Dangling Ligand Enables an Iron Hexa-Coordinated Molecular Catalyst for Water Oxidation by Photocatalysis

Photocatalytic water oxidation reaction by transition metal complexes remains a challenge because of their poor stability under irradiation, especially for earth-abundant metal catalysts. In this regard, iron-based water oxidation catalysts are prone to hydro-lysis and/or dissociate the ligands to form nanoparticles under a real catalytic condition. Herein, we describe a unique hexa-coordinated catalyst 1 [FeII (Py3tacn)Cl2] and its reference 2 [FeII(PhPy2tacn)Cl2] with a dangling pyridyl ligand and a phenyl group, respectively. We anticipated that the dynamically open and close coordination behaviors of the pyridyl ligand enabled balance of the reactivity and stability of catalyst 1. To our delight, the "open form" of catalyst 1 provided a free coordination site, and the "close form" guaranteed its molecular integrity, resulting in a water oxidation reaction with high efficiency and robustness. The turnover number and turnover frequency values of 2332 and 60 s-1 are the highest known to date among iron-based homogeneous water oxidation systems under visible light irradiation.