Synthesis, spectral characterization and study of thermal behavior kinetics by thermogravimetric analysis of metal complexes derived from salicylaldehyde and alkylamine

Journal of Molecular Structure 1142 (2017) 48e57

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Synthesis, spectral characterization and study of thermal behavior kinetics by thermogravimetric analysis of metal complexes derived from salicylaldehyde and alkylamine Brahim Bouzerafa b, *, Djouhra Aggoun a, **, Yasmina Ouennoughi a, Ali Ourari a, Ramiro Ruiz-Rosas c, Emilia Morallon c, Mohammad S. Mubarak d  Laboratoire d'Electrochimie, d'Ing enierie Mol eculaire et de Catalyse Redox (LEIMCR), Facult e de Technologie, Universit e Ferhat ABBAS S etif-1, 19000, Algeria b Laboratoire de Pr eparation, Modification et Application des Mat eriaux Polym eriques Multiphasiques (LMPMP), Facult e de Technologie, Universit e Ferhat ABBAS S etif-1, 19000, Algeria c Instituto Universitario de Materiales, Universidad de Alicante, Ap. 99, E-03080, Alicante, Spain d Department of Chemistry, the University of Jordan, Amman, 11942, Jordan a

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 June 2016 Received in revised form 11 April 2017 Accepted 11 April 2017

Cobalt(III)- and copper(II)-Schiff base complexes have been prepared by reaction of the bidentate Schiff base ligand (HL: 2-(4-methoxyphenyl)-1-iminosalicylidenethane) with cobalt(II) and copper(II) chlorides. Structures of the synthesized complexes have been characterized by various physicochemical techniques, such as IR and UVeVis spectroscopy, mass spectrometry, thermogravimetric analysis (TG/ DTG), and by elemental analysis. Additionally, the redox behavior of the cobalt(III) and copper(II) complexes has been examined by cyclic voltammetry at a glassy carbon electrode in DMF solutions. Thermogravimetric analysis has been employed to evaluate the thermal stability of the prepared complexes CoIII(L)3$1/2H2O, CuII(L)2, in addition to the previously synthesized HL and NiII(L)2. Furthermore, activation energies of the thermal decomposition were calculated using Kissinger, Ozawa and Coats-Redfern methods. The calculated activation energies were also useful to evaluate kinetic and thermodynamic parameters of the ligand and the corresponding metal complexes including DS, DH and DG. Calculated activation energies follow the order: Ea(NiII(L)2) > Ea(CuII(L)2) > Ea(CoIII(L)3$1/2H2O). © 2017 Published by Elsevier B.V.

Keywords: Bidentate Schiff base Spectroscopic characterization Cyclic voltammetry Thermogravimetric analysis Kinetic Thermodynamic parameters

1. Introduction Schiff bases have been extensively used as ligands in the synthesis of metal complexes [1]. In addition, these bases are important in the fields of medicinal and pharmaceutical chemistry [2]. Furthermore, Schiff base metal complexes were found to display interesting bioactivity and play a role as potent drugs in the area of pharmacology [3]. These complexes have been found as potential therapeutic substances [4], anticancer [5], growth-inhibiting agents for a wide range of bacterial strains [6], anti-fungal [7], and antimicrobial [8]. On the other hand, it is generally known that both copper and cobalt are involved in several human metabolisms. In

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (B. Bouzerafa), [email protected] (D. Aggoun). http://dx.doi.org/10.1016/j.molstruc.2017.04.029 0022-2860/© 2017 Published by Elsevier B.V.

the case of copper, it promotes chemical processes that are combined with other transition metals such as those of Mn(III) and Fe(III) currently involved in catalytic oxidation reactions [9]. In addition, these complexes have been commonly employed as agents for generating active oxygen species for DNA and novel potential DNA targeted as antitumor drugs [10]. The high stability of Schiff base complexes, with metals in different oxidation states, increased their applications in a wide range of fields. They coordinate to metal ions as tetradentate (NNOO), tridentate (NOO), and bidentate (NO) ligands [11e13]. This type of ligands and their metal complexes have been investigated due to their important properties as electrochemical catalysts [14,15]. Some Schiff bases have been employed as excellent homogeneous-and heterogeneous-phase catalysts [3]. In recent years, many areas of chemistry, including the use of spectroscopic techniques for characterization purposes, have significantly advanced. Recently, considerable attention has been

