Uranyl Ion Complexes with all-cis-1,3,5-Cyclohexanetricarboxylate

Article pubs.acs.org/crystal

Uranyl Ion Complexes with all-cis-1,3,5-Cyclohexanetricarboxylate: Unexpected Framework and Nanotubular Assemblies Pierre Thuéry*,† and Jack Harrowfield*,‡ †

CEA, IRAMIS, UMR 3299 CEA/CNRS, NIMBE, LCMCE, Bât. 125, 91191 Gif-sur-Yvette, France ISIS, Université de Strasbourg, 8 allée Gaspard Monge, 67083 Strasbourg, France

‡

S Supporting Information *

ABSTRACT: all-cis-1,3,5-Cyclohexanetricarboxylic acid (LH3) was reacted with uranyl nitrate under solvo-hydrothermal conditions, either alone or in the presence of additional metal cations (Na+, K+, Ni2+, Cu2+, or Tb3+), resulting in the crystallization of a series of eight complexes which were characterized by their crystal structures and luminescence properties. The six complexes [UO2(H2O)5][UO2(L)]2·2H2O·3THF (1), [Ni(bipy)2(H2O)2][UO2(L)]2·4H2O (2), [Ni(bipy)3][Ni(bipy)2(H2O)2][UO2(L)]4·5H2O (3), [Ni(H2O)6][UO2(L)]2·2H2O (4), [Cu(H2O)6][UO2(L)]2·2H2O (5), and [Tb(H2O)8][UO2(L)]3·8H2O (6) all contain the same {UO2(L)−}∞ anionic motif, in which the uranyl ion is tris-chelated by three L3− anions to give a two-dimensional assembly with hexagonal {63} topology. The reaction of uranyl nitrate alone with LH3 in water/N-methyl-2-pyrrolidone (NMP) yields the complex [(UO2)3(L)2(NMP)2] (7), which crystallizes as a three-dimensional framework. Finally, in the presence of Na+, K+, or even Kemp’s triacid (cis,cis-1,3,5trimethylcyclohexane-1,3,5-tricarboxylic acid), the complex [UO2(LH)] (8) is generated, the structure of which displays a wellresolved nanotubular species possibly associated with extremely disordered molecules or counterions of uncertain nature. These nanotubules have {63} topology and can be seen as resulting from the folding of the two-dimensional assembly present in the former complexes. Emission spectra measured in the solid state show the usual vibronic fine structure, with various degrees of resolution and quenching.

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INTRODUCTION In comparison to the polycarboxylate ligands based on the benzene ring platform, those in the corresponding alicyclic family containing the cyclohexane ring have been much less used for the synthesis of uranyl−organic coordination polymers, networks, or frameworks.1 The former group includes the benzene di-, tri-, tetra-, and hexacarboxylic/ate anions and derivatives such as naphthalenedicarboxylate, and it has given a wealth of uranyl complexes with original structures, from onedimensional (1D) chains to three-dimensional (3D) frameworks,2 which have recently been thoroughly reviewed.1e Nanotubular assemblies involving 1,2-benzenedicarboxylate are among the most unusual species obtained in this series2k (an analogous structure with the neptunyl cation has also been reported3). Few uranyl complexes with the simple cyclohexanecarboxylic acids have been reported, these being limited to 1D and 2D assemblies obtained with cyclohexane-1,3-dicarboxylic acid,4 and 2D uranyl−lanthanide(III) heterometallic species formed with all-cis-1,2,3,4,5,6-cyclohexanehexacarboxylic acid.5 An interesting feature of polycarboxylic acids based on the cyclohexane platform is the possibility of introducing substituents on the carbon atoms bearing the acid groups, which may induce conformational modifications through steric interactions. While acid groups in equatorial positions on the ring give an overall geometry close to that of the analogous aromatic counterpart, the presence of one or more axial acid groups results in an utterly different shape of the coordination © 2014 American Chemical Society

sites. This is illustrated by Kemp’s triacid (cis,cis-1,3,5-trimethylcyclohexane-1,3,5-tricarboxylic acid), in which the three acid groups are most commonly all axial on a chair conformation, resulting in a marked tendency to form closed species (metallacycles, polynuclear cages, nanotubules) with the uranyl ion,6 as well as by the monoester derivative of its cis,trans epimer, which gives an octanuclear uranyl cage.7 These results are notable since such species are not very frequently found in uranyl chemistry, with only a handful of other metallacycles8 and cages,9 the remarkable uranyl peroxide nanospheres,10 and nanotubular species based on phosphates and phosphonates,11 selenates,12 and carboxylates.2k,13 The novelty of the structures obtained with Kemp’s triacid, and also their sensitivity to the experimental conditions (solvent, presence of additional metal ions or nitrogen-containing ligands), prompted us to investigate the complexes formed by the uranyl ion with the unmethylated analogue, all-cis-1,3,5-cyclohexanetricarboxylic acid (LH3). All 90 crystal structures containing the all-cis form of this molecule referenced in the Cambridge Structural Database (CSD, 2014 release, Version 5.35)14 display the cyclohexane ring in the chair conformation with the three acid groups equatorial, so that quasiplanar uranyl species might be expected to form preferentially, as is the case with doubly deprotonated 1,3,5-benzenetricarboxylic Received: June 6, 2014 Revised: July 1, 2014 Published: July 1, 2014 4214

dx.doi.org/10.1021/cg500828s | Cryst. Growth Des. 2014, 14, 4214−4225

Crystal Growth & Design

Article

acid, which forms 1D ribbons.2c However, the eight complexes reported in the present work only partially support this simple view, and some unusual, and unexpected, architectures will be described. These complexes, most of them obtained under solvo-hydrothermal conditions and some in the presence of additional metal ions, were characterized by their crystal structures and, in most cases, by their luminescence properties.

