Alkane metathesis catalyzed by a well-defined silica-supported Mo imido alkylidene complex: [(≡SiO)Mo(=NAr)(=CHtBu)(CH2tBu)]

Article abstract of DOI:10.1002/anie.200602171

A two-functions-in-one catalyst precursor allows an olefin metathesis catalyst to be turned into an alkane metathesis catalyst. A silica-supported alkyl alkylidene molybdenum complex containing an ancillary imido ligand is a highly active olefin metathesis catalyst and also acts as a catalyst precursor for alkane metathesis. This system involves a single metal with dual properties and shows that ancillary ligands are compatible with alkane metathesis reactions. (Chemical Equation Presented).

Full text of DOI:10.1002/anie.200602171

Angewandte
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metathesis (interaction of an olefin and a carbene) was the
key carbon–carbon bond-cleavage and formation process and
that alkylidene hydrides are critical intermediates
[
4–7]
[8,9]
(Scheme 1).
Other systems have been reported
that
Scheme 1. Proposed active sites in alkane metathesis and possible
catalyst precursor 1a.
are based on a combination of dehydrogenation/hydrogena-
tion and olefin metathesis catalysts, with the most recent
[
9]
[10–13]
example using a mixture of homogenous catalysts.
The
[
14]
best catalysts prepared by SOMC are based on very simple
monoatomic systems: hydrides, alkyl alkylidenes, or alkyl
alkylidynes of Group 5 or 6 metals supported on oxides.
Although introducing ligands that play a key role in control-
ling the reactivity and the selectivity of catalysts has been a
long-term goal, all ancillary ligands introduced so far
Surface Organometallic Chemistry
DOI: 10.1002/anie.200602171
Alkane Metathesis Catalyzed by a Well-Defined
Silica-Supported Mo Imido Alkylidene Complex:
(ꢀSiO)Mo(=NAr)(=CHtBu)(CH tBu)]**
(
alkylidyne, pentamethylcyclopentadiene (Cp*), Cl) have
[5]
been detrimental to catalyst performances. Herein, we
report that a well-defined silica-supported alkyl alkylidene/
Mo complexcontaining an imido ligand ( 1a)
[
2
[
15]
is a good
[
5,6]
catalyst precursor for alkane metathesis (Scheme 1).
FrØdØric Blanc, Christophe CopØret,* Jean Thivolle-
Cazat, and Jean-Marie Basset*
When propane (540 equiv) is brought into contact with 1a
at 1508C in a batch reactor, it is converted into mainly ethane
(56.1%), butane (35.3%), and small quantities of methane
The conversion of alkanes at low temperatures has gained
interest over the past few years because more efficient routes
to chemicals are necessary to optimize oil resources.
(0.1%), isobutane (2.7%), and pentanes (5.9%). These
selectivities are constant over time (see the Supporting
Information), and noteworthy the selectivity in methane,
[
1,2]
[
16]
Within this context, we reported in 1997, the reaction of
alkane metathesis, which catalytically transforms a given
alkane into its lower and higher homologues using a silica-
probably produced by hydrogenolysis,
observed for other Group 6 supported systems.
is low as already
[
7,17]
Moreover,
À1
the turnover frequency is constant for 40 h (ca. 1 TONh ;
TON = turnover number), and then there is a reproducible
decrease of conversion (Table 1 and Figure 1). Note that this
well-defined catalyst precursor achieves about the same TON
[
3]
supported tantalum hydride prepared by surface organo-
metallic chemistry (SOMC). Based on kinetic and structure–
reactivity relationship studies, we have shown that p-bond
(55) in 120 h as the original tantalum hydride catalyst
supported on silica (TON = 60) despite the presence of
ancillary ligands.
[
*] F. Blanc, Dr. C. CopØret, Dr. J. Thivolle-Cazat, Dr. J.-M. Basset
Laboratoire de Chimie OrganomØtallique de Surface
UMR 9986 CNRS–ESCPE Lyon
Besides propane metathesis products, 2,2-dimethylpro-
pane (tBuCH ÀH; 0.82 equiv), 2,2-dimethylbutane (tBuCH À
43 bd du 11 Novembre 1918, 69616 Villeurbanne Cedex (France)
2
2
Fax: (+33)472431795
E-mail: coperet@cpe.fr
basset@cpe.fr
Me; 0.34 equiv), and 2,2-dimethylpentane (tBuCH ÀEt;
2
0
.10 equiv) are formed within 120 h (Table 2), whereas no
tBuCH ÀPr is detected (< 0.01% if any). Firstly, 2,2-dime-
2
[
**] F.B. is grateful to the French Ministry of Education, Research, and
Technology (MENRT) for a predoctoral fellowship. We thank R. R.
Schrock and A. Sinha for providing a sample of [Mo(=NAr)-
thylpropane is probably formed by exchange of the neopentyl
by a propyl ligand, either directly through s-bond metathesis
or in two steps through CÀH activation on the neopentylidene
(
=CHtBu)(CH tBu) ]. We are also indebted to the CNRS and ESCPE
2 2
Lyon for financial support.
unit followed by a decomposition of the penta-coordinated
complexthrough an a-H abstraction to regenerate an
alkylidene alkyl/Mo center (1b, R = Et, R’ = tBu;
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2006, 45, 6201 –6203
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6201