B. Bouzerafa et al. / Journal of Molecular Structure 1142 (2017) 48e57

paid to the use of thermogravimetric analysis (TG/DTG) as a valuable method to study the thermal stability of new compounds [16e20]. This strategy is often adopted in order to avoid problems, which can arise from increasing the temperature. Accordingly, significant research has been undertaken to understand the thermal characterization of various metal Schiff base complexes and the use of these techniques for identification purposes [21,22]. Additionally, the electrochemical behavior of complexes can provide useful information on catalytic processes because these are accompanied by changes in oxidation state of the metallic center and the structure of the complex [23,24]. We have recently described the synthesis and characterization of a bidentate Schiff base ligand (HL) along with its Ni(II) complex [25]. As a continuation of our work on Schiff base complexes, we describe, herein, the synthesis and characterization of cobalt(III) and copper(II) complexes (Scheme 1). In addition, the present study undertakes the thermal stability of Co(III), Cu(II), and Ni(II) complexes, along with the bidentate ligand HL via thermogravimetric analysis. Furthermore, the various kinetic and thermodynamic parameters of their decomposition steps were also calculated; this will shed some light on the stability of these complexes. Finally, the electrochemical behavior of the cobalt(III) and copper(II) complexes was investigated by cyclic voltammetry. 2. Experimental 2.1. Reagents and equipment Chemicals and reagents used throughout this work were purchased from commercial sources and used as received without further purification. Purity of synthesized compounds was checked with thin layer chromatography (TLC) using glass plates pre-coated with Merck silica gel 60 F254. High resolution mass spectral data

49

(HRMS) were acquired by electrospray ionization technique with the aid of a Bruker APEX-2 instrument. FT-IR spectra of synthesized complexes were recorded, as KBr discs, with a PerkinElmer 1000 FTIR Spectrophotometer in transmittance mode, whereas the UVevisible spectra were obtained using a Unicam UV-300 spectrophotometer. The C, H, and N percentages were determined with a LECO TruSpec Micro CHNS elemental micro-analyzer. We performed cyclic voltammetry experiments in DMF containing 0.1 M tetra-n-butylammonium tetrafluoroborate (TBABF4), on a Voltalab 40 Potentiostat-Galvanostat controlled by microcomputer. All measurements were carried out in a 5-cm3 Metrohm onecompartment cell. The working electrode was a glassy carbon of 3 mm in diameter, whereas a platinum wire served as the counter electrode. Potentials have been quoted with respect to the saturated calomel electrode (SCE). Thermogravimetric analyses (TG and DTG) for the prepared complexes were accomplished using a PerkinElmer TGA 7 analyzer apparatus. Measurements were done at heating rates of 5, 10, 15, and 20
C min1 under a nitrogen atmosphere over the temperature range of ambient-950
C. 2.2. Synthesis of the cobalt(III) and copper(II) Schiff base complexes The Schiff-base ligand, HL, was prepared according to a procedure described in earlier publications [25,26]. Synthesis of the cobalt(III) and copper(II) complexes of the Schiff base ligand was carried out according to the following general procedure: A hot solution of CoCl2$6H2O or CuCl2$2H2O (1.0 mmol) in 10 mL of methanol was added to 10 mL methanol solution of the Schiff base ligand (HL, 2 mmol). The mixture was heated, with stirring, at 70
C for 4 h. The complexes obtained were filtered, washed several times with methanol and then with diethyl ether, and finally dried in a desiccator over anhydrous CaO. Analytical results indicate that the cobalt and copper complexes have (1:3 and 1:2) metal-ligand

Scheme 1. Synthesis protocol of the cobalt(III) and copper(II) complexes.

