Poly[[diaquatetrakis(μ2-benzene-1,4-dicarbonitrile-κ2 N:N′)iron(II)] bis[tetrachloridoferrate(III)] nitromethane tetrasolvate]

In the title compound, {[FeII(C8H4N2)2(H2O)2][FeIIICl4]2·4CH3NO2}n, the FeII and FeIII ions are hexa- and tetracoordinated, respectively. Each unique benzene-1,4-dicarbonitrile molecule lies across a crystallographic inversion centre and bridges two FeII ions (each situated on an inversion centre), generating two-dimensional (4,4) square grid layers. The tetrachloridoferrate(III) anions and nitromethane solvent molecules lie between the square grid layers and are further link to the adjacent layers into a three-dimensional supramolecular structure through O—H⋯Cl and O—H⋯O hydrogen bonds.

In the title compound, {[Fe II (C 8 H 4 N 2 ) 2 (H 2 O) 2 ][Fe III Cl 4 ] 2 Á-4CH 3 NO 2 } n , the Fe II and Fe III ions are hexa-and tetracoordinated, respectively. Each unique benzene-1,4-dicarbonitrile molecule lies across a crystallographic inversion centre and bridges two Fe II ions (each situated on an inversion centre), generating two-dimensional (4,4) square grid layers. The tetrachloridoferrate(III) anions and nitromethane solvent molecules lie between the square grid layers and are further link to the adjacent layers into a three-dimensional supramolecular structure through O-HÁ Á ÁCl and O-HÁ Á ÁO hydrogen bonds.

Comment
The d 6 Fe(II) complexes exhibiting spin-crossover (SCO) transitions between a 1 A 1 low spin (S = 0) and a 5 T 2 high spin (S = 2) states are of interest due to their possible applications as molecular switches or materials for information storage (Kahn & Martinez 1998). Although the fundamental origin of the SCO phenomenon is molecular, SCO transitions in the crystal structure can be reversibly switched by external stimuli such as light, temperature and pressure as well as the existence of long or short range supramolecular interactions (Neville et al. 2007(Neville et al. , 2008Murray 2008). The title compound was obtained as a minor product in another an earlier from part of further study of how the nature of the two-connecting organodinitrile bridging ligands affect the SCO phenomenon in the Fe(II) complexes (Chainok et al. 2010).
A fragment containing the asymmetric unit with atom numbering and coordination environments of the metal centre of the title compound is shown in Fig. 1. The asymmetric unit contains one iron(II) cation (half-occupancy), one coordinated water molecule, half of two independent benzene-1,4-dicarbonitrile ligand, one tetrachloridoferrate(III) anion, and two nitromethane solvent molecules. The Fe II ion resides in an inversion center and is octahedrally coordinated by four nitrogen atoms from benzene-1,4-dicarbonitrile ligands in the equatorial plane and two equivalent terminal water molecules occupying the axial positions. The Fe-N and Fe-O bond lengths in the title compound, Table 1, are comparable to that observed for a high spin species in the corresponding Fe(II) complex containing the two-connecting organodinitrile ligands such as  (Chainok et al. 2010). Each benzene-1,4-dicarbonitrile ligand has crystallographically imposed inversion symmetry and is bound to two neighboring Fe II ions generating two-dimensional approximate square grid layers with (4,4) topology perpendicular to the c axis, with the dimension of 12.1307 (9) × 12.1554 (9) Å (Fe II ···Fe II distances across the benzene-1,4dicarbonitrile ligand), Fig. 2. The layers are packed to each other with an interlayer separation of 6.0777 (5) Å.
The Fe III atom is a tetrahedrally coordinated to four chloride anions. The bond lengths and angles around the Fe(III) ions are within the common ranges for this type of coordination environment (Chainok et al. 2010). The tetrachloridoferrate(III) anions and the nitromethane solvent molecules all lie in general position in the structure, and are included in the layers by eschewing the interpenetration of the networks. These guest molecules are then further linked to the adjacent two-dimensional layers into a three-dimensional supramolecular structure through O-H···Cl and O-H···O hydrogen bonds formed between the apically water molecules coordinated to the metal ions and Cl and O atoms of the guest molecules, Table 2.

Experimental
Single crystals of the title compound were obtained as the minor product during the synthesis of [Fe II (C 8 (Chainok et al. 2012) when traces of air and moisture are presented. Typically, FeCl 2 (62 mg, 0.5 mmol) and FeCl 3 (163 supplementary materials sup-2 mg, 1 mmol) were dissolved in 3 ml of CH 3 NO 2 to formed a yellow brown solution and this was pipetted into one side of the H-tube. Benzene-1,4-dicarbonitrile (192 mg, 1.5 mmol) was dissolved in 3 ml of CH 3 NO 2 to give a colorless solution and this was pipetted into the other side arm of the H-tube. The H-tube was then carefully filled with CH 3 NO 2 . Upon slow diffusion for two weeks, yellow block-shaped of the major product (ca. 60% yield based on FeCl 2 ) and pale yellow plate of the minor product (ca. 5% yield based on FeCl 2 ) single crystals were formed in the iron-containing side of the H-tube.

Refinement
The hydrogen atoms were placed in the geometrically idealized positions and constrained to ride on their parent atom positions with a C-H distances of 0.95 and 0.99 Å U iso (H) = 1.2U eq (C) and 0.98 Å for CH 3 [U iso (H) = 1.5U eq (C)]. The hydrogen atoms attached to oxygen atoms of the water molecules were located in a difference Fourier map and refined being in their as-found positions with a DFIX restraint of O-H distance at 0.90 Å, with U iso (H) = 1.2U eq (O). Fig. 1. Displacement ellipsoid plot of a fragment at the 50% probability level containing the asymmetric unit with atom numbering and coordination environments of the metal centers of the title compound.