Crystal structure of Ag2(μ-SCN)2(NH3)4

The molecular structure of Ag2(SCN)2(NH3)4 consists of [Ag(SCN)(NH3)2]2 dimers with an Ag⋯Ag separation of 3.0927 (6) Å.


Chemical context
The reactions of various silver salts with liquid ammonia and their products are in almost all cases still unknown. In textbooks, the formation of the linear diamminesilver(I) cation is often predicted without any structural evidence. In this contribution we want to report on the reaction and the product of AgSCN with liquid ammonia at 237 K. A dinuclear Ag I complex was obtained.

Structural commentary
All atoms are located on general sites. The silver atom Ag1 is surrounded by two ammine ligands (N2 and N3) with distances of 2.269 (2) and 2.248 (2) Å , respectively. These values are in good agreement with other reported Ag-N distances (Zachwieja & Jacobs, 1989). The thiocyanate anion coordinates with its soft sulfur atom to the silver atom at a distance of 2.5363 (6) Å , which is similar compared to those of pure AgSCN (Lindqvist, 1957). The S-C-N angle in this pseudo-halide anion is with 178.2 (2) almost linear. Two of the [Ag(SCN)(NH 3 ) 2 ] units are connected to each other via bridging S atoms of the thiocyanato ligands into a dimer located about a twofold rotation axis (Fig. 1). The resulting coordination polyhedron around Ag1 is that of a distorted tetrahedron where one short Ag-S distance [2.5363 (6) Å ] and a long one [3.0533 (7) Å ] are observed. Therefore, the bond towards the latter may be regarded as weaker. In the dimer, the two tetrahedra are connected through one edge into a double tetrahedron. It is interesting to note that the two SCN À anions point in the same direction as there is no center of inversion within the molecule but only the twofold rotation axis of the space-group type. The AgÁ Á ÁAg distance is short at ISSN 2056-9890 3.0927 (6) Å , and is clearly in the range of argentophilic interactions (Jansen, 1987;Zachwieja & Jacobs, 1989;Schmidbaur & Schier, 2015).

Supramolecular features
The dinuclear complexes are connected to others via hydrogen bonds between the ammine ligands (N2 and N3) as donors and the N1 and S1 atoms of the thiocyanato ligand as acceptors. A three-dimensional network is formed in which each [Ag(SCN)(NH 3 ) 2 ] unit is coordinated by four (Fig. 2) and the dimer by eight other molecules. Six are arranged like a hexagon around the central molecule with all SCN ligands pointing in the same direction. Two molecules reside above and below this fictitious plane and are shifted towards a corner of the hexagon whereby the SCN ligands point in the opposite direction. Each of these two molecules shows the same coordination as described above, and overall, an AB-stacking of the molecules along [001], similar to the hexagonal closest packing, is obtained. The crystal structure is shown in Fig. 3. It should be noted that no acceptor atom for the hydrogen atom H2A is present in the neighbourhood within the range of the sum of the van der Waals radii of H and N atoms. Numerical details of the hydrogen bonding are given in Table 1 The hydrogen bonds (dashed lines) present in the structure of the title compound as illustrated for one [Ag(SCN)(NH 3 ) 2 ] unit with the acceptor groups of four surrounding molecules. Displacement ellipsoids as in Fig. 1 The crystal structure of Ag 2 (SCN) 2 (NH 3 ) 4 viewed along [010] with hydrogen bonds (dashed lines). Displacement ellipsoids as in Fig.1. Table 1 Hydrogen-bond geometry (Å , ). (4) 3.672 (2) 166 (3) Symmetry codes: (i) Àx; Ày þ 1; Àz; (ii) x þ 1 2 ; y À 1 2 ; z; (iii) x; y À 1; z; (iv) Àx; y À 1; Àz þ 1 2 .

Figure 1
The dimeric [Ag(SCN)(NH 3 ) 2 ] 2 unit in the title compound. Displacement ellipsoids are shown at the 70% probability level and H atoms are drawn with an arbitrary radius. All non-labelled atoms are generated by symmetry code (Àx, y, Àz + 1 2 ).

Synthesis and crystallization
400 mg (2.41 mmol) of AgSCN were placed in a flame-dried Schlenk tube under argon. Approximately 0.5 ml of liquid ammonia were condensed into the reaction vessel. The reaction vessel was stored at 237 K. After two weeks, colorless crystals of suitable size for X-ray diffraction were obtained from the colorless solution. The formation of the title compound is shown in the scheme.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All hydrogen atoms of the ammine ligands were located from a difference Fourier map and were refined isotropically without further restraints.  SHELXL2014 (Sheldrick, 2015b) and SHELXLE (Hübschle et al., 2011); molecular graphics: DIAMOND (Brandenburg, 2015); software used to prepare material for publication: publCIF (Westrip, 2010).

Di-µ-thiocyanato-bis[diamminesilver(I)]
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.