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stoichiometry, respectively. The complexes are stable as solids or as solutions under atmospheric conditions. For CoIII(L)3$1/2H2O: Color: green; yield: 60%; FT-IR (KBr, cm1): 2920, 2840 (w, CH2 and CH3), 1613 (s, C]N), 1510 (s, C]C), 1242 (s, CeO), 527 (w, CoeO), 457 (w, CoeN); UVeVis: (lmax, nm, DMF): 392; Mass spectrum (m/z (%)): 828 (6.7%), 567 (100%), 256 (26.7%); Anal. Calc. (Found) for C48H49N3O6.5Co: C 69.39% (69.44), H 5.94% (5.93), N 5.06% (5.02). For CuII(L)2: Color: brown; yield: 50%; FT-IR (KBr, cm1): 2930, 2835 (w, CH2 and CH3), 1610 (nC]N), 1510 (s, C]C), 1247 (s, CeO), 512 (w, CueO), 459 (w, CueN); UVeVis: (lmax, nm, DMF): 305, 365; Mass spectrum (m/z (%)): 572 (65.51%), 317 (13.79%), 256 (100%); Anal. Calc. (Found) for C32H32N2O4Cu: C 67.17% (67.16), H 5.64% (5.72), N 4.90% (4.91). 3. Results and discussion 3.1. Chemistry Synthesis of the Schiff base ligand was carried out as described in a previous publication [25] shown in Scheme 1 is the reaction used to prepare the desired cobalt(III)- and copper(II)-Schiff base complexes. Structures of prepared complexes were confirmed with the aid of several spectroscopic techniques, including UVeVis, FTIR, and mass spectrometry, and by elemental analysis. Their elemental analyses are in agreement with those calculated from molecular formulas. 3.2. Spectroscopic properties High resolution mass spectral data of the synthesized cobalt(III) and copper(II) complexes suggest that they are mononuclear and coordinated to three ligands for the cobalt complex and only two for the copper complex (Fig. S1). The mass spectrum of the cobalt complex Co(L)3 displayed a molecular ion peak for which the measured high resolution (HRMS) data are in good agreement with the calculated value as suggested by the molecular formula; the peak corresponds to the protonated form [Mþ þ 6Hþ] at m/z 829.3019 and a relative abundance of 6.7%. In addition, the mass spectrum, the cobalt complex exhibits the more stable fragment at m/z 567.1691 (100%) which is consistent with the molecular formula of the structure proposed after removing one ligand to give rise to formation of another singly charged cobalt Co(L)2 complex. The third fragment observed in the spectrum corresponds to the mass of the ligand (256.1334 m/z, 26.66%). Therefore, these results confirm the proposed molecular structure of the cobalt complex as illustrated in Scheme 2. On the other hand, the molecular ion peak of the copper(II) complex with its two ligands entities was observed at m/z 572.1736 m/z (65.61%). The spectrum shows that the ligand moiety (256.1341 m/z, 100%) was the most stable fragment. The UVeVis spectra of 103 M solutions of the ligand HL and its cobalt(III) and copper(II) complexes in DMF were recorded at ambient temperature. In the UVeVis spectra of the ligand, an intense absorption band at around 314 nm was assigned to the n-p* transition of azomethine groups [25]. This band is present in the electronic absorption spectrum of the copper complex at 305 nm showing a shift of about 9 nm [27]. The spectrum of the copper complex also displays another absorption band at 365 nm, which corresponds to the metal-to-ligand charge transfer expressing the interactions between the ligand and its metallic center via coordination sites (NO) [28]. In addition, in the ligand spectrum, an absorption band is observed at around 410 nm, the region of which overlaps with the MLCT band of Cu complex and the LMCT band of Co complex; this band is attributed to the n-p* transition of the azomethine group. This band is not observed in the cobalt(III)

