Versatile Coordination Behavior of Salicylaldehydethiosemicarbazone in Ruthenium(II) Carbonyl Complexes: Synthesis, Spectral, X-ray, Electrochemistry, DNA Binding, Cytotoxicity, and Cellular Uptake Studies

The reaction of salicylaldehydethiosemicarbazone, [H-2-(Sal-tsc)], with an equimolar amount of [RuHCl(CO)(PPh3)(3)] has afforded two complexes, namely [Ru(H-Sal-tsc)(CO)Cl(PPh3)(2)] (1) and [Ru(Sal-tsc)(CO)(PPh3)(2)] (2), in one pot. The new complexes were separated and characterized by elemental analyses, various spectroscopic techniques (NMR, UV-vis, IR), X-ray crystallography, and cyclic voltammetry. In complex 1, the ligand coordinated in a bidentate monobasic fashion by forming an unusual strained NS four-membered ring in 32% yield. However, in 2, the ligand coordinated in a tridentate dibasic fashion by forming ONS five- and six-membered rings in 51% yield. Comparative biological studies such as DNA binding, cytotoxicity (MTT, LDH, and NO), and cellular uptake studies have been carried out for new ruthenium(II) complexes (1 and 2). From the DNA binding studies, it is inferred that the complex 1 exhibited electrostatic binding and 2 exhibited intercalative binding modes. On comparison of the cytotoxicity of the complexes in human lung cancer cells (A549) and liver cancer cells (HepG2), complex 2 exhibited better activity than 1; this may be due to the strong chelation and subsequent electron delocalization in 2 increasing the lipophilic character of the metal ion into cells.