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International audience; Electrochemical oxidation of the complex [Fe2(CO)4(κ2‐dmpe)(µ‐adtBn)] (adtBn = (SCH2)2NCH2C6H5, dmpe = Me2PCH2CH2PMe2) (1) has been studied by cyclic voltammetry (CV) in acetonitrile and in dichloromethane in presence of various substrates L (L =MeCN, trimethylphosphite, isocyanide). The oxidized species, [1‐MeCN](PF6)2, [1‐(P(OMe)3)2](PF6)2 and [1‐(RNC)4](PF6)2 (R = tButyl, Xylyl), have been prepared and characterized by IR, NMR spectroscopies and, excepted [1‐MeCN](PF6)2, by X‐ray diffraction analysis. The crystallographic structures of the new FeIIFeII complexes reveal in any case that the association of one additional ligand (P(OMe)3 or RNC) occurs and, according to the nature of the substrates, further substitutions of one or three carbonyl groups, by P(OMe)3 or RNC, respectively, arise. Density functional theory (DFT) calculations have been performed for elucidating and discriminating, in each case, the mechanisms leading to the corresponding oxidized species. Moreover, the different degree of ligand substitution in the diiron core has been theoretically rationalized.

Described are the syntheses of several Ni(μ-SR) 2 Fe complexes, including hydride derivatives, in a search for improved models for the active site of [NiFe]-hydrogenases. The nickel(II) precursors include (i) nickel with tripodal ligands: Ni(PS 3 ) â' and Ni(NS 3 ) â' (PS 3 3- = tris(phenyl-2-thiolato)phosphine, NS 3 3- = tris(benzyl-2-thiolato)amine), (ii) traditional diphosphine-dithiolates, including chiral diphosphine R,R-DIPAMP, (iii) cationic Ni(phosphine-imine/amine) complexes, and (iv) organonickel precursors Ni(o-tolyl)Cl(tmeda) and Ni(C 6 F 5 ) 2 . The following new nickel precursor complexes were characterized: PPh 4 [Ni(NS 3 )] and the dimeric imino/amino-phosphine complexes [NiCl 2 (PCHâ•N An )] 2 and [NiCl 2 (PCH 2 NH An )] 2 (P = Ph 2 PC 6 H 4 -2-). The iron(II) reagents include [CpFe(CO) 2 (thf)]BF 4 , [Cp∗Fe(CO)(MeCN) 2 ]BF 4 , FeI 2 (CO) 4 , FeCl 2 (diphos)(CO) 2 , and Fe(pdt)(CO) 2 (diphos) (diphos = chelating diphosphines). Reactions of the nickel and iron complexes gave the following new Ni-Fe compounds: Cp∗Fe(CO)Ni(NS 3 ), [Cp(CO)Fe(μ-pdt)Ni(dppbz)]BF 4 , [(R,R-DIPAMP)Ni(μ-pdt)(H)Fe(CO) 3 ]BAr F 4 , [(PCHâ•N An )Ni(μ-pdt)(Cl)Fe(dppbz)(CO)]BF 4 , [(PCH 2 NH An )Ni(μ-pdt)(Cl)Fe(dppbz)(CO)]BF 4 , [(PCHâ•N An )Ni(μ-pdt)(H)Fe(dppbz)(CO)]BF 4 , [(dppv)(CO)Fe(μ-pdt)] 2 Ni, {H[(dppv)(CO)Fe(μ-pdt)] 2 Ni]}BF 4 , and (C 6 F 5 ) 2 Ni(μ-pdt)Fe(CO) 2 (dppv) (DIPAMP = (CH 2 P(C 6 H 4 -2-OMe) 2 ) 2 ; BAr F 4 - = [B(C 6 H 3 -3,5-(CF 3 ) 2 ] 4 - )) Within the context of Ni-(SR) 2 -Fe complexes, these new complexes feature new microenvironments for the nickel center: Tetrahedral Ni, chirality, imine, and amine coligands, and Ni-C bonds. In the case of {H[(dppv)(CO)Fe(μ-pdt)] 2 Ni} + , four low-energy isomers are separated by ≤3 kcal/mol, one of which features a biomimetic HNi(SR) 4 site, as supported by density functional theory calculations

The electrochemical behavior of complexes [FeMo(CO) 5 (κ 2 -dppe)(μ-pdt)] (1) and [FeMo(CO) 4 (MeCN)( κ 2 -dppe)(μ-pdt)] (2), in the absence and in the presence of acid, has been investigated. The reduction of 1 follows at slow scan rates, in CH 2 Cl 2 -[NBu 4 ][PF 6 ] and acid-free media, an EC rev E mechanism that is supported by cyclic voltammetry (CV) experiments and digital CV simulations. In MeCN-[NBu 4 ][PF 6 ], the electrochemical reduction of 1 is the same as in dichloromethane and follows an ECE mechanism at slow scan rates, but with a positive shift of the redox potentials. In contrast, the oxidation of 1 is strongly solvent-dependent. In dichloromethane, the oxidation of 1 is reversible and involves a single electron, while in acetonitrile, it is irreversible at moderate and slow scan rates (v ≤ ca. 1 V s -1 ), and some chemical reversibility is apparent at higher scan rates (v = 10 V s -1 ). Density functional theory calculations revealed that the chemical step in the EC rev E mechanism corresponds to the dissociation of one PPh 2 end of the diphosphine ligand and the transfer of the semibridging CO to the Fe atom, similarly to the mechanism observed in the FeFe analogue complex. However, in the case of 1, the subsequent coordination of the phosphine ligand to the other metal is an unfavorable process.

