Synthesis, characterization and thermal degradation of 1-D coordination polymers of the type CuxZn1-x(dadb)·yH2O (dadb = 2,5-diamino-3,6-dichloro-1,4-benzoquinone; And x = 1.0, 0.5, 0.0625 and 0)

Article abstract of DOI:10.1016/j.tca.2010.07.019

New complexes of the type [{M_xM′_1-x(dadb)} _n·yH₂O] {where dadbH₂ = 2,5-diamino-3,6-dichloro-1,4-benzoquinone (1); M = Cu(II); M′ = Zn(II); x = 1 (2), 0.5 (4), 0.0625 (5), 0 (3); y = 0-2 and n = degree

Full text of DOI:10.1016/j.tca.2010.07.019

Thermochimica Acta 511 (2010) 17–26
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Thermochimica Acta
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Synthesis, characterization and thermal degradation of 1-D coordination
polymers of the type CuxZn1−x(dadb)·yH2O (dadb = 2,5-diamino-3,6-
dichloro-1,4-benzoquinone; and x = 1.0, 0.5, 0.0625 and 0)
R.L. Prasad∗, Anita Kushwaha, Deepshikha Singh
Department of Chemistry, Faculty of Science, BHU, Varanasi 221005, India
a r t i c l e i n f o
a b s t r a c t
New complexes of the type [{MxMꢀ1−x(dadb)}n·yH2O] {where dadbH2 = 2,5-diamino-3,6-dichloro-1,4-
benzoquinone (1); M = Cu(II); Mꢀ = Zn(II); x = 1 (2), 0.5 (4), 0.0625 (5), 0 (3); y = 0–2 and n = degree of
polymerization} were synthesized and characterized. Distinct identities of heterobimetallic complexes
are revealed from nature and position of their PXRD lines. Complexes under present study are hygro-
scopic in the temperature range RT–60 ◦C. The TGA, DTA and DSC thermograms suggest that complexes
first lose water molecules followed by loss of HCl molecules under 2nd step of thermal degrada-
tion which is catalyzed by Cu(II) ions, whereas elimination of CO under 3rd step is catalyzed by air.
Residue from 2 and 3 corresponds to 1/2 mole of metallic copper and zinc, respectively, under nitrogen
atmosphere, whereas, one mole of CuO and half mole of ZnO, respectively under air. Thermal degra-
dation pattern of the ligand moiety around Cu(II) have been observed to be different than that around
Zn(II) ion.
Article history:
Received 16 April 2010
Received in revised form 6 July 2010
Accepted 20 July 2010
Available online 27 July 2010
Keywords:
1-D coordination polymer
Metal complexes of 2,5-diamino-3,6-
dichloro-1,4-benzoquinone
Heterobimetallic complexes
Thermal analysis of 1-D polymers
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
cation as good conducting material as metallic/semiconducting
materials [8]. Therefore, study of thermal stability of such useful
The dadb molecule is reported to possess two paral-
lel conjugated strands (Fig. 1a) across the molecule [1–3].
Supramolecular structure (Fig. 1b) of the 2,5-diamino-3,6-dichloro-
1,4-benzoquinone (dadb) molecule [4] shows one dimensional
stacking through ␲–␲ interaction between ring carbon atoms of
adjacent molecules and exhibits metallic conductivity. Due to pres-
ence of donor atoms at each end of conjugated strands of the dadb
molecule, it is expected to form 1-D coordination polymer like that
of CA (chloranilic acid) [5] (Fig. 1c). Two metal ions coordinated
simultaneously to same conjugated dadb ligand are expected to
be in electronic communication with each other like Creutz-Taube
ion [6,7] (Fig. 1d). The extent of electronic communication depends
upon nature of metal ion and intermittent ligand [7]. During elec-
tronic communication between two metal ion, electron (charge
densities) will pass through intermittent coordinated ligand, i.e.
electron of metal ion will be delocalized on ligand. Consequently,
charge density on various atoms of the coordinated ligand may
vary upon variation of adjacent metal ion coordinated to ligand
in 1-D chain. Thus, coordination polymer of transition metal ions
with ␲-conjugated ligand dadb is expected to exhibit their appli-
materials is foremost important and is undertaken. Further, thin
film deposition (MOCVD) of metal and metal oxides have been
achieved by pyrolytic cleavage of metal complexes for their applica-
tion in electronic industries [9]. Metal/metal oxides formed under
different environment exhibit different characteristics, therefore,
thermal studies of the new title metal complexes were under
taken to predict the pyrolytic product which may be useful in
future as gas sensors [10], optical switch [11], magnetic stor-
age media [12], lithium batteries [13], solar cells [14] owing to
its photoconductive and photochemical properties and catalyst
[15].
Electronic environment on the ligand atoms changes by varying
the adjacent coordinated metal ion in 1-D coordination polymer,
therefore, it may be expected that the thermal degradation of the
ligand moiety of the complexes should be influenced by changing
the coordinated metal ion. This may change morphology, texture
and composition of useful pyrolytic product hence its properties
[16,17]. In view of above, present investigation was under taken
to study the thermal stability, degradation pattern, final pyrolytic
product and observe variation of degradation pattern and final
residue of heterobimetallic complexes w.r.t. their monometallic
complexes. Further, the reports on metal complexes of dadb are
lacking except one study on thermovolumetric [18] measurement
of cobalt(II), iron(II) and manganese(II) complexes of dadb under
∗
Corresponding author. Tel.: +91 542 6702487; fax: +91 542 2368127.
E-mail address: rlpjc@yahoo.co.in (R.L. Prasad).
0040-6031/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2010.07.019

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R.L. Prasad et al. / Thermochimica Acta 511 (2010) 17–26
Fig. 1. (a) Optimized molecular structure of 1. (b) 1-D stacking via ␲–␲ interactions. (c) Structure of [Cu(CA)]n. (d) Creutz-Taube ion.
nitrogen atmosphere upto 300 ◦C. In present communication syn-
thesis, characterization and thermal degradation pattern of 1-D
coordination polymers of the type CuxZn1−x(dadb) (x = 1.0, 0.5,
0.0625 and 0) is being reported.
