Synthesis and single-crystal structure of the pseudo-ternary compounds LiA[N(CN)₂]₂ (A = K or Rb)

2.1 Synthesis

Ag[N(CN)₂] was synthesized by blending an aqueous solution containing 200 mg (2.25 mmol) Na[N(CN)₂] (96 %, powder; Alfa Aesar, Ward Hill, MA, USA) with a second solution containing 400 mg (2.35 mmol) of AgNO₃ (≥99 %, Sigma-Aldrich, St. Louis, MO, USA), respectively. The gray precipitate was washed with deionized water and dried using an aspirator. The precipitate was mixed with 50 mg (1.2 mmol) LiCl and 90 mg (1.2 mmol) KCl (2.35 mmol) or 50 mg (1.2 mmol) LiCl and 145 mg (1.2 mmol) RbCl (all alkali metal chlorides: ≥99 %; Sigma-Aldrich) in 5 mL of deionized water. After stirring this mixture for 6 h, the resulting solution was filtered to remove the precipitated AgCl and allowed to evaporate at atmospheric conditions. In both cases, colorless, transparent, very thin, and fragile rectangular crystals were observed after a few days.

2.2 Crystallographic studies

Samples of the respective crystallized product were taken directly out of the Petri dish used for the evaporation and immersed in dried polybutene oil (M_n ~ 320, isobutylene > 90 %; Aldrich). Single-crystal selection took place with the help of a polarization microscope. A suitable crystal was 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 [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 by Xprep [7]. The program Shelxs-97 [8, 9], with the help of Direct Methods techniques, easily found the positions of potassium or rubidium. C, N, and Li positions 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 in Shelxl-97 [9, 10]. Doing further refinement cycles, the refinement converged and resulted in a stable model for the crystal structures. 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 and their alkali metal thiocyanate analogue.

Further details of the crystal structure investigations 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-430275 for LiK[N(CN)₂]₂ and CSD-430275 for LiRb[N(CN)₂]₂.

3.1 Crystal structure

The crystal structure of LiA[N(CN)₂]₂ is built from layers consisting of Li⁺ and A⁺ (A = K or Rb) ions and the dicyanamide anions connecting these cationic layers (Fig. 1). The [N(CN)₂] anions coordinate Li⁺ cations in a tetrahedral fashion (Fig. 2a), whereas A⁺ cations are surrounded by squashed square antiprisms formed by the very same anions (Fig. 2b). The coordination geometries (angles and bond lengths) are nearly ideal when using the radii given by Shannon [11] (Table 3). The layered structure is reflected in the plate-like habit of the crystals.

Fig. 1:

Unit cell of LiA[N(CN)₂]₂ as seen along the crystallographic b axis. Li⁺ and A⁺ are shown as displacement ellipsoids with white (Li⁺) or black octands (A⁺). Displacement ellipsoids are displayed at the 90 % probability level. Carbon atoms are displayed as black; nitrogen atoms as white spheres with fixed atomic radii.

Fig. 2:

Coordination of Li⁺ (left) and A⁺ cations (right), shown as displacement ellipsoids with white (Li⁺) or black octands (A⁺), by [N(CN)₂]⁻ anions. Carbon atoms are displayed as black, nitrogen atoms as white spheres with fixed atomic radii.