Pentapotassium dodecatungsto­borate(III) hexadecahydrate

The title compound, K₅[BW₁₂O₄₀]·16H₂O, contains a [BW₁₂O₄₀]⁵⁻ polyanion of 222 crystallographic symmetry, with a central tetrahedrally coordinated B^III atom surrounded by four groups of three edge-sharing octahedra (W₃O₁₃ subunits), which are linked in turn to each other and to the central BO₄ tetrahedron by shared O atoms at the vertices. There is a crystallographically unique B—O bond of 1.554 (10) Å, while the average W—O distances are 2.344 (17) Å for four coordinate O atoms, 1.917 (12) and 1.89 (2) Å for two coordinate O atoms within and connecting the W₃O₁₃ subunits, respectively, and 1.709 (8) Å for terminal O atoms. Not all of the K⁺ ions and H₂O groups were located.

S1. Comment

The condensation of [WO₄]^2- and borate has been known to produce heteropoly species for a long time. Klein (1883) reported two tungstoboric acid isomers which crystallized with tetragonal and hexagonal lattices. These were examined by Copaux (1909) and suggested to have W/B ratios of 12 and 14, respectively. Somewhat later Souchay (1951) reported the [BW₁₁O₃₉]^9- ion. Recently, the new tungstoborate species [BW₁₃O₄₆H₃]^8- has been identified in solution, and the hexagonal acid shown to have the composition H₂₁[B₃W₃₉O₁₃₂].69H₂O by an X-ray single-crystal study (Tézé et al., 1997). The quadratic acid is known to have the α-Keggin structure. As part of our studies of the structures and range of heteroatoms found for heteropolyoxo-molybdates and -tungstates, we have examined the structure of the α-[B₁₂O₄₀]^5- ion as the K⁺ salt to allow comparisons with our recent examination of the structures of α-[Co^IIW₁₂O₄₀]^6- and α-[Co^IIIW₁₂O₄₀]^5- (Nolan et al., 2000). Kraus (1936) reported unit-cell dimensions for K₅[BW₁₂O₄₀].18H₂O, which appear to be similar to the present compound (a = 19.0, c = 12.50 Å). The structure of the title compound consists of potassium cations, a [BW₁₂O₄₀]^5- anion and water molecules of crystallization. The geometry of the anion is based on the α-Keggin structure (Pope, 1983) and confirms the previous structural determination of the anion in the related compound K₅[BW₁₂O₄₀]·9H₂O (Yamase & Ishikawa, 1996). The anion exhibits a central tetrahedrally coordinated B^III, surrounded by four groups of three edge-sharing octahedra (W₃O₁₃ subunits) which are linked in turn to each other and to the central BO₄ tetrahedron by shared oxygen atoms at the vertices. A view of the anion is shown in Fig. 1. The central B^III sits at a site of 222 symmetry and is coordinated to four oxygen atoms with a B—O distance of 1.554 (10) Å. This bond distance may be compared with that of the B—O distance of 1.51 (3) or 1.52 (2) Å (this depends on the anion point symmetry) in [BW₁₁O₃₉Co^II(H₂O)]^6- (Weakley, 1984), the three unique B—O distances in H₂₁[B₃W₃₉O₁₃₂].69H₂O of 1.46 (5), 1.47 (3) and 1.58 (7) Å [average 1.50 (5) Å], although in the latter compound the boron sits in an environment intermediate between trigonal BO₃ and tetrahedral BO₄, and the B—O distances round for the two crystallographically unique [BW₁₂O₄₀]^5- ions (with 222 and 2 symmetry) of 1.46 (3), and 1.25 (6) and 1.79 (9) Å, respectively [average 1.49 (6) Å]. For tetrahedral coordination B—O bond lengths vary from 1.43 to 1.55 Å, with an average of 1.475 Å (Wells, 1984). The B—O bond length in the present compound is therefore at the upper limit of this range, and results from the relatively rigid polyoxotungstate framework of the [W₁₂O₄₀]^8- unit, which contains a central cavity in which the small B^III resides (Nolan et al., 2000). The O—B—O bond angles of the title compound vary from 108.8 (7) to 110.5 (7)°, and may be compared with values of 1110 (2) and 111 (2)° in the anion of 222 symmetry of K₅[BW₁₂O₄₀]·9H₂O, and 84 (6), 106 (4), 108 (4) and 134 (11)° for the lower symmetry 2 site. This latter site is obviously considerably distorted. The W—O bond lengths increase with increasing coordination, as found previously, with ranges of 2.320 (9)–2.358 (10) Å for four coordinate O atoms {K₅[BW₁₂O₄₀]·9H₂O, range for both sites, 2.33 (3)–2.47 (3) Å, 1.853 (10)–1.938 (10) Å for two coordinate O atoms {K₅[BW₁₂O₄₀]·9H₂O, 1.82 (3)–1.96 (4) Å}, and 1.699 (11)–1.719 (10) Å for the terminal O atoms {K₅[BW₁₂O₄₀]·9H₂O, 1.66 (3)–1.79 (5)Å}. For the title compound, the W—W distances within the W₃O₁₃ subunits average 3.317 (5) Å, while those connecting the W₃O₁₃ subunits average 3.641 (7) Å. these may be compared with values of 3.313 (3) and 3.645 (3), respectively, in K₅[BW₁₂O₄₀]·9H₂O. In terms of (average) bond distance comparisons, the [W₁₂O₄₀]^8- framework appears to be most similar to the polyoxotungstate framework of the [SiW₁₂O₄₀]^4- ion in the sequence of frameworks of the [XW₁₂O₄₀]^n- anions, where X = P^V, Si^IV, Co^III and Co^II, which exhibit progressively increasing size of the central heteroatom (Nolan et al., 2000). There are, however, some differences in that the W₃O₁₂ group (the W₃O₁₃ subunit minus the centrally bound oxygen atom) is slightly closer to the central boron, which is a consequence of the small size of the (formal) B^III, and necessitates that the angle at the O atoms bridging the W₃O₁₃ subunits is the smallest in the series of [XW₁₂O₄₀]^n- anions [149.3 (8)° for the B^III-containing anion versus a range of 154.9 (20) to 151.8 (8)° in the above sequence]. All K⁺-anion oxygen and K⁺-water oxygen distances are greater than 2.65 (1) Å, with those surrounding the fully occupied K1 ranging from 2.65 (1)–3.10 (1) Å and those surrounding the partially occupied potassium sites ranging from 2.65 (1)–3.39 (1) Å. These are similar to the distances found in K₆[Co^IIW₁₂O₄₀].16H₂O and K₅[Co^IIIW₁₂O₄₀].16H₂O (Nolan et al., 2000).

