Normal vs. Inverted Ordering of Reduction Potentials in [FeFe]-Hydrogenases Biomimetics: Effect of the Dithiolate Bulk.

Three hexacarbonyl diiron dithiolate complexes [Fe2 (CO)6 (µ-(SCH2 )2 X)] with different substituted bridgeheads (X=CH2 , CEt2 , CBn2 (Bn=CH2 C6 H5 )), have been studied under the same experimental conditions by cyclic voltammetry in dichloromethane [NBu4 ][PF6 ] 0.2 M. DFT calculations were performed to rationalize the mechanism of reduction of these compounds. The three complexes undergo a two-electron transfer whose the mechanism depends on the bulkiness of the dithiolate bridge, which involves a different timing of the structural changes (Fe-S bond cleavage, inversion of conformation and CO bridging) vs redox steps. The introduction of a bulky group in the dithiolate linker has obviously an effect on normally ordered (as for propanedithiolate (pdt)) or inverted (pdtEt2 , pdtBn2 ) reduction potentials. Et→Bn replacement is not theoretically predicted to alter the geometry and energy of the most stable mono-reduced and bi-reduced forms but such a replacement alters the kinetics of the electron transfer vs the structural changes.

Insights into Triazolylidene Ligands Behaviour at a Di-Iron Site Related to [FeFe]-Hydrogenases.

The behaviour of triazolylidene ligands coordinated at a {Fe2(CO)5(µ-dithiolate)} core related to the active site of [FeFe]-hydrogenases have been considered to determine whether such carbenes may act as redox electron-reservoirs, with innocent or non-innocent properties. A novel complex featuring a mesoionic carbene (MIC) [Fe2(CO)5(Pmpt)(µ-pdt)] (1; Pmpt = 1-phenyl-3-methyl-4-phenyl-1,2,3-triazol-5-ylidene; pdt = propanedithiolate) was synthesized and characterized by IR, 1H, 13C{1H} NMR spectroscopies, elemental analyses, X-ray diffraction ,and cyclic voltammetry. Comparison with the spectroscopic characteristics of its analogue [Fe2(CO)5(Pmbt)(µ-pdt)] (2; Pmbt = 1-phenyl-3-methyl-4-butyl-1,2,3-triazol-5-ylidene) showed the effect of the replacement of a n-butyl by a phenyl group in the 1,2,3-triazole heterocycle. A DFT study was performed to rationalize the electronic behaviour of 1, 2 upon the transfer of two electrons and showed that such carbenes do not behave as redox ligands. With highly perfluorinated carbenes, electronic communication between the di-iron site and the triazole cycle is still limited, suggesting low redox properties of MIC ligands used in this study. Finally, although the catalytic performances of 2 towards proton reduction are weak, the protonation process after a two-electron reduction of 2 was examined by DFT and revealed that the protonation process is favoured by S-protonation but the stabilized diprotonated intermediate featuring a {Fe-H⋯H-S} interaction does not facilitate the release of H2 and may explain low efficiency towards HER (Hydrogen Evolution Reaction).

Insights into the Two-Electron Reductive Process of [FeFe]H2 ase Biomimetics: Cyclic Voltammetry and DFT Investigation on Chelate Control of Redox Properties of [Fe2 (CO)4 (κ2 -Chelate)(µ-Dithiolate)].

The electrochemical reduction of complexes [Fe2 (CO)4 (κ2 -phen)(µ-xdt)] (phen=1,10-phenanthroline; xdt=pdt (1), adtiPr (2)) in MeCN-[Bu4 N][PF6 ] 0.2 m is described as a two-reduction process. DFT calculations show that 1 and its monoreduced form 1- display metal- and phenanthroline-centered frontier orbitals (LUMO and SOMO) indicating the non-innocence of the phenanthroline ligand. Two energetically close geometries were found for the doubly reduced species suggesting an intriguing influence of the phenanthroline ligand leading to the cleavage of a Fe-S bond as proposed generally for this type of complex or retaining the electron density and avoiding Fe-S cleavage. Extension of calculations to other complexes with edt, adtiPr bridge and even virtual species [Fe2 (CO)4 (κ2 -phen)(µ-adtR )] (R=CH(CF3 )2 , H) or [Fe2 (CO)4 (κ2 -phen)(µ-pdtR )] (R=CH(CF3 )2 , iPr) showed that the relative stability between both two-electron-reduced isomers depends on the nature of the bridge and the possibility to establish a remote anagostic interaction between the iron center {Fe(CO)3 } and the group carried by the bridged-head atom of the dithiolate group.

A Diiron Hydrogenase Mimic Featuring a 1,2,3-Triazolylidene.

