Rb(GaPO₄)₂(OH)(H₂O)·H₂O: a hydrated rubidium gallium phosphate analogue of GaPO₄·2H₂O and leucophosphite

Crystals of the title hydrated rubidium gallium phosphate, rubidium aqua-μ₃-hydroxo-di-μ-phosphato-digallium hydrate, were synthesized hydro­thermally at 453 K under autogenous pressure. The solid crystallizes in the monoclinic system and its structure was determined from single-crystal X-ray diffraction analysis. It is similar to dihydrated gallium phosphate, GaPO₄·2H₂O, which is isostructural with the mineral leucophosphite. The structure is built up from a three-dimensional anionic framework composed of corner-linked octameric Ga₄(PO₄)₄(OH)₂(H₂O)₂ units. The Ga atom is in an octahedral coordination. Connection of the Ga₄P₄ species generates eight-ring channels, in which are encapsulated the Rb⁺ cations and water mol­ecules.

S1. Comment

Microporous aluminium phosphates have been widely studied, because they find many potential applications in diverse areas such as catalysis, gas separation or ionic exchange. The replacement of Al by Ga leads to the production of open-framework solids. Some of these are isostructural with the aluminophosphate analogues and others exhibit novel three-dimensional topologies (Cheetham et al., 1999). For example, the gallium phosphates MIL-31 (Sassoye et al., 2000), cloverite (Estermann et al., 1991) or Ga₂(DETA)(PO₄)₂·2H₂O (DETA is?; Lin et al., 2001) are characterized by unique extra large open-frameworks delimited by 18-, 20- and 24-ring channels, respectively. While these materials are typically prepared by means of organic structure-directing agents (amines, tetraalkylammonium salts, etc.), there have been a few reports concerning the use of alkali cations for the formation of phosphate-based porous solids. To date, several gallium phosphates have been synthezized with Na (Attfield et al., 1995), K (Harrison et al., 1995) or Rb (Lii, 1996; Hammond & Zubieta, 1999) as cations. In these compounds, the alkali cations reside within small cavities delimited by six-, seven- or eight-membered rings.

In our recent work, we have focused our attention on the reactivity of alkali cations in the hydrothermal synthesis of open-framework gallium phosphates (Beitone et al., 2001). In this context, we describe here the synthesis and characterization of the title compound, Rb(GaPO₄)₂(OH)(H₂O)·H₂O, (I). \sch

The structure of (I) is similar to the hydrated gallium phosphate GaPO₄·2H₂O (Mooney-Slater, 1966), the gallium phosphates GaPO₄—C7 (Wang et al., 1989) and NH₄[Ga₂(PO₄)₂(OH)(H₂O)]·H₂O (Loiseau & Ferey, 1994), and the aluminophosphate AlPO₄-15 (Pluth et al., 1984). These structures all belong to the leucophosphite structural type (Moore, 1972; Dick & Zeiske, 1997) corresponding to a potassium iron phosphate.

The structure of (I) is built up from the connection by vertices of PO₄ tetrahedra and GaO₄(OH)(OH,H₂O) octahedra (Fig. 1). The two types of P atoms are tetrahedrally coordinated to O atoms, with typical P—O distances ranging from 1.515 (2) to 1.554 (2) Å. One of the two Ga atoms (Ga1) is connected to four O atoms and two OH (O9—H1) groups (in the cis position). The two Ga1—O9H distances [2.131 (2) and 2.141 (2) Å] are longer than the Ga1—O distances [Ga—O 1.905 (2)–1.925 (2) Å].

The Ga1 octahedra share an edge corresponding to the OH groups, each OH being also connected to the Ga atom Ga2. This second Ga atom is coordinated to four O atoms, one OH group (O9—H1) and one water molecule (H2—O10—H3). The water molecule attached to Ga2 is terminal [Ga2—O10 2.018 (2) Å], in the cis position of the OH group. The bond-valence sums (O'Keeffe & Brese, 1992) for atoms O9 and O10 are 0.997 and 0.459, respectively, omitting the O—H bonds. These calculations confirm the occurrence of a hydroxyl group (O9) and a water molecule (O10).

