Synthesis, characterisation and antibacterial activity of flavone-based Sn(iv) and Sb(iii) complexes

Three 3-hydroxyflavone ligands (L₁-L₃) and their corresponding Sn(iv) and Sb(iii) complexes (1-6) of general formula [Sn(L)₃]Cl (1-3) and [Sb(L)₂Cl] (4-6), where L is the monoanionic form of 3-hydroxy flavone (L₁ in 1 and 4), naphthyl flavone (L₂ in 2 and 5) or anthracenyl flavone (L₃ in 3 and 6), are synthesized and characterised using UV-visible, fluorescence, IR, NMR and high-resolution mass spectral studies. The compounds were evaluated against Staphylococcus aureus (MTCC 96), Escherichia coli (MTCC 111), Pseudomonas aeruginosa (MTCC 1688) and Klebsiella pneumoniae (MTCC 432). In general, coordination of the flavone ligands to Sn(iv) and Sb(iii) enhanced antibacterial activity relative to the parent ligands and the corresponding metal salts. Among the series, the Sb(iii) complexes were the most active, with complex 6 showing the highest potency, particularly against S. aureus, for which the minimum inhibitory concentration (MIC) was 2.9 µg mL^-1, comparable to ciprofloxacin under the tested conditions. The expanded MIC dataset also indicated a clear organism-dependent susceptibility pattern, with S. aureus being more susceptible than the tested Gram-negative bacteria. Checkerboard analysis further revealed synergistic interactions between the Sb(iii) complexes and ciprofloxacin against S. aureus. Mechanistic studies, including scanning electron microscopy, membrane permeability analysis and propidium iodide staining, indicated that the antibacterial action of complex 6 involves disruption and permeabilization of the bacterial membrane. In addition, the cytotoxic effects of the two most active complexes, 5 and 6, were assessed in human dermal fibroblast (HDF) cells by MTT assay. Both complexes maintained high HDF viability at their respective MICs against S. aureus, indicating that their anti-staphylococcal activity was achieved at concentrations that did not cause marked cytotoxicity under the tested conditions. These findings identify Sb(iii)-bound flavonoid complexes as promising scaffolds for further antibacterial investigation.