K₂[Fe^II₃(P₂O₇)₂(H₂O)₂]

The title compound, dipotassium diaqua­bis­(diphosphato)triferrate(II), K₂[Fe^II₃(P₂O₇)₂(H₂O)₂], was synthesized under solvothermal conditions. The crystal structure is isotypic with its Co analogue. In the structure, there are two crystallographically distinct Fe positions; one lies on an inversion center, the other on a general position. The first Fe²⁺ cation adopts a regular octa­hedral coordination with six O atoms, whereas the other is coordinated by five O atoms and a water mol­ecule. The [FeO₆] octa­hedron shares its trans-edges with an adjacent [FeO₅(H₂O)] octahedron; in turn, the [FeO₅(H₂O)] octa­hedron shares skew-edges with a neighbouring [FeO₆] octa­hedron and an [FeO₅(H₂O)] octa­hedron, resulting in a zigzag octa­hedral chain running along [001]. The zigzag chains are linked to each other by the P₂O₇ diphosphate groups, leading to a corrugated iron diphosphate layer, [Fe₃(P₂O₇)₂(H₂O)₂]^2-, parallel to (100). The inter­layer space is occupied by K⁺ cations, which adopt an eight-coordination to seven O atoms and one water mol­ecule from a neighbouring iron diphosphate layer. Thus, the K⁺ ions not only compensate the negative charge of the layer but also link the layers into a network structure.

Key indicators

• Single-crystal X-ray study
• T = 173 K
• Mean [sigma](P-O) = 0.003 Å
• R factor = 0.032
• wR factor = 0.074
• Data-to-parameter ratio = 13.0

checkCIF/PLATON results

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Alert level C
PLAT029_ALERT_3_C _diffrn_measured_fraction_theta_full Low .......      0.978
PLAT042_ALERT_1_C Calc. and Reported MoietyFormula Strings  Differ          ?
PLAT911_ALERT_3_C Missing # FCF Refl Between THmin & STh/L=  0.600          6
PLAT912_ALERT_4_C Missing # of FCF Reflections Above STh/L=  0.600        105
PLAT975_ALERT_2_C Positive Residual Density at 0.56A from O2     .       0.42 eA-3

Alert level G
PLAT004_ALERT_5_G Info: Polymeric Structure Found with Dimension .          2
PLAT005_ALERT_5_G No _iucr_refine_instructions_details in CIF ....          ?
PLAT793_ALERT_4_G The Model has Chirality at P2      (Verify) ....          R

   0 ALERT level A = Most likely a serious problem - resolve or explain
   0 ALERT level B = A potentially serious problem, consider carefully
   5 ALERT level C = Check. Ensure it is not caused by an omission or oversight
   3 ALERT level G = General information/check it is not something unexpected

   1 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   1 ALERT type 2 Indicator that the structure model may be wrong or deficient
   2 ALERT type 3 Indicator that the structure quality may be low
   2 ALERT type 4 Improvement, methodology, query or suggestion
   2 ALERT type 5 Informative message, check

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S1. Comment

A class of layered transition metal diphosphates with the general formula of A₂M₃(P₂O₇)₂(H₂O)₂ (A = K or NH₄, M = transition metal (II)) has recently attracted much attention. Since the first publication of K₂Co₃(P₂O₇)₂(H₂O)₂ (Lightfoot et al., 1990) in 1990, a few examples of ammonium substituted compounds, (NH₄)₂M₃(P₂O₇)₂(H₂O)₂ (M = Mn²⁺, Ni²⁺) (Chippindale et al., 2003; Wei et al., 2010), have been synthesized by using either solvothermal or ionothermal method. Their magnetic and ion exchange behaviors have also been discussed. Besides, a compound with mixed cations, Na(NH₄)[Ni₃(P₂O₇)₂(H₂O)₂], has also been reported (Liu et al., 2004). Although compounds incorporating with Mn²⁺, Co²⁺, and Ni²⁺ have been known, as a very close relationship to these elements, Fe²⁺ was not yet found in this class of compounds until very recently an ammonium iron diphosphate, (NH₄)₂[Fe₃^II(P₂O₇)₂(H₂O)₂], was reported by our group which was synthesized by using iron powder as iron source and applying the weak reductive agent pyridine as solvent (Liu et al., 2012). During our systematically investigation on iron compounds (Mi et al., 2004; Huang et al., 2012), we obtained a new member in this class of compounds with the formula of K₂[Fe^II₃(P₂O₇)₂(H₂O)₂].

