Poly[[tetra-μ3-acetato-hexa-μ2-acetatodiaqua-μ2-oxalato-tetralanthanum(III)] dihydrate]

The title compound, {[La4(CH3CO2)10(C2O4)(H2O)2]·2H2O}n, exhibits a two-dimensional layered structure with the oxalate and acetate ligands acting as bridges. The asymmetric unit contains two crystallographically independent lanthanum(III) ions, half of an oxalate ligand, five acetate ligands, one coordinated water molecule and one uncoordinated water molecule. The coordination numbers of the two La ions are 9 and 10. Adjacent layers of the structure, which extend parallel to (100), are linked by O–H⋯O hydrogen bonds and are also held together by van der Waals interactions between the CH3 groups of the acetate anions.

The title compound, {[La 4 (CH 3 CO 2 ) 10 (C 2 O 4 )(H 2 O) 2 ]Á2H 2 O} n , exhibits a two-dimensional layered structure with the oxalate and acetate ligands acting as bridges. The asymmetric unit contains two crystallographically independent lanthanum(III) ions, half of an oxalate ligand, five acetate ligands, one coordinated water molecule and one uncoordinated water molecule. The coordination numbers of the two La ions are 9 and 10. Adjacent layers of the structure, which extend parallel to (100), are linked by O-HÁ Á ÁO hydrogen bonds and are also held together by van der Waals interactions between the CH 3 groups of the acetate anions.
The interest in mixed-ligand lanthanide oxalates is due to: first, lanthanide ions have large size, high and variable coordination numbers and flexible coordination environments; second, oxalate, as a bisbidentate ligand, has a strong coordination ability to metal ions (Kustaryono et al., 2010;Roméro & Trombe, 1999;Yu et al., 2006;Ohba et al., 1993). Most of the known lanthanide oxalates were prepared by hydrothermal reactions of various oxalate salts or of mixtures containing free oxalic acid and other reagents (Zhang et al., 2009;Trombe et al., 2005), while in a few instances the oxalate was generated by chance via an in situ decomposition of organic reagents (Koner & Goldberg, 2009;Li et al., 2003;Mohapatra et al., 2009).
In one case a metal-assisted reduction of CO 2 was invoked to explain an unexpected oxalate formation (Min & Lee, 2002).
We report here the synthesis and crystal structure of a new lanthanum oxalate acetate, where the oxalate was formed in situ under hydrothermal conditions presumably by an redox reaction of copper(II) acetate under concomitant formation of Cu 2 O (see Experimental). When the reaction was carried out without copper(II) acetate, different products were obtained.
As shown in Fig. 1, the asymmetric unit of (I) contains two La ions, a half oxalate anion, five acetate groups, one coordinated water molecule, and one noncoordinated water molecule. The two crystallographically independent lanthanum atoms, La1 and La2, are nine-and ten-coordinated by oxygen atoms. The La1 ion is coordinated by seven acetate oxygen atoms (one acetate in chelating fashion) and two oxygen atoms O1 and O2 of a chelating centrosymmetric oxalate group.
The La-O bond distances vary from 2.476 (3) to 2.727 (2) Å (see supplementary materials). The La1-O bond distances of the oxalate group are 2.505 (2) Å and 2.519 (2) Å. The La2 ion is coordinated by nine acetate oxygen atoms (three acetate groups in chelating fashion) and by a water molecule (H 2 O13) in terminal position. The La2-O bond distances vary from 2.427 (3) to 2.802 (2) Å. The La-O bond distance of the coordinating water molecule is 2.517 (3) Å. All La-O bond distances of (I) are in the normal range for La(III) ions (Trombe & Roméro, 2000;Deng et al., 2009). In the crystal structure of (I), each centrosymmetric bisbidentate oxalate ligand bridges two neighbouring La1 ions. The acetate groups have three different coordination modes: Firstly, the acetate group is µ 3 -η 2 -η 2 -bridging (the group chelates one La and links with each oxygen to a second and third La). Such coordination has been observed previously (Zhang et al., 2009;Dan et al., 2006). Secondly, the acetate group chelates one La ion and binds with one of its two O to a second La. Such coordination mode of lanthanide acetates has been previously observed (Koner & Goldberg, 2009;Dan et al., 2006). Thirdly, an acetate ion bridges two metal ions with each oxygen bonded to one La. Such coordination mode has also been reported previously (Mazurek et al., 1985).
As shown in Fig. 2, the title compound possesses a two-dimensional polymeric layered structure parallel to (100). The layer contains as a characteristic feature 10-membered oval rings of La atoms linked by the acetate groups. Each of these rings is subdivided in two centrosymmetric halves by a central oxalate bridge, which reinforces the layer. While the central supplementary materials sup-2 parts of the layers are formed by the La ions and the carboxyl groups of acetate and oxalate anions, the outer parts of the layers are formed by the CH 3 groups of the acetate groups and by the La-coordinating water molecules. Therefore, perpendicular to (100) the layers are mutually held together by van der Waals contacts between the CH 3 groups and by the noncoordinating water molecule H 2 O14, each of which links two layers via one accepted and two donated O-H···O hydrogen bonds (Table   1). This leads to the formation of a three-dimensional supramolecular network shown in Fig. 3.
The mixture of copper acetate monohydrate (0.097 g, 0.5 mmol), lanthanum acetate hydrate (0.318 g, 1 mmol), borax (0.381 g, 1 mmol), acetic acid (0.2 ml) and deionized water (7 ml), was placed in a 23 ml Teflon reactor and stirred for 20 min in air, then heated at 453 K for 5 d, followed by cooling to room temperature at a rate of 5K/h. Colorless transparent X-ray quality single crystals of compound (I) and red single crystals of Cu 2 O were obtained, which were used for X-ray diffraction analysis.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.