Dipotassium dialuminium cyclo­octa­phosphate

Single crystals of the title compound, K₂Al₂P₈O₂₄, were obtained by solid-state reaction. The monoclinic structure is isotypic with that of the Ga^III analogue and is built of eight-membered phosphate ring anions P₈O₂₄⁸⁻ (2/m symmetry) isolated from each other and further linked by isolated AlO₆ octa­hedra ( symmetry) by sharing corners. Each AlO₆ octa­hedron is linked to four P₈O₂₄⁸⁻ rings in such a way that two rings are linked through bidentate diphosphate groups attached in the cis positions on two opposite parallel edges of the octa­hedron. The two other rings are linked via corner-sharing to the two remaining corners in the trans positions of the AlO₆ octa­hedron. Each P₈O₂₄⁸⁻ ring anion is linked to eight AlO₆ octa­hedra. More accurately, each ring anion is linked to four AlO₆ octa­hedra through bidentate diphosphate groups attached in the cis positions to the AlO₆ octa­hedron and to the four remaining octa­hedra by sharing corners. This three-dimensional linkage delimits channels running parallel to [001] in which the ten-coordinated K⁺ cations (2 symmetry) are distributed over two columns. These columns alternate with empty octa­gonally-shaped channels expanding through the P₈O₂₄⁸⁻ ring anions.

Related literature

The synthesis and an approximate unit cell with a slightly smaller β angle were reported for the title compound more than a quarter of a century ago (Grunze et al., 1983). The crystal structures of isotypic compounds determined from single-crystal data have been reported for K₂Ga₂P₈O₂₄ (Palkina et al., 1979) and K₂Mn₂P₈O₂₄ (Murashova & Chudinova, 1999). The isostructural potassium-containing cyclo­octa­phosphates K₂V₂P₈O₂₄ (Lavrov et al., 1981), K₂Fe₂P₈O₂₄ (Grunze et al., 1983) and K₂Cr₂P₈O₂₄ (Grunze & Chudinova, 1988) were reported without detailed structure analyses. For a review of the crystal chemistry of cyclo­octa­phosphates, see: Durif (1995, 2005). For potential applications of aluminophosphates, see: Cheetham et al. (1999); Hartmann & Kevan (1999). For background to distortion indices, see: Momma & Izumi (2008).

Experimental

Crystal data

• K₂Al₂P₈O₂₄

• M_r = 763.92

• Monoclinic, C 2/m

• a = 16.598 (2) Å

• b = 12.2150 (17) Å

• c = 5.0705 (7) Å

• β = 100.315 (4)°

• V = 1011.4 (2) ų

• Z = 2

• Mo Kα radiation

• μ = 1.31 mm⁻¹

• T = 296 K

• 0.30 × 0.10 × 0.08 mm

Data collection

• Bruker APEXII CCD diffractometer

• Absorption correction: multi-scan (SADABS; Bruker, 2008) T_min = 0.562, T_max = 0.748

• 9643 measured reflections

• 2844 independent reflections

• 2252 reflections with I > 2σ(I)

• R_int = 0.047

Refinement

• R[F² > 2σ(F²)] = 0.042

• wR(F²) = 0.124

• S = 1.07

• 2844 reflections

• 87 parameters

• Δρ_max = 1.40 e Å⁻³

• Δρ_min = −1.04 e Å⁻³

Table 1
Selected bond lengths (Å)

P1—O1^i	1.4861 (12)
P1—O3	1.4956 (12)
P1—O4	1.5701 (13)
P1—O7	1.5794 (8)
P2—O5	1.4622 (14)
P2—O2	1.4983 (12)
P2—O6	1.5957 (10)
P2—O4	1.6070 (13)
Symmetry code: (i) .

Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 1999), CaRine (Boudias & Monceau, 1998) and ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Supporting information

S1. Comment

The title compound, K₂Al₂P₈O₂₄, belongs to porous crystalline open-framework materials that gain growing interest for their potential applications as molecular sieves or catalysts (Cheetham et al. 1999; Hartmann & Kevan, 1999). The title cyclooctaphosphate is isotypic with K₂Ga₂P₈O₂₄ (Palkina et al., 1979) and K₂Mn₂P₈O₂₄ (Murashova & Chudinova, 1999). This structural family also includes K₂V₂P₈O₂₄ (Lavrov et al., 1981), K₂Fe₂P₈O₂₄ (Grunze et al., 1983) and K₂Cr₂P₈O₂₄ (Grunze & Chudinova, 1988), the structures of which have not yet been refined from X-ray diffraction data.

