TG-MS and TG-FTIR studies of imidazole-substituted coordination compounds: Co(II) and Ni(II)-complexes of bis(1-methylimidazol-2-yl)ketone

Article abstract of DOI:10.1016/j.tca.2012.05.013

Co(II) and Ni(II) coordination compounds with bis(1-methylimidazol-2-yl) ketone (BIK) of general formula M(BIK)₂X₂ (M = Co or Ni; X = Cl or NO₃) were synthesized and characterized by elemental analysis, by UV-vis and FTIR spectroscopies. Following our previous thermoanalytical studies on imidazole-substituted coordination compounds, the thermal behavior of the synthesized complexes was investigated by TG and DTG techniques: the thermal profile is always characterized by three substantial consecutive releasing steps for all the three complexes and the releasing supposed behavior is confirmed by EGA analysis (TG-MS and TG-FTIR). In particular, the first step is ascribed to the release of the two anions, followed by the loss of four methyl groups (side chains of the ligand) and two bridge-carbonyl groups. The residual Cobalt or Nickel tetra-imidazole complex decomposes in a final step to give the metal oxide as the final residue.

Full text of DOI:10.1016/j.tca.2012.05.013

Thermochimica Acta 543 (2012) 183–187
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TG–MS and TG–FTIR studies of imidazole-substituted coordination compounds:
Co(II) and Ni(II)-complexes of bis(1-methylimidazol-2-yl)ketone
S. Materazzia,∗, S. Vecchiob, L.W. Woc, S. De Angelis Curtisa
a Department of Chemistry, Sapienza University of Rome, P.le A. Moro, 5, I-00185 Rome, Italy
b Department S.B.A.I., Sapienza University of Rome, Via del Castro Laurenziano 7, I-00161 Rome, Italy
c Department of Chemistry, Illinois State University, Normal, IL, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Co(II) and Ni(II) coordination compounds with bis(1-methylimidazol-2-yl)ketone (BIK) of general
formula M(BIK)2X2 (M = Co or Ni; X = Cl or NO3) were synthesized and characterized by elemen-
tal analysis, by UV–vis and FTIR spectroscopies. Following our previous thermoanalytical studies on
imidazole-substituted coordination compounds, the thermal behavior of the synthesized complexes was
investigated by TG and DTG techniques: the thermal profile is always characterized by three substantial
consecutive releasing steps for all the three complexes and the releasing supposed behavior is confirmed
by EGA analysis (TG–MS and TG–FTIR). In particular, the first step is ascribed to the release of the two
anions, followed by the loss of four methyl groups (side chains of the ligand) and two bridge-carbonyl
groups. The residual Cobalt or Nickel tetra-imidazole complex decomposes in a final step to give the
metal oxide as the final residue.
Received 20 February 2012
Received in revised form 16 May 2012
Accepted 19 May 2012
Available online 28 May 2012
Keywords:
Coordination compounds
Bis(1-methylimidazol-2-yl)ketone
UV–vis
Evolved Gas Analysis
Thermal stability
TG–MS
© 2012 Elsevier B.V. All rights reserved.
TG–FTIR
1. Introduction
results are promising, though still far from those of established
catalyst systems [8–12].
Coordination compounds with nitrogen donor ligands like
substituted-imidazoles have been thoroughly investigated during
the last years. These compounds can be good models to simu-
late several industrial applications, like the behavior of manganese
compounds containing enzymes as catalysts for the polymeriza-
tion of olefins [1–12] and for washing and bleaching purposes.
The improvement of classical washing and bleaching processes is
both economically and ecologically relevant. The use of catalytically
active transition metal complexes, which can activate hydrogen
peroxide, might be a means of achieving these aims.
Several complexes with polydentate N-donor ligands have
been patented for this purpose [1–3]. Cobalt or Nickel complexes
are also important as model compounds for metal-containing
enzymes such as superoxide dismutase, catalases or extradiol-
cleaving catechol dioxygenases. In order to mimic the histidines,
many model complexes comprise bi- or tridentate N-donor ligands
such as 2,2ꢀ-bipyridine, hydrotris(pyrazol-1-yl)borate or triazacy-
clononane [4–7]. Cobalt (II), Manganese(II) or Ni(II) complexes
containing N-donor ligands have also been tested as polymeriza-
tion catalysts for olefines, especially ethene. The hitherto published
In this view, thermoanalytical characterizations are useful
to complete the information related to the metal-binding sites
and to relate their physico-chemical properties. In addition, the
Evolved Gas Analysis allows to confirm the proposed decompo-
sition steps. Several papers have been published by our group in
this field [13–22,24]. A thermoanalytical study of bis(imidazol-2-
yl)methane (BIM) complexes with cobalt, copper and nickel has
already been reported by our group, since BIM is the simplest poly-
imidazole ligand useful to model multihistidine coordination [17].
Bis(1-methylimidazol-2-yl)ketone (BIK) (Fig. 1) has been known
to be a versatile N,N ligand for many transition metals since its first
synthesis in 1977 [25].
In this work, following the experimental conditions reported in
the literature and in our previous work [24], the synthesis and the
thermoanalytical study of bis(1-methylimidazl-2yl)ketone(BIK)
complexes with divalent Copper or Nickel having a general for-
mula M(BIK)2X2 are reported (M = Co or Ni; X = Cl or NO3). The
thermal stability and the decomposition steps were determined by
thermogravimetry (TG) and derivative thermogravimetry (DTG).
The released products, due to the thermal decomposition, were
analyzed by on-line coupling a mass spectrometer and a FTIR
spectrometer to the thermobalance; the so obtained evolved gas
analysis (EGA–MS and EGA–FTIR) allowed to prove the proposed
decomposition steps.
∗
Corresponding author. Tel.: +39 06 49913616; fax: +39 06 40631.
E-mail address: stefano.materazzi@uniroma1.it (S. Materazzi).
0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.tca.2012.05.013