In the crystal structure of the title compound, there are two crystallographically distinct iron sites, one potassium, and two independent phosphorus (shown in Figure 1). Fe(2) lies on an inversion center (2a, -1) while Fe(1) on a general position (4e, 1). Fe(2) adopts a regular octahedral coordination with six oxygen atoms, whereas Fe(1) coordinates to five oxygen atoms and a water molecule in the form of Fe(1)O₅(OH₂). Each Fe(2)O₆ octahedron shares trans-edges with adjacent Fe(1)O₅(OH₂) octahedra, and in turn, Fe(1)O₅(OH₂) octahedron shares skew-edges with the neighboring Fe(2)O₆ and Fe(1)O₅(OH₂) octahedra, which finally leads to the formation of FeO₆-based zigzag chains parallel to [010] (see Figure 2a). Two independent phosphorus atoms adopt a four-fold coordination with one long P–O bond and three general P–O bonds which are often found in diphosphates. Every two PO₄ tetrahedra share the common O-vertex (O2) to constitute a P₂O₇ group which acts as a bidentate ligand to link FeO₆-based chains along [001], resulting in a undulating iron diphosphate layer, [Fe₃(P₂O₇)₂(H₂O)₂]^2–, parallel to the (100) (see Figure 2b). The layers stack along the a-axis direction in AAA fashion with the potassium atoms locating at the interlayer space (see Figure 2c). The potassium ion is 8-coordinated with seven oxygens and a water molecule from the neighbouring layers with one long bond and seven relatively short bonds (see Figure 2d). Bond-valence sum calculations (Brown, 2002) suggests both two iron atoms are in the 2+ valence state (Cald.: 2.008 v.u. for Fe(1) and 2.112 v.u. for Fe(2) and the potassium should be +1 oxidation (Cald.: 1.054 v.u. for K1). Comparing the crystal structure of NH₄- and K-compounds, both of them possess the same conformation of iron diphosphate layer, however, the interlayer distance of K-compound is relatively smaller than that of NH₄-compound identified by the a-value (9.1517 (16) Å for KFe-compound and 9.4131 (17) Å for NH₄Fe-compound) which is consistent with the ion radius of K⁺ and NH₄⁺.

Related literature

For background to iron compounds, see: Mi et al. (2004); Huang et al. (2012). For related structures, see: Chippindale et al. (2003) for (NH₄)₂[Mn₃(P₂O₇)₂(H₂O)₂]; Lightfoot & Cheetham (1990) for K₂Co₃(P₂O₇)₂(H₂O)₂; Liu et al. (2012) for (NH₄)₂[Fe₃(H₂O)₂(P₂O₇)₂]; Liu et al. (2004) for Na(NH₄)[Ni₃(P₂O₇)₂(H₂O)₂]; Wei et al. (2010) for (NH₄)₂[Ni₃(P₂O₇)₂(H₂O)₂]. For bond-valence calculations, see: Brown (2002).

S2. Experimental

The title compound has been synthesized under solvothermal condition. In a typical synthesis, 1 mmol Fe powder and 2.5 mmol KH₂PO₄ were added into the mixed solvent of 2 mL pyridine and 2 mL 1,2-propanediol. Then, 2 mL H₃PO₄ (85%) was dropped into the above solution to adjust pH value to 6-7. After stirred for 5 minutes, the mixture was transferred into a 15 mL Teflon-lined stainless steel autoclave which was heated at 436 K for 7 days and then was cooled down to room temperature. The final product, colourless block-like crystals, was washed with distilled water and filtrated by vacuum. The powder X-ray diffraction of the sample showed that it is isotypic to our recently reported compound [NH₄]₂[Fe^II₃(H₂O)₂(P₂O₇)₂].

S3. Refinement

The hydrogen atoms bonded to water (O8) were located from the difference Fourier maps and refined without applying any constraints on the distance of O–H and the displacement parameter of H atoms.