2.2.1. Metal complexes of the type [M(dadb)]n·xH2O [M = Cu2+
(2), Zn2+ (3)]
To a 0.414 g dadb (2 mmol) and 0.224 g KOH (4 mmol) dissolved
in 40 ml 50% ethanolic solution, a solution of metal salt (2 mmol)
dissolved in 15 ml distilled water was added slowly drop wise
with constant stirring in 30 min and continued stirring for further
5 h. Reaction mixture was heated for 30 min over water bath and
allowed to stand at room temperature for 2 h. The precipitated
complexes were filtered washed with distilled water thrice. The
precipitated complexes were purified by stirring for one hr in a
solvent mixture consist of 25% ethanol, 25% acetone, 5% DMF and
45% distilled water and filtered, washed with distilled water thrice
followed by ethanol and dried under vacuo over anhydrous CaCl2.
2. Experimental
2.1. Physico-chemical measurements
The elemental analysis (Carbon, hydrogen and nitrogen) was
performed on a Elemental Analyzer model Carlo Erba 1108. Mag-
netic susceptibility of the powdered samples was measured at room
temperature on a Cahn-Faraday electro balance using [CoHg(NCS)4]
as calibrant. The melting points of the complexes were determined
in open capillaries using Gallenkamp apparatus and are uncor-
rected. Electronic absorption spectra were recorded on a UV-1700
PHARMA SPEC UV–visible spectrophotometer in solid state as Nujol
mulls. IR spectra on KBr disc were recorded on a VARIAN 3100 FT-IR
spectrophotometer in the region 4000–400 cm−1. The X-ray pow-
der diffraction patterns were recorded on X-ray diffraction SEIFERT
RICH. SEIFERT and Co. Gmbh and Co. KG D-2070 Ahrensburg using
Cu K␣-radiation and Rigaku D. Max-B Powder X-ray diffractometer
with Cu K␣-radiation. The thermogravimetric analysis curves (TGA,
DTG and DTA) were recorded on Perkin Elmer, Diamond TG/DTA
and NETZSCH STA 409 C/CD. DSC thermograms were recorded on
Mettler Toledo TC 15 TA differential scanning calorimeter at rate of
5 ◦C/min under nitrogen atmosphere using Spec. pure grade indium
as standard by taking samples in closed lid aluminium pan. The vari-
able temperature electrical conductivity of the metal complexes
was measured using conventional two-probe technique on a Keith-
ley 236 source measure unit.
2.2.2. Synthesis of heterobimetallic complexes
2.2.2.1. Cu0.5Zn0.5(dadb)·xH2O (4). The heterobimetallic complex
Cu0.5Zn0.5(dadb)·xH2O was synthesized by adding a 10 ml homo-
geneous aqueous solution of ZnSO4·7H2O (0.287 g, 1 mmol) and
CuSO4·5H2O (0.250 g, 1 mmol) into a pink colored 50% ethano-
lic solution (40 ml) of dadb (0.414 g, 2 mmol) and KOH (0.224 g,
4 mmol) drop wise with constant stirring over a period of 1/2 h. The
reaction mixture turned to intense dark color solution. The reaction
mixture was further stirred for 5 h followed by digestion over water
bath for 30 min and cooled to room temperature. Subsequently
same procedures were followed as in Section 2.2.1.
2.2.2.2. Cu0.0625Zn0.9375(dadb)·xH2O (5). The heterobimetallic com-
plex Cu0.0625Zn0.9375dadb was synthesized by adding an aque-
ous solution of ZnSO4·7H2O (0.539 g, 1.88 mmol) + CuSO4·5H2O
(0.031 g, 0.12 mmol) into a pink colored 50% ethanolic solution
(40 ml) of dadb (0.414 g, 2 mmol) and KOH (0.224 g, 4 mmol) drop
wise with constant stirring over a period of 1/2 h. Rest procedure
was same as described in Section 2.2.1.
2.2. Synthesis of metal complexes
3. Results and discussion
All the chemicals used under present investigations were of
analytical reagent grade. Solvent were purified and dried prior to
the use by standard methods [19]. The ligand 2,5-diamino-3,6-
dichloro-1,4-benzoquinone (1) was prepared following literature
procedure [18,20].
Analytical data agree with composition of the complexes given
in Table 1. The solid complexes are stable in presence of air but
weakly hygroscopic. Complexes dried over CaCl2 under vacuo gain
some weight on exposure to air and extent of weight gain depends

R.L. Prasad et al. / Thermochimica Acta 511 (2010) 17–26
19
Table 1
Analytical data and physical properties of the complexes.
Compound
Compound no.
Color
Yield (%)
C
H
N
M.P.a (◦C)
>280
Cudadb·2H2O
Blackish green
Gray
65
84
66
72
24.24 (23.65)
22.95 (23.57)
23.93 (23.51)
25.54 (24.97)
1.97 (1.97)
1.56 (1.96)
1.73 (1.96)
1.78 (1.38)
8.63 (9.19)
8.74 (9.17)
8.70 (9.14)
9.24 (9.71)
2
Cu0.5Zn0.5dadb·2H2O
275–278a
277–280a
>300
4
Cu0.0625Zn0.9375dadb·2H2O
Gray
5
Zndadb·H2O
Dark pink
3
a
Decomposition temperature.
Table 2
X-ray powder diffraction data of the complexes.
Compound
Position of intense and distinct peaks (2Â)
I
II
III
IV
V
1
2
4
5
3
12.75
14.16
13.52
13.48
25.11, 26.59
25.44
–
24.85sh
23.84
27.46, 28.93, 29.80, 31.69, 45.53, 56.39, 66.38, 73.32
18.96
19.52
19.64
18.26
22.48
22.96
22.32
21.96
27.8
27.56
27.16
–
12.0, 13.84
sh = shoulder.
upon relative humidity present in air. They decompose in the tem-
perature range 275–300 ◦C. All the metal complexes are insoluble
in common organic solvents such as ethanol, methanol, acetone,
chloroform, dichloromethane, ether and benzene as well as highly
polar solvent like DMF and DMSO.
