A Pd-bisphosphine complex and organic functionalities immobilized on the same SiO2 surface: Detailed characterization and its use as an efficient catalyst for allylation

Article abstract of DOI:10.1039/c6cy00593d

A Pd-bisphosphine complex and several organic functionalities were immobilized on the same SiO₂ surface. The samples thus prepared were characterized by solid-state NMR, XPS, and XAFS. Based on the curve-fitting analysis of Pd K-edge EXAFS spectra, both the local environment of the immobilized Pd complexes and the interactions with the co-immobilized organic functions were discussed. The SiO₂-supported Pd-bisphosphine complex and DABCO exhibited excellent catalytic performance for the allylation of various nucleophiles: a TON of up to 106000 was obtained. Both the catalyst activation pathway and the reaction mechanism were also discussed on the basis of the structure of the used catalyst samples.

Full text of DOI:10.1039/c6cy00593d

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Pd-Bisphosphine Complex and Organic Functionalities Immobilized on the Same SiO2
Surface: Detailed Characterization and Its Use as an Efficient Catalyst for Allylation
Ken Motokura,1 Koki Saitoh,1 Hiroto Noda,1 Wang-Jae Chun,2 Akimitsu Miyaji,1 Sho
Yamaguchi,1 Toshihide Baba*,1
1
Department of Environmental Chemistry and Engineering, Tokyo Institute of
Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, 226-8502, Japan
E-mail: tbaba@chemenv.titech.ac.jp
2
Graduate School of Arts and Sciences, International Christian University, Mitaka,
Tokyo, 181-8585, Japan
ABSTRACT:
Pd-bisphosphine complex and several organic functionalities were immobilized on
the same SiO2 surface. The samples thus prepared were characterized by solid-state
NMR, XPS, and XAFS. Based on the curve-fitting analysis of Pd K-edge EXAFS
spectra, both the local environment of the immobilized Pd complexes and interactions
with the co-immobilized organic functions were discussed. The SiO2-supported
Pd-bisphosphine complex and DABCO exhibited excellent catalytic performance for
the allylation of various nucleophiles: a TON of up to 106000 was obtained. Both the
catalyst activation pathway and reaction mechanism were also discussed on the basis of
the structure of the used catalyst samples.
KEYWORDS: synergistic catalysis, palladium, organic base, bisphosphine, allylation
1

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Introduction
The palladium-catalyzed allylation of nucleophiles is one of the most efficient
protocols for constructing C-C bonds and introducing highly reactive allyl groups.[1]
Various homogeneous and heterogeneous palladium catalysts for allylation have been
reported.[2,3] Scheme 1 shows the previously reported Pd-catalyzed allylation systems
exhibiting very high turnover numbers (TONs). Uozumi and co-workers have
investigated the allylation of aromatic nucleophiles using both homogeneous[2a] and
heterogeneous[3a] catalysts with excellent TONs of 500 000 000 and 1250 000,
respectively. Moreover, Santelli and co-workers have reported a TON of 9800 000 for
the allylation of alkyl nucleophiles, such as active methylenes, using a homogeneous
Pd-Tedicyp catalyst in the presence of stoichiometric amount of NaH (Scheme 1).[2b]
The pre-activation of the alkyl C-H bond significantly enhances the reactivity of the
nucleophile; however, in this case, the formation of stoichiometric salt wastes cannot be
avoided. On the other hand, TONs of up to 2000 have been reported for Pd catalyst
systems using alkyl nucleophiles without pre-activation.[2,3]
For overcoming this problem, the concept, involving the synergistic activation of
both the allylating reagent and nucleophile by Pd species and catalytic amount of base,
respectively, should be effective.[4] This concept of synergistic activation has been
demonstrated by Muzart and co-workers in 1987 with the use of a heterogeneous
catalysis system.[3c] On the other hand, our group has also been continuously
developing a bifunctional catalytic surface composed of Pd- and an organic base, which
enables the synergistic activation of both allylating reagents and nucleophiles.[5,6]
Especially, a Pd-bisphosphine complex immobilized on SiO2 surface with
1,4-diazabicyclo[2.2.2]octane (DABCO) functionality was found to be a potentially
2

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highly active catalyst.[5c] The formation of stoichiometric waste can be avoided by the
use of a co-immobilized amine base, which serves as a catalyst for activating the
nucleophile. In addition, the close proximity of the Pd complex and organic base on the
same solid surface induces efficient bond formation between the activated substrate
molecules.
