Bis(2-trifluoromethyl-1H-benzimidazol-3-ium) naphthalene-1,5-disulfonate

The asymmetric unit of the title compound, C8H6F3N2 +·0.5C10H6O6S2 2−, consists of one 2-trifluoromethyl-1H-benzimidazol-3-ium cation and a half naphthalene-1,5-disulfate anion, which are linked by an N—H⋯O hydrogen bond. The anion sits across a centre of symmetry. The atoms of the benzimidazole ring are nearly coplanar (r.m.s. deviation of the fitted atoms = 0.0085 Å) and the triflouromethyl group lies out of this plane. In the crystal, the cations are linked to adjacent anions by N—H⋯O hydrogen bonds, forming a ladder structure parallel to the a axis in which the anions form the rungs. Adjacent ladders are linked by weak C—H⋯O interactions, forming sheets parallel to the ac plane.

The asymmetric unit of the title compound, C 8 H 6 F 3 N 2 + Á-0.5C 10 H 6 O 6 S 2 2À , consists of one 2-trifluoromethyl-1H-benzimidazol-3-ium cation and a half naphthalene-1,5-disulfate anion, which are linked by an N-HÁ Á ÁO hydrogen bond. The anion sits across a centre of symmetry. The atoms of the benzimidazole ring are nearly coplanar (r.m.s. deviation of the fitted atoms = 0.0085 Å ) and the triflouromethyl group lies out of this plane. In the crystal, the cations are linked to adjacent anions by N-HÁ Á ÁO hydrogen bonds, forming a ladder structure parallel to the a axis in which the anions form the rungs. Adjacent ladders are linked by weak C-HÁ Á ÁO interactions, forming sheets parallel to the ac plane.

Comment
Recently much attention has been devoted to crystals containing organic ions and inorganic ions due to the possibility of tuning their special structural features and their potential ferroelectrics properties (Fu et al., 2011;Zhang et al., 2010.). In our laboratory, the title compound has been synthesized and its crystal structure is herein reported.
Experimental 0.144 g (1 mmol) of 2-trifluoromethyl-1H-benzimidazol was firstly dissolved in 30 ml of ethanol, to which 0.288 g (1 mmol) of 1,5-naphthalene disulfonic acid was added to give a solution at the ambient temperature. Single crystals suitable for X-ray structure analysis were obtained by the slow evaporation of the above solution after 3 days in air.
The dielectric constant of the compound as a function of temperature indicates that the permittivity is basically temperature-independent (ε = C/(T-T 0 )), suggesting that this compound is not ferroelectric or there may be no distinct phase transition occurring within the measured temperature (below the melting point).

Refinement
H atoms were placed in calculated positions (N-H = 0.89 Å; C-H = 0.93Å for Csp 2 atoms and C-H = 0.96Å and 0.97Å for Csp 3 atoms), assigned fixed U iso values [U iso = 1.2Ueq(Csp 2 ) and 1.5Ueq(Csp 3 ,N)] and allowed to ride. The trifluoromethyl group is disordered over two sites. The site occupancies were refined and restraints were applied to the thermal parameters.
supplementary materials sup-2 Figures Fig. 1. The molecular structure of the title compound, showing the atomic numbering scheme with 30% probability displacement ellipsoids. Atoms with labels suffixed with an a are in a centrosymmetrically related part of the anion.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
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 > 2sigma(F 2 ) is used only for calculating R-factors(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.