A highly active and reusable heterogeneous ruthenium catalyst for olefin metathesis

Article abstract of DOI:10.1002/adsc.200505066

The synthesis and characterization of Ru complexes of the type [L ₂X₂Ru=CHR] (L = PPh₃, PCy₃, N-heterocyclic carbenes or their derivates; X = Cl) grafted on mesoporous SBA-15 material are described. In this heterogeneous catalyst the Ru complexes are anchored in the pore channels of mesoporous silica material SBA-15 to prevent the decomposition of the catalytic species. Compared with formal organic-supported heterogeneous catalysts, this heterogeneous catalyst shows relatively high catalytic activity in olefin metathesis reactions and can be used several times without any decrease in catalytic activities.

Full text of DOI:10.1002/adsc.200505066

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DOI: 10.1002/adsc.200505066
A Highly Active and Reusable Heterogeneous Ruthenium
Catalyst for Olefin Metathesis
Liang Li, Jian-lin Shi*
State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese
Academy of Sciences, 1295 DingXi Road, Shanghai 200050, Peopleꢀs Republic of China
Fax: (þ86)-21-5241-3122; e-mail: jlshi@sunm.shcnc.ac.cn
Received: January 30, 2005; Revised: May 16, 2005; Accepted: June 22, 2005
Abstract: The synthesis and characterization of Ru
complexes of the type [L X Ru¼CHR] (L¼PPh ,
2
2
3
PCy , N-heterocyclic carbenes or their derivates;
3
X¼Cl) grafted on mesoporous SBA-15 material
are described. In this heterogeneous catalyst the
Ru complexes are anchored in the pore channels of
mesoporous silica material SBA-15 to prevent the
decomposition of the catalytic species. Compared
with formal organic-supported heterogeneous cata-
lysts, this heterogeneous catalyst shows relatively
high catalytic activity in olefin metathesis reactions
and can be used several times without any decrease
in catalytic activities.
Scheme 1. Immobilized ruthenium catalysts 1, 2 and 3.
[
3]
one. Barrett and co-workers immobilized the catalyst
on vinyl polystyrene, 2, but recycling and reuse of it
[
4]
cause a dramatic decrease in activity. Very recently,
Schürer and co-workers introduced the saturated N-het-
erocyclic carbene ligands to attach ruthenium catalysts
onto polymer 3, unfortunately, its reaction activity was
Keywords: heterogeneous catalysis; immobilization;
mesoporous material; N-heterocyclic carbenes; ring-
closing metathesis; ruthenium
[
5]
also severely deteriorated upon reuse.
Through a study of the mechanism, Grubbs and Ul-
man have established that there was a slow but compet-
ing decomposition of the ruthenium catalytic species via
[6]
a bimolecular pathway during the catalytic reaction.
Based on this proposal we prepared complex 7 (shown
Ruthenium complexes of the type [L X Ru¼CHR] (L¼ in Scheme 2) and anchored it in the pore channels of in-
2
2
PPh , PCy , N-heterocyclic carbenes or their derivates; organic mesoporous silica material SBA-15 to prevent
3
3
X¼Cl) are highly efficient catalysts for olefin metathe- decomposition of the catalytic species.
sis. On account of their unique properties they are in-
Compared to polymers, inorganic silica material is a
creasingly used in organic synthesis as a homogeneous common support for the heterogenization of molecular
catalyst since being discovered by Grubbs and co-work- catalysts due to its excellent thermal and chemical sta-
[1]
ers. This precious catalyst exhibits high activity and se- bility. It has a rigid structure and does not swell in sol-
lectivity under relatively mild conditions giving few by- vent, and can be used at both high and low temperatures
[
7–10]
products, but it is difficult to separate from the mixtures and at high pressure.
SBA-15, a kind of inorganic
after reaction. The residual ruthenium complex may mesoporous silica material with large specific surface
2
cause problems such as olefin isomerization, decompo- area (690–1040 m /g), well-defined, tunable pore sizes
[11]
sition of the product, and increased toxicity of the final (47–300 Š) and thick framework walls (31–64 Š)
,
material. Therefore, it is necessary to immobilize it on provides a good opportunity for the immobilization of
solid materials to combine the advantages of conven- large catalytic species, and also for the promotion of cat-
tional heterogeneous catalysts with the versatility of ho- alytic conversions of bulky organic reactants.
mogeneous ones. Up to now, different approaches have
been developed in the heterogenization of this kind of cording to the literature using tri-block poly(ethylene
The mesoporous silica SBA-15 was synthesized ac-
[
2]
catalysts in organic materials. Earlier attempts by oxide)-poly(propylene oxide)-poly(ethylene oxide),
[11]
Grubbs and Nguyen to prepare polystyrene-supported EO PO EO , as a template in acidic conditions. Be-
2
0
70
20
catalyst 1 (see Scheme 1) achieved only limited success; fore being functionalized with the Ru complex, the out-
it was found to be less reactive than the homogeneous er surface of SBA-15 was modified in advance with Si-
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Liang Li, Jian-lin Shi
tion, meanwhile the characteristic peaks at 1498–
ꢀ
1
ꢀ1
1
660 cm and 750 cm , which could be related to the
CꢀC vibration of benzene and PꢀC vibration of PCy ,
3
respectively, appeared, indicating that complex 7 had
been loaded onto the support.
After the immobilization the hexagonal pore struc-
ture of SBA-15 remained intact. Figure 2 shows an
HR-TEM image of the sample after immobilization tak-
en with the electron beam parallel to pore channels. The
mesoporous channels are well ordered with a character-
istic hexagonal structure as suggested by the electron
diffraction pattern. The simultaneous EDS analysis, as
showing in Figure 3, indicates that Ru element exists in-
Scheme 2. Synthesis and immobilization of ruthenium car- side the pore channels of SBA-15. As the high surface
bene complexes.
area of SBA-15 material is mainly contributed by its
open pore system and the outer surface of the material
has been pre-coated with SiꢀCH groups, therefore, all
3
CH groups to prevent any deposition of the Ru complex of the catalytic functional groups prepared via this
3
on the outer surface, and afterwards the surfactant was
[
12]
removed by solvent extraction. The synthesis and im-
mobilization route to complex 7 is shown in Scheme 2.
Ligand 6 (in Scheme 2) was readily prepared from com-
[
13]
mercially available imidazole by N-alkylation with n-
BuBr and 1-chloro-3-trimethoxysilylpropane, followed
by the treatment with NaH in the presence of potassium
[14]
tert-butoxide. Complex 7 could be produced in high
yield by reaction between 6 and Grubbs catalyst in
[15]
THF solution. Immobilization of complex 7 in the
pore channels of SBA-15 was carried out through reflux-
ing it with SBA-15 in N,N-dimethylformamide (DMF)
solution under a nitrogen atmosphere.
The anchored ruthenium complex 7 in SBA-15 was
confirmed by FT-IR. The IR spectra of SBA-15 were re-
corded before and after immobilization (Figure 1). The
ꢀ
1
peak at 960 cm assigned as SiꢀOH stretching of the
surface silanols almost disappeared after immobiliza- Figure 2. HR-TEM image of SBA-15 after immobilization.
Figure 1. FT-IR spectra of SBA-15; a: before immobilization;
b: after immobilization.
Figure 3. EDS spectra of Ru-SBA-15 after immobilization.
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Adv. Synth. Catal. 2005, 347, 1745 – 1749

