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

DFT studies of activation of rac-Me₂Si(η⁵-C₉H₆)₂ZrMe₂ (rac-Me₂Si(η⁵-C₉H₆)₂=SBI) or SBIZrMeCl by a sheet model for methylaluminoxane (MAO) indicate formation of outer-sphere ion-pairs 3 (red UV-Vis spectrum) from contact ion-pairs 2 (blue spectrum) is favorable in agreement with experiment, while insertion of propene has a high barrier.

Abstract

Activation of SBIZrMe₂ or SBIZrMeCl and a sheet model for an active component of hydrolytic MAO, (MeAlO)₁₆(Me₃Al)₆, (16,6) has been studied by DFT. Contact ion-pair formation occurs through the intermediacy of SBIZrMe(Cl) or SBIZrMe₂ reacting with sheet 16,6 to furnish SBIZrMe-μ-X(MeAlO)₁₆(Me₃Al)₆ (2, X=Me, Cl). Contact ion-pairs 2 would be in equilibrium with heterodinuclear catalyst precursors [SBIZrMe₂AlMe₂][(MeAlO)₁₆(Me₃Al)₆X] (3 (X=Me, Cl) through reversible binding of Me₃Al at higher Al : Zr ratios. Calculations show that formation of ion-pairs 3 from contact ion-pairs 2 is more favourable for the SBIZr compared with the parent Cp₂Zr complexes. TD-DFT calculations were conducted on relevant SBIZr complexes to relate the results to earlier spectroscopic studies of catalyst activation using UV-Vis spectroscopy. Finally, propene insertion into ion-pairs 2, SBIZrMe-μ-MeB(C₆F₅)₃ (6) and [SBIZrMe][B(C₆F₅)₄] (7) was studied at M06-2X/TZVP level of theory. These studies suggest that contact ion-pairs 2 are significantly less reactive towards insertion than 6 or 7, in disagreement with experiment.

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