Uranyl compounds

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Uranyl nitrate , a representative of the uranyl compounds with a typical yellow color

Uranyl compounds are compounds that contain the uranyl (VI) ion (UO 2 2+ ) and / or the uranyl (V) ion (UO 2 + ). The uranyl ion is the most common form in uranium compounds . Solid uranyl compounds usually have a yellow color, with red, orange or green color components also occurring. Known connections are e.g. B. uranyl acetate and uranyl nitrate . They are water-soluble uranium salts and are commercially available. All uranyl compounds are very toxic and radioactive .

Deposits and minerals

Autunit from a quarry on Streuberg in Bergen (Vogtland) (Germany)

Uranyl compounds occur naturally in the form of uranyl minerals in oxidized parts of uranium deposits . Frequently occurring uranyl minerals are for example:

As early as 2005, more than 200 different uranium minerals were known. The website "Mineralienatlas" currently lists (as of 2016) over 300 uranium minerals. See also: List of uranium minerals

Uranyl minerals, which contain uranium in the +4 oxidation state, can help to understand the formation of uranium deposits and the interactions between water and the respective ore paragenesis that occur at the edges of uranium-rich deposits.

Oxidation of a uranium compound in air can lead to a uranyl compound. A contamination by uranyl ions has been found on and around exercise targets consistent with DU munitions were fired. Uranyl compounds are also released into the environment through leaks from containers for uranium hexafluoride . The uranium hexafluoride hydrolyzes, among other things, to uranyl fluoride and, with the components contained in the soil, forms other uranyl compounds in various compositions.

structure

Simplified representation of the uranyl ion. The configuration of two short U – O bonds prevents the closer approach of a third oxygen atom .

The geometric structure of the UO 2 2+ ion is linear. With these trans -oxo groups, the uranyl (VI) cation differs fundamentally from the oxo cations of group VI, molybdyl ( Mo O 2 2+ ) and tungstyl ( W O 2 2+ ), whose oxygen atoms are in cis - Configuration, d. H. angled, stand to each other. Denning was able to show in 2007 that the bonds between the oxygen atoms and the uranium atom, with a bond length of approx. 179  pm, formally correspond to a triple bond .

The uranyl (VI) ion showing the three-fold U – O bond ratio.

Due to the linear geometry of the UO 2 2+ cation, further ligands (L) can only be arranged in the equatorial plane . With natural uranyl minerals they are often at a distance of approx. 240 pm and are therefore considerably further away from the uranium atom than the uranyl oxygen atoms. Thus, as shown in the figure on the right, binding of a third oxidic oxygen atom is also excluded. This trans arrangement therefore means that the coordination geometry of the uranyl ion in complexes is either trigonal-bipyramidal, square-bipyramidal (octahedral), pentagonal-bipyramidal or hexagonal-bipyramidal. Natural examples of this coordination geometry are the minerals autunit (octahedral), oursinite (pentagonal-bipyramidal) and studtite (hexagonal-bipyramidal).

Coordination geometries of the uranyl ion (L = ligand)

In spite of this formal triple bond, the "classic" Lewis formula is used in this article, as in the specialist literature, as [O = U = O] 2+ .

properties

Uranium chemistry traditionally has to do with the liquid chemistry of the uranyl ion and is related to molecular chemistry. Uranium chemistry provides an important benefit in the production of uranium dioxide , which is used in the form of fuel pellets in light water reactors . The fissile material often breaks down chemically before it is used up.

Aqueous chemistry

Hydrolysis of the uranyl (VI) ion depending on the pH value

The aqueous chemistry of uranium is determined by the doubly positively charged uranyl (VI) ion UO 2 2+ ; the uranyl (V) ion UO 2 + is unstable in aqueous solution and breaks down to uranyl (VI) and uranium (IV). The uranyl (VI) cation can be viewed as the product of the hydrolysis of the hypothetical, sixfold positively charged uranium (VI) U 6+ ion, which can be formulated as follows:

[U (H 2 O) n ] 6+ → [UO 2 (H 2 O) 4 ] 2+ + 4 H + + ( n  -4) H 2 O

The driving force of this reaction is a decrease in the charge density on the uranium atom. This can also be seen in the number and type of equatorially coordinated ligands. Theoretical investigations show the relationship between charge density and bond lengths in uranyl (VI) complexes. With the coordination of stronger Lewis bases in the equatorial position, the positive charge density on the uranium atom is reduced and the bond length of the uranium atom to the axial uranyl oxygen atoms becomes longer. The stability with regard to the equatorially bound ligands increases in the following series: H 2 O <Cl - <F - <OH - <CO 3 2− <O 2 2− . The underlying physical interactions for the ligands aqua (H 2 O), tetrafluoro (F - ) 4 , tetrachloro (Cl - ) 4 and tetrahydroxo (OH - ) 4 are electrostatic in nature, whereas peroxo (O 2 2− ) and carbonato (CO 3 2− ) show clear covalences . The number of equatorially bound water molecules on the uranyl (VI) ion is often five. The charge density is reduced by further hydrolysis, for example when an H 2 O ligand gives up a proton and becomes an OH - ligand:

