Modification of mesoporous silicate SBA-15 with tris[bis(trimethylsilyl) amido]samarium and its utility in Tishchenko reaction

Article abstract of DOI:10.1016/j.jorganchem.2005.05.013

Treatment of mesoporous silicate SBA-15 with Sm[N(SiMe₃) ₂]₃ led to the formation of a novel organolanthanide/ inorganic hybrid material [SBA-15]Sm[N(SiMe₃)₂] _x via abstraction of N(SiMe₃)₂ by terminal silanol groups and subsequent surface silylation. The hybrid material was characterized by elemental analyses, IR spectroscopy, X-ray diffraction, and nitrogen sorption, indicating a successful tailoring inside the silicate SBA-15 and the maintenance of the well-ordered mesostructure. This hybrid material is a promising heterogeneous catalyst for the Tishchenko reaction, where it is superior to the homogeneous correspondent in deactivation behavior, reusability and relative tolerance to oxygen, particularly in the control of selectivity of mixed Tishchenko reaction due to the steric hindrance and the diffusion control derived from the surface confinement.

Full text of DOI:10.1016/j.jorganchem.2005.05.013

Journal of Organometallic Chemistry 690 (2005) 3783–3789
www.elsevier.com/locate/jorganchem
Modification of mesoporous silicate SBA-15
with tris[bis(trimethylsilyl)amido]samarium and its utility
in Tishchenko reaction
a
Yinghua Chen , Zhenyu Zhu , Jie Zhang , Jianzhong Shen , Xigeng Zhou
a
a
a
a,b,
*
a
Department of Chemistry, Molecular Catalysis and Innovative, Material Laboratory, Fudan University, Handan Road 220,
Shanghai 200433, PeopleÕs Republic of China
b
State Key Laboratory of Organometallic Chemistry, Shanghai 200032, PeopleÕs Republic of China
Received 3 March 2005; received in revised form 30 April 2005; accepted 10 May 2005
Available online 29 June 2005
Abstract
Treatment of mesoporous silicate SBA-15 with Sm[N(SiMe3)2]3 led to the formation of a novel organolanthanide/inorganic
hybrid material [SBA-15]Sm[N(SiMe3)2]x via abstraction of N(SiMe3)2 by terminal silanol groups and subsequent surface silylation.
The hybrid material was characterized by elemental analyses, IR spectroscopy, X-ray diffraction, and nitrogen sorption, indicating a
successful tailoring inside the silicate SBA-15 and the maintenance of the well-ordered mesostructure. This hybrid material is a
promising heterogeneous catalyst for the Tishchenko reaction, where it is superior to the homogeneous correspondent in deactiva-
tion behavior, reusability and relative tolerance to oxygen, particularly in the control of selectivity of mixed Tishchenko reaction due
to the steric hindrance and the diffusion control derived from the surface confinement.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Organolanthanide; Tishchenko reaction; SBA-15; Immobilization
1. Introduction
For example, it has been found that some organolanth-
anide catalysts can be grafted onto MCM-41 in terms of
the remaining catalytic activity and the spatial restric-
tion of mesopores can lead to high stereoselectivity of
the products [3]. As a new mesoporous material, SBA-
15 [4] is possessed of a two dimension hexagonal pore
structure, its texture with larger pore diameter and pore
volume compared to MCM-41 means more flexibility in
guest loading and less risk of the pore blockage [5],
which may ensure the attack of the substrates on the ac-
tive sites. The potential of such a structurally ordered
silica material to act as a versatile support material
has previously been demonstrated by SBA-15-grafted
organic moieties, inorganic complexes and organotran-
sition metal species [6], intriguing our great interest of
the corresponding immobilization of organolanthanides
onto this material.
There is currently considerable interest in studying
heterogenization of homogeneous catalysts for advanta-
geous recovering and recycling them from reaction sys-
tem [1]. Of particular interest has been the exploration
of immobilization of these homogeneous catalysts onto
porous inorganic solids to create organic–inorganic hy-
brid catalysts. Grafting the organolanthanide catalysts
onto porous silica materials by heterogeneously per-
formed silylamide route [2] proved to be an appealing
way to the heterogenization of homogeneous catalysts.
*
Corresponding author. Tel.: +86 216 564 3769; fax: +86 216 564
1740.
E-mail address: xgzhou@fudan.edu.cn (X. Zhou).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.05.013