Scheme 1. Homogeneous and heterogeneous Pd-catalyzed allylation reaction systems
exhibiting high turnover numbers. Nu: alkyl carbon nucleophile, R: leaving group such
as methyl carbonate.
In this paper, we fully investigate the synergistic catalysis between a Pd-bisphosphine
complex and organic functions immobilized on the same SiO2 surface, including the
3

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detailed catalyst structure, interaction between the Pd complex and organic functions,
and substrate scope as well as mechanism of allylation. In the case of a catalyst with
DABCO functionality, the highest TON based on Pd of up to 106000 was achieved
without the pre-activation of nucleophile (Scheme 1, this study).
Results and Discussion
Preparation of SiO2-supported Pd Complexes
SiO2-supported Pd-bisphosphine complex catalysts were prepared by a simple
silane-coupling reaction. As shown in Scheme 2, a Pd complex precursor (PP-Pd) was
prepared from a bisphosphine ligand having a triethoxysilane group and
1
1
1
[PdCl(η3-allyl)]2.[5c] H, 13C, 31P, H-1H COSY, H-13C HSQC NMR, and HRMAS
analyses clearly indicate the formation of the PP-Pd complex structure (Supporting
Information, Figure S1 and S2). The silane-coupling reaction between SiO2
(Aerosil300) and PP-Pd afforded the SiO2-supported Pd-bisphosphine complex
(SiO2/PP-Pd) (Scheme 3a).[5c,7] Other supported catalysts having organic functional
groups on the same surface were prepared from pre-functionalized SiO2 and PP-Pd. For
DABCO-immobilized SiO2, SiO2 was treated with chloropropyltrimethoxysilane to
afford SiO2/Cl, followed by quaternization between SiO2/Cl and DABCO, affording
SiO2/DABCO (Scheme S1, Supporting Information).[5c] SiO2 with methyl and n-hexyl
groups (SiO2/Me and SiO2/Hex, respectively) were also prepared by the silane-coupling
reaction (Scheme 4). Pre-functionalized SiO2 (SiO2/R) was treated with PP-Pd to afford
the corresponding SiO2-supported functional group (R) and Pd complex (SiO2/R/PP-Pd),
as shown in Scheme 3b-d.
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Scheme 2. Preparation of Pd-bisphosphine complex (PP-Pd).
Scheme 3. Preparation of SiO2/DABCO/PP-Pd, SiO2/PP-Pd, SiO2/Me/PP-Pd, and
SiO2/Hex/PP-Pd
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Scheme 4. Preparation of SiO2-supported organic functions (SiO2/Me and SiO2/Hex).
Characterization of SiO2-supported Pd Complexes
Table 1 summarizes the results obtained from the elemental analysis of the prepared
samples. For SiO2/DABCO, the nitrogen:chlorine ratio was ca. 2, indicative of the
formation of the quaternized DABCO structure.[5c] 13C CP/MAS NMR analysis was
employed for confirming the quantitative formation of quaternized DABCO from the
chloropropyl functionality of SiO2/Cl (Figure S3, Supporting Information).[5c] Similar
nitrogen, chlorine, and palladium contents were obtained for SiO2/PP-Pd,
SiO2/Me/PP-Pd, and SiO2/Hex/PP-Pd, implying that the PP-Pd complex structure
(N:Cl:Pd=1:1:1) on the SiO2 surface is maintained. As compared with that in
SiO2/PP-Pd, the nitrogen and chlorine content in SiO2/DABCO/PP-Pd increased
because of the immobilization of quaternized DABCO.
The coordination structure and carbon skeleton of the immobilized functions were
determined by solid-state 31P and 13C MAS NMR analysis, respectively. A strong signal
was observed at around 0 ppm in the 31P MAS NMR spectra of all supported
Pd-bisphosphine complex samples, implying that the Pd-bisphosphine complex
structure of PP-Pd is maintained (solution-state 31P NMR: 3 ppm) (Figure S4,
Supporting Information). Figure 1 shows the results obtained from the 13C NMR spectra.
