Synthesis, Characterization and Biological Investigations of Half-Sandwich Ruthenium(II) Complexes Containing Benzimidazole Moiety

Abstract

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.

Keywords: X-ray diffraction; antibacterial and antibiofilm activity; electrochemistry; molecular docking studies; ruthenium complexes; structural and spectroscopic studies.

Scheme 1

Synthetic route to the complexes [(η⁶-p-cymene)RuCl(bimCOO)] (1) and [(η⁶-p-cymene)RuCl₂(bim)] (2).

Scheme 2

Decarboxylation process of 1H-benzimidazole-2-carboxylic acid.

Figure 1

Molecular structure of complex 1 (a). Crystal packing with marked supramolecular interactions of C–H⋅⋅⋅Cl and C–H⋅⋅⋅π types forming zig-zag chains (view along z-axis) (b).

Figure 2

Molecular structure of complex 2 (a). Crystal packing with marked supramolecular interactions of C–H···Cl types constructing tetramer (view along c axis) (b).

Figure 3

The Hirschfeld surfaces highlight the relevant d_norm surface patches associated with the specific contacts for arene Ru(II) complexes.

Figure 4

The 2D fingerprint plots of all the intermolecular interactions for complexes 1 (a) and 2 (b) with percentage of interaction.

Figure 5

Cyclic voltammograms of complex 1 (a) and complex 2 (b) (each at a concentration of 1 mM) recorded in acetonitrile–ethanol mixture containing 0.1 M TBAPF₆. Lines (–) and (---) represent the curves of the ruthenium complexes and the ligand/supporting electrolyte, respectively (CV conditions: GCE, Ø = 2 mm, T = 25 °C).

Figure 6

P. aeruginosa PAO1 biofilm formation in the presence of Ru(II) precursor, free bimCOOH and arene Ru complexes (concentration of compounds—1 mM). The absorbance of the control was considered to represent 100% of biofilm formation (results were considered significant when compared to control; * p < 0.05. Data are presented as mean ± SD, n = 4).

Figure 7

Epifluorescence microscopy images of P. aeruginosa PAO1 biofilm treated with 1 mM of arene Ru(II) complexes. Biofilm was stained with nucleic acid stains using the FilmTracer™ LIVE/DEAD Biofilm Viability Kit (live cells are represented by the color green; dead cells are represented by the color red). The epifluorescence microscopy images were captured at 1000× magnification.

Figure 8

The best-scored poses obtained by molecular docking experiments for HSA with Ru(II) complex 1 (A) and Ru(II) complex 2 (D). HSA residues involved in forming interactions with compounds 1 and 2 are shown in panels (B,E), respectively. Furthermore, 2D ligand–protein interactions plots were generated for HSA with compound 1’s (C) and compound 2’s (F) best poses.