Thermal decomposition kinetics of some metal complexes of N,N-diethyl-N'-benzoylthiourea

Article abstract of DOI:10.1023/A:1010171230450

Thcrmogravimctry (TG) and differential thermal analysis (DTA) were performed on the complexes with general formula (M(DEBT)_n (where M=Fe, Co, Ni, Cu or Ru; n=2, or 3 and DEBT=N,N-di-cthyl-N'-bcnzoylthiourea). Derivative thermogravimctric (DTG) curves were also recorded in order to obtain decomposition data on the complexes. The complexes of Fe(III), Co(II), Ni(II), Cu(ll) and Ru(III) displayed two- or three-stage decomposition patterns when heated in a dynamic nitrogen atmosphere. Mass loss considerations relating to the decomposition stages indicated the conversion of the complexes to the sulfidcs or to the corresponding metal alone (Cu, Ru, NiS, CoS or FeS). Mathematical analysis of the TG and DTG data showed that the order of reaction varied between 0.395 and 0.973. Kinetic parameters such as the decomposition energy, the entropy of activation and the pre-exponcntial factor are reported.

Full text of DOI:10.1023/A:1010171230450

Journal of Thermal Analysis and Calorimetry, Vol. 61 (2000) 955–965
THERMAL DECOMPOSITION KINETICS OF SOME
METAL COMPLEXES OF
N,N-DIETHYL-N’-BENZOYLTHIOUREA
N. Özpozan1, H. Arslan2, T. Özpozan1, M. Merdivan3 and N. Külcü2
1Chemistry Department, Erciyes University, Kayseri
2Chemistry Department, Mersin University, Mersin
3Chemistry Department, Balikesir University, Balikesir, Turkey
(Received January 10, 1999; in revised form March 1, 2000)
Abstract
Thermogravimetry (TG) and differential thermal analysis (DTA) were performed on the complexes
with general formula (M(DEBT)n (where M=Fe, Co, Ni, Cu or Ru; n=2, or 3 and DEBT=N,N-di-
ethyl-N’-benzoylthiourea). Derivative thermogravimetric (DTG) curves were also recorded in order
to obtain decomposition data on the complexes. The complexes of Fe(III), Co(II), Ni(II), Cu(II) and
Ru(III) displayed two- or three-stage decomposition patterns when heated in a dynamic nitrogen at-
mosphere. Mass loss considerations relating to the decomposition stages indicated the conversion of
the complexes to the sulfides or to the corresponding metal alone (Cu, Ru, NiS, CoS or FeS). Mathe-
matical analysis of the TG and DTG data showed that the order of reaction varied between 0.395 and
0.973. Kinetic parameters such as the decomposition energy, the entropy of activation and the
pre-exponential factor are reported.
Keywords: kinetics, metal complexes
Introduction
N,N-Diethyl-N’-benzoylthiourea (DEBT) is a selective analytical reagent, especially
for the determination of transition metals in complex interfering matrices [1, 2]. The
complexation capacity of DEBT has been reported in several papers [3, 4]. However,
there is no information on their thermal decomposition in the literature.
In this study, thermoanalytical data (TG, DTA and DTG) were obtained on the
Fe(III), Co(II), Ni(II), Cu(II) and Ru(III) complexes of DEBT. The interpretation and
the mathematical analysis of these data and the energy and the entropy of activation
were based on the integral methods involving the Coats-Redfern equation [5, 6] and
the approximation methods involving the Horowitz-Metzger equation [7–9]. The
Horowitz-Metzger equation was also used to establish the order of reaction.
1418–2874/2000/ $ 5.00
Akadémiai Kiadó, Budapest
© 2000 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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ÖZPOZAN et al.: METAL COMPLEXES OF N,N-DIETHYL-N’-BENZOYLTHIOUREA
Experimental
Chemicals
DEBT and the complexes M(DEBT)n (where M=Fe(III), Co(II), Ni(II), Cu(II) or
Ru(III), n=2 or 3) were prepared by the methods described previously [4]. All chemi-
cals used in the syntheses were of analytical reagent grade or extra pure.
