Complexes in context: attempting to control the cellular uptake and localisation of rhenium fac-tricarbonyl polypyridyl complexes

Neutral and Cationic Re(I) Complexes with Pnictogen-Based Coligands and Tunable Functionality: From Phosphorescence to Photoinduced CO Release

In this work, we have explored Re(I) complexes featuring triphenylpnictogen (PnPh3, Pn = P, As, or Sb)-based coligands and bidentate (neutral or monoanionic) luminophores derived from 1,10-phenantroline (phen), as well as from 2-(3-(tert-butyl)-1H-1,2,4-triazol-5-yl)pyridine (H(N-tBu)). The effect of the increasingly heavy elements on the structural parameters, photoexcited-state properties, and electrochemical behavior as well as the hybridization defects and polarization of the Pn atoms was related to the charges of the main luminophores (i.e., phen vs N-tBu) and explored in terms of photoluminescence spectroscopy, X-ray diffractometry, and quantum-chemical methods. Therefore, an in-depth analysis of the bonding, crystal packing, excited-state energies, and lifetimes was assessed in liquid solutions, frozen glassy matrices, and crystalline phases along with a semiquantitative photoactivation study. Notably, by changing the main ligand from phen to N-tBu, an increase in radiative and radiationless deactivation rates (kr and knr, respectively) at 77 K together with a faster photoinduced CO release and fragmentation at room temperature was detected. In addition, a progressively red-shifted phosphorescence was observed with the growing atomic number of the pnictogen atom, along with a boost in kr and knr at 77 K. Down the Vth main group and upon coordination of the Pn atom to the Re(I) center, an increasingly prominent jump of s-orbital participation on the binding sxp3.00-orbitals of the Pn atoms is evidenced. Based on these findings, the ability of these complexes to act as tunable photoluminescent labels able to perform as light-driven CO-releasing molecules is envisioned.

Luminescent 11-{Naphthalen-1-yl}dipyrido[3,2-a:2',3'-c]phenazine-Based Ru(II)/Ir(III)/Re(I) Complexes for HCT-116 Colorectal Cancer Stem Cell Therapy

Due to a number of unpleasant considerations, marketed drugs have steadily lost their importance in the treatment of cancer. In order to find a viable cancer cell diagnostic agent, we therefore focused on metal complexes that displayed target adequacy, permeability to cancer cells, high standard water solubility, cytoselectivity, and luminescent behavior. In this aspect, luminescent 11-{naphthalen-1-yl} dipyrido [3,2-a:2',3'-c] phenazine based Ru(II)/Ir(III)/Re(I) complexes have been prepared for HCT-116 colorectal cancer stem cell therapy. Our study successfully established the possible cytotoxicity of IrL complex at different doses on HCT-116 colorectal cancer stem cells (CRCSCs). Additionally, an immunochemistry analysis of the complex IrL showed that the molecule was subcellularly localized in the nucleus and other regions of the cytoplasm, where it caused nuclear DNA damage and mitochondrial dysfunction. The level of BAX and Bcl-2 was further quantified by qRT-PCR. The expression of proapoptotic BAX showed increased expression in the complex IrL-treated cell compared to the control, indicating the potential of complex IrL for apoptotic induction. Upon further validation, complex IrL was developed as an inhibitor of autophagy for the eradication of cancer stem cells.

Rhenium(I) Complexes with Neutral Monodentate Coligands and Monoanionic 2-(1,2,4-Triazol-5-yl)pyridine-Based Chelators as Bidentate Luminophores with Tunable Color and Photosensitized Generation of 1O2: An Integrated Case Study Involving Photophysics and Theory

