Introduction

The thermal analysis techniques were extensively applied in studying of the thermal behavior of metal complexes [1]. In studies on the physical and chemical properties and the structures of solid complexes of transition metal ions with organic ligands, it is customary to investigate the thermal decomposition of these complexes so as to obtain useful data on the metal–ligand bonds [2–4] and stability trends. The thermal investigations on some derivatives of amines [5], sulfonamides [6], substituted thiourea salts [7], and complexes of bidentate ligands and others [8–13], have already been published.

The copper(II) complexes were characterized by different analytical and spectral methods. The thermal behaviors of the obtained complexes were also investigated. The thermal analysis techniques, such as thermogravimetry (TG), differential thermal analysis (DTA), and differential scanning calorimetry (DSC), were widely applied in studying the thermal behavior and the structure of metal complexes [14–25]. The results allowed us to acquire information concerning the structure of these compounds, including their thermal behavior and decomposition. Thermal analysis let us also to evaluate the presence of crystallization water molecules in the complexes and to determine the endothermic and/or exothermic effects connected with such processes as: dehydration, melting, crystallization, and decomposition. In view of the importance of copper(II) compounds and our interest in the chemistry of coordination compounds involving chelating coumarin base [26, 27], we report here the synthesis, characterization, and reactivity of copper(II) complexes containing coumarin base ligands with O, N, N, O, donor set, etc.

Experimental

Materials

All reagents were of analytical-reagent (AR) grade and were used without further purification. Solvents employed were purified by standard procedures prior to use [28]. 1,10-phenanthroline was purchased from Aldrich. Salicylaldehyde, ethyl acetoacetate, piperidine, chloroform, hexane, bromine, pyridine, toluene, cyclopentanone, cyclohexanone, α-tetralone, dimethyl formamide, and Cu(NO3)2·3H2O were purchased from the E. Merck (India) Limited, Mumbai. Acetic acid and EDTA were purchased from Sigma Chemical Co., India.

Instruments

Elemental analyses (C, H, N) were analyzed with the PerkinElmer, USA 2400-II CHN analyzer. FT–IR spectra (4000–400 cm−1) were recorded on Nicolet-400D spectrophotometer using KBr pellets. The magnetic moments were obtained by the Gouy’s method using mercury tetrathiocyanato cobaltate(II) as a calibrant (g = 16.44 × 10−6 c.g.s. units at 20 °C). Diamagnetic corrections were made using Pascal’s constant. The FAB mass spectrum of the complex was recorded at SAIF, CDRI, Lucknow with JEOL SX-102/DA-6000 mass spectrometer. A simultaneous TG/DTG had been obtained by a model 5000/2960 SDT, TA Instruments, USA. The experiments were performed in N2 atmosphere at a heating rate of 10 °C min−1 in the temperature range 50–800 °C, using Al2O3 crucible. The sample sizes are ranged in mass from 4.5 to 10 mg.

Synthesis of ligands (An) (n = 1–4)

The coumarin base ligands (An) (where n = 1–4) were synthesized according to the procedure described in the literature [29].

Synthesis of complexes (Cu(II)-n) (n = 1–4)

A water solution (100 mL) of Cu(NO3)2·3H2O (10 mmol) was added to dimethyl formamide solution (100 mL) of ligand (An) (10 mmol), followed by addition of 1,10-phenanthroline (10 mmol) in ethanol; the pH was adjusted to 4.5–6.0 by dropwise addition of 25% NaOH solution in water. The mixture was heated in a water bath for 3 h at 70 °C. The mixture was kept overnight at room temperature. The colored crystals obtained were washed with water, methanol, and finally with diethyl ether and dried in air.

Results and discussion

The following compounds have been studies:

 

Cu(II)-1 or C28H28CuN2O8

(A)

Cu(II)-2 or C28H25BrCuN2O7

(B)

Cu(II)-3 or C29H26CuN2O7

(C)

Cu(II)-4 or C32H26CuN2O6

(D)

Color, elemental analysis, and melting point of the complexes are presented in Table 1. The complexes are colored and stable in air. They are insoluble in water and in most organic solvents but soluble in DMSO.