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demineralized water (0.6 mL) were placed in a 15 mL tightly closed glass vessel and heated at 140 °C under autogenous pressure, giving light yellow crystals of complex 7 overnight (17 mg, 24% yield based on LH3). Anal. Calcd for C28H36N2O20U3: C, 23.44; H, 2.53; N, 1.95. Found: C, 23.39; H, 2.47; N, 1.76%. [UO2(LH)] (8). LH3 (44 mg, 0.20 mmol), Kemp’s triacid (13 mg, 0.05 mmol), UO2(NO3)2·6H2O (100 mg, 0.20 mmol), N-methyl-2pyrrolidone (0.3 mL), and demineralized water (0.4 mL) were placed in a 15 mL tightly closed glass vessel and heated at 140 °C under autogenous pressure, giving light yellow crystals of complex 8 (extremely thin needles or laths) in low yield within 3 days. Repeated attempts with different stoichiometries and/or longer heating periods did not lead to significant improvement in the yield. Compound 8 was also obtained when reacting LH3 (22 mg, 0.10 mmol) with UO2(NO3)2· 6H2O (100 mg, 0.20 mmol) in the presence of either KNO3 (20 mg, 0.20 mmol) or NaNO3 (19 mg, 0.22 mmol) in demineralized water (0.8 mL) at 140 °C, but the yield was even less in these cases. Crystallography. The data were collected at 150(2) K on a Nonius Kappa-CCD area detector diffractometer15 using graphitemonochromated Mo Kα radiation (λ = 0.71073 Å). The crystals were introduced into glass capillaries with a protective coating of ParatoneN oil (Hampton Research). The unit cell parameters were determined from 10 frames and then refined on all data. The data (combinations of φ- and ω-scans with a minimum redundancy of 4 for 90% of the reflections) were processed with HKL2000.16 Absorption effects were corrected empirically with the program SCALEPACK.16 The structures were solved by direct methods or Patterson map interpretation, expanded by subsequent difference Fourier synthesis, and refined by full-matrix least-squares on F2 with SHELXL-97.17 All nonhydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms bound to oxygen atoms were found on difference Fourier maps, except when indicated below. The carbon-bound hydrogen atoms were introduced at calculated positions in all compounds. All hydrogen atoms were treated as riding atoms with an isotropic displacement parameter equal to 1.2 times that of the parent atom (1.5 for CH3). Special details are as follows. Complex 3. The hydrogen atoms of the coordinated water molecule (O17) were found on a difference Fourier map but not those of the lattice water molecules. Complexes 4 and 5. The solvent water molecules were given 0.25 occupancy factors in order to retain acceptable displacement parameters and to account for the too short O···O contact between them, and they were refined with restraints on displacement parameters. The hydrogen atoms of the coordinated water molecules (O6 and O7) were found on a difference Fourier map (those of O7 being disordered over two sets of positions), but not those of the solvent water molecules. The acetonitrile solvate 5′ is isomorphous to 4 and 5, but the lower crystal quality prevented a satisfying structure refinement, so that only the crystal structure of 5 is described. Complex 6. Some solvent water molecules were given partial occupancy parameters in order to retain acceptable displacement parameters and/or so as to account for too close contacts. The hydrogen atoms of two coordinated water molecules only (O16 and O18) were found on a difference Fourier map. Complex 8. The contents of the large voids present are extremely disordered, and they could not be identified from any of the four data collections made on different crystals from three different batches. These voids may include a counterion (in which case no carboxylic proton would be retained) or extremely disordered water solvent molecules. The PLATON/SQUEEZE software18 was used so as to take into account the residual electron density in these voids. The potential solvent accessible void volume corresponds to 382 Å3 per formula unit, and the contribution of 237 electrons per uranium atom is added. This may correspond, for example, to half a UO2(H2O)52+ counterion (78 electrons) and about 16 water molecules. However, due to the uncertainty about the contents of the voids, the formula does not include the possible species present (but contains one carboxylic proton per uranium atom for charge balance). Crystal data and structure refinement parameters are given in Table 1. The molecular plots were drawn with ORTEP-319 and the polyhedral

EXPERIMENTAL SECTION

Syntheses. Uranium is a radioactive and chemically toxic element, and uranium-containing samples must be handled with suitable care and protection. UO2(NO3)2·6H2O (depleted uranium, R. P. Normapur, 99%), and Ni(NO3)2·6H2O were purchased from Prolabo, 2,2′-bipyridine (bipy) was from Fluka, and Cu(NO3)2·2.5H2O, Tb(NO3)3·5H2O, all-cis1,3,5-cyclohexanetricarboxylic acid (LH3), and Kemp’s triacid were from Aldrich. All were used without further purification. Elemental analyses were performed by MEDAC Ltd. at Chobham, UK. [UO2(H2O)5][UO2(L)]2·2H2O·3THF (1). LH3 (22 mg, 0.10 mmol), UO2(NO3)2·6H2O (50 mg, 0.10 mmol), THF (0.3 mL), and demineralized water (0.6 mL) were placed in a 15 mL tightly closed glass vessel and heated at 140 °C under autogenous pressure, giving light yellow crystals of complex 1 in low yield overnight. Repeated attempts with different stoichiometries and/or longer heating periods did not lead to a significant increase in the yield. [Ni(bipy)2(H2O)2][UO2(L)]2·4H2O (2). LH3 (22 mg, 0.10 mmol), UO2(NO3)2·6H2O (50 mg, 0.10 mmol), Ni(NO3)2·6H2O (29 mg, 0.10 mmol), 2,2′-bipyridine (16 mg, 0.10 mmol), N-methyl-2pyrrolidone (NMP, 0.3 mL), and demineralized water (1 mL) were placed in a 15 mL tightly closed glass vessel and heated at 140 °C under autogenous pressure, giving light yellow crystals of complex 2 in low yield overnight, mixed with an amorphous precipitate which was not further characterized. [Ni(bipy)3][Ni(bipy)2(H2O)2][UO2(L)]4·5H2O (3). LH3 (22 mg, 0.10 mmol), UO2(NO3)2·6H2O (50 mg, 0.10 mmol), Ni(NO3)2· 6H2O (15 mg, 0.05 mmol), 2,2′-bipyridine (16 mg, 0.10 mmol), N-methyl-2-pyrrolidone (0.3 mL), and demineralized water (1 mL) were placed in a 15 mL tightly closed glass vessel and heated at 140 °C under autogenous pressure, giving light pink crystals of complex 3 overnight (17 mg, 23% yield based on U). Anal. Calcd for C86H90N10Ni2O39U4: C, 34.93; H, 3.07; N, 4.73. Found: C, 34.12; H, 2.91; N, 4.56%. [Ni(H 2 O) 6 ][UO 2 (L)] 2 ·2H 2 O (4). LH 3 (11 mg, 0.05 mmol), UO2(NO3)2·6H2O (25 mg, 0.05 mmol), Ni(NO3)2·6H2O (15 mg, 0.05 mmol), acetonitrile (0.3 mL), and demineralized water (0.7 mL) were placed in a 15 mL tightly closed glass vessel and heated at 140 °C under autogenous pressure, giving light yellow crystals of complex 4 within 3 days (5 mg, 17% yield based on U). Anal. Calcd for C18H34NiO24U2: C, 18.49; H, 2.93. Found: C, 18.41; H, 2.87%. [Cu(H 2 O) 6 ][UO 2 (L)] 2 ·2H 2 O (5). LH3 (11 mg, 0.05 mmol), UO2(NO3)2·6H2O (25 mg, 0.05 mmol), Cu(NO3)2·2.5H2O (12 mg, 0.05 mmol), and demineralized water (0.8 mL) were placed in a 15 mL tightly closed glass vessel and heated at 140 °C under autogenous pressure, giving light yellow crystals of complex 5 in low yield within 1 week. When the synthesis was conducted in the additional presence of acetonitrile (0.3 mL), crystals of the complex [Cu(H2O)6][UO2(L)]2·CH3CN (5′) were obtained within 3 days (23 mg, 78% yield based on U). Anal. Calcd for C20H33CuNO22U2: C, 20.37; H, 2.82; N, 1.19. Found: C, 20.05; H, 2.84; N, 1.21%. [Tb(H2 O) 8 ][UO 2 (L)] 3 ·8H 2 O (6). LH 3 (22 mg, 0.10 mmol), UO2(NO3)2·6H2O (50 mg, 0.10 mmol), Tb(NO3)3·5H2O (44 mg, 0.10 mmol), and demineralized water (1.1 mL) were placed in a 15 mL tightly closed glass vessel and heated at 140 °C under autogenous pressure, giving light yellow crystals of complex 6 in low yield within 1 week. Repeated attempts with different stoichiometries and/or longer heating periods did not lead to significant improvement in the yield. [(UO2)3(L)2(NMP)2] (7). LH3 (22 mg, 0.10 mmol), UO2(NO3)2· 6H2O (100 mg, 0.20 mmol), N-methyl-2-pyrrolidone (0.4 mL), and 4215


