Electrochemical, mechanistic, and DFT studies of amine derived diphosphines containing Ru(ii)–cymene complexes with potent in vitro cytotoxic activity against HeLa and triple-negative breast cancer cells MDA-MB-231

Abstract

Complexes with general formula [RuCl(η⁶-p-cymene)(P–N^R–P)]X (R = CH₂Py (Py = pyridine) – [1a]⁺, CH₂Ph (Ph = phenyl) – [1b]⁺, Ph – [1c] and p-tol (p-tol = p-tolyl) – [1d]⁺; X = PF₆⁻ or BF₄⁻) were evaluated as cytotoxic agents against two cancer cell lines (HeLa and MDA-MB-231). All metal complexes are active in the range of concentrations tested (up to 100 μmol L⁻¹). The IC₅₀ (μmol L⁻¹) values for the metal complexes are lower than that found for cisplatin. The activities are up to 6- and 15-fold higher than cisplatin for HeLa and MDA-MB-231 cancer cell lines, respectively. Studies of DNA binding and DNA cleavage were performed. DNA binding studies revealed a modest hypochromic shift in the metal complexes electronic spectra, indicating a weak interaction with K_b values in the range of 1.7 × 10³–1.6 × 10⁴. Although the cleavage tests revealed that in the dark DNA is not a biological target for these metal complexes, upon blue light irradiation they are activated causing DNA cleavage. Electrochemical studies showed the presence of two independent redox processes, one attributed to the oxidation process of Ru²⁺ → Ru³⁺ (EC process) and the other one to the reduction of Ru²⁺ → Ru¹⁺, which is further reduced to Ru⁰ (ECE mechanism). In both processes, coupled chemical reactions were observed. DFT calculations were performed to support the electrochemical/chemical behavior of the complexes. The reactivity of complex [1b]BF₄ with CH₃CN was evaluated and two complexes were isolated [2b]BF₄ and [3b]BF₄. The complex mer-[RuCl(CH₃CN)₃(P–N^CH₂Ph–P)]BF₄ ([2b]BF₄) was isolated after refluxing the precursor [1b]BF₄ in CH₃CN. Isomerization of [2b]BF₄ in CH₃CN resulted in the formation of fac-[RuCl(CH₃CN)₃(P–N^CH₂Ph–P)]BF₄. An attempt to isolate the fac-isomer by adding diethyl ether was unsuccessful, and the complex [3b]BF₄ was observed as the major component. The complex [Ru₂(μ-Cl₃)(CH₃CN)₂(P–N^CH₂Ph–P)₂]BF₄ ([3b]BF₄) proved to be very stable and can be obtained from both the mer- and the fac-isomers. The molecular structures of [1b]BF₄ and [3b]BF₄ were solved by single-crystal X-ray diffraction.