Synthesis, spectral and thermal studies on pyrazolatebridged palladium(II) coordination polymers

Article abstract of DOI:10.1007/s10973-006-7863-0

Synthesis, spectroscopic characterization and thermal behavior of pyrazolate-bridged palladium complexes [Pd(μ-Pz)₂]_n (1), [Pd(μ-mPz)₂]_n (2), [Pd(μ-dmPz) ₂]_n (3), [Pd(μ-IPz)₂]

Full text of DOI:10.1007/s10973-006-7863-0

Journal of Thermal Analysis and Calorimetry, Vol. 87 (2007) 3, 789–792
SYNTHESIS, SPECTRAL AND THERMAL STUDIES ON PYRAZOLATE-
BRIDGED PALLADIUM(II) COORDINATION POLYMERS
A. V. G. Netto1*, Regina C. G. Frem1, A. E. Mauro1, Marisa S. Crespi1 and H. E. Zorel Jr.2
1Instituto de Química de Araraquara, UNESP, C.P. 355, 14801-970 Araraquara-SP, Brazil
2Faculdades Integradas Católicas de Palmas, Palmas, PR, Brazil
Synthesis, spectroscopic characterization and thermal behavior of pyrazolate-bridged palladium complexes [Pd(μ-Pz)2]n (1),
[Pd(μ-mPz)2]n (2), [Pd(μ-dmPz)2]n (3), [Pd(μ-IPz)2]n (4) {pyrazolate (Pz–), 4-methylpyrazolate (mPz–), 3,5-dimethyl-
pyrazolate (dmPz–), 4-iodopyrazolate (IPz–)} have been described in this work. The exobidentate coordination mode of pyrazolato
ligands in 1–4 was inferred on basis of IR spectroscopic evidences. TG investigations indicated that the introduction of substituents
at the 4 position in the pyrazolyl moiety into coordination polymers do not affect significantly their thermal stability, whereas at
the 3 and 5 position reduced the stability of the main chain. Metal palladium was the final product of the thermal decompositions,
which was identified by X-ray powder diffraction.
Keywords: coordination polymers, DTA, palladium(II), pyrazolates, TG
Introduction
Following up our interest in mono and polynuclear
systems involving transition metals and pyrazolyl-type
ligands [10–13] as well in their thermal behav-
ior [14–16], we describe in the present study the synthe-
sis, spectroscopic characterization and thermal studies
by TG and DTA on a series of palladium(II) pyrazolate
compounds [Pd(μ-Pz)2]n (1), [Pd(μ-mPz)2]n (2),
[Pd(μ-dmPz)2]n (3), [Pd(μ-IPz)2]n (4) {pyrazolate (Pz-),
4-methylpyrazolate (mPz-), 3,5-dimethylpyrazolate
(dmPz-), 4-iodopyrazolate (IPz-)} (Scheme 1).
A large area of coordination and supramolecular chem-
istry has been develop around with pyrazolate anions. In
particular, homoleptic pyrazolate complexes are attract-
ing interest in the recent years since they exhibit a rich
structural diversity (trimers, tetramers, hexamers or co-
ordination polymers may result depending on the reac-
tion conditions, the side chains at the pyrazolate ligand
and the geometry at the metal centres) [1–3], their po-
tential use as liquid crystals [4], antimicrobial drugs [5],
olefin cyclopropanation catalysts [6], phosphorescent
materials [7, 8] and CVD source reagents [9].
Experimental
Preparation of the complexes
Compounds 1–4 were synthesized as follows: 2 mL of
a methanolic mixture containing the appropriate
amounts of pyrazolyl ligand (0.61 mmols) and tri-
ethylamine (0.1 mL, 0.61 mmols) was added drop-
wise to a deep orange solution of [PdCl2(meCN)2]
(75 mg, 0.29 mmols) in 10 mL of MeOH affording a
clear suspension. The solids were separated by filtra-
tion, washed vigorously with MeOH and dried under
vacuum. This procedure, which differs from that pre-
viously reported by Minghetti et al. [17] and
Ardizzoia et al. [18], allowed the separation of the in-
soluble fine powders of the polymers. It is well
known that soluble cyclic trinuclear species of gen-
eral formulae [Pd3(μ-L)6]3 (L=pyrazolate ligand) are
also formed in the reaction media [19].
Scheme 1
*
Author for correspondence: adelino@iq.unesp.br
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2007 Akadémiai Kiadó, Budapest

