bk-1983-0211.ch025

25 Multiple Metal-Carbon Bonds in Catalysis RICHARD R. SCHROCK

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Massachusetts Institute of Technology, Department of Chemistry, Cambridge, M A 02139

The presence of alkoxide ligands slows down the rate of rearrangement of tantalacyclobutane rings and probably also speeds up the rate of reforming an alkylidene complex and an olefin. However, tantalum and niobium alkylidene complexes are not good olefin metathesis catalysts because either intermediate methylene complexes decompose rapidly, or because intermediate alkylidene ligands rearrange to olefins. Tungsten(VI) oxo and imido alkylidene complexes will metathesize olefins, probably because rearrangement processes involving a β-hydride are even slower as a result of the π-electron donor abilities of the oxo or imido ligand. Disubstituted acetylenes are metathesized by tungsten(VI) alkylidyne complexes containing t-butoxide ligands. When chloride ligands are present instead of t-butoxides, a tungstenacyclobutadiene complex can be isolated. It reacts with additional acetylene to give a cyclopentadienyl complex. Tanta lum neopentylidene hydride complexes react with ethylene to form new alkylidene hydride complexes in which many ethylenes have been incorporated into the alkyl chain. The polymer that slowly forms in the presence of excess ethylene is approximately a 1:1 mixture of even and odd carbon olefins in the range C50-C100. E.O. Fischer's discovery of (CO)5W[C(Ph)(OMe)] in 1964 marks the beginning of the development of the chemistry of metal-carbon double bonds (1). At about this same time the olefin metathesis reaction was discovered (2) but i t was not until about five years later that Chauvin proposed (3) that the catalyst contained an alkylidene ligand and that the mechanism consisted of the random reversible formation of all possible metallacyclobutane rings. Yet low oxidation state Fischer-type carbene complexes were found not to be catalysts for the metathesis of simple olefins. It is now 9

0097-6156/83/0211-0369$06.00/0 © 1983 American Chemical Society In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

INORGANIC C H E M I S T R Y : TOWARD T H E


370

21 ST

CENTURY

v i r t u a l l y c e r t a i n t h a t the a l k y l i d e n e chain t r a n s f e r mechanism i s c o r r e c t and t h a t the most a c t i v e c a t a l y s t s are d° complexes (count i n g the CHR l i g a n d as a d i a n i o n ) . In t h i s a r t i c l e I ' d l i k e f i r s t to t r a c e the events and f i n d i n g s which l e d to our concluding t h a t d ° a l k y l i d e n e complexes were r e s p o n s i b l e for the r a p i d c a t a l y t i c metathesis of o l e f i n s . Then I want to present some recent r e s u l t s concerning the p o l y m e r i z a t i o n of ethylene by an a l k y l i d e n e hydride c a t a l y s t , and f i n a l l y some r e s u l t s concerning the metathesis of acetylenes by tungsten(VI) a l k y l i d y n e complexes. We w i l l see t h a t an important feature of much of the chemistry of m u l t i p l e metal carbon bonds i s the r o l e played by a l k o x i d e , oxo, or other π-bonding l i g a n d s . Such observations are congruent with some recent ideas and r e s u l t s Chisholm discusses elsewhere i n t h i s volume concerning a l k o x i d e l i g a n d s i n organometallic chemistry. Tantalum and Niobium Neopentylidene Complexes (4) The f i r s t neopentylidene complex was prepared by the r e a c t i o n shown i n equation 1 (5). Although the exact d e t a i l s of t h i s r e a c Dentane Ta(CH CMe3) Cl2 + 2LiCH CMe3 • Ta(CHCMe3)(CH CMe3) -CMe4 2

3

2

2

(1)

3

t i o n are s t i l l unclear ( £ ) , i t i s almost c e r t a i n l y a v e r s i o n of what has come to be c a l l e d an α-hydrogen atom a b s t r a c t i o n r e a c t i o n . The best s t u d i e d example of α-hydrogen atom a b s t r a c t i o n i s the i n t r a m o l e c u l a r decomposition of Ta(T) -C5H5)(CH2CMe )2Cl2 to Ta(îi -C5H5)(CHCMe )Cl2 (7_). But the s i m p l e s t and most general i s the α-hydrogen atom a b s t r a c t i o n i n M(CH2CMe )2X3 M = Nb or Ta, X = Cl or Br) promoted by oxygen, n i t r o g e n , or phosphorus donor l i g a n d s ( 8 ) . The r e s u l t i n g octahedral molecules of the type M(CHCMe )L2X oTfered an i d e a l opportunity to study how a neopentylidene complex o f Nb or Ta r e a c t s with a simple o l e f i n . A complex such as Ta(CHCMe )(PMe )2Cl r e a c t s r e a d i l y with e t h y l e n e , propylene, or styrene to give a l l of the p o s s i b l e p r o ducts (up to four) which can be formed by rearrangement of i n t e r mediate metallacyclobutane complexes (two for s u b s t i t u t e d o l e f i n s ) by a β-hydride e l i m i n a t i o n process ( e . g . , equation 2) ( £ ) . We saw 5

3

5

3

3

3

3

3

3

3

+

(2)

a b s o l u t e l y no evidence for a m e t a t h e s i s - l i k e r e a c t i o n u n t i l we s t u d i e d the complexes M(CHCMe )(THF)2Cl (M = Nb or T a ) . In t h i s case we found low but r e p r o d u c i b l e y i e l d s (5-15%) of 3 , 3 - d i m e t h y l 1-butene upon r e a c t i n g M(CHCMe )(THF)2Cl with e t h y l e n e , and i n the case of c i s - 2 - p e n t e n e , -6 turnovers to 3-hexenes and 2-butenes. We reasoned~"tiïat the r a t e of metathesis of the MC r i n g was f a s t e r 3

3

3

3

3

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

25.

