Strongly ?? donating thiophenoxide in silica-supported tungsten oxo catalysts for improved 1-alkene metathesis efficiency

Article abstract of DOI:10.1021/om5013105

We report the synthesis of tungsten oxo alkylidene complexes bearing bulky thiophenoxide ligands [W(=O)(=CHtBu)(SHMT)₂(PMe₂Ph)] and [W(=O)(=CHtBu)(SHMT)₂] (SHMT = 2,6-dimesitylthiophenoxide) and their grafting on partially

Full text of DOI:10.1021/om5013105

Communication
pubs.acs.org/Organometallics
Strongly σ Donating Thiophenoxide in Silica-Supported Tungsten
Oxo Catalysts for Improved 1‑Alkene Metathesis Efficiency
Victor Mougel,† Margherita Pucino,† and Christophe Coperet*
́
Department of Chemistry and Applied Biosciences, ETH Zurich, Vladimir Prelog Weg 1-5, CH-8093 Zurich, Switzerland
̈
̈
S
* Supporting Information
ABSTRACT: We report the synthesis of tungsten oxo alkylidene
complexes bearing bulky thiophenoxide ligands [W(O)(CHtBu)-
(SHMT)2(PMe2Ph)] and [W(O)(CHtBu)(SHMT)2] (SHMT = 2,6-
dimesitylthiophenoxide) and their grafting on partially dehydroxylated
silica, affording the supported complexes [(SiO)W(O)(CHtBu)-
(SHMT)(PMe2Ph)] and [(SiO)W(O)(CHtBu)(SHMT)]. While
the molecular precursors are not significantly active in the metathesis of
alkenes, the grafted analogue without bound phosphine ligands displays
activity comparable to that of its aryloxide analogue [(SiO)W(O)(
CHtBu)(OHMT)] (OHMT = 2,6-dimesitylphenoxide). It is worth noting
that [(SiO)W(O)(CHtBu)(SHMT)] showed unprecedented activ-
ity in the metathesis of 1-alkenes, probably because of the lower stability of
metallacyclobutane intermediates.
dvances in alkene metathesis over the last 30 years have
transformed the way organic molecules are prepared.1−3
tylthiophenoxide)15,20 and the corresponding supported
systems.
A
Of d0 alkene metathesis catalysts,4−7 supported tungsten oxo
alkylidene complexes have recently emerged and been shown to
have unprecedented activities and stabilities.8,9 In particular,
silica-supported tungsten oxo alkylidene bearing phenolate
ligands catalyze the metathesis of internal olefins at loading as
low as 50 ppm. However, they are surprisingly sluggish for 1-
alkenes, because of the formation of very stable unsubstituted
metallacycles generated by the ethylene byproduct. This
reversible deactivation process could be circumvented by
using a reactor setup, allowing for the continuous removal of
ethylene.
The thiolate complex 1 was synthesized by salt metathesis of
[W(O)(CHtBu)Cl2(PMe2Ph)2]21,22 with 2 equiv of
potassium 2,6-dimesitylthiophenolate in 70% yield. A similar
synthesis was independently described by Schrock and
Hoveyda during the course of our studies.16 The phosphine-
free complex 2 was prepared by addition of tris-
(pentafluorophenyl)borane to a solution of 1 in pentane21
and was isolated as a crystalline yellow material by
recrystallization in pentane in 85% yield. X-ray diffraction
analysis shows that complex 1 adopts a distorted-trigonal-
bipyramidal geometry (Figure 1), similar to that of the related
phenoxide complex [W(O)(CHtBu)(OAr)2(PMe2Ph)]23
(Ar = 2,6-Ph2C6H3), in which the oxo ligand, C1 and C2 of the
DFT calculations have shown that dissymmetry at the metal
center (X ≠ Y) for d0 Schrock-type catalysts of general formula
(X)(Y)M(E)(CHR) (M = Mo, W, Re; E = O, NR, CR) not
only decreases the energy barrier for coordination of the olefin
substrate (the key step in metathesis) but also raises the energy
of the metallacyclobutane intermediates.10−14 In silica-sup-
ported phenolate tungsten oxo alkylidene complexes such as
[(SiO)W(O)(CHtBu)(OHMT)] and [(SiO)W(
O)(CHtBu)(dAdPO)] (OHMT = 2,6-dimesitylphenoxide,
dAdPO = 2,6-diadamantyl-4-methylphenoxide), the σ dona-
tions of the phenoxide and the surface siloxide ligands are very
similar. We thus reasoned that thiolate analogues15 would be
ideal targets, since they are isoelectronic with the corresponding
phenolates16−19 with the advantage of restoring the dissymme-
try at the metal center.