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S. Materazzi et al. / Thermochimica Acta 543 (2012) 183–187
Scheme 1. Synthesis of the complexes. M = Co or Ni; X = Cl or NO3.
3. Results and discussion
All the complexes were easily precipitated after mixing the
starting solutions; the complexes resulted with a good stoichiome-
try even if the yields were much lower than the supposed (around
45%) [23,24].
Fig. 1. The ligand bis(1-methylimidazol-2-yl)ketone (BIK).
Elemental analysis data of the complexes considered in this
study is summarized in Table 1. As a result of the good agree-
ment between experimental and calculated data a general formula
M(BIK)2X2 (where M = Co or Ni; X = Cl or NO3) can be hypothesized.
The Ni(BIK)2Cl2 and Co(BIK)2Cl2 complexes were also character-
ized by UV–vis and FTIR spectroscopies to be compared with the
corresponding literature data [23,24].
2. Experimental and methods
2.1. Materials
All the complexes showed IR bands (KBr) related to C O at
1633 cm−1 (m), 1478 cm−1 (w), 1463 cm−1 (w), and to C N at
1416 cm−1 (s), 1292 cm−1 (w), 1173 cm−1 (w). The UV–vis spec-
trum in acetonitrile showed a ꢀmax (log e) = 323 (4.51) nm. The
ligand BIK absorbs ultraviolet light at 318 nm (log e = 4.22). This
strong absorption results almost exclusively from an electron tran-
sition from the HOMO − 1 into the LUMO [23,24].
For all the complexes of general formula M(BIK)2X2, a 20 mL
methanol solution of 900 mg of BIK (about 4.8 mmol) was added to
2.4 mmol methanol solutions of respectively CoCl2, NiCl2, Co(NO3)2
or Ni(NO3)2. After few minutes of warm stirring, the solvent was
gently removed to precipitate the complexes. A similar procedure
has also been reported in the literature and previously applied for
the synthesis of the manganese-BIK complexes [23,24]. The reac-
tion is shown in Scheme 1.
Looking at the thermal behavior, with respect to the BIM com-
plexes, where only one main thermal decomposition step is present
[17], the BIK complexes are characterized by a first releasing
step of the anions, followed by the loss of four methyl groups
(side chains) and two bridge-carbonyl groups. The residual tetra-
imidazole cobalt or nickel compound decompose in a final step to
give the metal oxide as the final residue.
The TG curves of the two Co(BIK)2Cl2 and Ni(BIK)2Cl2 complexes
are reported in Fig. 2, while the TG curves of the two Co(BIK)2(NO3)2
and Ni(BIK)2(NO3)2 complexes are reported in Fig. 3, with their first
derivative curves. All the TG profiles evidence the described three
steps occurring in the temperature ranges 20–200 ◦C, 200–340 ◦C
and 340–450 ◦C, respectively.
2.2. Instrumental
Elemental analysis was performed by a VarioEl III CHN Analyzer;
UV–vis by a Perkin Elmer Lambda Series and FTIR measurements
by a Perkin Elmer 1760X instrument. TG curves were recorded
using a Perkin-Elmer TGA7 equipment. The samples investigated
of approximately 7–8 mg were heated in platinum crucibles in the
temperature range between 20 and 850 ◦C, under an atmosphere of
pure nitrogen or air (gaseous mixture of nitrogen and oxygen w−it1h
80 and 20%, v/v, respectively) under a flow rate of 100 mL min
.
Table 2 reports the calculated and the found mass loss for each
complex, and clearly shows that the experimental and the calcu-
lated results are in good agreement.
The thermal behavior was investigated using TG data at 10 ◦C min−1
because of its best resolution.
Mass spectra of the gases evolved during the experiments were
recorded by a simultaneous TG/DTA apparatus (STD 2960 simulta-
neous DTA–TGA, TA Instruments Inc., USA) using sealed crucibles
with a pinhole on the top. The gaseous species were analyzed by a
ThermoStar GDS 200 (Balzers Instrument) quadrupole mass spec-
trometer equipped with Chaneltron detector, through a heated
100% methyl deactivated fused silica capillary tubing. FTIR spectra
gases evolved during the experiments were recorded by a simulta-
neous Perkin Elmer system obtained by coupling the TGA7 and the
1760 FTIR before cited.
The following scheme summarizes the proposed TG releasing
steps (M = Co or Ni; X = Cl or NO3):
−2X
M(BIK)2X2−→ M(BIK)2−2C−O→−4MeM(Im)4−−4I→midMO
The EGA analysis performed by coupling the TG analyzer either
to MS or to FTIR spectrometers allowed to confirm the proposed
general decomposition mechanism.
In Fig. 4a, the mass spectra recorded in the TGA steps of the com-
plexes decomposition by TG–MS coupled analysis is reported: in
Table 1
Results of elemental analysis. Cobalt and Nickel have been determined by ICP-OES.
Complex
C %
H %
N %
Metal (Ni or Co) %
Found
Calculated
Found
Calculated
Found
Calculated
Found
Calculated
Co(BIK)Cl2
Ni(BIK)Cl2
Co(BIK)(NO3)2
Ni(BIK)(NO3)2
42.0
42.1
38.2
38.0
42.4
42.4
38.4
38.4
4.1
4.1
3.7
3.9
3.9
3.9
3.6
3.6
27.3
27.5
25.2
25.1
27.5
27.5
24.9
24.9
11.9
11.7
10.3
10.2
11.6
11.5
10.5
10.4

