2.2.1 Crystal 1

A plate-like pale-pink transparent single crystal has been mounted on a Nonius KappaCCD diffractometer with Mo Kα radiation (λ = 0.71073 Å).

The unit cell parameters found after 10 frames are: α = 11.3590(2) Å, b = 9.6162(2) Å, c = 9.9401(2) Å, β = 118.723(9)°. These parameters are the same that those already found for the Er₂(C₂O₄)₃(H₂O)₆·4 H₂O compound [7,15–20].

2.2.2 Crystal 2

A block-like pale-pink transparent single crystal has been mounted on a Enraf Nonius CAD-4 four circle diffractometer equipped with Mo Kα radiation and a graphite monochromator. Accurate unit cell parameters were derived from least-squares refinement of the setting angles of 25 machine-centred reflections in the range 4.59 ≤ θ ≤ 13.29° and have for values: a = 11.4145(14) Å, c = 8.893(2) Å. Data collection was performed in the ω–2 θ scan mode. Intensities of three standard reflections were measured every 60 min and showed no decrease during the data collection. The data were corrected for Lorentz and polarization effects and for absorption using the psi-scan method [21]. The structure was solved by direct methods and refined by full-matrix least-squares methods, which were performed using the SHELXS-97 and SHELXL-97 software packages [22, 23]. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms of water molecules have been localized and have not been refined.

2.2.3 Crystal 3

A diamond-like pale-pink transparent single crystal has been mounted on a Nonius KappaCCD diffractometer with Mo Kα radiation (λ = 0.71073 Å). The crystal data collection has been performed at room temperature.

A crystal-to-detector distance of 25.0 mm has been used and data collection strategy (determination and optimisation of the detector and goniometer positions) have been performed with the help of the COLLECT program [24] to measure Bragg reflections of the unique volume in reciprocal space until θ = 27.48°.

A total of 166 frames were recorded, using Δω = 2.0° rotation scans to fill the asymmetric zone of the reciprocal space (exposition time = 50 s deg^–1). Finally, 25 796 reflections have been indexed, Lorentz polarization corrected and then integrated in the trigonal symmetry (R3 point group) by the DENZO program of the KappaCCD software package [25].

Numerical absorption corrections have been performed using the analytical facilities [26] included in the WinGX program suite [27].

Structure determinations have been performed with the solving program SIR97 [28], that revealed all the non hydrogen atoms. All non-hydrogen atoms were refined anisotropically using the SHELXL program [23]. Hydrogen atoms of water molecules have not been localized.

Crystal and final structure refinement data of the three compounds are listed in Table 3.

3.1.1 Description of the structure of compound 1

This compound, isostructural to the already well-described lanthanide oxalate decahydrate, exhibits a 2D framework, based on an honeycomb-like molecular motif (Fig. 1).

3.1.2 Description of the structure of compound 2

This compound is original because it has never been characterized before. However the Y(III) containing homogenous compound (with K or Rb counter cations) has already been described in the literature [13]. In this paper its crystal structure had been solved on the basis of powder X-ray diffraction diagram. The crystal structure determination reported here, based on single crystal X-ray diffraction data, has allowed us to refine the space group determination (I4₁/amd) and the location of the water molecules.

The asymmetric unit, along with the atomic numbering scheme (Fig. 2), shows that the erbium(III) ions are eight coordinated by eight oxygen atoms from four oxalato groups, so forming a slightly distorted dodecahedron (Fig. 3). It can be noticed that this coordination polyhedron is free of water molecule. The distances between the Er(III) ion and the oxygen atoms belonging to oxalato groups are 2.318(1) and 2.357(1) Å. In Fig. 2, one can also notice that the O1 atom is also bounded to the K⁺ ion. The coordination polyhedron of the K⁺ ion is also a dodecahedron, formed by four oxygen atoms from four oxalato groups and four oxygen atoms from crystallization water molecules.

The crystal structure exhibits a 3D open framework. Along the c→ axis, the structure presents channels with square-cross sections, the erbium ions occupying the corners of the squares (Fig. 4). The distances between Er(III) ions are all 6.125(1) Å and so the surface of the square-cross section is 37 Å². These channels are filled with disordered crystallization water molecules. A deeper study of the location of the crystallization water molecules shows that they are forming two screws translated by $\overline{𝑐}$ and rotated by 45° from each other (Fig. 5). The distances between two oxygen atoms from adjacent crystallization water molecules belonging to a given screw are 2.901(1) Å.

The square cross-section channels can also be described as helixes, as shown in Fig. 6.

The crystal structure presents also channels spreading along the $\overline{𝑎}\mathrm{or}\overline{𝑏}$ axis. These channels present an irregular chair-like hexagonal section with six lanthanide ions localized at the corners and six oxalato groups forming the edges. Their surface is roughly 45 Å² large. They are filled with crystallization water molecules and with the potassium ions (Fig. 7).

3.1.3 Description of the structure of compound 3

This compound is, to the best of our knowledge, the first octadecahydrate lanthanide oxalate ever crystallographically characterized and the first lanthanide, only containing oxalate, exhibiting a 3D framework.

The asymmetric unit, along with the atomic numbering scheme (Fig. 8), shows that the erbium(III) ions are nine-coordinated by six oxygen atoms from three oxalato groups, according to a bidentate coordination mode, as well as by three coordination water molecules, so forming a tricapped trigonal prism (Fig. 9). The distances between the lanthanide ions and the bounded oxygen atoms are 2.415(10) Å and 2.275(65) Å for oxygen atoms belonging to water molecules and to oxalato groups respectively.

The projection view along the $\overline{𝑐}$ axis shows channels with hexagonal sections roughly 100 Å² large. Each section is formed by six lanthanide ions at the corners connected by six oxalato groups forming the edges. These channels are filled with crystallization water molecules (Fig. 10). One channel out of two is, actually, an helix spreading along the $\overline{𝑐}$ axis (Fig. 11). Each such channel is connected to three other helical channels in such a way that if one turns right, adjacent others turn left (Fig. 12). The connection between helixes is insured by a common Er(III)–C₂O₄^2––Er(III) molecular fragment. Six connected helixes form another hexagonal cross-section channel filled by crystallization water molecules too. The crystallization water molecules occupying these channels present bigger thermal agitation factors than those occupying the helical channels.

The views along the $\overline{𝑎}\mathrm{or}\overline{𝑏}$ axis are similar. The molecular skeleton can be described as a juxtaposition of channels with a square cross-section. These small channels are filled by crystallization water molecules (Fig. 13). The molecular motifs correspond to each other via the translation $1/3\overline{𝑐}-1/3\overline{𝑏}\left(\mathrm{or}1/3\overline{𝑐}-1/3\overline{𝑎}\right).$