Thermal behaviour of the homopolynuclear glyoxylate complex combinations with Cu(II) and Cr(III)

Article abstract of DOI:10.1023/A:1024565313792

Homopolynuclear complexes of Cu(II) respectively Cr(III) with the glyoxylate dianion, C₂H₂O₄^2-, have been studied in non-isothermal regime in air and nitrogen. The results of the non-isothermal analysis performe

Full text of DOI:10.1023/A:1024565313792

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Journal of Thermal Analysis and Calorimetry, Vol. 72 (2003) 515–524
THERMAL BEHAVIOUR OF THE
HOMOPOLYNUCLEAR GLYOXYLATE COMPLEX
COMBINATIONS WITH Cu(II) AND Cr(III)
M. Stefãnescu1, V. Sasca2* and M. Bîrzescu1
1University ‘Politehnica’ Timiºoara, Faculty of Industrial Chemistry and Environmental
Engineering, RO-1900 Timiºoara, Piaþa Victoriei nr. 2, Romania
2Timiºoara Branch of the Romanian Academy, RO-1900 Timiºoara, Bv. Mihai Viteazul nr. 24,
Romania
Abstract
Homopolynuclear complexes of Cu(II) respectively Cr(III) with the glyoxylate dianion, C2H2O24– ,
have been studied in non-isothermal regime in air and nitrogen.
The results of the non-isothermal analysis performed for the synthesised complexes,
Cu(C2H2O4) 0.5H2O, respectively [Cr2(OH)2(C2H2O4)2(OH2)4] 2H2O, correlated with the results of
the IR and TG analysis of the compounds obtained by thermal treatment from the initial complexes
and the results of the GLC and XR analysis have led to the establishment of the thermal decomposi-
tion mechanisms for the two studied complexes.
The decomposition mechanisms confirm the stoichiometric and structural formulae proposed
for the two synthesised homopolynuclear complexes.
Keywords: decomposition mechanisms, glyoxylates, homopolynuclear complexes, thermal analysis
Introduction
The homo- and heteropolynuclear complex combinations in which the cations are lo-
cated within the same coordination sphere are of a particular theoretical and practical
importance. Their study brings contributions to the development of the theory of
chemical bond and chemical species structure [1–3]. From the practical point of view
such combinations can be used for the generation of simple and mixed oxides and
metals with applicability in heterogeneous catalysis, electrocatalysis, obtaining of
thermoresistant pigments and various ferrites [4–6].
In this paper are presented studies carried out concerning some homopoly-
nuclear complex combinations with the glyoxylate dianion C2H2O24– as ligand which
may act in double bridge depending on the cation. From these combinations, by an
appropriately targeted thermal treatment oxides or mixtures of metal oxides in vari-
ous oxidation states and crystallinity degrees may be obtained [7, 8].
*
Author for correspondence: E-mail: vsasca@acad-tim.utt.ro
1388–6150/2003/ $ 20.00
Akadémiai Kiadó, Budapest
© 2003 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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STEFÃNESCU et al.: HOMOPOLYNUCLEAR GLYOXYLATE COMPLEX
The method of obtaining homopolynuclear complex combinations containing as
ligand the glyoxylate dianion is based on the hot oxidation reaction in aqueous solu-
tion of 1,2-ethanediol simultaneously with the formation in the solid state reaction
system of their glyoxylates. The oxidative medium is provided by introducing the
cations as nitrates [9, 10].
For this process the synthesis yield is practically 100%.
C2H4(OH)2+(2NO– +[M(H2O)x]2+)→MC2H2O4 yH2O+2NO+(x–y+2)H2O
3
2NO(g)+O2(g)→2NO2(g)
The thermal decomposition of glyoxylates in order to obtain oxides of remark-
able properties presents a series of advantages over other complex combinations or
conventional methods.
Thus, the combinations with carboxylic acid anions as ligands decompose at rela-
tively low temperatures and generate highly volatile decomposition products [11, 12].
In this paper the thermal behaviour of glyoxylate homopolynuclear complex
combinations formed by Cu(II), respectively Cr(III) is studied.
There are the decomposition mechanisms established using thermal analysis in air
and nitrogen, IR spectrometry, gas–liquid chromatography and diffractometric X-ray
analysis.
Experimental
The complex homopolynuclear combinations of Cu(II) and Cr(III) have been synthe-
sised according to the previous patent [9] and papers [10] using as reagents
1,2-ethanediol (Fluka), Cu(NO3)2 3H2O (Merck), and Cr2(NO3)3 9H2O (Merck).
After synthesis the Cu(II) and Cr(III) glyoxylates were purified by refluxing in a
water–acetone mixture and characterized by elemental analysis [Cu(II), Cr(III), C,
H], electronic diffuse reflectance, thermal analysis, IR spectroscopy, assigning the
composition formulae Cu(C2H2O4) 0.5H2O and [Cr2(OH)2(C2H2O4)2(OH2)4] 2H2O,
respectively [10, 13].
The thermal analysis of the obtained compounds has been carried out with a
model 1500D derivatograph (MOM, Hungary), in plate-type crucibles, at a heating
rate of 5°C min–1 in static air atmosphere and flowing nitrogen atmosphere of
100 mL min–1. The mass of the samples taken from each compound was 100.00 mg.
There have been studies for the thermal behaviour of both the initial complexes
and the intermediate compounds obtained by heating the glyoxylates at certain tem-
peratures for certain periods of time. The intermediate compounds were stabilized by
cooling in nitrogen in a Ugine–Eyraud balance.
The IR spectra of the synthesised complex combinations, intermediate com-
pounds or end-products have been recorded with a Specord M 80 spectrophotometer
(Carl Zeiss, Jena) in the range 4000–400 cm–1 at room temperature using the KBr pel-
let technique.
J. Therm. Anal. Cal., 72, 2003

