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

Abstract

New cage models (MeAlO)n(Me3Al)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(Me3Al)6Me]− towards chlorination, and loss of Me3Al is explored while reactivity of the neutrals towards formation of contact‐ and outer‐sphere ion pairs from Cp2ZrMe2 and Cp2ZrMeCl 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.

Sheet Models for Methylaluminoxane (MAO) Activators? A Theoretical Case Study involving rac‐Me2Si(η5‐C9H6)2Zr (SBIZr) Complexes

“…As in our previous studies with Cp 2 ZrMe 2 , [17] and Cp 2 ZrMeCl [23] one can form SBIZr contact ion‐pairs 2 featuring Zr‐μ‐X−Al interactions ( 2 a – c : X=Me, 2 d – e : X=Cl) with sheet 16,6 . The lowest E conformers are depicted in Figure 1 while energetic data at the M06‐2X [29] /TZVP [30] level of theory in gas phase, and MN15/def2‐TZVP in toluene continuum appear in Table S‐1 and Table 1, respectively.…”

“…Theory predicts formation of ion‐pairs 2 d – e is more favorable than 2 a – b from their respective neutral precursors. This is also true for outer‐sphere ion‐pairs 3 and 4 and was also seen in earlier studies of the Cp 2 Zr system [17,23] . Quantitative experimental data on this point is limited as L 2 ZrMeCl complexes are very infrequently used as catalyst precursors while earlier ESI‐MS work provides no direct insight into solution concentrations as ion intensity also depends on other factors [11,13] .…”

“…Higher level DLPNO‐CCSD(T) calculations for a smaller aluminoxane sheet model 10,5 , [23] show calculation of E using M06‐2X/TZVP over‐estimates the energy stabilization of two outer‐sphere π complexes, with respect to their contact ion‐pair precursors and monomer by ΔΔE=−12.0 ( syn ) to −17.6 ( anti ) kJ mol −1 in gas phase (Supporting Information, Table S‐5). In contrast, the transition structure featured better agreement with a deviation of ΔΔE=−3.6 kJ mol −1 .…”

“…One expects them to feature more weakly coordinating anions than our sheet model for [ 16,6 (Me)] − where insertion has a high barrier and e. g. binding of Me 3 Al to 2 in the case of the parent Cp 2 Zr system is quite unfavorable compared with experiment. Ironically, an isomeric cage model was no better as far as Me 3 Al binding is concerned [23] …”

“…Most recently, we compared cage and sheet structures for their ability to activate Cp 2 ZrMeCl or Cp 2 ZrMe 2 through halide, methide or Me 2 Al + abstraction [23] . These studies suggest that the lower energy sheet 16,6 was a better model for MAO activators, though cages may play a role as a source of labile structural Me 3 Al, [9,24] through facile dissociation from terminal μ 3 ‐O‐Al 2 Me 5 sites peculiar to cage structures.…”