NETTO et al.
Instrumentation
thermogravimetric data together with the IR spectros-
copy results confirmed the proposed formulas of the
synthesized compounds. The polymeric nature of the
obtained compounds was inferred by their intractability,
high stability and amorphous nature. The analytical data
are presented in Table 1.
Elemental analysis of carbon, nitrogen, and hydrogen
were performed on a CE Instruments, model
EA 1110 – CHNS-O microanalyser. Infrared spectra
were recorded on a Nicolet FTIR-Impact 400
spectrophotometer in the 4000–400 cm–1 spectral
range using KBr pellets. Thermal analyses (TG,
DTA) were carried out using a TA SDT 2960 model,
under dry synthetic air (flow rate: 100 mL min–1) tem-
perature up to 900°C and at heating rate of 10°C min–1
in α-alumina sample holders. The reference substance
was pure α-alumina in the DTA measurements. X-ray
powder diffraction patterns of the residues were ob-
tained using Zeiss HGZ4/B horizontal diffractometer
(G.D.R.) equipped with a proportional counter and
pulse height discriminator. The Bragg–Bretano ar-
rangement was adopted using CuKα radiation
(λ=1.541 ) and setting of 34 kV and 20 mA. The
peaks were identified using ICDD bases [20].
Infrared spectra
The most significant band of the complexes with their
tentative assignments are presented in Table 2. The
exobidentate coordination mode of the pyrazolate lig-
ands was clearly detected in the IR spectra of 1–4.
Firstly, the absence of the strong and broad νNH band at
ca. 3400–3200 cm–1 is an indication of the deprotona-
tion of the pyrazolyl ligands. Secondly, the shift of the
absorption band attributed to the ring breathing mode
to lower frequency (1491–1530 cm–1) when compared
with that one of the free ligand (1533–1596 cm–1) is
characteristic of exobidentate coordination mode [10].
Thirdly, the appearance of the expected two γCH bands
at 868 and 744 cm–1 for 1 agrees well with the
exobidentate character of pyrazole instead of three
ones for monodentate pyrazole complexes [10]. Tak-
ing into account that two γCH bands are expected for
monodentate 4-substituted-pyrazolyl derivatives [10],
the presence of only one γCH absorption in the spectra
of 2 (818 cm–1) and 4 (825 cm–1) also supports the
exobidentate coordination.
Results and discussion
Reactions of the [PdCl2(MeCN)2] precursor with
pyrazole-type ligands in the presence of the non-coordi-
nating weak triethylamine base lead to the pyrazolate-
bridged palladium coordination polymers 1–4, as intrac-
table and air stable solids. The elemental analyses and
Table 1 Results of chemical analyses of the compounds 1–4
Carbon/%
found
Nitrogen/%
found
Hydrogen/%
found
Compound
Color
Formula
calc.
29.96
35.77
40.49
14.64
calc.
23.29
20.86
18.88
11.38
calc.
2.51
3.75
4.76
0.82
[Pd(μ-Pz)2]n
1
2
3
4
white
white
C6H6N4Pd
C8H10N4Pd
30.10
35.96
40.25
14.56
23.49
21.05
19.01
11.42
2.49
3.58
4.94
0.88
[Pd(μ-mPz)2]n
[Pd(μ-dmPz)2]n
[Pd(μ-IPz)2]n
pale yellow C10H14N4Pd
white C6H4N4I2Pd
Table 2 Selected vibrational data (cm–1) of pyrazolate-bridged palladium(II) coordination polymers 1–4
ν/cm–1
Assignment
[Pd(μ-Pz)2]n (1)
[Pd(μ-mPz)2]n (2)
[Pd(μ-dmPz)2]n (3)
[Pd(μ-IPz)2]n (4)
3130w
1491mw
–
3128w
–
3128sh
1530m
1443sh
1420s, 1379m
1349ms
–
3124mw
1490w
–
νCH
ring breathing
δas(CH
1463mw
1381s
1323ms
–
)
3
1385m
–
1378m
–
νring+βring
νanel+βCH
βCH+νring
νring+βCH
βCH
1288mw
1184m
1062s
–
1299m
1165m
1066s
943m
825s
1177m
1084s
1015mw
818s
1152mw
1053m
–
βring
868w, 744s
624mw
763ms
651w
γCH
626m
612m
γring
790
J. Therm. Anal. Cal., 87, 2007

