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

The reactions of the dimeric complexes [RuX2(?6-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(?2N,O-Pro)(?6-p-cymene)] (X = Br, 1b; I, 1c; SCN, 1d) and [RuX(?2N,O-Hyp)(?6-p-cymene)] (X = Br, 2b; I, 2c; SCN, 2d), respectively. Alternatively, the one-pot, sequential addition of the appropriate a-amino carboxylate and X- salt to [RuCl2(?6-p-cymene)]2 led to [RuX(?2N,O-Pro)(?6-p-cymene)] (X = N3, 1e; NO2, 1f; CN 1g) and [Ru(N3)(?2N,O-Hyp)(?6-p-cymene)] (2e). Complexes [Ru(?3N,O,O'-O2CCH(NH2)(R)O)(?6-p-cymene)] (R = CH2, 3h; R = CHMe, 4h; R = CH2CH2, 5h) were prepared from the reaction of [RuCl2(?6-p-cymene)]2 with the appropriate a-amino acid and NaOH in refluxing isopropanol. Treatment of the l-serine (SerH2) derivative [RuCl(?2N,O-SerH)(?6-p-cymene)] (3a) with 1,3,5-triaza-7-phosphaadamantane (PTA) in water at reflux produced [Ru(?2N,O-Ser)(?P-PTA)(?6-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 a-amino acid complexes.