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Human 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 Mössbauer 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., A combined in cellulo Mössbauer‐ and EPR‐based approach is applied to address controversial aspects of Fe–S cluster‐binding properties of human anamorsin, a protein involved in the biogenesis of cytosolic [4Fe–4S] cluster‐binding proteins. We show that human anamorsin binds two [2Fe–2S] clusters in‐cell in two highly conserved cysteine‐rich motifs typical of the eukaryotic CIAPIN1 protein family, with different electron‐spin‐relaxation properties.

International audience; Multicomponent reactions are attracting strong interest as they contribute to the development of more efficient synthetic chemistry. Understanding their mechanism is thus an important issue to optimize their operation. However, it is also a challenging task owing to the complexity of the succession of molecular events involved. Computational methods have recently proven to be of utmost interest to help decipher some of these processes, and the development of integrated experimental and theoretical approaches thus appears as the most powerful means to understand these mechanisms at the molecular level. A good example is given by the synthesis of amidines which are important pharmaceutical compounds. Their synthesis requires the association of three components, often an alkyne, a secondary amine, and an organic azide as the nitrene precursor. We found that an alternative way is offered by an Fe-catalyzed combination of a hydrocarbon, a nitrile, and a nitrene which gives amidines in good yields under mild conditions. The efficiency of the transformation and the paucity of mechanistic information on these reactions prompted us to thoroughly investigate its mechanism. Several mechanistic scenarios were explored using experimental techniques, including radical trap and N-15 labeling studies, combined with density-functional theory (DFT) calculations of reaction profiles. This allowed us to show that the amidination reaction involves the trapping of an intermediate substrate cation by an Fe-released acetonitrile molecule pointing to a true multicomponent reaction occurring exclusively within the cage around the metal center. Moreover, the calculated energy barriers of the individual steps explained how amidination outweighs direct amination in these reactions. The perfect consistency between DFT results and specific experiments to validate them strongly supports these mechanistic conclusions and highlights the potency of this combined approach.

Both O2 and H2O2 can oxidize iron at the ferroxidase center (FC) of Escherichia coli bacterioferritin (EcBfr) but mechanistic details of the two reactions need clarification. UV/Vis, EPR, and Mössbauer spectroscopies have been used to follow the reactions when apo‐EcBfr, pre‐loaded anaerobically with Fe2+, was exposed to O2 or H2O2. We show that O2 binds di‐Fe2+ FC reversibly, two Fe2+ ions are oxidized in concert and a H2O2 molecule is formed and released to the solution. This peroxide molecule further oxidizes another di‐Fe2+ FC, at a rate circa 1000 faster than O2, ensuring an overall 1:4 stoichiometry of iron oxidation by O2. Initially formed Fe3+ can further react with H2O2 (producing protein bound radicals) but relaxes within seconds to an H2O2‐unreactive di‐Fe3+ form. The data obtained suggest that the primary role of EcBfr in vivo may be to detoxify H2O2 rather than sequester iron., The kinetics of E. coli bacterioferritin di‐ferrous ferroxidase centre reacting with O2 and H2O2 shows that H2O2 reacts circa 1000 times faster than O2 implying that the primary in vivo role of the protein is ROS detoxification rather than iron sequestering.

International audience; Two new symmetrical and unsymmetrical diiron(iii) complexes were synthesized and characterized by X-ray diffraction analysis, mass spectrometry, UV-visible and Mossbauer spectroscopies. They proved to be good catalysts for alkene and alkane oxidation reactions by H2O2 in acetonitrile solution, and interesting effects of both the nature and the symmetry of the complexes were observed on catalysis in the presence of water.

International audience; With the intention to investigate the redox properties of polynuclear complexes as previously reported for the pentamanganese complex [{Mn(II)(μ-bpp)3}2Mn(III)Mn(II)2(μ3-O)](3+) (2(3+)), we focused on the analogous pentairon complex that was previously isolated as all-ferrous. As Masaoka and co-workers recently published, aerobic synthesis leads to the [{Fe(II)(μ-bpp)3}2Fe(III)Fe(II)2(μ3-O)](3+) complex (1(3+)). This species exhibits in acetonitrile solution four reversible one-electron oxidation waves. Accordingly, the three oxidized species 1(4+), 1(5+), and 1(6+) with a 3Fe(II)2Fe(III), 2Fe(II)3Fe(III), and 1Fe(II)4Fe(III) composition, respectively, were generated by bulk electrolysis and isolated. Mössbauer spectroscopy allowed us to determine the spin states of all the iron ions and to unambiguously locate the sites of the successive oxidations. They all occur in the μ3-oxo core except for the 1(4+) to 1(5+) process that presents a striking electronic rearrangement, with both metals in axial position being oxidized while the core is reduced to the [Fe(III)Fe(II)2(μ3-O)](5+) oxidation level. This strongly differs from the redox behavior of the Mn5 system. The origin of this electronic switch is discussed.

