Cobalt–Imidazole Complexes: Effect of Anion Nature on Thermochemical Properties

A solvent-free method was proposed for the synthesis of hexaimidazolecobalt(II) nitrate and perchlorate complexes—[Co(C3H4N2)6](NO3)2 and [Co(C3H4N2)6](ClO4)2—by adding cobalt salts to melted imidazole. The composition, charge state of the metal, and the structure of the resulting complexes were confirmed by elemental analysis, XPS, IR spectroscopy, and XRD. The study of the thermochemical properties of the synthesized complexes showed that [Co(C3H4N2)6](NO3)2 and [Co(C3H4N2)6](ClO4)2 are thermally stable up to 150 and 170 °C, respectively. When the critical temperature of thermal decomposition is reached, oxidative two-stage gasification is observed. In this case, the organic component of the [Co(C3H4N2)6](NO3)2 complex undergoes almost complete gasification to form Co3O4 with a slight admixture of CoO, which makes it attractive as a component of gas-generation compositions, like airbags.


Introduction
To obtain a large volume of gaseous products within a short time, gas-generation compositions are used.They are in demand in fire extinguishing [1], autonomous systems for lifting sunken objects [2], car airbags [3], and systems for intensifying oil production, especially highly viscous types [4].The required components of gas-generation compositions are an oxidizer, a binder, and an energy additive (fuel).However, they can interact with each other during storage, leading to degradation in their performance.From this point of view, complex compounds of transition metals, which contain high-energy ligands and oxidizing anions, are more attractive.In addition, the metal can act as a catalyst, accelerating the thermal decomposition of the complex.
Currently, a significant number of metal-organic complexes of transition metals have been synthesized, but to be used in gas generators, they should meet strict requirements, including thermal stability, complete gasification, and resistance to external influences, and should not be transformed under the action of atmospheric oxygen or water vapor.Moreover, their synthesis should be simple, low-waste, and without the use of expensive equipment.The simplest way is the synthesis of metal-organic compounds without solvents by mixing solid metal salts with good chelating ligands [5], for example, ball mill use [6].However, it is difficult to achieve phase homogeneity, which reduces the yield of metal-organic compounds and requires further purification of the target product from The charge state of cobalt in the synthesized complexes with imidazole was determined using XPS.It can be seen from the XPS spectrum (Figure 1) that the binding energies of Co2p 3/2 were 781.4 and 781.3 eV for Co(C 3 H 4 N 2 ) 6 ](NO 3 ) 2 and [Co(C 3 H 4 N 2 ) 6 ](ClO 4 ) 2 , respectively.These values are very close to those for cobalt complexes, where the Co 2+ ion is coordinated by the nitrogen of the imidazole ring [10,11].In addition, there is a maximum of 786.6 eV, corresponding to the satellite of the ionic form of cobalt.The difference between the main peaks and the satellites is an important indicator of the oxidation state of the cobalt ion.A narrow separation of about 4-6 eV is a typical characteristic of Co 2+ , whereas a larger gap of about 9-10 eV is often found in Co 3+ [12].The separation between the main peak and the adjacent satellite peak was about 5 eV (Figure 1).Thus, despite the presence of anion oxidizers in the synthesized complexes, the oxidation of cobalt to Co 3+ does not occur, as was observed in the synthesis of complexes with imidazole [13] and ethylenediamine [14] in aqueous solutions in the presence of atmospheric oxygen.To determine the nature of the coordination between the central atom and the imidazole molecules, cobalt complexes were studied using diffuse reflectance UV-vis spectroscopy.The obtained spectra of the [Co(C3H4N2)6](NO3)2 and [Co(C3H4N2)6](ClO4)2 complexes (Figure 2) were almost identical.Hence, the type of coordination of the cation by the imidazole molecules is the same, despite the different nature of the anion.A detailed analysis of the spectra (Figure 2) showed that two regions of intense absorption could be distinguished in the wavelength range from 700 to 250 nm.Absorption below 450 nm corresponds to π→π* transitions in imidazole.Note that the complexes are characterized by a higher absorption intensity than that of the original imidazole.It is known [15] that a weakening of the hypochromic effect is observed due to a decrease in imidazole molecule interactions between each other, i.e., the imidazole molecules become isolated upon the complex formation.
The second intense absorption at 600-500 nm (Figure 2) can be attributed to d-d transitions of Co 2+ in high-spin octahedral complexes, with three transitions in the UV-vis spectra [16]: υ1 4 T2g---4 T1g 950-1250 nm weak υ2 4 A2g---4 T1g 420-520 nm weak (shoulder) υ3 4 T1g(P)---4 T1g 450-600 nm strong To determine the nature of the coordination between the central atom and the imid azole molecules, cobalt complexes were studied using diffuse reflectance UV-vis spectros copy.The obtained spectra of the [Co(C3H4N2)6](NO3)2 and [Co(C3H4N2)6](ClO4)2 com plexes (Figure 2) were almost identical.Hence, the type of coordination of the cation by the imidazole molecules is the same, despite the different nature of the anion.A detailed analysis of the spectra (Figure 2) showed that two regions of intense ab sorption could be distinguished in the wavelength range from 700 to 250 nm.Absorption below 450 nm corresponds to π→π* transitions in imidazole.Note that the complexes are characterized by a higher absorption intensity than that of the original imidazole.It is known [15] that a weakening of the hypochromic effect is observed due to a decrease in imidazole molecule interactions between each other, i.e., the imidazole molecules become isolated upon the complex formation.
The second intense absorption at 600-500 nm (Figure 2) can be attributed to d-d tran sitions of Co 2+ in high-spin octahedral complexes, with three transitions in the UV-vis spectra [16]: υ1 4 T2g---4 T1g 950-1250 nm weak υ2 4 A2g---4 T1g 420-520 nm weak (shoulder) υ3 4 T1g(P)---4 T1g 450-600 nm strong A detailed analysis of the spectra (Figure 2) showed that two regions of intense absorption could be distinguished in the wavelength range from 700 to 250 nm.Absorption below 450 nm corresponds to π→π* transitions in imidazole.Note that the complexes are characterized by a higher absorption intensity than that of the original imidazole.It is known [15] that a weakening of the hypochromic effect is observed due to a decrease in imidazole molecule interactions between each other, i.e., the imidazole molecules become isolated upon the complex formation.
