Mechanism of a Suzuki-Type Homocoupling Reaction Catalyzed by Palladium Nanocubes

Article abstract of DOI:10.1021/acscatal.6b03490

The trans-2-phenylvinylboronic acid homocoupling reaction catalyzed by palladium nanocubes (Pd-NCs) was investigated by kinetics, spectroscopy, and poisoning experiments. The reaction was evidenced to be sensitive to the presence of the base, which acts synergistically with the substrate molecules and assists the leaching of Pd oxide (PdO_x) species to the reaction medium. This species catalyzes the homocoupling reaction through the formation of Pd-O_x-B(OH)₂R pretransmetalation intermediates, via coordination with the vinylboronic acid molecules, involving an oxo-palladium-type interaction. The reaction rate was not enhanced by the saturation of the reaction medium with O₂, which is due to the oxidized nature of the Pd-NC surface.

Full text of DOI:10.1021/acscatal.6b03490

Subscriber access provided by UNIVERSITY OF SOUTH CAROLINA LIBRARIES
Article
Mechanism of a Suzuki-Type Homocoupling
Reaction Catalyzed by Palladium Nanocubes
Welman Elias, Aline Signori, Laize Zaramello, Brunno L.
Albuquerque, Daniela Coelho de Oliveira, and Josiel B Domingos
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03490 • Publication Date (Web): 11 Jan 2017
Downloaded from http://pubs.acs.org on January 12, 2017
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street
N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 9
ACS Catalysis
1
2
3
4
5
6
Mechanism of a Suzuki-Type Homocoupling Reaction Catalyzed by
Palladium Nanocubes
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
†
†
†
†
Welman C. Elias, Aline M. Signori, Laize Zaramello, Brunno L. Albuquerque, Daniela C. de
‡
†
Oliveira and Josiel B. Domingos *
†
LaCBio - Laboratory of Biomimetic Catalysis, Chemistry Department, Universidade Federal de Santa Catarina,
‡
Campus Trindade, Florianópolis - SC, 88040-900, Brazil. Laboratório Nacional de Luz Síncrotron, C. P. 6192,
Campinas-SP, 13083-970, Brazil.
ABSTRACT: The trans-2-phenylvinylboronic acid homocoupling reaction catalyzed by palladium nanocubes (Pd-
NCs) was investigated by kinetics, spectroscopy and poisoning experiments. The reaction was evidenced to be
sensitive to the presence of the base, which acts synergistically with the substrate molecules and assists the
leaching of Pd oxide (PdO ) species to the reaction medium. This species catalyzes the homocoupling reaction
x
through the formation of Pd-O -B(OH) R pre-transmetalation intermediates, via coordination with the vinyl
x
2
boronic acid molecules, involving an oxo-palladium-type interaction. The reaction rate was not enhanced by the
saturation of the reaction medium with O , which is due to the oxidized nature of the Pd-NC surface.
2
KEYWORDS: palladium, nanoparticles, nanocubes, vinyl boronic acid, homocoupling reaction, Suzuki, mechanism.
activity is attributed to the effect of the different
1
. INTRODUCTION
facets that compose the surface of the NPs and the
presence of O , in relation to the leaching of the
soluble catalytic Pd oxide species and its participation
in a homogeneous mechanism. Thus, with the aim
of enhancing our understanding of the mechanism
involved in Suzuki-type homocoupling reactions
catalyzed by Pd-NPs, we report herein an
investigation on the nature of the active species and
the effect of the reaction conditions, including the
influence of the base and the reaction atmosphere
The carbon-carbon bond forming homocoupling of
organoboron compounds, catalyzed by transition
metals, is highly versatile and plays a relevant role as
a synthesis methodology to produce symmetrical
functional
pharmaceuticals, fine chemicals and polymers.
Moreover, due to the mechanistic similarities, the
investigation of the homocoupling reaction of
boronic acids may provide useful information on the
Suzuki-Miyaura cross-coupling reaction. Most
mechanistic studies on the homocoupling of boronic
acids have been performed with the use of
organometallic palladium, but very limited effort has
been spent on reactions employing palladium
nanoparticles (Pd-NPs) as catalysts. Outstanding
catalytic activity and the possibility for recycling,
among other established benefits, make Pd-NPs an
interesting alternative to the use of organometallic
2
1
7
materials
leading
to
various
1
-6
(O2 vs. N ). A full kinetic/spectroscopic investigation,
2
associated with poisoning experiments, was
performed on the homocoupling reaction of trans-2-
phenylvinylboronic acid (PVBA) catalyzed by
palladium nanocubes (Pd-NCs) with different mean
sizes.
7
-9
2. EXPERIMENTAL SECTION
2.1.
polyvinylpyrrolidone (PVP 55 kDa), K PdCl ,
Materials
and
Methods.
The
L
1
0-13
complexes in coupling reactions.
