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Acta Cryst. (2012). E68, i5-i6    [ doi:10.1107/S1600536811052482 ]

An ammonium iron(II) pyrophosphate, (NH4)2[Fe3(P2O7)2(H2O)2], with a layered structure

B. Liu, X. Zhang, L. Wen, W. Sun and Y.-X. Huang

Abstract top

Diammonium diaquabis(phosphato)triferrate(II), (NH4)2[Fe3(P2O7)2(H2O)2], was synthesized under solvothermal conditions at 463 K. The crystal structure, isotypic to its Mn and Ni analogues, is built from iron pyrophosphate layers parallel to (100), which are linked by ammonium ions sitting in the interlayer space via O-H...O and N-H...O hydrogen bonds. There are two crystallographic Fe sites in the crystal structure, one at a special position (2a, \overline{1}), the other at a general position (4e, 1). The former Fe atom on the inversion centre is coordinated by six O atoms, forming an FeO6 octahedron, while the latter is coordinated by five phosphate O atoms and one water molecule, forming an FeO5(H2O) octahedron. Each FeO6 octahedron shares trans edges with two FeO5(H2O) octahedra, forming a linear trimeric unit. These trimers share the lateral edges of FeO5(H2O) with other trimers, forming a zigzag chain running along [010]. The zigzag chains are further linked by P2O7 groups into a layered structure parallel to (100).

Comment top

In the mineral kingdom, iron phosphates are one of the most important materials besides silicates and aluminates. The mineral cacoxenite is the most striking example in open-framework iron phosphate because of its gigantic cylindrical tunnels (diameter of 14.2 Å) which are only occupied by water molecules (Moore & Shen, 1983). Intrigued by the cacoxenite, a large amount of synthetic iron phosphates have been reported with 1-D, 2-D, and 3-D structures in last two decades (Lii et al., 1998; Alfonso et al., 2010; Mi et al., 2010). Here we report on a new ammounium iron(II) pyrophosphate, (NH4)2[FeII3(P2O7)2(H2O)2], with a layered structure.

The layered structure of the title compound is isotypic to (NH4)2[Mn3(P2O7)2(H2O)2] (Chippindale et al., 2003), K2[Co3(P2O7)2(H2O)2] (Lightfoot et al., 1990), (NH4)2[Ni3(P2O7)2(H2O)2] (Wei et al., 2010), and Na(NH4)2[Ni3(P2O7)2(H2O)2] (Liu et al., 2004). The asymmetric unit of the title compound (presented in Figure 1) shows that there are two crystallographically independent iron atoms with octahedral coordination and two distinct phosphorus atoms both with tetrahedral coordination. The two phosphate tetrahedra share the common O2-corner to form a P2O7 group. The Fe2 atom sits at the inversion center (0, 0.5, 0.5), while Fe1 atom at a general position. Fe2-octahedron shares its trans-edges to two Fe1-octahedra, while Fe1 shares cis-edges to a Fe1-octahedron and one Fe2-octahedron, resulting in zigzag chains running along [010] (Figure 2). The edge-sharing iron octahedral chains are intra- and inter-connected by P2O7 groups through common O-vertices to form flaty layers parallel to (100). The interlayer spaces are occupied by ammonium ions to compensate the negative charge of the iron pyrophosphate layers (Figure 3). Bond valence sum calculations suggest that both iron sites are in the 2+ oxidation state (2.05 for Fe1 and 1.95 for Fe2) (Brown & Altermatt, 1985) which are also consistent to the valence state of MnII, CoII, and NiII in the related compounds (Chippindale et al., 2003; Lightfoot et al., 1990; Wei et al., 2010; Liu et al., 2004).

Related literature top

For background to this compound, see: Moore & Shen (1983); Lii et al. (1998); Alfonso et al. (2010); Mi et al. (2010); Brown & Altermatt (1985). For related structures, see: Chippindale et al. (2003) for (NH4)2[Mn3(P2O7)2(H2O)2]; Lightfoot et al. (1990) for K2Co3(P2O7)2.2H2O; Liu et al. (2004) for Na(NH4)[Ni3(P2O7)2(H2O)2]; Wei et al. (2010) for (NH4)2[Ni3(P2O7)2(H2O)2].

