Advanced palladium-hybrid covalent organic framework: A highly efficient catalyst for sustainable Cascade synthesis of Chromeno[2,3-d] Pyrimidine-8-amines

Abstract

Covalent organic frameworks account for an emerging class of porous crystalline organic polymers characterized by their modular architecture, facilitating functionalization. The well-defined pores of COFs serve as ideal platforms for stabilizing metal species and ensuring superior accessibility. Incorporating substrate-binding sites within COFs enables the formation of host-guest interactions, thereby promoting synergistic effects that enhance catalytic performance. Polyimide covalent organic framework (PI-COF) is remarkably crystalline, an imide-based COF synthesized via a solvothermal reaction of cost-effective melamine and pyromellitic dianhydride. This COF serves as a robust platform for immobilizing palladium (Pd2+) ions, enabling PI-COF catalytic applications. The successful fabrication of (PI-COF) with palladium, i.e., Pd@PI-COF, has been comprehensively characterized using FTIR, PXRD, 13C(CP/MAS) NMR, BET, TGA, XPS, FE-SEM, HR-TEM, and ICP-OES. The synthesized Pd@PI-COF is utilized as a heterogeneous catalyst for developing a highly efficient protocol of one-pot synthesis of chromeno[2,3-d] pyrimidine-8-amine via four-component reactions. Pd@PI-COF serves as a sustainable catalyst, facilitating the reaction in ethanol under mild reaction conditions, achieving an exceptional yield up to 93 % in just 50 min. . The catalyst exhibits excellent recyclability and reusability, retaining its catalytic efficiency across four consecutive cycles with negligible loss in activity, while maintaining good turnover numbers (TON) and turnover frequencies (TOF).