title: Proton-coupled electron transfer in azobenzene/hydrazobenzene couples with pendant acid-base functions. Hydrogen-bonding and structural effects.
abstract: 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.
authors.affiliation: Laboratoire d'Electrochimie Moléculaire, UMR 7591, CNRS, Université Paris Diderot , Sorbonne Paris Cité, 15 rue Jean-Antoine de Baïf, F-75205 Paris Cedex 13, France.
authors.forename: Jean-Michel
authors.initials: JM
authors.lastname: Savéant
authors.forename: Cédric
authors.initials: C
authors.lastname: Tard