Substituted pyrazine-di-N-oxides as the mediators of catalytic oxidation of organic compounds

The mechanism of oxidation of 2,5-dimethyl-, 2,3,5,6-tetramethyl-, 2,3-dimethyl-5,6-cyclohexa, and 3-phenyl-5,6-cyclohexapyrazine-di-N-oxides is studied by cyclic voltammetry, quantum chemical simulations, and ESR electrolysis. The studies are carried out on electrodes of glassy carbon and Pt in 0.1 M LiClO₄ solutions in acetonitrile. ESR spectra of radical cations of substituted pyrazine-di-N-oxides are recorded. The effects of the temperature, oxygen, and the additions of water, pyridine, and acid on the shape of cyclic voltamograms and the intensity of ESR signals of pyrazine-di-N-oxides are studied. A quantum-chemical simulation of the reaction of pyrazine-di-N-oxide radical cations with acetonitrile is carried out. The oxidation of substituted pyrazine-di-N-oxides is described by the E₁C₁E₂C₂ mechanism, which includes the stage of the formation of a complex between the di-N-oxide radical cation and acetonitrile.     

Thermodynamic characteristics of the radical cation of pyridine N-oxide and its reaction with methanol

The thermodynamic characteristics of the pyridine N-oxide radical cation were obtained and the energetics of its hydration were estimated. The electrochemically generated pyridine N-oxide radical cations are capable of cleaving an H atom from methanol, which is a component of the medium.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2438–2441, November, 1989.We would like to express our deep appreciation to A. E. Shilov for his cooperation in the research and discussion of the results of the study.     

Electrochemical and ESR studies of the methane C-H bond activation in the presence of radical cations of pyrazine-di-N-oxide and its substituted derivatives

Activation of the methane C-H bond in the presence of electrochemically generated radical cations of pyrazine-di-N-oxide and also of 2,5-dimethyl- and 2,3,5,6-tetramethyl-pyrazine-di-N-oxides is studied by methods of cyclic voltammetry (CVA), quantum chemical simulations, and ESR electrolysis. The studies are carried out on glassy carbon (GC) and Pt electrodes in 0.1 M LiClO₄ solutions in acetonitrile. ESR spectra of radical cations of aromatic di-N-oxides in the absence and in the presence of methane are recorded. The changes in the shape CVA curves and the intensity of ESR signals of di-N-oxide radical cations observed in the presence of methane point to the activation of the methane C-H bond followed by its oxidation. The reaction of pyrazinedi-N-oxide at the methane C-H bond is simulated by quantum chemical methods. The obtained results are explained within the framework of the mechanism of overall two-electron oxidation of methane within its complex with an aromatic di-N-oxide radical cation.     

Effect of non-covalent interactions in 2,5-di-Me-pyrazine-di-N-oxide - methanol – Carbon nanotube electrocatalytic system

Catalytic oxidation of methanol (MeOH) in the absence of noble metals and noble metal oxides as catalysts, and the use of metal-free materials are inexpensive and attractive process for practical use in electrocatalysis, sensors, and in direct methanol fuel cells. In previous works, it was found that the use of single-walled (SWCNT) or multi-walled (MWCNT) carbon nanotube paper electrodes instead of GC increases the catalytic efficiency of organic compounds oxidation in the presence of aromatic di-N-oxides by several times. In this work, the effect of non-covalent interactions on the catalytic efficiency of MeOH oxidation in the presence of 2,5-di-Me-pyrazine-di-N-oxide (Pyr₁) in 0.1 M Bu₄NClO₄ solution in acetonitrile at SWCNT and MWСNT paper electrodes was studied by the methods of quantum chemical modeling, Raman spectroscopy, and using electrochemical data. New factors determined the features of mechanism of MeOH oxidation on CNT electrodes and lead to an increase in the catalytic efficiency of the electrode process in comparison with the GC electrode were established.     

