4-Aminopyridinium hydrogen succinate

In the title salt, C5H7N2 +·C4H5O4 −, the asymmetric unit comprises an aminopyridinium cation and a hydrogen succinate anion as protonation of the aromatic N atom of the 4-aminopyridine molecule has occurred. The crystal packing is stabilized by intermolecular O—H⋯O and N—H⋯O hydrogen bonds that lead to a two-dimensional array. Short C—H⋯O contacts are also present.


4-Aminopyridinium hydrogen succinate
H.-K. Fun, J. John, S. R. Jebas and T. Balasubramanian Comment 4-Aminopyridine (Fampridine) is used clinically in Lambert-Eaton myasthenic syndrome and multiple sclerosis because by blocking potassium channels it prolongs action potentials thereby increasing transmitter release at the neuromuscular junction (Judge & Bever, (2006); Schwid et al., 1997;Strupp et al., 2004). The structure of 4-aminopyridine has been reported (Chao & Schempp, 1977) as has a redetermination (Anderson et al., 2005). Succinic acid is a dicarboxylic acid and is a precursor for many chemicals of industrial importance (Zeikus et al., 1999;Song & Lee, 2006). Succinic acid derivatives are mostly being used in chemicals, food and pharmaceuticals (Sauer et al., 2008). The crystal structure of succinic acid has also been reported (Gopalan et al., 2000;Leviel et al., 1981). As an extension of our systematic study of hydrogen bonding patterns of 4-aminopyridine with carboxylic acids, the title compound (I) has been synthesized and the crystal structure determined.
The asymmetric unit of (I) (Fig. 1) contains a 4-aminopyridinium cation and a succinic acetate anion, indicating that proton transfer occurred during the co-crystallisation experiment. Protonation leads to the widening of C2-N2-C3 angle in the pyridine ring to 120.7 (2)°, compared to 115.25 (13)° in 4-aminopyridine (Anderson et al., 2005). This type of protonation has been observed in various 4-aminopyridine acid complexes (Bhattacharya et al., 1994;Karle et al., 2003). Otherwise, the bond lengths and bond angles in 4-aminopyridinium cation are comparable to the values reported earlier for 4-aminopyridine (Chao & Schempp, 1977;Anderson et al., 2005). The 4-aminopyridine ring is essentially planar with the maximum deviation from planarity being -0.011 (3) Å for atom C5. The bond lengths and bond angles of the succinic acetate are found to have normal values (Gopalan et al., 2000;Leviel et al., 1981).
The crystal packing is consolidated by O-H···O and N-H···O intermolecular hydrogen bonds (Table 1) supported by C-H···O contacts. An intramolecular N-H···O hydrogen bond stabilises the conformation of the molecule. The molecules aggregate to form a 2-D array parallel to the ab-plane (Fig. 2).

Experimental
Equimolar quantities of 4-aminopyridine (0.094 g, 1 mmol) and succinic acid (0.118 g, 1 mmol) were dissolved in ethanol (10 ml) and water (10 ml), respectively. The aqueous solution of succinic acid was added drop wise to the solution of 4-aminopyridine and stirred well for 4 h. The solution is refluxed at 343°K for 6 h. Colourless crystals were harvested after one month of solvent evaporation.

Refinement
The N-bound H atoms were located from the Fourier map and are allowed to refine freely (N-H = 0.85 -0.94 (3) Å). The O-bound H atom was located from the Fourier map and fixed in that position, with O-H = 1.09 Å, and allowed to refine with U iso (H) = 1.2U eq (O). All other H atoms were placed in calculated positions, with C-H = 0.93 -0.97 Å, and refined using a riding model with U iso (H) = 1.2U eq (C). Fig. 1. The molecular structure of (I), showing 50% probability displacement ellipsoids and the atom numbering scheme. The dashed line indicates hydrogen bonding.