Displacement of H₃CB(C₆F₅)₃^- Anions from Zirconocene Methyl Cations by Neutral Ligand Molecules:  Equilibria, Kinetics, and Mechanisms

The displacement of the MeB(C₆F₅)₃^- anion from seven different zirconocene methyl cations by neutral Lewis bases, such as dimethylaniline, benzyldimethylamine, and diⁿbutyl ether, was investigated by 1D and 2D NMR spectroscopy. Equilibrium constants for reactions with diⁿbutyl ether change by factors of less than 5 between the zirconocene contact ion pairs studied, despite substantial steric differences. Rate constants of this displacement reaction, however, change by a factor of more than 10⁵ between Me₂Si(C₅H₄)₂ZrMe⁺MeB(C₆F₅)₃^-, the most “open” complex, and rac-Me₂Si(2-Me-BzInd)₂ZrMe⁺MeB(C₆F₅)₃^-, the most highly substituted species studied. Kinetic and stereochemical data indicate that Lewis base−anion exchange proceeds by way of an associative mechanism, which occurs without side change of the zirconium-bound methyl group. DFT calculations support an associative substitution mechanism and propose five-coordinated reaction intermediates with the Lewis base coordinated to the central coordination site.