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     

Mechanistic investigation of xanthene oxidation by heterogeneous and homogeneous electron transfers

Anodic oxidation of xanthene is investigated in acetonitrile at a platinum electrode by means of cyclic voltammetric and exhaustive potentiostatic electrolysis techniques. On the voltammetric scale time, the process involves two electrons and leads to xanthydrol. The corresponding mechanism is an ECE (electron–deprotonation–electron) type electrode reaction, the rate-determining step being the deprotonation of the cation radical obtained after the first electron transfer. On the other hand the analysis of the oxidation by homogeneous redox catalysis is carried out, using three organic catalysts. This allows the determination of the rate constants of the homogeneous electron transfers between xanthene and catalysts, the xanthene cation radical deprotonation rate constant and the standard potential of the xanthene cation radical/xanthene couple.     

Two-electon-transfer redox systems: Part 6. Two-electron oxidation of hexakis(benzylthio)benzene—a study by electrolysis and cyclic voltammetry

The anodic oxidation of the oligo-thioether hexakis(benzylthio)benzene in a dichloromethane+NBu₄PF₆ electrolyte is investigated by several complementary techniques: cyclic voltammetry, ESR spectroscopy, and macroscopic electrolysis (including fractional electrolysis in combination with the determination of open circuit potentials, i.e. ‘potentiometric titration’). While ESR spectroscopy proves the formation of a π-delocalized radical cation, the electrochemical techniques indicate further oxidation to a dicationic stage, which undergoes follow-up reactions. Analysis of fractional electrolysis results clearly shows strong potential compression for the two electron transfer steps with a difference of formal potentials ΔE°=23 mV. This value is succesfully used to simulate the cyclic voltammograms of the starting compound.     

A comparative study of the surface electrochemical oxidation reactions of α-, β-, and γ-aminobutyric acid at platinum

The electrochemical oxidation reactions of α-, β- and γ-aminobutyric acid at a Pt electrode were investigated in aqueous solutions at pH 1 and pH 13 using steady-state current—potential measurements, cyclic voltammetry and open-circuit potential decay. The capacitance behaviour and the high Tafel slopes suggest the production of free radicals at the surface of the electrode accompanied by a second reaction involving loss of CO₂ which is the rate-determining step. The adsorbed intermediate species is either hydrolysed anodically to propionaldehyde or produces the aldehyde by the formation of a carbonium ion which is subsequently hydrolysed in solution. The mechanisms are compared with those observed in our earlier investigations of the electrochemical oxidation reactions of glycine and α- and β-alanine. Formation of propionaldehyde and other aldehyde products was confirmed by polarographic measurements, and no dimerized products were detected by gas chromatography. This behaviour differs from the dimerization process typical of the radical reactions associated with the Kolbe mechanism.     

Scanning electrochemical microscopy (SECM): Study of the formation and reduction of oxides on platinum electrode surfaces in Na2SO4 solution (pH = 7)

The formation and reduction of oxides on polycrystalline platinum were studied in a neutral solution with a scanning electrochemical microscope (SECM). Experiments were carried out with tip—substrate voltammetry where the faradaic current flowing to the tip is recorded while cycling the potential of the substrate, and with tip—substrate chronoamperometry where the faradaic tip current is recorded against time following the application of a potential step to the substrate. The tip current was made pH sensitive by holding the tip potential in a region where a pH dependent reaction occurs. Hydrogen evolution was used to probe pH decreases, oxygen evolution was used for the detection of pH increases and Pt oxide formation was used to detect both pH increases and decreases. The results showed that oxide formation occurs in two stages, each involving the transfer of electrons and the release of protons into the solution. During the first stage the release of H⁺ precedes the transfer of electrons. while in the second stage H⁺ release and electron transfer proceed simultaneously. Results are analysed in terms of the formation of PtOH during stage I and PtO during stage 2. However stage 2 behaves differently under slow potential changes and the release of protons lags behind the transfer of electrons. This is interpreted as the result of a place-exchange mechanism from PtOH to HOPt prior to stage 2, followed by the oxidation of HOPt to OPt during stage 2. Similarly, Oxide reduction was found to occur in two stages, each involving the transfer of electrons and the consumption of protons. During the first stage, the consumption of H⁺ precedes the transfer of electrons. The results suggest that during the transfer of electrons, protons diffuse from the outer layer of the oxide (OPt) into the inner layer to form HOPt. For the second stage results are analysed in terms of a place exchange mechanism from HOPt to PtOH running in parallel with the consumption of H⁺ followed by a surface reduction from PtOH₂⁺ to Pt metal.     