Instruments
TG/DTA experiments on the five solid complexes were carried out with a Shimadzu
DT-40 thermal analyser equipped with a simultaneous TG/DTA module under a dy-
namic nitrogen atmosphere (50 ml min–1) at a heating rate of 10 K min–1 in the tem-
perature range 298–1273 K. The mass of complexes was 5–10 mg in a platinum cru-
cible; α-Al2O3 was used as a reference substance.
X-ray powder diffraction analyses of the final residues were made with a Phil-
lips PW1010 model generator, using CuKα radiation (λ=1.5406 Å).
GC-MS analyses were carried out by means of a Hewlett-Packard 5890 HP model
gas chromatograph and a 5971 HP-1 capillary column at a heating rate of 15 K min–1.
Decomposition kinetics
The thermal behaviour of these chelates was studied in detail. The TG traces exhib-
ited two- and three-stage decomposition patterns. The mass losses and X-ray diffrac-
tion data indicated that the end-products were the sulfide or the metal itself.
Fig. 1 TG/DTG curves of Fe(DEBT)3, Co(DEBT)2, Ni(DEBT)2, Cu(DEBT)2 and
Ru(DEBT)3 in nitrogen
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957
Fig. 2 DTA curves of a – Ru(DEBT)3 and b – Fe(DEBT)3
Fig. 3 Coats-Redfern plots of A – Ru(DEBT)3; B – Ni(DEBT)2; C – Fe(DEBT)3;
D – Cu(DEBT)2 and E – Co(DEBT)2 for stage Ia
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The TG curves were redrawn as the rate of mass loss vs. temperature in order to
obtain DTG curves. Typical TG/DTG curves for all the metal chelates examined are
shown in Fig. 1, with the DTA curves of Ru(DEBT)3 and Fe(DEBT)3 in Fig. 2. The
DTG decomposition temperature ranges for the metal chelates are presented in Ta-
ble 1.
Table 1 Thermal decomposition data
Mass loss/%
Peak temp. Temp. ranges
Evolved
moiety
Substance
Stage
in DTG/K
in TG/K
TG
theoret.
23.27
23.27
23.27
18.66
11.5
Ia
493
530
608
732
–
441–505
505–553
553–617
617–751
>751
23.33
23.37
23.29
19.51
10.5
DEB
DEB
DEB
2SCN+CN
FeS
Ib
Fe(DEBT)3
Ic
II
Residue
Ia
493
577
802
–
451–503
503–595
595–884
>884
33.66
33.18
15.80
17.4
33.48
33.48
15.88
17.2
DEB
DEB
SCN+CN
CoS
Ib
Co(DEBT)2
Ni(DEBT)2
Cu(DEBT)2
Ru(DEBT)3
II
Residue
I
535
648
696
–
516–552
552–656
656–749
>749
67.03
11.07
4.83
66.97
10.97
4.92
2DEB
SCN
II
III
CN
Residue
17.1
17.1
NiS
Ia
476
535
969
–
427–488
488–551
551–959
>959
33.43
32.93
21.58
12.1
33.18
33.18
21.74
11.9
DEB
DEB
2SCN
Cu
Ib
II
Residue
Ia
446
476
748
–
326–458
458–571
571–802
>802
22.34
44.68
20.62
12.4
21.96
43.92
21.59
12.5
DEB
2DEB
3SCN
Ru
Ib
II
Residue
Two methods were used to evaluate the kinetic data from these TG traces: the
Coats-Redfern and Horowitz-Metzger methods. For a first-order process, the
Coats-Redfern equation may be written in the form
1–(1–α )1−n
T 2 (1−n)
AR
φE*
2RT
E*
E*
log
=log
1−
−
(1)
2.303RT
Since the term 1–2RT/E* is ~1, a plot of the left-hand side of Eq. (1) vs. 1/T was
drawn. The E* values were calculated from the first decomposition step of the slope in
Fig. 3 and the A value was determined from the intercept. The Horowitz-Metzger
equation may also be written in the form
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ÖZPOZAN et al.: METAL COMPLEXES OF N,N-DIETHYL-N’-BENZOYLTHIOUREA
959
1−(1−α 1−n
1−n
)
E*θ
RTs2
ln
=
(2)
where θ is the difference between the given temperature T and Ts, where Ts is the temper-
ature corresponding to the maximum decomposition rate. A plot of ln[(1–(1–α1–n))/(1–n)]
vs. θ was drawn and the slope E* was calculated for the curve, which was found to be lin-
ear. Typical plots for the first decomposition steps are given in Fig. 4.