A novel complex featuring a mesoionic carbene [Fe2(CO)5(trz)(µ-pdt)] (1) (trz = 1-phenyl-l,3-methyl,4-butyl-1,2,3-triazol-5-ylidene), was synthesized and spectroscopically and structurally characterized. The reductive behaviour of this compound in the presence and in the absence of acid (CH3CO2H) was examined by cyclic voltammetry (CV) that revealed the lack of efficient activity towards proton reduction.

Electrochemical and Theoretical Investigations of the Oxidatively Induced Reactivity of the Complex [Fe2 (CO)4 (κ2 -dmpe)(µ-adtBn )] Related to the Active Site of [FeFe] Hydrogenases.

Electrochemical oxidation of the complex [Fe2 (CO)4 (κ2 -dmpe)(µ-adtBn )] (adtBn =(SCH2 )2 NCH2 C6 H5 , dmpe=Me2 PCH2 CH2 PMe2 ) (1) has been studied by cyclic voltammetry (CV) in acetonitrile and in dichloromethane in the 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=tert-butyl, xylyl), have been prepared and characterized by IR and NMR spectroscopies and, except [1-MeCN](PF6 )2 , by X-ray diffraction analysis. The crystallographic structures of the new FeII FeII complexes reveal 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 to elucidate and discriminate, 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.

Influence of the Dithiolate Bridge on the Oxidative Processes of Diiron Models Related to the Active Site of [FeFe] Hydrogenases.

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, FeI FeII or FeII FeII , of the complexes.

New Fe(I) -Fe(I) complex featuring a rotated conformation related to the [2 Fe](H) subsite of [Fe-Fe] hydrogenase.

Rotated geometry: The first example of a dinuclear iron(I)-iron(I) complex featuring a fully rotated geometry related to the active site of [Fe-Fe] hydrogenase is reported.

Transfer of highly hydrophilic ions from water to nitrobenzene, studied by three-phase and thin-film modified electrodes.

The study by voltammetry of hydrophilic ion transfers across the interface between an aqueous solution and an immiscible organic solvent is limited by the presence of supporting electrolytes in both phases. Such a study is impossible for ions having a higher affinity for water than ions of the electrolytes. Indirectly, methods based on modified solid electrodes can be used; these are obtained by the deposition of an organic phase containing a molecule having redox properties, the modified electrode being in contact with an aqueous solution of the appropriate electrolyte. The three-phase electrode is very convenient for that purpose. However, this experimental tool also has its own limitation, due mainly to the redox species produced in the organic phase. The oxidized, or reduced, form of the redox molecule must have a very low affinity for water, as otherwise its transfer masks that of the ion under study. Ferrocene is almost useless because of the affinity of the ferrocenium cation for water, decamethylferrocene being a better choice. The present work illustrates how the use of lutetium bisphthalocyanines widely expands the possibilities, as these molecular sandwich complexes can be reduced as well as oxidized, the products of the reactions having a very low affinity for water. This made the determination of the Gibbs energy possible for the transfers of highly hydrophilic ions from water to nitrobenzene: Cl(-) (40 kJ mol(-1)), F(-) (57 kJ mol(-1)), H2PO4(-) (64 kJ mol(-1)). Nothing being really known about the transfer of F(-) or H2PO4(-) from water to organic solvents, these are the first values ever published. H(+), OH(-), and HSO4(-) have also been studied, showing that these species, which have a poor affinity for nitrobenzene, are prone to association reactions with the reduced or oxidized forms of the lutetium bisphthalocyanine.

Sb(III) oxidation by iodate in seawater: a cautionary tale.

Knowledge of antimony redox kinetics is crucial in understanding the impact and fate of antimony in the environment. The oxidation of Sb(III) with iodate was measured in 0.5 mol L(-1) NaCl solutions as a function of pH at environmentally significant concentrations of antimony and iodate. The oxidation of Sb(III) with iodate is pH dependent: no measurable oxidation is observed below pH 9. The undissociated Sb(OH)3 does not react with iodate and the formation of significant amounts of Sb(OH)4- is needed for the reaction to take place. It is thus unlikely that iodate oxidizes Sb(III) in seawater. Our results support that the observed presence of the thermodynamically unstable Sb(III) in oxic waters can be due to the kinetic stabilization of the trivalent state vis-à-vis some common abiotic oxidants at natural pH values. However, caution must be exercised because the presence of iodate in seawater favours fast oxidation of Sb(III) if water samples are acidified, as is the case in many analytical procedures.

Coadsorption of carbofuran and lead at the air/water interface. Possible occurrence of non-volatile pollutant cotransfer to the atmosphere.

The weak solubility of carbofuran allows adsorption at the air/water interface. Carbofuran-rich layers can then induce the coadsorption of metallic salts such as lead nitrate; on the other hand, when carbofuran is missing, no adsorption of this salt takes place. This phenomenon was quantitatively studied through surface tension measurements under concentration conditions close to the environmental ones. Heavy metal salt adsorbed about ten times more than carbofuran. Evidence was then provided that the simultaneous presence of both pollutants in water favours their adsorption and passing from water to the atmosphere through mechanisms such as bubbling.

