Enhancement of catalytic performance in asymmetric transfer hydrogenation by microenvironment engineering of the nanocage

Article abstract of DOI:10.1039/c0cc01401j

Ru-TsDPEN confined in the nanocage with an amphiphilic microenvironment can be ten times more active than that with a hydrophobic one in the transfer hydrogenation of acetophenone in HCOONa-H₂O, which is mainly due to the enhanced diffusion rat

Full text of DOI:10.1039/c0cc01401j

Chemical Communications
www.rsc.org/chemcomm
Volume 46 | Number 43 | 21 November 2010 | Pages 8073–8292
ISSN 1359-7345
COMMUNICATION
Qihua Yang and Can Li et al.
Enhancement of catalytic performance in asymmetric transfer hydrogenation
by microenvironment engineering of the nanocage
1359-7345(2010)46:43;1-D

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COMMUNICATION
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Enhancement of catalytic performance in asymmetric transfer
hydrogenation by microenvironment engineering of the nanocagew
Shiyang Bai, Hengquan Yang, Peng Wang, Jinsuo Gao, Bo Li, Qihua Yang* and Can Li*
Received 14th May 2010, Accepted 25th June 2010
DOI: 10.1039/c0cc01401j
Ru-TsDPEN confined in the nanocage with an amphiphilic
microenvironment can be ten times more active than that with
a hydrophobic one in the transfer hydrogenation of acetophenone
Choosing Ru-TsDPEN confined in SBA-16 as model
catalyst, the microenvironment engineering of SBA-16 with
confined Ru-TsDPEN was realized by using silylation agents
containing organic groups with different hydrophobic/
hydrophilic properties. In this case, the silylation agents were
used not only for reducing the pore entrance size but also for
adjusting the microenvironment of the nanocage of SBA-16.
2
in HCOONa–H O, which is mainly due to the enhanced
diffusion rates of reactants during the catalytic process.
Enzymes can be very active and selective for a range of chemical
reactions. Driven by the long-term desire to mimic enzymatic
catalysis, molecular capsules and self-assembled nano-
capsules have been designed and used as nanocages for
0
N-trimethoxysilylpropyl-N,N,N-tri-n-butylammonium (R ) with
an amphiphilic group of N,N,N-tri-n-butylammonium bromide
and propyltrimethoxysilane (R) with a lipophilic group of
propyl were chosen as silylation agents and the preparation of
the nanocages with different microenvironment is outlined
in Scheme 1. After adsorption of Ru-TsDPEN in the nanocages
of SBA-16, the pore entrance size of Ru-TsDPEN@SBA16
was adjusted with the desired amount of silylation reagent
(see ESIw). The catalyst was denoted as Ru-TsDPEN@SBA-n,
1
–7
catalysis.
Though there are several successful examples,
the efficiency of chemical catalysts in most nanocages is far
less than that of enzymes. The design and synthesis of nano-
cages based on solid materials are highly desirable for practical
applications.
Recently, our group reported an efficient method for
encapsulating transition metal complexes in the nanocage of
0
0
where n refers to the initial molar percentage of 100R /(R+R )
8
–11
SBA-16.
These nanocages exhibit high enantioselectivity
employed for the silylation.
0
and catalytic activity in asymmetric catalysis. Based on the
findings of the enzymes, the microenvironment of the cavity
where the binding site and catalytically active site exist plays
an important role in the unique catalytic performance of the
Based on the elemental analysis, the content of R on the
1
solid catalyst increases from 0 to 0.38 mmol g when the
ꢀ
0
0
molar percent ratio of R /(R+R ) in the initial mixture is
increased from 0 to 100, suggesting that the amount of the two
kinds of organic groups on the catalyst can be adjusted as
expected (Table 1). The content of encapsulated Ru-TsDPEN
in SBA-16 varies from 0.148 to 0.33 wt%, corresponding to
the number of Ru-TsDPEN in each nanocage of 2 to 4. The
1
2,13
enzymes.
