Pyrolytical characterization of transition metal complexes of cobalt, nickel, copper and zinc with ethylenediamine-N,N′ diacetate

Article abstract of DOI:10.1007/s10973-009-0184-3

A series of octahedral complexes, [M(EDDA)(H₂O)₂] ? H₂O (where, M⁺² = Co(II), Cu(II), Ni(II) and Zn(II); EDDA, ethylenediamine-N,-diacetate), was prepared and studied by means of thermogravimetry (TG) and differential thermal analysis (DTA). Their compositions were investigated by elemental analysis in order to ensure their purity and structural elucidation was based on spectral and magnetic properties. Thermal decomposition of these distorted octahedral complexes, [Ni(EDDA)(H ₂O)₂], [Co(EDDA)(H₂O)₂] ? H₂O, [Cu(EDDA)(H₂O)₂] ? H₂O and [Zn(EDDA)(H₂O)₂] ? H₂O came of in one, two, three and four steps, respectively, upon heating to 800 °C, with the loss of organic and inorganic fragments. Ligand decomposed in three steps. The thermal degradation of all the complexes in static air atmosphere started at temperatures lower than those observed for the free ligand degradation (Ni-complex being the only exception). The composition of intermediates formed during degradation was confirmed by microanalysis and IR spectroscopy. The residues corresponded to metal oxide except for Ni(II) and Zn(II) complexes. It was found that thermal stability of the complexes increased in the following sequence: Cu(II) ～ Co(II) < Zn(II) < Ni(II)

Full text of DOI:10.1007/s10973-009-0184-3

J Therm Anal Calorim (2010) 102:715–722
DOI 10.1007/s10973-009-0184-3
Pyrolytical characterization of transition metal complexes
of cobalt, nickel, copper and zinc with ethylenediamine-N,N0-
diacetate
S. Rehman Æ M. Arshad Æ K. Masud Æ
R. Afzal Æ U. Salma
Received: 19 March 2009 / Accepted: 22 April 2009 / Published online: 19 June 2009
´
´
Ó Akademiai Kiado, Budapest, Hungary 2009
Abstract A series of octahedral complexes, [M(EDDA)
(H2O)2] ꢀ H2O (where, M?2 = Co(II), Cu(II), Ni(II) and
Zn(II); EDDA, ethylenediamine-N,N0-diacetate), was pre-
pared and studied by means of thermogravimetry (TG) and
differential thermal analysis (DTA). Their compositions
were investigated by elemental analysis in order to ensure
their purity and structural elucidation was based on spec-
tral and magnetic properties. Thermal decomposition of
these distorted octahedral complexes, [Ni(EDDA)(H2O)2],
[Co(EDDA)(H2O)2] ꢀ H2O, [Cu(EDDA)(H2O)2] ꢀ H2O and
[Zn(EDDA)(H2O)2] ꢀ H2O came of in one, two, three and
four steps, respectively, upon heating to 800 °C, with the
loss of organic and inorganic fragments. Ligand decom-
posed in three steps. The thermal degradation of all the
complexes in static air atmosphere started at temperatures
lower than those observed for the free ligand degradation
(Ni-complex being the only exception). The composition of
intermediates formed during degradation was confirmed by
microanalysis and IR spectroscopy. The residues corre-
sponded to metal oxide except for Ni(II) and Zn(II) com-
plexes. It was found that thermal stability of the complexes
increased in the following sequence:
Cu(II) ꢁ Co(II)\Zn(II)\Ni(II)
Keywords EDDA ꢀ Transition metals ꢀ
Octahedral complexes ꢀ Pyrolytic study ꢀ Thermal stability
Introduction
In studies on the physical and chemical properties and the
structures of solid complexes of transition metal ions with
organic ligands, it is customary to investigate the thermal
decomposition of these complexes so as to obtain useful
data on the metal-ligand bonds [1–3] and stability trends.
The thermal investigations on some derivatives of amines
[4], sulfonamides [5], substituted thiourea salts [6], and
complexes of bidentate ligands and others [7–11], have
already been published.