However, in
-
2
4
recent years there has been intense debate regarding
the existence, and relative importance, of
ascorbic acid (AA), KBr, trans-2-phenylvinylboronic
acid (PVBA), trans,trans-1,4-diphenyl-1,3-butadiene
1
4
heterogeneous vs. homogeneous pathways, the
nature of the active species and the reaction
conditions that influence both. Thus, conclusions in
relation to the operating mechanism are frequently
(DPBT),
ethylenediaminetetraacetic
methylcyclohexene (MCH) were purchased from
Sigma-Aldrich and used as received without any
further purification. All solvents were spectroscopic
grade and ultrapure water was used in all systems.
Li CO ,
Na CO ,
3
K CO ,
2
Cs CO ,
2
2
3
2
3
3
acid
(EDTA)
and
1
3, 15-16
conflicting.
Moreover, the catalytic activity of
Pd-NPs in the cross-coupling reaction is closely
related to the intrinsic size and shape of the catalyst.
Nanoparticles with cubic shape, in Suzuki–Miyaura
reactions, for example, are more active than
cuboctahedron and octahedron structures under
aerobic conditions. This difference in the catalytic
2.2. Synthesis of Pd-NCs. The synthesis of Pd-NCs
was performed by adapting procedures described in
17-18
the literature.
1
In a two-neck round-bottom flask,
05 mg of PVP, 60 mg of AA, and 0.30 to 0.90 g of KBr
ACS Paragon Plus Environment

ACS Catalysis
Page 2 of 9
were dissolved in 8 mL of water and kept for 10 min
o
at 80 C under stirring. In the next step, 63 mg of
source (E = 1486.6 eV). The acquisition parameters
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
were Epass of 40 eV, energy step of 0.1 eV and
acquisition time of 1 s per point. The samples were
placed on stainless steel sample holders and
transferred under inert atmosphere to the XPS
analysis chamber where the internal residual pressure
K PdCl in 3 mL of water were added and the mixture
2
4
o
was aged for 4 h at 80 C. The resulting black
solutions were mixed with acetone and centrifuged to
collect the Pd-NCs as a fine powder.
−
8
was around 1×10 Torr. The binding energies (BE) of
Pd3d and C1s were referenced to the C1s peak at 284.5
eV, providing accuracy within ±0.2 eV.
2
.3. Characterization of Pd-NCs. All of the Pd-NCs
were characterized by transmission electron
microscopy (TEM) and X-ray photoelectron
spectroscopy (XPS). The Pd-NC15, used in the kinetic
investigations, was also characterized by small angle
X-ray scattering (SAXS) and X-ray absorption
spectroscopy (XAS). For the TEM analysis, a sample
of Pd-NCs was redispersed in water and dropped onto
a 200 mesh copper grid with carbon film and dried
naturally. The experiments were conducted on a
JEOL JEM-1011 microscope operating at 100 kV. SAXS
experiments were performed on the SAXS1 beamline
of the Brazilian Synchrotron Light Laboratory (LNLS
The flame atomic absorption spectrophotometry
(FAAS) analysis was carried out with an Hitachi Z-
8230 Polarized Zeeman spectrometer. The water
phase containing the Pd-NCs was centrifuged and the
supernatant was mixed with 10% (v:v) of an aqua
regia solution for the FAAS measurements.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
.4. Catalytic Activity Studies. All of the catalytic
activity experiments related to the homocoupling
reactions were performed in a biphasic system of
water and methylcyclohexane (H O/MCH, 1:1, v:v), at
2
o
–
Campinas, SP, Brazil). The solutions were loaded
8
0
C. The reaction kinetics of the boronic acid
into a temperature-controlled vacuum flow-through
cell composed of two mica windows separated by 1
homocoupling were investigated using UV-vis
spectrophotometry, by monitoring the appearance of
the product (DPBT) at 345 nm.
19
mm, normal to the beam. The collimated beam (λ =
1
.55 Å) crossed the sample through an evacuated
2
.5. Poisoning Experiments. The poisoning
flight tube and was scattered to a Pilatus 300K 2D
detector (Dectris). The incident beam was detected at
sample-to-detector distances of 500 mm and 3000
mm (silver behenate was used for the sample-to-
detector distance calibration). In all cases the 2D
images were found to be isotropic and they were
normalized using the FIT2D software developed by A.
20
Hammersley. Also, the resulting I(q) vs q scattering
curves were corrected by subtraction of the scattering
curve from the pure solvent curve and then placed on
an absolute scale using water as the standard. The
experiments were carried out using the optimized
reaction conditions and EDTA as the poisoning
-1
agent. In a quartz cuvette, 0.04 mmol L of Pd, 2.00
-1
mmol L of K CO , and 10 mmol L of EDTA were
-1
2
3
added to a mixture of 2 mL of H O/MCH (1:1). After 15
2
o
-1
min at 80 C, 0.4 mmol L of PVBA was added and
the reaction was monitored by UV-vis
spectrophotometry.
