4-Methyl-3,5-dinitrobenzoic acid–dimethyl sulfoxide (1/1)

The title complex, C8H6N2O6·C2H6OS, was predicted to illustrate an intermolecular hydrogen-bond motif between the carboxyl­ic acid and the sulfoxide funtionalities, based upon a previously published structure of an analogous complex. The predicted hydrogen-bond motif was observed, thereby indicating a certain robustness of this intermolecular interaction for crystal engineering purposes.

The crystallization was performed to evaluate the robustness of an intermolecular hydrogen bond involving an O-HÁ Á ÁO S contact between a carboxylic acid and a sulfoxide. This interaction was recently observed in the crystal structure of an analogous complex involving 3,5-dinitrobenzoic acid and DMSO (Abthorpe et al., 2005). This interaction also is found in 29 of a possible 37 instances in the Cambridge Structural Database (CSD Version 5.25 Update 3; Allen, 2002), when searching for structures which contain both a carboxyl group and a DMSO molecule among all organic structures for which three-dimensional coordinates have been determined. The hydrogen-bond interaction in the crystal structure is presented in Fig. 2.
The title complex packs in a monoclinic unit cell in the space group P2 1 /c. Crystal packing results in alternating sheets of acid and DMSO molecules stacking along [010]. (Figs. 3 and 4).
The experiment reported here represents a successful demonstration of the methodological approach of crystal engineering: observation of a particular heteromolecular hydrogen-bonding interaction, evaluation of the abundance of the interaction in the CSD, and application of this information to the design of a novel crystalline molecular complex. The demonstrated robustness of this hydrogen-bond motif indicates a potential utility for future crystal engineering experiment design.

Experimental
All starting components were obtained from Sigma Aldrich Ltd. 4-Methyl-3,5-dinitrobenzoic acid (64 mg) was dissolved in excess DMSO with gentle heating. The resulting solution was allowed to cool and evaporate slowly over a period of one week. From the solids that precipitated, a single crystal was harvested for subsequent XRD analysis.

Crystal
All H atoms bonded to carbon were positioned geometrically and refined using a riding model, with U iso = 1.5U eq for methyl H atoms and U iso (H) = 1.2U eq (carrier atom) for all other H atoms. The C-H distances of the methyl groups were fixed at 0.98 Å ; all other C-H distances were fixed at 0.95 Å . The O-H H atom was located in a difference Fourier map and refined isotropically.

Figure 4
The crystal packing (DIAMOND; Brandenburg, 1999)  Special details Experimental. The -COOH hydrogen atom was located and its position was refined satisfactorily. Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 > σ(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.