Effect of ligands on the thermolysis of the double complexes [Co(NH 3)6]2C2O4[Cu(C 2O4)2]2 and [Co(NH3) 6]Cl[Cu(C7H4O3)2]

Article abstract of DOI:10.1134/S0036023607070091

The thermolysis of the complexes [Co(NH₃)₆] ₂C₂O₄[Cu(C₂O₄) ₂]₂ (I) and [Co(NH₃)₆]Cl[Cu(C ₇H₄O₃)₂] (II) in air and hydrogen at 200, 350, and 500°C and the composition and properties of the thermolysis products are considered. The oxidative thermolysis of the complexes yields mixtures of cobalt and copper oxides, including mixed ones. The reductive thermolysis of the complexes yields a Co + Cu bimetallic powder in the case of compound I and a Co + Cu + C powder in the case of compound II. The thermal behavior of the complexes is governed by the nature of the ligand coordinated to the copper atom. The observed data are explicable in terms of the properties of this ligand. The chemistry of the oxidative and reductive thermolysis is discussed.

Full text of DOI:10.1134/S0036023607070091

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2007, Vol. 52, No. 7, pp. 1027–1032. © Pleiades Publishing, Inc., 2007.
Original Russian Text © D.P. Domonov, S.I. Pechenyuk, N.L. Mikhailova, A.T. Belyaevskii, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 7,
pp. 1104−1110.
COORDINATION
COMPOUNDS
Effect of Ligands on the Thermolysis of the Double Complexes
[Co(NH3)6]2C2O4[Cu(C2O4)2]2 and [Co(NH3)6]Cl[Cu(C7H4O3)2]
D. P. Domonov, S. I. Pechenyuk, N. L. Mikhailova, and A. T. Belyaevskii
Tananaev Institute of Rare Element and Mineral Chemistry and Technology, Kola Research Center,
Russian Academy of Sciences, ul. Fersmana 14, Apatity, Murmansk oblast, 184200 Russia
Received August 1, 2006
Abstract—The
thermolysis
of
the
complexes
[Co(NH3)6]2C2O4[Cu(C2O4)2]2
(I)
and
[Co(NH3)6]Cl[Cu(C7H4O3)2] (II) in air and hydrogen at 200, 350, and 500°C and the composition and properties
of the thermolysis products are considered. The oxidative thermolysis of the complexes yields mixtures
of cobalt and copper oxides, including mixed ones. The reductive thermolysis of the complexes yields a Co + Cu
bimetallic powder in the case of compound I and a Co + Cu + C powder in the case of compound II. The thermal
behavior of the complexes is governed by the nature of the ligand coordinated to the copper atom. The observed
data are explicable in terns of the properties of this ligand. The chemistry of the oxidative and reductive ther-
molysis is discussed.
DOI: 10.1134/S0036023607070091
Earlier, we reported the synthesis and properties of we report the thermolysis of the complexes and their
decomposition products.
a number of double complex salts, including
[Co(NH3)6]2ë2O4[Cu(C2O4)2]2 (I) [1]. The complex
[Co(NH3)6]Cl[Cu(C7H4O3)2] (II) was also described [2],
which is similar to complex I but has different ligands
at the copper atom and a different counter anion coun-
terbalancing the charge of the complex cation. Com-
plexes I and II are considered as starting compounds
for obtaining finely dispersed bimetallic products as
active components of catalysts. Both complexes, partic-
EXPERIMENTAL
The complexes were synthesized using published
procedures [1, 2], analyzed for the metals (central
atoms) and carbon (table), and identified by X-ray pow-
der diffraction. Thermal analysis was carried out in air
on an NTR-70 thermal analyzer equipped with a
PRT-1000 thermometer, a Pt-Pt/Rh thermocouple, a
ularly complex II, contain carbon-rich ligands. Here, PP-63 potentiometer, and a VT-1000 torsion balance.
Chemical analysis data for the thermolysis products of the complexes
Content, wt %
Compound
Reduction conditions
Cu : Co : C ratio
Cu
Co
C
[Co(NH3)6]2C2O4[Cu(C2O4)2]2 · 2H2O Initial complex
14.25
39.19
–
12.35
35.26
–
13.19
0.27
1 : 1 : 5
Air, 200°C
1 : 1 : 0.32
Air, 350°C
0.04
–
Air, 500°C
–
–
0.03
–
H2, 200°C
21.24
10.22
29.24
–
18.28
9.47
27.23
–
13.84
27.01
11.34
0.04
1 : 1 : 3.72
1 : 1 : 14
1 : 1 : 2.05
–
[Co(NH3)6]Cl[Cu(C7H4O3)2] · 5H2O Initial complex
Air, 200°C
Air, 350°C
Air, 500°C
–
–
0.01
–
1027

