Divalent transition metal complexes of 3,5-Pyrazoledicarboxylate

Article abstract of DOI:10.1007/s10973-005-6932-0

New divalent transition metal 3,5-pyrazoledicarboxylate hydrates of empirical formula Mpz(COO)₂(H₂O)₂, where M=Mn, Co, Ni, Cu, Zn and Cd (pz(COO)₂=3,5-pyrazoledicarboxylate), metal hydrazine complexes of the typ

Full text of DOI:10.1007/s10973-005-6932-0

Journal of Thermal Analysis and Calorimetry, Vol. 84 (2006) 2, 395–399
DIVALENT TRANSITION METAL COMPLEXES OF
3,5-PYRAZOLEDICARBOXYLATE
T. Premkumar and S. Govindarajan*
Department of Chemistry, Bharathiar University, Coimbatore 641 046, India
New divalent transition metal 3,5-pyrazoledicarboxylate hydrates of empirical formula Mpz(COO)2(H2O)2, where M=Mn, Co, Ni,
Cu, Zn and Cd (pz(COO)2=3,5-pyrazoledicarboxylate), metal hydrazine complexes of the type Mpz(COO)2N2H4 where M=Co, Zn
or Cd and Mpz(COO)2nN2H4×H2O, where n=1 for M=Ni and n=0.5 for M=Cu have been prepared and characterized by
physico-chemical methods. Electronic spectroscopic data suggest that Co and Ni complexes adopt an octahedral geometry. The IR
spectra confirm the presence of unidentate carboxylate anion (Dn=nasy(COO–)–nsym(COO–)>215 cm–1) in all the complexes and
bidentate bridging hydrazine (nN–N=985–950 cm–1) in the metal hydrazine complexes. Both metal carboxylate and metal hydrazine
carboxylate complexes undergo endothermic dehydration and/or dehydrazination followed by exothermic decomposition of or-
ganic moiety to give the respective metal oxides as the end products except manganese pyrazoledicarboxylate hydrate, which leaves
manganese carbonate. X-ray powder diffraction patterns reveal that the metal carboxylate hydrates are isomorphous as are those of
metal hydrazine complexes of cobalt, zinc and cadmium.
Keywords: hydrazine, IR spectra, metal complexes, 3,5-pyrazoledicarboxylic acid, thermal decomposition
Introduction
system we have recently shown that the imidazole-
dicarboxylic acid can form both mono- and dianionic
planar chelates with divalent transition metals under
appropriate experimental conditions and the nature of
decomposition of its complexes [12, 13].
Hydrazine is a versatile ligand and it offers the possi-
bility of different modes of coordination towards tran-
sition metal ions [1, 2]. It can function as a mono-
dentate [3] or bridging bidentate [4, 5] ligand. Thermal
reactivity of metal carboxylates with hydrazine is of in-
creasing interest, since they serve as precursors to
fine-particle oxide materials [6, 7]. Our recent interest
in hydrazine chemistry began with the preparation and
characterisation of metal complexes of nitrogen con-
taining heterocyclic dicarboxylic acids [8]. Hetero-aro-
matic dicarboxylates have been extensively used in co-
ordination chemistry; most reports are on pyrazines,
some of which have been reviewed [9]. Among het-
ero-aromatic dicarboxylic acids, 3,5-pyrazoledi-
carboxylic acid, pz(COOH)2, and 4,5-imidazoledi-
carboxylic acid, (H2imdc), are particularly of interest
as they have a number of attractive features. They are
sterically compact, planar and have multi-donor coor-
dination sites [10, 11]. In the five-membered diaza ring
Now pz(COOH)2 has been chosen in order to com-
pare the coordinating ability of carboxyl groups to that
of H2imdc, with respect to their position in the ring, with
divalent transition metal under same experimental con-
ditions. Although various metal complexes have been
synthesized by using pz(COOH)2 and other species as a
secondary ligand viz., tetrabutylammine [10, 14],
acetylacetone [15] and ethylenediamine-N,N’-di-acetic
and -propionic acids [16], the simple reaction between
the divalent transition metal ions and pz(COOH)2 has
not yet been carried out. Also, the interaction of
pz(COOH)2 and transition metals with hydrazine has
not been studied to date. In this paper we report for the
first time the preparation, spectroscopic and thermal
studies on divalent transition metal complexes of
3,5-pyrazoledicarboxylate and their hydrazine adducts.
Experimental
Preparation of Mpz(COO)2(H2O)2 (M=Mn, Co, Ni,
Cu, Zn and Cd)
Hot aqueous solution (50 cm3) containing 3,5-pyra-
zoledicarboxylic acid (1.74 g, 0.01 mol) was added
*
Author for correspondence: drsgovind@yahoo.co.in
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2006 Akadémiai Kiadó, Budapest

