Homogeneous Electron Transfer Catalysis of the Electrochemical Reduction of Carbon Dioxide. Do Aromatic Anion Radicals React in an Outer-Sphere Manner?

Electrochemically generated anion radicals of aromatic nitriles
and esters possess the remarkable property
to reduce carbon dioxide to oxalate with negligible formation of
carboxylated products. They may thus serve as
selective homogeneous catalysts for the reduction of CO2 in
an aprotic medium. The catalytic enhancement of the
cyclic voltammetric peaks of these catalysts is used to determine the
rate constant of the electron transfer from these
aromatic anion radicals to CO2 as a function of the
catalyst standard potential. Substituted benzoic esters
allowed
a particularly detailed investigation of the resulting
activation-driving force relationship. Using 14 different
catalysts
in this series made it possible to finely scan a range of reaction
standard free energies of 0.4 eV. Detailed analysis
of the resulting data leads to the conclusion that the reaction is not
a simple outer-sphere electron transfer. It rather
consists in a nucleophilic addition of the anion radical on
CO2, forming an oxygen (or nitrogen for the
nitriles)−carbon bond, which successively breaks homolytically, generating the
parent ester (or nitrile) and the anion radical
of CO2, which eventually dimerizes to oxalate.