The 13C NMR signals of the main carbon skeleton (a-e and Ph) of PP-Pd (a) were
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observed in SiO2/PP-Pd (b). These signals were also detected in other SiO2-supported
Pd complexes with other organic functions (SiO2/R/PP-Pd) (c-e). In addition, the signals
derived from the co-immobilized organic functions were observed: for example, in
SiO2/DABCO/PP-Pd (e), the signals of carbons next to the nitrogen atoms of
quaternized DABCO were detected at 46, 53, and 66 ppm (blue symbols).[5c]
Table 2 shows the XPS signals of SiO2-supported Pd complexes. Figure 2 shows the
XPS spectra of Pd3d, N1s, and Cl2p. As shown in Table 2 and Figure 2(A), the Pd3d5/2
signals of all samples were observed at 337.2-337.5 eV, corresponding to the Pd(II)
species with the phosphine ligand.[7] The signals of the P2p region were observed at
131.1-131.6 eV, also corresponding to the Pd-bisphosphine complex structure (Table 2)
[7]. For SiO2/DABCO/PP-Pd, two N1s signals were detected (399.6 and 402 eV,
respectively), while for other SiO2-supported Pd samples, only one signal was observed
(Table 2, Figure 2(B)), indicating the presence of tertiary and quaternized amine groups
in SiO2/DABCO/PP-Pd.[5c,8] The 1:2 atomic ratio of Pd and P was also detected by
XPS analysis.
For determining the detailed local structure of immobilized Pd complexes, Pd K-edge
XAFS analysis was conducted.[9] XANES, EXAFS, and the Fourier transform (FT) of
k3-weighted Pd K-edge EXAFS spectra (Figures S5-S7, Supporting Information) of
PP-Pd precursor, SiO2/PP-Pd, and SiO2/R/PP-Pd were almost identical.
Table
3
summarizes the curve-fitting results of EXAFS spectra. For
SiO2/DABCO/PP-Pd, the coordination number (N) and bond length (r) were evaluated
as 3.0 ± 0.5 and 2.26 ± 0.01 Å, respectively. This bond length was similar to those of the
Pd-P/Cl bonds in Pd(II)Cl2(PPh3)2 (average values: 2.32 Å).[10] These results clearly
support the proposed structure of the Pd complex on the SiO2 surface: two Pd-P and one
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Pd-Cl bonds. As shown in Table 3, other fresh SiO2-supported Pd complexes exhibited
similar N (2.6-2.9) and r values (2.26-2.27 Å), and these values were almost identical to
those of the PP-Pd precursor.
The above EXAFS analysis has revealed the maintenance of the local coordination
atoms in the first shell of all the Pd complexes immobilized on the SiO2 surface both
with and without the co-immobilized organic functions. The EXAFS results have
enlightened that no specific change is observed in the local structure of the immobilized
Pd complexes on the SiO2 surface both with and without co-immobilizers. Noteworthy,
however, is the clear changes in the Debye-Waller factors (∆σ2). As shown in Table 3,
the ∆σ2 values of the SiO2-supported samples were clearly affected by the
co-immobilized organic functions. Figure 3 (○) shows the dependence of the ∆σ2 value
on the size of the co-immobilized functions on the same SiO2 surface. The ∆σ2 value
increased with the increasing size of the co-immobilized molecule. As the Debye-Waller
factor is complementary with the coordination number (N), the ∆σ2 value was simulated
with a fixed coordination number N=3 (Figure 3; ▲): similar phenomena were also
observed. The Debye-Waller factor comprises contributions from both static and
dynamic disorder: σ2=σstatic2+σdynamic2. As shown in Table 3, the bond length (r) of the Pd
complex immobilized on the SiO2 surface was almost same, indicating σdynamic2, which
should be related to νPd-P/Cl, does not change by the size of the co-immobilized organic
functions.[11] In conclusion, the σstatic value can be expected to increase by the static
disorder of the Pd complex immobilized on the SiO2 surface by the steric effect
attributed to the co-immobilized organic functions. Figure 4 shows the schematic of the
order of the steric effect. The Pd complex and co-immobilized organic functions,
especially DABCO, existed in close proximity, suggesting their cooperation for a single
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catalytic transformation.
