Poly[[[diaqua­dioxouranium(VI)]-μ₃-nitriliotriacetato-κ³O:O′:O′′] trihydrate]

The basic units in the structure of the title compound, {[UO₂(C₆H₇NO₆)(H₂O)₂]·3H₂O}_n, are ribbons in which every UO₂²⁺ cation is coordinated in a monodentate manner to three tridentate-bridging nitriliotriacetate dianions. Hydrogen bonds bind the ribbons into a three-dimensional structure.

S1. Comment

In the field of human toxicology, internal contamination with actinides under either acute or chronic conditions has the potential to induce both radiological and chemical toxicity. At the molecular level, general understanding of the interactions engaged in actinide adducts, i.e. physical chemical mechanisms that drive the affinity of possible coordination sites for actinide cations still needs to be deepened. As a result, the intramolecular interactions of actinide elements with either smart chelates designed for coordination and bioorganic chemistry or naturally occurring chelating agents are relatively unknown.

Nitrilotriacetic acid [H₃nta, NH(CH₂COO)(CH₂COOH)₂] is a derivative of glycine commonly used in biochemistry and medicine. It behaves as a tri- or tetradentate aminocarboxylate chelate that is well known in complex transition metal cations. Lanthanides(III) can also form three-dimensional structures with nta³⁻ anions in various patterns (Wang et al., 2004). It can also be used as a protecting group for actinides(IV) (Np^IV and Pu^IV against hydrolysis at physiological pH).

The present investigation of the complexation of uranium by nitrilotriacetic acid is aimed at a broader understanding of the interaction of actinides with biological ligands.

The U atom in the title compound, (I), is seven-coordinate, its coordination polyhedron being a distorted pentagonal bipyramid with equatorial positions occupied by three O atoms of three Hnta²⁻ anions and two O atoms of coordinated water molecules (Fig. 1). Bond lengths in the U-atom environment are given in Table 1. The Hnta²⁻ anions act as tridentate bridging ligands, forming infinite ribbons with uranyl cations in the [010] direction (Fig. 2).

The central N atom in the Hnta²⁻ anion is protonated, preventing this atom from coordinating to U. This H atom forms three intramolecular hydrogen bonds, typical of nitrilotriacetate anions (Antsyshkina et al., 1997; Davidovich et al., 2002, Ilyukhin et al., 1998, 1999; Oliver et al., 1984), with the non-coordinated O atoms of the carboxylate groups acting as receptors (Fig. 1 and Table 2). This hydrogen bonding gives the anion nearly C₃ symmetry, as can be seen from the torsion angles given in Table 1. For this symmetric conformation of the anion the distances between the unligated O atoms of different carboxylate groups are greater than 3.5 Å; the shortest is O4···O8 [3.539 (5) Å]. Comparing this with the O···O distances in the equatorial plane of the uranyl cation [range 2.769 (5)–2.889 (5) Å, average 2.81 (2) Å], we infer that intramolecular hydrogen bonding can be a factor preventing the Hnta²⁻ anion from chelating U with the formation of an eight-membered ring. Nevertheless, single-carboxylate chelation with the formation of a four-membered ring still seems to be possible.

Coordinated and crystallization water molecules participate in a number of hydrogen bonds (Table 2). The O10—H10B···O6ⁱ hydorgen bonds link the ribbons into layers in the (002) planes. The hydrogen bonds in which the uncoordinated water molecules participate bind the layers into a three-dimensional structure. Only one hydrogen bond can be found in which the H₂O13 water molecule acts as a donor. This may be the reason that only one H atom of this molecule was located.

S2. Experimental

For the preparation of (I), a 0.1 M solution of Na₂Hnta (pH \sim 5, 0.125 ml) was added to a 0.025 M UO₂(NO₃)₂ solution (pH ~2, 0.5 ml). After several days of storage at room temperature, agglomerates of yellow needles appeared.

