Analysing the role of anions in the synthesis of catalytically active urea-based MOFs

Abstract

The synthesis of catalytically active metal–organic frameworks (MOFs) with copper(II) paddle-wheel clusters and urea–carboxylate linkers was achieved at room temperature in the presence of sulphate anions. The role of various anions in determining the MOF structure was analysed using X-ray diffraction. Structural analysis of the MOFs indicated that a two-fold interpenetrated rhombus grid (HI-101) was formed in the presence of sulphate anions, but a three-fold interpenetrated square grid network (HI-102) was obtained with nitrate and perchlorate anions. The experiment performed with various anions in the presence of sulphate anions also resulted in the formation of HI-101, indicating the selectivity of sulphate anions in the formation of the HI-101 framework. HI-101 proved to be an efficient catalyst for the cycloaddition of CO₂ at room temperature, the oxidation of primary alcohols to aldehydes and the methanolysis of epoxides, but the other MOFs were not catalytically active. Thus, the size, charge and affinity of sulphate anions play an important role in generating a two-fold interpenetrated rhombus grid, which is crucial for catalytic reactions. This study shows that anion-templated synthesis could generate a versatile urea-based MOF catalyst for CO₂ fixation and other reactions.