2-[(Pyridin-2-yl)amino]pyridinium 2,4,6-trinitrophenolate

In the cation of the title salt, C10H10N3 +·C6H2N3O7 −, the pyridine and pyridinium rings are linked by an intramolecular N—H⋯N hydrogen bond and are approximately coplanar, with a dihedral angle between their planes of 4.24 (6)°. In the crystal, the cations and anions are linked through N—H⋯O hydrogen bonds, forming supramolecular chains propagating along the c-axis direction. π–π stacking is observed between neighbouring chains, the centroid–centroid distances being 3.7638 (11) (between pyridinium rings) and 3.5331 (11) Å (between benzene rings).

In the cation of the title salt, C 10 H 10 N 3 + ÁC 6 H 2 N 3 O 7 À , the pyridine and pyridinium rings are linked by an intramolecular N-HÁ Á ÁN hydrogen bond and are approximately coplanar, with a dihedral angle between their planes of 4.24 (6) . In the crystal, the cations and anions are linked through N-HÁ Á ÁO hydrogen bonds, forming supramolecular chains propagating along the c-axis direction.stacking is observed between neighbouring chains, the centroid-centroid distances being 3.7638 (11) (between pyridinium rings) and 3.5331 (11) Å (between benzene rings).   Table 1 Hydrogen-bond geometry (Å , ).

. Synthesis and crystallization
The 2,2′-dipyridylamine (1.71 g, 0.01 mol) was added to picric acid (2.29 g, 0.01 mol) dissolved in ethanol (50ml). The obtained mixture was stirred for 24 hours, then this solution was left at room temperature for crystallization in the air.
The crystals grown within 24 hours were separated, washed with ethanol and air-dried.

Refinement
The NH hydrogen atoms were located from the difference Fourier map and refined isotropically. Other hydrogen atoms were positioned geometrically and were also constrained to ride on their parent atoms, with C-H = 0.95 Å, and U iso (H) = 1.2U eq (C).

Results and discussion
Picric acid is a common reagent widely used for isolation in crystalline state of various organic compounds, in particular, amines and alkaloids (Shriner et al., 1980). In laboratorial practice it is also used in analysis of amines and polycyclic hydrocarbons (In et al., 1997;Zaderenko et al., 1997).
In the present communication we report the molecular structure of 2-(2-pyridylamino)pyridinium picrate. The crystal structure of the title compound C 10 H 10 N 3 + ·C 6 H 2 N 3 O 7is ionic and consists of the protonated cation C 10 H 10 N 3 + and the deprotonated picrate anion C 6 H 2 N 3 O 7 -. The cation is approximately planar. The dihedral angle between two heterocyclic rings is 4.24 (6)  The molecular structure of the title compound, with atom labelling. Displacement ellipsoids are drawn at the 50% probability level.