The solid solution (Fe_0.81Al_0.19)(H₂PO₄)₃ with a strong hydrogen bond

Single crystals of the solid solution iron aluminium tris(dihydrogenphosphate), (Fe_0.81Al_0.19)(H₂PO₄)₃, have been pre­pared under hydro­thermal conditions. The compound is a new monoclinic variety (γ-form) of iron aluminium phosphate (Fe,Al)(H₂PO₄)₃. The structure is based on a two-dimensional framework of distorted corner-sharing MO₆ (M = Fe, Al) polyhedra sharing corners with PO₄ tetra­hedra. Strong hydrogen bonds between the OH groups of the H₂PO₄ tetra­hedra and the O atoms help to consolidate the crystal structure.

S1. Comment

Microporous materials find their origin in the discovery by Crönsted during the thirteenth century of the zeolitic property of the mineral stilbite. The zeolite family is made up of the aluminosilicate minerals with formula [Mⁿ⁺_x/n[(AlO₂)_x(SiO₂)_y]^x. wH₂O], where x indicates the number of Mⁿ⁺ cations necessary to compensate the negative charge of the whole framework. All these phases exhibit three-dimensional structures built up exclusively from corner-sharing TO₄ (T = Al, Si) tetrahedra, defining tunnels in which the Mⁿ⁺ cations and water molecules are located. Wilson et al. (1982) discovered a new family of compounds, the microporous aluminophosphates. Since 1992, the research groups of Cavellec (Cavellec et al., 1995; Cavellec, Riou & Férey, 1997; Cavellec, Férey & Grenèche, 1997; Riou-Cavellec et al., 1998) have been interested in the synthesis of these microporous materials. This work was followed by studies of microporous oxides by several groups (Debord et al., 1997; Lii & Huang, 1997a,b,c; Huang et al., 1998; Zima et al., 1998; Zima & Lii, 1998). Microporous materials derived from octahedral and tetrahedral frameworks currently boast an extensive chemistry and a number of them display useful properties as catalysts, sorbents and ionic exchangers (Davis & Lobo, 1992; Breck, 1974; Venuto, 1994).

Two polymorphs of Al(H₂PO₄)₃ have been reported to date. The α-form is hexagonal with cell parameters a = 7.849 (1) and c = 24.87 (3) Å (Reference?), and the hexagonal β-form has parameters a = 13.69 (1) and c = 9.135 (1) Å (Yoire, 1961), also found by Brodalla et al. (1981). The α-form is isostructural with Fe(H₂PO₄)₃ (Baies et al., 2006) and consists of a three-dimensional framework of corner-sharing FeO₆ and PO₂(OH)₂ tetrahedra. The synthesis of a new monoclinic variety of iron aluminium phosphate, (Fe_0.81Al_0.19)(H₂PO₄)₃ (γ-form), is reported in this work.

The compound (Fe_0.81 Al_0.19)(H₂PO₄)₃ is composed of a highly puckered sheet structure containing interconnected M₂P₂ units (M = Fe or Al) connected laterally by Fe–O–P mixed bridges to form two-dimensional layers perpendicular to the b axis (Fig. 1). The oligomeric M₂P₂ units are built up from alternating corner-sharing of octahedral MO₆ and tetrahedral PO₄ units. The MO₆ octahedra share six O atoms with adjacent P atoms, whereas the PO₄ tetrahedra share only two O atoms. The projection of the sheet is shown in Fig. 2, viewed down the [010] axis. The M—O distances in (Fe_0.81Al_0.19)(H₂PO₄)₃ have intermediate values between 1.944 (4) and 2.061 (4) Å, consistent with the occupation of Fe and Al valencies in these sites. The interatomic angles reveal distortions of the octahedra, varying from O6—Fe1—O2(x, y, z-1) = 86.66 (17)° to O1—Fe1—O6 = 178.84 (18)°. The dihydrogen phosphate ion, [H₂PO₄]^-, can be described as slightly distorted tetrahedra, with a mean value for the P—OH bond distances of 1.578Å and with P═O bond distances ranging from 1.504 (4) to 1.525 (4)Å. The O—P—O angles are in the range 101.6 (3)–118.1 (2)°.

