Antimicrobial Properties of Tris(homoleptic) Ruthenium(II) 2‑Pyridyl-1,2,3-triazole “Click” Complexes against Pathogenic Bacteria, Including Methicillin-Resistant Staphylococcus aureus (MRSA)

A series of tris­(homoleptic) ruthenium­(II)
complexes of 2-(1-R-1H-1,2,3-triazol-4-yl)­pyridine
“click”
ligands (R-pytri) with various aliphatic (R = butyl,
hexyl, octyl, dodecyl, and hexdecyl) and aromatic (R = phenyl and
benzyl) substituents was synthesized in good yields (52%–66%).
The [Ru­(R-pytri)3]2+(X–)2 complexes (where X– = PF6– or Cl–) were characterized
by elemental analysis, high-resolution electrospray ionization mass
spectrometry (HR-ESI-MS), 1H and 13C nuclear
magnetic resonance (NMR) and infrared (IR) spectroscopies, and the
molecular structures of six of the compounds confirmed by X-ray crystallography. 1H NMR analysis showed that the as-synthesized materials were
a statistical mixture of the mer- and fac-[Ru­(R-pytri)3]2+ complexes. These
diastereomers were separated using column chromatography. The electronic
structures of the mer- and fac-[Ru­(R-pytri)3]2+ complexes were examined
using ultraviolet–visible (UV-Vis) spectroscopy and cyclic
and differential pulse voltammetry. The family of R-pytri ligands and the corresponding mer- and fac-[Ru­(R-pytri)3]2+ complexes
were tested for antimicrobial activity in vitro against
both Staphylococcus aureus and Escherichia coli bacteria. Agar-based disk diffusion
assays indicated that two of the [Ru­(R-pytri)3]­(X)2 complexes (where X = PF6– and R = hexyl or octyl) displayed good antimicrobial activity against
Gram-positive S. aureus and no activity
against Gram-negative E. coli at the
concentrations tested. The most active [Ru­(R-pytri)3]2+ complexes ([Ru­(hexpytri)3]2+ and Ru­(octpytri)3]2+) were converted to the water-soluble chloride salts and screened
for their activity against a wider range of pathogenic bacteria. As
with the preliminary screen, the complexes showed good activity against
a variety of Gram-positive strains (minimum inhibitory concentration
(MIC) = 1–8 μg/mL) but were less effective against Gram-negative
bacteria (MIC = 16–128 μg/mL). Most interestingly, in
some cases, the ruthenium­(II) “click” complexes proved
more active (MIC = 4–8 μg/mL) than the gentamicin control
(MIC = 16 μg/mL) against two strains of methicillin-resistant S. aureus (MRSA) (MR 4393 and MR 4549). Transmission
electron microscopy (TEM) experiments and propidium iodide assays
suggested that the main mode of action for the ruthenium­(II) R-pytri complexes was cell wall/cytoplasmic membrane disruption.
Cytotoxicity experiments on human dermal keratinocyte and Vero (African
green monkey kidney epithelial) cell lines suggested that the complexes
were only modestly cytotoxic at concentrations well above the MIC
values.