Synthesis and thermal studies on iron(III) complexes of 4-N-(4′-antipyrylmethylidene)aminoantipyrine with varying counter ions

Article abstract of DOI:10.1016/S0040-6031(03)00309-5

Iron(III) complexes of the Schiff base 4-N-(4′ -antipyrylmethylidene)aminoantipyrine (AA) with counter ions, such as, perchlorate, nitrate, thiocyanate, chloride and bromide have been prepared and characterized by elemental analyses, electrical conductance in non-aqueous solvents, IR and electronic spectra, magnetic susceptibility measurements as well as by thermogravimetric analysis. The complexes have the general formulae [Fe(AA)₂(CO₄)](ClO₄)₂, [Fe(AA)2X2]X (X = NO₃^- or Br^-) and [Fe(AA)X₃] (X = SCN^- or Cl^-). In complexes, AA acts as a neutral bidentate ligand, coordinating through one of the carbonyl oxygens and azomethine nitrogen in perchlorate, nitrate and bromide complexes while the coordination of AA occurs in a neutral tridentate fashion through both carbonyl oxygens and azomethine nitrogen in thiocyanate and chloride complexes. In the perchlorate complex, one of the perchlorate ions is coordinated bidentately while in the nitrate complex two of the nitrate ions are coordinated monodentately to the metal ion. In the thiocyanate and chloride complexes all the anions are coordinated while in the bromide complex two of the bromide ions are coordinated. A high spin octahedral geometry is assigned to the iron(III) ion in all these complexes. The phenomenological, kinetic and mechanistic aspects of the nitrate, thiocyanate, chloride and bromide complexes were studied by TG and DTG techniques. The kinetic parameters like activation energy, pre-exponential factor and entropy of activation were also computed. The rate controlling process in all stages of decomposition is random nucleation with one on each particle (Mampel model) [Thermochim. Acta 2 (1971) 423].

Full text of DOI:10.1016/S0040-6031(03)00309-5

Thermochimica Acta 407 (2003) 73–84
Synthesis and thermal studies on iron(III) complexes of
4-N-(4ꢀ-antipyrylmethylidene)aminoantipyrine
with varying counter ions
N.T. Madhua, P.K. Radhakrishnanb, Matthias Grunerta,
Peter Weinbergera, Wolfgang Linerta,∗
Institute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt 9/163-AC, A-1060 Vienna, Austria
a
b
School of Chemical Sciences, Mahatma Gandhi University, Kottayam 686560, Kerala, India
Received 29 September 2002; received in revised form 25 March 2003; accepted 16 May 2003
Abstract
Iron(III) complexes of the Schiff base 4-N-(4ꢀ-antipyrylmethylidene)aminoantipyrine (AA) with counter ions, such as,
perchlorate, nitrate, thiocyanate, chloride and bromide have been prepared and characterized by elemental analyses, electrical
conductance in non-aqueous solvents, IR and electronic spectra, magnetic susceptibility measurements as well as by ther-
mogravimetric analysis. The complexes have the general formulae [Fe(AA)2(ClO4)](ClO4)2, [Fe(AA)2X2]X (X = NO3− or
Br−) and [Fe(AA)X3] (X = SCN− or Cl−). In complexes, AA acts as a neutral bidentate ligand, coordinating through one of
the carbonyl oxygens and azomethine nitrogen in perchlorate, nitrate and bromide complexes while the coordination of AA
occurs in a neutral tridentate fashion through both carbonyl oxygens and azomethine nitrogen in thiocyanate and chloride
complexes. In the perchlorate complex, one of the perchlorate ions is coordinated bidentately while in the nitrate complex two
of the nitrate ions are coordinated monodentately to the metal ion. In the thiocyanate and chloride complexes all the anions
are coordinated while in the bromide complex two of the bromide ions are coordinated. A high spin octahedral geometry is
assigned to the iron(III) ion in all these complexes. The phenomenological, kinetic and mechanistic aspects of the nitrate,
thiocyanate, chloride and bromide complexes were studied by TG and DTG techniques. The kinetic parameters like activation
energy, pre-exponential factor and entropy of activation were also computed. The rate controlling process in all stages of
decomposition is random nucleation with one on each particle (Mampel model) [Thermochim. Acta 2 (1971) 423].
© 2003 Elsevier B.V. All rights reserved.
Keywords: Iron(III) complexes; Counter ion; 4-N-(4ꢀ-Antipyrylmethylidene)aminoantipyrine; Fe(III) high spin complexes
1. Introduction
derivatives with 3d-metal ions [3] and their thermal
studies. In view of this, and as part of our continu-
ing interest on structural [1,4–6] and thermal [7,8]
aspects of antipyrine derivatives, we now report the
synthesis and thermal studies of a new series of
iron(III) complexes of a Schiff base antipyrine lig-
and (Fig. 1) with a variety of counter ions such as,
perchlorate, nitrate, thiocyanate, chloride and bro-
mide.
Antipyrine and its derivatives possess antibac-
terial [1] and antitumor [2] activities. Compara-
tively little is known on complexes of antipyrine
∗
Corresponding author. Tel.: +43-58801-15350;
fax: +43-58801-16299.
E-mail address: wlinert@mail.zserv.tuwien.ac.at (W. Linert).
0040-6031/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0040-6031(03)00309-5