The second intense absorption at 600-500 nm (Figure 2) can be attributed to d-d transitions of Co 2+ in high-spin octahedral complexes, with three transitions in the UV-vis spectra [16]: υ 1 4 T 2g ---4 T 1g 950-1250 nm weak υ 2 4 A 2g ---4 T 1g 420-520 nm weak (shoulder) The multiplet structure of the absorption in the region of 600-500 nm is due to close energy values for the transitions 4 T 1g (P)---4 T 1g and 4 A 2g ---4 T 1g , where the latter appears as a shoulder, since the υ 3 /υ 2 ratio ranges from 1.9 to 2.2 for octahedral Co 2+ complexes with different ligands [17,18].Summarizing the results of the UV-vis spectroscopy, we can conclude that the Co 2+ cation is in an octahedral environment of imidazole molecules that are isolated from each other.The anion nature in the complex has a negligible effect on the coordination of the central atom with the ligand.
The interaction between the cobalt ions and the imidazole molecules in the [Co(C 3 H 4 N 2 ) 6 ] (NO 3 ) 2 and [Co(C 3 H 4 N 2 ) 6 ](ClO 4 ) 2 complexes was described based on ATR FTIR spectroscopy data.According to Figure 3, the absorption bands of the υ 3 (NO 3 ) and υ 3 (ClO 4 ) modes [19] do not change their position or intensity.So, it can be argued that the anions remain in the outer coordination sphere of cobalt, as in the original salts Co(NO 3 ) 2 The multiplet structure of the absorption in the region of 600-500 nm is due to close energy values for the transitions 4 T1g(P)---4 T1g and 4 A2g---4 T1g, where the latter appears as a shoulder, since the υ3/υ2 ratio ranges from 1.9 to 2.2 for octahedral Co 2+ complexes with different ligands [17,18].Summarizing the results of the UV-vis spectroscopy, we can conclude that the Co 2+ cation is in an octahedral environment of imidazole molecules that are isolated from each other.The anion nature in the complex has a negligible effect on the coordination of the central atom with the ligand.
The interaction between the cobalt ions and the imidazole molecules in the [Co(C3H4N2)6](NO3)2 and [Co(C3H4N2)6](ClO4)2 complexes was described based on ATR FTIR spectroscopy data.According to Figure 3, the absorption bands of the υ3(NO3) and υ3(ClO4) modes [19] do not change their position or intensity.So, it can be argued that the anions remain in the outer coordination sphere of cobalt, as in the original salts Co(NO3)2•6H2O and Co(ClO4)2•6H2O.When forming the nearest cobalt environment, imidazole also does not undergo any structural rearrangements because the characteristic bands of the imidazole ring remain [20].However, the absorption bands of the N-H stretching vibrations are shifted to a higher-frequency region (from 3100 to 3300 cm −1 ).As was demonstrated in [21,22] based on calculation methods, such changes can be explained by the breaking of hydrogen bonds between imidazole molecules.The reason of their formation is the presence of a weakly acidic >N-H group in the imidazole ring and a nitrogen atom (-N=) with a lone electron pair capable of proton attaching.As a result of the complex formation, hydrogen bonds are broken and the N-H group becomes isolated, which shortens the bond length.
The interaction of imidazole with cobalt cations is also confirmed by the spectrum changes in the region of C-N bond vibrations (Figure 3).Indeed, the absorption bands of C-N bond stretching vibrations shift from 1670 to 1610 and 1630 cm −1 for the [Co(C3H4N2)6](NO3)2 and [Co(C3H4N2)6](ClO4)2 complexes, respectively (Figure 3).This indicates an increase in the C-N bond length due to the redistribution of electron density from nitrogen to metal.At the same time, a noticeable increase in the band intensity at 1060 ± 10 cm −1 relative to the other bands of imidazole ring is observed.This absorption band is related to the deformation vibrations of the (C=N-C) bond [20]; therefore, it can be assumed that the intensification of vibrations is due to the weakening of the bond between nitrogen and carbon.
Final confirmation of the formation of [Co(C3H4N2)6](NO3)2 and [Co(C3H4N2)6](ClO4)2 complexes upon the addition of cobalt salts to the imidazole melt in a stoichiometric When forming the nearest cobalt environment, imidazole also does not undergo any structural rearrangements because the characteristic bands of the imidazole ring remain [20].However, the absorption bands of the N-H stretching vibrations are shifted to a higherfrequency region (from 3100 to 3300 cm −1 ).As was demonstrated in [21,22] based on calculation methods, such changes can be explained by the breaking of hydrogen bonds between imidazole molecules.The reason of their formation is the presence of a weakly acidic >N-H group in the imidazole ring and a nitrogen atom (-N=) with a lone electron pair capable of proton attaching.As a result of the complex formation, hydrogen bonds are broken and the N-H group becomes isolated, which shortens the bond length.
The interaction of imidazole with cobalt cations is also confirmed by the spectrum changes in the region of C-N bond vibrations (Figure 3).Indeed, the absorption bands of C-N bond stretching vibrations shift from 1670 to 1610 and 1630 cm −1 for the [Co(C 3 H 4 N 2 ) 6 ](NO 3 ) 2 and [Co(C 3 H 4 N 2 ) 6 ](ClO 4 ) 2 complexes, respectively (Figure 3).This indicates an increase in the C-N bond length due to the redistribution of electron density from nitrogen to metal.At the same time, a noticeable increase in the band intensity at 1060 ± 10 cm −1 relative to the other bands of imidazole ring is observed.This absorption band is related to the deformation vibrations of the (C=N-C) bond [20]; therefore, it can be assumed that the intensification of vibrations is due to the weakening of the bond between nitrogen and carbon.Summarizing the results of the study with several physicochemical methods, we can conclude that imidazole melting with cobalt nitrate or perchlorate produces complexes with a CoN6 coordination environment.In this case, the Co 2+ ion interacts with the pyridine nitrogen atom, and the anions are located in the outer sphere of the cation.Thus, we have proposed in our work a novel solvent-free method for the synthesis of [Co(C3H4N2)6](NO3)2 and [Co(C3H4N2)6](ClO4)2 complexes.

