The reactions of a mother solution of RuCl₃ with benzimidazole derivatives 2-(2′-pyridyl)benzimidazole (2,2′-PyBIm, L₁) and 2-hydroxymethylbenzimidazole (2-CH₂OHBIm, L₂) yielded three novel ruthenium complexes: (H₂L₁)₂[Ru^IIICl₄(CH₃CN)₂]₂[Ru^IVCl₄(CH₃CN)₂]·2Cl·6H₂O (1), mer-[Ru^IIICl₃L₁(CH₃CN)]·L₁·3H₂O (2), and (HL₂)₄[Ru^IVCl₆]·2Cl·4H₂O (3). The isolated compounds were characterised by elemental analyses, UV-Vis and IR spectroscopy, and magnetic measurements. The nature of the ligands bound to the metal ions of these compounds and the experimental conditions significantly influenced the ruthenium complexes in different oxidation states. The N,N-donor ligand bound to the metal centre is a recognised stabiliser of the +III state of ruthenium, whereas the lack of ligand coordination promotes the formation of a mixed (Ru^III/Ru^IV) complex. In the case of complex 3, the absence of a N,O-donor ligand in the coordinate sphere facilitates the formation of the compound in a higher oxidation state. X-ray single crystal analyses revealed an octahedral geometry in each of the complexes. The crystal structure of ruthenium complexes is formed by a network of intermolecular classical and unconventional (C–H⋯π) hydrogen bonds. The most interesting feature of the supramolecular architecture of complexes is the existence of a very rare Cl⁻⋯π interaction and π⋯π stacking, which also contribute to structural stabilisation. Ruthenium compounds 2 and 3 behave as paramagnets with an octahedral geometry, corresponding to the presence of one or two unpaired electrons, respectively. The cyclic voltammetric data of complex 2 show three one-electron redox processes. The first redox couple is reversible, whereas the two other couples have a quasi-reversible nature. In the case of complex 3, two redox couples are reversible and the electrode processes are connected with exchange of one electron.

Half-sandwich Ru(II) complexes belong to group of biologically active metallo-compounds with promising antimicrobial and anticancer activity. Herein, we report the synthesis and characterization of arene ruthenium complexes containing benzimidazole moiety, namely, [(η⁶-p-cymene)RuCl(bimCOO)] (1) and [(η⁶-p-cymene)RuCl₂(bim)] (2) (where bimCOO = benzimidazole-2-carboxylate and bim = 1-H-benzimidazole). The compounds were characterized by ¹H NMR, ¹³C NMR, IR, UV–vis and CV. Molecular structures of the complexes were determined by SC-XRD analysis, and the results indicated the presence of a pseudo-tetrahedral (piano stool) geometry. Interactions in the crystals of the Ru complexes using the Hirshfeld surface analysis were also examined. In addition, the biological studies of the complexes, such as antimicrobial assays (against planktonic and adherent microbes), cytotoxicity and lipophilicity, were performed. Antibacterial activity of the complexes was evaluated against S. aureus, E. coli, P. aeruginosa PAO1 and LES B58. Cytotoxic activity was tested against primary human fibroblasts and adenocarcinoma human alveolar basal epithelial cells. Obtained biological results show that the ruthenium compounds have bacteriostatic activity toward Pseudomonas aeruginosa PAO1 strain and are not toxic to normal cells. A molecular docking study was applied as a predictive source of information about the plausibility of examined structures binding with HSA as a transporting system.

Antimicrobial resistance is a growing public health concern that requires urgent action. Biofilm-associated resistance to antimicrobials begins at the attachment phase and increases as the biofilms maturate. Hence, interrupting the initial binding process of bacteria to surfaces is essential to effectively prevent biofilm-associated problems. Herein, we have evaluated the antibacterial and anti-biofilm activities of three ruthenium complexes in different oxidation states with 2-pyridin-2-yl-1H-benzimidazole (L₁ = 2,2′-PyBIm): [(η⁶-p-cymene)Ru^IIClL₁]PF₆ (Ru(II) complex), mer-[Ru^IIICl₃(CH₃CN)L₁]·L₁·3H₂O (Ru(III) complex), (H₂L₁)₂[Ru^IIICl₄(CH₃CN)₂]₂[Ru^IVCl₄(CH₃CN)₂]·2Cl·6H₂O (Ru(III/IV) complex). The biological activity of the compounds was screened against Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa strains. The results indicated that the anti-biofilm activity of the Ru complexes at concentration of 1 mM was better than that of the ligand alone against the P. aeruginosa PAO1. It means that ligand, in combination with ruthenium ion, shows a synergistic effect. The effect of the Ru complexes on cell surface properties was determined by the contact angle and zeta potential values. The electric and physical properties of the microbial surface are useful tools for the examined aggregation phenomenon and disruption of the adhesion. Considering that intermolecular interactions are important and largely define the functions of compounds, we examined interactions in the crystals of the Ru complexes using the Hirshfeld surface analysis.

Pseudomonas aeruginosa biofilm-associated infections are a serious medical problem, and new compounds and therapies acting through novel mechanisms are much needed. Herein, the authors report a ruthenium(IV) complex that reduces P. aeruginosa PAO1 biofilm formation by 84%, and alters biofilm morphology and the living-to-dead cell ratio at 1 mM concentration. Including the compound in the culture medium altered the pigments secreted by PAO1, and fluorescence spectra revealed a decrease in pyoverdine. Scanning electron microscopy showed that the ruthenium complex did not penetrate the bacterial cell wall, but accumulated on external cell structures. Fluorescence quenching experiments indicated strong binding of the ruthenium complex to both plasmid DNA and bovine serum albumin. Formamidopyrimidine DNA N-glycosylase (Fpg) protein digestion of plasmid DNA isolated after ruthenium(IV) complex treatment revealed the generation of oxidative stress, which was further proved by the observed upregulation of catalase and superoxide dismutase gene expression.