The crystal structure of K₂M₂P₈O₂₄ is built up of 8-membered phosphate ring anions P₈O₂₄^8- (symmetry 2/m) (Fig. 1), further linked by isolated M^IIIO₆ octahedra to form the three-dimensional skeleton. Each MO₆ octahedron is linked to four ring anions P₈O₂₄^8-. Two ring anions are linked by corner-sharing in trans positions whereas the two others are connected in a bidentate fashion in cis positions on two opposite edges of the equatorial plane of the octahedron. This three-dimensional framework of K₂Al₂P₈O₂₄ delimits two kinds of channels expanding along the [001] direction (Fig. 2). The first channel is octagonally shaped since passing through the ring anions and is empty despite a size of ≈ 5.2 Å in diameter. The second channel, cross shaped, accommodates the K⁺ ions in a [6 + 4] coordination. The K⁺ ions are located over two columns shifted of about c/2 with respect to each other along the c axis. Thus they form two distinct K—K pairs with common square faces involving only O(5) oxygen atoms. In the first pair, corresponding to the shortest K—K distance (3.634 (9) Å) (Fig. 3), the four O(5) atoms delimit a pseudo-square face with O—O separations of 2.995 (2) and 3.064 (6) Å and O(5)—O(5)—O(5) angles of 90°. This pair corresponds to the two shortest K—O distances, viz. 2.7559 (15) and 2.8612 (15) Å. In the second pair with a K—K distance of 4.365 (7) Å, the four O(5) atoms at the vertices of the pseudo-square face (O—O separation 2.995 (2) and 3.139 (7) Å with O(5)—O(5)—O(5) angles of 90°), the respective K—O bonds lengths are 2.8612 (15) and 3.2790 (17) Å. The polyhedra surrounding three potassium cations engaged in two successive pairs (one long and one short) form a cluster within they share a common O(5)—O(5) edge with a contact distance of 2.995 (2) Å. This potassium-oxygen polyhedra packing also prevails in the Ga and Mn cyclooctaphosphate analogues but the K—O distances spread over larger ranges, viz. from 2.754 (28) Å to 3.359 (28) Å and from 2.738 (2) to 3.506 (2) Å, respectively. Thus the respective coordinations of the potassium cations can be regarded as being [6 + 4] and [8 + 2] for Ga and Mn cyclooctaphosphate.

A careful examination of the geometry of the M^IIIO₆ octahedra in this structural type shows that the distortion index (bond length) (Momma & Izumi, 2008) increases from Al to Mn (0.0117 for Al, 0.0202 for Ga and 0.0574 for Mn). The AlO₆ octahedron is only very slightly distorted with two shorter Al—O distances of 1.8523 (12) Å and four others very close to 1.90 Å (1.9013 (12) (× 2) and 1.9021 (11) (× 2) Å). The significant larger distortion of the MnO₆ octahedron is probably due to the Jahn-Teller effect associated with the d⁴ electronic configuration of Mn^III (Murashova & Chudinova, 1999).

Besides the structural family to which the title compound belongs, only another sodium and silver- containing cyclooctaphosphate, Ag₉NaP₈O₂₄(NO₃)₂.4H₂O, exhibits a ring anion with internal 2/m symmetry among the presently known cyclooctaphosphates (Durif, 1995, 2005). However, despite the common internal symmetry, the shape of the 8-membered ring anion present in this structure is very different from that of the title compound as shown in Fig. 4.

Related literature

The synthesis and an approximate unit cell with a slightly smaller β angle were reported for the title compound more than a quarter of a century ago (Grunze et al., 1983). The crystal structures of isotypic compounds determined from single-crystal data have been reported for K₂Ga₂P₈O₂₄ (Palkina et al., 1979) and K₂Mn₂P₈O₂₄ (Murashova & Chudinova, 1999). The isostructural potassium-containing cyclooctaphosphates K₂V₂P₈O₂₄ (Lavrov et al., 1981), K₂Fe₂P₈O₂₄ (Grunze et al., 1983) and K₂Cr₂P₈O₂₄ (Grunze & Chudinova, 1988) were reported without detailed structure alalyses. For a review of the crystal chemistry of cyclooctaphosphates, see: Durif (1995, 2005). For potential applications of aluminophosphates, see: Cheetham et al. (1999); Hartmann & Kevan (1999). For background to distortion indices, see: Momma & Izumi (2008).