complex. Furthermore, the complex displays a weak absorption band at a longer wavelength centered at 620 nm, which can be attributed to the DMSO-d6 transitions. For the cobalt complex, the spectrum exhibits an absorption band at 392 nm ascribed to the ligand-to-metal charge transfer (Fig. 1). Displayed in Fig. 2 are FT-IR spectra of HL and its cobalt(III) and copper(II) complexes. The FT-IR spectrum of the ligand (HL) shows broad stretching vibration bands between 3628 and 3320 cm1 and centered at 3431 cm1; these are assigned to the intramolecularly hydrogen bonded phenolic OeH stretching vibration (Fig. S2). In addition, a strong absorption band at 1630 cm1 attributed to the azomethine group (C]N) [29] was observed. The broad bands centered at 3431 cm1, attributed to the OeH vibrations, disappeared from spectra of the copper(II) and cobalt(III) complexes due to the coordination of metal ions to the phenoxy groups after their deprotonation. Moreover, the azomethine band in the complexes is shifted to lower frequencies (1613 and 1610 cm1 for the cobalt(III) and copper(II) complexes, respectively). This shift indicates that the nitrogen atom of the azomethine group is coordinated to the metal ion [30]. Additionally, the new bands at 457 cm1 and 459 cm1, and at 527 cm1 and 512 cm1 indicate coordination of metal ions to the azomethine and phenoxy groups. These are assigned to M-N and MO, respectively [31,32]. Listed in Table 1 are the important absorption bands for the ligand and its cobalt(III) and copper(II) complexes. 3.3. Thermogravimetric analysis 3.3.1. Thermal stability of CoIII(L)3·1/2H2O and CuII(L)2 Prepared complexes were subjected to thermogravimetric (TG) and differential thermogravimetric (DTG) analyses to obtain information about their thermal stabilities and to confirm or negate the presence of water in their structures. Measurements were performed at heating rates of 5, 10, 15, and 20
C min1 under a nitrogen atmosphere. Displayed in Fig. 3 are typical TG and DTG curves for the synthesized complexes CoIII(L)3$1/2H2O and CuII(L)2. (TG/DTG) curves performed at 10
C min1 for the cobalt(III) complex show four stages of decomposition, whereas only two stages were observed for the copper(II) complex. Given in Table 2 are the mass losses obtained from the TGA curves, along with species proposed to account for these mass losses; observed and calculated mass losses for each stage are in good agreement. In addition, Tmax in the DTG curve represent the temperature at which maximum mass loss is obtained. For the cobalt(III) complex, thermogravimetric analysis reveals that the decomposition of the cobalt complex starts at 170
C. At lower temperatures, the observed weight loss of 1.11% (calc. 1.08%) is attributed to the removal of half a molecule of hydration per formula unit. At higher temperatures, the compound gradually decomposes in the range of 170e328
C with a weight loss of 26.10% (calc. 26.07%) indicating the loss of two methoxyphenyl fragments from the complex. In the third stage, a weight loss of 36.86% (Calc. 37.76%) was obtained in the temperature range 328e428
C; this corresponds to the loss of C20H24O2N per formula unit. The last stage which takes place in the temperature range of 428e540
C corresponds to the remaining of the metallic oxide [1/ 2(Co2O3)] with some carbon as a residual species [33]. For the copper complex, its first stage decomposition took place in the temperature range from 190 to 330
C, with a weight loss corresponding to the fragment C20H24O2N2, whereas the second stage resulted in the expulsion of a C6H8O portion in the temperature range 330e499
C leaving the metallic oxide species CuO contaminated with carbon as residual species [34]. The observed overall weight losses are in conformity with the calculated values for the complex and are consistent with the thermal material decomposition.

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Scheme 2. Fragmentation ways observed for the CoIII(L)3$1/2H2O and CuII(L)2 complexes.

3.3.2. Determination of kinetic and thermodynamic parameters of thermal decomposition processes of studied complexes Mathematical treatment of the kinetic equations makes use of the following three methods of evaluation: (a) differential, (b) integral or (c) approximate [35]. The methods used in thermogravimetric data processing are those of Kissinger [36], Ozawa [37], and Coats-Redfern [38]. However, the most widely used methods to determine the apparent activation energy (Ea) and the preexponential factor (A) without prior knowledge of reaction order [36] are those of Kissinger and Ozawa. These two methods assume that the reaction rate at a given degree of conversion is only a function of temperature. The Kissinger's method [36] is a differential form, which can be used to determine the activation energy of the decomposition process. It makes use of the peak temperature at which the maximum reaction rate occurs. The equation involved in this process can be represented as follows:

ln Fig. 1. UVeVis spectra of 103 M DMF solutions of HL and its cobalt(III) and copper(II) complexes in DMF.

b 2 Tmax

! ¼ 

  Ea AR þ ln Ea RTmax

(1)

where, b is the heating rate (
C min1), Ea is the apparent activation

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Fig. 2. Infrared spectra of the HL, CoIII(L)3$1/2H2O and CuII(L)2.

energy (kJ mol1), Tmax is the peak temperature (
C) of the maximum rate (%. s1), A is the pre-exponential factor (s1), and R is the universal gas constant (J mol1 K1).   3 for a The plots of the left-hand side of Eq. (1) versus 10 Tmax

series of runs at different heating rates give straight lines; the activation energy Ea can be calculated from the slopes of these lines. The Ozawa's method [37], on the other hand, uses a logarithmic expression based on the integral isoconversional method [38]. It relies on the use of Doyle's approximation [39] for the considered integral, which will give the following equations:

Table 1 Infrared absorption bands of the Ligand (HL) and its cobalt(III) and copper(II) complexes.

