Comparing the Thermodynamic Properties and Reactivity of Fe^III- and Mn^III-Hydroxo Complexes

Mononuclear Fe^III- and Mn^III-hydroxo units are involved in proton-coupled electron-transfer (PCET) reactions in the lipoxygenase enzymes. While Fe and Mn forms of these enzymes utilize nearly identical active sites, there exist few synthetic model complexes where Fe^III- and Mn^III-hydroxo species share the same coordination sphere. In this work, we generated the Fe^III-hydroxo complex [Fe^III(OH)(dpaq)]⁺ (2) from the previously reported Fe^III-aqua complex [Fe^III(OH₂)(dpaq)]²⁺ (1) by base titration in both CH₃CN and H₂O. Although the Fe^III-aqua complex is low-spin (S = 1/2), X-band EPR data (g = 8.4 and 4.3) and a room-temperature solution magnetic moment (5.90 μ_B in CD₃CN) indicate that 2 is high spin (S = 5/2). From additional experiments, we determined O-H BDFEs for the Fe^II-aqua complex in CH₃CN (62.5 kcal mol^-1) and in H₂O (62.1 kcal mol^-1). These values are 15 kcal mol^-1 lower than the O-H BDFE of the analogous Mn^II-aqua complex, [Mn^II(OH₂)(dpaq)]⁺, in CH₃CN, highlighting a reduced oxidizing capability of the Fe^III-hydroxo species in PCET reactions. Comparative kinetic studies with 1,2-diphenylhydrazine reveal that 2 oxidizes this substrate 10-fold slower than the Mn^III-hydroxo analogue. These results show how metal identity affects the thermodynamics and reactivity of PCET by metal(III)-hydroxo complexes at parity of coordination sphere.