Study of Thermal Decomposition of the Cobalt Complexes
The principle of operation of gas-generation compositions is based on the release of a large volume of low-molecular-weight gases due to the thermochemical transformations of their components.This section presents data on the features of the thermal decomposition of the cobalt complexes synthesized in imidazole melt.It was noted that there were no endothermic effects on DSC curves (Figure 5) caused by the melting of imidazole at 90 °C [24] or the decomposition of cobalt salts at 200-300 °C [25], which minimized the presence of reagent impurities in the sample.Summarizing the results of the study with several physicochemical methods, we can conclude that imidazole melting with cobalt nitrate or perchlorate produces complexes with a CoN 6 coordination environment.In this case, the Co 2+ ion interacts with the pyridine nitrogen atom, and the anions are located in the outer sphere of the cation.Thus, we have proposed in our work a novel solvent-free method for the synthesis of [Co(C 3 H 4 N 2 ) 6 ](NO 3 ) 2 and [Co(C 3 H 4 N 2 ) 6 ](ClO 4 ) 2 complexes.

Study of Thermal Decomposition of the Cobalt Complexes
The principle of operation of gas-generation compositions is based on the release of a large volume of low-molecular-weight gases due to the thermochemical transformations of their components.This section presents data on the features of the thermal decomposition of the cobalt complexes synthesized in imidazole melt.It was noted that there were no endothermic effects on DSC curves (Figure 5) caused by the melting of imidazole at 90 • C [24] or the decomposition of cobalt salts at 200-300 • C [25], which minimized the presence of reagent impurities in the sample.Summarizing the results of the study with several physicochemical methods, we can conclude that imidazole melting with cobalt nitrate or perchlorate produces complexes with a CoN6 coordination environment.In this case, the Co 2+ ion interacts with the pyridine nitrogen atom, and the anions are located in the outer sphere of the cation.Thus, we have proposed in our work a novel solvent-free method for the synthesis of [Co(C3H4N2)6](NO3)2 and [Co(C3H4N2)6](ClO4)2 complexes.

