Electrocatalytic reduction of CO2 by aza-macrocyclic complexes of Ni(II), Co(II), and Cu(II). Theoretical contribution to probable mechanisms

The electrocatalyzed reduction of carbon dioxide was investigated with cobalt(II), nickel(II) and copper(II) complexes of hexaaza-macrocycle ligands. The macrocyclic ligands were obtained via the condensation of phenanthroline or bipyridine. The X-ray structure of a triflate derivative was determined and it showed the less common “CuN4O2” centrosymmetric octahedral configuration with apically elongated CuO bonds. A preference for a neutral, mononuclear, octahedral configuration is exhibited by the planar macrocycle with an extended π-electron system. Its structure is characterized by shorter CuN bonds than those of Cu complexes with saturated macrocycles. The reduction of the complexes has been studied by cyclic voltammetry and UV–Vis spectroscopy. The results show that the metal center in its lower oxidation state, i.e. M(I), is the active site. The metal center is oxidized to M(II) in the presence of CO2 with the concomitant formation of an adduct between the metal center and CO2. A further one electron reduction regenerates the catalyst and produces carbon monoxide and/or formic acid. A theoretical model is proposed for a simple mechanism that involves initial coordination of CO2 to the metal center.