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At least 271 records · Page 15Linked to original sources
PubMed2003

Electroreductive intramolecular cyclization of a bromo propargyloxy ester catalyzed by nickel(i) tetramethylcyclam electrogenerated at carbon cathodes in dimethylformamide.

Cyclic voltammetry and controlled-potential electrolysis have been employed to investigate and characterize the reductive intramolecular cyclization of ethyl 2-bromo-3-(3',4'-dimethoxyphenyl)-3-(propargyloxy)propanoate (1) promoted by (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane)nickel(I), [Ni(tmc)](+), electrogenerated at glassy carbon cathodes in dimethylformamide containing tetraalkylammonium salts. Cyclic voltammograms for reduction of [Ni(tmc)](2+) in the presence of 1 reveal that [Ni(tmc)](+) catalytically reduces 1 at potentials more positive than those required for direct reduction of 1. During controlled-potential electrolyses of solutions containing [Ni(tmc)](2+) and 1, catalytic reduction of the latter proceeds via one-electron cleavage of the carbon-bromine bond to form a radical intermediate that undergoes cyclization to afford 2-(3',4'-dimethoxyphenyl)-3-(ethoxycarbonyl)-4-methylenetetrahydrofuran (2). In the presence of a base (either electrogenerated or deliberately added as potassium tert-butoxide), 2 rearranges to give 2-(3',4'-dimethoxyphenyl)-3-(ethoxycarbonyl)-4-methyl-2,5-dihydrofuran (3). A mechanistic scheme is proposed to explain the results obtained by means of cyclic voltammetry and controlled-potential electrolysis.

Journal Article↗
PubMed2006

Catalytic reduction and intramolecular cyclization of haloalkynes in the presence of nickel(I) salen electrogenerated at carbon cathodes in dimethylformamide.

[reaction: see text] Pentylidenecyclopentane can be conveniently prepared in up to 86% yield via the catalytic reduction of 1-iodo- or 1-bromo-5-decyne by [[2,2'-[1,2-ethanediylbis(nitrilomethylidyne)]bis[phenolato]]-N,N',O,O']nickelate(I) electrogenerated at a carbon cathode in dimethylformamide containing tetramethylammonium tetrafluoroborate. This electrosynthesis can be accomplished at potentials for which the haloalkynes are electroinactive, and it can be completed within 30 min at room temperature. Attempts to synthesize pentylidenecyclobutane and pentylidenecyclohexane from 1-halo-4-nonynes and 11-halo-5-undecynes, respectively, under similar conditions afford the carbocycles in very low yields (2% and 6%, respectively). Other products derived from the various haloalkynes are dimers, alkynes, and 1-alkenynes. Dimers (alkadiynes) arising from 1-halo-4-nonynes and 11-halo-5-undecynes are formed in yields ranging from 80% to 89%, whereas icosa-5,15-diyne (the dimer obtained from a 1-halo-5-decyne) is found in significantly lower yield (</=13%). Alkynes and 1-alkenynes are produced in yields of 3-10% and 2-3%, respectively. A mechanistic scheme, involving alkyn-1-yl radicals arising from nickel(I) salen catalyzed cleavage of the carbon-halogen bond of each haloalkyne, is proposed to account for the formation of all products.

Journal Article↗
PubMed1996

Electrochemical Reduction of Mono- and Dihalothiophenes at Carbon Cathodes in Dimethylformamide. First Example of an Electrolytically Induced Halogen Dance.

