Thermal study of chromium and molybdenum complexes with some nitrogen and nitrogen-oxygen donors ligands

Article abstract of DOI:10.1007/s10973-005-7009-9

The complexes of chromium and molybdenum with salicylidene-2-aminophenol (shaH₂), salicylidene-2-aminoanisole (salanH₂), salicylidene-2-aminoaniline (salphenH₂) and biquinoline (biq) were studied using the thermogravimetri

Full text of DOI:10.1007/s10973-005-7009-9

Journal of Thermal Analysis and Calorimetry, Vol. 83 (2006)2, 385 –392
THERMAL STUDY OF CHROMIUM AND MOLYBDENUM COMPLEXES
WITH SOME NITROGEN AND NITROGEN–OXYGEN DONORS LIGANDS
A. A. Soliman1,2*, S. M. El-Medani2 and O. A. M. Ali3
1Department of Chemistry, Faculty of Science, UAEU, AlAin, P.O. Box 17551, United Arab Emirates
2Department of Chemistry, Faculty of Science, Cairo University (Fayoum), Fayoum, Egypt
3Chemistry Department, University College for Girls, Ain Shams University, Cairo, Egypt
The complexes of chromium and molybdenum with salicylidene-2-aminophenol (shaH2), salicylidene-2-aminoanisole (salanH2),
salicylidene-2-aminoaniline (salphenH2) and biquinoline (biq) were studied using the thermogravimetric techniques. The thermal
decomposition of all complexes was found to be first order reaction and the thermodynamic parameters corresponding to the
different decomposition steps were reported. Molybdenum complexes were found to be more thermally stable and the order of
stability was [Mo(CO)4(biq)]>[MoO(salphen)]>[MoO2(salphenH)2]>[MoO4(salan)2]>[MoO(sha)]. Similar trend was found for
chromium complexes where [Cr(CO)4(biq)]>[Cr(CO)2(salphen)] >[CrO2(CO)2(shaH2)]>[CrO2(CO)2(salan)2].
Keywords: chromium, kinetics of thermal decomposition, molybdenum, Schiff bases, thermogravimetric studies
Introduction
dene-2-aminoanisole (salanH2), N-salicylidene-2-
aminoaniline (salphenH2) and biquinoline (biq) were
prepared and their structures were reported [8–11]. The
molecular formulae were proved as Schemes 1–4.
In the use of transition metal carbonyls as reactive spe-
cies in homogeneous catalytic reactions such as hydro-
genation, hydroformylation and carbonylation, carbon
monoxide served simply as a ligand providing the com-
plex with the necessary reactivity and/or stability to al-
low reaction to ensue [1]. On the other hand, the pres-
ence of ligands having donor atom sets like N2O2 and N4
have been found to be useful catalysts especially for
epoxidation reactions [2]. In the same aspect a large
number of Schiff bases and their complexes have been
studied for their interesting and important properties,
e.g. their ability to reversibly bind oxygen [3], catalytic
activity in the hydrogenation of olefins [4], transfer of
an amino group [5], photochromic properties [6],
complexing ability towards some toxic metals [7]. In
previous work the preparation and the structures of
chromium and molybdenum with salicylidene-2-amino-
phenol (shaH2), salicylidene-2-aminoanisole (salanH2),
salicylidene-2-aminoaniline (salphenH2) and biquino-
line (biq) were reported [8–11]. In this article we throw
more light on the thermal stability of these complexes
and report the thermodynamic parameters of the differ-
ent decomposition steps of the complexes.
[CrO2(CO)2(shaH2)], [MoO(sha)],
[Cr2O2(CO)2(salan)2], [Mo2O4(salan)2],
[Cr(CO)2(salphen)], [MoO(CO)(salphen],
[MoO2(salphenH)2], [Cr(CO)4(biq)] and
[Mo(CO)4(biq)]
Measurements of the thermogravimetric analysis
(TG and DTG) were carried out under nitrogen
atmosphere with a heating rate of 10 K min–1 using a
Shimadzu DT-50 thermal analyzer.
Scheme 1
Kinetics of the decomposition of the complexes
Experimental
Determination of reaction order of decomposition
The Horowitz and Metzger [12] equation Cs=n1/1–n
where n is the order of the reaction and Cs is the mass
,
Chromium and molybdenum complexes with
N-salicylidene-2-hydroxyaniline (shaH2), N-salicyli-
*
Author for correspondence: ahmed.soliman@uaeu.ac.ae, ahmedsoliman2@hotmail.com
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2006 Akadémiai Kiadó, Budapest

