SYNTHESIS AND REACTIVITY OF PERMETHYLTANTALOCENE DERIVATIVES POSSESSING ETA-3-ALLYL, ETA-2-BUTADIENE, ETA-2-METHYLALLENE, AND ETA-1-ALKENYLIDENE LIGANDS - MODEL STUDIES FOR THE ROLE OF SURFACE VINYLIDENES IN THE FISCHER-TROPSCH HYDROCARBON-CHAIN-LENGTHENING PROCESS

The reactivity of Cp*2TaH3 (2) (Cp* = eta-5-C5Me5) toward a number of conjugated dienes has been investigated, and mechanistic pathways have been probed by deuterium-labeling studies. Treatment of 2 with allene at 50-degrees-C affords Cp*2Ta(eta-3-C3H5) (3), which, according to H-1 and C-13 NMR data, is most reasonably described as a tantalabicyclobutane complex. Warming at 80-degrees-C converts 3, via the intermediate Cp*2Ta(H)(HC = CMe), to the hydrido propenylidene species Cp*2Ta(H)[= C = C(H)Me] (4). Butadiene reacts with 2 to give sequentially Cp*2Ta(H)(eta-2-CH2 = CHCH = CH2) (8), Cp*2Ta(H)[eta-2-CH2 = C = C(H)Me] (9), Cp*2Ta(H)[= C = C(H)Et] (10), and Cp*2Ta(H)(MeC = CMe) (11). 9 exists in the form of two geometrical isomers in which the allene methyl substituent lies in the equatorial coordination plane of the [Cp*2Ta] fragment. Mechanistic studies have revealed the importance of beta-H elimination from sp2-hybridized carbon, allene roll, and 1,2-hydrogen shift processes. Treatment of Cp*2Ta(H)(= C = CH2) with excess CH2 = PMe3 leads to the formation of higher order linear-chain vinylidene homologues Cp*2Ta(H)[= C = C(H)R] (R = Me, Et). A mechanism involving the intermediacy of vinylidene species and the migration of-Sigma-alkenyl groups to a metal-bound methylene is proposed for the Fischer-Tropsch hydrocarbon-chain-lengthening process.