Xylyltrioxorhenium-the first arylrhenium(VII) oxide applicable as an olefin epoxidation catalyst

Article abstract of DOI:10.1039/c2cy20371e

The catalytic activity of (2,6-dimethylphenyl)trioxorhenium (XTO) for the epoxidation of cyclooctene is presented. Solutions of hydrogen peroxide and tert-butyl hydrogen peroxide (TBHP) in THF or CH₂Cl₂ were applied as oxidizing agen

Full text of DOI:10.1039/c2cy20371e

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Xylyltrioxorhenium – the first arylrhenium(VII) oxide
applicable as an olefin epoxidation catalyst†
Cite this: Catal. Sci. Technol., 2013,
3, 388
a
a
a
b
a
´
´
Stefan Huber, Mirza Cokoja, Markus Drees, Janos Mınk and Fritz E. Kuhn*
¨
The catalytic activity of (2,6-dimethylphenyl)trioxorhenium (XTO) for the epoxidation of cyclooctene is
presented. Solutions of hydrogen peroxide and tert-butyl hydrogen peroxide (TBHP) in THF or CH2Cl2
were applied as oxidizing agent–solvent mixtures. Cyclooctene oxide yields are higher with TBHP than
with H2O2. Application of non-coordinating solvents as reaction media increases both yields and
reaction rates. The possible active species is analyzed by 17O NMR and Raman spectroscopy.
Received 1st June 2012,
Accepted 23rd July 2012
DOI: 10.1039/c2cy20371e
www.rsc.org/catalysis
temperature and thus is a far better target molecule for
catalytic examinations. In 1994, catalytic activity of XTO
Introduction
In 1974 Wilkinson et al. reported the synthesis of [(CH3)4ReO],
the first high oxidation state alkyl oxo rhenium compound.1 Its
derivative (CH3)ReO3 methyltrioxorhenium(VII) (MTO) was the
first organorhenium(VII) oxide to be described.2 Only after the
synthesis of [(pentamethylcyclopentadienyl)trioxorhenium](VII),
reported independently by two different research groups in
19843 and the large scale synthesis and catalytic application
of MTO in 1989,4 however, the compound class R–ReO3(VII) was
systematically examined.5 Within a few years a plethora of
compounds were obtained.5,6 Nevertheless, it turned out that
all these derivatives, with the famous exception of MTO,
appeared to be either not catalytically active or of very limited
stability.5,7
Little attention has been paid to the aryl trioxorhenium
compounds of the type [Ar–ReO3] with respect to catalytic
applicability.8 Aryl ligands are available for the full range of
organic aromatic substitution chemistry.8b,c However, the
synthesis of [Ar–ReO3] is quite intricate since both the starting
materials and the products are at least in some cases highly
sensitive. Furthermore, most compounds of this class decompose
slowly when exposed to sunlight. Phenyltrioxorhenium, which is
obtained from Re2O7 and ZnPh2, is quite sensitive to moisture
and only stable at low temperatures (oÀ30 1C).8a,b Therefore, it is
not a good choice for catalytic applications. Its dimethyl derivative
xylyltrioxorhenium (XTO, 1), however, is stable in solution at room
in the epoxidation of olefins with aqueous H2O2 has been
mentioned.7a Since the compound is quite sensitive to moisture,
the observed poor catalytic activity is not surprising. In this work,
water-free oxidants (H2O2 in THF solution, TBHP in n-decane)
are applied in order to avoid the water-sensitivity of XTO. The
active species have also been investigated by 17O NMR and
Raman spectroscopy.
Experimental section
General
All preparations and manipulations were carried out using
standard Schlenk techniques or in an MBraun glove box under
a strictly oxygen- and water-free argon atmosphere. All reaction
vessels were protected from light. THF was of HPLC grade and
distilled over Na–benzophenone under argon before use.
