Surface versus molecular siloxy ligands in well-defined olefin metathesis catalysts: [{(RO)3SiO}Mo(=NAr)(=CHtBu)(CH2tBu)]

Article abstract of DOI:10.1002/anie.200503205

Surface organometallic chemistry: Similar electronic properties of molecular and surface siloxy ligands in 1 m and 1 were revealed by a structural investigation with solid-state NMR spectroscopy and a reactivity study in olefin metathesis. Despite similar initial turnover frequencies, the silica-supported catalyst 1 is more stable than 1 m and allows it to achieve higher turnovers, thus showing the advantage of site isolation on surfaces. (Figure Presented)

Full text of DOI:10.1002/anie.200503205

Angewandte
Chemie
DOI: 10.1002/anie.200503205
Communications
A well-defined Mo alkylidene com-
plex immobilized on silica, fully character-
ized by solid-state NMR spectroscopy, leads to better performances in
olefin metathesis than the corresponding nonsupported siloxy-con-
taining catalysts, probably because of the isolation of the active sites on
a supported system, which prevents decomposition through dimeriza-
tion. For more details, see the Communication by C. Copꢀret and co-
workerson the following pages.
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Alkene Metathesis
tion of a Mo alkylidene complex requires the use of well-
defined Mo alkyl alkylidene molecular precursors, which have
=
=
DOI: 10.1002/anie.200503205
become available only recently as [Mo( NAr)( CHtBu)-
(CH2tBu)2] (2; Ar= 2,6-diisopropylphenyl).[17–19] Since the
surface of silica partially dehydroxylated at 7008C (SiO2-(700)
Surface versus Molecular Siloxy Ligands in
Well-Defined Olefin Metathesis Catalysts:
)
can be compared to a bulky monodentate ligand,[16] we
investigated the reaction of 2 with SiO2-(700) in order to
generate a well-defined supported olefin metathesis catalyst.
When immersed in a solution of 2 in pentane, a SiO2-(700)
disk immediately turned yellow. After 2 h, the disk was
washed three times with pentane, dried under vacuum for 1 h
at 258C, and analyzed by IR spectroscopy. The IR peaks
associated with the surface silanol groups at 3747 cmÀ1 had
=
=
[{(RO)3SiO}Mo( NAr)( CHtBu)(CH2tBu)]**
FrØdØric Blanc, Christophe CopØret,*
Jean Thivolle-Cazat, Jean-Marie Basset,* Anne Lesage,
Lyndon Emsley,* AmritanshuSinha, and
Richard R. Schrock*
À
disappeared while new IR bands associated with n(C H) and
Dedicated to Professor Ei-ichi Negishi
on the occasion of his 70th birthday
À
d(C H) of hydrocarbyl ligands appeared in the regions of
3000–2700 and 1500–1350 cmÀ1, respectively (see Supporting
Information). Furthermore, the IR bands at 3065 and
We report herein the synthesis and characterization of
3027 cmÀ1 are attributed to n( C H) of the arylimido
= À
ꢀ
=
=
[( SiO)Mo( NAr)( CHtBu)(CH2tBu)] (1), a well-defined
heterogeneous catalyst for olefin metathesis, and its molec-
ular equivalents 1m and 1n.
group. During these steps the surface silanol groups reacted,
as their corresponding IR bands at 3747 cmÀ1 did not
reappear. Preparation of larger quantities of this solid was
performed by using calibrated silica (40–60 mesh) partially
dehydroxylated at 7008C. When a mixture of SiO2-(700)
(0.26 mmolgÀ1 SiOH) and 2 in pentane was stirred for 2 h at
258C, surface organometallic complex 1 was formed together
with 1 equivalent of 2,2-dimethylpropane (Scheme 1). The
We recently reported the preparation and characteriza-
tion of highly active heterogeneous olefin metathesis catalysts
[1–3]
ꢀ
ꢀ
=
[( SiO)Re( CtBu)( CHtBu)(CH2tBu)].
This system has
shown better activity than the best Re-based homogeneous
and heterogeneous catalysts and comparable initial activities
to Mo- and W-based homogeneous olefin metathesis catalysts.
