Transition metal complexes of Vanillin-4N-(2-pyridyl) thiosemicarbazone (H2VPT); thermal, structural and spectroscopic studies

Journal of Molecular Structure 969 (2010) 33–39

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Transition metal complexes of Vanillin-4N-(2-pyridyl) thiosemicarbazone (H2VPT); thermal, structural and spectroscopic studies Gaber Abu El-Reash, Usama El-Ayaan *, I.M. Gabr, El-Bastawesy El-Rachawy Department of Chemistry, Faculty of Science, Mansoura University, Mansoura 35516, Egypt

a r t i c l e

i n f o

Article history: Received 8 November 2009 Received in revised form 22 December 2009 Accepted 19 January 2010 Available online 10 February 2010 Keywords: Vanillin Thiosemicarbazone complexes Spectroscopy Thermal analysis

a b s t r a c t The present work carried out a study on the ligational behavior of the new ligand, Vanillin-4N-(2-pyridyl) thiosemicarbazone (H2VPT) 1 towards some transition metal ions namely, Mn2+, Co2+, Ni2+, Cu2+, Zn2+,Cd2+, Hg2+ and U6+. These complexes namely [Mn(HVPT)Cl] 2, [Co(VPT)(H2O)]2H2O 3, [Ni(HVPT)Cl(H2O)] 4, [Cu(HVPT)Cl(H2O)] 5, [Zn(VPT)(H2O)]H2O 6, [Cd(HVPT)Cl(H2O)] 7, [Hg(VPT)(H2O)]H2O 8 and [UO2(H2VPT)(OAc)2]H2O 9, were characterized by elemental analysis, spectral (IR, 1H NMR and UV–vis) and magnetic moment measurements. The suggested structures were confirmed by applying geometry optimization and conformational analysis. Thermal properties and decomposition kinetics of all compounds are investigated. The interpretation, mathematical analysis and evaluation of kinetic parameters (E, A, DH, DS and DG) of all thermal decomposition stages have been evaluated using Coats–Redfern equation. ESR spectra of [Cu(HVPT)Cl]H2O at room temperature show broad signal, indicating spin-exchange interactions between copper(II) ions. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Thiosemicarbazones and their metal complexes present a wide range of pharmacological applications as antimicrobial, antitumor and antiviral agents [1]. In many circumstances the activity increases upon coordination to metal ions [2–4]. Coordination chemistry of mixed hard–soft NS donor ligands is a field of current interest. The most important factor in this objective probably the design of ligands with an appropriate structural backbone. Thiosemicarbazones that are most widely studied are sulfur and nitrogen consisting ligands [5–7]. The real impetus towards coordination chemistry of thiosemicarbazone complexes is the wide range of biological properties depending on the parent aldehyde or ketone including antitumor [8,9] antibacterial and antifungal properties as well as their physicochemical effects [10,11]. In this paper we prepared the new ligand Vanillin-4N-(2-pyridyl) thiosemicarbazone (H2VPT) (Fig. 1) and studied its ligantional behavior towards some transition metal ions namely, Mn2+, Co2+, Ni2+, Cu2+, Zn2+,Cd2+, Hg2+ and U6+. We applied geometry optimization and conformational analysis to the free ligand and studied all possible structural isomers and have got the minimum energy with E- and E0 -isomers (Fig. 2). 1H

* Corresponding author. Present address: Department of Chemistry, College of Science, King Faisal University, P.O. Box 380, Hofuf 31982, Saudi Arabia. Tel.: +20 553901011; fax: +20 35886437. E-mail address: [email protected] (U. El-Ayaan). 0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2010.01.039