Cyclic voltammetry and controlled-potential electrolysis have been employed to probe the electrochemical reduction of a number of mono- and dihalothiophenes at carbon cathodes in dimethylformamide containing tetramethylammonium perchlorate. Reduction of 2-bromo-, 3-bromo-, 2-chloro-, 3-chloro-, and 2-iodothiophene gives rise to a single irreversible cyclic voltammetric wave for each compound that corresponds to the two-electron cleavage of the carbon-halogen bond, and thiophene is obtained as the only product. Cyclic voltammograms for the reduction of 2,3-dibromo-, 2,4-dibromo-, 2,5-dibromo-, 3,4-dibromo-, 2-bromo-5-chloro-, and 3-bromo-2-chlorothiophene each exhibit a pair of irreversible two-electron waves. Electrolyses of either 2,3-dibromo- or 2,4-dibromothiophene at potentials corresponding to the first voltammetric wave yield a two-to-one mixture of 3-bromo- and 3,4-dibromothiophene; under similar conditions, electrolyses of 2,5-dibromothiophene give a mixture of 2-bromo-, 3-bromo-, and 3,4-dibromothiophene, electrolyses of 2-bromo-5-chlorothiophene afford a mixture of 3-bromo-, 3,4-dibromo-, 3-bromo-2-chloro-, 4-bromo-2-chloro-, and 2-chlorothiophene, and electrolyses of 3-bromo-2-chlorothiophene yield 2-chlorothiophene. Aside from the last result, these product distributions appear to arise from an electrolytically induced halogen dance. When electrolyses of the dibromothiophenes and of 2-bromo-5-chloro- and 3-bromo-2-chlorothiophene are performed at potentials that correspond to the second voltammetric wave, thiophene is the only product obtained.

Journal Article↗
PubMed2005

Template-synthesized LiCoO2, LiMn2O4, and LiNi0.8 Co0.2 O2 nanotubes as the cathode materials of lithium ion batteries.

The first point of this work is to synthesize LiCoO2, LiNi0.8 Co0.2 O2, and LiMn2O4 nanotubes with the template of porous anodic aluminum oxide by thermal decomposition of sol-gel precursors. The as-synthesized materials were open-ended nanotubes with uniform shape and size based on the analysis of scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. An "in situ reaction from nanoparticle to nanotube" mechanism was discussed for the formation process of the nanotubes. The second point of this paper is to investigate the electrochemical properties of the as-synthesized nanotubes for the cathode materials of lithium ion batteries. It was found that the nanotube electrodes exhibited better reversibility and higher discharge capacities than that of their nanocrystalline counterparts. The reason for the improved electrochemical performance of the nanotube electrodes was also interpreted.

Journal Article↗
PubMed2005

Synthesis and electrochemical properties of nanostructured LiCoO2 fibers as cathode materials for lithium-ion batteries.

Nanostructured LiCoO2 fibers were prepared by the sol-gel related electrospinning technique using metal acetate and citric acid as starting materials. The transformation from the xerogel fibers to the LiCoO2 fibers and the nanostructure of LiCoO2 fibers have been investigated in detail. The LiCoO2 fibers with 500 nm to 2 mum in diameter were composed of polycrystalline nanoparticles in sizes of 20-35 nm. Cyclic voltammetry and charge-discharge experiments were applied to characterize the electrochemical properties of the fibers as cathode materials for lithium-ion batteries. The cyclic voltammogram curves indicated faster diffusion and migration of Li+ cations in the nanostructured LiCoO2 fiber electrode. In the first charge-discharge process, the LiCoO2 fibers showed the initial charge and discharge capacities of 216 and 182 (mA.h)/g, respectively. After the 20th cycle, the discharge capacity decreased to 123 (mA.h)/g. The X-ray diffraction and high-resolution transmission electron microscopy analyses indicated that the large loss of capacity of fiber electrode during the charge-discharge process might mainly result from the dissolution of cobalt and lithium cations escaping from LiCoO2 to form the crystalline Li2CO3 and CoF2 impurities.

Journal Article↗
PubMed2005

Indirect reduction of aryldiazonium salts onto cathodically activated platinum surfaces: formation of metal-organic structures.