Allylation Catalyzed by SiO2-supported Pd Complexes
The prepared SiO2-supported Pd complex were used for the allylation between ethyl
acetoacetate and allylmethylcarbonate. Table 4 summarizes the results. The reaction
bewteen 0.5 mmol of ethyl acetoacetate and 2.5 equivalent of allylmethylcarbonate
using 3.0 μmol of Pd in SiO2/DABCO/PP-Pd afforded allylated products in a total yield
of 99% with a TON of 300 (entry 1). With the use of the supported catalyst without
DABCO under the same reaction conditions, the TON decreased to 230 (entry 2). With
the use of other organic functions, such as methyl and hexyl groups (entries 3 and 4,
respectively), the product yield and TON did not increase as compared with those
observed with the use of SiO2/PP-Pd. These results clearly indicate that only DABCO
can enhance the allylation reaction. As shown in the above section, the Debye-Waller
factor (∆σ2) of SiO2/DABCO/PP-Pd and SiO2/Hex/PP-Pd were almost the same, while
the catalytic activity (TON) of SiO2/DABCO/PP-Pd was significantly higher than that
of SiO2/Hex/PP-Pd. This result indicates that the static disorder of the Pd complex
scarcely affects the catalytic activity.
The homogeneous Pd precursor (PP-Pd) exhibited catalytic activity similar to that
exhibited by the supported SiO2/PP-Pd catalyst (entry 5), indicating the catalytic
activity of the PP-Pd complex is maintained after the immobilization on the SiO2
surface. On the other hand, the addition of DABCO to the homogeneous PP-Pd catalyst
system did not enhance its catalytic activity (entry 6). Interestingly, the
co-immobilization of both PP-Pd and DABCO function on the SiO2 surface is necessary
for enhancing the catalytic activity.
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As the SiO2/DABCO/PP-Pd catalyst exhibited the highest activity for the allylation
reaction, the substrate scope was examined using this SiO2/DABCO/PP-Pd catalyst
under solvent-free conditions. Table 5 summarizes the results. Surprisingly, the reaction
of acetylacetone using 0.002 mol% of Pd in SiO2/DABCO/PP-Pd afforded the
corresponding allylated products in a total yield of 90%, with a TON of 106000 (entry
1). To the best of our knowledge, this TON value is more than two orders higher than
those of previously reported catalyst systems without the pre-activation of the alkyl C-H
bond of nucleophiles. Other nucleophiles, such as nitrile, diketone, ketoester, and even
diester, exhibited high reactivity, affording the corresponding diallylated prodcuts in
good to quantitative yields with a TON of greater than 11000 (entries 3, 4, 6, and 7,
respectively) except in the case of a steric bulk substrate (entry 5). For a ketoester with
one α-proton, mono allyaltion selectively proceeded, affording the corresponding
product in 98%, with a TON of 11700 (entry 6). Phenol was also found to be a reactive
nucleophile in this catalyst system (entry 8). In all cases, the reaction smoothly
proceeded in the presence of a very small amount of K2CO3.
To check the heterogeneous catalysis of the active SiO2/DABCO/PP-Pd, a hot
filtration test was conducted. After the removal of the solid catalyst at around a TON of
200, further allylation in the filtrate hardly occurred, suggesting that the reaction
proceeds on the solid surface. Regarding catalyst durability, after the completion of the
reaction of ethyl acetoacetate using SiO2/DABCO/PP-Pd, new substrates were added to
the reaction mixture. A further reaction proceeded to give almost quantitative yield of
the allylated products for at least 6 cycles.[5c]
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Catalyst Activation Pathway and Catalytic Reaction Mechanism
Characterization results indicated that the Pd species in the SiO2-supported
Pd-bisphosphine complexes is divalent with respect to chloride and the η1-allyl anion
[PP-Pd(η1-allyl)Cl]. Figure 5 shows the Pd K-edge XANES spectra of fresh
SiO2/DABCO/PP-Pd, SiO2/DABCO/PP-Pd-used, and several reference samples. The
XANES position of SiO2/DABCO/PP-Pd barely changed after the catalytic reaction,
indicating that the Pd species is still divalent after allylation. As shown in Table 2, the
XPS signal of Pd3d5/2 also supports the presence of Pd(II) after the reaction. These
results indicate that the robust structure of the Pd complex is stabilized by the
bisphosphine ligand, inhibiting the aggregation of Pd(0) particles during the catalytic
reaction.
