Crystals of ABHP are easily prepared by slow evaporation at room temperature of an aqueous solution of orthophosphoric acid (85 wt.% H₃PO₄) and the organic molecule: 2-Aminobenzamide (C₇H₈N₂O); in the molar ratio 1:1. Schematically the synthesis reaction is:

when the most of the solution is evaporated, prismatic crystals appears deep down the vessels. The crystals are stable under normal conditions of temperature and humidity.

The title compound has been studied by various physico-chemical methods: X-ray diffraction, Infrared spectroscopy and Thermal analysis.

A view of the structure projected along the b direction (Figure 2) shows that The inorganic entities have a layered organisation parallel to (a, c) plane, with an interplane period of c/2 = 7.972 Å. The organic cations are trapped in the interlayer spacing, and neutralize the negative charge of the inorganic layers. The asymmetric unit of the crystal structure consists of one phosphate anion and one organic cation.

Within the inorganic layer (Figure 3), The tetrahedron are associated in pairs, forming centrosymmetric finite clusters [H₄P₂O₈]²⁻. The P-P distance between two groups linked by four hydrogen bonds is 4.148 Å. The P-O distances range from 1.496(2) to 1.568(3) Å. These values are comparable to the reported data [11] -[14] . The longest P-O distances, 1.563(3) and 1.568(3) Å, are due to the presence of the acidic hydrogen atoms on the PO₄ tetrahedron. The average values of P-O distances and O-P-O angles are 1.534(3) Å and 109.39(15)˚. They are in good agreement with that generally observed in such anions in other phosphates [15] .

The calculated average values of distortion indices corresponding to the different angles and distances in the PO₄ tetrahedral [DI(OPO) = 0.019; DI(PO) = 0.021 and DI(OO) = 0.007], exhibit a pronounced distortion of the PO distances and OPO angles if compared to OO distances; so the phosphate group can be considered as a rigid regular arrangement of oxygen atoms, with the P atom displaced from their centroid [16] .

The interaction of the orthophosphoric acid with the organic molecule (C₇H₈N₂O) leads to the protonation of nitrogen grafted on the benzene ring and the formation of the (C₇H₉N₂O)⁺ cation. With regards to the organic cations arrangement, the (C₇H₉N₂O)⁺ groups are organized in opposition by creating, thus, a local inversion centre. Note that each two cations are associated with a hydrogen bonds N2-H2N2∙∙∙O5 to form a dimmer located in planes z = (n + 1)/2. Interatomic distances and angles in these groups spread within the respective ranges: 1.234(4) - 1.504(5) Å and 117.7(3) - 121.7(3)˚ (Table 3).

Hydrogen bonding play a significant role in the linking the organic molecules with the anionic sheets made by H₂PO₄ moieties. This interaction contributes to the cohesion of the structure. All the D (donor)-H∙∙∙A (acceptor) hydrogen bonds are listed in Table 4, with an upper limit of 2.149(4) Å for H∙∙∙A distance and a lower limit of 164(4)˚ for the D-H∙∙∙A bond angles. Thus, this atomic arrangement exhibits two types of intermolecular interaction: O-H∙∙∙O and N-H∙∙∙O, classified respectively as strong and weak hydrogen bonds. The structure studied in this work, contains a single hydrogen bond of the first type and the second type five. The only link O3-HO3∙∙∙O4 considered high [O3-HO3∙∙∙O4 = 2.554(3) Å], brings the two anionic species as a cluster (H₄P₂O₈)²⁻; the second type of hydrogen bonds connecting the inorganic clusters to organic dimmers. A three-dimensional frame work is then created.

The two curves corresponding to differential thermal analysis (DTA) and thermogravimetric analysis (TGA) in open air are given in Figure 4. The DTA curve shows an important endothermic melting peak at about 197˚C followed by a course of weak endothermic peaks in a wide temperature range [200˚C - 400˚C]. The TGA curve shows an important weight loss corresponding to the progressive pyrolysis of the organic molecule in this temperature range. The stability of the investigated anhydrous (C₇H₉N₂O)H₂PO₄ below this melting temperature (197˚C) can be explained by the different strong bonds observed by the X-ray diffraction.

