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 (Me₃ Al) in the presence of donors such as octamethyltrisiloxane (OMTS). Ionization by both methide ([Me]^- ) and [Me₂ Al]⁺ 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 [Me₂ Al]⁺ abstraction, while [Me]^- abstraction from Me₃ Al-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 (Me₃ Al)₄ Me]^- =[7,4]^- are especially stable compared to higher homologues, even though their neutral precursors are unstable.