Hoveyda-Grubbs catalyst confined in the nanocages of SBA-1: Enhanced recyclability for olefin metathesis

Article abstract of DOI:10.1039/c0cc03227a

Via a simple adsorption, the second generation Hoveyda-Grubbs catalyst was successfully immobilized on a mesoporous material SBA-1, leading to a highly recyclable solid catalyst for olefin metathesis.

Full text of DOI:10.1039/c0cc03227a

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Hoveyda–Grubbs catalyst confined in the nanocages of SBA-1:
enhanced recyclability for olefin metathesisw
Hengquan Yang,*a Zhancheng Ma,a Yuekui Wang,b Yunwei Wanga and Li Fanga
Received 13th August 2010, Accepted 20th September 2010
DOI: 10.1039/c0cc03227a
Via a simple adsorption, the second generation Hoveyda–Grubbs
catalyst was successfully immobilized on a mesoporous material
SBA-1, leading to a highly recyclable solid catalyst for olefin
metathesis.
17 mL of dry toluene containing 0.02 g of the second
generation Hoveyda–Grubbs catalyst (CAS No.: 301224-40-8).
After stirring at room temperature for 6 h under a N2
atmosphere, the solid was isolated by filtration, washed
thoroughly with hexane and dried under vacuum, giving the
greenish solid catalysts (ESIw, Fig. S19) denoted as H-G/w
(w = SiO2, MCM-41, SBA-15, SBA-16, FDU-1 and SBA-1).
The obtained solid catalysts were characterized with UV-vis,
FT-IR and N2 sorption. Taking the solid catalyst H-G/SBA-1
for example, its diffusion reflectance (DR) UV-vis spectrum
apparently shows three absorbance bands at 210, 260 and
380 nm, which are similar to the spectrum of the Ru complex
in a dichloromethane solution although the peak shifts are
observed (ESIw, Fig. S20). Compared with the FT-IR
spectrum of the parent SBA-1, a clear decrease in the intensity
of the isolated silanol vibration at 3740 cmÀ1 was observed in
the FT-IR spectrum of H-G/SBA-1, indicating that there were
certain chemical interactions of the Ru center with the surface
silanols (ESIw, Fig. S21). Determination of the Ru content
indicates that 67–90% of the Ru complexes initially added
were introduced onto the mesoporous materials and silica
(ESIw, Table S1). Among these investigated supports, SBA-1
and SBA-16 have the largest adsorption capacity for the Ru
complex and silica has the least adsorption capacity.
Olefin metathesis such as ring-closing metathesis (RCM) and
cross metathesis (CM) using
a commercially available
ruthenium catalyst (Hoveyda–Grubbs catalyst) has recently
become an important method to synthesize new olefins.1 In
view of ‘‘green’’ chemistry, simple catalyst separation, high
recyclability and low catalyst cost are key factors for practical
application in organic synthesis. Along these lines, much effort
has been made to immobilize the Ru complex catalysts on
solid matrices and polymers.2 However, the preparation of
these heterogeneous catalysts often consists of multiple steps
of synthesis and modification of the parent Ru complexes,
consequently making them less accessible for general use.
Moreover, many catalysts barely survive several recycles and
only a few systems have high recyclability (but usually with a
Ru loading up to 5 mol%). A highly recyclable catalyst
prepared with a simple procedure thus remains highly desirable.
Inspired by the high affinity of silica towards the Ru
complex catalysts,2i,3 we herein develop a solid catalyst for
olefin metathesis by a simple adsorption of the second
Fig. 1 shows the N2 sorption isotherms and pore size
distributions (inset of Fig. 1) of SBA-1 before and after
introduction of the Ru complex. H-G/SBA-1 exhibits the same
type of isotherms as the parent SBA-1. However, the pore
volume and pore size decrease from 0.44 cm3 gÀ1 and 1.37 nm
down to 0.37 cm3 gÀ1 and 1.12 nm after introduction of the Ru
complex, respectively. These results suggest that most of the
Ru complexes were located inside the nanopores of SBA-1.
With a series of solid catalysts in hand, we first examined
their recyclabilities with ring-closing metathesis (RCM) of
generation Hoveyda–Grubbs catalyst on
a mesoporous
material SBA-1. Such a solid catalyst exhibits a significantly
enhanced recyclability over this complex immobilized on silica
and other mesoporous materials. The impressive recyclability
may be attributed to the isolated nanocages of SBA-1 that can
efficiently prevent the complex from undergoing dimerization.
