Homogeneous Photochemical Water Oxidation by Biuret-Modified Fe-TAML: Evidence of Fe~Ⅴ(O) Intermediate

摘要

Water splitting, leading to hydrogen and oxygen in a process that mimics natural photosynthesis, is extremely important for devising a sustainable solar energy conversion system. Development of earth-abundant, transition metal-based catalysts that mimic the oxygen-evolving complex of photosystem Ⅱ, which is involved in oxidation of water to O_2 during natural photosynthesis, represents a major challenge. Further, understanding the exact mechanism, including elucidation of the role of active metal-oxo intermediates during water oxidation (WO), is critical to the development of more efficient catalysts. Herein, we report Fe~Ⅲ complexes of biuret-modified tetra-amidomacrocyclic ligands (Fe-TAML; 1a and 1b) that catalyze fast, homogeneous, photochemical WO to give O_2, with moderate efficiency (maximum TON = 220, TOF = 0.76 s~(-1)). Previous studies on photochemical WO using iron complexes resulted in demetalation of the iron complexes with concomitant formation of iron oxide nanoparticles (NPs) that were responsible for WO. Herein, we show for the first time that a high valent Fe~Ⅴ(O) intermediate species is photochemically generated as the active intermediate for the oxidation of water to O_2. To the best of our knowledge, this represents the first example of a molecular iron complex catalyzing photochemical WO through a Fe~Ⅴ(O) intermediate.