Electrocatalytic Proton Reduction by a Cobalt Complex Containing a Proton-Responsive Bis(alkylimdazole)methane Ligand: Involvement of a C-H Bond in H2 Formation.

Homogeneous electrocatalytic proton reduction is reported using cobalt complex [1](BF4 )2 . This complex comprises two bis(1-methyl-4,5-diphenyl-1H-imidazol-2-yl)methane (HBMIM Ph 2 ) ligands that contain an acidic methylene moiety in their backbone. Upon reduction of [1](BF4 )2 by either electrochemical or chemical means, one of its HBMIM Ph 2 ligands undergoes deprotonation under the formation of dihydrogen. Addition of a mild proton source (acetic acid) to deprotonated complex [2](BF4 ) regenerates protonated complex [1](BF4 )2 . In presence of acetic acid in acetonitrile solvent [1](BF4 )2 shows electrocatalytic proton reduction with a kobs of ≈200 s-1 at an overpotential of 590 mV. Mechanistic investigations supported by DFT (BP86) suggest that dihydrogen formation takes place in an intramolecular fashion through the participation of a methylene C-H bond of the HBMIM Ph 2 ligand and a CoII -H bond through formal heterolytic splitting of the latter. These findings are of interest to the development of responsive ligands for molecular (base)metal (electro)catalysis.

Reversible Redox Switching of Chromophoric Phenylmethylenepyrans by Carbon-Carbon Bond Making/Breaking.

Electrochromic organic systems that can undergo substantial variation of their optical properties upon electron stimulus are of high interest for the development of functional materials. In particular, devices based on radical dimerization are appropriate because of the effectiveness and speed of carbon-carbon bond making/breaking. Phenylmethylenepyrans are organic chromophores which are well suited for such purposes since their oxidation leads to the reversible formation of bispyrylium species by radical dimerization. In this paper, we show that the redox and spectroscopic properties of phenylmethylenepyrans can be modulated by adequate variation of the substituting group on the para position of the phenyl moiety, as supported by DFT calculations. This redox switching is reversible over several cycles and is accompanied by a significant modification of the UV-vis spectrum of the chromophore, as shown by time-resolved spectroelectrochemistry in thin-layer conditions.

Electrochemistry of Simple Organometallic Models of Iron-Iron Hydrogenases in Organic Solvent and Water.

Synthetic models of the active site of iron-iron hydrogenases are currently the subjects of numerous studies aimed at developing H2-production catalysts based on cheap and abundant materials. In this context, the present report offers an electrochemist's view of the catalysis of proton reduction by simple binuclear iron(I) thiolate complexes. Although these complexes probably do not follow a biocatalytic pathway, we analyze and discuss the interplay between the reduction potential and basicity and how these antagonist properties impact the mechanisms of proton-coupled electron transfer to the metal centers. This question is central to any consideration of the activity at the molecular level of hydrogenases and related enzymes. In a second part, special attention is paid to iron thiolate complexes holding rigid and unsaturated bridging ligands. The complexes that enjoy mild reduction potentials and stabilized reduced forms are promising iron-based catalysts for the photodriven evolution of H2 in organic solvents and, more importantly, in water.

A molecular material based on electropolymerized cobalt macrocycles for electrocatalytic hydrogen evolution.

An electrocatalytic material for the H2 evolution reaction (HER) in acidic aqueous solution has been prepared by electropolymerization of Co(ii) dibenzotetraaza[14] annulene (CoTAA). Chemical analysis by X-ray photoelectron spectroscopy (XPS) confirms that the structural integrity of the [Co(II)-N4] motif is preserved in the poly-CoTAA film. In acetate buffer solution at pH 4.6, an overpotential η = -0.57 V is required to attain a catalytic current density -ik = 1 mA cmgeom(-2). The faradaic efficiency of poly-CoTAA for the HER is 90% over a period of one hour of electrolysis, but there is a decrease of the apparent concentration of Co sites after prolonged H2 production, which we ascribe to partial demetallation of the poly-CoTAA film at negative potentials.

Electrochemistry of cytochrome c immobilized on cardiolipin-modified electrodes: a probe for protein-lipid interactions.

Electrochemistry of cytochrome c (cyt c) immobilized on a cardiolipin (CL)/phosphatidylcholine (PC) film supported on a glassy carbon electrode was investigated using variable-frequency AC voltammetry. At low ionic strength, we observed two redox-active subpopulations characterized by distinct values of potential (E1/2) and electron transfer rate constant (k(ET)). At high ionic strength, only one subpopulation was detected, consistent with the existence of very stable cyt c-CL adducts, most probably formed by hydrophobic interactions between the protein and the fatty acid (FA) chains carried by CL. This subpopulation exhibits a comparatively high k(ET) value (> 300 s(-1)) apparently changing with the structure of the FA chains of CL, i.e. 18:2(n - 6) or 14:0. Our study suggests that electrochemistry can be a useful technique for probing protein-lipid interactions, and more particularly the role played by the specific structure of the FA chains of CL on cyt c binding.

