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The behavior of [Fe(2)(CO)(4)(κ(2)-PNP(R))(μ-pdt)] (PNP(R) =(Ph(2)PCH(2))(2)NR, R=Me (1), Ph (2); pdt=S(CH(2))(3)S) in the presence of acids is investigated experimentally and theoretically (using density functional theory) in order to determine the mechanisms of the proton reduction steps supported by these complexes, and to assess the role of the PNP(R) appended base in these processes for different redox states of the metal centers. The nature of the R substituent of the nitrogen base does not substantially affect the course of the protonation of the neutral complex by CF(3)SO(3)H or CH(3)SO(3)H; the cation with a bridging hydride ligand, 1 μH(+) (R=Me) or 2 μH(+) (R=Ph) is obtained rapidly. Only 1 μH(+) can be protonated at the nitrogen atom of the PNP chelate by HBF(4)·Et(2)O or CF(3)SO(3)H, which results in a positive shift of the proton reduction by approximately 0.15 V. The theoretical study demonstrates that in this process, dihydrogen can be released from a η(2)-H(2) species in the Fe(I)Fe(II) state. When R=Ph, the bridging hydride cation 2 μH(+) cannot be protonated at the amine function by HBF(4)·Et(2)O or CF(3)SO(3)H, and protonation at the N atom of the one-electron reduced analogue is also less favored than that of a S atom of the partially de-coordinated dithiolate bridge. In this situation, proton reduction occurs at the potential of the bridging hydride cation, 2 μH(+). The rate constants of the overall proton reduction processes are small for both complexes 1 and 2 (k(obs) ≈4-7 s(-1)) because of the slow intramolecular proton migration and H(2) release steps identified by the theoretical study.

The reactivity of [Fe2(CO)4(κ2-LL)(μ-pdt)]+ complexes (pdt = S(CH2)3S, propanedithiolate) with different substrates L′ (L′ = CO, MeCN, P(OMe)3) was investigated electrochemically in order to assess the influence of the chelating ligand κ2-LL (LL = IMe-CH2-IMe (1+), dppe (2+); IMe = 1-methylimidazol-2-ylidene). This latter ligand is effectively shown to affect the reactivity of the cations in different ways: when L′ = CO, the adduct [Fe2(CO)4(μ-CO)(κ2-dppe)(μ-pdt)]+ (2-CO+) was clearly observed by cyclic voltammetry, whereas [Fe2(CO)4(μ-CO)(κ2-IMe-CH2-IMe)(μ-pdt)]+ (1-CO+) was not detected, although DFT calculations show that the energies of the products and the activation barriers to their formation are similar. When L′ = MeCN, the adducts X-MeCN+ with X = 1, 2 are both observed, but the formation is easier when LL = dppe. Finally, the reaction of [Fe2(CO)4(κ2-IMe-CH2-IMe)(μ-pdt)]+ (1+) with P(OMe)3 produces the disubstituted dication [Fe2(CO)2{P(OMe)3}2(κ2-IMe-CH2-IMe)(μ-CO)(μ-pdt)]2+ (42+) via a disprop...

Reaction of the base-containing diphosphine (Ph2PCH2)2NCH3 with [Fe2(CO)6(μ-S2)] (1) yielded at room temperature the novel compound [Fe2(CO)4(μ-κ1:κ1-SPPh2){μ-κ2:κ2-SCH2N(Me)CH2PPh2}] (2) resulting from S−S and P−C bond cleavage concomitant with P−S and C−S bond formation. Experiments at low temperature allowed the isolation of an intermediate species, [Fe2(CO)5(μ-κ1:κ1-SPPh2){μ-κ2:κ1-SCH2N(Me)CH2PPh2}] (3), differing from 2 by the coordination mode of the PNS ligand and the presence of one additional carbonyl group. When the reaction was performed in the presence of an excess of tBuNC, an analogous compound of 3 was obtained. The X-ray analysis of this species, 4, revealed that an isocyanide replaced a carbonyl ligand in the axial position at one iron center.

International audience; Two hexacarbonyl diiron compounds featuring dithiolate bridges with strong electron-withdrawing groups (CO2Me, 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.

