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

Displacement of the anion H3C-B(C6F5)(3)(-) from the zirconocene contact-ion pair [(C5H5)(2)Zr(CH3)(+)...(mu-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)(2)Zr(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 berate 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. (C) 1998 Elsevier Science B.V.