ECE Mechanism Research Articles

Mechanistic aspects of electro-oxidative generated triazolinediones in the presence of amines-EC' versus ECE mechanism

• Oxidation of urazoles in presence of amines proceeds through both ECE and EC' mechanisms. • A robust relation is observed between tendency for EC' and Hammett constant. • EC' and ECE mechanisms are sensitive to reaction conditions. Electrochemical oxidation of urazoles is studied in the presence of diverse amines to demonstrate competition of oxidative ring cleavage of urazoles, electrocatalytic oxidation of amines (EC' mechanism) and coupling reaction of amines and urazoles to produce products during ECE mechanism. The oxidized form of urazole is unstable due to the presence of carbonyl and azo moieties in the molecule. Instability is more critical with the presence of electron-withdrawing groups in the structure. The instability results in oxidative ring cleavage. However, the results reveal that triazolinediones (oxidized form of urazoles) promote a wide range of amine oxidations despite instability (EC' mechanism). Results of the current work indicate that depending on amine structure and also experimental conditions electrochemical oxidation of urazoles in the presence of amines can proceed via Michael addition reaction coupled with electrode process (ECE mechanism). The pathways are strongly influenced by reaction conditions. In this study, effect of different parameters such as pH, concentration ratio of amine to triazolinedione, substituent effects of triazolinedione and amine is studied to provide insight into effective parameters to proceed the reactions toward ECE or EC' mechanisms. Moreover, the effects of electronic and structure nature of amines are evaluated for benzylic, aromatic and aliphatic amines.

An eco-friendly electrochemical process for the formation of a new desloratadine derivative and its antibacterial susceptibility. Report of a new type of ortho-quinhydrone complex

In this work, the synthesis of a new derivative of desloratadine (DQ-TBC complex) was carried out for the first time by a green electrochemical method. In order to create optimal conditions for the electrochemical synthesis of DQ-TBC complex, electrooxidation of 4‑tert-butylcatechol (TBC) was studied in the absence and presence of desloratadine (DES). To discover the reaction mechanism, the effects of solution pH, concentration of DES and potential scan rates were investigated using cyclic voltammetry and chronoamperometry techniques. The results suggest that the electrooxidation of TBC in the presence of DES leads to DQ through an ECE mechanism. This compound (DQ) is converted to the ortho-quinhydrone complex (DQ-TBC complex) as the final product after participating in a noncovalent interaction (hydrogen bonding) with TBC. The desired product was successfully synthesized in an ethanol/water mixture in a divided cell, at room temperature using carbon electrode (five ordinary soft carbon rods with total area 38 cm2) as the anode. The final product was identified by cyclic voltammetry, FTIR, NMR, Mass spectroscopic and thermogravimetric methods. Moreover, the synthesized compound was tested for antibacterial activity. The results indicated that DQ-TBC complex shows promising antibacterial activity against Bacillus cereus and Staphylococcus aureus, gram-positive bacteria.

New insights on the electrochemical mechanism of epinephrine on glassy carbon electrode

The classic epinephrine’s ECE mechanism was revisited. The data obtained suggest that its mechanism is a multi-step irreversible electron transfer with a very fast intramolecular cyclization, an EECEE with a potential inversion, instead of a couple of two-electron processes in a single-step transfer. Moreover, the electrochemical and chemical steps are pH dependent. This conclusion is based on the combination of experimental data with Tafel analysis, digital simulations, and Gibbs energy and Randles-Sevcik predictions. A more detailed electrochemical mechanism was thus proposed to epinephrine’s electrooxidation.

Simulation of two electrode reactions coupled by the chemical reaction.

A model of an ECE mechanism consisting of two reversible electrode reactions coupled by the kinetically controlled, second order, reversible chemical reaction is developed for the staircase cyclic voltammetry on the rotating disk electrode. The relationship between the limiting current of the second wave and the rate constant of chemical reaction is investigated. The concentration of electroinactive component of the forward reaction influences the half-wave potential of the first wave and the limiting current of the second wave. The responses of the mechanisms with reversible and irreversible chemical reactions are compared.

