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Due to the great potential expressed by an anticancer drug candidate previously reported by our group, namely, Ru-sq ([Ru(DIP) 2 (sq)](PF 6 ) (DIP, 4,7-diphenyl-1,10-phenanthroline; sq, semiquinonate ligand), we describe in this work a structure-activity relationship (SAR) study that involves a broader range of derivatives resulting from the coordination of different catecholate-type dioxo ligands to the same Ru(DIP) 2 core. In more detail, we chose catechols carrying either an electron-donating group (EDG) or an electron-withdrawing group (EWG) and investigated the physicochemical and biological properties of their complexes. Several pieces of experimental evidences demonstrated that the coordination of catechols bearing EDGs led to deep-red positively charged complexes 1 - 4 in which the preferred oxidation state of the dioxo ligand is the uninegatively charged semiquinonate. Complexes 5 and 6 , on the other hand, are blue/violet neutral complexes, which carry an EWG-substituted dinegatively charged catecholate ligand. The biological investigation of complexes 1 - 6 led to the conclusion that the difference in their physicochemical properties has a strong impact on their biological activity. Thus, complexes 1 - 4 expressed much higher cytotoxicities than complexes 5 and 6 . Complex 1 constitutes the most promising compound in the series and was selected for a more in depth biological investigation. Apart from its remarkably high cytotoxicity (IC 50 = 0.07-0.7 μM in different cancerous cell lines), complex 1 was taken up by HeLa cells very efficiently by a passive transportation mechanism. Moreover, its moderate accumulation in several cellular compartments (i.e., nucleus, lysosomes, mitochondria, and cytoplasm) is extremely advantageous in the search for a potential drug with multiple modes of action. Further DNA metalation and metabolic studies pointed to the direct interaction of complex 1 with DNA and to the severe impairment of the mitochondrial function. Multiple targets, together with its outstanding cytotoxicity, make complex 1 a valuable candidate in the field of chemotherapy research. It is noteworthy that a preliminary biodistribution study on healthy mice demonstrated the suitability of complex 1 for further in vivo studies.

Increasing the Cytotoxicity of Ru(II) Polypyridyl Complexes by Tuning the Electronic Structure of Dioxo Ligands