Communications
Table 1: TONs and selectivities in the metathesis of various alkanes catalyzed by 1a in a batch reactor
after 120 h.
The selectivity of initiation products can
therefore be understood on the basis of the
model developed for olefin metathesis,
which is based on the minimization of
steric interactions in the metalla-cyclobu-
tane intermediates (or in their forma-
[
a]
[b]
Alkane
TON
Alkane selectivity
[
c]
[c]
[d]
[d]
C1
35.9
C2
n.a.
C3
C4
C5
C6
C7
[
e]
[f]
ethane
2
(
55
62.0
1.5/0.8
35.3/2.7
n.a./0.5
7.0/n.a.
n.o.
n.o.
0.6
n.o.
0.4)
[
18]
propane
butane
0.1
<0.1
1.0
56.1
11.1
35.9
n.a.
56.5
17.1
5.2/0.7
21.7/0.4
16.4/n.o
<0.1
2.0
tion); that is, by the relative position of
the substituents: typically substituents in the
(
9.9)
90
17.7)
3
7.7
[
1,3]-positions of the metalla-cyclobutane
(
intermediate generate less steric hindrance
than in the [1,2]-positions (Scheme 2). This
model is also consistent with the observed
product selectivity and explains the greater
selectivity of butane relative to that of
pentane when starting from 1c (R = R’ =
Et) and when the cross-metathesis of the
resulting propagating alkylidene hydride
isobutane
22.8
n.o.
(
0.7)
[
a] TON is expressed in mol of alkane transformed per mol of Mo. The values in parentheses are
conversions. [b] The selectivities are defined as the amount of product i over the total amount of
products (they do not significantly change with time and conversion). [c] Linear/branched alkanes.
[
d] Selectivity for the sum of all isomers. [e] n.a.=not applicable. [f] n.o.=not observed (<0.01% if
any).
Table 2: Cross-metathesis products in the metathesis of various alkanes
[
a]
catalyzed by 1 in a batch reactor after 120 h.
Alkane
tBuCH H tBuCH Me tBuCH Et tBuCH Pr tBuCH iPr
2 2 2 2 2
[
b]
ethane
propane
butane
1.01
0.82
0.88
0.35
0.34
0.10
0.20
n.o.
n.o.
n.o.
0.05
n.o.
n.o.
n.o.
n.o.
n.o.
0.10
0.14
0.04
isobutane 0.83
[a] The values given are equivalents of alkanes per Mo. [b] n.o. =not
observed (<0.01% if any).
Figure 1. Turnovers obtained (expressed in mol of propane converted
per mol of Mo) during the metathesis of propane catalyzed by 1a in a
batch reactor.
intermediate 2b (X = H, R = Et) and propene is involved.
Lower alkane homologues are formed using the same
elementary steps, that is, by exchanging ligands through CÀ
H activation on carbene intermediates and extrusion pro-
cesses (a-H abstraction). Although this mechanism is based
on a single-site mechanism, it is also possible to involve two
sites, one that generates the olefin and the other that carries
out the metathesis.
Scheme 2). Secondly, 2,2-dimethylbutane and 2,2-dimethyl-
pentane are probably obtained through cross-metathesis of
the neopentylidene ligand attached to the Mo center and a
propene moiety, which results from b-H transfer on the propyl
chain in 1b, followed by metathesis and subsequent similar
reverse steps (insertion of the olefin into a metal hydride and
exchange of the ligand by suc-
Butane is also converted efficiently into its lower and
higher homologues (TON = 90 in 120 h) and its cross-meta-
cessive CÀH activation on the
carbene ligand and a-H
abstraction). Note that the
alkane cross-metathesis prod-
ucts are obtained in a 3.4:1
ratio, which is very close to the
ratio of olefin cross-metathesis
products
3,3-dimethylbutene
and (E)-4,4-dimethyl-2-pen-
tene, obtained during the meta-
thesis of propene on 1a (that is,
2
.7:1), and this is consistent
with the proposed elementary
steps, and especially with olefin
metathesis being the key
carbon–carbon bond-formation
[
15]
step.
Scheme 2. Metathesis of propane catalyzed by 1.
6
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Angewandte
Chemie
thesis products (Table 1 and Table 2), whereas ethane is not.
This finding is in contrast to that observed for Ta-based
catalysts, for which an equimolar mixture of methane and
propane is obtained from ethane; however, it consistent with