SCHROCK

Multiple

Metal-Carbon

Bonds in

371

Catalysis

r e l a t i v e to the rate of rearrangement of the MC3 r i n g when an oxygen donor l i g a n d was present i n place of a phosphine l i g a n d . Therefore we prepared t-butoxy complexes such as Ta(CHCMe3)(0CMe3)2(PMe3)Cl i n order to see i f s e l e c t i v e metathesis of an i n c i p i e n t MC3 r i n g would r e s u l t . Ta(CHCMe3)(0CMe3)2(PMe3)Cl r e a c t s with e t h y l e n e , s t y r e n e , or 1-butene to give l a r g e l y t - b u t y l e t h y l e n e (equation 3 ) .


Ta=CHR •

Bu*CH=CH

M

2

(R = H,Ph,or Et) When styrene i s the o l e f i n the r e s u l t i n g benzylidene complex can be trapped i n the presence of a d d i t i o n a l PMe3 to give Ta(CHPh)(0CMe3)2(PMe3)2Cl. Neither the methylene nor the propylidene com p l e x could be observed, but i n the case of 1-butene we could t r a c e the fate of intermediate metallacyclobutane and a l k y l i d e n e com p l e x e s . Metathesis of 1-butene was not successful for two reasons. F i r s t , an intermediate β-ethylmetallacyclobutane complex rearranges t o 2-methyl-1-butene. Second, intermediate methylene complexes decompose by a b i m o l e c u l a r r e a c t i o n to give e t h y l e n e . In c o n t r a s t to the f a i l u r e to metathesize terminal o l e f i n s , i n t e r n a l o l e f i n s such as cis-2-pentene can be metathesized to the e x t e n t of ~50 t u r n o v e r s . The chain t e r m i n a t i n g r e a c t i o n i n t h i s case i s rearrangement of intermediate e t h y l i d e n e and propylidene complexes (equation 4). Both rearrangement of intermediate t r i s u b -

M=C'" CH R

•

CH =CHR 2

(R = H or Me)

(4)

N

2

s t i t u t e d metallacyclobutane complexes and b i m o l e c u l a r decomposition o f monosubstituted a l k y l i d e n e complexes must be slow enough to a l l o w a s i g n i f i c a n t number of steps i n the metathesis r e a c t i o n to proceed. Rearrangement of intermediate a l k y l i d e n e complexes then becomes the major t e r m i n a t i o n s t e p . There had been some evidence t h a t a l k o x i d e l i g a n d s slow down r e a c t i o n s which i n v o l v e e l i m i n a t i o n of a β-hydride from an a l k y l l i g a n d . α - O l e f i n s are dimerized to a mixture of h e a d - t o - t a i l and t a i l - t o - t a i l dimers by o l e f i n complexes of the type Ta(Ti -C5Me5)(CH2=CHR)Cl2 ( 1 0 K The β,β'- and a , β ' - d i s u b s t i t u t e d t a n t a l a c y c l o pentane complexes are intermediates i n t h i s r e a c t i o n . T h e i r decomposition i n v o l v e s the sequence shown i n equation 5. When one 5


372

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CHEMISTRY:

TOWARD

T H E 21ST

CENTURY

c h l o r i d e l i g a n d i n the c a t a l y s t i s replaced by a methoxide l i g a n d the rate of o l e f i n d i m e r i z a t i o n decreases by a f a c t o r of a p p r o x i mately 1 0 as a r e s u l t of the greater s t a b i l i t y of the t a n t a l a cyclopentane complexes. Although i t could not be proven, i t was f e l t t h a t the f i r s t step, β-hydride e l i m i n a t i o n , had been slowed down s i g n i f i c a n t l y . Therefore i t was f e l t t h a t the β - e l i m i n a t i o n process by which tantalacyclobutane complexes rearranged to o l e f i n s a t l e a s t would be slowed down by r e p l a c i n g two c h l o r i d e l i g a n d s i n Ta(CHCMe3)(PMe )2Cl w i t h t - b u t o x i d e l i g a n d s . The question as to whether the r a t e of metathesis of the TaC r i n g increases upon r e p l a c i n g c h l o r i d e by t - b u t o x i d e l i g a n d s i s s t i l l open. However, on the basis of some r e s u l t s we present l a t e r concerning acetylene metathesis i t seems l i k e l y t h a t t - b u t o x i d e l i g a n d s encourage reformation of the metal-carbon double bond. 2

3

3


3

O l e f i n Metathesis by Tungsten Oxo and Imido Complexes In r e t r o s p e c t i t i s not s u r p r i s i n g t h a t the niobium and t a n t a lum a l k y l i d e n e complexes we prepared are not good metathesis c a t a l y s t s since these metals are not found i n the " c l a s s i c a l " o l e f i n metathesis systems [2). Therefore, we set out to prepare some tungsten a l k y l i d e n e complexes. The f i r s t successful r e a c t i o n i s t h a t shown i n equation 6 (L = PMe3 or P E t ) These oxo 3

Ta(CHCMe )L Cl 3

2

3

+ W(0)(0CMe3)4

•

(6)

