Highly Efficient Visible-Light-Driven CO₂-to-CO Conversion by Coordinatively Unsaturated Co-Salen Complexes in a Water-Containing System

Abstract

The development of cost-effective catalysts for CO2 reduction is highly desired but remains a significant challenge. The unsaturated coordination metal center in a catalyst is favorable for the process of catalytic CO2 reduction. In this paper, two asymmetric salen ligands were used to synthesize two coordinatively unsaturated Co-salen complexes. The two Co-salen complexes exhibit an unsaturated coordination pattern and display high activity and CO selectivity for visible-light-driven CO2 reduction in a water-containing system. The photocatalytic performance of 2 is higher than that of 1 because the reduction potential of the catalytic CoII center and the energy barrier of the catalytic transition states of 2 are lower than those of 1, with turnover numbers (TONCO), turnover frequencies (TOF), and CO selectivity values of 8640, 0.24 s–1, and 97% for 2, respectively. The photocatalytic reduction of CO2 to CO for 2 is well supported by control experiments and density functional theory (DFT) calculations.

Electronic Effect Promoted Visible-Light-Driven CO2-to-CO Conversion in a Water-Containing System

“…In contrast to the catalyst, Ru-PS* cannot be quenched by the addition of sacrificial agent TEOA (Figure S13). Based on the above experimental results, it is clear that the excited-state Ru-PS* can be quickly quenched by catalysts probably following an oxidation quenching path for electron transfer in the photocatalytic process for CO 2 reduction as some reported catalytic systems. ,, …”

“…Based on the above experimental results, it is clear that the excited-state Ru-PS* can be quickly quenched by catalysts probably following an oxidation quenching path for electron transfer in the photocatalytic process for CO 2 reduction as some reported catalytic systems. 50,59,60 Based on the above arguments and the reported literature, we used density functional theory (DFT) calculations to better understand the catalytic mechanism of the catalytic reaction process. The proposed catalytic mechanisms for catalysts 2 and 4 are shown in Figure 3.…”

“…47,48 However, salen complexes have rarely been designed as catalysts for photocatalytic CO 2 reduction. 49,50 Recently, two salen complexes with coordinatively unsaturated metal centers have been reported as efficient catalysts for photoinduced CO 2 reduction in a water-containing system by our group. 49 To further improve the selectivity, activity, and stability of the catalysts for photochemical conversion of CO 2 in aqueous systems, we devote to develop new Co-salen complex catalysts with higher performance for photochemical reduction of CO 2 by regulating the electronic effect of the substituents on the ligands.…”

“…It is known that tuning redox properties of the active metal centers via coordination sphere chemistry achieved by ligand modification has been widely used to explore complexes with high performance for homogeneous catalytic reactions . Schiff base ligands which are easily synthesized and regulated − can stabilize different oxidation states of metal centers and control the performance of complexes in catalytic reactions. , Many Schiff base complexes have been synthesized and applied in the field of activation and conversion of small molecules. − In fact, salen complexes constructed by Schiff base ligands usually show unsaturated coordination pattern of the metal centers, which facilitate the formation of M–CO 2 adduct to activate and reduce CO 2 molecules. , However, salen complexes have rarely been designed as catalysts for photocatalytic CO 2 reduction. , Recently, two salen complexes with coordinatively unsaturated metal centers have been reported as efficient catalysts for photoinduced CO 2 reduction in a water-containing system by our group . To further improve the selectivity, activity, and stability of the catalysts for photochemical conversion of CO 2 in aqueous systems, we devote to develop new Co-salen complex catalysts with higher performance for photochemical reduction of CO 2 by regulating the electronic effect of the substituents on the ligands.…”

“…photocatalytic CO 2 reduction 49,50. Recently, two salen complexes with coordinatively unsaturated metal centers have been reported as efficient catalysts for photoinduced CO 2 reduction in a water-containing system by our group 49.…”