Competition of methyltrioxorhenium (MTO) with osmium tetroxide (OsO 4) for pyridines binding: Ligand binding assay

Article abstract of DOI:10.1016/j.poly.2010.12.026

Competition of methyltrioxorhenium (MTO) with osmium tetroxide (OsO ₄) toward L = pyridine and its derivatives, based on the equilibrium constant for the reaction OsO₄·L + MTO = MTO·L + OsO₄, has been measured. A successful correlation of log K _eq with the Hammett σ constants of the substituents on the ligands was realized. A negative reaction constant, obtained for the reactions, shows that a more positive charge expands on the pyridine nitrogen in the complex MTO·L as compared with the complex OsO₄·L. So, the rhenium center acts as a better electron acceptor than osmium center. The thermodynamic parameters have been obtained and an excellent linear relationship was observed between the enthalpy and entropy of the reactions.

Full text of DOI:10.1016/j.poly.2010.12.026

Polyhedron 30 (2011) 814–820
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Competition of methyltrioxorhenium (MTO) with osmium tetroxide (OsO4)
for pyridines binding: Ligand binding assay
Fatemeh Niroomand Hosseini a, , Katayoon Kamali , S. Masoud Nabavizadeh b
⇑
a
a
Department of Chemistry, Islamic Azad University, Shiraz Branch, Shiraz 71993-37635, Iran
Department of Chemistry, College of Sciences, Shiraz University, 71454 Shiraz, Iran
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Competition of methyltrioxorhenium (MTO) with osmium tetroxide (OsO ) toward L = pyridine and its
4
Received 18 November 2010
Accepted 16 December 2010
Available online 25 December 2010
derivatives, based on the equilibrium constant for the reaction OsO
measured. A successful correlation of log Keq with the Hammett constants of the substituents on the
ligands was realized. A negative reaction constant, obtained for the reactions, shows that a more positive
4
ꢀL + MTO = MTOꢀL + OsO
4
, has been
r
charge expands on the pyridine nitrogen in the complex MTOꢀL as compared with the complex OsO ꢀL. So,
4
Keywords:
Rhenium
Osmium
Equilibrium
Pyridines
the rhenium center acts as a better electron acceptor than osmium center. The thermodynamic parame-
ters have been obtained and an excellent linear relationship was observed between the enthalpy and
entropy of the reactions.
Ó 2010 Elsevier Ltd. All rights reserved.
1
. Introduction
Osmium tetroxide has been widely used for the cis-dihydroxyla-
The mono-peroxo complex A and the bis-peroxo complex B
shown in Scheme 2 are the active species for the catalytic oxidation
of methyltrioxorhenium with H O [19]. However, in the case of
2 2
tion of olefins under mild conditions, which is of fundamental
importance in nature. Many oxidation systems have been devel-
oped for this transformation [1]. In the first step of this transforma-
olefin epoxidation, depending on the constitution of the epoxide,
a hydrolytic ring opening catalyzed by the electropositive Re(VII)
center can also occur to give 1,2-diols [20]. This problem can be
overcome by replacing aqueous hydrogen peroxide with the
hydrogen peroxide–urea adduct as primary oxidant in non aque-
ous media [21] or by adding pyridine [19,22], 3-cyanopyridine
[23] or 2,2 -bipyridine (compounds 1 and 2 in Scheme 3) that re-
duce the Lewis acidity of the catalytic system and thus preserve
the epoxide from destruction [24].
Therefore, transition metal-based oxidants have been success-
fully used for oxidation of unsaturated organic compounds. It is
important to use environmentally friendly oxidants, such as
molecular oxygen or hydrogen peroxide, which give no waste
4
tion, OsO reacts with alkenes to yield a cyclic osmate ester
(Scheme 1). Subsequent reductive hydrolysis of ester in solution
gives the cis-1,2-diol. The presence of a tertiary amine that coordi-
nates to osmium accelerates the dihydroxylation reaction [2]. The
oxidation can be carried out under both catalytic and stoichiometric
conditions. The stoichiometric reaction is studied by Criegee who
has shown that pyridine accelerates the reaction considerably [3].
In addition, osmium tetroxide could be used in catalytic amounts
in the presence of stoichiometric amounts of a cooxidant, such as
hydrogen peroxide or N-methylmorpholine N-oxide (NMO), which
reoxidises osmium in situ [1c,4].
