Uranyl–organic one- and two-dimensional assemblies with 2,2′-bipyridine-3,3′-dicarboxylic, biphenyl-3,3′,4,4′-tetracarboxylic and bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic acids

Abstract

The complexes formed by uranyl ions with two aromatic and one alicyclic polycarboxylic acids have been crystallographically characterized. Three complexes were obtained under hydrothermal conditions with 2,2′-bipyridine-3,3′-dicarboxylic acid (H₂L¹), [UO₂(L¹)(H₂O)]·3H₂O (1), [UO₂(L¹)(DMF)]·0.5H₂O (2) and [UO₂(L¹)(H₂L¹)]·H₂O (3), all of which contain one-dimensional polymeric chains with the ligand being both chelating through one oxygen atom from each carboxylate group and bridging through the remaining donor atoms, the nitrogen atoms being uncoordinated. The last coordination site is occupied by either a water (1), a dimethylformamide (2), or a monodentate, zwitterionic H₂L¹ molecule (3). Biphenyl-3,3′,4,4′-tetracarboxylic acid (H₄L²) gives the complex [UO₂(H₂L²)(H₂O)₂]·2H₂O (4) under hydrothermal conditions, which is also a one-dimensional coordination polymer with uranyl chelation by only one carboxylate group from each aromatic ring. A two-dimensional assembly is finally obtained in the complex [(UO₂)₃(HL³)₂(H₂O)₆]·10H₂O (5), crystallized at room temperature, in which H₄L³ is the all-exo isomer of bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic acid. The three carboxylate groups in 5 are chelating, the ligand being thus a T-shaped node, and the layers are built from a tessellation of oblong twelve-membered rings arranged in herringbone fashion.

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Introduction

Complexation by polycarboxylates represents a prominent domain in the coordination chemistry of the uranyl ion,¹ with particular applications in uranyl–organic frameworks (UOFs) or coordination polymer synthesis, a field which has become widely investigated in recent years.² Most uranyl–organic assemblies involve polycarboxylates containing aromatic rings, and quite conformationally rigid as a result, such as phenyl- and naphthalenyl-,³ pyridyl-,⁴pyrazyl-,⁵ and pyrazolyl⁶ derivatives. More flexible, aliphatic or alicyclic ligands have also been used,⁷ as well as composite ligands comprising both an aromatic part and flexible arms.⁸ In a previous investigation of polycarboxylate ligands which had not before been associated with uranyl ions, some of them having been little used in coordination chemistry more generally, the uranyl complexes formed with the aromatic-based phenylsuccinic,^8d 1,3,5-benzenetriacetic,^8d,g and 1,2-phenylenedioxydiacetic^8g acids have been synthesized and crystallographically characterized, as well as those with the acetate-bearing, cyclohexyl-based ligand trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid.^8g All these ligands display some flexibility, imparted by the carboxylate-bearing arms, and give one- or two-dimensional assemblies with original architectures, some of them being of the bilayer type and one being a nanotubular arrangement.^8g Even with the ligand benzophenone-3,3′,4,4′-tetracarboxylic acid, which possesses a lesser degree of flexibility, it was possible to get a two-dimensional assembly of the ‘double floor’ type.^8d The present work is an extension of this investigation to two ligands in which two carboxylate-bearing aromatic rings can rotate with respect to one another around a central bond, 2,2′-bipyridine-3,3′-dicarboxylic acid (H₂L¹) and biphenyl-3,3′,4,4′-tetracarboxylic acid (H₄L²), represented in Scheme 1. These ligands only give one-dimensional assemblies, but it will be shown that a two-dimensional array can be obtained with the rigid, non-planar all-exo isomer of bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic acid (H₄L³). None of these ligands was used up to now in uranyl and, more generally, actinide chemistry; however, complexes of d-block and 4f metal ions with H₂L¹ and H₄L² are reported in the Cambridge Structural Database (CSD, Version 5.32).⁹ The third ligand, H₄L³, has seldom been used in coordination chemistry, since only the crystal structures of complexes with alkali (Li⁺–Cs⁺),¹⁰ alkaline-earth (Sr²⁺, Ba²⁺)^10,11 and Cu²⁺ ions¹² have been reported. It is well known that H₄L³ readily oxidizes to give the all-cis isomer of cyclohexanehexacarboxylic acid,¹³ which is why the complex involving this ligand was crystallized at room temperature, while the others were obtained under hydrothermal conditions.

