The History and Development of Organotransition Metal Chemistry

32 The History and Development of Organotransition Metal Chemistry

Downloaded by UNIV OF NORTH CAROLINA on February 18, 2015 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/ba-1967-0062.ch032

M A R V I N D. R A U S C H The University of Massachusetts, Amherst, Mass.

Organotransition metal compounds have been known for over a century, but it has been only during the past 15 years that most of the major advances in this area have been made. Organotransition metal compounds consist of organic molecules or ligands coordinated to transition metals by one or more carbon atoms. In the present sur vey, the formation, properties, structure, and reactions of various classes of these substances are discussed. Empha sis has been placed on important discoveries and theories that have stimulated the rapid progress of organotransition metal chemistry. Pertinent reviews on various aspects of this subject have also been cited.

/^\rganometallic compounds containing neutral, unsaturated organic molecules coordinated to certain transition metals were known long before the advent of Werner's theories on coordination compounds. Until about 1950, however, this aspect of coordination chemistry remained fal low, and the structure and bonding of the compounds known at that time were quite perplexing. The discovery of ferrocene and the development of a rational theory of bonding i n olefin-transition metal complexes i n the early 1950's prompted a vigorous research effort around the world that has attracted the attention of organic chemists, inorganic chemists, crystallographers, and scientists of many allied disciplines. Research in organo transition metal chemistry presently is proceeding at a very rapid rate, and the synthesis and properties of seemingly unlimited new types of "organometallic, T complexes" represent an important frontier in coordina tion chemistry. The purpose of the present survey is twofold. Firstly, because the Werner Symposium is partly historical, some of the more important dis coveries and theories that have led to the present high level of interest i n 486 In Werner Centennial; Kauffman, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1967.


32.

RAUSCH

Organotransition

487

Metals

organotransition metal chemistry are discussed. Some illustrative examples of synthetic methods and reactions of various types of organo transition metal compounds are also included. Secondly, reference is made to recent comprehensive reviews where the reader can learn more about various aspects of organotransition metal chemistry if he desires. This survey has therefore been prepared primarily for those persons who have little knowledge of this rather new branch of chemistry and who might like to learn more about it. It is difficult to systematize organotransition metal compounds be cause almost every week another "nonclassical" compound of this type seems to be reported. Nevertheless, one approach based on the coordina tion chemistry of these substances is to consider each organic molecule or radical as a ligand which is bonded to the transition metal v i a n carbon atoms. In other words, a singly occupied orbital from each carbon atom of the organic molecule or radical is involved in bonding with the metal, and the resulting organic ligand may be considered to be an n electron donor. Except i n a few instances, such as certain organometallic carbonium ions discussed below, each organic ligand should be considered as electrically neutral. Some examples of this generalization are as follows:

CHjCH

=

CHCH2

BUTENE (A MONOOLEFIN; n=2)

H~C = C~H | |

H-C=C-H CYCLOBUTADIENE (A DIENE; n= 4)

BENZENE (AN ARENE;n=6)

There is no reason, however, why an organic ligand must have an even number of carbon atoms involved in coordination with a transition metal. Organotransition metal compounds in which an orbital of a single carbon atom overlaps with an appropriate metal orbital are closely related to the well-known localized covalent or o--bonded compounds in organic chemis try and represent a case where n = 1. Various other organotransition metal compounds are now known in which the organic ligand consists of three, five, or seven carbon atoms bonded to the transition metal. C o m pounds of this sort in which odd numbers of electrons can be considered to be formally donated to a metal are often referred to as " e n - y l " derivatives. Typical organic ligands of this type are shown on the next page. Such ligands are best prefixed b y the Greek letter " i r " to clearly differen tiate them from or-bonded (n *= 1) ligands. Carbon monoxide is an organic molecule that deserves special mention in any discussion of organotransition metal chemistry, because many or ganic derivatives of the transition metals are derived from metal carbonyls

In Werner Centennial; Kauffman, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

488

WERNER CENTENNIAL

H

HC

CH

2

TT-CYCLOPENTADIENYL TT-CYCLOHEPTATRIENYL 2

(n = 5)


TT-ALLYI (n = 3)

(n=7)

and frequently contain carbon monoxide as a ligand. It was Werner who first suggested that carbonyl groups in metal carbonyls are attached directly to the metal atom rather than in chains. In the 1920's, Sidgwick (223) formulated the very useful Effective Atomic Number ( E A N ) Rule. A s applied to transition metals, this generalization simply postulates that these metals undergo chemical transformations b y transferring or sharing elec trons with ligands so that they can achieve a closed-shell configuration of the next higher inert gas in the periodic system. Using this scheme, carbon monoxide in metal carbonyls is best considered to be a two-electron donor. B y means of the previously mentioned system of "electron-bookkeep ing" for organic ligands, together with the Sidgwick E A N Rule, a large portion of experimental data on organotransition metal chemistry and metal carbonyl chemistry can be systematized. It must always be remem bered, of course, that any methods of systematization are only useful formalisms and may not account for each and every new compound dis covered. The factors that relate to bonding and structure in organotransi tion metal compounds are considerably involved, as is evidenced by recent theoretical treatments on the subject. In the present survey, a classifica tion of organotransition metal chemistry on the basis of organic ligands will be followed. Complexes ( n = 2)

Mono-olefin

Organotransition metal chemistry had its beginning i n 1827 when a Danish pharmacist named Zeise found that when platinum (IV) chloride was boiled in ethyl alcohol and potassium chloride subsequently added, a new compound having the composition K C l P t C ^ C ^ I L r H ^ O could be isolated (259, 260). The analysis of this compound, which is now formu lated as K[(C H )PtCl ]-H20 (Zeise's salt), was soon challenged by Liebig, and the analytical composition of this substance became the center of con siderable controversy (158, 159, 258, 261). I n 1869 Birnbaum (14) con firmed that ethylene exists in Zeise's salt when he synthesized this com pound by treating platinum(II) chloride in hydrochloric acid solution with ethylene, followed by the addition of potassium chloride. Since these early findings, a variety of other nonionic ethylene-platinum complexes such as I and I I have been prepared (6, 29). 2

4

3


32.

