Mechanism of Water Oxidation to Molecular Oxygen with Osmocene as Photocatalyst: A Theoretical Study

In the present work, photoinduced O₂ evolution from the [Cp₂Os–OH]⁺ complex in aqueous solution has been studied by the DFT, CASSCF, and CASPT2 methods. The CASPT2//CASSCF calculations predict that the S₃ state is initially populated and the subsequent deprotonation of [Cp₂Os–OH]⁺ proceeds very easily along the T₁ pathway as a result of the efficient S₃ → T₁ intersystem crossing. It is found that the O–O bond is formed via the acid–base mechanism, which is different from the direct oxo–oxo coupling mechanism suggested in the experimental study. Formation of the O–O bond is the rate-determining step and has an activation energy and activation free energy of 81.3 and 90.4 kcal/mol, respectively. This is consistent with the low quantum yield observed for generating molecular oxygen upon irradiation at 350 nm (∼ 82 kcal/mol). The O₂ release from an intermediate complex has to overcome a small barrier on the triplet pathway first and then pass through the triplet–singlet intersection, generating the O₂ molecules in either the lowest singlet or triplet state. The formed ³O₂ molecule can be converted into the ¹O₂ molecule by the heavy atom effect in the Os complexes, which is probably the reason only the ¹O₂ molecule was detected experimentally