210627-15-9Relevant academic research and scientific papers
Anion exchange in alkyl-zirconocene borate ion pairs. Are solvated alkyl-zirconocene cations relevant intermediates?
Beck,Lieber,Schaper,Geyer,Brintzinger
, p. 1483 - 1489 (2001)
Ion pairs of the type Cpx2ZrMe+···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 Cpx2ZrMe+···A - ion pairs and, hence, also in zirconocene-based catalyst systems for the polymerization of α-olefins.
