rac-Me2Si(2-Me-4-t-Bu-C5H2)2ZrMe+:: An alkyl zirconocenium cation stabilized by steric shielding against interaction with ancillary ligands

The sterically shielded cation rac-Me2Si(2-Me-4-t-Bu-C5H2)(2)ZrMe+, generated by reaction of rac-Me2Si(2-Me-4-t-Bu-C5H2)(2)ZrMe2 with methylalumoxane (MAO) or with dimethyl-anilinium 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)(2)Zr(mu-Me)(2)AlMe2+. Instead, the unsymmetric ternary adduct rac-Me2Si(2-Me-4-t-Bu-C5H2)(2)ZrMe(mu-Me)AlMe3-NMe2Ph+ is formed in the presence of dimethylaniline and trimethylaluminum. U`V/vis and H-1 NMR characteristics of hydrocarbon solutions containing rac-Me2Si(2-Me-4-t-Bu-C5H2)(2)ZrMe+...B(C6F5)(4)- or rac-Me2Si(2-Me-4-t-Bu-C5H2)(2)ZrMe(+...)MAOMe(-) indicate that cation-anion association in these ion pairs is unusually weak.