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Y. Chen et al. / Journal of Organometallic Chemistry 690 (2005) 3783–3789
The Tishchenko reaction is one of the most attractive
transformations of aldehydes, and the search for
cheaper and more effective catalysts is a current interest
[7–9]. Recently, lanthanide silylamide complexes
Ln[N(SMe3)2]3 have proved to be one of the most active
catalysts for Tishchenko reaction [10], but their weak-
ness for the oxygen and water-sensitivity and nonreus-
ability impedes them from the application in the
industry. Therefore, it would be of great interest to
determine whether the immobilization of lanthanide
bis(trimethylsily)amides onto the SBA-15 could yield
the hybrid materials which are not only active in the
Tishchenko reaction, but can be conveniently recovered
from reaction environments. Herein, we report the first
Sm[N(SiMe3)2]3-modified SBA-15 material and its cata-
lytic studies in Tishchenko reaction.
Fig. 1. The FT-IR spectra of SBA-15 (S1), hybrid material (S2) and
recovered material (S3).
2. Results and discussion
the absorption band at 3442 cmÀ1 assigned to the hydro-
gen bound silanols from S1 to S2 demonstrates a part
consumption of the silanol groups. The presence of the
trimethylsilyl moieties in S2 is characterized by the
appearance of three new peaks at 2964, 1392 and
2.1. Synthesis and characterization of the hybrid material
The SBA-15 material used in this study was synthe-
sized by employing nonionic block copolymer P123 as
a template described previously [11] and dehydrated at
200 ꢁC/10À3 mbar for 8 h prior to use. The immobilizing
procedure is readily carried out by the direct reaction of
SBA-15 and Sm[N(SiMe3)2]3 in hexane at ambient tem-
perature for several days (Scheme 1). The liberated silyl-
amine can further react with the residual silanol groups
and passivate them, which is consistent with the previ-
ous observation [12] and is in favor of decreasing the
side reactions in catalysis [13] and improve the hydro-
phobicity of the hybrid material [14]. The hybrid mate-
rial [SBA-15]Sm[N(SiMe3)2]x(S2) was characterized by
elemental analyses, IR spectroscopy, X-ray diffraction
and nitrogen sorption, indicating a successful tailoring
inside the silicate SBA-15 and the maintenance of the
well-ordered mesostructure.
846 cmÀ1
.
Fig. 2 shows the XRD patterns of the host material
S1 and the hybrid material S2. For S1, the observation
of four diffraction peaks that can be indexed to (100),
(110), (200), and (210) reflections indicates a well-
ordered two-dimensional (2D) mesostructure. The intro-
duction of the Sm[N(SiMe3)2]3 results in the reduction
of the (100) and (110) reflections and the disappearance
of the (200) and (210) reflections. This suggests that the
intensity reduction may be a consequence of the contrast
matching between the silicate wall and the organic moi-
eties in the cavities [15]. But the observance of (100) and
(110) peaks indicates the intact of the well-ordered mes-
oporous structure. From the wide-angle XRD pattern of
the S2 material, only a broad XRD reflection peak de-
rived from the diffraction of the amorphous wall of
the material is displayed, which implies a molecular-le-
ver disposal of the Sm[N(SiMe3)2]3 on the internal
surface.
As shown in Fig. 1, both S1 and S2 display a frame-
work bands for the mas(Si–O–Si) at 1080 cmÀ1 and a
strong peak at 804 cmÀ1 attributable to msym(Si–O–Si).
The considerable weakening of the relative intensity of
Si
Si
O
O
Si
Si
OH
OH
Sm
N(TMS)2
n-hexane
r.t.
SBA-15
+
+
Sm[N(TMS)2]3
NH3
SBA-15
Si OSm[N(TMS)2]2
Si
OSiMe3
Si OH
S2
S1
Scheme 1. Immobilization of Sm[N(SiMe3)2]3 onto mesoporous SBA-15.