NMR measurements confirm the presence of both isomers in solution. The thermal degradation kinetic parameters such as energy of activation (Ea) and the pre-exponential factor (A); and thermodynamic parameters like entropy (DS), enthalpy (DH) and activation energy (DG) for each step of degradation have been evaluated. 2. Experimental 2.1. Instrumentation and materials All starting materials were purchased from Fluka, Riedel and Merck and used as received. Elemental analyses (C, H and N) were performed on a Perkin–Elmer 2400 Series II Analyzer. Electronic spectra were recorded on a UV-UNICAM 2001 spectrophotometer using 10 mm pass length quartz cells at room temperature. Magnetic susceptibility was measured with a Sherwood Scientific magnetic susceptibility balance at 297 K. Infrared spectra were recorded on a Perkin–Elmer FTIR Spectrometer 2000 as KBr pellets and as Nujol mulls in the 4000–200 cm1 spectral range. 1H and 13 C NMR measurements at room temperature were obtained on a Jeol JNM LA 300 WB spectrometer at 250 MHz, using a 5 mm probe head in CDCl3. Thermogravimetric (TG) and differential (DTG) thermogravimetric analysis were performed on a DTG-50 Shimazu instrument at heating rate of 15 °C/min. ESR spectrum was obtained on a Bruker EMX spectrometer working in the X-band (9.78 GHz) with 100 kHz modulation frequency. The microwave power was set at 1 mW, and modulation

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G.A. El-Reash et al. / Journal of Molecular Structure 969 (2010) 33–39

(1.52 g, 10 mmol) [12]. The yellow crystals of H2VPT were removed by filtration, washed with ethanol and recrystallized from hot ethanol, m.p. 230, yield 2.75 g (91.0%). C14H14N4O2S (302.355) calcd.: C 55.6, H 4.66, N 18.5, S 10.6; found C 55.1, H 4.28, N 18.1, S 10.5% – 1 H NMR (DMSO-d6, 250 MHz, 24°C) d = 3.80 (s, 3H, OCH3), 11.02 (s, 2H, NH) 12.01 (s, OH), 15.01 (m, SH), 6.80–8.25 (phenyl and pyridyl protons, 7H).

2.2.2. Synthesis of metal complexes All complexes were prepared by refluxing H2VPT (0.30 g, 1.0 mmol) and the hydrated metal salts (1.0 mmol) e.g. chloride or acetate, in 30 ml ethanol for 2–3 h. The resulting solid complexes were filtered while hot, washed with ethanol followed by diethyl ether and dried in vacuo over CaCl2. Fig. 1. Structure of the free ligand (H2VPT).

2.3. Molecular modeling amplitude was set at 4 Gauss. The low field signal was obtained after 4 scans with a 10-fold increase in the receiver gain. A powder spectrum were obtained in a 2 mm quartz capillary at room temperature.

An attempt to gain a better insight on the molecular structure of ligand and its complexes, geometry optimization and conformational analysis has been performed by the use of MM+ [13] forcefield as implemented in hyperchem 7.5 [14].

2.2. Preparation of the free ligand H2VPT and its complexes 3. Results and discussion 2.2.1. Preparation of Vanillin-4N-(2-pyridyl)thiosemicarbazone (H2VPT) The ligand was prepared by boiling an ethanolic solution of 4(2-pyridyl)-3-thiosemicarbazide (1.68 g, 10 mmol) and Vanillin

The analytical and physical data of the ligand and metal complexes are listed in Table 1. The complexes are quite stable in air and insoluble in common organic solvents but soluble in dimethyl formamide (DMF) and dimethylsulfoxide (DMSO).

Fig. 2. Thione–thiol tautomeric structures of the free ligand (H2VPT). Table 1 Analytical and physical data and main IR spectral bands of H2VPT ligand and its metal complexes. Compound empirical formula

a py *

(F.Wt)

Color

(H2VPT), C14H14N4O2S

(302.352)

Yellow

[Cu(HVPT)Cl(H2O)], C14H15ClCuN4O3S [Ni(HVPT)Cl(H2O)], C14H15ClN4NiO3S [Co(VPT)(H2O)]2H2O, C28H30Co2N8O7S2 [Cd(HVPT)Cl(H2O], C14H15CdClN4O3S [Mn(HVPT)Cl(H2O)], C14H15ClMnN4O3S [Zn(VPT)(H2O)]H2O, C14H16N4O4SZn [Hg(VPT)(H2O)]H2O, C14H16HgN4O4S [UO2(H2VPT)(OAC)2]H2O C18H22N4O9SU

(418.358)

Green

(413.505)

Reddish-brown

(772.584)

Brown

(467.223)

Orange

(409.75)

Yellow

(401.760)