PREMKUMAR, GOVINDARAJAN
respective metal nitrate hydrates (e.g., 2.91 g of
water, alcohol and ether, and air-dried. Excess addi-
tion of hydrazine hydrate, that is more than 0.03 mol
reduced the Cu(II) to metallic copper.
Co(NO3)2×6H2O, 0.01 mol and manganese(II) acetate
tetrahydrate 2.45 g, 0.01 mol) in 25 cm3 of distilled
water. The micro-crystalline complexes deposited af-
ter 30 min were filtered off after standing overnight.
The compounds were washed with cold water and
ethanol, and air-dried.
Materials and measurements
All the chemicals were of commercial grade. Solvents
were distilled before use. The hydrazine content of
the complexes was determined volumetrically using a
standard KIO3 solution (0.025 M) under Andrews’
condition [17]. The metals after destroying the or-
ganic part and hydrazine by treatment with concen-
trated HNO3 and evaporating the excess HNO3, were
determined volumetrically by EDTA titration [17].
Instrumental details for elemental analysis, IR and
UV-Vis spectra, TG-DTA and XRD are the same as
described earlier [12, 13].
Preparation of Mpz(COO)2N2H4×nH2O (M=Ni, Co,
Zn and Cd)
To a hot aqueous solution (50 cm3) containing 3,5-
pyrazoledicarboxylic acid monohydrate (1.741 g,
0.01 mol) and hydrazine hydrate (4 cm3, 0.08 mol),
aqueous solution (50 cm3) of the respective metal nitrate
hexahydrates (e.g., 2.97 g of Zn(NO3)2×6H2O; 0.01 mol)
was added slowly with constant stirring. The resulting
micro-crystalline product formed was digested over a
water bath for about 1 h and isolated as above.
Results and discussion
Preparation of Cupz(COO)20.5N2H4×H2O
Compounds of empirical formula Mpz(COO)2(H2O)2
(M=Mn, Co, Ni, Cu, Zn and Cd) and metal hydrazine
complexes of the type Mpz(COO)2N2H4 (M=Co, Zn
or Cd) and Mpz(COO)2nN2H4×H2O (n=1 for M=Ni
and n=0.5 for M=Cu) have been prepared for which
the results of the analyses are given in Table 1. All the
complexes obtained are micro-crystalline, which are
stable in air and insensitive to light. They are insolu-
The title compound was prepared by the addition of
an aqueous solution (50 cm3) of copper nitrate tri-
hydrate (2.42 g, 0.01 mol) to an aqueous solution
(50 cm3) containing 3,5-pyrazoledicarboxylic acid
monohydrate (1.741 g, 0.01 mol) and hydrazine hy-
drate (1.5 cm3, 0.03 mol). A blue product, precipitated
rapidly, was filtered off and washed successively with
Table 1 Analytical data
Found (calculated)/%
Compound
D.pt./°C
Yield/%
Colour
hydrazine
–
metal
carbon
hydrogen nitrogen
22.00
(22.42)
23.98
(24.49)
2.42
(2.45)
11.03
(11.42)
Mnpz(COO)2(H2O)2
Copz(COO)2(H2O)2
Nipz(COO)2(H2O)2
Cupz(COO)2(H2O)2
Znpz(COO)2(H2O)2
Cdpz(COO)2(H2O)2
Copz(COO)2N2H4
140
209
187
119
135
137
198
116
138
205
186
87
95
95
93
95
85
95
97
79
85
80
white
peach
23.70
(23.66)
23.76
(24.09)
2.35
(2.41)
10.92
(11.24)
–
–
–
–
–
23.20
(23.59)
23.79
(24.11)
2.50
(2.41)
11.01
(11.25)
light green
light blue
white
24.80
(25.05)
22.99
(23.65)
2.28
(2.37)
11.21
(11.03)
24.90
(25.59)
22.89
(23.48)
2.28
(2.35)
10.28
(10.96)
36.80
(37.16)
19.07
(19.83)
2.02
(1.98)
8.94
(9.26)
white
13.60
(13.06)
23.80
(24.05)
23.95
(24.48)
2.37
(2.45)
22.83
(22.85)
pink
12.30
(12.18)
21.90
(22.34)
22.47
(22.83)
2.95
(3.04)
19.97
(21.31)
Nipz(COO)2N2H4×H2O
Cupz(COO)20.5N2H4×H2O
Znpz(COO)2N2H4
dark blue
blue
6.20
(6.36)
26.00
(25.25)
23.42
(23.84)
1.62
(1.59)
16.09
(16.69)
12.10
(12.72)
25.90
(25.99)
23.27
(23.86)
2.19
(2.39)
21.86
(22.26)
white
10.50
(10.72)
36.90
(37.65)
19.76
(20.09)
1.99
(2.01)
18.29
(18.76)
Cdpz(COO)2N2H4
white
396
J. Therm. Anal. Cal., 84, 2006

