Tricyanomethanides of the scandium group obtained from aqueous solution: syntheses, crystal structures and Raman spectra of Sc[C(CN)₃]₃(H₂O)₃, Y[C(CN)₃]₃(H₂O)₂ and La[C(CN)₃]₃(H₂O)₄

2.1 Source of material

Ag[C(CN)₃] was synthesized by blending an aqueous solution containing 1.13 g (10 mmol) of Na[C(CN)₃] (>98 %, TCI, Eschborn, Germany) with an aqueous solution of 2.04 g (12 mmol) Ag[NO₃] (≥99 %, Sigma-Aldrich, St. Louis, MO, USA). The resulting off-white precipitate was washed with deionized water and dried using an aspirator. 340 mg (3 mmol) of Ag[C(CN)₃] was added to 5 mL of demineralized water containing the respective rare-earth metal chloride (150 mg (0.99 mmol) of ScCl₃, 195 mg (1 mmol) of YCl₃, or 245 mg (1 mmol) LaCl₃). All anhydrous chlorides were obtained from ChemPur (Karlsruhe, Germany) with a purity of >98 %. After stirring the mixture for 6 h, the resulting solution was filtered to remove the precipitated AgCl and the excess of Ag[C(CN)₃]. The water was allowed to evaporate under ambient temperature and atmospheric conditions. After a few days, colorless and transparent, irregular crystals of the three title compounds were identified as the sole products in quantitative yield considering the employed amount of rare-earth metal trichloride.

2.2 Crystallographic studies

Single-crystal selection was carried out with the help of a polarization microscope, and suitable crystals were sealed into thin-walled glass capillaries (Hilgenberg, Malsfeld, Germany). Intensity data sets for (I) and (III) were collected with a STOE Stadivari diffractometer equipped with a Dectris Pilatus detector, a graded multilayer mirror monochromator and Mo-K_α radiation (λ = 71.07 pm) at room temperature. The collected intensity data was handled with the program package X-Area. ⁵ A multi-scan absorption correction was applied using STOE Lana. ⁶ A set of intensity data for (II) was collected with a Nonius Kappa-CCD diffractometer with graphite-monochromatized Mo-K_α radiation (λ = 71.07 pm) also at room temperature. The processing of the X-ray diffraction data was performed with the software package that came with the diffractometer. ⁷ The intensity data was corrected for Lorentz and polarization effects as well as for absorption with the program Habitus. ⁸ For all three title compounds the position of the respective rare-earth metal cation was obtained by using Direct Methods with the program Shelxs-97. ⁹ The positions of light atoms (including hydrogen) became apparent from the highest electron density on the difference Fourier map resulting from the first refinement cycles by full-matrix least-squares calculations on F ² (Shelxl-97 ¹⁰ ). The crystallographic data are listed in Table 1, atomic coordinates and equivalent isotropic displacement coefficients are shown in Table 2 and selected interatomic distances and angles of the title compounds can be found in Table 3.

Crystallographic data (including structure factors) for the three structures have been deposited with the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Further details of the crystal structure investigation may be obtained free of charge from the joint CCDC/ICSD Karlsruhe online deposition service: https://www.ccdc.cam.ac.uk/structures by quoting the deposition number CCDC 2386319 for Sc[C(CN)₃]₃(H₂O)₃ (I), CCDC 2422431 for Y[C(CN)₃]₃(H₂O)₂ (II) and CCDC 2422430 for La[C(CN)₃]₃(H₂O)₄ (III).

2.3 Calculation of the incremental volume sum according to Biltz

The incremental volume sum was calculated according to Biltz by adding up the respective volume increments of the respective trivalent cations (V(Sc³⁺) = 3.3 ų, V(Y³⁺) = 10.0 ų and V(La³⁺) = 13.3 ų), the crystal water (27 ų) (all volumes according to Biltz ¹¹ ) and the volume increment of the [C(CN)₃]^– anion (113(4) Å³, taken from ref. 12]). The thus obtained and the experimentally determined volumes of rare-earth metal tricyanomethanide hydrates are displayed in Table 4.

2.4 Raman spectroscopy

Raman spectroscopy was always performed with the single crystal used for the X-ray measurements on a XploRA spectrometer (Horiba, Kyoto, Japan) with 25 mW, excitation line at λ = 532 nm.