chemical formula M (g mol−1) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (g cm−3) μ(Mo Kα) (mm−1) F(000) reflns collcd indep reflns obsd reflns [I > 2σ(I)] Rint params refined R1 wR2 S Δρmin (e Å−3) Δρmax (e Å−3)

2 C38H46N4NiO22U2 1445.56 orthorhombic Pbcn 14.8810(13) 18.0065(15) 17.2267(8) 90 90 90 4616.0(6) 4 2.080 7.490 2760 218212 4381 3052 0.030 303 0.048 0.113 1.041 −1.79 2.35

1

C30H56O28U3 1578.84 orthorhombic Pbcn 14.1670(5) 18.6643(7) 16.8236(3) 90 90 90 4448.5(2) 4 2.357 10.987 2944 107402 6784 5286 0.026 277 0.029 0.065 1.021 −1.64 1.27

Table 1. Crystal Data and Structure Refinement Details C86H90N10Ni2O39U4 2957.22 monoclinic P2/c 15.2580(4) 18.0513(10) 16.9899(8) 90 95.344(3) 90 4659.1(4) 2 2.108 7.420 2824 168715 8833 7332 0.036 637 0.050 0.125 1.099 −1.54 2.14

3 C18H34NiO24U2 1169.22 orthorhombic Pnnm 10.0413(2) 15.5554(3) 10.0309(2) 90 90 90 1566.79(5) 2 2.478 11.001 1092 33556 2129 2037 0.018 124 0.032 0.083 1.270 −1.71 1.78

4 C18H34CuO24U2 1174.05 orthorhombic Pnnm 10.1436(5) 15.6398(7) 10.0394(4) 90 90 90 1592.69(12) 2 2.448 10.898 1094 40199 2171 1937 0.026 124 0.032 0.080 1.168 −1.47 1.62

5 C27H59O40TbU3 1896.75 orthorhombic Pnma 31.0331(13) 10.0992(3) 16.5477(7) 90 90 90 5186.2(3) 4 2.429 10.794 3528 101677 7051 5923 0.036 373 0.039 0.102 1.054 −2.83 2.39

6 C28H36N2O20U3 1434.68 triclinic P1̅ 8.9367(5) 9.4944(6) 11.9769(8) 83.967(4) 73.740(3) 67.598(3) 901.95(10) 1 2.641 13.519 654 55319 5498 4667 0.087 242 0.031 0.058 1.026 −2.32 2.35

7

C9H10O8U 484.20 hexagonal P63/m 20.7493(7) 20.7493(7) 10.0311(2) 90 90 120 3740.1(3) 6 1.290 6.524 1320 89111 2510 2225 0.037 91 0.032 0.080 1.062 −1.43 1.00

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Figure 1. Top left: View of complex 1. Displacement ellipsoids are drawn at the 50% probability level. Solvent molecules and carbon-bound hydrogen atoms are omitted. Hydrogen bonds are shown as dashed lines. Symmetry codes: i = x, 1 − y, z − 1/2; j = 1/2 − x, 1/2 − y, z − 1/2; k = x, 1 − y, z + 1/2; l = 1/2 − x, 1/2 − y, z + 1/2; m = 1 − x, y, 3/2 − z. Top right: View of the 2D assembly. Bottom: two views of the packing with sheets viewed face- or edge-on, respectively. The uranium coordination polyhedra are shown, and solvent molecules and hydrogen atoms are omitted in the last three views. representations with VESTA.20 The topological analyses were done with TOPOS.21 Luminescence Measurements. Emission spectra were recorded on solid samples using a Horiba-Jobin-Yvon Fluorolog spectrofluorimeter. Powdered complex was pressed between two silica plates which were mounted such that the faces were oriented vertically and at 45° to the incident excitation radiation. An excitation wavelength of 420 nm was used in all cases and the emissions were monitored between 450 and 650 nm.

