Antimony(III) and Bismuth(III) Halide Complexes Containing Thiazole-and Thiadiazole-Based Mercapto Ligands: Synthesis, Structural Characterization, and Biological Activity

Sulfur-rich heterocyclic ligands are widely employed as building blocks for biologically active main-group metal complexes owing to their versatile coordination behavior. In this study, four new antimony(III) and bismuth(III) halide complexes based on 2-mercapto-4-methylthiazole and 2-mercapto-5-methyl-1,3,4-thiadiazole ligands were synthesized, spectroscopically characterized, and evaluated for their biological activity. The crystal structures of complexes 1, 3, and 4 were determined by single-crystal X-ray diffraction, revealing distorted square-pyramidal coordination geometries with the ligands coordinated in their neutral thione form through the sulfur donor atom. Single-crystal X-ray diffraction analyses reveal distinct solid-state arrangements: complex 1 is monomeric, complex 3 forms a chloride-bridged polymer, and complex 4 forms bromide-bridged dimers. Time-dependent molar conductivity and UV–Vis measurements showed no substantial changes under the investigated conditions. The antimony(III) complexes exhibited antiproliferative activity against HeLa cells, whereas the bismuth(III) analogues were inactive within the investigated concentration range. The complexes also showed modest antibacterial effects in the disc-diffusion screening assay, with the most pronounced inhibition-zone responses observed against E. coli. Overall, the biological results reveal differences in activity among the investigated Sb(III) and Bi(III) complexes under the tested conditions.