Sonochemical synthesis of a new nano-plate lead(II) coordination polymer constructed of maleic acid

Article abstract of DOI:10.1016/j.ica.2010.04.024

A new nano-sized lead(II) coordination polymer of maleic acid (H ₂Mal), [Pb(u.7-Mal)]., (1), has been synthesized by sonochemical method and characterized by scanning electron microscopy, X-ray powder diffraction, elemental analyses and IR spectroscopy. The compound 1 was structurally characterized by single-crystal X-ray diffraction. Thermal stability of nano and bulk samples of compound 1 were studied and compared with each other. After calcination of nano-sized compound 1 at 600 ?C, pure phase microsized lead(II) oxide has been produced.

Full text of DOI:10.1016/j.ica.2010.04.024

Inorganica Chimica Acta 363 (2010) 2506–2511
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Sonochemical synthesis of a new nano-plate lead(II) coordination polymer
constructed of maleic acid
Leila Aboutorabi a, Ali Morsali b,
*
a Department of Chemistry, Payame Noor University, Abhar, Zanjan, Islamic Republic of Iran
b Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14155-4838, Tehran, Islamic Republic of Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
A new nano-sized lead(II) coordination polymer of maleic acid (H2Mal), [Pb(l7-Mal)]n (1), has been syn-
Received 27 February 2010
Received in revised form 6 April 2010
Accepted 13 April 2010
thesized by sonochemical method and characterized by scanning electron microscopy, X-ray powder dif-
fraction, elemental analyses and IR spectroscopy. The compound 1 was structurally characterized by
single-crystal X-ray diffraction. Thermal stability of nano and bulk samples of compound 1 were studied
and compared with each other. After calcination of nano-sized compound 1 at 600 °C, pure phase micro-
sized lead(II) oxide has been produced.
Available online 20 April 2010
Keywords:
Nano-plate
Ó 2010 Elsevier B.V. All rights reserved.
Lead(II) oxide
Maleic acid
Crystal structure
1. Introduction
offered for the preparation of coordination polymers. Some of them
are (1) slow diffusion of the reactants into a polymeric matrix, (2)
Within the field of coordination polymers, efforts to use metal
ions and organic spacers simultaneously have recently been devel-
oped [1–8]. During the past decades, a number of these com-
pounds, with interesting polymeric motifs, have been
successfully designed and synthesized. Some of them exhibit
encouraging potential for application in catalysis, nonlinear optics,
gas separation, magnetic properties, and molecular recognition [5].
Lead(II)–carboxylate systems containing mono-, di-, and polycar-
boxylate [9–12] have been studied extensively in the construction
of metal–organic networks. In contrast to inorganic nanomaterials,
specific syntheses of nano-structured coordination polymers
seems to be surprisingly sparse, and to date most investigations
on coordination polymers have been carried out only in the solid
state and studies of their properties were limited to investigations
at the macroscopic scale. The potential use of coordination poly-
mers as materials for nanotechnological applications would seem
to be very extensive as nanometer-scaled materials often exhibit
the new interesting size-dependent physical and chemical proper-
ties that can not be observed in their bulk analogous.
diffusion from the gas phase, (3) evaporation of the solvent at
ambient or reduced temperatures, (4) precipitation or recrystalli-
sation from a mixture of solvents, (5) temperature controlled cool-
ing and (6) hydrothermal synthesis. In this paper we describe a
simple synthetic sonochemical preparation of nano-structures of
a new coordination polymer, [Pb(l7-Mal)]n (1), {(H2Mal = maleic
acid}, and on their conversion into nano-structured lead(II) oxide
by calcination at moderately elevated temperature. Sonochemistry
is the research area in which molecules undergo a chemical reac-
tion due to the application of powerful ultrasound radiation
(20 kHz–10 MHz) [14]. Ultrasound induces chemical changes due
to cavitation phenomena involving the formation, growth, and
instantaneously implosive collapse of bubbles in a liquid, which
can generate local hot spots having a temperature of roughly
5000 °C, pressures of about 500 atm, and a lifetime of a few micro-
seconds [15]. These extreme conditions can drive chemical reac-
tions which have been developed to fabricate a variety of nano
compounds [14]. In recent years many kinds of nanomaterials have
been prepared by this method [15–25].
Although considerable effort has been dedicated to the con-
trolled synthesis of nanoscale particles of metals, oxides, sulfides,
and ceramic materials, little attention was focused to date on
nanoparticles of supramolecular compounds such as coordination
polymers [13]. Several different synthetic approaches have been
2. Experimental
All reagents and solvents for the synthesis and analysis were
commercially available and were used as received. A multiwave
ultrasonic generator (Sonicator_3000; Misonix, Inc., Farmingdale,
NY, USA) was used. IR spectra were recorded using Perkin–Elmer
597 and Nicolet 510P spectrophotometers. Microanalyses were
* Corresponding author. Fax: +98 2188009730.
E-mail addresses: morsali_a@yahoo.com, morsali_a@modares.ac.ir (A. Morsali).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.04.024

