title: Catalytic water splitting with an iridium carbene complex: a theoretical study.
abstract: 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 O-O 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. 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 Ir(OH)(+). The calculated mechanism fits well with the experimentally observed rate law: v = kobs[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.
authors.affiliation: Institute for the Organic Synthesis and Photoreactivity, National Research Council of Italy, Via P. Gobetti 101, 40129 Bologna (Italy). alessandro.venturini@isof.cnr.it.
authors.forename: Alessandro
authors.initials: A
authors.lastname: Venturini
authors.forename: Andrea
authors.initials: A
authors.lastname: Barbieri
authors.forename: Joost N H
authors.initials: JN
authors.lastname: Reek
authors.forename: Dennis G H
authors.initials: DG
authors.lastname: Hetterscheid
keywords: density functional calculations; iridium; oxidation; reaction mechanisms; water splitting