Synthesis, thermal and spectral studies of first-row transition metal complexes with Girard-T reagent-based ligand

Article abstract of DOI:10.1016/j.molstruc.2007.01.054

The complexing behaviour of 1-acetyltrimethyl ammonium chloride-4-benzoyl thiosemicarbazide (H₂GTBzIT) towards the following first-row transition metal ions namely, Cr(III), Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) have been examined by elemen

Full text of DOI:10.1016/j.molstruc.2007.01.054

Available online at www.sciencedirect.com
Journal of Molecular Structure 871 (2007) 14–23
www.elsevier.com/locate/molstruc
Synthesis, thermal and spectral studies of first-row transition
metal complexes with Girard-T reagent-based ligand
*
Usama El-Ayaan , I.M. Kenawy, Y.G. Abu El-Reash
Department of Chemistry, Faculty of Science, Mansoura University, Mansoura 35516, Egypt
Received 24 November 2006; received in revised form 22 January 2007; accepted 24 January 2007
Available online 1 February 2007
Abstract
The complexing behaviour of 1-acetyltrimethyl ammonium chloride-4-benzoyl thiosemicarbazide (H2GTBzIT) towards the following
first-row transition metal ions namely, Cr(III), Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) have been examined by elemental analysis, mag-
netic measurements, electronic, IR and 1H NMR. The proton-ligand ionization constants were determined potentiometrically using
Irving–Rossotti technique. The stability constants of complexes were also calculated and were found in agreement with the sequence
of stability constants of Irving and Williams. Thermal properties and decomposition kinetics of all complexes are investigated. The inter-
pretation, mathematical analysis and evaluation of kinetic parameters (E, A, DH, DS and DG) of all thermal decomposition stages have
been evaluated using Coats–Redfern and Horowitz–Metzger equations.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Synthesis; Spectroscopic; Thermal degradation kinetics; Girard-T derivative
1. Introduction
Finally, in view of the fact that these reagents and their
Schiff bases posses ligating atoms they have also been stud-
ied as ligands in coordination chemistry [5,6].
The water-soluble properties of Girard-T hydrazones
make the reagent potentially effective for isolation of car-
bonyls from mixtures soluble in non-polar solvents [1]. In
work on the chemistry of rancidity [2], Gaddis et al. have
found the reagent useful in learning about the distribution
of volatile and non-volatile carbonyls in rancid fat. This
work indicated possible usefulness of the reagent for inves-
tigation of the complex carbonyl anatomy of autoxidized
fats. Successful application depends on adaptation to
quantitative isolation of very small amounts of carbonyls,
and on devising means of controlling the reaction so as
to isolate at will free carbonyl and bound or potential car-
bonyl [3]. Recently, Girard-T reagent was applied as a very
simple and efficient classical method for the purification of
3-aminoestrone. By this rapid procedure the purity of
crude 3-aminoestrone was increased from 67% to 96% [4].
In this paper we report the syntheses, spectral and ther-
mogravimetric studies of new series of transition metal
complexes of 1-acetyltrimethyl ammonium chloride-4-ben-
zoyl thiosemicarbazide (H2GTBzIT). The decomposition
kinetics has been studied by employing the two methods
viz. Coats–Redfern and Horowitz–Metzger. Thermody-
namic parameters (DS, DH and DG), for each step of deg-
radation have been calculated using the standard
equations. In solution, ligand formation constants and sta-
bility constants of all studied complexes were determined
potentiometrically using Irving–Rossotti technique.
2. Theoretical
2.1. Thermal degradation kinetics
*
Non- isothermal methods have been extensively used for
the evaluation of kinetic parameters of decomposition
Corresponding author.
E-mail address: usama@mans.edu.eg (U. El-Ayaan).
0022-2860/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.01.054

U. El-Ayaan et al. / Journal of Molecular Structure 871 (2007) 14–23
15
reactions. The rate of decomposition process can be
2.1.2. Horowitz–Metzger equation
described as the product of two separate functions of tem-
perature and conversion.
The Horowitz–Metzger equation is an illustrative of the
approximation methods. These authors derived the
relation:
da
n
o
¼ kðT Þf ðaÞ
ð1Þ
1ꢀn
log ½1ꢀð1ꢀaÞ ꢁ=ð1ꢀnÞ ¼ Eh=2:303RT 2s for n ¼ 1 ð8Þ
dt
where a is the fraction decomposed at time t, k(T) is the
temperature dependent function and f(a) is the conversion
function dependent on the mechanism of decomposition. It
has been established that the temperature dependent func-
tion k(T) is of the Arrhenius type and can be considered as
the rate constant k:
When n = 1, the LHS of Eq. (8) would be log[ꢀlog(1 ꢀ a)].
