A Highly Adjustable Coordination System: Nanotubular and Molecular

Communication pubs.acs.org/crystal

A Highly Adjustable Coordination System: Nanotubular and Molecular Cage Species in Uranyl Ion Complexes with Kemp’s Triacid Pierre Thuéry* CEA, IRAMIS, UMR 3299 CEA/CNRS, SIS2M, LCCEf, Bât. 125, 91191 Gif-sur-Yvette, France S Supporting Information *

ABSTRACT: A unique example of topology tailoring in a uranyl−organic system is provided by the three-pronged Kemp’s tricarboxylate ligand. While unexceptional one-dimensional polymers are formed with uranyl alone under solvo-hydrothermal conditions, addition of Ni2+ ions yields the nanotubular species [(UO2)2Ni(L)2(H2O)4]∞, with a hydrophilic inner cavity lined by hydrated Ni2+ ions, and a hydrophobic outer surface. The further presence of 2,2′-bipyridine brings about the trapping of Ni2+ ions into counterions and formation of the octanuclear pseudocubic cage [(UO2)8(L)6(H2O)6]2−.

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species. An added benefit in the case of uranyl complexes is that solvents like NMP seem to provide a new means to avoid the formation of oxo/hydroxo species resulting from hydrolysis, a widespread feature of hydrous uranyl chemistry.5b−d An approach to overcome the natural tendency of uranyl ions to form quasi-planar species consists in the use of additional cations as bridges,5c and this is why nickel(II) ions were added in some of the present syntheses. The different routes to complexes 1−4 are summarized in Scheme 1.

anotubes and nanospheres are quite unusual objects in uranyl chemistry, although the former have been found among uranyl phosphonates,1 borate-phosphates,2 and selenates,3 and remarkable examples of the latter have been discovered in uranyl peroxides.4 In particular, among the many polycarboxylic acids which have been used in the synthesis of uranyl molecular complexes, coordination polymers, and frameworks,5 very few are able to form nanotubular species6 or discrete cyclic7 or cagelike structures.8 In the latter two cases, the curved shape of the bonding unit is a crucial parameter, as first shown with the monoester derivative of the cis,trans-epimer of Kemp’s triacid, which gives an octanuclear cage with additional peroxo ion bridging.8a Homoleptic octanuclear cages were later obtained with (1R,3S)-(+)-camphoric acid,8b and tri- and tetranuclear metallacycles with (2R,3R,4S,5S)-tetrahydrofurantetracarboxylic acid,7 while the saddle-shaped trans,trans,trans-1,2,3,4-cyclobutanetetracarboxylic acid yields a three-dimensional framework comprising both tetranuclear and octanuclear subunits.9 Interest remains high for such species, as witnessed by the recent report of very large capsules assembled from calixarene carboxylates.10 Kemp’s triacid (cis,cis-1,3,5-trimethylcyclohexane-1,3,5-tricarboxylic acid, LH3)11 is a promising candidate for the design of molecular cages. Although it is rarely used as a ligand for metal cations, it has been shown to subtend high-nuclearity manganese oxo clusters12 and planar octanuclear iron oxo clusters.13 Solvohydrothermal conditions with acetonitrile or N-methyl-2pyrrolidone (NMP) appeared to be suitable for the synthesis of uranyl complexes with LH3, and the four compounds [UO2(LH)]·CH3CN (1), [(UO2)3(L)2(NMP)2(H2O)]·2H2O (2), [(UO2)2Ni(L)2(H2O)4]·H2O (3), and [Ni(bipy)(H2O)4][(UO2)8(L)6(H2O)6]·H2O (4) (where bipy = 2,2′-bipyridine) could thus be obtained14 and crystallographically characterized.15 Although the primary reason for using organic solvents, either pure or mixed with water, is often to increase the solubility of the organic ligand, their presence may affect the geometry of the species formed in important ways, particularly in the case of coordinating solvents, and thus promote the formation of novel © 2014 American Chemical Society

Scheme 1. Syntheses of Complexes 1−4

Complexes 1 and 2 crystallized from mixtures of uranyl nitrate and LH3 in 2:1 water/acetonitrile or water/NMP, respectively. As shown in Figure 1, both are one-dimensional (1D) polymers, with the point (Schläfli) symbols {42.6} and {82.12}2{8}3, respectively. As in all the other complexes reported here, the uranium coordination geometry (pentagonal or hexagonal bipyramidal) and the U−O bond lengths are unexceptional. Two chelation modes of the ligand are encountered, either involving one carboxylate group (mode 1) or two (mode 2). In complex 1, the uranyl ion is chelated in both modes, giving rise to centrosymmetric dimeric subunits, and the carboxylic Received: November 15, 2013 Revised: January 7, 2014 Published: February 11, 2014 901

