Thermal decomposition of complexes of cadmium(II) and mercury(II) with triphenylphosphanes

Article abstract of DOI:10.1007/s10973-007-8529-2

A series of substituted triphenylphosphane complexes of the type CdL ₂X₂ (L= triorthotolylphosphane or trimetatolylphosphane; X=Cl^-, Br^- or I^-) and HgL₂X ₂ (L=triphenylphosphane or triorthotolylphosphane) was prepared fresh. The thermal decomposition was carried out in air with heating rate programmed at 10°C min^-1 and it revealed that the complexes with ortho derivative were less stable and the triphenylphosphane moiety leaves along with halogen in the first step. All the complexes were stable up to 210°C. However, the stability order of the tetrahedral complexes was X=Cl>Br. Values of n, E, lnA and ΔS ^# have been approximated and compared. Complexes having Br have higher E _a, lnA and ΔS ^# values than that having Cl.

Full text of DOI:10.1007/s10973-007-8529-2

Journal of Thermal Analysis and Calorimetry, Vol. 90 (2007) 3, 725–729
THERMAL DECOMPOSITION OF COMPLEXES OF CADMIUM(II) AND
MERCURY(II) WITH TRIPHENYLPHOSPHANES
R. K. Verma*, L. Verma, A. Bhushan and B. P. Verma
University Department of Chemistry, Magadh University, Bodh Gaya-824234, India
A series of substituted triphenylphosphane complexes of the type CdL2X2 (L= triorthotolylphosphane or trimetatolylphosphane;
X=Cl–, Br– or I–) and HgL2X2 (L=triphenylphosphane or triorthotolylphosphane) was prepared fresh. The thermal decomposition
was carried out in air with heating rate programmed at 10°C min–1 and it revealed that the complexes with ortho derivative were less
stable and the triphenylphosphane moiety leaves along with halogen in the first step. All the complexes were stable up to 210°C.
However, the stability order of the tetrahedral complexes was X=Cl>Br. Values of n, E, lnA and DS# have been approximated and
compared. Complexes having Br have higher Ea, lnA and DS# values than that having Cl.
Keywords: activation energy, cadmium(II), complexes, entropy of activation, mercury(II), non-isothermal solid state
decomposition, trimetatolylphosphane, triorthotolylphosphane, triphenylphosphane
Introduction
have been synthesized and somewhat studied but
complexes of cadmium and mercury with substituted
triarylphosphanes appear to have escaped attention of
the workers so far. Secondly, the catalysts may have to
be used at different temperatures and this created
interest in taking up the thermal decomposition of a
series of complexes of cadmium(II) and mercury(II)
{CdX2L2 (L=triorthotolylphosphane (TOTP) i.e., tri(2-
methylphenyl)phosphane or trimetatolylphosphane,
TMTP i.e., tri(3-methylphenyl)phosphane: X= Cl–, Br–
or I–) and HgX2L2 (L=triphenylphosphane or triortho-
tolylphosphane)(tolyl=methylphenyl)}. Variation in the
kinetic parameters that is observed [46, 47] with change
in the metal and the ligand, has been tested. The thermo-
gravimetric data have been subjected to mechanistic
analysis and 15 kinetic models have been tested.
Many authors investigated coordination compounds due
to their chemical, biological, environmental, ion-ex-
change and catalytic importance and their thermal,
spectral, microscopic and many other properties have
also been examined [1–39]. The catalytic applications
of a large number of phosphane based coordination
compounds in processes such as hydrogenation, hydro-
formylation, hydrosilylation, hydrocyanation and oligo-
merization [40–44] have established the tertiary
phosphanes particularly the triaryl ones among the
choice ligands. Triphenylphosphanes have two great
advantages over their trialkyl analogues. While the
trialkylphosphanes are nasty smelling liquids, triphenyl-
phosphane is an odourless solid with substantial stabil-
ity and so is the case with other triarylphosphanes.
Secondly, the greater s-electron donor ability (basicity)
of the trialkylphosphanes brings stability to the metal Experimental
complexes. The greater steric bulk of the arylphos-
Materials and methods
phanes and weaker bonding affinities with metals create
an electronic environment that is conducive to form-
ation of an empty or potentially reactive coordination
site on the metal. The (2-methylphenyl)diphenyl-
phosphane ligand, as part of a platinum catalyst has
been found to be enhancing selectivity to the branched
aldehyde in the hydroformylation of 2-butene [45].
Although the crowding affected the activity of the
catalyst, the substituted phosphanes have gained much
significance in recent times because of the steering
effect of ortho substitution of aryl phosphane. The
triaryl phosphane complexes of platinum group metals
A series of complexes of cadmium(II) and mer-
cury(II) {CdL2X2 (L=triorthotolylphosphane, TOTP
or trimetatolylphosphane,TMTP; X=Cl–, Br– or I–)
and HgL2X2 (L=triphenylphosphane or triorthotolyl-
phosphane)(tolyl=methylphenyl)}was prepared by
mixing hot solutions of the anhydrous halides (X=Cl,
Br, I) in absolute alcohol and that of the correspond-
ing phosphane derivatives in appropriate molar ratios,
followed by vigorous shaking on an electric shaker
for about an hour in each case.The formations of the
CdL2 I2 precipitate and the HgL2 X2 (L=TOTP) pre-
*
Author for correspondence: profrkverma@gmail.com
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2007 Akadémiai Kiadó, Budapest

