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Synthesis, spectral and thermo-kinetics explorations of Schiff-base derived metal complexes

  • Naushad Ahmad EMAIL logo , Manawwer Alam , Rizwan Wahab , Mukhtar Ahmed and Ashfaq Ahmad
Published/Copyright: October 20, 2020
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Open Chemistry
From the journal Open Chemistry Volume 18 Issue 1

Abstract

Schiff-base ligand, 2,6-bis(benzimino)-4-phenyl-1,3,5-triazine (L), and its transition metal complexes of Co(ii), Ni(ii), and Cu(ii) were synthesized by refluxing the reaction mixture and its analytical, spectral, and thermogravimetric characteristics were explored by various techniques: AAS, FT-IR, UV-vis, TG-DTG, CHNS/O, and VSM. It was observed that all the metal containing complexes are non-electrolytic, mononuclear, and paramagnetic in nature, confirmed by the molar conductance and magnetic susceptibility measurements. Optical spectral data were used to investigate the geometrical and spectral parameters of [Co(L)(ac)2], [Ni(L)(ac)2], [Cu(L)(ac)2], [Cu(L)(acac)2], and [Cu(L)(fmc)2] complexes. Simultaneous thermal analyses (TG-DTG) in nitrogen atmosphere reveal that the ligand decomposes in one step, [Co(L)(ac)2], [Ni(L)(ac)2], and [Cu(L) (ac)2] complexes are decomposed in three steps, whereas [Cu(L)(acac)2] and [Cu(L) (fmc)2] are decomposed in five and two steps, respectively. In addition, activation energy (Ea) and pre-exponential factor (ln A) were evaluated by TG-DTG decomposition steps of compounds using the Coats–Redfern formula. Enthalpy (∆H), entropy (∆S), and Gibbs free energy (∆G) of the as-prepared metal complexes were also speculated by various thermodynamic equations.

Keywords: Schiff-base; metal complexes; spectroscopic; kinetics; Coats–Redfern.

1 Introduction

Schiff base is one of the important classes of ligands, which have attracted special interest and are fast developing on account of their industrial, thermal, and biological applications. It is well documented that the combination of metal ions with Schiff bases provides flexible binding sites, which lead to materials with interesting properties such as reversibly binding oxygen capacity, catalytic conversion reaction: hydrogenation of olefins, transfer of the amino group, photochromic, sensing, magnetic, and optical properties [1,2,3,4,5,6]. Transition metal complexes of Schiff bases, fabricated from aldehydes and amines, have been widely explored. Because of the presence of both nitrogen and oxygen donor atoms in the Schiff base, they can easily coordinate with transition metal ions, producing enhanced thermally stable and colored metal complexes. They can serve as models of biologically important species and find applications in biomimetic catalytic reactions, materials chemistry and have a strong anticancer activity [7,8].

With the advancement of science and explosion of technologies, researchers have been showing an increasing interest in solid-state thermal kinetic studies of transition metal complexes containing both nitrogen and oxygen donor ligands [9,10]. Various thermal analysis techniques, such as thermogravimetric analysis (TGA), derivative thermogravimetry (DTG), differential scanning calorimetry (DSC), and differential thermal analysis (DTA), were extensively applied to investigate the thermal behavior of Schiff-base metal complexes such as activation energy, pre-exponential factor, and reaction order [11,12,13]. Among them, the TG-DTG technique is widely applied because of its simplicity and the information afforded by simple thermograms. On thermal treatment, these complexes undergo decomposition reactions and release heat, gaseous products, and solid metal oxide residues, which could be used in explosives, propellants, pyrotechnic compositions, and preparation of environmental sensors for the detection of trace-level different hazardous materials [14–16].

Thus, in the light of the above enormous applications, in the present manuscript, we discussed the synthesis, structural features, and kinetic parameters of the Schiff base derived from 2-pyridinecarboxaldehyde and 2,6-diamino-4-phenyl-1,3,5-triazine and its Co(ii), Ni(ii), and Cu(ii) acetates as well as acetylacetonate and trifluoromethanesulfonate Cu(ii) complexes. In the kinetic study on thermal decomposition, the Coats–Redfern equation is used for the calculation of kinetic parameters such as activation energy (E), enthalpy (∆H), entropy (∆S), and Gibbs free energy (∆G).

2 Experimental

2.1 Materials

2-Pyridinecarboxaldehyde, 2,6-diamino-4-phenyl-1,3,5-triazine, copper(ii) acetylacetonate [Cu(C5H7O2)2], copper(ii) trifluoromethanesulfonate [(CF3SO3)2Cu], copper(ii) acetate monohydrate [(CH3COO)2Cu·H2O], cobalt(ii) acetate tetrahydrate [(CH3COO)2Co·4H2O], and nickel(ii) acetate hydrate [(CH3COO)2Ni·H2O] reagent grades were purchased from Sigma-Aldrich. Solvents, such as dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethanol, methanol, diethyl ether, and acetone for synthesis and physical measurements, were obtained from Fluka and used without further purification.

2.2 Physical measurements

Elemental analyses of fabricated samples in powder form were performed on PerkinElmer 2400 Series II CHNS/O Elemental Analyzer. For the determination of metal percentages, atomic absorption spectroscopy (AAS), PerkinElmer 1100B spectrometer, was used after microwave digestion (TOPwave Analytik Jena microwave) of samples in the mixture of 65% HNO3 and 37% HCl. The FT-IR spectra were used for the determination of functional groups using the KBr disk method in the 4,000–400 cm−1 region on a Bruker Tensor 27 FT-IR spectrophotometer at room temperature. The electronic spectra of fabricated samples in DMF solution were recorded at room temperature on a UV-2550 spectrophotometer (Shimadzu, Japan). The 1H NMR spectrum of the Schiff-base ligand was recorded on a JEOL-GSX 300-MHz FX-1000 FT-NMR at room temperature using deuterated DMSO-d6 as a solvent and TMS as an internal standard. Chemical shifts are given in ppm, relative to TMS. Simultaneous TG-DTG of samples was carried out by SDT-Q600 (TA Instrument) in nitrogen atmosphere (100 mL min−1) at a heating rate of 20°C min−1 in the temperature range from room temperature to 800°C.

