Synthesis and crystal structure of poly[[bis(aqua-κO)tetrakis(μ-4,4′-bipyridine-κ2 N:N′)hexakis(3-chlorobenzoato)-κ5 O;κ2 O:O′-tricobalt(II)] methanol disolvate]

The synthesis, crystal structure and properties of a novel ladder-chain CoII coordination polymer constructed from 4,4′-bipyridine (4,4′-bipy) and m-chlorobenzoate (3-Clbenz), {[Co3(4,4′-bipy)4(3-Clbenz)6(H2O)2]·2CH3OH} n , are reported.


Chemical context
The exploration and synthesis of new one-dimensional coordination polymers based on transition metals and mixed Nand O-donating ligands such as 4,4 0 -bipyridine (4,4 0 -bipy) and benzoate derivatives have been intensively developed (Kaes et al., 2000;Saelim et al., 2020;Topor et al., 2021). The substituent groups at the benzoate ligands play an important role not only for electron densities on the aromatic ring, but also for flexible supramolecular interactions, resulting in various bulk physical properties, such as CO 2 adsorption (Takahashi et al., 2014(Takahashi et al., , 2015, photoluminescence (Lin, 2015) and conductivity (Islam et al., 2019). Among the reported compounds, the majority contain mixed 4,4 0 -bipy and para-substituted benzoate derivatives, but there is a limited number of examples containing meta-substituent benzoate ligands (Fang & Nie, 2011;Kar et al., 2011;Xin-Jian et al., 2013;Lin, 2015). We have therefore tried to expand investigations in this area by using various meta-substituted benzoate ligands containing hydroxy, nitro and halogen substituents. During this study, we employed 3-chlorobenzoate (3-Clbenz), which is expected to support crystal structures viaand halogenÁ Á Á intermolecular interactions, together with the 4,4 0 -bipy organic linker and have synthesized the new Co II coordination polymer {[Co 3 (4,4 0 -bipy) 4 (3-Clbenz) 6 (H 2 O) 2 ]Á2CH 3 OH} n , which has an interesting one-dimensional ladder-chain structure. This report describes the synthesis, crystal structure, spectroscopic and thermal properties of the title compound.

Structural commentary
The asymmetric unit of the title compound comprises two Co 2+ ions, three 3-Clbenz anions, two 4,4 0 -bipy molecules, one coordinated water molecule and one methanol solvate molecule as shown in Fig. 1. One of the Co 2+ ions (containing Co2), is situated at an inversion centre. One pyridine ring (C1-C5) and the methanol solvate molecule are disordered over two sets of sites with occupancies of 0.584 (19):0.416 (19) and 0.72 (3):0.28 (3), respectively. Both Co 2+ ions are six-coordinated and have octahedral environments. The Co1 ion is coordinated by three nitrogen atoms from three 4,4 0 -bipy bridging ligands and three oxygen atoms from the carboxylate groups of one monodentate and one bidentate 3-Clbenz ligands, providing a distorted octahedral geometry with angles O2-Co1-O1, O2-Co1-O3 and O2-Co1-N1 of 59.88 (6), 119.93 (7) and 148.87 (7) , respectively. The Co2 ion is coordinated by two nitrogen atoms from two 4,4 0 -bipy bridging ligands and four oxygen atoms from two monodentate 3-Clbenz ligands and two coordinated water molecules. The angles in its environment deviate from ideal values no more than by 3.5 . There is an intramolecular hydrogen bond in the coordination environment of Co2 between the aqua and 3-Clbenz ligands (see Table 1). The Co1 ions are connected by the 4,4 0 -bipy linkers into linear chains along the a-axis direction, and adjacent chains are linked via the Co2 ions by the 4,4 0 -bipy ligands, thus forming the ladder-chain structure shown in Fig. 2.

Supramolecular features
The crystal packing is stabilized by intermolecular interactions such as hydrogen bonds (classical O-HÁ Á ÁO and non-classical C-HÁ Á ÁO and C-HÁ Á ÁN), aromaticand ClÁ Á Á interactions (see Table 1 ,Figs. 3 and 4). The solvate methanol molecule forms hydrogen bonds to the non-coordinated O4 atom of the 3-Clbenz ligand at Co1 as an H-atom donor and to the coordinated water molecule at Co2 as an H-atom acceptor (see Figs. S1-S3 in the supporting information). Aromaticinteractions involving the phenyl rings of two 3-Clbenz ligands have an intercentroid Cg6Á Á ÁCg7 (1 + x, À1 + y, z) separation of 3.917 (2) Å ( Fig. 3) (Cg6 and Cg7 are the centroids of the C22-C27 and C29-C34 rings, respectively). There are also halogenÁ Á Á interactions between the 3-Clbenz ligands and  View of the ladder-chain structure along the a-axis direction. The hydrogen atoms located at carbon atoms and methanol solvate molecules are omitted for clarity.

Spectroscopic characterization
The FT-IR spectrum of the title compound (Fig. S4) has a characteristic broad peak centred at 3330 cm À1 assigned to the O-H stretching vibrations of coordinated water molecules and the methanol solvate. The strong and sharp peaks at about 1608 and 1382 cm À1 are attributed to the asymmetric and symmetric COO À stretching vibration of the monodentate 3-Clbenz ligands, and the peaks appearing at about 1557 and 1488 cm À1 are attributed to the asymmetric and symmetric COO À stretching vibration of the chelating 3-Clbenz ligand (Xin-Jian et al., 2013). The strong superimposed bands appearing at 1557 and 1488 cm À1 could be assigned to the C C/C N stretching vibration of the aromatic rings of the 3-Clbenz and 4,4 0 -bipy ligands. The medium-strong peaks in the region of 760 and 731 cm À1 are assigned to C-Cl vibration and C-H bending vibration of the 3-Clbenz ligands. In addition, the medium-strong peak at 1219 cm À1 is assigned to the weak C-N stretching vibration (Xin-Jian et al., 2013) and the bands between 1016 and 1145 cm À1 are assignable to the pyridine ring-breathing modes (Dey et al., 2011) of the 4,4 0bipy ligands. The characteristic C-H out-of-plane and inplane deformation bands for pyridine rings are observed at 808 and 631 cm À1 , and are shifted to a higher frequency as compared to the values observed for the free ligand (805 and 607 cm À1 ), suggesting coordinated 4,4 0 -bipy ligands (Seidel et al., 2011). The solid-state electronic spectrum ( Fig. S5) of the title compound shows d-d transitions with two broad bands at 489 and 1099 nm, assigned to the 3 : 4 T 1g ! 4 T 1g (P) and 1 : 4 T 1g ! 4 T 2g transitions, respectively (Fu et al., 2007;Piromchom et al., 2014). The results correspond to the typical d-d transitions for Co II in a distorted octahedral geometry, as confirmed by the X-ray structure.

PXRD and thermal analysis
The PXRD patterns ( Fig. S6) of the title compound used to check the phase purity show good accordance with its simulated PXRD pattern generated from the single-crystal X-ray diffraction data, confirming that the title compound has high phase purity. The TGA curve (Fig. S7) shows the thermal stability of the title compound below 325 C. The first complex step with a weight loss of 29.57% (calculated 30.88%) was found in the temperature range from 100 to 325 C, which was attributed to the loss of methanol molecule of crystallization, two coordinated water and three 3-Clbenz molecules. Then, the structure starts to collapse with a weight loss of 49.24% (calculated 49.44%) in the temperature range from 325-685 C that can be attributed to the removal of three remaining 3-Clbenz and three remaining 4,4 0 -bipy molecules. After that, the residual product is assumed to be CoO.

Figure 4
Packing

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (