pH dependent synthesis of Zn(ii) and Cd(ii) coordination polymers with dicarboxyl-functionalized arylhydrazone of barbituric acid: photoluminescence properties and catalysts for Knoevenagel condensation

Abstract

Four novel Zn(II) and Cd(II) compounds based on a dicarboxyl-functionalized arylhydrazone of barbituric acid ligand, 5-[2-{2,4,6-trioxotetrahydropyrimidin-5(2H)-ylidene}hydrazinyl]isophthalic acid (H₅L), namely [Zn(1κO:2κO′:3κO′′-H₃L)(H₂O)₂]_n (1), [Cd(1κO,O’:2κO′:3κO′′,O′′′-H₃L)(H₂O)₂]_n (2), [Zn(κN-H₃L)(H₂O)₃]·3H₂O (3) and [Cd(1κO,O′:2κN-H₃L)(H₂O)₃]_n·4nH₂O (4), have been synthesized at different pHs under hydrothermal conditions and characterized by elemental and thermogravimetric, FT-IR, and X-ray single-crystal diffraction analyses. Compounds 1 and 2 prepared in acidic medium are coordination polymers exhibiting double chain type one-dimensional (1D) structures, whereas 3 and 4 are obtained in basic medium. Compound 3 is a mononuclear Zn(II) complex containing three lattice water molecules, but in 4, Cd(II) cations are bridged by H₃L²⁻ ligands to form an interesting zig-zag type 1D coordination polymer. The crystal structures of these compounds are stabilized by strong hydrogen bonding interactions, forming two and three-dimensional networks. The pH value of the reaction system plays a crucial role in the ligand deprotonation and in the assembly of the metal–organic polymers. These compounds act as effective heterogeneous catalysts, under mild conditions, for the Knoevenagel condensation reaction of different aldehydes with malononitrile and can be recycled at least five times without losing activity. Further, compounds 1–4 exhibit luminescence properties in the solid state at room temperature.