SiO2-Supported Rh Catalyst for Efficient Hydrosilylation of Olefins Improved by Simultaneously Immobilized Tertiary Amines

Article abstract of DOI:10.1021/acscatal.7b01523

The simultaneous immobilization of a Rh complex and a tertiary amine on a SiO₂ surface afforded a highly active supported catalyst for the hydrosilylation of terminal olefins. The turnover number in the reaction using 0.00005 mol % of the Rh catalyst approached 1 900 000 over 24 h. A broad range of terminal olefins and hydrosilanes acted as good substrates, giving their corresponding hydrosilylation products in excellent yields. CSI-mass analysis of the precursor solution containing the Rh complex and tertiary amine indicated complexation between these two precursors. The resultant close positioning of the Rh complex and amine after attachment onto SiO₂ surface led to excellent catalysis.

Full text of DOI:10.1021/acscatal.7b01523

Letter
pubs.acs.org/acscatalysis
SiO2‑Supported Rh Catalyst for Efficient Hydrosilylation of Olefins
Improved by Simultaneously Immobilized Tertiary Amines
Ken Motokura,*,† Kyogo Maeda,† and Wang-Jae Chun‡
†Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology,
4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan
‡Graduate School of Arts and Sciences, International Christian University, Mitaka, Tokyo 181-8585, Japan
S
* Supporting Information
ABSTRACT: The simultaneous immobilization of a Rh complex and a
tertiary amine on a SiO2 surface afforded a highly active supported
catalyst for the hydrosilylation of terminal olefins. The turnover number
in the reaction using 0.00005 mol % of the Rh catalyst approached
1 900 000 over 24 h. A broad range of terminal olefins and hydrosilanes
acted as good substrates, giving their corresponding hydrosilylation
products in excellent yields. CSI-mass analysis of the precursor solution
containing the Rh complex and tertiary amine indicated complexation
between these two precursors. The resultant close positioning of the Rh
complex and amine after attachment onto SiO2 surface led to excellent
catalysis.
KEYWORDS: rhodium complex, amine, silica, hydrosilylation, concerted catalysis
he hydrosilylation of olefins is one of the most widely
used reactions for the synthesis of organosilicon
Rh−H) bond.1,6 In this case, electron donation to the Rh
complex may promote both the oxidative addition and insertion
steps. These facts encouraged us to develop a Rh complex on
T
compounds in research laboratories and industrial settings.1
Recently, supported Rh complexes have received considerable
attention as highly active catalysts for hydrosilylation reactions.
The high cost of Rh necessitates the development of reaction
systems with very low Rh loadings, as well as recyclable
catalysts. For example, Sawano and co-workers reported the
stabilization of a Rh-phosphine complex in a MOF channel.2
Furthermore, Marciniec and co-workers reported a SiO2-
supported Rh complex that is active for the hydrosilylation of
olefins.3 The reason for the excellent performance of supported
Rh complexes is mainly due to the isolated Rh center on the
support surface, which enables stabilization of the active Rh
complex. Accordingly, the reaction mechanism of Rh complex-
catalyzed hydrosilylation changes, because of its immobilization
on the SiO2 surface, resulting in high activity and stability of the
Rh complex.3
3
SiO2 with an electron-donating amino functionality for the
hydrosilylation. Herein, we report the novel catalyst with
coimmobilized tertiary amines in close proximity to the directly
attached Rh(I) complexes on a SiO2 surface. The simulta-
neously immobilized Rh complex and tertiary amine catalyst
(i.e., SiO2/Rh-NEt2) exhibited an excellent turnover number
and a wide substrate scope for the hydrosilylation of olefins
(see eq 1).
The preparation procedure of the SiO2-supported Rh
complex is shown in Scheme 1. As seen, treatment of SiO2
with [Rh(OH)cod]2 afforded the SiO2-supported Rh complex
(SiO2/Rh). The SiO2 support with both the Rh complex and
tertiary amine (SiO2/Rh-NEt2) was prepared from a mixed
solution of both precursors, as shown in Scheme 1B. The
formation of methanol and water in the supernatant after the
Several research groups,4 including ours,5 have developed
this catalyst design concept further in order to increase catalytic
activity; organic moieties are immobilized close to the metal
complex on the same support surface. To illustrate, good
performance for the 1,4-addition of phenylboronic acid was
demonstrated using a catalyst involving a diaminorhodium
complex and a tertiary amine on a SiO2 surface.5a The tertiary
amine attached to the SiO2 surface can activate phenylboronic
acid, which accelerates transmetalation between the RhOH
species and phenylboronic acid.5a
1
grafting reaction was detected by H NMR analysis. Results
from elemental analysis of the prepared samples are
summarized in Table 1. As seen, essentially the same Rh
content was detected from the samples. However, SiO2/Rh-
Usually, Rh-catalyzed hydrosilylations proceed via Rh−Si (or
Rh−H) bond formation by oxidative addition, coordination of
Rh to an olefin, and insertion of the olefin into the Rh−Si (or
Received: May 9, 2017
Revised: June 6, 2017
© XXXX American Chemical Society
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Letter
Scheme 1. Preparation of (A) SiO2/Rh and (B) SiO2/Rh-
NEt2
Figure 1. 13C NMR spectra for (a) SiO2/Rh-NEt2, (b) tertiary amine
precursor, and (c) [RhOH(cod)]2.