The crystal structure of (Fe_0.81 Al_0.19)(H₂PO₄)₃ is characterized by an extended hydrogen-bonding network. The layers are held together through strong hydrogen bonds between the terminal O atoms attached to the two-connected phosphate groups in adjacent layers. Analysis of the hydrogen bonds in (Fe_0.81Al_0.19)(H₂PO₄)₃ shows two different types of P—O—H···O—P bridges. Within the layer, adjacent H₂PO₄ ions are connected into chains by short hydrogen bonds with a distance of 2.614 (5) Å formed by one of the hydroxy groups, O3—H3···O6(x-1/2, -y+3/2, z+1/2). Adjacent layers are linked by longer hydrogen bonds, viz. O4—H4···O4(x, -y+1, z+1/2) [2.775 (6) Å], O12—H12···O12(x, -y+2, z-1/2) [3.056 (8) Å] and O11—H11···O12(x, -y+2, z+1/2) [3.223 (7) Å], which allow the layers to connect as observed in Fig. 1.

A comparison between (Fe_0.81Al_0.19)(H₂PO₄)₃ and the series of compounds (NH₄, H₃O, K)(Fe,Al)₃(HPO₄)₂(H₂PO₄)₆.4H₂O and [Al₂P₃O₁₀(OH)₂](C₆NH₈) is shown in Fig. 3. Detailed descriptions of their topology are also reported here. The aim of this comparison is to provide a review of possible approaches that can be used to establish the topology of microporous structures. For obvious reasons, we do not consider related octahedral–tetrahedral frameworks here. Most attention will be focused on network topology and the possibility of intercalating alkaline cations or organic molecules in the solid-state inorganic framework, which is important for both mineralogy and material sciences.

(Fe_0.81Al_0.19)(H₂PO₄) is considered an usual solid-state inorganic framework. A comparison between this compound and the series of compounds (NH₄, H₃O, K)(Fe,Al)₃(HPO₄)₂(H₂PO₄)₆.4H₂O is shown in Fig. 3(a) and (b). The common characteristic of these compounds is their bidimensionality. In (NH₄, H₃O, K)(Fe,Al)₃(HPO₄)₂(H₂PO₄)₆.4H₂O, the NH₄⁺, H₃O⁺ and K⁺ cations are located inside 12-sided polyhedra, which are generated by the corner-sharing MO₆ (M = Fe, Al) and H₂PO₄ units, while water molecules are located in the interlayer space (Mgaidi et al., 1999; Bosman et al., 1986; Anisimova et al., 1997). Fig. 3(a) and (c) show the comparison between (Fe_0.81Al_0.19)(H₂PO₄)₃ and the two-dimensional layered compound [Al₂P₃O₁₀(OH)₂](C₆NH₈). This structure contains macroanionic Al₂P₃O₁₀(OH)₂^- sheets that are charge-balanced by protonated 4-methylpyridine. The inorganic layers are constructed from alternating Al-centred units (AlO₄ and AlO₅) and P-centred units [PO₄, PO₃(OH), PO₂(═O)(OH)] with triply and doubly bridging phosphate groups (Yu et al., 2000). This comparison provides an example of the concept of scale chemistry (Férey, 2000). The cavities created by the framework, which are very small in typical solid-state inorganic frameworks and only able to accept alkaline cations or organic molecules, become larger and larger.

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S2. Experimental

The title compound was prepared from a reaction mixture of H₃PO₄ (4 mmol), FeO (5 mmol) and Al₂O₃ (5 mmol) in water (approximately 6 ml). The starting mixture was transferred and sealed into a 23 ml Teflon-lined stainless steel Parr autoclave under autogenous pressure, filled to approximately 25% volume capacity, and all reactants were stirred briefly before heating. The reaction mixture was heated at 343 K for 3 d to obtain (Fe_0.81Al_0.19)(H₂PO₄)₃, followed by slow cooling to room temperature. The product was filtered off, washed with deionized water and dried in air. A needle single crystal of (Fe_0.81Al_0.19)(H₂PO₄)₃ was selected under a polarizing microscope.

S3. Refinement

During refinement, the occupancy of the Fe site exhibited a significant deviation from full occupancy, indicating a substitution with Al; the final occupancies were constrained to sum to 1.0 and refined to 0.807 (7) and 0.193 (7), respectively, for Fe1 and Al1. The positions of all H atoms were located from a difference electron-density map and then refined with an O—H bond length restraint of 0.95 (5)Å and with U_iso(H) fixed at a value of 0.05 Ų.