0
products [25]. However, direct oxidation of the catalyst by O
2
or
Methyltrioxorhenium CH
methyl bond, is a reactant stable to hydrolysis, and can act in the
presence of H or O as a highly efficient and selective catalyst
for a number of reactions, such as epoxidation of alkenes [5], oxi-
dations of aromatic compounds [6], primary and secondary amines
3
ReO
3
or MTO, containing a metal–
2 2
H O
is often kinetically unfavored, since the energy barrier for
electron transfer can be high. The problem has been solved by
introduction of efficient electron transfer mediators (ETMs) be-
tween the substrate selective redox catalyst and the terminal oxi-
2
O
2
2
2 2 2
dant (O or H O ) [26]. Several compounds have been used as ETM
[
7], alkynes [8], anilines [9], sulfur compounds [10], phosphines,
in the hydrogen peroxide-based osmium-catalyzed dihydroxyla-
tion of various alkenes (see Scheme 4). The mechanism of these
oxidation systems is that the hydrogen peroxide reacts with a
cocatalyst (electron transfer mediator) to give a reactive peroxo
or hydroperoxide intermediate, which can efficiently reoxidize
Os(VI) to Os(VIII).
arsines, stibines [11], symmetric disulfides [12], alcohols [13],
ketones [14], halide ions [15], alkanes [16], aldehyde olefination
[
17] and olefin metathesis [18].
For the first time, Bäckvall and his coworkers [27] have reported
a novel and robust system for osmium-catalyzed asymmetric
⇑
Corresponding author. Tel.: +98 917 120 1790.
E-mail address: niroomand55@hotmail.com (F.N. Hosseini).
0
277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.12.026

F.N. Hosseini et al. / Polyhedron 30 (2011) 814–820
815
dihydroxylation of olefins by aqueous H
2
O
2
with methyltrioxorhe-
nium (MTO) as electron transfer mediator (ETM). The mechanism
has been shown in Scheme 5 where MTO acts as an ETM. In this
scheme, N-methyl morpholine (NMM) was also used as an addi-
O
O
O
HO
OH
N-donor O
O
O
Os
Os
O
Os
O
O
O
O
O
tional ETM to facilitate electron transfer from MTO to OsO
was demonstrated that in some cases direct electron transfer from
MTO to OsO is efficient without added NMM [28].
Considering the important applications of these two metal oxo
compounds, MTO and OsO , in catalytic activities and also concur-
rent application of them in H -based OsO -catalyzed dihydroxy-
4
. It
N-donor
N-donor
cyclic osmate ester
cis-diol
4
Scheme 1.
4
2
O
2
4
O
lation of alkenes, we have decided to measure their competition
and coordination ability toward Lewis bases such as pyridine and
its derivatives according to reaction shown in Scheme 6.
O
O
O
O
O
O
H O
2 2
H O
2
2
Re
Re
Re
O
O
H C
3
O
H O
2
H C
3
O
O
H C
3
OH2
2. Experimental
A
B
2.1. General
Scheme 2. Reaction of MTO with hydrogen peroxide.
Methyltrioxorhenium(VII) [29] and pyridines adducts of os-
mium tetroxide [2a,30] were synthesized according to literature
procedures. Osmium tetroxide was obtained from Merck. Pyridine
and its derivatives were purchased from Aldrich or Fluka and used
as received. Benzene (ACS grade, Aldrich) was used as the solvent
for UV–Vis spectroscopy.
Values of equilibrium constant (Keq) for reaction shown in
Scheme 6 were determined by UV–Vis spectrophotometric titra-
tion. The spectra were recorded using a Perkin–Elmer UV Lambda
25 spectrophotometer equipped with a circulating water thermo-
static system (EYELA NCB-3100 constant temperature bath).
Quartz cuvettes with 1 cm optical path lengths were used. Typi-
cally the equilibrium constant of the reaction was determined by
O
CH3
CH3
Re
O
O
O
O
L
L
O
Re
O
Re
O
H C
3
O
O
O
L
L
(
1)
(2)
(3)
Scheme 3. Lewis base adducts of MTO.
