Hydrogen Atom or Proton Coupled Electron Transfer? C-H Bond Activation by Transition-Metal Oxides

The C-H bond activation in oxidative dehydrogenation of propane by heterobimetallic oxide clusters (first-row transition metals), deposited on the zirconium oxide node of the NU-1000 metal organic framework, was investigated by multireference wave function theory. The redox-active part of the systems studied has the composition (CoO)(MO)(OH)₂ with M = Ti, Mn, Fe, Co, Ni, Cu, Zn. In this series, the energy of H transfer from propane to the metal oxide (ΔE) varies from -26 kcal/mol for M = Cu, Zn to 85 kcal/mol for M = Ti. This is accompanied by a change in the mechanism from hydrogen atom transfer, M²⁺(d) O^•- → M²⁺(d) OH^-, for M = Cu, Zn to proton coupled electron transfer, M(d) O^2- → M⁽(d) OH^-, for M = Ni, Co, Fe, Mn, Ti. Whereas for M = Ni (ΔE = -13 kcal/mol) Ni^+III is reduced to Ni^+II, for M = Co, Fe, Mn (ΔE = 1, 10, 6 kcal/mol, respectively) it is Co^+III that is reduced to Co^+II. For M = Ti, Ti maintains its +IV oxidation state and Co^+II is reduced to Co^+I.