VOLTAMMETRIC STUDY OF THE REDOX CHEMISTRY OF 2,3-DIHYDROXY-QUINOXALINE AND ITS ZINC-COMPLEXES IN NONAQUEOUS MEDIUM

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 a s solvents the monoanion of the ligand is obtained quantitatively afte r 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 fo rm exhibit the formation of stable complexes with zinc(II) whereas the dianionic species produces two coordination compounds having M:L stoi chiometric ratios of 1:1 and 1:2. The semiquinone form of the ligand b y itself shows the tendency to polymerize on the surface of the electr ode and it is not stabilized in the presence of this metal ion. A neut ral insoluble compound that passivates the electrode is formed when th e semiquinone and this metal ion interact. It was not possible to stud y this complex in detail but the electrochemical evidence indicates th at it has formed on the surface of the electrode. (C) 1998 Elsevier Sc ience Ltd. All rights reserved.