MOFs assembled from C3 symmetric ligands: structure, iodine capture and role as bifunctional catalysts towards the oxidation–Knoevenagel cascade reaction

Abstract

Three new Ni^II/Co^II-metal organic frameworks were self-assembled by the reaction of C₃ symmetric 1,3,5-tribenzoic acid (H₃BTC) and 2,4,6-tris(4-pyridyl)-1,3,5-triazine (4-TPT) ligands and Ni^II/Co^II salts under solvothermal conditions. Isomorphous MOF1 and MOF2 exhibit a 3D pillar-layer framework based on binuclear M₂(OH)(COO)₂ units connected by tritopic BTC³⁻ and 4-TPT ligands with a novel (3,5)-connected topology net. MOF3 displays a 3-fold interpenetrated 3D network exhibiting a (3,4)-connected topology net. The porous MOF3 can reversibly take up I₂. The activated MOFs contain both Lewis acid (Ni^II center) and basic (uncoordinated pyridyl or carboxylic groups) sites, and act as bifunctional acid–base catalysts. The catalytic measurements demonstrate that the activated MOF3 exhibits good activities for benzyl alcohol oxidation and the Knoevenagel reaction and can be recycled and reused for at least four cycles without losing its structural integrity and high catalytic activity. Thus, the catalytic properties for the oxidation–Knoevenagel cascade reaction have also been studied.