Transition metal complexes with pyrazole-based ligands. Part 8. Characterization and thermal decomposition of zinc(II) complexes with di- and trisubstituted pyrazoles

Article abstract of DOI:10.1023/A:1010135903480

We report the synthesis and the characterization (elemental analysis, FT-IR spectroscopy, thermal methods and molar conductivity measurements) of the mixed complexes of zinc with acetate and 3-amino-5-methylpyrazole, HL¹, [Zn(OAc)₂(HL¹)₂], or 3-amino-5-phenylpyrazole, HL², [Zn(OAc)₂(HL²)₂], or 4-acetyl-3-amino-5-methylpyrazole, HL³, [Zn(OAc)(L³)(HL³)]₂, with isothiocyanate and HL², [Zn(SCN)₂(HL²)₂], or HL³, [Zn(SCN)₂(HL³)₂], and with nitrate, isothiocyanate and 3,5-dimethyl-1-carboxamidinepyrazole, HL⁴, [Zn(NO₃)(NCS)(HL⁴)₂]. The thermal decomposition of the complexes is generally continuous resulting zinc oxide as end product, except [Zn(OAc)(L³)(HL³)]₂, in which case a well-defined intermediate was observed between 570-620 K. On the basis of the IR spectra and elemental analysis data of the intermediate a decomposition scheme is proposed.

Full text of DOI:10.1023/A:1010135903480

Journal of Thermal Analysis and Calorimetry, Vol. 63 (2001) 723 732
TRANSITION METAL COMPLEXES WITH
PYRAZOLE-BASED LIGANDS
Part 8. Characterization and thermal decomposition of
zinc(II) complexes with di- and trisubstituted pyrazoles
2
K. MØszÆros SzØcsØnyi1, V. M. Leovac1, . K. Ja imovi ,
1
V. I. ¨ e ljevi , A. KovÆcs3, G. Pokol4 and S. GÆl4
1University of Novi Sad, Faculty of Sciences, Institute of Chemistry, 21000 Novi Sad,
Trg D. Obradovi a 3, Yugoslavia
2Faculty of Metallurgy and Technology, Podgorica, Yugoslavia
3Research Group for Technical Analytical Chemistry of the Hungarian Academy of Sciences at the
Institute for General and Analytical Chemistry, Technical University of Budapest,
H-1521 Budapest, Hungary
4Institute for General and Analytical Chemistry, Technical University of Budapest,
H-1521 Budapest, Hungary
(Received July 13, 2000)
Abstract
We report the synthesis and the characterization (elemental analysis, FT-IR spectroscopy, thermal
methods and molar conductivity measurements) of the mixed complexes of zinc with acetate and
3-amino-5-methylpyrazole, HL1, [Zn(OAc)2(HL1)2], or 3-amino-5-phenylpyrazole, HL2,
[Zn(OAc)2(HL2)2], or 4-acetyl-3-amino-5-methylpyrazole, HL3, [Zn(OAc)(L3)(HL3)]2, with iso-
thiocyanate and HL2, [Zn(SCN)2(HL2)2], or HL3, [Zn(SCN)2(HL3)2], and with nitrate, isothio-
cyanate and 3,5-dimethyl-1-carboxamidinepyrazole, HL4, [Zn(NO3)(NCS)(HL4)2].
The thermal decomposition of the complexes is generally continuous resulting zinc oxide as
end product, except [Zn(OAc)(L3)(HL3)]2, in which case a well-defined intermediate was observed
between 570 620 K. On the basis of the IR spectra and elemental analysis data of the intermediate a
decomposition scheme is proposed.
Keywords: 4-acetyl-3-amino-5-methylpyrazole, 3-amino-5-methylpyrazole, 3-amino-5-phenyl-
pyrazole, FT-IR spectroscopy, thermal decomposition, zinc(II) complexes
Introduction
The biological activity of pyrazole and its derivatives is well known [1]. The utilisa-
tion of these compounds extends from the role as inhibitor of the alcohol dehydro-
genase [2, 3] to their potential antiallergic action [4]. Some pyrazole derivatives may
even inhibit the induction of mutations of some carcinogens [5].
1418 2874/2001/ $ 5.00
2001 AkadØmiai Kiad , Budapest
AkadØmiai Kiad , Budapest
'
Kluwer Academic Publishers, Dordrecht

