Synthesis, characterization, spectroscopic, crystal structures and solution studies of two coordination compounds of zinc(II) and iron(III) based on chelidamic acid and acridine

Two new proton transfer compounds, (acrH)2[Zn(hypydc)2]·10H2O 1 and (acrH)[Fe(hypydc)2]·4.5H2O 2 [where hypydcH2 = 4-hydroxypyridine-2,6-dicarboxylic acid (chelidamic acid) and acr = acridine] have been synthesized and characterized by elemental analysis, spectral (UV–Vis, IR), 1H NMR, 13C NMR spectroscopy and single crystal X-ray diffraction. The crystallographic analysis revealed that ZnII and FeIII atoms are coordinated by two N atoms and four O atoms from the carboxylate groups of (hypydc)2− ligands, forming a distorted octahedral geometry. In the anions two pyridine rings are inclined to one another by 88.10(5)° and 83.95(13)° for 1 and 2, respectively. The (H2O)n [n = 3–6] cyclic–linear water clusters connected two layers of anionic complexes in 1. The crystal packing of 1 show a 3D network formed through H-bonds involving water molecules, NH groups of acridinium and carboxylate anions. In 2 two types of rubout O–H⋯O hydrogen bond synthons, R 4 4 (16) and R 4 4 (18) link the anions and water molecules to form two-dimensional network parallel to bc plane. In crystal structures of both complexes extensive O–H⋯O, N–H⋯O and C–H⋯O hydrogen bonds as well as electrostatic forces, C–O⋯π and π–π stacking play important roles in stabilizing structures. The protonation constants of hypydc and acr, in all of probability protonated forms, the equilibrium constants for the hypydc–acr proton transfer system and the stoichiometry and stability of complexation of this system with Zn2+ and Fe3+ ions in 50% dioxane–50% water (V/V) solvent were investigated by potentiometric pH titration method. The stoichiometries of the most complex species in solution were compared with corresponding crystalline metal ion complexes 1 and 2.