Crystal structure of a novel one-dimensional zigzag chain-like cobalt(II) coordination polymer constructed from 4,4′-bipyridine and 2-hydroxybenzoate ligands

The synthesis, crystal structure and physical properties of a novel one-dimensional zigzag chain-like CoII coordination polymer, [Co2(2-OHbenz)4(4,4′-bpy)2.5(H2O)]n, constructed from 4,4′-bipyridine (4,4′-bpy) and 2-hydroxybenzoate (2-OHbenz) are reported.


Chemical context
The design and construction of new coordination polymers (CPs) is of current interest and attracts researchers in the fields of modern structural chemistry and materials science because of their potential applications in areas such as ionexchange, catalysis, sensors, magnetism, and non-linear optics (Dzhardimalieva & Uflyand, 2017;Loukopoulos & Kostakis, 2018;Horike et al., 2020). It is well known that the construction of CPs depends on a variety of factors such as the nature of metal ions and the organic ligands, the molar ratio of the reactants, and the reaction conditions e.g. reaction time, pH, solvents, and temperature (Kitagawa et al., 2004;Noro et al., 2009). The structure-property relationships of hybrid polymeric materials with 4,4 0 -bipyridine (4,4 0 -bipy) have been studied intensively (Biradha et al., 2006;Khrizanforova et al., 2020). This is because 4,4 0 -bpy is a rigid molecule that can link the metal centers to form a network with well-defined structures and also support the stability of the structures through ISSN 2056-9890 aromaticand C-HÁ Á Á interactions (Kaes et al., 2000). Furthermore, many researchers incorporate carboxylatebased ligands for the construction of CPs, giving rise to frameworks with a variety of dimensions and topologies (Gu et al., 2019;Horike et al., 2020). Benzoate and its derivatives have been widely used to construct the CPs because of the variety of their coordination modes, resulting in a variety of coordination geometries for the metal centers and interesting properties and applications of their CPs (Tong et al., 2000;Busskamp et al., 2007;Zhang et al., 2007;Song et al., 2009).
This work was undertaken as part of a search for new firstrow transition-metal coordination polymers constructed from 4,4 0 -bpy and carboxylate ligands. The Co II ion and hydroxybenzoate derivatives such as 2-hydroxybenzoate (2-OHbenz), 3-hydroxybenzoate (3-OHbenz) and 4-hydroxybenzoate (4-OHbenz) have been utilized for this. As a result, a Co II coordination polymer containing 4,4 0 -bpy and 2-OHbenz, [Co 2 (2-OHbenz) 4 (4,4 0 -bpy) 2.5 (H 2 O)] n , with a novel 1D alternating zigzag chain-like structure has been successfully synthesized and characterized and its crystal structure has been determined. Herein, we report the synthesis and crystal structure and physical properties of this compound.

Structural commentary
The asymmetric unit consists of two independent Co II atoms, two and a half of 4,4 0 -bpy ligands, four 2-OHbenz ligands and one water molecule (Fig. 1). Both Co II centers exhibit a distorted octahedral geometry with [CoN 2 O 4 ] and [CoN 2 O 3 O 0 ] chromophores for Co1 and Co2, respectively (Fig. 2). The Co1 ion is coordinated by two N atoms from two 4,4 0 -bpy ligands with different monodentate and bridging coordination modes in a trans-configuration, and four O atoms research communications Acta Cryst. (2020 The coordination environments of Co II centers in the title compound. The hydrogen atoms on the aromatic rings and phenol ring on 2-OHbenz ligands have been omitted for clarity. [Symmetry code: (i) Àx + 1, Ày, Àz + 1.] Table 1 Hydrogen-bond geometry (Å , ).

Supramolecular features
The extended structure of the title compound is consolidated by hydrogen bonds andstacking and C-HÁ Á Á interactions. The details of these weak interactions are summarized in Tables 1 and 2. The intermolecular interactions between the adjacent 1D zigzag chains are (i) C-HÁ Á ÁO hydrogen bonds between the benzene rings and hydroxyl groups of 2-OHbenz, (ii)stacking interactions between the bridging 4,4 0 -bpy and the terminal chelating 2-OHbenz and also between the phenyl rings of terminal chelating 2-OHbenz ligands, (iii) C-HÁ Á Á interactions between the C-H of the terminal mono- The intramolecular interactions in the title compound. The hydrogen atoms on aromatic rings have been omitted for clarity. Cg1, Cg2, Cg3 and Cg4 are the centroids of N1/C1-C5, N2/C6-C10, N3/C11-C15 and N4/ C16-C20 rings, respectively. [Symmetry code: (i) Àx + 1, Ày, Àz + 1.] Table 2 Analysis of short ring interactions (Å ).