Study of Thermal Decomposition of the Cobalt Complexes
The principle of operation of gas-generation compositions is based on the release of a large volume of low-molecular-weight gases due to the thermochemical transformations of their components.This section presents data on the features of the thermal decomposition of the cobalt complexes synthesized in imidazole melt.It was noted that there were no endothermic effects on DSC curves (Figure 5) caused by the melting of imidazole at 90 °C [24] or the decomposition of cobalt salts at 200-300 °C [25], which minimized the presence of reagent impurities in the sample.According to the data obtained (Figure 5), [Co(C 3 H 4 N 2 ) 6 ](NO 3 ) 2 and [Co(C 3 H 4 N 2 ) 6 ] (ClO 4 ) 2 complexes are thermally stable up to 150 and 170 • C, respectively.However, in-tense decomposition with a pronounced exothermic effect is observed above 200 • C. Based on DSC data, we calculated the critical temperature of thermal decomposition, where equilibrium is achieved between the heat loss of the sample and the exothermic effect of its thermochemical transformations.Hence, this parameter allows one to evaluate the thermal stability of substances, according to Xue's [26] method.The obtained values for the [Co(C 3 H 4 N 2 ) 6 ](NO 3 ) 2 and [Co(C 3 H 4 N 2 ) 6 ](ClO 4 ) 2 complexes were 227 and 253 • C, respectively.Thus, the perchlorate complex is more thermostable, and it is also characterized by a lower specific heat of combustion, despite the higher oxygen content (Table 1).
In general, the thermal decomposition of [Co(C 3 H 4 N 2 ) 6 ](NO 3 ) 2 and [Co(C 3 H 4 N 2 ) 6 ] (ClO 4 ) 2 complexes in the temperature range from 50 to 500 • C and at a heating rate of 5 • C/min proceeds through several stages by mass change, according to the change in the mass of the sample.In the case of the [Co(C 3 H 4 N 2 ) 6 ](NO 3 ) 2 complex, three stages of thermal decomposition can be distinguished, whereas for the [Co(C 3 H 4 N 2 ) 6 ](ClO 4 ) 2 complex, only two stages are clearly identified (Figure 5b).The third stage is weakly expressed due to the slow oxidation of the products and the incomplete decomposition of the ligand, probably due to the presence of chlorine.The first endothermic stage begins near 200 • C and reaches maximal mass loss at 210 ± 3 • C. As a result, the mass of the samples decreases by 12 ± 2%, which corresponds to the elimination of one imidazole molecule.Then, an additional mass loss of about 40% accompanied by the heat release is observed.Most likely, in this temperature region, the organic part of the complex is partially oxidized with both oxygen and nitrogen oxides, or chlorine formed during the decomposition of the nitrate anions (Figure 6) and perchlorate anions of the transition metals [27]: °C, respectively.Thus, the perchlorate complex is more thermostable, and it is also characterized by a lower specific heat of combustion, despite the higher oxygen content (Table 1).
In general, the thermal decomposition of [Co(C3H4N2)6](NO3)2 and [Co(C3H4N2)6](ClO4)2 complexes in the temperature range from 50 to 500 °C and at a heating rate of 5 °C/min proceeds through several stages by mass change, according to the change in the mass of the sample.In the case of the [Co(C3H4N2)6](NO3)2 complex, three stages of thermal decomposition can be distinguished, whereas for the [Co(C3H4N2)6](ClO4)2 complex, only two stages are clearly identified (Figure 5b).The third stage is weakly expressed due to the slow oxidation of the products and the incomplete decomposition of the ligand, probably due to the presence of chlorine.The first endothermic stage begins near 200 °C and reaches maximal mass loss at 210 ± 3 °C.As a result, the mass of the samples decreases by 12 ± 2%, which corresponds to the elimination of one imidazole molecule.Then, an additional mass loss of about 40% accompanied by the heat release is observed.Most likely, in this temperature region, the organic part of the complex is partially oxidized with both oxygen and nitrogen oxides, or chlorine formed during the decomposition of the nitrate anions (Figure 6) and perchlorate anions of the transition metals [27]: More detailed information on the composition of the released gases was obtained during the high-speed gasification of the complexes, close to real gas-generation conditions (Figure 7).It was found using dynamic mass spectrometry with simultaneous temperature measurement that no decomposition products of the complex are released during the endothermic stage, except for a small amount of water in the case of the  More detailed information on the composition of the released gases was obtained during the high-speed gasification of the complexes, close to real gas-generation conditions (Figure 7).It was found using dynamic mass spectrometry with simultaneous temperature measurement that no decomposition products of the complex are released during the endothermic stage, except for a small amount of water in the case of the [Co(C 3 H 4 N 2 ) 6 ](NO 3 ) 2 complex.Therefore, heat absorption is the result of melting, which begins at 215 and 190 • C for the [Co(C 3 H 4 N 2 ) 6 ](NO 3 ) 2 and [Co(C 3 H 4 N 2 ) 6 ](ClO 4 ) 2 complexes, respectively, under high-speed heating conditions.The first decomposition products of the complexes are identified at temperatures above 270 °C, and we can distinguish (1) low-temperature and (2) high-temperature stages according to the composition and intensity of the products released.It should be noted that these stages are not completely identical to the stages determined from the thermal analysis data (Figure 5), since the heating rates are very high.In this case, the (1) lowtemperature stage (Figure 7) includes stages I and II, determined from the results of the thermal analysis (Figure 5).At the (2) high-temperature stage, oxidative processes (stage III, thermal analysis data) occur, as evidenced by an abrupt increase in temperature.
At the (1) low-temperature stage, imidazole, N2, and/or CO are observed with an almost uniform temperature rise.The (2) high-temperature stage is characterized by intense heat release and a large amount of complete gasification products of the complex, mainly H2O, N2, СО, N2O, and CO2.In addition, in the case of the [Co(C3H4N2)6](NO3)2 complex, NO and NH3 are formed, but there is no oxygen, which is typical for the decomposition of a nickel complex of similar composition [9].For the [Co(C3H4N2)6](ClO4)2 complex, the release of oxygen was observed at 450 °C, and HCl was a minor product of its thermal decomposition.It can be assumed from the obtained dynamic mass spectrometry data that all the oxygen released during the decomposition of the nitrate anion [28] is spent on the oxidation of the organic ligand.Therefore, the degree of gasification of the nitrate complex is significantly higher than that of the perchlorate complex (Figure 5).As a result, the total mass loss of the nitrate complex was 85.5%, and the remaining mass of the sample (14.5%) was very close to the value corresponding to the formation of Co3O4 (13.6%).
IR spectroscopy and X-ray diffraction methods confirmed the formation of cobalt oxides during the thermal decomposition of the [Co(C3H4N2)6](NO3)2 and [Co(C3H4N2)6](ClO4)2 complexes (Figure 8).Indeed, two absorption bands at ~550 (Co 3+ vibration in the octahedral hole) and ~650 cm −1 (Co 2+ vibration in the tetrahedral hole), characteristic of Co3O4 spinel [29], were observed in the IR spectra of the samples after heat treatment in air (Figure 8a).The main reflections in the diffraction patterns of the thermal decomposition products of the [Co(C3H4N2)6](NO3)2 and [Co(C3H4N2)6](ClO4)2 complexes also belong to this phase (Figure 8b).The first decomposition products of the complexes are identified at temperatures above 270 • C, and we can distinguish (1) low-temperature and (2) high-temperature stages according to the composition and intensity of the products released.It should be noted that these stages are not completely identical to the stages determined from the thermal analysis data (Figure 5), since the heating rates are very high.In this case, the (1) low-temperature stage (Figure 7) includes stages I and II, determined from the results of the thermal analysis (Figure 5).At the (2) high-temperature stage, oxidative processes (stage III, thermal analysis data) occur, as evidenced by an abrupt increase in temperature.
At the (1) low-temperature stage, imidazole, N 2 , and/or CO are observed with an almost uniform temperature rise.The (2) high-temperature stage is characterized by intense heat release and a large amount of complete gasification products of the complex, mainly H 2 O, N 2 , CO, N 2 O, and CO 2 .In addition, in the case of the [Co(C 3 H 4 N 2 ) 6 ](NO 3 ) 2 complex, NO and NH 3 are formed, but there is no oxygen, which is typical for the decomposition of a nickel complex of similar composition [9].For the [Co(C 3 H 4 N 2 ) 6 ](ClO 4 ) 2 complex, the release of oxygen was observed at 450 • C, and HCl was a minor product of its thermal decomposition.It can be assumed from the obtained dynamic mass spectrometry data that all the oxygen released during the decomposition of the nitrate anion [28] is spent on the oxidation of the organic ligand.Therefore, the degree of gasification of the nitrate complex is significantly higher than that of the perchlorate complex (Figure 5).As a result, the total mass loss of the nitrate complex was 85.5%, and the remaining mass of the sample (14.5%) was very close to the value corresponding to the formation of Co 3 O 4 (13.6%).
IR spectroscopy and X-ray diffraction methods confirmed the formation of cobalt oxides during the thermal decomposition of the [Co(C 3 H 4 N 2 ) 6 ](NO 3 ) 2 and [Co(C 3 H 4 N 2 ) 6 ](ClO 4 ) 2 complexes (Figure 8).Indeed, two absorption bands at ~550 (Co 3+ vibration in the octahedral hole) and ~650 cm −1 (Co 2+ vibration in the tetrahedral hole), characteristic of Co 3 O 4 spinel [29], were observed in the IR spectra of the samples after heat treatment in air (Figure 8a).The main reflections in the diffraction patterns of the thermal decomposition products of the [Co(C 3 H 4 N 2 ) 6 ](NO 3 ) 2 and [Co(C 3 H 4 N 2 ) 6 ](ClO 4 ) 2 complexes also belong to this phase (Figure 8b).It should be noted that the gasification of the [Co(C3H4N2)6](NO3)2 and [Co(C3H4N2)6](ClO4)2 complexes is a nonequilibrium process, and along with the formation of Co3O4 and the oxidation of the organic ligand, reactions, which form the additional phases, occur in the samples.The analysis of the XRD data (Figure 8b) showed that it is not just Co3O4 that is formed during the thermal decomposition of the Co(C3H4N2)6](NO3)2 complex; the CoO phase also occurs, which has characteristic reflections at 42 and 61°.Note that they are absent in the diffraction patterns of the solid gasification products of the [Co(C3H4N2)6](ClO4)2 complex.However, the low degree of gasification of this complex (mass loss of about 58%) suggests the presence of amorphous impurities in the solid thermal decomposition products.Indeed, two regions of structureless absorption can be additionally distinguished in its IR spectrum at 1300-1600 cm −1 and 800-1100 cm −1 (Figure 8a).Higher-frequency absorption relates to the stretching vibrations of the C=C, C=N, and C-N bonds in carbon materials, including those containing nitrogen [30,31].The reason for absorption at 800-1100 cm −1 can be both the presence of perchlorate anions in the sample [32], and the deformation and rocking vibrations of the C-H and N-H bonds [33].However, there are no absorption bands in the IR spectrum at 3600-3000 cm −1 (Figure 8a) related to the stretching vibrations of the C-H and N-H bonds.Therefore, absorption at 800-1100 cm −1 is more likely due to the asymmetric stretching vibrations in the ClO4ˉ ion, and absorption at 1300-1600 cm −1 is associated with the presence of amorphous carbon doped with nitrogen resulting from the carbonization of the organic ligand of the [Co(C3H4N2)6](ClO4)2 complex.

