Some transition metal nitrate complexes with hexamethylenetetramine : PPPart LV. Preparation, X-ray crystallography and thermal decomposition

Article abstract of DOI:10.1007/s10973-007-8615-5

Three hexamethylenetetramine (HMTA) metal nitrate complexes such as [M(H₂O)₄(H₂O-HMTA)₂](NO ₃)?4H₂O (where M=Co, Ni and Zn) have been prepared and characterized by X-ray crystallography. Th

Full text of DOI:10.1007/s10973-007-8615-5

Journal of Thermal Analysis and Calorimetry, Vol. 91 (2008) 3, 971–977
SOME TRANSITION METAL NITRATE COMPLEXES WITH
HEXAMETHYLENETETRAMINE
Part LV. Preparation, X-ray crystallography and thermal decomposition
1
*
1
1
1
1
G. Singh , B. P. Baranwal , I. P. S. Kapoor , D. Kumar , C. P. Singh and R. Fröhlich
2
1
Department of Chemistry, DDU Gorakhpur University, Gorakhpur 273009, India
Organisch-Chemisches Institut, Universität Münster, 48149 Münster, Germany
2
2 4 2 2 3 2
Three hexamethylenetetramine (HMTA) metal nitrate complexes such as [M(H O) (H O-HMTA) ](NO )·4H O (where M=Co, Ni
and Zn) have been prepared and characterized by X-ray crystallography. Their thermal decomposition have been studied by using
dynamic, isothermal thermogravimery (TG) and differential thermal analysis (DTA). Kinetics of thermal decomposition was under-
taken by applying model-fitting as well as isoconversional methods. The possible pathways of thermolysis have also been proposed.
Ignition delay measurements have been carried out to investigate the response of these complexes under condition of rapid heating.
Keywords: crystal structure, hexamethylenetetramine, ignition delay, iso-conversional, isothermal TG, metal nitrate, model fitting
Introduction
Experimental
Materials
Nitrates are powerful oxidizing agents [1, 2] and decom-
pose exothermically at elevated temperatures. Of the
various classes of high energetic compounds (HECs),
transition metal complexes find application in explo-
sives, propellants and pyrotechnics. Moreover, transi-
tion metal complexes are potential burning rate modifier
for HTPB-AP propellants [3, 4]. These transition metal
complexes are used as precursor to obtain ultra fine
metal oxides, which have interesting electrical, mag-
netic and catalytic properties [5]. Thermal analysis pro-
vides realistic information about the thermal stability of
compounds. Recently, we have undertaken studies on
the thermolysis and kinetics of some transition metal ni-
trate and perchlorate complexes with 1,4-diaminobutane
The following chemicals were used as received; Cobalt
carbonate, nickel carbonate (Thomas Baker), zinc car-
bonate, 70% HNO
(Lancaster) and silica gel TLC grade (Qualigens).
(s.d.fine), hexamethylenetetramine
3
Preparation of the complexes
The hexahydrate metal nitrate complexes were ob-
tained by treating metal carbonates with 70% nitric
acid, washing the salts with petroleum ether, re-
crystallising from distilled water and dried over fused
calcium chloride. The HMTA metal nitrate com-
plexes were prepared by treating the aqueous solution
of metal nitrate hexahydrates with aqueous solution
of HMTA in appropriate stoichiometry.
[6, 7], propylenediamine [8, 9] ethylenediamine [4, 10]
and 1,6-diaminohexane ligand [11].
Tetraazaadamantane, urotropine or commonly
called hexamethylenetetramine (HMTA) in which
four nitrogen atoms are situated at the corners of a tet-
rahedron, is a ligand of polycyclicpolydentate type.
Its complexes in which HMTA acts as a monodentate
ligand [12], bidentate ligand [13, 14] and shows
non-chelating behaviour [15] (in low valent organo-
metallic complexes). It is used for preparing RDX.
Thus it was found interesting to prepare and charac-
terize the transition metal nitrate complexes contain-
ing intermolecular hydrogen bonded hexamethylene-
tetramine. The thermolysis of these complexes has
been done by TG and DTA techniques. The ignition
delay of these samples has also been measured. Kinet-
ics of thermolysis has been evaluated using both
model fitting as well as isoconversional methods.
M(NO
M(H O)
3
)
2
·6H
2
O+2C
6
H
(H O-HMTA)
12
N
](NO
4
+4H
)
2
O®
·4H O
2
[
2
4
2
2
3
2
where M=Co, Ni and Zn, respectively.
The complexes were recrystallized from aqueous
solutions and their purity was checked by thin layer
chromatography (TLC). The complexes were charac-
terized by X-ray crystallography.
Methods
X-ray crystallographic study
Crystals of the complexes were obtained by the
recrystallisation from aqueous solutions. The data
collection of the crystals were performed at low tem-
*
Author for correspondence: gsingh4us@yahoo.com
1
388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
©
2008 Akadémiai Kiadó, Budapest

