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 × 10^–3 {Ir(OH)₂ = [IrCp*(Me₂NHC)(OH)₂], where Me₂NHC = N-dimethylimidazolin-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 pathway exists wherein O₂ 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.

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 24.6 kcal mol⁻¹ fits well with the observed turnover frequency of 0.88 s⁻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.