Crystal structure of K0.75[Fe(II) 3.75Fe(III) 1.25(HPO3)6]·0.5H2O, an open-framework iron phosphite with mixed-valent Fe(II)/Fe(III) ions.

Single crystals of the title compound, potassium hexa-phosphito-penta-ferrate(II,III) hemihydrate, K0.75[Fe(II) 3.75Fe(III) 1.25(HPO3)6]·0.5H2O, were grown under mild hydro-thermal conditions. The crystal structure is isotypic with Li1.43[Fe(II) 4.43Fe(III) 0.57(HPO3)6]·1.5H2O and (NH4)2[Fe(II) 5(HPO3)6] and exhibits a [Fe(II) 3.75Fe(III) 1.25(HPO3)6](0.75-) open framework with disordered K(+) (occupancy 3/4) as counter-cations. The anionic framework is based on (001) sheets of two [FeO6] octa-hedra (one with point group symmetry 3.. and one with point group symmetry .2.) linked along [001] through [HPO3](2-) oxoanions. Each sheet is constructed from 12-membered rings of edge-sharing [FeO6] octa-hedra, giving rise to channels with a radius of ca 3.1 Å where the K(+) cations and likewise disordered water mol-ecules (occupancy 1/4) are located. O⋯O contacts between the water mol-ecule and framework O atoms of 2.864 (5) Å indicate hydrogen-bonding inter-actions of medium strength. The infrared spectrum of the compound shows vibrational bands typical for phosphite and water groups. The Mössbauer spectrum is in accordance with the presence of Fe(II) and Fe(III) ions.

Chemical context

Open-framework materials have been a major research topic in materials science during the last decades because of their potential applications (Barrer, 1982 ▸; Wilson et al., 1982 ▸; Davis, 2002 ▸, Adams & Pendlebury, 2011 ▸). Many efforts have been made to obtain porous materials using different oxoanions in combination with metals (Yu & Xu, 2010 ▸). The use of structure-directing agents or templates, not only organic but also inorganic, has also been extended in order to achieve this purpose. In this context, a new porous mixed-valent FeII/FeIII phosphitoferrate with lithium cations and an open-framework structure, Li1.43[FeII 4.43FeIII 0.57(HPO3)6]·1.5H2O, has been reported (Chung et al., 2011 ▸). This structure presents channels of ca 5.5 Å diameter along the [001] direction in which water mol­ecules and lithium ions are located. The same type of framework but with FeII cations and with ammonium counter-anions was reported recently for (NH4)2[FeII 5(HPO3)6] (Berrocal et al., 2014 ▸).

Here we report on the synthesis and crystal structure of isotypic K0.75[FeII 3.75FeIII 1.25(HPO3)6]·0.5H2O resulting from the replacement of lithium/ammonium by potassium. The iron cations in this compound are again in a mixed valence oxidation state of +II and +III.

Structural commentary

The asymmetric unit of K0.75[FeII 3.75FeIII 1.25(HPO3)6]·0.5H2O (Fig. 1 ▸) contains two Fe sites on special positions (6f and 4d) with site symmetries of .2. and .3., respectively, three O sites, one P site and one H site. In addition, disordered sites associated with a water mol­ecule (O1W) and the potassium counter-cation are present. The crystal structure is made up of two types of [FeO6] octa­hedra linked via edge-sharing into sheets parallel to (001). These sheets consist of 12-membered rings formed by six [Fe1O6] octa­hedra and six [Fe2O6] octa­hedra. In one of the FeO6 octa­hedra (Fe1), the Fe—O bond lengths range from 2.046 (2) to 2.179 (2) Å while in the [Fe2O6] octa­hedron, a more uniform bond-length distribution from 2.134 (2) to 2.143 (2) is observed. In order to assign the content of FeII and FeIII on these sites, a Mössbauer spectrum was recorded (Fig. 2 ▸). Three different components were observed, two doublets, corresponding to FeII cations, and a third doublet, corresponding to FeIII cations. The determined FeII/FeIII ratio is 3.1, in good agreement with the formula. According to bond-valence calculations (Brown, 2002 ▸), a clear assignment of which of the two iron sites carries the FeIII cations cannot be made. The calculated bond-valence sum for site Fe1 assuming FeII is 2.213 valence units (v.u.), while assuming FeIII gives 2.367. Corresponding values for the Fe2 site are 2.014 v.u. assuming FeII and 2.155 assuming FeIII. The O—Fe—O bond angles of the two [FeO6] octa­hedra are in the range between 78.10 (8) and 102.63 (7)° for cis- and between 175.77 (11) and 163.23 (8)° for the trans-angles.

