Combined static and dynamic density functional study of the Ti(IV) constrained geometry catalyst (CpSiH2NH)TiR+. 1. Resting states and chain propagation
Abstract
The resting state structure of the metallocene-alkyl cation, the coordination of the olefin to the preferred resting state structure, and the insertion process of the Ti-constrained geometry catalyst (CpSiH2NH)TiR+ have been studied with density functional theory. A combined static and dynamic approach has been utilized whereby 'static' calculations of the stationary points on the potential surface are meshed with first principles Car-Parrinello molecular dynamics simulations. The first molecular dynamics simulation specifically addressing the structure of a metallocene-alkyl cation is presented showing rapid interconversion between γ- and β-agostic conformations. Complementary static calculations show a small energetic preference for a γ-agostic resting state. Coordination of the olefin to the Ti-alkyl resting state is likely to result in the formation of a β-agostic π-complex which is highly favored energetically over other π-complexes that may initially form. The whole propagation cycle was studied from π-complex to subsequent π-complex. The propagation barrier corresponds to the insertion process which was calculated to have a free energy barrier of ΔG(paragraph)=24.3 kJ/mol at 300 K. The initial β-agostic interactions which stabilize the π-complex are replaced by α-agostic bonds which stabilize the insertion transition state. A study of the back-side insertion process reveals that it may be competitive with the front-side insertion process.