In this work, we describe the synthesis as well as structural, photophysical, and theoretical investigation of a new coordination chemical concept involving rhenium(I) complexes bearing monoanionic 1,2,4-triazolylpyridine-based bidentate chromophores. The X-ray diffractometric analysis of single crystals revealed particular packing features: the trifluoromethylated exemplar displayed two kinds of arrangements of the coordination centers, where the bidentate ligands at the edges of the unit cell are staggered parallel to each other, whereas those inside show antiparallel stacking with respect to the external ligands. On the other hand, the complexes bearing an adamantyl substituent yield a linear arrangement, where the bulky moiety of one luminophore points to the pyridine center of the adjacent ligand of the neighboring complex while including methanol molecules hydrogen-bonded to the triazolato unit. We observed that the photophysical properties of the complexes (photoexcited-state lifetimes, photoluminescence maxima and quantum yields) can be adjusted by tuning of the substitution pattern at the bidentate luminophore as well as by variation of the monodentate coligand. The photoluminescence spectra and photoexcited-state lifetimes of the crystalline phases were measured by phosphorescence lifetime micro(spectro)scopy. Interestingly, the vibrationally resolved emission spectra of the crystals closely resemble those of diluted frozen glassy matrixes at 77 K, in contrast with the broad bands observed in amorphous solids and in fluid solutions, where the charge-transfer character is enhanced. While the photoluminescence quantum yields (Φ_L) reach up to 15%, the complexes are able to attain up to 55% efficiency regarding the photosensitization of ¹O₂ (Φ_Δ), depending on the combination of luminophore and coligand. Theoretical calculations showed that the photoexcited triplet (T₁) state has a metal-ligand-to-ligand charge-transfer character, where promotion to the excited electronic configuration shortens the Re(I)-N bond involving the bidentate triazolylpyridine while stretching the three fac-CO-Re(I) bonds as well as the linkage to the axial monodentate coligand. The calculated vertical (E^vl) and 0-0 (E^(0-0)) radiative transition energies are in very good agreement with the experimental values (E_exp^lum).

Insight into achirality and chirality effects in interactions of an racemic ruthenium(II) polypyridyl complex and its Δ- and Λ-enantiomers with an RNA triplex

RNA triplexes have a variety of potential applications in molecular biology, diagnostics and therapeutics, while low stabilization of the third strand hinders their practical utilities under physiological conditions. In this regard, achieving the third-strand stabilization by binding small molecules is a promising strategy. Chirality is one of the basic properties of nature. To clarify achirality and chirality effects on the binding and stabilizing effects of RNA triplexes by small molecules, we report for the first time the RNA interactions of an racemic ruthenium(II) polypyridyl complex [Ru(bpy)₂(11-CN-dppz)]²⁺ (rac-Ru1) and its two enantiomers Δ/Λ-[Ru(bpy)₂(11-CN-dppz)]²⁺ (Δ/Λ-Ru1) with an RNA triplex poly(U-A*U) (where "-" represents Watson-Crick base pairing, and "*" denotes Hoogsteen base pairing, respectively) in this work. Research shows that although rac-Ru1 and its two enantiomers Δ/Λ-Ru1 bind to the RNA triplex through the same mode of intercalation, the binding affinity for enantiomer Δ-Ru1 is much higher than that for rac-Ru1 and enantiomer Λ-Ru1. However, compared to enantiomer Λ-Ru1, the binding affinity for rac-Ru1 does not show much of an advantage, which is slightly greater than that for the former. Thermal denaturation measurements reveal both rac-Ru1 and Δ-Ru1 to have a preference for stabilizing the third strand rather than the template duplex of the RNA triplex, while Λ-Ru1 stabilizes the RNA triplex without significant selectivity. Besides, the third-strand stabilizing effects by rac-Ru1 and Δ-Ru1 are not markedly different from each other, but more marked than that by Λ-Ru1. This work shows that the binding properties of the racemic Ru(II) polypyridyl complex with the RNA triplex are not simply an average of its two enantiomers, indicating potentially complicated binding events.

Understanding the photophysical properties of rhenium(I) compounds coordinated to 4,7-diamine-1,10-phenanthroline: synthetic, luminescence and biological studies

In the present study, the photophysical properties and preliminary time-dependent density functional theory (TD-DFT) data of new rhenium(i) polypyridyl compounds, fac-[Re(L)(Am2phen)(CO)3]0/+, where Am2phen = 4,7-diamine-1,10-phenanthroline and L = Cl and ethyl isonicotinate (et-isonic), provided new insights into excited-state deactivation through an unusual inversion between two metal-to-ligand charge-transfer excited states. In addition, their cellular uptake using breast cancer (MCF-7) and melanoma (SkMel-147 and SkMel-29) cell lines and bioactivity were investigated and their cell-killing mechanism and protein expression were also studied. Preliminary TD-DFT results showed that both compounds exhibited a strong and broad absorption band around 300-400 nm which corresponds to a combination of ILAm2phen and MLCTRe→Am2phen transitions, and a strong contribution of charge transfer transition MLCTRe→et-isonic for fac-[Re(et-isonic)(Am2phen)(CO)3]+ is also observed. In contrast to typical Re(i) polypyridyl complexes, the substitution of Cl with the et-isonic ligand showed a bathochromic shift of the emission maxima, relatively low emission quantum yield and fast lifetime. Photophysical investigation of the fac-[ReCl(et-isonic)2(CO)3] compound provided meaningful information on the excited state manifold of the fac-[Re(L)(Am2phen)(CO)3]0/+ complexes. As shown in the absorption profile, a remarkable inversion of the lowest-lying excited state takes place from the usually observed MLCTRe→Am2phen to the unusual MLCTRe→et-isonic. The lipophilicity of the positive-complex was higher than that of the non-charge compound and the same trend for the activity against cells was observed, in the absence of light. In addition, flow cytometry and Western Blot analyses showed an overexpression of pro-caspase-9, suggesting a caspase proteolytic cascade through an intrinsic-pathway apoptosis mechanism. The photophysical properties of these compounds reported herein provide new fundamental insights into the understanding of substituent groups on polypyridyl ligands which are relevant to practical development.