International audience; Green earth is a common green pigment based on celadonite and glauconite, used since Antiquity by artists. Two geological minerals, eight commercial green earth pigments and a sample taken from a historical location in Monte Baldo were characterized. A set of different techniques including X-Ray diffraction (XRD), scanning electron microscopy coupled to energy dispersive spectroscopy (SEM-EDS) and numerous spectroscopies: spectrophotocolorimetry, near and mid infrared, Raman, Mössbauer were used to identify the structure and composition of the different earths. The results highlight complex composition with the presence of various phases, which can be due to the pigment sampling at a different location in the same deposit. Mobile and non-invasive analyses were carried out in order to suggest a protocol for the identification of green earth in artworks, and more specifically to distinguish celadonite and glauconite. With the available mobile non-invasive techniques, and the above analyses on the raw pigments, the green area in Nicolas Poussin’s painting, Bacchanales d’enfants (Galleria Nazionale d’Arte Antica (GNAA), Rome) was examined as a case study.

Synthetic non-heme iron-oxo and iron-halide complexes promote selective halogenation of the sp3-C–H bonds via hydrogen atom abstraction and halide rebound phenomenon., Non-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.

Fe/S cluster biogenesis involves a complex machinery comprising several mitochondrial and cytosolic proteins. Fe/S cluster biosynthesis is closely intertwined with iron trafficking in the cell. Defects in Fe/S cluster elaboration result in severe diseases such as Friedreich ataxia. Deciphering this machinery is a challenge for the scientific community. Because iron is a key player, 57Fe-Mossbauer spectroscopy is especially appropriate for the characterization of Fe species and monitoring the iron distribution. This minireview intends to illustrate how Mossbauer spectroscopy contributes to unravel steps in Fe/S cluster biogenesis. Studies were performed on isolated proteins that may be present in multiple protein complexes. Since a few decades, Mossbauer spectroscopy was also performed on whole cells or on isolated compartments such as mitochondria and vacuoles, affording an overview of the iron trafficking. This minireview aims at presenting selected applications of 57Fe-Mossbauer spectroscopy to Fe/S cluster biogenesis.

Protein design is a powerful tool for interrogating the basic requirements for the function of a metal site in a way that allows for the selective incorporation of elements that are important for function. Rubredoxins are small electron transfer proteins with a reduction potential centered near 0 mV (vs normal hydrogen electrode). All previous attempts to design a rubredoxin site have focused on incorporating the canonical CXXC motifs in addition to reproducing the peptide fold or using flexible loop regions to define the morphology of the site. We have produced a rubredoxin site in an utterly different fold, a three-helix bundle. The spectra of this construct mimic the ultraviolet–visible, Mössbauer, electron paramagnetic resonance, and magnetic circular dichroism spectra of native rubredoxin. Furthermore, the measured reduction potential suggests that this rubredoxin analogue could function similarly. Thus, we have shown that an α-helical scaffold sustains a rubredoxin site that can cycle with the desired potential between the Fe(II) and Fe(III) states and reproduces the spectroscopic characteristics of this electron transport protein without requiring the classic rubredoxin protein fold.

International audience; An iron(II) borohydride complex ([(η1-H3BH)FeCl(NCCH3)4]) is employed as the precatalyst in iron-catalyzed radical cyclizations of unsaturated organic halides in the presence of NaBH4. Mechanistic investigations have established that the ligand bound to the metal center (acetonitrile versus ethylenebis(diphenylphosphine) (dppe)) plays a crucial role in the structure and reactivity of the active anionic iron(I) hydride ([HFeCl(dppe)2]−) and borohydride ([(η1-H3BH)FeCl(NCCH3)4]−) with unsaturated haloacetals. This work provides new insights into iron(I) hydride and borohydride species and their potential implication in single-electron processes.

Mammalian A-type proteins, ISCA1 and ISCA2, are evolutionarily conserved proteins involved in iron–sulfur cluster (Fe–S) biogenesis. Recently, it was shown that ISCA1 and ISCA2 form a heterocomplex that is implicated in the maturation of mitochondrial Fe4S4 proteins. Here we report that mouse ISCA1 and ISCA2 are Fe2S2-containing proteins that combine all features of Fe–S carrier proteins. We use biochemical, spectroscopic and in vivo approaches to demonstrate that despite forming a complex, ISCA1 and ISCA2 establish discrete interactions with components of the late Fe–S machinery. Surprisingly, knockdown experiments in mouse skeletal muscle and in primary cultures of neurons suggest that ISCA1, but not ISCA2, is required for mitochondrial Fe4S4 proteins biogenesis. Collectively, our data suggest that cellular processes with different requirements for ISCA1, ISCA2 and ISCA1–ISCA2 complex seem to exist.