Ta(0CMe ) Cl + W(0)(CHCMe )L Cl 3

4

3

2

2

a l k y l i d e n e complexes are octahedral species i n which the oxo and a l k y l i d e n e l i g a n d s are c i s to one another and the W(0)(CHCp) atoms a l l l i e i n the same plane ( 1 2 ) . This type of s t r u c t u r e cah be r a t i o n a l i z e d e a s i l y on the B a s i s of the f a c t t h a t the oxo l i g a n d i s an e x c e l l e n t π - e l e c t r o n donor ( 1 3 ) . Therefore, the oxo l i g a n d uses two of the a v a i l a b l e three d o r B T t a l s of π-type symmetry to bond to W, l e a v i n g only one to form the π-bond between W and the a l k y l i d e n e l i g a n d . Several d i f f e r e n t types of oxo neopentylidene complexes have been prepared i n c l u d i n g W(0)(CHCMe )(L)Cl , [W(0)(CHCMe3)L Cl] , and [W(0)(CHCMe3)L ] . C h a r a c t e r i z e a b l e oxo neopentylidene complexes have not y e t been prepared d i r e c t l y from oxo neopentyl complexes by α-hydrogen a b s t r a c t i o n r e a c t i o n s , although Osborn has presented some evidence t h a t they could be ( 1 4 ) . Another p o t e n t i a l l y important method of preparing oxo aTFylidene complexes i s by adding water or hydroxide to a l k y l i d y n e complexes (see l a t e r ) as shown i n equation 7 ( 1 5 ) . 3

+

2

[W(CCMe )Cl ]- + 2PEt 3

2

2+

2

4

3

+ Et N + H 0 3

2

—^

W(0)(CHCMe )(PEt ) Cl 3

3

2

(7) 2



25.

SCHROCK

Multiple

Metal-Carbon

Bonds in

Catalysis

Imido a l k y l i d e n e complexes were f i r s t prepared by a r e a c t i o n analogous to t h a t shown i n equation 6. Recently they have been prepared from imido a l k y l complexes by well-behaved α-hydrogen a b s t r a c t i o n r e a c t i o n s (16). Imido neopentylidene complexes seem to be more stable than oxo neopentylidene complexes, p o s s i b l y because the oxo l i g a n d i s s t e r i c a l l y more a c c e s s i b l e to Lewis a c i d s , i n c l u d i n g another tungsten c e n t e r . Oxo a l k y l i d e n e complexes react with o l e f i n s i n the presence of a t r a c e of A1C13 to give new a l k y l i d e n e complexes ( e . g . , b e n z y l i dene, methylene, e t h y l i d e n e ) (11a). Both terminal and i n t e r n a l o l e f i n s can be metathesized slowly i n the presence of aluminum c h l o r i d e . Probably the best c a t a l y s t s are the i o n i c s p e c i e s , [W(0)(CHCMe3)(PEt3)2Cl] AlCl4- and [W(0)(CHCMe3)(PEt3)2] (AlCl -)2 i n dichloromethane or chlorobenzene ( 1 7 ) . Of the order of 10-20 turnovers per hour for a day or more are p o s s i b l e with these c a t i o n i c c a t a l y s t s . These studies demonstrate that t r a n s a l k y l i d e n a t i o n i s p o s s i b l e with a d° tungsten a l k y l i d e n e complex that w i l l metathesize o l e f i n s s l o w l y , but c o n v i n c i n g l y . There i s s t i l l con s i d e r a b l e doubt concerning the r o l e of the Lewis a c i d . However, the f a c t that W(0)(CHCMe3)(PEt3)Cl2 (18) metathesizes o l e f i n s more r a p i d l y i n i t i a l l y than the s i x - c o o r d i n a t e complexes ( i n the presence of AICI3) e s t a b l i s h e s that a Lewis a c i d i s not r e q u i r e d . On the basis of these studies and some c a l c u l a t i o n s by Rappe and Goddard (19) i t would seem i n c o n t r o v e r t i b l e that the oxo l i g a n d prevents reduction of the metal and perhaps also enhances the rate of reforming an a l k y l i d e n e complex from a metallacyclobutane com p l e x . The next question was whether other strong π-donor l i g a n d s such as a l k o x i d e s could take over the oxo's function ( l i b ) . Osborn's discovery (14) that aluminum h a l i d e s bincTto oxo l i g a n d s i n tungsten oxo neopentyl complexes, and t h a t these com plexes decompose to give systems which w i l l e f f i c i e n t l y metathesize o l e f i n s , r a i s e d more questions concerning the r o l e of the Lewis a c i d . A subsequent communication (20) answered some of the ques t i o n s ; the aluminum h a l i d e removes Îiïe oxo l i g a n d and replaces i t w i t h two h a l i d e s to y i e l d neopentylidene complexes (equation 8 ) . +

2+

4

+ AIBr

Br 3

A d d i t i o n a l aluminum h a l i d e coordinates to an a x i a l h a l i d e to give a species which w i l l metathesize o l e f i n s extremely e f f i c i e n t l y . These studies demonstrate t h a t two a l k o x i d e ligands can take the place of an oxo l i g a n d and t h a t aluminum h a l i d e s , by c o o r d i n a t i n g t o a h a l i d e l i g a n d , can generate an e f f i c i e n t and l o n g - l i v e d c a t a lyst. I t i s p o s s i b l e that [W(CHR)(0R)2(Br)] AlBr4" i s responsible for the c a t a l y t i c a c t i v i t y , but at low c o n c e n t r a t i o n s , and i n the +


374

INORGANIC C H E M I S T R Y : TOWARD T H E 2 1 S T C E N T U R Y

presence of excess o l e f i n , bimolecular decomposition of the c a t i o n i c species should be slow. The Reaction of Tantalum Neopentylidene Hydride Complexes with Ethylene


