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

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journal contribution
posted 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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