C. Larabi et al.
Catalysis Communications 108 (2018) 51–54
Fig. 3. Catalytic activity and selectivity for propene metathesis over 2.
ν(CeH) and δ(CeH) of tert-butoxy fragments also appeared. In addition,
The structure of the supported species in 2, was studied by EXAFS
−
1
3
a broad band extending from 3700 to 3100 cm related to interactions
(k ·χ(k), Fig. S2 and Table 1). The results agree with a first coordination
between the tert-butoxy ligands and remaining silanols, was observed
sphere around W composed of ca. two oxo ligands at 1.74(1) Å and ca.
two oxygen atoms at 1.89(2) Å, which can be attributed to surface
siloxide ligands. The distances found for W]O and WeO bonds are in
[
15]. Qualitative GC analysis of the filtrate after washing revealed the
t
presence of BuOH, formed after silanolysis of the tert-butoxy tungsten
fragments. Elemental analysis of resulting material indicated a W and C
the usual range observed for bis-oxo siloxide tungsten molecular com-
i
%
content of 5.75%wt and 2.65%wt respectively. This corresponds to a
plexes studied by XRD, as for [W(]O)
2
(OSi( Pr)
3
)(bdt)] (1.72 to
(OSi
)] [20] (1.712 to 1.730 Å for W]O and 1.894(7) Å for
C/W molar ratio of 7.1, in line with the theoretical value of 8 for 1. In
1.735 Å for W]O and 1.886(4) Å for WeO) [19], [Cp*W(]O)
CH
2 3
WeO) or [(Me tacn)W(]O) (OSi( Pr) )] (1.719(1) for W]O and
2
1
13
(
2 6
C H
5
)
3
addition, the H MAS and C CP MAS NMR data show the presence of
i
+
1
13
tungsten tert-butoxy fragments, as reflected by the H and C peaks at
.4 and 26 ppm, respectively (Fig. 2). On the H MAS NMR spectrum,
3
1
1.888(2) Å for WeO) [21]. Similar parameters were obtained when
1
2
fitting the k ·χ(k) spectrum. The fit could be improved by adding fur-
the shoulder on the low field side of the main peak (about 2.5 ppm) is
assigned to the residual, interacting silanols. We note the absence of the
signal belonging to quaternary carbon atoms. From these combined
ther layers of back-scatterers, two oxygen atoms at 2.86(6) Å and ca.
two silicon atoms at 3.20(5) Å, attributed respectively to surface oxy-
gens and to silicon atoms of surface siloxides ligands of tungsten. The
W-O-Si angle deduced from this study, ca. 132 ± 5°, is rather small but
angles as small as 130.5 ± 1° have been measured for tungsten oxo
siloxide complexes [22]. The results obtained from the fit of the EXAFS
spectroscopic and analytical elements, it can be concluded that the
t
reaction of []W(O Bu)
2
4
] with the silica surface dehydroxylated at
t
00 °C proceeds by WeO cleavage with concomitant BuOH release,
leading to a bipodal surface species [(^SiO) W(]O)(O Bu) ]; 1
2 2
t
2 2
spectrum then agree with a (^SiO) W(]O) local structure for the
silica supported tungsten species present in 2.
(
Scheme 1).
The second step consists of the conversion of the supported complex
t
[
(
2 2
(^SiO) W(]O)(O Bu) ]; 1 at 300 °C (2 h) under high-vacuum
−5
10 mbar), which triggers the elimination of H on the β position from
3.2. Catalytic studies
oxygen and results in the formation of bipodal W oxo hydroxo tert-
butoxide [(^SiO)
t
2
W(]O)(OH)(O Bu)] with the release of 1 equivalent
The catalytic performances of 2 were probed in a flow reactor at
−
1
−1
−1
of isobutene. Further heating of the intermediate leads to the α-H ab-
straction, giving tert-butanol and formation of silica-supported W bis-
oxo species 2 (Scheme. S1).
4
20 °C and 1 bar (10 mlC3H6·min ; 10 molC3H6 mol
W
min ).
Importantly, after a rather long induction period (100h), the conversion
converges towards 30% (the conversion at equilibrium is about 45%)
and remained stable for at least two weeks, affording a cumulated TON
of 56,000 (Fig. 3). Furthermore, the selectivity in metathesis reaction
was > 99% (Fig. 3). The stoichiometry of propene metathesis should
generate a one to one molar ratio of ethene and butenes. However, from
the initiation step, the ethene to butenes molar ratio is constant with
time on stream (with a value of 1.06).
Qualitative GC analysis of the gas released after heating revealed
t
the presence of BuOH and 0.9 isobutene per W. DRIFT analysis of 2
reveals the disappearance of the alkyl vibrational bands
−
1
(
3000–2800 cm ), accompanied by re-appearance of isolated silanol
−1
groups at 3747 cm
(Fig. 1). Unfortunately, the characteristic W]O
vibration is masked by the network vibration of silica in the IR spec-
trum. Raman spectrum of 2 is presented in Fig. 1. It exhibits broad
−1
Raman features in the ranges of 400 to 500 and 800 to 900 cm and a
−
1
4. Conclusion
smaller band at 610 cm , corresponding to various breathing modes of
the siloxane rings within the silica network [16]. Indeed, we observed a
−
1
In conclusion, a first example of a well-defined bis-oxo tungsten
surface species composed only of bis-oxo and siloxy ligands has been
prepared and fully characterised. Without need of a co-catalyst, this
material demonstrated high, sustained activity and selectivity in pro-
pene metathesis. This represents a significant step to understand and
band centered at 985 cm , attributed to the combined features for
stretching vibrations of terminally bound bis-oxo ligands− in
2
(at-
1
[
6,16–18]. Importantly, no features at 270, 720, and 805 cm
tributed to W-O-W) were observed, suggesting the absence of oligo-
meric tungsten species. The influence of W surface concentration on the
band-edge energies determined from UV–vis spectra of 2 are presented
in Fig. S1. UV–vis diffuse reflectance spectrum of 2, revealed the pre-
sence of absorption band in the region of 230–300 nm accounting for
3 2
mimic the active species of the WO /SiO system, by capitalising on this
well-defined material. We have shown here that isolated organome-
tallic metal oxide fragments give rise to high and sustained performance
in propene metathesis. This was accomplished by using the surface
organometallic chemistry approach that allows for rational design of
heterogeneous catalysts and thus can significantly contribute to the
preparation of more efficient catalytic materials. Ongoing efforts are
now targeted at the design and characterisation of related bis-oxo
2
–
+6
charge transfer band from O to W ions in which isolated W centres
are in tetrahedral-coordination. This was confirmed by the band-edge
energies, E
isolated tetrahedral centres in Na
g
determination (3.85 eV) close to the one found for the
WO compound (E = 4.7 eV) [6,16].
2
4
g
53