3-Carboxymethyl-1,3-benzimidazolium-1-acetate monohydrate

The title compound, C11H10N2O4·H2O, has a zwitterionic structure, in which the benzimidazole ring system is planar, with a maximum deviation of 0.007 (3) Å. The carboxyl/carboxylate groups adopt a trans configuration. In the crystal structure, intermolecular O—H⋯O hydrogen bonds involving the hydroxy/oxide O atoms link the molecules into a one-dimensional chain. These chains are further linked by O—H⋯O hydrogen bonds involving the water molecules into a two-dimensional network. π–π contacts between the benzimidazole rings [centroid–centroid distance = 3.5716 (4) Å] lead to the formation of a three-dimensional supramolecular structure.

The title compound, C 11 H 10 N 2 O 4 ÁH 2 O, has a zwitterionic structure, in which the benzimidazole ring system is planar, with a maximum deviation of 0.007 (3) Å . The carboxyl/ carboxylate groups adopt a trans configuration. In the crystal structure, intermolecular O-HÁ Á ÁO hydrogen bonds involving the hydroxy/oxide O atoms link the molecules into a onedimensional chain. These chains are further linked by O-HÁ Á ÁO hydrogen bonds involving the water molecules into a two-dimensional network.contacts between the benzimidazole rings [centroid-centroid distance = 3.5716 (4) Å ] lead to the formation of a three-dimensional supramolecular structure.
In the molecule of the title compound ( Fig. 1), the benzimidazole ring system is planar with a maximum deviation of 0.007 (3) Å for atom N1, and the two carboxyl groups adopt a trans configuration with respect to the benzimidazole ring plane. The C-N bonds on the imidazolium rings are found to be within 1.323 (2)-1.391 (2) Å, which are between the C-N single and C=N double bonds, suggesting charge delocalization on the imidazolium rings. The torsion angles of C5-N1-C2-C1 [95.5 (2)°] and C6-N2-C4-C3 [-89.5 (2)°] are much smaller than the corresponding values in (II).
The lattice water molecules have site symmetries 2.
In the crystal structure, intermolecular O-H···O hydrogen bonds involving the hydroxy O atoms (Table 1) link the molecules into a one-dimensional chain (Fig. 2), in which they are further linked by O-H···O hydrogen bonds of lattice water molecules (Table 1) into a two-dimensional network (Fig. 3). The π···π contacts between the benzene rings of the benzimidazole groups (Fig. 4), Cg2···Cg2 i [symmetry code: (i) -x, 2 -y, -z, where Cg2 is centroid of the ring (C6-C11)] may further stabilize the structure, with centroid-centroid distance of 3.5716 (4) Å and lead to the formation of a three-dimensional supramolecular structure (Fig. 5).

Experimental
For the preparation of the title compound, benzimidazole (0.714 g,6 mmol) was added to an aqueous solution (35 ml) of iodoacetic acid (1.859 g, 10 mmol) and NaOH (0.405 g, 10 mmol). The resulting mixture was heated at reflux during which benzimidazole was gradually dissolved and the colorless solution changed to yellow. The pH was adjusted using saturated NaOH solution at 20 min intervals, keeping in the range of 8-9. When no pH change was detected, the solution was further refluxed for 30 min, cooled, acidified with hydrochloric acid until pH = 2-3. The brown precipitate formed was filtered and recrystallized using water during which the deep yellow solution changed to colorless. The colorless plate crystals were formed after 5 d.

Refinement
Atoms H3, H5 and H6 were located in a difference Fourier map and refined isotropically. The remaining H atoms were positioned geometrically with C-H = 0.93 and 0.97 Å for aromatic and methylene H atoms, respectively, and constrained to ride on their parent atoms, with U iso (H) = 1.2U eq (C).  Fig. 1. The molecular structure of the title molecule, with the atom-numbering scheme. Displacement ellipsoids are drawn at the 25% probability level.

Special details
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.