Association Reactions of the Anion Radicals of Some Hydroxyquinones: Evidence for Formation of π- and σ-Dimers As Well As a Neutral−Anion Radical Complex

The electrochemical reduction of nine quinones has been studied in acetonitrile and, in one case, in dimethyl sulfoxide. Included are seven hydroxyquinones plus 1,4-naphthoquinone, 8, and 9,10-anthraquinone, 9 . Each quinone is reduced in two steps, first to the anion radical and then, at more negative potentials, to the dianion. However, digital simulations show that the voltammetric data cannot be explained by these two reactions alone. For three of the hydroxyquinones, 2 − 4, plus 8 and 9, the fast disproportionation/comproportionation reaction connecting the neutral, anion radical, and dianion must be included along with a diffusion coefficient of the anion radical that is smaller than that of the neutral quinone and a still smaller diffusion coefficient for the dianion. Three other hydroxyquinones, 5 − 7, require, in addition, the formation of a neutral−anion radical complex. Finally, 5-hydroxy-1,4-naphthoquinone, 1, in acetonitrile involves both σ- and π-dimerization of the anion radical with subsequent reduction of the π-complex, whereas in dimethyl sulfoxide the σ-dimer is replaced by the aforementioned formation of the neutral−anion radical complex. The differences in behavior are discussed in terms of distribution of spin density in the anion radicals and intermolecular and intramolecular hydrogen bonding.