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Structure of the {U₁₃} polyoxo cluster U₁₃O₈Cl _x (MeO)_38-x (x = 2.3, MeO = methoxide).

ABSTRACT: The structure of a new type of polyoxo cluster complex that contains thirteen uranium atoms, {U₁₃}, is reported. The complex crystallized from methanol containing tetra-valent uranium (U^IV) with a basic organic ligand, and was characterized as di-chloridoocta-cosa-μ₂-methano-lato-octa-kis-(methano-lato)octa-μ₄-oxido-trideca-uranium, [U₁₃(CH₃O)_35.7Cl_2.3O₈] or [U₁₃(μ₄-O_oxo)₈Cl _x (MeO)_38-x ] (x = 2.3, MeO = methoxide) (I), by single-crystal X-ray diffraction. The characterized {U₁₃} polyoxo cluster complex (I) possesses a single cubic uranium polyhedron at the centre of the cluster core. To the best of our knowledge, this is the very first example of a polyoxo actinide complex that bears a single cubic polyhedron in its structure. The cubic polyhedron in I is well comparable in shape with those in bulk UO₂. The U-O bonds in the cubic polyhedron of I are, however, significantly shorter than those not only in bulk UO₂ but also in another analogue in the {U₃₈} cluster. This shortening of U-O bonds, together with BVS calculations and the overall negative charge (2-) of I, suggests that the central uranium atom in I, which forms the single cubic coordination polyhedron, is presumably oxidized to the penta-valent state (U^V) from the original tetra-valent state (U^IV). Complex I is, hence, the first example of a polyoxo cluster possessing a single cubic coordination polyhedron of U^V.

SUBMITTER: Fichter S

PROVIDER: S-EPMC8340969 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Coating system(s) will be required for Nb-silicide based alloys. Alumina forming alloys that are chemically compatible with the Nb-silicide based alloy substrate could be components of such systems. The intermetallic alloys Nb1.45Si2.7Ti2.25Al3.25Hf0.35 (MG5) and Nb1.35Si2.3Ti2.3Al3.7Hf0.35 (MG6) were studied in the cast, heat treated and isothermally oxidised conditions at 800 and 1200 °C to find out if they are αAl₂O₃ scale formers. A (Al/Si)alloy versus Nb/(Ti + Hf)alloy map, which can be considered to be a map for Multi-Principle Element or Complex Concentrated Nb-Ti-Si-Al-Hf alloys, and a [Nb/(Ti + Hf)]Nb5Si3 versus [Nb/(Ti + Hf)]alloy map were constructed making use of the alloy design methodology NICE and data from a previously studied alloy, and were used to select the alloys MG5 and MG6 that were expected (i) not to pest, (ii) to form αAl₂O₃ scale at 1200 °C, (iii) to have no solid solution, (iv) to form only hexagonal Nb₅Si₃ and (v) to have microstructures consisting of hexagonal Nb₅Si₃, Ti₅Si₃, Ti₅Si₄, TiSi silicides, and tri-aluminides and Al rich TiAl. Both alloys met the requirements (i) to (v). The alumina scale was able to self-heal at 1200 °C. Liquation in the alloy MG6 at 1200 °C was linked with the formation of a eutectic like structure and the TiAl aluminide in the cast alloy. Key to the oxidation of the alloys was the formation (i) of "composite" silicide grains in which the Nb₅Si₃ core was surrounded by the Ti₅Si₄ and TiSi silicides, and (ii) of tri-aluminides with high Al/Si ratio, particularly at 1200 °C and very low Nb/Ti ratio forming in-between the "composite" silicide grains. Both alloys met the "standard definition" of high entropy alloys (HEAs). Compared with HEAs with bcc solid solution and intermetallics, the VEC values of both the alloys were outside the range of reported values. The parameters VEC,  and  of Nb-Ti-Si-Al-Hf coating alloys and non-pesting Nb-silicide based alloys were compared and trends were established. Selection of coating alloys with possible "layered" structures was discussed and alloy compositions were proposed.

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Structure of the {U13} polyoxo cluster U13O8Cl x (MeO)38-x (x = 2.3, MeO = methoxide).

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Structure of the {U₁₃} polyoxo cluster U₁₃O₈Cl _x (MeO)_38-x (x = 2.3, MeO = methoxide).