1028
DOMONOV et al.
TG, wt %; DTA, rel. units
TG, wt %; DTA, rel. units
100
90
80
70
60
50
40
30
20
100
942
TG
870
883
90
80
70
871
856
880
DTA
DTA
658
498
572
60
483
448
384
354
346
50
40
30
293
246
218
281
219
305
272
228
189
154
170
24.10%
216
TG
32.39%
195
0
200
400
600
800
1000
T, °C
0
200
400
600
800
1000
T, °C
Fig.
1.
3 6 2
DTA
and
TG
curves
for
the
Fig.
2.
3 6
DTA
and
7 4 3 2
TG
curves
for
the
[Co(NH ) ]Cl[Cu(C H O ) ] complex in air.
[Co(NH ) ] ë O [Cu(C O ) ] complex in air.
2
4
2 4 2 2
solutions in Drechsel bottles. The absorbent solutions
were analyzed for carbon and nitrogen.
The heating rate was 9 K/min, and the sample size was
~0.05 g. Thermoanalytical curves for complexes I [1]
and II are presented in Figs. 1 and 2, respectively. Fur-
thermore, we identified the products of the thermolysis
of complexes I and II in air and hydrogen at 200, 350,
and 500°ë (Table 1, Figs. 3, 4). X-ray powder diffrac-
tion patterns were obtained on DRON-2 and DRF-2,2
diffractometers using graphite-monochromated CuKα
radiation.
RESULTS AND DISCUSSION
The thermolysis of both complexes in air gives rise
to pronounced endotherms between 170 and 250°ë.
Furthermore, the oxalate complex is characterized by
exotherms between 280 and 400°ë, showing no weight
loss above 300°ë. For the salicylate complex, the stron-
gest exotherm occurs at 600–660°ë and a slight weight
loss is observed up to 900°ë. The small weight loss step
at 900°ë is apparently due to the partial conversion of
cobalt(III) oxide into cobalt(II) oxide. Both complexes
lose nearly all of their carbon at 350°ë (table). A com-
parison between these and thermoanalytical data for
[Co(NH3)6]Cl3 [5] suggests that the endotherm near
200°ë, observed for all double salts containing the
[Co(NH3)6]3+ cation, arises from the elimination of
ammonia from the inner coordination sphere of cobalt.
The oxidation and reduction products of both com-
plexes were examined in order to determine their com-
position and surface micromorphology, identify the
oxidation and reduction by-products, and determine the
total decomposition and carbon burnout temperatures.
All products were characterized by X-ray diffraction,
and the reduction products were also examined with an
SEM-LEO-420 digital scanning electron microscope
running the accompanying software. The X-ray diffrac-
tion patterns from the products are presented in Figs. 3
and 4; the electron micrographs, in Figs. 5 and 6.
The density of the complexes was measured by the
picnometer method [3]. The immersion liquid was
chloroform, in which the complexes are insoluble. The
density of complex I was determined to be 1.81 g/cm3;
the density of complex II, 1.68 g/cm3.
According to X-ray diffraction data, the products of
the decomposition of both complexes in air are multi-
component mixtures. The decomposition product of the
oxalate complex contains CuO, Cu2O, and the mixed
oxides CuCo2O4 and Cu0.37Co2.63O4. The decomposition
product of the salicylate complex contains CuO, CuCl,
and Cu2Cl2O (whose thermal decomposition yields
CuCl2) [6]. In the diffraction patterns from the decom-
position products obtained in air at 200°ë (Fig. 3,
curves 1, 4), the reflections from crystalline compounds
are rather weak as compared to the strong background
of the amorphous phases. As the thermolysis tempera-
The complexes were reduced in a quartz tube heated
with an SNOL 0.2/1250 tubular resistance furnace [4],
which allowed visual observation of the process. The
reducing agent was hydrogen fed at a rate of 10 L/h.
The heating rate was 10 K/min. The reduction time was
2 h at 200 and 350°ë and 1 h at 500°ë. The products
were cooled in flowing hydrogen. The gaseous reduc-
tion products were trapped with 1.0 M HCl and NaOH ture is raised, the amorphous phases disappear (Fig. 3,
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 7 2007

