Synergistic effect of carboxyl-functionalized benzothiadiazole-based metal-organic frameworks for enhanced photocatalytic carbon‑nitrogen bond formation

The development of efficient photocatalysts for carbon‑nitrogen (CN) bond formation from aldehydes is highly desirable for the synthesis of functional organic molecules but remains a significant challenge. Herein, we report a novel imidazobenzothiadiazole-based tricarboxylate compound (H₂Ph-COOH, defined as "T-shaped" ligand), and successfully construct the first single-component metal-organic frameworks (MOFs, UiO-68-Ph-COOH) featuring an exposed carboxyl group. This MOFs was effectively employed in the photocatalytic synthesis of amides from aldehydes and amines. The carboxylated T-shaped ligand-based UiO-68-Ph-COOH exhibits outstanding optoelectronic properties and exceptional photocatalytic activity for amide synthesis at room temperature, achieving high yields (up to 92%) within 12 h, along with long-term durability and excellent stability. In comparison, control MOFs (UiO-68-Bt and UiO-68-Ph) derived from linear ligands showed markedly lower catalytic activity (10% and 16% yields, respectively) under identical reaction conditions. Mechanistic studies reveal that the exposed -COOH groups in UiO-68-Ph-COOH act as Brønsted acid sites, which promote the formation of key amino alcohol intermediate and concurrently facilitate the generation of superoxide radical (O₂^•-). This synergistic effect significantly improves the photocatalytic efficiency for amide synthesis. Additionally, UiO-68-Ph-COOH efficiently catalyzes the formation of benzothiazoles and benzimidazoles (up to 94% yield within 1.5 h). This work provides the first demonstration that a single-component MOFs can independently drive photocatalytic amide synthesis and reveals the exposed -COOH functionalization as a crucial design strategy for MOFs-based photocatalysts, thereby opening new avenues for designing efficient MOFs-based photocatalysis of CN bond formation.