Single Two-electron Step Research Articles

Dependence of Quinone Electrochemistry on Solvent Polarity and pH

We studied the polarographic reduction of quinones in aqueous/organic mixtures with dielectric constants (ε) from 78.4 down to 47. Added organic co-solvents were either protic or aprotic. The increase in polarographic half-wave potential, E 1/2, with declining ε was successfully fit to the Born equation down to ε ≈ 55. Cu2+ and ubiquinone0 were reduced in a single two-electron step that was more ε-sensitive when the added organic co-solvent was aprotic. Naphtho- and anthraquinone were reduced in two successive one-electron steps that were influenced identically by protic or aprotic organic co-solvents. The product of the first reduction wave was neutral semiquinone (·QH), which surprisingly, had to be protonated to ·QH2 + before reduction in the second wave. Except for r H+, radii derived from fitting our E 1/2 vs ε results to the Born equation were too small; in other words, non-electrostatic effects destabilized the oxidized species, greatly enhancing the Born electrostatic increase in E 1/2 with declining ε. Additionally, for ε < 55, we observed deviation from the Born equation, which may be due to changes in solvent structure and dynamics, and solvent-solute interactions. Finally, we studied quinones incorporated into phosphatidyl choline sonicated bilayer vesicles: Ubiquinone0 had two distinct irreversible two-electron reduction waves, one due to a population bound at the membrane surface, and another (whose E 1/2 was negatively shifted by 150 mV) due to a population localized in the membrane interior. Ubiquinone10 had a single irreversible two-electron reduction wave that was 250 mV more negative than the UQ0 membrane-interior population.

Free propargyl radical: Facile cathodic generation and covalent linkage to solid surfaces

The cathodic reduction of propargyl bromide in regular polar solvents such as N,N-dimethylformamide or acetonitrile (ACN) results in a two-electron scission of the C Br bond. In contrast to this, the use of solvents of higher dielectric permittivity like propylene carbonate (PC) containing tetraalkylammonium iodide as electrolyte permits the quasi instantaneous formation of the corresponding iodide that can be electrochemically cleaved in a different way. Using gold, platinum, palladium or copper under those conditions allows obtaining the propargyl radical at quite low potentials. Unexpectedly, glassy carbon (GC) as cathode material causes a neat splitting of two one-electron transfers (ET) usually observed for most of organic halides as a single two-electron step. A dense propargylation of these solid surfaces was noticed. The propargyl group surface coverage was checked by FTIR, by its derivatization with alkyl ferrocene using azide-yne click chemistry, and by bromine index (coulometric reduction of halogenated surface-grafted layers); its level was found to be (4–6) × 10 − 9 mol cm − 2 of the apparent starting electrode area.

Experimental evidence of formation of imines in the course of reduction of hydrazones

The reduction of hydrazones is generally suggested to proceed through a reductive cleavage of the nitrogen–nitrogen bond followed by a reduction of the carbon–nitrogen bond. This sequence of reduction processes is here supported for fluorenone (V) and benzophenone (VI) hydrazones as well as by a comparison of the reduction of fluorenone and benzophenone hydrazonium ions (I,III) with corresponding imines (II,IV). Another proof of the presence of imines as intermediates is the splitting of four-electron waves of hydrazones V and VI and hydrazonium ions I and VIII into two waves at pH < 2. This has been interpreted as due to differences in slopes d E 1/2/dpH and p K a-values of protonated hydrazine derivatives on one side and corresponding imines on the other. In this pH-range imines formed in reductions of VI and VIII are reduced in a single two-electron wave, those of I and V in two one-electron steps. Fluorenone imine (II) is sufficiently stable to allow recording of time-independent current–voltage curves between pH 6 and 11. In this pH-range the imine (II) is reduced in two one-electron steps. Benzophenone imine (IV) has been found stable between pH 4.6 and 12. At pH 4.6–8 the reduction of the imine IV takes place in a single two-electron step, at pH 8–12 in two one-electron steps. Final proof of the initial cleavage of the N–N bond is presented by comparison with the reduction of nitrones.