EFFECT OF LIGANDS ON THE THERMOLYSIS OF THE DOUBLE COMPLEXES
1029
I
CuO
Cu2O
CuCl
Co3O4
CuCo2O4
Cu0.37Co2.63O4
Cu2Cl2O
1
2
3
4
5
6
d/n, nm
Fig. 3. X-ray diffraction patterns from the products of (1–3) [Co(NH ) ] ë O [Cu(C O ) ]
2 4 2 2
and (4–6)
3 6 2
2
4
[Co(NH ) ]Cl[Cu(C H O ) ] decomposition in air at (1, 4) 200, (2, 5) 350, and (3, 6) 500°ë.
3 6
7 4 3 2
curves 2, 3, 5, 6). The chemical analysis of these prod-
ucts (table) indicates that, at 200°ë, carbon burns out
almost completely from the oxalate complex and only
partly from the salicylate complex. Therefore, carbon is
much less readily removable from complex II than
from complex I. The organic anions oxidize in air into
26[Co(NH3)6]Cl[Cu(C7H4O3)2]
+ 362.5O2 9CuO + 13CuCl2
+ 10Cu0.4Co2.6O4 + 156NH3 + 364CO2 + 104H2O.
In a hydrogen atmosphere, both complexes decom-
carbon dioxide (and water if they contain hydrogen, as pose to a lesser extent than they do in air at all temper-
in the case of the salicylate ion). The oxidative thermol-
ysis of these anions can be expressed by the following
chemical equations, in which the product composition
is slightly simplified:
atures examined. For the oxalate complex, the Co : Cu :
C ratio at 200°C indicates the removal of water and the
partial elimination of the ligands; the residue contains
about 2 oxalate groups per formula unit. We did not
detect any carbon in the alkali absorbent solution;
therefore, no ëé2 had evolved. In hydrogen, the oxalate
ion can turn into hydrocarbons and their derivatives,
[Co(NH3)6]2C2O4[Cu(C2O4)2]2
+ 2.5O2
CuO + CuCo2O4 + 12NH3 + 10CO2,
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 7 2007