[5,23,24] nature of these complexes. Distinguished peaks present
in the PXRD spectrum of each of the complexes 2–5 are divided into
five major ranges (I–V) (Table 2 and Fig. 2). There are two peaks in
the diffractogram of 3 in between 2Â values 12◦ and 14◦ whereas
other complexes 2, 4 and 5 exhibit only one peak in this region
which is most intense (I). For the heterobimetallic complexes 4 and
5, position of the most intense peak falls between corresponding
peak positions for the monometallic complexes 2 and 3. Second
intense peak (II) of heterobimetallic complexes 4 and 5 are observed
as broad and at higher 2Â values than those of corresponding peak
in the monometallic complexes 2 and 3. Third intense peak (III)
of the complex 4 is also obtained at higher 2Â value than those of
complexes 2 and 3. However, this peak of complex 5 is observed
at slightly lower 2Â value than that of 2 but higher than that of 3.
Position of fourth intense peak (IV) in 5 lies between those of 2 and
3 as shoulder whereas corresponding peak is absent in complex
4. Position of fifth peak in the heterobimetallic complexes 4 and
5 are very close to that of 2. The peak corresponding to fifth peak
between 27◦ and 28◦ in the complex 3 is very weak and is not well
3.1. IR spectra
The spectrum of the ligand dadb in KBr exhibits two band at
3308 and 3376 cm−1 due to ꢀs(NH2)opc and ꢀas(NH2)opc modes
of the NH2 groups [1]. The other characteristic ligand bands at
1667, 1616 and 1580 cm−1 are assigned to ꢀ(C O), ꢀ(C C ring)
and ␤NH2 modes of vibrations, respectively [1]. The metal com-
plexes of the dadb exhibit ꢀ(OH) stretching frequencies as a broad
band in the range 3421–3470 cm−1 which is absent in the parent
ligand. Presence of ꢀ(OH) stretching frequencies in the metal com-
plexes indicates the presence of water molecules in the complexes.
Metal complexes exhibit only one band corresponding to ꢀas(N H)
stretching vibration. This indicates that one of the hydrogen atoms
of NH2 groups is absent due to deprotonation. The decrease of the
wave number value for the NH stretching vibration of the metal
complexes as compared to that of the ligand dadb suggests that
nitrogen atom of the NH2 group is involved in coordination. Asym-
metric (C O) stretching vibration mode of the metal complexes
are observed in the range 1600–1647 cm−1 as medium intensity
band either as shoulder or as distinct peak, whereas, this peak is
observed as distinct peak at 1667 cm−1 in the ligand dadb. Thus
lowering of ꢀas(C O) frequency in the metal complexes indicate
coordinated nature of carbonyl group [21,22]. The ꢀ(C C) and ␤NH2
mode of vibrations of the ligand merge and yield a single broad band
into their metal complexes except in case of the Cudadb where
ꢀ(C C) is observed as distinct shoulder at 1542 and ␤NH2 mode at
1494 cm−1. Shifting of these bands to lower wave number in their
metal complexes as compare to that found in the free dadb ligand
support coordinated nature of the dadb ligand.
3.2. Powder XRD spectra
The powder XRD spectra of complexes recorded between 2Â
values 10◦ and 80◦ exhibit many lines suggesting the crystalline
Fig. 2. Powder X-ray diffraction pattern of the complexes 2, 3,4 and 5.

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R.L. Prasad et al. / Thermochimica Acta 511 (2010) 17–26
tentative thermal degradation pattern proposed for the metal com-
plexes under nitrogen are presented in Scheme 1(A) and (B), and
under air in Scheme 2(A) and (B).
Thomas Allmendinger [18] studied cobalt(II), iron(II) and man-
ganese(II) complexes of dadb thermo-volumetrically by heating
complexes up to 300 ◦C under nitrogen atmosphere and found that
quantitative amount of HCl gas is lost from cobalt and manganese
complexes but smaller amount of HCl is lost from iron complexes.
It was concluded that part of the chloride is consumed in com-
plexation with iron metal ions. Thus, it is obvious that there are
two possibilities during thermal degradation of metal complexes
of dadb: (i) all the liberated HCl molecules escape or (ii) one part
of the liberated HCl molecules react with metal ions to form metal
chlorides, liberate hydrogen and ligand residue while remaining
part of HCl escapes. Based on possibility (i) the Schemes 1A and
2A have been proposed for thermal degradation of metal com-
plexes under nitrogen and air atmospheres, respectively. Similarly
Schemes 1B and 2B have been proposed for thermal degradation
of metal complexes under nitrogen and air atmospheres on the
basis of possibility (ii). The weight losses calculated for various
steps of the thermal degradation of 2 under nitrogen atmosphere
on the basis of Scheme 1A and B are presented in S.I. Table 1(a).
Experimentally obtained weight losses from TGA curve with help
of corresponding DTG curve are also included in the table along
with calculated values (in parenthesis).
Fig. 3. UV–visible absorption spectra of the compounds 1–5.
marked. Thus, the nature and positions of X-ray diffraction peaks
of 4 and 5 are entirely different from those observed for 2 and 3.
These results indicate that the complexes 4 and 5 are new entities
rather than mixtures of 2 and 3.
First stage of weight loss corresponds to the loss of water
molecule under air as well as nitrogen atmosphere for all the com-
plexes. Loss of water molecules take place in the temperature range
RT–200 ◦C in most of the complexes and are associated with one or
two peaks in its DTG curve and corresponding endothermic peaks
in DTA thermogram of all the complexes. It is worth to be men-
tioned that the sample dried under vacuum gains some weight on
exposure to air by absorbing the water molecules. Further, presence
of exothermic peaks in the DTA curve for some of the complexes in
the temperature range RT–60 ◦C indicates absorption of moisture
by the samples on exposure to air. The amounts of water molecules
absorbed vary upon the variation of relative humidity present in the
air. Both the DTA and DSC curves for 3 (Fig. 4g) exhibit endothermic
curve corresponding to water loss in temperature range RT–200 ◦C.