S. Materazzi et al. / Thermochimica Acta 543 (2012) 183–187
185
Fig. 2. TG and DTG curves of Co(BIK)2Cl2 (red curves) and Ni(BIK)2Cl2 (blue curves) at a heating rate of 10 ◦C min−1 (100 mL min−1 air flow). (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 3. TG and DTG curves of Co(BIK)2(NO3)2 (grey curves) and Ni(BIK)2(NO3)2 (green curves) at a heating rate of 10 ◦C min−1 (100 mL min−1 air flow). (For interpretation of
the references to color in this figure legend, the reader is referred to the web version of the article.)
Table 2
Experimental mass loss percentages determined by TG and comparison with calculated ones, obtained by taking into account the proposed reaction mechanisms.
Compound
Exp. loss 1st TGA step (calculated)
Exp. loss 2nd TGA step (calculated)
res after 3rd TGA step (calculated)
Co(BIK)2Cl2
Ni(BIK)2Cl2
Co(BIK)2(NO3)2
Ni(BIK)2(NO3)2
−13.9% (−2Cl = −13.76%)
−14.0% (−2Cl = −13.85%)
−23.5% (−2NO3 = −22.13%)
−23.6% (−2 NO3 = −22.13%)
−23.2% (−4 × CH3; −2 × CO = −22.79%)
−23.0% (−4 × CH3; −2 × CO = −22.78%)
−20.5% (−4 × CH3; −2 × CO = −20.58%)
−21.1% (−4 × CH3; −2 × CO = −20.59%)
13.8% (res CoO = 14.73%)
14.0% (res NiO = 14.67%)
13.9% (res CoO = 13.30%)
13.8% (res NiO = 13.26%)

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S. Materazzi et al. / Thermochimica Acta 543 (2012) 183–187
Fig. 4. Evolved Gas Analysis (EGA) plots of the characteristic MS fragments and IR spectra: (a) EGA–MS of Ni and Co complexes; (b) EGA–FTIR of Ni and Co complexes –
second TG step (temperature range 200–340 ◦C).
the plot of the ion current related to the first TG step, fragments
at m/z 30 (NO+), 44 (N2O+) and 46 (NO2+) prove the release of
the nitrate ion, while the trace related to HCl (m/z = 36) prove the
release of the chloride ion. In the temperature range 220–320 ◦C
the fragments with m/z 44 (CO2+), 22 (CO22+), 18 (H2O+) and 17
(OH+) support the proposed release of CO (oxidated to CO2) and
the oxidation of the methyl groups to water and carbon dioxide.
The m/z = 26, detected when nitrogen flow purges the TGA furnace,
process, followed by the release of all the substitutions to give the
imidazole complex that decomposes in the final step.
The experimental results confirm that the thermal stability of
the complexes based on substituted imidazoles are only func-
tion of the metal coordination characteristics and of the side
chains.
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