Evaluation of Activation Energy for Thermal Decomposition of Cobalt Complexes
The evaluation of the activation energy (Ea) for the endothermic and exothermic stages of [Co(C3H4N2)6](NO3)2 and [Co(C3H4N2)6](ClO4)2 complex decomposition was performed using thermal analysis data by varying the heating rate β between 2.5, 5, and 10 °C•min −1 .The rate of 15 °C•min −1 was only used only for [Co(C3H4N2)6](NO3)2.The Kissinger model was chosen for the analysis of kinetic data as it does not require knowledge of the process mechanism [34], but thermal effects should occur at similar conversion values.(ClO 4 ) 2 complexes is a nonequilibrium process, and along with the formation of Co 3 O 4 and the oxidation of the organic ligand, reactions, which form the additional phases, occur in the samples.The analysis of the XRD data (Figure 8b) showed that it is not just Co 3 O 4 that is formed during the thermal decomposition of the Co(C 3 H 4 N 2 ) 6 ](NO 3 ) 2 complex; the CoO phase also occurs, which has characteristic reflections at 42 and 61 • .Note that they are absent in the diffraction patterns of the solid gasification products of the [Co(C 3 H 4 N 2 ) 6 ](ClO 4 ) 2 complex.However, the low degree of gasification of this complex (mass loss of about 58%) suggests the presence of amorphous impurities in the solid thermal decomposition products.Indeed, two regions of structureless absorption can be additionally distinguished in its IR spectrum at 1300-1600 cm −1 and 800-1100 cm −1 (Figure 8a).Higher-frequency absorption relates to the stretching vibrations of the C=C, C=N, and C-N bonds in carbon materials, including those containing nitrogen [30,31].The reason for absorption at 800-1100 cm −1 can be both the presence of perchlorate anions in the sample [32], and the deformation and rocking vibrations of the C-H and N-H bonds [33].However, there are no absorption bands in the IR spectrum at 3600-3000 cm −1 (Figure 8a) related to the stretching vibrations of the C-H and N-H bonds.Therefore, absorption at 800-1100 cm −1 is more likely due to the asymmetric stretching vibrations in the ClO 4 − ion, and absorption at 1300-1600 cm −1 is associated with the presence of amorphous carbon doped with nitrogen resulting from the carbonization of the organic ligand of the [Co(C 3 H 4 N 2 ) 6 ](ClO 4 ) 2 complex.