Related literature

For related literature, see: Casañ-Pastor, Gomez-Romero, Jameson & Baker (1991); Copaux (1909); Klein (1883); Kraus (1936); Nolan et al. (2000); Peterson & Levy (1957); Pope (1983); Souchay (1951); Tézé et al. (1997); Weakley (1984); Wells (1984).

S2. Experimental

A solution of H₃BO₃ (1.25 g) was added to one (25 ml) of Na₂MoO₄·2H₂O (25.0 g) with constant stirring, followed by 6 M HCl (15 ml). The pH of the resulting solution was adjusted to 6 and then the solution was refluxed for 24 h. The resulting precipitate of Na₁₀[H₂W₁₂O₄₂].nH₂O was filtered off. The pH of the filtrate was adjusted to 2 using 6 M HCl and was again refluxed for 30 min. Precipitation was effected by addition of KCl (5 g). The resulting product was then isolated by filtration and redissolved in water at pH 6 using 6 M HCl. Colourless needle-like crystals were obtained by slow evaporation of the solvent. IR (KBr disc): 1001m, 958 s (W—O asymmetric stretch), 920 s (B—O stretch), 906s h (W—O—W stretch), 807vs (W—O—W stretch), 507m cm^-1 (W—O—W bending). Thermogravimetric analysis was used to establish the actual water content of the bulk compound. Based on a loss of mass of 8.75% up to 573 K, there were 16.3 water molecules per formula unit, and so the composition was therefore close to K₅[BW₁₂O₄₀].16H₂O, although only 8 water molecules could be established by X-ray crystallography.

