Displacement of weakly coordinating anions from zirconocene alkyl cations by trialkyl phosphines: A model for olefin coordination in homogeneous Ziegler–Natta catalysis
✍ Scribed by Stefan Beck; Marc-Heinrich Prosenc; Hans-Herbert Brintzinger
- Publisher
- Elsevier Science
- Year
- 1998
- Tongue
- English
- Weight
- 366 KB
- Volume
- 128
- Category
- Article
- ISSN
- 1381-1169
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✦ Synopsis
¨¨Ä bstract 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 53 5 52 3 3 Ž . y x 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 .x q 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 mono-and bis-PMe 3 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. q 1998 Elsevier Science B.V.