[UO 2 (H 2 O) 4 ] 2+ → [UO 2 (H 2 O) 3 (OH)] + + H + ; p K S = about 4.2

In aqueous solution the uranyl (VI) ion can therefore be described as a weak acid.

With increasing pH , more polymeric ions form with the stoichiometry [(UO 2 ) 2 (OH) 2 ] 2+ and [(UO 2 ) 3 (OH) 5 ] + , before uranyl hydroxide [UO 2 (OH) 2 ] fails.

The uranyl (VI) ion can be reduced to the green uranium (IV) ion U 4+ using suitable reducing agents in the aqueous medium . The further reduction to uranium (III) ion U 3+ can take place by reducing uranium (IV) ions with zinc amalgam ; however, it is very easily oxidized in aqueous solution and is only stable in completely oxygen-free solutions.

Aqueous complex chemistry

Carbonato and hydroxo complexes of uranyl (VI) depending on the pH value

The Uranyl (VI) ion behaves in the sense of HSAB principle as the hard acid and forms weaker bonds to the weaker ligand (eg. Nitrogen bases ) than with hard ligands, such as fluoride  (F - ) or oxygen-containing bases, such as oxide  (O 2− ), hydroxide  (OH - ), carbonate  (CO 3 2− ), nitrate  (NO 3 - ), sulfate  (SO 4 2− ), phosphate  (PO 4 3− ) or carboxylate  (R – COO - ). Due to the good solubility of some uranyl carbonato complexes, the uranyl ion can be easily separated from other accompanying heavy metal ions, whereas uranyl phosphate complexes are not very soluble. Uranyl (VI) phosphates therefore also form an important group in uranium minerals (e.g. autunite , torbernite , uranocircite, etc.). Other uranium minerals consist of uranyl arsenate complexes (e.g. zeunerite ) and even more complex uranyl hydroxide phosphate complexes (e.g. renardite ). The important mineral carnotite should also be mentioned, which consists of a uranyl-vanadate complex of potassium.

Examples of basic uranyl (VI) complexes in aqueous solution:

  • UO 2 (OH) 2
  • UO 2 (CO 3 ) 2 2−
  • UO 2 (CO 3 ) 3 4−
  • UO 2 (OH) 4 2− , isolated as [Co (NH 3 ) 6 ] 2 [UO 2 (OH) 4 ] 3  · x H 2 O

biochemistry

Mushrooms

Uranyl compounds also play a role in biochemistry. With the fungi Aspergillus niger and Paecilomyces javanicus , for example, X-ray powder diffractometry could show that if they grow on a medium with an organic phosphate source (here: glycerol-2-phosphate (G2P)), these uranyl phosphates metaankoleit [(K 1,7 Ba 0.2 ) (UO 2 ) 2 (PO 4 ) 2  · 6 H 2 O or K (UO 2 ) (PO 4 ) · 3 H 2 O)], uramphite (NH 4 UO 2 PO 4  · 3 H 2 O), water-containing uranyl phosphate [(UO 2 ) 3 (PO 4 ) 2  · 4 H 2 O], water-containing potassium uranyl phosphate (KUO 2 PO 4  · 3 H 2 O) and chernikovite [(H 3 O) 2 (UO 2 ) 2 (PO 4 ) 2  · 6 H 2 O]. Likewise, the acidophilic fungus Coniochaeta fodinicola is able to bind uranium in the form of uranyl (VI) ions from the water of a uranium mine.

bacteria

An Citrobacter bacteria could be shown that they are able to absorb many times their own weight of uranyl and as an acidic uranyl phosphate (HUO 2 PO 4 ) at the cell surface by biomineralization to deposit. Several other types of bacteria are able to enzymatically reduce uranyl (VI). The mesophilic genera Geobacter , Shewanella , and Desulfotomaculum are able to gain energy for their growth from the reduction of uranyl (VI). Other bacteria, for example the thermophilic bacteria Thermus scotoductus , Pyrobaculum islandicum and Thermoanaerobacter sp. , can reduce soluble uranyl (VI) compounds, but this does not serve to generate energy. The thermophilic bacterium Thermoterrabacterium ferrireducens was able to show that it can also metabolize insoluble U (VI) compounds.