To obtain an excellent solid catalyst, a set of well-characterized
mesoporous materials including MCM-41, SBA-15, SBA-16,
FDU-1 and SBA-1 (ESIw, Table S1 and Fig. S1–S15) were
selected as supports because mesoporous materials have high
surface areas and ordered nanopores that can accommodate
metal complexes (commercially available silica was also
utilized as a support for comparison).4 The immobilization
procedure is very simple and includes only one step of
adsorption. A typical procedure for preparing heterogeneous
catalysts is as follows: 1.0 g of silica gel or mesoporous
material (evacuated at 398 K for 6 h) was dispersed in
a School of Chemistry and Chemical Engineering, Shanxi University,
Taiyuan, P.R. China. E-mail: hqyang@sxu.edu.cn;
Fax: +86-351-7011688; Tel: +86-351-7018371
b Key Laboratory of Chemical Biology and Molecular Engineering of
the Education Ministry, Institute of Molecular Science,
Shanxi University, Taiyuan, P.R. China
w Electronic supplementary information (ESI) available: Experimental
procedure, N2 sorption, XRD, FT-IR and UV-vis data. See DOI:
10.1039/c0cc03227a
Fig. 1 The N2 sorption isotherms of SBA-1 before and after
introduction of the second generation Hoveyda–Grubbs complex
catalyst. The inset is the corresponding pore size distribution.
c
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Table 1 The recycling results of RCM over different solid catalystsa
Reaction cycles (conv. %)b
1
2
3
4
5
6
7
8
9
60c 80
min min min min min min min min min
100 120 140 160 180 200 280
H-G/silica 99
H-G/MCM-41 96
98
98
99
98
98
99
98
98
97
99
96
85
99
99
76
91
97
79
51
99
99
24
63
56
37
20
99
99
22
20
H-G/SBA-15
H-G/SBA-16
H-G/FDU-1
H-G/SBA-1
98
99
98
99
Fig. 2 The profiles of the conversions versus time for the RCM of
diethyl diallylmalonate over H-G/SBA-1 and in the filtrate.
98
93
98
73
86
30
69
H-G/SBA-1(L) 98
a
Reaction conditions: 1 mmol (0.1 mmol mLÀ1), 2.5 mol% Ru with
prepared by only one step of adsorption, can also represent
one of the most recyclable solid catalysts for RCM of dienes in
view of the relatively low Ru loading (the comparison of
recyclability is shown in Table S3 of the ESIw).
b
respect to substrate, hexane as solvent, 25 1C. Determined with GC.
Reaction time.
c
To exclude any conversion contribution from the leached
Ru in the solution, we conducted a decisive experiment where
the conversion was monitored. The standard RCM of diethyl
diallylmalonate over H-G/SBA-1 was set up. After 70%
consumption of the reactant the reaction was stopped, and a
portion of the filtrate was immediately isolated through a
rapid centrifugation. The residual mixture and the filtrate were
continuously stirred under the same conditions as the standard
reaction. The profiles of conversions versus time over
H-G/SBA-1 and in the filtrate are displayed in Fig. 2. In the
presence of the solid catalyst the conversion was found to
further increase with time, while the conversion in the filtrate
does not show any increase (H-G/silica also shows the same
results). These results reveal that the conversion was truly
contributed by the solid catalyst instead of the leached Ru
complex in the filtrate. The Ru contents in the filtrate of the
reactions on these solid catalysts (after 60 min) were
determined with ICP-AES (ESIw, Table S2). For the first
reaction cycle, only 1–1.54% Ru complexes were leached into
the solution over the investigated solid catalyst. In the second
reaction cycle, the leaching level significantly deceased
and only 0.25% of Ru was found to leach from the reused
H-G/SBA-1. So the low leaching level of Ru further excludes
the conversion contribution by the leached Ru.
diethyl diallylmalonate under the same conditions. Table 1
summarizes the recycling results over different solid catalysts.
Hexane is chosen as the solvent because its lower polarity
favourably avoids leaching of the Ru complex from the solid
materials.2i Using the 2.5 mol% Ru catalyst (a relatively low
loading, compared with 5 mol% seen in the literature), the
fresh H-G/silica gave a 99% conversion within 60 min. For the
second and third reaction cycles, 98% conversions were
achieved within a slightly prolonged time (80–100 min).
