Synthesis, structural, morphological, and thermal decomposition kinetics of Iron (II) coordination polymer of sebacoyl bis (isonicotinoylhydrazone)

Accepted Manuscript Research paper Synthesis, structural, morphological, and thermal decomposition kinetics of Iron (II) coordination polymer of sebacoyl bis (isonicotinoylhydrazone) Mahejabeen Azizul Haque, Ratiram Gomaji Chaudhary, L.J. Paliwal PII: DOI: Reference:

S0020-1693(17)30090-7 http://dx.doi.org/10.1016/j.ica.2017.03.042 ICA 17502

To appear in:

Inorganica Chimica Acta

Received Date: Revised Date: Accepted Date:

17 January 2017 28 March 2017 30 March 2017

Please cite this article as: M.A. Haque, R.G. Chaudhary, L.J. Paliwal, Synthesis, structural, morphological, and thermal decomposition kinetics of Iron (II) coordination polymer of sebacoyl bis (isonicotinoylhydrazone), Inorganica Chimica Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.03.042

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Synthesis, structural, morphological, and thermal decomposition kinetics of Iron (II) coordination polymer of sebacoyl bis (isonicotinoylhydrazone) Mahejabeen Azizul Haque1*, Ratiram Gomaji Chaudhary2, L. J. Paliwal1 1

Post Graduate Teaching Department of Chemistry, Rashtrasant Tukadoji Maharaj Nagpur University, Nagpur, Maharshtra-440033, India 2 P.G. Department Chemistry, Seth Kesarimal Porwal College, Kamptee, Maharshtra-441001, India *Corresponding Author email-id: [email protected]

Corresponding Author: MAHEJABEEN AZIZUL HAQUE (Research Scholar) Post Graduate Teaching Department of Chemistry Rashtrasant Tukadoji Maharaj Nagpur University, Nagpur, Maharashtra -440033, India Mobile No. +917028308023 E-mail: [email protected]

Co-Author: RATIRAM GOMAJI CHAUDHARY (Assistant Professor) Post Graduate Department of Chemistry, Seth Kesarimal Porwal College Kamptee, Maharashtra-441001, India Mobile: +919860032754 Email: [email protected] LALITMOHAN J. PALIWAL (Professor) Post Graduate Teaching Department of Chemistry Rashtrasant Tukadoji Maharaj Nagpur University, Nagpur, Maharashtra -440033, India Mobile No. +9109822564859 E-mail: [email protected]

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Synthesis, structural, morphological, and thermal decomposition kinetics of Iron (II) coordination polymer of sebacoyl bis (isonicotinoylhydrazone) Mahejabeen Azizul Haque1*, Ratiram Gomaji Chaudhary2, L. J. Paliwal1 1

Post Graduate Teaching Department of Chemistry, Rashtrasant Tukadoji Maharaj Nagpur University, Nagpur, Maharshtra-440033, India 2 P.G. Department Chemistry, Seth Kesarimal Porwal College, Kamptee, Maharshtra-441001, India *To whom correspondence should be addressed. Email-id: [email protected]

Abstract: A new Iron (II) coordination polymer of sebacoyl bis (isonicotinoylhydrazone) (sebi) has been synthesized and characterized by various physicochemical methods viz. elemental analysis, fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermal analyses.

The geometry of

coordination polymer was determined by using magnetic susceptibility measurement and electronic spectra. The morphological behaviour was investigated by SEM and XRD. Further, the present article describes the comparative thermal stability and kinetic parameters of coordination polymer at different heating rates viz. 5, 7.5, 10, 15 and 20 ºC/min using various thermal analyses techniques like thermogravimetry (TG), derivative thermogravimetry (DTG), and differential thermal analysis (DTA). The TG/DTG/DTA curves of Iron (II) polymer at multiple heating rates were more or less similar in shape, which indicate that mass loss is independent of heating rate. The thermal decomposition occurs in four to five well-separated stages which involve initially the loss of water molecules in the first and second steps followed by decomposition of chelating ligand. Moreover, the kinetic parameters were evaluated at each stage of decomposition of Iron (II) coordination polymer on the basis of thermograms obtained at different heating rates. The kinetic triplet such as activation energy (Ea), order of reaction (n) and Arrhenius factor (A) were evaluated by using Coats-Redfern method. Keywords: Coordination polymer; Isonicotinic acid hydrazide; Electronic spectra; Morphology; Thermal analysis; Kinetic triplet 1.

