Infrared analysis (FTIR) of the TA, as well as the synthesized compounds, were made on a PerkinElmer Spectrum 100 spectrometer, with a resolution of 4 cm^-1, in 4000-600 cm^-1 region, using the Attenuated Total Reflectance (ATR) technique.

Simultaneous TG–DSC curves were obtained on a TG/DSC-1 from Mettler Toledo. The purge gas was air flow with the flow rate of 60 mL min^-1. A heating rate of 20 ºC min^-1 was adopted, with sample weighing about 7 mg in the temperature range of 30-1000 ºC. Alumina crucibles were used for this analysis.

For the synthesized compounds analysis, solutions of TA-Cu (II) and TA-Zn (II) were prepared using the concentrations of 0.16; 0.12; 0.08 and 0.04 g L^-1 using for each set of experiments a different solvent (Ethyl acetate, methanol and ethanol). The solutions were kept for 30 minutes on water-bath under 60ºC, to increase the solubility of the compounds. The resultant solution was transferred to a 25 mL volumetric flask. For each 2.50 mL of sample, 1.00 mL of DPPH solution (60 µg mL^-1) was added.

A negative control was prepared using the same procedure, without the addition of DPPH. This solution was used as a blank and its absorbance values were obtained by triplicate analysis.

A DPPH solution was left resting for 20 minutes, protected from light, to evaluate a possible degradation of the sample over time. The absorbance measures were made at 515 nm for methanol and 517 nm for ethanol and ethyl acetate, for each sample. The measures were realized 31 minutes after the preparation of the sample, and it was repeated after 35, 40, 50, 60, 70, 80 and 90 minutes, after the preparation of the samples [13].

From the analytical curve equation and data obtained from the proposed times for each evaluated concentration, the percentage and concentration of the reminiscent DPPH (%DPPH_REM), were calculated according to equation 1 [13].

(1)

[DPPH]_T=t represents DPPH concentration after the reaction with the sample and [DPPH]_T=0 is the initial concentration of DPPH (60 µL mL^-1) [12].

The absorbance values for all concentrations in 30, 60 and 90 minutes were converted in antioxidant activity % (%AA), according to equation 2 [13, 15Moreira, D.D.L.; Leitão, S.G.; Gonçalves, J.L.S.; Wigg, M.D.; Leitão, G.G. Antioxidant and antiviral properties of Pseudopiptadenia contorta (Leguminosae) and of quebracho (Schinopsis sp.) extracts. Quim. Nova, 2005, 28, 421-425.
[http://dx.doi.org/10.1590/S0100-40422005000300011] ].

(2)

Abs_control represents the initial absorbance of the DPPH solution, and Abs_sample represents the reactional mixture absorbance (DPPH + sample) [13, 15Moreira, D.D.L.; Leitão, S.G.; Gonçalves, J.L.S.; Wigg, M.D.; Leitão, G.G. Antioxidant and antiviral properties of Pseudopiptadenia contorta (Leguminosae) and of quebracho (Schinopsis sp.) extracts. Quim. Nova, 2005, 28, 421-425.
[http://dx.doi.org/10.1590/S0100-40422005000300011] ].

The FTIR spectra were made to characterize the compounds of TA-Cu (II) and TA-Zn (II). The obtained spectra were suggesting some vibrational bands of the compounds, and they allow to evaluate the types of interactions between the metal ion and TA.

The FTIR spectrum of the TA, (Fig. 1), indicate that the main vibrational bands on TA are: C=O, OH, C-O, H-C and C=C stretch, as well as C-O-H in-plane and out-of-plane bending. A wide band between 2870-3670 cm^-1 was assigned to O-H stretch from hydrogen bonds. The in-plane bending of C-O-H group occurs at 1610-1535 cm^-1, and the out-of-plane bending was assigned to 872 cm^-1. The characteristic peak at 1708 cm^-1 was assigned to the Carbonyl vibrations of TA. The attributions of the vibrational bands of TA agree with the literature [10Çakar, S.; Özacar, M. Fe-tannic acid complex dye as photosensitizer for different morphological ZnO based DSSCs, Spectrochim. Acta - Part A Mol. Biomol. Spectrosc., 2016, 163, 79-88.
[http://dx.doi.org/10.1016/j.saa.2016.03.031] [PMID: 27043871] ].

The FTIR of the TA-Cu (II) and TA-Zn (II) in 1:1 and 1:5 ratios, in buffered pH of 5.5 and 8.0 are shown in Fig. (2).

