Introduction

4-Imidazolecarboxylic acid is a derivative of imidazole which is of great biological importance ligand. Imidazole is a structural unit of constructive histidine, histamine, and it is a part of purines which are molecules showing important biological functions in the human body. Some of the imidazole derivatives possess anti-inflammatory, antituberculotic, antimicrobial, antimycotic, antiviral and even antitumor activity [1,2,3,4,5,6,7]. Tinidazole, ornidazole and metronidazole are known as antimicrobials and antiparasites since the mid-1960s [8]. Imidazole and its derivatives are used as fungicides due to their ability to inhibit the synthesis of ergosterol and the fungal cell membrane component, and to interfere with the synthesis of triglycerides and phospholipids [9].

The imidazole derivatives easily coordinate 3d-metal ions and are examined for their medical use [10, 11]. Many crystalline structures of 3d-metals complexes with derivatives of 4-imidazolecarboxylic acid have been described in the literature. A nitrogen atom of the imidazole ring participate in the coordination of metal by ligand (e.g., nitroimidazole complexes with copper, cobalt and zinc [12], 4-imidazoleacetate with manganese [13], 5-methyl-imidazolecarboxylate with copper [14], cadmium, nickel and cobalt [15], or complexes of 4,5-imidazoledicarboxylic acid with 3d metals [16, 17]). When the nitrogen atom is in close proximity to the carboxylic group, the metal is coordinated through nitrogen atom and carboxylic group. The five-membered chelate rings are then formed [16,17,18,19,20]. The chelate rings exert a stabilizing effect on the structure of these complexes [19, 20].

Transition metal complexes with imidazolecarboxylic acid derivatives were also tested for thermal stability. While manganese, cobalt and nickel complexes with 4,5-imidazoledicarboxylic acid decompose comparatively at a lower temperature (around 450 °C), zinc and cadmium complexes show a higher temperature of decomposition (around 620 °C) to give the end products [17].

Jabłońska-Wawrzecka and others investigated selected transition bivalent metal complexes with 5-carbaldehyde-4-methylimidazole. In the case of the complexes with Co(II), Mn(II), Cd(II) and Zn(II), the decomposition process was studied in nitrogen atmosphere, whereas for Ni(II) complex, the process was conducted in argon. The decomposition process occurred in three main stages, except for Zn(II) complex (two stages of decomposition were shown) in temperature range of 353–1163 K. The final products of decomposition process were CoO, MnO, Cd, ZnN4, NiN3 [21].

In our earlier work, we studied the structure and thermal properties of metal complexes with nitrogen-containing heteroaromatic acids, i.e., pyridinecarboxylic and pyrazinecarboxylic acids [22, 23]. In this work, the thermal properties of 4-imidazolecarboxylic acid complexes have been investigated. The structure of synthesized complexes was determined by means of infrared, Raman and UV–Vis spectroscopy and theoretical calculations.

Experimental

Sample preparation

The sodium salt of 4-imidazolecarboxylate was prepared by dissolving the appropriate weighed amount of acid (1 mmol) in aqueous solution of sodium hydroxide (10 mL, 0.1 mol L−1) in a stoichiometric molar ratio ligand/metal − 1:1. The reaction was carried out in a water bath at 70 °C. The zinc complex of 4-imidazolecarboxylic acid was received by addition the appropriate amount of an aqueous solution of zinc chloride (5 mL, 5 × 10−4 mol L−1) to a pre-prepared solution of sodium salt of 4-imidazolecarboxylic acid (containing 1 mmol of sodium salt). Then, solution was shaken on a thermostatic shaker at 70 °C for 1 h. In this mixture, the molar ratio of sodium salt of 4-imidazolecarboxylic acid to metal ion was 2:1. The solutions were left for 24 h at room temperature. The precipitates were then filtered, and the solution was rinsed with deionized water until all chloride ions were washed out. The solid substances have been dried for 24 h in an oven at 30 °C. For each complex (Mn, Co, Ni, Cu), the procedure was the same. The iron (III) complex was obtained by adding the solution iron (III) chloride (3.33 mL, 5 × 10−4 mol L−1) to a pre-prepared solution of sodium salt of 4-imidazolecarboxylic acid (containing 1 mmol of sodium salt). The further procedure was analogous to the synthesis of the other complexes. The yield of the synthesis process was about 60–70%.