In homogeneous catalysis, well-defined Mo complexes such as
[4–7]
=
=
[(RF6O)2Mo( NAr)( CHtBu)]
(RF6 = (CF3)2(CH3)C) are
ꢀ
much better catalysts than their Re equivalents [(RF6O)2Re(
CtBu)( CHtBu)].[8,9] However, preparing a supported equiv-
=
=
=
alent to [(RF6O)2Mo( NAr)( CHtBu)] has remained a chal-
lenge.[10–15] In surface organometallic chemistry,[16] the forma-
[*] F. Blanc, Dr. C. CopØret, Dr. J. Thivolle-Cazat, Dr. J.-M. Basset
Laboratoire de Chimie OrganomØtallique de Surface
UMR9986 CNRS—ESCPE Lyon
43 bd du 11 Novembre 1918, 69616 Villeurbanne Cedex (France)
Fax: (+33)4-7243-1795
E-mail: coperet@cpe.fr
basset@cpe.fr
Dr. A. Lesage, Prof. Dr. L. Emsley
Laboratoire de Chimie, UMR5532 CNRS—ENS Lyon
Laboratoire de Recherche ConventionnØ du CEA (23V/DSM 0432)
Ecole Normale SupØrieure de Lyon
46 AllØe d’Italie, 69364 Lyon Cedex 07 (France)
Fax: (+33)4-7272-8483
E-mail: Lyndon.Emsley@ens-lyon.fr
Scheme 1. Formation of surface complex 1 and its homogeneous
analogues 1m and 1n (R’=c-C5H9).
A. Sinha, Prof. Dr. R. R. Schrock
Department of Chemistry
Massachussetts Institute of Technology
Cambridge, MA 02139 (USA)
Fax: (+1)617-2537670
yellow solid thus obtained contains 1.7–2.1 wt% of Mo, which
corresponds to 0.17–0.21 mmol of Mo per gram of solids, in
agreement with consumption of most of the surface silanol
groups (70–80%). Moreover, the materials contain on
average 23 Æ 3C and 1.0 Æ 0.3N per grafted Mo (as obtained
from elemental analysis), consistent with the proposed
structure 1, for which 22C/Mo and 1N/Mo are expected.
Furthermore, the 1H magic-angle spinning (MAS) solid-state
NMR spectrum of 1 (Figure 1a) displays distinct signals at d =
E-mail: rrs@mit.edu
[**] F.B. is grateful to the French Ministry of Education, Research, and
Technology (MENRT) for a predoctoral fellowship. We thank B.
Fenet for recording solution-state 29Si{1H} NMRspectra. We are
also indebted to the CNRS, ESCPE Lyon, and the National Science
Foundation (R.R.S.) for financial support.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
=
À
11.7 ( CHtBu, 1H), 6.8 (Csp2 H, 3H), 3.5 (CHMe2, 2H), and
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Communications
=
(Cpara), 126 (Cmeta), 145 (Cortho), 153 (Cipso), and 279 ppm (
CHtBu); the signal of the alkylidene carbon atom at d =
ꢀ
=
=
279 ppm is barely observable. [( SiO)Mo( NAr)( *CHtBu)-
(*CH2tBu)] (1*), 33% 13C-labeled on the carbon atom a to
Mo, unambiguously displays signals for both the methine
=
À
(Mo CHtBu) and methylene carbon atoms (Mo CH2tBu) at
d = 279 and 56 ppm, respectively (Figure 2a). Furthermore,
using the 2D H–13C dipolar HETCOR NMR method[21,22]
1
(see Figure 2b and Supporting Information) with different
contact times (1 and 5 ms) allowed resolution and assignment
of all signals of the 13C spectrum (see Supporting Informa-
tion).[22,23] The carbene ( CHtBu) and methylene (CH2tBu)
=
signals gave correlations (dH [ppm] in F1; dC [ppm] in F2) at
(11.7; 279) and (1.9; 56), respectively. A cross-peak is
observed between the proton at d = 3.5 ppm (CHMe2) and
the carbon atom at d = 29 ppm, showing that CHMe2 (not
observed before) also appears at this chemical shift.