The synthesis and redox properties are presented for the electron-rich bis(monothiolate)s Fe(2)(SR)(2)-(CO)(2)(dppv)(2) for R = Me ([1](0)), Ph ([2](0)), CH(2)Ph ([3](0)). Whereas related derivatives adopt C(2)-symmetric Fe(2)(CO)(2)P(4) cores, [1](0)–[3](0) have C(s) symmetry resulting from the unsymmetrical steric properties of the axial vs equatorial R groups. Complexes [1](0)–[3](0) undergo 1e(−) oxidation upon treatment with ferrocenium salts to give the mixed valence cations [Fe(2)(SR)(2)(CO)(2)(dppv)(2)](+). As established crystallographically, [3](+) adopts a rotated structure, characteristic of related mixed valence diiron complexes. Unlike [1](+) and [2](+) and many other [Fe(2)(SR)(2)L(6)](+) derivatives, [3](+) undergoes C–S bond homolysis, affording the diferrous sulfido-thiolate [Fe(2)(SCH(2)Ph)(S)(CO)(2)(dppv)(2)](+) ([4](+)). According to X-ray crystallography, the first coordination spheres of [3](+) and [4](+) are similar, but the Fe–sulfido bonds are short in [4](+). The conversion of [3](+) to [4](+) follows first-order kinetics, with k = 2.3 × 10(−6) s(−1) (30 °C). When the conversion is conducted in THF, the organic products are toluene and dibenzyl. In the presence of TEMPO, the conversion of [3](+) to [4](+) is accelerated about 10×, the main organic product being TEMPO-CH(2)Ph. DFT calculations predict that the homolysis of a C–S bond is exergonic for [Fe(2)(SCH(2)Ph)(2)(CO)(2)(PR(3))(4)](+) but endergonic for the neutral complex as well as less substituted cations. The unsaturated character of [4](+) is indicated by its double carbonylation to give [Fe(2)(SCH(2)Ph)(S)(CO)(4)(dppv)(2)](+) ([5](+)), which adopts a bioctahedral structure.

The azadithiolate complex Fe2[(SCH2)2NMe](CO)6 reacts with borane to give an initial 1:1 adduct, which spontaneously decarbonylates to give Fe2[(SCH2)2NMeBH3](CO)5. Featuring a Fe–H–B three-center, two-electron interaction, the pentacarbonyl complex is a structural model for H2 complexes invoked in the [FeFe]-hydrogenases. The pentacarbonyl compound is a “σ complex”, where a B–H σ bond serves as a ligand for iron. The structure of this σ complex was characterized by variable-temperature NMR spectroscopy and X-ray crystallography. Complementary to the 1:1 borane adduct is the quaternary ammonium complex [Fe2[(SCH2)2NMe2](CO)6]+, which was also characterized. It represents a kinetically robust analogue of the N-protonated amine cofactor, as indicated by its mild reduction potential.

International audience; Electrochemical studies of [Fe2(CO)4(κ2-dmpe)(μ-dithiolate)] (dithiolate=adtBn, pdt) and density functional theory (DFT) calculations reveal the striking influence of an amine functionality in the dithiolate bridge on their oxidative properties. [Fe2(CO)4(κ2-dmpe)(μ-adtBn)] (1) undergoes two one-electron oxidation steps, with the first being partially reversible and the second irreversible. When the adtBn bridge is replaced with pdt, a shift of 60 mV towards more positive potentials is observed for the first oxidation whereas 290 mV separate the oxidation potentials of the two cations. Under CO, oxidation of azadithiolate compound 1 occurs according to an ECE process whereas an EC mechanism takes place for the propanedithiolate species 2. The dication species [1-CO]2+ resulting from the two-electron oxidation of 1 has been spectroscopically and structurally characterized. The molecular details underlying the reactivity of oxidized species have been explored by DFT calculations. The differences in the behaviors of 1 and 2 are mainly due to the presence, or not, of favored interactions between the dithiolate bridge and the diiron site depending on the redox states, FeIFeII or FeIIFeII, of the complexes.

