In the Laboratory
Microscale Synthesis of UO2Cl2(OPPh3)2 Francisco J. Arnáiz and Mariano J. Miranda Laboratorio de Química Inorgánica, Universidad de Burgos, 09001 Burgos, Spain
Only a few experiments involving uranium compounds, mainly centered on luminescence (1, 2) and equilibria in aqueous solution (3, 4), have been published in this Journal. In preparative handbooks there is also a dearth of welldescribed procedures to synthesize mononuclear neutral species suitable for further studies on uranyl compounds. UO2Cl2(OPPh 3) 2 is a useful starting material for the preparation of other anhydrous uranyl compounds via ligand replacement. It is a genuine monomeric neutral uranyl complex, also representative of octahedral trans,trans,transML02LI2LII2 species. The product, known since 1964 (5) and structurally characterized in 1978 (6 ), is obtained by treating UO2Cl 2 with OPPh 3 in anhydrous ethanol (7 ). In turn, UO2Cl2 is prepared by heating UCl4 with oxygen at 300– 350 °C (8), because UO2Cl2⭈xH2O is very difficult to dehydrate (9). (More recently, a cis form for UO 2Cl2(OPPh3)2 was structurally characterized. The crystals were isolated from a (CH3)3SiCl/UF5/OPPh3 acetonitrile solution (5:1:2) after standing in a glass apparatus for several weeks (10). However, at present, a reproducible, rational synthesis for the pure cis form is unavailable and we have obtained only the trans form in a variety of preparations starting from uranyl salts.) The method described here is very appropriate for teaching in the inorganic chemistry lab because: 1. It illustrates how some simple transformations can be made and how anhydrous species sometimes can be prepared from hydrated salts of cations very resistant to releasing coordinated water. 2. Precipitation begins after a short period of induction (from some seconds to more than 5 min, depending mainly on the temperature and HCl present in the solution), and the fine microcrystalline yellow precipitate settles readily. 3. The equipment for the synthesis is reduced to a minimum. 4. The product is air-stable, thus facilitating further manipulations. 5. All operations can be performed in a normal 3-h session.
Procedure CAUTION: Although there is no significant safety risk in handling uranium at microscale levels, uranium compounds are toxic and radioactive. Gloves should be worn throughout the process. Attention should be paid to local regulations on the use and disposal of radioactive compounds. In a medium-sized test tube place a spin bar, 0.1 g (0.24 mmol) of UO2(CH3CO2)2⭈2H2O, five drops of concentrated (~35%) HCl, and 1 g of methanol. Immerse the tube in a bath at 70–90 °C and boil the solution with stirring until evolution of vapor ceases. (CAUTION: Some toxic methyl chloride can be formed in this process. Use a well-ventilated hood). Dissolve the oily residue in 1 g of acetone and place the tube in a bath at 40–50 °C. Add a warm solution of 140 mg (0.50 mmol) of OPPh 3 in 2 g of acetone to the uranyl
solution and stir the mixture for 5 min. Wash the yellow microcrystalline solid twice in 2 mL of acetone by decanting, and dry it in air. The yield is 175 mg of UO 2Cl2(OPPh3)2 (81%). Anal. calcd for UO2Cl2(OPPh 3) 2: C, 48.2; H, 3.4. Found: C, 48.0; H, 3.4. The product melts at 297–298 °C. It is insoluble in diethyl ether and water, slightly soluble in acetone, somewhat more soluble in methanol and dichloromethane, and very soluble in dimethyl formamide and dimethyl sulfoxide. It can be manipulated in air and stored for months without special precautions. The IR spectrum presents numerous bands, that of 920 cm᎑1 assigned to νas(UO2); only one sharp band ν(PO), characteristic of a trans arrangement of two OPPh3 (11), is observed at 1065 cm᎑1—at lower frequency than that of free OPPh 3, as expected on coordination (12). Despite the stability of the compound in the solid state, in solution it displays the reactivity characteristic of labile complexes (readiness to undergo substitution reactions). For example, it dissolves in dimethyl sulfoxide with displacement of OPPh3 and reacts immediately with a solution of NaI in acetone (formation of NaCl is easily observed). Discussion The following points are suggested: 1. Actinide elements, although less markedly than lanthanide, form complexes in which the metal–ligand bonds are predominantly ionic and labile so that hydrochloric acid transforms acetate ion into acetic acid. The addition of excess methanol facilitates the removal, in a single run, of both water and acetic acid by forming methyl acetate and low boiling point azeotropes. 2. Triphenylphosphine oxide belongs to a class of ligands capable of displacing water from the coordination sphere of many cations and at the same time it is not sufficiently basic towards the proton to induce precipitation of uranyl oxhydrates, or uranates, or to stabilize anionic species such as [UO2Cl4]2᎑. 3. Uranium, in uranyl complexes, frequently coordinates five or six donor atoms from ligands in the equatorial plane because of its large ionic radius. The steric requirements of, and the hydrophobic environment created by, OPPh3 are likely determinant in precluding this behavior, so that no other molecules (as occur in many other complexes) are needed to fit the equatorial plane of uranium.
