Thermal study of cyclopalladated complexes of the type [Pd 2(dmba)2X2(bpe)]: (X=NO- 3, Cl-, N- 3, NCO-, NCS-; bpe=trans-1,2-bis(4-pyridyl)ethylen

Article abstract of DOI:10.1007/s10973-005-0903-3

The synthesis, characterization and thermal analysis of the novel cyclometallated compounds [Pd₂(dmba)₂Cl ₂(μ-bpe)] (1), [Pd₂(dmba)₂(N ₃)₂(μ-bpe)] (2), [Pd₂(dmba)

Full text of DOI:10.1007/s10973-005-0903-3

Journal of Thermal Analysis and Calorimetry, Vol. 82 (2005) 361–364
THERMAL STUDY OF CYCLOPALLADATED COMPLEXES OF THE
TYPE [Pd2(dmba)2X2(bpe)]
(X=NO–3 , Cl–, N–3 , NCO–, NCS–; bpe=trans-1,2-bis(4-pyridyl)ethylene)
E. T. de Almeida1,2, A. M. Santana1, A. V. G. Netto1, Claudia Torres1 and A. E. Mauro1*
1Instituto de Química de Araraquara-UNESP-C.P. 355, 14801-970 Araraquara-SP, Brazil
2Centro de Ciências da Educação e de Humanidades, CCEH, Universidade Católica de Brasília, UCB, EPCT–QS-07–Lote 1,
Águas Claras, 71966-700 Brasília-DF, Brazil
The synthesis, characterization and thermal analysis of the novel cyclometallated compounds [Pd2(dmba)2Cl2(µ-bpe)] (1),
[Pd2(dmba)2(N3)2(µ-bpe)] (2), [Pd2(dmba)2(NCO)2(µ-bpe)] (3), [Pd2(dmba)2(SCN)2(µ-bpe)] (4), [Pd2(dmba)2(NO3)2(µ-bpe)] (5)
(bpe=trans-1,2-bis(4-pyridyl)ethylene; dmba=N,N-dimethylbenzylamine) are described. The thermal stability of
[Pd2(dmba)2X2(µ-bpe)] complexes varies in the sequence 1>4>3>2>5. The final residues of the thermal decompositions were charac-
terized as metallic palladium by X-ray powder diffraction.
Keywords: cyclopalladated complexes, DTA, pseudohalides, TG
Introduction
Experimental
Preparation of the complexes
In the past few years, the cyclopalladated chemistry
have undergone spectacular advances mainly due to
their potential uses as technologically relevant mate-
rials (e.g. liquid crystals) [1], catalysts [2, 3], and
antitumoral drugs [4]. Despite the increased attention
on the cyclopalladated compounds, less attention ap-
pears to have been paid to their derivatives containing
linear pyridyl-donor ligands. Of particular interest is
to use pyrazine, 4,4’-bipyridine, and trans-1,2-bis(4-
pyridyl)ethylene) as bridges between metals, yielding
binuclear species as well supramolecular architec-
tures [5, 6]. We reasoned that unusual bimetallic com-
pounds could be generated by the cleavage of ortho-
palladated compounds of the type [Pd(dmba)(µ-X)]2
(X=bridging anionic group; dmba=N,N-dimethyl-
benzylamine) by the desired linear exobidentate
ligand such as trans-1,2-bis(4-pyridyl)ethylene (bpe).
In the framework of our current research on
cyclometallated compounds [7–12] as well in the co-
ordination and supramolecular chemistry of pseudo-
halides [13, 14], we report herein the synthesis, char-
acterization and thermal study of novel binuclear
cyclopalladated species of general formulae
The complexes [Pd(dmba)(µ-X)]2 (X=Cl–, N– , NCO–,
NCS–) and [Pd(dmba)(CH3CN)2](NO3) were prepared
3
as previously described [4, 7, 8].
[Pd2(dmba)2X2(µ-bpe)] (X=Cl– (1), N– (2),
3
NCO– (3), NCS– (4)) were synthesized for the first time
by adding a solution containing 0.354 mmol of
trans-1,2-bis(4-pyridyl)ethylene in CH2Cl2 to the re-
spective [Pd(dmba)(µ-X)]2 precursors (0.354 mmol)
dissolved
in
15
mL
of
CH2Cl2.
[Pd2(dmba)2(NO3)2(µ-bpe)] (5) was prepared as men-
tioned above but using the [Pd(dmba)(CH3CN)2](NO3)
precursor. The resulting clear suspensions were stirred
for 1 h and filtered off. The pale yellow powders ob-
tained were washed with CH2Cl2 and pentane and
dried in vacuum. All compounds showed poor solubil-
ity in most common solvents. Yields: 80–90%.
Instrumentation
Melting points were determined on a Mettler FP-2 mi-
croscope. Elemental analyses of carbon, nitrogen, and
hydrogen were performed on a microanalyser CE In-
struments, model EA 1110 – CHNS-O. Infrared spectra
were recorded as KBr pellets on a Nicolet FTIR-Im-
pact 400 spectrophotometer in the spectral range
4000–400 cm–1. Simultaneous thermal analysis
(TG-DTA) were carried out using a TA system model
SDT 2960, under flow of dry synthetic air
[Pd2(dmba)2X2(µ-bpe)] (X=Cl– (1), N– (2), NCO– (3),
3
SCN– (4), NO– (5); bpe=trans-1,2-bis(4-pyridyl)eth-
3
ylene; dmba=N,N-dimethylbenzylamine).
*
Author for correspondence: mauro@iq.unesp.br
1388–6150/$20.00
© 2005 Akadémiai Kiadó, Budapest
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands

DE ALMEIDA et al.
Table 1 Results of chemical analyses and melting points of the compounds 1–5
Carbon/%
Nitrogen/%
Hydrogen/%
Complex
m. p./°C
found
calc.
found
calc.
found
calc.
1
2
3
4
5
195.2 (dec.)
164.4 (dec.)
174.0 (dec.)
189.5 (dec.)
147.1 (dec.)
49.1
48.1
51.4
49.3
45.1
49.1
48.2
51.4
49.3
45.8
7.3
7.6
4.7
4.5
4.6
4.1
4.4
4.7
4.6
4.6
4.4
4.3
18.3
11.2
10.8
10.3
18.7
11.2
10.8
10.7
Table 2 IR spectral data (4000–400 cm–1) for compounds [Pd2(dmba)2X2(µ-bpe)] (X=Cl– (1), N3– (2), NCO– (3), SCN– (4), NO3– (5))
ν/cm–1
Complex
dmba
bpe
X
νCH
νCH3
νCH2
νCH
νring
ν
δ
1
2
3
4
5
3047 w
3050 w
3051 m
3046 m
3045 m
2907 m
2912 m
2907 w
2885 m
2905 m
2872 m
2844 w
2835 w
2792 w
2840 w
2984 w
2999 w
2987 w
2978 m
2999 m
1594 s
1606 s
1605 s
1573 m
1604 s
–
–
2036 s (νas), 1351 w (νs)
2203 s (νas), 1316 w (νs)
2134 s (νas)
–
581 w
472 w
836 s
1369 s
(100 mL min–1), temperature up to 1000°C and heating
rate of 10°C min–1, in α-alumina sample holders. The
reference substance was pure α-alumina in DTA mea-
surements. X-ray powder patterns of the residues were
obtained with a Zeiss HGZ4/B horizontal diffractometer
(G.D.R.) equipped with a proportional counter and
pulse height discriminator. The Bragg–Brentano
scheme was adopted using CuKα radiation (λ=1.541 Å)
and setting of 34 kV and 20 mA. The residues (Pd, PdO)
were identified using ICDD bases [15].
for the precursors. Therefore, the interaction between
bpe ligand and the precursors did not affect the integ-
rity of orthometallatted ring. Concerning to the anionic
ligands (X), the presence of terminal azide in (2) was
evidenced by the νasNNN band at 2036 cm–1 [13]
whereas the terminal N-bonded cyanato ligand in (3)
was inferred on basis of the νasNCO absorption at 2203
cm-1 [16]. Compounds (4) and (5) are characterized by
the existence of terminal S-bonded thiocyanato
(2134 cm–1, νasSCN) [13] and unidentate O-nitrato
groups (1369 cm–1, νsNO2) [8], respectively. The
bidentate coordination of trans-1,2-bis(4-pyridyl)eth-
ylene (bpe) in 1–5 was clearly evidenced by the ap-
pearance of additional IR bands at 2978–2999 cm–1
(νCH), and 1573–1606 cm–1 (νring). Taking into ac-
count the discussion of the IR data, the adjacent coor-
dination sites of the resulting cis-protected fragment
(dmba)Pd are occupied by one terminal anionic group
(X) and one 4-pyridyl ring from the bidentate bpe
ligand which acts as a bridge between two metallic
centers. The proposed molecular structure for com-
pounds 1–5 is shown in Fig. 1.
Results and discussion
The elemental analyses results and thermogravimetric
data, together with IR spectroscopy data, confirmed
the proposed binuclear formulae for the synthesized
compounds. The results of the analyses and melting
points are presented in Table 1.
Infrared spectra
The most important bands frequencies observed in the
IR spectra of the complexes along with their assign-
ments are presented in Table 2.
Thermogravimetric analysis
Comparison of the IR spectra of the precursors
[4, 7, 8] and 1–5 allowed us to gain some insight about
the molecular structures of these novel species. The
characteristic bands of the cyclometallated ring (dmba)
at 3045–3050 cm–1 (νCH), 2885–2912 cm–1 (νCH3),
and 2792–2844 cm–1 (νCH2) in IR spectra of 1–5 were
found unchanged when compared to those observed
TG and DTA curves for the compounds 1–5 are de-
picted in Fig. 2. Table 3 presents the results of the
thermal studies of these complexes together with the
assignments of each decomposition step.
In the range 165–459°C, the compounds 1–4 lose
their organic (i.e. bpe and dmba) and inorganic (X) lig-
362
J. Therm. Anal. Cal., 82, 2005