The electrochemical behaviour of the unsymmetrically disubstituted [Fe 2 (CO) 4 (κ 2 -dppe)(μ-pdt)(μ-H)] + ( 1μH + ) complex was studied by cyclic voltammetry (CV) in MeCN– and CH 2 Cl 2 –[NBu 4 ][PF 6 ] both in the absence and in the presence of acid (HBF 4 · Et 2 O or HOTs). In the absence of acid, 1μH + undergoes a reversible one-electron oxidation and two-successive one-electron reductions, the first of which is partially reversible. In contrast to symmetrical analogues, 1μH + does not regenerate its neutral precursor 1 upon reduction on the CV timescale. When acid is present, the reduction leads to catalytic proton reduction at a mild potential (ca. −1.3 V vs. Fc + /Fc) according to an original mechanism where 1μH + is the catalyst. A second proton reduction process at −1.5 V is also observed for both HBF 4 · Et 2 O and HOTs. The latter is proposed to be catalysed by the one-electron reduced form of the H-bridged complex, 1μH .

Electrochemical investigations on a structural analogue of the [2Fe](H) subsite of [FeFe]H(2)ases, namely, [Fe(2)(CO)(6){micro-SCH(2)N(CH(2)CH(2)- OCH(3))CH(2)S}] (1), were conducted in MeCN/NBu(4)PF(6) in the presence of HBF(4)/Et(2)O or HOTs. Two different catalytic proton reduction processes operate, depending on the strength and the concentration of the acid used. The first process, which takes place around -1.2 V for both HBF(4)/Et(2)O and HOTs, is limited by the slow release of H(2) from the product of the {2 H(+)/2 e} pathway, 1-2H. The second catalytic process, which occurs at higher acid concentrations, takes place at different potentials depending on the acid present. We propose that this second mechanism is initiated by protonation of 1-2H when HBF(4)/Et(2)O is used, whereas the reduction of 1-2H is the initial step in the presence of the weaker acid HOTs. The potential of the second process, which occurs around -1.4 V (reduction potential of 1-3H(+)) or around -1.6 V (the reduction potential of 1-2H) is thus dependent on the strength of the available proton source.

International audience; Novel asymmetrically substituted azadithiolate compounds [Fe2(CO)4(κ2-dppe){μ-SCH2N(R)CH2S}] (R = iPr, 1a; CH2CH2OCH3, 1b; CH2C6H5, 1c) have been synthesized by treatment of [Fe2(CO)6(μ-adt)] [adt = SCH2N(R)CH2S, with R = iPr, CH2CH2OCH3, CH2C6H5] with dppe (dppe = Ph2PCH2CH2PPh2) in refluxing toluene in the presence of Me3NO. 1a−c have been characterized by single-crystal X-ray diffraction analyses. The electrochemical investigation of 1a−c and of [Fe2(CO)4(κ2-dppe)(μ-pdt)] (1d) [pdt = S(CH2)3S] in MeCN− and THF−[NBu4][PF6] has demonstrated that the electrochemical reduction of 1a−d gives rise to an Electron-transfer-catalyzed (ETC) isomerization to the symmetrical isomers 2a−d where the dppe ligand bridges the iron centers. Compounds 2a−d were characterized by IR and NMR spectroscopy, elemental analysis, and X-ray crystallography for 2a.

International audience; To investigate the effects of CO-substitution on the thermodynamics and the kinetics of proton binding in benzenedithiolate-bridged diiron complexes, the monosubstituted derivative Fe2(bdt)(CO)5(P(OMe)3) (2) was prepared by a CO-displacement reaction catalyzed by electron transfer. Kinetic analysis, based on cyclic voltammetry at various scan rates and ligand concentrations, indicates that the substitution reaction follows an associative mechanism. As the hexacarbonyl parent 1, the monosubstituted complex 2 neither reacts with protons in solution, nor catalyzes the proton reduction when acetic acid is used as a proton source. In the presence of a stronger acid, such as toluenesulfonic acid, voltammetry of 2 exhibits a reduction peak, which does not have a counterpart in the voltammetry of 1. We tentatively ascribed this reduction peak to the occurrence of a CE process involving proton binding by 2 and the reduction of 2H+ in the vicinity of the electrode. Bulk electrolysis experiments indicated that the proton catalysis is rather slow, i.e. few turnovers per hour. However, the onset of proton reduction occurs at about −0.8 V vs. Fc+/0 in MeCN + Bu4NPF6 electrolyte.

