Development of a well-defined silica-supported tungstenocarbyne complex as efficient heterogeneous catalyst for alkyne metathesis

Article abstract of DOI:10.1016/j.jorganchem.2008.02.020

The interaction of [W({triple bond, long}C-tBu)(CH₂-tBu)(OAr)₂] (Ar = 2,6-iPr₂C₆H₃) (1), with the hydroxyl groups of a silica dehydroxylated at 700 °C leads to [({triple bond, long}SiO)W(OAr)₂({triple bond, long}C-tBu)] (2) which was characterized by IR, solid-state NMR and mass balance analysis. This well-defined surface species is an efficient catalyst for the metathesis of pent-2-yne.

Full text of DOI:10.1016/j.jorganchem.2008.02.020

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Journal of Organometallic Chemistry 693 (2008) 1733–1737
www.elsevier.com/locate/jorganchem
Communication
Development of a well-defined silica-supported
tungstenocarbyne complex as efficient heterogeneous
catalyst for alkyne metathesis
Nicolas Merle a, Mostafa Taoufik a, Morad Nayer a, Anne Baudouin a, Erwan Le Roux a,
b
a,
a
a
*
´
´ ´
Regis M. Gauvin , Frederic Lefebvre , Jean Thivolle-Cazat , Jean-Marie Basset
a Chimie Organome´tallique de Surface/Laboratoire de Chimie, Catalyse, Polyme`res et Proce´de´s (UMR-C2P2-5265 CNRS/ESCPE-Lyon/UCBL)
ESCPE Lyon, F-308-43, Boulevard du 11 Novembre 1918, F-69616 Villeurbanne Cedex, France
b
`
´
Unite´ de Catalyse et de Chimie du Solide (UMR 8181 CNRS/ENSCL), Groupe Syntheses Organometalliques et Catalyse, ENSCL-C7,
BP 90108, 59652 Villeneuve d’Ascq Cedex, France
Received 11 January 2008; received in revised form 18 February 2008; accepted 18 February 2008
Available online 23 February 2008
Abstract
The interaction of [W(„C–tBu)(CH2–tBu)(OAr)2] (Ar = 2,6-iPr2C6H3) (1), with the hydroxyl groups of a silica dehydroxylated at
700 °C leads to [(„SiO)W(OAr)2(„C–tBu)] (2) which was characterized by IR, solid-state NMR and mass balance analysis. This
well-defined surface species is an efficient catalyst for the metathesis of pent-2-yne.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Surface organometallic chemistry; Metallocarbyne; Solid-state NMR; Alkyne metathesis; Heterogeneous catalysis
1. Introduction
but the reaction proceeds usually at 80 °C [7,8]. Some other
highly active alkyne metathesis catalysts have been synthes-
ised by exchanging the amido ligand of molybdenum(VI)
alkylidyne amide complexes with 4-nitrophenol [9–11].
Unfortunately, as in the case of olefin metathesis, these cat-
alysts deactivate rapidly, apparently through a bimolecular
dimerization pathway leading to a Mo„Mo triple bond
[12,13]. To avoid such a reaction, Moore et al. reported
the grafting reaction of the molybdenum complex
[Mo(„C–tBu)(NAr(tBu))3] on amorphous silica pre-trea-
ted at 400 °C under dry O2, the purpose being to induce
minimal changes in the electronic structure of the complex
and to reduce its mobility [14]. This catalyst showed a high
catalytic activity and a good stability. We have recently
reported that the reaction of [W(„C–tBu)(CH2–tBu)3]
with highly dehydroxylated silica affords the well-defined
tungsten alkylidyne surface complex [(„SiO)W(„C–
tBu)(CH2–tBu)2] [15]. This system is active in olefin
metathesis but displays only a stoichiometric activity in
Alkene metathesis is now routinely used in organic syn-
thesis [1–3]. Some recent and elegant applications are the
synthesis of macrocycles by ring closing metathesis of a
diene [4]. However, alkene metathesis meets some limita-
tions as it gives mixtures of E and Z isomers, thus decreas-
ing its potential as a highly stereoselective synthetic
method. An approach to overcome this difficulty may be
found with alkyne metathesis [5]. Indeed, the newly formed
C„C bond could then be stereo-selectively reduced to
either E or Z, C@C isomers, or converted to another func-
tionality. Up to now, several alkyne metathesis catalysts
are known [5,6]. The catalyst developed by Schrock,
[(tBuO)3W(„C–tBu)], is one of the most extensively used,
*
Corresponding author. Fax: +33 472 431 795.