Fig. 4 Horowitz-Metzger plots of A – Ru(DEBT)3; B – Ni(DEBT)2; C – Fe(DEBT)3;
D – Cu(DEBT)2 and E – Co(DEBT)2 for stage Ia
Results and discussion
All complexes were studied by TG from ambient temperature to 1273 K under a ni-
trogen atmosphere. The temperature ranges, the percentage mass losses during the
decomposition, the maximum decomposition temperature (DTG max), and the theo-
retical percentage mass losses are listed in Table 1. The overall loss in mass from the
TG curves was 83% for [Ni(DEBT)2], 88% for [Cu(DEBT)2], 83% for [Co(DEBT)2],
90% for [Fe(DEBT)3], and 88% for [Ru(DEBT)3]. These complexes do not contain
Fig. 5 XRD curves of end-product of Ni(DEBT)2
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ÖZPOZAN et al.: METAL COMPLEXES OF N,N-DIETHYL-N’-BENZOYLTHIOUREA
any water molecule and there is no detectable change in the TG curves up to 373 K.
The thermal curves obtained for the five compounds are similar in character.
It was found from TG/DTA on the complexes of Co(II), Ni(II) and Cu(II), which
have a square planar structure, that they decompose in two or three stages. The
end-products remaining after pyrolysis were identified by X-ray powder diffracto-
metry. They consist of the metal sulfide or the metal itself. The diffraction pattern of
the end-product (NiS) of the pyrolysis of the Ni(II) complex can be seen in Fig. 5 as
an example.
For kinetic analysis of the pyrolysis reactions revealed by the TG analysis, a
computer program in BASIC was written [10]. The program accepts as data only the
percentage mass change and the related temperature, and it converts TG data to DTG
data. The same program calculates the degree of a pyrolytic reaction, the energy of
activation, the pre-exponential factor and the activation entropy through the kinetic
methods of Horowitz-Metzger and Coats-Redfern. The computer program was tested
with the known data for the pyrolysis of CaCO3.
The results of the analysis showed that the degrees of the main decomposition
reactions are close to unity, and the kinetic results obtained with the two methods are
in agreement with each other (Table 2).
When the activation energy of thermal decomposition and the ionic radius of
Co(II), Ni(II) or Cu(II) are compared, an inverse relationship is to be expected. As the
radius decreases, the activation energy should increase [6, 11]. The complexes of
Cu(II) and Co(II) decompose through similar mechanisms, and the activation energy
does in fact increase with decreasing radius.
Fig. 6 GC-MS chromatogram of Ru(DEBT)3
The ionic radii and the activation energies obtained by the Coats-Redfern
method for the first stage of decomposition of the Cu(II) and Co(II) chelates are given
below [12]:
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ÖZPOZAN et al.: METAL COMPLEXES OF N,N-DIETHYL-N’-BENZOYLTHIOUREA
961
E* =330.3 kJ mol–1 (rCu=69 pm)> E* =258.0 kJ mol–1 (rCo=74 pm)
Cu
Co
A decrease in radius of the metal ion allows the ligand to approach the central
metal atom more easily, resulting in a stronger bonding interaction between the metal
and the ligand. The strengthening of the bonding hampers cleavage of the bond and
E* increases [10, 11].