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N.T. Madhu et al. / Thermochimica Acta 407 (2003) 73–84
Fig. 1. 4-N-(4ꢀ-Antipyrylmethylidene)aminoantipyrine.
2. Experimental
For the nitrate and bromide complexes, the brown
solution so obtained after refluxing was cooled and
filtered. On partial evaporation of the solvent and sub-
sequent cooling, the complex was separated which was
repeatedly washed with hot benzene to obtain dark
brown solid of pure complexes. All the complexes
were then dried under vacuum over P4O10. (Warn-
ing: perchlorate salts with organic compounds are
potentially explosive. They should be handled in
small quantities and with caution.)
2.1. Materials
The metal salts were prepared from Analar ferric
carbonate and the respective 50% acids and crystalliz-
ing out the salts by evaporating the solution on a steam
bath. Ferric thiocyanate was prepared by the method
reported earlier [9].
2.2. Preparation of the ligand
2.4. Physical methods of analysis
The ligand, 4-N-(4ꢀ-antipyrylmethylidene)amino-
antipyrine (AA) was prepared and characterized as
previously described [4].
All the complexes were analyzed for metal and
halide content by the standard methods [10] and the
perchlorate content by the Kurz method [11]. The
electrical conductance of the complexes in acetoni-
trile, dimethylformamide, methanol and nitrobenzene
(10−3 M solutions) were measured at room temper-
ature using a Toshniwal conductivity bridge with a
dip type conductance cell (cell constant 0.9737, mea-
surement frequency 1591.5 s−1). The infrared spectra
of the ligands and the complexes were recorded in
the range of 4000–400 cm−1 on a Shimadzu IR 470
spectrophotometer and in the range of 400–200 cm−1
on a Perkin Elmer 983 IR spectrophotometer. Elec-
tronic spectra of the ligands and the complexes in
solid state (a paste with nujol) were measured in
the range of 200–1100 nm on a Shimadzu UV 160
A spectrophotometer. The elemental analyses (C, H
and N) were carried out on a Heraeus CHNO rapid
2.3. Preparation of complexes
The metal salt (1 mmol), Fe(ClO4)3·6H2O,
Fe(NO3)3·6H2O, Fe(SCN)3·6H2O, FeCl3·6H2O or
FeBr3·6H2O in methanol (10 ml), was added to a boil-
ing solution of 2.2 mmol of AA in acetone (100 ml)
for the perchlorate, nitrate and bromide complexes,
while 1.2 mmol of AA in methanol (100 ml) for the
thiocyanate and chloride complexes. The resulting
solution was boiled under reflux for about 2 h on
a water bath. For the perchlorate, thiocyanate and
chloride complexes, the solid product obtained on
refluxing was filtered and washed several times with
hot chloroform to remove the excess ligand. It was
then recrystallized from acetonitrile.