Mechanistic insights into the catalysis of electrochemical proton reduction by a diiron azadithiolate complex.

Cyclic voltammetry experiments and DFT calculations allowed us to establish a complete mechanism of the catalysis of electrochemical proton reduction by [Fe2(µ-SCH2N(H)CH2S)(CO)6] (Fe-adt) in acetonitrile. The proposed mechanism is fully consistent with the observed dependence of the voltammetric responses on the strength of the acid used as a proton source. Addition of moderately strong acids, such as CCl3CO2H (pK(a) = 10.7) or HOTs·H2O (pK(a) = 8.6), triggers the occurrence of new reduction events at potentials less negative than the reduction of Fe-adt, therefore ascribed to reduction of the protonated forms of the complex. Reduction of the N-protonated form seems to favor a tautomerization reaction leading to a Fe-H intermediate. On the other hand, addition of weak acids, such as ClCH2CO2H (pK(a) = 15.3), leads to direct protonation on the diiron site subsequently to reduction of the catalyst. A better understanding of the mechanism of proton reduction by the biologically relevant Fe-adt derivative could impact the design of improved catalysts inspired by FeFe-hydrogenase.

Tetranuclear iron complexes bearing benzenetetrathiolate bridges as four-electron transformation templates and their electrocatalytic properties for proton reduction.

Two tetranuclear iron-sulfur complexes, (µ,µ-pbtt)[Fe(2)(CO)(6)](2) (pbtt = benzene-1,2,4,5-tetrathiolato, 3) and (µ,µ-obtt)[Fe(2)(CO)(6)](2) (obtt = benzene-1,2,3,4-tetrathiolato, 4), were prepared from reaction of Fe(3)(CO)(12) and the corresponding tetramercaptobenzene in THF, respectively. Complexes 5 and 6, (µ,µ-pbtt)[Fe(2)(CO)(5)L(1)][Fe(2)(CO)(5)L(2)] (L(1) = CO, L(2) = PPyr(3) (Pyr = N-pyrrolyl), 5; L(1) = L(2) = PPyr(3), 6) were obtained by controlling CO displacement of 3 with PPyr(3). Molecular structures of 3-6 were determined by spectroscopic and single-crystal X-ray analyses. All-CO Fe(4)S(4) complexes 3 and 4 each display four-electron reduction processes in consecutive chemically reversible two-electron reduction events with relatively narrow potential spans in the cyclic voltammograms. Phosphine-substituted Fe(4)S(4) complexes 5 and 6 exhibit two consecutive two-electron reduction events, which are not fully reversible. The electrocatalytic properties of 3 and 4 for proton reduction were studied using a series of carboxylic acids of increasing strength (CH(3)COOH, CH(2)ClCOOH, CHCl(2)COOH, CCl(3)COOH, and CF(3)COOH). The mechanisms for electrochemical proton reduction to hydrogen catalyzed by complex 3 as a function of acid strength are discussed.

Electrochemical reduction and protonation of a biomimetic diiron azadithiolate hexacarbonyl complex: Mechanistic insights.

The electrochemical reduction and protonation of [Fe2(adtH)(CO)6] (1, adtH = SCH2N(H)CH2S) and [Fe2(pdt)(CO)6] (2, pdt = SCH2CH2CH2S) in the presence of moderately strong acid in acetonitrile was investigated by cyclic voltammetry (CV), focusing on the catalysis of hydrogen evolution reaction (HER) by a {2e-,2H+} pathway. The turnover frequencies at zero overpotential (TOF0) of the N-protonated product 1(H)+ and 2 for the HER were estimated from simulations of the catalytic CV responses at low acid concentration using a simple ECEC mechanism (two electrochemical and chemical steps). This approach confirmed that 1(H)+ is clearly a better catalyst than 2, pointing to a possible role of the protonable and biologically relevant adtH ligand in the enhancement of the catalytic performances. Density functional theory (DFT) calculations further suggested that, owing to a strong structural rearrangement in the course of the catalytic cycle, the HER catalysis by 1(H)+ only involves the iron center adjacent to the amine group in adtH and not the two iron centers as in 2. Since terminal hydride species (FeFe-H) are known to more easily undergo protonolyse to H2 than their bridging hydride isomers (Fe-H-Fe), this may explain here the enhanced activity of 1(H)+ over 2 for the HER.

A binuclear iron-thiolate catalyst for electrochemical hydrogen production in aqueous micellar solution.

