Crystal structure of poly[bis(μ-2-bromopyrazine)tetra-μ2-cyanido-dicopper(I)iron(II)]: a bimetallic metal-organic framework

In the title bimetallic metal–organic framework, {Fe(Brpz)2[Cu(CN)2]2}n, where Brpz is 2-bromopyrazine, the FeII cation is located on an inversion centre and has a slightly elongated octahedral FeN6 coordination environment. The CuI center has a fourfold CuC3N coordination environment with an almost perfect trigonal–pyramidal geometry. Copper(I) centers related by a twofold rotation axis are bridged by two carbon atoms from a pair of μ-CN groups, resulting in Cu2(CN)2 units that build up the coordination framework.

In the title metal-organic framework, [Fe(C 4 H 3 BrN 2 ) 2 {Cu(CN) 2 } 2 ] n , the Fe II cation is located on an inversion center and has a slightly elongated octahedral coordination environment [FeN 6 ], ligated by two pyrazine N atoms of symmetry-related bridging 2-bromopyrazine molecules in the axial positions and by four N atoms of pairs of symmetry-related cyanido groups in the equatorial positions. The Cu I center has a fourfold coordination environment [CuC 3 N], with an almost perfect trigonal-pyramidal geometry, formed by three cyanido C atoms and an N atom of a bridging 2-bromopyrazine molecule. Copper(I) centers related by a twofold rotation axis are bridged by two carbon atoms from a pair of -CN groups, resulting in Cu 2 (CN) 2 units. Each Cu 2 (CN) 2 unit is linked to six Fe II cations via a pair of linear CN units, the pair of -CN groups and two bridging 2-bromopyrazine ligands, resulting in the formation of a metal-organic framework, which is additionally stabilized by the short CuÁ Á ÁCu contacts of 2.4450 (7) Å .

Chemical context
The rational design and synthesis of cyanide-based coordination materials is of key interest today. By using different approaches for their design and tunable structures, composition and porosity, various exciting properties of these compounds, such as catalytic, photoluminescent, magnetic, electrical and other can be achieved (Zhang et al., 2015;Catala & Mallah, 2017). The cyanide anion is an important ligand in coordination chemistry as it can be used for stabilization of coordination materials formed by diverse transition metals. Cyanide-containing coordination materials of very different topologies have been proposed, although attention is frequently paid to heterometallic complexes. ISSN 2056-9890 The first class of cyanide-based metallic complexes emerged in the 18th century with the discovery of Prussian blue. Later its analogues containing two types of metals were synthesized. The inclusion of different metals instead of iron resulted in the occurrence of various attractive physical properties of these materials, which allowed their use as molecular sieves, for nanoscale devices, for hydrogen storage, etc. (Newton et al., 2011). Prussian blue analogues form networks with general formula AM A [M B (CN) 6 ] (A = alkali metal ion, M A and M B = transition metal ions) (Keggin & Miles, 1936). These complexes have a cubic structure in which the metallic centers are bridged in an M A -C N-M B fashion, forming threedimensional frameworks.
Another class of heterometallic cyanide coordination compounds that has attracted much attention is represented by f-d complexes composed of lanthanide(III) ions and dblock cyanometallates. This type of materials has been shown to have exceptional photoluminescent properties (Chorazy et al., 2017). In addition, polynuclear octacyanides form a different family of cyanide-based heterometallic complexes.
Compounds of this class can adopt very different geometries creating 0, 1, 2 or 3D assemblies. These materials are known for photomagnetism, molecular magnetism, and the ability to create chiral networks (Sieklucka et al., 2011).

Structural commentary
A fragment of the structure of the title compound, illustrating the sixfold coordination environment of atom Fe1, is shown in Fig. 1. Selected geometrical parameters are given in Table 1. The Fe II ion is located on an inversion centre and has a slightly elongated FeN 6 octahedral coordination environment. It is ligated by two N atoms of symmetry-related 2-bromopyrazine molecules in axial positions [Fe1-N3 = 1.980 (2) Å ] and by four N atoms of symmetry related cyanido groups in the equatorial positions [Fe1-N1 = 1.958 (2) and Fe1-N2 = 1.952 (2) Å ]. The Fe-N bond lengths clearly indicate that the Fe II center is in the low-spin state at the temperature of the experiment, i.e. 296 K. The deviation from an ideal octahedron of the Fe II center can be described by the octahedral distortion parameter AE|90 À | = 23.36 , where is a cis-N-Fe-N angle. Notably, this sum is higher than that expected for a low-spin Fe II ion. It is important to note, and should always Acta Cryst. (2018). E74, 1895-1898 research communications Table 1 Selected geometric parameters (Å , ).

Figure 2
A view of the coordination environment of the Cu atoms in the title compound, with atom labelling [symmetry codes: (i) Àx + 1, Ày, Àz + 1; be taken into account, that the octahedral distortion value cannot always be used to judge the spin state of a metallic center, it is rather a characteristic of the order level in the structure, the measurement temperature, etc. Atom Cu1 has a fourfold CuC 3 N coordination environment (Fig. 2, Table 1) with a 4 descriptor of 0.82, close to that for a perfect trigonal-pyramidal geometry ( 4 = 0.00 for squareplanar, 1.00 for tetrahedral and 0.85 for trigonal-pyramidal; Yang et al., 2007). It is ligated by three C atoms of the cyanido groups and an N atom of a bridging 2-bromopyrazine mol- Notably, the coordination to atom Fe1 occurs only via the more sterically accessible atom N3 of the pyrazine ring, while atom Cu1 binds to atom N4 of the pyrazine ring.

Supramolecular features
The crystal packing of the title compound is shown in Fig. 3. The coordination framework is made up of bridging 2-bromopyrazine ligands and Cu 2 (CN) 2 moieties (Fig. 2). The latter are formed by a pair of copper atoms, centered about a twofold rotation axis, being bridged by two carbon atoms from a pair of -CN groups. Each Cu 2 (CN) 2 unit is linked to six Fe II cations via a pair of linear CN units, the pair of -CN groups and two bridging 2-bromopyrazine ligands, resulting in the formation of a metal-organic framework (Fig. 3). The framework is additionally stabilized by the short Cu1Á Á ÁCu1 i contact of 2.4550 (7) Å , which is significantly shorter than the sum of the corresponding van der Waals radii, viz. 2.8 Å . Additionally, within the framework there are weak BrÁ Á Á contacts of 3.8298 (6) Å , that are of lone-pairÁ Á Á origin.

Synthesis and crystallization
Crystals of the title compound were obtained by slow diffusion within three layers in a 3 ml glass tube. The first layer was a solution of K[Cu(CN) 2 ] (7.8 mg, 0.05 mmol) in 1 ml of water; the second layer was a water/ethanol mixture (1:1, 1 ml); the third layer was a solution of Fe(ClO 4 ) 2 Á6H 2 O (9.1 mg, 0.025 mmol) and 2-bromopyrazine (8.0 mg, 0.05 mmol) in 0.5 ml of ethanol. After two weeks, brown crystals were formed in the middle layer. The crystals were kept under the mother solution prior to measurement.

poly[bis(µ-2-bromopyrazine)tetra-µ 2 -cyanido-dicopper(I)iron(II)]
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.08 e Å −3 Δρ min = −0.76 e Å −3 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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )