The first pseudo-ternary thiocyanate containing two alkali metals – synthesis and single-crystal structure of LiK₂[SCN]₃

2.1 Synthesis

All manipulations were performed under normal atmospheric conditions unless otherwise stated. Li[SCN] was obtained by exposing Li[SCN] · x H₂O (>98 %; Sigma-Aldrich, St. Louis, MO, USA) to 200 °C for 2 h under dynamic vacuum until the pressure was constant and below 200 Pa. Then the temperature was raised over 30 min to 300 °C. The sample was held at this temperature for 30 min, still under the dynamic vacuum. The water-free material obtained this way is extremely hygroscopic, colorless with a slight yellowish tint (a more intense yellow substance also appeared above the heated zone of the ampoule). K[SCN] (>99 %; Sigma-Aldrich, St. Louis, MO, USA) was kept for 2 h at 120 °C under dynamic vacuum. Both compounds were then transferred into an argon-filled glove box. For further experiments, the compounds were mixed in a 1:1, 2:1, and 1:2 molar ratio (overall mass in all cases, 350 mg).

Initially, the samples were heated under vacuum up to 350 K for 30 min, yielding some X-ray-amorphous, highly viscous material that dissolved easily in water and had then the odor of rotten eggs.

In follow-up attempts, the starting materials were filled in the aforementioned ratios into screw-top vials. Absolute ethanol (2 mL; Pharmco, Brookfield, CT, USA) was added to each vial. After dissolving the educts completely, the vials were placed in a drying furnace at 80 °C, which caused most of the solvent to evaporate. Even by applying reduced pressure, the solvent could not be removed completely, and an amorphous, colorless lump with adhering solvent drops remained. Cooling the products inside the sealed vials to 4 °C in a refrigerator produced for all starting compositions, transparent, very fragile, and extremely hygroscopic thin rectangular plates inside the solvent drops. These plates were inspected with single-crystal methods and turned out to be the title compound in all cases.

2.2 Crystallographic studies

A few single crystals of the title compounds were secured by unscrewing the vials containing the product samples and quickly immersing them in dried polybutene oil (M_n ~ 320, isobutylene >90 %; Sigma-Aldrich). The evaluation of the crystalline material took place under a polarization microscope. The selected specimens were mounted with 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 crystals stationary and protected from moisture. Intensity data sets 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 Apex2 [5] that came with the diffractometer. An empirical absorption correction was applied using Sadabs [6]. The intensity data were evaluated, and the input files for solving and refining the crystal structure were prepared with Xprep [7]. Reflection conditions led to the possible space groups Pna2₁ (no. 33) and Pnma (no. 62). Attempts to solve and refine the structure in the centric space group Pnma (no. 62) turned out to be unsuccessful. The E statistics (|E² – 1| = 0.745, with 0.976 expected for centric and 0.736 for non-centric space groups) hinted strongly toward Pna2₁ (no. 33). The program Shelxs-97 [8, 9] delivered for this space group with the help of Direct Methods the positions of K and S. The N, C, and Li positions were apparent from the positions of highest electron density on the difference Fourier maps resulting from the first refinement cycles by full-matrix least-squares calculations on F² in Shelxl-97 [10, 11]. Doing further refinement cycles, the refinement converged and resulted in a stable structural model. Additional crystallographic details are described in Table 1. Atomic coordinates, anisotropic, and equivalent isotropic displacement coefficients are shown in Table 2. Table 3 displays selected interatomic distances and angles of the title compound and of Li[SCN] [3] and K[SCN] [14].

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 depository number CSD-430274 for LiK₂[SCN]₃.

3.1 Crystal structure

The basic building block of this crystal structure is the Li⁺ cation coordinated tetrahedrally by three N atoms and one S atom (Fig. 1). These tetrahedra are linked by [SCN]^– anions (S2, C2, and N2 in Table 2 and Fig. 1) parallel to the c axis (also the 2₁ screw axis) forming columns (Fig. 1). These columns are packed nearly hexagonally with 8-fold coordinated K⁺ cations between these columns holding them together (Fig. 2).

Fig. 1:

Tetrahedral coordination of Li⁺ in LiK₂[SCN]₃ forming columns parallel to the crystallographic c axis. Nitrogen atoms are displayed as gray, carbon atoms as black, sulfur as white spheres, and lithium atoms with white octands. Displacement ellipsoids are drawn at the 90 % probability level.

Fig. 2:

View along the crystallographic c axis of LiK₂[SCN]₃. The same color code as in Fig. 1 is applied with the exception of K⁺, which is displayed with white octands.

All bond lengths are in the expected ranges when compared to the distances observed for Li[SCN] [3] and K[SCN] [14] (Table 3).