Figure 1. Ellipsoid plot (left, 50% probability) and chemical structure
(right) for [W(O)(CHtBu)(SHMT)2(PMe2Ph)] (1). Hydrogen
atoms are omitted for clarity.
Herein we report the synthesis, the grafting on partially
dehydroxylated silica at 700 °C (SiO2‑(700)), and the catalytic
performance of oxo alkylidene complexes with pendant thiolate
ligands [W(O)(CHtBu)(SHMT)2(PMe2Ph)] (1) and
[W(O)(CHtBu)(SHMT)2] (2) (SHMT = 2,6-dimesi-
Received: December 20, 2014
Published: January 26, 2015
© 2015 American Chemical Society
551
DOI: 10.1021/om5013105
Organometallics 2015, 34, 551−554

Organometallics
Communication
syn neopentylidene ligand, and the sulfur lie in the equatorial
plane while the phosphine and the second thiolate ligand are
facing trans to each other in axial positions. Worthy of note is
the significant difference between the axial W−S1 bond
distance (2.388(2) Å) and the equatorial W−S2 distance
(2.450(3) Å), highlighting the stronger trans influence of the
oxo in comparison to the phosphine ligand.
compositions of [(SiO)W(O)(CHtBu)(SHMT)-
(PMe2Ph)] (1@SiO2) and [(SiO)W(O)(CHtBu)-
(SHMT)] (2@SiO2) were confirmed by elemental analysis of
C, H, S, and P (see the Supporting Information for details).
1
The H magic angle spinning (MAS) NMR spectra of 1@
SiO2 and 2@SiO2 showed distinct resonances at 10.6 and 10.3
ppm, respectively, assigned to the alkylidene WCHtBu
signal. While for the OHMT complex [(SiO)W(O)(
CHtBu)(OHMT)] the proton chemical shift of the alkylidene
was very similar to that of its molecular precursor [W(O)(
CHtBu)(OHMT)2], indicative of the similar electronic proper-
ties of OHMT and surface siloxy ligands, proton signals are
significantly shifted by about 1.9 ppm for 1@SiO2 and 1.4 ppm
for 2@SiO2 with respect to their molecular precursors. This is
consistent with a stronger desymmetrization of the ligand set
upon grafting for the SHMT complexes with respect to their
phenoxide analogues. The 31P MAS NMR spectra of 1@SiO2
revealed a major signal at 8 ppm and a minor signal (5%) at 22
ppm. While the major signal matches the signal of the
coordinated phosphine in the molecular precursor, the minor
signal likely originates from the interaction of the phosphine
with surface silanols to afford a P(V) species.25 The 13C cross-
polarization (CP) MAS NMR spectra of [(SiO)W(O)(
CHtBu)(SHMT)(PMe2Ph)] and [(SiO)W(O)(
CHtBu)(SHMT)] contained signals from SHMT and
phosphine (for 1@SiO2) ligands (see the Supporting
Information for details). As generally observed for supported
tungsten alkylidene complexes at natural abundance, the
alkylidene signals were not observed in 13C spectra, but the
proton signals of the alkylidene groups combined with the
signals at 47 and 31 ppm for 1@SiO2 and 43 and 30 ppm for
2@SiO2 assignable to the β and γ carbons of the neo-
pentylidene group are clear indications of the presence of the
alkylidene moieties in the grafted compounds.
