A monoclinic polymorph of uranyl dinitrate trihydrate, [UO₂(NO₃)₂(H₂O)₂]·H₂O

Di­aqua­dinitratouranyl(VI) monohydrate is monoclinic (space group P2₁/c), in contrast to its triclinic polymorph. The main building block of the structure is the finite non-centrosymmetric [UO₂(NO₃)₂(H₂O)₂] cluster, which is a uranyl hexag­onal bipyramid that shares two non-opposite equatorial edges with the nitrate triangles, such that the two water mol­ecules are at neighbouring equatorial vertices. There is an interstitial water site in the structure, which is located between adjacent [UO₂(NO₃)₂(H₂O)₂] clusters.

S1. Comment

Single crystals of the monoclinic polymorph of [(UO₂)(H₂O)₂(NO₃)₂](H₂O) were obtained by evaporation of nitric acid in a platinum crucible with a uranyl borate glass during our attempts to synthesize novel uranyl borate compounds. The crystal structures of five uranyl nitrate compounds are known, viz. (UO₂)(NO₃)₂(H₂O)₂ (Dalley et al., 1971), triclinic [(UO₂)(H₂O)₂(NO₃)₂](H₂O) (Hughes & Burns, 2003), [(UO₂)(H₂O)₂(NO₃)₂](H₂O)₄ (Taylor & Mueller, 1965), [(UO₂)₂(OH)₂(H₂O)₃(NO₃)₂](H₂O) (Perrin, 1976) and [(UO₂)₃O(OH)₃(H₂O)₆](NO₃)(H₂O)₄ (Aberg, 1978). All of these structures involve finite clusters formed by uranyl polyhedra and nitrate groups with hydrogen bonding between neigbouring clusters provided by H₂O groups. The composition and geometric configuration of the clusters is related to the proportion of U and N atoms in the structure, as well as the hydration state. The compounds (UO₂)(NO₃)₂(H₂O)₂, [(UO₂)(H₂O)₂(NO₃)₂](H₂O) and [(UO₂)(H₂O)₂(NO₃)₂](H₂O)₄ all have a U:N ratio of 1:2 and they differ chemically only in their hydration states. Each contains the same cluster, [(UO₂)(H₂O)₂(NO₃)₂], formed by a uranyl hexagonal bipyramid that shares two opposite equatorial edges with nitrate groups. In this cluster, the remaining opposite equatorial vertices of the uranyl hexagonal bipyramid correspond to H₂O groups (Fig. 1a). In the crystal structure of [(UO₂)₂(OH)₂(H₂O)₃(NO₃)₂](H₂O), which has a U:N ratio of 1:1, two uranyl bipyramids of different (pentagonal and hexagonal) shape share an equatorial edge involving two OH groups, thus forming the [(UO₂)₂(OH)₂(H₂O)₃(NO₃)₂] cluster (Fig 1 b). The crystal structure of [(UO₂)₃O(OH)₃(H₂O)₆](NO₃)(H₂O)₄, which has a U:N ratio of 3:1, also contains uranyl bipyramids that share edges via OH groups and a central O atom, thus forming the [(UO₂)₃O(OH)₃(H₂O)₆] triuranyl cluster (Fig. 1c). H₂O groups are located at the unshared equatorial vertices of the uranyl bipyramids, and the nitrate group and the remaining H₂O groups are located in the interstitial space between the triuranyl clusters.

In this paper, we report the crystal structure of the monoclinic polymorph of [(UO₂)(H₂O)₂(NO₃)₂](H₂O). The U atom in this structure is part of a nearly linear uranyl ion, (UO₂)²⁺, which is coordinated by four O atoms and two H₂O groups arranged at the equatorial vertices of a hexagonal bipyramid, such that the two H₂O groups in the equatorial plane occupy neighboring vertices. Both symmetrically distinct nitrate triangles share an edge with the uranyl hexagonal bipyramid, thus forming the finite [(UO₂)(H₂O)₂(NO₃)₂] cluster (Fig. 2) and causing a distortion of the equatorial arrangement of the uranyl ion. Both nitrate chelate bite angles are about 50°, as expected for a four-membered chelate ring. The other L—U—L angles around the equatorial plane range between 61.7 (1) and 68.5 (2)° (Table 1). The [(UO₂)(H₂O)₂(NO₃)₂] cluster is non-centosymmetric, unlike the centrosymmetric cluster of the same composition in the crystal structure of the triclinic [(UO₂)(H₂O)₂(NO₃)₂](H₂O) (Fig. 1a and Hughes & Burns, 2003). There is one interstitial H₂O group in the monoclinic structure, which is located in the interstitial space between the uranyl nitrate clusters (Fig. 3).

