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Ionization of Cp2ZrMe2 and Lewis Bases by Methylaluminoxane: Computational Insights

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The interactions of the Lewis bases CO, octamethyltrisiloxane (OMTS) and 2,2’-bipyridine (bipy) with a sheet model for the principal activator (MeAlO)16(Me3Al)6 (16,6) in hydrolytic methylaluminoxane (MAO) were investigated by DFT. These studies reveal that OMTS and bipy form adducts with Me3Al prior to methide abstraction by 16,6 to form the ion-pairs [Me2Al(κ2-L)][16,6] (5: L = OMTS, 6: L = bipy, [16,6]− = [(MeAlO)16(Me3Al)6 Me]−) while CO simply binds to a reactive edge site without ionization. The binding and activation of Cp2ZrMe2 with 16,6 to form both neutral adducts 1 Cp2ZrMe2•16,6 and contact ion-pairs 4 and 7, both with formula [Cp2ZrMe][µ-Me(MeAlO)16(Me3Al)6], featuring terminal and chelated MAO-anions, respectively was studied by DFT. The displacement of the anion with either excess Cp2ZrMe2 or Me3Al was also studied, forming outer-sphere ion-pairs [(Cp2ZrMe)2µ-Me][16,6] (2) and [Cp2Zr(µ-Me )2AlMe2][16,6] (3). The theoretical NMR spectra of these species were compared to experimental spectra of MAO and Cp2ZrMe2 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

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