N.T. Madhu et al. / Thermochimica Acta 407 (2003) 73–84
75
analyzer. The magnetic moments were measured at
room temperature by the Gouy method using mer-
cury(II)tetrathiocyanatocobaltate(II) as the calibrant.
Thermogravimetric analyses were undertaken on a
Delta Series TGA 7 thermal analyzer in a nitro-
gen atmosphere (sample mass 10 mg, heating rate at
10 ◦C/min).
plexes, indicating the coordination of the azomethine
nitrogen [4].
In the perchlorate complex, the triply split band
maxima at 1140, 1115 and 1025 cm−1 are due to
the ν8, ν6, and ν1 vibrations, respectively, of biden-
tate coordination thereof (C2v) [13,14]. The band at
1085 cm−1 is assigned to the ν3 vibration of the unco-
ordinated perchlorate [13] ion (Td). In addition to this,
the vibrational frequencies at 940 and 638 cm−1 cor-
responding to the ν2 and ν3 vibrations, respectively,
of the perchlorate (C2v) ion and the band at 624 cm−1
due to the ν4 vibration of the perchlorate (Td) ion also
support the co-existence of both bidentate coordinated
and uncoordinated perchlorate ions in this complex
[13,14].
3. Results and discussion
The elemental analyses data (Table 1) suggest that
the complexes can be formulated: Fe(AA)2X3 (X =
ClO4−, NO3− or Br−) and Fe(AA)X3 (X = SCN− or
Cl−).
For the nitrate complex, the presence of coordinated
nitrate ions are indicated by two medium intensity
bands at 1420 and 1315 cm−1 due to the ν4 and ν1
vibrations, respectively, of the nitrate ion of C2v sym-
metry [15]. Since (ν4 − ν1) = 105 cm−1, the nitrate
ions are coordinated in a unidentate fashion [15]. Be-
sides a very strong band at 1380 cm−1 and a medium
intensity band at 825 cm−1 are due to the ν3 and ν2
vibrations, respectively, of the uncoordinated nitrate
ion (D3h) [16].
In the thiocyanate complex, a sharp band at
2050 cm−1 is indicative of N coordinated thiocyanate
ion [16]. Furthermore, the presence of bands at 850
and 480 cm−1 stands as additional evidence for N
coordinated thiocyanate ion [16].
3.1. Electrical conductance
The molar conductance (Table 1) of the perchlo-
rate complex in acetonitrile, dimethylformamide,
methanol and nitrobenzene fall in the range expected
for 1:2 electrolytes, while for the nitrate and bromide
complexes a 1:1 conducting nature [12] is observed.
The chloride and thiocyanate complexes behave as
non-electrolytes [12]. Thus, these may be formu-
lated as [Fe(AA)2(ClO4)](ClO4)2, [Fe(AA)2X2]X
−
−
(X = NO3 or Br−) and [Fe(AA)X3] (X = SCN or
Cl−).
3.2. IR spectra
In the far IR spectra halide complexes, the Fe–Cl
and Fe–Br stretching vibrations occurs [16] at 312 and
282 cm−1, respectively, which are absent in the ligand
spectrum.
These spectral observations along with the con-
ductance data indicate that one of the perchlorates,
two of the nitrates and bromides, all the thiocyanates
and chlorides are coordinated to the metal ion
in these complexes. In addition, the ν(Fe–O) and
ν(Fe–N) stretching vibrations are at about 550 and
440–450 cm−1, respectively, in complexes [4,5].
The important spectral data of AA and its com-
plexes are presented in Table 2. AA exhibits two
strong bands at 1643 and 1659 cm−1, corresponding to
2
the C1 O and C O stretching vibrations [4], respec-
=
=
tively (Fig. 2). The band ν(C1 O) of AA is shifted to
=
the 1620–1624 cm−1 region in the perchlorate, nitrate
and bromide complexes indicating coordination of
carbonyl oxygen [4] of C1 O. However, the band cor-
=
responding to ν(C2 O) remains practically unaltered
=
at about 1656 cm−1 indicating the non-coordination
of carbonyl oxygen [4] of C2 O. In the thiocyanate
=
and chloride complexes, the ν(C1 O) and ν(C O)
bands are shifted to 1618 and 1628 cm−1, respec-
tively, indicating the coordination of both the carbonyl
oxygens [6]. An intense band observed at 1596 cm−1
2
=
=
3.3. Electronic spectra
The spectra of AA and its complexes with tenta-
tive assignments are presented in Table 3. AA shows
two band maxima at∗28,570 and 40,485 cm−1 corre-
=
attributable to the C N stretching vibration [4] in
AA is shifted to about 1606 cm−1 in all these com-
sponding to n → ␲ and ␲ → ␲ transitions [4],
∗

