Characterization of the Fleeting Hydroxoiron(III) Complex of the Pentadentate TMC-py Ligand

Wei Min Ching, Ang Zhou, Johannes E.M.N. Klein, Ruixi Fan, Gerald Knizia, Christopher J. Cramer, Yisong Guo, Lawrence Que

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Abstract

Nonheme mononuclear hydroxoiron(III) species are important intermediates in biological oxidations, but well-characterized examples of synthetic complexes are scarce due to their instability or tendency to form μ-oxodiiron(III) complexes, which are the thermodynamic sink for such chemistry. Herein, we report the successful stabilization and characterization of a mononuclear hydroxoiron(III) complex, [FeIII(OH)(TMC-py)]2+ (3; TMC-py = 1-(pyridyl-2′-methyl)-4,8,11-trimethyl-1,4,8,11-tetrazacyclotetradecane), which is directly generated from the reaction of [FeIV(O)(TMC-py)]2+ (2) with 1,4-cyclohexadiene at 40 °C by H-atom abstraction. Complex 3 exhibits a UV spectrum with a λmax at 335 nm (ϵ ≈ 3500 M-1 cm-1) and a molecular ion in its electrospray ionization mass spectrum at m/z 555 with an isotope distribution pattern consistent with its formulation. Electron paramagnetic resonance and Mössbauer spectroscopy show 3 to be a high-spin Fe(III) center that is formed in 85% yield. Extended X-ray absorption fine structure analysis reveals an Fe-OH bond distance of 1.84 Å, which is also found in [(TMC-py)FeIII-O-CrIII(OTf)3]+ (4) obtained from the reaction of 2 with Cr(OTf)2. The S = 5/2 spin ground state and the 1.84 Å Fe-OH bond distance are supported computationally. Complex 3 reacts with 1-hydroxy-2,2,6,6-tetramethylpiperidine (TEMPOH) at 40 °C with a second-order rate constant of 7.1 M-1 s-1 and an OH/OD kinetic isotope effect value of 6. On the basis of density functional theory calculations, the reaction between 3 and TEMPOH is classified as a proton-coupled electron transfer as opposed to a hydrogen-atom transfer.

Original languageEnglish (US)
Pages (from-to)11129-11140
Number of pages12
JournalInorganic chemistry
Volume56
Issue number18
DOIs
StatePublished - Sep 18 2017

Bibliographical note

Funding Information:
This work was supported by the U.S. National Science Foundation (Grant Nos. CHE-1361773 to L.Q., CHE-1361595 to C.J.C., and CHE-1654060 to Y.G.) and the U.S. National Institutes of Health (Grant No. GM38767 to L.Q.). W.-M.C. acknowledges the Ministry of Science and Technology, Taiwan, for a postdoctoral fellowship, and J.E.M.N.K. thanks the Alexander von Humboldt Foundation for a Feodor Lynen Research Fellowship. 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). The authors acknowledge the Minnesota Supercomputing Institute (MSI) at the University of Minnesota for providing resources that contributed to the research results reported within this paper. R.F. and Y.G. also thank Prof. M. Hendrich for his assistance with the EPR instrument.

Publisher Copyright:
© 2017 American Chemical Society.

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