Transformation of a Cp*–Iridium(III) Precatalyst for Water Oxidation when Exposed to Oxidative Stress

The reaction of [Cp*Ir(bzpy)NO3] (1; bzpy = 2-ben- zoylpyridine, Cp* = pentamethylcyclopentadienyl anion), a competent water-oxidation catalyst, with several oxidants (H2O2, NaIO4, cerium ammonium nitrate (CAN)) was studied to intercept and characterize possible intermediates of the oxidative transformation. NMR spectroscopy and ESI-MS techniques provided evidence for the formation of many species that all had the intact Ir–bzpy moiety and a gradually more oxidized Cp* ligand. Initially, an oxygen atom is trapped in between two carbon atoms of Cp* and iridium, which gives an oxygen–Ir coordinated epoxide, whereas the remaining three carbon atoms of Cp* are involved in a h3 in- teraction with iridium (2 a). Formal addition of H2O to 2 a or H2O2 to 1 leads to 2 b, in which a double MeCOH functionalization of Cp* is present with one MeCOH engaged in an in- teraction with iridium. The structure of 2b was unambigu- ously determined in the solid state and in solution by X-ray single-crystal diffractometry and advanced NMR spectro- scopic techniques, respectively. Further oxidation led to the opening of Cp* and transformation of the diol into a dike- tone with one carbonyl coordinated at the metal (2c). A h3 interaction between the three non-oxygenated carbons of “ex-Cp*” and iridium is also present in both 2b and 2c. Iso- lated 2 b and mixtures of 2 a–c species were tested in water- oxidation catalysis by using CAN as sacrificial oxidant. They showed substantially the same activity than 1 (turnover fre- quency values ranged from 9 to 14 min-1).