3
. RESULTS AND DISCUSSION
3.1. Synthesis and Characterization of Pd-NCs. To
study the structure sensitivity of the Suzuki-type
homocoupling reaction, four different sizes of Pd
nanocubes (named Pd-NC11, Pd-NC15, Pd-NC18, and
Pd-NC23, according to the NC edge length) were
synthesized. Transmission electron microscopy
(TEM) was used to determine the edge length, shape
and size distribution of the Pd-NCs (Figures 1a and 1b
and Figures S1a-S1c in the Supporting Information).
Two perpendicular edges of each cube were measured
and plotted versus each other to confirm the cubic
morphology (Figures 1c and Figures S1d-S1f in the
I(q) vs q scattering profile of the NCs could be fitted
using the form factor of homogeneous spheres with a
power-law regime to adjust the background. The
fitting procedures and other analyses were performed
using the SASfit software, which makes use of the
least-squares fitting approach to minimize the chi
2
squared (χ ) parameter. The SASfit software package
was developed by J. Kohlbrecher and is available free
of charge.
2
1
Extended X-ray absorption spectra (EXAFS) at the Pd
K edge (24350 eV) for the as-prepared Pd-NCs were
collected using the LNLS XDS beamline. The spectra
were acquired at room temperature in transmission
mode with three ionization chambers using an Si (311)
Supporting
Information).
The
average
size
histograms showed a Gaussian distribution (Figure 1d
and Figures S1g-S1h in the Supporting Information)
and confirmed that the mean edge lengths of the Pd-
NCs were 11.8 ± 2.0, 14.5 ± 2.1, 18.9 ± 2.9, and 23.5 ± 2.6
nm for the Pd-NC11, Pd-NC15, Pd-NC18, and Pd-NC23,
respectively.
double-crystal monochromator and
a
toroidal
focusing mirror. A standard Pd foil was used to
perform the energy calibration for all scans. Five to
ten spectra were collected to improve the signal-to-
noise ratio. Each spectrum was acquired in the range
of 24150−25400 eV with 2-6 s/point. The data were
SAXS measurements were also obtained to confirm
the size, shape, and dispersity (σ) of the NCs. Figure
1e shows the SAXS profile for Pd-NC15, which was
22
processed using the Ifeffit and Demeter packages.
The XPS data for the samples were obtained with an
23-24
refined according to a spherical form factor. The
radius of gyration, RG, can be obtained at the low
SPECSLPVBA II (Phoibos-Hsa 3500 150,
channeltrons) SPECS spectrometer with an Al Kα
9
ACS Paragon Plus Environment

Page 3 of 9
ACS Catalysis
are given by (Eq. 1) and (Eq. 2), respectively:
limit of Q, according to the Guinier approximations.
The RG values for a dense sphere and a perfect cube
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 1. Pd-NC15 characterization: (a) TEM micrograph, (b) TEM Model for measurement of 2 cube edges, (c) side a vs. side b
plot, (d) average distribution histograms and Gaussian fit (black line), (e) SAXS data and corresponding curve fitting (red line),
and (f) Fourier-transformed EXAFS spectra for Pd-NC15 and Pd foil, and corresponding curve fitting (red line).
ꢄ
ꢆ
ꢅ
NC15 has a total of 38 shells (calculated using the
magic number model for a fcc cubic structure) and
ꢂ
ꢂ
ꢀ
ꢃ ꢀ ꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇ(Eq. 1)
25
ꢁ
the EXAFS analysis provides average values for the
oxidation state and coordination number for all Pd-
NC shells (the surface atoms on the Pd-NC15
represent only 7.8 % of the total atoms of the NCs),
these results could underestimate the amount of
oxidized Pd atoms on the NCs surface.
ꢈ
ꢂ
ꢉ
ꢂ
ꢀ ꢃ ꢊ
(Eq. 2)
ꢁ
ꢋ
where RS is the sphere radius and ec is the cube edge
length. Thus, the edge length of the cube can be
calculated by the sphere radius, using (Eq. 3):
ꢈꢂ
3.2.
PVBA
homocoupling
reaction.
The
ꢊ ꢃ ꢌ ꢀ
(Eq. 3)
ꢋ
ꢆ
ꢅ
investigation of the kinetics of the homocoupling
reaction of PVBA (Figure 2a) was performed with the
catalyst Pd-NC15 in a water/MCH (1:1) biphasic
medium, in the presence and absence of a carbonate
base. The high solubility of the homocoupling
product (DPBT) in the MCH solvent (Figure S2 in the
Supporting Information) allowed its appearance to be
selectively monitored by UV-vis spectroscopy at 345
nm (Figure 2). As can be seen in Figure 2b, the
reaction in the absence is much slower than in the
presence of the base, indicating that the base plays an
important role in the reaction. Thus, four different
carbonate bases were evaluated (Na CO , Li CO ,
The calculated RG
1
,
0.06 nm, 15.60 nm, and 0.19, respectively. These data
Rs
,
ec and
σ
values were 7.80 nm,
were in agreement with the results obtained by TEM.
