[acs symposium series] inorganic chemistry: toward the 21st century volume 211 || multiple...

25 Multiple Metal-Carbon Bonds in Catalysis

RICHARD R. SCHROCK

Massachusetts Institute of Technology, Department of Chemistry, Cambridge, MA 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 metathe-sized 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. Tantalum 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)9 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

0097-6156/83/0211-0369$06.00/0 © 1983 American Chemical Society

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

370 INORGANIC CHEMISTRY: TOWARD T H E 21 ST C E N T U R Y

v i r t u a l l y ce r t a in that the a lky l idene chain t ransfer mechanism i s cor rec t and that the most ac t ive ca ta lys t s are d° complexes (counting the CHR l igand as a d i an ion ) . In t h i s a r t i c l e I ' d l i k e f i r s t to trace the events and f indings which l ed to our concluding that d° a lky l idene complexes were responsible for the rapid 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 su l t s concerning the polymerizat ion of ethylene by an a lky l idene hydride c a t a l y s t , and f i n a l l y some resu l t s concerning the metathesis of acetylenes by tungsten(VI) a lky l idyne complexes. We w i l l see that an important feature of much of the chemistry of mul t ip le metal -carbon bonds i s the ro le played by a lkox ide , oxo, or other π-bonding l igands . Such observations are congruent with some recent ideas and resu l t s Chisholm discusses elsewhere i n t h i s volume concerning alkoxide l igands i n organometallic chemistry.

Tantalum and Niobium Neopentylidene Complexes (4)

The f i r s t neopentylidene complex was prepared by the react ion shown i n equation 1 (5). Although the exact d e t a i l s of t h i s reac-

Dentane Ta(CH 2CMe3) 3Cl2 + 2LiCH2CMe3 - • Ta(CHCMe3)(CH2CMe3)3 (1)

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 version of what has come to be c a l l e d an α-hydrogen atom abst ract ion r eac t ion . The best studied example of α-hydrogen atom abst ract ion i s the in t ramolecular decomposition of Ta(T)5-C5H5)(CH2CMe3)2Cl2 to Ta(îi 5-C5H5)(CHCMe 3)Cl2 (7_). But the simplest and most general i s the α-hydrogen atom abst rac t ion i n M(CH2CMe3)2X3 M = Nb or Ta, X = Cl or Br) promoted by oxygen, n i t rogen, or phosphorus donor l igands ( 8 ) . The r e s u l t i n g octahedral molecules of the type M(CHCMe 3)L2X 3

oTfered an ideal opportunity to study how a neopentylidene complex of Nb or Ta reacts with a simple o l e f i n .

A complex such as Ta(CHCMe 3 )(PMe 3 )2Cl 3 reacts r ead i ly with ethylene, propylene, or styrene to give a l l of the poss ible products (up to four) which can be formed by rearrangement of i n t e r mediate metallacyclobutane complexes (two for subst i tu ted o l e f i n s ) by a β-hydride e l imina t ion process ( e . g . , equation 2) ( £ ) . We saw

absolute ly no evidence for a meta thes is - l ike react ion u n t i l we s tudied the complexes M(CHCMe 3)(THF)2Cl 3 (M = Nb or Ta) . In t h i s case we found low but reproducible y i e l d s (5-15%) of 3 ,3-dimethyl-1-butene upon react ing M(CHCMe 3)(THF)2Cl 3 with ethylene, and i n the case of cis-2-pentene, -6 turnovers to 3-hexenes and 2-butenes. We reasoned~"tiïat the rate of metathesis of the MC3 r i ng was faster

-CMe4

+ (2)

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25. SCHROCK Multiple Metal-Carbon Bonds in Catalysis 371

r e l a t i v e to the rate of rearrangement of the MC3 r i ng when an oxygen donor l igand 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 ec t ive 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 reacts with ethylene, s tyrene, or 1-butene to give l a rge ly t -butyle thylene (equation 3 ) .