For first-order kinetics process the Horowitz–Metzger
equation maybe written in the form:
log½logðwa=wcÞꢁ ¼ Eh=2:303RT 2s ꢀ log 2:303
ð9Þ
where h = T ꢀ Ts, wc = wa ꢀ w, wa is the mass loss at the
completion of the reaction, and w is the mass loss up time
t. the plot of log[log(wa /wc)] versus h was drawn and found
to be linear from the slope of which E was calculated.
Other kinetic parameters DH, DS and DG were calculat-
ed as stated earlier in Section 2.1.1.
k ¼ AeꢀE=RT
ð2Þ
where R is the gas constant in (J molꢀ1 Kꢀ1). Substituting
Eq. (2) into Eq. (1), we get
da=dt ¼ ðA=ueꢀE=RT Þf ðaÞ
ð3Þ
where u is the linear heating rate dT/dt. On integration and
approximation, this equation can be obtained in the fol-
lowing form:
2.2. Molecular modeling
An attempt to gain a better insight on the molecular
structure of ligand and its complexes, geometry optimiza-
tion and conformational analysis has been performed by
the use of MM+ [11] forcefield as implemented in hyper-
chem 5.1 [12].
ln gðaÞ ¼ ꢀE=RT þ lnðAR=uEÞ
ð4Þ
where g(a) is a function of a dependent on the mechanism
of the reaction. The integral on the right hand side is
known as temperature integral and has no closed form
solution. Several techniques have been used for the evalua-
tion of temperature integral. Most commonly used meth-
ods for this purpose are the differential method of
Freeman–Carroll [7], integral method of Coats–Redfern
[8], the approximation method of Horowitz–Metzger [9].
In the present investigation, kinetic parameters have
been evaluated using the following two methods and results
obtained are compared with one another.
3. Experimental
3.1. Instrumentation and materials
All starting materials were purchased from Fluka, Rie-
del and Merk and used as received. Elemental analyses
(C, H, 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 suscep-
tibility was measured with a Sherwood Scientific magnetic
susceptibility balance at 297 K. Infrared spectra were
recorded on a Perkin-Elmer FT-IR Spectrometer 2000 as
KBr pellets and as Nujol mulls in the 4000–370 cmꢀ1 spec-
2.1.1. Coats–Redfern equation
The Coats–Redfern equation, which is a typical integral
method, can be represented as
Z
Z
1
T2
n
dað1 ꢀ aÞ ¼ ðA=uÞ
e
ꢀE=RT dt
ð5Þ
0
T1
1
tral range. H NMR measurements at room temperature
For convenience of integration the lower term T1 is usually
were obtained on a Jeol JNM LA 300 WB spectrometer
at 400 MHz, using a 5 mm probe head in CDCl3. Chemical
shifts are given in ppm relative to internal TMS (tetrameth-
ylsilane). Thermogravimetric (TG) and differential (DTG)
thermogravimetric analysis were performed on a DTG-50
Shimazu instrument at heating rate of 15 ꢁC/min.
taken as zero. This equation on integration gives,
ln½ꢀ lnð1 ꢀ aÞ=T2ꢁ ¼ ꢀE=RT þ ln½AR=uEꢁ
A plot of left-hand side (LHS) against 1/T was drawn. E
was calculated from the slope and A from the intercept val-
ue. The entropy of activation DS was calculated by using
the equation:
ð6Þ
3.2. Synthesis of 1-acetyl trimethylammonium chloride-4-
benzoyl thiosemicarbazide
DS ¼ R½lnðAh=kTsÞꢁ
ð7Þ
where k is the Boltzmann’s constant and h is the
Planck’s constant and Ts is the DTG peak temperature
[10].
The other kinetic parameters, DH and DG were comput-
ed using the relationships; DH = E ꢀ RT and
DG = DH ꢀ TDS.
Preparation of 1-acetyl trimethylammonium chloride-4-
benzoyl thiosemicarbazide (H2GTBzIT) was carried out by
refluxing (5.0 g, 30 mmol) of Girard-T with (4 ml, 30 mol)
of benzoyl isothiocyanate in 20 ml absolute ethanol over a
water bath for 4 h. The reaction mixture was left to cool till

16
U. El-Ayaan et al. / Journal of Molecular Structure 871 (2007) 14–23
white crystals were separated out. These were filtered off,
recrystallized from ethanol and finally dried in vacuum des-
iccators over anhydrous calcium chloride; m.p. 192.3.