Dichloromethane was dried according to literature methods.9
Yellow Re2O7,7a dixylyl zinc10 and 17O-labeled MTO11 were
synthesized according to the literature. A 3.3 mol LÀ1 water
free solution of H2O2 in THF was prepared by dissolving
aqueous H2O2 in THF followed by drying over MgSO4.7a
TBHP-D1 was obtained by mixing TBHP in n-decane with
10 equiv. of D2O, stirring overnight and subsequent drying
over molecular sieves. CDCl3 was degassed and stored over
molecular sieves prior to use. A 5.5 M solution of TBHP in n-decane
was obtained from Aldrich. MnO2, electrolytically precipitated, was
obtained from Acros. NMR spectra were obtained using a Bruker
Avance DPX-400 or Jeol JNM-GC-400 spectrometer. Mass
spectra were recorded on a Finnigan MAT 311 A and a MAT
90 spectrometer. Raman spectra were recorded on a Bio-Rad
FTS60A Dedicated FT-Raman spectrometer. Mid-IR spectra
were measured on a Varian FT-660 FTIR spectrometer using a
a Chair of Inorganic Chemistry/Molecular Catalysis, Catalysis Research Center,
¨
¨
Technische Universitat Munchen, Ernst-Otto-Fischer-Str. 1, D-85747, Garching bei
¨
Munchen, Germany. E-mail: fritz.kuehn@ch.tum.de; Fax: +49 89 289 13473;
Tel: +49 89 289 13081
b
´
Chemical Research Center, Hungarian Academy of Sciences, Pusztaszeri ut 59-67,
H-1025 Budapest, Hungary
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c2cy20371e
c
388 Catal. Sci. Technol., 2013, 3, 388--393
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micro ATR attachment. Microanalyses and mass spectroscopy Solvent effects were simulated with the PCM model15 as imple-
were performed at the TUM (Garching) laboratories.
mented in Gaussian 03. The solvent of choice was dichloromethane
with a dielectric constant of e = 2.02 and a relevant molecular radius
of 2.27 Å.
Improved preparation of 1
Dixylyl zinc (0.14 g, 0.50 mmol) was dissolved in 10 mL THF
and dropwise added to a solution of Re2O7 (0.48 g, 1.00 mmol)
in 25 mL THF, which was previously cooled to À78 1C. The
solution was slowly warmed to À5 1C and stirred for 2 h. The
solvent was removed under vacuum and the residue was
extracted with 6 Â 10 mL of cold (0 1C) n-hexane. After removing
the solvent under vacuum, yellow crystal needles of 1 (0.14 g,
0.41 mmol, 41% based on rhenium) were obtained, which
could be further purified by recrystallization in cold n-hexane.
The product was characterized by NMR and IR spectroscopy.
The obtained values are in good accordance with previously
published data.8b
Results and discussion
Synthesis and catalytic properties of XTO
The reaction of Re2O7 with dixylyl zinc yields compound 1 in
maximum 50% yield based on rhenium, since zinc perrhenate
is formed as a stoichiometric byproduct accounting for the
other 50% of the applied rhenium (see Scheme 1).8 The reaction is
very sensitive to the quality of the dirhenium heptoxide used. Only
highest quality grade, lemon-yellow Re2O7, produced by oxidation
of rhenium powder under oxygen gas and direct sublimation,
should be applied.7a Even tiny impurities decrease the yield
significantly. By lowering reaction temperatures and protecting
the reaction vessels from light to prevent decomposition, yields
of up to 40–45% based on rhenium can be obtained.
Previously, it had been stated that s-phenyltrioxorhenium
and s-mesityltrioxorhenium show no catalytic activity when
using aqueous H2O2 as an epoxidizing agent.7a However, no
examinations applying non-aqueous H2O2 or TBHP were carried
out. Fig. 1 summarizes the catalytic activity of 1 in the presence
of various oxidants and solvents.
Catalytic reactions
cis-Cyclooctene (0.341 g, 3.09 mmol), oxidant (6.19 mmol, TBHP
in decane, or H2O2 35% in water, or H2O2 in THF) and 1 (1 mol%,
0.031 mmol) as catalyst were applied. The reactions were carried
out in 4 mL CH2Cl2. The oxidant was added into a thermostated
reaction vessel and stirred for 24 h. The course of the reaction was
monitored by quantitative GC analysis; samples were taken every
5 min up to 30 min, then every hour for 3 h and a final sample was
taken after 24 h. For the destruction of the oxidant, a catalytic
amount of electrolytically precipitated MnO2 was added. After
the evolution of gas ceased, the resulting slurry was filtered and
dried over MgSO4. The external standards indane and p-xylene,
dissolved in iso-propanol, were added to the filtrate and the
resulting mixture was injected into a calibrated VarianWS gas
chromatograph. The conversion of cyclooctene and the formation
of cyclooctene oxide were calculated from a calibration curve
recorded prior to the catalytic reactions. The turnover frequencies
were determined by the initial-rate method using the conversion
of the first five minutes.
In general, oxidation reactions carried out using aqueous
H2O2 as an oxidant show no significant activity. Using THF as
solvent instead of water increases the activity, but the overall
activity remains low, most likely due to the formation of
water as a byproduct. Compound 1 reacts with water to yield
perrhenate, which is not active in olefin epoxidation in polar or
17O NMR-experiments
Scheme 1 Synthesis of xylyltrioxorhenium (1).