Recycling of Wastes The recovery of both uranium and OPPh3 from the mixture of residual portions of UO2Cl2(OPPh 3)2, washings, and KBr pellets is simple. The following scheme works satisfactorily and is suggested. Remove most of the organic solvents, add water, warm the suspension and add ammonia, stir for 15 minutes, cool the resulting suspension, recover the precipitate (mixture of “ammonium uranate” and OPPh3), and extract OPPh 3 with acetone.
JChemEd.chem.wisc.edu • Vol. 75 No. 11 November 1998 • Journal of Chemical Education
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In the Laboratory
Two simple alternatives to reuse the uranium are the following: 1. Heat the “ammonium uranate” at 800 °C for 2 hours to obtain U3O8 (or at 500 °C to obtain impure UO3) and use it as starting material for the preparation of the adduct. Dissolve it in boiling 6M HCl (usually 15–30 min is required), concentrate the solution nearly to dryness, add acetone and finally the OPPh3 solution. 2. Use the air-dried “ammonium uranate” as source of uranium assuming a composition of UO3⭈NH3⭈2H2O. Dissolve the uranate in the minimum amount of 12M HCl (1–2 drops for 100 mg of product), add acetone (a cloudy suspension results), separate the NH4Cl by filtration, and add OPPh 3 to the solution as above. The product obtained in this manner, on occasions, may be slightly contaminated with co-precipitated NH 4Cl (observed as a week band at 3147 cm ᎑1 characteristic of ammonium salts above, and sufficiently separated from those of aromatic C-H).
In both cases, if the HCl solution of uranyl chloride is not very concentrated, the subsequent acetone solution might contain sufficient water to precipitate OPPh 3 before, or at the time that, UO2Cl2(OPPh 3) 2 is formed. This event is evidenced in the IR spectrum of the sample (ν(P–O) of free OPPh3 at 1188 cm᎑1).
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Acknowledgments To reviewers for helpful comments, and to Dirección General de Enseñanza Superior (PB95-0832) and Junta de Castilla y León (Bu04/95) for financial support. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
Burrows, H. D.; Formosinho, S. J. J. Chem. Educ. 1978, 55, 125. Gil, J. M.; Gil, F. J. M. J. Chem. Educ. 1978, 55, 340. Williams, D. R. J. Chem. Educ. 1971, 48, 480. Tutem, E.; Apak, R.; Turgut, M. H.; Apak, V. J. Chem. Educ. 1991, 68, 569. Gans, P.; Smith, B. C. J. Chem. Soc. 1964, 4172. Bombieri, G.; Forsellini, E.; Day, J. P.; Azeez, W. I. J. Chem. Soc., Dalton Trans. 1978, 677. Day, J. P.; Venanzi, L. M. J. Chem. Soc. A 1966, 1363. Leary, J. A.; Suttle, J. R. Inorg. Synth. 1957, 5, 148. Hefley, J. D.; Matews, D. M.; Amis, E. S. Inorg. Synth. 1963, 7, 146. Akona, S. B.; Fawcett, J.; Holloway, J. H.; Russell, D. D.; Leban, I. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1991, C47(1), 45. See, e.g.: Clark, J. P.; Langford, V. M.; Wilkins, C. J. J. Chem. Soc. 1967, 792. See, e.g.: Kida, S.; Quagliano, J. V.; Walmsley, J. A.; Tyree, S. Y. Spectrochim. Acta 1963, 19, 189.
Journal of Chemical Education • Vol. 75 No. 11 November 1998 • JChemEd.chem.wisc.edu
