Spin States in Iron Porphyrins

Nakamura, Mikio

doi:10.1002/9781119129301.ch14

Cited by 6 publications

(7 citation statements)

References 97 publications

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“…The new band at 1591 cm –1 can be assigned to either a component vibration associated with the Pyr-1 linker component (Figure S7) or the v 10 marker band of the reduced Fe II species. The blue shift in the v 2 band (1551 to 1555 cm –1 ) suggests that when the Fe III surface species is reduced, the spin state changes from high-spin Fe III to a low-spin Fe II state, with a sixth axial ligand, possibly an H 2 O molecule from the solvent or a more complicated interaction with the buffer source itself. , The observation of this low-spin, six-coordinate Fe II complex suggests that the Pyr-1 linker bound axially to the Fe porphyrin unit pushes electron density to the iron center and promotes a low-spin state . However, beginning at −0.4 V vs Ag/AgCl, a shoulder peak at 1346 cm –1 emerges, corresponding to a high-spin Fe II species, , presumably not directly bound to the Pyr-1 modified surface or formed as a result of adventitious oxygen present within the system.…”

Section: Resultsmentioning

confidence: 99%

“…40,41 The observation of this low-spin, six-coordinate Fe II complex suggests that the Pyr-1 linker bound axially to the Fe porphyrin unit pushes electron density to the iron center and promotes a low-spin state. 42 However, beginning at −0.4 V vs Ag/AgCl, a shoulder peak at 1346 cm −1 emerges, corresponding to a high-spin Fe II species, 40,43 presumably not directly bound to the Pyr-1 modified surface or formed as a result of adventitious oxygen present within the system. The band fitting assignments as well as the trends of the v 4 species as a function of potential can be found in Figure S8 and S9.…”

Section: ■ Introductionmentioning

confidence: 99%

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Potential Induced Protonation of the Second Coordination Sphere in a Hangman-type Iron Porphyrin Complex Promotes HER: Insights via in Situ Raman Spectroelectrochemistry

et al. 2023

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The iron-based porphyrin complex containing a bispyridine-based hanging unit termed Py 2 XPFe was previously used as an effective catalyst for the reduction of protons to molecular hydrogen in solution. Here, the molecular compound was immobilized on a modified gold electrode surface and investigated by spectroelectrochemical methods under catalytic conditions. Immobilization of the Py 2 XPFe was facilitated using a pyridine-based amine linker molecule grafted to the gold electrode by electrochemical amine oxidation. The linker molecule denoted in this report as Pyr-1 allows for effective coordination of the iron porphyrin compound to the modified gold surface through axial coordination of the pyridine component to the Fe center. Resonance Raman spectroelectrochemistry was performed on the immobilized catalyst in pH 7 buffer at increasing cathodic potentials. This facilitates the electrochemical hydrogen evolution reaction (HER) while concurrently allowing for the observation of the v 4 , v 3 , and v 2 porphyrin marker bands, which are sensitive to oxidation and spin state changes at the metal center. The observed changes in these bands at decreasing potential indicate that the immobilized Py 2 XPFe exists in the formal highspin Fe III state before being reduced to the low-spin Fe II state resulting from axial interaction with the linker moiety. This Fe II state likely acts as the precatalyst for the HER reaction. Surfaced enhanced Raman spectroelectrochemistry was also conducted on the system as the gold electrode provides a sufficient surface enhancement effect so as to observe the bonding nature of the pyridine substituents within the second coordination sphere. As the potential is lowered cathodically, the pyridine ring breathing modes at 999 cm −1 are shown to increase in intensity due to protonation, which reach an intensity saturated limit whereat HER is conducted. This suggests that in pH 7 buffer, the increase in cathodic potentials facilitates protonation of the pyridine-based second coordination sphere. The extent to which protonation occurs can be viewed as a function of decreasing potential due to an increase in proton flux at the immobilized catalyst which, at the required onset potential for catalysis, aids in the reduction of protons to molecular hydrogen.

