Reactivity of Dimetallocycles - ACS Symposium Series (ACS

13 Reactivity of Dimetallocycles ANDREW F. DYKE, STEPHEN R. FINNIMORE, SELBY A. R. KNOX, PAMELA J. NAISH, A. GUY ORPEN, GEOFFREY H. RIDING, and GRAHAM E. TAYLOR

Downloaded by UNIV LAVAL on October 9, 2015 | http://pubs.acs.org Publication Date: June 11, 1981 | doi: 10.1021/bk-1981-0155.ch013

Department of Inorganic Chemistry, The University, Bristol BS8 1TS, England

The involvement of metallocycles in the catalysis of organic reactions is well established, and is exemplified by the studies of Wilke and co-workers in the field of organo-nickel chemistry (1) and by the role of metallocyclobutanes in the alkene metathesis reaction. Until very recently attention was given almost exclusively to the chemistry of metallocycles containing a single metal atom. There is now, however, a growing body of evidence prompting speculation (1-3) that metallocycles based on a dinuclear metal centre ("dimetallocycles") may be equally important. In seeking to understand catalysis of organic reactions by metal surfaces or by metal clusters it is clearly important to consider the dinuclear metal centre (4) and particularly the nature and reactivity of species co-ordinated at (or bridging) the centre. In this paper we describe studies on the synthesis and reactivity of di-iron and di-ruthenium metallocycles derived from alkynes. Their chemistry is marked by an ease of carbon-carbon bond-making and -breaking which provides access to a range of simple organic species (e.g. CCH , CHCH , CMe, CHMe, CMe , CHCHCHMe) bridging the dinuclear metal centre. Related studies with dimolybdenum and ditungsten complexes will be described briefly. 2

2

2

DI-IRON AND DIRUTHENIUM METALLOCYCLES. Under u.v. irradiation the well-known and readily available dimer [Fe (C0) (η-C H )2] reacts with alkynes RC R' to give the dimetallocycles [Fe (C0)(µ-C0){µ-C(0)CRCR'}(η-C H ) ] (1a-1f) in good yield (e.g. 1a, 42 %; 1d, 86 %) (5). Only (1g) (35 %) may be obtained in an analogous manner from [Ru (C0) (η-C H ) ], but on heating (1g) in toluene with other alkynes an unusual exchange occurs rapidly to provide the appropriate complex (1h-1k) in high yield. In the formation of (1) an alkyne and CO ligand have become linked and the exchangers notable for the breaking and 2

4

5

5

2

2

5

2

4

5

2

5

5

2

0097-6156/81 /0155-0259$05.00/0 © 1981 American Chemical Society

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


260

(la) (fb) (lc) (Id) (le) (If)

REACTIVITY OF

M M M M M M

= = = = = =

Fe, Fe, Fe, Fe, Fe, Fe,

R R R R R R

= = = = = =

R' R' R' RH, H,

= H = Me = Ph = C0 Me R' = Me R' = Ph

(lq) (Th) (Π) (Π) (flc) ~

2

M M M M M

= = = = =

METAL-METAL

Ru, Ru, Ru, Ru, Ru,

R R R R R

= = = = =

R* R" R' H, H,

BONDS

= Ph = H = Me R' = Me R* = Ph

regeneration of t h i s carbon-carbon bond. This behaviour i s matched by an unprecedented f l u x i o n a l i t y of d i m e t a l l o c y c l e s (V) derived from alkynes f o r which R = R". This has been c h a r a c t e r i s e d by v a r i a b l e temperature ti and C n.m.r. s p e c t r o scopy as being of the form i l l u s t r a t e d . It comprises a rapid breaking and re-forming of the ' a l k y n e ' - C O l i n k , i n v o l v i n g both carbons of the ' a l k y n e ' , with CO e f f e c t i v e l y entering and l e a v i n g the d i m e t a l l o c y c l e . Free energies of a c t i v a t i o n are in the range 67 (lg) to 85 (]a) kJ m o l " . The s t r u c t u r e of (Icj) was d e t e r mined^ by X-ray d i f f r a c t i o n , r e v e a l i n g that r e l a t i v e l y small atomic movements are a s s o c i a t e d with the f l u x i o n a l rearrangement. l

