Crystal structure of 2-ethyl-4-methyl-1-(2-oxido-3,4-dioxocyclobut-1-en-1-yl)-1H-imidazol-3-ium

In the crystal of the inner salt of the title compound, N—H⋯O and C—H⋯O hydrogen bonds form an (9) ring motif.


Chemical context
The study of the non-linear optical (NLO) properties of organic molecules and crystals are of great interest in physics, chemistry and applied technologies (Chemla et al., 1987). Certain classes of organic compounds exhibit very pronounced NLO and electro-optical (EO) effects. Their nonlinearity is based on the presence of molecular units containing strongly delocalized -electron systems with the donor and acceptor groups sited at opposite ends of the molecule (Bosshard et al., 1995;Kolev et al., 2008). The study of the development of new non-centrosymmetric single-crystal NLO materials to obtain efficient frequency doublers is the subject of crystal engineering. In this context, some squaric acid derivatives together with cyclobutenediones with proper substitution groups have been found to be of interest in terms of their high NLO responses (Kolev et al., 2008).
Squaric acid gives rise to two structurally different classes of derivatives, which can be described by the general molecular structures 1,3-N-squarenes and amine-containing molecule betaines (Gsä nger et al., 2014;Kolev et al., 2005). The squarenes shows photo-chemical, photo-conductive and NLO ISSN 2056-9890 properties and can therefore be used as electron acceptors in photo-sensitive devices (Lindsay & Singer, 1995). On the other hand, substituted betaines play an important role in NLO behavior due to their dipolar structures (Kolev et al., 2004). The conversion of the N2 atom of 2-ethyl-4-methylimidazole into the corresponding betaine squaric acid form provides a way of enhancing the charge-transfer transition at the molecular level.
This study reports a novel betain form of squaric acid with a 2-ethyl-4-methylimidazole molecule. The crystal structure, together with its NLO properties, are reported here.

Figure 2
The crystal packing of the title compound, illustrating the N-HÁ Á ÁO hydrogen bonds in the [010] direction together with weak C-HÁ Á ÁO hydrogen bonds.
HÁ Á ÁO and C-HÁ Á ÁO heteronuclear hydrogen bonds that form an R 2 2 (9) ring motif contribute as both donor and acceptor to the crystal packing (Table 1, Fig. 2). The NÁ Á ÁO distance should be in the region of 2.72-2.78 Å , The observed N1-H1Á Á ÁO1 i DÁ Á ÁA distance [2.680 (3) Å ; Table 1] corresponds to a [(+/À)CAHB] interaction. Looking at the NÁ Á ÁO distances in the symmetry-related hydrogen bonding between squarate ring systems, it can be seen that the interaction is slightly shorter than the relevant interval values and is symbolized as either N + -HÁ Á Á O 1\2À or N + -HÁ Á Á O À (+/À)CAHB (Korkmaz & Bulut, 2013). The C-HÁ Á ÁO (Table 1, Fig. 2) interactions correspond to weak hydrogen bonding with an electrostatic or dispersion character according to the classification of Jeffrey (1997). In the structure, the weak C-HÁ Á ÁO interactions are responsible for the connection between the ribbons. Therefore it can be said that the hydrogen bonds form the molecular assembly, producing a uni-dimensional construction in the supramolecular view, while the C-HÁ Á ÁO interactions extend this to bi-dimensionality.

Computational studies
We have applied computational methods to evaluate the compound in terms of NLO activity. The values of the dipole moment ( tot ), linear polarizability ( tot ) and first-order hyperpolarizability ( tot ) of the molecule were calculated at the DFT/B3LYP method level of 6-31++G(d,p) by using Gaussian 03W program (Frisch et al., 2004). Urea is accepted as a prototype molecule for non-linear optical materials and results were compared with its values (Pu, 1991). The calculation results for tot , tot and tot for urea at the same level are 3.8583 D, 4.9991 Å 3 and 3.2637 x 10 À31 cm 5 /esu, respectively. The obtained values of tot , tot and tot for the title compound are 14.8448 D, 22.2315 Å 3 and 6.8664 Â 10 À30 cm 5 / esu, respectively. These values are comparable with those for some of the pyridinium-betains of squaric acid (Kolev et al., 2008). The value of tot appears to be much greater than that of urea. This result clearly indicates that the title compound is a strong candidate to develop a non-linear optical material. This is a prerequisite for the design of efficient second-and third-order non-linear optical materials. It should be noted that the title compound crystallized in a centrosymmetric space group (P2 1 /n).

Synthesis and crystallization
The title compound was synthesized according to the procedure of Schmidt et al. (1984). Squaric acid (H 2 Sq; 1g, 8.7 mmol) and 2-ethyl-4-methylimidazole (0.96 g; 8.7 mmol) were dissolved in acetic anhydride (30 cm 3 ) in the molar ratio 1:1 and the solution was heated to 323 K using a controlled bath and stirred for 1 h. The reaction mixture was then cooled slowly to room temperature. The crystals formed were filtered, washed with water and methanol, and dried in air. A few days later, well-formed crystals were selected for X-ray studies. Elemental analysis for the compound (green, yield 48%) C 10 H 10 N 2 O 3 : calculated: C, 58.00; H, 5.11; N, 13.56%. Found: C, 58.25; H, 4.89; N, 13.59%. M.p. 544 K.

2-Ethyl-4-methyl-1-(2-oxido-3,4-dioxocyclobut-1-en-1-yl)-1H-imidazol-3-ium
Crystal data 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.