Pt(II) Rollover Cyclometalated Complexes Supported by 2,2'-Bipyridine N‑Oxide: Synthesis, Characterization, and...

Four platinum-(II) rollover cyclometalated complexes derived from 2,2'-bipyridine N-oxide (bpy^NO), [Pt-(bpy^NO-H)-(DMSO)-Me] (1a), [Pt-(bpy^NO-H)-(PPh₃)-Me] (2a), [Pt-(bpy^NO-H)-(DMSO)-Cl] (3a), and [Pt-(bpy^NO-H)-(PPh₃)-Cl] (4a), were synthesized, characterized, and evaluated for their antimicrobial and cytotoxic activities. The electron-poor nature of bpy^NO enables rollover C-H bond activation at room temperature. NMR and X-ray analysis (4a) revealed distinctive features, including an intramolecular interaction between the H^3' hydrogen and the N-oxide oxygen. A correlation between the direct ¹⁹⁵Pt-³¹P coupling constant and the donating properties of cyclometalated ligands has been found, allowing a useful scale of donor properties. A preliminary screening of antimicrobial activity against E. coli, K. pneumoniae, S. aureus multidrug resistant (MDR), S. pyogenes, P. aeruginosa, and C. albicans demonstrated selective activity. Complex 3a showed the most promising results against E. coli and multidrug-resistant S. aureus. Significantly, the complexes also exhibited biofilm inhibitory behavior; notably, complex 1a showed activity against K. pneumoniae. Additionally, preliminary cytotoxicity tests on human tumor (HT29) cells and normal (CCD 841 CoN) cells were performed. Among the investigated compounds, 1a, 2a, and 3a exhibited a significant reduction in tumor cell viability, whereas complex 4a displayed moderate activity. These results establish 2,2'-bipyridine N-oxide as a promising ligand scaffold for developing platinum complexes.