Reductive cleavage of disulfide bonds is an important step in many biological and chemical processes. Whether cleavage occurs stepwise or concertedly with electron transfer is of interest. Also of interest is whether the disulfide bond is reduced directly by intermolecular electron transfer from an external reducing agent or mediated intramolecularly by internal electron transfer from another redox-active moiety elsewhere within the molecule. The electrochemical reductions of 4,4′-bipyridyl-3,3′-disulfide (1) and the di-N-methylated derivative (22+) have been studied in acetonitrile. Simulations of the cyclic voltammograms in combination with DFT (density functional theory) computations provide a consistent model of the reductive processes. Compound 1 undergoes reduction directly at the disulfide moiety with a substantially more negative potential for the first electron than for the second electron, resulting in an overall two-electron reduction and rapid cleavage of the S–S bond to form the dithiolate. In contrast, compound 22+ is reduced at less negative potential than 1 and at the dimethyl bipyridinium moiety rather than at the disulfide moiety. Most interesting, the second reduction of the bipyridinium moiety results in a fast and reversible intramolecular two-electron transfer to reduce the disulfide moiety and form the dithiolate. Thus, the redox-active bipyridinium moiety provides a low energy pathway for reductive cleavage of the S–S bond that avoids the highly negative potential for the first direct electron reduction. Following the intramolecular two-electron transfer and cleavage of the S–S bond the bipyridinium undergoes two additional reversible reductions at more negative potentials.

Low-valent molybdenum dicarbonyl complexes with a diazabutadiene [mesDABR; [ArN═C(R)C(R)═NAr]; Ar = 2,4,6-trimethylphenyl (mes), R = H or CH3] ligand have been synthesized and fully characterized. The title complexes exhibit elongated DAB C–N and shortened C–C bond lengths over the free ligand and other zerovalent molybdenum complexes of DAB. Compared to known examples theoretically described as iminato π-radicals (L•–), the oxidation state assignment fits a molybdenum(II) description. However, Mo K-edge X-ray absorption spectroscopy indicates that the complexes are best described as molybdenum(0). This example demonstrates that caution should be exercised in assigning the oxidation state based on structural parameters alone. Cyclic voltammetry studies reveal an electrochemical–chemical process that has been identified by in situ Fourier transform infrared spectroelectrochemistry as cis-to-trans isomerization.

In 2001, Lehmann and Evans (J. Phys. Chem. B, 2001, 105, 8877–8884) reported that the electrochemical reduction of a hydrogen-bonded complex between a proton donor and the anion radical of 3,5-di-tert-butyl-1,2-benzoquinone in acetonitrile proceeded by a concerted proton-electron transfer (CPET) reaction in which electron transfer from the electrode and proton transfer from proton donor to the quinone moiety occurred concertedly. Support for this conclusion was based upon ruling out both of the competing two-step processes, electron transfer followed by proton transfer (EP) and proton transfer followed by electron transfer (PE). In the course of studies of related compounds it was decided to reinvestigate the reduction of 3,5-di-tert-butyl-1,2-benzoquinone. It was discovered that the earlier conclusion that a CPET reaction was occurring was tenable only for the particular electrolyte that was used, tetrabutylammonium hexafluorophosphate and for lower concentrations of the quinone. Even the small change of carrying out the reduction of the quinone in the presence of water with tetramethylammonium hexafluorophosphate as electrolyte, produced voltammograms with clear signatures that the process was EP rather than CPET. Even more dramatic effects were seen with cesium, potassium or sodium ions in the electrolyte. A general reaction scheme to explain results with all electrolytes will be presented.

Reduction of quinones in aprotic media proceeds in two steps: initial reduction to the anion radical, followed by reduction of the anion radical to the dianion. In the cases of 3,5-di-tert-butyl-1,2-benzoquinone, 1, and 3,6-di-tert-butyl-1,2-benzoquinone, 2, the second reduction peak seen in a cyclic voltammogram is much smaller than the first reduction peak. Similar effects are seen with other quinones, but none is so pronounced as with 1 and 2. Various attempts to explain this anomalous behavior have been presented in the past, and in the present work, new results have been obtained that rule out the previous explanations. It is shown that a series of addition reactions of the quinone dianion to the anion radical can adequately account for the observed behavior. This conclusion is supported by the ability to simulate the experimental voltammograms both in the absence of added water with no added sodium perchlorate and also with added sodium perchlorate up to a 1:1 ratio of sodium to quinone.

