Thermal decomposition of palladium(ii) pyrazolyl complexes: Part II

Article abstract of DOI:10.1007/s10973-005-0061-7

Thermal behavior of pyrazolyl complexes [PdCl₂(HPz)₂] (1), [PdCl₂(HdmPz)₂] (2), [Pd(SCN)₂(HPz) ₂] (3), [Pd(SCN)₂(HdmPz)₂] (4), [Pd(N ₃)₂(HdmPz)

Full text of DOI:10.1007/s10973-005-0061-7

Journal of Thermal Analysis and Calorimetry, Vol. 79 (2005) 339–342
THERMAL DECOMPOSITION OF PALLADIUM(II) PYRAZOLYL
COMPLEXES
Part II*
A. V. G. Netto1**, A. M. Santana1, A. E. Mauro1, Regina C. G. Frem1, E. T. de Almeida2,
Marisa S. Crespi1 and H. E. Zorel Jr.3
1Instituto de Química de Araraquara-UNESP-C.P. 355, 14801-970 Araraquara-SP, Brazil
2Universidade Católica de Brasília - UCB, Brasília-DF, Brazil
3Faculdades Integradas Católicas de Palmas, Palmas-PR, Brazil
Thermal behavior of pyrazolyl complexes [PdCl2(HPz)2] (1), [PdCl2(HdmPz)2] (2), [Pd(SCN)2(HPz)2] (3), [Pd(SCN)2(HdmPz)2] (4),
[Pd(N3)2(HdmPz)2] (5), [Pd(PzNHCO)2] (6) and [Pd(dmPzNHCO)2] (7) (HPz=pyrazole, HdmPz=3,5-dimethylpyrazole) has been studied
by TG and DTA. In general, the thermal stability of [PdX2(HL)2] (HL=HPz, HdmPz) compounds varies in the following order:
HdmPz>HPz as well, according to the trends X=Cl–>SCN–>NNN–. Except for 5, the [PdX2(HL)2] complexes showed higher thermal sta-
bility than the 6 and 7 chelates. No stable intermediates were isolated during the thermal decompositions because of the overlapping deg-
radation processes. The final products of the thermal decompositions were identified as metallic palladium by X-ray powder diffraction.
Keywords: DTA, palladium(II), pseudohalides, pyrazoles, TG
Introduction
The coordination chemistry of pyrazoles has been the
subject of intense researches in recent years due to
their versatility of bonding modes, yielding com-
pounds with large structural diversity and peculiar
properties [2, 3]. Particularly, mononuclear com-
plexes of the [PdCl2(HL)2] (HL=pyrazolyl ligands)
type have received considerable interest due to their
potential use as olefin polymerization catalysts [4]
and liquid crystals [5]. These facts have inspired us to
investigate the chemistry of Pd(II) pyrazolyl systems
containing pseudohalides [6–9] including their ther-
mal behavior. The thermal decomposition of the com-
plexes [PdCl2(HPz)2] (1), [PdCl2(HdmPz)2] (2),
Scheme 1
[Pd(SCN)2(HPz)2] (3), [Pd(SCN)2(HdmPz)2] (4),
[Pd(N3)2(HdmPz)2] (5), [Pd(PzNHCO)2] (6) and
[Pd(dmPzNHCO)2] (7) (HPz=pyrazole, HdmPz=3,5-
dimethylpyrazole) (Scheme 1) using TG and DTA are
described in this present study.
(5) were prepared according to the methods found
in the literature [7, 9, 10]. The [PdCl2(HdmPz)2] (2)
compound was prepared as follows: 3,5-dimethyl-
pyrazole (157 mg; 1.61 mmols) in 2 mL of CH3OH
was added to an orange solution of [PdCl2(MeCN)2]
(200 mg; 0.77 mmols) in 10 mL of CH3OH, resulting
a yellow suspension. The solid was isolated by filtra-
tion, washed with cold CH3OH, and dried under vac-
uum (yield 70%). Compounds [Pd(PzNHCO)2] (6)
and [Pd(dmPzNHCO)2] (7) were prepared in a differ-
Experimental
Preparation of the complexes
The complexes [PdCl2(HPz)2] (1), [Pd(SCN)2(HPz)2]
(3), [Pd(SCN)2(HdmPz)2] (4) and [Pd(N3)2(HdmPz)2]
*
**
Part I: see Ref. [1]
Author for correspondence: adelgodo@posgrad.iq.unesp.br
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2005 Akadémiai Kiadó, Budapest

