Spectroscopic and Reactivity Comparisons of a Pair of bTAML Complexes with Fe^VO and Fe^IVO Units

In this report we compare the geometric and electronic structures and reactivities of [Fe^V(O)]⁻ and [Fe^IV(O)]^2– species supported by the same ancillary nonheme biuret tetraamido macrocyclic ligand (bTAML). Resonance Raman studies show that the FeO vibration of the [Fe^IV(O)]^2– complex 2 is at 798 cm^–1, compared to 862 cm^–1 for the corresponding [Fe^V(O)]⁻ species 3, a 64 cm^–1 frequency difference reasonably reproduced by density functional theory calculations. These values are, respectively, the lowest and the highest frequencies observed thus far for nonheme high-valent FeO complexes. Extended X-ray absorption fine structure analysis of 3 reveals an FeO bond length of 1.59 Å, which is 0.05 Å shorter than that found in complex 2. The redox potentials of 2 and 3 are 0.44 V (measured at pH 12) and 1.19 V (measured at pH 7) versus normal hydrogen electrode, respectively, corresponding to the [Fe^IV(O)]^2–/[Fe^III(OH)]^2– and [Fe^V(O)]⁻/[Fe^IV(O)]^2– couples. Consistent with its higher potential (even after correcting for the pH difference), 3 oxidizes benzyl alcohol at pH 7 with a second-order rate constant that is 2500-fold bigger than that for 2 at pH 12. Furthermore, 2 exhibits a classical kinteic isotope effect (KIE) of 3 in the oxidation of benzyl alcohol to benzaldehyde versus a nonclassical KIE of 12 for 3, emphasizing the reactivity differences between 2 and 3.