CYCLOPALLADATED COMPLEXES OF THE TYPE [Pd2(dmba)2X2(bpe)]
473–803°C. The PdO further decomposed into Pd
(ASTM 05-0681) in the last mass loss of (5) at
803–841°C.
Taking into account the initial decomposition
temperatures, the thermal stability of the complexes
[Pd2(dmba)2X2(µ-bpe)] varies in the sequence
X=1>4>3>2>5. These thermoanalytical results agree
well with our previous studies [17], in which the high-
est thermal stability for chloro and thiocyanato-deriva-
tives of palladium was verified. The lowest stability of
(5) deserves further comments. According to HSAB
principle [18], soft Lewis acid forms more stable coor-
dination compounds with soft bases. As expected for a
Fig. 1 Proposed structure of the compounds 1–5 (X=Cl–, N3– ,
NCO–, SCN–, NO–3 )
ands, with uptake of O2, in 2–3 steps, yielding a mixture
of Pd (ASTM 05-0681) and PdO (ASTM 06-0515) [15].
Afterwards, the TG curves of 1–4 also exhibited a pro-
gressive mass gain up to ca. 800°C due to the oxidation
of the remaining Pd to PdO. The last mass loss is charac-
terized by the decomposition of PdO to Pd
(ASTM 05-0681) at ca. 840°C. The ligands of the com-
pound (5) are eliminated in the 147–473°C range, af-
fording pure PdO (ASTM 06-0515). After that, no mass
change in the TG curve of (5) was observed in the range
complex composed by a hard base (NO– and a soft
3
acid Pd(II)), the thermal decompostion of (5) begins at
the lowest temperature among those of the compounds
investigated. Finally, the known explosive characteris-
tic of azide contributes to the low thermal stability of
(2), as evidenced in our previous studies [17].
Table 3 Thermal analysis data for compounds 1–5
Complex
∆m/%
DTA peak/°C
Step
∆T/°C
Assignment
minitial
found
calculated
endo
exo
1
2
3
4
5
195–281
281–332
332–459
459–787
787–830
–35.18
–8.32
–27.01
3.36
–36.72
–9.63
–23.88
3.47
–
–
233
–
–2dmba
–2Cl
1
–
383
–
–1bpe, +0.2O2
+0.8O2
10.4976 mg
–
–3.83
29.02
–4.34
28.90
818
–
–1.0O2
residue
1
165–238
238–248
248–428
428–800
800–845
–5.22
–43.19
–21.70
3.10
–5.61
–41.68
–23.03
2.99
–
–
–
249
416
–
–1N3
2
–2dmba, –1N3
–1bpe, +0.3O2
+0.7O2
2
3
–
8.28618 mg
4
830
–
5
–3.99
29.00
–4.27
28.40
–
–1.0O2
residue
–2dmba
–1bpe
1
174–277
277–326
326–435
435–790
790–829
–36.38
–25.36
–8.26
2.72
–36.08
–24.31
–9.50
2.56
–
–
–
320
381
–
2
–2NCO,
+0.4O2
+0.6O2
–1.0O2
3
3
–
8.26233 mg
4
–
5
–4.09
28.63
–4.27
28.39
822
–
residue
–2dmba,
–1NCS
–1bpe,
1
189–266
266–334
334–813
813–833
–40.96
–30.62
4.52
–42.03
–29.51
2.86
–
–
–
332
–
2
4
–1NCS,
+0.3O2
+0.7O2
–1.0O2
3
–
5.46025 mg
4
–5.32
27.62
–4.10
27.22
821
–
residue
–2dmba,
–1bpe
–1N2O4
–
1
147–323
323–473
473–803
803–841
–57.23
–10.95
0.00
–57.33
–11.66
0.00
–
–
245
335
–
2
5
3
–
6.70398 mg
4
–4.55
27.27
–4.05
26.96
821
–
–1.0O2
residue
J. Therm. Anal. Cal., 82, 2005
363

DE ALMEIDA et al.
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The synthesis, characterization and thermal behavior
of the binuclear cyclometallated compounds
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3
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pounds varies in the sequence X=Cl– >SCN–>NCO–>
N– >NO– . Further studies involving the use of
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3
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Received: January 25, 2005
In revised form: February 18, 2005
Acknowledgements
DOI: 10.1007/s10973-005-6793-6
We thank the CNPq, CAPES and FAPESP for financial support.
364
J. Therm. Anal. Cal., 82, 2005