Structural Insights into the Nature of Fe 0 and Fe I Low-Valent Species Obtained upon the Reduction of Iron Salts by Aryl Grignard Reagents

International audienceMechanistic studies of the reduction of Fe III and Fe II salts by aryl Grignard reagents in toluene/tetrahydrofuran mixtures in the absence of a supporting ligand, as well as structural insights regarding the nature of the low-valent iron species obtained at the end of this reduction process, are reported. It is shown that several reduction pathways can be followed, depending on the starting iron precursor. We demonstrate, moreover, that these pathways lead to a mixture of Fe 0 and Fe I complexes regardless of the nature of the precursor. Mö ssbauer and 1 H NMR spectroscopies suggest that diamagnetic 16-electron bisarene complexes such as (η 4-C 6 H 5 Me) 2 Fe 0 can be formed as major species (85% of the overall iron quantity). The formation of a η 6-arene-ligated low-spin Fe I complex as a minor species (accounting for ca. 15% of the overall iron quantity) is attested by Mö ssbauer spectroscopy, as well as by continuous-wave electron paramagnetic resonance (EPR) and pulsed-EPR (HYSCORE) spectroscopies. The nature of the Fe I coordination sphere is discussed by means of isotopic labeling experiments and density functional theory calculations. It is shown that the most likely low-spin Fe I candidate obtained in these systems is a diphenylarene-stabilized species [(η 6-C 6 H 5 Me)Fe I Ph 2 ] − exhibiting an idealized C 2v topology. This enlightens the nature of the lowest valence states accommodated by iron during the reduction of Fe III and Fe II salts by aryl Grignard reagents in the absence of any additional coligand, which so far remained rather unknown. The reactivity of these low-valent Fe I and Fe 0 complexes in aryl−heteroaryl Kumada cross-coupling conditions has also been investigated, and it is shown that the zerovalent Fe 0 species can be used efficiently as a precursor in this reaction, whereas the Fe I oxidation state does not exhibit any reactivity

In cellulo Mossbauer and EPR studies bring new evidence to the long-standing debate on iron-sulfur cluster binding in human anamorsin

International audienceHuman anamorsin is an iron-sulfur (Fe-S)-cluster-binding protein acting as an electron donor in the early steps of cytosolic iron-sulfur protein biogenesis. Human anamorsin belongs to the eukaryotic CIAPIN1 protein family and contains two highly conserved cysteine-rich motifs, each binding an Fe-S cluster. In vitro works by various groups have provided rather controversial results for the type of Fe-S clusters bound to the CIAPIN1 proteins. In order to unravel the knot on this topic, we used an in cellulo approach combining Mossbauer and EPR spectroscopies to characterize the iron-sulfur-cluster-bound form of human anamorsin. We found that the protein binds two [2Fe-2S] clusters at both its cysteine-rich motifs

Mossbauer spectroscopic and computational investigation of an iron cyclopentadienone complex

International audience(Cyclopentadienone)iron carbonyl complexes have recently received particular attention for their use as catalysts in hydrogenation or transfer hydrogenation reactions including the N-alkylation of amines with alcohols. This is due to their easy synthesis from simple and cheap materials, air and water stabilities, and the crucial metal-ligand cooperation giving rise to unique catalytic properties. Here, we report a Mossbauer spectroscopic and computational investigation of such a complex and its corresponding activated species for dehydrogenation and hydrogenation reactions. This study affords a deeper understanding of the species formed by the reaction with Me$_3$NO and their distribution upon the added amount of an oxidant

Selective C–H halogenation over hydroxylation by non-heme iron(IV)-oxo

International audienceNon-heme iron based halogenase enzymes promote selective halogenation of the sp3-C–H bond through iron(IV)-oxo-halide active species. During halogenation, competitive hydroxylation can be prevented completely in enzymatic systems. However, synthetic iron(IV)-oxo-halide intermediates often result in a mixture of halogenation and hydroxylation products. In this report, we have developed a new synthetic strategy by employing non-heme iron based complexes for selective sp3-C–H halogenation by overriding hydroxylation. A room temperature stable, iron(IV)-oxo complex, [Fe(2PyN2Q)(O)]2+ was directed for hydrogen atom abstraction (HAA) from aliphatic substrates and the iron(II)-halide [FeII(2PyN2Q)(X)]+ (X, halogen) was exploited in conjunction to deliver the halogen atom to the ensuing carbon centered radical. Despite iron(IV)-oxo being an effective promoter of hydroxylation of aliphatic substrates, the perfect interplay of HAA and halogen atom transfer in this work leads to the halogenation product selectively by diverting the hydroxylation pathway. Experimental studies outline the mechanistic details of the iron(IV)-oxo mediated halogenation reactions. A kinetic isotope study between PhCH3 and C6D5CD3 showed a value of 13.5 that supports the initial HAA step as the RDS during halogenation. Successful implementation of this new strategy led to the establishment of a functional mimic of non-heme halogenase enzymes with an excellent selectivity for halogenation over hydroxylation. Detailed theoretical studies based on density functional methods reveal how the small difference in the ligand design leads to a large difference in the electronic structure of the [Fe(2PyN2Q)(O)]2+ species. Both experimental and computational studies suggest that the halide rebound process of the cage escaped radical with iron(III)-halide is energetically favorable compared to iron(III)-hydroxide and it brings in selective formation of halogenation products over hydroxylation

