Novel polymeric chelates of 8-hydroxyquinoline-dimethylolacetone

Article abstract of DOI:10.1007/s10973-007-8419-7

A new chelating copolymer (HQDMA) has been synthesized through copolymerization of 8-hydroxyquinoline and dimethylolacetone monomers in the presence of base as a catalyst. This newly developed copolymer ligand (H ₂L) has been used to prepare a series of five polymeric chelates (ML) by using Zn(II), Cu(II), Ni(II), Co(II) and Mn(II) metal ions. Both the parent ligand and its metal chelates have been systemically investigated in detail to elucidate the chemical structure and thermal behaviour by elemental analyses, spectral (IR and electronic) characterization, number-average molecular mass (M_n) determination and thermogravimetric analysis (TG). In addition to these, magnetic susceptibility measurements have also been carried out for studying geometry and metal-ligand stoichiometry of polymeric chelates. The chemical structure of polychelates on the basis of elemental and IR characterization suggests that the bidentate ligand (H₂L) coordinates to metal ions through oxygen atom of the phenolic hydroxyl group by replacing hydrogen atom and nitrogen of the quinoline ring. The studies of magnetic moments and electronic spectra reveal that all polychelates with octahedral geometry are paramagnetic in nature except that of Zn(II) chelate, which is diamagnetic. The thermogravimetric analysis of parent ligand and its metal chelates have shown remarkable difference in mode of thermal decomposition and their thermal stabilities. The kinetic parameter, energy of activation (E _a) of thermal decomposition has also been estimated by Broido method.

Full text of DOI:10.1007/s10973-007-8419-7

Journal of Thermal Analysis and Calorimetry, Vol. 92 (2008) 2, 505–512
NOVEL POLYMERIC CHELATES OF
-HYDROXYQUINOLINE-DIMETHYLOLACETONE
8
*
T. B. Shah , R. B. Dixit and B. C. Dixit
Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar 388120, India
A new chelating copolymer (HQDMA) has been synthesized through copolymerization of 8-hydroxyquinoline and
2
dimethylolacetone monomers in the presence of base as a catalyst. This newly developed copolymer ligand (H L) has been used to
prepare a series of five polymeric chelates (ML) by using Zn(II), Cu(II), Ni(II), Co(II) and Mn(II) metal ions. Both the parent ligand
and its metal chelates have been systemically investigated in detail to elucidate the chemical structure and thermal behaviour by ele-
mental analyses, spectral (IR and electronic) characterization, number-average molecular mass (M ) determination and
n
thermogravimetric analysis (TG). In addition to these, magnetic susceptibility measurements have also been carried out for studying
geometry and metal-ligand stoichiometry of polymeric chelates.
The chemical structure of polychelates on the basis of elemental and IR characterization suggests that the bidentate ligand
2
(H L) coordinates to metal ions through oxygen atom of the phenolic hydroxyl group by replacing hydrogen atom and nitrogen of
the quinoline ring. The studies of magnetic moments and electronic spectra reveal that all polychelates with octahedral geometry are
paramagnetic in nature except that of Zn(II) chelate, which is diamagnetic. The thermogravimetric analysis of parent ligand and its
metal chelates have shown remarkable difference in mode of thermal decomposition and their thermal stabilities. The kinetic pa-
a
rameter, energy of activation (E ) of thermal decomposition has also been estimated by Broido method.
Keywords: DMA, HQDMA, integral procedural decomposition temperature, number average molecular mass, polymeric che-
lates, thermogravimetric analysis
Introduction
opments of applications of coordination polymers and
polymer chelates of 8-hydroxyquinoline and its deriv-
atives in various areas of science and technology due
to their versatile chelating ability. They are com-
monly used for their diversity of applications such as
in waste water treatment for metal recovery, protec-
tive coatings, as water disinfectants, antifouling
paints, antimicrobial, surgical materials, gels and
ointment for medical uses, and biological activity
[6–8] due to their high thermal stability.