Fig. 7 Mass spectra of Ru(DEBT)3; a – the peak at tR=5 and b – the peak at tR=5.51
One molecule of diethylbenzamide (C6H5CON(C2H5)2; DEB) leaves the Cu(II)
and Co(II) complexes in the first decomposition stage. Two molecule of DEB leave
the Ni(II) complex in the first decomposition stage. The activation energy of this de-
composition stage for the Ni(II) complex is 160.3 kJ mol–1.
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The Fe(III) and Ru(III) complexes with octahedral structures, require energies
of 215.3 and 121.1 kJ mol–1, respectively, for the loss of 1 molecule of DEB in the
first stage. All the complexes under examination were analyzed via their GC
chromatograms and the mass spectra in the interval 333–533 K. Because of the simi-
larity in the decomposition processes of these metal chelates, for simplicity only the
GC chromatogram and the mass spectrum of Ru(DEBT)3 at different retention times
are given in Figs 6 and 7, respectively.
It is observed that the decrease in metal ionic radius on going from Ru(III) to
Fe(III) (with the same geometry) led to an increased activation energy of decomposi-
tion of the complex. The activation energies of decomposition of the Fe(III) and
Ru(III) complexes and the related radii of the metal ions can be compared:
E* =215.3 kJ mol–1 (rFe=64 pm)> E* =121.1 kJ mol–1 (rRu=69 pm)
Fe
Ru
The first mass loss observed in the TG curves of the complexes relates to the for-
mation of the metal thiocyanate. This occurs for Ni(II) in one step, and for Cu(II),
Co(II) and Ru(III) in two steps (Ia and Ib). The corresponding product for the Fe(III)
complex is formed in three steps (Ia, Ib and Ic). In general, the decomposition begin
at approximately 430 K, with a rapid mass loss above 66%. The calculated mass
losses show close agreement with the experimental values. Consequently, the mass
loss in this stage is attributed to the evolution of DEB.
Table 2 Kinetic data
Reaction
order
From Coats-
Redfern eqn.
From Horowitz-
Metzger eqn.
Complex
Stage
Ia
Parameters
Fe(DEBT)3
0.804
0.823
0.785
0.727
E*
A
215.3
7.35 1020
175.9
230.0
2.51 1022
179.7
∆S
r
E*
0.9991
0.9993
Ib
Ic
II
124.0
129.0
A
1.35 1010
–26.6
4.02 1010
–46.7
∆S
r
E*
0.9967
149.8
8 1010
0.9966
161.2
8.71 1011
A
∆S
r
E*
–41.3
–22.2
0.9991
199.6
0.9994
207.6
A
1.76 1012
5.60 1012
∆S
r
10.8
8.3
0.9943
0.9946
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ÖZPOZAN et al.: METAL COMPLEXES OF N,N-DIETHYL-N’-BENZOYLTHIOUREA
963
Table 2 Continued
Reaction
order
From Coats-
Redfern eqn.
From Horowitz-
Metzger eqn.
