Solid state thermal decomposition of metal complexes and its kinetic evaluation   has drawn much attention of many investigators, as it can be a photo type for investigating the fuel efficiency of solid propellants. Thermal decomposition kinetics analysis of solid propellants can be correlated to combustion characteristics of the fuels  . It is crucial to study the thermolytic characteristics on solid propellants so that one can get information on the potential hazard in their handling, storage and processing   . Kinetic studies are also useful in modelling and predicting the combustion characteristics of solid rocket propellants  . In the present investigation, we have thermally characterised five Fe(III) coordination complexes, which has been synthesised and characterised by the reported procedure  , and their thermal decomposition kinetics, based on various kinetic parameters like activation energy (E), entropy activation (∆S), pre-exponential factor (A), are evaluated by Coats-Redfern equation. This kinetic evaluation can be used as photo type for energetic characteristics of solid propellants.
GA and its Fe(III) complexes were prepared and characterised as reported earlier  . The ligand was prepared by simple condensation reaction between glyoxal and 4-aminoantipyrine in ethanolic medium. (Figure 1)  . A TGA-7 thermal analyser was used to record the thermogarams. The analysis was done under N2 atmosphere using around 10 mg of samples for each measurement. The kinetic evaluation of the thermal decomposition studies was done using a computer programme in Q-basic.
3. Results and Discussion
Based on the various analytical, physicochemical and spectral studies  , the complexes in the present studies can be formulated as [Fe(GA)(ClO4)](ClO4)2 and [Fe(GA)X2]X (where X = , SCN−, Cl− or Br−). A tetradentate coordination has been observed for through both carbonyl oxygens atoms and azomethine nitrogens in all complexes. One of the perchlorates, two of the nitrates are coordinated in a bidentate and monodentate fashion respectively in the corresponding complexes as evident from the infrared and molar conductance studies. In the thiocyanate complex, two of the thiocyanate ions are coordinated monodentately through nitrogen atoms. In the chloride and bromide complexes, both the halides are coordinated to the metal atom in the respective complexes. From the electronic spectra and the magnetic moment data, a spin free octahedral geometry is assigned to all the synthesized complexes (Figure 2). The TG and DTG curves of all five complexes are shown in the Figures 3-7.
Figure 1. Scheme for the synthesis of 1,2-Bis(imino-4’-antipyrinyl)ethane (GA).
Figure 2. Tentative structures of the Fe(III) perchlorate complex (left), and nitrate, thiocyanate, chloride and bromide complexes (X = , SCN−, Cl− or Br−) (right).
Figure 3. TG (red line) and DTG (blue line) of [Fe(GA)(ClO4)](ClO4)2.
Figure 4. TG (red line) and DTG (blue line) of [Fe(GA)(NO3)2]NO3.
Figure 5. TG (red line) and DTG (blue line) of [Fe(GA)(SCN)2]SCN.
Figure 6. TG (red line) and DTG (blue line) of [Fe(GA)Cl2]Cl.
The non-isothermal thermogravimetric analysis is generally used to investigate the thermal stability of solid state materials. In this method, the various kinetic parameters are calculated over an entire range of temperature in a continuous manner. The rate of a solid state reaction can be generally expressed by  .
(K = specific rate constant, α = the sample undergoing reaction and f(α) = conversion function).
Figure 7. TG (red line) and DTG (blue line) of [Fe(GA)Br2]Br.
In linear heating rate f, T = T0 + ft (T0―temperature of initiation)
Substituting (2) in (1)
where, K is the specific rate constant, which is temperature dependent, can be expressed by the Arrhenius equation
K = A e−E/RT (4)
(A―Pre-exponential factor, E―energy of activation, R―gas constant and T―temperature in Kelvin).
Substituting (4) in (3)
On integration, taking initial temperature as zero, this assumption is correct as no reaction occurs between T = 0 and T = Ti.
The temperature integral can be evaluated by Coats and Redfern equation  with Rainville function  . The temperature integral t is
where, T―temperature, A―pre-exponential factor, R―gas constant, ϕ―heating rate and E―activation energy.
On plotting L.H.S. of the equation, which includes g(α) versus 1/T, will be a straight line, from the slope and intercept are used to calculate the activation energy (E) and the pre-exponential factor (A).
The entropy of activation (DS) can be calculated by the following equation.
(k―Boltzmann constant, h―Planck’s constant and DS―Entropy of activation).
3.2. Phenomenological Aspects
Table 1, describes the phenomenological aspects on the thermal decomposition of the present complexes under study. The perchlorate complex having the formula [Fe(GA)(ClO4)](ClO4)2, undergoes a thermal decomposition in two stages in the temperature range 146˚C - 482˚C. The decomposition stage between 146˚C - 220˚C, with a DTG peak at 197˚C, having a mass loss of (24.52%) corresponds to the first stage. The corresponding mass loss happened due to the decomposition of three perchlorates into chlorides, which has been confirmed the chemical analysis and the infrared spectrum, by the intermediate compound analysis. The second stage having a mass loss of (54.72%), corresponds to the removal of a GA molecule occurs in the temperature range 220˚C - 482˚C. The corresponding mass loss is attributed to the removal of the ligand molecule GA. The corresponding DTG peak occurs at 369˚C. The final residue is anhydrous ferric chloride as confirmed by qualitative analysis.
Table 1. Phenomenological data for the thermal decomposition of Fe(III) complexes of GA.
