Synthesis and crystal structure of 4-fluorobenzylammonium dihydrogen phosphate, [FC6H4CH2NH3]H2PO4

The crystal structure of 4-fluorobenzylammonium dihydrogen phosphate, [FC6H4CH2NH3]H2PO4, consists of layers of 4-fluorobenzylammonium cations and dihydrogen phosphate anions that alternate along the c axis, connected by hydrogen bonds into a three-dimensional network.


Chemical context
A hybrid compound is a material that involves both organic and inorganic components blended in the solid state on the molecular scale. Such materials allow the combination of the intended properties of both the organic and inorganic components when they self-assemble in the crystal. The resulting properties do not simply consist of the sum of the individual contributions, since they also strongly depend on the nature of the interactions established by the different components within the structure. The nature of the interactions has been used to divide organic-inorganic hybrid materials into two different classes, both of them being of technological interest. In class I, organic and inorganic components are connected together through strong chemical covalent or iono-covalent bonds; in class II, the two components are assembled by weaker interactions, such as hydrogen bonds and/or van der Waals and Coulombic interactions.
In particular, in considering hybrid systems belonging to class II, derivatives from orthophosphoric acid (H 3 PO 4 ) are often associated with functionalized organic molecules (amines or amides) to produce organic-inorganic materials with potentially forceful hydrogen-bonding interactions between donor (D) and acceptor (A) components. Among these hybrid phosphates, the dihydrogen phosphates have received great interest over recent years. Indeed, these compounds can be considered the most stable organic phosphates and also the first to be studied in more detail. They have a technological interest in many realms, such as magnetism, electricity, optics and in biomaterials research (Adams, 1977;Hearn & Bugg, 1972). ISSN 2056-9890 In these compounds, the acidic dihydrogen phosphate anion H 2 PO 4 À , through the formation of O-HÁ Á ÁO hydrogen bonds, gives rise to various topologies of anionic substructures. In the crystal structure of 2-ammoniumbenzamide dihydrogen phosphate (Belghith et al., 2015), the H 2 PO 4 À tetrahedra are associated in pairs, forming centrosymmetric finite units, while in 2,3-dimethylanilinium dihydrogen phosphate (Rayes et al., 2004), they form a network composed of hydrogen-bonded chains. Two-dimensional anionic layers are observed in 4chloroanilinium dihydrogen phosphate (Dhaouadi et al., 2008) and in 2-methylpiperazinediium dihydrogen phosphate (Choudhury et al., 2000), while in the crystal structure of imidazolium dihydrogen phosphate (Blessing et al., 1986), the H 2 PO 4 À anions are linked by hydrogen bonds to form a threedimensional cage-type network, inside which the cations are trapped. The varieties of the observed arrangements suggest that selected packing architectures can be designed by choosing an appropriate amine.
In order to enrich the knowledge of such kinds of hybrid materials and to investigate the effect of hydrogen bonds on chemical and structural features, we report here synthesis and crystal structure analysis of the novel organic dihydrogen phosphate, (p-FC 6 H 4 CH 2 NH 3 ) + ÁH 2 PO 4 À .

Structural commentary
The title hybrid salt crystallizes in the Pbcn space group with one para-fluorobenzylammonium cation and one dihydrogen phosphate anion in the asymmetric unit ( Fig. 1). Analysis of the P-O bond lengths clearly reveals the double-bond character of the P-O2 interaction [1.492 (4) Å ], suggesting at the same time the possible protonation of the remaining O atoms showing longer bonds [P1-O1 = 1.561 (4), P1-O3 = 1.543 (4) and P1-O4 = 1.535 (4) Å ]. This is confirmed by the presence of electron density peaks close to these oxygen atoms, compatible in terms of height and distance from hydrogen atoms. However, the refinement showed half occupancy for two of the three hydrogen atoms, in agreement with charge neutrality and geometric considerations (both are disordered over two positions along the O-HÁ Á ÁH-O direction involving the same oxygen atom in two adjacent anions). This explains the shorter P-O3 and P-O4 bond lengths, when compared with P1-O1, revealing at the same time the composition of the resulting anion as H 2 PO 4 À . The organic cation exhibits a regular configuration, with distances and angles in accordance to literature data (Wang et al., 2015;Klapö tke et al., 2003).

Figure 1
The asymmetric unit of the title compound, with displacement ellipsoids drawn at the 50% probability level. The two half-filled H atoms have a site-occupation factor of 0.5.
inorganic supramolecular network (Fig. 3) along the c axis.
Within the organic network, the dipolar character of the 4-fluorobenzylammonium molecule leads to an alternating antiparallel molecular stacking along the a axis that prevents significantinteractions between the aromatic rings but promotes van der Waals interactions as the unique intermolecular interactions between the organic molecules.  (Dhaouadi et al., 2005). The main difference concerns the hydrogen atoms of the dihydrogen phosphate anion. These, ordered on two sites in the latter structure, are located over three positions for the title structure, two of which show half occupancy. In spite of this difference, the resulting anionic framework and the linking of the cations are analogous in both cases. A similarly organized anionic layer is formed by self-assembly of H 2 PO 4 À units in the structure of octane-1,8diammonium bis(dihydrogen phosphate) (Mrad et al., 2011). Although the amine used is of different nature, the compound crystallizes in the same space group Pbcn and, approximately similar to the present case, two hydrogen atoms were found to be shared along the O-H-O bonding direction involving two H 2 PO 4 À groups. The difference in the organic moiety is reflected in a different anchoring of the cations on the anionic layers, building in this case a three-dimensional hydrogenbonded network.

Synthesis and crystallization
Crystals of the title compound were grown by dissolving in water p-fluorobenzylamine (purity 99%, Sigma-Aldrich) and orthophosphoric acid (85% wt , d = 1.7 kg cm À3 ) in a 1:1 molar ratio. The resulting mixture was heated slightly (330 K Projections of the [FC 6 H 4 CH 2 NH 3 ]H 2 PO 4 structure along the a axis (left) and the b axis (right), showing the alternate stacking of inorganic and organic layers along the c axis.

Figure 2
A layer of H 2 PO 4 À anions, parallel to the ab plane, formed by hydrogen bonds displaying R 4 4 (16) graph-set ring motifs. The solution thus obtained was placed in a Petri dish and kept for crystallization at room temperature without disturbance. Single crystals of the title compound, suitable for X-ray diffraction analysis, were obtained after one week (yield 82%).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The H atoms were located in a difference Fourier map and refined as riding, with O-H = 0.82 Å , N-H = 0.89 Å , C-H = 0.93 and 0.97 Å . A rotating model was used for the OH and ammonium groups. The dihydrogen phosphate H atoms were refined with U iso (H) = 1.5U eq (O), those of the ammonium H atoms with U iso (H) = 1.5U eq (N), and the remaining ones with U iso (H) = 1.2U eq (C). Two H atoms were found to be disordered over two positions along the O-HÁ Á ÁH-O direction involving the same oxygen atom in two adjacent anions and refined with half occupancy. An outlier (524) was omitted in the last cycles of the refinement.  (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and VESTA (Momma & Izumi, 2011); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015b).

(I)
Crystal data C 7 H 9 FN + ·H 2 PO 4 − M r = 223.14 Orthorhombic, Pbcn a = 7.1630 (8)  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.