A co-crystal of ethylenediammonium bis(3,5-dinitrobenzoate) and 3,5-dinitrobenzoic acid

# 2005 International Union of Crystallography Printed in Great Britain – all rights reserved The co-crystal of ethylenediammonium bis(3,5-dinitrobenzoate) and 3,5-dinitrobenzoic acid, namely ethylenediaminium–3,5-dinitrobenzoate–3,5-dinitrobenzoic acid (1/2/2), C2H10N2 2+ 2C7H3N2O6 2C7H4N2O6, has as the asymmetric unit one 3,5-dinitrobenzoic acid molecule, one 3,5dinitrobenzoate ion and one-half of the ethylenediammonium ion, as this cation lies on an inversion centre. Each ethylenediammonium ion is hydrogen bonded to four benzoate ions and two benzoic acid molecules.

To measure the solubility of ethylenediammonium bis(3,5dinitrobenzoate) as a function of pH at 323 K, a suspension of the salt in water was prepared and allowed to equilibrate (Jones et al., 2005). In one experiment, the pH was found to be unusually low for a slurry of this salt and the experiment was stopped, but the sample continued to be held at 323 K. The cocrystals grew as pale-yellow prisms and were recovered on filtration of the slurry. Formation of these cocrystals was not observed in other solubility measurements at higher pH. Protonated 3,5-dinitrobenzoic acid is only expected to be present below pH 5 at 323 K (de Levie et al., 1999).
In the crystal structure, both a protonated and a deprotonated 3,5-dinitrobenzoic acid molecule are present in the asymmetric unit. The ethylenediammonium ion lies on an inversion centre so that only one-half of the ion is in the asymmetric unit. Fig. 1 shows the structure and atom labelling.
Each ethylenediammonium ion is hydrogen bonded to four benzoate ions and two benzoic acid molecules (Fig. 2). The crystal structure contains hydrogen-bonded chains of ethylenediammonium and benzoate ions along the a axis in the motif C 2 2 (6) (Fig. 3), hydrogen-bonded dimers of benzoate ions with benzoic acid molecules with an O-HÁ Á ÁO hydrogen bond through atom H7 in the motif D 1 1 (2), and dimers of ethylenediammonium ions hydrogen bonded to the carbonyl group of a benzoic acid molecule in the motif D 1 1 (2). The benzoate ions in this structure all lie in one plane and the benzoic acid molecules all lie in another orientation.

Figure 1
View of the asymmetric unit of (I), including the whole ethylenediaminium ion, which is on an inversion centre. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry code: (i) Àx, Ày, Àz.]

Figure 3
Unit cell contents, viewed along the c axis, showing hydrogen-bonded chains (dashed lines) along the a axis.
All H atoms attached to C and N atoms were fixed using a riding model, with C-H distances 0.93 Å (C Ar H) and 0.97 Å (CH 2 ), and N-H distances 0.89 Å . The U iso (H) values were set equal to 1.2U eq of the carrier atom for these H atoms. The hydroxy H atom was located in a Fourier difference map and the coordinates were refined with the O-H bond distance restrained to 0.82 (1) Å .
Data collection: COLLECT (Nonius, 2000); cell refinement: HKL SCALEPACK (Otwinowski & Minor, 1997); data reduction: HKL DENZO (Otwinowski & Minor, 1997) and SCALEPACK; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: WinGX (Farrugia, 1999 Special details Experimental. Solution pH was measured using an Accumet Basic AB15 pH meter with an Accumet glass calomel pH electrode and an ATC probe to compensate for temperature changes. 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.