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 & Müller, 1994, 2001; Müller 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 Paz et al., 2004; Sousa, Paz, Cavaleiro et al., 2004; Sousa, Paz, Soares-Santos et al., 2004), we came across the title compound, (I).

A search in the literature and in the Inorganic Crystal Structure Database (Belsky et al., 2002) shows that the [La(W₅O₁₈)₂]^9- polyoxoanion shares striking similarities with all complexes of the [Ln(W₅O₁₈)₂]^n- type, where Ln = Ce⁴⁺ (Peacock & Weakley, 1971; Iball et al., 1974), Ce³⁺ (Xue et al., 2002), Pr³⁺, Nd³⁺ (Ozeki & Yamase, 1994a), Sm³⁺ (Ozeki & Yamase, 1993, 1994a,b), Eu³⁺ (Sugeta & Yamase, 1993; Yamase et al., 1993), Gd³⁺ (Yamase & Ozeki, 1993; Ozeki & Yamase, 1994a; Yamase et al., 1994), Tb³⁺ (Ozeki & Yamase, 1994a; Ozeki et al., 1992), Dy³⁺ (Ozeki & Yamase, 1994a) and also with the actinide cation Th⁴⁺ (Griffith et al., 2000). Surprisingly, the structure containing La³⁺ cations has not been reported to date. We describe here the synthesis and crystal structure of Na₂(NH₄)₇[La(W₅O₁₈)₂]·16H₂O, determined in the space group C2/c at the low temperature of 100  (2)  K; this is also the first report of a complex of the [Ln(W₅O₁₈)₂]^n- type crystallizing with NH₄⁺ cations.

The [La(W₅O₁₈)₂]^9- polyoxoanion has crystallographic C₂ symmetry about an axis passing through the central La³⁺ cation and perpendicular to the vector containing the W1, La1 and W1ⁱ centres [Fig. 1; symmetry code: (i) 2  -  x, y,   -  z]. The two [W₅O₁₈]^6- anionic fragments are linked together via a central La³⁺ cation positioned in the lacuna of each anion (Fig. 1). This centre exhibits typical square antiprismatic coordination geometry, with La-O distances in the range 2.497  (6)-2.562  (6)  Å (Table 1 and Fig. 1). The degree of staggering between the upper and lower square faces of the antiprism is only ca 0.6° from ideal.

For the [W₅O₁₈]^6- moieties, the five crystallographically unique W centres exhibit distorted {WO₆} octahedral environments, in which the central W atom is displaced in the direction of the axial oxo ligand (average distance of displacement = 0.402  Å): W-O distances and O-W-O angles are in the ranges 1.724  (6)-2.324  (6)  Å and 74.5  (2)-179.0  (3)° [74.5  (2)-104.3  (3)° and 153.2  (2)-179.0  (3)° for cis and trans], respectively. The W-O distances can be divided into several groups according to the different types of O atoms (Table 3): O_I represent long bonds of the W-O1-W type (where O1 is the core O atom; see Fig. 1) found in the range 2.304  (6)-2.324  (6)  Å; O_II represent those connected to the W centres which are involved in edge-sharing of adjacent octahedra [1.890  (6)-2.031  (6)  Å]; O_III represent the lanthanum-bound O atoms (O15, O16, O17 and O18), and O_IV the terminal O atoms (O2, O8, O10, O12 and O14; see Fig. 1 and Table 3). As found in related compounds, pairs of short and long W-O_II bonds are observed (Table 3). This results from small displacements of the W centres, and also from the structural evidence that W1 is the statistically farthest W centre from any other: the WW distances for the W2W3W4W5 central square of [W₅O₁₈]^6- are in the range 3.264  (6)-3.291  (6)  Å, while W1W2-W5 distances are in the range 3.331  (6)-3.342  (6)  Å. It is interesting to note that the O1 core atom lies only 0.099  (6)  Å out of the plane of the equatorially bonded W2-W5 centres and in the direction of W1; the non-bonded La1O1 distance is 3.271  (6)  Å.

The anion charge is balanced by the presence of one Na⁺ and three and a half crystallographically unique NH₄⁺ cations, Na₂(NH₄)₇[La(W₅O₁₈)₂]. Interestingly, the Na⁺ cations in the crystal structure form {Na₂(H₂O)₁₀}²⁺ moieties, exhibiting a highly distorted octahedral coordination environment in which the average NaO_water contact distance is 2.372  Å (Table 2 and Fig. 2) and the Na1Na1ⁱⁱ distance is 3.411  (7)  Å [symmetry code: (ii)   -  x,   -  y, 1  -  z].

The polyoxoanion-supported lanthanum(III) complex anions, [La(W₅O₁₈)₂]^9-, pack closely in the ab plane in a typical brick-wall-like fashion, leading to several types of intersecting channels which accommodate the cations (Na⁺ and NH₄⁺) and the water molecules of crystallization (Figs. 3 and 4). These are, in turn, involved in an extensive hydrogen-bonded network composed of strong heteronuclear N⁺-HO and homonuclear O-HO interactions (not shown).