NETTO et al.
Fig. 2 — – TG and ××× – DTA curves for the complexes
[Pd(PzNHCO)2] (6) and [Pd(dmPzNHCO)2] (7)
Fig. 1 — – TG and ××× – DTA curves for the complexes
[PdCl2(HPz)2] (1), [PdCl2(HdmPz)2] (2),
[Pd(SCN)2(HPz)2] (3), [Pd(SCN)2(HdmPz)2] (4) and
[Pd(N3)2(HdmPz)2] (5)
sample holders. The reference substance was pure
a-alumina for DTA measurements. X-ray powder dif-
fraction patterns of the residues were obtained using a
Zeiss HGZ4/B horizontal diffractometer (G.D.R.)
equipped with a proportional counter and pulse height
discriminator. The Bragg–Bretano arrangement was
adopted using CuKa radiation (l=1.541 ) and set-
ting of 34 kV and 20 mA. The peaks were identified
using ICDD bases [12].
ent way from that reported by Hvastijová et al. [11].
To an orange solution of [PdCl2(MeCN)2] (200 mg;
0.77 mmols) in 10 mL of CH3OH was added
1.61 mmols of the respective pyrazolyl ligand {110 mg
of HPz (6); 155 mg of HdmPz (7)}. After stirring the
mixture for 5 min, KNCO (131 mg; 1.61 mmols), dis-
solved in 2 mL of H2O, was added dropwise, yielding a
clear suspension. The solid was isolated from the sus-
pension by filtration, washed with CH3OH, and dried
under vacuum. (Yield 75% (6), 70% (7)}.
Results and discussion
The elemental analyses and thermogravimetric data,
together with IR spectroscopy results, confirmed the
proposed formula for the obtained compounds. The
results of the analyses and melting points are summa-
rized in Table 1.
Instrumentation
Melting points were determined using a Mettler FP-2
microscope. Elemental analysis of carbon, nitrogen,
and hydrogen were performed on a CE Instruments
microanalyser, model EA 1110 – CHNS-O. Infrared
spectra were recorded in KBr pellets using a Nicolet
FTIR-Impact 400 spectrophotometer in the range
of 4000–400 cm–1. Thermogravimetry (TG) and dif-
ferential thermal analysis (DTA) were carried out us-
ing a TA instrument model SDT 2960, under a dry
synthetic air flow (100 mL min–1), temperature up
to 900°C and heating rate of 20°C min–1, in a-alumina
Infrared spectra
The neutral monodentate coordination of pyrazolyl lig-
ands in 1–5 was evidenced by the presence of intense
nNH bands in ca. 3100 cm–1 as well by the shift of the
ring breathing bands to lower frequencies
(1579–1516 cm–1) when compared to those ones of the
free ligands (1595–1558 cm–1) [8]. The existence of
340
J. Therm. Anal. Cal., 79, 2005