L. Aboutorabi, A. Morsali / Inorganica Chimica Acta 363 (2010) 2506–2511
2507
carried out using a Heraeus CHN-O-Rapid analyzer. Melting points
were measured on an Electrothermal 9100 apparatus and are
uncorrected. The thermal behavior was measured with a PL-STA
1500 apparatus. X-ray powder diffraction (XRD) measurements
were performed using a Philips diffractometer manufactured by
gold coating. The molecular structure plots were prepared using
MERCURY [26] and ORTEPIII [27].
To prepare the nano-plates of [Pb(l7-Mal)]n (1), a 50 ml of a
0.01 M solution of lead(II) nitrate dehydrate in MeOH was posi-
tioned in a high-density ultrasonic probe, and 50 ml of a 0.01 M
maleic acid in MeOH was added dropwisely to that solution. After
the end of the titration the solution remained in the bath for a se-
lected aging time at a selected temperature. The obtained precipi-
tates were filtered off, washed with water and then dried in air.
Product: d.p. >300 °C, yield: 0.257 g (80%). Anal. Calc. for
C4H2O4Pb: C, 14.95; H, 0.63. Found: C, 14.90; H, 0.70%. IR (KBr,
X’pert with monochromatized Cu K
a radiation and simulated
XRD powder patterns based on single crystal data were prepared
using the MERCURY [26]. Single crystal diffraction measurements
were made at 293(2) K using a Nonius kappa-CCD diffractometer.
The intensity data were collected using graphite monochromated
Mo K
a radiation. The structures were solved by direct methods
and refined by full-matrix least-squares techniques on F2. The sam-
cmÀ1
1504(vs), 1540(vs) and 1638(m).
) selected bands: 539(w), 617(m), 1292(m), 1411(vs),
ples were characterized with a scanning electron microscope with
MeOH
calcination
nano-structure of compound 1
by ultrasound
Pb(NO3)2 + H2Mal
PbO
water
single crystals of compound 1
by the branched tube method
Scheme 1. Materials produced and synthetic methods.
Fig. 1. The IR spectra of (a) bulk materials as synthesized of compound [Pb(
l
7-Mal)]n (1) and (b) nano-plats of compound [Pb(
l
7-Mal)]n (1) prepared by sonochemical
method.

