4-tert-Butylpyridinium picrate

In the title compound, C9H14N+·C6H2N3O7 −, the three nitro groups of the picrate anion are twisted out of the plane of the attached benzene ring; the dihedral angles are 32.8 (2), 10.5 (4) and 12.3 (4)°. The pyridinium cations and picrate anions are linked via bifurcated N—H⋯(O,O) hydrogen bonds. The ionic pairs are linked into a ribbon-like structure along [101] by 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 (In et al., 1997;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 owing to non-linear optical properties (NLO) (Shakir et al., 2009). This is due to the aggregation of the donor-acceptor molecules in a non-centrosymmetric manner which contributes to the bulk susceptibility from intermolecular charge transfer (Ashwell et al.,1995;Owen & White, 1976). We report here the crystal structure of the title salt.
The pyridinium ring of the cation (

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

Refinement
The N-bound H atom was located in a difference map and refined freely. C-bound H atoms were positioned geometrically (C-H = 0.93-0.96 Å) and allowed to ride on their parent atoms, with U iso (H) = 1.5U eq (C) for methyl H and 1.2U eq (C) for other H atoms. The displacement ellipsoids for the methyl carbons (C8-C10) of the tert-butyl group are elongated, suggesting possible disorder i.e free rotation of the tert-butyl group. Attempts to model the tert-butyl group as disordered over two sites did not give satisfactory results. Hence the original model was retained.
supplementary materials sup-2 Figures Fig. 1. The asymmetric unit of the title compound. Displacement ellipsoids are drawn at the 50% probability level.

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.