Table 1 Color, elemental analysis and melting point of the complexes
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IR spectra

Interpretation of infrared spectra of free An ligand and copper(II) complexes gave an idea about the mode of chelation and follow-up of the effect of coordination of metal ions on the vibration motions of free ligand. The IR spectra of the free ligand and its metal chelates were carried out within the mid-IR range 4000–400 cm−1. The IR spectrum of the free ligand shows a very strong band at ~3460 cm−1, which can be attributed to the aryl –OH group [30]. This band is shifted to lower wave numbers at ~3445 and ~3405 cm−1 in case of other metal complexes, respectively, but this band was absent in case of copper(II) complex which was assigned to the involvement of –OH group in the chelating. The shared aryl –OH group in coordination is confirmed by the hypochromic effect (decrease in intensity) of the υ(C–O) stretching band observed at ~1250 cm−1 in the free ligand and the complexes due to substituent’s or interactions with the molecular environment [30]. The IR spectrum of the free ligand revealed a very strong-to-strong bands at 1633 and 1598 cm−1 due to υ(C=N) of the 1,10-phenanthroline. This band is shifted to lower frequencies (30–38 cm−1) in the complexes assigned that it has been affected upon complexation via metal ions. In the 500–400 cm−1 region, the spectra of all complexes have a detected bands observed at ~550 and 468 cm−1 [31], which can be assigned to the υ(Cu–O) and υ(Cu–N) stretching vibrations, respectively. Therefore, the IR spectra indicate that An behaves as a uninegative bidentate and the coordination sites being ArOH of coumarin derivative ligand and C=N of the 1,10-phenanthroline.

Mass spectra

Mass spectrometric techniques offer comparative advantages for speed and productivity for pharmaceutical analysis [32]. The recorded mass spectra (Fig. 1) and the molecular ion peak for the complex Cu(II)-n have been used to confirm the molecular formula. The fragmentation pattern is given in supplementary material.

Fig. 1
figure 1

Mass spectra of Cu(II)-n complexes

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Cu(II)-1

The first peak at m/e 529 represents the molecular ion peak of the complex. The primary fragmentation of the complex takes place due to the loss of –OH2 and –OH group from the species (a) to give species (b) with peak at m/e 494. The species (b) further degrades with the subsequent loss of –C3H5 species forming species (C) with their peak at m/e 452. The species (c) further degrades with the subsequent loss of –CO2 species forming species (d) with their peak observed at m/e 408 represents the stable species with 99.6% abundance. The species (d) further degrades with the subsequent loss of –C6H4 species forming species (e) with their peak at m/e 305. The species (e) further degrades with the subsequent loss of –C6H5 species forming species (f) with peak at m/e 227. The species (f) further degrades with the subsequent loss of –C6H5 species forming species (g) with peak at m/e 156 and species (g) further degrades with the subsequent loss of –C5H3N species forming species (h) with peak at m/e 78.

Cu(II)-2

The first peak at m/e 607 represents the molecular ion peak of the complex. The primary fragmentation of the complex takes place due to the loss of –OH group from the species (a) to give species (b) with peak at m/e 590. The species (b) further degrades with the subsequent loss of –Br species forming species (C) with their peak at m/e 511. The species (c) further degrades with the subsequent loss of –C4H3 species forming species (d) with their peak at m/e 460. The species (d) further degrades with the subsequent loss of –C3H6 species forming species (e) with their peak at m/e 418. The species (e) further degrades with the subsequent loss of –C2H2 species forming species (f) with peak observed at m/e 392 represents the stable species with 99.2% abundance. The species (f) further degrades with the subsequent loss of –C4H3 species forming species (g) with peak at m/e 327. The species (g) further degrades with the subsequent loss of –OH2 species forming species (h) with peak at m/e 308 and species (h) further degrades with the subsequent loss of –C10H6N2 species forming species (I) with peak at m/e 154.

Cu(II)-3

The first peak at m/e 559 represents the molecular ion peak of the complex. The primary fragmentation of the complex takes place due to the loss of –OCH3 group from the species (a) to give species (b) with peak at m/e 528. The species (b) further degrades with the subsequent loss of –OH2 and –OH species forming species (C) with their peak at m/e 493. The species (c) further degrades with the subsequent loss of –C7H5N species forming species (d) with their peak at m/e 390. The species (d) further degrades with the subsequent loss of –C7H7O species forming species (e) with their peak observed at m/e 280 represents the stable species with 82.4% abundance. The species (e) further degrades with the subsequent loss of –C4H4 species forming species (f) with peak at m/e 232. The species (f) further degrades with the subsequent loss of –C5H4O2 species forming species (g) with peak at m/e 140.