Y. Chen et al. / Journal of Organometallic Chemistry 690 (2005) 3783–3789
3785
b
a
S1
S2
S3
0
10
20
30
40
50
0
1
2
3
4
5
6
2Theta(degrees)
2Theta(degrees)
Fig. 2. The small-angle powder XRD patterns of S1, S2 and S3 (a) and wide-angle powder XRD pattern of S2 (b).
The success of the introduction of Sm[N(SiMe3)2]x-
moiety onto the SBA-15 is also confirmed by the N2
adsorption–desorption isotherms for S1 and S2. Both
samples show type IV isotherm with a H1 hysteresis
loop, characteristic of mesoporous materials according
to the IUPAC classification. The parent material S1
indicates a significant increment for the absorption of
N2 between the relative pressures 0.65 and 0.70. The
hysteresis above the relative pressures 0.65 can be attrib-
uted to the capillary condensation. The incorporation of
Sm[N(SiMe3)2]x-moiety is characteristic in the isotherm
for its lower uptake of N2 and earlier onset of hysteresis
compared to the parent material. The calculated BET
surface areas and the pore diameter according to the
Barrett–Joyner–Halenda (BJH) method also show the
difference between the host material S1 and modified
material S2, the latter displays a significant drop in the
two indexes. Noticeably, the preservation of the type
IV isotherm is contrast to the observation in introduc-
tion of silylamides into the MCM-41 [16], which gave
only the type I isotherm. From Fig. 2, we can also find
that S2 exhibits a narrow pore size distribution similar
to S1, reflecting that Sm[N(SiMe3)2]x-moiety is uni-
formly dispersed inside the channel.
lytic system is slightly slower than that in the corre-
sponding homogeneous system due to the restricted
diffusion. For example, the yields are significantly lower
than those in the homogeneous cases when the S2-cata-
lyzed reactions are also quenched in 24 h. However, the
final yields are comparable to those in the homogeneous
cases upon prolonged reaction time. The poor yields of
the entries 11 and 12 may be attributed to the deactiva-
tion of the +M effect of the substituted phenyl on the
carbonyl and the inherent steric hindrance of the sub-
strate. Significantly, if butyl aldehyde was applied, the
S2 catalytic system gives a different selectivity of prod-
ucts from the homogeneous system. Trimers are gener-
ated as the main products in the heterogeneous
system, while a complex mixture of oligomers is ob-
served under the same conditions in the homogeneous
one (Scheme 2). This difference might be partly contrib-
uted to the surface or pore confinement of the hybrid
material which disfavors the further aggregation of the
trimeric intermediates with other aldehyde molecules.
The hybrid material displays greater stability to oxy-
gen compared with Sm[N(SMe3)2]3. The manipulations
involved the heterogeneous catalytic reactions allow to
be performed without the protection of N2 for a short
time, while the same procedure for Sm[N(SMe3)2]3 is
unfeasible. Moreover, the hybrid material is easily sepa-
rated from the reaction mixture by centrifugation and
can be reused. In most of the cases, the recovered mate-
rials keep basically the inherent activity in a second or
third catalytic run (Table 1). In contrast to the zeolite-
based organic transformation where the pore blockage
was usually observed [17], X-ray diffraction and nitro-
gen sorption analyses shows that the recovered material
(S3) from entry 2 maintains a well-ordered mesostruc-
ture and the same pore diameter with S2, indicating that
the pore blockage in these catalytic processes does not
occur under involved conditions due to the large pore
2.2. Catalytic activity in Tishchenko reaction
In order to elucidate whether the grafting process for
silylamides is practically necessary, and what advantages
it offers, we examined the parallel catalytic performances
of Sm[N(SMe3)2]3 and [SBA-15]Sm[N(SMe3)2]x in Tish-
chenko reaction. All the catalytic reactions were carried
out at ambient temperature and the catalysts (2.5 mol%
metal) were dissolved or suspended in the solvents,
respectively. Main results are summarized in Table 1.
A comparison of homogeneous and heterogeneous
catalysis demonstrates that the reaction rate in S2 cata-

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Y. Chen et al. / Journal of Organometallic Chemistry 690 (2005) 3783–3789
Table 1
Tishchenko reactions catalyzed by Sm[N(SiMe3)2]3 and hybrid material (S2)
Entry
1.
Catalyst
Substrate
PhCHO
Time (h)
24
Procuct
Yield (%)a
Sm[N(SiMe3)2]3
PhCOOCH2Ph
85
80(1st run)
2.
3.
S2
PhCHO
48
24
PhCOOCH2Ph
70(2nd run)
50(3rd run)
O
O
O
O
CHO
O
O
O
O
Sm[N(SiMe3)2]3
45
O
O
30(1st run)
30(2nd run)
16(3rd run)
CHO
O
4.
5.
S2
48
24
O
O
H
H
Sm[N(SiMe3)2]3
100
O
O
O
O
100(1st run)
95(2nd run)
92(3rd run)
H
H
6.
S2
48
O
O
O
O
H
H
H
H
7.
8.
Sm[N(SiMe3)2]3
82
24
48
24
48
Me
Me
Cl
Me
Me
Cl
Me
Me
O
O
O
O
O
78(1st run)
75(2nd run)
54(3rd run)
S2
Sm[N(SiMe3)2]3
S2
O
O
9.
85
Cl
O
O
82(1st run)
80(2nd run)
81(3rd run)
10.
Cl
Cl
Cl
OMe
O
O
OMe
O
O
OMe
OMe
MeO
MeO
OMe
H
H
O
O
11.
12.
Sm[N(SiMe3)2]3
10
5
24
48
OMe
OMe
MeO
MeO
OMe
S2
a Yields were determined by GC with an internal standard n-dodecane.
diameter and volume of SBA-15. However, the elemen-
tal analyses and IR spectroscopy of S3 suggest that the
bound silylamide ligand has been almost replaced by the
newly formed alkoxide ligand. This is consistent with the
previous proposed pathway for Tishchenko reaction
[18].

