Unconventional redox properties of hydroquinones with intramolecular OH-N hydrogen bonds

The redox (chemical and electrochemical) properties of several hydroquinones are reported in which the OH protons are engaged in intramolecular hydrogen bonds to a nitrogen-based acceptor (pyridine or amine). The 1,4-hydroquinones generally undergo reversible oxidation to quinones in which both OH protons have transferred to the pendant bases; the oxidation processes are generally chemically and electrochemically reversible, in stark contrast with normal hydroquinones, which are oxidized irreversibly (via proton loss) to quinones. The oxidation processes, believed to occur in concerted proton/electron transfer steps, are at much lower potentials for the hydrogen-bonded derivatives relative to unsubstituted derivatives. In contrast, isomeric 1,3-hydroquinones (resorcinols) are oxidized irreversibly at relatively high potentials. The stability of some of the 1,4-hydroquinone oxidized species permits their isolation and characterization both spectroscopically and structurally. Somewhat surprisingly, in the oxidized species in which the proton is now located on the nitrogen base, the characterization data indicate that there is no NH-O hydrogen bond. Relationships between the particulars of the redox properties of the hydroquinones (potentials, reversibility/stability) and molecular structure are discussed.