Nanoparticles Functionalised with Re(I) Tricarbonyl Complexes for Cancer Theranostics

Globally, cancer is the second (to cardiovascular diseases) leading cause of death. Regardless of various efforts (i.e., finance, research, and workforce) to advance novel cancer theranostics (diagnosis and therapy), there have been few successful attempts towards ongoing clinical treatment options as a result of the complications posed by cancerous tumors. In recent years, the application of magnetic nanomedicine as theranostic devices has garnered enormous attention in cancer treatment research. Magnetic nanoparticles (MNPs) are capable of tuning the magnetic field in their environment, which positively impacts theranostic applications in nanomedicine significantly. MNPs are utilized as contrasting agents for cancer diagnosis, molecular imaging, hyperfusion region visualization, and T cell-based radiotherapy because of their interesting features of small size, high reactive surface area, target ability to cells, and functionalization capability. Radiolabelling of NPs is a powerful diagnostic approach in nuclear medicine imaging and therapy. The use of luminescent radioactive rhenium(I), 188/186Re, tricarbonyl complexes functionalised with magnetite Fe3O4 NPs in nanomedicine has improved the diagnosis and therapy of cancer tumors. This is because the combination of Re(I) with MNPs can improve low distribution and cell penetration into deeper tissues.

Aspartic acid assisted one-step synthesis of stable CsPbX3@Asp-Cs4PbX6 by in situ growth in NH2-MIL-53 for ratiometric fluorescence detection of 4-bromophenoxybenzene

A molecularly imprinted ratiometric fluorescent sensor was synthesized for the detection of 4-bromophenoxybenzene (BDE-3) based on perovskite quantum dots and metal organic framework. First, aspartic acid (Asp) was introduced during the synthesis of perovskite CsPbX₃ for the formation of a core-shell structure of CsPbX₃@Asp-Cs₄PbX₆. Due to the protection of the shell layer Cs₄PbX₆, the stability of the core CsPbX₃ was improved significantly. Compared to CsPb(BrI)₃, the ultraviolet and thermal stabilities of CsPb(BrI)₃@Asp-Cs₄Pb(BrI)₆ were increased by 26 times and 32 times, respectively, and, compared to CsPbBr₃, these stabilities of CsPbBr₃@Asp-Cs₄PbBr₆ were increased by 3 times and 13 times, respectively. The water stabilities of CsPb(BrI)₃@Asp-Cs₄Pb(BrI)₆ and CsPbBr₃@Asp-Cs₄PbBr₆ were greatly improved too. Then, a ratiometric fluorescence sensor was constructed by in situ growth of CsPb(BrI)₃@Asp-Cs₄Pb(BrI)₆ in metal organic framework (NH₂-MIL-53) for the detection of BDE-3, in which the orange fluorescence of CsPb(BrI)₃@Asp-Cs₄Pb(BrI)₆ (614 nm) was regarded as the reference signal and the cyan fluorescence of NH₂-MIL-53 (494 nm) was used as the fluorescence response signal. To improve the selectivity of the sensor, the molecular imprinting polymer (MIP) was modified on the NH₂-MIL-53 and an imprinting factor of 3.17 was obtained. Under 365 nm light excitation, the fluorescent response signal at 494 nm was quenched gradually by BDE-3 in the range 11.4 to 68.5 nmol/L, while the reference signal at 614 nm remained unchanged. The limit of detection and limit of quantification were 3.35 and 11.2 nmol/L, respectively, and the fluorescent color of the sensor changed gradually from cyan to green to orange, which illustrated that the developed sensor has an ability to recognize BDE-3 specifically, a good anti-interference ability, and a sensitively visual detection ability. Moreover, the sensor was successfully applied to the BDE-3 detection in polyethylene terephthalate plastic bottle, polyvinyl chloride plastic bag, and circuit board with satisfactory recoveries (96.3-108.1%) and low relative standard deviations (5%). The preparation processes of NH₂-MIL-53, NH₂-MIL-53-CsPb(BrI)₃@Asp-Cs₄Pb(BrI)₆, and the MIP-NH₂-MIL-53-CsPb(BrI)₃@Asp-Cs₄Pb(BrI)₆ composites.

