Entropy of electrochemical formation ofp-semiquinones in dimethylformamide

Entropy of formation of anion radicals at the mercury electrode in dimethyl-formamide for 1,4-benzoquinone, 1,4-naphthoquinone, 9,10-anthraquinone, and 5,12-tetracenequinone has been determined from the temperature dependences of the reversible half-wave potentials using a nonisothermal cell. The linear relationship between the entropy and the calculated spin density at the oxygen atom of semiquinones was established. Similarly the linear correlation of the literature reaction entropy for electrooxidation of a few heteroaromatic compounds versus the experimental spin density at a heteroatom (N, S or Se) from ESR measurements was also observed.

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Die Entropie der elektrochemischen Bildung vonp-Semichinonen in Dimethylformamid

Zusammenfassung Die Bildungsentropie von Anionenradikalen an der Quecksilberelektrode wurde in Dimethylformamid für 1,4-Benzochinon, 1,4-Naphthochinon, 9,10-Anthrachinon und 5,12-Tetracenchinon aus der Temperaturabhängigkeit der reversiblen Halbwellenpotentiale unter Benutzung einer nichtisothermen Zelle bestimmt. Es wurde eine lineare Abhängigkeit zwischen der Entropie und der berechneten Spindichte am Sauerstoffatom der Semichinone festgestellt. Eine ähnliche lineare Korrelation konnte für die Literaturdaten von Reaktionsentropien der Elektrooxidation einiger heteroaromatischer Verbindungen gegenüber den experimentellen Spindichten am Heteroatom (N, S, Se; ESR-Messungen) beobachtet werden.

     

An electrochemical and DFT study on selected beta-diketiminato metal complexes

Selected homoleptic metal beta-diketiminates M(I)L and M(II)L2 [M(I) = Li or K, M(II) = Mg, Ca or Yb; L: L(Ph) = [N(SiMe3)C(Ph)]2CH, L(Bu(t)) = N(SiMe3)C(Ph)C(H)C(Bu(t))N(SiMe3), L* = [N(C6H3Pr(i)2-2,6)C(Me)]2CH] have been studied by cyclic voltammetry (CV). The primary reduction (E(p)red, the peak reduction potential measured vs. SCE in thf containing 0.2 M [NBu4][PF6] with a scan rate 100 mV s(-1) at a vitreous carbon electrode at ambient temperature) is essentially ligand-centred: E(p)red being ca. -2.2 V (LiL(Ph) and KL(Ph)) and -2.4 V [Mg(L(Ph))2, LiL(Bu(t)) and Ca(L(Ph))2], while LiL* is significantly more resistant to reduction (E(p)red = -3.1 V). These observations are consistent with the view that the two (L(Ph)) or single (L(Bu(t))) C-phenyl substituent(s), respectively, are available for -electron-delocalisation of the reduced species, whereas the N-aryl substituents of L* are unable to participate in such conjugation for steric reasons. The primary reduction process was reversible on the CV-time scale only for LiL(Bu(t)), Ca(L(Ph))2 and Yb(L(Ph))2. For the latter this occurs at a potential ca. 500 mV positive of Ca(L(Ph))2, consistent with the notion that the LUMO of Yb(L(Ph))2 has substantial metal character. The successive reversible steps, each separated by ca. 500 mV, indicate that there is strong electronic communication between the two ligands of Yb(L(Ph))2. The overall three-electron transfer sequence shows that the final reduction level corresponds to [Yb(II)(L(Ph))2-(L(Ph))3-]. DFT calculations on complexes Li(L(Ph))(OMe2)2 and Li2(L(Ph))(OMe2)3 showed that both HOMO and LUMO orbitals are only based on the ligand with a HOMO-LUMO gap of 4.21 eV. Similar calculations on a doubly reduced complex Yb[(mu-L(Ph))Li(OMe2)]2 demonstrated that there is a considerable Yb atomic orbital contribution to the HOMO and LUMO of the complex.     

