Dual Catalytic Cycle of H₂ and H₂O Oxidations by a Half-Sandwich Iridium Complex: A Theoretical Study

While hydrogenase and photosystem II enzymes are known to oxidize H₂ and H₂O, respectively, a recently reported iridium aqua complex [Ir^III(η⁵-C₅Me₅){bpy(COOH)₂}(H₂O)]²⁺ is able to oxidize both of the molecules and generate energies as in the fuel and solar cells (Ogo et al. ChemCatChem 2017, 9, 4024-4028). To understand the mechanism behind such an interesting bifunctional catalyst, in the present study, we perform density functional theory (DFT) calculations on the dual catalytic cycle of H₂ and H₂O oxidations by the iridium aqua complex. In the H₂ oxidation, we found that the H-H bond is easily cleaved in a heterolytic fashion, and the resultant iridium hydride complex is significantly stabilized by the presence of H₂O molecules, due to dihydrogen bond. The rate-determining step of this reaction is found to be the H₂O → H₂ ligand substitution with an activation energy of 10.7 kcal/mol. In the H₂O oxidation, an iridium oxo complex originating from an oxidation of the iridium aqua complex forms a hydroperoxide complex, where an O-O bond is formed with an activation energy of 21.0 kcal/mol. Such a relatively low activation barrier is possible only when at least two H₂O molecules are present in the reaction, allowing the water nucleophilic attack (WNA) mechanism to take place. The present study suggests and discusses in detail six reaction steps required for the dual catalytic cycle to complete.