Therefore, the catalytic efficiency of the solid
catalysts may be improved by tuning the microenvironment of
the nanocage. However, it has rarely been reported on tuning
the microenvironment of the nanocage where the molecular
catalysts are accommodated according to the requirement of
the properties of reactants and products.
2
ꢀ1
BET surface area of Ru-TsDPEN@SBA-0 is 411 m
g
,
2
ꢀ1
which is smaller than SBA-16, 916 m g . This is probably
due to the combined effect of the silylation and the occupa-
tion of Ru-TsDPEN in the nanocage (Table S1). However,
it is surprising to find that the BET surface area of
In this communication, we report that the catalytic activity
of Ru-TsDPEN confined in the nanocage can be greatly enhanced
by modifying the nanocage through microenvironment
engineering for asymmetric transfer hydrogenation.
Ru-TsDPEN@SBA-n samples (n 4 0), measured by N
2
2
ꢀ1
The diffusion rates of the reactants and products are
important factors for a catalytic reaction, especially when
the reactants and catalysts cannot be homogeneously mixed
together in a catalytic system. For example, mass diffusion is
one of the problems for asymmetric transfer hydrogenation of
prochiral ketones catalyzed by immobilized Ru-TsDPEN
using HCOONa solution as hydrogen source because of the
sorption, is below 30 m
g
, much lower than that of
Ru-TsDPEN@SBA-0 (Table S1). The occupied volume of
SBA-16 by Ru-TsDPEN cannot explain the extremely low
surface area. This is probably due to the fact that the existence
0
of the large organic group of R at the pore entrance prevents
1
4
2
H O–oil–solid triphase reaction system. This problem may
be solved by enhancing the diffusion rates of the reactants
during the catalytic process through modifying the micro-
environment of the solid catalyst.
State Key Laboratory of Catalysis, Dalian Institute of Chemical
Physics, Chinese Academy of Sciences, Dalian 116023, China.
E-mail: yangqh@dicp.ac.cn, canli@dicp.ac.cn;
Fax: +86-411-84694447; Tel: +86-411-84379552, +86-411-84379070
w Electronic supplementary information (ESI) available: Experimental
procedure, XRD, N sorption, UV–vis, the recycle of the catalyst and
2
Scheme 1 Illustration of encapsulation of Ru-TsDPEN in the nano-
the results of other aromatic ketones. See DOI: 10.1039/c0cc01401j
cage with different hydrophobic/hydrophilic microenvironment.
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N
2
from diffusing through the pore. The catalytic properties of
Ru-TsDPEN@SBA-0 exhibits much higher surface area and pore
volume than Ru-TsDPEN@SBA-n (n 4 0) (Table S1), suggesting
that the big difference in catalytic activity is not due to the textural
properties of the catalysts. Thus, the catalytic performance of
Ru-TsDPEN confined in SBA-16 may be influenced by the
microenvironment of the nanocage. The microenvironments of
Ru-TsDPEN@SBA-0 and Ru-TsDPEN@SBA-100 respectively
Ru-TsDPEN@SBA-n samples were tested for the asymmetric
transfer hydrogenation of acetophenone in HCOONa–H O
2
(
Table 1). Ru-TsDPEN@SBA-0 modified with hydrophobic
propyl group shows a conversion of 50% with 88% ee in 4 h. If
the phase transfer reagent, tetrabutylammonium bromide, was
added, 78% conversion with 92% ee could be obtained. This
result suggests that the low catalytic activity of the catalyst is
mainly due to the diffusion limitation in this oil/water/solid
tri-phase reaction system. A conversion of 90% with 92% ee
was obtained with Ru-TsDPEN@SBA-9 modified with
both propyl and N,N,N-tri-n-butylammonium bromide.
Ru-TsDPEN@SBA-38 shows a conversion of 499% with
0
modified with 100% R and 100% R were characterized
by benzene and water adsorption experiment (Fig. 1). As the
0
relative pressure (P/P ) increases, the adsorption amount of
Ru-TsDPEN@SBA-100 increases sharply. Under similar condi-
tions, Ru-TsDPEN@SBA-0 only shows sluggish increasing
tendency. After being normalized by surface area,
Ru-TsDPEN@SBA-100 can adsorb more water and benzene
than Ru-TsDPEN@SBA-0 at the same relative pressure.
