Tailored design of palladium species grafted on an amino functionalized organozinc coordination polymer as a highly pertinent heterogeneous catalyst

Article abstract of DOI:10.1039/c4ta03889d

The design of a highly active and stable heterogeneous palladium catalyst is gaining a lot of attention because of its increasing importance in the organic syntheses of commodity chemicals. Herein, we report the tailored synthesis of palladium species grafted on a highly stable amino functionalized organozinc coordination polymer (denoted as Pd/AZC) and its extraordinary catalytic performances in the Suzuki-Miyaura coupling (SMC) reaction. It achieved the highest turnover number of 2106 720 (>99% yield) in air among the most reported palladium catalysts for the SMC reaction of bromobenzene. The as-prepared Pd/AZC composite is also successfully applied for the catalysis of Mizoroki-Heck coupling, hydrogenation of nitro, and -CC- functional groups. Since the developed AZC support has thermal stability at least up to 573 K, it possesses high potential for grafting various metal species as catalytically active centers for a wide range of metal-catalyzed reactions. This journal is

Full text of DOI:10.1039/c4ta03889d

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Tailored design of palladium species grafted on an
amino functionalized organozinc coordination
polymer as a highly pertinent heterogeneous
catalyst†
Cite this: DOI: 10.1039/c4ta03889d
*
H. Choudhary, S. Nishimura and K. Ebitani
The design of a highly active and stable heterogeneous palladium catalyst is gaining a lot of attention
because of its increasing importance in the organic syntheses of commodity chemicals. Herein, we
report the tailored synthesis of palladium species grafted on a highly stable amino functionalized
organozinc coordination polymer (denoted as Pd/AZC) and its extraordinary catalytic performances in
the Suzuki–Miyaura coupling (SMC) reaction. It achieved the highest turnover number of 2 106 720
(>99% yield) in air among the most reported palladium catalysts for the SMC reaction of bromobenzene.
The as-prepared Pd/AZC composite is also successfully applied for the catalysis of Mizoroki–Heck
coupling, hydrogenation of nitro, and –C]C– functional groups. Since the developed AZC support has
thermal stability at least up to 573 K, it possesses high potential for grafting various metal species as
catalytically active centers for a wide range of metal-catalyzed reactions.
Received 28th July 2014
Accepted 7th September 2014
DOI: 10.1039/c4ta03889d
www.rsc.org/MaterialsA
in the presence of readily available organoboronic acids, the
SMC reaction has been extensively employed as a handy meth-
1. Introduction
odology in the syntheses of natural products, pharmaceuticals
and supra-molecular assemblies throughout the globe.8–13 The
homogeneous Pd catalytic processes have higher activity and
selectivity than their heterogeneous counterparts; however, they
have serious issues of recovery and reusability. The reasons for
the limited activity of the heterogeneous catalyst might be the
poor accessibility of reactants to the active sites and/or fast
deactivation by agglomeration of unstable Pd species into
inactive Pd particles. Thus, designing a highly active and stable,
heterogeneous Pd catalyst constitutes one of the important
objectives for the transition metal-catalyzed SMC reaction and
other catalytic reactions.14,15
We successfully fabricated such a coordination polymer to
gra drastically low palladium using a ligand design method-
ology. Herein, we present the synthesis, catalysis and structural
characterization of a monomeric palladium species graed on
an amino functionalized organozinc coordination polymer
(denoted as Pd/AZC) as a highly active and easily reusable
heterogeneous catalyst for the SMC reaction with a high turn-
over number (TON). Recently, Deraedt and Astruc compiled a
report on Pd nanoparticles (both homogeneous and heteroge-
neous) for the cross coupling reaction with concluding remarks
on forthcoming challenges of both stabilization and improve-
ment of catalytic efficiency.16 Various researchers have reported
TON in the range of 100–3 500 000 for haloarenes (the average
TON being 85 000 for bromobenzene) by homogeneous and
heterogeneous catalyses.17–30 Different supports such as
In recent decades, porous coordination polymers (PCPs) have
been an area of interest in many elds, namely energy, catalysis
and separation. PCPs are inorganic–organic hybrid advanced
materials with metal ions as the inorganic center and organic
ligands as the linker with well-dened channels/pores. The
prime reason of increasing attention on the PCP materials lies
in the ease of tuning their structural features and relative
properties for wide-range potential applications.1–3 For instance
in terms of catalytic performances, because of this tunability,
their structures can be precisely controlled to similarly incor-
porate active catalytic centers (in channels/pores) as enzymes
which are extremely active and selective catalysts owing to the
architectural active pockets in well-dened cavities.
