Synthesis and crystal structure of a cyanido-bridged copper(II)–silver(I) bimetallic complex containing a trimeric {[Ag(CN)₂]⁻}₃ anion, [Cu(Dach)₂-Ag(CN)₂-Cu(Dach)₂][Ag(CN)₂]₃ (Dach=cis-1,2-diaminocyclohexane)

2.1 Materials

CuCl₂·2H₂O was obtained from BDH Chemical Co., England. AgNO₃ was purchased from Panreac, Spain. Ethylenediamine (en) is a product of Merck Chemical Co., Germany. K[Ag(CN)₂] was prepared by reacting AgNO₃ with KCN in a 1:2 molar ratio.

2.2 Synthesis of 1

To a solution of 0.170 g (1 mmol) of CuCl₂·2H₂O in 10 mL water was added 0.224 g (2 mmol) of cis-1,2-diaminocyclohexane (Dach) in 15 mL methanol. The solution turned to purple blue color. After stirring for 15 min, 0.40 g (2 mmol) of K[Ag(CN)₂] in 15 mL distilled water was added and the reaction mixture was stirred for further 30 min at room temperature. After filtration, the filtrate was kept in air overnight. The blue crystals obtained in the filtrate were washed with methanol and finally dried in air. Yield 30%. M.p. 195–198°C. – IR (cm⁻¹): ν=3247, 3142 (N–H), 2927, 2855 (C–H), 2158, 2130 (C≡N), 1050 (C–N); δ=1587 (Dach, ν=3288 (N–H), 2920, 2851 (C–H), 1092 (C–N); δ=1591).

2.3 X-ray structure determination of 1

Single crystal data collection of 1 was performed at 296 K on a Bruker Kappa APEXII CCD diffractometer equipped with a four-circle goniometer and using graphite-monochromatized MoK_α radiation. The refinement and all further calculations were carried out using Shelxl-2014 [37], [38]. All H atoms were placed in calculated positions with C–H=0.97–0.98 Å, N–H=0.89 Å and were refined as riding with U_iso(H)=1.2U_eq(C, N). The N–C and C–C bond distances were constrained to 1.48–1.52 Å. All disordered atoms were treated as anisotropic with equal displacement parameters. In this way the refined occupancy of the alternative sites is 0.685(6):0.315(6). For molecular graphics, Platon [39] was used. Crystal data and details of the data collection are summarized in Table 1.

CCDC 1486594 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.

3.1 IR study

The title complex [Cu(Dach)₂-Ag(CN)₂-Cu(Dach)₂][Ag(CN)₂]₃ (1) was prepared by the reaction of CuCl₂ with cis-1,2-diaminocyclohexane (Dach) and K[Ag(CN)₂] mixed in a molar ratio of 1:2:2. In the IR spectrum of complex 1, a sharp C–N stretching peak of Dach was observed at 1050 cm⁻¹, while the ν(N–H) vibrations were observed at 3247 and 3142 cm⁻¹. The peak at 1587 cm⁻¹ can be assigned to NH₂ bending. The signals at 2927 and 2855 cm⁻¹ due to C–H stretching of Dach were also observed. The appearance of these bands indicates the coordination of the Dach ligand to the metal center. The formation of dicyanidoargentates is confirmed by the strong stretching bands in the region of 2170–2100 cm⁻¹. The ionic non-coordinated cyanide (e.g. KCN) displays this vibration at ν=2080 cm⁻¹, while AgCN and K[Ag(CN)₂] exhibit the same vibration at ν=2164 and 2140 cm⁻¹, respectively [40]. A value higher than 2080 cm⁻¹ points to the formation of the coordinative bond, Ag–CN, and the values higher than 2150 cm⁻¹ indicate the formation of the bridge Ag–CN–M and the polymeric nature of the structure. The title compound exhibits both these (C≡N) bands with medium and weak intensity at 2130 and 2158 cm⁻¹, respectively.

