbitals orthogonal to the metal-olcfin plane. A calculation indicates that interaction of this T*orbital with additional metal d orbitals could be significant. Thus the metal-acetylene bond could be strengthened beyond that of a metal-olefin bond. I n general it is found that t h r carbon-carbon bond of acetylenes is lengthened less upon coordination than that of olefins. This might be interpreted as an indication that acetylenes interact to a lesser degree. Yet, note that the correlation between bond length and bond strength is not linear. Thus the increases in going from a triplc bond to a double bond and then to a single bond arc 0.13 and 0.21 8, respectively. A smaller lengthening of an acetylenc. molrculc relative to an olefin nioleculc can still indicate a comparable change in bond order. As was noted in the discussion of olcfin complexes, a twisting of the C-C bond is usually observed in acetylene Complexes. This twisting is manifestc.d in a nonzero R-C-C-R torsion angle. Angles y and 6, defined the Sam(' way as for olefin complexes, should ideally be 0" and 180", respectively. Va1uc.s of y up to 9" have been observed, and the two 6 angles are not necessarily equal (Tablr I ) . As for olefin complexes, the differences in the 6's can be attributed to nonbonded intmactions, both intra- and intcrmolccular. The variations in y for various diphenylacetylene complexes can bc attributcd to minimization of rionhondcd contacts between the two phenyl rings. The contacts between ortho-hydrogen atoms would be rather close in a strictly planar molecule, and conjugation between the phenyl rings and the acetylene bridgcx is interfered with if the rings twist too much. The remaining method for relicf of the hydrogen atom contacts is twisting about the acetylene bond, Although this argumcnt seems plausible, a twist of 9" is observed in a dicyanoacetylenc complex (XXXVII, Table I),

Unsaturated Molecule-Metal Complexes

57

where steric crowding is minimal, and 0" (by symmetry) is observed in a bis (tert-butyl) acetylene complex ( X X X I X ), where nonbonded contacts might become important. 2. Diazene Complexes

The structures of two complexes ( X U and XLII, Table I) of the diphmyl-substituted diazenc., azobciizcwe, have been determined. Azobenzene is found to be capable of stronger a-backbonding than the isoelectronic traizs-diphenylethyleri(~.This cffect is manifested in several structural aspects. After t h r diffcrcncc. in nitrogen and carbon atomic radii is considered, it is found that t h r N=N bond is longer and the M-N bond is shorter than the respectivc hotids in the olefin complex. Angle y is 6.2( 13)O Icss in the azobenzme compl~~x, indicating a greater bending back of the phenyl rings. Molccular orbital calculatioiis ( 4 3 ) show that the highest occupied molecular orbitals of azobcnzenc and trans-stilbene are a t approximately the same energy, but the lowest unoccupied molecular orbital of azobcnzene is much lower than that of trans-stilbene. Thus azobenzene should be capable of better a-backbonding. The higher clcctroncgativity of the N=N bridge compared with that of a C=C bridge would also argue for better A-backbonding for a diaz(~iw. In the Ni(0) complexes of azobc~nz(mm,the n -+ a * transition observed in the visible spectrum provides an additional probe of the multiple bond (42, 4 4 ) . It is found that the transition shifts to higher energy as the electron density on the doublc bond is incrcased. When electron density is incrcascid by putting more electron-donating ligands on the nickel atom, the nickcl-azobcnzcnc bond is strcngthcncd. If, however, the electron density on the N=N bond is increased by putting elcctron-donating substituents on the phcnyl rings of the a z o b ~ n z ( ~ nthe r , coniplcx is destabilized. 3. Ketone Conzple.res Several complexes of hexafiiioroacc.toiie with d l o metals have berii prcpared. Based on 19FNMR data, thew complexes were thought to involve a ketonic C=O group involved in a a intcraction with the metal. This was confirmed by a structural detcrniination of the nickcl(0) bisphosphine complex (XLIII, Table I ) . In this structure thc CF, groups are bent back about as far as in typical halogenated olefin complexes. Interestingly, the oxygen atom and the carbon atom (with a larger atomic radius), are approximately equidistant from thc nickcl atom, whereas the phosphine ligand trans to the carbon atom shows a significant trans cffcct. This sug-

58

STEVEN D. ITTEL AND JAMES A. IBERS

gcsts that a CF3-substituted carbon atom may be a better ?r-acceptor than an oxygen atom, consistent with the spectroscopic results of Section 111, A,1.

4. Imine Complexes Few Ir-bonded imine complexes have been prepared and investigated structurally. The only complexes of nonbridging ?r-bonded C=N investigated structurally are a suhstitutcd ketenimine complex (XLIV), an iminium complex (XLV) , and a complex of the azine [(CF,) 2C=N-)z (XLVI) (see Table I ) . The ketenimine complex is unusual in that angle y for the complex is 11.9(10)" and the average 6 is 174". These angles are indicative of a coordinatcd triple bond, rather than thc expected double bond. This scemingly anomalous result can bc rationalized on the basis of a large contribution of resonance form B to the structure of the ketenimine. This

e/

N

N

*c

\ /

-C I C

111 +N f

A

-4,

B

form would be enhanced by the highly electron-withdrawing cyano groups. The -C=N-C (CN) portion of the complex is very nearly planar, and this should enhance electron withdrawal by the two cyano groups. The deviations from linearity, a , a t either end of the C-N bond are similar to thosc observed for acetylcnc complexes, thus lending additional support t o the triple-bond model of the kctcnimine. The ?r-iminium complex (iV-methylsalicylaldiminato)- (N-methylsalicylaldiminium) nickel(0) (XLV) is the first example of a nonchelating salicylaldimine complex. The nitrogen atom of the C=N double bond is made more electron-withdrawing by introduction of the positive charge. Thus, the greater bending back of the nitrogen substituents relative to the carbon substituents ( p = 55" and 77", respectively) is reasonable. The azine complex (XLVI), formed by the reaction of bis(trifluoromethyl) diazomethane with Pt (0) is a rather simple imine complex. The feature of note in this complex is angle y that represents one of the greatest observed deviations from planarity ( y = 127").

Unsaturated Molecule-Metal Complexes

59

IV SUMMARY The broad geometrical f(~aturc~s of the interaction bet\\ ccn unsaturated molecules and transition mvtal.; arc’ now well-drfincd as a result of a large number of structural studicxs. 111a grncral way these results can be rationalized by current, crudr bonding inodds. But if the ovcrall understanding of t h r bonding of unsaturated mol(1culvs to transition mctals is to bc improved, additional cxpwimentnl arid thcoretical work nerds to bp done. On the exprrimcntal side, a iiumhrr of factors must be considered. The diffraction results on a givcn conipouiid could be improved considerably if thci cxpcrirncnt wcw done at low tclmperatures so that the smearing effects of thermal motion could tw minimized. As yet, very few such studies have bcen performrd. Now that marc powerful nrutron sources have been developed, thrre is a w r y iniportant n w d for extensive neutron diffraction experiments oil these typcs of cwniplrxcs The scattering of neutrons docs riot follow a simple pattcrn \\ it11 atomic numbcr and, generally speaking the location of both the C or N atoms and the H atoms of an unsaturated molcculc in th(x presence of a traiisitiori metal could be made with much grc.atc.r precision using neutroiis. This is particularly true now that ligand systrms involving fewrr atoms are available (r.g., t-BuNC with 19 atoms versus PPh3with 34 atoms). But c v m the usual diffraction studicxs at room temperature could yield valuable information through n tcmatic approach. On(. nccds systematic studirs of thc samr urisaturatd molcculc bound to an ML, system in which first M and then L is varied. I d ( d l y one ncclds far more studies of systems in which two or mow different ol(4iiis are bound simultaneously t o the samr ML, system. Clearly such systcmatic studies are dependent on new syntheses. Although improvcmclnt of thv tlicwrctical models, especially in the direction of a przori calculations, prcvrrts formidablr computational problems, thew is the grneral trend that if r~liablc,interesting, and perhaps tantalizing observations are availabl(~,thcsc will serve as an incentive for theoreticians to procwd. Finally there is a dcspcratc. need for greater corrrlations of thrsc structural and theoretical studivs of thv nature of thc metal-unsaturated molecule interaction with other cxpcrirnentally derived quantities. We have in mind corrclations with spectroscopically derived quantitirs, such as stretching frcqucncies and Nh4R shielding parameters. But we also have in mind the most important problcm of the correlation of the metrical details of the bonding with the. reaction chcmistry. If the discovery and

60

STEVEN D. ITTEL A N D JAMES A. IBERS

utilization of ncw catalyst systems, so essential today in view of shifting patterns of frcdstocks and of encrgy considcrations, is to be anything but empirical, thrn an undm-standing of the relation betwwn the metal-unsaturatcd molccule interaction and thc rcaction chrmistry is of paramount importance. REFERENCES J. M. Baraban and J. A. McGinnety, Inorg. Chem. 13, 2864 (1974). J. M. Baraban and J. A. AZcGinnety, J . Amer. Chem. Soc. 97,4232 (1975). E. Benedetti, P. Corradini, and C. Pedone, J . Organomelal. Chem. 18, 203 (1969). G. Bombieri, E. Forsellini, C. Panattoni, R. Graziani, and G. Bandoli, J . Chem. SOC.A 1313 (1970). 5. I). J. Brauer and C. Kriiger, J . Organometal. Chem. 44, 307 (1972). 6. D. J. Brauer and C. Kriiger, .I. OrganonietnZ. Chem. 77, 423 (1974). 7. J. Browning and B. It. Penfold, Chem. Commun. 198 (1973). 8. J. Chatt and L. A. lhncanson, J . Chem. Soc. 2939 (1953). 9. P. T. Cheng, C. I). Cook, C. 1-1. Koo, S. C. Nyburg, and M. T. Shiomi, Acta Cr?lstaZlogr.,Sect B 27, 1904 (1971). 10. P. T. Cheng and S.C. Nyburg, Can. J . Chem. 50, 912 (1972). 11. M. R. Churchill and S. A. Bezman, Znorg. Chem. 11, 2243 (1972). 12. M. 11. Churchill and S. A. Bemian, Inorg. Chem. 12, 260 (1973). 13. J. Clemens, It. E. Ihvis, 11.Green, ,J. I). Oliver, and F. G. A. Stone, Chem. Conzmun., 10% (1971). 14. C. D. Cook, K. Y. Wan, U. Gelius, K. IIamrin, G . .Johansson, E. Olsson, €1. Siegbahn, C. Nordling, and K. Sieghban, J . Amer. Chern. Soc. 93, 1904 (1971). 15. C. A. Codson and T. H. Goodwin, J . Chem. Soc. 3161 (1963). 16. It. Countryman and B. I Me3SnBr > M(>#%iI,points out that rehybridization of tin orbitals is rather insignificant in this sckric>\.

B. Dimethyltin Dihalides Elcctron diffraction tmhniqncs showcd (131, 5 0 ) that in the gas phase McaSnXz ( X = CI, Br, I ) arc soincwhat distorted trtrahedra: MczSnClz (LCSnCl, 109.5"; LClSriCI, 107.5"; Sn-C, 2.11 A; Sn-CI, 2.33 A ) , MczSnBrz (Sn-C", 2.17 A; Sn-Br, 2.45 A), MczSnIz (53-1, 2.69 A ) . X-Ray analysis of crystallinc~hZ(~zSnFz showed that (129) in thc solid stat(, the compound is an associate containing six-coordinate tin. The structure is a two-dimcnsional infinitc net in which cvery tin atom is

lTeoSnS

6(CH,)

6("9Sn)

(ppin)

(pprn)

./(~L~SII-C;-'H) (frZ)

J(ll9Sn-l3C)

J(X-1II)

(lfz)

(HZ)

lIeaSriCI

0.61

158.0

58.1

386

131. 6

MerSnBr.

0.73

128.0

57.8

372

131.8

MeuSnI

0.88

38.6

57.2

-

132.1

72

V. S. PETROSYAN, N. S. YASHINA, A N D 0. A. REUTOV

bonded to 4 other tiri atoms by fluorines situated symmetrically in between. Methyl groups lie under and above the resulting plane and, thus, complcte t h r octahcdral structure. The Sn-C distance of 2.08 f 0.01 A is the short& knon n in organotin compounds and mas ascribed (129) to ionicity of the (SnF2) site. Irifrarcd spectra of the compound have shown, however, that v,(Sn-C') cannot be detected, whereas v(Sn-l?) is observed. Thus, covalcncc1 of the octahedral po1yInc.r ith trans-methyl groups is bcyond doubt (Table IV), (86, 22, 61, 83). The structure fits ell with the Mosshauer data (Table V) processed via a correlation (62) or a pointchargc mrthod leading t o A values for the trans-Ii2SnLe structures ( 4 8 ) . The IIt data of Tabl(. Ic' and the Mossbaucr data of Table V show that the association falls across thc. scrics M(+hiF2 > MczSnClz > MczSnBr2 > Me2Sn12 (105). A sharp differcnce bctn-(.en the Ric2SnF2structure and the structures of the other dihalidcs is vcxrificd by an X-ray study of MezSnC12( 4 1 ) . The tin environment 11 as shown to be intc.rmediatc between a tetrahedron and an octahedron, ou irig t o association of adjacmt molecules through Sn(3...Sn bridges. The structure consists of molecular chains, with tin and chlorine atoms h ~ i n gcoplanar in vach of thr chains. Mcthyl groups lie under and ahov(1the. plane. 'I'hc cliain has a zigzag shape. The bond lengths and anglrs arc listed in Tablc VI. The distorted octahrdron of ;1!I(+hiC12 agrcw also with the hiossbaucr quadrupolc splitting found for the compound (Table V) , lying between the tc.trahcdra1 (2.3 mni/scc) and octahrdral (4.1 mm/sec) valurs. Nuclear quadrupole r(wmancc 79Brarid *IBr spectra of solid Me2SnBrz display (119) n cakly split doublcts which suggest that u-electron density is equal in both the broinirics accurately to within the crystallographic splitting. The associatcb assumed on the basis of Mossbauer data (105) is thus not a fivr-coordinate bpccies, because othcrwisc a doublet of considerable splitting, with onc~o f the components corresponding to a bridgc bromine and the othor to a tcmiinal hrominc, would h a w arisen. Thc " I R parameters in Table VII, especially the J(lH-C-llgS n) couplings, imply that thc tiri rtihybridization in thc halides is significant and drpcndcnt on thc halogcm. As in the trimcthyltin halides, the s-electron contcmt in the Sn-C-H sit(. incrcasm across the series I < Br < C1.

C. Methyltin Trihalides Electron diffraction s h o w d (131) that, in the gas phase the trihalides have a slightly distorted tetrahedral structure: McSnC13 (Sn-C, 2.19 A; Sn-Cl, 2.32 A), 1CI(C3nBr3(Sn-Br, 2.45 A), MeSn13 (Sn-I, 2.68 A ) . No

Solid VibriLt,ion

IR

Solid

rc

IT{.

R.

Solution Solution in in henryeloLiquid zene hexane IR 111 II I

Sofid 1 It

Solution Solution in in hencyrlozcne hexane I11 1I I I e

COMPLEXES O F

X

Me

L hle

\I Sn-1,

x

/I

x x

L

x

\I/ sI1 / I \

x x

(Val hle hIe

\I

X

Me

\I Sn--X

Sn--X

x

Me I’

\I/

s x \I

Sn

Sn-Me

1, M e X

\I/

Sn

/ I \

x x

1,

0-b) kle >;

I /

Y Me

1, M e Me

\I

Ale-Sn

Sn-I,

I \ hZe I, (Ic) l l e Me

/I S Ile (IIC) Me 1 l e

I /

\I

I \

/I

X L (Id)

s

s x UId) S XIe

\I Sh-Me

S Me

\I X

X

I, hle hle

Sn-X

/I

I, (IVC)

(II I c )

\I/ Sn-L

Ale-Sn

L

\I/ sn /I\

S S

\I/ Sn L

/I\

L

X

X

?i

\I Sn-L

s11

/I\

S 3le X

I,

(Illd)

/I

S Me (IVd)

X h l e \Ie

\I/

Sn

X-ray methods that the trimethyltin chloride-pyridine complex has a 1:1 composition. It is a molecular adduct of trigonal bipyramidal structure, with three methyls lying on the equatorial plane and with the pyridine and a chlorine atom a t the axial positions. The Sn-CI distance found in the complex exceeds that in the initial halide.

79

Methyltin Halides

Y~,.(811-C)

Compound

(c.nP)

us(Sn-C)

(em-’)

v(Sri-X) (en-’)

These pioneer works were follon-c3d by numerous studies on complexes of trimethyltin halides, the data froni which are discussed in the following. 1. Coin pleaes with Pyridine and LY-Oxopyridines

The preceding results for thc trimcthyltin chloridc-pyridine complex ( 6 8 ) fit well with IR (8,9)and RIossbaucr data (1,101).Thc u,(Sn-C) : v,,(Sn-C) intensities ratio (Tablti X I I ) suggests a planar arrangement of the methyl groups, whereas u(Sn--Cl) suggests that the bond polarity is markrdly higher than in uncomplcxcd R/Ie3SnCl. The chloride, bromidcb, and iotlidc complexes probably possess similar structures sincc all three havc idcntical Sii-C stretching frequency ranges (Table X I I ) . An NQR 81Rrspectrum of MeXSnBr-Py points to a noticcable crystallographic nonc~pival(mcein the crystal cell and t o a higher Sn-Br bond ionicity comparc.d with McXSnBr. The PMR spectra of McsSnX I’y complexes dissolved in CHC13 implied that the compounds dissociated completely (145) in solution. Wc showed (116), however, in a study of t h r concentration and tcmperaturc dependence of J (lH-C-llgSn) in n4esSnX-pyridine-CHzClz mixtures, that the

-

80

V. S. PETROSYAN, N. S. YASHINA, AND 0. A. REUTOV

equilibrium MeqSnX

+ Py z====MeaSnX-Py

depends on the pyridine-to-CHzCl2 ratio and on the sample temperature, and is totally shifted toward the complex a t higher pyridine concentrations and lower temperatures. The limiting J(lH-C--”gSn) values observed (Table X I I I ) in the system reflect the s-clectron density content in the Sn-C-H site and allow one to deduce a series for the electron-donor ability of coordinating solvcnts. Pyridine occurs between weak and strong electron donors. Complexes of the type

-

(X = C1, Br; X’ = H, CH3, OCH3, C1, NOz) were studied both in the solid state and in solution (73, 7 6 ) . Their Y ( N 0) frequencies are lower than in the free ligands, implying coordination at the oxygen. The Y (Sn-X) frequencies (Table XII) show that complex formation raises the bond polarity.

J (llSSn-C--’H) L in ILleaSnX-La MeCN THF Acetone DMTAA l’yridine DMAA T M 15U I1M F DMSO kI M 1’T

(Hz)

MesSnCl

MeaSnRr

66.7 64.5 66.1 65.7 68.0 68.8 69.7 70.0 70.1 71.8

65.1 -

66.0 67.6 68.5 69.6 69.6 71.3

Abbreviations: THF, tetrahydrofuran; DlIF, N , N-dimethylforinamide; DMSO, dirnethylsulfoxide; HMPT, hrxametapol; DMTAA, diniethylthioacetarnide; DMAA, dirnethylacetamide; TMED, tetramethylethylenediamine.

Methyltin Halides

81

Spectrophotomctric data f o r the stability constants of complexes of Mc3SnCl with substituted pyridinc. N-oxides in acetonitrile show (76) that these constants correlate linearly with the n parameters of the y substituent s. No V,(Sn-C) was found in McSnBr-PyO. It was, therefore, interpreted (38) in terms of planarity of the C3Sn site, namely, structure I a (Table X I ) . 2. Complexes with Phosphine or Arsine Oxides Conductivities of the complexes in absolute ethanol were shown (38) to be very low when compared ~ i t htcbtramethylammonium bromidc as a reference. This finding was intorprc+d to indicate that molecular adducts exist in solution. The absence of v,(Sn-C) from the I R spectra and the J(lH-C-llSSn) magnitude (ca. 70 Hz) were interpreted in terms of planarity of thc C3Sn site and a noticeable rehybridization of the tin orbitals.

