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

Abstract

The established model cluster (AlOMe)₁₆(AlMe₃)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-<i>tert</i>-butyl-4-methylphenol), highlighted in previous experimental studies. The tendency of the three model Lewis acidic sites <b>A</b>-<b>C</b> to release neutral Al fragments (i.e., AlMe₂R; 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 <b>C</b> are most likely responsible for the activation capabilities of TMA-rich MAO, but TMA depletion destabilizes them, possibly inducing structural rearrangements. The remaining sites <b>A</b> and <b>B</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.