Bond-valence analysis was perfomed using the parameters of Burns et al. (1997) for ^[8]U⁶⁺ and Brese & O'Keeffe for N⁵⁺. The bond-valence sums at the U, N1 and N2 sites are 6.00, 4.89 and 4.91 valence units (v.u.), respectively, which are consistent with the formal valences of the corresponding ions. The bond-valence sums for atoms O9W to O11W range from 0 to 0.45 v.u., which is consistent with their assignment as H₂O groups. The bond-valence sums for the remaining O atoms range from 1.69 to 1.97 v.u. On the basis of the analysis of interatomic distances around the O atoms of the H₂O groups (Table 1), possible hydrogen-bond acceptors for the O9W-donor sites are atoms O11Wⁱ and O7; for atom O10W, the possible acceptors are atoms O11Wⁱⁱⁱ and O8ⁱ; and for atom O11W the possible acceptors are atoms O2^iv and O7.

S2. Experimental

A uranyl borate glass of undetermined composition was prepared by heating a mixture of NaBO₂(H₂O)₄ and UO₃ in the molar ratio 100:1 in a platinum crucible at 1366 K for 15 h. Concentrated nitric acid (69.2%) was added to the crucible after cooling to 293 K. Small pale-yellow crystals were recovered from the crucible walls after complete evaporation of the nitric acid at room temperature.

S3. Refinement

The positions of the atoms U and most of the O atoms were determined by direct methods. The remaining O and N atoms were located in difference Fourier maps during structure refinement. In the final cycle of refinement, the maximum difference Fourier peaks of 1.25 and −0.79 e Å⁻³ were at distances of 0.22 and 0.70 Å, respectively, from the U-atom position. H atoms were not included in the refinement. However, the electron-density distribution of the final difference Fourier map shows peaks of 0.71–0.55 e Å⁻³ that could be assigned to H atoms.

Computing details

Data collection: SMART (Bruker, 2001); cell refinement: SAINT-Plus (Bruker, 1999); data reduction: SAINT-Plus; program(s) used to solve structure: SHELXTL (Bruker, 1998); program(s) used to refine structure: SHELXTL; molecular graphics: ATOMS (Dowty, 2000); software used to prepare material for publication: SHELXTL NT (Bruker, 1998) and WinGX (Farrugia, 1999).

Figures

	Fig. 1. Finite clusters in the crystal structures of uranyl nitrate hydrates, viz. (a) [(UO2)(H2O)2(NO3)2] (Taylor & Mueller, 1965; Dalley et al., 1971; Hughes & Burns, 2003), (b) [(UO2)2(OH)2(H2O)3(NO3)2] (Perrin, 1976) and (c) [(UO2)3O(OH)3(H2O)6] (Aberg, 1978). Uranyl polyhedra are shown in gray, nitrate triangles are shown in black, O atoms of OH groups are shown as unfilled circles and O atoms of H2O groups are shown as filled circles.
	Fig. 2. The finite [(UO2)(H2O)2(NO3)2] cluster in the crystal structure of monoclinic [(UO2)(H2O)2(NO3)2](H2O). Displacement ellipsoids are shown at the 50% probability level.
	Fig. 3. The crystal structure of monoclinic [(UO2)(H2O)2(NO3)2](H2O), projected parallel to the (100) plane. Uranyl polyhedra are shown in gray, nitrate triangles are shown in black and O atoms of interstitial H2O groups are shown as gray spheres.

