Synthesis, characterization, biological activity, and QSAR studies of transition metal complexes derived from piperonylamine Schiff bases

A series of transition metal complexes of Co(II), Ni(II), Cu(II), and Zn(II) metal ions with various uninegative bidentate Schiff base ligands derived from condensation of piperonylamine and 2-hydroxy-1-naphthaldehyde/substituted salicylaldehyde derivatives were synthesized and were well characterized by different spectroscopic techniques (FT-IR, UV–Vis, NMR, fluorescence, mass, and ESR), molar conductance measurements, elemental analysis and by various physical studies like thermal (TGA, DTG, and DTA) and magnetic susceptibility measurements. The characterization data revealed that Schiff bases’ coordinates via azomethine nitrogen and deprotonated phenolic oxygen (NO) with metal centres in 2:1 molar ratio with octahedral geometrical arrangement for Co(II), Ni(II), and Zn(II) complexes and square planar geometry for Cu(II) complexes. The synthesized compounds were evaluated for their in vitro antimicrobial activities against two Gram-positive bacteria (Staphylococcus aureus and Streptococcus gordonii); two Gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa); and two fungal strains (Aspergillus niger and Candida albicans). Comparative biological study indicated that metal complexes were more noxious as compared to free ligands and Zn(II) complexes were found to be most potent antimicrobial agents among all the synthesized complexes. The complexes 23–30 showed excellent activity against tested microorganisms. Quantitative structure–activity relationship study of compounds was made for antimicrobial assessment which exposed that metal complexes with high molecular weight were more potent against E coli.