Non-Heme Tri-Iron(II) Complex as a Precursor for Dioxygen Activation and Oxygen Reduction Reaction Catalysis

Nonheme iron catalysts are attractive for bioinspired aerobic oxidations and oxygenations, as well as for the oxygen reduction reaction (ORR), although their potential in ORR remains largely underexploited. This study presents a rare example of such a system, displaying O₂ activation activity and ORR catalytic selectivity toward H₂O generation. The triiron(II) complex [Fe^IIL]₃(BF₄)₃, where L^- is a thiolate-containing N3S donor bis(2-pyridylmethyl)amine derivative, was fully characterized in the solid state and in MeCN solution, where it dissociates into the mononuclear solvent adduct [FeL(MeCN)]⁺. Upon exposure to O₂ in an acetonitrile solution, [FeL(MeCN)]⁺ generates a μ-oxo diiron(III), that further evolves to an adduct featuring an original {Fe₄OF₅}³⁺ core. In the presence of 2,6-lutidinium tetrafluoroborate (proton source), and octamethylferrocene (reducing agent), [FeL(MeCN)]⁺ catalyzes the ORR with exclusive production of H₂O. This system is finally discussed with respect to a few other reported nonheme Fe-based ORR catalysts operating under chemical conditions.