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

Organometallic compounds [Cp*Ir(κ 2 -N,O)X] (κ 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 ), NO 3 – ( b )) and [Ir(κ 3 -N,O,O)(1-κ-4,5-η 2 -C 8 H 13 )(MeOH)] (κ 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 Ce 4+ . They show similar TOF LT values (long-term TOF, 2.6–7.4 min –1 ) while TOF IN 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 1a, 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)NO 3 ] (bzpy = 2-benzoylpyridine; Δ E ≈ 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.