Cyclic voltammetric determination of free radical species from nitroimidazopyran: a new antituberculosis agent

PA-824 (2-nitro-6-(4-trifluoromethoxy-benzyloxy)-6,7-dihydro-5H-imidazo[2,1-b][1,3]oxazine) is being tested as antituberculosis drug. Little is known on the action mechanism of PA-824; however the reduction of the nitro group seems to be a key step in the metabolic activation, as is observed for the well-known bactericidal metronidazole. Consequently, this paper is focused on the cyclic voltammetric behavior of PA-824 with the aim of revealing the formation and stability of the corresponding nitro radical anion and its comparison with the metronidazole behavior.Both compounds PA-824 and metronidazole reveal, in aprotic medium (DMSO + 0.1 tetrabutylammonium hexafluorophosphate), a similar reduction pattern showing a well-resolved couple due to nitro reduction to form the corresponding nitro radical anion. The electrode reaction obeys an EC₂ mechanism with a dimerization reaction as the chemical step in aprotic medium. Using cyclic voltammetry theory for a dimerization reaction we have calculated the second-order decay constants, k_2,dim, and the half-life time, t_1/2, for the nitro radical anions formed from PA-824 and metronidazole. We have obtained k_2,dim values of 2.22 × 10² and 2.58 × 10⁴ M^?1s^?1 for metronidazole and PA-824, respectively. Our voltammetric results show that the PA-824 nitro radical anion requires more energy for formation (about 200 mV) and it is approximately 100 times less stable than the metronidazole radical anion.     

Voltammetric studies of aromatic nitro compounds: pH-dependence on decay of the nitro radical anion in mixed media

In this report we have chosen ethyl-m-nitrobenzoate (EMNB) as a prototype of a nitroaromatic compound in order to carry out a detailed cyclic voltammetric study focused on the coupled chemical reaction of the generated nitro radical anion. The study was carried out in mixed media (water+DMF) at different DMF contents and several pH values on both mercury and carbon electrodes. In order to study the coupled chemical reaction it was necessary to choose a narrow pH range between 8 and 10. The coupled chemical reaction follows second order kinetics and we have used Olmstead's procedure to calculate the second order rate constant k₂. The k₂ values are strongly pH dependent. Typical values of k₂=are 1.68×10⁴ l mol^?1 s^?1 and 1.15×10⁴ l mol^?1 s^?1 for 60% DMF, pH 9.5 on mercury and GCE, respectively. Considering an EMNB concentration of 0.1 mM the corresponding half life time values were 0.59 s and 0.86 s.     

Electrochemical reduction of C-4 nitrosophenyl 1,4-dihydropyridines and their parent C-4 nitrophenyl derivatives in protic media

A comprehensive study of the electrochemical reduction in protic media of parent C-4 nitrophenyl 1,4-DHPs and their corresponding nitrosophenyl 1,4-DHPs was carried out. The reduction potentials of synthesized aromatic nitro compounds were compared with those of the corresponding nitroso compounds, concluding that the latter were reduced at significant more positive potentials. Arylhydroxylamines formed in the 4-electrons reduction of nitrophenyl 1,4-DHPs at acid pH can be protonated and further reduced. At pH 12, in some compounds the 1-electron reduction to give the nitro radical anion could be distinguished from the cyclic voltammograms. Ortho-nitro derivatives were reduced at more negative potentials than other meta- and para-nitro derivatives. Nitrosophenyl 1,4-DHPs were reduced on Hg in a quasi-reversible process, involving 2- electrons, 2-protons to give the hydroxylamine derivative. Reduction potentials of the nitrosophenyl 1,4-DHPs having methoxycarbonyl- and ethoxycarbonyl-groups in the 3- and 5-positions were not significantly affected. However, a bulky substituent in those positions, such as the isopropyloxycarbonyl group, produced an increase in peak potential values, making reduction more difficult than the other derivatives.     

Reactions of the nitro radical anion of metronidazole in aqueous and mixed solvent: a cyclic voltammetric study

Cyclic voltammetry was used to investigate the electrochemical reduction of metronidazole (2-methyl-5-nitro-1H-imidazole-1-ethanol) at glassy carbon and gold electrodes at different pHs in aqueous solution as well as in mixed solvent viz., aqueous dimethyl formamide. The electrogenerated nitro radical anion undergoes a disproportionation reaction, the rate constant of which is dependent on the pH, solvent composition and electrode material. The interactions of the nitro radical anion with thymine and cytosine were also investigated using a cyclic voltammetric technique. Both the bases were found to react with metronidazole nitro radical anion. The rate constants for such reactions in aqueous solutions were 3.5 × 10³ and 3.0 × 10³ dm³ mol^?1 s^?1 for thymine and cytosine, respectively.     

