Cages versus Sheets: A Critical Comparison in the Size Range Expected for Methylaluminoxane (MAO)

New cage models (MeAlO)_n (Me₃ Al)_m (n=16, m=6 or 7) isomeric with previously reported sheet models for the principle activator found in hydrolytic MAO (h-MAO) are compared at M06-2X and MN15 levels of theory using density functional theory with respect to their thermodynamic stability. Reactivity of the neutrals or corresponding anions with formula [(MeAlO)₁₆ (Me₃ Al)₆ Me]^- towards chlorination, and loss of Me₃ Al is explored while reactivity of the neutrals towards formation of contact- and outer-sphere ion pairs from Cp₂ ZrMe₂ and Cp₂ ZrMeCl is examined. The results suggest on balance that a cage model for this activator is less consistent with experiment than an isomeric sheet model, although the latter are more stable based on free energy.