Preparing transition metal-organic frameworks based on oxalate-sulfate anions in deep eutectic solvents

Abstract By using deep eutectic mixture of choline chloride (ChCl) and oxalic acid (OA) with a molar ratio of 3:7 as reaction medium, three new transition metal-organic frameworks were synthesized, namely, K4[Fe3(OA)3(SO4)2] (1), K2[Fe(OA)(SO4)] (2), Na2[Co(OA)(SO4)]·H2O (3). Oxalate and sulfate anions were engaged in the construction of the frameworks as bridging ligands, and consequently led to the formation of two-dimensional anionic layer of 1 and three-dimensional anionic networks of 2 and 3. In the synthesis of these compounds, oxalic acid not only acted as hydrogen bond donor of DES solvent, but also acted as organic linker in the construction of the frameworks. These compounds were characterized by single-crystal and powder X-ray diffractions, elemental and thermogravimetric analysis. The catalytic activity of compounds 1 and 3 for oxidizing styrene was investigated.