Two lithium tricyanomethanide compounds: syntheses and single-crystal structure determination of LiK[C(CN)₃]₂ and Li[C(CN)₃]·½ (H₃C)₂CO

2.1 Synthesis

All manipulations were performed under normal atmospheric conditions. All chemicals were used as purchased. Single crystals of K[tcm] (Strem, > 98 %) were obtained by recrystallizing the compound from deionized water. Combining stoichiometric amounts of K[tcm] and LiCl (Aldrich, 99 %) dissolved in acetone precipitated KCl, which was filtered off. The solvent of the resulting clear solution was allowed to evaporate leaving transparent and colorless, small irregular, very hygroscopic chunks of Li[tcm]·½ (H₃C)₂CO. These crystals dissolved within a few minutes in the attracted water, leaving only solution droplets behind. Following the same procedures with acetonitrile as solvent, only very fine needles resulted, which are presumably Li[tcm]. These needles are sensitive even to slight pressure (e.g., touching them with a preparation needle) splitting up even more. This behavior is the reason why no X-ray or Raman data were obtained. Ethanol solutions of K[tcm] and LiCl yielded as main product the already known fine needles, but some transparent, colorless, thin plates were not hygroscopic and stable. These were found to be LiK[tcm]₂.

2.2 Crystallographic studies

Samples of the respective crystalline products 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 in the air. Intensity data were collected with a Bruker X8 Apex II diffractometer equipped with a 4 K CCD detector and using 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 program Shelxs-97 [7, 8] found the positions of potassium in K[tcm] and LiK[tcm]₂ and the nitrogen positions with the help of Direct Methods techniques. The positions of the carbon and hydrogen 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 in Shelxl-97 [9, 10]. 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 compound.

Further details of the crystal structure investigation 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 number CSD-428779 for K[tcm], CSD-428780 for LiK[tcm]₂ and CSD-428781 for Li[tcm]·½ (H₃C)₂CO.

3.1 Crystal structure of K[tcm]

The crystal structure of K[tcm] has been previously reported by two different groups, once as a preliminary report with the unit cell only [13], while the second includes atomic coordinates concentrating on the geometry of the [tcm] anion [14]. In K[tcm], the potassium cation is coordinated by seven nitrogen atoms forming a monocapped trigonal prism. The [tcm] anion shows only small deviations from D_3h symmetry. The crystal structure of K[tcm] is – as can be deduced from the plate-like habit of the crystals – layered (Fig. 1) with K⁺ and [tcm]^– stacked directly above each other along the cystallographic a axis with a small displacement of the successive layers (Fig. 2). All distances and angles are in the expected range (Table 3).

Fig. 1

View of the unit cell of K[tcm] parallel to the crystallographic b axis. Potassium atoms are shown as large, white spheres; carbon atoms, as small, black spheres; and nitrogen atoms as small, white spheres. Bonds to the potassium cation are drawn as broken thin lines, C–C and C–N bonds as thick lines.

Fig. 2

Perspective view of the unit cell of K[tcm] along to the crystallographic a axis. The same color code as for Fig. 1 applies.

3.2 Crystal structure of LiK[tcm]₂

In LiK[tcm]₂, lithium is coordinated in a tetrahedral fashion by four nitrogen atoms, while potassium is surrounded by six nitrogen atoms in a trigonal prismatic way (Fig. 3a and 3b). The trigonal prisms around potassium form columns parallel to the crystallographic c axis by sharing triangular faces (Fig. 4). These columns are connected by the [tcm] anions, which are bound to the tetrahedrally coordinated lithium atoms located in the voids in between the columns. In this perspective the layered nature of this compound also becomes obvious; the cations are all located at a ≈ ¼ and ¾.

Fig. 3

Coordination of lithium (3a) and potassium cations (3b) in LiK[tcm]₂ by nitrogen atoms. The same color code as for Fig. 1 applies except for Li being displayed as gray spheres.

Fig. 4

Perspective view of the unit cell of LiK[tcm]₂ along to the crystallographic c axis. The same color code as for Fig. 3 applies. The tetrahedra and trigonal prisms are displayed with a hatched pattern with the color of the respective atom.

3.3 Crystal structure of Li[tcm]·½(H₃C)₂CO

Looking at Li[tcm]·½ (H₃C)₂CO, one is reminded of Ag[tcm] [15, 16]. Both compounds share the same rarely adopted space group Ima2 (no. 46), but the unit cell parameters are rather different. Conversely, if the coordination of the metal atoms is taken into consideration, more similarities become obvious. Ag[tcm] is one of the rare examples of a trigonal planar coordination of silver [15, 16], which is prevented in Li[tcm]·½ (H₃C)₂CO by the acetone molecule connecting two crystallographically different Li atoms. Therefore, both lithium atoms are coordinated in a distorted tetrahedral fashion forming a corner shared Li₂N₆O_2/2 moiety (Fig. 5). These double-tetrahedra are stacked along the crystallographic a axis and connected by the [tcm] units, which also bind to the other double-tetraheda stacks. The coordination of lithium by acetone buckles these stacks opening channels parallel to the crystallographic a axis (and, therefore, to the aforementioned stacks). In these voids the nonpolar part of the acetone is located (Fig. 6).

Fig. 5

Coordination of lithium cations in Li[tcm]·½ (H₃C)₂CO by nitrogen and oxygen atoms.

Fig. 6

Perspective view of the unit cell of Li[tcm]·½ (H₃C)₂CO along to the crystallographic a axis. The same color code as for Fig. 5 applies. The tetrahedra are displayed with a hatched pattern with the color of the respective atom.