S3. Refinement

The standard reflection showed a 21% loss in intensity over the data collection, and a correction for this was applied. This may perhaps be associated with water loss from the crystal, leading to disorder of much of the H₂O molecules (see below). The systematic absences for the compound are unique for the space group P6₂22. Moreover, the compound is isomorphous and isostructural with K₅[Co^IIIW₁₂O₄₀].16H₂O (Nolan et al., 2000) and the atomic coordinates for the heavy atoms for this structure were therefore used for the present structure. The cations and water molecules were subsequently found using difference-Fourier methods. Four crystallographically unique sites containing potassium were located. One of these was fully occupied by potassium and located along a twofold axis, while the other three only exhibited partial occupancy by potassium together with the oxygen of a water molecule. A similar situation has been found previously in K₅[Co^IIIW₁₂O₄₀].16H₂O. Two of these latter sites were located along twofold axes while the third was in a general position. Based on the electron densities at these sites, new scattering factors were constructed from those of potassium and oxygen. These sites, OKW1, OKW2 and OKW3, had 83, 30 a nd 48% K, with the remainder O. Upon completion of the refinement, 4.92 potassium atoms (i.e. about the expected 5 per formula unit) and 8.08 water molecules had been located, the latter less than the 16 water molecules per formula unit suggested by thermogravimetric analysis. The remaining water molecules were highly disordered and correspond, assuming a volume per water molecule of about 40 ų (Peterson & Levy, 1957), to some 960 ų per unit cell which are located in the interanionic voids. This assumes that the composition of the crystal was typical of the bulk sample, with 16 water molecules per formula unit. This remaining water is zeolitic-like in nature, as was observed in the case of K₅[Co^IIIW₁₂O₄₀].16H₂O (Nolan et al., 2000). This study, performed on a crystal exposed to the atmosphere, allowed the location of only 3 water molecules per formula unit. For the related compound K₆[Co^IIW₁₂O₄₀].nH₂O, a crystal was sealed in a glass capillary to minimize efflorescence, and a subsequent X-ray study allowed the location of 11 water molecules per formula unit, while chemical analysis of an air-dried sample indicated only a trihydrated species (Casañ-Pastor et al., 1991). Attempted refinement in the enantiomeric space group P6₄22 gave a final R factor of 0.042 with 846 friedel pairs not averaged, significantly higher than the 0.036 observed for P6₂22. The maximum peak and minimum trough were located in the region of the W atoms, but not in chemically significant positions.

Computing details

Data collection: CAD-4 Software (Enraf-Nonius, 1989); cell refinement: CAD-4 Software; data reduction: local software (University of New South Wales); program(s) used to solve structure: SIR92 (Altomare, 1994); program(s) used to refine structure: RAELS (Rae, 1989); molecular graphics: ORTEPII (Johnson, 1976).

Figures

Fig. 1. The anion in K5[BW12O40].16H2O indicating the labelling of the atoms. Displacement ellipsoids are shown at the 50% probability level.

(I)

Crystal data

K5[BW12O40](H2O)16	Dx = 4.30 Mg m^−3
Mr = 3340.72	Mo Kα radiation, λ = 0.71073 Å
Trigonal, P6222	Cell parameters from 10 reflections
Hall symbol: P 62 2 (0 0 -2)	θ = 9–11°
a = 18.970 (4) Å	µ = 27.74 mm^−1
c = 12.414 (5) Å	T = 294 K
V = 3869 (2) Å^3	Prism, colourless
Z = 3	0.34 × 0.13 × 0.10 mm
F(000) = 4404.0

Data collection

Enraf-Nonius CAD-4 diffractometer	Rint = 0.047
ω–2θ scans	θmax = 25°
Absorption correction: analytical (de Meulenaer & Tompa, 1965)	h = 0→22
Tmin = 0.04, Tmax = 0.11	k = −22→0
5003 measured reflections	l = 0→14
2288 independent reflections	1 standard reflections every 30 min
2122 reflections with I > 3σ(I)	intensity decay: 21%

Refinement

Refinement on F	H-atom parameters not refined
R[F^2 > 2σ(F^2)] = 0.036	w = 1/[σ^2(F) + 0.0016F^2]
wR(F^2) = 0.048	(Δ/σ)max = 0.006
S = 1.54	Δρmax = 1.79 e Å^−3
2122 reflections	Δρmin = −2.70 e Å^−3
78 parameters

