Structure of the most stable methylaluminoxane anion [(MeAlO)16(Me3Al)6Me]− and its precursor

Abstract

Methylaluminoxane (MAO) serves as the predominant cocatalyst in olefin polymerization, yet the structure of its dominant anionic component observed in mass spectrometry has remained problematic despite the recent breakthrough crystallographic characterization of an MAO component reported (L. Luo et al., Science, 2024, 384, 1424–1428), which revealed sheet-like rather than cage-like structures. Here, we resolve this long-standing puzzle by introducing new structural models using hydrocarbon–aluminoxane structural analogy rather than traditional hydrolysis approaches. By extracting finite molecular segments from optimized infinite sheets using an ovalene-like framework, we identified neutral precursor structures that form the dominant MAO anion through [Me₂Al]⁺ abstraction. Our highest-level ab initio calculations reveal that the optimal structure produces an anionic species over 50 kJ mol⁻¹ more stable than previously reported models. Critically, our model exhibits notable structural similarities with the crystallographically characterized structure. These findings resolve the theory–experiment discrepancy and provide accurate structural foundations for understanding MAO activation mechanisms in industrial catalysis.