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-Me 2 Cp) 2 MR + MeB(C 6 F 5 ) 3 - where R = Me, CH 2 t Bu, CH 2 TMS, 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-Me 2 Cp) 2 M(R)Me + B(C 6 F 5 ) 3 → (1,2-Me 2 Cp) 2 MR + MeB(C 6 F 5 ) 3 -, enthalpies in toluene solution at 25 °C for M = Zr and R = Me, CH 2 TMS, 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 CH 2 TMS are −20.8(0.5) and −31.1(1.6) kcal/mol, respectively. Δ H ‡ reorg values for the reorganization process that interchanges the ion-pair enantiotopic sites for M = Zr and R = Me, CH 2 t Bu, CH 2 TMS, and CH(TMS) 2 are 22(1), 18(1), 17(1), and 9(2) kcal/mol, respectively. Corresponding Δ H ‡ reorg values for M = Hf and R = Me, CH 2 t Bu, and CH 2 TMS are >24, 12(3), and 15(2) kcal/mol, respectively. Δ H ‡ reorg values are highest in low dielectric constant solvents such as C 6 D 12 . Activation parameters for β-Me elimination in the complexes (1,2-Me 2 Cp) 2 MCH 2 t Bu + MeB(C 6 F 5 ) 3 - for M = Zr and Hf are found to be Δ H ‡ β - Me = 22.5(0.9) and 17.3(0.9) kcal/mol, and Δ S ‡ β - Me = 4.3(3.3) and −11.9(3.4) cal/mol·K, respectively.