TG, DTA Pyrolytic Analysis of Cobalt, Nickel, Copper, Zinc, and 5,8-Dihydroxy-1,4-Naphthoquinone Chelate Complexes

)e solid state reactions identified on the TG traces with correspondence to DTG peaks consequent to the nonisothermal decomposition of polymetallic chelates of the naphthazarin with Zn (II), Co (II), Ni (II), and Cu (II) over the temperature range ambient at 800°C have been studied kinetically following the Dave and Chopramethod as these solid state reactions exhibited their resemblance with the Freeman recommended reaction for kinetic studies. )e solid state reactions as described followed first order kinetics. )e kinetic data showed the very low value of Z for each of the solid state reaction in reference, concluding on the solid state reactions (the nonisothermal decomposition of polymetallic chelate of Zn (II), Co (II), Ni (II), and Cu (II) as slow reactions).


Introduction
Due to automation in the recent times, the instruments have become capable of self-operation, improving both accuracy and precision of measurements, as well as relinquishing both investigator's time and patience.
Besides other instruments, the thermal methods provide today the means of solving existing chemical as well as creating new ones. ese methods can provide rapid information concerning the thermal stability, composition of pyrolysis intermediates, and composition of the final product as a compound is heated to elevated temperature.
Borrel and Paris [1] carried out the synthesis and stoichiometry of some metal oxinate complexes and their associated thermal stability by using thermogravimetric analysis. e effect of alpha methyl substitution on the oxine ligand in Cu(II) and Zn(II) complexes [2] over the solubility products of Cu(II) and Zn(II) oxinates and methyloxinates was studied by potentiometric neutralization of acid solutions containing oxine or methyloxine and metallic cations [3]. e thermal stability and volatilization on vacuum of metallic chelates which are derivatives of 8-hydroxyquinoline have been studied by Charles and Langer [4]. Also, it had been observed that the temperature range of volatilization depends on metallic ion electronegativity for the divalent metal 8-hydroxyquinolinates. e thermal stability analysis of these complexes was studied by Wendlandt and Horton [5] using differential thermal analysis (DTA). ese 8-hydroxyquinolinates hydrates were also studied by Gore and Wendlandt [6] by using thermogravimetry, differential scanning calorimetry, and reflectance spectra. e crystalline structures study of this kind of complex [7][8][9] notifies monoclinic in the a and b forms for the copper (II) complexes regardless of the hydration degree. Zn (II) and Cd (II) complexes [10,11] have been found to be monoclinic, but it had been found that these structures depend on the hydration. e zinc (II) and cadmium (II) hydrated complexes show the same b form found for the copper (II) complexes. Several studies have indicated characteristic IR bands for these compounds [12][13][14][15][16][17][18]. An attractive group of natural 1,4-naphthoquinones is spinochrome, i.e., the pigments of echinoderms with naphthazarin 1 (5,8-dihydroxy-1,4-naphthoquinone) core [19][20][21][22]. Naphthoquinones in association with various metals have many medicinal properties [23][24][25][26]. e chelating agents are capable of chelating the metal cations having 2-4 valencies in line with the ligancy of metals involved, but the hydroxynaphthoquinone, namely, 5,8-dihydroxy-1,4-naphthoquinone (naphthazarin), a synthetic hydroxynaphthoquinone, appears to be able to form polymetallic chelates with different metal cations due to the presence of an additional hydroxyl group at carbon 8 as compared to its family member: 5-hydroxy-1,4-naphthoquinone (juglone) with the capability to chelate two metal cations initially forming the two six member rings with the progression of more six member ring formation in line with the metal ligancy involving either ligand molecule(s) or coordinated water molecules as the satisfaction of metal ligancy a prerequisite in the chelation process.
e literature survey has provided information on the little work performed so far on the chelating properties of naphthazarin molecule (Scheme 1).
is work centers around the synthesis of polymetallic chelates of different metal cations (Zn(II) Co(II), Ni(II), and Cu(II)) with naphthazarin and their pyrolysis mapping with emphasis on the detection of stability and instability zones, composition of the pyrolysis intermediates and the synthetic metal chelates, as well, and the kinetics of the nonisothermal decomposition of the polymetallic chelates involving the decomposition reactions detected on the pyrolysis traces with agreement to the type of reaction.
A (s) ⟶ B (s) + C (g). It was recommended by Freeman and Carrol [27] for the study of the kinetics of the reaction. e Dave and Chopra procedure [28] was applied to study the nonisothermal decomposition reactions kinetically.

Materials and Methods
e chemicals of high purity were used in the study of chelation of metal cations and naphthazarin at pH 6. For the synthesis of metal chelates, equal molar of aqueous metal salt solution and ethanolic solution of naphthazarin were mixed and buffered at pH 6. e resultant mixture was allowed to stand for a period of at least ten days. e crystals so formed were filtered, washed with double distilled water, and shade dried and finally bottled. e process of preparation of chelate complexes was carried out without using any external catalyst. e process takes place via the autocatalytic mechanism.
For pyrolysis mapping, the dried solid mass of metal chelate of naphthazarin with metal cations Zn (II)/Co (II)/Ni (II)/Cu (II) was subjected to thermal analysis in nitrogen environment (100/200 ml min −1 ) at 10°C min −1 over temperature range ambient at 800°C. e thermal database is given in Tables 1-4 (Dave and Chopra Method). Dave and Chopra [28] gave the accompanying expression to study the kinetics of nonisothermal decomposition reactions matching the solid state reaction of the type.A (s) ⟶ B (s) + C (g).