Table 1
Analytical and conductance data of the obtained complexes
Complex
Formula Weight of
weight the complex (%)
formed (g)
[Fe(AA)2(ClO4)](ClO4)2 1157.01 0.4223
Yield Found (calculated) (%)
Molar conductance (ꢀ−1 cm2 mol−1
)
Iron
Xa
C
H
N
PhNO2 MeCN DMF MeOH
73.00 4.9 (4.8) 25.7 (25.7) 47.7 (47.7) 3.9 (4.0) 12.1 (12.1) 51.6
262.9
160.6
77.9
73.6
130.9
146.0 178.6
[Fe(AA)2(NO3)2]NO3
[Fe(AA)(SCN)3]
[Fe(AA)Cl3]
1044.59 0.3663
631.48 0.4305
563.60 0.4007
1098.36 0.4389
70.15 5.4 (5.3)
68.18 8.9 (8.8)
–
–
52.7 (52.9) 4.3 (4.4) 17.4 (17.4) 19.2
89.6
63.5
40.1
75.1
94.5
51.2
47.6
96.6
49.3 (49.5) 3.5 (3.6) 17.5 (17.7)
9.8
3.8
71.10 9.9 (9.9) 18.9 (18.9) 49.0 (49.0) 4.1 (4.1) 12.4 (12.4)
79.92 5.1 (5.1) 21.9 (21.8) 50.3 (50.3) 4.2 (4.2) 12.8 (12.7) 34.9
[Fe(AA)2Br2]Br
a X = ClO4−, Cl− or Br−.

N.T. Madhu et al. / Thermochimica Acta 407 (2003) 73–84
77
Table 2
Important IR spectral bands (cm−1) of the obtained complexesa
AA
[Fe(AA)2(ClO4)](ClO4)2 [Fe(AA)2(NO3)2]NO3 [Fe(AA)(SCN)3] [Fe(AA)Cl3] [Fe(AA)2Br2]Br Assignment
1643 s 1620 s
1659 s 1655 s
1596 s 1606 s
1623 s
1656 s
1606 s
1618 s
1618 s
1606 s
1628 s
1628 s
1606 s
1624 s
1656 s
1606 s
ν(C1 O)
=
ν(C2 O)
=
=
ν(C N)
−
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
1140 m
1115 s
1025 m
940 w
638 w
1085 s
624 m
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
ν8—coordinated ClO4−
ν6—coordinated ClO4−
ν1—coordinated ClO4−
ν2—coordinated ClO4−
ν3—coordinated ClO4
−
ν3—ionic ClO4−
–
1420 m
1315 m
1380 s
825 m
–
ν4—ionic ClO4
−
ν4—coordinated NO3−
–
–
–
–
–
–
–
ν1—coordinated NO3
−
–
–
ν3—ionic NO3−
ν2—ionic NO3
ν(C–N)
2050 s
850 w
480 m
–
–
–
–
–
ν(C–S)
δ(NCS)
ν(Fe–Cl)
ν(Fe–Br)
ν(Fe–O)
ν(Fe–N)
–
–
–
312 m
–
550 w
450 w
–
–
282 m
550 w
440 w
550 w
445 w
555 w
450 w
550 w
440 w
a s: strong; m: medium; w: weak.
respectively. In complexes, both n → ␲∗ and ␲ → ␲
bands are found as blue shifted and appeared in the re-
gion 28,735–29,761 and 42,016–45,871 cm−1 regions,
respectively, compared to that of AA. All the com-
plexes exhibit a band in the 19,417–21,596 cm−1 range
∗
For the perchlorate complex, there is no mass loss
up to 205 ◦C indicating that either water or solvent
molecules are absent in this complex. Above 205 ◦C,
the thermal studies could not be conducted due its
explosive nature.
6
4
corresponding to the A1g → T1g transition consis-
tent with octahedral Fe(III) complexes [17,18]. Fur-
thermore, a strong band at 32,258 cm−1 in complexes
corresponds to charge transfer transition [4].
In the case of nitrate complex (Fig. 3), the decom-
position occurs in the 157–496 ◦C range. There is no
mass loss up to 157 ◦C revealing the absence of ei-
ther water or solvent molecules in this complex. The
first stage of decomposition starts at 157 ◦C and ends
at 241 ◦C. The observed mass loss (17.14%) is due to
the decomposition of three nitrate ions and which is
in agreement with a calculated mass loss of 17.79%.
This result is supported by the absence of nitrate ion
in the intermediate compound obtained at 241 ◦C as
confirmed by IR spectral analysis. The maximum rate
of mass loss occurs at 231 ◦C as indicated by the DTG
peak. The second stage starts at 241 ◦C and ends at
420 ◦C with the DTG peak at 348 ◦C. The correspond-
ing mass loss (37.14%) is attributed to the decompo-
sition of one ligand molecule. The third stage starts at
420 ◦C and comes to an end at 496 ◦C with DTG peak
at 473 ◦C and the corresponding mass loss (37.91%)
is due to the decomposition of the remaining ligand
3.4. Magnetic moment data
The molar magnetic moments of the complexes are
presented in Table 3. The values are in the 5.84–6.00
BM range, suggesting a high spin octahedral geometry
around Fe(III) ion [19,20].
Based on the results of this study, the structures
shown in Fig. 2 may be assigned to the complexes.
3.5. Phenomenological aspects
The TG and DTG curves of the complexes are given
in Figs. 3–6 and the corresponding thermal analysis
data is presented in Table 4.