Introduction

The design and synthesis of coordination polymers of transition metals with multidentate ligand have attracted substantial interest in supramolecular chemistry and material science due to their intriguing structural topologies and potential applications [1]. Especially, the coordination polymer of chelating ligand is an interesting topic in the branch of coordination chemistry due to its competing high thermal stability, supramolecular and scaffold structures. A vast range

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of coordination polymers with O and N-donor sites is a prime research area of material science for exploring the thermal, kinetics, optical, and mechanical properties [2, 3]. The literature survey reveals that the coordination polymers are well known for their high thermal stability and huge work has been reported on their synthesis, characterization and thermal studies [4-7]. Indeed, the coordination polymers play an important role in the field of material science, metallurgy, environmental protection, conductivity, luminescence, thermal insulation, catalysis, magnetism, nonlinear optics and coating materials [8-10]. Despite, the various factors which must be taken into consideration in developing suitable coordination polymer are nature of metal ion, ligand, solvent, temperature, and metal to ligand ratio. These metal–organic materials can be rationally synthesized and tuned by a suitable choice of organic ligands and metal ions to control their framework architecture and functionality. However, due to the complexity, coordination geometry of the metal centre and the coordination behavior of the organic ligands, it remains a challenge to design coordination polymers with interesting structure and excellent performance. During the past few decades, significant research has been embarking on the metal complexes of hydrazone type of ligands. Particularly, the hydrazones containing a pyridine ring moiety are of great interest due to their vast range of applications in biological and pharmacological sciences as anticonvulsant, antimicrobial, antitubercular, antitumor and analgesic agents [11-15]. Hydrazones have been extensively investigated due to their versatile chelating behaviour, structural flexibility and diverse range of applications [16-17]. The metal complexes of hydrazones including heterocyclic moieties containing nitrogen, oxygen and sulphur as hetero atoms have been studied extensively in order to establish a probable relationship between the chemical structure, thermal stability and biological activity [18-21]. The high thermal stability of coordination polymers might be due to the presence of chelation and heterocyclic ring formation. The introduction of metal ions into the polymer matrices enhanced the thermal stability and mechanical properties of polymers [22]. The stability of metal complexes with polydentate ligands depends on factors such as the number and type of donors present, their relative positions within the ligand, the nature of the ligand backbone, and the number and size of chelate rings formed on chelation. In symmetric bis-ligands, the donor atoms on the ring are widely separated, so that the ligand can coordinate with two or more metal ions at both ends giving chelate polymer [23-24]. Basically, the thermal analysis techniques were widely used to study thermal behaviour, decomposition pattern and structural identification of inorganic compounds, nanocomposites, nanomaterials, polymer blends, complexes and coordination polymers [25-26]. Also, the thermoanalytical techniques provided information regarding inner sphere or outer sphere of water in the coordination polymers, and to determine the endothermic or exothermic behaviour related to processes like dehydration, melting, crystallization and decomposition [27-29]. The extensive literature survey reveals that no work has been carried out on the synthesis and non-isothermal decomposition kinetics of coordination polymers of a divalent transition metal with sebacoyl bis (isonicotinoylhydrazone).

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In present study, we report the novel investigation on synthesis, structural, and morphological behaviour of sebacoyl bis (isonicotinoylhydrazone) and its Iron (II) coordination polymer. The main aim of this article is to introduce the thermal degradation behaviors and thermal stability of coordination polymer at multiple heating rates. Further, nonisothermal decomposition kinetics of Iron (II)- sebi coordination polymer have been evaluated by Coats-Redfern method. 2.