VERMA et al.
cipitate, however, were rapid. The crystals of the
[Cd(TMTP)2Cl2]are indicative of terminal M–Cl
bonds. These values support complexation involving
M–P bond, as these are lower than those occurring in
metal halides [48]. The vibration of the substituent
group in the ligand remains intact in its position even
in the complex. The vibrations of the aromatic ring
both in ligands and in their complexes for example,
show that the ring is intact. 3–4 peaks
(2880–2960 cm–1) in the tolyl ligand move only
slightly on complexation indicating that the methyl
group remains intact in the side chain. Peaks in the
ranges 670–735 cm–1 (P–Ph3), 672–745 cm–1 (o-tolyl)
and 680–785 cm–1 (m-tolyl) which are lowered by
5–10 cm–1 on complexation, have been assigned to
C–H out of plane vibrations. Those at
1568–1575 cm–1 have been assigned to C=C stretch-
ing vibrations and 3015–3050 cm–1 to C–H stretching
vibrations in the complexes. And, tetrahedral
structure seems plausible in all the seven cases.
complex formed were washed with ether and dried
over air suction and recrystallized. The triphenyl-
phosphane was of Eastman Kodak make. Cadmium
chloride crystals (CdCl2, 2.5H2O) were from Reanal
and other chemicals were of Merck/BDH make. An-
hydrous cadmium chloride and cadmium bromide
were obtained by heating crystals of cadmium chlo-
ride and cadmium bromide (CdBr2, 4H2O) in an air
oven at 150°C for a few hours. The stoichiometries of
the complexes were established on the basis of rou-
tine elemental analysis and molecular mass determi-
nation. Two series of diamagnetic complexes were
isolated viz.: (R3P)2 (MX)2 and (R3P)2·MX2 {M=Cd,
Hg; X=Cl, Br, I; R=phenyl, 2-methylphenyl-
(orthotolyl), 3-methylphenyl (metatolyl)}. However,
1:1 complexes of cadmium bromide could not be
isolated. The infrared spectra of the ligands and the
complexes were recorded on PerkinElmer 577 infra-
red spectrophotometer (200–4000 cm–1) in CsI-sup-
porting disc.
The crushed compound was loosely poured into
a sample holder of Stanton-Redcroft TG 750 Thermo-
balance and, dispersed well. As efforts to minimize
the local factors, only the freshly prepared samples
were used and small sample size (4–8 mg) was taken
in each case (to minimize the impact of heat and mass
transfer phenomena). The thermograms were re-
corded keeping heating rate programmed at 10 K
min–1 and using dry air as the purge gas. The residues
were analysed for percentage of metals and halogens
by dissolving them as usual.
All the complexes are stable upto 210°C (Fig. 1).
The TG data of the first step of decomposition were
subjected to kinetic analysis in each case. In the initial
analysis, the a–T plots of [Cd(TOTP)2Br2],
[Cd(TMTP)2Cl2],
[Cd(TMTP)2Br2]
and
Result and discussions
As expected, all the complexes were diamagnetic.
Although the nM–P frequencies could not be assigned
with certainty as they were expected to fall beyond
200 cm–1 , the shift in the P–C stretching frequencies
on the formation of complex suggests that the metal is
coordinating through the P of the ligand molecule.
The values 1450, 1410 and 1005 cm–1 , in PPh3 shift
to 1465, 1420 and 1015 cm–1 in [Hg(PPh3)2Cl2] and to
1460, 1412 and 1010 cm–1 in [Hg(PPh3)2Br2]
(Table 1). Similarly, the nP –C frequency values shifted
in [Hg(TOTP)2Br2] (1448, 1425 and 1010 cm–1) and
[Cd(TOTP)2Br2] (1445, 1428 and 1010 cm–1) from
1450, 1430 and 1020 cm–1. A little change was
observed in [Cd(TOTP)2I2] as well. In cases of
[Cd(TMTP)2Cl2] and [Cd(TMTP)2Br2], the values
(1455, 1425 and 1015 cm–1) and (1458, 1430 and
1020 cm–1) were assigned to changed P–C vibrations
(from 1450, 1428 and 1020 cm–1 in TMTP). The peak
at 225 cm–1 in [Hg(PPh3)2Cl2] and at 260 cm–1 in
Fig. 1 TG curves of the samples
726
J. Therm. Anal. Cal., 90, 2007