The study of polymeric ligand, polychelates, metal
complexes as well as of ionexchangers has attracted
the attention of many researchers because of their ver-
satile practical applications viz.; ion-exchangers in
pollution control for separation of heavy metal ions,
radio isotopes and in water treatment as well as for ca-
talysis in organic synthesis and in bioinorganic chem-
istry. Such polymers have unique property profile that
make them different from organic polymers with
some characteristics like strong bond which resists
cleavage reactions, multiple valences for attachment
of wide variety of ligands, high thermal stability and
ability to tailor made new polymeric structures with
endless variations [1–3].
Horrocks et al. [9] synthesized the first chelating
polymers of oxine by polycondensation of oxine with
formaldehyde using both acid and base catalysts and
subsequently chelated with different metal ions like
Zn(II), Cu(II), Ni(II), Co(II), Mn(II) in order to
achieve the combination of properties of both coordi-
nation polymers and those of metal chelates
crosslink’s such as flexibility, thermal stability and
ion exchange capacity. The study of thermal stability
of parent ligands and their chelates carried out by TG,
DSC, DTA and DTG methods. It is also reported that
polymer chelates of oxine-formaldehyde resins gave
polymeric chelates of better thermal stability by in-
corporation of aliphatic chain (–CH –) , directly at-
In this context, the interest in the coordination
compounds of oxine has increased considerably in the
last two decades. Most of the work has been done on
the monomeric chelates and papers have also been
published concerning polymeric ligands [4, 5]. Many
of the workers in the field of polymeric chelating lig-
ands have primarily interested in the synthesis of
chelateing resins and determining selectivity of che-
lating polymer with various metal ions. Literature sur-
vey has revealed the proliferation in intensive devel-
2
n
tached to oxine at 5- or 7-position [10].
Manolova et al. [11, 12] have published a series of pa-
*
Author for correspondence: dixits20002003@yahoo.co.in, dixits20002003@rediffmail.com
1
388–6150/$20.00
2008 Akadémiai Kiadó, Budapest
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
©

SHAH et al.
pers of polymers and copolymers of 8-hydroxy-
30 mL methanol was carried out in presence of alco-
holic solution of KOH (5 mL, 3.5% based on mass of
8-hydroxyquinoline) by refluxing at 80–900° for 8 h.
The colour of reaction mixture changes from yellow to
dark red and then to green. The reaction mixture was
poured into water and neutralized with dilute acid. The
resulting copolymer was filtered off and washed several
times with hot water followed by methanol in order to
remove unreacted 8-hydroxyquinoline. It was then
quinoline either present as end groups or as pendent
groups with a view to study their complexing ability
and biological activity.
Recently, many research workers have reported
the synthesis and characterization of various metal
complexes 8-hydroxyquinoline and mixed ligand
metal complexes. They have studied the struc-
ture-property relationship in terms of thermal stability
[
13–17]. Metal complexes of 8-hydroxyquinoline
air-dried
and
the
yield
was
88%.
have also been reported for their antimicrobial activ-
ity [18] and for fluorescent sensors [19].
8-hydroxyquinoline-dimethylolac- etone copolymer
was designated as HQDMA. The synthetic protocol for
the copolymerization is shown in Scheme 1.
On the basis of this documentations, the present
authors have made an attempt to synthesize a novel
8
-hydroxyquinoline based copolymeric ligand (H
namely 8-hydroxyquinoline-dimethylolac-
etone (HQDMA), which has yet not been reported. This
polymeric ligand has been prepared by the
copolymerization of 8-hydroxyquinoline with
2
L)
dimethylolacetone (DMA) in presence of base catalyst,
such that 5- and 7-position of 8-hydroxyquinoline are
blocked by dimethylolacetone and chelating sites of
phenolic –OH and ring nitrogen are to be free.
The polymeric metal chelates of HQDMA copol-
ymer ligand have been prepared with five different
transition metal ions viz.; Zn(II), Cu(II), Ni(II), Co(II)
and Mn(II). Both preformed copolymeric ligand
(H L) and their polymeric metal chelates (ML) have
2
been characterized further systematically as discussed
in the following section.
