Synthesis of Technetium Carboxylates: Wheel-Like Octanuclear Clusters (Tc8(µ-O)8(RCOO)16, Where R = CF3, C6H5)─Potential Nanobuilding Units for Tc-MOFs.

Herein, we studied the behavior of TcO4- in trifluoroacetic anhydride (TFAA) under visible light irradiation in situ by UV-vis spectroscopy. One carboxylate of Tc(VII) C2F3O5Tc (1) and two wheel-like carboxylate clusters of Tc(IV) Tc8(µ-O)8(CF3COO)16 (2, 3) and Tc8(µ-O)8(C6H5COO)16 (4) were synthesized and analyzed using pXRD, TGA, UV-vis spectroscopy, and SCXRD techniques. According to SCXRD, it was found that Tc(IV) trifluoroacetate exists in two crystalline modifications. By UV-vis spectroscopy and DFT calculations, it was shown that the primary compound in the reaction system is trifluoroacetate Tc(VII). A technetium trifluoroacetate(VII) and Tc intermediates of unidentified nature both show photosensitivity. The influence of intermolecular noncovalent interactions on the volatility of trifluoroacetate and benzoate Tc(IV) is shown. The main regularities of chemical transformations of technetium in nonaqueous solutions of carboxylates have been revealed. The obtained data on the kinetics of the process suggest that technetium in trifluoroacetic anhydride can simultaneously exist in the form of Tc(VII), Tc(VI), Tc(V), and Tc(IV). Under laser ionization or prolonged heating, the formation of the Tc(II,III)-cluster is observed.

Influence of the organic cation on the formation of hexahalotechnetates: X-ray, thermal and comparative analyses of non-covalent interactions.

In this work, we have reviewed non-covalent interactions in technetium hexahalide compounds and obtained eight new compounds of the CatnTcHal6 type, where Cat = dimethylammonium, tetramethylammonium, caffeinium, benzothiazolium, nicotinamidium, and pyrazolium, and Hal = Cl, Br. SCXRD studies were carried out for new compounds. In some compounds, halide anions and/or crystallization water were present. In the compounds obtained, an essential influence on the formation of structures and crystal packing is exerted by the molecules of crystallization water and halide ions. Diethylammonium and nicotinamidium compounds, whose structures do not contain other ions and contain sufficiently strong non-covalent interactions, best bind hexahalotechnetates. π-Stacking interactions, anion-π interactions, and halogen bonds were found in the structures. The percentage contribution of the H⋯Hal/Hal⋯H interactions in the transition from fluorine to bromine in TcHal62- anions decreases, while the contribution of interactions of other types increases. The greatest variety of interactions in anions is observed for compounds of caffeinium and nicotinamidium with TcBr62-. The paper considers the processes of thermolysis of some new and previously known CatnTcHal6 compounds with various cations. It is shown that the thermal stability of the compounds is only due to the properties of the organic cation and does not depend on the nature of the halogen. The proposed stages of the process of thermolysis of the TcHal62- anion, accompanied by the reduction of technetium to metal, have been established.

Route to Stabilization of Nanotechnetium in an Amorphous Carbon Matrix: Preparative Methods, XAFS Evidence, and Electrochemical Studies.

Technetium-carbon nanophases are obtained by thermal decomposition of pertechnetates with large organic cations under an argon atmosphere. Parallel carbonization of organic cations (hexamethyleneiminium and triphenylguanidinium), which occurs during the thermal decomposition of their pertechnetates, leads to the formation of X-ray amorphous solid products. An X-ray absorption fine structure study revealed that they have a crystal structure containing technetium-carbon bonds with a length of 1.76 Å. After subsequent annealing treatment at 1073-1673 K, the synthesized technetium-carbon phase has a cubic lattice with an a of 4.01 ± 0.03 Å. The products of thermal decomposition of the same perrhenates are also X-ray amorphous; however, unlike that of pertechnetates, the distance between rhenium and carbon atoms in them is significantly greater (2.14 Å). After subsequent annealing, they have a hexagonal lattice. The electrochemical properties of technetium-carbon nanophases prepared by thermal decomposition of pertechnetates with large organic cations are different from the properties of those prepared with metallic technetium. The oxidation of technetium carbide to its oxides at the electrode surface observed in the first anodic scan of cyclic voltammograms can be used for the deposition of noble metal nanoclusters under open-circuit conditions to prepare composite catalysts for the hydrogen evolution reaction. Nanotechnetium in the amorphous carbon matrix can also be a prospective material for reactor transmutation of technetium to stable isotopically pure ruthenium-100.

