Synthesis, spectral and thermal studies on dicarboxylate-bridged palladium(II) coordination polymers. Part i

Article abstract of DOI:10.1007/s10973-009-0252-8

This work describes the synthesis, IR and ¹³C CPMAS NMR spectroscopic as well the thermal characterization of the new dicarboxylate complexes [Pd₂(ox)₂(4,4'-bipy)]n (1), [Pd ₂(ox)₂(bpe)]_n (

Full text of DOI:10.1007/s10973-009-0252-8

J Therm Anal Calorim (2009) 97:123–126
DOI 10.1007/s10973-009-0252-8
Synthesis, spectral and thermal studies on dicarboxylate-bridged
palladium(II) coordination polymers. Part I
R. L. Fernandes Æ P. M. Takahashi Æ R. C. G. Frem Æ
A. V. G. Netto Æ A. E. Mauro Æ J. R. Matos
ICTAC2008 Conference
´
´
Ó Akademiai Kiado, Budapest, Hungary 2009
Abstract This work describes the synthesis, IR and 13C
CPMAS NMR spectroscopic as well the thermal charac-
terization of the new dicarboxylate complexes [Pd2(ox)2
(4,40-bipy)]n (1), [Pd2(ox)2(bpe)]n (2) and [Pd2(ox)2(pz)]n
(3) {ox = oxalate, bipy = 4,40-bipyridine, bpe = 1,2-bis
(4-pyridyl)ethane, pz = pyrazine}. TG experiments reveal
that compounds 1–3 undergo thermal decomposition in
three steps. Metal palladium was the final product of the
thermal decompositions, which was identified by X-ray
powder diffraction.
extensive use of this old but evergreen ligand are: (a) its
remarkable ability to mediate electronic effects between
metal centers affording compounds with a wide range of
magnetic properties [4, 5] and (b) the prevalence of its rigid
bis-bidentate mode providing some degree of predictability
with regard to the structural characteristics of the resulting
metal-oxalato networks.
Following up our interest in supramolecular systems
involving transition metals and N-based-ligands as well in
their thermal behavior [6, 7] we describe in the present
study the synthesis, spectroscopic characterization and
thermal studies on the series of palladium(II) oxalate
compounds [Pd2(ox)2(4,40-bipy)]n (1), [Pd2(ox)2(bpe)]n (2),
and [Pd2(ox)2(pz)]n (3) {ox = oxalate, bipy = 4,4’-bipyr-
idine, bpe = 1,2-bis(4-pyridyl)ethane, pz = pyrazine}.
Keywords 13C CPMAS NMR ꢀ IR ꢀ Oxalate ꢀ
Palladium(II) ꢀ Thermal behavior
Introduction
Research on metal-organic frameworks (MOFs) has rapidly
been expanding due to their interesting topologies and
potential applications as functional materials in catalysis
and gas adsorption [1, 2]. Particularly, dicarboxylic acids
and linear ligands bearing nitrogen (for instance 4,40-
bipyridine) have been used successfully as building blocks
in the construction of such interesting architectures due to
their ability to act as linkers between metals [3]. Con-
cerning the oxalate group, the main reasons for the
Experimental
Synthesis of the complexes
Compounds 1–3 were synthesized as follows: 3 mL of a
methanolic mixture containing the appropriate amount of
pyridil ligand (76.8 mmols) and sodium oxalate (51.6 mg,
38.5 mmol) was added dropwise to a deep orange solution
of [PdCl2(CH3CN)2] (100 mg, 38.5 mmols) in 10 mL of
MeOH. Yellow suspensions were obtained, filtered off and
the solids were washed with water, methanol and dried in
vacuum.
R. L. Fernandes ꢀ P. M. Takahashi ꢀ R. C. G. Frem (&) ꢀ
A. V. G. Netto ꢀ A. E. Mauro
´
Instituto de Quımica de Araraquara, UNESP, P.O. Box 355,
Methods
Araraquara 14801-970, SP, Brazil
e-mail: rcgfrem@iq.unesp.br
The elemental analysis of carbon, nitrogen and hydrogen
were performed on a CE Instruments, model EA 1110-
CHNS-O microanalyser.
J. R. Matos
Instituto de Quımica-USP-SP, P.O. Box 26077, Sa˜o Paulo, SP,
Brazil
´
123

