Crystal structure of 4-hydroxypyridin-1-ium 3,5-dicarboxybenzoate

A 1:1.4 molar equivalent of benzene-1,3,5-tricarboxylic acid cocrystallized with 4-hydroxypyridine yields the 4-hydroxypyridin-1-ium 3,5-dicarboxybenzoate salt.


Chemical Context
As a study in crystal engineering utilizing hydrogen bonding between disparate molecules (Desiraju, 2003), we have been investigating the cocrystallization of various pyridine compounds with benzene carboxylic acids (Staun & Oliver, 2012). From previous work, 4-hydroxypyridine undergoes hydrogen migration from the hydroxy O to the pyridine N atom, yielding 4-pyridone (Tyl et al., 2008). We were surprised to find that in the case of 4-hydroxypyridin-1-ium 3,5-dicarboxybenzoate, (I), an H atom is abstracted from one carboxylic acid group, yielding a pyridinium salt. This result allows for the hydroxy O and pyridine N atom to both act as hydrogen-bond donors, rather than the donor/acceptor situation of the 4pyridone species. These two molecules have been incorporated as linker species in metal-organic frameworks (Guo et al., 2011).

Structural Commentary
The structure of (I) shows that the 4-hydroxypyridine has abstracted an H atom from the benzenetricarboxylic acid, yielding a pyridinium cation and a carboxylate anion (Fig. 1). Bond distances about the pyridine ring show some localization of the bonds: C1-C2 and C4-C5 are slightly shorter than the ideal aromatic distance [1.367 (3)
Pertinent features of this extended network are an R 4 4 (28) ring comprised of 3,5-dicarboxybenzoate ions ( Fig. 2) (Bernstein et al., 1995). The carboxylic acid groups are involved in the hydrogen bonding within this ring. There is also an R 6 6 (44) ring of 3,5-dicarboxybenzoate ions, that incorporate a different chain of carboxylic acid groups. These rings are bridged by the 4-hydroxypyridinium cations resulting in the three-dimensional network. The hydrogen bonds within the structure are surprisingly strong, with O-HÁ Á ÁO and N-HÁ Á ÁO distances ranging from 2.533 (2) to 2.700 (2) Å ( Table 1).
The cations and anions form homogeneous -stacked columns parallel to the c axis, that is, 4-pyridinium cations stacking with other cations and 3,5-dicarboxybenzoate anions stacking with other anions. The centroid-to-centroid distances for both the pyridinium and the dicarboxybenzoate interactions are 3.6206 (13) Å , i.e. the c-axis spacing. The centroidto-perpendicular distances are 3.3629 (9) Å for the cation and 3.4372 (9) Å for the anion. Both measurements are within acceptedcontact ranges (see Table 2; Spek, 2009). Labeling scheme for (I). Displacement ellipsoids are depicted at the 50% probability level. The inter-ion hydrogen bond is shown as a dashed red line.

Synthesis and Crystallization
To a solution of benzene-1,3,5-tricarboxylic acid (0.035 g, 1.24 mmol) in MeOH (3 ml) in a 20 ml vial was added a solution of 4-hydroxypyridine (0.0218 g, 1.77 mmol) in MeOH (3 ml). The mixture was shaken vigorously, covered with perforated Parafilm and allowed to evaporate slowly over a period of 5 d, yielding colorless rod-like crystals.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Carboxylic, hydroxy, and pyridinium H atoms were initally located in a difference Fourier map. H atoms on the 4-hydroxypyridinium cation were refined freely. H atoms on the carboxylic acid groups were included with refined coordinates and atomic displacement parameters tied to that of the O atom to which they are bonded. C-H hydrogens were included in idealized positions riding on the C atom to which they are bonded, with C-H distances constrained to 0.95 Å and U iso (H) = 1.2 U eq (C). The compound is achiral, but crystallizes with a noncentrosymmetric, polar space group. The Flack x parameter refined to 0.20 (8), which suggests the possibility of a small amount of inversion twinnning (Parsons et al., 2013), but the strength of the anomalous signal is very weak. We compared both a model twinned by inversion and the untwinned model, and there was no significant difference. We therefore elected to model the structure without inclusion of a twin component.  program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: OLEX2 (Dolomanov et al., 2009), Mercury (Macrae et al., 2008) and POVRay (Cason, 2003); software used to prepare material for publication: publCIF (Westrip, 2010) and PLATON (Spek, 2009).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.