Crystal structure of poly[tetra-μ-cyanido-ethanolbis(2-iodopyrazine)digold(I)iron(II)]

The dicyanoaurate anions bridge the FeII cations to form polymeric chains propagating along the b axis.


Chemical context
Among all coordination compounds, cyanide-based complexes attract considerable attention. The cyanide group can be coordinated in either a monodentate or bridging way, connecting different metal ions, leading to the formation of one-, two-or three-dimensional frameworks. The variety of possible structures of cyanide-based complexes results in a variety of functional properties for these coordination materials, such as the ability to include small guest molecules (Klausmeyer et al., 1998), act as room-temperature magnets (Garde et al., 2002), display photomagnetic and magneto-optical properties (Mizuno et al., 2000;Mercurol et al., 2010) Keggin & Miles, 1936). Prussian blue analogues are very attractive because of their facile synthesis and the possibility to manipulate the magnetic ordering of the material by selecting appropriate spin sources (Ohkoshi et al., 1997).
Cyanometallate complexes are typically characterized by a low-spin state of the metal ions; however, the introduction of a complementary ligand with weak ligand field strength can lead to the formation of spin-crossover compounds. This type of compound is mostly represented by Hofmann clathrate analogues with general formula [M(L) x {M 0 (CN) 4 }] where M = Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Cd 2+ and Mn 2+ , M 0 = Ni 2+ , Pd 2+ , Pt 2+ and L is either a unidentate or bridging ligand. The first compound of this type reported by Hofmann & Hö chtlen (1903) was the [Ni(NH 3 ) 2 {Ni(CN) 4 }] clathrate, which is able to incorporate benzene or other aromatic molecules. In this structure, the bridging tetracyanonikelate anions contribute to the formation of infinite layers that propagate in the ab plane ISSN 2056-9890 (Powell & Rayner, 1949). However, the first Hofmann-clathrate analogue displaying spin-crossover behavior was [Fe(py) 2 {Ni(CN) 4 }] (Kitazawa et al., 1996). Later, different examples have been obtained for the modification of the original Hofmann clathrates, notably with di-or octacyanometallates (Gural'skiy et al., 2016b;Wei et al., 2016). Another modification method is the use of different organic ligands; for example, the inclusion of a bidentate ligand such as pyrazine leads to the formation of a three-dimensional network (Niel et al., 2001). Here we report a new cyanide-based compound with general formula [Fe(Ipz) 2 (EtOH){Au(CN) 2 } 2 ] in which the Fe II ions are stabilized in the high-spin state.

Structural commentary
The crystal structure of the title compound was determined at 296 K. It crystallizes in the triclinic P1 space group with two formula units per cell. The Fe II site has a distorted octahedral [FeN 5 O] coordination environment formed by two iodo-pyrazine N atoms, three dicyanoaurate N atoms and one ethanol O atom (Fig. 1). Two iodopyrazine molecules are coordinated in the cis configuration with the Fe-N distances of 2.216 (7) and 2.272 (7) Å (Table 1) indicating the high-spin state of the Fe II cation. One of the dicyanoaurate fragments is N-coordinated to the Fe II site in the form of an anion [Fe1-N2 = 2.096 (7) Å ], while the other two are coordinated in a trans configuration, further connecting the framework into a chain [Fe1-N1 = 2.105 (8) and Fe1-N5 = 2.096 (8) Å ]. The CN À anions bridge the Fe II and Au I cations in a quasi-linear mode with C1-Au1-C2 = 178.8 (3) and C3-Au2-C4 = 178.9 (3) . In addition, one of the coordination sites of the Fe II ion is occupied by an O-coordinated ethanol molecule nwith Fe1-O1 = 2.106 (6) Å , which is a typical value for Fe-O alcohol bonds. There is a deviation from an ideal octahedral geometry, AE|90 -Â| = 33.1 , where Â is the cis-N-Fe-N or cis-O-Fe-N angle in the coordination environment of Fe II . This value indicates a significant polyhedral distortion that can be explained by the Jahn-Teller effect and the presence of four different types of ligands.

Supramolecular features
The coordination framework is connected by bridging dicyanoaurate moieties into chains that propagate along the b-axis direction. In addition, the crystal packing is supported by NÁ Á ÁH-O hydrogen bonds (Fig. 2a, Table 2) in which H atoms from the ethanol hydroxyl group participate in weak interactions with the N atoms of the dicyanoaurate anions. The structure includes parallel-displacedinteractions with a distance of 3.381 (5) Å between the planes of the aromatic rings (Fig. 2b). Short Au Á Á Á Au distances of 3.163 (5) Å indicate intermolecular aurophilic interactions between the Au atoms of the monodentate and bridging dicyanoaurate moieties (Fig. 2c). The same type of aurophilic interaction was observed for a very similar Au-Fe-pyrazine complex, which displays high-temperature spin-transition behavior [AuÁ Á ÁAu (LS, 340 K) = 3.3886 (3) Å , AuÁ Á ÁAu (HS, 360 K) = 3.5870 (5) Å ; Gural'skiy et al., 2016a]. The AuÁ Á ÁAu distances in the above-mentioned structure are longer because they are defined by a three-dimensional framework of the complex; however, in the case of the title compound, the dicyanoaurate Table 1 Selected bond lengths (Å ).

Figure 1
A fragment of the molecular structure of the title compound showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level [symmetry codes:
anions are non-bridging and therefore are more flexible, which leads to the creation of aurophilic contacts that are closer to the optimum distance of 3 Å (Schmidbaur, 2000).

Synthesis and crystallization
Crystals of the title compound were obtained by the slowdiffusion method with three layers in a 5 ml tube. The first layer was a solution of K[Au(CN) 2 ] (29 mg, 0.1 mmol) in water (1 ml), the second layer was a water/ethanol mixture (1:1, 2.5 ml) and the third layer was a solution of Fe(OTs) 2 Á6H 2 O (OTs = toluenesulfonate) (50.6 mg, 0.1 mmol) and iodopyrazine (41.2 mg, 0.2 mmol) in ethanol (1 ml). After two weeks, yellow crystals grew in the middle layer; these were collected and kept under the mother solution prior to measurement.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All hydrogen atoms were placed geometrically at their expected calculated positions with C-H = 0.96 (CH 3 ), 0.97 (CH 2 ), 0.93 Å (C arom ), O-H = 0.859 (10) Å , and with U iso (H) = 1.2U eq (C) with the exception of methyl hydrogen atoms, which were refined with U iso (H) = 1.5U eq (C). The idealized CH 3 group was fixed using an AFIX 137 command that allowed the H atoms to ride on the C atom and rotate around th C-C bond.   program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg et al., 1999); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

Poly[tetra-µ-cyanido-ethanolbis(2-iodopyrazine)digold(I)iron(II)]
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.