Manganese(II) Oxidation and Mn(IV) Reduction in the Environment—Two One-Electron Transfer Steps Versus a Single Two-Electron Step

The chemistry of electron transfer processes are reviewed using a knowledge of orbital properties and available experimental data. Fe(H2O)6 2+ and Mn(H2O)6 2+ oxidation with O2 are discussed and compared. Mn(H2O)6 2+ oxidation by O2 occurs via an inner sphere process after complexation with inorganic (e.g.; OH−, increase pH) or organic ligands that replace water; whereas Fe(H2O)6 2+ at circumneutral pH occurs via an outer sphere mechanism. An outer sphere electron transfer process is symmetry forbidden for Mn(H2O)6 2+ based on analysis of the frontier molecular orbitals of the reactants. At higher pH, an inner-sphere process is also available for Fe(H2O)6 2+ oxidation as hydroxide and organic ligands replace water and bind Fe(II). The bonding of O2 to Fe(H2O)6 2+ in the precursor complex results in faster electron transfer for the inner sphere process than occurs in the outer-sphere process which occurs at lower pH. The bonding between the reactants in an inner sphere process is likely “end on” bonding for O2 to the metal with a bent M-O-O bond angle. Side-on bonding for O2 to the metal is possible and could lead to two-electron transfers from Mn(II) compounds but requires stabilization of the Mn-O2 bonding with organic ligands such as porphyrins. This would occur as an oxidative addition type reaction where the Mn(II) would give up two electrons to two different orbitals of O2 and increase its local coordination environment. For two-electron transfers during Mn(II) compound oxidation, multinuclear Mn complexes are required. One-electron transfers are more likely to occur during the oxidation of Mn(H2O)6 2+ by O2 and the reduction of MnO2 than two-electron transfers. Both soluble and solid phase Mn(III) species form as intermediates or stable species. From a microbiological viewpoint, Mn(III) compounds are ideal reagents as Mn(III) can act as an electron acceptor forming soluble Mn(H2O)6 2+ or as an electron donor forming insoluble MnO2. One-electron transfers are predicted based on the different spatial characteristics of the d z2 and d x2−y2 orbitals. The d z2 orbital has electron density on all three Cartesian coordinate axes (primarily the z axis) but the d x 2−y2 orbital has electron density only in the xy plane. Adding or losing two electrons simultaneously is not as likely a process but possible. However, O atom transfer can readily account for a two-electron transfer in MnO2 reduction. Better knowledge of the structures of Mn intermediates and of the types of reductant appears to be key for describing whether two one-electron transfer steps or a single two-electron step may be operative during MnO2 reduction.

Electrochemical reduction and oxidation of the antibiotic cefepime at a carbon electrode

The O-methyl oxime grouping in the antibiotic cefepime is reduced in aqueous buffer solution at pH<8 at a carbon electrode. The reduction of the protonated form of the oxime occurs in a single two-electron step. By comparison with anodic wave of 2-aminothiazole it was furthermore shown that the oxidation of cefepime involves the 2-amino group located on the thiazole ring in the side chain on C-7. Both reduction and oxidation peaks can be used for analytical purposes.

Electrochemical Growth of Ag2S on Ag(111) Electrodes. Coulometric and X-ray Photoelectron Spectroscopic Analysis of the Stepwise Formation of the First and Second Monolayers of Ag2S

Stepwise electrochemical growth of the first and second monolayers of Ag2S at a Ag(111) electrode in aqueous hydrosulfide (HS-) solutions is reported. The deposition of the first and second monolayers occurs at different electrode potentials and can be resolved in voltammetric experiments prior to the formation of a bulk Ag2S layer. Oxidative formation of the first Ag2S monolayer occurs by a kinetically facile two-electron, two-step mechanism, involving the one-electron oxidative adsorption of HS- (Ag + HS- ⇌ Ag−SH + e-) followed by a one-electron oxidative phase transition to yield a complete Ag2S monolayer (Ag−SH + Ag + OH- ⇌ Ag2S + H2O + e-). An irreversible wave corresponding to the formation of a second monolayer of Ag2S in a single two-electron step (2Ag + HS- + OH- ⇌ Ag2S + H2O + 2e-) is voltammetrically resolved at slow scan rates (<25 mV/s). Coulometric analysis of the voltammetric data demonstrates that the quantity of electricity (∼164 μC/cm2) consumed during the formation of the first and seco...