SINGH et al.
perature (223 K) using a Nonius Kappa CCD
Experimental
diffractometer, equipped with a rotating anode gener-
ator Nonius FR591. Programs used: data collection
Collect (Nonius B.V; 1998), data reduction and ab-
sorption correction Denzo-SMN [16]. The structures
were solved by direct methods (SHELXS-97) [17]
and refined by full-matrix least square method on all
Non-isothermal TG
TG studies on these complexes (mass 20 mg,
1
00–200 mesh) were undertaken in static air atmo-
–1
sphere at a heating rate of 5°C min using indigen-
ously fabricated TG apparatus [20]. Gold crucible
was used as sample holder. The curves of mass% vs.
temperature (°C) are given in Fig. 4.
2
F data using SHELXL-97 [18]. Hydrogen atoms
were in part placed on calculated positions and re-
fined riding, at the water molecules they were located
from a difference Fourier map and refined independ-
ent. Refinement with anisotropic thermal parameters
for non-hydrogen atoms led to the R-values of
100
Co
50
0
.036 (Co), 0.046 (Ni) and 0.033 (Zn). The crystal
structures (graphics done with Schakal [19]) of the
complexes are shown in Figs 1–3. The crystal param-
eters of the complexes are summarized in Table 1.
0
0
200
400
600
Temperature/°C
O21
O12
O22
1
00
50
0
O11
N10
Ni
O3*
N2
O13
C34*
N4*
O1
C24
N3A
C13*
C12
C23
C14
O2*
O1*
C14*
O2
N1
N4
N1*
Co1
C23*
N2*
C12*
C13
N3
C24*
C34
O13*
O3
N10*
O12*
O11*
O22*
0
200
400
600
O21*
Temperature/°C
Fig. 1 Crystal structure of cobalt complex
O21
Zn
O12
8
4
0
0
0
O22
O11
N10
O3*
N2
O13
C34*
N4*
O1
C24
N3A
C13*
C12
C23
C14
O2*
O1*
C14*
O2
N1
N4
N1*
Ni
C23*
N2*
C12*
C13
N3
C24*
C34
O13*
O3
N10*
O12*
O11*
0
300
600
O22*
Temperature/°C
O21*
Fig. 4 Non-isothermal TG of complexes in air atmosphere
Fig. 2 Crystal structure of nickel complex
DTA curve of complexes were obtained on Uni-
versal Thermal Analyser Instrument, Mumbai, in
O21
O12
–
1
O22
static air (heating rate of 10°C min ). DTA curves of
complexes are shown in Fig. 5.The TG and DTA
phenomenological data are summarised in Table 2.
O11
N10
O3*
N2
O13
C34*
N4*
O1
C24
N4
N3A
C13*
C12
C23
C14
O2*
O1*
C14*
O2
N1
N1*
Zn1
Isothermal TG
C23*
N2*
C12*
C13
N3
C24*
C34
O13*
O3
The isothermal TG on these complexes (mass 20 mg,
100–200 mesh) were carried out in static air atmo-
sphere using above said TG apparatus [20] at appropri-
ate temperatures and TG curves are presented in Fig. 6.
N10*
O12*
O11*
O22*
O21*
Fig. 3 Crystal structure of zinc complex
972
J. Therm. Anal. Cal., 91, 2008

