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}_<i>n</i> (<b>NUC-105</b>), with [Tm(COO)₂(H₂O)]_<i>n</i> 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 <b>NUC-105</b>, each of the four chains of [Tm(COO)₂]_<i>n</i> 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. <b>NUC-105a</b> exhibited great heterogeneous catalytic performance for CO₂ cycloaddition with epoxides under the reaction conditions (0.60 mol % <b>NUC-105a</b>, 5.0 mol % <i>n</i>-Bu₄NBr, 65 °C, 5 h), ensuring exclusive selectivity and high conversion rates. In addition, <b>NUC-105a</b>'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.