Oxidative addition reaction of diarylplatinum(ii) complexes with MeI in ionic liquid media: A kinetic study

Article abstract of DOI:10.1039/c0dt00282h

A kinetic study of the oxidative addition reaction of diarylplatinum(ii) complexes [Pt(p-MeC₆H₄)₂(NN)] (1a: NN = 1,10-phenanthroline (phen) and 1b: NN = 4,4-di-tert-butyl-2,2-bipyridine ( ^tBu₂bpy)) with MeI in ionic liquids 1-butyl-3- methylimidazolium bis(trifluoromethylsulfonyl) imide ([bmim][bta]) or 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄]) is described. The reactions were investigated as a function of MeI concentration and temperature under pseudo-first-order conditions using UV-vis spectroscopy techniques. In general, the oxidative addition reactions in ionic liquids followed an S_N2 mechanism, similar to that reported for the related reactions in conventional solvents, e.g. benzene or acetone. The reaction rates in different solvents followed the order acetone > ionic liquids > benzene. The trend in the values of k₂ clearly indicated that ionic liquids behave like conventional solvents and that no particular 'ionic liquid effect' was detected in this kind of reaction. The effect of solvent on the reactions was examined using a linear solvation energy relationship (LSER) based on the Kamlet-Taft solvent scale. The activation parameters, ΔH ^? and ΔS^?, were obtained for the reactions in each solvent and the investigation of enthalpy-entropy compensation confirmed that the mechanism operated in all solvents is similar. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c0dt00282h

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PAPER
www.rsc.org/dalton | Dalton Transactions
Oxidative addition reaction of diarylplatinum(II) complexes with MeI in ionic
liquid media: a kinetic study†
S. Masoud Nabavizadeh,*a Hamid R. Shahsavari,a Hajar Sepehrpour,a Fatemeh Niroomand Hosseini,b
Sirous Jamalic and Mehdi Rashidia
Received 9th April 2010, Accepted 2nd June 2010
First published as an Advance Article on the web 21st July 2010
DOI: 10.1039/c0dt00282h
A kinetic study of the oxidative addition reaction of diarylplatinum(II) complexes
[Pt(p-MeC6H4)2(NN)] (1a: NN = 1,10-phenanthroline (phen) and 1b: NN =
4,4¢-di-tert-butyl-2,2¢-bipyridine (tBu2bpy)) with MeI in ionic liquids 1-butyl-3-methylimidazolium
bis(trifluoromethylsulfonyl) imide ([bmim][bta]) or 1-butyl-3-methylimidazolium tetrafluoroborate
([bmim][BF4]) is described. The reactions were investigated as a function of MeI concentration and
temperature under pseudo-first-order conditions using UV-vis spectroscopy techniques. In general, the
oxidative addition reactions in ionic liquids followed an SN2 mechanism, similar to that reported for the
related reactions in conventional solvents, e.g. benzene or acetone. The reaction rates in different
solvents followed the order acetone > ionic liquids > benzene. The trend in the values of k2 clearly
indicated that ionic liquids behave like conventional solvents and that no particular ‘ionic liquid effect’
was detected in this kind of reaction. The effect of solvent on the reactions was examined using a linear
solvation energy relationship (LSER) based on the Kamlet–Taft solvent scale. The activation
parameters, DH‡ and DS‡, were obtained for the reactions in each solvent and the investigation of
enthalpy-entropy compensation confirmed that the mechanism operated in all solvents is similar.
ligand, have extensively been studied.2,4 The inner-sphere electron-
transfer/caged radical-pair mechanism becomes possible when a
formal two electron oxidation is available. This pathway can lead
Introduction
Oxidative addition reactions of square planar complexes of d8
metal ions such as Pt(II), Pd(II) and Ir(I) play a fundamental
to the SN2 product via cage collapse, but escape of the radical
pair gives free radicals which can form byproducts.2a Such a
role in many chemical processes, for instance, in biochemistry
and homogeneous catalysis.1 Detailed mechanistic studies have
demonstrated that such complexes usually proceed by an SN2
mechanism, however, sometimes the concerted three-centre mech-
anism is also a possibility.2 The concerted three-centre mechanism
is usually proposed in the oxidative addition of C–H and C–C
bonds,2b,2c and also we have proposed it in the oxidative addition
-
mechanism has been suggested for the oxidative addition of AuCl4
to [PtCl2(phen)].2d
On the other hand, ionic liquids (ILs) are ionic media resulting
from the combination of organic cations and various anions.