Kinetics of Nonisothermal Decomposition Reaction
where k is the specific rate constant, A is the total area under the DTG curve, a is the area at time t, n is the order of reaction, −dx/dt is the deviation from baseline (−dx/dt � 0). For, n � 1, equation (1) reduces to Equation (2) together with Arrhenius equation (3) where Z is the frequency factor and gives A straight line relationship is obtained on plotting log 10 k against T −1 , giving intercept as log 10 Z and slope (tan θ) as E/ 2.303 R (Figure 3). e samples of polymetallic chelates of naphthazarin with Zn (II) or Co (II) or Ni (II) or Cu (II) cations were run on EXSTAR TG/DTA 6300 in nitrogen (100/200 ml/min) atmosphere with reference weight of 10.500 mg and reference name alumina powder and temperature program as e sample weights ranging from 1 mg to 3 mg were employed. e small sample size within the limits of sensitivity of balance was so used as to ensure that the heating rate (10°C/min) could not depart from its constant value. e polymetallic chelates of naphthazarin with Zn (II), Co (II), Ni (II), and Cu (II) were mapped thermally, structurally, and compositionally employing the thermal database generated with the use of instrument "EXSTAR TG/DTA 6300." e nonisothermal decomposition reactions with correspondence to the well-defined sigmoids on TG traces had been kinetically studied employing the peaks on DTG trace with complete correspondence to sigmoids on TG traces.     e sample could not be further pyrolysed beyond 800°C due to the instrumental limitation. It showed that the sample could not be led to the complete combustion level, the stage of complete departure of organic matter from the sample mass, leaving behind ZnO (zinc oxide). e possible nonisothermal decomposition reactions identified on TG trace ( Figure 6) are described as follows:  is polymetallic chelate on its formation under the applied condition composed of additional 04 six membered rings in addition to the 02 six membered rings of the chelating agents ( Figure 3). e nonisothermal decomposition of the thermal mapping spectrum showed the thermal stability at initial states as ambience to 100°C (pdt: 100°C). Beyond 100°C, with the increase of temperature, the structural degeneration occurred, proceeding slowly to 169°C with the loss of 7H 2           e DTA spectrum analysis led to believe "No Visible deviation" from the baseline. e possible nonisothermal decomposition reactions identified on TG trace (Figure 7) are described as follows: 10       e analytical data on the pyrolysis journey on the polymetallic chelate of naphthazarin with nickel as shown by TG mapping are given in Table 3. e last plateau (545°C-800°C), beyond which the pyrolysis could not be extended (780°C), clearly indicated the incomplete combustion of the metal chelate in reference under the applied instrumental conditions. e 03 sigmoids traces on the TG spectrum had the correspondence to the peaks registered on DTG trace of the metal chelate. e instrument registered 2nd peak contiguous to the 3rd peak on DTG trace showing near correspondence to the sigmoid tracing on TG traces (Figure 9). e analytical data on the pyrolysis journey on the polymetallic chelate of naphthazarin with copper as shown by TG mapping are given in Table 4.

Kinetics and Solid State Reactions.
e I-XIII solid state reactions identified on the TG traces with correspondence to DTG peaks consequent to the nonisothermal decomposition of polymetallic chelates of the naphthazarin with Zn (II), Co (II), Ni (II), and Cu (II) over the temperature range ambient at 800°C have been studied kinetically following the Dave and Chopra method as these solid state reactions exhibited their resemblance with the Freeman recommended reaction for kinetic studies.
For each solid state reaction, the terms A, a, and −dx/dt at various T values employ DTG traces corresponding to sigmoids on TG trace. e plot of log k (where k � (−dx/dt)/(A − a)) against the reciprocal of absolute temperature (T) gave a straight line relationship justifying the assumption of order of reaction (n) as one.
Figures 10-21 represent the Dave and Chopra plots for n � 1 for different solid state reactions (I-XIII), giving slope (tanθ) as E/2.3R and intercept as log Z. e characteristics terms E and Z for solid state reactions (I-XIII) are tabulated in Table 5.
It is an established fact that the velocity rate increases with the rise of temperature according to collision theory for reactions, justifying more collisions among the involved molecules. is means that the Z (frequency factor or collisions number) value rises with more collisions among the molecules describing the reactions involved as fast reaction, but the lower values of Z may help conclude the reaction under study as slow in nature. e kinetic data showed the very low value of Z for each of the solid state reaction in reference (Table 5), concluding on the solid state reaction (I-XIII) (the nonisothermal decomposition of polymetallic chelate of Zn(II), Co(II), Ni(II), and Cu(II) as slow reactions). e DTG traces on the polymetallic chelates of naphthazarin with Zn (II), Co (II), Ni (II), and Cu (II) are shown in Figures 6-9, respectively.

Conclusion
e pyrolysis spectrum of each of the polymetal chelates exhibiting plateaus and sigmoids with correspondence on the DTG traces but with no responses on DTA traces have led us to conclude on the structures of the polymetal chelates tentatively.
e unidentified nonisothermal decomposition of polymetallic chelates of naphthazarin with resemblance to A (s) ⟶ B (s) + C (g) (reaction: sigmoid flanked by plateaus on the trace) has been studied kinetically applying the Dave and Chopra method and DTG traces. e solid state reactions described followed first order kinetics. e kinetic data showed the very low value of Z for each of the solid state reaction in reference, concluding on the solid state reactions (the nonisothermal decomposition of polymetallic chelate of Zn (II), Co (II), Ni (II), and Cu (II) as slow reactions).

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that there are no conflicts of interest.