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(mu-O)(OH2)(2)(TPA)(2)](4+) (1), [Fe-2(mu-O)(OH2)(2)(6-HPA)](4+) (2) and [Fe-2(mu-O)(OH2)(2)(BPMAN)](4+) (3), have been designed as electrocatalysts for water oxidation in 0.1 M NaHCO3 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 (FeFeIV)-Fe-IV state or via oxo-oxyl coupling pathway at the (FeFeV)-Fe-IV 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.