H
N
Me
titrating 3 ml a solution of OsO
4
ꢀpy in benzene with successive ali-
O
N
quots of a known concentration solution of MTO in the same sol-
vent at 25 °C. This procedure was repeated until the spectra
remained unchanged with further addition of MTO. The software
PSEQUAD [31] was used to analyze the multiwavelength absorbance
data to obtain the equilibrium constant.
O
O
O
O
V
N
O
N
Me
The same method was used at other temperatures and thermo-
dynamic parameters were obtained from Eq. (1).
Et
O
1
2
ꢁ
D
RT
H
DS
ln Keq
¼
þ
R
ð1Þ
Scheme 4. Electron transfer mediators (ETMs).
Me
N
O
O
O
Me Re
O
OsO
OsO
4
2
H O
O
NMM
O
Me Re
O
HO
OH
2 2
H O
3
Me
O
O
N
+
O
NMO
Scheme 5. Catalytic system for osmium-catalyzed dihydroxylation of olefins using H
2 2
O .
O
CH3
Re
CH3
Re
O
O
K
eq
O
O
Os
O
+
O
+
Os
O
O
O
O
O
O
O
L
L
Scheme 6. Ligand exchange between MTO and OsO4.

8
16
F.N. Hosseini et al. / Polyhedron 30 (2011) 814–820
2.2. Theoretical calculations
0.8
0
.4
GAUSSIAN03 [32] was used to fully optimize all the structures re-
ported in this paper at the B3LYP level of density functional theory
DFT). The effective core potential of Hay and Wadt with a double-n
valence basis set (LANL2DZ) [33] was chosen to describe Re and Os.
The 6-311++G basis set was used for other atoms. To evaluate and
ensure the optimized structures of the molecules, frequency calcu-
lations were carried out using analytical second derivatives. In all
cases only real frequencies were obtained for the optimized
structures.
0.2
0
0
0
0
.6
.4
.2
0
(
⁄⁄
-0.2
-0.4
-
0.47
-0.46
-0.45
-0.44
-0.43
Nitrogen Charge on L
3
. Results and discussion
3.1. Equilibrium constants and substituent effects
The equilibrium constants (Keq) for pyridine and five ring-
-0.2
4
substituted pyridines exchange between OsO and MTO (Scheme
6) are given in Table 1. The program PSEQUAD [31] is used to deter-
mine Keq by a global fit of absorbance-concentration data taken
at 14 wavelengths. The values of Keq are >1, with a magnitude that
increases as the Py ligand becomes more electron-donating. For
-
0.4
-0.4
-0.2
0
0.2
0.4
0.6
σ
3
example, the presence of CH group at the para position of pyridine
enhances the stability of MTO adducts by a factor of about 1.5 with
respect to Cl group. The adduct stability of MTO with pyridines re-
Fig. 1. The linear correlation between log(K
correlation between log(K /K ) and the charge of the nitrogen atom on the L
X H
ligands).
X
/K
H
) and r (the inset, plot showing a
spect to OsO
4
follows the order 4-methoxy > 4-methyl > 3-
methyl > 4-phenyl ꢂ 4-H > 3-chloro pyridine. The electronic effect
is examined more closely using the Hammett correlation for para
and meta substituted pyridine, as shown in Fig. 1. The linear corre-
culations are performed with the program suite GAUSSIAN03 [32] for
six different pyridine complexes. The calculated bond distances
lation between log(K
X
/K
H
) and
r, according to equation log(K
x
/
from DFT-optimized structures for the compounds OsO
4
ꢀL and
H
K ) = qr, shows that the equilibrium constant increases with the
MTOꢀL along with DFT-optimized structures for these complexes
are shown in Fig. 2. In each case the geometry at the metal is dis-
torted trigonal bipyramidal, but the bond lengths of the Os(VIII)–N
and Re(VII)–N are influenced significantly by the substituent on
the pyridine ring as is clear from Fig. 3 (M–N bond length versus
donating ability of the pyridine ligand. To confirm more this phe-
nomenon, theoretical studies using DFT calculations were per-
formed to calculate the charge on nitrogen atoms of the L ligands
[
32]. The data were used in connection with the equilibrium data
to confirm the electronic effect of the pyridines ligands on the
equilibrium behaviors. The graph of log(K /K ) for the reaction of
complexes OsO
ꢀL and MTO at 25 °C in benzene versus charges
pK
OsO
a
of pyridines). The M–N (M = Os or Re) bond lengths of the
4
ꢀL and MTOꢀL complexes apparently depend on the basicity
X
H
of the pyridine ligands. For example the Re–N bond length of the
4-methylpyridine complex is more than 0.043 Å shorter than the
corresponding bond length for the 3-chloropyridine complex
(2.445 Å versus 2.488 Å, respectively).