Electrochemical behaviour of 2-(4′-hydroxybenzeneazo)benzoic-acid at pyrolytic graphite electrodes

The electrochemical reduction of 2-(4′-hydroxybenzeneazo)benzoic acid (I) has been studied at pyrolytic graphite electrodes in the pH range 2.0–10.4. The cyclic voltammetric behaviour clearly indicated an ECE mechanism in acidic medium in which the two-electron two-proton reduction of I gives the hydrazo derivative. The acid catalysed disproportionation of the hydrazo intermediate was also studied in the pH range 2.0–6.0 and the value of k′/[H⁺] was found to be 1.4 × 10^?2 1 mol^?1 s^?1 The products of the reduction have been isolated and characterized using IR, melting point, mass and related techniques.     

Mechanistic study of the autoxidation of reduced flavin and quinone compounds

The mechanism of the autoxidation of reduced flavin and quinone compounds was investigated through semiquantitative analysis of voltammetric waves of the reduction of dioxygen (O₂) catalyzed by flavin and quinone adsorbed on electrode surfaces, where the redox equilibrium among the oxidized, (flavo)semiquinone and fully reduced states is easily controlled by the electrochemical method. The analysis provided evidence that the semiquinone radical plays a predominant role in the autoxidation. The generation of a superoxide anion radical as a product of the one-electron transfer from the semiquinone to O₂ was confirmed by the reduction of ferricytochrome c. The fact that the catalytic reduction wave of O₂ increases with pH was ascribed to the increase in the semiquinone formation constant. The mechanism of the reoxidation of reduced flavoprotein glucose oxidase with O₂ was also examined. The result supports the semiquinone-dependent one-electron transfer as the mechanism.     

Simultaneous electrochemical and quartz crystal microbalance studies of non-conducting microcrystalline particles of trans-Cr(CO)2(dpe)2 and trans-[Cr(CO)2(dpe)2]+ (dpe = Ph2PCH2CH2PPh2) attached to gold electrodes

Simultaneous cyclic voltammetric double potential step and electrochemical quartz crystal microbalance (EQCM) experiments on water insoluble trans-Cr(CO)₂(dpe)₂ and trans-[Cr(CO)₂(dpe)₂]X (dpe = Ph₂PCH₂CH₂PPh₂, X = Cl^?, Br^? and I^?), attached as an array of microcrystals, have been employed to probe mechanistic aspects of the redox chemistry of the [trans-Cr(CO)₂(dpe)₂]^+/0 process at the electrode |solid| solvent (electrolyte) interface in a variety of aqueous electrolytes. EQCM experiments show that the oxidation of solid trans-Cr(CO)₂(dpe)₂ involves the slow incorporation of non-solvated anions from the electrolyte solution into the solid. Interestingly, on the reverse scan of cyclic voltammetric experiments, EQCM data reveal that some but not all the anions are rapidly expelled from the crystal lattice. Double potential step experiments with the neutral chromium compound confirm that the oxidation reaction is a relatively slow process. The conclusion reached from all experiments is that the reduction process predominantly expels the anions that are relatively close to the solid|solution interface. EQCM investigations of trans-[Cr(CO)₂(dpe)₂]X compounds in electrolytes containing a different anion to that in the compound show that the anion originally in the salt is rapidly replaced by the anion in the aqueous electrolyte at open circuit potential, presumably via a rapid ion exchange process. The anion from the electrolyte is then expelled and incorporated into the solid during the reduction and oxidation steps respectively.     

The cathodic deprotection of the nitrobenzoyl group from phenyl nitrobenzoates in N,N-dimethylformamide

The reduction of phenyl benzoates with nitro substituents at the 2-, 3- and 4-positions of the benzoates in N, N-dimethylformamide is reported. The phenyl 4- and 3-nitrobenzoate are reduced in two cathodic steps. The first one, at about ?0.9 V vs. SCE, a reversible one-electron process, gives a rather stable anion radical. The second reduction step at potentials between ?1.5 and ?2.0 V vs. SCE leads to formation of the dianion, which decomposes giving free phenol in good yields ( > 80%). On the other hand, the phenyl 2-nitrobenzoate is reduced in one cathodic step. This step occurs at ?0.9 V with formation of an unstable anion radical which decomposes via C-O bond cleavage, giving phenol with a yield of ca. 80%. The mechanisms of the reduction of these compounds are discussed.     

