Preparation, characterization and thermolysis of metal nitrate complexes with 4,4'-bipyridine: Part 46

Article abstract of DOI:10.1007/s10973-005-7121-x

Two bis(bipyridine) polymeric metal nitrate complexes with 4,4'-bipyridine of simple formula like [M(bipy)₂](NO₃)2 xH₂O (where M=Co, Ni and Cu; x=4, 2 and 0, respectively) have been prepared and characterized. Their therma

Full text of DOI:10.1007/s10973-005-7121-x

Journal of Thermal Analysis and Calorimetry, Vol. 85 ( 2006) 2, 425–431
PREPARATION, CHARACTERIZATION AND THERMOLYSIS OF
METAL NITRATE COMPLEXES WITH 4,4’-BIPYRIDINE
Part 46.
1
G. Singh , C. P. Singh and R. Frohlich
*
1
2
1
Department of Chemistry, DDU Gorakhpur University, Gorakhpur 273009, India
2
Organisch-Chemisches Institut, Universit¬t Münster, 48149 Münster, Germany
Two bis(bipyridine) polymeric metal nitrate complexes with 4,4’-bipyridine of simple formula like [M(bipy) ](NO
2
3
)
2
2
⋅xH O (where
M=Co, Ni and Cu; x=4, 2 and 0, respectively) have been prepared and characterized. Their thermal decomposition has been undertaken
using simultaneous TG-DTG-DTA and DSC in nitrogen atmosphere and non-isothermal TG in air atmosphere. Isothermal TG has
been performed at decomposition temperature range of the complexes to evaluate the kinetics of decomposition by applying model-fit-
ting as well as isoconversional method. Possible mechanistic pathways have also been proposed for the thermolysis. Ignition delay
measurements have been carried out to investigate the response of these complexes under the condition of rapid heating.
Keywords: isoconversional method, mechanistic pathway, thermal decomposition
Introduction
(Thomas Baker), 70% nitric acid and methanol
Ranbaxy), ethanol (Hayman), 4,4’-bipyridine (Merck)
(
Nitrates are powerful oxidizing agents [1, 2] and de-
compose exothermically at elevated temperatures. Of
the various classes of high energetic compounds
and silica gel TLC grade (qualigens).
Preparation and characterization of 4,4’-bipyridine
metal nitrate complexes
(
HECs), transition metal complexes find applications
in explosives, propellant formulations and pyro-
technique compositions. Moreover, transition metal
complexes are known to use as potential burning rate
modifier for HTPB-AP propellants [3]. These transi-
tion metal complexes are also used as precursors to
obtain nano size metal oxides, which can have inter-
esting electrical, magnetic and catalytic proper-
ties [4]. Thermal analysis provides realistic informa-
tion about the thermal stability of compounds.
Recently we have undertaken studies on the thermo-
lysis and kinetics of some hexamine metal perchlor-
ates [5, 6], bis(ethylenediamine)metal nitrates [7],
bis(diethylenetriamine)metal nitrates [8] and
bis(propylenediamine)metal nitrates [9]. In continua-
tion of these studies, here we report the preparation,
characterization and thermolysis of the polymeric
transition metal nitrate complexes containing
Metal nitrates were prepared according to literature
methods [9]. 4,4’-Bipyridine metal nitrate complexes
were prepared by reacting an ethanolic solution of
corresponding metal nitrates with 4,4’-bipyridine in
appropriate stoichiometry.
M(NO
3
)
2
+2C10
H
8
N
2
+xH
2
O→
[M(C10
H
N
)
](NO
)
⋅xH
O
2
8
2
2
3
2
where M=Co, Ni and Cu; x=4, 2 and 0, respectively.
Precipitated complexes given in Table 1 were
washed with methanol, recrystallised from aqueous so-
lution, dried over fused silica and their purity was
checked by thin layer chromatography (TLC). The
complexes were characterized by standard gravimetric
methods [10], infrared (impact 400) [11, 12] mass
spectra (JEOL SX 102/DA-6000 mass spectrometer)
and elemental analysis (Table 1) (Heraeus Carlo
Erba 1108 Instrument). Mass spectra of the complexes
are presented in Fig. 1.
4
,4’-bipyridine. For the study of slow decomposition,
TG, DTA and DSC have been used. Kinetics of
thermolysis has also been evaluated.
Crystallographic study
Experimental
Crystals of the cobalt complex was obtained by the re-
crystallization from aqueous solution. The data collec-
tion of the crystal was performed at low temperature
Materials
The following chemicals were used as received; cop-
per carbonate (BDH), nickel and zinc carbonate
(198 K) using a Nonius Kappa CCD diffractometer,
*
Author for correspondence: gsingh4us@yahoo.com
1
388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
©
2006 Akadémiai Kiadó, Budapest

