Manganese-quinizarin-pyrazinecarboxylic acid mixed-ligand complex in aprotic media. Formation of a manganese(III)-semiquinone binuclear species

The monoanion of pyrazincarboxylic acid (PcA(-)), the dianion of quinizarin e (Qz(=)) and manganese(II) yield a soluble deep-blue complex in dimethylsu lphoxide whose Mn-II: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-potent ial electrolyses done at +0.30 V versus s.c.e. and +0.50 V versus s.c.e.. T he association between the metal ion and the ligands prevail and the oxidat ions finally produce a dark-red complex which possesses the same stoichiome try as the original deep-blue species. In the latter the metal ion is prese nt 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 versu s s.c.e. of a solution of the mixed-ligand complex indicates that it is bin uclear, generating a Mn-II-Mn-III 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 manganes e(III) and the semiquinone is reduced to the quinizarine dianion, indicatin g that the mixed-ligand formed exhibits intramolecular charge-transfer. Thi s is a good example of a binuclear species accumulating four oxidation equi valents after oxidation of both metal centers and the quinizarine ligands.