π-Sticked Metal‒Organic Monolayers for Single-Metal-Site Dependent CO₂ Photoreduction and Hydrogen Evolution Reaction

Hierarchical self-assembly of 2D metal‒organic layers (MOLs) for the construction of advanced functional materials have witnessed considerable interest, due to the increasing atomic utilizations and well-defined atom‒property relationship. However, the construction of atomically precise MOLs with mono-/few-layered thickness through hierarchical self-assembly process remains a challenge, mostly because the elaborate long-range order is difficult to control via conventional noncovalent interaction. Herein, a quadruple π-sticked metal‒organic layer (πMOL) is reported with checkerboard-like lattice in ≈1.0 nanometre thickness, on which the catalytic selectivity can be manipulated for highly efficient CO₂ reduction reaction (CO₂RR) and hydrogen evolution reaction (HER) over a single metal site. In saturated CO₂ aqueous acetonitrile, Fe-πMOL achieves a highly effective CO₂RR with the yield of ≈3.98 mmol g^‒1 h^‒1 and 91.7% selectivity. In contrast, the isostructural Co-πMOL as well as mixed metallic FeCo-πMOL exhibits a high activity toward HER under similar conditions. DFT calculations reveal that single metal site exhibits the significant difference in CO₂ adsorption energy and activation barrier, which triggers highly selective CO₂RR for Fe site and HER for Co site, respectively. This work highlights the potential of supramolecular π^…π interaction for constructing monolayer MOL materials to uniformly distribute the single metal sites for artificial photosynthesis.