Rhenium chemistry – Then and Now

Metallic Re3O2 with mixed-valence states

Rhenium (Re) shows the richest valence states from +2 to +7 in compounds, but its mixed-valence states are still missing thus far. In this work, we have explored the Re-O phase diagram with a wide range of stoichiometric compositions under high pressure through first-principles structural search calculations. Besides identifying two novel high-pressure phases of ReO2 and ReO3, we reveal two hitherto unknown Re-rich Re3O2 and O-rich ReO4 compounds. Re atoms in Re3O2 show mixed-valence states due to their inequivalent coordination environments, the first example in Re-based compounds. Electronic structure calculations demonstrate that the four discovered Re-O phases exhibit metallicity contributed by Re 5d electrons. Among them, ReO3 has a predicted critical temperature of up to 12 K at 50 GPa, derived from the interaction between Re 5d electrons and Re-derived low-frequency phonons. Our study points to new opportunities to disclose novel transition metal compounds with mixed-valence states.

Structural and Photophysical Trends in Rhenium(I) Carbonyl Complexes with 2,2':6',2″-Terpyridines

This is the first comprehensive review of rhenium(I) carbonyl complexes with 2,2':6',2″-terpyridine-based ligands (R-terpy)-encompassing their synthesis, molecular features, photophysical behavior, and potential applications. Particular attention has been devoted to demonstrating how the coordination mode of 2,2':6',2″-terpyridine (terpy-κ2N and terpy-κ3N), structural modifications of terpy framework (R), and the nature of ancillary ligands (X-mono-negative anion, L-neutral ligand) may tune the photophysical behavior of Re(I) complexes [Re(X/L)(CO)3(R-terpy-κ2N)]0/+ and [Re(X/L)(CO)2(R-terpy-κ3N)]0/+. Our discussion also includes homo- and heteronuclear multicomponent systems with {Re(CO)3(R-terpy-κ2N)} and {Re(CO)2(R-terpy-κ3N)} motifs. The presented structure-property relationships are of high importance for controlling the photoinduced processes in these systems and making further progress in the development of more efficient Re-based luminophores, photosensitizers, and photocatalysts for modern technologies.

Dinuclear Rhenium(I) Tricarbonyl Complexes as Anticancer Drug Candidates

Cancer is one of the deadliest diseases worldwide. Chemotherapy remains one of the most dominant forms for anticancer treatment. Despite their clinical success, the used chemotherapeutic agents are associated with severe side effect and pharmacological limitations. To overcome these drawbacks there is a need for the development of new types of chemotherapeutic agents. Herein, the chemical synthesis and biological evaluation of dinuclear rhenium(I) complexes as potential chemotherapeutic drug candidates are proposed. The metal complexes were found to be internalized by an energy dependent endocytosis pathway, primary accumulating in the mitochondria. The rhenium(I) complexes demonstrated to induce cell death against a variety of cancer cells in the micromolar range through apoptosis. The lead compound showed to eradicate a pancreatic carcinoma multicellular tumor spheroid at micromolar concentrations.

Synthesis and Characterization of Iridium(III) Complexes with Substituted Phenylimidazo(4,5-f)1,10-phenanthroline Ancillary Ligands and Their Application in LEC Devices

In this work, we report on the synthesis and characterization of six new iridium(III) complexes of the type [Ir(C^N)2(N^N)]+ using 2-phenylpyridine (C1-3) and its fluorinated derivative (C4-6) as cyclometalating ligands (C^N) and R-phenylimidazo(4,5-f)1,10-phenanthroline (R = H, CH3, F) as the ancillary ligand (N^N). These luminescent complexes have been fully characterized through optical and electrochemical studies. In solution, the C4-6 series exhibits quantum yields (Ф) twice as high as the C1-3 series, exceeding 60% in dichloromethane and where 3MLCT/3LLCT and 3LC emissions participate in the phenomenon. These complexes were employed in the active layer of light-emitting electrochemical cells (LECs). Device performance of maximum luminance values of up to 21.7 Lx at 14.7 V were observed for the C2 complex and long lifetimes for the C1-3 series. These values are counterintuitive to the quantum yields observed in solution. Thus, we established that the rigidity of the system and the structure of the solid matrix dramatically affect the electronic properties of the complex. This research contributes to understanding the effects of the modifications in the ancillary and cyclometalating ligands, the photophysics of the complexes, and their performance in LEC devices.