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the inside by the Ni(H2O)42+ groups, creating a hydrophilic environment, and on the outside by the hydrophobic carbon skeleton of the L3− ligands, while the uranyl ions occupy the median position; the walls are very thick, at ∼6 Å. The external and internal diameters are ∼18 and ∼6 Å, respectively, so that the diameter of the available internal free space (occupied by disordered water molecules) is ∼3.3 Å. As shown in the nodal representation of Figure 3, uranyl ions (point symbol {4.62})

Figure 1. View of the 1D polymers in 1 (top) and 2 (bottom, perpendicular (left) and parallel (right) to the chain axis). Hydrogen atoms are omitted.

group is uncoordinated. In 2, one L3− ligand chelates each of the three cations in mode 2 with all carboxylate groups in the axial position on the cyclohexane ring, thus generating subunits of three spatially close uranyl ions, and the other ligand connects three such subunits through mode 1 trichelation, with all groups equatorial. The chains in 2 have a tubular shape, but the internal space is negligible since the section is only ∼7 × 3 Å and NMP methyl groups protrude inside. When Ni(II) ions are added in the reaction mixture, a rearrangement occurs and while, for each ligand, two uranyl ions are chelated in mode 2, chelation of the third uranyl ion is disrupted by the presence of the nickel ions (Figure 2). The topological consequences are remarkable since 3 is a genuine tubular species. The neutral tubules are perfectly cylindrical and run along the [0 0 1] axis of the trigonal cell. They are lined on

Figure 3. Nodal representation of the nanotubular structure 3 (top) and the octanuclear cages obtained with Kemp’s tricarboxylate ligands as faces (complex 4, bottom left) and with (1R,3S)-(+)-camphorate ligands as edges (bottom right).8b Water ligands are excluded. Yellow: uranium; green: nickel; red: oxygen; and blue: centroid of the carboxylate ligand.

alone are geometrically sufficient to account for the cylinder formation, since the nickel ions ({6}) simply cut the eightmembered [(UO2)(L)]4 rings into two parts, and can thus be viewed as decorating groups directed inside (although their presence is essential for the formation of 3). Overall, uranyl ions are bound to three and Ni2+ ions to two L3− ligands, and the latter connect four metal centers with the point symbol {4.63.82}. Another rearrangement occurs when the reaction of LH3 with uranyl and nickel nitrates is performed in the presence of 2,2′bipyridine, giving compound 4. The Ni2+ ions are now trapped into disordered Ni(bipy)(H2O)42+ counterions, and the uranyl complex is a dianionic octanuclear cage. The asymmetric unit comprises two independent, but nearly identical, halves of the centrosymmetric (UO2)8(L)6(H2O)62− species (Figure 4). Chelation modes 1 and 2 are present here also, with atom U1 tris-chelated by three L3− anions in mode 1 and atoms U2, U3, and U4 chelated by one ligand in mode 2 and further bound to two oxygen atoms from two more ligands and a water molecule. Each uranyl ion is thus bound to three L3− anions (point symbol {43}), while each anion is bound to four cations ({44.62}). The eight cations are at the vertices of a distorted cube (Figure 3), with edge lengths in the range of 5.01−5.63 Å and a slight rhombohedral compression along the U1−U1i axis (8.80 Å compared to 9.24−9.33 Å for the other space diagonals); the six ligands cover the faces of this cube (if they are represented as a single point, as in Figure 3, the 14 vertices define a tetrakis hexahedron). The carboxylate groups are additionally hydrogen bonded to the water ligands. The internal cavity has an average diameter of ∼5.7 Å between uranyl oxo atoms (∼3 Å for available space) and, as shown by the spacefill representation of Figure 4, it is closed to the outside. This cavity contains one water molecule, whose protons are likely disordered, located at hydrogen

Figure 2. Top: view of complex 3 with solvent molecules and carbonbound hydrogen atoms omitted. Displacement ellipsoids are drawn at the 30% probability level. Symmetry codes: i = y, y − x, −z; j = x − y, x, 1 − z; k = −y, x − y, z; l = x − y, x, −z; m = y, y − x, 1 − z; n = y − x, −x, z. Bottom: view of the nanotubular structure viewed down the [0 0 1] axis. Yellow: uranium; green: nickel. 902

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planes. Hydrophobic interactions arising from segregation of the carbon skeleton on the outside of the closed objects are likely to play an important part in the formation of such species, as in some phosphonate-based tubules.1a,b In summary, Kemp’s triacid appears as a unique ligand in its complexes with the uranyl ion, giving access either to unexceptional 1D polymers or, upon addition of Ni2+ ions, to unusual nanotubular and molecular cage structures, thus displaying a remarkable and easily tailored structural diversity. These results open the way to the design of other nanostructures with this or similar ligands, which may not be restricted to complexes of the uranyl ion.

