Electrochemical and Spectroscopic Studies of 9,10‐Anthraquinone in a Room Temperature Molten Salt

Abstract

The electrochemical and spectroscopic properties of 9,10‐anthraquinone (AQ) in the low temperature :n‐butylpyridinium chloride (BuPyCl) molten salt system have been studied as a function of melt acidity. Infrared spectroscopic data indicate that AQ exists in the uncomplexed state in the basic melt (:1.0 BuPyCl). The electrochemical behavior in this region involves a single‐process two‐electron reduction (with slow electron transfer) of AQ to its dianion, the reduction mechanism probably proceeding by an ECE pathway. Oxidation of the dianion back to AQ occurs at a potential considerably positive (600 mV) of the potential for AQ reduction, thus indicating some interaction of the dianion with the melt. The complexation of AQ by in the acidic melt (:1.0 BuPyCl) produces as indicated both by infrared spectroscopy and chemical analysis. This complexation results in a shift in potential for the reduction process compared to the corresponding potential for AQ reduction in the basic melt of +1.4V. The reduction of also involves a single‐wave two‐electron process (with faster electron transfer than in the basic melt), thought to proceed by a disproportionation mechanism. Since the same separation in potentials for reduction and subsequent oxidation in the acidic melt was observed as that seen for AQ in the basic melt, some interaction of the dianion with the acidic melt is also evident. As the composition of the melt was varied through the neutral region (approx. :1.0 BuPyCl), in which the acidity is changing rapidly, an additional process due to reduction of was observed; by adjustment of melt acidity by small additions of or BuPyCl, a melt containing all three species AQ, , and could be obtained. Electrochemical studies of this system indicated that interconversion among the various species upon reduction is rather slow, an observation reflecting the low levels of present as well as the unbuffered nature of the melt in this region.