Methods

The products of dehydration and decomposition processes have been determined from the TG curves. Thermogravimetric analysis (TGA) was performed using a Mettler Toledo Star TGA/DSC1 unit. The process was conducted in an inert gas atmosphere (argon) and in an oxygen atmosphere. Samples (2–4 mg) were placed in aluminum pans and heated from 50 to 850 °C with a heating rate of 10 °C min−1. The TG, DTG and DSC curves were registered. The elemental analysis was carried out using the CHN 2400 Perkin Elmer Analyzer.

The FT-IR spectra were recorded with an Alfa (Bruker) spectrometer within the range of 400–4000 cm−1. Samples were measured in KBr matrix pellets and by the use of ATR technique. FT-Raman spectra of solid samples were recorded in the range of 400–4000 cm−1 with a MultiRam (Bruker) spectrometer. To calculate optimized geometrical structures of 4-imidazolecarboxylic acid, sodium salts, copper complex and cobalt complex, the quantum–mechanical method were used: density functional (DFT) hybrid method B3LYP with non-local correlation provided by Lee–Young–Parr expression. All calculations were carried out in a functional base 6-311++G(d,p). Calculations were performed using the Gaussian 09 package [24]. Experimental spectra were interpreted in terms of the calculated ones at DFT method in B3LYP/6-311++G(d,p) level and on the basis of the literature data [25, 26].

Theoretical wavenumbers were scaled according to the formula: νscaled = 0.98·νcalculated for B3LYP/6-311++G(d,p) level method [27]. Spectral assignments used the normal oscillation of the imidazole ring measured using DFT.

Results

IR and Raman spectra

The wavenumbers, intensities and assignments for the bands occurring in the vibrational spectra of the studied 4-imidazolecarboxylic acid and 4-imidazolecarboxylates were presented in Table 1. The normal vibrations for imidazole ring calculated at the DFT level are shown in Fig. 1. The spectrum assignment was based on literature data [25, 26] and theoretical calculations for 4-imidazolecarboxylic acid, its sodium salt, copper and cobalt complex. The monomer of 4-imidazolecarboxylic acid may be present in two conformations: s-cis (syn) and s-trans (anti) (Fig. 2). The more stable one is the anti-monomer.

Table 1 Wavenumbers/cm−1, intensities and assignments of bands occurring in the IR (KBr and DFT) and Raman spectra of 4-imidazolecarboxylic acid and sodium, manganese, nickel, cobalt, copper, zinc, iron 4-imidazolecarboxylates
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Fig. 1
figure 1

Normal modes of vibrations for imidazole (calculated B3LYP/6-311++G(d,p))

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Fig. 2
figure 2

The structures of 4-imidazolecarboxylic acid monomer (syn and anti conformers)

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In the spectra of 4-imidazolecarboxylic acid the characteristic bands derived from the vibrations of the carboxyl group νC=O at 1620 (in the IR spectra) and 1621 cm−1 (in Raman spectra) occurred. The replacement of the carboxylic group hydrogen with a metal ion brought about characteristic changes in the IR and Raman spectra of the metal 4-imidazolecarboxylates in comparison with the spectra of ligand. One can observe disappearance of bands of the symmetric and asymmetric valence vibrations of carboxylic group and appearance of bands of the asymmetric and symmetric vibrations of the carboxylate group νasym(COO), νsym(COO), as well as βasym(COO−), βsym(COO−), and disappearance or changes in positions and intensities of some aromatic bands (Table 1, Fig. 3).

Fig. 3
figure 3

IR spectra for 4-imidazolecarboxylic acid and their 3d-metal complexes

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In the IR spectra of 4-imidazolecarboxylates, the wavenumbers of asymmetric (νasymCOO−) and symmetric (νsymCOO−) stretching vibrations of carboxylate occur in the range of 1550–1637 and 1369–1429 cm−1, respectively (see Tables 1, 2). Asymmetric and symmetric deformation vibrations of the carboxylate group (βasymCOO−) and (βsymCOO−) are located, respectively, in the range of 483–531 and 817–851 cm−1 in the IR spectra of 4-imidazolecarboxylates.