Organic transformations such as C–Z (Z ¼ C, O, N, etc.)
coupling, hydrogenation, and oxidation have found potential
industrial applications using transition metals (such as Au, Pd,
Pt, etc.) catalysts. Among these, the Suzuki–Miyaura coupling
(SMC) reaction using the Pd catalyst4–7 has become more like a
ritual for the regioselective formation of C–C bonds in modern
organic chemistry.8–13 Because of the tolerance of the SMC
reaction for a wide range of substrates under milder conditions
School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1
Asahidai, Nomi, Ishikawa 923 1292, Japan. E-mail: ebitani@jaist.ac.jp
† Electronic supplementary information (ESI) available: Catalytic activity
optimization, reusability graph, detailed characterization of AZC (XAFS, XPS, IR
and XRD), proposed mechanism, and NMR & MS of organic compounds. See
DOI: 10.1039/c4ta03889d
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activated carbon or carbon nanotubes,31–34 metal organic moiety with a nitrogen containing functional group such as an
frameworks,21,29,35,36 zeolites,23,37–39 and mesoporous silica25,40–42 amine, and thereby we selected 2-aminoterephthalic acid (ATA)
have been utilized for the SMC reaction.43–45 The activities were as the organic linker for PCP. While zinc as the metal center was
observed as a function of easy access of the substrates and selected owing to its relative abundance, low cost and signi-
reagents to active species. Okumura et al. demonstrated that the cant catalytic characteristics. The obtained amino functional-
Pd/USY catalyst can give a TON of 13 000 000 using bromo- ized organozinc coordination polymer (AZC) may facilitate to
benzene as the substrate only under a H2 atmosphere without hold active Pd species with high dispersibility derived from
commenting on catalyst reusability.23
amino functionality and the porous framework of PCP
In this contribution, we designed an organozinc coordina- compounds.
tion polymer with amino functionality for efficient graing of
Pd species leading to highly dispersed and easily accessible
active centers. Because such PCPs have a regular microporous
structure and a signicant high surface area,1–3 the synthesized
organometallic coordination polymer is believed to possess a
highly porous structure that facilitates the access of substrates
to the rmly held active Pd species with amino moieties. The
catalytic system described here is supposed to be a promising
synthetic protocol for the development of advanced metal-
supported catalysts because of the following advantages: (i)
high catalytic activity and selectivity under an air atmosphere,
(ii) easy recovery and high reusability, and (iii) restricted homo-
coupling of phenyl boronic acid in the Pd-catalyzed SMC
reaction.
2.3 Catalyst synthesis
2.3.1 Synthesis of the amino-functionalized organozinc
coordination polymer (AZC). ATA (5 mmol) and Zn(NO3)2$6H2O
(5 mmol) in DMF (35 mL) were sealed in a 100 mL Teon lined
autoclave, and heated to 413 K (heating rate; 4 K minꢀ1) in a
programmable oven and maintained at the same temperature
for 24 h. The synthesis method involved the use of DMF as the
solvent which slowly dissociates at higher temperature to
deprotonate ATA, which in turn reacts with Zn(NO3)2$6H2O to
form the organozinc coordination polymer. The oven was
allowed to cool to room temperature slowly. The solid residue
obtained was ltered and washed with a small amount of DMF.
Brownish blocks were obtained and dried in vacuo before
grinding them to afford a light brown powder in a pestle mortar.
These dried brown powders were further treated with ethanol at
353 K for 24 h in order to remove DMF and ltered and dried in
vacuo to obtain “AZC”. Elemental analysis (%) for prepared AZC
was found as C, 37.2; H, 2.0; N, 6.2; Zn, 27.3. To account for the
result, a molecular formula was suggested Zn4(ATA)3(NO3)2 (or
2. Experimental
2.1 Materials
Phenylboronic acid, palladium chloride, zinc nitrate hexahy-
drate (Zn(NO3)2$6H2O), toluene, methyl isobutyl ketone, benzyl
alcohol, benzaldehyde, nitrobenzene, aniline, cinnamaldehyde,
3-phenylpropionaldehyde and standard solutions (1000 ppm) of
palladium and zinc were purchased from Wako Pure Chemical
Industries, Ltd. Potassium chloride, potassium carbonate
(K2CO3), lithium carbonate, cesium carbonate, calcium
carbonate, styrene, dimethylsulfoxide, acetone, N,N-dime-
thylformamide (DMF), methanol, acetonitrile, ethanol, N,N-
dimethylacetamide, zinc oxide, succinic acid and sodium
hydroxide were procured from Kanto Chemical Co., Inc. Tokyo
Chemical Industry Co., Ltd. supplied bromobenzene, maleic
anhydride and naphthalene whereas chlorobenzene was bought
from Junsei Chemical Co., Ltd. QuadraPure® TU, 1-methyl-2-
pyrrolidinone, 2-aminoterephthalic acid (ATA), tetrabuty-
lammonium bromide (TBAB), stilbene, 1,1-diphenylethylene
and zinc acetate (Zn(OAc)2) were obtained from Sigma-Aldrich,
Co. LLC.
C
24H15N5O18Zn4): C, 31.2; H, 1.6; N, 7.6; Zn, 28.4.