E-mail addresses: lefebvre@cpe.fr (F. Lefebvre), basset@cpe.fr (J.-M.
Basset).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.02.020

1734
N. Merle et al. / Journal of Organometallic Chemistry 693 (2008) 1733–1737
alkyne metathesis. The formation of the active species in
olefin metathesis (a metal–carbene) passes probably by a
a-hydrogen elimination and this reaction can also be
responsible of the lack of activity in alkyne metathesis
[16,17]. The modification of the metal coordination sphere
by replacing the two neopentyl moieties by ancillary
ligands should therefore be of considerable interest:
it could not only prevent the deactivation pathway by
a-hydrogen elimination, but it could also modify the per-
formances of the catalytic system through the modulation
of the steric and electronic parameters. We thus undertook
the preparation of such supported catalysts, and we report
here the synthesis and characterization of the new well-
defined silica-supported bisaryloxycarbyne catalyst,
[(„Si–O)W(„C–tBu)(OAr)2] (Ar = 2,6-iPr2C6H3). In con-
trast to the [(„SiO)W(„C-tBu)(CH2-tBu)2] complex, this
system is active in alkyne metathesis [18].
2.4. Reaction of acetonitrile with complex 2
The solid 2 was contacted with acetonitrile (80 Torr,
1.63 mol%) in a reactor at 25 °C. Small aliquots of the
gas phase were analyzed by gas chromatography as a func-
tion of time.
2.5. Metathesis of pent-2-yne
The solids 2 or 4 were contacted with penty-2-yne
(80 Torr, 1.63 mol% for 2 and 1.49 mol% for 4) in a reactor
at 80 °C. Small aliquots of the gas phase were analyzed by
gas chromatography as a function of time, allowing the
determination of the conversion and selectivities.
3. Results and discussion
When a disk of silica (Aerosil Degussa 200 m2 gÀ1) trea-
ted at 700 °C under vacuum (SiO2-(700)) is allowed to react
with a pentane solution of [W(„C–tBu)(CH2–tBu)(OAr)2]
(Ar = 2,6-iPr2C6H3) (1), the IR spectroscopy shows the
2. Experimental
consumption of the free silanol groups (m(OH) = 3747 cmÀ1
)
2.1. Preparation of SiO2-(700)
(Fig. 1). Simultaneously, two groups of bands appear in the
3100–2700 cmÀ1 and 1500–1300 cmÀ1 regions. These bands
are, respectively, assigned to m(CH) and d(CH) vibrations of
the hydrocarbyl and phenoxy ligands. Moreover, two
broad bands appear at 3710 cmÀ1 and 3607 cmÀ1. The
band at 3710 cmÀ1 is typical of residual surface hydroxyls
in interaction with perhydrocarbyl groups, while that at
3607 cmÀ1 is attributed to other silanol groups in interac-
tion with the aryloxy ligands of the grafted complex [19].
The free silanols do not reappear after washing of the unre-
acted complex with pentane; furthermore, the appearance
of m(CH) and d(CH) bands, and the presence of tBuCH3
evolved in the gas phase support a chemical grafting of
the tungsten complex on silica.
