Thermal study of nickel(II) pyrazolyl complexes

Article abstract of DOI:10.1007/s10973-005-0060-8

The thermal behavior of the pyrazolyl complexes [NiCl₂(HPz) ₄] (1), [Ni(NCS)₂(HPz)₄] (2), [NiCl ₂(HdmPz)₄]·2H₂O (3) and [Ni(NCS) ₂(HdmPz)₄]·2H₂O

Full text of DOI:10.1007/s10973-005-0060-8

Journal of Thermal Analysis and Calorimetry, Vol. 79 (2005) 335–338
THERMAL STUDY OF NICKEL(II) PYRAZOLYL COMPLEXES
P. M. Takahashi, A. V. G. Netto, A. E. Mauro and Regina C. G. Frem*
Instituto de Química de Araraquara-UNESP-C.P. 355, 14801-970 Araraquara-SP, Brazil
The thermal behavior of the pyrazolyl complexes [NiCl2(HPz)4] (1), [Ni(NCS)2(HPz)4] (2), [NiCl2(HdmPz)4]×2H2O (3) and
[Ni(NCS)2(HdmPz)4]×2H2O (4) (HPz=pyrazole, HdmPz=3,5-dimethylpyrazole) has been studied by thermogravimetry (TG) and
differential thermal analysis (DTA). The TG data indicated that the thermal stability of [NiX2(HL)4] (X=Cl, NCS) compounds var-
ies depending on the pyrazolyl ligand in the following order HL=HPz>HdmPz. From the thermal decomposition of 3 and 4 it was
possible to isolate the intermediate compounds [Ni(m-Cl)2(HdmPz)2] (3a) and [Ni(m-1,3-NCS)2(HdmPz)2] (4a), respectively. The
final products of the thermal decompositions of 1–4 were identified as NiO by X-ray powder diffraction.
Keywords: DTA, nickel(II), pseudohalides, pyrazoles, TG
Introduction
[NiCl2(HPz)4] (1) (338 mg; 0.84 mmol), dropwise
NaSCN was added (144 mg; 1.78 mmols) in 2 mL of
water, giving rise to a purple suspension. The stirring
was maintained for 1 h, and the compound was isolated
by filtration and dried under vacuum with a yield
of 50%. Compound [NiCl2(HdmPz)4]×2H2O (3) was ob-
tained from the addition of 332 mg (3.45 mmols)
of 3,5-dimethylpyrazole to an aqueous solution of
NiCl2×6H2O (200 mg; 0.84 mmol). After stirring and
warming at 60°C for 1 h, the solution was slowly evapo-
rated yielding blue crystals. The obtained yield
was 55%. Complex [Ni(NCS)2(HdmPz)4]×2H2O (4) was
prepared following the procedure: to a deep green aque-
ous solution of Ni(NO3)2×6H2O (200 mg; 0.84 mmol),
warmed at 50°C, NaSCN was added (144 mg;
1.78 mmols) in 2 mL of water. After 10 min, 3,5-di-
methylpyrazole (271 mg; 2.82 mmols) was dissolved
in 2 mL of CH3OH and was added dropwise affording a
blue suspension. The stirring was maintained for 5 h and
the compound was isolated by filtration and dried under
vacuum having a yield of 60%. On the basis of the TG
curves, the blue precursors 3 and 4 were heated in a fur-
nace from room temperature to 140 and 150°C, respec-
tively, yielding the yellow [Ni(m-Cl)2(HdmPz)2] (3a)
and the green [Ni(m-1,3-NCS)2(HdmPz)2] (4a) interme-
diate complexes.
Transition metal compounds containing pyrazole lig-
ands have received increasing attention due to their po-
tential application as catalysts, liquid crystals and
antitumoral drugs [1–3]. In supramolecular point of
view, the coordination versatility as well the hydrogen
bonding possibilities of the pyrazolyl ligands make
these systems good candidates to generate sophisticated
inorganic structures [4–8]. Extending our interests in the
coordination chemistry of transition metal complexes
containing N-based ligands and pseudohalides [4–8],
herein we report the results obtained on a systematic
thermal study on the complexes [NiCl2(HPz)4] (1),
[Ni(NCS)2(HPz)4] (2), [NiCl2(HdmPz)4]×2H2O (3) and
[Ni(NCS)2(HdmPz)4]×2H2O
(4)
(HPz=pyrazole,
HdmPz=3,5-dimethylpyrazole). It is worth mentioning
that analogous works dealing with the thermal behavior
of nickel(II) complexes containing heterocyclic ligands
have been recently published in the literature [9, 10].
Experimental
Preparation of the complexes
Compound [NiCl2(HPz)4] (1) was prepared from the
addition of 235 mg (3.45 mmol) of pyrazole to a deep
green solution containing 200 mg (0.84 mmols) of
NiCl2×6H2O, in water. After stirring for 1 h, the blue tur-
quoise suspension was filtered off and the solid was
washed and dried under vacuum. The yield was 70%.
Complex [Ni(NCS)2(HPz)4] (2) was synthesized by the
following procedure: to an aqueous solution of
Instrumentation
Elemental analyses of carbon, nitrogen, hydrogen,
and sulphur were performed on a microanalyser
CE Instruments, model EA 1110 – CHNS-O. Infrared
spectra were recorded on a Nicolet FTIR-Impact 400
spectrometer in the spectral range of 4000–400 cm–1
*
Author for correspondence: rcgfrem@iq.unesp.br
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2005 Akadémiai Kiadó, Budapest

