An Active Site Opening Mechanism in a (Pyridylamide)hafnium(IV) Ion Pair Catalyst: An Associative Mechanism

Abstract

We report a molecular dynamics (MD) study on the ion pair [{N–,N,C–}HfMe][MeB(C6F5)3], which is the active species of a (pyridylamide)hafnium(IV) complex for olefin polymerization. The present simulation model system is composed of an ion pair in explicit organic solvent (heptane) with and without ethylene monomers. MD simulations revealed an active site opening (ASO) mechanism, i.e., the associative active site opening (AASO), where the counteranion dissociates from the active site associatively with the monomer coordination to the active site. AASO also explains consistently the experimental fact that the activity of the catalyst increases when the counteranion has no Me groups. Further we show that AASO is the major mechanism of ASO by using the replica exchange molecular dynamics (REMD) method. In addition, we discuss two important factors for the AASO: one is the structural condition of the ion pair, and the other is the net stabilization energy. Finally, we conclude that both the ASO and the monomer coordination should be taken into account to accurately design the behavior of the ion pair and predict the activity.