Aerosil silica from Degussa (specific area of 200 m2 gÀ1),
was partially dehydroxylated at 700 °C under high vacuum
(10À5 Torr) during 15 h. The resulting solid had a specific
surface area of 180 m2 gÀ1 and contained 0.7 OH nmÀ2
as determined by reaction with CH3Li.
2.2. Preparation of 2 by impregnation of 1 onto SiO2-(700)
A mixture of [W(„C–tBu)(CH2–tBu)(OAr)2] (Ar = 2,6-
iPrC6H3) (1) (300 mg, 0.443 mmol) in pentane (5 mL) and
SiO2-(700). One gram was stirred at 25 °C during 5 h. After
filtration, the solid was washed five times with pentane and
all volatile compounds were condensed into another reac-
tor (of known volume) in order to quantify the neopentane
evolved during the grafting reaction. The resulting yellow
powder was dried under vacuum (10À5 Torr) to yield
1.094 g of 2. Analysis by gas chromatography indicated
the formation of 150 lmol of neopentane during the graft-
ing reaction (0.85 0.1 tBuCH3/W). Elemental analysis of
2: W, 3.05 wt% and C, 5.52 wt%.
3.0
2.0
1.0
2.3. Preparation of 4 by impregnation of 3 onto SiO2-(700)
A mixture of 3, [W(„C–tBu)(CH2–tBu)3] (115 mg,
0.25 mmol) in pentane (5 mL) and SiO2-(700). 0.8 g was stir-
red at 25 °C for 2 h. After filtration, the solid was washed
five times with pentane and all volatile compounds were
condensed into another reactor (of known volume) in order
to quantify the neopentane evolved during grafting. The
resulting brown powder was dried under vacuum
(10À5 Torr) to yield 0.84 g of 4. Analysis by gas chromatog-
raphy indicated the formation of 108 lmol of neopentane
during the grafting (0.9 0.1 tBuCH3/W). Elemental anal-
ysis of 2: W, 2.2 wt% and C, 2.2 wt%.
b)
0.0
a)
3400
2600
Wavenumbers (cm )
1800
-1
Fig. 1. Infrared spectra of the grafting reaction of [W(„C-tBu)(CH2-
tBu)(OAr)2] (Ar = 2,6-iPr2C6H3) (1) onto SiO2-(700) by impregnation
method: (a) silica partially dehydroxylated at 700 °C for 15 h; (b) same
sample after impregnation of 1 at 25 °C for 4 h followed by washing in
pentane and drying under vacuum.

N. Merle et al. / Journal of Organometallic Chemistry 693 (2008) 1733–1737
1735
To establish the mass balance, the reaction was carried
out by impregnation on larger quantities of silica with a
pentane solution of 1 (Table S1, entries 1, 2, 3 and 4, Sup-
plementary material). The resulting yellow solid contained
1.9 and 3.63 wt% of W and C, respectively, corresponding
to a C/W ratio of 29.1. These data are consistent with the
removal of one neopentyl group per grafted W and forma-
tion of a monografted surface complex as a major species,
which can be tentatively formulated as [(„SiO)(W„C–
tBu)(OAr)2] (2), (expected C/W ratio: 29) (Scheme 1).