These materials have attracted much attention recently in many
areas of science, particularly in green synthesis,5 mainly due
to the absence of any vapour pressure and interesting physical
characteristics. Many applications have already been developed
and new ones are added every day.6 They are non-flammable, have
high thermal stability7a and are emerging as novel replacements
for volatile organic compounds traditionally used as solvents
in many industrial processes. They are also used as catalysts in
organic reactions.7b In addition, ionic liquids can be recycled
after clean up and this offers great advantages. Despite these
developments, not much research has been carried out on the
details of the mechanisms of the reactions occurring in these
media, and possible changes that are involved in comparison
with usual organic solvents. Due to the unique properties of ionic
liquids, changes in reaction mechanisms could occur and may
account for some of the observed effects such as the acceleration
of reactions or a higher catalyst lifetime.8 In addition, some of
the ionic liquids can even act as co-catalysts and thus change
the reaction mechanism.8 Fig. 1 shows the chemical structures of
the most commonly used organic cations and anions that make
up the more popular ILs investigated. Amongst these, 1-butyl-
3-methylimidazolium (bmim) salts have been studied extensively
=
of the O–O bonds in R2O2 (R = H or C( O)Ph) to some
diarylplatinum(II) complexes [PtAr2(NN)] (in which Ar is Ph or
some simple substituted Ph groups and NN is a chelating bidentate
ligand such as bipyridine).3 The transition state has a five-
coordinate cationic character in the case of an associative (SN2)
mechanism, or forms a concerted three-centred six-coordinate
intermediate in the case of a concerted three-centre mechanism.
Another possibility for the mechanism of oxidative addition is
a radical pathway, although it has been rarely observed.2a The
kinetics of reaction of some alkyl halides with different mono and
binuclear organoplatinum(II) complexes, including those bearing
a bridging biphosphine ligand or having a chelating N,N or P,N
aDepartment of Chemistry, College of Sciences, Shiraz University, Shiraz,
71454, Iran. E-mail: nabavi@chem.susc.ac.ir; Fax: +98 711 2286008;
Tel: +98 711 2284822
bDepartment of Chemistry, Islamic Azad University, Shiraz Branch, Shiraz,
71993-37635, Iran
cDepartment of Chemistry, Persian Gulf University, Bushehr, 75169, Iran
† Electronic supplementary information (ESI) available: Abs–time curves
and Eyring plots. See DOI: 10.1039/c0dt00282h
7800 | Dalton Trans., 2010, 39, 7800–7805
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and the one with bis(trifluormethylsulfonyl)-amide, bta, as anion,
i.e. [bmim][bta], is a well-described ionic liquid with a relatively
low viscosity (as compared to other ionic liquids) and having a
melting point below 0 ◦C. Thus, the latter seems to be suitable for
kinetic investigations once obtained in colorless form.9
(Fig. 2 and 1S†). The absorbance time curves were analyzed by
pseudo-first-order methods. The pseudo first-order rate constants
(kobs) were evaluated by nonlinear least-squares fitting of the
absorbance-time profiles to a first-order equation (eqn (1)). A
plot of kobs versus [MeI] was linear (Fig. 3), and the slope gave the
second-order rate constant (k2).
At = A• + (A0 - A•)[exp(-kobst)]
(1)
Fig. 1 Chemical structures of some common ionic liquids.
The present investigation deals with the use of [bmim][bta]
or [bmim][BF4] ionic liquids as solvent for a typical oxidative
addition reaction of a metal complex [PtAr2(NN)] with MeI that
has been studied extensively during the past several decades using
more conventional aprotic solvents, such as benzene or acetone.
Substitution reactions in ionic liquid media have previously been
studied10 and in some cases a special ionic liquid effect has been
observed. van Eldick et al. have reported10b that the substitution
reaction of [Pt(terpyridine)Cl]+ with thiocyanate has the same
mechanism as found for molecular solvents, but the observed
rate constants showed a significant dependence on the nature of
ionic liquids. For example, the latter reaction in methanol is about
100 times faster than in [emim][OTf].10b Besides, Shaughnessy and
co-workers have reported the kinetic data for oxidative addition re-
actions or ligand substitution reactions involving Vaska’s complex
in ionic liquids.11 However, to the best of our knowledge, this work
presents for the first time a systematic temperature-dependence
study involving oxidative addition reaction in ionic liquid media.