4
on the L ligands, the inset of Fig. 1, is linear for the series of L li-
gands, pyridine, 4-methylpyridine, 3-methylpyridine, 3-chloropyr-
idine, 4-methoxypyridine and 3-phenylpyridine, and this confirms
that the equilibrium constants of the reactions correlates linearly
with the charges on the L ligands.
3.2. Sensitivity of metal center to ligand
It has been reported [34] that
q can be considered as a measure
of the Lewis acidity of the metal complexes bearing completely dif-
ferent ligands and having metals with different oxidation states. A
negative reaction constant, obtained for reaction shown in Scheme
To find the sensitivity of metal center, in OsO
idine and its derivatives, we have used equilibrium constants re-
ported for the reactions [M] + L = ML, where [M] = MTO or OsO
4
and MTO, to pyr-
4
6
(
q
< 0) means that a more positive charge expands on the pyri-
dine nitrogen in the complex MTOꢀL as compared with the complex
OsO
[35] (Eqs. (2) and (3)) to derive values of Kex that refer to reactions
(4) and (5). We choose pyridine as the constant reference point.
4
ꢀL. The rhenium center, so, acts as a better electron acceptor
than osmium center.
The Kex values, calculated from Eqs. (4) and (5) at 298 K, are col-
To consider the influence of the remote substituents on the
lected in Table 2. We then have constructed a plot of log Kex(OsO
4
)
structural properties of the OsO
4
ꢀL and MTOꢀL complexes, DFT cal-
against log Kex(MTO) for pyridines transfer. The result is shown in
Table 1
Equilibrium data (10 Keq/L mol 1)a for the reaction OsO
ꢁ
ꢀL + MTO = MTOꢀL + OsO
4
4
(L = pyridine and its derivatives) at different temperatures in benzene.
H (kJ mol 1
ꢁ
)
D
S (J mol
ꢁ1 ꢁ1
K )
Ligand
10 Keq at different temperatures
D
1
0 °C
20 °C
25 °C
30 °C
40 °C
Pyridine
111.1
157
143.8
96.9
165.1
111.0
82.4
104.9
100.1
64.3
120.1
81.7
73.3
87.1
85.1
53.8
99.9
72.7
61.5
76.4
73.0
45.1
88.4
60.0
50.1
54.9
54.4
31.3
65.7
47.3
ꢁ19.7 ± 0.7
ꢁ25.6 ± 0.6
ꢁ23.8 ± 0.3
ꢁ27.6 ± 0.3
ꢁ22.6 ± 0.4
ꢁ21.2 ± 0.7
ꢁ49.9 ± 2.3
ꢁ67.7 ± 2.4
ꢁ62.2 ± 1.0
ꢁ78.8 ± 1.1
ꢁ56.6 ± 1.5
ꢁ54.8 ± 2.3
4-Methylpyridine
3-Methylpyridine
3-Chloropyridine
4-Methoxypyridine
3-Phenylpyridine
a
Estimated error in Keq given based on least squares regression analysis is ±10ꢀ.

F.N. Hosseini et al. / Polyhedron 30 (2011) 814–820
817
Fig. 2. The DFT-optimized structures and calculated bond distances for the compounds OsO
4
ꢀL and MTOꢀL.
2.5
Table 2
Equilibrium constants for ligand exchange processes (Kex
T = 298 K.