Organotransition

RAUSCH

CI

C2H4

\ CI

Pt

/

CI

/

\

\

/

CI

489

Metals

PT

C2H4

/

\

\

/

CI

C2H4


I

Pt

/ \

CI

C2H4

II

In 1938 Kharasch and co-workers described a method generally applicable for preparing mono-olefin palladium complexes (147). P a l ladium (II) chloride reacted with warm benzonitrile to form the complex bis(benzonitrile)-palladium chloride, and the latter reacted directly with olefins such as ethylene, styrene, cyclohexene, etc., as follows: 2 (C H CN) PdCl2 + 2 Olefin -> [(Olefin)PdCl ] + 2 C H C N 6

5

2 2

2

6

B

T h i s method was later extended to dienes and represents an early example of an important synthetic route in organotransition metal chemistry wherein one neutral ligand displaces another from the coordination sphere of a transition metal. A major advance in theoretical aspects of organotransition metal chemistry came in 1951-1953 and has since become known as the DewarChatt concept of bonding in olefin-metal compounds. Winstein and Lucas (254) had attempted earlier to explain the nature of the bonding in olefinsilver complexes on the basis of resonance stabilization:

\

/

\

/C—c. -—/© / \ Ag

/

\

.c—cv - — / \ ©\ Ag

/ ,C = c

/

\ Ag ©

On the basis of molecular orbital theory, Dewar (64) divided the metalolefin bond into a a portion and a T portion. The former type of bond can result from a filled bonding olefin x orbital overlapped with a vacant 5s orbital of silver (III), while the latter type of bond can result from a filled

IT - TYPE BOND

0" - TYPE BOND

III

IV


490

WERNER CENTENNIAL

silver 4d orbital overlapped with the vacant ir* orbital (antibonding) of the olefin (IV). Chatt and Duncanson extended this concept to platinum-olefin com plexes such as Zeise's salt (29). In platinum-olefin complexes, the cr-type bond results from a filled w orbital of the olefin overlapped with a vacant 5d6s6p orbital of platinum, while the 7r-type bond results from a filled 5d6p orbital of platinum overlapped with the antibonding olefin ir* orbital (V). Downloaded by UNIV OF NORTH CAROLINA on February 18, 2015 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/ba-1967-0062.ch032

2

VI

This multiple, covalent bonding has often been referred to as "synergic," because w bonding tends to remove excess negative charge placed upon the metal by a bonding, and vice-versa. Thus, one type of bonding can be regarded as reinforcing the other type, and such a process accounts for the unusual stabilities of these substances. Chatt and Duncanson (29) proposed further a spatial arrangement of the atoms i n the anion [(C2H )PtCl ]~, as shown i n Structure V I . The carbon atoms forming the double bond in the olefin are regarded as being perpendicular to the plane formed by the metal atom and the other ligands. Subsequent x-ray crystallographic studies on Zeise's salt and related olefinmetal complexes have confirmed these earlier postulations (4, 63, 132, 4

256,

3

257).

U n t i l about 1957, only metals toward the end of the transition series, such as P d , P t , C u , A g , and H g , were known to form mono-olefin complexes. In 1959 Schrauzer (219, 220) prepared the first olefin complex of nickel, starting with nickel carbonyl and acrylonitrile: CN

/ H C=CH 2

Ni(CO) + 2 C H = C H — C N -> 4

1

Ni

2

+ 4 CO

T

HC=CH

2

/ NC


32.

RAUSCH

Organotransition

491

Metals


Spectral data indicate that ligand-metal bonding results primarily between the metal and the carbon-carbon double bond, and that the nitrile group interacts little if at all with the metal. Since 1959, many other complexes involving mono-olefins that contain electronegative groups have been described. Presumably, such a group being present significantly lowers the energy level of the olefin r* orbital, and thus back donation from the metal is facilitated. Ethylene complexes of transition metals near the beginning of the transition series are now known. A neutral complex was prepared in 1960 by the photochemical displacement of carbon monoxide by ethylene from cyclopentadienyl-manganese tricarbonyl (156):

+ C0

Using a somewhat different approach, Fischer and Fichtel (85) have ob tained a variety of ethylene-containing cationic complexes by reaction of 7r-cyclopentadienyl carbonyl halides of M o , W , and Fe with ethylene under pressure in the presence of aluminum chloride. The cations were isolated in the form of their salts with heavy anions such as P F " " , B(CeHs)i~, etc. Presumably the aluminum chloride serves as a halogen acceptor and "activates" a coordination site on the metal, facilitating coordination of ethylene. 8

In related studies, Green and co-workers (55, 108) prepared similar xethylene-containing cationic complexes directly from (7-ethyl derivatives by hydride abstraction techniques:


492

WERNER

CENTENNIAL

+ +

+ 0

3

CH CH


2

-

C BF

BF +0 C-H 4

3

4

3

The latter complex can be converted to the former by adding hydride ion from sodium borohydride. For the reader desiring more information on mono-olefin complexes of the transition metals, a number of excellent reviews are available. E a r l y studies on olefin-metal complexes (primarily of P t , P d , A g , and Hg) have been reviewed by Keller (146), Chatt (27), and Douglas (70). Two more recent reviews which comprehensively cover the literature up to 1961 have been prepared by G u y and Shaw (113) and by Bennett (11).

