Interaction of Neutral Donors with Methylaluminoxane

Methylaluminoxane (MAO) is a key activator for olefin polymerization catalysts, making its chemistry of ongoing interest. Strong and bidentate neutral donors such as 2,2-bipyridine are effective abstractors of the dimethylaluminum cation, [Me2Al](+), from methylaluminoxane (MAO), while monodentate, weaker donors such as THF appear most prone to adduct formation with both free and bound trimethylaluminum. The ionization process can be readily investigated using electrospray ionization mass spectrometry (ESI-MS) in fluorobenzene (PhF) solution. Complementary studies employing 1D and 2D H-1 NMR spectroscopic studies in [D-5]bromobenzene solution provide details on the extent of ionization vs. donor-acceptor complex formation for the different donors studied. DFT studies employing different neutral model precursors and mono- vs. didentate donors shed light on possible mechanisms for ion-pair formation.