The first coordination compound of deprotonated 2-bromonicotinic acid: crystal structure of a dinuclear paddle-wheel copper(II) complex

The copper(II) ion has a distorted square-pyramidal coordination environment, achieved by four carboxylate O atoms in the basal plane and the water molecule in the apical position. The pair of symmetry-related copper(II) ions are connected into a centrosymmetric paddle-wheel dinuclear cluster via four O,O′-bridging 2-bromonicotinate ligands. In the extended structure, the cluster molecules are assembled into an infinite two-dimensional hydrogen-bonded network lying parallel to the (001) plane via strong O—H⋯O and O—H⋯N hydrogen bonds, leading to the formation of various hydrogen-bond ring motifs: dimeric (8) and (16) loops and a tetrameric (16) loop.


Chemical context
Copper(II) carboxylates have been studied extensively because of their structural diversity and related possible applications. The origin of this diversity is in the variable donating ability of the carboxylate oxygen atoms and in the nature of the other coordinated ligands (Iqbal et al., 2013;Song et al., 2009). The structural diversity of copper(II) carboxylates depends strongly on the inclusion of additional coligands (e.g. hydroxy, alkoxy and azide ions), which are able to mediate magnetic coupling between the copper(II) ions and to enable ferromagnetic and antiferromagnetic interactions via their various bridging modes (Zhang et al., 2012;Ma et al., 2014).
Nicotinic acid has been widely used as a complexing agent for various metal ions and many crystal structures of its metal complexes (almost 900) have been reported and deposited in the Cambridge Structural Database (CSD, Version 5.40, searched October 2019; Groom et al., 2016). However, metal complexes of nicotinic acid derivatives have been much less explored. For example, no metal complexes of 2-bromonicotinic acid (2-BrnicH) have been reported so far.
Our goal was to prepare 2-bromonicotinate copper(II) complexes for the above-mentioned significance of copper(II) carboxylates. The syntheses were carried out in aqueous solution to ensure that water molecules (either coordinated and/or hydrated) would be present in their crystal structures, enabling the formation of hydrogen-bonded frameworks. Furthermore, we wanted to explore the type and occurrence of hydrogen bond motifs within the obtained frameworks.

Structural commentary
The asymmetric unit of 1 consists of a copper(II) ion coordinated by a water molecule and by two deprotonated Omonodentate 2-bromonicotinate ligands (Fig. 1). The coordination environment of the copper(II) ion can be described as a distorted square pyramid as amounts to 0 [ = ( À ) / 60 ( and are the largest angles), = 0 for an ideal square pyramid and 1 for an ideal trigonal bipyramid; Addison et al., 1984]. The basal plane of the pyramid is defined by four carboxylate O atoms [O2, O4, O3 i and O5 i ; symmetry code: (i) Àx + 1, Ày + 2, Àz + 1] from four 2-bromonicotinate ligands while its apical position is occupied by the aqua atom O1 (Fig. 2). The two symmetry-related copper(II) ions are connected into a centrosymmetric paddle-wheel dinuclear cluster [with a CuÁ Á ÁCu contact length of 2.6470 (11)  The dinuclear cluster of 1 with selected atoms labeled [symmetry code: (i) Àx + 1, Ày + 2, Àz + 1].

Figure 1
The asymmetric unit of 1, with the atomic numbering scheme. The displacement ellipsoids are drawn at the 40% probability level. coordination mode (Phetmung & Nucharoen, 2019). This CuÁ Á ÁCu interaction is slightly longer than the sum of the covalent radii of Cu atoms (2.64 Å ; Cordero et al., 2008). The CuÁ Á ÁCu contact length in 1 is also somewhat longer than those in related paddle-wheel copper(II) clusters with nicotinic acid derivatives Adhikari et al., 2016), but almost equal to that seen in the paddle-wheel copper(II) cluster with 2-chloronicotinic acid (Moncol et al., 2007). The Cu-O c and Cu-O w (c = carboxylate, w = water) bond lengths are comparable with literature values (Moncol et al., 2007;Adhikari et al., 2016).
The copper(II) ion in 1 is situated nearly at the center of the basal plane with an out-of-plane deviation of 0.209 (2) Å in the direction of the apical Cu1-O1 bond (Fig. 2). The squarepyramidal coordination environment around the copper(II) ion is distorted, as indicated by the angles for the cis

