Synthesis, single-crystal structure determination and Raman spectrum of Ca_2.57(4)Sr_0.43(4)Cl₂[CBN]

2.1 Synthesis

All manipulations were performed in a glove box under purified argon unless otherwise stated. To obtain the nominal composition Ca_1.5Sr_1.5Cl₂[CBN], 80 mg (2 mmol) Ca and 175 mg (2 mmol) Sr (both metals: 99.99%, distilled, dendritic pieces, Aldrich, St. Louis, MO, USA), 110 mg (1 mmol) CaCl₂ and 160 mg (1 mmol) SrCl₂ (both chlorides: 99.9%, Strem, Newburyport, MA, USA, powder <5 μ), 50 mg (2mmol) hexagonal BN (99+%, powder, Strem, Newburyport, MA, USA, degassed at 670 K under dynamic vacuum for 2 h) and 24 mg (2 mmol) graphite (99.999% powder, Aldrich, St. Louis, MO, USA, 325 mesh, degassed at 670 K under dynamic vacuum for 2 h) were arc-welded into a clean Ta container. The metal container was sealed into an evacuated silica tube. The tube was placed upright in a box furnace and heated to 900°C within 12 h. After 3 d reaction time the furnace was switched off and allowed to cool to room temperature. The product contained less than 10% (estimated by the different colors with the naked eye) transparent-red needles of the title compound, but needles of yellow Ca₃Cl₂[CBN] and orange Sr₃Cl₂[CBN] were also identified by the respective unit cell determined on different single crystals. Different starting stoichiometries did not increase the yield, but the ratio of yellow Ca₃Cl₂[CBN] and orange Sr₃Cl₂[CBN] changed according to the different educt ratios. Ca_2.57(4)Sr_0.44(4)Cl₂[CBN] decomposes quickly when exposed to moist air.

2.2 Crystallographic studies

Samples of the reaction mixture were removed from the glove box in polybutene oil (Aldrich, St. Louis, MO, USA, M_n~320, isobutylene>90%) for single-crystal selection under a polarization microscope, mounted in a drop of polybutene sustained in a plastic loop, and placed onto the goniometer in a cold stream of nitrogen [T=203(2)K] which froze the polybutene oil, thus keeping the crystal stationary and protected from oxygen and moisture in the air. Intensity data were collected with a BrukerX8ApexII diffractometer equipped with a 4 KCCD detector (Madison, WI, USA) and graphite-monochromatized MoK_α radiation (λ=71.073 pm). The intensity data were handled with the program package [6] that came with the diffractometer. An empirical absorption correction was applied using Sadabs [7]. The atomic coordinates of Ca₃Cl₂[CBN] [1] were used as starting model and were refined by full-matrix least-squares calculations on F² in Shelxl-97 [8], [9]. All alkaline earth metal positions showed displacement parameters about two to four times larger than those of the other atomic positions. Therefore, a mixed Ca²⁺/Sr²⁺ occupancy was introduced and refined for all crystallographic positions but were constrained to full occupancy. 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 and of some related compounds also containing the [C=B=N]⁴⁻ anion.

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 deposition number CSD-432247 for Ca_2.57(4)Sr_0.43(4)Cl₂[CBN].

2.3 Calculation of the incremental volume of the [C=B=N]^4–anion with Biltz volume increments

The molar volumes of several compounds (Table 3) containing the [C=B=N]⁴⁻ anion were used to determine the incremental volume of this anion by subtracting the Biltz volume increments for the respective monoatomic cations and anions [11], [12], [13] from the respective molar volume determined experimentally by X-ray methods. This volume appears not to be a function of the data acquisition temperature. The values obtained (Table 3) were averaged to yield V([C=B=N]⁴⁻)=30.5(6)cm³mol⁻¹ or 50.7(10)ų per molecule.

2.4 Raman spectroscopy

Red single crystals of Ca_2.57(4)Sr_0.44(4)Cl₂[CBN] sealed in thin-walled glass capillaries were used for the Raman-spectroscopic investigations, which 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=632.817nm, 50×magnification, 8×240s accumulation time). The Raman spectrum is displayed in Fig. 1 and the vibrational data are shown together with those of related compounds [3], [4], [5], [10] in Table 3.

Fig. 1:

Raman Spectrum of Ca_2.57(4)Sr_0.43(4)Cl₂[CBN]. On the vertical axis: Raman intensities in arbitrary units; wavenumbers on the horizontal axis are given in cm⁻¹.