3. Complexes with Sulfoxicles Trimcthyltin chloride and bromide form DMSO complexes of 1:1 composition, melting a t 49°C ( 7 3 ) and 63°C (112) respectively. The dccrease in v(S=O) ( A v is 95 em-' for R4v3SnC1.DMS0, and 45 cm-' for MesSnBr. BzzSO) may show (38,7 7 ) that the coordination is via the oxygen, whereas the absence of v,(Sn-C) from the complexes of Me3SnBr (Table X I I ) suggests a structure of type Ia. As the NQR *lBr frequency is s1iiftc.d on going from Me3SnBr to its complex with DMSO ( I I g ) , a strong charge transfer and an increase in the Sn-Br bond polarity may bc assumed. Studies of the concentration and tctmpcrature dcpendences of mixtures of DMSO and Me3SnX ( X = ('1, Rr) in CH2Clz gave limiting values for the J ('H-C-llgSn) constants (Table X I I I ) and the MeSSnX. DMSO stability constants (Table XIV). Thcsc~results suggcst that the values of 62.0 and 63.0 Hz, obtained csrlicr for CHC13solutions of Me3SnBr.DMSO (38) and Mc3SnBr-BzzS0 ( 7 7 ) , rc,spcctivcly, are nothing but the equilibrium values and do riot reflect structural specificity of the complexes formed. The J ('H-C-llgSn) valucis found for Me3SnX solutions in DMSO ( 4 ) exceed our va1uc.s (Table X I I I ) by 1.8 and 2.4 Hz, but the couplings reported ( 4 ) for Rlr3SnX solutions in CDC1, or for other methyltin halides are also 1.5 to 3.8 Hz grclatcr than the rrspective couplings reported elscwherc (66, 87, 116, 1 4 4 ) .

V. S. PETROSYAN, N. S. YASHINA, AND 0. A. REUTOV

82

TABLE XIV STABILITY CONSTANTS FOR Me3SnX.L COMPLEXES

Complexa

Solvent

LZethod

t("C)

HNDMR 113-{119sn) N M l l (ll9Sn) Calorimetry

+37 +20 +26 26 -30

acetone Acetone Isooctane CCla CH,Cl2

PMR

Acetone CFI,Cl,

rim

CCl, MeCN MeCN

Me3SnBr MeCY

Me3SnC1.acetone

Me&&

.acetone

PhfR

NMR(lIgS11)

+

+20

KP (molcs/ liter)

1.1 7.0 0.9 0.4 0.8

AH"

(kcal/ mole) 4.2 -

-6.0 -5.7 -

-30

3.0 0.7

1'MR NMR(1LgSn) HNDMR 'H- {119Sn]

+26 20 +20

+

0.5 2.7 2.1

MeCN

NRfR(llgSn)

120

3.5

-

MesSnCl -DMTAA

CCl4 CH,C12

PRlR PMR

+26 - 70

0.5 1.5

-5.9 -

MesSnBr - Py

cc1,

IR PMR

+27 - 30

1.8 36.0

-6.5

CII & I 2 CHZC12

PhlR

- 30

28.0

-

Me3SnCl.DMAA

CCl, cC14

IR PhlR

$27 +34

3.7 3.1

-7.9 -7.9

Me3SnC1-DMS0

cC14 CCl4 CCl* CH zC12

Calorimetry IR PMR PMR

+26 $27 34 +28

-8.2

+

9.1 8.3 6.2 2.8

MeaSnBr-DMSO

CH 2C12

PMR

+28

3.6

-

MeaSnCl.HMPT

Isooctane CHzCI,

PMR PMlt

+26 +28

384 102

MerSnBr-HMPT

CHzC12

I'M R

+28

99

MerSnCl-dioxan

Dioxan

HNDMR l€l-(IlsSn)

+20

MenSnCl. MeCN

-

MesSnCl Py

2.1

-

-48 -

4.1

-

-

-

-10.1 -

5.1

MesSnCl .DMF

CHsC12

PMR

-30

2.5

-

Me3SnBr .DMF

CHZC12

PMR

-30

3.1

-

Abbreviations: DMSO, dimethylsulfoxide; HMPT, hexametapol; DMF, N , N dimethylformamide; DMAA, dimethylacetamide; DMTAA, dimethylthioacetamide; HNDMR, heteronuclear double magnetic resonance.

83

Methyltin Halides

The data obtained, when :malyxcd as a whole, show that Mc3SnBrDMSO may be the strongest cortiplcx among molecular coniplexcs of trimethyltin halides with monodrntat r ligands.

4. Other Co,nplP.ces The molecular complexes of nirthylt i n halides with various electrondonating solvents h a w hccn givm a grmt deal of attention by a number of workers. Drago et al. (96, 14-16) eniplo~rd calorimetry arid I11 and PMR methods to mrasurr the stability const:mts and licats of formation of McBnCI complexcs with dinicthgl sulfoxidc, diintxthylacctamidc, DMF, acetonitrile, pgridine, and H M P T (see T:hlc Xl\'). Okavara and co-workers ( 9 1 ) ,\vho studied solvent effects on the PMR spectra, and on the ,,(Sn--C) :Vcls(Sn--also operate, on the whole, in solution. A collation o f NQR ( l l b ) ,Mijssbauer (113), and PMR (116) data suggcxsts t h a t in thc MezSnClzcomplrxcs the s-electron content in the Sn--C-H site, arid thc Sn-CI bond polarity arc greater than in the frec halides. As for the stcrcochemistriw of thc 1:2 complexes, thcy are mgular or slightly distortctd octahedra of thc I I I a or I I I b types, depending on the solvent applied.

C. Complexes o f Methyltin Trihalides In 1963, Beattie and McQuillan (8) studic.d th(. I R spectra of MeSnCI3 complexes with pyridinc, bipyridyl, and o-pheiianthrolinc, the compositions being 1 :2, I :1, and 1:I, rcspcctively. They obtained a rather complicated pattern of Sn-Cl vibrations and could make no structural assignment. Later it was shown (31, 81, 32) that I R spectroscopy is hardly applicable t o the study of complexes of this type. I n 1967, Wardell (151) studied the UV spectra of diethyl ether solutions of MeSnCl3 complexes with nitroanilines and measured the equilibrium constants. The data were interpreted in terms of 1:I complexes containing a five-coordinate tin (with 2 , s - or 2,4-diaminonitrobcnzcne) or a six-coordinate tin (with 3,4-diaminoriitrobcmzencx). In 1974, Barbieri and co-workers (107) synthesized the 1:1 complex of McSnC13 with N ,IT-cthylencbis (salicylidcneiminato) nickel and studied its Mossbauer, I R, and electronic spectra in the solid state, and its elec-

Methyltin Halides

91

tronic and PMR spectra in solutioii. Tlic data led to a structure of the Va or Vc type (Tablr XI). In 1973, we described riglit c~oniplcxc~s(111) of the type McSnX3.2L ( X = C1, Br; L = Py, DMF, I)hISO, IIRIPT) as 1 ~ 1 as 1 hleSnCl3-dioxan and 2McSnBr3.dioxan. Wr stutlicd (1I S ) the RIossbaucr spectra of these coniplexc~sand of frozcn solutioiih of RId3riX3 in DEE, DIME, T H F , acetone, arid TMED. The isomw shifts \\('re found to be very scnsitive to complex formation, uiilikr the. casrs of hlczSriXzarid AIr3SriX, and led to a scries for t h r clcctron-donor ability of the solvrnts ton ard RilcSnX3. The quadrupole splitting pat t c m found for lLlcSnXJ cornplexes is also very diff(wiit. Urilike Mc2SnX2,in \\ hich thc splitting increascs with electrondonor activity of the ligands, iii hl&i& it is greater for thc complexes with weaker donors. Thcsc. fact 5 ngrc'c 1% ith the present-day quadrupole splitting thcories (see Scctioii 11, 1)) and with thc NQEt spcctra of the complvxw ( 1 1 2 ) . The pattc.rii of V ( xprctrdt o give thc alkyl-substituted clustrr , RCCoa(C0)9. Howevvr, uh(m this reaction was carried out in dichloromethanc solution with tc,tramrthyltin, the product obtained (in 26% yield) was the acetyl derivat ivr, ralddiyd(1 in 74% yidd. Wr suggest that aluminum chloridc effects rclcasc. of t he aldchgdc from thc intcirmcdiat(* in thc first step in the rrduction of thc1 acylium ion, (OC)gCo3CCHOSiEt3+,via Cl- attack at silicon. In thc1 absence of aluminum chloride, furthcr rcductiori t o givc the methyl dcrivativr is possible. Thc conversion of chlororiicthylidynetricobalt iionacarbonyl t o the acylium ion by the actiori of aluminum chloride is a remarkable proccss. The rcaction docs not require rxt crnal carbon monoxide; it proceeds perfcctly satisfactorily undcr a nitrogcm :Ltniosphcrc. Tho CO function at the apical carbon atom of the products obtained thus was derived from carbon monoxide ligarids on cobalt in C1C'Co3(C0)9.This transfer of CO t o the apical carbon atom is vcry c+Kcicnt. T h r yic.lds given in Tahlc VIII are based on the amount of e l C C ~ ( c o ) gchargc>d.If one rccalculates thc yield of (OC)9C03CCOzCIIj in t h r first critry in Tablc VIII based on the available (OC)gC'03CCO+from t h r C'l('Co.~(CO) used [assuming the destruction of an amount of C:1Cdly incomplete and very possibly subject t o change. We suggest that initially a (OC)gCoaCC1.A1C13complex is formed in which substantial polarization of the C-C1 bond has occurred and which is capablc of Fricdrl-Crafts substitution on aromatic systems. Subsequent complexation of a second molccule of AlC'l, st>a carbon monoxide ligand [a known procms in nictal carbonyl chemistry (35, SS)] provides the activation for CO migration from cobalt to the elcctron-drficient apical carbon atom. This is not unrcasonablc sincc. it is known that in binuclear metal carbonyls t he bridging carbonyl ligands are stronger Lewis basic sitm than arc the trrmirial carbon monoxidcs (35, 3 6 ) . I n fact, aluminum alkyls h a w bccn rcxportc>dto promote a terminal t o bridging carbon monoxide ligarid shift in a binuclear ruthcnium complex ( 3 7 ) . Such a cobalt-tocarbon CO transft.r in our system, occurring eithw intra- or intrrmolecularly, would leave coordinativcly unsaturated cobalt atoms in cluster acylium ions which would require clfficient C'O transfer from other molecules in order t o obtain the. (Oc')gCo,JXO+ species in high yield. We are still working in this area in the hop(. of achieving a better understanding of this process. We not(. that systems that give the cluster acylium ion more rapidly are available. Thus, BrCCo3(CO) is converted to the yellow-brown acyliuni ion solution in dichloromethanc by an excess of aluminum chloride at a faster rat(. than is ClCX'03(CO)9.This transformation of ClCCo3(C;O)9 can be accelerated by using a larger excess of aluminum chloride (10 molar equivalents of AlC13 rather than 2 or 3) or by carrying out the ClCCo3(CO)9/3A1C13reaction in thc presence of iodomethane. I n the latter case, it appears that thc rate accclcration is duc. to the formation of the more reactive aluminum iodide rather than of ICCo3(CO) 9. The preparative utility of these systems has not yet bccn assessed in detail, but in all cases the yields of (OC)gCo3C"COzRon treatment of the solutions with methanol or ethanol were good. Robinson arid co-workers have claimed, without providing any experimental details, tha t boron trifluoride also converts BrCCoS(CO)g to the cluster acylium ion under a carbon monoxide atmosphere (38). They also

Alkylidynetricobalt Nonacarbonyl Complexes

119

report that the ClCCog(CO)g/AlClg reaction can be carried out in situ in the presence of the iiucleophilr ( o . g . , 1120, EtOH, PhOH) ( 3 8 ) .

IV

HIGHLY STABLE NONACARBONYL TRICOBALTCARBON-SUBSTITUTED CARBONIUM IONS Our discovery of the easily formed, very stable, and preparatively very useful (0C)gCo3CCO+ion snggclsted t o us that carbonium ions of type (OC)9C03CCItR’+niight provide another fruitful area of study. The electronic effects of the (OC)gCo,C clustcr were riot at all clear and one could entertain the possibility thatj this organometallic substituent might stabilize an adjacent positive. charge. A suitable carboiiiuni ion precursor was necdcd in ordcr to carry out such an investigation and at that time no such (OC)9C03Cderivativc w:ts available in useful amounts. Alcohols of type (OC) &oSCC (OH)RR’, as mcntioncd already, were available only in yields of 5% or less, and halidcs of typc (OC)gCo&C (X)RR’ were (and still are) unknown. The first problem, then, was to dcvthlop a more useful, high-yield alcohol synthesis. The reduction of a ketonc1 to a11 alcohol is a well-known organic reaction, and since ketones of typc (OC)9C03CC(0)R and the aldrhyde (OC) gCos CCHO could be preparcd in good yield, wc chose to cxaminc this route TABLE I X REDUC,TION OF RC(O)CCO~(CO)~ TO RCH2CCoa(C0)9BY Et3SiH/CF3C02Ha

R in I ~ C ( O ) C C O ~ ( C O )Et3SiH ~ (mmole)

(1

(mmole)

CFPCOzH (mmole)

RCHzCCo3(CO)g (5% yield)

7 7 7 7 8 8 5 5 5.2 7

6 6 6 6 8 9 6 5 6 6

C Z H S C C O ~ ( C(90) O)~ n-C3H;CCo3(CO)g (92) n-C4HyCCo3(CO)g (87) n-CaHiiCCos(C0)~(80) ~ - C ; I - I ~ , C C O ~ ( C(85) O)~ cyclo-cG€111CI-12cco3(co)9(75) ( C H ~ ) & H C H ~ C C O ~ ( (81) CO)~ C~H,CHZCCO~(CO)~ (82) ~ - C I I ~ C J I , C H & C O ~ ( C(78) O)~ p-BrCGHICH2CCo3(CO)~ (67)

From Seyferth et al. (39).

120

DIETMAR SEYFERTH

(39).The initial results were not encouraging. Attempted conversion of formylmethylidynetricobalt nonacarbonyl to the primary alcohol was unsuccessful. Treatment of this aldehyde with sodium borohydride in T H F a t reflux gave a mixture of CH3CCo3(CO)Yand H C C O ~ ( C O whereas )~, reaction with lithium aluminum hydride or sodium borohydride in benzene gave only HCCo3(C0)9.In view of the stability of some of these cluster complexes to strong acids, we turned our attention to a n acidic reducing system, tricthylsilane/trifluoroacetic acid, that had been developed by Russian workers [see Kursanov et al. (40)for a recent review]. This rcduction proceeds by initial protonation of the carbonyl compound, followed by reduction, via hydride transfer from the silicon hydride, of the protonated species. If one of the carbonyl substituents can stabilize an adjacent positive charge (e.g., an aryl group), the reduction proceeds past the alcohol stage to give a hydrocarbon:

7' +

R-C-OH,

R>CH, R

A

I

H

A number of clustrr-substituted ketones and the aldehyde all reacted with the Et3SiH/CF3C02Hto give the alkyl derivatives rather than the expected alcohols: Et&lH/CFCOzH

(OC)YCOICC (0)It

+

(OC) yCojCCH2R

(27)

The examples studicxd are given in Table IX. The product yields were excellent (75-927,) and, in fact, this reaction is the best available procedure for the preparation of such alkyl derivatives. The R in Eq. (27) may be primary or srcondary alkyl or aryl; when R becomes more bulky (e.g., Me&, (OC)gCo3C) the reduction fails. Although this reaction did not rcsult in the alcohols we rcquired for our furthrr studies, the results were of considrrable interest to us since they provided good indication that the (O C ) y C ~ 3 C - ~ ~ b s t i t u carbonium ted ions would be rather stable species. As mentioned, the complete reduction observed occurs only when a t least one of the substituents on the ketone carbonyl function is capable of stabilizing an adjacent positive charge. I n those cluster complexes in Table IX where R = H or alkyl, it must be the (OC)yCo3C substituent that is providing such stabilization. However, in order to pursue this question, we still required (OC)9Co3CC (OH)RR' compounds in useful amounts. Hydrosilylation of ketones and aldehydes converts these to silyl ethers whose hydrolysis gives alcohols. Phenylsilanes were found to add to benzo-

Alkylidynetricobalt Nonacarbonyl Complexes

121

phenone, but the reaction conditions (reaction temperatures of 220’-270°C) were not attractive ( 4 1 ) .Catalyzed hydrosilylations proceeded under much milder conditions, e.g., zinc chloride (&), HzPtCl6.6H2O ( 4 3 ) , (Ph3P)3RhCl (44). The hydrosilylntion of our cluster-substituted ketoncs was found t o occur under surprisingly mild conditions. It was sufficient to heat equimolar quantities of tric*thylsilanc and the ( OC)gCo3CC(0)R compounds in benzene at reflux, under a carbon monoxide atmosphcre for about 8 hours. The crude silyl cthw was converted to the alcohol by solution in concentrated sulfuric acid and subsequmt hydrolysis by pouring into an ice-water mixture: It

0

/I

I

+

(OC)~CO~C-CI? 1 :t 8 1 1 1

A

(OC)yC03CCHOStlCt3

(28)

It

I(

I

+/

(OC)Y(’O,C(‘I 1oSIl’:t3 ~ ( 0 c ) y C O 3 c c

(29)

\ I1 It

It

I

+/

(OC)~C~~CCH--OH

(OC),C~i,CC’

(30)

\ [I

It was found important t o carry out the hydrosilylation under a n atmosphere of carbon monoxidc in o r d w to obtain the good yields given in Table X for a number of these reactions. Thc quc.stion remains why these hydrosilylation reactions proceed so rcmlily under mild conditions in the absence of a catalyst. One possibility vhich we considered was that the cluster ketone provided its omii catalyst by way of minor dccomposition t o give mononuclear cobalt carbonyl intermc.diatcs which could bc thc actual catalytic species. However, RCCo,(CO), were not found t o catalyze the hydrosilylation of cyclohcxanone. A second possibility is that the C=O bond in these (OC)gCo3CC (0)R compounds is exceptionally reactive. Table X I lists t h e ketonic C=O stwtching frequencies of some of the (0C)9c103CC (0)R compounds that n-c’ have prepared. These are found to be in the range of 1560 t o 1645 cm-l, xt~much low-cr frequency than in dialkyl ketoncs (1725-1705 cm-l) or aryl kctoncs (1660-1700 cm-l), and this implies greater carbon-oxygen bond polarization :

(oc)gC(i~;+y-

I

_t

I

((~

I

-tTT’l’Fe-

(EtCO)2

0

(OC)~C(~~CC((’H,)L’~’~~:B(34)

(‘Ha

The carbonium ion salt obtained in this manncr rcactcd with methanol t o give the methyl ether, (OC)&o&C (CHI)20CH3,in 86% yield and with aniline to produce (OC)9C03CC(CHI)zNHC61-Isin 49y0 yield. At,tempted purification of the methyl cthcr by chromatography on pH4 silicic acid re-

125

Alkylidynetricobalt Nonacarbonyl Complexes

sultcd in formation of the alcohol, (OC)9C03CC(CH,) LIH, which is a furthcr indication of the easy accessibility of the tertiary carbonium ion. Reduction of isopropenyl-substituted cluster, by way of the carbonium ion, to ( 0 C )&oJCCH (CH,) could be effected with zinc amalgam in trifluoroacctic or concentrated hydrochloric acid. Similar protonation of (OC)&'oJCCH=CH2 ith HPV6/ ( EtCO) *O gave the (OC)&'03CHCH3+PF6- salt which we had p r e p a r d previously from the alcohol. In the case of ( 0seither a t the apical carbon atom, (OC)9C03CZ, or at the carbon atom a t o it, (OC‘)gCo3CCZILR’.Any functional group attached to the apical carbon atom is very sterically hindered and as a result will have rather limited chemical reactivity. Functional groups at the a-carbon atom mill be less hindrred, but sonic may be easily lost due t o the case with which a-carbonium ions are formed. Clearly, an investigation of the organo-functional chemistry at carbon atoms more remote from the cluster (OC)gCo3C-Y-CZRlt’ ( Y is a difunctional organic unit) should be of intersst and, perhaps, more useful in terms of possible applications. As has becn pointed out, very littlc is known concerning reaction mechanisms in the area of organocobalt cluster chemistry. The main reactions by which these complexes a m formed are only poorly understood, as are the reactions by which mcathylidyne- and halomethylidynetricobalt nonacarbonyls are alkylated and arylated. We can only guess about the mechanism of the remarkable aluminum chloride-induced conversion of (OC)gCo3CC1 t o (OC)&~CO+A1Cl4-,and the mechanisms of the decomposition and the oxidation of these clusters are not known. A b&tcr knowledge of mechanisms in organocobalt cluster chemistry most certainly would facilitate the development of the chemistry of these complexvs. Wc wers first drawn into studies on alkylidynetricobalt nonacarbony1 complexes because in thvse one is dealing with a carbon atom in a most unusual environment. Wc felt t ha t this novel class of complexes would show some rather interesting organic and organoinetallic chemistry and we

Alkylidynetricobolt Nonacorbonyl Complexes

141

have not been disappointed in this cxpcctation. We believe that more interesting chemistry of the RCCo3(CO)9 and related cluster complexes remains to be uncovered and we are continuing our efforts in this area.

ACKNOWLEDGMENTS

My pre- and postdoctoral co-workers who carried out the research reviewed here, are listed on the title page of this rhapter. I am indebted and grateful to them for their dedicated, enthusiastic, and skillful efforts and for their important contributions of original ideas that resulted in rapid development of organo-functional organocobalt cluster chemistry. My co-workers and I are grateful to the National Science Foundation for generous support of this work (NSF Grant G P 31429X).

