Crystal structure of catena-poly[[[tetraaquairon(II)]-trans-μ-1,2-bis(pyridin-4-yl)ethene-κ2 N:N′] bis(p-toluenesulfonate) methanol disolvate]

The 1,2-bis(pyridin-4-yl)ethene molecules bridge FeII cations to form polymeric chains running along the a axis.

In the title polymeric complex, {[Fe(C 12 H 10 N 2 ) 2 (H 2 O) 4 ](CH 3 C 6 H 4 SO 3 ) 2 Á-2CH 3 OH} n , the Fe II cation, located on an inversion centre, is coordinated by four water molecules in the equatorial positions and two 1,2-bis(pyridin-4yl)ethene molecules in the axial positions. This results in a distorted octahedral geometry for the [N 2 O 4 ] coordination polyhedron. The 1,2-bis(pyridin-4-yl)ethene molecules bridge the Fe II cations, forming polymeric chains running along the a-axis direction. Stabilization of the crystal structure is provided by O-HÁ Á ÁO hydrogen bonds; these are formed by coordinated water molecules as donors towards the O atoms of the methanol molecules and tosylate anions as acceptors of protons, leading to the formation of a three-dimensional supramolecular network. Weak C-HÁ Á ÁO hydrogen bonds are also observed in the crystal.

Chemical context
Transition metal complexes containing pyridine or substituted pyridines as ligands are of current interest due to their supramolecular arrangements and the probability of being spin-crossover compounds. Spin crossover (SCO), sometimes referred to as a spin transition or a spin equilibrium behaviour, is a phenomenon that occurs in some metal complexes wherein the spin state of a compound changes due to the influence of external stimuli such as temperature, pressure, light irradiation, magnetic field or guest effects (Gü tlich & Goodwin, 2004). Bridging N-donor ligands are often used to produce Fe-based SCO complexes; for example, pyrazine is known to form interesting three-dimensional frameworks with remarkable transition characteristics (Muñ oz & Real, 2011;. A variation of the aromatic N-donor ligand can lead to possible spin-state modulation in transition metal complexes (Gü tlich & Goodwin, 2004). In recent years, particular attention has been drawn to bridging ligands that are able to form analogues of Hoffman clathrates with a large pore size. These ligands include bridge-polydentate derivatives of pyridine and other azine ligands (Muñ oz & Real, 2011). Importantly, Fe-based SCO in analogues of Hoffman clathrates is known in complexes with 1,2-bis(pyridin-4-yl)ethene as a bridging N-donor ligand. Its complex with cyanoargentate as a co-ligand shows one of the largest thermal hysteresis (ca 95K wide) observed for spin-crossover complexes (Niel et al., 2002).
Here we report on the title new polymeric compound based on 1,2-bis(pyridin-4-yl)ethene in which Fe II ions are stabilized in the high-spin state.

Structural commentary
The Fe II cation has a distorted octahedral coordination environment [FeN 2 O 4 ], formed by two N atoms of 1,2-bis-(pyridin-4-yl)ethene and by four O atoms of four water molecules ( Fig. 1). Two 1,2-bis(pyridin-4-yl)ethene molecules are coordinated at the axial positions [with an Fe-N distance of 2.218 (2) Å ]. The equatorial positions of the Fe II cation are occupied by four O-coordinated water molecules with bond lengths Fe1-O1 = 2.114 (2) and Fe1-O2 = 2.077 (2) Å . The small difference in the lengths of the Fe-O bonds of 0.037 Å could be associated with a different participation of the water hydrogen atoms in hydrogen bonding. The metal-to-ligand distances clearly indicate the high-spin nature of the complex described herein.
The Fe II octahedral distortion parameter (the sum of the moduli of the deviations from 90 for all cis bond angles) is AE|90 À Â| = 28.15 (8), where Â is the cis-N-Fe-O and cis-O-Fe-O angles in the coordination environment of the Fe II atom. This value indicates a significant polyhedral distortion, which can be explained by the Jahn-Teller effect and the presence of different types of ligands.

Supramolecular features
The coordination structure is formed by binding 1,2-bis-(pyridin-4-yl)ethene fragments with Fe II cations into polymer chains that propagate along the a-axis direction. Stabilization in the crystal structure is ensured by O-HÁ Á ÁO hydrogen bonds (Fig. 2, Table 1): (i) H atoms of water molecules and the oxygen atoms of tosylate anions; (ii) H atoms of water molecules and methanol molecules; (iii) H atoms of the hydroxyl group of methanol with the tosylate anion. The compound contains two solvate molecules of methanol per Fe II cation. In the crystal lattice, each tosylate anion is connected with three water molecules of the complex cation, leading to the formation of a three-dimensional supramolecular network [1977][1978][1979][1980] research communications Table 1 Hydrogen-bond geometry (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: Fig. 2). In addition, weak C-HÁ Á ÁO hydrogen bonds are also observed in the crystal. A view of the packing is shown in Fig. 3.

Database survey
A survey of the Cambridge Structural Database confirmed that the structure of the title complex has not been reported previously. 41 structures are known with an Fe cation coordinated by four water O atoms and two N atoms from the pyridine fragment. The survey yielded the structure of one related compound, in which the Fe II cation has a distorted octahedral coordination environment [FeN 2 O 4 ], formed by two N atoms of 1,2-bis(pyridin-4-yl)ethene and by four O atoms of four water molecules; however, it contains 2,6-dioxo-1,2,3,6-tetrahydropyrimidin-4-olate as the anion and crystallizes in the orthorhombic Pbcn space group. In this analogue, Fe1-N1 = 2.2304 (2), Fe1-O2 = 2.1030 (2) and Fe1-O4 = 2.0908 (2) Å (Garcia et al., 2011), contrary to what is observed in the title compound.

Figure 2
Crystal structure of the title compound, showing hydrogen bonds as dashed lines. Colour key: violet Fe, yellow S, blue N, grey C and red O.

Figure 3
The crystal packing. Colour key: violet Fe, yellow S, blue N, grey C and red O.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All aromatic hydrogens and hydrogen atoms of the CH groups were placed in their expected calculated positions (C-H = 0.95 Å ) and refined as riding with U iso (H) = 1.2U iso (C). Methyl H atoms were placed in their expected calculated positions (C-H = 0.98 Å ) and refined as rotating groups with U iso (H) = 1.5U eq (C). Hydrogen atoms of the water molecules were assigned based on the difference-

Funding information
The work was supported by H2020-MSCA-RISE-2016 Project 73422.

catena-Poly[[[tetraaquairon(II)]-trans-µ-1,2-bis(pyridin-4-yl)ethene-κ 2 N:N′] bis(p-toluenesulfonate) methanol disolvate]
Crystal data [Fe(C 12  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.