Table 1. Elemental Analysis Results of SiO2-Supported Rh
Complexes
catalyst
SiO2/Rh
SiO2/Rh-NEt2
C (mmol g−1
)
N (mmol g−1
)
Rh (mmol g−1
)
4.3
7.0
0.46
0.43
0.6
NEt2 showed a larger amount of carbon and nitrogen, because
of the presence of the amine group.
The chemical structures of the attached Rh complex and
amine were characterized by solid-state nuclear magnetic
resonance (NMR), Fourier transform infrared (FT-IR) spec-
troscopy, and Rh K-edge X-ray absorption fine structure
(XAFS) measurements. Figure S1 in the Supporting
Information (SI) presents the 29Si MAS NMR spectra of
parent SiO2 and SiO2-support catalysts. After the attachment of
Rh, a decrease in Q2 (−90 ppm: Si(OH)2(OSi)2) and Q3
(−100 ppm: Si(OH)1(OSi)3) was detected, qualitatively. In the
case of SiO2/Rh-NEt2, the signals corresponding to the Q2 and
Q3 sites diminished. FT-IR analysis also revealed that the
intensity of ν(OH) stretching of SiOH (3747 cm−1)7
significantly decreased for supported catalysts, such as SiO2/
Rh and SiO2/Rh-NEt2 (see Figure S3 in the SI). The ν(OH)
stretching of the [Rh(OH)cod]2 precursor8 at 3585 and 3534
cm−1 also disappeared after attachment on the SiO2 surface (see
Figure S4 in the SI). These results clearly indicate that both
[Rh(OH)cod]2 and the tertiary amine with a Si(OMe)3 group
were immobilized by reaction with surface Si−OH groups on
the SiO2 surface. The 13C CP MAS NMR spectrum of SiO2/
Rh-NEt2 and the solution spectra of the Rh and amine
precursors are shown in Figure 1. A comparison of these
spectra indicates no significant change in the carbon chain of
the cyclooctadiene ligand and tertiary amine, suggesting that
the cyclooctadiene coordination structure3 and tertiary amine
skeleton in SiO2/Rh-NEt2 were maintained. FT-IR signals at
∼2700−3000 cm−1 (ν(CH)) and at 1464 cm−1 (δ(CH3))
(Figures S3 and S4) also support the NMR results. Figure 2
displays the Rh K-edge XANES and Fourier transform (FT) of
k3-weighted Rh K-edge EXAFS spectra of the SiO2-supported
Rh complexes, [Rh(OH)cod]2, and reference materials. These
XAFS results imply that the local structures of SiO2/Rh-NEt2,
SiO2/Rh, and [Rh(OH)cod]2 are almost identical; however,
they differ completely from Rh2O3 and Rh metal. The main
Figure 2. (A) Rh K-edge XANES and (B) Fourier transform of k3-
weighted Rh K-edge EXAFS spectra of (a) SiO2/Rh-NEt2, (b) SiO2/
Rh, (c) [RhOH(cod)]2, (d) Rh2O3, and (e) Rh0 foil. The k-range for
FT was k = 2.0−12 Å−1.
peak at 1.6 Å in the FT-EXAFS spectra can be assigned as Rh−
O or Rh−C, denoted as Rh−O/C bonds. The curve-fitting
results of the main peak using the Rh−O/C parameter, along
with the [Rh(OH)cod]2 crystal structure,8 are summarized in
Table 2. The peaks for supported Rh complexes were well-
fitted, with a coordination number of ∼6 and a Rh−O/C bond
length of 2.09 Å. The NMR, FT-IR, and XAFS results support
the proposed structures of SiO2/Rh and SiO2/Rh-NEt2 shown
in Scheme 1. Notably, the presence of the amine does not affect
the local structure of the Rh complex on the SiO2 surface.
