Monomolecular films of a nickel(II) pentaazamacrocyclic complex for the electrocatalytic oxidation of hydrogen peroxide at gold electrodes

Self-assembled monolayers (SAMs) of a redox active nickel(II) pentaazamacrocyclic complex 1 and mixed monolayers of 1 with ethyl disulfide have been fabricated on a gold electrode, and their electrochemical behavior has been studied by cyclic voltammetry in aqueous solutions of Na₂SO₄ and NaNO₃. The results demonstrate that the redox behavior as well as the electrocatalytic activity towards the oxidation of H₂O₂ of the SAM and mixed monolayers of 1 largely depend on the electrolyte anions, i.e. SO₄^2? and NO₃^?: the formal potential in the SO₄^2? electrolyte is about 220 mV less positive than that in the NO₃^? electrolyte, and the SAM and mixed monolayers of 1 possess an efficient electrocatalysis for the oxidation of H₂O₂ in the NO₃^? electrolyte, but not in the SO₄^2? electrolyte. In addition, a unique cyclic voltammogram with a sharp peak of inverted ‘V’ shape has been observed in the cathodic scan for the electrocatalytic oxidation of H₂O₂, largely depending on the concentration of the NO₃^? electrolyte anion and the solution pH. These electrolyte anion-dependent redox behaviors have been discussed on the basis of different coordinating tendencies of SO₄^2? and NO₃^? to the nickel(III) centre of the complex and a possible reaction mechanism for the observed electrocatalytic reaction.     

Electrochemical and spectral studies of self-assembled monolayer of 1,8,15,22-tetraaminophthalocyanatocobalt(II) on indium tin oxide surface

Self-assembled monolayer (SAM) of 1,8,15,22-tetraaminophthalocyanatocobalt(II) (4α-Co^IITAPc) was prepared on indium tin oxide (ITO) electrode by spontaneous adsorption from dimethylformamide (DMF) solution containing 4α-Co^IITAPc. The SAM of 4α-Co^IITAPc formed on ITO electrode was characterized by cyclic voltammetry, Raman and UV–visible spectroscopic techniques. The cyclic voltammogram (CV) of 4α-Co^IITAPc SAM shows two pairs of well-defined redox peaks corresponding to Co^III/Co^II and Co^IIIPc⁻¹/Co^IIIPc⁻². The surface coverage (Γ) was calculated by integrating the charge under the anodic wave corresponding to Co^II oxidation and it was found to be 2.25 × 10⁻¹⁰ mol cm⁻². Raman spectrum obtained for the SAM of 4α-Co^IITAPc on ITO surface shows strong stretching and breathing bands of Pc macrocycle, pyrrole ring and isoindole ring. Further, the –NH₂ bending mode of vibration was absent for the SAM of 4α-Co^IITAPc on ITO surface which indirectly confirmed that all the amino groups of 4α-Co^IITAPc are involved in bonding with ITO surface. UV–visible spectrum for the SAM of 4α-Co^IITAPc on ITO surface shows an intense B-band, Q-band and n–π^∗ transition with slight broadening when compared to that of 4α-Co^IITAPc in DMF.     

Improvement of alternariol monomethyl ether detection at gold electrodes modified with a dodecanethiol self-assembled monolayer

The electro-oxidation of alternariol monomethyl ether (AME), one of the main metabolites of the Alternaria genus mycotoxins, is studied at 1-dodecanethiol (DDT)-modified gold electrodes, in acetonitrile (ACN) – aqueous phosphate buffer solutions of different pH values, by using cyclic (CV) and square-wave (SWV) voltammetries. The AME voltammetric response at the bare electrode suffers from two drawbacks: it appears at potentials close to the onset of gold oxide formation, and it is hampered by a fouling of the electrode surface due to the accumulation of oxidized products. These shortcomings are circumvented by the use of DDT-coated electrodes, since the intervening monolayer inhibits gold oxide formation and surface passivation by the electrochemical products, without affecting the oxidation kinetics of AME significantly. Diagnostic criteria based on the voltammetric peak parameters show that the electrochemical behavior of AME at the modified electrode is mainly controlled by reactant diffusion from solution, with a weak adsorption of both the mycotoxin and its oxidation products at monolayer defects. Calibration curves were constructed from the AME square-wave voltammetric response and a detection limit of 9.1 × 10^?8 mol dm^?3 was determined, which is about three times smaller than a previous estimate at platinum and glassy carbon electrodes, and about fifty times smaller than the limit derived from measurements carried out at a polyphenol oxidase-modified carbon paste electrode.     

