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, low turnover number (TON), toxic solvents and homogeneous catalysts are often involved in this catalytic system, and the corresponding reactive mechanism are rarely reported. Herein, one novel porous copper-organic framework {[Cu2(L4-)(H2O)2]·3DMF·2H2O}n (1) (H4L = 2'-fluoro-[1,1':4',1''-Terphenyl]-3,3'',5,5''-tetracarboxylic acid) assembled by nano-sized [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 heterogeneous catalyst can effectively catalyze the cyclization of aziridines with CO2 without additional solvent, and the TON can reach to a record value of 90.5. Importantly, 1 displays excellent chemical stability, which can be recycled at least five times. The combination explorations of NMR, 13C isotope labeling experiments and density functional theory (DFT) calculations clearly uncover the mechanism of this aziridine/CO2 coupling reaction system, in which synergistic catalytic effect between 1 and TBAB can greatly reduce the reaction energy barrier from 51.7 to 36.2 kcal mol-1.