2.3.2 Synthesis of the palladium graed amino-function-
alized organozinc coordination polymer (Pd/AZC). About 0.5 g
of palladium chloride with 0.5 g potassium chloride were
weighed and suspended in 50 mL water. The solution was
sonicated with occasional stirring for 30 minutes to obtain a
homogeneous palladium stock solution with a concentration of
about 10 mg of PdCl2 per mL of the stock solution. In a typical
synthesis of 0.5 wt% Pd/AZC, a calculated amount of solution
(208 mL, if the concentration of stock solution is 10 mg PdCl2 per
mL) was taken in a round bottomed ask and 15 mL of water
was added. To this solution, 250 mg AZC was added and stirred
at room temperature for 6 h, and then at 353 K for 14 h. The
obtained brown colored solution was ltered, washed with
about 1 L of water and dried in vacuo at room temperature. The
dried powder was ground to obtain Pd/AZC (238 mg). Various
palladium graed AZC catalysts were denoted as xPd/AZC;
where the x is Pd content on support as (wt/wt) in theory.
2.2 Strategy for catalyst design
In general, the inclusion of facile access to the active species
and rm binding of the supported active species have proved as
an extra advantage to the heterogeneous catalyst. We really
2.4 Catalytic testing for the Suzuki–Miyaura coupling (SMC)
reaction
desired to fabricate such a heterogeneous material for the SMC All experiments to evaluate the catalytic activity were performed
reaction with features to hold the easily accessible active Pd under air in a round bottomed ask, unless mentioned. In a
rmly. The PCPs could be synthesized by controlled stitching of general reaction procedure, the catalyst was weighed into the
organic molecules with inorganic molecules especially the ask followed by the addition of a base, phenylboronic acid and
transition metals fascinated us. In principle, the Pd–N bonds the substrate (bromobenzene or chlorobenzene) in a molar ratio
are relatively strong, and thus we craved to choose an organic of 4 : 3 : 2. The solvent (5 mL) was added to disperse the
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reactants well and the ask was mounted on a preheated oil
bath at T K and the reaction was continued for t h. Aer the
passage of desired time, the ask was allowed to cool to room
temperature and naphthalene was added as an internal stan-
dard. The reaction mixture was stirred at room temperature,
diluted and the catalyst was ltered off using a Milex®-LG 0.20
mm. The obtained ltrate was analyzed by gas chromatography
(GC-17A, Shimadzu Co.) using an Agilent DB-1 column (30 m ꢁ
0.25 mm ꢁ 0.25 mm) attached to a FID detector. The conversion
and selectivity were determined with a calibration curve method
using naphthalene as an internal standard.
Recycling tests were performed to check the stability of the
Pd/AZC catalysts during the reaction. The catalyst was separated
from the reaction mixture by centrifugation. The supernatant
liquid was stored, and then analysis of products and the
leaching test of catalysts were performed. The residual catalyst
and base were washed by centrifugation with ethanol and dried
in vacuo overnight. Fresh substrates and reagents were added to
the catalyst, and then the reusability of the catalyst was veried
under similar reaction conditions.
In a typical case of isolation of 4-phenylbenzoic acid (Table 2,
entry 4), naphthalene was not added to the reaction mixture.
The reaction mixture was centrifuged and the resulting super-
natant liquid was ltered to remove any suspended solid
particles. The ethanol solution was concentrated followed by
the addition of water to precipitate the organics. The reaction
mixture containing water, ethanol and white solid were centri-
fuged again and the supernatant liquid was collected separately
to remove traces of bromobenzene (liquid). The solid was dis-
solved in a very small amount of ethanol and sufficient water
was added to achieve precipitation and was then centrifuged.
The mixture was le overnight to obtain colourless crystals to
record 1H NMR spectra.
2.5 Heterogeneity test and solid-phase poisoning test
2.5.1 Hot ltration test. 0.5 wt% Pd/AZC (1 mg) was
weighed into the ask followed by K2CO3 (4 mmol), phenyl-
boronic acid (3 mmol) and bromobenzene (2 mmol). Finally
ethanol (5 mL) was added to disperse the reactants and the
reaction mixture was heated at 353 K. Aer 20 minutes of
reaction progress, naphthalene (as an internal standard) was
added and the catalyst was separated at the reaction tempera-
ture by centrifugation followed by ltration using a Milex®-LG
0.20 mm. An additional amount of K2CO3 (3 mmol) was intro-
duced to the ltrate and the reaction was continued at 353 K for
an additional 40 minutes.
Furthermore, as a separate experiment 0.5 wt% Pd/AZC (1
mg) was weighed in to the ask followed by the addition of the
base, phenylboronic acid and bromobenzene in a molar ratio of
4 : 3 : 2 and ethanol (5 mL). The resulting solution was stirred at
353 K for 1 h. Aer 1 h of reaction progress, naphthalene was
added and subjected to GC analysis. The catalyst was removed
by centrifugation followed by ltration using a Milex®-LG 0.20
mm and fresh reagents (base, phenylboronic acid and bromo-
benzene in a molar ratio of 4 : 3 : 2) were added and the reac-
tion was continued for another 1 h at 353 K. The product
concentration was determined by GC analysis.