Experimental top

(NH4)2[FeII3(P2O7)2(H2O)2] has been synthesized under solvothermal conditions. 0.112 g Fe powder and 0.345 g NH4H2PO4 were added to a solution of 3 mL pyridine and 2 mL 1,2-dihydroxypropane (with a molar ratio of Fe : P : N = 2 : 3 : 3). Then 1.25 mL H3PO4 (85%) was added to adjust the pH value to 6-7. The mixture was transferred to a 15 mL Teflon-lined stainless steel autoclave (filling degree of 40%) and heated at 463 K for 5 days. After that, the solution was slowly cooled to room temperature and washed with distilled water several times. Light yellow block-like crystals were obtained as a single phase which has been confirmed by powder X-ray diffraction.

Refinement top

The hydrogen atoms bonded to water (O8) and nitrogen (N1) were located from the difference Fourier maps and refined without applying any restraints on the bond length. The displacement parameter of the hydrogen atoms (H1, H2) coordinated to O8 were refined with the common Uiso variables. The same treatment was applied to refine the displacement parameter of hydrogen atoms bonded to N1 (H3, H4, H5, H6).

Computing details top

Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT (Bruker, 2001); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2005) and ATOMS (Dowty, 2004); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008) and WinGX (Farrugia, 1999).

Figures top
[Figure 1] Fig. 1. Structural unit of (NH4)2[FeII3(P2O7)2(H2O)2], showing the coordination environments of Fe and P atoms. Thermal ellipsoids at the 50% probability level. Green sphere: Fe atom, purple sphere: P atom, red sphere: O atom, blue sphere: N atom, dark grey sphere: H atom. Symmetry codes: (i) x, –y+3/2, z+1/2; (ii) –x+2, –y+1, –z+1; (iii) –x+2, y+1/2, –z+1/2; (iv) x, –y+1/2, z+1/2
[Figure 2] Fig. 2. Polyhedral presentation of the iron pyrophosphate layer built from edge-sharing iron octhedral chains intra- and inter-connected by P2O7 groups. Green octahedron: FeO6, orange tetrahedron: PO4, green sphere: Fe atom, purple sphere: P atom, red sphere: O atom, dark grey sphere: H atom.
[Figure 3] Fig. 3. The crystal structure of (NH4)2[FeII3(P2O7)2(H2O)2] is built from zigzag chains of edge-sharing iron octahedra linked by P2O7 pyrophosphate groups to form layers parallel to (100), which are further linked by ammonium ions, sitting at the interlayer space, via the hydrogen bonds. Green octahedron: FeO6, orange tetrahedron: PO4, blue sphere: N atom, dark grey sphere: H atom.
Diammonium diaquabis(phosphato)triferrate(II) top
Crystal data top
(NH4)2[Fe3(H2O)2(P2O7)2]F(000) = 584
Mr = 587.55Dx = 2.730 Mg m−3
Monoclinic, P21/cMo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybcCell parameters from 4127 reflections
a = 9.4131 (17) Åθ = 2.2–28.3°
b = 8.1940 (15) ŵ = 3.55 mm−1
c = 9.3987 (17) ÅT = 173 K
β = 99.651 (3)°Block, light yellow
V = 714.7 (2) Å30.08 × 0.06 × 0.06 mm
Z = 2
Data collection top
Bruker Smart APEXI
diffractometer equipped with CCD area-detector		1652 independent reflections
Radiation source: fine-focus sealed tube1417 reflections with I > 2σ(I)
graphiteRint = 0.035
1265 images,φ=0, 90, 180°, and Δω=0.3°, χ= 54.74° scansθmax = 28.3°, θmin = 2.2°
Absorption correction: numerical
(SMART; Bruker, 2001)		h = −12→12
Tmin = 0.775, Tmax = 0.808k = −5→10
4127 measured reflectionsl = −12→12
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.040Hydrogen site location: difference Fourier map
wR(F2) = 0.085All H-atom parameters refined
S = 1.05 w = 1/[σ2(Fo2) + (0.0331P)2 + 1.7041P]
where P = (Fo2 + 2Fc2)/3
1652 reflections(Δ/σ)max < 0.001
135 parametersΔρmax = 0.55 e Å−3
0 restraintsΔρmin = −0.55 e Å−3
Crystal data top
(NH4)2[Fe3(H2O)2(P2O7)2]V = 714.7 (2) Å3
Mr = 587.55Z = 2
Monoclinic, P21/cMo Kα radiation
a = 9.4131 (17) ŵ = 3.55 mm−1
b = 8.1940 (15) ÅT = 173 K
c = 9.3987 (17) Å0.08 × 0.06 × 0.06 mm
β = 99.651 (3)°
Data collection top
Bruker Smart APEXI
diffractometer equipped with CCD area-detector		1652 independent reflections
Absorption correction: numerical
(SMART; Bruker, 2001)		1417 reflections with I > 2σ(I)
Tmin = 0.775, Tmax = 0.808Rint = 0.035
4127 measured reflectionsθmax = 28.3°
Refinement top
R[F2 > 2σ(F2)] = 0.040All H-atom parameters refined
wR(F2) = 0.085Δρmax = 0.55 e Å−3
S = 1.05Δρmin = −0.55 e Å−3
1652 reflectionsAbsolute structure: ?
135 parametersFlack parameter: ?
0 restraintsRogers parameter: ?
Special details top

