Amino[(1H-benzimidazol-2-yl)amino]methaniminium 4-methylbenzenesulfonate

The asymmetric unit of the title salt, C8H10N5 +·C7H7O3S−, consists of two amino[(1H-benzimidazol-2-yl)amino]methaniminium cations and two 4-methylbenzenesulfonate anions. The cations are each stabilized by intramolecular N—H⋯N hydrogen bonds between the free amino groups and the imine N atoms of the benzimidazole units, forming S(6) ring motifs. In the crystal, cations and anions are linked by N—H⋯O and C—H⋯O hydrogen bonds, forming a three-dimensional supramolecular framework. Two strong π–π stacking interactions [centroid–centroid distances = 3.4112 (14) and 3.4104 (14) Å] also occur between the centroids of the imidazole rings of like cations.


Comment
Guanidines are structurally novel molecules reported to exhibit remarkable biological and pharmacological activities, which are affected by the guanidine functionality (Han et al., 2008;Hannon & Anslyn, 1993). Guanidino-containing drugs such as metaiodobenzylguanidine, MIBG, and methylglyoxalbisguanylhydrazone, MGBG, were shown several decades ago to have antitumor properties and have been subjected to intense preclinical and clinical evaluation (Ekelund et al., 2001). Guanidines are also known as useful basic catalysts (Kovacevic & Maksic, 2001;Costa et al., 1998). The synthesis of guanidine derivatives has also attracted continued research interests in recent years, resulting in many new efficient synthetic methods and guanidinylation reagents for different classes of guanidine compounds (Wu et al., 2002;Hopkins et al., 2002;Kilburn et al., 2002;Manimala & Anslyn, 2002). Against this background, we report herein the the synthesis and crystal structure of the title compound.
As seen as in Fig. 1, the asymmetric unit contains two amino(1H-benzimidazol-2-ylamino)methaniminium) cations and two 4-methylbenzenesulfonate anions. The bond lengths in the title compound are within the normal range (Allen et al.,

Experimental
A mixture of 175 mg (1 mmol) 1-(1H-benzimidazol-2-yl)guanidine and 191 mg (1 mmol) of 4-methylbenzenesulfonyl chloride was heated under reflux in 50 ml ethanol together with few drops of triethylamine for 6 h. The solid product started to be deposited during heating and filtered off after completion. The crude solid was washed with ethanol and recrystallized to afford colourless plates suitable for X-ray difraction (M.p. 539-541 K).

Refinement
The C-bound H atoms were placed at geometrically idealized positions with C-H = 0.95 and 0.98 Å for aromatic and methyl H-atoms, respectively. The C-bound H-atoms were refined using a riding model with U iso (H) = 1.2U eq (C aromatic ) and suggests the orthorhombic space group Pbcn, but attempts to refine the structure in this space group resulted in an unsatisfactory model.

Figure 1
The asymmetric unit of the title compound with the atom numbering scheme. Displacement ellipsoids for non-H atoms are drawn at the 50% probability level.  View of the packing and hydrogen bonding (dashed lines) along the a axis of the title compound. Special details Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles Refinement. Refinement on F 2 for ALL reflections except those flagged by the user for potential systematic errors. Weighted R-factors wR and all goodnesses 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 observed criterion of F 2 > σ(F 2 ) is used only for calculating -R-factor-obs 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.