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)(2)SiMe2, 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)(6)Me](-). 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.

Automated summary

Hydrolysis of trimethylaluminum in polar solvents was monitored by ESI-MS, showing different anion distributions in hydrolytic methylaluminoxane (h-MAO). NMR spectroscopy revealed variations in activator contents of MAO synthesized under different conditions, with significantly different properties from non-hydrolytic MAO containing incorporated boron. The formation of oligomeric anions in MAO prepared by direct hydrolysis was observed, evolving into a mixture dominated by an anion with m/z 1375.