Further structural characterization of the Pd-NC15
catalyst, used in the kinetics studies, was performed
by XAS analysis. The data were collected on a
Synchrotron-based setup and the structural
parameters of the sample were obtained by the first-
shell fit of the EXAFS signal (Figure 1f) performed in
space from 1.8 to 5.6 Å. The data are presented with
no phase correction and the Pd foil was used as a
reference to establish the value of the reduction
2
factor S . The EXAFS fitting results (Table S1 in the
R
2
3
2
3
K CO3 and Cs CO ) in the reaction. The results
2 2 3
0
showed that the initial reaction rate (see Figure S4 in
the Supporting Information) is not affected by the
type of the carbonate counter ion and it reaches
saturation with an increase in the base concentration.
Supporting Information) showed that the intense Pd-
Pd bond signal for the Pd-NC15 sample presents
nearly the same distance (RPd-Pd) as the Pd foil
reference (2.73 Å and 2.74 Å, respectively). These
results clearly demonstrate that the Pd-NC15 sample
is formed of Pd(0) atoms. However, since the Pd-
The role of the base in Suzuki-type homocoupling
using boronic acids is still not clear and it may not be
ACS Paragon Plus Environment

ACS Catalysis
Page 4 of 9
2
, 7, 26
important.
In the case of Suzuki-Miyaura cross- the addition of PVBA (Figure 3), the initial rate was
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
coupling, the base is associated with three main types
of transmetalation pathway: (i) the hydroxide anions
react with the boronic acid to form the
trihydroxyboronate (the boronate pathway, Scheme
1a); (ii) the hydroxide reacts with palladium to form a
more reactive oxo–palladium species (oxo-palladium
1.4 times higher than the simultaneous addition of
the components and 2.3 times faster than in the case
when the PVBA was left to equilibrate with the base
for 30 min, prior to the addition of the Pd-NCs. These
results indicate that the catalyst is activated by the
base prior to the addition of the reactant, probably
due to the favorable formation of oxo-palladium-type
species.
pathway, Scheme 1b); or (iii) a combination of both
2
pathways.
7
Under aerobic conditions, the oxo-palladium pathway
in the Suzuki-type homocoupling of boronic acids is
associated with the formation of a palladium(II)
peroxo complex (Scheme 1c), which is initially formed
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1, 26
from the reaction of dioxygen with a Pd(0) species.
This complex then coordinates with the boronic acid
to form a reactive adduct which, through hydrolysis,
generates the oxo-palladium intermediate, in the
formation of the homocoupling product with a
second boronic acid molecule.
Scheme 1. Interaction types for the transmetalation
step: (a) boronate, (b) oxo-palladium and (c) peroxo-
palladium.
An established way to test for the formation of
peroxo-palladium species, as the origin of the catalytic
cycle for the aerobic homocoupling reaction of
boronic acids, is to run the reaction under an O or
2
N2 saturated atmosphere, which enhances or
completely inhibits the reaction, respectively.
1, 26, 33-34
Surprisingly, in our case, the saturation with
dioxygen did not improve the rate of the reaction,
and the dinitrogen did not decreased the reaction
rate compared with the reaction under air
Figure 2. (a) Time evolution of product UV-vis absorption
spectra and (b) kinetic profile at 345 nm. [Pd-NC15] = 0.04
atmosphere (Figure 4). Since Pd(0) is required for the
formation of
-1
-1
mmol L , [PVBA] = 0.40 mmol L and, when used, [K CO ] =
2
1
peroxo-palladium species, we
2
3
o
a
-1
.00 mmol L , in water/MCH (1:1) biphasic medium, at 80 C.
speculated that oxidized Pd rather than Pd(0) could
be the catalytic species under these particular
reaction conditions.
Previously reported experimental and theoretical
results indicate that the boronate pathway is faster
than the palladium hydroxo route in aqueous
2
8-29
medium.
pathway is slower than the oxo-palladium pathway
However, findings that the boronate
30-32
have also been reported.
Schimidt and coauthors, for example, observed that
the formation of biphenyl in a water/DMF (1:4)
medium was found to occur 1.3–2 times more rapidly
under conditions where neutral phenylboronic acid
was added to the pre-mixed solution of Pd(OAc) and
2
NaOAc, indicating that the oxo–palladium pathway
was more kinetically favored than the boronate
31
pathway. In our case, when the Pd-NCs were left to
equilibrate with the base for 15, 30 or 60 min, prior to
ACS Paragon Plus Environment

Page 5 of 9
ACS Catalysis
Pd(0) and PdO remains the same as that for Pd-NC15
x
1
2
3
4
5
6
7
8
9
(entries 5 to 7).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 3. Order of addition of the reaction components in
-
the PVBA homocoupling reaction. [Pd-NC15] = 0.04 mmol L
Figure 4. Vinyl boronic acid homocoupling reaction under
1
-1
-1
-1
, [PVBA] = 0.40 mmol L and [K CO ] = 2.00 mmol L , in
2
3
different atmospheres. [Pd-NC15] = 0.04 mmol L , [PVBA] =
o
-
1
-1
water/MCH (1:1) biphasic medium, at 80 C.
