REDOX CHEMISTRY OF 1,4-DIHYDROXY-9,10-ANTHRAQUINONE (QUINIZARINE) AND ITS MANGANESE(II) COMPLEXES IN DIMETHYLSULFOXIDE

The redox behaviour of the ligand 1,4-dihydroxy-9,10-anthraquinone (Quinizarine) and its oxidation-reduction products (semiquinones and dianion) has been studied in dimethylsulphoxide, (CH3)2SO. The combination of Quinizarine in the presence of two equivalents of tetraethylammonium hydroxide (TEAOH) with manganese(II) in a 1:2 and 1:1 mole ratio forms two complexes. The first one has a stability constant of 9.0 x 10(12) M-2, whereas the second one precipitates. On the other hand further oxidation by electrolysis of the solution of Quinizarine in the presence of the two equivalents of TEAOH yields a green solution of the corresponding semiquinone. The combination of this semiquinone with manganese(II) in a 1:1 mole ratio forms a violet complex. These complexes and their oxidation reduction products have been characterized by cyclic voltammetry, controlled-potential electrolysis and optical spectroscopy in dimethylsulphoxide solvent. The formation constant for the complex has been evaluated spectrophotometrically and turns out to be 8.9 x 10(5) M-1. These results may be relevant to the mechanism of the role of manganese in biological systems.