Thermal and spectroscopy investigation of cyclopalladated compounds of the type [Pd(C13H10N)(μ-X)]2, (X=H3CCOO, NCO, SCN, CN)

Article abstract of DOI:10.1023/A:1013968309561

The dimeric compound [Pd(bzan)(μ-OOCCH₃)]₂ (1) (bzan=N-benzylideneaniline) reacts with KX, in methanol/acetone (2:1), affording the analogous dimeric pseudohalogen-bridged species [Pd(bzan)(μ-X)]₂ [X(=NCO(2), SCN(3), CN(4)

Full text of DOI:10.1023/A:1013968309561

Journal of Thermal Analysis and Calorimetry, Vol. 67 (2002) 425–431
THERMAL AND SPECTROSCOPY INVESTIGATION
OF CYCLOPALLADATED COMPOUNDS OF THE
TYPE [Pd(C13H10N)(µ-X)]2, (X=H3CCOO, NCO, SCN, CN)
A. M. Santana, A. E. Mauro*, H. E. Zorel Jr., M. P. D. Mattioli and
V. A. de Lucca Neto
Instituto de Química – UNESP – C.P. 355, 14801-970, Araraquara, SP, Brazil
Abstract
The dimeric compound [Pd(bzan)(µ-OOCCH3)]2 (1) (bzan=N-benzylideneaniline) reacts with KX,
in methanol/acetone (2:1), affording the analogous dimeric pseudohalogen-bridged species
[Pd(bzan)(µ-X)]2 [X=NCO(2), SCN(3), CN(4)]. The compounds were characterized by elemental
analysis, infrared spectroscopy, NMR and thermogravimetric analysis. IR data for 2–4 showed
bands typical of coordinated pseudohalogen ligands clearly indicating the occurrence of the ex-
change reaction. Their thermal behaviour was investigated and suggested that their stability is influ-
enced by the bridging ligand. The thermal stability decreased in the order [Pd(bzan)(µ-CN)]2>
[Pd(bzan)(µ-SCN)]2>[Pd(bzan)(µ-OOCCH3)]2>[Pd(bzan)(µ-NCO)]2. X-ray results showed the for-
mation of Pd° as final decomposition product.
Keywords: cyclopalladated, IR spectroscopy, pseudohalogens, thermal analysis, X-ray diffraction
Introduction
Cyclometallated compounds constitute an important class of organometallic chemis-
try, nowadays largely investigated in many important laboratories. Particularly, there
has been a great interest in cyclopalladated with nitrogen donors due to their
application in a variety of fields such as synthesis of supramolecules [1], organic syn-
thesis [2], as anti-tumor drugs [3], for the design of new metalomesogens [4], cataly-
sis [5] and photochemistry [6]. Furthermore, we have recently shown that pseudo-
halogen-bridged cyclopalladated complexes are also good starting materials for
reactions with CS2 giving new heterocycles coordinated to the palladium atoms [7].
Although many palladium(II) coordination compounds have been studied by
thermoanalytical techniques, it is fact that cyclopalladated species, as such, have been
scarcely investigated so far [8]. So, we decided to synthesize new cyclopalladated
from [Pd(bzan)(µ-OOCCH3)]2 [9] by an anion exchange reaction. The compounds
*
Author for correspondence: E-mail: mauro@iq.unesp.br
1418–2874/2002/ $ 5.00
Akadémiai Kiadó, Budapest
© 2002 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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SANTANA et al.: CYCLOPALLADATED COMPOUNDS
prepared have the general formula [Pd(bzan)(µ-X)]2 [bzan=N-benzylideneaniline,
X=NCO(2), SCN(3), CN(4)] and were characterized by elemental analysis, infrared
and NMR spectroscopies and thermal analysis, simultaneous TG and DTA. All final
decomposition products were characterized by X-ray powder diffractometry. To the
best of our knowledge, this present study is one of the first ones dealing with a sys-
tematic thermal behaviour in a series of cyclopalladated-pseudohalogen complexes.
Experimental
Preparation of the complexes
[Pd(bzan)(µ-X)]2 (X=NCO, SCN, CN). To a solution of 0.1000 g (0.145 mmol) of
[Pd(bzan)(µ-OOCCH3)]2 in 45 cm3 of methanol/acetone (2:1) was added 0.290 mmol
of KX (0.0235 g of KNCO; 0.0282 g of KSCN or 0.0189 g of KCN). The mixtures
were stirred for 1 h and the yellow solids formed were filtered off, washed carefully
with water and diethyl ether. The compounds were recrystallized from dichloro-
methane/hexane (1:1) and then dried in vacuum. The yields were 90, 75 and 72% re-
spectively for [Pd(bzan)(µ-NCO)]2, [Pd(bzan)(µ-SCN)]2 and [Pd(bzan)(µ-CN)]2.
Instrumentation
Elemental analysis of carbon, nitrogen and hydrogen were performed at Instituto de
Química de Araraquara on an analyser CE Instruments, model EA 1110 – CHNS-O.
The infrared spectra (IR) were recorded on a Nicolet FTIR-400 spectrophoto-
meter in the spectral range 4000–400 cm–1, with the samples in the form of potassium
bromide pellets.
NMR of 1H and 13C spectra were obtained as DMSO-d6 solutions and referred to
the high field SiMe4 signal, on a Bruker AC-200 spectrometer.
Thermogravimetry (TG) and differential thermal analysis (DTA) were carried out
using a TA system model SDT 2960, under flow of dry synthetic air (100 cm3 min–1),
temperature up to 900°C and at heating rate of 5°C min–1, in α-alumina sample holders.
The reference substance was pure α-alumina in DTA measurements.
The X-ray powder patterns were obtained with a Zeiss HZG HGZ4/B horizontal
diffractometer (G.D.R.). The Bragg–Brentano arrangement was adopted using CuKα
radiation (λ=1.541 Å) and setting of 34 kV and 20 mA. The peaks were identified us-
ing ICDD data bases [10].
Results and discussion
The reactions of the cyclopalladated compound [Pd(bzan)(µ-OOCCH3)]2 and KX
(X=NCO, SCN, CN) in a molar ratio 1:2 occurred readily, leading to the species
[Pd(bzan)(µ-X)]2 (Scheme 1).
Elemental analysis for the synthesized compounds were in agreement with the
proposed formula and the results are present in Table 1.
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SANTANA et al.: CYCLOPALLADATED COMPOUNDS
427
i=KX, (X=NCO, SCN, CN), methanol/acetone (2:1), 1 h
Table 1 Elemental analysis data of compounds 1–4
C
H
N
Compound
found
51.00
51.31
48.38
57.27
calc.
51.52
51.16
48.78
57.78
found
3.57
3.02
2.90
3.20
calc.
3.75
3.07
2.92
3.22
found
4.11
8.48
8.02
8.91
calc.
4.12
8.52
8.12
8.96
[Pd(bzan)(µ-OOCCH3)]2
[Pd(bzan)(µ-NCO)]2
[Pd(bzan)(µ-SCN)]2
[Pd(bzan)(µ-CN)]2
The complexes were characterized by IR and NMR spectroscopies and thermo-
gravimetric analysis. The data given by those techniques are present as follows.
The pseudohalogen groups NCO, SCN and CN are versatile ligands which can
function as terminal monodentade or bridging groups. In this way, pseudohalogen
metal complexes have a remarkable structural diversity. In general, the vibrational
modes of pseudohalogen ligands are very characteristic and allow a clear identifica-
tion of their mode of coordination to the metallic centers. It is worthy to note that, for
the complex 2, the bands at 2170, νCN; 1330, νCO; 613 δNCO, cm–1 [11] are consis-
tent with bridging N-bonded cyanate. For the compound 3 the presence of bands at
2141, νCN; 840, νCS, 436, δNCS, cm–1 [11] shows that the thiocyanate is acting as a
bridging group and coordinated simultaneously through nitrogen and sulphur to Pd
atoms. Finally, the IR spectrum of 4 shows a band at 2153 cm–1, assigned to νCN
which gives evidence of cyanide bridging to Pd centers via carbon atom [8, 11].
1H as well as 13C spectra unambiguously showed the formation of the products.
1
In the H NMR the (–N=CH–), imine group appears, respectively, as a singlet, at
δ=7.61 for 1, 8.39 for 2, 8.46 for 3 and 8.39 for 4. In the same way, 13C spectra con-
firm the proposed structure showing the resonance signals for [Pd–C] at 147.2 for 1,
146.5 for 2, 147.6 for 3 and 147.7 for 4, which are characteristic of ortho-metallation
of the aromatic ring [8]. The 13C spectra also showed signals assigned to imine group
(–N=CH–) respectively, at 172.7 for 1, 176.1 for 2, 177.6 for 3 and 176.1 for 4 [12].
In order to get more information about these cyclopalladated complexes in the
solid state, we utilized thermal analysis and the results obtained are discussed below.
J. Therm. Anal. Cal., 67, 2002

