Structure, Dynamics, and Polymerization Activity of Zirconocenium Ion Pairs Generated with Boron-C6F5 Compounds and Al2R6

The activation reaction of the olefin polymerization precatalyst Cp*2ZrMe2 with a boron-C6F5 compound (B(C6F5)(3), [Ph3C][B(C6F5)(4)]) and an aluminum alkyl species (Al2Me6, (Al2Bu6)-Bu-i) is studied by NMR spectroscopy in order to determine the nature of the ion pairs that are formed preferentially. We show that a mixture of ion pairs with general formula Cp*Zr-2(Me)-mu-Me-E(C6F5)(3-x)R-x (E = Al, B; x = 0, 1; R = Me, Bu-i) (1, 2a/b, 4) is generated due to a rapid transfer of pentafluorophenyl groups from boron to aluminum. Therefore, the molecular ratio of the activators determines the final composition of the ion pairs present in solution. When the pentafluorophenyl group transfer is suppressed, the ion pair Cp*Zr-2-(mu-Me)(2)-AlMe2 (5) forms irrespective of the reagent ratio. The high dynamics of these solutions is demonstrated by DNMR studies. Gibbs free energies of activation were determined of 13.6(12) kcal mol(-1) at 298 K for the cocatalyst exchange of ion pair Cp*Zr-2(Me)-mu-Me-Al(C6F5)(2)Me (2a) and 13(2) kcal mol(-1) at 298 K for the methyl exchange in Al-2(C6F5)(x)Me6-x (x = 0, 1). Due to the stability of the ion pairs generated from the Cp*2ZrMe2 precatalyst at temperatures relevant for polymerization, correlations between activities in ethylene polymerization and the nature of the ion pairs can be established. All solutions containing the various ion pairs were found to be catalytically active in ethylene polymerization except that containing the ion pair 2a, which was attributed to the reduced Lewis acidity of the abstractor, as supported by DFT calculations.