Three metal organic frameworks of 2,4-dihydroxybenzoic acid: Synthesis, crystal structure and thermal behavior

For exploring the coordination forms of 2,4-dihydroxybenzoic acid, the reactions of 2,4-dihydroxybenzoic acid with sodium hydroxide and potassium hydroxide were studied. A potassium complex (Ⅰ), a sodium complex (Ⅱ) and a double-metal complex of potassium and cobalt (Ⅲ) for 2,4-dihydroxybenzoic acid were synthesized, and their single crystals were obtained firstly. We found it is impossible to obtain potassium complex of 2,4-dihydroxybenzoic acid when the molar ratio of 2,4-dihydroxybenzoic acid to KOH exceeds 1:1.8. Only monoacid complex of 2,4-dihydroxybenzoic acid can be obtained, and its two phenolic hydroxyl groups do not participate in reaction. The crystallographic studies show that complexes Ⅰ and Ⅱ all crystallized in monoclinic system, but complex Ⅲ crystallized in orthorhombic system. Besides, three complexes all formed three-dimensional metal organic framework (MOF) structures through K-O or Na-O coordination bonds. Two asymmetric independent K+ions with different coordination forms exist in complex Ⅰ, but Na+ ion in complex Ⅱ only presents one kind of coordination form. The coordination environments of K+ in complex Ⅲ is exactly consistent with K1+ in complex Ⅰ, but Co2+ in complex Ⅲ is coordinated only with water molecules. Meanwhile, the Hirshfeld surface analysis along with 2D fingerprint plots for the asymmetric unit of three complexes has been conducted to examine the contributions of the different intermolecular contacts. In addition, the thermal analysis studies indicate that three complexes all present three continuous endothermic decomposition processes. The thermal stability of complexes Ⅰ and Ⅱ are basically identical, but much higher than that of complex Ⅲ.