Preparation and thermal behaviour of divalent transition metal complexes of pyromellitic acid with hydrazine

Article abstract of DOI:10.1007/s10973-009-0433-5

Hydrazine forms two different types of complexes with divalent metal ions and pyromellitic acid (H₄pml) in aqueous medium: (i) hydrazinium complexes of formulae, (N₂H₅)₂M(pml) ?xH₂O, where x = 3 for M=Ni and x = 4 for M=Co or Zn, and (N₂H₅)₂Mn(H₂pml)₂, at pH 4.5, (ii) neutral hydrazine complexes with formulae, M₂(pml)(N ₂H₄) _n ?xH₂O where M=Co or Ni when n = 4 and x = 5 or 4 and M=Zn or Cd when n = 2, and x = 4 or 3 at pH 7, and M(H₂pml)(N₂H₄)?xH₂O where x = 4; M=Cu and x = 0; M=Hg, at pH 3, 7.5, respectively. All the complexes are insoluble in water, alcohol and ether. The N-N stretching frequency (990-1,007 cm^-1 for coordinated hydrazinium ion and 956-985 cm^-1 for bridged neutral hydrazine) indicates the nature of hydrazine present in the complexes. Simultaneously TG-DTA analysis indicates that hydrazinium complexes undergo dehydration and dehydrazination in a single step endothermally in the range of 289-300 °C whereas neutral hydrazine complexes undergo endothermic dehydration (~100 °C) followed by exothermic dehydrazination in the temperature range, 253-332 °C. The anhydrous metal carboxylates further decompose exothermally to leave the respective metal oxides or metal carbonates except zinc, which gives its oxalate as the end product. X-ray powder patterns indicate that even the complexes with the same formulation possess no isomorphism.

Full text of DOI:10.1007/s10973-009-0433-5

J Therm Anal Calorim (2010) 101:979–985
DOI 10.1007/s10973-009-0433-5
Preparation and thermal behaviour of divalent transition metal
complexes of pyromellitic acid with hydrazine
S. Vairam Æ T. Premkumar Æ S. Govindarajan
Received: 20 May 2009 / Accepted: 18 August 2009 / Published online: 15 September 2009
´
´
Ó Akademiai Kiado, Budapest, Hungary 2009
Abstract Hydrazine forms two different types of com-
plexes with divalent metal ions and pyromellitic acid
(H4pml) in aqueous medium: (i) hydrazinium complexes of
formulae, (N2H5)2M(pml)ꢀxH2O, where x = 3 for M=Ni
and x = 4 for M=Co or Zn, and (N2H5)2Mn(H2pml)2, at
pH 4.5, (ii) neutral hydrazine complexes with formulae,
M2(pml)(N2H4)nꢀxH2O where M=Co or Ni when n = 4 and
x = 5 or 4 and M=Zn or Cd when n = 2, and x = 4 or 3 at
pH 7, and M(H2pml)(N2H4)ꢀxH2O where x = 4; M=Cu and
x = 0; M=Hg, at pH 3, 7.5, respectively. All the complexes
are insoluble in water, alcohol and ether. The N–N
stretching frequency (990–1,007 cm-1 for coordinated
hydrazinium ion and 956–985 cm-1 for bridged neutral
hydrazine) indicates the nature of hydrazine present in the
complexes. Simultaneously TG-DTA analysis indicates
that hydrazinium complexes undergo dehydration and de-
hydrazination in a single step endothermally in the range of
289–300 °C whereas neutral hydrazine complexes undergo
endothermic dehydration (*100 °C) followed by exo-
thermic dehydrazination in the temperature range, 253–
332 °C. The anhydrous metal carboxylates further
decompose exothermally to leave the respective metal
oxides or metal carbonates except zinc, which gives its
oxalate as the end product. X-ray powder patterns indicate
that even the complexes with the same formulation possess
no isomorphism.