Scheme 1 The three carboxylic acids under study.

Experimental

Synthesis

Caution ! Because uranium is a radioactive and chemically toxic element, uranium-containing samples must be handled with suitable care and protection.

UO₂(NO₃)₂·6H₂O was purchased from Prolabo and 2,2′-bipyridine-3,3′-dicarboxylic acid, biphenyl-3,3′,4,4′-tetracarboxylic dianhydride and bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride from Aldrich. Elemental analyses were performed by Service de Microanalyse du CNRS at Gif-sur-Yvette, France. Very low yields prevented elemental analyses to be performed in several cases.

[UO₂(L¹)(H₂O)]·3H₂O (1). H₂L¹ (13 mg, 0.05 mmol), a 2-fold excess of UO₂(NO₃)₂·6H₂O (50 mg, 0.10 mmol), and demineralized water (2 mL) were placed in a 10 mL tightly closed glass vessel and heated at 120 °C under autogenous pressure. Light yellow crystals of complex 1 appeared in low yield within ten days.

[UO₂(L¹)(DMF)]·0.5H₂O (2). H₂L¹ (25 mg, 0.10 mmol), UO₂(NO₃)₂·6H₂O (50 mg, 0.10 mmol), dimethylformamide (0.3 mL), and demineralized water (2.5 mL) were placed in a 15 mL tightly closed glass vessel and heated at 180 °C under autogenous pressure. Light yellow crystals of complex 2 appeared in low yield within three days. Prolonged heating did not improve the yield significantly.

[UO₂(L¹)(H₂L¹)]·H₂O (3). H₂L¹ (25 mg, 0.10 mmol), UO₂(NO₃)₂·6H₂O (50 mg, 0.10 mmol), Eu(NO₃)₃·5H₂O (43 mg, 0.10 mmol), and demineralized water (1.5 mL) were placed in a 10 mL tightly closed glass vessel and heated at 120 °C under autogenous pressure. Light yellow crystals of complex 3 appeared within one month (8 mg, 21% yield based on the acid). Anal. calcd for C₂₄H₁₆N₄O₁₁U: C, 37.22; H, 2.08; N, 7.23. Found: C, 37.36; H, 1.91; N, 7.18%. The same complex is obtained when europium nitrate is replaced by caesium nitrate.

[UO₂(H₂L²)(H₂O)₂]·2H₂O (4). Biphenyl-3,3′,4,4′-tetracarboxylic dianhydride (30 mg, 0.10 mmol), UO₂(NO₃)₂·6H₂O (50 mg, 0.10 mmol), and demineralized water (2 mL) were placed in a 10 mL tightly closed glass vessel and heated at 180 °C under autogenous pressure. Light yellow crystals of complex 4 appeared in low yield within ten days. Prolonged heating did not improve the yield significantly.

[(UO₂)₃(HL³)₂(H₂O)₆]·10H₂O (5). Bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (25 mg, 0.10 mmol), UO₂(NO₃)₂·6H₂O (50 mg, 0.10 mmol), and NaOH (4 mg, 0.10 mmol) were dissolved in demineralized water (1 mL) upon heating. The solution was then left to slowly evaporate, giving light yellow crystals of complex 5 over a period of three days (10 mg, 18% yield based on U). Anal. calcd for C₂₄H₅₀O₃₈U₃: C, 17.36; H, 3.03. Found: C, 18.11; H, 2.95%.