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 > 2sigma(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 (Å2) top
xyzUiso*/Ueq
Fe10.86847 (6)0.86015 (7)0.50162 (6)0.00855 (16)
Fe21.00000.50000.50000.00833 (19)
P10.90578 (11)0.29865 (13)0.20925 (10)0.0077 (2)
P20.70438 (11)0.55532 (13)0.26344 (11)0.0085 (2)
O10.7060 (3)0.6456 (4)0.1229 (3)0.0126 (6)
O20.7509 (3)0.3679 (3)0.2350 (3)0.0092 (6)
O30.8740 (3)0.1331 (3)0.1398 (3)0.0105 (6)
O40.9971 (3)0.2951 (3)0.3592 (3)0.0102 (6)
O50.8132 (3)0.6192 (3)0.3887 (3)0.0099 (6)
O60.9625 (3)0.4199 (3)0.1104 (3)0.0099 (6)
O70.5538 (3)0.5385 (4)0.2972 (3)0.0126 (6)
O80.7301 (3)0.9906 (4)0.3371 (3)0.0168 (7)
H10.777 (6)1.016 (7)0.265 (6)0.033 (11)*
H20.647 (6)0.979 (7)0.296 (6)0.033 (11)*
N10.5548 (5)0.2761 (5)0.4867 (5)0.0203 (9)
H30.480 (7)0.206 (8)0.461 (7)0.050 (10)*
H40.632 (7)0.213 (8)0.485 (7)0.050 (10)*
H50.550 (7)0.323 (8)0.579 (7)0.050 (10)*
H60.550 (7)0.363 (8)0.430 (7)0.050 (10)*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Fe10.0091 (3)0.0087 (3)0.0078 (3)0.0007 (2)0.0012 (2)0.0006 (2)
Fe20.0095 (4)0.0079 (4)0.0074 (4)0.0003 (3)0.0008 (3)−0.0001 (3)
P10.0082 (5)0.0082 (5)0.0065 (5)0.0001 (4)0.0010 (4)−0.0001 (4)
P20.0084 (5)0.0095 (5)0.0075 (5)0.0010 (4)0.0005 (4)0.0003 (4)
O10.0145 (14)0.0132 (15)0.0109 (14)0.0029 (12)0.0045 (11)0.0007 (12)
O20.0085 (13)0.0095 (14)0.0099 (13)−0.0015 (11)0.0023 (11)−0.0022 (11)
O30.0157 (14)0.0086 (14)0.0070 (13)0.0002 (12)0.0015 (11)−0.0010 (11)
O40.0116 (14)0.0118 (15)0.0069 (13)0.0019 (12)0.0003 (11)−0.0003 (11)
O50.0112 (14)0.0085 (14)0.0084 (13)0.0018 (11)−0.0032 (11)−0.0022 (11)
O60.0141 (14)0.0057 (14)0.0108 (14)0.0011 (11)0.0048 (11)0.0003 (11)
O70.0087 (13)0.0170 (16)0.0124 (14)0.0010 (12)0.0026 (11)−0.0006 (12)
O80.0112 (15)0.0241 (19)0.0140 (15)−0.0017 (14)−0.0016 (12)0.0092 (13)
N10.017 (2)0.021 (2)0.021 (2)−0.0059 (17)−0.0030 (17)0.0012 (18)
Geometric parameters (Å, °) top