Computing details

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

Fig. 1. Structural unit of K2[Fe^II3(P2O7)2(H2O)2], showing the coordination environments of Fe, K, and P atoms. Thermal ellipsoids were drawn at the 50% probability level. Symmetry codes: (i) x, –y+3/2, z+1/2; (ii) –x+2, –y+1, –z+1; (iii) –x+2,–y+2, –z+1; (iv) –x+2, y+1/2, –z+1/2; (v) x, –y+1/2, z+1/2; (vi) –x+2, y–1/2, –z+1/2; (vii) x, –y+1/2, z–1/2; (viii) –x+1, y+1/2, –z+1/2; (ix) x+1, y–1/2, –z+1/2; (x) –x+1, –y+1, –z+1; (xi) x, y–1, z; (xii) x, –y+3/2, z–1/2; (xiii) x, y+1, z.

Fig. 2. Polyhedral presentation of the crystal structure of K2[Fe^II3(P2O7)2(H2O)2], (a) zigzag edge-sharing iron octahedral chain is built from FeO6 and FeO5(H2O) octahedra by sharing their trans- or skew-edges running along [010]; (b) P2O7 groups act as a bidentate ligand to link FeO6-based chains along [001] to form a corrugated iron diphosphate layer [Fe3(P2O7)2(H2O)2]^2–; (c) the iron diphosphate layers stack along [100] in AAA fashion with the potassium atoms locating at the interlayer space; (d) the K atom is 8-coordinated to seven oxygen atoms and a H2O molecule coming from adjacent iron diphosphate layers to form an irregular polyhedron. Purple octahedron: FeO6, orange tetrahedron: PO4, light grey sphere: K atom, dark grey sphere: H atom.

Dipotassium diaquabis(diphosphato)triferrate(II)

Crystal data

K2[Fe3(P2O7)2(H2O)2]	F(000) = 616
Mr = 629.66	Dx = 3.037 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 4006 reflections
a = 9.1517 (16) Å	θ = 2.3–28.3°
b = 8.1737 (15) Å	µ = 4.28 mm^−1
c = 9.3147 (17) Å	T = 173 K
β = 98.860 (3)°	Block, colourless
V = 688.5 (2) Å^3	0.09 × 0.09 × 0.08 mm
Z = 2

Data collection

Bruker SMART CCD area-detector diffractometer	1596 independent reflections
Radiation source: fine-focus sealed tube	1460 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.027
1265 images,ϕ=0, 90, 180 degree, and Δω=0.3 degree, χ= 54.74 degree scans	θmax = 28.3°, θmin = 2.3°
Absorption correction: multi-scan (SMART; Bruker, 2001)	h = −11→12
Tmin = 0.687, Tmax = 0.710	k = −10→10
4006 measured reflections	l = −12→8

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)] = 0.032	Hydrogen site location: difference Fourier map
wR(F^2) = 0.074	All H-atom parameters refined
S = 1.07	w = 1/[σ^2(Fo^2) + (0.0303P)^2 + 1.280P] where P = (Fo^2 + 2Fc^2)/3
1596 reflections	(Δ/σ)max < 0.001
123 parameters	Δρmax = 0.57 e Å^−3
0 restraints	Δρmin = −0.66 e Å^−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 > 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 (Ų)

	x	y	z	Uiso*/Ueq
Fe1	0.86324 (5)	0.85888 (6)	0.50352 (5)	0.00807 (13)
Fe2	1.0000	0.5000	0.5000	0.00754 (16)
P1	0.89966 (9)	0.30110 (10)	0.20949 (9)	0.00652 (18)
P2	0.69498 (9)	0.56235 (10)	0.26759 (9)	0.00744 (18)
K1	0.57440 (8)	0.27162 (10)	0.48169 (9)	0.01682 (19)
O1	0.6935 (3)	0.6515 (3)	0.1248 (3)	0.0111 (5)
O2	0.7397 (2)	0.3719 (3)	0.2360 (2)	0.0090 (5)
O3	0.8654 (2)	0.1362 (3)	0.1376 (2)	0.0084 (5)
O4	0.9925 (2)	0.2944 (3)	0.3595 (2)	0.0094 (5)
O5	0.8140 (2)	0.6235 (3)	0.3887 (3)	0.0103 (5)
O6	1.0389 (2)	0.9230 (3)	0.3891 (2)	0.0089 (5)
O7	0.5448 (2)	0.5477 (3)	0.3129 (3)	0.0120 (5)
O8	0.7250 (3)	0.9942 (3)	0.3420 (3)	0.0146 (5)
H1	0.770 (5)	1.027 (6)	0.278 (5)	0.023 (13)*
H2	0.645 (5)	0.991 (5)	0.302 (5)	0.013 (11)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Fe1	0.0087 (2)	0.0085 (2)	0.0070 (2)	0.00054 (17)	0.00131 (17)	0.00043 (17)
Fe2	0.0088 (3)	0.0075 (3)	0.0061 (3)	0.0003 (2)	0.0004 (2)	0.0000 (2)
P1	0.0070 (4)	0.0068 (4)	0.0058 (4)	0.0001 (3)	0.0008 (3)	−0.0002 (3)
P2	0.0072 (4)	0.0081 (4)	0.0069 (4)	0.0007 (3)	0.0007 (3)	−0.0007 (3)
K1	0.0152 (4)	0.0183 (4)	0.0164 (4)	−0.0009 (3)	0.0007 (3)	0.0013 (3)
O1	0.0113 (11)	0.0142 (12)	0.0083 (11)	0.0030 (9)	0.0029 (9)	0.0019 (9)
O2	0.0072 (11)	0.0086 (11)	0.0113 (12)	−0.0005 (8)	0.0015 (9)	−0.0025 (9)
O3	0.0105 (11)	0.0075 (11)	0.0071 (11)	−0.0004 (8)	0.0014 (9)	−0.0016 (9)
O4	0.0103 (11)	0.0096 (11)	0.0079 (11)	0.0005 (9)	0.0002 (9)	−0.0006 (9)
O5	0.0096 (11)	0.0089 (11)	0.0114 (12)	0.0017 (9)	−0.0014 (9)	−0.0020 (9)
O6	0.0091 (11)	0.0094 (12)	0.0082 (11)	−0.0001 (9)	0.0021 (9)	−0.0014 (9)
O7	0.0072 (11)	0.0134 (12)	0.0158 (13)	−0.0004 (9)	0.0028 (9)	−0.0008 (10)
O8	0.0097 (13)	0.0200 (14)	0.0140 (14)	0.0006 (10)	0.0009 (11)	0.0061 (10)

Geometric parameters (Å, º)

