Theoretical Elucidation of the Effect of Counteranions on the Olefin Polymerization Activity of (Pyridylamido)Hf(IV) Catalyst by QM and REMD Studies: MeB(C6F5)3– versus B(C6F5)4–

1-Octene insertion reaction by the cationic active species of (pyridylamido)­Hf­(IV) catalyst (HfCat+) was investigated, focusing on the effect of the counteranions (CAs), MeB­(C6F5)3– and B­(C6F5)4– on catalyst activity. Quantum mechanical (QM) calculation showed a remarkable difference in TS structures that HfCat+–MeB­(C6F5)3– complex forms an “inner-sphere” ion pair (ISIP), while HfCat+–B­(C6F5)4– forms an “outer-sphere” ion pair (OSIP). However, the activation free energies of 1-octene insertion by both complexes are almost identical to each other. Meanwhile, replica exchange molecular dynamics (REMD) calculation revealed that the time duration of the HfCat+ capturing 1-octene monomers throughout the REMD trajectories of HfCat+–B­(C6F5)4– complex was ∼2.5 times that of HfCat+–MeB­(C6F5)3–. This is because B­(C6F5)4– is more likely to dissociate from HfCat+ to form an OSIP than MeB­(C6F5)3– and thus allows monomers to approach HfCat+ more easily. Using these microscopic data, we have numerically evaluated the 1-octene polymerization reaction rate constants and succeeded in qualitatively reproducing the experimentally observed tendency of the polymerization reaction with B­(C6F5)4– faster than with MeB­(C6F5)3–. Finally, it is theoretically elucidated that the monomer capture process before the monomer insertion process determines the overall polymerization reaction rates in this catalytic system.