Abstract

The addn. of 2-​MBA-​ (2-​MBAH = thiosalicylic acid) to the insol. compd. formed between Mn(II) and Qz2-​ (H2Qz = quinizarine) forms an intense blue sol. new complex. The stoichiometry of this complex is Mn:Qz:2-​MBA = 1:1:2, in the presence of 4 equiv of base to neutralize the 2 hydroxylic protons of H2Qz and the carboxylic proton of each equiv. of 2-​MBAH. This mixed-​ligand complex is oxidized in several steps and a total of 4 equiv of charge per metal ion are transferred in this process. The complete oxidn. at +0.90 V vs. SCE produces the destruction of this mixed-​ligand complex. It is possible to oxidize sequentially at +0.35 and +0.65 V vs. SCE and the assocn. between the metal ion and the ligands prevails. These 2 oxidns. involve 1 equiv of charge each and give a dark red complex. The stoichiometry of this complex is maintained with respect to the original blue compd. and probably the metal ion is in the +3 oxidn. state, H2Qz was oxidized to its semiquinonic form and 2-​MBA-​ has not changed. The final process at +0.90 V vs. SCE corresponds to the oxidn. of 2-​MBA-​ which leads to the decompn. of the complex. The mixed-​ligand complex formed after the 2-​electron oxidn. process can be considered as a good model for biol. systems in which Mn plays a role as a catalyst for redox processes and ligands such as those studied here are present.