Scheme 1 Synthetic protocol for the formation of ligand
Experimental
2
(H L) and its polymeric metal chelates (ML)
Materials
Synthesis of polymeric metal chelates (ML)
All the chemicals used were of AR grade and were
obtained from Chiti Chem Corporation Baroda, India
and were purified when and where required.
Polymeric chelates were prepared by the following
method.
A dried copolymeric ligand (HQDMA) (2.27 g,
0.01 mol) was dissolved in 200 mL aqueous solution of
Synthesis of DMA monomer
formic acid (20%) and warmed in the temperature range
of 60–80°C on a water bath for 10–15 min. To this solu-
tion, a warm solution of metal acetate (0.005 mol) of
each of the metal ion in aqueous formic acid (50%) was
added dropwise with constant stirring. The reaction
mixture was made alkaline with dilute ammonia solu-
tion and further digested on water bath for an hour. Fi-
nally, the solid thus obtained was filtered off, washed
with hot water followed by acetone, DMF and air dried
at room temperature. The yields of all polymeric metal
chelates were almost quantitative.
The monomer a , a ¢-dimethylolacetone (DMA) was
prepared by the method reported in the patent litera-
ture with little changes in reaction conditions [20].
The procedure followed was: a mixture of acetone
(
1 mol, 60 mL), paraformaldehyde (0.66 mol, 60 g)
and 10 mL of 5% alcoholic KOH was heated at
0±2°C with continuous stirring for 10–15 min and
4
allowed it to cool at room temperature. The resulting
solution obtained was neutralized with formic acid,
giving colorless thick liquid. The yield was 96%.
Synthesis of HQDMA copolymer
Methods
Copolymerization of 8-hydroxyquinoline (0.05 mol,
Elemental analyses for C, H and N were carried out
on a Carlo Erba Elemental Analyzer (Italy). The
7.25 g) with dimethylolacetone (0.05 mol, 5.9 g) in
506
J. Therm. Anal. Cal., 92, 2008

POLYMERIC CHELATES OF 8-HYDROXYQUINOLINE-DIMETHYLOLACETONE
metal content of the polymeric chelates was carried
Non-aqueous conductometric titration of H2L
copolymeric ligand and its polymeric metal chelates
was carried out in pyridine against standard sodium
by decomposing a weighed amount of each polymeric
chelates with concentrated acid followed by standard
EDTA titration method [21]. Number average molec-
methoxide in pyridine as titrant. The M of each sam-
n
ular mass (M ) of ligand and its chelates was deter-
ple was calculated according to method reported in
literature [22] and data are reported in Table 1. The
n
mined by non-aqueous conductometric titration
method using sodium methoxide as titrant base and
pyridine as solvent.
degree of polymerization in the H L ligand was esti-
2
mated to about 8.
IR spectra of H L and the polymeric chelates were
2
scanned on a Nicolet-400D FTIR Spectrophotometer
in KBr. Magnetic susceptibility measurements of all
the polymeric chelates were carried out at room tem-
perature by the Gouy method using mercury
Infrared spectra
The IR spectra of copolymeric ligand (HQDMA) and
all of its polymeric metal chelates show that they re-
semble each other in their general shape. Comparison
of the IR spectra of copolymeric ligand (HQDMA)
and its polymeric metal chelates have shown some
important characteristic differences (Table 2).
tetrathiocyanatocobaltate(II) Hg[Co(NCS) ] as a
4
calibrant. The electronic spectra of the solid poly-
meric chelates were recorded on a Beckman DK-2A
Spectrophotometer using MgO as reference.
Thermal behaviour of parent ligand and its metal che-
lates were studied by TG on a DuPont 950
Thermogravimetric Analyzer in presence of air at the
The four characteristic fundamental frequencies
of 8-hydroxyquinoline ring in ligand are observed at
–
1650, 1565, 1500 and 1460 cm as expected on the
1
–
heating rate of 10°C min .
