New insight into 4-nitrobenzene diazonium reduction process: Evidence for a grafting step distinct from NO2 electrochemical reactivity

Abstract

International audienceElectrochemical and spectroscopic investigations were performed in order to clarify the mechanism of 4-nitrobenzene diazonium reduction on glassy carbon in protic medium. The number and nature of the electron transfer processes were found to be strongly correlated to the electrode surface state. On polished electrode two different reduction peaks were observed. Selective electrolyses realized at the corresponding potential definitely proved that the grafting process actually occurs at a potential distinct from NO2 electroreduction, this latter inducing the presence of the quasi-reversible NO/NHOH redox couple at the electrode surface. These results were confirmed by XPS analyses. Furthermore, voltammetric experiments using Fe(CN)63- showed that the electrochemical properties of the modified electrode strongly depend on the potential applied for grafting, which modulates the nitro group oxidation state. All the results suggested that the electrode functionalization was more efficient when grafting and NO2 reduction were performed separately

Functionalization of Carbon Materials by Reduction of Diazonium Cations Produced in Situ in a Brønstedt Acidic Ionic Liquid

“…Additional experiments were also performed by varying the concentration of 4‐nitroaniline in [BMIm][HSO 4 ] (5 and 50 m m , respectively), using cyclic voltammetry or chronoamperometry for completing the electrografting. The reported Γ values are in the range 2–10×10 −10 mol cm −2 (see Table S1), and these values are in fairly good agreement with the surface concentration values reported for nitrobenzene grafted layers either in conventional aqueous and organic electrolytes or in other ionic liquids values (reported values in the range 1.5–23×10 −10 mol cm −2 ) . They could be compared to the ideal theoretical value of 12×10 −10 mol cm −2 calculated for a close‐packed monolayer .…”

“…This redox system of rather low intensity does not significantly change during the five subsequent cycles (Figure S1 in the Supporting Information, SI). This behaviour has been already documented in a conventional electrolyte, as well as in an ionic liquid . The appearance of the redox system can be ascribed to the reversible oxidation of hydroxylamine moieties (‐NHOH) to nitroso groups (‐NO) [Eq.…”

“…], resulting from a partial reduction of the nitro substituents of the 4‐nitrobenzene diazonium chemisorbed onto the electrode surface [Eq. ]: …”

“…Four cycles were needed until the corresponding ‐NHOH/‐NO signal centered at 0.3 V (vs. SCE) exhibits a stable current intensity, suggesting that most of the attached nitro moieties have been transformed (Figure ). The signal of the quasi‐reversible oxidation of phenylhydroxylamine to nitrosophenyl groups in aqueous acidic electrolytes is often employed to assess for the grafting of nitrobenzene groups and to estimate the surface concentration of the electrografted organic layer …”

“…(2)], and the corresponding surface concentration cannot be evaluated because the oxidation of the amine does not occur in this potential window. Also the reduction of ‐NO 2 into ‐NHOH or into ‐NH 2 has been demonstrated to be highly sensitive to the solvent used and potential applied . Indeed, we have swept a GC ‐Ar‐NO 2 electrode (5 scans at 100 mV s −1 ) in aqueous 1 m H 2 SO 4 with a more extended potential window (0.6 to −0.8 V vs. SCE).…”