Figure 1

Asymmetric unit of K0.75[FeII 3.75FeIII 1.25(HPO3)6]·0.5H2O with displacement parameters drawn at the 50% probability level.

Figure 2

Mössbauer spectrum of the title compound showing the presence of FeII and FeIII. The fit was made with the NORMOS program (Brand et al., 1983 ▸).

The (001) iron oxide sheets are linked through phosphite groups whereby six anions share the innermost oxygen atoms of each ring (Fig. 3 ▸), forming 12-membered channels extending along [001]. The channels have a radius of about 3.1 Å. The P—O bond lengths of the anion range from 1.529 (2) to 1.541 (2) Å and are comparable with those of the two isotypic structures. The P—H distance in the title compound is 1.29 (4) Å, and the O—P—O bond angles range from 110.24 (11) to 114.32 (11)°.

Figure 3

Crystal structure of K0.75[FeII 3.75FeIII 1.25(HPO3)6]·0.5H2O in polyhedral representation, in a projection along [001]. Colour code: Fe1O6 octa­hedra are blue, Fe2O6 octa­hedra are magenta, HPO3 tetra­hedra are orange, O atoms are red and K+ ions are grey. Hydrogen-bonding inter­actions between O1 from the framework and O1W are shown with dashed lines.

The disordered potassium cations and water mol­ecules are located on special positions in the twelve-membered channels of the framework with site symmetries of 32. and 3.., respectively. The occupancy factors are 0.75 for potassium and 0.25 for the water mol­ecule. Although the hydrogen atoms of the water mol­ecule could not be located, the O⋯O distance of 2.864 (5) Å between the water O1W atom and the O1 atom of the framework indicates possible hydrogen-bonding inter­actions of medium strength. Because the O1W site is located on a threefold rotation axis, three hydrogen bonds with the inorganic skeleton with an angle of 113.42 (5)° are possible.

Synthesis and characterization

K0.75[FeII 3.75FeIII 1.25(HPO3)6]·0.5H2O was synthesized under mild hydro­thermal conditions and autogeneous pressure (10–20 bar at 343 K). The reaction mixture was prepared from 30 ml water, 2 ml of hypo­phospho­rous acid, 0.17 mmol of KOH and 0.37 mmol of FeCl3··6H2O. The mixture had a pH value of ≃ 3.0. The reaction mixture was sealed in a polytetra­fluoro­ethyl­ene (PTFE)-lined steel pressure vessel, which was maintained at 343 K for five days. This procedure allowed the formation of single crystals of the title compound with a dark green to black colour.

The IR spectrum (see supporting information for this submission) shows typical bands corresponding to the stretching and deformation mode of the water mol­ecules at 3235 and 2410 cm−1, respectively. The spectrum also shows the stretching and deformation modes of the P—H bond at 1750 cm−1. The bands corresponding to the symmetric (νs) and anti­symmetric (νas) stretching vibrational modes of the (PO3) groups appear at 930 and 1151 cm−1, whereas the symmetric (δs) and anti­symmetric (δas) deformation modes of this group are centred at 450 and 590 cm−1 (Nakamoto, 1997 ▸; Chung et al., 2011 ▸).

Thermogravimetric analysis of the title compound (see supporting information for this submission) shows a first mass-loss process of 1.05% between room temperature and 498 K. This mass loss corresponds to the removal of water (theoretical value: 1.13%). Between 498 K and 673 K, another mass loss of 0.45% takes place which could not be assigned to a chemical reaction. This second process is followed by a third continuous process associated with a considerable gain of mass due to the oxidation of the compound.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1 ▸. H atoms of the water mol­ecule were not modelled. The hydrogen atom of the phosphite group was located in a difference density map and was refined without any constraint. Potassium and water oxygen sites are located in the channels. The occupancy factors of both atoms were initially set taking into account the previous characterization (themogravimetric measurement, Mössbauer spectrum fit). Some trials to refine the occupancy factors of these atoms were made. However, the results were very similar to those initially set, with a slight increase of reliability factors. Therefore, for the final model the occupancy factors were fixed at 0.75 for the K1 and at 0.25 for the O1W site.