Toward a mechanistic understanding of Rhenium(VII) adsorption behavior onto aminated polymeric adsorbents: Batch experiments, spectroscopic analyses, and theoretical computations

Rhenium, a rare and critical metal, existing in the industrial wastewater has been aroused extensive interests recently, due to its environmental and resource issues. Chitosan, an easily available, low-cost and eco-friendly biopolymer, was prepared and modified by grafting primary, secondary, tertiary and quaternary amino groups, respectively. Adsorption behaviors and interactions between ReO4- and these four types of aminated adsorbents were investigated through batch experiments, spectroscopic analysis, and theoretical computations. Chitosan modified with secondary amines showed an extremely high uptake of ReO4- with 742.0 mg g-1, which was higher than any reported adsorbents so far. Furthermore, a relatively high adsorption selectivity for Re(VII), as well as the stable and facile regeneration of these aminated adsorbents revealed a promising approach for Re(VII) recovery in full-scale applications. The electrostatic attraction was illustrated to be the main adsorption mechanism by Fourier Transform Infrared Spectroscopy and X-ray Photoelectron Spectroscopy analyses. Significantly, the sub-steps of the adsorption process, encompassing the transformation of binding sites and the subsequent binding between these sites and the adsorbate, have been thoroughly investigated through the density functional theory (DFT) calculation method. This approach was firstly proposed to clearly demonstrate the differences in Re(VII) adsorption behavior onto four types of aminated adsorbents, resulting the importance of not only strong binding energy but also an appropriate binding spatial environmental for effective Re(VII) adsorption.

Rhenium-Pyrazolyl-Based Figure-Eight- and Z-Shaped Metallocycles: Self-Assembly, Solid-State Structures, Dynamic Properties in Solution, and Competitive Ligand-Induced Supramolecular Transformations into Rhenium-Pyridyl/-Benzimidazolyl/-Phosphine-Based Metallocycles/Acyclic Complexes

