Delocalized Frustrated Lewis Pairs in COF-Catalyzed N-Transfer for Urea Photosynthesis

Photosynthesis of urea through C─N coupling from CO₂ and nitrates (NO₃ ^-) represents a sustainable route for both carbon utilization and environmental remediation, yet remains fundamentally challenging due to competing parallel reduction reactions. Building upon hard-soft acid-base principles, we demonstrate that electronic delocalization modulation in iron (Fe)-keto oxygen frustrated Lewis pairs (FLPs) can effectively suppress these reductions while selectively promoting C─N coupling. To realize this strategy, we engineered a fluorinated Fe-β-ketoenamine-linked covalent organic framework (Fe/F─COF) photocatalyst, where F functionalization modulates the electronic structure to create electron-delocalized FLPs with precisely controlled Fe-keto oxygen spatial spacing at 3.4 Å. Experimental and computational results reveal that F-induced electron delocalization enhances Lewis acidity at Fe sites for NO₃ ^- activation and increases basicity at keto oxygen for the CO₂ activation. Furthermore, the electron-delocalized Fe center exhibits softened acid character that weakens the Fe-N bond in adsorbed *NH species, facilitating N-transfer for C─N coupling. Such spatial and electronic configuration lowers the C─N coupling barrier, favoring *NHCO intermediate formation and subsequent urea production. The Fe/F-COF achieves a high urea yield rate of 93 µmol g_cat. ^-1 h^-1 with an apparent quantum yield of 1.1% at 400 nm, surpassing most reported photocatalytic co-reduction coupling systems to date.