2.3 Synthesis of ligand [2,6-bis(benzimino)-4-phenyl-1,3,5-triazine]

A hot methanol solution of 2,6-diamino-4-phenyl-1,3,5-triazine (1.8 g, 0.01 mol) was added dropwise to a hot methanol solution of 2-pyridinecarboxaldehyde (2.14 g, 0.02 mol) and the reaction mixture was refluxed at 70°C for 6 h. The course of reaction was monitored by thin-layer chromatography (TLC). After completing the reaction, the reaction mixture was left to stand overnight. The resulted brown colored solid was filtered off, washed with ether and acetone, and dried under vacuum. The product is soluble in DMF and DMSO and insoluble in MeOH, EtOH, CHCl3, acetone, and benzene.

2.4 Synthesis of Co(ii), Ni(ii), and Cu(ii) complexes

A hot methanol solution of 2,6-diamino-4-phenyl-1,3,5-triazine (1.8 g, 0.01 mol) was added dropwise to a hot methanol solution of 2-pyridinecarboxaldehyde (2.14 g, 0.02 mol) to form a brown suspension. Then an appropriate amount of Co(ii), Ni(ii), and Cu(ii) (0.01 mol) metal salts was added, refluxed at 70°C for 4 h, and left to stand overnight at room temperature. The obtained complexes were filtered off, washed several times with cold EtOH, water, and ether, and then dried under vacuum.

  1. Ethical approval: The conducted research is not related to either human or animal use.

3 Results and discussions

Synthetic pathways for the preparation of ligand and its metal complexes are shown in Scheme 1. The Schiff-base derived Co(ii), Ni(ii), and Cu(ii) metal complexes are stable at room temperature and soluble in DMF, DMSO, THF, and CHCl3. The formation of products was recognized by FT-IR, NMR, UV-vis, AAS, and elemental analyses. Physical characterization, molar conductance, and elemental analyses of ligand and its metal complexes are listed in Table 1. The stoichiometry of ligand and its metal complexes was confirmed by their elemental analyses. The analytical data of metal complexes correspond with the general formula MLX2, where L = ligand; M = Co(ii), Ni(ii), or Cu(ii); and X = acetate, acetylacetonate, and trifluoromethanesulfonate ions. The observed molar conductance measured in DMF solution is 8.90–10.50 Ω−1 cm2 mol−1 and thereby indicates that all complexes are non-electrolytic and mononuclear in nature [16,17].

Scheme 1 Schematic preparation of the ligand and its metal complexes.
Scheme 1

Schematic preparation of the ligand and its metal complexes.

Table 1

Analytical and physical data of the Schiff base and its metal complexes

CompoundM.W.ColorStateM.P.YieldCFound (calculated) (%)Molar conductance (Ω−1 cm2 mol−1)
CHNOM
L365.39BrownLiquid1606570.31 (69.03)4.88 (4.14)27.08
C21H15N7−26.83
[CoL(ac)2] C25H21N7O4Co542.41BrownSolid3156455.33.8718.0711.7910.858.9
−55.36−3.9−18.08−11.8−10.87
[NiL(ac)2] C25H21N7O4Ni542.17Dark greenSolid3505955.333.8718.0711.810.867.5
−55.38−3.9−18.08−11.8−10.83
[CuL(ac)2] C25H21N7O4Cu547.02GreenSolid3106254.843.8317.9111.6911.616.75
−54.89−3.87−17.92−11.7−11.62
[CuL(acac)2] C31H29N7O6Cu659.15GreenSolid3255556.434.3914.8614.569.636.15
−56.49−4.43−14.87−14.56−9.94
[CuL(fmc)2] C24H15N7O4F6Cu642.95GreenSolid3355844.792.3315.249.959.887.95
−44.83−2.35−15.25−9.95−9.88

3.1 FT-IR study

The FT-IR spectral data containing relevant characteristic band regions of the ligand and its metal complexes are registered in Table 2. The ligand formation has been confirmed by the appearance of an azomethine ν(C–Na) band at 1,630 cm−1 and the absence of a ν(NH2) band at 3,400 cm−1 indicates that the Schiff-base condensation occurred between carbonyl groups of 2-pyridinecarboxaldehyde and amino groups of 2,6-diamino-4-phenyl-1,3,5-triazine [18,19]. The vibrations of 2-pyridinecarboxaldehyde ν(C–Nb) and 2,6-diamino-4-phenyl-1,3,5-triazine ν(C–Nc) groups of ligands are dictated at 1,558 and 1,605 cm−1, respectively. The spectrum of the ligand is also characterized by two medium bands at 608 and 413 cm−1, which are considered “in plane” and “out plane” deformation modes of 2-pyridinecarboxaldehyde rings, respectively [18]. A major negative shift in the ν(C–Na) band, which appeared in the 1,610–1,585 cm−1 region for complexes as compared to the ligand, endows that two azomethine nitrogens are coordinated with metal ions. Moreover, distinct bands in the far-IR region at 455–410 cm−1 were assigned the ν(M–N) band [20], which provides a direct evidence for the coordinated metal ion in the ligand frame. Co(ii) and Ni(ii) complexes have also shown negative shifts in the ν(C–Nb) band, which appeared at 1,543 and 1,545 cm−1, respectively, as compared to the ligand, suggesting that 2-pyridinecarboxaldehyde nitrogen is intricate in the coordination with metal ions. The two bands at 1,594–1,579 cm−1 and 1,378–1,367 cm−1 are assigned asymmetric and symmetric vibrations of acetate ions. The observed bands in these regions indicate that acetate groups are bonded to metal ions in a monodentate manner in case of [CuL(ac)2], [NiL(ac)2], and [CoL(ac)2] [21]. The bands in [CuL(acac)2] observed at 1,565 and 1,380 cm−1 regions, ascribed to νas(C–O) and νs(C–O) modes of coordinated acetylacetonate molecules, suggest that acetylacetonate ions are retained in the complex [22]. The vibrations of the triflate anion in [CuL(fms)2] are observed at 1,275, 1,035 νas(S–O), and (S–O); 1,220, 1,160νs(C–F), and νas(C–F) cm−1 are in agreement with the previous literature on monodentate coordination [23]. The other absorption bands corresponding to the aromatic ν(C–C), ν(C–H) and aliphatic ν(C–H) appeared at their proper positions. Thus, the FT-IR result indicates that the Schiff-base ligand in complexes is binding through azomethine and 2-pyridinecarboxaldehyde.

