Syntheses, single-crystal structure determination, and Raman spectra of Rb[C(CN)₃] and Cs[C(CN)₃]

2.1 Synthesis

All manipulations were performed under normal atmospheric conditions. All compounds were used as purchased. Single crystals were obtained by first dissolving 65 mg (0.5 mmol) K[tcm] (Strem, >98 %) and 52 mg (0.5 mmol) RbF or 76 mg (0.5 mmol) CsF (both Aldrich, 99 %) in 5 mL deionized water. The water was evaporated in a drying oven at 80 °C. 5 mL ethanol (Pharmco) was added to the remaining solid and stirred for 5 min. The resulting solution was filtered, and the solvent was allowed to evaporate at r. t. to dryness.

2.2 Crystallographic studies

Samples of the crystalline product were immersed in polybutene oil (Aldrich, M_n ∼ 320, isobutylene > 90 %) for single-crystal selection under a polarization microscope. Crystals were mounted in a drop of polybutene sustained in a plastic loop and placed onto the goniometer. A cold stream of nitrogen (T=203(2) K) froze the polybutene oil, thus keeping the crystal stationary and protected from oxygen and moisture. 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 manipulated with the program package [6] that came with the diffractometer. An empirical absorption correction was applied using Sadabs [7]. The program Shelxs-97 [8, 9] found the positions of the respective alkali metal with the help of Direct Methods techniques. The positions of the carbon and nitrogen atoms were apparent from the positions of highest electron density on the difference Fourier map resulting from the first refinement cycles by full-matrix least-squares calculations on F² with Shelxl-97 [10, 11]. Additional crystallographic details are described in Table 1. Atomic coordinates and equivalent isotropic displacement coefficients are shown in Table 2. Table 3 displays selected interatomic distances and angles of the title compounds.

Further details of the crystal structure investigations may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.de, http://www.fiz-karlsruhe.de/request_for_deposited_data.html), on quoting the depository numbers CSD-428450 for Rb[tcm] and CSD-428451 for Cs[tcm].

2.3 Raman spectroscopy

The single crystals used for the structure determinations were sealed in thin-walled glass capillaries. Raman spectroscopic investigations were performed on a microscope laser Raman spectrometer (Jobin Yvon, 4 mW, equipped with a HeNe laser with an excitation line at λ=632.817 nm, 50× magnification, 8 × 240 s accumulation time). The results are displayed in Fig. 1, and the exact frequencies and their assigned modes are shown in Table 4.

Fig. 1

Raman spectra of Rb[tcm] and Cs[tcm]. On the vertical axis, Raman intensities are displayed in arbitrary units. Numbers are given in cm^–1.

3.1 Raman spectrum

The frequencies obtained from the Raman spectra compare well with the reported Raman frequencies for K[tcm] [12]. Interestingly, one weak band around 973 cm^–1 is observed for all three compounds, but the symmetry considerations predict this mode to be active only for a C_3h symmetry of the [tcm] anion [12].

3.2 Crystal structure

K[tcm] crystallizes in space group P1̅ exhibiting a very low symmetry [3], whereas both title compounds show higher symmetry and are isopointal to calcite, CaCO₃, if one considers Ca atoms to be replaced by Rb or Cs atoms, respectively, and oxygen by cyanide units. Therefore, the compounds crystallize in a rocksalt derivative, where chloride is replaced by carbonate or [tcm] moieties, respectively. The anions are in both cases strictly planar with D_3h symmetry (as required by the symmetry of the space group R3̅c), but the anisotropic displacement parameters U₃₃ parallel to the c axis are slightly elongated (Table 2). The plane of the respective anion is perpendicular to the crystallographic c axis (Fig. 2). The alkali metal atoms of Rb[tcm] and Cs[tcm] are coordinated by six nitrogen atoms in an octahedral fashion, but the octahedra are squashed in the direction of the c axis (Fig. 3). It is interesting to note that the c lattice parameter of both compounds is essentially the same, whereas the a and b cell parameters differ according to the size of the respective alkali metal cation.

Fig. 2

View of the unit cell of Rb[tcm] perpendicular to the c axis. Rb is shown as gray, C as black, and N as white ellipsoids. The displacement ellipsoids are shown at the 90 % probability level. The octahedral coordination of Rb is displayed as open, gray hatched polyhedra.

Fig. 3

The octahedral coordination of Rb is displayed as an open, gray hatched polyhedron. The same color code and probability level were used as in Fig. 2.