Crystal structures of two 1:2 dihydrate compounds of chloranilic acid with 2-carboxypyridine and 2-carboxyquinoline

Crystal structures of hydrogen-bonded 1:2 dihydrate compounds of chloranilic acid with 2-carboxypyridine (I) and 2-carboxyquinoline (II) have been determined at 180 and 200 K, respectively. The base molecule in (I) is disordered over cationic and twitterionic states, while that in (II) is in a twitterionic form. In each crystal, the three components are linked by O—H⋯O and N—H⋯O hydrogen bonds, forming a layer structure.


Chemical context
Chloranilic acid, a dibasic acid with hydrogen-bond donor as well as acceptor groups, appears particularly attractive as a template for generating tightly bound self-assemblies with various pyridine derivatives as well as being a model compound for investigating hydrogen-transfer motions in O-HÁ Á ÁN and N-HÁ Á ÁO hydrogen-bond systems (Zaman et al., 2004;Molčanov & Kojić-Prodić, 2010;Seliger et al., 2009;Asaji et al. 2010). Previously, we have prepared three 1:1 compounds of chloranilic acid with 2-, 3-and 4-carboxypyridine and analysed the crystal structures in order to extend our study on D-HÁ Á ÁA hydrogen bonding (D = N, O or C; A = N, O or Cl) in chloranilic acid-substituted pyridine systems (Gotoh et al., 2006Tabuchi et al., 2005). In the present study, we have prepared a 1:2 compound of chloranilic acid with 2-carboxypyridine and also redetermined the structure of a 1:2 compound of chloranilic acid with 2-carboxyquinoline with higher precision than previously reported structure [Marfo-Owusu & Thompson, 2014; although the title and text in this reference refer to the 1:1 adduct of chloranilic acid with 2-carboxyqulinone, the reported structure is the 1:2 compound, the same as the present compound (II)]. The crystal structure of the anhydrous 1:2 compound of chloranilic acid with 2-carboxyquinoline was also reported by Marfo-Owusu & Thompson (2016).

Structural commentary
Compound (I) (Fig. 1) crystallizes with one-half of a chloranilic acid molecule, which is located on an inversion centre, one 2-carboxypyridine molecule and one water molecule in the asymmetric unit. In the crystal, the water molecule is disordered over two sites with equal occupancies of 0.5. The occupancies of the H atoms in the chloranilic acid molecule and the carboxy group of the 2-carboxypyridine molecule are also 0.5. The compound is, therefore, considered to be a disordered state over two forms, viz. bis(2-carboxypyridinium) chloranilate dihydrate, (A), and bis(pyridinium-2-carboxylate) chloranilic acid dihydrate, (B), as shown in the scheme and Fig. 2. In form (A), the water molecule acts as one N-HÁ Á ÁO hydrogen-bond acceptor and two O-HÁ Á ÁO hydrogen-bond donors (N1-H1Á Á ÁO5A, O5A-O9AÁ Á ÁO4 ii and O5A-H10AÁ Á ÁO2; symmetry code as in Table 1), while in form (B), the water molecule acts as the acceptor of N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds, and as two O-HÁ Á ÁO hydrogen-bond donors (N1-H1Á Á ÁO5B, O2-H2Á Á ÁO5B, O5B-H9BÁ Á ÁO4 ii and O5B-H10BÁ Á ÁO1 iii ; Table 1). The dihedral angle between the pyridine ring and the carboxy plane in the base molecule is 23.32 (15) .
The asymmetric unit of compound (II) consists of one-half of a chloranilic acid molecule, which is located on an inversion centre, one 2-carboxyquinoline molecule and one water molecule. In the crystal, the 2-carboxyquinoline molecule is in a

Figure 1
The molecular structure of compound (I), showing the atom-numbering scheme. Displacement ellipsoids of non-H atoms are drawn at the 50% probability level and H atoms are drawn as small spheres of arbitrary radii. The water molecule is disordered over two sites with equally occupancies. Atoms H2 and H3 have site-occupancy factors of 0.5.

Database survey
A search of the Cambridge Structural Database (Version 5.38, last update May 2017; Groom et al., 2016) for organic cocrystals of pyridinium-2-carboxylate (twitterionic form) gave six structures. For organic co-crystals of quinolinium-2carboxylate (twitterionic form), eight structures were found.

Synthesis and crystallization
Single crystals of compound (I) were obtained by slow evaporation of an acetonitrile solution (200 ml) of chloranilic acid (250 mg) with 2-carboxypridine (310 mg) at room temperature. Single crystals of compound (II) were obtained by slow evaporation from a methanol solution (150 ml) of chloranilic acid (310 mg) with 2-carboxyquinoline (520 mg) at room temperature.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The water molecule in compound (I) was found to be disordered over two sites in a difference-Fourier map. The occupancies were refined to 0.52 (2) and 0.48 (2) and then they were fixed at 0.5. The H atom in the carboxy group of the base molecule was also found in a  Computer programs: RAPID-AUTO (Rigaku, 2006), SHELXS97 (Sheldrick, 2008), SHELXL2016 (Sheldrick, 2015), ORTEP-3 for Windows (Farrugia, 2012), CrystalStructure (Rigaku, 2010) and PLATON (Spek, 2015). difference-Fourier map to be disordered between the adjacent carboxy groups, which are related by an inversion centre, and the occupancy was set to be 0.5. Since the N-bound H atom refined reasonably with an occupancy of 1, the occupancy of the H atom of the acid molecule was set to be 0.5 to balance the total charge of the compound. All other H atoms were found in a difference-Fourier map. The N-bound H atom was refined freely, while the positions of O-bound H atoms were refined, with O-H = 0.84 (2) Å and U iso (H) = 1.5U eq (O). For the water H atoms, distant restraints of HÁ Á ÁH = 1.37 (4) Å were also applied. C-bound H atoms were positioned geometrically (C-H = 0.95 Å ) and were treated as riding with U iso (H) = 1.2U eq (C). All H atoms in compound (II) were found in a difference- For both structures, data collection: RAPID-AUTO (Rigaku, 2006); cell refinement: RAPID-AUTO (Rigaku, 2006); data reduction: RAPID-AUTO (Rigaku, 2006); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2016 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: CrystalStructure (Rigaku, 2010) and PLATON (Spek, 2015).

Bis(2-carboxypyridinium) chloranilate dihydrate-bis(pyridinium-2-carboxylate) chloranilic acid dihydrate (1/1) (I)
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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (   where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.52 e Å −3 Δρ min = −0.45 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.