Experimental

2.1 Materials and measurements All the chemicals and solvents were used of analytical grade and without further purification. Isonicotinic acid hydrazide (Isoniazid) was purchased from Sd fine chem. Ltd. (India). Acetone, pyridine, and DMF were used after distillation. Elemental analyses were carried out using Elementar Vario EL III, CHN elemental analyzer. 1H NMR and 13C NMR spectra were recorded on a Bruker-Avance-II (400 MHz) spectrophotometer using DMSO-d6 as solvent and TMS as an internal standard. The FTIR was recorded as KBr pellets with Perkin Elmer-Spectrum RX-FTIR instrument. EI-MS spectrum was recorded on a LCQ ion trap mass spectrometer (Thermo Fisher, San Jose, CA, USA), equipped with an EI source. X-ray diffraction (XRD) of the powder samples were examined on a Bruker AXS D8 Advance diffractometer using Cu Kα radiation (λ=0.15406 nm). Scanning electron microscopy (SEM) micrographs were recorded with ZEISS EVO MA-10. UV-Vis-NIR diffuse reflectance spectra were scanned with a Varian, Cary 5000 spectrophotometer. Magnetic susceptibility measurements were carried out at room temperature on a Gouy’s balance using Hg[Co(SCN)4] as calibrant and diamagnetic corrections were made using Pascal’s constant. 2.2 Synthesis of ligand (sebi) Sebacoyl chloride was prepared by the standard method reported earlier in the literature [30]. Sebacoyl chloride (0.01 mol) was added to a cold solution of isoniazid (0.02 mol) in 50 mL acetone and 3 mL pyridine with constant stirring. The mixture was refluxed on water bath for about 5 h using guard tube of fused CaCl2 fitted with condenser and allowed to cool at room temperature. The completion of the reaction was monitored by thin layer chromatography. The reaction mixture was then poured slowly into ice cold water with constant stirring whereupon the off-white precipitate was formed. The obtained precipitate was filtered, washed several times with aqueous sodium bicarbonate solution to remove acid formed due to reaction of excess acid chloride with water, and finally dried. Further, the compound was purified by recrystallization using ethanol as a solvent. The yield of the ligand obtained was around 87% as an off-white crystalline solid and melting point is 181-183 °C. The reaction scheme for the synthesis of ligand is given in Scheme 1.

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Scheme 1. Synthesis of ligand 2.3 Synthesis of Iron (II) coordination polymer The Iron (II) coordination polymer was prepared by mixing hot solutions of Fe(II) acetylacetonate (0.02 mol) in a minimum quantity of DMF and sebi ligand (0.01 mol) in 25 ml DMF with constant stirring (Scheme 2). The reaction mixture was refluxed on an oil bath at 130-140 °C. The Iron (II) coordination polymer appeared within 10 h of refluxing on an oil bath. The reddish brown product obtained was filtered, washed thoroughly with hot DMF and then with alcohol followed by acetone, and finally dried in vacuum desiccators over silica gel blue as a desiccant.

Iron(II) acetylacetonate + sebi

DMF Reflux, 10 h

{[ Fe(II)2 (sebi) (H2O)4 ] . H2O }n

Scheme 2. Preparation of Iron (II) coordination polymer

2.3 Thermal analysis Thermogravimetric and differential thermal analyses were performed at different heating rates, β= 5, 7.5, 10, 15 and 20 °C min-1 in temperature range 30-1000 °C under nitrogen atmosphere. The kinetic parameters were calculated by means of TG/DTG/DTA data with the help of Microsoft office Excel Worksheet. For the calculation of activation energy we have assumed initial decomposition temperature (Ti), half decomposition temperature (Th), and final decomposing temperature (Tf). Table 4 depicts the degradation temperatures of the polymer at 5%, 10% and 20% and mass loss (T5, T10 and T20). 2.4 Kinetics Method The thermal analysis statistics were widely used to study the reaction mechanisms and kinetics of solids state materials [31-33]. The non-isothermal decomposition kinetics was studied from TG/DTG/DTA curves by Freeman–Carroll method [34], Sharp–Wentworth method [35], MacCallum–Tanner method [36], Ozawa equation [37], Chen–Liu (CL) method [38] and Coats–Redfern method (CR) [39] by various researchers. However, Coats–Redfern method (CR) method has been used to study the thermal decomposition process in non-isothermal condition. The kinetic parameters such as the pre-exponential Arrhenius factor (A), gas constant (R), activation energy (Ea), integral function g(a) and activation entropy were evaluated for each stages of thermal decomposition. For evaluation of kinetic parameters, we assume initial decomposition temperature (Ti), half decomposing temperature (Th) and final decomposition temperature (Tf). The matter

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released at each step of the degradation was identified through attributing the mass loss at given step to the similar mass calculated from molecular formula of investigated coordination compound. Evaluation of kinetics parameters from thermal decomposition curves for each stage was determined from generally accepted kinetic Arrhenius equation. The integral method developed by Coats-Redfern has been widely accepted as a reliable method. The kinetic parameters were evaluated using TG/DTG/DTA data under non-isothermal condition by Coats-Redfern equation.