Fe1—O1i2.055 (3)P1—O61.517 (3)
Fe1—O4ii2.092 (3)P1—O41.523 (3)
Fe1—O6iii2.108 (3)P1—O21.620 (3)
Fe1—O82.134 (3)P2—O71.510 (3)
Fe1—O6i2.185 (3)P2—O11.516 (3)
Fe1—O52.261 (3)P2—O51.518 (3)
Fe2—O52.127 (3)P2—O21.631 (3)
Fe2—O5ii2.127 (3)O8—H10.89 (6)
Fe2—O42.135 (3)O8—H20.82 (6)
Fe2—O4ii2.135 (3)N1—H30.91 (7)
Fe2—O3iv2.201 (3)N1—H40.89 (6)
Fe2—O3iii2.201 (3)N1—H50.96 (7)
P1—O31.514 (3)N1—H60.88 (7)
O1i—Fe1—O4ii93.89 (11)O6—P1—O4112.16 (16)
O1i—Fe1—O6iii167.57 (12)O3—P1—O2105.08 (15)
O4ii—Fe1—O6iii91.46 (11)O6—P1—O2106.26 (16)
O1i—Fe1—O889.59 (12)O4—P1—O2104.54 (15)
O4ii—Fe1—O8171.64 (12)O7—P2—O1111.99 (16)
O6iii—Fe1—O886.66 (12)O7—P2—O5113.84 (16)
O1i—Fe1—O6i92.29 (11)O1—P2—O5113.78 (17)
O4ii—Fe1—O6i93.09 (11)O7—P2—O2103.63 (16)
O6iii—Fe1—O6i76.20 (11)O1—P2—O2105.91 (15)
O8—Fe1—O6i94.37 (12)O5—P2—O2106.68 (15)
O1i—Fe1—O596.12 (11)P2—O1—Fe1v126.10 (17)
O4ii—Fe1—O580.21 (10)P1—O2—P2128.83 (18)
O6iii—Fe1—O595.84 (10)P1—O3—Fe2vi127.87 (17)
O8—Fe1—O591.87 (12)P1—O4—Fe1ii141.13 (18)
O6i—Fe1—O5169.56 (10)P1—O4—Fe2120.34 (16)
O5—Fe2—O5ii180.0Fe1ii—O4—Fe298.46 (11)
O5—Fe2—O497.61 (10)P2—O5—Fe2128.17 (16)
O5ii—Fe2—O482.39 (10)P2—O5—Fe1137.51 (16)
O5—Fe2—O4ii82.39 (10)Fe2—O5—Fe193.67 (10)
O5ii—Fe2—O4ii97.61 (10)P1—O6—Fe1vi121.76 (16)
O4—Fe2—O4ii180.0P1—O6—Fe1v132.09 (16)
O5—Fe2—O3iv92.14 (11)Fe1vi—O6—Fe1v103.80 (11)
O5ii—Fe2—O3iv87.86 (10)Fe1—O8—H1110 (4)
O4—Fe2—O3iv91.63 (10)Fe1—O8—H2134 (4)
O4ii—Fe2—O3iv88.37 (10)H1—O8—H2103 (5)
O5—Fe2—O3iii87.86 (10)H3—N1—H4103 (5)
O5ii—Fe2—O3iii92.14 (11)H3—N1—H5110 (5)
O4—Fe2—O3iii88.37 (10)H4—N1—H5114 (5)
O4ii—Fe2—O3iii91.63 (10)H3—N1—H6113 (5)
O3iv—Fe2—O3iii180.0H4—N1—H6114 (6)
O3—P1—O6112.80 (16)H5—N1—H6103 (5)
O3—P1—O4114.97 (16)
Symmetry codes: (i) x, −y+3/2, z+1/2; (ii) −x+2, −y+1, −z+1; (iii) −x+2, y+1/2, −z+1/2; (iv) x, −y+1/2, z+1/2; (v) x, −y+3/2, z−1/2; (vi) −x+2, y−1/2, −z+1/2.
Hydrogen-bond geometry (Å, °) top
D—H···AD—HH···AD···AD—H···A
