Mono and dinuclear Pd(II) complexes with pyrazole and imidazole-type ligands: Synthesis, characterization and thermal behaviour

Article abstract of DOI:10.1007/s10973-008-9667-x

The cyanate-bridged cyclopalladated compound [Pd(C²,N-dmba) (μ-NCO)]₂ (dmba=N,N-dimethylbenzylamine) reacts in acetone with pyrazole (pz), 3,5-dimethylpyrazole (dmpz), imidazole (imz) and 2-methylimidazole (mimz) to give [Pd₂

Full text of DOI:10.1007/s10973-008-9667-x

Journal of Thermal Analysis and Calorimetry, Vol. 97 (2009) 1, 57–60
MONO AND DINUCLEAR Pd(II) COMPLEXES WITH PYRAZOLE AND
IMIDAZOLE-TYPE LIGANDS
Synthesis, characterization and thermal behaviour
V. A. Lucca Neto, A. E. Mauro*, A. V. G. Netto, A. C. Moro and V. M. Nogueira
Instituto de Química de Araraquara, S±o Paulo State University – UNESP, C.P. 355, 14801-970 Araraquara – SP, Brazil
The cyanate-bridged cyclopalladated compound [Pd(C2,N-dmba)(m-NCO)]2 (dmba=N,N-dimethylbenzylamine) reacts in acetone with
pyrazole (pz), 3,5-dimethylpyrazole (dmpz), imidazole (imz) and 2-methylimidazole (mimz) to give [Pd2(C2,N-dmba)2(m-NCO)(m-pz)] (1),
[Pd2(C2,N-dmba)2(m-NCO)(m-dmpz)] (2), [Pd(C2,N-dmba)(NCO)(imz)] (3) and [Pd(C2,N-dmba)(NCO)(mimz)] (4), respectively.
The compounds were characterized by elemental analysis, IR spectroscopy and TG. The thermal decomposition of the compounds
occurs in three consecutive steps and the final decomposition products were identified as Pd(0) by X-ray powder diffraction.
The thermal stability order of the complexes is 2>3>1>4.
Keywords: cyclopalladated compounds, imidazoles, pyrazoles, spectroscopy, thermal analysis
Introduction
characterization and thermal behaviour of the
compounds [Pd2(C2,N-dmba)2(m-NCO)(m-pz)] (1),
[Pd2(C2,N-dmba)2(m-NCO)(m-dmpz)] (2),
[Pd(C2,N-dmba)(NCO)(imz)] (3) and
In the past few years, the cyclopalladated compounds
have undergone spectacular advances mainly due to
their important applications in fields such as organic
synthesis [1], new materials (e.g., metallomeso-
gens) [2], catalysis [3] and medical science (e.g., anti-
tumor drugs) [4]. The pseudohalide-bridged cyclo-
palladated dimers are also good starting materials for
reactions with nucleophiles like pyridines or
quinolinethiols affording new mono or dinuclear
compounds, respectively [5, 6]. We have also described
the molecular structures, obtained by X-ray crystal
diffraction of cyclopalladated species, which were
prepared from 1,3-dipolar cycloaddition reaction of
CS2 on azido bridge groups. These insertions
reactions are important working tools because the
complexes obtained are inaccessible through other
synthetic routes [7].
[Pd(C2,N-dmba)(NCO)(mimz)] (4).
Experimental
Preparation of the complexes
All the syntheses were carried out at room temperature.
All reagents were obtained from commercial suppliers
and employed without further purification. The starting
material [Pd2(C2,N-dmba)(m-NCO)]2 was prepared as
previously described [12].
[Pd2(C2,N-dmba)2(m-NCO)(m-pz)] (1),
[Pd2(C2,N-dmba)2(m-NCO)(m-dmpz)] (2),
[Pd(C2,N-dmba)(NCO)(imz)] (3) and
[Pd(C2,N-dmba)(NCO)(mimz)] (4). To a suspension
of 0.10 g (0.18 mmol) of [Pd(C2,N-dmba)(m-NCO)]2
in 20 mL of acetone was added 0.35 mmol of ligand
(0.024 g of pz; 0.034 g of dmpz; 0.024 g of imz;
0.029 g of mimz). After stirring the resulting colourless
solutions for 0.5 h, the solvent was removed under
reduced pressure and the white solids obtained were
washed with pentane and then dried in vacuum.
The yield was 85% in each case.
On the other hand, pyrazoles and imidazoles are
well known versatile ligands which can interact with
metallic centre in neutral monodentate, anionic
monodentate, or exo/endo-bidentate anionic fashions
[8]. Thus, these ligands represent a good choice for
designing new structures sustained by coordinate
and/or hydrogen bonds [9]. In addition, compounds
containing pyrazole or imidazole ligands [8, 10, 11]
have been successfully employed in catalysis, chemo-
therapy, electrochemical reduction of carbon dioxide,
as metallomesogens, and as models of active sites in
metalloenzymes. As a part of our research program in
this area, we report in the present study the synthesis,
Instrumentation
Elemental analyses of carbon, nitrogen and hydrogen
were performed on a microanalyzer CE Instruments,
*
Author for correspondence: mauro@iq.unesp.br
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
1388–6150/$20.00
© 2009 Akadémiai Kiadó, Budapest