Yellow

(536.960)

Yellow

(708.483)

Pink

azomethine pyridine New

% Calcd. (found) C

H

N

S

55.61 (55.10) 40.19 (39.00) 40.66 (40.10) 43.53 (42.99) 35.99 (35.79) 41.04 (41.08) 41.85 (40.99) 31.31 (31.10) 30.51 (30.35)

4.66 (4.28) 3.61 (3.59) 3.66 (3.61) 3.91 (3.62) 3.24 (3.10) 3.69 (3.59) 4.01 (3.88) 3.00 (2.87) 3.13 (2.96)

18.53 (18.11) 13.39 (13.22) 13.55 (13.33) 14.50 (14.23) 11.99 (10.55) 13.67 (13.99) 13.94 (13.73) 10.43 (10.12) 7.91 (7.55)

10.61 (10.28) 7.66 (7.55) 7.75 (7.52) 8.30 (8.20) 6.86 (6.28) 7.83 (8.00) 7.98 (7.79) 5.97 (5.89) 4.52 (4.34)

m(C@N)a

m(C@N)py

m(C@S)

d(OH)

m(C@N)*

m(CAS)

1641

1571

807

1384

–

–

1618

1569

–

1385

1590

660

1618

1551

–

1382

1592

600

1646

1571

–

–

1621

629

1616

1570

–

1371

1527

598

1633

1570

–

1391

1613

617

1615

1568

–

–

1575

614

1641

1555

–

–

1583

606

1647

1540

753

1382

–

–

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G.A. El-Reash et al. / Journal of Molecular Structure 969 (2010) 33–39

Fig. 5. Structure of [Co(VPT))(H2O)]2H2O.

4000 Fig. 3. Structure of [UO2(H2VPT)(OAC)2]H2O.

2000

3.1. NMR and IR spectra of the ligand (H2VPT)

3.2. NMR and IR spectra of complexes Important IR bands for the ligand and complexes with their tentative assignments are presented in Table 1. H2VPT behaves as neutral bidentate ligand coordinating via the nitrogen of pyridyl group and the thione sulfur atom. This behavior which is found in [UO2(H2VPT)(OAC)2]H2O (Fig. 3), is revealed by (i) the shift of m(C@N) to lower wave number and the clear change

0

Intensity

The NMR spectrum of the H2VPT in DMSO-d6 shows three signals at d = 9.63, 12.01 and 15.01 ppm relative to TMS which disappear upon adding D2O and can be assigned to NH4, OH and SH protons, respectively. The multiplets at 6.81–8.25 ppm are assigned to the phenyl and pyridyl ring protons, while the signal at 3.8 ppm is due to AOCH3 protons [15]. The presence of SH signal confirmed the presence of ligand in thione–thiol isomers (E- and E0 -forms) (Fig. 2). The IR spectrum of H2VPT (in KBr) shows three bands at 1614, 1604 and 1571 cm1 which are assigned to m(C@N)(azomethine), m(C@C)phenyl and [m(C@C) and m(C@N)] of the pyridyl ring, respectively [16]. The bands located at 1469, 1270, 920 and 807 cm1 assigned to thioamide I–IV vibrations have substantial contributions from m(C@N), d(CAH), d(NAH) and m(CAS) vibrations [17]. The bands at 3226 and 3147 cm1 are assigned to m(NH4) and m(NH2), respectively. The bands at 3520 and 1384 cm1 are assigned to m(OH) and d(OH) vibrations, respectively. The band observed at 1020 cm1 is due to m(NAN) vibration. The possibility of thione/ thiol tautomerism (HNAC@S/N@CASH) is ruled out, since no band characteristic for a thiol group (2500–2650 cm1) is found in the spectrum of the ligand.

-2000

-4000

-6000 2500

2750

3000

3250

3500

3750

4000

G (value) Fig. 6. X-band ESR spectrum of [Cu(HVPT)Cl(H2O)] at room temperature.

in both intensity and position of the thioamide IV bands and (ii) the position of the bands assigned to d(OH), m(NH) and m(CAO) remaining practically unchanged, indicating that NH and OH groups of the ligand are not involved in bonding. Three bands are observed at 904, 833 and 260 cm1 assigned to the asymmetric stretching frequency (m3), the symmetric stretching frequency (m1) and the bending vibration (m4), respectively, of the dioxouranium ion [18]. The force constant (F) for the bonding sites of m(U@O) is calculated by the method of McGlynn et al. [19].