α

(For n =1)

(1)

(For n

(2)

1)

Where α represent the fractional conversion, β is heating rate, A is the pre-exponential factor (min-1), Ea is the activation energy (kJ mol-1), R is the universal gas constant (8.314 J mol-1k-1), T is the temperature in Kelvin (K), n is the order of reaction and t is the time (min). Using Eq. 2, the Ea could be obtained from the slope of a line established from fitting the TG/DTG data:

vs. . The plot of

vs

would give a straight line [40]. While attaining the Eq. 2 the

order of reaction was not known beforehand, so first aim to assume tentative value of n. If the assumed reaction order adequately represents the reaction and line should be straight. If not, another reaction rate was assumed and the fitted line was examined for straightness [41-42]. 3. Results and discussion 3.1 Spectral characterization of ligand In the ESI mass spectrum of sebi, the molecular ion peak (M+) appeared at m/z = 441 amu, which corresponds to the molecular weight of the ligand (Figure S1). A (M+Na) molecular ion peak at m/z = 463 amu and a base peak at m/z = 177.50 amu is in good agreement with the empirical formula suggested by elemental analysis. 1H NMR of sebi (Figure S2) exhibits good signals at δ 10.61, 9.95, 8.72, 7.77, 2.18, 1.52 and 1.27 (ppm). The signals of NH protons appeared at 10.61 ppm and 9.95 ppm correspond to -NH-CO-py and -NH-CO-CH2, respectively. Peaks at 8.72 ppm (4H, d, J = 5.8 Hz, -CH-N-py ) and 7.77 (4H, d, J = 5.9 Hz, -CH-C-py) are due to protons of pyridine ring. Three multiplet peaks at 2.18 ppm (4H, m, C-CH2-CO), 1.52 ppm (4H, m, -CH2-C-CO) and 1.27 ppm (8H, m, -CH2-CH2-) are assigned to the presence of aliphatic chain. In 13C NMR of sebi (Figure S3), signals for -CH2- groups of aliphatic chain appeared at 24.44, 28.62 and 33.66 ppm. The resonance signals at 121.28, 139.50 and 150.02 ppm are attributed to carbon skeleton of pyridine ring. Signals at 163.74 and 174.47 ppm confirm the presence of carbonyl carbon of ligand. 3.2 Characterization of Iron (II) coordination polymer Iron (II) coordination polymer has been characterized by various physicochemical methods like elemental analysis, diffuse electronic and infrared spectral studies, magnetic susceptibility measurement, XRD, SEM, and thermal analysis.

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The analytical and physical properties of Iron (II) coordination polymer are listed in Table 1. The Iron (II) coordination polymer is stable towards air and moisture at room temperature. It is insoluble in all common organic and polar solvents like water, DMF and DMSO. The thermoanalytical data of the polymer are tabulated in Table 3-4. The comparative IR spectra of sebi and Iron (II) coordination polymer is depicted in Figure 1, and the spectral data are given in Table 2. Comparison of IR spectra of ligand and Iron (II) polymer revealed that the spectrum of Iron (II) polymer differs significantly from that of the ligand in some characteristic frequencies. The IR spectrum of ligand shows a band at 3292 cm-1 due to N-H frequency. The presence of methylenic (-CH2-) group is confirmed by presence of two strong bands at 2925 and 2847 cm-1 of CH2 asymmetric and symmetric stretching, respectively. While a band at 1499 cm1

is due to CH2 bending mode. The 1741 and 1578 cm-1 bands in the ligand may be attributed to >C=O and (C=N)

pyridine stretching, respectively [43]. A broad band observed at 3160 cm-1 may be assigned to ν (-OH) vibrations, suggesting the existence of several keto-enol tautomers in ligand as shown in Figure S4.

Figure 1. FTIR spectra of sebi ligand and Iron (II) coordination polymer A medium absorption band at 3160 cm-1 in ligand may be due to the intramolecular hydrogen-bonded enolic hydroxyl group. IR spectrum of Iron (II) indicates the disappearance of this absorption band suggesting deprotonation of –OH groups of ligand. Furthermore, the spectral band at 1253 cm-1 due to C-O stretching in the ligand, has also been shifted to higher frequency region at 1317 cm-1, suggesting formation of M-O bond by replacing the proton from the enolic hydroxyl group on chelation. Also, the band at 1741 cm-1 due to ν(C=O) in the ligand disappears in IR spectrum of Iron (II) suggesting coordination through oxygen atom of >C=O group of the ligand. In IR spectrum of ligand, a strong band at 1676 cm-1 is assigned to stretching vibration of exocyclic C=N-N=C group. Upon ligation of sebi with Iron (II), this band is shifted to a lower frequency region ~1591 cm-1 in Iron (II) polymer. This clearly suggests coordination of Iron (II) through nitrogen atom of exocyclic C=N-N=C group of the ligand [44]. Noteworthy two new bands appeared in

