Role of π-conjugation on the coordination behaviour, substitution kinetics, DNA/BSA interactions, and in vitro cytotoxicity of carboxamide palladium(II) complexes.

Treatments of N-(pyridin-2-ylmethyl)pyrazine-2-carboxamide (L1), N-(quinolin-8-yl)pyrazine-2-carboxamide (L2), N-(quinolin-8-yl)picolinamide (L3) and N-(quinolin-8-yl)quinoline-2-carboxamide (L4) with [PdCl2(NCMe)]2 afforded the corresponding Pd(ii) complexes, [Pd(L1)Cl] (PdL1); [Pd(L2)Cl] (PdL2); [Pd(L3)Cl] (PdL3); and [Pd(L4)Cl] (PdL4) in moderate yields. Structural characterisation of the compounds was achieved by NMR and FT-IR spectroscopies, elemental analyses and single crystal X-ray crystallography. The solid-state structures of complexes PdL2-PdL4 established the presence of one tridentate carboxamide and Cl ligands around the Pd(ii) coordination sphere, to give distorted square planar complexes. Electrochemical investigations of PdL1-PdL4 showed irreversible one-electron oxidation reactions. Kinetics reactivity of the complexes towards bio-molecules, thiourea (Tu), l-methionine (L-Met) and guanosine 5'-diphosphate disodium salt (5'-GMP) decreased in the order: PdL1 > PdL2 > PdL3 > PdL4, in tandem with the density functional theory (DFT) data. The complexes bind favourably to calf thymus (CT-DNA), and bovine serum albumin (BSA), and the order of their interactions agrees with the substitution kinetics trends. The in vitro cytotoxic activities of PdL1-PdL4 were examined in cancer cell lines A549, PC-3, HT-29, Caco-2, and HeLa, and a normal cell line, KMST-6. Overall, PdL1 and PdL3 displayed potent cytotoxic effects on A549, PC-3 HT-29 and Caco-2 comparable to cisplatin. All the investigated complexes exhibited lower toxicity on normal cells than cisplatin.