Ancillary Ligand and Olefin Substituent Effects on Olefin Dissociation for Cationic Zirconocene Complexes Bearing a Coordinated Pendant Olefin

A series of zirconocene complexes bearing 2,2-dimethyl-2-sila-4-pentenyl substituents (and methyl-substituted olefin variants) ((η^5-C_5H_5)_2Zr(CH_3)(CH_2SiMe_2CH_2CR^1═CR^2R^3) (R^1, R^2, R^3 = H, CH_3, 1, 5−7), (η^5-C_5H_4CMe_3)_2Zr(CH_3)(CH_2SiMe_2CH_2CH═CH_2) (2), {Me_2Si(η^5-C_5H_4)_2}Zr(CH_3)(CH_2SiMe_2CH_2CH═CH_2) (3), and {1,2-(SiMe_2)_2(η^5-C_5H_3)_2Zr(CH_3)(CH_2SiMe_2CH_2CH═CH_2) (4)) have been prepared. Methide abstraction with B(C_6F_5)_3 results in reversible coordination of the tethered olefin to the cationic zirconium center. The kinetics of olefin dissociation have been examined using NMR methods, and the effects of ligand variation for unlinked, singly [SiMe_2]-linked, and doubly [SiMe_2]-linked bis(cyclopentadienyl) arrangements have been compared (ΔG^⧧ values for olefin dissociation vary from 11.4 to 15.6 kcal·mol^(-1) measured over the temperature range 223−283 K). For the cation derived from 4 the kinetics for olefin dissociation and site epimerization (inversion at zirconium) can be distinguished. Additionally, with this ligand system competitive binding of the olefin and the [CH_3B(C_6F_5)_3] anion is observed. Methide abstraction from {1,2-(SiMe_2)_2(η^5-C_5H_3)_2}Zr(CH_3)(CH_2CMe_2CH_2CH═CH_2) results in rapid β-allyl elimination with loss of isobutene to cleanly afford the allyl cation [{1,2-(SiMe_2)_2(η^5-C_5H_3)_2}Zr(η^3-CH_2CH═CH_2)]^+.