Table 2 Spectral parameters (vibrations stretching the carboxylate anion) from the IR spectra of 4-imidazolecarboxylates and proposed type of coordination the metal to carboxylic group of the ligand
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On the basis of the differences between the wavenumbers of the asymmetric and symmetric stretching vibrations of the carboxylate group COO−, some assumption about the type of metal–ligand coordination might be stated [28, 29]. For the monodentate geometry, the bands for (νasymCOO−) and (νsymCOO−) in the complex are shifted to higher and lower wavenumbers, respectively, or (ΔνCOO−)studied complex ≫ (ΔνCOO−)sodium salt. This type of coordination has been observed for all complexes (Table 2).

In the aromatic ring of imidazole, the nitrogen-containing free electrons may be involved in coordinating the metal. In the complexes of manganese, iron, cobalt, copper, nickel and zinc, there is a monodentate type of coordination through the carboxylate anion. The metal is simultaneously coordinated by the ligand through the nitrogen atom (Figs. 4, 5). The changes in the wavenumbers of the band assigned to the vibrations of the nitrogen–carbon prove the engagement of nitrogen in metal coordination.

Fig. 4
figure 4

Theoretical structure of copper 4-imidazolecarboxylate (calculated in B3LYP/6-311++G(p,d))

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Fig. 5
figure 5

Theoretical structure of cobalt 4-imidazolecarboxylate (calculated in B3LYP/6-311++G(p,d))

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The coordination of metal through the nitrogen atom of the imidazole ring causes significant shift or disappearance of bands assigned to the C=N vibrations (bands 6, 7 and 8) from the IR spectra of complexes compared to the spectrum of ligand (Table 1, Fig. 3). For example, the band no. 7 originated from the CN stretching vibrations is shifted from 1446 (the IR spectra of acid) to 1456 cm−1 in the IR spectra of zinc complex, 1463 cm−1 in the IR spectra of cobalt, 1449 cm−1 in the IR spectra of copper and 1462 cm−1 in the IR spectra of iron. In the case of manganese and nickel complexes, this band disappears. The band no. 8 derived from the CN stretching vibrations is shifted from 1372 (in the spectra of acid) to 1333 cm−1 in the IR spectra of zinc complex, 1320 cm−1 in the IR spectra of manganese, 1336 cm−1 in the IR spectra of nickel, 1334 cm−1 in the IR spectra of copper and 1328 cm−1 in the IR spectra of iron. The intensity of this band in the IR spectra of complexes decreases in relation to the IR spectra acid. Many different 4-imidazolecarboxylic acid derivatives have coordinated the metals in analogous way [12, 13, 16, 17].

The structures and infrared vibration wavenumbers for the anhydrous copper complex (Fig. 4) and the cobalt dihydrate complex (Fig. 5) were calculated (DFT/B3LYP/6-311++) G(p,d). (The degree of hydration is in accordance with the thermal and elemental analysis). The calculations were used to detailed assignment of the infrared vibrational bands recorded for the studied complexes. When comparing the spectra of 4-imidazolecarboxylic acid with sodium and 3d-metal 4-imidazolecarboxylates, it has been observed the lower number of bands in the spectra of sodium salt compares the spectra of ligand, whereas in the spectra of the 3d metal complexes, the number of band is higher than in the IR spectra of ligand (Table 1). This means that sodium destabilizes the electronic charge distribution in the aromatic ring of 4-imidazolecarboxylic acid, while the 3d metals can have a stabilizing effect on the distribution of this charge. Analogous observations have been made for other ligands (aromatic and heteroaromatic acids)—the alkali metals disturb the aromatic system of ligands, while the 3d metals stabilize the system [22, 23, 30, 31].

