Well-Defined Silica-Supported Tungsten(IV)-Oxo Complex: Olefin Metathesis Activity, Initiation, and Role of Br?nsted Acid Sites

Article abstract of DOI:10.1021/jacs.9b09493

Despite the importance of the heterogeneous tungsten-oxo-based olefin metathesis catalyst (WO₃/SiO₂) in industry, understanding of its initiation mechanism is still very limited. It has been proposed that reduced W(IV)-oxo surface sp

Full text of DOI:10.1021/jacs.9b09493

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Article
Well-defined Silica-supported Tungsten(IV)-oxo Complex: Olefin
Metathesis Activity, Initiation and Role of Brønsted Acid Sites
Ka Wing Chan, Deni Mance, Olga V. Safonova, and Christophe Copéret
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b09493 • Publication Date (Web): 16 Oct 2019
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Well-defined Silica-supported Tungsten(IV)-oxo Complex: Olefin Me-
tathesis Activity, Initiation and Role of Brønsted Acid Sites.
Ka Wing Chan†, Deni Mance†, Olga V. Safonova§, Christophe Copéret*,†
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† ETH Zürich, Department of Chemistry and Applied Biosciences, Vladimir Prelog Weg 1-5, CH-8093 Zurich, Swit-
zerland
§ Paul Scherrer Institute, CH-5232 Villigen, Switzerland
ABSTRACT: Despite the importance of the heterogeneous tungsten-oxo based olefin metathesis catalyst (WO3/SiO2) in
industry, understanding of its initiation mechanism is still very limited. It has been proposed that reduced W(IV)-oxo sur-
face species act as precatalysts. In order to understand the reactivity and initiation mechanism of surface W(IV)-oxo species,
we synthesized a well-defined silica-supported W(IV)-oxo species (≡SiO)WO(OtBuF6)(py)3 (OtBuF6 = OC(CH3)(CF3)2; py =
pyridine) (F6@SiO2-700) via Surface Organometallic Chemistry (SOMC). F6@SiO2-700 was shown to be highly active in olefin
metathesis upon removal of pyridine ligands through the addition of tris(pentafluorophenyl)borane (B(C6F5)3) or thermal
treatment under high vacuum. The metathesis activity towards olefins with and without allylic C−H groups, namely β-
methylstyrene and styrene, respectively, was investigated. In the case of styrene, we demonstrate the role of surface OH
groups in initiating metathesis activity. In particular, the presence of strong Brønsted acidic surface OH sites, revealed by
15N-labeled pyridine, likely arises from the presence of adjacent W sites in the catalyst and assist styrene metathesis, an
olefin having no allylic C-H bond. In contrast, initiation of olefins containing allylic C−H group (e.g. β-methylstyrene) is
independent of surface OH group concentration and likely involves allylic C−H activation mechanism like the molecular
W(IV)-oxo species. This study indicates that initiation mechanisms depend on the olefinic substrates and reveals the syn-
ergistic effect of Brønsted acidic surface sites and reduced W(IV) sites in the initiation of olefin metathesis.
INTRODUCTION
temperature (70 °C) by organosilicon reductants.24-25 The
resulting M(IV) species initiate metathesis at 70 °C through
in-situ generation of M(VI) alkylidenes (Scheme 1b, an ex-
ample with W-based system). More recently, we have also
shown that molecular W(IV)-oxo complexes, e.g.
