Utilization of Guanidine-Based Ancillary Ligands in Arene–Ruthenium Complexes for Selective Cytotoxicity

[Image: see text] A family of three water-soluble half-sandwich arene–ruthenium complexes, depicted as C(1)–C(3), having the general formula [Ru(p-cymene)(L)Cl]Cl has been synthesized, where L represents (1H-benzo[d]imidazol-2-yl)guanidine (L(1)) or (benzo[d]oxazol-2-yl)guanidine (L(2)) or (benzo[d]thiazol-2-yl)guanidine (L(3)). The crystal structure of complex C(3) has been determined. The complexes show several absorption bands in the visible and ultraviolet regions, and they also show prominent emission in the visible region while excited near 400 nm. Studies on the interaction of ligands L(1)–L(3) and complexes C(1)–C(3) with calf thymus DNA reveal that the complexes are better DNA binders than the ligands, which is attributable to the imposed planarity of the ruthenium-bound guanidine-based ligand, enabling it to serve as a better intercalator. Molecular docking studies show that the complexes effectively bind with DNA through electrostatic and H-bonding interactions and partial intercalation of the guanidine-based ligands. Cytotoxicity studies carried out on two carcinoma cell lines (PC3 and A549) and on two non-cancer cell lines (BPH1 and WI-38) show a marked improvement in antitumor activity owing to complex formation, which is attributed to improvement in cellular uptake on complex formation. The C(1) complex is found to exhibit the most prominent activity against the PC3 cell line. Inclusion of the guanidine-based ligands in the half-sandwich ruthenium–arene complexes is found to be effective for displaying selective cytotoxicity to cancer cells and also for convenient tracing of the complexes in cells due to their prominent emissive nature.