Table 1

Experimental details

Crystal data
Chemical formula	K0.75[FeII 3.75FeIII 1.25(HPO3)6]·0.5H2O
M r	797.45
Crystal system, space group	Trigonal, P c1
Temperature (K)	100
a, c (Å)	10.1567 (5), 9.2774 (6)
V (Å3)	828.82 (8)
Z	2
Radiation type	Mo Kα
μ (mm−1)	5.14
Crystal size (mm)	0.29 × 0.05 × 0.04
Data collection
Diffractometer	Agilent SuperNova
Absorption correction	Analytical (CrysAlis PRO; Agilent, 2014 ▸)
T min, T max	0.423, 0.845
No. of measured, independent and observed [I > 2σ(I)] reflections	5132, 647, 618
R int	0.026
(sin θ/λ)max (Å−1)	0.664
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.026, 0.061, 1.19
No. of reflections	647
No. of parameters	54
H-atom treatment	All H-atom parameters refined
Δρmax, Δρmin (e Å−3)	0.8, −0.50

Computer programs: CrysAlis PRO (Agilent, 2014 ▸), OLEX2 (Dolomanov, 2009 ▸), SHELXL2014 (Sheldrick, 2015 ▸), DIAMOND (Brandenburg, 2001 ▸) and WinGX (Farrugia, 2012 ▸).

Crystal structure: contains datablock(s) I, 4R. DOI: 10.1107/S2056989015024007/wm5216sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989015024007/wm5216Isup2.hkl

Click here for additional data file.

Supporting information file. DOI: 10.1107/S2056989015024007/wm5216Isup4.tif

Click here for additional data file.

Supporting information file. DOI: 10.1107/S2056989015024007/wm5216Isup5.tif

CCDC reference: 1442401

Additional supporting information: crystallographic information; 3D view; checkCIF report

K0.75[FeII3.75FeIII1.25(HPO3)6]·0.5H2O	Dx = 3.195 Mg m−3
Mr = 797.45	Mo Kα radiation, λ = 0.71073 Å
Trigonal, P3c1	Cell parameters from 3002 reflections
Hall symbol: -P 3 2"c	θ = 2.3–28.0°
a = 10.1567 (5) Å	µ = 5.14 mm−1
c = 9.2774 (6) Å	T = 100 K
V = 828.82 (8) Å3	Prism, black
Z = 2	0.29 × 0.05 × 0.04 mm
F(000) = 779

Agilent SuperNova diffractometer	647 independent reflections
Radiation source: Nova (Mo) X-ray micro-source	618 reflections with I > 2σ(I)
Multilayer optics monochromator	Rint = 0.026
Detector resolution: 16.2439 pixels mm-1	θmax = 28.2°, θmin = 2.3°
ω scans	h = −13→12
Absorption correction: analytical (CrysAlis PRO; Agilent, 2014)	k = −9→13
Tmin = 0.423, Tmax = 0.845	l = −12→10
5132 measured reflections

Refinement on F2	Primary atom site location: iterative
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.026	Hydrogen site location: difference Fourier map
wR(F2) = 0.061	All H-atom parameters refined
S = 1.19	w = 1/[σ2(Fo2) + (0.0254P)2 + 2.2546P] where P = (Fo2 + 2Fc2)/3
647 reflections	(Δ/σ)max = 0.015
54 parameters	Δρmax = 0.8 e Å−3
0 restraints	Δρmin = −0.50 e Å−3

Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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 > 2σ(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.

