Crystal structure of catena-poly[[[trans-bis(acetonitrile-κN)diaquacobalt(II)]-μ-pyrazine-κ2 N:N′] dinitrate]

The central structural motif of the title coordination polymer, [Co(C4H4N2)(CH3CN)2(H2O)2(NO3)2]n, is a chain composed of CoII ions linked by bis-monodentate bridging pyrazine ligands through their N atoms. Nitrate anions are situated in the space between the CoII chains

The central structural motif of the title coordination polymer, [Co(NO 3 ) 2 (C 4 H 4 N 2 )(CH 3 CN) 2 (H 2 O) 2 ] n , is a chain composed of Co II ions linked by bis-monodentate bridging pyrazine ligands through their N atoms. The Co II ion is located on an inversion center and is additionally coordinated by two O atoms of water molecules and two N atoms of acetonitrile molecules. The resultant N 4 O 2 coordination sphere is distorted octahedral. The linear cationic chains extend parallel to the a axis and are aligned into layers parallel to the ac plane. Nitrate anions are situated in the space between the Co II chains and form O-HÁ Á ÁO hydrogen bonds with the coordinating water molecules, leading to a three-dimensional network structure. Weak C-HÁ Á ÁO hydrogen bonds are also present between pyrazine or acetonitrile molecules and the nitrate anions.

Chemical context
In the design of coordination polymers, the choice of bridging ligands between metal atoms plays an important role in the formation of the final structure and the resulting properties. During our investigations of the preparation conditions and magnetic properties of compounds with ladder-like structures, we have used pyrazine as a bis-monodentate bridging ligand to link paramagnetic metal cations. From the point of view of mediating magnetic interactions, the pyrazine molecule offers some advantages compared to other bidentate bridging ligands such as 4,4 0 -bipyridine. In some of the structures with the latter ligand, the two pyridine rings are not co-planar and therefore can magnetically isolate metal atoms (Losier & Zaworotko, 1996;Ruan et al., 2009;Seidel et al., 2011;Lehleh et al., 2013).
We herein report the preparation and structure of a pyrazine-bridged chain structure obtained by reacting pyrazine and cobalt(II) nitrate hexahydrate using acetonitrile as the solvent.

Structural commentary
The asymmetric unit of the title compound, [Co(C 4 H 4 N 2 )(CH 3 CN) 2 (H 2 O) 2 (NO 3 ) 2 ] n , contains one Co II cation located on an inversion center, one water molecule, one acetonitrile molecule, one nitrate anion, and one half of a pyrazine molecule, the latter being completed by inversion symmetry. The Co II cation exhibits an N 4 O 2 coordination set defined by two O atoms [O1, O1 ii ; symmetry code: (ii) À3 À x, 1 À y, Àz] of two coordinating water molecules, two N atoms (N2, N2 ii ) of two coordinating acetonitrile molecules, and two nitrogen atoms (N1, N1 ii ) of two bridging pyrazine molecules (Fig. 1). The two Co-O water bonds have a length of 2.0315 (8) Å , considerably shorter than the two Co-N acetonitrile bonds of 2.1263 (9) Å , and the two Co-N pyrazine bonds of 2.1493 (10) Å . The resulting coordination sphere is compressed octahedral with all bond lengths in good agreement with similar structures (Choudhury et al., 2002;Holman et al., 2005;Aşkin et al., 2015). In contrast to the N 2 O 4 Acta Cryst. (2016). E72, 151-154 research communications Figure 1 A fragment of the one-dimensional chain structure of the title compound with displacement ellipsoids drawn at the 50% probability level.

D-HÁ
coordination spheres observed more frequently in the structures of other Co-containing compounds (Choudhury et al., 2002;Holman et al., 2005;Hyun et al., 2011;Aşkin et al., 2015), the title structure exhibits an N 4 O 2 coordination sphere due to the inclusion of the solvent acetonitrile molecules in the coordination sphere of Co II . The bridging bis-monodentate pyrazine molecules link the Co II ions, forming linear chains extending parallel to the a axis. The distance between two symmetry-related Co II ions within a chain (symmetry code: 1 + x, y, z) is 7.0798 (3) Å , in good agreement with those reported for similar structures (Choudhury et al., 2002;Holman et al., 2005;Aşkin et al., 2015).

Supramolecular features
In the crystal, the cationic chains are arranged to form sheets parallel to the ac plane, and neighboring sheets are related by a glide plane. Nitrate ions are sandwiched in the space between the sheets and form columns parallel to the a axis. Each Co II chain is surrounded by six columns of nitrate ions that are related by the inversion centers located along the cationic chains. Each cationic chain is further surrounded by six other chains. This structural motif with alternating layers has been observed in similar structures (Choudhury et al., 2002;Yang et al., 2003;Holman et al., 2005;Aşkin et al., 2015). Co II chains in neighboring sheets interact through nitrate ions by forming O-HÁ Á ÁO hydrogen bonds where the donor O-H groups are provided by the coordinating water molecules and the acceptor oxygen provided by the nitrate ions. One of those hydrogen bonds is bifurcated. For numerical values and symmetry operators, see Table 1. Weak C-HÁ Á ÁO hydrogen bonds are also present between the C-H groups of bridging pyrazine and coordinating acetonitrile molecules, and the oxygen atoms of nitrate ions, linking Co II chains both within the same sheet and to adjacent sheets (Table 1, Fig. 2).

Synthesis and crystallization
The title compound was obtained by a slow diffusion method in an U-shaped glass tube.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 2. C-bound H atoms were calculated in geometrically idealized positions and refined riding on their parent atoms, with U iso (H) = 1.2U eq (C) (aromatic) and 1.5U eq (C) (methyl), and with C-H = 0.95 Å (aromatic) and 0.98 Å (methyl). The methyl H atoms were allowed to rotate around the corresponding C-C bond. H atoms bound to water molecules were found in a difference map and were freely refined.