For all FTIR spectra of TA-Cu (II) shown in Fig. (2), were observed a significant shift in the bands assigned to C=O stretching and O-H in-plane and out-of-plane bending. The C=O stretch shifted to 1567 cm^-1 (A₁), 1591 cm^-1 (A₂), 1583 cm^-1 (A₃) and 1594 cm^-1 (A₄). The C-O-H in-plane bending shifted to 1486-1423 cm^-1 (A₁), 1400-1309 cm^-1 (A₂), 1423-1345 cm^-1 (A₃) and 1425-1300 cm^-1 (A₄), and the C-O-H out-of-plane bending to 877 cm^-1 of the TA to 751 cm^-1 (A₁), 826 cm^-1 (A₂), 690 cm^-1 (A₃) and 823 cm^-1 (A₄).

Fig. (1) FTIR spectrum from TA. Where A: O-H stretching, B C=O stretching, C: O-H in-plane bending e D O-H out-of-plane bending.

Fig. (2) FTIR spectra of the synthesized compounds in buffered media. TA: Tannic acid; A1: TA-Cu(II), pH 5,5, in 1:1 ratio; A2: TA-Cu(II), pH 5,5 in 1:5 ratio; A3: TA-Cu(II), pH 8,0 in 1:1 ratio; A4: TA-Cu(II), pH 8,0 in 1:5 ratio; A5: TA-Zn(II), pH 5,5, in 1:1 ratio; A6: TA-Zn(II), pH 5,5 in 1:5 ratio; A7: TA-Zn(II), pH 8,0 in 1:1 ratio and A8: TA-Zn(II), pH 8,0 in 1:5 ratio.

The evaluation of the C=O stretch indicates a direct rate of its intensity with the amount of Cu (II) added in buffered pH of 5.5 and an inverse proportional in buffered media of pH 8.0 causing a band with high intensity in A₃.

The bands assigned to the in-plane bending were shifted when more metal ions were added. A reduction in the band related to the O-H stretch is observed, suggesting an attenuation of the strength of the intermolecular hydrogen bonds with water. These shifts indicate a new interaction of oxygen of the TA with Cu (II).

In TA-Zn (II) compounds in buffered pH of 5.5, in 1:1 and 1:5 ratio (A₅ and A₆), only the C-O-H in-plane bending (1579-1483 cm^-1 for A₅ and 1564-1483 cm^-1 for A₆) and out-of-plane bending, (826 cm^-1 in A₅ and A₆ compounds) bands were shifted. Differently, the same compounds in buffered pH of 8 (A₇ and A₈), showed that the C=O stretch when compared with the FTIR spectrum of TA, shifted from 1696 cm^-1 to 1549 cm^-1 (A₇) and 1597 cm^-1 (A₈), in-plane bending of C-O-H shifted to 1477-1390 cm^-1 (A₇) and 1477-1342 cm^-1 (A₈). Out-of-plane bending in C-O-H takes on wavenumber at 829 cm^-1 (A₇) and 690 cm^-1 (A₈). For this compound, using 1:1 ratio (A₇) it was observed an increase in the relative intensity of the C=O stretching and A₈ a higher intensity in O-H stretching region in 3670-2870 cm^-1.

The TA and the synthesized compounds FTIR spectra suggest coordination between the ligand and the metal ion. Distinct alterations are observed in the region assigned to the carbonyl group which was shifted to a lower wavenumber in the synthesized compound in comparison with the TA. This observation can indicate a coordination metal-TA occurs between metal ion and oxygen of the carbonyl group and hydroxyl groups.

The TG-DSC was used to evaluate the stoichiometric ratio of the metal ion to the TA, as well as the water content and the thermal behavior of the compounds TA-Cu (II), TA-Zn (II) and TA.

The simultaneous TG-DSC curves of TA are shown in Fig. (3). These curves show 4 steps of mass loss, with its thermal events shown in the DSC curve. The first mass loss occurs between 20 and 210 ºC, corresponding with the endothermic peak at 110 ºC, and assigned to the thermal dehydration of 11 molecules of water (Δm_TG=10.52%; Δm_Calcd.=10.43%). The second step of thermal decomposition corresponds to the dehydroxylation of TA between 210-266 ºC (Δm_TG=23.50%; Δm_Calcd.=22.37%). The third and fourth steps of mass loss are assigned to the oxidative degradation of TA between 266-330 °C and 330-640 °C (Δm_TG=65.98%; Δm_Calcd.=66.20%), with two exotherms between 420-590 °C and an exothermic peak at 640 ºC.

Fig. (3) Simultaneous TG-DSC curve of TA.