DECOMPOSITION OF CADMIUM(II) AND MERCURY(II) COMPLEXES
Table 1 Important IR frequencies and assignments
Compounds
IR frequencies/cm–1
Assignments
1
[Cd (TOTP)2Br2]
3020(s)[TOTP3020(s)]
2960–2880(m)(4 peaks)
C–H stretching
C–CH3
[TOTP2980–2900(w)(3 peaks)]
1562(s) [TOTP1570(s)]
1445(s),1428(s),1010
C=C
P–C stretching
[TOTP1450(s),1430(s),1020(s)]
1142(m),1050(w) [TOTP1145(s),1070(s)]
735(s),700(s),650(v.w.)
C–H in plane deformation
[TOTP745(s),705(s),672(s)]
C–H out of plane deformation
2
[Hg (TOTP)2Br2]
3020(s)
2980–2880(w)(3 peaks)
1568(s)
C–H stretching
C–CH3
C=C
1448(s),1425(s),1010 (s)
1140(s),1050(m)
738(s),695(s),662(m)
230(s)
P–C stretching
C–H in plane deformation
C–H out of plane deformation
Hg–X
3
[Cd (TMTP)2Cl2]
2990(m)[TMTP3040(s),3020(s)]
2880 (m)
C–H stretching
C–CH3
[TMTP2960–2888 (s and w)(4 peaks)]
1570(s) [TMTP1568(s)]
1455, 1425,1015(s)
C=C
P–C stretching
[TMTP1450(s), 1428(s),1020(s)]
1150(s),1080(s),1060(w)
[TMTP1148(m),1080(s),1060(m)]
760(s),672(s)[TMTP765(s),680(s)]
260(s)
C–H in plane deformation
C–H out of plane deformation
Cd–Cl
4
5
[Cd (TMTP)2Br2]
3020(m)
2980(w),2890 (m)
1570(s)
1458(s), 1430(m),1020(s)
1152(s),1090(s),1070(w)
762(s),675(s)
C–H stretching
C–CH3
C=C
P–C stretching
C–H in plane deformation
C–H out of plane deformation
[Cd (TMTP)2I2]
3020(s)
2920(v.w.), 2890(s)
1570(s)
C–H stretching
C–CH3
C=C
1455(s), 1430(m),1030(s)
1155(s),1085(s),1065(m)
770(s),672(s)
P–C stretching
C–H in plane deformation
C–H out of plane deformation
3038(s)[PPh3 3015(s)]
C–H stretching
C=C
P–C stretching
6
[Hg (PPh3)2Cl2]
1570(s),1555(w) )[PPh31560(s)]
1465(s),1420(s),1015(s)
[PPh31450(s),1410(s),1005 (s)]
1085(s),1055(s)[PPh31068(s),1050(s)]
735(s),703(s),682(s)
C–H in plane deformation
C–H out of plane deformation
[PPh3735(s),720(s),700(s),670(s)]
225(s)
Hg–Cl vibrations
7
[Hg(PPh3)2Br2]
3035(s)
C–H stretching
1565(w),1550(w)
1460(s),1412(s),1010 (s)
1080(s),1050(w)
730(s),695(s),678(s)
C=C
P–C stretching
C–H in plane deformation
C–H out of plane deformation
s=sharp; m=medium; w=weak; v.w=very weak
J. Therm. Anal. Cal., 90, 2007
727

