A Convoluted Polymeric Imidazole Palladium Catalyst: Structural Elucidation and Investigation of the Driving Force for the Efficient Mizoroki-Heck Reaction

Article abstract of DOI:10.1002/cctc.201500249

MPPI-Pd (7 mol ppm Pd), prepared from poly(N-isopropylacrylamide-co-N-vinylimidazole) and (NH₄)₂PdCl₄ by our molecular convolution method, promoted the Mizoroki-Heck reaction in water to give the corresponding coupling pro

Full text of DOI:10.1002/cctc.201500249

DOI: 10.1002/cctc.201500249
Full Papers
A Convoluted Polymeric Imidazole Palladium Catalyst:
Structural Elucidation and Investigation of the Driving
Force for the Efficient Mizoroki–Heck Reaction
Takuma Sato,[a] Aya Ohno,[a] Shaheen M. Sarkar,[a] Yasuhiro Uozumi,*[a, b] and
Yoichi M. A. Yamada*[a]
MPPI-Pd (7 mol ppm Pd), prepared from poly(N-isopropyl-
acrylamide-co-N-vinylimidazole) and (NH4)2PdCl4 by our molec-
ular convolution method, promoted the Mizoroki–Heck reac-
tion in water to give the corresponding coupling products
with high yield and reusability. To clarify why the catalyst was
so active, structural elucidation and temperature-dependent
absorption/discharge investigation were performed using X-ray
absorption fine structure analysis, Raman and far-IR spectrosco-
py, elemental analysis, and DFT calculations, which provided
the structure of cis-PdCl2L2 (L=imidazole moiety). Moreover,
the temperature-dependent absorption and discharge of aryl
iodides and water in MPPI-Pd were observed with near-IR spec-
troscopy and macroscopic observations. The efficient diffusion
of organic substrates into the internal nanospaces in MPPI-Pd
and the following discharge of the water molecules at 808C
should enhance the catalytic activity dramatically as the driv-
ing force of the reaction.
Introduction
The development of highly active and reusable polymer-sup-
ported metal catalysts is an important topic for synthetic or-
ganic chemistry, green chemistry, and medicinal chemistry as
well as for chemical and pharmaceutical processes.[1] These cat-
alysts can be applied to the chemical preparation of pharma-
ceutical compounds, electrofunctional materials, and organic
electroluminescent materials without contamination of metal
species. We have developed a highly active and reusable con-
voluted polymeric palladium catalyst (MPPI-Pd, 3) from poly(N-
isopropylacrylamide-co-N-vinylimidazole) (1) and (NH4)2PdCl4
(2) (Scheme 1), which was applied to the allylic arylation and
the Suzuki–Miyaura reaction in water.[2] The catalyst with 0.28–
66 molppmPd (0.000028–0.0066 mol% Pd) promoted allylic ar-
ylation and the Suzuki–Miyaura reaction with high recyclability.
To clarify why MPPI-Pd has a high catalytic activity, it is impor-
tant to investigate the structure of the Pd complex and the
mechanism of the absorption/discharge of organic substrates
(aryl halides, etc.) and water in MPPI-Pd.
Scheme 1. Preparation of MPPI-Pd.
The structural elucidation of insoluble and amorphous poly-
meric metal catalysts is quite difficult because XRD, mass anal-
ysis, and other analytical methods to characterize homogene-
ous catalysts are generally not useful because of the polymeric
nature of the catalysts. X-ray absorption fine structure (XAFS)
spectroscopy could be a powerful method to determine these
structures.[3] Although the determination of inorganic[4] and ho-
mogeneous[5,6] catalysts as well as metal-containing biomole-
cules[7] has been shown, the structural elucidation of polymeric
metal catalysts using XAFS is still evolving and challenging.[8]
Here, we found that MPPI-Pd [7 molppmPd (0.0007 mol%
Pd)] promoted the Mizoroki–Heck reaction efficiently in water
to give the corresponding coupling products with high yield
and reusability without hydrolysis of the products. We report
the structural elucidation of MPPI-Pd using X-ray absorption
near-edge structure analysis (XANES), extended X-ray absorp-
tion fine structure analysis (EXAFS), Raman and far-IR (FIR)
spectroscopy, and DFT calculations. Moreover, the tempera-
[a] Dr. T. Sato, A. Ohno, Dr. S. M. Sarkar, Prof. Dr. Y. Uozumi,
Dr. Y. M. A. Yamada
RIKEN Center for Sustainable Resource Science
2-1 Hirosawa, Wako, Saitama 351-0198 (Japan)
E-mail: ymayamada@riken.jp
[b] Prof. Dr. Y. Uozumi
Institute for Molecular Science (IMS)
5-1 Higashiyama, Myodaiji, Okazaki, Aichi 444-8787 (Japan)
E-mail: uo@ims.ac.jp
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500249.