On the mechanism of oxidative electropolymerization and film formation for phenanthroline-containing complexes of ruthenium

New experiments involving electrochemistry, Auger spectroscopy and elemental analysis of films and film-forming compounds, as well as X-ray crystallography and electrochemistry of a key model compound, have been used to elucidate the linkage structure and probable mechanism of formation of oxidatively generated polymeric and copolymeric films of phenanthroline-containing complexes of ruthenium. Film formation evidently involves indirect electrochemical activation of coordinated 1,10-phenanthroline to nucleophilic attack by the non-coordinated pyridyl nitrogen of a dipyridyl ligand singly coordinated to a second metal center. The resulting linkage is comprised of a carbon/nitrogen bond with an excess positive charge residing on the nitrogen atom. Electrochemical oxidation of a complexed metal ion (typically Ru(II)) drives the polymerization by: (a) rendering the 4 site of phenanthroline significantly electrophilic, and (b) providing an oxidizing equivalent for eventual H atom elimination and pyridinium ion formation via an internal electron transfer sequence.     

Mechanism and kinetic analysis of the photo-induced electron transfer mediated by a stacked metallotriporphyrin in planar lipid bilayers

The photo-induced transmembrane charge transfer across planar lipid bilayers sensitized by a stacked ZnCuZn triporphyrin and enclosed between two aqueous phases is shown to occur via monomers spanning the membrane. The first step of the proposed mechanism involves the oxidation of the excited state of the porphyrin ring which faces the aqueous oxidizing phase. The positive charge then shifts from the resulting radical cation toward the porphyrin ring facing the reducing phase. This mechanism for the intramolecular electron transfer is preferred to the one based on the formation of a charge-separated state such as P⁺—P^?—P. The reaction scheme selected is supported by fluorescence and UV—visible absorption data. The kinetic equations derived fit the experimental time course of the photocurrents and photopotentials. A numerical simulation provides a lower estimation of the rate constant of the photo-induced intramolecular electron-transfer step.     

Electrooxidation of quercetin at glassy carbon electrode studied by a.c. impedance spectroscopy

The present paper reports cyclic voltammetry and a.c. impedance spectroscopy studies on adsorption and electrooxidation of quercetin (3,3′,4′,5,7-pentahydroxyflavone) compound at glassy carbon electrode surface in 0.1 M sodium acetate–acetic acid buffer in 90% methanol solution. The resulted information provided support for a cascade electrooxidation mechanism, which process commences with oxidation of catechol hydroxyl groups and involves strongly adsorbed reaction intermediate. The significance of each oxidation step is explained through associated charge-transfer resistance (derived for all individual oxidation steps and electrosorption of quercetin) and capacitance parameters. This work also presents an original way to regenerate the surface of glassy carbon electrode (after being blocked by quercetin oxidation products) through voltammetric cycling over the potential range negative to the hydrogen reversible potential. The above is realized by means of in situ evolved hydrogen, which species is capable of electrochemically reducing products formed during the cascade electrooxidation reaction of quercetin.     

Thermodynamic and kinetic basicities of the carbon and oxygen ends of enolates. Their role in the reductive cleavage electrochemistry of α-substituted acetophenones

The electrochemical reductive cleavage of α-substituted acetophenones may follow a mechanism in which electron transfer and bond breaking are concerted, as with α-chloro-acetophenone, or a mechanism where the two steps are successive, as with, e.g. α-benzoyloxy–acetophenone. In both cases, the resulting phenacyl radical is immediately reduced, giving rise to the phenacyl enolate, the protonation of which is expected eventually to yield acetophenone. However, in cyclic voltammetry, the acetophenone wave, present at low scan rates, vanishes upon raising the scan rate. The disappearance of the wave is observed at lower scan rates when an acid, such as phenol, is added to the solution. This surprising behavior is the result of the oxygen end of the enolate being a thermodynamically weaker but kinetically faster base than its carbon end.     

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.     