The electrochemical reduction of 7 of the 10 isomeric dicyanonaphthalenes has been studied in N,N-dimethylformamide. The studied compounds were 1,2-, 1,3-, 1,4-, 2,3-, 1,8-, 2,6- and 2,7-dicyanonaphthalene. For most of the isomers, the one-electron reduction to the anion radical occurred as a simple, reversible reduction without complications from coupled chemical reactions. However, for 1,3-, 2,3- and 2,7-dicyanonaphthalene, the reduction was affected by a reversible dimerization of the anion radicals. Dimerization equilibrium constants were determined at various temperatures. DFT calculations of the anion radicals of all 10 isomers provided spin densities at ring carbons 1–8. The highest spin density for carbons 1–8 not bearing a cyano group was largest for the three isomers with detectable dimerization. Not only do these calculations suggest the site of dimerization, it was found that the free energy change for dimerization varies in an approximately linear fashion with spin density.Highlights► Study of seven isomers of dicyanonaphthalene reveals two new cases of reversible dimerization. ► Weak dimerization can be detected from the shape of the voltammogram. ► Systems with larger dimerization equilibrium constants show a separate oxidation peak for the σ-dimer. ► Dimerization equilibrium constants correlate with spin density at the site of dimerization.

Cyclic voltammetry has been employed to determine formation constants for the inclusion complexes formed from α- and β-cyclodextrin and three phenolic acids, protocatechuic (PTC), caffeic (CAF) and chlorogenic (CHL) acids. The method required the prior determination of the mechanism of oxidation of the phenolic acids in the absence of cyclodextrin. The study was conducted in unbuffered aqueous 1.0 M KCl using a glassy carbon working electrode. The voltammetric results could be accounted for equally well by the electron transfer/proton transfer/electron transfer/proton transfer (eHeH mechanism) or the electron transfer/proton transfer/proton transfer/electron transfer (eHHe mechanism). Upon addition of cyclodextrin to solutions of one of the phenolic acids, the anodic peak current for oxidation of the free acid was depressed as was the associated cathodic peak. There was also an increase of the anodic-to-cathodic peak potential separation. Simulation of the cyclic voltammograms allowed the extraction of the formation constant for inclusion of the phenolic acid in the cyclodextrin, Kf,4, as well as estimation of the formation constant for the oxidized phenolic acid (o-quinone) with the cyclodextrin, Kf,5. The diffusion coefficients of the complexes were also determined. Values of Kf,4 were determined under the same conditions using microcalorimetry and these results, along with literature values, were compared with those obtained by cyclic voltammetry. Underlying causes of the differences are discussed.Highlights► The mechanism of the electrochemical oxidation of three phenolic acids has been studied. ► The effect of the addition of α- and β-cyclodextrin on the electrochemistry of the phenolic acids was examined. ► Formation constants for the phenolic acid-cyclodextrin inclusion complexes were determined. ► The electrochemical results were in moderately good agreement with those obtained by microcalorimetry.

The [FeFe] hydrogenase enzyme active site inspired complexes [Fe2(μ-C6H4S2)(CO)5PTA] (1PTA) and [Fe2(μ-C6H4S2)(CO)4PTA2] (1PTA2) (PTA = 1,3,5-triaza-7-phosphaadamantane) were synthesized and characterized. The ability of 1PTA and 1PTA2 to catalytically produce molecular hydrogen in solution from the weak acid acetic acid was examined electrochemically and compared to previous studies on the all carbonyl containing analogue [Fe2(μ-C6H4S2)(CO)6] (1). Computational methods and cyclic voltammograms indicated that the substitution of CO ligands by PTA in 1 resulted in markedly different reduction chemistry. Both 1PTA and 1PTA2 catalytically produce molecular hydrogen from acetic acid, however, the mechanism by which 1 and 1PTA and 1PTA2 catalyze hydrogen differ in the initial reductive processes.

The reduction of trans  -azobenzene, Azb, has been studied in acetonitrile in the absence and presence of added water. The first step of reduction involves formation of the anion radical, Azb-Azb-, and in the second Azb-Azb- is reduced to Azb2− followed by rapid protonation by water to give the monoanion of hydrazobenzene, AzbH−. This second step of reduction was a candidate for a concerted proton–electron-transfer reaction (CPET) but it turned out instead to be electron transfer followed by proton transfer. From studies of the formal potential of the first step as a function of water concentration, formation constants for the 1:1 (Azb(H2O))(Azb(H2O)) and 2:1 (Azb-(H2O)2)(Azb-(H2O)2) hydrogen-bonded complexes were evaluated. Proton transfer from water to Azb-Azb- occurs leading to AzbHAzbH, which in turn is reduced to AzbH−. Kinetic analysis of the voltammetric results indicates that this proton transfer occurs within the 3:1 hydrogen-bonded complex, (Azb-(H2O)3)(Azb-(H2O)3), a species with a very small formation constant, 0.3 M−1. Two other derivatives, trans-4,4′-dichloroazobenzene and trans-4-dimethylaminoazobenzene were studied briefly and found to behave similarly to Azb.

