Crystal structure of a low-spin poly[di-μ3-cyanido-di-μ2-cyanido-bis(μ2-2-ethylpyrazine)dicopper(I)iron(II)]

In the title compound, {Fe(Etpz)2[Cu(CN)2]2}n, where Etpz is 2-ethylpyrazine, the low-spin FeII atom lies on an inversion centre and has an elongated octahedral [FeN6] coordination environment, where the axial positions are occupied by two 2-ethylpyrazine ligands and the equatorial positions are occupied by two pairs of symmetry-related cyanido groups. Each Cu2(CN)2 unit is connected to six FeII centres via two bridging 2-ethylpyrazine molecules and four cyanido groups, resulting in the formation of a polymeric three-dimensional bimetallic metal–organic framework, additionally stabilized by Cu⋯Cu contacts.

In the title metal-organic framework, [Fe(C 6 H 8 N 2 ) 2 {Cu(CN) 2 } 2 ] n , the low-spin Fe II ion lies at an inversion centre and displays an elongated octahedral [FeN 6 ] coordination environment. The axial positions are occupied by two symmetryrelated bridging 2-ethylpyrazine ligands, while the equatorial positions are occupied by four N atoms of two pairs of symmetry-related cyanide groups. The Cu I centre is coordinated by three cyanide carbon atoms and one N atom of a bridging 2-ethylpyrazine molecule, which form a tetrahedral coordination environment. Two neighbouring Cu atoms have a short CuÁ Á ÁCu contact [2.4662 (7) Å ] and their coordination tetrahedra are connected through a common edge between two C atoms of cyanide groups. Each Cu 2 (CN) 2 unit, formed by two neighbouring Cu atoms bridged by two carbons from a pair of -CN groups, is connected to six Fe II centres via two bridging 2-ethylpyrazine molecules and four cyanide groups, resulting in the formation of a polymeric three-dimensional metal-organic coordination framework.

Chemical context
The phenomenon of spin crossover (SCO) occurs in some metal complexes where the spin state of a compound changes as a result of the influence of external stimuli (temperature, pressure, light irradiation, magnetic field etc.) (Gü tlich & Goodwin, 2004). Analogues of Hofmann clathrates (Hofmann & Hö chtlen, 1903) are the most diverse SCO compounds with switchable properties because of their specific structural features. They are bimetallic two-or three-dimensional coordination frameworks formed by Fe II ions coordinated by cyanometallic anions [M(CN) x ] yÀ and N-donor heterocyclic ligands (Ohkoshi et al., 2014;Muñ oz & Real, 2011). Such frameworks have been prepared in forms of single crystals, thin films (Bell et al., 1994) and nanoparticles (Volatron et al., 2008), thus presenting a group of materials characterized by the presence of sharp and hysteretic SCO. A large variety of Hofmann-like polymeric SCO complexes originates from a set of available cyanometallates (formed by Ni, Pt, Pd, Ag, Au, Cu and Nb) and organic ligands, which potentially could promote the spin state change of Fe atoms (Muñ oz & Real, 2011). Pyridine (Kitazawa et al., 1996), aminopyridine (Liu et al., 2015), pyrazine (Niel et al., 2001), azopyridine (Agustí et al., 2008), pyrimidine (Niel et al., 2003) and some others have been reported as coligands in these frameworks. Among the above-mentioned azines, the simplest 2 -bridging system is pyrazine, which provides 1,4-binding and the formation of compact frameworks (Southon et al., 2009). Taking into account that the modification of pyrazine can influence not only the structure of a complex but also the spin state of Fe, and being inspired by a previously published structure with 2-bromopyrazine as a coligand and bridging cyanocuprates (Kucheriv et al., 2018), here we describe the crystal structure of a new Hofmann clathrate analogue of general formula [Fe(Etpz) 2 {Cu(CN) 2 } 2 ] n (where Etpz is 2-ethylpyrazine).
The coordination polyhedra of Fe and Cu atoms of the title compound and their relative positions are shown in Fig. 2. Six N atoms form a slightly elongated octahedral coordination environment of the Fe II ion. The deviation from an ideal octahedron of the Fe1 centre can be described by the octahedral distortion parameter AE|90 À | = 20.59 , where is a cis-N-Fe-N angle. The fourfold CuC 3 N coordination environment of the Cu I centre adopts a tetrahedral geometry. Two tetrahedra of neighboring Cu centres are connected through a common edge between two C atoms of cyanido groups. This common edge is perpendicular to the CuÁ Á ÁCu contact. Each Fe octahedron is surrounded by six double Cu-Cu edge-connected tetrahedra and is bound with them by four cyanido groups and two bridging pyrazine rings. At the same time, dicopper two edge-connected tetrahedra are linked to four Fe II ion octahedrons via cyanido bridges and to two Fe octahedra via pyrazine rings.   A fragment of the crystal structure of the title compound with atom labelling. Displacement ellipsoids are drawn at the 90% probability level [symmetry codes: The CuÁ Á ÁCu short contact is shown as a dashed line. Fig. 3 illustrates the crystal packing of the title compound. The unit cell contains four units of the title compound with empirical formula C 16 H 16 Cu 2 FeN 8 . The latter consists of bridging 2-ethylpyrazine ligands and Cu 2 (CN) 2 pairs, in which two Cu atoms, centred about a twofold rotation axis, are interconnected by two -CN groups through C atoms. The resulting polymeric three-dimensional metal-organic coordination framework is additionally stabilized by supramolecular CuÁ Á ÁCu contacts in each Cu 2 (CN) 2 unit.  Cui et al., 2001), the structure of which was described as a 3D network with two types of bridging cyanides. The CuÁ Á ÁCu distances are 2.5431 (11) and 2.5734 (13) Å , respectively, compared to 2.4662 (7) Å in the title MOF.