17O-Labeled MTO (0.015 g, 0.059 mmol) and XTO (0.020 g,
0.059 mmol) were dissolved in 0.4 mL of dry CDCl3 and stirred
for 10 min. Thereafter a 5.5 M solution of TBHP in n-decane was
added (1–400 equiv.). The solution was stirred for another 10 min at
À20 1C. 1H and 17O NMR spectra were recorded at À20 1C.
Raman experiments
XTO (10 mg, 0.029 mmol) was dissolved in 0.2 mL decane.
A 5.5 mol (1–10 equiv.) solution of TBHP or TBHP-D1 was added
and after short mixing, spectra were recorded at room temperature.
Computational details
All calculations have been performed with GAUSSIAN-03 using
the density functional/Hartree–Fock hybrid model Becke3LYP12
and the split valence triple-z (TZ) basis set 6-311+G**.13 Re
atoms have to be treated with an effective core potential, the
Hay–Wadt LANL2DZ basis set was chosen for this metal.14 All
optimizations were obtained without using constraint coordinates.
Fig. 1 Yields of cyclooctene epoxidation with compound 1 as catalyst precursor
with different oxidants (H2O2, TBHP) in various solvents (H2O, THF, CH2Cl2). All
reactions were performed at room temperature with
oxidant ratio of 1 : 100 : 200.
a catalyst/substrate/
c
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strongly coordinated in both cases, but the sterically more
demanding coordination sphere of XTO inhibits activation of
the catalyst–base adduct. Table 1 shows the turnover frequen-
cies (TOFs) obtained in the performed experiments. TOFs were
determined by the initial rate method, using the conversion in
the first 5 minutes, since no activation period could be
observed. Comparison of the obtained results shows the
same trends in temperature and solvent dependence given by
comparing the yields. In the case of using TBHP as oxidant,
using non-coordinating CH2Cl2 instead of coordinating THF
leads to an increase in the initial activity by one order of
magnitude. It can be assumed that THF coordinates to the Re
center reducing its Lewis acidity, i.e. the catalytic activity.
Fig. 2 Yields of the cyclooctene epoxidation with catalyst 1 in CH2Cl2 with the
oxidant TBHP at various temperatures with a catalyst/substrate/oxidant ratio of
1 : 100 : 200.
NMR spectroscopic studies
In order to elucidate the mechanism of the epoxidation of
cyclooctene catalyzed by 1, 17O-NMR studies were performed in
order to shed light on the reaction of XTO with TBHP and the
nature of the product. The oxygen atoms are suitable probes for
answering the question, whether Re-oxo-peroxo species are
formed or other reactions take place, analogously to the case
of MTO/H2O2. There are several methods known to enrich the
17O-content of 1 to obtain detectable signals. Due to the
high sensitivity to water it is not recommended to directly use
17O-enriched water for the labeling of 1, as is the case of MTO.
Other methods for enriching compound 1 with 17O include the
direct synthesis of 17O-labeled Re2O7 by oxidizing rhenium
metal powder with 17O enriched oxygen gas,9 and carrying
out the synthesis of XTO according to Scheme 1. Another
possibility involves a multistep reaction, where Re2O7 is hydro-
lyzed in 17O-labeled water and precipitated with AgNO3. The
resulting silver perrhenate would be reacted with (CH3)3SiCl
and THF to yield [Re2O7(THF)2].11 The third and most conve-
nient method, hence used in this work, draws on the ability of
water-stable methyltrioxorhenium (MTO) to exchange its
oxo ligands with (17O-enriched) water as well as with other
rhenium-oxo compounds.7b It is therefore straightforward to
label the easily available MTO and, in a second step, to transfer
17O-ligands to XTO. This method has also the advantage that
the 17O-NMR spectra can be calibrated on the resonance of
MTO. In all experiments 17O-labeled MTO is mixed with XTO in
a 1 : 1 ratio in CDCl3. After 10 min, a solution of TBHP in decane
is added and the 17O NMR spectrum is recorded at À20 1C.