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L. Aboutorabi, A. Morsali / Inorganica Chimica Acta 363 (2010) 2506–2511
Fig. 2. The XRD patterns of (a) simulated from single-crystal X-ray data of compound [Pb(l7-Mal)]n (1) and (b) nano-plates of compound [Pb(l7-Mal)]n (1).
To isolate single crystals of [Pb(l7-Mal)]n (1), maleic acid
Product: (0.165 g, yield 51%), m.p. >300 °C. Anal. Calc. for
(0.341 g, 3 mmol) and lead(II) nitrate dehydrate (1 g, 3 mmol) were
placed in the arm to be heated. Water was carefully added to fill
both arms, and then the arm to be heated was placed in a bath
at 60 °C. After 10 days, colorless crystals were deposited in the
cooler arm which were filtered off, washed with water and air
dried.
C4H2O4Pb: C, 14.95; H, 0.63. Found: C, 14.98; H, 0.65%. IR (KBr,
) selected bands: 539(w), 617(m), 1311(m), 1403(vs),
1504(vs), 1535(vs) and 1643(w).
cmÀ1
3. Results and discussion
The reaction between maleic acid (H2Mal) and lead(II) nitrate
using two different routes provided crystalline materials of the
general formula [Pb(l7-Mal)]n (1). Scheme 1 gives an overview of
the methods used for the synthesis of [Pb(
l7-Mal)]n (1) using the
two different routes.
The elemental analysis and IR spectra of the nano-plates and of
the single crystalline material are indistinguishable. The relatively
weak IR absorption bands around 2965 cmÀ1 are due to the C–H
modes involving the aliphatic hydrogen atoms (Fig. 1). The sym-
metric and asymmetric vibrations of the carboxylate group are ob-
served at 1404 and 1532 cmÀ1 as sharp absorption bands. The
D(masym–m
sym) value of 128 cmÀ1 indicates that the carboxylate
groups of ‘‘Mal2À” anions coordinate to the lead(II) centres in a
bridging mode [28].
Fig. 2 shows the simulated XRD pattern from single-crystal X-
ray data (see below) of compound 1 (Fig. 2a), in comparison with
Fig. 3. SEM photographs of [Pb(l7-Mal)]n (1) nano-flower formed by nano-plate
with different scale-bars (concentration of Pb+2 and Mal2À ions are 0.01 M in
MeOH).
Fig. 4. SEM photographs of [Pb(
and Mal2À ions are 0.01 M in water).
l
7-Mal)]n (1) nano-structure (concentration of Pb+2

L. Aboutorabi, A. Morsali / Inorganica Chimica Acta 363 (2010) 2506–2511
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Table 1
Crystal data and structure refinement [Pb(
l
7-Mal)]n (1).
Compound 1
Identification code
Empirical formula
Formula weight
C4H2O4Pb
321.25
T (K)
293(2)
Wavelength (Å)
Crystal system
Space group
0.71073
monoclinic
P21/c
Unit cell dimensions
a (Å)
b (Å)
c (Å)
9.8910(10)
6.9610(10)
8.2710(10)
111.13(5)
531.18(11)
4
4.017
31.686
560
0.25 Â 0.25 Â 0.08
2.21–30.12
À13 6 h 6 10
À9 6 k 6 9
À11 6 l 6 11
8341
b (°)
V (Å3)
Z
Density (calculated) (Mg/m3)
Absorption coefficient (mmÀ1
F(0 0 0)
)
Crystal size (mm3)
Theta range for data collection (°)
Index ranges
Reflections collected
Independent reflections
Absorption correction
Refinement method
1563
phi and omega scans
full-matrix least-squares on F2
1563/0/83
Fig. 5. A fragment of the two-dimensional network in compound [Pb(l7-Mal)]n (1),
H atoms are omitted for clarity.
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
1.054
Final R [I > 2
r
(I)]
Rl = 0.0311, wR2 = 0.0809
R1 = 0.0359, wR2 = 0.0840
4.062, À2.368
R indices (all data)
Largest difference peak, hole (e ÅÀ3
)
Table 2
Bond lengths (Å) and angles (°) for [Pb(
l7-Mal)]n (1).
Pb1–O1
Pb1–O2
Pb1–O4
Pb1–O3
Pb1–O4i
Pb1–O2ii
Pb1–O3i
2.52(1)
2.41 (1)
2.73(2)
2.61(1)
2.44(2)
2.69(1)
2.84(1)
O2–Pb1–O4i
81.17(2)
52.90(2)
79.98(2)
73.37(2)
72.97(2)
122.69(2)
141.82(2)
70.88(1)
95.94(2)
119.93(2)
80.52(2)
121.85(1)
126.23(2)
48.93(2)
136.42(1)
O2–Pb1–O1
O4i–Pb1–O1
O2–Pb1–O3
O4i–Pb1–O3
O1–Pb1–O3
O2–Pb1–O2ii
O4i–Pb1–O2ii
O1–Pb1–O2ii
O3–Pb1–O2ii
O2–Pb1–O4i
O4–Pb1–O4i
O1–Pb1–O4
O3–Pb1–O4
O2–Pb1–O4i
Fig. 6. Schematic representation of Pb(II) environment: in compound [Pb(
l7-Mal)]n
(1) (i: Àx, y + 1/2, Àz + 1/2. ii: Àx, Ày, Àz). Ellipsoids 30% probability.
Symmetry operations: i: Àx, y + 1/2, Àz + 1/2. ii: Àx, Ày, Àz.
120
100
60
the XRD pattern of a typical sample of compound 1 prepared by the
sonochemical process (Fig. 2b). Acceptable matches, with slight
differences in 2h, were observed between the simulated and exper-
imental powder X-ray diffraction patterns (Fig. 2b). This indicates
that the compound obtained by sonochemical process as nano-
places is identical to that obtained by single crystal diffraction.
The significant broadening of the peaks indicates that the particles
are of nanometer dimensions. Estimated by the Sherrer formula,
D = 0.891 k/bcos h, where D is the average grain size, k the X-ray
wavelength (0.15405 nm), and h and b the diffraction angle and
full-width at half maximum of an observed peak, respectively,
the average size of the particles is 60 nm, which is in agreement
with that observed by scanning electron microscopy, as shown in
Fig. 3.
TG
DTA
50
40
30
20
10
0
80
60
40
20
0
-10
800
0
200
400
600
Temperature(°C)
Fig. 7. Thermal behavior of compound [Pb(l7-Mal)]n (1) at nano-size.
Fig. 3 indicates the original morphology of the nano-plate flow-
er-like structure with the diameter varying between 40 and 70 nm.
The morphology of compound 1 prepared by the sonochemical
method (Fig. 3) is very interesting and it is composed of nano flow-
er-like formed by nano-plates with sizes of about 40–70 nm. The
different concentrations of lead(II) nitrate and ligand Mal2À

