Heptairon bis(phosphate) tetrakis(hydrogenphosphate)

A new iron hydrogen phosphate, heptairon bis­(phosphate) tetrakis­(hydrogen­phosphate), Fe₇(PO₄)₂(HPO₄)₄, has been prepared hydro­thermally and characterized by single-crystal X-ray diffraction. The compound has one Fe atom on an inversion centre and is isostructural with Mn₇(PO₄)₂(HPO₄)₄ and Co₇(PO₄)₂(HPO₄)₄. The structure is based on a framework of edge- and corner-sharing FeO₆, Fe₅ and PO₄ polyhedra, isotypic with that found in the mixed-valence iron phosphate Fe₇(PO₄)₆. The Fe atoms in the title compound are purely in the divalent state, just like the Co atoms in Co₇(PO₄)₂(HPO₄)₄, the necessary charge balance being maintained by the addition of H atoms in the form of bridging Fe-OH-P groups.

S1. Comment

Microporous materials containing transition metal elements are a focus of contemporary research due to their novel potential catalytic, electrical, optical and magnetic properties, which are not accessible to the main group elements (Cheetham et al., 1999). Hydrothermal and solvothermal methods have been used to synthesize many novel transition metal phosphates with open-framework structures, such as iron (Martin et al., 1994), cobalt (Cowley & Chipindale, 1999; Feng et al., 1997), nickel (Guillou et al., 1999; Escobal, Pizarro, Mesa, Arriotua & Rojo, 2000), manganese (Escobal, Pizarro, Mesa, Lezama et al., 2000; Fernandez et al., 2001) and molybdenum phosphates (Haushalter & Mundi, 1992). The frameworks of transition metal phosphates can be controlled by introducing organic templates to form one-dimensional (chain), two-dimensional (layered) and three-dimensional structures. Among these compounds, the iron phosphates are of interest because of their rich crystal chemistry and practical applications (Lii et al., 1998).

The title iron phosphate, Fe₇(PO₄)₂(HPO₄)₄, was synthesized by the hydrothermal method in the presence of ethylene glycol and imidazole. It is isostructural with Mn₇(PO₄)₂(HPO₄)₄ (Riou et al., 1987) and Co₇(PO₄)₂(HPO₄)₄ (Lightfoot & Cheetham, 1988). However, the method used to synthesize Mn₇(PO₄)₂(HPO₄)₄ and Co₇(PO₄)₂(HPO₄)₄, carried out with iron-containing starting materials, gave Fe₆^IIFe^III(PO₄)₃(HPO₄)₃ instead of Fe₇(PO₄)₂(HPO₄)₄ (Lightfoot & Cheetham, 1986).

As shown in Fig. 1, the crystal structure of the title compound consists of Fe—O polyhedra and PO₄ tetrahedra sharing edges or corners to form a three-dimensional open-framework. Of the four independent Fe atoms (Fig. 2), Fe1 is located at a crystallographic inversion centre and has an almost regular octahedral coordination environment, while atoms Fe2 and Fe4 display distorted octahedral coordination environments. Atom Fe3 is surrounded by five O atoms, to give a geometry which is best described as distorted trigonal-bipyramidal. The results of a bond-strength bond-length calculation (Brown & Shannon, 1973) suggest that all Fe atoms should be divalant, the valence sums for the four metal sites being 1.89, 2.22, 1.94 and 2.28, respectively.

All the phosphate groups in the title compound are almost tetrahedral. The P—O bond lengths are distributed in the range 1.508–1.565 Å. The P1—O4 and P3—O11 bond lengths of 1.559 (4) and 1.565 (5) Å, respectively, are significantly longer than the remaining P—O bond, which involves bridging Fe—HO—P groups. Although the shortest distance between O atoms is 2.477 Å, between O10 and O11, hydrogen bonds are expected to be formed between O11—H···O3 and O4—H···O7, if the angles between the respective atoms are considered (Table 2).

S2. Experimental

FeCl₂ (0.8 g), H₂O (6 ml) and ethylene glycol (EG; 11 ml), H₃PO₄ (0.8 ml, 85% wt), and imidazole (1.0 g) were placed, respectively, in a beaker with stirring; a homogeneous mixture was obtained. The mixture was hydrothermally treated in a Teflon-lined autoclave at 440 K for 6 d. The resulting single crystals were collected by filtration, washed with distilled water and dried in air at ambient temperature. Experiment proved that EG is necessary for the formation of crystals of good habit. EG has a higher viscosity than water, which may baffle the convection currents and help obtain a large good quality single-crystal (Lii et al., 1998).

