Received 10 December 2012
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-HO contacts. There is disorder (occupancy ratio 0.51:0.49) in the alkyl chain of one of the carboxylic acid molecules.
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).
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 data and figures for this paper are available from the IUCr electronic archives (Reference: MW2101 ).
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.
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