DIVALENT TRANSITION METAL COMPLEXES
ble in water and in most of the organic solvents. It is
nickel complexes) around 125°C. Such high tempera-
ture dehydration suggests the presence of two metal
bounded water molecules [21]. In the second step, the
isomeric anhydrous intermediate, (Mpz(COO)2), un-
dergoes a continuous decomposition to give the re-
spective metal oxide as the end product except manga-
nese complex, which leaves MnCO3 as the final resi-
due probably due to the partial ionic character of the
manganese compound [13]. This is seen as an exother-
mic multiplets with subtle differences. It is noteworthy
that the manganese 4,5-imidazoledicarboxylate di-
hydrate also leaves MnCO3 as the end product, unlike
others giving metal oxides [13]. Close observation of
the TG results shows that the metal carbonates are
formed as intermediates but the decomposition is con-
tinuous so that this cannot be quantified. However, the
TG of the zinc compound shows a distinct step for the
formation of ZnCO3 intermediate. The simultaneous
TG-DTA of the manganese and copper compounds are
shown in Figs 1 and 2, respectively, as representative
examples.
observed that only the 1:1:8 mole-ratio of metal salt,
3,5-pyrazoledicarboxylic acid and hydrazine hydrate,
respectively, yielded the hydrazine compounds. The
hydrazine content below eight mole-ratio always re-
sulted simple metal 3,5-pyrazoledicarboxylates, prob-
ably due to strong acidic nature [18] of the parent acid
which maintains the resulting solution pH<7.
The electronic spectra of cobalt carboxylate hy-
drate and its hydrazine complexes show a broad band
centered at 20,600 cm–1. This has been attributed to
4T1g(F)®4T1g (P) transition, which is characteristic of
octahedral geometry around the metal ion [19]. The
electronic spectra of nickel complexes exhibited max-
ima around 17,000 and 27,000 cm–1 corresponding to
3
3A2g®3T1g (F) and A2g®3T1g (P) d-d transitions,
which are characteristic of octahedral geometry [14].
The copper complex shows a single band at
2
12,890 cm–1, which is assigned to Eg®2T2g transi-
tion, as expected for a six-coordinate d9 species.
These transitions are characteristics of octahedral ge-
ometry of the metal atoms [19].
The simultaneous TG-DTA of metal 3,5-pyrazole-
dicarboxylate hydrazine adducts show that all the com-
plexes yield metal oxide as the final residue via the iso-
The IR spectra of all the metal carboxylate hy-
drates, copper and nickel hydrazine complexes show
strong bands in 3551–3370 cm–1 range assignable to
nOH stretching vibrations of coordinated and/or lattice
water molecules [20]. In this region the difference in
O–H absorption of these complexes is consistent with
the different water content and this is further substan-
tiated by the results of elemental and thermal analy-
ses. The N–H stretching vibrations of complexes ap-
pear in the region of 3333–2923 cm–1. The absorp-
tions in the range of 1640–1560 and 1394–1352 cm–1
correspond to the asymmetric and symmetric nCOO
frequencies of the metal bound carboxylates. The
large difference in nCOO frequencies (Dn>200 cm–1) is
indicative of monodentate coordination of both the
carboxylate groups of the 3,5-pyrazoledicarboxylate
ion. The bidentate bound hydrazines of metal
hydrazine complexes display a N–N stretching fre-
quency [4] in the range 982–951 cm–1. The IR band
centered at 1697 cm–1 in the spectrum of the free di-
carboxylic acid, assigned to stretching vibrations of
the non-ionized carboxyl group, is absent in the spec-
tra of both metal carboxylates and their hydrazine
complexes, confirming that the ligand is doubly ion-
ized in both the cases.
Fig. 1 Simultaneous TG-DTA of Mnpz(COO)2(H2O)2
Though the simultaneous TG-DTA curves of
metal 3,5-pyrazoledicarboxylate dihydrates appear
rather complex, there are two major steps of decompo-
sition except zinc compound that shows three distinct
steps. In the first step of the TG, two water molecules
are lost in the temperature range 85–230ºC. In DTA,
this decomposition is observed as a sharp endotherm
above 140ºC with a shoulder (except for cobalt and
Fig. 2 Simultaneous TG-DTA of Cupz(COO)2(H2O)2
J. Therm. Anal. Cal., 84, 2006
397