Y. Chen et al. / Journal of Organometallic Chemistry 690 (2005) 3783–3789
3787
OH
O
OH
Sm[N(SiMe3)2]3
O
O
O
O
+
+
etc.
CH3CH2CH2CHO
CH3CH2CH2CHO
O
+
O
O
OH
O
O
OH
S2
+
O
7%
78%
15%
Scheme 2. The reactions of butyl aldehyde catalyzed by Sm[N(SiMe3)2]3 and hybrid material (S2).
PhCHO + RCHO
PhCO2CH2Ph + PhCO2CH2R + RCO2CH2Ph + RCO2CH2R
(a)
(b)
(c)
(d)
R = fural, 2,3-dimethyoxybenzyl
Scheme 3. Mixed Tishchenko reactions catalyzed by Sm[N(SiMe3)2]3 and hybrid material (S2).
Table 2
Selectivity in mixed Tishchenko reactions catalyzed by Sm[N(SiMe3)2]3 and hybrid material (S2)
Catalyst
Substrate
Solvent
Time (h)
Selectivity a:b:c:da
Sm[N(TMS)2]3
S2
PhCHO/furfural (1/1)
PhCHO/furfural (1/1)
Toluene/hexane (1/1)
Toluene/hexane (1/1)
Toluene/hexane (1/1)
Toluene/hexane (1/1)
48
48
48
48
33:36:17:14
27:53:7:13
49:51:0.3:0
33:67:0:0
Sm[N(TMS)2]3
S2
PhCHO/2,3-dimethoxybenzaldehyde (1/1)
PhCHO/2,3-dimethoxybenzaldehyde (1/1)
a
Selectivity was determined by GC on the basis of area percentage.
To obtain additional insight into the potential use of
the hybrid material in the Tishchenko reaction, we
examined the catalytic behavior of S2 toward the mixed
Tishchenko reaction (Scheme 3). Significantly, when two
kinds of aldehydes (1:1) were utilized in this perfor-
mance, the yields of the cross products PhCO2CH2R
in the S2 catalytic system were obviously elevated as
compared with the corresponding homogeneous cataly-
sis (Table 2). The change of the selectivity could be
attributed to the spatial restriction and the diffusion
control of the surface confinement.
brid material displays greater stability to O2 compared
with Sm[N(SMe3)2]3, and it is easily separated from
the reaction mixture and can be reused twice at least
without significant reduction of activity.
4. Experimental
4.1. General
All syntheses and manipulations of compounds were
performed with rigorous exclusion of air and moisture,
using high-vacuum and glove-box techniques. Solvents
were freshly distilled under argon from Na/K alloy. The
dehydrated SBA-15 sample S1 was synthesized according
to the literature method [8]. Hexamethyldisilazane was
purchased from Aldrich and degassed before use. All
aldehydes were obtained from Aldrich and were degassed
and stored in glove-box. IR spectra data were obtained
with an AUATAR 360 FT-IR (Nicolet) analyzer. Nitro-
gen adsorption–desorption isotherms were measured
with a Micromeritics Tristar 3000 analyzer at 77 K. Sam-
ples were degassed prior to analysis under vacuum for 5 h
at ambient temperature. The Barrett–Emmett–Teller
(BET) method was utilized to calculate the surface areas.
The pore volume and pore-size distributions were calcu-
lated using the Barrett–Joyner–Halanda (BJH) method.
3. Conclusions
In summary, we have successfully modified the sili-
cate SBA-15 with samarium silylamide complex, giving
a novel hybrid material characteristic for the presence
of the reactive metal moieties and the maintenance of
the well-ordered mesoporous structure. This new hybrid
material [SBA-15]Sm[N(SiMe3)2]x is an efficient and
mild catalyst for Tishchenko reaction, especially for
mixed Tishchenko reaction it is complementary to the
corresponding homogeneous catalyst Sm[N(SiMe3)2]3
in selectivity due to the spatial restriction and the diffu-
sion control resulted from the ordered mesoporous
structure of the hybrid material. Furthermore, the hy-

