Sustainable catalysis: Rational Pd loading on MIL-101Cr-NH2 for more efficient and recyclable Suzuki-Miyaura reactions

Article abstract of DOI:10.1002/chem.201302621

Palladium nanoparticles have been immobilized into an amino-functionalized metal-organic framework (MOF), MIL-101Cr-NH₂, to form Pd@MIL-101Cr-NH₂. Four materials with different loadings of palladium have been prepared (denoted as 4-, 8-, 12-, and 16 wt %Pd@MIL-101Cr-NH ₂). The effects of catalyst loading and the size and distribution of the Pd nanoparticles on the catalytic performance have been studied. The catalysts were characterized by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared (FTIR) spectroscopy, powder X-ray diffraction (PXRD), N₂-sorption isotherms, elemental analysis, and thermogravimetric analysis (TGA). To better characterize the palladium nanoparticles and their distribution in MIL-101Cr-NH₂, electron tomography was employed to reconstruct the 3D volume of 8 wt %Pd@MIL-101Cr-NH₂ particles. The pair distribution functions (PDFs) of the samples were extracted from total scattering experiments using high-energy X-rays (60 keV). The catalytic activity of the four MOF materials with different loadings of palladium nanoparticles was studied in the Suzuki-Miyaura cross-coupling reaction. The best catalytic performance was obtained with the MOF that contained 8 wt % palladium nanoparticles. The metallic palladium nanoparticles were homogeneously distributed, with an average size of 2.6 nm. Excellent yields were obtained for a wide scope of substrates under remarkably mild conditions (water, aerobic conditions, room temperature, catalyst loading as low as 0.15 mol %). The material can be recycled at least 10 times without alteration of its catalytic properties.

Full text of DOI:10.1002/chem.201302621

FULL PAPER
DOI: 10.1002/chem.201302621
Sustainable Catalysis: Rational Pd Loading on MIL-101Cr-NH for More
2
Efficient and Recyclable Suzuki–Miyaura Reactions
[
a, b]
[a, c]
[a, b]
[a, c]
Vlad Pascanu,
Qingxia Yao,
Antonio Bermejo Gꢀmez,
Mikaela Gustafsson,
[a, c]
[
a, c]
[a, c]
[c]
Yifeng Yun,
Wei Wan,
Louise Samain, Xiaodong Zou,*
and
[
a, b]
Belꢁn Martꢂn-Matute*
Abstract: Palladium nanoparticles have
been immobilized into an amino-func-
tionalized metal–organic framework
sorption isotherms, elemental analysis,
and thermogravimetric analysis (TGA).
To better characterize the palladium
nanoparticles and their distribution in
lytic activity of the four MOF materials
with different loadings of palladium
nanoparticles was studied in the
Suzuki–Miyaura cross-coupling reaction.
The best catalytic performance was ob-
tained with the MOF that contained
8 wt% palladium nanoparticles. The
metallic palladium nanoparticles were
homogeneously distributed, with an
average size of 2.6 nm. Excellent yields
were obtained for a wide scope of sub-
strates under remarkably mild condi-
tions (water, aerobic conditions, room
temperature, catalyst loading as low as
0.15 mol%). The material can be recy-
cled at least 10 times without alteration
of its catalytic properties.
(
MOF), MIL-101Cr-NH , to form Pd@
2
MIL-101Cr-NH . Four materials with
MIL-101Cr-NH , electron tomography
2
2
different loadings of palladium have
been prepared (denoted as 4-, 8-, 12-,
was employed to reconstruct the 3D
volume of 8wt%Pd@MIL-101Cr-NH2
particles. The pair distribution functions
(PDFs) of the samples were extracted
from total scattering experiments using
high-energy X-rays (60 keV). The cata-
and 16wt%Pd@MIL-101Cr-NH ). The
2
effects of catalyst loading and the size
and distribution of the Pd nanoparti-
cles on the catalytic performance have
been studied. The catalysts were char-
acterized by using scanning electron
microscopy (SEM), transmission elec-
tron microscopy (TEM), Fourier-trans-
form infrared (FTIR) spectroscopy,
powder X-ray diffraction (PXRD), N2-
Keywords: cross-coupling · hetero-
geneous catalysts · metal–organic
frameworks · nanoparticles · palla-
dium
Introduction
ly benign and cost-effective methods for carbonÀcarbon or
carbonÀheteroatom bond-formating reactions is of para-
mount importance. Within this family of reactions, the
Suzuki–Miyaura cross-coupling reaction is one of the most
robust and versatile methods for the formation of the
carbon backbone of a target molecule, which is of obvious
relevance for further progress in various branches of this
field. A number of challenges remain to be overcome to
achieve high efficiency under mild reaction conditions (e.g.,
ambient temperature) in environmentally friendly solvents.
In addition, the use of cheaper and less-reactive substrates
Because of increasing concern regarding environmental
issues, there is a great demand for the development of effi-
cient sustainable technologies. In the area of organic synthe-
sis, the development of efficient heterogeneous catalytic
[1]
procedures has become of enormous importance, as shown
[2]
by the number of reports in this field in recent years. For
the industrial syntheses of building blocks and pharmaceuti-
cally relevant compounds, the elaboration of environmental-
(
e.g., aryl chlorides) also remains a challenge. Typical cata-
[
a] V. Pascanu, Dr. Q. Yao, Dr. A. Bermejo Gꢀmez, Dr. M. Gustafsson,
Y. Yun, Dr. W. Wan, Prof. X. Zou, Prof. B. Martꢁn-Matute
Berzelii Center EXSELENT on Porous Materials
Stockholm University, SE-10691 Stockholm (Sweden)
E-mail: xzou@mmk.su.se
lysts for these transformations can sometimes become very
expensive, owing to the necessity of using transition-metal
complexes in combination with different types of ligands.
Thus, their high cost and environmental impact are major
belen@organ.su.se
[3]
drawbacks for their large-scale application.
[
b] V. Pascanu, Dr. A. Bermejo Gꢀmez, Prof. B. Martꢁn-Matute
Department of Organic Chemistry, Arrhenius Laboratory
Stockholm University, SE-10691 Stockholm (Sweden)
Metal–organic frameworks (MOFs), also known as coor-
dination polymers, have gained wide interest over the last
decade. They have large surface area and are highly
[
c] Dr. Q. Yao, Dr. M. Gustafsson, Y. Yun, Dr. W. Wan, Dr. L. Samain,
Prof. X. Zou
Inorganic and Structural Chemistry
Department of Materials and Environmental Chemistry
Arrhenius Laboratory, Stockholm University
SE-10691 Stockholm (Sweden)
[4]
function
ACHTUNGTRENNGNUa lizable. MOFs have shown great potential in gas
[5]
uptake and separation, as well as in various biomedical ap-
[6]
plications, such as drug encapsulation and delivery. In ad-
dition, MOFs can be used as catalyst supports that allow for
the recycling of metallic species and also prevent the con-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201302621.
[4]
tamination of the final products. However, for a MOF to
Chem. Eur. J. 2013, 19, 17483 – 17493
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
17483

be a suitable catalytic support it must be stable under the re-
action conditions and allow no leaching of the active species,
whilst still allowing full accessibility of the starting materials
to the reaction site. However, this requirement is not always
a simple task. Gratifyingly, by a thoughtful design of the
linkers or subsequent postsynthetic modifications, it is
possible to carefully tailor the inside of the pores so that
they can better accommodate the catalyst and prevent it
from leaching. One of the materials that meets all of the cri-
whilst trying to perform the reactions under aerobic condi-
[18]
tions at lower temperatures.
