The chemistry of hydrazine derivatives - Thermal behavior and characterisation of hydrazinium salts and metal hydrazine complexes of 4,5-imidazoledicarboxylic acid

Article abstract of DOI:10.1016/S0040-6031(01)00756-0

New hydrazinium hydrogen-4,5-imidazoledicarboxylate (N₂H₅Himdc), its monohydrate, N₂H₅Himdc·H₂O, (H₂imdc = 4,5-imidazoledicarboxylic acid) and the metal complexes of the type M(imdc)0.5N₂H₄·H₂O, where M = Mn,Co or Cd and M(imdc)N₂H₄·H₂O, where M = Ni or Zn have been prepared. The compounds have been characterised by analytical, electronic and IR spectral and thermal studies. Electronic spectral data suggest that the Co and Ni complexes are of spin-free (high spin) type with octahedral geometry. The IR absorption bands of N-N stretching in the range 970-960 cm^-1 unambiguously prove the bidentate bridging nature of the N₂H₄ ligands. IR spectra also confirm the unidentate coordination behaviour of carboxylate (ν_asy = 1570 cm^-1 and ν_sym = 1390 cm^-1) groups of the imidazoledicarboxylate dianion. Simultaneous TG and DTA of the free acid show a single-step complete endothermic (281°C) decomposition whereas its anhydrous and hydrated hydrazinium salts show endotherm followed by exotherm decomposition. All the metal complexes undergo endothermic decomposition around 100°C with the loss of hydrazine and water, via the metal 4,5-imidazoledicarboxylate which further decomposes exothermically in the range 110-560°C to give the respective metal oxide as the end product.

Full text of DOI:10.1016/S0040-6031(01)00756-0

Thermochimica Acta 386 &2002) 35±42
The chemistry of hydrazine derivativesÐthermal behavior and
characterisation of hydrazinium salts and metal hydrazine
complexes of 4,5-imidazoledicarboxylic acid
T. Premkumar, S. Govindarajan*
Department of Chemistry, Bharathiar University, Coimbatore 641046, India
Abstract
New hydrazinium hydrogen-4,5-imidazoledicarboxylate &N2H5Himdc), its monohydrate, N2H5HimdcÁH2O, &H2imdc ˆ 4,5-
imidazoledicarboxylic acid) and the metal complexes of the type M&imdc)0.5N2H4ÁH2O, where M ˆ Mn,Co or Cd and
M&imdc)N2H4ÁH2O, where M ˆ Ni or Zn have been prepared. The compounds have been characterised by analytical,
electronic and IR spectral and thermal studies. Electronic spectral data suggest that the Co and Ni complexes are of spin-free
&high spin) type with octahedral geometry. The IR absorption bands of N±N stretching in the range 970±960 cmÀ1
unambiguously prove the bidentate bridging nature of the N2H4 ligands. IR spectra also con®rm the unidentate coordination
behaviour of carboxylate &nasy ˆ 1570 cmÀ1 and nsym ˆ 1390 cmÀ1) groups of the imidazoledicarboxylate dianion.
Simultaneous TGand DTA of the free acid show a single-step complete endothermic &281 8C) decomposition whereas its
anhydrous and hydrated hydrazinium salts show endotherm followed by exotherm decomposition. All the metal complexes
undergo endothermic decomposition around 100 8C with the loss of hydrazine and water, via the metal 4,5-
imidazoledicarboxylate which further decomposes exothermically in the range 110±560 8C to give the respective metal
oxide as the end product. # 2002 Elsevier Science B.V. All rights reserved.
Keywords: Hydrazinium; Hydrazine; 4,5-Imidazoledicarboxylic acid; Metal complexes; Thermal decomposition
1. Introduction
have been extensively used for investigations in coor-
dination chemistry, most reports are on pyrazines,
some of which have been reviewed [13]. We have
become particularly interested in the 4,5-imidazole-
dicarboxylic acid &H2imdc), since it has a number of
attractive features. They are sterically compact, planar
and have multi-donor coordination sites. The thermal
decomposition and IR spectral analysis of many metal
carboxylates such as formate [14,15], acetate [6,16],
propionate [17], oxalate [18,19], malonate [16,20],
succinate [20], benzoate and phthalate [21] with
hydrazine have been reported. However, to the best
of our knowledge, no work has been done on the
thermal property of metal hydrazine complexes as
well as simple hydrazinium salt of heteroaromatic
Hydrazine is a versatile ligand and it offers the
possibility of different modes of coordination towards
transition metal ions [1±3]. It can function as a mono-
dentate [4,5] or bridging bidentate [6±9] ligand. Ther-
mal reactivity of metal coarboxylates with hydrazine
is of increasing interest, since they serve as precursors
to ®ne-particle oxide materials [10±12]. Our recent
interest in hydrazine chemistry began with the pre-
paration and characterisation of metal complexes of
nitrogen containing heterocyclic, both mono and
dicarboxylic acids. Heteroaromatic dicarboxylates
* Corresponding author. Fax: ‡91-422-4222387.
0040-6031/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 4 0 - 6 0 3 1 & 0 1 ) 0 0 7 5 6 - 0