LUCCA NETO et al.
model EA1110–CHNS-O. Melting points were deter-
bands at 2215 and 2206 cm–1, respectively, is consistent
with terminal N-bonded cyanate [15]. The bidentate
coordination of pz and dmpz between [Pd(dmba)] moieties
in 1 and 2 was clearly evidenced by the appearance of
additional IR bands at 3131–3177 cm–1 (nCH), and
1525–1711 cm–1 (nring), as well as by the disappearance
of the bands at 3480–3540 and 1120–1140 cm–1 (nNH
and dNH, respectively), characteristics of the neutral
ligands [9]. Regarding to the IR spectra of 3 and 4, the
monodentate coordination of the imz and mimz
ligands was detected by the appearance of the bands
at 3134–3205 and 3069–3131 cm–1, attributed to the
vibrational modes nNH and nCH, respectively, of these
molecules. The proposed molecular structures for
compounds 1–4 are shown in Fig. 1.
mined on a Mettler FP-2 microscope. Infrared
spectra (IR) were recorded on a Nicolet Impact 400
spectrophotometer in the spectral range 4000–400 cm–1,
with a resolution of 4 cm–1, in KBr pellets. Thermal
analyses (TG) were carried out using a SDT 2960
system from TA Instruments, under dynamic flow of
dry synthetic air (50 mL min–1) at a heating rate of
20°C min–1 using a-alumina open crucibles for sample
and reference. The X-ray powder diffractograms were
obtained in a Siemens D5000 diffractometer, using
CuKa radiation (l=1.541 ) and setting of 40 kV and
30 mA. The residues (Pd, PdO) were identified using
ICDD bases [13].
Results and discussion
The reactions of the cyclopalladated compound
[Pd(C2,N-dmba)(m-NCO)]2 and pz, dmpz, imz and
mimz in the molar ratio 1:2 occur readily, leading to
the species [Pd2(C2,N-dmba)2(m-NCO)(m-pz)] (1),
[Pd2(C2,N-dmba)2(m-NCO)(m-dmpz)] (2),
[Pd(C2,N-dmba)(NCO)(imz)] (3) and
[Pd(C2,N-dmba)(NCO)(mimz)] (4). The elemental
analyses for the synthesized compounds were in
agreement with the proposed formula, Table 1.
The complexes were characterized by IR spectro-
scopy and thermogravimetric analysis. The data given
by these techniques are present as follows.
Fig. 1 Suggested structures for compounds 1–4
Thermogravimetric analysis
Infrared spectra
In the present paper, the main objectives of the
thermogravimetry (TG) is to enlight the influence of
the pyrazole and imidazole-type ligands in the initial
decomposition temperature of the complexes, as well
as to propose decomposition steps. The TG curves for
the dinuclear (1 and 2) and mononuclear compounds
(3 and 4) are shown in Figs 2 and 3, respectively.
Table 2 presents the thermal studies data of comp-
lexes 1–4, together with the assignment for each
decomposition step.
The characteristic bands of the cyclometallated ring at
3045–3054 cm–1 (nCHring), 2909–2917 cm–1 (nCH3) and
2853–2886 cm–1 (nCH2) in the infrared spectra of
compounds 1–4 were found unchanged when compared
to those observed for the starting compound [12].
Therefore, the interaction between pz, dmpz, imz and
mimz ligands and [Pd(C2,N-dmba)(m-NCO)]2 did not
affect the integrity of the orthometallatted ring, as
observed in other cyclopalladated compounds [14].
Concerning the NCO ligand, the presence of the
N-bridging cyanato group in 1 and 2 was evidenced
by the nCN band at 2172 and 2177 cm–1, respectively
[6, 15]. For the species 3 and 4, the presence of nCN
In the range 185–371°C, compounds 1 and 2 lose
the organic (dmba, pz and dmpz) and inorganic (NCO)
ligands, with uptake of O2, yielding a mixture of Pd
(ASTM 05-0681) and PdO (ASTM 06-0515) [13].
Table 1 Elemental analyses and melting points for complexes 1–4
Carbon/%
Hydrogen/%
Nitrogen/%
found
Complex Melting point/°C
found
calc.
found
calc.
calc.
1
2
3
4
195.2 (dec.)
230.0 (dec.)
178.6 (dec.)
193.4 (dec.)
45.24
46.42
43.80
46.06
44.76
46.62
44.52
46.10
4.75
4.89
4.41
4.98
4.61
5.05
4.60
4.97
11.22
11.22
15.42
14.21
11.86
11.32
15.98
15.36
J. Therm. Anal. Cal., 97, 2009
58

