Ionization of Cp 2 ZrMe 2 and Lewis Bases by Methylaluminoxane: Computational Insights.

The interactions of the Lewis bases CO, octamethyltrisiloxane (OMTS) and 2,2'-bipyridine (bipy) with a sheet model for the principal activator (MeAlO) 16 (Me 3 Al) 6 (16,6) in hydrolytic methylaluminoxane (MAO) were investigated by DFT. These studies reveal that OMTS and bipy form adducts with Me 3 Al prior to methide abstraction by 16,6 to form the ion-pairs [Me 2 Al(κ 2 -L)][16,6] (5: L=OMTS, 6: L=bipy, [16,6] - =[(MeAlO) 16 (Me 3 Al) 6 Me] - ) while CO simply binds to a reactive edge site without ionization. The binding and activation of Cp 2 ZrMe 2 with 16,6 to form both neutral adducts 1 Cp 2 ZrMe 2 ⋅16,6 and contact ion-pairs 4 and 7, both with formula [Cp 2 ZrMe][μ-Me(MeAlO) 16 (Me 3 Al) 6 ], featuring terminal and chelated MAO-anions, respectively was studied by DFT. The displacement of the anion with either excess Cp 2 ZrMe 2 or Me 3 Al was also studied, forming outer-sphere ion-pairs [(Cp 2 ZrMe) 2 μ-Me][16,6] (2) and [Cp 2 Zr(μ-Me) 2 AlMe 2 ][16,6] (3). The theoretical NMR spectra of these species were compared to experimental spectra of MAO and Cp 2 ZrMe 2 and found to be in good agreement with the reported data and assignments. These studies confirm that 16,6 is a very suitable model for the activators present in MAO but highlight the difficulty in accurately calculating thermodynamic quantities for molecules in this size regime.