Building C(sp3 ) Molecular Complexity on 2,2'-Bipyridine and 1,10-Phenanthroline in Rhenium Tricarbonyl Complexes

The reactions of [Re(N-N)(CO)₃ (PMe₃ )]OTf (N-N=2,2'-bipyridine, bipy; 1,10-phenanthroline, phen) compounds with tBuLi and with LiHBEt₃ have been explored. Addition to the N-N chelate took place with different site-selectivity depending on both chelate and nucleophile. Thus, with tBuLi, an unprecedented addition to C5 of bipy, a regiochemistry not accessible for free bipy, was obtained, whereas coordinated phen underwent tBuLi addition to C2 and C4. Remarkably, when LiHBEt₃ reacted with [Re(bipy)(CO)₃ (PMe₃ )]OTf, hydride addition to the 4 and 6 positions of bipy triggered an intermolecular cyclodimerization of two dearomatized pyridyl rings. In contrast, hydride addition to the phen analog resulted in partial reduction of one pyridine ring. The resulting neutral Re^I products showed a varied reactivity with HOTf and with MeOTf to yield cationic complexes. These strategies rendered access to Re^I complexes containing bipy- and phen-derived chelates with several C(sp³ ) centers.

Luminescent Rhenium(I)tricarbonyl Complexes Containing Different Pyrazoles and Their Successive Deprotonation Products: CO2 Reduction Electrocatalysts

Cationic fac-[Re(CO)₃(pz*H)(pypzH)]OTf (pz*H = pyrazole, pzH; 3,5-dimethylpyrazole, dmpzH; indazole, indzH; 3-(2-pyridyl)pyrazole, pypzH) were obtained from fac-[ReBr(CO)₃(pypzH)] by halide abstraction with AgOTf and subsequent addition of the corresponding pyrazole. Successive deprotonation with Na₂CO₃ and NaOH gave neutral fac-[Re(CO)₃(pz*H)(pypz)] and anionic Na{fac-[Re(CO)₃(pz*)(pypz)]} complexes, respectively. Cationic fac-[Re(CO)₃(pz*H)(pypzH)]OTf, neutral complexes fac-[Re(CO)₃(pz*H)(pypz)], and fac-[Re(CO)₃(pypz)₂Na] were subjected to photophysical and electrochemical studies. They exhibit phosphorescent decays from a prevalently ³MLCT excited state with quantum yields (Φ) in the range between 0.03 and 0.58 and long lifetimes (τ from 220 to 869 ns). The electrochemical behavior in Ar atmosphere of cationic and neutral complexes indicates that the oxidation processes assigned to Re^I → Re^II occurs at lower potentials for the neutral complex compared to cationic complex. The reduction processes occur at the ligands and do not depend on the charge of the complexes. The electrochemical behavior in CO₂ saturated media is consistent with CO₂ electrocatalyzed reduction, where the values of the catalytic activity [i_cat(CO₂)/i_cat(Ar)] ranged from 2.7 to 11.5 (compared to 8.1 for fac-[Re(CO)₃Cl(bipy)] studied as a reference). Controlled potential electrolysis for the pyrazole cationic (3a) and neutral (4a) complexes after 1 h affords CO in faraday yields of 61 and 89%, respectively. These values are higher for indazole complexes and may be related to the acidity of the coordinated pyrazole.

Identification of novel potent and non-toxic anticancer, anti-angiogenic and antimetastatic rhenium complexes against colorectal carcinoma

Combination therapy targeting both tumor growth and vascularization is considered to be a cornerstone for colorectal carcinomas (CRC) treatment. However, the major obstacles of most clinical anticancer drugs are their weak selective activity towards cancer cells and inherent inner organs toxicity, accompanied with fast drug resistance development. In our effort to discover novel selective and non-toxic agents effective against CRC, we designed, synthesized and characterized a series of rhenium(I) tricarbonyl-based complexes with increased lipophilicity. Two of these novel compounds were discovered to possess remarkable anticancer, anti-angiogenic and antimetastatic activity in vivo (zebrafish-human HCT-116 xenograft model), being effective at very low doses (1-3 μM). At doses as high as 250 μM the complexes did not provoke toxicity issues encountered in clinical anticancer drugs (cardio-, hepato-, and myelotoxicity). In vivo assays showed that the two compounds exceed the anti-tumor and anti-angiogenic activity of clinical drugs cisplatin and sunitinib malate, and display a large therapeutic window.

