Anomalous behavior in the two-step reduction of quinones in acetonitrile

Fabricating high-purity graphite disk electrodes as a cost-effective alternative in fundamental electrochemistry research

Graphite electrodes offer remarkable electrochemical properties, emerging as a viable alternative to glassy carbon (GCE) and other carbon-based electrodes for fundamental electrochemistry research. We report the fabrication and characterization of high-purity graphite disk electrodes (GDEs), made from cost-effective materials and a solvent-free methodology employing readily available laboratory equipment. Analysis of their physical properties via SEM, EDX and XPS reveals no metallic interferences and a notably high porosity, emphasizing their potential. The electrochemical performances of GDEs were found to be comparable to those of GCE. Immobilization of peptides and enzymes, both via covalent coupling and surface adsorption, was used to explore potential applications of GDEs in bioelectrochemistry. Enzyme activity could be addressed both via direct electron transfer and mediated electron transfer mechanism. These results highlight the interesting properties of our GDEs and make them a low-cost alternative to other carbon-based electrodes, with potential for future real-world applications.

Macrocyclic chemosensors with anthraquinone signaling unit built into ionophore. Experimental and computational studies (part I) - synthesis and effect of proton binding on spectrophotometric and electrochemical properties

Two macrocyclic chemosensors with anthraquinone signaling unit incorporated into ionophore system (via positions 1 and 8) have been synthesized and subsequently their physicochemical properties became the subject of our extensive research. First ligand, labeled in the paper as AQ-Ncrown is characterized by a cyclic structure of a crown ether, while second one AQ-Ncrypt includes an additional ethoxy bridge, which ensures the bicyclic character of a cryptand. The studied macrocycles possess both oxygen and nitrogen heteroatoms in the ionophore cavity. Dualistic (chromophore and electrophore) signaling nature of described compounds, makes them potentially attractive molecular recognition systems. The aim of our research was to synthesize and analyze the spectroscopic, acid-base and redox properties of aforesaid macrocycles. Furthermore, we have combined experimental approach together with theoretical investigations. The equilibrium structures of AQ-Ncrown and AQ-Ncrypt were determined with the use of DFT calculations. The sensitivity of studied macrocycles towards interactions with protons was scrutinized. The complete pH-spectrophotometric characteristic of studied ligands together with their protolytic forms and corresponding pK_a values were determined. The influence of medium (aprotic and protic solvent) on spectral effects was described. Furthermore, the molecular electrostatic potential maps for ligands and differential electron densities for their mono and dianions were calculated. The redox reactions was investigated at different pHs by cyclic voltammetry. Electrochemical results have presented intriguing phenomenon: the specific stabilization of the reduced form of the protonated molecules. The calculations have revealed that this is a consequence of barrierless intramolecular proton transfer (from the macrocycle cavity onto the anthraquinone moiety) that might occur during the reduction process in acidic medium.

Interplay and Competition Between Two Different Types of Redox-Active Ligands in Cobalt Complexes: How to Allocate the Electrons?

The field of molecular transition metal complexes with redox-active ligands is dominated by compounds with one or two units of the same redox-active ligand; complexes in which different redox-active ligands are bound to the same metal are uncommon. This work reports the first molecular coordination compounds in which redox-active bisguanidine or urea azine (biguanidine) ligands as well as oxolene ligands are bound to the same cobalt atom. The combination of two different redox-active ligands leads to mono- as well as unprecedented dinuclear cobalt complexes, being multiple (four or six) center redox systems with intriguing electronic structures, all exhibiting radical ligands. By changing the redox potential of the ligands through derivatisation, the electronic structure of the complexes could be altered in a rational way.

