Synthesis, structures, antimicrobial activity and biosafety evaluation of pyridine-2-formaldehyde-N-susbtituted-thiosemicarbazonates of copper(II)

Pyridine-2-formaldehyde-N1-substituted-thiosemicarbazones {(C5H4N4)HC2N3–N2(H)–C1(S)N1HR} (abbrev. HL1-R: R = Me, Et, Ph) with copper(I) halides in an acetonitrile–dichloromethane mixture (1 : 1, v/v) have yielded a novel series of CuII complexes with a central coordination core: [CuIIX(N,N,S-L1-R)] (R = Me, Et, Ph: X = I, 1–3; Br, 4–6, Cl, 7–9), which have been characterized with the help of analytical data, IR, UV-visible and ESR spectroscopy, ESI-mass spectrometry and single crystal X-ray crystallography. The thio-ligands coordinate to CuII as anions (L1-R)− through pyridyl nitrogen-N4, azomethine nitrogen-N3 and thiolato sulfur and CuII is further bonded to iodide, bromide or chloride. In order to explore their bio-activity, complexes (1–9) as well as the previously reported CuII complexes of 2-benzoylpyridine-N-substituted thiosemicarbazones {(C5H4N4)(C6H5)–C2N3–N2(H)–C1(S)–N1HR; HL2-R}, namely, [CuIIX(N,N,S-L2-R)] (R = Me, Et, Ph: X = I, 10–12; Br, 13–15; Cl, 16–18), have been evaluated for their antimicrobial potential against different microbial strains. The microbial strains investigated are Staphylococcus aureus (MTCC740), Klebsiella pneumoniae (MTCC530), Shigella flexneri (MTCC1457), Salmonella typhimurium 1 (MTCC98), Salmonella typhimurium 2 (MTCC1251), Escherichia coli (MTCC119), Staphylococcus epidermidis (MTCC435), methicillin resistant Staphylococcus aureus (MRSA) and a yeast culture Candida albicans (MTCC227). Several complexes tested have shown high antimicrobial activity; specifically, against methicillin resistant Staphylococcus aureus, Staphylococcus epidermidis and Salmonella typhimurium 2; the activity is found to be high at low minimum inhibitory concentration (MIC) values as compared to that of the standard Gentamicin. Using MTT cytotoxicity assay {MTT = 3-[(4,5-dimethylthiazol-2-yl)-2,5-diphenyl]tetrazolium bromide}, complexes tested were found to be biosafe with high cell viability (90–98%).