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 comple x whose Mn-II:Qz(2-):ACQ(-) stoichiometry is 1:1:2. This complex is ox idized 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 t he ligands prevails and the oxidations produce finally a dark-red comp lex of the same stoichiometry as the original deep-blue species. In th e latter the metal is in the oxidation state +3 and the quinizarine di anion has been oxidized to the semiquinone form. The monoanion of quin aldic acid is unchanged. The combination of the semiquinone of quiniza rine with Mn-II and quinaldic acid yields a species in which the metal ion presents magnetic characteristic of manganese((III)) and the semi quinone has been reduced to the dianion of quinizarine, indicating tha t intramolecular charge-transfer has occurred. The voltammetric behavi our of the Mn-III-semiquinone-quinaldic acid complex and its electroch emical 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 m ononuclear species of Mn-II. (C) 1997 Elsevier Science Ltd.