Crystal structure of catena-poly[[gold(I)-μ-cyanido-[diaquabis(2-phenylpyrazine)iron(II)]-μ-cyanido] dicyanidogold(I)]

Cyanide anions bridge FeII and AuI cations to form a one-dimensional polymeric compound with free dicyanoaurate anions.

In the title polymeric complex, {[Fe(CN) 2 (C 10 H 8 N 2 ) 2 (H 2 O) 2 ][Au(CN) 2 ]} n , the Fe II ion, which is located on a twofold rotation axis, has a slightly distorted FeN 4 O 2 octahedral geometry. It is coordinated by two phenylpyrazine molecules, two water molecules and two dicyanoaurate anions, the Au atom also being located on a second twofold rotation axis. In the crystal, the coordinated dicyanoaurate anions bridge the Fe II ions to form polymeric chains propagating along the b-axis direction. In the crystal, the chains are linked by O water -HÁ Á ÁN dicyanoaurate anions hydrogen bonds and aurophillic interactions [AuÁ Á ÁAu = 3.5661 (3) Å ], forming layers parallel to the bc plane. The layers are linked by offsetstacking interactions [intercentroid distance = 3.643 (3) Å ], forming a supramolecular metal-organic framework.

Chemical context
The design of functional materials based on coordination compounds is an important area of current scientific research. For example, metal-organic frameworks (MOFs), which consist of metal ions and organic ligand linkers, are studied intensively. Fe-based coordination polymers with N-donor bridging ligands are well known as compounds with switchable spin states (Niel et al., 2003;Gural'skiy et al., 2016;Kucheriv et al., 2016). This phenomenon is called spin crossover and can be observed in complexes of 3d 4 -3d 7 metal ions. Applying external stimuli, such as temperature, pressure, magnetic field, light irradiation or adding a guest can affect this kind of the compound and change their properties significantly (Gü tlich & Goodwin, 2004). The synthesis and crystallographic characterization of these complexes are of current interest because of the bistability of their magnetic, electrical, mechanical and optical properties (Senthil Kumar & Ruben, 2017). The parameters of these transitions could be controlled through a wide variety of available organic ligands and co-ligands. Complexes with metallocyanate bridges as co-ligands to Nbridging ligands form one of the largest family of spin-crossover compounds (Muñ oz & Real, 2011). Here we report on a new one-dimensional polymeric compound that is similar in its structure to switchable cyanometallates. It employs 2-phenylpyrazine as a ligand and Au(CN) 2À as co-ligands, while coordinated H 2 O molecules stabilize the Fe II ions in the highspin state.

Structural commentary
The structure of the title compound features a one-dimensional chain motif that runs parallel to the crystallographic b axis ( Figs. 1 and 2). The compound crystallizes in the monoclinic space group C2/c. Selected bond distances and bond angles are given in Table 1. The coordination sphere of the Fe II cation, atom Fe1, which is located on a twofold rotation axis, has a distorted octahedral environment [FeN 4 O 2 ]. It includes two 2-phenylpyrazine N atoms [Fe1-N3 = 2.223 (5) Å ] in axial positions, and two N atoms of cyano bridges and two water O atoms of water molecules [Fe1-O1 = 2.122 (4) Å ] in equatorial positions. The two CN À anions bridge the Fe II and Au I cations [Fe1Á Á ÁAu1 = 5.244 (3) Å ] to form a one-dimensional polymeric structure with bond lengths Fe1-N1 = 2.107 (5) Å and Fe1--N2 = 2.117 (6) Å ( Fig. 1 and Table 1).
The Fe II octahedral distortion parameter (the sum of the moduli of the deviations from 90 for all cis-bond angles) is AE|90 -Â| = 8.53 , where Â are the cis-N-Fe-O and cis-N-Fe-N angles in the coordination environment of the Fe II atom.

Supramolecular features
The crystal packing features different types of weak interactions (see Table 2  A view along the a axis of the crystal packing of the title compound. The hydrogen bonds (Table 2) and aurophillic interactions as shown as dashed lines. For clarity, the C-bound H atoms have been omitted.

Figure 1
A fragment of the molecular structure of the title compound, with the atom labelling Displacement ellipsoids are drawn at the 50% probability level. The Au1Á Á ÁAu2 interaction [3.5661 (3) Å ] is shown as a dashed line.

Database survey
A survey of the Cambridge Structural Database (Version 5.38; Groom et al., 2016) confirmed that the structure of the title complex has not been reported previously and revealed 41 Fe-Au CN-bridged frameworks supported axially by different coligands. There are 37 compounds with an octahedral FeN 6 environment. The coordination spheres of such compounds are formed by pyridine-azine ligands, substituted pyridines, saturated and substituted pyrazines, and pyrimidine (Clements et al., 2016;Arcís-Castillo et al., 2013;Agustí et al., 2008;Clements et al., 2014, Kosone & Kitazawa, 2016Niel et al., 2003). Nine such compounds have a stable low-or high-spin state and another 28 are complexes with a switchable spin state. There are also four compounds with an environment formed by the N atoms of organic ligands and water O atoms. The only compound with an FeN 5 O environment contains a pyridine-based N-donor ligand (Xu et al., 2014), while three compounds have an FeN 4 O 2 octahedral geometry. The bidentate bridging organoselenium triazole ligand and two different pyridine-based ligands were used to obtain these latter complexes (Seredyuk et al., 2007;Xu et al., 2014).

Synthesis and crystallization
Crystals of the title compound were prepared by the slow diffusion method between three layers in a 10 ml tube. The first layer was a solution of K[Au(CN) 2 ] (0.0058 g, 0.02 mmol) in water (2.5 ml), the second was a mixture of water/acetonitrile (1:2, 5 ml) and the third layer was a solution of 2-phenylpyrazine (0.0078 g, 0.05 mmol) and [Fe(OTs) 2 ]Á6H 2 O (0.0101 g, 0.02 mmol) (OTs = p-toluenesulfonate) in acetonitrile (2.5 ml) with 0.3 ml of water. After two weeks, yellow crystals grew in the second layer; these were collected and maintained under the mother solution until measured.

catena-Poly[[gold(I)-µ-cyanido-[diaquabis(2-phenylpyrazine)iron(II)]-µ-cyanido] dicyanidogold(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.