The effect of coimmobilized amines on the hydrosilylation
activity of the Rh complexes was examined in the reaction of
terminal olefins with 1,1,1,3,5,5,5-heptamethyltrisiloxane as the
hydrosilane, as shown in Table 3. In the reaction of 1-
hexadecene, a 96% yield of the hydrosilylation product was
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a
Table 2. Curve-Fitting Analysis of EXAFS Spectra for SiO2-Supported Rh Catalysts
b
c
d
sample
SiO2/Rh
shell
coordination number, N bond distance, r (Å) Debye−Waller factor, Δσ2 (× 10−3 Å2) inner potential correction, ΔE (eV)
Rh−C/O
Rh−C/O
(Rh−C)
(Rh−O)
5.5 0.8
2.09 0.01
2.10 0.01
5.33 0.23
4.23 0.23
−4.00 2.36
−4.40 2.36
SiO2/Rh-NEt2
[Rh(cod)OH]2
[Rh(cod)OH]2
5.7 0.8
e
e
f
4
2
2.09
f
2.07
a
Fourier transform and Fourier-filtering regions were limited, where Δk = 2.8−15 Å−1 and Δr = 1.1−2.0 Å, respectively. The goodness of curve fit is
b
c
in the range of 0.67−1.07. This is the bond distance between the absorber and backscatter atoms. The Debye−Waller factor (DW) value is relative
d
to the DW of the reference. The inner potential correction accounts for the difference in the inner potential between the sample and reference.
e
f
Data taken from ref 8. Average values were reported.
The scope of silanes compatible with the SiO2/Rh-NEt2-
catalyzed reaction is summarized in Chart 1. Simple
Table 3. Hydrosilylation Catalyzed by SiO2-Supported Rh
Complex
Chart 1. Substrate Scope for the SiO2/Rh-NEt2-Catalyzed
1
Hydrosilylation (Yield Was Determined by H NMR Using
an Internal Standard)
a
Yield (%)
catalyst
hydrosilylation
isomerization (cis:trans)
R = (CH2)12CH3
96 (15 min)
9 (30 min)
SiO2/Rh-NEt2
SiO2/Rh
4 (45:55)
1 (48:52)
<1
SiO2/NEt2
<1 (120 min)
R = Ph
SiO2/Rh-NEt2
SiO2/Rh
88 (60 min)
11 (60 min)
9 (6:94)
1 (16:84)
a
1
Determined by H NMR.
obtained in the presence of SiO2/Rh-NEt2 within 15 min,
whereas the yield was only 9% with SiO2/Rh for 30 min. The
catalyst with the tertiary amine also showed higher activity in
the reaction of allylbenzene. These results indicate that
hydrosilylation product yield was improved by the tertiary
amine immobilized on the same SiO2 surface with the Rh
complex. In both cases, a small amount of the olefin
isomerization product was detected.
Table 3 also shows that the turnover number (TON) of Rh
approached 2560 for the reaction of 1-hexadecene with SiO2/
Rh-NEt2. The addition of new substrates after the catalytic
reaction revealed that the catalyst could be reused at least 5
times (total TON = 13 000). To further increase the TON for
hydrosilylation, we examined the reaction using only 0.00005
mol % of Rh in SiO2/Rh-NEt2 (Scheme 2). Remarkably, the
TON increased to 1 900 000 over 24 h, which is significantly
higher than those of recently reported highly active Rh
catalysts, such as MOF-Rh (TON = 820 000 over 72 h)2 and
SiO2/Rh (TON = ca. 200 000 over 1 h × 10 cycles).3c
a
b
Isolated yield. 14% of 2-hexadecene (cis:trans = 64:36) was formed.
c
d
13% of 2-hexadecene (cis:trans = 38:62) was formed. 38% of 2-
e
hexadecene (cis:trans = 16:84) was formed. At 70 °C.
alkylsilanes, such as triethylsilane, were found to be good
substrates for the reaction, affording quantitative product yields.
The triethoxysilyl group could also be introduced to the
terminal olefin. High yields of the corresponding products were
obtained in the reactions of dimethylphenylsilane and
diphenylmethylsilane (82% and 71% yield, respectively).
However, hardly any of the hydrosilylation product was
obtained in the reaction of triphenylsilane. In these reactions
of phenylsilanes, olefin isomerization product (2-hexadecene)
formed as a byproduct. The ratio of the trans isomer of 2-
hexadecene increased with as the number of phenyl groups on
Si increased, indicating the formation of sterically hindered H−
Rh−Si(Ph)n species during the reaction. Chart 1 also
summarizes the scope of terminal olefins for the hydro-
silylation. As seen, not only aliphatic olefins but also
allylbenzene derivatives can be used as good substrates,
affording their corresponding hydrosilylation products in
good to excellent yields. Furthermore, terminal olefins having
Scheme 2. Large-Scale Hydrosilylation Catalyzed by
SiO2/Rh-NEt2
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Letter
synthetically useful functional groups, such as nitrile and
epoxide, were effectively converted to their corresponding
products with their functional groups left unchanged.
