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A large number of catalysts for hydrogen evolution reaction (HER) that are related (structurally and/or functionally) to the active site of [FeFe]-hydrogenases (H-cluster) has been proposed in the past few decades. Very recently, a novel recipe for HER catalysts has been added to the list. This consists in the combination of a diiron core, a ligand that can act as an intramolecular weak base to support protonation (both features that are common to the H-cluster), and a peculiar carbyne ligand that bridges the two metal centers. For instance, complex [Fe2{μ-CNMe2}(μ-CO)(CO)(CN)Cp2] (4) proved to be catalytically active towards HER. The novelty related to this family of biomimicry is not only structural, but also mechanistic. Indeed, an unprecedented (in the context of hydrogenase mimicry) ligand-based mechanism of HER, which avoids protonation at metal, has been dissected by DFT. In order to increase catalytic activity of this “non dithiolate based” system, a systematic search of ligand modification is therefore desirable. In this regard, herein we present results of replacing CN− (the proton shuttle in 4) by other ligands such as CNCH2Ph (5), PPh3 (6) or azide (7). The redox behavior of these compounds has been tested, in the absence and presence of a proton source. CV experiments combined with DFT calculations have revealed and rationalized critical points entailed by modifications of stereo-electronic features of 4. However, understanding the key factors underlying the absence of catalytic activity of these new compounds can be beneficial for the design of future improved biomimicry related to HER. As an example, properly tailored hydrocarbyl complexes replacing dithiolates at the Fe2 core might allow recovering the fruitful electronic properties of cyanides, when coordinated to metal centers.

Density functional theory (DFT) calculations on Fe(2)S(2)(CO)(6−2n)(PMe(3))(2n) for n = 0, 1, and 2 reveal that the most electron-rich derivatives (n = 2) exist as diferrous disulfides lacking an S–S bond. The thermal interconversion of the Fe(II)(2)(S)(2) and Fe(I)(2)(S(2)) valence isomers is symmetry-forbidden. Related electron-rich diiron complexes [Fe(2)S(2)(CN)(2)(CO)(4)](2−) of an uncertain structure are implicated in the biosynthesis of [FeFe]-hydrogenases. Several efforts to synthesize electron-rich derivatives of Fe(2)(μ-S(2))(CO)(6) (1) are described. First, salts of iron persulfido cyanides [Fe(2)(μ-S(2))(CO)(5)(CN)](−) and [Fe(2)(μ-S(2))(CN)(CO)(4)(PPh(3))](−) were prepared by the reactions of NaN(tms)(2) with 1 and Fe(2)(μ-S(2))(CO)(5)(PPh(3)), respectively. Alternative approaches to electron-rich diiron disulfides targeted Fe(2)(μ-S(2))(CO)(4)(diphosphine). Whereas the preparation of Fe(2)(μ-S(2))(CO)(4)(dppbz) was straightforward, that of Fe(2)(μ-S(2))(CO)(4)(dppv) required an indirect route involving the oxidation of Fe(2)(μ-SH)(2)(CO)(4)(dppv) (dppbz = C(6)H(4)-1,2-(PPh(2))(2), dppv = cis-C(2)H(2)(PPh(2))(2)). DFT calculations indicate that the oxidation of Fe(2)(μ-SH)(2)(CO)(4)(dppv) produces singlet diferrous disulfide Fe(2)(μ-S)(2)(CO)(4)(dppv), which is sufficiently long-lived as to be trapped by ethylene. The reaction of 1 and dppv mainly afforded Fe(2)(μ-SCH=CHPPh(2))(μ-SPPh(2))(CO)(5), implicating a S-centered reaction.

