Two-Dimensional Nanoporous {Tm2}-Organic Framework with Highly Defective Metal Sites: Preparation Strategy and Catalytic Studies

The invention of a catalyst to activate and convert inert and thermodynamically stable carbon dioxide (CO2) into high-value chemicals is a challenging task for achieving carbon neutrality and addressing energy crises. Herein, we report a highly robust nanoporous framework of {[Tm(PPDDC)(DMF)2]·4DMF·7H2O}n (NUC-169) self-assembled from our designed functional structure-oriented modules of 4,4’-(4-phenylpyridine-2,6-diyl)diisophthalic acid (H4PPDDC). In NUC-169, the exquisite combination of dinuclear [Tm2(COO)6] clusters and organic modules led to a 2D layer of [Tm(HPPDDC)]n with in-plane higher-order nanovoids (9.89 × 16.31 Å2), based on which a three-dimensional structure containing nanocage-like voids of 9.89 × 16.31 × 14.70 Å3 with the connection of hydrogen bonds. Furthermore, it is worth noting that it is very rare for a dual core unit to contain four coordinating solvent molecules, which implies that the activated state of NUC-169a will be a strong Lewis acidic material. In addition, multiple carboxyl groups and electron-donating pyridyl groups installed above and below the 2D surface further trained NUC-169a into a Lewis acid–base material. Catalytic experiments confirmed that the combination of 0.15 mol % NUC-169a and 5.0 mol % n-Bu4NBr displayed high catalytic performance on the coupling reactions of epoxides and CO2 into high-valued cyclic carbonates at 80 °C and 0.8 MPa CO2 pressure. Furthermore, Knoevenagel condensation can be efficiently catalyzed by NUC-169a. In those above two reactions, NUC-169a owns excellent heterogeneity, recyclability, and substrate tolerance. Compared to our previously reported 3D nanoprous MOFs, this work offers a simple and feasible synthesis strategy for preparing 2D MOFs by using the structure-oriented ligands containing the fragment of 4,4’-(4-phenylpyridine-2,6-diyl)diisophthalic acid, on which any desirable specific groups can be installed for obtaining a metal–organic skeleton with special properties.