Pd(II) COMPLEXES WITH PYRAZOLE AND IMIDAZOLE-TYPE LIGANDS
Table 2 Thermal analysis data for the compounds 1–4
Dm/mg
Complex
Step
DT/°C
calc.
found
Assignment
1
185–318
318–801
801–843
–4.66
+0.21
–0.42
2.74
–4.69
+0.33
–0.40
2.85
–2dmba, –pz, –NCO, +0.5O2
1
2
3
+0.5O2,
–1.0O2
Pd
(mi=7.61 mg)
residue
1
234–371
371–801
801–843
–3.26
+0.14
–0.27
1.78
–3.24
+0.21
–0.29
1.85
–2dmba, –dmpz, –NCO, +0.5O2
2
2
3
+0.5O2
–1.0O2
Pd
(mi=5.17 mg)
residue
1
189–441
441–809
809–847
–3.66
+0.12
–0.25
1.65
–3.65
+0.15
–0.27
1.67
–1dmba, –imz, –NCO, +0.25O2
3
2
3
+0.25O2
–0.5O2
Pd
(mi=5.44 mg)
residue
1
167–450
450–809
809–847
–5.17
+0.16
–0.33
2.20
–5.10
+0.15
–0.33
2.24
–1dmba, –mimz, –NCO, +0.25O2
4
2
3
+0.25O2
–0.5O2
Pd
(mi=7.54 mg)
residue
Fig. 3 TG curves for the complexes 3 and 4
Fig. 2 TG curves for the complexes 1 and 2
probably due to molecular distortions arising from the
presence of the mimz ligand.
Afterwards, the TG curves also exhibit a progressive
mass gain up to 801°C due to the partial oxidation of
the Pd to PdO. The last mass loss is characterized by
the decomposition of the remaining PdO to Pd
(ASTM 05-0681) at 843°C. Regarding to the TG
curves of the compounds 3 and 4, the ligands are
eliminated in the range 167–450°C, with uptake of
O2, affording a mixture of Pd (ASTM 05-0681) and
PdO (ASTM 06-0515). In the range 450–809°C the
partial oxidation of Pd to PdO is observed. The last
step corresponds to the decomposition of the
remaining PdO to Pd (ASTM 05-0681) at 809–847°C.
Thus, taking into account the initial decomposition
temperatures, the following thermal stability order
can be established: 2>3>1>4. It is suggested that the
lower thermal stability of 1 in comparison with that
observed in 2 can be the result of the more strained
metallocycle formed by the metal and the ligands (NCO
and pz). Finally, compound 4 is less stable than 3,
Conclusions
The results of the present investigation showed that
the compound [Pd(C2,N-dmba)(m-NCO)]2 reacts with
the nitrogen donor ligands giving products that are
easily isolated. For the cyclopalladated species
[Pd2(C2,N-dmba)2(m-NCO)(m-pz)] (1) and
[Pd2(C2,N-dmba)2(m-NCO)(m-dmpz)] (2), both NCO
and pyrazole groups act as bridging ligands between
two [Pd(dmba)] moieties. For compounds
[Pd(C2,N-dmba)(NCO)(imz)] (3) and
[Pd(C2,N-dmba)(NCO)(mimz)] (4), the cyanate and
imidazole ligands are coordinated to the palladium
atom in the monodentate coordination mode. The
thermoanalytical data showed that the thermal stability
of compounds 1–4 varies in the sequence 2>3>1>4.
J. Therm. Anal. Cal., 97, 2009
59

