Benzylammonium heptanoate–heptanoic acid (1/1)

The title salt, C7H10N+·C7H13O2 −·C7H14O2, is an unusual 2:1 stoichiometric combination of two carboxylic acid molecules and one amine. Although there are crystal structures of a number of 1:1 complexes reported in the literature, 2:1 acid amine complexes are rather uncommon. In this case, a proton is transferred between one acid molecule and the amine to give an acid anion and an ammonium cation whilst the other carboxylic acid remains protonated. The species interact strongly via electrostatic forces and hydrogen bonds. In addition we note that the N atom of the ammonium group makes four close contacts to surrounding O atoms. Three of these are hydrogen bonds with neighbouring acid anions while the fourth does not involve a hydrogen atom but is directed towards the carbonyl O atom of the protonated acid. Each of the acid anion O atoms accepts two hydrogen bonds from adjacent N atoms. There is also evidence of short C—H⋯O contacts. There is disorder (occupancy ratio 0.51:0.49) in the alkyl chain of one of the carboxylic acid molecules.

The title salt, C 7 H 10 N + ÁC 7 H 13 O 2 À ÁC 7 H 14 O 2 , is an unusual 2:1 stoichiometric combination of two carboxylic acid molecules and one amine. Although there are crystal structures of a number of 1:1 complexes reported in the literature, 2:1 acid amine complexes are rather uncommon. In this case, a proton is transferred between one acid molecule and the amine to give an acid anion and an ammonium cation whilst the other carboxylic acid remains protonated. The species interact strongly via electrostatic forces and hydrogen bonds. In addition we note that the N atom of the ammonium group makes four close contacts to surrounding O atoms. Three of these are hydrogen bonds with neighbouring acid anions while the fourth does not involve a hydrogen atom but is directed towards the carbonyl O atom of the protonated acid. Each of the acid anion O atoms accepts two hydrogen bonds from adjacent N atoms. There is also evidence of short C-HÁ Á ÁO contacts. There is disorder (occupancy ratio 0.51:0.49) in the alkyl chain of one of the carboxylic acid molecules.
Data collection: COLLECT (Nonius, 1998) Stable complexes formed between simple alkyl carboxylic acids and alkyl amines have been reported e.g. (Karlsson et al., 2000;Wood et al., 2012a;Wood et al., 2012b) most commonly from spectroscopic studies. Interestingly there also exist examples of 2:1 and 3:1 acid amine complexes, usually in an acid-rich environment (Sun et al., 2011;Kohler et al., 1981). No equivalent amine-rich complexes have yet been observed, although (Smith et al., 2001) and (Smith et al., 2002) have reported a diamine complex formed between methylamine and dnsa due to deprotonation of the phenolic group in the acid (Smith et al., 2002).
Such acid:amine complexes are generally considered to derive their stablity from the complete transfer of a proton from the acid to the amine with subsequent cation-anion electrostatic interaction and strong hydrogen-bond formation. In 2:1 or higher stoichiometry complexes, the hydrogen bond is considered to extend over the three (or more) species involved.
However, because there are very limited single crystal diffraction data, the exact form of the interactions are not clear.
In this paper the crystal structure of the 2:1 complex formed by two heptanoic acid molecules and benzylamine is reported. This complex results from the donation of a proton from one acid to the base, forming a carboxylate anion and an ammonium cation. This pair of ions, along with an additional protonated acid molecule, forms the structure illustrated in Figure 1.
All these species interact strongly by electrostatic forces and hydrogen bonding. Figure 2 illustrates the non-covalent interactions around one ammonium ion. There are three hydrogen bonds around each ammonium group nitrogen atom (indicated by dotted black lines in the Figure). Unexpectedly, an additional short contact is observed between the carbonyl group of a protonated acid and the nitrogen atom of the ammonium ion with no hydrogen atom. This interaction is also indicated by a black dotted line in Figure 2. An illustration of the two hydrogen bonds made by the protonated acid molecules is given in Figure 4. The nonprotonated oxygen forms a hydrogen bond with an adjacent carboxylate ion. The other carbonyl group appears to interact with the nitrogen of an ammonium ion.
The alkyl chain of the protonated acid group shows a significant degree of conformational disorder. The chain is not all trans but shows gauche conformers.

Experimental
Heptanoic acid and benzylamine, with purities of 99.8% and 99.7% respectively (titration and GC), were obtained from Sigma Aldrich and used without further purification. A small volume (approximately 1 mL) of each material was placed into two small vials and placed inside a larger vial with an inert atmosphere of nitrogen for a number of weeks, during which numerous crystals grew, particularly on a sample of polypropylene included as a nucleating surface, and around supplementary materials the top of the outer vial. Reaction of the amine with atmospheric CO 2 , was prevented by the inert atmosphere (Sun et al. 2011). The samples were measured at a temperature of 180 K.

Figure 1
Illustration of the title compound with atom labels.    Illustration of the two non-covalent bonds made by the protonated acid molecules indicated by black dotted lines.  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.29 e Å −3 Δρ min = −0.20 e Å −3 Special details Experimental. Part of the C 7 H 14 O 2 molecule is disordered over two sites. In the refinement, the disordered carbon atoms were assigned common isotropic displacement parameters and geometric constraints. 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. Part of the C 7 H 14 O 2 molecule is disordered over two sites. In the refinement, the disordered carbon atoms were assigned common isotropic displacement parameters and geometric constraints. The dataset presented is one of a number of separate datasets collected from different crystals. Even so, the crystal diffracted very poorly, a fact reflected in the low percentage of observed structure factors, the use of common isotropic displacement parameters for the disordered parts, and the 'unreasonable′ bond lengths. The SORTAV process was found to make no significant difference in this case which can be attributed to the Mo radiation and such a small crystal, the tiny mount and minimal oil, as one would expect. The SORTAV process was run as a check and to ensure that the equivalent reflections have been measured correctly -routine in our laboratory. The low percentage of observed reflections is due to the strenuous efforts which were made to measure everything possible with this poor crystal. The theta limit which was set for the data collection (27.5°) was probably higher than ideal but did ensure that nothing was missed. Again this was due to the very small, very poorly diffracting crystal.