Periodate as an Oxidant for Catalytic Water Oxidation: Oxidation via Electron Transfer or O-Atom Transfer?

Treatment of Ir(OH)(2) with sodium periodate in aqueous solution results in formation of dioxygen following the rate law v = k(obs)[Ir](0.65)[IO4](0.5), with k(obs) = 1.5 x 10(-3) {Ir(OH)(2) = [IrCp*(Me2NHC)(OH)(2)], where Me2NHC = N-dimethylimid-azolin- 2-ylidene and Cp* = cyclopentadienyl}. In situ ESI-MS experiments in combination with DFT calculations show that [Ir-III(IO3)](+) and [Ir-V(=O)(IO3)](+) species are present in the reaction mixture. On the basis of the presence of these species, a mechanistic pathway was calculated illustrating that water is not necessarily the source of the oxygen. A low-lying path-way exists wherein O-2 production proceeds via two consecutive O-atom-transfer reactions from periodate to the catalyst. The resulting iodite ligand is further oxidized to close the catalytic cycle. The rate-determining step in this process is formation of the O-O bond. For this transition a 21.8 kcal/mol barrier was found. This value fits very well with the observed turnover frequency of 0.27 s(-1). Although it is difficult to prove that this is the dominant pathway, these data clearly illustrate that one has to be very careful with interpretation of catalytic results in periodate-driven water oxidation reactions.