Synthesis, single-crystal structure determination and Raman spectrum of Cu[C(CN)₃]₂·2 NH₃

1.1 Synthesis

All manipulations were performed under normal atmospheric conditions. All compounds were used as purchased. Cu[tcm]₂ was obtained by blending stoichiometric amounts of aqueous K[tcm] (Strem, >98 %) and Cu(NO₃)₂·2.5 H₂O (Sigma Aldrich, ACS grade 98 %). After evaporation of the water, large single crystals of orange to brown Cu[tcm]₂ were manually selected and dissolved in 25 % NH₃(aq) (Fisher, analytical grade). The solvent was allowed to evaporate at r.t. under ambient conditions to dryness and next to a pale blue, amorphous ‘skin’, crystals of Cu[tcm]₂·2 NH₃ formed. Single-crystalline Ag[tcm] was obtained by mixing aqueous solutions of AgNO₃ (Mallinckrodt, analytical grade) with stoichiometric amounts of K[tcm]. Colorless material precipitated immediately. Single crystals of Ag[tcm] were obtained by recrystallizing the powder from 25 % NH₃(aq). Cu[tcm]₂ and Ag[tcm] are stable toward normal atmosphere; Cu[tcm]₂·2 NH₃ loses ammonia after a few hours without protection and forms amorphous, light-blue ‘skins’.

1.2 Crystallographic studies

Samples of the crystalline product 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 of 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 [11] that came with the diffractometer. An empirical absorption correction was applied using Sadabs [12]. The program Shelxs-97 [13, 14] found the positions of the respective metal atom and ammonia N (if contained in the respective compound). The positions of the carbon, nitrogen and hydrogen atoms (if contained in the respective compound) were apparent from the positions of highest electron density on the difference Fourier map resulting from the first refinement cycles by full-matrix least-squares calculations on F² in Shelxl-97 [15, 16]. 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 compounds.

Further details of the crystal structure investigations 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 numbers CSD-428560 for Ag[tcm], CSD-428561 for Cu[tcm]₂ and CSD-428562 for Cu[tcm]₂·2 NH₃.

1.3 Raman spectroscopy

The single crystals used for the structure determinations were sealed in thin-walled glass capillaries. Raman spectroscopic investigations were performed on a microscope laser Raman spectrometer (Jobin Yvon, 4 mW, equipped with a HeNe laser with an excitation line at λ = 632.817 nm, 50× magnification, 8 × 240 s accumulation time). The results are displayed in Fig. 1, the exact frequencies and their assigned modes are shown in Table 4.

Fig. 1

Raman spectra of Ag[tcm] and Cu[tcm]₂. On the vertical axis, Raman intensities are displayed in arbitrary units. Numbers are given in cm^–1.

2.1 Raman spectrum

The frequencies obtained from the Raman spectra (Figs. 1 and 2 and Table 4) compare well to the reported Raman frequencies for K[tcm] [19]. Interestingly, one weak band around 950–975 cm^–1 is observed for the potassium and the copper [tcm] compounds, but symmetry considerations predict this mode to be active only for a C_3v symmetry of the [tcm] anion and not for D_3h symmetry. The spectrum of Ag[tcm] shows a somewhat higher background. This makes it hard to distinguish low-intensity bands reliably from the background.

Fig. 2

Raman spectrum of Cu[tcm]₂·2 NH₃. On the vertical axis, Raman intensities are displayed in arbitrary units. Numbers are given in cm^–1. In the lower box, the spectrum is shown with different scaling of the Raman intensity (scaled up three times) to emphasize observed Raman modes with low intensities.

2.2 Crystal structure

The crystal structures of Cu[tcm]₂ [4, 5] and Ag[tcm] [20, 21] have already been discussed in detail, so we omit another discussion here, since these compounds were synthesized for three purposes: as educts, for comparison and to gather their Raman spectra. Nevertheless, compared to previously reported structural data, our structure refinements for Ag[tcm] and Cu[tcm]₂ are of better [4, 20] or at least comparable quality [5, 21].

In Cu[tcm]₂·2 NH₃, the [tcm] moiety is very close to planarity and ideal D_3h symmetry (Fig. 1 and Table 3); Raman data hint to lower C_3v symmetry. Each copper atom is coordinated in an octahedral fashion by two [tcm] anions and two trans-bonded ammonia molecules (Fig. 3). All bond lengths and angles are in a familiar and expected range (Table 3); only d(Cu–N₂) = 242.9(2) pm is a little longer than the other four Cu–N bonds, but this pattern is also found in Cu[tcm]₂ [d(Cu–N) = 247.1(3) pm] [4, 5]. Each [tcm] anion coordinates two Cu cations, therefore, chains are formed (Fig. 4), but only two of the cyanide groups (C1–N1 and C2–N2) form a bond to copper. These chains are packed and held in place only by van der Waals forces, since no specific bonding in between the chains (Fig. 5) is obvious. The packing pattern is interesting, since in principle these chains are just packed above each other with a little shift, but there are two different orientations of these chains present: one going roughly parallel to the crystallographic b axis and the other approximately parallel to [110]. Magnetic data have been measured for Cu[tcm]₂ [5] and show not quite unexpectedly indications for a very weak antiferromagnetic coupling, but even here the closest the metal atoms get is quite a distance to bridge with d(Cu–Cu) = 546.2 pm. The distances between copper cations of the title compound are all larger than 600 pm; therefore, even less exchange interactions are expected – which is also reflected in the very light blue color of the compound. Therefore, no measurements were done on Cu[N(CN)₂]₂·2 NH₃.

Fig. 3

Coordination sphere of the copper atoms in Cu[tcm]₂·2 NH₃. All atoms are marked with their labels as assigned in Table 2 except for the hydrogen atoms of the ammonia molecules (white small circles). The displacement ellipsoids are shown at the 90 % probability level.

Fig. 4

A chain consisting of the building blocks displayed in Fig. 3. One type of chain (Fig. 3a) packed together forms the structure (Fig. 3b). The same color code and probability level were used.

Fig. 5

Packing of the chains forming the structure. Ammonia molecules and non-connecting carbon and nitrogen atoms (C3 and N3) are omitted for clarity. Atoms are drawn as circles with fixed radii. The same color code and probability level were used.