Synthesis, single-crystal structure determination and Raman spectra of the tricyanomelaminates NaA₅[C₆N₉]₂ · 4 H₂O (A = Rb, Cs)

2.1 Synthesis

All manipulations were performed under normal atmospheric conditions. Na₃[TCM] was obtained by sealing gram portions of Na[dca] (Alfa Aesar, 96 %) under vacuum in silica tubes and heating the container up to 500 °C with subsequent annealing at this temperature for 6 h. The thus obtained Na₃[TCM] was dissolved with the respective alkali metal fluoride (Aldrich, 99 %) in a stoichiometric ratio of 2:5 (overall mass: 0.5 g) in 10 mL deionized, boiling water. The water was allowed to evaporate at r.t. leaving small cubes of NaF and brick-like cuboids of the title compounds behind.

An analog approach to synthesize NaK₅[TCM]₂ · 4 H₂O or similar mixed alkali metal compounds has been unsuccessful so far.

2.2 Crystallographic studies

Crystals of NaRb₅[TCM]₂ · 4 H₂O and NaCs₅[TCM]₂ · 4 H₂O were selected by their habit and immersed in polybutene oil (Aldrich, M_n ~ 320, isobutylene > 90 %) for single-crystal selection under a polarization microscope. Single crystals were mounted in a drop of polybutene sustained in a plastic loop, and placed onto the goniometer. A cold stream of nitrogen (T = 223(2) K) 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 manipulated with the program package [10] that came with the diffractometer. An empirical absorption correction was applied using sadabs [11]. The program shelxs-97 [12, 13] found the positions of the respective alkali metal(s) with the help of Direct Methods. The positions of the carbon and nitrogen atoms and of carbon, nitrogen and oxygen atoms, respectively, were apparent from the positions of the highest electron density on the difference Fourier maps resulting from the first refinement cycles by full-matrix least-squares calculations on F² with shelxl-97 [14, 15]. The positions of the hydrogen atoms could not be found and refined reliably. Doing further refinement cycles with all atoms being refined unrestrained the refinement converged and resulted in stable models for the respective crystal structure. Crystallographic details are described in Table 1. Atomic coordinates and equivalent isotropic displacement coefficients are shown in Table 2 for NaRb₅[TCM]₂ · 4 H₂O and in Table 3 for NaCs₅[TCM]₂ · 4 H₂O. Table 4 displays selected interatomic distances and angles of the title compounds.

Further details of the crystal structure investigation(s) 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-430375 for NaRb₅[TCM]₂ · 4 H₂O and CSD-430376 for NaCs₅[TCM]₂ · 4 H₂O.

2.3 Raman spectroscopy

The single-crystals of NaRb₅[TCM]₂ · 4 H₂O and NaCs₅[TCM]₂ · 4 H₂O 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, the exact frequencies and their assigned modes are shown in Table 5.

Fig. 1:

Raman spectra of NaRb₅[TCM]₂ · 4 H₂O and NaCs₅[TCM]₂ · 4 H₂O. On the vertical axis, Raman intensities are displayed in arbitrary units.

3.1 Raman spectra

The frequencies obtained from the Raman spectra of NaRb₅[TCM]₂ · 4 H₂O and NaCs₅[TCM]₂ · 4 H₂O compare well to the vibrational frequencies reported in the literature for Na₃[TCM] · 3 H₂O [2] or Rb₃ [TCM] · H₂O [5] (Table 5) and confirm therefore the presence of the tricyanomelaminate anion.

3.2 The crystal structures of NaRb₅[TCM]₂ · 4 H₂O and NaCs₅[TCM]₂ · 4 H₂O

NaRb₅[TCM]₂ · 4 H₂O and NaCs₅[TCM]₂ · 4 H₂O are found to be isotypic. Generally speaking, the crystal structure is somewhat typical for [TCM] containing compounds. It is built up from layers of sodium and rubidium or cesium cations, respectively, and of the [TCM] anion. The tricyanomelaminate anion consists of a six-membered triazine ring with three N–C–N substituents each bound to a carbon atom of the triazine ring (Fig. 2). One of the nearly linear substituents is turned by 180°, thereby reducing the molecular symmetry of the anion from C_3h to C_s – if one neglects the slight deviations from planarity. All C–N bond lengths and angles (Table 5) are in the expected range when compared to the lengths and angles of similar compounds. The sodium atom is coordinated in an octahedral fashion by four nitrogen and two trans-positioned water molecules. The four crystallographically independent Rb or Cs atoms, respectively, are eight-fold coordinated in an irregular fashion by nitrogen and/or oxygen atoms. The crystal water molecules are found both in the plane of the alkali metal cations, but also displaced from the cation layers (both up and down) completing the coordination sphere of the cations (Fig. 3).

Fig. 2:

The [TCM] anion as found in NaA₅[TCM]₂ · 4 H₂O (A = Rb or Cs).

Fig. 3:

Non-perspective view on the unit cell of NaA₅[TCM]₂ · 4 H₂O parallel to the crystallographic c axis (C, black circles; N, white circles; O, black circles; H, white circles; Na, white ellipsoids; Rb or Cs, gray ellipsoids).