VERMA et al.
Table 2 The models followed and the parameters obtained
Sl.
No.
Suggested mode
of decomposition
Temp. range/ DTG peaks/
K
Best fitted
mechanism
Ea/
kJ mol–1
DS#/
Compound
lnA
K
J K–1 mol–1
1
2
3
4
5
6
7
CdBr2(TOTP)2
CdBr2(TOTP)2®
CdBr2·TOTP
483–703
512, 533, 558 4D diffusion 418.23
91.6 511.7
23.6 –54.15
42.9 106.7
CdCl2(TMTP)2
CdBr2(TMTP)2
CdI2(TOTP)2
HgCl2(PPh3)2
HgBr2(PPh3)2
HgBr2(TOTP)2
CdCl2(TMTP)2® 513–693
CdCl·TMTP
533, 576,
635, 651
Second order 115
CdBr2(TMTP)2® 500–578
CdBr2·TMTP
543
3D diffusion 207.87
4D diffusion 140.65
2D diffusion 170.21
Second order 226.82
CdI2(TOTP)2®
CdI·TOTP
483–555
513–593
503–563
513–573
492, 523
534, 553
515, 542
555
28
–16.8
HgCl2(PPh3)2®
HgCl(PPh3)0.5
34.6 37.63
51.7 180
HgBr2(PPh3)2®
Hg·PPh3
266.98
57
223.8
Contracting
volume(R3)
HgBr2(TOTP)2®
HgBr·TOTP
[Hg(TOTP)2Br2] showed periods of induction (upto
a~0.25, 0.053, 0.25 and 0.04 respectively) indicating
that surface nucleation is taking place [49]. In other
cases, the a vs. T curves present the well known
sigmoidal shape of an autocatalytic process. The max-
imum da/dt occurs at a~0.5 in all the cases. After this
point, the curve presents a typical deceleratory behav-
iour and da/dt decreases as the heating time in-
creases.
Kinetic analysis of the TG data was carried out
under linear temperature programming, and the val-
ues of lnk were calculated using various f(a) values
[where lnk=ln{(da/dt)/fa)}]. Spreadsheets [50] were
prepared and the values of lnA and Ea were obtained
from the most linear plot of lnk vs. 1/T using the fol-
lowing relation (regression analysis was carried out to
test linearity):
plexes containing Cl is more stable than that having
Br. There is no change in thermal behaviour on
changing the ligand from tri(2-methylphenyl)phos-
phane (TOTP) to tri(3-methylphenyl)phosphane
(TMTP).
• Complexes having Br have higher Ea, lnA and DS#
values than that having Cl.
• Changing the halogen from Cl to Br in
[Cd(TMTP)2Cl2] brings slight change in onset tem-
perature which is accompanied by a sharp decrease
in DS# value and decrease in Ea and lnA values.
• Diffusion seems to be more preferred path in the
series.
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