Heteroleptic six-coordinate bismuth(iii) complexes with 2-acetylthiophene thiosemicarbazones: synthesis, characterization, and biological properties

Abstract

The present study aimed to prepare and then evaluate the antiproliferative and antibacterial activities of bismuth(III) thiosemicarbazone complexes. Five bismuth(III) halide complexes of general formulae [BiX₃(HL)₃] (X: Cl; 1 and 2) and [BiX₂(μ₂-X)(HL)₂]₂ (X: Br and I; 3,4 and 5), where HL is a thiosemicarbazone bearing an acetylthiophene moiety, have been synthesized and fully characterized, including their X-ray crystal structures. In all cases, the thiosemicarbazone ligand coordinated in a monodentate mode via the sulfur atom. Complexes 1 and 2 are mononuclear bismuth(III) complexes with octahedral (Oh) geometry around the metal ion formed by three sulfur and three chlorine atoms. However, the coordination mode of the ligands varied around the bismuth centers. Complex 1 contains one molecule with only a meridional orientation in its asymmetric unit, while complex 2 contains two molecules in its asymmetric unit, one with meridional orientation and the other with facial one. Two bismuth(III) atoms in dinuclear complexes (3, 4 and 5) were coordinated with each other by two halogen bridges. The bismuth(III) complexes 1–5 and their free ligands have been tested for their in vitro cytotoxic activity against human adenocarcinoma cervical cancer (HeLa) cells. The complexes showed promising results as antiproliferative agents for the HeLa cell line, with IC₅₀ values ranging from 4.5 ± 0.4 to 24.3 ± 1.7 μM. All these complexes were also tested against a series of bacteria, and they proved to be ineffective against Gram-positive bacteria (S. epidermidis and S. aureus) and moderately active against Gram-negative bacteria (P. aeruginosa and E. coli).