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

Organometallic compounds [Cp*Ir­(κ²-N,O)­X] (κ²-N,O = 2-pyridinecarboxylic acid, ion(−1) (<b>1</b>), 2,4-pyridinedicarboxylic acid, ion(−1) (<b>2</b>), 2,6-pyridinedicarboxylic acid, ion(−1) (<b>3</b>); X^– = Cl^– (<b>a</b>), NO₃^– (<b>b</b>)) and [Ir­(κ³-N,O,O)­(1-κ-4,5-η²-C₈H₁₃)­(MeOH)] (κ³-N,O,O = 2,6-pyridinedicarboxylic acid, ion(−2) (<b>4</b>)) are effective catalysts for the oxidative splitting of water to O₂ driven by Ce⁴⁺. 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 (<b>1</b> ≫ <b>2</b> > <b>3</b> > <b>4</b>), reaching a maximum value of 287 min^–1 (4.8 s^–1) for <b>1a</b>, which is the highest TOF value ever reported for an iridium catalyst. Voltammetric measurements indicate that the oxidative processes of compounds <b>1</b>–<b>4</b> are located at values substantially less positive than that of [Cp*Ir­(bzpy)­NO₃] (bzpy = 2-benzoylpyridine; Δ<i>E</i> ≈ 0.2–0.3 V), taken as reference catalyst for water oxidation. In particular, compound <b>3</b>, 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