Coordination properties of two new Schiff-base phenoxy-carboxylates and comparative study of their antioxidant activities

Abstract

Two novel tridentate Schiff-base ligands containing carboxylate groups were obtained from the condensation of 3-amino-4-hydroxybenzoic acid with either acetylacetone (H3L1) or salicylaldehyde (H3L2). H3L1 reacted with copper(II) acetate and pyridine (strongly coordinating solvent) to afford the C1 complex, while reaction of H3L2 with nickel(II) acetate and pyridine afforded two types of complexes C2 and C3. The two ligands and their metal complexes were fully characterized by elemental analysis, XRD and spectroscopic analysis. Upon its fully deprotonated state, (L1)3− coordinates in a keto-enamine fashion and acts as a ditopic binder through an ONO tridentate donor system and an O monodentate donor system (carboxylate). Upon its doubly deprotonated state, (HL2)2− ligand acts either as monotopic tridentate binder (i.e. enol-imine form with an ONO donor system that lead to complex C2) or as a ditopic ligand (ONO tridentate and O monodentate donor systems to afford complex C3). The molecular structure of all the compounds were determined by single crystal X-ray diffraction studies and revealed several important characteristics. In complex C1, the Cu(II) cations are five-coordinated with a slightly distorted square pyramidal geometry, while in C2 and C3 complexes, the Ni(II) ions are four and six–coordinated with a square planar and a slightly distorted octahedral geometry, respectively. Following this structural investigation, the antioxidant activities of the free ligands and their metal complexes were then investigated and demonstrated interesting antioxidant features especially for H3L2 and complex C3. Lastly, temperature dependent magnetic susceptibility measurements showed the presence of ferromagnetic interactions in C1 mediated through the phenoxido bridges.