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

• Published in: ChemSusChem 2017-11, Vol.10 (22), p.4503-4509
• Main Authors: Menendez Rodriguez, Gabriel, Gatto, Giordano, Zuccaccia, Cristiano, Macchioni, Alceo
• Format: Article
• Language: English
• Subjects: Benchmarking; Benchmarks; Catalysis; Catalysts; Chemical synthesis; Chlorides; Coordination Complexes - chemistry; Iridium; Iridium - chemistry; iridium complexes; Iridium compounds; Kinetics; Nanoparticles; Oxidation; Oxidation-Reduction; Periodic Acid; sodium periodate; Water - chemistry; water oxidation
• ISSN: 1864-5631; 1864-564X
• DOI: 10.1002/cssc.201701818

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. True nature: The activity of archetypal iridium water oxidation catalysts is evaluated under exactly the same experimental conditions, exploiting NaIO4 as electron acceptor. [Ir(OH)6]2− is found to be the most active catalysts reaching a record value for NaIO4‐driven water oxidation. Comparative kinetic analysis suggests that the active species has to be molecular in nature.