Carbonyl Complexes (n = 2) A s mentioned earlier, the rapid development of organotransition metal chemistry has been paralleled by and intimately connected with the de velopment of metal carbonyl chemistry. The discoveries of new metal carbonyls and the advances in understanding of the structures and bonding of these substances have been so profuse that they would require a special survey. Historically, however, it should be mentioned that M o n d and co workers (175) found as early as 1890 that nickel valves were corroded by hot gases containing carbon monoxide. Subsequent investigations showed that a stream of this gas passed over finely divided nickel yielded a colorless (and highly poisonous!) liquid having the formula N i ( C O ) 4 . N i + 4 CO - » Ni(CO)

4

This discovery was soon followed by isolation of a series of iron carbonyls with compositions F e ( C O ) , F e ( C O ) , and F e ( C O ) i . Since these important findings near the end of the last century, metal carbonyls of a majority of the transition metals have been isolated and characterized. M a n y of these compounds are " p u r e " metal carbonyls, containing only the metal and C O . M a n y other types are also known, including metal carbonyl-halides, -hydrides, -anions, -7r-cyclopentadienyls, etc. A number of earlier reviews dealing primarily with metal carbonyls has been published (169, 197). Fortunately, there are also two recent reviews on the subject by Abel (1) and Hileman (130). A useful, recent 5

2

9

3

2


32.

RAUSCH

Organotransition

493

Metals

review b y K i n g concerns the formation and reactions of metal carbonyl anions (148).


w Allyl (n = 3) and Related Complexes A l l y l derivatives of the transition metals can be classified as or-allyl or 7r-allyl. In the former, the allyl ligand contributes one electron (n = 1) to the allyl-metal bond, while in the latter the allyl ligand may be regarded as contributing three electrons (n = 3) to the allyl-metal bond. Probably the first transition metal complex containing a 7r-allyl ligand was described by Pritchard (198) in 1952, although the composition and structure of the product were not known precisely. The substance was reformulated as

H Co(CO) H+CH =CH-CH=CHo ^K d

;:

9

H

'

CH*

H

I

H

H

Xo

OC

H M

t CH? .Co. OC Q CO 0 0

v

c

'

CO

0

C H C o ( C O ) i n 1958 b y Jonassen and co-workers (142). Subsequent detailed investigations (5, 174, 176) have shown that two isomers are i n fact formed, and the formulation of each as 7r-allyl-cobalt complexes followed from considering their proton N M R spectra shown above. In 1960 several groups of workers developed a route to the parent complex, T-allylcobalt tricarbonyl (VII) (122, 123, 170) and also to TTallylmanganese tetracarbonyl (VIII) (143, 170). These syntheses were based on reactions of appropriate metal carbonyl anions with allyl halides as shown below. 4

7

3

+

-

.

f\

CH = CHCH Br + NaJCo(Co) ]-^[CH =CHCH -Co(CO) ] *H2C' | CH 2

2

4

2

2

4

:QO

OC

2

.Co | CO C 0 VII

In the latter reaction, the initially-formed

K

+

+I/2H

XVIII


2

32.

RAUSCH

Organotransition

503

Metals

Several years later, Grignard and Courtot (111) found that cyclopentadiene reacted with ethylmagnesium bromide, liberated ethane, and formed a new Grignard reagent, cyclopentadienylmagnesium bromide. ether-benzene

C H


6

6

+ C H MgBr 2

6

> C H MgBr + C H 6

6

2

6

Reactions of this type were subsequently extended to indene and fluorene —another pair of "acidic" hydrocarbons. The literature seems well-documented with many attempts during the period 1900-1950 to prepare organotransition metal compounds from Grignard reagents (usually methyl, phenyl, etc. derivatives) reacting with transition metal halides. I n retrospect, one can assume that cyclopentadienyl Grignard reagents or alkali metal derivatives were not i n cluded in these investigations, else the extensive developments that occurred in organotransition metal chemistry in the early 1950's might well have resulted many years before! Clearly, two of the most important discoveries relating to organo transition metal chemistry were described in 1951-52. Kealy and Pauson (144) were attempting to "couple" cyclopentadienylmagnesium bromide by means of ferric chloride during the course of research directed toward isolating the hydrocarbon fulvalene. Work-up of the reaction instead produced a remarkably stable, orange, organo-iron compound of composition C i o H i F e . This same substance was almost simultaneously described by Miller, Tebboth, and Tremaine (173) who isolated it from a vapor phase reaction of cyclopentadiene and iron at elevated tempera tures. Actually, the discovery was apparently known as early as 1948, but was not reported until 1952 since it was only incidental to other work. 0

XIX The unique "sandwich" structure soon proposed for this new com pound ( X I X ) , bis(cyclopentadienyl)iron, was subsequently fully confirmed by x-ray analysis (74, 75). The nature of the bonding between the cyclo-


504

WERNER CENTENNIAL


pentadienyl rings and the iron atom in this compound has been considerably disputed in recent years, and detailed arguments are contained i n reviews referred to at the end of this section. A subsequent important discovery by Woodward, Rosenblum, and Whiting {255) in 1952 indicated that this compound would undergo Friedel-Crafts substitution reactions. This re sult, together with the remarkable stability and spectral properties of

bis(cyclopentadienyl)iron, suggested that the compound represented a new type of aromatic system; hence, the name "ferrocene" was coined. It was soon noted that iron was by no means unique in forming wcyclopentadienyl derivatives, and research groups i n several countries set out to determine the nature and scope of cyclopentadienyl-metal chemis try. A t the present time, practically all the metals of the short transition series, as well as nearly all the metals and metalloids of the main group series, form one or more cyclopentadienyl compounds. In addition, cyclopentadienyl derivatives of over one-half the lanthanides have now been described, and even cyclopentadienyl derivatives of U , T h , and several Jrans-uranium elements are known. The present status of cyclo pentadienyl-metal chemistry is illustrated in part in Figure 1. Elements designated by shaded areas are known to form one or more cyclopentadienyl derivatives. Metal-cyclopentadienyl compounds of the ferrocene-type (e.g., the metallocenes) undergo essentially four different types of chemical reactions, and a typical example of each of these is illustrated below: Oxidation-Reduction

r


W)

32.

