4-Hydroxyanilinium 2-carboxyacetate

In the title compound, C6H8NO+·C3H3O4 −, the amino N atom is protonated, and one of the carboxyl groups is deprotonated to maintain the charge balance. In the crystal, classical N—H⋯O and O—H⋯O hydrogen bonds connect the ions into a two-dimensional network parallel to the ac plane. In addition, the structure is further stabilized by C—H⋯O and π–π interactions [centroid–centroid distance = 4.115 (2) Å].

In the title compound, C 6 H 8 NO + ÁC 3 H 3 O 4 À , the amino N atom is protonated, and one of the carboxyl groups is deprotonated to maintain the charge balance. In the crystal, classical N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds connect the ions into a two-dimensional network parallel to the ac plane. In addition, the structure is further stabilized by C-HÁ Á ÁO andinteractions [centroid-centroid distance = 4.115 (2) Å ].

Comment
Simple organic salts containing strong intrermolecular H-bonds have attracted an attention as materials which display ferroelectric-paraelectric phase transitions Huang et al., 1999;Zhang et al., 2001). With the purpose of obtaining phase transition crystals of organic salts, various organic molecules have been studied and a series of new crystal materials have been elaborated (Wang et al., 2002;Xue et al., 2002;Ye et al. ,2008). Herewith, we present the synthesis and crystal structure of the title compound, 4-hydroxyanilinium 2-carboxyacetate.
The crystal packing was further stabilized by aromatic π-π interactions between the benzene rings of the neighbouring cations with the Cg···Cg distances of 4.115 (2)Å (Cg is the centroide of the benzene ring) ( Fig. 2 and Table 1). Symmetry

Experimental
The malonic acid (10 mmol), 4-aminophenol (10 mmol) and ethanol (50 mL) were added into a 100 mL flask. The mixture was stirred at 333 K for 2 h, and then the precipitate was filtrated out. Colourless crystals suitable for X-ray diffraction were obtained by slow evaporation of the solution.

Refinement
All the H atoms attached to C atoms were placed into the idealized positions and treated as riding with C-H = 0.93Å

Computing details
Data collection: CrystalClear (Rigaku, 2005); cell refinement: CrystalClear (Rigaku, 2005); data reduction: CrystalClear (Rigaku, 2005); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure:  A view of the asymmetric unit with the atomic numbering scheme. The displacement ellipsoids were drawn at the 30% probability level. H atoms are presented as a small spheres of arbitrary radius.

Figure 2
The crystal packing of the title compound viewed along the a axis showing the H-bonding and π-π interactions (dotted line). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.31 e Å −3 Δρ min = −0.40 e Å −3 Special details Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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.