International audience; 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 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.

Mammalian A-type proteins, ISCA1 and ISCA2, are evolutionarily conserved proteins involved in iron–sulfur cluster (Fe–S) biogenesis. Recently, it was shown that ISCA1 and ISCA2 form a heterocomplex that is implicated in the maturation of mitochondrial Fe4S4 proteins. Here we report that mouse ISCA1 and ISCA2 are Fe2S2-containing proteins that combine all features of Fe–S carrier proteins. We use biochemical, spectroscopic and in vivo approaches to demonstrate that despite forming a complex, ISCA1 and ISCA2 establish discrete interactions with components of the late Fe–S machinery. Surprisingly, knockdown experiments in mouse skeletal muscle and in primary cultures of neurons suggest that ISCA1, but not ISCA2, is required for mitochondrial Fe4S4 proteins biogenesis. Collectively, our data suggest that cellular processes with different requirements for ISCA1, ISCA2 and ISCA1–ISCA2 complex seem to exist., The mitochondrial proteins ISCA1 and ISCA2 form a complex that is involved in the biogenesis of Fe–S clusters. Here the authors report that ISCA1 and ISCA2 interact differently with proteins of the Fe–S machinery and that under certain conditions, ISCA2 seems dispensable for Fe–S biogenesis.

International audience; Human mitoNEET (mNT) is the first identified Fe-S protein of the mammalian outer mitochondrial membrane. Recently, mNT has been implicated in cytosolic Fe-S repair of a key regulator of cellular iron homeostasis. Here, we aimed to decipher the mechanism by which mNT triggers its Fe-S repair capacity. By using tightly controlled reactions combined with complementary spectroscopic approaches, we have determined the differential roles played by both the redox state of the mNT cluster and dioxygen in cluster transfer and protein stability. We unambiguously demonstrated that only the oxidized state of the mNT cluster triggers cluster transfer to a generic acceptor protein and that dioxygen is neither required for the cluster transfer reaction nor does it affect the transfer rate. In the absence of apo-acceptors, a large fraction of the oxidized holo-mNT form is converted back to reduced holo-mNT under low oxygen tension. Reduced holo-mNT, which holds a [2Fe-2S](+)with a global protein fold similar to that of the oxidized form is, by contrast, resistant in losing its cluster or in transferring it. Our findings thus demonstrate that mNT uses an iron-based redox switch mechanism to regulate the transfer of its cluster. The oxidized state is the "active state," which reacts promptly to initiate Fe-S transfer independently of dioxygen, whereas the reduced state is a "dormant form." Finally, we propose that the redox-sensing function of mNT is a key component of the cellular adaptive response to help stress-sensitive Fe-S proteins recover from oxidative injury.

Fe/S cluster biogenesis involves a complex machinery comprising several mitochondrial and cytosolic proteins. Fe/S cluster biosynthesis is closely intertwined with iron trafficking in the cell. Defects in Fe/S cluster elaboration result in severe diseases such as Friedreich ataxia. Deciphering this machinery is a challenge for the scientific community. Because iron is a key player, 57Fe-Mössbauer spectroscopy is especially appropriate for the characterization of Fe species and monitoring the iron distribution. This minireview intends to illustrate how Mössbauer spectroscopy contributes to unravel steps in Fe/S cluster biogenesis. Studies were performed on isolated proteins that may be present in multiple protein complexes. Since a few decades, Mössbauer spectroscopy was also performed on whole cells or on isolated compartments such as mitochondria and vacuoles, affording an overview of the iron trafficking. Graphical abstract This minireview aims at presenting selected applications of 57Fe-Mössbauer spectroscopy to Fe/S cluster biogenesis