TRANSITION METAL NITRATE COMPLEXES WITH HEXAMETHYLENETETRAMINE
J. Therm. Anal. Cal., 91, 2008
973

SINGH et al.
Table 2 TG-DTA phenomenological data of the complexes under air atmosphere
TG
DTA
Complex
Step
Trange/°C
decomposition/%
peak temp./°C
nature
I
II
III
93–100
132–140
290–315
11.2
11.2
65.6
95
140
305
endo
endo
exo
1
I
II
III
95–110
120–145
320–340
11.2
11.2
65.0
105
140
330
endo
endo
exo
2
3
I
II
65–100
600–645
11.0
76.0
85
630
endo
endo
0
.2
0.8
Co
Co
0
.1
0
0
0
0
.6
.4
.2
.0
0.0
200
240
260
220
–0.1
0
0
0
200
400
600
280
Temperature/°C
0
50
100
Time/min
150
200
0.2
Ni
0.1
0.6
0
.5
.4
.3
0.2
.1
0.0
0.0
Ni
0
–0.1
0
2
2
30
50
270
290
200
400
600
Temperature/°C
0
310
0
.2
.1
.0
0.1
0.2
Zn
0
50
100
150
0
Time/min
0
0
.8
–
Zn
–
0.6
200
400
600
Temperature/°C
0.4
0.2
0.0
Fig. 5 DTA curve of complexes in air atmosphere
2
2
10
30
250
270
Ignition delay (D ) measurements
i
290
These studies were undertaken using tube furnace
TF) technique [2 1]. 20 mg samples (100–200 mesh)
(
0
50
100
were taken in an ignition tube (l~5 cm, d~0.5 cm) and
time interval between the insertion of the ignition
tube into the TF and the moment of ignition indicated
by the appearance of fumes with light and noise noted
with help of stop watch, gave the value of ignition de-
lay in seconds. The sample was inserted into the TF
with the help of a bent wire. The time for the insertion
of the ignition tube into the TF was kept constant
Time/min
Fig. 6 Isothermal TG of complexes
throughout each run. The accuracy of temperature
measurements of TF was ±1°C. Each run was taken
five times and mean D value are reported in Table 3.
i
974
J. Therm. Anal. Cal., 91, 2008

TRANSITION METAL NITRATE COMPLEXES WITH HEXAMETHYLENETETRAMINE
Table 3 Ignition delay activation energy for thermal ignition (E*) and correlation coefficient (r) for the complexes
i
D /s at temperature/°C
–1
Complex
E*/kJ mol
r
2
80±1
300±1
320±1
340±1
360±1
1
2
3
85
107
72
68
90
63
61
85
57
50
60
48
40
45
39
27.5
27.7
23.8
0.9984
0.9953
0.9997
Kinetic analysis
sented as [M(H O) (H O-HMTA) ](NO ) ·4H O,
2 4 2 2 3 2 2
where M=(Co, Ni or Zn), which is matched with the
empirical formula of the compound given in the text.
It is evident that all these complexes undergo de-
composition for involving more than one step, to give
metal oxide as residue and other various gaseous de-
composition products (Fig. 4). TG curve for the cobalt
complex shows that it decomposes in the three steps in
static air. Corresponding to these, three DTA peaks are
obtained (Fig. 5). In the first step (93–100°C) four of
the water molecules leave (~11.2% mass loss). In the
next second step (132–140°C) four water molecules
are coordinated with the metal leave (~11.2% mass
loss). In the third step (290–315°C), water, HMTA
molecules decomposes (~65.60% mass loss) and a res-
idue (~12%) of cobalt oxide is left. Thus the mechanis-
tic pathways of thermolysis of the cobalt complex may
be proposed as:
Isothermal TG data taken at appropriate temperatures
have been used to evaluate the kinetics of early
thermolysis. Kinetics is evaluated by using the model
fitting [22] as well as isoconversional method by
Vyazovkin and Wight [23]. The following equation is
found to hold under isothermal condition
–
lnta,i=ln[A/g(a)]–E
a
/RT
i
(1)
where a is the extent of conversion, E is the activa-
a
tion of energy at a particular a, R the gas constant and
T the absolute temperature. Thus, the values of E
i a
were evaluated at various a . The dependencies of Ea
i
on extent of conversion are presented in Fig. 8.
The kinetics of fast decomposition is evaluated
from the ignition delay (D ). The D data were found to
i i
fit in the following equation [21, 24]
E*/RT
D =Ae
i
(2)
93–110°C
[
Co(H
2
O)
[Co(H
4
(H
O)
2
O-HMTA)
(H O-HMTA)
2
](NO
3
)
2
·4H
+4H O
2
2
O¾ ¾¾ ®
where E* is the activation energy for ignition, A the
Arrhenius factor and T is the absolute temperature.
2
4
2
2
](NO
3
)
2
132–140°C
Co(H O) (H O-HMTA) ](NO ) ¾ ¾¾ ®
2 2 2 2 3 2
[
The values of E* were obtained from the slope of lnD
vs. 1/T (Fig. 7).
i
[
Co(H O-HMTA) ]·(NO ) +4H O
2
2
2
3 2
2
90–315°C
[
Co(H O-HMTA) ]·(NO ) ¾ ¾¾ ®
2 2 3 2
CoO+gaseous product
5
An endothermic peak at 95°C in DTA curves, is
obtained for the removal of four water molecules.
For the second step one sharp endotherm is obtained
due to removal of four water molecules coordinated
with metal. An exotherm is obtained at 305°C which
is due to exothermic decomposition of the dehy-
drated complex.
4
Ni
Co
Zn
3
.0014
0
0.0016
0.0018
0.002
–1
1
/T /K
Nickel complex decomposes in three steps
(Fig. 4). The first step (95–110°C) is due to loss of
four water molecules (~11.2% mass loss) giving one
endothermic peak at 105°C in DTA. This endotherm
is due to removal of four water molecules. In the sec-
ond step of the decomposition (120–145°C), four
water molecules which are coordinated to the metal,
leave (~11.2% mass loss). Corresponding to this de-
composition step an endotherm at 140°C in DTA
was obtained. In the third step, the dehydrated com-
plex decomposes between 320–340°C (~65% mass
loss) giving nickel oxide as residue and an
endotherm at 330°C in DTA (Fig. 5). This exotherm
in DTA is due to exothermic decomposition of ni-
Fig. 7 Plot of lnD
i
vs. 1/T for the elements shown
Results and discussion
The crystal structures of the complexes are shown in
Figs 1–3. The two HMTA molecules are attached with
the metal ion through water molecules. The bonding
between the HMTA and the water is clearly shown to
be hydrogen bonding. Each HMTA is attached by two
hydrogen bonds between nitrogen of HMTA and hy-
drogen of the water. The nitrate ion is also attached to
the water molecule via hydrogen bonding. Therefore
the molecular formula of a single crystal can be repre-
J. Therm. Anal. Cal., 91, 2008
975