Table 2

Infrared spectral data of the ligand and its complexes.

Compoundν(C–N)νasym(COO−)νsym (COO−)ν(C–C)aromν(C–H)aromν(C–H)aliν(M–N)ν(M–O)
L1,630a1,6203,068
1,558b
1,605c
[CoL(ac)2]1,585a1,5881,3671,6063,0422,919455515
1,543b
1,596c
[NiL(ac)2]1,595a1,5791,3721,6173,0482,910410545
1,545b
1,598c
[CuL(ac)2]1,612a1,5941,3781,6033,0552,920430536
1,555b
1,602c
[CuL(acac)2]1,610a1,6053,0602,925435509
1,560b
1,601c
[CuL(fms)2]1,615a1,6113,0532,909425543
1,559b
1,600c

Note: (a) azomethine, (b) pyridinecarboxaldehyde, and (c) 2,6-diamino-4-phenyl-1,3,5-triazine ring ν(C–N) bands.

3.2 NMR study

To determine the structure of a ligand, the 1HNMR spectrum of the ligand was performed in DMSO-d6. It did not show any signal corresponding to the primary amine group and aldehyde protons. The singlet peak at 7.49 was assigned to two N–CHa protons [24] and doublets at 8.36 to two N–CHb protons. The aromatic proton appears at 6.70, 6.87, and 7.48 ppm. The 13C NMR spectral data of the ligand confirm the 1H NMR spectral results. The peaks at 161.2, 153.5, and 157.2 ppm for (C–Na), (C–Nb), and (C–Nc) are observed. The aromatic ring carbon appears at 130.0, 128.1, 126.3 ppm and other 2-pyridinecarboxaldehyde and 2,6-diamino-4-phenyl-1,3,5-triazine carbon at 120.4, 137.1, 119.0, 146.2 and 161.2, 159.0, 77.3 ppm. Spectral analysis confirmed the formation of a Schiff-base condensation reaction between carbonyl groups of 2-pyridinecarboxaldehyde and amino groups of 2,6-diamino-4-phenyl-1,3,5-triazine.

3.3 UV-visible absorption spectral and magnetic measurements

The spectral parameters calculated by the Tanabe–Sugano diagram, band positions of absorption maxima, band assignments, and proposed geometry are listed in Table 3. The electronic spectrum of the ligand exhibits a band at 38,461 cm−1, which is allocated π–π*transition of the aromatic ring and shows another broad band at 26,315 cm−1, which is assigned n–π*/π–π* transition of the azomethine group. The hyperchromic shift in metal complex spectra clearly confirmed the coordination of ligand toward metal ions.

Table 3

Electronic spectral and magnetic moment data of metal complexes

CompoundsMagnetic moment (B.M)Spectral bands (cm−1)AssignmentsΔo (cm−1)Ba (cm−1)βGeometry
L26,315n–π*/π–π*
38,461π–π*
[CoL(ac)2]3.7516,0774T1g → 4A2g(F)12098.1864.150.771Oh
34,4824T1g → 4T1g(p)
[NiL(ac)2]2.7615,0153A2g(F) → 3T1g(F)6606.6600.60.556Oh
32,2583A2g(F) → 3T1g(p)
[CuL(ac)2]1.6815,6252B1g → 2EgSP
34,5022B1g → 2A1g
[CuL(acac)2]1.7715,5032Eg → 2T2gOh
[CuL(fms)2]1.4315,1052B1g → 2EgSP
33,8982B1g → 2A1g
  1. a

    B values used for Co(ii) 1,120 and Ni(ii) 1,080 cm−1

The electronic spectrum of the Co(ii) metal complex showed two bands at 16,077 and 34,482 cm−1 regions assignable to 4T1g(F) → 4A2g(F)(ν2) and 4T1g(F) → 4T1g(P)(ν3) transitions, respectively, which suggest an octahedral geometry around the Co(ii) ion (Figure 1a) [25]. On the basis of these assignments, the ligand field parameters calculated for the Co(ii) complex are Δo = 12098.1 cm−1, B = 864.15 cm−1, and β = 0.771. Similarly, the Ni(ii) spectrum revealed an octahedral geometry, which has been confirmed by the electronic transitions at 15,015 and 32,258 cm−1, which represent two 3A2g(F) → 3T1g(F)(ν2) and 3A2g(F) → 3T1g(P)(ν3) transitions, respectively (Figure 1b) [25]. The calculated spectral parameters are as follows: Δo = 6606.6 cm−1, B = 600.6 cm−1, and β = 0.556. In the electronic spectra of both the Co(ii) and Ni(ii) complexes, the first transition was not perceived due to being beyond the range of the used instrument which could have occurred in the near-IR region. Therefore, the reduced Racah parameter from the free ion and β values specified the covalent character of the metal-to-ligand σ bond. The observed magnetic moments 3.75 and 2.76 B.M. for Co(ii) and Ni(ii) complexes, respectively, pointed out the three and two unpaired electrons for Co(ii) and Ni(ii), respectively, which support octahedral geometry. Dark green [CuL(ac)2] shows spectral bands at 15,625 and 34,502 cm−1 assigned to 2B1g → 2Eg and 2B1g → 2A1g, respectively, indicative of square planar geometry (Figure 1c). Green [CuL(fmc)2] shows spectral bands at 15,105 and 33,898 cm−1 assigned to 2B1g → 2Eg and 2B1g → 2A1g, respectively, indicative of square planar geometry (Figure 1d). The spectrum of [CuL(acac)2] displays a broad band at 15,503 cm−1, which is consistent with the expected octahedral geometry, which corresponds to the 2Eg → 2T2g transition [26]. These Cu(ii) complexes exhibited a magnetic moment value very close to the spin value for an unpaired between 1.73 and 178 B.M.