724
M SZ` ROS SZ CS NYI et al.: ZINC(II) COMPLEXES
On the other hand, metal ions, especially transition metal ions, play an important
role in biological systems. For example, several enzymes are metal complexes where
the coordination environment of the catalytically active metal ion may alter the activ-
ity of a specific enzyme. Hence, structural characteristics of transition metal com-
plexes with bioactive ligands may be a valuable source of information. This work is a
part of our studies on transition metal complexes with pyrazole-based ligands. In our
previous paper [6] we reported on cadmium(II)-complexes with 3-amino-5-methyl-
pyrazole and various anions, while in [7] we described the sulphato and chloro com-
plexes of various cations (manganese, cobalt, copper and zinc) with the same ligand.
In this paper we report the synthesis, selected physico-chemical properties and the
thermal decomposition of acetato- and isothiocyanato complexes of zinc with differ-
ent pyrazole-based ligands. Special attention was paid on the dependence of the ge-
ometry and thermal properties of the complexes on the nature of the pyrazole ring
substituents and the coordinated anion.
Experimental
All pyrazole derivatives used were commercial products of the Aldrich Company ex-
cept for 4-acetyl-3-amino-5-methylpyrazole, HL3. The preparation of this ligand has
been described elsewhere [8]. The compound 3,5-dimethyl-1-carboxamidinepyra-
zole, HL4, was employed in the form of its nitrate salt.
The elemental analysis of the compounds was performed by standard micro-
methods in the Laboratory of Microanalysis at the Chemical Faculty of Belgrade.
The IR spectra of the compounds were recorded in the range of 4000 150 cm 1
on a Perkin Elmer System 2000 FT-IR spectrometer at room temperature using KBr
pellets in the mid-IR range and polyethylene pellets in the far-IR range. The spectra
were obtained with a resolution of 4 cm 1 and with a co-addition of 16 scans.
Thermal analysis was carried out using a DuPont 1090 TA system with samples
of ca. 5 mg. At the thermogravimetric measurements the samples were heated in a
platinum crucible. The thermogravimetric decomposition curves of the samples were
recorded in air and for particular compounds also in nitrogen atmospheres at a heat-
ing rate of 10 K min 1 up to 1000 K. Some of the measurements were repeated at a
heating rate of 2 and 5 K min 1. The DSC curves were registered up to 600 K using an
open aluminium pan as sample holder and an empty aluminium pan as reference. The
molar conductivity of freshly prepared 10 3 mol dm 3 solutions of the compounds in
an appropriate solvent was measured at room temperature using a digital conductivity
meter (Jenway 4010).
Results and discussion
The compounds were prepared by a reaction of hot solutions of zinc acetate and the
respective ligand in a suitable solvent (Table 1). The salt-to-ligand mole ratio was
1:2. By adding solid NH4SCN to the reaction mixture, the corresponding isothio-
J. Therm. Anal. Cal., 63, 2001

