Mixed ligand palladium(II) complexes of 6-methoxy-2-oxo-1,2-dihydroquinoline-3-carbaldehyde 4N-substituted thiosemicarbazones with triphenylphosphine co-ligand: synthesis, crystal structure and biological properties.

A series of new 6-methoxy-2-oxo-1,2-dihydroquinoline-3-carbaldehyde 4N-substituted thiosemicarbazone ligands (H2L1–H2L5) and their corresponding palladium(II) complexes [Pd(L1)(PPh3)] (1), [Pd(L2)(PPh3)] (2), [Pd(HL3)(PPh3)]Cl (3), [Pd(L4)(PPh3)] (4) and [Pd(L5)(PPh3)] (5), have been synthesized in order to evaluate the effect of terminal N-substitution in thiosemicarbazone moiety on coordination behaviour and biological activity. The new ligands and their corresponding complexes were characterized by analytical and various spectral techniques. The molecular structure of the complexes 2–5 were characterized by single crystal X-ray diffraction studies which revealed that the ligands H2L2, H2L4 and H2L5 are coordinated to palladium(II) as binegative tridentate (ONS2−) by forming six and five member rings whereas, the ligand H2L3 coordinated to Pd(II) as uninegative tridentate (ONS−). The interactions of the new complexes with calf thymus DNA (CT-DNA) have been evaluated by absorption and ethidium bromide (EB) competitive studies which revealed that complexes 1–5 could interact with CT-DNA through intercalation. Further, the interactions of the complexes with bovine serum albumin (BSA) were also investigated using UV-visible, fluorescence and synchronous fluorescence spectroscopic methods, which showed that the new complexes could bind strongly with BSA. Antioxidant studies showed that all the complexes have a strong antioxidant activity against 2-2′-diphenyl-1-picrylhydrazyl (DPPH) radical and 2,2′-azino-3-ethylbenzthiazoline-6-sulfonic acid diammonium salt (ABTS) cation radical. In addition, in vitro cytotoxicity of the complexes against human lung cancer (A549) cell line was assayed which showed that 4 has higher cytotoxic activity than the rest of the complexes and cisplatin.