Niobium neopentylidene complexes

R. R.Schrock, The Reaction of Niobium and Tantalum Neopentylidene Complexes with the Carbonyl Function, J. Am. Chem. Soc. 98, 5399-5400 (1976). 

A conceptual process for ethylene dimerization in the presence of tantalum or niobium based catalysts has been developed by MIT researchers [6-8]. The technology is based on a metal hydride-based homogeneous catalyst that selectively dimerizes ethylene to butene-1. The particular catalyst is neopentylidene complex of tantalum or niobium. The preparation of the homogeneous catalyst is rather a complex process the tantalum complex is prepared by reacting tri-neopentyl tantalum dichloride, Ta(CH2CMe3)3Cl2 and neopentyl lithium LiCH2CMe3 in octane solvent to yield thermally stable neopentylidene tantalum catalyst in quantitative yield. 

The Wittig-type olefination of carbonyl compounds is one of the characteristic reactions of carbene complexes. High-valent carbene complexes of early transition metals show ylide-like reactivity to vards carbonyl compounds. In 1976, Schrock first demonstrated that niobium and tantalum neopentylidene complexes 1 and 2, the typical nucleophilic Schrock-type carbene complexes, olefinate various carbonyl compounds including carboxylic acid derivatives [4]. 

Niobium neopentylidene complex, Nb[CH2C(CH3)3]3[CHC(CH3)3] (2), can also be prepared in a manner similar to that for the corresponding tantalum analogue 1... 

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