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IR spectra of coordination polymer at 692 and 616 cm-1 were due to coordination of Fe(II) metal ion with oxygen of >C=O group (M-O) and nitrogen of C=N-N=C group (M-N) respectively. Moreover, a broad absorption band was seen at 3205 cm-1 may be due the O-H group stretching vibration frequency of lattice water molecules, whereas a sharp bands at 846 cm-1 and 1412 cm-1 observed because of rocking and bending vibration modes of coordinated water molecule, hence significantly confirming an octahedral geometry of Iron (II) polymer.

Figure 2. Diffused reflectance spectra of Iron (II) coordination polymer

The diffused reflectance spectrum of Iron (II) coordination polymer was studied in the range 200-1980 nm and various crystal field parameters such as ∆ₒ, Racah interelectronic repulsion parameter (B and C) and Nephelauxetic ratio (β) have been calculated. The nature of ligand field around the metal ion has been deduced from the electronic spectra of the polymer. The electronic spectrum exhibits absorption bands at 11723 and 30488 cm-1 (Figure 2) which may be assigned to octahedral geometry. The first band at11723 cm-1 may be due to 5T2g (D) → 5Eg transition, while the other band at 30488 cm-1 may be due to charge transfer transition. The magnetic moment is found to be 5.37 B.M. which is in favour of six coordinated the high spin an octahedral geometry. The crystal field parameters obtained from electronic spectrum of this polymer are ∆ₒ = 11723 cm-1, B = 634 cm-1, C = 2534.72 cm-1 and β = 0.5978. The reduction of Racah parameter from the free-ion value of 1060 to 634 cm-1 and β value suggest the partial covalent nature of metal-ligand bond in the polymer [44-45]. On the basis of IR studies and by considering the potential donor atoms of the ligand, it can be concluded that the ligand is linked to Fe(II) ion at eight sites, thus acting as an octadentate ligand. On the basis of IR and electronic spectral studies the polymeric structure shown in Figure 3 is proposed for the Iron (II) polychelate.

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Figure 3. Structure of Iron (II) coordination polymer

Table 1. Analytical and physical data of Iron (II) coordination polymer and ligand Empirical

Formula

formula

weight a

sebi

C22H28N6O4

440.50

{[Fe2(sebi)(H2O)4]H2O}n

C22H34N6O9Fe2

638.20

Compound

Color Off white Reddish brown

Found (calculated) (%) c

Yield

DH b

%

(°C)

M

82.00

353

-

68.90

420

C

H

N

59.04

6.17

19.23

(59.93)

(6.35)

(19.06)

17.49

41.35

5.32

13.13

(17.50)

(41.36)

(5.33)

(13.16)

a

As calculated from the empirical formula of the repeating unit Half-Decomposition Temperature c Calculated values in parentheses. b

Table 2. FTIR spectral data of ligand and Iron (II) coordination polymer Compound

N-H

-OH

C=O

C=N (py)

C=N-N=C

C-O

C-N

M-O

M-N

ν(H2O)

δ(H2O)

ρr(H2O)

sebi

3292

3160

1741

1578

1676

1253

1323

-

-

-

-

-

Fe(II)-sebi

-

-

-

1560

1591

1317

-

692

616

3205

1412

846

3.2 Morphological behavior The morphology of Iron (II) coordination polymer has been investigated using X-ray diffraction and scanning electron microscopy techniques. The X-ray diffractogram of Iron (II) polymer (Figure 4(a)) exhibited some short, sharp peaks and also showed a halo pattern in the region 2θ = 10-80º. This indicates the semi-crystalline nature of the polymer. The SEM image of the polymer was recorded at energy of 10.00 kV with magnification of about 6.83 KX. The micrograph of

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polymer (Figure 4(b)) shows globular particles of varying sizes. The bigger particle has about 2 μm diameter whereas the smaller particles have diameter in the range of 0.25-0.5 μm.