On the other hand, the Pd-catalyzed allylation is widely accepted to proceed via the
formation of π-allylpalladium(II) and a Pd(0) species. For promoting the allylation, such
active Pd species should be form from the PP-Pd(η1-allyl)Cl structure on the SiO2
surface. As shown in Table 2 and Figure 2(C), the XPS signal position of Cl2p shifted
from 198.0 eV to 199.8 eV after the catalytic reaction, suggesting the formation of
potassium chloride.[12] The curve-fitting analysis of EXAFS spectra also indicates the
decrease of the Pd-P/Cl coordination number (N) of SiO2/DABCO/PP-Pd from 3.0 to
2.2 after the catalytic reaction (Table 3). These results are indicative of the removal of
Cl from the PP-Pd(η1-allyl)Cl complex. The reaction of SiO2/DABCO/PP-Pd with ethyl
acetoacetate and K2CO3 without allylmethylcarbonate afforded a small amount of the
allylated product. On the other hand, XPS and EXAFS curve-fitting analysis results
barely changed after the treatment of SiO2/DABCO/PP-Pd with only
allylmethylcarbonate (SiO2/DABCO/PP-Pd-treated, Table 2 and 3). These results
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indicate that the surface PP-Pd(η1-allyl)Cl complex first reacts with the nucleophile,
affording the allylated nucleophile, KCl, and monomeric Pd(0) (Scheme 5). Second, the
monomeric Pd(0) species immediately reacts with allylmethylcarbonate to afford
π-allylpalladium(II). The DABCO functionality immobilized on the SiO2 surface, which
is in close proximity to the PP-Pd complex, activates the nucleophile, followed by the
reaction between the activated nucleophile and π-allylpalladium(II) to form the allylated
product.
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Scheme 5. Proposed catalyst activation and catalytic reaction mechanism using
SiO2/DABCO/PP-Pd
Conclusion
In this study, Pd-bisphosphine complex and several organic functionalities
immobilized on the same SiO2 surface were characterized in detail by solid-state NMR,
XPS, and XAFS. The results obtained from the curve-fitting analysis of Pd K-edge
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EXAFS spectra indicate that the Debye-Waller factor increases with increasing size of
co-immobilized organic functions, suggesting that the immobilized functionalities are in
close proximity with the Pd-bisphosphine complex. The Pd complex and DABCO
functionality immobilized on the same SiO2 surface synergistically catalyzed the
allylation reaction: a TON of up to 106000 was achieved. EXAFS study of the used
catalyst indicated that the reaction proceeds via the formation of π-allylpalladium(II)
and a monomeric Pd(0) species. Synergism between immobilization methods for
well-defined molecules on solid surfaces and atomic-level characterization techniques
for these functionalized surfaces provides new opportunities for the creation of highly
efficient heterogeneous catalysts.
Acknowledgements
We thank Dr. Yohei Uemura (Institute for Molecular Science) for his support with
XAFS analysis. This study was conducted under the approval of the SAGA-LS
Advisory Committee (Proposal No. 1504037F). This study was supported by JSPS
KAKENHI (grant Nos. 25630362 and 15H04182) and JSPS Grant-in-Aid for Scientific
Research on Innovative Areas “3D Active-Site Science” (grant No. 26105003). H.N.
thanks Grant-in-Aid for JSPS Fellows (grant No. 26•12038).