Functional imaging of a model unicell: Spironucleus vortens as an anaerobic but aerotolerant flagellated protist

Advances in optical microscopy are continually narrowing the chasm in our appreciation of biological organization between the molecular and cellular levels, but many practical problems are still limiting. Observation is always limited by the rapid dynamics of ultrastructural modifications of intracellular components, and often by cell motility: imaging of the unicellular protist parasite of ornamental fish, Spironucleus vortens, has proved challenging. Autofluorescence of nicotinamide nucleotides and flavins in the 400-580 nm region of the visible spectrum, is the most useful indicator of cellular redox state and hence vitality. Fluorophores emitting in the red or near-infrared (i.e., phosphors) are less damaging and more penetrative than many routinely employed fluors. Mountants containing free radical scavengers minimize fluorophore photobleaching. Two-photon excitation provides a small focal spot, increased penetration, minimizes photon scattering and enables extended observations. Use of quantum dots clarifies the competition between endosomal uptake and exosomal extrusion. Rapid motility (161 μm/s) of the organism makes high resolution of ultrastructure difficult even at high scan speeds. Use of voltage-sensitive dyes determining transmembrane potentials of plasma membrane and hydrogenosomes (modified mitochondria) is also hindered by intracellular motion and controlled anesthesia perturbs membrane organization. Specificity of luminophore binding is always questionable; e.g. cationic lipophilic species widely used to measure membrane potentials also enter membrane-bounded neutral lipid droplet-filled organelles. This appears to be the case in S. vortens, where Coherent Anti-Stokes Raman Scattering (CARS) micro-spectroscopy unequivocally images the latter and simultaneous provides spectral identification at 2840 cm^-1. Secondary Harmonic Generation highlights the highly ordered structure of the flagella.

Optimization of aggregation-induced phosphorescence enhancement in mononuclear tricarbonyl rhenium(i) complexes: the influence of steric hindrance and isomerism

In order to improve the remarkable performance of a mononuclear tricarbonyl rhenium(i) complex (ReL1) that exhibits rare aggregation-induced phosphorescence enhancement (AIPE) behavior, two new complexes (ReL3 and ReL4) were prepared and investigated. They incorporate a 2-pyridyl-1,2,4-triazole (pyta) ligand connected to a 2-phenylbenzoxazole (PBO) moiety. Complex ReL3 differs from ReL1 by the presence of a bulky tert-butyl substituent, and ReL4 is an isomer where the PBO group is linked to the pyta ligand by its phenyl group. Theoretical calculations were in congruence with electrochemical and spectroscopic properties in solutions. Both new compounds exhibited strong AIPE and much better solid-state emission efficiency than ReL1, with photoluminescence quantum yields up to 55% for ReL4. Crystallographic data indicate that this increase in emission efficiency is due to optimum packing that prevents quenching. This work shows that minor structural changes may have major effects upon the solid-state spectroscopic properties and it provides a rational basis for accessing AIPE-active strongly emissive rhenium(i) complexes.

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?

Rhenium tricarbonyl complexes with arenethiolate axial ligands

Pyta and Tapy-based [Re(N^N)(CO)₃X] complexes with para-substituted benzenethiolates as axial ligand are reported along with their electrochemical and photophysical properties.

Design, Synthesis, and Reactivity of Multidentate Ligands with Rhenium(I) and Rhenium(V) Cores