International audience; A series of triiron clusters [Fe3(CO)5(2-dppe)(µ-pdtR2)(µ-pdt)] featuring different combination of dithiolate bridges have been prepared by using dinuclear and mononuclear precursors, [Fe2(CO)6(µ-xdt)] (xdt = pdt, pdtEt2, pdtBn2, adtBn) and [Fe(CO)2(2-dppe)(2-pdt)]. Novel triiron compounds, featuring sterically crowded dithiolate bridges (pdtEt2, pdtBn2), were spectroscopically and structurally characterized. Their protonation and redox behaviours are discussed. The effect of the dithiolate bridges on the electrochemical properties (oxidation and reduction) of the complexes [Fe3(CO)5(2-dppe)(µ-xdt)(µ-pdt)] has been examined by cyclic voltammetry and rationalized by DFT calculations. Studies of the protonation of these species and their proton reduction behaviour were considered.

It was recently discovered that some redox proteins can thermodynamically and spatially split two incoming electrons towards different pathways, resulting in the one-electron reduction of two different substrates, featuring reduction potential respectively higher and lower than the parent reductant. This energy conversion process, referred to as electron bifurcation, is relevant not only from a biochemical perspective, but also for the ground-breaking applications that electron-bifurcating molecular devices could have in the field of energy conversion. Natural electron-bifurcating systems contain a two-electron redox centre featuring potential inversion (PI), i. e. with second reduction easier than the first. With the aim of revealing key factors to tailor the span between first and second redox potentials, we performed a systematic density functional study of a 26-molecule set of models with the general formula Fe2 (μ-PR2 )2 (L)6 . It turned out that specific features such as i) a Fe-Fe antibonding character of the LUMO, ii) presence of electron-donor groups and iii) low steric congestion in the Fe's coordination sphere, are key ingredients for PI. In particular, the synergic effects of i)-iii) can lead to a span between first and second redox potentials larger than 700 mV. More generally, the "molecular recipes" herein described are expected to inspire the synthesis of Fe2 P2 systems with tailored PI, of primary relevance to the design of electron-bifurcating molecular devices.

International audience; The synthesis, characterization, and protonation of [Fe2(CO)6{(μ-SCH2)2(Et)P═O}] (1) using the moderately strong acid CF3CO2H (pKaMeCN = 12.7) are reported. Digital simulations of the cyclic voltammetry of 1 in the presence of CF3CO2H and DFT calculations have allowed us to obtain a detailed mechanistic picture of the processes underlying the catalytic hydrogen evolution reaction (HER) that 1 can mediate. Moreover, DFT has shed light on the role of the P═O functionality in the whole catalytic cycle of proton reduction. The reductive behavior of 1 features a double electron transfer with potential inversion, which is associated with deep structural rearrangement of the catalyst. The double reduction appears also functional to the intramolecular proton transfer from the P═O group to the diiron core, a crucial process for the H+/H– heterocoupling yielding H2. The key intermediate for the H2 formation and release is predicted to be a 3H+/3e– species, in which P═O is perfectly poised to shuttle protons from solution to the Fe–H–Fe moiety. Therefore, the R-P═O bridgehead installed in a dithiolato linker of a diiron core proves a valid and versatile alternative to the natural nitrogen-based Fe2 strap.

The potentials of redox systems involving nitrogen, oxygen, and metal ions of the first-row transition series have been computed according to the general approach of the grand canonical ensemble, which leads to the equilibrium value of the reduction potential via a (complete) sampling of configuration space at a given temperature. The approach is a single configuration approach in the sense that identical molecular structures are sampled for both the oxidized and reduced species considered in water solution. In this study, the solute and a cluster of 11-12 water molecules are treated explicitly at the same level of theory and embedded in a continuum solvent. The molecular energies are computed in the framework of the density functional theory. Our approach is basically different from the approach based on the ThermoDynamic Cycle involving gas-phase calculations of the electron affinity of the oxidized species, corrected by the differential hydration energy (obtained from continuum solvent models only) between oxidized and reduced forms. The calculated redox potentials are in agreement with the available experimental data much closer than other results so far presented in the literature. Our results are very satisfactory also in the case of the 3+/2+ redox states of the first-row transition metals, i.e., systems with a high positive charge for which enhanced effects of the solvent are expected.

