Synthesis, structures and antimicrobial activity of 5-nitro-salicylaldehyde-thiosemicarbazonates of zinc(II) coordinated to substituted bipyridines/phenanthrolines

Reactions of zinc(II) with 5-nitro-salicylaldehyde-N-1 -substituted thiosemicarbazones {(5-NO2-2-HO-C6H4)C-2(H)=N-3-(NH)-H-2-C-1(=S)-(NHR)-H-1; R = H, H2L1; Me, H2L2; Et, H2L3; Ph, H2L4 ) and 4,4'-dimethyl-2,2'-bipyridine (dm-bipy), 2,9-dimethyl-1,10-phenanthroline (dm-phen) and 3,4,7,8-tetramethyl-1, 10-phenanthroline (tm-phen) as co-ligands, have yielded complexes of stoichiometry, [Zn(L-n)(L)] {n = 1-4; L = dm-bipy, 1, 4, 7, 10; dm-phen, 2, 5, 8, 11; tm-phen, 3, 6, 9, 12) characterized by elemental analysis, infrared and electronic absorption spectroscopy and single crystal X-ray crystallography. Complexes 9 and 10 have distorted trigonal bipyramidal geometry (tau = 0.529-0.580), while complexes 5, 8 and 11 have distorted square pyramidal geometry (tau = 0.004-0.250). They displayed fluorescence bands at lambda(max) - 430-440 nm. In comparison to unsubstituted bipyridines/phenanthrolines, these zinc(II) complexes have shown higher antimicrobial activity with low minimum inhibitory concentration (MIC) against the clinical isolate methicillin resistant Staphylococcus aureus (MRSA), Gram positive bacteria, namely, Staphylococcus aureus (MTCC740), Enterococcus faecalis (MTCC439), Gram negative bacteria, namely, Klebsiella pneumonia 1 (MTCC109), Escherichia coli (MTCC119), Salmonella typhimurium 1 (MTCC98) and one yeast strain Candida albicans (MTCC227). These complexes tested were found to be cytotoxic to microorganisms (bactericidal/fungicidal). (C) 2018 Elsevier Ltd. All rights reserved.