78
N.T. Madhu et al. / Thermochimica Acta 407 (2003) 73–84
molecule. The mass losses in the second and third
stages are in agreement with a calculated mass loss of
38.42%. The decomposition completes at 496 ◦C and
the final residue is qualitatively proved to be anhy-
drous metal oxide.
For the thiocyanate complex (Fig. 4), there is no
mass loss up to 218 ◦C indicating that either water
or solvent molecules are absent in this complex. It
undergoes a two-stage decomposition process. The
first stage starts at 218 ◦C and ends at 455 ◦C with
the DTG peak at 321 ◦C. The corresponding mass loss
(39.28%) is attributed to the decomposition of half of
the ligand molecule and one thiocyanate ion and which
is in agreement with a calculated mass loss of 40.97%.
The IR spectrum of the residue after this stage shows
the presence of AA and thiocyanate ion indicating only
Fig. 2. Tentative structures of Fe(III) complexes of AA with perchlorate, nitrate, thiocyanate, chloride and bromide as counter ions.

N.T. Madhu et al. / Thermochimica Acta 407 (2003) 73–84
79
Fig. 2. (Continued ).
Table 3
Electronic spectral data and magnetic moments of the obtained complexes
Complex
AA
Band. max. (cm−1
)
Tentative assignments
µeff (BM)
∗
28570
40485
n → ␲
–
∗
␲ → ␲
∗
[Fe(AA)2(ClO4)](ClO4)2
[Fe(AA)2(NO3)2]NO3
[Fe(AA)(SCN)3]
28735
42372
32258
19417
n → ␲
5.85
5.92
5.94
5.95
6.00
∗
␲ → ␲
Charge transfer
6A1g
→
4T1g
∗
29761
43478
32258
19723
n → ␲
␲ → ␲
∗
Charge transfer
6A1g
→
4T1g
∗
28901
42735
32258
21276
n → ␲
␲ → ␲
∗
Charge transfer
6A1g
→
4T1g
∗
[Fe(AA)Cl3]
28985
42016
32258
20618
n → ␲
␲ → ␲
∗
Charge transfer
6A1g
→
4T1g
∗
[Fe(AA)2Br2]Br
28818
45871
32258
21596
n → ␲
␲ → ␲
∗
Charge transfer
6A1g
→
4T1g

80
N.T. Madhu et al. / Thermochimica Acta 407 (2003) 73–84
100
75
50
25
90
190
290
390
490
Temperature˚C
Fig. 3. TG and DTG curves of [Fe(AA)2(NO3)2]NO3 (nitrogen at a heating rate of 10 ◦C/min; —: TG; - - -: DTG).
partial removal of AA and thiocyanate at this stage.
The second stage starts at 455 ◦C and comes to an end
at 564 ◦C with the DTG peak at 529 ◦C. The corre-
sponding mass loss (50.70%) is due to the decomposi-
tion of the remaining half of the ligand molecule and
two thiocyanate ions and which is in agreement with a
calculated mass loss of 50.17%. The decomposition of
the complex completes at 564 ◦C and the final residue
is qualitatively proved to be anhydrous metal oxide.
For the chloride complex (Fig. 5), there is no mass
loss up to 260 ◦C indicating that either water or solvent
molecules are absent in this complex. It undergoes a
100
75
50
25
110
250
390
530
670
Temperature˚C
Fig. 4. TG and DTG curves of [Fe(AA)(SCN)3] (in nitrogen at a heating rate of 10 ◦C/min; —: TG; - - -: DTG).

