Neutral and cationic half-sandwich arene d6 metal complexes containing pyridyl and pyrimidyl thiourea ligands with interesting bonding modes: Synthesis, structural and anti-cancer studies

The reaction of [(p-cymene)RuCl2](2) and [Cp*MCl2](2) (M = Rh/lr) with benzoyl (2-pyrimidyl) thiourea (L1) and benzoyl (4-picolyl) thiourea (L2) led to the formation of cationic complexes bearing formula [(arene) M (L1)kappa Cl-2((N,S))](+) and [(arene) M (L2)kappa(2)(N,s)Cl](+) [(arene) = p-cymene, M = Ru, (1, 4); Cp*, M = Rh (2, 5) and Ir (3, 6)]. Precursor compounds reacted with benzoyl (6-picolyl) thiourea (L3) affording neutral complexes having formula [(arene) M (L3)kappa Cl-1((s))2] [arene = p-cymene, M = Ru, (7); Cp*, M = Rh (8), Ir (9)]. X-ray studies revealed that the methyl substituent attached to the pyridine ring in ligands L2 and L3 affects its coordination mode. When methyl group is at the para position of the pyridine ring (L2), the ligand coordinated metal in a bidentate chelating N, S- mode whereas methyl group at ortho position (L3), it coordinated in a monodentate mode. Further the anti-cancer studies of the thiourea derivatives and its complexes carried out against HCT-116, HT-29 (human colorectal cancer), Mia-PaCa-2 (human pancreatic cancer) and ARPE-19 (non-cancer retinal epithelium) cell lines showed that the thiourea ligands are inactive but upon complexation, the metal compounds displayed potent and selective activity against cancer cells in vitro. Iridium complexes were found to be more potent as compared to ruthenium and rhodium complexes.