Tantalum alkylidyne complexes

See also Scheme 17 for the template synthesis of tri(te/t-butyl)-Cp from a dimeric tantalum alkylidyne complex and 3,3-dimethyl-1-butyne. Schemes 8 and 9 show the syntheses following method 2. The 1,3,5-substi-tuted tri(/m-butyl)cyclopentadiene is obtained as the single isomer from both routes. 

Other (neutral) tantalum alkylidyne complexes formed by a hydrogen abstraction reactions followed, [33,34] although the alkylidyne ligand became much more prominent in the chemistry of Mo, W, and Re. 

The chemistry of alkylidene and alkylidyne complexes of early transition metals was developed by Schrock and co-workers and these complexes turned out to be of crucial importance to alkene and alkyne metathesis. Initially their research focused on tantalum complexes of the type CpTaCEIE, which after a-elimination (Figure 16.6) led to alkylidene complexes Cp(R)Cl2Ta=CHR [11]. 

Scheme 17. Formation of supracyclopentadienyl derivatives from the alkylidyne moiety in the dimeric tantalum complex and alkynes (13). Drawings of molecules are schematic.

In some cases, hydrogenation of the alkylidenes and alkylidynes reduces the metal-carbon multiple bonds to single bonds. The alkyhdene hgand in (29) is converted to an alkyl gronp when exposed to H2, leading to the formation of an interesting tantalum hthium bridging hydride complex. 

Using a procedure related to that for the preparation of tantalum catalysts, supported W-polyhydride species were prepared to evaluate their catalytic activity in alkane metathesis. Hydrogenolysis of the grafted, alkylidyne d°-complex W(=fBu)(CH2tBu)3 [52] led to W-polyhydride complexes (Scheme 2.8) [53]. 

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