1
basis of literature reports [23]. A broad band at
–
335–3550 cm and a medium band at 1420 cm are
1
–1
3
attributed to –OH stretching and bending vibration of
phenolic group of 8-hydroxyquinoline. One of the
significant difference that have been observed in the
IR spectrum of the parent ligand and that of its poly-
meric metal chelates is the presence of more broad-
Results and discussion
Characterization of HQDMA copolymer ligand
(H L) and its metal chelates
2
–
1
ened bands in the region of 3300–3600 cm in the IR
spectra polymeric chelates because of the formation
of a coordination bonds with the metal ions through
The physical and analytical data of HQDMA
copolymeric ligand and its metal chelates are fur-
nished in Table 1. The parent ligand (H L) is a brown
2
oxygen
-hydroxyquinoline as well as may be due to coordi-
nation of water molecule. Another noticeable differ-
ence is that the band due to the C–N stretching vibra-
atom
of
phenolic
group
of
colored powder and not melted upto 300°C. It is solu-
ble in acidic and basic solvents like acetic acid, for-
mic acid, DMF, pyridine, etc. as expected and, are
partially soluble in chloroform, DMSO and dioxane.
8
–
1
tion of 8-hydroxyquinoline at 1580 cm in the IR
spectrum of H L is shifted to a lower frequency
i.e. 1572–1560 cm . A weak band at 1110 cm cor-
responding to C–O–M stretching, and the two bands
The polymeric chelates of H L (HQDMA) with the
2
2
metal ions Zn(II), Cu(II), Ni(II), Co(II) and Mn(II)
vary in color from light green to dark brown. It is
completely insoluble in common organic solvents
–
1
–1
–
around 770 and 650 cm correspond to the M–N vi-
1
such as benzene, CCl
The results of elemental analyses of the
HQDMA copolymeric ligand (H L) are given in Ta-
ble 1 and are consistent with the proposed structure
shown in Scheme 1 with the molecular formula
. Examination of the metal content in each
polymeric chelates (Table 1) indicated metal to ligand
M/L) stoichiometry ratio as 1:2 suggesting that in the
4
, ethanol, DMF, THF, etc.
–
1
bration. Another medium band at 1420 cm in the
spectrum of the copolymeric ligand (HQDMA) is due
to the O–H inplane bending vibration of phenolic
group [23]. This band was appeared at
2
–
1
408–1400 cm in the IR spectra of polymeric metal
1
C H13NO
14 2
–
1
chelates indicating a negative shift of 10–15 cm
with decreased intensity supporting the coordination
of metal ion with oxygen atom. Further, the appear-
(
formation of polymeric metal chelates two ligands co-
ordinate with central metal atom at four coordination
sites and two water molecules at two coordination
sites. Hence, the general molecular formula of the re-
–
1
ance of two sharp new bands at 1110 and 502 cm
corresponding to the stretching vibration of C–O–M
bond and M–O bond [23] formed in chelation and the
coordination of ligand with metal through nitrogen
atom of the quinolinol ring can be evident from the
sulting polymeric chelates is C28H N O M·2H O.
24 2 4 2
The data of elemental analyses reported in Table 1 are
in agreement with the calculated values of C, H and N
based on the above mentioned molecular formula of
parent ligand as well as polymeric chelates.
–
negative shift (5–20 cm ) in characteristic stretching
1
–
frequency of C=C/C=N at 1630 and 1580 cm , re-
1
spectively in the IR spectra of chelates.
J. Therm. Anal. Cal., 92, 2008
507

SHAH et al.
508
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POLYMERIC CHELATES OF 8-HYDROXYQUINOLINE-DIMETHYLOLACETONE
J. Therm. Anal. Cal., 92, 2008
509

SHAH et al.
–
1
The two weak bands at 2940 and 2860 cm are
attributed to aliphatic stretching vibrations of
methylene groups (–CH –) [24] and a sharp band at
700–1710 cm attributed to the carbonyl group
=C=O), of dimethylolacetone group present in both
copolymeric ligand and of polymeric metal chelates.
In conclusion, all of these inferences of IR data
support our proposed structure of HQDMA (H L)
copolymeric ligand and its polymeric metal chelates
ML) as shown in Scheme 1.
Scrutinization of the results of thermogravimetric
analyses (TG) of copolymeric ligand reveals the two
steps degradation process (Fig. 1). A very slight de-
crease in mass loss (2–3%) depicted from the curve in
the temperature range 50–150°C for the copolymeric
ligand may be attributed to loosely hold moisture pres-
ent [29] in the polymeric ligand. The temperature range
for the first and the second step of thermal degradation
are observed at 150–350 and 350–650°C, respectively.