Significant progress in the
field of functionalized MOFs was reported in 2011 by Stock
and co-workers, who described, for the first time, a rapid
[24]
and facile method for the postsynthetic functionalization
of MIL-101Cr by covalently grafting amino groups onto the
terephthalate linkers. Bearing in mind the affinity of palladi-
um for nitrogen atoms, this modification constituted a very
important breakthrough, because it minimized the leaching
of metallic species. Indeed, Cao and co-workers followed
this idea and presented evidence of an increased stabiliza-
[7]
[8]
teria mentioned above is MIL-101Cr [(Cr F AHCUTNGRNEGUN( H O) O ACTHUNGTERNNUN[G (O C)-
3 2 2 2
C H -(CO )] ·nH O)], which was first reported in 2005 by
6
4
2
3
2
[
9]
Fꢃrey and co-workers. MIL-101Cr has a very high specific
tion effect of the nanoparticles in amino-functionalized
2
À1
[18,25]
surface area (BET, about 4100 m g ) and contains two
types of cages with diameters of 29 and 34 ꢄ, which have
pore apertures of 12 and 16 ꢄ, respectively. MIL-101Cr is
MIL-53Al-NH compared to MIL-53Al.
2
Over the past few years, our groups have undertaken on-
going research into the development of new and efficient
transition-metal-catalyzed processes for the formation of
carbonÀcarbon (CÀC) and carbonÀheteroatom (CÀX)
[
10]
stable in water,
even under very acidic conditions, and
can tolerate temperatures as high as 2758C in air. These
unique features of MIL-101Cr have attracted enormous at-
tention from researchers worldwide to explore its potential
[
26]
bonds
sis.
and the use of MOFs in heterogeneous cataly-
During our investigations, we became interested in
[
12,14]
[11]
in various applications, including catalysis.
understanding the parameters that affect the efficiency of
Suzuki–Miyaura cross-coupling reactions using Pd nanopar-
Supporting metallic nanoparticles inside a porous matrix
is one of the three possible ways of exploiting MOFs in cat-
ticles that are immobilized inside MIL-101Cr-NH . In partic-
2
[12]
alysis, along with supporting single-site metal complexes
or by making use of the structural metal atoms in the frame-
ular, we were intrigued by the fact that the reported palladi-
um loadings in the MOFs were often low, about 1–
[13,14]
[16,19,21,23a,25]
work as catalytically active sites.
The main advantage of
3 wt%,
with only a few exceptions with higher Pd
[20]
MOFs is that, by controlling the pore size, one can limit the
growth of the nanoparticles, thereby obtaining small and
well-distributed nanoparticles and avoiding agglomeration.
This result is achieved by the infiltration of a metallic pre-
cursor into the framework, followed by subsequent reduc-
tion, thus leading to the direct formation of active nanopar-
ticles in a controlled manner inside the pores. Recent re-
views by Dhakshinamoorthy and Garcia summarized the
use of metal nanoparticles that were immobilized inside
MOFs as heterogeneous catalysts for various reactions, in-
cluding condensation, hydrogenation, carbonÀcarbon cou-
loadings (5–6 wt%). Herein, we report our investigations
into the potential and limitations of MIL-101Cr-NH2 as
a catalyst support of Pd nanoparticles and its application in
[27]
Suzuki–Miyaura cross-coupling reactions. We investigated
how the loading of Pd into MIL-101Cr-NH affected the sta-
2
bility of the material and its performance as a catalyst and
we determined the optimal Pd loading into the framework.
Moreover, the leaching of palladium under different condi-
tions has been investigated. TEM and total scattering tech-
niques were employed to study the Pd nanoparticles in the
MIL-101Cr-NH2 crystals. The catalyst with the optimized
pling, and alcohol-oxidation reactions, as well as the synthe-
amount of Pd (8 wt% loaded) in MIL-101Cr-NH was sub-
2
[15]
sis of MeOH. Palladium nanoparticles have shown excep-
jected to a systematic screening of the reaction parameters,
thereby leading to efficient Suzuki–Miyaura coupling reac-
tions, with excellent yields for a large substrate scope, in-
cluding challenging heteroaromatic halides and bulky sub-
strates, under environmentally friendly conditions (water or
water/EtOH, ambient temperature, K CO ). Thus, herein,
[16]
tional ability to catalyze a rich variety of transformations.
As such, great efforts have been made to immobilize palla-
dium nanoparticles inside the robust MIL-101Cr to form
Pd@MIL-101Cr as a heterogeneous catalyst, which has been
[17,18]
used for Suzuki coupling reactions
or other palladium-
2
3
catalyzed reactions, such as the synthesis of indoles through
Sonogashira cross-coupling followed by a cycloaddition re-
we show the importance of tuning the loading of the nano-
particles in the material and of optimization of the reaction
conditions for efficient and recyclable catalysis by using
MOFs, which demonstrates their great potential as supports
of metal nanoparticles for practical use and large-scale ap-
plications.
[19]
action, the aqueous hydrogenation of phenols into cyclo-
[20]
hexanones, the synthesis of methyl isobutyl ketone from
[21]
[22]
acetone,
the hydrogenation of ketones,
and even
tandem processes, as cleverly illustrated by Corma and co-
[23]
workers.
As a proof of concept, in 2010, Jiang and co-workers pub-
[17]
lished the first report of a Suzuki coupling reaction of aryl
chlorides in water by using Pd@MIL-101Cr at 808C with
NaOMe as a base (1.5 equiv) and tetrabutylammonium bro-
mide (0.6 equiv) as a co-catalyst under an inert atmosphere.
From this starting point, other groups have been working on
the elaboration of this method to maintain high conversions
Results and Discussion
Synthesis and characterization of Pd@MIL-101Cr-NH2:
MIL-101Cr was synthesized according to a literature proce-
[9]
dure. The amino functionality was introduced into MIL-
[24]
101Cr in two steps, as reported by Stock and co-workers.
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Chem. Eur. J. 2013, 19, 17483 – 17493

Pd Loading on MIL-101Cr-NH for Suzuki–Miyaura Reactions
2
FULL PAPER
First, nitration of the phenyl rings was achieved by the treat-
ment of MIL-101Cr with HNO /H SO . In the second step,
3
2
4
reduction of the nitro groups by using SnCl ·2H O afforded
2
2
Pd@MIL-101Cr-NH . The quantitative functionalization of
2
MIL-101Cr was confirmed by both elemental analysis and
1
by H NMR spectroscopy after the digestion of MIL-101Cr-
NO and MIL-101Cr-NH (see the Supporting Information,
2
2
Figure S1, and the Experimental Section). Palladium load-
ings of 4, 8, 12, and 16 wt% were prepared by the impregna-
tion of MIL-101Cr-NH with the corresponding amount of
2
[
PdCl ACHTUNGTRENNUNG( MeCN) ] in dry CH Cl overnight, thus yielding
2 2 2 2
II
II
Pd @MIL-101Cr-NH , in which Pd was coordinated to the
amino groups. Subsequent reduction into Pd by using
2
0
NaBH4 afforded nanosized Pd particles inside the MIL-
0
1
01Cr-NH crystals, Pd @MIL-101Cr-NH .
2
2
Figure 1. PXRD patterns of MIL-101Cr, MIL-101Cr-NO
NH , and 4-, 8-, 12-, and 16wt%Pd@MIL-101Cr-NH . The palladium in
all four Pd-loaded samples is Pd .