M SZ` ROS SZ CS NYI et al.: ZINC(II) COMPLEXES
725
cyanato compound was formed, while the reaction of zinc nitrate, ammonium isothio-
cyanate and 3,5-dimethyl-1-carboxamidinepyrazole, HL4, resulted in the
[Zn(NO3)(NCS)(HL4)2] complex.
Table 1 Composition of the compounds and elemental analysis data
Found (calculated)/%
H
Solvent for
preparation
Compound*
Mr
C
N
37.60
(38.15)
6.00
(5.41)
22.10
(22.25)
[Zn(OAc)2(HL1)2]
1
2
3
4
5
6
377.76
501.90
499.97
MeOH
MeOH
MeOH
52.43
(52.64)
4.70
(4.83)
16.83
(16.75)
[Zn(OAc)2(HL2)2]
47.80
(48.04)
3.81
(3.64)
21.75
(22.42)
[Zn(NCS)2(HL2)2]
42.23
(41.85)
4.91
(5.03)
20.46
(20.92)
[Zn(OAc)L3(HL3)]2
[Zn(NCS)2(HL3)2]•EtOH
[Zn(NO3)(NCS)(HL4)2]
803.56 MeOH-DMF
505.99 MeOH-DMF
37.67
(37.98)
4.09
(4.79)
22.61
(22.15)
34.02
(33.80)
4.87
(4.37)
30.51
(30.33)
461.87
EtOH
*HL1=3-amino-5-methylpyrazole, HL2=3-amino-5-phenylpyrazole,
HL3=4-acetyl-3-amino-5-methylpyrazole, HL4=3,5-dimethyl-1-carboxamidinepyrazole
Selected physico-chemical properties of the compounds are given in Table 2.
The complexes are stable in air except for the [Zn(NCS)2(HL3)2]•EtOH solvate, which
loses the solvent at room temperature. Except the elemental analysis of this com-
pound all other measurements were carried out with the desolvated sample. All the
compounds are white crystals, well soluble in DMF, less soluble in MeOH and EtOH.
As is usual for zinc compounds, the complexes are diamagnetic. The molar conduc-
tivity of [Zn(NCS)(NO3)(HL4)2] corresponds to the value for an electrolyte of 1:1
type, probably because of the NO3 ↔ DMF exchange. The conductivity of the other
isothiocyanato complexes is somewhat lower as a consequence of the
[Zn(NCS)2(HL)2]+DMF ↔ [Zn(NCS)(DMF)(HL)2]++NCS
equilibrium, while the molar conductivity of the acetato complexes is significantly
lower than that of the thiocyanato complexes, indicating the stronger coordination of
the acetate anion.
The complex formation in these compounds is established through the pyridine
nitrogen atom of the pyrazole ring which in the case of the [Zn(OAc)2(HL2)2] [9] and
[Zn(OAc)(L3)(HL3)]2 [10] complexes has been proven by X-ray analysis. In the
[Zn(OAc)(L3)(HL3)]2 compound an exobidentate coordination of the pyrazolato an-
ion (L3) is established through both of the nitrogen atoms. The tetrahedral arrange-
ment around the zinc atom is achieved by dimer formation.
A single crystal of [Zn(OAc)2(HL2)2] can be obtained only in the presence of the
solvent molecule and is stable only in the mother liquor. Hence, the structure of the
J. Therm. Anal. Cal., 63, 2001

726
M SZ` ROS SZ CS NYI et al.: ZINC(II) COMPLEXES
[Zn(OAc)2(HL2)2]•2MeOH solvate has been determined by Lindemann s capil-
lary-mother liquor technique. The tetrahedral arrangement is accomplished by coor-
dination of the pyridine nitrogens of the two pyrazole ligands and the two acetato
groups.
Information on the coordination of the anions can be obtained from the IR spec-
tra. In the case of the acetate ion unidentate, bidentate (chelate) or bridging coordina-
tion may be expected. The above coordination types can be distinguished using the
∆ν=νa(COO) νs(COO) values of the carboxyl group: ∆ν>200 cm 1 is characteristic
on unidentate, ∆ν<100 cm 1 on bidentate coordination while 100<∆ν<200 cm 1 val-
ues refer to ionic or bridging complexes [11]. The ∆ν values of 158 cm 1 for sample 1
and 165 cm 1 for 2 are in agreement with a bridging coordination of the acetate ion in
these complexes where one of the oxygen atoms is coordinated to the metal while the
other one is probably involved in hydrogen bonding with an NH hydrogen. In the
case of 4, however, the ∆ν=254 cm 1 value refers to a unidentate coordination of the
acetato group.
Table 2 Molar conductivity, IR-absorbance of the anion and the geometry of the compound
λM/
νas and νs (ν) of the
Compound
Geometry
1
1
S cm2 mol
anion/cm
1
2
3
4
5
6
[Zn(OAc)2(HL1)2]
[Zn(OAc)2(HL2)2]
[Zn(NCS)2(HL2)2]
[Zn(OAc)L3(HL3)]2
[Zn(NCS)2(HL3)2]
[Zn(NO3)(NCS)(HL4)2]
3.8
3.0
1560, 1402
1562, 1397
2090 (CN )
1645, 1391
2090 (CN )
tetrahedral
tetrahedral
tetrahedral
tetrahedral
tetrahedral
octahedral
38.3
19.8
34.2
60.3
1500, 1250 (NO−3 )
2100 (CN )
The SCN group may be coordinated to the metal either through the nitrogen or
the sulphur. The criteria for determining the bonding type must be taken with caution.
In our case the position of the CN vibrational bands (the intensive IR-band below
2100 cm 1, ~2090 cm 1, 3; 2070 cm 1, 5; 2100 cm 1, 6), suggests that the coordinated
atom is nitrogen [11]. In the IR-spectrum of the [Zn(NO3)(NCS)(HL4)2] the coordina-
tion of the nitrate group is indicated by the bands at 1500 and 1250 cm 1 [11] not pres-
ent in the spectrum of the ligand. At the same time a very intensive band around 1380
cm 1 refers to the presence of an uncoordinated nitrate group [11]. However, as the
IR-spectra were recorded using KBr pellet technique, this band may appear because
of an NO−3 ↔ Br ion exchange between the compound and KBr, or it may belong to
the traces of NH4NO3 formed during the synthesis of the compound. This means that
both NO−3 and Br are coordinated.
Additional characteristic feature of the [Zn(NO3)(NCS)(HL4)2] complex is that
the HL4 ligand may be coordinated not only through the pyridine nitrogen atom, but
also through the imido-nitrogen of the carboxamidine group in agreement with a shift
of the ν(C=NH) vibration at 1700 cm 1 in the free ligand towards lower frequencies
J. Therm. Anal. Cal., 63, 2001

