Isolated Fe Sites in 2D Metal-organic Layers: Structural Regulation Governing Hydrocarbon Photooxidation

Incorporating Fe into the metal-oxo clusters of Zr-based metal-organic frameworks (MOFs) via metal-to-cluster charge transfer (MCCT) is an effective strategy to enhance light absorption and redox activity, thereby improving photocatalytic performance. However, precise control over Fe active sites, particularly the number of Fe atoms, their oxidation states, and coordination environments, remains challenging. This difficulty arises from the dynamic nature of terminal and bridging -OH/H₂O ligands on MOF nodes. In this study, three atomically defined Fe configurations (Fe₁, Fe₂, Fe₃) is constructed in a 2D triazine-based zirconium carboxylate MOF (ZrTATB) via postsynthetic modification. A distinct non-linear structure-performance relationship is revealed for photocatalytic toluene oxidation, with Fe₂ exhibiting optimal performance. Mechanistic studies reveal that Fe₂ achieves a balanced coordination environment. This balance enables efficient water activation (generating •OH) and selective C─H bond cleavage concurrently. Specifically, Fe₁ features four uncoordinated hydroxyls, adsorbs toluene too strongly. Fe_3, fully saturates its coordination sphere, binds substrates weakly. In contrast, Fe₂ preserves two free hydroxyls, achieving moderate adsorption strength and efficient •OH generation. Finally, water availability is found to govern product selectivity between benzaldehyde and benzoic acid, and reaction mechanism is accordingly proposed.