The substituted iron-thiolate complex [Fe(2)(µ-bdt)(CO)(4){P(OMe)(3)}(2)] (bdt=benzenedithiolate) is an active catalyst for electrochemical hydrogen production in aqueous sodium dodecyl sulfate solution, with a high apparent rate constant of 4×10(6) M(-1) s(-1). The half-peak potential for catalysis of proton reduction is less negative than -0.6 V versus the standard hydrogen electrode at pH 3. Voltammetric data are consistent with the rate of electrode reaction controlled by diffusion. A mechanism that begins with the rapid protonation of the iron-thiolate catalyst is proposed. The Faradaic efficiency in diluted HCl solutions is close to 100%, but the catalytic activity decayed after about twelve turnovers when electrolysis was carried out in the presence of acetic acid.

Multielectron-transfer templates via consecutive two-electron transformations: iron-sulfur complexes relevant to biological enzymes.

[FeFe] hydrogenase mimics: two polynuclear iron-sulfur complexes (1 and 2) were prepared and structurally characterized. They are potentially effective and stable multielectron-transfer relays for mediating four- and six-electron transformations via a cascade of reversible two-electron redox steps with relatively narrow potential spans.

Electrochemical hydrogen production in aqueous micellar solution by a diiron benzenedithiolate complex relevant to [FeFe] hydrogenases.

The diiron hydrogenase model Fe(2)(bdt)(CO)(6) (1, bdt = benzenedithiolate) was dispersed in aqueous micellar solution prepared from sodium dodecyl sulfate (SDS). Aqueous solution of 1 showed no sign of decomposition when left in contact with air over a period of several days. Current-potential responses recorded at a dropping mercury electrode over pH 7-3 were consistent with reduction of freely diffusing species. Catalysis of proton reduction was observed at pH < 6 with current densities exceeding 0.5 mA cm(-2) at an acid-to-catalyst ratio of 17. Bulk electrolysis at -0.66 V vs. SHE of solution of 1 at pH 3 confirmed the production of hydrogen with a Faradaic efficiency close to 100%. A mechanism involving initial reduction of 1 and subsequent proton-coupled electron transfer is proposed.

Oxidatively induced reactivity of [Fe2(CO)4(κ2-dppe)(µ-pdt)]: an electrochemical and theoretical study of the structure change and ligand binding processes.

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.

Influence of QD photosensitizers in the photocatalytic production of hydrogen with biomimetic [FeFe]-hydrogenase. Comparative performance of CdSe and CdTe.

Photocatalytic systems comprising a hydrogenase-type catalyst and CdX (X = S, Se, Te) chalcogenide quantum dot (QD) photosensitizers show extraordinary hydrogen production rates under visible light excitation. What remains unknown is the mechanism of energy conversion in these systems. Here, we have explored this question by comparing the performance of two QD sensitizers, CdSe and CdTe, in photocatalytic systems featuring aqueous suspensions of a [Fe2 (µ-1,2-benzenedithiolate) CO6] catalyst and an ascorbic acid sacrificial agent. Overall, the hydrogen production yield for CdSe-sensitized reactions QDs was found to be 13 times greater than that of CdTe counterparts. According to emission quenching experiments, an enhanced performance of CdSe sensitizers reflected a greater rate of electron transfer from the ascorbic acid (kAsc). The observed difference in the QD-ascorbic acid charge transfer rates between the two QD materials was consistent with respective driving forces for these systems.

Effect of electron-withdrawing dithiolate bridge on the electron-transfer steps in diiron molecules related to [2Fe](H) subsite of the [FeFe]-hydrogenases.

Two hexacarbonyl diiron compounds featuring dithiolate bridges with strong electron-withdrawing groups (CO(2)Me, tetrachloro-biphenyl) were synthesized and structurally characterized. Electrochemical study of these compounds demonstrates that such electron-withdrawing groups have a pronounced effect on both the reduction potentials and the electron transfer process. The reduced forms of these compounds catalyze the reduction of protons in dichloromethane. However, the tetrachloro-biphenyl derivative is the only one able to work in the potential range of its primary reduction process. A catalytic reaction scheme is proposed.

Investigation on the protonation of a trisubstituted [Fe(2)(CO)(3)(PPh(3))(kappa(2)-phen)(mu-pdt)] complex: rotated versus unrotated intermediate pathways.

The substitution of PPh(3) for a carbonyl group at the {Fe(CO)(3)} moiety in [Fe(2)(CO)(4)(kappa(2)-phen)(mu-pdt)] results in the formation of the trisubstituted complex [Fe(2)(CO)(3)(PPh(3))(kappa(2)-phen)(mu-pdt)] (2). Unlike its tetracarbonyl precursor, the protonation of 2 at low temperature does not afford any apparent transient terminal hydride species. Hydride formation for [Fe(2)(CO)(3)(L)(kappa(2)-phen)(mu-pdt)] (L = PPh(3), CO) species is also studied by density functional theory calculations, which show that activation barriers to give terminal and bridging hydrides can be remarkably close for this class of organometallic compounds.