B

1

In a d d i t i o n to providing an i n d i c a t i o n that d i m e t a l l o c y c l e s may have unexpectedly high l a b i l i t y , the complexes (V) are an e x c e l l e n t source of a range of d i n u c l e a r metal species containing bridging organic l i g a n d s . A s u b s t a n t i a l organic chemistry of d i i r o n and di-ruthenium centres i s in process of being e s t a b l i s h e d as a consequence. Scheme 1 summarises r e a c t i o n sequences evolved from the ethyne-derived dimetal l o c y c l e s ( U ) and (]Ji) and w i l l be described i n some d e t a i l . Comparable systems e x i s t f o r other complexes (1^). The sequence (1 )+(2)-*(2)+(4) i s unique to ruthenium, and to alkynes RC H (R = H7 MeT Ph) in that step (l)+(2) involves a hydrogen s h i f t . Up to about 100 °C ( l h ) undergoes the f l u x i o n a l breaking and regeneration of the ' a l k y n e ' - C O l i n k p r e v i o u s l y d e s c r i b e d , but i n b o i l i n g toluene (111 °C) i r r e v e r s i b l e cleavage of that bond o c c u r s , coupled with a hydrogen s h i f t , g i v i n g the y - v i n y l i d e n e complex [ R u ( C 0 ) ( y - C 0 ) ( y - C C H ) ( n - C H ) ] (2) as c i s - and trans-isomers i n 65 % y i e l d . On heating to the same temperature (]a) decomposes. A d d i t i o n of dry HBF^ to (2) r e s u l t s in p r o t o n a t i o r T a t the methylenic carbon and formation o f the 2

2

2

2

5

5

2


DYKE

13.

ET

Reactivity

AL.

of

261

Dimetallocycles

H

a

H

V

°C 111


M =

(la)

M =

Fe

(lh)

M =

Ru

H

(

1

\

C°

/

\

Ru

/ -Ru'

0ΎO Λ

Ru

(2)

H0 2

Me

CH„

I KU

-KU

0YÖ \

/

c 0

(5a)

M =

Fe

(5b)

M =

Ru

(3)

Me

^

H

M

- M

(4a)

M =

Fe

(4b)

M =

Ru

Scheme

_

1.


REACTIVITY OF

262

METAL-METAL

BONDS

y-methylcarbyne c a t i o n c i s - [ R u ( C O ) ( y - C O ) ( y - C M e ) ( n - C H ) 2 ] (3) q u a n t i t a t i v e l y (6). The c a t i o n i s a c i d i c and deprotonates i n the presence of moisture to regenerate (2). A distinctive f e a t u r e of (3J i s the very low f i e l d C ~ n . m . r . s h i f t of the y carbyne carbon (469.7 p.p.m.), which suggests carbonium ion character. As expected, t h i s carbon i s s u s c e p t i b l e to n u c l e o p h i l i c attack by hydride and (3) i s thereby converted to the y-methylcarbene complex [ R u ( C 0 ) ( y - C 0 ) ( y - C H M e ) ( n - C H ) ] (4b). However, on treatment of the l a t t e r with the hydride a b s t r a c t o r Ph C BFi '" the process i s not r e v e r s e d ; a b s t r a c t i o n occurs from the methyl group to form the y - v i n y l c a t i o n [ R u ( C 0 ) ( y - C 0 ) ( y - C H C H ) ( - C 5 H ) ] (5b). This complex, and i t s i r o n analogue, are best obtained d i r e c t l y and q u a n t i t a t i v e l y from (1) by p r o t o n a t i o n , a transformation which again h i g h l i g h t s the l a b i l i t y of the ' a l k y n e ' - C O carbon-carbon bond. Both (5a) and (5b) are f l u x i o n a l , undergoing a combination of c i s - t r a n s i n t e r conversion and r e o r i e n t a t i o n o f the y - v i n y l l i g a n d (7). Nucleo p h i l i c hydride attack on the y - v i n y l ligand of (5) occurs p r e f e r e n t i a l l y at the y - v i n y l i c carbon, a f f o r d i n g the y - m e t h y l carbene complexes (&). The i r o n analogue of (3) has been obtained by s u b j e c t i n g [ F e ( C 0 ) 4 ( n - C H ) J to methyl l i t h i u m and HBF s e q u e n t i a l l y (8), while dimanganese complexes r e l a t e d to (2) and (3) have been described r e c e n t l y (9). 2

