Design and Synthesis of a New Photoluminescent 2D Coordination Polymer Employing a Ligand Derived from Quinoline and Pyridine

Abstract

Application of organic ligand 2-(3-ethyl-pyrazin-2-yl)quinoline-4-carboxylate with N/O donor atoms enabled solvothermal synthesis of a 2D Cu(II) coordination polymer, {Cu(L)BF₄}_n (L = deprotonated 2-(3-ethyl-pyrazin-2-yl)quinoline-4-carboxylate). Both the ligand and its coordination polymer have been characterized. The condensed ring system of the applied ligand promotes the formation of coordination polymers rather than mononuclear species. The obtained 2D coordination polymer is photoluminescent with bathochromic/hypsochromic shifts in ligand absorption bands leading to a single absorption band at 465 nm. Density Functional Theory was employed to provide a theoretical description of the possible conformational changes within the ligand, with emphasis on the difference between the ligand conformation in its hydrochloride salt and in the polymer. Two models of polymer fragments were constructed to describe the electronic structure and non-covalent interactions. The Quantum Theory of Atoms in Molecules (QTAIM) was applied for this purpose. Using the obtained results, we were able to develop potential energy profiles for various conformations of the ligand. For the set of the studied systems, we detected non-covalent interactions, which are responsible for the spatial conformation. Concerning the models of polymers, electron spin density distribution has been visualized and discussed.

1. Introduction

Application of aminocarboxylate ligands with N/O-donor atoms and aromatic rings provides extensive possibilities for construction of 1-3D coordination polymers. The resulting products may display useful photochemical as well as magnetic properties. This is dependent on the used metal ions [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. Examples of Cu(II) coordination polymers have been reported with applications for gas absorption or ion exchange, for instance (the 4,4′–bpy 4,4′-bipyridine ligand has been utilized in the synthesis of [Cu(SiF₆)(4,4′–bpy)₂]_n [1], [{Cu(GeF₆)(4,4′–bpy)₂}·₈H₂O]_n [5], [{Cu(BF₄)₂(4,4′–bpy)(H₂O)₂}·4,4′–bpy]_n [6], (pzdc = pyrazine-2,3-dicarboxylate; dpyg = 1,2-Di(4-pyridyl)glycol) [Cu₂(pzdc)₂(dpyg)] [7], (tctfm-4,4′,4″,4‴tetracyanotetraphenylmethane) [{Cu(tctfm)}∙BF₄∙xC₆H₅NO₂] [8], {[Cu(4,4′-bpy)_1.5]·NO₃·1.5H₂O}_n [9], and (rot-N,N′-bis(3-pyridylmethyl)-1,5-diaminopentane dihydronitrate){[Cu(rot)(C₂O₄)0.5(H₂O)₂]·₃NO₃·20H₂O}_n [10].

Coordination polymers are also applied in heterogeneous catalysis. Compounds (where used 4,4′-bpy 4,4′-bipyridine, carboxylato co-ligand (formato or acetato){[Cu₃(4,4′–bpy)₃(μ–OOCH)₄(H₂O)₂](ClO₄)₂(H₂O)₆}_n, {[Cu(4,4′–bpy)(μ–OOCH)(NO₃)]}_n, {[Cu₂(4,4′–bpy)2(μ–OOCCH₃)₃](PF₆)(H₂O)}_n, being 2D coordination polymers, are effective catalysts to cyanosililation of aldehydes, which is a convenient synthesis method for cyanohydrins [11]. On the other hand, a compound with a ligand (bznbz, 1,3–bis–(5,6–dimetylobenzimidazol–1–il–metyleno)benzene){[Cu(bzmbz)₂]·(NO₃)₂}_n catalyzes degradation of azo dyes in the presence of hydroxyl radicals OH·, obtained from H₂O₂ solution containing Fe²⁺ ions under acidic conditions [12].

Another important field is luminescence of coordination polymers. In comparison to organic compounds used, among others, in the production of OLEDs, inorganic coordination compounds often display higher thermal stability, which allows for a wider application range. Depending on the used central metal ion and, in particular, its valence electrons configuration, luminescence properties of coordination polymers may be governed by excited states of inter-ligand charge transfer (IL), metal–ligand charge transfer (MLCT), ligand–metal charge transfer (LMCT), and ligand–ligand charge transfer (LLCT) character. One of the examples uses pyram—2-pirydyno-4-pirydylometyloamine {[Cu(PPh₃)(pyram)1.5]∙0.5CHCl₃∙ClO₄ compound in which, in comparison to the free ligand (λ_em = 460 nm), the absorption band is shifted toward lower energies with 10 times higher intensity [13].