SOLIMAN et al.
Scheme 3
Scheme 2
fraction of the substance present at the DTG peak
temperature; Ts, is given by:
Cs=(Ws–Wf)(W0–Wf)
(1)
and was used for the determination of the values of
the reaction order. Here Ws stands for the mass re-
maining at a given temperature Ts, i.e. the DTG peak
temperature, W0 and Wf are the initial and final masses
of the substance, respectively.
Integral method using the Coats–Redfern equation
For a first order process the Coats–Redfern equation
[13] may be written in the form:
Scheme 4
log(W / W )
⎡
⎤
f
r
log
=
T 2
⎢
⎥
⎣
⎦
(2)
E*
2.303RT
Approximation method using Horowitz–Metzger
equation
⎡ AR
2RT ⎤
⎞
⎛
= log
1–
–
⎜
⎝
⎟
⎥
θE*
E*
⎢
⎠
⎣
⎦
For the first order kinetic process, the Horowitz–
Metzger equation [12, 14] may be written in the form:
where Wf is the mass loss at the completion of the re-
action, W is the mass loss up to temperature T;
(Wr=Wf–W), R is the gas constant, E* is the activation
energy in J mol–1, q is the heating rate. Since
1–2RT/E*@1, a plot of the left hand side of Eq. (2) vs.
1/T was drawn which gave straight lines where E* and
A (Arrhenius constant) were calculated from the slope
and the intercept, respectively.
θE*
W
⎛
⎞
⎟
⎠
∞
log
=
– log2.303
(3)
⎜
⎜
2.303RT 2
W
r
⎝
s
where Ts=DTG peak temperature and q=T–Ts. A plot of
log[logW¥/Wr] vs. q will give a straight line and E* can
be calculated from the slope. The pre-exponential factor
C was calculated from the following equation [12, 13]:
C=(qE*/RT2)exp(E*/RT )
(4)
s
s
386
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CHROMIUM AND MOLYBDENUM COMPLEXES
The activation entropy DS*, the activation enthalpy
DH*and the free energy of activation DG* were calcu-
of the complex through successive elimination of two
CO groups. The other decomposition steps occurred
in the temperature ranges 544–806 K with a net mass
loss of 60.34% may be assigned to the elimination of
the Schiff base moiety leaving CrO2 as metallic resi-
due; CrO2 (23.74%).
lated using the following equations:
Ah
*
⎛
⎞
⎟
⎠
DS = 2.303 log
R
(5)
⎜
kT
⎝
DH*=E*–RT
(6)
(7)
[MoO(sha)] complex
DG*=DH*–TsDS*
[MoO(sha)] complex displayed also two decomposi-
tion steps in the temperature range 490–820 K. The
first decomposition step occurred in the temperature
range 490–700 K with a net mass loss of 28.50% corre-
sponding to the partial decomposition of the Schiff
base molecule through the elimination of a C6H4O
moiety. The second decomposition step occurred in the
temperature range 720–820 K with a mass loss
of 31.91% and corresponded to the removal of the rest
of the Schiff base molecule as C7H5N to give finally
the residue MoO2 (39.62%).
where k and h are the Boltzman and Planck constants,
respectively.
Results and discussion
The thermal studies of the chromium and molybdenum
complexes were carried out using the thermogravimetric
(TG) and differential thermogravimetric (DTG) tech-
niques. Typical TG and DTG plots for some complexes
were represented in Fig. 1. The temperature ranges of
decompositions along with the corresponding mass loss
of species are given in Tables 1–4.
[Cr2O2(CO)2(salan)2] complex
The decomposition of [CrO2(CO)2(salan)2] com-
plexes has been taken place in two steps. The first de-
composition step occurred in the temperature
range 392–502 K with a net mass loss of 13.39% was
assigned to the elimination of two carbonyls (2CO)
and the two methyl groups as ethane moiety (C2H6).
The second decomposition step is a composite one of
three successive and unresolved peaks and was found
in the temperature range 602–723 K. The mass loss
associated with this decomposition step was 63.0%
corresponding to the elimination of the rest of the
Schiff base molecule with the formation of Cr2O3 as
metallic residue (23.30%).
[Mo2O4(salan)2] complex
[Mo2O4(salan)2] decomposed in two separate steps
with two resolved and non overlapping DTG peaks.
The first decomposition peak was found to take place
in the temperature range 670–740 K with a net mass
loss of 30.0% which may be assigned for the partial
decomposition of the two bulk Schiff base molecules
via the equal elimination of two C6H4OCH3 species.
The second decomposition step found in the tempera-
ture range 1050–1240 K with a net mass loss of
70.0% may be assigned for the volatilization of the
rest of the complex including the metallic nuclei.
Fig. 1 TG and DTG plots of biquinoline complexes
[Cr(O)2(CO)2(shaH2)] complex
The TG plot of [Cr(O)2(CO)2(shaH2)] displayed four
successive decomposition steps which could be
treated as two decomposition steps for simplicity. The
first two steps were broad and existed over a wide
temperature range; 330–460 K, with a net mass loss
of 15.90% which may be due to partial decomposition
[Cr(CO)2(salphen)] complex
Chromium complex with salphen; [Cr(CO)2(salphen)]
decomposed in four successive steps in a wide range of
temperature; 340–1273 K. The first two steps with very
J. Therm. Anal. Cal., 83, 2006
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SOLIMAN et al.
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J. Therm. Anal. Cal., 83, 2006