S3. Refinement

The H atoms were placed in geometric positions, with O—H = 0.85 Å. Is this added text OK?

Computing details

Data collection: XSCANS (Bruker, 1997); cell refinement: XSCANS; data reduction: XSCANS; program(s) used to solve structure: SHELXTL (Sheldrick, 1997); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL.

Figures

	Fig. 1. A view of the packing plot for Fe7(PO4)2(HPO4)4 along the a direction, with dashed lines indicating hydrogen bonds.
	Fig. 2. A view of the structure of Fe7(PO4)2(HPO4)4 with 50% probability displacement ellipsoids, showing the atomic numbering scheme. The H atoms have been omitted for clarity.

Heptairon tetrakis(hydrogenphosphate) bis(phosphate)

Crystal data

H4Fe7O24P6	Z = 1
Mr = 964.80	F(000) = 468
Triclinic, P1	Dx = 3.723 Mg m^−3
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 6.528 (3) Å	Cell parameters from 25 reflections
b = 7.956 (4) Å	θ = 5.9–11.4°
c = 9.501 (4) Å	µ = 6.43 mm^−1
α = 104.03 (4)°	T = 293 K
β = 109.17 (2)°	Prism, black
γ = 101.66 (3)°	0.2 × 0.1 × 0.1 mm
V = 430.3 (3) Å^3

Data collection

Bruker P4 diffractometer	1537 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.040
Graphite monochromator	θmax = 27.5°, θmin = 2.4°
ω scans	h = −1→8
Absorption correction: ψ scan (North et al., 1968)	k = −9→9
Tmin = 0.282, Tmax = 0.526	l = −12→12
2420 measured reflections	3 standard reflections every 100 reflections
1921 independent reflections	intensity decay: none

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.043	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.106	H-atom parameters not refined
S = 1.03	w = 1/[σ^2(Fo^2) + (0.001P)^2 + 6.4359P] where P = (Fo^2 + 2Fc^2)/3
1849 reflections	(Δ/σ)max = 0.003
169 parameters	Δρmax = 1.22 e Å^−3
0 restraints	Δρmin = −0.94 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
Fe1	0.0000	0.0000	0.0000	0.0134 (3)
Fe2	0.38733 (15)	0.45498 (11)	0.11196 (10)	0.0120 (2)
Fe3	−0.27718 (15)	0.18673 (11)	−0.28076 (11)	0.0138 (2)
Fe4	−0.04910 (15)	0.28480 (12)	−0.51589 (11)	0.0145 (2)
P1	0.2285 (3)	0.1448 (2)	−0.22284 (18)	0.0103 (3)
P2	0.0853 (3)	0.5817 (2)	−0.17233 (18)	0.0107 (3)
P3	−0.5879 (3)	0.2307 (2)	−0.62669 (19)	0.0125 (3)
O1	0.0196 (7)	0.1730 (6)	−0.3368 (5)	0.0134 (9)
O2	0.2267 (7)	0.1886 (5)	−0.0558 (5)	0.0129 (8)
O3	0.5509 (7)	0.7509 (6)	0.2279 (5)	0.0128 (8)
O4	−0.2098 (8)	0.0608 (6)	−0.7238 (5)	0.0169 (9)
H4A	−0.1630	0.0984	−0.7876	0.020*
O5	0.3000 (7)	0.5378 (6)	−0.0860 (5)	0.0136 (9)
O6	0.0557 (8)	0.5458 (6)	−0.3451 (5)	0.0153 (9)
O7	0.1098 (8)	−0.2181 (6)	−0.0941 (6)	0.0196 (10)
O8	0.1257 (7)	0.5392 (6)	0.1668 (5)	0.0143 (9)
O9	−0.3767 (8)	0.2100 (6)	−0.5082 (5)	0.0173 (9)
O10	0.4774 (8)	0.3805 (6)	0.3099 (6)	0.0233 (10)
O11	0.2701 (8)	0.2904 (7)	−0.5300 (6)	0.0234 (10)
H11A	0.3285	0.2570	−0.4515	0.028*
O12	−0.2687 (7)	−0.0514 (6)	−0.2394 (5)	0.0151 (9)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Fe1	0.0128 (6)	0.0119 (6)	0.0149 (6)	0.0031 (5)	0.0053 (5)	0.0042 (5)
Fe2	0.0128 (4)	0.0119 (4)	0.0110 (4)	0.0043 (3)	0.0046 (3)	0.0032 (3)
Fe3	0.0141 (4)	0.0107 (4)	0.0174 (5)	0.0043 (3)	0.0071 (4)	0.0046 (3)
Fe4	0.0150 (4)	0.0137 (4)	0.0129 (4)	0.0031 (3)	0.0037 (4)	0.0050 (3)
P1	0.0094 (7)	0.0100 (7)	0.0103 (7)	0.0023 (5)	0.0037 (6)	0.0024 (5)
P2	0.0104 (7)	0.0108 (7)	0.0121 (7)	0.0038 (5)	0.0053 (6)	0.0043 (6)
P3	0.0101 (7)	0.0121 (7)	0.0118 (7)	0.0027 (5)	0.0016 (6)	0.0024 (6)
O1	0.011 (2)	0.016 (2)	0.013 (2)	0.0029 (16)	0.0041 (17)	0.0055 (17)
O2	0.012 (2)	0.0114 (19)	0.016 (2)	−0.0001 (15)	0.0089 (17)	0.0026 (16)
O3	0.010 (2)	0.015 (2)	0.013 (2)	0.0037 (16)	0.0048 (17)	0.0041 (16)
O4	0.019 (2)	0.014 (2)	0.018 (2)	0.0048 (17)	0.0089 (19)	0.0040 (18)
O5	0.015 (2)	0.0128 (19)	0.011 (2)	0.0031 (16)	0.0032 (17)	0.0033 (16)
O6	0.019 (2)	0.015 (2)	0.014 (2)	0.0064 (17)	0.0061 (18)	0.0074 (17)
O7	0.020 (2)	0.012 (2)	0.026 (3)	0.0043 (17)	0.011 (2)	0.0016 (18)
O8	0.012 (2)	0.014 (2)	0.015 (2)	0.0020 (16)	0.0054 (17)	0.0037 (17)
O9	0.016 (2)	0.024 (2)	0.011 (2)	0.0095 (18)	0.0035 (18)	0.0039 (18)
O10	0.020 (2)	0.022 (2)	0.025 (3)	0.0034 (19)	0.002 (2)	0.014 (2)
O11	0.019 (2)	0.032 (3)	0.017 (2)	0.013 (2)	0.004 (2)	0.005 (2)
O12	0.016 (2)	0.014 (2)	0.011 (2)	0.0027 (17)	0.0020 (17)	0.0035 (16)

