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

Ž .y wŽ . Ž .q Ž . Displacement of the anion H C–B C F from the zirconocene contact-ion pair C H Zr CH PPP m-H C – 3 6 5 3 5 5 2 3 3 Ž .yx B C F by a series of phosphines and formation of a bisphosphine complex by uptake of a second phosphine ligand have 6 5 3 been studied by NMR methods in C D solutions. Evidence is presented that associated, outer-sphere ion pairs 6 6 wŽ . Ž .Ž .xq Ž .y C H Zr CH PR H C–B C F predominate at zirconocene concentrations up to ca. 2 mM; higher aggregates 5 5 2 3 3 3 6 5 3 become apparent at higher zirconocene concentrations. Equilibrium constants for the formation of monoand 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 Ž . coordination in zirconocene-based polymerization catalysts: i The olefin substrate displaces the borate anion only from a Ž . small equilibrium fraction of the zirconocene alkyl cations present; ii the predominant fraction of the resulting zirconocene Ž . alkyl olefin cation remains in an outer-sphere association with the counteranion; iii mutual displacement of olefin and Ž . 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.