Rhenium(I)tricarbonyl core-based heteroleptic "figure-eight"- and Z-shaped metallocycles (1a-4a) of the general formula fac-[{(CO)3Re(μ-L)Re(CO)3}2(dppz)2] were self-assembled from Re2(CO)10, H2-L (H2-L = 5,8-dihydroxy-1,4-naphthaquinone (H2-dhnq) for 1a; 1,4-dihydroxy-9,10-anthraquinone (H2-dhaq) for 2a; 6,11-dihydroxy-5,12-naphthacenedione (H2-dhnd) for 3a; 2,2'-bisbenzimidazole (H2-bbim) for 4a), and bis(4-((pyrazolyl)methyl)phenylmethane) (dppz) via one-pot coordination-driven synthetic approach. The molecular structures of 1a and 4a were unambiguously confirmed by single-crystal X-ray diffraction (SC-XRD) methods. The metallocycles in the DMSO solution exist as an acyclic dinuclear-DMSO adduct of the general formula fac-[{(CO)3Re(μ-L)Re(CO)3}(DMSO)2] (1b, L = dhnq; 2b, L = dhaq; 3b, L = dhnd; 4b, L = bbim) and dppz, which are in dynamic equilibrium. The dynamic behavior of the rhenium-pyrazolyl bond in the solution state was effectively utilized to transform metallocycles 1a-4a into pyridyl/benzimidazolyl/phosphine donor-based heteroleptic metallocycles and acyclic dinuclear complexes (4-13). These include tetranuclear rectangles fac-[{(CO)3Re(μ-L)Re(CO)3}2(4,4'-bpy)2] (4 and 11, L = dhaq for 4 and bbim for 11), dinuclear metallocycles fac-[{(CO)3Re(μ-L)Re(CO)3}(dpbim)] (5-7 and 12; L = dhnq for 5, dhaq for 6, dhnd for 7, and bbim for 12), and dinuclear acyclic complexes fac-[{(CO)3Re(μ-L)Re(CO)3}(PTA)2] (8-10 and 13; L = dhnq for 8, dhaq for 9, dhnd for 10, and bbim for 13). These transformations were achieved through component-induced supramolecular reactions while treating with competitive ligands 4,4'-bipyridine (4,4'-bpy), bis(4-((1H-benzoimidazole-1-yl)methyl)phenyl)methane (dpbim), and 1,3,5-triaza-7-phosphaadamantane (PTA). The reaction mixture in the solution was analyzed using NMR and electrospray ionization mass spectrometry (ESI-MS) analysis. Additionally, crystal structures of 4, 6, and 13, which were obtained in the mixture of the solutions, were determined, providing unequivocal evidence for the occurrence of supramolecular transformation within the system. The results reveal that the size of the chelating ligand and the pyrazolyl donor angle of the ditopic ligand play crucial roles in determining the resulting solid-state metallacyclic architecture in these synthetic combinations. The dynamic behavior of the rhenium-pyrazolyl bond in the metallocycles can be utilized to transform into other metallocycles and acyclic complexes using suitable competing ligands via ligand-induced supramolecular transformations.

Trinuclear rhenium(I)-based metallocages as anticancer agents towards human cervical cancer cells

The first examples of spherical-shaped trinuclear rhenium(I) organometallic cages displaying cytotoxic, antimetastatic, antiproliferative and DNA-damaging behavior towards a human cervical (HeLa) cancer cell line are reported. The compact design of the metallocages facilitates their interactions with biosystems leading to comparable efficiency to that of the commonly used anticancer drug cisplatin.

Self-Assembly of Rhenium(I) Double-Stranded Helicate and Mesocate from Flexible Ditopic Benzimidazolyl/Naphthanoimidazolyl N-Donor and Rigid Bis-Chelating Hydroxyphenylbenzimidazolyl N∩OH-Donor Ligands: Synthesis, Characterization, and Photophysical and B-DNA Docking Studies

The self-assembly of three rheniumtricarbonyl core-based supramolecular coordination complexes (SCCs), fac-[Re(CO)3(μ-L)(μ-L')Re(CO)3] (1-3) was carried out using Re2(CO)10, rigid bis-chelating ligand (HO∩N-Ph-N∩OH (L1) (where HO∩N = 2-hydroxyphenylbenzimidazolyl), and flexible ditopic N-donor ligands (L2 = bis(3-((1H-benzoimidazol-1-yl)methyl)-2,4,6-trimethylphenyl)methane, L3 = bis(3-((1H-naphtho[2,3-d]imidazol-1-yl)methyl)-2,4,6-trimethylphenyl)methane, L4 = bis(4-(naphtho[2,3-d]imidazol-1-yl-methyl)phenyl)methane) via a one-pot solvothermal approach. In the solid state, the dinuclear SCCs adopt heteroleptic double-stranded helicate and meso-helicate architectures. The supramolecular structures of the complexes are retained in the solution based on the 1H NMR and electrospray ionization (ESI)-mass analysis. The spectral and photophysical properties of the complexes were studied both experimentally and using time-dependent density functional theory (TDDFT) calculations. All of the supramolecules exhibited emission in both solution and solid states. Theoretical studies were conducted to determine the chemical reactivity parameters, molecular electrostatic potential surface plots, natural population, and Hirshfeld analysis for complexes 1-3. Additionally, molecular docking studies were carried out for complexes 1-3 with B-DNA.