The X-ray structure of 2 shows a distorted tetrahedron with
the alkylidene in a syn configuration, as presented in Figure 2.
Figure 2. Ellipsoid plot (50% probability) and chemical structure
(right) for [W(O)(CHtBu)(SHMT)2] (2). Hydrogen atoms
were omitted for clarity.
The WO and WC bond lengths of 1.664(8) and
1.875(13) Å is typical of tungsten oxo alkylidene complexes.
The W−S1 bond length (2.345(2) Å) is essentially the same as
the W−S2 bond length (2.352(2) Å). In such complexes, the
opening of each face of the tetrahedron, defined by three
“basal” ligands and excluding the fourth “pivotal ligand”, has
been used to characterize the ease of access to the metal
center.13,24 In this complex the most open face is trans to the
alkylidene (339.65 vs 328.4° for an ideal tetrahedron), and the
approach of olefin on that face would not lead to the formation
of the metallacycle intermediate.14
The catalytic activity in alkene metathesis of all molecular
and grafted complexes was evaluated with cis-4-nonene, used as
a prototypical substrate (Table 1). The two molecular catalysts
Grafting 1 and 2 on SiO2‑(700) afforded the corresponding
materials 1@SiO2 and 2@SiO2 (Scheme 1). Monitoring the
a
Table 1. Homocoupling of cis-4-Nonene
catalyst loading,
mol %
time to equilibrium
conversion
Scheme 1. Grafting of 1 and 2 on SiO2‑(700) Yielding [(
SiO)W(O)(CHtBu)(SHMT)(PMe2Ph)] (1@SiO2) and
[(SiO)W(O)(CHtBu)(SHMT)] (2@SiO2)
b
catalyst
TOF3 min
0
1
0.1
0.1
0.1
0.1
0.02
no conv after 24 h
6% conv after 24 h
34% conv after 24 h
10 min
2
<0.1
<1
1@SiO2
2@SiO2
2@SiO2
138 (41)
85 (5)
2 h
a
b
Reactions were carried out in toluene at 30 °C. TOF at 3 min, given
in min−1 with the corresponding conversions in parentheses given in
percent.
show little to no activity under the conditions tested:
compound 1 was found to be inactive26 and only 6%
conversion was observed after 24 h with 2. Dramatic differences
were observed with the grafted catalysts. While 1@SiO2
showed little activity with only 34% conversion in 24 h, 2@
SiO2 reached equilibrium conversion in less than 10 min, with
an initial turnover frequency (TOF, determined at 3 min) of
138 min−1. At lower catalyst loading (0.02 mol %) equilibrium
conversion is reached in about 2 h.
grafting step by IR spectroscopy showed a significant decrease
of the isolated silanol band at 3747 cm−1, indicating partial
consumption of the silanol groups, similarly to what has been
observed for the OHMT complex [(SiO)W(O)(
CHtBu)(OHMT)].8 The broad band observed for both
materials around 3650 cm−1 indicated that some surface
silanols were interacting with the aromatic ligand of the grafted
complex. Mass balance analysis of the released thiol indicated
that around 50% and 40% of the surface silanols reacted with 1
and 2, respectively, in agreement with the W content of 1.98%
for 1 and 1.92% for 2 determined by elemental analysis. The
2@SiO2 also metathesizes ethyl oleate, an ester-containing
alkene, at 0.1 mol % catalyst loading with an initial TOF of 2
min−1, reaching equilibrium conversion in 36 h (Table 2). It is
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DOI: 10.1021/om5013105
Organometallics 2015, 34, 551−554

Organometallics
Communication
a
allows tailoring the catalyst to its potential application, as
exemplified here for 1-alkene metathesis.
