Electrochemical studies of weak carbon and nitrogen acids: Fluorene and p-cyanoaniline in dimethylformamide

The electrochemical reduction of fluorene and p-cyanoaniline in DMF at a platinum electrode is initially a one-electron process which affords the corresponding readical anions. In the absence of an added proton donor, decomposition of the radical anions occurs by carbonhydrogen bond cleavage to give the conjugate bases of the starting materials; the anions subsequently slowly abstract a proton from the tetraalkylammonium cation of the supporting electrolyte to regenerate the original electroactive species. In the presence of dimethylmalonate, both radical anions rapidly electron transfer to the added proton donor. Neither self-protonation nor protonation by the added donor was observed for either radical anion. In addition to proton abstraction, 9-fluorenyl anion reacts with oxygen to give fluorene and hydroxide ion. Abstraction of a proton from fluorene by the latter species then effects a chain reaction in which 9-fluorenyl anion is the chain-carrying species. Reduction of bifluorenyl occurs with carbon-carbon bond cleavage to give 9-fluorenyl anion as the initial product. Subsequent proton transfer from bifluorenyl to 9-fluorenyl anion then yields the final products, 9-bifluorenyl anion and fluorene, in equimolar amounts.     

Electrode reduction mechanism of aromatic nitriles in aprotic solvents: Benzonitrile

The electrode reaction mechanism of benzonitrile in anhydrous DMF has been studied by polarography, potential sweep voltammetry, macroscale electrolysis, e.p.r. spectrometry and kinetic analysis. The relatively stable radical anion formed by one-electron addition decays according to a first order kinetic law, with formation of benzene and CN^? as final products together with substantial amounts of the alkene and alkylamine corresponding to the tetraalkylammonium salt used as background electrolyte. The results are interpreted in terms of a mechanism involving protonation of the anion radical by the solvent and the back-ground electrolyte, followed by dismutation of the resulting neutral radical with the anion radical and elimination of CN^? ions from the anionic species thus formed. By phenol addition CN^? elimination is prevented and the reduction proceeds to 1-cyclohexene-1-nitrile or to cyclohexane-nitrile, depending on the reduction potential.     

Mechanism of Electroreduction of Peroxodiphosphate Anions

The kinetic regularities of peroxodiphosphate anion reduction on dropping mercury and 40% thallium amalgam electrodes are studied. The reaction rate is found to increase with an increase in the supporting electrolyte and peroxodiphosphate anion concentrations, with the adsorption of inorganic cations on the electrode, and when the electrode exhibits a more negative zero-charge potential. The data obtained are analyzed within the Frumkin theory of slow discharge.     

The Nature of the Active Centers of Methyl Methacrylate Anionic Electrochemical Polymerization

By the methods of classical and oscillographic polarography, electrolysis with controlled potential and spectrophotometry, the mechanism of electrochemical generation and the active centers nature of anionic polymerization of methyl methacrylate in dimethylformamide was investigated. It was shown that in dimethylformamide in the presence of tetraalkylammonium cations, methyl methacrylate carbanions exist as solvately divided ion pairs in which the electrostatic interaction is determined by the radius of the cation. It affects the chemical stages rate of the electrochemical generation of carbanions and appears by the anomalous shift of ζ 1/2 of polarographic reduction with the increasing of the cation radius of the supporting electrolyte. The replacement of the lithium cation with tetraalkylammonium changes the interaction type of methyl methacrylate carbanion with the counterion.     

Catalytic phenomena in the electrochemical reduction of triphenylphosphine and triphenylphosphine oxide in non-aqueous solvents with tetraalkylammonium supporting electrolytes

Polarography, cyclic voltammetry and coulometry reveal that a catalytic reduction of the tetraalkylammonium cation, R₄N⁺, of the supporting electrolyte is involved in the electrochemical reduction of triphenylphosphine (TPP) and its oxide (TPPO) in aprotic solvents such as acetonitrile, dimethylformamide and hexamethylphosphoramide. There is however progressive consumption of TPP and TPPO resulting in the final formation of phenyl substitution products (RPØ₂ and ROPØ₂). Comparison with the reduction of the BuPØ₃⁺ cation allows to propose the following mechanism which involves a chemical type catalytic process:

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Redox type catalytic mechanisms are discussed and shown to be unlikely. Values of the alkylation rate constant are derived from the polarographic or the coulometric data or from cyclic voltammetry according to its magnitude which varies with the solvent. TPP anion radical appears as more readily alkylable than TPPO anion radical.     