1030
DOMONOV et al.
I
Co
1
2
Cu
CuCl
3
4
5
6
d/n, nm
Fig.
4.
X-ray
diffraction
patterns
from
the
products
of
(1–3)
[Co(NH ) ] ë O [Cu(C O ) ]
3 6 2 2 4 2 4 2 2
and (4−6) [Co(NH ) ]Cl[Cu(C H O ) ] decomposition in hydrogen at (1, 4) 200, (2, 5) 350, and (3, 6) 500°ë.
3 6
7 4 3 2
including methane and methanol, and leave the system experiments. This is favorable for considerable coars-
as these compounds.All of the ammonia present in both
complexes is detectable as the ammonium ion in the
acid absorbent.
ening of the resulting metal particles, which is an unde-
sirable process in catalyst production [7].
The diffraction patterns from the products of the
thermolysis of both complexes in hydrogen are pre-
sented in Fig. 4. The diffraction pattern from the com-
plex I thermolysis product obtained at 200°ë shows
reflections from the initial complex (0.49, 0.42, and
0.39 nm [1]) and a reflection from elementary copper,
indicating that part of the crystals of I are intact. The
corresponding diffraction pattern from II shows reflec-
tions from CuCl, indicating that this double complex
decomposes at 200°C. For both complexes, the diffrac-
tion patterns from the decomposition products obtained
at 350°C exhibit reflections from the metals, which are
stronger for complex I than for complex II (Fig. 4,
curves 2, 5). Raising the decomposition (reduction)
temperature to 500°ë changes the diffraction patters
only slightly. The residual carbon does not give rise to
any reflections, even though it was reported that the
reduction of complex I yields the Co3C phase [8].
For the thermolysis of the salicylate ion in hydro-
gen, one should expect not carbon burnout, but a com-
plicated mechanism beginning with the dehydrogena-
tion of the benzene ring to carbon and then yielding
light hydrocarbons capable of removing carbon from
the system. In the thermolysis of the salicylate com-
plex, we observed the condensation of dark brown, res-
inous destruction products at the outlet (cold end) of the
quartz tube (see the description of the experimental
setup) and the formation of white dendrite-like crystals.
Thus, the reductive thermolysis of II yields a variety of
organic ligand decomposition products. For the salicylate
complex, the thermolysis residue contains 36.63 wt % C
at 200°ë, 14.69 wt % C at 350°ë, and 9.86 wt % C at
500°ë. Therefore, the complete decarbonization of
complex II would require much higher temperatures
and much longer reduction times than were used in our
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 7 2007

EFFECT OF LIGANDS ON THE THERMOLYSIS OF THE DOUBLE COMPLEXES
1031
The reflections from Co and Cu are abnormally
close together; in the temperature range examined, the
solid solubility of cobalt in copper does not exceed
4.9 at % and the solubility of copper in cobalt is no
higher than 1.7 at % [9]. The diffraction patterns did not
indicate the formation of any Co–Cu solid solution or
intermetallide.
Of particular interest are the electron micrographs
of the reductive thermolysis products of both com-
plexes. The reduction products obtained at 200 and
350°ë are irregularly shaped large crystals in the case
of complex I (Fig. 5a) and large tabular crystals in the
case of complex II (Fig. 6). In both cases, the crystal
size is several tens of nanometers. The crystals resulting
from I at 200 or 350°ë retain the shape of the parent
crystals, but the product seems to consist of two phases:
its micrograph indicates the presence of a finely dis-
persed phase with a particle size below 50 nm (Fig. 5a).
The product obtained by the thermolysis of II at 350°ë
appears as rounded lamellar intergrowths consisting of
platelike aggregates of ~50-nm particles (Figs. 6b, 6c).
The product resulting from the oxalate complex at
500°ë appears as large microporous agglomerates
retaining the shape of the parent crystals and consisting
of 100- to 200-nm particles, looking like broken stale
bread (Fig. 5b). As derived from the results of catalytic
tests [8], the size of copper particles in the product of
complex I reduction at 350°ë is 39 nm.
EHT = 10.00 kV
WD = 4 mm
Photo No. = 7139
Mag = 3.05 K X
Detector SE1
(‡)
1 µm
EHT = 20.00 kV
300 nm
WD = 5 mm
Photo No. = 7811
Mag = 10.05 K X
Detector SE1
(b)
The reduction product of the salicylate complex has
a cellular structure looking like cured foam (Fig. 6d).
The walls of the large cells are covered with smaller
cell “foam.” The foamlike structure of this reduction
product is likely due to the vigorous evolution of gas-
eous thermolysis products: the salicylate complex
yields at least 16 mol of gas per mole of (Co + Cu)
against 10 mol yielded by the oxalate complex.
Fig. 5. SEM images of the products of
[Co(NH ) ] ë O [Cu(C O ) ] reduction at (a) 350°ë
3 6 2
2
4
2 4 2 2
(1-µm-long marker bar) and (b) 500°ë (300-nm-long
marker bar).
that this complex loses weigh up to 900°ë, and for the
cellular structure of the reduction product.
The fact that the similar oxalate and salicylate com-
plexes show different thermal behaviors and their ther-
molysis products have different properties is due to the
different natures of the ligands coordinated to copper.
The oxalate ion is the anion of oxalic acid, which is
weak and comparatively unstable. Oxalic acid is a
strong reducing agent and decomposes readily upon
heating, yielding carbon dioxide. The salicylate ion is
the anion of salicylic acid, which is much more stable.
It has seven carbon atoms, while the oxalate ion has
only two ones. Furthermore, the salicylate ion is poorer
in oxygen and has a benzene ring. When heated in air,
it burns provided that the temperature is sufficiently
high and there is adequate oxygen access. As is noted
above, its thermal behavior in hydrogen is still more
The anion size is smaller in complex I than in com-
plex II, as is suggested by the higher density of I.
Therefore, the higher the molecular weight of the
organic ligand and the more complicated its structure,
the wider the decomposition temperature range of the
complex, particularly in a hydrogen atmosphere, the
more complicated the structure of the reduction prod-
uct, and the more problematic the removal of carbon
from this product. Hence, the simplest possible ligands
should be used in the synthesis of double complexes as
precursors of bimetallic nanopowders. Note that the
thermal analysis of the complexes in air is not informa-
tive in respect of their reduction with hydrogen and is,
therefore, of little use in their characterization as pre-
complicated. This accounts for the complicated shape cursors, since the thermolysis processes occurring in air
of the DTA curve of the salicylate complex, for the fact and in hydrogen have little in common.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 7 2007

