Crystal structure of 2-amino-5-nitropyridinium sulfamate

The title molecular salt, obtained by the reaction of sulfamic acid with 2-amino-5-nitropyridine, is the result of a proton transfer from sulfamic acid to the N atom of the pyridine ring. In the crystal, the cations and anions are linked by a number of N—H⋯O and N—H⋯N hydrogen bonds, forming sheets lying parallel to (100).


Structural commentary
The asymmetric unit of the title compound, Fig. 1, consists of a 2-amino-5-nitropyridin-1-ium cation and a sulfamate anion. The bond lengths and angles are within normal ranges and comparable with those in closely related structures (Babu et al., 2014a,b;Rajkumar et al., 2014). A proton transfer from the sulfamic acid to the pyridine atom N3 resulted in the formation of a salt. This protonation leads to the widening of the C5-N3-C1 angle of the pyridine ring to 122.9 (3) , compared with 115.25 (13) in unprotonated aminopyridine (Anderson et al., 2005). This type of protonation is observed in various aminopyridine acid complexes (Babu et al., 2014a,b;Rajkumar et al., 2014). In the sulfamate anion the S-O distances vary from 1.440 (3) (Babu et al., 2014a,b;Rajkumar et al., 2014). In contrast, in the solidstate structure of aminopyridinium, the C-N(H 2 ) bond is clearly longer than that in the ring (Nahringbauer & Kvick, 1977). The geometrical features of the aminopyridinium cation (N1/N3/C1-C5) resemble those observed in other 2-aminopyridinium structures (Babu et al., 2014a,b;Rajkumar et al., 2014) that are believed to be involved in amine-imine tautomerism (Ishikawa et al., 2002). However, previous studies have shown that a pyridinium cation always possesses an expanded C-N-C angle in comparison with pyridine itself (Jin et al., 2005).
In this atomic arrangement, one can distinguish the intercation-to-anion contact C5-H5Á Á ÁO3 (H5Á Á ÁO5 = 2.41 Å ), which induces the aggregation of the independent organic cation 2-amino-5-nitropyridinium. This kind of arrangement is also observed in the related structure of 2-amino-5-nitropyridinium hydrogen selenate (Akriche & Rzaigui, 2009). These pairs are located between the anionic layers to link them by various interactions. The geometric features of the organic cation are usual and comparable with values observed for other 2-amino nitropyridinium compounds (Akriche & Rzaigui, 2009). It is worth noticing that the C-NH 2 [1.317 (5) Å ] and C-NO 2 [1.449 (6) Å ] distances in the cations are, respectively, shortened and lengthened with respect to the same bond lengths [1.337 (4) and 1.429 (4) Å ] observed for 2-amino-nitropyridine (Aakeroy et al., 1998). All the 2-amino-nitropyridinium cations encapsulated in various anionic sub-networks show the same changes in the C-NH 2 and C-NO 2 distances, revealing a weak increase of bond character in the bond C-NH 2 and a decrease in the bond C-NO 2 .

Figure 2
The crystal packing of the title salt, viewed along the b axis. The hydrogen bonds are shown as dashed lines (see Table 1 for details; only the major components of the disordered nitro O atoms are shown).
forming an R 3 3 (22)ring motif. These motifs are further linked by N-HÁ Á ÁO hydrogen bonds, enclosing R 3 3 (8) loops, and forming sheets lying parallel to (100). Weak C-HÁ Á ÁO hydrogen bonds link the sheets, forming a three-dimensional structure ( Fig. 2 and Table 1). The identification of such supramolecular patterns will help us design and construct preferred hydrogen-bonding patterns of drug-like molecules.

Synthesis and crystallization
The starting material 2-amino-5-nitropyridine was obtained by treating 3-nitropyridine with ammonia in the presence of KMnO 4 . Colourless block-like crystals of the title salt were obtained by slow evaporation of a 1:1 equimolar mixture of 2amino-5-nitropyridine and sulfamic acid in methanol at room temperature.

Special details
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.  (6) O5-S1 1.460 (2)