Crystal structures of 2-aminopyridine citric acid salts: C5H7N2 +·C6H7O7 − and 3C5H7N2 +·C6H5O7 3−

2-Aminopyridine and citric acid mixed in 1:1 and 3:1 ratios in ethanol yielded crystals of two 2-aminopyridine citric acid salts, viz. C5H7N2 +·C6H7O7 − (I) and 3C5H7N2 +·C6H5O7 3− (II). Salt I is formed by the protonation of the pyridine N atom and deprotonation of the central carboxylic group of the acid, while in II all three carboxylic groups of the acid are deprotonated and the charges are compensated for by three 2-aminopyridinium cations.

2-Aminopyridine and citric acid mixed in 1:1 and 3:1 ratios in ethanol yielded crystals of two 2-aminopyridinium citrate salts, viz. C 5 H 7 N 2 + ÁC 6 H 7 O 7 À (I) (systematic name: 2-aminopyridin-1-ium 3-carboxy-2-carboxymethyl-2-hydroxypropanoate), and 3C 5 H 7 N 2 + ÁC 6 H 5 O 7 3À (II) [systematic name: tris(2aminopyridin-1-ium) 2-hydroxypropane-1,2,3-tricarboxylate]. The supramolecular synthons present are analysed and their effect upon the crystal packing is presented in the context of crystal engineering. Salt I is formed by the protonation of the pyridine N atom and deprotonation of the central carboxylic group of citric acid, while in II all three carboxylic groups of the acid are deprotonated and the charges are compensated for by three 2-aminopyridinium cations. In both structures, a complex supramolecular three-dimensional architecture is formed. In I, the supramolecular aggregation results from N amino -HÁ Á ÁO acid , O acid Á Á ÁH-O acid , O alcohol -HÁ Á ÁO acid , N amino -HÁ Á ÁO alcohol , N py -HÁ Á ÁO alcohol and C ar -HÁ Á ÁO acid interactions. The molecular conformation of the citrate ion (CA 3À ) in II is stabilized by an intramolecular O alcohol -HÁ Á ÁO acid hydrogen bond that encloses an S(6) ring motif. The complex threedimensional structure of II features N amino -HÁ Á ÁO acid , N py -HÁ Á ÁO acid and several C ar -HÁ Á ÁO acid hydrogen bonds. In the crystal of I, the common chargeassisted 2-aminopyridinium-carboxylate heterosynthon exhibited in many 2-aminopyridinium carboxylates is not observed, instead chains of N-HÁ Á ÁO hydrogen bonds and hetero O-HÁ Á ÁO dimers are formed. In the crystal of II, the 2-aminopyridinium-carboxylate heterosynthon is sustained, while hetero O-HÁ Á ÁO dimers are not observed. The crystal structures of both salts display a variety of hydrogen bonds as almost all of the hydrogen-bond donors and acceptors present are involved in hydrogen bonding.

Chemical context
Systematic structural and statistical analysis focusing on the identification of robust supramolecular synthons or patterns are essential for crystal engineering and the design of new solid-state structures with desired properties. Organic crystals, especially salts, are now considered as potential materials for optical applications because of their flexibility in molecular design (Jayanalina et al., 2015a), thermal stability and delocalized clouds of electrons (Jayanalina et al., 2015b). An analysis of the Cambridge Structural Database (Groom et al., 2016) by Bis & Zaworotko (2005) revealed that 77% of compounds that contain both the 2-aminopyridine and carb-oxylic acid moieties generate 2-aminopyridine-carboxylic acid supramolecular heterosynthons rather than carboxylic acid or 2-aminopyridine supramolecular homosynthons. In the absence of other competing functionalities, the occurrence of heterosynthons increased to 97%. Several salts and co-crystals containing 2-aminopyridine or 2-acetaminopyridine and a carboxylic acid moiety have been reported (Jayanalina et al., 2015a,b;Bis & Zaworotko, 2005;Aakerö y et al., 2006;Jasmine et al., 2015;Jin et al., 2001). In all of these reported structures, the charge-assisted 2-aminopyridinium-carboxylate or neutral 2-acetaminopyridine-carboxylic heterosynthon is observed, as suggested by statistical analysis. Keeping this in mind, the crystal structure analyses of two 2-aminopyridinium citrate salts, C 5 H 7 N 2 + ÁC 6 H 7 O 7 À (I) and 3C 5 H 7 N 2 + ÁC 6 H 5 O 7 3À (II), were undertaken in order to study the packing patterns and identify the supramolecular synthons present in each salt.
In the crystal of II, all of the strong hydrogen-bond donors and acceptors are utilized in a supramolecular association. Full details of the hydrogen-bonding interactions are given in Table 2, and illustrated in Figs. 2, 5 and 6. A number of the C ar -H groups are also involved in C-HÁ Á ÁO hydrogen bonds (Table 2) (Table 1). Table 2 Hydrogen-bond geometry (Å , ) for II.

Figure 4
A partial view along the b axis of the crystal packing of salt I, illustrating the layer-like structure. Red and blue dashed lines denote the various intermolecular interactions (Table 1).

Database survey
A survey of the Cambridge Structural Database (CSD, Version 5.39, last update May 2018; Groom et al., 2016) revealed 80 organic structures involving a citric acid moiety in the form of solvates/hydrates, salts/salt hydrates and co-crystals. 25 structures among these are salts/salt hydrates of citric acid (deprotonated to different extents) with various organic cations. It is observed that most of the organic citrates appear as their hydrates, with the exception of a few (including I and II). The most common hydrogen bonds observed in these hydrated salts are N amine -HÁ Á ÁO citric , N amine -HÁ Á ÁO water and O water -HÁ Á ÁO citric , forming different supramolecular architectures. In the absence of a water molecule, the most common hydrogen bonds are N amine -HÁ Á ÁO citric and O citric -HÁ Á ÁO citric . However, the nature of these supramolecular synthons varies from one structure to another, depending on the nature of the organic cations.
The crystal structure of 2-amino 5-chloropyridinium-ltartarate (Jayanalina et al., 2015b) shows that despite of the presence of other competing functionalities on the carboxylic acid (two alcoholic OH groups in tartaric acid), the most frequent 2-aminopyridinium-carboxylate heterosynthon is still observed. However, the presence of the alcoholic OH group in citric acid has resulted in a deviation from the regular trend as the charge-assisted 2-aminopyridinium-carboxylate heterosynthon is not observed in I; instead chains of N-HÁ Á ÁO hydrogen bonds and hetero O-HÁ Á ÁO dimers are observed. The 2-aminopyridinium-carboxylate heterosynthon is sustained in the crystal structure of II because of the nonavailability of the alcoholic OH group for intermolecular hydrogen bonding.
Hence, the study of the crystal structure of 2-aminopyridinium citrate, mixed in a 2:1 ratio, would be highly significant in understanding the packing-pattern trends observed in this family of salts. Unfortunately, despite a number of attempts, we have not been able to obtain goodquality single crystals of this salt.

Refinement details
Crystal data, data collection and structure refinement details are summarized in For both structures, data collection: APEX2 (Bruker, 2009); cell refinement: APEX2 and SAINT-Plus (Bruker, 2009); data reduction: SAINT-Plus and XPREP (Bruker, 2009); program(s) used to solve structure: SHELXT2016 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2016 (Sheldrick, 2015b); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL2016 (Sheldrick, 2015b). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.29 e Å −3 Δρ min = −0.30 e Å −3 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.