Guanidinium bromide–18-crown-6 (2/1)

In the title compound, 2CH6N3 +·2Br−·C12H24O6, the 18-crown-6 molecule lies about an inversion center, whereas the guanidinium cation and bromide anion are in general positions. The guanidinium cations link with the bromide anions and the crown ether molecules via N—H⋯O and N—H⋯Br hydrogen bonds, thus forming a three-dimensional network.

In the title compound, 2CH 6 N 3 + Á2Br À ÁC 12 H 24 O 6 , the 18crown-6 molecule lies about an inversion center, whereas the guanidinium cation and bromide anion are in general positions. The guanidinium cations link with the bromide anions and the crown ether molecules via N-HÁ Á ÁO and N-HÁ Á ÁBr hydrogen bonds, thus forming a three-dimensional network.

Yu-feng Wang Comment
Recent years, crown ethers have attracted much attention because of their wide application in catalysis, solvent extraction, separation of isotopes, host-guest and supramolecular chemistry (Clark et al., 1998). Several 18-crown-6 clathrates were discovered to be dielectric-ferroelectric materials (Fu et al., 2011), hence we designed the title compound in attempts to find new hydrogen-bonded dielectric materials. Dielectric-ferroelectric materials, comprising organic ligands, metal-organic coordination compounds and organic-inorganic hybrids almost show temperature dependence of their dielectric constants (Fu et al., 2009;Zhang et al., 2010;Zhang et al., 2008;Ye et al., 2006). Unfortunately, the study of temperature dependence of dielectric constant of the title compound indicates that the permittivity is basically temperature-independent below its melting point (395K-396K). Herein we descibe the crystal structure of this compound.
At room temperature (25°C), the single-crystal X-ray diffraction reveals that the asymmetric unit of the title compound consists of a guanidinium cation, a bromide anion and a half of 18-crown-6 molecule ( Fig. 1). The three NH 2 -groups of guanidinium interact with the oxygen atoms of crown ether molecule and with two bromide anions through two N-H···O and N-H···Br hydrogen bonds (Table 1), thus forming a three-dimensional network (Fig. 2).

Experimental
The hydrobromic acid (0.81 g, 10 mmol) and guanidinium carbonate (0.9 g, 5 mmol) were dissolved in 30 ml of water and the solution was combined with methanol solution of dibenzo-18-crown-6 (3.6 g 10 mmol). The mixture was stirred for 30 min to complete the reaction, and good quality blocky single crystals were obtained by slow evaporation of the filtrate after two weeks (the chemical yield 72%).

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

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq O2