Post-translational modifications of ribosomal proteins are important for the accuracy of the decoding machinery. A recent in vivo study has shown that the rimO gene is involved in generation of the 3-methylthio derivative of residue Asp-89 in ribosomal protein S12 (Anton, B. P., Saleh, L., Benner, J. S., Raleigh, E. A., Kasif, S., and Roberts, R. J. (2008) Proc. Natl. Acad. Sci. U. S. A. 105, 1826-1831). This reaction is formally identical to that catalyzed by MiaB on the C2 of adenosine 37 near the anticodon of several tRNAs. We present spectroscopic evidence that Thermotoga maritima RimO, like MiaB, contains two [4Fe-4S] centers, one presumably bound to three invariant cysteines in the central radical S-adenosylmethionine (AdoMet) domain and the other to three invariant cysteines in the N-terminal UPF0004 domain. We demonstrate that holo-RimO can specifically methylthiolate the aspartate residue of a 20-mer peptide derived from S12, yielding a mixture of mono- and bismethylthio derivatives. Finally, we present the 2.0 A crystal structure of the central radical AdoMet and the C-terminal TRAM (tRNA methyltransferase 2 and MiaB) domains in apo-RimO. Although the core of the open triose-phosphate isomerase (TIM) barrel of the radical AdoMet domain was conserved, RimO showed differences in domain organization compared with other radical AdoMet enzymes. The unusually acidic TRAM domain, likely to bind the basic S12 protein, is located at the distal edge of the radical AdoMet domain. The basic S12 protein substrate is likely to bind RimO through interactions with both the TRAM domain and the concave surface of the incomplete TIM barrel. These biophysical results provide a foundation for understanding the mechanism of methylthioation by radical AdoMet enzymes in the MiaB/RimO family.

The preparation of [FeIV(O)(MePy2tacn)]2+ (2, MePy2tacn = N-methyl-N,N-bis(2-picolyl)-1,4,7-triazacyclononane) by reaction of [FeII(MePy2tacn)(solvent)]2+ (1) and PhIO in CH3CN and its full characterization are described. This compound can also be prepared photochemically from its iron(II) precursor by irradiation at 447 nm in the presence of catalytic amounts of [Ru II(bpy)3]2+ as photosensitizer and a sacrificial electron acceptor (Na2S2O8). Remarkably, the rate of the reaction of the photochemically prepared compound 2 toward sulfides increases 150-fold under irradiation, and 2 is partially regenerated after the sulfide has been consumed; hence, the process can be repeated several times. The origin of this rate enhancement has been established by studying the reaction of chemically generated compound 2 with sulfides under different conditions, which demonstrated that both light and [Ru II(bpy)3]2+ are necessary for the observed increase in the reaction rate. A combination of nanosecond time-resolved absorption spectroscopy with laser pulse excitation and other mechanistic studies has led to the conclusion that an electron transfer mechanism is the most plausible explanation for the observed rate enhancement. According to this mechanism, the in-situ-generated [RuIII(bpy)3] 3+ oxidizes the sulfide to form the corresponding radical cation, which is eventually oxidized by 2 to the corresponding sulfoxide We acknowledge the European Commission for projects FP7-PEOPLE-2011-CIG-303522 (A.C.), FP7-PEOPLE-2010-ERG-268445 (J.L.-F.), FP7-PEOPLE-CIG-303522 (M.G.B.), and ERC-009StG-239910 (MC.); the Spanish Ministry of Science for Projects CTQ2012-37420-C02-01/BQU (MC.), CSD2010-00065 (MC.), and CTQ2011-27758 (J.P.P.); Generalitat de Catalunya for an ICREA Academia Award and Project 2009-SGR637 (M.C.); and Generalitat Valenciana for Project ACOMP/2013/008 (J.P.P.). The Spanish Ministry of Science is acknowledged for a Ramon y Cajal contract to A.C. and J.L.-F. J.M.L. acknowledges the support, in part, of Labex ARCANE (ANR-11-LABX-0003-01). The work at the University of Minnesota was supported by the US National Science Foundation (Grant CHE1058248 to L.Q) and the Dr. Venkateswarlu Pothapragada and Family Fellowship (to M.P.). XAS data were collected at beamline 9-3 of the Stanford Synchrotron Radiation Lightsource supported by the US-NIH and US-DOE. We thank Catexel for a generous gift of tritosyl-1,4,7-triazacyclononane

International audience; Superoxide reductase (SOR) is a non-heme iron metalloenzyme that detoxifies superoxide radical in microorganisms. Its active site consists of an unusual non-heme Fe(2+) center in a [His4Cys1] square pyramidal pentacoordination, with the axial cysteine ligand proposed to be an essential feature in catalysis. Two NH peptide groups from isoleucine 118 and histidine 119 establish hydrogen bonds involving the sulfur ligand (Desulfoarculus baarsii SOR numbering). To investigate the catalytic role of these hydrogen bonds, the isoleucine 118 residue of the SOR from Desulfoarculus baarsii was mutated into alanine, aspartate, or serine residues. Resonance Raman spectroscopy showed that the mutations specifically induced an increase of the strength of the Fe(3+)-S(Cys) and S-Cβ(Cys) bonds as well as a change in conformation of the cysteinyl side chain, which was associated with the alteration of the NH hydrogen bonding involving the sulfur ligand. The effects of the isoleucine mutations on the reactivity of SOR with O2 (*-) were investigated by pulse radiolysis. These studies showed that the mutations induced a specific increase of the pK a of the first reaction intermediate, recently proposed to be an Fe(2+)-O2 (*-) species. These data were supported by density functional theory calculations conducted on three models of the Fe(2+)-O2 (*-) intermediate, with one, two, or no hydrogen bonds involving the sulfur ligand. Our results demonstrated that the hydrogen bonds between the NH (peptide) and the cysteine ligand tightly control the rate of protonation of the Fe(2+)-O2 (*-) reaction intermediate to form an Fe(3+)-OOH species.