SINGH et al.
Non-isothermal TG
TG studies on these complexes (mass 30 mg,
00–200 mesh) were undertaken in static air at a heat-
1
–1
ing rate of 5°C min using indigenously fabricated TG
apparatus [16]. Gold crucible was used as sample
holder. The plots of mass% vs. temperature (°C) are
given in Fig. 3.
Simultaneous TG-DTG-DTA
Simultaneous TG-DTG-DTA curves on complexes
were obtained on a Mettler Toledo Star System in
–
1
inert nitrogen atmosphere (flow rate 100 mL min ) at
Fig. 1 Mass spectra of the complexes
Fig. 2 Crystal structure of the cobalt complex
equipped with
a
rotating anode generator
Nonius FR 591. Programmes used: data collection Col-
lect (Nonius B.V., 1998), data reduction
Denzo-SMN [13]. The structure was solved by direct
methods (Program SHELXS-97) [14] and refined by
2
full-matrix least squares method on all F data using
SHELXL-97 [15]. Hydrogen atoms were in part placed
on calculated positions and refined riding, at the water
molecules they were located from a difference Fourier
map and refined independent. Refinement with aniso-
tropic thermal parameters for non-hydrogen atoms and
isotropic parameters for hydrogen atoms led to the
R-value of 0.054. The crystal structure (graphics done
with Schakal [12]) of the complex is shown in Fig. 2,
details of the analysis and crystal parameters in Table 2.
Fig. 3 Non-isothermal TG of the complexes taken in static air
atmosphere
Table 1 Physical, elemental and spectral parameters of the complexes
–1
Observed (calculated)/%
IR, ν/cm
Complex
Color
pink
C–H C–H C=C C=N C–C
C
H
N
Metal
NO
3
–
M–N
Str.
Def.
825
Str.
Str.
Str.
4
2.9
4.41 16.41
9.2
[
[
[
Co(bipy)
2
](NO
](NO
](NO
3
)
2
⋅4H
2
O
3066
1608 1536 1220 1383
1635 1559 1281 1383
1603 1520 1141 1383
485
496
522
(
42.3) (3.7) (14.8) (10.4)
45.4 3.3 14.8 11.3
(44.7) (3.7) (15.6) (10.9)
6.1 3.9 16.9 11.2
47.4) (3.2) (16.6) (12.5)
sky
Ni(bipy)
2
3
)
2
⋅2H
2
O
3091
2990
871
826
blue
4
Cu(bipy)
2
3
)
2
blue
(
426
J. Therm. Anal. Cal., 85, 2006