Yield: 8.0 g (80.6 %) of (H2GTBZIT). Found: C, 46.90;
H, 5.65; N, 15.95; S, 9.59. Calcd. for C13H19ClN4O2S
(330.83): C, 47.2; H, 5.79; N, 16.94; S, 9.69%.
nal in the down-field region at d = 11.7 ppm assigned to the
coordinated (NH) proton and the signals of the other (NH)
protons shifted to the higher field region 7.9–7.6 ppm may
be due to break down of the hydrogen bond.
In the [Mn(H2GTBzIT)2Cl2] complex, the ligand
behaves as a neutral bidentate in both keto and enol form.
In the keto form, H2GTBzIT coordinate via CS in the thi-
one form and CO of benzoyl moiety but in the enol form
coordinate via the CS in the thione form and the nitrogen
of NH(1) group (Fig. 3). This mode of coordination is sup-
ported by the following observations: the shift of both
m(CO) benzoyl and m(CS) to lower wave number, m(CO)
of hydrazide moiety and m(OH) of the enolized benzoyl
moiety remains unchanged with simultaneous appearance
of new bands at 1643 and 1110 cmꢀ1 assignable to
m(C@N)and m(C–O), respectively.
3.3. Synthesis of metal complexes
All complexes were prepared by refluxing H2GTBzIT
(0.33 g, 1.0 mmol) and the hydrated metal salts (1.0 mmol),
e.g. chloride and 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.
4. Results and discussion
In the [Cr(HGTBzIT)Cl2(H2O)]Æ2H2O complex,
H2GTBzIT behaves as a mononegative tridentate coordi-
nating through the CO of hydrazide moiety in the keto
form, the nitrogen of the NH group and the enolic oxygen
of the benzoyl moiety with the displacement of a hydrogen
atom from the latter group as in (Fig. 4). This behavior is
revealed by the disappearance of m(CO) benzoyl moiety,
mNH(4) and the great change in both the intensity and
the position of m(CO) of the hydrazide moiety with appear-
ance of new bands assignable to m(C–O) and m(C@N).
H2GTBzIT behaves as a binegative tridentate in the
The analytical and physical data of (H2GTBzIT) ligand
and its complexes are listed in Table 1.
4.1. IR and NMR spectra of H2GTBzIT ligand and its
complexes
The principle IR bands of H2GTBzIT and its metal
complexes are listed in Table 2.
IR spectra (KBr) of H2GTBzIT, show a three bands at
3151, 3099 and 3061 cmꢀ1 assigned to mNH(2), mNH(4)
and mNH(1) modes, respectively. The two strong bands at
1689 and 1671 cmꢀ1 are attributed to m(CO) groups of
the hydrazide(h) and benzoyl(b) moieties, respectively.
Also, two bands at 706 and 1221 cmꢀ1 are assigned to
m(CS) and a combination of (mCS + mCN) [13,14], respec-
tively. The medium intensity band at 926 cmꢀ1 is attributed
to the m(N–N) vibration [15]. The sharp band at 3416 cmꢀ1
assigned to m(OH) indicates the presence of H2GTBzIT in
the keto–enol form through CO group of benzoyl moiety
(Fig. 1). This band is absent in the spectrum of (carboxym-
ethyl) trimethyl ammonium chloride hydrazide (GT).
The NMR spectrum of H2GTBzIT in DMSO-d6, shows
three signals at d = 8.053, 8.025 and 8.023 ppm relative to
TMS which disappear upon adding D2O and can be
assigned to NH(2), NH(4), NH(1) protons, respectively.
The multiple at (7.90–7.49 ppm) is assigned to phenyl ring
protons. The signal of CH2 protons appears shifted to
down field (5.05 ppm) because the presence of the quater-
nary nitrogen (N+) which act as a strong electron with-
drawing group.
[Ni(GTBzIT)(H2O)3]Æ2H2O
and
[Co2(HGTBzIT)Cl2-
(H2O)3]ÆH2O through the enolic oxygen of both carbonyl
groups with the displacement of hydrogen atoms and the
nitrogen of NH group. This behavior is revealed by the dis-
appearance of m(CO) benzoyl, m(CO) hydrazide, mNH(4)
and mNH(1) with appearance of new bands assignable to
m(C–O), mC@N(4) and mC@N(1). After heating the Ni(II)
complex in vacuum oven at 120 ꢁC for 6 h, the mass loss
is about 7.74% which suggests that a two water molecules
are uncoordinated.
In the [M3(HGTBzIT)(GTBzIT)Cl3(H2O)3] complexes
where M = Cu(II) or Zn(II), ligand behaves as a mononeg-
ative and binegative tridentate. In the binegative, the ligand
coordinates via the enolized oxygen of both CO with dis-
placement of hydrogen atoms and NH group. The mono-
negative ligand coordinate via the enolized (CO) of
hydrazide moiety with displacement of hydrogen atom,
NH and the oxygen of CO of benzoyl in Zn(II) complex
and the enolized oxygen of CO of benzoyl in Cu(II) com-
plex without displacement of hydrogen atom.
