Characterization of a d7 iron system. Tetraphenylporphineiron(I) anion

Cohen, Ilan; Ostfeld, David; Lichtenstein, Barry

doi:10.1021/ja00768a018

Cited by 61 publications

(18 citation statements)

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“…5). By comparison with the well-known spectrum of five-coordinate low-spin Co(II) (d 7 ) porphyrins and with an early study of an electrochemically reduced ferroheme, 34 this signal can be assigned unambiguously to low-spin Fe(I)-P450cam trapped in the geometry of the parent Fe(II)-P450cam. The g tensor indicates that the odd electron on Fe is in the d z 2 orbital, with g along the heme normal.…”

Section: Ferrous-p450cam Complexes: Cryoreduction To Fe(i)mentioning

confidence: 82%

Cryoreduction EPR and 13C, 19F ENDOR study of substrate-bound substates and solvent kinetic isotope effects in the catalytic cycle of cytochrome P450cam and its T252A mutant

Kim

Yang²,

Perera

et al. 2005

Dalton Trans.

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We recently used cryoreduction EPR/ENDOR techniques to show that a substrate can modulate the properties of both the monooxygenase active-oxygen intermediates and of the proton-delivery network which encompasses them. In the present report we use Q-band pulsed 19F ENDOR (Mims 3-pulse sequence) to examine the substrate binding geometries of camphor, through use of the 5,5'--difluorocamphor, and 13C ENDOR to examine the binding of 5-methylenyl camphor labeled with 13C at C11. These probes are examined in multiple states of the catalytic cycle of P450cam and its T252A mutant. As part of this investigation we further report a new cryoreduction reaction, the reduction of a ferroheme to the EPR-visible Fe(I) state, and use it to probe the substrate binding to the EPR-silent ferroheme state. Finally we report the solvent kinetic isotope effect on the decay of the camphor complex of the hydroperoxo-ferric intermediate, the first such measurement on an individual step within the P450cam reaction cycle. Following reduction of oxyferrous-P450cam, this step is the rate-limiting step in camphor hydroxylation, and its solv-KIE of 1.8 at 190 K establishes that it involves activation of the hydroperoxo moiety by transfer of the 'second' proton of catalysis. We suggest that the finding that the heme pocket can exist in multiple substates, including multiple substrate binding locations, even in P450cam, along with the established possibility that the hydroperoxo-ferriheme intermediate can react with substrate, may explain the formation of multiple products by P450s.

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Section: Ferrous-p450cam Complexes: Cryoreduction To Fe(i)mentioning

confidence: 82%

Cryoreduction EPR and 13C, 19F ENDOR study of substrate-bound substates and solvent kinetic isotope effects in the catalytic cycle of cytochrome P450cam and its T252A mutant

Kim

Yang²,

Perera

et al. 2005

Dalton Trans.

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“…From the very first times the catalytic or stoichiometric reactivity of doubly reduced Fe II porphyrin were investigated, until very recently, various spectroscopic techniques have indicated that the Fe II resonance form appears as largely predominant over the Fe 0 form (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21). Taking as an example the reduction of CO 2 to CO, the starting molecular reductant would be the Fe II resonance form of the doubly reduced Fe II porphyrin and the final product Fe II CO. From this perspective, the metal oxidation number seems to be invariant over the course of the reaction, and hence the ligand is not only noninnocent but also "redox active."…”

Section: Significancementioning

confidence: 99%

Ligand “noninnocence” in coordination complexes vs. kinetic, mechanistic, and selectivity issues in electrochemical catalysis

Costentin

Savéant

Tard

2018

Proc. Natl. Acad. Sci. U.S.A.

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The world of coordination complexes is currently stimulated by the quest for efficient catalysts for the electrochemical reactions underlying modern energy and environmental challenges. Even in the case of a multielectron-multistep process, catalysis starts with uptake or removal of one electron from the resting state of the catalyst. If this first step is an outer-sphere electron transfer (triggering a "redox catalysis" process), the electron distribution over the metal and the ligand is of minor importance. This is no longer the case with "chemical catalysis," in which the active catalyst reacts with the substrate in an inner-sphere manner, often involving the transient formation of a catalyst-substrate adduct. The fact that, in most cases, the ligand is "noninnocent," in the sense that the electron density and charge gained (or removed) from the resting state of the catalyst are shared between the metal and the ligand, has become common-place knowledge over the last half-century. Insistent focus on a large degree of noninnocence of the ligand in the resting state of the catalyst, even robustly validated by spectroscopic techniques, may lead to undermining the essential role of the metal when such essential issues as kinetics, mechanisms, and product selectivity are dealt with. These points are general in scope, but their discussion is eased by adequately documented examples. This is the case for reactions involving metalloporphyrins as well as vitamin B12 derivatives and similar cobalt complexes for which a wealth of experimental data is available.

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“…Because reduction of [FeTPP] can occur either at the metal or at the ligand, the exact nature of the electronic ground state of the reduced iron species [FeTPP] − and [FeTPP] 2– has long been controversial. A variety of spectroscopic studies, including Mössbauer, − electron paramagnetic resonance (EPR), − nuclear magnetic resonance (NMR), ,− X-ray, ,, UV/vis, ,,, and resonance Raman − experiments have been published, partially supporting different electronic ground state formulations. − Additionally, so far, resonance Raman experiments have been performed in solution, giving rise to solvent dependency of the electronic ground states. [FeTPP] and [FeTPP] − are even known to change their spin state upon axial ligation of solvent molecules .…”

Section: Introductionmentioning

confidence: 99%

Electronic Structure and Spin Multiplicity of Iron Tetraphenylporphyrins in Their Reduced States as Determined by a Combination of Resonance Raman Spectroscopy and Quantum Chemistry

Römelt

Bill

et al. 2018

Inorg. Chem.

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Iron tetraphenylporphyrins are prime candidates as catalysts for CO reduction. Yet, even after 40 years of research, fundamental questions about the electronic structure of their reduced states remain, in particular as to whether the reducing equivalents are stored at the iron center or at the porphyrin ligand. In this contribution, we address this question by a combination of resonance Raman spectroscopy and quantum chemistry. Analysis of the data allows for an unequivocal identification of the porphyrin as the redox active moiety. Additionally, determination of the spin state of iron is possible by comparing the characteristic shifts of spin and oxidation-state-sensitive marker bands in the Raman spectrum with calculations of planar porphyrin model structures.

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Characterization of a d7 iron system. Tetraphenylporphineiron(I) anion

Cited by 61 publications

References 1 publication

Cryoreduction EPR and 13C, 19F ENDOR study of substrate-bound substates and solvent kinetic isotope effects in the catalytic cycle of cytochrome P450cam and its T252A mutant

Cryoreduction EPR and 13C, 19F ENDOR study of substrate-bound substates and solvent kinetic isotope effects in the catalytic cycle of cytochrome P450cam and its T252A mutant

Ligand “noninnocence” in coordination complexes vs. kinetic, mechanistic, and selectivity issues in electrochemical catalysis

Electronic Structure and Spin Multiplicity of Iron Tetraphenylporphyrins in Their Reduced States as Determined by a Combination of Resonance Raman Spectroscopy and Quantum Chemistry

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