Combined static and dynamic density functional study of the Ti(IV) constrained geometry catalyst (CpSiH(2)NH)TiR(+) .1. Resting states and chain propagation

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 (CpSiH(2)NH)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 gamma- and beta-agostic conformations. Complementary static calculations show a small energetic preference for a gamma-agostic resting state. Coordination of the olefin to the Ti-alkyl resting state is likely to result in the formation of a beta-agostic pi-complex which is highly favored energetically over other pi-complexes that may initially form. The whole propagation cycle was studied from pi-complex to subsequent x-complex. The propagation barrier corresponds to the insertion process which was calculated to have a free energy barrier of Delta G(double dagger) = 24.3 kJ/mol at 300 K. The initial beta-agostic interactions which stabilize the pi-complex are replaced by alpha-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.