Hydrogen-Bonding Effects on the Reactivity of [X–Fe^III–O–Fe^IV═O] (X = OH, F) Complexes toward C–H Bond Cleavage

Complexes 1–OH and 1–F are related complexes that share similar [X–FeIII–O–FeIV═O]3+ core structures with a total spin S of 1/2, which arises from antiferromagnetic coupling of an S = 5/2 FeIII–X site and an S = 2 FeIV═O site. EXAFS analysis shows that 1–F has a nearly linear FeIII–O–FeIV 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–FeIV–O–FeIV═O]4+ core having individual S = 1 FeIV 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 FeIV═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.