124
R. L. Fernandes et al.
with IR spectroscopy and 13C CPMAS NMR results con-
firmed the proposed formula for the synthesized com-
pounds (see Scheme 1). The polymeric nature of the
obtained compounds was inferred by their intractability
and amorphous nature. The analytical data are presented in
Table 1.
Infrared spectra
The most significant bands of the complexes with their
tentative assignments are presented in Table 2. The bis-
chelating coordination mode of the oxalate ligand was
clearly detected in the IR spectra of 1–3 via the presence of
the characteristic bands at 1635–1645 cm-1 (masCOO-),
1315–1325 cm-1 (msCOO-) and 775–780 cm-1 (dO–C–O)
[9, 10]. IR experiments also revealed the coordination of
the linear bridging ligands in these complexes by the
appearance of the typical mCN and dCH bands at 1431–
1411 cm-1 and 815–833 cm-1, respectively [11, 12].
Scheme 1 Representation of the probable structural formulae of
the coordination polymers [Pd2(ox)2(L)]n (L = 4,40-bipyridine; 1,2-
bis(4-pyridyl)ethane; pyrazine)
Infrared spectra were recorded on a Nicolet FTIR-
Impact 400 spectrophotometer in the 4000–400 cm-1
spectral range using KBr pellets.
The 13C CP NMR spectra were obtained on a Varian
Unity INOVA spectrometer operating at 300 MHz for
carbon. Samples were packed in a silicon nitride rotor
(OD = 7 mm). Chemical shift values were referenced
externally to tetramethylsilane at 0 ppm.
13C CPMAS NMR spectra
The 13C CPMAS NMR results are given in Table 3. The
13C CPMAS NMR spectra of 1 and 2 clearly indicate the
coordination of oxalate ligand by the appearance of its
typical signal at 166.13 and 165.96 ppm, respectively [13].
TG experiments were carried out using a Shimadzu
system model TGA-50. The TG curves were registered
under flow of dry synthetic air (100 mL min-1), tempera-
ture up to 500 °C and heating rate of 20 °C min-1 in
platinum sample holders.
Table 2 Selected vibrational data (cm-1) of the palladium(II) com-
plexes 1–3
.
ꢁ
X-ray powder diffraction patterns of the residues were
measured using SIEMENS D-5000 diffractometer equip-
ped with a proportional counter and pulse height discrim-
inator. The Bragg-Bretano arrangement was adopted using
v
ð
cmꢁ1
Þ
1
2
3
Assignment
3409w
3043w
1606ms
1411s
815s
3415w
3045w
1605ms
1431m
833m
3440w
3038w
–
m(OH)
˚
m(CH)ar
m(CC)
CuKa radiation (k = 1.541 A) and setting of 34 kV and
20 mA. The peaks were identified using PDF bases [8].
1421s
815m
518m
1642ms
1319F
776m
m(CN)
d(CH)
Results and discussion
513m
518m
m(Pd–O)
mas(COO-)
ms(COO-)
d(O–C–O)
1636ms
1321s
776m
1643s
1321m
775m
Reactions of the [PdCl2(MeCN)2] precursor with pyridyl-
type ligands in the presence of oxalate groups lead to the
air stable dicarboxylate-bridged palladium solids 1–3. The
elemental analyses and thermogravimetric data, together
m medium, s strong, ms medium–strong, w weak
Table 1 Results of chemical analysis and melting points of compounds 1–3
Compound
Color
Formula
Carbon/%
Found
Nitrogen/%
Found
Hydrogen/%
Calc.
Calc.
Found
Calc.
[Pd2(ox)2(4,40bipy)]n 1
[Pd2(ox)2(bpe)]n 2
[Pd2(ox)2(pz)]n 3
Yellow
Yellow
Yellow
C14H8N2O8Pd2
C16H10N2O8Pd2
C8H4N2O8Pd2
30.3
32.9
20.6
30.85
33.65
20.49
4.9
4.8
6.0
5.14
4.91
5.97
2.2
2.1
0.9
1.48
1.76
0.86
123