N.T. Madhu et al. / Thermochimica Acta 407 (2003) 73–84
81
100
75
50
25
110
250
390
530
670
Temperature˚C
Fig. 5. TG and DTG curves of [Fe(AA)Cl2] (in nitrogen at a heating rate of 10 ◦C/min; —: TG; - - -: DTG).
two-stage decomposition process. The first stage starts
at 260 ◦C and ends at 573 ◦C with the DTG peak at
398 ◦C. The corresponding mass loss (34.49%) is at-
tributed to the decomposition of half of the molecule
of AA and which is in agreement with a calculated
mass loss of 35.61%. The IR spectrum of the residue
after this stage shows the presence of AA indicating
only partial removal of the ligand at this stage. The
second stage of decomposition, which starts at 573 ◦C,
is a continuous one. The steady mass loss observed
100
75
50
25
110
250
390
530
670
Temperature˚C
Fig. 6. TG and DTG curves of [Fe(AA)2Br2]Br (in nitrogen at a heating rate of 10 ◦C/min; —: TG; - - -: DTG).

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N.T. Madhu et al. / Thermochimica Acta 407 (2003) 73–84
Table 4
Phenomenological data for the thermal decomposition of the obtained complexes
Complex
Stage of decomposition
TG plateau (◦C)
DTG peak (◦C)
Found (calculated) (%)
[Fe(AA)2(NO3)2]NO3
I
II
III
157–241
241–420
420–496
231
348
473
17.14 (17.79)
37.14 (38.42)
37.91 (38.42)
[Fe(AA)(SCN)3]
[Fe(AA)Cl3]
I
II
218–455
455–564
321
529
39.28 (40.97)
50.70 (50.17)
I
II
260–573
573
398
–
34.49 (35.61)
Continuous
[Fe(AA)2Br2]Br
I
II
171–426
426
335
–
36.54 (36.54)
Continuous
in this stage may be due to the expulsion of the re-
maining part of the ligand molecule together with
the volatilization of the residue of anhydrous metal
chloride [21].
In the present investigation log[g(α)/T2] plotted
against 103/T gives straight lines whose slope and
intercept are used to evaluate the kinetic parameters
by the least squares method. The goodness of fit is
checked by calculating the correlation coefficient.
The entropy of activation (ꢁS) is also calculated
for each stage of thermal decomposition in these com-
plexes using the relationship:
The bromide complex (Fig. 6), undergoes a
two-stage decomposition process and there is no mass
loss up to 171 ◦C revealing the absence of either water
or solvent molecules in this complex. The first stage
of decomposition starts at 171 ◦C and ends at 426 ◦C
with the DTG peak at 335 ◦C. The corresponding
mass loss (36.54%) is due to the decomposition of
one molecule of AA and which in agreement with a
calculated mass loss of 36.54%. The second stage,
which begins at 426 ◦C, is a continuous one. The
steady mass loss observed in this stage may be due
to the expulsion of the remaining ligand molecule
together with the volatilization of the residue of an-
hydrous metal bromide [21].
kTs
A =
eꢁS/R
h
where A is the pre-exponential factor, k the Boltzmann
constant, Ts the peak temperature, ꢁS the entropy of
activation and R the gas constant. All the calculations
were done with the help of a computer program.
The values of various kinetic parameters calculated
are given in Table 5. The activation energies (E) in the
different stages of thermal decomposition of iron(III)
complexes are in the range of 28.25–249.95 kJ mol−1
.
The corresponding values of pre-exponential factor (A)
are in the range of 2.40 × 10−2–1.16 × 105 s−1. The
respective values of the entropy of activation (ꢁS) are
in the range of −281.81 to −153.02 J mol−1 range.
Neither the values of A nor the values ꢁS show a def-
inite trend along the series, but the energy of activa-
tion for the second-stage decomposition of nitrate and
thiocyanate complexes is higher than that of the first
stage. This may be due to the less steric strain in the
intermediate compound obtained after the first stage.
On comparing the activation energies of the second
stage of decomposition of the nitrate and thiocyanate
complex, the latter show remarkably higher value. This
may also be due to the less steric strain for the interme-
diate compound formed for the thiocyanate complex
3.6. Kinetic aspects
All the well characterized decomposition stages of
the nitrate, thiocyanate, chloride and bromide com-
plexes were selected for the study of the kinetics of
decomposition. The kinetic parameters like the acti-
vation energy (E) and the pre-exponential factor (A)
were calculated for all the well defined stages of de-
composition using Coats–Redfern equation [22]:
ꢀ
ꢁ
ꢀ
ꢁ
g(α)
T2
AR
2RT
E
log
= log
1 −
−
φE
E
2.303RT
where T is the temperature, A the pre-exponential fac-
tor, R the gas constant, ϕ the heating rate and E the
activation energy.