There is also much research on the electrical properties of coordination polymers. The reference materials are conductive metals with conductivity of 10⁴–10⁵ S cm^–1, normally increasing with a decrease in temperature. An important and interesting example is the Cu(I) coordination polymer with ligand dcnqi—diiminum N,N′–dicyano–1,4–benzoquinone [Cu(dcnqi)₂]_n with a structure comprising seven superposed diamond-type lattices [14]. At 295 K, monocrystals of this compound display electrical conductivity values from 800 to 1000 S cm^–1.

Herein are new insights into the coordination chemistry of a new ligand based on quinoline, functionalized with carboxylate and pyridyl groups, employed as a precursor of a novel two-dimensional coordination polymer. Introduction of a substituted pyrazine in this ligand results in increased coordination capabilities. The resulting Cu(II) polymer displays photoluminescent and antiferromagnetic properties. The experimental findings are supported by quantum-chemical simulations with the aid of DFT (Density Functional Theory) [15,16]. Based on theoretical investigations, it was possible to analyze conformational changes of the ligand as well as electronic density distribution. The network of non-covalent interactions was covered by means of Quantum Theory of Atoms in Molecules (QTAIM) [17,18].

2. Materials and Methods

2.1. Synthesis of Crystalline Materials

2-(3-Ethyl-pyrazin-2-yl)quinoline-4-carboxylate acid [LH] 1

In this study, 35 ml of double-distilled water and 10 ml of ethanol were combined in a flask (50 ml) equipped with a reflux condenser. To this mixture, isatin (1.22 g, 8.30 mmol; Sigma-Aldrich, Poznan, Poland), 2-acetyl-3-ethyl-pyrazine (0.80 g, 5.32 mmol; Aldrich), and KOH (2.24 g, 40.00 mmol; POCH, Gliwice, Poland) were added (Scheme 1). The mixture was stirred magnetically for a couple of minutes. It was then heated at 100 °C for 11 h. After cooling, 2 M HCl was added to adjust the pH to 4–5. A dark-yellow precipitate formed and was rinsed with distilled water. This procedure was continued until the precipitate’s color changed to light yellow. A powder was obtained as a result of drying this precipitate in a desiccator. This powder could not be crystallized from water. It was possible to dissolve it in 2 M HCl. This action yielded a crystalline hydrochloride salt.

Yield (pure ligand): 70% (based on isatin).

Elemental analysis calculated (%) for C₁₆ H₁₃ N₃ O₂

C 69.35 H 4.88, N 14.62; found: C 69.10, H 4.20, N 14.25.

IR (Nujol, cm⁻¹): 3401 s νsOH; 1621 s νsCOO; 1600 s ν(ring); 1584 s νasCOO; 1551 m, 1515 s δCNC; 1403 s νCC; 1344 m νsCOO; 1303 w, 1244 w νs COO/νCC; 1153 m νsCOO/δCOH; 1110 m νsOH; 1059 w νsCOO; 870 w νCN; 849 w νCN; 819 w νCN; 790 m δOCO; 765 m νCN/δOCO; 724 w δOCO; 680 w δ(ring); 663 m δCH; 642 m δCH; 591 m δCH; 520 w νCC; 492 w, 468 m δCNC; 434 w δCNC; 414 w δCNC; 278 w, 254 w, 247 w δCH; 227 w δC=C(ring); 203 w, 177 w, 150 w, 140 w, 133 w, 127 w, 121 w, 106 w, 100 w, 81 vw, 74 vw, 56 vw νasCOO.

1H NMR (500.13 MHz, methanol-d4, 300.0 K): δ = 1.43 (t, 3H, J = 7.5 Hz), 3.29–3.34 (m, 2H), 7.89–7.94 (m, 1H), 8.02–8.07 (m, 1H), 8.32 (d, 1H, J = 8.5 Hz), 8.73 (s, 1H), 8.80 (d, 1H, J = 1.7 Hz), 8.87 (s, 1H), 8.99 (d, 1H, J = 8.6 Hz) ppm.

13C NMR (125.77 MHz, methanol-d4, 300.0 K): δ = 13.6, 29.1, 124.2, 126.6, 127.1, 127.5, 128.0, 131.2, 133.5, 141.7, 143.9, 144.0, 146.6, 150.2, 158.5, 167.9 ppm.