S. Rehman ꢀ R. Afzal
Institute of Chemical Sciences, University of Peshawar,
Peshawar, NWFP, Pakistan
As an extension of our previous studies [12–22], the
present paper deals with pyrolysis of M(EDDA)(H2O)2 of
the same type of complexes with nitrogen-coordinating
ligand in order to understand the mechanism of disinte-
gration and the nature of degradation products. The char-
acterization of complexes was effected through elemental
analysis, spectral, magnetic, TG, DTG and DTA studies.
Attempts were also made to estimate the kinetic para-
meters from thermal studies employing Horowitz–Metzger
method [23].
M. Arshad (&)
Chemistry Division, Directorate of Science, PINSTECH, Nilore,
Islamabad, Pakistan
e-mail: marshads53@yahoo.com.sg
K. Masud
NLP, P.O. Box 1356, Nilore, Islamabad, Pakistan
U. Salma
PCSIR Laboratories, Jamrud Road, Peshawar, NWFP, Pakistan
123

716
S. Rehman et al.
Experimental
Reagents
minimum amount of water (20 mL) at 70 °C. After cool-
ing, filtering and washing as before, 1 g of light blue
crystals was obtained. The air-dried yield was 68%.
Salts of transition metals and other chemicals obtained
from standard source suppliers were of analytical grade and
used without further purification. Solvents were distilled
before use. The partial dehydration of metal salts was
carried out in vacuum oven for several hours around
80–100 °C.
Ethylenediamine-N,N0-diacetato(diaquo)Cu(II) hydrate
To a slurry of 1.21 g (0.0025 mol) of CuCO3, Cu(OH)2 in
25 mL of deaerated water on a steam bath, was added
1.76 g (0.01 mol) of H2EDDA. The mixture was heated
until the evolution of CO2 ceased, filtered and evaporated
to about 5 mL. The viscous mixture was filtered and the
resultant deep blue precipitate was washed with ethanol
and air dried; yield 0.4 g (40%).
Preparation of the ligand
Ethylenediamine-N,N0-diacetic acid (H2EDDA) was pro-
cured from market (Merck) and used without further
purification.
Ethylenediamine-N,N0-diacetato(diaquo)Zn(II) hydrate
To slurry of 0.2 g (0.016 mol) of ZnCO3 in 6 mL of
deaerated water on a steam bath, was added 0.32 g
(0.0018 mol) of H2EDDA dissolved in 4 mL of water. The
mixture was heated to 60 °C in nitrogen atmosphere and
evaporated to 2 mL. The viscous mixture was filtered and
white precipitate was washed with ethanol. Yield was
0.082 g (40%).
Preparation of complexes of ethylenediamine-N,N0-
diacetate (EDDA)
The complexes were synthesized as per reported method
[24].
Ethylenediamine-N,N0-diacetato(diaquo)Co(II) hydrate
Physicochemical techniques
The reaction was carried out in a three-necked flask using
deaerated water. The flask was flushed with nitrogen
throughout the reaction. During filtration, nitrogen was
drawn through the filter having a sealed connection. These
measures did not completely prevent contact with air but
were sufficient to block oxidation of the cobalt(II) complex
as evidenced by darkening of the product when these
precautions were not taken. To a stirred slurry of 1.3 g of
CoCO3 (0.01 mol) in 30 mL of water, 1.76 g of H2EDDA
was added. The reaction vessel was heated (60 °C) until
evolution of CO2 was complete (10 min), and the mixture
was then filtered rapidly through a medium-fritted disk.
The solution was placed in a vacuum-desiccator which was
evacuated until a large crop of pink crystals formed. The
product was filtered and washed with two 5-mL portions
of ice-cold water, alcohol and acetone. After drying in
the filter with nitrogen flush, the yield of [Co(EDDA)
(H2O)2] ꢀ H2O was recorded as 0.272 g, 68%. The dried
product appeared to be stable in air.
