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

Penta-ammine dioxide uranium(V) nitrate ammonia (1/1), [UO2(NH3)5]NO3·NH3, was obtained in the form of yellow crystals from the reaction of caesium uranyl nitrate, Cs[UO2(NO3)3], and uranium tetra-fluoride, UF4, in dry liquid ammonia. The [UO2]+ cation is coordinated by five ammine ligands. The resulting [UO2(NH3)5] coordination polyhedron is best described as a penta-gonal bipyramid with the O atoms forming the apices. In the crystal, numerous N-H⋯N and N-H⋯O hydrogen bonds are present between the cation, anion and solvent mol-ecules, leading to a three-dimensional network.

Introduction – Chemical context

Uranium chemistry in aqueous solution is dominated by the uranyl cation [UO2]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 [UO2]2+ uranyl cation, the uranyl cation [UO2]+ with penta­valent uranium disproportionates in aqueous solution into the [UVIO2]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 UV compound, [UO2(NH3)5]NO3·NH3, obtained from the reaction of UIV with UVI 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 UV 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+/Cu2+ (Woidy et al., 2015a ▸). However, the detailed reaction UVI + UIV → UV 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 [UO2]+ ion is coordinated by five ammine ligands (N1–N5) forming the complex penta­gonal–bipyramidal [UO2(NH3)5]+ cation which is shown in Fig. 1 ▸. The nitrate anion NO3 − consists of the nitro­gen atom N7 and the oxygen atoms O3–O5. An ammonia mol­ecule of crystallization (N6) is also observed in the structure.

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—H⋯N hydrogen-bonding interaction.

The U—O distances in the almost linear uranyl cation [O—U—O angle of 177.2 (1)°] are 1.861 (3) and 1.867 (3) Å, respectively. Such distances are slightly elongated compared to reported ones for uranyl compounds with penta­valent uranium (Berthet et al., 2003 ▸; Hayton & Wu, 2008 ▸), which are in the range 1.810 (4) to 1.828 (4) Å. However, U—O distances for uranyl cations [UO2]2+ with hexa­valent uranium are about 0.02 to 0.07 Å shorter. For the alkali metal uranyl nitrates, such as M[UO2(NO3)3] with M = K (Jouffret et al., 2011 ▸; Krivovichev & Burns, 2004 ▸), Rb (Barclay et al., 1965 ▸; Zalkin et al., 1989 ▸) and Cs (Malcic & Ljubica, 1961 ▸), the reported U—O distances are in the range 1.746 to 1.795 Å. In uranium(VI) compounds that contain the comparable penta­ammine dioxido uranium(VI) ion [UO2(NH3)5]2+, such as [UO2(NH3)5]Cl2·NH3, [UO2F2(NH3)3]2·2NH3 or [UO2(NH3)5]Br2·NH3, U—O distances in the range 1.768 (2) to 1.771 (3) Å were reported (Woidy et al., 2012 ▸, 2015b ▸); these are shortened by ca 0.1 Å compared to the uranyl ion presented here.

The nitro­gen atoms of the ammine ligands show U—N distances between 2.573 (3) and 2.629 (3) Å, which appear slightly elongated in comparison with the U—N distances determined for UVI compounds such as [UO2(NH3)5]Cl2·NH3 [2.505 (2)–2.554 (3) Å], [UO2(NH3)5]Br2·NH3 or [UO2F2(NH3)3]2·2NH3 [2.522 (3) to 2.577 (3) Å] (Woidy et al., 2012 ▸). In [UF4(NH3)4]·NH3 (Kraus & Baer, 2009 ▸), we observed an elongated U—N distance of 2.618 (5) Å due to the higher coordination number and different charge of the central atom.

The nitrate anion features no unexpected structural parameters and is practically identical compared to the nitrate anions of NaNO3 or KNO3. The N—O distances are 1.242 (5), 1.253 (4), and 1.254 (4) Å, the bond angles are 120° within the 3σ criterion [120.4 (3), 120.4 (3), and 119.2 (3)°] and therefore the anion is essentially planar.

