About alkali metal dicyanamides: syntheses, single-crystal structure determination, DSC/TG and vibrational spectra of KCs[N(CN)₂]₂ and NaRb₂[N(CN)₂]₃ · H₂O

2.1 Synthesis

All manipulations were performed under normal atmospheric conditions. All compounds were obtained by dissolving Na[dca] (Alfa Aesar, Ward Hill, MA, USA, 96 %) and the respective fluoride(s) (Aldrich, St. Louis, USA, 99 %) in 5 mL deionized water each, blending the solution and evaporating off the water at r. t. To the remaining solid 5 mL ethanol (Pharmco, Brookfield, CT, USA) was added and the mixture stirred for 5 min. The resulting solution was filtered and the solvent was allowed to evaporate from the filtrate at r. t.. To synthesize α-K[dca], KCs[dca]₂ and NaRb₂[dca]₃ · H₂O, 270 mg (3.0 mmol) Na[dca] and 120 mg (2.1 mmol) KF, 270 mg (3.0 mmol) Na[dca], 60 mg (1.0 mmol) KF and 300 mg (2.0 mmol) CsF, or 270 mg (3.0 mmol) Na[dca] and 210 mg (2.0 mmol) RbF were used, respectively.

2.2 Crystallographic studies

Samples of α-K[dca], KCs[dca]₂ and NaRb₂[dca]₃ · H₂O 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 graphite-monochromatized MoK_α radiation (λ = 71.073 pm). The intensity data were manipulated with the program package [7] that came with the diffractometer. An empirical absorption correction was applied using sadabs [8]. The program Shelxs-97 [9, 10] 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, oxygen and hydrogen 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 in Shelxl-97 [11, 12]. Doing further refinement cycles with all atoms being refined unrestrained (except for the hydrogen atom in NaRb₂[dca]₃ · H₂O) the refinement converged and resulted in stable models for the respective crystal structure. The site occupation factor (s.o.f.) for the hydrogen site in NaRb₂[dca]₃ · H₂O was restrained to ²/₃, because otherwise the refinement of the coordinates of the hydrogen atoms was not converging and the resulting moiety would be H₃O. 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-428975 for α-K[dca], CSD-428976 for KCs[dca]₂ and CSD-428977 for NaRb₂[dca]₃ · H₂O.

2.3 Raman and IR spectroscopy

Powder samples of α-Na[dca] as well as crystals of α-K[dca], KCs[dca]₂ and NaRb₂[dca]₃ · H₂O were sealed into thin-walled glass capillaries. 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 = 632.817 nm, 50 × magnification, 8 × 240 s accumulation time).

The IR spectra of KCs[dca]₂ and NaRb₂[dca]₃ · H₂O were obtained with a Bruker AFS 66 FT-IR instrument (Karlsruhe, Germany) with the KBr pellet technique (2 mg product being ground together with 400 mg dried KBr). The IR spectrum showed some absorptions typical for CO₂ in the region between 1300 and 1600 cm^–1 (asymmetric stretching mode) since the measurements were performed in normal atmosphere.

The Raman spectra of α-Na[dca] and α-K[dca] as well as the combined IR and Raman spectra of KCs[dca]₂ and NaRb₂[dca]₃ · H₂O are displayed in Figs. 1–3, the exact frequencies and their assigned modes are shown in Table 4.

Fig. 1:

Raman spectra of K[dca] and Na[dca]. On the vertical axis, Raman intensities are displayed in arbitrary units. Numbers are given in cm^–1.

Fig. 2:

Raman and IR spectrum of KCs[dca]₂. On the vertical axis, Raman intensities and IR transmissions are displayed in arbitrary units. All numbers are given in cm^–1.

Fig. 3:

Raman and IR spectrum of NaRb₂[dca]₃·H₂O. On the vertical axis, Raman intensities and IR transmissions are displayed in arbitrary units. All numbers are given in cm^–1.