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shown by the synthesis of 3D assemblies with various carboxylates, in which NMP is bound to uranyl.23a Complexes 2−6 were obtained in the presence of additional 3d block (Ni2+, Cu2+) or 4f (Tb3+) metal ions. Additional metal ions have often been used as a means of increasing the dimensionality of the species formed and thus overcoming the propensity of uranyl ions to generate quasi-planar architectures,1c,2q,25 but this does not occur in the present series, and they are present as counterions only. A minor but interesting point is that the uranium-containing anions studied here provide a means of crystallizing the UO22+ and Tb3+ cations as their simple (homoleptic) aqua complexes [the M(H2O)62+ cation, with M = Ni or Cu, is quite common, with 249 and 74 examples reported in the CSD, respectively, although it may be noted that the hexaaqua species is not considered to be the most stable aqua-ion of Cu2+]. Only 14 structures containing the Tb(H2O)x3+ (x = 8 or 9) cation26 are reported in the CSD, and 5 containing UO2(H2O)x2+ (x = 5 or 6).27 Crystal Structures. The uranium atoms in complexes 1−8 are in the usual pentagonal or hexagonal bipyramidal environments (both being present in 1 and 7), and the U−O bond lengths do not depart from the usual values as they can be retrieved from the CSD, so that comments on these geometric parameters will be succinct. In all of these complexes, the tricarboxylate ligand is in the chair conformation with all functional groups equatorial, which is the geometry found in all structures containing this molecule reported in the CSD, as indicated above. Geometries involving at least two groups in axial positions are very unfavorable due to steric interactions, as

RESULTS AND DISCUSSION

Synthesis. Except for complexes 5 and 6, which were synthesized under purely hydrothermal conditions, all the present compounds were obtained from solvo-hydrothermal syntheses at 140 °C (a value in the middle of the range of temperatures usually used in mild hydrothermal methods), with different organic components [acetonitrile, tetrahydrofuran (THF), N-methyl-2-pyrrolidone (NMP)]. All crystals were grown during the heating phase (not upon cooling), and their presence in the glass vials was checked visually. Using such mixtures of solvents has several advantages, such as enhancing the solubility of the reactants when this is necessary,22 and also preventing in some measure the formation of uranyl oxo/ hydroxo oligomeric species resulting from hydrolysis,6,23 which is widespread in uranyl aqueous chemistry.1,24 Indeed, all complexes 1−8 are completely devoid of hydrolysis products. Finally, potentially coordinating solvents such as NMP provide a means of generating novel species, which, somewhat counterintuitively, are not necessarily of lower dimensionality, as 4217



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well as electrostatic repulsion when the groups are anionic. This is at variance with the case of Kemp’s triacid, in which the most stable conformation corresponds to axial carboxylic acid groups (the destabilizing interactions being stronger for axial methyl groups)28 but for which other conformations can be found in deprotonated forms, and in particular in some of the uranyl complexes previously reported.6 The six compounds [UO2(H2O)5][UO2(L)]2·2H2O·3THF (1), [Ni(bipy)2(H2O)2][UO2(L)]2·4H2O (2), [Ni(bipy)3][Ni(bipy)2(H2O)2][UO2(L)]4·5H2O (3), [Ni(H2O)6][UO2(L)]2· 2H2O (4), [Cu(H2O)6][UO2(L)]2·2H2O (5), and [Tb(H2O)8][UO2(L)]3·8H2O (6) all contain the same polymeric structural motif {UO2(L)−}∞, and they differ by the counterions and the detailed shape and packing of the polymeric species. Complexes 1 and 2 are isomorphous, as well as 4 and 5. Representations of the metal environments, and of the polymeric assemblies and packings, are given in Figures 1−5 for compounds 1, 2, 3, 4, and 6, respectively (those for complex 5 do not differ in any significant way from those of 4). The asymmetric unit in 1 and 2 contains one UO2(L)− motif, and the metal center of the counterion (U or Ni) is located on a 2-fold rotation axis (Wyckoff position 4c of space group Pbcn). In 3, two crystallographically independent UO2(L)− motifs are associated with two slightly different counterions with the Ni atoms located on 2-fold rotation axes (2e and 2f in P2/c). The asymmetric unit in 4 and 5 contains only half a UO2(L)− motif, with the uranium atom (4g in Pnnm) being located on a mirror plane; the tricarboxylate ligand is bisected by a mirror plane, and the counterion has 2/m symmetry (2b). Finally, complex 6 displays a more complicated situation, with three independent uranyl ions, all located on a mirror plane (site 4c in Pnma) and three carboxylate ligands, all of them bisected by mirror planes, which form two separate groups containing either two (U1 and U2) or one (U3) UO2(L)− motifs, and one counterion with the Tb atom on a mirror plane also (4c). In all compounds 1−6, the uranyl ions (except that of the counterion in 1) are chelated by three carboxylate groups pertaining to different ligands, a bonding mode previously encountered in some complexes with the rare all-equatorial form of Kemp’s tricarboxylate, such as [Hbipy][UO2(L1)]· 0.5H2O·0.25THF where H3L1 is Kemp’s triacid,6b and also in the uranyl−lanthanide heterometallic complexes [UO2Ln(HL2)(H2O)7]·H2O (Ln = Pr, Eu, Tb, Er), where H6L2 is allcis 1,2,3,4,5,6-cyclohexanehexacarboxylic acid.5 In the latter case, the three carboxylate groups bound to uranyl ions are those in the 1, 3, and 5 positions and they are all equatorial, as in the present series. In all of these cases, a 2D assembly is formed, with the total point (Schläfli) symbol {63} (hexagonal binodal network, with all nodes trigonal). A similar topology is found in the complexes [Hbipy][UO2(HL3)]·1.5H2O and [Ni(bipy)3][UO2(HL3)]2·5H2O (H4L3 = all-trans isomer of 1,2,3,4-cyclobutanetetracarboxylic acid), in which only three functional groups of the ligand are coordinated,25a,29 as well as in d-block cation complexes with the present ligand L3−, although these subunits are assembled into a 3D framework in the latter case.30 The U−O(carboxylate) bond lengths in complexes 1−6 span the range 2.427(7)−2.509(7) Å [average 2.467(17) Å]. In all cases in which the 2,2′-bipyridine coligand is absent, the counterion is an homoleptic aqua complex hydrogen bonded to the anionic network and the water solvent molecules, while the bulky counterions Ni(bipy)2(H2O)22+ and Ni(bipy)32+ are found in 2 and 3 (the former also a hydrogen bond donor). It is notable that, although complexes of d