M SZ` ROS SZ CS NYI et al.: ZINC(II) COMPLEXES
727
(1670 cm 1). Such an N,N-coordination has been verified in the case of the
[Cu(NO3)2(HL4)2] complex by X-ray analysis [12]. A similar N,N-coordination has
been found also in 3,5-dimethyl-1-carboxamidepyrazole complexes with various cat-
ions [13, 14]. Based on the IR characteristics of the above complexes we can assume
an octahedral geometry for the [Zn(NO3)(NCS)(HL4)2] complex.
Thermal decomposition of the compounds
The thermal decomposition data are given in Table 3, the TG-DTG decomposition
curves of the acetato compounds are presented in Fig. 1 and those of the isothio-
cyanato complexes in Fig. 2. As can be seen, the thermal stability of the acetato com-
plexes is lower than the stability of the corresponding isothiocyanato complexes. The
thermal stability of the dimeric acetato compound is close to the stability of the
isothiocyanato complexes, while the stability of the nitrato-isothiocyanato complex is
higher than that of the acetato compounds, but lower than the stability of the isothio-
cyanato complexes.
Fig. 1 DSC curves of the samples: [Zn(OAc)2(HL1)2] ( ), [Zn(OAc)2(HL2)2] (---),
[Zn(OAc)L3(HL3)]2 (- - ), [Zn(NCS)2(HL2)2] ( ),
[Zn(NCS)2(HL3)2] ( - ), [Zn(NO3)(NCS)(HL4)]2 (- -)
J. Therm. Anal. Cal., 63, 2001

728
M SZ` ROS SZ CS NYI et al.: ZINC(II) COMPLEXES
The DSC curves presented in Fig. 3 give information on the heat effects of the
decomposition in the 300 600 K temperature range. The first sharp endothermic peak
in the acetato compounds refers to the melting of the sample. The decomposition
enthalpy of the following step depends on the anion: the decomposition of the iso-
thiocyanato complexes exhibits a small exothermic effect, while the decomposition
of the acetato complexes is an endothermic process.
Table 3 Thermal decomposition data of the compounds
Mass change,
found (calcd.)/%
Mass change,
found (calcd.)/%
Trange/K
Trange/K
Complex
1
2
3
4
5
6
[Zn(OAc)2(HL1)2]
[Zn(OAc)2(HL2)2]
[Zn(NCS)2(HL2)2]
[Zn(OAc)L3(HL3)]2
[Zn(NCS)2(HL3)2]
[Zn(NO3)(NCS)(HL4)2]
420 580
420 600
500 740
570 620
530 710
460 480
19.0 (22.80)*
15.0 (16.95)*
31.0 (27.74)3*
17.0 (17.32)2*
23.0 (27.98)3*
34.0 (34.57)3*
580 800
600 830
740 940
640 920
710 1000
480 1000
78.0 (78.46)
83.0 (83.78)
84.0 (83.72)
81.0 (79.74)
82.0 (82.30)
83.0 (82.38)
*According to Scheme 3, 2* According to Scheme 1, 3* According to Scheme 2
Fig. 2 TG and DTG curves of the samples: [Zn(NCS)2(HL2)2] ( ) ,
[Zn(NCS)2(HL3)2] (---), [Zn(NO3)(NCS)(HL4)]2 (- - )
J. Therm. Anal. Cal., 63, 2001