Evaluation of Activation Energy for Thermal Decomposition of Cobalt Complexes
The evaluation of the activation energy (E a ) for the endothermic and exothermic stages of [Co(C 3 H 4 N 2 ) 6] (NO 3 ) 2 and [Co(C 3 H 4 N 2 ) 6 ](ClO 4 ) 2 complex decomposition was performed using thermal analysis data by varying the heating rate β between 2.5, 5, and 10 • C•min −1 .The rate of 15 • C•min −1 was only used only for [Co(C 3 H 4 N 2 ) 6 ](NO 3 ) 2 .The Kissinger model was chosen for the analysis of kinetic data as it does not require knowledge of the process mechanism [34], but thermal effects should occur at similar conversion values.For this purpose, we identified the extreme temperatures for the endothermic (conversion 10 ± 2%) and exothermic (conversion 40 ± 3%) stages of [Co(C 3 H 4 N 2 ) 6] (NO 3 ) 2 and [Co(C 3 H 4 N 2 ) 6 ](ClO 4 ) 2 complex decomposition at all heating rates (Figure 9).From the obtained values, the activation energies for the first two stages were acquired (Table 2).As in the case of the critical temperature of thermal explosion, the [Co(C 3 H 4 N 2 ) 6 ](ClO 4 ) 2 complex is characterized by a higher activation energy than [Co(C 3 H 4 N 2 ) 6] (NO 3 ) 2 .
(a) (b)  Thus, the main stages of thermal decomposition of the [Co(C3H4N2)6](NO3)2 and [Co(C3H4N2)6](ClO4)2 complexes were analyzed, allowing us to evaluate their activation energies.The obtained results indicate the higher thermal stability of the perchlorate cobalt complex, characterized by a high critical temperature of thermal explosion and a low degree of gasification.Hence, this explains the low degree of its gasification and the higher content of carbon-and chlorine-containing impurities in the products of thermal decomposition.

Materials
For the synthesis of complex compounds, the following commercially available chemical reagents were used: Co(NO3)2•6H2O (GOST 4528-78, 97%),  Thus, the main stages of thermal decomposition of the [Co(C 3 H 4 N 2 ) 6 ](NO 3 ) 2 and [Co(C 3 H 4 N 2 ) 6 ](ClO 4 ) 2 complexes were analyzed, allowing us to evaluate their activation energies.The obtained results indicate the higher thermal stability of the perchlorate cobalt complex, characterized by a high critical temperature of thermal explosion and a low degree of gasification.Hence, this explains the low degree of its gasification and the higher content of carbon-and chlorine-containing impurities in the products of thermal decomposition.

