Pyridine-4-carbaldehyde–fumaric acid (2/1)

In the title co-crystal, 2C6H5NO·C4H4O4, two crystallographically different hydrogen-bonded trimers are formed, one in which the components occupy general positions, and one generated by an inversion centre. This results in the uncommon situation of Z = 3 for a triclinic crystal. In the formula units, molecules are linked by O—H⋯N hydrogen bonds.

In the title co-crystal, 2C 6 H 5 NOÁC 4 H 4 O 4 , two crystallographically different hydrogen-bonded trimers are formed, one in which the components occupy general positions, and one generated by an inversion centre. This results in the uncommon situation of Z = 3 for a triclinic crystal. In the formula units, molecules are linked by O-HÁ Á ÁN hydrogen bonds.

Comment
The co-crystallization process is widely used to obtain new solid forms of active pharmaceutical ingredients (API) with enhanced physiochemical properties such as stability, dissolution rate, and solubility without altering their pharmacological behavior (Aakeroy et al. 2006;Desiraju, 2003). The pyridine-4-carboxaldehyde and fumaric acid are widely used in the biological and medicinal fields. Pyridine-4-carboxaldehyde is used as a starting material for the preparation of cytokine suppressive drugs to treat arthritis (Boehm et al. 1996). Fumaric acid is of interest since it is known to form supramolecular assemblies with N-aromatic bases (Batchelor et al.2000) and is generally regarded as safe In the trimer, the neutral entities are held together via two (COOH) H···N (pyridine) hydrogen-bonds forming a complementary ADA array. The slightly corrugated aggregates are packed in stacks as shown in Fig. 2. The crystal packing is further stabilized by the weak C-H···O intermolecular interactions with participation of carbonyl oxygen atoms.

Experimental
Pyridine-4-carboxaldehyde (19.3 µL, 0.20 mmol) was dissolved in 5 ml of ethanol. To this solution was added fumaric acid (0.012 g, 0.10 mmol) in 5 mL of ethanol. The resulting solution was heated until the both compounds were dissolved completely and allowed to stand for slow evaporation. White prisms were obtained after 3 days. mp 215-220°C.

Refinement
The hydrogen atoms of carboxylic groups of O5, O7 and O9 were localized in the difference-Fourier map and refined

Figure 2
Crystal packing showing intermolecular hydrogen bonding interactions, view along c axis. 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.