36
T. Premkumar, S. Govindarajan / Thermochimica Acta 386 32002) 35±42
acids in general and H2imdc in particular. Hence,
keeping this in view, this work was undertaken to
study the thermal stability and coordinating ability of
hydrazine in the presence of H2imdc, on the com-
plexation and the results are given here.
2.91 g of Co&NO3)2Á6H2O, 0.01 mol; in the case of
Mn, manganese&II) acetate tetrahydrate 2.45 g,
0.01 mol was used) with constant stirring. The solid
products immediately formed were ®ltered off after
1 h and washed with water, alcohol and diethyl ether
and air-dried.
2. Experimental
2.3. Physico-chemical techniques
2.1. Preparation of hydrazinium hydrogen-4,5-
imidazoledicarboxylate and its monohydrate salt
All the chemicals used were pure commercial grade
and the solvents were distilled before use. The hydra-
zine content of the complexes was determined volume-
trically using a standard KIO3 solution &0.025 M) under
Andrews' condition [22]. The metals after destroying
the organic part and hydrazine by treatment with con-
centrated HNO3 and evaporating the excess HNO3,
were determined volumetrically by EDTA titration
[22]. The re¯ectance spectra of the powdered com-
plexes were recorded on a Shimadzu 240-A UV±VIS
spectrophotometerin the range 300±800 nm. IR spectra
were recorded as KBr pellets with a Perkin-Elmer 597-
model spectrophotometer in the 4000±400 cmÀ1 range.
Elemental analyses were performed on Perkin-Elmer
240 B CHN analyser. Simultaneous TG-DTA measure-
ments were carried out on a STA 1500 thermal analyser.
The experiments were carried out in air using platinum
cups as sample holders with 5±10 mg of the samples at
The anhydrous hydrazinium and its monohydrate
salts of H2imdc have been prepared by mixing hydra-
zine hydrate and the acid, in 1:1 and 2:1 molar ratio
&0.5 cm3, 0.01 mol and 1.1561 g, 0.01 mol; 1 cm3,
0.02 mol and 1.561 g, 0.01 mol), respectively, in
50 cm3 of distilled water. The resulting turbid solu-
tions were heated over waterbath for 1 h to get clear
solutions, which were kept for crystallisation in a
vacuum desiccator over calcium chloride. While the
anhydrous salt separated out after 24 h as white pow-
der, the monohydrated hydrazinium salt deposited as
white spongy crystals only after a couple of days. It is
worth mentioning that the 1:2 ratio of base and acid
also yielded the monohydrated salt. Both the salts are
soluble in hot water.
the heating rate of 10 8C minÀ1
.
2.2. Preparation of the metal complexes
An aqueous mixture &50 cm3) containing H2imdc
&1.561 g, 0.01 mol) and hydrazine hydrate &2 cm3,
0.04 mol) was added slowly to an aqueous solution
&50 cm3) of the respective metal nitrate hydrates &e.g.
3. Results and discussion
Dibasic acids with hydrazine are known to form
N2H4ÁH2A &N2H5HA) or &N2H6A), 2N2H4ÁH2A
Table 1
Analytical dataa
Compound
Yield &%) Colour
Found &calculated) &%)
Hydrazine
Metal
Carbon
Hydrogen
Nitrogen
HimdcN2H5
90
75
90
95
97
90
85
Dull white
White
17.40 &17.01)
15.10 &15.52)
6.40 &6.53)
±
30.13 &31.89)
28.96 &29.11)
23.97 &24.68)
23.95 &24.20)
22.53 &22.83)
23.42 &23.86)
19.27 &19.96)
3.98 &4.28)
4.23 &4.89)
1.98 &2.07)
1.96 &2.04)
2.56 &2.66)
1.87 &1.99)
1.53 &1.67)
28.96 &29.77)
19.86 &20.38)
16.23 &17.28)
16.83 &17.00)
19.93 &21.30)
21.93 &22.26)
13.21 &13.97)
HimdcN2H5ÁH2O
±
Mn&imdc)0.5N2H4ÁH2O
Co&imdc)0.5N2H4ÁH2O
Ni&imdc)N2H4ÁH2O
Zn&imdc)N2H4ÁH2O
Cd&imdc)0.5N2H4ÁH2O
Light yellow
Dark pink
Bluish violet
White
22.30 &22.60)
23.50 &23.85)
21.90 &22.34)
26.20 &25.99)
37.00 &37.28)
6.30 &6.47)
12.40 &12.72)
12.60 &12.72)
5.40 &5.31)
White
a H2imdc ˆ 4,5-imidazoledicarboxylic acid.