RAUSCH

Organotransition

505

Metals

Ligand Replacement


Ti'

Ti:

+ 2 HBr

-Br v

Br

+ 2 HCI

(AW)

Ring Addition

2

+ -

0 II $ CCI i

Co

Co

+

[CoC| H,J| CI 0

{92)

Ring Substitution

fo)

-h t C CI

AlCh

+ HCI

{208)

Reactions of the ring-substitution type have especially been studied in great detail. Ferrocene, for example, is known to undergo the following ring-substitution reactions (Fc = the ferrocenyl group): O Friedel-Crafts acylation Friedel-Crafts alkylation Mercuration Vilsmeier formylation Sulfonation

I Fc-H -> FcCR Fc-H -> Fc-R Fc-H -> Fc-HgX Fc-H -> Fc-CHO Fc-H - » Fc-SO,H



506

WERNER CENTENNIAL

Aminomethylation

Fc-H -> Fc-CH«N(CH,)«

Metalation

Fc-H -* F c - M

Arylation

Fc-H - » Fc-Ar

These reactions and their extensions have greatly broadened the scope of metallocene chemistry, especially since various other metallocenes also undergo certain of the above ring substitution reactions. A striking example concerns the relative ease of substitution reactions of the iron group metallocenes, namely, ferrocene, ruthenocene (RuCioHio), and osmocene (OsCioHio) (205, 206). One of the most fascinating areas of metal-cyclopentadienyl chemistry in recent years concerns the formation and relative stabilities of metallocenyl-carbonium ions. The ability of iron to stabilize cationic centers in certain ferrocene compounds was noted by Weliky and Gould (243) in 1957. In 1959 Richards and H i l l (213, 214, 215) reported the results of some kinetic studies relating to the relative rates of solvolysis of metallocenylmethylcarbinyl acetates. They determined that these solvolyses proceeded via a carbonium ion mechanism and found that the ferrocenyl acetate, for example, solvolyzed nearly seven times faster than did t r i phenylmethyl acetate under the same conditions. I t is now well-documented that carbonium ions containing adjacent metallocenyl groups possess unusual stability (20, 2 1 , 213, 214, 215, 237). Richards and co-workers (25, 213, 214, 215) propose that this stability stems from nonbonding electrons on the metal being directly coordinated with the carbonium ion center ( X X ) .

XX

XXI

Others contend that the stability can be more conventionally related to a resonance effect v i a the cyclopentadienyl rings and the metal ( X X I ) (103, 240). In either case, the positive charge which would develop on the exocyclic carbon atom would be delocalized considerably, which would aid the carbonium ion to form more easily. The differences between these i n terpretations are subtle but distinct, and the topic will no doubt be the subject of considerable future research.



Ca

Sr

Ba La

-

515

Ce Nd

Pr

Th

140 (43.6 140.5

? 299J

Ra Loa

133 137.4 13S

Gs

85.4 87.6

Kb

39.15 40.1

K

-

Sa

En

Gd Tb H o Er

Ta

Yb

So

Ti V

Cr Mil

Fe Co

Ni

339.5

U

Figure 1.

150.3 151.79 156 160 162 166

Nb M o

Ta w 183 181.'.

-

-

Ca

Zn

Rh Pd Ag Cd

Jn

Sn

79

Ga Ge 63.6 65.4 70

Se

Br

ci

Kr

A

Sb

Te

J

X

75.0 79.1 79.96 81.19

At

S

Ir

Pt

An

Hg TI Pb Bi

t

Pba Bia

191 193.0 194.6 197.2 300.3 3o4.1 906.9 108.5

Of

Tea

-

- -

- -

101.7 103.0 106 107 93 112.4 114 118.5 190 127 6196.95 198

Ru

Periodic table of the elements

9

Ac

171 173.0

Zr

89.0 90.7 94 96.0

Y

44.1 48.1 51.1 59.1 55.0 55 9 59.0 58 7

Al Si P

Mg

94 36 97.1 98.4 31.0 32.06 89.45 39.9

90

Na

Ne

93.05

Fl

9.1

14.04 16.00 19

c 19

B

n

Be

0

4

He

Li N

-

-

7.03

1.008

H

-



508

WERNER CENTENNIAL

Fortunately, the area of organotransition metal chemistry dealing with 7r-cyclopentadienyl-metal compounds is well reviewed. Two comprehen sive reviews covering the literature until 1959 have been written by W i l k i n son and Cotton (245) and by Fischer and Fritz (87). Several more recent reviews have been written by Birmingham (12, 13) and by Rausch (200, 202). The topic of metallocenes has been extensively reviewed by Rausch (199, 200, 202, 204), Plesske (196), and Little (160). The most recent review of metallocene chemistry is a two volume series by Rosenblum, the first volume of which has recently been published (217). Arene (n = 6) and Related Complexes Organometallic w complexes in which a benzene ring serves as the ligand were first isolated by Hein (124) as early as 1919, although it has only been in recent years that the composition and structure of these sub stances have been known with certainty. Hein found that phenylmagnesium bromide reacted with a slurry of chromium trichloride in ethyl ether at ice bath temperature and produced, following hydrolysis and addi tion of potassium iodide, a series of "polyphenylchromium iodides." The latter could be reduced electrolytically in liquid ammonia to yield such compounds as "tetraphenylchromium" and "triphenylehromium." ethyl ether

C H M g B r -f C r C l 6

5

3

o°c

HtO

•

> "polyphenylchromium iodides" KI redn.

(C,H,)«CrI

» (C,H,) Cr

(C,H ),CrI

> (CH6),Cr

6

4

I n 1953 Zeiss and Tsutsui {262, 266, 267) reinvestigated Hein's earlier studies. In 1954 together with Onsager they proposed ir-bonded "sandwich" structures for compounds of the type isolated earlier by Hein and co-workers. Thus, "tetraphenylchromium iodide" was reformulated

XXII

XXIII



32.

Organotransition

RAUSCH

509

Metals

as bis(biphenyl)chromium iodide ( X X I I ) ; "triphenylchromium iodide" was reformulated as benzene-biphenyl-chromium iodide ( X X I I I ) , etc. Analogous structures were suggested for the neutral organochromium com pounds obtained earlier. The formulation of Hem's compounds as organometallic, ir complexes was shown by Zeiss and Tsutsui to be consistent with chemical and magnetic data obtained by them and by previous investiga tors. I n 1956 Zeiss and Herwig isolated the parent bis (benzene) chromium cation as its tetraphenylboron salt ( X X I V ) and reduced the latter to the neutral complex bis(benzene)chromium ( X X V ) {263).

