[Os^IVCl₅(Hazole)]⁻ Complexes: Synthesis, Structure, Spectroscopic Properties, and Antiproliferative Activity

By exploring the Anderson type rearrangement reactions, osmium(IV) complexes of the general formula [cation]+[OsIVCl5(Hazole)]−, where [cation]+ = n-Bu4N+, Hazole = 1H-pyrazole (Hpz) (1), 1H-indazole (Hind) (2), 1H-imidazole (Him) (3), 1H-benzimidazole (Hbzim) (4), 1H,2,4-triazole (Htrz) (5), have been synthesized. To improve water solubility of tetrabutylammonium compounds, complexes with [cation]+ = Na+ [Hazole = Hpz 6), Hind (7), Htrz (8)] or H2azole+ [Hazole = Hpz (9), Hind (10), Htrz (11)] have been also prepared with the aim of testing them for cytotoxicity in cancer cells. In addition, the preparation of the complex {(n-Bu4N)2[OsIVCl6]}2[OsIVCl4(Him)2] (12) is also reported. The compounds have been comprehensively characterized by elemental analysis, electrospray ionization (ESI) mass spectrometry, spectroscopy (IR, UV−vis, 1D and 2D NMR), cyclic voltammetry, X-ray crystallography (1−6 and 12) and magnetic susceptibility (5). Complexes 6, 7, 9 are kinetically inert in aqueous solution and resistant to hydrolysis. Compounds 6−11 were found to possess modest antiproliferative acitivity in vitro against CH1 (ovarian carcinoma), A549 (non−small cell lung carcinoma), and SW480 (colon adenocarcinoma) cells with IC50 values in the 10−4 M concentration range. Replacement of azolium cations by sodium had significant effects; cytotoxicity increased in the case of the pyrazole system from 3 (A549) to the 5.5−fold (CH1).