Ruthenium Carbonyl Complexes with Azole Heterocycles – Synthesis, X‐ray Diffraction Structures, DFT Calculations, Solution Behavior, and Antiproliferative Activity

Abstract

The reactions of (H2azole)[trans-RuCl4(Hazole)2] [Hazole = 1H-pyrazole (Hpz), 1H-indazole (Hind), 1-methylindazole (1-Me-ind), or 1H-benzimidazole (Hbzim)] with 1:1 mixtures of 12 M HCl and formic acid resulted in the formation of trans-[RuCl4(CO)(Hazole)]– complexes, which were isolated and identified by X-ray diffraction as nBu4N[trans-RuCl4(CO)(Hpz)] (1a), nBu4N[trans-RuCl4(CO)(Hind)] (2a), [(Hind)2H][trans-RuCl4(CO)(Hind)] (2b), nBu4N[trans-RuCl4(CO)(1-Me-ind)] (3a), and nBu4N[trans-RuCl4(CO)(Hbzim)] (6a), whereas a prolonged reaction with 1H-benzimidazole afforded (nBu4N)2[trans-RuCl5(CO)] (7a). The reactions of [(DMSO)2H][trans-RuCl4(CO)(DMSO)] with different azole heterocycles in dichloromethane afforded five similar complexes, which were isolated as azolium salts (H2azole)[trans-RuCl4(CO)(Hazole)] [Hazole = 1H-indazole (2), 1-methylindazole (3), 2-methylindazole (4), 1H-imidazole (5), and 1H-benzimidazole (6)]. Complexes 2–6 were characterized by elemental analysis, high-resolution electrospray ionization (ESI) mass spectrometry, UV/Vis spectroscopy, and IR spectroscopy, and 3 was also characterized by X-ray diffraction. The pH of the solution was crucial for the stability of the obtained compound series. The interactions of 2–4 with the model proteins myoglobin and ubiquitin and the influence of the indazole tautomer identity on those processes were investigated by high-resolution mass spectrometry. The ruthenium(III) complexes interact with the proteins to form protein–RuII(CO) adducts, that is, metal reduction appears to be responsible for the release of the azole heterocycle but not CO. Moreover, the binding of the RuII(CO) unit to myoglobin reduces the observed global charge state of the adduct with respect to that of free myoglobin, which suggests that the Ru(CO) fragment can cross-link the protein backbone. Complexes 2–6 show low antiproliferative activity against the human cancer cell lines HeLa and MDA-MB-231 and the noncancerous cell line MRC-5, and this activity correlates with the low stability of complexes in aqueous media, especially at neutral or basic pH. The results are compared to previously reported data on ruthenium nitrosyl analogues and KP1019.