A detailed analysis has shown that some of the bands present in the spectrum of acid are shifted including the following ones: 6, αring (at about 1239 cm−1), 10, 11, 12, 13, 15, 16, 17, 19, φring (633 cm−1). A few bands were shifted toward lower values of the wavenumbers in the spectra of the complexes as compared to the acid spectrum. These bands are marked with numbers: 5, 8, 14, 18, 20.

The distribution of the electronic charge in the aromatic ring of the ligand depends on the nature of the metal–ligand bonding. In the case of ionic bonds formed between the ligand and alkali metals, the electronic system in ligand is disturbed. In transition metal complexes, the delocalized covalent bonds are formed between the metal and the ligand. This promotes the delocalization of the electronic charge in the aromatic ring and stabilization of the system. The delocalization of electrons in the central ion has also stabilizing effect on the aromaticity of ligand. A similar effect is observed in the lanthanide complexes [32, 33].

UV study

In the UV spectra of 4-imidazolecarboxylic acid, one maximum absorbance at 210 nm occurred in the range of 190–300 nm. In the spectra of the obtained transition metal complexes with 4-imidazolecarboxylic acid, the maximum of absorbance was observed at 225 (manganese 4-imidazolecarboxylate), 222 (nickel and zinc 4-imidazolecarboxylates), 220 (copper and cobalt 4-imidazolecarboxylates) and 209 nm (iron 4-imidazolecarboxylate). Slight hypochromic effect was observed (Fig. 6).

Fig. 6
figure 6

UV spectra for 4-imidazolecarboxylic acid and 3d-metal 4-imidazolecarboxylates

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The bands observed in the UV–Vis spectra correspond to the electron transitions π → π*. In the spectra of 4-imidazolecarboxylic acid complexes, bathochromic effect was observed (except for the iron complex). This demonstrates that under the influence of 3d metal the electronic system of ligand is stabilized (the aromaticity increases) [30, 31].

Thermal and elemental analysis

Table 3 shows the results of elemental analysis of synthesized complexes, color of complexes and percent yield of synthesis. The complexes of 4-imidazolecarboxylic acid with transition metal ions have the general formula: M(C3N2H3COO)x·nH2O where M = Mn, Fe, Co, Ni, Cu, Zn, x = 3 for iron complex, x = 2 for other complex and n= 0–3. Manganese complex was white similarly as the ligand; other complexes were colorful. Synthesis was carried out several times to optimize the synthesis and give reaction yield in the range of 70–85%.

Table 3 Elemental analysis for manganese, iron, cobalt, nickel, copper and zinc 4-imidazolecarboxylate
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The TG/DTG curves registered in an oxygen atmosphere and argon atmosphere are shown in Fig. 7. The data characteristic for the thermal decomposition of the complexes is collected in Table 4. The thermal decomposition of 4-imidazolecarboxylic acid is a one-step process. The complete decomposition of the ligand occurs at about 270 °C. Generally, the complexes decompose in the two ground stages: the first stage is dehydration of the complexes with the formation of anhydrous compounds, and the second stage is the decomposition of anhydrous compounds connected with oxidation of organic ligands and formation of metal oxides (metal carbonate in the case of decomposition of the manganese complex)—as residues. Similar decomposition products were obtained in the case of thermal decomposition of 3d metal complexes with 4,5-imidazolecarboxylic acid, i.e., metal oxides: CoO, NiO, CuO, ZnO and Fe2O3, and in the case of thermal decomposition of the manganese complex, manganese carbonate MnCO3 was obtained probably due to the partial ionic character of the manganese compound [17]. The anhydrous compounds decompose in one step which corresponds to the sharp exothermic peaks in DSC curve (Fig. 8). They can be attributed to oxidation of organic matter and the gaseous products that arise during thermal decomposition [22, 23]. If the thermal decomposition is conducted in an inert gas atmosphere (argon), other end products are created.

