Benzylammonium hepta­noate–hepta­noic acid (1/1)

Mary H Wood; Stuart M Clarke

doi:10.1107/S1600536813003012

. 2013 Feb 6;69(Pt 3):o346–o347. doi: 10.1107/S1600536813003012

Benzylammonium heptanoate–heptanoic acid (1/1)

Mary H Wood ^a, Stuart M Clarke ^a,^*

PMCID: PMC3588549 PMID: 23476539

Abstract

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.

Related literature

For spectroscopic studies of acid–amine complexes, see: Karlsson et al. (2000 ▶); Kohler et al. (1981 ▶); Smith et al. (2001 ▶, 2002 ▶); Klokkenburg et al. (2007 ▶). For recent diffraction studies of acid–amine complexes, see: Jefferson et al. (2011 ▶); Sun et al. (2011 ▶); Wood & Clarke (2012a ▶,b ▶).

Experimental

Crystal data

C₇H₁₀N⁺·C₇H₁₃O₂ ⁻·C₇H₁₄O₂
M _r = 367.52
Monoclinic,
a = 25.5516 (5) Å
b = 6.3250 (1) Å
c = 27.9899 (6) Å
β = 90.639 (1)°
V = 4523.27 (15) Å³
Z = 8
Mo Kα radiation
μ = 0.07 mm⁻¹
T = 180 K
0.23 × 0.05 × 0.05 mm

Data collection

Nonius KappaCCD diffractometer
Absorption correction: multi-scan (SORTAV; Blessing, 1995 ▶) T _min = 0.910, T _max = 1.000
22723 measured reflections
5085 independent reflections
2353 reflections with I > 2σ(I)
R _int = 0.060

Refinement

R[F ² > 2σ(F ²)] = 0.068
wR(F ²) = 0.238
S = 0.98
5085 reflections
232 parameters
10 restraints
H-atom parameters constrained
Δρ_max = 0.29 e Å⁻³
Δρ_min = −0.20 e Å⁻³

Data collection: COLLECT (Nonius, 1998 ▶); cell refinement: SCALEPACK (Otwinowski & Minor, 1997 ▶); data reduction: DENZO (Otwinowski & Minor, 1997 ▶) and SCALEPACK; program(s) used to solve structure: SIR92 (Altomare et al., 1994 ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ▶); molecular graphics: Mercury (Macrae et al., 2008 ▶); software used to prepare material for publication: SHELXL97.

Supplementary Material

Click here for additional data file.^{(23.8KB, cif)}

Crystal structure: contains datablock(s) I, global. DOI: 10.1107/S1600536813003012/mw2101sup1.cif

e-69-0o346-sup1.cif^{(23.8KB, cif)}

Click here for additional data file.^{(249.1KB, hkl)}

Structure factors: contains datablock(s) I. DOI: 10.1107/S1600536813003012/mw2101Isup2.hkl

e-69-0o346-Isup2.hkl^{(249.1KB, hkl)}

Click here for additional data file.^{(8.1KB, cml)}

Supplementary material file. DOI: 10.1107/S1600536813003012/mw2101Isup3.cml

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Table 1. Hydrogen-bond geometry (Å, °).

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N1—H1A⋯O1ⁱ	0.91	1.87	2.773 (3)	171
N1—H1B⋯O2ⁱⁱ	0.91	1.96	2.823 (3)	158
N1—H1C⋯O2	0.91	1.90	2.781 (3)	164
O3—H3⋯O1	0.84	1.78	2.610 (3)	172
C1—H1D⋯O4ⁱⁱⁱ	0.99	2.60	3.542 (4)	160
C3—H3A⋯O2ⁱ	0.95	2.59	3.456 (4)	152

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Symmetry codes: (i) Inline graphic ; (ii) ; (iii) .

Acknowledgments

We thank the Department of Chemistry, the BP Institute and the Oppenheimer Trust for financial and technical assistance, and Dr J. E. Davies for collecting and analysing the X-ray data.

supplementary crystallographic information

Comment

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.

Figure 3 illustrates the four hydrogen bonds around each carboxylate ion. Interestingly there are two hydrogen bonds evident for each oxygen (indicated by dotted black lines in the Figure).

An illustration of the two hydrogen bonds made by the protonated acid molecules is given in Figure 4. The non-protonated 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 the top of the outer vial. Reaction of the amine with atmospheric CO₂, was prevented by the inert atmosphere (Sun et al. 2011). The samples were measured at a temperature of 180 K.