2
, MIL-101Cr-
The synthesized material was washed with water and
2
2
0
EtOH to eliminate all of the residues. The purity of the four
final materials (4-, 8-, 12-, and 16wt%Pd@MIL-101Cr-NH2)
was checked by energy-dispersive spectroscopy (EDS) that
was connected to a SEM, elemental analysis, and by induc-
tively coupled plasma-optical emission spectrometry (ICP-
OES; Supporting Information, Table S2). The actual Pd con-
tents, as determined by ICP-OES, for the samples of 4-, 8-,
The weight losses of the synthesized samples upon heating
in air were studied by using TGA (Supporting Information,
Figure S4). MIL-101Cr was stable up to 3008C. The thermal
stability of MIL-101Cr seemed to be slightly weakened after
amino-functionalization and even more so after immobiliza-
tion of the Pd nanoparticles. The amount of water and sol-
vent molecules in the pores differed between the samples, as
shown by the different weight losses during heating.
1
2-, and 16wt%Pd@MIL-101Cr-NH were 4.38, 8.06, 11.29,
2
and 15.07 wt%, respectively. Importantly, no traces of either
Sn from SnCl or Na from the reducing reagent were found
2
in any of the Pd@MIL-101Cr-NH samples.
2
The functionalization of the terephthalate linker first with
nitro groups and then with amino groups was also followed
by FTIR spectroscopy. The FTIR spectra and vibration fre-
quencies of our synthesized samples, as shown in the Sup-
porting Information, Figures S2 and S3 and Table S3, corre-
N -sorption measurements were performed for as-synthe-
sized MIL-101Cr, amino-functionalized MIL-101Cr-NH2,
and 4-, 8-, 12-, and 16wt%Pd@MIL-101Cr-NH2 samples.
2
The N -adsorption isotherms, the BET surface areas, and
2
the pore volumes are shown in Figure 2 and Table 1. The
[24]
sponded well with literature values. Strong bands at 1540
BET surface areas of MIL-101Cr and MIL-101Cr-NH are
2
À1
2
À1
and 1357 cm , which corresponded to NÀO stretches, were
2966 and 2869 m g , respectively, which are in agreement
with the reported values. Compared with the parent MIL-
[24]
observed in the spectrum of MIL-101Cr-NO (Supporting
2
Information, Figure S3). These bands disappeared after re-
101Cr-NH , the BET surface area and pore volume de-
2
duction of the nitro groups and a clear NÀH bending fre-
creased with higher Pd loading (Table 1). The adsorption
À1
quency band at 1593 cm appeared instead. The CÀN
isotherms of all samples show stepwise N uptake, at p/p0
2
À1
stretch at 1258 cm in MIL-101Cr-NH was clearly seen in
2
the spectrum. All of the frequency bands of the MIL-101Cr
framework remained unchanged after the incorporation and
reduction of palladium.
The structure of MIL-101Cr was verified by powder X-ray
diffraction. The PXRD patterns of our synthesized samples
of MIL-101Cr matched with those reported in the litera-
[9]
ture. The structure of MIL-101Cr remained intact after all
of the postsynthetic modification steps and no obvious
changes in the PXRD patterns were detected, which con-
firmed that MIL-101Cr-NH was chemically and hydrother-
2
mally stable and that it could withstand harsh conditions,
such as those applied during the nitration step (with mix-
II
tures of concentrated acids) and during the reduction of Pd
by using excess NaBH (Figure 1). The systematic changes
4
in the relative peak intensities in the PXRD patterns with
different Pd loadings, for example, those within the regions
2
Figure 2. Nitrogen-adsorption isotherms of MIL-101Cr, MIL-101Cr-NH ,
2
q=5–68 and 8–98, indicated that Pd had been incorporated
and 4-, 8-, 12-, and 16wt%Pd@MIL-101Cr-NH
performed at 77 K.
2
. The measurements were
into the pores of MIL-101Cr.
Chem. Eur. J. 2013, 19, 17483 – 17493
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
17485

X. Zou, B. Martꢁn-Matute et al.
Table 1. Specific surface areas and pore volumes of MIL-101Cr and the
functionalized MIL-101Cr materials.
Material
BET surface area
Total pore volume
ACHTUNGTRENNUNG[ m g ]
2
À1
3
À1
A
H
U
G
R
N
N
]
MIL-101Cr
MIL-101Cr-NH
2966
2869
1831
1623
1396
1321
1.403
1.389
0.870
0.767
0.678
0.625
2
4
8
1
1
wt%Pd@MIL-101Cr-NH
wt%Pd@MIL-101Cr-NH
2wt%Pd@MIL-101Cr-NH
6wt%Pd@MIL-101Cr-NH
2
2
2
2
Figure 4. 2D section of the 3D tomographic image of two Pd@MIL-
1
01Cr-NH
tron tomography from a tilt series of 83 TEM images with a tilt range of
18 and a tilt step of 0.58. For the tilt series and the full tomogram, see
2
crystals with 8wt% Pd loading, as reconstructed by the elec-
values up to 0.25, thus indicating the existence of different
types of cage.
4
the Supporting Information, Movies S1 and S2, respectively.
SEM analysis showed that the MIL-101Cr crystals were
formed as octahedra, with sizes between 0.3 and 1.0 mm
(
Supporting Information, Figure S5). TEM analysis indicat-
S2). Electron diffraction and high-resolution transmission
electron microscopy (HRTEM) images show that the Pd
nanoparticles are crystalline (Supporting Information, Fig-
ed that the octahedral morphology of MIL-101Cr was pre-
served throughout all post-modification steps, which include
nitration, reduction of the nitro groups, introduction of Pd ,
and reduction of Pd into Pd . The sizes of the Pd particles
ranged from 20 to 30 ꢄ, which were comparable to the pore
diameters of unfunctionalized MIL-101Cr (29 and 34 ꢄ,
II
[28]
ure S6).
II
0
A better technique to study the size and distribution of
the Pd nanoparticles is X-ray total scattering, because TEM
only gives some local information and does not necessarily
represent the nature of the bulk samples. Owing to their
small particle sizes and the low concentrations of the Pd
nanoparticles, no Bragg peaks for the Pd nanoparticles
could be clearly seen in the PXRD patterns that were re-
corded on an in-house powder X-ray diffractometer. There-
fore, we recorded the total scattering intensities of the 4-, 8-,
[9]
Figure 3). However, on increasing the Pd loading in MIL-
01Cr-NH from 11.3 to 15.1 wt%, the size of the Pd nano-
1
2
particles increased up to about 8 nm (Figure 3). These larger
Pd nanoparticles were found on the surfaces of the
1
6wt%Pd@MIL-101Cr-NH2 crystals, as shown by TEM.
Electron tomography was performed on crystals of
wt%Pd@MIL-101Cr-NH to study the distribution of the
8
12-, and 16wt%Pd@MIL-101Cr-NH samples at the beam-
2
2
Pd nanoparticles. The reconstructed 3D tomographic images
show that the nanoparticles are embedded inside the crystals
and are homogeneously distributed throughout the crystals
line P02.1, Petra III, DESY, by using a high-energy synchro-
tron source (60 keV). The pair distribution function (PDF),
G(r), was obtained by performing a Fourier transform of the
total scattering intensities, including both Bragg peaks and
(
Figure 4 and the Supporting Information, Movies S1 and
[29]
diffuse scattering.