Supramolecular features
The extended structure of 1 features strong O-HÁ Á ÁO and O-HÁ Á ÁN hydrogen bonds, weak C-HÁ Á ÁO hydrogen bonds (Table 1) (17) ]. The strong hydrogen bonds link the cluster molecules into an infinite twodimensional hydrogen-bonded network lying parallel to the (001) plane (Fig. 3), with the anion-interactions consolidating the layered network. The layers are assembled into a three-dimensional network by the C-HÁ Á ÁO bonds.
There are some distinctive hydrogen-bonded ring motifs within the layered network of 1 (Fig. 4). The dimeric R 2 2 (8) motif is formed between symmetry-related molecules (indicated in brown and green) via two water molecules and two carboxylate O atoms, the dimeric R 2 2 (16) motif is formed between symmetry-related molecules (indicated in brown and blue) via two water molecules and two pyridine N atoms, while the tetrameric R 4 4 (16) motif is formed by symmetry-related molecules (indicated in blue, brown, red and green) via two water molecules and two pyridine N atoms (Fig. 4). The water molecules participate in the formation of motifs as both singleand double-proton donors [single in the R 2 2 (8) and R 2 2 (16) motifs and double in the R 4 4 (16) motif], while the carboxylate O and pyridine N atoms participate as single-proton acceptors exclusively. These hydrogen-bonded motifs in 1 are quite different from those in the crystal structures of related paddlewheel copper(II) clusters with nicotinate derivatives Adhikari et al., 2016). This difference is not surprising in the case of copper(II) clusters with 2-ethoxynicotinate and 2-(naphthalen-2-ylmethylsulfanyl)nicotinate because the presence of water molecules of crystallization drastically affects the crystal packing Adhikari et al., 2016). The difference in the hydrogen-bonded ring motifs in the case of the copper(II) cluster with 2-methoxynicotinate  can be attributed to the different supramolecular arrangement of the cluster molecules, which are connected into a hydrogen-bonded chain, as opposed to a hydrogen- Symmetry codes: (i) x þ 1; y; z; (ii) Àx þ 1; Ày þ 1; Àz þ 1; (iii) x; Ày þ 3 2 ; z þ 1 2 .  The distinctive hydrogen-bonded ring motifs (represented by dotted lines) found within the layered network of 1, viz. the dimeric R 2 2 (8) and R 2 2 (16) motifs and the tetrameric R 4 4 (16) motif. The various symmetryrelated cluster molecules are shown in blue, brown, red and green (see text).

Figure 3
bonded network in the case of 1. Furthermore, it seems that the substituents in the nicotinate derivatives (methoxy group versus bromine atom) have a great influence on the supramolecular assemblies and on the hydrogen-bond motif types in the respective crystal packings because of the difference in the proton-acceptor abilities of the two substituents.

Hirshfeld surface analysis
The Hirshfeld surface analysis of 1 was performed using CrystalExplorer17.5 (Wolff et al., 2012). Normalized contact distances, d norm , were plotted with standard color settings: regions highlighted in red represent shorter contacts, while longer contacts are shown in blue (Fig. 5). The fingerprint plots show distances from each point on the Hirshfeld surface to the nearest atom inside (d i ) and outside (d e ), and are presented for all contacts and for the contributions of two primary contacts, O-HÁ Á ÁO and O-HÁ Á ÁN hydrogen bonds (Fig. 6). The percentage contributions of all other selected contacts are presented as a pie chart (Fig. 6).

PXRD and thermal analysis
The PXRD analysis was used to confirm the bulk content of 1 (see Fig. S1 in the supporting information). The experimental and calculated PXRD traces of 1 are in very good agreement, confirming the phase purity of 1.
The thermal stability of 1, as determined from the TG curve, is up to 140 C (Fig. S2 in the supporting information). The two coordinated water molecules (observed mass loss 3.9%, calculated 3.7%) were released at 176 C (endothermic peak at the DSC curve). The thermal decomposition of 1 continues via two consecutive steps (observed mass losses 10.6% and 24.7%) in the temperature range of 190-390 C (exothermic peak at 195 C), which corresponds to the release of approximately one and a half 2-bromonicotinate ligands (calculated mass loss 31.2%). The decomposition finishes with the release of another two 2-bromonicotinate ligands (observed mass loss 46.7%, calculated 41.6%) in the final step (temperature range of 390-600 C). The observed residue (13.3%) at 600 C, remained after total decomposition of 1, corresponds to CuO. The experimental mass fraction of copper (10.7%) matches nicely with the calculated mass fraction (13.1%).

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 25-800 C, under an oxygen flow of 50 mL min À1 on an Mettler-Toledo TGA/DSC 3+ instrument. Approximately 2 mg of the sample was placed in a standard alumina crucible (70 ml).

Synthesis and crystallization
2-Bromonicotinic acid (0.0502 g; 0.2485 mmol) was dissolved in distilled water (5 ml) with the addition of a drop of concentrated ammonia solution and then mixed and stirred with an aqueous copper(II) chloride dihydrate solution (0.0220 g; 0.1290 mmol in 2 ml of distilled water). The pH of the obtained solution was adjusted to 6-7 by adding an ammonia solution dropwise. The clear solution was left to evaporate slowly at room temperature for a month until blue crystals of 1, suitable for X-ray diffraction measurements, were obtained, which were collected by filtration, washed with ethanol and dried in vacuo. Yield: 0.0209 g (17%). Selected IR bands (   Hirshfeld surfaces on the molecule of 1. Regions highlighted in red represent shorter contacts, while longer contacts are blue.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The C-bound H atoms were placed geometrically (C-H = 0.93 Å ) and refined as riding atoms. The water-molecule H atoms were found in difference-Fourier maps and refined with the O-H distances restrained to an average value of 0.82 Å using DFIX and DANG instructions. The constraint U iso (H) = 1.2U eq (carrier) was applied in all cases. The highest difference peak is 1.00 Å away from Br2 and the deepest difference hole is 0.78 Å away from the same 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 the University of Split institutional funding.  CrystalExplorer17.5 (Wolff et al., 2012); software used to prepare material for publication: SHELXL2018/3 (Sheldrick, 2015b).

Tetrakis(µ-2-bromonicotinato-κ 2 O:O′)bis[aquacopper(II)](Cu-Cu)
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.