The IR spectrum of the [Cd(H2GTBzIT)Cl2-
(H2O)]Æ2H2O complex shows that the ligand behaves as a
neutral tridentate via the carbonyl oxygen (CO) of both
hydrazide and benzoyl moieties in the keto form and the
nitrogen of the NH group (Fig. 2). The mode of complex-
ation is suggested by the shift of both m(CO) and m(NH)
vibrations to higher wavenumber. Also, the m(CS) shifted
to higher wavenumber indicating that (CS) is out of coor-
dination. The NMR spectrum of this complex shows a sig-
4.2. Electronic spectra and magnetic moments of complexes
Electronic spectra were measured in 10ꢀ3 M dimethyl
sulfoxide (DMSO) solution of all studied complexes. The
band positions, magnetic moments and calculated ligand
field parameters are given in Table 3.
The electronic spectrum of [Cr(HGTBZIT)Cl2-
(H2O)]Æ2H2O shows two strong absorption bands at
16,447 (m1) and 22,075 cmꢀ1 (m2). We could not observe

U. El-Ayaan et al. / Journal of Molecular Structure 871 (2007) 14–23
17
the expected m3 band on our instrument which might be
hidden below m2. The three spin-allowed transitions for
chromium(III) in an octahedral field are as follows:
4A2g(F) fi 4T2g(F) (m1), 4A2g(F) fi 4T1g(F) (m2) and
4A2g(F) fi 4T1g(P) (m3). The m1 transition is a direct mea-
surement of the ligand field parameter 10 Dq. From m1
and m2, the value of B and b can be calculated. In addition,
the leff-value can be taken as additional evidence for the
octahedral geometry [16].
The electronic spectrum of the [Mn(H2GTBZIT)2Cl2]
complex shows a strong band at 24,134 cmꢀ1 which is
6
assigned to the A1g fi 4T2g(D) transition. The other char-
acteristic bands for d–d transitions are difficult to recognize
in this complex and thus the ligand field parameters could
not be calculated.
The nickel complex namely, [Ni(GTBZIT)(H2O)3].
2H2O shows spectral bands in the 14,577–17,391 and
3
26,246 cmꢀ1 regions, assignable to the A2g fi 3T1g(F) (m2)
3
and A2g fi 4T1g(P) (m3) transitions, respectively indicating
octahedral geometry.
The electronic spectrum of the blue complex,
[Co2(HGTBzIT)Cl2(H2O)3]ÆH2O exhibits an intense band
4
at 14,663 cmꢀ1 assignable to the A2(F) fi 4T1(P) transi-
tion and a shoulder at 16,260 cmꢀ1 due to spin coupling,
indicating tetrahedral geometry for this complex. The blue
color as well as the magnetic moment of 3.77 B.M. are a
further indication for the tetrahedral geometry [17].
The lowering in leff (1.46 B.M.) of the trinuclear copper
complex, [Cu3(H GTBZIT)(GTBZIT)Cl3(H2O)3] may be
attributed to the covalent nature of the copper–sulphur
bond. The electronic spectra show bands at 12,658,
20,000, 25,974 and 27,777 cmꢀ1 regions. These band posi-
tions are in agreement with those generally observed for
square-planar copper(II) complexes [18,19].
4.3. Potentiometric studies
4.3.1. Determination of the proton-ligand ionization
constants
The ionization constants of 1-acetyltrimethyl ammoni-
um chloride-4-benzoyl thiosemicarbazide (H2GTBzIT)
was determined potentiometrically using Irving-Rossotti
technique [20] at different temperatures (295, 308 and
318 K) and at constant ionic strength of 0.08 M KCl. At
constant temperature of 295 K, the effect of different ionic
strength on the proton-ligand ionization constant was stud-
ied using five different KCl concentrations. The ligand was
titrated against carbonate-free sodium hydroxide
(0.0105 M) solution.
The average number of protons associated with the
ligand ꢀnA was calculated at different pH-values using
Irving–Rossotti equation [20].
