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Abstract

Complexes 1–OH and 1–F are related complexes that share similar [X–Fe^III–O–Fe^IV═O]³⁺ core structures with a total spin S of 1/2, which arises from antiferromagnetic coupling of an S = 5/2 Fe^III–X site and an S = 2 Fe^IV═O site. EXAFS analysis shows that 1–F has a nearly linear Fe^III–O–Fe^IV core compared to that of 1–OH, which has an Fe–O–Fe angle of ∼130° due to the presence of a hydrogen bond between the hydroxo and oxo groups. Both complexes are at least 1000-fold more reactive at C–H bond cleavage than 2, a related complex with a [OH–Fe^IV–O–Fe^IV═O]⁴⁺ core having individual S = 1 Fe^IV units. Interestingly, 1–F is 10-fold more reactive than 1–OH. This raises an interesting question about what gives rise to the reactivity difference. DFT calculations comparing 1–OH and 1–F strongly suggest that the H-bond in 1–OH does not significantly change the electrophilicity of the reactive Fe^IV═O unit and that the lower reactivity of 1–OH arises from the additional activation barrier required to break its H-bond in the course of H-atom transfer by the oxoiron(IV) moiety.