Abstract

The combination of manganese(II) with quinizarine and the monoanion of quinaldic acid in dimethylsulfoxide yields a soluble deep-blue complex whose MnII : Qz2- : AcQ- stoichiometry is 1 : 1 : 2. This complex is oxidized in two steps each involving one equivalent of charge per metal ion as indicated by controlled-potential electrolyses at +0.30 V vs S.C.E. and +0.45 V vs S.C.E. The association between the metal ion and the ligands prevails and the oxidations produce finally a dark-red complex of the same stoichiometry as the original deep-blue species. In the latter the metal is in the oxidation state + 3 and the quinizarine dianion has been oxidized to the semiquinone form. The monoanion of quinaldic acid is unchanged. The combination of the semiquinone of quinizarine with MnII and quinaldic acid yields a species in which the metal ion presents magnetic characteristic of manganese(III) and the semiquinone has been reduced to the dianion of quinizarine, indicating that intramolecular charge-transfer has occurred. The voltammetric behaviour of the MnIII-semiquinone-quinaldic acid complex and its electrochemical reduction indicate that it is binuclear which upon reduction at -0.20 V vs S.C.E. generates a MnII-MnIII mixed-valence species. The latter is in turn reduced at -0.95 V vs S.C.E. producing probably a mononuclear species of MnII. © 1997 Elsevier Science Ltd.