Electrochemical behaviour of iron in molten enamel by means of cyclic,square wave and hydrodynamic voltammetry

The electrochemical behaviour of iron present in molten enamel is studied at a platinum disc electrode with cyclic and square wave voltammetry and at a platinum rotating disc electrode using chronoamperometry. Fe(III) is reduced in two steps to a Pt–Fe alloy, which disturbs the oxidation behaviour of Fe(II) considerably. Proof is shown for the formation of the alloy. Indications were found that the chemical equilibrium between Fe(III) and Fe(II) in homogeneous solution is attained slowly, which means that Fe(III) actual concentrations are measured instead of analytical concentrations. A consequence is that the diffusion coefficients determined with cyclic and/or square wave voltammetry and reported in the literature must be corrected. This was shown by using rotating disc chronoamperometry.     

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.     

Hydrogen bond complexation of dimethyl-[1-butyl-2,4-dioxo(1H,3H)pyrimido]tetrathiafulvalene with aminopyridine derivatives probed by cyclic voltammetry

The cyclic voltammetric response of dimethyl-[1-butyl-2,4-dioxo(1H,3H)pyrimido]tetrathiafulvalene 1 is studied in the presence of aminopyridine derivatives. The first oxidation potential of 1 increases with an increase in 2,6-di(N-acetylamino)pyridine concentration, with the maximum shift being 30 mV. This is assigned to the formation of a multihydrogen-bond complex in the neutral state with the binding constant estimated to be ca. 1000 M^?1. An irreversible following chemical reaction is suggested for the explanation of the complicated behaviour of the second redox wave of 1 in the presence of 2,6-di(N-acetylamino)pyridine.     

Square-wave voltammetry of quasi-reversible electrode processes with coupled homogeneous chemical reactions

The electrochemical behaviour of systems complicated by electrode kinetic and quasi-reversible preceding or following homogeneous chemical reactions under square-wave voltammetry (SWV) conditions is analysed theoretically. The results are discussed in detail, considering the influence of rate and equilibrium constants, together with experimentally controlled parameters such as f and E_sw. Both kinetic stages act synergistically for the case of a CE mechanism, but the EC reaction scheme exhibits more complex behaviour, especially for reversible and quasi-reversible electrochemical reactions, which present a minimum response of current for the quasi-reversible chemical reactions. These curves are characteristic for each system, providing not only the bases for their distinction but also for the extraction of kinetic and thermodynamic information.     

Impedance voltammetry of electro-dimerization mechanisms: Application to the reduction of the methyl viologen di-cation at mercury electrodes and aqueous solutions

The faradaic impedance for an electrode mechanism with a reversible homogeneous dimerization reaction following the electron transfer step is derived. The chemical reaction shows up in the frequency dependence of the faradaic impedance and admittance in a similar way as deduced by Sluyters-Rehbach and Sluyters (J. Electroanal. Chem. 23 (1989) 457; J. Electroanal. Chem. 26 (1990) 237) for a homogeneous first-order chemical reaction. Two limiting cases can be distinguished in which the general expression reduces to the simpler Randles or pseudo-Randles expression. Under those conditions, the presence of the dimerization reaction can be inferred from the potential dependence of the impedance parameters. The theory is applied to the reduction of the methyl viologen di-cation at mercury electrodes in aqueous solution. The rate and the equilibrium constants for the dimerization reaction and the standard potential for the electron transfer step are obtained from the Warburg coefficient, while the potential dependence of the irreversibility coefficient allows the calculation of the standard rate constant and the transfer coefficient for the electron transfer step.     

Investigation of the electrochemical reduction mechanism of 1-[N-(2-pyridyl)aminomethylidene]-2(1H)-naphthalenone (PN)

The electrochemical reduction mechanism of 1-[N-(2-pyridyl)aminomethylidene]-2(1H)-naphthalenone (PN) was investigated by using various electrochemical techniques in 0.1 M tetrabutylammonium tetrafluoroborate (TBATFB) in acetonitrile at glassy carbon (GC) electrode. The number of electrons transferred and diffusion coefficient of the compound were estimated by using an ultramicroelectrode (UME). PN has two reduction peaks in a cyclic voltammogram. Each of them corresponds to a one-electron transfer. In this medium and at the GC electrode surface, the first peak was observed at about ?1.8 V (vs. Ag/Ag⁺) which is more stable and well defined compared to the second peak. It was determined by using cyclic voltammetry that PN is electroreduced by an EC mechanism. It was also explained by multicycle CV experiments and a dopamine test as well as by Raman spectroscopy that the homogenous chemical reaction is dimerization. The EC mechanism was also examined and the kinetics of this process were calculated by digital simulation.     