PALLADIUM(II) PYRAZOLYL COMPLEXES
Table 1 Results of elemental analyses and melting points of the compounds 1–7
Carbon/%
Nitrogen/%
found
Hydrogen/%
Complex
m.p./°C
found
calc.
calc.
found
calc.
1
2
3
4
5
6
7
203–204
>250
23.14
32.34
26.60
34.42
31.42
29.02
37.84
22.99
32.50
26.78
34.74
31.38
29.42
37.66
17.82
14.93
23.50
19.92
36.41
25.35
22.20
17.87
15.16
23.43
20.26
36.59
25.73
21.96
2.87
4.40
2.12
4.13
4.55
2.73
4.10
2.57
4.36
2.25
3.89
4.21
2.47
4.21
149–150
206–207
>250
>250
>250
Table 2 Thermal analysis data for compounds 1–7
Dm/%
DTA peaks/°C
Complex
Step
DT/°C
Assignment
obt.
calc.
endo
exo
1
2
3
4
181–359 –44.36
359–584 –17.31
–44.33
–17.13
0.51
–5.10
33.95
206, 214, 300
–
563
–
–2Cl–, –HPz
–HPz, 0.45O2
0.05O2
–
–
826
1
584–805
805–842
0.46
–5.20
33.59
–
–0.50O2
residue
1
2
3
238–372 –69.72
–69.91
3.03
–4.33
28.79
279
–
814
362
–
–
–2Cl–, –2HdmPz, 0.15O2
0.35O2
–0.50O2
372–784
784–845
2.75
–4.45
28.58
2
3
residue
1
2
3
4
132–371 –35.27
371–578 –33.05
–35.17
–33.39
1.78
–3.56
29.66
164
–
–
811
183
488
–
–SCN–, –HPz
–SCN–, –HPz, 0.2O2
0.3O2
578–794
794–840
1.64
–3.72
29.60
–
–0.5O2
residue
1
2
3
4
5
188–275 –37.87
275–436 –12.70
436–496 –22.16
–37.18
–14.00
–21.63
1.54
–3.08
25.65
236, 247
–SCN–, –HdmPz
–SCN–
–HdmPz, 0.2O2
0.3O2
–0.5O2
–
–
458
–
–
–
4
5
496–792
792–831
1.48
–3.21
25.54
813
residue
1
2
3
4
121–253 –26.65
253–332 –42.70
–25.12
–44.15
1.25
–4.18
27.80
–
–
–
819
200
308
–
–HdmPz
–HdmPz,–2N–3 , 0.35O2
0.15O2
332–803
803–845
1.56
–4.55
27.66
–
–0.50O2
residue
1
2
3
4
129–299 –11.80
299–363 –51.31
–13.18
–51.79
2.44
–4.89
32.58
–
–
–
817
–
353
–
–HNCO
–Pz–, –PzNHCO–, 0.25O2
0.25O2
6
7
363–803
803–852
2.58
–6.67
32.80
–
–0.50O2
residue
1
2
3
90–333
333–815
815–847
–69.58
2.49
–4.83
28.08
–70.10
2.08
–4.18
27.80
–
–
827
329
–
–
–2dmPzNHCO–, 0.25O2
0.25O2
–0.50O2
residue
terminal S-bonded thiocyanato groups in 3 and 4 was
detected by the nasSCN bands at 2117 and
2112 cm–1 [7, 9], respectively, whereas the terminal co-
ordination of azido ligand in 5 was inferred on basis of
the nasNNN band at 2044 cm–1 [9]. The formation of
3,5-dimethylpyrazole ligands in the chelates 6 and 7,
originated from the 1,3-dipolar cycloaddition reaction
between the Pd(HL) moiety and NCO group, was con-
firmed by the presence of a strong nCO band at
ca. 1710 cm–1 [10]. The disappearance of the usual
nasNCO absorption in the 2250–2200 cm–1 range, char-
the
1-carbamoylpyrazole
and
1-carbamoyl-
J. Therm. Anal. Cal., 79, 2005
341

NETTO et al.
acteristic of terminally coordinated NCO group [13],
gave further evidence to the formation of the chelated
addition products 6 and 7.
creased thermal stability for [PdX2(HL)2] complexes in
the order HdmPz>HPz and X=Cl–>SCN–>NNN–. In
general, the [PdX2(HL)2] type complexes showed
higher thermal stability than the chelates
[Pd(LNHCO)2] (L=Pz, dmPz). The thermoanalytical re-
sults presented in this work will be important for further
investigations dealing with the application of these com-
pounds in catalytic systems and as antitumor agents.
Thermal analysis
The TG and DTA curves for the compounds 1–5
and 6–7 are shown in Figs 1 and 2, respectively. Ta-
ble 2 lists the results of the thermal studies of these
complexes together with the assignments of each de-
composition step. The TG curves of these compounds
showed a similar thermal degradation pattern in which
the ligands are initially released in 1, 2 or 3 stages, to-
gether with uptake of O2, leading to a mixture of
Pd (ASTM 05-0681) and PdO (ASTM 06-0515) [12].
The slight mass increase up to ca. 800°C is ascribed to
the oxidation of the remaining Pd to PdO. Finally, the
decomposition of PdO to Pd (ASTM 05-0681) is com-
pleted at ca. 840°C.
Acknowledgments
The authors are thankful to CNPq, CAPES and FAPESP for
their financial support.
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Conclusions
The thermal behavior of [PdCl2(HPz)2] (1),
[PdCl2(HdmPz)2]
[Pd(SCN)2(HdmPz)2] (4), [Pd(N3)2(HdmPz)2] (5),
[Pd(PzNHCO)2] (6) and [Pd(dmPzNHCO)2] (7) has
been described in this work. TG studies showed in-
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(2),
[Pd(SCN)2(HPz)2]
(3),
342
J. Therm. Anal. Cal., 79, 2005