—

A few years ago I v i n , Rooney, and Green made a provocative suggestion for which there was only tenuous experimental support ( 2 1 ) . They suggested t h a t s t e r e o s p e c i f i c propylene p o l y m e r i z a t i o n b y ~ 7 i e g l e r - N a t t a c a t a l y s t s could be explained by a mechanism i n v o l v i n g r e a c t i o n of the o l e f i n with an a l k y l i d e n e l i g a n d i n an a l k y l i d e n e hydride c a t a l y s t . We set out to t e s t t h i s proposal by preparing and studying tantalum a l k y l i d e n e hydride complexes (22). One of these, Ta(CHCMe )(H)(PMe )3Cl2, reacted r e a d i l y with e t h y l ene to give n e i t h e r products of rearrangement nor metathesis o f an intermediate tantalacyclobutane complex. High b o i l i n g products were formed but we could not obtain c o n s i s t e n t r e s u l t s . However, r e s u l t s using Ta(CHCMe )(H)(PMe ) l2 were c o n s i s t e n t and r e p r o ducible (23). Ta(CflCMe3)(H)(PMe ) l2 i s probably a pentagonal bipyramidal complex c o n t a i n i n g an a x i a l neopentylidene l i g a n d with the phosp h i n e s , one i o d i d e , and the hydride i n the pentagonal plane ( c f . Ta(CCMe )(H)(dmpe)2(ClAlMe ) ( 2 4 ) ) . The H NMR s i g n a l for the hydride l i g a n d i s a c h a r a c t e r i s t i c o c t e t at δ 8.29 w h i l e the broad a l k y l i d e n e α-proton s i g n a l i s found at δ - 1 . 7 6 . On a d d i t i o n of a l i m i t e d quantity of ethylene v i r t u a l l y i d e n t i c a l H NMR patterns appear at δ 7.74 and δ -0.73 c o n s i s t e n t with formation of a new complex, T a ( C H R ) ( H ) ( P M e ) l 2 ; -50% of the o r i g i n a l Ta(CHCMe )(H)(PMe )3l2 remains. The v o l a t i l e s formed on treatment of t h i s mixture with CF3CO2H c o n s i s t of neopentane (-50%), and the alkanes Me3C(CH2CH2) CH3 where η = 1, 2, 3, and 4 (-50% t o t a l y i e l d ) , c o n s i s t e n t with h y d r o l y s i s of Ta[CH(CH2CH2) CMe ](H)(PMe )3l2. When CD2CD2 i s used the new product i s Ta(CDR)(D)(PMe3)3l2. When excess ethylene i s added to Ta(CHCMe3)(H)(PMe3)3l2 a pale green polymer slowly forms which weighs approximately four times the o r i g i n a l weight of Ta(CHCMe3)(H)(PMe3)3l2 a f t e r two days; a t t h i s p o i n t any f u r t h e r increase of the weight of the polymer i s n e g l i g i b l e . H y d r o l y s i s of the pale green polymer y i e l d e d a white organic polymer which was shown by f i e l d desorption mass s p e c t r a l s t u d i e s to c o n s i s t of approximately a 1:1 mixture of even and odd carbon o l e f i n s i n the range C S Q - C I O O Therefore, the pale green polymer i s l a r g e l y o r g a n i c . By s i m i l a r l y studying the polymer prepared from Ta(CDCMe3)(D)(PMe3)3l2 we showed t h a t only the odd carbon polymers increased by two mass u n i t s . Therefore, most of the even carbon polymers must be p o l y e t h y l e n e . The mechanism o f chain t r a n s f e r i s at present unknown, but the preceding r e s u l t suggests t h a t i t i s not metathesis of metallacyclobutane r i n g s . There are two ways of viewing the r e a c t i o n between Ta(CHCMe3)(H)(PMe3)3l2 and e t h y l e n e . The one shown i n equation 9 (nonessen3

3

3

3

3

3

3

l

3

3

l

3

3

3

3

n

n

3

3


25.

Metal-Carbon

Multiple

SCHROCK

375

Catalysis

Bonds in

H

I

C9H4

Ta=CHCMe

3

— • TaCH CMe3

TaCH CH CH CMe3

2

2

2

•

2

Ta=CHCH CH CMe3


2

2

t i a l ligands omitted) contains i n p a r t the c l a s s i c a l Cossee-type step (25) where ethylene " i n s e r t s " i n t o the tantalum(III)-neopentyl and subsequent t a n t a l u m ( I I I ) - a l k y l bonds. I t cannot y e t be r u l e d out since magnetization t r a n s f e r experiments show t h a t the a l k y l i dene α-proton and the hydride l i g a n d i n Ta(CHR)(H)(PMe3)3l exchange r e a d i l y , most l i k e l y by forming Ta(CH R)(PMe3)3l . The a l t e r n a t i v e shown i n equation 10 i s analogous to t h a t proposed by I v i n , Rooney and Green. We do not think i t w i l l be easy to d i s t i n g u i s h between these two p o s s i b i l i t i e s , i f i t i s p o s s i b l e at a l l . But since a l k y l i d e n e ligands i n other tantalum(V) complexes r e a c t r a p i d l y with o l e f i n s , and since there are few examples of i s o l able t r a n s i t i o n metal a l k y l complexes t h a t r e a c t r e a d i l y with ethylene ( 2 6 ) , we feel t h a t the second a l t e r n a t i v e i s more p l a u s i b l e . 2

