π-Stacking and hydrogen bonding in catena-poly[[(4,4′-bipyridine-κN)­di­oxo­uranium(VI)]-di-μ-hydroxo]

The title compound, [UO₂(OH)₂(C₁₀H₈N₂)]_n, was obtained under hydro­thermal conditions. The U atom is seven-coordinated and its environment is penta­gonal bipyramidal, with the oxo atoms in axial positions, and one N atom and four hydroxide groups in the equatorial plane. The hydroxide ions are bridging, which results in the formation of infinite chains with the bipyridine mol­ecules alternately located on either side. Neighbouring chains inter­penetrate so that each bipyridine ligand is involved both in hydrogen bonds with two hydroxide ions and in π-stacking with its two neighbours from the next chain.

S1. Comment

Dioxouranium(VI) (uranyl) hydroxides and hydrates are known to organize as layered materials with the general formula [(UO₂)_x(O)_y(OH)_z].nH₂O (Weller et al., 2000). In particular, the α, β and γ forms of the polymorphic uranyl hydroxide UO₂(OH)₂, prepared under hydrothermal conditions, are composed of sheets in which the uranyl equatorial environment is either hexagonal or square planar (Roof et al., 1964; Bannister & Taylor, 1970; Taylor, 1971; Taylor & Hurst, 1971; Siegel et al., 1972). The complex [UO₂(OH)₂(4,4'-bipy)], (I), was also obtained under hydrothermal conditions, but coordination of the bipyridine molecule disrupts the planar arrangement of uranyl hydroxide to give an unusual mono-dimensional polymer. It is to be noted that a search of the Cambridge Structural Database (CSD; Version 5.27; Allen, 2002) gives no example of 4,4'-bipyridine coordinated to the uranyl ion. The first examples were indeed reported recently in a family of uranyl complexes with aliphatic carboxylates formed in the presence of bipyridines (Borkowski & Cahill, 2006).

The asymmetric unit in (I) comprises one uranyl ion, two hydroxo groups and one monodentate 4,4'-bipyridine molecule (Fig. 1). A polymeric chain running along the a axis is formed through metal coordination to the images of the hydroxo groups generated by the helicoidal binary axis. The U atom is thus in the usual pentagonal–bipyramidal environment and it is displaced by 0.0242 (14) Å only from the mean equatorial plane defined by atoms O3, O4, O3ⁱ, O4ⁱ and N1 [r.m.s. deviation 0.082 Å; symmetry code: (i) x - 1/2, -y + 1/2, -z + 2]. The average U1—O(oxo) and U1—O(µ-hydroxo) bond lengths are 1.795 (4) and 2.346 (4) Å, respectively (Table 1). The former is not significantly larger than the average value for comparable bonds in the CSD [1.77 (3) Å, 738 structures], whereas the latter is in perfect agreement with the value of 2.34 (3) Å for double hydroxide bridges reported in the CSD (18 structures). It is to be noted that, whereas dinuclear uranyl complexes with a double hydroxide bridge are quite common, no example of a monodimensional polymer based on this motif is present in the CSD. The U1—N1 bond length, 2.614 (7) Å, is larger than the average value for the other U—N(4,4'-bipyridine) bonds reported [2.57 (2) Å; Borkowski & Cahill, 2006]. The latter comprises both mono- and bidentate bipyridines, but this does not seem to have an influence on the U—N bond lengths.

The chains directed along the a axis display successive 4,4'-bipyridine molecules pointing on either side along the c axis, which gives a planar, double comb-like assemblage (Fig. 2). Neighbouring chains along the c axis interpenetrate in such a manner that the 4,4'-bipyridine molecules are roughly superimposable when viewed down the a axis, with a centroid offset of about 1.1 Å. The two aromatic rings in each molecule make a dihedral angle of 33.4 (2)°, but the coordinated (N1/C1–C5) and uncoordinated (N2/C6–C10) rings of neighbouring molecules are nearly parallel, with a dihedral angle of 2.9 (4)°. The presence of π-stacking interactions between these latter rings is indicated by the inter-centroid distances Cg1···Cg2ⁱ = 3.838 Å and Cg1···Cg2ⁱⁱ = 3.718 Å, where Cg1 and Cg2 are the centroids of the N1/C1–C5 and N2/C6–C10 rings, respectively [symmetry codes: (i) x – 1/2, 1/2 – y, 1 – z; (ii) x + 1/2, 1/2 – y, 1 – z] (the corresponding distances between planes are 3.48 and 3.60 Å, respectively). The shortest interatomic contacts are at about 3.6 Å on either side, and thus slightly larger than twice the out-of-plane van der Waals radius of a C atom (1.7 Å; Bondi, 1964). These distances and offsets seem large, but they are compatible with the usual parallel-displaced stacking geometry (Meyer et al., 2003). Further linking of neighbouring chains along the c axis is ensured by hydrogen bonds between the hydroxo groups and the uncoordinated N atoms (Table 2). All these weak interactions result in the formation of a two-dimensional framework parallel to the ac plane, in which ribbons of organic spacers and chains of uranyl hydroxide alternate. These sheets are superimposed along the b axis so that the projection of the U atoms of one sheet lie between the 4,4'-bipyridine molecules of the other, with no notable inter-sheet interaction, apart from van der Waals ones.

