Zirconocene allyl complexes : dynamics in solution, reaction with aluminum alkyls, B(C 6 F 5) 3 -induced propene insertion, and density-functional calculations on possible formation and reaction pathways

Abstract

The complex (C5H5)2Zr(Me)(methallyl) (1) was prepared and studied as a model for Zr-bound allyl species which are likely to arise in zirconocene-based polymerization systems. 1 undergoes allyl−alkyl exchange with trimethyl aluminum (TMA) or with methyl alumoxane (MAO) at rates that are proportional to the Al concentration, but remain 1−2 orders of magnitude below those of typical olefin insertions. Its perfluorotriphenylborane adduct 2, i.e., the contact ion pair (C5H5)2Zr(methallyl)+H3CB(C6F5)3-, has been characterized with regard to the rearrangement dynamics of its allyl ligand. Propene reacts with 2, at rates which are substantially lower again than those of cationic Zr−alkyl species, under insertion between Zr and one of the allylic termini. Scrambling of deuterium from an allylic CD2 terminus over several C atom positions next to the unsaturated chain end indicates rather extensive metal migration within an initial olefin insertion product. Density-functional calculations indicate that insertion of propene directly into an η3-coordinated Zr−allyl unit occurs with lower activation energy than insertion into an η1-bound Zr−allyl species and that the lowest-energy pathway for the reactivation of a cationic zirconocene allyl species is its reconversion to the corresponding Zr−alkyl species by molecular hydrogen.