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SANTANA et al.: CYCLOPALLADATED COMPOUNDS
Thermal behaviour
Nowadays, it must be pointed out that thermogravimetry has become an important tech-
nique for studies of organometallic compounds, like the cyclopalladated species [8]. Par-
ticularly in the series [Pd(bzan)(µ-X)]2 (X=OOCCH3(1), NCO(2), SCN(3), CN(4)) the
application of thermogravimetry has its main goals in revealing the influence of bridging
group X on the initial decomposition temperature and on the thermal decomposition
mechanisms. The steps, initial and final temperatures (°C), partial mass losses (%) for the
thermal decompositions of all compounds in dry synthetic air, are given in Table 2.
The TG curves for these compounds are shown in Fig. 1. As proved in accor-
dance with X-ray powder diagrams with final products with ICDD data bases, all
compounds decompose giving Pd, card 05–0681 [10].
Fig. 1 TG (—), DTG ( ) and DTA (- - -) curves for compounds 1–4
We observed that compounds 1, 2, 3 and 4 decompose in two steps, the first one be-
ing concerned with acetate or pseudohalogen elimination, followed by pyrolysis of bzan.
The first mass loss of 1 occurs between 123–290°C, corresponding to, by mass calcula-
tion, two acetate groups and 0.6 bzan mol. The second step, 290–438°C, suggests a mass
loss which was attributed to the final elimination of bzan, and uptake of O2. It was noted a
mass gain in the temperature range 438–807°C, attributed to PdO formation, which de-
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SANTANA et al.: CYCLOPALLADATED COMPOUNDS
429
composed above 807°C affording Pd° [13]. The first mass loss of 2 observed between
106–228°C, suggests elimination of two NCO groups and 0.26 bzan mol. The second
step, in the range 228–372°C, is due to the final elimination of bzan molecules and up-
take of O2, giving a mixture of Pd° and PdO. There is a mass gain between 372–800°C,
attributed to the formation of PdO, which decomposed above 800°C giving Pd° [13]. The
first mass loss of 3, in the temperature range 144–251°C, is due to elimination of 1.5 SCN
mol. The second step, between 251–542°C, corresponds, by mass calculation, to the loss
of two bzan mols and of 0.5 SCN mol, as well as gain of O2. It was observed between
542–800°C an uptake of O2, assigned to the oxidation of Pd° to PdO. As before, there is a
decomposition of PdO into Pd° above 800°C [13]. The first decomposition step of 4, be-
tween 231–278°C, suggests mass loss attributed to elimination of two CN groups and
1.36 bzan mol. The second step, 278–401°C, is assigned to the elimination of 0.64 bzan
mol, and gain of O2, giving a mixture of Pd° and PdO. In the last event, between
401-817C, occurs a mass gain forming PdO, that decomposes to Pdo, as before [13].
The thermal decomposition steps proposed are supported by IR spectra of the
residues corresponding to the first mass losses of 1, 2 and 4, which show no absorp-
tion bands corresponding to the vibrational modes of bridging groups. In addition, the
presence of both Pd° and PdO in the residue of the second step was clearly demon-
strated by X-ray diffraction data.
Table 2 Thermogravimetric analysis (TG) and differential thermal analysis (DTA) data for com-
pounds 1–4
Found/
Calc./
Peaks/°C
Compound
mi/mg
∆T/ Ti–Tf/
°C
∆m/mg
%
exo
endo
[Pd(bzan)(µ-OOCCH3)]2
3.96
–1.30
–1.23
+0.18
–0.16
123–290
290–438
438–807
807–824
32.83
33.99
4.54
32.82
33.99
4.63
280
420
–
–
–
–
802
4.04
4.63
–
[Pd(bzan)(µ-NCO)]2
5.03
–1.00
–2.30
+0.12
–0.22
106–228
228–372
372–800
800–834
19.88
47.72
2.39
19.94
47.71
2.43
196
352
–
–
–
–
818
4.34
4.38
–
[Pd(bzan)(µ-SCN)]2
6.53
–0.82
–3.60
+0.14
–0.28
144–251
251–542
542–800
800–893
12.56
55.13
2.14
12.64
56.50
2.32
250
510
–
–
–
–
820
4.29
4.64
–
[Pd(bzan)(µ-CN)]2
3.72
–1.77
–0.49
+0.04
–0.19
231–278
278–401
401–817
817–892
47.58
18.29
1.08
47.57
18.45
1.54
264
385
–
–
–
–
822
5.11
5.12
–
mi – initial mass, Ti – initial temperature, Tf – final temperature, ∆T – variation of temperature,
∆m – variation of mass, found/% – experimental percentage, calc./% – calculated percentage
J. Therm. Anal. Cal., 67, 2002