M SZ` ROS SZ CS NYI et al.: ZINC(II) COMPLEXES
729
Fig. 3 TG and DTG curves of the samples: [Zn(OAc)2(HL1)2] ( ),
[Zn(OAc)2(HL2)2] (---), [Zn(OAc)L3(HL3)]2 (- - -)
The decomposition up to 600 K is independent on the nature of the gas carrier. In
argon, the decomposition was not completed up to 1300 K. In air, the shape of the
TG-DTG curves of the acetato complexes above 700 K refer to the burning of the
sample, while the isothiocyanato complexes do no show such an effect. The final
product of the decomposition in air is always ZnO.
During the decomposition of the compound with 4-acetyl-3-amino-5-methyl-
pyrazole, [Zn(OAc)(L3)(HL3)]2, an intermediate was formed which was stable in the
range of 570 620 K. By freezing-out technique the intermediate was isolated and its
composition was determined by elemental analysis. The IR spectrum of the intermediate
was also recorded. The most characteristic changes with respect to the spectrum of
[Zn(OAc)(L3)(HL3)]2 are the disappearance of the NH2 stretching vibrations and the ap-
pearance of a new NH stretching vibrational band at 3363 cm 1. In the 1000 1700 cm 1
range the C=O stretching band of the acetato group shifted to the lower wavenumbers by
11 cm 1 and the C O stretching band (1406 cm 1) lost intensity, while in the far-IR region
the positions of the ZnO and ZnN stretching bands are significantly altered. These
changes resemble our recent experience on 3-aminopyrazole complexes [6], where the
two pyrazole moieties of a complex got connected by an NH bridge formed from the
J. Therm. Anal. Cal., 63, 2001

730
M SZ` ROS SZ CS NYI et al.: ZINC(II) COMPLEXES
two NH2 groups parallel with an NH3 elimination in the decomposition procedure. In the
case of the [Zn(OAc)(L3)(HL3)]2 compound the NH3 elimination is probably followed
with the loss of two acetato groups. The mass loss of 17.0% (calcd 16.20%) and the ele-
mental analyses data (found (%) C-41.4; H-4.6; N-23.0, calcd (%): C-41.03; H-4.49;
N-20.80) may support a decomposition mechanism presented by Scheme 1:
At the decomposition of the acetato complexes with 3-amino-5-methylpyrazole and
3-amino-5-phenylpyrazole also an NH3 elimination takes place. However, in these com-
plexes probably the splitting of the pyrazole ring occurs during the decomposition. As the
decomposition of these compounds is continuous, it is difficult to propose the decompo-
sition mechanism. We can assume an analogy with the decomposition of sulphato M(II)
complexes with 3-amino-5-methylpyrazole [6] where the stable intermediates were ob-
served and identified. Comparison of the TG-DTG curves of the [Zn(OAc)2(HL1)2] and
[Zn(OAc)2(HL2)2] compounds with the corresponding ones from [6], as well as the mass
loss during the decomposition suggest that the decomposition of these complexes takes
place in a similar way as in [6] (Scheme 2):
No intermediate formation was observed in the case of the decomposition of the
isothiocyanato complexes either. The mass loss (Table 1) up to the second minimum
of the DTG curve may correnspond to Scheme 3.
J. Therm. Anal. Cal., 63, 2001

