Organic cage-anchored ultrafine palladium nanoparticles enable direct C(sp2)-H arylation of benzothiazole

Controlled synthesis of ultrafine palladium nanoparticles (Pd NPs) remains a significant challenge, as their high surface energy often leads to rapid aggregation, reducing catalytic activity and stability. Here, we report a cryptand-like porous organic amine cage as a template as well as stabilizer for the fabrication of ultrafine Pd NPs. The cage was synthesized through imine condensation of terephthaldehyde with tris(2-aminoethyl)amine, followed by reductive amination, affording a robust amine-linked cryptand (PAC) with multiple coordination sites. Subsequent addition of Pd(OAc)₂, followed by the in situ reduction, generated well-dispersed Pd NPs (size ∼4 nm) confined within the intermolecular cage space as evident from FT-IR, UV-vis and BET surface area analyses. The hybrid PAC-supported Pd nanomaterial (Pd/PAC) was comprehensively characterised by using multiple analytical techniques UV-vis, FT-IR, BET, PXRD, FE-SEM, HR-TEM, and XPS. The Pd/PAC hybrid catalyst efficiently promotes the direct C(sp²)-H arylation of benzothiazole with haloarenes, affording diverse 2-aryl benzothiazole derivatives in moderate to excellent yields (72-85%) with a broad substrate scope, with excellent recyclability up to 6 cycles with no Pd leaching as evident from ICP-MS analysis of the filtrate under hot conditions. Further, DLS study of the reaction mixture confirms the retention of the particulate nature of Pd NPs during the course of the reaction, indicating the heterogenous behavour of the catalyst. Moreover, mechanistic insights were obtained through DFT analysis, which supported a plausible pathway involving oxidative addition and reductive elimination during the catalytic cycle. Notably, to the best of our knowledge, this study presents the first example of an organic cage-stabilized palladium nanoparticle-catalyzed C-H activation of benzothiaozole.