WO(OtBuF6)2(py)3 (OtBuF6 = OC(CH3)(CF3)2; py = pyri-
dine), initiate metathesis in the presence of olefins and
Lewis acid activators, e.g. tris(pentafluorophenyl)borane
(B(C6F5)3) (Scheme 1c).26 Detailed mechanistic studies have
shown that initiation of the W(IV) species takes place via
allylic C−H bond activation of the olefin followed by pro-
ton transfers, which is promoted by pyridine to generate
the metallacyclobutane. Despite the comparable catalytic
performance of these W(IV) systems to well-defined alkyli-
dene complexes, the initiation of W(IV) only leads to the
formation of a small amount of active species in solution
(few percent of the total W species). The low initiation ef-
ficiency can be ascribed to the endergonic nature of this
initiation process, as shown by DFT calculations, com-
bined with the fast decomposition of the low-coordinated
W(IV) and W(VI) intermediates generated in solution. We
thus reasoned that generating the corresponding well-de-
fined silica-supported W(IV)-oxo complex could shed light
on the initiation mechanism and improve the catalytic per-
formance by preventing some possible deactivation path-
ways in solution, such as bimolecular decomposition.13, 27-32
Olefin metathesis is a widely used reaction for applica-
tions ranging from petrochemicals to pharmaceuticals.1-13
Owing to the high and increasing demand for propene, the
tungsten-based heterogeneous catalyst (WO3/SiO2) is used
in the Olefin Conversion Technology (OCT) process, one
of the largest industrial metathesis processes, producing
propene via ethenolysis of butenes.4 Typically, WO3/SiO2
operates at high temperature (> 400 °C),14 but various pre-
treatment methods, such as thermal treatment under re-
ducing conditions, have been shown to increase the activ-
ity at low temperatures.14-22 However, despite years of stud-
ies, the molecular-level understanding of the activation
process remains very limited and the nature of active spe-
cies is still under debate.14, 21 Several studies have reported
the formation of oxygenates (e.g. acetone, formaldehyde
and acetaldehyde) during the activation of WO3/SiO2 with
propene.20, 22-23 It has been proposed that the formation of
oxygenates is consistent with the formation of reduced
W(IV) species that are precursors of the active sites,17, 19, 22
while other studies favor the pseudo-Wittig mechanism
that generates the W(VI) alkylidenes directly (Scheme
1a).18
Recently, our group has shown that molecularly-defined
silica-supported metal-oxo species and the related
MO3/SiO2 (M = Mo or W) can be activated at low
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Scheme 1. State of the Art in the Initiation of W-based Olefin Metathesis Catalysts. a) WO3/SiO2. b) Molecularly-
defined Supported W-oxo Species. c) W(IV) Molecular Models.
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Here, we report the synthesis and the reactivity of well-
defined silica-supported W(IV)-oxo species
Additional υ(C−H) bands of the alkoxide and pyridine lig-
ands are observed in the region of 3200-2800 cm-1 and 1630-
1360 cm-1. The υ19b and υ8a ring-breathing vibrations modes
of pyridine ligands also appear in the region of 1630-1440
cm-1, indicating the presence of pyridine on the surface. El-
emental analysis of F6@SiO2-700 reveals 3.82 wt% of W (i.e.
0.6 Wnm-2) and the composition of 19 C, 17 H, 4 N and 6 F
per W center. These data are consistent with the formation
of (≡SiO)WO(OtBuF6)(py)3 as shown in Scheme 1. Further
analysis of F6@SiO2-700 by X-ray Absorption Near Edge
Structure (XANES) spectroscopy at both W LI and LIII edge
(Figure S2-S3) shows that F6@SiO2-700 shares the same
edge energy and spectroscopic features as F6, indicating
retention of the oxidation state and geometry upon graft-
ing. The lack of pre-edge feature in the W LI edge XANES
spectrum further supports that the grafted species remains
in an octahedral geometry. Extended X-ray Absorption
Fine Structure (EXAFS) fitting analysis of the first coordi-
nation sphere of F6@SiO2-700 is also consistent with the
structure (≡SiO)WO(OtBuF6)(py)3 (Table 1). 1H Magic An-
gle Spinning (MAS) NMR spectrum of F6@SiO2-700 displays
two signals at δ 1.95 and 1.69 ppm (Figure S6) indicating
the presence of ─CH3(CF3)2 groups in two different envi-
ronments. Additional proton resonances at δ 8.88 and 7.71
ppm indicate the presence of pyridine ligands. Similarly,
19F MAS NMR spectrum shows a broad signal at ca. δ -81
ppm corresponding to the ─CF3 groups (Figure S7). These
chemical shifts are similar to the reported F6 molecular an-
alogue.26
(≡SiO)WO(OtBuF6)(py)3 (OtBuF6 = OC(CH3)(CF3)2; py =
pyridine) prepared via Surface Organometallic Chemistry
(SOMC).12, 33-35 We demonstrate that the supported W(IV)
precatalyst is active in olefin metathesis upon activation
with B(C6F5)3 or thermal treatment, which removes the co-
ordinated pyridine ligands. The activated surface species
displays one order of magnitude higher catalytic activity
than the molecular analogue, and, in contrast to the mo-
lecular species, it also catalyzes the self-metathesis of ole-
fins without any allylic C−H group, such as styrene. We
demonstrate that the metathesis activity of styrene is
linked to the presence of surface OH groups that open
other initiation pathways involving additional proton-
transfer steps in the in-situ formation of alkylidene species.