Tetrazole-Substituted isomeric ruthenium polypyridyl complexes for low overpotential electrocatalytic CO2 reduction

Introducing tetrazole moiety to the ligand framework of two isomeric ruthenium catalysts, cis/trans-[Ru(tpy)(mtzp)(CH3CN)]2+ (tpy = 2,2′:6′,2′'-terpyridine, mtzp = 2-(1-methyl-1H-tetrazol-5-yl)pyridine), for the electrochemical reduction of CO2 to CO has altered the catalytic pathway with significantly low overpotential (0.37 V) compared to its analogous catalysts. Without manipulating steric effects, only the electronic nature of tetrazole moiety enables CO2 binding to ruthenium center to form metallocarboxylate intermediate just after one-electron reduction. This is the first synthesized isomeric pair of ruthenium complex follow ECE (E = electron transfer, C = chemical reaction) mechanism for electrocatalytic reduction of CO2. By successful characterization of the Ru–CO intermediate with the help of 13C NMR, spectro-electrochemical studies and analysis of byproducts formed during the electrocatalysis, a mechanism of CO2 reduction has been established in presence of water and anhydrous conditions which is further supported by density functional theory (DFT).

Adsorptive Stripping Differential Pulse Voltammetric Determination of Clozapine in Biological Samples Using a Hydrophobic Ionic Liquid Modified Electrode

An ionic liquid-based carbon paste electrode (ILCPE) was used for the investigation of the electrochemical behavior of clozapine (CLZ) in acidic media using cyclic voltammetry and adsorptive stripping differential pulse voltammetry. Comparative experiments were carried out using carbon paste electrode. These studies have revealed that the oxidation of CLZ is facilitated at ILCPE with an obvious ECE mechanism. Sensitive and quantitative measurement of CLZ was established under optimum conditions. The anodic peak current was linear to CLZ concentration in the two ranges of 0.2–1 and 1–10 μM with sensitivity of 0.5455 and 0.0572 μA/μM, respectively. The detection limit was 9 × 10–4 μM (S/N = 3). This method was successfully applied to CLZ detection in drug tablets and biological fluids without pretreatment steps and proved to be reliable compared with the British Pharmacopoeia official method.

Mechanistic Insights into Co and Fe Quaterpyridine-Based CO2 Reduction Catalysts: Metal-Ligand Orbital Interaction as the Key Driving Force for Distinct Pathways.

Both [CoII(qpy)(H2O)2]2+ and [FeII(qpy)(H2O)2]2+ (with qpy = 2,2':6',2″:6'',2‴-quaterpyridine) are efficient homogeneous electrocatalysts and photoelectrocatalysts for the reduction of CO2 to CO. The Co catalyst is more efficient in the electrochemical reduction, while the Fe catalyst is an excellent photoelectrocatalyst ( ACS Catal. 2018, 8, 3411-3417). This work uses density functional theory to shed light on the contrasting catalytic pathways. While both catalysts experience primarily ligand-based reductions, the second reduction in the Co catalyst is delocalized onto the metal via a metal-ligand bonding interaction, causing a spin transition and a distorted ligand framework. This orbital interaction explains the experimentally observed mild reduction potential and slow kinetics of the second reduction. The decreased hardness and doubly occupied dz2-orbital facilitate a σ-bond with the CO2-π* in an η1-κC binding mode. CO2 binding is only possible after two reductions resulting in an EEC mechanism (E = electron transfer, C = chemical reaction), and the second protonation is rate-limiting. In contrast, the Fe catalyst maintains a Lewis acidic metal center throughout the reduction process because the metal orbitals do not strongly mix with the qpy-π* orbitals. This allows binding of the activated CO2 in an η2-binding mode. This interaction stabilizes the activated CO2 via a π-type interaction of a Fe-t2g orbital and the CO2-π* and a dative bond of the oxygen lone pair. This facilitates CO2 binding to a singly reduced catalyst resulting in an ECE mechanism. The barrier for CO2 addition and the second protonation are higher than those for the Co catalyst and rate-limiting.

Initial degradation mechanism of salicylic acid via electrochemical process

• The hydroxylation of salicyclic acid was systematically investigated with DFT calculations. • Calculated standard reduction potential indicates that forming of salicyclic acid radical is lower than formation of . OH radical. • The degradation of Salicylic acid follows the ECE mechanism. • The attack of hydroxyl anion HO − was more favourable on the position C3 and C5 with lower activation barriers. • The findings suggest the favorable formation of 2,3dHBA acid and 2,5dHBA as observed in experimental work. Salicylic acid is a common chemical used in the pharmaceutical industry. Much attention has been paid in recent years to the elimination of this contaminant from the water by electrochemical oxidation. However, there is still a debate about its intermediate products during the degradation process. The goal of this work was to provide further insight into the degradation of salicylic acid by the Density Functional Theory calculations. The detailed mechanism shall be investigated along with the most suitable intermediate products during the process. The results provide an explanation for the experimental observation of 2,3-dihydroxybenzoic acid and 2,5-dihydroxybenzoic acid over other derivatives.