T. Premkumar, S. Govindarajan / Thermochimica Acta 386 32002) 35±42
37
Table 2
IR spectral data &cmÀ1
)
Compound
H2imdc
n&OH) of acid/watera
n&NH)
n
of acid
nasy&OCO)
nsym&OCO)
n&N±N)
ꢀCˆO†
3423
3173
2955
3146
2974
3148
2974
3214
3327
3313
3288
3284
1660
1585
±
1382
±
±
±
±
±
N2H5Himdc
3328
1600
1521
±
1380
±
977
±
±
1610
N2H5HimdcÁH2O
3463 &b)
3330
1525
±
1386
±
974
±
±
±
±
±
±
±
Mn&imdc)0.5N2H4ÁH2O
Co&imdc)0.5N2H4ÁH2O
Ni&imdc)N2H4ÁH2O
3382 &b)
3393 &b)
3393 &b)
3423 &b)
3423 &b)
1571
1572
1575
1573
1570
1386
1391
1388
1389
1380
962
967
970
962
960
Zn&imdc)N2H4ÁH2O
Cd&imdc)0.5N2H4ÁH2O
a b: broad.
&N2H5)2A and N2H4Á2H2A &N2H5HAÁH2A) types of
salts &H2A-dibasic acid). The hydrazinium salts of
these types with different dibasic acids have been
reported [21,23,24]. The best known of these are only
N2H4ÁH2A and 2N2H4ÁH2A with dicarboxylic acids.
But, only the former type of salt is obtained with
H2imdc. The 2:1 mole ratio aqueous solutions of
hydrazine hydrate and H2imdc always resulted in
the formation of hydrazinium hydrogen-4,5-imidazo-
ledicarboxylate monohydrate instead of dihydrazi-
nium salt. The reason for this could be a strong
internal hydrogen bond between the COOH and
COOÀ groups of the bicarboxylate anion [16,25].
The metal complexes were highly insoluble in water
and other common organic solvents. All of them are
monohydrated and stable in air &Table 1).
3.1. Electronic spectra
The electronic spectra of the cobalt complex shows
a strong band around 22,730 cmÀ1 which is assigned to
4
the 4T1gꢀP†! T1g transition of typical six-coordinated
Fig. 1. Simultaneous TG-DTA of H2imdc.

38
T. Premkumar, S. Govindarajan / Thermochimica Acta 386 32002) 35±42
Fig. 2. Simultaneous TG-DTA of N2H5Himdc.
cobalt&II) complex [26]. The nickel complex exhibits
two bands at 14,124 and 22,123 cmÀ1, which are
ascribable to 3A2g ! T1g and 3A2g ! T1g&P) transi-
tions, respectively, characteristic of octahedral geo-
metry [26].
3.2. Infrared spectra
3
3
In Table 2 the main bands in the infrared spectra of
thefree acid, itssaltsandmetal complexes arelisted and
areassignedonthebasisofearlierstudies[8,27]. TheIR
Fig. 3. Simultaneous TG-DTA of N2H5HimdcÁH2O.