Electrochemistry of Quinones with Respect to their Role in Biomedical Chemistry

Quinones are ubiquitous in nature and form one of the largest class of antitumor agents approved for clinical use. They are known to be efficient in inhibiting cancer cells growth. Under physiological conditions they can undergo non-enzymatic one-electron reduction to give the moderately toxic species of semiquinone radical-anion. Thus, electrochemical study of quinones might provide a basic knowledge on semi-quinone radicals formation in both in vivo and in vitro under different media. Several processes are outlined briefly and discussed in the present article. Previously we investigated the electrochemical and spectral properties of ω-N-quinonyl amino acids. Such quinone-bearing peptides are known to be cytotoxic and of potential clinical significance. We were able to prove that the ω-amino quinonyl compounds are very effective in producing stable semiquinone radicals. Moreover, a direct relation was found between the first reduction potentials of the quinonyl moiety and their reactivity towards the ω-amino acids. In order to increase our knowledge of such amino quinonyl compounds and enlarge the arsenal of such cytotoxic compounds, a series of N,N-diquinonyl amines (1-6) bearing an internal proton (stems from the NH moiety) were synthesized. Their electron-transfer capabilities were probed by cyclic voltammetry measurements, in dichloromethane. It was found that the acidic NH group linking the two quinonyl moieties undergoes an initial electrochemical reduction step and generates a nitride anion. This step is followed by further reductions to yield quasi-stable semiquinone radicals and polyanions, Since these acidic diquinones (1-6) serve also as a source of internal proton donors even in non-polar medium, they might cause protonation of basic radical-anions and polyanion intermediates during the various electrochemical stages. The processes are demonstrated and discussed by analyzing different mechanistic schemes. The successful generation of relatively stable semiquinone radicals is a prerequisite for the manifestation of site directed antitumor activity by these bis-quinonyl amino derivatives. Based on the values of their redox potentials some of them could be promising candidates for clinical development.

Visualization and quantitation of electronic communication pathways in a series of redox-active pillar[6]arene-based macrocycles

While oxidized pillar[5]arenes with 1-5 benzoquinone units are known, very few examples of oxidized pillar[6]arenes have been reported. We describe here the synthesis, characterization and electrochemical behavior of a series of macrocyclic hosts prepared by the stepwise oxidation of 1,4-diethoxypillar[6]arene, resulting in high-yield and high-purity isolation of two constitutional isomers for each macrocycle, in which two, three or four 1,4-diethoxybenzene units are replaced by benzoquinone residues. A careful structural comparison with their counterparts in the pillar[5]arene framework indicates that the geometries of the macrocycles are better described as non-Euclidean hyperbolic hexagons and elliptic pentagons, respectively. A comprehensive computational study to determine anisotropic induced current density (ACID) allows us to visualize and quantify through-space and through-bond communication pathways along the macrocyclic belt. Experimental and simulated voltammetric data, as well as UV-vis spectra, of the new macrocycles afford insights into the various electronic communication pathways in these compounds.

Hydrogen bonding and protonation effects in amino acids' anthraquinone derivatives - Spectroscopic and electrochemical studies

Six novel amino acid chromophores were synthesized and their spectroscopic, acid-base, and electrochemical properties are discussed in this work. In studied compounds, selected amino acid residues (l-Aspartic acid, l-Glutamic acid, l-Glutamine, l-Histidine, l-Lysine, l-Arginine) are attached to the 1-(piperazine) 9,10-anthraquinone skeleton via the amide bond between the carboxyl group of amino acid and nitrogen atom of the piperazine ring. All derivatives have been characterized using a variety of spectroscopic techniques (mass spectrometry, 1HNMR, UV-Vis, IR spectroscopy), acid-base (electrochemical and UV-Vis) titrations, and cyclic voltammetry methods. Basing on observed experimental effects, supported by quantum chemical simulations, the structure-properties links were established. They are indicative of the specific interactions within and/or in-between amino acid side groups, which are prone to form both, intra- and intermolecular hydrogen bonds as well as electrostatic interactions with the anthraquinone system.

