1,2,3-Triazoles and their metal chelates with antimicrobial activity

The emergence of drug-resistant bacterial and fungal pathogens has highlighted the urgent need of innovative antimicrobial therapeutics. Transition metal complexes with biologically active ligands (coumarins, terpyridines, triazoles, uracils, etc.) have long been investigated for antimicrobial activity. 1,2,3-Triazoles and their molecular derivatives are well known for a plethora of physiological activities, including antibacterial and antifungal. The aim of the present mini-review is to inform the reader about research conducted on potential antimicrobial 1,2,3-triazole complexes with transition metals. What the authors find surprising is how little such research and experimentation has actually been performed and reported in scientific literature. The goal is to highlight research efforts up to now and impress upon the reader the vast perspectives for novel, effective medicinal substances hidden in this yet unexplored field.

Synthesis and anti-microbial activity of a new series of bis(diphosphine) rhenium(V) dioxo complexes

Rhenium-based metallodrugs have recently been highlighted as promising candidates for new antibiotics to combat multi-drug resistant (MDR) pathogens. A new class of rhenium(V) dioxo complexes were prepared from readily accessible diphosphine ligands, and have been shown to possess potent activity against Staphylococcus aureus (S. aureus) and Candida albicans (C. albicans) alongside low human cell toxicity.

Anticancer and Antibiotic Rhenium Tri- and Dicarbonyl Complexes: Current Research and Future Perspectives

Organometallic compounds are increasingly recognized as promising anticancer and antibiotic drug candidates. Among the transition metal ions investigated for these purposes, rhenium occupies a special role. Its tri- and dicarbonyl complexes, in particular, attract continuous attention due to their relative ease of preparation, stability and unique photophysical and luminescent properties that allow the combination of diagnostic and therapeutic purposes, thereby permitting, e.g., molecules to be tracked within cells. In this review, we discuss the anticancer and antibiotic properties of rhenium tri- and dicarbonyl complexes described in the last seven years, mainly in terms of their structural variations and in vitro efficacy. Given the abundant literature available, the focus is initially directed on tricarbonyl complexes of rhenium. Dicarbonyl species of the metal ion, which are slowly gaining momentum, are discussed in the second part in terms of future perspective for the possible developments in the field.

Combatting AMR: A molecular approach to the discovery of potent and non-toxic rhenium complexes active against C. albicans-MRSA co-infection

Antimicrobial resistance (AMR) is a major emerging threat to public health, causing serious issues in the successful prevention and treatment of persistent diseases. While the problem escalates, lack of financial incentive has lead major pharmaceutical companies to interrupt their antibiotic drug discovery programs. The World Health Organisation (WHO) has called for novel solutions outside the traditional development pathway, with emphasis on new classes of active compounds with non-classical mechanisms of action. Metal complexes are an untapped source of antibiotic potential owing to unique modes of action and a wider range of three-dimensional geometries as compared to purely organic compounds. In this study, we present the antimicrobial and antifungal efficacy of a family of rhenium tricarbonyl diimine complexes with varying ligands, charge and lipophilicity. Our study allowed the identification of potent and non-toxic complexes active in vivo against S. aureus infections at MIC doses as low as 300 ng/mL, as well as against C. albicans-MRSA mixed co-infection. The compounds are capable of suppressing the C. albicans morphogenetic yeast-to-hyphal transition, eradicating fungal-S. aureus co-infection, while showing no sign of cardio-, hepato-, hematotoxiciy or teratogenicity.

