Uranium Pentafluoride

In the following procedures, /3-uranium pentafluoride is conveniently prepared by the photochemical reduction of uranium hexafluoride, in a manner similar to an earlier smaller-scale preparation.9 Pentaethoxyuranium is prepared directly from 0-UFs and sodium ethoxide in ethanol. The preparation of hexafluoro-uranium salts from /3-UFs in nonaqueous solvents is described in a procedure that avoids the use of hydrofluoric acid common to previous methods.10,11 

A 1-L quartz flask (shown diagrammatically in Fig. 1) (the checker finds that the flasks of this size can be quite fragile. Caution should be exercised in their use) is 

STOPCOCK ADAPTER (CONNECTION TO HIGH VACUUM SYSTEM) 

Uranium pentafluoride prepared in the above maimer is the low-temperature, form, which can be identified by its characteristic X-ray powder pattern. The single rystal structure has been determined tetragonal symmetry, space group I42d(Z = 8), with eight-coordination and a geometry intermediate between a dodecahedron and a square antiprism. Uranium pentafluoride is air sensitive and disproportionates in water to give a precipitate of uranium tetrafluoride and a solution of uranyl fluoride. Consequently, UFs must be handled in an inert atmosphere. The near-infrared-visible spectrum of uranium pentafluoride in acetonitrile has been determined and is perhaps the most convenient characterization. 

---

Uranium pentafluoride [13775-07-0], UF, has been isolated under different conditions, leading to two different modifications, d and p. The former is a grayish white soHd, which is synthesized from the interaction of UF [7783-81-5] and HBr or by heating UF [10049-14-6] and UF to 80—100°C. The yellowish white P-modification is also obtained by reacting UF and UF, but at higher temperatures (150—200°C). The two different modifications of UF have both been stmcturaHy characterized. The a-form consists of infinite chains of octahedral UF units. The P-form has eight-coordinate uranium atoms with the fluorides in a geometry between dodecahedral and square antiprismatic. 

Bismuth pentafluoride is an active fluorinating agent. It reacts explosively with water to form ozone, oxygen difluoride, and a voluminous chocolate-brown precipitate, possibly a hydrated bismuth(V) oxyfluoride. A similar brown precipitate is observed when the white soHd compound bismuth oxytrifluoride [66172-91 -6] BiOF, is hydrolyzed. Upon standing, the chocolate-brown precipitate slowly undergoes reduction to yield a white bismuth(Ill) compound. At room temperature BiF reacts vigorously with iodine or sulfur above 50°C it converts paraffin oil to fluorocarbons at 150°C it fluorinates uranium tetrafluoride to uranium pentafluoride and at 180°C it converts Br2 to bromine trifluoride, BrF, and bromine pentafluoride, BrF, and chlorine to chlorine fluoride, GIF. It apparently does not react with dry oxygen. 

The pentafluorides MoFs (yellow), ReFs (green), OsFs (blue), and UFS (pale yellow-green) are extremely moisture sensitive and must be handled and stored in a dry box. The samples can be stored in Kel-F bottles. With the exception of UF5 these compounds have sufficient vapor pressure so that they can be sublimed. Uranium pentafluoride, on the other hand, is a nonvolatile solid at room temperature. The melting points for these compounds are MoFs, 65° ReFs, 47° OsFs, 70°. The infrared spectra (Nujol mull) show the following broad bands MoFs, 740, 693, 653, 520 cm"1 ReFs, 720, 691, 660, 530 cm 1 OsFs, 710, 690, 655, 530 cm 1 UFS, 620, 565, 510, 405 cm"1. More detailed spectroscopic and powder diffraction data have been summarized elsewhere.5 7... 

Pentavalent. The pentafluorides of Pa, U, and Np have been reported. The synthetic routes to obtain the pentafluorides are given in equations (11) and (12). Uranium pentafluoride, UF5, gives two different modifications that have been structurally characterized, a (grayish white solid) and (yellowish white). The a-phase, which is produced from the reaction of UFs with HBr or mildly heating a mixture of UFs and UF4, has infinite chains of UFe units. The /3-phase produced by heating a mixture of UFe and UF4 to... 

Sodium Hexafluorouranate(V) Uranium pentafluoride (2.00 g) and dried NaF (0.25 g) are combined in a 250-mL flask in a drybox. The flask is then attached to a vacuum line, and approximately 150 mL of dry acetonitrile is vacuum transferred into the flask. (The acetonitrile can be dried by distillation from P4O10 or CaH2.) The mixture is allowed to warm to room temperature. With stirring, the NaF gradually dissolves over a period of 12 hours. The solvent is removed in vacuo, quantitatively yielding NaUF6 in the rhombohedral form (X-ray powder diffraction). The potassium salt is prepared similarly. 

Asprey, L. B. Paine, R. T. "One Electron Reduction Synthesis of Uranium Pentafluoride," Chem. Commun. 1973, 921. 

Halstead, G. W. Eller, P. G. Asprey, L. B. Salazar, K. V. "Convenient Multi gram Synthesis of Uranium Pentafluoride and Uranium Pentaethoxide," Inorg. Chem., 1978, 17, 2967. 

The fluorine content thus is obtained entirely from low-cost hydrogen fluoride. This reaction, because of its potential large-scale utility, has therefore been the subject of a number of investigations. Ferris (28) has examined the side reactions and thermodynamics of the oxidation in the temperature range 600-900°. In addition to the expected products UFe and UO2F2, Ferris observed the formation of uranium pentafluoride. Its formation was postulated to be the result of a side reaction between UF and UF4. Detailed experiments indicated also that the reaction... 

Uranium pentafluoride reacts violently, giving UF4 or UOF2 ... 

The hexafluorouranate products are all air-sensitive, pale blue-green powders. The reaction of [UFg] with water produces [UFs] and [U02F4] whose presence in the product may be determined by the visible-near-infrared spectrum in acetonitrile. The characteristic fundamental V3 mode of [UFe] is found at 525 cm" in the infrared spectrum of the salts. Reactions of tetra-alkylammonium fluorides with uranium pentafluoride in acetonitrile yield the corresponding [UF ]" salts, but these salts were found to be unstable under vacuum after removal of acetonitrile. They can be prepared less conveniently in HF, however. The uranium in NaUFe is six oordinate in solution and in the solid state, whereas in KUFj the uranium is six-coordinate in solution and eight-coordinate in the solid state. The exact geometry of the isolated... 

---