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 (NH2 −). 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 N⋯N distance for N—H⋯N hydrogen bonds. If one assumes that CO3 2− is present instead of NO3 −, then a ‘regular’ [UVIO2]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 CO3 2− (Zemann, 1981 ▸) and 1.250 Å for NO3 − (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 [UO2(NH3)5]2+ ions, we conclude that the compound should contain UV atoms in form of [UO2]+ ions.

Supra­molecular features

The crystal structure of the title compound is shown in Fig. 2 ▸. The ammonia mol­ecule of crystallization (N6) acts as an acceptor of an N—H hydrogen bond with an ammine ligand (N2). It forms also two disparate N—H⋯O hydrogen bonds 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 [UO2(NH3)5]+ cations via N—H⋯O 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 [UO2(NH3)5]+ cations. The linear UO2 + 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 ▸.

Figure 2

Crystal structure of [UO2(NH3)5]NO3·NH3 viewed along [010]. Displacement ellipsoids are shown at the 70% probability level.

Table 1

Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N1—H1A⋯O4i	0.91	2.43	3.166 (4)	138
N1—H1A⋯O4ii	0.91	2.47	2.996 (4)	117
N1—H1B⋯O1iii	0.91	2.25	3.079 (4)	151
N1—H1C⋯O2iv	0.91	2.12	3.006 (4)	165
N2—H2A⋯O4i	0.91	2.49	3.220 (5)	138
N2—H2B⋯N6	0.91	2.14	3.024 (5)	164
N2—H2C⋯O4ii	0.91	2.36	3.232 (5)	160
N3—H3A⋯O2v	0.91	2.27	3.136 (5)	159
N3—H3B⋯O1vi	0.91	2.34	3.151 (4)	149
N3—H3C⋯O5vii	0.91	2.52	3.142 (5)	126
N4—H4A⋯O1vi	0.91	2.37	3.219 (4)	156
N4—H4B⋯O2v	0.91	2.26	3.086 (4)	150
N4—H4C⋯O3	0.91	2.55	3.253 (5)	134
N5—H5A⋯O5iii	0.91	2.14	3.048 (5)	176
N5—H5B⋯O3	0.91	2.44	3.063 (5)	126
N5—H5B⋯O5	0.91	2.59	3.394 (5)	147
N5—H5C⋯O1iii	0.91	2.37	3.273 (4)	171
N6—H6A⋯O4vii	0.86 (7)	2.50 (7)	3.342 (6)	167 (7)
N6—H6B⋯O3vi	0.81 (8)	2.32 (8)	3.102 (6)	162 (7)

Symmetry codes: (i) ; (ii) ; (iii) ; (iv) ; (v) ; (vi) ; (vii) .

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[UO2(NO3)3] and 27 mg (0.09 mmol, 1 eq.) UF4 were placed in a reaction flask under argon atmosphere. After cooling to 195 K ca 10 ml NH3 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 [UF4(NH3)4]·NH3 were observed (Kraus & Baer, 2009 ▸) next to colourless crystals of CsNO3, 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.

Table 2

Experimental details

Crystal data
Chemical formula	[U(NH3)5]NO3·NH3
M r	434.24
Crystal system, space group	Orthorhombic, P b c a
Temperature (K)	123
a, b, c (Å)	15.7497 (2), 7.7375 (1), 18.8126 (2)
V (Å3)	2292.57 (5)
Z	8
Radiation type	Mo Kα
μ (mm−1)	14.17
Crystal size (mm)	0.24 × 0.21 × 0.04
Data collection
Diffractometer	Oxford Diffraction Xcalibur3
Absorption correction	Multi-scan (CrysAlis RED; Oxford Diffraction, 2009 ▸)
T min, T max	0.166, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	88079, 6635, 5051
R int	0.045
(sin θ/λ)max (Å−1)	0.892
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.033, 0.090, 1.04
No. of reflections	6635
No. of parameters	136
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å−3)	5.60, −3.79

