Distinct methylalumoxane(MAO)-derived Me–MAO− anions in contact with a zirconocenium cation—a 13C-NMR study

Abstract

Zirconocenium cations of the type [(MeC₅H₄)₂ZrMe]⁺, formed by excess methylalumoxane (MAO) from (MeC₅H₄)₂ZrCl₂ or (MeC₅H₄)₂ZrMe₂ with ¹³C-labelled ring ligands, are found to form ion pairs with two types of anions, Me–MAO_A⁻ and Me–MAO_B⁻, which differ in their coordinative strengths: More strongly coherent ion pairs [(MeC₅H₄)₂ZrMe⁺⋯Me–MAO_B⁻] are converted to more easily separable ion pairs [(MeC₅H₄)₂ZrMe⁺⋯Me–MAO_A⁻] by a sufficient excess of MAO. These react with Al₂Me₆ to form outer-sphere ion pairs containing the cationic AlMe₃ adduct [(MeC₅H₄)₂Zr(µ-Me)₂AlMe₂]⁺; formation of the more easily separable ion pairs might be required also for polymerisation catalysis.