The study of nickel(II) and cobalt(II) complexes with a chiral salen derivative as catalysts for the electrochemical cyclisation of unsaturated 2-bromophenyl ethers

Reductions of Ni(II) and Co(II) complexes with the Jacobsen ligand (a chiral, substituted salen) in the absence and in the presence of allyl 2-bromophenyl ethers in N,N^′-dimethylformamide were investigated with the aid of cyclic voltammetry and constant-current and controlled-potential electrolyses. The complexes were effective catalysts for the reduction of these organic substrates and cyclic compounds were obtained in good yields under appropriate experimental conditions. Modest asymmetric induction was observed.     

Novel studies on the electrochemical oxidation of 2-[4-(N,N-dimethylamino)phenyl]-6-methyl benzothiazole (DPMB) in acetonitrile at platinum electrodes

The electrochemical oxidation mechanism of 2-[4-(N,N-dimethylamino)phenyl]-6-methyl benzothiazole (DPMB) is studied in a 0.1 M N(C₄H₉)₄ClO₄ + acetonitrile (ACN) reaction medium by cyclic (CV) and square wave voltammetries (SWV) as well as by controlled potential bulk electrolysis at platinum electrodes. The primary radical cation formed by the one electron oxidation of DPMB undergoes a deprotonation process, which is the rate-determining step, followed by a radical–radical coupling. On the other hand, an initial quasi-reversible monoelectronic charge transfer mechanism is inferred from cyclic and square wave voltammograms recorded at scan rates and frequencies higher than 0.4 V s^?1 and 40 Hz, respectively. Diffusion coefficients of DPMB at different temperatures were calculated from the quasi-reversible convoluted cyclic voltammograms. DigiSim® and COOL software were used to fit the quasi-reversible cyclic and square wave voltammetric responses, respectively. Formal potentials, formal rate constants and positive transfer coefficients at different temperatures were evaluated from the fitting of cyclic voltammograms. The experimental activation parameters were also determined. The effects of the analytical concentration of the reagent and the temperature, as well as the addition of trifluoracetic acid and a strong base such as lutidine on the electrochemical responses are discussed. A general reaction mechanism as well as probable structures for dimeric products are proposed.Besides, the presence of an acid–base equilibrium in DPMB solutions is also studied by employing UV–Vis spectroscopic measurements at different trifluoracetic acid concentrations. An apparent value of (1.5 ± 0.2) × 10³ M^?1 was estimated for the DPMB basic constant at 20.0 °C     

Spectral and electrochemical studies of axial ligand binding reactions of carbonylruthenium(II) meso-tetramesitylporphyrin

The spectral and electrochemical properties of carbonylruthenium(II) meso-tetramesitylporphyrin, Ru(CO)(TMP) in the presence of imidazole, N-methylimidazole and hydroxide anion as axial ligands were investigated. Spectrophotometric titration of Ru(CO)(TMP) of the nitrogeneous bases caused the absorption spectrum to shift to longer wavelengths. Larger shifts in wavelength were observed in the titration using tetrabutylammonium hydroxide. The formation constants for these ligands coordinated to the ruthenium center were calculated. The effect of axial ligand ligation caused the decrease of vibrational frequency of CO as detected from FT-IR spectroscopy. The CO stretching frequency (ν_CO) of Ru(CO)TMP in CH₂Cl₂ is 1940 cm^?1, which is lowered to 1936 and 1913 cm^?1 upon coordination of nitrogenous bases and hydroxide anion, respectively. Cyclic voltammetry of Ru(CO)(TMP) in CH₂Cl₂ showed an irreversible reduction wave at ?1.63 V and two reversible oxidations at E_1/2 = 0.78 and 1.27 V vs. Ag|AgCl, respectively. Addition of imidazole and hydroxide into Ru(CO)(TMP) solution caused shifts in the redox potential and accordingly, the binding constants of the ligands to the one- and two-electron oxidized ruthenium porphyrins were estimated and compared. Spectroelectrochemical methods were used to characterize the above electron-transfer reactions.     

