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 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)4Me]- = [7,4]- are especially stable compared to higher homologues, even though their neutral precursors are unstable.