RESULTS AND DISCUSSION
Synthesis
(≡SiO)WO(OtBuF6)(py)3. Grafting of WO(OtBuF6)2(py)3
(F6) on silica partially dehydroxylated at 700 °C (SiO2-700
and
characterization
of
)
in benzene for 24 h afforded a dark blue material F6@SiO2-
700 (Scheme 2). Analysis of the combined supernatant and
washings of the material by 1H NMR spectroscopy revealed
the consumption of F6 and the release of HOtBuF6 (0.72
equiv. per grafted W center) suggesting that grafting oc-
curred via protonolysis. Furthermore, no free pyridine was
observed in the washings.
Scheme 2. Preparation of Well-defined Silica-sup-
ported W(IV)-Oxo Species
Table 1. EXAFS Fit Parameters for the W LIII-edge Spec-
trum of F6@SiO2-700
neighbor
Na
1*
2*
3*
σ2 (Å2)b
R (Å)c
O
O
N
0.002 (2)
0.003 (2)
0.005 (3)
1.73 (1)
2.00 (3)
2.16 (4)
b
c
aNumber of specified neighbors. Debye-Waller factor. Dis-
tance between W metal center to the specified neighbors.
*Fixed parameters in the fit.
Comparison of the IR spectra of F6@SiO2-700 and SiO2-700
reveal the disappearance of isolated silanol υ(O−H) at 3747
cm-1 upon grafting (Figure S1), while a broad red shifted
band appears at 3700 cm-1 indicating the interactions be-
tween remaining surface silanols and the grafted species.
Catalytic activity. We then examined the catalytic activ-
ity of F6@SiO2-700 towards self-metathesis of cis-4-nonene
at 70 °C. In the absence of activation, this material displays
no metathesis activity, similar to what was observed with
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the F6 molecular compound.26 However, upon the addition
compared to F6 (Table 2; Figure S15). Noteworthy,
F6@SiO2-700 also catalyzes the self-metathesis of styrene
while F6 does not, under similar reaction conditions (Fig-
ure S16). These results indicate that immobilization of the
precatalyst on silica enables efficient metathesis for sub-
strate without allylic C−H group, indicating the contribu-
tion of alternative initiation mechanism and possibly the
involvement of surface OH groups, which are not present
in the molecular system (vide infra).14, 42-43
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of B(C6F5)3 (3 equiv per W) to remove the coordinated pyr-
idine ligands, F6@SiO2-700 catalyzes self-metathesis of cis-
4-nonene (1000 equiv per W) at 70 °C reaching equilibrium
conversion (51 %) within 3 h (Figure S13). To quantify the
amount of pyridine that was abstracted by B(C6F5)3,
F6@SiO2-700 was contacted with B(C6F5)3 (3 equiv per W)
in the absence of olefin, and the amount of py-B(C6F5)3
quantified suggests that at least 2.2 equiv of pyridine (per
W) was removed. Alternatively, thermal treatment of
F6@SiO2-700 at 400 °C under high vacuum also removes the
coordinated pyridines, resulting in a metathesis active ma-
terial F6@SiO2-700-thermal, without the need of any activator.