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DTA curves of 1, 2, 3, and 4 are given in Fig. 1 with DTA data summarized in Table
2. Two exothermic peaks are observed in DTA curves of all compounds, besides a weak
endothermic peak observed at 216°C in DTA curve of 1, indicative of partial melting of
this compound, which is in agreement with its melting range of 210–220°C [9]. The first
exothermic peak is strongly associated with the loss of the bridging ligand and the second
is strongly associated with the loss of bzan ligand, in accordance with the TG data of the
compounds 1–4. In addition, all DTA curves show an endothermic peak corresponding to
thermal decomposition of PdO to Pd° in the range of 800–850°C [13].
Conclusions
Summarizing the previous results, it was observed that all species decompose in two
steps. For all them the first step is concerned with the total or partial elimination of the
bridging groups, followed by thermal decomposition of bzan. Furthermore, taking into
account the initial temperature of the decomposition process, it is possible to establish the
following thermal stability order: [Pd(bzan)(µ-CN)]2>[Pd(bzan)(µ-SCN)]2>
[Pd(bzan)(µ-OOCCH3)]2>[Pd(bzan)(µ-NCO)]2. The greater thermal stability of
[Pd(bzan)(µ-CN)]2 relies on the basis of eletronic grounds as strong Pd–CN bonds may
result from the back-bonding of appropriate full metal orbitals to empty π*–CN orbitals.
The thermal data obtained in the present work will be extremely valuable for further stud-
ies involving the application of the compounds 1, 2, 3 and 4 in homogeneous catalysis,
such as in the carbonilation of amines.
* * *
The authors wish to acknowledge CNPq, FAPESP, FINEP and CAPES for financial support.
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