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

Research output: Contribution to journalArticle

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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 - Jan 1 2017

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Isotopes
Ligands
Electrospray ionization
Atoms
ligands
X ray absorption
Ground state
Density functional theory
Paramagnetic resonance
Protons
Hydrogen
Rate constants
Stabilization
distribution (property)
Spectroscopy
Thermodynamics
Ions
sinks
molecular ions
isotope effect

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Characterization of the Fleeting Hydroxoiron(III) Complex of the Pentadentate TMC-py Ligand. / Ching, Wei Min; Zhou, Ang; Klein, Johannes E.M.N.; Fan, Ruixi; Knizia, Gerald; Cramer, Christopher J.; Guo, Yisong; Que, Lawrence.

In: Inorganic Chemistry, Vol. 56, No. 18, 01.01.2017, p. 11129-11140.

Research output: Contribution to journalArticle

Ching, Wei Min ; Zhou, Ang ; Klein, Johannes E.M.N. ; Fan, Ruixi ; Knizia, Gerald ; Cramer, Christopher J. ; Guo, Yisong ; Que, Lawrence. / Characterization of the Fleeting Hydroxoiron(III) Complex of the Pentadentate TMC-py Ligand. In: Inorganic Chemistry. 2017 ; Vol. 56, No. 18. pp. 11129-11140.
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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 M{\"o}ssbauer 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 {\AA}, 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 {\AA} 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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T1 - Characterization of the Fleeting Hydroxoiron(III) Complex of the Pentadentate TMC-py Ligand

AU - Ching, Wei Min

AU - Zhou, Ang

AU - Klein, Johannes E.M.N.

AU - Fan, Ruixi

AU - Knizia, Gerald

AU - Cramer, Christopher J.

AU - Guo, Yisong

AU - Que, Lawrence

PY - 2017/1/1

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N2 - 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.

AB - 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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