Comparative study of the thermodynamics and kinetics of the ion transfer across the liquid/liquid interface by means of three-phase electrodes.

A comparative study of the behavior of different sorts of three-phase electrodes applied for assessing the thermodynamics and kinetics of the ion transfer across the liquid/liquid (L/L) interface is presented. Two types of three-phase electrodes are compared, that is, a paraffin-impregnated graphite electrode at the surface of which a macroscopic droplet of an organic solvent is attached and an edge pyrolytic graphite electrode partly covered with a very thin film of the organic solvent. The organic solvent contains either decamethylferrocene or lutetium bis(tetra-tert-butylphthalocyaninato) as a redox probe. The role of the redox probe, the type of the electrode material, the mass transfer regime, and the effect of the uncompensated resistance are discussed. The overall electrochemical process at both three-phase electrodes proceeds as a coupled electron-ion transfer reaction. The ion transfer across the L/L interface, driven by the electrode reaction of the redox compound at the electrode/organic solvent interface, is independent of the type of redox probe. The ion transfer proceeds without involving any chemical coupling between the transferring ion and the redox probe. Both types of three-phase electrodes provide consistent results when applied for measuring the energy of the ion transfer. Under conditions of square-wave voltammetry, the coupled electron-ion transfer at the three-phase electrode is a quasireversible process, exhibiting the property known as "quasireversible maximum". The overall electron-ion transfer process at the three-phase electrode is controlled by the rate of the ion transfer. It is demonstrated for the first time that the three-phase electrode in combination with the quasireversible maximum is a new tool for assessing the kinetics of the ion transfer across the L/L interface.

A sterically stabilized FeI-FeI semi-rotated conformation of [FeFe] hydrogenase subsite model.

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 [Fe(I)Fe(I)] 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(κ(2)-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 [Fe(I)Fe(I)] hydrogenase model complex showing a semi-rotated geometry without the need of stabilization via agostic interactions (Fe···H-C).

The effect of the presence of trace metals on the oxidation of Sb(III) by hydrogen peroxide in aqueous solution.

Despite its importance for understanding the behaviour of antimony in the environment, the oxidation kinetics of Sb(III) with natural oxidants is still not well understood. We have studied the oxidation of Sb(III) by hydrogen peroxide on a time scale of hours in the presence of some trace metals, Cu(II), Mn(II), Zn(II) and Pb(II), under pH and concentration conditions close to natural ones. The effects that these trace metals have on Sb(iii) oxidation by hydrogen peroxide vary. Zn(II) had no catalytic effect at all, but Cu(II), Mn(II) and Pb(II) did, though their effects were not uniform. Cu(II) significantly accelerated the reaction, which remained first-order with respect to Sb(III) at any Cu(II) concentration tested. Pb(II) and Mn(II) also enhanced the reaction rates, but the apparent order of the reaction with respect to Sb(III) changed to two. The trace metal effect observed was concentration dependent for Pb(II). The addition of the hydroxyl radical scavenger 2-propanol suggests that the trace metal catalytic effect observed involves the action of hydroxyl radicals, but that they are not responsible for the oxidation of Sb(III) by H2O2 in the absence of trace metals. The fact that Sb(III) can be oxidized by hydroxyl radicals present in water, even if it is not capable of producing them, has important environmental implications because hydroxyl radicals are known to be abundant in many natural waters such as seawater, humic-rich surface waters or rainwater.

Kinetic studies on Sb(III) oxidation by hydrogen peroxide in aqueous solution.

Knowledge of antimony redox kinetics is crucial in understanding the impact and fate of Sb in the environment and optimizing Sb removal from drinking water. The rate of oxidation of Sb(III) with H2O2 was measured in 0.5 mol L(-1) NaCl solutions as a function of [Sb(III)], [H2O2], pH, temperature, and ionic strength. The rate of oxidation of Sb(III) with H2O2 can be described by the general expression: -d[Sb(III)]/dt= k[Sb(III)][H2O2][H+](-1) with log k = -6.88 (+/- 0.17) [kc min(-1)]. The undissociated Sb(OH)3 does not react with H2O2: the formation of Sb(OH)4- is needed for the reaction to take place. In a mildly acidic hydrochloric acid medium, the rate of oxidation of Sb(III) is zeroth order with respect to Sb(III) and can be described by the expression -d[Sb(III)]/dt = k[H2O2][H+][Cl-] with log k = 4.44 (+/- 0.05) [k. L2 mol(-2) min(-1)]. The application of the calculated rate laws to environmental conditions suggests that Sb(III) oxidation by H2O2 may be relevant either in surface waters with elevated H2O2 concentrations and alkaline pH values or in treatment systems for contaminated solutions with millimolar H2O2 concentrations.