Proton-Coupled Electron Transfer in Azobenzene/Hydrazobenzene Couples with Pendant Acid–Base Functions. Hydrogen-Bonding and Structural Effects

Electron transfer in azobenzene derivatives bearing two carboxylic acid groups is coupled with intramolecular proton transfer in a stepwise manner in the title 2e(-) + 2H(+) redox couple. The presence of the pendant acid-base functions pushes the redox chemistry of the azo/hydrazo couple toward positive potentials by as much as 0.75 V. This is essentially the result of H-bonding of one of the nitrogen atoms by the neighboring carboxylic group and H-bonding of one carboxylate by the neighboring protonated nitrogen atom. The two electron-transfer reactions, particularly the second one, are accompanied by strong structural changes, which results in the occurrence of a square scheme mechanism in which electron transfer and structural change are not concerted. These are typical phenomena that are likely to be encountered when attempting to boost proton-coupled electron-transfer stoichiometric or catalytic processes by installing pendant acid-base functionalities in the close vicinity of the reacting center.