Electrochemical and chemical formation of a low-barrier proton transfer complex between the quinone dianion and hydroquinone

The electrochemistry of the 1,4-benzoquinone and hydroquinone in acetonitrile and dimethylsulfoxide was reviewed to explain the nature of a broad reversible signal that appears during the reduction of quinones with small amounts of proton donors and that cannot be explained in the framework of the classical mechanisms of quinone reduction. Cyclic voltammetry and NMR experiments as well as electronic structure calculations were performed to show that under specific conditions, the anion QH^- disproportionates into a face-to-face dianionic quinhydrone associated by strong intermolecular hydrogen bonds. This complex explains the mentioned broad signal and the results presented show that it can be formed during the reduction of benzoquinone in the presence of stoichiometric amounts of weak proton donors such as acetic acid and hydroquinone, or by half-deprotonation of hydroquinone. An analysis of the energetic barriers of the hydroxyl protons involved in the complex is also presented in this work.