Figure 1 Electronic spectra of (a) [CoL(ac)2], (b) [NiL(ac)2], (c) [CuL(ac)2], and (d) [CuL(fms)2] metal complexes.
Figure 1

Electronic spectra of (a) [CoL(ac)2], (b) [NiL(ac)2], (c) [CuL(ac)2], and (d) [CuL(fms)2] metal complexes.

3.4 Thermogravimetric study

To make certain the proposed formula of compounds, thermal degradation patterns were investigated under nitrogen flow. The simultaneous TG-DTG curves are shown in Figure 2 and temperature ranges, mass losses, residues, and maximum decomposition steps are listed in Table 4. The total mass losses from the TG curves are found to be 63.09% for L, 85.14% for [CoL(ac)2], 84.58% for [NiL(ac)2], 77.27% for [CuL(ac)2], 74.28% for [CuL(acac)2], and 82.78% for [CuL(fms)2]. The TG curves showed that the thermal decomposition of ligand takes place in one step, while the metal complexes undergo two to five decomposition steps eliminating different fragments in the temperature range of 50–800°C, confirmed by the DTG curves. The decomposition pattern of metal complexes indicates the detachment of chelates and leaving respective metal oxides at final temperature. It was observed that the attached anionic groups in metal salts, electronegativity, and atomic radius of metals influenced thermal stability and pattern of the decomposition process [27].

Figure 2 Simultaneous TG-DTG thermograms of (a) L, (b) [CoL(ac)2], (c) [NiL(ac)2], (d) [CuL(ac)2], (e) [CuL(acac)2], and (f) [CuL(fms)2] complexes in nitrogen atmosphere.
Figure 2

Simultaneous TG-DTG thermograms of (a) L, (b) [CoL(ac)2], (c) [NiL(ac)2], (d) [CuL(ac)2], (e) [CuL(acac)2], and (f) [CuL(fms)2] complexes in nitrogen atmosphere.

Table 4

Thermoanalytical (TG-DTG) data of the ligand and its metal complexes

CompoundStepTemp. range (°C)Mass loss (%)DTGmax (°C)DTGmax (%)Total mass loss (%)Residue (%)
LI200–52557.73396.629.3363.0936.91
[CoL(ac)2] I50–2504.28112.622.3885.1414.86
II250–36023.27305.8217.34
III360–56545.41422.2148.96
[NiL(ac)2] I50–25028.17208.3213.5484.5815.42
II250–36023.08283.5636.96
III360–60027.6458.0666.32
[CuL(ac)2]I50–2252.62102.351.5777.2722.73
II225–3254.04292.235.21
III325–56554.95441.1133.04
[CuL(acac)2]I50–2009.2387.043.2374.2825.72
II200–3356.5290.4512.27
III335–41523.98384.5927.28
IV415–55518.8509.4652.08
V455–6659.52583.1361.09
[CuL(fms)2]I50–1804.88125.862.8582.7817.22
II180–44562.98351.6735.91

The anhydrous ligand shows thermal stability up to 200°C and then degrades until it reaches 525°C, which is accompanied by a 57.73% mass loss, leaving more than 35% residue (Figure 2a). This step corresponds to the loss of two 2-pyridinecarboxaldehyde moieties. The first step occurs at a maximum temperature of 396.60°C.

The TG-DTG thermograms for Co(ii) and Ni(ii) complexes undergo a three-step decomposition (Figures 2b and c). The first stage of degradation occurred between 50 and 250°C in Co(ii) and Ni(ii) complexes, which are accompanied by mass losses of 4.28% and 28.17%, respectively. This step corresponds to the loss of moisture in Co(ii) and triazine moieties in Ni(ii) complexes. The first decomposition step occurred at maximum temperatures of 112.62°C and 208.32°C. The second decomposition step occurred between 250 and 360°C, which are associated with 23.27% for Co(ii) and 23.08% for Ni(ii) complexes due to the loss of acetate groups. The second decomposition step occurs at maximum temperatures of 305.82°C and 283.56°C. The third decomposition step within the ranges of 360–565°C in Co(ii) and 360–600°C in Ni(ii) complexes is accompanied by mass losses of 45.41% and 27.6%, which are assigned to the loss of C12H10N2 and C6H5N as evolved components. The third decomposition step occurs at maximum temperatures of 422.21 and 458.06°C.

In contrast to Co(ii) and Ni(ii) complexes, Cu(ii) complexes showed multiple decompositions at various temperature ranges due to the nature of coordinated groups. [CuL(ac)2], [CuL(acac)2], and [CuL(fms)2] showed three, five, and two successive steps of decomposition, respectively. The thermal degradation of the [CuL(ac)2] occurs mainly in three degradation stages (Figure 2d). The first stage of degradation occurred between 50 and 225°C, which is accompanied by a mass loss of 2.62%. This step corresponds to the loss of methanol solvent. The first step of decomposition occurs at a maximum temperature of 102.35°C. The second major step of mass loss between 225 and 325°C corresponds to the loss of carbon monoxide (CO) gas. The second step of decomposition occurs at a maximum temperature of 292.23°C. In the third major step, triazine and acetate groups were evolved in the temperature range of 325–565°C with a 54.95% weight loss, leaving CuO as a residue. The third step of decomposition occurs at a maximum temperature of 441.11°C. The thermal degradation of the [CuL(acac)2] occurs in five degradation stages (Figure 3e). The first stage of degradation occurred between 50 and 200°C and shows a mass loss of 8.23% and Tmax of 87.04°C, corresponding to the degradation. This step corresponds to the loss of the C3H4O component. The second step of mass loss occurred between 200 and 335°C, which is accompanied by a mass loss of 6.50% and Tmax of 290.45°C, corresponding to the loss of CO2 gas. The third major step of mass loss occurred between 335 and 415°C, which is accompanied by a mass loss of 23.98% and Tmax of 384.59°C, corresponding to the loss of triazine moiety. The last two minor steps correspond to decompositions of 2-pyridinecarboxaldehyde and the residual ligand. [CuL(fms)2] shows a two-step degradation (Figure 3f). In the first step, 4.88% was observed between 50 and 180°C with the evolution of SOx gas. The first step of decomposition occurs at a maximum temperature of 125.86°C. The second major step occurred between 180 and 445°C resulting in a 62.98% mass loss. This step corresponds to the elimination of ligand and leaving bluish black CuS as a residue. The second decomposition stage occurs at a maximum temperature of 351.67°C.