Crystallography

The data were collected at 150(2) K on a Nonius Kappa-CCD area detector diffractometer¹⁴ using graphite-monochromated Mo-Kα radiation (λ 0.71073 Å). The crystals were introduced in glass capillaries with a protecting “Paratone-N” oil (Hampton Research) coating. The unit cell parameters were determined from ten frames, then refined on all data. The data (combinations of φ- and ω-scans giving complete datasets up to θ = 25.7° and a minimum redundancy of 4 for 90% of the reflections) were processed with HKL2000.¹⁵ Absorption effects were corrected empirically with the program SCALEPACK.¹⁵ The structures were solved by direct methods with SHELXS-97 and subsequent Fourier-difference synthesis and refined by full-matrix least-squares on F² with SHELXL-97.¹⁶ All non-hydrogen atoms were refined with anisotropic displacement parameters. The DMF molecule in 2 is disordered over two positions which have been given occupancy parameters constrained to sum to unity, and refined to values close to 0.5; the lattice solvent molecule (O8) has been given an occupancy parameter of 0.5 in order to retain an acceptable displacement parameter and so as to account for its too short contact with one of the DMF positions; restraints on bond lengths and some angles had to be applied for the disordered DMF molecule. In all structures, the hydrogen atoms bound to oxygen and nitrogen atoms were found on Fourier-difference maps (except for those of four solvent water molecules in 5) and those bound to carbon atoms were introduced at calculated positions; all were treated as riding atoms with an isotropic displacement parameter equal to 1.2 (OH₂, NH, CH) or 1.5 (CH₃) times that of the parent atom.

Crystal data and structure refinement details are given in Table 1 and selected bond lengths and angles in Table 2. The molecular plots were drawn with SHELXTL¹⁶ and Balls & Sticks.¹⁷†

Table 1 Crystal data and structure refinement details

	1	2	3	4	5
Chemical formula	C12H14N2O10U	C15H14N3O7.5U	C24H16N4O11U	C16H16O14U	C24H50O38U3
M/g mol^−1	584.28	594.32	774.44	670.32	1660.73
Crystal system	Triclinic	Triclinic	Monoclinic	Triclinic	Monoclinic
Space group	P1̄	P1̄	P21/c	P1̄	P21/n
a/Å	9.2235(10)	9.1796(6)	8.9172(6)	5.5628(3)	13.2312(4)
b/Å	9.8300(10)	9.9326(6)	27.7936(18)	6.5429(3)	12.9824(7)
c/Å	10.5498(13)	10.5607(6)	10.6743(5)	13.8049(9)	25.0960(13)
α/°	84.376(7)	88.852(4)	90	99.877(4)	90
β/°	65.630(6)	65.327(4)	113.777(4)	91.181(4)	96.853(3)
γ/°	86.336(8)	84.468(3)	90	111.977(3)	90
V/Å^3	866.81(17)	870.70(10)	2421.0(3)	457.06(5)	4280.0(3)
Z	2	2	4	1	4
D calc/g cm^−3	2.239	2.267	2.125	2.435	2.577
μ(MoKα)/mm^−1	9.416	9.368	6.778	8.959	11.443
F(000)	544	554	1472	316	3096
Reflections collected	22 943	37 545	56 757	21 604	104 646
Independent reflections	3212	3294	4576	1728	8100
Observed reflections [I > 2σ(I)]	3001	3172	3943	1720	6172
R int	0.064	0.058	0.037	0.050	0.072
Parameters refined	227	265	361	142	589
R 1	0.028	0.026	0.021	0.021	0.035
wR2	0.069	0.066	0.048	0.050	0.070
S	1.026	1.088	1.022	1.058	1.016
Δρmin/e Å^−3	−1.64	−1.32	−1.02	−1.55	−1.83
Δρmax/e Å^−3	1.08	1.37	0.78	0.63	1.12

