Mechanisms of Alcohol Oxidation Catalyzed by Co(III)-Oxo Cubanes: A Computational Study

We present a computational study on the reaction mechanisms for thermal alcohol oxidation at Co-oxo cubane [Co4O4] complexes in acetonitrile solution, using hybrid DFT in combination with DCOSMO-RS. These complexes serve as molecularly well-defined models for the interfacial reactivity of extended Co oxide surfaces. A key focus is placed on comparing different possible reaction pathways to obtain a detailed mechanistic picture. To this end, the alcohols iso-propanol and benzyl alcohol, together with the oxidants O2, H2O2, and tert-butyl hydroperoxide, are selected as model systems. Clear mechanistic differences are observed between oxidation with O2 and the hydroperoxides (ROOH). For ROOH, two catalytic cycles are identified: one for ROOH decomposition, generating ROO•, and one for alcohol oxidation, consuming ROO•. In contrast, for O2, no efficient activation via homolytic pathways with reasonable energy barriers is found, suggesting that alcohol oxidation with O2 may additionally involve radical chain processes. Overall, the calculations indicate a strong preference for homolytic over heterolytic pathways. For the cobalt atoms, we find that oxidation states higher than IV do not appear to play a significant role.