These two steps of thermal degradation in ligand can be
explained as; the first step of decomposition may be due
to minor degradation that has occurred in aliphatic
bridge attached to oxine moiety. The mass loss occurred
in this step is equal to 11.58% (cal. 12.43%) based on
the assumption of dimethylolacetone of copolymeric
ligand. While the second fast decomposition step of
degradation is due to complete degradation of the ligand
with 97.5% mass loss. Hence, these results suggest the
following probable stages of thermal degradation;
2
–
1
1
(
2
(
Magnetic moments and electronic spectra
Magnetic moments (meff) of the polymeric metal che-
lates are given in Table 2. Examination of these data
reveals that all the polymeric metal chelates other
than that of Zn(II) are paramagnetic, while that of
Zn(II) is diamagnetic in nature. The results of elec-
tronic spectra (Table 2) and that of magnetic moment
indicate the octahedral geometry for Cu-HQDMA,
Ni-HQDMA, Co-HQDMA and Mn-HQDMA poly-
meric metal chelates [25–27].
loss of moisture
[H L]×nH O¾ ¾ ¾¾ ® [H L]+nH O
(
i)
2
2
2
2
50–150°C
8
HQ+loss of aliphatic chain
(
ii) [H L]¾ ¾¾ ®
150–350°C
2
Thermogravimetric studies
–CH
–CH –CO–CH –CH –
2 2 2 2
For the preliminary comparison of diverse polymeric
materials, the methods, which has so far proved most
versatile is the thermogravimetric analysis (TG) in the
dry nitrogen atmosphere because it gives the true
value of residual mass as function of temperature and
time. However, in TG, the analytical results are not
only dependent upon the types of thermal decomposi-
tion reaction, but they are also affected by the proce-
dural details such as sample size, particle size, atmo-
sphere employed, heating rate etc. and therefore TG
results are highly empirical and trivial. In order to fur-
nish such results in a more definite order, the two
methods ‘differential procedural decomposition tem-
perature (PDT)’ and ‘integral procedural decomposi-
tion temperature (IPDT)’ are used. In the first method,
PDT is estimated from the peak maxima of DTG,
where as IPDT can be obtained from the normalized
data of curve, a method reported in literature [28] and
(iii)
8HQ¾ ¾¾ ® 97.5% loss
3
50–650°C
Similarly, scrutinization of the results of
thermogravimetric analyses (TG) of polymeric metal
chelates reveals the single step degradation process
Fig. 1). Initially mass loss occurred below 150°C in
(
thermal degradation of all of the polymeric chelates
may be due to mass loss of hydrated water, where as
that obtained in the temperature range 150–200°C
might be due to presence of metal coordinated two
water molecules [29]. This water loss is remarkably
high about 7% and it is in agreement with the calcu-
lated mass loss (7.5%) assuming the presence of two
coordinated water molecules and also satisfies the six
coordinated sites of metal atom. All the polymeric
chelates undergo thermal degradation in a single step
in the temperature range of 200–550°C in contrast to
two steps thermal degradation of parent polymeric
ligand. The temperature at which the thermal degra-
the other parameters T
0
, T10 and IPDT are estimated
for studying the thermal stability. Accordingly, in the
present study, the thermal stability was estimated in
terms IPDT and PDT by using the equation:
IPDT=
area under the peak of normalized curve
T0 +
(T - T )
f
0
total area of the curve
where, T =initial temperature, T =final temperature.
0 f
These data of T , T , PDT and IPDT of thermal de-
0 10
composition of parent ligand and its chelates are
given in Table 3.
