Positional Effects of a Pyridyl Group in Zn(II) Coordination Polymers on the Selective Dye Adsorption Properties

Three 1D zig-zag chain coordination polymers {[Zn(L1)](I)2}n (CP1), {[Zn(L2)](I)2}n (CP2) and {[Zn(L3)](I)2}n (CP3) were synthesized under the self-assembly conditions, where L1 = 1,2-bis(pyridin-3-ylmethylene)hydrazine; L2 = 1,2-bis(1-(pyridin-3-yl)ethylidene)hydrazine; L3 = 1,2-bis(1-(pyridin-4-yl)ethylidene)hydrazine. The molecular structures of CP1, CP2 and CP3, were determined by single crystal X-ray diffraction studies. Ligands L1 and L2 differ from L3 in terms of position of nitrogen atom on the pyridyl group, while L1 and L2 differs from each other with respect to the presence of hydrogen or methyl group on the imine carbon. These minor structural differences in the ligands has affected the electronic environment and the chemical nature of the chains formed in these Zn(II) coordination polymers. Hirshfeld surface (HS) analysis has been performed on CP1, CP2 and CP3 in order to determine the intermolecular interactions present in the crystal lattice and it is in well agreement with the single crystal XRD analysis of the hydrogen bonding interactions. Selective dye adsorption behavior was observed for CP3, which showed considerable adsorption of Methyl Orange (MO) and sodium salt of Methyl Red (MR) whereas CP1 and CP2 do not show any adsorption properties at all. Further it was observed that adsorption by CP3 was selective only for an anionic dye - Methyl Orange (MO) and sodium salt of Methyl Red (MR) whereas cationic dyes such as Methylene Blue (MB), Malachite Green (MG), Rhodamine-B (RB) were not adsorbed by CP3. These studies showed the importance of chemical nature of the ligands in coordination polymers on the adsorption properties. Although being 1D chain without any pores, CP3 was capable of holding the dyes in between the neighboring chains due to the electronic effects resulted from the coordination of the ligand L3 with the Zn(II) center.