The Rb analogue of grimselite, Rb₆Na₂[(UO₂)(CO₃)₃]₂(H₂O)

The crystal structure of the Rb analogue of grimselite, rubidium sodium uranyl tricarbonate hydrate, Rb₆Na₂[(UO₂)(CO₃)₃]₂(H₂O), consists of a uranyl hexagonal bipyramid that shares three non-adjacent equatorial edges with carbonate triangles, resulting in a uranyl tricarbonate cluster of composition [(UO₂)(CO₃)₃)]. These uranyl tricarbonate clusters form layers perpendicular to [001] and are interconnected by NaO₈ polyhedra. The title compound is isostructural with grimselite, with a reduced occupancy of the H₂O site (25% versus 50% in grimselite).

S1. Comment

The compound Rb₆Na₂[(UO₂)(CO₃)₃]₂(H₂O) was obtained by evaporation of a solution containing UO₂(NO₃)₂(H₂O)₆, RbNO₃ and Na₂CO₃ at ambient temperature and pressure. Rb₆Na₂[(UO₂)(CO₃)₃]₂(H₂O) is structurally analogous to the mineral grimselite, K₃Na[(UO₂)(CO₃)₃](H₂O) (Li & Burns, 2001). Each contains the uranyl tricarbonate cluster, [(UO₂)(CO₃)₃], and crystallizes in space group P62c. However, these two compounds differ in their hydration state.

There is one symmetrically distinct U^VI atom in the structure of Rb₆Na₂[(UO₂)(CO₃)₃]₂(H₂O). This atom is part of a linear uranyl ion, (UO₂)²⁺, which is coordinated by six equatorial O atoms, forming a uranyl hexagonal bipyramid. Three non-adjacent equatorial edges of the uranyl hexagonal bipyramid share edges with CO₃ groups, giving the uranyl tricarbonate cluster, [(UO₂)(CO₃)₃] (Fig 1). These clusters share edges with NaO₈ hexagonal bipyramids, to form heteropolyhedral sheets parallel to (001) (Fig. 2). These sheets are stacked along [001], with voids in the resulting framework containing Rb cations and H₂O molecules. The H₂O sites are 25% occupied in the structure of Rb₆Na₂[(UO₂)(CO₃)₃]₂(H₂O), whereas they are 50% occupied in the structure of grimselite. This difference in H₂O content may be attributable to the larger size of the Rb⁺ cation.

Bond-valence sums were calculated for each ion using the parameters of Brese & O'Keeffe (1991), giving the following values: 6.18 for U, 4.15 for C, 1.29 for Rb and 1.15 for Na (Burns et al., 1997). The bond-valence sum for Rb is based on its coordination environment when the O5 site is occupied and the partial occupancy of this site may, therefore, represent a way to limit the overbonding of the Rb site. For atoms O1, O2, O3 and O4, the bond-valence sums are within the normal range (1.88–2.25). The bond-valence sum for atom O5 (0.32) is consistent with an H₂O molecule.

S2. Experimental

Transparent yellow crystals of Rb₆Na₂[(UO₂)(CO₃)₃]₂(H₂O), up to 1 mm in maximum dimension, were synthesized from a solution containing Na₂CO₃ (0.026 g), RbNO₃ (0.111 g) and UO₂(NO₃)₂(H₂O)₆ (0.123 g) in ultra-pure water (25 ml). Following evaporation in an open beaker for 28 d at 293 K under ambient pressure, crystals were recovered from the beaker walls.

S3. Refinement

The positions of the U, Rb, Na, C and most O atoms were determined by direct methods. The remaining O atoms were located by subsequent difference Fourier syntheses. The locations of the highest peak and deepest hole in the difference Fourier map are 0.75 Å from U and 0.51 Å from Rb, respectively. H-atom positions were not determined in the refinement. However, atom O2 is a possible hydrogen-bond acceptor, as it is the nearest O atom to O5 [O2···O5 3.122 (9) Å].

Computing details

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

Figures

	Fig. 1. A view of the [(UO2)(CO3)3] clusters in Rb6Na2[(UO2)(CO3)3]2(H2O), shown with 50% probability displacement ellipsoids.
	Fig. 2. The heteropolyhedral layer of NaO8 hexagonal bipyramids (grey) and [(UO2)(CO3)3] clusters (UO8 shown in white, CO3 shown in black), projected along [001] (unit cell shown as dashed black lines).

