Replacement of chlorides with dicarboxylate ligands in anticancer active Ru(II)-DMSO compounds: a new strategy that might lead to improved activity.

A series of new Ru(II)-DMSO complexes containing dicarboxylate ligands (dicarb), namely, oxalate (ox), malonate (mal), methylmalonate (mmal), dimethylmalonate (dmmal), and succinate (suc), have been synthesized and structurally characterized. These compounds were prepared from the known Ru(II)-Cl-DMSO anticancer complexes cis,fac-[RuCl2(DMSO-S)3(DMSO-O)] (1) and trans-[RuCl2(DMSO-S)4] (2) and from the chloride-free precursor fac-[Ru(DMSO-S)3(DMSO-O)3][CF3SO3]2 (3), with the aim of assessing how the nature of the anionic ligands influences the biological activity of these species. Basically, the investigated ligands can be divided into two groups. The reaction of either 1 or 2 with K2(dicarb) (dicarb = ox, mal, mmal) yielded preferentially the mononuclear species [K]fac-[RuCl(DMSO-S)3(eta2-dicarb)] (dicarb = mal, 6; mmal, 9; ox, 14) that contains a chelating dicarboxylate unit and a residual chloride. Likewise, when 3 was used as a precursor, the neutral mononuclear species fac-[Ru(DMSO-O)(DMSO-S)3(eta2-dicarb)] (dicarb = mal, 7; mmal, 10; ox, 16), which contains a DMSO-O ligand in the place of Cl-, was obtained. On the contrary, K2(suc) and K2(dmmal) yielded preferentially the dinuclear species [fac-Ru(DMSO-S)3(H2O)(mu-dicarb)]2 (dicarb = dmmal, 11; suc, 13), with two bridging dicarboxylate moieties. The two water molecules in anti geometry have strong intramolecular H-bonding with the non-coordinated oxygen atoms of the carboxylate groups. The solid-state X-ray structural data showed that the preferential binding mode of the investigated dicarboxylates, either bridging (mu) or chelating (eta2), is dictated mainly by steric reasons. Oxalate, unlike the other dicarboxylates, has also the bridging bis-chelate (eta4,mu) coordination mode available: this was found in the dinuclear species [{fac-RuCl(DMSO-S)3}2(eta4,mu-ox)] (15) and [{fac-Ru(DMSO-O)(DMSO-S)3}2(eta4,mu-ox)][CF3SO3]2 (17). We also isolated the unprecedented neutral metallacycle, [fac-Ru(DMSO-S)3(eta3,mu-ox)]4 (18), in which each oxalate unit has one unbound oxygen atom. The new complexes were thoroughly characterized by 1-D (1H and 13C) and 2-D (H-H- COSY and HMQC) NMR spectroscopy in solution and by IR spectroscopy in the solid state. The molecular structures of 10 compounds, 6-11, 13, 15, 17, and 18, were determined by X-ray crystallography. The behavior of selected complexes in aqueous solution was investigated by 1H NMR spectroscopy.