LUCCA NETO et al.
7 A. M. Santana, A. E. Mauro, E. T. Almeida, A. V. G.
The high stability of 2 can be explained by the less
tensioned metallocycle formed between the metallic
centres and bridging ligands (dmpz and NCO).
Compound 4 is less stable than 3, probably due to
molecular distortions arising from the presence of the
2-methylimidazole ligand.
Netto, S. I. Klein, R. H. A. Santos and J. R. Zóia, J. Coord.
Chem., 53 (2001) 163.
8 A. V. G. Netto, R. C. G. Frem and A. E. Mauro, Quím.
Nova, 31 (2008) 1208.
9 A. V. G. Netto, R. C. G. Frem, A. E. Mauro,
E. T. de Almeida, A. M. Santana, J. de Souza Jr. and
R. H. A. Santos, Inorg. Chim. Acta, 350 (2003) 252.
10 M. J. Bloemink, H. Engelking, S. Karentzopoulos,
B. Krebs and J. Reedijk, Inorg. Chem., 35 (1996) 619.
11 A. V. G. Netto, R. C. G. Frem, A. E. Mauro,
R. H. A. Santos and J. R. Zoia, Trans. Metal Chem.,
27 (2002) 279.
Acknowledgements
The authors wish to acknowledge CNPq, FAPESP and
CAPES for the financial support.
12 V. A. de Lucca Neto, A. E. Mauro, A. C. F. Caires,
S. R. Ananias and E. T. de Almeida, Polyhedron,
18 (1998) 413.
References
13 Powder Diffraction File of the Joint Committee on Powder
Diffraction Standards. Sets 1-32, published by the
International Center of Diffraction Data, Swarthmore, PA
19081, USA 1982.
1 S. R. Ananias, A. M. Santana, A. E. Mauro,
V. A. de Lucca Neto and E. T. de Almeida, Quím. Nova,
26 (2003) 53.
14 E. T. de Almeida, A. M. Santana, A. V. G. Netto,
C. Torres and A. E. Mauro, J. Therm. Anal. Cal.,
82 (2005) 361.
2 D. J. Saccomando, C. Black, G. W. V. Cave, D. P. Lydon
and J. P. Rourke, J. Organomet. Chem., 601 (2000) 305.
3 A. C. F. Caires, A. E. Mauro, A. C. Moro, A. O. Legendre
and S. R. Ananias, Quím. Nova, 29 (2006) 750.
4 A. S. Abu-Surrah and M. Kettunen, Curr. Med. Chem.,
13 (2006) 1337.
15 K. Nakamoto, Infrared and Raman Spectroscopy of
Inorganic and Coordination Compounds,
Wiley Interscience, New York 1986, p. 484.
5 S. R. Ananias, A. E. Mauro, K. Zutin, C. M. C. Picchi and
R. H. A. Santos, Trans. Metal Chem., 29 (2004) 284.
6 A. C. Moro, A. E. Mauro, S. R. Ananias, A. Stevanato and
A. O. Legendre, J. Therm. Anal. Cal., 87 (2007) 721.
ICTAC 2008
DOI: 10.1007/s10973-008-9667-x
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