2

5

5

5

5

13

2

2

2

+

3

+

+


2

2

2

n

5

2

2

5

5

2

4

Χ,-Ray d i f f r a c t i o n studies have been completed on each of the species depicted i n Scheme ~ι r• ·•• -sf \ ^/ * \-Zjl•» \JJ ι * \ / ' \£*r These reveal a c l e a r preference f o r a c i s arrangement of the c y c l o pentadienyls throughout. For (4a) and (5a) the methyl and methylene groups of the b r i d g i n g " o r g a n i c figands are anti with respect to the c i s c y c l o p e n t a d i e n y l s , presumably on s t e r i c grounds. Protonation of (2) has l i t t l e e f f e c t on the metal-metal d i s t a n c e ( i t lengthens by~about 0.02 A)but the metal-carbon distance decreases by c a . 0.1 A, i n keeping with d e l o c a l i s a t i o n of charge to the metal atoms and p a r t i a l M=C double bond c h a r a c t e r . Bridging y-methylcarbene i n ($a) i s i n d i c a t e d to be a l e s s e f f e c t i v e π-acceptor than bridging CO by c o n s i d e r a t i o n of the Fe-C distances [1.986(3) c . f . 1.902(3) A ] . Hydride attack on (5b) o c c u r s , u n l i k e (5a), not only at the 3 - v i n l y n i c carbon but a l s o at the α - c a r b o n of the y - v i n y l , g i v i n g a very low y i e l d of the ethene complex , [ R u ( C 0 ) ( C H ) ( y - C 0 ) ( - C H ) ] (6). V a r i a b l e temperature Ή n.m.r. spectroscopy r e v e a l s the complex to be f l u x i o n a l , under going both c i s - t r a n s i n t e r c o n v e r s i o n and ethene r o t a t i o n . F o r t u n a t e l y , t h i s i n t e r e s t i n g s p e c i e s , whose chemistry i s under study, may be obtained i n 70 % y i e l d by e x p l o i t i n g once more the l a b i l i t y of ( l q ) , which s u f f e r s r a p i d displacement of d i p h e n y l acetylene by ethene in b o i l i n g toluene (!Q). The exchange i s reversed when ( £ ) i s i r r a d i a t e d with u.v. l i g h t i n the presence of diphenyl a c e t y l e n e . It may be noted that the formation of (6) from 2

2

i +

2

n

5

5

2


13.

DYKE

ET

Reactivity

AL.

of

263

Dimetallocycles

(6)


(10) represents a conversion of ethyne to ethene at the diruthenium centre. In p r i n c i p l e , a d d i t i o n may occur c i s or trans and t h i s f e a t u r e i s being i n v e s t i g a t e d . In essence, Scheme 1 describes two pathways by which ethyne may be converted to y-methylcarbene at a d i n u c l e a r metal c e n t r e . In t h i s connection i t i s i n t e r e s t i n g to note that studies of the chemisorption of ethyne on metal surfaces have led to suggestions that y-methylcarbene or y-methylcarbyne are formed as surface s p e c i e s , perhaps v i a y - v i n y l i d e n e (1J-13). The sequence (la)-* ( S ä M f e ) "lay be achieved f o r a v a r i e t y of complexes ( 1 J , ~ providing to date y-carbenes as summarised i n Scheme 27 Given the considerable choice of alkynes and n u c l e o p h i l e s a v a i l a b l e there i s c l e a r l y scope f o r the preparation of many more. We are c u r r e n t l y developing an e q u a l l y promising route to ycarbenes, derived uniquely from a l l e n e s (10). For unsubstituted a l l e n e the synthesis i s presented i n Scheme 3. It i s c l e a r from r e s u l t s described e a r l i e r that (lg) i s an e f f i c i e n t source of R u 2 ( C 0 ) 3 ( n - C H 5 ) " and i t again l i b e r a t e s diphenyl acetylene when n

5

2

R (i)

H

+

(11) H

R R R R

= = = =

R' = H H, R' = Me Me, R' = H R' = Me

y-C(H)Me y-C(H)Et y-CMe y-C(Me)Et 2

Scheme

2.