Related literature

For related literature, see: Allen (2002); Bannister & Taylor (1970); Bondi (1964); Borkowski & Cahill (2006); Meyer et al. (2003); Roof et al. (1964); Siegel et al. (1972); Taylor (1971); Taylor & Hurst (1971); Weller et al. (2000).

S2. Experimental

Uranyl nitrate hexahydrate (135 mg, 0.269 mmol) and 4,4'-bipyridine (42 mg, 0.269 mmol) were dissolved in water (3 ml). The solution was placed in a tightly sealed vessel and heated at 453 K for two days under autogenous pressure. Crystals of (I) were formed during slow cooling of the solution.

S3. Refinement

The H atoms bound to O atoms were found in a difference Fourier map and introduced as riding atoms, with U_iso(H) values of 1.2U_eq(O). All other H atoms were introduced at calculated positions as riding atoms, with C—H bond lengths of 0.93 Å and U_iso(H) values of 1.2U_eq(C). The absolute structure was determined from the value of the Flack parameter, which was refined together with the other parameters.

Computing details

Data collection: COLLECT (Nonius, 1998); cell refinement: HKL-2000 (Otwinowski & Minor, 1997); data reduction: HKL-2000; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Bruker, 1999); software used to prepare material for publication: SHELXTL (Bruker, 1999) and PLATON (Spek, 2003).

Figures

	Fig. 1. A view of compound (I), with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level [symmetry codes: (i) x - 1/2, -y + 1/2, -z + 2; (ii) x + 1/2, -y + 1/2, -z + 2.]
	Fig. 2. The packing through weak interactions in (I). Atoms are represented as spheres for clarity, C-bound H atoms are omitted and hydrogen bonds are shown as dashed lines [symmetry codes: (i) x - 1/2, -y + 1/2, -z + 2; (ii) x, y, z + 1.]

catena-poly[[(4,4'-bipyridine-κN)dioxouranium(VI)]-di-µ-hydroxo]

Crystal data

[UO2(OH)2(C10H8N2)]	F(000) = 832
Mr = 460.23	Dx = 2.714 Mg m^−3
Orthorhombic, P212121	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2ac 2ab	Cell parameters from 14902 reflections
a = 7.3964 (4) Å	θ = 2.9–25.7°
b = 10.8144 (7) Å	µ = 14.42 mm^−1
c = 14.0800 (11) Å	T = 110 K
V = 1126.23 (13) Å^3	Irregular, translucent light yellow
Z = 4	0.10 × 0.10 × 0.06 mm

Data collection

Nonius KappaCCD area-detector diffractometer	2123 independent reflections
Radiation source: fine-focus sealed tube	1901 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.068
two ϕ and one ω scan with 2° steps	θmax = 25.7°, θmin = 2.9°
Absorption correction: multi-scan (SCALEPACK; Otwinowski & Minor, 1997)	h = −8→9
Tmin = 0.213, Tmax = 0.421	k = −13→13
14902 measured reflections	l = −17→17

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.033	H-atom parameters constrained
wR(F^2) = 0.086	w = 1/[σ^2(Fo^2) + (0.0482P)^2 + 4.3193P] where P = (Fo^2 + 2Fc^2)/3
S = 1.10	(Δ/σ)max = 0.001
2123 reflections	Δρmax = 2.66 e Å^−3
155 parameters	Δρmin = −1.41 e Å^−3
0 restraints	Absolute structure: Flack (1983), 875 Friedel pairs
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.02 (3)

Special details

Experimental. crystal-to-detector distance 40 mm

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.