International audience; The reduction of the Fe(II)Fe(II) complex [Fe2(CO)2{P(OMe)3}2(κ2-IMe-CH2-IMe)(μ-CO)(μ-pdt)]2+ (2P2+; pdt = S(CH2)3S), which is a synthetic model of the H cluster of the [FeFe] hydrogenases in its inactive state, has been investigated electrochemically and theoretically (by density functional theory, DFT) in order to determine the mechanisms, intermediates, and products of the related processes. The electrochemical reduction of 2P2+ occurs according to an ECE-type reaction where the intervening chemical step is the loss of one P(OMe)3 ligand. This outcome, which is based on cyclic voltammetric experiments, is strongly supported by DFT calculations that provide additional information on the intermediates and the energetics of the reactions involved. The electrochemical reoxidation of the neutral product of the reduction follows an EEC process where the chemical step is the binding of P(OMe)3 to a dicationic intermediate. DFT calculations reveal that this intermediate has an unusual geometry wherein one of the two C–H bonds of a side methylene from the pdt group forms an agostic interaction with one Fe center. This interaction is crucial to stabilize the 32e− diferrous center and concomitantly to preserve Fe(II) from binding of weakly coordinating species. Nonetheless, it could be displaced by a relatively stronger electron donor such as H2, which could be relevant for the design of new oxidation catalysts.

A dithiolate-bridged diiron complex containing succinimide ester has been grafted on glassy carbon electrodes modified with either amine or aminophenyl groups. Covalent attachment of the complex through amide binding is confirmed by the linear dependence of the reduction peak current on the voltammetric scan rate. Attachment using the aminophenyl-modified electrode leads to better behaved cyclic voltammograms. The effect of addition of acid on the voltammetric response is also reported. Keywords: Amide bonding, CME, Diazonium, Hydrogenases, Organometallic models, Succinimide ester

International audience; Studies of the protonation of [Fe2(CO)4(κ2-PNP)(μ-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 μ-hydride species 3. Variable-temperature NMR experiments have been performed to clarify the processes involved.

The electrochemical reduction of [Mo2Cp2(CO)4{μ-η2:η3-HC⋮C−C(R1)(R2)}]+ complexes has been investigated by cyclic voltammetry, controlled-potential electrolysis, and coulometry. On the cyclic voltammetry time scale, the complexes with R1 = H, R2 = H (1+), Me (2+), Et (3+) undergo an irreversible or a quasi-reversible one-electron reduction whereas the analogues with R1 = H, R2 = Fc (4+) and R1 = Me, R2 = Me (5+) and Ph (6+) reduce in a single-step, reversible or quasi-reversible, two-electron process. Two different chemical reactions are involved in the overall reduction mechanism. The first chemical step is assigned as a structural rearrangement, responsible for slowing down the heterogeneous electron transfer. Extended Huckel MO calculations indicate that in the case of the complexes with R1 = H, R2 = Fc and R1 = Me, R2 = Me or Ph, a small increase in the distance between one metal center and the carbon of the C(R1)(R2) group could trigger the two-electron transfer process. The second chemical reaction ...

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 (see figure). Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The reactivity, towards nucleophiles and electrophiles, of dimolybdenum allenylidene complexes of the type [Cp2Mo2(CO)4(μ,η2(4e)-CCCR1R2)] (Cp=η5-C5H5) has been investigated. The nucleophilic attacks occur at the Cγ carbon atom, while electrophiles affec the Cα atom. Variable temperature solution 1H NMR studies show a dynamic behavior of these complexes consisting of an equilibrium between two enantiomers with a symmetrical [Cp2Mo2(CO)4(μ-σ,σ(2e)-CCCR1R2)] transition state. Extended Huckel MO calculations have been carried out on the model [Cp2Mo2(CO)4(μ,η2-CCCH2]. The calculated charges of the allenylidene carbon atoms suggest that the electrophilic attacks are under charge control, while the nucleophilic attacks are rather under orbital control.

The ionic complexes [Cp2Mo2(CO)4{μ-η2:η3-HCCC(H)(Fc)}][BF4] (1) and [{Cp2Mo2(CO)4(μ-η2:η3-HCCC(H)}2Fc][{Bf4}2] (5) react with NaHg in toluene solution. The results are interpreted in terms of a radical mechanism involving intramolecullar intermolecular coupling reactions.

The monoallenylidene complex [Cp2Mo2(CO)4{μ−σ:η2−CCC(Fc)(H)}] (5) and the diallenylidene complex [{Cp2Mo2(CO4(μ−σ:η2−CCC(H))}2Fc] (6) have been obtained from the corresponding monocarbenium ion [Cp2Mo2(CO)4{μ−η2:η3−HCCC(H)(Fc)}][BF4] and the dicarbenium ion [{Cp2Mo2(Co)4(μ−η2:η3−HCCC(H)}2Fc] [{BF4}2] (4) respectively, and their spectroscopic data are determined.

A selective abstraction of the acetylenic proton of cationic allynyl complexes [MoCp(CO)22(μ-η2,η3-HCCCR1R2)][BF4](Cpη5-C5H5) by using the acetylide LiCCC(CH3)CH2 gives rise to allenylidene complexes [Cp2Mo2(CO)4(μ,η2-CCCR1R2)] R2 = C6H5 (6c). Compounds 3b and 4b exist as two isomers while for 6c the formation reaction is stereoselective. The crystal structure of [Cp2Mo2(CO)4(μ,η2-CCC6H10)] (1b) has been determined.

