Synthesis, characterization and anticancer activity of dinuclear ruthenium(II) complexes linked by an alkyl chain

A series of 1,10-phenanthroline-based dinucleating bridging ligands BL1-3 and their dinuclear ruthenium Ru(II) complexes [(bpy)(2)Ru(BL1-3)Ru(bpy)(2)](PF6)(4) (bpy = 2,2'-bipyridine; PF6 = hexafluorophosphate) have been synthesized and characterized using elemental analysis, infrared, nuclear magnetic spectroscopy and mass spectrometry methods. Their photophysical and electrochemical properties were also studied. The symmetrical nature of the BLs enabled the formation of dinuclear Ru(II) complexes 1-3 with equivalent metal centers. The complexes 1-3 exhibited the spin-allowed singlet metal-to-ligand charge transfer ((MLCT)-M-1; d(pi)-pi*) transition at approximate 460 nm and Ru(II) metal centered emission at around 600 nm in solution at room temperature. The emission profile and emission maxima are similar and independent of the excitation wavelength for each complex. The complexes 1-3 undergo metal centered oxidation and the E-1/2 values for the Ru(II)/Ru(III) redox couple are 0.9 and 1.4 V versus a saturated calomel electrode. The cytotoxicity of these complexes in vitro was evaluated using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The results indicated that the Ru(II) complexes 1-3 exhibited significant dose-dependent cytotoxicity to cervical cancer (HeLa), gastric cancer (SGC-7901) and gastric cancer (BGC-823) tumour cell lines. It is worth noting that the cytotoxicity of the complexes 1-3 increased with the increase of methylene of the BLs. The DNA quenching constants were determined to be 1.34 x 10(5), 1.46 x 10(5) and 2.98 x 10(5) mol(-1) L for 1, 2 and 3, respectively, indicating that there were different interactions between complexes 1-3 with DNA.