rubidium sodium uranyl tricarbonate hydrate

Crystal data

Rb6Na2[(UO2)(CO3)3]2(H2O)	Dx = 3.808 Mg m^−3
Mr = 1476.94	Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P62c	Cell parameters from 1623 reflections
Hall symbol: P -6c -2c	θ = 3.3–33.8°
a = 9.4316 (7) Å	µ = 23.95 mm^−1
c = 8.3595 (8) Å	T = 297 K
V = 643.99 (9) Å^3	Anhedral, yellow
Z = 1	0.15 × 0.12 × 0.12 mm
F(000) = 660

Data collection

Bruker APEX CCD area-detector diffractometer	787 reflections with I > 2σ(I)
ϕ and ω scans	Rint = 0.073
Absorption correction: empirical (using intensity measurements) via ψ-scan (SHELXTL-NT; Bruker, 1998)	θmax = 34.6°, θmin = 2.5°
Tmin = 0.044, Tmax = 0.056	h = −14→14
12787 measured reflections	k = −14→14
958 independent reflections	l = −13→13

Refinement

Refinement on F^2	w = 1/[σ^2(Fo^2) + (0.052P)^2] where P = (Fo^2 + 2Fc^2)/3
Least-squares matrix: full	(Δ/σ)max < 0.001
R[F^2 > 2σ(F^2)] = 0.034	Δρmax = 2.04 e Å^−3
wR(F^2) = 0.085	Δρmin = −1.69 e Å^−3
S = 0.97	Extinction correction: SHELXTL-NT (Bruker, 1998), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
958 reflections	Extinction coefficient: 0.044 (8)
40 parameters	Absolute structure: Flack (1983), with 407 Friedel pairs
0 restraints	Absolute structure parameter: −0.02 (4)

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	Occ. (<1)
U	0.6667	0.3333	0.75	0.02461 (13)
Rb	0.2764 (2)	0.0	0.0	0.0602 (4)
Na	0.3333	0.6667	0.75	0.0369 (12)
C	0.9878 (15)	0.6164 (13)	0.75	0.0330 (19)
O1	0.9642 (8)	0.4689 (15)	0.75	0.046 (2)
O2	0.8564 (9)	0.6250 (10)	0.75	0.042 (2)
O3	0.6667	0.3333	0.9628 (10)	0.0434 (17)
O4	1.1262 (10)	0.7388 (11)	0.75	0.043 (2)
O5	0.0	0.0	0.198 (4)	0.078 (14)	0.25

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
U	0.02406 (16)	0.02406 (16)	0.0257 (2)	0.01203 (8)	0.0	0.0
Rb	0.0726 (7)	0.0552 (8)	0.0471 (7)	0.0276 (4)	0.0049 (3)	0.0099 (6)
Na	0.0437 (19)	0.0437 (19)	0.023 (2)	0.0219 (10)	0.0	0.0
C	0.035 (5)	0.025 (4)	0.042 (5)	0.017 (4)	0.0	0.0
O1	0.026 (3)	0.037 (5)	0.069 (4)	0.011 (4)	0.0	0.0
O2	0.029 (4)	0.028 (4)	0.062 (5)	0.010 (3)	0.0	0.0
O3	0.049 (3)	0.049 (3)	0.032 (3)	0.0247 (14)	0.0	0.0
O4	0.029 (4)	0.039 (4)	0.059 (6)	0.014 (4)	0.0	0.0
O5	0.10 (2)	0.10 (2)	0.026 (14)	0.052 (11)	0.0	0.0

Geometric parameters (Å, º)