0
.40 mmol L and [K CO ] = 2.00 mmol L , in water/MCH
2 3
o
(1:1) biphasic medium, at 80 C.
The boronic acid homocoupling reaction mediated by
Pd(II) was first reported by Moreno-Manãs and co-
These results indicate that the use of a base and
boronic acid can assist the leaching of the PdOx
present on the surface of the Pd-NCs, exposing the
core Pd(0) atoms. This type of assistance has been
35
authors. In contrast to oxidative homocoupling, the
initial transmetalation occurs through acid-base
reactions of Pd(II) atoms with two boronic acid
molecules and, subsequently, the homocoupling
product and Pd(0) atoms are formed through a
reductive process. These Pd(0) atoms are thus
assumed to be the species responsible for the
continuation of the catalytic cycle.
previously reported, e.g.
, Fang and co-authors
observed that Pd centers, or clusters, leached from
Au@Pd-NPs in the presence of the components of a
Suzuki-Miyaura cross-coupling reaction, i.e., 4-
bromoanisole with phenyl boronic acid, in water and
38
In this context, the surface oxidation state of the Pd
atoms in the Pd-NC15 catalyst was evaluated by X-ray
photoelectron spectroscopy (XPS) experiments,
before and after the reaction completion, with and
without a base. Figure 5 shows the Pd 3d spectra and
their peak positions for the samples: (a) before the
reaction in the absence of a base, (b) before the
reaction in the presence of a base for 15 min, (c) after
with the base K
2
CO .
3
Moreover, it has been
observed that the oxidation state of the Pd has an
effect on the extent of leaching (oxidized Pd(II) is
3
9-40
prone to leaching).
Collins and co-authors has
shown that {100} facets present superior catalytic
activity compared with {111} facets on the surface of
Pd-NPs, due to the surface Pd oxide species being
17
more readily leached.
1
1
3 h of reaction in the absence of a base, and (d) after
3 h of reaction in the presence of a base. The Pd 3d5/2
In fact, in our case, 5.0 ± 0.2 ppm of Pd was detected
in the reaction supernatant after the removal of the
Pd-NCs. This represents around 13% of the total Pd
leached to the reaction medium, relative to the total
amount of Pd atoms present in the Pd-NC15, and is
energy in the region of 337.0 to 337.6 eV (Table 1) can
be attributed to different types of Pd oxide species
(
PdO ), while the peak at 335.2 eV corresponds to
x
36
Pd(0) species. In Figure 5 it can be noted that
without a base, the surface of the NCs is composed
only of PdOx species, before and after 13 h of
homocoupling reaction (Figure 5a and 5c).
4
5% higher than the value obtained in the absence of
a base. The simultaneous presence of the base and
boronic acid increases the total Pd leaching to 18%.
The experimental XPS and leaching results indicate
that the homogeneous PdO species leached from the
In the presence of a base, the broad peaks (Figure 5b
and 5d) of the spectra were deconvoluted and the
peaks related to Pd(0) and PdO species were present,
x
surface of the NCs could be related to the rate
enhancement though coordination to PVBA
molecules. However, in contrast to the findings
x
3
7
before and after the reaction completion. These XPS
results are summarized in Table and they
1
7
1
reported by Collins and co-authors, the leaching of
Pd oxides from the NCs was not enhanced by the
saturation of the reaction medium with O . As seen
demonstrate that the surface of the Pd-NC15 in the
presence of a base, before the reaction, is comprised
of 62% Pd(0) and 38% PdO (entry 3); and this
2
x
above, the reaction rate is virtually the same as that
obtained with an inert atmosphere. This is probably
due to the oxidized nature of the Pd-NC15 surface, as
observed in the XPS analysis (Figure 5a).
composition remains after the reaction completion
(entry 4). For the different sizes of Pd-NCs, analyzed
after the homocoupling reaction, the ratio between
ACS Paragon Plus Environment

ACS Catalysis
Page 6 of 9
for 15 min, (c) Pd-NC15 in absence of K CO after 13 h of
2
3
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
reaction, and (d) Pd-NC15 in presence of K CO after 13 h of
2 3
reaction.
After the catalytic cycle, the leached atoms could
return to the NCs surface, and eventually reoxidize or
form new clusters (Scheme 2). Indeed, the TEM
analysis of the reaction medium, before and after the
reaction completion, in the presence and absence of a
base (Figure S6 in the Supporting Information),
showed the existence of small clusters of Pd-NCs
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
after the reaction completion when K CO3 was
2
present in the reaction medium.
Moreover, if this mechanism is operating,
sequestration of the soluble oxidized Pd atoms
should inhibit the reaction. Therefore, a poisoning
test was performed, with the addition of an excess
amount of EDTA in the medium for the reaction with
the Pd-NC15 catalyst or the K PdCl4 salt, for
2
comparison purposes (Figure 6). In both experiments,
the addition of EDTA completely inhibited the
reaction. This result strongly supports oxidized Pd as
the active species of the reaction and the Pd-NCs
acting as a reservoir for this species.