The connection of the gallium octahedra forms a tetrameric unit which is linked to four phosphate groups. Two of them (P2) have bonds to three adjacent Ga atoms, whereas the two other (P1) connect to two Ga centres only. This results in the formation of an octameric Ga₄(PO₄)₄(OH)₂(H₂O)₂ cluster, corresponding to the basic building unit encountered in the leucophosphite structural type. This moiety was also observed in the extra large pore gallium phosphate, Ga₄(HPO₄)(PO₄)₃(OH)₃·4N₂C₄H₁₄·6H₂O, whose structure exhibits channels bounded by 20-ring windows (Chippindale et al., 1999; Walton et al., 2000). The arrangement of these blocks, connected to each other through all the remaining free O atoms (except O10, which is terminal), describes a three-dimensional framework delimiting eight-ring channels running along [010] (Fig. 2) and [001].

The Rb⁺ cation (Rb1) and a water molecule (OW) are encapsulated within the [010] and [001] tunnels, respectively. The Rb⁺ cation is coordinated to ten O atoms, two of them belonging to the water molecules of O10 and OW. The solvent water molecule OW is hydrogen bonded to O9 via H1 [O9—H1···OW 2.25 (1) Å], and to the ligand water molecule O10 via atom H2 [O10—H2···OW 1.988 (1) Å].

An aluminophosphate containing Rb⁺ cations and which is isostructural with leucophosphite has recently been described by Nandini Devi & Vidyasagar (1999), but this compound differs in the A:M ratio (A is Rb and M is Al or Ga). This ratio was observed to be 3:4 in the aluminophosphate, whereas it is 1:2 in the present gallium-based phase, (I).

Table 1. Principal interatomic distances (Å).

S2. Experimental

The title compound was prepared hydrothermally from a mixture of gallium oxide, phosphoric acid, rubidium hydroxide and deionized water in the molar ratio 0.5:1:0.5:80. This mixture was sealed in a Teflon-lined Parr autoclave and then heated for 36 h at 453 K under autogeneous pressure. The pH was 2 during the synthesis. After cooling to room temperature, the solid was separated from the liquid phase by filtration, washed with water then dried in air. A single-crystal of (I) was optically selected for the diffraction study and glued to a glass fibre.

Computing details

Data collection: SMART (Siemens, 1994); cell refinement: SMART; data reduction: SHELXTL (Sheldrick, 1994); program(s) used to solve structure: SHELXTL; program(s) used to refine structure: SHELXTL; molecular graphics: DIAMOND (Brandenburg, 1996); software used to prepare material for publication: SHELXTL.

Figures

	Fig. 1. Displacement ellipsoid plot (50% probability) of the Ga4(PO4)4(OH)2(H2O)2 building unit of (I) with the Rb cations and the free water molecule OW.
	Fig. 2. A polyhedral projection of the structure of (I) along [010], showing the eight-ring channels incorporating the Rb cations (large grey circles). Dark grey octahedra denote GaO4(OH)(OH,H2O) units, light grey tetrahedra denote PO4 groups and small open circles denote H atoms.

Rubidium aqua-µ₃-hydroxo-di-µ-phosphato-digallium hydrate

Crystal data

Rb(GaPO4)2(OH)(H2O)·H2O	F(000) = 888
Mr = 467.89	Dx = 3.501 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yn	Cell parameters from 5336 reflections
a = 9.6384 (5) Å	θ = 2.7–29.6°
b = 9.6723 (5) Å	µ = 11.93 mm^−1
c = 9.7512 (5) Å	T = 293 K
β = 102.465 (1)°	Octahedral, colourless
V = 887.63 (8) Å^3	0.24 × 0.20 × 0.16 mm
Z = 4

Data collection

Siemens SMART CCD area-detector diffractometer	2228 independent reflections
Radiation source: fine-focus sealed tube	2069 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.020
ω scans	θmax = 29.6°, θmin = 2.7°
Absorption correction: multi-scan (SORTAV; Blessing, 1995)	h = −9→13
Tmin = 0.098, Tmax = 0.148	k = −12→11
5966 measured reflections	l = −12→12

