Abstract:

CO₂, as a greenhouse gas, has excessive emissions that lead to many environmental problems and is a rich and cheap C1 resource. Effective utilization and transformation of CO₂ has become an important means of achieving carbon neutrality. Oxazolidinones are important intermediates in pharmaceutical chemistry that can be synthesized by carboxylation cyclization of CO₂ with propargyl amines or cycloaddition of CO₂ with aziridines. Owing to CO₂'s high stability, these reactions typically require harsh conditions, such as high temperatures or pressures. It is desirable, but challenging, to find a catalyst that can catalyze these two types of reactions under relatively mild conditions. Metal-organic frameworks (MOFs) are an emerging class of heterogeneous catalysts that with great potential in the catalytic conversion of CO₂ to value-added products because of their attractive features, such as abundant metal active sites, inherent porosity, and easy functionalities. Herein, a unique three-dimensional (3D) MOF, {(CH₃NH₂CH₃)₂[Co₃(BCP)₂]·6H₂O·4DMF}_n (1) (H₄BCP: 5-(2,6-bis(4-carboxyphenyl)pyridin-4-yl) isophthalic acid; DMF: N,N'-dimethylformamide), was synthesized using carboxylic acid ligands and cobalt salts via a solvothermal method. According to structural analysis, [Co₃] clusters as secondary building units (SBU) are bridged by BCP⁴⁻ ligands, forming an anion framework with flu topology, and dimethylamine cations act as counter ions in the pores. The framework has rectangular channels of approximately 0.4 nm × 0.9 nm along the a-axis direction, exhibiting its porous property. Infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS) characterizations proved the coordination interaction between the carboxyl groups in the ligands and the metal ions. The powder X-Ray diffraction (PXRD) test further confirmed the phase purity of the synthesized samples. PXRD and thermogravimetry (TG) analyses indicated that 1 possessed good solvent and thermal stabilities. The catalytic experiments revealed that 1 could effectively catalyze CO₂ with aziridines or propargyl amines to prepare oxazolidinones. In the cycloaddition of CO₂ with aziridines, 1 can facilitate the reaction under relatively mild conditions compared to other reported MOF-based catalysts. It shows excellent universality for substrates with various substitutions on the N atom or benzene ring. Investigation of the mechanism indicated that the coordination interaction of cobalt metal sites with the nitrogen atoms of aziridines can activate the substrates. For the carboxylative cyclization of CO₂ with propargylic amines, this catalyst also has a broad substrate scope. Control experiments and nuclear magnetic resonance (NMR) tests suggest that Lewis acid metal sites are responsible for the high catalytic efficiency achieved by activating the alkyne groups. Moreover, 1 showed good reusability in both reactions. Compound 1 represents a new catalyst that enables "two birds with one stone" in the catalytic synthesis of oxazolidinones using CO₂.

Key words: Metal-organic framework, Bifunctional catalyst, CO₂ utilization, Oxazolidinone, Reusability