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
B	0.5000 (1)	0.0000 (1)	0.5000 (1)	0.0229 (2)
W1	0.37332 (4)	0.00723 (4)	0.30232 (5)	0.0311 (1)
W2	0.56844 (4)	0.14873 (4)	0.30205 (5)	0.0295 (2)
W3	0.44594 (4)	0.15078 (4)	0.49093 (5)	0.0298 (2)
O123	0.4809 (6)	0.0551 (6)	0.4274 (8)	0.0243 (2)
O2	0.6199 (7)	0.2225 (6)	0.2064 (8)	0.0374 (4)
O1	0.3008 (7)	−0.0106 (7)	0.2090 (9)	0.0409 (2)
O3	0.4190 (7)	0.2230 (7)	0.5132 (11)	0.0382 (4)
O12	0.4675 (7)	0.0910 (7)	0.2268 (8)	0.0329 (2)
O32	0.5236 (7)	0.2046 (6)	0.3797 (8)	0.0309 (2)
O21	0.5934 (7)	0.0669 (6)	0.2612 (8)	0.0300 (2)
O13	0.3112 (6)	−0.0673 (6)	0.4078 (8)	0.0304 (2)
O23	0.6489 (6)	0.1798 (6)	0.4084 (8)	0.0285 (2)
O31	0.3678 (7)	0.0918 (6)	0.3806 (9)	0.0333 (2)
K1	0.3687 (2)	0.7374 (3)	0.0000 (1)	0.040 (1)
OKW1	0.5000 (1)	0.0000 (1)	0.0448 (13)	0.069 (7)
OKW2	0.4701 (11)	0.3488 (11)	0.3425 (14)	0.123 (7)
OKW3	0.1305 (11)	0.261 (2)	0.0000 (1)	0.205 (9)
OW1	0.1995 (16)	0.399 (3)	0.5000 (1)	0.051 (9)
OW2	0.725 (3)	0.0000 (1)	0.0000 (1)	0.058 (9)
OW3	0.204 (3)	0.0000 (1)	0.5000 (1)	0.075 (9)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
B	0.0263 (2)	0.0190 (3)	0.0209 (3)	0.0095 (1)	0.0000 (1)	0.0000 (1)
W1	0.0347 (2)	0.0270 (2)	0.0293 (2)	0.0137 (1)	−0.0071 (2)	0.0002 (1)
W2	0.0369 (2)	0.0225 (2)	0.0244 (2)	0.0114 (1)	0.0014 (1)	0.0035 (2)
W3	0.0348 (3)	0.0235 (2)	0.0327 (2)	0.0156 (2)	−0.0002 (1)	0.0002 (1)
O123	0.0284 (2)	0.0200 (2)	0.0225 (2)	0.0105 (1)	−0.0007 (1)	0.0005 (1)
O2	0.0489 (4)	0.0267 (3)	0.0288 (4)	0.0130 (2)	0.0046 (2)	0.0078 (4)
O1	0.0437 (3)	0.0366 (5)	0.0394 (4)	0.0179 (2)	−0.0159 (4)	−0.0004 (1)
O3	0.0449 (7)	0.0289 (4)	0.0470 (5)	0.0230 (5)	0.0005 (1)	−0.0004 (1)
O12	0.0428 (3)	0.0273 (3)	0.0252 (2)	0.0151 (2)	−0.0045 (2)	0.0030 (1)
O32	0.0389 (3)	0.0219 (2)	0.0306 (3)	0.0142 (1)	−0.0012 (1)	0.0029 (1)
O21	0.0367 (2)	0.0251 (2)	0.0234 (2)	0.0117 (1)	0.0049 (1)	0.0019 (1)
O13	0.0281 (2)	0.0262 (2)	0.0335 (3)	0.0110 (1)	−0.0044 (1)	−0.0007 (1)
O23	0.0308 (2)	0.0207 (2)	0.0277 (3)	0.0082 (2)	0.0025 (1)	0.0020 (1)
O31	0.0356 (2)	0.0287 (3)	0.0378 (3)	0.0176 (3)	−0.0053 (1)	0.0013 (1)
K1	0.042 (2)	0.050 (3)	0.031 (3)	0.025 (2)	0.000 (2)	0.0000 (1)
OKW1	0.097 (9)	0.039 (8)	0.067 (9)	0.031 (9)	0.0000 (1)	0.0000 (1)
OKW2	0.127 (9)	0.141 (9)	0.106 (9)	0.073 (9)	0.013 (9)	0.026 (9)
OKW3	0.205 (9)	0.205 (9)	0.205 (9)	0.102 (9)	0.0000 (1)	0.0000 (1)
OW1	0.051 (9)	0.051 (9)	0.051 (9)	0.026 (8)	0.0000 (1)	0.0000 (1)
OW2	0.058 (9)	0.058 (9)	0.058 (9)	0.029 (9)	0.0000 (1)	0.0000 (1)
OW3	0.075 (9)	0.075 (9)	0.075 (9)	0.038 (8)	0.0000 (1)	0.0000 (1)