Ta=CHR • Bu*CH=CH2 M (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 addi t ional PMe3 to give Ta(CHPh)-(0CMe3)2(PMe3)2Cl. Neither the methylene nor the propylidene complex could be observed, but i n the case of 1-butene we could trace the fate of intermediate metallacyclobutane and a lky l idene complexes . Metathesis of 1-butene was not successful for two reasons. F i r s t , an intermediate β-ethylmetallacyclobutane complex rearranges to 2-methyl-1-butene. Second, intermediate methylene complexes decompose by a bimolecular react ion to give ethylene.

In contras t to the f a i l u r e to metathesize terminal o l e f i n s , i n t e rna l o l e f i n s such as cis-2-pentene can be metathesized to the extent of ~50 turnovers. The chain terminating react ion i n t h i s case i s rearrangement of intermediate ethylidene and propylidene complexes (equation 4). Both rearrangement of intermediate t r i s u b -

M=C'" • CH2=CHR (R = H or Me) (4) N CH 2 R

s t i t u t e d metallacyclobutane complexes and bimolecular decomposition of monosubstituted a lkyl idene complexes must be slow enough to a l low a s i g n i f i c a n t number of steps i n the metathesis react ion to proceed. Rearrangement of intermediate a lky l idene complexes then becomes the major termination step.

There had been some evidence that alkoxide l igands slow down react ions which involve e l imina t ion of a β-hydride from an a l k y l l i g a n d . α -Olef ins are dimerized to a mixture of head- to- ta 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 5-C5Me5)-(CH2=CHR)Cl2 (10K The β,β'- and a,β ' - d i subs t i tu ted t an t a l acyc lo -pentane complexes are intermediates i n t h i s r eac t ion . Their decomposition involves the sequence shown i n equation 5. When one

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372 I N O R G A N I C C H E M I S T R Y : T O W A R D T H E 21ST C E N T U R Y

ch lo r ide l igand i n the c a t a l y s t i s replaced by a methoxide l igand the rate of o l e f i n d imer izat ion decreases by a factor of approxi mately 10 2 as a r e su l t of the greater s t a b i l i t y of the t an t a l a -cyclopentane complexes. Although i t could not be proven, i t was f e l t that 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 that the β -e l imina t ion process by which tantalacyclobutane complexes rearranged to o l e f in s at l eas t would be slowed down by replac ing two ch lo r ide l igands i n Ta(CHCMe3)(PMe 3)2Cl 3 wi th t-butoxide l igands . The question as to whether the rate of metathesis of the TaC 3 r i ng increases upon rep lac ing ch lor ide by t-butoxide l igands i s s t i l l open. However, on the basis of some resu l t s we present l a t e r concerning acetylene metathesis i t seems l i k e l y that t-butoxide l igands encourage reformation of the metal-carbon double bond.

O le f in Metathesis by Tungsten Oxo and Imido Complexes

In retrospect i t i s not su rp r i s ing that the niobium and tantalum a lky l idene complexes we prepared are not good metathesis ca ta 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 lky l idene complexes. The f i r s t successful react ion i s that shown i n equation 6 (L = PMe3 or PE t 3 ) These oxo

Ta(CHCMe 3 )L 2 Cl 3 + W(0)(0CMe3)4 • (6)

Ta(0CMe 3 ) 4 Cl + W(0)(CHCMe 3 )L 2 Cl 2

a lky l idene complexes are octahedral species i n which the oxo and a lky l idene l igands are c i s to one another and the W(0)(CHCp) atoms a l l l i e i n the same plane (12). This type of s t ructure cah be r a t i o n a l i z e d e a s i l y on the Bas is of the fact that the oxo l igand i s an exce l l en t π -e lec t ron donor (13). Therefore, the oxo l igand uses two of the ava i l ab l e three d orBTtals of π-type symmetry to bond to W, leaving only one to form the π-bond between W and the a lky l idene l i g a n d . Several d i f fe ren t types of oxo neopentylidene complexes have been prepared inc lud ing W(0)(CHCMe 3 )(L)Cl 2 , [W(0)(CHCMe3)L 2 Cl] + , and [W(0)(CHCMe3)L 2 ] 2 + . Characterizeable oxo neopentylidene complexes have not yet been prepared d i r e c t l y from oxo neopentyl complexes by α-hydrogen abst rac t ion reac t ions , although Osborn has presented some evidence that they could be (14) . 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 lky l idyne complexes (see l a t e r ) as shown i n equation 7 (15).