ðm3 Þ2 ¼ ð1307Þ2 ðF U—O Þ=14:1 The FU–O value is 6.75 m dynes Å1, the U–O bond distance is calculated with the help of the equation [20] shown below.

Fig. 4. Structure of [Cd(HVPT))Cl(H2O)](tetrahedral) and [Ni(HVPT))Cl(H2O)](square-planar).

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G.A. El-Reash et al. / Journal of Molecular Structure 969 (2010) 33–39

RU—O ¼ 1:08F 1=3 þ 1:17 The U–O bond distance (1.74 Å) falls in the usual region as reported earlier [7,12]. Acetate chelates show two bands at 1635 and 1315 cm1 assignable to masym(COO) and msym(COO), respectively. This difference exhibit D value [masym(COO)–msym(COO)] equal to 320 cm1 which is much greater than ionic complexes and indicates a monodentate acetate groups [21,22]. The NMR spectrum of this complex shows new signals in the region d = 2.1–2.7 ppm assigned to the CH3 protons of the acetate groups. Also, the presence of signals at 9.37, 9.59 and 11.09 ppm assigned to NH2, NH4 and OH protons, respectively, are similar to that observed in the H2VPT spectrum which confirms the neutral behavior of the ligand. In [Cu(HVPT)Cl(H2O)], [Ni(HVPT)Cl(H2O)], [Cd(HVPT)Cl(H2O)] and [Mn(HVPT)Cl(H2O)], H2VPT behaves as a mononegative bidentate ligand in the thiol form, coordinating via the deprotonated SH and C@N (azomethine) and the nitrogen of the pyridine ring. This behavior is revealed by (i) the position of the bands assigned to m(OH), d(OH) and m(CAO) remaining practically unchanged, indicating that OH group does not coordinate to the metal ion, (ii) the absence of m(NH) and thioamide IV m(CS) bands with simultaneous appearance of new bands due to m(SAC@N) and m(CAS) vibrations confirm the deprotonation of the SH. In the spectra of [Ni(HVPT)Cl(H2O)] and [Cd(HVPT)Cl(H2O)] (Fig. 4), the presence of a weak signal at 10.5 ppm assigned to OH proton suggests that the deprotonation takes place through the thiol form. In [Zn(VPT)(H2O)]H2O, [Co(VPT)(H2O)]2H2O and [Hg(VPT)(H2O)]H2O, H2VPT behaves as a binegative tridentate ligand (Fig. 5) coordinating via the deprotonation of both OH and CS groups. The third centre is the CN of azomethine in the zinc complex and the CN of pyridine ring in the Co and Hg-complexes. This behavior is revealed by (i) the disappearance of m(NH), m(OH) and m(CS) bands with simultaneous appearance of new bands assigned to m(SAC@N) and m(CAS) frequencies, (ii) the shift of m(C@N)(azomethine) in the Zncomplex and the [m(C@C) + m(C@N)] of the pyridine ring in the Co and Hg-complexes to lower wavenumbers and (iii) the shift of m(CAO) and m(NAN) to lower wavenumbers. The absence of signals due to acetate, SH and OH protons in the NMR spectrum of zinc complex confirmed the binegative behavior of the ligand.

parameter, According to Hathaway et al. [24], if the value of G is greater than 4, the exchange interaction between copper(II) centers in the solid state is negligible, whereas when it is less than 4, a considerable exchange interaction is indicated in the solid complex. The calculated G value is less than 4 suggesting that there are copper–copper exchange interactions. The tendency of A|| to decrease with an increase of g|| is an index of an increase of the tetrahedral distortion in the coordination sphere of copper [25]. In order to quantify the degree of distortion of the copper(II) complexes, we selected the f factor g||/A|| obtained from the ESR spectra. Although the f factor, which is considered an empirical index of tetrahedral distortion. Its value range between 105 and 135 for square-planar complexes, depending on the nature of the coordinated atoms. In the presence of a tetrahedrally distorted structure the values can be much larger [26]. For the complex, the g||/A|| quotient is 130, evidence in support of the square-planar geometry with no appreciable tetrahedral distortion. Molecular orbital coefficients, a2 (A measure of the covalency of the in-plane r-bonding between a copper 3d orbital and the ligand orbitals) and b2 (covalent in-plane p-bonding), were calculated by using the following equations [27,28].