S3. Refinement

The H atoms were located from difference Fourier syntheses, except for one H atom of one of the three solvent water molecules. The H atoms of CH₂ groups were refined as riding in idealized positions (C—H = 0.99 Å), with displacement parameters equal to 1.2 times those of the attached C atoms. The H atoms of water molecules were refined with restraints on the O—H distances and H—O—H angles, the displacement parameters being equal to 1.2 times those of the attached O atoms. For the H atom attached to the N atom, only its orientation was fixed during refinement. The largest electron density peaks in the final difference Fourier synthesis are 2.02 e Å⁻³ (1.06 Å from atom O5 and 1.73 Å from atom U) and 1.69 e Å⁻³ (1.59 Å from H3B), and the deepest hole is 0.87 Å from atom U.

Computing details

Data collection: COLLECT (Nonius, 1998); cell refinement: HKL Package (Otwinowski & Minor, 1997); data reduction: HKL Package; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997a); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997a); molecular graphics: SHELXTL (Sheldrick, 1997b); software used to prepare material for publication: SHELXTL.

Figures

	Fig. 1. A view of [UO2(Hnta)(H2O)2]·3H2O, showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are represented by circles of arbitrary size. Dashed lines indicate the hydrogen-bonding interactions. [Symmetry code: (i) −x + 1/2, y + 1/2, −z + 1/2; (ii) −x + 1/2, y − 1/2, −z + 1/2.]
	Fig. 2. A [UO2(Hnta)(H2O)2]n ribbon in the structure of [UO2(Hnta)(H2O)2]·3H2O.

Poly[[[diaquadioxouranium(VI)]-µ₃-nitriliotriacetato-κ³O:O':O''] trihydrate]

Crystal data

[U(C6H7O6)O2(H2O)2]·3H2O	F(000) = 1024
Mr = 549.24	Dx = 2.536 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yn	Cell parameters from 16104 reflections
a = 10.8858 (3) Å	θ = 2.5–27.4°
b = 9.9530 (2) Å	µ = 11.35 mm^−1
c = 13.3275 (3) Å	T = 120 K
β = 95.0178 (15)°	Parallelepiped, yellow
V = 1438.45 (6) Å^3	0.10 × 0.06 × 0.04 mm
Z = 4

Data collection

Nonius KappaCCD area-detector diffractometer	3240 independent reflections
Radiation source: fine-focus sealed tube	2599 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.065
ϕ– and ω scans	θmax = 27.4°, θmin = 2.5°
Absorption correction: multi-scan (MULABS in PLATON; Spek, 2003)	h = −14→13
Tmin = 0.487, Tmax = 0.635	k = −12→12
16104 measured reflections	l = −16→17

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.029	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.061	H atoms treated by a mixture of independent and constrained refinement
S = 1.03	w = 1/[σ^2(Fo^2) + (0.0254P)^2] where P = (Fo^2 + 2Fc^2)/3
3240 reflections	(Δ/σ)max = 0.061
221 parameters	Δρmax = 2.02 e Å^−3
16 restraints	Δρmin = −1.71 e Å^−3

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.

Refinement. 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.

