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]+ ion, 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]+ ion, while [Me]− ion 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. Hydrolysis of trimethylaluminum (Me3Al): Viewed through the lenses of electrospray ionization mass spectrometry and density functional theory, methylaluminoxane activators adopt sheet structures while the larger sheets form chelated anions on reaction with neutral donors. Calculated ion‐pair stabilities in polar media agree well with experimental spectra collected during formation of activators.