Uranyl (V)

Crystal structure of the diisobutylaluminum uranyl (V) Pacman complex

The one-electron reduction of the uranyl (VI) -dication (UO 2 2+ ) leads to the representation of the uranyl (V) -monocation (UO 2 + ). This is unstable in aqueous solution and disproportionate to U 4+ and UO 2 2+ :

UO 2 + + 4 H + → U 4+ + UO 2 2+ + 2 H 2 O

The synthetic preparation of uranyl (V) complexes was therefore not possible for a long time. In 2003, however, the first uranyl (V) compound was presented and examined by X-ray crystallography . Berthet et al. synthesized by chance the cation [UO 2 (OPPh 3 ) 4 ] + (Ph = phenyl) in the form of the triflate salt. The uranyl (V) ion differs from uranyl (VI) in its longer U – O bonds and a higher Lewis basicity of the uranyl oxygen atoms, due to the lower charge density on the uranium atom. Its electronic configuration is therefore [Rn] 5 f 1 , i.e. H. it has a single f -electron and is therefore paramagnetic . Since then, various ligand systems have been tested in which uranyl (V) complexes can be prepared in a targeted manner, with the exclusion of atmospheric oxygen and moisture. The increased basicity of the uranyl oxygen atoms often leads to the coordination of heteroatoms , for example in a polymeric uranyl (V) complex {[UO 2 py 5 ] [KI 2 py 2 ]} n (py = pyridine), its equatorial Level of pyridine ligands and its uranyl oxygen atoms are coordinated by K + ions. Similarly, the one-electron reduction of uranyl (VI) -Pacman complexes can be carried out with the help of diisobutylaluminum hydride , so that a uranyl (V) -aluminum (III) complex is formed in which the aluminum atom is directly bound to the uranyl oxygen .

Uranyl compounds

The following uranyl (VI) compounds are known, among others:

Uranyl nitrate and uranyl acetate are soluble uranium salts. These salts are commercially available and have the same toxicity as other heavy metal nitrates and acetates.

synthesis

Laboratory synthesis of uranyl (VI) compounds can be done in several ways, e.g. B. the oxidation of UCl 4 at 300-350 ° C to UO 2 Cl 2 . However, it has proven to be useful to convert the inexpensive uranyl salts such as uranyl acetate, which are often found in old chemical stocks, into corresponding hydrolysis- and air-sensitive uranyl halides (UO 2 X 2 (X = (F), Cl, Br, I)), uranyl triflate (UO 2 (OTf) 2 ) or uranylbis (hexamethyldislilyamide) (UO 2 {N (Si (CH 3 ) 3 ) 2 } 2 ).

The synthesis of uranyl chloride from uranyl acetate takes place, for example, by stirring the acetate in an excess (exc. ​​= Excess) of concentrated hydrochloric acid in a stream of nitrogen to dryness. The crude product is dissolved in tetrahydrofuran (THF) and an excess of chlorotrimethylsilane (TMSCl) is added to remove residual water. The solvent is then removed, the product is dried in vacuo and stored under a nitrogen atmosphere.

UO 2 (CH 3 COO) 2 + exc. HCl (aq) → UO 2 Cl 2 (aq) 3 + 2 CH 3 COOH
UO 2 Cl 2 (aq) 3 + exc. THF + exc. TMSCl → UO 2 Cl 2 (thf) 3 + 6 HCl + 3 (TMS) 2 O

Spectroscopy

Uranyl nitrate under ordinary light (above) and under UV light (below)

Infrared spectroscopy

The uranyl ion is characterized by typical IR bands that lie between 920 and 980 cm −1 and are assigned to the asymmetric O = U = O stretching vibration. The symmetrical stretching vibration can be found with the help of Raman spectroscopy at 860 cm −1 . In uranyl (V) complexes these are shifted to lower wavenumbers (897 to 912 cm −1 ). The analysis of these vibrations is an important aid in the characterization of uranyl complexes, as their values ​​are antiproportional to the donor strength of the equatorial ligands.

Electromagnetic Spectroscopy

The uranyl (V) ion is, due to its single f -electron, suitable to be investigated with the help of electron spin resonance (ESR). Likewise, this free electron in uranyl (V) complexes creates a paramagnetic shift in the resonances in NMR spectroscopy .

The magnetic properties of uranyl (V) complexes can also be determined by SQUID magnetometry.