SINGH et al.
trate ion from the dehydrated complex. Thus the
pathways for the thermolysis of nickel complex may
be proposed as:
model fitting method. The value of activation energy
varies with extent of conversion (a) (Fig. 8). The
value of E for Zn complex is higher than that of Ni
and Co complex. Except in higher a range, E for Ni is
greater than Co indicating the greater stability of Ni
complex over Co complex.
All these complexes are quite stable at room
temperature but they ignite with noise, light and fume
on sudden high heating. Ignition delay data (Table 3)
shows that time for ignition at a fixed temperature is
in the order Ni>Co>Zn. This shows that Ni complex
is more stable than Co because of higher nuclear
charge. Zn complex is less stable due to completely
filled valence shell d-orbital.
9
0–101°C
[
Ni(H O) (H O-HMTA) ](NO )·4H O¾ ¾¾ ®
2 4 2 2 3 2
[
Ni(H O) (H O-HMTA) ](NO ) +4H O
2
2
4
2
2
3 2
120–145°C
Ni(H O) (H O-HMTA) ](NO ) ¾ ¾¾ ®
[
2
4
2
2
3 2
[
Ni(H O-HMTA) ](NO ) +4H O
2
2
3 2
2
3
20–340°C
[
Ni(H O-HMTA) ](NO ) ¾ ¾¾ ®
2 2 3 2
NiO+gaseous products
Zn complex decomposes in two steps in static
air atmosphere. In the first step (65–100°C), four
water molecules leave the complex (~11% mass
loss) giving an endotherm at 85°C in DTA (Fig. 5).
In the second step (600–645°C), the dehydrated
complex decomposes endothermically (~76% mass
loss). An endotherm is also obtained in DTA curve at
Conclusions
~
630°C (Fig. 5). Zinc oxide was ultimately obtained
HMTA is coordinated with water molecule through
hydrogen bonding. The nitrate ion is also attached to
HMTA through water. The thermal stability of the
complexes was found in the order Ni>Co>Zn by igni-
tion delay.
and the proposed pathways for the decomposition of
zinc complex may be given as:
6
5–100°C
[
Zn(H O) (H O-HMTA) ](NO ) ·4H O¾ ¾¾ ®
2 4 2 2 3 2 2
[
Zn(H O) (H O-HMTA) ](NO ) +4H O
2
2
4
2
2
3 2
6
00–645°C
[
Zn(H O) (H O-HMTA) ](NO ) ¾ ¾¾ ®
2 4 2 2 3 2
ZnO+gaseous product
References
Similar pathways have been suggested for the
thermal decomposition of nitrate and perchlorate com-
plexes of transition metals with 1,4-diaminobutane,
propylenediamine, ethylenediamine and nitrate com-
plexes of 1,6-diaminohexane ligands [5–11]. In the
present case, there is no indication for the formation of
mono ligand intermediate. This may be attributed to
the polycyclic chain of the HMTA molecules.
1 B. T. Encyclopedia of Explosives and related Items,
Vol. 1, Picatiny Aresenel, New Jersey.
2 J. H. Koper, O. G. Jansen and P. J. Vanden,
Berg. Explosive Stoff, 8 (1970) 181.
3
4
5
G. Singh, I. P. S. Kapoor and D. K. Pandey, J. Energ. Mater.,
0 (2002) 223.
G. Singh and D. K. Pandey, Propellants, Explos. Pyro.,
8 (2003) 5.
S. Y. Sawant, K. R. Kannan and V. M. S. Verneker,
2
2
The kinetic analysis applying model fitting ap-
proach using isothermal TG data, gives an average ac-
th
13 National Symposium on Thermal Analysis BARC,
Mumbai, India 2002.
–
1
tivation energy 39.3, 31.05 and 63.6 kJ mol upto
a=0.75, 0.55 and 0.70, respectively. The isothermal
TG data shows that the solid-state decomposition of
these complexes is not as simple as indicated by the
6 G. Singh, C. P. Singh and S. M. Mannan, J. Hazarad. Mater.,
122 (2005) 111.
7
G. Singh, C. P. Singh and S. M. Mannan,
Thermochim. Acta, 437 (2005) 21.
G. Singh and D. K. Pandey, Combust. Flame,
8
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
1
35 (2003) 135.
9 G. Singh and D. K. Pandey, J. Therm. Anal. Cal.,
2 (2005) 353.
8
1
1
0 G. Singh, S. Prem Felix and D. K. Pandey,
Thermochim. Acta, 411 (2004) 61.
1 G. Singh, C. P. Singh and S. M. Mannan, J. Hazard. Mater.,
A135 (2006) 10.
12 W. Strohmeier and J. F. Guttenberger, Chem. Ber.,
6 (1963) 2112.
3 I. S. Ahuja, C. L. Yadava and S. Tripathi, Ind. J. Chem.,
1989) 167.
9
Ni
Co
Zn
1
1
(
0
0.2
0.4
0.6
0.8
4 I. S. Ahuja and S. Tripathi, Chemistry Education,
October–December 1990.
Fig. 8 Dependence of activation energy (E) on the extent of
conversion (a) for the complexes of the elements shown
15 A. Luttringhaus and W. Kullick, Tetrahedron Lett.,
10 (1959) 13.
976
J. Therm. Anal. Cal., 91, 2008