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L. Aboutorabi, A. Morsali / Inorganica Chimica Acta 363 (2010) 2506–2511
Fig. 8. XRD pattern PbO prepared by calcination of compound [Pb(l7-Mal)]n (1).
solution (both of 0.001 and 0.01 M in MeOH and water) were
tested, however, appropriate nano-size of compound 1 obtained
under concentration of 0.01 M in MeOH. According to Fig. 4, the
particles obtained under concentration of 0.01 M in water have
large sizes and the morphology of compound is formed from mix-
ture of nano-plate and nano-wires. The IR spectrum and XRD pat-
425 °C is suggested to be PbO (observed 68.70, calcd: 69.45%).
Nano-sized plates of compound 1 are less stable and start to
decompose at 240 °C. Detectable decomposition of the nanoparti-
cles of 1 thus starts about 95° earlier than that of its bulk counter-
parts, probably due to the much higher surface to volume ratio of
the nano-sized particles, as more heat is needed to annihilate the
lattices of the single crystals. The DTA curve displays three exo-
thermic effects at 350, 392 and 425 °C for the single crystals of
compound 1 (Fig. S2). The difference between the two maximum
intensities in the DTA curves also indicate, in agreement with the
TGA results, the lower stability of the nano-plates when compared
with their single crystal counterparts (Fig. 7).
Micro-crystals PbO have been generated by thermal decomposi-
tion of nano-sized compound 1. The final product upon calcination
of compound 1 is, based on their XRD patterns (Fig. 8), PbO. The
phase purity of the as-prepared tetragonal PbO micro-crystals are
completely obvious and all diffraction peaks are perfectly indexed
to the orthorhombic PbO structure with the PbO lattices with the
parameters of a = 5.8931(4) Å, b = 5.4904(4) Å, c = 4.7528(4) Å,
Z = 4 in Pbcm for PbO (JCPDS card file No. 77-1971). No character-
istic peaks of impurities are detected in the XRD pattern.
tern of
a typical sample of compound 1 prepared by the
sonochemical process under concentration of 0.01 M in water is
also same with the crystalline sample. This point shows that the
reaction in two different solvents produces the same product but
the morphology and size of compounds are difference.
Determination of the structure of compound 1 by X-ray crystal-
lography shows the complex in the solid state (Figs. 5, 6 and S1) to
be a 2D polymeric network. A view of the coordination environ-
ment around the lead(II) ion in compound 1 is shown in Fig. 6,
and Table 1 lists selected bond distances and angles. The title com-
plex crystallizes in monoclinic with space group P21/c. Single-crys-
tal X-ray diffraction analysis of reveals that the lead(II) ion in
compound 1 is coordinated by seven oxygen atoms of ‘‘Mal2À” an-
ions, resulting in a seven-coordinate complex with a PbO7 chromo-
phore. The one of carboxylate moiety of the ‘‘Mal2À” ligand acts as
both bidentate, and bridging group (totally tridentate) in a
l
-1,3
Fig. 9 shows SEM images of PbO micro-crystals, produced by
calcination of nano-sized compound 1. However, the precursor
mode, where two oxygen atoms of the carboxylate group biden-
tately coordinate to a lead(II) ion, creating a four-membered che-
late ring, and one of them also bridges two adjacent lead(II) ions.
The second of carboxylate moiety of the ‘‘Mal2À” ligand acts as
both bidentate, and bridging group (totally tetradentate) in a
l-
2,4 mode, yielding a two-dimensional network. The intrachain dis-
tance between two neighboring lead(II) ions is 4.221 Å.
The lead(II) ion in 1 is seven-coordinate and displays hemidi-
rected geometry with a stereo-chemically active lone pair as ex-
pected (Fig. 6). Additionally, the O2–Pb–O4i angle of 136.42(14)°
suggests that there is a hole in the coordination sphere of 1 due
to a lone pair-bond pair repulsion. The observed shortening of this
Pb–O bond [bond distances for Pb1–O2 and Pb1–O2ii of 2.412(4)
and 2.686(4) Å (Table 2)] supports the presence of this feature [29].
To examine the thermal stability of the nano-sized plates and
the single crystals of compound 1, thermal gravimetric (TG) and
differential thermal analyses (DTA) were carried out between 20
and 600 °C under nitrogen flow (Figs. 7 and S2). Compound 1 at
bulk material does not melt and is stable up to 335 °C at which
temperature it begins to decompose. Decomposition of the ‘‘Mal2À
”
ligand takes place between 335 and 425 °C with three exothermic
effects at 350, 392 and 425 °C. The solid residue formed at around
Fig. 9. SEM photograph of PbO microparticles produced by calcinations of nano-
structured compound [Pb(l7-Mal)]n (1).