The microanalyses of C, H and N were performed with a
CHN analyzer, Carlo Erba Model 1106. Metal ions and
residues were determined with an XRF-500 link system
England. Ultraviolet and visible absorption spectra of
complexes (4 9 10-2 M solution) were recorded with a
Cary Model 14 spectrophotometer in the range of 200–
900 nm using various solvents. The degradation products
(including gases) were identified through ATI Mattson,
Infinity Series FTIR, Model No. 9495, USA. The magnetic
susceptibility data on the complexes were determined by
the Gouy’s method at room temperature [25]. The ther-
moanalytical (TG-DTA-DTG) measurements were carried
out with NETZSCH simultaneous Thermal Analyzer STA
429. Samples were contained in an alumina crucible Al2O3
(8 mm dia 9 10 mm depth) with central base recess. The
crucible was then adjusted on palladium/ruthenium cruci-
ble support platform, which gave a proportional signal to
the recorder and computer interface to plot the mass-loss of
sample against temperature. Studies were made in static
atmosphere of air using 20–35 mg samples at a heating rate
of 10 °C/min. The activation energy (E°) and order of
reaction (n) were evaluated using the Horowitz–Metzger
method. A plot of Ln Ln Wo - Wf/W - Wf against h
resulted in a straight line. The activation energy was cal-
culated from its slope, which was equal to E°/RT2s, whereas
order of reaction was determined by using the relation
Ethylenediamine-N,N0-diacetato(diaquo)Ni(II)
To slurry of 1.76 g (0.01 mol) of H2EDDA in 40 mL of
deaerated water on a steam bath, were added 2.62 g
(0.01 mol) of NiCl2 ꢀ 6H2O and 0.8 g (0.2 mol) of NaOH
with stirring. The solution was evaporated to about 10 mL,
cooled and filtered. The crude product was washed with
ethanol, which weighed 1.30 g. This was dissolved in a
123

Pyrolytical characterization of transition metal complexes
717
Table 1 Kinetic parameters of
Complex
Temperature
range (°C)
Activation
energy
(E°, KJ/mol)
Order of
reaction
(n)
thermal decomposition of ligand
and metal complexes of
H2EDDA
H2EDDA
210–643
50–460
284–340
50–495
80–450
22.59
5.57
11.2
5.22
6.80
3
C6H16N2O7Co
C6H14N2O6Ni
C6H16N2O7Cu
C6H16N2O7Zn
1.5
2
1.5
0.5
Table 2 Analytical data of
Complex
Color
Pink
Carbon (%)
Found
Hydrogen (%)
Found
Nitrogen (%)
Found
EDDA complexes
C6H16N2O7Co
C6H14N2O6Ni
C6H16N2O7Cu
25.6 (25.1)a
26.7 (26.8)
24.8 (24.7)
29.8 (29.9)
5.87 (5.62)
5.15 (5.25)
5.81 (5.53)
4.90 (4.98)
9.61 (9.76)
10.6 (10.4)
9.75 (9.60)
11.5 (11.6)
Light blue
Dark blue
White
a
Calculated values are given in
parentheses
C6H16N2O7Zn
Table 3 Values of magnetic
Complex
leff
B.M.
m (kK)
Assignment
Oh
moment and electronic
absorption spectra for EDDA
complexes
4
C6H16N2O7Co
5.0
8.95 (6.0)
19.9 (11.7)
9.90 (12.6)b
13.2 (1.9)
16.4 (4.2)
27.2 (7.2)
14.9 (11.8)a
–
4T1g ? T2g
4
4T1g ? T2g(P)
3
C6H14N2O6Ni
3.21
3A2g ? T2g
1
? Eg
3
? T1g(F)
3
? T1g(P)
C6H16N2O7Cu
1.76
–
–
a
Molar extinction coefficient
C6H16N2O7Zn
Diamagnetic
b
Molar absorptivity (e)
between reaction order and concentration at maximum
slope (Table 1).