Figure 5. XPS spectra of Pd 3d core level for Pd-NC15: (a) Pd-
NC15 in absence of K CO , (b) Pd-NC15 in presence of K CO
3
2
3
2
Table 1. Binding energy and PdO /Pd(0) ratios for the Pd-NCs measured by XPS analysis.
x
Entry
Sample
Binding Energy (eV)
% Relative Atomic Ratio
PdO /Pd(0)
x
PdOx
Pd(0)
3
Pd5/2
3Pd3/2
342.8
3Pd5/2
-
3Pd3/2
-
[a]
1
Pd-NC15
337.6
337.3
337.2
337.0
336.7
336.3
335.9
100/0
100/0
38/62
30/70
30/70
31/69
28/72
[
b]
2
3
4
5
6
7
Pd-NC15
342.6
342.3
341.9
341.8
341.3
341.0
-
-
[c]
Pd-NC15
335.3
335.2
334.7
334.7
334.2
340.3
340.2
339.9
339.7
339.4
[d]
Pd-NC15
Pd-NCs11
Pd-NCs18
Pd-NCs23
[d]
[d]
[d]
[a]
[b]
[c]
[d]
In absence of K CO before reaction, in absence of K CO after 13 h of reaction, in presence of K CO before reaction, in
2
3
2
3
2
3
-1
-1
presence of K CO after 13 h of reaction. [Pd-NC15] = 0.04 mmol L , [PVBA] = 0.40 mmol L and, when used, [K CO ] = 2.00
2
3
2
3
-1
o
mmol L , in water/MCH (1:1) biphasic medium, at 80 C.
ACS Paragon Plus Environment

Page 7 of 9
ACS Catalysis
Figure 7. Structure sensitivity analysis of Pd-NCs with
different edge lengths in the vinyl boronic acid
1
2
3
4
5
6
7
8
9
-
1
homocoupling reaction. [Pd-NC15] = 0.04 mmol L , [PVBA] =
-
1
-1
0.40 mmol L and [K CO ] = 2.00 mmol L , in water/MCH
2
3
o
(1:1) biphasic medium, at 80 C.
Based on the results, three possible scenarios could
explain the interaction of these PdO species with the
x
boronic acid in a pre-transmetalation step, as shown
in Scheme 3: (A) a oxo-palladium-type pathway, (B) a
boronate-type pathway, and (C) an acid-base
reaction.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Considering that the boron center has high
38
oxophilicity and that the reaction was favored by
the conditions were the Pd-NCs were let to stand in
the presence of the base prior to the addition of the
boronic acid (Figure 3), it appears that not only the
Figure 6. Effect of EDTA addition on the initial reaction rate
(r ) using K PdCl and Pd-NCs as catalysts, in the presence of
o
2
4
-
1
leaching of PdO species is favored by the reaction
x
a base. [Catalysts] = 0.04 mmol L (concentration based on
-
1
conditions, but the interaction of this catalytic
species with the boron center, following an oxo-
palladium-type pathway, is also favored (Scheme 3A).
the total amount of Pd), [PVBA] = 0.40 mmol L , [K CO ] = 2
2
3
-
1
-1
mmol L and, when used, [EDTA] = 10 mmol L ) in a
o
water/MCH (1:1, v:v) biphasic medium, at 80 C.
Another result supporting the mechanism involving a
catalytic homogeneous pathway is revealed by the
structure sensitivity plots shown in Figure 7. In these
plots, the normalization of the initial rate for the total
surface atoms concentration or for the defect atoms
concentration (edge and corner atoms) shows a linear
increase with the size of the NCs, indicating that
there is no atom type preference on the surface of the
NCs.
The results reported above establish that the catalytic
efficiency of the NCs investigated is due to PdO leaching,
x
which is enhanced by the synergistic action of the base
and the boronic acid (Scheme 2). The corresponding
catalytic efficiencies are remarkable, since the amounts of
leached Pd species are in the ppm range. Although the
FAAS technique does not differentiate between PdO and
x
Pd(0) species, under the reaction conditions studied,
Pd(0) is most likely not involved at the early stage of the
reaction or is much less active than the PdO species.
x
Scheme 2. Pd-NCs catalysis of the vinyl boronic acid
homocoupling reaction.
ACS Paragon Plus Environment

ACS Catalysis
Page 8 of 9
reaction conditions in the presence of a base when
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
compared to the reaction without a base, in
agreement with the proposed mechanism presented
in Scheme 2.
4. CONCLUSIONS
The results reported herein provide evidence that the
PVBA
homocoupling
reaction
involves
coordination of the boronic acid molecules to
the
palladium oxides species (PdO ) through an oxo-
x
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
palladium-type transmetalation pathway in the
presence of a base. This is in agreement with a recent
publication by Thomas and Denmark, who showed
that the presence of Pd-O-B moieties, in pre-
transmetalation intermediates, is highly favorable for
Scheme 3. Mechanism pathways for the
transmetalation of boronic acids with oxidized
palladium species.
