Crystal structure of the co-crystal fac-triaquatris(thiocyanato-κN)iron(III)–2,3-dimethylpyrazine (1/3)

The FeIII complex located on a threefold rotation axis links with dimethylperazine molecules via O—H⋯N hydrogen bonds,forming a three-dimensional supramolecular framework.

In the crystal of the title compound, [Fe(NCS) 3 (H 2 O) 3 ]Á3C 6 H 8 N 2 , the Fe III cation is located on a threefold rotation axis and is coordinated by three N atoms of the thiocyanate anions and three water molecules in a fac arrangement, forming a slightly distorted N 3 O 3 octahedron. Stabilization within the crystal structure is provided by O-HÁ Á ÁN hydrogen bonds; the H atoms from coordinating water molecules act as donors to the N atoms of guest 2,3dimethylpyrazine molecules, leading to a three-dimensional supramolecular framework.

Chemical context
In the large family of coordination compounds, materials showing a tunable character of their physical properties (e.g., electrical, magnetic, optical etc) are of special interest. Attempts to design compounds with such tunability have revealed the possibility to target the property of interest through the rational choice of ligands in transition metal complexes. For instance, variation of the aromatic N-donor ligand can lead to possible spin-state modulation of transition metals. In certain cases, these complexes can even possess spin crossover behaviour (transition between low and high spin states of a metal). The phenomenon of spin transition, which is one of the most known examples of molecular bistability, can be provoked by some external stimuli (temperature, pressure, light, magnetic field, absorption of some compounds) and is followed by a change of the optical, magnetic and electric properties (Gü tlich & Goodwin, 2004).
One of the simplest bridging N-donor ligands in the design of coordination polymers is pyrazine. This ligand is known for the formation of not only low-dimensional chains and sheets but also of some more complicated architectures, such as [Ag(pz)](CB 11 H 12 ) [CB 11 H 12 À is the monocarba-closo- ISSN 2056-9890 dodecaborate(À) anion], which exhibits a three-dimensional structure made up of checkerboard sheets of silver cations and anions connected by pillars of bridging pyrazine ligands (Cunha-Silva et al., 2006). In addition, pyrazine is able to construct Hofmann clathrates -spin crossover compounds with general formula [Fe II M II (pz)(CN) 4 ] 1 where M = Ni, Pd or Pt (Niel et al., 2001). A combination of pyrazine ligands with thiocyanates instead of tetracyanidometalates leads to the two-dimensional coordination polymer [Fe(pz) 2 (NCS) 2 ] 1 with an antiferromagnetic exchange between the metal cations (Real et al., 1991). In this context, we attempted to synthesize an Fe II thiocyanate complex with 2,3-dimethylpyrazine; however, the exposure of the starting material [Fe(OTs) 2 ]Á6H 2 O (OTs = p-toluenesulfonate) to the oxygen in the air led to the oxidation of Fe II and to the formation of the title compound.

Structural commentary
In the crystal structure of the title compound, the Fe III cation is located on a threefold rotation axis and is in an octahedral coordination environment formed by three N atoms of the thiocyanate anions and three O atoms of water molecules arranged in a fac configuration (Fig. 1). The distance between the Fe III ion and the N atoms [2.025 (4) Å ] is longer than that between the Fe III ion and the O atoms [2.034 (3) Å ] and therefore the FeN 3 O 3 octahedron is slightly distorted. These structural features are typical for related compounds (Shylin et al., 2013(Shylin et al., , 2015 Table 1 Hydrogen-bond geometry (Å , ). (3) 2.745 (4) 172 (8)

Figure 1
The molecular structure of the title compound, with displacement ellipsoids drawn at the 50% probability level. [Symmetry codes: (i) Ày + 1,

Figure 2
Crystal structure of the title compound, showing hydrogen bonds as dashed lines. H atoms not involved in hydrogen bonding have been omitted for clarity. Colour key: bronze Fe, yellow S, blue N, grey C and red O.

Supramolecular features
In the title compound, the crystal packing is stabilized by O-HÁ Á ÁN hydrogen bonds (Table 1): the H atoms from coordinating water molecules act as donors to the N atoms of guest 2,3-dimethylpyrazine molecules. The compound contains three guest molecules of pyrazine per Fe III cation. In the crystal lattice, each molecule of the complex is attached to six molecules of pyrazine, while each pyrazine is connected with two water molecules of the host complexes, leading to the formation of a three-dimensional network (Fig. 2).

Synthesis and crystallization
Crystals of the title compound were obtained by the slowdiffusion method between three layers, the first layer being a solution of [Fe(OTs) 2 ]Á6H 2 O (0.096 g, 0.2 mmol) and NH 4 SCN (0.046 g, 0.6 mmol) in water (10 ml), the second being a water/methanol mixture (1/1, 10 ml) and the third a solution of 2,3-dimethylpyrazine (0.065 g, 0.6 mmol) in methanol (3 ml). After two weeks, red plates grew in the second layer; they were collected, washed with water and dried in air, yield 0.028 g (23%).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All hydrogen atoms connected to C and O atoms were placed in their expected calculated positions and refined as riding with C-H = 0.98 (CH 3 ), 0.95 (C arom ), O-H = 0.80 (3) Å , and with U iso (H) = 1.2U iso (C) with the exception of methyl hydrogen atoms, which were refined with U iso (H) = 1.5U eq (C).