Spectroscopic (IR, Raman, NMR), thermal and theoretical (DFT) study of alkali metal dipicolinates (2,6) and quinolinates (2,3)

2.1 Sample preparation

The alkali metal salts of 2,3-pyridinedicarboxylic and 2,6-pyridinedicarboxylic acids were prepared by dissolving appropriate weighed amount of particular acids in aqueous solution of alkali metal hydroxides in a stoichiometric ratio ligand:metal – 1:2. 0.1 mol of 2,3-pyridinedicarboxylic acid, was added to 20 mL of the previously prepared metal hydroxide solutions having a concentration of 0.1 M. The synthesis of alkali metal salts of 2,6-pyridinedicarboxylic acid was performed in a similar manner. The mixtures were heated on a steam bath with stirring until the acid dissolved in sodium hydroxide. Then, water was evaporated on a water bath and dried in an oven at 70 °C for 24 h. All chemicals were purchased from Sigma–Aldrich.

2.2 Measurement and calculation

The FT-IR spectra were recorded with an Alfa (Bruker) spectrometer within the range of 400–4000 cm⁻¹. Samples in the solid state were measured in KBr matrix pellets and in ATR technique. FT-Raman spectra of solid samples were recorded in the range of 400–4000 cm⁻¹ with a MultiRam (Bruker) spectrometer. The resolution of the spectrometer was 1 cm⁻¹. The ¹H and ¹³C NMR spectra of D₂O solution of studied compounds were recorded with a Bruker Avance II 400 MHz unit at room temperature. TMS was used as an internal reference. To calculate optimized geometrical structures of 2,3-, 2,6-pyridinedicarboxylic acid and lithium, sodium and potassium salts quantum–mechanical methods were used: density functional (DFT) hybrid method B3LYP with non-local correlation provided by Lee–Young–Parr expression. All calculations were carried out with functional base 6-311++G(d,p). Calculations were performed using the Gaussian 09 (Frisch et al., 2009) package. Experimental spectra were interpreted in terms of DFT method calculations in B3LYP/6-311++G(d,p) level and literature data (McCann and Laane, 2008; Karabacak et al., 2015). Theoretical wave numbers were scaled according to the formula: ν_scaled = 0.98 · ν_calculated for B3LYP/6-311++G(d,p) level method (Rode et al., 2001). Chemical shifts (δi) were calculated by subtracting the appropriate isotopic part of the shielding tensor (σi) from that of TMS (σ_TMS): δi = σ_TMS − σi (ppm). The isotropic shielding constants for TMS calculated using the DFT method at the same level of theory were equal to 31.8201 ppm and 182.4485 ppm for the ¹H nuclei and the ¹³C nuclei, respectively. The products of dehydration and decomposition processes were determined from the TG curves. Thermogravimetric analysis (TGA) was performed on a Mettler Toledo Star TGA/DSC1 unit. Argon was used as a purge gas (20 ml min⁻¹). Samples between 2 and 4 mg were placed in aluminum pans and heated from 50 °C to 900 °C with a heating rate of 10 °C/min.