Fig. 7
figure 7

TG/DTG curves of 4-imidazolecarboxylic acid and 4-imidazolecarboxylates (in oxygen atmosphere and argon atmosphere)

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Table 4 Thermoanalytical results (TG, DTG, DSC) for complexes of 4-imidazolecarboxylates (process in oxygen atmosphere)
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Fig. 8
figure 8

DSC curves of 4-imidazolecarboxylic acid and 4-imidazolecarboxylates (in oxygen atmosphere and argon atmosphere)

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Thermogravimetric studies and elemental analysis showed that the complexes tested were hydrated (except for the copper complex). The degree of hydration was—one particle of water in the zinc and iron complexes, two water molecules in the cobalt complex and three molecules in the manganese and nickel complex. In the first stage, the endothermic processes of dehydration of complexes occur in only one step (for the hydrated complexes). For majority complexes, dehydration temperatures are higher than 100 °C, what can point to the coordination character of water molecules.

The endothermic process of dehydration at low temperature with the onset temperatures of water evaporation at 90 °C was observed for manganese and iron complexes. This value shows the outer-sphere character of water molecules in the structure of manganese and iron complex. Zinc complex loses one molecule water in the temperature range 170–200 °C; cobalt complex loses two molecules water in 180–200 °C. Iron complex loses one molecule water in the temperature range 100–150 °C; nickel complex lose three molecules water in 120–180 °C. The manganese complex loses water in two stages. In the first stage, it loses one molecule of water in the temperature range of 90–150 °C; in the second stage it loses two molecules of water in the range of 170–270 °C. In the second stage of thermal decomposition (conducted in oxygen atmosphere), the anhydrous complexes are decomposed to the corresponding metal oxides (metal carbonate in case of decomposition of the manganese complex). During this process, decomposition products such as carbon monoxide and carbon dioxide and degradation products of the imidazole ring are released.

The TG and DTG curves of manganese complex are shown Fig. 7b. The DSC curves are shown in Fig. 8. MnL2·3H2O decomposes (in oxygen atmosphere) in three stages in temperature range of 90–390 °C. In a first and second step, followed by dehydration of the complex, the mass loss is 6.5 and 16.50% (calculated 6.10, 16.31%) for the first and second stage of dehydration, respectively. In the third stage of thermal decomposition, the imidazole ring and the carboxylate group are decomposed (temperature range is 280–390 °C). Gaseous decomposition products are released and the final product is MnCO3 manganese carbonate. The mass loss is 66.92% (calculated 66.50%). The final product is formed above 390 °C. Thermal decomposition of the manganese complex (in argon atmosphere) takes place in several stages. The final product probably is manganese carbonate and residual carbon from the decomposition of carboxylic group.

The TG and DTG curves of CoL2·2H2O complex are shown in Fig. 7d. DSC curves are shown in Fig. 8. The thermal decomposition of the cobalt complex occurs in three stages. In the first stage, dehydration takes place, whereby two molecules of water are detached. This process takes place between 170 and 200 °C. The mass loss is 10.57% (calculated 11.35%). At the next stage of thermal decomposition (at 255–370 °C), the imidazole ring disintegrates. (The mass loss is 54.73%.) The final stage is the thermal decarboxylation (375–420 °C) leading to the final product—CoO cobalt oxide (mass loss 75.41%—measured, 76.37—calculated).

The decomposition of the cobalt complex in argon atmosphere also occurs in several stages (Fig. 7d). After the dehydration step, the imidazole ring disintegrates and the decomposition takes place in subsequent steps. The final product of the thermal decomposition carried out at temperature higher than 850 °C is cobalt oxide and carbon residue, which has not been burned.

The TG and DTG curves of NiL2·3H2O complex are shown in Fig. 7e. DSC curves are shown in Fig. 8. The thermal decomposition of the nickel complex also takes place in three stages. In the first stage, dehydration takes place at 120–180 °C (mass loss 16.62%—measured, 16.13—calculated). In the second stage, the thermal decomposition of the imidazole ring (240–365 °C) (mass loss 54.50%) occurs. In the third stage of the thermal decomposition, the final product—nickel oxide NiO (370–420 °C) (mass loss 77.72%—measured, 77.69—calculated)—occurs. The thermal decomposition process carried out in an inert gas atmosphere takes place in several stages. These are: dehydration, thermal decomposition of the imidazole ring, decomposition of carbonates. The final product of the process (about 850 °C) is nickel oxide and carbon residue.