Complex
Stage
Ia
Parameters
Co(DEBT)2
0.794
0.817
0.924
0.973
0.775
0.705
0.807
0.897
0.398
E*
A
258.0
7.24 1025
271.4
259.8
6.7 1025
245.3
∆S
r
E*
0.9972
41.9
0.9986
44.4
Ib
II
I
A
34.4
64.4
∆S
r
E*
–194.2
0.9978
261.5
–215.2
0.9985
270.7
A
1.74 1015
6.2 1015
∆S
r
E*
68.1
49.3
0.9981
160.4
7.9 1013
0.9975
166.0
1.71 1014
Ni(DEBT)2
A
∆S
r
E*
42.4
22.7
0.9899
340.2
6.5 1025
0.9890
346.7
1.8 1026
II
III
Ia
Ib
II
A
∆S
r
E*
270.6
251.4
0.9750
79.6
0.9734
88.9
A
2466.7
–158.7
0.9520
330.3
12923
–173.4
0.9602
359.0
∆S
r
E*
Cu(DEBT)2
A
1.50 1034
3.7 1037
∆S*
r
E*
430.7
470.3
0.9940
78.7
0.9285
84.5
187.2 104
–129.5
0.9946
124.9
8.9 105
–139.0
0.9800
A
44.4 104
–115.5
0.9930
111.77
3.2 107
–79.9
∆S*
r
E*
A
∆S*
r
0.9991
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ÖZPOZAN et al.: METAL COMPLEXES OF N,N-DIETHYL-N’-BENZOYLTHIOUREA
Table 2 Continued
Reaction
order
From Coats-
Redfern eqn.
From Horowitz-
Metzger eqn.
Complex
Stage
Ia
Parameters
Ru(DEBT)3
0.827
0.943
0.642
E*
121.1
1.5 1012
9.6
131.8
2.7 1013
8.82
A
∆S*
r
E*
0.9960
67.8
28.8 103
–138.2
0.9886
118.9
0.9972
60.6
12.8 103
–170.4
0.9924
136.5
Ib
II
A
∆S*
r
E*
A
1.5 106
–105.35
0.9952
2.7 107
–109.9
0.9967
∆S*
r
Units of parameters: E*=kJ mol–1, A=s–1, ∆S*=J mol–1 K–1, r=correlation coefficient of the linear plot,
n=order of reaction
The decomposition of the Cu(II), Co(II), Fe(III) and Ru(III) complexes to metal
thiocyanate are observed to occur in one, two or three steps; the first mass loss corre-
sponds to the elimination of 1 DEB molecule, followed by the elimination of the re-
maining DEB molecules according to the oxidation numbers of the metals.
The activation entropies of pyrolysis of the complexes are all positive in the first
decomposition stage. The endothermic peaks observed in the DTA studies stem from
the stability of the reactant.
The activation energy of decomposition involving elimination of the second
DEB molecule from the Co(II) and Cu(II) complexes increases with decreasing metal
ionic radius. Only the activation energies of Co(II) and Cu(II) are compared here, be-
cause of the similarities of their second decomposition mechanisms, in which they
both decompose in two steps, with elimination of 1DEB molecule in each step. The
E* values are 41.9 kJ mol–1 for the Co(II) and 78.7 kJ mol–1 for the Cu(II) complex.
The E* value for elimination of the second DEB from the Fe(III) complex is
124.0 kJ mol–1. The corresponding value is 67.8 kJ mol–1 for the Ru(III) complex,
where 2 molecules of DEB (the second and the third) are evolved simultaneously.
149.8 kJ mol–1 is necessary to eliminate the third and last molecule of DEB in the de-
composition of the Fe(III) chelate. The second stage, including the elimination of
SCN and CN groups, starts at 550–600 K, after the elimination of all DEB molecules
in the first stage. The loss of these groups reveals different behaviour for each chelate
in the second stage, but all groups leave in one step. The moieties evolved in this
stage are 1SCN radical from the Ni(II) chelate, 2SCN radicals from the Cu(II) che-
late, 2SCN and 1CN radicals from the Fe(III) chelate, 1SCN and 1CN radicals from
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ÖZPOZAN et al.: METAL COMPLEXES OF N,N-DIETHYL-N’-BENZOYLTHIOUREA
965
the Co(II) chelate and 3SCN radicals from the Ru(III) chelate. Of all these chelates,
only that of Ni(II) undergoes a third decomposition stage, in which a CN radical is
evolved, leaving NiS behind. Finally, NiS, CoS, FeS, Ru and Cu are the end-products
of the pyrolytic decompositions.
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