Computing details

Data collection: CAD-4 EXPRESS (Duisenberg, 1992; Macíček & Yordanov, 1992); cell refinement: CAD-4 EXPRESS (Duisenberg, 1992; Macíček & Yordanov, 1992); data reduction: MolEN (Fair, 1990); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures

	Fig. 1. The structure of (Fe0.81 Al0.19)(H2PO4)3. Dashed lines show the intra and inter-layer hydrogen bonds.
	Fig. 2. The sheet structure of (Fe0.81 Al0.19)(H2PO4)3, viewed along the b axis, showing the oligomeric M2P2 units (M = Fe or Al) connected laterally by Fe–O–P mixed bridges.
	Fig. 3. (a) The stacking sheets of (Fe0.81Al0.19)(H2PO4)3. (b) The structure of the series of compounds (NH4, H3O, K)(Fe,Al)3(HPO4)2(H2PO4)6.4H2O, with the water molecules lying in the interlayer space. (c) The two-dimensional layered compound [Al2P3O10(OH)2](C6NH8), with the 4-methylpyridine lying in the interlayer space.

Iron aluminium tris(dihydrogenphosphate)

Crystal data

(Fe0.81Al0.19)(H2PO4)3	F(000) = 682
Mr = 341.32	Dx = 2.495 Mg m^−3
Monoclinic, Cc	Melting point: Al0.19 Fe0.81 H6 O12 P3 K
Hall symbol: C -2yc	Mo Kα radiation, λ = 0.71073 Å
a = 11.700 (1) Å	Cell parameters from 25 reflections
b = 15.590 (1) Å	θ = 2.2–26.9°
c = 5.030 (1) Å	µ = 1.98 mm^−1
β = 98.00 (1)°	T = 293 K
V = 908.6 (2) Å^3	Parallelepiped, grey
Z = 4	0.15 × 0.15 × 0.1 mm

Data collection

Enraf–Nonius CAD-4 diffractometer	1043 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.000
Graphite monochromator	θmax = 26.9°, θmin = 2.2°
ω/2θ scans	h = −14→14
Absorption correction: ψ scan (North et al., 1968)	k = −19→0
Tmin = 0.703, Tmax = 0.754	l = −6→0
1104 measured reflections	2 standard reflections every 120 min
1104 independent reflections	intensity decay: 0.4%

Refinement

Refinement on F^2	Hydrogen site location: inferred from neighbouring sites
Least-squares matrix: full	H atoms treated by a mixture of independent and constrained refinement
R[F^2 > 2σ(F^2)] = 0.029	w = 1/[σ^2(Fo^2) + (0.0495P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.074	(Δ/σ)max = 0.001
S = 1.04	Δρmax = 0.55 e Å^−3
1104 reflections	Δρmin = −0.64 e Å^−3
166 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
10 restraints	Extinction coefficient: 0.0129 (12)
Primary atom site location: structure-invariant direct methods	Absolute structure: Flack H D (1983), Acta Cryst. A39, 876-881
Secondary atom site location: difference Fourier map	Absolute structure parameter: 0.00 (3)

Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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.

A needle-shaped single crystal of (Fe_0.81Al_0.19)(H₂PO₄)₃ with dimensions given in crystal data table was selected under a polarizing microscope. Diffraction data were collected at room temperature on an automated diffractometer using graphite-monochromated Mo Kα. Details of crystal data, intensity collection, and some features of the structure refinement are reported in crystal data table. Corrections for Lorentz and polarization effects were done and also for absorption with the empirical ψ scan method (North et al., 1968). The structures were solved by Patterson methods SHELXS97 (Sheldrick, 1997) in the Cc space group, which allowed to obtain the positions of Fe and phosphorus atoms. The refinement of the crystal structure was performed by full matrix least-squares based, on F², using SHELXL97 program (Sheldrick,1997), obtaining the oxygen atoms.