Moreover, quantitative GC analyses revealed that
0.9 equiv. of 2,2-dimethylpropane per grafted W evolved
during the grafting reaction, which is in agreement with
the cleavage of about one neopentyl group in 1 and forma-
tion of one Si–O–W bond, as already observed with other
molecular complexes [15,20–22]. The W loading is rela-
tively low but it can be increased by increasing the reaction
time. Thus, the behaviour of complex 1 is completely differ-
ent from that of the perhydrocarbyl complexes of Ti, Zr,
Hf or Ta, for which the grafting reaction takes place within
30 min to 1 h under similar conditions [20–23]. This obser-
vation is consistent with the presence in 1 of bulky aryloxy
groups around the metal which prevents efficient protonol-
ysis of the W–C bond by a surface silanol. Therefore, while
SiO2-(700) contains 0.26 mmol of surface hydroxyl groups
per gram, the W elemental analysis shows that only
0.10 mmol (around 40%) are consumed during the grafting
reaction. This observation is in agreement with the pres-
ence of residual silanols, as observed by IR spectroscopy
(vide supra). Note that the grafted organometallic frag-
ment {W(„C–tBu)(OAr)2} has a projected surface area
Fig. 2. (a) 1H MAS NMR spectrum of 2. The spectrum was recorded with
eight scans and a relaxation delay of 2 s. (b) 13C CP-MAS NMR spectrum
of 2. The spectrum was recorded with 50000 scans, a relaxation delay of
2 s and a CP contact time of 2 ms. An exponential line broadening of
80 Hz was applied before Fourier transformation.
2
˚
of about 100 A , which would afford a maximum W load-
ing of 5.9 wt%.
1
Solid 2 was further characterized by solid-state H and
13C NMR spectroscopy. The H MAS NMR spectrum of
1
and methyl groups, respectively) and to the carbynic t-
butyl moiety (51 and 31 ppm for the quaternary carbon
and the methyl groups, respectively). Unfortunately,
despite extended measurement time, it has not be possible
to detect the resonance of the carbynic atom (a resonance
should be expected at ca. 300 ppm), probably due to the
fact that, (i) it is not surrounded by proton and, (ii) the sig-
nal is probably relatively broad due to the interaction with
the metal. However the observation of the signal at 51 ppm
(52.11 ppm in 1) and the absence of any signal of a methy-
2 (Fig. 2a) displays various signals assigned to the aromatic
protons (d = 6.9 ppm), the isopropyl methyne protons
(d = 3.5 ppm), and the methyl groups of the isopropyl
and neopentylidyne groups (d = 1.0 ppm, with a shoulder
at 0.8 ppm). The 13C CP-MAS spectrum shows seven
resolved peaks (Fig. 2b), which can be assigned to the aro-
matic carbons (163, 136 and 122 ppm for the ipso, ortho
and undistinguished meta and para carbons, respectively),
to the isopropyl groups (27 and 21 ppm, for the methyne
O
O
W
H
O
O
Si
O
O
pentane, 5 h, 25 °C
O
O
Si
+
O
O
O
Si
Si
O
W
-tBuCH3
O
O
O
O
O
O
SiO2-(700)
Surface silanols
1
2
Scheme 1.

1736
N. Merle et al. / Journal of Organometallic Chemistry 693 (2008) 1733–1737
O
O
W
t
Bu
O
+
O
O
Si
Si
O
O
O
O
O
O
O
O
W
W
N
t
Bu
C
N
O
O
+
O
O
O
Si
O
Si
Si
Si
O
O
O
O
O
O
O
O
O
O
W
t
Bu
2
O
+
O
O
Si
Si
O
O
O
O
Scheme 2.
lenic carbon (like in 1) at ca. 74 ppm support the above
proposed structure [18].
W
2
+
80 °C
Furthermore, these assignments were confirmed by
1H–13C 2D HETCOR NMR (see Fig. S1 Supplementary
material) which gave correlation cross-peaks (dH [ppm]
Scheme 3.
1
in F1; dC [ppm] in F2) between H and 13C spectra for
However when pent-2-yne (80 Torr, 61 equiv. per W atom)
was contacted with the above material (1.63 mol%) at
80 °C in a batch reactor, but-2-yne and hex-3-yne were
formed in a 1/1 ratio and the thermodynamic equilibrium
(corresponding to 30% conversion of pent-2-yne) was
reached after 140 min (Scheme 3 and Fig. S2 in Supplemen-
tary material). The initial turnover frequency was
0:32 mol mol WÀ1 minÀ1. Interestingly, no oligomerization
of the alkyne substrate or of the reaction products was evi-
denced either by GC after extraction of the solid or via the
mass balance. This behaviour is different from that of sim-
ilar homogeneous catalysts for which formation of heavy
products is problematic [18].