The electrochemical reduction of nine quinones has been studied in acetonitrile and, in one case, in dimethyl sulfoxide. Included are seven hydroxyquinones plus 1,4-naphthoquinone, 8, and 9,10-anthraquinone, 9. Each quinone is reduced in two steps, first to the anion radical and then, at more negative potentials, to the dianion. However, digital simulations show that the voltammetric data cannot be explained by these two reactions alone. For three of the hydroxyquinones, 2−4, plus 8 and 9, the fast disproportionation/comproportionation reaction connecting the neutral, anion radical, and dianion must be included along with a diffusion coefficient of the anion radical that is smaller than that of the neutral quinone and a still smaller diffusion coefficient for the dianion. Three other hydroxyquinones, 5−7, require, in addition, the formation of a neutral−anion radical complex. Finally, 5-hydroxy-1,4-naphthoquinone, 1, in acetonitrile involves both σ- and π-dimerization of the anion radical with subsequent reduction of the π-complex, whereas in dimethyl sulfoxide the σ-dimer is replaced by the aforementioned formation of the neutral−anion radical complex. The differences in behavior are discussed in terms of distribution of spin density in the anion radicals and intermolecular and intramolecular hydrogen bonding.

A new synthetic method for annulating hydroquinones to Fe2S2(CO)6 moieties is reported. Piperidine catalyzed a multistep reaction between Fe2(μ-SH)2(CO)6 and quinones to afford bridged adducts in 26−76% yields. The hydroquinone adducts undergo reversible two-electron reductions. In the presence of acetic acid, hydrogen is produced catalytically with these adducts at potentials more negative than that of the initial reversible reduction. Spectroscopic studies suggest the presence of intramolecular hydrogen bonding between the phenolic OH groups and the adjacent sulfur atoms. Computations, which are in good agreement with the electrochemical studies and spectroscopic data, indicate that the hydrogen bonding is most important in the reduced forms of the catalysts. This hydrogen bonding lowers the reduction potential for catalysis but also lowers the basicity and thereby the reactivity of the catalysts.

The electrochemical reduction of the title compounds proceeds in two steps in N,N-dimethylformamide. The first step is a one-electron reduction to form the anion radical of the benzophenone, a reaction which is affected by the presence of hydroxylic additives such as water, ethanol, or methanol because of hydrogen-bond formation between the additive and the anion radical. Formation constants for these complexes have been evaluated for both protiated and deuterated versions of the additives. It was found that the formation constants for the protiated additives were about 20% larger than for the deuterated additives. For benzophenone, the second step of reduction in the presence of water proceeds by a concerted proton−electron transfer reaction with the electron passing from the electrode to the hydrogen-bonded complex with proton transfer from water to the oxygen atom of benzophenone anion radical. This conclusion is supported by data that rule out alternative mechanisms, by the small value of the apparent value of the transfer coefficient and by agreement between experimental and calculated reorganization energies for the process. On the other hand, the second stage of reduction of 4-cyanobenzophenone proceeds by initial electron transfer followed by proton transfer from water to the dianion. The overall mechanism includes formation of 1:1, 1:2, and 1:3 dianion:water complexes with the proton transfer occurring within the 1:3 complex. 4,4′-Dicyanobenzophenone shows little evidence for proton transfer to the dianion, but the formation of the dianion:water complexes is important.

In 2001, Lehmann and Evans (J. Phys. Chem. B, 2001, 105, 8877–8884) reported that the electrochemical reduction of a hydrogen-bonded complex between a proton donor and the anion radical of 3,5-di-tert-butyl-1,2-benzoquinone in acetonitrile proceeded by a concerted proton-electron transfer (CPET) reaction in which electron transfer from the electrode and proton transfer from proton donor to the quinone moiety occurred concertedly. Support for this conclusion was based upon ruling out both of the competing two-step processes, electron transfer followed by proton transfer (EP) and proton transfer followed by electron transfer (PE). In the course of studies of related compounds it was decided to reinvestigate the reduction of 3,5-di-tert-butyl-1,2-benzoquinone. It was discovered that the earlier conclusion that a CPET reaction was occurring was tenable only for the particular electrolyte that was used, tetrabutylammonium hexafluorophosphate and for lower concentrations of the quinone. Even the small change of carrying out the reduction of the quinone in the presence of water with tetramethylammonium hexafluorophosphate as electrolyte, produced voltammograms with clear signatures that the process was EP rather than CPET. Even more dramatic effects were seen with cesium, potassium or sodium ions in the electrolyte. A general reaction scheme to explain results with all electrolytes will be presented.

The [FeFe] hydrogenase enzyme active site inspired complexes [Fe2(μ-C6H4S2)(CO)5PTA] (1PTA) and [Fe2(μ-C6H4S2)(CO)4PTA2] (1PTA2) (PTA = 1,3,5-triaza-7-phosphaadamantane) were synthesized and characterized. The ability of 1PTA and 1PTA2 to catalytically produce molecular hydrogen in solution from the weak acid acetic acid was examined electrochemically and compared to previous studies on the all carbonyl containing analogue [Fe2(μ-C6H4S2)(CO)6] (1). Computational methods and cyclic voltammograms indicated that the substitution of CO ligands by PTA in 1 resulted in markedly different reduction chemistry. Both 1PTA and 1PTA2 catalytically produce molecular hydrogen from acetic acid, however, the mechanism by which 1 and 1PTA and 1PTA2 catalyze hydrogen differ in the initial reductive processes.