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

journal contribution

posted on 2017-05-08, 15:51 authored by Santanu Pattanayak, Andrew J. Jasniewski, Atanu Rana, Apparao Draksharapu, Kundan K. Singh, Andrew Weitz, Michael Hendrich, Lawrence Que, Abhishek Dey, Sayam Sen GuptaIn 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 FeO 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 FeO complexes. Extended X-ray absorption fine structure analysis of**3**reveals an FeO 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**.