The first coordination compound of 6-fluoronicotinate: the crystal structure of a one-dimensional nickel(II) coordination polymer containing the mixed ligands 6-fluoronicotinate and 4,4′-bipyridine

A one-dimensional nickel(II) coordination polymer with the mixed ligands 6-fluoronicotinate (6-Fnic) and 4,4′-bipyridine (4,4′-bpy), namely, [Ni(H2O)2(6-Fnic)2(4,4′-bpy)·3H2O]n, (1), was prepared. The nickel(II) ion in 1 is octahedrally coordinated by the O atoms of two water molecules, two O atoms from O-monodentate 6-fluoronicotinate ligands and two N atoms from bridging 4,4′-bipyridine ligands.


Chemical context
The design of coordination polymers relies on the concepts of crystal engineering (Desiraju, 2007(Desiraju, , 2013 and has become a prominent field of research in recent years for many reasons including the functional properties shown by coordination polymers, their aesthetics and many possible applications such as catalysis, gas storage and separation, magnetism, luminescence and 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 multifunctionality of the organic ligands used as building blocks in the assembly of coordination polymers is reflected in the position and coordination ability of their donor atoms and/or groups and is the main factor in the design of unusual and unexpected architectures with novel topologies and properties. The main challenge is to control the formation of a coordination polymer with the desired molecular and crystal structure, which is particularly affected by the experimental conditions such as the choice of solvents, starting metal salts, additional ligands, temperature, hydrothermal conditions, pH value (Li et al., 2016;Zhou et al., 2016;Gu et al., 2016). ISSN 2056-9890 Various aromatic carboxylic acids with additional functional groups have often been used in the construction of coordination polymers because of the variety of their coordination modes (often unpredictable) and their potential for forming supramolecular interactions (Gu et al., 2016(Gu et al., , 2018Wang et al., 2016;Zhang et al., 2019). Fluorine-substituted aromatic carboxylic acids are good candidates for the design of functional coordination polymers showing higher thermal stability as well as stability towards oxidation (Peikert et al., 2015;Yuan et al., 2016).
Although metal complexes with nicotinate have been wellstudied and documented [almost 900 crystal structures in the Cambridge Structural Database (CSD, Version 5.40, searched January 2020; Groom et al., 2016)], metal complexes of its fluorinated analogues (e.g. 5-fluoronicotinate) have been much less studied (around 30 crystal structures in the CSD). On the other hand, no metal complexes of other fluorinated analogues of nicotinate (e.g. 2-fluoronicotinate, 4-fluoronicotinate, 6-fluoronicotinate) have been reported so far.
Our goal was to prepare nickel(II) coordination polymers as the nickel(II) ion is relatively abundant, with a large ionic radius and defined stereochemistry, showing a high ligandfield stabilization energy, which enables the formation of nickel(II) coordination polymers with diverse topologies and high stabilities (Liu et al., 2019). We opted for nickel(II) coordination polymers with mixed ligands: 6-fluoronicotinate (6-Fnic) as the main ligand and 4,4 0 -bipyridine (4,4 0 -bpy), a well-established, bridging N-donor ligand, frequently used in the design of nickel(II) coordination polymers, as the supporting ligand.
In this work, we report the synthesis and characterization of the first metal complex with 6-fluoronicotinate -the onedimensional nickel(II) coordination polymer {[Ni(6-Fnic) 2 -(4,4 0 -bpy)(H 2 O) 2 ]Á3H 2 O} (1). The synthesis was carried out in a mixture of water and ethanol in the hope that the coordinated water molecules would complete the coordination sphere around the nickel(II) ion and participate in the formation of various hydrogen-bond motifs within the hydrogen-bonded framework, along with the anticipated lattice water molecules. Furthermore, we wanted to explore the effect of the probable weak intermolecular interactions involving the aromatic F atoms (for example C-HÁ Á ÁF interactions) on the assembly of the polymeric chains in the crystal packing.

Structural commentary
As the nickel(II) ion and the lattice water molecule (atom O4) are situated on an inversion center and a twofold axis, respectively, the asymmetric unit of 1 consists of one half of a nickel(II) ion, one coordinated water molecule, one fluoronicotinate ligand, one half of a 4,4 0 -bipyridine ligand and one and a half lattice water molecules. The nickel(II) ion is octahedrally coordinated by two 6-fluoronicotinate O atoms (O2 and O2 i ) and by two 4,4 0 -bipyridine N atoms (N2 and N2 i ) in the equatorial position, whilst two water molecule O atoms (O1 and O1 i ) are bound in the axial positions [symmetry code: (i) Àx + 1 2 , Ày + 3 2 , Àz]. In this way, a trans isomer is formed (N2 i -Ni1-N2 = 180 ) (Fig. 1). The 6-fluoronicotinate ligands are bound to the nickel(II) ion via their carboxylate O atoms in an O-monodentate fashion, whilst the 4,4 0 -bipyridine ligands act as bridge and, thus, connect the symmetry-related nickel(II) ions into an infinite one-dimensional polymeric chain extending in the [110] direction (Fig. 2). There are three lattice water molecules per repeating polymeric unit, {[Ni(6-Fnic) 2 (4,4 0 -bpy)(H 2 O) 2 ]Á3H 2 O}.

