Redox chemistry of 1,2-dihydroxynaphthalene, 1,2-naphthosemiquinone and 1,2-naphthoquinone and their complexes with manganese(II) and manganese(III)

Cyclic voltammetry and controlled-potential electrolyses were carried out on solutions of 1,2-naphthoquinone (1,2-NQ), 1,2-dihydroxynaphthalene [1,2-Di(OH)-Cat] and their reduction and oxidation products in dimethyl sulfoxide. A study of the interaction of these species with Mn-II and Mn-III has been undertaken. The complexes Mn-III(Cat(2-))3(3-), Mn-II(Cat(2-))2(2-) and Mn-II(Cat(2-)), where Cat(2-) represents the dianion of 1,2-dihydroxynaphthalene, were obtained in solution and the species Mn(II)SQ has been detected only on the surface of the electrode. An attempt to obtain the latter complex quantitatively caused intramolecular charge-transfer yielding the more stable Mn-III-catechol complex. The Mn-III-semiquinone complex is unstable and the ligand is easily oxidized. The charge-transfer reaction between Mn-III(Cat(2-))3(3-) and Mn-II(Cat(2-))2(2-) is observed at -0.54/-0.52 V versus s.c.e. whereas the Mn-II complexes are reduced beyond the electrochemical window (at a potential more negative than -2.00 V versus s.c.e). On the other hand, all these species are destroyed when the coordinated ligand is oxidized. U.v.-vis. spectroscopy and magnetic susceptibility measurements were performed on the compounds in solution in order to characterize and to confirm the oxidation states of the metal ion in the species.