Synthesis of {Cu(L)BF₄}_n (L = (L = deprotonated 2-(3-ethyl-pyrazin-2-yl)quinoline-4-carboxylate) 2.

Double-distilled water was applied, and Cu(BF₄)₂⋅H₂O and 2-(3-ethyl-pyrazin-2-yl)quinoline-4-carboxylate chloride dihydrate (organic ligand) were used for the synthesis (Scheme 2).

Teflon-lined pressure reactor Berghof 100 was fed with 2-(3-ethyl-pyrazin-2-yl)quinoline-4-carboxylate chloride dihydrate (0.350 g, 1.17 mmol), 10 cm³ double distilled water, and Cu(BF₄)₂ • xH₂O (anhydrous basis - 0.20 g, 0.843 mmol anhydrous basis).

The reactor was closed and flushed with nitrogen gas for 20 minutes and subsequently heated. During the first day, heating was carried out at 100 °C. For the next 4 days the reactor temperature was at 145 °C. Then, the reactor was turned off and slowly cooled down at 5 °C per hour. The coordination polymer product formed green–blue crystals at 73% yield. When other Cu(II) sources were used, no crystalline products were obtained. The reagent quantities have been optimized for the highest yields.

IR [KBr, cm⁻¹]: 3431 s ν(CH), 2262 m ν(CH), 2011 w ν(OH), 1618 m ν(C=O), 1559 m ν_as(COO), 1517 m δ(CNC), 1407 m ν(CC)/ν(CN), 1380 m ν_s(COO), 783 m ν(CC), 650 m δ(CH)/ν(COO), 630 m δ(CH), 533 w ν(CC), 481 m ν(CC), 470 s ν_s(CuN)/ν_s(CuO).

Elemental analysis calculated (%) for {Cu(L)BF₄}_n: C 44.83 H 3.02, N 9.79; found: C 44.20, H 2.98, N 9.10.

2.2. IR Spectroscopy

IR spectra were recorded with a Bruker VERTEX 70 FTIR spectrometer (Bruker, Billerica, MA, USA) for the starting organic ligand and its Cu(II) polymer.

2.3. X-Ray Diffraction Studies

2.3.1. X-Ray Diffraction Experiment

Diffraction data were collected on a Rigaku Oxford Diffraction XtaLAB Synergy-R DW diffractometer equipped with a HyPix ARC 150° Hybrid Photon Counting (HPC) detector using CuKα (λ = 1.5418 Å) at 100 K. Data collection, cell refinement, data reduction, and analysis were carried out with the CRYSALIS^PRO (Rigaku Oxford Diffraction Ltd., Abingdon, UK, 2020). An analytical absorption correction was applied with the use of CRYSALIS^PRO RED [19].

CCDC numbers of compounds 1 (2443543 ligand) and 2 (2443544 2D polymer) contain the supplementary crystallographic data for this paper. These data are provided free of charge by The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Selected X-ray data are shown in Tables S1–S11.

2.3.2. Crystal Structure Refinement

Both crystal structures were solved with Intrinsic Phasing using the SHELXT program and refined using SHELXL (2015 release) [20,21] with anisotropic thermal parameters for non-H atoms. In the final refinement cycles, all H atoms were treated as riding atoms in geometrically optimized positions.

2.4. Photoluminescence Spectra

The photoluminescence spectra of the applied organic ligand and for the title polymer for solid crystalline samples anchored to quartz tubes were recorded with an FLSP920 Spectrometer from Edinburgh Instruments, equipped with a 450 W Xenon arc lamp as an excitation source.

2.5. Studies of the Magnetic Properties

Magnetic susceptibility data were recorded on a Quantum Design MPMS-XL5 SQUID magnetometer over the 300–1.8 K temperature range. Magnetic data were corrected for diamagnetic contributions, which were estimated from Pascal’s constants, and for temperature-independent paramagnetism, estimated at 60 × 10⁻⁶ emu/mol for the Cu²⁺ ion.

2.6. NMR

NMR spectra were recorded on a Bruker 500 MHz device. For results, see Figure S1a,b.

