Theoretical study of the interactions between cations and anions in group IV transition-metal catalysts for single-site homogeneous olefin polymerization

Density functional theory has been used to investigate the interaction between a series of cationic polymerization catalysts and their anionic counterions. The catalyst systems include (NPR₃)₂TiMe⁺, (Cp)(NCR₂)TiMe⁺, (Cp)SiR₂NR′)TiMe⁺, (Cp)OSiR₃TiMe⁺, and (Cp)NPR₃TiMe⁺. The counterions studied are B(C₆F₅)₄^-, MeB(C₆F₅)₃^-, TMA-MAOMe^-, and MAOMe^-, where TMA = trimethylaluminum and MAO = methylalumoxane. Two simplified model structures, which have been proposed as the counterions for the active (TMA-MAOMe^-) and dormant (MAOMe^-) ion pairs in single-site catalysts activated by MAO, were used for the last two counterions. The interaction between the cation and anion will be discussed in terms of ion-pair formation and separation energies. Full quantum-mechanical (QM) calculations demonstrate that, for the same catalyst system but different anions, the ion-pair separation energies increase in the order B(C₆F₅)₄^- < MeB(C₆F₅)₃^- < TMA-MAOMe^- < MAOMe^-. For the same counterion but different cations, the (NPR₃)₂TiMe⁺ system has the lowest separation energy. Increasing the size of the R group decreases the ion-pair separation energy. Combined quantum-mechanical (QM) and molecular-mechanical (MM) models (QM/MM) for MeB(C₆F₅)₃^- and TMA-MAOMe^- have also been developed and examined by comparing the ion-pair formation and separation energies to the full QM results. The QM parts of MeB(C₆F₅)₃^- and TMA-MAOMe^- are represented by MeBCl₃^- and MeBMe₂Cl^-, respectively. The other parts of the anions are replaced by MM atoms. Preliminary studies on olefin insertion reactions for the (NPH₃)₂TiMe-μMe-A (A = B(C₆F₅)₃ and TMA-MAO) systems suggest that the QM/MM models satisfactorily reproduce the behavior of the ion-pair system in the insertion process.