Electrochemical reduction of CO2 catalysed by transition metal complexes

Cobalt and nickel complexes of N,N'-1,2-phenylenebis(salicylideneiminato) (L = salophen) have been investigated by cyclic voltammetry and controlled-potential electrolysis as potential catalysts for the electrochemical activation of CO2. In the case of Ni(II)L, no catalysis was observed. The first electron uptake is ligand-based, yielding radical anion Ni(II)L- which rapidly dimerizes. On the contrary, the electron transfer to Co(II)L is metal-centered giving Co(I)L- complex which in MeCN containing LiClO4 as background electrolyte appears to catalyze the CO2 reduction to CO and carbonate, in a potential region where direct CO2 reduction does not occur. Mechanistic investigations point to the interaction of CO2 with Co(I)(L)Li to give a precursor complex containing a head-to-tail CO2 dimer. Further reduction of the latter yields a cobalt-carbonyl complex which releases quite slowly the final CO2 reduction product CO.