Optical spectroscopy

The uranyl (VI) ion has a characteristic absorption band at 25,000 cm −1 (400 nm), which corresponds to blue-violet light. As a result, many uranium compounds appear with the complementary color yellow. Frequently uranyl complexes also have a bright green fluorescence under UV light , which can also be used as an analytical tool by means of laser-fluorescence techniques, such as in the TRLFS ( T IME R esolved L aser-Induced F luorescence S pectroscopy). Due to its free f -electron, the uranyl (V) ion has the basic term 2 F, which is further split into 2 F 7/2 and 2 F 5/2 , so that four transitions are often observed in the electromagnetic and near-infrared spectrum can.

use

Laboratory applications

Soluble uranyl salts such as uranyl acetate are used for negative contrasting in transmission electron microscopy (TEM). After spreading a DNA molecule, this can also be made visible in the TEM using uranyl acetate. A solution of uranyl acetate in glacial acetic acid and a solution of magnesium acetate in glacial acetic acid are used in combined form for the detection of sodium . Pale yellow octahedra or dodecahedra with a rhombic structure are formed. It is the sparingly soluble compound magnesum sodium triuranyl nonaacetate [MgNa (UO 2 ) 3 (CH 3 COO) 9  · 9 H 2 O].

Industrial applications

An industrially important complex is uranyl nitrate, [UO 2 (NO 3 ) 2 ] · 2 H 2 O, which coordinates six donor atoms in the equatorial plane, four of which are oxygen atoms from the two bidentate nitrate ions and two oxygen atoms from the two coordinated water molecules. The uranium atom therefore has a hexagonal-bipyramidal coordination geometry . Charge-neutral complexes such as uranyl nitrate can also be extracted into organic solvents such as diethyl ether . The previously coordinated water molecules are displaced by diethyl ether molecules. The complex becomes hydrophobic and changes into the organic phase. Since nitrate forms stronger complexes with the actinides than with the lanthanoids and transition metals , this synergy effect is used in the processing of nuclear fuel by extracting uranyl together with plutonyl (VI) (PuO 2 2+ ).

Specifically, in the processing of nuclear fuel, the uranyl nitrate is extracted with tributyl phosphate (TBP, (CH 3 CH 2 CH 2 CH 2 O) 3 PO) as the ligand and kerosene as the solvent. In a later step, treatment with nitric acid results in the nitrato complex [UO 2 (NO 3 ) 4 ] 2− , which is more soluble in water and can thus be recovered. The uranyl nitrate is obtained as a solid by evaporating the water.

Health and environmental hazards

Uranyl compounds
GHS
labeling
06 - Toxic or very toxic 08 - Dangerous to health 09 - Dangerous for the environment
danger
H and P phrases 330-300-373-411
?

Uranyl compounds are highly toxic compounds and should not get into the body. They cause severe kidney damage and kill the cells of the kidney canals (tubules) through which the primary urine flows. They can also cause leukemia . Usually the kidneys, liver, lungs and brain are damaged. Accumulations of uranyl ions in human tissue, including the germ cells, cause hereditary diseases and are the cause of diseases of the immune system in white blood cells . Uranyl compounds are also powerful neurotoxins .

All uranyl compounds are radioactive . The activity depends on the isotopic composition of the uranium. The toxicity of soluble uranyl salts is proportional to the rate at which they are absorbed into human tissue when incorporated.

Individual evidence

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  3. ↑ PC Burns: U 6+ Minerals and Inorganic Compounds: Insights into an expanded structural hierarchy of crystal structures In: The Canadian Mineralogist , 2005, Volume 43, pp. 1839-1894. (Abstract)
  4. ↑ Search for minerals on Mineralienatlas.de
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  7. ↑ Assessment of the Public Health Impact From the Accidental Release of UF 6 at the Sequoyah Fuels Corporation Facility at Gore, Oklahoma ( Memento of the original from September 15, 2006 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 3.1 MB); Docket No. 40-8027. License No. SUB-1010. Manuscript Completed: March 1986; Date Published: March 1986. Ad Hoc Interagency Public Health Assessment Task Force, US Nuclear Regulatory Commission; Washington, DC 20555. @1@ 2Template: Webachiv / IABot / www.gl.iit.edu
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literature

  • Ingmar Grenthe, Janusz Drożdżynński, Takeo Fujino, Edgar C. Buck, Thomas E. Albrecht-Schmitt, Stephen F. Wolf: Uranium. In: Lester R. Morss, Norman M. Edelstein, Jean Fuger (Eds.): The Chemistry of the Actinide and Transactinide Elements. Springer, Dordrecht 2006, ISBN 1-4020-3555-1 , pp. 253-698, doi: 10.1007 / 1-4020-3598-5_5 .

Web links

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