Synthetic pathways to a range of potentially N3O‐tetradentate ligands designed to coordinate to rhenium cores, as well as their coordination behaviors towards different rhenium cores (oxidation states +I and +V) are investigated. Two functionalized N‐{[1‐(4‐R)‐1H‐1,2,3‐triazol‐4‐yl]methyl}‐2‐(pyridin‐2‐ylmethoxy)aniline derivatives L1H (R = methyl acetate) and L2H (R = 4‐nitrophenyl) act exclusively as bidentate ligands and lead to the formation of mononuclear tricarbonylrhenium(I) complexes of the general formula [(LH)Re(CO)3Cl] with L = L1 or L2. Both complexes are characterized by 1H NMR and 13C NMR, FTIR spectroscopy, electrospray ionization mass spectrometry, and in the case of [(L2H)Re(CO)3Cl], single‐crystal X‐ray diffraction. The rhenium is coordinated by three carbonyl groups, a chlorine atom and two nitrogen atoms of a triazole group, and a nitrogen of the aniline ring of the ligand, respectively. A theoretical study shows complex [(L2H)Re(CO)3Cl] is the most stable structural isomer. In addition, the oxorhenium(V) complex [(L3)ReO] is isolated and fully characterized after the reaction of the ReV precursor [ReOCl3(PPh3)2] with L3H3 [methyl 2‐(4‐{[2‐(2‐hydroxyphenylamino)‐2‐oxoethylamino]methyl}‐1H‐1,2,3‐triazol‐1‐yl)acetate]. Its corresponding 99mTc complex was achieved with a good radiochemical yield (> 90 %). The convenient synthesis of this ligand, coupled with its high affinity for [ReO]3+ and [99mTcO]3+ cores, make it a promising chelator for biomedical applications.

Simultaneously Inducing and Tracking Cancer Cell Metabolism Repression by Mitochondria-Immobilized Rhenium(I) Complex

Mitochondrial metabolism is essential for tumorigenesis, and the development of cancer is usually accompanied by alternations of mitochondrial function. Emerging studies have demonstrated that targeting mitochondria and mitochondrial metabolism is an effective strategy for cancer therapy. In this work, eight phosphorescent organometallic rhenium(I) complexes have been synthesized and explored as mitochondria-targeted theranostic agents, capable of inducing and tracking the therapeutic effect simultaneously. Complexes 1b-4b can quickly and efficiently penetrate into A549 cells, specifically localizing within mitochondria, and their cytotoxicity is superior to cisplatin against the cancer cells screened. Notably, complex 3b [Re(CO)₃(DIP) (py-3-CH₂Cl)]⁺ containing thiol-reactive chloromethylpyridyl moiety for mitochondria immobilization shows higher cytotoxicity and selectivity against cancer cells than other Re(I) complexes without mitochondria-immobilization properties. Mechanistic studies show that complexes 1b-4b induce a cascade of mitochondria-dependent events including mitochondrial damage, mitochondrial respiration inhibition, cellular ATP depletion, reactive oxygen species (ROS) elevation, and caspase-dependent apoptosis. By comparison, mitochondria-immobilized 3b causes more effective repression of mitochondrial metabolism than mitochondrial-nonimmobilized complexes. The excellent phosphorescence and O₂-sensitive lifetimes of mitochondria-immobilized 3b can be utilized for real-time tracking of the morphological changes of mitochondria and mitochondrial respiration repression during therapy process, accordingly providing reliable information for understanding anticancer mechanisms.

Technetium(I) Complexes of Bathophenanthrolinedisulfonic Acid

Bathophenanthrolinedisulfonate (BPS) complexes of technetium(I) of the type [Tc(CO)₃(BPS)(L)]ⁿ (L = imidazole derivatives) were synthesized and evaluated both in vitro and in vivo. [^99mTc(CO)₃(BPS)(MeIm)]^- (MeIm = 1-methyl-1H-imidazole) was prepared in near-quantitative yield using a convenient two-step, one-pot labeling procedure. A targeted analogue capable of binding regions of calcium turnover associated with bone metabolism was also prepared. Here, a bisphosphonate was linked to the metal through an imidazole ligand to give [^99mTc(CO)₃(BPS)(ImAln)]^2- (ImAln = an imidazole-alendronate ligand) in high yield. The technetium(I) complexes were stable in vitro, and in biodistribution studies, [^99mTc(CO)₃(BPS)(ImAln)]^2- exhibited rapid clearance from nontarget tissues and significant accumulation in the shoulder (7.9 ± 0.2% ID/g) and knees (15.1 ± 0.9% ID/g) by 6 h, with the residence time in the skeleton reaching 24 h. A rhenium analogue, which is luminescent and has the same structure, was also prepared and used for fluorescence labeling of cells in vitro. The data reported demonstrate the potential of this class of compounds for use in creating isostructural optical and nuclear probes.

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.

Photo-induced cytotoxicity and anti-metastatic activity of ruthenium(ii)–polypyridyl complexes functionalized with tyrosine or tryptophan

The synergestic effect of oxygen, light, and photosensitizer has found application in photodyanmic therapy (PDT).

