REDOX CHEMISTRY OF 3,4-DIHYDROXY-2-BENZOIC ACID, ITS OXIDATION-PRODUCTS AND THEIR INTERACTION WITH MANGANESE(II) AND MANGANESE(III)

The redox chemistry of the ligand 3,4-dihydroxybenzoic acid (3,4-DHBA) has been studied in dimethylsulphoxide and the conditions for the for mation of the corresponding semiquinone and quinone have been determin ed. The manganese(II) and manganese(III) complexes with the different forms of this ligand have been characterized by cyclic voltammetry, UV -vis spectroscopy and magnetic susceptibility measurements. Neither th e neutral nor the monoanionic form of the ligand show the formation of complexes with the metal ions. The dianion can be oxidized electroche mically to the corresponding semiquinone, at a more positive potential due to the presence of a deactivating group, forming a ''peroxo-type' ' dimer. The subsequent oxidation of this species generates the corres ponding quinone. The dianion and the semiquinone forms of the ligand p roduce manganese(II) and manganese(III) complexes with 1:2 stoichiomet ry, which is favoured by the ortho position of the hydroxide groups. T hese results may be relevant for the development of models for biologi cal systems.