2

H

2

H

Ta=CHCH CH CMe3 2

2

(10)

One of the most i n t e r e s t i n g aspects of the mechanism shown i n equation 10 i s the l a s t s t e p , an α - e l i m i n a t i o n r e a c t i o n to give the new a l k y l i d e n e hydride complex. Our r e s u l t s do not imply t h a t β - e l i m i nation to give an o l e f i n hydride intermediate i s r e l a t i v e l y slow. I t i s p o s s i b l e t h a t although K > K j , k i > k (equation 11), i . e . , β-elimination i s s t i l l faster. I f t h i s i s t r u e , i t must also 2

2

H ^

TaCH CH R 2

CHR

"k"

-1

2

k

Ta=CHCH R 2

(11)

-2

be true t h a t the o l e f i n hydride complex i s r e l a t i v e l y s t a b l e toward displacement of CH =CHR by ethylene under the r e a c t i o n c o n d i t i o n s which we employ. 2


376


These r e s u l t s at l e a s t demonstrate t h a t ethylene can be p o l y merized by an a l k y l i d e n e hydride c a t a l y s t , probably by forming a metallacyclobutane hydride i n t e r m e d i a t e . The extent to which t h i s i s r e l e v a n t to the more c l a s s i c a l Z i e g l e r - N a t t a p o l y m e r i z a t i o n systems (27) i s unknown. Recent r e s u l t s i n l u t e t i u m chemistry ( 2 8 ) , where a l k y l i d e n e hydride complexes are thought to be u n l i k e l y , provide strong evidence for the c l a s s i c a l mechanism.


Tungsten(VI) A l k y l i d y n e Complexes and Acetylene Metathesis On the basis of the f a c t t h a t tungsten(VI) a l k y l i d e n e complexes w i l l metathesize o l e f i n s one might p r e d i c t t h a t acetylenes should be metathesized by tungsten(VI) a l k y l i d y n e complexes ( 2 9 ) . Acetylene metathesis i s not unknown, but the c a t a l y s t s are i n F F f i c i e n t and poorly understood (30, 31). The f i r s t tungsten(VI) aTFylTcTyne complex was prepared i n low y i e l d (-20%) by r e a c t i n g WCle * h s i x equivalents of neopentyl l i t h i u m ( 3 2 ) . Three e q u i v a l e n t s of the l i t h i u m reagent are used simply to reduce W(VI) to W ( I I I ) . Therefore the y i e l d i s l i m i t e d and the mechanism by which W(CCMe )(CH?CMe ) forms obscure. A higher y i e l d route to W(CCMe )(CH2CMe ) c o n s i s t s of the r e a c t i o n shown i n equation 12 ( 3 3 ) . Reproducible y i e l d s of 50-70% can be obtained on a r e l a t i v e l y l a r g e scale (30 g ) . The mechanism by which W(CCMe3)(CH2CMe3)3 forms v i a t h i s route i s only s l i g h t l y w i

3

3

3

3

3

3

ether W ( 0 M e ) C l + 6NpMgCH CMe 3

3

2

3

•

W(CCMe )(CH CMe ) 3

2

3

(12)

3

b e t t e r understood; the methoxide l i g a n d s are b e l i e v e d to prevent r e d u c t i o n of tungsten(VI) and so allow a tungsten(VI) n e o p e n t y l i dene complex to form. I t i s f e l t t h a t once a neopentylidene complex forms, formation of a neopentylidyne complex would be f a s t . Since both W(0Me ) (CH2CMe ) and W(0Me)2Np4 can be prepared, and shown not to be converted i n t o W(CCMe )(CH2CMe ) under the r e a c t i o n c o n d i t i o n s , the c r u c i a l intermediate most l i k e l y s t i l l contains some h a l i d e ( s ) . W(0Me)2(CH2CMe )2Cl2 i s an i n t e r e s t i n g p o s s i b i l i t y since W(0CH2CMe )2(CH2CMe )2Br2 i s a p l a u s i b l e p r e c u r s o r to W(CHCMe )(0CH2CMe )2Br2 (equation 8 ) . W(CCMe )Np r e a c t s with three equivalents of HC1 i n ether or dichloromethane i n the presence of NEt4Cl to y i e l d blue [NEt4][W(CCMe )Cl4] q u a n t i t a t i v e l y ( 3 4 ) . I f 1,2-dimethoxyethane i s present instead of NEt4Cl, the product i s purple W(CCMe )(dme)Cl . E i t h e r reacts smoothly with three equivalents of L i X to give W(CCMe )X (X = 0CMe , SCMe , NMe2)· A l l are thermally s t a b l e , s u b l i m a b l e , monomeric pale y e l l o w to w h i t e , c r y s t a l l i n e s p e c i e s . W(CCMe )(0CMe ) r e a c t s r a p i d l y with symmetric acetylenes to give the new a l k y l i d y n e complexes shown i n equation 13. W(CPh)(0CMe ) i s orange and W(CCH2CH CH )(0CMe ) i s w h i t e . Both can be sublimed. The l a t t e r i s an important species since i t 3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

2

3

3

3


3

25.

