Luminescent imidazolium carboxylate supported aggregate and infinite coordination networks of copper and zinc

Abstract

The new copper dimer [LCu(DMF)]₂(NO₃)₄(H₂O)(DMF)₂ (4), where L = [{1,1′-(CH₂)₂-C₁₄H₈)-3,3′-(CH₂CO₂)₂}{(HCN)₂CH}], and porous coordination polymers [{L₂Cu(OH₂)₂}₂Br₂]_x (5) and [{L₂Zn(OH₂)₂}₂Br₂]_x (6) have been isolated from reactions of luminescent imidazolium carboxylate ligand, LH₂Br₂ (3) and the corresponding metal precursors. The reaction between Cu(NO₃)₂·3H₂O and LH₂Br₂ (3) in DMF at 100 °C yielded bluish green crystals of tetracationic discrete copper dimer 4, the structure of which contains a rare tetracationic [(DMF)Cu(II)]₂ dimer unit that is bridged by four carboxylates of two L in a “paddle-wheel” structure. When the reaction was carried out in the presence of a water–ethanol–methanol mixture, light green crystals of 5 were obtained. Molecule 5 comprises two-dimensional (2D) porous coordination polymeric sheets consisting of unique symmetrical dinuclear [(C(O)O)Cu(OH₂)₂(O(O)C)₂]₂ building blocks, which are connected by imidazolium anthracene spacers. The infinite 2D porous coordination polymeric sheets are further linked by significant intermolecular hydrogen-bonding interactions by bromide anions to form a three-dimensional supramolecular framework. Interestingly, the reaction between zinc dust and LH₂Br₂ (3) in H₂O at room temperature gave similar structural features to those in 5, though they differ in terms of C–O bond distances and M–O–C angles. The solution-state UV-visible absorption spectra of 2–6 in water exhibits the comparable absorption pattern with decrease in the intensity of absorption from 5, 4, 3, 6 and 2, while the solid-state UV-visible absorption spectra of 2–6 are significantly different from the solution-state UV-visible absorption spectra. The considerable change in the fluorescent emission was observed upon complexation of 3 with corresponding metal precursors and the fluorescent emission was shifted towards the red region in the order of 2, 3, 6, 4 and 5 in water.