Displacement of weakly coordinating anions from zirconocene alkyl cations by trialkyl phosphine : a model for olefin coordination in homogeneous Ziegler-Natta catalysis

Abstract

Displacement of the anion H3C–B(C6F5)3− from the zirconocene contact-ion pair [(C5H5)2Zr(CH3)+⋯(μ-H3C)–B(C6F5)3−] by a series of phosphines and formation of a bisphosphine complex by uptake of a second phosphine ligand have been studied by NMR methods in C6D6 solutions. Evidence is presented that associated, outer-sphere ion pairs [(C5H5)2Zr(CH3)(PR3)]+ H3C–B(C6F5)3− predominate at zirconocene concentrations up to ca. 2 mM; higher aggregates become apparent at higher zirconocene concentrations. Equilibrium constants for the formation of mono- and bis-PMe3 complexes have been determined; the latter are found to be highly sensitive to steric perturbations. These data, together with density-functional estimates for reaction enthalpies, lead to the following qualitative conclusions with regard to olefin coordination in zirconocene-based polymerization catalysts: (i) The olefin substrate displaces the borate anion only from a small equilibrium fraction of the zirconocene alkyl cations present; (ii) the predominant fraction of the resulting zirconocene alkyl olefin cation remains in an outer-sphere association with the counteranion; (iii) mutual displacement of olefin and counteranion from the Zr center is slow compared to typical olefin insertions; (iv) uptake of a second olefin under formation of a cationic zirconocene alkyl diolefin complex is rather unlikely even at elevated olefin concentrations.