A heterocyclic N'-(4-(diethylamino)-2-hydroxybenzylidene)-4-oxopiperidine-1-carbohydrazide Schiff base ligand and its metal complexes: Synthesis, structural characterization, thermal behavior, fluorescence properties, and biological activities

Abstract

A new heterocyclic hydrazone Schiff base ligand, N'-(4-(diethylamino)-2-hydroxy benzylidene)-4-oxopiperidine-1-carbohydrazide, (H₂L) was derived by a condensation reaction of 4-oxopiperidine-1-carbohydrazide with 4-(diethylamino)-2-hydroxybenz-aldehyde. The ligand reacts with chloride salts of chromium(III), manganese(II), iron(III), cobalt(II), nickel(II), copper(II) and zinc(II) to form metal complexes of [Cr(L)(Cl)(H₂O)₂], [Mn(HL)(Cl)(H₂O)₂], [Fe(L)(Cl)(H₂O)₂], [Co(HL)(Cl)(H₂O)₂], [Ni(HL)(Cl)(H₂O)₂], [Cu(HL)(Cl) (H₂O)₂], [Zn(L)(H₂O)], respectively. The structure of the hydrazone ligand was confirmed by elemental analysis and spectroscopic techniques, viz., FT-IR, ¹H NMR, ¹³C NMR, and LC-MS spectroscopy. The newly synthesized ligand behaves as a tridentate ONO donor towards Cr, Mn, Fe, Co, Ni, Cu, and Zn metal ions. The spectral, magnetic moment, and thermal data indicate the octahedral geometry for all metal complexes except for Zn, which has tetrahedral geometry with 1:1 stoichiometry (M:L). ESR study revealed that π-bonding covalency is much stronger than the σ-bonding with axial distortion in the structure. The molar conductivity data suggested the nonelectrolytic nature of the complexes. The powder X-ray diffraction patterns suggest the nanocrystalline nature of the compounds. The SEM micrograph of the ligand significantly differs from its Ni(II) complex indicating coordination of Ni(II) ion to the ligand. The intense fluorescence emitted in the region of λ_Excitation 521 to 524 nm due to the functional fluorophores of the ligand and its manganese (II), chromium(III), cobalt(II), and zinc(II) complexes. Various kinetic parameters such as E_a, ∆S, ∆H, and ∆G of various decomposition steps were calculated from TGA diagrams using Coats-Redfern method and the thermal stability order was found to be Cr < Fe < Co < Mn = Cu < Zn < Ni. The antibacterial and antifungal activities of the ligand and its divalent and trivalent metal complexes were performed against the various pathogens viz. Escherichia coli, Salmonella typhi, Staphylococcus aureus, Bacillus subtilis, Candida albicans, and Aspergillus niger with reference to standard antibiotics viz. ofloxacin, azithromycin, and fluconazole. All metal complexes showed promising biological activity as compared with their parent ligand and may be used as a potential antimicrobial candidate in biological science.