Role of the (pseudo)halido ligand in ruthenium(ii) p-cymene α-amino acid complexes in speciation, protein reactivity and cytotoxicity

Abstract

The reactions of the dimeric complexes [RuX₂(η⁶-p-cymene)]₂ (X = Br, I, SCN) with L-proline (ProH) and trans-4-hydroxy-L-proline (HypH), in methanol in the presence of NaOH, afforded [RuX(κ²N,O-Pro)(η⁶-p-cymene)] (X = Br, 1b; I, 1c; SCN, 1d) and [RuX(κ²N,O-Hyp)(η⁶-p-cymene)] (X = Br, 2b; I, 2c; SCN, 2d), respectively. Alternatively, the one-pot, sequential addition of the appropriate α-amino carboxylate and X⁻ salt to [RuCl₂(η⁶-p-cymene)]₂ led to [RuX(κ²N,O-Pro)(η⁶-p-cymene)] (X = N₃, 1e; NO₂, 1f; CN 1g) and [Ru(N₃)(κ²N,O-Hyp)(η⁶-p-cymene)] (2e). Complexes [Ru(κ³N,O,O′-O₂CCH(NH₂)(R)O)(η⁶-p-cymene)] (R = CH₂, 3h; R = CHMe, 4h; R = CH₂CH₂, 5h) were prepared from the reaction of [RuCl₂(η⁶-p-cymene)]₂ with the appropriate α-amino acid and NaOH in refluxing isopropanol. Treatment of the L-serine (SerH₂) derivative [RuCl(κ²N,O-SerH)(η⁶-p-cymene)] (3a) with 1,3,5-triaza-7-phosphaadamantane (PTA) in water at reflux produced [Ru(κ²N,O-Ser)(κP-PTA)(η⁶-p-cymene)]Cl ([3i]Cl). The products were isolated in good to excellent yields, and were characterized by elemental analysis, IR and multinuclear NMR spectroscopy. The structures of 1f and 2b–e were ascertained by X-ray diffraction studies. The behaviour of the complexes in water and cell culture medium was investigated by multinuclear NMR and UV-Vis spectroscopy, revealing a considerable influence of the monodentate ligand on the aqueous chemistry. Complexes 1d–e, 2d–e, 3h, 4h and [3i]Cl, showing substantial inertness in aqueous media, were assessed for their cytotoxicity towards A2780 and A2780cisR cancer cell lines and the noncancerous HEK 293T cell line. A selection of compounds was also investigated for Ru uptake in A2780 cells and interactions with cytochrome c as a model protein. Combined, these studies provide insights into the previously debated role of the ‘leaving’ ligand on the biological activity of Ru(II) arene α-amino acid complexes.