Anion exchange in alkyl-zirconocene borate ion pairs. Are solvated alkyl-zirconocene cations relevant intermediates?

Ion pairs of the type (Cp2ZrMe+)-Zr-x. . .A(-) containing various ansa-zirconocene methyl cations in contact with Me-B(C6F5)(3)(-) or B(C6F5)(4)(-) anions have been studied with regard to their anion exchange kinetics by 2D-NMR methods in benzene or toluene solutions. The results-acceleration of anion exchange by added Li+. . . Me-B(C6F5)(3)(-), substantial nonproductive exchange between added and Zr-bound Me-B(C6F5)(3)(-) anions, an increase of exchange rates at increased zirconocene concentrations, and the exclusively entropic origin of this rate increase-all indicate that anion exchange occurs by way of ion quadruples or higher ionic aggregates, rather than via dissociation to solvent-separated ions. These findings imply that solvent-separated (i.e. anion-free) alkyl zirconocene cations are unlikely to be relevant intermediates in reaction systems containing (Cp2ZrMe+)-Zr-x. . .A(-) ion pairs and, hence, also in zirconocene based catalyst systems for the polymerization of alpha -olefins.