On the reduction of oxygen in dimethylformamide

The first reduction process in dimethylformamide containing 0.1 M tetraethylam-moniumperchlorate was studied. The techniques of polarography, controlled potential coulometry and potential sweep voltammetry were applied. The results obtained are consistent with the hypothesis that some chemical reactions are involved in the overall electrode process. In particular of the two chemical reactions considered, i.e. the protonation of the superoxide ion and that of its disproportion, the data seem to favour the first reaction over the second one. Evidences that the tetraethylammonium ion of the supporting electrolyte works as proton donor are provided.     

An electrochemical study of the incorporation of cobalt macrocyclic complexes into nafion films

Cobalt complexes of the macrocyclic ligand, 2,3,9,10-tetramethyl-1,4, 8,11-tetraazacyclotetradeca-1,3,8,10-tetraene (TIM) and its hydroxy derivatives, were incorporated in Nafion films by electrochemical cycling and by ion exchange in aqueous solution. Cyclic voltammetric studies show that the redox potentials of the Co(TIM) complexes undergo a significant anodic shift when incorporated in the films. Electrochemical studies also indicate that, while the Co(TIM) complexes reside in hydrophilic regions of the Nafion film, the metallated porphine complex, cobalt tetrakis(pentafluorophenyl)porphine, is probably immobilized in hydrophobic regions. The incorporation of the complexes was also followed by electronic absorption spectrophotometry.     

Cyclopentadienylnickel thiolate complexes: synthesis, molecular structures and electrochemical detection of sulfur dioxide adducts

Simple reactions between Ni(η⁵-C₅H₅)(PR₃)Br and the Schiff-base thiols, 4-HSC₆H₄NC(H)C₄H₂SBr-4^′ (1) and 4-HSC₆H₄NC(H)C₄H₃S (2), or organothiols, HSC₆H₄F-4 and HSC₆H₄NH₂-4, produced cyclopentadienylnickel thiolates of the formulae, Ni(η⁵-C₅H₅)(PR₃)(SC₆H₄NC(H)C₄H₂SBr-4) (3), Ni(η⁵-C₅H₅)(PR₃)(SC₆H₄NC(H)C₄H₃S) (4) or Ni(η⁵-C₅H₅)(PR₃)(SC₆H₄X-4) (R=Ph, X=F (6) or NH₂ (7) and R=Bu, X=F (5) or NH₂ (8)) which were characterized by a combination of analytical techniques. Complexes 3, 6 and 7 were structurally characterized by X-ray crystallography, showing that they possess the familiar trigonal geometry around the nickel center. These complexes react with sulfur dioxide, with 5, 6, 7 and 8 exhibiting substantial differences between the redox potentials of the pre- and post-SO₂ compounds to suggest that these complexes can be developed as potentiometric SO₂ sensors.     

水对胆绿素在二甲基甲酰胺中电化学氧化反应的影响

本文采用现场时间分辨光谱电化学技术结合循环伏安电化学方法对胆绿素(BV)在纯二甲基甲酰胺(DMF)和DMF-H~2O混合溶剂中的电化学氧化反应进行了较为系统的研究, 并提出了相应的反应机理。实验中发现, 在有机溶剂DMF中引入水不但促进了BV氧化反应的进行, 而且还使BV的氧化反应由在DMF中的ECEC历程转变为ECCECC反应过程。     

Comments on the mechanism of the electrochemical reduction of sulphur in dimethylformamide

This paper presents a clarification of the electrochemical behaviour of sulphur and polysulphide ions in non-aqueous solvents following the recent publication of a paper [Electrochem. Commun. 3 (2001) 514] devoted to the mechanism of the electrochemical reduction of sulphur in dimethylformamide.     

