Thermal studies of chromium, molybdenum and ruthenium complexes of chloranilic acid

Article abstract of DOI:10.1016/S0040-6031(00)00486-X

Chromium, molybdenum and ruthenium complexes of chloranilic acid (H₂CA) were investigated by the thermogravimetric (TG) technique. The TG plot of Cr(H₂CA)₃ showed three decompositions in the temperature range 336-802°K. On

Full text of DOI:10.1016/S0040-6031(00)00486-X

Thermochimica Acta 359 (2000) 37±42
Thermal studies of chromium, molybdenum and
ruthenium complexes of chloranilic acid
Ahmed A. Solimana, Saadia A. Alib, Mostafa M.H. Khalilc,
Ramadan M. Ramadanc,*
aChemistry Department, Faculty of Science, Cairo University, Giza, Egypt
bChemistry Department, University College for Girls, Ain Shams University, Cairo, Egypt
cChemistry Department, Faculty of Science, Ain Shams University, Cairo, Egypt
Received 22 February 2000; accepted 28 February 2000
Abstract
Chromium, molybdenum and ruthenium complexes of chloranilic acid (H2CA) were investigated by the thermogravimetric
(TG) technique. The TG plot of Cr(H2CA)3 showed three decompositions in the temperature range 336±8028K. On the other
hand, the TG plot of MoO3(HCA) complex displayed successive weight losses in four steps covering the temperature range
350±12788K. The thermal decomposition of the cluster compound Ru3(CO)10(m-H)(HCA) occurred in three steps within the
decomposition temperature range 302±7648K. Calculation of the reaction order of decompositions of the three complexes
revealed that they follow ®rst-order kinetics. The thermodynamic parameters for the different decomposition steps of the
complexes were also determined from their DTG plots. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: Chromium complex; Molybdenum complex; Ruthenium complex; Thermogravimetric technique; Thermal decomposition
1. Introduction
arrangements [5±7]. Also, reactions of chrysenequi-
none with the cluster compounds M3(CO)12, MˆRu
and Os, yielded the quinone derivatives M(CO)3
(chrysenequinone) [8]. On the other hand,
M3(CO)12 reacted with 2,3-dihydroxyquinoxaline
(DQ) in dimethylsulfoxide to give the complex
M(CO)2(DQ)(DMSO). The IR and NMR spectro-
scopy showed that these complexes have trigonal
bipyramidal structures with the two CO molecules
differently bonded to the metal center; axially in the
ruthenium complex and equatorially in the osmium
complex [9].
In a preceding paper, we have described the reac-
tions of M(CO)6, MˆCr and Mo, as well as the
reaction of the cluster compound Ru3(CO)12 with
2,5-dichloro-3,6-dihydroxy-p-quinone (chloranilic
acid) [11]. The existence of these complexes in
Direct reactions of quinones with metal carbonyls
are commonly used for the preparation of binary
quinone- and semiquinone complexes [1±9]. The
importance of quinone complexes is related to the
use of o-benzoquinones in oxidative-addition reac-
tions to basic metals [3] and their relevance to metal
catalyzed biological redox systems [10]. Reactions
of the chromium and molybdenum hexacarbonyls
with chrysenequinone and chrysenequinonemonox-
ime ligands gave several semiquinone derivatives.
The spectroscopic and electrochemical investigation
of these complexes illustrated interesting structural
* Corresponding author. Fax: ‡20-2-4831836.
E-mail address: (rrandan)@asunet.shams.eun.eg (R.M. Ramadan)
0040-6031/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 0 4 0 - 6 0 3 1 ( 0 0 ) 0 0 4 8 6 - X