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

	x	y	z	Uiso*/Ueq
U	0.617473 (17)	0.705417 (17)	0.327790 (14)	0.01074 (7)
O1	0.5419 (3)	0.6416 (3)	0.4305 (3)	0.0161 (8)
O2	0.6979 (3)	0.7639 (3)	0.2268 (3)	0.0160 (8)
O3	0.4276 (3)	0.7658 (3)	0.2330 (3)	0.0151 (8)
O4	0.2905 (3)	0.6285 (3)	0.2961 (3)	0.0173 (8)
O5	−0.0625 (3)	0.4240 (3)	0.1211 (3)	0.0160 (8)
O6	−0.0431 (3)	0.5496 (3)	0.2622 (3)	0.0182 (8)
O7	−0.0575 (3)	1.0253 (3)	0.2809 (3)	0.0163 (8)
O8	0.0676 (4)	0.8691 (4)	0.3533 (3)	0.0231 (9)
O9	0.7827 (3)	0.7945 (4)	0.4450 (3)	0.0170 (8)
H9A	0.798 (5)	0.877 (2)	0.454 (4)	0.020*
H9B	0.848 (3)	0.751 (4)	0.460 (4)	0.020*
O10	0.7652 (3)	0.5354 (3)	0.3694 (3)	0.0157 (8)
H10A	0.767 (5)	0.466 (3)	0.404 (3)	0.019*
H10B	0.818 (4)	0.529 (5)	0.328 (3)	0.019*
O12	0.7413 (5)	0.3297 (4)	0.4932 (3)	0.0393 (13)
H12A	0.774 (7)	0.252 (4)	0.495 (5)	0.047*
H12B	0.726 (7)	0.345 (6)	0.554 (2)	0.047*
O13	0.9759 (4)	0.6448 (5)	0.5289 (3)	0.0317 (10)
H13A	0.991 (6)	0.642 (7)	0.5909 (16)	0.038*
N	0.0961 (4)	0.7338 (4)	0.1901 (3)	0.0102 (9)
H1	0.1005 (15)	0.705 (3)	0.250 (5)	0.03 (2)*
C1	0.2203 (5)	0.7937 (5)	0.1763 (4)	0.0126 (11)
H1A	0.2206	0.8899	0.1952	0.015*
H1B	0.2368	0.7873	0.1046	0.015*
C2	0.3204 (5)	0.7204 (5)	0.2409 (4)	0.0142 (11)
C3	0.0655 (5)	0.6148 (5)	0.1245 (4)	0.0140 (11)
H3A	0.1419	0.5653	0.1130	0.017*
H3B	0.0265	0.6448	0.0584	0.017*
C4	−0.0223 (5)	0.5230 (5)	0.1752 (4)	0.0136 (11)
C5	−0.0031 (5)	0.8380 (5)	0.1815 (4)	0.0131 (11)
H5A	−0.0849	0.7948	0.1695	0.016*
H5B	0.0086	0.8991	0.1245	0.016*
C6	0.0050 (5)	0.9166 (5)	0.2802 (4)	0.0132 (11)
O11	0.8345 (4)	1.0573 (4)	0.4641 (3)	0.0208 (9)
H11A	0.876 (4)	1.067 (6)	0.5206 (19)	0.025*
H11B	0.884 (4)	1.073 (6)	0.420 (3)	0.025*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
U	0.00950 (11)	0.00953 (10)	0.01327 (11)	0.00028 (8)	0.00147 (7)	0.00067 (8)
O1	0.015 (2)	0.0121 (18)	0.022 (2)	0.0000 (16)	0.0055 (16)	0.0017 (15)
O2	0.013 (2)	0.0127 (18)	0.022 (2)	−0.0014 (15)	0.0025 (16)	0.0004 (15)
O3	0.0096 (19)	0.0140 (18)	0.021 (2)	−0.0009 (15)	−0.0043 (15)	0.0027 (15)
O4	0.018 (2)	0.0127 (18)	0.021 (2)	0.0013 (16)	0.0013 (16)	0.0045 (16)
O5	0.015 (2)	0.0133 (18)	0.019 (2)	−0.0065 (15)	−0.0010 (16)	0.0025 (15)
O6	0.021 (2)	0.0177 (19)	0.017 (2)	−0.0059 (16)	0.0061 (16)	−0.0031 (16)
O7	0.020 (2)	0.0127 (18)	0.0162 (19)	0.0036 (16)	0.0042 (16)	0.0014 (15)
O8	0.029 (2)	0.023 (2)	0.017 (2)	0.0111 (19)	−0.0040 (18)	−0.0025 (17)
O9	0.012 (2)	0.0134 (18)	0.024 (2)	0.0029 (16)	−0.0075 (16)	0.0002 (17)
O10	0.014 (2)	0.0118 (18)	0.021 (2)	0.0037 (16)	0.0039 (16)	0.0043 (16)
O12	0.072 (4)	0.024 (2)	0.024 (2)	0.012 (2)	0.019 (3)	0.0070 (19)
O13	0.027 (2)	0.048 (3)	0.019 (2)	0.004 (2)	−0.003 (2)	0.006 (2)
N	0.010 (2)	0.010 (2)	0.011 (2)	−0.0016 (17)	0.0024 (17)	−0.0003 (17)
C1	0.011 (3)	0.010 (2)	0.016 (3)	−0.004 (2)	−0.003 (2)	0.001 (2)
C2	0.014 (3)	0.013 (3)	0.015 (3)	0.001 (2)	0.002 (2)	−0.002 (2)
C3	0.014 (3)	0.010 (2)	0.018 (3)	−0.003 (2)	0.002 (2)	−0.002 (2)
C4	0.010 (3)	0.012 (2)	0.018 (3)	0.001 (2)	−0.002 (2)	0.003 (2)
C5	0.012 (3)	0.015 (2)	0.012 (3)	0.001 (2)	0.001 (2)	0.002 (2)
C6	0.013 (3)	0.013 (3)	0.014 (3)	−0.002 (2)	0.003 (2)	0.001 (2)
O11	0.026 (2)	0.024 (2)	0.013 (2)	−0.0034 (18)	0.0011 (18)	−0.0003 (17)