Introduction
A survey of literature indicates that the chemistry of pyro-
mellitic acid (1,2,4,5-benzene tetracarboxylic acid denoted
as H4pml), one of the major products of oxidation of coal,
has been extensively studied [1]. The crystal structure of its
dihydrate was investigated by Takusagawa et al. [2]. Owing
to the various ways of its coordination through carboxylate
ions, its use as a ligand is of recent interest [3]. Despite the
versatility in ligation of its anion, in some cases, for instance
in hexaaquacobaltate(II) dihydrogen pyromellitate, it
behaves so indifferently that it does not involve in coordi-
nation [4]. This is because of the hydrogen bonding held
between the carboxylate anions with water, which prevents
it in the participation of coordination. The salts of the acid
with monovalent ions such as Na?, K?, and NH4?, divalent
ions viz., Mn2?, Fe2?, Co2?, Cu2?, Zn2? and UO22? ions,
and trivalent ions, Al3? and Tl3? have been reported and
characterised by IR and Raman spectroscopy studies [5].
The study reveals that it behaves as mono-, di-, tri-, and
tetra-anions in their compounds. Lanthanide complexes with
pyromellitic acid have also been reported along with their
complexation thermodynamics [6, 7]. The luminescence
studies and absorption methods indicate that the pyro-
mellitate forms 1:1 lanthanide complexes with carboxylates
in the ortho position, in bidentate fashion. Recently a study
of thermochemical properties of lanthanide complexes with
pyromellitic acid has also been reported [8].
Keywords Hydrazine ꢀ Hydrazinium ligand ꢀ IR spectra ꢀ
Pyromellitic acid ꢀ Thermal analysis
Hydrazine, N2H4, a base with a lone pair of electrons
each on their nitrogen atoms act as a mono- or bi-dentate
bridging ligand, and its protonated (N2H5?) form serves as
a monodentate ligand. Several hydrazinium and hydrazine
complexes with transition/lanthanide metal carboxylates
are found in the literature [9–15]. Our research group has
long been involved in the study of coordination ability and
S. Vairam ꢀ T. Premkumar ꢀ S. Govindarajan (&)
Department of Chemistry, Bharathiar University,
Coimbatore 641 046, India
e-mail: drsgovind@yahoo.co.in
123

980
S. Vairam et al.
thermal stability of hydrazine in the presence of various
carboxylic acids with metals (complexation). We have
recently reported the thermal behaviour of various hydra-
zinium salts and metal hydrazine complexes of nitrogen
containing heterocyclic carboxylic acids such as 2-pyra-
zinecarboxylic acid, 2,3-pyrazinedicarboxylic acid [16],
4,5-imidazoledicarboxylic acid [17] and 3,5-pyrazoledi-
carboxylic acid [18]. In earlier reports, we have also
reported the preparation and thermal reactivity of hydra-
zinium 2,n-pyridinedicarboxylates (n = 3, 4, 5 and 6) [19]
and metal dipicolinate complexes of hydrazine [20].
Encouraged by these results obtained in our laboratory in
association with our extensive knowledge in thermal
behaviour of other metal carboxylate and/or metal hydra-
zine carboxylate system [21–25], we thought of extending
this study to understand the coordination ability and ther-
mal reactivity of hydrazine in the presence of aromatic
carboxylic acids, which contain higher number of carbox-
ylic acid groups, with divalent transition metals and also to
understand the thermal behaviour of these complexes. In
this study, we selected pyromellitic acid (1,2,4,5-benzene
tetracarboxylic acid), which has four carboxylic groups and
eight sites for potential coordination to a metal atom.
Though, a range of simple metal complexes of pyromellitic
acid have been reported [4–7], there appears to be no report
on transition metal hydrazine compounds of pyromellitic
acid and their thermal properties. Therefore, in this article
we report the coordinating properties of the acids and
thermal characterisation of their metal complexes.
beaker. A clear solution of pH 4.5 obtained was heated
over a waterbath at 80 °C for about 45 min. A micro-
crystalline solid product formed was allowed to stand for
about 1 h. The product was filtered, washed with ethyl
alcohol and diethyl ether, and dried over anhydrous CaCl2
in a desiccator.