38
A.A. Soliman et al. / Thermochimica Acta 359 (2000) 37±42
different structural arrangements, depending on the
central metal atom, along with their higher thermal
stability has prompted us to carry out a detailed
thermal analysis to throw more light on their compo-
sition.
ture of DTG peak), and Wi and Wf are the initial and
®nal weights of the substance. The order of the
decomposition reaction (n) can then be calculated.
1=1
n
Cs ˆ ꢀn†
(2)
2.2. Calculation of kinetic and thermodynamic
parameters
2. Experimental
2.2.1. Integral method using the Coats±Redfern
Chromium and molybdenum hexacarbonyls as well
as triruthenium dodecacarbonyl were supplied from
Aldrich. 2,5-dichloro-3,6-dihydroxy-p-quinone, chlo-
ranilic acid (H2CA), was purchased from Sigma. All
the solvents used were of analytical reagent grade
and puri®ed by distillation according to the standard
methods. Cr(H2CA)3, MoO3(HCA) and Ru3(CO)10(m-
H)(HCA) were prepared as described before [11].
Measurements of the thermogravimetric analysis
(TG and DTG) were carried out under nitrogen atmo-
sphere with a heating rate of 108C/min using a Shi-
madzu DT-50 thermal. The mass spectra of the
complexes were performed on Finnigan MAT SSQ
7000 spectrometer. Table 1 lists data for the most
abundant peaks in the mass spectra in m/z units (m is
the mass of the fragment ion and z is its charge).
equation
For a ®rst-order process, the activation energy E* in
J mol 1 can be calculated from the following equation
[14]:
ꢀ
ꢁ
log ꢀW1=ꢀW1 W††
log
T2
ꢀ
ꢂ
where W is the mass loss at the completion of the
ꢃꢁ
AR
fEÃ
2RT
EÃ
EÃ
ˆ log
1
(3)
2:303RT
1
decomposition reaction, W the mass loss up to tem-
perature T, R the gas constant and f the heating rate.
Since 1 2RT/E*1, the plot of the left-hand side of
Eq. (3) against 1/T would give a straight line. E* was
then calculated from the slope and the Arrhenius
constant, A, was obtained from the intercept.
2.1. Determination of reaction order of
decomposition
2.2.2. Approximation method using Horowitz±
Metzger equation
For the ®rst-order kinetic process, the Horowitz±
Metzger equation [12,13] may be written in the form
The reaction order of decomposition of the chro-
mium, molybdenum and ruthenium complexes were
calculated from their DTG plots [12,13]. The weight
fraction of the substance present at the DTG peak (Cs)
can be determined from the following relation:
ꢀ
ꢁ
W1
Wr
yEÃ
2:303RTs2
log log
ˆ
log 2:303
(4)
where Ts is the DTG peak temperature and yˆT Ts.
line and E* can be calculated from the slope. The
pre-exponential factor C was calculated from the
ꢀWs Wf†
A plot of log [log W /Wr] versus y will give a straight
Cs ˆ
(1)
ꢀWi Wf†
where Ws is the weight remaining at Ts (the tempera-
1
Table 1
Selected mass spectral data of the three complexes
Complex
m/z valuesa
Cr(H2CA)3
MoO3 (HCA)
679.0, 605.15, 518.0, 459.9, 397.4, 378.15, 347.0, 309.4, 206.15, 159.95, 122.0, 45.95
670.4, 582.2, 474.2, 399.9, 385.9, 352.1, 336.0, 300.0, 242.0, 181.0, 144.0, 100.5, 58.0
1045, 961.8, 929.4, 887.5, 854.5, 792.0, 769.4, 721.0, 598.2, 530.8, 492.0, 470.4, 354.8,
291.6, 370.0, 350.0, 314.1, 72.0, 56.0
Ru3(CO)10(m-H) (HCA)
a Most abundant peaks.