Under the same reaction conditions with cis-4-nonene,
while a short induction period of ca. 30 min is observed in
the case of F6@SiO2-700-thermal, equilibrium conversion is
reached within 2 h (Figure S13). The high temperature ther-
mal treatment used could also lead to additional surface
reactions including the formation of surface sites with dif-
ferent reactivities.36-37 While no py-B(C6F5)3 is present in
F6@SiO2-700-thermal, surface oxygen atoms that can act as
Lewis base could play a similar role in proton transfers.38-40
It is noteworthy that F6@SiO2-700 displays a significantly
higher catalytic activity compared to its F6 homogeneous
analogue (Table 2). This difference in activity points to the
9
Effects of surface OH densities. In order to investigate the
possible involvement of surface OH groups in facilitating
initiation, the molecular complex F6 was grafted on silica
with varying initial surface OH densities, that can be
achieved through controlling the dehydroxylation temper-
atures of the supports (SiO2-x, x = dehydroxylation temper-
atures in °C), namely SiO2-500 (0.46 mmolOH/g; 1.4 OH/nm2)
and SiO2-200 (0.84 mmolOH/g; 2.5 OH/nm2) yielding
F6@SiO2-500 and F6@SiO2-200, respectively. The catalytic
activity of these materials was then compared to F6@SiO2-
700, which has an initial OH density of 0.31 mmolOH/g (0.9
OH/nm2) on SiO2-700. In all materials, the weight loadings
of W were kept the same, i.e. with a density of ca. 0.6 Wnm-
2, yielding materials with different densities of remaining
silanols. In addition, a small amount of bis-grafted W sites
(up to ca. 27 % of total surface W) could be present in
F6@SiO2-200, based on mass balance analysis (see ESI for
details). IR spectra of the grafted materials (Figure 1) indi-
cate that no isolated silanol (3747 cm-1) remains, while
broad OH bands – associated with OH groups interacting
with the grafted species – appear at 3690-3700 cm-1. The
intensities of OH bands increase with the initial OH den-
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advantage of site isolation in the supported F6@SiO2-700
,
which likely prevents bimolecular deactivation of low-co-
ordinated reaction intermediates, thus improving catalyst
stability.41
Table 2. Catalytic Activities of Supported and Molecu-
lar Species in Olefin Metathesisa
catalysts
mol substrate
%
rate
conv. at
sity: F6@SiO2-200 > F6@SiO2-500 > F6@SiO2-700.
c
(min-1)f 6 hg
F6@SiO2-700
0.1
Cis-4-nonene
14 d
Equil.;
3 h
Equil.;
2 h
47%
86%
59%
F6@SiO2-700- 0.1
Cis-4-nonene
28 d,h
b
thermal
F6
0.3
1
2
Cis-4-nonene
1-nonene
β-methylsty-
rene
2 d
F6@SiO2-700
F6@SiO2-700
2 d
0.4 e
F6@SiO2-700
F6
2
2
Styrene
β-methylsty-
rene
0.1 e
40%
<0.01 e,h 12%
a Batch reactor, 70 °C, in the presence of B(C6F5)3 (3 equiv per
W). b Without B(C6F5)3. c Catalyst loadings based on W mol %.
d 1.0 M toluene solution. e 0.2 M toluene solution. f Rate of me-
tathesis (mol of product/mol of W/min) at conversion < 20%.
g Conversion at 6 h unless otherwise noted by giving the time
reaching equilibrium conversion. h Following an induction pe-
riod (ca. 30 min), the metathesis starts and reaches a maxi-
mum rate after 60 min.
Figure 1. IR spectra of F6@SiO2-200 (red; top), F6@SiO2-500
(blue; middle) and F6@SiO2-700 (black; bottom).
The catalytic activities of F6@SiO2-700, F6@SiO2-500 and
F6@SiO2-200 in the self-metathesis of trans-β-methylsty-
rene and styrene in the presence of B(C6F5)3 were then in-
vestigated. The rates of self-metathesis of trans-β-me-
thylstyrene are similar for all three materials, while the
rates of styrene self-metathesis parallel the increasing OH
densities of the materials (Figure 2). The rate of styrene
metathesis catalyzed by F6@SiO2-200 is approximately 2.5
times higher compared to F6@SiO2-700. The increases in ac-
tivities with increasing OH densities and the lack of sty-
rene metathesis activity in F6 molecular system further
We then investigated the metathesis activity towards
other olefins including 1-nonene, trans-β-methylstyrene
and styrene. The results are summarized in Table 2. In the
presence of B(C6F5)3, self-metathesis of 1-nonene catalyzed
by F6@SiO2-700 reaches 86% conversion at 6 h (Figure S14).
