Abstract

The redox behaviour of the ligand Lapachol has been studied in dimethylsulphoxide, and the conditions for the formation of the corresponding semiquinone have been determined. The combination of this semiquinone with manganese(II) in a 1:1 mole ratio forms a green complex which has been characterized by cyclic voltammetry, controlled-potential electrolysis, optical spectroscopy and magnetic susceptibility measurements. The metal centre in this complex has the characteristics of manganese(III), indicating an intramolecular charge-transfer between the metal and the ligand. The formation constant for the [MnIII(TA)] complex (TA represents the trianion of the fully reduced ligand) has been evaluated voltammetrically and turns out to be 1.8 × 1015 M−. The direct interaction of manganese(III) with semiquinone leads to disproportionation yielding manganese(II) and the free ligand. These results may be relevant to the mechanism of the oxygen evolution reaction in photosystem II of green plant photosynthesis.