Analytical Sciences

Mechanistic Study on the Electrochemical Reduction of 9,10-Anthraquinone in the Presence of Hydrogen-bond and Proton Donating Additives

Jiro KATSUMI,* Tatsushi NAKAYAMA,** Yukihiro ESAKA,*,** and Bunji UNO*,**

*Graduate School of Drug Discovery and Medical Information Sciences, Gifu University, Yanagido, Gifu 501-1193, Japan
**Gifu Pharmaceutical University, Daigaku-nishi, Gifu 501-1196, Japan

The electrochemical reduction of 9,10-anthraquinone (AQ) was investigated in CH₃CN in both the absence and presence of the hydrogen-bond and proton donating additives, CH₃OH, CH(CF₃)₂OH, phenol, 4-methoxyphenol, 4-cyanophenol, 2,4,6-trichlorophenol, and benzoic acid (BA). Three clearly different types of electrochemical behavior were observed with increasing concentrations of the additives, and were simulated to analyze the reaction mechanisms. Type I was observed for weakly interacting additives, such as CH₃OH, characterized by positive shifts of the two well-separated reduction waves, corresponding to the formation of AQ^•− and AQ²⁻, with no loss of reversibility. The second wave shifted more strongly, and finally merged with the first. These behaviors are explained by the association of AQ²⁻ with the additives via strong hydrogen-bonding. Type II is attributed to a reduction mechanism involving quantitative formation of strong hydrogen-bonded complexes of AQ²⁻ with additives, such as CH(CF₃)₂OH, phenol and 4-methoxyphenol, showing a reversible or quasireversible two-electron reduction wave with increasing concentrations of the additives. The behavior of Type III, observed in the presence of strongly interacting additives, such as 2,4,6-trichlorophenol and BA, is characterized by a voltammogram composed of the 2-electorn cathodic and the broad anodic waves without keeping reversibility, facilitated by proton transfer in the hydrogen-bonded complexes, AQ^•−-BA and AQ²⁻-BA. The effects of hydrogen-bonding and protonation on the electrochemistry of AQ have been systematically demonstrated in terms of the potentials and reaction pathways of the various species, which appear in quinone-hydroquinone systems.

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