2.5.2 Solid-phase poisoning test. The catalyst poison
candidate was added to the reaction ask before the addition of
the reagents for the SMC reaction of bromobenzene. The
amount of QuadraPure® TU was used four times of that
required to bind total Pd as indicated by the manufacturer
(scavenging limit; 0.19 mmol Pd per gram QuadraPure TU). As a
control experiment aq. PdCl2 (47 nmol Pd) was used as the
catalyst with two equivalents of QuadraPure TU (twice the
amount required to bind all the palladium in the reaction
mixture).
2.6 Characterization
Table 1 The SMC reaction with the Pd/AZC catalysta
The structure was analyzed by powder X-ray diffraction (PXRD)
patterns with a SmartLab (Rigaku Co.) using Cu Ka radiation (l
¼ 0.154 nm) at 40 kV and 30 mA in the range of 2q ¼ 4–60ꢂ. The
diffraction patterns were analyzed with the database in the joint
committee of powder diffraction standards (JCPDS). For
inductively coupled plasma atomic emission spectroscopy (ICP-
AES) analysis, an ICPS-7000 ver. 2 (Shimadzu Co.) was employed
to quantify the actual Pd amount and to evaluate Pd leaching, if
any, during the reaction. Contents of Pd in the catalyst and/or
the reaction medium were estimated by a calibration curve
method. A H-7100 (Hitachi, Ltd.) operating at 100 kV was
utilized to acquire the morphology of the catalyst by trans-
mission electron microscopy (TEM). The samples for TEM
measurements were dispersed in water or ethanol, and the
supernatant liquid was dropped onto a copper grid before
drying in vacuo overnight. Scanning electron microscopy (SEM)
micrographs were obtained using an S-4100 (Hitachi, Ltd.). The
electronic state of Pd and Zn was analyzed by X-ray photoelec-
tron spectroscopy (XPS). The XPS experiments were conducted
on an AXIS-ULTRA DLD spectrometer system (Shimadzu Co.
and Kratos Analytical Ltd.) using an Al target at 15 kV and 10 mA
Entry
X
R
Catalyst
Pdb/mmol
Time/h Yieldc/%
1
Br
H
0.5 Pd/AZC
1.4
0.5
>99
2
23.5 ꢁ 10ꢀ2
93
3d
4
0.05 Pd/AZC 4.7 ꢁ 10ꢀ3
48
1.0
>99
0.5 Pd/AZC
4.7 ꢁ 10ꢀ2
77, 75,e 85f
5g
6h
7h,i
8h,i,j
1.4
1.4
5.6
2.8
0
Cl NO2 3 Pd/AZC
Cl
3.0
4.0
6.0
>99
18
34
H
a
Reaction conditions: substrate (2 mmol), phenylboronic acid (3
mmol), K2CO3 (4 mmol), ethanol (5 mL), round bottomed ask, 353
b
K, air. Actual amount of Pd used in the reaction estimated by ICP-
c
AES. Determined by GC using naphthalene as the internal standard
on the basis of bromobenzene. Bromobenzene (10 mmol). 1st
d
e
reuse. 2nd reuse. Without bromobenzene. Substrate (0.5 mmol),
f
g
h
i
phenylboronic acid (0.75 mmol), K2CO3 (1 mmol), ethanol, 353 K. In
the presence of tetrabutylammonium bromide (TBAB;
1 mmol).
j
Ethanol : DMF (1 : 1), 50 mL Teon lined autoclave, 373 K.
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Table 2 The SMC reaction of aryl halides and boronic acids with the Pd/AZC catalysta
Entry
X
R1
H
R2
H
R3
Time/h
1.0
Conv.b/%
Yieldb/%
1
2
3
Br
H
OCH3
F
COOH
H
OCH3
F
H
OCH3
F
H
80
83
91
84
>99
26
47
99
>99
90
82
77
69
92
4c
5d
6d
7d
8
3.0
73f,g
>99
19g
32g,h
96
Cl
I
NO2
H
H
H
0.3
1.0
9
68
10
89
11e
COOH
OH
3.0
76g
a
Reaction conditions: aryl halide (2 mmol), boronic acid (3 mmol), K2CO3 (4 mmol), 0.5 Pd/AZC (1 mg), ethanol (5 mL), round bottomed ask, 353
K, air. b Determined by GC using naphthalene as the internal standard on the basis of aryl halides. c Aryl halide (1 mmol), boronic acid (1.5 mmol),
d
e
K2CO3 (2 mmol), 0.5 Pd/AZC (5 mg). Aryl halide (0.5 mmol), boronic acid (0.75 mmol), K2CO3 (1 mmol), 3 Pd/AZC (5 mg). 0.5 Pd/AZC (5 mg).
f
Isolated yield. g Product was conrmed by NMR and GC-MS (see the ESI). h 4,40-Diuorobiphenyl was observed.