Synthesis and crystallization
A solution of 4,4 0 -bpy (0.1562 g, 1.0 mmol) in MeOH (5 mL) was slowly added into a solution of Co(NO 3 ) 2 Á6H 2 O (0.2910 g, 1.0 mmol) in a 4:1 mixture of methanol and water (10 mL). The resulting solution was stirred for 20 min. Next, a solution of 2-OHbenzH (0.1382 g, 1.0 mmol) in methanol (5 mL) was slowly added dropwise and stirred over a period of 15 min. After that, the mixture was filtered. The filtered solution was left to stand without disturbance and allowed to slowly evaporate in the air. After five days, red crystals suitable for single crystal X-ray diffraction were obtained [56.18% yield based on cobalt(II) salt]. Elemental analysis; calculated for C 53 H 42 Co 2 N 5 O 13 : C 59.06, H 4.21, N 6.50%; found: C 59.14, H 3.99, N 6.41%. IR (KBr, /cm À1 ): 3087s, 1595s, 1485s, 1460s, 1459s, 1413m, 1389s, 1359s, 1308w, 1252m, 1218m, 1143w, 1068w, 1029w, 871w, 814s, 749s, 701w, 671w, 633w, 530w. The IR spectrum of the title compound (see Fig. S1 in the supporting information) shows a characteristic broad peak centered at 3087 cm À1 , which is assigned to OH stretching vibrations of the water molecule and the hydroxyl groups of 2-OHbenz. Strong and sharp peaks at 1595 and 1485 cm À1 can be assigned as the asymmetric and symmetric COO À stretching vibrations of the chelating 2-OHbenz ligands, respectively. Peaks in the region of 600-1000 cm À1 are assigned to CH bending of the aromatic rings in the ligands (Zhu et al., 2016).
The solid-state electronic spectrum of the title compound ( Fig. S2) shows two broad bands in the visible region with the main peak centered about 515 nm (19.42 kK), which can be assigned to the 3 : 4 T 1g ! 4 T 1g (P) transition. There is a small peak as a shoulder at around 655 nm (15.27 kK), assigned to the 2 : 4 T 1g ! 4 A 2g transition and a broad band centered about 1095 nm (13.24 kK), which can be assigned to the 1 : 4 T 1g ! 4 T 2g transition. The characteristic bands of this electronic spectrum correspond to a distorted octahedral geometry for Co II compounds as confirmed by the X-ray structure (Piromchom et al., 2014).
The PXRD pattern of the title compound ( Fig. S3) was used to check the phase purity of the bulk sample in the solid state. The measured PXRD pattern of the title compound closely matches the simulated pattern generated from the singlecrystal X-ray diffraction data, confirming the title compound is pure.
The TGA curve shown in (Fig. S4) demonstrates the thermal stability of the title compound up to 160 C. The first weight-loss step of 27.37% is observed from 160 to 277 C and can be attributed to the loss of coordinated water and two 2-OHbenz molecules. The next step weight-loss step of 25.7% observed from 277 to 356 C corresponds to the loss of a coordinated 2-OHbenz molecule. Finally, the weight loss of about 36.33% from 356 to 520 C can be assigned to the removal of two and half of the 4,4 0 -bpy ligands. The residual product is assumed to be CoO.
The solid-state photoluminescent properties of the title compound and free ligands were investigated at room temperature. As shown in Fig. S5, the emission spectra of the free ligands 4,4 0 -bpy and 2-OHbenzH ( ex = 340 nm) exhibit strong emission bands at 425 and 439 nm, respectively. However, no detectable emission can be observed for the title compound ( ex = 340 nm). This complete PL quenching is the result of the low energy d-d transitions in the partially filled metal ion centers found for Co II compounds described above and reported elsewhere (Yang et al., 2012;Zhu et al., 2014).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. C-bound H atoms were positioned geometrically and refined using a riding model, with C-H = 0.93 Å and the U iso (H) = 1.2U eq (C). O-bound H atoms were located in a difference electron-density map, and were refined with bond-length restraints of O-H = 0.84 (1)  program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

bipyridine-κ 2 N:N′-[aquahemi(µ-4,4′-bipyridine-κ 2 N:N′)(2-hydroxybenzoato-κO)(2-hydroxybenzoatoκ 2 O,O′)cobalt(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.