Spatial Orientation-Dependent Double Hydrogen Bond/Photoredox Cooperative Catalysis in Metal-Organic Frameworks

Catalyst activity is typically the primary priority in designing a catalytic system. The higher catalyst activity is envisioned to confer more powerful and effective catalytic performance. Here, we report a heterogeneous double hydrogen bond/photoredox synergistic catalysis, wherein the flexibility and spatial orientation of catalytic centers, rather than catalyst activity, play a dominant role in catalytic performance. We synthesized six metal-organic frameworks (MOFs) with photocatalytic and double hydrogen bond donor (DHBD) centers. MOF-F-TU featuring the relatively flexible (F) thiourea (TU) center has proved most effective in promoting dehalogenation and reductive coupling reactions. In contrast, MOF-R-SQA1 with a stronger but rigid (R) squaramide (SQA1) site exhibits no catalytic activity. Mechanistic studies rationalize this unique phenomenon by implying a cooperative activation of the substrate via its double hydrogen bonding with DHBD and π-π overlapping with photocatalytic center, which is closely associated with the spatial arrangement of synergistic catalytic centers. MOF-F-TU also outperforms homogeneous counterparts, displaying turnover numbers of up to 9500.