CHROMIUM AND MOLYBDENUM COMPLEXES
broad DTG peaks was found in the temperature
loss of 66.61% which corresponded to the elimination
of the biquinoline moiety. The metallic residue
(19.98%) remained after the decomposition was at-
tributed to CrO2 species.
range 340–733 K with a total mass loss of 26.04%
(mass=109.98) which may be related to the partial de-
composition of the complex through the removal of two
carbonyl groups and part of the ligand as C2H2N2. The
last decomposition steps found in the temperature
range 740–1273 K with a mass loss of 54.06%
(mass=228.29) may be assigned for the removal of the
rest of the ligand molecule leaving CrO2 as the metallic
residue.
[Mo(CO)4(biq)] complex
The thermal decomposition of [Mo(CO)4(biq)] was
reported [11]. The complex was further heated up
to 1145 K. A new decomposition step appeared in the
temperature range 982–1145 K. The mass losses for
the five decomposition steps with the corresponding
mass losses and the suggested species eliminated are
tabulated in Table 4.
[MoO(CO)(salphen)] complex
[MoO(CO)(salphen)] complex decomposed in four
thermal decomposition steps within the whole tempera-
ture range 353–1273 K. The first two merged decompo-
sition step with a relatively sharp DTG peak was found
in the temperature range 353–733 K. The mass loss cor-
responded to this step was 22.91% (mass=104.11)
which may be due to the removal of CO and C6H4 moi-
eties. The third decomposition step was broaded over
the temperature range 733–1083 K with a net mass loss
of 34.15% (mass=155.18). This mass loss may be as-
signed for the decomposition of major parts of the lig-
ands in the form of C6H4O and C5H3. The rest of the
ligand (C3H3+O2+N2) was removed in the final step in
the temperature range 1090–1273°C leaving Mo as the
metallic residue.
Kinetics of thermal decomposition
The calculated values of DE*, A, DS*, DH* and DG* for
the decomposition steps are given in Tables 5–8.
[CrO2(CO)2(shaH2)] complex showed consider-
able thermal stability which is reflected from the mod-
erately high values of the activation energy averaged to
60.71 kJ mol–1. On the other hand, [MoO(sha)] was
found to be comparatively more stable which is re-
flected from the relatively higher activation energy
ranging from 92.94–149.60 kJ mol–1 (average values)
which may be explained on the basis that molybdenum
complex is less sterically hindered with no carbonyl
moiety which often decomposed at lower temperatures
compared with the organic moiety coordinated to the
metal [15].
[MoO2(salphen)2] complex
[MoO2(salphen)2] decomposed thermally in four
steps within the temperature range 350–1268 K. The
first two decomposition steps, found in the tempera-
ture range 350–653 K, with a mass loss of 21.28
(M=161.45) was assigned for partial decomposition
of the ligand with the removal of two O2 molecules in
addition to one molecule N2 and C5H10 moiety. The
third step found in the temperature range 660–1083 K
with a mass loss of 49.34% (mass=374.37) may be as-
signed for further decomposition of the ligand moiety
with the removal of two C12H13 and N2+O2 molecules
(net mass of 374.40). The rest of the ligand moiety
was removed in the last decomposition step in the
temperature range 1093–1268 K with a mass loss
of 16.28% (mass=123.50). The metallic residue re-
mained at the end of decomposition was assigned as
metallic Mo (13.10).
[CrO2(CO)2(salan)2] showed a weak thermal sta-
bility which is reflected from the very low activation en-
ergy of the sum of the decomposition steps ranging from
37.10–40.73 kJ mol–1 with a sum of 38.92 kJ mol–1. On
contrary, [Mo2O4(salan)2], showed high thermal stabil-
ity which is reflected from the relatively very high en-
ergy of activation ranging from 257.80–381.99 kJ mol–1
with a sum of 639.79 kJ mol–1.
The complexes of salphen with chromium and mo-
lybdenum showed high thermal stability which is re-
flected from their energies of activation ranging
from
16.97–79.73,
21.22–255.12,
21.93–
210.76 kJ mol–1, with sums of 96.70, 324.12,
309.44 kJ mol–1 for [Cr(CO)2(salphen)], [MoO(salphen)]
and [MoO2(salphenH)2], respectively.
[Cr(CO)4(biq)] complex showed moderate
thermal stability as reflected from the activation
energy of the different decomposition steps ranging
from 33.53 to 263.55 kJ mol–1. [Mo(CO)4(biq)]
complex showed a wide variation of activation
energies of the decomposition steps ranging from
36.59 to 606.09 kJ mol–1.
[Cr(CO)4(biq)] complex
The complex decomposed in three steps, the first one
occurred in the temperature range 295–367 K, with a
mass loss of 6.65% (mass=22.95) which is consistent
with the elimination of CO group. The second and
third decomposition steps (498–752 K) with net mass
In general and based on the sum of the energies of
activation, the molybdenum complexes were found to
be more stable than chromium complexes. The order of
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389

