Synthesis, spectroscopy, structures and antimicrobial activity of mixed-ligand zinc(ii) complexes of 5-nitro-salicylaldehyde thiosemicarbazones

Abstract

A series of new complexes of zinc(II) with 5-nitro-salicylaldehyde-N¹-substituted thiosemicarbazones {5-NO₂-C₆H₄(2-OH)-C²(H)N³–N²H–C¹(S)–N¹HR; H₂L¹–NHR, R = H, Me, Et, Ph: respective thio-ligands} using bipyridines/phenanthrolines (L′) as co-ligands have been synthesized and characterized using elemental analysis, infrared, NMR, electronic absorption and fluorescence spectroscopy and single crystal X-ray crystallography. Thio-ligands coordinate to the metal center as di-anions (O,N,S-L-NHR)²⁻ in their complexes, [Zn(κ³-O,N,S-L-NHR)(κ²-N,N-L′)] {R, (N,N-L′): H, bipy 1, phen, 2; Me, bipy 3, phen 4; Et, bipy 5, phen, 6; Ph, bipy, 7, phen 8}. The geometries of complexes fall into three categories: distorted trigonal bipyramidal (1-molecule II, 5, 7); distorted square pyramidal (4, 6 and 8-molecule II) and geometry in between square pyramidal and trigonal bipyramidal (1-molecule I, 8-molecule II). These zinc(II) complexes have shown significant antimicrobial activity against Staphylococcus aureus (MTCC740), methicillin resistant Staphylococcus aureus (MRSA), Klebsiella pneumoniae (MTCC109), Shigella flexneri (MTCC1457), Salmonella typhimurium (MTCC741) and Candida albicans (MTCC227). The activity against MRSA and S. aureus is a significant development in antimicrobial investigations vis-à-vis lack of activity of gentamicin against MRSA or low antimicrobial activity against S. aureus as per literature reports. All complexes are active against K. pneumoniae and S. typhimurium, while complexes reported in the literature were found to be inactive. Complexes tested were found to be cytotoxic to microorganisms (bactericidal/fungicidal).