Figure 2. Top: View of complex 2. Displacement ellipsoids are drawn at the 30% probability level. Solvent molecules and carbon-bound hydrogen atoms are omitted. Symmetry codes: i = 1/2 − x, 3/2 − y, z + 1/2; j = 1/2 − x, y + 1/2, z; k = 1/2 − x, 3/2 − y, z − 1/2; l = 1/2 − x, y − 1/2, z; m = 1 − x, y, 3/2 − z. Middle and bottom: Two views of the packing with sheets viewed face- or edge-on, respectively. Solvent molecules and hydrogen atoms are omitted. Uranium coordination polyhedra are shown in yellow and those of nickel in green.

block30,31 and 4f cations32 with this tricarboxylic/ate ligand are known, coordination to uranyl ions is preferred in the present 4218



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Figure 3. View of complex 3. Displacement ellipsoids are drawn at the 50% probability level. Solvent molecules and carbon-bound hydrogen atoms are omitted. Symmetry codes: i = x, 1 − y, z − 1/2; j = x, y − 1, z; k = x, 1 − y, z + 1/2; l = x, y + 1, z; n = −x, y, 1/2 − z. The packing is closely related to that in complex 2 (views are given as Supporting Information).

cases, which may be ascribed both to the intrinsic high affinity of carboxylates for uranyl ions and also to the probable high stability of the hexagonal uranyl-organic network, which is consistently the outcome of the crystallization process. In all cases, the packing of the hexagonal arrays is such that uranyl ions are superimposed upon L3− ligands when viewed perpendicular to the sheets, and it gives rise to channels with an approximate diameter of ∼7 Å, which are occupied by the aqua complexes in compounds 1 and 4−6, while the bipy-containing counterions in 2 and 3 are offset, so that only the bipy aromatic rings protrude into the channels. Although the topology of the sheets is unchanged throughout the series, the degree of corrugation is highly variable. In complexes 1, 2, and 3, the sheets are undulating (particularly so in 2 and 3), and they are stacked so that interlayer channels are formed, with a section of ∼5 Å × ∼9 Å in 1 and ∼7 Å × ∼11 Å in 2 and 3. These channels contain the counterions in 2 and 3, while the latter are offset in 1, a situation reversed with respect to that relative to the hexagonal channels. The strong corrugation in 2 and 3 may thus result from structure-directing (“template”) effects exerted by the bulky counterions. The layers in 4 and 5 display an approximate triangular wave shape and there are no interlayer channels. The same shape, but closer to planarity, is found in 6; however, in this case, the sheets are separated in groups of three, with the counterions located between the central (containing U3) and the lateral layers (containing U1 and U2). Small channels with a section of ∼5 Å × ∼7 Å are formed between these groups of three, which are occupied by solvent water molecules. It is notable that the orientation of the undulations with respect to the 2D network is different in complexes 1−3 and 4−6, as is shown in Figures 1−5. The complex [(UO2)3(L)2(NMP)2] (7) has been obtained under solvo-hydrothermal conditions with N-methyl-2-pyrrolidone (NMP) as the organic component, as complexes 2 and 3, but in the absence of additional metal cations. NMP is a common solvent thermally stable at the temperatures typical of mild solvothermal conditions (a marked difference with N,Ndimethylformamide), but it is only recently that it was first used in this context.6a,23 As indicated above, it is frequently bound to uranyl cations in the final products, which is a means of generating species different from those obtained under purely

Figure 4. Top: View of complex 4. Displacement ellipsoids are drawn at the 50% probability level. Solvent molecules and carbon-bound hydrogen atoms are omitted. Hydrogen bonds are shown as dashed lines. Symmetry codes: i = x, y, −z; j = 3/2 − x, y − 1/2, 1/2 − z; k = 3/2 − x, y − 1/2, z − 1/2; l = x, y, 1 − z; m = 3/2 − x, y + 1/2, 1/2 − z; n = 1 − x, 1 − y, −z; o = 1 − x, 1 − y, z. Middle and bottom: Two views of the packing, with sheets viewed face- or edge-on, respectively. Solvent molecules and hydrogen atoms are omitted. Uranium coordination polyhedra are shown in yellow and those of nickel in green.

hydrothermal conditions. Interestingly, it enabled isolation of a 3D framework with terephthalate, which usually gives 1D or 2D 4219



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Figure 6. Top: View of complex 7. Displacement ellipsoids are drawn at the 50% probability level. Symmetry codes: i = 1 − x, −y, 1 − z; j = −x, −y, 1 − z; k = x + 1, y, z; l = 1 − x, −y, −z; m = x, y + 1, z; n = x − 1, y, z; o = x, y − 1, z. Bottom: View of the 3D framework. Hydrogen atoms are omitted in both views.

Complex 7 crystallizes in the space group P1̅, and the asymmetric unit contains two independent uranyl ions, with U1 on an inversion center and U2 in general position, one L3− ligand and one coordinated NMP molecule (Figure 6). Atom U1 is eight-coordinate (hexagonal bipyramidal environment), with two chelating and two monodentate carboxylate groups in the equatorial plane, and atom U2 is sevencoordinate (pentagonal bipyramidal environment), with four carboxylate oxygen atoms from four different ligands and one NMP oxygen atom in the equatorial plane. The U1−O (chelating carboxylate) bond lengths are 2.601(3) and 2.529(4) Å, the former, with atom O4, being larger due to the bridging nature of this donor atom. The U−O (monodentate carboxylate) bond lengths span the range 2.363(3)−2.383(2) Å [average 2.369(7) Å], while the U2−O4 bond length is slightly larger, at 2.476(2) Å. The U2−O(NMP) bond length of 2.365(4) Å is within the range previously found, 2.325(3)−2.438(6) Å [average 2.38(3) Å for 13 values].23,33 Both U1 and U2 are thus 4-fold nodes, each of them connecting four L3− ligands, while

Figure 5. Top: View of complex 6. Displacement ellipsoids are drawn at the 50% probability level. Solvent molecules and carbon-bound hydrogen atoms are omitted. Hydrogen bonds are shown as dashed lines. Symmetry codes: i = x, −y − 1/2, z; j = x, 1/2 − y, z; k = x, y, z + 1; l = x, 1/2 − y, z + 1; m = 1/2 − x, y − 1/2, z + 1/2; n = 1/2 − x, 1 − y, z + 1/2; o = x, y, z − 1; p = 1/2 − x, 1 − y, z − 1/2; q = x, 3/2 − y, z; r = x, y − 1, z − 1; s = x, y + 1, z. Middle: View of one group of three sheets viewed face-on. Bottom: View of the packing with sheets viewed edge-on. The uranium coordination polyhedra are shown in yellow and those of terbium in blue.