Figure 1 Mixed ellipsoid and polyhedral representation of the polyoxoanion-supported lanthanum(III) complex anion, [La(W5O18)2]9-, showing the labelling scheme for selected atoms and emphasizing the square antiprismatic coordination environment for the central La3+ cation. Atoms belonging to the asymmetric unit are represented with ellipsoids drawn at the 50% probability level. [Symmetry code: (i) 2  -  x, y,   -  z.]

Figure 2 Schematic representation of the cationic {Na2(H2O)10}2+ moieties. The Na1Na1i distance is 3.411  (7)  Å [symmetry code: (i)   -  x,   -  y, 1  -  z].

Figure 3 Polyhedral representation of the crystal packing of Na2(NH4)7[La(W5O18)2]·16H2O, viewed along the a direction.

Figure 4 Polyhedral representation of the crystal packing of Na2(NH4)7[La(W5O18)2]·16H2O, viewed towards the (,11,1) plane.

Experimental

All chemicals were purchased from Aldrich and used without further purification. Na₂WO₄·2H₂O (9.90  g, 30  mmol) and H₃BO₃ (0.15  g, 2.43  mmol) were dissolved in hot distilled water (ca 21  ml, 363-373  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(NO₃)₃ (3.24  mmol) in 1  M CH₃COOH (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 NH₄Cl (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^-1): (N⁺-H, from NH₄⁺) = 1401 (s), _as(W-O_IV, terminal W-O stretch) = 931 (s), _as(W-O_II-W, edge-shared W-O-W stretching mode) = 840 (s) and 789 (s).

Crystal data

Na2(NH4)7[La(W5O18)2]·16H2O Mr = 3013.94 Monoclinic, a = 11.784  (2)  Å b = 14.838  (3)  Å c = 29.143  (6)  Å = 93.26  (3)° V = 5087.4  (18)  Å3 Z = 4 Dx = 3.935  Mg  m-3 Mo K radiation Cell parameters from 1014 reflections = 2.7-28.7° = 23.47  mm-1 T = 100  (2)  K Plate, white 0.35 × 0.21 × 0.06  mm

Data collection

Bruker SMART CCD1000 diffractometer Thin-slice and scans Absorption correction: multi-scan (SADABS; Sheldrick, 1997) Tmin = 0.045, Tmax = 0.333 21  307 measured reflections 5183 independent reflections 4577 reflections with I > 2(I) Rint = 0.069 max = 26.4° h = -14 14 k = -18 18 l = -36 36

Refinement

Refinement on F2 R[F2 > 2(F2)] = 0.031 wR(F2) = 0.081 S = 1.08 5183 reflections 326 parameters H-atom parameters not defined w = 1/[2(Fo2) + (0.0169P)2 + 65.9468P] where P = (Fo2 + 2Fc2)/3 (/)max = 0.002 max = 1.66  e  Å-3 min = -2.16  e  Å-3

Table 1 Selected geometric parameters (Å, °) La1-O15 2.497  (6) La1-O18 2.511  (6) La1-O16 2.530  (6) La1-O17 2.562  (6) O15-La1-O15i 151.4  (3) O15-La1-O18 72.7  (2) O15-La1-O18i 133.7  (2) O18-La1-O18i 74.3  (3) O15-La1-O16i 84.66  (19) O18-La1-O16i 151.71  (19) O15-La1-O16 72.61  (19) O15i-La1-O16 84.7  (2) O18-La1-O16 112.58  (19) O16i-La1-O16 75.0  (3) O15-La1-O17 112.8  (2) O15i-La1-O17 74.8  (2) O18-La1-O17 70.99  (19) O18i-La1-O17 85.41  (19) O16i-La1-O17 135.25  (19) O16-La1-O17 72.16  (19) O15-La1-O17i 74.8  (2) O17-La1-O17i 150.5  (3) Symmetry code: (i) .

Table 2 Contact distances (Å) Na1O1W 2.346  (8) Na1O2W 2.402  (7) Na1O3W 2.321  (7) Na1O4W 2.328  (7) Na1O5W 2.379  (8) Na1iiO5W 2.456  (7) Symmetry code: (ii) .

Table 3 W-O bond-distance categories (Å) for the [W5O18]6- anionic fragment present in (I) Category Range Average Range W-OI 2.304  (6)-2.324  (6) 2.314 0.020 W-OII (short) 1.890  (6)-1.963  (6) 1.927 0.073 W-OII (long) 2.022  (6)-2.031  (6) 2.027 0.009 W-OIII 1.776  (6)-1.790  (6) 1.783 0.014 W-OIV 1.726  (6)-1.734  (6) 1.730 0.008

The distinction between water molecules and NH₄⁺ cations proved to be very difficult. In order to balance the anion charge, three and a half NH₄⁺ 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 NH₄⁺ 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.

Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT; program(s) used to solve and refine structure: SHELXTL (Bruker, 2001); molecular graphics: DIAMOND (Brandenburg, 2001); software used to prepare material for publication: SHELXTL.