A.A. Soliman et al. / Thermochimica Acta 359 (2000) 37±42
39
following equation [12,13]:
cluster compound Ru3(CO)12 reacted with chloranilic
acid in benzene to give the dark brown complex
[Ru3(CO)10(m-H)(HCA)]. The appearance of the
stretching frequency of the coordinated carbonyl of
ligand at 1370 cm 1 suggested that it was bound to the
metal as a catechol [11].
The thermal studies of the three complexes were
carried out using the thermogravimetric (TG) and
differential thermogravimetric (DTG) techniques.
Typical TG and DTG plots for the three complexes
are given in Fig. 1. The plots exhibited well-de®ned
and non-overlapping decomposition steps. The tem-
perature ranges of decompositions along with the
corresponding mass loss of species are given in
Table 1. Comparison between the mass spectral data
of the complexes with the thermally decomposed
species was performed to assist in proposing the
thermal decomposition schemes.
ꢂ
ꢃ
ꢂ
ꢃ
fEÃ
RTs2
EÃ
C ˆ
exp
(5)
RTs
The activation entropy S*, activation enthalpy H* and
the free energy of activation G* were calculated using
the following equations:
ꢂ
ꢃ
Ah
kT
Sà ˆ 2:303 log
Hà ˆ Eà RT
R
(6)
(7)
(8)
Gà ˆ Hà TsSÃ
where k and h are Boltzman and Planck constants,
respectively.
3. Results and discussion
The TG plot of Cr(H2CA)3 complex showed that
it was decomposed in three steps. The ®rst step,
occurred in he temperature range 336±4598K, was
consisted of two unresolved peaks with a net weight
loss of 23.50% (Fig. 1). The percentage weight loss
was consistent with the elimination of a water mole-
cule and two chlorine molecules. On the other hand,
the two decomposition peaks appeared in the tem-
perature ranges 464±549 and 554±8028K were well
separated with a net weight loss of 17.92 and 51.00%,
respectively, and corresponded to material decompo-
sition to give ®nally a metallic chromium (Table 2).
The residual masses obtained from the three decom-
position steps of the complex were found to be
Reaction of chromium hexacarbonyl with chlora-
nilic acid yielded the tris derivative Cr(H2CA)3. On
the other hand, the corresponding reaction of
Mo(CO)6 with H2CA in air gave the tri-oxo complex
MoO3(HCA). The spectroscopic studies of the two
complexes revealed that the chromium complex has an
octahedral arrangement, with chloranilic acid bonded
as a semiquinone, while the molybdenum one is
trigonal bipyramid, with the ligand coordinated to
two equatorial sites [11]. The proton of the coordi-
nated hydroxyl group of the MoO3(HCA) was lost,
probably via an oxidative addition which would result
in the formation of a molybdenum (V) species. The
Table 2
Thermal data for chromium, molybdenum and ruthenium complexes of chloranilic acid
Molecular
formula
Molecular DTGmax Decomposition Weight
weight
Mass loss found Eliminated
(calculated) species
Solid decomposi-
tion product (%)
(8K)
temperature (8K) loss (%)
[C18H6O12C16Cr] 678.95
386
498
669
336±459
464±549
554±802
23.50
17.92
51.00
160.00 (159.83) 2C12, H2O
122.00 (122.08) C6H2O3
7.58
(Cr)
347.00 (345.05) C6O4C12, C6H2O4
[C6HO7Cl2Mo]
352.94
327,379 350±670
14.52
16.38
28.30
51.50 (51.45)
58.00 (56.02)
O, Cl
2 CO
40.80
812
712±860
(MoO3)
1063
979±1278
100.50 (100.50) C4HOCl
[C16H2O14Cl2Ru3] 792.29
330
526
692
302±416
440±583
685±764
9.1
44.2
70
72.00 (72.02) CO, CO2
39.6
350.00 (350.08) C6HO3Cl2 and RuH(CO)2 Ru2C4O4
56 (56.02) 2CO