VN
ꢀnA ¼ Y ꢀ
ðV 0 ꢀ V ÞTL
Where Y is the ionizable protons in the ligand, V is the vol-
ume of the alkali at desired pH. V0 is the initial volume of

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U. El-Ayaan et al. / Journal of Molecular Structure 871 (2007) 14–23
Table 2
Infrared data (cmꢀ1) of H2GTBzIT ligand and its metal complexes
Compound
IR (cmꢀ1
)
m(CO)h
m(CO)b
mCS
m(OH)
m(N–N)
m(C–O)
m(C@N)
H2GPBzIT
[Mn(H2GTBzIT)2Cl2]
[Cr(HGTBzIT)Cl2(H2O)]Æ2H2O
[Co2(HGTBzIT)Cl2(H2O)3]ÆH2O
1689
1691
1670
–
1671
1665
–
706
703
714
711
3416
3416
–
926
923
971
945
1080 sh
1100
1115
1105
1115
1070
1110
1105
1120
1110
1120
–
1640 sh
1643
1621
1638
1601
–
–
[Zn3(HGTBzIT)(GTBzIT)Cl3(H2O)3]
[Ni(GTBzIT)(H2O)3]Æ2H2O
–
1655 sh
712
713
710
711
–
924
934
937
921
1645
–
–
–
1636
1605
1626
1601
–
[Cu3(HGTBzIT)(GTBzIT)Cl3(H2O)3]
–
–
3331
–
[Cd(H2GTBzIT)Cl2(H2O)]Æ2H2O
1711
1676
b, benzoyl moiety.
h, hydrozide moiety.
sh, shoulder.
Fig. 1. Structure of the ligand (H2GTBzIT).
Fig. 2. Structure of [Cd(H2GTBzIT)Cl2(H2O)]Æ2H2O.
Fig. 4. Structure of [Cr(HGTBzIT)Cl2(H2O)]Æ2H2O.
the titrated mixtures (25 ml in this study), TL is the initial
ligand concentration, and N is the concentration of the
alkali.
The proton-ligand formation curves are obtained by
plotting ꢀnA versus pH. Same study was applied for other
two temperatures namely, 308 and 318 K.
From these curves, the maximum ꢀnA value was found to
be ꢂ1 indicating the presence of one dissociable amide pro-
ton in the ligand.
The proton-ligand stability constant (pKa) of
H2GTBzIT was determined by interpolation at half-ꢀnA val-
Fig. 3. Structure of [Mn(H2GTBzIT)2Cl2].

U. El-Ayaan et al. / Journal of Molecular Structure 871 (2007) 14–23
19
Table 3
Magnetic Moment, Electronic bands and ligand field parameters of the complexes derived from H2GTBzIT
Compounda
Band position (cmꢀ1
)
Dq
B
b
leff (B.M.)
[Cr(HGTBzIT)Cl2(H2O)]Æ2H2O
[Mn(H2GTBzIT)2Cl2]
16,447, 22,075, 25,641, 27,933
24,134, 25,413
14,577, 17,391, 19,342, 26,246
14,663, 16,260, 19,120
1644
–
759.4
–
–
539.1
–
1021
–
–
0.59
–
0.98
–
3.89
7.30
3.40
3.77
1.46
[Ni(GTBzIT)(H2O)3]Æ2H2O
[Co2(HGTBzIT)Cl2(H2O)3]ÆH2O
[Cu3(HGTBzIT)(GTBzIT)Cl3(H2O)3]
a
12,658, 20,000, 25,974, 27,777
–
Electronic spectra were measured in 10ꢀ3 M dimethyl sulphoxide (DMSO) solution of all studied complexes.
Table 4
ues, i.e. at ꢀnA = 0.5 from the ꢀnA–pH curves. These values
Thermodynamic parameters and association constant for H2GTBzIT
(0.08 M KCl) at different temperatures
are confirmed by the method of least square by plotting
logðꢀnA=1 ꢀ ꢀnAÞ versus pH as shown in (Fig. 5).
The thermodynamic properties of solutions are not only
useful for estimating the feasibility of reaction in solution,
but they also offer one of the better methods for investigat-
ing the theoretical aspects of solution [21].
Temperature pka
(K)
DGꢁ
ꢀTDSꢁ(kJ molꢀ1
)
DHꢁ(kJ molꢀ1
)
(kJ molꢀ1
)
295
308
318
5.30
5.05
4.85
29.94
29.78
29.53
14.73
14.57
14.32
15.21
The standard free energy (DGꢁ) associated with the acid
dissociation can be calculated from the equilibrium con-
stants of the reaction, as follow:
(a) pka values decrease with increasing temperature, i.e.
the acidity of the ligand increases with increasing
temperature Table 4.
(b) The positive values of DH indicate that the dissocia-
tion processes are endothermic in nature and enhance
with the rise of temperature.
