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 redu ced in two successive one-electron steps, which are electrochemically reversible at low scan rates. At -0.72 V vs S.C.E. the semiquinone ani on radical is formed and at - 1.42 V vs S.C.E. a second reduction to t he corresponding dianion is observed. A mixture of zinc(II) and the di anion of 2,3-dimethoxy-1,4-naphthoquinone at a mole ratio of 1:1 in Me 2SO yields a stable complex. The zinc(Il)-semiquinone complex is forme d only on the surface of the electrode because under these conditions this radical ligand disproportionates producing the corresponding quin one and the dianion. The different forms of the ligand and the zinc(Il )-dianion complex have been characterized by cyclic voltammetry, UV-vi s spectroscopy and magnetic susceptibility measurements. The stability constants for the 1:1 zinc(Il)-dianion and zinc(II)-semiquinone compl exes are large, but it was not possible to determine their exact value as their voltammetric behaviour was irreversible. However the high st ability of the dianion complex causes the disproportionation of the li gand in the semiquinone complex.