In case of 2 the broad endothermic peak is present in its DTA
curve, however, DSC curve recorded for the sample of 2 (Fig. 4g)
dried under vacuo over CaCl2 exhibits no peak in the tempera-
ture range 40–200 ◦C indicating the presence of non-coordinated
nature of water molecules. Therefore, it may be concluded that
water molecules present on/near to surface of sample are lost at
lower temperature and as the complexes are polymeric therefore,
crystalline water molecules trapped inside the polymeric matrix
are lost at higher temperatures. The DTG and DTA peaks observed
above 100 ◦C are probably due to delayed elimination of trapped
crystalline water molecules.
In second step of thermal degradation two HCl molecules are
expected to be lost under Scheme 1A, whereas under Scheme 1B it
is assumed that one out of two moles of HCl molecules are liberated
on heating the complex 2, i.e. one mole of HCl molecules react with
copper ions to form half mole of CuCl2 and hydrogen gas molecules
and other half of the HCl molecules (one mole) escapes [18]. Due
to above reaction, half of ligand fragments are expected to remain
bonded with metal, whereas other half of the ligand moiety become
free from metal. The free ligand fragment obtained is expected to
lose 1/2 moles of (CN)2 and H2 molecules. The net weight loss under
Scheme 1A and B are calculated to be 37.63% and 34.60%, respec-
tively from 2 under nitrogen atmosphere. Calculated weight loss
following Scheme 1B from 2 falls on the curvature of the DTG peak
suggesting that Scheme 1B being followed for degradation of this
complex is unlikely. However, the observed weight loss 37% from
TGA curve coinciding with slowest rate of weight loss in the DTG
3.3. Magnetic moments and electronic spectra
Magnetic and solid state electronic spectra of the ligand 1 and
metal complexes 2–5 are given in Fig. 3 and Table 3. The ꢁeff value
of complex 2 is slightly less than that expected from one unpaired
electron probably due to antiferromagnetic interaction between
neighbouring copper centres [5]. Other heterobimetallic complexes
show magnetic moment corresponding to one unpaired electron
[25,26]. Zinc metal complex 3 is diamagnetic. On the basis of nature
and position of the peaks a square-planar geometry may be tenta-
tively proposed of the complex 2 [26,27]. Due to similarity of peak
positions of first and second peaks of 4 and 5 with those of 2 a
square-planar structure around copper metal ion in heterobimetal-
lic complexes may be proposed.
3.4. Thermal analysis
Thermal studies of the monometallic and heterobimetallic com-
plexes of dadb in various mole ratios of metal ions have been carried
out under nitrogen and air atmosphere. The thermogravimetric
analyses (TGA, DTG and DTA) of complexes were recorded in the
temperature range RT–1000 ◦C and are presented in Fig. 4a–f. DTG
curves of all the complexes under nitrogen and air atmospheres are
given S.I. Fig. 1a and b, respectively. Differential scanning calorime-
try (DSC) curves for complexes 3 and 2 have been represented in
Fig. 4g. The weight losses in various temperature ranges on thermal
degradation of the complexes under nitrogen and air atmospheres
are given in supplementary information (S.I.) Table 1(a)–(f) along
with position of peaks in DTG and DTA thermograms. Plausible
Table 3
Magnetic moment data and electronic absorption bands of the complexes.
Compound
Magnetic
moment (BM)
–
1.63
2.03
ꢂmax (cm−1) as nujol mull
1
2
4
5
3
16722, 22573, 32051, 36316, 37736, 46083
16502, 23148, 31847
16892,27933,40983, 46511
17544, 25316, 26455, 28818, 30488
19011, 25316, 32051, 36765
1.92
Diamagnetic

R.L. Prasad et al. / Thermochimica Acta 511 (2010) 17–26
21
Fig. 4. (a) TGA and DTG thermograms of 2 and 3 under nitrogen atmosphere. (b) TGA and DTG thermograms of 2 and 3 under air atmosphere. (c) TGA curve of the complexes
2, 3, 4 and 5 under nitrogen atmosphere. (d) DTA curve of the complexes 2, 3, 4 and 5 under nitrogen atmosphere. (e) TGA curve of the complexes 2, 3, 4 and 5 under air
atmosphere. (f) DTA curve of the complexes 2, 3, 4 and 5 under air atmosphere. (g) DSC curves of the complexes 2 and 3 and (h) PXRD pattern of residue of 2 and CuO.

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R.L. Prasad et al. / Thermochimica Acta 511 (2010) 17–26
Scheme 1. Thermal degradation of metal complexes under nitrogen atmosphere.
Scheme 2. Thermal degradation of metal complexes under air atmosphere.

R.L. Prasad et al. / Thermochimica Acta 511 (2010) 17–26
23
curve matches with weight loss calculated for this complex follow-
ing Scheme 1A indicating Scheme 1A being followed during thermal
degradation of 2 under nitrogen atmosphere.
copper metal and 2 mole of carbon from 2 under nitrogen atmo-
sphere is found to be exothermic, however, it is expected to be
endothermic. This points toward some exothermic reaction tak-
ing place during with loss of copper metal and 2 mole of carbon.
Further, the volatilization of copper metal is not possible in this
temperature range, however, polymeric organometallic compound
expected to be formed (Scheme 1a) may break into volatile cop-
per acetylide. Presence of exothermic peak in DTA may arise due
to explosion of copper acetylide after volatilization in vicinity of
crucible at this temperature range. This heat is expected to aid in
evaporation of 2 mole of carbon which is otherwise expected to
be endothermic process under nitrogen atmosphere. Weight loss
corresponding to the loss of 2 mole of carbon under air does not
exhibit any indication of heat flow in its DTA curve. Finally, the
residue obtained under nitrogen atmosphere at 700 ◦C corresponds
to 1/2 mole of metallic copper, whereas that under air to one mole
of CuO. The DTA curve under nitrogen exhibits endothermic peaks
corresponding to water loss (109 ◦C), however, exothermic peaks
are observed corresponding to the loss of HCl molecules (264 ◦C).