Recent Development of Rhenium-Based Materials in the Application of Diagnosis and Tumor Therapy

Rhenium (Re) is widely used in the diagnosis and treatment of cancer due to its unique physical and chemical properties. Re has more valence electrons in its outer shell, allowing it to exist in a variety of oxidation states and to form different geometric configurations with many different ligands. The luminescence properties, lipophilicity, and cytotoxicity of complexes can be adjusted by changing the ligand of Re. This article mainly reviews the development of radionuclide 188Re in radiotherapy and some innovative applications of Re as well as the different therapeutic approaches and imaging techniques used in cancer therapy. In addition, the current application and future challenges and opportunities of Re are also discussed.

Fundamentals of Rhenium-188 Radiopharmaceutical Chemistry

The β^- emitter, rhenium-188 (¹⁸⁸Re), has long been recognized as an attractive candidate for targeted cancer radionuclide therapy (TRNT). This transition metal shares chemical similarities with its congener element technetium, whose nuclear isomer technetium-99m (^99mTc) is the current workhorse of diagnostic nuclear medicine. The differences between these two elements have a significant impact on the radiolabelling methods and should always receive critical attention. This review aims to highlight what needs to be considered to design a successful radiopharmaceutical incorporating ¹¹⁸Re. Some of the most effective strategies for preparing therapeutic radiopharmaceuticals with ¹⁸⁸Re are illustrated and rationalized using the concept of the inorganic functional group (core) and a simple ligand field theoretical model combined with a qualitative definition of frontiers orbitals. Of special interest are the Re(V) oxo and Re(V) nitrido functional groups. Suitable ligands for binding to these cores are discussed, successful clinical applications are summarized, and a prediction of viable future applications is presented. Rhenium-188 decays through the emission of a high energy beta particle (2.12 MeV max energy) and a half-life of 16.9 h. An ideal biological target would therefore be a high-capacity target site (transporters, potential gradients, tumour microenvironment) with less emphasis on saturable targets such as overexpressed receptors on smaller metastases.

Rhenium(I)-Based Neutral Coordination Cages with a Spherical Cavity for Selective Recognition of Fluoride

Neutral heteroleptic trinuclear coordination cages containing a preorganized well-defined small spherical endocavity, which is made up of electron-deficient bowl frameworks, three coordination-induced neutral polar C-H donors, and a phenyl motif, were self-assembled via a one-pot approach; the endocavity accommodates fluoride selectively in the presence of other halides.

Applications of Low-Valent Transition Metalates: Development of a Reactive Noncarbonyl Rhenium(I) Anion