Influence of a pendant amine in the second coordination sphere on proton transfer at a dissymmetrically disubstituted diiron system related to the [2Fe]H subsite of [FeFe]H2ase.

Studies of the protonation of [Fe2(CO)4(kappa2-PNP)(mu-pdt)] (1; PNP = (Ph2PCH2)2NCH3) by HBF4.Et2O showed that the nature of the reaction product depends on whether the reaction is conducted in acetone or in dichloromethane. In acetone, an N-protonated form, 2, is isolated. Tautomerization of 2 in CH2Cl2 gives rise to a mu-hydride species 3. Variable-temperature NMR experiments have been performed to clarify the processes involved.

First insights into the protonation of dissymetrically disubstituted di-iron azadithiolate models of the [FeFe]H2ases active site.

Dissymetrically disubstituted di-iron azadithiolate complexes [Fe2(CO)4(kappa 2-LL){mu-SCH2N(iPr)CH2S}] (LL = dppe, phen) protonate exclusively at the N atom of the bridge, like the hexacarbonyl precursor but in contrast to symmetrically disubstituted analogues; substitution of dppe for two CO groups noticeably increases the kinetics of the electrocatalytic proton reduction process.

New nitrosyl derivatives of diiron dithiolates related to the active site of the [FeFe]-hydrogenases.

Nitrosyl derivatives of diiron dithiolato carbonyls have been prepared starting from the precursor Fe(2)(S(2)C(n)H(2n))(dppv)(CO)(4) (dppv = cis-1,2-bis(diphenylphosphinoethylene). These studies expand the range of substituted diiron(I) dithiolato carbonyl complexes. From [Fe(2)(S(2)C(2)H(4))(CO)(3)(dppv)(NO)]BF(4) ([1(CO)(3)]BF(4)), the following compounds were prepared: [1(CO)(2)(PMe(3))]BF(4), [1(CO)(dppv)]BF(4), NEt(4)[1(CO)(CN)(2)], and 1(CO)(CN)(PMe(3)). Some of these substitution reactions occur via the addition of 2 equiv of the nucleophile followed by the dissociation of one nucleophile and decarbonylation. Such a double adduct was characterized crystallographically in the case of [Fe(2)(S(2)C(2)H(4))(CO)(3)(dppv)(NO)(PMe(3))(2)]BF(4). This result shows that the addition of two ligands causes scission of the Fe-Fe bond and one Fe-S bond. When cyanide is the nucleophile, nitrosyl migrates away from the Fe(dppv) site, yielding a Fe(CN)(2)(NO) derivative. Compounds [1(CO)(3)]BF(4), [1(CO)(2)(PMe(3))]BF(4), and [1(CO)(dppv)]BF(4) were also prepared by the addition of NO(+) to the di-, tri-, and tetrasubstituted precursors. In these cases, the NO(+) appears to form an initial 36e(-) adduct containing terminal Fe-NO, followed by decarbonylation. Several complexes were prepared by the addition of NO to the mixed-valence Fe(I)Fe(II) derivatives. The diiron nitrosyl complexes reduce at mild potentials and in certain cases form weak adducts with CO. IR and EPR spectra of 1(CO)(dppv), generated by low-temperature reduction of [1(CO)(dppv)]BF(4) with Co(C(5)Me(5))(2), indicates that the SOMO is located on the FeNO subunit.

Electronic and molecular structure relations in diiron compounds mimicking the [FeFe]-hydrogenase active site studied by X-ray spectroscopy and quantum chemistry.

Synthetic diiron compounds of the general formula Fe2(µ-S2R)(CO)n(L)6-n (R = alkyl or aromatic groups; L = CN- or phosphines) are versatile models for the active-site cofactor of hydrogen turnover in [FeFe]-hydrogenases. A series of 18 diiron compounds, containing mostly a dithiolate bridge and terminal ligands of increasing complexity, was characterized by X-ray absorption and emission spectroscopy in combination with density functional theory. Fe K-edge absorption and Kß main-line emission spectra revealed the varying geometry and the low-spin state of the Fe(i) centers. Good agreement between experimental and calculated core-to-valence-excitation absorption and radiative valence-to-core-decay emission spectra revealed correlations between spectroscopic and structural features and provided access to the electronic configuration. Four main effects on the diiron core were identified, which were preferentially related to variation either of the dithiolate or of the terminal ligands. Alteration of the dithiolate bridge affected mainly the Fe-Fe bond strength, while more potent donor substitution and ligand field asymmetrization changed the metal charge and valence level localization. In contrast, cyanide ligation altered all relevant properties and, in particular, the frontier molecular orbital energies of the diiron core. Mutual benchmarking of experimental and theoretical parameters provides guidelines to verify the electronic properties of related diiron compounds.