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Crystal structure of the bis(cyclohexylammonium) succinate succinic acid salt adduct

The title salt adduct comprises two cyclohexylammonium cations, one succinate anion and one molecule of succinic acid, linked together through intermolecular hydrogen-bonding interactions giving a two-dimensional layer-like self-assembly lying parallel to (010).

The crystal structure of the title salt adduct, 2C 6 H 14 N + ÁC 4 H 4 O 4 2À ÁC 4 H 6 O 4 , consists of two cyclohexylammonium cations, one succcinate dianion and one neutral succinic acid molecule. Succinate dianions and succinic acid molecules are self-assembled head-to-tail through O-HÁ Á ÁO hydrogen bonds and adopt a syn-syn configuration, leading to a strand-like arrangement along [101]. The cyclohexylammonium cations have a chair conformation and act as multidentate hydrogen-bond donors linking adjacent strands through intermolecular N-HÁ Á ÁO interactions to both the succinate and the succinic acid components. This results in two-dimensional supramolecular layered structures lying parallel to (010).

Chemical context
In the field of crystal engineering, dicarboxylic acids constitute very suitable building blocks which can act as polydirectional synthons and thus present numerous possibilities for molecular assembly through the formation of hydrogen-bonded networks (Ivasenko & Perepichka, 2011). Furthermore, the additional involvement of amines, via the formation of ammonium cations, significantly increases the potential for linkage and the topological diversity (Yuge et al., 2008;Lemmerer, 2011). Some papers dealing with spectroscopic studies on quaternary ammonium hydrogenoxalates have been reported from our laboratory (Gueye & Diop, 1995). In the scope of our current studies on the interactions between quaternary ammonium salts of carboxylic acids and halogenidotin(IV) complexes (Gueye et al., 2014), the reaction involving cyclohexylamine and succinic acid was initiated and led to the isolation of the title organic salt adduct 2C 6 H 14 N + ÁC 4 H 4 O 4 2À ÁC 4 H 6 O 4 , (I), the structure of which is reported herein.

Structural comments
The asymmetric unit of (I) contains two cyclohexylammonium cations, one succinate dianion and one molecule of succinic acid (Fig. 1 Bruno et al., 2004;Du et al., 2009;Zhang et al., 2011;Froschauer & Weil, 2012), it is interesting to note that the carbon-oxygen bond distances recorded for the succinic acid [C1-O1 = 1.2974 (17) (17)] are very similar. In general, a more pronounced difference in length is expected between the C O bond and the C-OH bond of succinic acid (in the range of 0.1 Å ), while for the succinate dianion the deviation between the C-O bonds is narrowed (in the range of 0.01 Å ). Thus, to confirm more accurately the nature of the components of (I), namely the presence of distinct succinic acid and succinate species, electron-density mapping has been performed (Fig. 2). It follows that the location of the acidic protons is clearly established, confirming unambiguously the composition of (I). Moreover, the relative equalizing of the carbon-oxygen bonds can be explained by the contribution of concomitant N-HÁ Á ÁO interactions involving all oxygen atoms of succinic acid and the succinate dianion with surrounding cyclohexylammonium cations. The average C-C-C-O torsion angle, calculated on 616 succinic acids, is equal to 171 (12) with a deviation of the mean equal to 0.4 , whereas the average torsion angle calculated on 964 succinate acids is equal to 167 (12) with a deviation of the mean also equal to 0.4 . These results match the torsion angles found in (I) for succinic acid: 154.09 (16), 156.32 (12), 159.25 (17) and 161.07 (12) but those found for the succinate anion are rather different: 121.41 (15), 121.78 (17), 151.8 (2) and 152.14 (13) .

Figure 2
Electron-density mapping around C 4 H 6 O 4 and C 4 H 4 O 4 2À , showing the precise location of acidic protons.

Figure 3
Crystal packing of (I) viewed along the a axis, showing the infinite strands based on succinate-succinic acid hydrogen-bonding interactions and linked through the cyclohexylammoninum cations into sheets. Intermolecular hydrogen bonds are shown as dashed blue lines. H atoms not involved in hydrogen bonding are omitted for clarity. Colour code: C dark grey, H light grey, O red, N blue. actions linking the succinate-succinic acid strands, giving twodimensional supramolecular layers lying parallel to (010) (Fig. 3).

Synthesis and crystallization
The title compound was obtained by reacting cyclohexylamine (5.76 mL) with succinic acid (5.0 g) in a molar ratio of 2:1, in 50 mL of water, at 298 K. The resulting clear solution was allowed to evaporate at 298 K leading after a few days to colourless block-like crystals suitable for an X-ray crystal structure determination.

Special details
Experimental. SADABS (Bruker, 2014) was used for absorption correction. wR2(int) was 0.0455 before and 0.0417 after correction. The ratio of minimum to maximum transmission is 0.9524. The λ/2 correction factor is 0.0015. 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.