This function describes the statistical
distribution of all of the interatomic distances in a sample
and, thus, is particularly suitable for investigating nanoparti-
[30]
cles, which usually give very broadened or even no Bragg
peaks by using conventional PXRD. The PDFs of the 4-, 8-,
1
2-, and 16wt%Pd@MIL-101Cr-NH samples were extract-
2
ed from the total X-ray scattering signal truncated at a maxi-
À1
mum Q value (Qmax) of 19 ꢄ (Figure 5; Supporting Infor-
mation, Figure S7). For comparison, the PDFs that were cal-
culated from the structural model of MIL-101Cr and from
the structure of an ideal Pd nanoparticle (space group
¯
Fm3m) with a diameter of 25 ꢄ are also shown in Figure 5
for the detailed procedure see the Supporting Information).
The peaks in the observed PDFs of the 4-, 8-, 12-, and
6wt%Pd@MIL-101Cr-NH samples correspond to the in-
(
1
2
teratomic distances in both MIL-101Cr and the Pd nanopar-
ticle. For example, the peak at 2.0 ꢄ corresponds to the
nearest mean CrÀO distance, which is not present in the cal-
culated PDF of the pure Pd nanoparticle (Figure 5, top).
The peak at 2.9 ꢄ corresponds to the shortest distance be-
tween two Pd atoms in the Pd nanoparticle. The “ripples”
on either side of the first peak in the calculated PDF of the
Pd nanoparticle originate from termination effects that are
Figure 3. TEM images of samples of 4-, 8-, 12-, and 16wt%Pd@MIL-
01Cr-NH with different Pd loadings (4.4, 8.1, 11.3, and 15.1 wt% Pd,
respectively). Insets show the size distributions of the Pd particles as his-
tograms, each of which was formed from about 100 Pd particles in several
crystals in the TEM images. The x axis in the histograms is the size in nm
and the y axis is the number of particles.
1
2
À1
due to the finite Qmax of 19 ꢄ that was used for the calcu-
17486
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ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 17483 – 17493

Pd Loading on MIL-101Cr-NH for Suzuki–Miyaura Reactions
2
FULL PAPER
The PDF of 12wt%Pd@MIL-101Cr-NH is similar to that of
2
1
6wt%Pd@MIL-101Cr-NH , except that the former attenu-
2
ated at a shorter distance (r). This result indicates that the
size of the Pd nanoparticles in 16wt%Pd@MIL-101Cr-NH2
is slightly larger than that in 12wt%Pd@MIL-101Cr-NH2.
On the other hand, the PDFs of the 4- and 8wt%Pd@
MIL-101Cr-NH samples are very similar at r>4.5 ꢄ and at-
2
tenuate at much lower r values compared to the PDFs of
1
2- and 16wt%Pd@MIL-101Cr-NH . This result indicates
2
that the sizes of the Pd particles are similar in 4- and
8
1
wt%Pd@MIL-101Cr-NH2 and are smaller than those in
2- and 16wt%Pd@MIL-101Cr-NH . It is worth mentioning
2
that the PDF can also tell if there is agglomeration of the
nanoparticles in the sample. Because no peaks are observed
at large r values (>20 ꢄ), it can be concluded that there is
no significant agglomeration of Pd nanoparticles in the four
2
Figure 5. PDFs of samples of 4-, 8-, 12-, and 16wt%Pd@MIL-101Cr-NH ,
compared with the calculated PDFs of MIL-101Cr (bottom) and of a Pd
Pd@MIL-101Cr-NH samples.
2
nanoparticle (diameter: 25 ꢄ, top). For the sake of clarity, the PDFs are
À2
vertically stacked by 10 ꢄ
.
Suzuki–Miyaura cross-coupling reaction with Pd@MIL-
1
01Cr-NH : In an initial series of reactions, we screened the
2
reactivity of p-bromoanisol (1a) with different organoboron
reagents and bases (Table 2). A similar screening of the re-
activity of p-bromotoluene is shown in the Supporting Infor-
mation (Table S3). Unless otherwise specified, water was
used as the solvent and the reactions were performed at
room temperature. Based on the characterization of the dif-
ferent catalysts, screening of the reaction conditions was per-
lations and can be neglected. Whereas the peaks in the cal-
culated PDF of the Pd nanoparticle are relatively sharp,
those above 2.5 ꢄ in the calculated PDF of MIL-101Cr are
rather broad, owing to multiple overlapping of different
atom–atom correlations of similar distances. The peak at
1
.3 ꢄ results from overlapping of the nearest CÀC and CÀO
distances in MIL-101Cr. The integrated intensity of a well-
defined PDF peak yields the coordination number of that
formed with 8wt%Pd@MIL-101Cr-NH (3 mol% Pd load-
2
ing). This material had small Pd nanoparticles, which ranged
from 20 to 30 ꢄ. In contrast to 12- and 16wt%Pd@MIL-
[
28]
specific atomic pair. For example, the integrated intensity
of the first peak at 2.9 ꢄ in the calculated PDF of the pure
Pd nanoparticle will yield 12, which corresponds to the coor-
dination number of Pd. In case of multielement samples,
such as MIL-101Cr and 4-, 8-, 12-, and 16wt%Pd@
101Cr-NH , the 8wt% catalyst did not show agglomeration
2
of palladium on its surface. Moreover, 8wt%Pd@MIL-
101Cr-NH contained a more favorable wt%Pd per gram of
2
material than 4wt%Pd@MIL-101Cr-NH , which is desirable
2
MIL-101Cr-NH , the peak intensities in the PDF are multi-
from an atom-economy viewpoint. Nevertheless, the catalyt-
ic activity of all of the catalysts has been compared (see
below).
2
plied by a weighting factor to take into account the different
scattering factors of the chemical species. Thus, in the ob-
served PDFs of 4-, 8-, 12-, and 16wt%Pd@MIL-101Cr-NH2,
the peak intensities can be related to the multiplicity and
abundance of the atomic pairs, as well as to atomic scatter-
ing factors. By comparing the PDFs of the 4-, 8-, 12-, and
Influence of the base: The nature of the base in a Suzuki–
Miyaura coupling reaction greatly affects the efficiency of
[31]
the reaction. In general, the reactivities of different bases
in the coupling reactions of aryl iodides and bromides de-
1
6wt%Pd@MIL-101Cr-NH samples (Figure 5), a change in
2
À
2À
À
the relative peak intensities can be observed; in particular,
with higher Pd loading, the intensity of the peak at 2.9 ꢄ,
which corresponds to the PdÀPd distance, increases in com-
creased in the order: OH >CO @AcO . At high concen-
3
trations, bases can lead to the formation of unreactive boro-
À
À
nates (ArB(OH) ) and, thus, the [OH ]/ ACHTUNGTERNNNU[G ArB(OH) ] ratio
2
3
parison with that of the peak at 2.0 ꢄ, which is assigned to
needs to be controlled. Moreover, the countercations can
have an unexpected deceleration effect on the transmetala-
tion step, by coordinating to the hydroxo ligand of
the CrÀO distance. This result is reasonable because, with
higher Pd loading, the number of Pd atoms and, hence, the
number of PdÀPd distances of 2.9 ꢄ is expected to increase,
whereas the number of CrÀO distances of 2.0 ꢄ in the MIL-
[31]
A
H
U
G
R
N
U
G
With these arguments in
2
mind, we screened a wide selection of bases and observed
1
01Cr-NH should remain constant. When the PDFs of the
the stability of the 8wt%Pd@MIL-101Cr-NH catalyst, as
2
2
1
2- and 16wt%Pd@MIL-101Cr-NH samples are compared
shown in Table 2. When PhB(OH) (2a) and Cs CO were
2
2
2
3
to the PDF of the 25 ꢄ Pd nanoparticles (calculated), one
can see that the peaks at r>4.5 ꢄ mainly originate from the
Pd nanoparticles. In fact, the PDF of 16wt%Pd@
used, 4-methoxybiphenyl (3a) was obtained in quantitative
yield within only 6 h (Table 2, entry 1). We were particularly
interested in finding a less-expensive base than Cs CO that
2
3
MIL-101Cr-NH can be almost fully described by the struc-
ture of the Pd nanoparticles with an average size of 25 ꢄ.
could afford comparably high yields without harming the
material. Thus, a range of other bases was tested (Table 2,
2
Chem. Eur. J. 2013, 19, 17483 – 17493
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
17487

X. Zou, B. Martꢁn-Matute et al.
Table 2. Suzuki–Miyaura coupling reactions catalyzed by Pd@MIL-101Cr-NH
mization of the reaction conditions.