,
Eqs. (4) and (5)) at
2
2
2
2
.48
.46
.44
.42
L
K
ex (MTO)
4
Kex (OsO )
M=Re
4
3
3
4
3
-Methylpyridine
-Methylpyridine
-Chloropyridine
-Methoxypyridine
-Phenylpyridine
1.93 ± 0.10
1.27 ± 0.04
0.13 ± 0.01
3.68 ± 0.15
0.65 ± 0.03
1.72 ± 0.24
1.24 ± 0.16
0.17 ± 0.02
2.71 ± 0.38
0.75 ± 0.10
M=Os
0
.6
.3
0
y = -0.0068734 + 0.83269x R= 0.99811
0
2.4
2
.38
2
3
4
5
6
7
pK of ligand
a
K
Fig. 3. Relationship between M–N(L) bond length (M = Os or Re) and pK
L = pyridine and its derivatives) for OsO
ꢀL and MTOꢀL complexes.
a
of L
-
-
-
0.3
0.6
0.9
(
4
Fig. 4 and the correlation is fully acceptable. The slope of the line is
.83 ± 0.03, that is, the equilibrium of ligand exchange at the os-
0
mium center is less sensitive to the ligand than at the rhenium
center.
3.3. Temperature profiles
-
1
-0.8 -0.6 -0.4 -0.2
0
0.2
0.4
0.6
log Kex (MTO)
In order to have a better understanding of the thermodynamics
of reaction shown in Scheme 6, it is useful to consider the enthalpic
and entropic contributions to these reactions. The temperature
Fig. 4. Correlation of the equilibrium constants (at 298 K) for exchange of pyridines
at a rhenium center vs. the equilibrium constants for pyridines exchange at an
osmium center, both on logarithmic scales.
dependence of the reactions between OsO
4
ꢀPy and MTO are studied

8
18
F.N. Hosseini et al. / Polyhedron 30 (2011) 814–820
CH3
Re
CH3
O
O
[
MTO.L]
Re
O
K(MTO.L)
=
+
L
ð2Þ
ð3Þ
ð4Þ
ð5Þ
O
O
[MTO] [L]
O
L
O
O
O
[
OsO
4
.L]
+
L
O
4
K(OsO .L)
=
Os
O
Os
L
O
O
[OsO
4
] [L]
O
CH3
Re
CH3
O
O
O
O
K(MTO.L)
O
+
L
Re
O
+
Py
Kex (MTO) =
K(MTO.Py)
Py
O
L
O
O
O
K(OsO .L)
4
Os
Py
O
+
L
Os
L
O
+
Py
Kex (OsO ) =
4
O
O
K(OsO .Py)
4
over the temperature range of 283–313 K. The
D
H and
D
S values
On the other hand, the reactions are accompanied by an entropy
for these reactions are evaluated by applying a linear ln Keq least-
squares analysis according to the van’t Hoff equation (Fig. 5 and
Eq. (1)) and presented in Table 1. In general, the pyridines transfer
production indicating that the entropy of MTOꢀL complexes are
systematically lower than the entropy of OsO
4
ꢀL complexes in ben-
zene solution. However it seems that the entropy changes for the
from OsO
greater than
4
to MTO is not entropy controlled as T
D
S is in general not
reactions can be positive when the corresponding MTOꢀL com-
D
H. For more investigations, a series of DFT calcula-
plexes compare to OsO
4
ꢀL. To investigate the intrinsic entropy
tions has been performed and enthalpy and entropy changes dur-
ing the ligand exchange reactions were determined. The results
show that the values of calculated enthalpies are more negative
for reactions with more electron donating substituent on pyridine
ring (i.e. 4-methoxypyridine) than those for electron withdrawing
group on L (i.e. 3-chloropyridine).
changes of the reactions, DFT calculations have been done in the
gas phase and found that S values are positive, although no cor-
D
relation found between the calculated entropy changes and the ba-
sicity of the pyridine ligands. This confirms again that the ligand
exchange process, shown in Scheme 6, is enthalpy controlled, not
entropy controlled.
3
(
a)
-18
(b)
y = 273.57 x - 6.55
2
.5
2
(
c)
-20
(
d)
-
-
-
-
22
24
26
28
K
1
.5
1
0.0031
0.0032
0.0033
T
0.0034
0.0035
0.0036
-
1
-1
/ K
-
0.08 -0.075 -0.07 -0.065 -0.06 -0.055 -0.05 -0.045
-1
-1
Fig. 5. van’t Hoff plots for ligand exchange between MTO and OsO
4
[(a) 4-
ΔS / kJ mol K
methoxypyridine, (b) 3-methylpyridine, (c) pyridine and (d) 3-chloropyridine] in
benzene.
