Measurement of barriers for alkene dissociation and for inversion at zirconium in a d0 zirconium-alkyl-alkene complex

The beta-allyl zirconacyclobutane complex Cp*Zr-2[CH2CH(CH2CH=CH2)CH2] (7) reacted rapidly with B(C6F5)(3) in CD2Cl2 at -78 degrees C to form the zwitterionic d(0) zirconium(IV) chelate complex Cp*Zr-2[eta(1),eta(2)-CH2CH[CH2B(C6F5)(3)]CH2CH=CH2] (2a and 2b). Low-temperature H-1, C-13, TOCSY1D, and NOESY1D NMR spectroscopy of 2 established the bending of the tethered alkene to the d(0) metal center. A dynamic NMR study of the interconversion of 2a and 2b allowed measurement of the alkene dissociation energy (Delta G(double dagger) = 10.5 (2a to 2b) and 10.3 (2b to 2a) kcal mol(-1)), but the complex decomposed before the barrier for site epimerization at the zirconium center could be determined. Reaction of 7 with [(C6H5)(2)(CH3)NH][B(C6F5)(4)] led to the formation of two isomeric d(0) zirconium(IV)-alkyl-alkene chelates Cp*Zr-2[eta(1),eta(2)-CH2CH(CH3)CH2CH=CH2] [B(C6F5)(4)] (8a and 8b). This more thermally stable zirconium alkyl-alkene complex allowed the measurement of barriers associated with decomplexation of the alkene (Delta G(double dagger) = 10.7 and 11.1 kcal mol(-1)) and site epimerization at the zirconium center (Delta G(double dagger) =14.4 kcal mol(-1)) by line shape analysis of variable-temperature H-1 and C-13 NMR spectra.