40
A.A. Soliman et al. / Thermochimica Acta 359 (2000) 37±42
Fig. 1. TG and DTG plots of Cr, Mo and Ru complexes.

A.A. Soliman et al. / Thermochimica Acta 359 (2000) 37±42
41
matched masses obtained from the fragmentation
patterns of the complex in its mass spectrum are
illustrated in Scheme 2.
The TG plot of the cluster compound Ru3(CO)10-
(m-H)(HCA) showed that it decomposed in three steps
within the temperature range 302±7648K The percen-
tage weight loss in the ®rst decomposition peak was
attributed to the elimination of two CO and an oxygen
atom. This is consistent with the mass spectrum of the
complex. The second decomposition peak occurred at
440±5838K could be attributed to a material decom-
position involving the loss of a volatile mononuclear
ruthenium carbonyl species. Scheme 3 illustrates the
expected decomposed species with their matched m/z
values obtained from the fragmentation pattern of the
complex in its mass spectrum.
If the maximum temperature of the ®rst decom-
position peak in the DTG plots of the investigated
complexes (Table 2) is taken as a measure of their
thermal stabilities, it can be concluded that: the chro-
mium and molybdenum complexes have more higher
thermal stability than the ruthenium one. This can be
attributed to the higher af®nity of both chromium and
molybdenum to coordinate oxygen donor moieties.
On the other hand, the carbonyl groups in the cluster
Scheme 1. The thermal decomposition steps of Cr(H2CA)3 complex.
consistent with the masses of the fragmentation
patterns in its mass spectrum (Scheme 1).
The thermogravimetric studies of the MoO3(HCA)
complex showed that it was decomposed in three steps
(Fig. 1). The successive weight losses were observed
in the temperature range 350±12788K that ended with
the formation of a stable metal oxide MoO3 species
(Table 2). The ®rst decomposition peak occurred at
350±4078K was due to a loss of an oxygen; more
conveniently from the ligand moiety. The following
two decomposition peaks were due loss of a chlorine
and two CO species. The expected decomposition
species obtained from the three steps along with their
Scheme 2. The thermal decomposition steps of MoO3(HCA) complex.
Scheme 3. The thermal decomposition steps of Ru3(CO)10(m-H)(HCA) complex.

42
A.A. Soliman et al. / Thermochimica Acta 359 (2000) 37±42
Table 3
The kinetic and thermodynamic data of the thermal decompositions of the three complexes
1
1
1
1
1
Complex
Decomposition E* (kJ mol
)
R2a
A (s
)
S* (JK 1 mol
CR HM
190.56
)
H* (kJ 1 mol
CR HM
3195
)
G* (kJ 1 mol
CR HM
124.82 127.18 0.33
)
Cs
temperature
(8K)
CRb
HMc
CR
HM
CR
HM
Cr (H2CA)3
MoO3(HCA)
464±549
554±802
37.52 36.10 0.99 0.98 1157.25 1068.00
40.24 40.28 0.98 0.98 152.94 151.10
191.23 33.38
209.95 34.67
209.84
34.72 175.06 175.17 0.31
712±860
123.59 123.72 0.95 0.97 2.1Â107 2.1Â107
113.09
221.02
113.27 116.83
216.67 55.54
116.97 208.66 208.94 0.34
56.07 290.48 286.40 0.29
979±1278
64.38 64.91 0.99 0.96 63.41
106.90
Ru3(CO)10(m-H)
(HCA)
302±416
23.38 23.39 0.99 0.98 228.60
1282.90
200.60
186.29 20.63
20.61 86.84
82.08 0.31
440±583
685±764
52.35 53.00 0.97 0.99 14416.9 41999.0
42.08 43.09 0.96 0.99 194.29 193.81
170.00
208.14
161.00 47.97
208.15 36.32
48.60 137.42 133.37 0.30
37.34 180.35 181.39 0.29
a
Correlation coef®cient of the Arrhenius plots.
Coats±Redfern method.
b
c
Horowitz±Metzger method.
compound Ru3(CO)10(m-H)(HCA) are more easily
lost on heating. The mass spectrum of the complex
showed a fragmentation pattern with a successive loss
of CO groups [11].
References
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The estimated values of Cs for the thermal decom-
position of the three complexes were found in the
range of 0.29±0.37 (Table 3). This range indicated
that the decomposition reactions of the complexes
followed ®rst-order kinetics [12]. The values of acti-
vation energies, as well as the Arrhenius constants are
given in Table 3. The values obtained from applying
either Coats±Redfern or Horowitz±Metzger equations
were comparable, Table 3. (The correlation coef®-
cients of the Arrhenius plots of the thermal de-
composition steps were found to be in the range
0.95±0.99 which indicated good ®tness of the linear
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of the Ru3(CO)10(m-H) (HCA) complex [9] On the
other hand, the negative values of S* may indicate that
the decomposition reactions were slower than the
normal ones.
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