The behaviour of μ-enyne complexes [CP2MO2(CO)4(μ-HCCR)] [CP2MO2(CO)4  [Mo(η5-C5H5)(CO)2]2; R  C(CH3)CH2 (1); R  CHCHCH3 (2;) R  C6H9 (3)] and of the corresponding μ-η2,η3-allenyl [Cp2Mo2(CO)4(μ-η2,η3-HCCR′)][BF4] [R′  C(CH3)2 (4); R′  CHCH2CH3 (5); R′  C6H10 (6)] towards Na/Hg has been studied and compared with that of the vinylacetylene and protonated vinylacetylene species [Cp2Mo2(CO)4(μ-HCCCHCH2)] (11) and [Cp2Mo2(CO)4(μ-η2,η3 -HCCCHCH3)]-[BF4] (14), respectively. It appears that when the Cγ carbon atom of the μ-enyne bears a hydrogen atom (complexes 2 and 11), dimerization occurs leading to tetrametallic species [[{Cp2Mo2(CO)4}(μ-HCCCH2CHCH3)]2] (8) and [[{Cp2Mo2(CO)4}(μ-HCCCH2CH2)]2] (12). In the other cases, μ-σ,η3 allylic species such as [Cp2Mo2(CO)4 {μ-σ,η3-HC ⋯ CH ⋯ C(CH3)2}] (7) are formed. Reaction of Na/Hg with most of the μ-η2,η3 allenyl complexes regenerates the parent μ-enyne compounds. When the “C+” carbon atom bears a hydrogen atom, carbon-carbon coupling leading to a dimerization is favoured. The crystal structure of tetrametallic [[{Cp2Mo2(CO)4}{μ-HCCCH(CH2CH3)}]2] (10) obtained from 5 has been determined.

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.

International audience; Six-membered heterocycles, 1,4-dithiane and 1,4-thioxane, were treated with [Fe2(CO)6(μ-pdt)] (pdt = propanedithiolate) in order to synthesise [2Fe3S] complexes with well positioned heteroatoms as potential base. Monosubstituted compounds of general formula [Fe2(CO)5L(μ-pdt)] (L = 1,4-dithiane, 1,4-thioxane) were mainly obtained and characterised by 1H NMR, IR, elemental analysis and X-ray diffraction studies. Electrochemical behaviour of these monosubstituted derivatives was investigated by cyclic voltammetry. An original tetranuclear cluster with a [4Fe8S] core was also isolated as a minor product and structurally characterised.

A new dissymmetrically disubstituted diiron dithiolate species, [Fe(2)(CO)(4)(κ(2)-P(Ph)(2)N(Ph)(2))(μ-pdt)] (pdt = S(CH(2))(3)S), was prepared by using a flexible cyclic base-containing diphosphine, 1,3,5,7-tetraphenyl 1,5-diaza-3,7-diphosphacyclooctane (P(Ph)(2)N(Ph)(2) = {PhPCH(2)NPh}(2)). Preliminary investigations of proton and electron transfers on the diiron system have been done.

The purpose of the present study was to evaluate the use of a non-innocent ligand as a surrogate of the anchored [4Fe4S] cubane in a synthetic mimic of the [FeFe] hydrogenase active site. Reaction of 2,3-bis(diphenylphosphino) maleic anhydride (bma) with [Fe 2 (CO) 6 (µ-pdt)] (propanedithiolate, pdt = S(CH 2 ) 3 S) in the presence of Me 3 NO-2H 2 O afforded the monosubstituted derivative [Fe 2 (CO) 5 (Me 2 NCH 2 PPh 2 )(µ-pdt)] ( 1 ). This results from the decomposition of the bma ligand and the apparent C–H bond cleavage in the released trimethylamine. Reaction under photolytic conditions afforded [Fe 2 (CO) 4 (bma)(µ-pdt)] ( 2 ). Compounds 1 and 2 were characterized by IR, NMR and X-ray diffraction. Voltammetric study indicated that the primary reduction of 2 is centered on the bma ligand.

International audience; The possibility of using hydrogenase models based on Fe2(μ-NRN) complexes was investigated. The use of benzo-[c]-cinnoline (BC = N2C12H8) bridge allows for an unprecedented control of the electron-transfer process in diiron complexes. We were able to modify the potential gap between the two one-electron reduction steps of [Fe2(BC)(CO)6] by changing the solvent properties. The catalyst exhibits modest catalytic activity for proton reduction. However, there is a considerable opportunity for improvement considering the wide range of azo-bridged diiron described in the literature.