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.024	All H-atom parameters refined
wR(F^2) = 0.062	w = 1/[σ^2(Fo^2) + (0.0383P)^2 + 2.8975P] where P = (Fo^2 + 2Fc^2)/3
S = 0.96	(Δ/σ)max < 0.001
2228 reflections	Δρmax = 2.47 e Å^−3
166 parameters	Δρmin = −1.57 e Å^−3
5 restraints	Extinction correction: Larson (1970), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0047 (5)

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 on F² for ALL reflections except for 0 with very negative F² or flagged by the user for potential systematic errors. Weighted R-factors wR and all goodnesses of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The observed criterion of F² > σ(F²) is used only for calculating _R_factor_obs 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
Rb1	0.48171 (4)	−0.19401 (4)	0.10195 (4)	0.02112 (11)
Ga1	0.11207 (3)	0.04460 (3)	0.13059 (3)	0.00674 (10)
Ga2	0.40026 (3)	−0.22615 (3)	−0.31484 (3)	0.00652 (10)
P1	0.20901 (8)	−0.18425 (7)	0.36638 (8)	0.00570 (15)
P2	0.20474 (8)	−0.02534 (8)	−0.14872 (8)	0.00585 (15)
O1	0.2601 (2)	−0.1674 (2)	−0.4749 (2)	0.0105 (4)
O2	0.5547 (2)	−0.2670 (2)	−0.1622 (2)	0.0116 (4)
O3	0.1962 (2)	0.2172 (2)	0.1963 (2)	0.0109 (4)
O4	0.2197 (2)	−0.0406 (2)	0.2980 (2)	0.0089 (4)
O5	0.3189 (2)	−0.1173 (2)	−0.1850 (2)	0.0102 (4)
O6	−0.0554 (2)	0.0876 (2)	0.2000 (2)	0.0089 (4)
O7	0.2883 (2)	−0.3819 (2)	−0.2826 (2)	0.0089 (4)
O8	0.2396 (2)	−0.0084 (2)	0.0132 (2)	0.0087 (4)
O9	0.0021 (2)	−0.1414 (2)	0.0576 (2)	0.0074 (4)
O10	0.5081 (3)	−0.0569 (3)	−0.3536 (3)	0.0182 (5)
OW	0.3146 (4)	0.1475 (3)	−0.4591 (3)	0.0297 (7)
H1	0.057 (5)	−0.192 (5)	0.043 (6)	0.032 (14)*
H2	0.461 (5)	0.003 (4)	−0.394 (5)	0.023 (12)*
H3	0.586 (3)	−0.042 (5)	−0.331 (5)	0.019 (12)*
H4	0.278 (6)	0.143 (7)	−0.393 (5)	0.046 (17)*
H5	0.272 (9)	0.098 (8)	−0.521 (7)	0.10 (3)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Rb1	0.0184 (2)	0.0293 (2)	0.0161 (2)	0.00658 (13)	0.00485 (13)	0.00735 (13)
Ga1	0.0062 (2)	0.0086 (2)	0.0056 (2)	−0.00047 (11)	0.00176 (12)	−0.00082 (11)
Ga2	0.0062 (2)	0.0072 (2)	0.0060 (2)	−0.00004 (11)	0.00101 (11)	0.00047 (11)
P1	0.0060 (3)	0.0057 (3)	0.0052 (3)	0.0001 (2)	0.0006 (3)	0.0004 (2)
P2	0.0061 (3)	0.0061 (3)	0.0057 (3)	0.0004 (2)	0.0023 (3)	−0.0002 (2)
O1	0.0115 (10)	0.0118 (10)	0.0066 (10)	0.0027 (8)	−0.0013 (8)	0.0003 (8)
O2	0.0097 (10)	0.0156 (11)	0.0086 (10)	0.0041 (8)	0.0003 (8)	−0.0010 (8)
O3	0.0138 (11)	0.0095 (10)	0.0102 (10)	−0.0037 (8)	0.0046 (8)	−0.0007 (8)
O4	0.0103 (10)	0.0064 (9)	0.0089 (10)	−0.0012 (8)	−0.0006 (8)	0.0031 (7)
O5	0.0097 (10)	0.0113 (10)	0.0098 (10)	0.0037 (8)	0.0026 (8)	−0.0015 (8)
O6	0.0072 (10)	0.0095 (10)	0.0103 (10)	−0.0013 (8)	0.0026 (8)	−0.0024 (8)
O7	0.0115 (10)	0.0067 (9)	0.0105 (10)	−0.0011 (8)	0.0066 (8)	−0.0013 (8)
O8	0.0076 (9)	0.0121 (10)	0.0064 (10)	0.0019 (8)	0.0016 (8)	−0.0013 (8)
O9	0.0067 (9)	0.0094 (10)	0.0067 (9)	0.0018 (8)	0.0029 (8)	−0.0002 (8)
O10	0.0090 (11)	0.0178 (12)	0.0271 (14)	−0.0033 (9)	0.0022 (10)	0.0099 (10)
OW	0.047 (2)	0.0206 (14)	0.028 (2)	−0.0063 (13)	0.0237 (14)	−0.0061 (12)