a2 ¼ ðAk =0:036Þ þ ðg k  2:0023Þ þ 3ðg ?  2:0023Þ=7 þ 0:04 b2 ¼ ðg k  2:0023ÞE=  8ka2 where k = 828 cm1 for the free copper ion and E is the electronic transition energy. As a measure of the covalency of the in-plane r-bonding a2 = 1 indicates complete ionic character, whereas a2 = 0.5 denotes 100% covalent bonding, with the assumption of negligibly small values of the overlap integral. The b2 parameter gives an indication of the covalency of the in-plane p-bonding. The smaller the b2, the larger the covalency of the bonding. The values of a2 and b2 for the complexes indicate that the inplane r-bonding and in-plane and p-bonding are appreciably covalent. For the square-planar geometry complexes, the lower values of b2 compared to a2 indicate that the in-plane p-bonding is more covalent than the in-plane r-bonding. These data are well consistent with other reported values [29]. 3.3. Electronic spectra and magnetic moment measurements Electronic spectra were measured in 103 M dimethyl sulfoxide (DMF) solution of all studied complexes. The tentative assignments

3.3. ESR spectroscopy To obtain further information about the stereochemistry and the site of the metal ligand bonding and to determine the magnetic interaction in the metal complexes, ESR spectrum of the copper complex [Cu(HVPT)Cl(H2O)] were recorded in the solid state (Fig. 6). The spin Hamiltonian parameters of the complex with Cu(II), S = 1/2, I = 3/2, were calculated and are summarized in Table 2. The room temperature solid state ESR spectra of these complexes are quite similar and exhibit an axially symmetric g-tensor parameters with g||>g\ > 2.0023 indicating that the copper site has 2 a dx  y2 ground-state characteristic of square-planar, square pyramidal or octahedral stereochemistry [23]. In axial symmetry the g-values are related by the expression, G = (g||  2)/(g\  2) = 4, where G is the exchange interaction Table 2 ESR data of [Cu(HVPT)Cl]H2O complex at room temperature. Complex

g||

g\

A||  104 (cm1)

g||/A||

G

a2

b2

[Cu(HVPT)Cl]H2O

2.28

2.08

175

130

3.5

0.83

0.81

Table 3 Magnetic moment, electronic bands and ligand field parameters of the complexes derived from H2VPT. Compounda

Band position (cm1)

H2VPT

35,211, 32,051, 27,932 p ? p* n ? p*

[Cu(HVPT)Cl(H2O)]

16,233 24,038

2

[Ni(HVPT)Cl(H2O)]

16,666 20,792 25,125

1

16,556 23,321

2

24,691 25,773

6

[UO2(H2VPT)(OAc)2] H2O 23,474 27,624 17,543

1

[Co(VPT)(H2O)]2H2O [Mn(HVPT)Cl(H2O)]

Assignment

leff (B.M.) –

T2g ? 2E1g (G) 2.07 d ? p*

A1g ? 1A2g Diam. A1g ? 1B1g Spin-forbidden

1

B2g ? 2Eg B2g ? 2A1g

2.3

2

A1 ? 4E A1 ? 4A1

6.5

6

R + g ? 2 P4 n?p*

Diam.

a Electronic spectra were measured in 103 M dimethyl sulphoxide (DMF) solution of all studied complexes.