N.T. Madhu et al. / Thermochimica Acta 407 (2003) 73–84
83
Table 5
Kinetic parameters for the thermal decomposition of the obtained complexes
Complex
Stage
E (kJ mol−1
)
A (s−1
)
ꢁS (J K mol−1
)
[Fe(AA)2(NO3)2]NO3
I
II
III
47.65
48.67
225.48
0.59
0.17
−253.61
−265.35
−155.53
1.16 × 105
[Fe(AA)(SCN)3]
I
II
36.74
249.95
5.56 × 10−2
−274.65
1.69 × 105
−153.02
[Fe(AA)Cl3]
I
I
50.82
28.25
0.15
−267.19
−281.81
[Fe(AA)2Br2]Br
2.40 × 10−2
Table 6
Correlation coefficients calculated using the nine forms of g(α) for the obtained complexes
No. Form of g(α)
Correlation coefficient (r)
Nitrate complex
Thiocyanate complex
Stage III Stage I Stage II
Chloride complex, Bromide complex,
stage I
stage I
Stage I
Stage II
1
2
3
4
5
6
7
8
9
α2
−0.9875 −0.9939 −0.9783 −0.9883 −0.9883
−0.9936 −0.9986 −0.9887 −0.9947 −0.9940
−0.9764 −0.9942 −0.9739 −0.9657 −0.9862
−0.9464
−0.9684
−0.8715
−0.9776
−0.9992
−0.9980
−0.9938
−0.9748
−0.9868
−0.9893
−0.9949
−0.9606
−0.9966
−0.9994
−0.9982
−0.9911
−0.9953
−0.9976
α + (1 − α) ln(1 − α)
[1 − (1 − α)1/3
]
2
[1 − (2/3)α] − (1 − α)2/3 −0.9956 −0.9990 −0.9926 −0.9966 −0.9958
−ln(1 − α)
−0.9996 −0.9999 −0.9992 −0.9993 −0.9997
−0.9993 −0.9998 −0.9991 −0.9987 −0.9996
−0.9991 −0.9988 −0.9989 −0.9961 −0.9995
−0.9955 −0.9989 −0.9939 −0.9959 −0.9963
−0.9976 −0.9994 −0.9973 −0.9981 −0.9981
[−ln(1 − α)]1/2
[−ln(1 − α)]1/3
1 − (1 − α)1/2
1 − (1 − α)1/3
compared to that of nitrate complex. But, the negative
values of entropy of activation indicate that the acti-
vated complex has a more ordered structure than the
reactants [7,8].
stages. In the present investigation, the highest value
of correlation coefficient is obtained for
g(α) = −ln(1 − α)
in all stages of decomposition of the present com-
plexes. Hence the mechanism of decomposition is the
random nucleation with one nucleus on each particle.
This represents the ‘Mampel model’ [7,8,23].
3.7. Mechanistic aspects
The assignment of the mechanism of thermal
decomposition is based on the assumption that the
form of g(α) depends on the reaction mechanism. In
the present investigation, nine forms of g(α), sug-
gested by Satava [23], are used to enunciate the
mechanism of thermal decomposition in each stage.
The correlation coefficient for all these nine forms
were calculated and the form of g(α) for which the
correlation has a maximum value is chosen as the
mechanism of reaction (Table 6). We have calculated
the kinetic parameters of the thermal decomposition
of iron(III) nitrate, thiocyanate, chloride and bro-
mide of AA using all the well defined decomposition
4. Conclusion
Iron(III) complexes of the Schiff base ligand AA,
have been synthesized and characterized by various
physicochemical studies and an octahedral geometry
is assigned to all the complexes.
The TG results show that though the stoichiometry
of the nitrate and bromide complexes is the same,
the bromide complex is more stable than the nitrate
complex. The nitrate complex undergoes a three-stage

