REDOX CHEMISTRY AND SPECTROSCOPY OF THE MANGANESE-QUINIZARINE-THIOSALICYLIC ACID MIXED-LIGAND COMPLEX IN APROTIC MEDIUM

The addition of the monoanion of thiosalicylic acid to the insoluble c ompound formed between manganese(II) and the dianion of quinizarine fo rms a very intense blue soluble new complex. The stoichiometry of this complex is Mn:Qz:2-MBA = 1:1:2, in the presence of 4 equivalents of b ase in order to neutralize the two hydroxylic protons of quinizarine a nd the carboxylic proton of each equivalent of 2-MBA. This mixed-ligan d complex is oxidized in several steps and a total of 4 equivalents of charge per metal ion are transferred in this process. The complete ox idation at +0.90 V vs S.C.E. produces the destruction of this mixed-li gand complex. However, it is possible to oxidize sequentially at +0.35 and +0.65 V vs S.C.E. and the association between the metal ion and t he ligands prevails. These two oxidations involve 1 equivalent of char ge each and lead to the formation of a dark red complex. The stoichiom etry of this complex is maintained with respect to the original blue c ompound and it is proposed that the metal ion is in the +3 oxidation s tate, the quinizarine has been oxidized to its semi-quinonic form and the monoanion of thiosalicylic acid has not changed. The final process at +0.90 V vs S.C.E. corresponds to the oxidation of the monoanion of the latter which leads to the decomposition of the complex. It is con cluded that the mixed-ligand complex formed after the two-electron oxi dation process can be considered as a very good model for biological s ystems in which it is known that manganese plays an important role as a catalyst for redox processes and ligands such as those studied here are present.