Tetraethylammonium 7,12-dicyano-1-carba-closo-dodecaborate

In the title compound, C8H20N+·C3H10B11N2 −, the carborane anion cage displays nearly-perfect Cs symmetry, with the two CN groups lying on a noncrystallographic mirror plane that bisects the cage. In the crystal, the anions form extended chains along the a-axis direction, with C—H⋯N hydrogen bonds linking consecutive anions. The C N bond lengths (and B—C N angles) in the nitrile moities are 1.1201 (19) Å, 178.60 (15)° and 1.1433 (17) Å, 179.45 (15)°, similar to those observed in organic nitriles. A hydrogen bond between a methylene H atom of the cation and the N atom in one of the nitrile groups of the anion is the closest contact between the anion and cation, at 2.52 Å.

In the title compound, C 8 H 20 N + ÁC 3 H 10 B 11 N 2 À , the carborane anion cage displays nearly-perfect C s symmetry, with the two CN groups lying on a noncrystallographic mirror plane that bisects the cage. In the crystal, the anions form extended chains along the a-axis direction, with C-HÁ Á ÁN hydrogen bonds linking consecutive anions. The C N bond lengths (and B-C N angles) in the nitrile moities are 1.1201 (19) Å , 178.60 (15) and 1.1433 (17) Å , 179.45 (15) , similar to those observed in organic nitriles. A hydrogen bond between a methylene H atom of the cation and the N atom in one of the nitrile groups of the anion is the closest contact between the anion and cation, at 2.52 Å .

Comment
Derivatives of the 1-carba-closo-dodecaborate(1-) carborane anion, e.g. CB 11 H 12 -, have been recognized as exceptionally weakly-coordinating anions (Reed, 1998) and they have been used to prepare the strongest Brønsted acids known (Juhasz et al., 2004). This family of carborane anions has further potential uses in pharmaceuticals, in optical and electronic materials, and in catalysts for industrial-scale chemical reactions. A relatively small number of synthetic reactions have been developed for producing new derivatives of CB 11 H 12 -, and derivatives bearing CN groups on boron were unknown until very recently (Rosenbaum et al., 2013). In the present report, we describe the crystal structure of the tetraethylammonium salt of the dicyanated carborane anion, 7,12-(CN) 2 -closo-CHB 11 H 9 -. In the crystal structure, the carborane anion cluster has nearly perfect C s symmetry, with the two CN groups lying on a mirror plane that bisects the cluster. The carborane anions pack to form extended chains. The closest contact between consecutive anions is hydrogen bond with a length of 2.406 Å from hydrogen on C1 of one cluster to nitrogen in the CN group on B12 of the next. A weak interaction between the anion and cation is indicated; the closest contact between these involves a methylene hydrogen on the tetraethylammonium cation at 2.519 Å from the nitrogen atom in the CN group on B7 of the carborane anion. The C≡N bond distances (and B-C≡N angles) are 1.1201 (19) Å, 178.60 (15)° and 1.1433 (17) Å, 179.45 (15)° for the CN groups on B12 and B7, respectively. These bond lengths are similar to those observed in organic nitriles (Allen et al., 1987).

Experimental
For the synthesis and spectroscopic characterization of the title compound, see (Rosenbaum et al., 2013). Colorless crystals of the compound suitable for X-ray diffraction were obtained by the slow evaporation over 10 days of a saturated solution of the compound in a 1:3 acetonitrile/water solution. The crystallization procedure was performed under normal atmosphere at 25 °C using reagent grade acetonitrile and deionized water.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. All H-atoms were positioned and

Figure 2
Packing diagram for tetraethylammonium 7,12-dicyano-1-carba-closo-dodecaborate.  Special details Experimental. Absorption correction: CrysAlisPro, Agilent Technologies, Empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm. 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.