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Fe1	0.46063 (8)	0.75449 (5)	0.23372 (15)	0.0096 (2)	0.807 (7)
Al1	0.46063 (8)	0.75449 (5)	0.23372 (15)	0.0096 (2)	0.193 (7)
P1	0.37457 (11)	0.63448 (9)	0.7039 (2)	0.0120 (3)
P2	0.53144 (11)	0.88680 (8)	−0.2360 (3)	0.0130 (3)
P3	0.20471 (11)	0.84607 (8)	0.0622 (2)	0.0133 (3)
O1	0.3675 (3)	0.6782 (2)	0.4335 (8)	0.0176 (8)
O2	0.4614 (3)	0.6688 (2)	0.9336 (8)	0.0159 (8)
O3	0.2543 (3)	0.6315 (3)	0.8050 (8)	0.0198 (8)
O4	0.4093 (4)	0.5375 (3)	0.6550 (9)	0.0228 (9)
O5	0.4751 (4)	0.8412 (3)	−0.4858 (8)	0.0230 (9)
O6	0.5559 (3)	0.8332 (3)	0.0183 (8)	0.0171 (8)
O7	0.6462 (3)	0.9328 (3)	−0.2820 (9)	0.0222 (9)
O8	0.4504 (3)	0.9632 (3)	−0.1694 (8)	0.0188 (8)
O9	0.1072 (3)	0.7987 (3)	−0.1014 (9)	0.0262 (10)
O10	0.3224 (3)	0.8093 (3)	0.0430 (8)	0.0184 (8)
O11	0.1871 (4)	0.8523 (3)	0.3654 (9)	0.0246 (9)
O12	0.2029 (4)	0.9444 (3)	−0.0223 (12)	0.0297 (9)
H3	0.203 (6)	0.618 (5)	0.671 (15)	0.050*
H4	0.418 (7)	0.510 (6)	0.800 (13)	0.050*
H7	0.678 (7)	0.927 (5)	−0.435 (13)	0.050*
H8	0.438 (6)	0.996 (5)	−0.315 (14)	0.050*
H11	0.152 (6)	0.903 (4)	0.403 (19)	0.050*
H12	0.222 (7)	0.959 (5)	−0.192 (11)	0.050*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Fe1	0.0074 (3)	0.0132 (4)	0.0080 (3)	−0.0001 (3)	0.0002 (2)	0.0000 (3)
Al1	0.0074 (3)	0.0132 (4)	0.0080 (3)	−0.0001 (3)	0.0002 (2)	0.0000 (3)
P1	0.0111 (6)	0.0142 (6)	0.0109 (6)	−0.0013 (5)	0.0020 (5)	−0.0005 (5)
P2	0.0130 (6)	0.0157 (6)	0.0103 (6)	−0.0010 (5)	0.0023 (5)	−0.0009 (5)
P3	0.0093 (6)	0.0165 (6)	0.0138 (6)	−0.0008 (5)	0.0010 (5)	−0.0003 (5)
O1	0.0169 (18)	0.0207 (19)	0.0145 (19)	−0.0021 (15)	−0.0005 (16)	0.0063 (15)
O2	0.0129 (17)	0.0198 (18)	0.0145 (18)	−0.0012 (15)	0.0003 (15)	−0.0035 (16)
O3	0.0102 (17)	0.035 (2)	0.0147 (18)	−0.0014 (16)	0.0020 (15)	0.0018 (17)
O4	0.027 (2)	0.020 (2)	0.021 (2)	0.0025 (16)	0.0028 (18)	−0.0005 (17)
O5	0.027 (2)	0.026 (2)	0.015 (2)	−0.0036 (16)	−0.0007 (18)	−0.0060 (17)
O6	0.0119 (18)	0.025 (2)	0.0144 (19)	−0.0034 (15)	0.0015 (16)	0.0028 (15)
O7	0.0193 (19)	0.027 (2)	0.022 (2)	−0.0041 (17)	0.0100 (17)	−0.0006 (18)
O8	0.0196 (19)	0.0198 (19)	0.0179 (18)	0.0010 (15)	0.0055 (17)	−0.0009 (16)
O9	0.0140 (18)	0.041 (3)	0.023 (2)	−0.0044 (18)	−0.0008 (17)	−0.0111 (19)
O10	0.0107 (17)	0.027 (2)	0.0180 (18)	0.0023 (15)	0.0026 (15)	0.0007 (17)
O11	0.0197 (19)	0.038 (2)	0.017 (2)	−0.0033 (17)	0.0055 (17)	−0.0053 (18)
O12	0.031 (2)	0.022 (2)	0.039 (2)	0.002 (2)	0.0131 (19)	0.006 (2)

Geometric parameters (Å, º)