Analysis of the Surface Oxidation Process on Pt Nanoparticles on a Glassy Carbon Electrode by Angle-Resolved, Grazing-Incidence X-ray Photoelectron Spectroscopy

We have analyzed the surface oxidation process of Pt nanoparticles that were uniformly dispersed on a glassy carbon electrode (Pt/GC), which was adopted as a model of a practical Pt/C catalyst for fuel cells, in N₂-purged 0.1 M HF solution by using angle-resolved, grazing-incidence X-ray photoelectron spectroscopy combined with an electrochemical cell (EC-ARGIXPS). Positive shifts in the binding energies of Pt 4f spectra were clearly observed for the surface oxidation of Pt nanoparticles at potentials E > 0.7 V vs RHE, followed by a bulk oxidation of Pt to form Pt(II) at E > 1.1 V. Three types of oxygen species (H₂O_ad, OH_ad, and O_ad) were identified in the O 1s spectra. It was found for the first time that the surface oxidation process of the Pt/GC electrode at E < ca. 0.8 V (OH_ad formation) is similar to that of a Pt(111) single-crystal electrode, whereas that in the high potential region (O_ad formation) resembles that of a Pt(110) surface or polycrystalline Pt film.

Visible-light-induced oxidant and metal-free dehydrogenative cascade trifluoromethylation and oxidation of 1,6-enynes with water

Generally, oxy-trifluoromethylation in olefins is achieved using oxidants and transition metal catalysts. However, labile olefins remain unexplored due to their incompatibility with harsh reaction conditions. Here, unprecedented light-induced oxidant and metal-free tandem radical cyclization-trifluoromethylation and dehydrogenative oxygenation of 1,6-enynes have been achieved using a photoredox catalyst, CF3SO2Na, and phenanthrene-9,10-dione (PQ), Langlois' reagent (CF3SO2Na) and water as the oxygen source. This benign protocol allows for access to various CF3-containing C3-aryloyl/acylated benzofurans, benzothiophenes, and indoles. Moreover, the oxidized undesired products, which are inherently formed by the cleavage of the vinylic carbon and heteroatom bond, have been circumvented under oxidant free conditions. The mechanistic investigations by UV-visible and ESR spectroscopy, electrochemical studies, isotope labelling and density functional theory (DFT) suggest that light induced PQ produced a CF3 radical from CF3SO2Na. The generated CF3 radical adds to the alkene, followed by cyclization, to provide a vinylic radical that transfers an electron to PQ and generates a vinylic cation. Alternatively, electron transfer may occur from the CF3-added alkene moiety, forming a carbocation, which would undergo cationic cyclization to generate a vinylic carbocation. The subsequent addition of water to the vinylic cation, followed by the elimination of hydrogen gas, led to the formation of trifluoromethylated C3-aryloyl/acylated heterocycles.

Cooperative reduction by Ln2+ and Cp*− ions: synthesis and properties of Sm, Eu, and Yb complexes with 3,6-di-tert-butyl-o-benzoquinone

Reactions of lantanocenes LnCp₂*(thf)_n with the title o-quinone result in either dinuclear (Sm³⁺,Yb³⁺) or trinuclear mixed-valent (Eu²⁺/Eu³⁺) catecholates.

Revealing the thermodynamic driving force for ligand-based reductions in quinoids; conceptual rules for designing redox active and non-innocent ligands

Metal and ligand-based reductions have been modeled in octahedral ruthenium complexes revealing metal-ligand interactions as the profound driving force for the redox-active behaviour of orthoquinoid-type ligands. Through an extensive investigation of redox-active ligands we revealed the most critical factors that facilitate or suppress redox-activity of ligands in metal complexes, from which basic rules for designing non-innocent/redox-active ligands can be put forward. These rules also allow rational redox-leveling, i.e. the moderation of redox potentials of ligand-centred electron transfer processes, potentially leading to catalysts with low overpotential in multielectron activation processes.