This publication is part of a Special Issue on Palladium Catalysis. To view
the complete issue, visit:
http://onlinelibrary.wiley.com/doi/10.1002/cctc.v7.14/issuetoc
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ture-dependent absorption and discharge of aryl iodides and
water in MPPI-Pd was investigated by utilizing near-IR (NIR)
spectroscopy and macroscopic observations to show the dy-
namic mechanism that enhances the catalytic activity.
Table 2. Mizoroki–Heck reaction with 7 molppmPd of 3.[a]
Entry
R1 4
R2 5
Product 6
Yield [%]
Results and Discussion
1
2
3
4
5
6
7
Ph 4a
CO2Bu 5a
5a
5a
5a
5a
5a
5a
5a
Ph 5b
5b
5b
5b
6a
6b
6c
6d
6e
6 f
6g
6h
6i
6j
6k
6l
6m
6n
6o
6p
96
99
96
96
95
99
94
96
96
94
96
93
95
96
94
95
MPPI-Pd-catalyzed Mizoroki–Heck reaction
4-F3CC6H4 4b
4-O2NC6H4 4c
4-AcC6H4 4d
4-MeC6H4 4e
4-MeOC6H4 4 f
3-pyridyl 4g
1-naphthyl 4h
4a
Here, MPPI-Pd was utilized to catalyze the Mizoroki–Heck reac-
tion in water to show its high catalytic activity and generality.
It is an important challenge to develop highly active and reus-
able polymeric metal catalysts as this coupling is a useful reac-
tion in synthetic organic chemistry to supply pharmaceutical,
agricultural, and functional materials.[9,10,11,12] In particular, since
Buchmeiser and Wurst reported a highly active polymer-sup-
ported Pd catalyst for the Mizoroki–Heck reaction,[12j] the devel-
opment of highly active and reusable polymer-supported
Pd catalysts has been watched with interest. Moreover, the
Mizoroki–Heck reaction of alkyl acrylates in water is challeng-
ing because hydrolysis is a fatal side reaction.
8b
9
10
11
12
13
14
15
16
4b
4-NCC6H4 4i
4e
4 f
4a
4e
5b
CONH-iPr 5c
5c
5c
4 f
[a] Conditions: 4 (1 mmol), 5 (2 mmol), 3 (7 nmol Pd; 0.01 mg), Et3N
(3 mmol), TBAB (0.20 mmol), H2O (1.5 mL), 1008C, 17 h; [b] 3 (20 nmol
Pd), 48 h.
The reusability of MPPI-Pd was investigated in the Mizoroki–
Heck reaction of iodobenzene (4a) and butyl acrylate (5a)
with triethylamine and tetrabutylammonium bromide (TBAB)
at 1008C for 17 h in water without the use of organic solvents,
which affords butyl cinnamate (6a) in 99% yield (Table 1). The
(5c) was a suitable reagent for the reaction and gave the cor-
responding a,b-unsaturated amides 6n–p in 94–96% yield (En-
tries 14–16).
Table 1. Reusability of MPPI-Pd in the in-water Mizoroki–Heck reaction of
4a and 5a.[a] Yields given in %.
