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., 2008 ▸, 2012 ▸; Setifi et al., 2002 ▸, 2013 ▸, 2014 ▸; Addala 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 ▸, 2009 ▸). In an attempt to prepare such an iron(II) complex using hydro­thermal synthesis, we obtained instead the title compound [Fe(pyim)(H₂O)₄]SO₄, (I), where pyim is 2-(pyridin-2-yl)-1H-imidazole.

Fig. 1 ▸ shows the asymmetric unit of (I). The main building units in the crystal structure of (I) are octa­hedral [Fe(pyim)(H₂O)₄]²⁺ complex cations and [SO₄]²⁻ 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)°.

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)(H₂O)₄]SO₄ complex where dmbpy is 5,5′-dimethyl-2,2′-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 −5.2 (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)°; Tinant et al., 2010 ▸]. In the present complex, all four Fe—O bond lengths, ranging from 2.1191 (18) to 2.1340 (17) Å, are longer than the corres­ponding ones in the [Fe(dmbpy)(H₂O)₄]SO₄ complex, which range from 2.079 (2) to 2.110 (2) Å.

The crystal packing of (I) is stabilized by a complex hydrogen-bonding network involving the coordinating water mol­ecules and the imidazole fragment as donors to the O acceptors atoms of the sulfate anion. Each cationic [Fe(pyim)(H₂O)₄]²⁺ unit is surrounded by five [SO₄]²⁻ anions. Similarly to the crystal structure of [Fe(dmbpy)(H₂O)₄]SO₄, pairs of axially and equatorially coordinating water mol­ecules bind to pairs of O acceptor atoms from the same [SO₄]²⁻ group, forming eight medium-strength inter­actions (Table 1 ▸). These hydrogen bonds arrange the complex mol­ecules into layers parallel to the ab plane (Fig. 2 ▸). Additional N—H⋯O and C—H⋯O hydrogen bonds involving the donors from the aromatic ligand inter­connect adjacent layers into a three-dimensional arrangement (Fig. 3 ▸). The vicinity of aromatic rings in the inter-layer region gives rise to C—H⋯π [H3⋯Cg1ⁱ = 3.033 Å; C3—H3⋯Cg1ⁱ = 117°; symmetry code: (i) = −x + , y + , z; Cg1 is the centroid of the imidazole ring] and weak π–π inter­actions [Cg1⋯Cg2ⁱⁱ = 3.821 Å, the shortest inter­atomic distance N3⋯C2ⁱⁱ = 3.325 (1) Å; symmetry code: (ii) = −x + 1, −y + 1, −z + 1; Cg1 and Cg2 are the centroids of the imidazole and pyridine rings, respectively]. C—H⋯O inter­actions are also observed (Table 1 ▸).

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%).

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.