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STEFÃNESCU et al.: HOMOPOLYNUCLEAR GLYOXYLATE COMPLEX
517
The volatile products resulting from the thermal decomposition were analysed
by gas–liquid chromatography with a GCHF 18.3 instrument, on a 3 mm×3 m classi-
cal column filled with Porapak Q (80–100 mesh).
As end-products the metal oxides have been evidenced by diffraction analysis
using a Dron 3 X-ray diffractometer with the CoKα radiation.
Results and discussion
Important information concerning the structure of the complex combinations synthe-
sised have been obtained from the analysis of the diffuse reflectance electronic spec-
tra and IR vibrational spectra, together with the results of the elemental analysis [10].
According to its structure, the glyoxylate dianion C2H2O24– acts as a double bridge
ligand, determining a polynuclear structure of the complex combinations formed.
The studies carried out for the Cu(II) glyoxylate type combination,
[Cu(C2H2O4) 0.5H2O] led to the following possible structure formula [10]:
Scheme 1
A square-plane coordination geometry has been established for the Cu(II) ion, in
good agreement with literature data [14].
Fig. 1 Thermal curves of the complex combination Cu(C2H2O4) 0.5H2O; –– – air, --- – nitrogen
J. Therm. Anal. Cal., 72, 2003

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STEFÃNESCU et al.: HOMOPOLYNUCLEAR GLYOXYLATE COMPLEX
The study of the thermal decomposition mechanism allowed the confirmation of
the stereochemical and structure formulae of the glyoxylate type complex homo-
polynuclear combination of Cu(II).
In Fig. 1 the thermal conversion curves for the Cu(II) glyoxylate in air respec-
tively nitrogen are presented which supported the delimitation of the steps in the de-
composition process in air according to Fig. 2 [15].
From Fig. 1 one may note that:
– irrespective of the working atmosphere, the degradation of the ligand begins
after the loss of the crystallization water;
– for the thermal conversion in air, under the working conditions established, the
final residue corresponds to the reaction stoichiometry, with the formation of CuO, at
the temperature of 300°C;
– in the thermal conversion in nitrogen it is evidenced the reduction of Cu(II) to
Cu(I) or even Cu(0).
The samples subjected to thermal analysis (Fig. 2) have been also analyzed by
IR spectrometry (Fig. 3), which allowed a better evidence of the steps in the thermal
conversion. From the TG curves and IR spectra of the samples treated at 130°C, re-
spectively 160°C, it follows that the crystallization water has been mostly lost up to
130°C and after this temperature the decomposition of the ligand from the superficial
strata of the sample begins, according to the reactions:
120°C
2Cu(C2H2O4 ) 0.5H2O
→ 2Cu(C2H2O4)+H2O
(1)
(2)
2Cu(C2H2O4) 0.5H2O 120–180°→C Cu2O+3CO+CO2+2H2O
Both in the 130 and 160°C IR spectra, besides the specific bands for the glyoxylate
ligand there occurs a band at 610 cm–1 which is assigned to the Cu(I)–O bond from Cu2O
which confirms the occurrence of the reaction (2).
Fig. 2 TG curves of the initial (i) and thermally treated (130, 160, 220, 350°C) Cu(II) glyoxylate
J. Therm. Anal. Cal., 72, 2003