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)
(1), 2,4-pyridinedicarboxylic acid, ion(−1) (2),
2,6-pyridinedicarboxylic acid, ion(−1) (3); X– = Cl– (a), NO3– (b)) and [Ir­(κ3-N,O,O)­(1-κ-4,5-η2-C8H13)­(MeOH)] (κ3-N,O,O = 2,6-pyridinedicarboxylic acid,
ion(−2) (4)) are effective catalysts for the oxidative
splitting of water to O2 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 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)­NO3] (bzpy = 2-benzoylpyridine; ΔE ≈ 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.