Refinement. Structure solved by direct methods and subsequent Fourier-difference synthesis. All non-hydrogen atoms were refined with anisotropic displacement parameters. The H atoms bound to O atoms were found on a Fourier-difference map and all the others were introduced at calculated positions. All were treated as riding atoms with an isotropic displacement parameter equal to 1.2 times that of the parent atom. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

The absolute structure was determined from the value of the Flack parameter, which was refined together with the other parameters.

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

	x	y	z	Uiso*/Ueq
U1	0.12272 (4)	0.24487 (3)	0.95909 (2)	0.01885 (14)
O1	0.1161 (10)	0.0794 (6)	0.9718 (5)	0.0251 (15)
O2	0.1297 (11)	0.4078 (5)	0.9374 (4)	0.0232 (14)
O3	0.4268 (8)	0.2249 (6)	0.9123 (5)	0.0236 (16)
H3	0.4712	0.2532	0.8605	0.028*
O4	0.3196 (7)	0.2609 (7)	1.0891 (5)	0.0268 (15)
H4	0.2869	0.2413	1.1407	0.032*
N1	0.1216 (11)	0.2327 (7)	0.7737 (5)	0.0191 (16)
N2	0.1070 (12)	0.2269 (9)	0.2699 (6)	0.029 (2)
C1	0.1657 (14)	0.1307 (9)	0.7260 (8)	0.028 (3)
H1	0.1972	0.0607	0.7606	0.034*
C2	0.1674 (14)	0.1232 (10)	0.6282 (9)	0.029 (3)
H2	0.1995	0.0501	0.5976	0.034*
C3	0.1192 (14)	0.2289 (8)	0.5760 (8)	0.020 (2)
C4	0.0827 (16)	0.3382 (10)	0.6250 (9)	0.029 (3)
H4A	0.0610	0.4113	0.5922	0.035*
C5	0.0794 (14)	0.3359 (10)	0.7238 (9)	0.029 (3)
H5	0.0473	0.4073	0.7565	0.035*
C6	0.1200 (14)	0.2273 (10)	0.4678 (8)	0.026 (2)
C7	0.0688 (13)	0.1214 (9)	0.4189 (8)	0.021 (2)
H7	0.0385	0.0492	0.4511	0.025*
C8	0.0642 (17)	0.1265 (11)	0.3213 (9)	0.038 (3)
H8	0.0291	0.0557	0.2887	0.045*
C9	0.1615 (15)	0.3257 (12)	0.3201 (9)	0.036 (3)
H9	0.2002	0.3946	0.2862	0.043*
C10	0.1647 (14)	0.3330 (10)	0.4176 (10)	0.028 (3)
H10	0.1956	0.4060	0.4485	0.034*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
U1	0.01501 (19)	0.02211 (19)	0.0194 (2)	−0.0010 (3)	0.00060 (15)	−0.00082 (18)
O1	0.022 (4)	0.029 (3)	0.024 (4)	0.001 (4)	0.014 (5)	0.002 (3)
O2	0.021 (3)	0.030 (3)	0.019 (4)	−0.005 (4)	−0.003 (4)	0.005 (3)
O3	0.019 (3)	0.038 (4)	0.013 (3)	0.002 (3)	−0.004 (3)	−0.003 (4)
O4	0.017 (3)	0.042 (4)	0.022 (3)	−0.001 (3)	0.004 (3)	0.004 (4)
N1	0.022 (4)	0.018 (3)	0.017 (4)	−0.005 (5)	0.002 (4)	−0.001 (3)
N2	0.023 (4)	0.045 (6)	0.019 (5)	0.005 (5)	−0.004 (4)	−0.009 (4)
C1	0.024 (7)	0.027 (5)	0.034 (7)	0.003 (4)	0.006 (5)	0.006 (5)
C2	0.028 (7)	0.030 (5)	0.027 (6)	0.003 (4)	0.002 (5)	−0.003 (5)
C3	0.012 (4)	0.016 (4)	0.032 (5)	−0.009 (5)	−0.007 (5)	0.004 (4)
C4	0.035 (8)	0.028 (5)	0.024 (6)	0.007 (5)	0.002 (6)	0.003 (5)
C5	0.028 (7)	0.030 (5)	0.028 (7)	0.013 (4)	−0.002 (5)	−0.002 (5)
C6	0.019 (5)	0.029 (5)	0.029 (6)	−0.001 (5)	0.012 (6)	0.004 (5)
C7	0.016 (5)	0.029 (5)	0.017 (5)	0.003 (4)	0.002 (4)	0.003 (5)
C8	0.046 (8)	0.038 (6)	0.030 (7)	0.004 (5)	−0.006 (6)	−0.010 (6)
C9	0.017 (6)	0.059 (8)	0.031 (7)	0.015 (5)	0.001 (5)	0.004 (6)
C10	0.016 (6)	0.030 (6)	0.038 (7)	0.002 (4)	0.006 (5)	−0.004 (6)

