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

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-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., 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 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.