Ionization of Cp₂ZrMe₂ and Lewis Bases by Methylaluminoxane: Computational Insights

Abstract

The interactions of the Lewis bases CO, octamethyltrisiloxane (OMTS) and 2,2’‐bipyridine (bipy) with a sheet model for the principal activator (MeAlO)16(Me3Al)6 (16,6) in hydrolytic methylaluminoxane (MAO) were investigated by DFT. These studies reveal that OMTS and bipy form adducts with Me3Al prior to methide abstraction by 16,6 to form the ion‐pairs [Me2Al(κ2‐L)][16,6] (5: L=OMTS, 6: L=bipy, [16,6]−=[(MeAlO)16(Me3Al)6 Me]−) while CO simply binds to a reactive edge site without ionization. The binding and activation of Cp2ZrMe2 with 16,6 to form both neutral adducts 1 Cp2ZrMe2⋅16,6 and contact ion‐pairs 4 and 7, both with formula [Cp2ZrMe][μ‐Me(MeAlO)16(Me3Al)6], featuring terminal and chelated MAO‐anions, respectively was studied by DFT. The displacement of the anion with either excess Cp2ZrMe2 or Me3Al was also studied, forming outer‐sphere ion‐pairs [(Cp2ZrMe)2μ‐Me][16,6] (2) and [Cp2Zr(μ‐Me)2AlMe2][16,6] (3). The theoretical NMR spectra of these species were compared to experimental spectra of MAO and Cp2ZrMe2 and found to be in good agreement with the reported data and assignments. These studies confirm that 16,6 is a very suitable model for the activators present in MAO but highlight the difficulty in accurately calculating thermodynamic quantities for molecules in this size regime.

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

“…It is almost impossible to distinguish these processes experimentally as e. g., a Lewis base might first abstract Me 3 Al from neutral 16,7 and then form an ion-pair via methide abstraction. [24] In this paper we report new cage models (MeAlO) 16 (Me 3 Al) m (m = 6 or 7) for high MW MAO that are isomeric with our earlier sheet model and are shown here to be less stable from a thermodynamic perspective. Moreover, we critically compare their reactivity towards ion-pair formation with metallocene complexes, [24] reactivity towards chlorinating agents, and towards loss of Me 3 Al, especially for the corresponding anions.…”

“…[24] In this paper we report new cage models (MeAlO) 16 (Me 3 Al) m (m = 6 or 7) for high MW MAO that are isomeric with our earlier sheet model and are shown here to be less stable from a thermodynamic perspective. Moreover, we critically compare their reactivity towards ion-pair formation with metallocene complexes, [24] reactivity towards chlorinating agents, and towards loss of Me 3 Al, especially for the corresponding anions. [4,20] To our knowledge this is the first study that compares the reactivity of these two different motifs in the size range suggested by experiment and we find significant differences in behaviour.…”

“…We had basically formed this opinion earlier in that formation of outer-sphere ion-pairs 2 and contact ion-pairs 1 using our sheet model for 16,6 was predicted as being unfavorable on a free energy basis, even in toluene continuum [24] and yet these species are readily detected in solution by 1 H or 13 C NMR spectroscopy. [41] The MN15/def2-TZVP method, which requires 4-5 times the amount of CPU per optimization cycle, provides results that are in closer agreement with higher levels of theory.…”

“…We also include comparable data for some other isomeric structures, sheet i-16,7 which forms reversibly from sheet 16,6 through binding of Me 3 Al at its most reactive site [24] and also cage i-16,6 which forms from cage 16,7 by reversible loss of Me 3 Al, again from the site most reactive towards [Me 2 Al] + abstraction (a, Figure 1). Details of both processes will be discussed below.…”

“…[1,37] We have studied this fundamental process, i. e., ion-pair formation using our sheet model for 16,6 and Cp 2 ZrMe 2 just recently. [24] In that work, we focused on the formation of contact vs. outer-sphere ion-pairs 1 and 2 in equilibrium with each other through reversible binding of Me 3 Al (eq. 1, X=Me) in toluene polar continuum.…”