The corresponding molecular complexes 1m and 1n, with
a molecular monosiloxy equivalent, were prepared by using
(RO)3SiOH in place of a siloxy group of the silica surface
(Scheme 1).[24–26] The reaction of 2 with 1 equivalent of [(c-
C5H9)7Si7O12SiOH] in C6D6 or pentane for 2 h at 258C
= =
[(c-C5H9)7Si7O12SiO)Mo( NAr)(
quantitatively
gave
CHtBu)(CH2tBu)] (1m) and tBuCH3. The closeness of the
NMR data of 1m to those of 1 fully supports the assignments
(see Supporting Information). Complex 1m shows resonances
at d = 12.05 and 281.2 ppm (1JCH = 110 Hz) for the alkylidene
1
ligand in the H and 13C NMR spectra, respectively, which is
Figure 1. Solid-state NMRspectra of 1. a) 1H NMRspectrum
acquired at a MAS frequency of nR =12.5 kHz with eight scans and
a recycle delay of 2 s. The 1H rf field was 83 kHz. b) 2D 1H–1H
NMRspectrum recorded at a MAS frequency of nR =10.0 kHz with
the DUMBO-1 homonuclear dipolar decoupling sequence[20] in t1.
c) Trace parallel to F1 extracted along F2 at dH =1.9 ppm.
0.9 ppm (CH3, 30H). The signal of the methylene
protons in the position a to Mo is only observed
as a shoulder centered around d = 1.9 ppm. The
presence of relatively sharp signals for a solid-
1
state NMR spectrum (Dn= = 390 Hz) shows that
2
1H–1H dipolar coupling is weak, the ligands
undergo rapid restricted motion, and the species
1
are dispersed. A 2D H–1H homonuclear corre-
lation NMR spectrum recorded with the
DUMBO-1 homonuclear dipolar decoupling
sequence[20] in t1 resulted in a slight improvement
1
1
in the line width of the H NMR spectrum (Dn = =
2
260 Hz; Figure 1b), in contrast to the much larger
effect that is observed for bulk materials. This
results, however, in a better resolution of the
signal at d = 1.9 ppm (Figure 1c).
Figure 2. Solid-state NMRspectra acquired at a MAS frequency of nR =12.5 kHz.
a) 13C CP MAS NMRspectrum of 1* recorded with TPPM-15[31] 1H decoupling at
nH1 =83 kHz with 56000 scans. The recycle delay was set to 1 s, and the contact time
for the CP step was 1 ms. An exponential line broadening of 80 Hz was applied
before Fourier transformation. b) 2D 1H–13C dipolar HETCORsolid-state NMR
spectrum of 1* with a contact time of 1 ms (short-range HETCOR). A total of 32 t1
were collected with 4096 scans each. Left: Projection along F2. Asterisks denote
spinning side bands.
The 13C cross-polarization (CP) MAS spec-
trum contains 11 resolved peaks (see Figure 2a
and Supporting Information), which can be
tentatively assigned as follows: d = 22 (CH-
=
(CH3)2), 29 ( CHC(CH3)3), 31 (CH2C(CH3)3),
=
36 (CH2CMe3), 46 ( CHCMe3), 56 (CH2tBu), 122
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Chemie
consistent with a syn-alkylidene complex.[17] A similar reac-
tion carried out over 15 h between 2 and [(tBuO)3SiOH]
Table 1: Olefin metathesis activity at room temperature for 1, 1m, and
2.[a]
=
=
yielded [{(tBuO)3SiO}Mo( NAr)( CHtBu)(CH2tBu)] (1n),
which displayed similar NMR data and whose X-ray crystal
structure (not shown) showed it to be the expected syn
isomer.