The data indicate that there is not any special ‘ionic liquid effect’
in the reactions under study in contrast with such effects which
have been previously reported for substitution reactions.10b
Fig. 2 Changes in the UV-vis spectrum during the reaction of complex
[Pt(p-MeC6H4)2(phen)], 1a, (3 ¥ 10-4 M) and MeI in [bmim][bta] at T =
25 ◦C: (a) initial spectrum (before adding MeI) and (b) spectrum at t =
30 s; successive spectra were recorded at intervals of 45 s.
Experimental
1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide
([bmim][bta]) and 1-butyl-3-methylimidazolium tetrafluoroborate
([bmim][BF4]) were purchased from Solvent Innovation, and the
precursor complexes [Pt(p-MeC6H4)2(phen)], 1a,3,4g (phen = 1,10-
phenanthroile) and [Pt(p-MeC6H4)2(tBu2bpy)], 1b,4f,4g (tBu2bpy
= 4,4¢-di-tert-butyl-2,2¢-bipyridine) were made by the known
methods. The Pt(IV) complexes were made using the appropri-
ate platinum(II) complex and CH3I in ionic liquid media and
the isolated compounds were characterized by comparing their
NMR and UV-Vis spectra with those of the previously prepared
materials.4f,4g Kinetic studies were carried out by using a Perkin-
Elmer Lambda 25 spectrophotometer with temperature control
using an EYELA NCB-3100 constant-temperature bath.
Fig. 3 Plots of first-order rate constants for the reaction of complex
[Pt(p-MeC6H4)2(phen)], 1a, with MeI at different temperatures (a, 15 ◦C;
b, 20 ◦C; c, 25 ◦C; d, 30 ◦C, e, 40 ◦C) versus the concentration of MeI in
[bmim][bta].
Kinetic studies of the oxidative addition reactions
The same method was used at other temperatures (see Table 1),
and activation parameters were obtained from the Eyring equation
(eqn (2) and Figure 2S).
In a typical experiment, a solution of [Pt(p-MeC6H4)2(phen)], 1a,
in [bmim][bta] (1.4 ml, 3 ¥ 10-4 M) in a cuvette was thermostated at
25 ◦C and a known excess of MeI was added using a microsyringe.
Concentration range used for MeI was 0.01–0.1 M. After rapid
stirring, the absorbance at l = 430 nm was collected with time
k2
T
kB
h
DS‡ DH‡
⎛
⎜
⎜
⎞
⎟
⎠
⎛
⎜
⎜
⎞
⎟
⎟
+
ln
= ln
−
⎟
⎟
⎟
⎟
⎟
⎜
⎝
⎜
⎝
⎠
R
RT
(2)
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Table 1 Second-order rate constantsa and activation parametersb for reaction of [Pt(p-MeC6H4)2(NN)], 1, with MeI in different solvents
k2/L mol-1 s-1 at different temperatures
Complex
NN
Solvent
10 ◦
C
15 ◦
C
20 ◦
C
25 ◦
C
30 ◦
C
40 ◦
C
DH‡/kJ mol-1
DS‡/J K-1 mol-1
1a
1a
1a
1a
1b
1b
1b
1b
phen
phen
phen
phen
acetonec
0.45
0.05
0.56
0.06
0.10
0.36
0.84
0.12
0.14
0.29
0.71
0.08
0.14
0.46
1.08
0.16
0.87
0.10
0.21
0.61
1.43
0.19
0.28
0.54
1.07
0.13
0.27
0.71
1.84
0.23
0.36
0.64
28.9 2.0
32.2 2.2
41.6 0.4
33.0 1.7
35.3 0.4
29.2 0.8
42.5 0.5
37.9 1.9
-149
-156
-119
-139
-123
-161
-114
-123
7
7
1
6
1
3
2
6
benzenec
[bmim][bta]
[bmim][BF4]
acetoned
0.42
tBu2bpy
tBu2bpy
tBu2bpy
tBu2bpy
0.64
0.10
0.09
benzened
[bmim][bta]
[bmim][BF4]
0.52
1.12
a Estimated errors in k2 values are 5%. b Activation parameters were calculated from the temperature dependence of the second order rate constant in
the usual way using Eyring equation. c From ref. 4g. d From ref. 4f.
The plots of calculated pseudo first order rate constants, kobs
,
Results and discussion
against the concentration of MeI, are linear with a zero intercept
under all conditions showing a first-order dependence of the rate
on the concentration of MeI (Fig. 3).