The close agreement (within 2%) between the observed
and the calculated values (Table 3) gives strong support for
the expected octahedral geometry of the complex [27]. The
electronic absorption spectrum of pink [Co(EDDA)
(H2O)2] ꢀ H2O is characteristic of those observed for Co(II)
octahedral complexes [28]. The band assignments are
awarded on the basis of Oh symmetry (Table 3) and the
effective magnetic moment, 5.0 BM, is typical of octahe-
dral cobalt(II) complexes [27]. The blue complex of Cu(II)
with EDDA exhibits single, symmetric absorption bands in
the red region (Table 3). The Cu(II)-EDDA complex
exhibits a leff of 1.76 BM. In the case of [Cu(EDDA)
(H2O)2] ꢀ H2O, the chelate structure tends to constrain the
acetate oxygen along the z2 orbital; or, alternately, if the
chelate were planar, the acetate would tend to be pulled
away from the metal ion due to the expected strain in the
framework analogous to that found for the in-plane acetate
of Co(EDDA)- [29] and the energy of the x2 - y2 orbital
would, therefore, be lowered. In case of complex,
Results and discussion
Composition and structure
The results of the elemental analyses are presented in
Table 2, which indicated the expected composition of the
complexes. The EDDA afforded a standard by which
deviations from an octahedral arrangement of ligand may
be determined. The data from electronic absorption spectra
of complexes are summarized in Table 3. The spectro-
chemical behavior of nickel complex is believed to reflect
the average coordination geometry. [Ni(EDDA)(H2O)2] is
demonstrated by the calculation of energies of the
3
3
3
3A2g ? T1g (F) and A2g ? T1g (p) transition from the
3
3A2g ? T2g transition which has an energy of 10Dq [26].
123

718
S. Rehman et al.
Fig. 3 Thermal curves of [Ni(EDDA)(H2O)2]
Fig. 1 Thermal curves of EDDA
Fig. 2 Thermal curves of [Co(EDDA)(H2O)2] ꢀ H2O
Fig. 4 Thermal curves of [Cu(EDDA)(H2O)2] ꢀ H2O
123

Pyrolytical characterization of transition metal complexes
719
[Zn(EDDA)(H2O)2] ꢀ H2O, conductance data indicate
presence of non-ionic species. The IR data show the
attachment through the same sites as those found for Ni, Cu
and Co complexes. Considering the analytical, IR and
conductance data, the octahedral geometry is assigned,
where EDDA is acting as a tetradentate ligand [24]. Water
molecules are the part of octahedral structure.
Thermal properties
The thermal decomposition curves (TG, DTG and DTA)
are given in Figs. 1, 2, 3, 4, 5, while the TG mass-loss data,
DTG and DTA peak temperatures are presented in Table 4.
The ligand (H2EDDA) shows a three-step pyrolysis. The
first stage starts around 210 °C and terminates at 258 °C.
For this stage DTG and DTA peaks appear at 220 °C and
228 °C, respectively. Two species evolve, i.e., carbon
monoxide (1 mole) and hydrogen (3 moles). This stage
accounts for 20% mass-loss. The intermediate, formed at
the completion of first step, resists further degradation (the
conjugated system exhibits resonance stabilization) to
some extent. The rate of slow decomposition from 258 °C
to 372 °C (5% mass-loss) confirms this observation. The
second stage comes to an end around 440 °C. The overall
mass-loss is 35%. One mole each of carbon dioxide and
Fig. 5 Thermal curves of [Zn(EDDA)(H2O)2] ꢀ H2O
Table 4 Thermoanalytical results for the ligand and complexes, [M(EDDA)(H2O)2]
Complex
TG, Temperature
range (°C)
Stage
Mass loss (%)
DTA
DTG
Evolved moiety
Calc.
Found
H2EDDA
210–258
258–440
440–643
50–200
I
19.3
35.2
45.4
7
20
35
45
(-)228
(?)419
(?)606
(-)80
220, 398, 600
CO, 3H2
CO2, H2O
2HCN, C2H2
H2O
II
III
I
C6H16N2O7Co
6.3
80
200–460
II
68
67.6
(-)212, (-)255,
(?)423
209, 238, 322,
390
3H2O, C2H2, HCN, H2,
CO, NH3
[460
Res.
25
26.1
78.2
21.8
52.1
–
–
CoO
C6H14N2O6Ni
C6H16N2O7Cu
284–340
[340
50–250
I
79.2
20.8
52
(-)324
341
CO2, H2O, C2H4, C2H6,
H2, N2 Ni
Res.
