Displacement of H<sub>3</sub>CB(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub><sup>-</sup> Anions from Zirconocene Methyl Cations by Neutral Ligand Molecules:  Equilibria, Kinetics, and Mechanisms

The displacement of the MeB(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub><sup>-</sup> anion from seven different zirconocene methyl cations by neutral Lewis bases, such as dimethylaniline, benzyldimethylamine, and di<sup>n</sup>butyl ether, was investigated by 1D and 2D NMR spectroscopy. Equilibrium constants for reactions with di<sup>n</sup>butyl 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 10<sup>5</sup> between Me<sub>2</sub>Si(C<sub>5</sub>H<sub>4</sub>)<sub>2</sub>ZrMe<sup>+</sup>MeB(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub><sup>-</sup>, the most “open” complex, and <i>rac</i>-Me<sub>2</sub>Si(2-Me-BzInd)<sub>2</sub>ZrMe<sup>+</sup>MeB(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub><sup>-</sup>, 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.