Geometric parameters (Å, º)

U—O1	1.775 (3)	O12—H12A	0.852 (19)
U—O2	1.767 (3)	O12—H12B	0.851 (19)
U—O3	2.402 (4)	O13—H13A	0.83 (2)
U—O9	2.443 (4)	N—C3	1.493 (6)
U—O10	2.367 (3)	N—C5	1.493 (6)
U—O5^i	2.372 (3)	N—C1	1.504 (6)
U—O7^ii	2.361 (3)	N—H1	0.84 (7)
O3—C2	1.265 (6)	C1—C2	1.515 (7)
O4—C2	1.236 (6)	C1—H1A	0.9900
O5—C4	1.275 (6)	C1—H1B	0.9900
O5—U^ii	2.372 (3)	C3—C4	1.522 (7)
O6—C4	1.230 (6)	C3—H3A	0.9900
O7—C6	1.278 (6)	C3—H3B	0.9900
O7—U^i	2.361 (3)	C5—C6	1.526 (7)
O8—C6	1.233 (6)	C5—H5A	0.9900
O9—H9A	0.842 (19)	C5—H5B	0.9900
O9—H9B	0.836 (19)	O11—H11A	0.848 (19)
O10—H10A	0.827 (19)	O11—H11B	0.849 (19)
O10—H10B	0.841 (19)
O2—U—O1	177.52 (16)	C5—N—C1	111.5 (4)
O2—U—O7^ii	84.96 (14)	C3—N—H1	106.1 (12)
O1—U—O7^ii	94.56 (14)	C5—N—H1	106.8 (12)
O2—U—O10	92.41 (15)	C1—N—H1	105.7 (12)
O1—U—O10	85.11 (14)	N—C1—C2	110.5 (4)
O7^ii—U—O10	75.33 (13)	N—C1—H1A	109.6
O2—U—O5^i	94.05 (14)	C2—C1—H1A	109.6
O1—U—O5^i	87.77 (14)	N—C1—H1B	109.6
O7^ii—U—O5^i	144.04 (13)	C2—C1—H1B	109.6
O10—U—O5^i	140.51 (13)	H1A—C1—H1B	108.1
O2—U—O3	88.75 (15)	O4—C2—O3	127.4 (5)
O1—U—O3	93.41 (15)	O4—C2—C1	118.7 (5)
O7^ii—U—O3	71.83 (12)	O3—C2—C1	113.9 (4)
O10—U—O3	146.90 (12)	N—C3—C4	109.4 (4)
O5^i—U—O3	72.22 (12)	N—C3—H3A	109.8
O2—U—O9	88.89 (15)	C4—C3—H3A	109.8
O1—U—O9	90.11 (15)	N—C3—H3B	109.8
O7^ii—U—O9	144.74 (12)	C4—C3—H3B	109.8
O10—U—O9	70.28 (12)	H3A—C3—H3B	108.2
O5^i—U—O9	70.95 (12)	O6—C4—O5	128.1 (5)
O3—U—O9	142.82 (12)	O6—C4—C3	117.7 (5)
C2—O3—U	128.7 (3)	O5—C4—C3	114.3 (4)
C4—O5—U^ii	129.0 (3)	N—C5—C6	107.6 (4)
C6—O7—U^i	139.2 (3)	N—C5—H5A	110.2
U—O9—H9A	125 (3)	C6—C5—H5A	110.2