264

REACTIVITY OF

METAL-METAL

BONDS


heated with a l l e n e to y i e l d the unusual complex (7), which has l o s t the metal-metal bond. This i s regained on p r o t o n a t i o n , when the y-1-methyl v i n y l c a t i o n (8) i s formed. Hydride attack on

Scheme 3.

(8) then occurs s p e c i f i c a l l y at the 3-vinyl carbon, generating the μ - d i m e t h y l c a r b e n e complex (9) in 60 % y i e l d o v e r a l l . Substituted a l l e n e s are a v a i l a b l e and we a n t i c i p a t e being able to convert these in the same manner (e.g. buta-1,2-diene to y-C(Me)Et and penta-2,3-diene to y - C E t ) . 2

The t r a n s i t i o n metal chemistry of y-carbenes (14) i s of i n t e r e s t because of the p o s s i b l e involvement of such species i n F i s c h e r - T r o p s c h synthesis (15) and alkene metathesis (16,17). However, apart from y - C H and one example of y-CHMe, simple hydro carbon species have, u n t i l the work described here, been g e n e r a l l y u n a v a i l a b l e and the r e a c t i v i t y of y-carbenes i s e f f e c t i v e l y unexplored. An opportunity was t h e r e f o r e presented f o r such study, in which carbon-carbon bond formation has taken precedence. 2


13.

DYKE E T A L .

Reactivity

of

265

Dimetallocycles

The μ - c a r b e n e complexes are s t r u c t u r a l l y r e l a t e d to the carbonyl-bridged forms of the dimers [M (C0)i (n-C H5)2j and they r e a c t s i m i l a r l y with alkynes under u.v. i r r a d i a t i o n . Reactions of the μ-methylcarbene complexes (4,) are summarised in Scheme 4. The new complexes are derived by l i n k i n g of the carbene and alkyne


2

t

5

R = R' = H, Me, C0 Me R = H, R' = Me R = Me, R' = Ph 9

Scheme 4. and may be viewed as ' i n s e r t i o n ' products. Obtained in 50 - 90 % y i e l d s , they were c h a r a c t e r i s e d s t r u c t u r a l l y through an X-ray d i f f r a c t i o n study of [ F e ( C 0 ) ( y - C 0 ) { y - C H ( M e ) C ( C 0 M e ) } ( n - C H ) ] (18). Bridging ligands of t h i s type have been reported r e c e n t l y (17,19)» but were not obtained in t h i s way. The s t r u c t u r e of the complexes (9) i s s t r i k i n g l y r e l a t e d to that of the complexes [V). However, whereas the ' a l k y n e ' - C O bond in (Y) i s very l a b i l e the ' a l k y n e ' - c a r b e n e carbon-carbon bond in (jjjI i s not. Each of (JJ) i s s t e r e o c h e m i c a l ^ r i g i d and s t a b l e at temperatures up to 100 °C. Slow transformation of the X!-ray compound does occur, in b o i l i n g toluene over several days to produce [ F e ( C O ) { C H C ( C 0 M e ) } ( n - C H ) ] (10). This e x t r a o r d i n a r y 2

2

2

2

2

2

2

2

5

5

2

2

2


5

5

2

REACTIVITY

266

OF

METAL-METAL

BONDS

r e a c t i o n comprises a double β - e l i m i nation from the CMe group, bringing the methyl carbon i n t o c o - o r d i n a t i o n .