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.

New Preparative Approach to Purer Technetium-99 Samples-Tetramethylammonium Pertechnetate: Deep Understanding and Application of Crystal Structure, Solubility, and Its Conversion to Technetium Zero Valent Matrix.

99Tc is one of the predominant fission products of 235U and an important component of nuclear industry wastes. The long half-life and specific activity of 99Tc (212,000 y, 0.63 GBq g-1) makes Tc a hazardous material. Two principal ways were proposed for its disposal, namely, long-term storage and transmutation. Conversion to metal-like technetium matrices is highly desirable for both cases and for the second one the reasonably high Tc purity was important too. Tetramethylammonium pertechnetate (TMAP) was proposed here as a prospective precursor for matrix manufacture. It provided with very high decontamination factors from actinides (that is imperative for transmutation) by means of recrystallisation and it was based on the precise data on TMAP solubility and thermodynamics accomplished in the temperature range of 3-68 °C. The structure of solid pertechnetates were re-estimated with precise X-ray structure solution and compared to its Re and Cl analogues and tetrabutylammonium analogue as well. Differential thermal and evolved gas analysis in a flow of Ar-5% H2 gas mixture showed that the major products of thermolysis were pure metallic technetium in solid matrix, trimethylammonium, carbon dioxide, and water in gas phase. High decontamination factors have been achieved when TMAP was used as an intermediate precursor for Tc.

Thiourea as a Stabilizer of Reduced Forms of Technetium─Tc(III) and Tc(IV): Experimental and Theoretical Studies of Complexes.

This paper presents synthetic methods for the preparation of Tc(III) and Tc(IV) coordination compounds with thiourea. We have shown that the main product of the synthesis is the complex [TcTu5X]X2, (Tu = (NH2)2CS, X = Cl, Br) and not [TcTu6]Cl3·4H2O, as previously thought. Tu2[TcX6]X2·3H2O is the main technetium-containing byproduct of the reaction. All reaction products, including byproducts, were characterized by X-ray diffraction analysis. We also measured the solubility for the obtained Tc(III) complexes. This research work considers the process of thermolysis of the obtained Tc(III) complexes and shows that the presence of sulfur atoms in the coordination sphere can inhibit the process of metal formation in an argon-hydrogen medium. The analysis of nonvalent interactions in Tc(III) complexes showed that the main contribution is made by van der Waals interactions of the H···H type (40.8-42.3%) and hydrogen bonds Hal···H/H···Hal and H···S/S···H, which are 41.6-44.5% in total. As the temperature decreases, the proportion of H···H contacts and H bonds decreases, and when the halogen (Cl by Br) is replaced, the proportion of H bonds increases and the proportion of van der Waals interactions decreases.

Chemical Oxidative Polymerization of Methylene Blue: Reaction Mechanism and Aspects of Chain Structure.

The kinetic regularities of the initial stage of chemical oxidative polymerization of methylene blue under the action of ammonium peroxodisulfate in an aqueous medium have been established by the method of potentiometry. It was shown that the methylene blue polymerization mechanism includes the stages of chain initiation and growth. It was found that the rate of the initial stage of the reaction obeys the kinetic equation of the first order with the activation energy 49 kJ × mol-1. Based on the proposed mechanism of oxidative polymerization of methylene blue and the data of MALDI, EPR, and IR spectroscopy methods, the structure of the polymethylene blue chain is proposed. It has been shown that polymethylene blue has a metallic luster, and its electrical conductivity is probably the result of conjugation over extended chain sections and the formation of charge transfer complexes. It was found that polymethylene blue is resistant to heating up to a temperature of 440 K and then enters into exothermic transformations without significant weight loss. When the temperature rises above 480 K, polymethylene blue is subject to endothermic degradation and retains 75% of its mass up to 1000 K.