Haloperoxidase Activity of Manganese Peroxidase fromPhanerochaete chrysosporium

Manganese peroxidase (MnP) fromPhanerochaete chrysosporiumexhibits haloperoxidase activity at low pH. In the presence of hydrogen peroxide, MnP oxidizes bromide and iodide as measured by the formation of tribromide and triiodide complexes and the halogenation of various organic substrates. The optimum pHs for bromide and iodide oxidation are 2.5 and 3.0, respectively. Transient-state kinetic studies show that the reaction between MnP compound I and bromide or iodide occurs via a single two-electron step process, obeying second-order kinetics. The second-order rate constants for MnP compound I reduction by bromide and iodide are (4.1 ± 0.2) × 103and (1.1 ± 0.1) × 105m−1s−1, respectively, at pH 3.0. MnP brominates a variety of aromatic substrates, including veratryl (3,4-di-methoxybenzyl) alcohol (I) to produce of 2-bromo-4, 5-dimethoxybenzyl alcohol (II). MnP also hydrobrominates cinnamic acid (VI) to produce 2-bromo-3-hydroxyphenylpropionic acid (VII). With 3,4-dimethoxycinnamic acid (III) as the substrate, two bromination products are identified:trans-2-bromo-1-(3,4-dimethoxyphenyl) ethylene (IV) and 2-bromo-3-(3,4-dimethoxyphenyl)-3-hydroxypropionic acid (V). MnP also brominates 1,3-dicarbonyl compounds such as monochlorodimedone and malonic acid. Incubation of MnP with bromide and H2O2in the absence of organic substrates results in enzyme inactivation. MnP binds halides to produce characteristic optical difference spectra. From these spectra, apparent dissociation constants at pH 3.0 are determined to be 0.13, 20, and 45 mmfor fluoride, chloride, and bromide, respectively.

Investigation of the Polarographic Properties and Potential Carcinogenity of Some Hydroxyurea Derivatives by DC Polarography

Polarographic reduction was studied for a series of 7 urea derivatives and the results were used to assess their potential carcinogenity. The polarographic reduction was examined in absolutely anhydrous dimethylformamide by DC polarography. In the conditions applied, the majority of the compounds was reduced within a single two-electron step, only biuret and its formyl derivative were reduced in two one-electron steps. The potential carcinogenity of the substances was assessed based on the tg α value of the slope of dependence of the polarographic wave height on the concentration of α-lipoic acid added as a test substance. For hydroxyurea, which is the only substance in this series for which a carcinogenic activity has been demonstrated, the tg α parameter attained a value of 0.290. Still higher values were obtained for the formyl derivatives - formylbiuret (0.362) and 2-carbamoyl-1-formylguanidine (0.510). So high tg α values warn of a significant potential carcinogenity. The other substances studied exhibited considerably lower tg α values, indicating that their potential carcinogenity will be low.

Electrochemical and spectroelectrocemical studies of dihydro-tetra-azapentacene as a model of polyazaacene

5,14-dihydro-5,7,12, 14-tetraazapentacene (5, 14-dihydro-quinoxalino[2, 3-b]phenazine) has been synthesized and studied as a model for polyazaacene with respect to the electrochemical reduction and oxidation in an aprotic electrolyte. While reduction consists of two separate one-electron steps, the oxidation is a single two-electron step. Both processes are irreversible due to chemical follow-up reactions and may be classified as ECE and EC mechanism, respectively. The reduction is followed by a self-protonation reaction. The oxidation is accompanied by proton loss. Using in situ uv/vis spectroscopy it has been proved that the intermediate products reform the pristine compound in the reverse potential scan. The influence of the self-protonation reaction on the reduction process can be restricted by lowering the concentration of the substance in solution as well as by increasing the scan rate. It can be excluded completely by addition of proton donors. In the presence of trifluoroacetic acid a complex is formed, the reduction of which is reversible. The oxidation of that complex remains irreversible at these conditions.