However, beginning with the fourth reaction cycle, the
conversion was observed to decrease down to 76%. In the
fifth reaction cycle, the conversion further dropped to 24%. In
the case of MCM-41 being used as the support, the fresh
H-G/MCM-41 afforded a 96% conversion. For the second
and the third reaction cycles, 98% and 97% conversions were
obtained, respectively, and the results are comparable with
those of H-G/silica. From the fourth reaction cycle on, the
conversions were much higher than those of H-G/silica but
decreased during the following reaction cycles. Similar to these
two solid catalysts, the conversions for the Ru complexes
immobilized on SBA-15, SBA-16 and FDU-1 were also
observed to significantly decrease especially after three
reaction cycles.
Different to these above solid catalysts, H-G/SBA-1
exhibited an impressive recyclability. Under the same condi-
tions, the fresh H-G/SBA-1 gave a 99% conversion. From the
second to fourth reaction cycles, the conversions were kept at
99% (within 80–120 min). Remarkably, for the fifth reaction
cycles, the conversion over H-G/SBA-1 was much higher than
the conversions over other catalysts and a 99% conversion was
achieved. Furthermore, for the sixth and seventh reaction
cycles, 98% conversions were still obtained. The eighth
reaction cycle gave an 86% conversion. For the ninth
reaction cycle, as high as 69% conversion was still afforded
(within 280 min), which is much higher than that of the fifth
reaction cycles over H-G/silica. Obviously, among these solid
catalysts, the H-G/SBA-1 exhibited an outstanding recyclability.
Compared with the results over the reported catalysts
(prepared by grafting on silica or polymers), H-G/SBA-1,
Over the past decade, much effort has been made to
understand Grubbs type catalyst deactivation and many
pathways have been proposed. One of the major pathways
for deactivation was considered to involve the less active
bimetallic species that were formed via dimerization of the
mononuclear Ru complexes during catalytic reaction.2a,c,5
After both excluding the leached Ru contribution and
confirming the location of the complex inside the nanocages,
in view of the same chemical nature of these selected support
surfaces we suppose that the unique pore structure of SBA-1 is
a crucial factor for the high recyclability because the isolated
nanocages may be helpful for preventing the formation of the
dinuclear Ru complex by spatial limitations. To test this
assumption, the ground-state geometries of both mono- and
dinuclear ruthenium complexes were optimized at the
DFT/B3LYP/3-21G* level (see ESIw) using the Gaussian03
c
8660 Chem. Commun., 2010, 46, 8659–8661
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included in Table 2. Diallyl ether was smoothly converted to
the cyclized product with a conversion of 99% within 1.5 h
over H-G/SBA-1 (Table 2, entry 1). Di(4-butenyl)-substituted
diethylmalonate was full cyclized within 1.5 h (Table 2,
entry 2). A satisfactory conversion was also obtained for
1,4-diallylbutyne as a substrate (Table 2, entry 3). For allyl
substituted silane, a moderate conversion was achieved
(Table 2, entry 4). N,N-Diallylbenzamide was smoothly
converted to the cyclic olefin with a good conversion
(Table 2, entry 5). For linear dienes, moderate conversions
were also afforded (Table 2, entries 6 and 7). Further,
H-G/SBA-1 shows moderate to good activity for olefin homo-
dimerization in cross metathesis (ESIw, Table S4).
Via a simple and straightforward adsorption, the second
generation Hoveyda–Grubbs catalyst was successfully
immobilized on a mesoporous material SBA-1, leading to an
active solid catalyst for olefin metathesis. Such a catalyst
shows significantly enhanced recyclability, compared with this
complex immobilized on other mesoporous materials and
silica. The solid catalyst can be used nine times, representing
one of the most recyclable solid catalysts. The high
recyclability was attributed to the isolated nanocage with a
suitable size that can efficiently prevent the mononuclear
ruthenium complex from dimerization by spatial limitations.
This strategy for stabilizing the mononuclear metal complex
by the nanocage may be applicable to other catalytic systems.
We acknowledge the Natural Science Foundation of China
(20903064), State Key Laboratory of Catalysis Foundation
(Dalian Institute of Chemical Physics, N-09-02), New
Teacher Foundation from Education Ministry of China
(200801081035) and Shanxi Natural Science Foundation for
Youths (2009021009).
