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

Abstract

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 λ_max = 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.