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Four complexes containing the [UO2(oda)2]2− anion (oda is oxydiacetate) are reported, namely dipyridinium dioxidobis(oxydiacetato)uranate(VI), (C5H6N)2[U(C4H4O5)2O2], (I), bis(2-methylpyridinium) dioxidobis(oxydiacetato)uranate(VI), (C8H8N)2[U(C4H4O5)2O2], (II), bis(3-methylpyridinium) dioxidobis(oxydiacetato)uranate(VI), (C8H8N)2[U(C4H4O5)2O2], (III), and bis(4-methylpyridinium) dioxidobis(oxydiacetato)uranate(VI), (C8H8N)2[U(C4H4O5)2O2], (IV). The anions are achiral and are located on a mirror plane in (I) and on inversion centres in (II)–(IV). The four complexes are assembled into three-dimensional structures via N—HO and C—HO interactions. Compounds (III) and (IV) are isomorphous; the [UO2(oda)2]2− anions form a porous matrix which is nearly identical in the two structures, and the cations are located in channels formed in this matrix. Compounds (I) and (II) are very different from (III) and (IV): (I) forms a layered structure, while (II) forms ribbons.
We recently published a study of complexes featuring the nine-coordinate chiral [Ln(oda)3]3− anion (where Ln = Pr, Eu, Gd or Dy, and oda = oxydiacetate) (Lennartson & Håkansson, 2009a ). In the case of Na5[Er(oda)3](H2O)6(BF4)2, which crystallizes in the Sohncke space group (Flack, 2003 ) R32, it was possible to crystallize a whole sample as one enantiomerically pure single crystal. This represents the first example of the preparation of enantiomerically pure bulk quantities of a nine-coordinate complex displaying only achiral ligands. Since all precursors [diglycolic acid, erbium(III) chloride hexahydrate, sodium hydroxide, sodium bicarbonate, sodium tetrafluoroborate and water] were achiral, the overall synthesis may be regarded as a case of absolute asymmetric synthesis (Feringa & van Delden, 1999 ; Mislow, 2003 ). Progressing from the oxydiacetate lanthanide complexes we have examined the corresponding actinide complexes, and four complexes containing the dioxidobis(oxydiacetato)uranate(VI) anion, [UO2(oda)2]2−, are presented in this paper.
Only six crystal structures of uranyl complexes containing the oxydiacetate ligand are listed in the Cambridge Structural Database (CSD, Version 5.30 of May 2009; Allen, 2002 ). In the absence of coordinating ligands, uranyl oxydiacetate forms a coordination polymer (Bombieri et al., 1974 ), which undergoes spontaneous resolution (Jacques et al., 1984 ; Perez-Garcia & Amabilino, 2007 ) on crystallization. The other structures published are oxodiacetatodi(pyridine oxide)dioxouranium(VI) (Bombieri et al., 1973 ), di(1,3,5,7-tetraazaadamant-1-ium)di(μ2-hydroxo)di(oxodiacetato)tetraoxodiuranium(VI) dihydrate (Jiang et al., 2002 ) and three structures containing the [UO2(oda)2]2− anion (Bombieri et al., 1973 ; Jiang et al., 2002 ).
Dipyridinium dioxidobis(oxydiacetato)uranate(VI), (I), bis(2-methylpyridinium) dioxidobis(oxydiacetato)uranate(VI), (II), bis(3-methylpyridinium) dioxidobis(oxydiacetato)uranate(VI), (III), and bis(4-methylpyridinium) dioxidobis(oxydiacetato)uranate(VI), (IV), all form yellow crystals from aqueous solution. None of the crystal structures includes water, neither coordinated to the U atom nor as co-crystallized water.
The uranyl moieties in compounds (I)–(IV) are linear, as expected, and are coordinated by two oxydiacetate ligands, giving rise to complex [UO2(oda)2]2− anions. The anions differ somewhat between the four compounds. In (I), the [UO2(oda)2]2− anion is located on a mirror plane bisecting both oxydiacetate ligands (Fig. 1 ). The two oxydiacetate ligands are coordinated differently to the central U atom. One is virtually planar, and atoms O3 and O5 are both coordinated to the central U atom. The other ligand deviates considerably from planarity and, since the U1—O8 distance is probably too long to be considered a U—O bond, it is best described as a bidentate ligand. The [UO2(oda)2]2− anions in (II), (III) and (IV) are very similar (Figs. 2 –4 ), with the central U atoms located on crystallographic inversion centres and with the oxydiacetate ligands virtually planar. Both types of coordination mode have been reported previously (Jiang et al., 2002 ). Selected geometric parameters for (I)–(IV) are compared in Table 1 . The[UO2(oda)2]2− anions are achiral, in contrast with the propeller-shaped [Ln(oda)3]3− anions, but this does not exclude the possibility of a chiral crystal structure, since achiral molecules may assemble into chiral supramolecular structures (Matsuura & Koshima, 2005 ; Lennartson & Håkansson, 2009b ). However, compounds (I)–(IV) form centrosymmetric crystals.
