Methylaluminoxane Reactivities and Anionic Structures: From Small Oligomers to Large Sheets

The structure and reactivity of small methylaluminoxane (MAO) species (MeAlO)_n(Me₃Al)_m (n=1-8) have been investigated using DFT (M06-2X), MP2, and CCSD(T) calculations. This hierarchy of methods reveals that DFT artificially stabilizes structures containing 4-coordinate oxygen atoms while higher-level calculations demonstrate a clear preference for structures with 3-coordinate oxygen and 4-coordinate aluminum centers. Analysis of ionization pathways shows these neutral MAO molecules form anions through either methide or Me₂Al⁺ abstraction, with the latter mechanism dominant for sheet structures (n=5-8). Smaller species (n=1-4, m ≥ n) show minimal reactivity toward either ionization pathway. The resulting anions consistently adopt sheet structures with 3-coordinate oxygen and 4-coordinate aluminum at all levels of theory. We introduce a comprehensive stability metric incorporating both ionization energies and neutral precursor stability, which successfully explains the distribution of anions observed in ESI-MS spectra during Me₃Al hydrolysis. Extension of this analysis to larger species, including the recently isolated and characterized sheet (MeAlO)₂₆(Me₃Al)₉ (Luo et al., Science, 2024, 384, 1424-1428), reveals unexpected trends in reactivity.