The rate constant kobs can be expressed as a function of [MeI],
namely, kobs = k2[MeI], in which k2 is the second-order rate
constant.
The kinetic study of the oxidative addition reaction of [Pt(p-
MeC6H4)2(NN)] (NN = 1,10-phenanthroline, phen, or 4,4¢-di-tert-
t
butyl-2,2¢-bipyridine, Bu2bpy) with MeI was performed in ionic
liquids [bmim][bta] or [bmim][BF4] and also in molecular solvents
(acetone and benzene), in order to study the solvent dependence
of the reaction. The reactions are described in Scheme 1.
The activation parameters were determined from the effect of
temperature on k2. The enthalpy and entropy of activation, DH‡
and DS‡, were obtained from Eyring plots (see Fig. 2S†). Table 1
summarizes the values of k2, DH‡ and DS‡ for the studied reactions
of complexes 1a and 1b in the selected solvents. It should be noted
that the reactions in the ionic liquid showed the same behavior
as that found for other more conventional solvents as a function
of the experimental variables. The large negative values of DS‡
obtained for the reaction in all solvents including [bmim][bta]
are typical of oxidative addition by a common SN2 mechanism
which involves nucleophilic attack of the metallic centre at the
methyl group of MeI and formation of the cationic intermediate
[(p-MeC6H4)2(NN)Pt-Me]+ I-. Although an inner-sphere electron-
transfer/caged radical-pair mechanism is a possibility, MeI addi-
tion is thought to be an SN2 reaction on the basis of its clean
second-order kinetics and the failure of radical traps to inhibit it.
Also no byproduct has been detected by the reaction of complexes
1 with MeI in ionic liquids, thus further supporting the SN2
mechanism. Also the DH‡/DS‡ compensation plots obtained for
reactions of complexes 1a and 1b with MeI in different solvents
(see Fig. 5) give a good straight line. It may be taken as an evidence
for operation of a common (associative, second-order) mechanism
in this series of reactions. The reactions of organoplatinum(II)
complexes [PtR2(NN)], in which R = Me or Ar and NN is various
diimine ligands such as 2,2¢-bipyridine and 1,10-phenanthroline,
with MeI have been extensively investigated.2a,2b,4g,4h In all cases,
an SN2 mechanism has been suggested. A secondary a-deuterium
KIE study involving the reaction of MeI/CD3I with some
organoplatinum(II) complexes has also been reported to confirm
the operation of the SN2 mechanism in the oxidative addition
of MeI to some organoplatinum(II) complexes.4f,4g So it is well
established that these reactions proceed by an SN2 mechanism.
The reaction rates of MeI with complex 1b is slightly greater
than that with complex 1a for the studied solvents (with the
exception of [bmim][BF4]). A slight increase in the rate is observed
on going from phen to tBu2bpy in the arylplatinum(II) complexes
Scheme 1
The solvent dependence of the oxidative addition reactions was
studied by dissolving the complexes 1 in the selected solvents
followed by rapid mixing with a known excess of MeI. Typical
examples of the observed spectral changes and kinetic traces (Abs-
time curves) are shown in Fig. 2 and 1S (ESI†), respectively. The
kinetic traces recorded for the reactions in the ILs (Fig. 1S) as well
as in molecular solvents, benzene or acetone, showed excellent
fits to a single-exponential function (eqn (1)); this confirms the
pseudo-first-order behavior of the reactions as shown in Scheme 1.
It is well established that the oxidative addition reactions of
organoplatinum(II) complexes with MeI usually proceed by an
SN2 mechanism and follows the well known second order rate
law:2,4
Rate = -d[Pt(p-MeC6H4)2(NN)]/dt = k2[complex][MeI]
(3)
Under pseudo-first order condition, [MeI]0 ꢀ [complex]0, eqn (3)
is simplified to:
Rate = kobs[complex]
(4)
7802 | Dalton Trans., 2010, 39, 7800–7805
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Table 2 The property parameters of solventsa and the second-order rate constants (k2/L mol-1 s-1) for the reaction of [Pt(p-MeC6H4)2(NN)], 1, with
MeI in different solvents at 25 ◦
C
N
c
d
d
t
Solvent
erb
ET
a
b
p*d
k2 (NN = Bu2bpy)
k2 (NN = phen)
Acetone
Benzene
[bmim][bta]
[bmim][BF4]
20.7
2.30
11.6
11.7
0.355
0.111
0.642
0.673
0.08
0.00
0.62
0.73
0.43
0.10
0.24
0.73
0.71
0.55
0.98
1.09
1.43
0.19
0.28
0.54
0.87
0.10
0.21
0.61
a Data from ref. 12 and 14. b Dielectric constant. c Polarity. d Kamlet–Taft parameters.