(N2H5)2Mn(H2pml)2
The above procedure was followed with manganous ace-
tate, pyromellitic acid and hydrazine hydrate in the same
molar ratio to prepare the title compound which formed
only after 6 days.
Preparation of hydrazine complexes
M2pml(N2H4)nꢀxH2O where M=Co, Ni, Zn or Cd; n = 2
or 4; x = 3, 4 or 5
These complexes were prepared by adding metal nitrate
(e.g. 0.291 g of Co(NO3)2ꢀ6H2O in 10 mL of distilled
water), to an aqueous solution (50 mL) containing pyro-
mellitic acid (0.254 g) and 99–100% hydrazine hydrate
(3 g) in the molar ratio 1:1:6 at room temperature. A solid
product formed immediately in the reaction mixture at pH
7, was digested over waterbath at 80 °C for 10 min and the
resulting product was filtered, washed with distilled water,
ethyl alcohol and diethyl ether. The compound was dried
over CaCl2 in a desiccator.
O
C
O
C
CuH2pmlN2H4ꢀ4H2O
Similar synthetic procedure was followed to prepare this
complex by using metal nitrate, pyromellitic acid and
hydrazine hydrate with the molar ratio 1:1:3. On vigorous
stirring, a clear solution became turbid and a dark blue
solid appeared slowly. The solid products were digested
over a waterbath for about 10 min and were isolated and
washed as above.
HO
HO
OH
OH
C
O
C
O
1,2,4,5-benzene tetracarboxylic acid or Pyromellitic acid (H4pml)
Experimental
HgH2pmlN2H4
Preparation of hydrazinium complexes
An aqueous solution (80 mL) of the ligand containing
pyromellitic acid (0.508 g, 2 mmol) and hydrazine hydrate
(1 g, 2 mmol) was added carefully to an aqueous solution
(10 mL) of mercuric nitrate (0.333 g, 1 mmol) with vigo-
rous stirring (so as to avoid reduction to mercury). A dull
white precipitate formed immediately was digested for
about 2 h on a waterbath. The resulting solid products were
filtered, washed with absolute alcohol followed by diethyl
ether and dried over anhydrous CaCl2 in a desiccator.
(N2H5)2MpmlꢀxH2O where M=Co, Ni or Zn and x = 3 or 4
These complexes were prepared by adding the ligand
solution, which was obtained by mixing an aqueous solu-
tion of pyromellitic acid (0.508 g, 2 mmol in 40 mL of
distilled water) and 99–100% hydrazine hydrate (0.3 g,
6 mmol) to a metal nitrate solution (e.g. 0.291 g of
Co(NO3)2ꢀ6H2O, 1 mmol in 10 mL of distilled water) in a
123

Preparation and thermal behaviour of divalent transition metal complexes
Physicochemical techniques
981
The stoichiometry of the complexes was fixed by chemical
analysis. Hydrazine content was determined by titrating
against standard KIO3 (0.025 M) under Andrews condi-
tions [26]. Metal contents were determined by titrating
with 0.01 M EDTA [26], after decomposing the com-
plexes with concentrated HNO3. Mercury was estimated
gravimetrically by precipitating HgS. IR spectral data of
the complexes were recorded as KBr pellets using a Perkin
Elmer 597 spectrometer. Electronic spectra in the UV–
visible region were obtained using a Hitachi Perkin Elmer
20/200 recording spectrophotometer by dispersing the
solid samples in Nujol. The magnetic susceptibility of the
complexes was measured (298 K) using a vibrating sample
magnetometer (VSM E.G&G model 155) in the magnetic
field from 2 to 10 KG and appropriate corrections were
also made. ESR spectrum was recorded at room temper-
ature using a Jeol 100 TES spectrometer. The simulta-
neous TG-DTA experiments were carried out in air using
STA 1500 thermal analyser and NETZSCH-Geratebeau
GmbH thermal analyser at the heating rate of 10 °C/min.