Geometric parameters (Å, º)

Fe1—O7	2.121 (5)	P1—O2	1.545 (4)
Fe1—O2	2.174 (4)	P1—O4^iv	1.557 (4)
Fe1—O12	2.248 (4)	P2—O5	1.530 (4)
Fe2—O10	2.047 (5)	P2—O6	1.535 (5)
Fe2—O5	2.085 (4)	P2—O7^v	1.536 (5)
Fe2—O2	2.121 (4)	P2—O8^ii	1.537 (4)
Fe2—O8	2.125 (4)	P3—O10^vi	1.509 (5)
Fe2—O5^i	2.126 (5)	P3—O12^vii	1.526 (4)
Fe2—O3	2.202 (4)	P3—O9	1.536 (5)
Fe3—O12	2.033 (4)	P3—O11^viii	1.567 (5)
Fe3—O8^ii	2.046 (4)	O3—P1^i	1.532 (4)
Fe3—O9	2.108 (5)	O3—Fe3^ii	2.126 (4)
Fe3—O3^ii	2.126 (4)	O4—P1^iv	1.557 (4)
Fe3—O1	2.187 (4)	O5—Fe2^i	2.126 (5)
Fe4—O1	2.069 (4)	O6—Fe4^iii	2.100 (4)
Fe4—O4	2.087 (5)	O7—P2^ix	1.536 (5)
Fe4—O6^iii	2.100 (4)	O8—P2^ii	1.537 (4)
Fe4—O6	2.115 (5)	O8—Fe3^ii	2.046 (4)
Fe4—O11	2.124 (5)	O10—P3^x	1.509 (5)
Fe4—O9	2.132 (5)	O11—P3^xi	1.567 (5)
P1—O3^i	1.532 (4)	O12—P3^vii	1.526 (4)
P1—O1	1.534 (4)
O7^xii—Fe1—O7	180.0	O11—Fe4—O9	165.75 (19)
O7—Fe1—O2	91.31 (17)	O3^i—P1—O1	111.2 (2)
O7—Fe1—O2^xii	88.69 (17)	O3^i—P1—O2	114.2 (2)
O2—Fe1—O2^xii	180.0	O1—P1—O2	110.2 (2)
O7—Fe1—O12	91.16 (18)	O3^i—P1—O4^iv	106.9 (2)
O2—Fe1—O12	87.35 (16)	O1—P1—O4^iv	108.8 (2)
O7—Fe1—O12^xii	88.84 (18)	O2—P1—O4^iv	105.3 (2)
O2—Fe1—O12^xii	92.65 (16)	O5—P2—O6	108.0 (2)
O12—Fe1—O12^xii	180.0	O5—P2—O7^v	109.9 (3)
O10—Fe2—O5	178.53 (18)	O6—P2—O7^v	110.2 (3)
O10—Fe2—O2	97.14 (19)	O5—P2—O8^ii	110.8 (2)
O5—Fe2—O2	84.33 (17)	O6—P2—O8^ii	109.3 (2)
O10—Fe2—O8	89.75 (19)	O7^v—P2—O8^ii	108.6 (3)
O5—Fe2—O8	89.95 (17)	O10^vi—P3—O12^vii	111.2 (3)
O2—Fe2—O8	105.93 (17)	O10^vi—P3—O9	111.2 (3)
O10—Fe2—O5^i	94.42 (19)	O12^vii—P3—O9	111.6 (3)
O5—Fe2—O5^i	85.42 (17)	O10^vi—P3—O11^viii	107.4 (3)
