3,5-Dinitrobenzoic acid–dimethyl sulfoxide (1/1)

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.

The title complex, C 7 H 4 N 2 O 6 ÁC 2 H 6 OS, involves an intermolecular hydrogen-bond motif between the carboxyl and the sulfoxide groups. This motif has precedence in the Cambridge Structural Database, and is shown here to be more favourable than a possible carboxylic acid homodimer.

Comment
As part of ongoing research into the crystallization preferences of crystalline complexes, the growth of a previously reported three-component crystalline complex (Pedireddi et al., 1996) was attempted. The solvent dimethyl sulfoxide (DMSO) was used to aid dissolution of the components. In several instances, the crystallization unexpectedly resulted in single crystals of the title adduct, (I). The asymmetric unit consists of one molecule each of 3,5-dinitrobenzoic acid (3,5-DNBA) and DMSO (Fig. 1).
Previously reported crystallizations of 3,5-DNBA indicate a dimorphic nature for this substance. Two monoclinic polymorphs have been reported (Prince et al., 1991) and both exhibit the common carboxylic acid dimer hydrogen-bond motif. The interaction between 3,5-DNBA and DMSO described here involves a hydrogen bond between the carboxyl moiety of the acid and the sulfoxide group of the solvent (Fig. 2). It is interesting, therefore, that the title complex represents the disruption of the carboxylic acid dimer by the introduction of the sulfoxide moiety upon complexation.
A search of the Cambridge Structural Database (CSD, Version 5.25, Update 3; Allen, 2002) reveals precedence for this acid-DMSO interaction. Searching for structures which contain both a carboxyl moiety and a DMSO molecule among all organic structures for which three-dimensional coordinates have been determined resulted in 37 hits. Of those, 29 complexes exhibit an O-HÁ Á ÁO S hydrogen bond which is shorter than the sum of the van der Waals radii of the two O atoms. The apparent substantial likelihood of the formation of this motif indicates a potential utility for crystal engineering experimental design.
Crystal packing results in what may be perceived as alternating sheets of 3,5-DNBA and DMSO stacking along [001] (Figs. 3 and 4).

Experimental
All starting components were obtained from Sigma Aldrich Ltd. 3,5-Dinitrobenzoic acid (357 mg) and 0.5 equivalents of anthracene (150 mg) were heated to reflux in benzene (ca 50 ml). To dissolve the solids fully, a small quantity of DMSO (ca 2 ml) was added to the slurry. The resulting solution was allowed to cool and evaporate slowly over a period of 24 h. Crystals were observed before all the solvent had evaporated; a single crystal was harvested from this saturated solution for X-ray diffraction analysis.

Crystal data
178 parameters H atoms treated by a mixture of independent and constrained refinement All H atoms bonded to C atoms were placed geometrically and refined using a riding model. The U iso values for methyl H atoms were taken as 1.5U eq of the carrier atom. For all other H atoms, U iso (H) = 1.2U eq (carrier atom). The C-H distances of the methyl groups were organic papers    Data collection: COLLECT (Nonius, 1998); cell refinement: SCALEPACK (Otwinowski & Minor, 1997); data reduction: SCALEPACK and DENZO (Otwinowski & Minor, 1997); program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: XP (Sheldrick, 1993) and DIAMOND (Brandenburg, 1999); software used to prepare material for publication: SHELXL97. 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq O1 0.41031 (17