Multiple

SCHROCK

Metal-Carbon

W(CCMe )(0CMe3)3 + RCECR

•

3

Bonds in Catalysis

W(CR)(0CMe3)3 + RC=CCMe3

(13)


R = Ph or Pr (excess RC=CR required) proves t h a t β-hydrogen atoms are t o l e r a t e d i n the a l k y l i d y n e l i g a n d , and t h a t the bulk o f the s u b s t i t u e n t i n W(CR)(00^3)3 i s probably not a f a c t o r i n determining whether W(CR)(0CMe3)3 decomposes to W2(0CMe3)ç and RC=CR, or not. (So f a r we have not observed t h i s r e a c t i o n . ) We have shown t h a t the s t o i c h i o m e t r i c r e a c t i o n i s f i r s t order i n tungsten and f i r s t order i n acetylene over a wide range of concentrations ( 3 5 ) . W(CCMe3)(0CMe3)3 r e a c t s r a p i d l y wvth unsymmetric acetylenes to give the i n i t i a l metathesis products, RC=CCMe3 and/or R'CECCMes, and the symmetric acetylenes c a t a l y t i c a l l y . The most impressive i s the metathesis of 3-heptyne where the value for k ( M " s e c " ) i s between 1 and 10. Therefore, i n neat 3-heptyne (-1 M) at 25° the number of turnovers i s of the order of several per second. I f we assume t h a t W(YI) or Mo(YI) a l k y l i d y n e s i t e s or complexes are r e s p o n s i b l e for the r e l a t i v e l y slow metathesis i n the known heterogeneous (30) and homogeneous (31) systems, then i t becomes c l e a r t h a t the c o n c e n t r a t i o n of a c t i v e species on the surface or i n s o l u t i o n must be extremely s m a l l . W(CCMe3)(0CMe3)3 i s not the only a l k y l i d y n e complex which w i l l metathesize a c e t y l e n e s . W(CCMe3)(NMe2)3 w i l l a l s o , although the data so f a r have not been q u a n t i t a t e d . A l l others (W(CCMe3)Np3, [W(CCMe3)Cl4]", W(CCMe )(dme)Cl3, and W(CCMe3)(SCMe3)3) w i l l not. Of these, we have s t u d i e d the r e a c t i o n between the h a l i d e complexes and a l k y l acetylenes most c l o s e l y . A d d i t i o n of excess 2-butyne to W(CCMe3)(dme)Cl3 y i e l d s r e d , paramagnetic, s o l u b l e W d ^ - C s l ^ B u * ) (MeC=CMe)Cl2> and orange, paramagnetic, [W(* -C5Me4But)Cl4]2, each i n -50% y i e l d ( 3 6 ) . The i d e n t i t y of W(n -C5Me4But)(MeC=CMe)Cl2 was proven by an x-ray s t r u c t u r a l study which showed i t to be s i m i l a r to Ta(n -C5Me5)(PhC=CPh)Cl2 (37) and r e l a t e d s p e c i e s . The W(Y) dimer and W(III) acetylene complex probably form by d i s p r o p o r t i o n a t i o n of some intermediate W(IY) complex. What we were most i n t e r e s t e d i n was whether any i n t e r mediates not c o n t a i n i n g a T ^ - C S I ^ B U * l i g a n d could be i s o l a t e d . W(CCMe3)(dme)Cl3 r e a c t s with one e q u i v a l e n t of 2-butyne to give a v i o l e t complex whose C NMR data are c o n s i s t e n t with i t being a tungstenacyclobutadiene complex (equation 14) ( 3 6 ) . In p a r t i c u l a r , two s i g n a l s are found at 268 and 263 ppm (cT7 335 f o r the neopentylidyne α-carbon atom i n W(CCMe3)(dme)Cl3) and a t h i r d 1

1

3

5

5

5

1 3

W(CBu )(dme)CI + MeC=CMe f

(14)

3

Me


378

INORGANIC

CHEMISTRY:

TOWARD T H E 2 1 S T C E N T U R Y

a t 151 ppm. An x-ray s t r u c t u r a l study showed t h a t i t i s indeed a tungstenacyclobutadiene complex, a t r i g o n a l bipyramidal monomer w i t h a x i a l c h l o r i d e l i g a n d s and a planar WC3 r i n g . Perhaps the most s u r p r i s i n g feature of t h i s molecule i s the f a c t t h a t VI·"Co (2.12Â) i s l e s s than a t y p i c a l W i V D - C ^ y i bond d i s t a n c e . This i s probably the reason why the C - C o - C angle i s so large ( 1 1 9 ° ) , and c o u l d be the reason why the α - s u b s t i t u e n t s are bent away from the m e t a l . There i s c l e a r l y plenty of room f o r a d d i t i o n a l 2-butyne to coordinate to tungsten, probably between C l q and C ( / C l q - W - C » 1 4 0 ° ) , to produce a WC5 r i n g which then c o l l a p s e s to a c y c l o p e n t a d i e n y l system (equation 15). As a r e s u l t of t h i s process the metal a

a


e

a

e

a

lit

(15)

(not observed)

(disproportionates)

i s reduced from W(YI) to W(IV). Perhaps at l e a s t one r o l e of a t - b u t o x i d e l i g a n d i s to make the metal more d i f f i c u l t to reduce. I t probably a l s o d e s t a b l i z e s the tungstencyclobutadiene complex r e l a t i v e to the a l k y l i d y n e complex so t h a t the actual c o n c e n t r a t i o n of a tungstenacyclobutadiene t r i - t - b u t o x i d e complex i s s m a l l . I n t e r e s t i n g l y , one t - b u t o x i d e l i g a n d can be added i n the e q u a t o r i a l p o s i t i o n but i f a d d i t i o n of a second i s attempted, only t r i - t b u t o x y a l k y l i d y n e complexes r e s u l t (equation 16).