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Alkylidynetricobalt Nonacarbonyl Complexes

143

53. 1). Seyferth, G. H. Williams, arid I>. 1). Traficante, J . Amer. Chem. Soc. 96, 604 (1974). 54. G. A. Olah, E. B. Baker, J. C. ICvans, W. S. Tolgyesi, J. S. Rfclntyre, and I. J. Bastien, J . Amer. Chem. Soc. 86, 1360 (1964). 55. G. A. Olah, Science 168, 1298 (1970). -56. J. J. Dannenberg, RI. K. Levenlxrg, :tiid J. H. Richards, Tetrahedron 29, 1575 (1973), and earlier references cited therein. 57. G. IS. Williams, D.1) . Traficante, arid 1). Seyferth, J . Organometal. Chem. 60, C53 (1973). 58. .J. B. Stothers, “Carbon-13 NAZI?. Spectroscopy.” Academic Press, New York, 1972. 59. 1). Seyferth, C. H. I,;sctibsrh, nnd 11. 0. Scstle, b. Orgcc.nonieln/. (‘hciii. 97, Cll (1975). 60. See Stothers (58), pp. 217-23s. 61. C. G. Kreiter and V. Forniacrk, Aicgew. Cliem. 84, 155 (1972). 62. F. H. Kijhler, 1%.J. Kalder, and 15. 0. Fischer, J . Organometal. Chem. 85, C19 (1975). 63. 13. 0. Fischer, G. Kreis, C. (4. Kreiter, J. Rliiller, G. Huttner, and H. Lorenz, Angelo. Chem. 85, 018 (1973). 64. T. G. Traylor, 11. J. Uerwin, .J. Jerkunicx, and hl. 1,. Hall, Pure Appl. Chern. 30, 599 (1972). 65. A. J. Deeming, S. ITasso, M. Underhill, A. J. Canty, 13. F. G. Johnson, W. G. Jackson, J. Lewis, and T . W. hlatheson, .I. Chem. Soc., Chem. Commun. 807 (1974). 66. B. E. Rlann, Advan. Organomelnl. (‘hem. 12, 135 (1974). 67. E. 0. Fischer, Angela. Chem. 86, (is1 (1974). 68. I. U. Khand, G. R. Knox, P. L. Pauson, and W. E. Watts, J . Organometal. Chem. 73, 383 (1974). 69. A. T. Wehman, unpublished work. 70. K. Tominaga, N. Yamagami, and II. Wakamatsu, Tetrahedron Lett. 2217 (1970). 71. G. Albanesi and E. Gavezotti, Atti Accad. Naz. Lincei, Rend., CI. Sci. Fis. Mat. Nut. 41, 497 (1966). 72. R. S. Dickson and G. R. Tailby, Amt. J . Chem. 23, 229 (1970). 73. C. Hoogzand and W. Hubel, in “Organic Syntheses via Metal Carbonyls” (I. Wender and P. Pino, eds.), Vol. I , pp. 343-371. Wiley (Interscience), New York, 1968. 74. C. H. Bamford, C. G. Eastmond, and W. It. Maltman, Trans. Faraday Soc. 60, 1432 (1964). 75. G. PAlyi, F. Baumgartner, and I. Czajlik, J . Organometal. Chern. 49, C85 (1973). 76. P. A. Elder and B. FI. Robinson, J . Organometal. Chem. 36, C45 (1972). 77. T. W. hlatheson and B. 1%.Robinson, J . Chem. Sac. A 1457 (1971). 78. V. G. Albano, P. Chini, S. RIartinengo, M. Sansoni, and I).Strumbolo, J . Chem. Soc., Chem. Commun. 299 (1974). 79. G. P&lyi, F. Piacenti, and L. Mark6, Itiorg. Chim. Acta Rev. 4, 109 (1970). 80. B. R. Penfold and 13. 13. Robinson, Accounts Chem. Res. 6, 73 (1973). 81. T. I. Voyevodskaya, I. hl. Pribytkova, and Yu. ,4.Ustynyuk, J . Orgariometal. Chem. 37, 187 (1972). 82. A. J . Deeming and M. Underhill, J . C‘hern. Soc., Chem. Commun. 277 (1973); J . Chem. Soc., Dalton Trans. 1415 (1974). 83. A . J . Canty, B. F. G. Johnson, J. Lewis, and J. R. Norton, J . Chem. Soc., Chem. Commun. 1331 (1972). 84. W. S. Sly, J . Amer. Chem. Soc. 81, 18 (1959).

144

DIETMAR SEYFERTH

85. W. Hubel, in “Organic Syntheses via Metal Carbonyls” (I. Wender and P. Pino, eds.), Vol. 1, pp. 273-342. Wiley (Interscience), New York, 1968. 86. D. Seyferth, M.0. Nestle, and A . T. IVehman, J . Amer. Chem. Soc. 97, 7417 (1975). 87. K. M. Nicholas and R. Pettit,, J. OrganometaL Chem. 448, C21 (1972). 88. (a) Y. Iwashita, F. Tamura, and A. Nakamura, Inorg. Chem. 8, 1179 (1969); (b) Y. Iwashita, ibid. 9, 1178 (1970); (c) Y. Iwashita, A. Ishikawa, and M. Kainosho, Spectrochim. Acta, Part A 27, 271 (1971). 89. L. J. Todd and J. R. Wilkinson, J . Organometal. Chem. 80, C31 (1974).

Ten Years of Metallocarboranes KENNETH P. CALLAHAN Metcalf Research laboratory Departrnenf of Chemistry Brawn Universify Providence, Rhode Island

M. FREDERICK HAWTHORNE Departrnenf o f Chernisfry* Univerrify of California Lor Angeles, Californio

I. Introduction . 11. h3etallocarboranes: Syntiiet,ic h1t:tliods. . A. Preparation from nitlo-Cnrt)orane Anions . B. Preparation by l’olyhetlral l+:xpnrision C. Preparation by I’olyhedrd Contraction . 1 ) . Preparation h y l’olyhedrnl Sul,rog:Lt,ion . E. Preparation by Thermal llct.al Trmsfer . . 111. Twelve-Vertex ~\letallocarborltii(,s -4.llonometallic Complexes with Ttlcntictd Carborane Ligands B. Monoinetallic Complexes \\it11 1)iffcrent Carborane Iigands C. Mixed-Ligand Complexes D. Bimetallic Complexes IV. Thirteen-Vertex ~.letallocarl)or:Lllrs V. Fourteen-Vertex L~etsllocarhoraties VI. Eleven-Vertex lletallocsrhoratIcs A. Llonometallic Complexes R. Bimetallic Complexes VII. Ten-Vertex Met:illocarboraites VIII. Nine-Vertex nZetsil1ocarhoranc.s . IX. Oxidative Addition to B-H Ilonds X. Metallocarboranes in Homogcricvus ( h t d y s i s . References

.

.

. . . . .

. . .

14.5 150 150 151 152 153 153 155 155 161 163 166 167 171 171 171 173 175 178 180 182 183

I INTRODUCTION I n the early 1960s, the chemistry of the boron hydrides had been extended not only to include a remarkable number of new parent boranes having diverse structures, but, the polyhedral BloHlo2-and B12H12~-ions * Contribution

So. 3453.. 145

146

KENNETH P. CALLAHAN AND M. FREDERICK HAWTHORNE

and two isomeric CzBloHIzcarborancs as well.' Both BI0Hlo2-and 1,2- (or ortho-) C~BloHlz were obtained from decaborane (14), BI0Hl4,whereas the 1,7- (or mela-) isomer of CzBloHlz was prepared by thermal rearrangemcnt of the 1,2-isomer at 400"to 600C" (Fig. 1) (31,39, 57, 65). As later work would show (47, 7 7 ) ,an entire serics of polyhedral B,,Hn2and isoelectronic CzB,-zH, carboranes were synthetically accessible for n = 6 through 12. The generally decreased chemical reactivity of these polyhedral species over that of thc boron hydridcs suggested that they

1.12 -C2BDH12

O B H O C H OH FIG.1 . Structures and numbering of the three isomeric irosahedral carboranes, and the degradation of 1,2-C2Bl0H12 to 7 , 8-CzBgH1z-. The bridging hydrogen is shown in one of the two equivalent bridging positions. Numbering of polyhedral positions follows the latest IUPAC-approved scheme as published in Pure A p p l . Chenr., 30, 683 (1972). The following definitions apply to the various descriptions of polyhedra used in the text: closo refers to a borane, carborane, or metallocarborarie polyhedron that has a closed, fully triangulated geometry ; nido refers to a polyhedron the geometry of which can be described as a closo polyhedron from which one vertex (frequently one of high roordination number) has been removed; commo refers to metallocarborane complexes in which one vertex, generally a transition metal, is shared between t,wo polyhedra. Coordination numbers of polyhedral vertices are calculated from the number of nearest-neighbor atoms and do not imply the presence of discrete 2-electron chemical bonds between these atoms.

Ten Years of Metallocarboranes

147

were probably stabilized by t hrcic-dirnciisional electron delocalization and hence were representative “aromaticJ’ members of the boron hydridc family. Indeed, Lipscomb (71, ’72) arid co-workers have carried out a series of molecular orbital treat m m t s of selected polyhedral ions and carboranes which adequately espleins t hv bonding present in these species. I n all cases, a polyhedron having 11 number of vertices requires n 1 clectroii pairs d(1localizcd in ail cqual number of extended bonding orbitals to achieve polyhedral cage bonding. Although the icosahcdral 1 ,2- and 1 ,7-(?2B1&12 carborancls n-erc’ found to be quite stablc at high teinpcmturcs and toward most common reagents (31, 41, 5 7 ) , strong base in thc~prcscmcb of a protonic solvciit caused a specific degradation reaction (97) which cleanly removed a BH vertex from the icosahedron to produccx thv corresponding (’ZBSH12- ion (Fig. 1) :

+

1,2-C?BloHIr

+ 110- + 21tOH ----t7,8-C2139Hl?- + I 3 ( O I L ) ~+ H ?

(1)

It was correctly assumcd and latcr drtclrmined unequivocally (64) that in each of these ttto reactions t h r BH v d e x that mas removed was always one of the two equivalent vclrtices found as the nearest neighbors of the two equivalent C“H vertices. T h c fact that the carbon atoms in polj hcdral surfaces of C2B,-2Hn carboranc1.sare electronic counterparts of boron atoms in the corresponding isoelectronic B,,H,*- polyhedral ions (62) requires these carbon atoms t o resemhlv C+, a species present in carbonium ions. Consequently, the B H vertices which are nearest ncighbois of two such carbon atoms will be activatcd for nucleophilic attack by base through the advent of a strong inductivcl effcct. The assumption that the isomeric 7 ,8- and 7 ,9-C2B9Hle-ionsstructurally resembled eleven-particle fragments of an icosahedron, coupled with their known empirical formulas, suggclsted that the twelfth hydrogen atom was prescnt in a three-center B-H-B bridge bond located in the periphery of the open five-membered fact. (see Fig. 1). Simplified molecular orbital considerations suggested t ha t the rcinoval of this bridge hydrogen atom as a proton would generate a 7,8- or 7 J9-C2BgH112ion having G delocalized electrons in the open five-mcbmbcred face. The orbitals in which these G electrons were distributed could, t o a first approximation, be considered as sp3-like and pointed toward the unoccupied vertex of the original carborane icosahedron. This disposition of dc1ocalizc.d electrons ( Fig. 2 ) should closely resemble the ubiquitous cyclopcntadicnide ion, a constituent in a large number of organomctallic compounds. As a result of this observation, the twelfth hydrogen atom in the 7 ,8-C2BgH12-ion was successfully removed by treatment n i t h strong bases such as sodium hydride (55). Reaction of the resulting 7 ,S-C’2ByHn2-ion with iron(I1) chloride produced

148

KENNETH P. CALLAHAN AND M. FREDERICK HAWTHORNE -2

FIG.2 . Schematic representation of the spa-like bonding orbitals in (a) 7 , 8-C2B9111,*and (h) 7,9-C2B91IIlZ-.

the first metallocarborarie in a manner analogous to the synthesis of ferrocene (56) :

NaH

7,8-CsB91T12-

Fe(I1) . ,

7,8-C2T39H112-

--H2

i l ,2-C2B~Hll)2Fe(II)2(2)

By using the C5H5-ligand in conjunction with the 7 , 8-C2&,H112- ligand, the mixed-ligand species CjHjFc(1,2-CzBgHll)- was obtained ( 4 4 ) . Oneelectron oxidation of this anion formed the unchargcd C',H5Fc ( 1,2-C'&&1) species containing formal iron(III), an analog of the ferriciriium ion. Subsequent single-crystal X-ray diffraction studies on this derivative (104) substantiatcd the bclicf that the iron atom was indeed occupying the twelfth vertex of an icosahedron. I n this manner metallocarborane chemistry was brought into bcing and thc first wedding of transition metals with carborancs into hybrid cluster compounds was accomplished. Following thew events, the chemistry of the metallocarboranes was rapidly expanded through the use of many of the transition metals and certain main group mrtals such as Be (82, 83), Al, and Ga ( 1 0 2 ) .A boron vertex may be reinserted as well ( 5 3 ) . In addition, carborane ligands other than the original 7,s- and 7,9-CzB9Hll2- ions were attached to metals, often in conjunction with a wide varic.ty of truly organic ligands (47'). More recent developments have led to the synthesis of metallocarboranes that contain more than a single transition metal in a polyhedral surface, and these transition metals nccd not be identical ( 3 ) . With very few exceptions, the gross geometries of polyhedral metallocarborancs may be correlated with the total number of vrrticrs present in the polyhedron. Metal, carbon, boron, or other nonhydrogen elements are counted as vertices and thc.ir total number equated with the value of n in R,Hn2- ions. In nearly every case the approximate geometry of t h r poly-

Ten Years of Metallocarboranes

149

hedral metallocarborane will coincide with that of the corresponding B,H,2- ion, when known ( 4 ) . Table I lists these geometries as a function of n. The thermal polyhedral rearrangement of 1 ,2- to 1 ,7-CzB,,HI2 already mentioned (see Fig. 1) is but one rxample of a reaction commonly observed throughout the polyhedral carborane and metallocarborane families. At the present timc, the mechanism of tlicse interesting rearrangements rcmains obscure, although several schemes have been advanced to explain experimental results ( 4 6 ) .Polyhedral rearrangements in metallocarboranes occasionally occur with great facility in comparison to the energy required to effect the 1,2- to 1,7-C2BloHlzisomerixation, and are important aspects of the physical and chemical propcrtics of metallocarboranes. In this review, we treat in dcpth thc synthesis, structures, properties, and reactions of 7-bonded metallocarboranes. Our survey is restricted to complcxcs of 2-carbon carborancs and to specics that havc between nine and fourteen total polyhedral vchces. Coverage of metal complexes of other hetrroboranes is availahh. in Grimes’s book (41) and in Todd’s review (9.3). The recent work of Grimes and his group has concentrated on metallocarboranes having fewer than nine vertices (42, 75, 7 6 ) . Our approach to the subject has been to divide thc metallocarboranes according to the size of the polphcdron. Starting with twclvc-vertex compounds, which constitute the majority of the effort, we proceed t o the larger polyhedra, so far unknown in the B,Hn2- and C2B,-2H, series, and then t o the lower polyhedra. Furthcr subdivisions within each polyhedral size include synthesis, structurcs, and propertics of monometallic complexes, rcactions of monometallirs, bimetallic preparations and reactions, and, in two instances, trimetallic compounds. TAT3T,I< I

CORRELATIOV OF (:Ro\\ I’OI.YHEDRAI, GEOMETRY WITH T O T ~YrL M D C R OF V E R T I ~ E S Total vertice5, n 12 11 10

9 8

7 6

Observed geometry Icosahedron (ktadecahedron I3ieapped square antiprism Tricapped trigonal prism I hdecahedron Pen tagonal bipyramid ()ctdiedron

150

KENNETH P. CALLAHAN AND M. FREDERICK HAWTHORNE

Metallocarborancs have only been known for 10 years, and research into their synthesis and characterization has involved a small number of workers. Conscquent?ly,practical applications of these unique compounds havc not been rapidly forthcoming. Itcccnt work has shown catalytic activity in certain of t h e x compouiids, however, arid may signify future commercial value and industrial importance of mctallocarboranes.

I1 METALLOCARBORANES: SYNTHETIC METHODS Five major synthetic routes are now available for the preparation of metallocarborancs, although only oiic was well established in 1969. The recently developed synthetic methods have allowed the preparation of more complex and diverse compounds and have greatly expanded the field of metallocarborsne chemistry. Thcse preparative methods are discussed in some detail in this section, for the synthesis of all the known metallocarboranes have been accomplished by one or more of these routes.

A . Preparation from nido-Carborane Anions As mentioned previously, the first metallocarborane synthesized, ( 1 , 2-C2BBHl1) zFe(11)2- (Fig. 3 ) , was prepared in a manner similar to the synthesis of ferrocene, e.g., reaction of anhydrous FeClz with the nidocarborane dianion 7,8-CzB9H1I2-,which itself was formed from 1,2-CzBloH12 2-

FIG.3. Structure of the first metallocarhorane ever synthesized, ( 1 , 2-C2B9Hll)zFe(II)2-.

Ten Years of Metallocarboranes

151

by degradation with strong base [Eq. (2)I. This synthetic method, with some modifications, has been used to prepare a wide varicty of monometallic twelve-vertex metalloc,drboranes. The reaction conditions for this type of preparation generally involve nonaqueous solvents, such as tctrahydrofuran (T H F ) or diethyl ether, and rigorous exclusion of air and watcr. Some metallocarboranes, however, may be prepared in high yicld by reaction of a mctal salt and the nido monoanion, CzBsH12-, in strong aqueous base : 7,S-C213JIll-

+ 011-

2 (7,&C2B9111,2-1

4-11'"

-H20

-

+ 7,8-C2BgHIl2-

(1,2-CJ39Hil)211n-'

(3)

(4)

In these instances, it is belicwd that the strong base deprotonatcs the monoanion to a small extent, permitting complexation to occur. This synthetic approach proved valuable for the preparation of lower monometallocarboranes as wcll : the C&H1I2- ion, prepared from 6,8C2B7H13,was found to react with mctal ions, losing one equivalent of hydrogen gas, to form metallocarboranes of the type ( C Z B ~ H ~ ) ~ (38). M"-~

B. Preparation by Polyhedral Expansion Polyhedral expansion, which was first reported in 1970 (Fig. 4) (16), entails the reduction of a closo-carborane with a strong reducing agent,

FIG.4. Polyhedral expansion of 1,7-C&,H,.

152

KENNETH P. CALLAHAN AND M. FREDERICK HAWTHORNE

such as an alkali metal, followed by reaction with a transition metal reagent. All reactions must be performed in nonaqueous solvents under nitrogen. It is believed ( 4 ) that the reduction step results in an opening of the closed carborane polyhedron, producing a nido dianionic species, which then reacts with a transition metal ion in a fashion similar to that just discussed :

-

+ 2Sa 2 (C?Bn-?ITn?-)+ >In+ C?Bn--2Hn

CzB,-zHn2-

(CzB,-,H,)

(5) 231n-4

(6)

Although a number of different metallocarboranes have been synthesized in high yield by this reaction (18, 94),the actual chemistry is frequently much more complex than implied by Eqs. ( 5 ) and (6). For example, products containing greater and fewer numbers of boron atoms than present in the carborane starting matwials have been isolated from polyhedral expansion rcactions. The polyhedral expansion reaction appears t o be a general synthetic method for metallocarboranes; all the known closo-carboranes have been found t o produce metal-containing compounds when subjected t o the reduction-complexation operations of this synthetic scheme. Moreover, metallocarboranes containing more than one transition metal may be prepared by the polyhedral expansion of monometallocarboranes. Examples of this synthetic route will be described in following sections. I t should be noted that in the polyhedral expansion process, as idealized in Eqs. (5) and (6), the product metallocarborane has one more vertex than was present in the carborane starting material-hence the origin of the descriptive phrase “polyhedral expansion.” By contrast, when metallocarboranes are prepared by reaction with CzBgH112- ions, which are prepared from the icosahedral C2BlOHl2carboranes, twelve-vertex metallocarboranes result.

C. Preparation by Polyhedral Contraction The polyhedral contraction route to metallocarborancs consists of the degradative removal of a polyhcdral boron atom of a metallocarborane followed by oxidative closure of the resulting nido-metallocarborane complex to a closo species having one fewer vertex than present in the starting material (68): 1 onC,H~CO(I,~-C~R~H,,) C,H.CO(~,~-CPRHH~O) (7)

2

1x202

The polyhedral contraction process is thus complementary to polyhedral

Ten Years of Metallocarboranes

153

expansion, in that the formcr d(.creascs polyhedral size (Fig. 5) whereas the latter iiicrcascs the number of polyhedral vertices. Polyhedral contraction is not as general a synthrtic method as is polyhedral expansion, since some metallocarboranes urdlrgo complete decomposition upon attempted partial degradation, and side rcactioiis are frequently a difficulty in polyhedral contraction. NCvert8hdvssJthis is a valuable route to new complexes if the proper reaction conditions can effected.

