Solvothermal synthesis of tetranuclear Zinc(II) schiff base complexes: halogen-driven structural modulation and selective antifungal and antibacterial properties against gram-positive and gram-negative bacteria

Abstract

Two novel tetranuclear Schiff base zinc(II) complexes, [Zn4(L1)4(bpp)2](DMF)2(H2O) (1) and [Zn4(L2)4(bpp)2](DMF)2 (2) were successfully synthesized under solvothermal conditions (bpp = 1,3-di(4-pyridyl)propane, L1\L2 = 5‑chloro\bromo‑salicylaldehyde methylthiosemicarbazone). The complexes were synthesized and later characterized by X-ray single crystal diffraction, powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA). Comparative crystallography demonstrated localized halogen substitution effects: bromo/chloro exchange induced significant yet confined geometric changes at ligand sites without perturbing zinc coordination geometries, underscoring framework robustness. The antimicrobial activities of both complexes were evaluated in vitro against four commercially available microbial strains: the fungal strain Candida albicans (C. albicans), the Gram-negative bacteria strains Escherichia coli (E. coli) and Pseudomonas aeruginosa (P. aeruginosa), and the Gram-positive bacteria strain Staphylococcus aureus (S. aureus). Notably, both complexes demonstrated significant antimicrobial selectivity toward different microorganisms. Particularly, both complexes showed pronounced antimicrobial activity against S. aureus. This study provides evidence of tetranuclear Zn(II)-thiosemicarbazone clusters as Gram-positive-targeted antimicrobial agents, offering new insights into metalloantibiotic design through strategic halogen substitution and multinuclear assembly.