Investigation of the electrochemical reduction mechanism of 1-[N-(2-pyridyl)aminomethylidene]-2(1H)-naphthalenone (PN)

The electrochemical reduction mechanism of 1-[N-(2-pyridyl)aminomethylidene]-2(1H)-naphthalenone (PN) was investigated by using various electrochemical techniques in 0.1 M tetrabutylammonium tetrafluoroborate (TBATFB) in acetonitrile at glassy carbon (GC) electrode. The number of electrons transferred and diffusion coefficient of the compound were estimated by using an ultramicroelectrode (UME). PN has two reduction peaks in a cyclic voltammogram. Each of them corresponds to a one-electron transfer. In this medium and at the GC electrode surface, the first peak was observed at about ?1.8 V (vs. Ag/Ag⁺) which is more stable and well defined compared to the second peak. It was determined by using cyclic voltammetry that PN is electroreduced by an EC mechanism. It was also explained by multicycle CV experiments and a dopamine test as well as by Raman spectroscopy that the homogenous chemical reaction is dimerization. The EC mechanism was also examined and the kinetics of this process were calculated by digital simulation.     

Synthesis and electrochemical investigation of oligo-(arylenevinylenes) intended for the preparation of two-dimensional polymer networks

Four phenylenevinylene oligomers (1–4) have been synthesized and characterized by cyclic voltammetry, absorption spectroscopy and photoluminescence (PL). Two of the oligomers contain a tri-substituted central benzene ring, while one contains a tetra-substituted central ring. These oligomers were prepared in an effort to increase hole and electron mobility in organic materials via extension of conjugation to two dimensions. Optical and electrochemical evidence is given for significant interactions of the alkoxy-substituted styryl units of 3 that are attached to the central benzene ring in meta positions. The tetra-substituted oligomer (4) also exhibits strong interactions between the pair of para substituted arms. The photoluminescence (PL) quantum yield for the tetra-substituted oligomer is high, indicating that it is an excellent candidate for the active layer in organic light-emitting diodes.     

Ligand-catalysed metal ion reduction. Voltammetric determination of rate and formation constants for the nickel complex with 6-mercaptopurine-9-d-riboside

The catalytic reduction of nickel ions on the hanging mercury drop electrode in the presence of traces of 6-mercaptopurine-9-d-riboside (MPR) was investigated by linear scan voltammetry in the physiological pH range. A nickel complex is reduced at potentials more positive than the reduction of the hydrated ion. The reduction of the complex produces a voltammetric pattern which is typical for the electrode processes controlled by a parallel chemical reaction. The theoretical treatment for the regeneration of the reactant by a redox reaction was adapted to the investigated electrode process. The kinetic equation thus derived was used for the determination of both the rate (k₁=3.11×10⁶ mol^?1 dm³ s^?1) and formation (log β₁=3.40) constants for the reducible complex by means of a multiparametric curve fitting approach. Under the same conditions traces of MPR catalyze the oxidation of Ni amalgam producing both an increase of the anodic peak current and the shift of this peak towards more negative potentials.     

