Density functional benchmarks for methylaluminoxane: successes, biases, and transferability limits

Methylaluminoxane (MAO) is the most widely used cocatalyst for olefin polymerization, yet its molecular structure remains incompletely understood. Computational methods are essential for elucidating the structure and reactivity of MAO, but their accuracy has been difficult to assess. Benchmarking based on trimethylaluminum (TMA) dimerization has proven insufficient because it omits the oxygen environments central to MAO chemistry. Here, we benchmark 25 density functional methods against DLPNO-CCSD(T) and RI-MP2 reference calculations for a set of MAO oligomers, evaluating geometry, electronic energy, vibrational frequencies, and thermodynamic properties. We identify energetic biases toward specific structural motifs (µ₄-O and µ-Me) in ten functionals, with errors ranging from under 5 kJ mol^-1 to over 45 kJ mol^-1. ωB97X-D4 and MN15 show the largest errors and should be avoided for pure MAO systems. Two functionals, ωB97X-V and ωB97M-D4, accurately reproduce reference electronic energies without structural bias. For vibrational properties, quasi-harmonic treatment effectively reduces method-dependent entropy variations, and thermochemical corrections prove robust across functionals. However, extending these methods to metallocene-MAO ion pairs reveals distinct system-dependent behavior that warrants further investigation. Together, these findings highlight that functional performance is system-dependent, with significant implications for computational studies of catalyst activation mechanisms.