Electro-assisted naproxen adsorption followed by its electrodegradation and simultaneous electroreactivation of the activated carbon electrode

In this work, the adsorption of a pharmaceutical pollutant, Naproxen (NPX), on activated carbon electrodes was improved through electroadsorption treatment under anodic polarization. After that, the contaminant was electrochemically degraded favoring the reactivation of the adsorbent material. The electroadsorption tests were performed with selected anodic potentials up to 1.1 V vs. NHE. The NPX removal increased from 14.2 mg/g (32.3%), during adsorption without electric potential, to 18.3 mg/g (42.7%) and 20.3 mg/g (47.1%) at 0.7 and 1.1 V vs. NHE, respectively, at an initial NPX concentration of 10 mg/L, in 0.05 M KNO3 solutions at pH 7. The presence of NPX on the electrode was confirmed by differential pulse voltammetry. The electrochemical oxidation of NPX followed an ECE mechanism confirmed by the analysis of voltamperometric parameters. The degradation mechanism was studied at different electrolysis times whose products were extracted by solvent and analyzed by HPLC-MS. Additionally, the reuse of the electrode was studied by the analysis of its textural properties, surface chemical analysis and in a new electroadsorption process. It was observed that the reactivated electrode increased its adsorption capacity up to 38 mg NPX/g (83 %) at 0.7 V and 41 mg NPX/g (84 %) at 1.1 V.

Redox Properties and Reactivity of Organic Trisulfides in Reactions with Alkenes

The redox reactions of organic trisulfides containing various hydrocarbon groups with alkenes in aprotic solvents were studied. The electrooxidation of trisulfides proceeds irreversibly by the ECE mechanism, with formation of sulfur-centered RS+ and RSS+ intermediates. The generated cations enter into electrophilic addition reactions with alkenes, forming asymmetric di- and monosulfides. The electrochemical reduction of trisulfides leads to the formation of a radical anion, which is fragmented into the RSS anion and RS radical. In the presence of acetic acid, the cathodic activation of trisulfides is accompanied by the formation of alkyl and phenyl hydrodisulfides (RSSH).

Voltammetric study of triazole antifungal agent terconazole on sp3 and sp2 carbon-based electrode materials

Terconazole belongs to synthetic triazole antifungal agents, inhibiting biosynthesis of ergosterol or other sterols and thus disrupting cell wall synthesis in fungi. In this study, electrochemical behavior of terconazole was studied for the first time using cyclic voltammetry and differential pulse voltammetry on boron doped diamond electrode (BDD) with different surface treatments leading to O-terminated, H-terminated and polished surfaces, as well as on glassy carbon (GCE) and basal-plane pyrolytic graphite electrodes (PGE). Terconazole is not reducible within the potential window on these electrode materials. It is oxidizable in aqueous solutions in pH range from 2.0 to 12.0 in two steps due to the presence of piperazine ring in its molecule, possessing two redox centers at the tertiary nitrogen atoms, by an ECE mechanism. Optimal conditions were found for determination of terconazole in 0.1 mol L−1 phosphate buffer pH 7.2. Using differential pulse voltammetry, the lowest detection limit of 0.40 μmol L−1 and excellent repeatability (RSD 1.1%) were achieved on O-terminated BDD when in-situ anodic activation was used prior to each scan. This is beneficial in comparison with GCE and PGE; their necessity of ex-situ polishing worsens the signal repeatability (RSD ~ 4%) and detection limits reaching 0.50 μmol L−1 and 1.23 μmol L−1, respectively.

Anion Radical of Carbonyl Compounds as Electrochemically Generated Base in Henry Reactions: 1,2-Acenaphthenedione

Using 1,2-Acenaphthenedione as an example, cyclic voltammetry, chronoamperometry, coulometry, electrolysis and digital simulation were used to show that electroreduction of carbonyl compounds in the presence of nitromethane in 0.1 M Bu4NClO4/DMF proceeds via the ECE mechanism gives rise nitromethane anion initiating cyclic nitroaldol (Henry) process leading to β-nitroalcohols.

Heterobimetallic complexes of IrM (M = FeII, CoII, and NiII) core and bridging 2-(diphenylphosphino)pyridine: electronic structure and electrochemical behavior.