MPPI-Pd was applied to the more challenging Mizoroki–
Heck reaction of the less reactive substrate aryl bromide and
chloride without the use of organic solvents (Scheme 2). When
First use
99
Second use
98
Third use
97
Fourth use
95
Fifth use
96
[a] Conditions: 4a (1 mmol), 5a (2 mmol), 3 (330 nmol Pd, 0.5 mg), Et3N
(3 mmol), TBAB (0.20 mmol), water (1.5 mL), 1008C, 17 h.
reused catalyst promoted the reaction efficiently to give 6a in
98, 97, 95, and 96% yield in the second to fifth consecutive
runs. Inductively coupled plasma atomic emission spectrosco-
py (ICP-AES) indicated that Pd was not present in the reaction
mixture. This result as well as our previous studies, which in-
clude the hot leaching test,[2] indicate that MPPI-Pd works as
a heterogeneous catalyst.
Scheme 2. Mizoroki–Heck reaction of aryl bromide and chloride.
the reaction of bromobenzene (7) and 5a was performed with
66 molppmPd (0.0066 mol%) of MPPI-Pd in the presence of
NaOAc and TBAB at 1408C for 20 h, we were pleased to find
that the reaction proceeded smoothly to give 6a in 82% yield.
The reaction of 4’-chloroacetophenone (8) and 5a was per-
formed under similar conditions to afford the coupling product
6d in 85% yield.
The in-water Mizoroki–Heck reaction of various aryl iodides
4
and olefins
5
was performed with 7 molppmPd
(0.0007 mol% Pd; Table 2). The reaction of electron-deficient
substrates 4b–d proceeded smoothly to give the correspond-
ing butyl cinnamates 6b–d in 96–99% yield with a turnover
number (TON) of approximately 140000 (Entries 2–4). Electron-
rich materials 4e–f were converted readily to the correspond-
ing coupling products 6e–f in 95 and 99% yield under similar
conditions (Entries 5–6). The reaction of a substrate that bears
a pyridine ring 4g afforded the product 6g in 94% yield
(Entry 7). The coupling with styrene (5b) was also performed
in water under similar conditions to give the corresponding
stilbenes in 93–96% yield (Entries 9–13). N-Isopropylacrylamide
Structural elucidation of MPPI-Pd
It is important but difficult to clarify the structure of MPPI-Pd
to consider its high catalytic activity. First, the coordination
structure of MPPI-Pd was investigated to examine the structure
of the Pd complex. The Pd K-edge XANES spectrum of MPPI-Pd
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vibrations in cis-PdIICl2L2.[13,14] The FIR spectrum supported
these observations (Figure 2c and d). Based on these results,
cis-PdCl2L2 (L=imidazole moiety of the polymeric ligand) was
the most likely local structure. Therefore, the structure of cis-
PdCl2(1-methylimidazole)2 was optimized by DFT calculation at
the level of B3LYP/def2-TZVP (Figure 3).[15] The fitting analysis
of the Pd K-edge Fourier-transformed extended X-ray absorp-
tion fine structure (FT-EXAFS) spectrum of MPPI-Pd was per-
formed using the DFT model (Figure 4). The best fitting result
was obtained for the parameters given in Table 3. The scatter-
ing paths of PdÀCl, PdÀN (3-N in 1-methylimidazole), and
PdÀC (likewise 2-C and 4-C) were included in the fitting R
range. The good agreement of the experimental data of FT-
EXAFS with the DFT model was consistent with the results of
vibrational spectroscopy. These results suggested that the co-
Table 3. Fitted parameter values of FT-EXAFS with the DFT model of
PdCl2(1-methylimidazole)2.[a]
R-range
k-range
R factor
0.012
DE
DR(Cl)
À1
1.2–2.9 Š
3–15 Š
+2.85 eV
+0.01 Š
DR(N)
DR(C)
s2(Cl)
s2(N)
s2(C)
2
2
2
À0.07 Š
À0.10 Š
0.004 Š
0.005 Š
0.003 Š
Figure 1. First derivative XANES spectra of MPPI-Pd and some reference
compounds [(NH4)2PdCl2 and PdCl2 for PdII; Pd(PPh3)4 and Pd black for Pd0].
