Neopentylidene complex tantalum

A series of tantalum imido complexes has been synthesized by the addition of imines or nitriles to tantalum neopentylidene complexes as shown in Eq. (76). 

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... 

The Reaction of Tantalum Neopentylidene Hydride Complexes with Ethylene... 

As with all previous complexes, tris(neopentyl)neopentylidene tantalum(V) reacts... 

To follow the mode of grafting of an organometallic species onto Si02, the tris (neopentyl)neopentylidene tantalum complex has been used to show the reactivity of its groups with silanol groups of silica (dehydroxylated at 300 to 700 °C)... 

Tantalum is the first element of subgroup Ya for which both pentaalkvl and ylidic compounds could be completely characterized and fully investigated. Not only was it possible to prove the existence of Ta(CH3)6 and some of its complexes (S3), but the tantalum ylide, trisneopentyl neopentylidene tantalum, could also be isolated (82a). This ylide is the product of a reaction between trisneopentyltantalum dichloridc and neo-... 

Schrock carbenes are typically generated by a-hydrogen elimination from a high-valent, early metal, dialkyl complex concomitant with or followed by elimination of alkane. The first S5mthesis of an alkylidene by Schrock is summarized in Equation 13.13. Addition of neopentyl zinc to tantalum pentachloride generated tris(neopentyl)dichlorotantalum. Subsequent addition of neopentyllithium did not generate the homoleptic neopentyltantalum compound. Instead, this reaction generated the first alkylidene complex, tris(neopentyl) tantalum neopentylidene. ... 

The use of a silica-supported, tantalum alkyhdene as a precursor for alkane metathesis was found to result in a stoichiometric, alkane cross metathesis in which an initial pendant alkyl-alkylidene group was transformed to produce the desired, active species [76, 90]. This reaction was later observed to work with additional, well-defined systems supported on alumina [58] and sihca-alumma [53]. As mentioned previously, this transformation does not occur when the surface organometallic precursor bears no alkyl group. Exposing these supported, metal neopentyl, neopentylidene, and neopentyhdyne complexes to alkane at 150 °C produced alkanes containing a neopentyl fragment (CH jiBu) via cross metathesis. Propane metathesis with these alkyl-alkyhdene surface complexes typically generates stoichiometric amounts of dimethylpropane, 2,2-dimethylbutane, and 2,2-dimethylpentane (Scheme 2.11). 

Basset and his group have observed that propane and propene metathesis give similar Cn+i/Cn+2 ratios of cross-metathesis products on silica-supported tantalum-neopentylidene catalyst at 150°C. The olefin-metathesis activity of these Schrock-type supported complexes results from the presence of the silyloxy ligand (vide infra) - Organometallic complexes are bound to silica or alumina by reaction of soluble complexes and involve die formation of one or several bonds between the central metal and the oxygen atom of the oxide support. 

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