Biguanidinium dichloride

The asymmetric unit of the title compound, C2H9N5 2+·2Cl−, is composed of one diprotonated biguanidinium cation and two chloride anions. The diprotonated cation consists of two planar halves twisted by 36.42 (6)°. The ions are associated in the crystal structure by extensive hydrogen bonding into a three-dimensional network; the diprotonated biguanidinium cation is hydrogen bonded to the chloride anions.

The asymmetric unit of the title compound, C 2 H 9 N 5 2+ Á2Cl À , is composed of one diprotonated biguanidinium cation and two chloride anions. The diprotonated cation consists of two planar halves twisted by 36.42 (6) . The ions are associated in the crystal structure by extensive hydrogen bonding into a three-dimensional network; the diprotonated biguanidinium cation is hydrogen bonded to the chloride anions.

Comment
Biguanidine derivatives, characterized by multiple hydrogen-bond donor sites, are good candidates to be coupled in the crystal with carefully selected molecules having multiple hydrogen-bond acceptor sites (Portalone & Colapietro, 2004, 2007. As a part of a more general study of multiple-hydrogen-bonding DNA/RNA nucleobases as potential supramolecular reagents, this paper reports the crystal structure of the title compound, (I), BIGH 2 2+ 2Cl − . The asymmetric unit of (I) (Fig.   1) consists of a diprotonated biguanidinium cation (BIGH 2 2+ ) and two chloride anions; protonation occurs at the bridge N atom and the imino N atom of the biguanidine molecule. The structure of the delocalized cation is very similar to those previously reported for the carbonate (BIGH 2 )CO 3 and the sulfates (BIGH 2 )SO 4 ·2H 2 O and (BIGH 2 )SO 4 ·H 2 O (Pinkerton & Schwarzenbach, 1978), the dinitrate (BIGH 2 )NO 3 , the diperchlorate (BIGH 2 )2ClO 4 , the bis-dinitramide (BIGH 2 )(DN) 2 and (BIGH 2 )(DN) 2 ·H 2 O (Martin et al., 1997). BIGH 2 2+ is composed of two planar halves sharing the atom N(1). These two planar parts are twisted with respect to each other by 36.42 (6)°.
The C-N terminal bond lengths are shorter due to delocalization of π-electron density through the planar fragments. The lack of complete planarity of the cation is due to steric interaction between the hydrogen atoms. This interaction induces a strain in the molecule which is manifested by the opening of the angle at the bridging N atom [C1-N1-C2, 127.9 (1)°].
The weakening of the bridges bonds is due to the lowered basicity of the bridge N atom on protonation [pk a I = 11.5; pk a II = 2.9 (Kurzer & Pitchfork, 1968) (Ernst, 1977). Analysis of the crystal packing of (I) (Fig. 2) shows that the structure is stabilized by ten hydrogen bonds N-H···Cl − involving all protons (Table 1) which account for the relatively high density (D x = 1.61 Mg m −3 ).

Experimental
Biguanide (0.1 mmol, Sigma Aldrich at 98% purity) was dissolved in water (9 ml) and heated under reflux for 2 h. After cooling a solution to an ambient temperature, while stirring, HCl (6 mol L −1 ) was added dropwise until the pH = 2. Crystals suitable for single-crystal X-ray diffraction were obtained by slow evaporation of the solvent after a few days.

Refinement
All H atoms were found in a difference map and refined isotropically.  (18)