Synthesis of (3-nitro-2-oxo-benzaldehyde thiosemicarbazonato)-zinc(II) complexes: the position of nitro group in phenyl ring alters antimicrobial activity against K. pneumoniae 1, S. typhimurium 2, MRSA and C. albicans .

The basic purpose of this investigation was to explore the effect on antimicrobial activity of a nitro group at an ortho- versus para-position in the 2-hydroxy-phenyl ring of nitro-salicylaldehyde-N-substituted thiosemicarbazone (X-NO2-stscH2-N1HR, X = 3 or 5; stsc-stands for salicyladehyde thiosemicarbazone) complexes with zinc. Reactions of zinc(ii) acetate with 3-nitro-salicylaldehyde-N-substituted thiosemicarbazones (3-NO2-stscH2-N1HR) and 2,2-bipyridine, or 1,10-phenanthroline as co-ligands, yielded complexes of stoichiometry, [Zn(3-NO2-stsc-N1HR)(N,N-L)] {L, R: bipy, H, 1; Me, 2; Et, 3; Ph, 4; phen, H, 5; Me, 6; Et, 7; Ph, 8}. The thio-ligands coordinate to the metal as dianions (deprotonation of -OH and -N2H moieties) through O, N3 and S donor atoms in distorted trigonal bipyramid geometry (4, 5, 7: τ = 0.718-0.576) or in distorted square pyramid geometry (8: τ = 0.349). ESI-mass spectrometry supported the formation of molecular ion peaks. Complexes displayed fluorescence with λmax = 438-473 nm. It was found that these five-coordinated [Zn(3-NO2-stsc-N1HR)(N,N-L)] complexes showed high antimicrobial activity against methicillin resistant S. aureus (MRSA), Klebsiella pneumoniae 1, Salmonella typhimurium 2 and C. albicans vis-à-vis that of 5-NO2-stscH2-N1HR zinc complexes reported earlier. However, in comparison, the antimicrobial activity of 5-nitro complexes against S. aureus was high relative to 3-nitro complexes in the present case.