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

Ion pairs of the type (Cp2ZrMe+)-Zr-x. . .A(-) containing various ansa-zirc onocene methyl cations in contact with Me-B(C6F5)(3)(-) or B(C6F5)(4)(-) an ions have been studied with regard to their anion exchange kinetics by 2D-N MR methods in benzene or toluene solutions. The results-acceleration of ani on exchange by added Li+. . . Me-B(C6F5)(3)(-), substantial nonproductive e xchange 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 occ urs by way of ion quadruples or higher ionic aggregates, rather than via di ssociation to solvent-separated ions. These findings imply that solvent-sep arated (i.e. anion-free) alkyl zirconocene cations are unlikely to be relev ant intermediates in reaction systems containing (Cp2ZrMe+)-Zr-x. . .A(-) i on pairs and, hence, also in zirconocene based catalyst systems for the pol ymerization of alpha -olefins.