3.1 The tricyanomethanide [C(CN)₃]^– anion

The rigid [C(CN)₃]^– anion (often dubbed as tcm^–) as basic building block in all three title compounds exhibits only small deviations from ideal D _3h point symmetry (Table 3, Figures 1–3). The C–C bond lengths ranging from 138.5(2) to 141.0(3) pm are consistent with 141 pm obtained from DFT data for the free [C(CN)₃]^– anion; the C≡N bond lengths are found in the interval from 113.5(2) to 116.0(3) pm being somewhat shorter than the distance of 117 pm calculated with DFT methods for the free [C(CN)₃]^– anion. ¹³ These bond lengths range between the distances expected for a C–C single (154 pm) and a C=C double bond (133 pm) ¹² or a C=N double (122 pm) and a C≡N triple bond (111 pm), ¹⁴ respectively, indicating a highly delocalized electronic system with so-called Y aromaticity. ¹⁵

Figure 1:

Structure and coordination of the [C(CN)₃]^– anions in Sc[C(CN)₃]₃(H₂O)₃ (I).

Figure 2:

Structure and coordination of the [C(CN)₃]^– anions in Y[C(CN)₃]₃(H₂O)₂ (II).

Figure 3:

Structure and coordination of the [C(CN)₃]^– anions in La[C(CN)₃]₃(H₂O)₄ (III).

3.2 Crystal structure of Sc[C(CN)₃]₃(H₂O)₃ (I)

Compound (I) crystallizes in the triclinic space group P 1 ‾ (no. 2) with the lattice parameters a = 964.73(7), b = 1024.26(7), c = 1096.48(8) pm, α = 70.767(3), β = 64.936(3), γ = 62.481(3)° for two formula units per unit cell (Table 1). Its crystal structure contains a crystallographically unique Sc³⁺ cation coordinated by three water molecules and four star-shaped tricyanomethanide anions [C(CN)₃]^– forming a distorted pentagonal bipyramid [ScO₃N₄] with their grafting oxygen and nitrogen atoms. Two seven-fold coordinated Sc³⁺ cations are assembled to a dinuclear unit (Figure 4), which represents the entire molecular and crystal structure of Sc[C(CN)₃]₃(H₂O)₃. In this centrosymmetric dimer, two (tcmA)^– and (tcmB)^– anions work as terminal ligands with N1A and N1B, while two (tcmC)^– anions link the two Sc³⁺ centers with N1C and N2C (Figure 1) to keep them at a distance of 796.9(1) pm. The molecular dimers   ∞ 0 {(Sc[C(CN₃]₂(H₂O)₃)₂} are arranged with alternating barycenters as shown in Figure 5, held together by a vast hydrogen bond system and some Y-aromatic π-stacking. The former consists of hydrogen bonds between water molecules and nitrogen atoms of [C(CN)₃]^– anions, which are not involved in the Sc³⁺ coordination (H1A···N3C: 204 pm, H1B⋯N3A: 206 pm, H2A···N2A: 243 pm, H2B⋯N2B: 182 pm, H3A···N2A: 212 pm, H3B⋯N3B: 196 pm). Interestingly, even shorter Sc³⁺···Sc³⁺ distances occur between two dimers (619.4(1) pm, 746.2(1) pm and 779.8(1) pm plus 838.5(1) pm) than within these dimers (796.9(1) pm, vide supra).

Figure 4:

Quasi-molecular dimers in the crystal structure of Sc[C(CN)₃]₃(H₂O)₃ (I).

Figure 5:

View at the crystal structure of Sc[C(CN)₃]₃(H₂O)₃ (I) as seen along [010].

3.3 Crystal structure of Y[C(CN)₃]₃(H₂O)₂ (II)

Compound (II) crystallizes in the orthorhombic space group P2₁2₁2₁ (no. 19) with the lattice parameters a = 1011.06(7), b = 1172.54(8), c = 1419.21(9) pm and four formula units per unit cell (Table 1). The crystallographically unique Y³⁺ cation, is coordinated by two H₂O molecules and six [C(CN)₃]^– anions. The attached oxygen and nitrogen atoms are assembled like a distorted trigonal dodecahedron [YO₂N₆] (Figure 6). Only one of the three independent [C(CN)₃]^– anions (tcmA)^– acts as a terminal ligand with N1A, while the remaining two, namely (tcmB)^– and (tcmC)^–, link the Y³⁺ centers to create a three-dimensional framework. Thereby, (tcmB)^– is grafted with N1B and N3B, but (tcmC)^– even with N1C, N2C and N3C (Figure 2) in order to provide connections into all three spatial directions (d(Y³⁺···Y³⁺) = 757.4(1) pm (2×), 791.5(1) pm (2×), 866.3(1) pm (2×), 876.9(1) pm (2×) and 882.2(1) pm (2×)). Owing to a smaller number of water molecules, the hydrogen bonding system between water molecules and nitrogen atoms of [C(CN)₃]^– anions, which are not involved in the Y³⁺ coordination (H1A···N2A: 203 pm, H1B⋯N3A: 194 pm, H2A···N2A: 233 pm, H2B⋯N2B: 193 pm), remains rather simple.