3.1 IR and Raman Spectra

The wave numbers, intensities and assignments of the bands occurring in the IR (KBr and ATR) and Raman spectra of 2,3-pyridinedicarboxylates are presented in Table 1 and 2,6-pyridinedicarboxylates are presented in Table 2. The spectral assignments were done on the basis of the literature data (McCann and Laane, 2008; Karabacak et al., 2015) and calculated IR wave numbers of studied compounds. Theoretical spectra for acids and lithium, sodium, potassium salts were calculated by DFT/B3LYP method on 6-311++G^∗∗ basis. Calculated normal vibrations were characterized by computer animation. A good correlation between experimental and theoretical IR and Raman spectra was noted. The correlation coefficients for the IR spectra are higher than 0.990. Normal vibrations of the aromatic ring were given by Varsanyi (1973). The changes of intensities and wave numbers of the bands of the aromatic system in the case of salts were discussed, in comparison with the free ligands. Substitution of the metal ion in the two groups of carboxylic acids results in the disappearance of the characteristic vibration of the carboxyl group, such as v(C⚌O) in 2,3-PDA present at 1707, 1622 cm⁻¹ in the IR_KBr spectrum, 1706 cm⁻¹ in the IR_ATR spectrum, 1698 cm⁻¹ in Raman, in the 2,6-PDA acid present at 1701 cm⁻¹ in IR_KBr, 1693 and 1634 cm⁻¹ in IR_ATR and 1644 cm⁻¹ in Raman; v(C—O) in 2,3-PDA present at 1307 cm⁻¹ in IR_KBr, 1307 cm⁻¹ in IR_ATR, 1309 cm⁻¹ in Raman, in 2,6-PDA present at 1328 cm⁻¹ in IR_KBr, 1331 cm⁻¹ in IR_ATR and 1328 cm⁻¹ in Raman. Vibration bands of the hydroxyl group i.e. v(OH), β(OH) and γ(OH) disappear as well. The disappearance of the bands characteristic of the carboxyl group indicates that the two carboxyl groups are being substituted by the metal ion. In the spectra of salts there appear bands assigned to vibrations of the carboxylate anion (the metal to ligand ratio was 2:1). In the alkali metal salts with 2,3-PDA these new vibrations give characteristic wide and intense two bands responsible for the ν_as(COO⁻) (1626–1590 and 1579–1575 cm⁻¹ in IR_KBr spectra, 1623–1587 and 1573–1562 cm⁻¹ in IR_ATR, 1633–1623 and 1581–1571 cm⁻¹ Raman spectra) and ν_sym(COO⁻) (1408–1382 cm⁻¹ in IR_KBr spectra, 1403–1372 cm⁻¹ in IR_ATR, 1409–1386 cm⁻¹ in Raman spectra) stretching of the carboxylic anion. The bands assigned to the symmetric in-plane deformation of the carboxylic anion (β_sym: 843–830 cm⁻¹ IR_KBr spectra, 843–823 cm⁻¹ IR_ATR, 846–823 cm⁻¹ Raman spectra) and asymmetric in-plane deformation of the carboxylic anion (β_as, observed only in the case of lithium salt, 497 IR_KBr). Bands derived from the carboxylate anion vibration in the spectra of the alkali metal salt of the 2,6-PDA occur at somewhat higher wave numbers than it is the case of the spectra of 2,3-PDA salts. In the alkali metal salts with 2,6-PDA these vibrations gave characteristic wide and intense two bands responsible for the ν_as(COO⁻) (1656–1646 and 1612–1609 cm⁻¹ in IR_KBr spectra, 1658–1631 and 1614–1669 cm⁻¹ in IR_ATR, 1665–1636 and 1619 cm⁻¹ in Raman spectra) and ν_sym(COO⁻) (1384–1378 cm⁻¹ in IR_KBr spectra, 1385–1365 cm⁻¹ in IR_ATR, 1397–1386 cm⁻¹ in Raman spectra) stretching of the carboxylic anion. The bands assigned to the symmetric in-plane deformation of the carboxylic anion, β_sym occur at 827–816 cm⁻¹ in IR_KBr, 826–813 cm⁻¹ in IR_ATR, 826–815 cm⁻¹ in Raman. Asymmetric in-plane deformation of the carboxylic anion was not observed in the 2,6-PDA salts.

The bands derived from aromatic ring vibrations of the studied acids and their salts with alkali metals are present in the entire spectral range. As compared to the spectra of acids, in the spectra of the salts a decrease in the wave numbers can be seen and the intensity of vibrations of the aromatic system is reduced. A number of bands present in the acid spectra disappear in the salts upon the metal ion substitution at the carboxyl group and as well some additional bands emerge that aren’t present in the spectra of the ligands. Based on the comparison of the wave numbers and intensities of the aromatic ring vibration bands (vibration of C—C and C—H bonds in the aromatic ring) in the ligand and salts, some conclusions can be drawn on the influence of metal on a disturbance or stabilization of the aromatic ring. The disturbance of this system is indicated by the reduction in the number and intensity of the bands derived from the aromatic system vibrations and/or their shift toward lower wave numbers in the IR and Raman spectra of the complexes compared to the spectrum of a given acid; this is due to the reduction in the force constants and polarization of C—C and C—H bonds in the ring. It was observed that in the IR and Raman spectra more bands present on the ligand disappear in the case of 2,3-PDA salts than it is in the case of 2,6-PDA salts. In addition, salts of 2,6-PDA reveal more new ligand bands than 2,3-PDA salts. The wave numbers of some bands present in the spectra of the salts tested decrease in the series Li → Na → K → Rb → Cs. These changes are not regular as it is observed for the spectra of alkali metal salts with monocarboxy-pyridinecarboxylic acids.

3.2 NMR spectra

Chemical shifts of the signals coming from protons in the ¹H NMR spectra of 2,3- and 2,6-PDA alkali metal salts take lower values than the corresponding ones for acids (Tables 3 and 4). An aromatic pyridine ring is disturbed, resulting in a change in the electron density around the protons of the aromatic ring. The values for the chemical shifts of aromatic protons labeled H3, H4 and H5 in the spectra of acid and its salts 2,6-PDA overlap to give a single band in the spectrum. For 2,3-PDA acid and its salts the three aromatic protons give its spectral image of three clearly separated signals. Decreases in proton chemical shifts for H4 and H5 in 2,3-PDA acid salts are small, and the decreases in proton chemical shifts for H3 are much larger and similar in size as those in 2,6-PDA salts.