[W(CCMe 3 )Cl 4 ] - + 2PEt 3 + E t 3 N + H 20 — ^ (7)

W(0) (CHCMe 3 ) (PEt 3 ) 2 Cl 2

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25. SCHROCK Multiple Metal-Carbon Bonds in Catalysis

Imido a lky l idene complexes were f i r s t prepared by a react ion analogous to that shown i n equation 6. Recently they have been prepared from imido a l k y l complexes by well-behaved α-hydrogen abs t rac t ion reactions (16). Imido neopentylidene complexes seem to be more stable than oxo neopentylidene complexes, poss ibly because the oxo l igand i s s t e r i c a l l y more access ib le to Lewis ac ids , i nc lud ing another tungsten center .

Oxo a lky l idene complexes react with o l e f in s i n the presence of a trace of A1C13 to give new a lkyl idene complexes ( e . g . , b e n z y l i -dene, methylene, ethylidene) (11a). Both terminal and in te rna 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 ca ta lys t s are the i o n i c species , [W(0)(CHCMe3)(PEt3)2Cl] +AlCl4- and [W(0)(CHCMe3)(PEt3)2] 2 +(AlCl 4-)2 i n dichloromethane or chlorobenzene (17). Of the order of 10-20 turnovers per hour for a day or more are possible with these ca t -i o n i c c a t a l y s t s . These studies demonstrate that t r ansa lky l idena -t i o n i s possible with a d° tungsten a lkyl idene complex that w i l l metathesize o l e f i n s s lowly , but conv inc ing ly . There i s s t i l l cons iderable doubt concerning the ro le of the Lewis a c i d . However, the fact that W(0)(CHCMe3)(PEt3)Cl2 (18) metathesizes o l e f in s more r ap id ly i n i t i a l l y than the s ix-coordina te complexes ( in the presence of AICI3) es tabl ishes that a Lewis acid i s not requi red . On the basis of these studies and some ca l cu la t ions by Rappe and Goddard (19) i t would seem incon t rove r t ib l e that the oxo l igand prevents reduction of the metal and perhaps also enhances the rate of reforming an a lky l idene complex from a metallacyclobutane comp l e x . The next question was whether other strong π-donor l igands such as alkoxides could take over the oxo's function ( l i b ) .

Osborn's discovery (14) that aluminum hal ides bincTto oxo l igands i n tungsten oxo neopentyl complexes, and that these complexes 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 , ra ised more questions concerning the ro le of the Lewis a c i d . A subsequent communication (20) answered some of the quest i o n s ; the aluminum hal ide removes Î i ïe oxo l igand and replaces i t wi th two hal ides to y i e l d neopentylidene complexes (equation 8 ) .

Addi t iona l aluminum hal ide coordinates to an ax ia l hal ide to give a species which w i l l metathesize o l e f in s extremely e f f i c i e n t l y . These studies demonstrate that two alkoxide l igands can take the place of an oxo l igand and that aluminum ha l ides , by coordinat ing to a hal ide 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 ca ta l y s t . I t i s poss ible 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 concentrat ions, and i n the

+ AIBr 3

Br

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374 INORGANIC CHEMISTRY: TOWARD T H E 21ST 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 (21) . They suggested that s te reospec i f ic propylene polymerizat ion by~7iegler-Nat ta ca ta lys t s could be explained by a mechanism i n v o l v i n g react ion of the o l e f i n with an a lky l idene l igand i n an a lky l idene hydride c a t a l y s t . We set out to tes t t h i s proposal by preparing and studying tantalum a lky l idene hydride complexes (22). One of these, Ta(CHCMe 3)(H)(PMe 3)3Cl2, reacted r ead i ly with e t h y l ene to give nei ther products of rearrangement nor metathesis of an intermediate tantalacyclobutane complex. High b o i l i n g products were formed but we could not obtain consis tent r e s u l t s . However, r e su l t s using Ta(CHCMe 3 )(H)(PMe 3 ) 3 l2 were consis tent and reproducib le (23).