Platinum cups were employed as sample holders and
alumina as reference. The temperature was ambient to
700 °C.
Results and discussion
All the complexes obtained are polycrystalline solids,
which are stable in air and insensitive to light. They are
insoluble in water and in organic solvents like ethanol,
acetone and chloroform. The analytical data of the com-
plexes are compatible with the proposed compositions
(Table 1).
Electronic and ESR spectra
The cobalt complexes of pyromellitic acid register a band
4
in the range of 19,800–20,835 cm-1, due to T1g(F) ?
4
4
4A2g(F) and T1g(F) ? T1g(P) transitions of typical six
coordinated Co(II) complexes [27]. The corresponding
nickel complexes show two bands in the regions 10,560–
10,822 and 17,212–17,730 cm-1 which are ascribable to
3
3
4A2g ? T1g, T1g(p) transitions, respectively, of octahe-
dral Ni(II) complexes. The Cu complex shows absorptions
at 21,354 (intense), 17,721(weak) and 12,136 (weak) cm-1
corresponding to the excitation of electrons, zx ? x2 - y2,
zx ? xy and zx ? z2 orbitals, respectively, implying the
complex in square planar geometry. The magnetic
moments of Ni and Co complexes lie in the range of 3.2–
3.3 and 4.9–5.1 BM, respectively.
123

982
S. Vairam et al.
resultant endotherm with low intensity around 300 °C,
followed by a strong exothermic peak in the range of 475–
535 °C (483 °C for Co, 474 °C for Ni and 535 °C for Zn).
The intermediates formed during decomposition of all
these complexes are anhydrous metal dihydrogencarboxy-
lates (shown in Fig. 2a–c as representative examples). This
is understood on comparison with the TG-DTA curve
(Fig. 2a) of nickel hydrogencarboxylate. The final products
are the corresponding metal carbonates except nickel and
zinc, which leave mixture of carbonates and oxides and
oxalate as the end products, respectively.
200
300
400
100
Magnetic field/mT
Fig. 1 ESR spectrum of Cu(H2pml)(N2H4)ꢀ4H2O
(N2H5)2Mn(H2pml)2
The axial ESR spectrum for copper complex shown in
Fig. 1 has features at g = g\ = 2.11 which clearly indi-
k
cates that the Cu(II) ion is in isotropic environment.
This complex undergoes two-step decomposition. First step
mass loss is dehydration from the carboxyl groups forming
anhydride followed by a continuous dehydrazination
between 250 and 311 °C showing successive endothermic
peaks at 250 and 303 °C forming the manganese pyrom-
ellitate intermediate as in the previous case. In the second
step, the remaining manganese pyromellitate intermediate
undergoes sharp exothermic decomposition at 483 °C to
give manganese carbonate as the final residue.
IR spectra
The important IR absorption frequencies of the compounds
are given in Table 1. The hydrazinium complexes of
cobalt, nickel, manganese and zinc display a N–N
stretching frequency [28] in the range of 1,008–991 cm-1
and the hydrazine complexes show in the range of 985–
946 cm-1, corroborating the presence of N2H5? and N2H4,
respectively. All the hydrated complexes display a broad
peak in the region of 3,600–3,400 cm-1 indicating the O–H
stretching band of water. All the complexes show absorp-
tion in the range of 1,664–1,560 cm-1 corresponding to
C=O asymmetric and 1,370–1,352 cm-1 corresponding to
C=O symmetric, difference being [200 cm-1 unequivo-
cally revealing monodental coordination of carboxylate
group [29]. The complexes of manganese, copper and
mercury show absorption at 1,680–1,707 cm-1 indicating
the presence of free carboxyl group in them.
