Effect of hydroxylic additives on the electrochemical reduction of some azobenzenes

The reduction of trans  -azobenzene, Azb, has been studied in acetonitrile in the absence and presence of added water. The first step of reduction involves formation of the anion radical, Azb-Azb-, and in the second Azb-Azb- is reduced to Azb2− followed by rapid protonation by water to give the monoanion of hydrazobenzene, AzbH−. This second step of reduction was a candidate for a concerted proton–electron-transfer reaction (CPET) but it turned out instead to be electron transfer followed by proton transfer. From studies of the formal potential of the first step as a function of water concentration, formation constants for the 1:1 (Azb(H2O))(Azb(H2O)) and 2:1 (Azb-(H2O)2)(Azb-(H2O)2) hydrogen-bonded complexes were evaluated. Proton transfer from water to Azb-Azb- occurs leading to AzbHAzbH, which in turn is reduced to AzbH−. Kinetic analysis of the voltammetric results indicates that this proton transfer occurs within the 3:1 hydrogen-bonded complex, (Azb-(H2O)3)(Azb-(H2O)3), a species with a very small formation constant, 0.3 M−1. Two other derivatives, trans-4,4′-dichloroazobenzene and trans-4-dimethylaminoazobenzene were studied briefly and found to behave similarly to Azb.