^aSymmetry operators: (i): −x + 3/2, Y − 1/2, −Z + 3/2; (ii): X − 1/2, −Y + 1/2, Z + 1/2; (iii): −X + 1, −Y + 1, −Z + 2; (iv): −X + 1, −Y + 1, −Z + 1.

The normal modes of vibration in an isolated PO₄ tetrahedron with an ideal T_d symmetry have been widely studied [17] . The n₁(A₁) and n₃(F₂) symmetric and asymmetric stretching modes are observed in 1200 - 900 cm⁻¹ region, whereas the n₂(E) and n₄(F₂) symmetric and asymmetric bending modes are distinguished in the 600 - 400 cm⁻¹ domain [18] [19] . The group-theoretical analysis shows that the number of normal modes of PO₄ tetrahedron is given by the representation: G_int = A₁ + E + 2F₂. The localisation of two protons on two of the oxygen atoms reduces the ideal symmetry from T_d to C_2v. The correlation of group to subgroup shows that these modes can be divided into 2A₁ + B₁ + B₂ stretching and 2A₁ + A₂ + B₁ + B₂ bending vibration in the C_2v symmetry of H₂PO₄ group. However, in the crystal, The H₂PO₄ tetrahedron has the lower local symmetry C₁, and therefore anisotropic crystal fields may lift degeneracy and allow inactive modes to be active.

Infrared absorption spectrum (Figure 5) of the title compound shows vibration bands characteristic of (C₇H₉N₂O)⁺ cations and anions. Its interpretation is made in terms of internal modes of two atomic groups, PO₂ and P(OH)₂, included in anion. The two stretching vibrations, asymmetric and symmetric of PO₂ group, are observed respectively at 1150 and 1074 cm⁻¹; while those related to P(OH)₂ occur, as two intense bands, at 940 and 874 cm⁻¹ [20] . Then we attribute the two very intense bands at 1094 cm⁻¹ and 958 cm⁻¹ and the two intense bands at, 880 cm⁻¹ and 853 cm⁻¹ to these four vibrations. The splitting of F₂ stretching

mode of PO₄ into two very intense components at 1094, and 958 cm⁻¹ corroborate the symmetry lowering of H₂PO₄ in the solid state. Bending modes of H₂PO₄ group are observed at lower frequencies. The two medium bands at 748 cm⁻¹ and 552 cm⁻¹ correspond respectively to the rocking ρ(PO₂) and to the bending δ(OHPOH) vibrations; whereas the strong band at 665 cm⁻¹ is attributed to the wagging ω(PO₂) vibration. The two bands, strong at 495 cm⁻¹ and medium at 434 cm⁻¹ correspond to the torsion τ(PO₂) and to the bending δ(O-P-O) vibrations. The two strong bands observed at 1239 cm⁻¹ and 853 cm⁻¹ are assigned to δ(P-O-H) in plane bending and γ(P-O-H) out of plane bending modes [21] .

The presence of a strong band at 1689 cm⁻¹ is assigned to the stretching vibration modes of C=O groups. Abroad band extending from 3300 cm⁻¹ to 1966 cm⁻¹ is observed in the IR spectrum. This broad band must be due the symmetric and asymmetric stretching modes of NH₃, NH₂, CH and OH. The stretching and bending modes of groups appears via the medium band at 3127 cm⁻¹ and the strong band at 1554 cm⁻¹ respectively. The presence of the strong band at 1619 cm⁻¹ correspond to the stretching vibration modes of C=C groups.

Crystallographic data for the structural analysis have been deposited at the Cambridge Crystallographic Data Centre, CCDC No. 1419172. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 IEZ, UK (fax: +44-1226-336033; e-mail: deposit@ccdc.cam).