International audience; 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.

International audience; This review focuses on electron and proton transfers involving hexacarbonyl and substituted {2Fe2S} complexes inspired from the active site of the [FeFe]-hydrogenases ([FeFe]H2ases), and on different mechanisms by which the synthetic models catalyze the reduction of protons. Selected aspects of the mechanisms proposed for the biological process on the basis of DFT calculations are also briefly presented.

Depending on the substituent (Tol or CF3) withdrawing power, reactions of the alkyne RCCH with the bis(acetonitrile) compound [Mo2(C5H5)2(μ-SCH3)3(CH3CN)2](BF4) (1) gave either the “flyover” metallacycle complex 4 or the thiophenium derivative 3. In 3 the thiophenium ligand shows a new μ-η2:η2 mode of coordination. A single-crystal structure of 3, the first complex with binding of thiophenium to a binuclear site, was obtained.

International audience; During the last decade, the structure of the subsite of [FeFe]H2ases (or Fe-only hydrogenases) has inspired organometallic chemists to find out whether it was possible to build synthetic molecules, based on sulfur, iron, carbon monoxide and cyanide or surrogates, having the same efficiency for hydrogen production/uptake as that of the enzyme. A short nonexhaustive overview of recent advances in this chemistry is presented

International audience; A few recent electrochemical studies of diiron models of the iron-only hydrogenases' active site are summarized. Emphasis is put on the reduction mechanisms of hexacarbonyl complexes and on the different mechanisms of proton reduction that may operate depending on the nature of the complex and the strength of the acid. An attempt is made to discuss the thermodynamic and kinetic limitations of proton reduction processes supported by these compounds.

Metal complexes with N-Heterocyclic carbene (NHC) ligands are widely used in organometallic chemistry. 1a , 1b , 1c NHC are known to be good donor groups, and the incorporation of functionality is ...

International audience; Reaction of the bis(nitrile) complex [Mo2Cp2(μ-SMe)3(NCMe)2](BF4) (1) with dimethylpropargylic alcohol, HCtriple bond; length of mdashCCMe2(OH), at room temperature in dichloromethane produced good yields of the μ-alkynol species [Mo2Cp2(μ-SMe)3{μ-CHCCMe2(OH)}](BF4) (2a) through replacement of the two acetonitrile ligands in 1 by the alkynol. The NMR spectra of 2a indicate a μ-η1:η1 coordination mode for the alkyne which is thereby incorporated into a dimetallacyclobutene ring like that found here by X-ray diffraction (XRD) analysis of the related complex [Mo2Cp2(μ-SMe)3(μ-η1:η1-CHCCO2Me)](BPh4) (2b). When 2a was stirred with Et3N at room temperature in dichloromethane, deprotonation gave high yields of the μ-3-hydroxyalkynyl derivative [Mo2Cp2(μ-SMe)3{μ-η1:η2-CCCMe2(OH)}] (3), together with small amounts of the already-known vinylacetylide [Mo2Cp2(μ-SMe)3{μ-η1:η2-CCC(Me)CH2}] (4) resulting from dehydration of 3. Treatment of 3 with 1 equiv. of HBF4 · OEt2 in diethyl ether at room temperature gave the 3-hydroxyvinylidene derivative [Mo2Cp2(μ-SMe)3{μ-η1:η2-CCHCMe2(OH)}](BF4) (5) as the major product, together with other minor products [Mo2Cp2(μ-SMe)3{μ-η1:η2-CCHC(Me)CH2}](BF4) (6), [Mo2Cp2(μ-SMe)3(μ-η1:η2-CCCMe2)](BF4) (7), [Mo2Cp2(μ-SMe)3(μ-η1:η2-CCH2)](BF4) (8), [Mo2Cp2(μ-SMe)3{μ-η1:η2-CCH(CHMe2)}](BF4) (9) and [Mo2Cp2(μ-SMe)3(μ-O)](BF4) (10). The vinylidene (6) and allenylidene (7) species resulted from dehydration of the 3-hydroxyvinylidene complex 5 whereas the vinylidene derivative 8 was formed by deketonisation of 5. When 3 reacted with a large excess of HBF4 · OEt2 in dichloromethane, the 3-isopropylvinylidene complex 9 was obtained nearly quantatively via a Hradical dot radical process. When left for several days CD2Cl2 solutions of 5 afforded mainly the vinylidene species 8 by deketonisation and the side-oxoproduct [Mo2Cp2(μ-SMe)3(μ-O)](BF4) (10) by hydrolysis or reaction with oxygen. Addition of nucleophiles (H−, OMe−, OH−, SMe−) to the allenylidene complex [Mo2Cp2(μ-SMe)3(μ-η1:η2-CCCPh2)](BF4) (11) resulted in the formation of the corresponding μ-acetylide derivatives [Mo2Cp2(μ-SMe)3(μ-η1:η2-CCCRPh2)] [R = H (12), OMe (16a), OH (17), SMe (16b)], which by further reaction with tetrafluoroboric acid afforded either the vinylidene species [Mo2Cp2(μ-SMe)3{μ-η1:η2-CCH(CRPh2)}](BF4) when R = H (13), or the starting complex 11 when R is a leaving group (OMe). Reaction of 13 with Na(BH4) gave the μ-alkylidyne complex [Mo2Cp2(μ-SMe)3(μ-η1-CCH2CPh2H)] (14) by nucleophilic attack of H− at the Cβ carbon atom of the vinylidene chain. Proton addition at Cα in 14 led to the formation of a μ-vinylidene compound 15 containing an agostic C–H bond. New complexes have been characterised by elemental analyses and spectroscopic methods, supplemented for 2b and 3 by X-ray diffraction studies.