2
; opti- of this reaction. Potential problems owing to pore
blocking in the case of insufficient solubility of the
base would also be avoided. We found that several
bases, such as iPr NH, iPr EtN, and Et N, afforded
[
a]
2
2
3
the cross-coupled product (3a) in very high yields
Table 2, entries 8–10). However, the stability of the
(
Entry
ArBr
Boron
(equiv)
Base
Solvent
t
Yield
[%]
[
b]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[h]
nanoparticles with these bases was compromised. A
comparative leaching study was performed by using
one inorganic base (Cs CO ) and one organic base
1
2
3
4
5
6
7
8
9
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
1c
1a
1a
1a
1a
1a
1a
1a
1a
PhB(OH)
2a (1.5)
2a (1.5)
2a (1.5)
2a (1.5)
2a (1.5)
2a (1.5)
2a (1.5)
2a (1.5)
2a (1.5)
2a (1.5)
2a (1.5)
2a (1.5)
2a (1.5)
2a (1.5)
2a (1.2)
2
(2a, 1.5)
Cs
NaOH
Na CO
CO
2
CO
3
water
water
water
water
water
water
water
water
water
water
water
water
water
water
water/EtOH
water
water
water
water
water
water
water
water
6
6
6
6
6
6
6
6
6
6
6
6
6
6
0.5
4
4
4
6
3
3
3
3
>99
>99
71
82
94
2
3
2
3
(
iPr NH) in the coupling reaction of compound 1a
2
K
2
3
with compound 2b in water at room temperature.
After completion of the reaction (6 h), the mixture
BaCO
CsF
NaOAc
3
71
–[c] was filtered through a short pad of Celite and the
iPr
iPr
2
NH
EtN
89
86
83
–
3
>99
84
>99
68
99
83
>99
58
56
33
36
Pd content in the filtrate was analyzed by ICP-OES
analysis. For the reaction with Cs CO , a remarkable
2
2
3
10
11
12
13
14
15
16
17
18
19
20
21
22
23
3
Et N
stability of the catalyst was observed and only
.17 ppm Pd was detected in the filtrate. Higher Pd
C
2
H
4
A
H
U
G
R
N
U
G
2
)
2
0
Cs
Cs
Cs
Cs
Cs
Cs
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
content was detected in the filtrate when iPr NH
2
was used (5.64 ppm), which, although still relatively
low, was not negligible. This higher leaching was
a consequence of the high affinity of palladium for
nitrogenated ligands. Furthermore, if a chelating
base (ethylenediamine) was used (Table 2,
entry 11), all of the palladium was extracted from
the support and the material lost its catalytic activi-
ty. Experiments that were performed in the absence
of base showed conversions from 3% to 21%
PhBpin (2b, 1.2)
PhBF K (2c, 1.2)
3
2b (1.2)
2b (1.2)
2b (1.2)
2b (1.2)
2b (1.2)
K
K
K
K
K
2
2
2
2
2
CO
CO
CO
CO
CO
3
3
3
3
3
[
[
[
d]
e]
f]
[
(
3
a] Unless otherwise stated, the organoboron reagent (0.12 or 0.15 mmol), aryl halide
0.1 mmol), base (0.2 mmol), and 8wt%Pd@MIL-101Cr-NH
mol% Pd loading) were suspended in deionized water (2 mL). The mixture was the presence of free amino groups in the frame-
(4 mg, 0.003 mmol, (Table 2, entry 12). This result may be attributed to
2
stirred vigorously at RT for 6 h in a glass vial in air (see the Supporting Information);
work, which can act as basic functionalities.
1
[
b] yields determined by H NMR spectroscopy with 1,2,4,5-tetrachloro-3-nitroben-
zene as an internal standard; [c]>95% conversion into anisole (i.e., dehalogenation);
d] 4wt%Pd@MIL-101Cr-NH (3 mol% Pd loading); [e] 12wt%Pd@MIL-101Cr-NH
3 mol% Pd loading); [f] 16wt%Pd@MIL-101Cr-NH (3 mol% Pd loading).
Solvent and substrate polarity: Under the conditions
shown in Table 2, entry 1, we compared the reactivi-
ty of compound 1a with those of p-bromotoluene
[
(
2
2
2
(
1b) and electron-poor bromide 1c (Table 2, en-
tries 13 and 14). Surprisingly, in repeated experi-
entries 2–12). Sodium hydroxide also afforded excellent re-
ments, the neutral (1b) and electron-rich substrates (1a)
reached full conversion within 6 h, whereas the electron-
poor compound 1c was converted into the corresponding bi-
phenyl in only 84% yield. This unexpected reactivity, which
was opposite to that in organic solvents, owing to the elec-
tronic nature of these substrates, may be attributed to the
different solubilities of compounds 1a–1c in water. Indeed,
when water was replaced by a mixture of EtOH and water
(1:1, in which differences in solubility were minimized), bro-
mide 1c afforded the corresponding coupling product in ex-
cellent yield within only 30 min (Table 2, entry 15).
sults (Table 2, entry 2); however, MIL-101Cr-NH was not
2
stable under these strongly basic conditions (Supporting In-
[24]
formation, Figure S8).
For this reason, carbonates were
preferred because the release of hydroxy anions into the
aqueous medium was slower. This property prevented the
[32]
decomposition of the framework during the catalysis and
À
also allowed for better control over the [OH ]/
A
H
U
G
R
N
U
G
2
ratio. Other carbonates, such as Na CO , K CO , and
2
3
2
3
BaCO3 afforded compound 3a in 71%, 82%, and 94%
yield, respectively (Table 2, entries 3–5). CsF gave a compa-
rable yield to those that were obtained with carbonates
(
Table 2, entry 6). Acetates have been shown to be a poor
Organoboron reagents: Next, we evaluated other organobor-
on derivatives. To clearly observe any differences in their re-
activities, the reactions were stopped after only 4 h and only
1.2 equivalents of the organoboron reagents were used
(Table 2, entries 16–18). Pinacol phenylboronate (2b) af-
forded the highest yield, followed by phenyltrifluoroborate
potassium salt (2c) and phenylboronic acid (2a). The lower
reactivity of compound 2a can be explained by its prefer-
ence to form boroxine trimers, which are inactive under
choice of base, because they lead to a slow transmetalation
[31]
step. Although various amounts of the coupling product
3a) were detected in several experiments with NaOAc as
(
the base, most of the starting material underwent dehaloge-
nation to produce anisole, which occurred faster than the
coupling reaction (Table 2, entry 7). We also tested
a number of organic bases (Table 2, entries 8–11) because
their liquid nature may facilitate the large-scale application
17488
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ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 17483 – 17493

Pd Loading on MIL-101Cr-NH for Suzuki–Miyaura Reactions
2
FULL PAPER
[
33]
these mild reaction conditions. Thus, we decided to con-
tinue our investigation by using pinacolate ester 2b. Further-
more, the use of compound 2b compensated for the loss of
reactivity that was observed when Cs CO was replaced by
only 2 h in water/EtOH, in which the solubility of the sub-
strate was increased (Table 3, entry 4). Interestingly, the re-
action of 9-bromoanthracene (1e) proceeded very slowly,
even in water/EtOH after a prolonged reaction time
(Table 3, entry 5). This result may be due to the large size of
this substrate, which results in limited access to 8wt%Pd@
2
3
K CO and quantitative yields could be obtained with the
latter base by using only 1.2 equivalents of PhBpin (Table 2,
entry 19).