[a] The coordination number of Pd was fixed by the model structure.
was compared with that of PdII
species ((NH4)2PdCl4 and PdCl2)
and Pd0 species (Pd(PPh3)4 and
Pd black). The first derivative of
the XANES spectrum of MPPI-Pd
was more similar to the refer-
ence compounds for PdII than
those of Pd0 (Figure 1). This
result suggested strongly that
the oxidation state and the coor-
dination geometry of the Pd
center in MPPI-Pd was square-
planar PdII. Although Pd0 species
were also detected in MPPI-Pd,[2]
this result suggested that the
major species in MPPI-Pd is PdII.
Elemental analysis of MPPI-Pd
(Supporting Information) implied
that two Cl atoms and possibly
two N atoms coordinate to the
Pd center. The Raman spectrum
showed a broad peak near n˜ =
290 cmÀ1, and two peaks were
revealed at n˜ =287/295 cmÀ1 in
the second derivative spectrum
(Figure 2a and b). The set of
split peaks in the latter spectrum
Figure 2. Low-frequency Raman and FIR spectra of MPPI-Pd: a) Raman spectrum, b) second derivative of Raman
spectrum, c) FIR spectrum, and d) second derivative of FIR spectrum. Black arrows indicate split PdÀCl stretching
correspond to the symmetric/
asymmetric ClÀPdÀCl stretching vibrations.
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Figure 3. DFT-optimized geometry of the model structure, cis-PdCl2(1-meth-
ylimidazole)2, for MPPI-Pd at the level of B3LYP/def2-TZVP. For simplicity, the
polymer ligands 1 were substituted for 1-methylimidazole (green: Pd,
yellow: Cl, blue: N, gray: C, white: H). Selected internuclear distances:
PdÀCl=2.31 Š, PdÀN=2.08 Š.
Figure 5. NIR spectra of MPPI-Pd (top left) and 1 (bottom left) in the range
of n˜ =6400–6000 cmÀ1. Overtones of CÀH stretching vibrations (denoted as
b and c, see also Figure 7) of imidazole moiety in 1 (center) and MPPI-Pd
(right). Imidazoles are shown in red.
mer 1 was soluble/insoluble in water below/over 358C). Water-
swollen MPPI-Pd shrank in water if it was heated gradually
from 23 to 808C, whereas it swelled more if it was cooled from
23 to 58C (Figure S3). In contrast, MPPI-Pd absorbed 4a at
808C to swell to 1.4 times larger in diameter, whereas it ab-
sorbed 4a but did not swell at 238C.
As we believe that the dynamic phenomena of the tempera-
ture-dependent absorption and discharge pathway is one of
the driving forces that promotes efficient reaction, we investi-
gated MPPI-Pd in water and 1-iodonaphthalene (4h)[17] at 23
and 808C by NIR spectroscopy.
Figure 4. k3-Weighted FT-EXAFS of MPPI-Pd (solid line) and the best fit with
the DFT-optimized geometry of cis-PdCl2(1-methylimidazole)2 (dashed line).
NIR spectroscopy is one of the most powerful methods to
survey the changes in the intermolecular interaction and the
microenvironment of insoluble polymer catalysts through the
overtone and combination bands derived from XÀH (X=C, N,
O, etc.) vibrations because of the high transmittance of NIR
light and sufficient spectral resolution compared with middle-
IR (MIR) spectroscopy.[18] The NIR experiment was performed
using the above-mentioned 10 mm disks of solid samples.
The NIR spectra of MPPI-Pd and 1 and their assignment in
the range of n˜ =4000–7000 cmÀ1 are shown in Figure 7. The
spectra of both MPPI-Pd and 1 were quite similar to that of
poly(N-isopropylacrylamide) (PNIPAM) except for the absorp-
ordination structure of the Pd center in MPPI-Pd was cis-
PdCl2L2 (L=imidazole moiety in the polymeric ligand 1). More-
over, a comparison of the NIR spectra of MPPI-Pd and 1 indicat-
ed clearly that almost all of the imidazole moieties in MPPI-Pd
contributed to the coordination to the Pd atoms because the
overtones of the CÀH stretching vibrations derived from imida-
zoles were shifted significantly to higher wavenumbers after
complexation with Pd2+ (Figure 5).
tion bands in the range of n˜ =6000–6300 and 5000–5300 cmÀ1
.
