Sheet Models for Methylaluminoxane (MAO) Activators? A Theoretical Case Study involving rac-Me 2 Si(η 5 -C 9 H 6 ) 2 Zr (SBIZr) Complexes.

Activation of SBIZrMe 2 or SBIZrMeCl and a sheet model for an active component of hydrolytic MAO, (MeAlO) 16 (Me 3 Al) 6 , (16,6) has been studied by DFT. Contact ion-pair formation occurs through the intermediacy of SBIZrMe(Cl) or SBIZrMe 2 reacting with sheet 16,6 to furnish SBIZrMe-μ-X(MeAlO) 16 (Me 3 Al) 6 (2, X=Me, Cl). Contact ion-pairs 2 would be in equilibrium with heterodinuclear catalyst precursors [SBIZrMe 2 AlMe 2 ][(MeAlO) 16 (Me 3 Al) 6 X] (3 (X=Me, Cl) through reversible binding of Me 3 Al 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 Cp 2 Zr 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(C 6 F 5 ) 3 (6) and [SBIZrMe][B(C 6 F 5 ) 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.