I
–
–
(?)90, (?)144,
(-)210, (?)238
108, 150, 217
3H2O, CO2, CO, C2H2
250–325
325–495
[495
II
10
11
27
6
9.9
10.6
27.3
6.1
(?)320
(?)445
–
315
440
–
H2, HCN
CH3NH2
CuO
III
Res.
I
C6H16N2O7Zn
80–140
140–180
180–315
315–450
[450
(?)105
(?)160
(?)182
(?)340
–
98
H2O
II
12
18
9.3
54.7
12.2
18.2
9.1
161
285
420
–
2H2O
III
IV
Res.
C2H2, CO
HCN,
54.4
ZnO3H7C2N
Res. residue, calc. calculated
(-) = Endothermic, (?) = Exothermic
123

720
S. Rehman et al.
water is given off. DTG peak is found at 398 °C while
DTA peak is noted at 419 °C. The intermediate produced
between second and final stage does not manifest any
stability and disintegrates to hydrogen cyanide (two moles)
and acetylene (one mole). The third stage ends at 643 °C
accompanied by 45% mass-loss. There is one DTG peak
(600 °C) and one DTA peak (606 °C) for this stage. The
whole degradation process leaves no residue. The pyrolysis
of the ligand proceeds in the following way:
52.1% ascribed to the elimination of 3 moles of H2O, one
mole each of acetylene, CO and CO2. This stage ends around
250 °C and is marked by three DTG peaks (108, 150,
217 °C) which suggest that although in TG there is only one
fall but the elimination of certain entities from the complex
occurs along with the formation of very unstable
intermediates. Four peaks for DTA in this region (50–
250 °C) support above-drawn conclusion. The second step
which commences at 250 °C and terminates at 325 °C,
shows a mass-loss of 9.9%. The mainframe of complex,
which broke down in the first step, experiences elimination
of H2 and HCN. Single peaks, each for DTG and DTA, are
found in this region. The third stage exhibits a mass-loss of
10.6% in the temperature zone of 325–495 °C. A mole of
methylamine breaks up from the already destroyed
framework of the complex. Single peaks for DTG (440 °C)
and DTA (445 °C) are observed for this stage. Above
495 °C, the residue is identified as CuO and was stable. The
equations for the degradation stages are as follows:
C6N2O4H12 ! CO " þ3H2 " þC5N2O3H6 I stage
C5N2O3H6 ! CO2 " þH2O " þC4N2H4 II stage
C4N2H4 ! 2HCN " þC2H2 " III stage
The complex, [Co(EDDA)(H2O)2] ꢀ H2O, starts to lose
mass around 50 °C. The first stage which accounts for 6.3%
mass-loss is attributed to the loss of water (outside the
coordination sphere) leaving behind the non-hydrated
complex. A single peak each for DTG and DTA in this
region (both at 80 °C) substantiates the TG results. The
remaining complex begins decomposing from 200 °C and up
to 460 °C the whole structure is destroyed. In this stage, many
small molecules (as a result of the pyrolysis) are released such
as H2O, C2H2, HCN, CO, NH3. For this step, three DTA
peaks (212, 255, 423 °C) and four DTG peaks (209, 238, 322
and 390 °C) are observed which clearly show that at least
three highly unstable intermediates are also formed during
this degradation stage. This stage accounts for 67.6% mass-
loss. The residue that is stable after 460 °C is identified as
CoO. The two-stage degradation is summarized as under:
C6H16N2O7Cu ! 3H2O " þCO2 " þCO " þC2H2 "
þ C2H8N2OCu I stage
C2H8N2OCu ! H2 " þHCN " þCH5NOCu II stage
CH5NOCu ! CH3NH2 " þCuO III stage
The decomposition for the complex, [Zn(EDDA)
(H2O)2] ꢀ H2O, commences around 80 °C and first step
accounts for 6.1% mass-loss which is attributed to the
elimination of H2O (outside coordination sphere). The
second stage starts around 140 °C and shows a mass-loss of
12.2%. Two moles of water detach and leave the octahedral
complex. Single peaks for DTA and DTG are observed (160
and 161 °C) as was the case for first step (105 and 98 °C,
respectively). The third step which starts from 180 °C and
endsat315 °C, exhibitsamass-lossof18.2% whichismarked
with the elimination of acetylene and carbon monoxide.
