Detection of an η-Alkene Intermediate of the Type [Cp2Zr(Me)(η1-alkene)]+: The Role of Such Species in Metallocene Catalyst Deactivation to Allylic Species

The zirconocene complex [Cp 2 ZrMe] + reacts with 2,4-dimethyl-1-pentene to give methane and the allyl complex [Cp 2 Zr(η 3 -CH 2 C(CH 2 CHMe 2 )CH 2 )] + . The reaction involves as an intermediate the unprecedented alkyl η 1 -alkene complex [Cp 2 Zr(Me)(η 1 -CH 2 CMeCH 2 CHMe 2 )] +, in which C(2) is essentially carbocationic and the C(1)−C(2) bond order is sufficiently low that rotation about it occurs. The hydrogen atoms of the methyl group on C(2) are sufficiently acidic that proton transfer to the Zr−Me group occurs to generate the allylic product. It seems likely that the chemistry described here provides a rationale for deactivation of propene polymerization catalysts since the normal products of chain transfer, polymers with vinylidene end groups, should form allylic species in the same way.