Crystal structure and Hirshfeld surface analysis of 2-aminopyridinium hydrogen phthalate

New proton-transfer single crystals of 2-aminopyridinium phthalate were obtained with the aim of synthesizing a new non-linear optical (NLO) material, which is achieved with the help of extensive hydrogen-bonding interactions between the ions.

Aminopyridine and phthalic acid are well known synthons for supramolecular architectures for the synthesis of new materials for optical applications. The 2-aminopyridinium hydrogen phthalate title salt, C 5 H 7 N 2 + ÁC 8 H 5 O 4 À , crystallizes in the non-centrosymmetric space group P2 1 . The nitrogen atom of the -NH 2 group in the cation deviates from the fitted pyridine plane by 0.035 (7) Å . The plane of the pyridinium ring and phenyl ring of the anion are oriented at an angle of 80.5 (3) to each other in the asymmetric unit. The anion features a strong intramolecular O-HÁ Á ÁO hydrogen bond, forming a self-associated S(7) ring motif. The crystal packing is dominated by intermolecular N-HÁ Á ÁO hydrogen bonds leading to the formation of 2 1 helices, with a C(11) chain motif. They propagate along the b axis and enclose R 2 2 (8) ring motifs. The helices are linked by C-HÁ Á ÁO hydrogen bonds, forming layers parallel to the ab plane. Hirshfeld surface analysis and two-dimensional fingerprint plots were used to investigate and quantify the intermolecular interactions in the crystal.

Chemical context
Crystal engineering and the design of supramolecular architectures are of significant interest owing to the technological applications of the resulting materials in the electronics and optical industries. Supramolecular interactions such as chargeassisted hydrogen bonds andinteractions play an important role in crystal engineering as they lead to directional molecular recognition events between molecules or ions, and therefore mediate self-assembly of well-defined supramolecular networks (Guelmami et al., 2007;Prakash et al., 2018;Siva et al., 2017). Amine-based materials are particularly important as they are synthesized by the condensation of the corresponding aldehydes and amines and exhibit strong intermolecular hydrogen bonds between the electronegative acceptor and the N atom of the imine moiety. Pyridinium families are now considered to be potential materials for optical applications because of their flexibility in molecular design, strength and thermal stability, which are derived from delocalized clouds of electrons. Another electronic field of research related to 2-aminopyridinium salts is focused on their optical limiting and frequency-conversion applications (Liu et al., 2015;Siva et al., 2019). The present work is a part of a structural study of new proton-transfer compounds of 2-aminopyridine with phthalic acid and the corresponding hydrogen-bonding interactions. The hydrogen bonding present in the crystal of the title salt was substantiated by Hirshfeld surface analysis.

Structural commentary
The molecular structure of the title salt is shown in Fig. 1. Protonation on the N-atom site of the pyridine ring, atom N11, is confirmed by the elongated C-N bond distances [C11-N11 = 1.341 (8) Å and C15-N11 = 1.357 (9) Å ] and the enlarged C11-N11-C15 bond angle of 122.3 (6) . The nitrogen atom of the -NH 2 group in the cation deviates from the pyridine ring plane (r.m.s. deviation = 0.0062 Å ) by 0.035 (7) Å . The planes of the pyridinium ring of the cation and the phenyl ring of the anion are oriented at a dihedral angle of 80.5 (3) in the asymmetric unit. In the anion the twisting of the carboxyl planes out of the benzene ring is negligible [planes O21/O22/C27 and O23/O24/C28 are inclined to the benzene ring (C21-C26) by 1.3 (8) and 0.7 (7) , respectively], because of the strong O22-H22AÁ Á ÁO23 intramolecular hydrogen bond ( Fig. 1 and Table 1), which makes a self-associated S(7) ring motif.

Supramolecular features
2-Aminopyridine and phthalic acid are known materials for structure-extension properties, which connect the molecules in the supramolecular assembly. These supramolecular synthons are crystallized together not only to study the molecular structure but also the crystal packing via intermolecular interactions. This structure-extension property of the synthon molecules is generally exploited for possible non-centrosymmetric materials, which are desired as they possess many applications. The structure extension of the molecules is possible by linear (chain C motifs) and cyclic (ring R motifs) hydrogen-bonding associations. This was accomplished in the title compound, which exhibits non-linear optical (NLO) properties, because of the extensive intermolecular interactions.
The packing of the ions in the crystal is dominated by N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds (Table 1) The crystal packing of the title salt viewed along the a axis. Hydrogen bonds are shown as dashed lines (Table 1). For clarity, the C-bound H atoms have been omitted.

Figure 1
The asymmetric unit of the title salt, with atom labelling and 50% probability displacement ellipsoids.

Figure 3
Packing of the title salt viewed along the c axis. Hydrogen bonds are shown as dashed lines (Table 1). For clarity, H atoms not involved in hydrogen bonding have been omitted. anion hetero-synthon is formed via two N-HÁ Á ÁO hydrogen bonds (N11-H1NÁ Á ÁO24 i and N12-H12AÁ Á ÁO23 i ), that enclose an R 2 2 (8) ring motif ( Fig. 2 and Table 1). These heterosynthons are linked by a further N-HÁ Á ÁO hydrogen bond (N12-H12BÁ Á ÁO22 ii ), to form 2 1 helices, with a C(11) chain motif, that propagate along the b-axis direction. The helices are linked by C-HÁ Á ÁO hydrogen bonds, forming layers lying parallel to the ab plane ( Fig. 3 and Table 1). There are no significant C-HÁ Á Á orcontacts present in the crystal (PLATON; Spek, 2009).

Hirshfeld surface analysis
The Hirshfeld surface analysis (Spackman & Jayatilaka, 2009) and the associated two-dimensional fingerprint plots (McKinnon et al., 2007) were performed with Crystal-Explorer17 (Turner et al., 2017). The Hirshfeld surface is colour-mapped with the normalized contact distance, d norm , from red (distances shorter than the sum of the van der Waals radii) through white to blue (distances longer than the sum of the van der Waals radii).
The Hirshfeld surface (HS) of the title salt, mapped over d norm in the colour range of À0.7098 to 1.1914 arbitrary units, is given in Fig. 4. The short interatomic contacts, i.e. the donors and acceptors of the hydrogen bonds (Table 1) Hirshfeld surface for the title salt mapped over d norm , in the colour range À0.7098 to 1.1914 au.

Synthesis and crystallization
A 1:1 mixture of 2-aminopyridine and phthalic acid was heated to 313 K and stirred for 1 h before being poured into a petri dish and kept undisturbed for 25 days. Colourless blockshaped single crystals were obtained by the slow evaporation of a methanol and water (v:v = 20:80%) solution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The OH and NH H atoms were located in a difference-Fourier map and refined freely. The    program(s) used to solve structure: SHELXT2014 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: Mercury (Macrae et al., 2008) and PLATON (Spek, 2009); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015b) and PLATON (Spek, 2009).

2-Aminopyridinium hydrogen phthalate
Crystal data 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.