A lanthanum(III) complex with a lacunary polyoxotungstate: Na2(NH4)7[La(W5O18)2].16H2O

# 2005 International Union of Crystallography Printed in Great Britain – all rights reserved The crystal structure of a lanthanum polyoxotungstate complex, viz. heptaammonium disodium decatungstolanthanate hexadecahydrate, Na2(NH4)7[La5O18)2] 16H2O, has been determined by single-crystal X-ray diffraction at 100 (2) K in the space group C2/c. The [La(W5O18)2] 9 polyoxoanion has the central La cation located on a twofold rotation axis. The close packing of the polyoxoanion-supported lanthanum(III) complexes with Na and NH4 + cations leads to the formation of several intersecting undulating channels, where the water molecules of crystallization are located and involved in strong hydrogen bonds.

The crystal structure of a lanthanum polyoxotungstate complex, viz. heptaammonium disodium decatungstolanthanate hexadecahydrate, Na 2 (NH 4 ) 7 [La5O 18 ) 2 ]Á16H 2 O, has been determined by single-crystal X-ray diffraction at 100 (2) K in the space group C2/c. The [La (W 5 O 18 ) 2 ] 9À polyoxoanion has the central La 3+ cation located on a twofold rotation axis. The close packing of the polyoxoanion-supported lanthanum(III) complexes with Na + and NH 4 + cations leads to the formation of several intersecting undulating channels, where the water molecules of crystallization are located and involved in strong hydrogen bonds.

Comment
Polyoxometalates (POMs) are a unique type of compound showing remarkable structural diversity and potentially interesting applications in catalysis, non-linear optical and magnetic materials, liquid crystals and biomedical materials (Pope & Mü ller, 1994Mü ller et al., 1998, and references therein;Pope, 1983). In the course of our research on the synthesis and structural characterization of novel functional materials containing POMs (Almeida Sousa, Paz, Cavaleiro et al., 2004;Sousa, Paz, Soares-Santos et al., 2004), we came across the title compound, (I).
The anion charge is balanced by the presence of one Na + and three and a half crystallographically unique NH 4 + cations, Na 2 (NH 4 ) 7 [La(W 5 O 18 ) 2 ]. Interestingly, the Na + cations in the crystal structure form {Na 2 (H 2 O) 10 } 2+ moieties, exhibiting a highly distorted octahedral coordination environment in inorganic papers Mixed ellipsoid and polyhedral representation of the polyoxoanionsupported lanthanum(III) complex anion, [La(W 5 O 18 ) 2 ] 9À , showing the labelling scheme for selected atoms and emphasizing the square antiprismatic coordination environment for the central La 3+ cation. Atoms belonging to the asymmetric unit are represented with ellipsoids drawn at the 50% probability level. [Symmetry code: (i) 2 À x, y, 3 2 À z.]

Experimental
All chemicals were purchased from Aldrich and used without further purification. Na 2 WO 4 Á2H 2 O (9.90 g, 30 mmol) and H 3 BO 3 (0.15 g, 2.43 mmol) were dissolved in hot distilled water (ca 21 ml, 363-inorganic papers   Polyhedral representation of the crystal packing of Na 2 (NH 4 ) 7 [La(W 5 O 18 ) 2 ]Á16H 2 O, viewed towards the (8,11,1) plane. 373 K), and the final pH was adjusted to 7.1 using a 6 M aqueous solution in HCl. After 10 min, a solution of La(NO 3 ) 3 (3.24 mmol) in 1 M CH 3 COOH (ca 5.4 ml) was added dropwise, and the resulting mixture was stirred thoroughly at 363 K for 30 min. The temperature was then slowly dropped to 343 K, after which an aqueous solution of NH 4 Cl (12 g, 224 mmol) was added dropwise. The resulting solution was allowed to stand at ambient temperature for 24 h and then filtered. The collected solid was recrystallized from warm distilled water, giving good quality white crystals suitable for X-ray diffraction. Selected FT-IR data (cm À1 ): (N + -H, from NH 4 + ) = 1401 (s), as (W-O IV , terminal W-O stretch) = 931 (s), as (W-O II -W, edgeshared W-O-W stretching mode) = 840 (s) and 789 (s).   (7) Symmetry code: (ii) 1 2 À x; 1 2 À y; 1 À z. The distinction between water molecules and NH 4 + cations proved to be very difficult. In order to balance the anion charge, three and a half NH 4 + cations have been selected, taking into consideration FT-IR data and geometrical aspects, such as charge proximity and the number of neighbours with which hydrogen bonding might occur. Since the number of possible hydrogen bonds in which the water molecules and NH 4 + cations could be involved is quite large, no attempt was made either to find or to place geometrically the H atoms in these groups. The highest peak in the final difference Fourier map was located 1.25 Å from O4 and the deepest hole 0.94 Å from W1.
We are grateful to the Fundaçã o para a Ciê ncia e Tecnologia (FCT, Portugal) for their general financial support under the POCTI programme (supported by FEDER).

Special details
Experimental. (See detailed section in the paper) 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq  (6)