Novel phenanthroline-based ruthenium complex as a promising antimicrobial agent

A novel ruthenium(II) compound, cis-[Ru(phen)₂(4-bzpy)(Cl)](PF₆) (complex I), where phen = 1,10-phenanthroline and 4-bzpy = 4-benzoylpyridine, was synthesized and fully characterized using electrochemical and spectroscopic methods, and its structure was determined by single-crystal X-ray diffraction. Density functional theory (DFT) and time-dependent DFT calculations were performed to shed light on the electronic structure and nature of the vibrational and electronic transitions. The photochemical behavior of complex I in aqueous solution showed that irradiation with blue light (453 nm) promoted the release of the coordinated 4-bzpy ligand originating cis-[Ru(phen)₂(H₂O)(Cl)]⁺ ion, as identified by NMR and electronic spectroscopy. Moreover, complex I exhibited a great ability to cleave DNA molecules upon blue light irradiation, which was associated with the production of reactive oxygen species (singlet oxygen and superoxide anion). In this study, we also investigated the antimicrobial activity of complex I along with a similar compound, cis-[Ru(bpy)₂(4-bzpy)(Cl)](PF₆) (complex II), their precursors [Ru(bpy)₂Cl₂] and [Ru(phen)₂Cl₂], and the free ligand 4-bzpy. These two ruthenium complexes I and II have a common auxiliary ligand 4-bzpy, but distinct chelating ligands (phenanthroline or 2,2'-bipyridine, bpy). Notably, both complexes showed promising antibacterial activity against Gram-positive bacterial strains of Staphylococcus aureus and Staphylococcus epidermidis. However, complex I showed a superior antibacterial effect compared with complex II, supporting the important role of the phen ligand, likely providing greater lipophilicity to this compound.