FeIFeI Fe2(S2C3H6)(CO)6(µ-CO) (1a–CO) and its FeIFeII cationic species (2a+–CO) are the simplest model of the CO-inhibited [FeFe] hydrogenase active site, which is known to undergo CO photolysis within a temperature-dependent process whose products and mechanism are still a matter of debate. Using density functional theory (DFT) and time-dependent density functional theory (TDDFT) computations, the ground state and low-lying excited-state potential energy surfaces (PESs) of 1a–CO and 2a+–CO have been explored aimed at elucidating the dynamics of the CO photolysis yielding Fe2(S2C3H6)(CO)6 (1a) and [Fe2(S2C3H6)(CO)6]+ (2a+), two simple models of the catalytic site of the enzyme. Two main results came out from these investigations. First, a–CO and 2a+–CO are both bound with respect to any CO dissociation with the lowest free energy barriers around 10 kcal mol−1, suggesting that at least 2a+–CO may be synthesized. Second, focusing on the cationic form, we found at least two clear excited-state channels along the PESs of 2a+–CO that are unbound with respect to equatorial CO dissociation.

In view of the depletion of fossil fuel reserves and climatic effects of greenhouse gas emissions, Ni,Fe-containing carbon monoxide dehydrogenase (Ni-CODH) enzymes have attracted increasing interest in recent years for their capability to selectively catalyze the reversible reduction of CO2 to CO (CO2 + 2H+ + 2e- ⇌ CO + H2O). The possibility of converting the greenhouse gas CO2 into useful materials that can be used as synthetic building blocks or, remarkably, as carbon fuels makes Ni-CODH a very promising target for reverse-engineering studies. In this context, in order to provide insights into the chemical principles underlying the biological catalysis of CO2 activation and reduction, quantum mechanics calculations have been carried out in the framework of density functional theory (DFT) on different-sized models of the Ni-CODH active site. With the aim of uncovering which stereoelectronic properties of the active site (known as the C-cluster) are crucial for the efficient binding and release of CO2, different coordination modes of CO2 to different forms and redox states of the C-cluster have been investigated. The results obtained from this study highlight the key role of the protein environment in tuning the reactivity and the geometry of the C-cluster. In particular, the protonation state of His93 is found to be crucial for promoting the binding or the dissociation of CO2. The oxidation state of the C-cluster is also shown to be critical. CO2 binds to Cred2 according to a dissociative mechanism (i.e., CO2 binds to the C-cluster after the release of possible ligands from Feu) when His93 is doubly protonated. CO2 can also bind noncatalytically to Cred1 according to an associative mechanism (i.e., CO2 binding is preceded by the binding of H2O to Feu). Conversely, CO2 dissociates when His93 is singly protonated and the C-cluster is oxidized at least to the Cint redox state.

International audience; The so-called FeFe hydrogenases catalyze H2 production and oxidation at a dinuclear inorganic active site. Some of them can be natively purified in an overoxidized, O2-resistant “Hinact” state, recently identified by Rodríguez-Maciá et al. as the product of the reaction of the enzyme with sulfide [Rodríguez-Maciá, P.; J. Am. Chem. Soc. 2018, 140, 9346]. We used a combination of direct electrochemistry experiments with the FeFe hydrogenase from Chlamydomonas reinhardtii, site-directed mutagenesis, molecular dynamics and density functional theory (DFT) calculations to describe the mechanism of inhibition: the diffusion of the inhibitor in the enzyme and its subsequent reaction at the active site H-cluster. We conclude that hydrogen sulfide (H2S) inhibits the enzyme noncompetitively, in a first step by replacing a conserved water molecule that is involved in proton transfer, and then binding to the active site as a hydrosulfide ligand (HS–). DFT calculations with the PBE0-D3 functional successfully describe the redox state of the cubane fragment of the H-cluster in the resulting “Htrans” state. Our experimental and theoretical results are consistent with the reactivation involving the reduction of the H-cluster in the Htrans state, followed by the potentiometric or catalytic reoxidation of the enzyme. This mechanism reconciles all experimental observations, and we suggest that it is common to all FeFe hydrogenases. In addition, we observe that the hydrogenases from Megasphaera elsdenii, Clostridium acetobutylicum (CaI), and Clostridium pasteurianum (CpI) are also inhibited by sulfide, but with very slow kinetics. Although sulfide inhibition is fully reversible, we observed an irreversible inactivation by polysulfide contaminants, which should be avoided if the hydrogenase is exposed to sulfide to prepare samples that are protected from air, e.g., for transport or storage.*