U—O9—H9B	122 (4)	N—C5—H5B	110.2
H9A—O9—H9B	108 (3)	C6—C5—H5B	110.2
U—O10—H10A	135 (3)	H5A—C5—H5B	108.5
U—O10—H10B	113 (3)	O8—C6—O7	125.2 (5)
H10A—O10—H10B	109 (3)	O8—C6—C5	118.0 (4)
H12A—O12—H12B	105 (3)	O7—C6—C5	116.7 (5)
C3—N—C5	112.5 (4)	H11A—O11—H11B	106 (3)
C3—N—C1	113.5 (4)
O2—U—O3—C2	156.8 (4)	C5—N—C1—C2	146.7 (4)
O1—U—O3—C2	−22.0 (4)	C1—N—C5—C6	−77.4 (5)
O7^ii—U—O3—C2	71.7 (4)	C3—N—C1—C2	−84.9 (5)
O10—U—O3—C2	64.3 (5)	C5—N—C3—C4	−79.8 (5)
O5^i—U—O3—C2	−108.6 (4)	N—C1—C2—O3	−179.9 (4)
O9—U—O3—C2	−116.7 (4)	N—C3—C4—O5	174.2 (4)
U—O3—C2—O4	−4.4 (8)	N—C5—C6—O7	167.7 (4)
U—O3—C2—C1	173.4 (3)	N—C1—C2—O4	−2.0 (6)
C1—N—C3—C4	152.3 (4)	N—C3—C4—O6	−7.8 (7)
U^ii—O5—C4—O6	−29.4 (8)	N—C5—C6—O8	−14.3 (6)
U^ii—O5—C4—C3	148.4 (3)	U^i—O7—C6—O8	146.2 (4)
C3—N—C5—C6	153.7 (4)	U^i—O7—C6—C5	−36.0 (7)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O9—H9A···O11	0.84 (2)	1.84 (2)	2.683 (5)	175 (6)
O9—H9B···O13	0.84 (2)	1.92 (2)	2.735 (6)	164 (6)
O10—H10A···O12	0.83 (2)	1.84 (2)	2.656 (5)	167 (5)
O10—H10B···O6^iii	0.84 (2)	1.82 (2)	2.634 (5)	163 (6)
O11—H11A···O8^iv	0.85 (2)	1.85 (2)	2.673 (5)	163 (6)
O11—H11B···O7^iii	0.85 (2)	2.07 (3)	2.819 (5)	148 (5)
O12—H12A···O11^v	0.85 (2)	2.10 (2)	2.932 (6)	166 (6)
O12—H12B···O4^vi	0.85 (2)	2.04 (2)	2.890 (5)	173 (6)
O13—H13A···O3^vii	0.83 (2)	2.27 (5)	2.953 (5)	140 (6)
N—H1···O4	0.84 (7)	2.24 (3)	2.655 (6)	111 (2)
N—H1···O6	0.84 (7)	2.22 (3)	2.614 (5)	109 (2)
N—H1···O8	0.84 (7)	2.19 (3)	2.599 (5)	110 (2)

Symmetry codes: (iii) x+1, y, z; (iv) −x+1, −y+2, −z+1; (v) x, y−1, z; (vi) −x+1, −y+1, −z+1; (vii) x+1/2, −y+3/2, z+1/2.