Olefin
1
1m
TOF
[sÀ1 [b]
2
TOF
[sÀ1 [b]
t
t
TOF
[sÀ1 [b]
t
]
[h][c]
]
[h][c]
]
[h][c]
1
1-Octene
Ethyl oleate
0.06
0.04
=
6
0.06
0.03
1
0.02
0.005
48[d]
[e]
When 1 is contacted with 1350 equivalents of propene at
258C (batch reactor), equilibrium is reached within 20 min
(around 30% conversion, E/Z-butene ratio = 2.6) with a
1
24
–
[a] Experimental conditions: 1% Mo, 258C, 0.5m solution of olefin in
toluene under Ar atmosphere. All reactions were monitored by gas
chromatography. [b] Initial TOF measured after 5 min of reaction and
expressed in moles of substrate per mole of Mo per second. [c] Time to
reach equilibrium (around 50% conversion). [d] 40% conversion.
[e] Deactivation after 2% conversion.
turnover frequency (TOF) of 1.0 mol(molMo
)
À1 sÀ1, which is
ꢀ
ꢀ
=
four times faster than with [( SiO)Re( CtBu)( CHtBu)-
(CH2tBu)] (TOF = 0.25 mol(molRe)
À1 sÀ1).[1,2] Moreover,
roughly 0.7 equivalents of a 2.7:1 mixture of 3,3-dimethylbu-
tene and 4,4-dimethyl-2-pentene is formed, as previously
observed for [( SiO)Re( CtBu)( CHtBu)(CH2tBu)],[1,2] and
shows that initiation is almost quantitative, in agreement with
a well-defined system. Moreover, the ratio of cross-metathesis
products is in full agreement with the following model: the
favored pathway involves reaction intermediates that mini-
mize the interaction between the alkyl substituents
(Scheme 2).[27]
longer lifetime under catalytic conditions, which shows that
the effect of active-site isolation prevents some deactivation
pathways such as dimerization of reactive intermediates.[30]
Moreover, the formation of about 0.7 equivalents of cross-
metathesis product shows that most of the surface complex
generates the active species, in agreement with a well-defined
heterogeneous catalyst.
ꢀ
ꢀ
=
Experimental Section
See Supporting Information for full experimental procedures, the IR
and 13C CP MAS NMR spectra of 1, and the 13C CP MAS NMR
spectra, 2D 1H–13C dipolar HETCOR spectra with 1- and 5-ms
contact times, and a full assignment of 1*.
Received: September 8, 2005
Published online: January 16, 2006
Keywords: carbene ligands · heterogeneous catalysis ·
.
metathesis · molybdenum
[1] M. Chabanas, A. Baudouin, C. CopØret, J.-M. Basset, J. Am.
Chem. Soc. 2001, 123, 2062.
[2] C. CopØret, New J. Chem. 2004, 28, 1.
[3] X. Solans-Monfort, E. Clot, C. CopØret, O. Eisenstein, J. Am.
Chem. Soc. 2005, 127, 14015.
[4] R. R. Schrock, Acc. Chem. Res. 1986, 19, 342.
[5] J. S. Murdzek, R. R. Schrock, Organometallics 1987, 6, 1373.
[6] R. R. Schrock, Chem. Rev. 2002, 102, 145.
Scheme 2. Application of the metallacyclobutane model propene
metathesis to explain the favored k1,3 pathway.
Moreover, the self-metathesis of 0.5m solutions of 1-
octene and ethyl oleate in toluene in the presence of 1% of 1
gives the equilibrated mixture in 10 and 60 min, respectively
(Table 1). The respective initial TOFs are 0.06 and 0.04 mol
[7] F. J. Feher, T. L. Tajima, J. Am. Chem. Soc. 1994, 116, 2145.
[8] R. Toreki, R. R. Schrock, J. Am. Chem. Soc. 1990, 112, 2448.
[9] R. Toreki, G. A. Vaughan, R. R. Schrock, W. M. Davis, J. Am.
Chem. Soc. 1993, 115, 127.
´
(molMo
)
À1 sÀ1, which is very close to those obtained for
[10] W. A. Herrmann, A. W. Stumpt, T. Priermeier, S. Bogdanovic, V.
Dufaud, J.-M. Basset, Angew. Chem. 1996, 108, 2978; Angew.
Chem. Int. Ed. Engl. 1996, 36, 2803.
