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

K0.7[FeII 3.7FeIII 1.3(HPO3)6]·5H2O was synthesized under mild hydrothermal conditions. The open-framework phosphite contains channels extending along [001] in which disordered potassium cations and water molecules are located.

oxoanions. Each sheet is constructed from 12-membered rings of edge-sharing [FeO 6 ] octahedra, giving rise to channels with a radius of ca 3.1 Å where the K + cations and likewise disordered water molecules (occupancy 1/4) are located. OÁ Á ÁO contacts between the water molecule and framework O atoms of 2.864 (5) Å indicate hydrogen-bonding interactions of medium strength. The infrared spectrum of the compound shows vibrational bands typical for phosphite and water groups. The Mö ssbauer 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.  (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 Fe II cations and with ammonium counteranions was reported recently for (NH 4 ) 2 [Fe II 5 (HPO 3 ) 6 ] (Berrocal et al., 2014).
Here we report on the synthesis and crystal structure of isotypic K 0.75 [Fe II 3.75 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 [FeO 6 ] octahedra linked via edge-sharing into sheets parallel to (001). These sheets consist of 12-membered rings formed by six [Fe1O 6 ] octahedra and six [Fe2O 6 ] octahedra. In one of the FeO 6 octahedra (Fe1), the Fe-O bond lengths range from 2.046 (2) to 2.179 (2) Å while in the [Fe2O 6 ] octahedron, a more uniform bond-length distribution from 2.134 (2) to 2.143 (2) is observed. In order to assign the content of Fe II and Fe III on these sites, a Mö ssbauer spectrum was recorded (Fig. 2 ] 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.
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) .
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 OÁ Á ÁO 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.  Mö ssbauer spectrum of the title compound showing the presence of Fe II and Fe III . The fit was made with the NORMOS program (Brand et al., 1983).

Figure 3
Crystal (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 FeCl 3Á Á6H 2 O. 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 À1 , respectively. The spectrum also shows the stretching and deformation modes of the P-H bond at 1750 cm À1 . The bands corresponding to the symmetric ( s ) and antisymmetric ( as ) stretching vibrational modes of the (PO 3 ) groups appear at 930 and 1151 cm À1 , whereas the symmetric ( s ) and antisymmetric ( as ) deformation modes of this group are centred at 450 and 590 cm À1 (Nakamoto, 1997;Chung et al., 2011).
Thermogravimetric analysis of the title compound (see supporting information for this submission) shows a first massloss 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, Mö ssbauer 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.  (Agilent, 2014); cell refinement: CrysAlis PRO (Agilent, 2014); data reduction: CrysAlis PRO (Agilent, 2014); program(s) used to solve structure: OLEX2 (Dolomanov, 2009); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2001); software used to prepare material for publication: WinGX (Farrugia, 2012). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.015 Δρ max = 0.8 e Å −3 Δρ min = −0.50 e Å −3 Special details 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 F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.