Ta(CflCMe3)(H)(PMe 3) 3l2 i s probably a pentagonal bipyramidal complex containing an ax ia l neopentylidene l igand with the phos-phines, one i od ide , and the hydride i n the pentagonal plane ( c f . Ta(CCMe 3)(H)(dmpe)2(ClAlMe 3) (24)) . The lH NMR signal for the hydride l igand i s a c h a r a c t e r i s t i c octet at δ 8.29 while the broad a lky l idene α-proton signal i s found at δ -1 .76 . On addi t ion 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 lH NMR patterns appear at δ 7.74 and δ -0.73 consis tent with formation of a new complex, Ta(CHR)(H)(PMe 3 ) 3 l2 ; -50% of the o r i g i n a l Ta(CHCMe 3)(H)-(PMe 3 )3l2 remains. The v o l a t i l e s formed on treatment of t h i s mixture with CF3CO2H cons i s t of neopentane (-50%), and the alkanes Me3C(CH2CH2)nCH3 where η = 1, 2, 3, and 4 (-50% to t a l y i e l d ) , cons is tent with hydro lys i s of Ta[CH(CH2CH2) nCMe 3](H)(PMe 3)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 af ter two days; at t h i s point any further increase of the weight of the polymer i s n e g l i g i b l e . Hydrolysis of the pale green polymer y i e lded a white organic polymer which was shown by f i e l d desorption mass spectral s tudies to cons 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 rge ly organic . By s i m i l a r l y studying the polymer prepared from Ta(CDCMe3)(D)(PMe3)3l2 we showed that only the odd carbon polymers increased by two mass u n i t s . Therefore, most of the even carbon polymers must be polyethylene. The mechanism of chain t ransfer i s at present unknown, but the preceding r e su l t suggests that i t i s not metathesis of metallacyclobutane r i n g s .

There are two ways of viewing the react ion between Ta(CHCMe3)-(H)(PMe3)3l2 and ethylene. The one shown i n equation 9 (nonessen-

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25. S C H R O C K Multiple Metal-Carbon Bonds in Catalysis 375

H I C9H4 Ta=CHCMe3 — • TaCH2CMe3 TaCH2CH2CH2CMe3 •

Ta=CHCH2CH2CMe3

t i a l l igands omitted) contains i n part the c l a s s i c a l Cossee-type step (25) where ethylene " inser t s" in to the tantalum(III)-neopentyl and subsequent t an ta lum(I I I ) -a lky l bonds. I t cannot yet be ruled out since magnetization t ransfer experiments show that the a l k y l i dene α-proton and the hydride l igand i n Ta(CHR)(H)(PMe3)3l 2

exchange r e a d i l y , most l i k e l y by forming Ta(CH 2 R)(PMe3)3l 2 . The a l t e rna t i ve shown i n equation 10 i s analogous to that 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 ngu i sh between these two p o s s i b i l i t i e s , i f i t i s poss ible at a l l . But since a lky l idene l igands i n other tantalum(V) complexes react r ap id ly 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 lky l complexes that react r ead i ly with ethylene (26) , we feel that the second a l t e rna t ive i s more p l a u s i b l e .