Geometric parameters (Å, º)

B—O123	1.554 (10)	W2—O2	1.719 (10)
B—O123^i	1.554 (10)	W2—O12	1.908 (10)
B—O123^ii	1.554 (10)	W2—O32	1.917 (10)
B—O123^iii	1.554 (10)	W2—O21	1.905 (10)
W1—O123	2.354 (10)	W2—O23	1.876 (11)
W1—O1	1.699 (11)	W3—O123	2.358 (10)
W1—O12	1.938 (10)	W3—O3	1.708 (10)
W1—O13	1.853 (10)	W3—O32	1.902 (10)
W1—O31	1.923 (11)	W3—O31	1.914 (11)
W1—O21^i	1.876 (10)	W3—O13^ii	1.923 (10)
W2—O123	2.320 (9)	W3—O23^iii	1.891 (10)
O123—B—O123^i	109.1 (7)	O12—W2—O23	161.8 (4)
O123—B—O123^ii	110.5 (7)	O32—W2—O21	160.8 (4)
O123—B—O123^iii	108.8 (7)	O32—W2—O23	89.3 (4)
O123^i—B—O123^ii	108.8 (7)	O21—W2—O23	87.1 (5)
O123^i—B—O123^iii	110.5 (7)	O123—W3—O3	169.8 (5)
O123^ii—B—O123^iii	109.1 (7)	O123—W3—O32	74.3 (4)
O123—W1—O1	170.4 (4)	O123—W3—O31	75.2 (4)
O123—W1—O12	74.7 (4)	O123—W3—O13^ii	84.9 (4)
O123—W1—O13	85.7 (4)	O123—W3—O23^iii	85.3 (4)
O123—W1—O31	75.1 (4)	O3—W3—O32	98.6 (5)
O123—W1—O21^i	85.3 (4)	O3—W3—O31	97.5 (5)
O1—W1—O12	99.0 (5)	O3—W3—O13^ii	102.3 (5)
O1—W1—O13	100.8 (5)	O3—W3—O23^iii	102.3 (5)
O1—W1—O31	97.7 (5)	O32—W3—O31	87.7 (5)
O1—W1—O21^i	102.0 (5)	O32—W3—O13^ii	159.1 (4)
O12—W1—O13	160.2 (4)	O32—W3—O23^iii	90.3 (4)
O12—W1—O31	88.4 (5)	O31—W3—O13^ii	89.1 (5)
O12—W1—O21^i	88.0 (5)	O31—W3—O23^iii	160.2 (4)
O13—W1—O31	90.0 (5)	O13^ii—W3—O23^iii	85.8 (4)
O13—W1—O21^i	86.8 (4)	B—O123—W1	124.9 (5)
O31—W1—O21^i	160.3 (5)	B—O123—W2	125.8 (5)
O123—W2—O2	169.7 (5)	B—O123—W3	125.0 (5)
O123—W2—O12	76.1 (4)	W1—O123—W2	90.2 (3)
O123—W2—O32	75.0 (4)	W1—O123—W3	89.7 (3)
O123—W2—O21	85.9 (4)	W2—O123—W3	90.2 (3)
O123—W2—O23	85.8 (4)	W1—O12—W2	118.9 (5)
O2—W2—O12	97.3 (5)	W2—O32—W3	120.5 (5)
O2—W2—O32	97.2 (5)	W2—O21—W1^i	148.4 (6)
O2—W2—O21	102.0 (5)	W1—O13—W3^ii	150.3 (6)
O2—W2—O23	100.9 (5)	W2—O23—W3^iii	149.3 (6)
O12—W2—O32	88.2 (5)	W1—O31—W3	120.0 (6)
O12—W2—O21	89.4 (5)

Symmetry codes: (i) −x+1, −y, z; (ii) x−y, −y, −z+1; (iii) y−x+1, y, −z+1.