Synthesis, spectral analysis, DFT-assisted studies, in vitro antioxidant and antimicrobial activity of transition metal complexes of hydrazone ligands derived from 4-nitrocinnemaldehyde

Here, we report a new series of Co(II), Ni(II), Cu(II) and Zn(II) metal complexes (1–20) derived from hydrazone ligands [Nʹ-((E)-3-(4-nitrophenyl)allylidene)benzohydrazide] HL, [Nʹ-((E)-3-(4-nitrophenyl)allylidene)nicotinohydrazide] HL, [Nʹ-((E)-3-(4-nitrophenyl)allylidene)isonicotinohydrazide] HL, [4-methyl-Nʹ-((E)-3-(4-nitrophenyl)allylidene)benzohydrazide] HL which were obtained from condensation reaction of 4-nitrocinnemaldehyde with benzo/ nicotinic/ iso-nicotinic/ p-toluic hydrazide. The structure elucidation was deduced from spectral and physicochemical techniques like H NMR, C NMR, FT-IR, UV–Vis, ESR, SEM, TGA/DTG, XRD, HRMS, molar conductance measurements and elemental analysis. Also, DFT (B3LYP/LanL2DZ) computations have been conducted for ligand and its metal complexes to study the structural and energetics behaviour. The spectral and physicochemical techniques suggested octahedral geometry of all the afforded metal complexes with hydrazone ligands in (1 M: 2L) molar ratio having molecular formula of the type [M(L)(HO)]. Thermal analysis suggested that the complexes were stable up to 190 °C and that water molecules are coordinated to metal ions in complexes. XRD data revealed crystalline nature of compounds and scanning electron microscopy (SEM) suggested that metal complexes have different surface morphology as compared to hydrazone ligands. In vitro antioxidant potential of synthesized compounds was assessed and investigation revealed that metal complexes have more efficiency of decolourizing the DPPH solution as compared to ligands. To ascertain the therapeutic aspects of synthesized compounds, in vitro antimicrobial assessment was performed on four bacterial strains (Bacillus subtilis, Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli) and two fungal strains (Candida albicans, Aspergillus niger) taking ciprofloxacin and fluconazole as reference drugs, respectively. Compounds 11, 12, 15, 16, 19 and 20 were found to be potent antimicrobial agent among the tested series. Thus, this investigation gave a vision into the structural-based modification of hydrazones and their complexation with metals could enhance biological activities.4 Graphical abstract: [Figure not available: see fulltext.]