Relevance of Chemical vs Electrochemical Oxidation of Tunable Carbene Iridium Complexes for Catalytic Water Oxidation

Based on previous work that identified iridium(III) Cp* complexes containing a C,N-bidentate chelating triazolylidene-pyridyl ligand (Cp* = pentamethylcyclopentadienyl, C5Me5-) as efficient molecular water oxidation catalysts, a series of new complexes based on this motif has been designed and synthesized in order to improve catalytic activity. Modifications include specifically the introduction of electron-donating substituents into the pyridyl unit of the chelating ligand (H, a; 5-OMe, b; 4-OMe, c; 4-tBu, d; 4-NMe2, e), as well as electronically active substituents on the triazolylidene C4 position (H, 8; COOEt, 9; OEt, 10; OH, 11; COOH, 12). Chemical oxidation using cerium ammonium nitrate (CAN) indicates a clear structure-activity relationship with electron-donating groups enhancing catalytic turnover frequency, especially when the donor substituent is positioned on the triazolylidene ligand fragment (TOFmax = 2500 h(-1) for complex 10 with a MeO group on pyr and a OEt-substituted triazolylidene, compared to 700 h(-1) for the parent benchmark complex without substituents). Electrochemical water oxidation does not follow the same trend, and reveals that complex 8b without a substituent on the triazolylidene fragment outperforms complex 10 by a factor of 5, while in CAN-mediated chemical water oxidation, complex 10 is twice more active than 8b. This discrepancy in catalytic activity is remarkable and indicates that caution is needed when benchmarking iridium water oxidation catalysts with chemical oxidants, especially when considering that application in a potential device will most likely involve electrocatalytic water oxidation.