Observed exothermic peaks during the loss of CO (351 ◦C), (CN)2
(502 ◦C), and copper acetylide and 2 mole of carbon (633 ◦C) are
Experimentally observed weight loss corresponding to the loss
of two CO molecules under 3rd step of decomposition matches with
calculated value following Scheme 1A and that calculated under
Scheme 1B falls on the curvature of DTG peak. Calculated weight
loss under 4th step of the thermal degradation following Scheme 1B
and A are calculated to be 8.29 and 16.58%, respectively. Weight
loss obtained from TGA curve for this step matches with that cal-
culated for Scheme 1A. At end of 4th step the residue obtained
corresponds to the expected composition CuC2 and rule out the
possibility of thermal degradation under Scheme 1B. This may lose
two mole of carbon (7.65%) and yield a metallic copper residue
∼20.28% as shown in Scheme 1A2. However, TGA curve shows the
weight loss of 18.4% leaving behind a residue of 9% which is very
close to 1/2 mole of copper metal consistent with Scheme 1A1. This
indicate that under 5th step of thermal degradation 1/2 mole of cop-
per metal is evaporated/volatilized along with two mole of carbon
probably by forming copper acetylide [28] (however it is expected
to be unstable and explode at that temperature).
Thermal degradation of 2 under air is also expected to follow
either of the two fragmentation pattern shown by Scheme 2A and
B. The calculated weight losses corresponding to the Scheme 2A
and B for 2 under air are given in S.I. Table 1(b) in parenthesis pre-
ceded by observed values from TGA curve. Apart from water loss
under 1st step of thermal degradation, the losses under 2nd step in
air is also taking place at almost same temperature as in the case of
that under nitrogen atmosphere (Fig. 4b). The loss of CO molecules
at third step of degradation under air starts at much lower temper-
ature even before the completion of HCl loss. Temperature ranges
for the losses of HCl and CO molecules under 2nd and 3rd steps,
respectively, are so close to each other that they yield one broad
peak in their DTG (S.I. Fig. 1b) and DTA curves (Fig. 4f). The experi-
mental data of weight loss at end of third step of degradation does
not match with that calculated by Scheme 2B, however, it matches
with that calculated from Scheme 2A. Weight loss from the TGA
curve indicates that thermal degradation under IVth step, i.e. loss
of (CN)2 molecule and Vth step, i.e. absorption of oxygen are tak-
ing place simultaneously. Weight loss under this combined step
also matches with degradation Scheme 2A. The loss of two mole of
carbon at last step of thermal degradation is also consistent with
degradation Scheme 2A for the thermal degradation of 2 under air
atmosphere. The residual mass corresponds with one mole of CuO.
Powder XRD spectrum (Fig. 4h) of residue of 2 matches with X-ray
diffractogram of freshly prepared CuO following literature proce-
dure [29] and JCPDS file of CuO (5-661) confirms the formation of
CuO as residue under air atmosphere.
unexpected because breaking of CO,
C N and C2 molecules from
ligand residue should be endothermic under nitrogen atmosphere.
The DTA curve under air exhibits an endothermic peak correspond-
ing to the loss of water and an strong exothermic peak combined
together for the loss of HCl and CO molecules where tempera-
ture goes upto 297 ◦C. Next step of degradation is expected to be
endothermic corresponding to loss of (CN)2 molecules. Its DTA
and TGA curves (Fig. 4e and f) show that loss of this step starts
from ∼285 ◦C indicating non-ideal nature of these curves. Thus,
the temperature of the sample increased above the programmed
temperature due to the exothermic effect in second and third step
followed by a decrease of such temperature in endothermic forth
step of degradation, producing an effect in the shape (backward
curvature) of TGA and DTA curves. It appears that time-temperature
profiles seem to be affected by the strong exothermic character of
the previous processes. The DTA curve shows slightly but gradually
increasing exothermic peak from 297–372 ◦C with peak maxima at
372 ◦C due to absorption of oxygen molecules (fifth step of degra-
dation) accompanied with forth step. Thus it can be concluded that
loss of (CN)2 molecules starts ∼285 ◦C and absorption of oxygen
starts at relatively higher temperature with highest rate ∼370 ◦C
The amount of residue consistent with one mole of CuO indicates
that absorbed oxygen converts copper ion into CuO. However, there
is no indication of heat flow during the loss of C2 molecule under
air atmosphere.
The weight loss observed (30.2%) upto 2nd step of degrada-
tion of 3 (S.I. Table 1(c)) is found to be intermediate between
those calculated following Scheme 1A (31.55%) and Scheme 1B
(28.26%). The weight loss observed (39.40%) from TGA curve upto
3rd step of weight loss (loss of one CO molecule) also falls at
intermediate position between those of Scheme 1A (41.26%) and
B (37.97%). Therefore, based on above data no conclusion can be
drawn. Weight loss upto forth step of degradation calculated by
Scheme 1A2 (69%) falls on the curvature of DTG peak ∼710 ◦C and
weight loss calculated by Scheme 1A1 (84.5%) also falls on the cur-
vature of the DTG curve, therefore, it may be concluded that none of
the Scheme 1A1 or 1A2 is applicable for the thermal degradation of
3 under nitrogen atmosphere. However, experimentally observed
weight loss (80.8%) upto 4th step of degradation matches with cal-
culated weight loss on the basis of Scheme 1B1 (80.34%). It is worth
to be mentioned that boiling point of ZnCl2 is 732 ◦C [30], there-
fore, it may volatilize in the temperature range (400–825 ◦C) during
4th step of thermal losses. Observed weight loss (7.7%) from TGA
curve under fifth step fits well with weight loss calculated from
Scheme 1B (8.32%). Thus, weight loss upto 4th step, weight loss dur-
Second stage of thermal degradation of 2 starts ∼200 ◦C under
air as well as nitrogen atmosphere. The loss of CO molecule at third
stage of decomposition of 2 under the air starts at lower temper-
ature (276 ◦C) than that under nitrogen atmosphere (315 ◦C). Thus
it appears that rate of elimination of CO molecule is accelerated
under air atmosphere, whereas elimination of HCl molecule take
place at almost same temperature under both atmospheres. Fourth
stage of decomposition under nitrogen atmosphere the weight loss
corresponds to the loss of (CN)2 molecule in the temperature range
425–550 ◦C, whereas under air atmosphere loss of (CN)2 molecule
take place at lower temperature (300–420 ◦C) accompanied by a
gain of 1/2 mole of oxygen to form CuO. Fifth stage of decom-
position under nitrogen atmosphere the weight loss corresponds
to the loss of 1/2 mole of copper and one mole of C2 molecule,
whereas in the presence of air the weight loss is consistent with
the loss of 2 mole of carbon. The DTA curve corresponding to the
loss of 1/2 mole of copper and 2 mole of carbon under nitrogen
atmosphere exhibits a broad exothermic peak at 633 ◦C. Loss of

24
R.L. Prasad et al. / Thermochimica Acta 511 (2010) 17–26
ing 5th step and overall residue obtained after thermal degradation
under nitrogen atmosphere match with the weight loss calculated
on the basis of Scheme 1B1. It may be concluded that 3 follows the
degradation patterns shown by Scheme 1B under nitrogen atmo-
sphere.
