Reactivity Trends of Lewis Acidic Sites in Methylaluminoxane and Some of Its Modifications

The established model cluster (AlOMe) 16 (AlMe 3 ) 6 for methylaluminoxane (MAO) cocatalyst has been studied by density functional theory, aiming to rationalize the different behaviors of unmodified MAO and TMA-depleted MAO/BHT (TMA = trimethylaluminum; BHT = 2,6-di- tert -butyl-4-methylphenol), highlighted in previous experimental studies. The tendency of the three model Lewis acidic sites A – C to release neutral Al fragments (i.e., AlMe 2 R; R = Me or bht) or transient aluminum cations (i.e., [AlMeR] + ) has been investigated both in the absence and in the presence of neutral N-donors. Sites C are most likely responsible for the activation capabilities of TMA-rich MAO, but TMA depletion destabilizes them, possibly inducing structural rearrangements. The remaining sites A and B , albeit of lower Lewis acidity, should be still able to release cationic Al fragments when TMA-depleted modified MAOs are treated with N-donors (e.g. [AlMe(bht)] + from MAO/BHT). These findings provide tentative interpretations for earlier observations of donor-dependent ionization tendencies of MAO and MAO/BHT and how TMA depleted MAOs can still be potent activators.