Synthetically tunable iridium(III) bis-pyridine-2-sulfonamide complexes as efficient and durable water oxidation catalysts

A series of phenylene-linked iridium bis-pyridine-2-sulfonamide (bpsa) complexes substituted with electron-donating and electron-withdrawing groups and a new bpsa complex containing naphthalene linkage were synthesized and investigated as efficient and robust water oxidation catalysts. It was found that the oxidation potentials of these substituted complexes are within a broad range, which reflects their effectively tuned electronic structures. Under various water oxidation conditions, diverse kinetics behaviors are observed. The reaction mechanism strongly depends on the structure of the catalyst, and the nature of the oxidant as well as their ratios and concentrations. As evidenced by cyclic voltammetry and DFT calculations, the addition of electron-donating substituents greatly destabilizes the HOMO of the catalyst and thus facilitates the initial oxidation process. On the other hand, the limited driving force provided by the oxidized state of an electron-rich catalyst also impedes efficient water oxidation reactions. The robustness of a water oxidation catalyst is critical, and long-term reactions have exhibited turnover numbers (TONs) of up to 16,200. (C) 2016 Elsevier B.V. All rights reserved.