(by a factor of about 1.3 from phen to tBu2bpy) probably due to
more electron releasing character of tBu2bpy ligand as compared
to that of phen ligand, which makes the platinum centre in
the [Pt(p-MeC6H4)2(tBu2bpy)] complex more electron rich than
the platinum centre in [Pt(p-MeC6H4)2(phen)] complex, towards
oxidative addition reactions. This has recently been confirmed
using density functional calculations, based on the charge on
nitrogen atoms of the tBu2bpy ligand (-0.438) and phen (-0.429).4g
The same behavior has been reported for the reaction of MeI
multi-parameter method, based on that of Kamlet et al.,15 was
performed using two solvent parameters. Good multi-parametric
correlations were obtained when more than one solvent parameter
were considered. The general relationship can be expressed by:
ln k2 = (ln k2)◦ + aX1 + bX2
where (ln k2)◦ is the value of ln k2 in a solvent for which the
properties Xi are zero for all i values, X1 and X2 are different
solvent parameters, and a and b are the coefficients of X1 and X2,
respectively, which can be obtained by multiple linear regression
analysis.15
The change of the k2 values for complex 1a was correlated at
first with each of the solvent parameters as follows: er, dielectric
constant of the solvent; ET N , its polarity; a, the hydrogen bond
donation (HBD) ability; b, its hydrogen bond acceptance (HBA)
or electron pair donation ability to form a coordinative bond;
p* is its polarity/polarizability parameter. The correlations were
found to be unsatisfactory (correlation coefficients were R2 = 0.80,
0.21, 0.06, 0.67 and 0.18 for er, ET N , a, b and p*, respectively).
Correlations of rate constants of complex 1a with the solvent
parameters of Kamlet et al. are given in Table 3 with multiple
correlation coefficients R2 in the range 0.44–0.98. As is clear from
Table 3, multi-parameter correlations using two parameters gave
further improvement, with the following most useful form:
t
with [Pt(p-MeC6H4)2(4,4¢-R2-bpy)] (where R = H, Me or Bu)
complexes.4f
As is clear from Tables 1 and 2, the reaction rate order obtained
for oxidative addition reaction of complexes 1 with MeI in different
solvents is: k2 (acetone) > k2 ([bmim][BF4]) > k2 ([bmim][bta]) >
k2 (benzene). Our goal is to find a correlation between k2 values
and solvent parameters.
Different solvent parameters have been proposed over the years
to give various solvent properties.12 They have been used to explain
discrepancies observed between various properties of a given
solute as the solvents are changed. In many cases, it has been
demonstrated that solute properties are dependent on more than
one solvent parameter.13
A method was applied for the treatment of the experimental
results in which the rate constant of the oxidative addition reaction
of complexes 1 with MeI in different solvents was first correlated
separately with each one of the solvent parameters to evaluate
them for their ability to provide a reasonable explanation (the
solvents used and their parameters are given in Table 2). Then,
to give an independent interpretation of the kinetic results in
different solvents, the linear solvation energy relationship (LSER)
ln k2 = (ln k2)◦ + aer + bb
which includes er and b parameters (R2 = 0.98). The fitting results
N
are shown in Fig. 4. These results suggest that the influence of ET
a and p* terms on the k2 values may be considered negligible.
