Synthesis, Structure and in Vitro Biological Screening of Palladium(II) Complexes of Functionalised Salicylaldimine Thiosemicarbazones as Antimalarial and Anticancer Agents

A series of mononuclear salicylaldiminato(thiosemicarba-zone)palladium(II) complexes of general formula [Pd(saltsc-R)PPh3], {H(2)saltsc-R = salicylaldehyde thiosemicarbazone; R = H (5), 3-tert-butyl (6), 3-methoxy (7), 5-chloro (8)} have been synthesized. The palladium complexes were prepared by the reaction of the appropriate salicylaldimine thiosemicarbazone with Pd(PPh3)(2)Cl-2. All complexes were characterised by a range of spectroscopic and analytical techniques. The molecular structures of 6-8 have been determined by single-crystal X-ray diffraction analysis. The salicylaldimine thiosemicarbazones coordinate to palladium in a tridentate manner, through the phenolic oxygen, imine nitrogen and thiolate sulfur, forming five-and six-membered chelate rings within their structures. The fourth coordination site for these square-planar complexes is occupied by PPh3. Biological activities of the thiosemicarbazone ligands and palladium complexes have been investigated toward the WHCO1 oesophageal cancer cell line and against two strains of the malaria parasite Plasmodium falciparum, W2 (chloroquine-resistant) and D10 (chloroquine-sensitive). The palladium(II) complexes show enhanced in vitro antiplasmodial activity in comparison with their thiosemicarbazone ligand precursors. On the other hand, in vitro anticancer activity studies on oesophageal cancer cell lines revealed a decrease in activity upon coordination of palladium to the thiosemicarbazone ligand.