SOLIMAN et al.
390
J. Therm. Anal. Cal., 83, 2006

CHROMIUM AND MOLYBDENUM COMPLEXES
J. Therm. Anal. Cal., 83, 2006
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SOLIMAN et al.
4 G. Henrici-Olive and S. Olive, ‘The Chemistry of the
thermal stability according to the coordinated ligands
was found to be biq>salphenH2>salanH2>shaH2. This
can be explained on the basis that biq (NN donor) is
less sterically hindered than the other complexes. On
the other hand salphenH2 is strong NNOO tetradentate
ligand compared with the NOO tridentate salanH2 and
shaH2. This trend is found to be biq>salphenH2
>shaH2> salanH2 for chromium complexes.
Catalyzed Hydrogenation of Carbon Monoxide’, Springer,
Berlin 1984, p. 152.
5 H. Dugas and C. Penney, ‘Bioorganic Chemistry’,
Springer , New York 1981, p. 435.
6 J. D. Margerum and L. J. Miller, ‘Photochromism’,
Interscience, Wiley, (1971), p. 569.
7 W. J. Sawodny and M. Riederer, Angew. Chem.,
Int. Edn. Engl., 16 (1977) 859.
The entropy change, DS*, for the formation of the
most of the activated complexes from the starting re-
actants, is in most cases of negative values. The nega-
tive sign of the DS* suggests that the degree of struc-
tural ‘complexity’ (arrangement, ‘organization’) of
the activated complexes was lower than that of the
starting reactants and the decomposition reactions are
slow reactions [16].
8 S. M. El-Medani, J. Coord. Chem., 57 (2004) 115.
9 O. A. M. Ali, M. H. Khalil, G. M. Attia and R. M. Ramadan,
Spectroscopy Letters, 36 (2003) 71.
10 S. M. El-Medani, J. Coord. Chem., 57 (2004) 115.
11 A. O. Youssef, M. H. Khalil, R. M. Ramdan and
A. A. Soliman, Trans. Met. Chem., 28 (2003) 331.
12 H. H. Horowitz and G. Metzger, Anal. Chem.,
35 (1963) 1464.
13 A. W. Coats and J. P. Redfern, Nature, 201 (1964) 68.
14 M. Nath and P. Arora, Synth. React. Inorg. Met. Org. Chem.,
23 (1993) 1523.
The values of Cs for the thermal decomposition
of the complexes are in the range 0.23–0.33 which in-
dicates that the decomposition follows first order ki-
netics [12, 14].
15 A. A. Soliman, S. A. Ali, M. M. H. Khalil and
R. M. Ramadan, Thermochim. Acta, 359 (2000) 37.
16 L. T. Valaev and G. G. Gospodinov, Thermochim. Acta,
370 (2001) 15.
References
Received: March 20, 2005
Accepted: August 30, 2005
1 J. P. Collman and L. S. Hegedøs, Principles and Applica-
tion of Organotransition Metal Chemistry, University Sci-
ence Book, California 1980.
DOI: 10.1007/s10973-005-7009-9
2 T. Katsuki, Coord. Chem. Rev., 140 (1995) 189.
3 R. D. Jones, D. A. Summerville and F. Basolo, Chem. Rev.,
79 (1979) 139.
392
J. Therm. Anal. Cal., 83, 2006