diaquadinitratouranyl(VI) hydrate

Crystal data

[UO2(NO3)2(H2O)2]·H2O	F(000) = 800
Mr = 448.1	Dx = 3.281 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
a = 7.1797 (15) Å	Cell parameters from 2067 reflections
b = 8.954 (3) Å	θ = 2.7–25.4°
c = 14.301 (4) Å	µ = 17.95 mm^−1
β = 99.401 (7)°	T = 293 K
V = 907.1 (4) Å^3	Plate, pale yellow
Z = 4	0.20 × 0.10 × 0.06 mm

Data collection

Bruker APEX CCD area-detector diffractometer	2219 reflections with I > 2σ(I)
ω scans	Rint = 0.054
Absorption correction: empirical (using intensity measurements) (XPREP; Bruker, 1997)	θmax = 34.4°, θmin = 2.7°
Tmin = 0.171, Tmax = 0.375	h = −6→11
9795 measured reflections	k = −13→14
3706 independent reflections	l = −20→22

Refinement

Refinement on F^2	H-atom parameters not defined
Least-squares matrix: full	w = 1/[σ^2(Fo^2)-1.0571P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.030	(Δ/σ)max = 0.001
wR(F^2) = 0.059	Δρmax = 1.25 e Å^−3
S = 1.00	Δρmin = −0.79 e Å^−3
3706 reflections	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
128 parameters	Extinction coefficient: 0.00110 (6)
0 restraints

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
U	0.11836 (3)	0.20766 (2)	0.37909 (2)	0.03531 (8)
N1	0.4627 (7)	0.0728 (6)	0.3287 (4)	0.0503 (14)
N2	−0.1763 (8)	0.2507 (5)	0.2118 (4)	0.0434 (13)
O1	0.0347 (6)	0.0274 (4)	0.3900 (3)	0.0517 (12)
O2	0.2085 (6)	0.3912 (4)	0.3704 (3)	0.0568 (12)
O3	−0.1956 (6)	0.2888 (4)	0.2953 (3)	0.0503 (11)
O4	0.3248 (6)	0.1142 (5)	0.2674 (3)	0.0533 (12)
O5	−0.0138 (6)	0.2004 (4)	0.2055 (3)	0.0483 (11)
O6	0.4507 (6)	0.1091 (5)	0.4129 (3)	0.0595 (12)
O7	0.5920 (6)	−0.0001 (5)	0.3044 (4)	0.0681 (15)
O8	−0.3020 (7)	0.2615 (4)	0.1433 (3)	0.0554 (12)
O9W	0.2592 (6)	0.2126 (4)	0.5471 (3)	0.0556 (11)
O10W	−0.1030 (7)	0.3130 (4)	0.4781 (3)	0.0670 (14)
O11W	0.3919 (6)	0.0018 (4)	0.0818 (3)	0.0533 (12)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
U	0.03720 (13)	0.03301 (11)	0.03622 (13)	0.00012 (11)	0.00752 (9)	0.00116 (11)
N1	0.032 (3)	0.048 (3)	0.069 (5)	−0.004 (2)	0.002 (3)	0.008 (3)
N2	0.044 (3)	0.038 (3)	0.051 (4)	−0.001 (2)	0.015 (3)	0.003 (2)
O1	0.044 (3)	0.038 (2)	0.076 (4)	−0.0037 (19)	0.019 (2)	0.008 (2)
O2	0.068 (3)	0.037 (2)	0.068 (3)	0.001 (2)	0.020 (3)	0.007 (2)
O3	0.056 (3)	0.054 (2)	0.045 (3)	0.011 (2)	0.020 (2)	−0.001 (2)
O4	0.046 (3)	0.062 (3)	0.053 (3)	0.010 (2)	0.012 (2)	−0.003 (2)
O5	0.045 (3)	0.060 (3)	0.041 (3)	0.010 (2)	0.010 (2)	0.000 (2)
O6	0.043 (3)	0.077 (3)	0.054 (3)	0.008 (2)	−0.004 (2)	0.005 (3)
O7	0.040 (3)	0.059 (3)	0.110 (5)	0.010 (2)	0.025 (3)	0.003 (3)
O8	0.052 (3)	0.065 (3)	0.044 (3)	0.005 (2)	−0.008 (2)	−0.001 (2)
O9W	0.061 (3)	0.058 (3)	0.045 (3)	0.009 (2)	0.001 (2)	0.001 (2)
O10W	0.079 (4)	0.089 (3)	0.037 (3)	0.030 (3)	0.020 (3)	0.008 (2)
O11W	0.052 (3)	0.056 (3)	0.051 (3)	−0.004 (2)	0.007 (2)	0.007 (2)