TAKAHASHI et al.
Table 1 Results of chemical analysis of nickel(II) compounds
Carbon/%
found
Nitrogen/%
Hydrogen/%
Sulfur/%
Complex
calc.
found
calc.
found
calc.
found
calc.
1
35.51
37.92
43.71
37.42
44.30
38.76
35.85
37.60
43.66
37.31
44.37
39.25
27.41
31.08
20.37
17.02
23.33
22.33
27.88
31.33
20.37
17.41
23.57
22.89
4.21
3.34
6.52
5.06
5.63
4.64
4.01
3.61
6.61
5.02
5.95
4.40
–
–
2
13.33
–
14.34
–
3
3a
4
–
–
10.98
16.44
10.77
17.46
4a
(KBr pellets) and on a Perkin Elmer Spectrum 2000 in
the spectral range of 700–50 cm–1 (CsI pellets). Ther-
mal analyses (TG) and differential thermal analy-
sis (DTA) were carried out using a TA system model
SDT 2960, under flowing of dry synthetic air
(100 mL min–1), up to 900°C at a heating rate of
20°C min–1, in a-alumina sample holders. The refer-
ence substance was pure a-alumina for DTA mea-
surements. X-ray powder patterns of the residues
were obtained with a SIEMENS D-5000 diffracto-
meter equipped with a proportional counter and pulse
height discriminator. The Bragg–Bretano arrange-
ment was adopted using CuKa radiation (l=1.541 )
and setting of 34 kV and 20 mA. The peaks were
identified using PDF bases [11].
of 3 showed a nNi–Cl absorption at 222 cm–1 whereas
for 3a this band is shifted to 206 cm–1. Similarly, the
displacement of nasSCN band from 2077 (4)
to 2118 cm–1 (4a) is a strong evidence of the change of
the coordination mode of thiocyanate group from ter-
minal to end-to-end fashion [12].
Thermoanalytical results
The TG and DTA curves for the complexes 1–4 are il-
lustrated in Fig. 1. Table 2 lists the results of the ther-
mal studies of these compounds together with the as-
signments of each decomposition stage based on mass
calculation. Therefore the groups indicated at the
right column of the Table 2 do not correspond neces-
sarily to the gaseous final products of decomposition.
With exception of the TG curve obtained for 3, the
Results and discussion
The elemental analyses and thermogravimetric data,
together with IR spectroscopy results, confirmed the
proposed formulae for the synthesized compounds.
The results of the analyses are presented in Table 1.
Infrared spectra
The neutral monodentate coordination of pyrazolyl lig-
ands in 1–4 was evidenced by the presence of intense
nNH bands from 3420 to 3217 cm–1 as well by the shift
of the ring breathing bands to lower frequencies
(1574–1515 cm–1) when compared to those ones of the
free ligands (1595–1558 cm–1) [7, 8]. The presence of
hydrogen bonds can also be evidenced by the unfold-
ing of the nNH band. The existence of terminal
N-bonded thiocyanate groups in 2 and 4 was detected
by the nasSCN bands at ca. 2077 cm–1 [12]. Besides the
typical bands of pyrazole molecules, the IR spectra of
3a and 4a showed some differences when compared to
those ones recorded for 3 and 4. According some au-
thors, nickel(II) compounds containing chloro ligand
coordinated in a terminal fashion exhibit nNi–Cl bands
in a higher frequency than those ones observed for
chloro-bridged species [13]. In fact the far-IR spectrum
Fig. 1 — – TG and ××× – DTA curves for the complexes 1–4
336
J. Therm. Anal. Cal., 79, 2005