D. Preparation by Polyhedral Subrogation Synthetic polyhedral subrogation for the preparation of polymetallocarboranes from mononictallocarboranes is an off shoot of polyhedral contraction in that, after degradativc removal of a BH vertex, a transition metal ion is reacted with the niclo-metallocarboranc produced rather than with an oxidizing agent. In this way, a new transition metal vertex is incorporated into the polyhedral framcwork without a change in the number of vertices between reactant and product (Fig. 6) : 1 . OH

This method is useful for the synthesis of metallocarboranes containing two similar or differcnt transition mrtal vertices (19, 23) but has not yet been explored to determine its full potential.

E. Preparation by Thermal Metal Transfer A newly discovered syrithctic route ( 2 7 ) to bimetallocarboranes, thermal metal transfer may prove to be a highly valuable synthetic method but, as

154

KENNETH P. CALLAHAN AND M. FREDERICK HAWTHORNE

EtOH I

I1

II

12

O C H

Ow FIG.6. I'olyhedral subrogation of C&Co(l , ~ - C J ~ I , H , with Z ) Fe(I1).

yet, has been solidly established for only one system (Fig. 7 ) . The method, which involves the pyrolysis of a metallocarboranc through a hot tube in vucuo, was found t o produce bimctallocarborancs having one vertex greater than was prescnt in the starting material:

heat

C~EI,CO (CZB,Hio)

( C X , ) ZCOZ( CzBJIm)

(9)

Even more intriguing was the observation that the same products were formed upon pyrolysis of ionic cobalticinium salts of commo-metallocarboranc ions : (CjHg) &of (C,BJI,,) 2 C o - k ICjHs) ZCOL( C ~ U ~ H ~ O )

(10)

This production of neutral bimctallocarboranes from ionic monometallic

FIG.7. Thermal metal transfer as a synthetic route to bimetallocarboranes.

155

Ten Years of Metallocarboranes

precursors was unprccedcnted, and further examples of this type of rcaction are being sought. Spencer, Green, and Stone (58) have rccently described a new synthetic approach to metallocarboranc~sin which an organometallic transition mctal complrx is thermally reacted \I itli a neutral closo-carborane, resulting in the incorporation of the metal iiito the polyhedron: 1 ,8- (('FI1) ,C2U,H,

+ (1 ,3-H3SbCI3(acac)(19), by X-ray crystallography.

Configurational isomcrs in cyuilibrium in solution werc also found for the two series of hexacoordinatc antimony compounds, RzSbXz (acac) (It = aryl) and CH3SbC14L (I, = PyO or 4-CH3PyO). The P M R spectrum of (C6HS)2SbC12(acac)at 22O in CDC13 shows two sets of acetylacetonate methyl (A,B) and y-protons (C,D), as shown in Fig. 1. However, the spectrum at -30" of thc solution freshly prepared below -30" in dichloromcthane shows only one methyl resonance (A in Fig. 2 ) . With increasing temperature, a new peak ( B in Fig. 2 ) begins to appear and the intcnsity ratio B/A increases rapidly to the equilibrium value. From these observations, togcthcr with t,hc results of benzene-induced solvent shifts of methyl protons of both tolyl- and acetylacetonate groups in (p-CH3CsH4)z-

ROKURO OKAWARA AND YOSHIO MATSUMURA

190

B

6.5

5.5

-'

5.0

2.5

2.0

1.5

b(ppm)

FIG. 1 . Proton magnetic resonance spectrum of (CGH5)2SbC12(acac) in CDCIBa t 22".

SbClz(acac) , we proposed that isomerixation of the trans-phenyl (11) (peak A) to cis-phenyl (111) (peak B) isomer takes place and that the two isomers are in equilibrium in ~ o l u t i o n .In ~ the solid state, this compound gave two crystal forms. Both forms show the same PMR spectral properties on solution as described in the foregoing. The X-ray struc-

ture determination revealed, however, that both have trans-phenyl structures. The only difference is in the dihedral angle of the two phenyl ring planes as shown in Fig. 3 (20, 2 1 ) . A trans-methyl structure was also indicated by an X-ray structural study for the methyl analog, (CH3)2SbBrz (acac ) (22). 3 The effect of substituents X and Y on the equilibrium was also studied for (p-YCBHa)zSbX2(acac)(X = F, C1, Br; Y = NO2, C1, CH3, C&O) (11).

191

Organoantimony Chemistry

A

22"

4

FIG.2 . hlethyl proton resonanws of (C611j)&Clz(acac) in dichloromethane a t various temperatures. (a) The solution \V:E freshly prepared below -30". The figures in parentheses represent the time (in minutes) elapsed from the preparation of the solution to the measurements. (b) The solut,iori was kept a t room temperature for 24 hours, and ample time mas allowed for each mwsurement.

For monomethylantimony adducts, CH3SbC14L (L = PyO or 4CH3PyO) in solution, two isomeric forms (VI) and (VII) were also suggested to be in equilibrium from the solvent-dependent P M R spectra (9). From the PMR and I R spectra of (CH3)&3bC13L (L = DMSO, HMPA, TPPO, or PyO) in solution, an octahedral geometry with trans-methyl configuration (VIII) was suggested (8).

192

ROKURO OKAWARA AND YOSHIO MATSUMURA

FIG.3. The molecular structure of (C61-I~)2ShC1~(acnc). The dihedral angle of two plienyl ring planes: (IV), 84.6" (20);(V), 38" ( 2 1 ) .

(VI)

(VII)

(VnI)

It is notable that the existence of configurational isomers in solution has been established for these hexacoordinatc organoantimony compounds, since among hexacoordinate organotin compounds, the structures of which have been extensively studied, there are few reports of such isomer^.^

111 TRIORGANOSTIBINE SULFIDE I n contrast to the well-studied triorganophosphine oxide or sulfide, very little work had been done on the analogous organoantimony compounds. Recently two configurational isomers have been isolated from dimethyltin N , N'bis(salicyla1dehyde)ethylenediiminate (63).

193

Organoantimony Chemistry

250

260

270

280

290

300

310

320

330

Wavelength (nm)

FIG.4. Ult,raviolet spectra of l2H5)(i-C3H7) ( C s H j )SbI was obtained by the reaction of (CH3)) with ( C 2 H j 30BF4 ) in methylcne chloride, followed by (i-C3H7) ( C 6 H 5 Sb treatment with KI in methanol. Onc cnantiomcr of this stibonium iodide was 0btainc.d by way of Ag-( - ) -dibenzoylhydrogcntartrate ( D B H T ): (CH,) (C2115)( z - C ~ I I(C811r,)ShI ~) Ag--(-)-DBHT [aID18-

-

66.5" (c, 1.02 i n inc~thanol)3[ a ] o Z 6

+ 4.10" ( c , 0.63 in methanol)

(8)

This cnantiomcr is optically stable both in thc solid and in solution.

C. Coordination Behavior with Transition Metal Carbonyls 1. Reaction of Bis(c1iorqanostibino)methane with Metal Carbonyls

In the substitution reactions of the metal hcxacarbonyls, M (CO)6 ( M = Cr, Mo, W) , with bis (diorganostibino)mcthane, we found that bis (diarylstibino) mcthanc acts as a monodentatc ligand and bis (dimcthylstibino) mcthanc as a bridging ligarid to form pcritacarbonyl complexes XVI and XVII ( 4 5 ) . Furthermore, doubly bridged tctracarbonyl complexcs (XVIII) were obtained from the rcaction of M(CO)r(dicne) with bis(diarylstibino) methanc ( 4 6 ). I 3 This coordination behavior of bis (diorganostibino)methane sccms to be different from that of the phosphinc analog l2 The calculated barrier to the pyramidal inversion for trimethylstihine was reported to he 26.7 (61) and 25 kcal/mole ( 6 2 ) . l 3 Diene = 1,h-cyclooctadiene (C8H12)or 2,5-norbornadiene (C~HS).

20 1

Organoantimony Chemistry

(R = C,H, , P-CH3C,H4 ; M = C r , Mo) (xvII1)

in the sense that bis(diphcnylphosphin0) methane acts as a chelating ligand in M o ( C 0 )4[ (CsHj)d']L'H2 (55). However, a recent report (56) suggests the possibility that onr bis (diphenylstibino) methane acts as a chelating ligand in new monomrric compounds, Mo (CO) 4[ (C6H5)zSb]zCHz and M O ( C O ) ~[(C6H5)zSb]zC:€Iz]z. ( Bis (diorganostibino) methane rcacted with dicobalt octacarbonyl, under mild conditions, to give a dinuclcar complex (XIX) containing both bridging carbonyl groups and bridging stibinomethane. The substitution reaction of compound XIX with diarylacetylene gave complex X X ( 4 7 ) . 0

0

>hylcarbanion a t the ar-

230

HUBERT SCHMIDBAUR

sonium center can compete successfully. Once formcd, pentamethylarsenic is stable and easily recovered in the work-up of the reaction mixture: LiCHz

LiCHx

- LiCl

2 . Properties, Spectra, and Structure

A t room temperature, pentamethylarsorane is a colorless liquid of a characteristic odor which resembles the analogous antimony compound. It crystallizes below -6°C and can bc sublimed under reduced pressure a t -10°C. The (CH3)gAs is a monomer in benzene solution and shows a molecular ion in the mass spectrum with very low intensity. The vibrational spectra, infrared and Raman, could be assigned to a trigonalbipyramidal skeleton. There are striking similarities to the spectra of Sb(CH3)b ( 1 3 ) . The proton ISMR spectrum appears as a singlet over a temperature range of +35" to -95°C and indicates nonrigid behavior. This pseudorotation phenomenon is charactcristic for pentacoordinate molecules. The lH signal of the methyl resonance of pentakis-p-tolylarsorane also remains unsplit even a t -90°C ( 2 0 ) . Compound (CH,) 5 A is~ slowly hydrolyzed by water, and tetramethylarsoniurn hydroxide arid some trinicthylarsenic oxide are formed (37). With methanol, tetramethylmethoxyarsorane is generated with evolution of methane (68). It is iritercsting to note that (CH3) AS and (CH3) 3AsCH2 both lead to identical products with these protic reagents:

(cH,),As

-

j[:H Hzo

- CH,

((CH,),As]OH

(CH,),AsOCH,

(42a) (cH,),As

=a, ( 4 2 ~

CH,OH

More generally speaking, these are only two examples for a larger series of reactions which are currently under investigation (89). - CH4 (CH3)sAs €IX [(CH,),As]X (CI-I~)~ASCH~ HX

+

+

t

(43)

Group V Pentaalkyls and Alkylidene Trialkyls

23 1

The thermal decomposition of (CHs)& a t 100°C leads to quantitative yields of trimethylarsine, methane, and ethylene, as followed by gas chromatography. Only traces of ethane were detectable. It is, therefore, assumed that the compound is decomposed via the ylide, which is known to be unstable under these conditions: heat

I-

~(CH~)SA -2(CH3)3;2~=CH2 S

+ 2CI-I4

(44)

fast

very slow

heat

2(CII,)3As

+ C,IL

(45)

2C2He

Thus, (CH3) AS is a t least kinct,ically much more stable than the corresponding ylide, arid this relationship should give pentamethylphosphorane a good possibility for isolation if it can be prepared. There is no information available, however, on whether the decomposition occurs via a polar or a radical mechanism.

v ANTIMONY YLIDES AND PENTAALKYLSTIBORANES

A. Antimony Ylides

It is obvious from the literature summarized in recent reviews (11, 86) that very little information is available on ylidic compounds of antimony. Moreover, the fcw compounds synthesized were all taken from the aryl series, whereas those of the alkyl series appear to be completely unknown. Even the aryl antimony ylides were of very limited thermal stability and could only be isolated in special rases ( I 1) . It is, therefore, to be expectctl that a compound such as trimethylmethylene stiborane, (CH3)3SbCH2, should be relatively unstable and attempts to synthesize this spcvies would have to be carried out a t low temperature. Another difficulty arises from the experimental fact (104) that all conventional methods of synthesis for stibonium ylides lead to pentaalkylstiboranes instead of ylides (see Introduction). Thus mcthylation of (CH3)J3bX or (CHI) 3SbX2halides by orgariometallic reagents of lithium, magnesium, aluminum, or zinc invariably yield (CHs) &b as the sole product. I n a search for other possible routes t o trimethylmethylene stiborane

HUBERT SCHMIDBAUR

232

the transylidation method in the combination with phosphorus ylides was investigated very recently (69). Although (CH3) sPCH2 was found t o convert tetrainethylstibonium salts into ( CH3) S b , the corresponding ethylidene phosphorane reacted in a different rnanner-(CH,) 3Sb was the only antimony-containing product. As expected for a transylidation reaction, tetraethylphosphonium salt was found in a quantitative yield. This result can be explained in terms of ((CH,)aSb]Cl

+ (CzH,)31’=CHCH,

(CH,),Sh=CH,

heat

+ [(C2H,),P]Cl

L

(CH&Sb

+ (CH,),

(46)

in which the transitory cxistencc of antimony ylide is proposed. Unfortunately i t has not yet been possible to obtain inore direct evidence for this intermediate, even whcn the reaction was carried out a t -25°C. I n principle, hov-ever, processes of this type seem to be very promising because they use ylides as very strongly basic but nonalkylating reactants, which are to be preferred over the strongly alkylatirig organometallic reagents. Further studies arc required, a t even lower temperatures, with othcr solvents and a variety of ylides. This section is not cornplctc without mentioning the hydrogen-deuterium exchange experiment by Doehring and Hoffmann ( l a ) , which indicated an enhanred acidity of the hydrogens in a tetramethylstibonium salt. I n the corresponding equilibrium, stiboriium ylides play the role of strong bases :

=

(CI-I,),Sb@

(CIT,),Sb@-CH,e

5

]+H@

(47)

(ClTs)jSb=CH2

As would be expected, thc proton exchange is much slower than in tetramethylphosphoniuni and -arsonium salts ( l a ) .

B.

Pentaalkylstiboranes

1. Preparation

Pentamethylantimony was the first RsSb species t o be obtained by Wittig and Torssell in 1955 (104). Various other derivatives of this type have since been synthesized, and a range of preparative methods has been described. Among these are the alkylation of R4SbX, RSSbX2, and SbX6 species with organometallic compounds of lithium, magnesium, aluminum,

Group V Pentaalkyls and Alkylidene Trialkyls

and zinc, as follows: KSbX

I

1 W) does not reflect the trend in a-basicity or M-C bond stability ( W > Mo). An essentially concerted trans-insertion mechanism is inferred, which is supported inter alia by the low kinetic deuterium isotope effect ( k H / l c D = 1).

+

255

Acetylene and Allene Complexes

The concerted trans insertion formally belongs to a thermally allowed ma] reaction utilizing the acetylene a, orbital. A nonpolar fourcentered heteroatomic transition state with a skewed disposition of participating U- and ?r-bonds may bc postulated:

+

The geometry in the transition state readily explains the preferential formation of the conformational isomer a (46) (Scheme 2).

Y-,,,,

Isomer a

Isomer b Scheme 2

It is interesting to observe different mechanisms for the apparently similar insertion reactions betwccn Cp,MoH2 and fluoroacetylenes. Thus, Cp2MoHz acts as a a-base for CF3C=CH with a polar triple bond, but in the essentially concerted trans insertion it may behave as a c2*component against a nonpolar bulky fluoroacetylcne, CF3C=CCF3. The observed discriminating behavior toward these fluoroacetylenes contrasts sharply with the nondiscrimination of typical N-bases ( R2NH or &PH) (66) in reactions of the fluoroacetylenes. Specificity of transition metal complexes in reactions with apparently similar organic substrates is thus of interest and deserves further study.

256

SEI OTSUKA AND AKIRA NAKAMURA

1. Metalocyclization

Metal +acetylene complexes react with further molecules of acetylenes in two different ways, namely ligand exchange or substitution (Scheme 3 ) . R'

R' C- CR'

R' Scheme 3

Insertion initially gives metalocyclopentadicnes which may further give rise to larger metalocyclic complexes. Factors determining the reaction paths are not clear a t present. In general many metal acetylene complexes of d a d 0metals (70-89), e.g., Fe ( 3 , Y O ) , Co (10,7l), Ni ( 7 2 ) ,RU ( 7 3 , 7 4 ) , R h (75-78), Pd (83-84), and Ir ( 8 6 ) ,react with excess acetylene to give metalocyclopentadicne complcxes or acetylene oligomers (87). The following are some examples:

PPhs

CpC(

c' c+

R

\

RCECR R=CO,CH,

R

R'

R

&Pd---lll CYR

cR'

R=CO,CH, RCGCR

=

& P d g R R R

I n these complexes, repulsive interaction (c') is operating in addition t o interactions (a) and (b) (see Fig. 1).Thermal excitation then either causes

257

Acetylene and Allene Complexes

thcsc complexes to lose the acetylene or gives rise to a thermally excited molecule in which a change in coordination state (to a monohapto state) may occur (73, 89). A similar thermal activation has been proposed (90) for soine insertion reactions of dioxygen complexes, L2MO2 ( M = Ni, Pd, Pt), where the orbital interaction scheme is similar except for the occupancy of the dioxygen rr*orbital:

/

c

M---'''

thermal excitation =

+

M-C

I

+c- -M-C'

ti

.

,cI

I

The radical or ionic character dcpcnds on the identity of the metal, the effective metal oxidation statc, and the auxiliary ligands. These combined effects determine the reactivity, stereo- and regioselectivity toward acetylene insertion. For example, thc reaction of CpCo(PPh3) (RC=CR') with asymmetric acetylenes, RC=Clt', gives a mixture of isomeric products:

PPh,

I

R

R-CGC-R'

cpco R

R'

Isomer a

R'

R

Isomer b

R'

R

Isomer c

(i) R = CO,CH, , R' = CH,: a. 9%; b. 50% (ii) R = P h , R' = CO,CH,: a. 13%; b. 20%

A predominance of isomer b and the absence of isomer c indicates the direction of polarization in the metal +acetylene moiety (91). A zwitterionic intermediate, Cp (PPh3)Co+--C (CHI)=e-CO2CH3, is implied for case (i) ; much less polarization with some radical character would account for the isomer distribution in case (ii) . Recently, a similar polar monohaptoacetylene intermediate was invoked to explain the novel addition of

258

SEI OTSUKA AND AKIRA NAKAMURA

CF3C=CCF3 to a C-H

bond of an Ru-alkenyl complex (92)l:

I

R

R

R = CO,CH,

For reactions of cationic Pt-acetylene complexes (93), another polar monohaptoacetylene complex (C) may be postulated:

I'

/

L3Pt + C '\

Y

Me-Pt--ill

CYR

1 c\R

-

\

( D)

( C)

L I

+

L

R L I

Me -Pt -C, I

L

,OMe CHR,

Thus, most of the electrophilic reactions of [PtCHS(L)z(RC=CR)]+ can be explained with the intermediate mechanistically indistinguishable from the platinieed carbonium ion model (D) proposed by Chisholm and Clark. Since the positive @-carbonhas a vacant pr-orbital, the rearrangement t o alkoxycarbene complexes can be regarded as a carbonium ion rearrangement. I n contrast to the later transition metal complexes, electron-deficient complexes of the earlier transition metals, e.g., CpV( CO), (RC=CR) (94), are mostly inert to acetylene cyclization. Thus, bis- or trisacetylene com1 The molecular structure of the product [Ru.C(C02Me) :C(CO2Me)C(CF3) k(CFs)H(PPhs)(+CaH5)]has been fully confirmed by an X- ray diffraction study (L. E. Smart, J . Chem. SOC.,Dalton Trans., in press).

259

Acetylene and Allene Complexes

plexcs, e.g., M (RCECR) 3 ( CO) (95-97), are prepared by thermal and photochemical reactions. The absence of stable bis or tris complexes of d8-d10metals may be attributed to the facile metalocyclization. Thus, interaction (c) (see Fig. 1) or (c') appmrs to impart this distinction. In molybdenum acetylene complexcs, ('pJ10 (RC=CR) (46, 4 7 ) , intermediate between the two cases, the relevant d ~ orbital , is partially occupied by interaction with the E l , (Cp) orbital,

resulting in a weak attractive interaction (c) . Indeed, these acetylene complexes are inert to acetylene oligomerization. A further reaction of metalocyclopentadiene complexes with acetylenes leads t o metalocycloheptatriene complexes by metalocyclic enlargement ( 3 , 10, 98) or to benzene derivatives by reductive elimination (57, 70, 73, 7'7, 8,2,98):

$ M

metalocyclic enlargement

\

aromatization

~

$M

or 1

cis, cis, cis

isomer

cis,trans, cis

isomer

For example, reaction of excess CFBC-CCR with Pt ( PEt3) gave ( Et3P)%Pt[q2-Cs (CF3)61, which probably formed from a +acetylene complex through a platinacyclopentadiene complex (98).2 Rerently, Stone et al. [ J . Chem. Soc., ('herti. Commm., 723 (1975)] have established a reverse pathway for reactions of this kind. Certain complexes of Pt(0) react with Ce(CF& with cleavage of a C C bond of the benzene derivative to give a platinacyclohepta-cis, cis, cis-triene.

