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 FeO 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.
Bibliographical noteFunding Information:
The work performed at the CSIR-National Chemical Laboratory was supported by SERB, New Delhi (Grant No. SERB/EMR/2014/0016 to SSG). The work performed at the Univ. of Minnesota was supported by the U.S. National Science Foundation (Grant No. CHE-1361773 to L.Q.). XAS data were collected on Beamlines 7-3 and 9-3 at the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, which is supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. Use of Beamlines 7-3 and 9-3 is supported by the DOE Office of Biological and Environmental Research and by the National Institutes of Health, National Institute of General Medical Sciences (including P41GM103393). S.P. and K.K.S. thank UGC-India for a fellowships. S.P. and S.S.G. thank Dr. A. Das (CSIR-NCL) for use of the electrochemical facility at CSIR-NCL.
© 2017 American Chemical Society.