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Table 2 Environment of the uranium atoms in compounds 1–5: selected bond lengths (Å) and angles (°)^a

a Symmetry codes are those given in the figure legends for 1–4; 5: ′ = 3/2 − x, y + 1/2, 1/2 − z; ′′ = 2 − x, −y, −z; ′′′ = 1/2 − x, y − 1/2, 1/2 − z; ′′′′ = 1 − x, −y, −z. b Labels A and B correspond to the two crystallographically independent complexes.
1	U–O1	1.763(4)	O1–U–O2	176.45(13)
	U–O2	1.748(4)	O3–U–O4′	77.66(12)
	U–O3	2.376(3)	O4′–U–O5′	79.32(11)
	U–O4′	2.346(3)	O5′–U–O6′′	72.50(11)
	U–O5′	2.355(3)	O6′′–U–O7	65.18(11)
	U–O6′′	2.404(3)	O7–U–O3	67.65(11)
	U–O7	2.430(4)
2	U–O1	1.764(3)	O1–U–O2	179.10(14)
	U–O2	1.753(3)	O3–U–O4′	75.15(12)
	U–O3	2.392(3)	O4′–U–O5′	78.33(12)
	U–O4′	2.350(4)	O5′–U–O6′′	72.18(12)
	U–O5′	2.373(3)	O6′′–U–O7	67.51(12)
	U–O6′′	2.399(3)	O7–U–O3	68.22(13)
	U–O7	2.435(4)
3	U–O1	1.760(2)	O1–U–O2	175.89(9)
	U–O2	1.767(2)	O3–U–O4′	71.10(7)
	U–O3	2.383(2)	O4′–U–O7	67.64(7)
	U–O4′	2.445(2)	O7–U–O6′′	70.06(7)
	U–O5	2.371(2)	O6′′–U–O5	73.83(7)
	U–O6′′	2.401(2)	O5–U–O3	78.90(8)
	U–O7	2.337(2)
4	U–O1	1.772(3)	O1–U–O1′	180
	U–O2	2.455(3)	O2–U–O3	52.19(9)
	U–O3	2.506(2)	O3–U–O6	64.60(10)
	U–O6	2.444(3)	O6–U–O2′	63.51(9)
5 ^b	U1–O1A	1.771(4)	O1A–U1–O2A	179.47(19)
	U1–O2A	1.758(4)	O4A–U1–O5A	51.46(13)
	U1–O4A	2.532(4)	O5A–U1–O10A′	62.14(14)
	U1–O5A	2.471(4)	O10A′–U1–O11A′	51.95(12)
	U1–O10A′	2.468(4)	O11A′–U1–O12A	65.37(13)
	U1–O11A′	2.521(4)	O12A–U1–O13A	65.60(13)
	U1–O12A	2.468(4)	O13A–U1–O4A	65.65(12)
	U1–O13A	2.449(4)
	U2–O3A	1.754(4)	O3A–U2–O3A′′	180
	U2–O6A	2.470(4)	O6A–U2–O7A	52.18(14)
	U2–O7A	2.511(4)	O7A–U2–O14A	65.74(17)
	U2–O14A	2.462(6)	O14A–U2–O6A′′	62.12(16)
	U3–O1B	1.767(4)	O1B–U3–O2B	178.58(19)
	U3–O2B	1.758(4)	O4B–U3–O5B	51.55(12)
	U3–O4B	2.537(4)	O5B–U3–O10B′′′	62.65(13)
	U3–O5B	2.479(4)	O10B′′′–U3–O11B′′′	51.70(12)
	U3–O10B′′′	2.475(4)	O11B′′′–U3–O12B	66.38(13)
	U3–O11B′′′	2.506(4)	O12B–U3–O13B	64.38(13)
	U3–O12B	2.454(4)	O13B–U3–O4B	63.64(12)
	U3–O13B	2.437(4)
	U4–O3B	1.763(4)	O3B–U4–O3B′′′′	180
	U4–O6B	2.446(4)	O6B–U4–O7B	52.00(14)
	U4–O7B	2.546(4)	O7B–U4–O14B	67.38(13)
	U4–O14B	2.459(4)	O14B–U4–O6B′′′′	66.22(14)

