Thermal decomposition of cadmium thiourea coordination compounds

Article abstract of DOI:10.1023/A:1012306512566

Thermal decomposition of cadmium thiourea coordination compounds was considered. Cadmium sulfide is the final product for all the compounds, whereas the composition of other products of the thermolysis substantially depends on the nature of the acido ligand or the outer-sphere anion. Thermal stability parameters of the coordination compounds under study and effective activation energies of their thermolysis were determined, and a mechanism of the thermolysis was proposed.

Full text of DOI:10.1023/A:1012306512566

Russian Journal of General Chemistry, Vol. 71, No. 4, 2001, pp. 495 499. Translated from Zhurnal Obshchei Khimii, Vol. 71, No. 4, 2001,
pp. 533 537.
Original Russian Text Copyright
2001 by Semenov, Naumov.
Thermal Decomposition of Cadmium Thiourea
Coordination Compounds
V. N. Semenov and A. V. Naumov
Voronezh State University, Voronezh, Russia
Received December 7, 1999
Abstract Thermal decomposition of cadmium thiourea coordination compounds was considered. Cadmium
sulfide is the final product for all the compounds, whereas the composition of other products of the thermolysis
substantially depends on the nature of the acido ligand or the outer-sphere anion. Thermal stability parameters
of the coordination compounds under study and effective activation energies of their thermolysis were deter-
mined, and a mechanism of the thermolysis was proposed.
Alongside with conventional physical procedures
for production of semiconductor films of cadmium
sulfide by vacuum and cathode spraying, chemical
procedures find increasing application. One of them
is the method of spraying of solutions of thiourea
coordination compounds onto a heated support [1].
An important stage of this procedure is the thermal
decomposition of the thiourea complex with CdS
formation.
deposition, but also to control the amount of active
admixtures in CdS semiconductor films.
The derivatograms of almost all the coordination
compounds under study (Figs. 1 and 2) show similar
endo- and exo- effects associated with melting and de-
composition of the complexes and also with trans-
formations of their decomposition products. The
similar shapes of the derivatograms for the halide
complexes under study and [Cd(thio) Cl ] [3] suggest
2
2
that the thermolyses products are also similar. These
are gaseous H S, NH CN, and NH , and also inter-
mediate solid ammonium and guanidine thiocyanates
and the more thermally stable melam and melem.
Previous studies of reactions of cadmium chloride
and acetate with thiourea and of the mechanism of
thermolysis of the resulting coordination compounds
showed [2, 3] that the latter process is rather com-
plicated and that, apart from cadmium sulfide, various
solid and gaseous products can be formed, depending
on temperature. It is necessary to point out that dif-
ferent authors differently explain the nature of the
elementary act of the process and that no data on the
mechanisms of thermal decomposition of other cad-
mium thiourea complexes are available. The thermal
decomposition is considered to be due to mere
thermal excitation of C S bonds [4], but at the same
time the belief was stated that the excitation of this
bond results from fluctuation of a negative charge
localized on the complex during cleavage of hydro-
gen bonds in the crystal of the coordination compound
on its melting [5].
2
2
3
The thermal stability of cadmium thiourea com-
plexes depends on the nature of the acid residue and
on the composition of the inner sphere. Comparison
of the decomposition onset point of the complexes
(
see table) allows us to arrange them in the following
order by increasing thermal stability:
[
Cd(thio) (CH COO) ], [Cd(thio) ](NO ) , [Cd(thio) ]F ,
2 3 2 4 3 2 4 2
[
Cd(thio) I ], [Cd(thio) Br ], [Cd(thio) SO ],
2 2 2 2 2 4
[
Cd(thio) Cl ], [Cd(thio) ]SO .