T. Premkumar, S. Govindarajan / Thermochimica Acta 386 32002) 35±42
39
977±974 cmÀ1 are indicative of the presence of N2H5
ion in the salt [23].
‡
spectra of the H2imdc and its salt show the absorption
frequencies in the region 3423±3328 cmÀ1, due to O±H
stretching of the free COOH group. The carboxyl
stretching frequencies of free carbonyl groups in these
cases register theirabsorptionbands intheregion1660±
1610 cmÀ1. In addition to 3330 cmÀ1, there is a broad
band at 3463 cmÀ1 in the monohydrate salt, indicating
the presence of water molecule. The IR frequencies in
the region 1386±1380 and 1525±1521 cmÀ1 for these
simple salts are assigned for symmetric and asymmetric
stretching frequencies of the carboxylate ions, respec-
tively. The N±N stretching frequencies in the range
The infrared spectra of the metal complexes show a
broad band in the region 3423±3382 cmÀ1 due to O±H
stretching frequency of water molecule. In these com-
plexes, the asymmetric and symmetric stretching fre-
quencies of the carboxylate ions are seen in the ranges
1575±1570 and 1391±1380 cmÀ1, respectively, with
an average separation Dn &nasyÀnsym) of 185 cmÀ1
,
indicating the monodentate coordination behaviour of
both carboxylate groups [28] in the dianion. The N±N
stretching frequency of these complexes appears in the
Table 3
Thermal decomposition dataa
Compound
DTA peak
temperature &8C)
Thermogravimetry &TG)
Decomposition phenomena/
decomposition product
Temperature
Mass loss &%)
Found
range &8C)
Calculated
H2imdc
281&‡)
272&‡)
270±300
250±300
99.00
62.00
100.00
64.85
Complete decomposition
N2H5Himdc
Dehydrazination and decar-
boxylation
328&À)
422&À)sh
525&À)
300±600
99.00
100.00
Complete decomposition
N2H5HimdcÁH2O
246&‡)
271&‡)d
289&‡)d
322&À)
522&À)
Dehydration,
dehydrazination and
decarboxylation
240±300
300±600
66.00
99.00
67.00
100.00
Complete decomposition
Mn&imdc)0.5N2H4ÁH2O
Co&imdc)0.5N2H4ÁH2O
Ni&imdc)N2H4ÁH2O
100&‡)
55±120
13.50
66.00
13.99
64.23
Mn&imdc)
MnO2
382&À)
120±500
100&‡)
399&À)
105&‡)
165&À)
257&À)
305&À)
378&À)
50±125
125±465
15.00
69.00
13.76
67.05
Co&imdc)
Co3O4
50±170
20.00
19.03
Ni&imdc)
170±500
50±110
73.00
16.00
71.57
18.55
NiO
Zn&imdc)N2H4ÁH2O
90&‡)
Zn&imdc)
380&À)
450&À)
486&À)
500&À)
110±560
55±105
69.00
11.00
67.60
11.27
ZnO
Cd&imdc)0.5N2H4ÁH2O
91&‡)
Cd&imdc)
400&À)
412&À)sh
438&À)
105±560
55.00
57.41
CdO
a &‡): endotherm; &À): exotherm; sh: shoulder; d: doublet.

40
T. Premkumar, S. Govindarajan / Thermochimica Acta 386 32002) 35±42
range 970±960 cmÀ1 which is observed for the biden-
tate bridging hydrazine molecule [8].
3.3.3. Hydrazinium hydrogen-4,5-
imidazoledicarboxylate monohydrate
3N2H5HimdcÁH2O)
3.3. Thermal studies
Almost similar modes of decomposition as the
anhydrous salt but for a endothermic doublets &271
and 289 8C) with a shoulder around 246 8C due to
dehydration, which is clearly different from the anhy-
drous salt. The TGalso shows &Fig. 3) a two-step
decomposition, with dehydration, dehydrazination
and decarboxylation, all taking place in a single-step,
in the temperature range 240±300 8C to form imida-
zole which further decomposes exothermically as
observed in the anhydrous salt.
3.3.1. 4,5-imidazoledicarboxylic acid 3H2imdc)
The DTA of the H2imdc shows a distinct endotherm
&Fig. 1) at 281 8C due to complete decomposition of
the acid into gaseous products. The TGshows distinct
single-step decomposition between 270 and 300 8C
with almost 100 % weight loss. This decomposition
temperature of the free acid is in good agreement with
the literature decomposition temperature [29].
3.3.2. Hydrazinium hydrogen-4,5-
imidazoledicarboxylate 3N2H5Himdc)
3.3.4. Metal complexes
The thermal data of the complexes are summarised
in the Table 3. The simultaneous TG-DTA of the
complexes shows that they follow the same two-step
decomposition with the isomeric metal imidazoledi-
carboxylate as intermediate. The formation of the
intermediate is accompanied by a mass loss corre-
sponding to the loss of a water molecule and hydra-
zine. This loss is observed as an endotherm around
100 8C, in all the compounds except nickel, which
shows an endotherm at 105 8C followed by an
exotherm at 165 8C. In the second step, the intermedi-
ate undergoes a continuous decomposition to give the
The simultaneous TGand DTA traces of this salt are
given in Fig. 2. The TGof the salt shows two-step
decomposition accordance with DTA showing a sharp
endotherm followed by a broad exothermic multiplets.
The endotherm at 272 8C is assigned to decorboxyla-
tion along with dehydrazination to give imidazole as
an intermediate, which further undergoes continuous
exothermic decomposition in the range 300±600 8C to
gaseous products. This complete decomposition is in
good agreement with TGweight loss, which is almost
100%.
Fig. 4. Simultaneous TG-DTA of Co&imdc)0.5N2H4ÁH2O.

T. Premkumar, S. Govindarajan / Thermochimica Acta 386 32002) 35±42
41
Fig. 5. Simultaneous TG-DTA of Ni&imdc)N2H4ÁH2O.
respective metal oxide as the end product. This is seen
Acknowledgements
as a broad exotherm with subtle differences. The
simultaneous TG-DTA of the cobalt and nickel com-
pounds are shown in Figs. 4 and 5, respectively, as
representative examples.
T. Premkumar thanks the Bharathiar University,
Coimbatore, India, for the award of a University
Research Fellowship.
4. Conclusions
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interesting to note that nickel and zinc form mono-
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