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Results and discussion

Three complexes were obtained from the diacid H₂L¹, [UO₂(L¹)(H₂O)]·3H₂O (1), [UO₂(L¹)(DMF)]·0.5H₂O (2) and [UO₂(L¹)(H₂L¹)]·H₂O (3), which were obtained under different conditions. Complexes 1 and 2 can be considered to be isomorphous, the only differences being the replacement of the aqua ligand in 1 by a dimethylformamide ligand in 2 (the metal ions retaining the same local geometry), and a different number of solvent water molecules. In both cases, the asymmetric unit comprises one uranyl ion and one deprotonated, dianionic ligand (Fig. 1). The metal atom is only bound to oxygen atoms, the two nitrogen atoms of the ligand being uncoordinated, which is not unexpected owing to the particular affinity of uranyl for oxygen donors. The ligand is chelating through atoms O4 and O5, pertaining to different carboxylate groups, the two remaining atoms, O3 and O6, being monodentate. The ligand is thus bound to three metal atoms, while the latter is connected to three different (L¹)²⁻ anions, the chelating one being trans with respect to the aqua/DMF ligand and the other two on each side of the latter. The average U–O(carboxylate) bond length (including both complexes) is unexceptional, at 2.37(2) Å [average U–O(carboxylate) bond length from the CSD, 2.42(7) Å], as well as the U–O(water) and U–O(DMF) bond lengths of ca. 2.43 Å [average values from the CSD, 2.43(5) and 2.39(5) Å, respectively]. The two aromatic rings are strongly tilted with respect to one another, with a dihedral angle of 74.95(14)° in 1 and 75.63(18)° in 2 permitting chelation by the two carboxylate groups while keeping them sufficiently far apart from each other. This bonding mode results in the formation of a one-dimensional polymer running along the c axis direction, with the total point symbol (4², 6) for the binodal net, as given by the TOPOS software.¹⁸ The packing index, estimated with PLATON,¹⁹ amounts to 0.66 in 1 and 0.72 in 2 (0.55 and 0.68, respectively, with solvent excluded).

Fig. 1 Views of the isomorphous complexes [UO₂(L¹)(H₂O)]·3H₂O (1) (top) and [UO₂(L¹)(DMF)]·0.5H₂O (2) (middle). Displacement ellipsoids are drawn at the 30% probability level. Solvent water molecules and carbon-bound hydrogen atoms are omitted. Only one position of the disordered DMF molecule is shown for 2. Symmetry codes: ′ = 1 − x, 2 − y, 1 − z; ′′ = x, y, z + 1; ′′′ = x, y, z − 1. Bottom: view of the polymeric chains in 1. The solvent molecules and hydrogen atoms are omitted and the uranium coordination polyhedra are represented.