The ions in (I) are associated by N—HO and C—HO interactions (Table 2 ). Classical N—HO interactions form a short contact between atoms H1 and O7 within the asymmetric unit. Due to symmetry, each [UO2(oda)2]2− anion will interact with two pyridinium cations. The C—HO interactions involving H5O4(1 − x, −y, 1 − z), H7O4(1 − x, −y, 2 − z) and H8O7(x, y, 1 + z) give rise to layers in the bc plane (Fig. 5 ). These layers are further associated into a network structure (Fig. 6 ) by two sets of C—HO interactions, viz. H2AO2(−1 + x, y, z) and H4AO1(1 + x, y, z).
Introducing a methyl group in the 2-position on the pyridinium cation, i.e. on going from (I) to (II), dramatically alters the crystal packing (Table 3 ). The 2-picolinium cation in (II) binds two [UO2(oda)2]2− anions through N—HO and C—HO interactions. Two sets of interactions, viz. H1O3(−x, −y, −z) and H10O5, connect the 2-picolinium cation to one anion, and a third interaction, H8O6(1 − x, 1 − y, −z), introduces connections to a second anion. As seen in Fig. 7 , these interactions give rise to infinite ribbons. Sets of ribbons are partly stacked in a similar fashion to the strakes in a ship’s hull, giving rise to layers. The layers are stacked into a three-dimensional structure, where ribbons in adjacent layers are orthogonal; a schematic drawing is presented in Fig. 8 .
The crystal structure of the analogous 3-picolinium complex, (III), is different from both (I) and (II) (Table 4 ). N—HO and C—HO interactions in (III) give rise to a three-dimensional network structure (Fig. 9 ). The [UO2(oda)2]2− anions in (III) form a porous matrix with channels running parallel to the crystallographic a and c axes. These channels are occupied by the 3-picolinium cations. A view along the a axis is presented in Fig. 10 .
Compound (IV) is isomorphous with (III). The matrices formed by the anions are almost identical, forming the same type of channels. The orientations of the cations occupying these channels differ between the two structures, and the intermolecular interactions in (IV) (Table 5 ) are of course different from those in (III), as depicted in Fig. 11 .
In the case of the [Ln(oda)3]3− complexes, spontaneous resolution did not occur for Na3[Ln(oda)3](H2O)6, which crystallized in the polar space group Cc. Addition of certain salts led to more complex structures, of which Na3NH4[Ln(oda)3](SCN)(H2O)4 is racemic and Na5[Ln(oda)3](H2O)6(BF4)2 undergoes spontaneous resolution. It appears that the presence of BF4 − is essential for spontaneous resolution to occur in this system. Preliminary studies show that recrystallization of (I) from water in the presence of inorganic salts leads to cocrystallization in certain cases, and the formation of a chiral supramolecular structure may be observed at a future date.
For the preparation of uranyl oxydiacetate, diglycolic acid (0.13 g, 1.0 mmol) and sodium bicarbonate (0.17 g, 2.0 mmol) were dissolved in water (5 ml). A solution of uranyl nitrate hexahydrate (0.50 g, 1 mmol) in water (5 ml) was added. The solution was heated to reflux and a yellow precipitate formed. The mixture was cooled to ambient temperature and the precipitate collected by filtration, washed with water (3 × 5 ml) and acetone (3 × 5 ml), and dried by suction (yield 0.35 g, 87%). For the preparation of (I), pyridine (0.3 ml) and water (1.0 ml) were added to a mixture of uranyl oxydiacetate (0.35 g, 0.82 mmol) and diglycolic acid (0.11 g, 0.82 mmol). The mixture was heated until a clear solution was obtained. Yellow crystals of (I) formed on cooling to ambient temperature (yield 0.33 g, 54%). Compounds (II)–(IV) were prepared in an analogous manner, substituting pyridine by 2-, 3- and 4-picoline, respectively.
The N-bound H atoms were located in difference Fourier maps and refined isotropically, with the N—H distances in (I) and (IV) restrained to 0.90 (2) Å. The C-bound H atoms were included in calculated positions, with C—H = 0.93 (aromatic), 0.96 (methyl) or 0.97 Å (methylene), and refined using a riding model, with U iso(H) = 1.5U eq(C) for the methyl groups and 1.5U eq(C) for the remainder. A few strong low-angle reflections were excluded since these caused saturation of the image plate.
For all compounds, data collection: CrystalClear (Rigaku, 2000 ); cell refinement: CrystalClear; data reduction: CrystalClear; program(s) used to solve structure: SIR92 (Altomare et al., 1993 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ); molecular graphics: ORTEP-3 (Farrugia, 1997 ) and PLUTON (Spek, 2009 ); software used to prepare material for publication: SHELXL97.
Crystal structure: contains datablocks I, II, III, IV, global. DOI: 10.1107/S0108270109053839/gg3221sup1.cif
Structure factors: contains datablocks I. DOI: 10.1107/S0108270109053839/gg3221Isup2.hkl
Structure factors: contains datablocks II. DOI: 10.1107/S0108270109053839/gg3221IIsup3.hkl
Structure factors: contains datablocks III. DOI: 10.1107/S0108270109053839/gg3221IIIsup4.hkl
Financial support from the Swedish Research Council (VR) is gratefully acknowledged.
Supplementary data for this paper are available from the IUCr electronic archives (Reference: GG3221). Services for accessing these data are described at the back of the journal.