Abstract

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 × 1012 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 × 105 M−1. These results may be relevant to the mechanism of the role of manganese in biological systems.