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.