Electrochemical reduction of Fe(2)(mu-pdt)(CO)(6) 1 (pdt = propane-1,3-dithiolate) leads initially to a short-lived species, 1-, then subsequently to two-electron reduced products, including a CO-bridged diiron compound, 1B. The assignment of the redox level of 1- is based on EPR and UV-vis spectra together with the observation that a CO-saturated solution of 1- decays to give 1 and 1B. Hydride reduction of 1 also results in formation of 1B via a relatively long-lived formyl species, 1(formyl). Despite its involvement in hydride transfer reactions, 1B is formulated as [Fe(2)(mu-S(CH(2))(3)SH)(mu-CO)(CO)(6)](-) based on a range of spectroscopic measurements together with the Fe-Fe separation of 2.527 A (EXAFS). Electrocatalytic proton reduction in the presence of 1 in moderately strong acids has been examined by electrochemical and spectroelectrochemical techniques. The acid concentration dependence of the voltammetry is modeled by a mechanism with two electron/proton additions leading to 1H(2), where dissociation of dihydrogen leads to recovery of 1. Further reduction processes are evident at higher acid concentrations. Whereas free CO improves the reversibility of the electrochemistry of 1, CO inhibits electrocatalytic proton reduction, and this occurs through side reactions involving a dimeric species formed from 1-.

The known complex {Cp(PPh3)2Ru}2(μ-C⋮CC⋮C) (3-Ph) and its PMe3-substitution product {Cp(PPh3)(PMe3)Ru}2(μ-C⋮CC⋮C) (3-Me) have been shown by cyclic voltammetry to undergo a series of four stepwise one-electron oxidation processes. Successive oxidation potentials (V) for the first three reversible processes of 3-Ph (3-Me) are −0.23 (−0.26), +0.41 (+0.33), and +1.03 (+0.97); the fourth, irreversible oxidation at +1.68 (+1.46) V is accompanied by chemical transformation followed by further oxidation. Chemical oxidation of 3-Ph with 1 or 2.5 equiv of AgPF6 in CH2Cl2/1,2-dimethoxyethane gave the one- and two-electron oxidized species [3-Ph][PF6] and [3-Ph][PF6]2, respectively. The chemical and electrochemical studies have been complemented by a series of detailed spectroelectrochemical experiments to obtain spectral data associated with the 3 n + (n = 0−4) species from 1500 to 40 000 cm-1, without necessitating the isolation of each individual species. Theoretical techniques have been employed in order to probe the structure of the conjugated all-carbon ligand at each stage of oxidation. Both the metal centers and the carbon atoms of the C4 bridge are affected, with removal of electrons housed in MOs delocalized over all atoms of the Ru−C4−Ru chain. Comparison of models with different ligand surroundings suggests that molecules containing strong electron-donating ligands should be more easily oxidized.

The synthesis and characterisation of the first {2Fe2S}cluster bearing both CO and CN ligands is described; the iron atoms are linked by the bridging 1,3-propanedithiolate unit that has been identified in the crystallographic structure of the {2Fe2S} sub-unit of the H-centre of the all-iron hydrogenase from Desulfovibrio desulfuricans.

Structure determinations of the caesium alums CSM~~~[SO~],*~~H,O, M = V, Cr, Mn, Fe, Co, Al, Ga, or In, have been carried out at 295(1) K by X-ray diffraction methods. The data obtained include ( i ) MIrL0 distances for the metals in the [M(0H,),l3+ species [V-0, 1.992(6) ; Cr-0, 1.959(3) ; Mn-0, 1.991 (6) ; Fe-0, 1.995(4) ; Co-0, 1.873(5) ; AI-0, 1.877(3) ; Ga-0, 1.944(3) ; and In-0, 2.1 12(4) A], (ii) an estimate of 1 .34 A for the effective co-ordinated radius for the water molecule in the cation, and (id) the proposal of a new criterion for the classification of the different alum types, based on the disposition of the water molecules about the univalent cation.

