Metallosalen-based polymers for reduction of carbon dioxide

Abstract

In this research, a series of novel asymmetric copper- and nickel-salens bearing thiophene-based substituents were successfully synthesized and characterized by nuclear magnetic resonance spectroscopy and mass spectrometry. Homogeneous electrocatalytic activities of these monomers toward reduction of CO2 were investigated using cyclic voltammetry technique. Electrochemical studies showed these salen monomers are able to serve as catalysts in the electrochemical reduction of CO2 due to the significantly increase of the current observed under CO2-saturated condition, compared with those found under the N2-saturated one. Results also revealed that insertion of the carbon-carbon triple-bond between the thiophene-based substituents and the salen core led to the lower required reduction potential, while the additional thiophene rings did not significantly influence in this aspect. Furthermore, addition of 3% of H2O as a proton source resulted in even higher current enhancement in the reduction of CO2. Electropolymerization of the target monomers through their thiophene-based units gave desirable metallosalen-based polymers in most cases. In the polymerization process, results showed that the introduction of bithiophenyl rings led to the lower required oxidation potential, while the derivatives containing carbon-carbon triple-bond spacers required higher oxidation potential than those having the thiophene-based substituents directly linked to the salen cores. However, further optimization of the polymerization condition is required to obtain more efficient films for the reduction of CO2.