Fe1—O5^i	1.945 (4)	P3—O10	1.507 (4)
Fe1—O10	1.958 (4)	P3—O11	1.570 (4)
Fe1—O1	1.979 (4)	P3—O12	1.590 (4)
Fe1—O9^ii	1.980 (4)	O2—Al1^i	2.017 (4)
Fe1—O2^iii	2.017 (4)	O2—Fe1^i	2.017 (4)
Fe1—O6	2.063 (4)	O3—H3	0.87 (5)
P1—O1	1.513 (4)	O4—H4	0.84 (5)
P1—O2	1.525 (4)	O5—Al1^iii	1.945 (4)
P1—O3	1.563 (4)	O5—Fe1^iii	1.945 (4)
P1—O4	1.593 (4)	O7—H7	0.90 (5)
P2—O5	1.512 (4)	O8—H8	0.89 (5)
P2—O6	1.521 (4)	O9—Al1^iv	1.980 (4)
P2—O7	1.567 (4)	O9—Fe1^iv	1.980 (4)
P2—O8	1.587 (4)	O11—H11	0.93 (5)
P3—O9	1.504 (4)	O12—H12	0.94 (5)
O5^i—Fe1—O10	91.98 (17)	O7—P2—O8	103.7 (2)
O5^i—Fe1—O1	92.76 (18)	O9—P3—O10	114.1 (2)
O10—Fe1—O1	92.04 (17)	O9—P3—O11	111.7 (2)
O5^i—Fe1—O9^ii	90.27 (19)	O10—P3—O11	109.3 (2)
O10—Fe1—O9^ii	174.9 (2)	O9—P3—O12	110.2 (3)
O1—Fe1—O9^ii	92.37 (17)	O10—P3—O12	109.1 (3)
O5^i—Fe1—O2^iii	174.10 (19)	O11—P3—O12	101.6 (3)
O10—Fe1—O2^iii	90.69 (17)	P1—O1—Fe1	139.6 (2)
O1—Fe1—O2^iii	92.40 (16)	P1—O2—Al1^i	136.6 (2)
O9^ii—Fe1—O2^iii	86.67 (18)	P1—O2—Fe1^i	136.6 (2)
O5^i—Fe1—O6	88.21 (17)	Al1^i—O2—Fe1^i	0.00 (5)
O10—Fe1—O6	87.29 (17)	P1—O3—H3	108 (6)
O1—Fe1—O6	178.84 (18)	P1—O4—H4	111 (7)
O9^ii—Fe1—O6	88.25 (17)	P2—O5—Al1^iii	156.0 (3)
O2^iii—Fe1—O6	86.66 (17)	P2—O5—Fe1^iii	156.0 (3)
O1—P1—O2	118.1 (2)	Al1^iii—O5—Fe1^iii	0.00 (4)
O1—P1—O3	111.5 (2)	P2—O6—Fe1	135.7 (2)
O2—P1—O3	107.4 (2)	P2—O7—H7	123 (6)
O1—P1—O4	105.7 (2)	P2—O8—H8	107 (6)
O2—P1—O4	107.0 (2)	P3—O9—Al1^iv	169.0 (3)
O3—P1—O4	106.4 (2)	P3—O9—Fe1^iv	169.0 (3)
O5—P2—O6	116.7 (2)	Al1^iv—O9—Fe1^iv	0.00 (3)
O5—P2—O7	112.0 (3)	P3—O10—Fe1	146.5 (3)
O6—P2—O7	108.1 (2)	P3—O11—H11	112 (6)
O5—P2—O8	108.9 (2)	P3—O12—H12	119 (5)
O6—P2—O8	106.4 (2)

Symmetry codes: (i) x, y, z+1; (ii) x+1/2, −y+3/2, z+1/2; (iii) x, y, z−1; (iv) x−1/2, −y+3/2, z−1/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O3—H3···O6^v	0.86 (7)	1.94 (7)	2.614 (5)	134 (7)
O3—H3···O7^v	0.86 (7)	2.42 (8)	3.207 (6)	152 (7)
O4—H4···O4^vi	0.84 (7)	1.95 (7)	2.775 (6)	167 (8)
O7—H7···O3^vii	0.91 (7)	1.91 (7)	2.765 (6)	156 (7)
O8—H8···O8^viii	0.89 (7)	1.92 (7)	2.764 (6)	159 (7)
O11—H11···O12^ix	0.92 (7)	2.47 (6)	3.220 (7)	139 (6)
O12—H12···O12^viii	0.94 (6)	2.23 (7)	3.055 (8)	146 (7)

Symmetry codes: (v) x−1/2, −y+3/2, z+1/2; (vi) x, −y+1, z+1/2; (vii) x+1/2, −y+3/2, z−3/2; (viii) x, −y+2, z−1/2; (ix) x, −y+2, z+1/2.