Ligand-Induced High-Deficient 2D {Zn2}‑Organic Material: High Catalytic Activity on CO2‑Epoxide Cycloaddition and DFT Calculation

Due to diffusion barriers, the catalytic applications of porous metal–organic materials in organic synthesis are currently limited to relatively small substrates. Therefore, a synthesis strategy consisting of dimensionality reduction, active site addition, and defect engineering was adopted to develop two-dimensional (2D) metal–organic materials by using the designed organic linker 4,4′-(4-(4-(trifluoromethoxy)phenyl)pyridine-2,6-diyl)diisophthalic acid (H4TPPD), based on which a highly robust 2D [Zn2(COO)6(DMF)4]-based framework of {[(CH3)2NH2][Zn(HTPPD)(DMF)2]·2H2O}n (NUC-141) was self-assembled. The thermally activated NUC-141a has the following distinctive merits: (i) higher-order in-plane nanoscale pores of ca. 15.1 × 10.2 × 8.9 Å3; (ii) functionalized by four kinds of acid–base active sites, including highly defected Zn2+ sites, trifluoromethoxy, free carboxyl, and pyridyl. Due to its multiple high-density catalytic sites, NUC-141a exhibits a higher catalytic performance in the coupling reactions of CO2 and various epoxides than most reported 2D and three-dimensional (3D) metal–organic framework (MOF)-based materials. Meanwhile, NUC-141a exhibits high turnover number (TON) values, selectivity, and chemical stability in coupling reactions. Density functional theory (DFT) calculations using double-defected ZnO6 as a catalytic unit confirmed that the energy barriers for overcoming ring-opening, CO2 insertion, and ring-closing steps were 5.4, 15.3, and 29.2 kcal/mol, respectively, all of which were significantly lower than those of monodefected ZnO7, saturated ZnO8, and most reported MOF catalysts. In addition, NUC-141a exhibits high separation performance for CO2/CH4 mixtures at 273 K and 100 kPa, with separation selectivity of 49 (1:99, v/v), 38 (10:90, v/v), and 23 (50:50, v/v). Therefore, this work not only demonstrated through a series of DFT calculations that more highly defected metal sites in MOFs have higher catalytic performance but also provided an effective synthesis strategy for preparing 2D functional MOFs.