Figure 3 Coats–Redfern plots for major decomposition of (a) L, (b) [CoL(ac)2, II step], (c) [CoL(ac)2, III step], (d) [NiL(ac)2], (e) [CuL(ac)2], (f) [CuL(acac)2], and (g) [CuL(fms)2] complexes.
Figure 3

Coats–Redfern plots for major decomposition of (a) L, (b) [CoL(ac)2, II step], (c) [CoL(ac)2, III step], (d) [NiL(ac)2], (e) [CuL(ac)2], (f) [CuL(acac)2], and (g) [CuL(fms)2] complexes.

3.5 Thermal kinetics investigation

Non-isothermal methods have been widely applied for the calculation of kinetic parameters of solid-state decomposition reactions by the thermal technique (TG-DTG). The decomposition rate is the product of two separate functions: temperature and conversion, which are represented by the following mathematical equation:

(1)ν=dαdt=kTfα=Aexp−EaRTfα

where α is the fraction decomposed at time t, f(α) is a temperature-independent function of conversion whose function is dependent on the mechanism of decomposition, k(T) is a temperature-dependent function, A is the frequency factor, assumed to be independent of temperature, Ea is the apparent activation energy, T is the absolute temperature, and R is the gas constant (8.314 J/mol K). In case of non-isothermal conditions, the linear heating rate of β = dT/dt, equation (1) can be written as:

(2)dαdT=Aβfαexp−EaRT

After the integration and approximation, equation (2) can be represented by

(3)gα= −EaRT+lnARβEa

where g(α) is a function of α dependent on the mechanism of reaction. This is a basic equation for the evolution of kinetic parameters. The integral on the right-hand side is known as the temperature integral and it has no closed-form solution. Herein, kinetic information has been assessed using the Coats–Redfern method [28] and shows the best linearity of data. It can be represented as:

(4)gα=∫0αdα/fα=Aβ∫T0Texp−EaRTdT

where T0 is the onset temperature. No conversion occurs before T0, therefore, T0 can be set equal to zero for convenience. This equation on integration gives:

(5)ln−ln1−αT2=−EaRT+lnARβEa

A plot of the left-hand side against 1,000/T was drawn. The Ea values were calculated from the slope and A values from the intercept. The other thermodynamic parameters of the decomposition reaction, such as enthalpy (∆H), entropy (∆S), and Gibbs free energy (∆G), were calculated using the following equations:

(6)ΔH=Ea−RT
(7)ΔS=R[ln(Ah/kT)]
(8)ΔG=ΔH−TΔS

where k is Boltzmann’s constant, h is Planck’s constant, and T is the DTG peak temperature. The decomposition reactions are classified as exothermic (∆H < 0 or ∆G < 0) and/or endothermic (∆H > 0 or ∆G > 0) on the basis of whether they give off or absorb heat energy. The reaction for which ∆G is positive and ∆S is negative is considered as unfavorable or non-spontaneous slow decomposition reaction. The kinetic parameters are listed in Table 5 and linearization curves of major decomposition stage of synthesized compounds are shown in Figure 3(a–g). The linear correlation coefficients (r2) of thermal decomposition steps were found to lie in the range of 0.99–1, showing a good fit with the linear function.

Table 5

Thermal kinetic parameters of the ligand and its metal complexes

CompoundStageE (kJ mol−1)ln A (min−1)∆H (kJmol−1)∆S (kJmol−1K−1)∆G (kJ)r2
L I30.8861.14727.588−0.237121.870.998
[CoL(ac)2]I10.416−2.7489.480−0.25938.7240.994
II25.5951.67223.058−0.23193.7650.999
III28.5832.55925.073−0.226120.710.998
[NiL(ac)2]I28.2944.07026.562−0.20869.8990.997
II9.974−1.2187.617−0.25479.8180.997
III6.694−2.3022.885−0.267125.4760.998
[CuL(ac)2]I3.040−6.3772.189−0.28931.7730.999
II19.212−0.84516.782−0.25190.3600.997
III43.523.4.66939.856−0.209132.200.995
[CuL(acac)2]I13.9920.47413.268−0.23834.0370.998
II5.733−3.9553.319−0.27783.9421
III63.3959.43460.197−0.168125.0360.998
IV5.506−3.2871.270−0.276142.2390.999
V9.105−2.3024.257−0.2694.5260.997
[CuL(fms)2]I6.126−4.2905.079−0.27339.4910.998
II66.18110.82763.257−0.156118.220.999

From the results, it has been observed that all decomposition steps exhibited a straight line using the method of Coats–Redfern to plot the left-hand side vs 1,000/T and suggesting first-order and endothermic reaction because of the positive values of enthalpy and Gibbs free energy. Taking the major decomposition stage as a criterion, the data show that Ea for metal complexes are higher values than other degradation stages, indicating that the rate of decomposition for this stage is difficult and proceeding through high endothermic stages than other stages. The activation energy in terms of thermal stability of Co(ii), Ni(ii), and Cu(ii) complexes is expected to increase in relation with decreasing their radius [29], the smaller size of Cu(ii) permits a closer approach of the ligand, as compared to Co(ii) and Ni(ii). Hence, the Ea value for Cu(ii) complexes is higher than that of others, that is, thermal stability of Cu(ii) complexes is higher than that of others. The values of entropies are negative for all of the compounds, which indicate that the decomposition reactions are slower than normal and activated complexes have a more ordered structure (non-spontaneously) than reactants.