The effect of glassy carbon surface oxides in non-aqueous voltammetry: the case of quinones in acetonitrile

Glassy carbon (GC) electrodes are well-known to contain oxygenated functional groups such as phenols, carbonyls, and carboxylic acids on their surface. The effects of these groups on voltammetry in aqueous solution are well-studied, but there has been little discussion of their possible effects in nonaqueous solution. In this study, we show that the acidic functional groups, particularly phenols, are likely causes of anomalous features often seen in the voltammetry of quinones in nonaqueous solution. These features, a too small second cyclic voltammetric wave and extra current between the two waves that sometimes appears to be a small, broad third voltammetric wave, have previously been attributed to different types of dimerization. In this work, concentration-dependent voltammetry in acetonitrile rules out dimerization with a series of alkyl-benzoquinones because the anomalous features get larger as the concentration decreases. At low concentrations, solution bimolecular reactions will be relatively less important than reactions with surface groups. Addition of substoichiometric amounts of naphthol at higher quinone concentrations produces almost identical behavior as seen at low quinone concentrations with no added naphthol. This implicates hydrogen bonding and proton transfer from the surface phenolic groups as the cause of the anomalous features in quinone voltammetry at GC electrodes. This conclusion is supported by the perturbation of surface oxide coverage on GC electrodes through different electrode pretreatments.

Probing of the pH-Dependent Redox Mechanism of a Biologically Active Compound, 5,8-Dihydroxynaphthalene-1,4-dione

The redox behaviour of a potential anticancer organic compound, 5,8-dihydroxynaphthalene-1,4-dione (DND), was investigated in 1 : 1 buffered aqueous ethanol using cyclic, differential pulse, and square wave voltammetry. The redox processes were found to occur in a pH-dependent diffusion-controlled manner. Presence of an α-hydroxyl group stabilised semiquinone radical of DND, formed by the gain of 1 e– and 1 H+, prevented the second step reduction, which is in contrast to the general mechanism previously reported for quinines in protic and aprotic media. In addition, our results supported an independent oxidation and reduction process. Square wave voltammetry provided evidence about the reversible and quasi-reversible nature of oxidation and reduction peaks. Based on the voltammetric results, the electrode reaction mechanism of DND was proposed. Parameters including pKa, transfer coefficient, diffusion coefficient, and electron transfer rate constant were evaluated. The values of pKa obtained from cyclic voltammetry and ultraviolet-visible spectroscopy not only agreed with each other, but also with reported values of structurally related compounds evaluated by other techniques.

Remarkable sensitivity of the electrochemical reduction of benzophenone to proton availability in ionic liquids

The reduction of benzophenone was investigated in five different ionic liquids by using transient cyclic voltammetry, near steady-state voltammetry, and numerical simulation. Two reversible, well-resolved one-electron-reduction processes were observed in dry (≤20 ppm water, ca. 1 mM)) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Bmpyrd][NTf(2)]) and 1-butyl-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide ([Bmpipd][NTf(2)]), which did not contain any readily available proton source. Upon addition of water, the second process became chemically irreversible and shifted to a more positive potential by approximately 600 mV; moreover, the two reduction processes merged into a single two-electron proton-coupled process when about 0.6 M H(2)O was present. This large dependence of potential on water content, which was not observed in molecular solvents (electrolyte), was explained by a reaction mechanism that incorporated protonation and hydrogen-bonding interactions of the benzophenone dianion with as many as seven water molecules. In the three imidazolium-based ionic liquids used herein, the first benzophenone-reduction process was again reversible, whilst the second reduction process became chemically irreversible owing to the availability of the C2-H imidazolium protons in these ionic liquids. The reversible potentials for benzophenone reduction were remarkably independent of the identity of the ionic liquids, thereby implying either weak interactions with the ionic liquids or relatively insignificant differences in the levels of ion-pairing. Thus, the magnitude of the separation of the potentials of the reversible first and irreversible second reduction processes mainly reflected the proton availability from either the ionic liquid itself or from adventitious water. Consequently, voltammetric reduction of benzophenone provides a sensitive tool for the determination of proton availability in ionic liquids.