Geometric parameters (Å, º)

Rb1—O6^i	2.847 (2)	P1—O3^i	1.535 (2)
Rb1—O2	2.900 (2)	P1—O4	1.554 (2)
Rb1—O8	2.923 (2)	P1—Rb1^ix	3.6926 (8)
Rb1—O5	2.991 (2)	P2—O5	1.515 (2)
Rb1—O3^i	3.001 (2)	P2—O6^v	1.541 (2)
Rb1—O7^ii	3.017 (2)	P2—O7^xi	1.549 (2)
Rb1—O1^ii	3.229 (2)	P2—O8	1.551 (2)
Rb1—OW^iii	3.276 (4)	P2—Rb1^iv	3.6398 (8)
Rb1—O10^iv	3.437 (3)	P2—Rb1^viii	3.9553 (8)
Rb1—O5^iv	3.570 (2)	O1—P1^xii	1.528 (2)
Rb1—P2	3.5981 (8)	O1—Rb1^viii	3.229 (2)
Rb1—P2^iv	3.6398 (8)	O2—P1^vii	1.528 (2)
Ga1—O3	1.905 (2)	O3—P1^vi	1.535 (2)
Ga1—O4	1.921 (2)	O3—Rb1^vi	3.001 (2)
Ga1—O8	1.921 (2)	O5—Rb1^iv	3.570 (2)
Ga1—O6	1.925 (2)	O6—P2^v	1.541 (2)
Ga1—O9	2.131 (2)	O6—Rb1^vi	2.847 (2)
Ga1—O9^v	2.141 (2)	O7—P2^iii	1.549 (2)
Ga1—Rb1^vi	3.8767 (5)	O7—Rb1^viii	3.017 (2)
Ga2—O2	1.905 (2)	O9—Ga1^v	2.141 (2)
Ga2—O1	1.917 (2)	O9—Ga2^ix	2.163 (2)
Ga2—O7	1.918 (2)	O9—H1	0.75 (3)
Ga2—O5	1.937 (2)	O10—Rb1^iv	3.437 (3)
Ga2—O10	2.018 (2)	O10—H2	0.79 (3)
Ga2—O9^vii	2.163 (2)	O10—H3	0.75 (3)
Ga2—Rb1^viii	4.0145 (5)	OW—Rb1^xi	3.276 (4)
P1—O2^ix	1.528 (2)	OW—H4	0.80 (3)
P1—O1^x	1.528 (2)	OW—H5	0.81 (3)
O6^i—Rb1—O2	110.43 (6)	O4—Ga1—O6	97.68 (9)
O6^i—Rb1—O8	136.29 (6)	O8—Ga1—O6	163.67 (9)
O2—Rb1—O8	102.58 (6)	O3—Ga1—O9	175.48 (9)
O6^i—Rb1—O5	145.31 (6)	O4—Ga1—O9	93.44 (8)
O2—Rb1—O5	53.72 (6)	O8—Ga1—O9	84.94 (9)
O8—Rb1—O5	49.22 (6)	O6—Ga1—O9	84.57 (9)
O6^i—Rb1—O3^i	55.74 (6)	O3—Ga1—O9^v	90.44 (9)
O2—Rb1—O3^i	142.66 (7)	O4—Ga1—O9^v	178.30 (9)
O8—Rb1—O3^i	80.63 (6)	O8—Ga1—O9^v	83.27 (9)
O5—Rb1—O3^i	115.19 (6)	O6—Ga1—O9^v	83.81 (9)
O6^i—Rb1—O7^ii	81.18 (6)	O9—Ga1—O9^v	87.52 (8)
O2—Rb1—O7^ii	89.14 (6)	O2—Ga2—O1	172.76 (10)
O8—Rb1—O7^ii	128.02 (6)	O2—Ga2—O7	94.66 (10)
O5—Rb1—O7^ii	124.23 (6)	O1—Ga2—O7	92.50 (10)