Polarographic Reduction and Potential Carcinogenity of Synthetic 1,3,5-Triazine Bases and Nucleosides

DC polarographic parameters were measured for a series of 15 synthetic 5-aza compounds derived from cytosine, cytidine, uracil and uridine in nonaqueous (dimethylformamide) solutions. The substances in aprotic media are reduced in a single two-electron step at the mercury drop electrode, except for 5,6-dihydro derivatives of 5-azauracil and 5-azauridine which are reduced in two steps. α-Lipoic acid was added to the solutions of the substances, and the slopes tg α of the plots of diffusion current of the substances versus a-lipoic acid concentration, which can serve as an index of potential carcinogenic activity of the substances measured, were determined. The tg α values of all the compounds studied are low as compared to related substances whose carcinogenic activity has been proved. 5-Azacytidine and 5-azauracil are exceptions exhibiting tg a values of 0.295 and 0.400, respectively. For the former compound, this is consistent with the WHO classification as "probably carcinogenic to humans".

Electrochemistry of end-capped oligothienyls. New insights into the polymerization mechanism and the charge storage, conduction and capacitive properties of polythiophene

The kinetics of anodic coupling to dimers of thiophene oligomers ( n = 3–5), methyl protected at one α-terminal position, is second order in oligomer concentration and evidences high activation enthalpies and negative activation entropies. Activation free energies are linearly related to the inverse of the oligomer length n (the dimerization rate decreases as n is increased). Thin films of methyl end-capped thiophene oligomers ( n = 4, 6, 8 and 10) display reversible oxidations from a single one-electron step (tetramer) to a single two-electron step (octamer and decamer) through two separate one-electron steps (hexamer). ESR indicates strong magnetic dimerization for the one-electron-oxidized hexamer. The close resemblance of the electrochemical and ESR behaviour of the hexamer with that of polythiophene suggests that oxidation of the latter occurs via hexameric spin-dimerized polarons. The conductive and capacitive properties of the end-capped oligomers ( n = 6, 8 and 10) were investigated by in situ conductivity and chronopotentiometry. While conductivity of octamer and decamer is displayed at the two-electron (bipolaron) state, the hexamer, insulating at this state, is conducting at the mixed-valence polaron-bipolaron state; capacitive responses are evidenced at the bipolaron state for the octamer and decamer only. The difference of conductive and capacitive behaviour between the hexamer and the higher oligomers is explained by charge localization in hexameric segments.

Electrohydrodimerization of 4-chloro-3-formyl-2H(1)-benzopyrans in ethanolic solutions: A comparative study

The electrochemical reduction of two β-chlorovinylaldehydes, 4-chloro-3-formyl-2H(1)-benzopyran and β-chlorocinnamaldehyde, was studied in buffered ethanolic solutions. The protonated form of the aldehydes was reduced at pH < 5.00 in a single two-electron step while two- and three-step reductions were observed in the pH range 5.00–7.00 and at pH > 7.00, respectively. It was found that hydrogenoly- sis of the carbon-chlorine bond takes place first, in preference to that of the carbon-carbon double bond and that of the aldehydic group. Macroscale electrolysis of 4-chloro-3-formyl-2H(1)-benzopyran and its substituted compounds in 60% (v/v) ethanol + water mixtures afforded head-to-tail electrohydrodimers whereas the macroelectrolysis of β-chlorocinnamaldehyde gave only oily products.

ChemInform Abstract: Mechanism of Electrooxidation of Substituted Phenols in Aqueous Solutions: Some Podophyllotoxin Derivatives as Models

Etoposide (1) and teniposide (2) proved to be suitable models for oxidation of phenols, because radicals formed by the one-electron oxidation of their phenolates do not dimerize. Mechanism (1)-(6) indicates in which pH range the oxidation occurs in a single two-electron step and in which in two one-electron transfers. The phenolates are more easily oxidized than phenols. Due to a rapid formation of the phenolate ions at the electrode surface, its oxidation is preferred even at pH 3, about seven pH units below the pKa value. Phenoxenium ions formed in the two-electron oxidation eliminate CH3O− ions and form ortho-quinones.