Rhenium tetrazolato complexes coordinated to thioalkyl-functionalised phenanthroline ligands: synthesis, photophysical characterisation, and incubation in live HeLa cells

Three new complexes of formulation fac-[Re(CO)3(diim)L], where diim is either 1,10-phenanthroline or 1,10-phenanthroline functionalised at position 5 by a thioalkyl chain, and L is either a chloro or aryltetrazolato ancillary ligand, were synthesised and photophysically characterised. The complexes exhibit phosphorescent emission with maxima around 600 nm, originating from triplet metal-to-ligand charge transfer states with partially mixed ligand-to-ligand charge transfer character. The emission is relatively long-lived, within the 200-400 ns range, and with quantum yields of 2-4%. The complexes were trialed as cellular markers in live HeLa cells, along with two previously reported rhenium tetrazolato complexes bound to unsubstituted 1,10-phenanthroline. All five complexes exhibit good cellular uptake and non-specific perinuclear localisation. Upon excitation at 405 nm, the emission from the rhenium complexes could be clearly distinguished from autofluorescence, as demonstrated by spectral detection within the live cells. Four of the complexes did not appear to be toxic, however prolonged excitation could result in membrane blebbing. No major sign of photobleaching was detected upon multiple imaging on the same cell sample.

Luminescent rhenium(I) tricarbonyl complexes with pyrazolylamidino ligands: photophysical, electrochemical, and computational studies

New pyrazolylamidino complexes fac-[ReCl(CO)3(NH[double bond, length as m-dash]C(Me)pz*-κ(2)N,N)] (pz*H = pyrazole, pzH; 3,5-dimethylpyrazole, dmpzH; indazole, indzH) and fac-[ReBr(CO)3(NH[double bond, length as m-dash]C(Ph)pz*-κ(2)N,N)] are synthesized via base-catalyzed coupling of the appropriate nitrile with pyrazole, or via metathesis by halide abstraction with AgBF4 from a bromido pyrazolylamidino complex and the subsequent addition of LiCl. In order to study both the influence of the substituents present at the pyrazolylamidino ligand, and that of the "sixth" ligand in the complex, photophysical, electrochemical, and computational studies have been carried out on this series and other complexes previously described by us, of the general formula fac-[ReL(CO)3(NH[double bond, length as m-dash]C(R')pz*-κ(2)N,N)](n+) (L = Cl, Br; R' = Me, Ph, n = 0; or L = NCMe, dmpzH, indzH, R' = Me, n = 1). All complexes exhibit phosphorescent decays from a prevalently (3)MLCT excited state with quantum yields (Φ) in the range between 0.007 and 0.039, and long lifetimes (τ∼ 8-1900 ns). The electrochemical study reveals irreversible reduction for all complexes. The oxidation of the neutral complexes was found to be irreversible due to halido-dissociation, whereas the cationic species display a reversible process implying the ReI/ReII couple. Density functional and time-dependent density functional theory (TD-DFT) calculations provide a reasonable trend for the values of emission energies in line with the experimental photophysical data, supporting the (3)MLCT based character of the emissions.

Toward hypoxia-selective rhenium and technetium tricarbonyl complexes

With the aim of preparing hypoxia-selective imaging and therapeutic agents, technetium(I) and rhenium(I) tricarbonyl complexes with pyridylhydrazone, dipyridylamine, and pyridylaminocarboxylate ligands containing nitrobenzyl or nitroimidazole functional groups have been prepared. The rhenium tricarbonyl complexes were synthesized with short reaction times using microwave irradiation. Rhenium tricarbonyl complexes with deprotonated p-nitrophenyl pyridylhydrazone ligands are luminescent, and this has been used to track their uptake in HeLa cells using confocal fluorescent microscopy. Selected rhenium tricarbonyl complexes displayed higher uptake in hypoxic cells when compared to normoxic cells. A (99m)Tc tricarbonyl complex with a dipyridylamine ligand bearing a nitroimidazole functional group is stable in human serum and was shown to localize in a human renal cell carcinoma (RCC; SK-RC-52) tumor in a mouse.

