"Tailored pincer and chelating ligands for transition-metal complexes: from design to applications"

The rational design of the metal coordination sphere through tailored ligands represents a fundamental strategy to modulate the reactivity and biological properties of transition-metal complexes, opening the way to diverse applications in medicine and materials science. While the development of non-platinum agents is primarily driven by the need to overcome cancer drug resistance and mitigate the side effects of classical chemotherapeutics, the versatility of these molecular architectures also offers a unique opportunity to tackle the rising global threat of antimicrobial resistance. In this context, my PhD research focused on the design, synthesis, and characterization of novel coinage metal and first-row transition metal complexes supported by tailored chelating and pincer ligands, investigating their potential as anticancer, antimicrobial agents and exploring their broader therapeutic versatility. The research strategy relied on the modification of established ligand scaffolds, namely -diketones, Schiff bases, scorpionates, and N-Heterocyclic Carbenes (NHCs). A crucial aspect of this work was the selection of specific phosphane coligands with varying lipophilic, steric, and electronic properties. These ligands were employed to stabilize the metal centers in the +1 oxidation state while also allowing fine-tuning of the complexes’ solubility and biological behavior. In the first stage of my work, I explored the coordination chemistry of sterically hindered and fluorinated -diketones. A series of homoleptic complexes involving 3d-transition metals (Mn, Fe, Co, Ni, Cu, Zn) was synthesized and fully characterized. The introduction of bulky substituents and fluorinated moieties was exploited to enhance the lipophilicity and stability of the complexes. Initial screening of the 3d-metal series highlighted the potential of the Cu2+ derivatives. This prompted the development of analogous heteroleptic, phosphane- stabilized, Cu+ and Ag+ complexes, and a comparative study revealed that the Cu+ derivatives outperformed cisplatin in both 2D and 3D cell culture models.1,2 In parallel, I developed a synthetic platform for N,O-donor ligands based on pyrazole moieties supported by 3-substituted acetylacetone scaffolds, expanding the library of available chelators for copper coordination.3 A substantial part of my doctoral project was dedicated to the family of heteroscorpionate ligands, specifically bis(pyrazolyl)acetates. First, parallel investigations on simple aliphatic ester derivatives (isopropyl) were conducted to validate the intrinsic pharmacological potential of the metal-ligand scaffold. These studies, performed using 2D MTT assays on different cancer cell lines and human colon cancer 3D spheroids, confirmed that the copper- phosphane core is the primary driver of cytotoxicity, acting through a mechanism distinct from that of cisplatin, which involves disruption of redox homeostasis.4 Building on this robust backbone, I developed a bioconjugation strategy specifically for targeting Glioblastoma, a highly aggressive brain tumor. Initial functionalization with amantadine yielded complexes that demonstrated remarkable cytotoxicity, proven to be strictly copper- dependent, and the ability to enhance chemosensitivity to temozolomide, the standard-of- care drug.5 Subsequently, the design evolved to include memantine. The incorporation of memantine was strategically designed to introduce selectivity toward glioma cells by exploiting the known affinity of the memantine moiety for NMDA receptors, alongside its neuroprotective properties.6 Notably, the most promising Cu+ complexes exhibited a distinct mechanism of action compared to classical platinum drugs, potentially involving cuproptosis, a copper-dependent regulated cell death pathway. Furthermore, to exploit complementary antitumor mechanisms, I synthesized the corresponding Ag+ phosphane complexes with these ligands embedding a chelating core and a bioactive moiety,7 expanding the therapeutic potential of this class of compounds. Extending the investigation to applications beyond oncology, I explored two distinct therapeutic paths. First, Cu2+, Cu+, and Ag+ complexes with phenoxy-ketimine Schiff base ligands exhibited promising antibacterial activity.8 Second, in a collaborative study on burn injury treatment, selected fluorinated -diketonate and amantadine-functionalized copper complexes were evaluated for their protective effects against thermal trauma, showing a significant ability to mitigate systemic physiological damage and improve cell survival.9 During my visiting research period at the University of Texas at Arlington (USA), under the supervision of Prof. H. V. R. Dias, I addressed a specific gap in the literature regarding mono-pyrazolyl and mono-pyridyl borate architectures. I successfully synthesized novel fluorinated boron-phenylated ligands and their copper(I)-triphenylphosphina derivatives. Single-crystal X-ray diffraction analysis allowed us to elucidate the structural flexibility of these systems, highlighting the influence of the ligand scaffold on the metal coordination environment. Biologically, these complexes proved effective against cisplatin-resistant cells, suggesting a mechanism of action distinct from platinum drugs, likely involving the disruption of intracellular redox homeostasis.10 Finally, the research encompassed advanced delivery strategies and organometallic synthesis. In the field of nanotechnology, I contributed to the development of an anisotropic nano-drug delivery system, conjugating copper(I)-PTA complexes with gold nanorods.11 In the late stage of the project, I focused on the synthesis of a zwitterionic, carboxylate- functionalized bis-N-heterocyclic pre-carbene ligand and the corresponding dinuclear Ag+ and Au+ bis-NHC complexes. These binuclear architectures were designed to exploit a potential synergistic effect between the metal centers, aiming for enhanced stability and controlled metal-ion release; these species are currently under evaluation as potential anticancer agents. The structural features, stoichiometry and purity of the synthesized species were extensively investigated both in solution (1H-, 11B-, 19F-, 13C-, 31P-NMR and ESI-MS) and in the solid state (FT-IR, elemental analysis, melting point and single-crystal XRD). Furthermore, advanced characterization techniques such as Synchrotron Radiation-induced X-ray Photoelectron Spectroscopy (SR-XPS) and X-ray absorption fine structure (XAFS) combined with DFT modelling were employed to deeply probe the electronic structure and the local geometry around the metal centers. These comprehensive analyses confirmed the proposed molecular architectures and highlighted the pivotal role of the metal oxidation state and ligand design in determining the biological outcome.