METAL NITRATE COMPLEXES WITH 4,4’-BIPYRIDINE
Table 2 Crystal data and summary of intensity and structure
refinement data
isothermal TG data have been done using the both
model fitting method [9, 14] as well as model free
isoconversional method [15]. Dependencies of acti-
vation energy (E) on the extent of conversion (α) for
the complexes are shown in Fig. 11.
Empirical formula
20 6 10
C H24CoN O
CDC deposit no.
Colour
245665
pink
Formula mass
Temperature/K
λ/
567.38
198
Ignition delay (D
measurement
) and ignition temperature (IT)
i
0.71073
hexagonal
Crystal system
Space group
Crystal size/mm
These studies were undertaken using tube furnace (TF)
technique [19] as reported earlier. 20 mg samples
1
P6 (No. 169)
0.40×0.30×0.20
a=13.653(1) , α=90°
b=13.653(1) , β=90°
c=22.535(1) , γ=120°
Cell constants
Molecules per unit cell, Z
6
–
3
Calculated density/g cm
1.554
7.75
–
1
μ/cm
F (000)
1758
graphite
Monochromater
0
θ range for data collection
1.72 to 26.30
–
13≤h≤17, –13≤k≤14,
17≤l≤28
Limiting indices
–
Fig. 4 Simultaneous TG-DTG-DTA curves of the cobalt com-
1
4402/4292
plex taken in nitrogen atmosphere
Reflections collected/unique
Data/restraints/parameters
[
R(int)=0.034]
292/11/380
1.151
2
Goodness-of-fit on F
2
R1=0.054, wR =0.086
Final R indices [I>2σ(I)]
R indices (all data)
2
R1=0.072, wR =0.090
–
3
Largest diff. peak and hole/e
0.47 and –0.33
–
1
a heating rate of 10°C min . The curves are shown in
Figs 4–6 for cobalt, nickel and copper complexes
respectively. TG-DTG and DTA phenomenological
data are summarized in Table 3.
DSC studies
Fig. 5 Simultaneous TG-DTG-DTA curves of the nickel com-
plex taken in nitrogen atmosphere
DSC curves were undertaken in inert atmosphere
–
flowing nitrogen at a rate of 50 mL min ) on Mettler
1
(
Toledo Star system (sample mass 4 mg, heating
–
rate 10°C min ). DSC curves are shown in Fig. 7 and
1
data are summarized in Table 3.
Isothermal TG
The isothermal TG on these complexes (mass 30 mg,
1
00–200 mesh) were carried out in static air using the
above said TG apparatus [13] at appropriate tempera-
tures. The isothermal TG curves are presented in
Figs 8–10 for cobalt, nickel and copper complexes,
respectively in which the extent of conversion (α) is
plotted vs. time (min). Kinetic analysis from the
Fig. 6 Simultaneous TG-DTG-DTA curves of the copper com-
plex taken in nitrogen atmosphere
J. Therm. Anal. Cal., 85, 2006
427

SINGH et al.
Fig. 8 Isothermal TG curves of the cobalt complex in static air
atmosphere
Fig. 9 Isothermal TG curves of the nickel complex in static air
atmosphere
Fig. 10 Isothermal TG curves of the copper complex in static
Fig. 7 DSC curves of the complexes taken in flowing nitrogen
atmosphere
air atmosphere
(100–200 mesh) were taken in an ignition tube
Results and discussion
(I=5 cm, d=0.4 cm) and time interval between the in-
sertion of the ignition tube into the TF and the moment
of ignition indicated by the removal of fumes with light
and noise, noted with the help of a stop watch, gave the
value of ignition delay in s. The accuracy of tempera-
ture measurement of TF was ±1°C. At a selected tem-
perature, this experiment was repeated four times and
D values are reported in Table 3. D data were found to
The analytical data presented in Table 1, indicate a
good agreement between observed and calculated per-
centage values of each elements, which confirms the
metal complex formation. Complexes were found to be
polymeric, as indicated by crystal structure of cobalt
complex (Fig. 1) and presence of peaks at higher m/z
than monomer in the mass spectra of all the three com-
plexes (Fig. 2). On the other hand, IR spectral data (Ta-
ble 1) also signalize the proposed molecular formula of
the complexes. ν(M–N) and ν(C=N) peak confirms the
metal to ligand bond formation. It is evident that all
these complexes undergo decomposition for more than
i
i
fit in following equation
E*/RT
1
/D =Ae
i
where E* is the activation energy for ignition and T is
the absolute temperature. Typical plots of logD vs.
i
1
/T are given in Fig. 12.
428
J. Therm. Anal. Cal., 85, 2006

