Benzene-1,3,5-tricarboxylic acid–pyridinium-2-olate (1/3)

The asymmetric unit of the title compound, C9H6O6·3C5H5NO, contains one benzene-1,3,5-tricarboxylic acid molecule (BTA) and three pyridin-2-ol molecules each present in the zwitterion form. In the crystal, these entities are linked through O—H⋯O− and N+—H⋯O− hydrogen bonds, forming sheets parallel to (10-1). These layers contain macrocyclic rings of composition [BTA]2[pyol]6 and with graph-set notation R 6 8(44), which are stacked along c through π–π interactions [inter-centroid distances = 3.536 (2)–3.948 (3) Å]. They are interconnected by N+—H⋯O− hydrogen-bonded chains of pyridin-2-ol molecules running parallel to c, forming a three-dimensional network. There are also C—H⋯O hydrogen bonds present which reinforce the three-dimensional structure.

This work was supported financially by the Universidad Autó noma de Sinaloa (PROFAPI 2012/048). The authors are also grateful to the Autonomous State University of Morelos (CIQ-UAEM) for access to the X-ray diffraction facilities of the Chemical Research Center.

Comment
The engineering and design of novel materials via non-covalent synthesis has developed as a very attractive and potential area of research because of its importance in molecular recognition (Aakeröy et al., 2010;Li et al., 2014), pharmaceutical chemistry (Karki et al., 2009) and materials chemistry (Yan et al., 2012). Aromatic carboxylic acids form reliable supramolecular synthons for the construction of novel organic networks by hydrogen bonding and π-π interactions (Desiraju, 1995), and numerous studies have focused on hydrogen bonding between carboxylic acids and pyridine derivatives (Bhogala et al., 2005;Shattock et al. 2008;Yu, 2012). Herein, we report on the solid-state structure of a 1:3 co-crystal formed between benzene-1,3,5-tricarboxylic acid and pyridin-2-ol.

Experimental
A solution of 4-hydroxypyridine (0.050 g, 0.525 mmol) and benzene-1,3,5-tricarboxylic acid (0.055 g, 0.262 mmol) in a solvent mixture of THF and DMF (7.5 ml, 2:1, v/v) was stirred for 30 min at room temperature, giving a clear transparent solution. Upon slow evaporation of the solvents during approximately 30 days, yellow crystals were obtained.
Spectroscopic data for the title compound are available in the archived CIF.

Refinement
The C-bound H atoms were positioned geometrically and treated as riding atoms: C-H = 0.95 Å with U iso (H) = 1.2U eq (C). The H atoms bonded to O and N were initially located in a difference Fourier map. They were refined with an X-H distance restraint of 0.840 (1) Å with U iso (H) = 1.5U eq (O,N).  The molecular structure of the title compound, with atom labelling. The displacement ellipsoids are drawn at the 50% probability level.  A view along the a axis of the crystal packing of the title compound, showing the two-dimensional hydrogen bonded sheets parallel to (10-1) (see Table 1 for details).   Table 1 for details; H atoms not involved in hydrogen bonding have been omitted for clarity).

Special details
Experimental. Spectroscopic data for the title compound: IR (KBr, cm -1 ): 3442, 3103, 3050, 1704, 1690, 1624, 1613, 1468, 1282, 1193, 1094, 1024. 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. 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 > σ(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.