4-Aminopyridinium picrate

In the title compound, C5H7N2 +·C6H2N3O7 −, the 4-aminopyridinium cation is essentially planar (r.m.s. deviation = 0.002 Å). The three nitro groups in the picrate anion are twisted away from the attached benzene ring [dihedral angles = 24.1 (1), 9.3 (3) and 21.4 (1)°]. In the crystal structure, the ions are linked into a three-dimensional network by N—H⋯O and C—H⋯O hydrogen bonds.


Comment
It is well known that picric acid forms charge transfer molecular complexes with a number of aromatic compounds such as aromatic hydrocarbons and amines through electrostatic or hydrogen bonding interactions Zaderenko et al., 1997). The bonding of donor-acceptor picric acid complexes strongly depends on the nature of partners. Some of the picric acid complexes crystallize in centrosymmetric space group though they possess non-linear optical (NLO) properties (Shakir et al., 2009). This is due to the aggregation of the donor and acceptor molecules in a non-centrosymmetric manner which contribute to the bulk susceptibility from intermolecular charge transfer process (Ashwell et al., 1995;Owen & White, 1976).

Experimental
Equimolar solutions of 4-aminopyridine in methanol and picric acid in methanol were mixed together. The solution was stirred well for 1 h and the precipited salt was filtered off. The salt was repeatedly recrystallised from methanol to get single crystals suitable for X-ray analysis.

Refinement
N-bound H atoms were located in a difference map and refined isotropically. C-bound H atoms were positioned geometrically (C-H = 0.93 Å) and allowed to ride on their parent atoms, with U iso (H) = 1.2 U eq (C). Fig. 1

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 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.