Crystal structure of tetraaqua[2-(pyridin-2-yl)-1H-imidazole-κ2 N 2,N 3]iron(II) sulfate

The Fe—O and Fe—N bond lengths in the title compound, [Fe(pyim)(H2O)4]SO4, where pyim is 2-(pyridin-2-yl)-1H-imidazole, are markedly different than in the related structure of [Fe(dmbpy)(H2O)4]SO4 where dmbpy is 5,5′-dimethyl-2,2′-bipyridine.

In the title compound, [Fe(C 8 H 7 N 3 )(H 2 O) 4 ]SO 4 , the central Fe II ion is octahedrally coordinated by two N atoms from the bidentate 2-(pyridin-2-yl)-1H-imidazole ligand and by four O atoms of the aqua ligands. The largest deviation from the ideal octahedral geometry is reflected by the small N-Fe-N bite angle of 76.0 (1) . The Fe-N coordination bonds have markedly different lengths [2.1361 (17) and 2.243 (2) Å ], with the shorter one to the pyrimidine N atom. The four Fe-O coordination bond lengths vary from 2.1191 (18) to 2.1340 (17) Å . In the crystal, the cations and anions are arranged by means of medium-strength O-HÁ Á ÁO hydrogen bonds into layers parallel to the ab plane. Neighbouring layers further interconnect by N-HÁ Á ÁO hydrogen bonds involving the imidazole fragment as donor group to one sulfate O atom as an acceptor. The resulting three-dimensional network is consolidated by C-HÁ Á ÁO, C-HÁ Á Á andinteractions.

Chemical context
Polynitrile anions have recently received considerable attention in the fields of coordination chemistry and molecular materials (Benmansour et al., 2010). These organic anions are of interest due to their ability to act towards metal atoms with various coordination modes and for their high degree of electronic delocalization (Miyazaki et al., 2003;Atmani et al., 2008;Benmansour et al., 2008Benmansour et al., , 2012Setifi et al., 2002Setifi et al., , 2013Setifi et al., , 2014Addala et al., 2015).
We are interested in using these anionic ligands in combination with other neutral bridging co-ligands to explore their structural features and properties relevant to the field of molecular materials exhibiting the spin crossover (SCO) phenomenon (Dupouy et al., 2008(Dupouy et al., , 2009. In an attempt to prepare such an iron(II) complex using hydrothermal synth-  Fig. 1 shows the asymmetric unit of (I). The main building units in the crystal structure of (I) are octahedral [Fe(pyim)(H 2 O) 4 ] 2+ complex cations and [SO 4 ] 2À anions. The distorted octahedral environment of the central Fe II ion is defined by two N donor atoms of the pyim ligand and by the O atoms of two water molecules in the equatorial plane, while the two remaining water molecules coordinate at the axial sites. The bite angle N1-Fe-N2 of 76.04 (7) shows the most significant deviation from the ideal octahedral geometry, with the other coordination angles deviating by 0.21 (7) to 11.91 (7) .

Structural commentary
The Fe-N coordination bonds with the chelate ligand have markedly different lengths, Fe-N1 = 2.243 (2) and Fe-N2 = 2.1361 (17) Å , which are also dissimilar to those in the previously reported [Fe(dmbpy)(H 2 O) 4 ]SO 4 complex where dmbpy is 5,5 0 -dimethyl-2,2 0 -bipyridine (Belamri et al., 2014.) comprising a nearly symmetrical dipyridyl ligand [Fe-N = 2.176 (3) Å on average]. The torsion angles within the approximately planar five-membered chelate ring of (I) vary from 0.6 (3) to À5.2 (2) and reflect a more pronounced deviation from planarity in comparison with the dmbpy Fe II complex that exhibits a maximal torsion angle of 2.0 (3) . The dihedral angle of 5.5 (1) between the aromatic rings of the pyim ligand is within the range of the values reported for the eight independent molecules in the crystal structure of the non-coordinating ligand [1(1) to 17 (1)

Supramolecular features
The crystal packing of (I) is stabilized by a complex hydrogenbonding network involving the coordinating water molecules and the imidazole fragment as donors to the O acceptors atoms of the sulfate anion. The molecular structure of (I), with atom labels and displacement ellipsoids at the 50% probability level. Hydrogen bonds are shown as double dashed lines. Table 1 Hydrogen-bond geometry (Å , ).

Synthesis and crystallization
The title compound was obtained under hydrothermal conditions from a mixture of iron(II) sulfate heptahydrate (28 mg, 0.1 mmol), 2-(pyridin-2-yl)-1H-imidazole (15 mg, 0.1 mmol) and potassium tricyanomethanide KC(CN) 3 (26 mg, 0.2 mmol) in water-ethanol (4:1 v/v, 20 ml). The mixture was transferred to a Teflon-lined autoclave and heated at 423 K for 48 h. The autoclave was then allowed to cool to ambient temperature. Block-like yellow crystals of (I) were collected by filtration, washed with water and dried in air (yield 58%).

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms bonded to C atoms were placed at geometrically calculated positions and refined using a riding model. C-H distances were fixed to 0.93 Å for aromatic C atoms, with U iso (H) = 1.2U eq (C). The H atoms attached to O and N atoms were located in a difference Fourier map and were refined isotropically.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.