in an energy range of 0–1200 eV. The binding energies were
calibrated with the O 1s level (531.0 eV) as an internal standard
reference. X-ray absorption spectroscopy (XAS) was performed
with a transmission and/or uorescence mode at a BL-9C in
KEK-PF under the approval of the Photon Factory Program
Advisory Committee (Proposal no. 2013G586) and BL01B1 in
SPring-8 under the approval of the Japan Synchrotron Radiation
Research Institute (JASRI) (Proposal no. 2012B1610 and
2013B1478). The obtained XAS spectra were analyzed with
Rigaku REX2000 soware (ver. 2.5.92). The IR measurements of
Fig. 1 (a) SEM micrograph of AZC showing irregular blocks. (b) TEM
samples were carried out on a PerkinElmer Spectrum 100 FT-IR
spectrometer. Nuclear Magnetic Resonance (NMR) spectra were
recorded on a 400 MHz Bruker (AVANCE III 400) using DMSO-d6
as the solvent with TMS as the internal standard. A gas Chro-
matogram (GC-17A, Shimadzu Co.) coupled to a Mass Spec-
trometer (QP5000, Shimadzu Co.) (GC-MS) was employed to
obtain the mass fragmentation spectra of synthesized
compounds.
images of AZC at different magnifications.
3. Results and discussion
3.1 Morphology and crystallinity of AZC and Pd/AZC
Solvothermal treatment of Zn(NO3)2$6H2O with ATA in DMF at
413 K, followed by ethanol treatment afforded a brownish
material (denoted as AZC). The SEM (Fig. 1a) and TEM (Fig. 1b)
analyses of the sample captured porous blocks with an irregular
size and shape having thread/tube like features at the edges.
The AZC and Pd/AZC consisted of a phase with PXRD patterns
observed at 2q ¼ 11.2, 14.4, 18.1, 20.3, 23.6, 25.8 and 27.2ꢂ
corresponding to (111), (210), (220), (310), (320), (400) and (411)
planes, respectively (Fig. 2), which were much different from the
Fig. 2 XRD patterns of AZC (black) and Pd/AZC (red).
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PXRD patterns of IRMOF-3 and MIL-53 reported, previously.2,46 (Table 2, entry 7). The cross-coupling of 4-carboxyphenylboronic
Requisite amounts of PdCl2 were loaded on AZC by an adsorp- acid with bromobenzene required longer reaction times and
tion method to afford xPd/AZC, where the x is the Pd content higher amounts of palladium to accomplish decent yields
(wt%) in theory. The Pd graing process on AZC preserved its (Table 2, entry 4). The 2 mmol iodobenzene quickly cross-
original structure as evidenced by XRD (Fig. 2). The co-existence coupled with phenylboronic acid to produce >90% biphenyl
of agglomerates and large crystals of impurities such as ZnO, yields within 20 minutes catalyzed by 47 nmol of palladium,
PdCl2 and PdO in these materials were excluded on the basis of whereas in contrast, aryl iodides with hydroxyl and carboxyl
TEM images and XRD patterns.
functional groups necessitated longer reaction times to reach
high conversions (Table 2, entries 8, 11).
3.2 Catalytic activity of Pd/AZC for the SMC reaction
3.3 Investigation of the localized Pd/AZC structure around
the Zn atom
The catalytic activity of Pd/AZC was investigated for the SMC
reaction and was found to be highly active for both bromo- and
chloro-benzenes (Table 1). All experiments were performed
under an air atmosphere. High activities for bromobenzene
were realized at a shorter reaction time even with low Pd loading
(Table 1, entries 1–2). Although some of the current reports have
demonstrated a highly efficient SMC reaction in aqueous
media, we found our catalytic system to be fruitful in the
ethanol solvent (see ESI, Table S1†). While optimizing the
activity of Pd/AZC in ethanol, a remarkable TON value of
2 106 720 was reached with 4.7 nmol of Pd for 10 mmol of
bromobenzene (Table 1, entry 3, and also see Table S2 in the
ESI†). The highly active Pd/AZC could keep its potential at least
up to recycling 7 runs without any signicant loss of activity
(Table 1, entry 4, and Fig. S1 in ESI†). The control experiment
without bromobenzene (Table 1, entry 5) pronounced the
catalytic activity to be restricted to cross-coupling under the
present reaction conditions.
To account for the high activity and propose a suitable mecha-
nistic pathway for the SMC reaction over Pd/AZC, the structural
features were further characterized by spectroscopic methods.
The local structure around zinc in AZC and palladium in Pd/AZC
was investigated by XAS measurements. The Zn K-edge X-ray
absorption near-edge structure (XANES) spectrum of AZC was
similar to the Zn(II) salts of Zn(OAc)2 and Zn(NO3)2$6H2O
(Fig. 3a); 58% of Zn(OAc)2 with 42% of Zn(NO3)2$6H2O was
estimated by deconvolution of XANES features (see the ESI,
Fig. S2†), indicating the Zn(II) species as the integral part of
electronic states and/or the local structure in AZC.
