Hydrogen bonding complexes in the quinone-hydroquinone system and the transition to a reversible two-electron transfer mechanism

• Electrochemical reduction of 1,4-benzoquinone in presence of hydroquinone as hydrogen bonding proton donor.
• X-Ray structures of anions arising from the 1,4-benzoquinone and hydroquinone.
• Formation of a quinhydrone type dianionic complex.
• Adsorption of anionic quinone-hydroquinone complexes and its stabilization by the supporting electrolyte cations.
• Reversible two-electron transfer behaviour in the quinone hydroquinone system.

The electrochemical reduction of 1,4-benzoquinone in acetonitrile in the presence of hydroquinone as a hydrogen bonding proton donor was studied. Different association modes and mechanistic regimes were found depending on the hydroquinone concentration. At stoichiometric concentrations, the formation of a dianionic quinhydrone linked by two-equivalent strong hydrogen bonds, was found. At higher concentrations of hydroquinone, the structure of this complex changes to another in which the monoprotonated quinone dianion is stabilized by interaction with hydroquinone and the supporting electrolyte cation. This complex is adsorbed on the glassy carbon electrode during its formation and the amount deposited is influenced by the concentration and size of the electrolyte cations. However, by further increasing of the hydroquinone concentration, the complex adsorption is totally inhibited and the cyclic voltammetry behaviour becomes reversible with the transfer of two-electrons.

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