Materials
For the synthesis of complex compounds, the following commercially available chemical reagents were used: Co(NO 3 ) 2 •6H 2 O (GOST 4528-78, 97%), 2CoCO The synthesis was performed within a ceramic crucible on the heated surface of an IKA-C-Mag HS4 tile (IKA, Staufen, Germany) at a predetermined temperature of 120 • C. First, 0.01 moles of cobalt salts ((Co(NO 3 ) 2 •6H 2 O or Co(ClO 4 ) 2 •6H 2 O)) was added to the melted imidazole C 3 H 4 N 2 (0.06 mole, melting point = 89-91 • C), and an active interaction between the reagents was observed with the formation of blue humid powder that quickly dried and became pink-violet while being stirred.Afterwards, the obtained complex compound was dried in a desiccator under P 2 O 5 and in a vacuum centrifuge (30 • C, 4 h).The yield was 97 ± 1%.

Characterization of Complex Compounds and Solid Products of Combustion
The Co content was found using inductively coupled plasma atomic emission spectrometry, employing an Optima 4300 DV (PerkinElmer, Waltham, MA, USA).
The carbon, hydrogen, and nitrogen contents were calculated with the use of an automatic CHNS-analyzer EURO EA 3000 (Euro Vector S.p.A., Castellanza, Italy).The samples (0.5-2 mg) were burned in a vertical reactor in a dynamic regime at 1050 • C in the flow of a helium-oxygen mixture.
The oxygen balance of the complexes was calculated using the following equation: where C, H, and O are the number of carbon, hydrogen, and oxygen atoms per molecule, respectively.M is 4 3 , assuming that the main product of cobalt oxidation is Co 3 O 4 .The heat of the combustion of the complexes was determined in a Semimicro Calorimeter Parr 6725 with a Calorimetric Thermometer 6772 (Parr, Moline, IL, USA).
Diffuse reflectance UV-vis spectra were recorded in air at room temperature on a Varian Cary 100 instrument (Agilent, Santa Clara, CA, USA) equipped with a standard diffuse reflectance attachment.Samples in the form of powder were placed in the cell equipped with a quartz window.
The infrared spectra were obtained by attenuated total reflection infrared spectroscopy (ATR FTIR) employing an Agilent Cary 600 spectrometer (Agilent Technologies, Santa Clara, CA, USA) with a Gladi ATR attachment (PIKE Technologies, Madison, WI, USA) in the range of wavenumbers 250-4000 cm −1 without a preliminary sample preparation.
The powder X-ray diffraction (XRD) patterns of the synthesized complexes and the solid products of their combustion were obtained in the 2θ range from 10 to 70 • with a step of 0.02 • and speed of 2 • /min using an ARL X'tra (Thermo, Ecublens, Switzerland) diffractometer equipped with a linear detector Mythen2R 1D (Dectris, Baden, Switzerland).CuK α radiation (λ = 1.5418Å) was used.The composition of the gasification products was identified by the Rietveld method [35].The average sizes of the coherent scattering regions (CSRs) were calculated using the Scherer formula by reflexes 311 for Co 3 O 4 and 200 for CoO.The diffraction profile was described using the Fityk program, and the Pseudo Voigt function was used.
Crystallographic data (excluding structure factors) for the structures in this paper have been deposited at the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK.Copies of the data can be obtained free of charge by quoting the depository numbers CCDC-1142518 for [Co(C 3 H 4 N 2 ) 6 ](NO 3 ) 2 and CCDC-1251528 for [Co(C 3 H 4 N 2 ) 6 ](ClO 4 ) 2 [23].
The XPS spectra of the samples were recorded with an accuracy of ±0.1 eV and a depth of about 5 nm using a SPECS photoelectron spectrometer with a PHOIBOS-150-MCD-9 hemispheric analyzer and a FOCUS-500 monochromator (SPECS Surface Nano Analysis GmbH, Berlin, Germany) (AlK α radiation, 150 W, hν = 1486.74eV).The binding energy (BE) scale of the spectrometer was pre-calibrated using Au4f 7/2 (84.0 eV) and Cu2p 3/2 (932.6 eV) core-level peaks.The samples were supported on a conducting scotch.The sample charge was taken into account using C1s lines (284.8 eV).The individual spectra of elements allowed us to determine their electronic state and to calculate the ratios of different forms of cobalt present on the catalyst surface, taking into account the element sensitivity coefficients [36].
The composition of the gaseous products from the combustion was analyzed by the dynamic mass spectral thermal analysis (DMSTA) method, using a time-of-flight mass spectrometer with a molecular beam sampling system MSCh-4 (Plant Of Scientific Instrumentation, Sumy, USSR) under a flow of Ar (5 mL•min −1 ).The average heating rate ranged from 100 to 245 • C•s −1 .The sample weight was 1-5 mg.The delay between measurements was 0.04 s.The identification of mass spectral signals was carried out using the mass spectra of individual substances from the NIST database [37].The kinetic parameters of the thermal decomposition of [Co(C 3 H 4 N 2 ) 6 ](NO 3 ) 2 and [Co(C 3 H 4 N 2 ) 6 ](ClO 4 ) 2 were calculated using the Kissinger method [34] based on the DSC data for different heating rates.The Kissinger equation is used to find the activation energy of non-isothermal processes without the assumptions of kinetic models: where β is the heating rate; T m is the temperature of the peak of the reaction rate; E is the activation energy of the reaction; R is the universal gas constant; and A is the preexponential factor.In addition, a comparison of the thermal stability of the synthesized complexes was completed based on the critical temperature of thermal decomposition using the method provided by Xue et al.
where T e is the onset temperature as a function of β, and T 0 is the onset temperature extrapolated to β = 0 using the 3rd-order polynomial fit [26].

