Crystal structures and hydrogen bonding in the co-crystalline adducts of 3,5-dinitrobenzoic acid with 4-aminosalicylic acid and 2-hydroxy-3-(1H-indol-3-yl)propenoic acid

The crystal structures of the co-crystal adducts of 3,5-dinitrobenzoic acid with 4-aminosalicylic acid (a 2:2:0.4-hydrate) and with 2-hydroxy-3-(1H-indol-3-yl)propenoic acid (a 1:1:1 d 6-DMSO solvate) show, respectively, polymeric and hexamolecular hydrogen-bonded and π–π-bonded structures


Structural commentary
In the co-crystal of 3,5-DNBA with 4-aminosalicylic acid, (I) (Fig. 1), the asymmetric unit consists of two PASA molecules (A and B), two 3,5-DNBA molecules (C and D) and a partially occupied water molecule of solvation (O1W), with site occupancy = 0.4. However, what is most unusual in this structure is the presence of not four homodimers in the unit cell, but two homodimers (centrosymmetric PASA A-A i and 3,5-DNBA C-C ii pairs), as well as two heterodimer B-D pairs (for symmetry codes, see Table 1). All dimers are formed through the common cyclic R 2 2 (8) ring motif. Present in the PASA molecules are the expected intramolecular salicylic acid phenolic O-HÁ Á ÁO carboxyl hydrogen bonds, also present in the parent acid (Montis & Hursthouse, 2012).

Figure 2
Molecular conformation and atom-naming scheme for adduct (II), with displacement ellipsoids drawn at the 40% probability level. Inter-species hydrogen bonds are shown as dashed lines.
This unit is essentially planar with a dihedral angle of 4.97 (7) between the indole ring of A and the benzene ring of B. With the HIPA molecule there is a maximum deviation from the least-squares plane of the 15-atom molecule of 0.120 (2) Å (C6A). The planar conformation of the acid side chain in this molecule is maintained by the presence of delocalization extending from C2A of the ring to O14A of the carboxylic acid group [torsion angle C11A-C12A-C13A-O14A = À177.43 (16) ]. This is also found in the parent acid, which has the similar enol configuration as in (II) [corresponding torsion angle 170.0 (3) ] with an E orientation and in the crystal forms a centrosymmetric homodimer with an R 2 2 (8) hydrogen-bond motif (Okabe & Adachi, 1998).
In the adducts (I) and (II), the 3,5-DNBA molecules are essentially planar with the exception of the C3-nitro groups of the C molecule in (I), and the B molecule in (II), where the defining C2-C3-N3-O32 torsion angles are 158.2 (3) and 168.39 (17) , respectively. The overall torsion angle range for the remaining groups in both (I) and (II) is 170.8 (3)-179.2 (2) . These minor deviations from planarity are consistent with conformational features of both polymorphs of the parent acid 3,5-DNBA (Prince et al., 1991)

Figure 4
The centrosymmetric hydrogen-bonded heterohexameric structure of the adduct (II) in the unit cell, viewed down a. For symmetry code (i), see Table 2.

Synthesis and crystallization
The title co-crystalline adducts (I) and (II) were prepared by dissolving equimolar quantities of 3,5-dinitrobenzoic acid and the respective acids 4-aminosalicylic acid [for (I)] or (1Hindol-3-yl)propenoic acid [for (II)] in ethanol and heating under reflux for 5 min after which room-temperature evaporation of the solutions gave for (I), yellow prisms and for (II), a red powder. This latter compound was dissolved in d 6deuterated DMSO and solvent diffusion of water into this solution gave red prisms of (II). Specimens were cleaved from both prismatic crystals for the X-ray analyses.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms on all potentially interactive O-H and N-H groups in all molecular species were located by difference-Fourier methods and positional and displacement parameters were refined for all but those of the phenolic O2A and O2B groups and on the disordered   used to refine structure: SHELXL97 (Sheldrick, 2008) within WinGX (Farrugia, 2012); molecular graphics: PLATON (Spek, 2009); software used to prepare material for publication: PLATON (Spek, 2009).

(I) 4-Amino-2-hydroxybenzoic acid-3,5-dinitrobenzoic acid-water (2/2/0.4)
Crystal data Special details Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles 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.