Theoretical study of metallocene catalyst activation and propene insertion with new sheet models for...

Theoretical calculations involving new sheet models (MeAlO)₁₆(Me₃Al)₇ (16,7) show these aluminoxane-based activators function to activate metallocene catalysts by [Me₂Al]⁺ abstraction. However, this process involves more steps than the conventional methide abstraction. Based on MN15/def2-TZVP calculations, the formation of outer-sphere ion-pairs (OSIP) [rac-Me₂Si(η⁵-indenyl)₂ZrMe₂AlMe₂][16,6] ([SBIZrMe₂AlMe₂][16,6] 2) would be favorable in toluene. These outer-sphere ion-pairs are favored by the Me₃Al-dissociation equilibrium to the extent that detecting contact ion-pairs SBIZrMe-μ-Me-16,6 (1) would be rather difficult, in disagreement with many spectroscopic experiments. This discrepancy can be explained by invoking competing catalyst activation that involves 16,7 sheets vs. other reactive components of MAO that function via methide abstraction (neutral sheets like 16,6 or especially large cages like 23,7). The key difference however, is that the ion-pairs formed via methide abstraction would have to be largely unreactive to olefin insertion, and this is certainly true of either sheet 16,6 or cage 23,7. Contact ion-pairs SBIZrMe-μ-Me-16,6 (1) derived from 16,7 sheets have very different reactivities towards monomer insertion; we explore the ramifications of this two-state behavior in the context of propagation competing with dormant vs. active ion-pair interconversion.