Figure 1 . 14 Figure 1 .
Figure 1.XPS spectra of Co2p for [Co(C 3 H 4 N 2 ) 6 ](NO 3 ) 2 and [Co(C 3 H 4 N 2 ) 6 ](ClO 4 ) 2 complexes.To determine the nature of the coordination between the central atom and the imidazole molecules, cobalt complexes were studied using diffuse reflectance UV-vis spectroscopy.The obtained spectra of the [Co(C 3 H 4 N 2 ) 6 ](NO 3 ) 2 and [Co(C 3 H 4 N 2 ) 6 ](ClO 4 ) 2 complexes (Figure2) were almost identical.Hence, the type of coordination of the cation by the imidazole molecules is the same, despite the different nature of the anion.
Materials 2024, 17, x FOR PEER REVIEW 5 of 14 amount was obtained by the powder XRD method (Figure 4).It was found that all peaks in the XRD pattern of the complexes containing the coordination site of CoN6 matched the theoretical powder pattern of [Co(C3H4N2)3](NO3)2 and [Co(C3H4N2)3](ClO4)2 plotted from the CIF [23].(a) (b)

Figure 6 .
Figure 6.Thermal decomposition of Co(NO3)2•6H2O in air with a heating rate of 10 °C•min −1 : (a) thermal analysis data obtained by TG and DTA; (b) TG-FTIR data of released gases.

Figure 6 .
Figure 6.Thermal decomposition of Co(NO 3 ) 2 •6H 2 O in air with a heating rate of 10 • C•min −1 : (a) thermal analysis data obtained by TG and DTA; (b) TG-FTIR data of released gases.

Figure 7 .
Figure 7.The composition of gaseous products during thermal decomposition of (a) [Co(C3H4N2)6](NO3)2 and (b) [Co(C3H4N2)6](ClO4)2 complexes in argon with a high rate of heat.Method of dynamic mass spectrometry with simultaneous temperature measurement.(dωi/dtchange in i-component fraction in the gas phase with time).

Figure 7 .
Figure 7.The composition of gaseous products during thermal decomposition of (a) [Co(C 3 H 4 N 2 ) 6 ](NO 3 ) 2 and (b) [Co(C 3 H 4 N 2 ) 6 ](ClO 4 ) 2 complexes in argon with a high rate of heat.Method of dynamic mass spectrometry with simultaneous temperature measurement.(dω i /dt-change in i-component fraction in the gas phase with time).

Figure 8 .
Figure 8.(a) ATR FTIR and (b) XRD data obtained after thermal treatment of [Co(C 3 H 4 N 2 ) 6 ](NO 3 ) 2 and [Co(C 3 H 4 N 2 ) 6 ](ClO 4 ) 2 complexes at 500 • C. It should be noted that the gasification of the [Co(C 3 H 4 N 2 ) 6 ](NO 3 ) 2 and [Co(C 3 H 4 N 2 ) 6 ](ClO 4 ) 2 complexes is a nonequilibrium process, and along with the formation of Co 3 O 4 and the oxidation of the organic ligand, reactions, which form the additional phases, occur in the samples.The analysis of the XRD data (Figure8b) showed that it is not just Co 3 O 4 that is formed during the thermal decomposition of the Co(C 3 H 4 N 2 ) 6 ](NO 3 ) 2 complex; the CoO phase also occurs, which has characteristic reflections at 42 and 61 • .Note that they are absent in the diffraction patterns of the solid gasification products of the [Co(C 3 H 4 N 2 ) 6 ](ClO 4 ) 2 complex.However, the low degree of gasification of this complex (mass loss of about 58%) suggests the presence of amorphous impurities in the solid thermal decomposition products.Indeed, two regions of structureless absorption can be additionally distinguished in its IR spectrum at 1300-1600 cm −1 and 800-1100 cm −1 (Figure8a).Higher-frequency absorption relates to the stretching vibrations of the C=C, C=N, and C-N bonds in carbon materials, including those containing nitrogen[30,31].The reason for absorption at 800-1100 cm −1 can be both the presence of perchlorate anions in the sample[32], and the deformation and rocking vibrations of the C-H and N-H bonds[33].However, there are no absorption bands in the IR spectrum at 3600-3000 cm −1 (Figure8a) related to the stretching vibrations of the C-H and N-H bonds.Therefore, absorption at 800-1100 cm −1 is more likely due to the asymmetric stretching vibrations in the ClO 4

Table 2 .
Activation energy of different stages of thermal decomposition acquired by Kissinger method and the critical temperature of thermal explosion of [

Table 2 .
Activation energy of different stages of thermal decomposition acquired by Kissinger method and the critical temperature of thermal explosion of [Co(C 3 H 4 N 2 ) 6 ](NO 3 ) 2 and [Co(C 3 H 4 N 2 ) 6 ](ClO 4 ) 2 .