compared with SiO2/NEt2 → Rh.9 The reason for the similar
activities of SiO2/Rh → NEt2 and SiO2/Rh-NEt2 remains
unclear, because differences in the interaction environment
between “homogeneous−homogeneous” and “surface−homo-
geneous” species are not easy to assess. Conclusively, the
interaction between the Rh complex and tertiary amine is a key
factor for the preparation of a highly active catalyst for
concerted catalysis. EXAFS analysis of the used SiO2/Rh-NEt2
catalyst indicated the recovery of the initial Rh coordination
structure (Figure S8), highlighting the good stability of the Rh
complex during catalysis. Investigations regarding the inter-
action mode between the Rh complex and tertiary amine in the
precursor solution and characterization of the reaction
mechanism by in situ analysis are now underway.10
In summary, SiO2/Rh-NEt2 was found to be an excellent
heterogeneous catalyst for the hydrosilylation of terminal
olefins. The TON approached 1 900 000 over 24 h, and a
variety of silanes and olefins were shown to be effective
substrates. The effect of the surface tertiary amine was strongly
affected by the immobilization procedure. CSI-mass analysis
suggests that the interaction between the two precursors before
and/or during the immobilization step is mandatory to improve
the product yield and TON.
After hot filtration of the SiO2/Rh-NEt2 catalyst at ca. 30%
conversion, the reaction completely stopped. In addition, the
catalytic performance decreased for the system using a mixture
of diethylbutylamine, a homogeneous amine, and SiO2/Rh,
compared with SiO2/Rh-NEt2. These results indicate that the
hydrosilylation reaction occurs at the solid surface. Thus, the
Rh complex and tertiary amine are likely to be located in close
proximity to each other on the SiO2 surface for a concerted
catalysis. As shown above, the EXAFS results of the catalyst
with and without the tertiary amine are almost identical,
suggesting no direct interaction between the Rh metal center
and the tertiary amine. However, CSI-mass analysis of the
precursor solution containing [Rh(OH)cod]2 and diethylami-
nopropyltrimethoxysilane showed several signals that are
assigned as complexes containing Rh-cyclooctadiene and the
tertiary amine (see Figure S7 in the SI). The interaction
between the Rh complex and tertiary amine in the precursor
solution strongly suggests that these two functionalities are
positioned closely on the SiO2 surface, enabling the concerted
activation of substrates.
If the interaction between the Rh complex and tertiary amine
before immobilization influences the catalysis, the preparation
procedure should strongly affect catalytic activity. To verify the
assumption, we examined bifunctional catalysts synthesized via
a sequential immobilization procedure, that is, SiO2/Rh →
NEt2 and SiO2/NEt2 → Rh. No significant changes of the Rh
coordination environment were observed in these catalysts by
XAFS and FT-IR analysis (see Figures S5 and S8 in the SI).
The catalytic reaction results are summarized in Figure 3. In the
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.7b01523.
■
S
Experimental section, product characterization, NMR,
XAFS, FT-IR, and CSI-mass spectra (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: motokura.k.ab@m.titech.ac.jp.
■
ORCID
Ken Motokura: 0000-0002-0066-5139
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
XAFS measurements were conducted under the approval of the
Photon Factory Advisory Committee (Proposal No.
2016G025). This study was supported by the JSPS KAKENHI
(Grant Nos. 25630362 and 15H04182), JSPS Grant-in-Aid for
Scientific Research on Innovative Areas (Grant Nos. 16H01010
and 26105003), cooperative research program of Institute for
Catalysis, Hokkaido University (Grant No. 17A1002), and
Challenging Research Award from Tokyo Tech. We would like
to thank Dr. Yohei Uemura (Institute for Molecular Science)
for his support with XAFS analysis. We also appreciate Dr.
Kosuke Suzuki, Prof. Kazuya Yamaguchi, and Prof. Noritaka
Mizuno (The University of Tokyo) for their help with the CSI-
mass measurement, and Ms. Chiaki Tsuneishi and Dr. Shinsuke
Inagi (Tokyo Tech) for their help with FT-IR measurement.
We thank Prof. Toshihide Baba (Tokyo Tech) for his generous
support and unrestricted access to analytical facilities.
Figure 3. Effect of preparation procedure of SiO2-supported Rh
catalysts on activity for hydrosilylation.
case of the sequential immobilization of the Rh complex onto
SiO2/NEt2 (SiO2/NEt2 → Rh), the product yield increased
(59%), compared with SiO2/Rh (11%). The initial immobiliza-
tion of Rh (SiO2/Rh → NEt2) induced a catalytic performance
(86%) that was comparable with that observed for SiO2/Rh-
NEt2 (88%). It can be expected that the attachment of Rh on
the weakly acidic SiO2 surface may slightly enhance its
electrophilicity, resulting in its interaction with the nucleophilic
amine molecule. This interaction would enhance the close
immobilization of the two species in SiO2/Rh → NEt2,
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