Organometallic Iridium Catalysts Based on Pyridinecarboxylate Ligands for the Oxidative Splitting of Water

Organometallic compounds [Cp*Ir(kappa(2)-N,O)X] (kappa(2)-N,O = 2-pyridinecarboxylic acid, ion(-1) (1), 2,4-pyridinedicarboxylic acid, ion(-1) (2), 2,6-pyridinedicarboxylic acid, ion(-1) (3); X- = Cl- (a), NO3- (b)) and [Ir(kappa(3)-N,O,O)(1-kappa-4,5,-eta(2)-C8H13)(MeOH)] kappa(3)-N,O,O = 2,6-pyridinedicarboxylic acid, ion(-2) (4)) are effective catalysts for the oxidative splitting of water to O-2 driven by Ce4+. They show similar TOFLT values (long-term TOF, 2.6-7.4 min(-1)) while TOFIN values (initial TOF) strongly depend on the catalyst (1 >> 2 > 3 > 4), reaching a maximum value of 287 min(-1) (4.8 s(-1)) for la, which is the highest TOF value ever reported for an iridium catalyst. Voltammetric measurements indicate that the oxidative processes of compounds 1-4 are located at values substantially less positive than that of [Cp*Ir(bzpy)NO3] (bzpy = 2-benzoylpyridine; Delta E approximate to 0.2-0.3 V), taken as reference catalyst for water oxidation. In particular, compound 3, having a pendant -COOH moiety in close proximity to an iridium coordination site, as shown by the structure determined by single-crystal X-ray diffraction, exhibits several low-potential oxidation processes.