2
3
MIL-101Cr-NH , and the catalysis mainly occurred at the Pd
2
nanoparticles on the external surfaces of the MOF catalyst
particles. Importantly, heteroaromatic substrates 1 f and 1g
could also be coupled in high yields (Table 3, entries 6–7).
However, a lower yield was obtained for 2-bromopyridine
(1h; Table 3, entry 8). Slightly higher leaching of palladium
was observed for substrates that contained heteroatoms:
3.10 ppm Pd was found in the filtrate after the reaction of
bromopyridine (1g), whereas only 0.17 ppm was found after
the coupling of compound 1a (see above). The coupling re-
actions of aryl chlorides required more elevated tempera-
tures, which, disappointingly, were more favorable for the
formation of byproducts (that is, dehalogenation (ArH) or
homocoupling (ArÀAr) products). Activated aryl chlorides
Comparison of Pd@MIL-101Cr-NH catalysts with different
Pd loadings: With the optimized conditions in hand, we
compared the activities of the 4-, 8-, 12-, and
2
1
2
6wt%Pd@MIL-101Cr-NH2 catalysts (Table 2, entries 20–
3). The reactions were analyzed before full conversion had
occurred (3 h) by using a Pd loading of 3 mol%. Whereas
the reactions with 4- and 8wt%Pd@MIL-101Cr-NH afford-
2
ed very similar and satisfactory yields (Table 2, entries 20
and 21), the reactions with 12- and 16wt%Pd@MIL-101Cr-
NH afforded significantly lower yields (Table 2, entries 22
2
and 23). This behavior was in agreement with the TEM and
PDF analyses, which showed that the nanoparticles in the
12- and 16wt%Pd@MIL-101Cr-NH2 catalysts were larger
1i and 1j successfully reacted in water and they returned
good-to-excellent yields with little or no formation of unde-
sired byproducts (Table 3, entries 9 and 10). However, if
EtOH was used as a co-solvent, both activated and inacti-
vated substrates 1j–1m returned very good conversions but
only fair yields, with the concomitant formation of signifi-
cant amounts of homocoupling and dehalogenation products
(Table 3, entries 10–13).
with broader size distributions. The poorer performance of
the MOFs with higher Pd loadings may be explained by an
increased difficulty in the diffusion of the substrates and the
products through these materials, owing to pore blocking.
Leaching tests: The stability and performance of the immo-
bilized nanoparticles were evaluated according to the ap-
[34]
proach reported by Sheldon et al. A hot-filtration test was
performed by stirring the 8wt%Pd@MIL-101Cr-NH cata-
Recycling tests: The recyclability of 8wt%Pd@MIL-101Cr-
NH2 was investigated in the reaction of p-bromotoluene.
The experiments were performed on a larger scale (1 mmol)
than in previous investigations to facilitate separation and
minimize handling errors. After each run, the solid catalyst
was separated by centrifugation, washed with water and
EtOH (twice each), dried under vacuum, and reused. As
shown in Figure 6, the catalyst remained highly active, even
2
lyst in boiling water for 15 min, followed by filtration
through a pad of Celite. The filtrate was subsequently used
as the solvent for the reaction of compound 1a with com-
pound 2a under similar reaction conditions to those in
Table 2, entry 1, but in the absence of the palladium catalyst.
1
After 6 h, only 3% conversion was detected by H NMR
spectroscopy. The Pd leached into solution after catalysis
when carbonates were used as the base was only 0.17 ppm
[35]
after 10 cycles. Interestingly, the PXRD pattern after the
(
see above). Together with the high recyclability of the cata-
tenth cycle was different from that of MIL-101Cr-NH , but
2
lyst (see below), these results clearly demonstrate the stabil-
ity of the Pd nanoparticles in MIL-101Cr-NH2.
similar to that of another phase, presumably MIL-88B-NH
2
[36]
(Supporting Information, Figure S9). This phase-transfor-
mation phenomenon is currently under investigation in our
laboratories. TEM analysis showed the presence of some
limited aggregation of Pd nanoparticles on the surfaces of
the MOF catalysts after the 10th cycle (Supporting Informa-
tion, Figure S10). Because the size of the nanoparticles re-
mained unchanged, the aggregation did not seem to signifi-
cantly affect the efficiency of the catalyst. Elemental analy-
sis of the recycled 8wt%Pd@MIL-101Cr-NH2 material
showed an overall loss of 0.96 wt% Pd after 10 runs. Yields
of between 97% and >99% were obtained in the recycling
runs. Based on these excellent results, we decreased the Pd
loading by 20 times to 0.15 mol% and performed the reac-
tions on the same scale and under similar conditions to
those in Table 2, entry 19, and the reaction proceeded to
completion within 4 h. This result indicates that Pd@MIL-
Substrate scope: Based on this series of observations
(
Table 2), we screened
a variety of substrates with
8
wt%Pd@MIL-101Cr-NH as the catalyst (3 mol% Pd load-
2
ing) and K CO as the base, because K CO is environmen-
2
3
2
3
tally benign and about 20-times cheaper than Cs CO . More-
2
3
over, potassium has been shown to be the countercation
that infers the least-intense decelerating effect in the trans-
[31]
metalation step. These results are shown in Table 3. The
reactions were either performed in water or in water/EtOH
mixtures (1:1 v/v). Aryl bromides 1a–1c were easily coupled
within short reaction times (6 h in water, 30 min in water/
EtOH) at room temperature (Table 3, entries 1–3). 2-
Bromo ACHTUNGTRENNUNGn aphthalene (1d), which showed very low solubility
in water at ambient temperatures, afforded compound 3d in
only 22% yield. However, 97% yield was obtained after
101Cr-NH is a highly efficient catalyst.
2
Chem. Eur. J. 2013, 19, 17483 – 17493
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
17489

X. Zou, B. Martꢁn-Matute et al.
Table 3. Suzuki coupling reactions of various aryl bromides and chlorides catalyzed by 8wt%Pd@MIL-101Cr-
NH .
2
Conclusion
[
a]
In summary, we have provid-
ed evidence for a strong rela-
tionship between the amount
of palladium that is loaded
into MIL-101Cr-NH and the
catalytic properties of the cor-
responding system. The MIL-
2
Entry
Substrate
Product
Yield
water, 6 h) [%]
Yield
[
b]
[b]
(
(water/EtOH, 0.5 h) [%]
1
01Cr-NH2
material
was
1
2
3
4
5
>99 (94)
95
found to be an excellent sup-
port for palladium nanoparti-
cles. The size, distribution,
and reactivity of palladium
nanoparticles were shown to
be influenced by the amount
of palladium metal that was
impregnated into the frame-
work. The nanoparticles were
investigated by using both
conventional and advanced
methods, such as electron to-
mography and PDF analysis.
The best performance was ob-
served for MIL-101Cr-NH2
that contained about 8 wt%
palladium nanoparticles. This
result indicates that an opti-
mal ratio was achieved be-
tween the amount of loaded
catalyst and the number of
pores that remained accessi-
ble for the diffusion of the
substrates and the products.