  Ea log p E=RT y  2:315  RTmax

Absorption band, ʋ (cm

1

(2)

)

Species

ʋ(C]N)

ʋ(OeH)

ʋ(C]C)

ʋ(CeH)

ʋ(MeO)

ʋ(MeN)

HL CoIII(L)3$1/2H2O CuII(L)2

1636.0 1613.0 1610.0

3431.0 e e

1574.0 1510.0 1510.0

2934.0 2923.0 2934.0

e 527.0 512.0

e 457.0 459.0

and,

  AEa Ea  2:315  0:4567 log b ¼ log RgðaÞ RTmax

(3)

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expansion and the assumption ( 2RT=E ≪1 Þ; which gives an a 1   Z T Ea dT A. approximate value for the following integral exp RT T0 After rearrangement of the kinetic expression given in g(f), the following equation can be obtained:

    gðfÞ AR Ea ¼ log  log 2 bEa 2:303RT T

(5)

If the thermal decomposition of the complexes follows a first order reaction, gðfÞ will be -logð1  fÞ:   3 versus 10 T gives a straight line, The plot of log logð1fÞ 2 T where Ea and A can be calculated from the slope and the intercept, respectively. The kinetic and thermodynamic parameters of the samples, such as the activation energy (Ea), enthalpy of activation (DH), entropy of activation (DS), and Gibbs free energy (DG) were calculated using Eqs. (6)e(8).

  Ah kT

DS ¼ 2:303R log

(6)

where h and k are Planck and Boltzmann's constants, respectively.

Fig. 3. TG and DTG curves of (A): CoIII(L)3$1/2H2O and (B): CuII(L)2 performed at 10
C min1 under nitrogen atmosphere.

DH ¼ Ea  RT

(7)

DG ¼ DH  T DS

(8)

Depicted in Fig. 4 are typical TG and DTG curves of HL,

Table 2 Results obtained from thermogravimetric decomposition of the HL, CoIII(L)3$1/2H2O and CuII(L)2 complexes.

III

Co (L)3$1/2H2O

CuII(L)2

Number of steps

Temp. range/
C

Tmax/
C

Mass loss/% Found (Calcd.)

Assignments

Residue

Step1 Step2 Step3 Step4 Step1 Step2

115-170 170e328 328-428 428-540 190-330 330-499

147.9 275.8 382.5 485.4 288.5 375.2

1.11 (1.08) 26.10 (26.07) 36.86 (37.76) 15.15 (14.39) 56.38 (56.70) 17.02 (16.79)

Loss Loss Loss Loss Loss Loss

1/2(Co2O3) þ 8C

In Eq. (3), b is the heating rate (
C min1), Ea represents the apparent activation energy (kJ mol1), A is the pre-exponential factor, whereas (s1), g(a) is the integral form of function, Tmax is the maximum temperature (
C) at the weight loss rate (%.s1), and R is the gas constant (J mol1 K1). Finally, a is the degree conversion, which represents the decomposed amount of the sample at time t and is defined in terms of changes of weight of the sample as follows:

f ¼

w0  wt w0  wf

(4)

Moreover, 1- a is the concentration of the material, which remains to react.where wt is the weight of the sample at a particular temperature, w0 and wf are the initial and final weights of the reaction, respectively. Thus, for a ¼ constant, the plots of log b versus 103/Tmax, may be obtained from differential curves, recorded at several heating rates which will give straight lines. Their slopes can be used to evaluate the apparent activation energy and the pre-exponential factor. The Coats-Redfern's method [40] uses the asymptotic series

of of of of of of

1/2 H2O C14H14O2 C20H24O2N C6H10N2O0.5 C20H24O2N2 C6H8O

CuO þ 6C

CoIII(L)3$1/2H2O, CuII(L)2, and NiII(L)2 samples at different heating rates from 5 to 20
C min1. Results from these reveal that the thermal decomposition curves of the samples are quite similar. The selected steps of the thermal pattern of decomposition of the ligand and its complexes are used to calculate both the kinetic and thermodynamic parameters. The activation energy Ea, the preexponential factor A, enthalpy of activation (DH), entropy of activation (DS), and the Gibbs free energy change (DG) are evaluated from TG and DTG data by using the Coats-Redfern's method. The calculated kinetic thermal degradation parameters are listed in Table 3. As expected, Ea, DH, and DG values are positive, whereas DS values are negative, indicating non-spontaneous chemical processes of the thermal decomposition of HL, CoIII(L)3$1/2H2O, CuII(L)2, and NiII(L)2 samples. Furthermore, the activation energy (Ea) values in the different stages ranged from 67.20 to 148.77 kJ mol1, whereas the respective values of the preexponential factor (A) varied from 8.41  106 to 5.85  1010 s1. The values corresponding to the entropy of activation (DS) were between 118.93 and 44.07 J mol1 K1. The corresponding values of the enthalpy of activation (DH) were in the range between