Cl Bu* Bu*0-W^-Me C

'

M e

,

Λ

Ο

• •

WtCRKOBu^

(16)

(R = Bu or Me) f

In view of the s e n s i t i v i t y of o l e f i n metathesis c a t a l y s t s to f u n c t i o n a l groups we were somewhat s u r p r i s e d to f i n d t h a t acetylene metathesis c a t a l y s t s are apparently much more t o l e r a n t of func t i o n a l groups than o l e f i n metathesis c a t a l y s t s . For example, W(CCMe3)(0CMe3)3 w i l l metathesize 3-heptyne i n the presence of many e q u i v a l e n t s of a c e t o n i t r i l e , e t h y l a c e t a t e , phenol, t r i e t h y l ami ne, or i n t e r n a l o l e f i n s ( 3 5 ) . Consequently, W(CCMe3)(0CMe3)3 w i l l metathesize some f u n c t i o n a l i z e d a c e t y l e n e s . P r e l i m i n a r y s t u d i e s show t h a t i t w i l l metathesize EtC=CCH2NMe2, and t h a t i t w i l l cross metathesize Me3SiOCH2C=CCH20SiMe3 with 3-hexyne. However, f i r s t attempts to metathesize EtC=CCH2Cl, H0CH2C=CCH20H with 3-hexyne, or EtC=CC02Me f a i l e d . There are several p o s s i b l e reasons why and we are i n the process of attempting to f i n d out i n d e t a i l what they are.


25.

SCHROCK

Multiple

Metal-Carbon

Bonds in

Catalysis

379


Literature Cited

1.

Fischer, E.O.; Maasböl, A. Angew Chem. Int. Ed. Eng. 1964, 3, 580.

2.

(a) Grubbs, R.H. Prog. Inorg. Chem. 1978, 24, 1-50; (b) Katz, T.J. Adv. Organomet. Chem. 1977, 16, 283-317; (c) Calderon, N.; Lawrence, J. P.; Ofstead, E.A". Adv. Organomet. Chem. 1979, 17, 449-492; (d) Rooney, J.J.; Stewart, A. Spec. Period. Rep.: Catal. 1977, 1, 277. Hérrison, J.L.; Chauvin, Y. Makromol. Chem. 1970, 141, 161.

3. 4.

Schrock, R.R. in "Reactions of Coordinated Ligands"; Braterman, P.S., Ed.; Plenum: New York, in press.

5.

Schrock, R.R. J. Am. Chem. Soc. 1974, 96, 6794.

6.

Schrock, R.R.; Fellmann, J.D. J. Am. Chem. Soc. 1978, 100, 3359.

7.

Wood, CD.; McLain, S.J.; Schrock, R.R. J. Am. Chem. Soc. 1979, 101, 3210. Rupprecht, G.A.; Messerle, L.W.; Fellmann, J.D.; Schrock, R.R. J. Am. Chem. Soc. 1980, 102, 6236.

8. 9.

Rocklage, S.M.; Fellmann, J.D.; Rupprecht, G.A.; Messerle, L.W.; Schrock, R.R. J. Am. Chem. Soc. 1981, 103, 1440.

10

McLain, S.J.; Sancho, J.; Schrock, R.R. J. Am. Chem. Soc. 1980, 102, 5610. 11. (a) Schrock, R.R.; Rocklage, S.; Wengrovius, J.; Rupprecht, G.; Fellmann, J. J. Molec. Catal. 1980, 8, 73; (b) Wengrovius, J.H.; Schrock, R.R. Organometallics 1982, 1, 148. 12. Churchill, M.R.; Rheingold, A.L. Inorg. Chem. 1982, 21, 1357. 13. Griffith, W.P. Coord. Chem. Rev. 1970, 5, 459. 14. Kress, J.; Wesolek, M.; Le Ny, J.; Osborn, J.A. J. Chem. Soc. Chem. Commun. 1981, 1039. 15. Rocklage, S.M.; Schrock, R.R.; Churchill, M.; Wasserman, H.J. Organometallics, in press.


380


16. Pedersen, S.F.; Schrock, R.R. J. Am. Chem. Soc., in press. 17. Wengrovius, J.H.; Ph.D., Thesis, Massachusetts Institute of Technology, 1981. 18. Wengrovius, J.; Schrock, R.R.; Churchill, M.R.; Missert, J.R.; Youngs, W.J. J. Am. Chem. Soc. 1980, 102, 4515.


19. Rappé, A.K.; Goddard, W.A. Ill J. Am. Chem. Soc. 1982, 104, 448; 1980, 102, 5114. 20. Kress, J.; Wesolek, M.; Osborn, J.A. J. Chem. Soc. Chem. Commun. 1982, 514. 21. Ivin, K.J.; Rooney, J.J.; Stewart, C.D.; Green, M.L.H.; Mahtab, R. J. Chem. Soc. Chem. Commun. 1978, 604. 22. Fellmann, J.D.; Turner, H.W.; Schrock, R.R. J. Am. Chem. Soc. 1980, 102, 6608. 23. Turner, H.W.; Schrock, R.R. J. Am. Chem. Soc. 1982, 104, 2331. 24. Churchill, M.R.; Wasserman, H.J.; Turner, H.W.; Schrock, R.R. J. Am. Chem. Soc. 1982, 104, 1710. 25. Cossee, P. J. Catal. 1964, 3, 80. 26. An example of a complex that reacts slowly with ethylene in a manner consistent with insertion of ethylene into the metal alkyl bond is Co(n5-C5H5)(PPh)Me2: Evitt, E.R.; Bergman, R.G. J. Am. Chem. Soc. 1979, 101, 3973. 3