S2. Experimental

Single-crystals of the title compound were obtained by solid state reaction, from the reagents K₂CO₃, Al₂O₃ and (NH₄)H₂PO₄ in the molar ratio K / P / Al = 57 / 34 / 9. The mixture has progressively been heated up to 873 K over a period of 12 h. Then the temperature was slowly decreased down to 723 K at the rate of 5 K h^-1 and maintained at this value for 12 h. Then a new cooling step down to 573 K at the rate of 5 K h^-1 was carried out before the furnace was switched off. Single-crystals of K₂Al₂P₈O₂₄ were extracted from the batch by washing with hot water in order to remove the excess of P₂O₅. A translucent colorless needle of the title compound was used for the structure refinement.

S3. Refinement

The highest residual peak in the final difference Fourier map was located 0.74 Å from atom P2 and the deepest hole was located 0.69 Å from atom P1.

Computing details

Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 1999), CaRine (Boudias & Monceau, 1998) and ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures

	Fig. 1. ORTEP-3 view (Farrugia, 1997) of the centrosymmetric (P8O24)^8- ring anion. Displacement ellipsoids are drawn at the 50% probability level. Symmetry codes: (i) -x, -y + 1, -z + 1; (iv) -x, y, -z; (v) -x, -y + 1, -z; (vi) x - 1/2, -y + 1/2, z; (ix) -x + 1/2, -y + 1/2, -z + 1; (xi) x, -y, z.
	Fig. 2. Projections of the 8-membered ring anions P8O24^8-along [010] (left) and [001] (right).
	Fig. 3. Partial view showing the packing of the K—O polyhedra along the c axis in the ball and stick (left) and polyhedral (right) representation.
	Fig. 4. Comparison of the 8-membered ring anions P8O24^8- (both with internal 2/m symmetry) in K2Al2P8O24 (top) and Ag9NaP8O24(NO3)2.4H2O (bottom).

Dipotassium dialuminium cyclooctaphosphate

Crystal data

K2Al2P8O24	F(000) = 752
Mr = 763.92	Dx = 2.508 Mg m^−3
Monoclinic, C2/m	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2y	Cell parameters from 3017 reflections
a = 16.598 (2) Å	θ = 3.3–37.7°
b = 12.2150 (17) Å	µ = 1.31 mm^−1
c = 5.0705 (7) Å	T = 296 K
β = 100.315 (4)°	Needle, colourless
V = 1011.4 (2) Å^3	0.30 × 0.10 × 0.08 mm
Z = 2

Data collection

Bruker APEXII CCD diffractometer	2844 independent reflections
Radiation source: fine-focus sealed tube	2252 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.047
Detector resolution: 8.3333 pixels mm^-1	θmax = 38.7°, θmin = 4.1°
ω and ϕ scans	h = −29→28
Absorption correction: multi-scan (SADABS; Bruker, 2008)	k = −18→20
Tmin = 0.562, Tmax = 0.748	l = −8→8
9643 measured reflections

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	Primary atom site location: structure-invariant direct methods
R[F^2 > 2σ(F^2)] = 0.042	Secondary atom site location: difference Fourier map
wR(F^2) = 0.124	w = 1/[σ^2(Fo^2) + (0.0738P)^2 + 0.0398P] where P = (Fo^2 + 2Fc^2)/3
S = 1.07	(Δ/σ)max < 0.001
2844 reflections	Δρmax = 1.40 e Å^−3
87 parameters	Δρmin = −1.04 e Å^−3

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 F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² 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 (Ų)

