Synthesis and crystal structure of a 6-chloronicotinate salt of a one-dimensional cationic nickel(II) coordination polymer with 4,4′-bipyridine

The title compound is a 6-chloronicotinate salt of a one-dimensional cationic nickel(II) coordination polymer with 4,4′-bipyridine. The nickel(II) ion in the polymeric cation is octahedrally coordinated by four water molecule O atoms and by two 4,4′-bipyridine N atoms. The 4,4′-bipyridine ligands act as bridges, connecting the symmetry-related nickel(II) ions into polymeric chains along the b-axis direction. In the extended structure, these chains, the anions and the water molecules of crystallization are assembled into a three-dimensional network via strong O—H⋯O and O—H⋯N hydrogen bonds


Chemical context
Functional coordination polymers have attracted great interest in recent years, mostly due to their aesthetics and many interesting properties such as catalytic, magnetic and luminescent, potential for use in gas storage and separation, molecular sensing (Mueller et al., 2006;Bosch et al., 2017;Zhang et al., 2015;Zeng et al., 2014Zeng et al., , 2016Douvali et al., 2015;Xu et al., 2017;Zhou et al., 2017). The organic ligands, used as building blocks in the construction of coordination polymers, need to be multifunctional, which is evident from the position, coordination ability and steric hindrance of their donor atoms and/or groups. The design of functional coordination polymers with the desired structures is not always straightforward and is strongly dependent on the experimental conditions including the type of solvents, starting metal salts, additional ligands, temperature, hydrothermal conditions and pH value (Li et al., 2016;Zhou et al., 2016;Gu et al., 2016). Aromatic carboxylic acids with additional functional groups have become popular in the design of coordination polymers. The main reasons are the many possible and unpredictable coordination modes of this type of ligand and their affinity for participation in supramolecular interactions (Gu et al., 2016(Gu et al., , 2018Wang et al., 2016;Zhang et al., 2019).
The metal complexes of chlorinated analogues of the nicotinate anion (e.g. 2-chloronicotinate and 5-chloronicotinate) have not been particularly well-studied [as of March 2020, there are around 20 crystal structures in the CSD (Groom et al., 2016) for each ligand]. Furthermore, no metal complexes of the 4-chloronicotinate anion have been reported. The crystal structures of only three metal complexes of 6-chloronicotinate (6-Clnic) are known so far (Xia et al., 2012a,b;Li et al., 2006). Recently, we have reported the synthesis, crystal structure and properties of a one-dimensional nickel(II) coordination polymer with mixed ligands: 6-fluoronicotinate as the main ligand and 4,4 0 -bipyridine (4,4 0bpy) as the supporting ligand (Politeo et al., 2020).
In a continuation of our work on coordination polymers with mixed ligands, we set out to prepare a similar coordination polymer with 6-chlorinicotinate and 4,4 0 -bipyridine, as we did with 6-fluoronicotinate (Politeo et al., 2020). Therefore, we carried out the synthesis and crystallization under the same experimental conditions (in a mixture of water and ethanol and with the same molar ratios of the nickel(II) sulfate and ligands), in hope that the analogous nickel(II) coordination polymer could be obtained. We also wanted to examine the influence of the possible weak intermolecular interactions involving the chlorine atoms (e.g. C-HÁ Á ÁCl interactions) on the assembly of the polymeric chains in the crystal packing, especially since the analogous C-HÁ Á ÁF interactions were not found in the crystal packing of the nickel(II) coordination polymer with 6-fluoronicotinate (Politeo et al., 2020). Unfortunately, we were not able to prepare the desired nickel(II) coordination polymer under these experimental conditions, but instead we obtained a 6-chloronicotinate salt of a onedimensional cationic nickel(II) coordination polymer with 4,4 0 -bipyridine, namely the title compound, {[Ni(4,4 0bpy)(H 2 O) 4 ](6-Clnic) 2 Á4H 2 O} n , (1).
between two water molecules of crystallization and two 6chloronicotinate anions (indicated in blue and green); each 6chloronicotinate anion is linked via a single carboxylate O atom. The tetrameric R 4 4 (10) motif is formed between the [Ni(4,4 0 -bpy)(H 2 O) 4 ] 2+ } n cation, a 6-chloronicotinate anion (indicated in red and green, respectively) and two water molecules of crystallization; the cation participates in this motif via a coordinated water O atom and the 6-chloronicotinate anion via both carboxylate O atoms. The dimeric R 2 2 (8) motif is formed between the {[Ni(4,4 0 -bpy)(H 2 O) 4 ] 2+ } n cation and the 6-chloronicotinate anion (indicated in red and brown, respectively); the cation is involved in this motif via two coordinated water O atoms and the 6-chloronicotinate anion via both carboxylate O atoms. Finally, the pentameric R 4 5 (16) motif is composed of the {[Ni(4,4 0 -bpy)(H 2 O) 4 ] 2+ } n cation, two 6-chloronicotinate anions (indicated in red, green and pink) and two water molecules of crystallization; the cation participates in this motif via two coordinated water O atoms, one 6-chloronicotinate anion (shown in green) via both carboxylate O atoms and the pyridine N atom and the other 6chloronicotinate anion (shown in pink) via its carboxylate O atom only (Fig. 4). Both coordinated water molecules and water molecules of crystallization participate in the formation of motifs as single-and double-proton donors [coordinated water molecules as single-proton donors in the R 4 5 (16) and R 2 2 (8) motifs and double-proton donors in the R 4 4 (10) motif only; water molecules of crystallization as single-proton donors in the R 4 5 (16) motifs and R 4 4 (10) motifs and doubleproton donors in the R 4 5 (16) and R 2 4 (8) motifs]. The water molecules of crystallization also participate in some of these motifs [R 4 5 (16) and R 4 4 (10)] as single-proton acceptors. The 6chloronicotinate pyridine N atoms act as single-proton acceptors in the R 4 5 (16) motif only, whilst the carboxylate O atoms act as both single-and double-proton acceptors [single in the R 4 5 (16), R 2 2 (8) and R 4 4 (10) motifs and double in the R 4 5 (16) and R 2 4 (8) motifs]. Two weak C-HÁ Á ÁO interactions are also observed (Table 1).
There are no weak C-HÁ Á ÁCl interactions in the extended structure of 1; we hoped that these interactions could have an impact on the assembly of the polymeric chains within the hydrogen-bonding framework of 1: the polymeric chains do not contain the 6-chloronicotinate ligands, but the uncoordinated 6-chloronicotinate anions could still participate in these interactions. However, the possible C-HÁ Á ÁCl interactions are most probably hindered by the extensive hydrogen bonding, involving strong O-HÁ Á ÁO and O-HÁ Á ÁN hydrogen bonds, which is reflected in the formation of various hydrogenbonded motifs. This was expected because of the participation of the water molecules of crystallization in the crystal packing of 1, since the compound was crystallized from a mixed waterethanol solution.