XPS analysis of AZC also indicated that the Zn has an
oxidation state of +2 observed at 1021.4 eV (see the ESI,
Fig. S3†).49 The k3-weighted extended X-ray absorption ne
structure (EXAFS) of AZC at the Zn K-edge suggested a high
similarity in EXAFS oscillation to Zn(OAc)2; merging estimation
The activity was not limited to bromobenzene; the reaction
of activated chlorobenzene (p-nitrochlorobenzene) also affor-
ded 4-nitrobiphenyl with >99% yield (>99% conv.) in the
absence of additives (Table 1, entry 6). However, we realized that
chlorobenzene could not be coupled effectively with arylboronic
acid under the present reaction conditions (Table S3 in the
ESI†), whereas a higher conversion of chlorobenzenes (58.4%)
was observed at 353 K in the presence of TBAB with 18% yield of
biphenyl (Table 1, entry 7). In fact, chloroarenes required larger
Pd amounts in comparison to bromoarenes. Employment of
ethanol and DMF as solvents with TBAB as the additive
agent47,48 could improve the yield of biphenyl moderately in an
autoclave reactor (Table 1, entry 8); the implementation of such
reaction parameters in an autoclave could afford high conver-
sion of chlorobenzene with decent yields (30–34%) of biphenyl
(see the ESI, Table S3†).
For future interests, not only the various aryl halides but also
their reactivity with diverse boronic acids was subjected to the
SMC reaction using the Pd/AZC catalyst. These reactions were
also attempted and are tabulated in Table 2.
The reactivities of aryl halides in general were found to
decrease with the use of phenylboronic acids (Table 2, entries 1,
5, 8; R3 ¼ H) to urophenylboronic acids (entries 3, 7, 10;
R3 ¼ F) to methoxyphenylboronic acids (Table 2, entries 2, 6, 9;
R3 ¼ OCH3). Because of the high reactivity of uorophenylbor-
onic acid, a large excess of the palladium catalyst and low
reactivity of aryl chloride produced 4,40-diuorobiphenyl as the
Fig. 3 (a) Normalized XANES (b) EXAFS spectra at the Zn K-edge of
references and AZC. (c) FT of the k3-weighted EXAFS spectrum of AZC
(solid line) and Zn(OAc)2 (dashed line) at the Zn K-edge. The inset
homocoupling product of uorophenylboronic acid in traces shows the proposed local structure around Zn in AZC.
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indicated that the AZC has structural features of Zn(OAc)2 the XPS of Pd/AZC in the Pd 3d region: Pd 3d3/2 (342.2 eV) and
(above 80%) mixed with Zn(NO3)2$6H2O (Fig. 3b, and see the 3d5/2 (336.9 eV), split each individual peak into two peaks at
ESI, Fig. S4†). A broad peak in the Fourier transforms (FT) of 341.4 eV and 342.3 eV for Pd 3d3/2 whereas 335.6 eV and 337.1
˚
EXAFS of AZC appeared at 1.3–1.7 A (solid line, Fig. 3c) and eV for Pd 3d5/2; corresponding to Pd(II) in the square planar
which was at lower distance of Zn–O coordination compared to organopalladium complex and oxygen-coordinated Pd(II) states
Zn(OAc)2 (dashed line, Fig. 3c). This shi towards lower (Fig. 4b).51,52 The Pd K-edge XAS studies of the Pd/AZC were also
bond distance in AZC was supposedly accounted due to differ- performed, and its oxidation states as well as co-ordination of
ences in the side-chain of RCOO– (R ¼ CH3 in Zn(OAc)2 and Pd species were estimated. Pd K-edge XANES characteristics
C6H3(NH2)(COOH) in AZC) and structural complexity relating to
higher bond strength of Zn–O.
3.4 Investigation of the localized Pd/AZC structure around
the Pd atom
The IR analysis of AZC and Pd/AZC demonstrated that ATA was
stable under synthesis conditions forming a palladium–amine
complex assembly (N–H and Pd–N stretching was observed)
(Fig. 4a and S5 in the ESI†). The appearance of two peaks (at 477
and 496 cmꢀ1) for Pd–N in IR spectra supports the loading of Pd
on AZC through the palladium–amine complex with cis geom-
etry.50 In the Pd 3d XPS shown in Fig. 4b, the peaks at higher
binding energy (than that of Pd foil) distinguished the oxidized/
ionic species of palladium in Pd/AZC. Deconvolution analysis of
Fig. 4 (a) IR spectrum of AZC (black) and Pd/AZC (red) demonstrating
the two weak Pd–N stretching observed for Pd/AZC. (b) XPS of Pd 3d Fig. 5 (a) Normalized XANES, (b) k3-weighted EXAFS and (c) FT of
in Pd/AZC.
k3-weighted EXAFS of references and Pd/AZC at Pd K-edge XAS.