mic peak observed in the DTA curve at 633 ◦C corresponds to forth
step of thermal degradation under nitrogen atmosphere. Forth step
of decomposition of complex 3 under air exhibits an exothermic
DTA peak at 394 ◦C whereas complex 2 exhibits at 372 ◦C. This
indicates that this step of thermal degradation undergoes at lower
temperature in presence of copper ions as compared to that of zinc
ion. It is worth to be mentioned that absorption of oxygen also
take place in complex 2 under forth step of degradation whereas
oxygen absorption takes place under fifth step of degradation of
complex 3. The ZnCl2 and two mole of carbon are lost under fifth
stage of decomposition accompanied with absorption of 1/4 moles
of oxygen to form half mole of ZnO. The residual mass after ther-
mal degradation of 3 under air corresponds to 1/2 mole of ZnO,
whereas that in case of complex 2 it corresponds to one mole of CuO.
Fifth stage of decomposition under air takes place at much lower
temperature for the complex 3 as compare to that under nitro-
gen atmosphere. At fifth step of thermal degradation of 3 under
nitrogen atmosphere, weight loss corresponds to the loss of two
mole of carbon in the temperature range 825–1100 ◦C accompa-
nied with endothermic peak at ∼1043 ◦C. Thus two mole of carbon
are lost at higher temperature from 3 as compare to that from 2
under nitrogen atmosphere. The residual mass corresponds to 1/2
moles of metals from both the complexes 2 and 3 under nitrogen
atmosphere.
Thermogravimetric data (S.I. Table 1(c–d)) of the complexes
4 and 5 indicate that water is lost at first stage of degradation
like 2 and 3. The thermal degradation in subsequent steps for the
heterobimetallic complexes have been interpreted assuming that
degradation of ligand moiety around copper and zinc metal ions
follow the degradation pattern of 2 and 3, respectively. Thus, in
case of complex 4, it is assumed that one mole of HCl liberated
molecule react with all the zinc ions present and converts it into
ZnCl2 and ligand moiety around copper ion follows the degra-
dation pattern of scheme A as in case of complex 2. In case of
complex 5, experimental weight loss data matches with calcu-
lated data assuming that half of zinc ions react with HCl, i.e. if
follows the pattern of scheme B as in complex 3 and copper ions
do not react with HCl and ligand moiety around copper follows
the pattern of scheme A as in complex 2. Obviously, all the com-
plexes absorb oxygen and form metal oxides as residue under air
atmosphere.
Second step of thermal degradation of heterobimetallic com-
plexes (4 and 5) under both atmosphere (nitrogen and air)
corresponds to the losses of half mole of each of (CN)2 and
H2 molecules whereas in cases of complex 2 two moles of HCl
molecules and in case of complex 3 this step is consistent with loss
of one mole of HCl and half mole of each of (CN)2 and H2 molecules.
From TGA and DTG thermograms of 4 and 5 it is obvious that 2nd
step of degradation starts at lower temperature than those for the
2 and 3 under both atmospheres. Thus it appears that bimetallic
complexes are different from those of monometallic complexes
2 and 3. In case of 3, there is no DTG/DTA peak in the tempera-
ture range 200–300 ◦C indicating major losses under 2nd step of
decomposition to take place at higher temperature (>300 ◦C). The
DTA peak for 2 and 4 in the temperature range 200–300 ◦C under
both atmosphere (nitrogen and air) are exothermic, whereas for
the complex 5 it is endothermic. In case of the complex 4 one HCl
molecules are eliminated from the ligand moiety around copper
metal ions which are expected to react with zinc metal ions to
form 1/2 moles of ZnCl2 and hydrogen molecules, both of these
steps are expected to be exothermic. Residual part of ligand moiety
which becomes free from metal under same step of thermal degra-
dation loses 1/2 moles of (CN)2 molecules and is also expected to
be endothermic step. Therefore, the heat flow is expected to be
resultant of all the processes under this step and is observed as
weakly exothermic DTA peak for the complex 4 in the both envi-
Theoretically calculated weight loss (31.55%) upto 2nd step of
degradation (S.I. Table 1(d) and Fig. 4b) following Scheme 2A falls
on the curvature of DTG peak corresponding to the loss of CO
molecules, therefore, the pattern of Scheme 2A being followed
for the degradation of 3 under air is unlikely. The experimentally
observed weight loss (28%) upto 2nd step of thermal degradation is
consistent with calculated value following Scheme 2B (28.26%). Cal-
culated weight loss upto third step of thermal degradation (for the
loss of one CO molecule) following Scheme 2A (41.26%) is incon-
sistent with experimentally observed weight loss (37.3%) falling
on the curvature of the DTG curve for the next step of degrada-
tion. However, weight loss obtained from TGA and DTG curves
(Fig. 4b) upto 3rd step of degradation matches with calculated
weight loss by Scheme 2B (37.97%). These results indicate that
unlike complex 2 the complex 3 does not follow the degradation
pattern shown by Scheme 2A rather it follows the Scheme 2B. Cal-
culated weight loss at end of 4th step of degradation following
Scheme 2A falls on curvature of the DTG curve for the subsequent
step of thermal degradation. However, weight loss calculated fol-
lowing Scheme 2B matches with weight loss obtained from TGA
curve corroborated by the DTG curve. Weight loss upto end of 5th
and 6th steps of degradation are calculated to be 71.78% by both
Scheme 2A and B2 which is inconsistent with weight loss observed
from TGA curve (86.1%) for 3 under air. However, weight loss
calculated by Scheme 2B1 (85.89%) matches with experimentally
observed from TGA curve. Complex 3 leaves a residue correspond-
ing to 1/2 mole of ZnO (13.9%, theoretical 14.11%) consistent with
expected residue from the Scheme 2B1. Thus, it may be concluded
that 3 under air follows the thermal degradation pattern presented
under Scheme 2B. Finally, it may be summarized that 3 leaves a
residue corresponding to 1/2 mole of metallic zinc under nitrogen
and 1/2 mole of ZnO under air atmosphere on thermal degrada-
tion, whereas 2 provides a residue corresponding to 1/2 mole of
metallic copper under nitrogen and one mole of CuO under air
atmosphere.