Cu(II)-4

The first peak at m/e 579 represents the molecular ion peak of the complex. The primary fragmentation of the complex takes place due to the loss of –C7H7O group from the species (a) to give species (b) with peak at m/e 472. The species (b) further degrades with the subsequent loss of –OH2 and –OH species forming species (C) with their peak at m/e 437. The species (c) further degrades with the subsequent loss of –C2H2 species forming species (d) with their peak at m/e 411. The species (d) further degrades with the subsequent loss of –C4H6 species forming species (e) with their peak observed at m/e 362 represents the stable species with 99.6% abundance. The species (e) further degrades with the subsequent loss of –C8H6 species forming species (f) with peak at m/e 260. The species (f) further degrades with the subsequent loss of –C5H4O2 species forming species (g) with peak at m/e 168 and species (g) further degrades with the subsequent loss of –C4H4 species forming species (h) with peak at m/e 116.

Electronic spectra and magnetic moments

Copper(II) complexes (d9 system) are known for their varieties of structures due to their various coordination numbers. Six coordinated copper(II) complexes possesses distorted octahedral geometry. The spectra of copper complexes are very difficult to assign even with relatively simple ligands because of the breadth of absorption band even at low temperature. The different coordination number having wide range of geometry. Generally for square pyramidal copper(II) complexes, bands observed in the region 9000–10000, 11500–16000, and 15000–19000 cm−1 have been ascribed to the transition 2B1 → 2A1, 2B1 → 2B2, and 2B1 → 2E, respectively. The diffuse reflectance spectra of square pyramidal copper(II) complexes Cu(II)-n have been taken in solid state. The spectra of copper(II) complexes exhibit a three band observed in the 9050, 13500, and 16600 cm−1 [33–35]. These bands are characteristic of copper(II) d-d transition in tetragonal field in which copper(II) atom is in distorted square pyramidal coordination environment. The copper(II)-d9 system and its compounds are expected to have magnetic moment close to be spin only value 1.73 B.M. Generally the magnetic moments of copper(II) in any of its geometry lies around 1.8 B.M. at room temperature. The magnetic moment value of copper(II) complexes obtained in the range of 1.76–1.92 B.M., which are close to the spin-only values expected in S = ½ system (1.73 B.M.) and may be indicative of five coordinating copper(II) complexes and consistent with the presence of single unpaired electron [36, 37].

Calculation of activation thermodynamic parameters of the decomposed complexes

Each decomposition process follows the trend

$$ {\text{Solid-}}1\mathop{\longrightarrow}\limits^{\text{heat}}{\text{Solid-}}2 + {\text{Gas}} $$

This process comprises several stages. The method reported by Freeman–Carroll [38] has been adopted. Plots of [Δlog(dw/dt)/Δlogwr] versus [Δ(1/T)/Δlogwr] were linear for all of the decomposition steps. The energy of activation E a was calculated from the slopes of these plots for a particular stage and the order of reactions (n) determined from the intercept, showing first order reaction over the entire range of decomposition for all of the complexes. A typical plot for the thermal degradation of Cu(II)-1 is shown in Fig. 2.

Fig. 2
figure 2

Freeman–Carroll plot for thermal degradation of Cu(II)-1 complex

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The thermal behavior of the prepared complexes

In the following paragraphs, the thermal behavior of the synthesized complexes, characterized on the basis of TG/DTG and DTA methods, is described.

Cu(II)-1

TG/DTG and DTA curves of the complex Cu(II)-1 are represented in Figs. 3 and 4, respectively. The thermal decomposition of the complex Cu(II)-1 take place in three stages. The thermal dehydration of this complex takes place in a single step between 40 and 190 °C, with a mass loss is 4.75% (calc. 4.84%). The maximum rate of mass loss is indicated by the DTG peak at 185 °C. Three mole of lattice water molecules and (one mole water + one mole hydroxy) of coordinated molecules are removed in this stage of dehydration. The total loss of two water and one hydroxyl molecules is a first-order reaction and the value of the energy of activation for the dehydration process is 3.16 kJ mol−1. The second stage, which occurs in the temperature range of 315–335 °C with a DTG peak observed at 325 °C, corresponds to the decomposition of the Ph ligand. The observed mass loss is 40.67% (calc. 44.63). The third stage is related to the decomposition of A1 ligand in the temperature range of 500–750 °C with well-separated DTG peaks at 550 °C, accompanied by a mass loss of 44.92% (calc. 44.60). The overall mass loss observed is 90.34% as compared to the theoretical value 90.43%. The end product estimated as CuO, has the observed mass of 9.49% compared to the calculated value of 9.55%.

