Browsing by Author "del Valle, MA"

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    Manganese-quinizarin-pyrazinecarboxylic acid mixed-ligand complex in aprotic media. Formation of a manganese(III)-semiquinone binuclear species
    (KLUWER ACADEMIC PUBL, 1999) Bodini, ME; del Valle, MA; Copia, G; Jara, F
    The monoanion of pyrazincarboxylic acid (PcA(-)), the dianion of quinizarine (Qz(=)) and manganese(II) yield a soluble deep-blue complex in dimethylsulphoxide whose Mn-II:Qz(=):PcA(-) stoichiometry has been established as 1:1:1. This mixed-ligand complex is oxidized in two steps, each involving one equivalent of charge per complex present, as indicated by controlled-potential electrolyses done at +0.30 V versus s.c.e. and +0.50 V versus s.c.e.. The association between the metal ion and the ligands prevail and the oxidations finally produce a dark-red complex which possesses the same stoichiometry as the original deep-blue species. In the latter the metal ion is present in oxidation state +3 with the quinizarine dianion as the corresponding semiquinone of the oxidized quinizarine. The monoanion of pyrazincarboxylic acid remains unchanged. Controlled-potential electrolysis at -0.20 V versus s.c.e. of a solution of the mixed-ligand complex indicates that it is binuclear, generating a Mn-II-Mn-III mixed-valence species. The latter is, in turn, reduced at -1.60 V versus s.c.e. producing probably a mononuclear of manganese(II) species. If manganese(II) is combined with the semiquinone of quinizarine the metal ion exhibits the magnetic characteristic of manganese(III) and the semiquinone is reduced to the quinizarine dianion, indicating that the mixed-ligand formed exhibits intramolecular charge-transfer. This is a good example of a binuclear species accumulating four oxidation equivalents after oxidation of both metal centers and the quinizarine ligands.
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    Thin amorphous platinum films photochemically obtained, and their potential use as modified electrodes
    (ELSEVIER SCIENCE SA, 2002) Tejos, M; Schrebler, R; Diaz, FR; del Valle, MA
    Thin amorphous nanostructured Pt films have been photochemically obtained by means of direct UV radiation (254 nm) of an amorphous Pt[CH3(CH2)(8)COCHCOC4H9](3)K film deposited on Si(100) and on ITO glass through the spin-coating technique. The product of this photolysis was analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and microanalysis. After that, the film was subject to cycles for 1 h, between -0.30 and 1.20 V, in a 0.50 M H2SO4 electrolytic solution. It may be observed that the peak potential shifts in approximately 0.04 V towards less positive values for the modified system, if compared with the platinum bulk. It only showed one peak in the hydrogen discharge if compared with the two peaks in the Pt bulk. Based on these data, the ITO/Pt modified system was tested as an electrodic substrate for aniline polymerization in 0.50 M H2SO4. The polymeric deposit was obtained at lower potentials than those required when the Pt bulk is used as an electrode. (C) 2002 Elsevier Science B,V, All rights reserved.
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    Voltammetric study of the redox chemistry of 2,3-dihydroxy-quinoxaline and its zinc complexes in non-aqueous medium
    (PERGAMON-ELSEVIER SCIENCE LTD, 1998) Bodini, ME; del Valle, MA; Copia, GE; Soto, C
    The electrochemical behaviours of 2,3-dihydroxy-quinoxaline (2,3-DH(2)Qx) and the complex species formed with zinc(II) have been studied in non-aqueous media. Using dimethylsulphoxide or N,N-dimethylformamide as solvents the monoanion of the ligand is obtained quantitatively after reduction of one of the hydroxylic protons. The reduction potential for the hydroxylic proton is -1.93 V vs S.C.E. in DMSO and -1.68 V vs S.C.E. in DMF, reflecting the different donor number of the solvents. If acetonitrile is used, the simultaneous reduction of both hydroxylic protons is observed. The oxidation to the semiquinone is observed at -0.12 V us S.C.E. in DMSO and DMF whereas it appears at -0.02 V us S.C.E. in acetonitrile.