Self-protonation mechanism in the electrochemical reduction of Jatropholone

The electrochemical reduction mechanism of jatropholone (JOH) and derivatives (JOAc) was investigated by cyclic voltammetry, polarography, coulometry and controlled potential electrolysis. It involves at the first wave potential, a self-protonation mechanism, whereby the two-electron reduction product (JOHH^?), an aromatic ketone, is formed together with the conjugate base of the former compound, JO^?. The unusual stoichiometry of the cathodic process at the first wave, 1 mol electron mol^?1, indicates that JOHH^? is not further protonated. The decreased basicity of the anion is due to resonance stabilisation. The second wave is related to the reduction of JO^? and further reduction of JOHH^?, that has a reminiscent electroactive group. In the latter case, the background electrolyte is liable to be the proton donor, through Hoffman elimination. The complete reduction involves 4 e^? and 4 protons. Electrolysis performed near the first wave potential led to the almost exclusive reduction of the exocyclic double bond, without dimerization.     

Structure and electrochemical behavior of a flavin sulfide monolayer adsorbed on gold

The electrochemical behavior and structure of monolayers of the synthetic flavin 10-(3′-methylthiopropyl)-isoalloxazinyl-7-carboxylic acid adsorbed on gold is reported. The redox behavior of the compound is quasi-reversible. The surface concentration is estimated to be 1.5×10^?10 mol cm^?2. An electron transfer rate constant of 340 s^?1 is estimated for the cathodic process and a value of 540 s^?1 is estimated for the anodic process. The reduction potential of the monolayer is found to shift with pH as expected for a 2e^?, 2H⁺ process. The monolayers have also been characterized by X-ray photoelectron spectroscopy (XPS) and low voltage field emission secondary electron microscopy (LVFESEM). The most likely orientation would have the long axis parallel to the surface with the carboxyl group exposed to the solution. A comparison of the C1s XPS spectra at glancing and normal emission indicates that the carboxyl group is at the film surface. The LVFESEM images indicate that the molecules pack in domains that do not follow the topology of the gold grains. Semi-quantitative examination of the micrographs shows that 10–15% of the gold surface is uncovered. The bare gold substrate catalyzes the oxidation of NADH; the presence of the flavin film reduces the observed catalysis by the gold substrate.     

Catalysis by immobilized redox enzymes. Diagnosis of inactivation and reactivation effects through odd cyclic voltammetric responses

Odd cyclic voltammetric responses, with an inverted peak appearing on the reverse scan, have been recently reported for the catalysis of immobilized enzymes involved in a direct electron transfer at the electrode surface and implicated in a chemical inactivation/redox reactivation mechanism. In this work, it is shown that this twisted reverse trace behavior can be related rigorously and quantitatively to such a reaction scheme by means of a minimal number of dimensionless parameters. As a prelude, the requirements for ‘pure catalytic’ conditions to be achieved are established quantitatively. It is also shown that simple irreversible or reversible inactivation does not entail the appearance of twisted reverse traces. The quantitative analysis of the inactivation/reactivation mechanism does not lead to a closed form expression of the current responses, but rather requires the numerical resolution of the pertinent differential equations. This approach may be readily extended to virtually any kind of mechanism, including more complex reactions schemes, distance-dependent electron transfer kinetics, the use of immobilized or free-moving redox cosubstrates, consideration of substrate mass transport limitations, etc.     

Electrochemical reduction of N-thioamidoimidates: application to the synthesis of thiazolo[5,4-d]thiazoles

The reduction of N-thioamidoimidates (1) has been examined in aprotic media at a mercury electrode. As shown by cyclic voltammetry at fast scan rates and controlled potential electrolysis, an overall irreversible one electron transfer is followed by a rapid second order chemical reaction leading to a dimer which involves to thiazolo[5,4-d]thiazole by intramolecular cyclization.