Crystal structure of bis(propane-1,3-diaminium) hexafluoridoaluminate diaquatetrafluoridoaluminate tetrahydrate

In the crystal structure of bis(propane-1,3-diammonium) hexafluoridoaluminate diaquatetrafluoridoaluminate tetrahydrate, two different environments of the Al3+ cations are observed, namely, AlF6 and AlF4(H2O)2.

The title compound, (C 3 H 12 N 2 ) 2 [AlF 6 ][AlF 4 (H 2 O) 2 ]Á4H 2 O, was obtained by a solvothermal method in ethanol as solvent and with aluminium hydroxide, HF and 1,3-diaminopropane as educts. The asymmetric unit contains a quarter each of two crystallographically independent propane-1,3-diammonium dications, [AlF 6 ] 3À and [AlF 4 (H 2 O) 2 ] À anions and four water molecules. The cations, anions and three of the independent water molecules are situated on special positions mm, while the fourth water molecule is disordered about a mirror plane. In the crystal, intermolecular N-HÁ Á ÁF and O-HÁ Á ÁF hydrogen bonds link the cations and anions into a three-dimensional framework with the voids filled by water molecules, which generate O-HÁ Á ÁO hydrogen bonds and further consolidate the packing.

Chemical context
Hybrid organic-inorganic fluoride compounds are composed of both organic and inorganic moieties. The search for new compounds in this class of materials is still intense due to their applications in many domains such as gas storage, catalysis, separation, ion-exchange and biomedicine (Horcajada et al., 2012;Stock & Biswas, 2012). Various hybrid materials containing fluorine organic ligands have been described in the literature (Ben Ali et al., 2007). The dimensionality of the metal fluoride entities are 0D (isolated polyanions) (Adil, Ben Ali et al., 2006;Fourquet et al., 1987) , 1D (chains) or 2D (layers) (Adil et al., 2010). The structural architecture of hybrid materials mainly depends on the metal and an organic part. However, other physical and physicochemical factors affect the resulting products such as the synthesis method (temperature, concentration, time of heating etc.) (Su et al., 2010). This work is a continuation of an exploration of chemical systems including metal fluoride and amine, and the study of their structures.

Structural commentary
The asymmetric unit of the title compound contains aluminum atoms located in two crystallographically independent sites with different environments, [Al2F 6 ] and [Al1F 4 (H 2 O) 2 ], and two independent 1,3-propane diamine (dap) dications (Fig. 1). The Al-F distances in the two octahedra range from 1.768 (2) ISSN 1600-5368 to 1.809 (3) Å while the Al1-OW1 distance is longer [1.944 (4) Å ]. The [AlF 6 ] octahedron is regular whereas [AlF 4 (H 2 O) 2 ] exhibits a pronounced distortion due to the strong influence of the crystal field created by the heteroligands (F À /H 2 O). The value of the calculated valences (3.08 for Al1 and 3.01 for Al2) of the individual Al 3+ cations (Brese & O'Keeffe, 1991) is in good agreement with the theoretical value, whereas those for the F À anions are equal to 0.5. These anions complete their valence by establishing strong hydrogen bonds.

Supramolecular features
Each [AlF 4 (H 2 O) 2 ] octahedron is linked via N-HÁ Á ÁF or O-HÁ Á ÁF hydrogen bonds (Table 1) to one type of the organic cations ( Fig. 2), with the formation of infinite chains parallel to the a axis. These chains are linked to each other by the AlF 6 3À dications and form infinite (H 2 dap)[AlF 4 (H 2 O) 2 ] layers parallel to the ac plane (Fig. 3) . These layers are connected by the second organic cations and form a three-dimensional framework showing cavities, which are filled with the lattice water molecules.

Figure 2
The environment of the AlF 4 (H 2 O) 2 octahedron. Dashed lines denote hydrogen bonds.

Figure 3
The crystal packing of the title compound, viewed approximately along [001].

Synthesis and crystallization
The title compound was prepared from a starting mixture of AlF 3 (0.5 g) in 40% HF (1.5 ml) and ethanol (5 ml). 1,3-Diaminopropane (0.54 ml) was added and mild hydrothermal conditions (463 K) were applied in a Teflon-lined autoclave (25 ml). The resulting product was washed with ethanol and dried in air giving colourless single crystals.

Bis(propane-1,3-diaminium) hexafluoridoaluminate diaquatetrafluoridoaluminate tetrahydrate
Crystal data Extinction correction: WinGX (Farrugia, 2012), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.0022 (7) 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 > σ(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.