Reinvestigation of a former concerted proton-electron transfer (CPET), the reduction of a hydrogen-bonded complex between a proton donor and the anion radical of 3,5-di-tert-butyl-1,2-benzoquinone

In 2001, Lehmann and Evans (J. Phys. Chem. B, 2001, 105, 8877-8884) reported that the electrochemical reduction of a hydrogen-bonded complex between a proton donor and the anion radical of 3,5-di-tert-butyl-1,2-benzoquinone in acetonitrile proceeded by a concerted proton-electron transfer (CPET) reaction in which electron transfer from the electrode and proton transfer from proton donor to the quinone moiety occurred concertedly. Support for this conclusion was based upon ruling out both of the competing two-step processes, electron transfer followed by proton transfer (EP) and proton transfer followed by electron transfer (PE). In the course of studies of related compounds it was decided to reinvestigate the reduction of 3,5-di-tert-butyl-1,2-benzoquinone. It was discovered that the earlier conclusion that a CPET reaction was occurring was tenable only for the particular electrolyte that was used, tetrabutylammonium hexafluorophosphate and for lower concentrations of the quinone. Even the small change of carrying out the reduction of the quinone in the presence of water with tetramethylammonium hexafluorophosphate as electrolyte, produced voltammograms with clear signatures that the process was EP rather than CPET. Even more dramatic effects were seen with cesium, potassium or sodium ions in the electrolyte. A general reaction scheme to explain results with all electrolytes will be presented.

Electrochemical Reduction of Quinones in Different Media: A Review

The electron transfer reactions involving quinones, hydroquinones, and catechols are very important in many areas of chemistry, especially in biological systems. The therapeutic efficiency as well as toxicity of anthracycline anticancer drugs, a class of anthraquinones, is governed by their electrochemical properties. Other quinones serve as important functional moiety in various biological systems like electron-proton carriers in the respiratory chain and their involvement in photosynthetic electron flow systems. The present paper summarizes literatures on the reduction of quinones in different solvents under various conditions using different electrochemical methods. The influence of different reaction conditions including pH of the media, nature of supporting electrolytes, nature of other additives, intramolecular or intermolecular hydrogen bonding, ion pair formation, polarity of the solvents, stabilization of the semiquinone and quinone dianion, catalytic property, and adsorption at the electrode surface, are discussed and relationships between reaction conditions and products formed have been presented.

The Effect of H-bonding and Proton Transfer on the Voltammetry of 2,3,5,6-Tetramethyl-p-phenylenediamine in Acetonitrile. An Unexpectedly Complex Mechanism for a Simple Redox Couple

The voltammetry of 2,3,5,6-tetramethyl-p-phenylenediamine, H₂PD, has been studied and compared to that of its isomer N,N,N'N'-tetramethyl-p-phenylenediamine, Me₂PD. Both undergo two reversible electron transfer processes in acetonitrile that nominally correspond to 1e- oxidation to the radical cations, Me₂PD⁺ and H₂PD⁺, and a second 1e- oxidation at more positive potentials to the quinonediimine dications, Me₂PD²⁺ and H₂PD²⁺. While the voltammetry of Me₂PD agrees with this simple mechanism, that of H₂PD does not. The second voltammetric wave is too small. UV/Vis spectroelectrochemical experiments indicate that the second wave does correspond to oxidation of H₂PD⁺ to H₂PD²⁺ in solution. The fact that the second wave is not present at all at the lowest concentrations (5 µM), and that it increases at longer times and higher concentrations, indicates that H₂PD⁺ is not the initial solution product of the first oxidation. A number of lines of evidence suggest instead that the initial product is a mixed valent, H-bonded dimer between one H₂PD in the the full reduced, fully protonated state, H₄PD²⁺, and another in the fully oxidized, fully deprotonated state, PD. A mechanism is proposed in which this dimer is formed on the electrode surface through proton transfer and H-bonding. Once desorbed into solution, it breaks apart via reaction with other H₂PD's, to give 2 H₂PD⁺, which is the thermodynamically favored species in solution.