[Fe2(S2C3H6)(CO)5{P(OMe)3}] (2) and [Fe2(S2C3H6)(CO)4{P(OMe)3}2] (3) were selectively prepared by the electrochemical reduction of [Fe2(S2C3H6)(CO)6] (1) in the presence of trimethyl phosphite ligand. Electrochemical data indicate a CO-displacement reaction catalyzed by electron transfer. Complexes 2 and 3 were characterized by X-ray crystallography, which shows that in the solid state, the trimethyl phosphite ligands lie in the apical configuration. NMR suggests that multiple isomers exist in solution. Both 2 and 3 exhibit a primary reduction step that involves two electrons. No stable hydride derivatives could be isolated by the reaction of 2 or 3 with strong acid. However, for both 2 and 3, proton reduction catalysis was detected at a potential less negative than that of the reduction of the diiron complex working as catalyst. The proposed mechanism involves a protonation of the diiron complex in the vicinity of the electrode, which is triggered by the more facile reduction of the protonated form (CE mechanism). The protonation and reduction steps are followed by a bimolecular reaction that regenerates the starting complex. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Reactions of the di-iron complex [Fe 2 (μ-S) 2 (CO) 6 ] 2− with carboxy-functionalized dihalide derivatives (XCH 2 ) 2 R (X = Cl, R = NC 6 H 4 CH 2 CO 2 CH 3 ; X = Br, R = C 6 H 3 COOH, C 6 H 3 COON(COCH 2 ) 2 ) gave new functionalized dithiolate di-iron complexes [Fe 2 (μ-SRS)(CO) 6 ] (R = (CH 2 ) 2 NC 6 H 4 CH 2 CO 2 CH 3 ( 1 ), (CH 2 ) 2 C 6 H 3 COOH ( 2 ), (CH 2 ) 2 C 6 H 3 COON(COCH 2 ) 2 ( 3 )) in low yields. The azadithiolate complex 1 has been characterized by a single crystal X-ray diffraction analysis and studied by electrochemical methods.

The complexes [Fe2(CO)6{μ-SCH2N(R) CH2S}] (R = CH2CH2OCH3, 1a; R = iPr, 1b) and [Fe2(CO)6(μ-pdt)] 2 (pdt = S(CH2)3S) are structural analogues of the [2Fe] H subsite of [FeFe]H2ases. Electrochemical investigation of 1 and 2 in MeCN-[NBu4][PF6] under Ar and under CO has demonstrated that the reduction can be resolved into two one-electron transfer steps by using fast scan cyclic voltammetry. At slow scan rates the reduction of 1 tends towards a two-electron process owing to the fast disproportionation of the anion, while the two-electron reduction of 2 is clearly favoured in the presence of CO. Substitution of a CO ligand in 2 by a N-heterocyclic carbene results in the destabilisation of the anion. Thus, in MeCN-, thf- or CH 2Cl2-[NBu4][PF6], the electrochemical reduction of Fe2(CO)5LNHC(μ- pdt)] 3 (LNHC = 1,3-bis(methyl)-imidazol-2-ylidene, 3a; 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene, 3b) occurs in a single-step, two-electron process at moderate scan rates; under appropriate conditions this process can be separated into two one-electron steps. Density Functional Theory calculations successfully rationalize the effects of the S-to-S linkage on the electrochemistry of the complexes. © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

International audience; Treatment of [Fe2(μ-pdt)(CO)6] [pdt = S(CH2)3S] with dppe (Ph2PCH2CH2PPh2) in refluxing toluene affords the asymmetric complex [Fe2(μ-pdt)(CO)4(dppe)] (1). Protonation of 1 with HBF4−Et2O in CH2Cl2 gives at room temperature the μ-hydrido derivative [Fe2(μ-pdt)(CO)4(dppe)(μ-H)](BF4) (2). Monitoring the reaction by 1H, 31P, and 13C NMR at low temperature reveals unambiguously that the process of the protonation of 1 implies terminal hydride intermediates.

