Ethylenediamine-palladium(II) complexes with pyridine and its derivatives: synthesis, molecular structure and initial antitumor studies

Abstract

The synthesis of four mononuclear palladium complexes of general formula [Pd(en)Cl(L)]NO 3(en = ethylenediamine; L = pyridine ( I), 4-methylpyridine ( II), 4-hydroxypyridine ( III) or 4-aminopyridine ( IV)) has been achieved. The structure of these compounds was studied by elemental analysis, IR, far-IR and 1H NMR; complex I was analyzed by X-ray diffraction. The crystal of [Pd(en)(pyridine)Cl]NO 3 is monoclinic, space group P2 1/ c ( a = 7.990(2), b = 16.058(3), c = 9.846(2) Å, β = 103.81(3)°, Z = 4, R = 0.067, R w = 0.066). The Pd(II) atom exhibits an approximately square planar coordination with bond lengths in the range 2.017–2.042 Å for Pd-N and 2.320 Å for Pd-Cl. In order to determine the donor strength of the aromatic pyridine ligands, the stability constants of binary complex ML 2+ (M = [Pd(en)(H 2O) 2] 2+; L = pyridine, 4-Me-pyridine, 4-OH-pyridine and 4-NH 2-pyridine) were determined by potentiometric pH titration in aqueous solution ( T = 25°C, I = 0.1 mol 1 −1 NaNO 3). The results show that the stability constants of the binary complexes systematically increase with increasing p K a of the pyridines. The above four palladium complexes, [Pt(en)(pyridine)Cl]NO 3 and cis-diamminedichloroplatinum(II) ( cis-DDP) were assayed for cytotoxicity in vitro against the human leukemia cell line HL-60, and compounds I, II, III and cis-DDP show significant cytotoxic activity against HL-60.