Voltammetry in the presence of ultrasound: surface and solution processes in the sonovoltammetric reduction of nitrobenzene at glassy carbon and gold electrodes

The application of power ultrasound in electrochemistry may promote or usefully modify electrode reactions. In the work reported here the four-electron reduction of nitrobenzene in alkaline (pH 13) aqueous media was studied as a model system. The electrochemical reduction is known to follow a complex mechanism [E. Laviron, A. Vallat and R. Meunier-Prest, J. Electroanal. Chem., 379 (1994) 427], involving protonations as well as a dehydration step; both surface and solution pathways for this reduction may be observed [C. Nishihara and H. Shindo, J. Electroanal. Chem., 221 (1987) 245], depending on the nature and state of the electrode. Both under silent and ultrasonic conditions nitrobenzene is reduced on glassy carbon electrodes in a chemically reversible one-electron process followed by an irreversible three-electron reduction. At sufficiently negative potentials the reduction process remains overall four-electron, with phenylhydroxylamine as the major product even at the high current densities obtained with intense ultrasound. Glassy carbon electrodes are shown to be suitable for kinetic studies, although damage, as endorsed by an increase in roughness and capacitance and probably initiated by mechanical damage at very short electrode–horn distances, was detected by a.c. impedance, voltammetric and atomic force microscopy (AFM) methods. On gold electrodes a more complicated mechanism due to a surface reaction pathway arises. A comparison of sonovoltammetric and rotating disk voltammetric results gives evidence for the homogeneous pathway being dominant under applied ultrasound conditions. The transition between surface and solution pathways is mass flux as well as concentration dependent, and an estimate for the rate of the surface catalyzed reaction for a concentration of 1.44 mM nitrobenzene (k = 4±2 × 10^?2 cms^?1), attributed to protonation of the nitrobenzene radical anion, is obtained from combined rotating disk and sonovoltammetric data. Damage to the gold surface as monitored by various techniques is small, but manifests itself by an interesting decrease in double layer capacitance.     

Cathodic reduction of 2-nitronaphthothiophen-4,9-quinone: evidence of catalysis by proton donors and its simulation

2-Nitronaphtho[2,3-b]thiophen-4,9-quinone (1) is biologically active. The reducible groups have conjugate interaction. Electrochemical experiments (cyclic voltammetry and electrolysis) were performed in order to verify possible intramolecular electron transfer or secondary redox systems and to gain insight into the redox behaviour to help in the understanding of its trypanocidal mechanism of action. Cyclic voltammograms of 1 at the Hg electrode, in DMF+TBAP or DMF+TEAP showed the presence of at least three waves, the two first related to quinone reduction and the third one relative to a catalytic process. After cathodic reduction, at potentials close to the third electron uptake, protons from adventitious water or ammonium quaternary salts can be reduced. Hydrogen formation, with the regeneration of the quinone dianion could be the cause of its catalytic nature. This effect is more pronounced with TEAP. Macroscale electrolyses reinforce the findings. This reaction can be hampered by addition of electrophiles to the medium. Simulated curves fit the experimental ones well. The fourth wave, present at fast scan rates, where the catalysis is not effective, is related to further reduction of the nitro radical anion to the hydroxylamino derivative. At the time scale of cyclic voltammetry, no intramolecular electron transfer was observed. The biological activity of 1 is, possibly, related to the very electrophilic quinone group, generating reactive radical oxygen species through redox cycling.     