(a)

Fig. 4 (a) Powder XRD & (b) SEM image of Iron (II) coordination polymer

3.3 Thermal analysis Thermal analysis technique is widely used to determine the thermal degradation behaviours, decomposition temperature, thermal stability and kinetics triplet of solid state compounds. It is also useful to identify the lattice and coordinated water molecules in the present study. The isoconversional methods in non-isothermal thermogravimetric experiments are valuable for evaluation of kinetic parameters. The mass loss at every stage of thermal decomposition curve is identified and is compared with the similar mass calculated from molecular formula of the polymer under investigation. The thermal decomposition behaviour is influenced by some essential factors such as the mass of investigating polymer and the multiple heating rates. In order to study the influence of multiple heating rates on the thermal decomposition behaviour of coordination polymer, thermogram of Iron (II) coordination polymer has been recorded at different heating rates viz. 5, 7.5, 10, 15 and 20 °C min-1 under nitrogen atmosphere over temperature range 30-1000 °C. The literature survey reveals that there are only few articles reported on evaluation of thermodynamics kinetics at multiples heating rates [24-25]. The TG/DTG curves for Iron (II) coordination polymer at various heating rates are shown in Figure 5. All the TG curves obtained at different heating rates are approximately similar in shape indicating that mass loss is independent of heating rate. 3.3.1. Thermal degradation behaviour of Iron (II) coordination polymer The thermograms of sebi and Iron (II) coordination polymer are shown in Figure 5, which depict mass loss during heating from ambient temperature to 1000 °C. It reveals that the ligand decomposes in three stages, while the polymer decomposes in five consecutive stages. The half-decomposition temperatures of ligand and polymer were found to be

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353 °C and 420 °C, respectively suggesting higher thermal stability of polymer as compared to a ligand. In the case of the polymer, the initial mass loss is due to moisture and trapped solvent, while the major mass loss at higher temperature is attributed to decomposition of an organic moiety of polymer in several steps, finally leading to the formation of iron oxide residue.

Figure 5. TG curves of sebi and its Iron (II) coordination polymer The various TG-DTG curves at multiple heating rates viz. 5, 7.5, 10, 15 and 20 °C min-1 under nitrogen atmosphere have been shown in Figure 6. In the present study of Iron (II) coordination polymer, the structural transformation in each stage was investigated with the help of thermogravimetric analyses under controlled multiple heating rates.

Figure 6. TG and DTG curves of Iron (II) coordination polymer

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Table 3. TG/DTG/DTA data and assignments of Iron (II) coordination polymer

I II

DTGmax TDTG 75 248

Temperature range (ºC) 31-110 110-260

Weight loss (%) Obs/Calc 3.0/2.8 11.4/11.3

1 H2O (lattice water) 4 H2O (coordinated water)

III

329

260-400

38.8/38.7

56.5% ligand

IV

479

400-520

9.9/10.0

14.6% ligand

V

756

700-820

8.2/8.1

11.8% ligand

Stages

Assignment

A close and detailed investigation of all thermal curves Figure 6 suggested that thermal decomposition of the polymer occurs in five prominent stages. The non-isothermal thermograms indicated first stage of thermal degradation in temperature range 31-110 °C with TDTG peak at 75 °C corresponds to loss of one lattice water molecule with the mass loss of 3.0% (calc. 2.8%). The effective decomposition temperatures of Iron (II) coordination polymer are shown in Table 4. The activation energy (Ea) for the first step was found to be in the range 9-15 kJmol-1. The second stage decomposition due to loss of coordinated water molecules was observed in temperature range 110-260 °C and the corresponding activation energy was calculated in the range 40-100 kJmol-1. The mass loss of 11.4% (calc. 11.3%) corresponds to loss of two coordinated water molecules. The third, fourth and fifth stages of decomposition may be due to loss of organic moiety and the energy of activation for all these stages (40-200 kJmol-1) have been tabulated in Table 5. The third stage of degradation in temperature range 260-400 °C involved mass loss 38.8% (calc. 38.7%) of 56.5% ligand with strong TDTG peak at 329 °C. However, in fourth stage degradation was observed in temperature range 400-520 °C as a result of mass loss 9.9% (calc. 10.0%) for 14.6% ligand with TDTG peak at 479 °C. The fifth stage of thermal degradation in temperature range 700-820 °C showed mass loss 8.2% (calc. 8.1%) of 11.8% ligand with corresponding TDTG peak at 756 °C. Eventually no further mass loss was observed above 820 °C because of formation of stable iron oxide, perhaps indicated a good thermal stability of Iron (II) polymer confirming that strong interaction or chemical bonding exist between polymeric ligand and Fe (II) metal ion. The multiple DTG/DTA peaks of the polymer suggest that polymer has multiple step decomposition patterns and hence undergo decomposition in different stages. The DTG peaks indicate that higher the heating rate, higher will be the temperature of observed DTG peaks. However, mass loss in the polymer at every stage does not change significantly at different heating rates. Thermal stability was investigated on the basis of various kinetic parameters such as activation energy (Ea), frequency factor (A) and correlation coefficient (r2) for different stages at multiple heating rates and are summarized in Table 6.