TRANSITION METAL NITRATE COMPLEXES WITH HEXAMETHYLENETETRAMINE
1
6 Z. Otwinowski and W. Minor, Methods Enzymol.,
22 S. Vyazovkin and C. A. Wight, J. Phys. Chem.,
A101 (1997) 8279.
276 (1997) 307.
1
7 G. M. Sheldrick, Acta Cryst. A, 46 (1990) 467.
8 G. M. Sheldrick, SHELXL-97, Program for Crystal Structure
Refinement, University of Gottingen, Germany 1997.
9 E. Keller, SCHAKAL, A computer program for the
graphic representation of molecular and crystallographic
models, University of Freiburg, Germany 1997.
23 N. Semenov, Chemical Reactions, Clarendon Press,
Oxford 1935, Chapter 18.
1
24 E. S. Freeman and S. Gorden, J. Phys. Chem.,
60 (1956) 867.
1
2
0 G. Singh and R. R. Singh, Res. Ind., 23 (1978) 92.
Received: July 4, 2007
Accepted: July 30, 2007
21 G Singh, I. P. S. Kapoor and S. K. Vasudeva, Ind. J. Technol.,
29 (1991) 589.
DOI: 10.1007/s10973-007-8615-5
J. Therm. Anal. Cal., 91, 2008
977