Low-valent transition metalates─anionic, electronic-rich organometallic complexes─comprise a class of highly reactive chemical reagents that find integral applications in organic synthesis, small-molecule activation, transient species stabilization, and M-E bond formation, among others. The inherent reactivity of such electron-rich metal centers has necessitated the widespread use of strong backbonding ligands, particularly carbonyls, to aid in the isolation and handling of metalate reagents, albeit sometimes at the expense of partially masking their full reactivity. However, recent synthetic explorations into transition-metalate complexes devoid of archetypic back-bonding ligands have led to the discovery of highly reactive metalates capable of performing a variety of novel chemical transformations.Building on our group's long-standing interest in reactive organometallic species, a series of rational progressions in early-to-middle transition-metal chemistry ultimately led to our isolation of a rhenium(I) β-diketiminate cyclopentadienide metalate that displays exceptional reactivity. We have found this Re(I) metalate to be capable of small-molecule activation; notably, the complex reversibly binds dinitrogen in solution and can be utilized to trap N₂ for the synthesis of functionalized diazenido species. By employing isolobal analogues to N₂ (CO and RNC), we were able to thoroughly monitor the mechanism of activation and conclude that the metalate's sodium counterion plays an integral role in promoting dinitrogen activation through a novel side-on interaction. The Re(I) metalate is also used in forming a variety of M-E bonds, including a series of uncommon rhenium-tetrylene (Si, Ge, and Sn) complexes that display varying degrees of multiple bonding. These metal tetrylenes act to highlight deviations in chemical properties within the group 14 elements. Our metalate's utility also applies to metal-metal bond formation, as demonstrated through the synthesis of a heterotetrametallic rhenium-zinc dimer. In this reaction, the Re(I) metalate performs a dual role as a reductant and metalloligand to stabilize a transient Zn₂²⁺ core fragment. Finally, the metalate displays unique reactivity with uranium(III) to yield the first transition metal-actinide inverse-sandwich bonds, in this case with three rhenium fragments bound through their Cp moieties surrounding the uranium center. Notably, throughout these endeavors we demonstrate that the metalate displays reactivity at multiple locations, including directly at the rhenium metal center, at a Cp carbon, through a Cp-sandwich mode, or through reversibly bound dinitrogen.Overall, the rhenium(I) metalate described herein demonstrates utility in diverse applications: small-molecule activation, the stabilization of reduced and/or unstable species, and the formation of unconventional M-E/M-M bonds or heterometallic complexes. Moving forward, we suggest that the continued discovery of noncarbonyl, electron-rich transition-metal anions featuring new or unconventional ligands should produce additional reactive organometallic species capable of stabilizing unique structural motifs and performing novel and unusual chemical transformations.

Theoretical Prediction of Rhenium Separation from Ammonium Perrhenate by Phonon-Photon Resonance Absorption

Rhenium (Re) is an extremely rare and precious element that is mainly used in the construction of aerospace components and satellite stations. However, an efficient and simple method for preparing Re has yet to be devised. To this end, we investigated the vibrational spectrum of ammonium perrhenate (NH₄ReO₄) using the CASTEP code based on first-principles density functional theory. We assigned the infrared (IR) absorption and Raman scattering spectra for NH₄ReO₄ using a dynamic process analysis of optical branch normal modes. We examined the IR-active peaks of Re-related vibrational modes in detail and found that the typical IR peak at approximately 914 cm^-1 is due to the Re-O bond stretching. Thus, we posit that strong terahertz laser irradiation of NH₄ReO₄ at 914 cm^-1 will lead to sufficient resonance absorption to cleave its Re-O bonds. This method could potentially be used to efficiently separate Re from its oxides.

Rhenium-catalysed reactions in chemical synthesis: selected case studies

This Perspective presents and discusses a selection of examples that reinforce the enabling and distinctive reactivity provided by homogeneous rhenium catalysis in chemical synthesis. Specifically, the ability for lower oxidation...

Accessible triplet excited states in the photoisomerization of allenes with extended conjugation

A series of bidentate allene- and enyne-containing ligands have been synthesized and the photochemical properties of their rhenium(I) complexes have been studied. These complexes exhibit facile isomerization of the conjugated double bonds upon ambient light exposure. Simulations unveiled a very efficient intersystem crossing and the consequent key role of the triplet states in the observed photochemistry of these substrates upon rhenium(I) complexation.

Synthesis of tetranuclear rhenium(I) tricarbonyl metallacycles

Re(I) tricarbonyl complexes have received much attention due to their attractive photochemical, electrochemical, and biological properties. Beyond simple mononuclear complexes, multinuclear assemblies offer greater structural diversity and properties. Despite previous reports on the preparation of di-, tri-, or tetranuclear Re(I) tricarbonyl assemblies, the synthesis of these supramolecular structures remains challenging due to overall low yields or tedious purification protocols. Herein, the facile preparation and characterization of tetranuclear Re(I) tricarbonyl metallacycles with a square geometry is reported using a tetrazole-based ligand. The synthesis of the metallacycle was optimized using different metal precursors, solvents, temperatures, and reagent concentrations. Finally, the scope of suitable tetrazole-based ligands was explored to produce several tetranuclear Re(I) tricarbonyl-based metallacycles.

