Mixed‐Ligand Complexes of a Schiff Base, 8‐Hydroxyquinoline and Oxalic Acid with Cu(II), Ni(II), Zn(II), and Fe(III) Ions: Pyrolytic Products and Biological Activities

Binary mononuclear complexes of the Schiff base ligand 1,4‐di(hydroxybenzylidene)thiosemicarbazide (H2L) and the transition metal ions Cu(II), Ni(II), Zn(II), and Fe(III) were prepared in the presence of various molar ratios of LiOH. The binary complexes react with 8‐hydroxyquinoline (8‐HOqu) and oxalic acid (H2Ox) to form mixed‐ligand complexes. The mixed ligand complexes were prepared by using various molar ratios of LiOH. The thermal studies explored the stability of the mixed‐ligands in their complexes as the isolated thermal products also contain the mixed ligands. All of the binary and mixed‐ligand complexes have octahedral configurations except [CuHL]Cl and [ZnL], which have square‐planar geometrics. The Schiff base ligand (H2L) is coordinated to the central metal atom as a neutral, monoanionic and/or dianionic tetradentate ONNO ligand in the binary complexes and in the mixed‐ligand complexes. The mixed‐ligand, 8‐HOqu behaves as a neutral or monoanionic bidentate ligand. The oxalato ligand coordinates as a dianionic bidentate or a bridged bidentate ligand towards Fe(III) ions. All complexes and the corresponding thermal products were isolated and their structures were elucidated by elemental analyses, conductance, IR and electronic absorption spectra, magnetic moments, 1H NMR and TG‐DSC measurements as well as by mass spectroscopy. The free ligand H2L and its metal complexes showed higher antibacterial activity than some of the investigated antibiotics, doxycillin, sulperazon, septrin, cefobid, nitrofurantion, and erythromycin.