Synthesis and characterization of peroxo complexes of uranium(VI) with aroylhydrazone ligands

The uranium(VI) peroxo complexes containing aroylhydrazones ligands having composition [UO(O₂)L-L(NO₃)₂]·H₂O (where L-L = Benzoic acid[1-(Furan-2-yl)methylene] hydrazide, Benzoic acid[(thiophene-2-yl)methylene] hydrazide, Benzoic acid[1-(thiophene-2-yl)ethylidene] hydrazide, Benzoic acid(phenylmethylene) hydrazide, Benzoic acid[1-(anisol-3-yl)methylene] hydrazide and Benzoic acid[(p-chlorobenzyl)methylene] hydrazide are reported. The complexes were characterized by various physico-chemical techniques, viz. elemental analysis, molar conductivity, magnetic susceptibility measurements, infra red, electronic, mass spectral and TGA/DTA studies. These studies revealed that complexes are non-electrolytes and diamagnetic in nature. The ligands are bound to metal in a bidentate mode. Thermal analysis results provide conclusive evidence for the presence of water molecules in the complexes. Mass spectra confirm the molecular mass of the complexes. Antifungal activity of complexes revealed enhanced activity of complexes as compared to the corresponding ligands.     

Mechanism of the electrochemical reduction of 2-halo-5-nitrothiophenes in dimethylformamide

The reduction of 2-bromo-5-nitrothiophene and 2-iodo-5-nitrothiophene in dimethylformamide (DMF) was studied by means of classical and commutated polarography, EPR spectroscopy, a rotating platinum disk electrode with a ring, and preparative electrolysis. It was established that, depending on the nature of the halogen, their anion radicals may undergo further reduction to 2-nitrothiophene anion radicals or decomposition with splitting out of a halide anion and conversion to nitrothienyl radicals, which were identified on the ring electrode. The latter are capable of undergoing dimerization to 2,2-dinitro-5,5-dithienyl. The spectra of the anion radicals of 2-nitrothiophene and 2,2-dinitro-5,5-dithienyl were recorded by means of EPR. A mechanism for the reduction of halonitrothiophenes is proposed.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 195–198, February, 1981.     

An electrochemical study of antineoplastic gallium, iron and ruthenium complexes with redox noninnocent alpha-N-heterocyclic chalcogensemicarbazones

The electrochemical properties of a series of alpha-N-heterocyclic chalcogensemicarbazones (HL), namely, thiosemicarbazones, selenosemicarbazones, and semicarbazones, and their gallium(III), iron(III), and ruthenium(III) complexes with the general formula [ML(2)][Y] (M = Ga, Fe or Ru; Y = PF(6)(-), NO(3)(-), or FeCl(4)(-)) were studied by cyclic voltammetry. The novel compounds were characterized by elemental analysis, a number of spectroscopic methods (NMR, UV-vis, IR), mass spectrometry and by X-ray crystallography. All complexes show several, mostly reversible, redox waves attributable to the reduction of the noninnocent chalcogensemicarbazone ligands at lower potentials (<-0.4 V vs NHE) than the metal-centered iron or ruthenium redox waves (>0 V vs NHE) in organic electrolyte solutions. The cyclic voltammograms of the gallium complexes display at least two consecutive reversible one-electron reduction waves. These reductions are shifted by approximately 0.6 V to lower potentials in the corresponding iron and ruthenium complexes. The electrochemical, chemical, and spectroscopic data indicate that the ligand-centered reduction takes place at the CH(3)CN double bond. Quantum chemical calculations on the geometric and electronic structures of 2-acetylpyridine (4)N,(4)N-dimethylthiosemicarbazone (HL(B)), the corresponding metal complexes [Ga(L(B))(2)](+) and [Fe(II)(L(B))(2)], and the one-electron reduction product for each of these species support the assignment of the reduction site and elucidate the observed order of the ligand-centered redox potentials, E(1/2)([Fe(II)(L)(2)]) < E(1/2)(HL) < E(1/2)([Ga(L)(2)](+)). The influence of water on the redox potentials of the complexes is reported and the physiological relevance of the electrochemical data for cytotoxicity as well as for ribonucleotide reductase inhibitory capacity are discussed.     

