Crystal structure of catena-poly[[(dimethyl sulfoxide-κO)(pyridine-2,6-dicarboxylato-κ3 O,N,O′)nickel(II)]-μ-pyrazine-κ2 N:N′]

A one-dimensional NiII coordination polymer has been prepared via solvothermal synthesis using dimethyl sulfoxide as solvent. The coordination polymer forms double-chains along [010] and exhibits π–π stacking and C—H⋯π interactions forming the interior of the double-chains, separated from a C—H⋯π hydrogen-bonding network in the space between the double-chains.


Chemical context
In general,interactions are considered important mechanisms for molecular recognition and may function as structure-directing factors in the design and preparation of coordination polymers. However,interactions are not always observed in the final coordination polymer simply by using starting materials containing aromatic moieties. During our investigation of the rational design and synthesis of coordination polymers, we have previously reported a dinuclear Ni II complex obtained by reacting 2,6-pyridine dicarboxylic acid and nickel carbonate using water as solvent (Liu et al., 2011). The intermolecular force between the dinuclear complexes is dominated by hydrogen bonding. We recently repeated the synthesis of this compound using dimethyl sulfoxide (DMSO) as solvent under solvothermal conditions and obtained the title compound. We herein report its synthesis and structure which exhibits bothstacking and C-HÁ Á Á interactions involving two different aromatic molecules, viz. pyridine and pyrazine. ISSN 2056-9890

Structural commentary
The asymmetric unit contains two half Ni II complexes with mirror symmetry (denoted A and B), where each of the Ni II atoms is coordinated by a 2,6-pyridine-dicarboxylic acid dianion, a pyrazine molecule, and a DMSO ligand (Fig. 1). The tridentate 2,6-pyridine-dicarboxylate anion coordinates to Ni II in a meridional fashion via the pyridine nitrogen atom and two carboxylate oxygen atoms; the DMSO molecule coordinates to Ni II through its oxygen atom and the pyrazine ligands through their N atoms. Thus each Ni II is in an N 3 O 3 coordination environment. Individual Ni II complexes are linked along the axial positions by bis-monodentate bridging pyrazine molecules to form a linear chain parallel to [010] and propagated through mirror symmetry elements passing through the Ni II atoms, the anions, and bisecting both the pyrazine ligands and the DMSO molecules along the S O bonds. In the chains, the Ni-Ni distance across bridging pyrazine is 7.0296 (4) Å , i.e. the length of the b axis.

Supramolecular features
In the crystal, two Ni II chains form a double-chain structure viastacking between their pyridine moieties (Fig. 2). Two stacked pyridine rings in the double-chain structure are separated by a centroid-to-plane distance of 3.5148 (2) Å . This separation distance is half of the Ni-Ni distance, indicating that the formation ofstacking in the double-chain structure may have been promoted by coordinative bonding distances across bridging pyrazine ligands. A search in the literature returned only a few other examples of coordination polymers exhibiting similar structural features (Zheng et al., 2000;Nawrot et al., 2015). Within the double-chain, twostacked pyridine moieties are also parallel-shifted by 1.50422 (8) Å , consistent with values obtained from computational studies (Huber et al., 2014). Althoughstacking interactions are prevalent among systems composed of discrete aromatic molecules, it is not always observed in coordination polymers synthesized from aromatic starting materials. The title structure thus provides an interesting example for further investigation on the interplay between coordinative bonding andstacking as a potential strategy for incorporatingstacking in the design and synthesis of coordination polymers.
Accompanying thestacking interaction described above, there is also a T-shaped C-HÁ Á Á interaction between the pyridine C4-H4 group and the bridging pyrazine molecule (Tiekink & Zuckerman-Schpector, 2012), contributing additional stability to the double-chain structure. The   concurrence of both parallelstacking and T-shaped C-HÁ Á Á interactions in crystal structures is known in the literature, but primarily among systems of discrete aromatic molecules (Tiekink & Zuckerman-Schpector, 2012). We are aware of only one other example of a coordination polymer exhibiting this feature (Felloni et al., 2010). In the C -HÁ Á Á configuration of the title structure, the centroid-to-centroid distance between pyridine and pyrazine is 4.8389 (2) Å , which includes the pyridine C4-H4 bond length of 0.95 Å and a distance of 2.53310 (12) Å from the pyridine H4 atom to the centroid of the pyrazine ring. Although the title structure is a coordination polymer, these distances are in good agreement with results of computational studies performed on discrete aromatic molecules (Mishra & Sathyamurthy, 2005;Hohenstein & Sherrill, 2009;Huber et al., 2014).
In contrast to thestacking and C-HÁ Á Á interactions forming the interior of the double-chains, the exterior of the double-chains is mainly occupied by polar DMSO molecules and carboxylate groups. As a result, a network of C-HÁ Á ÁO hydrogen bonds exists in the space between the double-chains (Fig. 3), linking double-chains to form a three dimensional network. Double-chains of molecule B are linked by C21B-H21AÁ Á ÁO2B ii to form sheets parallel to (001). Double-chains of molecule A are linked by C21A-H21EÁ Á ÁO2A i/iv , C12A-H12AÁ Á ÁO1A i , C21A-H21DÁ Á ÁO4A iii , and C22A-H22DÁ Á ÁO4A iii hydrogen bonds to form sheets extending along the same direction. Thus, alternating sheets with an ABAB pattern can be observed. Two neighboring sheets are connected via C11A-H11AÁ Á ÁO5B and C11B-H11BÁ Á ÁO5A hydrogen bonds to form a three-dimensional network. The hydrogen-bond lengths and angles are summarized in Table 1.
In summary, a separation of dissimilar interactions can be observed between the non-covalent lipophilicstacking and C-HÁ Á Á interactions in the interior of the double-chains and the polar hydrogen bonds in the exterior of the doublechains, further stabilizing the crystal structure.

Synthesis and crystallization
Anhydrous NiCO 3 (0.67 mmol, 79.15 mg), 2,6-pyridine dicarboxylic acid (0.67 mmol, 111.41 mg), and pyrazine (1.00 mmol, 80.09 mg) were dissolved in 10 ml dimethyl sulfoxide. The resulting mixture was transferred into a stainless steel autoclave which was heated at 373 K for 24 h and cooled to room temperature at a cooling rate of 0.1 K per minute.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were positioned geometrically (C-H = 0.93/1.00 Å ) and allowed to ride with U iso (H)= 1.2/1.5U eq (C). Methyl H atoms were allowed to rotate around the corresponding C-C bond. There are two disordered parts, both of which are in molecule A. The carboxylate atom O2A sits just outside of the mirror plane (occupancy 0.5) and one of the DMSO methyl groups is disordered over two positions in a ratio of 0.54 (2):0.46 (2). The C atom of this group was refined with isotropic displacement parameters.

catena-Poly[[(dimethyl sulfoxide-κO)(pyridine-2,6-dicarboxylato-κ 3 O,N,O′)nickel(II)]-µ-pyrazine-κ 2 N:N′]
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.