Four benzimidazole-based ZnII/CdII polymers extended by aromatic polycarboxylate coligands: synthesis,structure, and luminescence

Four benzimidazole (bim)-based metal complexes, {Zn₂(bim)₄(btec)] · DMF} _n (1), {Zn(bim)(btc)] · Htea} _n (2), Zn(bim)₂(bdc)] _n (3), and {Cd₃(bim)₄(H₂O)₆(btc)₂] ? 2H₂O} _n (4) (H₄btec = 1,2,4,5-benzenetetracarboxylic acid, H₃btc = 1,3,5-benzenetricarboxylic acid, H₂bdc = 1,4-benzenedicarboxylic acid, and tea = triethylamine), have been obtained by the introduction of benzene-based polycarboxylate as coligand and are structurally characterized by single-crystal X-ray diffraction, elemental analysis, IR spectra, thermogravimetric curves, and luminescence spectra. Controlled by the number and position of the carboxylates attached to the aromatic ring, all the four complexes exhibit polymeric structures from 1-D chain or ribbon for 1, 3, and 4 to 2-D layer for 2. In contrast, the neutral bim is a terminal ligand to complete the metal coordination sphere and it also helps to assemble the low-dimensional coordination skeleton into a high-dimensional ordered supramolecular architecture by N–H ··· O hydrogen-bonding and π–π stacking interactions. Additionally, the complexes exhibit strong emissions originating from the bim-based intraligand as well as photo-induced interligand charge transfer upon cation binding, suggesting potential applications as luminescent materials.