The rate of self-metathesis of trans-β-methylstyrene cata-
lyzed by F6@SiO2-700 is an order of magnitude higher
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point to the involvement of surface OH groups in the ini-
tiation step with styrene, an olefin without an allylic C−H
bond (vide infra).
cycle species (e.g. olefin π complex and metallacyclopen-
tane) that can serve as an olefin reservoir for subsequent
cross-metathesis reactions have prevented us from obtain-
ing a reliable quantification of the active alkylidene spe-
cies.
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Figure 2. Rates of metathesis (min-1) (at conversion < 20%) of
F6, F6@SiO2-700, F6@SiO2-500 and F6@SiO2-200 for trans-β-
methylstyrene (blue) and styrene (red).
Investigations of reaction intermediates formed upon re-
action of F6@SiO2-200 with styrene. To investigate the reac-
tion intermediates formed on the surface, F6@SiO2-200 was
exposed to 13C-dilabeled styrene-α,β-13C2 in the presence of
B(C6F5)3 in toluene at 70 °C for 1 h. The supernatant was
then removed, and the solid was washed with toluene, fol-
Figure 3. 13C-13C SQ-DQ MAS NMR experiment acquired on a
600 MHz spectrometer at 12kHz MAS of F6@SiO2-200 after
contacting with styrene-α,β-13C2 under reaction conditions.
Asterisk (in red) indicates spinning sideband.
13
lowed by drying under high vacuum. C solid-state NMR
spectrum of the resulting solid reveals a major signal at ca.
δ 65 ppm (Figure S18), which indicates the presence of eth-
ylene π complex formed from the ethylene released during
styrene metathesis. This assignment is further confirmed
by the 13C-13C single-quantum double-quantum SQ-DQ ex-
periment (Figure 3), in which a strong self-correlating sig-
nal at δ 130 ppm in the DQ dimension and δ 65 ppm in the
SQ dimension was observed. Noticeably, this self-correlat-
ing signal appears to be very broad, indicating a distribu-
tion of surface W ethylene π complexes likely associated
with slightly different environments. In addition, two in-
tense signals at δ 65 and 85 ppm that correlate with each
other (Figure 3) are assigned to the b- and a-carbons of the
styrene π complex, respectively. The signals at δ 75 ppm (α-
C) and 36 ppm (β-C) are also correlated in the SQ-DQ ex-
periment and are attributed to the α- and β- carbons of the
unsubstituted metallacyclopentane. The observation of
both olefin π complexes and metallacyclopentane as the
major species is in line with what was observed in the F6
molecular system.26 However, the absence of evidence for
alkylidenes or metallacyclobutanes is noteworthy; it im-
plies that the number of active species formed is likely
small, possibly pointing out the low efficiency of the initi-
ation process due to the competing and favorable for-
mation of olefin π complexes and metallacyclopentane.26
Finally, two additional signals at δ 61 and 19 ppm are as-
signed to the presence of ethoxy group (EtO−) on the sur-
face,44-45 which arise from the protonation of ethylene by
surface OH groups. Such species are not observed on silica
(see ESI for details) suggesting the presence of highly
Brønsted acidic OH groups in F6@SiO2-200 under reaction
conditions.