4 Conclusions

The Schiff-base ligand was synthesized via condensation of 2-pyridinecarboxaldehyde and 2,6-diamino-4-phenyl-1,3,5-triazine. The metal complexes of the prepared ligand were synthesized with Co(ii), Ni(ii), and Cu(ii) salts. The observed result indicates that the synthesized metal complexes show an octahedral and square planar geometry. The decomposition process of all complexes occurs in multi-steps and the ligand undergoes a one-step decomposition, and thermal degradation reactions follow first-order reactions. In all metal complexes, the activation energy for the major step is greater than those of the other minor steps. The entropy of all compounds was negative, whereas enthalpy and Gibbs free energy were positive for all compounds. We also conclude that Cu(ii) complexes are more stable than other synthesized metal complexes.

Acknowledgments

The authors are grateful to the Researchers Supporting Project number (RSP-2020/113), King Saud University, Riyadh, Saudi Arabia for the support.

  1. Conflict of interest: Authors declare no conflict of interest.

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Received: 2020-03-17
Revised: 2020-08-16
Accepted: 2020-08-25
Published Online: 2020-10-20

© 2020 Naushad Ahmad et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/chem-2020-0168
Keywords for this article
Schiff-base; metal complexes; spectroscopic; kinetics; Coats–Redfern.
Creative Commons
BY 4.0