Electrochemistry of podophyllotoxin derivatives: Part II. Oxidation of teniposide and some epipodophyllotoxin derivatives

Oxidation of the anions of teniposide (I) and its aglycone (III) occurs over the pH range from 3 to 12 in two one-electron waves. The loss of the first electron yields a radical, the second a phenoxenium ion. The latter is converted by nucleophilic attack involving hydroxide ions into an ortho-quinone. In the cis-hydroxy acids derived from teniposide (IIa) and etoposide (IIb), the oxidation occurs at pH < 9.5 in a single two-electron step. This difference in behaviour is attributed to a facilitated oxidation of the radical formed in the first electron transfer of the open chain compounds. The primary products of the two-electron oxidation of the hydroxy acids IIa and IIb undergo chemical transformations which do not involve the formation of an ortho-quinone. The difference in reactivity of the product of the one-electron oxidation between the lactones (I, III) and the corresponding hydroxy acids (IIa, IIb) results in the two-electron oxidation of hydroxy acids in the physiological pH range, and in the formation of a relatively stable radical in solutions of the lactones. The absence of a stable radical as well as the impossibility of an ortho-quinone in solutions of the hydroxy acids may be responsible for the smaller antineoplastic activities of these open chain compounds when compared to the corresponding lactones.

A study of the polarographic reduction of methaqualone

Methaqualone [2-methyl-3- o-tolyl-4(3H)-quinazolinone] is reduced at pH 1.5–5 in a single two-electron step. At pH 1.5–3, wave i 1 appears and is gradually replaced by wave i 2 which predomiantes in the solution between pH 4.5 and 6.5. At pH > 5, the height of wave i 2 decreases in the shape of a dissociaton curve with increasing pH. For both waves, the diprotonated form of methaqualone (I a) is reduced to 1,2,-dihydromethaqualone as the final product. In wave i 1, the monoprotonated form I b is further protonated at the electrode surface before it accepts the first electron; in wave i 2, the free base form I accepts two protons at the electrode surface before the first electron uptake. Polarographic curves are complicated by the presence of three waves of catalytic hydrogen evolution. Wave i 1, cata appears at pH < 5, waves i 2,cata at −1.5 V and i 3,cata at −1.7 V at pH > 5. Citrate buffers pH 1.5–3 or Britton-Robinson buffers at pH 2.6–3.6 are most suitable for quantitative work with eitehr d.c. polarography or differential pulse polarography.

Polarographic reduction of aldehydes and ketones: Part XXV. Electroreduction of pyridinecarboxaldehydes in aqueous buffered solutions

The mechanisms of electroreduction of 2-, 3-, and 4-pyridinecarboxaldehydes have been studied in aqueous buffered solutions of pH between 2 and 12 by means of dc and differential pulse polarography, linear-sweep voltammetry and controlled-potential electrolysis. Spectroscopic measurements confirmed that these compounds in aqueous solutions exist in equilibria involving both the hydrated and unhydrated forms. The degree of hydration depends strongly on protonation both of the pyridine ring and of the side-chain. Dehydration reactions producing the aldehyde species occur prior to the electron (and possibly the proton) transfer. The loss of water can take place from a protonated geminal diol form or from a neutral diol, depending on pH. Alternatively, the geminal diol anion can eliminate hydroxide ion. A diprotonated pyridinecarboxaldehyde is the electroactive species at pH < 7, the monoprotonated and unprotonated forms are reduced at higher pH-values. For the 2- and 4-aldehyde base-catalyzed dehydration occurs at pH > 8 manifested by an increase of the total limiting current. In this reaction the rate-determining step is the dissociation of a proton from the geminal diol forming a geminal diol anion, which is followed by a rapid elimination of a hydroxide ion to yield the reducible aldehyde. Reaction schemes involving sequences of hydration-dehydration and acid-base reactions as well as electron transfers at mercury electrodes have been proposed for the three isomers. The predominating processes in individual pH-ranges have been pointed out. Electroreduction of the 3-aldehyde occurs up to pH 10 in a single two-electron step. The 2- and 4-aldehydes are reduced between pH 2 and 12 in two, one-electron steps. Electrodimerization occurs at the potential of the limiting current of the first one-electron wave for the 2- and 4-isomer, similarly as for the 3-isomer at pH > 10, where its reduction occurs in a single one-electron wave. Surface dimerization or reaction of the electrolysis products with the parent aldehyde was considered in interpretation of the reduction of the 4-pyridinecarboxaldehyde at pH > 7. Electropolymerization accompanies the reduction of 2-pyridinecarboxaldehyde in alkaline media.