METAL NITRATE COMPLEXES WITH 4,4’-BIPYRIDINE
Table 3 TG-DTA phenomenological of the complexes under nitrogen atmosphere
TG
DTA
DSC
Complex
Step
Trange /°C
Decomposition/%
Nature
Trange /°C
Nature
Trange /°C
I
II
78–91
9.5
endo
endo
–
95.9
–
endo
endo
endo
exo
89.2
125.6
266
155–187
235–258
286–310
10.07
11.38
42.2
[Co(bipy)
2
](NO
3
)
2
2
⋅4H O
III
IV
225
307.5
exo
306
I
125–209
273–316
10.0
60.3
endo
exo
124
318
endo
exo
122
326
[Ni(bipy)
2
](NO
3
)
2
2
⋅2H O
II
I
187–218
305–324
32.1
45.4
endo
exo
213
332
endo
exo
208
333
[Cu(bipy)
2
3 2
](NO )
II
nism of thermolysis of the cobalt complex may be pro-
posed as follows.
9
5°C, 190–312°C
[
Co(C10
H
8
N
2
)
2
](NO
3
)
2
⋅4H O ⎯ ⎯ ⎯ ⎯ ⎯ →
2
[
Co(C H N ) ](NO ) +4H O
10 8
2 2
3 2
2
[
Co(C H N ) ](NO ) →[Co(C H N )](NO ) +bipy
1
0
8
2 2
3 2
10
8
2
3 2
3
20°C
[
Co(C H N )](NO ) ⎯ ⎯⎯ →CoO+gaseous products
10
8
2
3 2
In DTA and DSC curve of this complex (Fig. 4), an
endothermic peak at 97°C is obtained for the removal of
four water molecules. For the second step decomposi-
tion, one sharp endotherm is obtained due to the re-
moval of half of the bipyridine molecule. For the fourth
step decomposition, an exotherm is observed at 307°C
which is due to the exothermic decomposition of the
Fig. 11 Dependencies of activation energy (E) on the extent of
conversion (α) for the complexes of the elements shown
residue [Co(bipy)](NO ) . However, there is no DTA
3 2
peak for the third step mass loss, but in DSC this step
appears as an endothermic process at ~196°C (Table 3).
In air as well as in nitrogen atmosphere, Ni com-
plex decomposes in two steps (Figs 3 and 5). The first
step is the loss of two water molecules (~10% mass
loss) giving an endothermic peak at 124°C in DTA and
DSC. The dehydrated complex decomposes be-
tween 273–320°C (~60.3% mass loss) giving nickel
oxide as residue and an endotherm at 318°C (Figs 5
and 7; Table 3). Thus, the plausible mechanism for the
thermolysis of nickel complex may be proposed as
Fig. 12 Plot of lnD
i
vs. 1/T for the complexes of the elements
shown
1
25–209°C
[
Ni(bipy)
2
](NO
3
)
2
⋅2H O ⎯ ⎯ ⎯ ⎯ →
2
one stage, finally giving metal oxides as residue and
other various gaseous decomposition products. TG
curve for the cobalt complex shows that it decomposes
in four steps in static air as well as in flowing nitrogen
atmosphere (Figs 3 and 4). Corresponding to these four
steps, four DTG peaks are obtained. In the very first
step (78–98°C) four of the water molecules leave out
[
Ni(bipy) ](NO ) +2H O
2
3 2
2
273–320°C
[
Ni(bipy) ](NO ) ⎯ ⎯ ⎯ ⎯ →NiO+gaseous products
2
3 2
Copper complex also decomposes in two steps in
both, static air as well as in flowing nitrogen atmosphere
and gives two corresponding DTG peaks (Figs 3 and 6).
In the first step (187–218°C), one bipyridine ligand
leaves the molecule (~32% mass loss) giving an
endotherm at 213°C in DTA and at 208°C in DSC
curve. In second step (305–324°C) the residue
(
~13% mass loss). The middle two steps (155–258°C)
are due to the removal of one bipyridine molecule
~30% mass loss). In fourth step (235–258°C), residue
ignited with light and a sharp mass loss is observed
~40% mass loss) which is due to the exothermic de-
composition of [Co(bipy)](NO residue. Finally a
(
[
Cu(bipy)](NO ) ignited exothermically and for this
3 2
(
mass loss (~45.4%) an exothermic peak at ~332°C was
obtained in DTA and DSC curves (Figs 6 and 7; Ta-
ble 2). Finally, the residue correspond to copper oxide.
)
3 2
residue (~15%) of cobalt oxide is left. Thus the mecha-
J. Therm. Anal. Cal., 85, 2006
429