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  11. Design, Synthesis and Characterization of Novel Isoxazole Tagged Indole Hybrid Compounds
  12. Comparison of kinetic and enzymatic properties of intracellular phosphoserine aminotransferases from alkaliphilic and neutralophilic bacteria
  13. Green Organic Solvent-Free Oxidation of Alkylarenes with tert-Butyl Hydroperoxide Catalyzed by Water-Soluble Copper Complex
  14. Ducrosia ismaelis Asch. essential oil: chemical composition profile and anticancer, antimicrobial and antioxidant potential assessment
  15. DFT calculations as an efficient tool for prediction of Raman and infra-red spectra and activities of newly synthesized cathinones
  16. Influence of Chemical Osmosis on Solute Transport and Fluid Velocity in Clay Soils
  17. A New fatty acid and some triterpenoids from propolis of Nkambe (North-West Region, Cameroon) and evaluation of the antiradical scavenging activity of their extracts
  18. Antiplasmodial Activity of Stigmastane Steroids from Dryobalanops oblongifolia Stem Bark
  19. Rapid identification of direct-acting pancreatic protectants from Cyclocarya paliurus leaves tea by the method of serum pharmacochemistry combined with target cell extraction
  20. Immobilization of Pseudomonas aeruginosa static biomass on eggshell powder for on-line preconcentration and determination of Cr (VI)
  21. Assessment of methyl 2-({[(4,6-dimethoxypyrimidin-2-yl)carbamoyl] sulfamoyl}methyl)benzoate through biotic and abiotic degradation modes
  22. Stability of natural polyphenol fisetin in eye drops Stability of fisetin in eye drops
  23. Production of a bioflocculant by using activated sludge and its application in Pb(II) removal from aqueous solution
  24. Molecular Properties of Carbon Crystal Cubic Structures
  25. Synthesis and characterization of calcium carbonate whisker from yellow phosphorus slag
  26. Study on the interaction between catechin and cholesterol by the density functional theory
  27. Analysis of some pharmaceuticals in the presence of their synthetic impurities by applying hybrid micelle liquid chromatography
  28. Two mixed-ligand coordination polymers based on 2,5-thiophenedicarboxylic acid and flexible N-donor ligands: the protective effect on periodontitis via reducing the release of IL-1β and TNF-α
  29. Incorporation of silver stearate nanoparticles in methacrylate polymeric monoliths for hemeprotein isolation
  30. Development of ultrasound-assisted dispersive solid-phase microextraction based on mesoporous carbon coated with silica@iron oxide nanocomposite for preconcentration of Te and Tl in natural water systems
  31. N,N′-Bis[2-hydroxynaphthylidene]/[2-methoxybenzylidene]amino]oxamides and their divalent manganese complexes: Isolation, spectral characterization, morphology, antibacterial and cytotoxicity against leukemia cells
  32. Determination of the content of selected trace elements in Polish commercial fruit juices and health risk assessment
  33. Diorganotin(iv) benzyldithiocarbamate complexes: synthesis, characterization, and thermal and cytotoxicity study
  34. Keratin 17 is induced in prurigo nodularis lesions
  35. Anticancer, antioxidant, and acute toxicity studies of a Saudi polyherbal formulation, PHF5
  36. LaCoO3 perovskite-type catalysts in syngas conversion
  37. Comparative studies of two vegetal extracts from Stokesia laevis and Geranium pratense: polyphenol profile, cytotoxic effect and antiproliferative activity
  38. Fragmentation pattern of certain isatin–indole antiproliferative conjugates with application to identify their in vitro metabolic profiles in rat liver microsomes by liquid chromatography tandem mass spectrometry
  39. Investigation of polyphenol profile, antioxidant activity and hepatoprotective potential of Aconogonon alpinum (All.) Schur roots
  40. Lead discovery of a guanidinyl tryptophan derivative on amyloid cascade inhibition
  41. Physicochemical evaluation of the fruit pulp of Opuntia spp growing in the Mediterranean area under hard climate conditions
  42. Electronic structural properties of amino/hydroxyl functionalized imidazolium-based bromide ionic liquids
  43. New Schiff bases of 2-(quinolin-8-yloxy)acetohydrazide and their Cu(ii), and Zn(ii) metal complexes: their in vitro antimicrobial potentials and in silico physicochemical and pharmacokinetics properties
  44. Treatment of adhesions after Achilles tendon injury using focused ultrasound with targeted bFGF plasmid-loaded cationic microbubbles
  45. Synthesis of orotic acid derivatives and their effects on stem cell proliferation
  46. Chirality of β2-agonists. An overview of pharmacological activity, stereoselective analysis, and synthesis
  47. Fe3O4@urea/HITh-SO3H as an efficient and reusable catalyst for the solvent-free synthesis of 7-aryl-8H-benzo[h]indeno[1,2-b]quinoline-8-one and indeno[2′,1′:5,6]pyrido[2,3-d]pyrimidine derivatives
  48. Adsorption kinetic characteristics of molybdenum in yellow-brown soil in response to pH and phosphate
  49. Enhancement of thermal properties of bio-based microcapsules intended for textile applications
  50. Exploring the effect of khat (Catha edulis) chewing on the pharmacokinetics of the antiplatelet drug clopidogrel in rats using the newly developed LC-MS/MS technique
  51. A green strategy for obtaining anthraquinones from Rheum tanguticum by subcritical water
  52. Cadmium (Cd) chloride affects the nutrient uptake and Cd-resistant bacterium reduces the adsorption of Cd in muskmelon plants
  53. Removal of H2S by vermicompost biofilter and analysis on bacterial community
  54. Structural cytotoxicity relationship of 2-phenoxy(thiomethyl)pyridotriazolopyrimidines: Quantum chemical calculations and statistical analysis
  55. A self-breaking supramolecular plugging system as lost circulation material in oilfield
  56. Synthesis, characterization, and pharmacological evaluation of thiourea derivatives
  57. Application of drug–metal ion interaction principle in conductometric determination of imatinib, sorafenib, gefitinib and bosutinib
  58. Synthesis and characterization of a novel chitosan-grafted-polyorthoethylaniline biocomposite and utilization for dye removal from water
  59. Optimisation of urine sample preparation for shotgun proteomics
  60. DFT investigations on arylsulphonyl pyrazole derivatives as potential ligands of selected kinases
  61. Treatment of Parkinson’s disease using focused ultrasound with GDNF retrovirus-loaded microbubbles to open the blood–brain barrier
  62. New derivatives of a natural nordentatin
  63. Fluorescence biomarkers of malignant melanoma detectable in urine
  64. Study of the remediation effects of passivation materials on Pb-contaminated soil
  65. Saliva proteomic analysis reveals possible biomarkers of renal cell carcinoma
  66. Withania frutescens: Chemical characterization, analgesic, anti-inflammatory, and healing activities
  67. Design, synthesis and pharmacological profile of (−)-verbenone hydrazones
  68. Synthesis of magnesium carbonate hydrate from natural talc
  69. Stability-indicating HPLC-DAD assay for simultaneous quantification of hydrocortisone 21 acetate, dexamethasone, and fluocinolone acetonide in cosmetics
  70. A novel lactose biosensor based on electrochemically synthesized 3,4-ethylenedioxythiophene/thiophene (EDOT/Th) copolymer
  71. Citrullus colocynthis (L.) Schrad: Chemical characterization, scavenging and cytotoxic activities
  72. Development and validation of a high performance liquid chromatography/diode array detection method for estrogen determination: Application to residual analysis in meat products
  73. PCSK9 concentrations in different stages of subclinical atherosclerosis and their relationship with inflammation
  74. Development of trace analysis for alkyl methanesulfonates in the delgocitinib drug substance using GC-FID and liquid–liquid extraction with ionic liquid
  75. Electrochemical evaluation of the antioxidant capacity of natural compounds on glassy carbon electrode modified with guanine-, polythionine-, and nitrogen-doped graphene
  76. A Dy(iii)–organic framework as a fluorescent probe for highly selective detection of picric acid and treatment activity on human lung cancer cells
  77. A Zn(ii)–organic cage with semirigid ligand for solvent-free cyanosilylation and inhibitory effect on ovarian cancer cell migration and invasion ability via regulating mi-RNA16 expression
  78. Polyphenol content and antioxidant activities of Prunus padus L. and Prunus serotina L. leaves: Electrochemical and spectrophotometric approach and their antimicrobial properties
  79. The combined use of GC, PDSC and FT-IR techniques to characterize fat extracted from commercial complete dry pet food for adult cats
  80. MALDI-TOF MS profiling in the discovery and identification of salivary proteomic patterns of temporomandibular joint disorders
  81. Concentrations of dioxins, furans and dioxin-like PCBs in natural animal feed additives