Structural Insights into the Nature of Fe⁰ and Fe^I Low-Valent Species Obtained upon the Reduction of Iron Salts by Aryl Grignard Reagents

Mechanistic studies of the reduction of Fe^III and Fe^II salts by aryl Grignard reagents in toluene/tetrahydrofuran mixtures in the absence of a supporting ligand, as well as structural insights regarding the nature of the low-valent iron species obtained at the end of this reduction process, are reported. It is shown that several reduction pathways can be followed, depending on the starting iron precursor. We demonstrate, moreover, that these pathways lead to a mixture of Fe⁰ and Fe^I complexes regardless of the nature of the precursor. Mössbauer and ¹H NMR spectroscopies suggest that diamagnetic 16-electron bisarene complexes such as (η⁴-C₆H₅Me)₂Fe⁰ can be formed as major species (85% of the overall iron quantity). The formation of a η⁶-arene-ligated low-spin Fe^I complex as a minor species (accounting for ca. 15% of the overall iron quantity) is attested by Mössbauer spectroscopy, as well as by continuous-wave electron paramagnetic resonance (EPR) and pulsed-EPR (HYSCORE) spectroscopies. The nature of the Fe^I coordination sphere is discussed by means of isotopic labeling experiments and density functional theory calculations. It is shown that the most likely low-spin Fe^I candidate obtained in these systems is a diphenylarene-stabilized species [(η⁶-C₆H₅Me)­Fe^IPh₂]⁻ exhibiting an idealized <i>C</i>_2<i>v</i> topology. This enlightens the nature of the lowest valence states accommodated by iron during the reduction of Fe^III and Fe^II salts by aryl Grignard reagents in the absence of any additional coligand, which so far remained rather unknown. The reactivity of these low-valent Fe^I and Fe⁰ complexes in aryl–heteroaryl Kumada cross-coupling conditions has also been investigated, and it is shown that the zerovalent Fe⁰ species can be used efficiently as a precursor in this reaction, whereas the Fe^I oxidation state does not exhibit any reactivity

Intercepting a transient non-hemic pyridine N-oxide Fe(iii) species involved in OAT reactions

International audienceIn the context of bioinspired OAT catalysis, we developed a tetradentate dipyrrinpyridine ligand, a hybrid of hemic and non-hemic models. The catalytic activity of the iron(iii) derivative was investigated in the presence of iodosylbenzene. Unexpectedly, MS, EPR, Mossbauer, UV-visible and FTIR spectroscopic signatures supported by DFT calculations provide convincing evidence for the involvement of a relevant Fe-III-O-N-Py active intermediate

.Single-Ion Magnetic Behaviour in an Iron(III) Porphyrin Complex: A Dichotomy Between High Spin and 5/2-3/2 Spin Admixture

International audienceA mononuclear iron(III) porphyrin compound exhibiting unexpectedly slow magnetic relaxation, which is a characteristic of single-ion magnet behaviour, is reported. This behaviour originates from the close proximity (approximate to 550 cm(-1)) of the intermediate-spinS=3/2 excited states to the high-spinS=5/2 ground state. More quantitatively, although the ground state is mostlyS=5/2, a spin-admixture model evidences a sizable contribution (approximate to 15 %) ofS=3/2 to the ground state, which as a consequence experiences large and positive axial anisotropy (D=+19.2 cm(-1)). Frequency-domain EPR spectroscopy allowed them(S)= |+/- 1/2&RightAngleBracket;->|+/- 3/2&RightAngleBracket; transitions to be directly accessed, and thus the very large zero-field splitting in this 3d(5)system to be unambiguously measured. Other experimental results including magnetisation, Mossbauer, and field-domain EPR studies are consistent with this model, which is also supported by theoretical calculations

Single Asparagine to Arginine Mutation Allows PerR to Switch from PerR Box to Fur Box

Fur family proteins, ubiquitous in prokaryotes, play a pivotal role in microbial survival and virulence in most pathogens. Metalloregulators, such as Fur and PerR, regulate the transcription of genes connected to iron homeostasis and response to oxidative stress, respectively. In <i>Bacillus subtilis</i>, Fur and PerR bind with high affinity to DNA sequences differing at only two nucleotides. In addition to these differences in the PerR and Fur boxes, we identify in this study a residue located on the DNA binding motif of the Fur protein that is critical to discrimination between the two close DNA sequences. Interestingly, when this residue is introduced into PerR, it lowers the affinity of PerR for its own DNA target but confers to the protein the ability to interact strongly with the Fur DNA binding sequence. The present data show how two closely related proteins have distinct biological properties just by changing a single residue

Proton-Coupled Intervalence Charge Transfer: Concerted Processes

The kinetics of proton-induced intervalence charge transfer (IVCT) may be measured electrochemically by generating one of the members of the IVCT couple in situ and following its conversion by means of the electrochemical signature of the other member of the couple. In the case of the diiron complex taken as an example, the reaction kinetics analysis, including the H/D isotope effect, clearly points to the prevalence of the concerted proton–intervalence charge transfer pathway over the stepwise pathways. A route is thus open toward systematic kinetic studies of proton-induced IVCT aiming at uncovering the main reactivity parameters and the factors that control the occurrence of concerted versus stepwise pathways