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Spectroscopic Studies of Synthetic Methylaluminoxane; Structure of Methylaluminoxane Activators.

Hydrolysis of trimethylaluminum (Me 3 Al) in polar solvents can be monitored by electrospray ionization mass spectrometry (ESI-MS) using the donor additive octamethyltrisiloxane [(Me 3 SiO) 2 SiMe 2 , 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 (Me 3 Al) 6 Me] - . Theoretical calculations predict that sheet structures with composition (MeAlO) n (Me 3 Al) m are favored 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 Me 3 Al.