Figure 6:

Coordination pattern of Y³⁺ in the crystal structure of Y[C(CN)₃]₃(H₂O)₂ (II).

3.4 Crystal structure of La[C(CN)₃]₃(H₂O)₄ (III)

Compound (III) crystallizes in the monoclinic space group C2/c (no. 15) with the lattice parameters a = 2843.17(19), b = 789.74(5), c = 1812.31(12) pm and β = 106.938(3)° for eight formula units per unit cell (Table 1). Once again, the crystal structure exhibits just a singular RE ³⁺ cation, which carries four water molecules and five [C(CN)₃]^– anions. The resulting [LaO₄N₅] polyhedron resembles a tricapped trigonal prism with O1, O4 and N1A as caps (Figure 7), where the N1A cap represents the nitrogen atom of a terminally attached [C(CN)₃]^– anion, namely (tcmA)^–. The remaining two [C(CN)₃]^– anions serve as bidentately grafting ligands, bridging two La³⁺ cations each, (tcmB)^– with N1B and N2B as well as (tcmC)^– with N1C and N3C (Figure 3). In this way the connectivity leads to an infinite chain propagating along the [101] direction as shown in Figure 7 with La³⁺···La³⁺ distances of 789.9(1) pm (2×). It is worth mentioning that shorter contacts between two La³⁺ cations are found again, such as 649.2(1) pm (1×), but also longer ones, with the shortest of them being 876.9(1) pm (1×). The considerable amount of water molecules provides an extended hydrogen bond system between them and nitrogen atoms of the [C(CN)₃]^– anions, which are not involved in the La³⁺ coordination (H1A···N3A: 214 pm, H1B⋯N2A: 211 pm, H2A···N3A: 215 pm, H2B⋯N3B: 201 pm, H3B⋯N2C: 207 pm, H4A···N2C: 221 pm, H4B⋯N2A: 205 pm), but also attractive contacts among each other (H3A···O4: 233 pm).

Figure 7:

Coordination pattern of La³⁺ in the crystal structure of La[C(CN)₃]₃(H₂O)₄ (III).

3.5 Biltz volume sums versus experimentally determined volumes

In general, the sum of the volume increments is smaller than the volumes determined experimentally by crystallographic means (Table 4). The deviations range between 1.3 and 5.4 %. Since the employed method is rather simple, with no claims to be of highest accuracy, the results can nevertheless be regarded as an indication for the correctness of the respective composition. Only the volume calculated for La[C(CN)₃]₃(H₂O)₅ ² is with more than 7.4 % larger than the experimentally determined volume. Since the crystallographic data reported in the doctoral thesis ² are not unambiguous, if taken just for themselves, the comparison with the data newly acquired by us raises some questions about the reliability of the reported data.

3.6 Raman spectra

The frequencies obtained from the Raman spectra of the three title compounds (Figure 8 and Table 5) are very similar to those obtained from IR spectra of other Ln[C(CN)₃]₃(H₂O) _n ² examples, corroborating the presence of tricyanomethanide anions. Especially the C≡N stretching modes found around 2200 cm⁻¹ are clearly visible and typical for the tricyanomethanide anion next to the less prominent lines around 700 cm⁻¹, representing the C–C≡N in-plane mode of [C(CN)₃]^–. The broad humps around 3200 cm⁻¹ are characteristic for the presence of crystal water.

Figure 8:

Raman spectra of Sc[C(CN)₃]₃(H₂O)₃ (I), Y[C(CN)₃]₃(H₂O)₂ (II) and La[C(CN)₃]₃(H₂O)₄ (III). On the vertical axis, Raman intensities are displayed in arbitrary units. Wavenumbers are given in cm⁻¹.