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

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.

3.1 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–N^a) 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–N^b) and 2,6-diamino-4-phenyl-1,3,5-triazine ν(C–N^c) 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–N^a) 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–N^b) 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 ν_as(C–O) and ν_s(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 ν_as(S–O), and (S–O); 1,220, 1,160ν_s(C–F), and ν_as(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.

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–CH^a protons [24] and doublets at 8.36 to two N–CH^b 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–N^a), (C–N^b), and (C–N^c) 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.

The electronic spectrum of the Co(ii) metal complex showed two bands at 16,077 and 34,482 cm⁻¹ regions assignable to ⁴T_1g(F) → ⁴A_2g(F)(ν₂) and ⁴T_1g(F) → 4T_1g(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 Δ_o = 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 ³A_2g(F) → ³T_1g(F)(ν₂) and ³A_2g(F) → ³T_1g(P)(ν₃) transitions, respectively (Figure 1b) [25]. The calculated spectral parameters are as follows: Δ_o = 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 ²B_1g → ²E_g and ²B_1g → ²A_1g, 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 ²B_1g → ²E_g and ²B_1g → ²A_1g, 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 ²E_g → ²T_2g 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.

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.

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 T_max 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 T_max 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 T_max 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 SO_x 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.

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:

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, E_a 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:

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

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:

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:

A plot of the left-hand side against 1,000/T was drawn. The E_a 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:

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.

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 E_a 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 E_a 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.