H H

One of the most i n t e r e s t i ng aspects of the mechanism shown i n equation 10 i s the l a s t step, an α -e l imina t ion react ion to give the new a lky l idene hydride complex. Our resu l t s do not imply that β -e l imi 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 poss ib le that although K 2 > K j , k i > k 2 (equation 11), i . e . , β -e l imina t ion i s s t i l l f a s te r . I f t h i s i s t rue , i t must also

Ta=CHCH2CH2CMe3 (10)

H

TaCH2CH2R ^ Ta=CHCH2R (11) CHR "k"

-1 k - 2

be true that the o l e f i n hydride complex i s r e l a t i v e l y stable toward displacement of CH2=CHR by ethylene under the react ion condi t ions which we employ.

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376 INORGANIC CHEMISTRY: TOWARD THE 21ST C E N T U R Y

These resu l t s at l ea s t demonstrate that ethylene can be po lymerized by an a lky l idene hydride c a t a l y s t , probably by forming a metallacyclobutane hydride intermediate. The extent to which t h i s i s relevant to the more c l a s s i c a l Z ieg le r -Nat ta polymerizat ion systems (27) i s unknown. Recent resu l t s i n lu te t ium chemistry (28) , where a lky l idene 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 lky l idyne Complexes and Acetylene Metathesis

On the basis of the fact that tungsten(VI) a lky l idene complexes w i l l metathesize o l e f i n s one might predic t that acetylenes should be metathesized by tungsten(VI) a lky l idyne complexes (29). Acetylene metathesis i s not unknown, but the ca ta lys 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 react ing WCle w i * h s i x equivalents of neopentyl l i t h i u m (32). Three equivalents 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 3)(CH?CMe 3) 3 forms obscure. A higher y i e l d route to W(CCMe 3)(CH2CMe 3) 3 cons is t s of the react ion shown i n equation 12 (33). 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 large scale (30 g ) . The mechanism by which W(CCMe3)(CH2CMe3)3 forms v ia t h i s route i s only s l i g h t l y

ether W(0Me) 3 Cl 3 + 6NpMgCH2CMe3 • W(CCMe 3)(CH 2CMe 3) 3 (12)

bet ter understood; the methoxide l igands are bel ieved to prevent reduction of tungsten(VI) and so allow a tungsten(VI) neopenty l i dene complex to form. I t i s f e l t that once a neopentylidene complex forms, formation of a neopentylidyne complex would be f a s t . Since both W(0Me 3) 3(CH2CMe 3) 3 and W(0Me)2Np4 can be prepared, and shown not to be converted in to W(CCMe 3)(CH2CMe 3) 3 under the react i o n cond i t ions , 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(CH2CMe3)2Cl2 i s an i n t e r e s t i ng p o s s i b i l i t y since W(0CH2CMe3)2(CH2CMe3)2Br2 i s a p l aus ib l e precursor to W(CHCMe3)(0CH2CMe3)2Br2 (equation 8 ) .

W(CCMe3)Np3 reacts 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 3)Cl4] quan t i t a t i ve ly (34). I f 1,2-dimethoxyethane i s present instead of NEt4Cl, the product i s purple W(CCMe 3)(dme)Cl 3. E i t he r reacts smoothly with three equivalents of LiX to give W(CCMe 3)X 3 (X = 0CMe 3, SCMe 3, NMe2)· A l l are thermally s t ab le , subl imable, monomeric pale ye l low to white, c r y s t a l l i n e species .

W(CCMe 3)(0CMe 3) 3 reacts rap id ly with symmetric acetylenes to give the new a lky l idyne complexes shown i n equation 13. W(CPh)(0CMe 3) 3 i s orange and W(CCH2CH 2CH 3)(0CMe 3) 3 i s whi te . Both can be sublimed. The l a t t e r i s an important species since i t

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25. SCHROCK Multiple Metal-Carbon Bonds in Catalysis

W(CCMe3)(0CMe3)3 + RCECR • W(CR)(0CMe3)3 + RC=CCMe3 (13)

proves that β-hydrogen atoms are to le ra ted i n the a lky l idyne l i g a n d , and that the bulk of the subst i tuent i n W(CR)(00^3)3 i s probably not a fac tor i n determining whether W(CR)(0CMe3)3 decomposes to W2(0CMe3)ç and RC=CR, or not. (So far we have not observed t h i s r eac t ion . ) We have shown that the s to ich iomet r ic reac t ion 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 (35).