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

Catalytic H2 oxidation has been dissected by means of DFT into the key steps common to the Fe2 unit of both the [FeFe]-hydrogenase cofactor and selected biomimics. The aim was to elucidate the molecular details underlying the very different performances of the two systems. We found that the better enzyme performance is based on a single iron atom that is maintained electron-poor, favoring H2 binding, although embedded within a highly electron-rich cofactor, ensuring a facile oxidation of the Fe2 -H2 adduct. This is due to 1) CN- coordinating to both iron atoms, due to their amphipathic Lewis acid/base properties, and 2) the 4Fe4S subunit further withdrawing electrons from the Fe2 core. Preserving a moderate electron deficiency at a single iron also helps the cofactor preserve hydride affinity, which favors H2 cleavage. Such valuable characteristics allow the biocatalyst to turnover close to equilibrium conditions. All previous biomimicry has shown, in contrast, the impossibility to properly balance the two apparently contrasting aforementioned requisites, although evident progress has been made by the H2 -ase community. Disclosure of the differences identified could inspire the design of novel biomimics, for instance, reconsidering the use of CN- in the catalyst architecture. Indeed, in the presence of bases normally employed in oxidative catalysis, undesired stable protonation at coordinated CN- , which affects the opposite process (proton reduction), could be overcome.

Fundamental reactions of imino-phosphine ligands were elucidated through studies on Ph2PC6H4CH=NC6H4-4-Cl (PCHNArCl) complexes of iron(0), iron(I), and iron(II). The reaction of PCHNArCl with Fe(bda)(CO)3 gives Fe(PCHNArCl)(CO)3 (1), featuring an η2-imine. DNMR studies, its optical properties, and DFT calculations suggest that 1 racemizes on the NMR time scale via an achiral N-bonded imine intermediate. The N-imine isomer is more stable in Fe(PCHNArOMe)(CO)3 (1OMe), which crystallized despite being the minor isomer in solution. Protonation of 1 by HBF4·Et2O gave the iminium complex [1H]BF4. The related diphosphine complex Fe(PCHNArCl)(PMe3)(CO)2 (2), which features an η2-imine, was shown to also undergo N protonation. Oxidation of 1 and 2 with FcBF4 gave the Fe(I) compounds [1]BF4 and [2]BF4. The oxidation-induced change in hapticity of the imine from η2 in [1]0 to κ1 in [1]+ was verified crystallographically. Substitution of a CO ligand in 1 with PCHNArCl gave Fe[P2(NArCl)2](CO)2 (3), which contains the tetradentate diamidodiphosphine ligand. This C-C coupling is reversed by chemical oxidation of 3 with FcOTf. The oxidized product of [Fe(PCHNArCl)2(CO)2]2+ ([4]2+) was prepared independently by the reaction of [1]+, PCHNArCl, and Fc+. The C-C scission is proposed to proceed concomitantly with the reduction of Fe(II) via an intermediate related to [2]+.