The third complex obtained with the same ligand was isolated from an experiment involving a mixture of uranyl and europium nitrates, the latter being added with the aim to get a heterometallic complex, and it was also obtained in the presence of uranyl and caesium nitrates; the lanthanide or caesium ions are however absent from the compound formed. The structure of 3 is somewhat different from those of 1 and 2, which can be due either to the presence of the additional ions or to differences in the concentration of the reactants; however, complex 3 was not obtained when concentrations were varied in the absence of the additional ions. The asymmetric unit in 3 comprises one uranyl ion and two ligands, one of them fully deprotonated and the other retaining its two protons. One of the latter protons is displaced from the carboxylic group to a nitrogen atom, thus giving a zwitterionic carboxylate/pyridinium species (Fig. 2). The bonding mode of the deprotonated ligand is identical to that encountered in 1 and 2, while the monodentate H₂L¹ molecule replaces the aqua/DMF ligand of the former species. The average U–O(carboxylate) bond length is 2.39(4) Å, the bond with the monodentate ligand being the shortest. The dihedral angles between the aromatic rings are 73.75(8)° in the deprotonated molecule and 77.49(10)° in the neutral one, both values being thus close to those in 1 and 2. A one-dimensional polymer analogous to those in 1 and 2 is formed, running along the c axis direction and with the total point symbol (4², 6) for the binodal net. The bulky terminal H₂L¹ ligands are arranged in rows separating the chains, the whole packing displaying undulated sheets when viewed down the c axis. The protonated atom N4 is hydrogen bonded to the atom N1 of the proximate molecule in the same chain [N4⋯N1′′ 2.815(4) Å, N4–H⋯N1′′ 162°; ′′ = 2 − x, 1 − y, 1 − z], while O9 is hydrogen bonded to the solvent water molecule [O9⋯O11 2.540(3) Å, O9–H⋯O11 148°], the latter being in its turn bound to atoms N3 and O8 from two other chains. The packing index amounts to 0.71 (or 0.69 with solvent excluded).

Fig. 2 Top: view of the complex [UO₂(L¹)(H₂L¹)]·H₂O (3). Displacement ellipsoids are drawn at the 50% probability level. The solvent molecule and carbon-bound hydrogen atoms are omitted. Symmetry codes: ′ = 2 − x, 1 − y, −z; ′′ = 2 − x, 1 − y, 1 − z. Middle: view of the packing of chains. Bottom: view of the packing down the chain axis. The solvent molecules and hydrogen atoms are omitted in the last two views.

Uncomplexed carboxylic groups are also present in the complex [UO₂(H₂L²)(H₂O)₂]·2H₂O (4). The asymmetric unit comprises half a uranyl ion, with the uranium atom located on an inversion centre, half a centrosymmetric (H₂L²)²⁻ ligand, one coordinated and one free water molecules (Fig. 3). The uranium atom is in a hexagonal bipyramidal environment, being bound to two chelating carboxylate groups and two aqua ligands. The average U–O(carboxylate) bond length of 2.48(3) Å is larger than in 1–3, thus reflecting the larger coordination number, and the chelation is slightly asymmetric. Very simple chains of alternate uranyl and (H₂L²)²⁻ moieties are formed, which run along the c axis. Atom O5 is hydrogen bonded to the solvent water molecule [O5⋯O7 2.618(4) Å, O5–H⋯O7 163°], while the water molecules, both complexed and free, are bound to uranyl oxo or carboxylic/ate oxygen atoms, and ensure the inter-chain connections. The packing is quite compact, with no free space present (packing index 0.76, or 0.70 with solvent excluded).

Fig. 3 Top: view of the complex [UO₂(H₂L²)(H₂O)₂]·2H₂O (4). Displacement ellipsoids are drawn at the 50% probability level. The hydrogen bonds are shown as dashed lines. The carbon-bound hydrogen atoms are omitted. Symmetry codes: ′ = 1 − x, 1 − y, 1 − z; ′′ = 1 − x, 1 − y, −z. Bottom: view of the packing of chains. The solvent molecules and hydrogen atoms are omitted.

The di-aromatic carboxylic acids H₂L¹ and H₄L² thus give only one-dimensional coordination polymers with uranyl ions under the conditions used, the former displaying a chelating (through two carboxylate groups) and bridging coordination mode and the latter a bridging, bis-chelating mode with two uncomplexed carboxylic groups which is also found in some of the d-block metal ion complexes with this ligand reported in the CSD. The low connectivity obtained with the tetracarboxylic acid H₄L² is particularly unfortunate since, for example, the closely related benzophenone-3,3′,4,4′-tetracarboxylic acid gives a two-dimensional assembly of the ‘double floor’ type, in which the two crystallographically independent ligands are bound to either five or six uranium atoms.^8d The present result contrasts also with the coordination mode of (HL²)³⁻ in 4f metal ion complexes, in which six cations are bound to each ligand, all carboxylic/ate groups being involved.²⁰ In the present case, attempts at deprotonating the two remaining carboxylic groups in the (H₂L²)²⁻ ligand by adding a base in the reaction medium proved unsuccessful since no crystalline material was recovered.