International audience; Reaction of [Fe2{μ-S(CH2)3S}(CO)6] (1) at room temperature with the N-heterocyclic carbenes IMe-(CH2)2-L (IMe = 1-methylimidazol-2-ylidene, L = NMe2, SMe) afforded the pentacarbonyl carbene derivatives [Fe2{μ-S(CH2)3S}(CO)5{IMe-(CH2)2-NMe2}] (2a) and [Fe2{μ-S(CH2)3S}(CO)5{IMe-(CH2)2-SMe}] (2b). Reaction of 1 with IMe-CH2-IMe at room temperature provided the dimer [{Fe2(μ-S(CH2)3S)(CO)5}2{μ-(IMe-CH2-IMe)}] (3) together with the chelated bis-NHC complex [Fe2{μ-S(CH2)3S}(CO)4{IMe-CH2-IMe}] (4a) as the major product. The analogous reaction of 1 with IMe-(CH2)2-IMe yielded the chelated bis-NHC complex [Fe2(μ-S(CH2)3S)(CO)4{IMe-(CH2)2-IMe}] (4b). Addition of HBF4 to compound 4a afforded the stable bridging hydride complexes [Fe2(μ-H){μ-S(CH2)3S}(CO)4{IMe-CH2-IMe}](BF4) (5a,b) with NHC ligands in a basal/basal and basal/apical mode of coordination in 5a,b, respectively. The molecular structures of 2a, 3, 4a,b, and 5a were confirmed by X-ray diffraction studies. Low-temperature NMR studies on the protonation of 4a showed spectroscopic evidence for the formation of a very unstable terminal hydride and a bridging hydride species with a NHC ligand having a non classical mode of coordination via a C-4(5) bond. Cyclic voltammetry revealed that 4a is a catalyst for proton reduction.

Fe 2 (bdt)(CO) 6 ( 1 ) (bdt = benzenedithiolate) exhibits a chemically reversible two-electron reduction at −1.27 V vs. Fc +/0 in CH 3 CN + Bu 4 NPF 6 electrolyte. The increase in height of the reduction wave of complex 1 upon increasing the concentration of toluenesulfonic acid (p K a ∼ 8 in CH 3 CN) is consistent with a catalysis of the proton reduction reaction ( k cat ∼ 500 M −1 s −1 ). The reaction scheme, derived from the simulation of the voltammetric responses, involves the initial formation of the dianion 1 2− followed by two successive reactions with acid. The last protonation step is the rate determining step in the catalytic cycle. Using experiments conducted with acetic acid (p K a ∼ 23 in CH 3 CN), we further shown that the intermediate species 1 H − is a significantly weaker base than the dianion 1 2− .

cited By 115; International audience; The reaction of [Fe2(μ-S(CH2)3S)(CO) 6] with an excess of 1,3-dimethylimidazol-2-ylidene (LMe) affords the mono- and disubstituted species [Fe2(μ-S(CH 2)3S)-(CO)5(LMe)] (1) and (Fe 2(μ-S(CH2)3(CO)4(L Me)2) respectively. The structures of 1 and 2 have been determined by X-ray crystallography. © 2005 American Chemical Society.

The bis(phosphido)-bridged complex [Fe(2)(mu-PPhH)(2)(CO)(6)] (1) undergoes double deprotonation to give the phosphorus-centered dianionic derivative [Fe(2)(mu-PPh)(2)(CO)(6)](2)(-) (2) which in turn reacts with the tertiary base RN(CH(2)Cl)(2) to give [Fe(2)[(PPhCH(2))(2)NR](CO)(6)] (3) in moderate yield. Treatment of 3 with HBF(4).Et(2)O affords the N-protonated species [Fe(2)[(PPhCH(2))(2)NHR](CO)(6)] BF(4) (4). The structural changes to the heavy atom skeleton of 3 arising from protonation are slight, the most obvious being a ca. 0.03 Alengthening of the N-C bonds.

