Bis(4-methylimidazolium) succinate succinic acid solvate

In the title compound, 2C4H7N2 +·C4H4O4 2−·C4H6O4, the asymmetric unit consists of two 4-methylimidazolium cations, one succinate dianion and one netrual succinic acid molecule and within the latter components, the C—O, C=O and C O bonds are clearly evidenced from their relative distances. In the crystal structure, the individual components are linked by intermolecular N—H⋯O, O—H⋯O and C—H⋯O hydrogen bonds into a two-dimensional network parallel to the (101) plane in which R 3 3(9), R 3 3(12) and R 4 4(18) hydrogen-bond motifs are present.


S1. Comment
In recent years, research on co-crystal or organic salts has been expanded rapidly owing to their potential application in the preparation of active pharmaceutical ingredients (Aakeröy et al., 2007;Childs & Hardcastle, 2007;Childs et al., 2007). In this paper, we report an organic salt formed by 4-methyl-imidazole and succinic acid in 95% methanol solution at room temperature, namely bis(4-methyl-imidazolium) succinicate succinic acid, (I).
In (I), the asymmetric unit is composed of two 4-methylimidazolium cations, one succinicate dianion and one netrual succinic acid molecule. The title compound can be regarded as an organic salt according to the definition of Aakeröy and Salmon (2005). One of the succinnic acid molecules is dually deprotonated, leading to a dianion ( Fig. 1) which can be evidenced to an extent by the variations of the carboxyl C-O, C ═O and C ÛO bond distances (Table 1).
In the crystal structure, by a combination of four N-H···O and two O-H···O hydrogen bonds (Table 2) molecules in (I) are linked into a two-dimensional network parallel to the (101) plane ( Fig.2) in which R 3 3 (9), R 3 3 (12) and R 4 4 (18) hydrogen-bonding motifs are present (Bernstein et al., 1995). Within the network, several weak C-H..O interactions are present. No other interactions, such as C-H···π or π···π are observed in (I).

S2. Experimental
All the reagents and solvents were used as obtained without further purification. A 1:2 molar amounts of succinic acid (0.2 mmol, 23.6 mg) and 4-methyl-imidazole (0.4 mmol, 32.8 mg) were dissolved in 95% methanol (20 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.
Plate crystals of (I) suitable for single-crystal X-ray diffraction analysis were grown by slow evaporation of the solution at the bottom of the vessel.

S3. Refinement
H atoms bonded to C atoms were located in difference maps and subsequently treated as riding, with C-H = 0.93 Å (aromatic), 0.97Å (methylene), 0.96Å (methyl), U iso (H) = 1.2U eq ( aromatic and methylene C) and 1.5U eq ( methyl C). H atoms bonded to N and O atoms were also found in 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)    where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.28 e Å −3 Δρ min = −0.25 e Å −3 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.