International audience; FeFe hydrogenases catalyze H-2 oxidation and formation at an inorganic active site (the "H-cluster"), which consists of a [Fe-2(CO)(3)(CN)(2)(dithiomethylamine)] subcluster covalently attached to a Fe4S4 subcluster. This active site is photosensitive: visible light has been shown to induce the release of exogenous CO (a reversible inhibitor of the enzyme), shuffle the intrinsic CO ligands, and even destroy the H-cluster. These reactions must be understood because they may negatively impact the Use of hydrogenase for the photoproduction of H-2. Here, we explore in great detail the reactivity of the excited states of the H-duster under catalytic conditions by examining, both experimentally and using TDDFT calculations, the simplest photochemical reaction: the binding and release of exogenous CO. A simple dyad model can be used to predict which excitations are active. This could be used for probing other, aspects of the photoreactivity of the H-cluster.

The [FeFe] hydrogenase is a highly sophisticated enzyme for the synthesis of hydrogen via a biological route. The rotated state of the H-cluster in the [FeIFeI] form was found to be an indispensable criteria for an effective catalysis. Mimicking the specific rotated geometry of the [FeFe] hydrogenase active site is highly challenging as no protein stabilization is present in model compounds. In order to simulate the sterically demanding environment of the nature's active site, the sterically crowded meso-bis(benzylthio)diphenylsilane (2) was utilized as dithiolate linker in an [2Fe2S] model complex. The reaction of the obtained hexacarbonyl complex 3 with 1,2-bis(dimethylphosphino)ethane (dmpe) results three different products depending on the amount of dmpe used in this reaction: [{Fe2(CO)5{μ-(SCHPh)2SiPh2}}2(μ-dmpe)] (4), [Fe2(CO)5(κ2-dmpe){μ-(SCHPh)2SiPh2}] (5) and [Fe2(CO)5(μ-dmpe){μ-(SCHPh)2SiPh2}] (6). Interestingly, the molecular structure of compound 5 shows a [FeFe] subsite comprising a semi-rotated conformation, which was fully characterized as well as the other isomers 4 and 6 by elemental analysis, IR and NMR spectroscopy, X-ray diffraction analysis (XRD) and DFT calculations. The herein reported model complex is the first example so far reported for [FeIFeI] hydrogenase model complex showing a semi-rotated geometry without the need of stabilization via agostic interactions (Fe⋯H-C). This journal is

The one-electron oxidation of the diiron complex [Fe(2)(CO)(4)(κ(2)-dppe)(μ-pdt)] (1) (dppe = Ph(2)PCH(2)CH(2)PPh(2); pdt = S(CH(2))(3)S) has been investigated in the absence and in the presence of P(OMe)(3), by both electrochemical and theoretical methods, to shed light on the mechanism and the location of the oxidatively induced structure change. While cyclic voltammetric experiments did not allow to discriminate between a two-step (EC) and a concerted, quasi-reversible (QR) process, density functional theory (DFT) calculations favor the first option. When P(OMe)(3) is present, the one-electron oxidation produces singly and doubly substituted cations, [Fe(2)(CO)(4-n){P(OMe)(3)}(n)(κ(2)-dppe)(μ-pdt)](+) (n = 1: 2(+); n = 2: 3(+)) following mechanisms that were investigated in detail by DFT. Although the most stable isomer of 1(+) and 2(+) (and 3(+)) show a rotated Fe(dppe) center, binding of P(OMe)(3) occurs at the neighboring iron center of both 1(+) and 2(+). The neutral compound 3 was obtained by controlled-potential reduction of the corresponding cation, while 2 was quantitatively produced by reaction of 3 with CO. The CO dependent conversion of 3 into 2 as well as the 2(+) ↔ 3(+) interconversion were examined by DFT.

In conventional hydrogen fuel cells the rapid interconversion of protons and electrons to hydrogen requires catalysis by expensive metals, usually platinum. In the living world enzymes catalyse this same reaction at extraordinary rates using abundant metals. Tard et al. now report the chemical synthesis of the iron–sulphur structure at the heart of the hydrogenase protein. The resulting iron–sulphur framework functions as an electrocatalyst for proton reduction, a potentially important step towards new materials to replace platinum in the anodes of fuel cells. The metal-sulphur active sites of hydrogenases catalyse hydrogen evolution or uptake at rapid rates. Understanding the structure and function of these active sites—through mechanistic studies of hydrogenases1,2,3,4, synthetic assemblies5,6,7,8,9,10,11,12 and in silico models13,14,15—will help guide the design of new materials for hydrogen production or uptake16. Here we report the assembly of the iron-sulphur framework of the active site of iron-only hydrogenase (the H-cluster), and show that it functions as an electrocatalyst for proton reduction. Through linking of a di-iron subsite to a {4Fe4S} cluster, we achieve the first synthesis of a metallosulphur cluster core involved in small-molecule catalysis. In addition to advancing our understanding of the natural biological system, the availability of an active, free-standing analogue of the H-cluster may enable us to develop useful electrocatalytic materials for application in, for example, reversible hydrogen fuel cells. (Platinum is currently the preferred electrocatalyst for such applications, but is expensive, limited in availability and, in the long term, unsustainable17.)