polymers with uranyl ions, and also with 2,5-thiophenedicarboxylate and nitrilotriacetate, with the presence of channels containing the NMP molecules as a common feature.23a In the present case, the difference between complexes 7 and 1, the latter also obtained in the absence of other metal ions but in the presence of THF as a cosolvent, is particularly significant. 4220



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Figure 7. Top left: View of complex 8. Displacement ellipsoids are drawn at the 50% probability level. Symmetry codes: i = x, y, 1/2 − z; j = x − y, x − 1, z − 1/2; k = x − y, x − 1, 1 − z; l = x, y, 3/2 − z; m = y + 1, y − x + 1, 1 − z. Top right: View of a nanotubule with the half part at the rear removed for clarity. Bottom left: View of the nanotubular 1D coordination polymer. Bottom right: View of the packing down the c axis. Hydrogen atoms are omitted in all views.

presence of both LH3 and Kemp’s triacid, or purely hydrothermal in the presence of KNO3 or NaNO3. This suggests that none of the additional ligand, cations or solvent is present in the final species, which points to the possibility of a UO2(H2O)52+ counterion, as in complex 1 but highly disordered. In the absence of more conclusive information, one proton was assumed to be retained by the ligand for charge balance. The asymmetric unit in 8 contains one uranyl ion located on a mirror plane (site 6h of space group P63/m) and one tricarboxylic/ate ligand bisected by a mirror plane, as in complexes 4−6 (Figure 7). As in the 2D structures of complexes 1−6, the uranyl ion is chelated by three carboxylate groups from three ligands, with U−O bond lengths in the range 2.446(3)−2.465(4) Å [average 2.456(8) Å], but, in contrast with the previous cases, there is no formation of a 2D assembly. Instead, 8 crystallizes as a nanotubular species with the same connectivity as the 2D polymer (point symbol {63}, similar to that for a carbon nanotube). The flattened wave shape found for example in compounds 4−6, with its alternation of parts directed upward and downward, is replaced by a circular arrangement which enables the sheet to close upon itself after six repeats of the UO2(LH) fragment (the directions of the undulations and of the folding are different if the structures of 1−3 are considered, but identical for 4−6, which is why 4 is chosen as an illustration in Figure 8). It is notable that all carboxylate-based uranyl nanotubules reported up to now have trigonal or hexagonal symmetry,2k,6a,13 which is not the case in

the tricarboxylate is bound to six metal atoms, with one carboxylate group chelating and bridging, and the other two bridging bidentate in approximately syn,syn (O6, O7) or syn,anti (O8, O9) fashion. This connectivity, completely different from that in complexes 1−6, gives rise to the formation of a 3D framework with the total point symbol {44.62}3{46.69}2 (first symbol for the metal cations and second for the L3− ligands). Channels running along the a axis and with a section of ∼5 Å × ∼8 Å are occupied by the NMP molecules, a situation analogous to that found with other polycarboxylates,23a which may be indicative of a structure-directing effect exerted by these molecules. As a consequence, the framework is devoid of accessible free space, with a Kitaigorodski packing index (KPI, estimated with PLATON18) of 0.70, and it contains no solvent molecules. The last complex in this series, [UO2(LH)] (8), is the one displaying the most unexpected structure. As indicated in the Experimental Section, the four diffraction data collections made on different crystals from three different batches did not enable to determine the nature of the counterions and/or solvent molecules which are obviously present in the large voids, so that the formula of the compound is necessarily incomplete. However, the main features of the polymeric assembly are unambiguously determined and are of sufficient originality to deserve a detailed discussion. Complex 8 was crystallized in low yield as very thin needles or laths under two different sets of conditions, either solvo-hydrothermal (with NMP) and in the 4221



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Figure 8. Top: Nodal representations of the 2D assembly in 4 (left) and of the nanotubular species in 8 (right). Yellow: uranium, red: oxygen, blue: centroid of the tricarboxylate ligand. Bottom: Representation of the metal−ligand sequence in 4 and 8, viewed down the folding axis.

only be highly conjectural, particularly considering the uncertainties about the counterions and the molecules possibly included in the tubules in 8. It can be surmised that the additional molecules (Kemp’s triacid) or cations (K+ or Na+) present during the synthesis of 8 exert some kind of structuredirecting effect, different from those of the aqua complexes or bipy-containing counterions in 1−6. Similar effects were hypothesized in the case of uranyl selenate tubules, for which cylindrical micelles formed by butylammonium cations have been proposed as templating species.12a However, for such template effects to be efficient, the sheets must display a sufficient degree of geometric flexibility. Some flexibility arises in the present case from rotation of the uranyl equatorial plane with respect to the mean plane of the ligand (the plane defined by the three carbon atoms of the cyclohexane ring bearing the carboxylate substituents is considered in the following). The dihedral angles between these planes in 8 are 57.71(19)° for two of the uranium atoms bound to the ligand and 2.5(3)° for the third uranium atom. Different sets of values are measured in the other complexes, either close to one another [29.80(13), 37.70(11), and 26.94(14)° in 1] or with two small angles and one large [6.0(6), 7.3(5), and 52.0(3)° in 2; comparable values for one ligand in 3], two large angles and one small [53.0(2) (twice) and 10.1(4)° in 4; analogous values for 5 and the same tendency, albeit less marked, in 6]. These differences result in the different degree and shape of the undulations in complexes 1−6. In agreement with the identity of the directions of undulations and folding (see above), the dihedral angles in 8 appear to follow the same trend as in 4−6, which are the other cases in which the ligand has mirror symmetry. Luminescence Properties. Emission spectra under excitation at a wavelength of 420 nm in the solid state were recorded for all compounds except 2, for which a sufficient amount of crystals could not be isolated from the amorphous precipitate. Representative spectra are given in Figure 9 (and the others are given as Supporting Information). While the