Single peaks for DTG and DTA are observed (285 and
182 °C, respectively). The fourth stage begins around 315 °C
and loss of 9.1% mass is observed. HCN is eliminated. As was
the case with above three stages, single peaks for DTA and
DTG were noticed (340 and 420 °C, respectively). The
residue was stable above 450 °C and it is evident that a part of
the overall framework of the complex remains intact. The
results obtained are in accordance with the literature [30–36].
The path of decomposition may be given as:
C6H16N2O7Co ! H2O " þC6H14N2O6Co I stage
C6H14N2O6Co ! CO2 " þ3H2O " þ2C2H2 "
þ HCN " þNH3 " þCoO (residue)
II stage
The complex, [Ni(EDDA)(H2O)2], starts losing mass
around 284 °C and shows a single-step decomposition. The
ligand framework splits in unusual fashion and gives out
O2, CO, CO2, ethylene, ethane, hydrogen, nitrogen and
water leaving behind nickel as residue. This stage exhibits
a mass-loss of 78.2%. This result is also supported by DTG
and DTA curves (single peaks). It is interesting to note that
in complexation with nickel, the ligand decomposes in a
single step, however, when degraded alone, it pyrolyzed in
three stages. The route, this complex follows, is:
C6H14N2O6Ni ! H2O " þCO2 " þCO " þC2H4 "
þ C2H6 " þH2 " þN2 " þO2 "
þ Ni (residue)I stage
C6H16N2O7Zn ! H2O " þC6H14N2O6Zn I stage
C6H14N2O6Zn ! 2H2O " þC6H10N2O4Zn II stage
C6H10N2O4Zn ! C3H8N2O3Zn þ C2H2 " þCO "
III stage
The complex, [Cu(EDDA)(H2O)2] ꢀ H2O, begins losing
mass around 50 °C. The first step shows a mass-loss of
123

Pyrolytical characterization of transition metal complexes
721
Ali (Sr. Tech. IAD, PINSTECH) are acknowledged for extending
technical assistance.
C3H8N2O3Zn ! HCN " þZnO3H7C2N ðResidue)
IV stage
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H2O but less stable than [Ni(EDDA)(H2O)2] when
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3. HCN gas was detected in the pyrolysis of [Zn(ED-
DA)(H2O)2] ꢀ H2O, [Cu(EDDA)(H2O)2] ꢀ H2O, and
[Co(EDDA)(H2O)2] ꢀ H2O whereas it could not be
found in the degradation of [Ni(EDDA)(H2O)2].
4. For complexes, Ni was the residue in case of
[Ni(EDDA)(H2O)2] whereas CuO and CoO were found
as residue for [Cu(EDDA)(H2O)2] ꢀ H2O and [Co(ED-
DA)(H2O)2] ꢀ H2O, respectively. In case of [Zn(ED-
DA)(H2O)2] ꢀ H2O, part of the main framework of the
complex remained intact as residue. For oxide residues,
oxygen was taken from degrading complexes.
5. The complexes show the following order of thermal
stability: [Cu(EDDA)(H2O)2] ꢀ H2O * [Co(EDDA)
(H2O)2] ꢀ H2O\[Zn(EDDA)(H2O)2] ꢀ H2O\[Ni(ED-
DA)(H2O)2].
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7. The ligand, despite being organic in nature, unfolds
remarkable stability when judged from the temperature
range of its complete degradation, i.e., 210–643 °C.
This may be attributed to the formation of one of the
intermediates which is considered to be resonance-
stabilized and seems to impart thermal stability as
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8. There was no residue in case of ligand.
9. The activation energies of complexes show that
[Ni(EDDA)(H2O)2] possesses the highiest thermal
stability whereas [Cu(EDDA)(H2O)2] ꢀ H2O is the
least thermally stable among these complexes.
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Acknowledgment The authors are indebted to PINSTECH, Nilore
Islamabad for providing the facility of thermonalytical techniques.
Messrs Nadeem Ahmad, Adeel Khattak (CD, PINSTECH) and Nehad
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