Platinum phases of general formula [Pt(n-), M+, MX] can be electrogenerated from cathodic polarization in dry dimethylformamide containing a supporting electrolyte, MX. The reaction of these electrogenerated Pt phases as reducing agent with aryldiazonium salts was investigated for preparing controlled metal-organic interfaces and characterizing the reactivity of the "reduced platinum phases". In a two-step process, the "reduced platinum phase" locally reacts with aryldiazonium salts, leading to the attachment of aryl groups onto the metal surface in the previously modified areas. Detailed experiments using cyclic voltammetry, X-ray photoelectron spectroscopy (XPS), and in situ electrochemical atomic force microscopy (EC-AFM) were carried out to follow the reaction in solution with the example of NaI as supporting electrolyte (MX = NaI). These studies demonstrate the irreversible attachment of aryl groups onto the platinum electrode. Comparison between the direct electroreduction of aryldiazonium compounds (4-nitrophenyl- and 4-bromophenyldiazonium) on a platinum electrode and their reaction with [Pt2-, Na+, NaI] suggests that a similar general mechanism is responsible for the grafting. However in the second case, no applied potential is required to stimulate the binding thanks to the reductive properties of [Pt2-, Na+, NaI]. Competitive reduction of the organic layer and growth of the layer were observed and analyzed as a function of the injected charge used to initially produce [Pt2-, Na+, NaI]. Similar reactions are highly probable with other MX salts owing to the redox properties observed for this type of platinum phase ([Pt(n-), M+, MX]).

Journal Article↗
PubMed2006

Cathodic electrodeposition of mixed molybdenum tungsten oxides from peroxo-polymolybdotungstate solutions.

Mixed molybdenum tungsten trioxide films of varying stoichiometry (MoxW1 - xO3, 0 < x < 1) were prepared by cathodic electrodeposition on indium tin oxide (ITO)-coated glass substrates from aqueous peroxo-polymolybdotungstate solutions. Electrochemical quartz crystal microbalance (EQCM), cyclic voltammetry, and chronocoulometry were used to gain insight into the electrodeposition mechanism. The compositional and structural properties were characterized for MoxW1 - xO3 films deposited at intermediate potentials (-0.35 V vs Ag/AgCl) and sintered at 250 degrees C using energy-dispersive spectroscopy, X-ray diffraction, and Raman spectroscopy. These studies reveal that films consist of homogeneously mixed MoxW1 - xO3, with an enriched Mo content ranging in composition from 0.4 < x < 0.7 depending upon the mol % Mo present in the deposition solution. Chronoamperometry and spectroelectrochemical measurements were conducted to estimate lithium ion diffusion coefficients and coloration efficiencies for the mixed metal oxide films in 1 M LiClO4/propylene carbonate. The subtle interplay between structural and compositional properties due to the uniform mixing of Mo and W oxide components shows that electrochromic and lithium ion transport properties are moderately enhanced relative to those of single-component WO3 and MoO3 and demonstrate improved structural stability over pure MoO3 polymorphs during electrochemical cycling.

Cations↗
PubMed2003

A microbial fuel cell with improved cathode reaction as a low biochemical oxygen demand sensor.

Mediator-less microbial fuel cells (MFC) enriched with oligotrophic microbes were optimized through enhancement of cathode reaction and lowering O2 diffusion into the anode compartment as a low BOD sensor. The optimization of the MFC has greatly improved the maximum current and coulomb yield. The oligotroph-type MFC could be used as a low BOD sensor with high operational stability, good repeatability and reproducibility.

Bacterial Physiological Phenomena↗
PubMed2005

Microwave devices: carbon nanotubes as cold cathodes.

To communicate, spacecraft and satellites rely on microwave devices, which at present are based on relatively inefficient thermionic electron sources that require heating and cannot be switched on instantaneously. Here we describe a microwave diode that uses a cold-cathode electron source consisting of carbon nanotubes and that operates at high frequency and at high current densities. Because it weighs little, responds instantaneously and has no need of heating, this miniaturized electron source should prove valuable for microwave devices used in telecommunications.

Journal Article↗
PubMed1992

Determination of trace amounts of estriol and estradiol by adsorptive cathodic stripping voltammetry.

Estriol and estradiol are electroinactive in the potential range from -200 to -1000 mV versus a silver-silver chloride electrode at a mercury electrode. The conversion of these estrogens into electroactive nitro derivatives of estrogens, which are used for voltammetric determination, was studied. Such nitro derivatives give a well defined cathodic stripping wave at -600 mV in pH 10.5 borate buffer. Estriol and estradiol are determined in the ranges 1 x 10(-9)-1.5 x 10(-6) and 5 x 10(-9)-2 x 10(-6) mol dm-3, respectively, by differential-pulse adsorptive stripping voltammetry at a hanging mercury drop electrode. Some steroids, such as estrone, interfere because the three estrogens have almost the same molecular structure and have similar nitro derivatives, but progesterone does not interfere and is reduced at significantly more negative potentials than the nitrated estrogens. It can be determined simultaneously with estriol or estradiol. A method was developed for the assay of estriol in pharmaceutical preparations.