Fig. 3
figure 3

Final structure and thermal scheme of Cu(II)-1 complex

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Fig. 4
figure 4

TG/DTG and DTA curves of Cu(II)-1 complex

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Cu(II)-2

The Cu(II)-2 complex undergoes decomposition in three stages (Figs. 5 and 6). The thermal dehydration of this complex takes place in a single step between 40 and 200 °C, with a mass loss of 9.67% (calc. 9.41). The maximum rate of mass loss is indicated by the DTG 190 °C. Two lattice and (one mole water + one mole hydroxy) are removed in this stage of dehydration. The total loss of two lattice and (one mole water + one mole hydroxy) of coordinated water molecules is a first-order reaction and the value of energy of activation for the dehydration process is 12.28 kJ mol−1. The second stage occurs in the temperature range 265–350 °C, with the mass loss (39.40%) is due to the removal of Ph ligand from the complex (calc. 39.81%). The DTG peak corresponding to this stage is found at 330 °C. The third stage is related to the decomposition of A2 ligand, taking place in the temperature range 360–420 °C with a DTG peak at 400 °C and being accompanied by a mass loss of 41.78% (calc. 41.49%). The overall mass losses are observed to be 90.85%, which is in very good agreement with the calculated value of 90.71%. The final residue, estimated as CuO has the observed mass 9.12% as against the calculated value of 9.27%.

Fig. 5
figure 5

Final structure and thermal scheme of Cu(II)-2 complex

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Fig. 6
figure 6

TG/DTG and DTA curves of Cu(II)-2 complex

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Cu(II)-3

Figures 7 and 8 in this complex takes place in a single step between 40 and 230 °C, with a mass loss of 4.53% (calc. 4.54%). The maximum rate of mass loss is indicated by the DTG 220 °C. One lattice and (one mole water + one mole hydroxy) are removed in this stage of dehydration. It was found that the loss of one lattice and (one mole water + one mole hydroxy) is a first-order reaction, and the value of the energy of activation for the dehydration process is 5.25 kJ mol−1. After dehydration, the crystal structure of the complex is distorted. In the second stage between 500 and 600 °C, the Ph ligand degrades as one maximum in the DTG curve at 570 °C with a mass loss of 38.45% (calc. 38.50%). The third stage, which occurs in the temperature range of 710–800 °C with DTG peak observed at 810 °C, corresponds to the decomposition of A3 ligand. The observed mass loss (48.08%) is coinciding with the calculated value of 48.22%. The overall mass losses are observed to be 91.06%, which is in very good agreement with the calculated value of 91.26%. The final residue, estimated as CuO, has the observed mass 8.67% as against the calculated value of 8.90%.

Fig. 7
figure 7

Final structure and thermal scheme of Cu(II)-3 complex

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Fig. 8
figure 8

TG/DTG and DTA curves of Cu(II)-3 complex

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Cu(II)-4

The thermal decomposition of the complex Cu(II)-4 undergoes in three stages (Figs. 9 and 10). The first stage is related to the liberation of one lattice and (one mole water + one mole hydroxy) are removed in this stage of dehydration. It was found that the loss of one lattice and (one mole water + one mole hydroxy) is a first-order reaction, and the value of the energy of activation for the dehydration process is 4.01 kJ mol−1. In the second stage between 420 and 480 °C, the Ph ligand degrades as one maximum in the DTG curve at 470 °C with a mass loss of 37.45% (calc. 37.83%). The third stage, which occurs in the temperature range of 690–730 °C with DTG peak observed at 710 °C, corresponds to the decomposition of A4 ligand. The observed mass loss (49.09%) is coinciding with the calculated value of 48.88%. The overall mass losses are observed to be 90.95%, which is in very good agreement with the calculated value of 91.17%. The final residue, estimated as CuO, has the observed mass 8.68% as against the calculated value of 8.83%.

Fig. 9
figure 9

Final structure and thermal scheme of Cu(II)-4 complex

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Fig. 10
figure 10

TG/DTG and DTA curves of Cu(II)-4 complex

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Boltzman and Plank constants, respectively. There was no definite trend in the values of the entropy of activation. However, the negative value of the entropy of activation indicated that the activated complex has a more ordered structure than the reactants, and the reactions are slower than normal [39]. The kinetic paramerters, especially the energy of activation (E a) values, are helpful in assigning the strength of complexes [40]. The calculated E a values of the investigated complexes for the first dehydration step were in the range of 3.16–12.28 kJ mol−1 (Table 2). Based on the E a values, the thermal stabilities of complexes in the decreasing order are Cu(II)-2 > Cu(II)-3 > Cu(II)-4 > Cu(II)-1.

Table 2 Kinetic parameters of complexes
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