3.1 Raman spectrum

The Raman spectrum of the title compound is of low quality since the crystals recovered from the single-crystal measurement already showed signs of decompositions due the sensitivity of the compound towards air and moisture. Additionally, the polybutene oil used during the X-ray measurement shows with some lines marked in each case with an asterisk in Fig. 1. Other crystals sealed in capillaries taken directly out of protective atmosphere also decomposed quickly maybe due to a somewhat contaminated atmosphere inside the glove box. Nevertheless, the frequencies obtained compare well to those reported by Somer [10] and others for Ca₁₅[CBN]₆[C₂]₂X₂ (X=½O[3], X=F[4] and X=H[5]) and confirm therefore the presence of the carbido–nitrido–borate (Table 3).

3.2 The incremental volume of the [C=B=N]^4–

The obtained value (Table 3) of V([C=B=N]⁴⁻)=30.5(6) cm³ mol⁻¹ or 50.7(10) ų per molecule is considerably smaller than the calculated 39 cm³mol⁻¹ or 64.8 ų per molecule obtained from tabulated data [11], [12], [13]. This is probably due to the shorter C=B and B=C bond lengths owing to the double bond character of both bonds.

3.3 Crystal structure

Materials with M₃X₂[CBN] stoichiometry adopt a layered structure. A characteristic feature of these compounds are puckered layers with the sequence X-M-X (M=Ca or Sr, X=Cl or Br [1], [2]) sandwiching the [C=B=N]⁴⁻ anions (Fig. 2). This peculiar anion is bent and shows only very low C_s symmetry. Carbon atoms are coordinated in a distorted octahedral fashion by five metal atoms and boron, while the nitrogen atoms show a distorted tetrahedral coordination environment formed by three metal atoms and the boron atom (Fig. 3). The light atoms could be distinguished unambiguously by the X-ray data – not only by their coordination environment, but also by their respective distance to the central boron atom and significantly worse quality factors of the refinement when atoms were exchanged. The bending of the triatomic moiety is in between the angles reported for Ca₃Cl₂[CBN] and Sr₃Cl₂[CBN] (as one would expect), but the bond lengths found for the title compound are smaller than both of the aforementioned compounds. This might be just an artifact, because the data for the structure determination of the title compound were acquired at lower temperature and the precision of the crystallographic coordinates especially for light atoms such as C, B, and N is increased, while at room temperature the standard deviations are quite large.

Fig. 2:

Perspective view of the unit cell of Ca_2.57(4)Sr_0.43(4)Cl₂[CBN] along the crystallographic c axis. Displacement ellipsoids are displayed at the 95% probability level.

Fig. 3:

Coordination of the [CBN]⁴⁻ anion. The same color code as in Fig. 2 is applied.

In compounds crystallizing with the M₃Cl₂[CBN] structure, all three metal atoms can be can considered to be coordinated in the form of distorted octahedra. While for the M2 and M3 positions this environment is formed by three chlorine atoms and three end-on coordinating light atoms of three different [CBN]⁴⁻ moieties, the M1 position is surrounded by four Cl atoms and two different side-on coordinating anions with boron and carbon within bonding distance to the alkaline earth metal (Fig. 4). The side-on coordination by two triatomic moieties is probably responsible for the comparably high strontium content on the M1 position, since Sr²⁺ is a larger cation than Ca²⁺ with higher coordination needs. This coordination could also be an explanation for the significant deviation from linearity of the [CBN]⁴⁻ anion by more than 8°, as the coordination number of M1 is increased by the bending. In accordance with these thoughts, the position with the highest coordination number contains the highest concentration of strontium. Nevertheless, the doping level for all positions has the same order of magnitude.

Fig. 4:

Coordination of the three M²⁺ positions. The same color code as in Fig. 2 is applied.

3.4 Conclusion

The synthesis and the structural and vibrational properties of another compound containing the [C=B=N]⁴⁻ anion is reported. The low yields are probably due to the narrow temperature regime where the three-atomic anion does not decompose into smaller anions, and the metals and the halides form a reactive flux. Ca_2.57(4)Sr_0.43(4)Cl₂[CBN] shows the typical structural and spectroscopic features of a compound containing the [C=B=N]⁴⁻ anion. No cationic ordering was observed, but an indication for a slight preference of strontium for the position with the highest coordination number is observed. The incremental volume was calculated from experimental data to be 30.5(6) cm³ mol⁻¹ or 50.7(10) ų per molecule – more than 20% less than the sum of the respective Biltz volumes.