Uranium precursors

The first and thus far only silsesquioxane complex of an actinide element is [Cy7Si70i2]2U (100). This colorless, nicely crystalline uranium(VI) compound is formed upon reaction of 3 with any uranium precursor, e.g., UCI4 in the presence of NEt3. In all cases oxidation of uranium to the hexavalent oxidation state is observed. The best synthetic route leading to 100 in ca. 80% yield is the reaction of 3 with uranocene as outlined in Scheme 33. 

In deeper waters the deficiency of from its uranium precursor is dramatic (Table 5.5) because this thorium isotope has a very long half hfe (c. 75 000 y) and thus particle scavenging is much more effective at removal than the ingrorvth toward secular equilibrium with Bacon and Anderson (1982) showed that depth profiles of dissolved and particulate °Th could be used to demonstrate the dynamic relationship of metal exchange between particulate and dissolve forms. They argued that the thorium-uranium isotope pair could be used as a tracer of particle removal rates for those metals that faU in the category of adsorbed in Fig. 1.3. 

Synthesis of Highly Reactive Uranium Precursors with Monomeric, Chelating, and Macrocyclic Ligands... 

Edehnann etal. recently reported the reaction of disilanol (32) with Ce [N(SiMe3)2]3 in the presence of excess pyridine to surprisingly produce the product of oxidation, heptacoordinate cerium(IV) siloxide (33), in yields of 67% (Scheme 7). Interestingly, the low oxidation state uranium precursors, (C0T)2U (COT = yjg-CgHg), and UCI4 react with... 

The isotope molybdenum-99 is produced in large quantity as the precursor to technetium-99y, a radionucleide used in numerous medical imaging procedures such as those of bone and the heart (see Medical imaging technology). The molybdenum-99 is either recovered from the fission of uranium or made from lighter Mo isotopes by neutron capture. Typically, a Mo-99 cow consists of MoO adsorbed on a lead-shielded alumina column. The TcO formed upon the decay of Mo-99 by P-decay, = 66 h, has less affinity for the column and is eluted or milked and either used directly or appropriately chemically derivatized for the particular diagnostic test (100). 

Mill tailings are another form of nuclear waste. The residue from uranium ore extraction contains radium, the precursor of short-Hved radon and its daughters. Piles of tailings must be properly covered. 

As previously stated, uranium carbides are used as nuclear fuel (145). Two of the typical reactors fueled by uranium and mixed metal carbides are thermionic, which are continually being developed for space power and propulsion systems, and high temperature gas-cooled reactors (83,146,147). In order to be used as nuclear fuel, carbide microspheres are required. These microspheres have been fabricated by a carbothermic reduction of UO and elemental carbon to form UC (148,149). In addition to these uses, the carbides are also precursors for uranium nitride based fuels. 

Bis(r78-cyclooctatetraene)uranium(IV) and its precursor, dipotassium cyclo-octatrienediide, are air and water sensitive. A knowledge of inert-atmosphere techniques is essential to conduct this synthesis successfully.7 ( Caution. Dry dipotassium cyclooctatrienediide may react explosively with air therefore it always should be handled under an inert atmosphere or in vacuum.)... 

I he atomic wcighi varies because of natural variations in the isotopic composition of the element, caused by the various isotopes having different origins - I h is the end product of the thorium decay scries, while Ph and " Pb arise Irom uranium as end products of the actinium and radium series respectively. Lead-204 has no existing natural radioactive precursors. Electronic configuration l.v 2s lfc22/j"3v 3//,3i/l"4v- 4/, 4l/" 4/ IJ5v- 5/ "5t/l"bv />-. Ionic radius Pb I.IX A. Pb 1 0.7(1 A. Metallic radius 1.7502 A. Covalent radius (ip i 1.44 A. First ionization potential 7.415 cV second. 14.17 eV. Oxidation... 

The preparation of monocyclo-octatetraenyl uranium(IV) alkoxide complexes from reaction of uranium cyclo-octatetraene precursors with alcohols has been reported [318]. A very unusual monomeric complex, U(OTeF5)6, which can be con-... 

This is a convenient, facile, and high-yield preparative route for quantitative preparation of the complexes above no special equipment is required. UI3(THF)4 crystallizes in a P2x/c space group. It is mononuclear with pentagonal-pyramidal coordination geometry around the central uranium atom. This compound is stable until 75°C, then THF molecules are removed by steps, forming UI3 at 162°C. Lewis base adducts of uranium tri-iodide, such as UI3(THF)4 above, are synthetically useful precursors for trivalent uranium chemistry (see Sec. 5.3.2) [360]. 

Schiff base and related complexes of uranium and thorium are widely described in recent literature and covered in a review [463]. Those of U(VI) have a practical use as catalytic organic oxidants [460] or as part of a polystyrene-supported chelating resin [464,465]. Among other Schiff base precursors, salicylaldehyde [466] and triethylenetetramine [464], 3-formylsalicylic acid and o-hydroxybenzylamine [465], or salicylaldehyde and l-amino-2-naphthol-4-sulfonic acid [467] were used. In the example of Schiff base complexes, kinetics of formation of U(VI) complexes and their pK values were studied [468]. 

The fact that neutrons can be detected with reasonably high efficiency and with minimal interferences from other radiations permits the practical determination of fissionable species such as isotopes of uranium and thorium by delayed neutron counting. The known delayed neutron emitter precursors are all short lived and the irradiated samples are counted with 10BF3-filled proportional counters immediately after irradiation without any separation chemistry. 

Ceramic membranes were first developed in the 1940s for uranium isotope enrichment processes. Important progress has been made since that time, mainly due to the improved knowledge of the physicochemical properties of the membrane precursors. Most CMR studies concern alumina membranes other oxides such as silica, titania, or zirconia are much less frequently mentioned. 

In order to be used as nuclear fuel, carbide microspheres are required. These microspheres have been fabricated by a carbothermic reduction of UO3 and elemental carbon to form UC. In addition to these uses, the carbides are also precursors for uranium nitride based fuels. 

In a much earlier patent, the removal of organics from exhaust gases by oxidation over a supported uranium oxide catalyst was reported by Hofer and Anderson [39]. The catalyst was 4% U3O8 supported on alumina spheres. The authors used the incipient wetness technique to impregnate alumina with uranyl nitrate solution. In this case the catalyst precursors were calcined at 700°C for 3 h to decompose the uranium salt. The use of other uranium compounds as starting materials was mentioned and these included uranyl acetate, uranium ammonium carbonate and uranyl chloride. The alumina-supported catalyst had a surface area of ca 400m g and further added components, such as copper, chromium and iron, were highlighted as efficient additives to increase activity. 

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