Synthesis and application of a cobalt(II)-incorporated hydrazone-linked covalent organic framework for the synthesis of cyclic carbonates and oxazolidinones via CO2 fixation under solvent-free conditions.

Herein, a hydrazone-linked covalent organic framework (TFPB-OD) was constructed using 1,3,5-tris(4-formylphenyl)benzene (TFPB) and oxalyldihydrazide (OD) monomeric units. Co(II) was embedded via post-synthetic modification, yielding a well-coordinated, nitrogen- and oxygen-rich porous framework, Co(II)-TFPB-OD, for the synthesis of an active Lewis acidic center-containing heterogeneous catalyst. Both metal-containing and metal-free hydrazone-linked COFs were thoroughly characterised using different characterization techniques, such as FT-IR spectroscopy, PXRD, BET surface area analysis, XPS, TGA, SEM, and TEM, to establish a comparative study. These techniques confirmed the successful formation of a crystalline, micro-porous Co(II)-TFPB-OD COF with Co(II) distributed uniformly with a surface area of around 212.65 m2 g-1. This metal COF was used as a catalyst for the formation of cyclic carbonate from oxiranes via thermal CO2 fixation and oxazolidinone via three-component cyclization between oxiranes, amines and CO2. Both reactions were carried out under solvent-free conditions using atmospheric CO2 pressure. For cyclic carbonate, a substrate yield of up to 96% was achievable, and for oxazolidinone, the yield varied up to 82%. The catalyst exhibited excellent recyclability up to 5 consecutive cycles, and the reused COF was characterized again via PXRD, SEM, BET surface area analysis, FT-IR spectroscopy, and XPS to confirm the recyclability and stability of the hydrazone-linked COF.