rac-Me 2 Si(2-Me-4-t-Bu-C 5 H 2) 2 ZrMe + : an alkyl zirconocenium cation stabilized by steric shielding against interaction with ancillary ligands

Abstract

The sterically shielded cation rac-Me₂Si(2-Me-4-t-Bu-C₅H₂)₂ZrMe⁺, generated by reaction of rac-Me₂Si(2-Me-4-t-Bu-C₅H₂)₂ZrMe₂ with methylalumoxane (MAO) or with dimethylanilinium or trityl perfluorotetraphenylborate, differs from other, more open zirconocene methyl cations in that it does not form the symmetric AlMe₃ adduct rac-Me₂Si(2-Me-4-t-Bu-C₅H₂)₂Zr(μ-Me)₂AlMe₂⁺. Instead, the unsymmetric ternary adduct rac-Me₂Si(2-Me-4-t-Bu-C₅H₂)₂ZrMe(μ-Me)AlMe₃−NMe₂Ph⁺ is formed in the presence of dimethylaniline and trimethylaluminum. UV/vis and ¹H NMR characteristics of hydrocarbon solutions containing rac-Me₂Si(2-Me-4-t-Bu-C₅H₂)₂ZrMe⁺***B(C₆F₅)4- or rac-Me₂Si(2-Me-4-t-Bu-C₅H₂)₂ZrMe⁺***MAOMe^- indicate that cation−anion association in these ion pairs is unusually weak.