Glycine–phthalic acid (1/1)

In the title compound, C2H5NO2·C8H6O4, the glycine molecule exists as a zwitterion (2-azaniumylethanoate) with a positively charged amino group and a negatively charged carboxylate group. In the crystal, N—H⋯O and O—H⋯O hydrogen bonds link the components into layers parallel to the ab plane. The central part of each layer is composed of hydrogen-bonded glycine zwitterions, while phthalic acid molecules interact with the zwitterions in such a way that benzene rings protrude up and down from the layer.

In the title compound, C 2 H 5 NO 2 ÁC 8 H 6 O 4 , the glycine molecule exists as a zwitterion (2-azaniumylethanoate) with a positively charged amino group and a negatively charged carboxylate group. In the crystal, N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds link the components into layers parallel to the ab plane. The central part of each layer is composed of hydrogen-bonded glycine zwitterions, while phthalic acid molecules interact with the zwitterions in such a way that benzene rings protrude up and down from the layer.
TB thanks the University Grants Commission (UGC) for the award of a Research Fellowship under the Faculty Improvement Programme (FIP). We are grateful to Professor Helen Stoeckli-Evans, University of Neuchâ tel, Switzerland, for measuring the X-ray diffraction data. ST thanks the management of SASTRA University for their encouragement.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: CV5360).

Comment
As part of our studies on amino acids and carboxylic acids interactions (Sharma et al. 2006;Selvaraj et al., 2007), we report here the crystal structure of the title cocrystal of glycine and phthalic acid, (I).
The asymmetric unit of (I) contains one glycine molecule and one phthalic acid molecule (Fig. 1). The glycine molecule exists as a zwitterion with a positively charged amino group and a negatively charged carboxylate group as found in glycine-trimesic acid complex (Herbstein et al., 1981) and glycine-glutaric acid cocrystal (Losev et al., 2011), where glutaric acid exists as a neutral molecule. The phthalic acid exists as a neutral molecule with both carboxylic acid groups being unionized. The stoichiometry between the glycine and phthalic acid is 1:1.
The crystal packing is stabilized by a network of N-H···O and O-H···O hydrogen bonds (Table 1). As illustrated in In (I), the zwitterionic glycine has one donor atom capable of forming three hydrogen bonds, and one of them forms bifurcated hydrogen bonds, while neutral phthalic acid can also forms three hydrogen bonds through two acceptors (Table   1). In the crystal structure, the zwitterionic glycines are arranged in linear arrays along [010] direction. In each array, adjacent glycines are connected by a N1···O2 hydrogen bond which can be described as a head-to-tail sequence having a graph-set motif of C5 (Bernstein et al., 1995) (Fig. 3). In contrast to (I), no head-to-tail sequence was observed in glycine-glutaric acid cocrystal (Losev et al., 2011). As observed in many binary complexes of amino acids complexed with carboxylic acids, the neutral molecules in the complex do not interact among themselves. However, here, phthalic acid molecule is interconnected by zwitterionic glycines via two intermolecular N1···O3 hydrogen bonds. The glycine amino group acts as donor for 1-substituted carboxylic O3 atoms of the phthalic acid molecules emanating from different phthalic acids layers. Another carboxylic O5 atom acts as acceptor for an intermolecular hydrogen bond with the amino group of a glycine. The 2-substituted carboxylic group of the phthalic acid molecules in two different layers are interconnected by glycines. One carboxylic group in one layer interacts with the glycine in one layer, while its symmetryrelated equivalents in the adjacent layers interacts with the glycine in the neighbouring layer [C 1 2 (4) graph-set motif]. The donor atoms (O4 and O6) of the phthalic acid molecule participate in intermolecular short and linear O-H···O hydrogen bonds with the carboxylate group of glycine. These hydrogen bonds produce C 2 2 (11) chains that run parallel to the a axis.

Experimental
The title complex was prepared by dissolving glycine and phthalic acid in a stoichiometric ratio in double distilled water.
The resulting solution was heated to ca 50° C and the title cocrystal was obtained by a slow cooling method from an aqueous solution.

Refinement
The H-atoms bound to nitrogen and oxygen were located from difference electron density maps and isotropically refined.
All the remaining H atoms were placed in geometrically idealized positions (C-H = 0.95-0.99 Å) and constrained to ride on their parent atoms, with U iso (H) = 1.2U eq (C).

Figure 1
A content of asymmetric unit of (I) showing the atomic-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level.   where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.22 e Å −3 Δρ min = −0.19 e Å −3 Extinction correction: SHELXL97 (Sheldrick, 2008), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.0086 (15) 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. 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 > 2sigma(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.