Crystal structure of [UO2(NH3)5]NO3·NH3

The title compound, [UO2(NH3)5]NO3·NH3, contains a pentagonal–bipyramidal [UO2(NH3)5]+ cation, a nitrate anion and an ammonia molecule of crystallization.


Introduction -Chemical context
Uranium chemistry in aqueous solution is dominated by the uranyl cation [UO 2 ] 2+ , with the uranium atom in the hexavalent oxidation state. The most prominent representatives are the well-known uranyl nitrates and uranyl halides. In contrast to the [UO 2 ] 2+ uranyl cation, the uranyl cation [UO 2 ] + with pentavalent uranium disproportionates in aqueous solution into the [U VI O 2 ] 2+ cation and a tetravalent uranium species. Only under controlled conditions (Kraus et al., 1949) and in organic solvents (Arnold et al., 2009) are uranyl cations with pentavalent uranium observable. Here we report on the crystal structure of a U V compound, [UO 2 (NH 3 ) 5 ]NO 3 ÁNH 3 , obtained from the reaction of U IV with U VI species in anhydrous liquid ammonia. The compound is not stable at temperatures above ca 238 K due to the loss of ammonia of a still unknown amount. Despite several efforts, we have not yet been able to reproduce the synthesis of the compound.
Obviously, the two uranium compounds used as educts reacted in a comproportionation reaction in order to form the U V compound reported here. It is possible that the redox potentials in liquid ammonia are reversed compared to aqueous solutions, leading to a comproportionation. Such changes of electrochemical potentials are not uncommon and, for example, are known for the system Cu/Cu + /Cu 2+ (Woidy et al., 2015a). However, the detailed reaction U VI + U IV ! U V is still unclear, and despite some efforts we were not able to elucidate further reaction products which must be present (e.g. fluoride containing ones).

Results and discussion -Structural commentary
All atoms in the structure of the title compound reside on general Wyckoff positions 8c of space group Pbca. The pentavalent uranium atom U1 and the oxygen atoms O1 and O2 form an uranyl cation. This [UO 2 ] + ion is coordinated by five ammine ligands (N1-N5) forming the complex pentagonal-bipyramidal [UO 2 (NH 3 ) 5 ] + cation which is shown in Fig. 1. The nitrate anion NO 3 À consists of the nitrogen atom N7 and the oxygen atoms O3-O5. An ammonia molecule of crystallization (N6) is also observed in the structure.
As we are not able to completely explain the formation of the title compound from the educts, the question arises whether the cation is not simply a 'regular' uranyl(VI) cation. It is obvious that no second nitrate anion is present in the structure. Due to chemical reasoning, the ammonia molecule of crystallization also cannot be an amide anion (NH 2 À ). As ammine ligands are bound to the uranium cation, some of their electron density is transferred to the Lewis-acidic U atom, which leads to a weakening of the N-H bonds and therefore to an acidification of these protons. So, an amide anion residing next to an acidified ammine ligand is not a plausible assumption, especially since the ammonia molecule of crystallization shows an usual NÁ Á ÁN distance for N-HÁ Á ÁN hydrogen bonds. If one assumes that CO 3 2À is present instead of NO 3 À , then a 'regular' [U VI O 2 ] 2+ ion would also result. However, if one refines the occupancy of the N atom of the nitrate anion, an occupancy of 1.00 (2) is observed, whereas if the occupancy of the C atom of a putative carbonate anion is refined, an occupancy of 1.30 (2) is obtained. Comparing the atomic distances of the trigonal-planar anion with the mean distances from the literature, 1.284 Å for CO 3 2À (Zemann, 1981) and 1.250 Å for NO 3 À (Baur, 1981), it is most likely that in our case a nitrate anion is present. In summary, all these points indicate that the central atom is an N atom of a nitrate anion. Together with the observation of slightly elongated U-O and U-N bond lengths in comparison to similar [UO 2 (NH 3 ) 5 ] 2+ ions, we conclude that the compound should contain U V atoms in form of [UO 2 ] + ions.

Figure 1
The molecular components of the title compound. Displacement ellipsoids are shown at the 70% probability level. The dashed line corresponds to a N-HÁ Á ÁN hydrogen-bonding interaction.
to two symmetry-equivalent nitrate anions; the third H atom (H6C) is not involved in hydrogen-bond formation. The nitrate anion is hydrogen-bonded to five symmetry-related [UO 2 (NH 3 ) 5 ] + cations via N-HÁ Á ÁO hydrogen bonds and two symmetry-related ammonia molecules of crystallization. The nitrate anions lie parallel to the ac plane and are arranged in columns running parallel to the b axis (Fig. 2). The oxygen atoms of the uranyl cation act as acceptors of hydrogen bonds from four (O1) and three (O2) ammine ligands of two symmetry-related [UO 2 (NH 3 ) 5 ] + cations. The linear UO 2 + cations are also arranged parallel to the b axis. Overall, a three-dimensional hydrogen-bonded network results. Numerical details of the hydrogen bonding interactions are compiled in Table 1.

Synthesis and crystallization
The purity of the used educts was evidenced by powder X-ray diffraction and IR spectroscopy. 50 mg (0.09 mmol, 1 eq.) Cs[UO 2 (NO 3 ) 3 ] and 27 mg (0.09 mmol, 1 eq.) UF 4 were placed in a reaction flask under argon atmosphere. After cooling to 195 K ca 10 ml NH 3 were added to the reaction mixture resulting in a clear yellow solution and a green solid residue. Yellow single crystals of the title compound were obtained during storage at 233 K and were selected under cold perfluoroether oil (Kottke & Stalke, 1993). Additionally, emerald green crystals of [UF 4 (NH 3 ) 4 ]ÁNH 3 were observed (Kraus & Baer, 2009) next to colourless crystals of CsNO 3 , both evidenced by determination of their unit-cell parameters.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The structure was solved by the heavy-atom method and all other atoms were located from difference Fourier maps. In case of the hydrogen atoms of nitrogen atoms N1-N5, their positions were refined using a riding model with N-H = 0.91 Å and U eq (H) = 1.5U iso (N). The hydrogen atoms of the ammonia molecule of crystallization were refined freely. The maximum and minimum residual electron densities are located close to the U atom at distances of 0.58 and 0.04 Å , respectively.      (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015) and SHELXLE (Hübschle et al., 2011); molecular graphics: DIAMOND (Brandenburg, 2012); software used to prepare material for publication: publCIF (Westrip, 2010). Extinction correction: SHELXL2014 (Sheldrick, 2015), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.00070 (7) Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.