Active and Reusable Heterogeneous Ru Catalyst for Olefin Metathesis
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scheme, as we believe, should have been confined in the Table 1. Recycling and reuse of the mesoporous silicon-
bounded Ru complex in ring-closing metathesis of diallylma-
lonic acid diethyl ester.
pore channels.
The N adsorption/adsorption isotherms of these ma-
2
terials are shown in Figure 4. The type IV isotherm
curves with a well defined step clearly indicate that these
materials possess a mesoporous structure. After immo-
bilization the BET surface area of SBA-15 decreased
Cycle
1
2
3
4
5
Conversion (35 min, %)
98
96
96
95
95
2
2
from 734 m /g to 233 m /g and the average pore size de-
creased from 7.4 nm to 5.2 nm due to the loading of the decrease in catalytic activity along with the cycle times.
Ru complex in the mesoporous structure. However, the For this heterogeneous catalyst, the catalytic species
pore size was still large enough to permit the diffusion of were covalently anchored on the inner surface of the
large organic molecule in it.
pores in SBA-15, and it is very difficult, as we believe,
The catalytic activity of 8 was tested through standard for them to come to contact and react with each other,
ring-closing metathesis (RCM) diallylmalonic acid di- so thematerial exhibiteda high catalytic activity evenaf-
[
16]
ethyl ester (Scheme 3), and the results are shown in ter being reused for several times. The element analysis
Table 1. At 458C in CH Cl , the RCM product was al- by the ICP technique indicated that there was no Ru el-
2
2
most equally formed after 35 min reaction for each cy- ement in the reaction mixture within the detection lim-
cle, and could be isolated as a colorless oil after filtration its. The immobilized catalyst showed very high stability
and concentration. The first cycle catalytic activity is against leaching of the active species into the liquid
very similar to the corresponding homogeneous catalyst phase under the given reaction conditions. This feature,
[
9]
(
about 92% conversion after 25 min ), which means which is important for a heterogeneous catalyst system,
that the reaction is, most probably, not diffusion-con- is mainly ensured by the relatively strong interaction be-
trolled due to the high porosity of the SBA-15 material. tween the silane coupling agent and the Ru center and
In order to test its recycle and reuse capability, after by the covalent grafting between the organic ligands
the first reaction, all the catalysts were separated by fil- and the inorganic mesoporous materials.
tration and thoroughly washed, and then reused under
In Table 2 are listed the results of other metathesis re-
the same conditions. The reaction conversion data (Ta- actions catalyzed by 5 mol % of supported catalyst 8 for
ble 1) show that the immobilized catalyst can be repeat- 10 h. It shows that this immobilized catalyst has a high
edly used at high catalytic activity without any apparent catalytic activity for olefin metathesis reactions.
Table 2. Result of metathesis reactions utilizing the immobi-
[a]
lized catalyst.
Entry
Substrate
Product
Yield [%]
100
1
2
3
98
Figure 4. Nitrogen adsorption-desorption isotherms; a: be-
fore immobilization; b: after immobilization
100
4
82
[a]
All reactions have been performed under standard condi-
tions employing 5 mol % of catalyst at 458C in CH Cl for
2
2
Scheme 3. Ring-closing metathesis (RCM) reaction of diallyl-
malonic acid diethyl ester.
10 h. E¼ester group, Ts¼p-toluenesulfonyl, Tr¼triphe-