41
the transfer of B-aryl groups to palladium. The
leaching of the catalytic species PdO to the reaction
Considering the simplified mechanism shown in
Scheme 2 and the fact that the reaction presented a
non-linearly dependence on the PVBA concentration
x
medium is synergistically assisted by the
base/boronic acid molecules and the Pd-NCs act as a
reservoir for this species. Due to the oxidized nature
of the Pd-NC surface, the saturation of the reaction
medium with O did not enhanced the reaction rate.
2
Such an enhancement would be expected for a
reduced surface, through the preferential adsorption
(Figure 8), the rate law of product formation, using a
steady-state approach (see Supporting Information),
can be expressed by (Eq. 4):
ꢓ
ꢍꢎꢏꢐ
ꢍꢑ
ꢒ ꢎꢏꢔꢕꢖꢐ ꢎꢏꢍꢐ
ꢓ
ꢃ
(Eq. 4)
ꢗꢘꢙꢙꢚꢎꢏꢔꢕꢖꢐꢓ
of O
leaching of Pd oxides in the presence of a base and
onto {100} Pd-NC facets and the sequential
2
1
boronic acid molecules, or through the formation of
7
where Kapp is a combined equilibrium constant of the
formation of intermediates and k2 is the catalytic
constant for the product formation.
1
, 33
catalytic peroxo like species.
ASSOCIATED CONTENT
Supporting Information
Complementary microscopy, spectroscopy and kinetic data
are included in the SI. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Phone: +55 (48) 3721 3620. Fax: +55 (48) 3721 6850. E-mail:
josiel.domingos@ufsc.br
ACKNOWLEDGMENTS
We are grateful to CNPq and CAPES for financial support
received for this study under the Science Without Borders
Program (SWE 107700/2014-1). This work was also supported by
the Brazilian Synchrotron Light Laboratory (LNLS) under
proposals SAXS1 – 17701/19004 and XAFS1 – 17702/19010 (beam
time usage) and the Central Laboratory of Electron Microscopy
Figure 8. Plot of initial reaction rate (r ) for the vinyl boronic
o
acid homocoupling reaction as a function of [PVBA] in the
-
1
presence of a base. [Pd-NC15] = 0.04 mmol L , [K CO ] = 2
2
3
-1
o
mmol L , in water/MCH (1:1, v:v) biphasic medium, at 80 C.
The red line represents the nonlinear fit using (Eq. 4).
(LCME) at UFSC (TEM analysis).
REFERENCES
In fact, the experimental kinetics data could be well
fitted by applying (Eq. 4), and the reaction rate
constants were determined using a non-linear fitting
1
.
Adamo, C.; Amatore, C.; Ciofini, I.; Jutand, A.; Lakmini, H.
J. Am. Chem. Soc. 2006, 128, 6829-6836.
2
2
3
.
Karanjit, S.; Ehara, M.; Sakurai, H. Chem.-Asian J. 2015, 10,
procedure. Based on these results, rate constants (k2
-6
)
397-2403.
.
-2
-1
-1
of 9.2 x 10 min and 4.5 x 10 min were determined
for the reaction in the presence (Figure 8) and
Tyagi, D.; Binnani, C.; Rai, R. K.; Dwivedi, A. D.; Gupta, K.;
Li, P.-Z.; Zhao, Y.; Singh, S. K. Inorg. Chem. 2016, 55, 6332-6343.
absence of
a
base (Figure S7 in Supporting
4.
1967.
Vogler, T.; Studer, A. Adv. Synth. Catal. 2008, 350, 1963-
Yamamoto, Y. Synlett 2007, 2007, 1913-1916.
Information), respectively. This reveals that the
catalytic activity is 20,000 times higher for the
5
.
ACS Paragon Plus Environment

Page 9 of 9
ACS Catalysis
6.
Zhou, L.; Xu, Q. X.; Jiang, H. F. Chin. Chem. Lett. 2007, 18,
24.
Disch, S.; Wetterskog, E.; Hermann, R. P.; Korolkov, D.;
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1043-1046.
Busch, P.; Boesecke, P.; Lyon, O.; Vainio, U.; Salazar-Alvarez, G.;
Bergström, L.; Brückel, T. Nanoscale 2013, 5, 3969-3975.
7.
Zheng, G. C.; Kaefer, K.; Mourdikoudis, S.; Polavarapu, L.;
Vaz, B.; Cartmell, S. E.; Bouleghlimat, A.; Buurma, N. J.; Yate, L.; de
Lera, A. R.; Liz-Marzan, L. M.; Pastoriza-Santos, I.; Perez-Juste, J. J.
Phys. Chem. Lett. 2015, 6, 230-238.
25.
Schneider, W. F.; Werth, C. J. ACS Catal. 2013, 3, 453-463.
26. Lakmini, H.; Ciofini, I.; Jutand, A.; Amatore, C.; Adamo, C.
J. Phys. Chem. A 2008, 112, 12896-12903.
Shuai, D.; McCalman, D. C.; Choe, J. K.; Shapley, J. R.;
8.