2.4 DSC/TG measurements

8.277 mg of KCs[dca]₂ and 7.417 mg NaRb₂[dca]₃ · H₂O were placed on DSC/TG pans made out of alumina. This setup was introduced into a Netzsch STA 449C instrument (Selb, Germany) under a constant stream of pure argon. After flushing the material at room temperature for ten minutes, each sample was heated with 2 K min^–1 up to 700 °C (Figs. 4 and 5).

Fig. 4:

Graph of the DSC/TG on KCs[dca]₂.

Fig. 5:

Graph of the DSC/TG on NaRb₂[dca]₃·H₂O.

3.1 Optical spectra

The frequencies obtained from the IR and Raman spectra of the title compounds compare well to the vibrational frequencies reported in the literature for NaCs[dca]₃ [2] (Table 4) and confirm therefore the presence of the dicyanamide anion. The IR spectrum of NaRb₂[dca]₃ · H₂O shows only very weak indications for the presence of water. These can not be reliably distinguished from the background. This might be due to the fact that NaRb₂[dca]₃ · H₂O loses water already at comparably low temperatures (see DSC/TG measurements).

3.2 DSC/TG measurements

Slight mass fluctuations at the beginning of the measurements at low temperatures are probably due the hygroscopic nature of the dicyanamide compounds and adhering water incorporated from moisture out of the air. Below 200 °C, the DSC/TG measurements show only endothermic effects for both compounds; one at 157.9 °C for KCs[dca]₂ and two at 98.0 °C and 185.2 °C for NaRb₂[dca]₃ · H₂O. The endothermic effect with a mass loss of approximately 3.5 % at 98.0 °C for NaRb₂[dca]₃ · H₂O can be explained by partial loss of water which is lower than the theoretically expected mass loss of 6.5 % for losing all the water. This might be due the partial loss of water that already occurs at r. t.

The other endothermic effect observed below 200 °C is due to the melting of the compounds. Melting of K[dca] and Rb[dca] was observed at 230 °C and 190 °C, respectively [4]. Additionally, for NaCs₂[dca]₃ [2], Na[dca] [3], K[dca] and Rb[dca] [4] a large and broad exothermic peak in the range between 300–400 °C was observed for each compound indicating the trimerization of the [dca] anion to the cyclic [C₆N₉]^3– moiety. In the case of NaRb₂[dca]₃ · H₂O, this trimerization peak is easily seen and identified at 311 °C probably indicating the formation of Rb₃[C₆N₉] [4] which is reported to happen in the same temperature range. The trimerization can not reliably be confirmed by the data acquired for KCs[dca]₂, since the exothermic peak found here in this temperature range is too small to allow such a conclusion.

Above 400 °C, the decomposition of KCs[dca]₂ and NaRb₂[dca]₃ · H₂O starts to occur. This kind of behavior was already observed for Na[dca] [3], K[dca] and Rb[dca] [4].

3.3 The crystal structure of α-K[dca]

The structure has already been described in detail, but a rough sketch is given here for comparison with KCs[dca]₂. In α-K[dca], each potassium cation is surrounded by four [dca] anions and coordinated in a quadratic antiprismatic fashion (Fig. 6) with K–N distances between 280–305 pm. The crystal structure itself is – as reflected by the layered habit of the crystals – formed by alternating layers of [dca] and potassium ions (Fig. 7).

Fig. 6:

Coordination of K⁺ in α-K[dca]. Carbon atoms are shown as black, nitrogen atoms as white and potassium atoms as light gray circles. K–N bonds are drawn as broken lines, virtual N–N connections are supposed to emphasize the coordination polyhedron.

Fig. 7:

View of the unit cell of α-K[dca] parallel to the crystallographic b axis. The same color code as in Fig. 6 applies, only the coordination sphere of one potassium atom was drawn with closed faces.

