Synthesis and Characterization of the New Dicyanamide LiCs 2 [N(CN) 2 ] 3

: Crystals of LiCs 2 [N(CN) 2 ] 3 were obtained from the reaction of stoichiometric amounts of aqueous solutions of LiCl and CsBr with Ag[N(CN) 2 ]. X-ray single-crystal structure analysis showed that LiCs 2 [N(CN) 2 ] 3 crystallizes isotypically to NaCs 2 [N(CN) 2 ] 3 and adopts the hexagonal space group P 6 3 / m (No. 176), with a = 6.8480(8), c = 14.1665(17) Å, and Z = 2. The IR and Raman spectra of the title compound exhibit modes typical for the dicyanamide anion.


Introduction
Nitrogen-based materials are interesting not only for research purposes but also for industrial applications. Whether used as fertilizers, high-performance steel coatings, or lithium-ion battery materials [1,2], the research on nitrogen compounds has advanced and recently focused on complex nitrogen-containing compounds opposed to simple nitrides [3]. One interesting inorganic moiety is the boomerang-shaped dicyanamide anion [N(CN) 2 ] − which is often dubbed as [dca] − . This [dca] species has been found to allow for a rich diversity of compounds, simply due to its chemical stability as well as its ability to coordinate metal ions through terminal and/or bridging nitrogen atoms. Whenever coordination with all three nitrogen atoms of the [dca] anion occurs, ferromagnetic and antiferromagnetic transition-metal compounds with a rutile-like structure result [4]. On the other hand, [dca] − also forms one-dimensional (1D)-and two-dimensional (2D)-structures, which makes this anion interesting for crystal engineering [5]. Moreover, some dicyanamides show promise as water-oxidation catalysts [6], whilst Li [dca] has been suggested as a potential lithium-ion battery material [7]. In total, there is an enormous variety of pseudo-binary dicyanamide compounds available with examples known for ammonium [8], alkali metals [9][10][11][12], alkaline-earth metals [13], transition metals [4][5][6][14][15][16][17][18][19], and rare-earth metals [20][21][22]. Additionally, a number of pseudo-ternary dicyanamides has also been reported with KCs[dca] 2 [23] [25]. We here report the synthesis and single-crystal structure determination of the new pseudo-ternary compound LiCs 2 [dca] 3 according to its IR and Raman spectra.

Structural Description and Discussion
LiCs 2 [dca] 3 crystallizes isotypically to NaCs 2 [dca] 3 [25] in the hexagonal space group P6 3 /m (No. 176) with a = 6.8480(4), c = 14.1665(17) Å, and Z = 2. The lattice parameters of LiCs 2 [dca] 3 are smaller than those of NaCs 2 [dca] 3 (a = 7.0001(4), c = 14.4929(7) Å) due to the smaller cationic size of lithium compared to sodium. The boomerang-shaped dicyanamide anion exhibits bond lengths and angles consistent with those given in the literature: the bond length of d(C-N1) = 1.16 Å of the terminal angles consistent with those given in the literature: the bond length of d(C-N1) = 1.16 Å of the terminal C-N pairs indicates a triple bond, while the central C-N pairing with d(C-N2) = 1.31 Å is found to be in the expected distance range of a C-N single bond. The angles of the [dca] anion are also typically found for such a moiety with ∡(N1-C-N2) = 172.1° and ∡(C-N1-C) = 119.8° ( Figure 1, Table 1).

LiCs2[dca]3 NaCs2[dca]3 [25]
Li-N1 (6×  [9]. These octahedra are connected with each other by sharing three [N(CN)2] − and they are bonded by the terminal nitrogen atoms. In this way columns are formed parallel to the crystallographic c axis. These columns are packed hexagonally, forming channels hosting the cesium cations ( Figure 1). NaCs2[dca]3 was reported to incorporate coordination spheres around cesium with twelve nitrogen atoms from nine [dca]-groups and with two different distances [25]. A closer look at the structure of NaCs2[dca]3 reveals that there are four different d(Cs-N) ( Table 1). The calculated valence bond sum (VBS [26]) confirms that all twelve nitrogen atoms are part of the coordination sphere for Cs + in NaCs2[dca]3. This is not the case for the title compound. Calculations via VBS analysis reveals that the coordination sphere of Cs + contains nine nitrogen atoms of nine different dicyanamides with three different distances ( Figure 2, Table 1), generating a tri-capped trigonal prism according to IUPAC nomenclature. Two cesium atoms share six of these groups. Half of these anions coordinate the Cs + via their bridging nitrogen with d(Cs-N2 = 3.52 Å ); for the other half, it was found that six nitrogen atoms coordinate terminally to the cesium cation with d(Cs-N1 = 3.25 Å ). The coordination sphere is completed by three terminal nitrogen atoms in the layers below or above the Cs + with d(Cs- The thermal ellipsoids correspond to 90% probability using the refined ADPs.