Synthesis, spectral and thermal studies
125
Table 3 13C CPMAS NMR
data (ppm) for compounds
1 and 2
Numbering scheme
Compound
Chemical (shift/ppm)
Assignment
C=O
1
166.13
0
CA, CA
153.02
0
Cq, Cq
147.76
0
126.40, 122.51
CB, CB
2
165.96
153.10
146.38
131.25
120.89
C=O
0
CA, CA
0
Cq, Cq
–CH=CH–
0
CB, CB
13
The C signals at 153.02 (CA, CA ), 147.76 (Cq), 126.40
0
0
and 122.51 ppm (CB, CB ) agree well with the presence of
4,40-bipyridine in 1 (see numbering scheme in Table 3).
For the complex 2, the coordination of 1,2-bis(4-pyri-
dyl)ethane (bpe) is evidenced by the aromatic signals at
0
0
153.10 (CA, CA ), 146.38 (Cq), and 120.89 ppm (CB, CB )
as well the resonance of –CH=CH– moiety at 131.25 ppm
[14]. The 13C CPMAS NMR spectrum of 3 could not be
registered due to its lower yield synthesis.
Thermogravimetric analysis
The TG curves for the complexes 1–3 are illustrated in
Fig. 1. Table 4 lists the results of the thermal studies of
these compounds together with the assignments of each
decomposition stage based on mass calculation. Therefore,
the groups indicated at the right column of the Table 2 do
not correspond necessarily to the gaseous final products of
decomposition.
The TG curve of 1 indicates that this compound is
thermally stable up to 167 °C, then undergoes two mass
losses y oxidative thermal breakdown of the polymer
backbone, giving rise to Pd0 (ASTM 05-0681) [8] as res-
idue at 444 °C. A slight mass increase of 2.8%, between
444 and 835 °C is ascribed to the partial oxidation of Pd0 to
PdO which further degrades to metallic palladium in the
last mass loss (3.3%) at 835–867 °C. The final mass per-
cent residue of 39.30% agrees well with the calculated
amount of Pd (39.05%).
Fig. 1 TG curves for the complexes 1–3
metallic Pd is noticed in the last mass loss (2.6%) between
824 and 873 °C (Calcd. 2.77%).
Compound 3 decomposed faster than the coordination
polymers 1 and 2. The TG curve indicates that 3 is ther-
mally stable up to 130 °C, then decomposes into Pd0 in
three steps. The first step between 130 and 373 °C is
accompanied by 51.7% mass loss, which is attributed to the
elimination of oxalate and pirazine ligands and uptake of
O2 (Calcd. 51.21%) yielding a mixture of Pd0 (05-0681)
and PdO (ASTM 06-0515) [8]. A progressive mass gain of
3.1% up to 724 °C is ascribed to the formation of addi-
tional PdO which further decomposed into Pd (Calcd.
45.38%, Found 45.7%) at 724–883 °C.
Compound 2 showed a similar thermal behavior to 1.
The first step between 191 and 474 °C is accompanied by
two mass losses which are attributed to the elimination of
oxalate and bpe ligands, yielding Pd0 as residue (ASTM
05-0681) [8]. The increase of mass (2.3%) observed at
474–824 °C is probably due to the partial oxidation of Pd0
to PdO by uptake of O2. The decomposition of PdO to
123

126
R. L. Fernandes et al.
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Complex Step
DT (°C) Dm (%)
Assignment
Obt.
Calc.
1
2
3
1
167–444 -60.2 -60.9 -1 bipy; -2 ox
pound: Fe2(C2O4)Cl4(DMF)4 (DMF
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= dimethylformamide).
2
444–835
835–867
?2.8
-3.3
39.3
?3.2 ?0.5 O2
-3.2 -0.5 O2
39.1 Pd
3
´
´
´
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1
191–474 -61.9 -63.0 -1 bpe; -2 ox
2
474–824
824–873
?2.3
-2.6
37.8
?2.8 ?0.5 O2
-2.8 -0.5 O2
37.0 Pd
3
Residue
1
130–373 -51.7 -51.2 -1 pz; -2 ox
2
373–717
717–887
?3.1
-5.7
45.7
?3.4 ?0.5 O2
-6.8 -1 O2
45.4 Pd
3
Residue
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´
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This work describes the synthesis, spectroscopic charac-
terization and thermal studies of the new palladium(II)
dicarboxylate complexes [Pd2(ox)2(4,40-bipy)]n (1),
[Pd2(ox)2(bpe)]n (2) and [Pd2(ox)2(pz)]n (3). From the
inspection of TG curves of these neutral [Pd2(ox)2(L)]n
complexes, it is noticed that the stability varies according
to the linear bridging groups, following the order bpe C
4,40-bipy [ pz.
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bpy)2(H2O)2](ClO4)4ꢀ(4,40-H2Bpy) (M = CdII, ZnII and bpy =
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Acknowledgements The authors acknowledge CNPq, FAPESP and
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