The oxygen resonance originating from ReQO of 1 appears
at 820 ppm (Fig. 3, bottom spectrum).22 Upon addition of
TBHP, the signal of 1 at 820 ppm is slightly shifted to a broader
signal at 819 ppm and a new signal at 726 ppm appears (Fig. 3,
middle). The half width of the peak at 819 ppm is ca. 50 Hz,
which is significantly broader than that of the peaks at 726 ppm
(ca. 35 Hz) and 820 ppm (ca. 30 Hz), which indicates that this
line is broadened by hydrogen exchange between terminal oxo
ligands and hydroxo ligands. A weak signal at 65 ppm of tert-
butanol (reported at 60 ppm20) is visible.
non-polar solvents. Changing the oxidant to TBHP in THF strongly
increases the catalytic activity. Changing the solvent from coordi-
nating THF to non-coordinating CH2Cl2 leads to overall epoxide
yields of 460% and notably higher initial activities. In all cases no
byproducts were detected by GC; especially no ring opening of the
epoxide was observed. Thus, yields are equivalent to conversion.
The reactions are, however, limited by catalyst decomposition.
Similar experiments using the urea–hydrogen-peroxide adduct
(UHP) as an oxidant in CH2Cl2 do not show any measurable
activity, most likely due to the poor solubility of UHP in CH2Cl2.
The activity of the system XTO/TBHP in olefin epoxidation is
noteworthy. To the best of our knowledge, molecular organyl
rhenium oxide complexes could so far only be activated by H2O2,
whereas TBHP leads either to no reaction or to decomposition to
perrhenate.5,16,17 TBHP, on the other hand, is known to be an
efficient oxidant for some molecular Mo catalysts.18 The catalytic
system XTO/TBHP apparently follows a somewhat different
reactivity pattern than MTO/H2O2. Fig. 2 shows the dependence
of the product yields on the reaction temperature. With increasing
reaction temperature, both TOFs and yields increase. A tempera-
ture optimum can be found around 35 1C. Further increase in the
reaction temperature (55 1C, CHCl3 as solvent) does not lead to a
further increase in catalytic activity.
Adding a base such as tert-butylpyridine, which is used in
MTO catalysis to increase catalyst performance and to inhibit
epoxide ring opening,19 almost completely inhibits the catalytic
activity of XTO. In both MTO and XTO the a-carbon of the
catalyst is shifted by 5 ppm in 13C NMR upon coordination of
tert-butylpyridine. Therefore, the base seems to be equally
Table 1 Comparison of the epoxide yields and catalyst activities depending on
the oxidant, solvent and temperature (after 24 h reaction time)
Oxidant
Solvent
Temperature
Yielda [%]
TOF [hÀ1
]
H2O2
H2O2
TBHP
TBHP
TBHP
TBHP
H2O
THF
THF
CH2Cl2
CH2Cl2
CH2Cl2
RT
RT
RT
RT
0 1C
35 1C
B1%
16%
22%
60%
54%
77%
o1
3
40
316
126
364
Upon increasing the concentration of TBHP the resonance
at 819 ppm disappears and the resonance at 726 ppm becomes
stronger. The signal for the monoperoxo complex of MTO,
a
After 24 h reaction time.
c
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Table 2 Characteristic Raman bands of 2 bands of deuterated species are
shown in brackets
Raman band [cmÀ1
]
Mode
917 m (917 m)
883 vs (884 vs)
998 m (757 m)
995 s (744 s)
556 m (520 m)
538 w (510 w)
nA (ReO2)
nS (ReO2)
d(ReOH(D))
Re–OH(D) stretch
Table 3 Characteristic Raman bands of complex 3
Raman band [cmÀ1
]
Mode
977 m
917 m
885 vs
510 m
451 m
(O–O) stretch
nA (ReO2)
Fig. 3 17O NMR spectra of the reaction of 1 with 17O-labeled MTO and various
amounts of TBHP in decane (bottom: before addition of TBHP; middle: 200 equiv.
TBHP; top: 400 equiv. TBHP).
nS (ReO2)
nS (Re(O–O))
nA (Re(O–O))
products, including tert-butyl methyl ether, methanol, methane
and ethene.17
Raman spectroscopic studies
To further shed light on the active species, Raman spectra were
measured. In order to identify characteristic OH bands, all
experiments were performed with TBHP (tbutyl-OOH) as well as
with TBHP-D1 (tbutyl-OOD). Other than 17O NMR, Raman
spectroscopy is fast enough to resolve the difference between
the oxo and hydroxo ligands of 2. In the first experiments, one
equivalent of TBHP/TBHP-D1 was added to a concentrated
solution of 1 in decane. A pale red solution formed, which
was very unstable and formed a black residue within few
minutes at room temperature. Table 2 shows the characteristic
bands of 2 and their corresponding vibrational modes.