Theory of the exaltation effect and the effect of correlation exaltation of migration current

A theory has been developed for the effect of migration current exaltation (increase of the limiting current of cation reduction due to a conjugated reaction of reduction of a neutral substance). The effect of correlation exaltation of currents observed during parallel reactions of cation reduction in the absence of an indifferent electrolyte is also considered. The theory of these effects is based on the analysis of the corresponding sets of electrodiffusion equations. The obtained expression for exaltation current differs from the result obtained by the approximate method of Heyrovsky, in which a linear relationship is postulated between the migration current and total current and which does not account for the important role played by reduction products of the neutral substance in migration transfer.     

Charged states of Sc3N@C68: an in situ spectroelectrochemical study of the radical cation and radical anion of a non-IPR fullerene

The redox behavior of Sc 3N@C 68 is studied systematically by means of electrochemistry, in situ ESR/Vis-NIR spectroelectrochemistry, and detailed theoretical treatment. Formation of the negatively and positively charged paramagnetic species for the same trimetallic nitride endohedral fullerene is demonstrated for the first time. The electrochemical study of Sc 3N@C 68 exhibits two electrochemically irreversible but chemically reversible reduction steps and two reversible oxidation steps. A double-square reaction scheme is proposed to explain the observed redox reaction at cathodic potentials involving the reversible dimerisation of the Sc 3N@C 68 monoanion. The spin state of the radical cation and the radical anion is probed by ESR spectroscopy, indicating that in both states, the large part of the unpaired spin is delocalized on the fullerene cage. The charged states of the non-isolated pentagon rule fullerene are characterized furthermore by in situ absorption spectroscopy. The interpretation of experimental data is supported by the density functional theory (DFT) calculations of the spin distribution in the anion and cation radicals of Sc 3N@C 68 and time-dependent DFT calculations of the absorption spectra of the charged species.     

Application of the pulse radiolysis technique to reactions of organic radical ions

The nanosecond pulse radiolysis technique has been applied to study the rate constants for charge transfer and substitution reactions of radical ions. Electron transfer from biphenyl anion to styrene derivatives shows a correlation with the reduction potential of the acceptors expected from the Marcus theory. The positive charge transfer from biphenyl cation to the same acceptors shows a much larger rate constant, suggesting a considerable shift of the free energy relationship to the positive side of G^o. The substitution reaction of fluorenone anion with organic halides shows an S_N2 character, while that of diethyl fumarate shows electron transfer nature. The dimerization of radical anions has been proven for benzophenone and fluorenone, when their lifetime of the parent anions are prolonged by countercations.     

Anion sensitive voltammetry of fullerene C60 dissolved in 1,2-dichlorobenzene deposit in contact with aqueous electrolyte

Electroreduction of C₆₀ dissolved in hydrophobic solvent deposited on the electrode surface was studied. A microliter amount of C₆₀ and tetrahexylammonium perchlorate solution in 1,2-dichlorobenzene was deposited on basal plane pyrolytic graphite electrode and this electrode was immersed into an aqueous solution. The voltammetry shows three consecutive reduction–oxidation steps. The redox potential of first electroreduction step is sensitive on anion but not on cation present in the aqueous phase. This parameter also depends on electrolyte concentration in the aqueous and organic phase. It is proposed that electroreduction of C₆₀ is preceded by anion exchange and followed by anion expulsion to the aqueous phase. Similar anion effect on the redox potential is also observed for unsupported deposit indicating importance of initial partitioning of electrolyte into the organic phase.     

Behavior of electrogenerated bases in room-temperature ionic liquids

The reductive electrochemistry of substituted benzophenones in the aprotic room-temperature ionic liquid (RTIL) 1-butyl-1-methylpyrrolidinium bistriflimide occurs via two consecutive one-electron processes leading to the radical anion and dianion, respectively. The radical anion exhibited electrochemical reversibility at all time-scales whereas the dianion exhibited reversibility at potential sweep rates of >or=10 V s(-1), collectively indicating the absence of strong ion-paring with the RTIL cation. In contrast, reduction in 1-butyl-3-methylimidazolium bistriflimide is complicated by proton-transfer from the [Bmim] cation. At low potential sweep rates, reduction involves a single two-electron process characteristic of either an electrochemical, chemical, electrochemical (ECE) or disproportion-type (DISP1) mechanism. The rate of radical anion protonation in [Bmim] is governed by basicity and conforms to the Hammett free-energy relation. At higher potential sweep rates in [Bmim][NTf2], reduction occurs via two consecutive one-electron processes, giving rise to the partially reversible generation of the radical anion and the irreversible generation of the dianion, respectively. Also, the redox potentials for the reversible parent/radical anion couples were found to be a linear function of Hammett substituent constants in both RTIL media and exhibited effectively equivalent solvent-dependent reaction constants, which are similar to those for reduction in polar molecular solvents such as acetonitrile or alcohols.     