  82. Structure and some physicochemical and functional properties of water treated under ammonia with low-temperature low-pressure glow plasma of low frequency
  83. Mesoscale nanoparticles encapsulated with emodin for targeting antifibrosis in animal models
  84. Amine-functionalized magnetic activated carbon as an adsorbent for preconcentration and determination of acidic drugs in environmental water samples using HPLC-DAD
  85. Antioxidant activity as a response to cadmium pollution in three durum wheat genotypes differing in salt-tolerance
  86. A promising naphthoquinone [8-hydroxy-2-(2-thienylcarbonyl)naphtho[2,3-b]thiophene-4,9-dione] exerts anti-colorectal cancer activity through ferroptosis and inhibition of MAPK signaling pathway based on RNA sequencing
  87. Synthesis and efficacy of herbicidal ionic liquids with chlorsulfuron as the anion
  88. Effect of isovalent substitution on the crystal structure and properties of two-slab indates BaLa2−xSmxIn2O7
  89. Synthesis, spectral and thermo-kinetics explorations of Schiff-base derived metal complexes
  90. An improved reduction method for phase stability testing in the single-phase region
  91. Comparative analysis of chemical composition of some commercially important fishes with an emphasis on various Malaysian diets
  92. Development of a solventless stir bar sorptive extraction/thermal desorption large volume injection capillary gas chromatographic-mass spectrometric method for ultra-trace determination of pyrethroids pesticides in river and tap water samples
  93. A turbidity sensor development based on NL-PI observers: Experimental application to the control of a Sinaloa’s River Spirulina maxima cultivation
  94. Deep desulfurization of sintering flue gas in iron and steel works based on low-temperature oxidation
  95. Investigations of metallic elements and phenolics in Chinese medicinal plants
  96. Influence of site-classification approach on geochemical background values
  97. Effects of ageing on the surface characteristics and Cu(ii) adsorption behaviour of rice husk biochar in soil
  98. Adsorption and sugarcane-bagasse-derived activated carbon-based mitigation of 1-[2-(2-chloroethoxy)phenyl]sulfonyl-3-(4-methoxy-6-methyl-1,3,5-triazin-2-yl) urea-contaminated soils
  99. Antimicrobial and antifungal activities of bifunctional cooper(ii) complexes with non-steroidal anti-inflammatory drugs, flufenamic, mefenamic and tolfenamic acids and 1,10-phenanthroline
  100. Application of selenium and silicon to alleviate short-term drought stress in French marigold (Tagetes patula L.) as a model plant species
  101. Screening and analysis of xanthine oxidase inhibitors in jute leaves and their protective effects against hydrogen peroxide-induced oxidative stress in cells
  102. Synthesis and physicochemical studies of a series of mixed-ligand transition metal complexes and their molecular docking investigations against Coronavirus main protease
  103. A study of in vitro metabolism and cytotoxicity of mephedrone and methoxetamine in human and pig liver models using GC/MS and LC/MS analyses
  104. A new phenyl alkyl ester and a new combretin triterpene derivative from Combretum fragrans F. Hoffm (Combretaceae) and antiproliferative activity
  105. Erratum
  106. Erratum to: A one-step incubation ELISA kit for rapid determination of dibutyl phthalate in water, beverage and liquor
  107. Review Articles
  108. Sinoporphyrin sodium, a novel sensitizer for photodynamic and sonodynamic therapy
  109. Natural products isolated from Casimiroa
  110. Plant description, phytochemical constituents and bioactivities of Syzygium genus: A review
  111. Evaluation of elastomeric heat shielding materials as insulators for solid propellant rocket motors: A short review
  112. Special Issue on Applied Biochemistry and Biotechnology 2019
  113. An overview of Monascus fermentation processes for monacolin K production
  114. Study on online soft sensor method of total sugar content in chlorotetracycline fermentation tank
  115. Studies on the Anti-Gouty Arthritis and Anti-hyperuricemia Properties of Astilbin in Animal Models
  116. Effects of organic fertilizer on water use, photosynthetic characteristics, and fruit quality of pear jujube in northern Shaanxi
  117. Characteristics of the root exudate release system of typical plants in plateau lakeside wetland under phosphorus stress conditions
  118. Characterization of soil water by the means of hydrogen and oxygen isotope ratio at dry-wet season under different soil layers in the dry-hot valley of Jinsha River
  119. Composition and diurnal variation of floral scent emission in Rosa rugosa Thunb. and Tulipa gesneriana L.
  120. Preparation of a novel ginkgolide B niosomal composite drug
  121. The degradation, biodegradability and toxicity evaluation of sulfamethazine antibiotics by gamma radiation
  122. Special issue on Monitoring, Risk Assessment and Sustainable Management for the Exposure to Environmental Toxins
  123. Insight into the cadmium and zinc binding potential of humic acids derived from composts by EEM spectra combined with PARAFAC analysis
  124. Source apportionment of soil contamination based on multivariate receptor and robust geostatistics in a typical rural–urban area, Wuhan city, middle China
  125. Special Issue on 13th JCC 2018
  126. The Role of H2C2O4 and Na2CO3 as Precipitating Agents on The Physichochemical Properties and Photocatalytic Activity of Bismuth Oxide
  127. Preparation of magnetite-silica–cetyltrimethylammonium for phenol removal based on adsolubilization
  128. Topical Issue on Agriculture
  129. Size-dependent growth kinetics of struvite crystals in wastewater with calcium ions
  130. The effect of silica-calcite sedimentary rock contained in the chicken broiler diet on the overall quality of chicken muscles
  131. Physicochemical properties of selected herbicidal products containing nicosulfuron as an active ingredient
  132. Lycopene in tomatoes and tomato products
  133. Fluorescence in the assessment of the share of a key component in the mixing of feed
  134. Sulfur application alleviates chromium stress in maize and wheat
  135. Effectiveness of removal of sulphur compounds from the air after 3 years of biofiltration with a mixture of compost soil, peat, coconut fibre and oak bark
  136. Special Issue on the 4th Green Chemistry 2018
  137. Study and fire test of banana fibre reinforced composites with flame retardance properties
  138. Special Issue on the International conference CosCI 2018
  139. Disintegration, In vitro Dissolution, and Drug Release Kinetics Profiles of k-Carrageenan-based Nutraceutical Hard-shell Capsules Containing Salicylamide
  140. Synthesis of amorphous aluminosilicate from impure Indonesian kaolin
  141. Special Issue on the International Conf on Science, Applied Science, Teaching and Education 2019
  142. Functionalization of Congo red dye as a light harvester on solar cell
  143. The effect of nitrite food preservatives added to se’i meat on the expression of wild-type p53 protein
  144. Biocompatibility and osteoconductivity of scaffold porous composite collagen–hydroxyapatite based coral for bone regeneration
  145. Special Issue on the Joint Science Congress of Materials and Polymers (ISCMP 2019)
  146. Effect of natural boron mineral use on the essential oil ratio and components of Musk Sage (Salvia sclarea L.)
  147. A theoretical and experimental study of the adsorptive removal of hexavalent chromium ions using graphene oxide as an adsorbent
  148. A study on the bacterial adhesion of Streptococcus mutans in various dental ceramics: In vitro study
  149. Corrosion study of copper in aqueous sulfuric acid solution in the presence of (2E,5E)-2,5-dibenzylidenecyclopentanone and (2E,5E)-bis[(4-dimethylamino)benzylidene]cyclopentanone: Experimental and theoretical study
  150. Special Issue on Chemistry Today for Tomorrow 2019
  151. Diabetes mellitus type 2: Exploratory data analysis based on clinical reading
  152. Multivariate analysis for the classification of copper–lead and copper–zinc glasses
  153. Special Issue on Advances in Chemistry and Polymers
  154. The spatial and temporal distribution of cationic and anionic radicals in early embryo implantation
  155. Special Issue on 3rd IC3PE 2020
  156. Magnetic iron oxide/clay nanocomposites for adsorption and catalytic oxidation in water treatment applications
  157. Special Issue on IC3PE 2018/2019 Conference
  158. Exergy analysis of conventional and hydrothermal liquefaction–esterification processes of microalgae for biodiesel production
  159. Advancing biodiesel production from microalgae Spirulina sp. by a simultaneous extraction–transesterification process using palm oil as a co-solvent of methanol
  160. Topical Issue on Applications of Mathematics in Chemistry
  161. Omega and the related counting polynomials of some chemical structures
  162. M-polynomial and topological indices of zigzag edge coronoid fused by starphene
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