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Fig. 4. TG and DTG curves of the HL, CoIII(L)3$1/2H2O, CuII(L)2 and NiII(L)2 at different heating rates obtained in nitrogen atmosphere where a is the degree conversion parameter and (1-a) is the concentration of the material which remains to react.

Table 3 Kinetic and thermodynamic parameters for the thermal decomposition of the samples: HL, CoIII(L)3$1/2H2O, CuII(L)2 and NiII(L)2 obtained from Coats-Redfern method at 10
C min1 in nitrogen atmosphere. Sample HL NiII(L)2 CoIII(L)3$1/2H2O CuII(L)2

Tmax/
C 269.9 375.2 276.0 384.0 289.4

DT/
C 221.5e285.4 243.6e338.9 255.6e301.8 356.7e395.7 274.4e308.9

103/Ti0/K1 1.792 1.455 1.488 1.369 1.627

Slope 4.34±0.04 7.77±0.09 3.51±0.06 3.65±0.04 5.34±0.09

Intercept 7.77±0.07 11.31±0.14 5.23±0.10 5.00±0.06 8.69±0.016

62.7 and 97.5 kJ mol1. The resulting values of the free energy of activation (DG) varied between 115.8 and 142.6 kJ mol1. The calculated activation energy Ea values of the ligand (HL) and its

Ea/kJ mol1 83.2 148.8 67.3 69.9 102.1

A/s1 9

3.0x10 2.1x108 1.4x107 8.4x106 5.8x1010

DS/J K1 mol1

DH/kJ mol1

DG/kJ mol1

R2/%

68.5 92.1 113.4 118.9 44.1

78.6 77.8 62.7 64.47 97.5

115.8 137.5 124.9 142.6 122.2

99.69 98.44 98.41 99.39 99.68

complexes show that the thermal stability follow the order: Ea(NiII(L)2)>Ea(CuII(L)2)>Ea(CoIII(L)3$1/2H2O) which is not in harmony with the electropositive character of the metal ions. In

Table 4 Summaries of the Ea values of thermal decomposition performances of different samples obtained from Kissinger and Ozawa's method. Samples

HL NiII(L)2 CoIII(L)3$1/2H2O CuII(L)2

a

0.15e0.80 0.15e0.70 0.05e0.50 0.55e0.70 0.05e0.60

Tmax (K)

Kissinger

Ozawa

5

10

15

20

E/kJ mol1

R2/%

Ea/kJ mol1

R2/%

527.1 630.3 532.33 646.3 553.3

540.8 648.4 548.5 656.1 562.3

558.6 651.9 560.4 664.6 571.3

562.3 655.5 570.9 668.2 576.3

80.2 170.9 89.9 219.0 146.7

94.55 93.20 99.98 99.86 97.99

57.9 (a ¼ 0.475) 166.4 (a ¼ 0.425) 128.0 (a ¼ 0.425) 159.9 (a ¼ 0.625) 89.2 (a ¼ 0.325)

97.44 94.79 98.36 95.03 98.06

The fat for the Alpha conversion means the interval where we have a linear adjustment in the method of Kissinger and that of Ozawa.