NICKEL(II) PYRAZOLYL COMPLEXES
Table 2 Thermal analysis data for nickel(II) compounds
Dm/%
DTA peaks/°C
Complex
Step
DT/°C
Assignment
obt.
calc.
endo
exo
1
2
3
–49.98
–16.49
–13.88
19.65
–50.82
–16.94
–13.66
18.58
–3HPz
–HPz
166–287
287–406
406–800
244
326, 345
–
–
–
700
1
–2Cl–,
+0.5O2
residue
1
2
3
4
–29.30
–31.46
–18.03
–4.93
–30.45
–30.45
–18.81
–3.59
–2HPz
–2HPz
160–216
216–300
300–450
450–800
208
258
–
–
–
2
3
–NCS–, CN–
–S2–, +0.5O2
384
532, 770
–
residue
16.28
16.70
–2H2O,
–2HdmPz
–2Cl–,
–HdmPz
–2CH3
–HPz,
1
2
3
4
–41.04
–28.71
–6.74
–9.43
14.08
–41.50
–30.36
–5.47
–9.10
13.57
70–160
160–290
290–465
465–800
131
247
344
–
–
–
–
657
residue
+0.5O2
1
2
3
4
5
–2.99
–32.56
–33.41
–14.69
–2.81
–3.03
–35.32
–32.29
–14.12
–2.70
–H2O
–H2O,
–2HdmPz
–2HdmPz
–NCS–, CN–
S2–, +0.5O2
40–93
93–176
176–292
292–505
505–750
80
165
230
–
–
–
–
371
–
4
–
residue
13.54
12.54
TG-DTA curves of compounds 1, 2 and 4 showed a
similar thermal degradation behavior in which the
pyrazolyl ligands are firstly eliminated followed by
the pyrolysis of the anionic groups together with up-
take of O2, leading to the formation of NiO residue
(ASTM 78-0643) [11]. With regard to the thermal be-
havior of 3, the last two stages of the decomposition
probably involve the formation of a carbonaceous res-
idue during the gradual degradation of the remaining
HdmPz ligand. The TG-DTA curves of the isothio-
cyanato-complexes 2 and 4 also showed that NiS
(ASTM 77-1624) [11] is formed after the release of
the ligands. The following step is characterized by the
oxidation of NiS to NiO (ASTM 78-0643) [11].
The analysis of TG curves also revealed the higher
thermal stability of pyrazole-based systems (1 and 2)
than those observed for [NiX2(HdmPz)4] (3 and 4) prob-
ably because of the increased steric hindrance of 3,5-di-
methylpyrazole. It is also worth mentioning that no sta-
ble intermediate could be isolated from the heating of 1
and 2 due to the overlapping steps.
heating of the precursor samples 3 and 4 there were
two pyrazole ligands released accompanied by a sub-
sequent structural rearrangement in which the termi-
nal X-groups changed their coordination modes to the
bridging fashion, affording the coordination polymers
3a and 4a. Therefore, this work showed the potential
use of thermal data as a tool for alternative synthetic
routes of polymeric metal compounds.
Acknowledgments
We thank the CNPq, CAPES, FAPESP and FINEP for finan-
cial support.
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