other families of uranyl-based tubular compounds. The nanotubules in 8, directed along the c axis and centered on the 63 axes, have external and internal diameters of ∼20 and ∼13 Å (∼10 Å for available internal space), respectively, and they are thus much wider than the uranyl−nickel(II) heterometallic nanotubules obtained with Kemp’s tricarboxylate, which have a large external diameter of ∼18 Å, but very thick walls limiting the internal diameter to ∼6 Å.6a To the best of our knowledge, this is the first case in which a uranyl carboxylate complex can be crystallized as either a 2D network or a 1D nanotubular species with the same topology. The closed species can be seen as resulting from the folding of a sheet, for example, that of the sheets in 4 in a direction parallel to the wave crests, as shown by the nodal representation in Figure 8 (of course, this folding is only a geometric description and the mechanism of formation of the nanotubules in 8 may not involve intermediate planar species). Sheet-rolling mechanisms have previously been identified in several inorganic nanotube systems,34 and one has been proposed in the case of uranyl selenate nanotubules.12a,b This suggests that the wellknown tendency of uranyl ions to generate quasi-planar assemblies with organic as well as inorganic linkers may well be an asset for the synthesis of nanotubular compounds. As an aside, it may be worth making some comments on the choice of terms to designate these compounds. The use of the term “nanotube” is restricted to species in which the individual tubular moieties are free-standing, and it is not applicable to tube-shaped units coexisting in crystal form and that cannot be separated from one another. This is the reason for the choice of the slightly different term “nanotubule” which seems to have gained wide acceptance, particularly for uranyl-containing species,11c−e,12 although it may not be universally accepted; unfortunately, the adjective “tubular” is the same for “tube” and “tubule”. The question that arises is to know why the present uranyl− tricarboxylic/ate system crystallizes as a 2D assembly in complexes 1−6 and as a nanotubular species in 8. The answer can 4222



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solid, although subtle differences in the uranyl coordination sphere appear to be of importance in that the resolution in the spectrum of complex 8 is inferior, despite the fact that again a hexagonal bipyramidal species only is present in the solid. The well resolved but different spectrum seen for complex 6, which contains both uranyl and terbium(III) centers, and where once more the U(VI) is present in a hexagonal bipyramidal form only, may contain some weak contributions from the Tb(III) center (emission maxima at 491 and 540 nm in particular). It is well-known that transition metal ions, Cu(II) in particular, can quench uranyl ion emission,37 due to energy transfer to the d−d absorption band and nonradiative decay, and the weak intensity of the uranyl emission from complex 5 can probably be explained in this way, although the differences between the spectra for complexes 3 and 4 indicate that the presence of 2,2′bipyridine ligand on Ni(II) has an important effect (perhaps independently of the metal ion) and that the simple Ni(II) aqua-ion may not be a particularly effective quencher. The simple procedure used to obtain the spectra does not facilitate a precise quantitative comparison of emission intensities, and single crystal measurements might be justified in some cases.

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CONCLUSIONS Following previous work on the uranyl complexes with Kemp’s triacid, we have investigated the species formed under various experimental conditions with its unmethylated analogue, all-cis1,3,5-cyclohexanetricarboxylic acid (LH3). While the conformation of the former acid, with three axial carboxylic acid groups, is conducive to the formation of closed uranyl-containing species, as previously shown,6 the most stable conformation of the latter, with three equatorial functional groups, would seem destined to give only 1D or 2D planar assemblies. However, the geometry of uranyl−organic species is notoriously largely unpredictable, and this proved both true and fortunate in the present case. As could be expected, the six complexes 1−6, which were obtained under hydrothermal or solvo-hydrothermal conditions in the presence of different organic solvents and/ or additional metal ions, crystallize as 2D anionic assemblies with a {63} honeycomb topology, and they differ by the nature and position of the metal-containing counterions, the orientation and the magnitude of the corrugation, and the packing of the sheets. In the presence of uranyl ions alone and with N-methyl-2-pyrrolidone (NMP) as the organic solvent component, complex 7 can be isolated, which crystallizes as a 3D framework with channels occupied by the coordinated NMP molecules. This result is further evidence for the interest of NMP as a solvent in the synthesis of uranyl−organic species,6a,23 and it is here observed once more that solvent coordination does not necessarily result in a dimensionality decrease. The most interesting structure in this series, notwithstanding the uncertainties as to the nature of the counterions present, is that of complex 8, obtained in water/ NMP and in the presence of either Kemp’s triacid, K+ or Na+ as additional species. This complex crystallizes as a 1D assembly with a nanotubular geometry, a quite unusual result in uranyl chemistry, particularly when it involves carboxylate ligands. A uranyl−nickel(II) heterometallic nanotubular complex with Kemp’s tricarboxylate has previously been synthesized, in which the shape of the ligand results in a hydrophobic outer surface and very thick tube walls. The geometry in 8 is very different, with analogous inner and outer surfaces, thin walls, and a larger internal space of ∼10 Å in diameter. From a topological viewpoint, these nanotubules can be seen as resulting from the

Figure 9. Solid state emission spectra of complexes 1, 4, and 6. Excitation wavelength 420 nm.

vibronic progression in the ∼460−600 nm range which characterizes the emission spectra of uranyl complexes has a similar form in most cases, with typically six peaks corresponding to the S11 → S00 and S10 → S0ν (ν = 0−4) electronic transitions,35 the actual band positions are known to be sensitive to various factors, including the number of donor atoms in the equatorial plane of the uranium coordination sphere.36 Thus, the rather poorly resolved luminescence of complexes 1 and 7 may perhaps be understood in terms of the presence of both hexagonal and pentagonal bipyramidal U(VI) centers in the solids. The well-resolved spectrum of 4 (with maxima at 462, 481, 501, 523, 547, and 573 nm) is consistent with the presence of hexagonal bipyramidal species only in the 4223



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folding of the {63} hexagonal planar network, closing upon itself after six repeats of the basic unit. Structure-directing effects exerted by the additional species present during the synthesis may be operative in the formation of this complex, which shows that it is worthwhile to investigate such systems under a wide range of different experimental conditions, with the possibility of original architectures turning up when least expected. Luminescence measurements correlate well with the determined structures, both in regard to the equatorial coordination of the uranyl centers and to the presence of other metal ions within the lattice (and possibly to their particular form of coordination).