Adsorption↗
PubMed1995

Determination of cobalt in blood using cathodic stripping voltammetry.

A method was developed to determine low levels of cobalt in whole blood, serum and plasma. Samples of blood (0.2 ml) were mineralized at 160 degrees C in the presence of concentrated nitric acid. Residual organic matter was destroyed by digestion using ultraviolet irradiation after dilution with water. The cobalt concentration was determined by catalytic cathodic stripping voltammetry (CSV) preceded by adsorptive collection of cobalt complexed with diphenylglyoxime (DPG). The optimized analytical conditions for the CSV analysis included a DPG concentration of 0.5 mumol l-1, 0.05 mol l-1 ammonium chloride buffer (pH 9.3), 0.15 mol l-1 nitrite, a deposition potential of -0.75 V, an adsorption time of 30-120 s and a negative potential scan using the differential-pulse modulation. The limit of detection was 40 pg of cobalt in 0.2 ml of blood, which was limited by the blank level of cobalt in the reagents after purification.

Animals↗
PubMed2001

The influence of chlorine on the intensity of metal atomic lines emitted by an electrolyte cathode atmospheric glow discharge.

The effect of different matrix anions in the solution on the intensity of metal atomic lines was investigated. A significant increase in intensity was found for chloride anions compared with nitrate and sulfate anions. This effect was even greater when the appropriate acids were applied. A further enhancement of the metal line intensities could be observed when HCl was used in the solution phase and simultaneously elemental chlorine was mixed with atmospheric air at levels up to 6-10 vol.%. This double effect was especially high for the Cu, Ni and Pb resonant atomic lines at higher chlorine-to-air ratios in the gas phase, and the W-anode tip was destroyed by chemical burning. The application of volatile organic chlorine compounds (carbon tetrachloride and chloroform) in the gas phase, even without any acidification, also caused an enhancement of the metal line intensities. The experimental results can be attributed to the different rates of the ion-ion (positive metal ion-negative chloride ion) and the positive metal ion-electron recombination processes taking place in the cathode dark space of the discharge plasma, yielding neutral metal atoms for excitation. This study is important for the on-line measurement of heavy metals in liquids.

Journal Article↗
PubMed2001

Catalytic hydrogen evolution in cathodic stripping voltammetry on a mercury electrode in the presence of cobalt(II) ion and phenylthiourea or thiourea.

The system Co(II)-phenylthiourea (PTU)-borax buffer was investigated by cathodic stripping voltammetry (CSV) at a hanging mercury drop electrode. The results of the voltammetric measurements showed that the presence of both PTU and Co(II) gives rise to a new irreversible peak at about -1.5 V. Based upon our previous results obtained in the study of other sulfur compounds and the sulfide ion itself, the peak was ascribed to the catalytic hydrogen evolution superimposed on the reduction of the coordinated Co(II) ion. The catalyst itself is a Co(II) complex with the sulfide ion produced by the decomposition of the analyte during the deposition step. The influence of PTU and cobalt concentration, accumulation conditions and stripping parameters was investigated and complementary data on thiourea are included. The results showed that the measurement of the catalytic hydrogen evolution peak current can be used as a basis for a simple, accurate and rapid method for the determination of PTU within the concentration range 10-100 nM. The catalytic method is relatively free of interferences and could be a suitable alternative for cases in which the stripping peak due to mercury ion reduction in the accumulated mercury compound is disturbed by some interference.

Journal Article↗
PubMed2002

Cathodically activated nucleophilic aromatic substitution of hydrogen: a novel electrochemical mechanism.

Cathodically activated nucleophilic aromatic substitution of hydrogen (SNArH) is reported for the first time; the 1,3,5-trinitrobenzene radical anion reacts with the nucleophile N-methylformamide leading to high yields of the sigma H-complex radical anion; this intermediate can be easily oxidised electrochemically by means of a three-electron mechanism giving rise to the nucleophilic aromatic substitution product (NASH product) in good yield.

Journal Article↗