Highly Efficient Conversion of Aziridines and CO₂ Catalyzed by Microporous [Cu₁₂] Nanocages

The conversion of CO₂ as a C1 source into value-added products is an attractive alternative in view of the green synthesis. Among the reported approaches, the cyclization reaction of aziridines with CO₂ is of great significance since the generated N-containing cyclic skeletons are extensively found in pharmaceutical chemistry and industrial production. However, a low turnover number (TON) and homogeneous catalysts are often involved in this catalytic system. Herein, one novel copper-organic framework {[Cu₂(L^4-)(H₂O)₂]·3DMF·2H₂O} (1) (H₄L = 2'-fluoro-[1,1':4',1″-Terphenyl]-3,3″,5,5″-tetracarboxylic acid) assembled by nanosized [Cu₁₂] cages was successfully synthesized and structurally characterized, which exhibits high CO₂/N₂ selectivity due to the strong interactions between CO₂ and open Cu(II) sites and ligands in the framework. Catalytic investigations suggest that 1 as a heterogeneous catalyst can effectively catalyze the cyclization of aziridines with CO₂, and the TON can reach a record value of 90.5. Importantly, 1 displays excellent chemical stability, which can be recycled at least five times. The combination explorations of nuclear magnetic resonance (NMR), ¹³C-isotope labeling experiments, and density functional theory (DFT) clearly uncover the mechanism of this aziridine/CO₂ coupling reaction system, in which 1 and tetrabutylammonium bromide (TBAB) can highly activate the substrate molecule, and the synergistic catalytic effect between them can greatly reduce the reaction energy barrier from 51.7 to 36.2 kcal/mol.