O2—Fe2—O5^i	91.33 (17)	O12^vii—P3—O11^viii	109.5 (3)
O8—Fe2—O5^i	161.61 (17)	O9—P3—O11^viii	105.7 (3)
O10—Fe2—O3	98.10 (18)	P1—O1—Fe4	137.1 (3)
O5—Fe2—O3	80.43 (17)	P1—O1—Fe3	121.6 (2)
O2—Fe2—O3	164.22 (17)	Fe4—O1—Fe3	98.34 (18)
O8—Fe2—O3	78.23 (16)	P1—O2—Fe2	121.2 (2)
O5^i—Fe2—O3	83.45 (16)	P1—O2—Fe1	121.3 (2)
O12—Fe3—O8^ii	139.30 (18)	Fe2—O2—Fe1	116.2 (2)
O12—Fe3—O9	123.63 (18)	P1^i—O3—Fe3^ii	137.2 (3)
O8^ii—Fe3—O9	94.01 (18)	P1^i—O3—Fe2	123.8 (2)
O12—Fe3—O3^ii	105.10 (18)	Fe3^ii—O3—Fe2	97.46 (17)
O8^ii—Fe3—O3^ii	81.74 (17)	P1^iv—O4—Fe4	137.5 (3)
O9—Fe3—O3^ii	98.00 (17)	P2—O5—Fe2	130.4 (3)
O12—Fe3—O1	85.19 (17)	P2—O5—Fe2^i	135.0 (3)
O8^ii—Fe3—O1	89.19 (17)	Fe2—O5—Fe2^i	94.58 (17)
O9—Fe3—O1	77.31 (17)	P2—O6—Fe4^iii	132.5 (3)
O3^ii—Fe3—O1	169.52 (16)	P2—O6—Fe4	124.9 (2)
O1—Fe4—O4	104.60 (18)	Fe4^iii—O6—Fe4	101.85 (19)
O1—Fe4—O6^iii	166.02 (17)	P2^ix—O7—Fe1	155.3 (3)
O4—Fe4—O6^iii	88.49 (18)	P2^ii—O8—Fe3^ii	127.3 (3)
O1—Fe4—O6	89.64 (17)	P2^ii—O8—Fe2	127.1 (3)
O4—Fe4—O6	163.75 (18)	Fe3^ii—O8—Fe2	102.52 (18)
O6^iii—Fe4—O6	78.15 (19)	P3—O9—Fe3	136.6 (3)
O1—Fe4—O11	92.02 (19)	P3—O9—Fe4	122.3 (3)
O4—Fe4—O11	89.63 (19)	Fe3—O9—Fe4	98.88 (18)
O6^iii—Fe4—O11	83.04 (19)	P3^x—O10—Fe2	142.7 (3)
O6—Fe4—O11	97.80 (18)	P3^xi—O11—Fe4	150.2 (3)
O1—Fe4—O9	79.42 (17)	P3^vii—O12—Fe3	119.0 (3)
O4—Fe4—O9	81.65 (18)	P3^vii—O12—Fe1	129.7 (3)
O6^iii—Fe4—O9	107.82 (18)	Fe3—O12—Fe1	110.78 (19)
O6—Fe4—O9	93.55 (18)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O4—H4A···O7^iv	0.85	1.71	2.539 (7)	164
O11—H11A···O3^i	0.85	2.03	2.850 (7)	162

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