2 2 4 4
Examination of the derivatograms of thiourea com-
plexes of cadmium halides (Fig. 1) shows that the
chloride [3] and bromide complexes are closest to
each other in the decomposition pattern. The com-
plicated nature of thermal transformations of the
fluoride complex in the range 500 600 K is connected
with the effect of the outer-sphere fluorine. The ten-
dency of fluorine to form strong hydrogen bonds
F H NH with thiourea amino groups (as indicated
The aim of this work was to study features of
thermal decomposition of a number of coordination
compounds formed by cadmium salts with thiourea
(
(
thio) and effect of the chemical nature of the anions
2
F , Cl , Br , I , SO , NO , and CH COO ) on the
4
3
3
process. The knowledge of these features will make it
possible not only to select optimal regimes of sulfide
by a long-wave shift of the
absorption band at
NH
3100 cm 1 [6]) and also the presence of the F ion in
1
070-3632/01/7104-0495$25.00 2001 MAIK Nauka/Interperiodica

496
SEMENOV, NAUMOV
(b)
(a)
(c)
DTG 483
9
8
00
00
4
98
T, K
7
53
DTG
DTA
DTG
DTA
5
4
6
63
660
7
00
DTA
533
453
13
473
5
53
600
4
4
53
5
513
53
393
m
m0
5
4
00
00
453
,
%
3
40
TG
TA
TG
TA
TG
TA
0
0
3
00
3
6
0
0
1000 2000
0
1000 2000
0
1000 2000 3000
t, s
Fig. 1. Derivatograms of halide coordination compounds: (a) [Cd(thio) ]F , (b) [Cd(thio) Br ], and (c) [Cd(thio) I ].
4
2
2
2
2 2
(a)
(b)
(c)
9
00
443
T, K
DTG
DTA
DTG
DTG
8
7
6
5
00
00
00
00
4
83
5
58
513
653
DTA
DTA 513
6
03
493
4
473
98
m
6
58
3
23
,
%
4
4
93
28
m0
408
3
TG
73
TG
TA
TG
TA
0
400
00
TA
3
3
6
0
0
0
1000 2000
0
1000 2000
0
1000 2000
3
000
t, s
Fig. 2. Derivatograms of coordination compounds with oxygen-containing anions: (a) [Cd(thio) ](NO ) , (b) [Cd(thio) SO ],
4
3 2
2
4
and (c) [Cd(thio) ]SO .
4
4
the outer sphere of the complex favor hydrogen
fluoride formation on decomposition. Hydrogen
halides are also liberated on thermolysis of others
halide-containing complexes, but the process involv-
ing [Cd(thio) ]F is qualitatively distinguished from
to decomposition products by their multiple bonds to
give new compounds, which are not formed by decom-
position of thiourea and other coordination com-
pounds. The resulting fluorocarbons are not oxidized
by air oxygen and are low-volatile. Actually, an IR
spectral study of solid decomposition residues shows
4
2
the above cases, first, by that here the F ion is not
bound directly to the cadmium ion and, second, by a that the greatest amount of admixtures remains upon
peculiar chemical behavior of HF. The latter can add decomposition of the fluoride complex. The purest
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 4 2001

THERMAL DECOMPOSITION OF CADMIUM THIOUREA COORDINATION COMPOUNDS
497
cadmium sulfide is obtained from the complex
Cd(thio) Cl ].
Temperature parameters and activation energies of thermal
decomposition of cadmium thiourea coordination com-
pounds
[
2
2
Special attention should be paid to [Cd(thio) I ].
2
2
Two endo-effects without weight change (393 and
13 K) are detected in the DTA curve. This fact can
be connected with isomerization of iodide complexes.
