ZINC(II) COMPLEXES WITH THE REDUCTION PRODUCTS OF 2,3-DIMETHOXY-1,4-NAPHTHOQUINONE IN DIMETHYL-SULFOXIDE REDOX CHEMISTRY AND SPECTROSCOPY

In dimethylsulphoxide the 2,3-dimethoxy-1,4-naphthoquinone (Q) is reduced in two successive one-electron steps, which are electrochemically reversible at low scan rates. At -0.72 V vs S.C.E. the semiquinone anion radical is formed and at - 1.42 V vs S.C.E. a second reduction to the corresponding dianion is observed. A mixture of zinc(II) and the dianion of 2,3-dimethoxy-1,4-naphthoquinone at a mole ratio of 1:1 in Me2SO yields a stable complex. The zinc(Il)-semiquinone complex is formed only on the surface of the electrode because under these conditions this radical ligand disproportionates producing the corresponding quinone and the dianion. The different forms of the ligand and the zinc(Il)-dianion complex have been characterized by cyclic voltammetry, UV-vis spectroscopy and magnetic susceptibility measurements. The stability constants for the 1:1 zinc(Il)-dianion and zinc(II)-semiquinone complexes are large, but it was not possible to determine their exact value as their voltammetric behaviour was irreversible. However the high stability of the dianion complex causes the disproportionation of the ligand in the semiquinone complex.