Nanoporous {Pb₃}-Organic Framework for Catalytic Cycloaddition of CO₂ with Epoxides and Knoevenagel Condensation

Because of increasingly serious environmental problems and resource shortages, chemically fixing surplus CO 2 into value-added products has gradually become a challenging and hot research topic, in which the preparation of zeolite-like metal–organic frameworks (MOFs) with rich Lewis acid–base sites and nanopores is the cornerstone. Herein, the butterfly-shaped [Pb 3 (COO) 6 (H 2 O) 2 (N pyridine ) 2 ] cluster, polynitro tritopic carboxylic acid of 2,4,6-tri(4-carboxy-2-nitrophenyl)-1,3,5-trinitrobenzene (H 3 TCNT), and 2,4,6-tri(pyridin-4-yl)-1,3,5-triazine (TPT) engender a highly robust microporous framework of [Pb 3 (TCNT) 2 (TPT)(H 2 O) 2 ] n (NUC-91) with rectangular nanochannels (15.28 × 12.16 Å 2 ) along the b axis. Because of extremely rich functional sites such as Lewis acidic sites of Pb 2+ ions and Lewis basic sites of free nitrogen atoms on the inner surface of void volumes, activated NUC-91a as a heterogeneous catalyst can effectively catalyze the cycloaddition of CO 2 with various epoxides under mild conditions. For substrates 2-methyloxirane, 2-fluorooxirane, 2-ethyloxirane, 2-(trifluoromethyl)oxirane, oxiran-2-ylmethanol, 2-vinyloxirane, and 2-phenyloxirane, the transformation to related cyclic carbonates could reach 99% with turnover number (TON) and turnover frequency (TOF) of 825 and 206 h –1, respectively. Moreover, Knoevenagel condensation reactions of aldehydes and malononitrile could be efficiently effected by NUC-91a. Therefore, this work provided a simple strategy for effectively prefunctionalizing widely used ligands, which can be employed to design highly catalytic metal–organic frameworks to facilitate the capture and conversion of CO 2 .