Heavy metal complexes of 4-chlorodipicolinic acid - structural, spectral and thermal correlations

The ligand, 4-chloro-2,6-dipicolinic acid (H₂PDA-Cl), and its two heavy metal complexes, [Ag(HDPA-Cl)(H₂DPA-Cl)·2H₂O] (1) and {[Cd(μ₂-H₂O)(H₂O)(PDA-Cl)]}_n (2), were prepared and then characterized by single-crystal X-ray diffraction. In addition, spectral and thermal correlations with structural results complete their solid-state description and facilitate complex 3 ([Pb(DPA-Cl)]) composition determinations. Complex 1 crystallizes in a monoclinic space group C2/c and each DPA-Cl ligand is tridentate to Ag(I) through the pyridine N and two monodentate carboxyl O atoms. The carboxy group with carbonyl C1 is semideprotonated and forms a symmetric hydrogen bond with a carboxy group of a neighboring complex. Complex 2 crystallizes in a triclinic lattice with space group P$.$ The Cd(II) ions are seven-coordinate and the coordination polyhedra can be described as a distorted pentagonal bipyramid. IR data are consistent with monodentate coordination of the carboxylate to Ag(I), Cd(II), and Pb(II) and observed wavenumber shifts confirm PDA-Cl ligand coordination to Pb(II) in 3. Thermal stability of anhydrous complexes indicates the metal–ligand interactions. The thermal stability of prepared compounds is reflected by the strength of interaction between metal–ligand and hydrogen bonds.