New (N,O) Schiff bases of 2-hydroxynaphthaldehyde and their homoleptic Zn(II) and Cu(II) complexes – Synthesis, structural characterization, Hirshfeld surface analysis and antimicrobial activity studies

Abstract

New Schiff bases L1H and L2H have been synthesized by condensation of 2-hydroxynaphthaldehyde with 2,3-dimethoxyaniline and 2-(3,4-dimethoxyphenyl) ethan-1-amine respectively. Their complexes are [M(L1-2)2] where, M: Zn (1 and 2) and Cu (3 and 4). They were prepared by the reaction of ligands with Zn(OAc)2·2H2O and CuCl2·6H2O in the molar ratios of 2:1 (L:M). Both ligands and their four complexes were characterized by CHN analysis, FT-IR and UV–Visible spectroscopy. In addition, the ligands and complexes 1 and 2 were characterized by 1H and 13C{1H} NMR spectroscopy. The molecular structures of the four compounds L1H, L2H, 3 and 4 were determined by single crystal X-ray diffraction. In all complexes, the deprotonated (L1 and L2) ligands coordinated to M(II) ions hence, acting as bidentate (NO) donors using imine nitrogen (>CN) and phenolate oxygen (PhO−) atoms. The ligands (L1H and L2H) and complexes (1–4) were screened in different dosages for antimicrobial activity by the agar well diffusion method against the pathogenic bacterial strains E. coli and E. aerogenes and a fungal strain C. albicans. These compounds showed moderate antibacterial activity but, significant antifungal activity. The antimicrobial activity results also revealed that the metal complexes were more active than their corresponding free ligands. The zinc complexes (1–2) showed higher antibacterial activity than copper ones. Surprisingly, the compounds related to 3,4-dimethoxy aniline (L1H, 1 and 3) showed higher activity than those containing 2-(3,4-dimethoxyphenyl)ethan-1-amine (L2H, 2 and 4) especially towards E. coli. Unlike the antibacterial activity, the antifungal activity of L1H was higher than that of its complexes and that of L2H was lower than that of L1H as well as its complexes.