Tricarbonylrhenium(I) complexes with the N-methylpyridine-2-carbothioamide ligand – Synthesis, characterization and cytotoxicity studies

A new series of tricarbonyl complexes of rhenium(I) in the ‘2 + 1’ system with the bidentate ligand N-methylpyridine-2-carbothioamide (NC5H4-CS-NH-CH3, LH(Me)NS) and a monodentate ligand, being either an anion (Cl, Br, I or SCN) or a neutral molecule (3,5-dimethylpyrazole (Hdmpz) or imidazole (Him)), was synthesized. The use of mixed ligands leads to the formation of neutral or cationic (in the form of PF6− salts) tricarbonylrhenium(I) complexes: [Re(CO)3(LH(Me)NS)X] (X = Cl, Br, I, NCS) (complexes 1–4) and [Re(CO)3(LH(Me)NS)Y]+ (Y = Hdmpz, Him) (5 and 6), respectively. In case of the [Re(CO)3(LH(Me)NS)NCS] complex two polymorphic forms (4a and 4b) have been distinguished. Crystal structure of all complexes was determined by single-crystal X-ray diffraction method and the results were compared with the molecular structures obtained from DFT calculations. The compounds were characterized by FT-IR, NMR, UV-Vis and HPLC techniques. IR and UV-Vis spectra were also simulated by DFT and TD-DFT methods. Cytotoxicity of the complexes was estimated using human ovarian cancer cell line (A2780), its cisplatin resistant cell line (A2780cis) and non-cancerous human embryonic kidney cells (Hek-293). The toxicity of newly synthesized complexes was comparable to cisplatin when tested against both cancerous cell lines (IC50 = 2–49 μM), but lower than cisplatin towards non-cancerous cells (IC50 = 6–63 μM). Among them, the complexes with chloride and iodide anions exhibit remarkable cytotoxicities.