Synthesis, Structures, and Photoluminescence Properties of Five Novel Zinc(II) and Cadmium(II) Coordination Polymers based on Different Zwitterionic Pyridine Ligands

A series of five new Zn-II and Cd-II mixed-ligand coordination polymers, namely, {[Zn(L-1)(4,4'-bpy)]center dot(ClO4)center dot 2H(2)O} (1), {[Zn(L-2)(4,4'-bpy)(0.5)]center dot(ClO4)} (2), {[Zn(L-3)(4,4'-bpy)]center dot(NO3)center dot 2H(2)O} (3), {[Cd(L-4)(4,4'-bpy)(0.5)(NO3)]center dot 5H(2)O} (4), and {[Zn(L-4)(4,4'bpy)]center dot Cl center dot H2O} (5) [4,4'-bpy = 4,4'-bipyridine, L-1 = 4-carboxy-1-(4carboxybenzyl)pyridin-1-ium chloride, L-2 = 3-carboxy-1-(4-carboxybenzyl)pyridin-1-ium chloride, L-3 = 4-carboxy-1-(3-carboxybenzyl)pyridin-1-ium chloride, and L-4 = 3-carboxy-1-(3-carboxybenzyl)pyridin-1-ium chloride], were obtained by the reactions of the 4,4'-bipyridine with four dicarboxylate zwitterionic pyridine ligands. Single-crystal X-ray structural analyses reveal that the five complexes demonstrate different molecular frameworks coming from various coordination modes and flexibilities of different dicarboxylate zwitterionic pyridine ligands and central metal atoms. Mononuclear twofold dinuclear 2D twofold interpenetrating net for 2, four-coordinate mononuclear twofold interpenetrating 2D layer for 3, mononuclear 2D layer arranged in parallel and with large grids for 4, and twofold trans interpenetrating 2D network for compound 5. The structural diversities in 1-5 indicate that the nature of the ligands and the presence of different metal atoms have a great influence on central metal coordination modes and the structural topologies of the metal-organic molecular architectures. In addition, pi center dot center dot center dot pi stacking interactions also play important roles in the final crystal packing and supramolecular frameworks. The powder X-ray diffraction, elemental analysis, and photoluminescence properties of 1-5 were studied, which show that architectures play an important role in emission bands and intensities.