U—O3^i	1.779 (8)	Na—O4^vii	2.368 (9)
U—O2^ii	2.418 (8)	Na—O3^viii	2.401 (8)
U—O1	2.433 (6)	Na—O1^ix	3.018 (6)
Rb—O2^iii	2.822 (6)	C—O4	1.237 (14)
Rb—O1^iv	2.897 (9)	C—O2	1.281 (14)
Rb—O4^v	2.992 (7)	C—O1	1.295 (17)
Rb—O5^vi	3.086 (19)
O2^ii—U—O2	120.0140	O4^v—Rb—O3^xiv	122.42 (18)
O2^ii—U—O2^x	120.0080	O4^xiii—Rb—O3^xiv	62.6 (2)
O2—U—O2^x	119.9780	O5^vi—Rb—O3^xiv	117.8 (3)
O2^ii—U—O1	66.9 (4)	O5—Rb—O3^xiv	124.3 (2)
O2—U—O1	53.1 (4)	O3^iv—Rb—O3^xiv	104.83 (5)
O2^x—U—O1	173.0 (4)	O2^iii—Rb—O5^viii	118.3 (3)
O2^ii—U—O1^ii	53.1 (4)	O2^xi—Rb—O5^viii	56.2 (3)
O2—U—O1^ii	173.1 (4)	O1^iv—Rb—O5^viii	173.7 (2)
O2^x—U—O1^ii	66.9 (4)	O1^xii—Rb—O5^viii	89.4 (5)
O2^ii—U—O1^x	173.1 (4)	O4^v—Rb—O5^viii	58.0 (3)
O2—U—O1^x	66.9 (4)	O4^xiii—Rb—O5^viii	116.2 (3)
O2^x—U—O1^x	53.1 (4)	O5^vi—Rb—O5^viii	11.8 (10)
O1—U—O1^x	119.990 (5)	O5—Rb—O5^viii	76.49 (16)
O1^ii—U—O1^x	119.988 (5)	O3^iv—Rb—O5^viii	120.03 (19)
O2^iii—Rb—O2^xi	173.5 (3)	O3^xiv—Rb—O5^viii	112.0 (2)
O2^iii—Rb—O1^iv	55.8 (2)	O2^iii—Rb—O5^xv	56.2 (3)
O2^xi—Rb—O1^iv	129.9 (2)	O2^xi—Rb—O5^xv	118.3 (3)
O2^iii—Rb—O1^xii	129.9 (2)	O1^iv—Rb—O5^xv	89.4 (5)
O2^xi—Rb—O1^xii	55.8 (2)	O1^xii—Rb—O5^xv	173.7 (2)
O1^iv—Rb—O1^xii	93.5 (4)	O4^v—Rb—O5^xv	116.2 (3)
O2^iii—Rb—O4^v	92.07 (17)	O4^xiii—Rb—O5^xv	58.0 (3)
O2^xi—Rb—O4^v	87.52 (17)	O5^vi—Rb—O5^xv	76.49 (16)
O1^iv—Rb—O4^v	118.3 (2)	O5—Rb—O5^xv	11.8 (10)
O1^xii—Rb—O4^v	67.1 (2)	O3^iv—Rb—O5^xv	112.0 (2)
O4^v—Rb—O4^xiii	172.9 (3)	O3^xiv—Rb—O5^xv	120.04 (19)
O2^iii—Rb—O5^vi	110.4 (4)	O5^viii—Rb—O5^xv	88.2 (8)
O2^xi—Rb—O5^vi	63.6 (4)	O4^vii—Na—O4^x	120
O1^iv—Rb—O5^vi	164.8 (6)	Na^viii—O3—90	1_455
O1^xii—Rb—O5^vi	101.1 (6)	Na^xvi—O3—180	10_554
O4^v—Rb—O5^vi	64.8 (4)	Na^ix—O1—73.2 (3)	1_455
O4^xiii—Rb—O5^vi	108.8 (4)	Na^ix—O1—46.8 (3)	3_675
O2^xi—Rb—O5	110.4 (4)	Na^ix—O1—166.7 (3)	2_655
O5^vi—Rb—O5	64.7 (11)	Na^ix—O1—90	10_554
O2^iii—Rb—O3^iv	56.01 (19)	O4^vii—Na—O1^vii	46.8 (3)
O2^xi—Rb—O3^iv	128.96 (19)	O4^x—Na—O1^vii	73.2 (3)
O1^iv—Rb—O3^iv	55.78 (17)	O3^viii—Na—O1^vii	90
O1^xii—Rb—O3^iv	74.14 (19)	O3^xvi—Na—O1^vii	90
O4^v—Rb—O3^iv	62.6 (2)	O1^ix—Na—O1^vii	120
O4^xiii—Rb—O3^iv	122.42 (18)	O4^x—Na—O1^x	46.8 (3)
O5^vi—Rb—O3^iv	124.3 (2)	O3^viii—Na—O1^x	90
O5—Rb—O3^iv	117.8 (3)	O3^xvi—Na—O1^x	90
O2^iii—Rb—O3^xiv	128.95 (19)	O1^ix—Na—O1^x	120
O2^xi—Rb—O3^xiv	56.01 (19)	O4—C—O2	122.9 (10)
O1^iv—Rb—O3^xiv	74.13 (19)	O4—C—O1	122.5 (11)
O1^xii—Rb—O3^xiv	55.77 (17)	O2—C—O1	114.6 (10)

Symmetry codes: (i) x, y, −z+3/2; (ii) −x+y+1, −x+1, z; (iii) −x+1, −x+y, −z+1; (iv) y, x−1, −z+1; (v) x−1, y−1, z−1; (vi) y, x, −z; (vii) x−1, y, z; (viii) y, x, z−1/2; (ix) −x+y+1, −x+2, z; (x) −y+1, x−y, z; (xi) −y+1, x−y, z−1; (xii) −x+y+1, −x+1, z−1; (xiii) x−y, −y+1, −z+1; (xiv) x, y, z−1; (xv) x, y, −z+1/2; (xvi) y, x, −z+2.