O8—H1···O3vii0.89 (6)1.87 (6)2.734 (4)162 (5)
O8—H2···O7viii0.82 (6)2.00 (6)2.785 (4)159 (5)
N1—H3···O1ix0.91 (7)1.86 (7)2.717 (5)156 (6)
N1—H4···O2iv0.89 (6)2.51 (6)2.966 (5)112 (5)
N1—H4···O8x0.89 (6)2.56 (7)3.310 (6)142 (5)
N1—H5···O7xi0.96 (7)1.99 (7)2.859 (5)150 (5)
N1—H6···O70.88 (7)1.91 (7)2.791 (5)174 (6)
Symmetry codes: (vii) x, y+1, z; (viii) −x+1, y+1/2, −z+1/2; (ix) −x+1, y−1/2, −z+1/2; (iv) x, −y+1/2, z+1/2; (x) x, y−1, z; (xi) −x+1, −y+1, −z+1.
Table 1
Selected geometric parameters (Å, °) top
Fe1—O1i2.055 (3)P1—O61.517 (3)
Fe1—O4ii2.092 (3)P1—O41.523 (3)
Fe1—O6iii2.108 (3)P1—O21.620 (3)
Fe1—O82.134 (3)P2—O71.510 (3)
Fe1—O6i2.185 (3)P2—O11.516 (3)
Fe1—O52.261 (3)P2—O51.518 (3)
Fe2—O52.127 (3)P2—O21.631 (3)
Fe2—O5ii2.127 (3)O8—H10.89 (6)
Fe2—O42.135 (3)O8—H20.82 (6)
Fe2—O4ii2.135 (3)N1—H30.91 (7)
Fe2—O3iv2.201 (3)N1—H40.89 (6)
Fe2—O3iii2.201 (3)N1—H50.96 (7)
P1—O31.514 (3)N1—H60.88 (7)
P1—O2—P2128.83 (18)Fe2—O5—Fe193.67 (10)
Fe1ii—O4—Fe298.46 (11)
Symmetry codes: (i) x, −y+3/2, z+1/2; (ii) −x+2, −y+1, −z+1; (iii) −x+2, y+1/2, −z+1/2; (iv) x, −y+1/2, z+1/2.
Table 2
Hydrogen-bond geometry (Å, °) top
D—H···AD—HH···AD···AD—H···A
O8—H1···O3v0.89 (6)1.87 (6)2.734 (4)162 (5)
O8—H2···O7vi0.82 (6)2.00 (6)2.785 (4)159 (5)
N1—H3···O1vii0.91 (7)1.86 (7)2.717 (5)156 (6)
N1—H4···O8viii0.89 (6)2.56 (7)3.310 (6)142 (5)
N1—H5···O7ix0.96 (7)1.99 (7)2.859 (5)150 (5)
N1—H6···O70.88 (7)1.91 (7)2.791 (5)174 (6)
Symmetry codes: (v) x, y+1, z; (vi) −x+1, y+1/2, −z+1/2; (vii) −x+1, y−1/2, −z+1/2; (viii) x, y−1, z; (ix) −x+1, −y+1, −z+1.
Acknowledgements top

This work was supported by the National Natural Science Foundation of China (No. 40972035), the Natural Science Foundation of Fujian Province of China (No. 2010 J01308) and the Scientific and Technical Project of Fujian Province of China (No. 2009 J1009).

references
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