Crystal structure of benzene-1,3,5-tricarboxylic acid–4-pyridone (1/3)

A 5:1 mixture of 4-hydroxypyridine with benzene 1,3,5-tricarboxylic acid in methanol yields the title hydrogen-bonded framework compound. This compound crystallizes in the orthorhombic space group Pna21 and is a polymorph of the same stoichiometric species, reported in Cc.


Structural commentary
The dihedral angles formed by the carboxylic acid moieties with respect to the benzene ring are 2.95 (16), 6.23 (10) and 10.28 (18) . These are comparable with those for the previously reported polymorph of this compound [3.9 (2), 9.3 (2), and 13.3 (2) ; Campos-Gaxiola et al., 2014]. It should be noted that the 4-hydroxypyridine has undergone rearrangement from a hydroxypyridine to the pyridone form of the molecule as previously observed (Tyl et al., 2008). The 4-pyridone C-O bond distances range from 1.280 (8) to 1.295 (8) Å . These distances are comparable with previously reported examples of this molecule (Staun & Oliver, 2012;Tyl et al., 2008). Inspection of the bond distances about each pyridone ring shows a slight tendency for the C-C bonds to the nitrogen [1.347 (12) to 1.371 (11) Å ] to be shorter than those to the carbonyl carbon [1.410 (11) to 1.421 (10) Å ]. This supports the proposed formal, localized double bond along the 'edges' of the pyridone ring.
Two of the three 4-pyridone rings are co-planar with the benzene tricarboxylic acid moiety, similar to that of the previously reported structure (Campos-Gaxiola et al., 2014). The remaining 4-pyridone is essentially perpendicular to this plane, also similar to the Campos-Gaxiola structure (Table 1).
three molecules, the resulting architecture is a three-dimensional hydrogen-bonded network. The BTC, N1 and N2 pyridone molecules form a graph-set R 6 8 (44) ring that is parallel with the ab plane (Macrae et al., 2008). This corresponds with that observed by Campos-Gaxiola et al. The BTC and N3 pyridone form an R 5 5 (30) ring that is perpendicular to the previous ring. Further inspection of this network reveals that there are two independent, interpenetrating networks (Fig. 2). The BTC molecules in the two networks form typical slipped --stacks [C g Á Á ÁC g = 3.592 (5) Å , C g Á Á Áperp = 3.302 (4) Å ; C g represents the center of gravity of the ring, perp is the shortest perpendicular distance; Spek, 2009]. Other potentialcontacts are beyond 4 Å . Due to the efficient packing of these molecules there is a significant number of close C-HÁ Á ÁO contacts, primarily between pyridone carbon atoms and carboxylic acid oxygen atoms, with one notable example being a contact from C9 to O3 v [symmetry code:

Comparison with the structure of the monoclinic polymorph
Inspection of an overlay of the two structures reveals some differences between the two polymorphs ( Fig. 3). The orientation of the carboxylic acid groups of the BTC in the title compound has one 'reversed' with respect to the others, while the Campos-Gaxiola structure has all three oriented in the same direction, forming a propeller-like motif about the BTC. This results in a change in the hydrogen-bonding motif, reversing the orientations of the pyridone moieties. Perhaps the most prominent structural change is the orientation of the pyridone perpendicular to the plane of the BTC. In the title compound the pyridone rings are oriented with planes that are parallel to each other along the channels they occupy and are related by the screw axis parallel to the c axis. The perpendicular pyridone rings in the Campos-Gaxiola structure alternate their orientation along the channel, related by the cglide. The change in hydrogen-bonding directionality is propagated to the orientation of the N1 and N2 pyridone chains. Examining the orientation of the carbonyl of the pyridone in these two chains reveals that the Campos-Gaxiola structure has the N1 and N2 chains oriented with the carbonyl Space-filling views displaying the interpenetrating networks (a) along the a axis; (b) along the c axis.

Figure 3
Overlay of the title compound (red) with the Campos-Gaxiola (light green) structure. The BTC moiety is used as the target for overlay. The view is along the c axis of both structures. Non-H atoms depicted as arbitrary spheres, H atoms as short sticks.
along the a-axis forming a 'parallel' alignment of the adjacent pyridone chains; again the c-glide is the cause for this arrangement. The N1 and N2 chains in the title compound adopt an 'anti-parallel' orientation with carbonyls in one chain being oriented in the opposite direction to the next chain, again a function of the screw axis. This is highlighted in Fig. 3 with the pyridone chain on the left of the figure showing an overlap of the pyridone rings between the two structures and the chain on the right of the figure showing the opposite orientation of the pyridone rings.

Synthesis and crystallization
The compound was formed by dissolving 4-hydroxypyridine (0.112 g, 1.18 mmol) in methanol (3 mL) and benzene 1,3,5tricarboxylic acid (0.052 g, 0.24 mmol) in methanol (3 mL). The two solutions were combined and allowed to evaporate over 5 d yielding crystals suitable for diffraction studies. The crystallization process yields crystals of both the previously reported 1:1 co-crystal (Staun & Oliver, 2015) and those of the title compound. Presumably the differences in solvent composition and time for crystallization can yield one polymorph over the other. Several crystallization attempts were made using the methodology described herein (slow evaporation from methanol) and all yielded mixtures of the 1:1 and the 3:1 co-crystals reported herein. No evidence of the Campos-Gaxiola structure was observed within the crystals examined (reported as colorless rectangular prisms).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Where possible, hydrogen atoms were initially located from a difference Fourier map and were subsequently refined using a riding model with C-H = 0.95 Å , N-H = 0.88 Å and O-H = 0.84 Å . U iso (H) was set to 1.2U eq (C/N) and 1.5U eq (O). The reliability for the correct enantiomorph of the space group is low, due to the use of Mo K radiation with a light atom structure. Analysis of the Flack x [0.1 (10); Flack, 1983], Hooft y [0.2 (10); Hooft et al., 2008] and Parsons z [À0.2 (12); Parsons et al., 2013] parameters tends to indicate that the correct enantiomorph of the space group and absolute structure has been determined (Flack & Bernardinelli, 1999). Since these values are not close to zero the model could be refined as a racemic twin. However, this does not yield new or useful information and we have retained the standard model.   program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015); molecular graphics: OLEX2 (Dolomanov et al., 2009) and Mercury (Macrae et al., 2008); software used to prepare material for publication: publCIF (Westrip, 2010).

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.