LiCs 2 [dca] 3 NaCs 2 [dca] 3 [25]
Li-N1 (6×) Li + is octahedrally coordinated by six terminal nitrogen atoms of six different [dca] moieties with d(Li-N1) = 2.28 Å. This distance is in good agreement with Li-N distances for the octahedrally coordinated lithium cation in Li[dca] (2.22-2.29 Å) [9]. These octahedra are connected with each other by sharing three [N(CN) 2 ] − and they are bonded by the terminal nitrogen atoms. In this way columns are formed parallel to the crystallographic c axis. These columns are packed hexagonally, forming channels hosting the cesium cations ( Figure 1). NaCs 2 [dca] 3 was reported to incorporate coordination spheres around cesium with twelve nitrogen atoms from nine [dca]-groups and with two different distances [25]. A closer look at the structure of NaCs 2 [dca] 3 reveals that there are four different d(Cs-N) ( Table 1). The calculated valence bond sum (VBS [26]) confirms that all twelve nitrogen atoms are part of the coordination sphere for Cs + in NaCs 2 [dca] 3 . This is not the case for the title compound. Calculations via VBS analysis reveals that the coordination sphere of Cs + contains nine nitrogen atoms of nine different dicyanamides with three different distances ( Figure 2, Table 1), generating a tri-capped trigonal prism according to IUPAC nomenclature. Two cesium atoms share six of these groups. Half of these anions coordinate the Cs + via their bridging nitrogen with d(Cs-N2 = 3.52 Å); for the other half, it was found that six nitrogen atoms coordinate terminally to the cesium cation with d(Cs-N1 = 3.25 Å). The coordination sphere is completed by three terminal nitrogen atoms in the layers below or above the Cs + with d(Cs-N1  Atoms not involved have been greyed out. The thermal ellipsoids correspond to 90% probability using the refined ADPs.

Vibrational Spectra
The frequencies obtained from the IR and Raman spectra of the title compound confirm the presence of the [dca] moiety and agree very well to the IR/Raman data of the isostructural NaCs2[dca]3 [25] (Figure 3, Table 2). The IR spectrum was measured under atmospheric conditions, therefore it shows the presence of water due the hygroscopic nature of the title compound.  The thermal ellipsoids correspond to 90% probability using the refined ADPs.

Vibrational Spectra
The frequencies obtained from the IR and Raman spectra of the title compound confirm the presence of the [dca] moiety and agree very well to the IR/Raman data of the isostructural NaCs 2 [dca] 3 [25] (Figure 3, Table 2). The IR spectrum was measured under atmospheric conditions, therefore it shows the presence of water due the hygroscopic nature of the title compound.  Atoms not involved have been greyed out. The thermal ellipsoids correspond to 90% probability using the refined ADPs.

Vibrational Spectra
The frequencies obtained from the IR and Raman spectra of the title compound confirm the presence of the [dca] moiety and agree very well to the IR/Raman data of the isostructural NaCs2[dca]3 [25] (Figure 3, Table 2). The IR spectrum was measured under atmospheric conditions, therefore it shows the presence of water due the hygroscopic nature of the title compound.    3 . All numbers are given in cm −1 .

Synthesis
All synthetic steps involving AgNO 3  Colorless, transparent crystals suitable for X-ray diffraction were selected and measured.

Single-Crystal Diffraction
Suitable single crystals were mounted on glass fibers. Intensity data were collected with a Bruker SMART APEX CCD detector (Bruker AXS GmbH, Karlsruhe, Germany) equipped with an Incoatec microsource (Mo-Kα 1 radiation, λ = 0.71073 Å, multilayer optics). Temperature control was achieved using an Oxford Cryostream 700 (Oxford Cryosystems Ltd, Oxford, United Kingdom) at 100 K. Collected data were integrated with SAINT+ [27] and multi-scan absorption corrections were applied with SADABS [28]. The structure was solved by charge-flipping methods (Superflip [29]) and refined on F 2 as implemented in Jana2006 [30]. More crystallographic details can be found in Tables 3-5

Infrared and Raman Spectra
For the recording of the IR spectrum, an ALPHA II FT-IR-spectrometer (Bruker Optik GmbH, Ettlingen, Germany) equipped with an ATR Platinum Diamond measuring cell was used. All measurements were performed within the range of 4000 to 400 cm −1 .
Raman-spectroscopic investigations were performed on a microscope laser Raman spectrometer (Jobin Yvon, Unterhaching, Germany, 4 mW, equipped with a HeNe laser with an excitation line at λ = 632.82 nm, 50× magnification, 2 × 40 s accumulation time). The Raman spectrum was recorded on a crystal sealed in a thin-walled glass capillary.

Conclusion
The compound LiCs 2 [dca] 3 was synthesized, its crystal structure determined, and its IR and Raman spectra were reported. The acquired data of the vibrational spectra as well as the structural results are similar to the data of the previously reported alkali metal dicyanamides NaCs 2 [dca] 3, Li[dca] and Cs[dca], although the smaller Li + changes the coordination of Cs + .