Metal-Alkyl Group Effects on the Thermodynamic Stability and Stereochemical Mobility of B(C₆F₅)₃-Derived Zr and Hf Metallocenium Ion-Pairs

Abstract

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, CH2tBu, 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, CH2tBu, 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, CH2tBu, 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)2MCH2tBu+ 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.