Complete X-ray single-crystal structure determination and Raman spectrum of NH₄[C(CN)₃]

2.1 Synthesis

Ag[tcm] was synthesized by blending an aqueous solution containing 200 mg (2.25 mmol) K[tcm] (96%, powder, Alfa Aesar, Ward Hill, MA, USA) with a solution containing 400 mg (2.35 mmol) of AgNO₃ (≥99%, Sigma-Aldrich, St. Louis, MO, USA). The off-white precipitate was washed with deionized water and dried using an aspirator. The precipitate was mixed with 50 mg (1.2 mmol) NH₄Cl (≥99%, Sigma-Aldrich, St. Louis, MO, USA) in 5 mL of deionized water. After stirring this mixture for 6 h, the resulting solution was filtered to remove the precipitated AgCl. The water was evaporated in a drying oven at 80°C. The material crystallizes in form of plates and is not hygroscopic and appears to be stable over periods of several weeks.

2.2 Crystallographic studies

Samples of NH₄[tcm] taken out of the drying oven were immersed in dried polybutene oil (Aldrich, St. Louis, MO, USA, M~320, isobutylene>90%) for single-crystal selection under a polarization microscope, mounted in a drop of polybutene sustained in a plastic loop, and placed onto the goniometer in a cold stream of nitrogen [T=203(2) K] which froze the polybutene oil, thus keeping the crystal stationary and protected from oxygen and moisture in the air. Intensity data were collected with a Bruker X8 Apex II diffractometer equipped with a 4 K CCD detector and graphite-monochromatized MoK_α radiation (λ=71.073 pm). The intensity data were handled with the program package [8] that came with the diffractometer. The intensity data were corrected for Lorentz and polarization effects, but due to the low X-ray absorbance of NH₄[tcm], no other absorption correction was applied. Systematic absences led to the space group P2₁/c. The program Shelxs-97 [9], [10] found the carbon and nitrogen positions with the help of Direct Methods. The hydrogen positions were apparent from the positions of the highest electron density on the difference Fourier map resulting from the first refinement cycle by full-matrix least-squares techniques with Shelxl-97 [11], [12]. Doing further refinement cycles, the refinement converged and resulted in a stable model for the crystal structure. Additional crystallographic details are described in Table 1. Atomic coordinates and equivalent isotropic displacement coefficients are shown in Table 2. Table 3 displays the anisotropic displacement parameters and Table 4 shows selected distances and angles.

Further details of the crystal structure investigation may be obtained from FIZ Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.de) on quoting the deposition number CSD-432839 for NH₄[tcm].

2.3 Raman spectroscopy

Transparent colorless single crystals of NH₄[tcm] sealed in thin-walled glass capillaries were used for the Raman-spectroscopic investigations, which were performed on a microscope laser Raman spectrometer (Jobin Yvon, Unterhaching, Germany), 4 mW, equipped with a HeNe laser with an excitation line at λ=632.817 nm, 50× magnification, and 8×240 s accumulation time. The Raman spectrum is displayed in Fig. 1 and the vibrational data are shown together with those of related compounds [7], [13] in Table 4.

Fig. 1:

Raman spectrum of NH₄[tcm]. Both insets are enlarged by the factor of 25 on the vertical axis.

3.1 Raman spectrum

The Raman spectrum of the title compound shows frequencies indicating unambiguously the presence of the ammonium and tricyanomethanide ions (Table 4). Weak bands indicating the presence of the NH₄⁺ cation are observed in the expected region around 2800–3400 cm⁻¹. Modes with very weak intensities around 959 cm⁻¹ and 980 cm⁻¹ are observed for the anion which is predicted to be Raman-active only for C_3h symmetry [13]. Anyhow, the observed intensities of these modes might be seen as indication, but not as proof for a deviation of the anion from the ideal D_3h symmetry (see paragraph ‘Crystal structure’).

3.2 Crystal structure of NH₄[tcm]

The four hydrogen atoms of the cation exhibit N–H bond lengths around 80 pm. This is shorter than expected, but this is likely an artifact of the structure determination method using X-rays for locating electron densities. All four H atoms are located near a direct line drawn from the central nitrogen atom of NH₄⁺ to the closest nitrogen atom belonging to different [tcm] anions. These N–N distances are all near 300 pm. Within a distance below 350 pm, four more N–N contacts complete the coordination sphere of the ammonium cation in NH₄[tcm] forming a distorted cube (Fig. 2, left; Table 3).

Fig. 2:

Coordination of the ammonium cation (left) and the tricyanomethanide anion (right). Thick black lines indicate covalent bonding, the thin lines indicate hydrogen bonds towards nitrogen.

The [tcm]⁻ anion is surrounded by four ammonium ions, in which N1 is coordinated by two cations, and each N2 and N3 by one cation. The [tcm] moiety is slightly distorted by this unsymmetric coordination, but equivalent bonds have the same bond length within three times the standard deviations (Fig. 2, right; Table 3). The central C0 atom is slightly out of the plane defined by N1, N2 and N3 by a little less than 0.15 pm, but this is not more than one standard deviation of any bond length determined in this work.

NH₄⁺ and [tcm]⁻ ions are stacked above each other along the crystallographic b axis (Fig. 3). These stacks of NH₄⁺ and [tcm]⁻ ions alternate along the crystallographic a axis forming layers. Double layers form parallel to the (ab) plane in which the [tcm]⁻ ions of each monolayer are tilted about 30° due to the 2₁/c screw axis symmetry with respect to the crystallographic c axis (Fig. 4) to achieve maximal packing density within the double layers.

Fig. 3:

Perspective view of the unit cell of NH₄[tcm] along to the crystallographic b axis. Thick black lines indicate covalent bonding, the thin lines indicate hydrogen bonds towards nitrogen.

Fig. 4:

Non-perspective view perpendicular to the (bc) plane. Thick black lines indicate covalent bonding, the thin lines indicate hydrogen bonds towards nitrogen.