According to [6], the neutral complex [Cd(thio) I ] is
formed in the synthesis of iodide thiourea complexes,
and, as a result of the greater stability of the complex
ion [CdI4]2 in solution [7], the bis-complex
Temperature, K
4
onset
of
weight
loss
decom- Ea,
position kJ/mol
(exo-
Compound
melting
endo-
effect)
2
2
(
effect)
[
Cd(thio) ][CdI ] is also formed. Owing to the strong
4 4
[Cd(thio) ]F2
433
483
463
443
453
483
453
453
433
408
473
493
473
523
498
483
443
443
498
513
231.0
190.7
251.0
194.6
679.0
806.6
813.3
845.2
4
trans-effect of thiourea [8], the bis-complex is ther-
mally transformed to the neutral complex: [Cd(thio)4]
[
Cd(thio) Cl ]
2
2
[Cd(thio) Br ]
2
2
[
CdI4]
2[Cd(thio)2I2].
[Cd(thio) I ]
2
2
[
[
[
[
Cd(thio) (CH COO) ] 423
It should be noted that the melting points (453 K)
2 3 2
Cd(thio) ](NO )
433
473
488
in all the derivatograms of the halide complexes,
given in Fig. 1 coincide with the melting point of
thiourea, whereas in the case of a mixed chloride
complex [3] the melting peaks of the complex (483 K)
and thiourea are detected separately. This fact can be
explained by expulsion of one thiourea molecule into
the outer sphere as a result of thermal decomposition
of the complex. This is confirmed by the observation
in the IR spectra of molten complex [Cd(thio) I ] of
4
3 2
Cd(thio) SO ]
2
4
4
Cd(thio) ]SO4
tion. According to [8], the C S bond energy in thio-
urea is 100.5 kJ/mol. Coordination of thiourea de-
creased this energy, and, consequently, the E for the
thermal decomposition should be lower. However, we
obtaiined high activation energies (see table) indi-
cative of a complicated character of the process and
of the influence of a number of additional solid-phase
interactions on its energy. Mixed complexes with
inner-sphere Cl and I ligands decompose via inter-
mediate formation and thus with lower E . The com-
plex [Cd(thio) Cl ] has a strong intramolecular hydro-
gen bond Cl H NH, and thus the corresponding E is
the lowest.
a
2
2
the CS, CN, and
bands of uncoordinated thio-
HNH
1
urea at 634, 732, 1086, and 1472 cm [6]. The de-
tachment and expulsion of one thiourea molecule from
the inner sphere of the complex [Cd(thio) I ] are
connected with mutual influence of the ligands, pri-
marily steric. The immediate environment of the cad-
mium atoms in the [Cd(thio) Hlg ] complexes has C
symmetry. By the immediate environment we mean a
system of atoms directly bound with the complex-
forming ion, i.e., Cd(S) (Hlg) . The arrangement of
iodine atoms, having a considerable covalent radius,
in the tops of the distorted tetrahedron is sterically
hindered, and this is the reason for the expulsion of
one thiourea molecule upon thermal excitation of the
complex. We could detect the intermediate compound
2
2
a
2
2
2
2
2v
a
Examining thermolysis of the complexes with
oxygen-containing acido ligands (Fig. 2) one can note
that with the nitrate complex conditions are created
for oxidation of organic products at fairly low tem-
peratures. This complex decomposes with explosion,
and the presence of such a strong oxidant as NO3
which evolves oxygen at the decomposition point of
the complex [9], provides not only removal of organic
products, but also oxidation of CdS to sulfate. The IR
spectra at temperatures above 443 K show bands at
2
2
[Cd(thio)I ] with reduced coordination number by IR
2
spectroscopy: It shows CS, CS, and
6
bands at
CN
1
12, 708, 1093 cm , respectively.
Thus, the liberation of cadmium sulfide from thio-
05, 602, 624, and 1100 cm 1 due to sulfate absorp-
4
urea complexes with Cl and I ligands can be con-
sidered as a process involving an intermediate with
reduced coordination number. Offering this scheme
for the two above complexes, we do not exclude that
it works with other thiourea coordination compounds.