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addition, the positive values of (DH) demonstrate that the decomposition is an endothermic process [41]. The positive DG values corresponding to the different steps of thermal decomposition process suggest non-spontaneous processes. The obtained energy values of the thermal decomposition pattern of the complexes revealed that these materials have a high thermal stability; this is due to the covalent character of their bonds [42]. On the other hand, the negative values of DS of thermal decomposition of the synthesized complexes demonstrate that they are more ordered than the ligand itself [43]. The linear curves obtained via the Kissinger and Ozawa methods for HL, CoIII(L)3$1/2H2O, CuII(L)2, and NiII(L)2 are shown in Fig. S3 and S4, whereas the obtained Kinetic parameters are summarized in Table 4. In addition, the values of kinetic parameters obtained by using Ozawa and Kissinger methods seem to be more compatible than those obtained by using Coats-Redfern's method [44]. 3.4. Electrochemical behavior of the cobalt(III) and copper(II) complexes Cyclic voltammetry was employed to explore the electrochemical behavior of the complexes CoIII(L)3$1/2H2O and CuII(L)2. Displayed in Fig. 5A and B are cyclic voltammograms for a 1 mM solution of cobalt(III) complex in DMF containing 0.1 M TBABF4 recorded at a scan rate of 100 mV s1 at a glassy carbon electrode in the potential range from 0 to 1.6 V and 2.0 to þ0.6 V, respectively, vs. SCE. The voltammograms show three reduction waves with peak potentials of 0.88, 1.12, and 1.86 V vs. SCE. The first wave may be attributed to the reduction of Co(III) to Co(II), whereas the second reduction wave is ascribed to the reduction of the Co(II) complex to its Co(I) species [45]. At more negative potentials, a reduction wave with a peak potential of 1.86 V, along with other smaller ones, were observed; these waves are most likely associated with reduction of the ligand [46]. In addition, two anodic waves with peak potentials of 0.09 and þ0.22 V were observed. The first wave is ascribed to the oxidation of Co(I) to Co(II) [47], whereas the second may be related to the irreversible redox couple of Co(III)/Co(II) system, these results are in agreement with the literature [48]. Exhibited in Fig. 5C is a cyclic voltammogram for a 1 mM solution of the complex CuII(L)2 in at a glassy carbon electrode, in DMF containing 0.1 M TBABF4 as supporting electrolyte, recorded at a scan rate of 100 mV s1. The CV of this complex shows three reduction peaks observed at þ0.19, 0.35, and 1.04 V vs. SCE. The wave with peak potential (Epc ¼ þ0.19 V) may be attributed to the reduction of Cu(III) species to Cu(II), whereas the wave with the peak potential (Epc ¼ 0.35 V) can be attributed to reduction of ligand. The third reduction wave (1.04 V) can be attributed to reduction of Cu(II) to Cu(I) (Fig. 5c) [49]. When the scan was reversed, three oxidation peaks with peak potentials þ0.18, þ0.49, and þ1.25 V. The first peak (Epa ¼ þ0.18 V) is associated with the ligand. The second wave (þ0.49 V) was assigned to the oxidation of Cu(II) to Cu(III), and the last wave can be ascribed to the oxidation of the azomethine group of the Schiff base ligand. Furthermore, the electrochemical behavior of the copper (II) complex was carried out between 0.2 and þ1.0 V vs. SCE as shown in Fig. 5D. The main feature of this voltammogram is the Cu(III)/ Cu(II) [50] redox system at þ0.51 V for the anodic peak and þ0.32 V for the cathodic one. The peak separation (DEp ¼ Epa - Epc) of 190 mV was observed, suggesting that this redox system is not Nernstian, but it is rather quasi-reversible. Additionally, the evolution of the anodic and cathodic current peaks for the Cu(III)/Cu(II) redox system at different scan rates have been analyzed by CV between 0.2 and þ1.0 V vs. SCE (Fig. 5D). These voltammograms show a continuous increase of anodic and cathodic peak currents

Fig. 5. Cyclic voltammograms of 1 mM of CoIII(L)3$1/2H2O on GC electrode in DMF solutions and 0.1 M TBABF4 cycling from - 2.0 to þ0.6 V vs. SCE (A) and between 0.0 and - 1.6 V vs. SCE (B); Cyclic voltammograms of 1 mM of CuII(L)2 on GC electrode in DMF solutions and 0.1 M TBABF4 cycling from - 1.6 to þ1.4 V vs. SCE (C) and between 0.2 and þ1.0 V vs. SCE (D) at 100 mV s1 and at various scan rates.