Database survey
A search through the CSD for the Fe ion ligated by four N C-Cu and two azines gave 15 hits, which are all bimetallic MOFs with pyrimidine, cyanopyridine and fluoro-, chloro-, bromo-and iodopyridine as ligands.
A search through the CSD for 2-ethylpyrazine gave 20 hits, in most of which 2-ethylpyrazine molecule binds to Cu, Ag, Mn or Rh ions. In the majority of compounds containing copper, the 2-ethylpyrazine serves as a bridging ligand between two Cu atoms in MOFs. An example closely related to the title structure is catena-[( 3 -cyano)tris( 2 -cyano)bis-

Synthesis and crystallization
Crystals of the title compound were obtained by a slow diffusion within three layers in a 3 ml glass tube. The first layer was a solution of K[Cu(CN) 2 ] (9.3 mg, 0.06 mmol) in 1 ml of H 2 O; the second layer was an H 2 O/EtOH mixture (1:1, 1 ml); the third layer was a solution of Fe(ClO 4 ) 2 Á6H 2 O (10.9 mg, 0.03 mmol) and 2-ethylpyrazine (6.5 mg, 0.06 mmol) in 0.5 ml of EtOH. After two weeks, brown crystals were formed in the middle layer. The crystals were kept under the mother solution prior to measurement.  Computer programs: SAINT and APEX (Bruker, 2013), SHELXS (Sheldrick, 2008), SHELXL (Sheldrick, 2015) and OLEX2 (Dolomanov et al., 2009).

Figure 3
A view normal to the ac plane of the crystal structure of the title compound showing the CuÁ Á ÁCu contacts as dashed lines. Ethyl substituents of 2-ethylpyrazine rings and H atoms have been omitted for clarity. Colour code: Fe dark red, Cu green, C grey, N blue.

poly[di-µ 3 -cyanido-di-µ 2 -cyanido-bis(µ 2 -2-ethylpyrazine)dicopper(I)iron(II)]
Crystal data 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.