Dynamic properties of metallocenium ion pairs in solution by atomistic simulations

We report a molecular dynamics study of the dynamics and energetic of the [H₂Si(Cp)₂ZrMe⁺][MeB(C₆F ₅)₃^-], IP1, and [Me₂Si(Cp) ₂ZrMe⁺][B(C₆F₅)₄ ^-], IP2, ion pairs in benzene. The metrical parameters obtained for the IP1 ion pair are in excellent agreement with the NMR data reported for the strictly related [Me₂Si(Cp)₂ZrMe⁺][MeB(C ₆F₅)₃^-] ion pair (J. Am. Chem. Soc. 2004, 126, 1448). This validates the molecular modeling protocol we developed. Simulation of the IP2 ion pair suggests that the counterion oscillates between two geometries characterized by a different coordination pattern of the F atoms to the Zr cation. In one case the B(C₆F₅)₄ ^- coordinates to the metal with two F atoms of the same aryl ring, whereas in the other case two F atoms of different aryl rings are involved in the coordination. Strong solvent reorganization occurs around IP1 and IP2, as well as around the two isolated cations. In the case of the two ion pairs solvent is never coordinated directly to the metal, whereas in the absence of the counterion one benzene molecule is coordinated to the metal through a cation-π interaction. Free energy calculations result in ion pair free energies of separation of 36.8 and 23.3 kcal/mol for IP1 and IP2, respectively. Simulations with the Zr-B distance fixed at values > 7 Å have been also performed. This mimics the situation occurring after counterion displacement by an inserting monomer molecule during olefin polymerization by the title catalysts.