W(CCMe3)(0CMe3)3 reacts rap id ly 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 " 1 sec" 1 ) 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 that W(YI) or Mo(YI) a lky l idyne s i t e s or complexes are responsible 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 that the concentration of ac t ive species on the surface or i n so lu t ion must be extremely s m a l l .

W(CCMe3)(0CMe3)3 i s not the only a lky l idyne complex which w i l l metathesize acetylenes. W(CCMe3)(NMe2)3 w i l l a l s o , although the data so far have not been quant i ta ted . A l l others (W(CCMe3)Np3, [W(CCMe3)Cl4]", W(CCMe3)(dme)Cl3, and W(CCMe3)(SCMe3)3) w i l l not. Of these, we have studied the react ion between the hal ide complexes and a l k y l acetylenes most c l o s e l y . Addi t ion of excess 2-butyne to W(CCMe3)(dme)Cl3 y i e l d s red , paramagnetic, soluble W d ^ - C s l ^ B u * ) -(MeC=CMe)Cl2> and orange, paramagnetic, [W(* 5-C5Me4But)Cl4]2, each i n -50% y i e l d (36). The i d e n t i t y of W(n5-C5Me4But)(MeC=CMe)Cl2 was proven by an x-ray s t ruc tura l study which showed i t to be s i m i l a r to Ta(n5-C5Me5)(PhC=CPh)Cl2 (37) and re l a t ed species . The W(Y) dimer and W(III) acetylene complex probably form by d ispropor t ionat ion of some intermediate W(IY) complex. What we were most in teres ted i n was whether any i n t e r mediates not containing a T ^ - C S I ^ B U * l igand could be i s o l a t e d .

W(CCMe3)(dme)Cl3 reacts with one equivalent of 2-butyne to give a v i o l e t complex whose 1 3 C NMR data are consis tent with i t being a tungstenacyclobutadiene complex (equation 14) (36). In p a r t i c u l a r , two s ignals are found at 268 and 263 ppm (cT7 335 for the neopentylidyne α-carbon atom i n W(CCMe3)(dme)Cl3) and a t h i r d

R = Ph or Pr (excess RC=CR required)

W(CBu f)(dme)CI3+ MeC=CMe (14)

Me

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at 151 ppm. An x-ray s t ruc tura l study showed that i t i s indeed a tungstenacyclobutadiene complex, a t r igona l bipyramidal monomer with ax ia l ch lo r ide l igands and a planar WC3 r i n g . Perhaps the most su rp r i s ing feature of t h i s molecule i s the fact that VI·"Co (2.12Â) i s less than a t y p i c a l W i V D - C ^ y i bond dis tance. This i s probably the reason why the C a - C o - C a angle i s so large ( 1 1 9 ° ) , and could be the reason why the α-subst i tuents are bent away from the meta l . There i s c l e a r l y plenty of room for addi t ional 2-butyne to coordinate to tungsten, probably between C l e q and C a ( / C l e q - W - C a » 140° ) , to produce a WC5 r i ng which then col lapses to a cyclopenta-dienyl system (equation 15). As a r e su l t of t h i s process the metal

lit

(15)

(not observed) (disproportionates)

i s reduced from W(YI) to W(IV). Perhaps at l eas t one ro le of a t -butoxide l igand i s to make the metal more d i f f i c u l t to reduce. I t probably also destabl izes the tungstencyclobutadiene complex r e l a t i v e to the a lky l idyne complex so that the actual concentration 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 ng ly , one t-butoxide l igand can be added i n the equatorial pos i t i on but i f addi t ion of a second i s attempted, only t r i - t -butoxyalkyl idyne complexes r e su l t (equation 16).