Weight loss in second stage of thermal degradation of 3 under
both atmosphere (nitrogen and air) are completed almost at the
same temperature ∼335 ◦C and corresponds to the loss of one
mole HCl molecule and 1/2 mole of (CN)2 and hydrogen molecules
(Schemes 1B and 2B). The loss of HCl molecules begins at rela-
tively higher temperature (300 ◦C) in 3 as compare to that in 2
(∼255 ◦C) under both environments. Weight loss corresponds to
the loss of one CO molecule at third stage of decomposition of com-
plex 3 under nitrogen as well as air atmospheres, however, two
moles of the CO molecules are lost under third step of decompo-
sition of complex 2. The corresponding DTG peak is obtained at
380 and 338 ◦C under nitrogen and air atmosphere, respectively,
for 3. From these results it can be concluded that air accelerates
the loss of CO molecules. Further, third step of decomposition
takes place at higher temperatures for the complex 3 under both
atmospheres than those of corresponding decomposition for the
complex 2.
Weight loss under fourth step of decomposition of complex 3
corresponds to the loss of half mole of (CN)2 and one mole of CO
molecules under air, whereas under nitrogen atmosphere evapo-
ration of ZnCl2 is also associated along with above (weight loss
at V step in Scheme 2B). This step of decomposition is completed
at lower temperature under air than that of under nitrogen atmo-
sphere. The losses of CO and (CN)2 molecules from 2 under nitrogen
atmosphere exhibit exothermic peaks in its DTA curve but in case of
3 there is no indication of exothermic peaks, however an endother-

R.L. Prasad et al. / Thermochimica Acta 511 (2010) 17–26
25
ronment. In case of complex 5 the amount of weight loss observed
from TGA and DTG thermograms under second step of thermal
degradation is consistent with elimination of 1/2 mole of (CN)2
and hydrogen molecules. Further the loss under 2nd step is taking
place with very slow speed in case of heterobimetallic complexes,
whereas in case of 3rd step the loss is taking place with relatively
faster speed as is evident from TGA and DTG curves (Fig. 4c and e).
There are strong peaks in DTG curves of the complexes 2 (∼263 ◦C)
and 3 (∼314 ◦C) corresponding to 2nd step of thermal degradation
under both atmosphere. Presence of weak peaks at 262 and 253 ◦C
under nitrogen and 273 and 262 ◦C under air atmospheres in the
DTG curves of complexes 4 and 5 (S.I. Fig. 1a and b), respectively,
supports the above interpretations. Presence of weak endothermic
peak in the DTA thermograms of complex 5 at 255 and 267 ◦C under
nitrogen and air atmospheres, respectively (Fig. 4d and f) unlike
other complexes are consistent with DTG thermogram. Thus, as
the amount of copper is lowered in heterobimetallic complexes the
amount of heat evolved during the thermal degradation is reduced
in the temperature range 200–300 ◦C and finally it becomes weakly
endothermic as in case of 5. In third stage of degradation involves
the loss of one mole of each HCl and CO molecules from complex 4
and one mole of HCl and 1/2 mole of CO molecules from complex 5
under nitrogen atmosphere. Under air atmosphere the weight loss
is consistent with the loss of one mole of HCl and 1/2 mole of CO
molecules from former complex whereas only one mole of HCl from
the latter complex. However in case of 2 and 3 this step involves
the loss of two and one moles of CO molecules, respectively under
both atmospheres. DTA curves show that this step is exothermic in
thermal degradation of 2 as well as heterobimetallic complexes 4
and 5 under both atmospheres because there is loss of HCl along
with CO molecules. The weight loss at forth step of degradation of
complexes 4 and 5 under nitrogen atmosphere matches with cal-
culated losses of remaining CO, (CN)2, and half of copper metal is
being lost as CuC2 and appropriate amount of zinc metal as ZnCl2
assuming the part of complex containing copper ions undergoes
degradation in same way as complex 2 and remaining part con-
taining zinc ions as complex 3. Under air atmosphere weight loss
at forth step of degradation corresponds to the loss of remaining
CO and half mole of (CN)2 molecules of complex 4, whereas in case
of the complex 5 the weight loss corresponds to the loss of two
CO molecules. Under fifth step of degradation the weight loss from
complex 4 match with loss of 1/4 mole of ZnCl2 and remaining 1.5
mole carbon under nitrogen and loss of 1/2 mole of ZnCl2 and (CN)2
molecules, two mole carbon and absorption of 1/4 mole of oxygen
under air atmosphere. The residue obtained after thermal degra-
dation of complex 4 corresponds to 1/4 mole of metallic copper
under nitrogen atmosphere and 1/2 mole of CuO under air. Simi-
larly weight losses under fifth step of thermal degradation match
with losses of 3/8 mole of ZnCl2, and remaining amount of carbon
under nitrogen and losses of 1/2 mole of (CN)2 and ZnCl2 molecules
and two mole of carbon and absorption of 1/4 mole of O2 molecule
under air atmosphere. The residue obtained at end of thermal
degradation corresponds to the mixture of metallic copper and zinc
under nitrogen and mixture of oxides of copper and zinc under air
atmosphere.
moisture by the samples on exposure to air. The amounts of water
molecules absorbed depend upon the relative humidity present in
the air.