Computer programs: CrysAlis CCD and CrysAlis RED (Oxford Diffraction, 2009 ▸), SHELXL97 (Sheldrick, 2008 ▸), SHELXL2014 (Sheldrick, 2015 ▸), SHELXLE (Hübschle et al., 2011 ▸), DIAMOND (Brandenburg, 2012 ▸) and publCIF (Westrip, 2010 ▸).

Crystal structure: contains datablock(s) I. DOI: 10.1107/S2056989016016480/wm5331sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989016016480/wm5331Isup2.hkl

CCDC reference: 1510182

Additional supporting information: crystallographic information; 3D view; checkCIF report

[U(NH3)5]NO3·NH3	Dx = 2.516 Mg m−3
Mr = 434.24	Melting point: not measured K
Orthorhombic, Pbca	Mo Kα radiation, λ = 0.71073 Å
a = 15.7497 (2) Å	Cell parameters from 44787 reflections
b = 7.7375 (1) Å	θ = 2.8–39.2°
c = 18.8126 (2) Å	µ = 14.17 mm−1
V = 2292.57 (5) Å3	T = 123 K
Z = 8	Plate, colourless
F(000) = 1592	0.24 × 0.21 × 0.04 mm

Oxford Diffraction Xcalibur3 diffractometer	6635 independent reflections
Radiation source: Enhance (Mo) X-ray Source	5051 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.045
Detector resolution: 16.0238 pixels mm-1	θmax = 39.3°, θmin = 3.1°
phi– and ω–rotation scans	h = −27→27
Absorption correction: multi-scan (CrysAlis RED; Oxford Diffraction, 2009)	k = −8→13
Tmin = 0.166, Tmax = 1.000	l = −32→32
88079 measured reflections

Refinement on F2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: mixed
R[F2 > 2σ(F2)] = 0.033	H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.090	w = 1/[σ2(Fo2) + (0.0532P)2 + 3.9914P] where P = (Fo2 + 2Fc2)/3
S = 1.04	(Δ/σ)max = 0.003
6635 reflections	Δρmax = 5.60 e Å−3
136 parameters	Δρmin = −3.79 e Å−3
0 restraints	Extinction correction: SHELXL2014 (Sheldrick, 2015), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
Primary atom site location: heavy-atom method	Extinction coefficient: 0.00070 (7)

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.

	x	y	z	Uiso*/Ueq
U1	0.11450 (2)	0.22849 (2)	0.80178 (2)	0.00916 (4)
O1	0.11836 (14)	−0.0112 (4)	0.79457 (13)	0.0145 (4)
O2	0.10583 (16)	0.4688 (4)	0.80629 (15)	0.0180 (5)
N1	0.26344 (17)	0.2174 (4)	0.86194 (16)	0.0140 (5)
H1A	0.257174	0.197530	0.909327	0.021*
H1B	0.290520	0.320039	0.855278	0.021*
H1C	0.294646	0.130809	0.842227	0.021*
N2	0.0977 (2)	0.2180 (5)	0.93878 (17)	0.0182 (6)
H2A	0.111922	0.110928	0.954839	0.027*
H2B	0.042857	0.241370	0.950589	0.027*
H2C	0.132433	0.298114	0.959019	0.027*
N3	−0.04957 (19)	0.2180 (4)	0.81645 (19)	0.0185 (6)
H3A	−0.070849	0.128341	0.790693	0.028*
H3B	−0.072525	0.318832	0.800654	0.028*
H3C	−0.062761	0.203054	0.863146	0.028*
N4	0.0394 (2)	0.2335 (4)	0.67697 (19)	0.0188 (6)
H4A	0.003321	0.325238	0.674759	0.028*
H4B	0.009622	0.133855	0.670795	0.028*
H4C	0.079243	0.243219	0.642176	0.028*
N5	0.2334 (2)	0.2756 (5)	0.70995 (18)	0.0190 (6)
H5A	0.214265	0.346174	0.674728	0.029*
H5B	0.248928	0.172025	0.691072	0.029*
H5C	0.278944	0.325155	0.731585	0.029*
N6	−0.0713 (3)	0.3021 (8)	1.0069 (3)	0.0327 (9)
H6A	−0.110 (4)	0.227 (8)	1.013 (4)	0.033 (19)*
H6B	−0.095 (4)	0.381 (10)	0.986 (4)	0.05 (2)*
H6C	−0.056 (6)	0.312 (12)	1.037 (5)	0.06 (3)*
N7	0.2550 (2)	0.0309 (4)	0.55493 (17)	0.0204 (6)
O3	0.1861 (2)	0.0340 (5)	0.58716 (17)	0.0320 (7)
O4	0.25705 (19)	0.0485 (4)	0.48871 (15)	0.0266 (6)
O5	0.3231 (2)	0.0085 (5)	0.58813 (17)	0.0311 (7)