The TG and DTG curves of FeL3·H2O are shown in Fig. 7c. DSC curves are shown in Fig. 8. In the first stage of the thermal decomposition of the iron complex (in oxygen atmosphere), the dehydration occurs. At a temperature of 100–150 °C, one molecule of the water is disconnected (mass loss 5.65%—measured, 4.14—calculated). In the second, final product Fe2O3 (iron oxide) (295–315 °C) is formed. The mass loss in this process is 78.95%—measured, 80.25%—calculated. The thermal decomposition of the iron complex is carried out in argon atmosphere in several stages. After thermal dehydration (above 400 °C), there is a multistage decomposition of the complex. The thermal decomposition of the iron complex leads to the same products (Fe2O3) when heated in an aerobic and anaerobic atmosphere. The TG and DTG curves of CuL2 are shown in Fig. 7f. DSC curves are shown in Fig. 8. The anhydrous copper complex undergoes thermal decomposition in a one step. The final product of copper complex decomposition is copper oxide CuO. (This product is formed in the temperature range 330–370 °C.) The mass loss in this process is 72.10%—measured, 72.16%—calculated. Heating the copper complex under an argon atmosphere leads to the formation of CuO copper oxide and carbon residue. The TG and DTG curves of ZnL2·H2O are shown in Fig. 7g. DSC curves are shown in Fig. 8. The zinc complex undergoes thermal decomposition (in oxygen atmosphere) in several stages. In the first stage, thermal dehydration occurs (170–200 °C) (mass loss 5.89%—measured, 6.72—calculated). In subsequent steps, the thermal decomposition of the imidazole ring and subsequent decomposition of the zinc carbonate with the release of carbon oxides are followed. The final product of the process is zinc oxide. (This product is formed in the temperature range 540–580 °C) (mass loss 73.37%—measured, 74.11—calculated.) During the thermal decomposition of the zinc complex in an argon atmosphere, dehydration occurs in the first stage. The next step is the thermal decomposition of the imidazole ring. The final product is zinc carbonate.

The most thermally stable complex is the iron complex. The decomposition process of iron 4-imidazolecarboxylate starts above 275 °C. The least stable is manganese complex. (The decomposition starts above 180 °C).

Conclusions

  1. 1.

    The thermal stability of selected 3d transitional complexes (with manganese, iron, cobalt, nickel, copper and zinc) was compared. The study was conducted in an oxygen atmosphere and in an inert gas atmosphere (in argon). Composition of complexes and their structure (type of the ligand–metal coordination) was determined using spectroscopic methods (infrared and Raman spectroscopy). A detailed analysis of infrared spectra and UV spectra of the complexes in aqueous solutions was performed to determine the effect of metals on the change in the electronic charge distribution of the ligand under complexation. Theoretical calculations for imidazole, 4-imidazolecarboxylic acid, its sodium salt, copper and cobalt complexes have been performed for the assignment and analysis of the infrared and Raman spectra of studied compounds.

  2. 2.

    Based on the spectroscopic studies, the ligand (4-imidazolecarboxylic acid) has been found to coordinate the metal by the carboxylate group (monodentate type of coordination) and the nitrogen atom of imidazole ring.

  3. 3.

    On the basis of spectroscopic studies (IR, Raman) and theoretical calculations, it was found that 3d-transition metals influence the electronic charge distribution of the aromatic ring of ligand. Under the influence of the 3d metals, the aromaticity of 4-imidazolecarboxylic acid ligand increases.

  4. 4.

    Based on the elemental and thermal analysis, it was found that the complexes were hydrated (except for the copper complex).

  5. 5.

    The products of thermal decomposition of investigated complexes conducted in the oxygen atmosphere at 50–850 °C are metal oxides. (In the case of thermal decomposition of the manganese complex, manganese carbonate MnCO3 was obtained). In the case of an anaerobic process of the thermal decomposition, the final products can be metal oxides and unburned carbon residues.

  6. 6.

    The most thermally stable complex is the iron complex. The process of its decomposition begins above 275 °C. The least stable is manganese complex. (The decomposition starts above 180 °C.)