Spectroscopic and Reactivity Comparisons of a Pair of bTAML Complexes with FeV=O and FeIV=O Units

Santanu Pattanayak, Andrew J. Jasniewski, Atanu Rana, Apparao Draksharapu, Kundan K. Singh, Andrew Weitz, Michael Hendrich, Lawrence Que, Abhishek Dey, Sayam Sen Gupta

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Abstract

In this report we compare the geometric and electronic structures and reactivities of [FeV(O)] and [FeIV(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 [FeIV(O)]2- complex 2 is at 798 cm-1, compared to 862 cm-1 for the corresponding [FeV(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 [FeIV(O)]2-/[FeIII(OH)]2- and [FeV(O)]/[FeIV(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.

Original languageEnglish (US)
Pages (from-to)6352-6361
Number of pages10
JournalInorganic chemistry
Volume56
Issue number11
DOIs
StatePublished - Jun 5 2017

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    Pattanayak, S., Jasniewski, A. J., Rana, A., Draksharapu, A., Singh, K. K., Weitz, A., Hendrich, M., Que, L., Dey, A., & Sen Gupta, S. (2017). Spectroscopic and Reactivity Comparisons of a Pair of bTAML Complexes with FeV=O and FeIV=O Units. Inorganic chemistry, 56(11), 6352-6361. https://doi.org/10.1021/acs.inorgchem.7b00448