The molar ratio of adsorbed benzene to water was 0.92 and
9
2% ee in 3 h. It needed only 2.5 h to convert all the aceto-
phenone to the desired product when Ru-TsDPEN@SBA-100
was used as catalyst. Also, the filtration reaction was performed.
After a reaction for 10 min, Ru-TsDPEN@SBA-100 was
filtered off and the filtrate was further stirred for 3 h under
similar conditions. No further increase of the conversion was
observed, suggesting that the asymmetric reaction is mainly
catalyzed by Ru-TsDPEN confined in the nanocage.
1.63
respectively
for
Ru-TsDPEN@SBA-100
and
Ru-TsDPEN@SBA-0 at P/P
0
of 0.95, which indicates that
the nanocage modified with N,N,N-tri-n-butylammonium
bromide favors both the adsorption of water and benzene. It
can be concluded from the above characterization that
Ru-TsDPEN@SBA-0 has lipophilic surface properties and
Ru-TsDPEN@SBA-100 has amphiphilic surface properties.
In other words, the microenvironment of Ru-TsDPEN@
SBA-100 is compatible for guest molecules with both
hydrophilicity/hydrophobicity and the microenvironment of
Ru-TsDPEN@SBA-0 is mainly compatible for the hydrophobic
guest molecules.
The TOF of the catalysts increases sharply when the molar
0
ratio of R in the initial mixture increases from 0 to 100%. The
TOF of Ru-TsDPEN@SBA-100 is about 10 times higher than
that of Ru-TsDPEN@SBA-0 and comparable to that of
ꢀ
1
homogeneous one (TOF of 140 h ). As far as we know,
Ru-TsDPEN@SBA-100 shows the highest catalytic activity
among the Ru-TsDPEN/porous-silica catalysts ever reported
1
5–17
for the asymmetric transfer hydrogenation of ketones.
Because the transfer hydrogenation reaction involves
ꢀ
ꢀ
The fact that Ru-TsDPEN@SBA-100 with low surface area
shows high catalytic activity suggests that the pore entrance
size of Ru-TsDPEN@SBA-100 is large enough for the free
diffusion of the reactants and products. This result is not
ketone and HCOO , the diffusion of HCOO into the nano-
cage is also a very important factor influencing the catalytic
ꢀ
performance. Due to the difficulty in detecting HCOO , the
negatively charged alizarin red was chosen as probe molecule
ꢀ
consistent with that obtained by N
2
sorption characterization.
(alizarin red has similar negative charge to HCOO and
This is probably due to the fact that the N,N,N-tri-n-butyl-
ammonium bromide aggregates at the pore entrance will swing
to let the guest molecules diffuse freely through the pore
entrance on contact with solution.
the adsorption amount can be easily estimated by UV-vis
spectroscopy) to investigate the adsorption capacities of
Ru-TsDPEN@SBA-100 and Ru-TsDPEN@SBA-0 for anion.
