Radical abstraction vs. oxidative addition mechanisms for the activation of the S–H, O–H, and C–H bonds using early transition metal oxides

Abstract

Quantum chemical calculations are performed to study the S–H, O–H, and C–H bond activation of H₂S, H₂O, and CH₄ by bare and ligated ZrO⁺ and NbO⁺ units. These representative oxides bear low energy oxo and higher energy oxyl units. S–H and C–H bonds are readily activated by metal oxyl states (radical mechanism), but the O–H bond is harder to activate with either the oxyl or oxo states. Our results suggest that known practices for the C–H bond activation can be applied to S–H, but not to O–H bonds. The identified trends are rationalized in terms of the HS–H, HO–H, and H₃C–H dissociation energies to the homolytic or heterolytic fragments. We also found that these dissociation energies drop to about half after coordination of H₂S or H₂O to the metal oxide unit. In addition, chlorine ligands are shown to stabilize the higher energy oxyl states of the transition metal oxygen unit enhancing the reactivity of the formed complexes.