Figure 3
There are some distinctive hydrogen-bonded ring motifs within the two-dimensional network of 1 (Fig. 4). The octameric R 8 8 (24) motif is formed between six lattice water molecules and two symmetry-related polymeric chains (indicated in blue and green), which are linked via two 6-fluoronicotinate pyridine N atoms and two carboxylate O atoms. The hexameric R 6 8 (16) motif is formed between four lattice water molecules and two symmetry-related polymeric chains (indicated in blue and red), which are linked via two coordinated water molecules and two carboxylate O atoms, while the intramolecular S 1 1 (6) motif is formed within the polymeric chain (indicated in red) via a coordinated water molecule and a carboxylate O atom (Fig. 4). Both coordinated and lattice water molecules participate in the formation of motifs as single-and double-proton donors [coordinated water molecules as single-proton donors in the S 1 1 (6) motif and doubleproton donors in the R 6 8 (16) motif; lattice water molecules as single-proton donors in the R 8 8 (24) and R 6 8 (16) motifs and double-proton donors in the R 8 8 (24) motif]. The 6-fluoronicotinate pyridine N atoms act as single-proton acceptors exclusively, while carboxylate O atoms act as both single-and double-proton acceptors [single in the S 1 1 (6) and R 8 8 (24) motifs and double in the R 6 8 (16) motif].
Although there are many reported nickel(II) coordination polymers containing bridging 4,4 0 -bipyridine and pyridinedicarboxylate ligands, there are only two structurally similar one-dimensional nickel(II) polymers with 4,4 0 -bipyridine and pyridinecarboxylate (i.e. picolinate; Li et al., 2009) or fluorinated benzoate (2,6-difluorobenzoate; Yuan et al., 2016) ligands. The polymeric chains are assembled with lattice water molecules in the crystal packing of the picolinate polymer (Li et al., 2009) or with the solvated ethanol molecules in the crystal packing of the 2,6-difluorobenzoate polymer (Yuan et al., 2016). The discussed hydrogen-bond motifs in 1 are completely different from those observed in the crystal packings of these similar polymers, except for the intramolecular S 1 1 (6) motif, which is also present in the packing of the 2,6-difluorobenzoate polymer (Yuan et al., 2016). The reason for the different hydrogen-bond motifs may be due to the different arrangement of the lattice water molecules (primarily connected to each other into a layered network and not extensively connected to the polymeric chains) in the packing of the picolinate polymer (Li et al., 2009), and the fact that the ethanol O atoms are solely proton acceptors (not being able to participate in extensive hydrogen bonding as water molecules) in the packing of the 2,6-difluorobenzoate polymer (Yuan et al., 2016).
Unfortunately, there are no weak intermolecular interactions involving the aromatic F atoms; we hoped these interactions could have an effect on the supramolecular assembly of the polymeric chains in 1. The reason for the lack of such interactions may be the extensive hydrogen bonding, comprising strong O-HÁ Á ÁO and O-HÁ Á ÁN hydrogen bonds, that hinders weak C-HÁ Á ÁF supramolecular interactions. Indeed, the crystallization from an aqueous solution enabled the participation of the lattice water molecules in the extended structure of 1, enhancing the number of O-HÁ Á ÁO and O-HÁ Á ÁN hydrogen bonds in the hydrogen-bonded network and leading to the formation of the anticipated hydrogen-bond motifs.

PXRD and thermal analysis
The PXRD analysis was used to confirm the bulk content of 1 (Fig. 5). The experimental and calculated PXRD spectra of 1 are in very good agreement.
The thermal stability of 1, as determined from the TG curve, is only up to 40 C (Fig. S1 in the supporting information). Both the coordinated (two) and lattice (three) water molecules were released in the same step (observed mass loss 14.5%, calculated 15.4%). The two small endothermic peaks in the DSC curve (63 and 115 C) suggest that the process of the water evolution is not straightforward and that the water molecules are differently bound in 1 (coordinated vs lattice). Indeed, the polymeric chains and lattice water molecules are assembled into a hydrogen-bonded three-dimensional structure (see Supramolecular features). It is therefore not surprising that the release of some water molecules affects the whole hydrogen-bonded structure and leads to its complete collapse in a single, not well-resolved thermal step. The The distinctive hydrogen-bonded ring motifs (represented by the dotted lines) found within the two-dimensional network of 1 viewed along the [110] direction, viz. octameric R 8 8 (24), hexameric R 6 8 (16) and intramolecular S 1 thermal decomposition of 1 continues in a broad step (observed mass loss 56.7%) in the wide temperature range of 150-570 C (without any well-defined peaks in the DSC curve), 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, ChemPUR) 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 an Mettler-Toledo TGA/DSC 3+ instrument. Approximately 2 mg of the sample were placed in a standard alumina crucible (70 ml).

Synthesis and crystallization
6-Fluoronicotinic acid (0.0495 g, 0.3508 mmol) was dissolved in distilled water (5 ml), 4,4 0 -bipyridine (0.0276 g, 0.1767 mmol) was dissolved in ethanol (2 mL) and nickel(II) sulfate heptahydrate (0.0517 g, 0.1841 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 evaporate slowly at room temperature for approximately three weeks until light-blue crystals of 1, suitable for X-ray diffraction measurements, were obtained. These were collected by filtration, washed with their mother liquor and dried in vacuo. Yield: 0.0483 g (45%). Selected IR  Table S1 in the supporting information).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. C-bound atoms were positioned geometrically and refined using a riding model [0.93 Å , U iso (H) = 1.2U eq (C) for aromatic H atoms]. Water H atoms were found in difference-Fourier maps, O-H distances were restrained to an average value of 0.82 Å using DFIX and DANG instructions and they were refined isotropically [U iso (H) = 1.2U eq (O)].

catena-Poly[[diaquabis(6-fluoropyridine-3-carboxylato-κO)nickel(II)]-µ-4,4′-bipyridine-κ 2 N:N′] trihydrate]
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.