The c o - o r d i n a t i o n of the bridging ligand i n ( £ ) may be represented in three ways; that shown e a r l i e r a 'y-allyl' mode (9b) and a ' y - v i n y l c a r b e n e ' representation ( % ) . The l a t t e r appears to have some v a l i d i t y on the basis of bond lengths (the

y-C i s e q u i d i s t a n t from both metals) and n.m.r. d a t a , and suggested that the v i n y l s u b s t i t u e n t of the carbene might be released from complexation. This was achieved when under 100 atm. of CO [Fe (C0)2(y-C0){y-CH(Me)C2H }(n-C5H5)2] (13) was formed as shown, in high y i e l d . Heating or i r r a d i a t i n g (1J) reverses the process. 2

2

(£>

(U>

There are i m p l i c a t i o n s f o r both alkyne polymerisation and alkene metathesis i n the above observations. The sequence (4)->(£)->( 1J) comprises the transformation of one y-carbene to another yjha an alkyne i n s e r t i o n followed by rearrangement. If one envisages a molecule of alkyne in the r o l e played by CO i n the (2)+(]l) conversion then the sequence can be taken a step f u r t h e r through i n s e r t i o n of alkyne i n t o the new y-carbyne. Successive i n s e r t i o n s and rearrangements of t h i s type then provide a mechanism


13.

DYKE E T A L .

Reactivity

of

267

Dimetallocycles


fo alkyne polymerisation at a d i n u c l e a r metal c e n t r e , i n i t i a t e d by a y-carbene. This i s l a i d out in Scheme 5. A scheme f o r alkyne polymerisation i n i t i a t e d by carbenes co-ordinated at a mononuclear metal centre has been postulated (20), and r e c e n t l y i t was shown that such carbenes do i n i t i a t e polymerisation (21).

etc Scheme 5. The a b i l i t y of a y-carbene to r e a c t with an unsaturated hydrocarbon and form an enlarged d i m e t a l l o c y c l e encourages s p e c u l a t i o n over t h e i r r o l e in such processes as alkene metathesis and FischerTropsch s y n t h e s i s . In Scheme 6 a p o s s i b l e mechanism f o r metat h e s i s i n i t i a t e d by a y-carbene i s presented, owing much to other workers (17,22). Reactions of y-carbenes with alkenes are under i n v e s t i g a t i o n i n our l a b o r a t o r y . Recently P e t t i t has observed that the y-methylene complex [ F e ( C 0 ) ( y - C H ) ] generates propene when subjected to a pressure of ethene and has a l s o suggested the intermediacy of a three-carbon d i m e t a l l o c y c l e (23). 2

8

2

DIMOLYBDENUM AND DITUNGSTEN METALLOCYCLES. Most metathesis c a t a l y s t s involve molybdenum or tungsten and in view of our s p e c u l a t i o n over a p o s s i b l e r o l e f o r y-carbenes in the process i t was of i n t e r e s t to attempt the preparation of such complexes of these metals. In an adaptation of the successful route described e a r l i e r , the y-ethyne complex (12) (and analogous



268

REACTIVITY OF

METAL-METAL

BONDS

Scheme 6.

complexes of other alkynes) was treated with HBF^ and NaBH^ in sequence (24). The r e s u l t s are summarised in Scheme 7. Protonation does y i e l d a y - v i n y l c a t i o n , but a d d i t i o n of NaBHi* regenerates (]2) rather than form the desired y-methyl carbene complex ( ] 4 ) . ~ Other n u c l e o p h i l e s such as c h l o r i d e , a c e t a t e , and t r i f l u o r o a c e t a t e attacked molybdenum, g i v i n g the complexes (15) with the y - v i n y l r e t a i n e d . A d d i t i o n of the appropriate a c i d HX to (12) provides ( ] § ) d i r e c t l y . An X.-ray d i f f r a c t i o n study has been completed on the t r i f l u o r o a c e t a t e of (IJj). U.v. i r r a d i a t i o n of [ W ( C 0 ) ( y - C H 5 ) 2 ] in the presence of dimethyl acetylene d i c a r b o x y l a t e e f f e c t s alkyne-CO l i n k i n g as in the d i - i r o n and diruthenium systems, to provide 2

6

5


13.

