Crystal structure of catena-poly[[[diaqua[1,2-bis(pyridin-4-yl)ethene]{4-[2-(pyridin-4-yl)ethenyl]pyridinium}gold(I)iron(II)]-di-μ-cyanido] bis[dicyanidogold(I)] 1,2-bis(pyridin-4-yl)ethene dihydrate]

In the title compound, the FeII ion is coordinated in a distorted octahedral [FeN4O2] environment by two dicyanoaurate anions, two water molecules and two partially protonated 1,2-di(4-pyridyl)ethylene molecules. Dicyanoaurate anions bridge the FeII cations, forming infinite chains, which propagate along the a-axis direction. The chains are connected via aurophillic interactions with two non-coordinated dicyanoaurate anions for each FeII ion.


Chemical context
Iron(II) complexes exhibiting spin-crossover (SCO) properties attract considerable attention because of their fascinating ability to change multiple physical properties (magnetic, optical, mechanical, etc.) under the influence of external stimuli (Gü tlich & Goodwin, 2004). These materials can be integrated into various devices as switches, triggers, chemical sensors, etc. (Suleimanov et al., 2015). For these reasons, new SCO materials, which undergo transition with defined temperature, hysteresis and abruptness are strongly desired. There are several classical approaches as how to modulate the SCO characteristics of complexes, among them the introduction of slightly modified ligands and co-ligands to obtain new SCO compounds and inclusion of some guest molecules to already existing complexes (Ni et al., 2017).
Fe II Hofmann clathrate (Hofmann & Hö chtlen, 1903) analogues represent one of the biggest classes of SCO coordination compounds. They are cyanobimetallic complexes of general formula [Fe(L) n {M(CN) x } y ] in which the Fe II ions are connected by bridging cyanometallic anions into infinite layers (n = 2 for monodentate ligands and n = 1 for bismonodentate ligands). These layers are supported by N-donor aromatic ligands (L = pyridine, diazines and their substituted analogues). Di-, tetra-and octacyanometallic (x = 2, y = 2: M = Cu, Ag, Au; x = 4, y = 1: M = Ni, Pd, Pt) anions have been introduced to develop coordination compounds of this kind. It has been shown that the inclusion of guest molecules can ISSN 2056-9890 significantly influence the temperature, completeness and step character of spin transition in complexes belonging to this class (Ohtani & Hayami, 2017). In order to develop new SCO Hofmann clathrate analogues with voids big enough to incorporate bulky guest molecules, some bis-monodentate pyridine-based ligands have been introduced, such as 4,4 0 -bipyridine (Yoshida et al., 2013), bis(4-pyridyl)acetylene (Bartual-Murgui et al., 2011), bis(4-pyridyl)ethylene (Muñ oz-Lara et al., 2012), etc.
Here we describe the crystal structure of a new cyanometallic Fe II complex with bpe of general formula [Fe(bpe)(Hbpe)Au(CN) 2 ](Au(CN) 2 ) 2 ÁbpeÁ2H 2 O in which the Fe II ions are stabilized in the high-spin (HS) state.

Figure 1
A fragment of the crystal structure of the title compound showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level. The guest bpe molecule and disorder of the [Au(CN) 2 ] À counter-ions are not shown for clarity. Symmetry-generated atoms are not labelled.
H1AÁ Á ÁO2 = 156 ] and weak hydrogen bonds with free di- Hydrogen-bonding parameters are summarized in Table 1.

Database survey
A search of the Cambridge Structural Database (CSD version 5.40, last update February 2019; Groom et al., 2016) revealed that the current structure has never been published before. 101 cyanometallic structures containing Fe-N C-Au fragments were found. These hits include multiple temperature-dependant measurements, which were conducted to study the spin-crossover characteristics of Fe II complexes. For example, these hits include a three-dimensional framework

Synthesis and crystallization
Crystals of the title compound were prepared by the slow diffusion method between three layers in a 3 ml tube. The first layer was a solution of K[Au(CN) 2 ] (0.03 mmol) in water (0.5 ml), the second was a mixture of water/ethanol (1:2, 1.5 ml) and the third layer was a solution of 1,2-di(4-pyridyl)ethylene (0.05 mmol) and [Fe(OTs) 2 ]Á6H 2 O (0.01 mmol; OTs = p-toluenesulfonate) in ethanol (0.5 ml) with 0.2 ml of water. After two weeks, red crystals grew in the second layer; the crystals were collected and kept in the mother solution prior to measurement.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The hydrogen atoms were placed at their expected calculated positions (C-H = 0.93, N-H = 0.86, O-H = 0.92-0.96 Å ) and refined as riding for the guest water molecules (O2, O3) and aromatic rings, and as rotating for the coordinated water molecule (O1) with U iso (H) = 1.2U iso (C), U iso (H) = 1.2U iso (N), U iso (H) = 1.5U iso (O). U aniso values for all C and N atoms in the guest dicyanoaurate anions and the O2 and O3 water molecules were constrained to be equal using the EADP command. Distances N3A-C3A and N2A-C2A were restrained to a target of 1.15 Å and distances Au2A-C3A and Au2A-C2A were restrained to a target of 1.99 Å using the DFIX command. The following distances were restrained to be equal using the SADI command: C2A-N2A and C2B-N2B; Au1-C2A and Au1-C2B; C3A-N3A and C3B-N3B; Au1-C3A and Au1-C3B; C2A-C3A and C2B-C3B.

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.