Abstract

The monoanion of pyrazincarboxylic acid (PcA-), the dianion of quinizarine (Qz=) and manganese(II) yield a soluble deep-blue complex in dimethylsulphoxide whose MnII:Qz= :PcA- stoichiometry has been established as 1:1:1. This mixed-ligand complex is oxidized in two steps, each involving one equivalent of charge per complex present, as indicated by controlled-potential electrolyses done at +0.30 V versus s.c.e. and +0.50 V versus s.c.e.. The association between the metal ion and the ligands prevail and the oxidations finally produce a dark-red complex which possesses the same stoichiometry as the original deep-blue species. In the latter the metal ion is present in oxidation state +3 with the quinizarine dianion as the corresponding semiquinone of the oxidized quinizarine. The monoanion of pyrazincarboxylic acid remains unchanged. Controlled-potential electrolysis at -0.20 V versus s.c.e. of a solution of the mixed-ligand complex indicates that it is binuclear, generating a MnII-MIII mixed-valence species. The latter is, in turn, reduced at -1.60 V versus s.c.e. producing probably a mononuclear of manganese(II) species. If manganese(II) is combined with the semiquinone of quinizarine the metal ion exhibits the magnetic characteristic of manganese(III) and the semiquinone is reduced to the quinizarine dianion, indicating that the mixed-ligand formed exhibits intramolecular charge-transfer. This is a good example of a binuclear species accumulating four oxidation equivalents after oxidation of both metal centers and the quinizarine ligands.