Reactivity

ET AL.


DYKE

of

269

Dimetallocycles

0 (15) X = C l , C0 Me, C 0 C F 2

2

3

Scheme 7.

[W (C0) i-C(0)C (C0 Me)2}(n-C5H5) ] (16) whose s t r u c t u r e has been e s t a b l i s h e d by X-ray d i f f r a c t i o n (25.) ~ The bridging u n i t i n ( \ § ) i s s u b t l y d i f f e r e n t from that i n (1J, being an η : η l i g a n d r a t h e r than η : η . The new carbon-carbon~bond i s e a s i l y broken on warming to 50 ° C , when CO i s e j e c t e d to produce the y-alkyne complex (1J). Other alkynes r e a c t with [ W ( C 0 ) ( n - C H ) ] to a f f o r d analogues of (17) d i r e c t l y , probably via_ thermally very unstable species o f type (16). In c o n t r a s t to the complexes (1J, protonation of [\6) occurs with r e t e n t i o n of the ' a l k y n e ' - C O l i n k , so that the development of chemistry l i k e that from (JJ appears unlikely. 2

l+

2

2

2

2

1

2

3

2

6

5

5


2


270

REACTIVITY

OF M E T A L - M E T A L

BONDS

SUMMARY D i m e t a l l o c y c l e s have been discovered which e x h i b i t high r e a c t i v i t y with respect to carbon-carbon bond-making and -breaking processes. They allow the synthesis of a v a r i e t y of simple but important hydrocarbon ligands bridging a d i n u c l e a r metal c e n t r e . μ-Carbene complexes are r e a d i l y a v a i l a b l e by several routes and t h e i r r e a c t i o n s have i m p l i c a t i o n s f o r both alkyne polymerisation and alkene metathesis. A s u b s t a n t i a l chemistry o f organic species co-ordinated a t d i n u c l e a r metal centres i s i n prospect, with s i g n i f i c a n c e f o r metal surface chemistry and c a t a l y s i s .

LITERATURE CITED. 1. Wilke, G., Pure Appl.Chem., (1978), 50, 677. 2. Chisholm, M.H. and Cotton F.A., Acc.Chem.Res., (1978),11,356. 3. Knox, S.A.R., Stansfield, R.F.D., Stone, F.G.A., Winter, M.J., and Woodward, P., J.Chem.Soc.,Chem.Commun., (1978), 221. 4. Muetterties, E.L., Rhodin, R.N., Band, E . , Brucker, C.F., and Pretzer, W.R., Chem.Rev., (1979), 79, 91. 5. Dyke, A.F., Knox, S.A.R., Naish, P.J., and Taylor, G.E., J.Chem.Soc.,Chem.Commun., (1980), 409. 6. Davies, D.L., Dyke, A.F., Endesfelder, Α., Knox, S.A.R., Naish, P.J., Orpen, A.G., Plaas, D., and Taylor, G.E., J.Organometallic Chem., (1980), 198, C43. 7. Dyke, A.F., Knox, S.A.R., Naish, P.J., and Orpen, A.G., J.Chem.Soc.,Chem.Commun., (1980), 441, 8. Nitay, Μ., Priester, W., and Rosenblum, M., J.Amer.Chem.Soc., (1978), 100, 3620. 9. Lewis, L.N., Huffman, J.C., and Caulton, K.G., J.Amer.Chem. Soc., (1980), 102, 403. 10. Dyke, A.F., Knox, S.A.R., and Naish, P.J. J.Organometallic Chem., (1980), in press. 11. Ibach, H., Hopster, H., and Sexton, B., Appl.Surf.,Sci., (1977), 1, 1. 12. Kesmodel, L.L., Dubois, L.H., and Somorjai, G.A., Chem.Phys. Lett., (1978), 56, 267.


13. dyke et al. 13. 14. 15. 16. 17. 18. 19.


20. 21. 22. 23. 24. 25.

Reactivity of Dimetallocycles

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Received December 4, 1980.


Reactivity of Dimetallocycles - ACS Symposium Series (ACS

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