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Table 1. Elemental analysis of SiO2-supported materialsa
sample
C (mmol g-1)
N (mmol g-1) Cl (mmol g-1) Pd (mmol g-1)
SiO2/Cl
3.8
6.2
-
0.8
0.6
0.3
0.7
0.3
0.3
-
SiO2/DABCO
SiO2/PP-Pd
1.4
0.4
1.1
0.3
0.3
-
11.9
0.28
0.17
0.18
0.25
SiO2/DABCO/PP-Pd 11.5
SiO2/Me/PP-Pd
SiO2/Hex/PP-Pd
9.6
10.8
a Part of the data in this table were previously reported in ref.[5c]
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Table 2. XPS analysis of SiO2-supported Pd catalysts
binding energy (eV)
Sample
Pd 3d5/2
337.5
337.2
337.2
337.4
337.5
P 2p
N 1s
Cl 2p
SiO2/PP-Pd
131.4
131.6
131.1
131.2
131.6
131.6
-
399.6
399.6, 402
399.2
399.6
399.9, 402
399.6, 402
-
197.9
198.0
197.6
198.0
199.8
198.7
199.2
199.2
SiO2/DABCO/PP-Pd
SiO2/Me/PP-Pd
SiO2/Hex/PP-Pd
SiO2/DABCO/PP-Pd-used
SiO2/DABCO/PP-Pd-treateda 337.4
b
PdCl2
338.1
-
KCl c
-
-
Reference: Si 2p (103.4 eV).
a SiO2/DABCO/PP-Pd catalyst treated with allylmethylcarbonate.
b Data from ref. [7]
c Data from ref. [12]
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Table 3. Curve-fitting analysis of EXAFS spectra for SiO2-supported PP-Pd complex
catalystsa
Sample
shell
N b
r c
∆σ2 d
(Å2×10–3) (eV)
2.80
∆E e
Rf f
(%)
1.95
(Å)
2.26
SiO2/PP-Pd
Pd-P/Cl 2.6
–2.07
±2.03
–0.61
±1.98
–6.81
±2.09
–3.11
±2.00
–5.89
±1.91
–8.75
±2.17
–4.71
±2.04
±0.4 ±0.01 ±0.20
Pd-P/Cl 3.0 2.26 5.05
±0.5 ±0.01 ±0.20
Pd-P/Cl 2.6 2.27 3.63
±0.4 ±0.01 ±0.20
Pd-P/Cl 2.9 2.27 4.86
±0.4 ±0.01 ±0.20
Pd-P/Cl 3.0 2.29 2.64
(fix) ±0.01 ±0.03
Pd-P/Cl 2.2 2.25 4.86
±0.3 ±0.01 ±0.20
SiO2/DABCO/PP-Pd-treated h Pd-P/Cl 2.7
2.26 5.62
±0.4 ±0.01 ±0.20
SiO2/DABCO/PP-Pd
SiO2/Me/PP-Pd
3.05
1.28
2.87
1.28
2.15
2.28
SiO2/Hex/PP-Pd
PP-Pd (THF solution) g
SiO2/DABCO/PP-Pd-used
i
Pd(II)Cl2(PPh3)2
(Pd-P)
2
2
2.34 j
2.30 j
(Pd-Cl)
a
Fourier transform and Fourier-filtering region were limited, where ∆k = 2.8 ~ 15 Å-1.
and ∆r = 1.4 ~ 2.2 Å, respectively.
b Coordination number.
c Bond distance between absorber and backscatter atoms.
d The Debye-Waller factor (DW), which is relative to the DW of the reference.
e
The inner potential correction accounts for the difference in the inner potential
between the sample and reference
f A goodness of curve fit.
g Homogeneous THF solution of the PP-Pd complex.
h SiO2/DABCO/PP-Pd catalyst treated with allylmethylcarbonate.
i Data from ref. [10]
j Average value was reported.
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Table 4. The allylation catalyzed by SiO2-supported Pd complexes a
entry catalyst
conversion (%) yield (mono/di) (%) TON (×102)b
1
2
3
4
5
SiO2/DABCO/PP-Pd
>99
98
83
90
97
83
18 / 81
40 / 49
62 / 21
55 / 31
48 / 42
53 / 22
3.0
SiO2/PP-Pd
2.3
1.7
2.0
2.2
1.6
SiO2/Me/PP-Pd
SiO2/Hex/PP-Pd
PP-Pd
6 c
PP-Pd + DABCO
a
Reaction conditions: ethyl acetoacetate (0.50 mmol), allylmethylcarbonate (1.25
mmol), Pd catalyst (Pd: 3.0 μmol), K2CO3 (0.25 mmol), toluene (2 mL), 70 oC, 60 min.
b
TON was calculated as follows: TON = (moles of nucleophile)×[(yield of the
mono-product) + 2×(yield of the di-product)] / (moles of Pd).
c 6μmol of DABCO was added.