The complexes Fe2(pdt)(CNR)6 (pdt(2-) = CH2(CH2S(-))2) were prepared by thermal substitution of the hexacarbonyl complex with the isocyanides RNC for R = C6H4-4-OMe (1), C6H4-4-Cl (2), Me (3). These complexes represent electron-rich analogues of the parent Fe2(pdt)(CO)6. Unlike most substituted derivatives of Fe2(pdt)(CO)6, these isocyanide complexes are sterically unencumbered and have the same idealized symmetry as the parent hexacarbonyl derivatives. Like the hexacarbonyls, the stereodynamics of 1-3 involve both turnstile rotation of the Fe(CNR)3 as well as the inversion of the chair conformation of the pdt ligand. Structural studies indicate that the basal isocyanide has nonlinear CNC bonds and short Fe-C distances, indicating that they engage in stronger Fe-C π-backbonding than the apical ligands. Cyclic voltammetry reveals that these new complexes are far more reducing than the hexacarbonyls, although the redox behavior is complex. Estimated reduction potentials are E1/2 ≈ -0.6 ([2](+/0)), -0.7 ([1](+/0)), and -1.25 ([3](+/0)). According to DFT calculations, the rotated isomer of 3 is only 2.2 kcal/mol higher in energy than the crystallographically observed unrotated structure. The effects of rotated versus unrotated structure and of solvent coordination (THF, MeCN) on redox potentials were assessed computationally. These factors shift the redox couple by as much as 0.25 V, usually less. Compounds 1 and 2 protonate with strong acids to give the expected μ-hydrides [H1](+) and [H2](+). In contrast, 3 protonates with [HNEt3]BAr(F)4 (pKa(MeCN) = 18.7) to give the aminocarbyne [Fe2(pdt)(CNMe)5(μ-CN(H)Me)](+) ([3H](+)). According to NMR measurements and DFT calculations, this species adopts an unsymmetrical, rotated structure. DFT calculations further indicate that the previously described carbyne complex [Fe2(SMe)2(CO)3(PMe3)2(CCF3)](+) also adopts a rotated structure with a bridging carbyne ligand. Complex [3H](+) reversibly adds MeNC to give [Fe2(pdt)(CNR)6(μ-CN(H)Me)](+) ([3H(CNMe)](+)). Near room temperature, [3H](+) isomerizes to the hydride [(μ-H)Fe2(pdt)(CNMe)6](+) ([H3](+)) via a first-order pathway.

The effects of simple thiolates vs chelating dithiolates on M-M bonding, redox potentials, and synthetic outcomes have been probed experimentally and computationally. Nickelocene (Cp2Ni) has long been known to react with simple thiols to give diamagnetic Cp2Ni2(SR)2 with planar Ni2S2 cores and long Ni- - -Ni distances. Ethane- and propanedithiol (edtH2 and pdtH2, respectively) instead give di-, tri-, and pentanickel complexes, with nonplanar Ni2S2 cores. The 36e Cp2Ni2(pdt) (1pdt) adopts a symmetrical butterfly Ni2S2 structure. Variable-temperature NMR spectra indicate that 1pdt possesses a thermally accessible triplet state (ΔG = 2.65(5) kcal/mol) in equilibrium with a diamagnetic ground state. DFT calculations indicate that the singlet-triplet gap is highly sensitive to the nonplanarity of the Ni2S2 core. The calculations further reveal that only the high-spin form of 1pdt features Ni-Ni bonding, which is unprecedented. Cp2Ni3(pdt)2 (2pdt), which derives from 1pdt, crystallized as cis and trans isomers, both with a central Ni(pdt)22- unit that is S,S-chelated to two CpNi+ centers. Reaction of Cp2Ni with 1,2-ethanedithiol (H2edt) and 1,2-benzenedithiol (bdtH2) exclusively gave the trinickel species 2edt and 2bdt, which are structurally analogous to cis-2pdt. Solutions of 2edt are unstable, depositing crystals of Cp2Ni5(edt)4 (3edt). Cyclic voltammetric studies show that the Ni2 species oxidize readily to give the mixed-valence cations [1pdt]+ and [1edt]+. Crystallographic and EPR analyses indicate that these cations are delocalized mixed-valence Ni(II)-Ni(III) species. Oxidation of Cp2Ni2(SEt)2, which features a planar Ni2S2 core, afforded a mixed-valence cation, showing the pyramidal Ni2S2 core observed in [1pdt]0/+ and [1edt]0/+. Although not obtained from the Cp2Ni/H2edt reaction, the neutral complex 1edt was obtained by reduction of [1edt]+. Variable-temperature NMR measurements and DFT calculations indicate that the triplet is further stabilized in this highly pyramidalized species.