Phototoxic Activity and DNA Interactions of Water-Soluble Porphyrins and Their Rhenium(I) Conjugates

In the search for alternative photosensitizers for use in photodynamic therapy (PDT), herein we describe two new water-soluble porphyrins, a neutral fourfold-symmetric compound and a +3-charged tris-methylpyridinium derivative, in which either four or one [1,4,7]-triazacyclononane (TACN) units are connected to the porphyrin macrocycle through a hydrophilic linker; we also report their corresponding tetracationic Re(I) conjugates. The in vitro (photo)toxic effects of the compounds toward the human cell lines HeLa (cervical cancer), H460M2 (non-small-cell lung carcinoma), and HBL-100 (non-tumorigenic epithelial cells) are reported. Three of the compounds are not cytotoxic in the dark up to 100 μm, and the fourfold-symmetric couple revealed very good phototoxic indexes (PIs). The intracellular localization of all derivatives was studied in HeLa cells by confocal fluorescence microscopy. Although low nuclear localization was observed for some of them, it still prompted us to investigate their capacity to bind both quadruplex and duplex DNA; we observed significant selectivity in the tris-methylpyridinium derivatives for G-quadruplex interactions.

Luminescent Re(I) terpyridine complexes for OLEDs: what does the DFT/TD-DFT probe reveal?

The electronic structure and spectroscopic properties of a series of rhenium(I) terpyridine complexes were investigated using density functional theory (DFT) and time dependent density functional theory (TD-DFT) methods. The influence of different substituent groups on the optical and electronic properties of Re(I) terpyridine complexes has also been explored. The reorganization energy calculations show that the substituted Re(I) terpyridine complexes are better electron transport materials with high quantum efficiency in OLED devices due to their high electron transport mobility and low λ(electron) values, whereas the unsubstituted complex shows relatively balanceable charge transfer abilities with the higher efficiency in organic light emitting devices (OLEDs). An NBO analysis reveals that n→σ* interactions are mainly responsible for the ground state stabilization of all the complexes. QTAIM results show that in all cases, Re-CO bonds are shared type transient interactions as reported in the other metal ligand complexes. The absorption is associated with (1)MLCT/(1)LLCT/(1)ILCT character while the emission transition has (3)MLCT/(3)LLCT/(3)ILCT character as revealed by a natural transition orbital (NTO) analysis. The higher quantum yields reported for the complexes 4-6 are found to be closely related to both its smaller ΔE(S1-T1), higher μ(S1), E(T1) and moderate (3)MLCT character. The calculated results show that Re(I) terpyridine complexes, particularly complexes 4-6, are suitable candidates for OLED materials.

Synthesis, characterization and cellular location of cytotoxic constitutional organometallic isomers of rhenium delivered on a cyanocobalmin scaffold

Constitutional isomers based on vitamin B₁₂ and a fluorescent rhenium diimine complex were prepared, characterized, tested against PC-3 prostate cancer cells and investigated via IR spectromicroscopy for cellular uptake by live 3T3 fibroblasts.

Platinum trimethyl bipyridyl thiolates – new, tunable, red- to near IR emitting luminophores for bioimaging applications

[PtMe₃(S-C₆H₄-4-CO₂Me)(bpy)]ⁿ⁺ exemplifies a new class of luminophores, absorbing in the visible and emitting into the NIR, with good cell uptake making it suitable for fluorescence microscopy imaging.

Development of a cyclometalated iridium complex with specific intramolecular hydrogen-bonding that acts as a fluorescent marker for the endoplasmic reticulum and causes photoinduced cell death

Cyclometalated iridium complexes have important applications as phosphorescent probes for cellular imaging due to their photophysical properties. Moreover, these properties also make them potential candidates as photosensitizers for photodynamic therapy (PDT) of tumors and skin diseases. Treatment of MCF7 breast carcinoma cells with a heteroleptic phosphorescent cyclometalated iridium(III) complex C2 followed by confocal imaging indicates that the complex selectively localizes and exhibits high fluorescence in the endoplasmic reticulum. In an unprecedented approach, systematic alteration of functional groups or the metal core in C2 to synthesize a series of iridium(III) complexes (C1–C10) and an organometallic rhenium complex C11 with an imidazolyl modified phenanthroline ligand has indicated the functional groups and their interactions that are responsible for this selective localization. Remarkably, the exposure of the cells treated with C2 to irradiation at 405 nm for one hour led to membrane blebbing and cell death, demonstrating a photosensitizing property of the compound.

Metal-carbonyl units for vibrational and luminescence imaging: towards multimodality

Metal-carbonyl complexes are attractive structures for bio-imaging. In addition to unique vibrational properties due to the CO moieties enabling IR and Raman cell imaging, the appropriate choice of ancillary ligands opens up the opportunity for luminescence detection. Through a classification by techniques, past and recent developments in the application of metal-carbonyl complexes for vibrational and luminescence bio-imaging are reviewed. Finally, their potential as bimodal IR and luminescent probes is addressed.