M SZ` ROS SZ CS NYI et al.: ZINC(II) COMPLEXES
731
However, the decomposition of these samples is continuous, thus an intermedi-
ate could not be isolated.
Conclusions
A tetrahedral geometry is characteristic of all of the complexes, except the complex
with 3,5-dimethyl-1-carboxamidinepyrazole, [Zn(NO3)(NCS)(HL4)2] which has an
octahedral configuration. The geometry of these complexes is determined above all
by the nature of the substituent on the pyrazole ring. As there is no possibility for a
stabilization by ligand field stabilization energy (LFSE), the stereochemistry of the
Zn-complexes depends on the cation size and on steric factors of the ligand. In these
complexes, the ring substituents with additional donor atoms may serve for formation
of complexes with a higher coordination number. So, 3,5-dimethyl-1-carboxamidine-
pyrazole is coordinated to the zinc atom through the N,N-atoms leading to an octahe-
dral structure in the [Zn(NO3)(NCS)(HL4)2] complex.
The thermal stability of the compounds is mostly anion dependent. However, the
decomposition mechanism depends not only on the anion type but also on the
pyrazole ring substituent. The thermal decomposition of the complexes with 3-amino
substitution takes place by losing of NH3-molecule ([6] or H2O, [7]) which may be
followed by a splitting of one of the pyrazole rings as is presented in Scheme 2. In the
case of the acetato or isothiocyanato complexes there is a possibility of the intermedi-
ate stabilization through an additional bond formation via the anion (Scheme 3 and
[7]). The stability of the acetato intermediates is most probably a consequence of the
stability of the bridge. This proposition is also supported by the DSC curves of the
compounds: while the second step of the decomposition of the isothiocyanato com-
plexes exibits a small exothermic effect, the corresponding decomposition step of the
acetato compounds is an endothermic process.
References
1 S. Trofimenko, Chem. Rev., 93 (1993) 943.
2 I. Bertini, G. Lanini, C. Luchinat, C. Haas, W. Maret and M. Zeppezauer, Eur. Biophys. J.,
14 (1987) 431.
3 L. A. Clejan, D. R. Koop and A. I. Cederbaum, Drug. Metab. Dispos., 17 (1989) 694.
4 L. J. Huang, S. C. Kuo, J. P. Wang, K. Ishii and H. Nakamura, Chem. Pharm. Bull., (Tokyo),
42 (1994) 2036.
5 M. Moriya, T. Harada and Y. Shirasu, Cancer Lett., 17 (1982) 147.
6 K. MØszÆros SzØcsØnyi, E. Z. Ivege , V. M. Leovac, A. KovÆcs, G. Pokol and ¦. K. Ja imovi ,
J. Therm. Anal. Cal., 56 (1999) 493.
7 K. MØszÆros SzØcsØnyi, E. Z. Ivege , V. M. Leovac, L. S. Vojinoviº, A. KovÆcs, G. Pokol,
J. MadarÆsz and ¦. K. Ja imovi , Thermochim. Acta, 316 (1998) 79.
8 A. Hergold-Brundic, B. Kaitner, B. Kamenar, V. M. Leovac, E. Z. Ivege and N. Jurani ,
Inorg. Chim. Acta, 188 (1991) 151.
J. Therm. Anal. Cal., 63, 2001

732
M SZ` ROS SZ CS NYI et al.: ZINC(II) COMPLEXES
9 ¦. K. Ja imovi , Z. D. Tomi , G. A. Bogdanovi , E. Z. Ivege and V. M. Leovac, Acta Cryst.,
C55 (1999) 1769.
10 Z. D. Tomi , . K. Ja imovi , V. M. Leovac and V. I. ¨ e ljevi , Acta Cryst., (submitted for
publication).
11 K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds,
Wiley-Interscience, New York 1997.
12 ¦. K. Ja imovi , characterization and thermal decomposition of zinc(II) complexes with di-
and trisubstituted pyrazoles, Ph. Thesis, University of Novi Sad, Novi Sad 1999.
13 F. Valach, J. Kohout, M. Dunaj-JurŁo, M. HvastijovÆ and J. Ga o, J. Chem. Soc., Dalton
Trans., (1979) 1867.
14 N. Saha and K. M. Datta, J. Indian Chem. Soc., 59 (1982) 728.
J. Therm. Anal. Cal., 63, 2001