(S)-2-(1-Hydroxyethyl)benzimidazolium dihydrogen phosphate

The asymmetric unit of the title compound, C9H11N2O+·H2PO4 −, is built up from a 2-(1-hydroxyethyl)benzimidazolium cation and a dihydrogen phosphate anion which are connected by an N—H⋯O hydrogen bond. The cation is roughly planar, the dihedral angle between the rings being only 1.4 (2)°. The S configuration is deduced from the synthetic pathway and supported by the refinement of the Flack parameter. Intermolecular O—H⋯O and N—H⋯O hydrogen bonds build up a three-dimensionnal network.

The author gratefully acknowledges financial support by the start-up fund of Southeast University.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: DN2468).

Comment
The benzimidazoles, benzothiazoles, and benzoxazoles can be utilized as not only a wide variety of biologically active and medicinally significant compounds but also as advanced materials including non-linear optics (NLO), organic light-emitting diodes (OLED), and liquid crystals (Rodembusch et al., 2004;Gong et al., 2005;Chen, 2005;Belmar et al., 1999).
The title compound is built up from a dihydrogen phosphate anion and a (1H-benzimidazol-2-yl)ethanolium cation which are connected by a N-H···O hydrogen bond (Fig. 1).The S absolute configuration is deduced from the synthetic pathway and supported by the refinement of the Flack parameter (Flack, 1983). The phenyl ring and imidazole ring are roughly planar, making a dihedral angle of only1.4°. All bond lengths and angels are normal.
The molecules are connected via O-H···O and N-H···O hydrogen bonds making a three dimensionnal network (Table   1, Fig. 2).

A solution of phosphoric acid (1 mmol) in water was added to a methanol solution of L-(-)-1-(1H-Benzimidazol-2-yl)ethanol
(1 mmol), and then the mixture was stirred for half an hour at room temperature. The mixture was then filtered and the filtrate was evaporated at room temperature for a period of one month.Crystals suitable for X-ray diffraction analysis were obtained then. L-(-)-1-(1H-Benzimidazol-2-yl)ethanol was synthesized by the reaction of Benzene-1, 2-diamine and Ethyl L-(-)-lactate(R. Xia, et al., 2008).

Refinement
All H atoms attached to C and O atom were fixed geometrically and treated as riding with C-H = 0.93-0.98 Å, O-H = 0.82 Å and with U iso (H) = 1.2Ueq(C) or 1.5Ueq(C, O) for methyl and hydroxyl.
The su on the Flack parameter is rather high, however the value of the parameter agrees with the S configuration.
Moreover, inverting the configuration leads to the value 0.88 close to 1.

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 > 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.