Metal-alkyl group effects on the thermodynamic stability and stereochemical mobility of B(C₆F₅)₃-derived Zr and Hf metallocenium ion-pairs

This paper reports significant dependence of ion-pair formation energetics and stereomutation rates upon the metal-bound alkyl substituent (R) and the solvent dielectric constant in the metallocenium series (1,2-Me₂Cp)₂MR⁺ MeB(C₆F₅)₃^- where R = Me, CH₂(t)Bu, CH₂TMS, CH(TMS)₂, and M = Zr and Hf, as determined by reaction titration calorimetry and dynamic NMR spectroscopy. For the ion-pair forming reaction, (1,2-Me₂Cp)₂M(R)Me + B(C₆F₅)₃ → (1,2-Me₂Cp)₂MR⁺ MeB(C₆F₅)₃^-, enthalpies in toluene solution at 25 °C for M = Zr and R = Me, CH₂TMS, and CH(TMS)₂ are -24.6(0.8), -22.6(1.0), and -59.2(1.4) kcal/mol, respectively. Corresponding values for M = Hf and R = Me and CH₂TMS are -20.8(0.5) and -31.1(1.6) kcal/mol, respectively. ΔH((+))(reorg) values for the reorganization process that interchanges the ion-pair enantiotopic sites for M = Zr and R = Me, CH₂(t)Bu, CH₂TMS, and CH(TMS)₂ are 22(1), 18(1), 17(1), and 9(2) kcal/mol, respectively. Corresponding ΔH((+))(reorg) values for M = Hf and R = Me, CH₂(t)Bu, and CH₂TMS are >24, 12(3), and 15(2) kcal/mol, respectively. ΔH((+))(reorg) values are highest in low dielectric constant solvents such as C₆D₁₂. Activation parameters for β-Me elimination in the complexes (1,2-Me₂Cp)₂MCH₂(t)Bu⁺ MeB(C₆F₅)₃^- for M = Zr and Hf are found to be ΔH((+))(β-Me) = 22.5(0.9) and 17.3(0.9) kcal/mol, and ΔS((+))(β-Me) = 4.3(3.3) and -11.9-(3.4) cal/mol·K, respectively.