Bifunctional metal–organic frameworks afforded by postsynthetic modification for efficient cycloaddition of CO2 and epoxides

Abstract

AbstractBifunctional ionic metal–organic frameworks (MOFs) containing Lewis acid sites (unsaturated metal sites) and halide ions (Cl−, Br−, and I−) have attracted increasing attention due to their extra high activity for the cycloaddition of CO2 with epoxides. Herein, a novel microporous MOF (1‐Eu), namely, [Eu3(L)2(HCOO)(H2O)5]·14H2O (H4L = 2,6‐di(2,4‐dicarboxyphenyl)‐4‐(pyridine‐4‐yl)pyridine), has been synthesized by using a new bipyridyl‐based tetracarboxylate ligands (H4L). Structural analyses show that 1‐Eu is a 3D framework in which 1D chains with alternating triple and single carboxylate bridges are interlinked by the L ligands and contains microporous channels with uncoordinated pyridyl N atoms, which are easy to be modified by N‐methylation. Therefore, three bifunctional N‐methylation 1‐Eu MOFs, 1‐Eu‐MeX (X = Cl−, Br−, and I−), were successfully prepared from the 1‐Eu MOF by a postsynthetic modification (PSM) method. 1‐Eu‐MeX can efficiently catalyze the cycloaddition reaction without any cocatalyst and solvent. Among them, the 1‐Eu‐MeI catalyst displays the highest catalytic performance. Our work thus represents a rare demonstration of ionic MOFs as heterogeneous catalysts for efficient CO2 fixation with epoxides. More significantly, 1‐Eu‐MeX are the first reported Eu‐based ionic MOFs with the bipyridyl‐based tetracarboxylate ligand.