,
Table 3 Parametric solvent coefficients of the reaction of [Pt(p-MeC6H4)2(NN)], 1, with MeI obtained from the different multi-parametric equations
(LSER)a
Complex [Pt(p-MeC6H4)2(phen)], 1a
Complex [Pt(p-MeC6H4)2(tBu2bpy)], 1b
LSER equation
(ln k2)◦
a
b
R2
(ln k2)◦
a
b
R2
ln k2 = (ln k2)◦ + aer
ln k2 = (ln k2)◦ + aET
ln k2 = (ln k2)◦ + aa
ln k2 = (ln k2)◦ + ab
ln k2 = (ln k2)◦ + ap*
-2.49(0.55)
-1.89(1.18)
-1.36(0.88)
-2.25(0.65)
-2.52(2.21)
-2.64(0.90)
-2.62(0.81)
-2.78(0.27)
-3.03(1.43)
-2.15(1.07)
-2.20(0.79)
-1.53(1.95)
0.12(0.04)
1.72(2.35)
0.65(1.83)
3.03(1.49)
1.68(2.57)
0.11(0.06)
0.12(0.06)
0.09(0.02)
0.11(0.06)
3.34(2.74)
3.84(2.23)
3.81(2.73)
0.80
0.21
0.06
0.67
0.18
0.82
0.83
0.98
0.83
0.68
0.76
0.72
-2.07(0.40)
-1.08(1.16)
-0.74(0.81)
-1.54(0.82)
-1.15(2.15)
-1.87(0.58)
-1.95(0.55)
-2.16(0.55)
-1.68(1.03)
-1.29(1.28)
-1.45(0.87)
-0.28(2.22)
0.11(0.03)
0.63(2.32)
-0.17(1.68)
1.98(1.86)
0.42(2.50)
0.12(0.04)
0.12(0.04)
0.10(0.04)
0.12(0.04)
2.74(3.26)
3.25(2.45)
3.35(3.11)
0.87
0.04
0.01
0.36
0.01
0.90
0.89
0.90
0.89
0.44
0.64
0.54
N
ln k2 = (ln k2)◦ + aer + bET
ln k2 = (ln k2)◦ + aer + ba
ln k2 = (ln k2)◦ + aer + bb
ln k2 = (ln k2)◦ + aer + bp*
ln k2 = (ln k2)◦ + ab + bET
ln k2 = (ln k2)◦ + ab + ba
ln k2 = (ln k2)◦ + ab+ bp*
0.50(1.74)
0.41(1.12)
1.82(0.62)
0.75(1.70)
-0.49(2.77)
-0.99(1.61)
-1.21(2.96)
-0.68(1.12)
-0.39(0.77)
0.56(1.26)
-0.54(1.22)
-1.18(3.31)
-1.56(1.77)
-2.12(3.37)
N
N
a Values in parentheses are standard deviations.
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Fig. 4 Plot showing a correlation between the experimental rate constants (ln k2) for the reaction of (a) complex 1a and (b) complex 1b with MeI at
25 ◦C in different solvents and calculated rate constants based on Kamlet–Taft LSER fit; ln k2 = (ln k2)o + aer + bb equation.
Fig. 5 Plot of DS‡ vs. DH‡ for the oxidative addition of MeI with (a) complex 1a and (b) complex 1b in different solvents.
From the results presented in Table 3, it can be concluded
that the dielectric constant of the solvent is the most important
parameter in this system (R2 = 0.80 for ln k2 versus er plot).
Acetone, with the highest dielectric constant, shows the largest
value for k2. The oxidative addition reaction of complex 1a
with MeI is nearly 7 times faster in acetone than in benzene.
This, of course, strongly supports the operation of the SN2-type
mechanism in the reactions because in the latter mechanism, in
which after each MeI attack a polar transition state is expected,
the rate should be faster in a more polar acetone solvent than
in benzene with a significantly lower dielectric constant. Solvents
that are strong hydrogen bond donors (larger b) accelerate the
reaction. This effect becomes evident when comparing the pair
[bmim][bta]/[bmim][BF4]. For both ionic liquids the values of
er, ET N , a and p* barely differ but the higher value of b for
[bmim][BF4] leads to a significant increase (nearly 2 times) of
the k2 value. It should be noted that the reactions in the ionic
liquid showed the same behavior as that found for other more
conventional solvents as a function of the experimental variables.
The trend in the values of k2 clearly indicates that the ionic
liquid behaves like acetone and that no drastic acceleration or
deceleration of the oxidative addition of MeI can be observed.
Enthalpy-entropy compensation effect
Table 1 presents the values obtained for the activation enthalpy
(DH‡) and entropy (DS‡) in all the solvents. The DH‡/DS‡
compensation plots for reactions of complexes 1a and 1b with
MeI in different solvents are shown in Fig. 5. For each complex
a good straight line is obtained and although the number of the
points is not ample, as it is it may be taken as an evidence for
operation of a common (associative, second-order) mechanism in
this series of reactions.16
Conclusions
The oxidative addition reaction of diarylplatinum(II) complexes
with MeI was chosen as an experimental system for investigating
the effect of ionic liquids (as solvent) on the reaction mechanism.