XXIV

XXV

Independent of these investigations, Fischer and co-workers reasoned that the compound bis (benzene) chromium ( X X V ) , being isoelectronic with the "sandwich" compound ferrocene, should be capable of formation and study. In a research program initiated in 1954, Fischer and Hafner {90, 91) discovered that chromium trichloride, reduced by aluminum powder at 150°C. in the presence of benzene and aluminum chloride (Friedel-Craftstype method), formed a bis (benzene) chromium salt. The latter was readily reduced to the parent substance bis (benzene) chromium ( X X V ) by alkaline sodium dithionite solution. 3CrCl + 2A1 4- A1C1 + 6 C H -> 3[(C H ) Cr]+ [AICIJ" 3

3

2[(C H ) Cr]+ + S 0 ~ + 4 0 H 6

6

2

2

4

2

6

6

6

6

2

2(C H ) Cr + 2S0 " + 2 H 0 6

6

2

3

2

2

The "sandwich" structure of bis(benzene)chromium ( X X V ) pre pared by this new route was soon confirmed by Weiss and Fischer {242). Moreover, Fischer and Seus {98) prepared bis (phenyl) chromium iodide ( X X I I ) by the Friedel-Crafts-type method, starting with biphenyl in place of benzene, and were able to show that the product obtained in this manner was identical to bis (biphenyl) chromium iodide ( X X I I ) isolated via Hein's Grignard route. A longstanding and vexing problem i n coordination chemistry was thus finally solved.



510

WERNER

CENTENNIAL

The Friedel-Crafts method is the most versatile route available for preparing arene-metal complexes, having been applied successfully to nearly a dozen transition metals. The method has definite limitations, however, with regard to the type of arene unit used, and only a l k y l - and aryl-substituted benzenes in general can be successfully employed. The discoveries by Zeiss and Tsutsui and by Fischer and Hafner were unquestionably of great importance for further expanding and developing organotransition metal chemistry. B y 1956, it had become apparent to many investigators that the "sandwich" structure found i n the compound ferrocene a few years earlier was not necessarily unique, and that other carbocyclic and even heterocyclic ring systems might be capable of serving as ligands in organometallic, w complexes. Another important advance in organotransition metal chemistry was reported in 1957 by Zeiss and Herwig (264, 265). These workers found that reaction of phenylmagnesium bromide with chromium trichloride in tetrahydrofuran ( T H F ) , rather than in ethyl ether, yielded a red, crystal line compound, triphenylchromium-tris-tetrahydrofuranate ( X X V I ) . THF

C r C l + 3C«H MgBr 3

6

> (C«H ) Cr.3 T H F + 3MgBrCl 6

3

XXVI This compound could be subsequently converted to arene-metal complexes by treating it with ethyl ether and hydrolysis. The substance probably contains three phenyl groups c-bonded to the chromium atom, as well as three solvent molecules occupying the remaining coordination sites around the metal in an octahedral-type arrangement. Such a "c-bonded," organochromium compound was, of course, the original goal of Hein and co workers many years before. These earlier attempts failed partly because of choice of solvent. Tetrahydrofuran is known to be more basic than ethyl ether and evidently forms a stronger dative bond with the metal, thus stabilizing triphenylchromium as a hexacoordinate complex. Zeiss and co-workers further found that acetylenes displace T H F molecules in the coordination sphere of chromium and other metals in analogous complexes. The acetylenes subsequently condense in cyclic arrangements, and such cyclic condensation reactions have provided impor tant new routes to both arene-metal complexes and uncomplexed arenes. Further details on these novel reactions are contained i n the reviews at the end of this section. B y 1957, a variety of both 7r-cyclopentadienyl and arene-metal com plexes was known, and it was interesting to determine whether "mixedsandwich" complexes containing both of these ligands around the same could be produced. Two early examples of such complexes, are given below:



32.

RAUSCH

Organotransition

511

Metals

Y e t another major advance i n arene-metal chemistry was made in 1957 by Fischer and Ofele (95), who found that a sealed tube reaction involving chromium hexacarbonyl, bis (benzene) chromium, and benzene produced the "half-sandwich" compound benzene-chromium tricarbonyl ( X X V I I ) .

BENZENE

Cr(C0) + ( C H ) Cr 6

6

6

2

SEALED TUBE

^ ii/

o r T ^co c o XXVII

The centrosymmetric structure proposed for benzene-chromium tricarbonyl was subsequently confirmed b y x-ray analysis (8, 52). Following this initial discovery, several additional routes to benzenechromium tricarbonyl and related complexes were developed, the most con venient of which involves the simple refluxing of the metal carbonyl and arene with concurrent loss of carbon monoxide (96, 97, 184,186,187). reflux, with or

Arene + M(CO)

> (Arene)M(CO) + 3CO

6

3

without solvent

M

» Cr, Mo, W

Arene = benzene, toluene, fluorobenzene, phenol, methyl benzoate, etc. A wide variety of arene-metal tricarbonyl complexes are now known, 'and their chemistry largely waits to be developed. The effects of coordination


512

WERNER

CENTENNIAL


on the reactivity of the arene ligand might be expected to be quite pro nounced, and already a number of notable developments have been described i n the literature. Thus, while benzene-chromium tricarbonyl undergoes Friedel-Crafts acylation readily (17, 8 1 , 216), nucleophilic-type reactions such as the facile conversion of fluorobenzene-chromium t r i carbonyl ( X X V I I I ) to anisole-chromium tricarbonyl ( X X I X ) also occur in high yield (17, 186,187).