L. Aboutorabi, A. Morsali / Inorganica Chimica Acta 363 (2010) 2506–2511
2511
with morphology of nano-plates gives micro-crystals PbO and the
morphology of the PbO particles is quite different from compound
1. This point may be due to complete decomposition and breakup
of the compound with change of morphology of nano-structure.
ta_request/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/
j.ica.2010.04.024.
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A
new Pb(II) coordination polymer, {[Pb(l7-Mal)]n (1),
H2Mal = maleic acid} has been synthesized using a thermal gradi-
ent approach and by sonochemical irradiation. Compound 1 was
structurally characterized by single-crystal X-ray diffraction. The
crystal structure of compound 1 consists of a two-dimensional
polymer and shows the coordination number in the Pb(II) ions is
seven. Reduction of the particle size of the coordination polymers
of 1 to a few dozen nanometers results in lower thermal stability
when compared to the single crystalline samples. Calcination un-
der air produces micro-sized particles of PbO. It is interesting to
note that this precursor is one of the few samples prepared by son-
ication as an alternative synthetic procedure to form nano-sized
particles of a coordination polymer. This method of preparation
may have some advantages such as: it takes place with shorter
reaction times, produces better yields and it also is likely to pro-
duce nano-sized particles of the coordination polymer.
Acknowledgements
This work was supported by the Payame Noor and Tarbiat Mod-
ares Universities.
Appendix A. Supplementary material
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Madison, Wisconsin, USA, 2000–2003.
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Compounds, 5th ed., John Wiley and Sons, New York, 1997.
[29] L. Shimoni-Livny, J.P. Glusker, C.W. Bock, Inorg. Chem. 37 (1998) 1853.
CCDC 660710 contains the supplementary crystallographic data
for 1. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/da-