(S)-Alanine ethyl ester tetracyanidoborate, (C5H12NO)[B(CN)4]

The asymmetric unit of the title molecular salt contains two cations and two anions, which are linked by N—H⋯N hydrogen bonds in the extended structure.

The title molecular salt, C 5 H 12 NO + ÁC 4 BN 4 À or (C 5 H 12 NO)[B(CN) 4 ], was obtained as single crystals by slow evaporation of a solution of the compound in acetonitrile over several weeks. The asymmetric unit contains two (S)-alanine ethyl ester cations and two tetracyanidoborate anions, which are linked by N-HÁ Á ÁN hydrogen bonds. The compound exhibits a relatively low melting point of 110 C and shows a solid-solid phase transition near room temperature (T s-s = 29 C) on the basis of DSC measurements.

Structure description
For more than 20 years, ionic liquids as salts with low melting points have attracted great interest because of their unique properties and applications. These properties include for instance large liquid ranges, broad electrochemical windows as well as low vapour pressures (Hallett & Welton, 2011;Welton, 1999). The title compound acts as a first example of a low-melting chiral substance in our ongoing efforts to investigate tetracyanidoborate-based ionic liquids (Bernsdorf et al., 2009;Flemming et al., 2010;Siegesmund et al., 2017).

data reports
In the extended structure, the shortest hydrogen-bond contacts are found between the N-bonded H atoms of the cations (N9 and N10) and the N atoms of the tetracyanidoborate anions: the shortest NÁ Á ÁN donor-acceptor distance is 2.920 (3) Å (Table 1). Fig. 2 shows the packing of the ions within and around the unit cell.

Synthesis and crystallization
The title compound, (C 5 H 12 NO)[B(CN) 4 ], was obtained in high purity as a colorless solid on a multi-gram scale from the salt metathesis of (S)-alanine ethyl ester hydrochloride and K[B(CN) 4 ] in acetonic solution at room temperature. (S)-Alanine ethyl ester hydrochloride (2.0 g, 13.0 mmol) was added in one portion to a vigorously stirred solution of K[B(CN) 4 ] (2.2 g, 14.3 mmol) in 100 ml acetone at room temperature and was further stirred overnight. The precipitate was filtered off and the solvent of the filtrate was removed in vacuum. The residue was dissolved in a minimum amount of dichloromethane, filtered again and the solvent was removed in vacuum. The final product was obtained as a colourless solid in high yield (2.8 g, 91%); m.p. = 110 C, T s-s = 29 C. The thermal behaviour was determined by means of differential scanning calorimetry (DSC) in the temperature range from À100 to 200 C with a heating rate of 10 K min À1 . Analytical data for C 9 H 12 BN 5 O 2 % (calc.): C 46.43 (46.39); H 5.25 (5.19); N 26.53 (30.05).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Sixteen reflections were omitted from the refinement because their intensities were affected by the beam stop. Details can be found in the refine_ special_- Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
A view of the unit-cell contents in projection down the b axis.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.