M2pml(N2H4)nꢀxH2O where M=Co, Ni, Zn or Cd; n = 2
or 4; x = 3, 4 or 5: the DTA curves show endothermic peaks
in the range of 100–130 °C corresponding to the first stage
mass loss as shown by TG curves. This mass loss is equiv-
alent to the mass of corresponding water molecules present in
the complexes. The values of observed mass loss are com-
parable with the calculated values (Table 2). The high tem-
perature elimination of water molecules indicates that lattice
water molecules are strongly held by hydrogen bonding.
The second stage of decomposition occurs in the range
of 160–375 °C showing broad exothermic peaks in the
range of 240–332 °C (240 °C for Co, 253 °C for Ni,
286 °C for Zn and 332 °C for Cd) due to the loss of
hydrazine molecules. This exothermic elimination implies
that hydrazine molecules are involved in bridging coordi-
nation [9–11]. The intermediates are the corresponding
pyromellitate for cobalt, zinc and cadmium, and oxalate for
nickel complex. Finally, all the complexes undergo com-
plete exothermic decomposition in the range of 400–
500 °C to form their stable oxides except cadmium which
gives its carbonate as the final products.
Thermal studies
The thermal data of the complexes are listed in Table 2.
The compositions of the intermediates and the final prod-
ucts are those which best fit with the observed mass losses
in the TG studies. Thermogravimetric results are in good
agreement with the corresponding DTA data.
(N2H5)2MpmlꢀxH2O where M=Co, Ni or Zn
and x = 3 or 4
CuH2pmlN2H4ꢀ4H2O
These complexes are stable up to 260 °C and lose their
water and N2H4 molecules after 270 °C. The high tem-
perature dehydration may be due to strong hydrogen
bonding between carboxylate groups and water. Exother-
mic dehydrazination and endothermic dehydration merge
with each other in the range of 270–485 °C showing the
This complex shows two-step decomposition, where first
step is dehydration of a molecule of water showing endo-
thermic peak at 100 °C. The second step is the elimination
of water and hydrazine molecule showing exothermic peak
at 269 °C resulting in the formation of CuCO3 as the
residue.
123

Preparation and thermal behaviour of divalent transition metal complexes
983
Table 2 Thermal data
Complex
DTA
Thermogravimetry (TG)
Decomposition products
Temp./°C
Temp. range/°C
Mass loss/%
Obs.
Calc.
(N2H5)2Co(pml)ꢀ4H2O
(N2H5)2Ni(pml)ꢀ3H2O
(N2H5)2Zn(pml)ꢀ4H2O
(N2H5)2Mn(H2pml)2
289(?)
483(-)
300(?)
474(-)
296(?)
535(-)
250(-)
303(-)
483(-)
100(?)
150(?)
240(-)
399(-)
113(?)
253(-)
407(-)
127(?)
286(-)
505(-)
110(?)
332(-)
471(-)
100(?)
269(-)
312(-)
195(?)
242(-)
530(-)
570(-)
79(?)
270–370
370–500
284–380
380–700
262–484
484–700
250–311
23.5
71.6
27.5
77.6
22.5
66.0
16.0
23.3
73.4
27.5
77.4
22.9
66.2
16.1
(N2H5)Co(Hpml)
CoCO3
Ni(H2pml)
1/2 NiCO3 ? 1/2 NiO
(N2H5)Zn(Hpml)
ZnC2O4
Mn(C6H2 (COOH)(CO)2O(COO))2
311–500
60–175
81.0
15.2
81.5
15.4
MnCO3
Co2(pml)(N2H4)4ꢀ5H2O
Co2(pml)(N2H4)4
175–375
375–650
60–160
35.0
69.0
13.0
46.5
74.7
14.0
26.0
68.0
8.9
37.2
72.6
12.7
48.3
73.7
13.9
26.3
68.5
9.1
Co2(pml)
Co3O4
Ni2(pml)(N2H4)4ꢀ4H2O
Zn2(pml)(N2H4)2ꢀ4H2O
Cd2(pml)(N2H4)2ꢀ3H2O
Cu(H2pml)(N2H4)ꢀ4H2O
Hg(H2pml)(N2H4)
Ni2(pml)(N2H4)4
NiC2O4
160–318
318–600
60–208
NiO
Zn2(pml)(N2H4)2
Zn2pml
208–347
347–600
80–120
ZnO
Cd2(pml)(N2H4)2
Cd2(pml)(N2H4)
CdCO3
120–430
430–650
80–300
14.4
47.0
27.0
14.5
45.1
24.8
Cu(H2pml)
300–340
200–350
72.0
58.5
70.5
58.2
CuCO3
Mercury
350–600
52–330
85.0
2.8
–
Volatilisation and phase changes
Hg[Hgpml]
Hg[Hgpml]ꢀH2O
2.7
–
152(?)
