Voltammetric and spectroscopic study of the manganese-quinizarine-quinaldic acid mixed-ligand complex in dimethylsulfoxide

The combination of manganese(II) with quinizarine and the monoanion of quinaldic acid in dimethylsulfoxide yields a soluble deep-blue complex whose Mn-II:Qz(2-):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 Mn-II 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 Mn-III-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 Mn-II-Mn-III mixed-valence species. The latter is in turn reduced at -0.95 V vs S.C.E. producing probably a mononuclear species of Mn-II. (C) 1997 Elsevier Science Ltd.