Second step of thermal degradation (loss of HCl or (CN)2
molecules and hydrogen) take place at almost same temperature
under both (nitrogen and air) atmosphere. However, the second
step of degradation take place at lower temperature in complex
2 than that of complex 3, i.e. by changing the metal ion from
zinc to copper the thermal degradation process is accelerated. It
may be concluded that copper (II) metal ion having unpaired elec-
tron catalyzed the removal of HCl molecules. Third step of thermal
degradation (loss of CO molecules) are completed at lower tem-
perature in air than that under nitrogen atmosphere for all the
complexes. Thus it appears that oxygen helps in removal of CO
molecules under air. Under air atmosphere CO is expected to be
converted into CO2 and liberates heat. It is this heat which acceler-
ates the rate of removal of other fragments at lower temperature
under air as compared to that under nitrogen atmosphere from all
the complexes 2, 3, 4 and 5. Fourth step of decomposition involves
loss of (CN)2 molecules under nitrogen and absorption of oxygen
in addition to the loss of (CN)2 under air from complex 2, whereas
in case of complex 3, in addition to above losses the left over
CO elimination take place in air and under nitrogen atmosphere
evaporation of zinc chloride also take place this step. Thermal
degradation is completed at lower temperature under air than that
under nitrogen like third step for all the complexes studied by
thermo-gravimetrically. Forth step of degradation is completed at
lower temperature in case of copper complex 2 than that of zinc
complex 3 which is more distinct under nitrogen atmosphere. Fifth
step of decomposition involves the loss of half mole of metallic cop-
per and two mole of carbon under nitrogen atmosphere whereas in
case of air atmosphere only two mole of carbon is lost from complex
2. In case of zinc complex 3, the loss of two mole of carbon takes
place under nitrogen whereas under air atmosphere the loss of two
mole of carbon is accompanied with loss of ZnCl2 and absorption
of oxygen. Fifth step is endothermic under nitrogen, whereas, it
is weakly exothermic under air atmosphere. Presence of exother-
mic peak in DTA curve under air atmosphere presumably arises
due to reaction of carbon atoms with oxygen under air to form
CO and CO2 in the temperature range 450–600 ◦C and liberating
heat. Due to this liberated heat the thermal degradation (mainly
the loss of C2) under fifth step is also completed at much lower
temperature under air. However, there is no indication of exother-
mic peak in case of complex 4 and 5. The loss of zinc chloride, a
fraction of (CN)2 molecules are associated under this step in the
complex 4 and 5 and they are expected to be endothermic, there-
fore, overall effect is weakly endothermic. In case of complex 2
the residue corresponds to half mole of copper metal under nitro-
gen atmosphere, whereas same sample under air yields a residue
corresponding to one mole of CuO. These result indicate that half
mole of copper is lost/evaporated under nitrogen atmosphere prob-
ably by forming a complex with C2 molecules (acetylide moiety).
Sublimation/evaporation of metallic copper is unlikely around this
temperature range, however, polymeric organometallic compound
expected to be formed during thermal degradation of the complex
may break into volatile copper acetylide. Presence of exothermic
peak at 633 ◦C in DTA curve is expected to arise due to explosion
of copper acetylide just after volatilization in vicinity of crucible at
this temperature range. This heat may aid in evaporation of car-
bonacious residue, otherwise, it should be endothermic process
under nitrogen atmosphere and loss of it should be completed
at much higher temperature as in the case of complex 3. In case
of complex 3 the residue obtained from TGA curve corresponds
to the half mole of metallic zinc and zinc oxide under nitrogen
and air atmosphere, respectively. These observations are consis-
tent with degradation pattern proposed under scheme A and B for
4. Conclusion
First stage of weight loss corresponds to the loss of water
molecule under air as well as nitrogen atmosphere for all the
complexes. DSC thermogram of complex 2 dried over calcium
chloride under vacuo for a period of 20 days exhibits no peak
in the temperature range 40–240 ◦C indicating the presence of
non-coordinated nature of water molecules, i.e. crystalline water.
Presence of exothermic peaks in the DTA curve of some of the com-
plexes in the temperature range RT–60 ◦C indicate absorption of

26
R.L. Prasad et al. / Thermochimica Acta 511 (2010) 17–26
the complex 2 and 3, respectively. The residue obtained from ther-
mal degradation of the heterobimetallic complex 4 corresponds
to 1/4 mole of metallic copper (5.3%) and 1/2 mole of copper
oxide (12.62%) under nitrogen and air atmospheres, respectively.
These results indicate that ligand moiety (dadb) around copper and
zinc metal ions undergo degradation according to scheme A and
B, respectively, in heterobimetallic complex 4. Further, since the
copper and zinc metal ions are expected to be in alternate posi-
tion in the polymeric chain, therefore, HCl molecules liberated by
the heterobimetallic complexes from copper sites completely react
with zinc metal ions and form ZnCl2 and subsequent fragmenta-
tion follows. Thus, all the zinc metal ions are expected to form
zinc chloride which is evaporated/volatilized whose boiling point
[30] is 732 ◦Cs and the residue corresponds to copper metal/copper
oxide depending upon the environment under which degradation
is taking place. For the equimolar mixture of 2 and 3, the final
residue is expected to be ∼10.55% consistent with composition 1/4
mole of copper and 1/4 mole of zinc metal ion following respec-
tive degradation pattern of complexes 2 and 3, respectively, under
nitrogen atmosphere. Similarly under air atmosphere the residue
is expected to be consistent with composition 1/2 mole of copper
oxide and 1/4 mole of zinc oxide following respective degrada-
tion pattern of the complexes 2 and 3. These thermal degradation
results do not match with residue obtained from the complex 4
further confirms the distinct identity of heterobimetallic complex
4 rather than equimolar mixture of 2 and 3. Similarly, residue
obtained from degradation of complex 5 also can be explained. A
careful observation of TGA and DTA curves indicate that each of the
complexes follows its characteristics thermal degradation curves
depending upon amount of respective metal ion contribution
into it.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tca.2010.07.019.
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