	x	y	z	Uiso*/Ueq
K	−0.09371 (4)	0.5000	0.25194 (13)	0.02290 (13)
Al	0.2500	0.2500	0.0000	0.00771 (13)
P1	0.19023 (2)	0.12098 (3)	0.46962 (8)	0.00720 (9)
P2	0.07782 (2)	0.27631 (3)	0.16477 (8)	0.00835 (9)
O1	0.28060 (8)	0.36965 (10)	0.2366 (2)	0.0118 (2)
O2	0.14059 (7)	0.28394 (11)	−0.0132 (2)	0.0132 (2)
O3	0.24968 (7)	0.15349 (10)	0.2942 (2)	0.0105 (2)
O4	0.10711 (7)	0.18408 (11)	0.3873 (2)	0.0122 (2)
O5	0.05469 (9)	0.37739 (12)	0.2866 (3)	0.0184 (3)
O6	0.0000	0.21471 (15)	0.0000	0.0140 (3)
O7	0.16028 (11)	0.0000	0.3993 (3)	0.0125 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
K	0.0299 (3)	0.0147 (3)	0.0240 (3)	0.000	0.0046 (2)	0.000
Al	0.0085 (3)	0.0083 (3)	0.0064 (3)	0.0001 (2)	0.0016 (2)	0.0006 (2)
P1	0.00956 (16)	0.00518 (17)	0.00672 (15)	0.00032 (12)	0.00110 (11)	0.00033 (11)
P2	0.00850 (17)	0.00632 (18)	0.01023 (16)	0.00094 (12)	0.00164 (12)	0.00031 (12)
O1	0.0186 (5)	0.0084 (5)	0.0078 (4)	−0.0009 (4)	0.0006 (4)	−0.0006 (3)
O2	0.0095 (5)	0.0177 (6)	0.0130 (5)	0.0020 (4)	0.0033 (4)	0.0049 (4)
O3	0.0104 (5)	0.0111 (5)	0.0105 (5)	0.0011 (4)	0.0037 (3)	0.0033 (4)
O4	0.0108 (5)	0.0125 (6)	0.0138 (5)	0.0033 (4)	0.0034 (4)	0.0051 (4)
O5	0.0247 (6)	0.0105 (6)	0.0198 (6)	0.0053 (5)	0.0030 (5)	−0.0045 (5)
O6	0.0096 (7)	0.0092 (8)	0.0217 (8)	0.000	−0.0011 (6)	0.000
O7	0.0177 (7)	0.0051 (7)	0.0129 (7)	0.000	−0.0021 (5)	0.000

Geometric parameters (Å, º)