The catalyst 8wt%Pd@MIL-
>99 (96)
>99
84
22
0
99 (96)
[
c]
97 (94)
[
d]
19
–
6
99 (89)
7
8
9
99 (92)
47
–
95 (90)
–
[
[
[
e]
99 (95)
70 (62)
52 (47)
101Cr-NH displayed remark-
2
able activity in the Suzuki–
Miyaura cross-coupling reac-
tion. Aryl and heteroaryl bro-
mides and activated aryl
chlorides were successfully
coupled within short reaction
times under mild conditions
f,e]
h,e]
[g,e]
10
11
12
13
54
[i,e]
50
(
8
ambient temperature and
08C, respectively) in benign
[
[
j,e]
l,e]
[k,e]
4
3
44
solvents (water and EtOH).
Our investigations have also
shown that the solubility and
hydrophobicity of the sub-
strates significantly affect
their ability to access the cata-
lyst pores in water, but this
limitation is minimized by em-
ploying EtOH as a co-solvent.
The excellent recyclability of
[m,e]
36
[
(
a] Unless otherwise stated, the pinacol phenylboronate (0.12 mmol), aryl halide (0.1 mmol),
0.2 mmol), and 8wt%Pd@MIL-101Cr-NH (4 mg, 0.003 mmol, 3 mol% Pd loading) were suspended in either
deionized water (2 mL) or water/EtOH (1:1). Then, the mixture was stirred vigorously at RT in air for 6 h
water) or 30 min (water/EtOH) in a glass vial (see the Supporting Information); [b] yield determined by
2 3
K CO
2
(
1
H NMR spectroscopy with 1,2,4,5-tetrachloro-3-nitrobenzene as an internal standard; yield of the isolated
product is given in parentheses; [c] 2 h; [d] 16 h; [e] 808C; [f] 96% conversion, 13% homocoupling; [g] 89%
conversion, 9% homocoupling, 16% dehalogenation; [h] 68% conversion, 13% homocoupling; [i] 89% con-
version, 12% homocoupling, 15% dehalogenation; [j] 98% conversion; toluene was the main byproduct;
the
8wt%Pd@MIL-101Cr-
[
k] 100% conversion, 12% homocoupling, 32% dehalogenation; [l] 44% conversion; anisole was the main by-
NH catalyst makes it a prom-
2
product; [m] 79% conversion, 10% homocoupling, 23% dehalogenation.
17490
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ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 17483 – 17493

Pd Loading on MIL-101Cr-NH for Suzuki–Miyaura Reactions
2
FULL PAPER
and the Cr and Pd contents were determined by inductively coupled
plasma-optical emission spectrometry (ICP-OES) on a Varian Vista
MPX ICP-OES at Medac Ltd, Chobham, UK. High-energy X-ray diffrac-
tion experiments were carried out at a fixed energy of approximately
6
2
0 keV at a wavelength of 0.2073 ꢄ (determined by using a CeO stan-
dard) at the beamline P02.1 of Petra III, Desy, Hamburg. The 2D diffrac-
tion images were integrated into a linear scattering signal by using the
[
41]
Fit2D software.
The pair distribution function (PDF), G(r), was ob-
tained by sine Fourier transform of the normalized scattering intensity to
À1
[42]
a maximum Q value, Qmax4 =19 ꢄ , by using the PDFgetX2 software.
The PDF of the Pd nanoparticles was calculated with the PDFgui soft-
[
43]
ware
(for a detailed procedure, see the Supporting Information).
H NMR spectra were recorded at 400 MHz; C NMR spectra were re-
corded at 100 MHz on a Bruker Advance spectrometer. H and C NMR
1
13
1
13
chemical shifts (d) are reported in ppm relative to tetramethylsilane, with
the solvent resonance as an internal standard (CDCl
3 H
: d =7.26 ppm,
d
C
=77.16 ppm; [D ]DMSO: =2.50 ppm, =39.5 ppm). Coupling
6
d
H
d
C
constants (J) are given in Hz. High-resolution mass spectra (HRMS)
were recorded on a Bruker microTOF ESI-TOF mass spectrometer.
2 3
Synthesis of Pd@MIL-101Cr-NH : Samples of MIL-101Cr (Cr F-
[
9,44]
A
H
U
G
R
N
U
G
2
O)
2
O
A
H
U
G
R
N
U
G
2
)-C
6
H
4
-(CO
2
)]
3
·nH
2
O),
MIL-101Cr-NO
2
(Cr
(Cr
3
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(H
2
O)
O)
2
O-
O-
Figure 6. Recycling experiments on a 1 mmol scale; reaction time of each
run: 0.5 h.
[
24]
A
H
U
G
R
N
N
2
)-C
)-C
2
-(CO
-(CO
2
)]
)]
3
·nH
·nH
2
2
O),
and MIL-101Cr-NH
2
3
ACHTUNGTRENNUNG( H
2
2
[
24]
A
H
U
G
R
N
N
2
2
2
3
O) were prepared according to literature
with four different loadings
procedures. Samples of Pd@MIL-101Cr-NH
2
of Pd nanoparticles that were intended to be 4, 8, 12, and 16 wt% Pd
were prepared by using analogous procedures, as given below.
ising candidate for large-scale Suzuki–Miyaura cross-cou-
pling reactions. This thorough study of the structural and
catalytic properties of metallic nanoparticles that are sup-
ported on a MOF material can stand as a model for detailed
investigations of other related heterogeneous systems, in
search for a better understanding of the underlying mecha-
nism of heterogeneous catalysis with MOFs.
Synthesis of MIL-101Cr-NH
wt%Pd@MIL-101Cr-NH
2 2 2 2
-PdCl : For MIL-101Cr-NH -PdCl affording
8
2
, the following procedure was used: A mix-
À4
ture of MIL-101Cr-NH
2
(0.240 g, 3.15ꢆ10
mol), [PdCl
2 2
ACHTUNGTRENNUNG( MeCN) ]
À4
(0.055 g, 2.11ꢆ10 mmol), and anhydrous CH
2
2
Cl (30 mL) was stirred
overnight at RT (18 h). The solid was obtained by centrifugation, washed
with anhydrous CH Cl (2ꢆ10 mL), and dried at 408C for 2 h under
vacuum. To obtain samples with 4, 12, and 16 wt% Pd loading, the
amount of [PdCl (MeCN) ] that was used to afford 8 wt% Pd loading
2
2
2
A
H
U
G
E
N
N
2
was multiplied by 0.5, 2, and 3, respectively; the amount of solvent was
kept the same.
Synthesis of 8wt%Pd@MIL-101Cr-NH
added portion-wise to a mixture of MIL-101Cr-NH
2
: NaBH
4
(0.076 g, 2 mmol) was
-PdCl (0.140 g) and
Experimental Section
2
2
EtOH (30 mL) at 08C. The green mixture became darker after about
2 min. The reaction was stirred for an additional 2 h at 08C to complete
the reduction. The solid was obtained by centrifugation, washed twice
with water (45 mL) and once with EtOH (45 mL), and the dark-green
solid was dried at 858C overnight (18 h) in air.
General: All chemicals and solvents were used as received without fur-
ther purification. Powder X-ray diffraction (PXRD) was performed in
the Bragg–Brentano geometry on a PANalytical XꢅPert PRO diffractom-
eter that was equipped with a Pixel detector and CuKa1 radiation (l=
1
.5406 ꢄ). The samples were dispersed over zero-background Si plates.
