Characterization of the Fleeting Hydroxoiron(III) Complex of the Pentadentate TMC-py Ligand
journal contributionposted on 31.08.2017, 19:19 by Wei-Min Ching, Ang Zhou, Johannes E. M. N. Klein, Ruixi Fan, Gerald Knizia, Christopher J. Cramer, Yisong Guo, Lawrence Que
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.
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Pentadentate TMC-py Ligand Nonhemeisotope distribution patternhydrogen-atom transfer1.84Fe IIIOHUV spectrumelectrospray ionization mass spectrumbondtheory calculationsExtended X-ray absorptionλ maxwell-characterized examplesComplex 3hydroxoironTEMPOHproton-coupled electron transferFe IVH-atom abstractionstructure analysisComplex 3 exhibitsisotope effect valueground statecomplex