Speciation of Uranyl Complexes in Ionic Liquids by Optical Spectroscopy Peter Nockemann, Kelly Servaes, Rik Van Deun, Kristof Van Hecke, Luc Van Meervelt, Koen Binnemans,* and Christiane Go 1 rller-Walrand Katholieke UniVersiteit LeuVen, Department of Chemistry, Celestijnenlaan 200F bus 2404, B-3001 LeuVen (Belgium) Received September 6, 2007 Uranyl complexes dissolved in room-temperature ionic liquids have diagnostic absorption and emission spectra which reflect the molecular symmetry and geometry. In particular, the characteristic vibrational fine structure of the absorption spectra allows identification of the molecular symmetry of a uranyl complex. The concept of speciation of uranyl complexes is illustrated for the hydrated uranyl ion, the tetrachloro complex [UO 2 Cl 4 ] 2- , the trinitrato complex [UO 2 (NO 3 ) 3 ] - , the triacetato complex [UO 2 (CH 3 COO) 3 ] - , and the crown ether complex [UO 2 (18-crown- 6)] 2+ in imidazolium and pyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquids. The competition between 18- crown-6 and small inorganic ligands for coordination to the uranyl ion was investigated. The crystal structures of the hydrolysis product [(UO 2 ) 2 (µ 2 -OH) 2 (H 2 O) 6 ] [UO 2 Br 4 ](18-crown-6) 4 and imidazolium salt [C 6 mim] 2 [UO 2 Br 4 ] are described. Introduction Liquid-liquid extraction is being used for the reprocessing of spent nuclear reactor fuels. 1-4 Uranium and plutonium are separated from fission products and heavier actinides to recover the fissile 235 U and 239 Pu radionuclides as well as the fertile 238 U radionuclide. The best known nuclear reprocessing process is the PUREX process (plutonium and uranium recoVery by extraction). 5,6 After dissolution of the used nuclear fuel rods in concentrated nitric acid, uranium (as the uranyl ion) and plutonium (as the Pu 4+ ion) are extracted into the organic phase which consists of 30% tributylphosphate (TBP) and kerosene. The fission products and heavy actinides (neptunium, americium, curium) remain in the aqueous phase. In subsequent process steps separation of uranium and plutonium can be achieved. Ionic liquids could offer an alternative for the organic phase in this type of liquid-liquid extraction processes. 7-13 Advantages of ionic liquids include a low vapor pressure and low inflammability. Moreover, boron-containing ionic liquids (for instance, tetrafluoroborate salts) are strongly neutron absorbing, so that the risks of criticality accidents during the reprocessing of nuclear fuel can be considerably reduced. Recent experiments have shown that the mechanism of extraction of metal species from an aqueous phase into an ionic liquid is not necessarily the same as the mechanism for extraction into a conventional organic phase. 14,15 Knowledge of the solvation of metal ions in ionic liquids is of prime importance for understanding extraction processes involving ionic liquids. Several authors * To whom correspondence should be addressed. Fax: +32 16 32 79 92. E-mail: Koen.Binnemans@chem.kuleuven.be. (1) Campbell, D. O.; Burch, W. D. J. Radioanal. Nucl. Chem. 1990, 142, 303-320. (2) Baumgartner, F. Kerntechnik 1978, 20, 74-85. (3) Musikas, C. Inorg. Chim. Acta 1987, 140, 197-206. (4) Nash, K. L. ACS Symp. Ser. 2006, 933, 21-40. (5) Choppin, G. R. SolV. Extr. Res. DeV. Jpn. 2005, 12,1-10. (6) McKibben, J. M. Radiochim. Acta 1984, 36,3-15. (7) Gutowski, K. E.; Bridges, N. J.; Cocalia, V. A.; Spear, S. K.; Visser, A. E.; Holbrey, J. D.; Davis, J. H.; Rogers, R. D. ACS Symp. Ser. 2005, 902, 33-48. (8) Baston, G. M. N.; Bradley, A. E.; Gorman, T.; Hamblett, I.; Hardacre, C.; Hatter, J. E.; Healy, M. J. F.; Hodgson, B.; Lewin, R.; Lovell, K. V.; Newton, G. W. A.; Nieuwenhuyzen, M.; Pitner, W. R.; Rooney, D. W.; Sanders, D.; Seddon, K. R.; Simms, H. E.; Thied, R. C. ACS Symp. Ser. 2002, 818, 162-177. (9) Stepinski, D. C.; Young, B. A.; Jensen, M. P.; Rickert, P. G.; Dzielawa, J. A.; Dilger, A. A.; Rausch, D. J.; Dietz, M. L. ACS Symp. Ser. 2006, 933, 233-247. (10) Visser, A. E.; Rogers, R. D. J. Solid State Chem. 2003, 171, 109- 113. (11) Cocalia, V. A.; Gutowski, K. E.; Rogers, R. D. Coord. Chem. ReV. 2006, 250, 755-764. (12) Binnemans, K. Chem. ReV. 2007, 107, 2592-2614. (13) Cocalia, V. A.; Jensen, M. P.; Holbrey, J. D.; Spear, S. K.; Stepinski, D. C.; Rogers, R. D. Dalton Trans. 2005, 1966-1971. (14) Jensen, M. P.; Neuefeind, J.; Beitz, J. V.; Skanthakumar, S.; Soder- holm, L. J. Am. Chem. Soc. 2003, 125, 15466-15473. (15) Jensen, M. P.; Beitz, J. V.; Neuefeind, J.; Skanthakumar, S.; Soder- holm, L. ACS Symp. Ser. 2005, 901, 18-31. Inorg. Chem. 2007, 46, 11335-11344 10.1021/ic701752j CCC: $37.00 © 2007 American Chemical Society Inorganic Chemistry, Vol. 46, No. 26, 2007 11335 Published on Web 11/29/2007