Cl Bu* , Λ Ο •

B u * 0 - W ^ - M e • WtCRKOBu^ (16)

C ' M e (R = Bu f or Me)

In view of the s e n s i t i v i t y of o l e f i n metathesis ca ta lys t s to functional groups we were somewhat surprised to f ind that acetylene metathesis ca ta lys t s are apparently much more to le ran t of funct 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 equivalents of a c e t o n i t r i l e , e thylaceta te , phenol, t r i e t h y l ami ne, or in te rna l o l e f in s (35). Consequently, W(CCMe3)(0CMe3)3 w i l l metathesize some func t iona l ized acetylenes. Prel iminary studies show that i t w i l l metathesize EtC=CCH2NMe2, and that 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 possible reasons why and we are i n the process of attempting to f ind out i n de ta i l what they are .

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25. S C H R O C K 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.

3. Hérrison, J.L.; Chauvin, Y. Makromol. Chem. 1970, 141, 161. 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. 8. Rupprecht, G.A.; Messerle, L.W.; Fellmann, J.D.; Schrock, R.R.

J. Am. Chem. Soc. 1980, 102, 6236. 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.

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380 INORGANIC C H E M I S T R Y : TOWARD T H E 21ST C E N T U R Y

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)(PPh3)Me2: Evitt, E.R.; Bergman, R.G. J. Am. Chem. Soc. 1979, 101, 3973.

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.

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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 of Mi n n e s o t a : Does the s t a b i l i t y of the 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 imply t h a t t h e " s p e c i a l e f f e c t " of the OR l i g a n d i s t o enhance the r a t e of de c o m p o s i t i o n of t h i s i n t e r m e d i a t e ?

R.R. Schrock: We b e l i e v e so, but the t - b u t o x i d e l i g a n d seems t o be a unique a l k o x i d e . We have prepared t u n g s t e n a c y c l o b 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 are s t a b l e toward 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 complex.

M.H. Chisholm, I n d i a n a U n i v e r s i t y : I sh o u l d l i k e t o make a p r e d i c t i o n . In view of the a b i l i t y of t u n g s t e n t o form m u l t i p l e bonds w i t h carbon, as i s w e l l i l l u s t r a t e d by your 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 the emergence of a new c l a s s of 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 (C1* ) as a l i g a n d . The probable modes of bonding f o r c a r b i d e should compliment t h a t found f o r oxo and n i t r i d o complexes.

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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 ta: Can you polymeri z e p r o p y l e n e w i t h your t a n t a l um c a t a l y s t ?

R.R. Schrock: No. P r o p y l e n e r e a c t s to g i v e p r i m a r i l y propane and a complex of the type T a ( C H C M e 3 ) ( C H 2 P M e 2 ) ( P M e 3 ) 2 I 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 : Is t h e r e any evidence f o r an exchange of a l k y l i d y n e l i g a n d of the s o r t shown below?

L 2'Ci, 3WECr + L 2CJt 3WECR' * L 2'Cfc 3WECR' + L 2C£ 3W=CR

R.R. Schrock: No. The r e q u i r e d l a b e l l i n g experiment has not been done.

A . J . C a r t y , Guelph-Waterloo C e n t r e : What are the f r e q u e n c i e s of the metal-carbon ( V ( M E C ) ) s t r e t c h i n g f r e q u e n c i e s i n the t u n g s t e n a l k y l i d y n e compounds? One might expect these to be q u i t e high i n view of the x-ray data showing s h o r t MEC bonds and evidence of m u l t i p l e bond r e a c t i v i t y .

R.R. Schrock: There are bands at ca. 1 3 0 0 cm i n the 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 complexes which might be a s s i g n e d t o WEC s t r e t c h i n g modes analogous t o those observed by F i s c h e r i n complexes of the type ( X ) ( C 0 ) 5 W E C R ( X = h a l i d e ) , but we have not done the a p p r o p r i a t e l a b e l l i n g or Raman s t u d i e s t o c o n f i r m the assignments.

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[acs symposium series] inorganic chemistry: toward the 21st century volume 211 || multiple...

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