cited By 110; International audience; The reduction of Fe2(μ-SC6H4S)(CO) 6 (1) at glassy carbon electrode appears chemically reversible (E1/2 = -1.44 V vs. Fc in Bu4NPF6-CH 2Cl2, ks,h ∼ 5 × 10-3 cm s-1) and actually consists of two overlapping one-electron transfers with the second transfer slightly more favorable than the first (E 02-E01 ∼ 35 mV). Apparently, the reduction of Fe2(μ-SCH2C6H 4CH2S)(CO)6 (2) is also a two-electron process (E1/2 = -1.68 V vs. Fc in Bu4NPF6-CH 2Cl2), but 22- decomposes on the voltammetric time scale. For both 1 and 2, the electrochemical reduction of the neutral cluster generates intermediate species which further catalyze proton reduction in acid-containing electrolyte (i.e., HBF4 in Bu4NPF 6-CH2Cl2). Unfortunately voltammetry also reveals a catalytic cycle for electrochemical proton reduction either slow or complicated by side reactions. © 2003 Elsevier B.V. All rights reserved.

The bis(phosphido)-bridged complex [Fe 2 (μ-PPhH) 2 (CO) 6 ] (1) undergoes double deprotonation to give the phosphorus-centered dianionic derivative [Fe 2 (μ-PPh) 2 (CO) 6 ] 2 - (2) which in turn reacts with the tertiary base RN(CH 2 Cl) 2 to give [Fe 2 {(PPhCH 2 ) 2 NR}-(CO) 6 ] (3) in moderate yield. Treatment of 3 with HBF 4 .Et 2 O affords the N-protonated species [Fe 2 {(PPhCH 2 ) 2 NHR}(CO) 6 ] BF 4 (4). The structural changes to the heavy atom skeleton of 3 arising from protonation are slight, the most obvious being a ca. 0.03 A lengthening of the N-C bonds.

cited By 33; International audience; Reactions of the bis(nitrile) compound [Mo2Cp2 (MeCN)2(μ-SMe)3](BF4) (1) with terminal alkynes in a 1:1 ratio in dichloromethane at room temperature led to the alkyne adduct [Mo2Cp2(μ-SMe)3(RCCH)] (BF4) (2: R = Tol (2a), Ph (2b), CH3C=CH2 (2c), nPr (2d), CO2Me (2e), CF3 (2f)). Compounds 2a-d were readily deprotonated with Et3N to give neutral acetylide derivatives [Mo2Cp2(μ-η1:η2- C≡CR)(μ-SMe)3] (3). Protonation of 3 afforded exclusively the vinylidene complexes [Mo2Cp2(μ-η1: η2-C=CHR) (μ-SMe)3](BF3) (4). Reaction of 1 with an excess of terminal alkyne RCCH in dichloromethane gave either the six-membered metallacycle compounds [Mo2Cp2(μ- η2:η4-CR=CHCR=CHSMe)(μ-SMe)2] (BF4) (5) or the S-methylthiophenium derivatives [Mo2Cp2(μ-η2:η4- C4H2R2SMe)(μ-SMe)2] (BF4) (6), depending on the nature of the R groups. Further reactions of the alkyne adducts 2 with isocyanide and carbon disulfide led to the vinyl-thioether complexes [Mo2Cp2(μ- η1:η3-CR=CR′-SMe)-(μSMe)2 (RNC)](BF4) (7, 8) and to their CS2 adducts [Mo2Cp2(S2C-CR′=CRSMe)(μ- SMe)2]-(BF4) (9, 10), which arise from regioselective C-S coupling. A mechanism is proposed for the formation of the cyclic thiometalla compounds 5, 9, and 10 and of the thiophenium species 6 which assigns a key intermediate role to vinyl-thioether species. The molecular structures of 3a, 4b, and 7a have been established by X-ray diffraction studies.

The cover picture shows a schematic view of the diiron active site of [FeFe] hydrogenases that catalyze the reversible H+/H2 conversion. Attempts to have a better understanding of the activity of this natural system and to design new organometallic electrocatalysts for the production of H2 have allowed new insight into the chemistry of sulfur-rich diiron carbonyls. Details are presented in the Microreview by P. Schollhammer et al. on p. 4671 ff.

International audience; Dissymetrically disubstituted di-iron azadithiolate complexes [Fe2(CO)4(κ2-LL){μ-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.

Treatment of [Fe(2)(micro-pdt)(CO)(6)] (pdt = S(CH(2))(3)S) with 1,10-phenanthroline (phen) in refluxing toluene affords the asymmetric complex [Fe(2)(micro-pdt)(CO)(4)(phen)] (1); the protonation of with HBF(4).OEt(2) in CD(2)Cl(2) at 203 K has been monitored by (1)H NMR.