Redetermination of di-μ-hydroxido-bis[diaquachloridodioxidouranium(VI)] from single-crystal synchrotron data

The title compound, [(UO2)2Cl2(OH)2(H2O)4], was obtained unintentionally as the product of an attempted reaction between uranium(VI) oxide dihydrate, UO3·2H2O, and hydrogen bis(trifluoromethylsulfonyl)imide (HTf2N), in an experiment to obtain crystals of uranyl bis(trifluoromethylsulfonyl)imide, UO2(Tf2N)2·xH2O. The structure consists of neutral dimers of uranyl (UO2 2+) units, double bridged by OH− anions. Each uranyl unit is surrounded by one Cl and four O atoms, which form an irregular pentagon, in a plane perpendicular to the linear uranyl groups. The coordination geometry around each U atom can be considered to be distorted pentagonal-bipyramidal. In the crystal structure the uranyl dimers are connected to each other by hydrogen-bonding interactions [O⋯Cl = 3.23 (1) Å].

The title compound, [(UO 2 ) 2 Cl 2 (OH) 2 (H 2 O) 4 ], was obtained unintentionally as the product of an attempted reaction between uranium(VI) oxide dihydrate, UO 3 Á2H 2 O, and hydrogen bis(trifluoromethylsulfonyl)imide (HTf 2 N), in an experiment to obtain crystals of uranyl bis(trifluoromethylsulfonyl)imide, UO 2 (Tf 2 N) 2 ÁxH 2 O. The structure consists of neutral dimers of uranyl (UO 2 2+ ) units, double bridged by OH À anions. Each uranyl unit is surrounded by one Cl and four O atoms, which form an irregular pentagon, in a plane perpendicular to the linear uranyl groups. The coordination geometry around each U atom can be considered to be distorted pentagonal-bipyramidal. In the crystal structure the uranyl dimers are connected to each other by hydrogenbonding interactions [OÁ Á ÁCl = 3.23 (1) Å ].
The FWO-Vlaanderen and KULeuven are gratefully acknowledged for financial support. The ESRF is acknowledged for providing 9 shifts of beam time at the Swiss-Norwegian Beam Lines (SNBL, BM01A, project HS-3496) between June 10th and June 13th, 2008. We would like to thank Professor K. Binnemans and Dr P. Nockemann for useful discussions.
Redetermination of di--hydroxido-bis[diaquachloridodioxidouranium(VI)] from single-crystal synchrotron data D. Huys, R. Van Deun, P. Pattison, L. Van Meervelt and K. Van Hecke Comment Uranyl bis(trifluoromethylsulfonyl)imide is a useful starting material for the study of the spectroscopic properties of uranyl complexes in ionic liquids (Nockemann et al., 2007;Binnemans, 2007). The presence of chloride ions in the final product was surprising, because no chloride had been added to the reaction mixture. The chloride contamination of the reaction mixture can probably be attributed to chloride impurities in the aqueous hydrogen bis(trifluoromethylsulfonyl)imide solution. It should be noted that the presence of chloride traces in the HTf 2 N solution is not surprising, since the bis(trifluoromethylsulfonyl)imide anion can be synthesized by reaction between trifluorometylsulfonylamide and trifluoromethylsulfonyl chloride. Moreover, the uranyl ion has a strong tendency to form chloro complexes when both chloride ions and bis(trifluoromethylsulfonyl)imide ions are present (Nockemann et al., 2007). The acidity of coordinated water molecules, especially those in bridging positions, easily leads to the formation of hydroxo dimers or oligomers in neutral aqueous medium (Åberg, 1970;Tsushima et al., 2007).
However, in the latter structure, no meaningful anisotropic refinement could be carried out for the positions of the oxygen atoms (Åberg, 1969).
The asymmetric unit consists of uranyl dimers, double bridged by OH --anions. Each uranyl cation is surrounded by one chlorine and four oxygen atoms, which form an irregular pentagon, in a plane perpendicular to the linear uranyls. The coordination geometry around each uranium atom can be considered as a distorted pentagonal bipyramid ( Figure 2). As in the structure of Åberg, no hydrogen atoms could be unambiguously located on the water molecules, nor on the hydroxyl anions.
Only one hydrogen bond is observed between one of the lateral coordinating oxygen atoms O4 and the Cl2-atom of a subsequent dimer (3.23 (1) Å), which additionally links the dimers together in the [001] direction (c-direction).

Experimental
Uranium(VI)oxide dihydrate UO 3 .2H 2 O (966 mg, 3 mmol), suspended in 10 ml of deionized water. To this suspension, 2 equivalents (2.4 g) of a 80% solution of hydrogen bis(trifluoromethylsulfonyl)imide (from IoLiTec) was added and the mixture was refluxed while stirred for 1 h. After leaving the solution to cool to room temperature, the excess of UO 3 was filtered off and the clear liquid was evaporated to dryness using a rotary evaporator. The remaining thick slurry was further dried at 60 °C using a Schlenk apparatus for 36 h, yielding 885 mg of a glassy residue. A small solid sample was removed from the batch and the remaining solid was redissolved in a minimal amount of water, while slightly heating using a heat gun. The liquid was transferred to a small crystallization dish and cooled to 5 °C in a refrigerator. A very small amount of the dry solid product was then carefully added to the solution and the crystallization dish was put in a desiccator at room temperature. After 10 weeks, small hygroscopic crystals were obtained.

Refinement
No hydrogen atoms could be unambiguously located on the water molecules, nor on the hydroxyl anions. Fig. 1. Coordination geometry of the title compound, showing the atom-labelling scheme of the asymmetric unit and 50% probability displacement ellipsoids.