One of the major advances in medical science has been the development of antimicrobials; however, a consequence of their widespread use has been the emergence of drug-resistant populations of microorganisms. There is clearly a need for the development of new antimicrobials – but more importantly, there is the need for the development of new classes of antimicrobials, rather than drugs based upon analogues of known scaffolds. Due to the success of the platinum anticancer agents, there has been considerable interest in the development of therapeutic agents based upon other transition metals – and in particular ruthenium(II/III) complexes, due to their well known interaction with DNA. There have been many studies of the anticancer properties and cellular localisation of a range of ruthenium complexes in eukaryotic cells over the last decade. However, only very recently has there been significant interest in their antimicrobial properties. This review highlights the types of ruthenium complexes that have exhibited significant antimicrobial activity and discusses the relationship between chemical structure and biological processing – including site(s) of intracellular accumulation – of the ruthenium complexes in both bacterial and eukaryotic cells.

The redox potentials for successive oxidations in a series of ligand-bridged dinuclear ruthenium complexes are shown to be dependent on the identity of the anion used as the electrolyte in the electrochemical measurements. Since the differences between these redox potentials (ΔE_ox) are often used to calculate the comproportionation equilibrium constants (K_c) in mixed-valence species—and therefore the extent of inter-metal communication between the metal centres—the results demonstrate the need for extreme care in comparison of ΔE_ox data and of K_c values derived from them.

Mixed-valence chemistry has a long and rich history which is characterised by a strong interplay of experimental, theoretical and computational studies. The intervalence charge transfer (IVCT) transitions generated in dinuclear mixed-valence species (particularly of ruthenium and osmium) have received considerable attention in this context, as they provide a powerful and sensitive probe of the factors which govern electronic delocalisation and the activation barrier to intramolecular electron transfer. This tutorial review discusses classical, semi-classical and quantum mechanical theoretical treatments which have been developed over the past 35 years for the analysis of IVCT absorption bands. Particular attention is drawn to the applicability of these models for the analysis of mixed-valence complexes which lie between the fully localised (Class II) and delocalised (Class III) limits in the “localised-to-delocalised” (Class II–III) regime. A clear understanding of the complex interplay of inter- and intramolecular factors which influence the IVCT process is crucial for the design of experimental studies to probe the localised-to-delocalised regime and in guidance of the development of appropriate theoretical models.

The minimum inhibitory concentrations (MIC) of a series of synthetic inert polypyridylruthenium(II) complexes against four strains of bacteria – Gram positive Staphylococcus aureus (S. aureus) and methicillin-resistant S. aureus (MRSA), and Gram negative Escherichia coli (E. coli) and Pseudomonas aeruginosa (P. aeruginosa) – have been determined. The results demonstrate that for the dinuclear ruthenium(II) complexes ΔΔ/ΛΛ-[{Ru(phen)₂}₂{μ-bb_n}]⁴⁺ {where phen = 1,10-phenanthroline; bb_n = bis[4(4′-methyl-2,2′-bipyridyl)]-1,n-alkane (n = 2, 5, 7, 10, 12 or 16)} the complexes linked by the bb₁₂, bb₁₄ and bb₁₆ ligands are highly active, with MIC values of 1 μg mL⁻¹ against both S. aureus and MRSA, and 2–4 and 8–16 μg mL⁻¹ against E. coli and P. aeruginosa, respectively. The mononuclear complex [Ru(Me₄phen)₃]²⁺ showed equal activity (on a mole basis) against S. aureus compared with the Rubb₁₂, Rubb₁₄ and Rubb₁₆, but was considerably less active against MRSA and the two Gram negative bacteria. For the dinuclear Rubb_n family of complexes, the antimicrobial activity was related to the octanol–water partition coefficient (logP). However, the highly lipophilic mononuclear complex Δ-[Ru(phen)₂(bb₁₆)]²⁺ was significantly less active than Rubb₁₆, highlighting the importance of the dinuclear structure. Preliminary toxicity assays were also carried out for the ΔΔ isomers of Rubb₇, Rubb₁₀, Rubb₁₂ and Rubb₁₆ against two human cells lines, fresh red blood cells and THP-1 cells. The results showed that the dinuclear ruthenium complexes are significantly less toxic to human cells compared to bacterial cells, with the HC₅₀ and IC₅₀ values 100-fold higher than the MIC for the complex that showed the best potential – ΔΔ-Rubb₁₂.

To determine the in vitro susceptibility and cellular uptake for a series of dinuclear ruthenium(II) complexes [{Ru(phen)₂}₂{μ-bb_n}]⁴⁺ (Rubb_n), and the mononuclear complexes [Ru(Me₄phen)₃]²⁺ and [Ru(phen)₂(bb₇)]²⁺ against Staphylococcus aureus, methicillin-resistant S. aureus, Escherichia coli and Pseudomonas aeruginosa. The in vitro susceptibility was determined by MIC and MBC assays, and time–kill curve experiments, while the cellular uptake was evaluated by monitoring the fluorescence of the complexes remaining in the supernatant of the cultures after incubation for various periods of time, flow cytometry and confocal microscopy. Rubb₁₂ and Rubb₁₆ are highly active, with MIC and MBC values of 1–2 mg/L (0.5–1 μM) for the two Gram-positive strains and 2–4 mg/L for E. coli and 16–32 mg/L for P. aeruginosa. Rubb₁₆ showed equal or better activity (on a molar basis) to gentamicin and ampicillin for all strains apart from P. aeruginosa. The relative MBC to MIC values indicated that Rubb₁₂ and Rubb₁₆ are bactericidal, and from the time–kill curve experiments, the ruthenium complexes can kill the bacteria within 2–6 h. The cellular uptake studies demonstrated that the observed antimicrobial activity is correlated with the level of uptake of the ruthenium complexes. Confocal microscopy confirmed the cellular uptake of Rubb₁₆, and tentatively suggested that the ruthenium complex is localized in the bacteria. The inert dinuclear ruthenium(II) complexes Rubb₁₂ and Rubb₁₆ have potential as new antimicrobial agents. The structure of the dinuclear ruthenium complexes can be readily further modified in order to increase their selectivity for bacteria over human cells.