Crystal structure of NH4[La(SO4)2(H2O)]

The structure of (NH4)[La(SO4)2(H2O)] comprises LaO9 polyhedra and SO4 tetrahedra, which are linked by common edges and vertices, forming a three-dimensional network with the hydrogen-bonded NH4 + ions in the cavities.

The principal building units in the crystal structure of ammonium aquabis-(sulfato)lanthanate(III) are slightly distorted SO 4 tetrahedra, LaO 9 polyhedra in the form of distorted tricapped trigonal prisms, and NH 4 + ions. The La 3+ cation is coordinated by eight O atoms from six different sulfate tetrahedra, two of which are bidentate coordinating and four monodentate, as well as one O atom from a water molecule; each sulfate anion bridges three La 3+ cations. These bridging modes result in the formation of a three-dimensional anionic [La(SO 4 ) 2 (H 2 O)] À framework that is stabilized by O-HÁ Á ÁO hydrogenbonding interactions. The disordered ammonium cations are situated in the cavities of this framework and are hydrogen-bonded to six surrounding O atoms.

Chemical context
Three-dimensional framework materials are characterized by their structural diversity. They are of continuing interest as a result of their technologically important properties and potential applications in catalysis, ion-exchange, adsorption, intercalation, and radioactive waste remediation (Szostak, 1989;Cheetham et al., 1999;Rosi et al., 2003;Ok et al., 2007). Many materials showing such functional features contain structurally versatile cations, in particular heavier metal cations with large coordination spheres. Among many other cations, lanthanide cations have been used widely, since they exhibit high coordination numbers and can show a large topological diversity in the resulting framework structures (Bataille & Louë r, 2002;Wickleder, 2002;Yuan et al., 2005). One of the most promising synthetic methods for the preparation of compounds with framework structures is the hydrothermal (or solvothermal) reaction technique (Feng et al., 1997;Natarajan et al., 2000) in which mineralizers such as acids or bases are introduced to increase the solubility and reactivity of the reagents (Laudise, 1959;Laudise & Ballman, 1958). Moreover, organic or inorganic templates are used to direct the topologies of the framework structures and the concomitant physical and chemical properties of the products (Szostak, 1989;Breck, 1974;Barrer, 1982). Thus, we have tried to utilize the hydrothermal technique to react a lanthanide cation (La 3+ ) with sulfuric acid in the presence of NH 4 OH and 3-aminobenzoic acid as a template to prepare higher dimensional framework materials. However, in the present case the organic template was not incorporated in the resultant crystal structure of the title compound, NH 4 [La (SO 4   Sulfates with an A + :Ln 3+ (A + = alkaline ions, Ln 3+ = lanthanide ions) ratio of 1:1 are one of the best investigated groups among hydrous ternary sulfates. They crystallize either as monohydrates (Blackburn & Gerkin, 1995;Barnes, 1995;Iskhakova et al., 1985a) or tetrahydrates (Eriksson et al., 1974), and in few cases also as dihydrates (Kaučič et al., 1985;Iskhakova & Trunov, 1985). The tetrahydrates are mainly found for the bigger monovalent ions Cs + , NH 4 + , and Rb + . For the smaller A + ions such as Na + , the monohydrate becomes dominant.

Structural commentary
The structure of the title compound comprises LaO 9 polyhedra and SO 4 tetrahedra as the principal building units (Fig. 1), forming an anionic [La(SO 4 ) 2 (H 2 O)] À framework by sharing common edges and vertices (Fig. 2). The NH 4 + counter-cations are situated in the cavities of this framework.
The La 3+ cation is coordinated by eight O atoms from six different sulfate tetrahedra. Two tetrahedra are in a bidentate coordination mode and four tetrahedra are in a monodentate mode. The distorted tricapped trigonal-prismatic coordination sphere is completed by one O atom from a water molecule. The La-O bond lengths, ranging from 2.472 (3) to 2.637 (3) Å with 2.496 (3) Å to the water molecule, and the O-La-O angles, ranging from 53.55 (8) to 145.43 (9) , are similar to the analogous distances found in NaLa(SO 4 ) 2 ÁH 2 O (Blackburn & Gerkin, 1995). The ninefold coordination of La 3+ in NH 4 [La(SO 4 ) 2 (H 2 O)] is typical for the majority of monohydrated alkali rare earth sulfate complexes and of rare earth complexes in general. For early members of the rare earth sulfate series, the coordination number of nine is realized, e.g. for Ce, Pr, La and Nd (Blackburn & Gerkin, 1994, 1995Iskhakova et al., 1985bIskhakova et al., , 1988. For later members of the sulfate series, such as Gd (Sarukhanyan et al., 1984b), the coordination number decreases to eight, presumably in association with the lanthanide contraction. There are two sulfur atoms (S1, S2) in the asymmetric unit of the title compound, both with very similar S-O bond lengths in the ranges 1.465 (3) (19) for S1 and 104.70 (16)-111.52 (17) for S2, reflect the distortion of the two sulfate tetrahedra. Each SO 4 anion bridges three La 3+ cations (Fig. 2).

Supramolecular features
The bridging modes of the O atoms result in the formation of a three-dimensional anionic framework, stabilized by O-HÁ Á ÁO hydrogen-bonding interactions between the aqua ligand and the two SO 4 tetrahedra (Table 1)  The principal building units, LaO 9 polyhedra and SO 4 tetrahedra, in the crystal structure of (NH 4 )[La(SO 4 ) 2 (H 2 O)], showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level.

Figure 2
The connection of LaO 9 polyhedra and SO 4 tetrahedra in the crystal structure of (NH 4 )[La(SO 4 ) 2 (H 2 O)], viewed along the a axis.
ammonium cation could not be located, the NÁ Á ÁO distances between 2.865 (5) and 3.036 (5) Å strongly suggest N-HÁ Á ÁO hydrogen bonds of medium strength (Table 1). It appears most likely that the number of O atoms (six) in the vicinity of the N atom is the reason for the disorder of the ammonium cation.

Synthesis and crystallization
The title compound was obtained during the attempted preparation of a complex resulting from the hydrothermal reaction of La 2 O 3 (0.1 g, 1 mmol) with 37%wt sulfuric acid and 3-aminobenzoic acid (0.048 g, 1 mmol) in the presence of NH 4 OH in 10 ml water. The mixture was kept in a 23 ml Teflon-lined steel autoclave at 433 K for 3 d. After this treatment, the autoclave was cooled slowly to room temperature. Slow evaporation of the solvent at room temperature led to the formation of prismatic colourless crystals of the title compound.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 R-factors(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.