Analysis of the Substituent Effect on the Reactivity Modulation during Self-Protonation Processes in 2-Nitrophenols

A voltammetric and spectroelectrochemical ESR study of the reduction processes of five substituted 4-R-2-nitrophenols (R = −H, −OCH3, −CH3, −CN, −CF3) in acetonitrile was performed. In the potential range considered here (−0.2 to −2.5 V vs Fc+/Fc), two reduction signals (Ic and IIc) were detected; the first one was associated with the formation of the corresponding hydroxylamine via a self-protonation pathway. The voltammetric analysis at the first reduction signal showed that there are differences in the reduction pathway for each substituted 4-R-2-nitrophenol, being the E 1/2 values determined by the inductive effect of the substituent in the meta position with respect to the nitro group, while the electron-transfer kinetics was determined by the protonation rate ( ) of the anion radical electrogenerated. However, at potential values near the first reduction peak, no ESR signal was recorded from stable radical species, indicating the instability of the radical species in solution. Nevertheless, an intense ESR spectrum generated at the second reduction peak was detected for all compounds, indicating the monoelectronic reduction of the corresponding deprotonated 4-R-2-nitrophenols. The spin-coupling hyperfine structures revealed differences in the chemical nature of the electrogenerated radical; meanwhile, the −CF3 and −CN substituents induced the formation of a dianion radical structure, and the −H, −CH3, and −OCH3 substituents provoked the formation of an anion radical structure due to protonation by acetonitrile molecules of the initially electrogenerated dianion radical. This behavior was confirmed by analyzing the ESR spectra in deuterated acetonitrile and by performing quantum chemical calculations of the spin densities at each site of the electrogenerated anionic radicals.