Reduction of Dinitrobenzene Derivatives in Ethaline: Hydrogen-Bonding Interactions through the Inversion of Potentials in a Deep Eutectic Solvent

The reduction of dinitro-benzene derivatives have been examined in Ethaline containing a low amount of water (less than 0.2 %) chosen as a prototypical example of Type III-DES. Analyses of the voltammograms reveal a situation of potential inversion (i. e. the second electron transfer is easier and occurs at a least negative potential) for the reductions of 1,2- and 1,4-dinitrobenzene contrarily to the reduction of dinitrobenzene in a classical organic solvent. This is ascribed to H-Bond interactions in Ethaline that are different for a delocalized radical anion and the dianion in a molecule bearing two conjugated redox groups. Indeed, the inversion of potential is not observed for the 1,3-dinitrobenzene reduction. Reduction potentials in Ethaline are less negative than those in organic solvent (acetonitrile, DMF), that falls in line with the H-bonding stabilization of the electrogenerated charged species in Ethaline. The potential inversion is not observed in ethylene glycol (with 0.2 mol L-1 Et4N+ Cl-) that is one of the components of Ethaline suggesting that choline chloride plays a major role in the H-bonding/ion-pairing stabilization. Interestingly and despite these stabilizations, the charge transfer kinetics remain comparable with those measured in common organic solvents (k(s) around 0.13 cm s(-1)).