Electrochemical and ESR-study of the mechanism of organic compound oxidation in the presence of mediators—Radical cations of substituted pyrazin-di-N-oxydes

The mechanism of methanol, ethanol, diethyl ether, triethyl-o-formate, cyclohexanol, and cyclohexane oxidation in the presence of electrochemically generated radical cations of 2,5-dimethyl-, 2,3,5,6-tetramethyl-, 2,3-dimethyl-5,6-cyclohexa-, and 3-phenyl-5,6-cyclohexapyrazin-di-N-oxydes as mediators was studied by cyclic voltammetry, ESR-electrolysis, and gas chromatography. The studies were carried out at glassy-carbon-, Pt-, and Au-electrodes in 0.1 M LiClO₄ solutions in acetonitrile and methanol, the alcohol being used as a solvent and substrate simultaneously. ESR-spectra of the radical cations of the pyrazin-di-N-oxydes were recorded. Effects of temperature, oxygen, admixtures of water and acid, and the nature of substrate and solvent on the shape of the cyclic voltammograms and intensity of the ESR-spectra of the pyrazin-di-N-oxydes are elucidated. By comparing experimental and calculated voltammograms, the rate constants for the interaction between the pyrazin-di-N-oxydes and the substrates C-H-bonds are determined. Mechanism of the ultimate two-electron catalytic oxidation of the organics as a constituent of complexes, formed with the radical cations of the mediators (pyrazin-di-N-oxydes), is suggested.     

Electroreduction of dinitrogen in a Ti(III)?Mo(III)-guanidine system

Dinitrogen reduction on a mercury cathode has been studied in an alkaline methanol solution with guanidine buffer (pK=13.6) in the presence of homogeneous catalysts (alkoxy complexes of Mo(III) and Ti(III). As is found, dinitrogen electroreduction occurs on the electrode surface. The reaction rate is determined by the rate of electron transfer.

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(pK=13,6) - Ti(III) Mo(III). , . .

     

Electrochemical and ESR studies of au-protein from Micrococcus luteus

Au-protein from Micrococcus luteus, with and without Au inactive center, and chloroauric acid (HAu ^IIICl₄·4H₂O) with the addition of rutin, catechol, and riboflavin have been studied by means of electrochemistry and ESR. The redox potentials for Au-protein, as well as for the complexes Au-rutin and Au-catechol, have been measured, and ESR spectra of complexes Au-rutin and Au-catechol have been recorded. It has been shown that the Au atom binds to Au-protein via OH-groups of rutin. Flavin does not participatein gold binding. Au-protein is characterized by two peaks of cyclic voltammogram, −0.37 and −0.54 V. Au-protein with these potentials is able to function in the electron-transport chain of membranes between flavoproteins and quinones.     

Electrochemical and ESR studies of the oxidation mechanism of pyrazine-di-N-oxides in the presence of methanol and its deuterated derivatives

The mechanism of oxidation of pyrazine-, 2,5-di-Me-, and 2,3,5,6-tetra-Me-pyrazine-di-N-oxides in the presence of methanol and its deuterated derivatives (CH₃OD, CD₃OD), i.e., compounds exhibiting the high energy of C-H bond dissociation, is studied by the methods of cyclic voltammetry, ESR electrolysis, and quantum chemical modeling. The study is carried out on a glassy carbon (GC) electrode in acetonitrile and on an Au electrode in solutions of different alcohols (methanol and its deuterated derivatives CH₃OD, CD₃OD). In alcohol solutions, the ESR spectra of radical cations and radical anions of the tested aromatic di-N-oxides are observed. The quantum chemical simulation of the reaction of the pyrazine-di-N-oxide radical cation with the MeOH C-H bond is carried out. The results obtained are explained within the framework of the E₁C₁E₂C₂ mechanism for a two-stage electrode process determined by the catalytic current of the second electrode stage. The overall two-electron catalytic oxidation of an alcohol within its complex with the pyrazine-di-N-oxide radical cation is proposed.     

Radical Cation of Pyrazine-di-N-oxide as a Mediator in Electrocatalytic Oxidation of Organic Compounds

The mechanism of oxidation of cyclohexanol, methanol, diethyl ether, triethyl orthoformate, and cyclohexane in the presence of a mediator—electrochemically generated radical cation of pyrazine-di-N-oxide (PyrDNO)—is studied on glassy carbon and platinum in a 0.1 M LiClO₄ solution in acetonitrile employing cyclic voltammetry, ESR electrolysis, and gas chromatography. Effect of temperature, additives of acid and water, oxygen, and the nature of the substrate and solvent on the shape of cyclic voltammograms and intensity of ESR signal of PyrDNO is examined. ESR spectra for radical cations and anions of PyrDNO with g factors equal to, respectively, 2.0090 and 2.0031 are recorded. A mechanism for the overall two-electron catalytic oxidation of an organic substance, which involves a stage in which it complexes with the radical cation of PyrDNO, is suggested.