Benchmarking Water Oxidation Catalysts Based on Iridium Complexes: Clues and Doubts on the Nature of Active Species

Water oxidation (WO) is a central reaction in the photo- and electro-synthesis of fuels. Iridium complexes have been successfully exploited as water oxidation catalysts (WOCs) with remarkable performances. Herein, we report a systematic study aimed at benchmarking well-known Ir WOCs, when NaIO4 is used to drive the reaction. In particular, the following complexes were studied: cis-[Ir(ppy)2(H2O)2]OTf (ppy=2-phenylpyridine) (1), [Cp*Ir(H2O)3]NO3 (Cp*=1,2,3,4,5-pentamethyl-cyclopentadienyl anion) (2), [Cp*Ir(bzpy)Cl] (bzpy=2-benzoylpyridine) (3), [Cp*IrCl2(Me2-NHC)] (NHC=N-heterocyclic carbene) (4), [Cp*Ir(pyalk)Cl] (pyalk=2-pyridine-isopropanoate) (5), [Cp*Ir(pic)NO3] (pic=2-pyridine-carboxylate) (6), [Cp*Ir{(P(O)(OH)2}3]Na (7), and mer-[IrCl3(pic)(HOMe)]K (8). Their reactivity was compared with that of IrCl3⋅n H2O (9) and [Ir(OH)6]2− (10). Most measurements were performed in phosphate buffer (0.2 m), in which 2, 4, 5, 6, 7, and 10 showed very high activity (yield close to 100 %, turnover frequency up to 554 min−1 with 10, the highest ever observed for a WO-driven by NaIO4). The found order of activity is: 10>2≈4>6>5>7>1>9>3>8. Clues concerning the molecular nature of the active species were obtained, whereas its exact nature remains doubtfully.