Crystal structure of diaquabis(4-cyanopyridine-κN)bis(thiocyanato-κN)iron(II) 4-cyanopyridine disolvate

The crystal structure of the title compound consists of discrete octahedral complexes and additional cyanopyridine solvate molecules that are linked by intermolecular O—H⋯N and C—H⋯N hydrogen bonding into a three-dimensional network.


Chemical context
Thiocyanate anions are versatile ligands that can coordinate in different modes to metal cations. In most cases the anionic ligands are terminally N-bonded to the metal cation but there are also several examples for a -1,3 bridging mode (Werner et al., 2015;Palion-Gazda et al., 2015). The latter coordination is of special interest if the compounds contain paramagnetic metal cations because then cooperative magnetic properties can be expected (Palion-Gazda et al., 2015). In this context, we have reported on several compounds with one-or two-dimensional structures based on Mn, Fe, Co or Ni as metals, thiocyanate ligands and different N-donor co-ligands that show different magnetic properties (Suckert et al., 2016;Rams et al., 2017;. Whereas compounds with a terminal coordination of the anionic ligands can usually be synthesized straightforwardly, compounds with bridging ligands are sometimes difficult to obtain from solution. Therefore, we have developed an alternative procedure which is based on thermal decomposition of precursors with a terminal NCS coordination that frequently transform into the desired polymeric compounds on heating. In the course of our investigations on the synthesis of coordination polymers with iron as metal, thiocyanate ligands and 4-cyanopyridine as co-ligands, we obtained the title compound which was identified by single crystal X-ray diffraction. Unfortunately, all samples were always contaminated with a second unknown crystalline phase, preventing any further investigations.

Structural commentary
The asymmetric unit of [Fe(NCS) 2 (C 6 H 4 N 2 ) 2 (H 2 O) 2 ]Á-2C 6 H 4 N 2 contains one Fe II cation that is located on an inversion centre, one thiocyanate anion, one water molecule and two 4-cyanopyridine molecules (Fig. 1). Discrete centrosymmetric [Fe(NCS) 2 (C 6 H 4 N 2 ) 2 (H 2 O) 2 ] complexes are formed, in which the Fe II cations are octahedrally coordinated by two N-bonded thiocyanate anions, two (pyridine)N-bonded 4-cyanopyridine ligands and two water molecules, each of them in a trans-position (Fig. 1). The disparate bond lengths are similar to those in related thiocyanate compounds. The distortion of the octahedron is also reflected by the deviation of the bond angles from ideal values. The structure contains additional 4-cyanopyridine solvate molecules that are located in the cavities of the structure.

Supramolecular features
The discrete complexes are linked into chains parallel to [101] by centrosymmetric pairs of intermolecular C-HÁ Á ÁN hydrogen bonds between the cyano group of the coordinating 4-cyanopyridine ligand and one of the pyridine H atoms (Fig. 2, Table 1). These chains are further linked by the 4-cyanopyridine solvate molecules through intermolecular O-HÁ Á ÁN hydrogen bonding. One water H atom is hydrogen-bonded to the N atom of the cyano group and the other H atom to the pyridine N atom of another 4-cyanopyridine solvate molecule. Since all water H atoms are involved in hydrogen bonding, each of the complexes is surrounded by four 4-cyanopyridine ligands, of which two are hydrogen-bonded via the cyano group, whereas the other two are hydrogen-bonded via the pyridine N atom (Fig. 3, Table 1). This arrangement leads to a three-dimensional network structure. It is noted that there are additional short contacts between the thiocyanate anions and the pyridine H atoms of the coordinating 4-cyanopyridine ligand of a neighbouring complex, which is indicative of weak C-HÁ Á ÁS hydrogen bonding (Table 1). Table 1 Hydrogen-bond geometry (Å , ). x; Ày þ 3 2 ; z À 1 2 ; (iii) Àx þ 1; Ày þ 1; Àz þ 2; (iv) x À 1; Ày þ 3 2 ; z þ 1 2 .

Figure 1
The discrete complex and the solvent molecule of the title compound with labeling and displacement ellipsoids drawn at the 50% probability level.

Figure 2
Part of the crystal structure of the title compound in a view along the b axis with emphasis on the connection of discrete complexes and solvent molecules by intermolecular hydrogen bonding (dashed lines).

Database survey
In the Cambridge Structure Database (Version 5.38, last update 2016; Groom et al., 2016), five structures of coordination polymers with 4-cyanopyridine and thiocyanate as ligands are reported, in which the metal cations are solely connected through -1,3 bridging thiocyanate anions. Two of these compounds contain copper, two cadmium and one is a bimetallic compound in which copper and mercury are present. The two copper-containing compounds are built up of chains, in which the cations are either tetrahedrally (Lin et al., 2004) or octahedrally (Machura et al., 2013a) coordinated. In the bimetallic compound the cations are linked into a threedimensional structure (Machura et al., 2013b), whereas the two cadmium-containing compounds exhibit either onedimensional or three-dimensional coordination networks (Chen et al., 2002).

Synthesis and crystallization
Iron(II) chloride tetrahydrate, potassium thiocyanate and 4-cyanopyridine were obtained from Alfa Aesar and used without further purification. 29.8 mg iron(II) chloride tetrahydrate (0.15 mmol) and 29.2 mg KSCN (0.30 mmol) were reacted with 62.5 mg 4cyanopyridine (0.60 mmol) in 1.5 ml water at room temperature. After two days, single crystals suitable for structure analysis were obtained. The batch contained a small amount of an additional crystalline phase that could not be identified.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Hydrogen atoms of the water molecule were located from a difference map, and C-bound hydrogen atoms were refined in calculated positions [C-H = 0.95 Å and O-H = 0.84 Å ] with U iso (H) = 1.2U eq (C) [1.5 for U eq (O)] using a riding model (O-H hydrogen atoms were allowed to rotate but not to tip).  (Sheldrick, 2015), DIAMOND (Brandenburg, 2014) and publCIF (Westrip, 2010).

Figure 3
The crystal structure of the title compound in a view along the a axis. Intermolecular hydrogen bonding is shown as dashed lines. SHELXL2014 (Sheldrick, 2015); molecular graphics: XP (Sheldrick, 2008) and DIAMOND (Brandenburg, 2014); software used to prepare material for publication: publCIF (Westrip, 2010).

Diaquabis(4-cyanopyridine-κN)bis(thiocyanato-κN)iron(II) 4-cyanopyridine disolvate
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.