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, 2009
a
![[triangle]](/corehtml/pmc/pmcents/rtrif.gif)
). In the case of Na
5[Er(oda)
3](H
2O)
6(BF
4)
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, [UO
2(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(μ
2-hydroxo)di(oxodiacetato)tetraoxodiuranium(VI) dihydrate (Jiang
et al., 2002
) and three structures containing the [UO
2(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 [UO
2(oda)
2]
2− anions. The anions differ somewhat between the four compounds. In (I)
, the [UO
2(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 [UO
2(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[UO
2(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, 2009
b
). However, compounds (I)
–(IV)
form centrosymmetric crystals.
| Table 1Selected geometric parameters (Å, °) for (I)–(IV)
|
The ions in (I)
are associated by N—H
O and C—H
O interactions (Table 2). Classical N—H
O interactions form a short contact between atoms H1 and O7 within the asymmetric unit. Due to symmetry, each [UO
2(oda)
2]
2− anion will interact with two pyridinium cations. The C—H
O interactions involving H5
O4(1 −
x, −
y, 1 −
z), H7
O4(1 −
x, −
y, 2 −
z) and H8
O7(
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—H
O interactions,
viz. H2
AO2(−1 +
x,
y,
z) and H4
AO1(1 +
x,
y,
z).
| Table 2Hydrogen-bond geometry (Å, °) for (I)
|
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 [UO
2(oda)
2]
2− anions through N—H
O and C—H
O interactions. Two sets of interactions,
viz. H1
O3(−
x, −
y, −
z) and H10
O5, connect the 2-picolinium cation to one anion, and a third interaction, H8
O6(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.
| Table 3Hydrogen-bond geometry (Å, °) for (II)
|
The crystal structure of the analogous 3-picolinium complex, (III)
, is different from both (I)
and (II)
(Table 4). N—H
O and C—H
O interactions in (III)
give rise to a three-dimensional network structure (Fig. 9). The [UO
2(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.
| Table 4Hydrogen-bond geometry (Å, °) for (III)
|
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.
| Table 5Hydrogen-bond geometry (Å, °) for (IV)
|
In the case of the [Ln(oda)
3]
3− complexes, spontaneous resolution did not occur for Na
3[Ln(oda)
3](H
2O)
6, which crystallized in the polar space group
Cc. Addition of certain salts led to more complex structures, of which Na
3NH
4[Ln(oda)
3](SCN)(H
2O)
4 is racemic and Na
5[Ln(oda)
3](H
2O)
6(BF
4)
2 undergoes spontaneous resolution. It appears that the presence of BF
4
− 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.