[28]
ꢀ
ꢀ
=
[( SiO)Re( CtBu)( CHtBu)(CH2tBu)].
The initial rates
with the corresponding molecular complex 1m are similar,
but the reaction times needed to reach the same conversion
(thermodynamic equilibrium) are much longer, which shows
that, in this case, decomposition is faster. On the other hand,
the quasi-inactivity of molecular complex 2 illustrates the
activating effect of the siloxyl group.[3,29]
In conclusion, we have shown that the molecular and
surface complexes 1 and 1m are electronically very similar in
terms of both their comparable NMR data and initial TOFs in
olefin metathesis. Nonetheless, the supported catalyst has a
[11] F. Blanc, M. Chabanas, C. CopØret, B. Fenet, E. Herdtweck, J.
Organometal. Chem. 2005, 690, 5014.
[12] K. C. Hultzsch, J. A. Jernelius, A. H. Hoveyda, R. R. Schrock,
Angew. Chem. 2002, 114, 609; Angew. Chem. Int. Ed. 2002, 41,
589.
[13] R. M. Kroell, N. Schuler, S. Lubbad, M. R. Buchmeiser, Chem.
Commun. 2003, 2742.
[14] S. J. Dolman, K. C. Hultzsch, F. Pezet, X. Teng, A. H. Hoveyda,
R. R. Schrock, J. Am. Chem. Soc. 2004, 126, 10945.
[15] M. Mayr, D. Wang, R. Kroell, N. Schuler, S. Pruehs, A.
Fuerstner, M. R. Buchmeiser, Adv. Synth. Catal. 2005, 347, 484.
Angew. Chem. Int. Ed. 2006, 45, 1216 –1220
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www.angewandte.org
1219

Communications
[16] C. CopØret, M. Chabanas, R. Petroff Saint-Arroman, J.-M.
Basset, Angew. Chem. 2003, 115, 164; Angew. Chem. Int. Ed.
2003, 42, 156.
[17] A. Sinha, R. R. Schrock, Organometallics 2004, 23, 1643.
[18] D. Ehrenfeld, J. Kress, B. D. Moore, J. A. Osborn, G. Schoettel, J.
Chem. Soc. Chem. Commun. 1987, 129.
[19] G. Schoettel, J. Kress, J. A. Osborn, J. Chem. Soc. Chem.
Commun. 1989, 1062.
[20] A. Lesage, D. Sakellariou, S. Hediger, B. Elena, P. Charmont, S.
Steuernagel, L. Emsley, J. Magn. Reson. 2003, 163, 105.
[21] S. Hediger, A. Lesage, L. Emsley, Macromolecules 2002, 35,
5078.
[22] M. Chabanas, A. Baudouin, C. CopØret, J.-M. Basset, W. Lukens,
A. Lesage, S. Hediger, L. Emsley, J. Am. Chem. Soc. 2003, 125,
492.
[23] R. Petroff Saint-Arroman, M. Chabanas, A. Baudouin, C.
CopØret, J.-M. Basset, A. Lesage, L. Emsley, J. Am. Chem.
Soc. 2001, 123, 3820.
[24] F. J. Feher, T. A. Budzichowski, Polyhedron 1995, 14, 3239.
[25] H. C. L. Abbenhuis, Chem. Eur. J. 2000, 6, 25.
[26] R. Duchateau, Chem. Rev. 2002, 102, 3525.
[27] J. L. Bilhou, J. M. Basset, R. Mutin, W. F. Graydon, J. Am. Chem.
Soc. 1977, 99, 4083.
[28] M. Chabanas, C. CopØret, J.-M. Basset, Chem. Eur. J. 2003, 9,
971.
[29] D. G. H. Ballard, Adv. Catal. 1973, 23, 263.
[30] L. P. H. Lopez, R. R. Schrock, J. Am. Chem. Soc. 2004, 126, 9526.
[31] S. Hediger, B. H. Meier, N. D. Kurur, G. Bodenhausen, R. R.
Ernst, Chem. Phys. Lett. 1994, 223, 283.
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