4-Aminopyridinium 4-nitrobenzoate 4-nitrobenzoic acid

The asymmetric unit of the title compound, C5H7N2 +·C7H4NO4 −·C7H5NO4, consists of an aminopyridinium cation, a 4-nitrobenzoate anion and a neutral 4-nitrobenzoic acid molecule. The pyridine ring forms dihedral angles of 64.70 (5)° and 70.37 (5)°, respectively, with the benzene rings of 4-nitrobenzoic acid and 4-nitrobenzoate. In the crystal structure, the cations, anions and the neutral 4-nitrobenzoic acid molecules are linked by O—H⋯O and N—H⋯O hydrogen bonds, forming a two-dimensional network parallel to (001). Adjacent networks are cross-linked via C—H⋯O hydrogen bonds and π–π stacking interactions [centroid–centroid distances 3.6339 (6) and 3.6566 (6) Å].


4-Aminopyridinium 4-nitrobenzoate 4-nitrobenzoic acid
C. K. Quah, S. R. Jebas and H.-K. Fun Comment 4-Aminopyridine (Fampridine) is used clinically in Lambert-Eaton myasthenic syndrome and multiple sclerosis because by blocking potassium channels, it prolongs the action potentials thereby increasing transmitter release at the neuromuscular junction (Judge et al., 2006;Schwid et al., 1997;Strupp et al., 2004). The crystal structure of 4-aminopyridine has been reported (Chao & Schempp, 1977;Anderson et al., 2005). As an extension of our systematic study of hydrogen bonding patterns of 4-aminopyridine with aromatic carboxylic acids, we report here the crystal structure of the title compound.
The asymmetric unit of the title compound contains one 4-aminopyridinium cation, one 4-nitrobenzoate anion and one 4-nitrobenzoic acid molecule. A proton transfer from the carboxyl group of 4-nitrobenzoic acid to atom N2 of 4-aminopyridine resulted in the formation of ions. This lead to the widening of C8-N2-C12 angle of the pyridine ring to 120.86 (9)°, compared to 115.25 (13)° in the unprotonated 4-aminopyridine (Anderson et al., 2005). This type of protonation is observed in various 4-aminopyridine acid complexes (Bhattacharya et al., 1994;Karle et al., 2003). The bond lengths and angles of the 4-aminopyridne are comparable to the values reported earlier for 4-aminopyridine (Chao & Schempp, 1977;Anderson et al., 2005). The bond lengths and angles of the 4-nitrobenzoic acid is found to be normal (Andrau & White, 2003).
Experimental 4-Aminopyridine and 4-nitrobenzoic acid were mixed in equimolar ratio in methanol and warmed in a water bath for 2 h.
Colourless single crystals were obtained after a week on slow evaporation.

Refinement
Atom H1N2 was located from a difference map and was refined with the N-H distance restrained to 0.85 (1)

Special details
Experimental. The data was collected with the Oxford Cyrosystem Cobra low-temperature attachment. 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 Rfactors(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.