Fig. 1 Curves of HQDMA and its polymeric chelates
510
J. Therm. Anal. Cal., 92, 2008

POLYMERIC CHELATES OF 8-HYDROXYQUINOLINE-DIMETHYLOLACETONE
dation initiated in a series of five metal chelates is al-
most same as in the polymer ligand, however it oc-
curred very fast in chelates as compared to that in par-
ent ligand. This evidence may suggest that the parent
ligand is thermally more stable than its polymeric
metal chelates and can be explicable on the basis of
either the thermal decomposition of metal chelates
may catalytically be induced by the metal ions [30] or
the metal atom in chelates might have developed a
considerable strain in the ring of chelates. Further, the
mode of thermal degradation in each of the polymer
chelates is found to be similar with difference in rate
of thermal decomposition, which may be due to dif-
ferent catalytical activity of central metal atom pres-
ent in each of the polymeric chelates. In conclusion,
different stages on the mode of thermal decomposi-
tion and the order of thermal stability of metal che-
lates may be assigned as;
Fig. 2 Normalized curves of v – L and
˜ – ZnL
Further, it is reported in literature that polymers
may decompose back to either the monomer or may
produce unsaturated hydrocarbons from chain frag-
ments of varying length like alkanes, alkenes and
dienes. The kinetics of this rapid process is approxi-
mately first order [32]. On the basis of this assump-
tion several methods are reported in literature for the
study of kinetics of thermogravimetry. Sharp [33] has
made a critical comparison of three methods namely
Freeman and Caroll [34], Coat and Redferm [35] and
Achar and Brindley [36], and reported that the later
two methods can lead to satisfactory kinetic parame-
ters of thermal degradation. Later on, Broido [37] has
proposed a very simple and sensitive graphical meth-
ods for estimation of kinetic parameters. He proposed
the following equation;
(
A) Mode of thermal decomposition of metal
chelates:
(
i)
loss of moisture
ML(H O) ] ×nH O¾ ¾ ¾¾ ® [ML(H O) ] +nH O
[
2
2
n
2
2
2
n
2
5
0–110°C
loss of coordinated
water molecules
(
(
ii) [ML(H O) ] ¾ ¾ ¾ ¾ ® [ML] +2H O
110–200°C
2
2
n
n
2
ln[ln(1/y)]=(E
a
/R)(1/T)+constant
iii)
[ML] ¾ ¾¾ ® metal oxide
200–550°C
n
where the energy of activation ‘E ’ is computed from
a
(
B) The order of thermal stability of metal che-
the slope of the plot of ln[(ln(1/y)] vs. (1/T) and re-
lates:
ported in Table 3.
Zn-HQDMA>Mn-HQDMA>Co-HQDMA@
Ni-HQDMA>Cu-HQDMA
slope=–E /R
a
–1
where R is a gas constant (1.987 kcal mol ), T=temper-
ature and Y=fraction of numbers of initial molecules.
The energy of activation of parent ligand is
A comparative study of thermal stability of poly-
meric chelates has also been carried out with a view to
examine structure property relationship [28, 31]. For
–
11.57 kcal mol for polymeric ligand and those of
1
–
1
this purpose the thermal parameters such as T , T10
0
,
metal chelates are in the range of 7.4–9.3 kcal mol ,
further supporting the higher thermal stability of
copolymeric ligand than polymeric metal chelates.
PDT, IPDT are estimated and are furnished in Table 3.
The significant difference is observed in the tempera-
tures corresponding to T10, PDT and IPDT. The values
of these temperatures are higher in the case of polymeric
ligand as compared to those of its metal polymeric che-
lates indicating that parent polymeric ligand is thermally
more stable than its polymeric chelates. The selected
normalized curve of HQDMA and Zn-HQDMA poly-
meric chelates are shown in Fig. 2. Scrutinization of
PDT and IPDT values of polymeric chelates have indi-
cated that the Cu-HQDMA polymeric chelates have the
least thermal stability, whereas Zn-HQDMA polymeric
chelate is the most stable of the five metal polymeric
chelates having thermal stability comparable to that of
parent copolymeric ligand (HQDMA). Thus, the results
of thermogravimetric analyses of polymeric ligand and
its polymeric chelates reveal that the polymeric ligand is
thermally more stable than its polymeric metal chelates.
Acknowledgements
The authors are thankful to Professor J. S. Parmar, Head, De-
partment of Chemistry, Sardar Patel University, for providing
the necessary research facilities.
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