Catalytic Water Splitting with an Iridium Carbene Complex: A Theoretical Study

Catalytic water oxidation at Ir(OH)(+) (Ir=IrCp*(Me2NHC), where Cp*=pentamethylcyclopentadienyl and Me2NHC=N,N-dimethylimidazolin-2-ylidene) can occur through various competing channels. A potential-energy surface showing these various multichannel reaction pathways provides a picture of how their importance can be influenced by changes in the oxidant potential. In the most favourable calculated mechanism, water oxidation occurs via a pathway that includes four sequential oxidation steps, prior to formation of the OO bond. The first three oxidation steps are exothermic upon treatment with cerium ammonium nitrate and lead to formation of Ir-V(O)(O-.)(+), which is calculated to be the most stabile species under these conditions, whereas the fourth oxidation step is the potential-energy-determining step. OO bond formation takes place by coupling of the two oxo ligands along a direct pathway in the rate-limiting step. Dissociation of dioxygen occurs in two sequential steps, regenerating the starting material Ir(OH)(+). The calculated mechanism fits well with the experimentally observed rate law: v=k(obs)[Ir][oxidant]. The calculated effective barrier of 24.6kcalmol(-1) fits well with the observed turnover frequency of 0.88s(-1). Under strongly oxidative conditions, OO bond formation after four sequential oxidation steps is the preferred pathway, whereas under milder conditions OO bond formation after three sequential oxidation steps becomes competitive.