Site-Specific Asymmetric Coordination Engineering in Defective Metal-Organic Frameworks Stabilizes Cu(I) Active Sites...

Solar-driven CO₂ reduction by means of single-atom photocatalysis holds great promise for simultaneously addressing carbon neutrality and producing valuable chemicals. The coordination-field engineering of a single-atom (SA) site essentially provides an intriguing strategy to steer the CO₂ reduction pathway. We report herein the site-specific confinement of Cu SA with an asymmetric coordination microenvironment in defective MIL-125-NH₂ to facilitate photocatalytic CO₂-to-CH₃OH conversion. This is achieved by precisely anchoring a flexible N-heteromacrocyclic ligand of tetraxetan (DOTA) on unsaturated Ti-oxo clusters to repair the missing-linker defect in heat-treated MIL-125-NH₂, followed by Cu-DOTA chelation leveraging ultrahigh formation constant to stabilize Cu SA. Under scavenger-free conditions, the photocatalyst exhibits a methanol yield of 229.0 μmol·g^-1·h^-1 and an excellent product selectivity of up to 95.9%. Mechanistic studies reveal that the asymmetric coordination environment promotes the formation of stable Cu(I) over Cu(II), which lowers the energy barrier of the key reaction intermediates and suppresses byproduct formation, thereby enabling high selectivity toward the product CH₃OH.