However, we have yet found any experimental evi-
dence for this assumption, apparently because of a
low stability of the intermediates.
tion. The exo-effects at 443 and 483 K and the endo-
effects at 493 and 513 K are attributable to formation
of oxidation products which are removed as gases
according to the DTG curve. It is necessary to note
that such organic admixtures as guanidine thiocyanate,
melam, and melem were not found in the solid residue.
Thus, taking account of the resutls of analysis of
gaseous products, we can propose the following
general scheme of thermal decomposition of the com-
From thermogravimetric curves we could estimate
the apparent activation energies (E ) of the decomposi- plex [Cd(thio) ](NO ) .
a
4
3 2
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 4 2001

498
SEMENOV, NAUMOV
anions and to their involvement in redox processes.
F
The highest values of E for the sulfate and nitrate
a
complexes are connected with the redox reactions that
accompany the thermal decomposition.
3
EXPERIMENTAL
4
5
5
The IR spectra were recorded on a UR-20 spectro-
photometer in the range 400 4000 cm 1 (Nujol mulls)
in cells with KBr windows.
7
8
4
1
2
6
3
The coordination compounds were synthesized
from aqueous solutions by the procedures [2, 6,
2.3 10 /T, K 1
3
2
.0
2.1
2.2
1
4
0 12] using the salts CdF , CdCl 2.5H O, CdBr
2 2 2 2
Fig. 3. Linearization of the TG curves for thermolysis of
the coordination compounds (1) [Cd(thio) Cl ],
H O, CdI , Cd(CH COO) 2H O, Cd(NO ) 4H O,
2
2
3
2
2
3 2
2
2
2
and CdSO 8/3H O of analytically pure grade and
4
2
(
(
[
2) [Cd(thio) ]F , (3) [Cd(thio) I ], (4) [Cd(thio) Br ],
4 2 2 2 2 2
thiourea of ultrapure grade. The resulting compounds
were identified by IR spectroscopy based on published
data [6, 11, 12].
5) [Cd(thio) ](NO ) , (6) [Cd(thio) (CH COO) ], (7)
4
3 2
2
3
2
Cd(thio) SO ], and (8) [Cd(thio) ]SO .
2
4
4
4
8
[Cd(thio) ](NO )
7 CdS + CdSO + 18NH3
Identification of the CdS formed was carried out
4
3 2
4
on a DRON-1 diffractometer with CuK radiation.
+
18HCNS + 14H CN + 16NO + 2O + 6H S + 8H O.
2 2 2 2 2 2
Thermal analysis of the coordination compounds
was performed on a Paulik Paulik Erdey-3427 deri-
vatograph. The heating rate was 10 deg/min for all
the complexes apart from [Cd(thio) ](NO ) which
was heated at a rate of 2.5 deg/min. Weighed samples
of 40 mg were used.
Similar redox processes take place on thermo-
lysis of [Cd(thio) SO ] and [Cd(thio) ]SO complexes,
2
4
4
4
the main oxidants here being sulfate ions. However,
4
3 2
2
4
the weaker oxidizing power of SO ions compared
with NO turns the process on a somewhat different
3
pathway. This is evidenced by the observation of a
number of exo- and endo-effects in the range 470
The apparent activation energies E of thermolysis
were calculated from the TG curves. It was therewith
a
6
60 K, different from the corresponding effects in the
assumed that the process as a whole is described by
derivatograms of the nitrate complex. The low-tem-
perature effect at 323 K at constant weight for the
neutral complex [Cd(thio) SO ] can be assigned to a
kt
the first-order kinetic equation m = m e . Here m is
0
the weight of the reactant, m is its initial weight, and
0
2
4
2
k is the rate constant. As the deviation h of the TG
curve from zero line is proportional to the weight of
change in the dentacity of the SO ligand.
4
The rather high decomposition point [Cd(thio)4]
consumed reactant [(hmax
hmax is the full deviation], we can deduce the follow-
ing expression from Arrhenius’s equation:
h)/hmax = m/m , where
0
SO is explained by the fact that the inner coordina-
4
2
+
tion sphere of [Cd(thio)4] is saturated with thiourea.
