ansa-metallocene derivatives : X. Hydrogenolysis of the zirconium-alkyl bond in bridged and unbridged alkyl zirconocene derivatives. Evidence for direct and ring-mediated hydrogen transfer reactions

Abstract

The rate of hydrogenolytic alkane liberation from permethylzirconocene neopentyl halide compounds, (C5(CH3)5)2Zr(X)CH2C(CH3)3, X = F, Cl, Br), is greatly reduced if the ring ligands are interconnected by an ethylene bridge, as in C2H4(C5(CH3)4)2Zr(X)CH2C(CH3)3. Hydrogenolysis of the corresponding permethylzirconocene neopentyl hydride compounds (X = H), on the other hand, is too fast for kinetic measurements at room temperature, even with the ethylene-bridged derivative. These observations, and the inverse kinetic isotope effect observed for reaction with D2, are interpreted with the assumption that H2-induced alkane liberation from permethylzirconocene alkyl halides proceeds via an indirect ring-mediated hydrogen transfer reaction which is feasible only with freely rotating ring ligands; hydrogenolysis of permethylzirconocene alkyl hydrides, on the other hand, apparently occurs without such limitation, by direct H2-to-alkyl hydrogen transfer.