Electrogenerated Bases: The Role of Weak Electroinactive Proton Donors and the Effect of Electrocatalysis on the Redox Behavior of Azobenzene

Article abstract of DOI:10.1021/jo00218a028

The stepwise electrochemical reduction of PhN=NPh in DMF-0.1 M (CH3)4NPF6 can give, depending upon proton availability, three anionic products.The basicities of these electrogenerated bases increase in the order PhN=NPh^- radical < PhNHN^-Ph < PhN^-N^-Ph.Water (pK_a^Me2SO=31.6), which is capable only of protonating PhN^-N^-Ph, causes the reversible PhN=NPh^- radical/PhN^-N^-Ph redox couple to shift to more positive potential.From the magnitude of the potential shift and the reported value of pK_a^DMF(H2O) = 34.7, pK_a^DMF(PhNHN^-Ph) is estimated to be 38.1 Ph2C=NNH2 and PhNHNHPh both protonate PhN^-N^-Ph irreversibly.The PhNHN^-Ph that is produced in this reaction is subsequently consumed in an electrocatalytic cycle which involves PhN=NPh as the oxidant, the less easily oxidized PhNHN^-Ph as the reductant, and proton transfer from PhNHNPh to the conjugate base of the added proton donor as the rapid, irreversible follow-up reaction.The anodic peak for the irreversible oxidation of PhNHN^-Ph is discernible only at scan rates in excess of about 1 V/s at -51 deg C.Protonation of PhNHN^-Ph by both diethyl malonate (pK_a^Me2SO=16.4) and fluorene (pK_a^Me2SO=22.6) and of PhN=NPh^- radical by diethyl malonate is irreversible and affords PhNHNHPh and the conjugate base of the added proton donor as the principal electroactive products.