Crystal structures of the two salts 2-methyl-1H-imidazol-3-ium nitrate–2-methyl-1H-imidazole (1/1) and 2-methyl-1H-imidazol-3-ium nitrate

The title salts, C4H7N2 +·NO3 −·C4H6N2, (I), and C4H7N2 +·NO3 −, (II), are composed of hydrogen-bonded chains along [001] and [100] for (I) and (II), respectively.

The title salts, C 4 H 7 N 2 + ÁNO 3 À ÁC 4 H 6 N 2 , (I), and C 4 H 7 N 2 + ÁNO 3 À , (II), were obtained from solutions containing 2-methylimidazole and nitric acid in different concentrations. In the crystal structure of salt (I), one of the -NH H atoms of the imidazole ring shows half-occupancy, hence only every second molecule is in its cationic form. The nitrate anion in this structure lies on a twofold rotation axis. The neutral 2-methylimidazole molecule and the 2-methyl-1H-imidazol-3-ium cation interact through N-HÁ Á ÁN hydrogen bonds to form [(C 4 H 6 N 2 )Á Á Á(C 4 H 7 N 2 ) + ] pairs. These pairs are linked with two nitrate anions on both sides through bifurcated N-HÁ Á Á(O,O) hydrogen bonds into chains running parallel to [001]. In the crystal structure of salt (II), the C 4 H 7 N 2 + cation and the NO 3 À anion are both located on a mirror plane, leading to a statistical disorder of the methyl H atoms. The cations and anions again interact through bifurcated N-HÁ Á Á(O,O) hydrogen bonds, giving rise to the formation of chains consisting of alternating anions and cations parallel to [100].

Chemical context
While targeting the synthesis of new Sn IV complexes, crystals of the salt C 4 H 7 N 2 + ÁNO 3 À , (II), were obtained serendipitously by mixing trimethyltin acetate with 2-methylimidazole in the presence of nitric acid. In the dynamic of seeking new ammonium salts soluble in organic solvents that can be used for further metallorganic syntheses, we have initiated the targeted preparation of this salt. However, by variation of the ratio between nitric acid and 2-methylimidazole we also obtained crystals of compound (I), C 4 H 6 N 2 ÁC 4 H 7 N 2 + ÁNO 3 À , and report the two structures in this communication.

Structural commentary
The asymmetric unit of salt (I) consists of a 2-methylimidazole moiety in a general position and part of a nitrate anion. The ISSN 2056-9890 anion is completed by application of twofold rotation symmetry. The hydrogen atom H1 attached to N1 of the imidazole ring has a statistical occupancy of 0.5, thus leading to a 1:1 mixture of a 2-methyl-1H-imidazol-3-ium cation and a neutral 2-methylimidazole molecule in the crystal applying symmetry operation (i) 1 À x, y, 1 2 À z (Fig. 1). In the nitrate anion, the N-O bond lengths [1.2433 (11)-1.2774 (19) Å ], are in a typical range (see, for example, Diop et al., 2013) and indicate some delocalization over the two oxygen atoms O1 and O1 i . The longer N-O distance is observed for atom O2 involved in the stronger of the two observed N-HÁ Á ÁO hydrogen bonds (Table 1). The imidazole ring is planar with a maximum deviation of 0.005 (1) Å . The asymmetric unit of salt (II) consists of an ordered 2-methyl-1H-imidazol-3-ium cation and a nitrate anion (Fig. 2), both lying on a mirror plane.
In the two structures, the O-N-O angles have normal values close to 120 and their sum (360 ) reflect a perfect trigonal-planar geometry for each of the nitrate anions. For the 2-methyl-1H-imidazol-3-ium cations and for the neutral 2-methylimidazole molecule, the N-C distances involving C2, the C atom that carries the methyl group, are equal within 0.01 Å , and their values are consistent with double-bond character, as previously observed (Diop et al., 2015).

Figure 2
The molecular components of salt (II), showing the atom labelling and displacement ellipsoids drawn at the 50% probability level. H atoms are drawn as spheres of arbitrary radius and hydrogen bonds are shown as dashed lines.

Figure 3
Partial view of the packing in the crystal structure of (I), showing a chain of hydrogen-bonded molecules. Only one of the statistically disordered H-atom positions between the imidazole rings is shown.

Synthesis and crystallization
All chemicals were purchased from Aldrich (Germany) and were used as received. Single crystals suitable for X-ray studies of (II) were first obtained by serendipity when a mixture of 2-methylimidazole and concentrated nitric acid was added to trimethyltin acetate in methanol. Colourless single crystals of (I) were obtained after slow evaporation at room temperature of an aqueous solution consisting of 2-methylimidazole and concentrated nitric acid in a 2:1 ratio. Compound (II) can also be prepared in a similar way by changing the ratio between 2-methylimidazole and nitric acid to 1:1.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. For (I), all H atoms were clearly discernible from difference Fourier maps and were freely refined. Half-occupancy of H1 is required for structural reasons and was indicated by the values of the residual density peaks found in the difference Fourier map ( Partial view of the packing in the crystal structure of (II), showing a chain made up of hydrogen-bonded nitrate anions and 2-methyl-1H-imidazol-3ium cations.  for an occupancy factor of 1 and 0.5, respectively). For (II), the H atoms bound to C were placed in calculated positions and then refined using a riding model with C-H = 0.95 Å (aromatic) and 0.98 Å (methyl) and U iso (H) = 1.2 and 1.5U eq (C), respectively. As a result of the mirror symmetry of the 2-methyl-1H-imidazol-3-ium cation, the methyl H atoms are statistically disordered over two positions. H atoms bound to N atoms were located from a difference Fourier map and were freely refined.

Special details
Experimental. X-ray crystallographic data for I were collected from a single-crystal sample, which was mounted on a loop fiber. Data were collected using a Bruker Venture diffractometer equipped with a Photon 100 CMOS Detector, a Helios MX optics and a Kappa goniometer. The crystal-to-detector distance was 4.0 cm, and the data collection was carried out in 1024 x 1024 pixel mode. 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.