As shown in Fig. 2, more than 90% of alizarin red can be
adsorbed by Ru-TsDPEN@SBA-100 within 2.5 h. Only
less than 60% of alizarin red could be adsorbed by
Ru-TsDPEN@SBA-0 even after an adsorption for 25 h. This
indicates that the positively charged quaternary ammonium
Because the asymmetric transfer hydrogenation is first order
in the concentration of catalyst and ketone substrate, the
influence of the concentration of Ru-TsDPEN in the nano-
1
8
cage on the catalytic performance could be excluded. Also,
a
Table 1 Asymmetric transfer hydrogenation of acetophenone catalyzed by Ru-TsDPEN@SBA-n
0
ꢀ1
b
c
c
R content mmol g Ru content wt% Number of Ru-TsDPEN in each cage Time h Conv. % Ee % TOF h
ꢀ1d
Catalysts
e
e
88 (92) 12
Ru-TsDPEN@SBA-0
0
0.148
0.315
0.330
0.230
—
B2
B4
B4
B3
—
4
4
3
2.5
1.5
10
50 (78)
90
499
499
499
89
Ru-TsDPEN@SBA-9 0.16
Ru-TsDPEN@SBA-38 0.29
Ru-TsDPEN@SBA-100 0.38
Ru-TsDPEN
Ru-TsDPEN/MCM-41
92
92
92
93
93
32
74
126
140
—
—
—
f
—
—
a
b
O and a S/C ratio of 100 in 0.24 mL water. See the
Reactions were carried out at 40 1C, using 0.1 mmol of substrate, 0.5 mmol HCOONaꢂ2H
2
c
d
e
Supporting Information for detail. Analysis by GC. Calculated at the conversion of B30%. Data in parentheses refers to the catalytic results
of Ru-TsDPEN@SBA-0 with the addition of 0.0016 mmol of TBNBr during the catalytic process. Data from ref. 15.
f
8
146 | Chem. Commun., 2010, 46, 8145–8147
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Table 2 Asymmetric transfer hydrogenation of sodium o-acetylphenol
and o-acetylphenol
a
Cat.
R
Time h
Conv. %
Ee %
0
Ru-TsDPEN
2 -ONa
2 -ONa
13
13
13
3
6
6
0
—
0
Ru-TsDPEN@SBA-0
Ru-TsDPEN@SBA-100
Ru-TsDPEN
Ru-TsDPEN@SBA-0
Ru-TsDPEN@SBA-100
16
98
97
26
B98
B90
90
90
90
90
0
2 -ONa
2 -OH
Fig. 1 Adsorption isotherms of (A) water and (B) benzene on
a) Ru-TsDPEN@SBA-100 and (b) Ru-TsDPEN@SBA-0 at 293 K.
0
(
0
2 -OH
2 -OH
0
ion can efficiently enhance the accumulation of the substrate of
ꢀ
a
o
Reactions were carried out at 40 C, using 0.1 mmol of ketone,
.5 mmol HCOONaꢂ2H O and a S/C ratio of 100 in 0.24 mL water.
HCOO in the nanocage. The existence of N,N,N-tri-n-butyl-
0
2
ammonium bromide cannot only provide a suitable micro-
environment for the substrate with different lipophilicity but
also can accumulate negatively charged substrate around the
active site.
nanocage mainly contribute to the high catalytic activity of
encapsulated Ru-TsDPEN. This study also shows that high
performance solid catalysts could be expected by encapsulating
the metal complexes in the nanocage with a desirable
microenvironment.
The authors would like to thank the National Basic
Research Program of China (2009CB623503, 2010CB833300,
2
008DFB50130), and the National Natural Science Founda-
tion of China (20921092).
Notes and references
1
2
3
4
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Fig. 2 Adsorption capacity of (a) Ru-TsDPEN@SBA-100 and
(b) Ru-TsDPEN@SBA-0 for alizarin red versus time.
In order to further elucidate the effect of the microenvironment
on the catalytic activity, substrates with different hydrophobic/
hydrophilic properties were used for the transfer hydrogena-
2
tion in HCOONa–H O (Table 2). For the water-soluble
5
6
A. W. Kleij and J. N. H. Reek, Chem.–Eur. J., 2006, 12, 4219.
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o-acetylphenol in NaOH solution), no reaction was observed
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of Ru-TsDPEN in the reaction system. Ru-TsDPEN@SBA-0
shows 16% conversion. It is encouraging to find that
Ru-TsDPEN@SBA-100 shows a conversion of 98% with 90%
ee under similar reaction conditions. When the substrate
was shifted from sodium o-acetylphenol to o-acetylphenol,
Ru-TsDPEN showed a conversion of 98% with 90% ee in 3 h.
Ru-TsDPEN@SBA-0 gave a conversion of 26% with 90% ee in
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2
mainly attributed to the very special microenvironment of the
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19
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1
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This journal is ꢁc The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 8145–8147 | 8147