84
N.T. Madhu et al. / Thermochimica Acta 407 (2003) 73–84
decomposition while the bromide complex decom-
poses in two stages. The thiocyanate and chloride
complex, the molecular compositions of which are
different from that of nitrate and bromide complexes
follow a two-stage decomposition pattern and undergo
decomposition at higher temperatures compared to
that of nitrate and bromide complexes. The chloride
complex decomposes at higher temperature than the
thiocyanate complex. The chloride complex is the
most stable and the nitrate complex is the least stable
among the series.
There is no definite trend observed either in the
value of A or in the value of ꢁS in the different stages
of decomposition among the series. But the activation
energy can be correlated to the steric strain occurred
for the intermediate compound.
[2] C.J. Alice, C.P. Prabhakaran, Trans. Met. Chem. 15 (1990)
449.
[3] A.M. Donia, F.A. El-Saied, Polyhedrdron 21 (1988) 2149.
[4] N.T. Madhu, P.K. Radhakrishnan, Trans. Met. Chem. 25
(2000) 287.
[5] N.T. Madhu, P.K. Radhakrishnan, Synth. React. Inorg.
Met.-Org. Chem. 30 (2000) 1561.
[6] N.T. Madhu, P.K. Radhakrishnan, Synth. React. Inorg.
Met.-Org. Chem. 31 (2001) 315.
[7] M.K.M. Nair, P.K. Radhakrishnan, J. Therm. Anal. Calorim.
52 (1998) 475.
[8] C.R. Vinodkumar, M.K.M. Nair, P.K. Radhakrishnan, J.
Therm. Anal. Calorim. 61 (2000) 143.
[9] A.N.S. Ram, S. Sarasukutty, C.P. Prabhakaran, Curr. Sci.
(India) 45 (1976) 514.
[10] A.I. Vogel, A Text Book of Quantitative Inorganic Analysis,
ELBS, London, 1961.
[11] E. Kurz, G. Kober, M. Berl, Anal. Chem. 30 (1958)
1983.
[12] W.J. Geary, Coord. Chem. Rev. 7 (1971) 81.
[13] R.P. Scholer, A.E. Merbach, Inorg. Chim. Acta 15 (1975)
15.
[14] B.J. Hathaway, A.E. Underhill, J. Chem. Soc. (1961)
3091.
The mechanism for the solid state thermal decom-
position for all the complex is found to be random
nucleation with one nucleus on each particle. This rep-
resents the—Mampel model.
[15] K. Nakamoto, Infrared and Raman Spectra of Inorganic and
Coordination Compounds, Wiley, New York, 1986.
[16] B.M. Gatehouse, S.E. Livingstone, R.S. Nyholm, J. Inorg.
Nucl. Chem. 8 (1958) 75.
Acknowledgements
[17] K.K. Narang, V.P. Singh, Synth. React. Inorg. Met.-Org.
Chem. 23 (1993) 971.
[18] A.B.P. Lever, Inorganic Electronic Spectroscopy, 2nd ed.,
Elsevier, Amsterdam, 1984.
[19] M.S. Islam, M.M. Uddin, Synth. React. Inorg. Met.-Org.
Chem. 21 (1991) 1093.
[20] F.A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry,
3rd ed., Wiley, New Delhi, 1992.
Thanks for financial support due to the “Fonds
zur Förderung der Wissenschaftlichen Forschung in
Österreich” (project 15874-N03), and the European
Science Foundation ESF within the project “Molecu-
lar magnets” are gratefully acknowledged.
References
[21] J.A. Dean (Ed.), Langes Hand Book of Chemistry, 13th ed.,
McGraw-Hill, New York, 1976.
[22] A.W. Coats, J.P. Redfern, Nature 201 (1964) 68.
[23] V. Satava, Thermochim. Acta 2 (1971) 423.
[1] M. Alaudeen, A. Abraham, P.K. Radhakrishnan, Proc. Ind.
Acad. Sci. (Chem. Sci.) 107 (1995) 57.