Electrochemical characteristics of a cobalt pentacyanonitrosylferrate film on a modified glassy carbon electrode and its catalytic effect on the electrooxidation of hydrazine

Electrochemically prepared thin films of cobalt pentacyanonitrosylferrate (CoPCNF) were used as surface modifiers for glassy carbon electrodes. The electrochemical behavior of a CoPCNF-modified glassy carbon electrode was studied by cyclic voltammetry; the modified electrode shows one pair of peaks with a surface-confined characteristic in 0.5 M KNO₃ as supporting electrolyte. The effect of different alkali metal cations in the supporting electrolyte on the behavior of the modified electrode was studied and the transfer coefficient (α) and charge transfer rate constant (k_s) for the electron transfer between the electrode and modifier layer were calculated. The experimental results show that the peak potential and peak current vary with different alkali metal cations, but anions such as Cl^?, NO₃^?, CH₃COO^?, H₂PO₄^?/HPO₄^2? and SO₄^2? at 0.5 M concentration have no effect on the peak potential and peak current. An extensive study showed that the response of the modified electrode is not affected within a pH range of 2–8. The CoPCNF films on glassy carbon electrodes show excellent electrocatalytic activity toward the oxidation of hydrazine in 0.5 M KNO₃. The kinetics of the catalytic reaction were investigated by using cyclic voltammetry, rotating disk electrode (RDE) voltammetry and chronoamperometry. The average value of the rate constant for the catalytic reaction and the diffusion coefficient were evaluated by different approaches for hydrazine.     

Substituent and solvent dependence of the one-electron reduction of 5-substituted-N-methylisatins in aprotic solvents

Potentials for the one-electron reduction of nine 5-substituted N-methylisatins were measured in ten aprotic solvents by cyclic voltammetry. Best-fit Hammett σ substituent constants were determined and used in Hammett plots to determine reaction constants, ρ, for all solvents. Correlations of solvent polarity parameters with the Hammett ρ values and with measured reduction potentials suggest that empirical solvent parameters that reflect the strength of solvent molecular cohesive forces and solvent Lewis acidity are important predictors of the ability of solvent to facilitate the one-electron reduction of isatins in aprotic media by stabilizing the radical anion reduction products.     

Novel studies on the electrochemical oxidation of 2-[4-(N,N-dimethylamino)phenyl]-6-methyl benzothiazole (DPMB) in acetonitrile at platinum electrodes

The electrochemical oxidation mechanism of 2-[4-(N,N-dimethylamino)phenyl]-6-methyl benzothiazole (DPMB) is studied in a 0.1 M N(C₄H₉)₄ClO₄ + acetonitrile (ACN) reaction medium by cyclic (CV) and square wave voltammetries (SWV) as well as by controlled potential bulk electrolysis at platinum electrodes. The primary radical cation formed by the one electron oxidation of DPMB undergoes a deprotonation process, which is the rate-determining step, followed by a radical–radical coupling. On the other hand, an initial quasi-reversible monoelectronic charge transfer mechanism is inferred from cyclic and square wave voltammograms recorded at scan rates and frequencies higher than 0.4 V s^?1 and 40 Hz, respectively. Diffusion coefficients of DPMB at different temperatures were calculated from the quasi-reversible convoluted cyclic voltammograms. DigiSim® and COOL software were used to fit the quasi-reversible cyclic and square wave voltammetric responses, respectively. Formal potentials, formal rate constants and positive transfer coefficients at different temperatures were evaluated from the fitting of cyclic voltammograms. The experimental activation parameters were also determined. The effects of the analytical concentration of the reagent and the temperature, as well as the addition of trifluoracetic acid and a strong base such as lutidine on the electrochemical responses are discussed. A general reaction mechanism as well as probable structures for dimeric products are proposed.Besides, the presence of an acid–base equilibrium in DPMB solutions is also studied by employing UV–Vis spectroscopic measurements at different trifluoracetic acid concentrations. An apparent value of (1.5 ± 0.2) × 10³ M^?1 was estimated for the DPMB basic constant at 20.0 °C     

Electrochemically induced ageing of polyaniline monitored by the changes in its voltammetric response

Experimental results for the ageing of polyaniline films of different thicknesses in 3.7 M H₂SO₄ are presented. The ageing process was monitored by measuring the changes in the voltammetric response. The relation between the changes in the voltammetric peak parameters and the extent of the ageing process is analyzed employing a formal representation of the anodic voltammetric wave. The model considers a contribution of capacitive current together with an electrochemically reversible faradaic process, with interactions between the redox centres. The analysis shows that during the ageing process, only interaction energies between the reduced redox centres change and they become more attractive. Also, it is shown that the extent of the ageing process may adequately be represented by the relative change of the voltammetric peak current. On the other hand, the change of the peak potential is not a direct measure of the extent of the ageing process. It is defined the so-called apparent number of exchanged electrons, n_ap, which shows the same ageing time dependence as the peak current does. The ageing process is interpreted by an Elovich type of kinetic characterized by a pseudo zero order rate constant, k₀, and a parameter, β, that indicates how fast the activation free energy changes with the extent of the process. Values of β and k₀ are given for different polymer thicknesses.     

