Coordination Polymers Based On Benzimidazole-5,6-Dicarboxylic Acid:Preparation,Crystal Structures And Properties

To design and synthesis novel coordination polymers have provoked significant interestsowing to their variety of intriguing structures and fascinating properties,such as luminescence,magnetism, catalysis, ion exchange and gas adsorption. Up to now, researchs choosendifferent ligands with transition metal ions and lanthanide ions to construct various newfunctional coordination polymers. The coordination mode of the organic ligands and metalions is the key affect factor for constructing coordination polymers,while the molar ratio ofthe reagents,solvent, pH value of the solution, reacting time and temperature have anassistance effect on the final structure. The aromatic N-and O-containing heterocycliccarboxylic acid has multiple coordinate sites,can be favor of synthesizing new coordinationpolymers. In this paper, we choose benzimidazole-5,6-dicarboxylic acid as the main ligand,2’2-bipy and oxalate as the auxiliary ligand with different transition metals and lanthanideions to generate seven new coordination polymers. The benzimidazole-5,6-dicarboxylic acidligand possesses the below properties:（1） As a rigid ligand and possessing two potentiallycoordination carboxyl groups, it is beneficial for thermal stability to construct the extendedM–O–M networks.（2） It has two nitrogen atoms of imidazole ring and four oxygen atoms oftwo carboxyl groups,which all can be regarded as hydrogen bonding acceptors or donors toform abundant hydrogen bonds. In addition, the relative large p-conjugated system in thebenzimidazole ring is beneficial to form p-p stacking. These supramolecular interactions arealso good for stabilizing and improving the thermal stabilities.（3） The carboxyl groups whichusually adopt diverse coordination modes such as terminal monodentate, chelating to onemetal center, bridging bidentate or tridentate to two metal centers, etc. as well as the nitrogenatom may result the more interesting architectures. It has two-COOH and one-NH that maybe completely or partially deprotonated to generate H2bidc-, Hbidc2-and bidc3-anionicdepending on the pH value, therefore, the ligand has varitries of coordination modes togenerate new metal organic frameworks.Seven compounds have been synthesized under hydrothermal method by changing themolar ratio of reactants, pH value, solvent, reaction time and temperature. Their structureshave been characterized by X-ray diffraction, elemental analysis, IR spectra and, fluorescenceproperties and thermogravimetric analysis. The experiment results as follows:1. We choose benzimidazole-5,6-dicarboxylic acid as the main ligand, oxalate as theauxiliary ligand with Pr^（Ⅲ）, Nd^（Ⅲ）, Sm^（Ⅲ）, Eu^（Ⅲ）, Gd^（Ⅲ）ions under solvethermal method, 2. namely:{[Pr（Hbidc）（ox）_1/2（H₂O）]·H₂O}n（1）,{[Nd（Hbidc）（ox）_1/2（H₂O）]·H₂O}n（2）,{[Sm（Hbidc）（ox）_1/2（H₂O）]·H₂O}n（3），[Eu（Hbidc）（ox）_1/2]（n4）,[Gd（Hbidc）（ox）_1/2]（n5）.Thecompounds1-5are3D layer structures. Compounds1-3are isostructural, the centralPr1（III） is nine-coordinated in distorted tricapped trigonal prism environments.Compounds4and5are isostructural, the coordination polyhedron around the centralEu1（III） ion can be visualized as a slightly distorted dodecahedral geometry.Thestructures of compounds1-5are strengthened for the π–π stacking. In addition,fluorescence properties in solid state for3and4have been investigated, respectively.Compared with the emission spectrum of H3bidc, the emission band of3and4showred shift.3. We choose benzimidazole-5,6-dicarboxylic acid as the main ligand,2.2’-bipy as theauxiliary ligand with Ni^（II）, Cu^（II）ions under solvethermal method, namely:{[Ni₃（bidc）₂（2,2’-bpy）]·2H₂O}n（6）and {[Cu₃（Hbidc）₂·2H₂O]·5H₂O}n（7）.Compounds6and7are2D frameworks, in compound6the intermolecularhydrogen bonds further link the adjacent2D layers to generate a3D supramolecularframework.