	U11	U22	U33	U12	U13	U23
U1	0.00711 (5)	0.00894 (6)	0.01145 (5)	0.00013 (3)	−0.00104 (3)	−0.00027 (3)
O1	0.0137 (10)	0.0142 (12)	0.0157 (11)	−0.0007 (8)	−0.0016 (8)	−0.0011 (8)
O2	0.0176 (11)	0.0096 (11)	0.0267 (14)	0.0021 (8)	−0.0032 (9)	−0.0019 (9)
N1	0.0093 (10)	0.0178 (14)	0.0147 (11)	0.0020 (9)	−0.0012 (8)	−0.0006 (10)
N2	0.0140 (11)	0.0250 (16)	0.0156 (12)	0.0012 (10)	0.0010 (10)	−0.0005 (11)
N3	0.0111 (11)	0.0222 (16)	0.0221 (14)	−0.0006 (10)	−0.0004 (10)	0.0003 (11)
N4	0.0184 (13)	0.0214 (15)	0.0167 (12)	−0.0025 (11)	−0.0054 (11)	0.0006 (11)
N5	0.0167 (13)	0.0230 (16)	0.0172 (13)	−0.0017 (11)	0.0026 (10)	−0.0014 (11)
N6	0.0188 (15)	0.049 (3)	0.030 (2)	0.0031 (17)	0.0034 (15)	0.002 (2)
N7	0.0263 (16)	0.0193 (15)	0.0158 (13)	−0.0001 (12)	−0.0030 (11)	−0.0010 (11)
O3	0.0275 (14)	0.044 (2)	0.0242 (15)	0.0026 (14)	0.0024 (12)	−0.0076 (14)
O4	0.0347 (16)	0.0330 (17)	0.0121 (11)	0.0024 (13)	−0.0015 (11)	0.0020 (11)
O5	0.0269 (14)	0.0409 (19)	0.0255 (15)	−0.0051 (13)	−0.0095 (12)	0.0083 (14)