3.4 The crystal structure of KCs[dca]₂

KCs[dca]₂ resembles β-Rb₂[dca]₂ [3, 4] both in symmetry (C2/c, no. 15) and lattice parameters (KCs[dca]₂: a = 1382.7(2), b = 998.1(1) and c = 1455.4(2) pm with β = 118.085(4) ° vs. β-Rb₂[dca]₂: a = 1381.56(7), b = 1000.02(1) and c = 1443.28(2) pm with β = 116.8963(6) °). β-Rb₂[dca]₂ contains two crystallographically independent rubidium atoms which are both coordinated by seven nitrogen atoms with distances in the range between 302–314 pm. These polyhedra form neither a channel nor a layer structure. In KCs[dca]₂ the potassium cation is coordinated by seven nitrogen atoms with distances in the range 287–301 pm (Fig. 8) comparing well to distances found for α-K[dca]. These polyhedra pair up by edge-sharing forming a channel structure in which the eightfold coordinated cesium atoms are located with Cs–N distances between 312 and 363 pm (Fig. 9).

Fig. 8:

The coordination of K⁺ in KCs[dca]₂. The same color code as in Fig. 6 applies.

Fig. 9:

View of the unit cell of KCs[dca]₂ perpendicular to the crystallographic b axis parallel to the (101) direction. The coordination polyhedra of the potassium atoms are shown with closed faces, Cs⁺ cations are displayed as large white spheres.

3.5 The crystal structure of NaRb₂[dca]₃ · H₂O

The resemblance between NaRb₂[N(CN)₂]₃ · H₂O and the already known compound NaCs₂[N(CN)₂]₃ [2] is striking. Both compounds adopt the same space group (P6₃/m, no. 176) and the cell parameters are very similar with a = 705.98(7) and c = 1462.89(12) pm for NaRb₂[N(CN)₂]₃ · H₂O and a = 705.1(1) and c = 1450.7(3) pm for NaCs₂[N(CN)₂]₃, but the length of the c axis of NaRb₂[N(CN)₂]₃ · H₂O is significantly shorter than the c axis of NaCs₂[N(CN)₂]₃ – despite the fact that the ionic radius of rubidium is smaller than that of cesium. This can be understood with a closer look at the coordination of the cations. In both compounds, Na⁺ is octahedrally coordinated by terminal nitrogen atoms of the [dca] anions (Figs. 10a and 10b) forming columns parallel to the crystallographic c axis. These columns are packed hexagonally forming channels hosting the respective heavy alkali metal cations (Fig. 11). In NaCs₂[N(CN)₂]₃, the cesium atom is coordinated by seven terminal nitrogen atoms while the rubidium atom in NaRb₂[N(CN)₂]₃ · H₂O is coordinated sevenfold by six terminal nitrogen atoms and one oxygen atom of the water molecule (Figs. 12a and 12b). This explains the difference of the c axes of both compounds. The incorporated water molecule ‘expands’ the structure of NaRb₂[N(CN)₂]₃ · H₂O in the c direction compared to NaCs₂[N(CN)₂]₃, while the a axes of both compounds are next to identical. Via the formation of hydrogen bonds to the nitrogen atoms of [dca] anions [d(H···N) = 218 pm] belonging to neighboring columns (Fig. 10b), this water molecule is responsible for the bonding and the packing of these columns (Fig. 13). The site occupation factor of the hydrogen atom was restrained to ²/₃ because this occupation is in accordance with the correct stoichiometry for H₂O.

Fig. 10:

Coordination of a sodium atoms in NaCs₂[dca]₃ (a) and in NaRb₂[dca]₃·H₂O (b). The same color code as in Fig. 6 applies; instead of K⁺, Na⁺ is here displayed as light gray sphere.

Fig. 11:

Perspective view of the unit cell of NaRb₂[dca]₃·H₂O parallel to the crystallographic c axis. Hydrogen atoms (belonging to the H₂O) are shown as small black spheres. The hydrogen position has the occupancy of ²/₃.

Fig. 12:

Coordination of cesium atoms in NaCs₂[dca]₃ (a) and of rubidium atoms in NaRb₂[dca]₃·H₂O (b). The same color code as in Fig. 6 applies; Cs⁺ and Rb⁺ are displayed as large white spheres.

Fig. 13:

Coordination of H₂O in NaRb₂[dca]₃·H₂O. The same color code as in Fig. 12 applies.