O3^i—Rb1—O7^ii	118.51 (6)	O2—Ga2—O5	87.72 (9)
O6^i—Rb1—O1^ii	75.60 (6)	O1—Ga2—O5	93.30 (9)
O2—Rb1—O1^ii	47.14 (6)	O7—Ga2—O5	89.89 (9)
O8—Rb1—O1^ii	146.77 (6)	O2—Ga2—O10	88.14 (10)
O5—Rb1—O1^ii	100.38 (6)	O1—Ga2—O10	84.72 (10)
O3^i—Rb1—O1^ii	130.88 (6)	O7—Ga2—O10	176.86 (10)
O7^ii—Rb1—O1^ii	52.53 (6)	O5—Ga2—O10	88.79 (11)
O6^i—Rb1—OW^iii	90.47 (7)	O2—Ga2—O9^vii	87.22 (9)
O2—Rb1—OW^iii	82.23 (7)	O1—Ga2—O9^vii	92.00 (9)
O8—Rb1—OW^iii	66.07 (7)	O7—Ga2—O9^vii	88.31 (8)
O5—Rb1—OW^iii	59.01 (7)	O5—Ga2—O9^vii	174.47 (9)
O3^i—Rb1—OW^iii	64.89 (7)	O10—Ga2—O9^vii	93.25 (10)
O7^ii—Rb1—OW^iii	165.12 (7)	O2^ix—P1—O1^x	107.80 (13)
O1^ii—Rb1—OW^iii	113.52 (7)	O2^ix—P1—O3^i	112.05 (13)
O6^i—Rb1—O10^iv	94.31 (6)	O1^x—P1—O3^i	112.33 (13)
O2—Rb1—O10^iv	146.19 (6)	O2^ix—P1—O4	110.46 (12)
O8—Rb1—O10^iv	71.08 (6)	O1^x—P1—O4	107.60 (12)
O5—Rb1—O10^iv	114.90 (6)	O3^i—P1—O4	106.52 (12)
O3^i—Rb1—O10^iv	70.52 (6)	O5—P2—O6^v	111.70 (13)
O7^ii—Rb1—O10^iv	71.76 (6)	O5—P2—O7^xi	108.87 (12)
O1^ii—Rb1—O10^iv	124.17 (6)	O6^v—P2—O7^xi	109.54 (12)
OW^iii—Rb1—O10^iv	121.45 (7)	O5—P2—O8	106.93 (12)
O6^i—Rb1—O5^iv	115.39 (6)	O6^v—P2—O8	110.71 (12)
O2—Rb1—O5^iv	100.67 (6)	O7^xi—P2—O8	109.01 (12)
O8—Rb1—O5^iv	84.57 (6)	Ga1—O9—H1	107 (4)
O5—Rb1—O5^iv	98.69 (5)	Ga1^v—O9—H1	112 (4)
O3^i—Rb1—O5^iv	116.64 (6)	Ga2^ix—O9—H1	98 (4)
O7^ii—Rb1—O5^iv	43.46 (5)	Ga2—O10—H2	115 (4)
O1^ii—Rb1—O5^iv	88.24 (5)	Rb1^iv—O10—H2	76 (4)
OW^iii—Rb1—O5^iv	150.27 (7)	Ga2—O10—H3	129 (4)
O10^iv—Rb1—O5^iv	46.48 (5)	Rb1^iv—O10—H3	78 (4)
O3—Ga1—O4	88.71 (9)	H2—O10—H3	116 (5)
O3—Ga1—O8	98.82 (10)	Rb1^xi—OW—H4	83 (5)
O4—Ga1—O8	95.41 (9)	Rb1^xi—OW—H5	73 (7)
O3—Ga1—O6	91.21 (9)	H4—OW—H5	109 (8)

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