Recently, Pettit and co-workers (184) have shown that the relative rate of solvolysis of benzyl chloride-chromium tricarbonyl is about a million times faster than for uncomplexed benzyl chloride under analogous SNI conditions. The area of carbonium ion formation and stabilization in arene-metal systems thus represents another fertile field for future investigation. The topic of arene-metal chemistry has been the subject of several reviews. The most recent review by Fischer and Fritz (88) was published in 1961, while a still earlier review by these authors appeared i n 1959 (87). Another review published in this same year was written by Wilkinson and Cotton (245), while an excellent source of information on the subject was prepared i n 1960 by Zeiss (262). w-Cycloheptatrienyl

( n = 7) and Related Complexes

Following the successful isolation and characterization of many 71-cyclopentadienyl- and arene-metal complexes as described above, it was only natural for investigators to attempt to form metal 7r-complexes con taining C 7 - and larger ring systems. It seemed entirely reasonable at this point that complexes containing the well-known cycloheptatrienyl (tropylium) ion might be capable of formation and isolation. In 1958 Abel, Bennett, and Wilkinson (2, 8) found that cycloheptatriene, when refluxed with either molybdenum or chromium hexacarbonyl, yielded complexes of the type ( C H ) M ( C O ) 3 ( X X X ) . In these complexes, the cycloheptatriene ring system can be regarded as a six ?r-electron donor. 7

8



32.

RAUSCH

Organotransition

513

Metals

C 0 XXX The structure of cycloheptatriene-molybdenum tricarbonyl ( X X X , M = M o ) was subsequently confirmed by x-ray analysis (76). The six olefinic carbon atoms were found to be approximately planar and symmetrically bonded to the metal, with the methylene carbon bent away from the latter. Carbon-carbon ring distances indicated alternate double and single bonds i n the planar portion of the molecule, rather than a delocalized 7r-electron system, as was originally suggested. The analogous complex ( C H ) W ( C O ) 3 was subsequently described i n the literature (62, 164). Although Able et al. (2, 8) had originally set out to prepare 7r-cycloheptatrienyl complexes of metals, the cycloheptatriene complexes they actually obtained served as key intermediates i n forming the former com plexes. I n 1958 Dauben and Honnen (61) reported that cycloheptatrienemolybdenum tricarbonyl reacted with triphenylmethyl tetrafluoroborate in methylene chloride solution with abstraction of hydride ion from the molybdenum complex. The reaction products, obtained i n nearly quan titative yields, were triphenylmethane and the 7r-cycloheptatrienyl complex [ ( 7 r - C H ) M o ( C O ) 3 ] B F 4 - . 7

8

7

+

7

This significant finding was subsequently extended to the analogous chromium and tungsten derivatives (61, 62, 179, 180) and has been useful in synthesizing still other types of " e n - y l " metal ir complexes. It should also be mentioned that organometallic, ir complexes, reputed to be [ ( 7 r - C H ) P t B r ] and ( 7 r - C H ) P t B r , were also described i n 1958 (89). These complexes were obtained from reaction of platinum (IV) bromide 7

7

2

2

7

7

3


514

WERNER

with cycloheptatriene and with tropylium bromide, respectively. They were assigned 7r-cycloheptatrienyl structures primarily on the basis of com parisons of their infrared spectra with the spectrum of tropylium bromide. In 1959 M u n r o and Pauson (180) announced some preliminary find ings in what is the most detailed study thus far undertaken concerning the reactions of 7r-cycloheptatrienyl-metal derivatives. The reactions of the cations [ ( 7 r - C H ) M ( C O ) ] ( M = C r or M o ) with nucleophilic reagents proceeded by three different routes: (1) normal addition to the sevenmembered ring (177); (2) reductive dimerization of the starting cation (179); (3) ring contraction to form benzene-metal tricarbonyls (178). The latter reaction results, for example, when the nucleophile is cyclopentadienide ion or diethyl malonate ion, and experiments have shown that the 7r-complexed ring is derived entirely from the 7r-cycloheptatrienyl ligand. 7


CENTENNIAL

7

3

+

C

C

0

M=Cr,Mo

0

It has also been established that this novel type of rearrangement proceeds b y initial attack of the nucleophile on the 7r-complexed ring to form sub stituted cycloheptatriene-metal tricarbonyl intermediates, although -nechanistie details are still rather obscure. A variety of organometallic, w complexes are now known, i n which both 7r-cyclopentadienyl and 7r-cycloheptatrienyl ligands are bonded simul taneously to the same transition metal, providing a series of unique "mixed sandwich" complexes. The first of these, 7r-cyclopentadienyl-7r-cycloheptatrienyl-vanadium ( X X X I ) , was described by K i n g and Stone (153)

XXXI


32.

RAUSCH

Organotransition

515

Metals


in 1959, and its structure was subsequently confirmed b y x-ray analysis (80). Several novel routes to additional, mixed, ring complexes of this type are outlined below:

iso-C H MgBr + CrCI + C H + C H 3

7

3

5

6

7

8

(ftfc

II

H0

Na S 0

2

RCCI+.

2

2

W)

4

+ K0H R=CH

R=CH ,C H

3 6 5 ,C

H

3

M=Cr,Mn

6

5

M=Cr

Initial attempts to prepare 7r-cyeloheptatrienyl-iron tricarbonyl com plexes were not successful—the products often being 7r-cycloheptadienyliron tricarbonyl derivatives (60). I n 1964 Mahler, Jones, and Pettit (162) reported the synthesis of a complex [ ( C H ) F e ( C O ) ] " B F ~ by the following route: 7

H OCrh

Fe (C0) 2

9

1

7

3

h

4

[(CH)Fe(C0)^j BF4 7

7

It was postulated that three strong C - H stretching frequencies in the infra red spectrum of this complex reflect a lower symmetry of the C H - l i g a n d than does the analogous series of complexes, [ ( 7 r - C H ) M ( C O ) ] B F ~ , mentioned above. The structure of the iron complex seems best repre sented b y X X X I I rather than b y X X X I I I , the iron atom being bonded 7

7

7


7

3

+

4

516

WERNER

CENTENNIAL

to five carbons of the ring and leaving one double bond formally not coor dinated to the metal. The fact that only a single sharp peak is observed in the N M R spectrum of the [ ( C H 7 ^ 6 ^ 0 ) 3 ] + cation is believed to be due to a rapid valence tautomerism postulated previously by Dickens and Lipscomb (65, 66, 67) for cyclooctatetraene-iron tricarbonyl (vide infra). 7


+ ETC.