Three complexes based on an Ir-M (M = FeII, CoII, and NiII) heterobimetallic core and 2-(diphenylphosphino)pyridine (Ph2PPy) ligand were synthesized via the reaction of trans-[IrCl(CO)(Ph2PPy)2] and the corresponding metal chloride. Their structures were established by single-crystal X-ray diffraction as [Ir(CO)(μ-Cl)(μ-Ph2PPy)2FeCl2]·2CH2Cl2 (2), [IrCl(CO)(μ-Ph2PPy)2CoCl2]·2CH2Cl2 (3), and [Ir(CO)(μ-Cl)(μ-Ph2PPy)2NiCl2]·2CH2Cl2 (4). Time-dependent DFT computations suggest a donor-acceptor interaction between a filled 5dz2 orbital on iridium and an empty orbital on the first-row metal atom, which is supported by UV-vis studies. Magnetic moment measurements show that the first-row metals are in their high-spin electronic configurations. Cyclic voltammetry data show that all the complexes undergo irreversible decomposition upon either reduction or oxidation. Reduction of 4 proceeds through an ECE mechanism. While these complexes are not stable to electrocatalysis conditions, the data presented here refine our understanding of the bonding synergies of the first-row and third-row metals.

Influence of Pyridine on the Multielectron Redox Cycle of Nickel Diethyldithiocarbamate.

Two-electron (2e-)-transfer reactions for monometallic complexes of first-row transition metals are uncommon because of the tendency of these metals to proceed through sequential one-electron (1e-)-transfer pathways. For this chemistry to be observed, structural changes upon electron transfer are often needed to shift the 1e- redox potentials to a condition of potential inversion where 2e- transfer becomes favorable. Nickel(II) dithiocarbamate complexes take advantage of these conditions to drive 2e- oxidation from NiII to NiIV. Here, we have studied the electrochemistry of NiII(dtc)2, where dtc- is N,N-diethyldithiocarbamate in an acetonitrile solvent as a function of the scan rate and added pyridine to gain further insight into the mechanism for its 2e- oxidation to [NiIV(dtc)3]+. The scan rate dependence revealed evidence for an ECE mechanism in which the chemical step constituted ligand exchange between [NiIII(dtc)2]+ and NiII(dtc)2. A pseudo-first-order rate constant for this reaction of 34 s-1 was obtained at 1 mM NiII(dtc)2. The addition of pyridine to the electrolyte solution showed pronounced changes to the cyclic voltammetry (CV) that were consistent with the formation of a pyridine-bound NiIII complex, [NiIII(dtc)2(py)2]+, which was stable at high scan rates but decomposed to [NiIV(dtc)3]+ at low scan rates. The observed decomposition rate constant was well modeled with two parallel decay pathways, one through the dipyridine [NiIII(dtc)2(py)2]+ and another through a monopyridine [NiIII(dtc)2py]+. Overall, these data point to a mechanism for oxidation from NiII(dtc)2 to [NiIV(dtc)3]+ that proceeds through an undercoordinated [NiIII(dtc)2]+ complex, which can be trapped on the time scale of CV experiments using pyridine ligands. These studies provide insight into how we may be able to control 1e- versus 2e- redox chemistry using the coordination environment and nickel oxidation state.

Simulation of Cyclic Voltammograms of Electrode Reactions with Coupled Homogeneous Chemical Reactions Using the Method of Lines

The aim of this work is to motivate the use of the method of lines to simulate cyclic voltammograms and to circumvent the use of highly specialized, expensive, commercial software for modeling complex mechanisms of electrochemical reactions coupled to chemical reactions for educational purposes. The method of lines consists of discretizing the partial diffusion equations of the species participating and transform them into a set of ordinary differential equations, from which the concentration profiles of the species are calculated and current is evaluated from the first Fick’s law. Here, we apply this strategy to simulate three well-known electrode reactions mechanisms, one of them is a redox reversible reaction, other one is a preceding reaction mechanism, CE and a third one an ECE mechanism. Results are similar to the ones reported in the original papers published by Nicholson and Shain and others. This strategy could be used as a suitable tool to teach students cyclic voltammetry in a more hands on, active manner, as well as, an alternative to research. Since new possible mechanisms can be proposed, solved and compared with experimental cyclic voltammograms.

Electrochemical Derivatization of Acetaminophen for Indirect Determination of Eflornithine Using β‐CD Modified Glassy Carbon Electrode

AbstractIn the present work, the oxidation of acetaminophen in the absence and presence of eflornithine was electrochemically investigated by means of cyclic voltammetry at a glassy carbon electrode (GCE). Our results indicate that N‐acetyl‐p‐benzoquinone imine (NAPQI) produced from two‐electron electrochemical oxidation of acetaminophen participates in a Michael addition reaction with eflornithine via an ECE mechanism. This fact was used for the determination of eflornithine using differential pulse voltammetry (DPV) technique on the surface of β‐Cyclodextrin modified glassy carbon (β‐CD/GC) electrode. β‐CD/GC electrode was prepared through an electrodeposition procedure and characterized by Fourier‐transform infrared spectroscopy (FT‐IR), Cyclic Voltammetry (CV), Field Emission Scanning Electron Microscopy (FESEM) and Energy‐dispersive X‐ray spectroscopy (EDS) techniques. Under optimum conditions, the β‐CD/GC electrode showed a good linearity as a function of the eflornithine concentration over the range from 5 to 100 μM with detection limit and quantification limit of 1.94 and 5.8 μM, respectively. Finally, the proposed protocol was confirmed to be successful in determination of eflornithine in human urine samples with good recovery, ranging from 97.2 % to 104.8 %.

