Synthesis and crystal structure of a two-dimensional CoII coordination polymer: poly[(μ3-3-carboxybenzoato)[μ2-5-(pyridin-4-yl)-1H,2′H-3,3′-bi[1,2,4-triazole]]cobalt(II)]

The divalent CoII atom is six-coordinated by three N atoms from two symmetrical 5-(pyridin-4-yl)-1H,2′H-3,3′-bi[1,2,4-triazole] (H2pyttz) ligands and three O atoms from three symmetrical 3-carboxybenzoate (Hbdic) ligands, leading to a distorted {CoN3O3} octahedral coordination environment. Two CoII cations are linked by four bridging carboxylate groups to generate a dinuclear [Co2(CO2)4] unit.


Chemical context
In recent years, the design and synthesis of coordination polymers (CPs) or metal-organic frameworks (MOFs) have attracted great interest because of their fascinating architectures and potential applications in areas such as gas storage and separation, catalysis, fluorescence, magnetism, molecular recognition, conductivity etc (Kitagawa et al., 2004;Zhou et al., 2012;Cavka et al., 2014;Zhang et al., 2014;Huang et al., 2017;Nath et al., 2016;Ni et al., 2017;Yi et al., 2016;Sun et al., 2016). It is well known that organic ligands play a crucial role in the rational design and synthesis of coordination polymers (Li & Sato, 2017;Sun & Sun, 2015). Among the many organonitrogen ligands, the rigid 5-(pyridin-4-yl)-1H,2 0 H-3,3 0bi(1,2,4-triazole) ligand (H 2 pyttz) attracted our attention for the following reasons. First, the H 2 pyttz ligand possesses seven potential N-donor coordination sites and can exhibit various coordination modes. Second, the uncoordinated N atoms are helpful for the construction of hydrogen bonds. The hydrogen bonds not only increase the diversity of coordination polymer structures, but also enhance their stability. With an increasing interest in H 2 pyttz organometallic systems, we report herein on the synthesis and crystal structure of the title compound [Co(C 8 H 5 O 4 )(C 9 H 6 N 7 )] n , (I).

Structural commentary
The asymmetric unit of (I) contains one independent Co II cation, one partially deprotonated Hpyttz À ligand and one partial deprotonated Hbtc À ligand. Notably, the deprotonated Hbtc À ligand adopts two different coordination modes. The deprotonated carboxylate group has a bis(monodentate) coordination mode to bridge two Co II centers while the undeprotonated carboxylic group adopts a monodentate mode. As shown in Fig. 1, the Co II cation is six-coordinated to three carboxylic oxygen atoms from three symmetrical Hbtc À ligands and three nitrogen atoms from two symmetrical Hpyttz À ligands in a distorted [CoN 3 O 3 ] octahedral coordination geometry. Four bridging carboxylate groups link two Co II cations to generate a dinuclear [Co 2 (CO 2 ) 4 ] unit, which is further connected into an infinite chain along the b-axis direction. There exist eight-and 16-membered metallamacrocycles in the chain structure. In the 16-membered metallamacrocycle, the dihedral angle between the two aromatic rings is 0 , indicating the parallel orientation of the two aromatic rings.
The other infinite linear chain is along the a-axis direction with a CoÁ Á ÁCo distance of 6.5825 (5) Å and Co-Co-Co angle of 180.00 and it is also generated through the coordination between the Hpyttz À ligands and the Co II cations. In the complex, the Hpyttz À ligand is almost coplanar, with dihedral angles of 7.48 (4), 6.87 (4) and 4.43 (4) between the pyridine and the two triazole rings, respectively. Finally, these two kinds of chains are cross-linked, by sharing the Co II cations, into a two-dimensional network.

Supramolecular features
In the crystal, adjacent two-dimensional networks are packed parallel to each other in an Á Á ÁAAAAÁ Á Á fashion (Fig. 2). It should be noted that the carboxylic oxygen atom O3 and the uncoordinated nitrogen atom N7 in adjacent networks interact with each other and form strong O3-H3Á Á ÁN7 hydrogen bonds (Table 1), which further link the two-dimensional networks into a three-dimensional supramolecular architecture.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms attached to C and N atoms were placed in calculated positions (C-H = 0.93 Å , N-H = 0.86 Å ) and refined as riding atoms with U iso (H) = 1.2U eq (C,N), respectively. The carboxyl H atom was located in the difference Fourier-map and refined isotropically with U iso (H) = 1.5U eq (O).

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 )
x y z U iso */U eq