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Table 5. The allylation catalyzed by SiO2/DABCO/PP-Pd under solvent-free
conditionsa
entry nucleophile
(mmol) Pd
K2CO3 Time Yield
(mono/di) (%)
TON b
(μmol) (mmol) (h)
1 c
2
12.6
9.0
0.21
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
0.025
0.13
0.13
0.13
0.13
0.13
0.13
0.13
0.13
216
31 <1 / >99
<1 / 93
3 / 87
106000
12000
11200
11800
6400
3
9.0
5
4
9.0
48 <1 / 98
54 83 / 12
24 <1 / 98
31 <1 / 95
5
9.0
6
9.0
11700
11400
11700
10100
7 d
8
9.0
18.0
18.0
113
98 / -
9
124 84 / -
a
Reaction conditions: nucleophile, allylmethylcarbonate (2.5 equiv.), K2CO3,
SiO2/DABCO/PP-Pd, 70 C, neat. Yield was determined by 1H NMR analysis using an
o
b
internal standard.
TON was calculated as follows: TON = (moles of
nucleophile)×[(yield of the mono-product) + 2×(yield of the di-product)] / (moles of Pd).
c 80 oC. d 100 oC.
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●
●
Ph
*
*
●
(e)
(d)
(c)
*
*
●
●
●
●
*
*
*
●
*
*
EtO
EtO
*
a
b
d
c
e
(b)
CHCl
3
EtO
Ph
EtO
a
b
e
d
c
f
(a)
200
150
100
50
0
13C Chemical Shift / ppm
Figure 1. 13C NMR spectra of (a) CDCl3 solution of PP-Pd, (b) SiO2/PP-Pd,[5c] (c)
SiO2/Me/PP-Pd, (d) SiO2/Hex/PP-Pd, and (e) SiO2/DABCO/PP-Pd.[5c] 13C CP/MAS
NMR was conducted for SiO2-supported materials. (*) indicate spinning side bands.
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(B)
(A)
NR3
2000
1000
N+R4
(d)
(b)
(c)
(b)
(a)
(a)
348 346 344 342 340 338 336 334 332
Binding Energy / eV
408
404
400
396
392
Binding Energy / eV
(C)
500
(c)
(b)
(a)
206
204
202
200
198
196
194
192
Binding Energy / eV
Figure 2. XPS spectra of (A) Pd 3d of (a) SiO2/DABCO/PP-Pd, (b) SiO2/PP-Pd, (c)
SiO2/Me/PP-Pd, and (d) SiO2/Hex/PP-Pd, (B) N 1s of (a) SiO2/DABCO/PP-Pd and (b)
SiO2/PP-Pd, and (C) Cl 2p of (a) SiO2/DABCO/PP-Pd, (b) SiO2/DABCO/PP-Pd-treated,
and (c) SiO2/DABCO/PP-Pd-used.
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5.5
5.0
4.5
4.0
3.5
3.0
2.5
R=DABCO
R=Me
R=Hex
(none)
0
2
4
6
8
10
Size of co-immobilized molecule / Å
Figure 3. Debye-Waller factor (∆σ2) of k3-weighted Pd K-edge EXAFS spectra of
SiO2-supported Pd complexes (SiO2/R/PP-Pd) plotted against the size of the
co-immobilized molecule on the same SiO2 surface (○). The ∆σ2 values determined by
the simulation with a fixed coordination number value (N=3) are also shown (▲). Size
of the co-immobilized molecule is the distance between oxygen and the edge atom of
the silane coupling reagents, which have quaternized DABCO, n-hexyl, and methyl
groups.
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≈
<
<
SiO2 surface
Figure 4. Order of the steric effect around immobilized Pd-bisphoshine complexes
estimated from their Debye-Waller factors (∆σ2) of k3-weighted Pd K-edge EXAFS
spectra.
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Figure 5. Pd K-edge XANES spectra of PdIIO (red line), SiO2/DABCO/PP-Pd (bold
orange line), SiO2/DABCO/PP-Pd-used (black solid line), Pd0 foil (blue solid line), and
Pd0(PPh3)4 (blue dash line).
26

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Graphical Abstract
Synergistic
Catalysis
<
<
SiO2 surface
Steric Effect Determined by Pd K-edge
EXAFS analysis
27