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(Fig. 5a), EXAFS (Fig. 5b), and FT of EXAFS (Fig. 5c) of Pd/AZC of Pd on hydroxyapatite (HAP) with Pd coordinated to four
were very different from the Pd foil but had resemblance to the oxygens in a Ca-decient site of HAP. A sound similarity in the
Pd(II) state of Pd(OAc)2 (Fig. 6a). The literature survey suggested FT of Pd/AZC and PdHAP54 at Pd K-edge were observed.
the species to possess a square planar geometry close to that of
Pd(OAc)2 or [Pd(NH3)4]Cl2.53 The FT of EXAFS exhibited a single
3.5 Proposed structure of Pd/AZC
˚
peak in the region of 1.4–1.8 A (Fig. 6a), resulting from the
The local structure around the Pd atom was proposed as shown
backscattering of the adjacent nitrogen and/or oxygen atoms
in Fig. 7. Neither any characteristics peak for the Pd–Pd bond,
nor any signicant signals at higher distances were observed in
(Fig. 5b and c). The inverse FT of the peak was well-tted using
Pd–N (CN ¼ 1.8) and Pd–O (CN ¼ 2.2) shells in the range of k ¼
FT, supporting the monomeric atomically dispersed Pd ions
with AZC. A thoughtful conclusion of these characteristics data
inspired us to propose the structures of AZC and Pd/AZC as
described in Fig. 7. A detailed and minute study with higher
accuracy for structural determination is subject to further
investigation and ongoing work.
ꢀ1
4–12 A (Fig. 6b and Table 3). Mori et al.54 reported the loading
˚
3.6 Heterogeneity of the Pd/AZC catalyst during the SMC
reaction
The Pd K-edge XANES and EXAFS analysis of the catalyst aer
the SMC reaction (of bromo- and chlorobenzene) suggested that
the catalyst conserved its original structural features (Fig. 5a–c).
EXAFS oscillation of the spent catalyst was indistinguishable
from the fresh catalyst. Besides, the FT of k3-weighted EXAFS at
the Pd K-edge denied the presence of any Pd–Pd shell in the
spent catalyst, rather bespoke the ionic Pd species with signals
for Pd–N/Pd–O shells. XRD patterns of the catalyst aer the
reaction also supported the integrity of the catalyst aer the
reaction (see the ESI, Fig. S6†). The ICP-AES analysis conrmed
that neither the supported Pd nor the assembled Zn leached
during the SMC reaction under our reaction conditions on the
ppm order.
The hot-ltration test is a useful method for establishing the
heterogeneity of the catalytic reaction. Aer 20 minutes of SMC
reaction progress, the catalyst was quickly ltered off (see
Section 2.5) and the ltrate along with an additional base was
stirred for another 40 minutes at 353 K. The GC analysis showed
a high initial rate of reaction with 60% biphenyl yield (62%
conv.) within 20 minutes of the reaction with just 1 mg of 0.5
wt% Pd/AZC. The SMC reaction did not proceed aer the
separation of the solid catalyst aer 20 minutes. Additionally, in
a separate experiment when fresh reactants (bromobenzene,
phenylboronic acid and base with the molar ratio of 2 : 3 : 4)
were added to the ltrate obtained by centrifugation and
ltration with the lter (0.2 mm) aer 1 h of the SMC reaction,
further reaction did not take place. These results indicate that
the catalysis by Pd/AZC is heterogeneous.
Fig. 6 (a) FT of the k3-weighted EXAFS spectrum of Pd/AZC (solid line)
and Pd(OAc)2 (dashed line) at the Pd K-edge. (b) The inverse FT of Pd/
AZC performed in the range of 4–12 A˚ꢀ1. The dashed orange line
shows the results of a curve-fitting analysis.
Table 3 Curve-fitting results for Pd K-edge EXAFSa
c
d
2
˚
˚
Shell
CNb
R /A
DW /A
As described by various researchers,55–59 the solid-phase
poisoning tests were performed to ascertain the heterogeneity
of Pd/AZC in the SMC reaction. Richardson and Jones reported
that two equivalents of QuadraPure TU which are required for
binding all the palladium could completely shutdown the
reactivity of solid catalysts that operate through the leaching
mechanism.56 In our case, however, it was found that the use of
even four equivalents of QuadraPure TU could not affect the
catalytic activity of Pd/AZC. The control experiment with aq.
PdCl2 and two equivalents of QuadraPure TU afforded no
product with 4% bromobenzene conversion. It conrms that no
Pd–N
Pd–O
1.8
2.2
2.050
2.026
0.0004
0.0104
a
The curve tting was performed using the McKale method. FT of the
k3-weighted EXAFS spectrum (k3c(k)) of Pd/AZC was performed in the
ꢀ1
˚
range of k ¼ 3–13 A with a window functionꢀf1or 20, and the inverse
˚
FT was examined in the ranges of k ¼ 4–12 A and R ¼ 1.258–1.872
˚
A with a window function for 10. The obtained results were under the
R
factor of 0.041%; which was dened by the formula
P
2
ðk3cobs ꢀ k3ccal
Þ
.
b
c
Coordination number. Interatomic distance.
