Iron Catalyzed Water Oxidation: O-O Bond Formation via Intramolecular Two Oxo Interaction.

Herein, we reported 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). All results indicate that 1 and 2 are the molecular catalysts and the O-O bond formation via oxo-oxo coupling rather than WNA pathway. However, complex 3 displays negligible catalytic activity in both cyclic voltammetry and bulk electrolysis. DFT calculations suggested that the anti to syn isomerization of the two high-valent Fe=O moieties has to take place via the axial rotation of one Fe=O around the Fe-O-Fe center. This is followed by the O-O bond formation via an oxo-oxo coupling pathway with a barrier of 19.2 kcal/mol at the Fe(IV)Fe(IV) state or via oxo-oxyl coupling pathway with a barrier of 12.7 kcal/mol at the Fe(IV)Fe(V) state for 2 . The alternative water nucleophilic attack pathway has a much higher barrier of 23.8 kcal/mol. Importantly, the rigid ligand of BPMAN based diiron(III) complex 3 limits the anti to syn isomerization and axial rotation of the Fe=O moiety, which has a barrier of over 30 kcal/mol and thus accounts for the negligible catalytic activity.