Extremely High Turnover Number and Turnover Frequency of an Iron Chlorin Complex for Photocatalytic Water Oxidation

Highly active water oxidation catalysts based on earth-abundant elements are strongly demanded for practical application of artificial photosynthesis. An iron complex with a chlorin ligand derived from chlorophyll-a demonstrated the highest turnover frequency (= 21.3 ± 0.3 s–1) and turnover number (= 6920 ± 110) among homogeneous catalysts reported for photocatalytic water oxidation utilizing [RuII(bpy)3]2+ (bpy: 2,2′-bipyridine) and S2O82– as a photosensitizer and a sacrificial electron acceptor, respectively. The molecular nature of the Fe chlorin complex during the photocatalytic water oxidation was confirmed by dynamic light scattering analysis and electrospray ionization time-of-flight mass spectrometry. Also, kinetic studies suggested that the rate-determining step is O–O bond formation via coupling of two high-valent iron–oxo species formed by three-electron oxidation of the mononuclear Fe chlorin unit with water. The high planarity of the chlorin ligand enables the high-valent iron–oxo species to approach another iron–oxo unit closely, thereby facilitating O–O bond formation.