The decisive factor influencing the electron charge distribution on the pyridine ring of 2,3-PDA and 2,6-PDA is the distribution of carboxyl groups linked to the ring. In the case of dipicolinic acid, the carboxyl groups are arranged symmetrically with respect to the nitrogen atom, with the consequence that the electronic charge around the carbon atoms is arranged symmetrically in the molecule. This can be observed as the equal values of the chemical shifts of carbons C2⚌C6 and C3⚌C5 in the ¹³C NMR spectrum. In the case of 2,3-PDA, the carboxyl groups are asymmetrically attached to the aromatic ring, thereby the values for the chemical shifts of the carbons are asymmetrically distributed. Substitution of the carboxyl groups with an alkali metal atom induces the changes in the electronic charge distribution (implying the changes in the chemical shifts of the carbon atoms in the observed ¹³C NMR spectra). Changes in the values of the chemical shifts for the carbon atoms in the spectra of 2,6-PDA salts occur in a symmetrical manner, i.e. the same increase in the chemical shifts of carbon C2 and C6, and the same decrease in chemical shifts for carbons C3 and C5 can be observed. In the case of 2,3-PDA salts, changes in the chemical shifts of the carbons (indicating the redistribution of the electronic charge in the molecules) are not symmetrical. In the series of salts of both acids the same direction of changes in chemical shifts is observed, i.e. an increase in the value of the signals for the carbon atoms C2 and C6 (decrease in electron density) and a decline in the value of the signals for C4 and C5 (increase in electron density) are found. Otherwise, for the C3 carbon atom of the 2,3-PDA salts an increase in chemical shifts relative to acid can be seen, whereas for the 2,6-PDA salts chemical shift values decrease. In conclusion, the electronic charge of the pyridine ring in the case of 2,6-PDA is distributed more evenly than in 2,3-PDA. Alkali metals perturb the electronic system to much higher degree, if the alkali metal atom is substituted to the carboxyl groups of 2,3-PDA than to those of 2,6-PDA.

3.3 Thermogravimetric study

Thermogravimetric studies of the 2,3- and 2,6-pyridinecarboxylate alkali metal salts (Tables 5 and 6) showed that the degrees of hydration of the 2,6-PDA alkali metal salts were very similar in the series (Li—Na—K—Rb—Cs). In the case of 2,3-PDA salts, the degree of hydration of each salt was also similar. The salts prior to the studies were dried for 24 h at 70 °C. Dehydration occurs in a single step for the salts tested (all hydrated salts). For the 2,6-PDA salts the dehydration process begins at 100 °C. Sodium and potassium salts of 2,6-PDA loose water in the temperature range of 100–210 °C, and all other salts in the range of 100–150 °C. The process for the dehydration of the 2,3-PDA salts starts at slightly higher temperatures (above 130 °C). After dehydration, in subsequent steps the process of decomposition of the tested compounds occurs. Both acids are degraded completely in a one-step process of decomposition at about 250 °C. Studies on the decomposition of salts in the range of 70–890 °C showed that the decomposition proceeds via formation of alkali metal carbonates and in a later step in the case of certain salts, to alkali metal oxides. In the first step in a temperature range from ca 370 to 500 °C beside carbonates some residues of organic carbon are found as well that are formed during the decomposition of the aromatic ring. This can be seen as a pitch curve on TG and DTA graphs (Fig. 3). In a further step of the decomposition process the organic carbon residues are oxidized to carbon dioxide and a product in the form of carbonate is formed. In the case of sodium salts the produced carbonates are stable in a range of temperatures, while for the other salts a decomposition of carbonates to the oxides of alkali metals occurs. Comparing the curves illustrating the thermal decomposition of the alkali metal salts of the two studied acids (Fig. 3) one can observe that salts of 2,6-PDA are dehydrated at a slightly lower temperature than the salts of 2,3-PDA. In contrast, the decomposition of the aromatic ring occurs at a slightly lower temperature in the case of 2,3-PDA salts than for those of 2,6-PDA. Based on the decomposition curve, it can be concluded that the 2,3-pyridinedicarboxylates are thermally less stable than the 2,6-pyridinedicarboxylates of the alkali metals.

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Figure 3

TG/DTA curves of 2,3-pyridinedicarboxylic acid (A) and sodium salts (B), 2,6-pyridinedicarboxylic acid (C) and sodium salts (D).

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