1-(2-Methyl-5-nitrophenyl)guanidinium picrate

In the crystal structure of the title salt, C8H11N4O2 +·C6H2N3O7 −, the pictrate anion participates in extensive hydrogen bonding with the guanidinium ion group of the cation, linking the molecules through N+—H⋯O− hydrogen bonds and intermolecular N—H⋯O and C—H⋯O interactions. These hydrogen-bonding configurations involve two three-centre/bifurcated bonds [N—H⋯(O,O)] that are observed between two N atoms from the guanidinium ion group of the cation and the o-NO2 and phenolate O atoms of the picrate anion. In addition, π–π interactions also contribute to the crystal packing, with a centroid-to-centroid distance of 3.693 (6) Å and a slippage angle of 1.614°. A significant number of conformational differences are observed between the salt in the crystal structure and the models obtained by density functional theory (DFT) calculations of the geometry-optimized structure.

In the crystal structure of the title salt, C 8 H 11 N 4 O 2 + Á-C 6 H 2 N 3 O 7 À , the pictrate anion participates in extensive hydrogen bonding with the guanidinium ion group of the cation, linking the molecules through N + -HÁ Á ÁO À hydrogen bonds and intermolecular N-HÁ Á ÁO and C-HÁ Á ÁO interactions. These hydrogen-bonding configurations involve two three-centre/bifurcated bonds [N-HÁ Á Á(O,O)] that are observed between two N atoms from the guanidinium ion group of the cation and the o-NO 2 and phenolate O atoms of the picrate anion. In addition,interactions also contribute to the crystal packing, with a centroid-to-centroid distance of 3.693 (6) Å and a slippage angle of 1.614 . A significant number of conformational differences are observed between the salt in the crystal structure and the models obtained by density functional theory (DFT) calculations of the geometryoptimized structure.
MTS thanks the University of Mysore for the use of their research facilities. RJB acknowledges the NSF MRI program (grant No. CHE-0619278) for funds to purchase the X-ray diffractometer.

Comment
Guanidines, important compounds that have many biological, chemical and medicinal applications (Berlinck, 2002;Heys et al., 2000), have received increasing interest as medicinal agents with antitumour, antihypertensive, antiglaucoma and cardiotonic activities (Laeckmann et al., 2002;Kelley et al., 2001;Moroni et al., 2001). Due to their strong basic character, they can be considered as super-bases that readily undergo protonation to generate resonance-stabilized guanidinium cations (Ishikawa & Isobe, 2002). Guanidine is used in variety of supramolecular recognition processes across the spectrum of organic, biological and medicinal chemistry (Orner & Hamilton, 2001) with special interest motivated by their potential applications in non-linear optics (Zyss et al., 1993).
supplementary materials sup-2 A density functional theory (DFT) geometry optimization molecular orbital calculation (Schmidt & Polik, 2007) was performed on the C 8 H 11 O 2 N 4 + .C 6 H 2 N 3 O 7 cation-anion pair of the title molecule, (I), with the GAUSSIAN03 program package (Frisch et al. 2004) employing the B3LYP (Becke three parameter Lee-Yang-Parr) exchange correlation functional, which combines the hybrid exchange functional of Becke (Becke, 1988(Becke, , 1993 with the gradient-correlation functional of Lee, Yang and Parr (Lee et al. 1988) and the 3-21G basis set (Hehre et al. 1986). Starting geometries were taken from X-ray refinement data. The dihedral angle between the dihedral planes of the guanidinium ion [(CH 5 N 3 ) + ] and the bonded 2-methyl-5-nitrophenyl group in the cation decreases by 7.1 (2)° to 70.9 (1)° while the mean plane of the 5-nitro group decreases by 5.0 (3)° to become planar with the benzyl ring. In the picrate anion, the twist of the mean planes of

Experimental
The title compound was synthesized by adding a solution of picric acid (0.92 g, 2 mmol) in 10 ml of methanol to a solution of 1-(2-methyl-5-nitrophenyl)guanidine (0.45 g, 2 mmol) in 10 ml of methanol (Scheme 2). A yellow colour developed and the solution was allowed to evaporate slowly at room temperature.The yellow colour compound formed was filtered off, washed several times with diethyl ether, and then dried over CaCl 2 (yield: 64.2%). Crystals for X-ray studies were grown by slow evaporation of dimethyl formamide solution. The melting range was found to be 382-385 K. Analysis found    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 Rfactors(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.