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-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.