Influence of metal complex formation on the antimicrobial activity of nifuroxazide: Spectroscopic, electrochemical, and DFT studies

[M(HL)₂] complexes (M = Co(II) (1), Ni(II) (2), and Cu(II) (3); H₂L = 4-hydroxybenzoic[(5-nitro-2-furanyl)methylene]hydrazide acid, nifuroxazide) were synthesized, characterized (by elemental analysis, TG, IR, UV-vis., EPR, magnetic, and conductance measurements) and tested for their antimicrobial activities. H₂L is a mono-negative bidentate ligand via hydrazone N and C-O^- forming intermediate complexes between tetrahedral and square-planar geometries ("flattened" tetrahedron, D_2d symmetry) for 1 and 2, as well as square-planar for Cu(II) complex 3. Natural bond orbital analysis revealed that the interaction of oxygen with metal ion is the main factor which determines the stability of 1-3 as the binding energy decreases with an increase in the M-O bond length. Time-dependent density functional theory calculations were carried out to understand the electronic transitions in related experimental observations. The reduction potential values of the nitro group are affected by the metal center. Ni(II) complex 2 displayed the highest activity among the tested complexes against Escherichia coli with a MIC₅₀ value of 0.098 μmol mL^-1 compared with 0.131 (1) and 0.117 μmol mL^-1 (3).