The thermoanalytical results for the synthesized compounds are shown in Table 1. The results suggest that the stoichiometry of the compounds are according to the general formula (M)_xL_n^.H₂O, where M is the metal ion, L is TA and n is the hydration water number. The stoichiometry of these compounds shows that the relationship M-L of the precipitate is not related to the initial concentration of the metal solution, but is related to a maximum amount of metal for each ligand molecule.

Table 1
Thermoanalytical data from the Cu(II) and Zn(II) compounds in buffered pH of 5.5 and 8.

In comparison with the literature results, it can be observed that the buffered system (pH = 5.5 and 8.0) helped in the formation of zinc compounds, which was not obtained in other papers, due to the formation of zinc hydroxide [10Çakar, S.; Özacar, M. Fe-tannic acid complex dye as photosensitizer for different morphological ZnO based DSSCs, Spectrochim. Acta - Part A Mol. Biomol. Spectrosc., 2016, 163, 79-88.
[http://dx.doi.org/10.1016/j.saa.2016.03.031] [PMID: 27043871] ].

Although the proportions of 1:1 and 1:5 were controlled in the experiments, a stoichiometric relation up to 5 times was observed for zinc compounds and up to 10 times in copper compounds in comparison with the pre-established quantities. This fact is due to the presence of 10 carboxylic and 20 hydroxyl groups in each TA, increasing the coordination sites of TA.

The increase of the pH improved the stoichiometric relationship between zinc and TA, on the other hand the same relationship decreased for copper compounds, as the pre-established relationship in pH 8.0, maintained its values close to the ones obtained for other situations, indicating that the copper interaction with TA is stronger than the other studied compounds. The TA dehydroxylation in metal-compounds could not be calculated, due to the presumable interaction with the hydroxyl group with the metal, which increases its thermal stability and causes this event to overlap with the oxidative degradation of the remaining TA molecule.

Each compound has shown a very characteristic TG – DSC profile. Although final temperatures of the thermal decomposition of the compounds have been different, the final residue formed was the same, as indicated in Table 2 and Fig. (4).

Table 2
Thermal decomposition results of AT-Cu (II) and AT-Zn (II) compounds obtained at pH 5.5 and 8.

The A₁ compound has shown the molecular formula of Cu_10.5L^.15H₂O and has shown that the thermal decomposition occurs in four consecutive steps (Table 2). DSC curve showed three exothermic peaks at 235 °C, 293 °C and 344 °C relative to TA oxidation.

The A₂ compound had its molecular formula determined as Cu_9.5L^.11H₂O, and it has shown a similar thermal decomposition in comparison with A₁. The thermal decomposition of A₂ shows the same number of steps of A₁ but final temperature higher than A₁, besides an exothermic peak at 450 ºC which indicates the oxidation of the organic matter.

The A₃ has the molecular formula of Cu_7.5L^.20H₂O, has shown that the thermal decomposition occurs consecutively in five steps of mass losses with an exothermic peak at 515 ºC. The compound synthesized in buffered pH of 8.0 (A₄) with the molecular formula of Cu_5.5L^.10H₂O had its thermal decomposition in five steps more defined in comparison with the compound synthesized at buffered pH of 5.5. The DSC curve of A₄ was observed four exothermic peaks at 208 ºC, 424 ºC, 505 ºC and 630 ºC.

Fig. (4) Simultaneous TG-DSC curves of TA-Cu(II) and TA- Zn(II).

The TA-Zn compound with 1:1 ratio and buffered pH of 5.5 (A₅) had its molecular formula determined as Zn_6.5L^.15H₂O and had its thermal decomposition in three consecutive steps of mass loss with an exothermic peak at 290 ºC and an exotherm between 400-490 ºC, indicating the oxidation of the organic matter. The A₆ presented the molecular formula of Zn₈L^.9.5H₂O and had shown a different behavior, with its thermal decomposition in 6 steps of mass loss with exothermic peaks at 140 °C, 335 °C and 390 °C and two endothermic peaks at 332 °C, 385 ºC and 476 °C.

The TA-Zn compound with 1:5 ratio and buffered pH of 5.5 (A₇) with the molecular formula of Zn₁₄L^.21.H₂O had its thermal decomposition in three steps of mass loss with an endothermic peak at 219 ºC and two exothermic peaks at 391 ºC and 391 ºC. This compound synthesized in buffered pH of 8.0, with the molecular formula of Zn₂₅L^.28.5H₂O, had its thermal decomposition in four steps of mass loss with an endothermic peak at 160 ºC.

The interpretation of each thermal event in these curves is shown in Table 2.

The dehydration are related to the elimination of hydration water, decomposition and oxidative degradation are observed in Table 2. The dehydroxylation/thermal decomposition and oxidative burn are related to the organic matter in TA.