nylmethyl.
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Liang Li, Jian-lin Shi
In conclusion, through introducing SiꢀOMe groups to Synthesis of 7
one of the ligands, the Ru carbene complex has been
Asolution of RuCl (¼CHPh)(PCy ) (0.606 mmol) inTHF was
2
3 2
mainly covalently anchored on the inner pore surface
of the inorganic mesoporous material SBA-15. After
the immobilization, the decomposition of the catalytic
treated with a solution of 5 (0.606 mmol) in THF (20 mL) at
788C and slowly warmed to room temperature. The solution
was filtered and the solvent removed. The complex was washed
ꢀ
species has been effectively prevented and durable cat- with pentane (3ꢁ20 mL), and the resulting orange-brown sol-
alytic activity has been achieved. This method can also id was dried under vacuum; yield: 350 mg (70%); anal. calcd. for
be used for the immobilization of other heterogeneous
metal complex catalysts.
C H65Cl N O PSiRu: C 54.41, H 7.96, N 3.43; found: C 54.53, H
38 2 2 3
1
7
.92, N 3.32%; H NMR (CDCl ): d¼0.82 (m, 2H, SiCH ), 1.28
3
2
(
t, 3H, CH CH ), 1.12–1.91(all m, 30H of PCy ), 1.81 (m, 2H,
3
2
3
CH CH ), 1.89 (m, 2H, SiCH CH ), 2.01 (m, 2H, CH CH
3
2
2
2
3
2
CH ), 2.38 (m, 3H, CH of PCy ), 3.61 (s, 9H, OCH ), 7.12 (t,
2
3
3
2
H, NCH CH CH Si), 7.21 (t, 2H, NCH CH CH CH ),
2 2 2 2 2 2 3
7
.30–7.48 (m, 5H, C H ), 7.70, 7.72 (all s, 1H, NCHCHN),
6
5
3
1
Experimental Section
20.32 (s, 1H, Ru-CH); P NMR (CDCl
PCy ).
): d¼34.88 (s Ru-
3
3
Modification SBA-15’s Outer Surface with SiꢀCH
3
Groups
Synthesis of 8
SBA-15 (1.5 g) was dispersed in dry toluene (50 mL) at 708C
Asolution of complex 7 (816 mg, 0.1 mmol) in dimethylforma-
mide (DMF) (2 mL), was added to a well-stirred DMF suspen-
sion (40 mL) of the inorganic support (modified SBA-15 dried
at 3008C/0.1 mmHg for 3–4 h, 1 g ) and the mixture was stirred
at 1308C under a nitrogen atmosphere for 24 h. The solid was
then filtered and Soxhlet-extracted for 6 h to remove the re-
maining non-supported complex from the heterogenized cata-
lyst.
under a N atmosphere, then (CH ) SiCl (10 mL) was added
2
3 3
under stirring. The mixture was filtered after being continuous-
ly stirred for 24 h, followed by drying in vacuum at 908C for
more than 8 h. After modification of the external surface, the
surfactant template was removed by solvent extraction using
refluxing ethane under stirring for 2 days.
Synthesis of 5
Ring-Closing Metathesis
A250-mL, round-bottom flask was charged with N-butylimi-
dazole (9.7 g, 75 mmol), 31 g (75 mmol) of 1-chloro-3-trime-
thoxysilylpropane, a stir bar, and 100 mL of toluene. The mix-
ture was stirred at 1108C for about 12 h and then separated
through a funnel, and the oil-like sample was washed with
toluene (2ꢁ20 mL); Yield: 16.7 g (70%); anal. calcd. for
C H ClN O Si: C 48.52, H 8.09, N 8.71; found: C 48.71, H
10 mmol of diallylmalonic acid diethyl ester were dissolved in
50 mL of CH Cl and 1 mol % of the ruthenium complex was
2
2
added. The suspension was stirred at 458C for the desired
time. The results of the reaction was determined with GC-
MS and ICP.
13
26
2
3
1
8
.15, N 8.50%; H NMR (CDCl ): d¼0.65 (m, 2H, SiCH ),
3
2
0
.91 (t, 3H, CH CH ), 1.35 (m, 2H, CH CH ), 1.89 (m, 2H,
3
2
3
2
SiCH CH ), 2.05 (m, 2H, CH CH CH ), 3.57 (s, 9H, OCH ),
2
2
3
2
2
3
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.16 (t, 2H, NCH CH CH Si), 7.25 (t, 2H, NCH CH CH CH ),
2 2 2 2 2 2 3
7.48,7.63 (s, 2H, NCHCHN).
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