[Fe(GA)(NO3)2]NO3, the nitrate complex, decomposes in three stages. The thermal decomposition stage between 142˚C - 264˚C with a mass loss of 27% corresponds to the first stage. The mass loss happens at this stage corresponds to the decomposition of three nitrate ions. This observation was confirmed by the by the infrared spectral analysis of the intermediate residue at 264˚C. The second decomposition stage between 264˚C - 497˚C with a mass loss (31%) occurs, corresponding to the decomposition of half a molecule of ligand  . The partial removal of the ligand at this stage is confirmed by the infrared spectrum of the residue after this stage the third stage, which happens between 497˚C - 910˚C. A mass loss of (31.50%), occurring at this stage, is due to the removal of the remaining part of GA. The corresponding DTG peaks were observed at 192, 325 and 583˚C for the first, second and third stages of the decomposition, respectively. The decomposition gets completed at 910˚C and the final residue is qualitatively proved to be anhydrous metal oxide.
The thiocyanate complex, [Fe(GA)(SCN)2]SCN, decomposes in three stages between the temperature range 190˚C - 801˚C. The thermal decomposition between 190˚C - 313˚C, the DTG peak at 303˚C having a mass loss of (31%) is owing to the removal of half a molecule of GA  , corresponds to the first stage. This observation is confirmed by the infrared spectrum of the residue after this stage. The second stage having a mass loss of 25.87%, is due to decomposition of three thiocyanate ions occurs between 313˚C - 515˚C, having a DTG peak at 453˚C. Absence of thiocyanate ions above 515˚C, is confirmed by infrared spectral analysis of the intermediate residue at this stage. The decomposition stage between 515˚C - 801˚C with the DTG peak at 636˚C, corresponds to the third stage. The mass loss (32.02%) occurs at this stage is owing to the removal of the remaining ligand molecule. The decomposition gets completed at 801˚C, leaving behind anhydrous metal oxide as final residue as proved qualitatively.
[Fe(GA)Cl2]Cl, the chloride complex decomposes in two stages. The thermal decomposition occurs between 211˚C - 400˚C corresponds to the first stage. The mass loss (36.01%), occurring at this stage is owing to the decomposition of half a molecule GA  , and this observation was also confirmed by the infrared spectral analysis of the intermediate residue. The DTG peak of this stage occurs at 307˚C. The decomposition stage between 400˚C to 874˚C with the DTG peak at 637˚C corresponds to the second stage. The mass loss (36.17%) occurring at this stage is attributed to the decomposition of the remaining GA molecule, leaving behind anhydrous ferric chloride as confirmed by qualitative analysis.
[Fe(GA)Br2]Br, the bromide complex, undergoes two-stage decomposition. The first stage starts at 240˚C and ends at 408˚C, with a mass loss (29.10%) is due to the decomposition of half a molecule of GA  . The partial removal of GA at this stage is confirmed by the infrared spectral analysis of the residue at this stage. The corresponding DTG peak of this stage is observed at 350˚C. The decomposition, which starts at 408˚C and ends at 856˚C, corresponds to the second stage (DTG peak at 620˚C). A mass loss (29%) occurs at this stage is due to the removal of the remaining half of the ligand molecule. Anhydrous ferric bromide, left behind as final residue, which has been confirmed by qualitative analysis.
3.3. Kinetic Aspects
Activation energy (E), pre-exponential factor (A) and entropy of activation (DS) for the thermal decomposition of Fe(III) complexes are given in Table 2. On comparing the various kinetic parameters, lower activation energy was observed in second stage as compared to first thermal decomposition stage. The same trend has been observed for pre-exponential factor (A) and entropy of activation (DS). The present results show that the starting materials having higher Activation energy (E), are more stable than the intermediate products however; the intermediate products possess well-ordered chemical structure due to their highly negative entropy of activation (DS) values.
3.4. Comparison of the Kinetic Parameters with Structure and Thermal Stability
The activation energies (E) of iron(III) complexes vary in the range 31.75 to 82.23 kJ∙mol−1. The corresponding values of pre-exponential factor (A) of these complexes come in the range 1.56 × 10−2 to 66.20 S−1 while the respective values of entropy of activation (DS) of these complexes fall in the range −288.61 to −213.83 J∙mol−1. As the decomposition proceeds, it has been observed that there is a decrease in the value of all the kinetic parameters as we move from first stage to the second stage may be due to the more ordered structure of intermediate compound attained after the first decomposition stage.
Based on the activation energies obtained from first and second stages of
Table 2. Kinetic parameters for the thermal decomposition of Fe(III) complexes containing different counter anions
thermal decomposition of these complexes; perchlorate and nitrate complexes were found to be highly stable respectively, while chloride complex was found to be least stable in both stages. The negative value of the entropy of activation also indicates that the activated complexes are more ordered than reactants      . Even though there is a common GA molecule is present in all the complexes, there is a variation in the different kinetic parameters. This variation could be due to the effect of counter anions on the thermal decomposition of the complexes. Hence, herein we report that the counter anions play an important role on the thermal decomposition kinetic parameters of the complexes and this result was found in line with our previous reports     .
4. Conclusion and Summary
A comparative thermal decomposition kinetic investigation on Fe(III) complexes of 1,2-Bis(imino-4’-antipyrinyl)ethane (GA)), with varying counter anions ( , , SCN−, Cl−, and Br−), by using Coats-Redfern equation has been done by thermogravimetric traces. The kinetic parameters (activation energy (E), pre-exponential factor (A) and entropy of activation (∆S)) were quantified and compared. The present study proves that the counter anions play an important role on the thermal decomposition kinetic parameters of the complexes. The studies also give an insight into the energetic characteristics of solid propellants.
*The authors wish in to be known that, in their opinion, the first four authors should be regarded as joint first authors.
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