DGꢃ ¼ ꢀ2:303RT pka
While the standard enthalpy change (DHꢁ) of dissociation
can be estimated from the use of Van’t Hoff equation:
(c) The negative values of DS are owing to the increasing
of the order as a result of solvation process, which
can be explained as the sum of the bound solvent
molecules with the dissociated ligand being more
than the solvent molecules originally accompanying
the un-dissociated form.
(d) DG values for the dissociation constants are positive,
i.e. the dissociation processes are non-spontaneous.
(e) The correlations of pka values versus 1/T are linear in
all cases, suggesting that DCp values for the dissocia-
tion processes are zero over the studied temperature
range (295–318 K).
d ln K DHꢃ
dT
¼
RT2
By plotting pka versus 1/T, from the slope the enthalpy
change (DHꢁ) for the dissociation of the ligand can be
calculated.
Also, the standard entropy change (DSꢁ) of dissociation
can be calculated using the following well known equation:
DGꢃ ¼ DHꢃ ꢀ TDSꢃ
The stoichiometric thermodynamic functions are summa-
rized in Table 4. From these data the following conclusions
can be obtained:
We also studied the effect of differing the ionic strength
on the proton-ligand stability constant and the result indi-
cate that the values of pka of the ligand decrease as the ion-
ic strength increases due to inert salt effect (Table 5).
1.5
1
4.3.2. Determination of stability constants of H2GTBzIT
complexes
First-row transition metal complexes with H2GTBzIT
ligand have been studied potentiometrically. These include,
Cu(II), Co(II), Ni(II), Zn(II), Fe(III) in addition to Cd(II).
The titrations were performed at different ionic strength
0.5
0
4
4.5
5
5.5
6
6.5
7
7.5
-0.5
-1
Table 5
-1.5
-2
The association constants of H2GTBzIT
R2 = 0.992
Method
pka (M)
0.04
0.08
0.12
0.16
0.20
pH
Mid-point (half-method)
Least square method
5.0
5.05
5.2
5.3
5.4
5.4
5.4
5.4
5.4
5.4
Fig. 5. Relation between logðꢀnA=1 ꢀ ꢀnAÞ versus pH for proton–
H2GTBzIT, 0.20 M KCl and at 295 K.

20
U. El-Ayaan et al. / Journal of Molecular Structure 871 (2007) 14–23
Co2þ < Ni2þ < Cu2þ > Zn2þ
values ranging from (0.04–0.20 M KCl) and at temperature
of 295 K.
In agreement with the will known sequence of stability con-
stants of Irving and Williams [23], clearly this order reflects
the changes in heats of complexation along the series and it
arises from the influence of the polarization ability of the
metal ions, as measured by the ratio of charge to ionic
radius.
Moreover, Irving and Williams series is a consequence
of crystal-field stabilization energy which varies in the
order of Co2+ < Ni2+ < Cu2+ > Zn2+, i.e. the splitting
caused by the ligand in the energy level of the d-electrons
diminishes the total energy of the system, thus causes in
stabilization.
An inspection of the titration curves for H2GTBzIT
reveals that the metal-ligand titration curves are below
and well separated from the ligand titration curves. This
indicates that the complexation processes were occurred
with liberation of hydrogen ions.
The stability constant are evaluated from the formation
curves drawn between n and pL, where n is the average
number of ligand attached per metal ion and pL is the free
ligand exponent, n values can be calculated from the fol-
lowing equation:
T L ꢀ ½Lꢁ
ꢀn ¼
TM
Also the results show that the trivalent metal ions are
more stable than that of divalent ones, as a result of
increasing the oxidation state.
where TL and TM are the initial concentrations of the li-
gands and the metal, respectively, and [L] is the ligand con-
centration at equilibrium. Therefore, the difference between
TL and [L] is the concentration of the bound ligand to the
metal ion. The above equation can rewrite as follow:
4.4. Thermal analysis
ðV 2 ꢀ V 1ÞNo
In studying the decomposition kinetics, many methods
mentioned in literature were used [24–27]. In each case
the least square plots were drawn. The first few points that
did not fall on the straight line were discarded. These types
of deviations were reported by several researches [28]. This
is explained as due to the failure by obeying as first order
kinetics always by the solids in their decomposition in the
early stages. The TG/DTG curves (Fig. 6) of cadmium
complex, [Cd(H2GTBzIT)Cl2(H2O)]Æ2H2O are presented
as a representative example.
The kinetic parameters evaluated by Coats–Redfern
(CR) and Horowitz–Metzger (HM) methods are listed in
Tables 7 and 8, respectively. In both methods, (CR and
HM) the left hand side of equations 6 and 9 are plotted
against 1000/T and h, respectively (Fig. 7). From the results
obtained, the following remarks can be pointed out: (i) The
energy of activation (E) values increases on going from one
decomposition stage to another for a given complex, indi-
n ¼
ðV 0 þ V 1ÞnATM
where V2 is the volume of alkali required to reach the de-
sired pH in the mixture of complex solution, TM is the ini-
tial metal ion concentration.