the absence of methane in the metathesis of other alkanes
[4] C. CopØret, O. Maury, J. Thivolle-Cazat, J.-M. Basset, Angew.
Chem. 2001, 113, 2393 – 2296; Angew. Chem. Int. Ed. 2001, 40,
331 – 2234.
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[
5] E. Le Roux, M. Chabanas, A. Baudouin, A. de Mallmann, C.
CopØret, E. A. Quadrelli, J. Thivolle-Cazat, J.-M. Basset, W.
Lukens, A. Lesage, L. Emsley, G. J. Sunley, J. Am. Chem. Soc.
(
propane and butane). Finally, 1a does not catalyze the
2004, 126, 13391 – 13399.
metathesis of 2-methylpropane, a branched alkane, under the
same reaction conditions, despite the presence of a reasonable
amount of one of the initiation products. Therefore, the
propagation step is probably hindered by the olefin-meta-
thesis step, which would require a disfavored [2+2] cyclo-
addition between a disubstituted carbene and a disubstituted
olefin.
[6] J. M. Basset, C. CopØret, L. Lefort, B. M. Maunders, O. Maury,
E. Le Roux, G. Saggio, S. Soignier, D. Soulivong, G. J. Sunley, M.
Taoufik, J. Thivolle-Cazat, J. Am. Chem. Soc. 2005, 127, 8604 –
8605.
[
7] E. Le Roux, M. Taoufik, C. CopØret, A. de Mallmann, J.
Thivolle-Cazat, J.-M. Basset, B. M. Maunders, G. J. Sunley,
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2005, 44, 6755 – 6758.
In conclusion, the well-defined Mo/imido surface complex
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[
9] A. S. Goldman, A. H. Roy, Z. Huang, R. Ahuja, W. Schinski, M.
1
a belongs to a new type of silica-supported catalyst precursor
for the metathesis of linear alkanes at moderate temperatures
1508C). The product selectivity clearly shows that the
Brookhart, Science 2006, 312, 257 – 261.
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Chem. Commun. 1996, 2687 – 2688.
[
(
products are formed by p-bond metathesis as a key carbon–
carbon bond-breaking and -forming step. Nonetheless, this
system involves a single metal with dual properties in contrast
to the Goldman–Brookhart system, which relies on mixing
two catalysts. More importantly, this result shows for the first
time that ancillary ligands, an imido group in this case, can be
used. Further development of alkane metathesis catalysts
through the investigation of structure–reactivity relationships
are currently underway.
[
[
11] C. M. Jensen, Chem. Commun. 1999, 2443 – 2449.
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Experimental Section
General procedure: All experiments were carried out under dry
oxygen-free Ar using either high-vacuum lines (1.34 Pa) or glovebox
techniques. [(ꢀSiO)Mo(=NAr)
19] A. Sinha, R. R. Schrock, Organometallics 2004, 23, 1643 – 1645.
(
=CHtBu)(CH tBu)] (1a) was obtained according to a previously
2
[15]
reported procedure: [Mo(=NAr)
(
=CHtBu)(CH tBu) ]
[
19]
(Ar= 2,6-diisopropylphenyl) was impreg-
2
2
nated on silica that had been partially dehydroxylated at 7008C.
The alkanes were dried and deoxygenated before use by passing them
through a mixture of freshly regenerated molecular sieves (3 Š) and
R-3-15 catalysts (BASF).
Procedure for the metathesis reaction of alkanes in a batch
reactor: A mixture of catalyst precursor 1a (13.5 mmol) and dry
alkane (540–580 Torr, 500–540 equiv) were heated at 1508C in batch
reactor of known volume (220 mL). Aliquots were drawn and
analyzed by gas chromatography over time (HP5890 apparatus,
KCl/Al O3 on
a fused-silica column, 50 m ” 0.32 mm) and the
2
products identified by GC–mass-spectrometric analysis. The selectiv-
ities S are defined as the amount of product i over the total amount of
i
products.
Received: May 31, 2006
Published online: August 17, 2006
Keywords: alkanes · alkylidene ligands · imido ligands ·
.
metathesis · molybdenum
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[3] V. Vidal, A. ThØolier, J. Thivolle-Cazat, J.-M. Basset, Science
1997, 276, 99 – 102.
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