Electrocatalytic oxidation of NADH at graphite electrodes modified with osmium phenanthrolinedione

We report here a detailed study concerning the electrochemical behavior of Os(4,4′-dimethyl, 2,2′-bipyridine)₂(1,10-phenanthroline 5,6-dione) complex, adsorbed on spectrographic graphite, and about its electrocatalytic activity for NADH oxidation. Cyclic voltammetric measurements, performed in aqueous phosphate buffer solutions, at different scan rates and pH values, allowed us: (i) to relate the redox response of the o-quinone ligand (phendione) to that of the Os(II) central ion; (ii) to confirm that, in aqueous solutions, the phendione based redox process globally involves two electrons and two protons; (iii) to estimate the rate constant for the heterogeneous electron transfer corresponding to the phendione redox couple (k_s≈20.1 s^?1). The second order rate constant for electrocatalytic oxidation of NADH (k_1,[NADH]=0=1.9×10³ M^?1 s^?1, at pH 6.1) as well as its pH dependence (from pH 5.5 to 8.1) were evaluated from RDE experiments, using both Koutecky–Levich and Lineweaver–Burk data interpretations.     

Anion recognition and electrochemical characteristics of the self-assembled monolayer of nickel(II) azamacrocyclic complex

Anionic recognition of the self-assembled monolayer of dinickel(II) (2,2-bis(1,3,5,8,12-pentaazacyclotetradec-3-yl)-diethyl disulfide) perchlorate (1) was studied electrochemically. The dinickel(II) complex 1 adsorbs on gold electrodes from methanol solutions and yields stable, self-assembled electroactive monolayers (SEMs); the SEM of 1 shows a reversible redox wave at 0.82 V in aqueous 0.1 M NaNO₃ corresponding to the Ni^3+/2+ redox reaction. The surface coverage, Γ, of the self-assembly of 1 determined by cyclic voltammetry is constant (Γ=(1.4±0.08)×10^?10 mol cm^?2) with change in the deposition time (2–36 h) and the concentration of 1 in methanol solution (0.2–5 mM) and is equivalent to a monolayer coverage of the nickel macrocyclic complex. The capacitance of the monolayer of 1 was determined from the double-layer capacitance measurements by chronoamperometry; the monolayer of 1 is assigned to be well-solvated by observing that the dielectric constant of the self-assembly domain (?_film=74) is nearly equal to that of water (?_water=78). Electrochemical investigations reveal that the monolayer of 1 can sense electrochemically various non-electroactive anions, NO₃^?, CF₃COO^?, SO₄^2?, H₂PO₄^?, HPO₄^2?, ClO₄^?, PF₆^? and SCN^?, from the variation of the formal potential, E°′, in aqueous solutions of different anions. The E°′ of the monolayer of 1 is 0.82 V in aqueous 0.1 M NaNO₃ and shifts to a less positive potential, 0.55 V, in aqueous 0.1 M Na₂SO₄; the shift in the E°′ was reversible on exchanging the monolayer of 1 between 0.1 M NaNO₃ and 0.1 M Na₂SO₄. The shift in the E°′ of the monolayer has been explained by an axial coordination of electrolyte anions with the trivalent nickel ion. The redox reaction of the SEM of 1 is not observed in aqueous solutions of 0.1 M NaClO₄ and 0.1 M NaSCN; but the redox activity was retained on changing the monolayer electrode to an aqueous solution of 0.1 M Na₂SO₄ or 0.1 M NaNO₃. The monolayer of 1 could detect electrochemically the biologically important phosphate anion, adenosine triphosphate (ATP), at submillimolar concentrations; on addition of 1 mM ATP, the formal potential of the monolayer shifts towards the less positive potential region by about 250 mV. The CVs of the SEM of 1 were recorded in aqueous solutions containing different concentrations of NaH₂PO₄ or Na₂SO₄, keeping the ionic strength of the electrolyte solution constant with added NaNO₃. The E°′ of the monolayer shifts to the less positive potential region with an increase in the concentration of H₂PO₄^? or SO₄^2? anion in solution phase, and the analysis of cyclic voltammetric results reveals that the nickel(III) complex forms a 1:1 complex with SO₄^2? anion but a 1:2 complex with H₂PO₄^? anion.     