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Table 4. Thermal decomposition temperature of Iron (II) coordination polymer Compound

TG a

Ti

sebi Sebi-Fe (II)

200

310

b

Tmax

302, 357, 401.5 75, 248, 329, 479, 756

c

d

e

470

353

261

276

294

0

850

420

160

225

253

27.00

Tf

Th

T5%

f

T10%

g

T20%

h

Residue %

a

= initial degradation temperature for ligand in polymer; b= peak temperature; c= final step decomposition temperature; d= half decomposition temperature; e-g= temperature corresponding to 5%, 10% and 20% weight loss; h= residue left after TGA analysis at 950 ºC. 3.4. Kinetics study of Fe (II) coordination polymer Universally accepted Coats-Redfern method has been successfully used to investigate the thermal degradation behaviour, thermal stability, kinetics mechanism and to decide whether water molecules present are either in the inner or outer sphere of newly synthesized coordination polymer at multiple heating rates. Table 6 indicated that the first stage of decomposition required low Ea, which may be assigned to the rupture of weaker bonds, i.e. lattice water. In contrast, increase in decomposition conversion for stronger bonds required higher Ea values. Therefore, the fractional conversion values (α) were calculated at each stage (Supplementary source file Table S1) and resulting data suggested a different mechanism for each step in the decomposition process. Plots of fractional conversion (α) versus temperature (K) for first, second and third degradation stages are displayed in Figures 7-9 and for fourth and fifth degradation stages are showed in Figure S5 and S6. Figures 7-9 reveal that the plotted curves shifted to higher temperature with increasing heating rates, signifying that ‘α’ value increases with increase in temperature. Further, the CR plot for each stage was plotted by taking ln(g(α))/T2) versus 1/T and displayed in Figures 10-12 for first, second and third stages, while those for fourth and fifth stages are given in Supplementary Figures S7 and S8. It gives a straight line having slope –Ea/R; thus, activation energies were determined from the obtained slope. The plots of activation energy versus fractional conversion at multiple heating rates have been shown in Figure 13 (Table 5).

Figure 7. Plot of fractional conversion versus temperature for first stage decomposition of Iron (II) coordination polymer

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Figure 8. Plot of fractional conversion versus temperature for second stage decomposition of Iron (II) coordination polymer

Figure 9. Plot of fractional conversion versus temperature for third stage decomposition of Iron (II) coordination polymer From Figure 13 and Table 5 it can be concluded that, as the reaction proceeds, there is a initial increase of activation energy up to the third stage and then it smoothly decreases for the fourth stage. However, the Ea for the final stage of decomposition further increases. In the initial stage of the decomposition process, the Ea is 9-15 kJmol-1 at α=0.0340.051. It increases fairly, reaching about 40-60 kJmol-1at α=0.214-0.365 with a maximum value of 50-95 kJmol-1 at α=0.4-0.6 which corresponds to third decomposition stage. Then, there is a slight decrease in the Ea value i.e. in the range 35-90 kJ mol-1 at α=0.6-0.9 and lastly rises sharply to 80-110 kJmol-1at α=0.9-1.0. This confirms that thermal decomposition process is a complex mechanism involving parallel and multi-step reactions.

