Two-Dimensional Nanoporous {Tb2}–Organic Framework with Acid–Base for CO2-Epoxide Cycloaddition and Deacetalization–Knoevenagel Condensation

Abstract

Functional two-dimensional metal–organic frameworks (2D MOFs) with nanoporous structures have received great attention in terms of heterogeneous catalytic performance, because of unimpeded mass transfer and more easily accessible active sites. However, the controllable preparation of 2D self-supporting metal–organic nanosheets with hierarchical porosity is still a challenge. In this context, we designed a structure-oriented organic linker of 4,4′-(4-(4-bromophenyl)pyridine-2,6-diyl)diisophthalic acid (H₄BrPDC), based on which a chemo-robust 2D terbium(III)-organic material of {[Tb(HBrPDC)(DMF)₂]·2DMF·2H₂O} (NUC-145Tb) with dinuclear [Tb₂(COO)₆(DMF)₄] as inorganic nodes was self-assembled under the solvothermal conditions. After thermal activation with the removal of lattice and associated molecules, activated NUC-145Tb-a with the higher-order in-plane nanopores of 10.33 × 12.84 Ų is full of multifarious catalytic sites such as highly defected acidic Tb³⁺ sites, basic pyridine groups, and Brønsted acidic–COOH H-bond donors (HBDs), implying that the infrequent NUC-145Tb-a is a promising heterogeneous catalyst. A series of tests have shown that NUC-145Tb-a exhibits excellent catalytic performance in the coupling cycloaddition reaction of CO₂ and epoxide substrates under comparatively moderate reaction conditions. In addition, because of acidic sites (metal sites and uncoordinated carboxyl groups) and basic sites (bromine groups and free pyridine), activated NUC-145Tb-a exhibits high catalytic activity in the deacetalization–Knoevenagel condensation reaction of acetals and malononitrile under lukewarm conditions. Therefore, this work not only reports a 2D {Tb₂}–organic catalytic compound but also, more importantly, provides a class of organic carboxylic ligands with broad application prospects in constructing 2D functional metal–organic materials.