K—O5^i	2.7559 (15)	Al—O3	1.9021 (11)
K—O5^ii	2.7559 (15)	P1—O1^ix	1.4861 (12)
K—O5	2.8612 (15)	P1—O3	1.4956 (12)
K—O5^iii	2.8612 (15)	P1—O4	1.5701 (13)
K—O2^iv	2.9493 (14)	P1—O7	1.5794 (8)
K—O2^v	2.9493 (13)	P2—O5	1.4622 (14)
K—O3^vi	3.2431 (13)	P2—O2	1.4983 (12)
K—O3^vii	3.2431 (13)	P2—O6	1.5957 (10)
K—O5^iv	3.2790 (17)	P2—O4	1.6070 (13)
K—O5^v	3.2790 (17)	P2—K^v	3.4928 (7)
K—P2^iv	3.4927 (7)	O1—P1^ix	1.4861 (12)
K—P2^v	3.4927 (7)	O2—K^v	2.9494 (13)
Al—O2^viii	1.8523 (12)	O3—K^x	3.2431 (13)
Al—O2	1.8523 (12)	O5—K^i	2.7559 (15)
Al—O1^viii	1.9013 (12)	O5—K^v	3.2790 (17)
Al—O1	1.9013 (12)	O6—P2^iv	1.5957 (10)
Al—O3^viii	1.9021 (11)	O7—P1^xi	1.5793 (8)
O5^i—K—O5^ii	65.84 (6)	O5—K—P2^v	106.81 (4)
O5^i—K—O5	99.41 (4)	O5^iii—K—P2^v	58.01 (3)
O5^ii—K—O5	66.08 (6)	O2^iv—K—P2^v	119.65 (3)
O5^i—K—O5^iii	66.08 (6)	O2^v—K—P2^v	25.13 (2)
O5^ii—K—O5^iii	99.41 (4)	O3^vi—K—P2^v	129.83 (3)
O5—K—O5^iii	63.13 (6)	O3^vii—K—P2^v	74.69 (2)
O5^i—K—O2^iv	147.08 (5)	O5^iv—K—P2^v	78.81 (3)
O5^ii—K—O2^iv	82.35 (4)	O5^v—K—P2^v	24.68 (2)
O5—K—O2^iv	73.61 (4)	P2^iv—K—P2^v	102.94 (2)
O5^iii—K—O2^iv	130.89 (4)	O2^viii—Al—O2	180.0
O5^i—K—O2^v	82.35 (4)	O2^viii—Al—O1^viii	90.00 (6)
O5^ii—K—O2^v	147.08 (5)	O2—Al—O1^viii	89.99 (6)
O5—K—O2^v	130.89 (4)	O2^viii—Al—O1	89.99 (6)
O5^iii—K—O2^v	73.61 (4)	O2—Al—O1	90.01 (6)
O2^iv—K—O2^v	126.97 (6)	O1^viii—Al—O1	180.00 (5)
O5^i—K—O3^vi	109.11 (4)	O2^viii—Al—O3^viii	91.53 (5)
O5^ii—K—O3^vi	72.48 (4)	O2—Al—O3^viii	88.47 (5)
O5—K—O3^vi	112.63 (4)	O1^viii—Al—O3^viii	91.12 (5)
O5^iii—K—O3^vi	171.88 (4)	O1—Al—O3^viii	88.88 (5)
O2^iv—K—O3^vi	49.77 (3)	O2^viii—Al—O3	88.47 (5)
O2^v—K—O3^vi	112.88 (4)	O2—Al—O3	91.52 (5)
O5^i—K—O3^vii	72.48 (4)	O1^viii—Al—O3	88.88 (5)
O5^ii—K—O3^vii	109.11 (4)	O1—Al—O3	91.12 (5)
O5—K—O3^vii	171.88 (4)	O3^viii—Al—O3	180.0
O5^iii—K—O3^vii	112.63 (4)	O1^ix—P1—O3	116.38 (7)
O2^iv—K—O3^vii	112.88 (4)	O1^ix—P1—O4	109.98 (7)
O2^v—K—O3^vii	49.77 (3)	O3—P1—O4	110.70 (7)
O3^vi—K—O3^vii	70.64 (5)	O1^ix—P1—O7	109.34 (8)
O5^i—K—O5^iv	154.67 (6)	O3—P1—O7	109.20 (8)
O5^ii—K—O5^iv	114.04 (5)	O4—P1—O7	99.98 (8)
O5—K—O5^iv	61.04 (5)	O5—P2—O2	117.73 (9)
O5^iii—K—O5^iv	89.66 (4)	O5—P2—O6	111.73 (8)
O2^iv—K—O5^iv	47.65 (3)	O2—P2—O6	107.39 (6)
O2^v—K—O5^iv	98.27 (4)	O5—P2—O4	111.39 (8)
O3^vi—K—O5^iv	94.08 (3)	O2—P2—O4	108.11 (7)
O3^vii—K—O5^iv	126.82 (4)	O6—P2—O4	98.73 (7)
O5^i—K—O5^v	114.04 (5)	O5—P2—K^v	69.45 (6)
O5^ii—K—O5^v	154.67 (6)	O2—P2—K^v	56.71 (5)
O5—K—O5^v	89.66 (4)	O6—P2—K^v	101.18 (5)
O5^iii—K—O5^v	61.04 (5)	O4—P2—K^v	157.99 (5)
O2^iv—K—O5^v	98.27 (4)	P1^ix—O1—Al	133.95 (8)
O2^v—K—O5^v	47.65 (3)	P2—O2—Al	138.36 (8)
O3^vi—K—O5^v	126.82 (4)	P2—O2—K^v	98.17 (6)
O3^vii—K—O5^v	94.08 (3)	Al—O2—K^v	113.58 (5)
O5^iv—K—O5^v	54.36 (5)	P1—O3—Al	136.54 (8)
O5^i—K—P2^iv	155.10 (4)	P1—O3—K^x	120.51 (6)
O5^ii—K—P2^iv	93.25 (3)	Al—O3—K^x	101.05 (4)
O5—K—P2^iv	58.01 (3)	P1—O4—P2	132.20 (8)
O5^iii—K—P2^iv	106.81 (4)	P2—O5—K^i	141.08 (8)
O2^iv—K—P2^iv	25.13 (2)	P2—O5—K	134.77 (8)
O2^v—K—P2^iv	119.65 (3)	K^i—O5—K	80.59 (4)
O3^vi—K—P2^iv	74.69 (2)	P2—O5—K^v	85.87 (6)
O3^vii—K—P2^iv	129.83 (3)	K^i—O5—K^v	114.04 (5)
O5^iv—K—P2^iv	24.68 (2)	K—O5—K^v	90.34 (4)
O5^v—K—P2^iv	78.81 (3)	P2—O6—P2^iv	123.73 (12)
O5^i—K—P2^v	93.25 (3)	P1^xi—O7—P1	138.66 (12)
O5^ii—K—P2^v	155.10 (4)

Symmetry codes: (i) −x, −y+1, −z+1; (ii) −x, y, −z+1; (iii) x, −y+1, z; (iv) −x, y, −z; (v) −x, −y+1, −z; (vi) x−1/2, −y+1/2, z; (vii) x−1/2, y+1/2, z; (viii) −x+1/2, −y+1/2, −z; (ix) −x+1/2, −y+1/2, −z+1; (x) x+1/2, y−1/2, z; (xi) x, −y, z.