Iron‐Catalyzed Water Oxidation: O–O Bond Formation via Intramolecular Oxo–Oxo Interaction

Herein, we report the importance of structure regulation on the O−O bond formation process in binuclear iron catalysts. Three complexes, [Fe 2 (μ‐O)(OH 2 ) 2 (TPA) 2 ] 4+ ( 1 ), [Fe 2 (μ‐O)(OH 2 ) 2 (6‐HPA)] 4+ ( 2 ) and [Fe 2 (μ‐O)(OH 2 ) 2 (BPMAN)] 4+ ( 3 ), have been designed as electrocatalysts for water oxidation in 0.1 M NaHCO 3 solution (pH 8.4). We found that 1 and 2 are molecular catalysts and that O−O bond formation proceeds via oxo–oxo coupling rather than by the water nucleophilic attack (WNA) pathway. In contrast, complex 3 displays negligible catalytic activity. DFT calculations suggested that the anti to syn isomerization of the two high‐valent Fe=O moieties in these catalysts takes place via the axial rotation of one Fe=O unit around the Fe‐O‐Fe center. This is followed by the O−O bond formation via an oxo–oxo coupling pathway at the Fe IV Fe IV state or via oxo–oxyl coupling pathway at the Fe IV Fe V state. Importantly, the rigid BPMAN ligand in complex 3 limits the anti to syn isomerization and axial rotation of the Fe=O moiety, which accounts for the negligible catalytic activity.