In [Cd(thio) ]F and [Cd(thio) ](NO ) , the thermal
4
4
4
3 2
1
/t
ln{ln[hmax/(hmax
h)] }
stability of such sphere is essentially reduced by the
aggressive outer-sphere NO and F ions, while with
3
A
Ea/RT.
2
such a soft oxidant as SO this does not occur.
4
Here A is a constant. Linearization, according to this
The complex [Cd(thio) (CH COO) ] thermolyzes
2
3
2
expression, of the TG curve in the F 1/T coordinates
similarly to [Cd(thio) Cl ]. A difference consists in
1/t
2
2
(
where F = ln{ln[hmax/(hmax h)] } (Fig. 3) gives
that here, apart from usual thermolysis products,
ketene, acetone, and carbon dioxide are also formed
2] as a result of decomposition of the acetate ligand.
The latter starts to decompose at 443 K, as judged
from the disappearance of its characteristic bands from
the IR spectrum. This fact seems to be responsible for
the rather low thermal stability of the acetate complex.
the required activation energy. This procedure for
determination of the activation energy in nonisother-
mal conditions was offered by Zivcovic and Dobovi-
sek [13].
[
REFERENCES
1
. Ugai, Ya.A., Semenov, V.N., and Averbakh, E.M.,
Poluchenie i svoistva tonkikh plenok (Production and
Properties of Thin Films), Kiev: Naukova Dumka,
1982, pp. 58 60.
The high apparent activation energies of the com-
plexes with oxygen-containing ligands point to
cleavage of the acido ligands and the outer-sphere
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 4 2001

THERMAL DECOMPOSITION OF CADMIUM THIOUREA COORDINATION COMPOUNDS
499
2
3
. Khodzhaev, O.F., Azizov, T.A., and Ergenshbaev, D.,
Zh. Obshch. Khim., 1976, vol. 46, no. 5, pp. 971 976.
8. Vorob’ev-Desyatovskii, N.V., Kukushkin, Yu.N., and
Sibirskaya, V.V., Koord. Khim., 1985, vol. 11, no. 10,
pp. 1299 1328.
. Ugai, Ya.A., Semenov, V.N., and Averbakh, E.M.,
Zh. Obshch. Khim., 1986, vol. 56, no. 9, pp. 1945
9. Ugai, Ya.A., Neorganicheskaya khimiya (Inorganic
1
950.
Chemistry), Moscow: Vysshaya Shkola, 1989, p. 256.
4
5
6
7
. Dutault, F. and Lahaye, J., Bull. Soc. Chim. Fr., 1980,
vol. 1, no. 5 6, pp. 236 240.
10. Rokabado Mekado, T.R., Akimov, V.M., and Molod-
kin, A.K., Zh. Neorg. Khim., 1978, vol. 25, no. 12,
pp. 3381 3384.
. Makurin, Yu.N. and Zhelonkin, N.A., Neorg. Mater.,
1
994, vol. 30, no. 2, pp. 279 280.
11. Kharitonov, Yu.Ya., Brega, V.D., and Ablov, A.V.,
Zh. Neorg. Khim., 1971, vol. 16, no. 2, pp. 572 573.
. Ugai, Ya.A., Semenov, V.N., and Averbakh, E.M.,
Zh. Prikl. Khim., 1988, vol. 61, no. 11, pp. 2409 2414.
12. Semenov, V.N. and Kisnadat, K., Zh. Prikl. Khim.,
1
990, vol. 63, no. 1, pp. 31 35.
. Lur’e, Yu.Yu., Spravochnik po analiticheskoi khimii
(
Reference Book on Analytical Chemistry), Moscow:
13. Zivcovic, Z.D. and Dobovisek, B., Thermochim. Acta,
Khimiya, 1989, p. 311.
1979, vol. 32, pp. 205 211.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 4 2001