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

• Published in: Angewandte Chemie International Edition 2021-05, Vol.60 (22), p.12467-12474
• Main Authors: Zhang, Hong‐Tao, Su, Xiao‐Jun, Xie, Fei, Liao, Rong‐Zhen, Zhang, Ming‐Tian
• Format: Article
• Language: English
• Subjects: artificial photosynthesis; Bonding; Catalysts; Catalytic activity; Coordination compounds; Coupling (molecular); Electrocatalysts; Iron; iron catalysts; Isomerization; Oxidation; O−O bond formation; Rotation; Sodium bicarbonate; water oxidation
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.202100060

Herein, we report the importance of structure regulation on the O−O bond formation process in binuclear iron catalysts. Three complexes, [Fe2(μ‐O)(OH2)2(TPA)2]4+ (1), [Fe2(μ‐O)(OH2)2(6‐HPA)]4+ (2) and [Fe2(μ‐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 FeIVFeIV state or via oxo–oxyl coupling pathway at the FeIVFeV 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. Three binuclear Fe complexes are tested as molecular water oxidation catalysts. O−O bond formation can be achieved via intramolecular oxo–oxo interaction rather than by water nucleophilic attack (WNA), and the catalytic performance can be effectively controlled by the framework of the ligand.