2.7. Computational Methodology

The starting geometry of the analyzed ligand was extracted from the crystal structure (see text above) and modified accordingly to prepare suitable structures for further modeling. The same procedure was applied to prepare small and larger models of the polymer. The quantum-chemical simulations were performed based on Density Functional Theory (DFT) [15,16]. The simulations were carried out using M06 functional [22] and the def2-TZVP basis set for the ligand and the small model of the polymer in order to optimize the geometry of the systems studied [23,24]. Concerning the larger model of the polymer, the same functional but SDD basis set was used [25,26,27]. Harmonic vibrational calculations were performed as well to confirm that the obtained structures correspond with the minima on the Potential Energy Surface (PES). Next, the conformational analysis was performed for the ligand. Three torsional angles were scanned, and potential energy profiles were obtained. The method called “scan with optimization” was applied for this purpose. The procedure is based on geometry optimization of the remaining parts of the molecules while the chosen dihedral angle is scanned (in our case, the increment was set to 10°). The wavefunctions for further electron density studies were generated on the basis of the above-mentioned levels of theory. For this part of the simulations, the Gaussian 16, Rev. C.01 suite of programs was used [28]. The electronic structure analysis was carried out on the basis of the Quantum Theory of Atoms in Molecules (QTAIM) [15,16]. The presence of bond and ring critical points (BCPs and RCPs) allowed the detection of non-covalent interactions responsible for the molecular spatial constitution. It provided a qualitative picture of the electron density partitioning. The AIMAll program was employed for the QTAIM analysis, topological analysis, and results presentation [29]. The figures showing the obtained data were prepared using the VMD 1.9.3 [30] and GaussView [31] programs.

3. Results and Discussion

3.1. Crystal Structure

In this study, 2-(3-ethyl-pyrazin-2-yl)quinoline-4-carboxylate was used for the synthesis. This ligand was isolated as hydrated chloride (Scheme 3), able to form coordination polymers with its N/O donor atoms and additionally displaying photoluminescent properties. The ligand obtained in Pfitzinger synthesis is not crystalline; therefore, for its structure determination, it was crystallized from aqueous solution in the form of chloride salt.

The resulting crystal structure 1 comprises protonated ligand cations, chloride counterions, and water molecules (Figure 1, Supplementary Tables S1–S6). The ligand protonated atom is the pyridyl N atom, whereas the quinoline N atom remains not protonated. The carboxylic group is twisted by 38.93(3)° (O2 C14 C1 C2 torsion angle). The carboxylic H atom is a donor to a hydrogen bond to chloride ion O1–H1B···Cl1 [1 − x, 1 − y, 1 − z] (Table S10). On the other hand, for the 2-ethylpyrazine moiety bonded to the quinoline ring via the C10 atom, the N1-C8-C10-C13 and C9-C8-C10-N2 angle values are 16.14(2) and 13.13(3)°, respectively. The orientation of quinoline and pyrazine rings is affected by the pyrazin ethyl group, constituting some kind of steric hindrance. The angle between quinoline and pyrazine ring planes is 12.08(3)°. The presence of water molecules and chloride ions results in the formation of a 3D hydrogen bonding network.

Figure 2 displays graph set motifs that can be distinguished in the crystal structure 1.

The Cu(II) coordination polymer was obtained in solvothermal synthesis in a Berghoff reactor in accordance with the scheme below.

A 2D Cu(II) coordination polymer with a single Cu²⁺ linked with donor atoms N/O of aminocarboxylate ligand (Figure 3) was obtained via a solvothermal route. This compound is displayed in Scheme 4 with the formula {Cu(L) BF₄}_n.

Each Cu²⁺ ion is tetracoordinate with three N atoms and one carboxylate O atom in its coordination sphere (the relevant bond lengths are Cu(1)−N(3) 1.9226(19); Cu(1)−O(1)^{−x,−1/2+y,1/2−z} 2.4470(19); Cu(1)−N(1)^{x,1/2−y,−1/2+z} 2.0375(19); and Cu(1)−N(2)^{x,1/2−y,−1/2+z} 2.0309(19) Å, Table S8). The Cu²⁺ lies in a special position on a symmetry center so that only one coordinated N and O atom are symmetrically independent. The coordination sphere is in the shape of a trigonal pyramid. Additionally, in the crystal structure, C–H···O and C–H···F hydrogen bond types can be found (Table S11).

The polymer forms a 2D network along the [100] and [010] directions (Figure 4).

Within this network, weak stacking interactions between aromatic rings are observed with a centroid-to-centroid distance of 3.349(3) Å, as illustrated in Figure 5.