(ipa, ipc) with the increase of the scan rate. In this case, it can be observed that peak separation values are relatively far from reaching 60 mV, despite the fact that the scan rates were varied from 40 to 400 mV s1. Therefore, oxidation of the copper complex

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corresponds to a one-electron charge transfer, which is compatible with a redox system that is not Nernstian. The ipa/ipc ratios are systematically higher than unity. These results permit us to conclude that the redox couple Cu(III)/Cu(II) is compatible with a slow electrochemical system. Therefore, this is consistent with an electrochemical process usually governed by a diffusion regime [50]. 4. Conclusion In the present work we have synthesized Co(III) and Cu(II) complexes of a ligand derived from salicylaldehyde and an alkylamine. These two complexes have been characterized with different spectroscopic techniques, such as IR, UVeVis, and mass spectrometry, and by elemental and thermal analysis. Activation energies of the decomposition of HL, CoIII(L)3$1/2H2O, CuII(L)2 and NiII(L)2 were calculated using the Kissinger, Ozawa and CoatsRedfern methods. TG curve of CoIII(L)3$1/2H2O shows two steps in the pattern of thermal decomposition situated in the ranges of conversion 0.72e0.95 and 0.42e0.64 with Ea ¼ 89.9 and 219.0 kJ mol1, respectively. On the other hand, the TG of CuII(L)2 complex shows one thermal degradation in the degree of conversion between 0.35 and 0.95 with an Ea ¼ 146.7 kJ mol1. The higher values of the activation energy of the studied complexes reflect a reasonable high thermal stability. Furthermore, thermodynamic parameters calculated using Coats-Redfern's method afforded negative values of entropy of activation, which shows a higher order achieved in the complexes rather than the ligand. These negative values express a change in the order due to the complexity of the complex thermal decomposition process. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.molstruc.2017.04.029. References [1] A. Gulcu, M. Tumer, H. Demirelli, R.A. Wheatley, Cd(II) and Cu(II) complexes of polydentate Schiff base ligands: synthesis, characterization, properties and biological activity, Inorg. Chim. Acta 358 (2005) 1785e1797. [2] E. Yousif, E. Rentschler, N. Salih, J. Salimon, A. Hameed, M. Katan, Synthesis and antimicrobial screening of tetra Schiff bases of 1,2,4,5-tetra (5-amino1,3,4-thiadiazole-2-yl)benzene, J. Saudi Chem. Soc. 18 (2014) 269e275. [3] A. Ourari, Y. Ouennoughi, D. Aggoun, M.S. Mubarak, E.M. Pasciak, D.G. Peters, Synthesis, characterization, and electrochemical study of a new tetradentate nickel(II)-Schiff base complex derived from ethylenediamine and 50 -(Nmethyl-N-phenylaminomethyl)-20 -hydroxyacetophenone, Polyhedron 67 (2014) 59e64. [4] M.A. Ali, M.H. Kadir, M. Nazimuddin, S.M.M. Majumder, M.T.H. Tarafder, M.A. Khair, Synthesis, characterization and antifungal properties of some foorcoordinate nickel(II) and four and five coordinate copper(II) complexes containing tridentate thiosemicarbazones and heterocyclic bases, Indian J. Chem. A 27 (1988) 1064e1067. [5] H.-Q. Chang, L. Jia, J. Xu, T.-F. Zhu, Z.-Q. Xu, R.-H. Chen, T.-L. Ma, Y. Wang, W.N. Wu, Syntheses, crystal structures, anticancer activities of three reduce Schiff base ligand based transition metal complexes, J. Mol. Struct. 1106 (2016) 366e372. [6] S.O. Bahaffi, A.A. Abdel Aziz, M.M. El-Naggar, Synthesis, spectral characterization, DNA binding ability and antibacterial screening of copper(II) complexes of symmetrical NOON tetradentate Schiff bases bearing different bridges, J. Mol. Struct. 1020 (2012) 188e196. [7] S.N. Pandeya, D. Sriram, G. Nath, E. De Clercq, Synthesis, antibacterial, antifungal and anti-HIV evaluation of Schiff and Mannich bases of isatin derivatives with 3-amino-2-methylmercapto quinazolin-4(3H)-one, Pharm. Acta Helv. 74 (1999) 11e17. [8] H.A.R. Pramanik, P.C. Paul, P. Mondal, C.R. Bhattacharjee, Mixed ligand complexes of cobalt(III) and iron(III) containing N2O2-chelating Schiff base: synthesis, characterisation, antimicrobial activity, antioxidant and DFT study, J. Mol. Struct. 1100 (2015) 496e505. [9] S. Zakavi, S. Talebzadeh, S. Rayati, Catalytic activity of Mn(III) and Fe(III) complexes of meso-tetra(n-propyl)porphyrin in oxidation of olefins: mesoalkyl substituent in comparison with the alkenyl and aryl ones, Polyhedron

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