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Y. Chen et al. / Journal of Organometallic Chemistry 690 (2005) 3783–3789
0.8
Powder X-ray diffraction (XRD) patterns were recorded
with a Bruker D4 powder X-ray diffractometer using Cu
Ka radiation. Elemental analyses were carried out on a
Rapid CHN–O analyzer and a S4 EXPLORER X-ray
Spectrometer. Gas–liquid chromatograms were per-
formed on a HP 6890 series chromatograph equipped
with a FID detector and a HP 5793 mass-spectrometer
detector using He as a carrier gas.
600
550
500
450
400
350
300
250
200
150
100
50
S1
S2
S3
0.6
0.4
0.2
0.0
0
2
4
6
8
10
12
14
16
Pore Diameter(nm)
4.2. Immobilization of Sm[N(SiMe3)2]3 onto SBA-15
Tris[bis(trimethylsilyl)amido]samarium (0.52 g) was
dissolved in n-hexane (10 ml), and was quickly added
to a suspension of the dehydrated SBA-15 (0.78 g) in
n-hexane (10 ml). After stirring at ambient temperature
for 72 h, the reaction mixture was centrifuged and the
solution was decanted. The precipitate was washed sev-
eral times with n-hexane (20 ml) and dried in vacuum for
3 h to give S2 as a pale-yellow powder. Removal of sol-
vent from the solution left 0.04 g unreacted silylamide.
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure(P/P0)
Fig. 3. N2 adsorption–desorption isotherms at 77 K and pore size
distribution profiles of SBA-15 (S1), hybrid material (S2) and
recovered material (S3).
was filtered through the MgSO4 pad. The resulting dry
organic layer was then analyzed by GLC.
S1: White powder, BET surface area: 463.7 m2/g.
BJH pore diameter: 7.74 nm. IR (KBr): m = 3442 (s),
4.3.2. Condensation reactions of aldehydes catalyzed by
the hybrid material
162s9 (m), 1080 (s), 804 (s) cmÀ1
.
S2: Pale yellow powder, BET surface area: 273.4
m2/g. BJH pore diameter: 6.96 nm. Anal. Found: C,
13.21; H, 3.72; N, 1.96; Sm, 16.1%. IR (KBr):
m = 3417 (m), 2964 (m), 1627 (m), 1525 (m), 1392
The hybrid material (2.5 mol% metal) suspended in n-
hexane (2 ml) was stirred in a reaction flask. The sub-
strate (0.1 g) dissolved in toluene (2 ml) was added to
the catalyst. After stirring for 24 h, the catalyst was sep-
arated from the reaction mixture by centrifugation and
washed several times with hexane. The combined organic
layer was analyzed by GLC. The recovered material was
dried in vacuum and reused for the next run.
(m), 1080 (s), 846 (s), 806 (s) cmÀ1
.
S3:Yellow powder, BET surface area: 270.9 m2/g.
BJH pore diameter: 6.65 nm. Anal. Found: C,
16.29; H, 1.62; N, 0.48; Sm, 14.6%. IR (KBr):
m = 3411 (m), 2964 (m), 2921 (m), 2852 (m), 1594
(m), 1542 (m), 1421 (m), 1087 (s), 804 (m) cmÀ1
.
4.3.3. Mixed Tishchenko reaction
A mixture of two different aldehydes in 1:1 molar ra-
tio were dissolved in toluene (2 ml) and added to the cat-
alyst in n-hexane (2 ml) under the protection of N2.
After the reaction was completed, the mixture was
quenched, extracted and analyzed as described above
(see Fig. 3).
4.3. General catalytic procedure
The catalytic reactions were carried out under the
protection of nitrogen. The qualitative and quantitative
analysis of the reactants and the products was made by
GLC. Yields were determined by GLC based on the rel-
ative area of GLC signals referred to the internal stan-
dard (n-dodecane). Selectivity is represented by
product distribution based on the relative area of GLC
signals. Standard lipid products were prepared by the
literature procedures [7].
Acknowledgments
We thank the National Natural Science Foundation
of China and the Research Fund for the Doctoral Pro-
gram of Higher Education of China and the New Cen-
tury Distinguished Scientist of National Education
Ministry of China for financial support.
4.3.1. Condensation reactions of aldehydes catalyzed by
Sm[N(SiMe3)2]3
The catalyst (2.5 mol% metal) dissolved in n-hexane
(2 ml) was stirred in a reaction flask. The substrate
(0.1 g) dissolved in toluene (2 ml) was directly added
to the catalyst. After stirring for 24 h, the reaction mix-
ture was quenched with 2 ml of water, and extracted
with CH2Cl2 (3 · 2 ml). The combined organic extract
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