Defect Engineering in a Nanoporous Thulium–Organic Framework in Catalyzing Knoevenagel Condensation and Chemical CO2 Fixation

Defect engineering is an extremely effective strategy for modifying metal–organic frameworks (MOFs), which can break through the application limitations of traditional MOFs and enhance their functionality. Herein, we report a highly robust nanoporous thulium­(III)–organic framework, {[Tm2(BDCP)­(H2O)5]­(NO3)·3DMF·2H2O} n (NUC-105), with [Tm­(COO)2(H2O)] n chains and [Tm2(COO)4(H2O)8] dinuclears as metal nodes and 2,6-bis­(2,4-dicarboxylphenyl)-4-(4-carboxylphenyl)­pyridine (BDCP) linkers. In NUC-105, each of the four chains of [Tm­(COO)2] n and the two rows of [Tm2(COO)4(H2O)8] units is unified by the organic skeleton, resulting in a rectangular nanochannel with dimensions of 15.35 Å × 11.29 Å, which leads to a void volume of 50%. It is worth mentioning that the [Tm2(COO)4(H2O)8] cluster is very rare in terms of its higher level of associated water molecules, implying that the activated host framework can serve as a strong Lewis acid. NUC-105a exhibited great heterogeneous catalytic performance for CO2 cycloaddition with epoxides under the reaction conditions (0.60 mol % NUC-105a, 5.0 mol % n-Bu4NBr, 65 °C, 5 h), ensuring exclusive selectivity and high conversion rates. In addition, NUC-105a’s strong catalytic impact on the Knoevenagel condensation of aldehydes and malononitrile can be attributed to the collaboration between the drastically unsaturated Lewis acidic Tm3+ centers and Lewis basic pyridine groups.