IR spectroelectrochemistry of Fe4{Me(CH2S)3}2(CO)8 (4Fe6S) in the nu(CO) region shows that the neutral and anion forms have all their CO groups terminally bound to the Fe atoms; however, for the dianion there is a switch of the coordination mode of at least one of the CO groups. The available structural and nu(CO) spectra are closely reproduced by density-functional theory calculations. The calculated structure of 4Fe6S2- closely mirrors that of the diiron subsite of the [Fe-Fe] hydrogenase H cluster with a bridging CO group and an open coordination site on the outer Fe atom of pairs of dithiolate-bridged Fe0FeII subunits connected by two bridging thiolates. Geometry optimization based on the all-terminal CO isomer of 4Fe6S2- does not give a stable structure but reveals a second-order saddle point ca. 11.53 kcal mol(-1) higher in energy than the CO-bridged form. Spectroelectrochemical studies of electrocatalytic proton reduction by 4Fe6S show that slow turnover from the primary reduction process (E1/2'=-0.71 V vs Ag/AgCl) involves rate-limiting protonation of 4Fe6S- followed by reduction to H:4Fe6S-. Rapid electrocatalytic proton reduction is obtained at potentials sufficient to access 4Fe6S2-, where the rate of dihydrogen elimination from the FeIIFeII core of 4Fe6S is ca. 500 times faster than that from the FeIFeI core of Fe2(mu-S(CH2)3S)(CO)6. The dramatically increased rate of electrocatalysis obtained from 4Fe6S over all previously identified model compounds appears to be related to the features uniquely common between it and the H-cluster, namely, that turnover involves the same formal redox states of the diiron unit (FeIFeII and Fe0FeII), the presence of an open site on the outer Fe atom of the Fe0FeII unit, and the thiolate-bridge to a second one-electron redox unit.

Intermediates formed during reduction of Fe(2)(mu-PPh(2))(2)(CO)(6) (1) in the presence of protons have been identified by spectroelectrochemical, continuous-flow, and interrupted-flow techniques. The mechanism for electrocatalytic proton reduction suggested by these observations yields digital simulation of the voltammetry in close agreement with measurements conducted in THF over a range of acid concentrations. The mechanism for electrocatalytic proton reduction involves initial formation of the dianion, 1(2-), which is doubly protonated prior to further reduction and dihydrogen elimination. The IR spectra of the singly and doubly protonated forms of 1(2-) indicate structures corresponding to [FeH(CO)(3)(mu-PPh(2))(2)Fe(CO)(3)](-) (1H-) and FeH(CO)(3)(mu-PPh(2))(2)FeH(CO)(3) (1H(2)). The thiolato and dithiolato analogues of 1 exhibit electrocatalytic proton reduction associated with the two-electron reduction step, and this implies that the corresponding two-electron reduced doubly protonated species is unstable with respect to dihydrogen elimination. The stability of 1H(2) is most likely to be due to the weak interactions between the iron centers of the flattened [2Fe2P] core. Whereas 1H(2) is stable in the absence of a reducing potential, 1H- rearranges rapidly to a product previously described as [Fe(2)(mu-PPh(2))(mu-CO)(PHPh(2))(CO)(5)](-) (1H-(W)). Another protonation product of 1(2-), previously formulated as [Fe(2)(mu-PPh(2))(2)(mu-CO)H(CO)(5)](-), has been reformulated as [Fe(2)(mu-PPh(2))(mu-CO)(CO)(6)](-) (2) on the basis of a range of spectroscopic measurements. Solution EXAFS measurements of 1, 1(2-), 1H-(W), and 2 are reported, and these yield model-independent Fe-Fe distances of 2.61 (1), 3.58 (1(2-)), 2.58 (1H-(W)), and 2.59 A (2). The presence of an Fe-Fe bond for both 1H-(W) and 2 is a key aspect of the proposed structures, and this strongly supports the deductions based on spectroscopic evidence. The fits of the solution EXAFS to different structural models give statistics in agreement with the proposed structures.

The reduction chemistry of (mu-bridge)[Fe(CO)3]2 [bridge = propane-1,3-dithiolate (1) and ethane-1,2-dithiolate (2)] is punctuated by the formation of distinct products, resulting in a marked difference in CO inhibition of electrocatalytic proton reduction. The products formed following reduction of 2 have been examined by a range of electrochemical, spectroelectrochemical, and spectroscopic approaches. Density functional theory has allowed assessment of the relative energies of the structures proposed for the reduction products and agreement between the calculated spectra (IR and NMR) and bond distances with the experimental spectra and EXAFS-derived structural parameters. For 1 and 2, one-electron reduction is accompanied by dimerization, but the structure, stability, and reaction with CO of the dimer is different in the two cases, and this is responsible for the different CO inhibition response for electrocatalytic proton reduction. Calculations of the alternate structures of the two-electron, one-proton reduced forms of 2 show that the isomers with terminally bound hydrides are unlikely to play a significant role in the chemistry of these species. The hydride-transfer chemistry of the 1B species is more reasonably attributed to a hydride-bridged form. The combination of experimental and computational results provides a solid foundation for the interpretation of the reduction chemistry of dithiolate-bridged diiron compounds, and this will underpin translation of the diiron subsite of the [FeFe] hydrogenase H cluster into an abiological context.

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