Electrochemical behaviour of graphite cathodes in the presence of tetraalkylammonium cations

A graphite cathode in N,N-dimethylformamide with tetraalkylammonium salts as supporting electrolyte was found to be able to accept reversibly a rather large charge in a way somewhat akin to the formation of anion radicals and ion pairs of polycondensed aromatic hydrocarbons. The charged graphite has certain similarities with an amalgam, it can thus be used as an insoluble chemical reducing agent. The charging of the graphite depends on the size of the cation, in the presence of very large cations the cathode disintegrates. The nature of the charge is discussed on the basis of i-E curves and coulometric results.     

Medium effect in the electroreduction of nitromesitylene

One-electron reduction of nitromesitylene to the corresponding radical anion has been studied at a hanging mercury drop electrode in various perchlorate salt solutions in five organic solvents: dimethylsulfoxide, dimethylacetamide, propylene carbonate, hexamethylphosphoramide and N-methylformamide. Standard redox potentials, diffusion coefficients, standard rate constants and transfer coefficients have been evaluated from cyclic voltammetry measurements. The results obtained are compared with the literature data for dimethylformamide and acetonitrile solutions.The standard rate constants were found to depend on the cation of the supporting electrolyte as well as on the solvent. It is shown that the rate constants corrected for both ion-pair formation and the double layer effect cannot be described by classical theories of heterogeneous electron transfer. It is shown that the dynamic dielectric properties of the solvent, described by the dielectric relaxation time, influence the rate of the heterogeneous charge transfer. The greater the dielectric relaxation time of the solvent, the smaller is the reaction rate.     

An Amphoteric Switch to Aromatic and Antiaromatic States of a Neutral Air‐Stable 25π Radical

Ever since the discovery of the trityl radical, isolation of a stable and neutral organic radical has been a synthetic challenge. A (4n+1)π open‐shell configuration is one such possible neutral radical but an unusual state between aromatic (4n+2)π and antiaromatic (4n)π electronic circuits. The synthesis and characterization of an air‐ and water‐stable neutral 25π pentathiophene macrocyclic radical is now described. It undergoes reversible one‐electron oxidation to a 24π antiaromatic cation and reduction to a 26π aromatic anion, thus confirming its amphoteric behavior. Structural determination by single‐crystal X‐ray diffraction studies revealed a planar configuration for the neutral radical, antiaromatic cation, and aromatic anion. In the solution state, the cation shows the highest upfield chemical shift ever observed for a 4nπ system, while the anion adhered to aromatic nature. Computational studies revealed the delocalized nature of the unpaired electron as confirmed by EPR spectroscopy.     

Photoinduced Electron Transfer Reaction between Poly-guanylic Acid (5`) with Anthraquinone-2-sulfonate

TheinteractionofquinonephotonucleasewithDNAhasbeenwidelystUdied.Anthraquinonederivatives,inparticularthatofanhraquinone-2-sulfonatehasbeenusedascleavingagentforduPlexDNA1-5.Howevef,directobservationofexcitedionpairsofbiomoleculesespeciallytheStabilizedradicalcationofbiomoleculeishamPeredbytheoverwhelmingtransientabsorPtionofhydrogenbondedradicalanionofquinone.lnthiswork,theinteractionofpolylG]withtripletanthraquinone-2-sulfonateinCH,CN-H:O(97f3)viaelectrontransferreactionhasbeenachieved…     

Spectroscopic study on anion recognition properties of calix[4]pyrroles: effects of tetraalkylammonium cations

The solution binding properties of calix[4]pyrroles with anion (added as tetraalkylammonium salts) were investigated using UV-vis spectroscopic techniques. The obvious red-shift of absorption maximum band of calix[4]pyrrole in EtOH in the presence of the tetramethylammonium (TMA(+)) or tetraethylammonium (TEA(+)) salts were observed. These results displayed in electronic absorption spectra indicated calix[4]pyrrole receptors linking anionic species through multiple hydrogen bonding interactions are capable of using the periphery electron-rich "walls" for selectively binding electron-deficient tetraalkylammonium cation subunits by cation-pi charge-transfer interaction. It was seen that the stability of the calix[4]pyrrole-anion complex depends strongly on the cation. The meso-alkyl groups of the calix[4]pyrrole, the affinity for the anion subunits and the structure of tetraalkylammonium cations have considerable effects on the formation of cation-pi charge-transfer interaction.     

