Synthesis and crystallographic characterization of Cu[SeCN]

2.1 Synthesis of α-Cu[SeCN]

Our synthetic procedure follows the synthesis reported by Söderbäck. ² Cu[SO₄]·5H₂O (500 mg, 2.0 mml, 1.2 eq.) was dissolved in 30 mL warm (70 °C) deionized water and combined with 0.5 mL concentrated hydrochloric acid. Na₂[SO₃] (160 mg, 1.7 mmol, 1.0 eq.) was separately dissolved in 20 mL warm deionized water (70 °C), combined with 1 mL of diluted (8:1) hydrochloric acid, and added to the solution of CuSO₄·5H₂O. The resulting blue solution was then precipitated with a solution of K[SeCN] (303 mg, 2.1 mmol, 1.2 eq.) in 20 mL deionized water. The resulting brownish suspension was filtered through a paper filter. The brown filter cake was subsequently washed with 50 ml portions of water, ethanol and diethyl ether, and dried in a desiccator under reduced pressure and over silica gel. The product was obtained in form of a grey/brown microcrystalline powder (254 mg, 1.5 mmol, 75 %, see Figure 1). Elemental analysis: calc. for Cu[SeCN]: C 7.13, N 8.31; found C 7.37, N 8.23.

Figure 1:

Freshly synthesized Cu[SeCN].

2.2 Crystal structure of Cu[SeCN]

The structure of α-Cu[SeCN] was refined from powder X-ray diffraction data. The structural model from copper(I)thiocyanate, α-Cu[SCN] ⁹ (Figure 2, Table S1), was used as a starting point as the reflection positions and intensities were very similar. However, the reflection positions were slightly shifted to lower angles due to larger lattice parameters. Rietveld refinement corroborated the model and, together with elemental analysis, proved its purity. α-Cu[SeCN] crystallizes in the α-Cu[SCN] type in the orthorhombic space group Pbca (№ 61), with lattice parameters a = 7.6625(1), b = 6.9628(1), c = 11.2821(2) Å, V = 601.93(1) ų, and 8 formula units in the unit cell (Figure 3). It contains only one crystallographically independent Cu[SeCN] unit with all atoms occupying the general Wyckoff-position 8c (site symmetry 1).

Figure 2:

Rietveld refinement of α-Cu[SeCN]. Black crosses display measured data, the red line represents the refined model, green bars indicate Bragg positions and the grey line displays the difference plot. Further data is compiled in Table S1.

Figure 3:

Unit cell of Cu[SeCN]: (a) view along [001]; (b) view along [010]; (c) view along [100]. Cu: bronze, Se: magenta, C: white, N: blue.

The Cu atom is surrounded by an almost ideal trigonal pyramid consisting of 3 Se atoms on the basal plane and 1 N atom on the upper vertex (csm = 0.244512, ¹⁰ Figure 4a). The distances between the Cu and N atom, and Cu and Se atoms are 1.9960(14) Å and 2.448(14)–2.482(13) Å, respectively. Both values agree with the sum of the ionic radii of the corresponding atoms (∑_Cu–Se = 2.58 Å; ∑_Cu–N = 2.06 Å). ¹¹ The [SeCN]⁻ is almost linear with ∡_Se–C–N = 179.91(13)°.

Figure 4:

Excerpts of the crystal structure of Cu[SeCN]. (a) Coordination around Cu atom: Cu[Se₃N] trigonal pyramid; (b) view along [100]: different orientation of polyhedra along the [001] axis (orange and green); (c) view along [001]: alternated chains of different orientated pyramids around Cu atom (orange and green). Cu: bronze, Se: magenta, C: white, N: blue.

The [Se₃N] pyramids around the Cu atoms show different orientation along the [001] axis. Pyramids of the same orientation (orange or green, resp.) are connected with each other via common Se vertices and form chains along [010] (Figure 4b). The pyramids of different orientation are also connected with each other by common Se atoms and form layers within the (110) plane (Figure 4c).

Along [100] the Cu and Se atoms form layers of hexagonal [CuSe]₃ rings (Figure 5). The layers are stacked in an AA′ sequence, where every other layer is shifted along [010] and connected by common [SeCN]⁻ anions along [001]. The Se–Cu bond lengths within one ring correspond to the interatomic Cu–Se distance within one Cu[Se₃N] tetrahedron and lies between 2.448(14)–2.482(13) Å.

Figure 5:

Crystal structure of Cu[SeCN]. (a, b) View along [010] and [100]: layers of [CuSe]₃ units connected with each other by common [SeCN] anions; (c) view along [001]: [CuSe]₃ hexagonal rings, C and N atoms are slightly transparent for better visibility. Cu: bronze, Se: magenta, C: white, N: blue.

In Figure 6, a comparison of the experimental and calculated crystal structures is shown. Here, both Cu[SeCN] units almost overlap, indicating the plausibility of the refined structure. In Table 1 experimental and calculated lattice parameters are shown. Our calculations show only a very small deviation from the measured lattice parameters. Comparing our refined model with to the predicted structure of Wijeyasinghe, ⁵ we find deviations of 0.8–1.7 % for the lattice parameters and 3.6 % for the unit cell volume.

Figure 6:

Comparison of experimental (black border) and calculated (blue border) structure of α-Cu[SeCN]. Cu: bronze, Se: magenta, C: white, N: blue.