Furthermore, characteristic tubular arrangements are formed in the crystal structure (Figure 6a,b).

Figure 7 further displays the formation of channels along [100], whereas Figure 8 visualizes a layered arrangement of molecules along [001].

3.2. Photoluminescence Properties

Luminescence properties of the ligand and Cu(II) CPs were tested in solid state at room temperature. In the ligand spectrum (Figure 9), emission bands observed at 425 nm can be interpreted in terms of π π* transitions. For the coordination polymer (Figure 10), the absorption band is shifted to 465 nm and could be interpreted as a ligand–metal charge transfer (LMCT) band with delocalization of electron density from the ligand via its N atoms to Cu(II) d orbitals. Similar effects have been observed for copper(II) triazolate coordination polymer with cyanide as a co-ligand, reported by Zhu et al. [32]. The photoluminescence spectrum of the Cu(II) Schiff base complexes was described by Rani et al., who also noted a shift in polymer spectrum toward higher wavelengths [33]. For binuclear copper complexes with carboxylate ligands, photoluminescence properties have also been reported. Carboxylate ligands are donors of electron pairs, which lead to the auxochromic effect, increasing the intensity of light absorption [34]. Zink et al. reported on dinuclear copper(I) halide complexes with P-N ligands and proposed to apply these compounds for singlet harvesting in OLEDs [35].

3.3. Magnetic Properties

Figure 11 shows selected Cu···Cu distances whereas Figure 12 denotes model of antiferromagnetic interactions transmitted via ligand.

Magnetic susceptibility values for the coordination polymer were determined with a SQUID magnetometer at a 1.8–300 K temperature range. Based on the χ_mT experimental curve, at 300 K the corresponding value is 0.425 cm³ K mol⁻¹ (Figure 13), pointing out that there are weak antiferromagnetic interactions between Cu²⁺ ions, transmitted via bonds within the ligand. In order to quantify these interactions based on spin Hamiltonian H = –2JS₁S₂, the Bleaney–Bowers Equation (1) was used [36], taking into account mainly the nearest-neighbor interactions:

$${\chi }_{\mathrm{m}}={\displaystyle \frac{N{\beta }^2{g}^2}{3kT}}{\left[1+{\displaystyle \frac{1}{3}}\exp \left({\displaystyle \frac{-2J}{kT}}\right)\right]}^{-1}$$

(1)

Knowing the two-dimensional crystal structure of 1, a molecular field model correction (2) was introduced [37]:

$${\chi }_{\mathrm{m}}^{\mathrm{corr}}={\displaystyle \frac{{\chi }_{\mathrm{m}}}{1-\frac{2zJ\prime {\chi }_{\mathrm{m}}}{N{\beta }^2{g}^2}}}$$

(2)

In the above equation, χ_m is defined as molar susceptibility, zJ’ characterizes magnetic interactions between Cu(II) ions in the crystal lattice, N is the Avogadro number, g = the spectroscopic splitting factor, β = the Bohr magneton, and k = the Boltzmann constant. On the χ_mT curve, the corresponding values decrease under 50 K, which may be caused by additional weak antiferromagnetic interactions between Cu(II) ions, supported by Neel temperature at 2.5 K. The following values resulted from application of the Bleaney–Bowers model: 2J = −1.07 ± 0.02 cm^–1, zJ′ = –0.2 ± 0.1cm^–1, and R = 3.66 10⁻³ (R = ∑[(χ_mT)_obsd − (χ_mT)_cald]² ∑(χ_mT)_obsd]²). Figure 12 illustrates a model of antiferromagnetic interactions between Cu²⁺ ions.

3.4. Molecular Modeling

Density Functional Theory (DFT) was employed for the theoretical description of the discussed ligand and two polymer models. The first relevant question is the conformational preference of the ligand, since the crystal structures of the ligand (in the hydrochloride form) and the polymer exhibit rotation of the pyrazine ring with respect to the quinoline skeleton. Initial optimization of the two models yielded the structures shown in Figure 14a,b. The structure from the left panel, corresponding to the free ligand, is more stable than the structure from the right panel, corresponding to the ligand coordinating the copper cation in the polymer. The difference is, however, only 5.0 kcal·mol⁻¹, indicating that neither strong intramolecular attractive forces within the ligand nor significant steric hindrances exist. The lack of strong intramolecular interactions will be shown in Figure 18, on the basis of the QTAIM analysis, where the ligand exhibits only weak C-H···O and C-H···N contacts. The rotation of the pyrazine ring seems not hindered, and the form allowing better coordination capabilities and overall crystal packing is preferred in the polymer.