G.A. El-Reash et al. / Journal of Molecular Structure 969 (2010) 33–39

of the significant electronic spectral absorption bands of H2VPT and its metal complexes are given in Table 3. The electronic spectrum of the ligand show three bands at 35,211, 32,051 and 27,932 cm1. The strong band at 35,211 cm1 is due to p ? p* transition of the aromatic pyridine group. The bands at 32,051 and 27,932 cm1 are assignable to the n ? p* transitions of the azomethine and thioamide groups, respectively. These bands appear to be shifted to higher energy in the spectra of complexes, suggesting the coordination of imino nitrogen and the sulfur of thioamide group with the central metal ion. The electronic spectra are dominated by intense intra-ligand charge transfer. The magnetic moment of [Mn(HVPT)Cl(H2O)] complex, (6.5 B.M.) indicate the presence of five unpaired electrons, as expected for high spin 3d5 system [12]. The electronic spectra provide evidence for tetrahedral structure in two ways. Firstly, the fact that the spectra at 24,814 cm1 is clearly observed indicates tetrahedrally coordinated manganese(II), and secondly tetrahedral complexes where the laporte restriction is not so rigid generally exhibit spectra with molar extinction coefficient in the 1–10 l mol1 cm1 range [30]. The magnetic moment (2.3 B.M) for [Co(VPT)(H2O)]2H2O falls in the range reported for a square-planar structure. The spectrum shows two bands at 16,556 and 22,321 cm1 assignable to 2 B2g ? 2Eg and two 2B2g ? 2A1g, respectively. The diamagnetic reddish-brown nickel complex [Ni(HVPT)Cl (H2O)], exhibit a broad band in the region 15,870–20,830 cm1 (m2) which may be assigned to 1A1g ? 1A2g and 1A1g ? 1B1g. The other band at ca. 25,125 cm1 (m3) may be assigned to spin-forbidden transition. The fact that no band is observed below 10,000 cm1 indicates a square-planar stereochemistry for this complex [31]. The electronic spectra of [Cu(HVPT)Cl(H2O)] exhibits two bands: an asymmetric broad band at 16,233 cm1 and a more intense band at 24,038 cm1. The latter band may be assigned to ligand metal charge transfer transition. The asymmetric band is assigned to the

37

transition 2T2g ? 2Eg. The band position and the magnetic moment value (2.07 B.M) can be taken as evidence for the square-planar configuration. The electronic spectrum of [UO2(H2VPT)(OAc)2]H2O shows two bands at 23,474 and 27,624 cm1 assignable to 1R+g ? 2P4 and charge transfer n ? p* respectively. A third band at 17,543 cm1 due to electronic transitions from apical oxygen atom to the f-orbitals of uranyl atom is characteristic of the uranyl moiety [30]. 3.4. Thermal analysis The nature of water molecules in the complexes was assigned thermogravimetrically. Thermal data showed that the crystal water molecules are volatilized within the 75–125 °C temperature range, while coordinated water molecules are removed in the 120– 200 °C temperature range. Thermogravimetric (TG) and differential thermogravimetric (DTG) analysis of studied complexes are observed in Fig. 7. In this study we use the integral method of Coats and Redfern to evaluate the kinetic parameters and study the thermal behavior of complexes using the following equations:

" # 1  ð1  aÞ1n

  E AR þ ln for n – 1 RT uE ð1  nÞT      lnð1  aÞ E AR ln þ ln for n ¼ 1 ¼ RT uE T2

ln

2

¼

ð1Þ ð2Þ

where A, is the pre-exponential factor. The correlation coefficient, r, was computed using the least square method for different values of n, by plotting the left-hand side of equations (1), (2) versus 1000/T, Fig. 8. The n value which gave the best fit (r ffi 1) was chosen as the order parameter for the decomposition stage of interest. From the intercept and linear

Fig. 7. TG and DTG of some metal complexes.

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G.A. El-Reash et al. / Journal of Molecular Structure 969 (2010) 33–39

Fig. 8. Coats–Redfern plots for [Cd(HVPT)Cl]H2O complex, where Y=[ln(1  a)/T2].