Fe1—O1^i	2.058 (2)	P1—O2	1.628 (2)
Fe1—O4^ii	2.103 (2)	P2—O7	1.503 (2)
Fe1—O8	2.121 (3)	P2—O1	1.515 (2)
Fe1—O6	2.127 (2)	P2—O5	1.527 (2)
Fe1—O6^iii	2.169 (2)	P2—O2	1.648 (2)
Fe1—O5	2.214 (2)	K1—O1^vii	2.685 (2)
Fe2—O5^ii	2.109 (2)	K1—O7	2.740 (3)
Fe2—O5	2.109 (2)	K1—O7^viii	2.771 (3)
Fe2—O4^ii	2.124 (2)	K1—O2^iv	2.859 (2)
Fe2—O4	2.124 (2)	K1—O3^iv	2.929 (2)
Fe2—O3^iv	2.210 (2)	K1—O2	3.043 (2)
Fe2—O3^v	2.210 (2)	K1—O8^ix	3.047 (3)
P1—O3	1.516 (2)	K1—O7^vii	3.339 (3)
P1—O6^vi	1.520 (2)	O8—H1	0.82 (5)
P1—O4	1.521 (2)	O8—H2	0.76 (4)
O1^i—Fe1—O4^ii	95.66 (9)	O1^vii—K1—O7	94.87 (7)
O1^i—Fe1—O8	89.59 (10)	O1^vii—K1—O7^viii	90.91 (7)
O4^ii—Fe1—O8	172.30 (10)	O7—K1—O7^viii	86.72 (8)
O1^i—Fe1—O6	167.91 (9)	O1^vii—K1—O2^iv	119.53 (7)
O4^ii—Fe1—O6	89.92 (9)	O7—K1—O2^iv	143.75 (7)
O8—Fe1—O6	86.00 (10)	O7^viii—K1—O2^iv	81.99 (7)
O1^i—Fe1—O6^iii	94.22 (9)	O1^vii—K1—O3^iv	170.43 (7)
O4^ii—Fe1—O6^iii	91.94 (9)	O7—K1—O3^iv	94.29 (7)
O8—Fe1—O6^iii	93.29 (10)	O7^viii—K1—O3^iv	86.85 (7)
O6—Fe1—O6^iii	74.83 (10)	O2^iv—K1—O3^iv	50.94 (6)
O1^i—Fe1—O5	96.59 (9)	O1^vii—K1—O2	110.62 (7)
O4^ii—Fe1—O5	80.63 (9)	O7—K1—O2	50.46 (7)
O8—Fe1—O5	93.20 (10)	O7^viii—K1—O2	132.06 (7)
O6—Fe1—O5	94.90 (9)	O2^iv—K1—O2	118.20 (8)
O6^iii—Fe1—O5	167.42 (9)	O3^iv—K1—O2	77.53 (6)
O5^ii—Fe2—O5	180.0	O1^vii—K1—O8^ix	91.01 (8)
O5^ii—Fe2—O4^ii	97.39 (9)	O7—K1—O8^ix	112.28 (8)
O5—Fe2—O4^ii	82.61 (9)	O7^viii—K1—O8^ix	160.66 (8)
O5^ii—Fe2—O4	82.61 (9)	O2^iv—K1—O8^ix	80.33 (7)
O5—Fe2—O4	97.39 (9)	O3^iv—K1—O8^ix	88.08 (7)
O4^ii—Fe2—O4	180.0	O2—K1—O8^ix	64.50 (7)
O5^ii—Fe2—O3^iv	87.34 (9)	O1^vii—K1—O7^vii	48.08 (6)
O5—Fe2—O3^iv	92.66 (9)	O7—K1—O7^vii	89.37 (3)
O4^ii—Fe2—O3^iv	90.54 (9)	O7^viii—K1—O7^vii	138.28 (9)
O4—Fe2—O3^iv	89.46 (9)	O2^iv—K1—O7^vii	121.35 (7)
O5^ii—Fe2—O3^v	92.66 (9)	O3^iv—K1—O7^vii	134.87 (7)
O5—Fe2—O3^v	87.34 (9)	O2—K1—O7^vii	70.56 (6)
O4^ii—Fe2—O3^v	89.46 (9)	O8^ix—K1—O7^vii	49.70 (7)
O4—Fe2—O3^v	90.54 (9)	P2—O1—Fe1^x	123.83 (14)
O3^iv—Fe2—O3^v	180.0	P1—O2—P2	128.09 (15)
O3—P1—O6^vi	112.74 (13)	P1—O3—Fe2^vi	127.45 (13)
O3—P1—O4	114.99 (13)	P1—O4—Fe1^ii	142.06 (15)
O6^vi—P1—O4	111.86 (13)	P1—O4—Fe2	119.89 (13)
O3—P1—O2	104.68 (12)	Fe1^ii—O4—Fe2	98.04 (9)
O6^vi—P1—O2	106.47 (13)	P2—O5—Fe2	129.55 (14)
O4—P1—O2	105.14 (13)	P2—O5—Fe1	135.27 (14)
O7—P2—O1	113.58 (13)	Fe2—O5—Fe1	95.14 (9)
O7—P2—O5	113.47 (14)	P1^v—O6—Fe1	121.29 (13)
O1—P2—O5	113.57 (14)	P1^v—O6—Fe1^iii	130.73 (13)
O7—P2—O2	103.75 (13)	Fe1—O6—Fe1^iii	105.17 (10)
O1—P2—O2	105.48 (13)	H1—O8—H2	102 (4)
O5—P2—O2	105.80 (13)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O8—H1···O3^xi	0.82 (5)	1.90 (5)	2.716 (4)	171 (5)
O8—H2···O7^xii	0.76 (4)	1.95 (4)	2.696 (4)	164 (4)

Symmetry codes: (xi) x, y+1, z; (xii) −x+1, y+1/2, −z+1/2.