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.

Alzheimer's disease (AD) involves a number of factors including an anomalous interaction of copper with the amyloid peptide (Aβ), inducing oxidative stress with radical oxygen species (ROS) production through a three-step cycle in which O2 is gradually reduced to superoxide, oxygen peroxide and finally OH radicals. The purpose of this work has been to investigate the reactivity of 14 different Cu(ii)-Aβ coordination models with the aim of identifying on an energy basis (Density Functional Theory (DFT) and classical Molecular Dynamics (MD)) the redox competent form(s). Accordingly, we have specifically focused on the first three steps of the cycle, i.e. ascorbate binding to Cu(ii), Cu(ii) → Cu(i) reduction and O2 reduction to O2-. Compared to the recent literature, our results broaden the set of possible redox competent metallopeptide forms responsible for ROS production. Indeed, in addition to the three-coordinated species containing one His ligand, a N-terminal amine group and the carboxylate side chain of the Asp1 residue of Aβ already proposed, we found two other Cu-Aβ coordination modes involving two histidines.

The photochemistry of the [FeFe] hydrogenases model [Fe-2(edt)(CO)(4)(PMe3)(2)] (edt=1-2 ethane-dithiolate, (1) is investigated at Time-Dependent Density Functional Theory (TDDFT) level, focusing on the effect of the phosphine ligands on the early stages of the photodynamic of the system compared to that of the all CO model Fe-2(pdt)(CO)(6) (2) (pdt=1-3 propanedithiolate). We observed a role of the FeS charge transfer for the lower energy singlet transitions, unveiling a photoisomerization pathway between the lowest energy forms while the higher energy excitations are likely involved in the CO dissociation pathways. TDDFT shows that the average Fe-CO bond elongation in 1 is shorter than that observed in 2, providing the electronic structure rationale on the observation that diiron dithiolates are more photo-stable with respect to the CO photolysis than the all CO model. This is relevant for catalyst photo-stability and is an advantageous and thus desirable feature for practical applications of photo-hydrogen evolution

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.

DFT has been used to investigate viable mechanisms of the hydrogen evolution reaction (HER) electrocatalyzed by [Fe2(CN){μ-CN(Me)2}(μ-CO)(CO)(Cp)2] (1) in AcOH. Molecular details underlying the proposed ECEC electrochemical sequence have been studied, and the key functionalities of CN- and amino-carbyne ligands have been elucidated. After the first reduction, CN- works as a relay for the first proton from AcOH to the carbyne, with this ligand serving as the main electron acceptor for both reduction steps. After the second reduction, a second protonation occurs at CN- that forms a Fe(CNH) moiety: i.e., the acidic source for the H2 generation. The hydride (formally 2e/H+), necessary to the heterocoupling with H+ is thus provided by the μ-CN(Me)2 ligand and not by Fe centers, as occurs in typical L6Fe2S2 derivatives modeling the hydrogenase active site. It is remarkable, in this regard, that CN- plays a role more subtle than that previously expected (increasing electron density at Fe atoms). In addition, the role of AcOH in shuttling protons from CN- to CN(Me)2 is highlighted. The incompetence for the HER of the related species [Fe2{μ-CN(Me)2}(μ-CO)(CO)2(Cp)2]+ (2+) has been investigated and attributed to the loss of proton responsiveness caused by CN- replacement with CO. In the context of hydrogenase mimicry, an implication of this study is that the dithiolate strap, normally present in all synthetic models, can be removed from the Fe2 core without loss of HER, but the redox and acid-base processes underlying turnover switch from a metal-based to a ligand-based chemistry. The versatile nature of the carbyne, once incorporated in the Fe2 scaffold, could be exploited to develop more active and robust catalysts for the HER.