	x	y	z	Uiso*/Ueq	Occ. (<1)
Fe1	0.62108 (5)	0	0.25	0.00709 (16)
Fe2	0.6667	0.3333	0.33159 (7)	0.00659 (18)
P1	0.70307 (8)	0.11196 (8)	0.58780 (7)	0.00748 (18)
O3	0.6916 (2)	0.1569 (2)	0.4316 (2)	0.0110 (4)
O2	0.3954 (2)	−0.1473 (2)	0.3120 (2)	0.0095 (4)
O1	0.8210 (2)	0.1342 (2)	0.1437 (2)	0.0124 (4)
K1	1	0	0.25	0.0247 (5)	0.75
O1W	1	0	0.063 (2)	0.033 (4)	0.25
H1	0.645 (4)	−0.033 (4)	0.592 (4)	0.011 (9)*

	U11	U22	U33	U12	U13	U23
Fe1	0.0070 (2)	0.0067 (3)	0.0075 (3)	0.00336 (14)	−0.00016 (10)	−0.00031 (19)
Fe2	0.0062 (2)	0.0062 (2)	0.0074 (3)	0.00310 (11)	0	0
P1	0.0072 (3)	0.0087 (3)	0.0068 (3)	0.0042 (3)	0.0006 (2)	0.0003 (2)
O3	0.0112 (10)	0.0147 (10)	0.0083 (9)	0.0075 (8)	0.0018 (7)	0.0024 (8)
O2	0.0091 (9)	0.0079 (9)	0.0113 (9)	0.0041 (8)	0.0023 (7)	−0.0004 (7)
O1	0.0153 (10)	0.0095 (10)	0.0140 (10)	0.0073 (9)	−0.0003 (8)	0.0002 (8)
K1	0.0151 (7)	0.0151 (7)	0.0439 (15)	0.0075 (3)	0	0
O1W	0.018 (5)	0.018 (5)	0.062 (12)	0.009 (2)	0	0