PXRD and thermal analysis
The experimental and calculated PXRD traces of 1 (Fig. 5) match nicely, indicating the phase purity of the bulk of 1.
Compound 1 is thermally stable only up to 40 C (Fig. S1 in the supporting information). Both the coordinated (four) and uncoordinated (four) water molecules were released in the same step (observed mass loss 20.3%, calculated 21.4%), with a pronounced endothermic peak in the DSC curve at 90 C. The thermal decomposition of 1 continues in a broad step (observed mass loss 55.2%) in the wide temperature range of 145-590 C (with two small peaks in the DSC curve at 216 and 480 C), which probably corresponds to the complete degradation of 1. The remaining residue at 600 C is most probably NiO.

Materials and methods
All chemicals for the synthesis were purchased from commercial sources (Merck) and used as received without further purification. The IR spectrum was obtained in the range 4000-400 cm À1 on a Perkin-Elmer Spectrum Two TM FTIR spectrometer in the ATR mode. The PXRD trace was recorded on a Philips PW 1850 diffractometer, Cu K radiation, voltage 40 kV, current 40 mA, in the angle range 5-50 (2) with a step size of 0.02 . Simultaneous TGA/DSC measurements were performed at a heating rate of 10 C min À1 in the temperature range 25-600 C, under a nitrogen flow of 50 ml min À1 on a Mettler-Toledo TGA/DSC 3+ instrument. Approximately 2 mg of sample was placed in a standard alumina crucible (70 ml).

Synthesis and crystallization
6-Chloronicotinic acid (0.0525 g, 0.3332 mmol) was dissolved in distilled water (5 ml) using an ultrasonic water bath, 4,4 0bipyridine (0.0244 g, 0.1562 mmol) was dissolved in ethanol (2 ml) and nickel(II) sulfate heptahydrate (0.0446 g, 0.1588 mmol) was dissolved in distilled water (2 ml). The solutions of the two ligands were first mixed together under stirring. The resulting solution was then slowly added to the nickel(II) sulfate solution under stirring. The pH of the final solution was adjusted to 7 by adding an ammonia solution dropwise. The obtained, clear solution was left to slowly evaporate at room temperature for approximately three weeks until light-green crystals of 1, suitable for X-ray diffraction measurements, were obtained, which were collected by filtration, washed with their mother liquor and dried in vacuo. Yield: 0.0496 g (46%  1579, 1539, 1419, 1388, 1360 [(C-C), (C-N)] (Fig. S2, Table S1 in the supporting information).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. C-bound H atoms were positioned geometrically and refined using riding model [C-H = 0.93 Å , U iso (H) = 1.2U eq (C) for the aromatic H atoms]. The H atoms belonging to the water molecules were found in the difference-Fourier maps. The O-H distance was restrained to an average value of 0.82 Å using DFIX and DANG instructions. The isotropic U iso (H) values were also fixed [U iso (H) = 1.2U eq (O)].
The highest difference peak is 0.86 Å away from the O4 atom and the deepest difference hole is 0.84 Å away from the Cl1 atom.

Funding information
This research was supported by a Grant from the Foundation of the Croatian Academy of Sciences and Arts for 2019 and by University of Split institutional funding.  CrysAlis PRO (Rigaku OD, 2018); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: Mercury (Macrae et al., 2020); software used to prepare material for publication: SHELXL2018/3 (Sheldrick, 2015b).

catena-Poly[[[tetraaquanickel(II)]-µ-4,4′-bipyridine-κ 2 N:N′] bis(6-chloronicotinate) tetrahydrate]
Crystal data [Ni(C 10  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.