Geometric parameters (Å, º)

U—O1	1.738 (3)	N2—O3	1.271 (6)
U—O2	1.778 (4)	O2—O11W^i	3.007 (6)
U—O9W	2.450 (5)	O7—O9W^ii	2.921 (6)
U—O10W	2.481 (4)	O7—O11W	3.272 (7)
U—O3	2.482 (5)	O8—O10W^iii	3.028 (5)
U—O4	2.494 (4)	O9W—O11W^iv	2.746 (5)
U—O5	2.509 (4)	O9W—O7^ii	2.921 (6)
U—O6	2.516 (4)	O10W—O11W^v	2.705 (6)
N1—O7	1.230 (6)	O10W—O8^iv	3.028 (5)
N1—O6	1.265 (6)	O11W—O10W^vi	2.705 (6)
N1—O4	1.267 (7)	O11W—O9W^iii	2.746 (5)
N2—O8	1.223 (7)	O11W—O2^vii	3.007 (6)
N2—O5	1.268 (6)	O11W—O7	3.272 (7)
O1—U—O2	178.6 (2)	O9W—U—O6	65.86 (14)
O1—U—O3	90.87 (17)	O9W—U—O10W	68.51 (15)
O2—U—O3	90.45 (17)	O10W—U—O3	63.18 (15)
O1—U—O4	89.77 (15)	O10W—U—O4	174.90 (15)
O2—U—O4	90.25 (15)	O10W—U—O5	113.63 (15)
O1—U—O5	89.11 (18)	O10W—U—O6	134.36 (15)
O2—U—O5	92.17 (17)	O7—N1—O6	125.3 (6)
O1—U—O6	89.30 (16)	O7—N1—O4	120.1 (6)
O2—U—O6	89.60 (17)	O6—N1—O4	114.6 (5)
O1—U—O9W	91.26 (17)	O8—N2—O5	122.6 (5)
O2—U—O9W	87.44 (17)	O8—N2—O3	123.5 (5)
O1—U—O10W	92.23 (15)	O5—N2—O3	113.9 (6)
O2—U—O10W	87.87 (15)	O9W^ii—O7—O11W	139.57 (18)
O3—U—O4	112.12 (14)	O11W^iv—O9W—O7^ii	113.6 (2)
O3—U—O5	50.45 (12)	O11W^v—O10W—O8^iv	87.10 (16)
O3—U—O6	162.44 (13)	O10W^vi—O11W—O9W^iii	107.4 (2)
O4—U—O5	61.70 (14)	O10W^vi—O11W—O2^vii	121.36 (17)
O4—U—O6	50.32 (14)	O9W^iii—O11W—O2^vii	129.64 (18)
O5—U—O6	112.00 (13)	O10W^vi—O11W—O7	119.81 (16)
O9W—U—O3	131.68 (13)	O9W^iii—O11W—O7	105.36 (16)
O9W—U—O4	116.15 (15)	O2^vii—O11W—O7	61.70 (13)
O9W—U—O5	177.82 (12)

Symmetry codes: (i) −x+1, y+1/2, −z+1/2; (ii) −x+1, −y, −z+1; (iii) x, −y+1/2, z−1/2; (iv) x, −y+1/2, z+1/2; (v) −x, y+1/2, −z+1/2; (vi) −x, y−1/2, −z+1/2; (vii) −x+1, y−1/2, −z+1/2.