SINGH et al.
Table 4 Ignition delay D
i
, activation energy for thermal ignition (E*) and correlation coefficient (r) for the complexes
D
i
/s at temperature/°C
–1
Complex
E*/kJ mol
r
3
75±1
400±1
425±1
450±1
475±1
[
[
[
Co(bipy)
2
](NO
](NO
](NO
3
)
2
⋅4H
2
O
74
123
105
55
102
88
48
80
71
42
65
62
35
49
53
28.6
36.8
27.8
0.9907
0.9944
0.9986
Ni(bipy)
2
3
)
2
2
⋅2H O
Cu(bipy)
2
3 2
)
Thus, the plausible mechanistic pathways for the
thermolysis of this complex may be proposed as
to correlate particular activation energy for a particu-
lar process in thermolysis.
To examine the effect of rapid heating on the
1
87–218°C
[
Cu(bipy) ](NO ) ⎯ ⎯ ⎯ ⎯ →
2
3 2
complexes, D data reported in Table 4 indicates that
i
[
Cu (bipy)](NO ) +bipy
3 2
the time required for the thermal ignition at a particular
temperature, increases in the order Co<Cu<Ni com-
plex. Thus, showing the relative thermal stability of the
complexes in the same order under the condition of
rapid heating. Activation energy for the thermal igni-
tion of the complex (Table 4) was found to be in the in-
creasing order as Co<Cu<Ni complex which is also in
agreement with the thermal stability order of the com-
plexes. The values of activation energies for slow ther-
mal decomposition (E) and rapid decomposition (E*)
are not same, but the increasing order is same in both
the cases. Less stability of the cobalt complex may be
attributed to the less nuclear charge and greater ionic
3
05–324°C
[
Cu(bipy)](NO ) ⎯ ⎯ ⎯ ⎯ →CuO+gaseous products
3
2
Monobipyridine cobalt and copper nitrate is
formed as intermediates and separation of stages is due
to the relatively higher thermal stability of this mono-
ligand intermediate in comparison to that of nickel com-
plexes. The formation and stability of such monoligand
intermediate compounds have also been seen earlier
during the thermal studies of bis(ethylenediamine)cop-
per chloride/bromide monohydrates [20], bis(ethylene-
diamine)copper nitrate [9, 21] and bis(ethylenedi-
amine)metal perchlorate complexes [22].
The analysis of the kinetics from isothermal TG,
using model fitting methods lead to uncertainty.
However, values of activation energy E, obtained
from different models for particular samples are
nearly equal irrespective of the equations used. An
average value of E equal to 45.6, 116.3 and
2+
size of Co . Moreover, due to higher nuclear charge
2+
2+
and small size of Cu than Ni the stability of Cu
complex should be greater as compared to Ni complex.
However, the thermal stability of Ni complexes over
Cu complexes is also reported in earlier studies, where
–
9.5 kJ mol have been evaluated for cobalt, nickel
1
[Ni(en) ](ClO ) have greater thermal stability than
2 4 2
5
[
Cu(en) ](ClO ) [22].
2 4 2
and copper complexes, respectively. In case of nickel
complex, the value of E is much higher than cobalt
and copper complexes because in this case, the E is
calculated for the loss of two bipyridine and two
water molecules, while in case of cobalt and copper, E
is calculated for the loss of one bipyridine ligand
along with water molecules.
Conclusions
The thermal studies carried out on the two bipyridine
metal complexes using TG-DTA, DSC (in flowing ni-
trogen) and non-isothermal TG in air, indicates that all
these complexes decomposes in multisteps, last step of
which are rapid and exothermic. Both, nickel and cop-
per complex decompose in two steps, whereas cobalt
complex decomposes in four steps. Application of
model fitting method for the kinetic analysis from iso-
thermal data, fails to explain the complexity of thermo-
lysis as it gives single value of activation energy which
can not be assigned to a particular process. On the
other hand, isoconversional method describes well the
complexity of even single step decomposition that
yields a series of E values as a function of extent of
conversion. The ignition delay and E* have been found
to be maximum for the nickel complex and minimum
for the cobalt and copper complexes.
Application of isoconversional method to the
isothermal TG data indicates that the decomposition
of these complexes is not simple, as indicated by
model fitting methods. Values of E for these com-
plexes are found to vary with α. As we have just stud-
ied the kinetics of the removal of water molecules and
one bipyridine ligand (~30% mass loss), Fig. 10 rep-
resents the dependencies of activation energies on α
up to 0.30. The E for nickel complex in this α range is
lower than the cobalt copper complexes. For these
complexes, E decreases in α range 0.09 to 0.21 which
may be due to the predominance of exothermic
changes over endothermic changes. However, the
variation of activation energy with extent of conver-
sion is changing at every stage, which may be the re-
sult of many competeting effects and is very difficult
430
J. Therm. Anal. Cal., 85, 2006

METAL NITRATE COMPLEXES WITH 4,4’-BIPYRIDINE
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431