The Oxygen evolving complex of plant photosystem II is made of a manganese cluster that gives rise to a low temperature EPR multiline signal in the S2 oxidation state. The origin of this EPR signal has been addressed with respect to the question of the magnetic couplings between the electron and nuclear spins of the four possible Mn ions that make up this complex. Considering Mn(III) and Mn(IV) as the only possible oxidation states present in the S2 state, and no large anisotropy of the magnetic tensors, the breadths of the EPR spectra calculated for dimers and trimers with S = ½ have been compared with that of the biological site. It is concluded that neither a dinuclear nor a trinuclear complex made of Mn(III) and Mn(IV) can be responsible for the multiline signal; but that, by contrast, a tetranuclear Mn complex can be the origin of this signal. The general shape of the experimental spectrum, its particular hyperfine pattern, the positions of most of the hyperfine lines and their relative intensities can be fit by a tetramer model described by the following six fitting parameters: g ≈ 1.987, A1 ≈ 122.4 10-4 cm-1, A2 ≈ 87.2 10-4 cm-1, A3 ≈ 81.6 10-4 cm-1, A4 ≈ 19.1 10-4 cm-1 and δH = 24.5 G. A second model described by parameters very close to those given above except for A4 ≈ 77.5 10-4 cm-1 gives an equally good fit. However, no other set of parameters gives an EPR spectrum that reproduces the hyperfine pattern of the S2 multiline signal. This demonstrates that in the S2 state of the oxygen evolving complex, the four manganese ions are organized in a magnetic tetramer.

International audience; The two pentadentate amino-pyridine ligands L5(2) and L5(3) (L5(2) and L5(3) stand for the N-methyl-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine and the N-methyl-N,N',N'-tris(2-pyridylmethyl)propane-1,3-diamine, respectively) were used to synthesize four mononuclear Mn(II) complexes, namely [(L5(2))MnCl](PF6) (1(PF6)), [(L5(3))MnCl](PF6) (2(PF6)), [(L5(2))Mn(OH2)](BPh4)2 (3(BPh4)2), and [(L5(3))Mn(OH2)](BPh4)2 (4(BPh4)2). The X-ray diffraction studies revealed different configurations for the ligand L5(n) (n = 2, 3) depending on the sixth exogenous ligand and/or the counterion. Solid state high-field electron paramagnetic resonance spectra were recorded on complexes 1-4 as on previously described mononuclear Mn(II) systems with tetra- or hexadentate amino-pyridine ligands. Positive and negative axial zero-field splitting (ZFS) parameters D were determined whose absolute values ranged from 0.090 to 0.180 cm(-1). Density-functional theory calculations were performed unraveling that, in contrast with chloro systems, the spin-spin and spin-orbit coupling contributions to the D-parameter are comparable for mixed N,O-coordination sphere complexes.

