Crystal structure of bis(2-aminoanilinium) hydrogen phosphate

In the title compound, the hydrogen phosphate anions are linked by O—H⋯O hydrogen bonds into chains parallel to [100]. The inorganic anionic chains and the organic cations are linked by N—H⋯O and N—H⋯N hydrogen bonds, forming a two-dimensional supramolecular network extending parallel to (001).


Chemical context
Inorganic-organic hybrid compounds are of current interest due to their fascinating architectures and potential applications in crystal engineering and supramolecular chemistry (Singh et al., 2011;Direm et al., 2015). Among the explored hybrid compounds, organic phosphates formed as a result of the reaction with inorganic oxy acids such as orthophosphoric acid (H 3 PO 4 ) and organic amines and amides are particularly interesting. Organic monohydrogen (HPO 4 2À ) and dihydrogen phosphate (H 2 PO 4 À ) compounds provide a class of materials with numerous practical and potential uses in various fields such as biomolecular sciences, catalysis, liquidcrystal-material development, ferroelectrics, non-linear optical and supramolecular studies (Khan et al., 2009;Evans et al., 2008;Balamurugan et al., 2010). Non-covalent interactions, such as hydrogen bonding and other weak interactions, represent the basic set of tools for the construction of elaborate supramolecular architectures of organic or inorganic-organic compounds. In this respect, the potential of monohydrogen and dihydrogen phosphate anions as useful building blocks has been investigated structurally (Shylaja et al., 2008;Oueslati et al., 2007;Jagan et al., 2015;Trojette et al., 1998;Soumhi & Jouini, 1995). Here we report the structure and the self-assembled supramolecular architecture exhibited through the formation of O-HÁ Á ÁO, N-HÁ Á ÁO and N-HÁ Á ÁN hydrogen bonds in bis(2-aminoanilinium) hydrogen phosphate. ISSN 2056-9890

Figure 2
Partial packing diagram of the title compound showing the formation of an organic-inorganic supramolecular ladder through N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds extending along [100]. The formation of rings with R 2 4 (8) and R 3 3 (10) graph-set motifs is also shown. Hydrogen atoms not involved in hydrogen bonding are omitted for clarity. the crystal structure ( Fig. 4), adjacent organic-inorganic layers are separated by a distance equal to the length of the c axis.

Database Survey
A CSD database search (ConQuest 1.17; Groom & Allen, 2014) showed 48 entries for hydrogen phosphate salts formed with various amino cations. It is interesting to observe that most of the reported structures of hydrogen phosphate salts are hydrated (33 structures) compared to the reported structures of dihydrogen phosphate and phosphate salts. Most of the hydrogen phosphate structures reported contain alkyl cations (Ilioudis et al., 2002;Mrad et al., 2012;Li et al., 2010), in which the alkyl cations encapsulated between chains of hydrogen phosphate are flexible with respect to the nature of the cations, which may induce a change in physical properties (Baouab & Jouini, 1998). As observed in the title compound, in the crystal structure of 2-aminoanilinium dihydrogen phosphate (CSD refcode: SAYWAQ; Trojette et al., 1998), the dihydrogen phosphate anions form chains, which are bridged by 2-aminoanilinium cations through N-HÁ Á ÁO hydrogen bonds, generating a two-dimensional inorganic-organic network. Conversely, in the crystal structure of 1,2-phenylenediammonium bis(dihydrogen phosphate) (ZAYPAQ; Soumhi & Jouini, 1995), the anions form inorganic sheets interlinked by 1,2-phenylenediammonium cations, thus generating a three-dimensional inorganic-organic framework.
This can be attributed to the double protonation of the cations in ZAYPAQ compared to the title compound and SAYWAQ. In the crystal structure of 2-aminoanilinium perchlorate monohydrate (KAJGUY; Raghavaiah et al., 2005), the 2-aminoanilinium cation, the perchlorate anion and the lattice water molecule assemble into a unique hydrogen-bonded supramolecular framework, forming alternate hydrophobic and hydrophilic zones.

Synthesis and crystallization
The title compound was prepared by dissolving in water o-phenylenediamine and orthophosphoric acid in a 2:1 molar ratio. The resulting mixture was stirred continuously for 3 h with constant heating maintained at 333 K. The solution was then cooled, filtered and kept for crystallization without any disturbance. Good diffraction-quality crystals were obtained after one week.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The hydrogen atoms associated with the N and O atoms were localized in a difference electron-density map and refined with the N-H and O-H distances constrained to values of 0.90 (2) and 0.85 (1) Å , respectively. All other hydrogen atoms were placed in calculated positions and allowed to ride on their parent atoms, with C-H = 0.93 Å and U iso (H) = 1.2U eq (C). Data collection: APEX2 (Bruker, 2012); cell refinement: APEX2 and SAINT (Bruker, 2012); data reduction: SAINT and XPREP (Bruker, 2012); program(s) used to solve structure: SHELXS2014 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and Mercury (Macrae et al., 2008); software used to prepare material for publication: PLATON (Spek, 2009).

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.