4-Carboxy-2-methyl-1H-imidazol-3-ium-5-carboxylate monohydrate

In the title compound, C6H6N2O4·H2O, one carboxyl group is deprotonated and one imidazole N atom is protonated. The organic molecule, excluding methyl H atoms, is essentially planar, with an r.m.s. deviation of 0.0156 (1) Å. In the crystal structure, intermolecular N—H⋯O hydrogen bonds link molecules into chains along the b axis; these chains are further linked via O—H⋯O hydrogen bonds involving the water O atoms and carboxyl O atoms, generating a two-dimensional supramolecular framework.

The author thanks Jiangxi Science and Technology Normal University for supporting this study.
As shown in Fig. 1, the asymmetric unit consists of a neutral C 6 H 6 N 2 O 4 molecule and one water molecule. The organic molecule, excluding methyl hydrogen atoms, is essentially planar, with an r.m.s. deviation of 0.0156 (1) Å. The C1-containing carboxylate group forms an intramolecular hydrogen bond with the neighboring C5-containing carboxyl group.
In the crystal structure, intermolecular N-H···O hydrogen bonds link the molecules into chains along the b axis; these chains are further linked via O-H···O hydrogen bonds involving the water O atoms and carboxyl O atoms, generating a two-dimensional supramolecular framework (Fig. 2).

Experimental
The title compound was synthesized according to a revised procedure (Anderson et al., 1989). 2-Methylimidazole (3.0 g) was added to a mixture of concentrated sulfuric acid (40 ml) and water (30 ml) at 363 K. This was followed by the careful addition of powdered potassium dichromate (22 g). After 30 min the mixture was poured into ice-cold water. The white precipitates were collected by filtration, and washed with water. Recrystallization from hot water afforded colorless block crystals of the title compound. Yield: 1.8 g (44%).

Refinement
The carboxyl and water H atoms were located in a difference Fourier map and refined with U iso (H) = 1.5U eq (O). The O-H distances of the water molecule were restrained to 0.85 (1) Å; however, that of the carboxyl was refined freely. All other H-atoms were positioned geometrically and refined using a riding model with C-H (methyl) = 0.96 Å, N-H = 0.86 Å; U iso (H) = kU eq (carrier atom), where k = 1.2 for N and 1.5 for C.
supplementary materials sup-2 Figures   Fig. 1. The molecular structure of the title compound, with displacement ellipsoids drawn at the 30% probability level. Hydrogen atoms are drawn as spheres of arbitrary radius.   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.