The electronic properties of the isostructural series of heterotrinuclear thiophenolate-bridged complexes of the general formula [LFeMFeL](n+) With M = Cr, Co and Fe where L represents the trianionic form of the ligand 1,4,7-tris(4-tertbutyl-2-mercaptobenzyl)-1,4,7-triazacyclononane, synthesized and investigated by a number of experimental techniques in the previous work(1), are subjected now to a theoretical analysis. The low-lying electronic excitations in these compounds are described within a minimal model supported by experiment and quantum chemistry calculations. It was found indeed that various experimental data concerning the magnetism and electron delocalization in the lowest states of all seven compounds are completely reproduced within a model which includes the electron transfer between magnetic orbitals at different metal centers and the electron repulsion in these orbitals (the Hubbard model). Moreover, due to the trigonal symmetry of the complexes, only the electron transfer between nondegenerate orbital, a(1), originating from the t(2g) shell of each metal ion in a pseudo-octahedral coordination, is relevant for the lowest states. An essential feature resulting from quantum chemistry calculations, allowing to explain the unusual magnetic properties of these compounds, is the surprisingly large value and, especially, the negative sign of the electron transfer between terminal iron ions, beta'. According to their electronic properties the series of complexes can be divided as follows: (1) The complexes [LFeFeFeL](3+) and [LFeCrFeL](3+) show localized valences in the ground electronic configuration. The strong antiferromagnetic exchange interaction and the resulting spin 1/2 of the ground-state arise from large values of the transfer parameters. (2) In the complex [LFeCrFeL](+), due to a higher energy of the magnetic orbital on the central Cr ion than on the terminal Fe ones, the spin 3/2 and the single unpaired a(1) electron are almost localized at the chromium center in the ground state. (3) The complex [LFeCoFeL](3+) has one ground electronic configuration in which two unpaired electrons are localized at terminal iron ions. The ground-state spin S = 1 arises from a kinetic mechanism involving the electron transfer between terminal iron ions as one of the steps. Such a mechanism, leading to a strong ferromagnetic interaction between distant spins, apparently has not been discussed before. (4) The complex [LFeFeFeL](2+) is characterized by both spin and charge degrees of freedom in the ground manifold. The stabilization of the total spin zero or one of the itinerant electrons depends on beta', i.e., corresponds to the observed S = 1 for its negative sign. This behavior does not fit into the double exchange model. (5) In [LFeCrFeL](2+) the delocalization of two itinerant holes in a(1) orbitals takes place over the magnetic core of chromium ion. Although the origin of the ground-state spin S = 2 is the spin dependent delocalization, the spectrum of the low-lying electronic states is again not of a double exchange type. (6) Finally, the complex [LFeCoFeL](2+) has the ground configuration corresponding to the electron delocalization between terminal iron atoms. The estimated magnitude of the corresponding electron transfer is smaller than the relaxation energy of the nuclear distortions induced by the electron localization at one of the centers, leading to vibronic valence trapping observed in this compound. ispartof: Journal of the American Chemical Society vol:125 issue:41 pages:12615-12630 ispartof: location:United States status: published

The X-band EPR spectra of the IR sensitive untreated PSII and of MeOH- and NH 3 -treated PSII from spinach in the S 2 -state are simulated with collinear and rhombic g - and Mn-hyperfine tensors. The obtained principal values indicate a 1Mn(III)3Mn(IV) composition for the Mn 4 cluster. The four isotropic components of the Mn-hyperfine tensors are found in good agreement with the previously published values determined from EPR and 55 Mn-ENDOR data. Assuming intrinsic isotropic components of the Mn-hyperfine interactions identical to those of the Mn-catalase, spin density values are calculated. A Y-shape 4 J -coupling scheme is explored to reproduce the spin densities for the untreated PSII. All the required criteria such as a S =1/2 ground state with a low lying excited spin state (30 cm −1 ) and an easy conversion to a S =5/2 system responsible for the g =4.1 EPR signal are shown to be satisfied with four antiferromagnetic interactions lying between −290 and −130 cm −1 .

