Are Methylaluminoxane Activators Sheets?

Density functional theory calculations on neutral sheet models for methylaluminoxane (MAO) indicate that these structures, containing 5-coordinate and 4-coordinate Al, are likely precursors to ion-pairs seen during the hydrolysis of trimethylaluminum (Me3Al) in the presence of donors such as octamethyltrisiloxane (OMTS). Ionization by both methide ([Me](-)) and [Me2Al](+) abstraction, involving this donor, were studied by polarizable continuum model calculations in fluorobenzene (PhF) and o-difluorobenzene (DFB) media. These studies suggest that low MW, 5-coordinate sheets ionize by [Me2Al](+) abstraction, while [Me](-) abstraction from Me3Al-OMTS is the likely process for higher MW 4-coordinate sheets. Further, comparison of anion stabilities per mole of aluminoxane repeat unit (MeAlO)(n), suggest that anions such as [(MeAlO)(7)(Me3Al)(4)Me](-)=[7,4](-) are especially stable compared to higher homologues, even though their neutral precursors are unstable.

Automated summary

Density functional theory calculations on neutral sheet models for methylaluminoxane suggest that 5-coordinate and 4-coordinate aluminum structures may be precursors to ion pairs observed during the hydrolysis of trimethylaluminum. The studies show that low molecular weight 5-coordinate sheets mainly ionize through [Me2Al](+), while higher molecular weight 4-coordinate sheets are more likely to ionize through [Me](-) from Me3Al-OMTS. Additionally, comparisons of anion stabilities per mole of aluminoxane repeat unit indicate that certain anions are especially stable compared to higher homologues, despite their unstable neutral precursors.