Design, synthesis and in vivo evaluation of 3-arylcoumarin derivatives of rhenium(I) tricarbonyl complexes as potent antibacterial agents against methicillin-resistant Staphylococcus aureus (MRSA)

We have prepared a series of ten 3-arylcoumarin molecules, their respective fac-[Re(CO)₃(bpy)L]⁺ and fac-[Re(CO)₃(L⁀L)Br] complexes and tested all compounds for their antimicrobial efficacy. Whereas the 3-arylcoumarin ligands are virtually inactive against the human-associated pathogens with minimum inhibitory concentrations (MICs) > 150 μM, when coordinated to the fac-[Re(CO)₃]⁺ core, most of the resulting complexes showed remarkable antibacterial potency. Several rhenium complexes exhibit activity in nanomolar concentrations against Gram-positive pathogens such as Staphylococcus aureus strains, including methicillin-resistant S. aureus (MRSA) and Enterococcus faecium. The molecules do not affect bacterial cell membrane potential, but some of the most potent complexes strongly interact with DNA, indicating it as a possible target for their mode of action. In vivo studies in the zebrafish model showed that the complexes with anti-staphylococcal/MRSA activity were non-toxic to the organism even at much higher doses of the corresponding MICs. In the zebrafish-MRSA infection model, the complexes increased the survival rate of infected fish up to 100% and markedly reduced bacterial burden. Moreover, all rescued fish developed normally following the treatments with the metallic compounds.

Recent Studies on the Antimicrobial Activity of Transition Metal Complexes of Groups 6–12

Antimicrobial resistance is an increasingly serious threat to global public health that requires innovative solutions to counteract new resistance mechanisms emerging and spreading globally in infectious pathogens. Classic organic antibiotics are rapidly exhausting the structural variations available for an effective antimicrobial drug and new compounds emerging from the industrial pharmaceutical pipeline will likely have a short-term and limited impact before the pathogens can adapt. Inorganic and organometallic complexes offer the opportunity to discover and develop new active antimicrobial agents by exploiting their wide range of three-dimensional geometries and virtually infinite design possibilities that can affect their substitution kinetics, charge, lipophilicity, biological targets and modes of action. This review describes recent studies on the antimicrobial activity of transition metal complexes of groups 6–12. It focuses on the effectiveness of the metal complexes in relation to the rich structural chemical variations of the same. The aim is to provide a short vade mecum for the readers interested in the subject that can complement other reviews.

Photophysical and Electrocatalytic Properties of Rhenium(I) Triazole-Based Complexes

A series of [Re(N^N)(CO)3(Cl)] (N^N = diimine) complexes based on 4-(pyrid-2-yl)-1,2,3-triazole (1), 1-benzyl-4-(pyrimidin-2-yl)-1,2,3-triazole (2), and 1-benzyl-4-(pyrazin-2-yl)-1,2,3-triazole (3) diimine ligands were prepared and their photophysical and electrochemical properties were characterized. The ligand-based reduction wave is shown to be highly sensitive to the nature of the triazole-based ligand, with the peak potential shifting by up to 600 mV toward more positive potential from 1 to 3. All three complexes are phosphorescent in solution at room temperature with λmax ranging from 540 nm (1) to 638 nm (3). Interestingly, the complexes appear to show inverted energy-gap law behaviour (τ = 43 ns for 1 versus 92 ns for 3), which is tentatively interpreted as reduced thermal accessibility of metal-centred (3MC) states from photoexcited metal to ligand charge transfer (3MLCT) states upon stabilisation of the N^N-centred lowest unoccupied molecular orbital (LUMO). The photophysical characterisation, supported by computational data, demonstrated a progressive stabilization of the LUMO from complex 1 to 3, which results in a narrowing of the HOMO–LUMO energy gap (HOMO = highest occupied molecular orbital) across the series and, correspondingly, red-shifted electronic absorption and photoluminescence spectra. The two complexes bearing pyridyl (1) and pyrimidyl (2) moieties, respectively, showed a modest ability to catalyse the electroreduction of CO2, with a peak potential at ca. −2.3 V versus Fc/Fc+. The catalytic wave that is observed in the cyclic voltammograms is slightly enhanced by the addition of water as a proton source.

Light-Activated Rhenium Complexes with Dual Mode of Action against Bacteria

New antibiotics and innovative approaches to kill drug-resistant bacteria are urgently needed. Metal complexes offer access to alternative modes of action but have only sparingly been investigated in antibacterial drug discovery. We have developed a light-activated rhenium complex with activity against drug-resistant S. aureus and E. coli. The activity profile against mutant strains combined with assessments of cellular uptake and synergy suggest two distinct modes of action.

