3-Amino-1-methylpyrazin-1-ium iodide

In the cation of the title compound, C5H8N3 +·I−, the C—N(H2) bond distance [1.338 (8) Å] is at the lower end of the range for aryl amines. In the crystal structure, cations and anions are linked via N—H⋯I hydrogen bonds, forming centrosymmetric four-component clusters.

For the synthesis and characterization of the title compound, see : Foucher et al. (1993). Additional preparative details of similar compounds are given by Goto et al. (1968). For related structures, see Chao et al. (1976); Foucher et al. (1989); Kazheva et al. (2006). For the crystal structure of 3-amino-1methylpyrazin-1-ium chloride, see the following paper. For comparative bond-distance data, see: Allen et al. (1987).

Comment
The title compound, (I), is prepared in moderate yield from the reaction of 2-aminopyrazine with methyl iodide in carbon tetrachloride (Foucher et al., 1993). The proximity of the amine group to one of the diazine nitrogen atoms makes it an ideal chelating ligand to metals and geometrically suggests the possibility for amine-imine tautomerism.
The molecular structure of (I) is shown in Fig. 1. The cation in (I) is the amine tautomer and resembles closely in terms of bond angles and bond lengths, other aromatic 1,4-diazines (Foucher et al., 1989). In a comparison, the major structural difference between 2-aminopyrazine (Chao et al., 1976) and (I) is observed in the C5-N4-C3 angle which is 121.3 (5)° in (I) and is 116.6 (1) in 2-aminopyrazine. These two structures are characterized by short amine-ring bond distances [1.338 (8) Å for C6-N7 in (I) and 1.341 (1) Å in 2-aminopyrazine] compared to typical bond lengths of sp 2 (C)-NH 2 bond lengths, i.e. 1.36 Å (Allen et al., 1987). These short bond lengths are suggestive of a considerable degree of double bond character, where the lone pair of the amine participates in the resonance of the ring π system. In the crystal structure, cations and anions are linked via intermolecular N-H···I hydrogen bonds to form centrosymmetric four component clusters (Fig. 2).

Experimental
General procedures for the synthesis of this type of compound are given by Goto et al. (1968) and Kazheva et al. (2006).
The title compound was prepared by the slow addition of an excess of methyl iodide (16 mmol) to a refluxing solution of the 2-aminopyrazine (7.9 mmol) in CCl 4 for 12 h. The crude products were filtered off and recrystallized from a 4:1 ethanol/water mixture giving crystals suitable for X-ray analysis. Yield 1.12 g, 60%. Characterization by NMR agreed with previous literature (Foucher et al., 1993).

Refinement
H atoms bonded to C atoms were placed in calculated positions with C-H = 0.95 and 0.98 Å, and included in a ridingmotion approximation with U iso (H) = 1.2U eq (C) or 1.5U eq (C methyl ). H atoms bonded to the amine-N atom were refined independently but with the N-H distance refined as a free variable [SHELXL (Sheldrick, 2008) Fig. 1. The asymmetric unit of (I) with displacement ellipsoids drawn at the 30% probability level. The dashed line indicates a hydrogen bond. Fig. 2. Part of the crystal structure of (I) with hydrogen bonds shown as dashed lines.