The surface species 4, [(„SiO)W(„C–tBu) (CH2–tBu)2]
[15] prepared by grafting reaction of W(„C–tBu)(CH2-
tBu)3 (3), on silica dehydroxylated at 700 °C was fully inac-
tive in the same reaction conditions, even if the initiation
products 4,4 dimethylpent-2-yne (Me–C„C–tBu) and 2,2
dimethyl hex-3-yne (Et–C„C–tBu) (0.45 equiv. per W)
were observed as in the case of species 2. The substitution
of the two aryloxy groups in complex 2 by two neopentyl
ligands in complex 4, allowed probably a a-hydrogen
migration from the neopentyl to form bis-carbene species
[16,17] which has been proved to be active in olefin metath-
esis [15] but led to fast deactivation of the catalyst in alkyne
metathesis reaction.
the methyl (CH(CH3)2) (1.0; 21) and („CC(CH3)3) (1.0;
31), the isopropyl (CH(CH3)2) (3.5; 27), and the aromatic
groups (CH-para, CH-meta) (6.9; 122). Accordingly, no
correlation was observed for the peaks centered at 51 and
163 ppm, thus confirming their respective assignments as
tert-butyl quaternary and aromatic ipso carbons.
We have then studied the stoichiometric and catalytic
reactivities of 2. Treatment of 2 with acetonitrile (10 equiv.)
in gas phase at 80 °C led to the evolution of 0.8 equiv. of
4,4 dimethylpent-2-yne (GC–MS) per grafted tungsten
and to a red solid tentatively formulated as [(„Si–
O)W(„N)(OAr)2] (Scheme 2), on the basis of the known
reactivity of molecular analogues [18,24]. Further charac-
terizations of this complex are in progress and will be
reported in a forthcoming paper.
High oxidation state carbynic group 6 derivatives are
known to catalyze the metathesis of internal alkynes
[9,25]. Thus, the reaction of 2 with an excess of pent-2-
yne was performed in a batch reactor at room temperature.
After reaction the solid turned to orange and 0.65 equiv.
per tungsten of a 1/1 mixture of 4,4 dimethylpent-2-yne
(Me–C„C–tBu) and 2,2 dimethylhex-3-yne (Et–C„C–
tBu) had evolved. These products arise from the interaction
between pent-2-yne and the carbyne ligand, namely the ini-
tiation step for alkyne metathesis. The 1/1 ratio is consis-
tent with
a comparable stability of both possible
4. Conclusions
metallacyclobutadiene intermediates [26], which is not
influenced by 1,2- or 1,3-interactions due to the planarity
of the cycle, at the difference of alkene metathesis [27].
The above results show that at room temperature only a
stoichiometric reaction is observed with pent-2-yne.
According to elemental and mass balance analyses, and
to IR and solid state NMR spectroscopies, complex 1 reacts
with SiO2-(700) leading to [(„SiO)W(„C–tBu)(OAr)2] (2),

N. Merle et al. / Journal of Organometallic Chemistry 693 (2008) 1733–1737
1737
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as the major surface species. This surface complex exhibits
high catalytic activity and good stability in metathesis of
pent-2-yne in contrast to [(„SiO)W(„C–tBu)(CH2–tBu)2]
(4), probably because deactivation by a-hydrogen migration
is prevented. The system is also resistant to alkyne polymer-
ization, which is problematic for similar homogeneous cat-
alysts. In addition, the surface species 2 reacts with
acetonitrile, a reaction which can be considered as the first
step of nitrile-alkyne cross-metathesis [28].
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Appendix A. Supplementary material
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Basset, G. Sunley, B.M. Maunders, Organometallics 25 (2006) 1569–
1577.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/
j.jorganchem.2008.02.020.
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