In Figure 15, the conformational analysis results are presented. We have investigated three dihedral angle rotations—the dihedral angle responsible for the carboxylic group rotation and two dihedral angles associated with the pyrazine ring rotation (N1-C8-C10-C13 and C9-C8-C10-C13). As it is shown in Figure 15, the carboxylic group exhibits free rotations; therefore, the obtained potential energy profile is symmetric. The most favorable conformation is the starting one obtained from the geometry optimization (see Figure 15 a,b,c). The degree of interaction of the carboxylic π electrons with the delocalized aromatic system is indicated by the height of the rotation barrier, 3.5 kcal·mol⁻¹, which is not large. Concerning the pyrazine ring rotation, there is a steric hindrance visible in both presented charts, leading to a 9.5 kcal·mol⁻¹ barrier for rotation. These two charts are similar, belonging to the same rotation, but with different conditioning during the scan. However, the most stable conformation in both cases is the one obtained from the geometry optimization and crystal structure (where the carboxylic and ethyl groups are located on the opposite sides).

In the next step, the models of polymers were investigated. We have constructed two models with two and three ligands attached to the Cu ion (denoted as small and large models, respectively). The obtained structures after the quantum-chemical simulations at the DFT/M06/def2-TZVP and DFT/M06/SDD are presented in Figure 16a,b.

For the polymers, spin density distribution analysis was performed, and the obtained results are presented in Figure 17a–d.

The spin density is not located only on the Cu metal center. For the small model, the spin density is located only on one of the two ligands, specifically the moiety binding by two nitrogen atoms. In the case of the larger model, the spin density is distributed to all three ligands, but its larger values are limited to the immediate vicinity of the binding nitrogen atoms. The relatively small range of the spin density distribution for the large model corresponds well with the also rather small experimentally derived exchange interaction, 2J = −1.07 ± 0.02 cm^–1.

In order to complete our theoretical study, the topology analysis based on electron density partitioning was carried out. The obtained molecular graphs for all studied systems are presented in Figure 18. An application of the QTAIM theory enabled the detection of Bond and Ring Critical Points (BCPs and RCPs) in the case of the studied ligand and the small model of the polymer. Concerning the large model of the polymer, cage points (CPs) were found as well. The dotted lines indicate the presence of non-covalent interactions present in the studied molecules. As it is shown, the ligand spatial conformation is stabilized by interaction between hydrogen atoms from the ethyl group and quinoline ring with oxygen atoms from the carboxylic group and the nitrogen atom from the quinoline ring. Concerning the small model of the polymer, six non-covalent interactions were found. It is worth underlining that a BCP was found between the Cu ion and one of the hydrogen atoms from the ethyl group. In addition, being somehow in the middle, another hydrogen atom from the ethyl group is interacting with a hydrogen atom from the quinoline moiety. A stabilizing interaction between the oxygen atom from the carboxylic group and one of the hydrogens from the quinoline ring was also detected. Concerning the large model of the polymer, the presence of the third ligand introduced additional non-covalent interactions. As it is shown in Figure 18, two ligands of the polymer are in parallel arrangement. They interact with the Cu ion via oxygen from the carboxylic group and two nitrogen atoms from the pyrazine and quinoline moieties of the second ligand. As it is presented in Figure 18a–f, there is a network of non-covalent interactions between the parallelly arranged ligands, in line with their π-electron-rich nature, supporting their stacked spatial conformation in the studied model.

4. Conclusions

A new copper(II) 2D coordination polymer was synthesized via the solvothermal reaction between the luminescent aminocarboxylate ligand and a copper(II) precursor. This study enriches our knowledge of structural and luminescent properties of coordination polymers [38,39,40,41] with ligands derived from quinolinecarboxylic acid, possibly inspiring future research in this field. A holistic approach has been taken from design of the synthesis, through characterization of the product properties till computational modelling.

The DFT modeling of the ligand has revealed its preferred conformation and explained the conformational flexibility of the pyrazine ring so that the ligand can easily adapt its structure to provide better metal binding in the polymer. The spin density distribution maps do not exhibit long-range interactions away from the metal center. Interaction analysis within the QTAIM methodology reveals key factors for stabilization of the arrangement of ligands in the model.