DFT studies, spectral and biological activity evaluation of binary and ternary sulfamethazine Fe(III) complexes

[FeL₃]·H₂O (1) and [FeL₂Q]·3H₂O (2) (HL = sulfamethazine and HQ = 8-hydroxyquinoline) have been synthesized, characterized (elemental analysis, FT IR, UV–vis, TGA, magnetic and conductivity), and tested for their antibacterial activity against Staphylococcus aureus and Escherichia coli. Theoretical calculations involving geometry optimization, natural orbital analysis, electronic spectra, and molecular electrostatic potential have been done at the DFT/B3LYP level of theory. The high 3d-electron contribution of 6.71 is accounted to L → d_Fe charge transfer. Coordination of HL to Fe(III) in 1 led to a significant decrease in the antibacterial activity, and presence of a secondary ligand in 2 completely abolished it.