PREMKUMAR, GOVINDARAJAN
meric metal pyrazoledicarboxylate intermediate as ob-
served in the metal hydrazine 4,5-imidazoledi-
carboxylates [12]. All of them show endothermic de-
hydrazination followed by endo- and exothermic de-
composition of the metal organic moieties in multiple
steps. However, between nickel and copper complexes,
the former undergoes endothermic dehydration at
116°C, whereas the latter at 140°C indicating the pres-
ence of a lattice and a coordinated water molecule, re-
spectively. As a representative example, the decomposi-
tion pattern of nickel complex, in air, is given below on
the basis of mass losses in the TG curve.
endo (116°C), 70–130°C
Nipz(COO)2N2H4×H2O ¾¾¾¾¾¾®
Nipz(COO)2N2H4+H2O
Fig. 4 Simultaneous TG-DTA of Cupz(COO)2·0.5N2H4·H2O
(i)
(ii)
endo (171°C), 130–195°C
2Nipz(COO)2N2H4 ¾¾¾¾¾¾¾®
2Nipz(COO)2+N2+2NH3+H2O
endo (245 and350°C), 195–370°C
Nipz(COO)2 ¾¾¾¾¾¾¾¾®
NiCO3+N2+CO2+H2O
(iii)
(iv)
exo (450°C) and endo (505°C), 370–600°C
NiCO3 ¾¾¾¾¾¾¾¾¾¾®
NiO+CO2
The simultaneous TG-DTA of the nickel and
copper hydrazine complexes is shown in Figs 3 and 4,
respectively, as examples. Our effort, to isolate the in-
termediates was unsuccessful due to their continuous
decomposition as evident from the TG. Hence, we
have tried to assign the possible intermediates as ob-
served from the TG mass losses, which are in agree-
ment with the calculated mass losses.
X-ray powder diffractogram of metal 3,5-pyrazole-
dicarboxylate hydrates and their hydrazine adducts has
been recorded in order to know the isomorphism among
the set of complexes. Except with variation in peak in-
tensity, the patterns (Fig. 5) of the former compounds
are convincingly comparable suggesting them to be
isomorphous. Similarly, the latter hydrazine complexes
of cobalt, zinc and cadmium (Fig. 6) seem to reveal
isomorphism among them. However, the nickel and
Fig. 5 X-ray powder diffraction patterns of Mpz(COO)2(H2O)2
Fig. 6 X-ray powder diffraction patterns of metal hydrazine
complexes
Fig. 3 Simultaneous TG-DTA of Nipz(COO)2N2H4·H2O
398
J. Therm. Anal. Cal., 84, 2006

DIVALENT TRANSITION METAL COMPLEXES
3 A. Ferrari, A. Braibanti, G. Bigliardi, A. M. Lanfredi and
copper hydrazine compounds are not isomorphous with
the above complexes indicating the structural differ-
ences between them as their composition themselves is
different. The fact that these compounds were isolated
as powders and not as single crystals prevented X-ray
structure determination.
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Conclusions
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• Based on the physico-chemical results it is ob-
served that pz(COOH)2 is present as a dianion in
both the set of complexes. On the other hand,
H2imdc is behaving as a monoanion in the
M(Himdc)2×nH2O [13] complexes and dianion in
the hydrazine complexes as reported earlier [12].
The reason for the dianionic behaviour of the for-
mer acid, in both set of complexes, may be due to
its easy solubility and also the lack of internal hy-
drogen bonding between the carboxylic groups
[18] as against the latter.
• Six coordination has been proposed for all the
metal complexes considering tetradentate coordi-
nation of pyrazoledicarboxylate dianion and bi-
dentate coordination of neutral hydrazine moiety in
the case of hydrazine adduct complexes and mono-
dentate coordination of water molecules in metal
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Accepted: March 29, 2005
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DOI: 10.1007/s10973-005-6932-0
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