Zhu, Y.; Hosmane, N. S. Coord. Chem. Rev. 2015, 293–294,
357-367.
27.
Lennox, A. J. J.; Lloyd-Jones, G. C. Chem. Soc. Rev. 2014, 43,
9
6
.
Dhital, R. N.; Sakurai, H. Asian J. Org. Chem. 2014, 3, 668-
412-443.
28.
84.
Lima, C. F. R. A. C.; Rodrigues, A. S. M. C.; Silva, V. L. M.;
1
1
0.
1.
Pérez-Lorenzo, M. J. Phys. Chem. Lett. 2012, 3, 167-174.
Fihri, A.; Bouhrara, M.; Nekoueishahraki, B.; Basset, J.-M.;
Silva, A. M. S.; Santos, L. M. N. B. F. ChemCatChem 2014, 1291-1302.
29. Ortuño, M. A.; Lledós, A.; Maseras, F.; Ujaque, G.
ChemCatChem 2014, 6, 3132-3138.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Polshettiwar, V. Chem. Soc. Rev. 2011, 40, 5181-5203.
12.
Astruc, D.; Lu, F.; Aranzaes, J. R. Angew. Chem., Int. Ed.
30.
Carrow, B. P.; Hartwig, J. F. J. Am. Chem. Soc. 2011, 133,
2005, 44, 7852-7872.
2116-2119.
31. Schmidt, A. F.; Kurokhtina, A. A.; Larina, E. V. Russ. J.
Gen. Chem. 2011, 81, 1573-1574.
1
3.
Bej, A.; Ghosh, K.; Sarkar, A.; Knight, D. W. RSC Adv. 2016,
6, 11446-11453.
1
4.
Cano, R.; Schmidt, A. F.; McGlacken, G. P. Chem. Sci. 2015,
32.
Amatore, C.; Jutand, A.; Le Duc, G. Chem.--Eur. J. 2011, 17,
6, 5338-5346.
2492-2503.
33.
15.
Fairlamb, I. J. S.; Lee, A. F., In C-H and C-X Bond
Tsunoyama, H.; Sakurai, H.; Ichikuni, N.; Negishi, Y.;
Functionalization: Transition Metal Mediation, The Royal Society of
Chemistry: 2013; pp 72-107.
Tsukuda, T. Langmuir 2004, 20, 11293-11296.
34. Parrish, J. P.; Jung, Y. C.; Floyd, R. J.; Jung, K. W.
Tetrahedron Lett. 2002, 43, 7899-7902.
1
6.
973-4985.
7. Collins, G.; Schmidt, M.; Dwyer, C. O.; Holmes, J. D.;
McGlacken, G. P. Angew. Chem., Int. Ed. 2014, 53, 4142-4145.
8. Jin, M.; Liu, H.; Zhang, H.; Xie, Z.; Liu, J.; Xia, Y. Nano Res.
010, 4, 83-91.
9. Cavalcanti, L. P.; Torriani, I. L.; Plivelic, T. S.; Oliveira, C.
L. P.; Kellermann, G.; Neuenschwander, R. Rev. Sci. Instrum. 2004,
5, 4541-4546.
20. Hammersley, A. P. Scientific software FIT2D,
http://www.esrf.eu/computing/scientific/FIT2D: 2009.
1. Kohlbrecher, J. Software package SASfit for fitting small-
angle scattering curves,
http://kur.web.psi.ch/sans1/SANSSoft/sasfit.html: 2009.
Balanta, A.; Godard, C.; Claver, C. Chem. Soc. Rev. 2011, 40,
4
1
35.
Moreno-Mañas, M.; Pérez, M.; Pleixats, R. J. Org. Chem.
1996, 61, 2346-2351.
36.
NIST X-ray Photoelectron Spectroscopy Database, Version
1
2
1
4.1. National Institute of Standards and Technology: Gaithersburg,
2012.
37.
Bernardi, F.; Fecher, G. H.; Alves, M. C. M.; Morais, J. J.
Phys. Chem. Lett. 2010, 1, 912-917.
7
38.
Fang, P.-P.; Jutand, A.; Tian, Z.-Q.; Amatore, C. Angew.
Chem., Int. Ed. 2011, 50, 12184-12188.
39.
Dams, M.; Drijkoningen, L.; De Vos, D.; Jacobs, P. Chem.
2
Commun. (Cambridge, U. K.) 2002, 1062-1063.
40.
2005, 231, 245-253.
41. Thomas, A. A.; Denmark, S. E. Science 2016, 352, 329-332.
Okumura, K.; Nota, K.; Yoshida, K.; Niwa, M. J. Catal.
22.
Ravel, B.; Newville, M. J. Synchrotron Radiat. 2005, 12, 537-
541.
23.
Disch, S.; Wetterskog, E.; Hermann, R. P.; Wiedenmann,
A.; Vainio, U.; Salazar-Alvarez, G.; Bergström, L.; Brückel, T. New J.
Phys. 2012, 14, 13025-13036.
Table of Contents Graphic
ACS Paragon Plus Environment