PYRAZOLATE-BRIDGED PALLADIUM(II) COORDINATION POLYMERS
Thermal analysis
down of the polymer backbone affording a mixture of
Pd (ASTM 05-0681) and PdO (ASTM 06-0515) [20]. A
slight mass increase of 4.03% between 371–799°C is as-
cribed to the formation of additional PdO which further
degrades to Pd (ASTM 05-0681) in the last mass loss
(6.33%) at 799–847°C. The final residue of 44.60%
agrees well with the calculated amount of Pd (44.23%).
The TG-DTA curves for compound 2 indicated
that the first step (125–336°C) is accompanied
by 11.15% mass loss assigned to the decomposition
of the methyl side chains (calcd. 11.20%). Such elimi-
nation of methyl substituents does not affect the sta-
bility of the main chain until 336°C, because the mass
loss is only ca. 11% at this temperature, indicating
that the polymer backbone of 2 is very stable be-
low 336°C. A further heating to 406°C resulted in an
abrupt mass loss of 49.22% associated to the degrada-
tion of the main polymeric chain, yielding Pd as a
residue (calcd. 39.61%, found 39.70%).
TG and DTA curves obtained for compounds 1–4 are
shown in Fig. 1 and the results of the thermal investi-
gations are listed in Table 3.
The TG curves showed that the amorphous sol-
ids 1–4 decomposed without melting. Compound 1 is
thermally stable up to 301°C, then undergoes a subse-
quent mass loss caused by the oxidative thermal break-
Compound 3 started to degrade at lower tempera-
ture (256°C) than 1. Afterwards, a mass loss of 60.84%
is caused by the thermal breakdown of the main chain,
leading to a mixture of Pd (ASTM 05-0681) and PdO
(ASTM 06-0515) [20]. A progressive mass gain
of 2.15% up to 813°C is ascribed to the formation of
additional PdO which further decomposed to Pd
(calcd. 35.87%, found 35.52%) at 813–8595°C.
Compound 4 decomposed more slowly than the
other coordination polymers. A small mass loss of
10.37% upon heating of the sample in the range of
114–298°C is attributed to the partial release of iodine
at the 4 position on the pyrazolyl ring. Similarly to the
thermal behavior of 2, the main chain of 4 remains sta-
ble until 298°C. A further increase in the temperature
Fig. 1 — – TG and ⋅⋅⋅ – DTA curves of the 1 – [Pd(μ-Pz)2]n,
2 – [Pd(μ-mPz)2]n, 3 – [Pd(μ-dmPz)2]n, 4 – [Pd(μ-IPz)2]n
complexes
Table 3 Representative thermoanalytical data of compounds 1–4
Δm/%
DTA peaks/°C
Assignment
Complex
Step
ΔT/°C
obt.
calc.
endo
exo
1
301–371
371–799
799–847
–53.10
+4.03
–6.33
44.60
–53.11
+3.99
–6.65
44.23
–
–
362
–
–2Pz, +0.2O2
+0.3O2
2
3
1
817
–
–0.5O2
residue
1
125–336
336–406
406–817
817–854
–11.15
–49.22
+5.92
–5.85
39.70
–11.20
–49.19
+5.96
–5.96
39.61
–
–
–
383
–
–2CH3
–2Pz
2
2
3
4
3
4
+0.5O2
–0.5O2
832
–
residue
1
256–364
364–813
813–859
–60.84
+2.15
–5.79
35.52
–60.90
+2.16
–5.39
35.87
–
–
302
–
–2dmPz, +0.3O2
+0.2O2
2
3
832
–
–0.5O2
residue
1
114–298
298–525
525–822
822–852
–10.37
–66.77
+1.67
–2.79
21.74
–10.36
–66.73
+1.61
–2.93
21.61
–
–
–
–0.2I2
–2Pz–, –0.8I2, +0.2O2
+0.25O2
2
390, 513
3
4
–
–
–
834
–0.45O2
residue
J. Therm. Anal. Cal., 87, 2007
791

NETTO et al.
4 S. J. Kim, S. H. Kang, K. M. Park, H. Kim, W. C. Zin,
to 525°C resulted in the breakdown of the polymer
backbone caused by a considerable mass loss of
66.77%, affording a mixture of Pd (ASTM 05-0681)
and PdO (ASTM 06-0515) [20]. The partial oxidation
of the Pd content in the mixture is related to the slight
mass gain of 1.67% between 525–822°C. The decom-
position of PdO to Pd (calcd. 21.61%, found 21.74%)
occurred at 822–852°C.
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Synthesis, characterization and thermal behavior of
[Pd(μ-Pz)2]n (1), [Pd(μ-mPz)2]n (2), [Pd(μ-dmPz)2]n (3),
[Pd(μ-IPz)2]n (4) have been described in this work. TG
studies showed that the introduction of substituents at
the 4 position on the pyrazolyl moiety into coordination
polymers does not affect significantly their thermal sta-
bility. While the unsubstituted polymer 1 is stable up
to 301°C the decomposition of the main polymeric
chain started at about 336°C (2) and 298°C (4) when
methyl groups and iodine, respectively, are attached to
these polymers. On the other hand, the coordination
polymer 3 started to degrade at a temperature lower than
the other species, probably due to the steric hindrance
introduced by methyl groups at 3 and 5 position, adja-
cent to the coordination sites of the pyrazolyl ring.
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We thank the CNPq, CAPES and FAPESP for financial support.
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J. Therm. Anal. Cal., 87, 2007