Synthesis, structural characterisation and solution chemistry of ruthenium(III) triazole-thiadiazine complexes

Two ruthenium(III) complexes structurally similar to the anticancer compound NAMI were prepared: Na[RuCl4(DMSO)(L1)] ( 1) and Na[RuCl4(DMSO)(L2)] ( 2), where L1 and L2 are differently functionalised triazole-thiadiazine ligands. To facilitate the crystallisation of the complex anions, Na+ was substituted with the [bis(triphenylphosphoranylidene)ammonium] cation (PPN+), allowing the X-ray characterisation of PPN[RuCl4(DMSO)(L1)].2H2O ( 1a.2H2O) and PPN[RuCl4(DMSO)(L2)].3H2O ( 2a.3H2O), respectively. The two compounds undergo stepwise hydrolytic processes, as assessed by means of UV-vis and 1H NMR spectroscopy. The first hydrolytic step consists of the replacement of a chloride anion with a water molecule, with a half-life of 50 min ( 1) and 110 min ( 2), while the subsequent hydrolytic steps are more complicated to describe since more than one product is generated at the same time. The redox potential of the Ru(III)/Ru(II) couple (0.31 V for 1 and 0.28 V for 2) suggests that these complexes can be reduced in the intracellular environment, in agreement with the activation by reduction mechanism proposed for NAMI and NAMI-A. 1 and 2 were tested on a human cancer cell line derived from a fibrosarcoma (HT1080), and on non-cancerous primary human fibroblasts (HF), where they showed a modest inhibitory effect.