260

SEI OTSUKA AND AKIRA NAKAMURA

A related reaction with Ni[P (0Me)J.i gave unexpectedly a cis,trans, cisnickelacyclohcptatriene complex that must be constructed with a transinsertion process a t some stage of its formation (98) :

SR

R

Ni[P(OMe),],

+

CF,C-CCF,

-

[(MeO),P],Ni

R

R

(R = CF,)

2. Metalococyclization of Acetylenes with CO or RNC

Metalocycles arc also formed by the reaction of acetylenes with metal carbonyls or with isonitrile complexes ( 3 , 73, 99-102). Their formation may involve monohaptoacetylene intermediates.

M(C0)

-

0

II

0

I1

RCZCR

/I

0

NR

Some of these metalocycles have been confirmed by spectroscopic data and by an X-ray analysis (103) and are important intermediates in catalytic cocyclization with carbon monoxide or with isonitriles (see following).

26 1

Acetylene and Allene Complexes

N / R‘

N

\

R‘

C. Catalytic Reactions 1. Activation of Acetylene by Coinplexatioii

Studics on elementary reactions of acctylcnes with metal complcxcs are now beginning to shed some light on tlw nature of “activation” caused by complexation. This activation is not a simple process. Many low-valent d*-dIO metal complexes and also som(’ rarly transition metal compounds with higher oxidation state ( &d2 complexes) are capable of activating acetylenes. As already describcd, in thc former complexes, interaction (c’) would lead t o activation of an +acctylcne ligand to a n 7’-acetylenc having some radical as well as some anionic character:

$-Acetylene complex

ql-Acety lene complex

In t h e latter complexes, strong r-donor interaction (a) and weak r-back donation (b) (see Fig. 1) would lrad t o thc formation of apparently similar +acetylene complexes by thrmmal activation. Here the species, however, have some cationic character as manifrstcd by their preferential reactions with electron-donating acety1tmc.s (63, 104) :

In sharp contrast t o thew activations, an q2-acetylme complex is stabilized when all the interactions [ ( a ) , (b) , arid (c) ] arc bonding, as in some electron-deficient d6 complexcs, c’.g.,W (RC=CR) (CO) (95-97).

262

SEI OTSUKA AND AKIRA NAKAMURA

2. Catalytic Cyclooligomerization

Acetylenes are catalytically cyclized to benzenes and cyclooctatetraenes (70, 105-107). Small amounts of styrenes, vinylcyclooctatetraenes, naphthalenes, and azulenes are also formed in some instances (108-110). Some elementary steps in these reactions have already been discussed. A plausible reaction path for the cyclization is in Scheme 4 (111).

Scheme 4

3. Linear Oligomerization Acetylenes are also oligomerized to mono- or divinylacetylenes, or dienylacetylenes by Ni(0) (lid), Rh(1) ( I I S ) , or Pd(1I) (114) complexes (Scheme 5 ) . Meriwether et al. (119) proposed hydrido-u-alkynylnickel complexes as active intermediates in the catalytic linear oligomerization. Subsequent insertion of acetylene into an M-u-alkynyl bond has been assumed. ML?I

RCGCH

-

R'C

III---MLm

HNC

thermal excitation

I q'-Acetylene complex

Scheme 5

Acetylene and Allene Complexes

263

Conversion of an +acetylene complex to the hydridoalkynyl complex will lead to linear oligomerization or polymerization. The tendency of some Iih or Pd complexes to form hydridooalkynyl complexes explains their catalytic activity toward linear oligomcrization. Recently, Hagihara et al. ( 115) examined the reaction of preformed hydrido-a-alkynyl complcxes, M H (U-CECPh) Lf,with Mc02C'C-CC02Me and found cis inscrtion into the M-H bond. It R

+

t i a n s - ~ I € I ( C ~ C ~ ' h ) L LItC'-C11

-

I

I

Iruns-M(C=CHh) (-C=CII)L,

31 = Pd, Pt; 1, = 1'b:t3; 12 = CO&le

They also found a novel stercosclective linear trimerization of PhCFCH with a Pd (Ph) (PBu3)2 catalyst. 4. Catalytic Cocyclixation with Isocyanides

Cocyclization of acetylene with isocyanides gives interesting new cyclic compounds (103, 116). The reaction patterns are generally similar to the cocyclization with carbon monoxide which is already known (103, 117 ) . Low-valent nickel, palladium, or cobalt complexes are active in the following reactions (102, 10.9) for which intervention of acetylene complexes has been suggested : RCECR

+

R'NC

Ni, Pd, o r

Co complexes

= -N

EN-++q \ N g N / N\

N-

Recently, Yamazaki et al. (10.9) carried out stoichiometric reactions of cobalt-acetylene complexes with isocyanides and isolated the expected intermediate metalocyclic complexes (Scheme 6 ) . Another interesting rcactiori is the formation of aminopyrrolc derivatives from t-BuNC and various acctylenes (118). The catalysts include various N i ( 11) and Ni (0)phosphine complexes:

t-B"

Based on the formation of a C ~ C O ( C N R ' ) ~ ( R C ~ Ccomplex R) from acetylene and isocyanides (103), the paths shown in Scheme 7 are proposed.

264

SEI OTSUKA A N D AKIRA NAKAMURA

Y

RCACR I

R

~l-~,

R

R = 2,6-Dimethylphenyl Ph R'

R'-N

R

R

R l N q N R l N I R'

Scheme 6

5. Catalytic Cocyclizatioit with Heterounsaturntion

Yamazaki et al. (91, 119) and Bonnemann et al. (120) have recently reported catalytic syntheses of substituted pyridines from acetylenes and nitrilcs. Various cobalt complrxrs serve as active catalysts, in particular, c p C 0 ( P P h 3 ) ~(91) 01 Co(C8HI2)(C8HI3)(120). Similar reactions of acetylenes with CS2 or RNCS also give new heterocycles (91) :

Ni(R'NC),(RC-CR)

-

1

R'NC

\

N

&LN. Ni

NH-R' R'

Scheme I

Acetylene and Allene Complexes

RCECR

+

R’CN

RCGCR

+

CS,

265

A

-)I+f C’

/I

S

RCECR

+

R’NCS

/I S

Thc following intermediate coniplcx has also been isolated (121)

R

This structure gives support for thc proposal of an ionic monohaptoacety+ lcne complex, Cp (PPh3) Co--C ( R ) =(%,as an activated precursor for the reaction with CS2.

111 ALLENE COMPLEXES A. Structure and Bonding Since the compilation of cornplcxcs in 1972 (11-13), a few have been reported: M(PPh3)2(allenc) (122, 123) ( M = Ni, Pd, P t ; allene is CHZ=C=CH2, CH~=C=CMP~, PhCH=C=CHPh) ; CpzFc (CO)[CHF=C=C ( CH3)SnCl] (1.24, 125) ; and PtCH3( HBpz3) (&C=C= CR’2) (HBpz3 = tripyrazolylborato; It = R’ = CHI or R = H, R’ = CH3) (126). Single-crystal X-ray diffraction data arc shown in Table VI. As in olcfin complexes, there arc two types of coordination-one containing an allcnc double bond perpendicular t o the equatorial molecular plane and the other containing the ligand in the plane. I n the former type, we find labile complexes, e.g., RhI ( PPh3),(CH2=C=CH2) (136) and fluxional

tu

0. 0.

TABLE VI STRUCTURAL

PARAMETERS OF

TRANSITION METAGALLENE COMPLEXESa

C (1);C(2) Compound

01)

(A)

Perpendicular b

1.40 (1) 1.41(1) 1.373(8) 1.377 (8) 1.37 (1) 1.41(1) 1.3.5(6) 1.37(3) 1.48(5) 1.44(4) 1.430 (11) 1.44(2) S(l)-y2)

1.30 (1) 1.29(1) 1.325(8) 1.321(9)

Perpendicular*

Rhz(acac)z(CO)z(CHz=C=CHz)c Perpendicular * R~I(PP~,),(CHZ=C=CHZ)~ [PtC12(RiezC=C=CMe,)]2 P t (PPh3)z(CII?=C=CHz) Pt(PPh3)z(CHz=C=CHMe) Pt(PPh3)z(CHz=C=CMez) Pd (PPh3)s(CIIz=C=CH?)

C (2)-C (3)

Structure

Perpendicular* Perpendicularb In-plane* In-plane* In-plane * In-plane*

0%)

1.72(5) 1.65 (3)

1.34(7) 1.36(3) 1.31(5) 1.32(4) 1.316 (11) 1.32(2) C(2),S(3) (11) 1.54(5) 1.63(3)

C (1)-C (2)C(3)(”) 153.3 (6) 132.6(6) 147.2(6) 148.9 (6) 144.5 ( 6 ) 158 (4) 151(2) 142 (3) 146 (3) 140.8(8) 148 (1) s(1)-C (2)s(3)(“1 136 (1.5) 140 (2)

M-C(l)

M-C(2)

Ref.

2.13(1) 2.13(1) 2.177 (6) 2.176(6) 2.12(1) 2.14(1) 2.17(4) 2.2.5(2) 2.13(3) 2.12(3) 2.107 (8) 2.12(1)

2.07(1) 2.06(1) 2.027(5) 2.033 (5) 2.05 (1) 2.06 (1) 2.04 (4) 2.07(2) 2.03 (3) 2.05(3) 2.049 (7) 2.07 (1)

127

130 131 132 1.33

blGS(1) 2.33 (1) 2.305 (11)

IiGC(2) 2.06 (4) 2.00 (3)

1.34 135

128 127 129

128

Carbon disulfide complexes are included for comparison. The C=C bond length of free molecules ranges from 1.300 to 1.312 ,i. refers to the molecular structure containing a coordinated double bond perpendicular t o the niolecular plane, and in-plane to that containing the double bond lying in the molecular plane. c Each of the two double bonds acts as a monodentate olefin and thus the bent allene bridges the two nietal atoms. The bromo analog, RhBr(PPh~)z(CHz=C=CIIz), has quite similar structural parameters (Kasai et al., unpublished). a

* Perpertdzcular

Acetylene and Allene Complexes

267

ones, e.g., Pt2C14(Me2C=C=C.Rle2)2(1S7, 138). In the latter are found both labile, e.g., P ~ ( P P ~ ~ ) ~ ( C ' H F = = C ' = C(139) H ~ ) and inert compounds, e.g., Pt (PPh,),(CH2=C=CH2) (1.29) (see following). I n general the degree of clongation of the double bond upon coordination parallels the degree of bending of the allene molecule. The Dewar-ChattDuncanson molecular orbital model of the mctal-olcfin bond would account for these features. The central carbon-metal distance is shorter than the other carbon-metal distance and may be explained by an additional interaction between a filled metal d-orbital and the olefiri ?r*-orbital with the uncoordinated double bond. However, this view, although attractive, appears not to be supported by the fact that the M-C ( 2 ) distance remains nearly constant, within standard (wm, rc.gardless of the number of methyl substitucnts a t the uricoordiriat cd double-bond carbon in Pt (PPh3)2(allene). The X-ray bond distanccs may not be sensitive to the electronic variation or they may simply bc a reflection of the atomic radii susceptible to the change in hybridization. I n thr case of Pt ( PPh3) ( CH2=C=CMe2), a nonbonding rtpulsion exists h t w w n thc phosphine ligand and one of the methyl substitucnts, as the diffcrcnce Fourier map indicates the particular methyl group t o be a hindered rotator ( 1 3 2 ) . The repulsion is also reflected in the two P-metal-C angles. This stcric factor may be responsible for the apparent irregularity in the bmding [C (1)-C (2)-C ( 3 ) angle] and, hence, for the absence of a linear corrcllation between the angle and the distance of the coordinated double-bond [C' ( 1)-(" ( 2 )] in Pt (PPh3) (allene) . A series of nickrl triad complexes ML2(allene) (Table VII) were prepared (192, 193, 139) and studicd in solution by means of 'H NMR spectroscopy. Consistent with a planar niolecular structure,

the 1H NMR spectrum shows three signals with the proton signal a t site a highest field, and the signal at site c lowest field. The following features are conspicuous: (i) large Pt-H coupling constants, (ii) fairly strong P-H coupling, in particular the long-range coupling J ~ - H (23-37 C Hz) ; (iii) J p a - H o +J p b - ~ " (-10 Hz) , Jpb-Hb (

fast

(1)

Ni-L

Scheme 8

The monomer insertion was assumed to take place via electrophilic attack of allene at the o-allyl-metal bond rather than a t the n-ally1 end. I n support of this notion is thc preferential formation of Complex V from IV and complex VIII from VII (Scheme 8), indicating that the insertion site is the carbon end of an extended pn-system in complexes I V and VII, where negative-charge localization is enhanced compared to the isolated allyl system. The ligand effect may also be reasonably interpreted assuming participation of the anionic end of the allyl group in the insertion. It is known that a-donor ligands stabilize high oxidation states of metals. Here the role of a tertiary phosphine ligand is, contrary to that of electronwithdrawing phosphites, to shift the n-a allyl equilibrium toward the

Acetylene and Allene Complexes

277

o-form, requiring an increase in formal oxidation statc of the metal. Electrophilic attack of allene is thcn facilitated to give higher oligomers. However, the predominant formation of complex I1 from V and of complex I11 from VIII suggests that hlocltage of a coordination site by a ligand L in complex V is effective in raising the potential barrier for allene coordination (hence the insertion). In addition the free energy of activation for thermal decomposition of complex V should be low as the kinetics indicate. Thus, further insertion of allcnc into V to produce pentamcr I11 becomes a minor reaction path (Tablr VIII) (123) compared to the formation of complex 11. The foregoing interpretation is consistent with the observed relative rate, i.e., pentamerizatiori with nakcd Ni (0) > tetramerization with Ni(0)-PR3 > trimcrization with Ni(O)-P(OR)3. 2. Rhodium Oligomerixation

The reaction path of alleiici oligomerization on Rh (I) differs somewhat from that on Ni(0) species. The rnoiiomer complex can be isolated. Complex RhCl (PPh,) 3 with a stoichionictric amount of allcne gives RhCl(PPh3)2(C,H4) (X) (136). Thc h o m o or iodo analog is prepared from complex X. The structure of the iodo compound has been determined (129). I n the absrnce of strongly coordinating substances such as phosphorus ligands, RhCl speciw take up 5 moles of allcnc to give the cyclopentamer complex RhCl(( 'ljHzo) (XIV) (150). For example, [RhCl(CzH4)2]2 is a good sourcc for t h r production of RhCl species. Lower allene oligomers could not be detectc,d in this reaction. Dimer and tetramer complexes havc been obtained with Rh(1) having a chelating anion, e.g. acetylacetonate(acac) (149). Thus thc low-temperature ( - 78°C) reaction of allene with Kh (acac) ( C2H4) prodiiccs unstable Rh (acac) (C3H4)3 of unknown structure which givcrs, upon treatment with pyridine, very stable Rh (acac)py2( C6H8) ( X I I ) . Thr structure of complex XI1 has been cstablished by a n X-ray study. Hcnce the unstable compound is believed to have a rhodacyclopentanc unit ( X I ) . The unstable five-membered ring ( X I ) is apparently stabilized in complex X I 1 which may be regardcd as an octahcdral Rh(II1) (d6) complex. The unstable Rh(acac) (C3H4)I is a precursor of the tctramer complcx Rh(acac) (C12H16) ( X I I I ) , which is also directly obtainable from the room-temperature reaction of allerie with Rh (acac) ( C1H4) in pentanc. Thc other p-diketonato complex, Rh (dpd)-

2 78

SEI OTSUKA AND AKIRA NAKAMURA

(C12H16) (dpd = 1,3-diphenylpropane-l, 3-dionato) mas also made. The structure of complex XI11 was established by an X-ray study (151). The reaction path from the unstable complex containing ( XI) to complex XI11 remains t o be elucidated. The structure of pentamer ligand of XIV, derivable from the tctramcr ligand of XIII, is different from the nickel

pentamer 111. The key step determining the pentamer structure is thus the tetramer stage. Complex XI11 is best described as an Rh (111)complex. The Rh(II1) ion apparently prefers coordination of the allylic group conjugated with a double bond so that the negative charge localization a t the ally1 end matches with the high metal oxidation state. Rhodium (I)-phosphine systems lead to catalytic tetramerization. For example, the system [RhC1(C2Ha)2]2 with 1 to 2 moles of PPh3 is effective in the selective formation of an interesting spiro compound (XV) (152) free from other isomers. Although the detailed reaction path is unknown due to the inaccessibility of the intermediate complexes, the formation of (XV) may be visualized from a tetramer complex as follows:

Acetylene and Allene Complexes

279

REFERENCES

1. Bowden, F. L., and Lever, A. B. P., Organometal. Chein. Rev. 3, 227 (1968). 2. Kemmitt, R. D. W., MI‘P Znt. Ref).Sci., Znorg. Chein., Ser. 1 6 (part 2), 227 (1972). 3. Hiibel, W., in “Organic Syntheses via RIetal Carbonyls” (I. Wender and P. Pino, eds.), Vol. 1, p. 273. Wiley (Interscience), New York, 1968. 4. Hartley, F. 11., Chenz. Reu. 69, 799 (1969); 73, 163 (1973); Angew. Chenz. 84, 667 (1972); “Chemistry of Platiiiuni and Palladium,” p. 361. Applied Science Publ., London, 1973. 5 . Heck, It. F., “Organotransition XIetaI Chemistry,” pp. 167-200. Academic Press, New York, 1974. 6. Green, Pvl. L. I%, i i i “0rganomet:tllic Compounds” (G. E. Coates, K . Wade, and M. L. H. Green, eds.), Vol. 2, pp. 288-333. Methuen, London, 1968. 7. Rlaitlis, P. M., “Organic Chemistry of Palladium,” Vol. 1, pp. 110-130; Vol. 2, pp. 31, 47-58. Academic Press, New York, 1971. 8. Jolly, P. W., and Wilke, G., “0rg:rnic Chemistry of Nickel,” Vol. 1, pp. 305-315. Academic Press, New York, 1974. 9. Pettit, L. l)., and Barnes, D. S., Fortschr. Chem. Forsch. 28, 85 (1972). 10. I)ickson, It. S., and Fraser, P. .J., L4duai~. Organometal. Chenz. 12, 323 (1973). 11. Shaw, B. L., and Stringer, A. ,J., Inorg. Chim. Acta Rev. 7, 1 (1973). 12. Baker, lt., Chem. Reu. 73, 487 (1973). 13. Jolly, P. W., and Wilke, G., “The Organic Chemistry of Nickel,” Vol. 2. Academic Press, New York, (1975). 14. Nelson, J. H., Wheelock, K. S., Cusachs, L. C., and Jonassen, H. B., J . A,mer. Cheni. SOC. 91, 7005 (1969); Inorg. Cheni. 1 1 , 422 (1972); Wheelock, K . S., Nelson, J. II., Cusachs, L. C., and Jonassen, H. B., J . Amer. Chem. Soc. 92, 5110 (1970); Nelson, J. I%., and Jonassen, 11. I.,and Graziarri, It., it,” I’rogrrss ill Coordination Chemistry” (%i.Cttis, ed.), p. 310. Elsevier, London, lO(i8. 21. Ilavies, B. W., Puddephatt, li. ,J., and Pnyne, N. C., Can. J . Chem. 50, 2276 (1972). 22. Davies, B. W., and Payne, N. C., Inorg. Chern. 13, 1843 (1974). 23. Davies, G. It., Hewertson, W., hhis, 11. I€. B., Owston, P. G., and Patel, C. G., J . Chem. Soc. A 1873 (1970). 24. McGinnety, J. A., J . Chem. Soc., Dalton Trans. 1038 (1974). 25. Jacobson, S., Carty, A. J., Mathieu, &.I.,and Palenik, G. J., J . Amer. Chem. SOC. 96, 4330 (1974). 26. Dickson, It. S., and lbers, J. A., J . Organometal. Chem. 36, 191 (1972). 27. Nesmeyanov, A. N., Gusev, A. I., Pasynskii, A. A,, Anisimov, K. N., Kolobova, N. E., and Struchkov, Yu. T., Chein. Coinmun. 739 (1969). 28. Kirchner, R. M., and Ibers, J. A., J . Amer. Chem. Soc. 95, 1095 (1973). 29. Laine, R. M., Moriarty, It. E., and Bau, li., J . Amer. Chem. SOC.94, 1402 (197’2) 30. Davidson, G., Organometal. Chem. Rev. 8, 342 (1972): see Maitlis (Y), Vol. I, pp. 120-121.

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31. 32. 33. 34. 35. 36.