2.3 Thermal behaviour of α-Cu[SeCN]

The results of the DSC-TGA measurement alongside high-temperature X-ray diffraction data are illustrated in Figures 7 and 8. According to diffraction data, α-Cu[SeCN] shows thermal stability up to 285 °C. At 290 °C only a few strong reflections were obtained, which coincides with the corresponding DSC-TGA data (Figures 7 and 8, area I shown in red). Next, in the range between 290 and 320 °C crystalline CuSe ¹² and CuSe₂ ¹² were detected (Figures 7 and 8, area II shown in yellow). Its formation is accompanied by the weight loss of 13.3 %, which can probably be assigned to the loss of dicyan (CN)₂ (15.4 %). The endothermic signal at T_onset = 303 °C in the DSC data can likely also be assigned to the endothermic decomposition of α-Cu[SeCN] into a combination of CuSe, CuSe₂ and (CN)₂. In the third step in the range between 325 and 370 °C pure CuSe ¹³ was observed (Figures 7 and 8, area III shown in green). The weight loss here is ∼5 %. In the region between ∼375 and 430 °C the second decomposition process with the residue decomposing at 382 °C takes place. At 375–650 °C Cu₂Se ¹⁴ was detected in the diffraction data (Figures 7 and 8, area IV shown in blue). The DSC-TGA data show in this region two weight loss steps: The first begins in the green range at 350 °C and ends at 425 °C and is 19.5 %. Here, also an endothermal signal with T_onset = 426 °C was observed and can perhaps be assigned to the liberation of elemental selenium (23.4 %). The last weight loss is continuous, starts at 425 °C and goes to the final measurement temperature of 1000 °C, and is 18.4 %.

Figure 7:

DSC (red) and TGA (black) graph of α-Cu[SeCN].

Figure 8:

High-temperature powder X-ray diffraction of α-Cu[SeCN].

2.4 Raman spectroscopy of Cu[SeCN]

The crystal structure of α-Cu[SeCN] was optimized with the DFT-PBE0 method, and the Raman spectrum was calculated. The calculated vibrational frequencies have been obtained within the anharmonic approximation, and they are systematically over-estimated in comparison to the experimental frequencies (Figure 9).

Figure 9:

Raman spectrum of α-Cu[SeCN] in the region between 0 and 3,000 cm⁻¹ (black: collected with 532 nm laser; red: calculated).

In α-Cu[SeCN] (4 non-equivalent atom positions in the unit cell) are 48 Raman active vibrational modes. Selected computed and collected frequencies are shown in Table 2. The spectrum is dominated by the asymmetric stretching vibration (ν_{as, stretch}) of the selenocyanate anion, which in this case is a combination of the B _2g (calc: 2,275 cm⁻¹) and A _g (calc: 2,273 cm⁻¹) modes. The B _1g and B _3g modes (both 2,265 cm⁻¹) of the same vibration are found at slightly lower energies which probably results in the observed broadening of the signal towards the low energy side. The displacement of the Se atom within these vibration modes is only minor, so that the vibration can be approximated as the stretching vibration of the CN moiety. The respective symmetric stretching vibration (ν_{s, stretch}) can be found at 631–633 cm⁻¹ (B _3g , B _2g , B _1g ) in the calculated spectra. In contrast to the vibration discussed above, here the Se atom is part of the vibration mode with significant displacement. Lattice vibrations can be observed at low wavenumbers.

2.5 Attempted synthesis of Cu[SeCN]₂

The synthesis of Cu[SeCN]₂ was described by W. Crookes in 1851 and Söderbäck in 1974. ^{1 , 2} In both cases, Cu²⁺ solutions were precipitated with K[SeCN] yielding brown powders that transform to black solids within short times. However, analytical data is missing safe for elemental analysis in one case. ² Later, Bergstrom ¹⁵ repeated the synthesis but was unable to replicate the described results.

In this work, we repeated the syntheses from literature. Cu[NO₃]·3H₂O (211 mg, 0.87 mmol, 1.0 eq) and K[SeCN] (251 mg, 1.75 mmol, 2.0 eq.) were dissolved in 20 mL deionized water, respectively. Combining of the solutions lead to the formation of brown precipitate. The suspension was stored at 4 °C for 16 h and subsequently filtered through paper filter. The obtained black solid was washed with water, ethanol and diethyl ether, and dried on air. The results of elemental analysis match those of Cu[SeCN]. The obtained powder pattern shows a high amorphous part and matches neither Cu[SeCN] nor any compounds isotypic to Cu[SCN] or Cu[SCN]₂. In the Raman spectrum the band at 2,118 cm⁻¹ and 508 cm⁻¹ was assigned to the vibrations of a [SeCN]⁻ anion, however, do not provide any statements about the cation.

The low stability of several copper(II) pseudohalide compounds can be described on the example of copper(II) cyanide, Cu[CN]₂. Cu^II[CN]₂ is an unstable compound that, upon storage or in solution, gives Cu^I[CN] and cyanogen. ¹⁶ The more stable latter compound can be synthesized by the reaction of any copper(II) salt with cyanuric acid or potassium cyanide in an aqueous solution. After a few minutes in the reaction solution, the decomposition process can be observed by a colour change towards green and accompanying formation of gaseous cyanogen, (CN)₂. ¹⁷ This is somewhat unsurprising as it aligns with reported standard electrode potentials of +0.373 V for (CN)₂ (1) and Cu²⁺ +0.153 V (2). ¹⁸