We have observed that the studied reactions proceeded according
to an SN2 mechanism in ionic liquids. The results here show that
ionic liquids are solvents with a range of different properties
that can be described using the Kamlet–Taft approach in the
studied reactions. The application of the Kamlet–Taft approach in
LSERs showed that there is no any significant “ionic liquid effect”,
7804 | Dalton Trans., 2010, 39, 7800–7805
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since the same correlation was obtained considering all solvents,
including common organic solvents (benzene and acetone) and
ionic liquids and no strong acceleration or deceleration of the
oxidative addition of MeI can be observed. The determined
thermodynamic parameters also indicate that mechanism of the
reactions is the same in all the related solvents.
2009, 11, 843; (g) F. Li, R. Liu, J. Wen, D. Zhao, Z. Sun and Y. Liu,
Green Chem., 2009, 11, 883; (h) T. Welton, Chem. Rev., 1999, 99, 2071;
(i) P. Wasserscheid and W. Keim, Angew. Chem. Int. Ed, 2000, 39, 3773;
(j) R. Sheldon, Chem. Commun., 2001, 2399; (k) V. Conte, B. Floris, P.
Galloni, V. Mirruzzo, A. Scarso, D. Sordi and G. Strukul, Green Chem.,
2005, 7, 262; (l) M. Deetlefs and K. R. Seddon, Green Chem., 2010, 12,
17.
6 (a) M.-C. Tseng, M.-J. Tseng and Y.-H. Chu, Chem. Commun., 2009,
7503; (b) M. Fabris, V. Lucchini, M. Noe`, A. Perosa and M. Selva,
Chem.–Eur. J., 2009, 15, 12273; (c) C. Zhao, D. R. MacFarlane and
A. M. Bond, J. Am. Chem. Soc., 2009, 131, 16195; (d) T. Tsuda, S.
Seino and S. Kuwabata, Chem. Commun., 2005, 44, 6792; (e) D. W.
Himmelberger, L. R. Alden, M. E. Bluhm and L. G. Sneddon, Inorg.
Chem., 2009, 48, 9883; (f) C. Kohlmann, L. Greiner, W. Leitner, C.
Wandrey and S. Lu¨tz, Chem.–Eur. J., 2009, 15, 11692; (g) O. N. Van
Buu, A. Aupoix, N. D. T. Hong and G. Vo-Thanh, New J. Chem., 2009,
33, 2060; (h) J. McGinley, V. McKee, H. Toftlund and J. M. D. Walsh,
Dalton Trans., 2009, 8406.
7 (a) U. Doman´ska and R. Bogel-Łukasik, J. Phys. Chem. B, 2005, 109,
12124; (b) R. M. Pagni, in R. D. Rogers, et al. (eds) Green industrial
applications of ionic liquids, 2003, Kluwer, Amsterdam.
8 J. Dupont, R. F. deSouza and P. A. Z. Suarez, Chem. Rev., 2002, 102,
3667.
9 (a) J. G. Huddleston, A. E. Visser, W. M. Reichert, H. D. Willauer, G. A.
Broker and R. D. Rogers, Green Chem., 2001, 3, 156; (b) P. Bonhoˆte, A.
Dias, N. Papageorgiou, K. Kalyanasundaram and M. Gra¨tzel, Inorg.
Chem., 1996, 35, 1168.
10 (a) C. F. Weber, R. Puchta, N. J. R. v. E. Hommes, P. Wasserscheid and
R. van Eldik, Angew. Chem., Int. Ed., 2005, 44, 6033; (b) P. Illner, S.
Begel, S. Kern, R. Puchta and R. van Eldik, Inorg. Chem., 2009, 48,
588; (c) P. Illner, A. Zahl, R. Puchta, N. v. E. Hommes, P. Wasserscheid
and R. van Eldik, J. Organomet. Chem., 2005, 690, 3567; (d) I. Correia
and T. Welton, Dalton Trans., 2009, 4115.
11 (a) S. J. P’Pool, M. A. Klingshirn, R. D. Rogers and K. H. Shaughnessy,
J. Organomet. Chem., 2005, 690, 3522; (b) M. D. Sliger, S. J. P’Pool,
R. K. Traylor, J. McNeill III, S. H. Young, N. W. Hoffman, M. A.
Klingshirn, R. D. Rogers and K. H. Shaughnessy, J. Organomet. Chem.,
2005, 690, 3540.
