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, [H2-(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.