Mechanism of alcohol oxidation by trans-dioxoruthenium(VI): The effect of driving force on reactivity

The effect of driving force on the rate of oxidation of alcohols by trans-[RuVILO2]2+{L1 = (2,2′-bipyridine)2; L2 = N,N′-dimethyl-6,7,8,9,10,11,17,18-octahydro-5H-dibenzo[en][1,4,8,12] dioxadiaza-cyclopentadecine; L3 = N,N′-dimethyl-N,N′-bis(2-pyridylmethyl)propylenediamine; L4 = meso-2,3,7,11,12-pentamethyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17), 13,15-triene; L5 = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane} with E°(RuVI-RuIV) ranging from 0.66 to 1.01 V vs. saturated calomel electrode has been investigated. In most cases the complexes behave as two-electron oxidants being reduced to trans-[RuIVL(O)(H2O)]2+. The rate constants (k2) for alcohol oxidation increase with E° of the ruthenium oxidant. The slopes of the linear free-energy plots of log k2 vs. E° for benzyl alcohol and propan-2-ol are -14.7 and -16.9 V-1 respectively. The oxidation is accompanied by large kinetic α-C-H bond isotope effects and negative ΔS‡ values, suggesting association of Ru=O and the α-C-H bond in the transition state. For trans-[RuVIL2O2]2+ the existence of a linear free-energy relationship between log k2 and the ionization energies of the alcohols and the large negative ρ values in Hammett plots for the oxidation of substituted benzyl alcohols indicate a charge-transfer mechanism. A common mechanism involving either a hydride or hydrogen atom abstraction is proposed.