Evidence for mixed-ion clusters in metallocene catalysts: Influence on ligand exchange dynamics and catalyst activity

Alkylzirconium ion pairs [L2Zr-CH2SiMe3+center dot center dot center dot B(C6F5)(4)(-)], as models of catalytic zirconium polymeryl species, form mixed-ion clusters [{LZrR+}(n){CPh3+}(m){X-}((n+m))], as shown by pulsed field-gradient spin-echo NMR measurements. Higher ion aggregation leads to enhanced rates of site epimerization. This rate enhancement is used to probe previously inaccessible kinetic barriers for the "chain swinging" process. The presence or absence of such a barrier has implications for catalytic activity and response to ionic activators.