N-alkylation of amines with alcohols is a method of choice for accessing compounds of pharmaceutical interest in a more sustainable way. This field of catalysis is now largely dominated by the use of molecular complexes in homogeneous phase. The proposed mechanism is based on hydrogen auto-transfer (HAT). Among the two main families of iron-based catalysts that have been developed, we have chosen to focus more particularly on iron(cyclopentadienone) type catalysts. These catalysts have several advantages: air stability and easy synthesis in few steps, as well as the possibility of modulating catalytic activity via ligand modification. Iron(cyclopentadienone) complexes have been shown to be effective catalysts but several challenges remain. The main problem in terms of substrates concerns the use of primary alcohols bearing electron-withdrawing groups but also secondary alcohols. In addition, they often require the use of high temperatures and/or unwanted additives. Reactions are most often carried out at solvent reflux or in closed vials for reaction times of up to days for some. This highlights the presence of a high-energy barrier within at least one stage of the catalytic cycle, severely limiting the application of these molecular catalysts to develop new reactivities. In addition, these molecular catalysts operate under homogeneous conditions making their recycling difficult and reducing the atom economy of the process as a whole. The development of efficient catalysts that can be recycled and can operate in milder conditions remains a challenge.As a first step, an in-depth mechanistic study on the functioning of known molecular complexes of iron (cyclopentadienone)carbonyl is conducted. It has led to a better understanding of the chemical activation step of the catalyst by shedding light on the formation of new iron species.In a second step, this same strategy was applied in the development of a new multi-catalytic process with the aim of synthesizing fluorinated pharmaceutical compounds. The results suggest that neither the mode of activation – i.e. chemical and/or thermal – nor the type of complex [LFe(CO)3] or [LFe(CO)2(NCPh)], has any influence on yields.Finally, the grafting of these catalysts on a graphene surface was studied. Two types of grafting were considered: covalent grafting and supramolecular grafting. Our choice finally fell on supramolecular grafting. For this purpose, a synthetic pathway to access functionalized complexes with a perylene-type anchor has been developed. This strategy was established with the aim of developing a heterogeneous and therefore recyclable version of the catalyst., La N-alkylation d’amines avec des alcools constitue une méthode de choix pour accéder à des composés d’intérêt pharmaceutique de manière plus durable. Ce domaine de catalyse est désormais largement dominé par l’emploi de complexes moléculaires en phase homogène. Le mécanisme proposé se base sur l’auto-transfert de dihydrogène (HAT). Parmi les deux grandes familles de catalyseurs à base de fer qui ont été développées, nous avons choisi de nous intéresser plus particulièrement aux catalyseurs de type fer(cyclopentadiénone). Ces catalyseurs présentent plusieurs avantages : stabilité à l’air et synthèse aisée en peu d’étapes ainsi que la possibilité de moduler l’activité catalytique via la modification des ligands. Ces catalyseurs se sont montrés efficaces mais plusieurs défis restent à relever. Le principal problème en terme de substrats concerne l’utilisation d’alcools primaires portant des groupements électroattracteurs mais aussi les alcools secondaires. Par ailleurs, ils nécessitent souvent l’usage de températures élevées et/ou d’additifs non désirés. Les réactions sont le plus souvent effectuées à reflux du solvant ou dans des vials clos pour des temps de réaction pouvant atteindre des jours pour certaines. Ceci met en évidence la présence d’une barrière énergétique élevée au sein d’au moins une étape du cycle catalytique, limitant fortement l’application de ces catalyseurs moléculaires pour développer de nouvelles réactivités. De plus, ces catalyseurs moléculaires opèrent en conditions homogènes rendant leur recyclage difficile et diminuant l’économie d’atomes du procédé dans son ensemble. Le développement de catalyseurs efficaces pouvant être recyclés et pouvant opérer en conditions plus douces reste un défi à relever.Dans un premier temps, une étude mécanistique approfondie sur le fonctionnement des complexes moléculaires connus de fer(cyclopentadiénone)carbonyle est menée. Elle a permis de mieux comprendre l'étape d'activation par voie chimique du catalyseur en mettant en lumière la formation de nouvelles espèces de fer.Dans un second temps, cette même stratégie a été appliquée dans le développement d’un nouveau procédé multi-catalytique dans le but de synthétiser des composés pharmaceutiques fluorés. Les résultats suggèrent que ni le mode d’activation – à savoir chimique et/ou thermique – ni le type de complexe [LFe(CO)3] or [LFe(CO)2(NCPh)], n’a d’influence sur les rendements.Enfin, le greffage de ces catalyseurs sur une surface de graphène a été étudié. Deux types de greffage ont été envisagés : le greffage covalent et le greffage supramoléculaire. Notre choix s’est finalement porté sur le greffage supramoléculaire. Dans ce but, une voie de synthèse permettant d’accéder aux complexes fonctionnalisés avec une ancre de type pérylène a été développée. Cette stratégie a été établie dans le but de développer une version hétérogène et donc recyclable du catalyseur.

