Journal
INORGANIC CHEMISTRY
Volume 56, Issue 11, Pages 6352-6361Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.inorgchem.7b00448
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Funding
- SERB, New Delhi [SERB/EMR/2014/0016]
- U.S. National Science Foundation [CHE-1361773]
- U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences [DE-AC02-76SF00515]
- DOE Office of Biological and Environmental Research
- National Institutes of Health, National Institute of General Medical Sciences [P41GM103393]
- UGC-India
- Division Of Chemistry
- Direct For Mathematical & Physical Scien [1361773] Funding Source: National Science Foundation
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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 angstrom, which is 0.05 angstrom 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.
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