Crystal structure and magnetic study of the complex salt [RuCp(PTA)2-μ-CN-1κC:2κN-RuCp(PTA)2][Re(NO)Br4(EtOH)0.5(MeOH)0.5]

A new RuII-ReII complex salt, μ-cyanido-κ2 C:N-bis-[(η5-cyclo-penta-dien-yl)bis(3,5,7-tri-aza-phosphaadamantane-κP)ruthenium(II)] tetra-bromido-(ethanol/methanol-κO)nitrosylrhenate(II), [Ru(CN)(C5H5)2(C6H12N3P)4][ReBr4(NO)(CH4O)0.5(C2H6O)0.5] or [RuCp(PTA)2-μ-CN-1κC:2κ2 N-RuCp(PTA)2][Re(NO)Br4(EtOH)0.5(MeOH)0.5] (PTA = 3,5,7-tri-aza-phosphaadaman-tane) was obtained and characterized by single-crystal X-ray diffraction, elemental analysis and infrared spectroscopy. The title salt was obtained by liquid-liquid diffusion of methanol/DMSO solutions of (NBu4)[Re(NO)Br4(EtOH)] and [(PTA)2CpRu-μ-CN-1κC:2κ2 N-RuCp(PTA)2](CF3SO3). The RuII and ReII independent moieties correspond to a binuclear and mononuclear complex ion, respectively. A deep geometrical parameter analysis was performed, and no significant differences were found with earlier reports containing similar mol-ecules. The magnetic properties were investigated in the temperature range 2.0-300 K, and the complex behaves as a quasi-magnetically isolated spin doublet with weak anti-ferromagnetic inter-actions.

The role of substituted pyridine Schiff bases as ancillary ligands in the optical properties of a new series of fac-rhenium(i) tricarbonyl complexes: a theoretical view

Over the last few years, luminescent Re(i) tricarbonyl complexes have been increasingly proposed as fluorophores suitable for fluorescence microscopy to visualize biological structures and cells. In this sense, incorporating an asymmetrical pyridine Schiff base (PSB) as the ancillary ligand strongly modifies the staining and luminescent properties of Re(i) tricarbonyl complexes. In this work, we analyzed two series of Re(i) tricarbonyl complexes with their respective PSB ligands: (1) fac-[Re(CO)₃(2,2′-bpy)(PSB)]¹⁺ and (2) fac-[Re(CO)₃(4,4′-bis(ethoxycarbonyl)-2,2′-bpy)(PSB)]¹⁺, where the PSB exhibits substitutions at positions 4 or 6 in the phenolic ring with methyl or halogen substituents. Thus, we performed computational relativistic DFT and TDDFT studies to determine their optical properties. The ten complexes analyzed showed absorption in the visible light range. Furthermore, our analyses, including zero-field splitting (ZFS), allowed us to determine that the low-lying excited state locates below the ³LLCT states. Interestingly, seven of the ten analyzed complexes, whose corresponding PSB harbors an intramolecular hydrogen bond (IHB), exhibited luminescent emission that could be suitable for biological purposes: large Stokes shift, emission in the range 600–700 nm and τ in the order of 10⁻² to 10⁻³ s. Conversely, the three complexes lacking the IHB due to two halogen substituents in the corresponding PSB showed a predicted emission with the lowest triplet excited state energy entering the NIR region. The main differences in the complexes' photophysical behavior have been explained by the energy gap law and time-resolved luminescence. These results emphasize the importance of choosing suitable substituents at the 4 and 6 positions in the phenolic ring of the PSB, which determine the presence of the IHB since they modulate the luminescence properties of the Re(i) core. Therefore, this study could predict Re(i) tricarbonyl complexes' properties, considering the desired emission features for biological and other applications.

Over the last few years, luminescent Re(i) tricarbonyl complexes have been increasingly proposed as fluorophores suitable for fluorescence microscopy to visualize biological structures and cells.