The thesis matter concerns the valence inversion in FeIIFeIII induced by deprotonation of a FeII ligand. This study is of strong interest owing to the fact this process can be described as an electron transfer induced by a proton transfer. Protons and electrons transfers play essential roles in numerous catalytic or biologic reactions and therefore understanding whether they occur in a sequential or concerted manner is presently a major endeavor. The first part of the thesis is devoted to the characterization of the first system possessing this original property. It is based on binuclear complex FeIIFeIII where the two Fe ions are bridged by a dicarboxylate and a phenoxide. The ferric ion is bound by a bis-2-picolylamine group and the ferrous ion by a similar group where a pyridine has been replaced by aniline. Deprotonation of this FeII bound aniline induces the valence inversion, the resulting anilide being bound to the FeIII ion. The aniline complex was isolated with the aniline in trans position with respect to the bridging phenoxide, but it is not stable in solution and isomerizes, the aniline group moving to a cis position upon exchange with a pyridine. The same phenomenon was observed for the anilide complex obtained through deprotonation. This phenomenon was studied by combining UV-visible, 1H-RMN and Mössbauer spectroscopies, and the thermodynamic and kinetic characteristics of the isomerization were determined. The link between the electron transfer and the proton transfer were studied by electrochemical techniques. Thorough studies by cyclic voltammetry and isotopic labeling showed that in this system the electron and proton transfers are concerted. The second section of the thesis was aimed at studying the factors susceptible to influence the electron transfer, namely the redox potentials of the two Fe sites, and the proton transfer, namely the pKa of the protic ligand. To achieve it, new complexes were prepared by modifying either the protic ligand, the aniline being replaced by a benzimidazole, or the Fe binding group, substitution of bis-2-picolylamine by bis-(2-methyl-N-methylbenzimidazole)amine. Model complexes incorporating these changes but deprived of the protic ligand were also obtained to assess their influence on the redox properties of the Fe ions. The study of the influence of redox properties was considered first. The substitution of the group complexing FeIII has not a strong influence on the structure of the protonated complex which still exists as two isomers. By contrast, after deprotonation a single isomer exists. The spectroscopic properties are mostly unchanged which shows that the electronic structure of the system is not altered significantly. The study of the influence of the acidicity was then conducted. Two complexes differing by the nature of the FeIII bound group were considered. Replacing aniline by benzimidazole does not change significantly the structural properties of the system, but the valences of the Fe ions are less localized than in the original complex. The deprotonation of benzimidazole occurs and leads to a chromophore that differs from the preceding, revealing the difference in ligands. However, a preliminary electrochemical study reveals a behavior similar to that of the original complex., Le sujet de la thèse concerne l’étude de l’inversion de valence dans des complexes FeIIFeIII induite par la déprotonation d’un ligand de FeII. L’intérêt de cette étude vient de ce que le processus considéré peut être décrit comme un transfert d’électron induit par un transfert de proton. Les transferts de protons et d’électrons sont au cœur de nombreuses réactions catalytiques ou biologiques et la compréhension des mécanismes de leur mise œuvre, séquentielle ou couplée, est un enjeu très actuel.La première partie de la thèse est consacrée à la caractérisation du premier système présentant cette propriété originale. Il est fondé sur un complexe binucléaire FeIIFeIII dont les deux ions Fe sont pontés par un dicarboxylate et un phénolate. Le FeIII est lié par un groupe bis-2-picolylamine et le FeII par un groupe similaire dans lequel une pyridine a été remplacée par une aniline. C’est la déprotonation de ce groupe aniline lié au FeII qui provoque l’inversion de valence, l’anilinate formé étant lié au FeIII.Le complexe aniline est isolé avec le groupe aniline en position -trans par rapport au phénolate pontant mais il possède la particularité de ne pas être stable en solution et de s’isomériser, le groupe aniline passant en position -cis par échange avec une pyridine. Le même phénomène est observé pour le complexe anilinate formé par déprotonation. Ce phénomène a été étudié en combinant les spectroscopies UV-visible, RMN du proton et Mössbauer et les caractéristiques thermodynamiques et cinétiques de l’isomérisation ont été déterminées.La relation entre le transfert d’électron et le transfert de proton a été étudiée par des techniques électrochimiques. Des études approfondies par cyclovoltamétrie et marquage isotopique ont montré que dans ce système les transferts d’électron et de proton sont concertés.La deuxième partie de la thèse est consacrée à l’étude des facteurs qui sont susceptibles d’influencer le transfert d’électron, c’est-à-dire les potentiels rédox des deux sites à Fe, et le transfert de proton, c’est-à-dire le pKa du ligand protique. Pour ce faire, de nouveaux complexes ont été préparés en modifiant soit le ligand protique, l’aniline étant remplacée par un benzimidazole, soit le groupe complexant le Fe, remplacement de la bis-2-picolylamine par la bis-(2-méthyl-N-méthylbenzimidazole)amine. Des complexes présentant ces modifications mais dépourvus du ligand acide ont été synthétisés pour étudier les effets de ces modifications sur les propriétés rédox des ions Fe.L’étude de l’influence des propriétés rédox a été menée dans un premier temps. La substitution du groupe complexant le FeIII n’a pas de conséquence importante au plan structural pour le complexe protoné qui existe toujours sous la forme de deux isomères. Par contre, après déprotonation un seul isomère existe. Les propriétés spectroscopiques sont peu altérées ce qui montre que la structure électronique du système n’est pas bouleversée.L’étude de l’influence des propriétés acides a été menée ensuite. Deux complexes ont été étudiés qui diffèrent par la nature du groupe complexant le FeIII. La substitution de l’aniline par le benzimidazole ne change pas de façon notable les propriétés structurales du système mais on note cependant que les valences des deux ions sont moins localisées. La déprotonation du benzimidazole est observée et conduit à un chromophore sensiblement différent du précédent illustrant les différences de ligand.L’étude électrochimique préliminaire révèle un comportement similaire au premier complexe étudié.