However, among other tetracarboxylic acids which have been tried, the non-planar all-exo isomer of bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic acid (H₄L³) gave an interesting complex, [(UO₂)₃(HL³)₂(H₂O)₆]·10H₂O (5). This compound was not obtained from H₄L³ itself (this reaction did not yield any crystalline material), but from its dianhydride, which generates the tetra-acid in situ. The dianhydride was also used as a starting material in the case of the alkali (Li⁺–Cs⁺) and alkaline-earth (Ba²⁺) metal ion complexes previously described.¹⁰ Other cases have been reported in which a complex could be obtained from a ligand generated in situ from a parent molecule, but not when the ligand itself was used as a reactant, for example, in the UO₂²⁺/Zn²⁺ complexes with phosphonoacetate ligands.²¹ During the synthesis of complex 5, the solution was only briefly heated, then left to slowly evaporate, so as to avoid the tetracarboxylic acid to be oxidized into the all-cis isomer of cyclohexanehexacarboxylic acid,¹³ as previously observed in the presence of lanthanide ions.^13cH₄L³ is a quite rigid molecule, in which the four carboxylic groups can only rotate around the bond linking them to the central cyclic moiety. These four groups are located on the same side of the C₆ ring to which they are attached, which is in the boat conformation, and the whole molecule thus possesses some curvature. The asymmetric unit in 5 contains two crystallographically independent but nearly identical motifs (labeled A and B), with two uranyl ions in the general position (U1, U3), two with the uranium atom located on an inversion centre (U2, U4), two (HL³)³⁻ ligands, six coordinated and ten free water molecules (Fig. 4). Each of the three carboxylate groups in each ligand is chelating one uranium atom in a slightly asymmetric way (Table 2), with an average U–O(carboxylate) bond length of 2.50(3) Å, identical to that in complex 4, in keeping with similar coordination mode and number. The metal atoms U1 and U3 are bound to two chelating groups in cis positions in the uranyl ion equatorial plane and to two adjacent aqua ligands, while the two chelating groups are in trans positions, as well as the two aqua ligands, in the case of U2 and U4. Each uranyl ion is thus bound to two (HL³)³⁻ ligands, while each ligand connects three cations as a T-shaped node. Each of the two independent motifs in the asymmetric unit extends as a two-dimensional network parallel to the (1 0 1) plane. These assemblies consist in a tessellation of oblong [(UO₂)(HL³)]₆ rings in which four ligands define the corners through bonding by two adjacent carboxylate groups, two uranyl ions with cis bis-chelation (U1, U3) define the shortest sides and two uranyl ions (one cis and one trans) and the ligand bonded to them through opposite carboxylate groups define each of the largest sides. These twelve-membered rings are attached to their neighbours in herringbone fashion, with each of the ligand nodes pertaining to three rings. The total point symbol for the binodal net is (12³)₂(12)₃, the first symbol corresponding to the anions and the second to the cations. Whereas eight of the nodes defining the largest sides are approximately coplanar, the uranyl ion defining the small side and one of the corners are much displaced with respect to the ring, so that the layers appear rugged. As a result, when viewed edge-on down the b axis, the independent layers are partly superimposed. The arrangement is quite compact, with a packing index of 0.73 (0.59 with solvent excluded). The two uncomplexed carboxylic groups point towards the ring centre and they are hydrogen bonded to solvent water molecules.