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Figure 10. CR plot for evaluation of Ea of the first stage of Iron (II) in nitrogen atmosphere

Figure 11. CR plot for evaluation of Ea of the second stage of Iron (II) in nitrogen atmosphere

Figure 12. CR plot for evaluation of Ea of the third stage of Iron (II) in nitrogen atmosphere

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Figure 13. Plot of Ea versus α for all stages of Iron (II) coordination polymer at heating rates 5, 7.5, 10, 15 and 20 ºC/min From Figure 13 it is clearly observed that initially activation energy increases as the fractional conversion increases; then it falls down though fractional conversion is high, and finally there is sudden increase in activation energy which reveals that initial and final stages of decomposition are slow processes, while fourth stage is fast because of weakly bonded organic fragments. However, first stage has minimum value of activation energy while the activation energy value increases smoothly giving maximum value at final stage. Table 5. Activation energy and fractional conversion of Iron (II) coordination polymer at multiple heating rates Stages

β=5 ºC/min Ea/kJmol

β=7.5 ºC/min

1

α

Ea/kJmol

1

β=10 ºC/min

α

Ea/kJmol

1

β=15 ºC/min

α

Ea/kJmol

1

β=20 ºC/min

α

Ea/kJmol1

α

I

13.7929

0.0371

12.1467

0.0341

9.9186

0.0507

14.6492

0.0461

9.4696

0.0421

II

40.3644

0.3652

48.2877

0.2461

59.2641

0.2138

55.8950

0.3617

59.2289

0.2661

III

49.5763

0.5354

60.4178

0.5797

78.2846

0.3798

59.2123

0.5551

55.1467

0.5750

IV

44.2720

0.8240

72.8306

0.8337

38.9594

0.6992

87.5713

0.8184

73.0301

0.7952

V

102.884

0.9665

88.1866

0.9872

81.9926

0.9475

93.1084

0.9614

87.0309

0.9350

Table 6. Kinetic parameters of Iron (II) coordination polymer in non-isothermal condition at multiple heating rates using CR equation Stages

n

I

β=5 ºC/min

β=7.5 ºC/min

lnA

2

lnA

2

r

r

1.1

3.4751

0.991

15.1872

II

2.1

10.1688

0.984

III

2.5

12.2244

IV

2.6

V

1.4

β=10 ºC/min lnA

2

r

0.994

2.3256

11.9543

0.992

0.988

14.4978

10.7455

0.989

26.8111

0.996

β=15 ºC/min

β=20 ºC/min

lnA

2

r

lnA

r2

0.997

3.8541

0.996

7.3788

0.995

21.7571

0.992

13.5789

0.994

14.1878

0.986

0.988

16.8048

0.988

13.9842

0.993

13.0089

0.996

15.6329

0.999

8.4591

0.998

17.7972

0.998

15.0092

0.996

13.7315

0.993

12.3633

0.998

13.8137

0.995

12.8837

0.988

16

3.

Conclusions

On the basis of physicochemical characterization and thermal studies, the ligand sebi acts as octadentate ligand binding to Fe(II) through nitrogen and oxygen atoms of ligand. In each repeat unit of coordination polymer there are four 5membered chelate rings along with four molecules of water of coordination which suggest octahedral stereochemistry to Fe(II) coordination polymer. Due to formation of 5-membered four chelate rings, the Fe(II) coordination polymer achieves more thermal stability as compared to the ligand. One lattice water and four coordinated water molecules per repeat unit of Fe(II) polymer were confirmed on the basis of thermal analyses. The activation energy was calculated for each stage of thermal degradation curve using Coats-Redfern method. A significant sigmoid curve was obtained by plots of activation energy versus fractional conversion in all the multiple heating rates of thermal decomposition. The CR data reveals that the activation energy depends on degree of fractional conversion. The kinetics study in non-isothermal conditions suggested that Iron (II) coordination polymer was highly thermally stable. The results supported that first and second step of decomposition were accompanied by lower activation energies while third and consecutive steps of decomposition by higher activation energies indicating slow degradation of ligand. Final residue obtained corresponds to stable iron oxide. Acknowledgements The authors are thankful to SAIF Chandigarh, Punjab University, Punjab and STIC, Cochin, Kerala for recording various spectroscopic and thermal analyses. The authors are also grateful to Department of Physics, Rashtrasant Tukadoji Maharaj Nagpur University, Nagpur, for SEM analyses. References [ 1]

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18

HIGHTLIGHTS

The synthesized sebacoyl bis(isonicotinoylhydrazone) (sebi) ligand acts as an octadentate ligand. The new Iron (II) coordination polymer has high spin octahedral geometry. Iron (II) coordination polymer achieves more thermal stability as compared to the ligand due to formation of four 5-membered chelate rings in each repeat unit. A significant sigmoid curve was obtained by plots of activation energies versus fractional conversion. Investigated kinetic parameters values using Coats-Redfern method suggested more thermal stability for higher stages than lower stages of thermal decomposition at different heating rates.

19