Technetium and rhenium in five-coordinate symmetrical and dissymmetrical nitrido complexes with alkyl phosphino-thiol ligands. Synthesis and structural characterization

Luminescence Color and Intensity Changes of Nitridorhenium(V) Complexes Induced by Protonation/Deprotonation on the Bidentate Azolylpyridine Ligands

Tricyanidonitridorhenium(V) complexes with azolylpyridines, namely, [ReN(CN)3(H-N2py)]- (1-H, H-N2py = 2-(3-pyrazolyl)pyridine) and [ReN(CN)3(L)]2- (2-a, L = 2-[1,2,3]-triazol-4-yl-pyridine anion (N3py-), and 3-a, that is, L = 2-(tetrazol-5-yl)-pyridine anion (N4py-)), were newly synthesized and characterized. The structures of the new complexes were determined by single-crystal X-ray analysis. The 1-H complex includes two geometrical isomers in which an isomer is the conformation with the pyridyl (py) and pyrazolyl (pyrz) moieties of H-N2py occupying the trans site to the nitrido (the ax site) and the trans site to the cyanido (the eq site), respectively, in a bidentate fashion; the other isomer is the py and pyrz moieties coordinated to the eq and ax sites. In 2-a and 3-a, the triazolyl (trz) and tetrazoly (tetrz) moieties in N3py- and N4py- occupy the eq site, and the py moieties in N3py- and N4py- coordinate to the ax site. The complex 1-H is deprotonated upon the addition of 1,8-diazabicyclo[5.4.0]undec-7-one or NaOH to produce [ReN(CN)3(N2py)]2- (1-a), and 2-a is protonated upon the addition of p-toluene sulfonic acid (TsOH) to give [ReN(CN)3(H-N3py)]- (2-H) in DMSO. The protonation reaction does not occur for 3-a with TsOH in DMSO. All the complexes show one-electron redox waves of the Re(VI)/Re(V) and azolylpyridine ligand-centered processes in 0.1 M (n-C4H9)4NPF6-DMSO. All the complexes exhibit photoluminescence in DMSO and in the crystalline phase at 296 K. The emissive excited states of the complexes in DMSO were assigned to MLCT with a spin triplet nature. The emission band shifts to shorter and longer wavelengths upon protonation and deprotonation of the coordinated azolylpyridines, respectively. The emission color and intensity changes of 2-H and 2-a in the presence of acidic and basic vapors were investigated.

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.

Synthesis, Structures, and Photoluminescent Properties of Tricyanidonitridorhenium(V) Complexes with Bipyridine-Type Ligands

Tricyanidonitridorhenium(V) complexes with 2,2'-bipyridine (bpy) derivatives in which the 4 and 4' positions were substituted by X, [ReN(CN)₃(X₂bpy)]^- (X = NMe₂, NH₂, OMe, Me, Cl, and Br), were newly synthesized and characterized. The structures of the new complexes were determined by single-crystal X-ray analysis. UV-vis spectra of the complexes in dimethyl sulfoxide (DMSO) showed that the peak maximum wavelengths of rhenium-to-π* bpy-type-ligand charge transfer were in the range of 474-542 nm. Cyclic voltammograms in n-(C₄H₉)₄NPF₆-DMSO showed one-electron oxidation and reduction waves corresponding to the Re(VI/V) and X₂bpy^0/- processes, respectively. The new complexes and [ReN(CN)₃bpy]^- showed photoluminescence in the crystalline phase at 295 and 80 K and in DMSO at 295 K. The origin of the emission in DMSO was attributed to the triplet nature of the rhenium-to-π* bpy-type-ligand charge-transfer transition. Density functional theory calculations showed that the highest occupied and lowest unoccupied molecular orbitals were primarily localized on the d_xy orbital of the rhenium and π* orbitals of the bpy-type ligand, respectively.

Nitrido Technetium-99 m Core in Radiopharmaceutical Applications: Four Decades of Research

The knowledge on element 43 (Tc) of the periodic table, built over the years through the contributions given by the close relationship between chemistry and nuclear medicine, allowed the development of new and increasingly effective radiopharmaceuticals useful both as perfusion and target specific imaging agents for SPECT (single photon emission tomography). Among the manifold Tc-compounds, Tc(V) nitrido complexes played a relevant role in the search for new technetium-99m radiopharmaceuticals, providing efficient labeling procedures that can be conveniently exploited for the design and synthesis of agents, also incorporating small organic molecules or peptides having defined structural features. With this work, we present an overview of four decades of research on the chemistry and on the nuclear medicine applications of Tc(V) nitrido complexes.

Metal complexes containing natural and and artificial radioactive elements and their applications

Recent advances (during the 2007–2014 period) in the coordination and organometallic chemistry of compounds containing natural and artificially prepared radionuclides (actinides and technetium), are reviewed. Radioactive isotopes of naturally stable elements are not included for discussion in this work. Actinide and technetium complexes with O-, N-, N,O, N,S-, P-containing ligands, as well π-organometallics are discussed from the view point of their synthesis, properties, and main applications. On the basis of their properties, several mono-, bi-, tri-, tetra- or polydentate ligands have been designed for specific recognition of some particular radionuclides, and can be used in the processes of nuclear waste remediation, i.e., recycling of nuclear fuel and the separation of actinides and fission products from waste solutions or for analytical determination of actinides in solutions; actinide metal complexes are also usefulas catalysts forcoupling gaseous carbon monoxide, as well as antimicrobial and anti-fungi agents due to their biological activity. Radioactive labeling based on the short-lived metastable nuclide technetium-99m (^99mTc) for biomedical use as heart, lung, kidney, bone, brain, liver or cancer imaging agents is also discussed. Finally, the promising applications of technetium labeling of nanomaterials, with potential applications as drug transport and delivery vehicles, radiotherapeutic agents or radiotracers for monitoring metabolic pathways, are also described.

Excited-state characteristics of tetracyanidonitridorhenium(V) and -technetium(V) complexes with N-heteroaromatic ligands

Six-coordinate tetracyanidonitridorhenium(V) and -technetium(V) with axial N-heteroaromatic ligands, (PPh4)2[MN(CN)4L] [M = Re, L = 4-(dimethylamino)pyridine (dmap), 3,5-lutidine (lut), 4-picoline (pic), 4-phenylpyridine (ppy), pyridine (py), 3-benzoylpyridine (3bzpy), 4,4'-bipyridine (bpy), pyrazine (pz), 4-cyanopyridine (cpy), or 4-benzoylpyridine (4bzpy); M = Tc, L = dmap, lut, pic, py, pz, or cpy] were synthesized and characterized. The crystal structures of 11 complexes were determined by single-crystal X-ray analysis. All of the complexes showed photoluminescence in the crystalline phase at room temperature. The emission maximum wavelengths (λ(em)) of the rhenium complexes with dmap, lut, pic, ppy, or py were similar to one another with a quite high emission quantum yield (Φ(em)): λ(em) = 539-545 nm, Φ(em) = 0.39-0.93, and emission lifetime (τ(em)) = 10-45 μs at 296 K. The emission spectra at 77 K exhibited vibronic progressions, and the emissive excited state is characterized as (3)[(d(xy))(1)(dπ*)(1)] (dπ* = d(xz), d(yz)). On the other hand, the emission maximum wavelength of the rhenium complex with 3bzpy, bpy, pz, cpy, or 4bzpy was significantly dependent on the nature of the axial ligand in the crystalline phase: λ(em) = 564-669 nm, Φ(em) ≤ 0.01-0.36, and τ(em) = 0.03-13.3 μs at 296 K. The emission spectra at 77 K in the crystalline phase did not show vibronic progressions. The emissive excited state of the rhenium complex with bpy, pz, cpy, or 4bzpy is assignable to originate from the metal-to-N-heteroaromatic ligand charge-transfer (MLCT)-type emission with a spin-triplet type. The change in the excited-state characteristics of rhenium complexes by the N-heteroaromatic ligand is a result of stabilization of the π* orbital of the N- heteroaromatic ligand to a lower energy level than the dπ* orbitals. The emission spectral shapes of technetium complexes were almost independent of the nature of the N-heteroaromatic ligand with λ(em) = 574-581 nm at room temperature. The different emission characteristics between the pz and cpy coordinate rhenium complexes and the technetium analogues would be due to stabilization of technetium-centered orbitals compared with the rhenium ones in energy.

Dithiocarbamate complexes as radiopharmaceuticals for medical imaging

Over the past 30 years dithiocarbamate ligands have found application in radiopharmaceutical metal-ligand complexes to image a range of disease states. The vast majority of research and applications, and the widest range of complex structures, have involved radionuclides of technetium and rhenium. Considering the extent of coordination chemistry of dithiocarbamate ligands described elsewhere in this issue, the extent of radiopharmaceutical application with metallic radionuclides is surprisingly narrow. Here we summarise the types of radiopharmaceutical complexes studied and the uses, and potential uses, to which they have been put in nuclear medicine.