Brønsted acidic sites in F6@SiO2-200 probed by 15N-pyri-
dine. The observation of surface EtO− group (Figure 3)
upon contacting F6@SiO2-200 with styrene-α,β-13C2 under
reaction conditions suggests the presence of strong
Brønsted acidic sites, which are further evidenced by using
15
15N-labeled pyridine (15N-py) as a probe molecule. N-la-
beled F6@SiO2-200 (15N-F6@SiO2-200) was synthesized and
its 15N Cross-polarization (CP) MAS NMR spectrum shows
15
15
similar N resonance signals compared to the N-labeled
molecular precursor (15N-F6) (Figure S19) along with a
small peak at δ 293 ppm indicating a small amount of 15N-
py that is H-bonded to the OH groups on the surface, likely
released upon grafting.46-47 Upon activation of the 15N-
F6@SiO2-200 with 3 equiv of B(C6F5)3 at 70 °C, followed by
the subsequent addition of extra 15N-py, a shift in the major
15N signals to lower chemical shifts, compared to 15N-
F6@SiO2-200, was observed. This result indicates that some
of the changes in the coordination environment of the W
centers after the removal of 15N-py ligands by B(C6F5)3 are
irreversible and that the surface remaining 15N-py-B(C6F5)3
adducts formed likely contribute to the signal observed.48
Notably, a small peak at δ 207 ppm also appeared, which is
attributed to pyridinium (15N-pyH+) (Figure S19).46-47 The
presence of 15N-pyH+ is further supported by the peak ob-
served at 1533 cm-1 in the IR spectrum corresponding to a
15
combination of vibration modes involving N+−H (Figure
S20).49-51 The formation of 15N-pyH+ by protonating the 15N-
py indicates the presence of strong Brønsted acidic site
upon addition of B(C6F5)3 to 15N-F6@SiO2-200. Notably, such
Brønsted acidic sites are absent on SiO2-200, despite the
presence of pyridine and B(C6F5)3 (Figure S20), indicating
the low-coordinated W center is likely responsible for the
formation of the strong Brønsted acid site by interacting
with the OH group in close proximity.
We also attempted to quantify the amount of active al-
kylidenes formed by cross-metathesis reactions between
styrene and cis-5-decene. However, the presence of off
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Proposed initiation mechanism for the formation of al-
formation of a specific initiation product, which may be
due to strong adsorption on the surface and further reac-
tions. Although there is no direct initiation product ob-
served in F6@SiO2-x during the generation of alkylidene
species, we propose that olefins with allylic C−H groups
likely follow the allylic C−H activation mechanism and
subsequent proton transfers to generate the metallacyclo-
butane intermediates, as reported in the molecular F6 sys-
tem.26 Since increasing OH density does not lead to an in-
crease in the metathesis activity with the trans-β-me-
thylstyrene (Figure 1), this suggests the presence of com-
peting initiation mechanisms, e.g. allylic C−H bond activa-
tion that depend on the olefinic substrates.
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3
4
5
6
7
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kylidenes. Based on the above observations: i) the increase
activity in styrene metathesis parallels the increase in sur-
face OH density (Figure 1), and ii) the presence of strong
Brønsted acidic sites in the supported system due to the
presence of low-coordinated W centers as shown by the
15N-py experiments, we propose that for olefins having no
allylic C−H group, e.g. styrene, surface OH groups can as-
sist the initiation by protonating the olefins or the W cen-
ter as shown in Scheme 3.14, 42-43 Protonation of the W-co-
ordinated olefin by surface OH groups can directly lead to
the formation of tungsten−carbon bond, a tungsten alkyl
species. Alternatively, protonation of the free olefin can
yield transient carbocations that can either generate sur-
face alkoxy groups that are observed experimentally (Fig-
ure 3) or further react with the W(IV) centers forming the
tungsten alkyl species. Another possibility would be proto-
nating the W center to generate W−H species that can un-
dergo insertion with an olefin to yield tungsten alkyl spe-
cies. Subsequent deprotonation of the tungsten alkyl spe-
cies by the silanolate ligand leads to the formation of an
alkylidene.