Direct electrochemical reduction of a bromo-propargyloxy ester at vitreous carbon cathodes in dimethylformamide

Cyclic voltammograms for the reduction of ethyl 2-bromo-3-(3^′,4^′-dimethoxyphenyl)-3-(propargyloxy)propanoate (1) at glassy carbon electrodes in dimethylformamide containing tetraalkylammonium salts exhibit three prominent waves corresponding to cleavage of the carbon–bromine bond and to subsequent reduction of ethyl trans-3-(3^′,4^′-dimethoxyphenyl)-prop-2-enoate (4). Controlled-potential electrolyses of 1 at potentials corresponding to reduction of the carbon–bromine bond afford 4 as the major product with an average yield of 56%. In the presence of a proton donor (1,1,1,3,3,3-hexafluoro-2-propanol), the quantity of 4 decreases slightly, and 2-(3^′,4^′-dimethoxyphenyl)-3-(ethoxycarbonyl)-4-methyl-2,5-dihydrofuran (3) is obtained in moderate amount (26%). We propose a mechanistic scheme whereby the major products are formed via a combination of one- and two-electron processes.     

Conductance study of ammonium complexes with several crown ethers and cryptands in nitrobenzene,acetonitrile and dimethylformamide solutions

The formation of ammonium complexes with several crown ethers and cryptands in nitrobenzene, acetonitrile and dimethylformamide solutions was investigated by conductometry at 25°C. Stability constants of the resulting 1:1 complexes sere determined from the molar conductance-mole ratio data and found to vary in the order DC18C6>18C6>DB30C10>DB21C7>DB24C8>DB18C6>15C5>B15C5>12C4, in the case of crown complexes, and in the order C222>C221>C211>C22>C21 for the ammonium cryptates. The stabilities of the complexes varied inversely with the Gutmann donicity of the solvents. Influences of the number of members in the macrocycle, nature of the substituents in the polyether ring, cavity size and dimensionality, conformations of the free and complexed ligands and number of N⁺–H bonds available for hydrogen bonding are discussed.     

An electrochemical study of enzymatic oligonucleotide digestion

This paper describes the synthesis and application of a novel ferrocene (Fc) label that can be efficiently attached to oligonucleotides. We demonstrate how pulse electrochemical methods can be used to measure very low concentrations of ferrocene label and, importantly, show good electroanalytical discrimination between a labelled oligonucleotide and an enzyme digested labelled oligonucleotide, in which the ferrocene label nucleotide conjugate has been released. Real time in situ analysis gives a much greater understanding of the process. Potential applications include the detection of specific nucleic acid sequences and measurement of nuclease activity.     

N-Heterocyclic carbene adducts of orthopalladated triarylphosphite complexes

Carbene adducts of orthopalladated triarylphosphite complexes have been synthesised and characterised. The structures of three of these complexes were determined by single-crystal X-ray analysis. The complexes are active in the Suzuki coupling of a range of aryl bromide substrates.     

Synthesis and study of catalysts of electrochemical oxygen reduction reaction based on polymer complexes of nickel and cobalt with Schiff bases

The article presents the results of studies of new nanosize catalysts of electrochemical oxygen reduction reaction (ORR) obtained using the method of thermal decomposition of polymer complexes of nickel and cobalt with tetradentate (N₂O₂) Schiff bases. The catalysts are characterized using the methods of thermogravimetry, electrochemical quartz microgravimetry, scanning electron microscopy with X-ray microanalysis, XPS. The ORR process on electrodes modified by the above catalysts was studied using the voltammetry and rotating disk electrode techniques. The obtained catalysts manifested high specific activity per initial polymer mass (more than 600 mA/mg).