Chemotherapy remains one of the dominant treatments to cure cancer. However, due to the many inherent drawbacks, there is a surge for new chemotherapeutic drugs. More specifically, the discovery of new drug candidates able to overcome severe side effects, the occurrence of resistance and the inefficacy toward metastatic tumours is highly desirable. In this work, we designed a new chemotherapeutic drug candidate against cancer, namely [Ru(DIP)2(sq)]PF6 (Ru-sq) (DIP = 4,7-diphenyl-1,10-phenanthroline; sq = semiquinonate ligand). The aim was to combine the great potential expressed by Ru(II) polypyridyl complexes and the singular redox and biological properties associated to the catecholate moiety. Several pieces of experimental evidence (e.g., X-ray crystallography, electron paramagnetic resonance, electrochemistry) demonstrate that the semiquinonate is the preferred oxidation state of the dioxo ligand in this complex. The biological activity of Ru-sq was then scrutinised in vitro and in vivo, and the results highlight the tremendous potential of this complex as a chemotherapeutic agent against cancer. Ru-sq was notably found have a much higher cytotoxic activity than cisplatin on several cell lines (i.e. in the nanomolar range), and, contrary to cisplatin, to have mitochondrial disfunction as one of its modes of action. The multicellular targets of Ru-sq could potentially be the key to overcome one of the main drawbacks of cisplatin i.e. the occurrence of resistance. Moreover, Ru-sq exhibited impressing activity on Multi Cellular Tumour Spheroids (MCTS) model, leading to a growth inhibition of the tumour even 13 days after treatment (20 μM). Very importantly, using two different in vivo models, it could be demonstrated that this compound is extremely well-tolerated by mice and has a very promising activity, curing, in some cases, tumour-bearing mice.<br>

Due to the great potential expressed by an anticancer drug candidate previously reported by our group, namely, Ru-sq ([Ru(DIP)2(sq)](PF6) (DIP: 4,7-diphenyl-1,10phenanthroline, sq: semiquinonate ligand), we describe in this work a structure-activity relationship (SAR) that involves a broader range of derivatives resulting from the coordination of different catecholate-type dioxo ligands to the same Ru(DIP)2 core. More in detail, we chose catechols carrying either electron-donating or electronwithdrawing groups EDG or EWG and investigated the physico-chemical 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 physico-chemical 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 of the series and was selected for a more in-depth biological investigation. Apart from its remarkably high cytotoxicity (IC50 = 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 of 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 4 outstanding cytotoxicity, make complex 1 a valuable candidate in the field of chemotherapy research. Noteworthy, a preliminary biodistribution study on healthy mice demonstrated the suitability of complex 1 for further in vivo studies.

A series of [Re(N^N)(CO)(X)] (N^N = diimine and X = halide) complexes based on 4-(2-pyridyl)-1,2,3-triazole (pyta) and 1-(2-pyridyl)-1,2,3-triazole (tapy) diimine ligands have been prepared and electrochemically characterized. The first ligand-based reduction process is shown to be highly sensitive to the nature of the isomer as well as to the substituents on the pyridyl ring, with the peak potential changing by up to 700 mV. The abilities of this class of complexes to catalyze the electroreduction and photoreduction of CO were assessed for the first time. It is found that only Re pyta complexes that have a first reduction wave with a peak potential at ca. -1.7 V vs SCE are active, producing CO as the major product, together with small amounts of H and formic acid. The catalytic wave that is observed in the CVs is enhanced by the addition of water or trifluoroethanol as a proton source. Long-term controlled potential electrolysis experiments gave total Faradaic yield close to 100%. In particular, functionalization of the triazolyl ring with a 2,4,6-tri-tert-butylphenyl group provided the catalyst with a remarkable stability.