Electrochemical characterization of cefadroxil β-lactam antibiotic and Cu(II) complex formation

The electrochemical behaviour of cefadroxil, a first-generation β-lactam antibiotic, was studied at glassy carbon electrodes in aqueous media over a wide range of pH. The first oxidation process is of the phenol moiety and follows an ECE mechanism, generating catechol and resorcinol derivatives as sub-products, which are then reduced and oxidized in subsequent cycles. The sulphur heteroatom present in the cyclic structure close to the β-lactam moiety is oxidized in two steps generating sulphoxide and sulphone. This process was identified from direct comparison with amoxicillin, which has a similar molecular structure, although they belong to different classes of β-lactam antibiotics. For amoxicillin, oxidation of the sulphur heteroatom occurred at more positive potentials, most likely due to structural difficulties in stabilizing the charged oxidized species. Formation of a complex between copper (II) and each of the antibiotics was studied by cyclic voltammetry. Finally, determination of cefadroxil in commercial samples was successfully carried out.

Theory of staircase cyclic voltammetry of two electrode reactions coupled by a chemical reaction

Two reversible electrode reactions that are connected by either reversible or totally irreversible chemical reactions are theoretically analysed by staircase cyclic voltammetry. The dependence of peak potentials on the thermodynamic and kinetic parameters is calculated. If the mechanism is permanently in equilibrium, the stability constant of the reversible chemical reaction can be determined. Furthermore, the critical kinetic parameter is determined and its application to the measurement of the forward rate constant of the chemical reaction is demonstrated. Also, the influence of the kinetics of electrode reactions is discussed. Keywords: ECE mechanism; Cyclic voltammetry; Theory

FeMo Heterobimetallic Dithiolate Complexes: Investigation of Their Electron Transfer Chemistry and Reactivity toward Acids, a Density Functional Theory Rationalization.

The electrochemical behavior of complexes [FeMo(CO)5(κ2-dppe)(μ-pdt)] (1) and [FeMo(CO)4(MeCN)(κ2-dppe)(μ-pdt)] (2), in the absence and in the presence of acid, has been investigated. The reduction of 1 follows at slow scan rates, in CH2Cl2-[NBu4][PF6] and acid-free media, an ECrevE mechanism that is supported by cyclic voltammetry (CV) experiments and digital CV simulations. In MeCN-[NBu4][PF6], the electrochemical reduction of 1 is the same as in dichloromethane and follows an ECE mechanism at slow scan rates, but with a positive shift of the redox potentials. In contrast, the oxidation of 1 is strongly solvent-dependent. In dichloromethane, the oxidation of 1 is reversible and involves a single electron, while in acetonitrile, it is irreversible at moderate and slow scan rates ( v ≤ ca. 1 V s-1), and some chemical reversibility is apparent at higher scan rates ( v = 10 V s-1). Density functional theory calculations revealed that the chemical step in the ECrevE mechanism corresponds to the dissociation of one PPh2 end of the diphosphine ligand and the transfer of the semibridging CO to the Fe atom, similarly to the mechanism observed in the FeFe analogue complex. However, in the case of 1, the subsequent coordination of the phosphine ligand to the other metal is an unfavorable process.

Electroorganic synthesis of disulfonamide substituted p-benzoquinone by hydroquinone electrochemical oxidation

This study illustrates electrochemical behavior of hydroquinone and 4-amino-6-chloro­benzene-1,3-disulfonamide in the phosphate buffer solution evaluated by cyclic voltammetry. It was found that the peak of the hydroquinone oxidation potential in the presence of 4-amino-6-chlorobenzene-1,3-disulfonamide is shifted to more positive values compared to hydroquinone alone. Based on these results, the electrochemical synthesis of new disulfonamide substituted p-benzoquinone is proposed and carried out via electro­chemical oxidation of hydroquinone in the presence of 4-amino-6-chlorobenzene-1,3-di­sulfonamide in the electrolytic cell. It has been concluded that hydroquinone is converted into disulfonamide substituted p-benzoquinone via an ECE mechanism. The successful electrochemical synthesis was conducted in the water/ethanol mixture under green conditions without any toxic reagents or solvents and with high atom economy.