P
2
ðk3cobs
Þ
d
Debye–Waller factor.
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Fig. 7 Proposed structure of AZC on the basis of spectroscopic characterization. Ball and stick model structure of (a) Pd/AZC; (b) local structure
around palladium and zinc in Pd/AZC; (c) (i) side and (ii) top view of the local structure of palladium in Pd/AZC. (d)–(f) Simplified structure of (a)–
(c). Carbon (gray), oxygen (red), nitrogen (blue), palladium (brown), and zinc (white). The hydrogen atoms are not shown for simplicity in the ball
and stick model.
homogeneous Pd species catalyze the SMC reaction under our
conditions. These experiments authenticated the signicant
heterogeneous nature of the Pd/AZC catalyst.
3.7 Proposed reaction pathway
A general reaction mechanism by divalent Pd has been
considered and sketched accordingly (see the ESI, Scheme S1†).
An oxidative addition of bromobenzene on the exposed Pd(II)
center of Pd(O)2(NH3)2 followed by ligand exchange with the
base and arylboronic acid are considered as sequential steps.
Reductive elimination of biphenyl regenerates the active naked
Pd and thereby the hetero-coupling of bromoarenes and aryl-
boronic acid takes place consecutively. Since the active Pd
species is generated, no further regeneration step is required
and thus the product with a high TON is afforded.
Even aer two decades of the SMC reaction, it is still a matter
of discussion that whether the genuine catalysis by the hetero-
geneous catalyst is heterogeneous or homogeneous. Some
researchers have held the atom-leaching mechanism as the
main cause of high activity for such cross-coupling reac-
tions.55,60,61 The non-leaching of the palladium species into the
reaction medium is supposed to be the reason for low activity
(34% biphenyl yield) of chlorobenzene in the present case. As
discussed above, no black-colored Pd or reduced Pd was
observed aer the SMC reaction over Pd/AZC and this reects
the stability of heterogeneous Pd/AZC. The results also indicate
that the Pd precipitation during the reaction and its effect on
catalytic activity of Pd species could be negligible with enduring
AZC as the support. The strong affinity of Pd to N and interac-
tion of ionic Pd with oxygen could prevent the active Pd species
from leaching or undergo agglomeration under the reaction
conditions.
3.8 Catalytic scope of Pd/AZC for other organic reactions
The Pd/AZC catalyst was also used for various Pd-catalyzed
reactions as shown in Table 4. We found that the present Pd/
AZC catalyst efficiently catalyzed the Mizoroki–Heck reaction of
bromobenzene and styrene (Table 4, entry 1). The catalyst also
efficaciously promoted hydrogenation reactions of the –NO2
group (in nitrobenzene), and the –C]C– group (in
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Table 4 Scope of catalysis by Pd/AZC
high turnover numbers (TON ¼ 2 106 720) without any addi-
tives under atmospheric conditions. The SMC reaction of other
aryl halides and aryl boronic acids could also be efficaciously
catalyzed by Pd/AZC under similar conditions. The character-
ization evidences the stability and durability of the Pd/AZC
catalyst, highlighting it as a candidate for a potential industrial
catalyst.
The Pd/AZC also successfully catalyzed the Mizoroki–Heck
coupling, hydrogenation of nitro, and –C]C– functional
groups. The high thermal stability, reect its high potential for
graing various metal species as catalytically active centers for a
wide range of metal-catalyzed reactions.
Entry Substrate
1b
Product
Time/h Conv.a/% Yielda/%
24
97
84
2c
3c
2.5
>99
>99
6
6
>99
>99
>99
98
4c,d,e
Acknowledgements
The authors are thankful to Prof. Dr Yuki Nagao (JAIST) for
helpful discussion on the AZC morphology and Prof. Dr Tetsuya
Shishido (Tokyo Metropolitan Univ.) for support in XAS
measurements. HC extends his thanks to the Japan Society for
the Promotion of Science (JSPS) for the fellowship. This work is
partially supported by JSPS KAKENHI Grant Numbers 26-12396
(Grant-in-Aid for JSPS Fellows) and 25820392 (Grant-in-Aid for
Young Scientists (B)).
a
Determined by GC using naphthalene as the internal standard.
Reaction conditions: bromobenzene (3.75 mmol), styrene (4.5
b
mmol), K2CO3 (4.5 mmol), 0.5 Pd/AZC (25 mg), 1-methyl-2-
pyrrolidinone (5 mL), 403 K, N2 ow. Reaction conditions: substrate
(1 mmol), 3 Pd/AZC (25 mg), ethanol (5 mL), H2 balloon (1 atm),
353 K. Water was used instead of ethanol. The conversion and
yield were determined using HPLC (Waters 600).
c
d
e
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