Electrochemistry of coordination compounds : XII. Electrochemical studies of d 8 σ-organometallic rhodium and iridium complexes

The electrochemical behaviour of d 8 organometallic rhodium and iridium complexes, M(R)(CO)(PPh 3) 2 (R = alkyl, aryl), has been investigated with a mercury electrode in acetonitrile. The reduction proceeds via a single two-electron step, no (or only small) differences in the reduction potential being noted between rhodium and iridium. The comparison with previous findings suggests that this unusual result is related to a specific influence of the organic ligand in promoting the reduction process in such a way that the actual redox molecular orbital can be mainly of ligand-orbital character. Since (i) specific medium effects could not be studied and (ii) studies of the effects of substituents in the benzene ring of some rhodium organometallic complexes were inconclusive, it was not possible to establish the pathways for the electron uptake.

One- vs. two-electron oxidations of tetraarylethylenes in aprotic solvents

The oxidation of dimethylaminophenyl substituted ethylenes and tetra-p-anisyl ethylene at a platinum electrode in methylene chloride and acetonitrile solutions was investigated by cyclic and rotating disk electrode voltammetry, potential step chronocoulometry, controlled potential coulometry and e.s.r. spectroscopy. The results show that the oxidations occur by either two one-electron steps or by a single two-electron step depending upon the structure of the molecule. Factors influencing the reaction path are discussed.

One- vs. two-electron oxidations of tetraarylethylenes in aprotic solvents

The oxidation of dimethylaminophenyl substituted ethylenes and tetra- p-anisyl ethylene at a platinum electrode in methylene chloride and acetonitrile solutions was investigated by cyclic and rotating disk electrode voltammetry, potential step chronocoulometry, controlled potential coulometry and e.s.r. spectroscopy. The results show that the oxidations occur by either two one-electron steps or by a single two-electron step depending upon the structure of the molecule. Factors influencing the reaction path are discussed.

Electrochemical behaviour of 1,1′-dioxy-2,2′-diphenyl-Δ3.3′-bi-3H-indole in dimethylformamide. effect of protonating agents of various strengths

The electrochemical reduction of 1,1′-dioxy-2,2′-diphenyl-Δ 3.3′ -bi-3H-indole (I) in dimethylformamide (DMF) solution has been studied at mercury and platinum electrodes in the presence and absence of protonating agents. Voltammetric, controlled-potential coulometric, e.s.r. and u.v. spectroscopic techniques were employed in the study. In an aprotic medium (DMF with tetraethylammonium perchlorate as supporting electrolyte) the reduction proceeds in two reversible one-electron steps: first to a very stable anion radical, identified by e.s.r. spectrum, and then to the di-anion. In the presence of a protonating agent (BH) (p K BH d >15) the second reduction step shifts to more positive potentials, while the first is unchanged. With a large amount of (BH) a single two-electron step is observed, the corresponding 1,1′-di-hydroxy-2,2′-diphenyl-3,3′-bi-indole (II) being the reduction product. In the presence of a stronger protonic source (AH) (p K AH d <15) a new step, less negative than the two original bi-nitrone (I) reduction steps, appears. With the increase of (AH) concentration the new step increases and the two normal steps decrease. In the presence of a sufficient amount of (AH), the bi-nitrone (I) reduction exhibits a single two-electron step. The potentials of this overall reduction process depend linearly on the p K AH d values of acids investigated. A multi-step scheme suggested for the electrochemical reduction of (I) explains the various experimental results obtained.