3.2 Crystal structure description

The molecular structure of 1 along with the crystallographic numbering scheme is illustrated in Fig. 1. Selected bond lengths and bond angles are gathered in Table 2. The crystal structure of the complex consists of a trinuclear complex cation and three [Ag(CN)₂]⁻ anions. Each trinuclear complex ion is constituted of two [Cu(Dach)₂]²⁺ units joined by cyanido groups of a bridging [Ag(CN)₂]⁻ anion. Each copper atom in [Cu(Dach)₂]-NC-Ag-CN-[Cu(Dach)₂]³⁺ is pentacoordinated in a distorted square pyramidal environment. The basal plane of the square pyramid is defined by four amine nitrogen atoms of Dach, while the apical position is occupied by a nitrogen atom from the bridging cyanido group at a distance of 2.180(6) Å. The angles around copper show significant distortions from the normal values of square pyramidal geometry. The cis N–Cu1–N angles vary from 84.5(4)° to 101.1(7)°, while the trans N–Cu1–N angles are in the range of 146.4(7)°–176.4(4)°. The central [Ag(CN)₂]⁻ unit is symmetric and exhibits a regular linear environment around Ag1. While the N–C–Ag1 moiety is nearly linear (175.1(7)°), the Cu1–N–C part is bent with a bond angle of 155.2(7)°. The cyclohexyl ring of Dach attains a chair conformation. One of the Dach molecules of each [Cu(Dach)₂]²⁺ unit is disordered over two positions. Due to this disorder, one of the Cu–N(Dach) distances (Cu–N3B=2.23(2) Å) is somewhat longer than the others, which vary from 1.98(3) to 2.045(12) Å and are in agreement with the average values reported for similar complexes [11], [26], [27], [28], [29], [33], [35] (Table 2). A similar dinuclear structure has been reported for [Cu₂(bpca)₂(H₂O)₂{Ag₂(CN)₃}] (bpca=bis(2-pyridylcarbonyl)amide anion) [33].

Fig. 1:

The molecular structure of [Cu(Dach)₂-Ag(CN)₂-Cu(Dach)₂][Ag(CN)₂]₃ (1) with displacement ellipsoids drawn at the 40% probability level. The H atoms are drawn as spheres with arbitrary radii. The minor part of the disordered Dach atoms is omitted for clarity. Symmetry codes: i=1–x, 1–y, 1–z and ii=–x, –y, 2–z.

The positive charge of the trinuclear complex ion is neutralized by three non-coordinated [Ag(CN)₂]⁻ anions, which are self-associated through argentophilic interactions to form a trimer, {[Ag(CN)₂]⁻}₃. Such trimers have been detected earlier in chromium(III) and cobalt(III) complexes [9]. The Ag–Ag distance (3.2296(8) Å) is below the sum of the van der Walls radii of two silver atoms (3.44 Å), which is considered as the upper limit for viable argentophilic interactions [6], [7]. The strong argentophilic interactions contribute to the packing forces of the crystal structure. The three Ag atoms of the trimer lie on the same line with an Ag2–Ag3–Ag2 angle of 180°. There exist two types of Ag(I) centers in the trimer; the central Ag(3) atom is equidistant from the two peripheral Ag(2) atoms by symmetry. Ag(3) shows a distorted square planar geometry, while the other two display a T-shaped environment. The C–Ag3–C fragment of the central [Ag(CN)₂]⁻ ion is linear, while that of C–Ag2–C shows a slight deviation from linearity (∠C–Ag–C=177.6(3)°). The Ag–C distance (2.036 Å) of the central anion is slightly shorter than that of the terminally bound ones (Ag–C=2.044(15)–2.066(9) Å).

The trinuclear complex cation and the non-coordinated [Ag(CN)₂]⁻ anions are linked to each other through electrostatic forces and hydrogen bonds to form irregular chains. In the crystal packing of the components, the ions are H-bonded via N–H or C–H hydrogens of Dach and nitrogen atoms of non-coordinated [Ag(CN)₂]⁻ ions. The nitrogen atom (N8) of the central [Ag(CN)₂]⁻ ion mainly forms bonds with C–H hydrogen atoms because of its perpendicular orientation with respect to the other two [Ag(CN)₂]⁻ ions. Because of the close proximity of N8 to the Dach ring, the C–H···N hydrogen bonds are of comparable strength to those of N–H···N. A view of the packing of the complex units showing hydrogen bonds is given in Fig. 2. Further details of hydrogen bonding are given in Table 3.

Fig. 2:

Packing diagram of 1 showing H bonding interactions.

The present study describes the use of dicyanidoargentate(I) as a bridging ligand for the preparation of a new trinuclear copper(II)–silver(I) complex, [Cu(Dach)₂-Ag(CN)₂-Cu(Dach)₂][Ag(CN)₂]₃ (1), the crystal structure of which was resolved. The copper atom in 1 adopts a distorted square pyramidal geometry having cyanide nitrogen at the apical position. The non-coordinated [Ag(CN)₂]⁻ anions self-assemble to form trimers and stabilize the structure through H-bonding.