Fig. 6.
D
H–D
S compensation plot for pyridines exchange between OsO
4
and MTO.

F.N. Hosseini et al. / Polyhedron 30 (2011) 814–820
[3] R. Criegee, Justus Liebigs Ann. Chem. 522 (1936) 75.
819
3
.4.
DH–DS compensation plot
[
4] (a) H.S. Singh, in: W.J. Mijs, C.R.H.I. de Jonge (Eds.), Organic Syntheses by
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D
H and DS are a
(
b) A.H. Haines, in: B.M. Trost, I. Fleming (Eds.), Comprehensive Organic
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[
(
(
b) D. Sica, D. Musumeci, F. Zollo, S. De Marino, J. Chem. Soc., Perkin Trans. 1
2001) 1889;
strated by a linear correlation between
the linear plot of the enthalpy change versus the entropy change
is called the compensation temperature (T ). Chemical reactions
D
H and
D
S. The slope of
(c) D. Musumeci, D. Sica, Steroids 67 (2002) 661;
(
(
(
d) A.M. Al-Ajlouni, J.H. Espenson, J. Am. Chem. Soc. 117 (1995) 9243;
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called isokinetic or isoequilibrium processes. An enthalpy–entropy
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(
(
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Tc is the Gibbs free energy change at temperature T
tion means that, at temperatures close to T , changes in
G remains independent of the
D
H ¼ T
c
DS þ
D
G c
. This equa-
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T
where
DG
c
Sundermeyer, Acc. Chem. Res. 37 (2004) 645;
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D
set by changes in
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(
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[
Thermodynamic parameters indicate that the enthalpy–entropy
compensation effect holds in general for ligand exchange between
(
[
MTO and OsO
4
. As shown in Fig. 6, an acceptable linear relationship
S for this reaction is observed. The graph of en-
[
between H and
D
D
(
thalpy versus entropy values for reactions shown in Scheme 6 in
benzene generates a highly linear trend and can be expressed by
(
[
[
[
[
[
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H ¼ T
The value of T
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D
S ꢁ 6:55 kJ=mol
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(
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ˇ
(
(
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415.
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(
6
c
observed compensation temperature T must be significantly dif-
[
ferent from the mean experimental temperature. For the reactions
shown in Scheme 6 with pyridine and its derivatives, the mean
experimental temperature is significantly different from the ob-
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(
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served T
effect between
rather than simply an artifact of statistical correlation.
c
(298 versus 273 ± 24 K). Furthermore the compensation
D
H and S may be indicative of chemical fact
D
(
b) J.H. Espenson, J. Chem. Soc., Chem. Commun. (1999) 479. and references
therein.
[
[
20] W.A. Herrmann, R.W. Fischer, D.W. Marz, Angew. Chem., Int. Ed. Engl. 30
(
1991) 1638.
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4
. Conclusion
(
Equilibrium constants for reactions in which a series of pyridine
6
4
ligands transfer from OsO to MTO have been evaluated. The ex-
[
[
23] C. Coperét, H. Adolfsson, K.B. Sharpless, Chem. Commun. (1997) 1565.
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243.
change reaction is governed by electronic effect and the values of
equilibrium constants correlate with the Hammett reaction con-
stant. The enthalpy and entropy of the reactions were measured
and an acceptable linear relationship between
observed.
[
[
25] (a) C. Döbler, G. Mehltretter, M. Beller, Angew. Chem., Int. Ed. 38 (1999) 3026;
(
(
b) C. Döbler, G. Mehltretter, U. Sundermeier, M. Beller, J. Am. Chem. Soc. 122
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DH and DS is
(c) B.M. Choudary, N.S. Chowdari, S. Madhi, M.L. Kantam, Angew. Chem., Int.
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Acknowledgment
(
1
b) S.Y. Jonsson, K. Färnegårdh, J.E. Bäckvall, J. Am. Chem. Soc. 123 (2001)
365.
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[
[
27] S.Y. Jonsson, H. Adolfsson, J.E. Bäckvall, Chem. Eur. J. 9 (2003) 2783.
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Financial support of the Islamic Azad University, Shiraz Branch,
Iran (Grant No. 89.495) is gratefully acknowledged.
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[
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