Crystal structure of (NH4)2[FeII 5(HPO3)6], a new open-framework phosphite

(NH4)2[FeII 2(HPO3)6] exhibits an open-framework structure with channels in which disordered ammonium cations are situated.


Chemical context
Research in the area of solids exhibiting open-framework structures continues to be exciting because of their numerous potential applications (Barrer, 1982;Hagrman et al., 1999). Prior to the early 1980s when nanoporous aluminium phosphates were first reported by Flanigen and co-workers, aluminosilicate-based zeolites (Wilson et al., 1982) and closely related systems represented the predominant class of materials with open-framework structures. In the meantime, a plethora of activities and efforts have been undertaken for the synthesis of numerous other compounds with open-framework structures of different dimensionalities (Yu & Xu, 2006).

Structural commentary
Tha asymmetric unit of (NH 4 ) 2 [Fe 5 (HPO 3 ) 6 ] is displayed in Fig. 1 The iron oxide sheets are linked through phosphite groups in which six anions share the most interior oxygen atoms of each ring (Fig. 2), forming 12-membered channels along [001] with a radius of about 3.1 Å . The phosphorus(III) atom of the complex oxoanion is located on a general position of this space group. The P-O bond lengths of the anion range from 1.514 (3) to 1.538 (3) Å , and the P-H distance is 1.28 (5) Å , with O-P-O bond angles from 110.28 (17) to 114.29 (17) .

Supramolecular features
The ammonium cations are located in the 12-membered channels of the framework structure. Although no hydrogen atoms of the cations could be located due to the positional disorder, NÁ Á ÁO contacts of 2.67 (6), 2.85 (7), 2.87 (8)

Synthesis and characterization
(NH 4 ) 2 [Fe II 5 (HPO 3 ) 6 ] 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 phosphorous acid, 0.17 mmol of NH 4 OH and 0.37 mmol of FeCl 3 . The mixture had a pH value of ' 6.0. The reaction mixture was sealed in a polytetrafluoroethylene (PTFE)-lined steel pressure vessel, which was maintained at 343 K for five days. This procedure apparently caused reduction of iron(III) to iron(II) and led to the formation of single crystals of the title compound with a dark-green colour. All crystals appeared to be twinned. The presence of ammonium cations in the title compound was confirmed by infra-red spectroscopy, showing bands at 3190 and 1450 cm À1 . Characteristic bands of the phosphite P-H group were also observed at 2510 and 1050 cm À1 (Nakamoto, 1997).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. The title crystal was confirmed to be twinned by merohedry using the TwinRotMap option in PLATON (Spek, 2009). The twin element is a 180 -rotation around the <110> direction, or any other equivalent representations of the coset decomposition of the 6/mmm holohedry under crystal class 3m1. The twin law (010/100/001) was used during the refinements, and the twin volume of the second component refined to a value of 0.079 (1)%. The asymmetric unit of (NH 4 ) 2 [Fe II 5 (HPO 3 ) 6 ], with displacement parameters drawn at the 50% probability level.
The hydrogen atom of the phosphite group was located in a difference map and restrained to be equidistant to the three oxygen atoms of the group, and a fixed isotropic displacement parameter with a value equal to 1.2U eq of the parent P atom was assigned.
The ammonium cation is equally disordered around a threefold rotation axis along (00z) and was refined with two positions, N1 and N2. The occupancy factors of N1 and N2 were initially freely refined, but since they refined close to the expected value of 1/6, this value was fixed during the last cycles. Because the ellipsoids of these atoms were very elongated, ISOR commands of SHELXL2014 (Sheldrick, 2008) were used to achieve more regular displacements. This command restrains the U ij components of anisotropically refined atoms to behave approximately isotropically within a standard uncertainty. H atoms belonging to the disordered ammonium atoms were not considered in the final model.

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 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.