Ruthenium(II)-arene complexes with monodentate aminopyridine ligands: Insights into redox stability and electronic structures and biological activity

The synthesis and spectroscopic characterization of four ruthenium(II) arene complexes with monodentate pyridine derivatives ([(η6–p-cymene)RuCl2L]: L = 2-aminopyridine, 2-methylaminopyridine, 2-benzylaminopyridine, and pyridine) are reported. Full characterization was undertaken using 1H and 13C NMR spectroscopy, vibrational and electronic spectroscopies and crystallography (2-methylaminopyridine derivative). UB3LYP//(6-31 + G(d),SPK-DZCD) density functional theory calculations determined the molecular and electronic structures. Cyclic voltammetry determined a large electrochemical stability window (>2.2 V) extending well beyond the physiological E°. Interactions with CT-DNA and BSA, and activity against four cell lines (HeLa, B16F10, HEp-2 and Vero) were evaluated. The 2-methylaminopyridine shows weak cytotoxicity (IC50 = 346 μmol L−1) towards HeLa cells. All the complexes interact with DNA at relatively high concentrations as determined by UV–vis spectroscopic titration. Results of circular dichroism spectroscopy, ethidium bromide competition, fluorescence spectroscopy and DNA viscosity measurements identify electrostatic interactions between partly hydrolyzed cationic complexes and the phosphate backbone of DNA as the most likely interaction mode. Slower rates of hydrolysis may be the origin of lower cytotoxicity for κ1 these complexes.