2-Methylimidazolium hydrogen maleate

Molecules in the title compound, C4H7N2 +·C4H3O4 −, are linked by intermolecular N—H⋯O hydrogen bonds into one-dimensional chains parallel to [101]. These chains are in turn linked by an R 2 2(8) motif, formed by weak C—H⋯O hydrogen bonds, into corrugated sheets running parallel to (10). These sheets are further linked by weak intermolecular C—H⋯O hydrogen bonds, forming a three-dimensional network. Intramolecular N—H⋯O and O—H⋯O interactions are also present.

Molecules in the title compound, C 4 H 7 N 2 + ÁC 4 H 3 O 4 À , are linked by intermolecular N-HÁ Á ÁO hydrogen bonds into onedimensional chains parallel to [101]. These chains are in turn linked by an R 2 2 (8) motif, formed by weak C-HÁ Á ÁO hydrogen bonds, into corrugated sheets running parallel to (101). These sheets are further linked by weak intermolecular C-HÁ Á ÁO hydrogen bonds, forming a three-dimensional network. Intramolecular N-HÁ Á ÁO and O-HÁ Á ÁO interactions are also present.

Comment
As part of the continuing studies on the synthesis of co-crystal or organic salts involving imidazole (Liu & Meng, 2006), the crystal structure of title compound (I) is reported. It was obtained by mixing a 2:1 molar amounts of 2-methylimidazole and 2-maleic acid and in 95% methanol solution at room temperature.
According to Aakeröy and Salmon (2005) complex (I) is an organic salt. In (I), one of the carboxyl protons is transferred to the imidazole N atom, forming a 1:1 anhydrous organic adduct. The two carboxyl groups in the maleate anion are hydrogen-bonded to each other via atom H3A located approximately at the mid-point of atoms O1 and O3 ( Fig.1).

Experimental
All the reagents and solvents were used as obtained without further purification. A 1:2 molar amounts of maleic acid (0.1 mmol, 11.6 mg) and 2-methylimidazole (0.2 mmol, 16.4 mg) were dissolved in 95% methanol (10 ml). The mixture was stirred for half an hour at room temperature and then filtered. The resulting solution was kept in air for one week. Blockshaped crystals suitable for single-crystal X-ray diffraction analysis were grown by slow evaporation of a solution of (I).

Refinement
H atoms bonded to C atoms were located in difference maps and subsequently treated as riding modes, with C-H=0.93 Å, U iso (H) = 1.2U eq (C) and C-H=0.96 Å, 1.5U eq (C) for methyl H atoms. H atoms bonded to N and O atoms were also found in the difference maps and their distances were refined freely (see Table 1 for the distances), and the U iso (H) values being set k times of their carrier atoms (k=1.2 for N and 1.5 for O atoms)

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 > 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.