Acknowledgements
We thank the Shiraz University Research Council, and the Iran
National Science Foundation (Grant No. 88001391) for financial
support.
References
1 (a) B. Lippert, Cisplatin. Chemistry and Biochemistry of a Leading
Anticancer Drug, Wiley-VCH, Weinheim, 1999; (b) U. Fekl and K. I.
Goldberg, Adv. Inorg. Chem, 2003, 54, 260.
2 (a) L. M. Rendina and R. J. Puddephatt, Chem. Rev., 1997, 97, 1735;
(b) C. M. Anderson, R. J. Puddephatt, G. Ferguson and A. J. Lough,
J. Chem. Soc., Chem. Commun., 1989, 1297; (c) C. M. Anderson, M.
Crespo, M. C. Jennings, A. J. Lough, G. Ferguson and R. J. Puddephatt,
Organometallics, 1991, 10, 2672; (d) A. Peloso, J. Chem. Soc., Dalton
Trans., 1976, 984.
3 M. Rashidi, S. M. Nabavizadeh, R. Hakimelahi and S. Jamali, J. Chem.
Soc., Dalton Trans., 2001, 3430.
4 (a) S. Jamali, S. M. Nabavizadeh and M. Rashidi, Inorg. Chem.,
2005, 44, 8594; (b) S. Jamali, S. M. Nabavizadeh and M. Rashidi,
Inorg. Chem., 2008, 47, 5441; (c) S. J. Hoseini, S. M. Nabavizadeh, S.
Jamali and M. Rashidi, Eur. J. Inorg. Chem., 2008, 5099; (d) S. M.
Nabavizadeh, S. J. Hoseini, B. Z. Momeni, N. Shahabadi, M. Rashidi,
A. H. Pakiari and K. Eskandari, Dalton Trans., 2008, 2414; (e) M.
Rashidi, N. Shahabadi and S. M. Nabavizadeh, Dalton Trans., 2004,
619; (f) M. Rashidi, S. M. Nabavizadeh, A. Akbari and S. Habibzadeh,
Organometallics, 2005, 24, 2528; (g) S. Habibzadeh, M. Rashidi,
S. M. Nabavizadeh, L. Mahmoodi, F. Niroomand Hosseini and R. J.
Puddephatt, Organometallics, 2010, 29, 82; (h) S. M. Nabavizadeh, E. S.
Tabei, F. Niroomand Hosseini, N. Keshavarz, S. Jamali and M. Rashidi,
New J. Chem., 2010, 34, 495.
5 (a) A. Xu, J. Wang and H. Wang, Green Chem., 2010, 12, 268; (b) Z.
Baa´n, Z. Finta, G. Keglevich and I. Hermecz, Green Chem., 2009,
11, 1937; (c) J. Mutschler, T. Rausis, J.-M. Bourgeois, C. Bastian, D.
Zufferey, I. V. Mohrenz and F. Fischer, Green Chem., 2009, 11, 1793;
(d) H. Wang, Z. Li, Y. Liu, X. Zhang and S. Zhang, Green Chem., 2009,
11, 1568; (e) Z. P. Rosol, N. J. German and S. M. Gross, Green Chem.,
2009, 11, 1453; (f) C. Wang, W. Zhao, H. Li and L. Guo, Green Chem.,
12 Y. Marcus, Chem. Soc. Rev., 1993, 22, 409.
13 (a) A. Taha and A. M. Kiwan, New J. Chem., 2001, 25, 502; (b) W. R.
Fawcett, J. Phys. Chem., 1993, 97, 9540.
14 (a) H. Weingrtner, Angew. Chem., Int. Ed., 2008, 47, 654; (b) P. Wasser-
scheid and T. Welton (eds), Ionic Liquids in Synthesis, Wiley VCH,
Weinheim, 2008.
15 (a) J. A. Kamlet, J. L. M. Abboud, M. H. Abraham and R. W. Taft,
J. Org. Chem., 1983, 48, 2877; (b) J. L. M. Abboud and R. Notario,
Pure Appl. Chem., 1999, 71, 645.
16 (a) L. Liu and Q.-X. Guo, Chem. Rev., 2001, 101, 673; (b) J. H.
Espenson, Chemical Kinetics and Reaction Mechanisms McGraw-Hill,
New York, 2nd edn, 1995.
This journal is
The Royal Society of Chemistry 2010
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