Abstract

When mixts. of Mn(II) and 2-​hydroxy-​1,​4-​naphthoquinone (Lawsone) with 1:1 and 1:2 mol ratios are reduced in aprotic solvents, stable [MnIII(ONO2)​] and [MnIV(ONO2)​2]​2-​ complexes are formed, resp. The ONO2 represents the trianion of the fully reduced ligand. These complexes and their redox products were characterized by cyclic voltammetry, controlled-​potential electrolysis, optical spectroscopy, and magnetic susceptibility measurements in DMSO and MeCN solvents. On the basis of these results, a self-​consistent redox mechanism is presented for the interconversion of the various species of Mn(II)​-​Lawsone systems with mole ratios 1:1 and 1:2. The formation const. for the [MnIII(ONO2)​] complex was evaluated voltammetrically to be 4 × 1014 M-​1. The direct interaction of Mn(III) with semiquinone leads to disproportionation, yielding Mn(II) and the free ligand. These results may be relevant to the mechanism of the O evolution reaction in photosystems II of green-​plant photosynthesis.