2-Cyano-1-methylpyridinium nitrate

In the title compound, C7H7N2 +·NO3 −, all atoms except the methyl H atoms lie on a crystallographic mirror plane. The interlayer distance, including that between aligned N atoms from alternating cations and anions in adjacent layers, is exceptionally short at 3.055 (1) Å. Two-dimensional C—H⋯O hydrogen-bonded networks link cations to anions, while C—H⋯N interactions link cations within each layer. Anion–π interactions with the cations assist in binding the layers together.

In the title compound, C 7 H 7 N 2 + ÁNO 3 À , all atoms except the methyl H atoms lie on a crystallographic mirror plane. The interlayer distance, including that between aligned N atoms from alternating cations and anions in adjacent layers, is exceptionally short at 3.055 (1) Å . Two-dimensional C-HÁ Á ÁO hydrogen-bonded networks link cations to anions, while C-HÁ Á ÁN interactions link cations within each layer. Anion-interactions with the cations assist in binding the layers together.

Experimental
Data collection: APEX2 (Bruker, 2010); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT and CELL_NOW (Sheldrick, 2008b); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008a); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008a); molecular graphics: SHELXTL (Sheldrick, 2008a); software used to prepare material for publication: SHELXTL. with all atoms except for the methyl H atoms lying in the mirror planes (Koplitz et al., 2003;Mague et al., 2005). More recently, Cui and Wen reported that 2-cyanoanilinium nitrate also crystallizes in flat layers of two-dimensional networks with only a few atoms, including nitrate O atoms and ammonium H atoms, protruding from the mirror planes (Cui & Wen, 2008). The similarities between cations suggested a systematic study of analogous cyanopyridinium and anilinium salts both to look for other layered structures resembling graphite and to investigate trends in crystal architecture with variations in anion as well as relative ring position of the cyano and pendant groups. Of the eight possibilities investigated so far, the title compound is the only additional layered structure discovered to date.
2-Cyano-N-methylpyridinium nitrate crystallizes in the same space group as 2-cyanoanilinium nitrate. However, even though the cations of these two compounds are isomers that differ only by the interchange of one carbon and one nitrogen atom from the ring and pendant group, the distribution of anions relative to cations in the crystal differs markedly. In particular, the nitrate ions in the present structure lie wholly in the mirror plane such that the N3-O1 bond of one anion is oriented with O1 lying directly over the centroid of the pyridinium ring in the adjacent layer and N3 lying directly over the pyridinium nitrogen (N1) at a distance of 3.055 (1) Å. This close contact is likely the result of electrostatic cationanion attraction with the orientation reinforced by an anion-π interaction (Frontera et al., 2011). Calculated densities are 1.401 g-cm -3 for 2-cyanoanilinium nitrate and 1.531 g-cm -3 for 2-cyano-N-methylpyridinium nitrate with the greater density of the latter attributable to the anion lying wholly in the mirror plane rather than perpendicular to it. This is reflected in the much shorter b axis of the unit cell [6.101 (1) versus. 6.563 (1) Å] which is the stacking direction in the pyridinium salt. In the anilinium salt, only one nitrate N-O bond is coplanar with the cation rings while the other two oxygen atoms are disposed on either side of the mirror and form N-H···O hydrogen bonds with the pendant ammonium groups of the cations in adjacent layers. These strong hydrogen bonds are likely responsible for the different orientation of the anion in the anilinium salt which leads to a larger interlayer spacing.

Refinement
H-atoms were placed in calculated positions (C-H = 0.95 -0.98 Å) and included as riding contributions with isotropic displacement parameters 1.2 -1.5 times those of the attached carbon atoms. Because both ions sit on the mirror plane, the methyl group H atoms are disordered across the mirror. Trial refinements with both the one-component reflection file extracted from the full data set with TWINABS and with the full two-component file showed that use of the former provided a better refinement.

Special details
Experimental. The diffraction data were obtained from 3 sets of 400 frames, each of width 0.5 °. in omega, colllected at phi = 0.00, 90.00 and 180.00 °. and 2 sets of 800 frames, each of width 0.45 ° in phi, collected at omega = -30.00 and 210.00 °. The scan time was sec/frame. Analysis of 576 reflections having I/σ(I) > 15 and chosen from the full data set with CELL_NOW (Sheldrick, 2008b) showed the crystal to belong to the orthorhombic system and to be twinned by a 180 ° rotation about c. The raw data were processed using the multi-component version of SAINT under control of the two-component orientation file generated by CELL_NOW. Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 R-factors(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. Symmetry codes: (i) x, y, z−1; (ii) x+1/2, −y+1/2, −z+3/2; (iii) x+1/2, −y+1/2, −z+1/2; (iv) −x+1/2, −y+1, z−1/2; (v) x, y, z+1.