Synthesis, characterization and in vitro cytotoxicity of the first palladium(II) oxalato complexes involving adenine-based ligands

The first [Pd(Ln)2(ox)] xH2O oxalato(ox) complexes involving 2-chloro-N6-(benzyl)-9-isopropyladenine (L1; complex 1), 2-chloro-N6-(4-methoxybenzyl)-9-isopropyladenine (L2; 2), 2-chloro-N6-(2,3-dimethoxybenzyl)-9-isopropyladenine (L3; 3), 2-chloro-N6-(2,4-dimethoxybenzyl)-9-isopropyladenine (L4; 4), and 2-chloro-N6-(4-methylbenzyl)-9-isopropyladenine (L5; 5) have been synthesized by the reactions of potassium bis(oxalato)palladate(II) dihydrate, [K2Pd(ox)2]·2H2O, with the mentioned organic compounds (H2ox = oxalic acid; x = 0 for 1–3 and 5 or 2 for 4). Elemental analyses (C, H, N), FTIR, Raman and NMR (1H, 13C, 15N) spectroscopies, conductivity measurements and thermal studies (thermogravimetric and differential thermal analyses, TG/DTA) have been used to characterize the prepared complexes. The molecular structures of [Pd(L2)2(ox)] (2) and [Pd(L5)2(ox)]·L5·Me2CO (5·L5·Me2CO) have been determined by a single crystal X-ray analysis. The geometry of these complexes is slightly distorted square-planar with two appropriate Ln (n = 2 or 5) molecules mutually arranged in the head-to-head (2) or head-to-tail (5) orientation. The Ln ligands are coordinated to the central Pd(II) ion via the N7 atoms. The same conclusions regarding the binding properties of L1–L5 ligands can be made based on multinuclear NMR spectra. In vitro cytotoxicity of the complexes 1–5 has been evaluated against human chronic myelogenous leukaemia (K562) and human breast adenocarcinoma (MCF7) cancer cell lines. Significant cytotoxicity has been determined for the complexes 3 (IC50 = 6.2 μM) and 5 (IC50 = 6.8 μM) on the MCF7 cell line, which is even better than that found for the well-known and widely-used platinum-bearing antineoplastic drugs, i.e. oxaliplatin and cisplatin.