Fe

i

0

+

i

Fe (C0)

(CO),

3

XXXIII

XXXII

A variety of other 7r-eycloheptatrienyl and related metal complexes is known, as well as complexes in which 1,3-cycloheptadiene (C7H10) serves as a ligand. For details, refer to a recent comprehensive review by Fischer and Werner (100) and/or two earlier reviews by these authors (99) and by Bennett (11). Cyclooctatetraene and Related Complexes Cyclooctatetraene, C H , is a monocyclic organic ring system which contains four conjugated double bonds. A number of organometallic, T complexes containing cyclooctatetraene is now known in which the latter donates from two to seven T electrons to a transition metal. N o complex is presently known in which cyclooctatetraene formally donates all eight 7r electrons to the same transition metal, although both chemical and physical evidence indicates that the remaining "uncoordinated" double bonds in many cyclooctatetraene-metal complexes are greatly effected by complexing of adjacent, conjugated double bonds. 8

8

M e t a l , 7r complexes of cyclooctatetraene were first described by Reppe et al. (211) in 1948. Silver and copper complexes were obtained during the synthesis of this hydrocarbon from acetylene. These complexes of cyclooctatetraene have since been investigated in considerable detail by several groups of investigators (50, 114, 167, 168). A n x-ray analysis of the complex [ ( C H ) A g ] + N 0 - ( X X I V ) indicates that the silver ion is coordinated to two nonadjacent bonds of cyclooctatetraene but is situated at differences from each. [ ( C H ) A g ] units are, furthermore, joined together by weak bonds. 8

8

3

8

8

+


32.

517

Organotransition Metals

RAUSCH

\

I


p-~Ag

XXXIV The two remaining double bonds of the ligand possibly interact to some extent with silver ions because the infrared spectrum of the complex shows no "free" double bonds, and solutions of the complex show only a single proton N M R signal (105). In 1953 Jensen (141) reported that the reaction between cyclo octatetraene and K ( P t C l ) produced the complex ( C 8 H ) P t C l . He like wise described the iodo derivative, ( C H ) P t I . A " t u b " conformation was postulated for the cyclooctatetraene ligand i n these structures. Sub sequent, detailed, infrared and N M R studies of these and analogous palladium complexes indicate substantial 7r-electron derealization (105). In 1959 a remarkable class of compounds containing cyclooctatetraene coordinated to iron tricarbonyl units was discovered independently by three research groups (165, 166, 182, 207). Two principal products were obtained having the compositions ( C H ) F e ( C O ) and ( C H ) F e ( C O ) , of 2

4

8

8

8

C , H + Fe(CO) 8

heat or 6

2

2

8

3

8

8

2

G

(C,H )Fe(CO) + C H,Fe (CO), 8

U.V.

8

3

8

2

which the latter complex could be prepared from the former by treating it with excess iron pentacarbonyl.

XXXV

XXXVI


518

WERNER

The binuclear iron complex ( C H ) F e 2 ( C O ) 6 had been expected from the reaction, but the " c h a i r " conformation ( X X X V ) , which was subse quently found for this substance, was entirely unexpected (65, 66, 67). In this complex, each end of the cyclooctatetraene ligand behaves as a butadiene-type (n = 4) ligand, and bond distance measurements indicate very little w-w interaction between the two halves of the ring. The proton N M R spectrum of the complex in solution exhibits two resonances of equal intensity, while the infrared spectrum is very similar to the spectrum of butadiene-iron tricarbonyl and similar diene complexes (105). The properties and reactions of the complex ( C H ) F e ( C O ) proved to be even more unusual (165, 166, 182, 207). It might be assumed a priori that this complex possesses two uncoordinated double bonds. Treatment of the complex with bromine in carbon tetrachloride, however, did not cause a halogen to be added, and several attempts catalytically to hydrogenate the complex failed to result in hydrogen uptake. It should be mentioned that both cyclooctatetraene and cyclooctadienes undergo these reactions readily. Initial attempts to form Diels-Alder adducts by means of maleic anhydride under normal conditions were not successful, but recently use of the more reactive dienophile tetracyanoethylene has led to positive results (59, 222). Initial infrared studies indicated that no olefinic C = C stretch ing vibrations were present in the region expected for "free" olefinic, double bonds. Most striking of all was the observance of a single, sharp proton resonance signal in the N M R spectum of ( C H ) F e ( C O ) in a variety of solvents and at temperatures as low as — 60°C. Subsequently, Dickens and Lipscomb (65, 66, 67) conducted an x-ray analysis of this complex and found that the cyclooctatetraene ligand pos sessed a novel "dihedral" conformation ( X X X V I ) . The F e ( C O ) group is bound to only one-half of the ring, and this portion of the complex is similar to the geometry in butadiene-iron tricarbonyl. Six of the eight carbon atoms are approximately planar, and this plane forms an angle of 41° with the butadiene-type fragment. Bond distances and angles as well as calculations of overlap integrals indicate appreciably more T T - T interac tion between the butadiene-type portions of the molecule than in the dinuclear-iron complex ( X X X V ) . In view of the structure exhibited by ( C H ) F e ( C O ) in the crystal state, the apparent equivalence of the ligand protons of this complex in solution could be explained in several ways: (1) a negligible chemical shift of various protons; (2) a differing geometry in solid and solution states; and (3) a dynamic effect (65,66,67). Recent studies point to the latter effect as being operative in ( C H ) F e ( C O ) (9). In other words, the F e ( C O ) group is assumed to be rapidly rotating around the cyclooctatetraene ring in solution, and the rate of this rotation is great enough to cause proton equivalency on the N M R time scale of the various valence tautomeric structures: 8


CENTENNIAL

8

8

8

3

8

8

3

3

8

8

8

8

3

3


3

32.

RAUSCH

Organotransition


\

519

Metals

Fe(CO),

ETC.