383(-)
560(-)
330–600
82.6
Volatilisation and phase changes
HgH2pmlN2H4
complex decomposes in the low temperature (242 °C) due
to the presence of hydrazine (Fig. 2d, e).
The curve of this complex reveals that it melts and
decomposes to metallic mercury showing endothermic
peaks at 195 and 242 °C in DTA. Subsequently it further
volatilizes accompanying phase changes showing exo-
therms at 530 and 578 °C to give 15% residue. This is quite
similar to the decomposition of oxalato complex of mer-
cury [30]. Further, while comparing the thermal decom-
position of the pyromellitate complex of mercury,
Hg[Hgpml]ꢀH2O, which decomposes exothermally at
383 °C after its dehydration. It is found that the hydrazine
Our effort to isolate the intermediates was unsuccessful
due to their continuous decomposition as evident from the
TG. Hence, we have tried to assign the possible interme-
diates as observed from the TG mass losses, which are in
agreement with the calculated mass losses.
X-ray diffraction studies
The X-ray powder patterns indicate that no isomorphism
exists among the complexes. However, the hydrazinium
123

984
S. Vairam et al.
Fig. 2 Simultaneous TG-DTA
of a Ni(H2pml)ꢀ12H2O,
b (N2H5)2Nipmlꢀ3H2O,
c Ni2pml(N2H4)4ꢀ4H2O,
d Hg[Hgpml]ꢀH2O and
e Hg(H2pml)ꢀN2H4
a
b
Exo
ΔT
120
80
40
0
Exo
ΔT
TG
100
80
60
40
20
120
80
40
0
100
80
60
40
20
TG
Endo
Endo
DTA
DTA
100
–40
800
700
0
400
500 600
0
200
400
600
800
200 300
Temperature/°C
Temperature/°C
d110
100
90
25
20
Exo
TG
Exo
c 100
TG
ΔT
ΔT
80
80
60
15
10
5
Endo
Endo
70
60
50
DTA
40
40
DTA
30
0
20
200
400
600
10
–5
Temperature/°C
0
600
100 200 300 400 500
Temperature/°C
700 800
110
Exo 16
14
e
TG
100
90
80
70
60
50
40
30
20
10
ΔT
12
10
Endo
8
6
4
2
0
–2
DTA
0
100 200
300
500
600 700
400
Temperature/°C
800
complexes having the similar formulation, (N2H5)2M(pml)ꢀ
xH2O, seem to possess similarity to some extent in their
diffractograms.
electronic spectra, magnetic moment and thermal studies
of the complexes suggest that the Mn, Co, Ni and Zn
(hydrazinium) complexes are of octahedral geometry with
coordination number 6, and Cu is of square planar
geometry, and Zn (hydrazine), Cd and Hg of tetrahedral
geometry with coordination number 4.
Conclusions
The reaction of metal nitrate with hydrazine in the presence
of pyromellitic acid form hydrazinium complexes at pH 4
and neutral hydrazine complexes at around 7. Pyromellitate
behaves as tetra anion in the case of Co, Ni, Zn and Cd
complexes as well as dianion in the case of Cu and Hg. In
the hydrazine complexes, hydrazine acts as neutral,
bridging ligand for zinc, cadmium and mercury, and mono
dentate ligand for copper. The insoluble nature of all
complexes suggests a polymeric structure via pyromellitate
and hydrazine moieties.
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