Crystal structure of 2-methyl-1H-imidazol-3-ium hydrogen oxalate dihydrate

In the title molecular salt 2-methyl-1H-imidazol-3-ium hydrogen oxalate dihydrate, N—H⋯(O,O) and O—H⋯O hydrogen bonds link the components into a bilayer-like assembly.


Chemical context
Imidazolium-type building blocks are useful in the field of crystal engineering (MacDonald et al., 2001). With many possibilities of substitution (involving various positions around the five-membered ring) and via the propagation of multidirectional hydrogen-bonding interactions, they easily lead to the self-assembly of poly-dimensional packing networks. In 2010, Callear and co-workers described various topologies based on imidazolium/dicarboxylic acid combinations and showed the crystal-packing effects of substitution in the imidazole ring (Callear et al., 2010). In this context, and for some time, our group has focused on the contribution of the 2-methylimidazolium cation as a co-crystal in organic  and organic-inorganic hybrid salts Diop, Diop, Plasseraud & Maris, 2015. Continuing our ongoing studies in this field, we report herein the crystal structure of a new hydrated organic salt, namely 2-methyl-1H-imidazol-3-ium hydrogen oxalate dihydrate, (I), isolated by reacting 2-methyl-1H-imidazole and oxalic acid in a 1:1 molar ratio in water.

Supramolecular features
Hydrogen-bonding interactions are listed in Table 1 and illustrated in Fig. 2. Both N-H groups of the imidazolium cation are involved in asymmetric bifurcated N-HÁ Á Á(O,O) hydrogen bonds with two distinct neighbouring hydrogen oxalate anions, which initiates the propagation of an infinite ribbon along the b-axis direction. Considering the orientation of the methyl groups of the cations along the ribbon, the sequence can be described as 'isotactic'. The cations and anions are positioned alternately and are almost coplanar [dihedral angle between adjacent species = 1.15 (9) ].
As well as the cation-to-anion links, the OH group of the anion acts as a hydrogen-bond donor with one molecule of water, which is also the donor for hydrogen-bond interactions with (i) a second molecule of water and (ii) an O atom of a hydrogen oxalate anion involved in a neighbouring ribbon. The second water molecule also bridges two distinct hydrogen oxalate anions through two O-HÁ Á ÁO hydrogen bonds. Thus, all the O atoms of the hydrogen oxalate anions are involved in the hydrogen-bonding network.
The supramolecular arrangement depicted in Fig. 2 relies on the contributions of the four components of (I) and can be described as resulting from three levels of organization: (i) C 4 H 7 N 2 + and HC 2 O 4 À assembled in infinite ribbons; (ii) parallel ribbons of C 4 H 7 N 2 + /HC 2 O 4 À connected together by water molecules, which leads to a staircase-sheet structure; (iii) sheets stacked in pairs which can be described as a twodimensional bilayer-like arrangement propagating in (101). This final organization is again induced by the formation of hydrogen-bonding interactions between the water molecules contained in each sheet. The inter-sheet distance is about 3.4 Å . Interestingly, all the methyl substituents of the imidazolium rings are oriented in the same direction along the c axis. Thus, the isotacticity observed at the ribbon level is also extended across the supramolecular network. The molecular structure of (I), showing the atom labelling. Displacement ellipsoids are draw at the 50% probability level. Table 1 Hydrogen-bond geometry (Å , ).  (2)  167 Symmetry codes: (i) x; y À 1; z; (ii) Àx þ 1 2 ; y À 1 2 ; Àz þ 1 2 ; (iii) Àx þ 1; Ày þ 1; Àz þ 1; (iv) x À 1 2 ; Ày þ 3 2 ; z À 1 2 .

Figure 2
The

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2    program(s) used to solve structure: SHELXS2013 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).  (Bruker, 2014) was used for absorption correction. wR2(int) was 0.0622 before and 0.0548 after correction. The Ratio of minimum to maximum transmission is 0.9269. The λ/2 correction factor is 0.00150. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.40 e Å −3 Δρ min = −0.23 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.