Fig. 4 Top: view of molecule A in the complex [(UO₂)₃(HL³)₂(H₂O)₆]·10H₂O (5). Displacement ellipsoids are drawn at the 30% probability level. The solvent molecules and carbon-bound hydrogen atoms are omitted. Symmetry codes: ′ = 3/2 − x, y + 1/2, 1/2 − z; ′′ = 2 − x, −y, −z; ′′′ = 3/2 − x, y − 1/2, 1/2 − z. Middle: view of the two-dimensional assembly down the a axis. Bottom: view of the packing with the sheets parallel to (1 0 1) viewed edge-on (yellow: U1, U2; green: U3, U4). The solvent molecules and hydrogen atoms are omitted.

Conclusions

The complexes formed by uranyl ions with three polycarboxylic acids, either aromatic with two (H₂L¹) or four (H₄L²) carboxylic groups attached or alicyclic with four acid groups (H₄L³), were investigated. Both H₂L¹ and H₄L² comprise two aromatic rings (pyridyl and phenyl, respectively) held together by a single bond. In their uranyl ion complexes obtained under hydrothermal conditions, the pyridyl nitrogen atoms in H₂L¹ and one carboxylate group on each aromatic ring in H₄L² are left uncoordinated, so that both ligands possess only one carboxylate donor group on each aromatic ring. The different positions of these groups result in different coordination modes: in complexes 1–3, (L¹)²⁻ is chelating one cation through one oxygen atom from each carboxylate group, and it is further bridging through the two remaining atoms, while, in complex 4, (H₂L²)²⁻ is doubly chelating through the two carboxylate groups located in the 3 and 3′ positions, which are divergent since the molecule sits on an inversion centre. One-dimensional coordination polymers are formed with both ligands, a situation very frequent with the uranyl ion, as exemplified by the complexes recently reported with various polycarboxylates,^{3d,f,g,4g,5a,7h,8g,22} carboxyphosphonates,²³squarate²⁴ or hydroxamate²⁵ ligands. However, it is notable that the full complexing potential of H₄L² is not reached, in contrast to what is observed with the lanthanide ion complexes with this ligand,²⁰ and also with the uranyl complex of the related ligand benzophenone-3,3′,4,4′-tetracarboxylic acid.^8d Another tetracarboxylic acid, H₄L³, finally gave complex 5, which is a two-dimensional coordination polymer based on twelve-membered, non-planar [(UO₂)(HL³)]₆ rings. The three carboxylate groups in 5 are chelating, the remaining carboxylic function being left uncoordinated. It is well known that uranyl ions are prone to give one- or two-dimensional assemblies with quasi-planar geometries, due to the presence of the two oxo groups which enforce an equatorial positioning of the other ligands. Particularly clear-cut examples of this are to be found with polycarboxylates which are themselves able to display donor atoms sets close to planarity, for example, 2-pyridylacetic,^8a 1,2-phenylenedioxydiacetic,^8g nitrilotriacetic,²⁶ benzene-1,3,5-tricarboxylic,^3b and pyrazinedicarboxylic⁵ acids, all of which give planar, ribbon-like polymers, or phenylsuccinic,^8d tartaric,^7g citric and citramalic,²⁷ pyridinedicarboxylic,^4g and various aliphatic dicarboxylic acids,^7d,f,m which yield planar two-dimensional assemblies. In all these cases, the uranyl equatorial plane is close to the average plane of the assembly. This is at variance with the geometry observed in 5, in which the uranyl equatorial planes are strongly tilted with respect to the layer mean plane, while the coordination sites of the ligand are not all directed in-plane. The restriction of the assembly formed to a two-dimensional geometry arises here from the presence of the terminal water ligands which prevent the extension of the polymer in a direction transverse to the layers to give a three-dimensional framework, and it may thus be viewed as accidental and not as a result of the geometry of the components (the large number of coordinated water molecules in this case is likely a consequence of the synthesis having been performed at room temperature).

Acknowledgements

The Direction de l'Energie Nucléaire of the CEA is thanked for its financial support through the Basic Research Program RBPCH.

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Footnote

† CCDC reference numbers 832118–832122. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c1ce05772c

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