Charged states of α,ω-dicyano β,β'-dibutylquaterthiophene as studied by in situ ESR UV-vis NIR spectroelectrochemistry

The influence of the molecular structure on the stabilization of charged states was studied in detail by in situ ESR UV-vis NIR spectroelectrochemistry at a novel α,ω-dicyano substituted β,β'-dibutylquaterthiophene (DCNDBQT) and the electrochemically generated cation and anion radicals have been proved for the first time. The voltammetry of DCNDBQT results in two separate oxidation steps with the reversible first one. The experimental absorption maxima at 646 and 1052 nm together with the calculated ones (by DFT method) as well as an ESR signal at the first anodic step prove the presence of a radical cation. Three additional optical bands (554, 906, and 1294 nm for CT-transition) can be attributed to the formation of cation radical dimer. The dicationic structure formed in the second oxidation step is not stable. The stabilization proceeds via a dimer formation in two chemical follow-up reactions. The existence of the dimeric structures was proved by ex situ MALDI TOF mass spectrometry. As the substitution by cyano groups opens the route to cathodic reductions, DCNDBQT shows a single quasi-reversible reduction step. Here, the in situ ESR UV-vis NIR spectroelectrochemical measurements and theoretical calculations let us confirm the electrochemical generation of an anion radical. As we found a low number of anion radicals by quantitative ESR spectroelectrochemistry and an appearance of additional bands in the UV-vis NIR absorption spectra, the formation of dimeric structures must be considered and was corroborated by mass spectrometry. The role of dimerization in the reaction mechanism of the DCNDBQT oxidation and reduction are discussed in general. The experimental results were interpreted using the quantum chemical calculations based on density functional theory.     

The effect of the alkyl chain length of the tetraalkylammonium cation on CO2 electroreduction in an aprotic medium

The effect of tetraalkylammonium cation (TAA⁺) chain length on CO₂ electroreduction is investigated on an Ag electrode in a DMF solution. Linear sweep voltammetric (LSV) studies show that the onset potentials of CO₂ reduction move to increasingly negative potentials upon increasing the alkyl chain length of the tetraalkylammonium cation of the supporting electrolyte. Density functional theory (DFT) computations indicate that the onset potential of CO₂ reduction is dependent on the strength of ion pairing between TAA⁺ cation and the electrogenerated CO₂^.−. CO₂ disturbance experiments reveal that the peak current of CO₂ reduction is determined by the quantity of TAA⁺ cation adsorbed on the Ag surface.     

The effect of some organic surfactants upon the reduction rate of In3+ in perchloric medium on mercury

The effect of Triton X-100, dodecylammonium, tetrabutylammonium, and dodecylsulphate upon the reduction rate of In³⁺ to indium amalgam in 0.1 M HClO₄ was investigated, both in the presence and in the absence of 1 M NaClO₄. After having accounted for the blocking effect upon the kinetics of In³⁺ reduction via the factor (1??), where ? is the surface coverage by the surfactant, the remaining electrostatic effect was compared with that predicted by the Frumkin theory for diffuse-layer effects. For this purpose the Gouy-Chapman-Stern theory was used to calculate the potential _d at the outer Helmholtz plane from measured values of the charge density q_M on the metal and of the charge density q_i due to the adsorbed surfactant. With the exception of Triton X-100, the electrostatic effect predicted by the Frumkin theory combined with the Gouy-Chapman-Stern theory was found to be much greater than that observed experimentally. This result was explained by the inadequacy of the Gouy-Chapman-Stern model of the double layer in the presence of bulky surface-active ions.     

Electrochemical Approach to the Radical Anion Formation from 2′‐Hydroxy Chalcone Derivatives

Three 2′‐hydroxy chalcone derivatives were electrochemically reduced to the radical anion by a reversible one‐electron transfer followed by a chemical dimerization reaction. Under suitable conditions of the medium, the one‐electron reduction produces very well resolved cyclic voltammograms due to the formation of the radical anion. By using appropriately the wide versatility of the cyclic voltammetric technique, was possible to study the generation of the radical anion and its stability.