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CONCLUSION
We have shown that well-defined W(IV) oxo species
prepared via
(≡SiO)WO(OtBuF6)(py)3 (F6@SiO2-700
)
SOMC is active in olefin metathesis upon activation by
B(C6F5)3 or thermal treatment. F6@SiO2-700 shows an order
of magnitude higher catalytic activity compared to the F6
molecular analogue, confirming the advantage of site iso-
lation when reactive intermediates are formed. Further-
more, F6@SiO2-700 also initiates the self-metathesis of sty-
rene in sharp contrast to the reported F6 suggesting that
alternative initiation mechanisms are involved in sup-
ported species. The fact that increasing surface OH densi-
ties of the supports further increases the rates of styrene
metathesis suggests a surface proton assisted initiation
mechanism for olefins without allylic C−H group. This is
also consistent with the generation of strong Brønsted
acidic surface sites in the activated catalysts. For substrates
with allylic C−H groups e.g. β-methylstyrene, initiation via
allylic C−H activation is still likely involved, as evidenced
by the lack of dependence of metathesis activity with OH
density. This study suggests that initiation mechanisms for
generating W-based olefin metathesis catalysts from
W(IV)-oxo species depend on the olefinic substrates. Fur-
thermore, these findings reveal the participation of the sur-
face functionalities of oxide support, namely the surface
Brønsted acid sites. It is noteworthy that initiation of me-
tathesis catalysts (whether involving allylic C−H activation
or not) involves a key proton transfer step. This also indi-
cates that oxide supports in heterogeneous olefin metath-
esis catalysts might not always be inert, even for a support
like silica. Surface Brønsted acidic sites formed in the pres-
ence of W can be responsible for metathesis activity by
opening alternative initiation processes. This study helps
to further understand the effects of Brønsted acidity in het-
erogeneous metathesis catalysts.
Scheme 3. Proposed Initiation Mechanisms for Olefin
Without Allylic C−H Group.
The proposed carbocation intermediates could also lead to
an indirect initiation pathway involving dimerization of
styrene followed by the allylic C−H activation.52-53 How-
ever, the corresponding dimer does not initiate metathesis
as shown in the molecular F6 system (see ESI for details),
precluding the possibility of the latter initiation pathway.
The feasibility of the proposed initiation mechanism in
Scheme 3 is also supported by DFT calculations, which sug-
gest the initiation pathway, especially the formation of
tungsten alkyl from W(IV) species is thermodynamically
favorable. The calculations also show that subsequent
deprotonation of the alkyl ligand would be thermoneutral
in specific environments, i.e. with a slightly elongated
W−O silanolate bond, which is only possible for a small
amount of surface strained sites (see ESI for details). This
likely contributes to the formation of only a small amount
of active sites associated with low initiation efficiency. In
order to obtain more insight into the initiation mecha-
nism, we also tried to track the initiation product formed
in the process. However, unlike the reported homogeneous
F6 system,26 monitoring the metathesis of either β-me-
thylstyrene or styrene by GC-MS does not reveal the
ASSOCIATED CONTENT
Supporting Information. The Supporting Information is
available free of charge on the ACS Publications website. Full
experimental and computational details and catalytic data.
(PDF)
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Page 6 of 8
19.
Howell, J. G.; Li, Y.-P.; Bell, A. T., Propene Metathesis over
A Study of Active Site
AUTHOR INFORMATION
Supported Tungsten Oxide Catalysts:
Formation. ACS Catal. 2016, 6, 7728-7738.
20.
Propylene Metathesis by Supported WOx/SiO2 Catalysts. ACS Catal.
2017, 7, 573-580.
21.
State Metal–Carbon Double Bonds. Organometallics 2017, 36, 1884-
1
2
3
4
5
6
7
8
Corresponding Author
* ccoperet@ethz.ch
Lwin, S.; Wachs, I. E., Catalyst Activation and Kinetics for
ACKNOWLEDGMENT
Schrock, R. R.; Copéret, C., Formation of High-Oxidation-
The authors are grateful to the Swiss National Foundation
(SNF) for financial support of this work (grant no.
200021L_157146). DM acknowledges support from the ETHZ
Postdoctoral Fellowship Program and from the Marie Curie
Actions for People COFUND Program.
1892.
22.
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9
23. Lwin, S.; Li, Y.; Frenkel, A. I.; Wachs, I. E., Nature of WOx
Sites on SiO2 and Their Molecular Structure–Reactivity/Selectivity
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Relationships for Propylene Metathesis. ACS Catal. 2016, 6, 3061-3071.
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H.; Tsurugi, H.; Mashima, K.; Safonova, O.; Copéret, C., Low
Temperature Activation of Supported Metathesis Catalysts by
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