The pL value can be calculated by the following
equation:
P
j¼j
bjHj
ðV 2 þ V 0Þ
j¼0
pL ¼
ꢄ
T L ꢀ ꢀn
V 0
and for monobasic acid the following simplified form of
equation can successfully apply [22],
pL ¼ pka ꢀ pH ꢀ logf½HAꢁinit ꢀ ½OHꢀꢁg
The stoichiometric stability constants are evaluated using
the half-method in which the following equation was
applied:
Log K ¼ logð1=½LꢁÞn¼0:5
ðThe pK values are evaluated at n ¼ 0:5Þ
The experimental stability constant values (Table 6) for
complexation of the investigated ligands were found fol-
lows the order:
Table 6
The stability constants of metal ions–H2GTBzIT complexes at different
ionic strength of KCl and at 295 K
Complex
pK (M)
0.04
0.08
0.12
0.16
0.20
Cu(II)–H2GTBzIT
Co(II)–H2GTBzIT
Ni(II)–H2GTBzIT
Cd(II)–H2GTBzIT
Zn(II)–H2GTBzIT
Fe(III)–H2GTBzIT
4.84
3.6
4.14
4.14
2.90
4.98
4.98
–
4.42
4.42
2.40
4.98
4.70
3.58
4.00
4.00
2.32
4.98
4.70
3.30
4.00
4.00
2.30
4.98
4.70
3.30
4.00
4.00
2.30
4.84
Fig. 6. TG and DTG of [Cd(H2GTBzIT)Cl2(H2O)]Æ2H2O.

U. El-Ayaan et al. / Journal of Molecular Structure 871 (2007) 14–23
21
Table 7
Kinetic parameters evaluated by Coats–Redfern equation
Compound
Stage
T (K)
A (Sꢀ1
)
E (kJ molꢀ1
)
DH (kJ molꢀ1
)
DS (kJ molꢀ1 Kꢀ1
)
DG (kJ molꢀ1
)
[Cd(H2GTBzIT)Cl2(H2O)]Æ2H2O
1st
2nd
3rd
478
555
837
5.52 · 105
11.9
72.8
37.4
230.3
68.8
32.8
223.3
ꢀ0.139
ꢀ0.229
ꢀ0.020
ꢀ0.049
ꢀ0.086
ꢀ0.094
135
160
240
1.66 · 1012
[Co2(HGTBzIT)Cl2(H2O)3]ÆH2O
1st
2nd
3rd
614
709
878
33.6 · 109
47.4 · 107
22.2 · 107
147.5
159.8
178.4
142.4
153.9
171.0
173
215
254
[Cr(HGTBzIT)Cl2(H2O)]Æ2H2O
1st
2nd
583
658
17.9 · 104
83.9
217.8
86.1
212.4
ꢀ0.150
166
185
21.1 · 1014
0.042
[Mn(H2GTBzIT)2Cl2]
1st
2nd
3rd
541
659
866
27.7 · 102
82.1 · 105
17.1 · 104
11.6 · 107
85.0 · 109
32.3 · 1010
29.4 · 105
6.66
32.3 · 103
59.9
116.4
127.8
55.4
110.9
120.6
ꢀ0.184
ꢀ0.119
ꢀ0.154
ꢀ0.096
ꢀ0.042
ꢀ0.032
ꢀ0.127
ꢀ0.237
ꢀ0.168
155
189
254
[Ni(GTBzIT)(H2O)3]Æ2H2O
1st
2nd
3rd
545
655
772
106.4
162.7
202.0
101.9
157.3
196.0
154
185
221
[Zn3(HGTBzIT)(GTBzIT)Cl3(H2O)3]
1st
2nd
3rd
578
733
921
95.8
50.0
123.9
91.0
43.9
116.2
164
217
271
Table 8
Kinetic parameters evaluated by Horowitz–Metzger equation
Compound
Stage
T (K)
A (Sꢀ1
)
E (kJ molꢀ1
)
DH (kJ molꢀ1
)
DS (kJ molꢀ1 Kꢀ1
)
DG (kJ molꢀ1
)
[Cd(H2GTBzIT)Cl2(H2O)]Æ2H2O
1st
2nd
3rd
478
555
837
5.40 · 105
12.2
87.3
34.8
228.0
83.4
30.1
221.1
ꢀ0.139
ꢀ0.229
ꢀ0.019
ꢀ0.049
ꢀ0.086
ꢀ0.095
150
157
237
1.71 · 1012
[Co2(HGTBzIT)Cl2(H2O)3]ÆH2O
1st
2nd
3rd
614
709
878
34.0 · 109
50.1 · 107
19.2 · 107
144.4
137.6
191.6
139.3
131.7
184.2
173
192
268
[Cr(HGTBzIT)Cl2(H2O)]Æ2H2O
1st
2nd
583
658
16.2 · 104
91.0
208.1
79.0
202.6
ꢀ0.150
174
175
22.3 · 1014
0.042
[Mn(H2GTBzIT)2Cl2]
1st
2nd
3rd
541
659
866
26.3 · 102
80.4 · 105
15.6 · 104
10.9 · 107