Temperature-dependent absorption and discharge
As the overtone and combination bands of PNIPAM in the NIR
region have been fully characterized by Wu et al.,[19] we fol-
lowed their assignment in relation to the absorption bands de-
rived from the PNIPAM-like structure in 1 and assigned the ab-
sorption bands around n˜ =6000–6300 cmÀ1 exclusively to the
overtone of the CÀH stretching vibration of imidazoles in
MPPI-Pd and 1 (Figures 5 and 7, bandsb and c). The prominent
absorption bands in the range of n˜ =5000–5300 cmÀ1 were at-
tributed to the combination of bending and symmetric/asym-
metric stretching of water in the polymer matrix (Figure 7,
bandg).
Not only the elucidation of the local structure near the Pd
complex but also the analysis of the thermally dependent dy-
namic structure of MPPI-Pd is important to find the reason for
its high catalytic activity. During the catalytic reaction, we ob-
served the temperature-dependent high absorption/discharge
ability for organic molecules and water at 23 and 808C.
Thus, a disk of MPPI-Pd (30 mg, 1 10 mm, prepared by
using a 110 mm mold at 20 MPa for 10 min under vacuum at
238C) repelled water at 808C, whereas at 238C it swelled in
water (Figure 6). Notably, MPPI-Pd deformed in water at 808C,
and its surface was apparently dry. This is probably because 1,
a polymeric component of MPPI-Pd, shows a lower critical so-
lution temperature (LCST) phase transition[16] at 358C (the poly-
The 110 mm disk of MPPI-Pd was heated and cooled under
aerobic conditions that included moisture to observe the ab-
sorption and discharge of water to/from MPPI-Pd. First, MPPI-
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Figure 7. NIR spectra of MPPI-Pd and 1. Selected peak assignment:
a) 2”n(NH)f; b, c) 2”n(CH)imidazole; d) 2”nas(CH3); e) 2”nas(CH2); f) 2”ns(CH3);
g) ns/as(OH)water+d(OH)water; h) n(NH)b+Amide II; i) Amide B+Amide II;
j) n(NH)b+Amide III; k) nas(CH3)+das(CH3); l) ns(CH3)+das(CH3); m) nas(CH3)+CH3
skeletal; n) nas(CH3)+CH3 rocking; o) nas(CH3)+CH2 rocking.
Figure 8. Heating and cooling of MPPI-Pd to see the adsorption and desorp-
tion of water; a) as-prepared MPPI-Pd, b) heated at 808C for 30 min, c) after
30 min at 238C, d) after 2 h at 238C, e) after 6 h at 238C.
adsorbed water from the air at 238C reversibly. Notably, the
[2”n(CH)imidazole] band of MPPI-Pd did not significantly change
during the heating and cooling process (Figure 5). Conse-
quently, the local structure of the Pd-imidazole complex in
MPPI-Pd was thermally stable.
Figure 6. Morphology of MPPI-Pd in water and 4a at 23 and 808C from 0 to
4 h.
The heating and cooling experiment was examined in the
presence of 4h. When MPPI-Pd was heated at 808C for 30 min,
and 4h (ꢀ2 mL) was placed directly on the disk of MPPI-Pd at
808C, 4h soaked into it readily, and further changes in the rela-
tive intensity of the combination bands (n˜ =4000–4200 cmÀ1)
were observed (Figure 9).[20] The changes were restored within
6 h at 238C. In contrast, when 4h was added to as-prepared
MPPI-Pd at 238C, no significant spectral change was observed
in terms of the water (n˜ =5000–5300 cmÀ1) and combination
bands (n˜ =4000–4200 cmÀ1; Figure S5).