Robust Binding of Disulfide-Substituted Rhenium Bipyridyl Complexes for CO2 Reduction on Gold Electrodes

Heterogenization of homogenous catalysts on electrode surfaces provides a valuable approach for characterization of catalytic processes in operando conditions using surface selective spectroelectrochemistry methods. Ligand design plays a central role in the attachment mode and the resulting functionality of the heterogenized catalyst as determined by the orientation of the catalyst relative to the surface and the nature of specific interactions that modulate the redox properties under the heterogeneous electrode conditions. Here, we introduce new [Re(L)(CO)₃Cl] catalysts for CO₂ reduction with sulfur-based anchoring groups on a bipyridyl ligand, where L = 3,3′-disulfide-2,2′-bipyridine (SSbpy) and 3,3′-thio-2,2′-bipyridine (Sbpy). Spectroscopic and electrochemical analysis complemented by computational modeling at the density functional theory level identify the complex [Re(SSbpy)(CO)₃Cl] as a multi-electron acceptor that combines the redox properties of both the rhenium tricarbonyl core and the disulfide functional group on the bipyridyl ligand. The first reduction at −0.85 V (vs. SCE) involves a two-electron process that breaks the disulfide bond, activating it for surface attachment. The heterogenized complex exhibits robust anchoring on gold surfaces, as probed by vibrational sum-frequency generation (SFG) spectroscopy. The binding configuration is normal to the surface, exposing the active site to the CO₂ substrate in solution. The attachment mode is thus particularly suitable for electrocatalytic CO₂ reduction.

Ru(II) coordination compounds of N-N bidentate chelators with 1,2,3 triazole and isoquinoline subunits: Synthesis, spectroscopy and antimicrobial properties

Bidentate chelators 1-(1-benzyl-1,2,3-triazol-4-yl)isoquinoline and 3-(1-benzyl-1,2,3-triazol-4-yl)isoquinoline were prepared from benzyl bromide and trimethylsilylethynylisoquinoline precursors using a tandem deprotection/substitution/CuAAC synthetic approach. Each chelator is capable of forming a stable 3:1 Ru(II) coordination compound, which forms as a geometric isomer mixture. These Ru(II) complexes possess unique MLCT absorbance signatures at 450/472 nm (1-isomer) and 367 nm (3-isomer) relative to their constituent chelating units. Minimum inhibitory concentration values as low as 0.4 μM are observed for Ru(II) complexes against representative Gram-positive bacteria Bacillus subtilis and Staphylococcus epidermidis. Comparing the MIC values of these isoquinoline compounds with analogous 2-(1-benzyl-1,2,3-triazol-4-yl)pyridine compounds shows a 2.5- to 40-fold improvement in potency. This study establishes that increased hydrophobicity introduced at the central chelating units of Ru(II) coordination compounds can be a useful means by which to optimize antimicrobial activity that is complimentary to the variation of peripheral substituent identity at the chelator's N1 triazole position.

Coordination-Driven Self-Assembly of Triazole-Based Apoptosis-Inducible Metallomacrocycles

Ru(II)-metallomacrocycles containing 4-pyridyl-1,2,3-triazole moiety were realized by coordination-driven self-assembly. All new compounds were characterized by electrospray ionisation mass spectrometry, elemental analysis, and 1H and 13C NMR spectroscopic techniques. The molecular structure of metallomacrocycle 8 was determined by single-crystal X-ray crystallography. The anticancer activities of metallomacrocycles 5-8 were evaluated by cytotoxicity, cell cycle analysis, and related protein expression. Metallomacrocycle 7 showed the highest cytotoxicity in HepG2 human hepatocellular carcinoma cells. In addition, apoptotic HepG2 cells were analyzed when metallomacrocycle 7 was treated. Our results suggest that metallomacrocycle 7 induces liver cancer cell death by increasing apoptosis and cell cycle arrest and that it has potential use as an agent for the treatment of human hepatocellular carcinoma.

A CuAAC Click Approach for the Introduction of Bidentate Metal Complexes to a Sulfanilamide-Derived Antibiotic Fragment

A simple "click-chemistry" approach was employed in order to functionalize the known antibiotic fragment sulfanilamide with a bidentate pyridyl-triazole pocket, which allowed for the synthesis of ruthenium(II) and rhenium(I) carbonyl chloride complexes. Six new complexes were prepared and comprehensively characterized, including five single crystal X-ray structures, photophysical characterization, and testing for antimicrobial activity. Interestingly, functionalization of the pyridine ring with an ortho-hydroxymethyl group resulted in a greater than 100-fold increase in the rate of ligand release in a dimethylsulfoxide solution. Subsequent studies indicated this process could be further accelerated by irradiation with 265 nm light. Structural characterization of four of the complexes indicates that this is the result of a lengthening and weakening of the Re-N_Pyridine bond (average (L^tri) = 2.19 Å vs L^triOH = 2.25 Å) due to the steric influence of the hydroxymethyl group. The organometallic rhenium(I) pyridyl-triazole functionality maintains its characteristic fluorescent properties despite the presence of the sulfonamide moiety. Two of the compounds showed modest antimicrobial activity against methicillin-resistant Staphylococcus aureus, whereas the structurally similar sulfamethoxazole alone showed no activity under the same conditions.

Pyridylbenzimidazole based Re(i)(CO)3 complexes: antimicrobial activity, spectroscopic and density functional theory calculations

fac-[ReBr(CO)3(L1,2)] L1 = 1-ethyl-2-(pyridin-2-yl)benzimidazole (1) and L2 = 1-[(pyridin-2-yl) benzimidazole]-propyl-sulfonic acid (2), fac-[Re2Br2(CO)6L3] (3) L3 = 1,1'-(hexane-1,6-diyl)bis[2-(pyridin-2-yl)1H-benzimidazole] and fac-[ReBr(CO)3(L4,5-κ2N1N2)] (L4 = 2,6-bis(benzimidazol-2'-yl)pyridine (4) and L5 = 2,6-bis(1-ethyl-benzimidazol-2'-yl)pyridine (5) were synthesized and fully characterized using different spectrocopic and analytical tools. The spectrocopic data showed coordination of L1-3 to fac-ReBr(CO)3 via the benzimidazole and pyridine N-atoms. For 4 and 5, the absence of a two-fold axis of symmetry for L4,5 in the 1H NMR spectra reflect the κ2N1,N2 mode of coordination. The electronic properties of 1-5 were investigated by time-dependent density functional theory calculations in the singlet and triplet states. The ligands and their Re(i) complexes were assessed for their potential antimicrobial activity. Compound 5 was screened against non-malignant cell line (noncancerous human embryonic kidney cell line (HEK293)) as well as evaluated for its blood compatibility.

Computational investigations of click-derived 1,2,3-triazoles as keystone ligands for complexation with transition metals: a review

In recent years, metal complexes of organo 1,2,3-triazole click-derived ligands have attracted significant attention as catalysts in many chemical transformations and also as biological and pharmaceutical active agents. Regarding the important applications of these metal-organo 1,2,3-triazole-based complexes, in this review, we focused on the recently reported investigations of the structural, electronic, and spectroscopic aspects of the complexation process in transition metal complexes of 1,2,3-triazole-based click ligands. In line with this, the coordination properties of these triazole-based click ligands with transition metals were studied via several quantum chemistry calculations. Moreover, considering the complexation process, we have presented comparative discussions between the computational results and the available experimental data.

Functional metal complexes from CuAAC “click” bidentate and tridentate pyridyl-1,2,3-triazole ligands

This Frontiers article examines the use of “click” complexes for the development of catalysts, anti-cancer and anti-bacterial agents and emissive materials.

The unsuspected influence of the pyridyl-triazole ligand isomerism upon the electronic properties of tricarbonyl rhenium complexes: an experimental and theoretical insight

How can the intimate nature of the triazole moiety govern the geometry and luminescence properties of a rhenium complex?

Exploring the cellular uptake and localisation of phosphorescent rhenium fac-tricarbonyl metallosurfactants as a function of lipophilicity

The cellular distribution of amphiphilic rhenium(i) complexes is tuned by the nature of the axial donor.

Crystal structure of the cis and trans polymorphs of bis-[μ-2-(1,3-benzo-thia-zol-2-yl)phenolato]-κ3N,O:O;κ3O:N,O-bis-[fac-tri-carbonyl-rhenium(I)]

The title dinuclear complex, [Re2(C13H8NOS)2(CO)6], crystallizes in two polymorphs where the 2-(1,3-benzo-thia-zol-2-yl)phenolate ligands and two carbonyl groups are trans- (I) or cis-arranged (II) with respect to the [Re2O2(CO)4] core. Polymorphs I and II exhibit a crystallographically imposed centre of symmetry and a twofold rotation axis, respectively. The structures may be described as being formed by two octa-hedrally distorted metal-coordinating units fused through μ-oxido bridges, leading to edge-sharing dimers. The crystal packing is governed by C-H⋯O hydrogen-bonding inter-actions, forming chains parallel to the c axis in I and a three-dimensional network in II.

Influence of bidentate ligand donor types on the formation and stability in 2 + 1 fac-[MI(CO)3]+ (M = Re, 99mTc) complexes

In the last two decades, a number of chelate strategies have been proposed for the fac-[M^I(CO)₃]⁺ (M = Re, ^99mTc) core in radiopharmaceutical applications. However, the development of new ligands/complexes with improved function and in vivo performance has been limited in recent years. Expanding on our previous studies using the 2 + 1 labeling strategy, a series of bidentate ligands (neutral vs. anionic) containing an aromatic amine in combination with monodentate pyridine analogs or imidazole were explored to determine the influence of the bidentate and monodentate ligands on the formation and stability of the respective complexes. The 2 + 1 complexes with Re and ^99mTc were synthesized in two steps and characterized by standard radio/chemical methods. X-ray characterization and density functional theory analysis of the Re 2 + 1 complexes with the complete bidentate series with 4-dimethylaminopyridine were conducted, indicating enhanced ligand binding energies of the neutral over anionic ligands. In the ^99mTc studies, anionic bidentate ligands had significantly higher formation yields of the 2 + 1 product, but neutral ligands appear to have increased stability in an amino acid challenge assay. Both bidentate series exhibited improved stability by increasing the basicity of the pyridine ligands.

Recent development of luminescent rhenium(i) tricarbonyl polypyridine complexes as cellular imaging reagents, anticancer drugs, and antibacterial agents

This Perspective summarizes recent advances in the biological applications of luminescent rhenium(i) tricarbonyl polypyridine complexes.

Oxidatively Locked [Co₂L₃]6+ Cylinders Derived from Bis(bidentate) 2-Pyridyl-1,2,3-triazole "Click" Ligands: Synthesis, Stability, and Antimicrobial Studies

A small family of [Co₂(Lpytrz)₃]6+ cylinders was synthesised from bis(bidentate) 2-pyridyl-1,2,3-triazole "click" ligands (Lpytrz) through an "assembly-followed-by-oxidation" method. The cylinders were characterised using ¹H, 13C, and DOSY NMR, IR, and UV-Vis spectroscopies, along with electrospray ionisation mass spectrometry (ESMS). Stability studies were conducted in dimethyl sulfoxide (DMSO) and D₂O. In contrast to similar, previously studied, [Fe₂(Lpytrz)₃]4+ helicates the more kinetically inert [Co₂(Lpytrz)₃]6+ systems proved stable (over a period of days) when exposed to DMSO and were even more stable in D₂O. The triply stranded [Co₂(Lpytrz)₃]6+ systems and the corresponding "free" ligands were tested for antimicrobial activity in vitro against both Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) microorganisms. Agar-based disk diffusion and Mueller-Hinton broth micro-dilution assays showed that the [Co₂(Lpytrz)₃]6+ cylinders were not active against either strain of bacteria. It is presumed that a high charge of the [Co₂(Lpytrz)₃]6+ cylinders is preventing them from crossing the bacterial cell membranes, rendering the compounds biologically inactive.