Voltammetric study of the redox chemistry of 2,3-dihydroxy-quinoxaline and its zinc complexes in non-aqueous medium

Abstract

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.

Neither the protonated ligand nor its quinonic form exhibit the formation of stable complexes with zinc(II) whereas the dianionic species produces two coordination compounds having M:L stoichiometric ratios of 1:1 and 1:2.

The semiquinone form of the ligand by itself shows the tendency to polymerize on the surface of the electrode and it is not stabilized in the presence of this metal ion. A neutral insoluble compound that passivates the electrode is formed when the semiquinone and this metal ion interact. It was not possible to study this complex in detail but the electrochemical evidence indicates that it has formed on the surface of the electrode. (C) 1998 Elsevier Science Ltd. All rights reserved.