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

摘要

Catalytic water oxidation at lr(OH)~+ (lr = lrCp*-(Me_2NHC), where Cp* = pentamethylcyclopentadienyl and Me_2NHC = 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 O-O bond. The first three oxidation steps are exothermic upon treatment with cerium ammonium nitrate and lead to formation of lr_v(=O)(O~·)~+, which is calculated to be the most stabile species under these condi tions, whereas the fourth oxidation step is the potential-energy-determining step. O-O 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 lr(OH)~+. The calculated mechanism fits well with the experimentally observed rate law: v=K_(obs)[Ir][oxidant]. The calculated effective barrier of 24.6 kcal mol~(-1) fits well with the observed turnover frequency of 0.88 s~(-1). Under strongly oxidative conditions, O-O bond formation after four sequential oxidation steps is the preferred pathway, whereas under milder conditions O-O bond formation after three sequential oxidation steps becomes competitive.