Iron-N-heterocyclic carbene complexes as efficient electrocatalysts for water oxidation under acidic conditions.

The development of stable, Earth-abundant, and high-activity molecular water oxidation catalysts under acidic and neutral conditions remains a great challenge. Here, the use of N-heterocyclic carbene (NHC)-based iron(III) complex 1 {[phenyl(tris(3-methylimidazol-1-ylidene))borate]2Fe(III)}+ as a catalyst for water oxidation under acidic and neutral conditions was investigated. Two iron(II) carbene complexes, 2 {[2,6-bis(3-methylimidazolium-1-yl)pyridine]2Fe}2+ and 3 {[2,6-bis(3-methylimidazolium-1-yl)pyridine-4-carboxylic acid]2Fe}2+, were also used for comparison. A series of experiments demonstrate that complex 1 has excellent performance in terms of both catalytic activity and stability. In addition, the faradaic efficiency and turnover frequency (TOF) reach 95.0% and 2.8 s-1, respectively. An overpotential of ca. 490 mV is obtained at pH 1.5. Density functional theory (DFT) calculations indicate that dehydrogenation is the potential-determining step (PDS) in water oxidation. Complex 1 has a lower free energy barrier in this process than 2 and 3. High-valent Fe species are further proven in 1 by spectroelectrochemical measurements, which are crucial in promoting water oxidation. This study is expected to contribute to the development of homogeneous water oxidation catalysis under acidic and neutral conditions.