Crystal structure of a looped-chain CoII coordination polymer: catena-poly[[bis(nitrato-κO)cobalt(II)]bis[μ-bis(pyridin-3-ylmethyl)sulfane-κ2 N:N′]]

The reaction of cobalt(II) nitrate with bis(pyridin-3-ylmethyl)sulfane ligand, afforded a one-dimensional looped polymeric chain. The CoII cation displays a distorted octahedral geometry coordinated by four pyridine N atoms from four individual ligands and two O atoms from two monodentate nitrate anions. Two symmetry-related ligands are connected by two symmetry-related CoII cations, forming a 20-membered cyclic dimer. These cyclic dimers are connected to each other by sharing CoII atoms, forming a looped chain. In the crystal, adjacent looped chains are connected by intermolecular π–π stacking interactions and C—H⋯π and C—H⋯O hydrogen bonds, resulting in the formation of a three-dimensional supramolecular architecture.

The asymmetric unit of the title compound, [Co(NO 3 ) 2 (C 12 H 12 N 2 S) 2 ] n , contains a bis(pyridin-3-ylmethyl)sulfane (L) ligand, an NO 3 À anion and half a Co II cation, which lies on an inversion centre. The Co II cation is six-coordinated, being bound to four pyridine N atoms from four symmetry-related L ligands. The remaining coordination sites are occupied by two O atoms from two symmetry-related nitrate anions in a monodentate manner. Thus, the Co II centre adopts a distorted octahedral geometry. Two symmetry-related L ligands are connected by two symmetry-related Co II cations, forming a 20-membered cyclic dimer, in which the Co II atoms are separated by 10.2922 (7) Å . The cyclic dimers are connected to each other by sharing Co II atoms, giving rise to the formation of an infinite looped chain propagating along the [101] direction. Intermolecular C-HÁ Á Á (HÁ Á Áring centroid = 2.89 Å ) interactions between one pair of corresponding L ligands and C-HÁ Á ÁO hydrogen bonds between the L ligands and the nitrate anions occur in the looped chain. In the crystal, adjacent looped chains are connected by intermolecularstacking interactions [centroid-tocentroid distance = 3.8859 (14) Å ] and C-HÁ Á Á hydrogen bonds (HÁ Á Áring centroid = 2.65 Å ), leading to the formation of layers parallel to (101). These layers are further connected through C-HÁ Á ÁO hydrogen bonds between the layers, resulting in the formation of a three-dimensional supramolecular architecture.
Over the last two decades, numerous one-dimensional coordination polymers have been developed, not only because of their fascinating architectures but also their potential applications as functional materials (Furukawa et al., 2014;Silva et al., 2015). In this area of research, dipyridyl-type molecules as organic building blocks have been widely used to construct diverse one-dimensional self-assembled coordination polymers with intriguing structural topologies (Leong & Vittal, 2011;Wang et al., 2012). Our group has also developed several one-dimensional coordination polymers with fascinating topologies such as zigzag (Lee et al., 2013;Moon et al., 2016), helical , double helical (Lee et al., 2015), looped chain (Ju et al., 2014) and ribbon-type double-stranded (Moon et al., 2017;Park et al., 2010) structures using dipyridyl-type ligands. In an extension of our research, the title compound was prepared by the reaction of cobalt(II) nitrate with bis(pyridin-3-ylmethyl)sulfane (L) as a ISSN 2056-9890 flexible dipyridyl-type ligand, synthesized using a literature procedure (Park et al., 2010;Lee et al., 2012). Herein, we report the crystal structure of the title compound, which adopts a one-dimensional looped-chain structure.

Structural commentary
As illustrated in Fig. 1, the asymmetric unit of the title compound consists of one Co II cation located on an inversion centre, one (pyridin-3-ylmethyl)sulfane ligand, L, and one NO 3 À anion. The Co II cation is coordinated by four pyridine N atoms from four symmetry-related L ligands. In addition, the Co II cation binds to two O atoms of two symmetry-related monodentate nitrate anions, forming a distorted octahedral CoN 4 O 2 coordination. Selected bond lengths and angles around the Co1 atom are listed in Table 1. The N1-and N2pyridine rings coordinated to the Co II centre are tilted by 70.75 (7) with respect to each other ( Fig. 1).
Two symmetry-related L ligands bridge two Co II atoms, resulting in the formation of a 20-membered cyclic dimer with a CoÁ Á ÁCo separation of 10.2922 (7) Å . The cyclic dimers are connected by sharing Co II atoms, leading to the formation of an infinite looped chain propagating along the [101] direction. An intermolecular C7-H7BÁ Á Á Cg2 i interactions [HÁ Á Á = 2.89 Å ; Table 2; yellow dashed lines in Fig. 1; Cg2 is the centroid of atoms N2/C8-C12; symmetry code: (i) Àx, Ày + 1, Àz] between one pair of corresponding L ligands and several C-HÁ Á ÁO hydrogen bonds between the L ligands and the NO 3 À anions (Table 2; black dashed lines in Fig. 1) contribute to the stabilization of the looped chain.

Synthesis and crystallization
The L ligand was synthesized according to a literature method (Park et al., 2010;Lee et al., 2012). Crystals of the title compound were grown by slow evaporation of a methanol/ H 2 O (2:1) solution of the L ligand with Co(NO 3 ) 2 Á6H 2 O in a 2:1 molar ratio.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The supramolecular layer formed by intermolecularstacking interactions (yellow dashed lines) and C-HÁ Á Á hydrogen bonds (black dashed lines) between the looped chains. H atoms not involved in intermolecular interactions have been omitted for clarity.

catena-Poly[[bis(nitrato-κO)cobalt(II)]bis[µ-bis(pyridin-3-ylmethyl)sulfane-κ 2 N:N′]]
Crystal data [Co(NO 3 ) 2 (C 12 H 12 N 2 S) 2 ] M r = 615.54 Triclinic, P1 a = 8.1620 (7)  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.