Defect Engineering in a Nanoporous Thulium–Organic\nFramework in Catalyzing Knoevenagel Condensation and Chemical CO<sub>2</sub> Fixation

Defect\nengineering is an extremely effective strategy for modifying\nmetal–organic frameworks (MOFs), which can break through the\napplication limitations of traditional MOFs and enhance their functionality.\nHerein, we report a highly robust nanoporous thulium­(III)–organic\nframework, {[Tm<sub>2</sub>(BDCP)­(H<sub>2</sub>O)<sub>5</sub>]­(NO<sub>3</sub>)·3DMF·2H<sub>2</sub>O}<sub><i>n</i></sub> (<b>NUC-105</b>), with [Tm­(COO)<sub>2</sub>(H<sub>2</sub>O)]<sub><i>n</i></sub> chains and [Tm<sub>2</sub>(COO)<sub>4</sub>(H<sub>2</sub>O)<sub>8</sub>] dinuclears as metal nodes and 2,6-bis­(2,4-dicarboxylphenyl)-4-(4-carboxylphenyl)­pyridine\n(BDCP) linkers. In <b>NUC-105</b>, each of the four chains of\n[Tm­(COO)<sub>2</sub>]<sub><i>n</i></sub> and the two rows\nof [Tm<sub>2</sub>(COO)<sub>4</sub>(H<sub>2</sub>O)<sub>8</sub>] units\nis unified by the organic skeleton, resulting in a rectangular nanochannel\nwith dimensions of 15.35 Å × 11.29 Å, which leads to\na void volume of 50%. It is worth mentioning that the [Tm<sub>2</sub>(COO)<sub>4</sub>(H<sub>2</sub>O)<sub>8</sub>] cluster is very rare\nin terms of its higher level of associated water molecules, implying\nthat the activated host framework can serve as a strong Lewis acid. <b>NUC-105a</b> exhibited great heterogeneous catalytic performance\nfor CO<sub>2</sub> cycloaddition with epoxides under the reaction\nconditions (0.60 mol % <b>NUC-105a</b>, 5.0 mol % <i>n</i>-Bu<sub>4</sub>NBr, 65 °C, 5 h), ensuring exclusive selectivity\nand high conversion rates. In addition, <b>NUC-105a</b>’s\nstrong catalytic impact on the Knoevenagel condensation of aldehydes\nand malononitrile can be attributed to the collaboration between the\ndrastically unsaturated Lewis acidic Tm<sup>3+</sup> centers and Lewis\nbasic pyridine groups.