1032
DOMONOV et al.
EHT = 20.00 kV
300 nm
WD = 5 mm
Photo No. = 7024
Mag = 10.02 K X
Detector SE1
EHT = 20.00 kV
1 µm
WD = 5 mm
Photo No. = 7010
Mag = 3.05 K X
Detector SE1
(a)
(b)
EHT = 20.00 kV
100 nm
WD = 5 mm
Photo No. = 7001
Mag = 29.01 K X
Detector SE1
EHT = 20.00 kV
300 nm
WD = 5 mm
Photo No. = 7069
Mag = 10.12 K X
Detector SE1
(c)
(d)
Fig. 6. SEM images of the products of [Co(NH ) ]Cl[Cu(C H O ) ] reduction at (a) 200°ë (300-nm-long marker bar), (b, c) 350°C
3 6
7 4 3 2
(1-µm- and 100-nm-long marker bars), and (d) 500°C (300-nm-long marker bar).
4. S. I. Ginzburg, K. A. Gladyshevskaya, N. A. Ezerskaya,
ACKNOWLEDGMENTS
et al., in The Manual on the Chemical Analysis of the
Platinum-Group Metals and Gold (Nauka, Moscow,
1965) [in Russian].
This study was supported by the Presidium of the
Russian Academy of Sciences (program “Fundamental
Problems of the Physics and Chemistry of Nanosized
Systems and Nanomaterials,” subprogram “Organic
and Organic–Inorganic Hybrid Nanosized Systems and
Materials Based on These Systems for Information
Technologies”) and from Academician Kalinninkov’s
Scientific School (grant no. NSh 4383.2006.3).
5. S. I. Pechenyuk, D. P. Domonov, D. L. Rogachev, and
A. T. Belyaevskii, Zh. Neorg. Khim. 52 (7) (2007)
[Russ. J. Inorg. Chem. 52 (7), 1033 (2007)].
6. ASTM Diffraction Data Cards and Alphabetical and
Grouped Numerical Index of X-ray Diffraction Data
(Philadelphia, 1946–1969).
7. A. A. Khasin, Ross. Khim. Zh. 47 (6), 36 (2003).
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