Raman and UV–vis spectroscopic study of polyaniline membranes containing a bulky cationic additive

Electrically conducting soluble polyaniline (PANI), containing different amounts of a bulky lipophilic cationic additive, tridodecylmethylammonium chloride (TDMACl), was studied by Raman (λ_exc=780 nm) and UV–vis spectroscopy. PANI was made simultaneously electrically conducting and soluble with bis[4-(1,1,3,3-tetramethylbutyl)phenyl]phosphoric acid in dichloromethane. The PANI membranes were prepared by drop casting on glassy carbon or ITO substrates. Raman and UV–vis measurements were carried out in a 0.1 M CaCl₂ solution at potentials between 400 and ?600 mV (vs. SCE) at pH 6, or alternatively at the open circuit potential at pH 10. The results of Raman, UV–vis and cyclic voltammetric measurements confirm that the incorporation of TDMACl into the PANI membrane facilitates the oxidation and reduction of PANI.     

An in situ UV-vis and IR spectroelectrochemical study of the deposition of a hydroquinone anion salt on platinum electrodes from dichloromethane solutions

Cyclic voltammetry with a platinum electrode of hydroquinone (BQH₂) solutions in dichloromethane, containing tetrabutylammonium perchlorate supporting electrolyte, shows a sharp asymmetric irreversible oxidation peak at about ?0.3 V (SCE). This feature is seen, in addition to the expected features in this system, when the cycle is extended to potentials more negative than ?0.6 V (SCE). Cyclic voltammetry, in situ UV-vis and infrared spectroelectrochemistry have shown that hydroquinone anion (BQH^?) is formed at negative potentials and this appears to arise via surface decomposition of hydroquinone to p-benzosemiquinone (BQH) followed by reaction of BQH with the p-benzoquinone radical anion (BQ^?). The sharp asymmetric peak in the cyclic voltammograms is due to oxidation of the hydroquinone anion in the insoluble tetrabutylammonium salt on the electrode surface. The oxidation of BQH^? appears to occur via disproportionation of (BQH) and leads to BQH₂ and p-benzoquinone (BQ) as the products.     

Voltammetric characterisation of the radical anions of 4-nitrophenol, 2-cyanophenol and 4-cyanophenol in N,N-dimethylformamide electrogenerated at gold electrodes

The electrochemical reductions of 4-nitrophenol, 2-cyanophenol and 4-cyanophenol were studied in N,N-dimethylformamide (DMF) at gold macro- and microdisk electrodes. It was inferred at the gold macroelectrode that the reduction of the parent molecule to the corresponding radical anion is chemically irreversible due to a rapid protonation process in which the basic radical anion reacts with the acidic parent compound. The use of microdisk electrodes and fast scan cyclic voltammetry (144 kV s⁻¹) for the reduction of each of the three compounds of interest failed to outrun the kinetics of the protonation reaction, suggesting a fast process with a second-order rate constant in excess of 1 × 10⁷ M⁻¹ s⁻¹.     

Electrochemical reduction of (5-etoxycarbonylmethylidene-4-oxothiazolidine-2-ylidene)-N-phenylethanone in aprotic medium: A spectroelectrochemical approach

Electrochemical reduction of (5-etoxycarbonylmethylidene-4-oxothiazolidine-2-ylidene)-N-phenylethanone has been studied in dimethylsulfoxide (DMSO) by cyclic and linear voltammetry with both, stationary and rotating electrode coupled with UV–Vis and EPR spectroelectrochemistry in order to identify the intermediates and to elucidate the reaction mechanism. The results point out an ECE mechanism, with deprotonation of the substrate by the electrogenerated base (EGB) anion radical (self-protonation) as the chemical step following the first electron transfer. The proposed reduction mechanism is supported by DigiSim simulations. The EPR spectrum recorded during the electrochemical reduction in DMSO at the potential of the second redox couple is well resolved, confirming the reduction of (5-etoxycarbonylmethylidene-4-oxothiazolidine-2-ylidene)-N-phenylethanone to its corresponding dianion radical. Solvent dependent semiempirical PM3-MO calculations allow a rationalization of the experimental results in terms of the electronic structure and reactivity of the intermediate species.