Poly[bis(μ2-N,N-dimethylformamide-κ2 O:O)bis(μ4-thiophene-2,5-dicarboxylato-κ4 O:O′:O′′:O′′′)dicobalt(II)]

The asymmetric unit of the title three-dimensional metal–organic hybrid compound comprises two cobalt(II) cations, one residing on a twofold axis and the other on a centre of inversion, one thiophene-2,5-dicarboxylate (tdc2−) ligand and one coordinated dimethylformamide (DMF) solvent molecule. A pair of carboxyl and one DMF connect the adjacent cobalt(II) cations into an infinite chain, leading to a rod-spacer framework with rhombus-window channels, yet no residual solvent-accessible voids are present because the coordinated DMF are oriented into the potential channels.

The asymmetric unit of the title three-dimensional metal-organic hybrid compound, [Co 2 (C 6 H 2 O 4 S) 2 (C 3 H 7 NO) 2 ] n , comprises two cobalt(II) cations, one residing on a twofold axis and the other on a centre of inversion, one thiophene-2,5-dicarboxylate (tdc 2À ) ligand and one coordinating dimethylformamide (DMF) solvent molecule. Both of the cobalt(II) cations exhibit an octahedral coordination environment from the four carboxyl O atoms of the tdc 2À anions in a 4 -1 : 1 : 1 : 1 fashion and two O atoms from DMF. A pair of carboxyl O atoms and one DMF molecule connect the adjacent cobalt(II) cations into an infinite chain, leading to a rod-spacer framework with rhombus-window channels, yet no residual solvent-accessible voids are present because the coordinating DMF molecules are oriented into the potential channels.

Structure description
Facing the timetable for a carbon-neutral future, electrochemical redox reactions are the cornerstones of large-scale storage and chemical conversion of renewable clean energy in the future, in which electrocatalytic water splitting plays a central role (Seh et al., 2017;Cheng et al., 2022). Metal-organic frameworks (MOFs), a class of crystalline and highly porous frameworks usually constructed from 3d metal ions and organic ligands (Yin et al., 2015), provide great opportunities for the preparation of new electrocatalysts for water splitting. Benefitting from outstanding designability and regulation for the composition and structure of MOFs, 3d-metal-based electrocatalysts with excellent electrocatalyst performance can be obtained from both highly stable MOFs and nanocomposites derived from the thermal or chemical reaction of the MOF precursor (Zhu et al., 2018). In a previous study, we discovered alkali-induced in situ formation of amorphous Ni x Fe 1-data reports x (OH) 2 from a linear [M 3 (COO) 6 ]-based MOF template for overall electrochemical water splitting (Yin et al., 2015).
In parallel work, thiophene-2,5-dicarboxylic acid (H 2 tdc) and the cobalt ion were chosen to construct MOFs for potential electrochemical applications. The H 2 tdc ligand is a typical di-topic linker comparable to terephthalic acid that has strong coordination ability. In fact, there are 366 polymeric structure records from a total of 409 compounds constructed from H 2 tdc, based on a Cambridge Structural Database analysis (CSD version 5.4.1;December 2021;Groom et al., 2016), suggesting its suitability for MOF assembly. In addition, there are carbon and sulfur elements stemming from the thiazole ring backbone, facilitating the generation of sulfurcontaining nanocomposites for electro-catalysis. On this occasion, the title compound was obtained during the synthetic exploration of new three-dimensional rod-spacer MOFs of [Co 2 (tdc) 2 (DMF) 2 ] n in a solvothermal reaction.
There have been reports about the isostructural Mn II compound, yet no other metal-based MOF has been described (Tan et al., 2013).
The title compound ( Fig. 1) crystallizes in the monoclinic space group C2/c. The asymmetric unit comprises two cobalt(II) cations (one resides on a twofold axis and the second on an inversion centre), one full tdc 2À ligand, and one coordinating DMF molecule Each of the cobalt(II) cations exhibits a octahedral coordination geometry by the four carboxyl O atoms from the tdc 2À anions in a 4 -1 : 1 : 1 : 1 fashion and two O atoms from DMF. The calculated continuous shape measures (CShM) value for Co1 and Co2 are 0.338 and 0.240, respectively, indicating only quite a small coordination distortion from a regular octahedron. A pair of carboxyl and one DMF link adjacent cobalt(II) cations into infinite chains via C-HÁ Á ÁO hydrogen bonds (Table 1, Fig. 2). In particular, the DMF ligand adopts a 2 -bridging mode to link adjacent metal ions. Compared to its usual role as a terminally bound ligand, such coordination behaviour is rare but has been observed in some known MOFs (Fritzsche et al., 2019). As a result, a rod-spacer framework with rhombuswindow channels is formed through the tdc 2À linkage of neighbouring chains. However, no solvent-accessible voids were noted because the coordinating DMF molecule is oriented into the channels and fully occupies any potential void space. The compound is thermally stable up to 260 C under an N 2 atmosphere by thermogravimetric analysis. Thermogravimetric analysis: the mass of the compound remains stable until 250 C, followed by an obvious mass loss of 23.7% corresponding to the loss of coordinating DMF (calculated 26.8%) in the range of 250-310 C, and then thermal decomposition of the framework with residuals of 34.3% from 400-800 C, much higher than the theoretical data for decomposition products of Co 3 O 4 (calculated 27.7%) or CoO (calculated 26.0%), suggesting the formation of carbonand sulfur-rich nanocomposites.

Synthesis and crystallization
A solution of H 2 tdc (0.2 mmol, 34.4 mg) and CoCl 2 Á6H 2 O (0.2 mmol, 47.6 mg) in DMF (dimethyl formamide, 15 ml) was stirred in air with a magnetic stirrer, generating a purple transparent solution after stirring for 5 min. The reaction solution was transferred to a hydrothermal reaction vessel containing 25 ml of a polytetrafluoroethylene liner, followed by heating at 140 C for 48 h. The reaction vessel was cooled to room temperature at a rate of 10 C per hour. The precipitate was washed and filtered to obtain a large amount of light- Table 1 Hydrogen-bond geometry (Å , ).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Atoms C3 and C4 of the thiazole ring and the C atoms of the coordinating DMF are disordered over two sets of sites with occupancy ratios of 0.550 (17):0.450 (17) nd 0.855 (5):0.145 (5), respectively. Disorder treatment and restraints for the displacement parameters of the thiazole ring and coordinated DMF were applied. Disorder was treated as follows: two adjacent carbon atoms C3, C4 in the thiazole ring were split into two parts, and the C7, C8, C9 atoms in the DMF were split into two positions also, followed by SIMU restraints for these atoms and subsequent refinements, resulting in lower, acceptable Rfactors and refinement.

data-2
IUCrData (2022). 7, x220775 Refinement. The single-crystal diffraction data were collected on a on a Bruker APEX-II CCD diffractometer (Mo-Kα, λ = 0.71073?Å), with the APEX-II software for data reduction and analysis (Bruker 2016). The dataset of a selected singlecrystal of (I) were collected at 298?K. The structure was solved by direct methods and refined by full-matrix leastsquares method on F2 using SHELX algorithms in Olex2 (Sheldrick 2008;Dolomanov et al., 2009). All non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were generated geometrically.

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