Modification of methylaluminoxane-activated ansa-zirconocene catalysts with triisobutylaluminum-transformations of reactive cations studied by NMR spectroscopy

When triisobutylaluminum (AliBu3) is added to solutions containing methylaluminoxane (MAO) and rac-[Me2Si(ind)2ZrCl2] (ind: indenyl) in C6D6, NMR spectra show that methyl-bridged mixed-alkylaluminum dimers Al(μ-Me)2Me4−xiBux predominate. These dimers react with MAO under partial transfer of isobutyl groups and induce a conversion of the initially prevailing cationic trimethylaluminum adduct rac-[Me2Si(ind)2Zr(μ-Me)2AlMe2+] to rac-[Me2Si(ind)2Zr(μ-Me)2AlMeiBu+] and rac-[Me2Si(ind)2Zr(μ-Me)2AliBu2+]. These species are unstable and release isobutene under formation of zirconocene hydrides