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Section: Resultsmentioning

confidence: 99%

Section: ■ Introductionmentioning

confidence: 99%

Potential Induced Protonation of the Second Coordination Sphere in a Hangman-type Iron Porphyrin Complex Promotes HER: Insights via in Situ Raman Spectroelectrochemistry

et al. 2023

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“…9a,d,e,h In our calculations, a quintet ground state 10 was found for the intermediate A, which is in line with previously reported experimental observations. 11 Starting from the resting state intermediate A, coordination of diazirine 1 via one of the nitrogen lone pairs results in the formation of intermediate B in a closed-shell singlet (css) spin state via spin-crossover (Figure 1). The subsequent ring-opening of the diazirine motif via transition state TS(B-D) has an activation free energy of 34.3 kcal mol −1 on the closed-shell singlet surface to lead to intermediate D. The isomerization via TS(B-D) takes place on the css surface and does not represent a stable stationary point in other spin states.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

“…We initiated our investigation by studying the formation of phenyldiazomethane ( 3 ) via isomerization of 3-phenyl-3 H -diazirine ( 1 ) at the B3LYP-D3(BJ)/def2-TZVP+CPCM(Et 2 O)//B3LYP-D3(BJ)/6-31G(d),LANL2DZ(Fe) level of theory . In the calculations, an iron(II) porphyrin complex bearing an imidazole ligand in the axial position was employed as a model for the histidine-coordinated iron(II) heme active site in the enzyme, which is formed in the presence of Na 2 S 2 O 4 as reductant. , The implicit CPCM solvation model with Et 2 O as the solvent was employed to simulate the hydrophobic environment found inside the active site of the enzyme. ,,, In our calculations, a quintet ground state was found for the intermediate A , which is in line with previously reported experimental observations . Starting from the resting state intermediate A , coordination of diazirine 1 via one of the nitrogen lone pairs results in the formation of intermediate B in a closed-shell singlet (css) spin state via spin-crossover (Figure ).…”

Section: Resultsmentioning

confidence: 99%

Iron Heme Enzyme-Catalyzed Cyclopropanations with Diazirines as Carbene Precursors: Computational Explorations of Diazirine Activation and Cyclopropanation Mechanism

Rogge,

Zhou,

Porter

et al. 2024

J. Am. Chem. Soc.

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The mechanism of cyclopropanations with diazirines as air-stable and user-friendly alternatives to commonly employed diazo compounds within iron heme enzyme-catalyzed carbene transfer reactions has been studied by means of density functional theory (DFT) calculations of model systems, quantum mechanics/molecular mechanics (QM/MM) calculations, and molecular dynamics (MD) simulations of the iron carbene and the cyclopropanation transition state in the enzyme active site. The reaction is initiated by a direct diazirine-diazo isomerization occurring in the active site of the enzyme. In contrast, an isomerization mechanism proceeding via the formation of a free carbene intermediate in lieu of a direct, one-step isomerization process was observed for model systems. Subsequent reaction with benzyl acrylate takes place through stepwise C−C bond formation via a diradical intermediate, delivering the cyclopropane product. The origin of the observed diastereo-and enantioselectivity in the enzyme was investigated through MD simulations, which indicate a preferred formation of the cis-cyclopropane by steric control.

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“…Fe IV O porphyrins also exclusively adopt the S = 1 configuration of the metal center. 158 The reactivity of iron( iv )–oxo complex 42 in C–H bond activation and further details on the electronic properties of 42 are discussed in 4.4.…”

Section: Reactivitymentioning

confidence: 99%

Cyclic iron tetra N-heterocyclic carbenes: synthesis, properties, reactivity, and catalysis

Schlachta

Kühn

2023

Chem. Soc. Rev.

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Spin States in Iron Porphyrins

Cited by 6 publications

References 97 publications

Potential Induced Protonation of the Second Coordination Sphere in a Hangman-type Iron Porphyrin Complex Promotes HER: Insights via in Situ Raman Spectroelectrochemistry

Potential Induced Protonation of the Second Coordination Sphere in a Hangman-type Iron Porphyrin Complex Promotes HER: Insights via in Situ Raman Spectroelectrochemistry

Iron Heme Enzyme-Catalyzed Cyclopropanations with Diazirines as Carbene Precursors: Computational Explorations of Diazirine Activation and Cyclopropanation Mechanism

Cyclic iron tetra N-heterocyclic carbenes: synthesis, properties, reactivity, and catalysis

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