Table 2. Catalytic Activity of 2@SiO2
catalyst loading,
time to equilibrium
conversion
b
substrate
mol %
TOF3 min
c
ASSOCIATED CONTENT
ethyl
0.1
2 (0.6%) 36 h (500 TON)
■
oleate
S
* Supporting Information
d
1-nonene
0.1
27 (9%)
37 (4%)
5 h (>93% conv., 935
TON)
Text, figures, tables, and CIF files giving synthetic details,
catalytic reactions, and the X-ray structures of 1 and 2. This
material is available free of charge via the Internet at http://
pubs.acs.org.
d
1-nonene
0.02
48 h (>93% conv., 4650
TON)
a
b
Reactions were carried out in toluene at 30 °C. TOF at 3 min, given
in min−1 with the corresponding conversions given in parentheses.
c
Turnover numbers (TON) calculated taking into account only the
products from productive metathesis: i.e., 4-octene and 5-decene.
Maximum conversion under the conditions employed (closed vial
AUTHOR INFORMATION
■
d
Corresponding Author
*E-mail for C.C.: ccoperet@inorg.chem.ethz.ch.
only opened for sampling).
Author Contributions
†These authors contributed equally.
Notes
worth noting that 2@SiO2 quantitatively converts 1-nonene in
less than 5 h with an initial TOF of about 27 min−1, in contrast
to the case for [(SiO)W(O)(CHtBu)(OHMT)], which
deactivated very quickly (maximum conversion 66% after 24 h)
and presented a surprisingly low initial TOF of 13 min−1 under
the same reaction conditions. This high activity and stability
allowed us to decrease the catalyst loading to 0.02 mol %,
reaching full conversion in about 48 h without the need to
continuously remove ethylene.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Mr. Maxence Valla for assistance with the solid-state
NMR measurements and Mr. Wolfram Gruning for fruitful
̈
discussions. We acknowledge Matthew P. Conley for insightful
information on the formation of metallacyclobutane with
phenoxide complexes. V.M. was supported by an ETH
The high activity of 2@SiO2 toward 1-alkene led us to
investigate the formation of metallacycles by reaction with 13C-
ethylene and their characterization by solid-state NMR, as
previously developed in our group.9,27,28 In contrast to all of the
supported tungsten neopentylidene metathesis catalysts inves-
tigated so far, no mono-13C-labeled 3,3-dimethylpropene was
released when 2@SiO2 was contacted with 15 equiv of bis(13C-
labeled) ethylene, and the solid-state NMR of the catalyst after
exposure was identical with that of the parent compound. It is
noteworthy that no signals from methylidene and/or metal-
lacyclobutane species were observable. In addition, the catalytic
activity of this compound in cis-4-nonene metathesis was
unchanged to prior to exposure to ethylene (similar initial TOF
and time to equilibrium; see the Supporting Information for
details). Under the same conditions, the phenoxide analogue
[(SiO)W(O)(CHtBu)(OAr)] (OAr = OHMT or
dAdPO9) quantitatively reacted with 13C-ethylene to afford an
SP-metallacycle as the major surface species and the compound
formed by exposure to ethylene shows significantly reduced
activity toward cis-4-nonene. This observation allows us to
conclude that the formation of unsubstituted metallacycles in
the presence of ethylene is much less favored for 2@SiO2 than
for its phenoxide analogue and could explain its highest activity
with 1-alkenes.
In summary, we prepared thiolate tungsten oxo alkylidene
molecular and supported complexes. The grafted phosphine-
free surface complex [(SiO)W(O)(CHtBu)(SHMT)]
showed unprecedented activity in the metathesis of 1-alkenes, a
reaction that has proved challenging even with state of the art
supported tungsten oxo alkylidene catalysts due to their
propensity to form stable unsubstituted metallacycles in the
presence of ethylene. Desymmetrization of the ligand set of
tungsten oxo alkylidene catalysts has successfully lead to
improve 1-alkene metathesis catalysts, probably because it
disfavors the formation of very stable metallacycles. These
results highlight the fact that a fine tuning of the structure and
electronic properties of supported alkene metathesis catalysts
fellowship (cofunded ETH Zurich-Marie Curie action for
people, FEL-08 12-2).
̈
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