Upon increasing the TBHP amount to 10 equiv., a yellow
solution formed, which is stable at room temperature for
several hours. Raman experiments show that the Re–OH bands
(998, 995, 556 and 538 cmÀ1), previously observed in the initial
spectrum, almost fully disappear and the characteristic bands
of a Re-peroxo moiety, found at 510, 451 and 977 cmÀ1, appear.
The Re(O)2 vibrational bands at 917 and 885 cmÀ1 are still
visible, indicating the formation of a mono-peroxo complex.
Table 3 summarizes the most important bands and their
vibrational modes of 3 in solution.
Scheme 2 Possible active species of XTO (R = xylyl). s-type-coordinated TBHP 2,
monoperoxo complex 3.
which is dominant at low H2O2 concentrations, is reported to
appear at 763 ppm.20
Scheme 2 shows two possible complexes that may be the
actual active species. Complex 2 is a linear TBHP–XTO complex
which is proposed in analogy to the active species often
discussed in [MoO2] catalysis with TBHP.19 Species 3 is
proposed in analogy to the monoperoxo species of MTO, which
is long known to be one of the catalytically active species of
MTO.20
In order to assign this observation, the 17O NMR shifts of
compounds 2 and 3, relative to MTO at 829 ppm, were calcu-
lated by DFT methods. Due to the fact that only ReQO atoms
can be exchanged with 17O by MTO, not all of the predicted
peaks are visible in the real NMR experiments. A 17O shift of
721 ppm (and 707 ppm, which cannot be observed) for the
ReQO in the monoperoxo complex of XTO 3 was calculated. On
the other hand a shift of 819 ppm (and 693 ppm, which cannot
be observed) is predicted for the ReQO moiety in the linear
s-type-coordinated XTO–TBHP complex 2. This shift can only
be observed at lower concentrations of TBHP. Hence a mono-
peroxo species like 3 seems to be the major species under
catalytic conditions, where low catalyst amounts meet high
oxidant concentrations.
The Raman experiments prove and extend the results of
NMR experiments. At low concentrations, the unstable red
compound 2 is dominant. It is responsible for the quick
decomposition of the catalyst. At higher concentrations of
TBHP the stable, yellow mono-peroxo species 3 is formed,
which, in analogy to MTO, should also be the catalytically active
species (Scheme 3).
Upon adding 400 equiv. TBHP, two broad peaks around 745
and 765 ppm emerge, indicating that MTO can be transformed
Decomposition of the catalyst
to CH3-Re-peroxo complexes by TBHP, but in significantly When treated with TBHP, XTO appears to form Re-peroxo
smaller yields than for 1 (Fig. 3).21 Furthermore, MTO decom- species as is the case with MTO and H2O2. However, unlike
poses in the presence of TBHP to perrhenate and several other MTO, XTO exhibits catalytic activity in olefin epoxidation with
c
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Scheme 3 Possible activation pathway of 1 in the presence of TBHP.
TBHP as oxidant. Finally it also decomposes to perrhenate, Acknowledgements
however, significantly slower than MTO.
SH is grateful to the TUM Graduate School for financial
support.
In 1999, Brittingham and Espenson suggested a radical
pathway for the decay of the TBHP adduct of MTO, based
on product quantifications and kinetic measurements.22 Pro-
duct examinations in our case of the final reaction mixture by
NMR also show the presence of perrhenate and bisxylyl, a
typical radical decomposition product. Radical decomposition
of XTO is further amplified by thermal and light
induced cleavage of the aryl–rhenium bond. Most probably
due to steric hindrance, decomposition of XTO in the presence
of the bulky TBHP is significantly slower than decomposition of
MTO, therefore catalytic activity can be observed before
decomposition.
Periana et al. have discussed a non-radical decomposition
mechanism of phenyltrioxorhenium23a and methyltrioxo-
rhenium23b,c when using H2O2 in some detail using DFT
methods. A key step in this mechanism is the insertion of
oxygen into the rhenium alkyl(aryl)-bond. This mechanism
might be responsible for the instant decomposition of the
catalyst in aqueous H2O2 solution and the fast decomposition
of the catalyst in H2O2 in THF. However, the main decomposi-
tion products of this pathway, 2,6-dimethylphenol and
2,6-dimethylphenol tert-butyl ether, could not be observed
in 1H NMR experiments after treating XTO with TBHP. Anyway,
the whole decomposition mechanism is not yet well under-
stood. Accordingly, a detailed experimental and theoretical
study of the decomposition pathways of XTO when using TBHP
is currently being performed in our laboratories.
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