Pd was heated at 808C for 30 min and then allowed to cool at
238C. The sample was continuously exposed to air over 6 h at
238C and 35% relative humidity. During the process, reversible
changes in the NIR spectra were observed (Figure 8). Thus, the
heating of as-prepared MPPI-Pd at 808C provided a decreased
absorbance of water (n˜ =5000–5300 cmÀ1) and a change in the
intensity of the combination bands in the range of n˜ =4000–
4200 cmÀ1 ([nas(CH3)+CH2 rocking], [nas(CH3)+CH3 rocking], and
[nas(CH3)+CH3 skeletal]). The changes were restored within 6 h
at 238C in air. A similar result was also obtained for 1 (Fig-
ure S4). These results indicated that both MPPI-Pd and 1 desor-
bed water molecules at an elevated temperature (808C) and
Meanwhile, small wavenumber shifts of overtone bands of
the CÀH asymmetric stretching of CH3 and CH2 ([2”nas(CH3)],
[2”nas(CH2)]) in the range of n˜ =5800–6000 cmÀ1, in which CH3
derived from the N-isopropyl group and CH2 derived from the
main chain of the polymer, were also observed. The wavenum-
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sulting composite was left at 238C in air, and further wave-
number shifts were observed (Figure 10, from (f) to (i)). Finally,
the bands of [2”nas(CH2)] and [2”nas(CH3)] after 6 h (Figure 10,
(i)) almost corresponded with that of the composite at 238C
(Figure 10, (j)).
The key results are: (1) When MPPI-Pd was heated/cooled to
80/238C, the bands of [2”nas(CH2)] and [2”nas(CH3)] shifted re-
versibly. The results indicate that water was desorbed/ad-
sorbed reversibly at a high/low temperature. This corresponds
to the morphology of MPPI-Pd in water at 80/238C. (2) When
4h was added to MPPI-Pd at 808C, the intensity of the combi-
nation bands of CH2 and CH3 (n˜ =4000–4200 cmÀ1) changed
clearly (Figure 9). In contrast, when 4h was added to MPPI-Pd
at 238C, no significant spectral change was observed. These re-
sults suggested that organic substrates and reagents diffused
preferably into the polymer matrix at 808C rather than 238C.
This also corresponds to the morphology of MPPI-Pd in iodo-
benzene at 80/238C.
Figure 9. Heating and cooling of MPPI-Pd with the addition of 1-iodonaph-
thalene (4h, 1-INaph) at 808C; a) as-prepared MPPI-Pd, b) heated at 808C for
30 min, c) 1-iodonaphthalene was added at 808C, d) after 30 min at 238C,
e) after 2 h at 238C, f) after 6 h at 238C.
From these results as well as the observation in Figure 6, we
propose and summarize a plausible mechanism of the temper-
ature-dependent absorption/discharge of water and hydropho-
bic organic molecules (Figure 11). As the changes of the [2”
nas(CH3)] and [2”nas(CH2)] bands were reversible and as the
polymer component in MPPI-Pd comprises both a hydrophobic
main chain and amphiphilic amide moieties (represented by
CH2 and CH3, respectively), it is proposed that MPPI-Pd has two
distinct domains: a chain domain and a nanospace domain, in
which the hydrophobic organic molecules (substrates and re-
actants) and water can be absorbed. Water is ab-
sorbed in MPPI-Pd at 238C and is discharged at 808C
because of the properties of the polymer 1, which
has a LCST at 358C. When substrates and reactants
are introduced at 808C, they are absorbed well espe-
cially in the nanospace domain in which water has
been expelled. In contrast, when they are added at
238C, they are less absorbed, and water still exists in
the nanospace domain in MPPI-Pd. If we take into ac-
count the aspects mentioned above, the efficient dif-
fusion of organic substrates into the internal nano-
spaces in MPPI-Pd, which were generated by the
packing of water surrounded by hydrated NIPAM
units at 238C and the following discharge of the
water at 808C, would enhance the catalytic activities
dramatically as the driving force of the reaction.
