Synthesis, characterization, and antifungal and antioxidant evaluation of novel ethylenediamine-derived Schiff bases and their Ni(II), Co(II), Cd(II), and Cu(II) complexes

Abstract

Three Schiff base ligands derived from condensation of ethylenediamine with benzaldehyde, salicylaldehyde, and vanillin, namely (1E,1'E)-N,N'-(ethane-1,2-diyl)bis(1-phenylmethanimine) (BBE), 4,4'-((1E,1'E)-(ethane-1,2-diylbis(azaneylylidene))bis(methaneylylidene))bis(2-methoxyphenol) (BHE), and 2,2'-((1E,1'E)-(ethane-1,2-diylbis(azaneylylidene))bis(methaneylylidene))diphenol (BME), were synthesized and complexed with Co(II), Cu(II), Ni(II), and Cd(II) ions. To the best of our knowledge, these specific metal complexes have not been reported previously, and their biological activities have not been evaluated. The ligands and complexes were characterized by melting point determination, solubility tests, molar conductivity, UV--visible and FTIR spectroscopy, magnetic susceptibility, X-ray fluorescence, powder X-ray diffraction (XRD), and thermogravimetric/differential thermal analyses (TGA/DTA). Low molar conductivity values (10.5--12.0 Omega^-1 cm² mol^-1) indicate non-electrolytic behavior, while FTIR and X-ray fluorescence studies indicate coordination through azomethine nitrogen and oxygen donor atoms. XRD patterns show crystalline structures with sharp diffraction peaks, and thermal analyses indicate multistep decomposition and good thermal stability. Biological analysis indicates increased antifungal activity of the metal complexes over the free ligands, with lower minimum inhibitory concentrations; some complexes also showed improved 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity (IC50 = 0.68101 mg/mL). The first-row transition metals are biologically relevant, while Cd(II) was included only for comparative structure--activity evaluation despite its known toxicity. This study provides insight into Ni(II) and Cd(II) complexes through integrated structural, thermal, and biological characterization.