U1—O1	1.861 (3)	N3—H3B	0.9100
U1—O2	1.867 (3)	N3—H3C	0.9100
U1—N5	2.573 (3)	N4—H4A	0.9100
U1—N2	2.592 (3)	N4—H4B	0.9100
U1—N3	2.600 (3)	N4—H4C	0.9100
U1—N1	2.606 (3)	N5—H5A	0.9100
U1—N4	2.629 (3)	N5—H5B	0.9100
N1—H1A	0.9100	N5—H5C	0.9100
N1—H1B	0.9100	N6—H6A	0.86 (7)
N1—H1C	0.9100	N6—H6B	0.81 (8)
N2—H2A	0.9100	N6—H6C	0.63 (9)
N2—H2B	0.9100	N7—O3	1.242 (5)
N2—H2C	0.9100	N7—O5	1.253 (4)
N3—H3A	0.9100	N7—O4	1.254 (4)
O1—U1—O2	177.20 (11)	H2A—N2—H2B	109.5
O1—U1—N5	93.92 (11)	U1—N2—H2C	109.5
O2—U1—N5	86.72 (12)	H2A—N2—H2C	109.5
O1—U1—N2	92.56 (11)	H2B—N2—H2C	109.5
O2—U1—N2	88.76 (12)	U1—N3—H3A	109.5
N5—U1—N2	138.26 (11)	U1—N3—H3B	109.5
O1—U1—N3	90.52 (11)	H3A—N3—H3B	109.5
O2—U1—N3	87.34 (11)	U1—N3—H3C	109.5
N5—U1—N3	143.01 (11)	H3A—N3—H3C	109.5
N2—U1—N3	78.00 (11)	H3B—N3—H3C	109.5
O1—U1—N1	88.25 (10)	U1—N4—H4A	109.5
O2—U1—N1	94.52 (10)	U1—N4—H4B	109.5
N5—U1—N1	68.98 (10)	H4A—N4—H4B	109.5
N2—U1—N1	70.06 (10)	U1—N4—H4C	109.5
N3—U1—N1	147.94 (10)	H4A—N4—H4C	109.5
O1—U1—N4	87.96 (11)	H4B—N4—H4C	109.5
O2—U1—N4	89.59 (11)	U1—N5—H5A	109.5
N5—U1—N4	74.08 (11)	U1—N5—H5B	109.5
N2—U1—N4	147.40 (11)	H5A—N5—H5B	109.5
N3—U1—N4	69.40 (11)	U1—N5—H5C	109.5
N1—U1—N4	142.49 (10)	H5A—N5—H5C	109.5
U1—N1—H1A	109.5	H5B—N5—H5C	109.5
U1—N1—H1B	109.5	H6A—N6—H6B	104 (7)
H1A—N1—H1B	109.5	H6A—N6—H6C	103 (10)
U1—N1—H1C	109.5	H6B—N6—H6C	121 (10)
H1A—N1—H1C	109.5	O3—N7—O5	120.4 (3)
H1B—N1—H1C	109.5	O3—N7—O4	120.4 (3)
U1—N2—H2A	109.5	O5—N7—O4	119.2 (3)
U1—N2—H2B	109.5

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1A···O4i	0.91	2.43	3.166 (4)	138
N1—H1A···O4ii	0.91	2.47	2.996 (4)	117
N1—H1B···O1iii	0.91	2.25	3.079 (4)	151
N1—H1C···O2iv	0.91	2.12	3.006 (4)	165
N2—H2A···O4i	0.91	2.49	3.220 (5)	138
N2—H2B···N6	0.91	2.14	3.024 (5)	164
N2—H2C···O4ii	0.91	2.36	3.232 (5)	160
N3—H3A···O2v	0.91	2.27	3.136 (5)	159
N3—H3B···O1vi	0.91	2.34	3.151 (4)	149
N3—H3C···O5vii	0.91	2.52	3.142 (5)	126
N4—H4A···O1vi	0.91	2.37	3.219 (4)	156
N4—H4B···O2v	0.91	2.26	3.086 (4)	150
N4—H4C···O3	0.91	2.55	3.253 (5)	134
N5—H5A···O5iii	0.91	2.14	3.048 (5)	176
N5—H5B···O3	0.91	2.44	3.063 (5)	126
N5—H5B···O5	0.91	2.59	3.394 (5)	147
N5—H5C···O1iii	0.91	2.37	3.273 (4)	171
N6—H6A···O4vii	0.86 (7)	2.50 (7)	3.342 (6)	167 (7)
N6—H6B···O3vi	0.81 (8)	2.32 (8)	3.102 (6)	162 (7)