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70. Hoogzand, C., and Hiibel, W., “Organic Syntheses via Metal Carbonyls” (I. Wender, and P. Pino, eds.), Vol. 1, p. 343. Wiley, New York, 1968. 71. Yamazaki, H., and Hagihara, N., J . Organometal. Chem. 7, P22 (1967). 72. Zeiss, H. I%, and Tsutsui, M., J . Amer. Chem. Sac. 81, 6090 (1959). 73. Burt, R., Cooke, M., and Green, M., J . Chem. SOC.A 2981 (1970). 74. Sears, C. J., Jr., and Stone, F. G. A., J . Organometal. Chem. 11, 644 (1968). 75. Collman, J. P., and Kang, J. W., J . Amer. Chem. Sac. 89, 844 (1967). 76. Brateman, L. R., Maitlis, P. M., and Dahl, L. F., J . Amer. Chem. Sac. 91, 7292 (1969). 77. Kang, J. W., Childs, R. F., and Maitlis, P. M., J . Amer. Chem. Sac. 92, 721 (1970). 78. Clarke, B., Green, M., and Stone, F. G. A., J . Chem. SOC.A 951 (1970). 79. Mague, J. T., and Wilkinson, G., Znorg. Chem. 7, 542 (1968). 80. Collman, J. P., Cawse, J. N., and Kang, J. W., Inorg. Chem. 8, 2574 (1969). 81. Mague, J. T., Znorg. Chem. 9, 1610 (1970); Mague, J. T., Nutt, M. 0. and Gause, E. H., J . Chem. Soc., Dalton Trans. 2578 (1973). 82. McVey, S., and Maitlis, P. M., J . Organometal. Chem. 19, 169 (1969); Kang, J. W., McVey, S., and Maitlis, P. M., Can. J . Chem. 46, 3189 (1968). 83. Moseley, K., and Maitlis, P. M., J . Chern. Sac., Dalton Trans. 169 (1974). 84. Ito, Ts., Hasegawa, S., Takahashi, T., and Ishii, Y., Chem. Commun. 629 (1972); Roe, I>. M., Calvo, C., Krishnalnnchari, M., Moseley, K., and Maitlis, P. M., J . Chem. Soc., Chem. Commun. 436 (1973). 85. Ashley-Smith, J., Green, M., and Wood, 1). C., J . Cheni. Sac. A 1847 (1970). 86. Collman, J. P., Kang, J. W., Little, W. F., and Sullivan, M. F., Irwrg. Chem. 7, 1298 (1968). 87. Singer, H., and Wilkinson, G., J . Chem. SOC.A 849 (1968). 88. Kern, R. J., Chem. Commun. 706 (1968). 89. Clemens, J., Green, M., Kuo, RI. C., Fritchie, C. J., Mague, J. T., and Stone, F. G. A., Chem. Commun. 53 (1972). 90. Otsuka, S., Nakamura, A,, Tatsuno, Y., and Miki, M., J . Amer. Chem. SOC.94, 3761 (1972). 91. Wakatsuki, Y., and Yamazaki, H., Tetrahedron Lett. 4549 (1974). 92. Blackmore, T., Bruce, M. I., Stone, F. G. A,, Davis, It. E., and Gartza, A., Chem. Commun. 852 (1971). 93. Chisholm, R.I. H., and Clark, H. C., J . Amer. Chem. SOC.94,1532 (1972); Chisholm, M. H., Clark, H. C., and Hunt,er., 1). II., Chem. Commun. 809 (1971). 94. Tsumura, R., and Hagihara, N., Ui~11.Chem. SOC.Jap. 38, 1901 (1965). 95. King, R. B. and Fronzaglia, A , , Cheni. Commun. 547 (1965). 96. Tate, D. P., Augl, J. M., Ritchey, W. M., Ross, B. L., and Grosselli, J. G., J. Amer. Chem. SOC.86, 3261. 97. Strohmeier, W. and von Hobe, I)., Z. Naturforsch. B 19, 959 (1964). 98. Browning, J., Green, M., Perifold, B. It., Spencer, J. L., and Stone, F. G. A., J . Chem. Soc., Chem. Commrcn. 31 (1973); J . Chem. Sac., Dalton Trans. 97 (1974). 99. Baddley, W. H., Chem. Comnaun.762 (1972). 100. Greatrex, It., Greenwood, N. N., and Pauson, P. L., J . Organometal. Chem. 13, 533 (1968). 101. Kaska, W. C., and Kimball, M. E., Inorg. Nucl. Chem,. Lett. 4, 719 (1967). 102. See Heck (b), pp. 238-255. 103. Yamazaki, H., Aoki, K., Yamamoto, Y., and Wakatsuki, Y., Abstr. 22nd Symp. Oignnonzetal. Chenr. Jnp., p. 20813 (1974); J . Amer. Chem. Soc. 97, 3546 (1975). 104. Hubert,, A. J., and Dale, J., J . Chem. SOC.3160 (1965).

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105. Schrauzer, G. N., Aduan. Organometal. Chem. 2, 1 (1964). 106. Bird, C. W., “Transition Metal Intermediates in Organic Syntheses,” pp. 1-29. Academic Press, New York, 1967. 107. lleppe, W., Schlichting, O., Klager, K., and Toepel, T., Ann. Chem. 560, 1 (1948). 108. SchrBder, G., “Cyclooctaletraen,” pp. 12-15. Verlag Chemie, Weinheim, 1965. 109. Cope, A. C., and Fenton, J . Amer. Chem. Soc. 73, 1195 (1951). 110. Craig, L. E., and Larrabee, L., J . Amer. Chem. Soc. 73, 1191 (1951). 111. Naknmura, A., Mem. Inst. Sci. I n d . Res., Osnka Univ. 19, 81 (1962); Chem. Abstr. 59, 8786 (1963). 112. Meriwether, L. S., Colthup, €3. C., Kennerly, G. W., and Iteusch, 11. N., J . Org. Chem. 26, 5155 (1961); Meriwether, L. S., Leto, &I. F., Colthup, E. C., and Kennerly, G. W., J. Orq. Chem. 27, 3930 (1962). 113. Brown, C. K., Georgion, I)., and Wilkirison, G., J . Chem. SOC.A 3120 (1971). 114. Tohdn, Y., Sonogashira, K., and Hagihara, N., unpublished result. 115. Tohda, Y., Sonogashira, K., and Hagihara, N., J . Chem. SOC.,Chem. Commun. 54 (1975). 116. Suzuki, Y., and Takixawa, T., J . Chem. SOC.,Chem. Commun. 837 (1972). 117. See Bird (106), pp. 174-1‘31; Thompson, D. T., and Whyman, It., in “Transition Metals in Homogeneous Catalysis” ((2. N. Ychrauxer, ed.), p. 147. Dekker, New York, 1971. 118. Jaut.elat, M., and Ley, K., Synthesis 593 (1970); Otsuka, S., Nakamura, A., and Yamagata, T., presented at Symp. Org. Synthetic Chenz., 1971. 119. Yamaxaki, II., and Wakatsuki, Y., Tetrahedron Lett. 3383 (1973). 120. Uonrieniann, H., Angew. Chem. 85, 1024 (1973); Bhnnernann, H., Brinkmann, R., and Schenkluhn, H. Synthesis 575 (1074). 121. Wakatsuki, Y., Yamazaki, H., and Iwasaki, H., J . Amer. Chem. Soc. 95, 5781 (1973). 122. Otsuka, S., Nakamura, L4.,and Tani, K., J . Organometal. Chem. 14, P30 (1968). 123. Otsuka, S., Tani, K., and Yamagata, T., J . Chem. SOC.,Dalton Trans. 2491 (1973). 124. Lichtenberg, 1). W., and Wojcicki, A,, J . A m r . Chem. Soc. 94, 8271 (1972). 125. Benaim, J., Merour, J. Y., and lioustan, J. L., Compt. R e d . Acad. Sci., Ser. C 272, 789 (1972). 126. Clark, I-I. C., arid hlanzer, L. E., J . Amer. Chem. Soc. 95,3812 (1973); Inorg. Chem. 13, 7996 (1974). 127. Itacanelli, P., Psntini, G., Immirzi, A,, Allegra, G., and Porri, L., Chem. Commun. 361 (1969). 128. Hewitt, T. G., and De Bocr, J. J., J . Chem. Soc. A 817 (1971). 129. Kashiwagi, T., Yasuoka, N., Kasai, N., and Kakudo, hf., Chem. Commun. 361 (1969); Technology Rep. Osaka Univ. 24, 3.55 (1074). 130. Kadonaga, ll.,Yasuoka, S . , and Kasai, S . , Chem. Commtn. 1597 (1971); Kashiwagi, T., Yasuoka, X., Kasui, S . ,and Iiakudo, >I., 2’whnol. R e p . , Osaka I’nizl. 24, 1188 (1974). 131. Okamoto, K., Yasuoka, N., and Kasai, N., unpublishcd. 132. Yasuoka K.,RPorita, hl., Kai, Y., and Kasai, N., J , Organotrcctal, Chem. 90, 111 (19751, 133. Okamoto, K., Kai, Y., Yasuoka, N., arid Kasai, N., J . Organometal. Chem. 65, 427 (1974). 134. Mason, It., and Rae, A. I. hl., J , Chem, Soc. d 1767 (1970). 135. Kashiwagi, T., Yasuoka, N., Ueki, T., Kasai, N., Kakudo, M., Takahashi, S., and Hagihara, N., Bull. Chem. Soc. J a p . 41, 296 (1968).

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136. Otsuka, S., Nakamura, A., nntl Tani, K., Kogyo KagaAu Zasshi ( J . Chem. Soc. Jap., Ind. Chern. Sect.) 70, 2007 (1967). 137. Vrieze, K., Volger, H. C., Gronert, PIT., and Praat, A. P., J . Organometal. Chem. 16, P19 (1969). 138. Vrieze, K., Volger, II. C., and Praat, -4.P., J . Organornefal. Chevz. 21, 467 (1970). 139. Otsuka, S., and Tani, K., unpublished. 140. For cxaniplc see Weinstein, B., atrd Fcnsclau, A. II., J . Chem. SOC.368 (1967); J . Org. Chew. 32, 2278, 2988 (1067); Fischer, H., “The Chemistry of Alkenes” (S.Patni, ed.), p. 1025. Interscience, New York, 1964. 141. Dolhier, W. It., ,Jr., and Ilai, S.-II., J . A m e r . Chem. Sac. 92, 1774 (1970). 142. Woodward, It. B., and Hoffmanii, It., “The Conservation of Orbital Symmetry,” pp. 163-166. Academic Press, New York, 1970. 143. Hoover, F. W., and Lindscy, li. V., J . Org. Chem. 34, 3039 (1969). 144. Otsuka, S., Nakamura, A,, Uetla, S.,and Tani, K., Chern. Comnzun.863 (1971). 145. Englert, M., Jolly, P. W., arid Wilke, G., Angew. Chern. 83, 84 (1971); ibid. 84, 120 (1972). 146. Nakamurz, A,, H u l l . Chem. SOC.,J n p . 39, 543 (1966); Xakamura, A., and Hagitiara, N., J . Organotnetal. Cheru. 3, 480 (1965). 147. Otsuka, S., Nakamura, A,, and Tnni, K., J . Chem. Sac. A 2248 (1968). 148. Hughes, 11. P., and Powell, J., J . Organotnetal. Chem. 20, P17 (1969). 149. Ingrosso, G., Immirzi, A,, and Porri, I,., J . Organometul. Chem 60, C35 (1973). 150. Otsuka, S., Tani, K., and Nakamura, A., J . Chem. SOC.A 1404 (1969). 151. Pantini, G., Itacanelli, P., Immiwi, A , , and Porri, L., J . Organometal. Chern. 33, C17 (1971). 152. Otsuka, S., Nakamura, A., and hIinamida, H., Chem. Commun. 191 (1969).

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High Nuclearity M e t a l Carbonyl Clusters P. CHINI, G. LONGONI, and V. G. ALBANO lsfifufo

I. Introduction . 11. Structural Data in the Solid State 111. Structural Data in Solution . IV. Syntheses . . V. Methods of Separation . VI. Reactivity . A. Reduction . B. Oxidation . C. Ligand Substitution . D. Oxidative Addition . VII. Iron Derivatives . VIII. Ruthenium Derivatives . IX. Osmium Derivatives . . X. Cobalt Derivatives . XI. Rhodium Derivatives, . XII. Iridium Derivatives . . XIII. Nickel Derivatives . XIV. Platinum Derivatives. . XV. Bonding Theories . References .

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319 320 322 322 323 324 325 325 327 . 332 . 333 . 334 . 336 . 341

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I INTRODUCTION I n the last 5 years a t least eight reviem concerning polynuclear metal carbonyls have been publishcd ( 1G,18,34,4l , 86, 87, 91, 108), and it may appear unlikely that any new niaterial could be added a t this time. Nevertheless, we have willingly acccytcd thc. invitation of the Editors to present a comprehensive review which, besidc the material published up to June 1974, also summarizes the most, rclevant of our recent results, as yet only published as preliminary notcs. I n order to present a fresh view, we have confined ourselves to compounds containing 5 or more metal atoms. The slow rate of publication in this area is mainly due to the number of steps required by this research, i.e., synthesis, crystallization, structural identification, and chemical characterization. 205

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I n 1943, Hieber and Lagally reported that the reaction of anhydrous rhodium trichloridc with carbon monoxide a t SO'C, under pressure, and in the presence of silver and copper as halogen acccptors, gavc a black crystalline product which, on t he basis of clcnieiital analysis, was formulated as Rh,(CO)11 ( 7 5 ) . Thc exact nature of this compound was established 20 years latcr by Dahl using three-dimensional X-ray analysis which led to its reformulation as Iih6(CO)16( 5 3 ) . This discovery can bc rcgardcd as the birthday of the chemistry of high riuclearity clusters. I n 1962, Dahl had also structurally characterized Fez(CO) 15C,the first high riuclcarity carbide ( 26). This compound was originally prepared in cxtrcmcly low yields (0.57,) by the reaction of FC3(C0)12 with substituted acetylrnes, and, probably dur to the pcwliarity of this synthesis, was considered for some h i e much more a curiosity rather than bring recognized as the precursor of today's large fanlily of carbidc-carbonyl clusters. Finally the hexanuclcar diariion [C'O~(CO),512- was the first anionic high nuclearity cluster to bc isolated ( 3 3 ) .Its discovery in 1967 prompted cxtension of such investigations to other transition mrtals and originated the present chemistry of thc high nuclearity anionic clustcrs. More than fifty different examples of high nuclcarity carbonyl clusters (HNCC) arc prcscntly kiiou-n, all of which contain Group VIII transition metals (Table I). I n post-transition metals thc incrcased separation between the ( n - 1 ) cl and ns-np orbitals is probably responsible for thc low stability of their bonds with the highly a-acidic carbon monoxidc ligand; a number of high nuclrarity clusters with lcss a-acidic ligands, such as tertiary phosphines (17, 50) or organic donor groups (51, 7 2 ) , is known, however. Approximate calculations on some of the more crowded clustcrs, such as [Fe, (CO) 13]'-, Fe6('20)l;C, and Ilus(CO)1BH2, show that, at the Icvc.1 of the carbon atoms, about 96'g of the available surface is occupied ( 9 5 ) . This figure seems very high particularly if one takes into consideration tha t thc distribution of the carbonyl groups is not homogeneous. The high nuclcarity clusters of the transition metals that precede Group VIII are, thercfore, expected t o be destabilized by steric crowding, although some carbides and mixed nitrosyl-carbonyl derivatives should bc sterically possible.

II STRUCTURAL DATA IN THE SOLID STATE The main bond distances found in HNCC are reported in Table 11, which has been divided into three sections, corresponding to the three

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287

transition pcriods, in order t o niakc a comparison of the data easier. Such comparison should be done with sonir caution since the data reported are mean values and since there arc' thc usual uncertainties due to discrepancies among sterically equivalent intcractions (packing cffects, thermal motion, and crystal disorder) . Moreovcr, bccausc thc expanded orbitals of the lowvalcrit transition metals can suff'er considerable dimcnsional variation on changing the electron density at thc m&l atoms, only large differences, or comparison between strictly rclated species, are meaningful.

TABLE I1 BONDDISTANCES IN HIGHNUCLEIRITY METALCARBONYL CLUSTERS

ac-o (8)

(A) Nuclearity

Formula

Idealized and crystallographic symmetry

W

n ~AI-V

(Ax)Teririinai

Edge Face Edge Face bridging bridging Terminal bridging bridging

E

Ref.

(a) Iron, cobalt, and nickel 5 6 6 6 6 8 5

2.64 2.67 2.51 2.50 2.50

5

5 6 9

13. loc 2.34a 3.10c 2.38d 2.770 2. 44d 2.70‘

1.76 1.70 1.74 1.70 1.76 1.72 (1. 76a \1.86b 1 .s9a

-

1.90 -

1.94 1.84 1.91n

-

-

-

-

1.17 -

1.19 1.21 1.15

1.17 1.18 1.15 1.17 1.13 1.16 (1.16“ \1.04b 1.01a

1.07O

-

1.18 1 . 13a

-

26 49 2 3 8 IS 95

-

1.85~

1.82~

1.050

1 . 14a

1.75

1.90

1.13

1.17

-

28

-

-

-

-

-

95

5 0

5

0

>

Z CJ

.
ly long, and the C--0 distances correspondingly short, indicating predoniinantly dative bonding. Deformation and other bonding peculiarities arc often found in HPU’CC, and our present ability mcwly t o describe most of these phenomena gives a n idea of the present, state of thc theory. Prcliminary data 011 the salt [NEt&Mo2Nil( CO) iridicatcl a structure with DShsymmctry bastid on an cquatorial Nii (CO) rhonibus with two Mo(COj4 groups placed above and below the plane (111).It is probable that thcre arc only tcrniinal carbonyl groups.

29 1

High Nuclearity Metal Carbonyl Clusters

Q 0

0

FIG.1. Schematic molecular structrires of the pentanuclear high nuclearity metal carbonyl clusters and of OS~(CO)~S.

As shown by the structurm so far discussed, and as previously pointed out by King ( g o ) ,a triangular network of metal atoms is the most common basic unit in transition metal clust(m, suggesting that bonding between metal atoms in a triangular network is not confincd to the edges but can also occur within the trianglc itself. This hypothesis is reasonable bccause some orbital overlap could still occur at the center of the triangles (1.155 times the metallic radius), whereas there can be little bonding interaction

292

P. CHINI, G. LONGONI, AND V.

G.ALBANO

FIG.2. Schematic molecular structures of some high nuclearity metal carbonyl clusters based on octahedra of metal a t o m (section a).

through the ccnter of a square, where the vertex-center distance is 1.414 times the metallic radius. A good example of the tendency toward triangulated polyhedra is given by the bicapped tetrahedron present in O S ~ ( C O (104). ) ~ ~ In this cluster (complex I V in Fig. l ) , each of the two osmium atoms shared by the basal triangles is directly bonded to 5 other osmium atoms. I n spite of theoretical considerations ( g o ) , the presence of pentaconnected vertices is common in HNCC, indicating bonds of metallic type and implying considerable delocalization. The 0s-0s distances orthogonal t o the symmetry axis, between the atoms of greater metallic character (tetra- and pentazonnected vertices), are considerably shorter (2.74 8 as compared to 2.84 A ) .

High Nuclearity Metal Carbonyl Clusters

b

293

d

FIG.3. Schematic representation of t,he molecular structure of the dianion [Rhn(CO) 3 0 1 ~ .