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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; experimental details; Scheme of bonding modes; and additional views of complexes 1−4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 4. Top: view of the asymmetric unit (half an octanuclear cage) in one of the two independent molecules in complex 4 with carbon-bound hydrogen atoms omitted. Hydrogen bonds are shown as dashed lines. Displacement ellipsoids are drawn at the 50% probability level. Symmetry code: i = 1 − x, 2 − y, 1 − z. Bottom: view of the octanuclear cage showing the uranium coordination polyhedra (left) and spacefill model (right), with hydrogen atoms omitted.

REFERENCES

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bonding distance from the eight oxo groups [2.652(16)−2.95(2) Å]. Among the uranyl−carboxylate octanuclear cages reported to date, one comprises additional peroxo bridges,8a and only that built from (1R,3S)-(+)-camphorate is homoleptic.8b Due to the latter ligand possessing only two carboxylate groups and being bound to two cations only, twelve anions are necessary to hold the cubic array of eight uranyl ions, and the ligands span the cube edges in this case, as shown in Figure 3 (point symbol {83}2{8}3). As a result, the cage is more open and the cavity diameter is larger, at ∼7 Å (∼4.3 Å for available space). These two structures thus illustrate two ways of building uranyl-based cubic cages, with either di- or tritopic ligands (the latter acting as 4-fold nodes). The topology in 3 corresponds to a tessellation of fourmembered rings as found in 4 and eight-membered rings as found in the camphorate complex, the latter halved by the bridging Ni2+ ions. Obviously, with its three-pronged geometry, Kemp’s tricarboxylate is well-adapted to the formation of curved topologies. The three carboxylate groups, all in axial positions in 3 and 4, make dihedral angles of 55−82° with the average cyclohexane plane and of 62−89° with one another (with two lower values of 36 and 39° in 3), which indicates that, notwithstanding the overall rigidity, slight adjustments are possible, particularly through carboxylate rotation around the bond with the cyclohexane ring and also through the variable dihedral angles between carboxylate groups and the uranyl average equatorial 903

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(13) Takemura, Y.; Okui, Y.; Kure, B.; Nakajima, T.; Tanase, T.; Mikuriya, M.; Takahashi, M. Inorg. Chim. Acta 2011, 379, 100. (14) Complexes 1−4 were synthesized under solvo-hydrothermal conditions in 2:1 water/acetonitrile (1, 4) or water/N-methyl-2pyrrolidone (2, 3) at 140 °C (2−4) or 180 °C (1). (15) Crystal data for 1: C14H19NO8U, M = 567.33, triclinic, space group P1̅, a = 8.5846(5), b = 8.7581(4), c = 11.2696(7) Å, α = 79.944(4), β = 85.037(4), γ = 76.391(3)°, V = 809.95(8) Å3, Z = 2. Refinement of 221 parameters on 4945 independent reflections out of 48773 measured reflections (Rint = 0.066) led to R1 = 0.027, wR2 = 0.055, S = 1.010, Δρmin = −2.09, Δρmax = 1.25 e Å−3. Crystal data for 2: C34H54N2O23U3, M = 1572.88, triclinic, space group P1̅, a = 9.7255(4), b = 14.1936(6), c = 16.6406(5) Å, α = 89.960(2), β = 86.398(3), γ = 85.849(2)°, V = 2286.50(15) Å3, Z = 2. Refinement of 591 parameters on 13939 independent reflections out of 134737 measured reflections (Rint = 0.052) led to R1 = 0.027, wR2 = 0.055, S = 0.966, Δρmin = −1.75, Δρmax = 1.25 e Å−3. Crystal data for 3: C24H40NiO21U2, M = 1199.33, trigonal, space group R3̅, a = b = 33.4807(14), c = 15.8968(4) Å, V = 15432.3(16) Å3, Z = 18. Refinement of 457 parameters on 6498 independent reflections out of 130398 measured reflections (Rint = 0.038) led to R1 = 0.057, wR2 = 0.114, S = 1.134, Δρmin = −1.70, Δρmax = 3.61 e Å−3. Crystal data for 4: C82H120N2NiO63U8, M = 4104.75, triclinic, space group P1̅, a = 14.9055(3), b = 21.9044(6), c = 23.1437(6) Å, α = 92.1930(13), β = 106.7162(14), γ = 107.1126(14)°, V = 6854.0(3) Å3, Z = 2. Refinement of 1588 parameters on 25977 independent reflections out of 394554 measured reflections (Rint = 0.054) led to R1 = 0.040, wR2 = 0.119, S = 1.049, Δρmin = −4.05, Δρmax = 3.81 e Å−3.

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