Meanwhile, the framework structure of MPPI-Pd de-
rived from the molecular convolution of polymer
chains with Pd2+ helps to maintain the thermally de-
pendent structure based on hydrogen bonding be-
tween the NIPAM units and water.
bers of the [2”nas(CH3)] (x axis) and [2”nas(CH2)] bands (y axis)
of MPPI-Pd under various conditions are summarized in
Figure 10. If MPPI-Pd was heated and cooled between 23 and
808C in air (as shown in Figure 8), water was desorbed and
readsorbed, and the [2”nas(CH2)] and [2”nas(CH3)] bands
changed reversibly (Figure 10, from (a) to (e), blue arrows).
However, the addition of 4h to MPPI-Pd at 808C (as shown in
Figure 9) led to different changes in the [2”nas(CH3)] and [2”
nas(CH2)] bands (Figure 10, from (b) to (f), red arrows). The re-
Furthermore, as water was discharged at 808C
from MPPI-Pd and hydrophobic organic molecules
were absorbed in MPPI-Pd, this phenomenon should
help the Mizoroki–Heck reaction with 5a in water.
Thus, the Mizoroki–Heck reaction with 5a should
proceed smoothly without the hydrolysis of 5a to
afford the corresponding cinnamic acid esters in high
yield without the formation of free cinnamic acids.
Figure 10. Plot of [2”nas(CH2)] versus [2”nas(CH3)]; a) as-prepared MPPI-Pd, b) heated at
808C for 30 min, c) cooled at 238C after 30 min, d) after 2 h, e) after 6 h, f) 1-iodonaph-
thalene added after heating at 808C for 30 min, g) cooled at 238C after 30 min, h) after
2 h, i) after 6 h, j) 1-iodonaphthalene added at 238C without preheating.
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Keywords: EXAFS spectroscopy
·
Mizoroki–Heck
reaction · nanostructures · palladium · structure
elucidation
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Conclusions
We elucidated the coordination structure of a highly active
and reusable polymeric palladium catalyst MPPI-Pd, prepared
from
poly(N-isopropylacrylamide-co-N-vinylimidazole)
and
(NH4)2PdCl4 by our molecular convolution method, which pro-
vided the coordination structure of cis-PdCl2L2 (L=imidazole
moiety of the polymeric ligand). The Mizoroki–Heck reaction
proceeded with 7 molppmPd (0.0007 mol% Pd) of MPPI-Pd in
water to give the corresponding coupling products in >93%
yield with a turnover number of approximately 140000. More-
over, temperature-dependent absorption and discharge of aryl
iodides and water in MPPI-Pd were observed by near-IR spec-
troscopy and macroscopic observations to investigate the
reason why MPPI-Pd has high catalytic activity. We believe the
elucidation and investigation of the catalytic structure and
driving force for the organic transformation will help to create
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Acknowledgements
We thank Dr. Tetsuo Honma, Dr. Masafumi Takagaki (JASRI,
SPring-8), and Prof. Hikaru Takaya (Kyoto University) for XAFS
measurements (SPring-8, BL14B2, 2012B1868). We are grateful to
JASCO Corporation (Tokyo, Japan) for the measurement of
Raman spectra (Mr. Takeo Soejima) and to JASCO Engineering
(Tokyo, Japan) for the measurement of FIR spectra (Ms. Misa
Aoyama). We also thank RIKEN for elemental (Dr. Yoshio Sakagu-
chi) and ICP-AES (Ms. Chieko Kariya) analyses. We gratefully ac-
knowledge the financial support from JST ACT-C, JST CREST, JSPS
(#24550126, #20655035, and #2105), the Takeda Science Founda-
tion, the Naito Foundation, and RIKEN.
ChemCatChem 2015, 7, 2141 – 2148
www.chemcatchem.org
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Full Papers
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[17] As 4a is volatile at 808C, we could not measure the NIR spectrum of
4a with MPPI-Pd at 808C. Instead, 4h was used although MPPI-Pd ab-
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tion.
Received: March 9, 2015
Published online on June 1, 2015
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www.chemcatchem.org
2148
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim