Pivotal role of 99Tc NMR spectroscopy in solid-state and molecular chemistry

The radioelement Technetium (element 43) pertains to various domains including the nuclear enterprise (i.e., spent nuclear fuel (SNF) reprocessing and nuclear waste remediation) and nuclear medicine (i.e., development of new imaging agents) as well as to the fundamental science of transition metals (i.e., chemical trends in catalytic properties). One method that can provide critical information to improve knowledge in these domains is 99Tc nuclear magnetic resonance (NMR) spectroscopy. The review, presented here, summarizes the pivotal role of 99Tc NMR spectroscopy over the past two decades and presents prospects of the method to tackle challenges in Tc chemistry.

Intramolecular Re···O Nonvalent Interactions as a Stabilizer of the Polyoxorhenate(VII)

The first polyoxorhenate(VII) compound, pyrazolium polyoxorhenate ((C4N2H5)2Re4O15), and two new rhenium(VII) and technetium(VII) salts have been synthesized and studied. The structure of Tc2O7 has been reinvestigated. The [Re4O15]2- polyoxoanion contains four Re(VII) atoms: one with an octahedral environment and three with a tetrahedral environment. Polyoxorhenate is formed in the presence of a buffering agent, pyrazole, the latter maintaining pH = 2.5 during the formation of crystals. The [Re4O15]2- polyoxoanion has novel stoichiometry and the cis-conformation, likely due to the stabilizing intramolecular nonvalence interactions. For the first time, intramolecular interactions of the Re···O, Re···μ-O, and O···O are described (previously known were only intermolecular ones). In all of the compounds, intermolecular Re···O interactions are observed, which, however, in other compounds, do not lead to the formation of polyoxometalates. The Hirshfeld surface analysis showed that the main contribution to intermolecular interactions is made by the O···H/H···O contacts, van der Waals interactions of the H···H for cations, and the O···O for anions. DFT calculations of the [Re4O15]2- geometry, compared with the crystallographic data, revealed a deviation in the angles. Mass spectroscopy of the red polyoxometalate [Tc20O68]4- was carried out for the first time. Comparison of the results of MALDI and LI for the first known polyoxometalates of the manganese subgroup made it possible to find general patterns of oligomerization for rhenium and technetium compounds. The ESI-MS and LI-MS methods applied to solution and crystals Re compounds made it possible to prove rhenium being able to form not only [Re4O15]2- but also heavier polyoxoanions.

Rapid determination of 99Tc in water samples using Ti(OH)3-TcO2 co-precipitation and TK200 resin by liquid scintillation counting

A novel method for the determination of ⁹⁹Tc in water samples was developed using stable Re as a chemical yield tracer and TiCl₃ as a reducing agent. The influences of several experimental parameters, including TiCl₃ concentration, HCl concentration and reaction time, on the reduction of TcO₄^- and ReO₄^- as well as Ti(OH)₃-TcO₂-ReO₂ co-precipitation were investigated. Tc(VII) and Re(VII) retained on TK200 resin were effectively eluted by 5 mL of 1 mol/L NH₄SCN, which can be directly mixed with the scintillation cocktail for liquid scintillation counting. The results show that the chemical behaviors of Tc and Re are very consistent in the whole procedure. The decontamination factors of potential interferences from β-emitting nuclides mainly released from nuclear fuel reprocessing plants were also evaluated, and the minimum detectable activity concentration was calculated to be 0.08 Bq/L for ⁹⁹Tc in water samples with a counting time of 2 h.

A 70-Year-Old Mystery in Technetium Chemistry Explained by the New Technetium Polyoxometalate [H7 O3 ]4 [Tc20 O68 ] ⋅ 4H2 O

[H₇ O₃ ]₄ [Tc₂₀ O₆₈ ] ⋅ 4H₂ O [1] was prepared from an aqueous Tc₂ O₇ solution concentrated over anhydrous H₂ SO₄ . [Tc₂₀ O₆₈ ]^4- is the first polyanionic species to be reported for Tc. The unit cell contains one centrosymmetric [Tc₂₀ O₆₈ ]^4- polyanion as well as hydronium ions and water molecules. The core of the structure consists of four Tc(V)O₆ octahedra that form a square Tc₄ O₄ ring. The four Tc(V)O₆ octahedra are decorated by sixteen Tc(VII)O₄ tetrahedra. Calculations show the bonding within the Tc₄ O₄ ring to consist of a 3-center bond formed between each neighboring pair of Tc atoms and their bridging oxygen. Calculations also indicate that a strong d→d electronic transition at 513 nm is the origin of the red color of [1]. The characterization of red HTcO₄ solutions by X-ray absorption spectroscopy has complemented the description of this compound in aqueous solution. The formation mechanisms in solution, including the possible role of technetium's radioactivity in the formation of [1], are discussed.

Technetium Nitrido-Peroxo Complexes: An Unexplored Class of Coordination Compounds

The purpose of this work was to further expand the chemistry of mixed technetium nitrido-peroxo complexes, a still poorly explored class of compounds containing the Tc(VII) moiety, [99gTc][Tc(N)(O2)2]. A number of novel complexes of the formula [99gTc][Tc(N)(O2)2(L)] with bidentate ligands (L) (where L = deprotonated alanine, glycine, proline) were prepared by reacting a solution of nitrido-technetic(VI) acid with L in the presence of a source of H2O2. Alternatively, the complex [99gTc][Tc(N)(O2)2X]− (X = Cl, Br) was used as a precursor for substitution reactions where the halogenide ion was replaced by the bidentate ligand. The new complexes were characterized by elemental analysis and mass spectroscopy. The preparation of the analogous [99mTc][Tc(N)(O2)2] moiety, radiolabeled with the metastable isomer Tc-99m, was also studied at a no-carrier-added level, using S-methyl-N-methyl-dithiocarbazate as the donor of the nitrido nitrogen atoms.