Two new pH-controlled metal–organic frameworks based on polynuclear secondary building units with conformation-flexible cyclohexane-1,2,4,5-tetracarboxylate ligand

By regulating the pH values, two new zinc(II) coordination polymers, formulated as [Zn4(μ7-CTAI)(μ3-OH)(μ2-OH)3(H2O)2]n·2n(H2O) (1), [Zn2(μ7-CTAII)(H2O)3]n (2) have been prepared by a flexible ligand, cyclohexane-1,2,4,5-tetracarboxylic acid (H4CTA) under hydrothermal conditions. Complex 1 exhibits a two-dimensional binodal (3,6)-connected topological network containing rare octanuclear zinc(II) clusters as the secondary building units (SBU1). Complex 2 displays a three-dimensional binodal (4,8)-connected topological network containing unusual Zn4(COO)6 secondary building units (SBU2). And importantly, the conformation of H4CTA in 1 exhibits (a,e,e,a) fashion and transforms to thermodynamically more stable conformation (e,a,e,e) type in 2 by pH-controlled. In addition, both of the complexes show strong photoluminescence at room temperature, and may be good candidates for potential luminescence materials.