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(4) Thuéry, P. CrystEngComm 2009, 11, 232. (5) Thuéry, P. Cryst. Growth Des. 2010, 10, 2061. (6) (a) Thuéry, P. Cryst. Growth Des. 2014, 14, 901. (b) Thuéry, P. Cryst. Growth Des. 2014, 14, 2665. (7) Thuéry, P.; Nierlich, M.; Baldwin, B. W.; Komatsuzaki, N.; Hirose, T. J. Chem. Soc., Dalton Trans. 1999, 1047. (8) (a) Thuéry, P.; Villiers, C.; Jaud, J.; Ephritikhine, M.; Masci, B. J. Am. Chem. Soc. 2004, 126, 6838. (b) Unruh, D. K.; Gojdas, K.; Libo, A.; Forbes, T. Z. J. Am. Chem. Soc. 2013, 135, 7398. (9) (a) Thuéry, P.; Masci, B. Supramol. Chem. 2003, 15, 95. (b) Thuéry, P.; Masci, B. Cryst. Growth Des. 2008, 8, 3430. (c) Thuéry, P. Cryst. Growth Des. 2009, 9, 4592. (d) Pasquale, S.; Sattin, S.; Escudero-Adán, E. C.; Martínez-Belmonte, M.; de Mendoza, J. Nature Commun. 2012, 3, 785. (10) (a) Burns, P. C.; Kubatko, K. A.; Sigmon, G.; Fryer, B. J.; Gagnon, J. E.; Antonio, M. R.; Soderholm, L. Angew. Chem., Int. Ed. 2005, 44, 2135. (b) Qiu, J.; Burns, P. C. Chem. Rev. 2013, 113, 1097 and references therein. (11) (a) Poojari, D. M.; Grohol, D.; Clearfield, A. Angew. Chem., Int. Ed. 1995, 34, 1508. (b) Aranda, M. A. G.; Cabeza, A.; Bruque, S.; Poojari, D. M.; Clearfield, A. Inorg. Chem. 1998, 37, 1827. (c) Adelani, P. O.; Albrecht-Schmitt, T. E. Angew. Chem., Int. Ed. 2010, 49, 8909. (d) Adelani, P. O.; Albrecht-Schmitt, T. E. Inorg. Chem. 2011, 50, 12184. (e) Wu, S.; Wang, S.; Diwu, J.; Depmeier, W.; Malcherek, T.; Alekseev, E. V.; Albrecht-Schmitt, T. E. Chem. Commun. 2012, 48, 2334. (f) Adelani, P. O.; Cook, N. D.; Babo, J. M.; Burns, P. C. Inorg. Chem. 2014, 53, 4169. (12) (a) Krivovichev, S. V.; Kahlenberg, V.; Tananaev, I. G.; Kaindl, R.; Mersdorf, E.; Myasoedov, B. F. J. Am. Chem. Soc. 2005, 127, 1072. (b) Krivovichev, S. V.; Kahlenberg, V.; Kaindl, R.; Mersdorf, E.; Tananaev, I. G.; Myasoedov, B. F. Angew. Chem., Int. Ed. 2005, 44, 1134. (c) Albrecht-Schmitt, T. E. Angew. Chem., Int. Ed. 2005, 44, 4836. (d) Krivovichev, S. V. Eur. J. Inorg. Chem. 2010, 2594. (13) Thuéry, P. Inorg. Chem. Commun. 2008, 11, 616. (14) (a) Allen, F. H. Acta Crystallogr., Sect. B 2002, 58, 380. (b) Bruno, I. J.; Cole, J. C.; Edgington, P. R.; Kessler, M.; Macrae, C. F.; McCabe, P.; Pearson, J.; Taylor, R. Acta Crystallogr., Sect. B 2002, 58, 389. (15) Hooft, R. W. W. COLLECT; Nonius BV: Delft, The Netherlands, 1998. (16) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307. (17) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112. (18) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7. (19) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565. (20) Momma, K.; Izumi, F. J. Appl. Crystallogr. 2008, 41, 653. (21) (a) Blatov, V. A.; Shevchenko, A. P.; Serezhkin, V. N. J. Appl. Crystallogr. 2000, 33, 1193. (b) Blatov, V. A.; O’Keeffe, M.; Proserpio, D. M. CrystEngComm 2010, 12, 44. (22) Thuéry, P.; Masci, B.; Harrowfield, J. Cryst. Growth Des. 2013, 13, 3216. (23) (a) Thuéry, P.; Harrowfield, J. Cryst. Growth Des. 2014, 14, 1314. (b) Thuéry, P.; Harrowfield, J. CrystEngComm 2014, 16, 2996. (24) Knope, K. E.; Soderholm, L. Chem. Rev. 2013, 113, 944. (25) See, for example: (a) Thuéry, P. CrystEngComm 2013, 15, 6533. (b) Thuéry, P.; Rivière, E. Dalton Trans. 2013, 42, 10551. (26) (a) Harrowfield, J. M. In Met. Ions Biol. Syst.; Sigel, A., Sigel, H., Eds.; Marcel Dekker: New York, 2003; Vol. 40, Chapter 4, pp 105− 159. (b) Cotton, S. A.; Harrowfield, J. In The Rare Earth Elements: Fundamentals and Applications; Atwood, D. A., Ed.; John Wiley & Sons Ltd.: Chichester, UK, 2012; pp 55−63. (27) (a) Rogers, R. D.; Kurihara, L. K.; Benning, M. M. J. Incl. Phenom. 1987, 5, 645. (b) Deshayes, L.; Keller, N.; Lance, M.; Nierlich, M.; Vigner, J. D. Acta Crystallogr., Sect. C 1994, 50, 1541. (c) Thuéry, P. Cryst. Growth Des. 2008, 8, 4132. (d) Thuéry, P. CrystEngComm 2009, 11, 1081. (28) (a) Kemp, D. S.; Petrakis, K. S. J. Org. Chem. 1981, 46, 5140. (b) Rebek, J.; Marshall, L.; Wolak, R.; Parris, K.; Killoran, M.; Askew, B.; Nemeth, D.; Islam, N. J. Am. Chem. Soc. 1985, 107, 7476.

ASSOCIATED CONTENT

S Supporting Information *

Tables of crystal data, atomic positions and displacement parameters, anisotropic displacement parameters, and bond lengths and bond angles in CIF format; additional views of the crystal structure of 3, and excitation spectra of compounds 3, 5, 7, and 8. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.T.). *E-mail: harrowfi[email protected] (J. H.). Notes

The authors declare no competing financial interest.

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REFERENCES

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Uranyl Ion Complexes with all-cis-1,3,5-Cyclohexanetricarboxylate

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