1-(2-Carboxyethyl)-3-(carboxylatomethyl)-2-ethylbenzimidazol-1-ium monohydrate

In the title compound, C14H16N2O4·H2O, three substituent groups are located on the same side of the benzimidazole ring plane. In the crystal, O—H⋯O hydrogen bonds and π–π stacking between parallel imidazole rings [centroid–centroid distance = 3.858 (4) Å] link the molecules into a three-dimensional supramolecular structure.


Comment
Benzimidazole and its derivatives are strongly coordinating agents and form stable complexes with various metals, which find various application in supramolecular coordination complexes, catalytic systems and soft materials (Chakrabarty et al., 2011;Cook et al., 2012;Wang et al., 2009;Wei et al., 2012). Meanwhile, the coordination chemistry of dicarboxylates has gained great attention for a variety of reasons, such as good conformation freedom, various coordination modes and so on (Wang et al., 2010;Wu et al., 2012). Herein we report the crystal structure of 1-(2-acetoxy)-3-(3-propionyloxy)-2-ethyl-3H-benzimidazolium salt and the molecular structure is shown in Fig 1. In the imidazolium ring, the bond lenghs range from 1.338 (2) to 1.397 (2) Å, in good agreement with the presence of conjugated double bonds and indicating a zwitterionic structure (Chen & Huang, 2006). In the U-shaped molecule, the two carboxyl group run almost perpendicular to the benzimidazolium plane in the same orientation. The N1-C10-C11 and N2-C12-C13 angles are 112.57 (14) and 113.06 (15)°, respectively. A dimer is formed by benzimidazolium salt connected to neighboring molecule through O3-H3A ···O1 hydrogen bonds (Fig 2). In addition, the dimer makes full use of two water molecules as bridging molecules to form various O-H···O hydrogen bonds to generate the wide hydrogen-bond ribbon (Fig 3).
Obviously, water molecules, as a kind of linking unit, play an important role in constructing this structure. The distance between the donor and acceptor of the hydrogen bond is in the range of 2.474 (2)-2.851 (3) Å. π-π stacking is observed between parallel imidazole rings of adjacent molecules [1-x, 2-y, -z], the centroids distance being 3.858 (4) Å.

Refinement
Water H atoms were located in a difference Fourier map and refined isotropically. Other H atoms were positioned geometrically and refined as riding atoms, with C-H = 0.93 and O-H = 0.82 Å, U iso (H) = 1.5U eq (C,O).  The molecular structure of the title compound with atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level.  Hydrogen-bonded linking pattern of the wide hydrogen-bonded ribbon in the crystal structure of the title compound.

Figure 3
Packing diagram of the title compound. All hydrogen atoms bonded to carbon are omitted for clarity.

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