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)(2)MR+ MeB(C6F5)(3)(-) where R = Me, (CH2Bu)-Bu-t, 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)(2)M(R)Me + B(C6F5)(3) --> (1,2-Me2Cp)(2)MR+ MeB(C6F5)(3)(-), enthalpies in toluene solution at 25 degreesC 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. DeltaH double dagger (reorg) values for the reorganization process that interchanges the ion-pair enantiotopic sites for M = Zr and R = Me, (CH2Bu)-Bu-t, CH2TMS, and CH(TMS)(2) are 22(1), 18(1), 17(1), and 9(2) kcal/mol, respectively. Corresponding DeltaH double dagger (reorg) values for M = Hf and R = Me, (CH2Bu)-Bu-t, and CH2TMS are >24, 12(3), and 15(2) kcal/mol, respectively. DeltaH double dagger (reorg) values are highest in low dielectric constant solvents such as C6D12 Activation parameters for beta -Me elimination in the complexes (1,2-Me2Cp)(2)(MCH2Bu+)-Bu-t MeB(C6F5)(3)(-) for M = Zr and Hf are found to be DeltaH double dagger (beta -Me) = 22.5(0.9) and 17.3(0.9) kcal/mol, and DeltaS double dagger (beta -Me) = 4.3(3.3) and -11.9(3.4) cal/mol(.)K, respectively.