27. Boor, J. Jr. "Ziegler-Natta Catalysts and Polymerizations"; Academic: New York, 1979. 28. Watson, P.L. J. Am. Chem. Soc. 1982, 104, 337-339. 29. Katz, T.J. J. Am. Chem. Soc. 1975, 97, 1592-1594. 30. (a) Panella, F.; Banks, R.L.; Bailey, G.C. J. Chem. Soc. Chem. Commun. 1968, 1548-1549; (b) Moulijn, J.Α.; Reitsma, H.J.; Boelhouwer, C. J. Catal. 1972, 25, 434-436. 31. (a) Mortreux, Α.; Delgrange, J.C.; Blanchard, M.; Labochinsky, Β. J. Molec. Catal. 1977, 2, 73; (b) Devarajan, S.; Walton, O.R.M.; Leigh, G.J. J. Organomet. Chem. 1979, 181, 99-104.


25. SCHROCK Multiple Metal-Carbon Bonds in Catalysis

381

32. Clark, D.N.; Schrock, R.R. J. Am. Chem. Soc. 1978, 100, 6774. 33.

Schrock, R.R.; Clark, D.N.; Sancho, J.; Wengrovius, J.H.; Rocklage, S.M.; Pedersen, S.F. Organometallics, in press.

34. Wengrovius, J.H.; Sancho, J.; Schrock, R.R. J. Am. Chem. Soc. 1981, 103, 3932.


35.

Sancho, J.; Schrock, R.R. J. Molec. Catal. 1982, 15, 75-79.

36. Schrock, R.R.; Pedersen, S.F.; Churchill, M.R.; Wasserman, H.J. J. Am. Chem. Soc., in press. 37. Smith, G.; Schrock, R.R.; Churchill, M.R.; Youngs, W.J. Inorg. Chem. 1981, 20, 387. RECEIVED August 1 1 ,

1982

Discussion W.L. G l a d f e l t e r , U n i v e r s i t y o f M i n n e s o t a : Does t h e s t a b i l i t y of t h e t r i c h l o r o t u n g s t e n a c y c l o b u t a d i e n e complex i m p l y t h a t t h e " s p e c i a l e f f e c t " o f t h e OR l i g a n d i s t o enhance t h e r a t e o f decomposition of t h i s intermediate?

R.R. S c h r o c k : We b e l i e v e s o , b u t t h e t - b u t o x i d e ligand seems t o be a u n i q u e a l k o x i d e . We have p r e p a r e d tungstenacyclob u t a d i e n e complexes c o n t a i n i n g o t h e r a l k o x i d e l i g a n d s which a r e s t a b l e t o w a r d d e c o m p o s i t i o n t o an a l k y l i d y n e c o m p l e x .

M.H. Chisholm, Indiana University: I should like to make a p r e d i c t i o n . I n v i e w o f t h e a b i l i t y of tungsten t o form m u l t i p l e bonds w i t h c a r b o n , a s i s w e l l i l l u s t r a t e d by y o u r work w i t h a l k y l i d e n e and a l k y l i d y n e l i g a n d s , I b e l i e v e we s h a l l soon see t h e emergence o f a new c l a s s o f t u n g s t e n compounds i n c o r p o r a t i n g c a r b i d e (C * ) a s a l i g a n d . The p r o b a b l e modes o f b o n d i n g f o r carbide should compliment t h a t found f o r oxo and n i t r i d o c o m p l e x e s . 1


382

INORGANIC C H E M I S T R Y :

TOWARD THE

W.L. G l a d f e l t e r , U n i v e r s i t y of M i n n e s o t a : i z e p r o p y l e n e w i t h your t a n t a l u m c a t a l y s t ?

21ST

Can you

CENTURY

polymer-

R.R. S c h r o c k : No. Propylene reacts to g i v e primarily p r o p a n e and a complex of t h e t y p e T a ( C H C M e ) ( C H P M e ) ( P M e ) I . 3

2

2

3

2

2


J.M. B u r l i t c h , C o r n e l l U n i v e r s i t y : I s t h e r e any e v i d e n c e f o r an exchange of a l k y l i d y n e l i g a n d o f t h e s o r t shown below? L 'Ci, WECr 2

3

+

R.R. Schrock: n o t b e e n done.

L CJt WECR' 2

No.

3

The

*

L 'Cfc WECR' 2

required

3

+

labelling

L C£ W=CR 2

3

experiment

has

A.J. C a r t y , G u e l p h - W a t e r l o o C e n t r e : What a r e t h e f r e q u e n cies of the metal-carbon (V(MEC)) stretching frequencies i n t h e t u n g s t e n a l k y l i d y n e compounds? One m i g h t e x p e c t t h e s e t o be q u i t e h i g h i n v i e w o f t h e x - r a y d a t a showing s h o r t M E C bonds and e v i d e n c e of m u l t i p l e bond r e a c t i v i t y .

R.R. S c h r o c k : T h e r e a r e bands a t c a . 1 3 0 0 cm i n t h e IR s p e c t r a of s e v e r a l a l k y l i d y n e c o m p l e x e s w h i c h m i g h t be a s s i g n e d t o WEC s t r e t c h i n g modes a n a l o g o u s t o t h o s e o b s e r v e d by F i s c h e r i n c o m p l e x e s o f t h e t y p e ( X ) ( C 0 ) W E C R ( X = h a l i d e ) , but we have n o t done t h e a p p r o p r i a t e l a b e l l i n g o r Raman s t u d i e s t o c o n f i r m the assignments. 5


bk-1983-0211.ch025

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