Scheme 1 The proposed mechanism for preventing the formation of
dinuclear Ru complex by the isolated nanocage of SBA-1.
program package. Based on the optimized geometries, the
three-dimensional sizes of the mono- and dinuclear ruthenium
complexes are estimated to be 1.764 Â 1.370 Â 1.047 nm3
and 2.016 Â 1.450 Â 0.969 nm3, respectively (ESIw, Fig. S22
and S23). Obviously, the dinuclear ruthenium complex is
larger in molecular size. The cage size of SBA-1 was
determined with N2 to be 1.37 nm (it is should be noted that
this size is generally considered to be underestimated because
of the use of a cylinder model).6 The nanocage with such a
pore size can accommodate the mononuclear Ru complex, as
shown in Scheme 1. However, the dinuclear ruthenium
complex and the transition state for its formation (surely with
larger size than the dinuclear complex) can not be held and the
dimerization was thus inhibited. To further confirm the
assumption, we used SBA-1(L) with a larger pore size
(1.85 nm, Fig. S16–S18 in the ESIw) as the support to prepare
a solid catalyst H-G/SBA-1(L). The recyclability of H-G/SBA-1(L)
was also tested (Table 1, the last line). The conversions over
H-G/SBA-1(L) were lower than those over H-G/SBA-1 from
the sixth reaction cycle onward.
To further examine the substrate scope of H-G/SBA-1, the
RCM of selected dienes were conducted. The results are
Table 2 RCM reactions of various dienes over H-G/SBA-1a
Notes and references
1 (a) L. Yet, Chem. Rev., 2000, 100, 2963; (b) M. E. Maier, Angew.
Chem., Int. Ed., 2000, 39, 2073; (c) J. Prunet, Angew. Chem., Int. Ed.,
2003, 42, 2826.
b
Entry Substrates
1
Products
Time (h) Conv.%
2 (a) C. Copereta and J. M. Basseta, Adv. Synth. Catal., 2007, 349, 78;
´
1.5
1.5
99
99
(b) D. P. Allen, M. M. Van Wingerden and R. H. Grubbs, Org.
Lett., 2009, 11, 1261; (c) J. Lim, S. S. Lee and J. Y. Ying, Chem.
Commun., 2010, 46, 806; (d) X. Elias, R. Pleixats, M. W. C. Man and
J. J. E. Moreau, Adv. Synth. Catal., 2007, 349, 1701;
(e) J. S. kingsbury, J. P. A. Harrity, P. J. Bonitatebus Jr. and
A. H. Hoveyda, J. Am. Chem. Soc., 1999, 121, 791; (f) G. Y. Liu,
H. Y. He and J. H. Wang, Adv. Synth. Catal., 2009, 351, 1610;
(g) J. O. Krause, S. H. Lubbad, O. Nuyken and M. R. Buchmeiser,
Macromol. Rapid Commun., 2003, 24, 875; (h) D. Fischer and
S. Blechert, Adv. Synth. Catal., 2005, 347, 1329; (i) B. V. Berlo,
K. Houthoofd, B. F. Sels and P. A. Jacobs, Adv. Synth. Catal., 2008,
350, 1949; (j) H. Clavier, K. Grela, A. Kirschning, M. Mauduit and
S. P. Nolan, Angew. Chem., Int. Ed., 2007, 46, 6786; (k) H. Clavier,
N. Audic, J. C. Guillemin and M. Mauduit, J. Organomet. Chem.,
2005, 690, 3585.
2
3
4
1.5
1.5
90
37
5
6
4
91
3 J. S. Kingsbury, J. P. A. Harrity, P. J. Bonitatebus and
A. H. Hoveyda, J. Am. Chem. Soc., 1999, 121, 791.
4 (a) M. J. Kim and R. Ryoo, Chem. Mater., 1999, 11, 487;
(b) H. Q. Yang, J. Li, J. Yang, Z. M. Liu, Q. H. Yang and C. Li,
Chem. Commun., 2007, 1086; (c) H. Q. Yang, L. Zhang, L. Zhong,
Q. H Yang and C. Li, Angew. Chem., Int. Ed., 2007, 46, 6861.
5 S. H. Hong, A. G. Wenzel, T. T. Salguero, M. W. Day and
R. H. Grubbs, J. Am. Chem. Soc., 2007, 129, 7961.
1.5
1.5
53
48
7
Reaction conditions: 1 mmol (0.1 mmol mLÀ1), 2.5 mol% Ru with
a
b
6 P. Kowalczyk, M. Jaroniec, K. Kaneko, A. P. Terzyk and
P. A. Gauden, Langmuir, 2005, 21, 10530.
respect to substrate, hexane as solvent, 25 1C. Determined with GC.
c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 8659–8661 8661