Geometric parameters (Å, º)

U1—O1	1.799 (7)	C1—C2	1.380 (17)
U1—O2	1.790 (6)	C1—H1	0.9300
U1—O3	2.353 (6)	C2—C3	1.405 (15)
U1—O4	2.345 (7)	C2—H2	0.9300
U1—O3^i	2.343 (6)	C3—C4	1.396 (14)
U1—O4^i	2.343 (6)	C3—C6	1.523 (15)
U1—N1	2.614 (7)	C4—C5	1.391 (18)
U1—U1^ii	3.8750 (3)	C4—H4A	0.9300
U1—U1^i	3.8750 (3)	C5—H5	0.9300
O3—U1^ii	2.343 (6)	C6—C10	1.384 (14)
O3—H3	0.8563	C6—C7	1.390 (15)
O4—U1^ii	2.343 (5)	C7—C8	1.376 (17)
O4—H4	0.7952	C7—H7	0.9300
N1—C1	1.332 (13)	C8—H8	0.9300
N1—C5	1.355 (12)	C9—C10	1.375 (18)
N2—C9	1.343 (15)	C9—H9	0.9300
N2—C8	1.342 (16)	C10—H10	0.9300
O1—U1—O2	175.9 (3)	N1—C1—C2	123.7 (10)
O3—U1—O4	68.4 (2)	N1—C1—H1	118.2
O4—U1—O3^i	76.7 (2)	C2—C1—H1	118.2
O3^i—U1—O4^i	68.6 (2)	C1—C2—C3	118.2 (10)
O4^i—U1—N1	72.9 (2)	C1—C2—H2	120.9
N1—U1—O3	73.7 (2)	C3—C2—H2	120.9
U1—O3—U1^ii	111.2 (3)	C4—C3—C2	118.7 (10)
U1—O4—U1^ii	111.5 (3)	C4—C3—C6	120.3 (9)
O2—U1—O3^i	90.7 (3)	C2—C3—C6	120.9 (9)
O1—U1—O3^i	92.6 (3)	C5—C4—C3	118.9 (10)
O2—U1—O4^i	90.2 (3)	C5—C4—H4A	120.6
O1—U1—O4^i	88.7 (3)	C3—C4—H4A	120.6
O2—U1—O4	92.4 (3)	N1—C5—C4	121.9 (10)
O1—U1—O4	90.7 (3)	N1—C5—H5	119.0
O4^i—U1—O4	145.28 (16)	C4—C5—H5	119.0
O2—U1—O3	90.9 (3)	C10—C6—C7	119.4 (11)
O1—U1—O3	87.9 (3)	C10—C6—C3	120.2 (9)
O3^i—U1—O3	145.15 (16)	C7—C6—C3	120.3 (10)
O4^i—U1—O3	146.2 (2)	C8—C7—C6	118.0 (11)
O2—U1—N1	83.1 (2)	C8—C7—H7	121.0
O1—U1—N1	92.8 (3)	C6—C7—H7	121.0
O3^i—U1—N1	141.0 (2)	N2—C8—C7	124.4 (11)
O4—U1—N1	141.7 (2)	N2—C8—H8	117.8
U1^ii—O3—H3	111.8	C7—C8—H8	117.8
U1—O3—H3	124.9	N2—C9—C10	125.1 (12)
U1^ii—O4—H4	123.3	N2—C9—H9	117.4
U1—O4—H4	120.4	C10—C9—H9	117.4
C1—N1—C5	118.5 (9)	C9—C10—C6	117.3 (11)
C1—N1—U1	123.0 (7)	C9—C10—H10	121.3
C5—N1—U1	118.5 (6)	C6—C10—H10	121.3
C9—N2—C8	115.5 (10)

Symmetry codes: (i) x−1/2, −y+1/2, −z+2; (ii) x+1/2, −y+1/2, −z+2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O3—H3···N2^iii	0.86	2.10	2.938 (11)	164
O4—H4···N2^iv	0.80	2.26	3.015 (11)	159

Symmetry codes: (iii) x+1/2, −y+1/2, −z+1; (iv) x, y, z+1.