Table 4 Temperature of decomposition, and the kinetic parameters of free ligand and its metal complexes. Compound

Step

T (K)

Aa (S1)

(H2VPT)

1st 2nd

521 780

59.24 0.057

[Ni(HVPT)Cl(H2O)]

1st 2nd

642 810

[Cu(HVPT)Cl(H2O)]

1st 2nd 3rd

[Mn(HVPT)Cl(H2O)]

Eb (kJ mol1)

DHb (kJ mol1)

DSc (kJ mol1 K1)

DGb (kJ mol1)

39.88 27.63

35.55 21.14

0.223 0.285

152.2 243.4

2.6  104 4.8  105

70.70 121.08

65.36 114.32

0.194 0.153

189.9 238.2

510 660 800

24.9 3.9  103 4.29  1010

38.87 74.55 194.06

34.63 69.06 187.41

0.231 0.191 0.058

117.8 195.2 233.7

1st 2nd 3rd

563 685 856

366.3 238.5 1.2  105

49.45 65.54 120.39

44.77 59.85 113.28

0.209 0.215 0.165

162.7 207.1 254.5

[Co(VPT)(H2O)]2H2O

1st 2nd 3rd 4th

300 463 623 838

4.6  105 5.4  103 1.4  105 1.7  103

48.2 54.4 79.2 98.7

45.7 50.6 74.1 91.7

0.144 0.185 0.161 0.200

89.2 136.5 174.5 259.3

[Hg(VPT)(H2O)]H2O

1st 2nd 3rd

539 714 904

1.2  104 825.5 6.0  105

65.5 76.9 144.3

61.0 71.0 136.7

0.180 0.205 0.152

158.2 217.1 274.1

[Cd(HVPT)Cl(H2O)]

1st 2nd 3rd 4th

333 553 795 981

124.0 7.2  105 6.0  103 1.2  104

28.2 85.7 97.1 124.8

25.4 81.1 90.5 116.6

0.214 0.146 0.189 0.185

96.7 162.0 240.8 298.5

[Zn(VPT)(H2O)]H2O

1st 2nd 3rd 4th

339 608 749 870

422.5 2.01 1.6  107 2.9  108

31.4 34.7 136.7 180.5

28.6 29.7 130.5 173.2

0.204 0.253 0.123 0.100

97.7 183.7 222.6 260.4

[UO2(H2VPT)(OAc)2]H2O

1st 2nd 3rd 4th

483 559 668 799

4.7  107 9.9  104 3.31 3.9  105

91.8 82.8 41.0 120.9

87.8 78.1 35.5 114.2

0.110 0.163 0.250 0.154

141.0 169.0 202.4 237.5

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slope of such stage, the A and E values were determined. The other kinetic parameters, DH, DS and DG were computed using the relationships; DH = E  RT, DS = R[ln(Ah/kT)] and DG = DH  TD S, where k is the Boltzmann’s constant and h is the Planck’s constant. The kinetic parameters are listed in Table 4. The following remarks can be pointed out: (i) all complexes decomposition stages show a best fit for (n = 1) indicating a first order decomposition in all cases. Other n values (e.g. 0, 0.33, and 0.66) did not lead to better correlations. (ii) The value of DG increases significantly for the subsequently decomposition stages of a given complex. This is due to increasing the values of TDS significantly from one stage to another which overrides the values of DH. Increasing the values of DG of a given complex as going from one decomposition step subsequently to another reflects that the rate of removal of the subsequent ligand will be lower than that of the precedent ligand [32,33]. This may be attributed to the structural rigidity of the remaining complex after the expulsion of one and more ligands, as compared with the precedent complex, which require more energy, TDS, for its rearrangement before undergoing any compositional change. (iii) The negative values of activation entropies DS indicate a more ordered activated complex than the reactants and/or the reactions are slow [34]. (iv) The positive values of DH mean that the decomposition processes are endothermic. 4. Conclusion A new series of complexes (2–8) were prepared from the novel ligand Vanillin-4N-(2-pyridyl)thiosemicarbazone (H2VPT). Geometry optimization and conformational analysis have been performed and the perfect agreement with spectral studies allow for suggesting the exact structure of all studied complexes. The stability of complexes was explained and kinetic parameters (E, A, DH, DS and DG) of all thermal decomposition stages have been evaluated using Coats–Redfern method. References [1] H. Beraldo, D. Gambino, Mini-Rev. Med. Chem. 4 (2004) 31. [2] I.C. Moreira, J.P. Speziali, N.L. Mangrich, A.S. Takahashi, J.A. Beraldo, H. J. Braz. Chem. Soc. 17 (2006) 1571.

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