Spectroscopic Studies of Synthetic Methylaluminoxane: Structure of Methylaluminoxane Activators

Hydrolysis of trimethylaluminum (Me3Al) in polar solvents can be monitored by electrospray ionization mass spectrometry (ESI‐MS) using the donor additive octamethyltrisiloxane [(Me3SiO)2SiMe2, OMTS]. Using hydrated salts, hydrolytic methylaluminoxane (h‐MAO) features different anion distributions, depending on the conditions of synthesis, and different activator contents as measured by NMR spectroscopy. Non‐hydrolytic MAO was prepared using trimethylboroxine. The properties of this material, which contains incorporated boron, differ significantly from h‐MAO. In the case of MAO prepared by direct hydrolysis, oligomeric anions are observed to rapidly form, and then more slowly evolve into a mixture dominated by an anion with m/z 1375 with formula [(MeAlO)16(Me3Al)6Me]−. Theoretical calculations predict that sheet structures with composition (MeAlO)n(Me3Al)m are favoured over other motifs for MAO in the size range suggested by the ESI‐MS experiments. A possible precursor to the m/z 1375 anion is a local minimum based on the free energy released upon hydrolysis of Me3Al. Hydrolysis of Me6Al2 using hydrated salts, or reaction of Me6Al2 with trimethylboroxine affords methylaluminoxane (MAO) with different ESI‐MS spectra, NMR spectra and residual Me3Al content. Unlike commercial MAO, which upon treatment with donors, consists mainly of a single anionic species [(MeAlO)16(Me3Al)6Me]−=[16,6]−, synthetic MAO can differ significantly in its ion‐pair speciation. Theory suggests the ion‐pair precursors have sheet structures.