Two new cadmium(II) coordination polymers based on imidazole-containing ligands: synthesis, structural characterization and fluorescence properties

The judicious selection of suitable ligands is vitally important in the construction of novel metal-organic frameworks (MOFs) with fascinating structures and interesting properties. Recently, imidazole-containing multidentate ligands have received much attention. Two new Cd^II coordination frameworks, namely, poly[tris{μ-1,4-bis[(1H-imidazol-1-yl)methyl]benzene-κ²N³:N^3'}tetrakis(nitrato-κ²O,O')dicadmium], [Cd₂(NO₃)₄(C₁₄H₁₄N₄)₃]_n, (I), and poly[[bis{μ₃-1,3,5-tris[(1H-imidazol-1-yl)methyl]benzene-κ³N³:N^3':N^3''}cadmium] hexafluorosilicate], {[Cd(C₁₈H₁₈N₆)₂](SiF₆)}_n, (II), have been synthesized and characterized by elemental analysis, IR spectroscopy and single-crystal X-ray diffraction. In polymer (I), the 1,4-bis[(1H-imidazol-1-yl)methyl]benzene ligand bridges Cd²⁺ ions with a distorted seven-coordinated pentagonal bipyramidal geometry, forming a one-dimensional ladder chain, and the nitrate anions coordinate to the Cd²⁺ ions in a terminal bidentate fashion. In the crystal, adjacent chains are further connected by C-H...O hydrogen bonds to generate a two-dimensional (2D) supramolecular structure. Polymer (II) exhibits a 2D layered structure in which 1,3,5-tris[(1H-imidazol-1-yl)methyl] benzene ligands join Cd²⁺ centres having a six-coordinated octahedral structure. The layers are connected by hexafluorosilicate anions via C-H...F hydrogen-bond interactions, giving rise to a three-dimensional supramolecular network structure in the solid state. In addition, powder X-ray diffraction (PXRD) patterns were recorded, thermogravimetric analyses (TGA) carried out and fluorescence properties investigated.