Fe1—O1	2.046 (2)	P1—H1	1.29 (4)
Fe1—O1i	2.046 (2)	O2—P1vii	1.534 (2)
Fe1—O2	2.096 (2)	O2—Fe2viii	2.134 (2)
Fe1—O2i	2.096 (2)	O1—P1ix	1.529 (2)
Fe1—O3i	2.179 (2)	O1—K1	2.935 (2)
Fe1—O3	2.179 (2)	K1—O1x	2.935 (2)
Fe1—K1	3.8486 (6)	K1—O1xi	2.935 (2)
Fe2—O2i	2.134 (2)	K1—O1xii	2.935 (2)
Fe2—O2ii	2.134 (2)	K1—O1xiii	2.935 (2)
Fe2—O2iii	2.134 (2)	K1—O1i	2.935 (2)
Fe2—O3	2.143 (2)	K1—Fe1xii	3.8486 (6)
Fe2—O3iv	2.143 (2)	K1—Fe1x	3.8486 (6)
Fe2—O3v	2.143 (2)	K1—K1xiv	4.6387 (3)
P1—O1vi	1.529 (2)	K1—K1xv	4.6387 (3)
P1—O2vii	1.534 (2)	O1W—O1Wxv	1.17 (4)
P1—O3	1.541 (2)
O1—Fe1—O1i	97.50 (12)	O1xi—K1—O1xii	63.23 (8)
O1—Fe1—O2	167.07 (8)	O1x—K1—O1xiii	63.23 (8)
O1i—Fe1—O2	89.81 (8)	O1xi—K1—O1xiii	109.30 (4)
O1—Fe1—O2i	89.81 (8)	O1xii—K1—O1xiii	78.54 (8)
O1i—Fe1—O2i	167.07 (8)	O1x—K1—O1	109.30 (4)
O2—Fe1—O2i	85.10 (11)	O1xi—K1—O1	78.54 (8)
O1—Fe1—O3i	90.97 (8)	O1xii—K1—O1	109.30 (4)
O1i—Fe1—O3i	91.82 (8)	O1xiii—K1—O1	171.12 (8)
O2—Fe1—O3i	78.11 (8)	O1x—K1—O1i	78.54 (8)
O2i—Fe1—O3i	98.73 (7)	O1xi—K1—O1i	109.30 (4)
O1—Fe1—O3	91.82 (8)	O1xii—K1—O1i	171.12 (8)
O1i—Fe1—O3	90.97 (8)	O1xiii—K1—O1i	109.30 (4)
O2—Fe1—O3	98.73 (7)	O1—K1—O1i	63.23 (8)
O2i—Fe1—O3	78.11 (8)	O1x—K1—Fe1xii	140.73 (4)
O3i—Fe1—O3	175.77 (11)	O1xi—K1—Fe1xii	31.62 (4)
O1—Fe1—K1	48.75 (6)	O1xii—K1—Fe1xii	31.62 (4)
O1i—Fe1—K1	48.75 (6)	O1xiii—K1—Fe1xii	94.44 (4)
O2—Fe1—K1	137.45 (6)	O1—K1—Fe1xii	94.44 (4)
O2i—Fe1—K1	137.45 (6)	O1i—K1—Fe1xii	140.73 (4)
O3i—Fe1—K1	92.11 (5)	O1x—K1—Fe1x	31.62 (4)
O3—Fe1—K1	92.11 (5)	O1xi—K1—Fe1x	140.73 (4)
O2i—Fe2—O2ii	85.13 (8)	O1xii—K1—Fe1x	94.44 (4)
O2i—Fe2—O2iii	85.13 (8)	O1xiii—K1—Fe1x	31.62 (4)
O2ii—Fe2—O2iii	85.13 (8)	O1—K1—Fe1x	140.73 (4)
O2i—Fe2—O3	78.10 (8)	O1i—K1—Fe1x	94.44 (4)
O2ii—Fe2—O3	93.44 (7)	Fe1xii—K1—Fe1x	120
O2iii—Fe2—O3	163.22 (8)	O1x—K1—Fe1	94.44 (4)
O2i—Fe2—O3iv	163.23 (8)	O1xi—K1—Fe1	94.44 (4)
O2ii—Fe2—O3iv	78.10 (8)	O1xii—K1—Fe1	140.73 (4)
O2iii—Fe2—O3iv	93.44 (7)	O1xiii—K1—Fe1	140.73 (4)
O3—Fe2—O3iv	102.63 (7)	O1—K1—Fe1	31.62 (4)
O2i—Fe2—O3v	93.44 (7)	O1i—K1—Fe1	31.62 (4)
O2ii—Fe2—O3v	163.22 (8)	Fe1xii—K1—Fe1	120
O2iii—Fe2—O3v	78.10 (8)	Fe1x—K1—Fe1	120
O3—Fe2—O3v	102.63 (7)	O1x—K1—K1xiv	109.64 (4)
O3iv—Fe2—O3v	102.63 (7)	O1xi—K1—K1xiv	70.36 (4)
O1vi—P1—O2vii	112.13 (11)	O1xii—K1—K1xiv	109.64 (4)
O1vi—P1—O3	114.32 (11)	O1xiii—K1—K1xiv	70.36 (4)
O2vii—P1—O3	110.24 (11)	O1—K1—K1xiv	109.64 (4)
O1vi—P1—H1	105.9 (16)	O1i—K1—K1xiv	70.36 (4)
O2vii—P1—H1	105.6 (16)	Fe1xii—K1—K1xiv	90
O3—P1—H1	108.1 (16)	Fe1x—K1—K1xiv	90
P1—O3—Fe2	135.47 (12)	Fe1—K1—K1xiv	90
P1—O3—Fe1	123.82 (12)	O1x—K1—K1xv	70.36 (4)
Fe2—O3—Fe1	98.26 (8)	O1xi—K1—K1xv	109.64 (4)
P1vii—O2—Fe1	127.45 (12)	O1xii—K1—K1xv	70.36 (4)
P1vii—O2—Fe2viii	130.31 (12)	O1xiii—K1—K1xv	109.64 (4)
Fe1—O2—Fe2viii	101.19 (8)	O1—K1—K1xv	70.36 (4)
P1ix—O1—Fe1	129.84 (13)	O1i—K1—K1xv	109.64 (4)
P1ix—O1—K1	124.86 (11)	Fe1xii—K1—K1xv	90
Fe1—O1—K1	99.63 (7)	Fe1x—K1—K1xv	90
O1x—K1—O1xi	171.12 (8)	Fe1—K1—K1xv	90
O1x—K1—O1xii	109.30 (4)	K1xiv—K1—K1xv	180