Highly Robust {Ln4}-Organic Frameworks (Ln = Ho, Yb) for Excellent Catalytic Performance on Cycloaddition Reaction of Epoxides with CO2 and Knoevenagel Condensation

Abstract

Due to the high electron charge, large ion radius, and plentiful outer hybrid orbitals of LnIII cations, microporous Ln-MOFs can be used as Lewis acidic catalysts with high catalytic activity for a variety of organic reactions, which prompts us to explore cluster-based nanoporous Ln-MOFs by employing structure-oriented ligands. Herein, the exquisite combination of coplanar [Ln4(μ3–OH)2(μ2–HCO2)(H2O)2] clusters (abbreviated as {Ln4}) and the structure-oriented multifunctional ligand of 2,6-bis(2,4- dicarboxylphenyl)-4-(4-carboxylphenyl)pyridine (H5BDCP) led to two isomorphic nanoporous frameworks of {(Me2NH2)[Yb4(BDCP)2(μ3–OH)2(μ2–HCO2)(H2O)2]·5DMF·H2O}n (NUC-38Yb) and {(Me2NH2)[Ho4(BDCP)2(μ3–OH)2(μ2– HCO2)(H2O)2]·6DMF·3H2O}n (NUC-38Ho). To the best of our knowledge, NUC-38Ho and NUC-38Yb are rarely reported {Ln4}-based three-dimensional (3D) frameworks with embedded hierarchical triangular-microporous and hexagonal-nanoporous channels, which are shaped by six rows of coplanar {Ln4} clusters and characterized by plentiful coexisting Lewis acid–base sites on the inner wall including open LnIII sites, Npyridine atoms, μ3–OH, and μ2–HCO2. Catalytic experiments performed using NUC-38Yb as the representative exhibited that NUC-38Yb possessed a high catalytic activity on the cycloaddition reactions of epoxides with CO2 under mild conditions, which can be ascribed to its structural advantages including nanoscale channels, rich bifunctional active sites, large surface areas, and chemical stability. Moreover, NUC-38Yb, as a heterogeneous catalyst, could greatly accelerate the Knoevenagel condensation reactions of aldehydes and malononitrile. Hence, this work paves the way for the construction of functional Ln-cluster-based nanoporous metal–organic frameworks (MOFs) by elaborately designing functional ligands with transnormal connection modes.