Fe(CO):

In some additional studies concerning cyclooctatetraene-iron carbonyl chemistry, Keller, Emerson, and Pettit (US) have isolated two additional isomers having the composition (C8Hg)Fe2(CO)6, for which Structures X X X V I I and X X X V I I I have been proposed on the basis of infrared, N M R , and Mossbauer measurements.

(C0) Fe

Fe(CO):

(C0) Fe

Fe(CO),

3

3

XXXVII Another important development in cyclooctatetraene-metal chemistry has concerned the protonation of C H - m e t a l derivatives (59, 221). The complex ( C H ) F e ( C O ) is readily protonated i n strong acids to yield salts of the type [(C H )Fe(CO) ]+ X " , where X = CI, C10 , or B F . The proton N M R spectrum indicates the complex contains the bicyclo[5.1.0]-octadienyl-iron tricarbonyl cation ( X X X I X ) , and this formulation is supported b y the observation that when the tetrafluoroborate salt of 8

8

8

8

(C0) Fe 3

XXXIX

8

3

9

3

BF

4

+NaBH

4

4

(C0) Fe

4

3

XL

this cation is subsequently reduced b y sodium borohydride, bicyclo[5.1.0]-octa-2,4-diene-iron tricarbonyl ( X L ) is formed. I n contrast, the protonation of the cyclooctatetraene complex ( C H ) M o ( C O ) (in which the C H ligand serves as a six, 7r-electron donor) i n concentrated sulfuric acid produces a complex which is best regarded as a homotropylium derivative (253). 8

8

8

3

8


520

WERNER CENTENNIAL


Cyclooctatetraene also forms interesting complexes with cobalt, rhodium, nickel, and other transition metals, but these will not be elaborated on here. It should also be mentioned that other eight-membered ring systems, such as 1,5-cyclooctadiene, 1,3,5- and 1,3,6-cyclooctatrienes, etc., form a variety of metal, T complexes. The most recent survey of cyclooctatetraene and related metal, w complexes is the review by Fischer and Werner {100), as well as earlier reviews by these authors {99), and Bennett {11). Complexes Containing

Nine-, 10- and 12-Membered Ring Systems

A number of organometallic, w complexes containing monocyclic ring systems larger than eight carbon atoms has been reported i n which these rings formally donate two, four, or six w electrons to a transition metal. A n early study along these lines was made by K i n g and Stone {152, 154), who investigated the reaction of bicyclo[4.3.0]-nona-2,4,8-triene (8,9-dihydroindene) ( X L I ) with molybdenum hexacarbonyl. The reaction product, C H i o M o ( C O ) , seems best formulated as a cyclononatetraene complex ( X L I I ) , rather than as a derivative of the parent ring system ( X L I ) which, i n fact, is a valence tautomer of cyclononatetraene. Formulation of C H i M o ( C O ) as X L I I allows one formally uncoordinated double bond in the organic ligand, and the complex was found to take up one molecule of hydrogen readily. A n analogous tungsten complex was also prepared by these workers. 3

9

9

0

3

XLI

XLII

B o t h silver nitrate and molybdenum hexacarbonyl react with the nine-membered ring system cis,m,eis-cyclonona-l,4,7-triene to form com plexes of the types ( C H i ) - 3 A g N 0 and C H i M o ( C O ) , respectively {288). I n the assigned structures, based on N M R and related data, the silver nitrate complex has the silver ions associated with the "outer" irorbital lobes ( X L I I I ) , while the molybdenum tricarbonyl complex has the molybdenum bonded to the "inner" 7r-orbital lobes ( X L I V ) . I n both complexes, a "crown" conformation of the ligand is assumed. Only a very limited amount of research has been published concerning 10-carbon, monocyclic olefins as ligands. I n 1955 Cope and co-workers 9

2

3

9

2

3


32.

RAUSCH

Organotransition

521

Metals

0


XLIII

XLIV

(51) prepared silver complexes of both cis- and Jrans-cyclodecene. In 1965 Jonassen and co-workers (283) reported the synthesis of silver and copper complexes of the dienes m,Jrans-cyclodeca-l,5-diene and m,cis-cyclodeca1,6-diene. Although a detailed structural investigation was not under taken, these workers postulate that the cyclodeca-l,5-diene complex ( C i H i ) C u C l might have the dimeric structure ( X L V ) , in analogy to the known structure of cycloocta-l,5-diene-copper chloride (239). 0

6

XLV Two isomeric 1,5,9-cyclododecatrienes, namely, trans,trans,cisC w H i s ( X L V I ) and trans,trans,trans-Q> 12H18 ( X L V I I ) , are formed in good yield by the cyclic trimerization of butadiene using certain Ziegler-type catalysts (247, 250, 251, 252). The formation of these 12-membered ring hydrocarbons probably proceeds via metal 7r-complexed intermediates. When the cyclic triene ( X L V I I ) is treated with nickel acetylacetonate and

XLVI

XLVII

XLVIII

aluminum alkyls, a very air-sensitive complex of composition C ^ H i s N i is formed. The infrared spectrum of this complex indicates that all three double bonds participate i n bonding with the metal. The complex decom poses thermally at about 140°C. with formation of a nickel mirror and


522

WERNER

cyclododecatriene, while reaction with hydrogen yields nickel and cyclododecane. Because the metal atom i n C ^ H i s N i is two electrons short of an inert gas configuration, it might be expected to react readily with elec tron donors. Such is the case, and both carbon monoxide and triethylphosphine adducts are known. On the basis of its properties and reactions, a structure such as X L V I I I has been proposed for C i H i N i . Cyclododecatrienes likewise form w complexes with transition metals such as silver, copper, palladium, etc. Details of these studies, as well as the work on nickel complexes described above, are contained i n the recent monograph by Fischer and Werner (100). 2


CENTENNIAL

8

Metal Complexes ( n = 1)

a-Bonded Organotransition

E a r l y attempts to prepare stable, isolable, o--bonded organic deriva tives of the transition metals were almost uniformly unsuccessful, except for certain organometallic derivatives of platinum and gold. It has already been mentioned that the "

The History and Development of Organotransition Metal Chemistry

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