The octahedron, another triangulated polyhedron, is the most common type in HNCC; about half of the compounds reported in Table I contain octahedra or deformed octahedra of metal atoms. The structures of some octahedral HNCC are reported in Figs. 2-4. The high symmetry of octahedral RuG(CO)~,H,is illustrated in Fig. 2 by structure V ( 4 8 ) .Tricoordinatiori toward carbon monoxide is maintained in Rus(CO)17C, structure V I ( l l S ) , and in the analogous RUG(CO)14 (nirsitylene) C (103) by formation of a carbonyl bridge. The same tendency is also evident in structurc VII, which has been found in the anion [FeG(CO) &]'- ( 4 9 ) , although hcrc the bridges are very unsymmetrical and one of the iron atoms is bondcd to 4 carbon monoxides. All of the other octahedral clusters bearing sixtc~mcarbonyl groups, and also some of their derivatives, have the much more syinmctrical structurc VIII. Typical examples of this stereochemistry are RhG(CO)16( 5 3 ) and the anion [Rhs(CO)i,I]- ( 6 ) . Comparison among the octahedral structures VI-XI (Figs. 2-4) shows that the number of CO ligarids coordinated to each metal atom increases from three to five. This indicates that the metal-ligand and metal-metal systems of bonds are largely independent, a fact which is of considerable theoretical interest (see Section XV) . The formal coordination numbers of the metal atoms in these clustcrs

P. CHIN!, G. LONGONI, AND

294

Q

V. G. ALBANO

PP

n

0

CNi6(C0)6(pz-CO163Z~ (C3vl (XIII

FIG.4. Schematic molecular structures of some high nuclearity metal carbonyl clusters based on octahedra of metal a t o m (section b).

are not only variable but are also unusually high (34, 106). However, their stereochemical significance cannot be compared with that usually accepted in simple compounds, because in HNCC part of the bonds are metallic in character and cannot be represented as simple clectron pair bonds. The related unsymmetrical distribution of this high number of bonds along the directions of the quaternary axes of the octahedra is readily

High Nuclearity Metal Carbonyl Clusters

2 95

apparent in structurc VIII, n h c w thcrc arc two bonds (terminal CO’s) pointing out from the cluster and six bonds (two bridging CO’s and four mctal-metal bonds) pointing ton ard thc cluster. In structure XI the corresponding distribution is 1:8. Thcsc unsymmetrical distributions of the bonds obviously require the prtwncc. of some counterbalancing electron density in the proper directions. The bond distances in specios llhG((’O)16and [Rh6(CO)ljl]- are very similar, the only exception bciiig the terminal CO group accompanying thc iodide ligand. The lth-C distance is 0.02 greater, and C-0 length 0.08 8 shorter, than in the remailling invtal carbonyl fragments, in agrccmerit n i t h a locally lolwr backdotration. Although the dioctahcdral anion [llhls( CO) 30]2-, structure I X of Fig. 3, exhibits mean values for the I)ontlitig interactions similar to those found in the two preceding species, an c.xatnination of the individual distances shows considerable deformations (5). For example, pcrpcndicular to the twofold axis joining thc two tnoivtic~s,the equatorial planes of t,he octahedra are rectangularly dcformcd ni th mean edges 2.68 and 2.54 A. Thcsc deformations h a w been t e nt a t iv~ly~xplainedin several ways (5, 4 0 ) . The infrared spcctra of the solids suggest t hat Co6(CO) 16 and IrG(CO)16 arc isostructural with l t h G ( C o Ib) (36, 96) , and for the former compound this hypothesis is confirmed by thc isomorphism of the crystals ( I ) . The same structure is probably prcwnt it1 the dianion [Hh,(CO) 1,(CN)J2(45). Tetracoordinatioii to\\ ard carbon monoxide is niaintained in the [co,(c0)1,]’- dianion (compkx X in Fig. 4) ( 2 ) and, probably, in the homologous anions [Rh6(CO) j]2- (99) and [IrG(CO)153’- (96). The progressive lengthening of thc Co-C distances (1.74-1.90-2.00 8) for the sequence, term inal-etl!le brzdying-face bridging, and tlic parallel increase in the C-0 values (1.15-1.17-1.19 A ) , are in agreement both 1 1itli the increasing mu1ticentc.r character and related steric request of the mvtal-carbonyl interaction, a i d with the lowering of thc C=O stretching absorptions obscxrved in the Ilt qwctra ( 3 4 ) . Structure XI (Fig. 4) is conimoii to the isoclectronic anions of the salts [NMe~]s[CoaNiz(CO)ll] ( 8 ) , IhcticalFeG(CO) 17C have failed. Finally, 1-electron oxidation of [Fe2Rc(CO)12]- with tropiliuni bromide has been reported t o give a neutral mixcd-metal cluster formulated as [ F C ~ R ~ ( C Oon ) ~the ~ ] ~basis of elemental analyses ( 6 7 ) ;however, its IR spectrum, which shows carbonyl absorptions quit(. similar to those of the starting material, is inconsistent with such a formulation.

Vlll RUTHENIUM DERIVATIVES Reduction of Ru3(CO) 12 depends primarily on the experimental conditions. Reduction with (>ithersodium borohydridc or sodium amalgam in T H F , or with methanolic potassium hydroxide, followed by acidification, has been rrported t o give thc tetranuclear species Ru4(CO)13H2 and R U . , ( C O ) ~ ~( 8H2~) . On t h c x other hand, reduction in T H F with carbonylmctalatcs such as [Rlii (0 5]- ) and [Cpl'"l(CO) 3]- gives, aftcr acidification, a more complicated mixture, which has h e n successfully separated into its components by srlcctivci rxtraction. After elimination of some tctranuclear hydridc derivatives by dissolution in light petroleum, the sparingly solublc hexaniiclear Rug(CO) 18H2 was isolatcld by extraction in dichloromethane ( 4 8 ) . Its parcnt dianion, [Ru6(CO) 18]2-, although prcdictcd theorctically ( 1 2 0 ) ,has been neither isolatcld nor observed even in solution. Ruthenium carbide-carbonyl clusters h a w been obtained through pyrolysis reactions. Thus, by heating 12u4(CO)12H1 at 130°C in the prescrice of ethylene (10-12 atin), trace quantities of Ituj(CO)& have been obtained along with higher yields (30%) of I ~ U ~ ( C O )(59). ~ ~ CThc latter is, however, bcttcr synthesized by direct pyrolysis of Ru3(CO) 12 in di-nbutyl ether (85). Heating Itu3(CO)l2 in aromatic hydrocarbons, such as benzene, toluene, 1?1-~ylcn(~, or nirsitylenc, gives a mixture of RUG(CO) 17C and of thcrAr-arene derivatives HuG(CO)14(arene)c', which in the last solvent is the major product (58,83). Monosubstitutcd derivatives RuG(CO) 1 6 ( L) C [L = PPh3, P (p-FC'eHd)s, AsPha] have bwn obtained by boiling Ru6(CO) 17C in hcxanc with cxc(w tertiary phosphincs or arsiiics (85).

High Nuclearity Metal Carbonyl Clusters

325

IX OSMIUM DERIVATIVES Pyrolysis at 200°C of Osa(C'O)12 in a scaled, evacuated tube afforded a mixture of at least se wn diffcrcwt carbonyl clusters which could be scparated by thin-layer chroniatographj . In addition t o some unrcactcd O S ~ ( C O ) ~the ~ , new compounds, O S ~ ( C O ) ~O~S, ~ ( C O ) ~OsB(CO)18, ~, Os8(C O ) Z ~and , Os8(CO)zlC', w ( w identified by mass spectroscopy (58); the last compound was originally formulated as Os5(CO) ( 6 1 ) .Further pyrolysis of Os6(CO)18 at 255°C' givcs thc pentanuclear carbide dcrivativc, in 40y0 yicld ( 5 9 ). Os5(CO) I n the presence of trace quatititics o f water, pyrolysis of Osl(CO)12 at 230°C results in a mixture of hkdrid(1 carbonyl clusters. Thc new dcmva' tives, O S ; ( C O ) ~ ~ HOs5(CO)16He, ~, O S ~ ( C O ) ~ ~and H ~ ,O S ~ ( C O ) ~ ~Hz, (C) along with the known compl(bxc1i O h l (('0) loH(OH) , OsI(C0) 13H2, and Osl(C0) 12H4 ( G O ) , have bccm scparatcd by thin-layer chromatography and identified by mass spcctroscopy, as twforr. The IR spcctra of all of t h ( w (,ompounds do not shorn absorptions due to bridging carbonyl groups, and thc NRIR spectra indicate that th(1 hydrogen atoms arc always in hritlging positions. Future structural determinations of such an imprcssivc, swics of polynuclcar derivatives will make a considcrablc contribution t o tlw chmiistry of high nuclcarity clusters. Until now only thr structurcs of Os,,(('O) 18 and O S ~ ( C Ohave ) ~ ~been dctermined (104).

X COBALT DERIVATIVES Scheme 1 summarizes thr syrithcws, arid the most significant reactions, of the [ C ' O ~ ( C O ) ~ ~dianion, ]~which can he considered the key species in cobalt HNCC chemistry.

326

P. CHINI, G. LONGONI, AND

V. G. ALBANO

Yellow-green [Co, (CO)1j]2- is conveniently synthesized in high yields (80-90%) by merely heating under vacuum an cthanolic solution of [Co ( EtOH) z][Co (CO).J2, obtained by the reaction of Coz (CO) with ethanol ( 3 5 ) . An altcrnativc route t o this could be the reduction of Co4(CO)12 with alkali metals ( 3 7 ) ,

although this is less coiivmient both because of the simultaneous formation of a large quantity of tctracarbonylcobaltate and of the further facile reduction to the tetra-anion [Co,(CO) ,1y- according t o Eq. ( 2 2 ) . Cobaltocene behaves as a toluene-soluble pseudoalkali metal, and in this nonpolar solvent ionic products precipitate out as soon as they are formed. Thus, the reduction of Coa (CO)12 with cobaltocenc in toluene gives a n intermediate that analyzes as [CoCp2][Co4 (CO) 10-111, and which is probably dimcric. Unfortunately, its lability has prevented further characterization (37). Protonation of [Co6 (CO) 15]2- a t - 70°C gives a new unstable species, for which thc analyl ical and spectral data agree with the formula [Co,(CO)i&]- (33). A logical extension of the synthesis of [Co,(CO) 13-J- from [Co(EtOH).] [Co (CO) ,I2 has been successfully applied to the preparation of mixedmetal clusters. Thermal decomposition of [Ni( EtOH) z][Co (CO)q]t prepared in situ gives the red hexanuclcar dianion [Ni2C04(CO)1J2-, through the following redox condensation and redistribution processes (44):

+

+

~ [ K ~ ( E ~ O H ) , ] [ C O ( C O2[Co(CO)a]---t2[?;1C03(CO)ii])~]~ 22:IZtOH

+ 2CO (45)

=[N12C04(CO),4]z-+ Co2(CO)s

2[NiCo3(CO),,]-

(46)

The whole process can be represented schematically by 2L1*+

+ GR.l--2Rf4-~;ZlaZ-

The deep red tetra-anion [Co,(CO) direct reduction of Co4(CO) 12 ( 3 7 ):

14]4-

(11

+

$12

(47)

is conveniently synthesized by

=

Id, Sa, I coiisidcrcd a definite intvrmediate. In fact, it has a characteristic and rc~produciblcIR spectrum, and at -70°C it has a peculiar magiictic resoriancc spectrum consisting of two doublets (191.7 and 208.3 ppin) and a rnultiplct (247 ppm) ( 7 4 ) .

High Nuclearity Metal Carbonyl Clusters

33 1

As shown in Scheme 4, thc chemistry of the violct dianion [Rh12(CO)30]2is relatively simple: the main fcaturcl is the easr of rupture of the Rh-Rh bond connecting the two octahcdrn. The best synthesis of the green hexanuclear dianion [Kh,j( CO) 1512- involvos the reduction of ILh6(CO)16under nitrogrn with a stoichiometric amount of alkali (99) :

+

I i l 1 6 ( ~ o ) , f i 40H-

-

Nz,Me011

+ c o r + 2H,O

[lths(C0)1a12-

(59)

This should bc contrasted with the reduction undrr carbon monoxide, which is complicated because of concurrent degradation and redox condensation reactions [reprcscnted by Eqs. (6) and (14)] arid which gives rise to the heptanuclear anion [I& (C'O) 16y-. Dianioii [Rh6(CO)15]2- is qnitcl rvactive, as shown in Scheme 5. Most of thrsc reactions, as wcll as thr. prckparations of the [Rh7(CO)16?- and [Ith6(CO) 14y-anions, Eq. ( 3 ) and (24), h a w already bccn discussed in previous parts of this review. Although the [Rh12(CO)so]z- dianion is modcratcly air-stable, solutions of thc. more reduced anions [Rh6(CO) 13y-, [Rh7(CO) 16]3-, and [1Zh6(CO) 1 4 1 4 - arc cxtrernely air-sensitive.

Occasionally during the synthesis of the [IthT(CO) trianion, low yields of a yellow anion, which was formulated on thc basis of elemental analyses as [Rh3 (CO) lo]-, w w also obtained (39). Subsequent X-ray structural investigation showcd it to bv [Ithe( CO) &I2- and, furthermore, suggested that its casual formation could be dur to the accidental presence in the reaction medium of trace amounts of chloroform ( 7 ) . As a result, this compound is now readily available in high yields (80-90%) by deliberate addition of small amounts of chloroform to the reaction mixture. The following reaction accounts for thc apparent stoichiometry :

332

P. CHINI, G. LONGONI, AND V. G. ALBANO

The mechanism probably involves a multistep reaction, with initial formation of [Rh7(C0)l6-J3-, degradation to [Rh(CO) 4]-, and condensation of this anion with the chloroform (10, 23). The [Rh6(CO)&]2dianion is the precursor of a wide series of carbide-carbonyl derivatives. Scheme 6 shows the compounds so far structurally characterized and the necessary conditions for thcir syntheses (9: I S ) .

Several other species have been isolated in the crystalline state and are presently awaiting definite characterization. Great difficulties arise from the fact that these derivatives are only sparingly soluble in inert solvents and often react with polar solvcnts [see for instance Eq. (37)]. The octanuclear carbide Rh,(CO j l& polymerizes T H F a t room temperature (102).

XI1 IRIDIUM DERIVATIVES Unlike cobalt and rhodium, the chemistry of polynuclear iridium carbony1 derivatives has not been studicd in detail (15a). Reduction of Irq(CO) 12 under carbon monoxide with K&O3 in methanol gives the yellow tetranuclear hydride derivative [Ir4 (CO)llH]-, whereas under nitrogen the brown dianion [Irs(CO)20]2- has been isolated as a tetraalkylammonium salt (97). It has been suggestcxd that the structure of the dianion could result from the linking of two iridium tetrahedra, although its formulation so far is based only on elemental analyses. Clearly such a n interesting compound deserves further chemical and structural characterization. The reduction of Ir4(CO)12by sodium metal in THF under carbon monoxide gives hexanuclear [Ire( CO) Its I R spectrum compares well ] ~ - [Rh6(CO)1512-. As with those of the analogous dianions [ C O ~ ( C O ) ~ Sand previously shown [see Eq. (29) 1, dianion [IT6( CO) 15]2- reacts with acetic

High Nuclearity Metal Carbonyl Clusters

333

acid under carbon monoxide t o givc red crystals of Ir6(CO)16(96). This hexanuclear species is much less stable than Irl (CO) 12, and its chemistry has not yet been studied.

Xlll NICKEL DERIVATIVES There is spectroscopic evidencc that the reduction of Ni(CO)4 gives rise to a large number of products, most of which have still t o be isolated and characterized. Results obtaiiwd so far from a recent reinvestigation of this chemistry (95) suggest that all of the formulations previously reported in the literature are incorrect (76, 77, f 1 6 ) . Reduction under nitrogen of tc%racarbonyl nickel with alkali metals or sodium and lithium amalgams ( 7 f ) in THF, or with alkali hydroxides in methanol, gives a mixture of the dianions [Ni,(CO)12]2- and [NiG(CO)1J2(68, 95). The final composition of thv reaction mixture greatly depends on the experimental conditions owing to the easily reversed equilibrium : "1,(C0),2]2-

+ NI(C'o)4===[S1,(CO),,]'- + 4co

(61)

The yellow pentanuclear dianion, [Ni, (CO)12]2-, is rather labile and has been isolated in a pure state only as the bis (triphenylphosphine)iminium salt by crystallization under carbon monoxide in anhydrous solvents. I n wet solvents, it reacts readily with carbon monoxide to give a mixture of tetracarbonylnickel and an unstable hydride derivative presently formulated as [Ni(CO)3H]- ( T = 18.3), by comparison of its I R spectrum with those of [Ni (CO)3x1- ( X = C'I, Hr, I) (31). This reaction contrasts with that of [Ni,(C0)12]2- with water under nitrogen when the red dianion [Nis(CO) 1212- is formed. Thc. rvaction proceeds with formation of tetracarbonylnickel, hydrogen and traces of carbon monoxide and its stoichiometry is believed to be the following: 3[N1s(C0)1,]'-

+ 2H2O -2[Y16(COj,i]'-

+ Hi + 20H- + aNl(co),

(62)

The mechanism should involve an ~ a s yinitial protonation t o give a hydride intermediate which t h m loscs hydrogen and condenses to [Nis(CO) 1~]~-. Equation (62) explains why prcxcipitation from the original reaction mixture by addition of an aqueous solution of tctraalkylammonium salts always gives the red hexanuclcar dianion [NiG(CO) 12]2- (yields up to 60%). A comparison of its I R spoctrum with that reported in the literature'for [Ni4(C0)9]2- (76) strongly suggests that the latter should be re-

334

P. CHINI, G. LONGONI, AND V. G. ALBANO

formulated as [&(CO) 12]*-. Our present kriowlcdge of the reactivity of this hexanuclcar dianion is summarized in Scheme 7. Dianion [Ni6(CO)1 2 7 reacts slowly under vacuum with Ni(CO)4, to give the dark-red cnneanuclcar [Ni,(CO) 1J2-. The latter may, however, be better synthesized by oxidation of the hcxanuclear complex with a stoichiomctric amount of Ni( E t 0H ) &'lz. Hydrolysis of [Ni6(cO)12]2- takes place only in an acidic medium arid gives, depending on thc pH, two hydride derivatives, presently formulated as [Nix(CO)n4H2]2and [Nill(C'O)zoH2]2-, but not yet structurally characterized. Such formulations are bascd on elemental analyses and, in the case of violrt [Nill(CO)20Hz7-, also on a molecular weight calculated from unit cell m d density measurements.

It is also worth noting that the ease of hydrolysis decreases in the series: Nis > Ni6 > Nis > Nill, as expected for a progressively higher delocalization of charge and consequent lowering of the nucleophilicity of the cluster. Finally mixed-metal clusters with formulas [M2Ni3(CO)16]2- (M = Cr, Mo, W) and [M2Ni4(CO) 1 3 ( M = Mo) have been isolated by Ruff by with Ni (CO) condensation of the corresponding dianions [Mz (CO) in refluxing T H F (yields 30-687,) (1I 1 ) .

XIV

PLATINUM DERIVATIVES An insoluble compound formulated as [Pt (GO)J n has been obtained by Booth and Chatt both by hydrolysis of Pt (CO)&lz in benzene and by carbonylation of Na2PtCI4in ethanol (19, 20). More recently, reductive carbonylation of Na2PtCl6.6HzO in methanol, in the presence of alkali acetates or hydroxide, has been shown to proceed

335

High Nuclearity Metal Carbonyl Clusters

according to the following reaction schc.me (29,95) :

-[rt(co)cl,]-- o w , CO

OH-, CO

[Pt(C(>)L]nor [i’t3(CO)6ln2-

[PtCl$

OIL-, CO

OII-,

co

[PtlS(C0)38]2-

(7L

> 6)

OH-, CO

[€’t15~~Y))dol~-

olive-green

yc~llo\\-grceii

o w , CO

OH-, CO

[l’tl?(co)2412-

[ I’t ,((Y )), s]z-

[l’t ,(CO) 1212-

blue-green

Vl(J1f’t-Ihelead mirror was not formed, but a white precipitate, MeaPbX, separated and the spectrum of R was obtained ( 4 2 ) . The problem of radical recombination is overcome if the radicals are isolated from one another by a n inert matrix. This usually lcads to anisotropic spectra, but the adamantane technique, in which the trapped radicals are free to rotate and thereby give isotropic spectra, has been successfully applied to the study of &n(Me).C13-, ( n = 0, 1, 2 , or 3) and &Me3 (143). Othcr solid state techniques that have brrn used to generate Group I V radicals include y irradiation for 6eMcs using GcMe4 in a matrix (1457, and a procedure involving a rotating cryostat, for &Me3 and i)bMe3 from ClMMe3 and Na ( I S ) . Transicnt cationic species, c.g., [ P b h h l t , are accessible by one-electron oxidation, e.g., from PblLlc4-[IrC16]z- (129). b. Structure of Radicals. Whercas the mcthyl radical 6Ha is planar, there are now much data suggesting that all other Group IV radicals show varying degrees of deviation from planarity. The main line of evidence uses the hyperfinc coupling of the unpaired electron to those isotopes of the central atom that possess nonzero nuclear spin (29Si,73Ge, lI7Sn, and lI9Sn).For a planar radical, the unpaired electron occupies a pure p , orbital which has a node a t the ccntral atom, whereas for a pyramidal radical the odd clcctron is in a hybrid orbital. The magnitude of the isotropic coupling depends on the amount of s character in the orbital containing the unpaired electron. Consequently, the more pyramidal the radical, the more s character in the orbital of the odd electron and, hence, the greater is the hyperfine coupling. I n general the greater the diff erence in electronegativity between the central atom and thc atoms bonded to it, thc more pyramidal the radical. For cxample, in the series $iMe,(SiMe3)3-,, as methyl groups are replaced by less electroncgative SiMe3 groups there is a trcnd toward the more planar structures ( 4 1 ) .From thc a ( M ) data of Table I11 and on MH3,$1hTe3, or MCl,, trends are summarized in Scheme 1. c. Stability of Radicals. The trityl radical, 6Ph3, is perhaps the bestknown stablc radical of Group IV and was long thought to cxist in solution equilibrium with its symmetrical dimcr, hexaphenylethane. It has now been established (138),however, that therc is the following equilibrium:

Dimerization is presumably prevented by crowding around the ccntral carbon. Morc reccnt results, including those of Sections 11, A, 2 and 111, demonstrate the importance of steric hindrance to dimerization in con-

Free Radicals

CH3, CP\le3

All show marked deviations from planarity ilpproximately planar

Iiicreabiiigly pyramidal

Approximately tetrahedral

a Electronegativity a d s in opposition t o a steric effect; hoaever the former is clearly dominant despite the considerable bulk of It and It'.