Electro-oxidation of phenol and its derivatives on poly-Ni(OH)TPhPyPc modified vitreous carbon electrodes

The electrochemical oxidation of phenol and its derivatives using poly-nickel hydroxy tetraphenoxypyrrolephthalocyanine (poly-Ni(OH)TPhPyPc) modified vitreous carbon electrodes are described. The films were formed by the electro-transformation of the electropolymerized pyrrole-substituted phenoxyphthalocyanine poly-NiTPhPyPc modified electrode in aqueous 0.1 M NaOH solution. The poly-Ni(OH)TPhPyPc films showed better stability and resistance to electrode fouling compared to poly-NiTPhPyPc and unmodified electrodes. The resistance to surface fouling and stability can be attributed to the structure of the ring substituent on the phthalocyanine macrocycle and to the particular O–Ni–O bridged architecture of the nickel phthalocyanine film.     

A comparative evaluation on the voltammetric behavior of boron-doped diamond (BDD) and glassy carbon (GC) electrodes in different electrolyte media

Voltammetric responses of boron-doped diamond (BDD) and glassy carbon (GC) electrodes on the anodic oxidation of two dissimilar compounds namely 2,6-dimethoxyphenol and 1,3,5-trimethylbenzene in acidic, neutral and basic media have been explored. Cyclic voltammetric (CV) analysis reveals that the BDD electrode shows wider cathodic potential window and lower background current in all the media than the GC. However, in the anodic side, the window is wide only in aqueous acidic medium and the background limit for both the electrodes is similar in both neutral and alkaline media containing solvents other than water. Further, the anodic oxidation of 2,6-dimethoxyphenol takes place at less potential on the GC when compared to the BDD and the oxidation peak current is also higher on the former electrode. CV results show that no anodic peak appears for the oxidation of 1,3,5-trimethylbenzene on the GC electrode in any of the above media, whereas in the acidic medium, a broad anodic wave appears on the BDD.     

Electrochemical and thermal studies of the ternary complex of Cu(II) with saccharin and nicotinamide

The electrochemical behaviour and thermal decomposition of a ternary complex of Cu(II) with saccharin and nicotinamide have been investigated by means of voltammetric (square-wave and cyclic voltammetry), spectroscopic and thermoanalysis (TG, DTA, DTG) measurements. The electrochemical data show that the ternary complex undergoes reduction at a more negative potential (?0.310 V) when compared to the binary complexes; Cu(II)–nicotinamide (a shoulder at ?0.164 V) and Cu(II)–saccharinate (?0.252 V). ESR spectra (room temperature) of the complex showed a broad signal (g_⊥=2.064) due to the weak anti-ferromagnetic coupling between the Cu atoms. ESR and magnetic susceptibility data suggest that the structure of the complex is square–pyramidal in the solid state. The thermal behaviour of the complex from ambient temperature up to 1000 °C in a static air atmosphere was studied. The decomposition pathway of the complex is interpreted in terms of the structural data. A possible mechanism for the decomposition of the complex is proposed.     

The electrode reactions of Fe(CN)63− and Fe(CN)64− ions studied at temperatures below the melting point of stoichiometric electrolytes

The electrode reactions of Fe(CN)₆^3? and Fe(CN)₆^4? ions have been studied at temperatures below the melting point of stoichiometric electrolytes. Tetramethylammonium cation hydrates: (CH₃)₄NOH·nH₂O(n = 5, 7.5, 10) and (CH₃)₄NF·4H₂O were selected as electrolytes. The redox active ion with higher electric charge, Fe(CN)₆^4?, is more stable at temperatures below as well as above the melting point of these electrolytes. However, the temperature dependence of the redox potential of the Fe(CN)₆^3?/Fe(CN)₆^4? couple is more pronounced in these conditions. In the limited temperature range below the electrolyte melting point the kinetics of the reaction are controlled by the rate of reactant transport towards the electrode surface. The apparent diffusion coefficient of redox active ions does not change substantially at temperatures around the electrolyte melting point. The activation energy of reactant transport is twice as large in frozen than in liquid electrolyte. It has been concluded that the motion of the redox active ions is restricted to a limited volume—the intergrain space of the electrolyte. This conclusion is supported by results of experiments performed in a cell filled with chemically inert beads and liquid electrolyte.