Tetranuclear and 1D Polymeric Cd(II) Complexes with a Tetrapyridyl Imidazolidine Ligand: Synthesis, Structure, and Fluorescence Sensing Activity

Two novel coordination compounds of cadmium, [Cd2L1(ssal)2(H2O)3]2·10H2O (1) and {[Cd2L1(dca)4(H2O)]·H2O}n (2) (where L1 = 2,2′,2″-(1-(pyridin-2-ylmethyl)imidazolidine-2,4,5-triyltripyridine), ssal = sulfosalicylate, and dca = dicyanamide), have been synthesized under reflux conditions. The nitrogen-rich ligand L1 was formed in situ by Cd(II)-mediated coupling cycloaddition of the Schiff base N-(2-pyridylmethyl)pyridine-2-carbaldimine: thus, through a Cd(II)-catalyzed reaction. In the two species sulfosalicylate or dicyanamide anions act as a coligand. The single-crystal X-ray structural analyses revealed that 1 is a discrete centrosymmetric tetranuclear complex crystallizing with 10 lattice water molecules that favor the formation of a 3D supramolecular structure through H bonds. On the other hand complex 2 is a coordination polymer built by bridging dca anions connecting −[Cd1(L1)Cd2]– building blocks. The polymers are further connected by H bonds involving the coordinated and lattice water molecules with terminal dca N atoms of symmetry-related chains. The luminescent properties of complexes 1 and 2 were investigated in various organic solvents, and both complexes show a highly selective sensitive response toward nitroaromatic compounds, especially toward picric acid (PA). The possible quenching mechanism of the process has been validated by the combined effect of energy and electron transfer mechanisms as well as electrostatic interactions.