84.8 · 109
30.3 · 1010
27.7 · 105
6.33
35.0 · 103
72.8
124.5
143.4
68.3
119.0
136.1
ꢀ0.184
ꢀ0.119
ꢀ0.154
ꢀ0.096
ꢀ0.042
ꢀ0.032
ꢀ0.127
ꢀ0.237
ꢀ0.167
168
197
269
[Ni(GTBzIT)(H2O)3]Æ2H2O
1st
2nd
3rd
545
655
772
113.7
164.3
228.2
109.2
158.8
221.8
162
186
247
[Zn3(HGTBzIT)(GTBzIT)Cl3(H2O)3]
1st
2nd
3rd
578
733
921
108.5
51.4
81.2
103.7
45.3
73.6
177
219
228
cating that the rate of decomposition decreases in the same
order. Generally stepwise stability constants decreases with
an increase in the number of ligand attached to a metal ion.
Conversely, during decomposition reaction the rate of
removal of remaining ligands will be smaller after the
expulsion of one or two ligands. (ii) The values of DG
increases significantly for the subsequently decomposition
stages due to increasing the values of (TDS) from one stage
to another. 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 neg-
ative DS values for the decomposition steps indicate that all
studied complexes are more ordered in their activated
states. (iv) the positive DH values mean that the decompo-
sition processes are endothermic.
Conclusion
In solid state, the stability of complexes was examined
and the kinetic parameters were evaluated using the
Coats–Redfern method (a typical integral method) and
the Horowitz–Metzger (an approximation method). The

22
U. El-Ayaan et al. / Journal of Molecular Structure 871 (2007) 14–23
-11
-11.5
-12
0
1st stage, CR
1st stage, HM
-0.1
-0.2
-0.3
)]
]
2
γ
α
-0.4
-0.5
-0.6
-0.7
-0.8
-0.9
-1
/w
)/T
α
-12.5
-13
w
-
1
(
o(g
l
ln
-
[
o[g
ln
l
-13.5
R2 = 0.984
15
R2 = 0.988
2.1
-14
1.95
-20
-15
-10
-5
0
5
10
20
2
2.05
2.15
2.2
1000/T (K-1)
θ (Κ)
0.1
0.05
0
-11
-11.2
-11.4
-11.6
-11.8
-12
2nd Stage, HM
2nd Stage, CR
]
2
)]
γ
α
)/T
α
/w
-
-0.05
-0.1
1
(
w
ln
-
[
-12.2
-12.4
-12.6
-12.8
o(g
ln
l
g
o
l
R2 = 0.9922
1.72 1.74
-0.15
-0.2
R2 = 0.9938
60
1.58
1.6
1.62
1.64
1.66
1.68
1.7
1.76
1.78
1000/T (K-1)
20
30
40
50
70
θ (Κ)
0.4
0.2
0
-8
-10
-12
-14
-16
-18
-20
3rd stage, HM
3rd stage, CR
-0.2
)]
γ
]
T
2
/
-0.4
α
)/
w
(
α
-
g
-0.6
-0.8
-1
1
(
l
[
ln
g
-
[
lo
ln
R2 = 0.999
40
-1.2
-1.4
R2 = 0.9948
1.28 1.3
-60
-40
-20
0
20
60
θ (Κ)
1.12
1.14
1.16
1.18
1.2
1.22
1.24
1.26
1000/T (K-1)
Fig. 7. Coats–Redfern (CR) and Horowitz–Metzger (HM) plots for [Cd(H2GTBzIT)Cl2(H2O)]Æ2H2O.
[4] I.-I. Radu, D. Poirier, L. Provencher, Tetrahedron Lett. 43 (2002)
7617.
result shows that the kinetic parameters (E, DH, DS and
DG) evaluated by both methods are in a very good
agreement.
In solution, the stability constant of complexes was
found follows the order: Co2+ < Ni2+ < Cu2+ > Zn2+, in
agreement with the will known sequence of stability con-
stants of Irving and Williams.
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