Crystalline mixed-valence copper supramolecular isomers for electroreduction of CO2 to hydrocarbons

Abstract

Supramolecular isomers can serve as effective model systems to explore the stable self-assembly process of specific structures, but it is often difficult to capture multiple variants in different stable states. Here, four variants (CuL-1, CuL-2, CuL-3, and CuL-4) assembled from the same Cu^ICu^IIL-metalloligand motif form different structures in which the dihedral angles of adjacent Cu^ICu^IIL-metalloligands (9°, 33°, 84°, 180°) increase gradually. CuL-2, CuL-3 and CuL-4 belong to supramolecular isomers. The relevant experimental and calculative results demonstrate that the larger the dihedral angle of adjacent Cu^ICu^IIL-metalloligands, the higher the stability of the structural variant. Moreover, the metastable structures (CuL-1, CuL-2, and CuL-3) can undergo single crystal-to-single crystal transformation into the most stable variant (CuL-4). Additionally, based on the unique mixed-valence copper structure distribution characteristics and strong alkali stability, CuL-4 is applied as an electrocatalyst for CO₂ electroreduction (CO₂RR), in which CO₂ can be reduced to high-value hydrocarbons with a total faradaic efficiency of 85.02% (FE_CH4 = 67.76%, FE_C2H4 = 17.26%) at −1.0 V (j = 477.42 mA cm⁻²). As far as we know, CuL-4 is better than most crystalline CO₂RR catalysts. Furthermore, the corresponding DFT calculation proves that the independent Cu(I) sites are mainly responsible for CH₄ generation, while the synergistic two adjacent Cu(II) sites are inclined to produce C₂H₄. Significantly, crystalline CuL-4 can serve as a well-defined model electrocatalyst to explore the role of evenly distributed mixed-valence copper active sites in reducing CO₂ to hydrocarbons.