SCHEME 1. Variations in geometry around t,he central metal M in some Group I V radicals &lX3[X = 11, CI, R, or XR;;It = (lIe3Si)2CII;R' = MeaSi]

tributing to radical stability. 0thc.r factors, such as the nonavailability of disproportionation pathways and t h r possibility of delocalization into silicon d orbitals, may contribute t o stability. 2.

Stable Group IV Element-Centered Radicals

This work (44,46, 142) originated in an attempt to establish whether the interesting compound SriItz [R = (MeaSi)&H] (47) (formally analogous t o a carbene) exists in a singlet or triplet ground state. No evidence was found from ESR spectra of liquid or frozen ( - 110"*) solutions for the latter, but a weak signal near g = 2 was detected. It had quartct structure, suggesting $nR3, since the threc equivalent a protons of 8n[CH(SiMea)2]3 would give rise to a 1:3: 3 : 1 quartet. Unambiguous assignment, through ' n satellites, was not then possible due to the low detection of I17Sn and "!S concentration of the species. The presence of the radical was initially attributed to reaction of Snlh with traces of oxygen, but later work showed that irradiation of the solution with UV or visible light caused ft dramatic

* Trmperatures

are all Centigr:de unless otherwise noted.

ELECTRON S P I N RESONAXEP A R A h l E T E R S

TABLE I11 STABLE GROUPIv TRIS(ALKYL) ASD

FOR THE

TRIS(A4hlIDO)

R.4DICALSa

or

Radicalb

Reactantsc

Radiationd Solvent

uv

SiRB

-

GeC12 diox/LiR

g

a(?l')e

2.0027

0.48

19.3

CsHs

2.0078

0.38

9.2

SnC12/LiR j

uv Or ViS.

CsHs

Ge(NR;)3

GeClz-diox/LiiYR;

UV

n-CeH1,

1.9991

Sn(SR;) 3

SnC12/LiNR;

uv

n-CsH14

1.9912

Ge(SBu 1R')3"

Ge(ICButR')2

n-C,Hi,

1.9998

1.29

Sn(NButRf)p

Sn(NBulRf)z

uv uv

n-CsHi,

1.9928

1.27

SnR3

SnR2

UV

n-CsHi4

2.0094

+ 6Sn(NRi)zq

Stability a t 20" in solutionh

u(M)cJ.g

C,H,

SnR3

r

7

4HY

Synthesis

tli2

17.1 317.6 ( l l F h ) 342.6 (1lg9n)

{

17.3

tliz f112

tliz f1/2

169.8 ("7Sn) 177.6 (Il9Sn)

-

10 min a t 30"

1 year

> 5 months

-

Q

rn W --I

Unchanged after 4 months

169.8 (ll'Sn) t l l z 177.6 (119Sn) 1.06

-

5 Q

3 months 5 minp 5 minp

e

Data from Refs. 44, 46, 14%. = (Me3Si)&H; R’ = MeaSi. c R = (Me3Si)zCH, R’ = MeaSi. d Under Ar or a vacuum. In mT. f BSi, 76Ge, 1%n, or 119Sn. 0 Corrected for second-order shift (Eq. (7). Based on ESR signal strength of a light-protected, sealed sample. i None required. j Or SnR2. Or Ge(NR:)2. Or Sn( N&’)2. 81-2.016; 911-1.994. n Reference 14%. Reference 95. The reason for the lower stahility of these radicals compared with M(SRL), is uncertain; from 9 and a(Ge) values on the Ge radicals, bond angles are probably similar; the generation of & I ( S B U ~ Rwas ’ ) ~not as clean as of &I(YRiIa, and other paramagnetic. species were sometimes detected, possibly KUu tR’. This was an attempt a t a crossover experiment, possibly relevant to the mechanism of photolysis, Eqs. (8)-(11); ii trace of Sn(SR;)a was also present, but not detected with 1 : 1 SnR2:Sn(NR:)z, nor any mixed alkyl-amido-radical (142). Irradiation of Sn(XR:)2 with Sn(CsH,-q)z yielded Sn(SR:)(C;H;-q), but no paramagnetic species. Radical formation was also not detected by UV irradiation of Sn(CsH6-q)z,SnIz, SnCl(NR:), or Zn(NR,’)z. a

bR

O

s 0

n p.

358

M. F. LAPPERT AND P. W. LEDNOR

increase in the signal strength. Neither type of irradiation caused deposition of a tin mirror, and it was also demonstrated that (i) heat did not cause any increase in signal strength (irradiation of a sample without concomitant cooling raises the temperature considerably), and (ii) a sample prepared in the dark gave n o ESIt signal, but irradiation generated the radical. The intensity o f the signal obtaincd on irradiation allow-1.d idcntification of satellite pcaks due t o 117Sn and l%n, confirming formulation of the radical as $11113 ( 4 6 ) . Estcnsioii t o thc related f i e & and to isoelcctronic amidcs, ii1 (Mi:) (M = GP or Sn; R' = McBSi) \\as carricd out b y reacting thc nietal(I1) chloride nith the apprcq)riatc. lithium rwgent and irradiating a solution of the product or, alternatively, GrIZ2 (93) or M ( S K 3 2 (92). For the Sicc.1itcw.d radical, 8iR3 (S(Y Fig. 1),a diffcrent routc was r q u i r d sinw suitablr Si (11) species are unknown, excclpt as short-lived intermediates. Compound Si2CI, was reacted with LiR ni th the view to forming K3SiSiX, (X = R3, C13, %PI, or RC12) which would thcn be expected to fragment readily to gilt3. This radical n as obtained from SizClsand Lilt, followed by irradiation, but the compound isolated from thr wactioii, (SiC'12R)2,suggestcd a diff ( w r i t mechanism for radical formation, pwhaps via R2SiC1SiC'Is. IN situ UV irradiation of a solution of PbH2 ( 4 7 ) at 20" gave a complex spectrum containing lines attributable to ( Mc3Si)&H, other paramagnctic species, and a lead mirror. [Assignment was confirmed by generating the samc radical from the low-tcnipcrature ( -40") irradiation of ( ButO)2 and (Me3Si)2CH2: doublct of multiplcts, a ( a - H ) = 1.89 mT, a(-y-H) = 0.037 niT.1 Irradiation of a solution of Pb(NKi)2 (92) with visiblc or UV light at 20", or UV at -40", gave no signals. Some decomposition of the sample appeared to occur. The main feature of thc ESR spectra (e.g., Fig. 1) of these metal-centered radicals is a inultipht arising from the coupling of the unpaircd electron to three equivalent protons (quartet) or three equivalent nitrogen riuclci (septet). For dilute solutions of the amido radicals, the septets showed further structure, attributed t o partially resolved proton coupling. Under conditions of higher gain, satellite lines from those isotopes of the central atom that possess nonzero spin were obsc>rved(nucleus, percent abundance, nuclear spin: "Si, 4.7, 3; 73Ge,7.6, 4;117Sn,7.7, +; and l19Sn, 8.7, $). The low abundance of thcsc: isotopes makes detection of the satellite lines difficult, but the intensity of these was increased by using high microwave poww (c.g., 50 mW) and high modulation amplitude (e.g., 0.5 mT). (For C-centered radicals, values such as these can lead to saturation or loss in resolution, rcspcctivcly.) For h R 3 , the satellite lines were very broad but could be sharpened by an increase in temperature. The width of the lines

359

Free Radicals

is attributed to incomplete averaging of the anisotropic contribution to the and hyperfine tensors, caused b y slow tumbling of the radical. Raising the tenipcrature increases the rntc of tumbling so that the anisotropy is averaged to zero and the spectrum becoines isotropic. Measurement of the q va1uc.s [rclativc t o polycrystallirie diphenylpicrylhydraxyl ( D P P H ) ] and thck CY proton [ a ( H ) ] or nitrogen [a(N)] couplings, was straightfor\\ ard, hiit determination of the central atom hyperfine coupling was not, and requires further comment. It is s w n from the spectra (c.g., Fig. 1) t h a t the satellite lines are not synimctrical about the central niultiplct; the satellites are shifted downfield, but not b y equal amounts. This is a second-order effect and results from thc breakdown of the “high-field approximation” in the theory of coupling constants. This approximation assumes no coupling between the spin of the electron and the spin of the nucleus, so that when the formcr is reversed, the latter rclmains unc.haiiged. Under conditions of low magnetic field or large hyperfine coupling, this is no longer true and results in a nonlinear divergence of energy levcls as the field increases and, hence, an unequal separation of lines in thc spectrum. (More rigorous explanations of this effect are given in Refs. 84 and 8 . ) Corrected values of the coupling constants arc obtained from the observed spacings by application of t h e Breit-Rabi equation (for I = i), n(M)

=

2Ho(Ho - H d 2Ho - I i k

a(J1)

=

2Ho(Hz - Ha) 2Ho - H I

(7)

where a ( M ) is the corrected coupling constant, H o the field position of the central line, and Hk or Hl the field position of the low- or high-field satellite, respectively (104). Since each nucleus (*%, “’Sn, and lI9Sn) gives rise to a pair of lines (at H k and H l ) , two values of t h e couplings are obtained in

360

M. F. LAPPERT AND

P. W. LEDNOR

each case, which serves as an intrrnal check. For the germanium-centered radicals, thc Brcit-Rabi equation was used in the form of Ref. 168. Computer analysis providcs the position of thc satellite lines using a trial value of thc coupling constant and the observed ficld position of thc central multiplet. Thc value of the coupling constant is varied until the calculated line positions agrce with the measured ones. The results are included in Table 111. Stability nieasuremcnts on thc radicals AX3 were madc using samples in sealed tubes, protected from light and stored a t ambient temperature. Spectra wcrr rccordcd pcriodically, and stabilitics cstimatcd from the decrease in signal strength (we Table 111). T h r radical &It3decayed in benzene with a half-life of about 10 minutes a t ca. 30". The decay curve was measurcd using the spectrometer to plot peak hcight against time. Thc value of tllz remained constant over five half-lives, thereby showing that the radical decayrd with first-order or pseudo-first-ordcr kinctics. The decay of &R3 appeared to bc revcrsiblr in benzcnc but not in hexane. Irradiation of the sample in C6H6caused formation of the radical up to a constant maximum intmsity; shutting off thc light led to complete decay. This cycle of formation-dccay was repeated several times but does not prove unambiguously that the radical decayed rcversibly. Electron spin resonarm' data obtaincd on MR3 and a ( N R 4 ) 3 arc listed in Table 111. For comparison, rcsults on MMe3 are: &Me& solution, g, = 2.0031, a ( H ) = 0.634 mT, a ( M ) = 18.3 mT ( 1 5 ) ; GcMe3, matrix, g,,, = 2.0101, a(H) = 0.53 mT, a(M) = 8.47 mT ( 1 4 3 ) ;&Me3, matrix, g,,, = 2.0163, a(H) = 0.25 mT, a ( M ) = 153.0 (lI7Sn) and 161.1 (Il9Sn) mT ( I S ) ; i)bMe3, matrix, g,*, = 2.0389, a ( M ) = 185.0 mT ( I S ) . The mechanism for 3x3 formation from photolysis of MX2 [MXz = GeR2, SnIh, Gc(NRi)p, or Sn(NR:)2] may follow cither of two routes (shown for MX2 = SnItz) ( 4 6 ) , SnIt*hv R It

+ SnR,

-

+ SnIt

SnR3

or SnRz (Snit,)*

+ SnRz

-

(SnRz) *

SnR3

+ SnR

(10)

(11)

I n the first mechanism, an M-X bond is homolyzed and the resultant radical X is trapped by another molecule of MXz. [Unsuccessful attempts to provide evidence for this proposition were experiments (i)-(iii) ; the radical precursors ( B U ~ O Nand ) ~ AIBN, as well as the inhibitor galvinoxyl, reacted with SnRz at 20" to give diamagnetic solutions; whereas, in ex-

36 1

Free Radicals

pcrimcnt (iv), P h 3 e did not react (1.42).] Alternatively, MX3 is formed from a bimolecular reaction bctwecn an excited state of MXZ (possibly triplet) and a ground state MX2. Both mechanisms require formation of a M ( I ) species: since no metal devrlops on photolysis, MX must react with solvent or form a soluble diamagnetic oligomer. It is possible that SIR, is formcd according to the following reactions:

c1 c1 SiiC16

+ GLiR-R-Si-Si-It

I

1

I

I

c1 c1 I

/

1

1

+ It-Si-Si-Cl

(‘1 c1 isolattd

( + other products?)

(12)

R C1 pxtulste d

c1 (‘1 I

,

I / 11 c:1

2SiR1”Y-SilL

+ 1 (Sin), -

n

Supporting evidence for Eq. (13) is provided by (109) /(si’fe*)4\

MezSi-

x s i M e z Si?fe2

+ (SiMe2)

I)

(15)

The postulated SiMcz was trappcid as an insertion product, but SizMeswas not isolated (which might h a w brcn expected if the silylene itself photolyzed t o &Me3). I n situ irradiation of (SiMez)6(toluene, -60”) did not lead t o ESR detection of $iMc3 (142). However, for Sinz the bulky R groups may stabilize the silylenc sufficiently for photolysis to biR3 to be favored over alternative reactions. There is precedent for such rcdox reactions in transition metal chemistry, e.g. ( S ) , (TiX3)2-TiS4

+ -n1 ( T I X ~ ) ~ - X

=

NMei

although photochemical disproportions are less common (25) (but see Section 11, B) . The g values for MRs increase down the group (Table 111), the same trend as is found for &IMe3 and MH3. It is attributed to the parallel increase in the M-spin-orbit coupling constant, because, in general, Ag is proportional t o E/AE, where Ag is the difference between the measured g and the free-spin value of 2.0023, E is the spin-orbit coupling constant,

362

M. F. LAPPERT AND P. W. LEDNOR

and AE is the diffcrcncc bctwccli ground-state and excited-state energies. However, Q [ & ( N K ~ ) ~ > ] Q [ ~ ( N R . ~ (Table ) ~ ] 111); cf., y(6eC13) > g($nC13). These findings may be due to delocalization of the unpaired electron into ligand T-type orbitals (142, 243). Anisotropy in valuchs was only found for $nlt3 (Table 111). The Q values of the aniide radicals arc less than those of corresponding alkyls, coiisistcnt with the former having niorc pyramidal structures; bcnding of an x f X 3 radical mixes exeittd states into the Lvave function for the unpaired electron and, thus, loncm the Q valuc.. The methine splittings a(H) for the radicals ~‘I[CH(Sih4e3)2]3arc close to those for XI (CH3)3. The niost notable fcaturc is the large diffcrcncc between C(CH3)3[a(H) = 2.25 mT] and the analogous Si, Gr, and Sn species [a(H) = 0.634, 0.53, and 0.275 inT, rcspectivcly; see Tablc 1111. This differcncc is a t t r i h u t d to (i) the niorc pyramidal gwmctry of the Si, Ge, Sn, (and Pb) radicals; (ii) th(x reluctance of thc heavier clcnicnts to form niultiplc bonds; and (iii) thc greater size of the heavier elements cornpared with C:. All t hrec factors reduce hyperconjugative coupling, which is believed to bc thc main cause of such splittings in simple alkyl radicals (6). 111 this procms there is a contribution to thc bonding from a structure in which thc uripaircd clcctron couples with onr of the electrons in a C-H bonding orbital, H

H. M-C

(16)

-M=C

The extreme longevity of radicals 6TR3 and Fh(NRi)3 (Tablc 111) must be mainly due to stcric hindrance to dimerization (44, 4 6 ) . The low values of the M-H bond strengths [D(M-I-I): C‘, 104; Si, 81; Ge, 73; Sn, 70 kcal mole-’] (110) do not favor H abstraction from the C-H bonds of the solvent, but the shorter lifetimes of the $11 (NR:) and Ge (NK;) radicals in hcxane compared to hnK3 and &R3 in beiizerie may reflect the ease of H abstraction from thc two solvents. A third factor, for t h r tris(alky1) radicals is the low probability of disproportionation, such as that shown for a carbon-centered radical,

because stable, double-bonded compounds of the heavier Group I V clcments arc unknown.

363

Free Radicals

The first-order decay of radical &R3 in benzene implies reaction with solvent or an intramolecular rearrangement, such as

SiMe,

CH,-Si(Me,)-C(H)

I

(SiMe,)-Si[

CH(SiMe,),],

Nonradical products

It is interesting that the stability, unlike the geometry, of these radicals k X 3 is so sensitive to steric effrcts. Bulky groups X thus hinder the formation of a four-coordinate X3hl-MX3 or X3M-H and favor the threecoordinate A X 3 ( 4 4 ) .A similar vffect has been noted for transition metal MR3 and M (NR:) 3 complexes ( 4 8 ) .

B. Transition Metal Compounds Many paramagnetic organo-transition metal complexes are stable under ambient conditions. Relevant ligands include CO, It-, olefin, q-C5Hs-, or q-arene, and compounds may bc iirutral, t'.g., [V(CO)J, [Cr (CH2SiMe3)4], [Cr { CH (SiMe3)2 131, and Ta('lz ( C5H5-q)2 1 , anionic, c.g., [Os3 (CO) &, [Cr ( CH2SiMe3)J-,and [Tir12 ( (':H5-q) 2]-, or cationic, e.g., [Cr (ArHq ) 2 ] + . These owe thcir stability, in part, t o electron delocalization for .rr-bonded ligands; whereas for t l i v alhyls, it is duc to stmic effects (effectively making t h r mrtal coordinativrly saturated) and the use of ligands that do not allow normal dccomposition pathways to br accessible (e.g., p elimination from M--R if R- Iias no P-hydrogen). Various recrnt reviews are available (48, 49, 55, 79, 80, 8 4 ) , but a representative serirs of compounds is included in Table IV. 13xpcriincntal results are conccrncd with preparative methods (e.g., all.;ali-nictal or elcctrochemical reduction in donor solvents for radical anions), and bulk magnetic susceptibility, ESlt measurements, or clrctronic spvctra. Thcsc have, in grneral, becn unexceptional. For vxample, many of t h c h alkyls of first-row transition mctals have magnetic moments c l o s ~to spin-only valurs, and ESIt spectra arc' consistent n ith clectronic structure and gcomctry, e.g., tetrahedral and trigonal for thc local Cr environment of ('rl *213LltO

+

R U ~ O [i'tn,(i~~r:)~l-[i't~t(o~~ (PR:) ~)

+ iz

(42)

with the latter representing an SH2 process at Pt (11). Confirmation of this result was sought by attempting to synthesize a Pt(1I)-t-butoxide, as in Ey. (42), in various ways: photolytically from Bu402 using trans-[PtMe( I3r) ( PPh3)2] or cis-[PtMe2 ( PEt3)21; thermally from ( B U ~ O Nand ) ~ ~ i s - [ P t M c ~ ( P l l ~()R~ ]= Et or P h ) ; or by an attempted chain reaction using BulOCl and cis-[PtMez(PR3)2] [cf. Eqs. (28) and (29)1.These were unsuccessful. However, compounds containing Pt (11)-0 bonds are rare and largely limited t o complexes having chelating ligands ( 1 4 ) ,which may achicve kinetic stability by imposing a conformation on the alkoxide that is unfavorable t o normal facile decomposition pathways, such as p elimination. Clearly further work is required using a system suitable for monitoring preparative-scale experiments with ESR studies.

M. F. LAPPERT A N D P. W. LEDNOR

378

C I

FIG.3. The ESIt spectrum generated from eis-[Pt;1\len(PBu;)~], (puQN)z, and BuLNO in CJI, at 40" [a = Buf(fiO)Me,b = Bu'(NO)OBu', c = Bu:NO].

' PhS

+ [Pt (CR2SiMe3)2(P;1IeJ'h)

t

23

[Pt (CH2SMe3)SPh (I'MezPh) (XVII) Propagation

%

hle3S1&,

+ Ph2Sz-Me3SiCH2SPh

2]

+ hk3fhCHZ

+ PhS

+ (XVI1)-[Pt (Sl'h) 2(I'MezI'h) + hk3SICfTZ ,hfe3Sl&~ + P I I ~ S ~ - M ~ ~ S I C H ~ S P + ~ PhS PhS

21

(44) (45) (46) (47)

This mechanism was established by identifying the products of the reaction from [Pt ( CH2SiMe3) (PMc2Ph)2] and PhzS2 a t 60" in C6H6 as trans[Pt (SPh)2 (PMezPh)z] and MeaSiCHzSPh and showing that the reaction only took place undcr similar conditions at a reasonable rate in the presence of ( B u ~ O N as ) ~ a free-radical initiator; additionally, in the cis[PtMe2(PEt3)&Ph& system, in the absence of initiator but with nitrosodurene as spin trap, a weak signal of the spin adduct Ar(Me)fiO was

379

Free Radicals

BJON=N-OBU'

kT

r 2 ButO

N,

Bu