Ancillary ligand and olefin substituent effects on olefin dissociation forcationic zirconocene complexes bearing a coordinated pendant olefin

A series of zirconocene complexes bearing 2,2-dimethyl-2-sila-4-pentenyl su bstituents (and methyl-substituted olefin variants) ((eta (5)-C5H5)(2)Zr(CH 3)(CH2SiMe2CH2CR1=(CRR3)-R-2) (R-1, R-2, R-3 = H, CH3, 1, 5 - 7), (eta (5)- C5H4CMe3)(2)Zr(CH3)(CH2SiMe2CH2CH=CH2) (2), {Me2Si(eta (5)-C5H4)(2)}- Zr(CH 3)(CH2SiMe2CH2CH=CH2) (3), and {1,2-(SiMe2)(2)(eta (5) -C5H3)(2)Zr(CH3)(CH2 SiMe2CH2CH= CH2) (4)) have been prepared. Methide abstraction with B(C6F5)( 3) results in reversible coordination of the tethered olefin to the cationi c zirconium center. The kinetics of olefin dissociation have been examined using NMR methods, and the effects of ligand variation for unlinked, singly [SiMe2] -linked, and doubly [SiMe2] -linked bis(cyclopentadienyl) arrangem ents have been compared (DeltaG(double dagger) values for olefin dissociati on vary from 11.4 to 15.6 kcal . mol(-1) measured over the temperature rang e 223-283 K). For the cation derived from 4 the kinetics for olefin dissoci ation and site epimerization (inversion at zirconium) can be distinguished. Additionally, with this ligand system competitive binding of the olefin an d the [CH3B(C6F5)(3)] anion is observed. Methide abstraction from {1,2-(SiM e2)(2)(eta (5)-C5H3)(2)}Zr(CH3)(CH2CMe2CH2CH=CH2) results in rapid P-allyl elimination with loss of isobutene to cleanly afford the allyl cation [{1,2 -(SiMe2)(2)(eta (5)-C5H3)(2)}Zr(eta (3)-CH2CH=CH2)](+).