Macrocyclic [N₄²⁻] Coordinated Nickel Complexes as Catalysts for the Formation of Oxalate by Electrochemical Reduction of Carbon Dioxide

The macrocyclic nickel complexes shown in Figure 1 are able to catalyze the electrochemical reduction of CO2 to oxalate. In the case of the complexes with R2 = COOEt or COMe, the overall reaction can be interpreted in terms of an outer-sphere electron-transfer reaction (6) followed by a dimerization of the CO2•- radical anions (7), but the variation of the electron-transfer rate constants with the standard potentials points to a coordinative interaction between the complexes and the CO2 molecule. Complexes without COOEt or COMe substitution in the R2 position undergo a fast deactivation reaction (first order with respect to [CO2]) that is even visible in the time scale of the cyclic voltammetric experiments. The results of the cyclic voltammetric investigations could be confirmed in preparative-scale electrolyses where the complex Ni−Etn(Me/COOEt)Etn proved to be the most active and persistent redox catalyst for the electrochemical reduction of CO2 to oxalate that has been described so far.