Digestion of MIL-101Cr: In a typical experiment, the material (10 mg)
was suspended in an aqueous solution of NaOH (2.0m, 1 mL) and ultra-
sonicated for 30 min. The water was evaporated under vacuum and the
SEM and energy-dispersive spectroscopy (EDS) were performed on
a JEOL-7000F field-emission scanning electron microscope operating at
1
2 kV. The samples were precoated with a thin carbon film. The presence
1
residue was analyzed by H NMR spectroscopy in deuterated DMSO
of possible byproducts was investigated by using EDS. TEM observations
were performed on a JEOL TEM (JEM-2100F) operating at 200 kV. The
MIL-101Cr samples were dispersed in absolute EtOH and then treated
by ultrasonication for 2 min. A droplet of the suspension was transferred
onto a copper grid and dried in air. A tilt series of TEM images for elec-
tron tomography was recorded with a tilt step of 0.58 and an angular
(
Supporting Information, Figure S1).
General procedure for the Suzuki–Miyaura cross-coupling reactions:
Unless otherwise stated, pinacol phenylboronate (24.5 mg, 0.12 mmol),
the aryl bromide (0.1 mmol),
wt%Pd@MIL-101Cr-NH (4 mg, 0.003 mmol) were suspended in deion-
ized water (2 mL) and the mixture was stirred vigorously at RT for 6 h.
Then, the mixture was extracted into CH Cl (4 mL), the organic phase
K
2
CO
3
(27.6 mg, 0.2 mmol), and
8
2
[
37,38]
range of 418 (83 images in total). TomoJ
was employed for the tomo-
2
2
graphic reconstruction. The images were aligned by using the Pd particles
[
39]
was separated, the volatile compounds were evaporated, and the residue
was analyzed by H NMR spectroscopy. To isolate the final product, the
reaction was repeated on a larger scale (ꢆ4) and the crude reaction mix-
ture was purified by column chromatography on silica gel (pentane or
pentane/EtOAc mixtures).
themselves as markers. Ten iterations of the ART algorithm were per-
1
[
40]
formed in TomoJ with a relaxation coefficient of 0.1. ImageJ was used
to preprocess the tilt series of TEM images. Thermogravimetric analysis
(
TGA) was performed under a flow of air between 20 and 5008C with
À1
a heating rate of 28Cmin on a thermogravimetric analyzer (PERKIN
ELMER TGA 7) that was equipped with a platinum pan. N
isotherms were recorded at 77 K on a Micromeritics ASAP2020 analyzer.
The samples for N adsorption were first exchanged with absolute EtOH
2
-adsorption
2
before activation. The samples were degassed by heating at 1208C over-
night prior to the sorption measurements. Specific surface areas were cal-
Acknowledgements
culated from the data in the adsorption branch at p/p
total pore volume was calculated from the uptake at p/p
0
=0.05–0.30. The
=0.99. FTIR
0
This project was supported by the Knut and Alice Wallenberg Founda-
tion, the Swedish Research Council (VR), the Swedish Governmental
Agency for Innovation Systems (VINNOVA) through the Berzelii
spectroscopy was performed on a Varian 670-IR spectrometer. Elemental
analysis was performed on a Carlo Erba Flash 1112 elemental analyzer
Chem. Eur. J. 2013, 19, 17483 – 17493
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
17491

X. Zou, B. Martꢁn-Matute et al.
Center EXSELENT, and the Knut and Alice Wallenberg Foundation.
B.M.-M. was supported by VINNOVA through a VINNMER grant.
W.W. was supported by a postdoctoral grant from the Carl-Trygger Foun-
[15] For reviews, see: a) A. Dhakshinamoorthy, H. Garcia, Chem. Soc.
Rev. 2012, 41, 5262–5284; b) A. Dhakshinamoorthy, M. Opanasen-
ˇ
ko, J. Cejka, H. Garcia, Catal. Sci. Technol. 2013, 3, 2509–2540.
dation. L.S. was supported by The Consortium for Crystal Chemistry
[16] For a review, see: K. C. Nicolaou, P. G. Bulger, D. Sarlah, Angew.
Chem. 2005, 117, 4516–4563; Angew. Chem. Int. Ed. 2005, 44, 4442–
4489.
[17] B. Yuan, Y. Pan, Y. Li, B. Yin, H. Jiang, Angew. Chem. 2010, 122,
4148–4152; Angew. Chem. Int. Ed. 2010, 49, 4054–4058.
[18] Y. Huang, Z. Zheng, T. Liu, J. Lu, Z. Lin, H. Li, R. Cao, Catal.
Commun. 2011, 14, 27–31.
3
(
C ), Rçntgen–ꢄngstrçm Cluster. The electron microscopy facility was
supported by the Knut and Alice Wallenberg Foundation. The authors
acknowledge Petra III, Desy, for the use of the synchrotron facility (I-
2
0120456 EC) and Dr. Jozef Bednarcik for his assistance in using the
Beamline P02.1.
[
[
[
19] H. Li, Z. Zhu, F. Zhang, S. Xie, H. Li, P. Li, X. Zhou, ACS Catal.
011, 1, 1604–1612.
20] H. Liu, Y. Li, R. Luque, H. Jiang, Adv. Synth. Catal. 2011, 353,
107–3113.
21] Y. Pan, B. Yuan, Y. Li, D. He, Chem. Commun. 2010, 46, 2280–
282.
2
[
[
[
1] a) M. Zhang, J. Guan, B. Zhang, D. Su, C. Williams, C. Liang, Catal.
Lett. 2012, 142, 313–318; b) M. M. Dell’Anna, M. Mali, P. Mastroril-
li, A. Rizzuti, C. Ponzoni, C. Leonelli, J. Mol. Catal. A 2013, 366,
3
1
2
86–194; c) B. J. Borah, D. K. Dutta, J. Mol. Catal. A 2013, 366,
02–209; d) A. Kumbhar, S. Jadhav, S. Kamble, G. Rashinkar, R.
2
[
[
22] J. Hermannsdçrfer, R. Kempe, Chem. Eur. J. 2011, 17, 8071–8077.
23] a) F. G. Cirujano, F. X. Llabrꢃs i Xamena, A. Corma, Dalton Trans.
Salunkhe, Tetrahedron Lett. 2013, 54, 1331–1337.
2] a) K. Sawai, R. Tatumi, T. Nakahodo, H. Fujihara, Angew. Chem.
2
012, 41, 4249–4254; b) A. Arnanz, M. Pintado-Sierra, A. Corma,
2
008, 120, 7023–7025; Angew. Chem. Int. Ed. 2008, 47, 6917–6919;
M. Iglesias, F. Sꢇnchez, Adv. Synth. Catal. 2012, 354, 1347–1355.
b) N. Zhang, D. J. Hoffman, N. Gutsche, J. Gupta, V. Percec, J. Org.
Chem. 2012, 77, 5956–5964; c) B. H. Lipshutz, S. Ghorai, Aldrichi-
mica Acta 2012, 45, 3–16; d) U. Yilmaz, S. Deniz, H. Kucukbay, N.
Sireci, Molecules 2013, 18, 3712–3724.
[
[
[
24] S. Bernt, V. Guillerm, C. Serre, N. Stock, Chem. Commun. 2011, 47,
2
838–2840.
25] Y. Huang, S. Liu, Z. Lin, W. Li, X. Li, R. Cao, J. Catal. 2012, 292,
11–117.
26] a) A. Bartoszewicz, M. Livendahl, B. Martꢁn-Matute, Chem. Eur. J.
1
3] a) For a review, see: J. Magano, J. R. Dunetz, Chem. Rev. 2011, 111,
2
177–2250; b) N. Marion, P. de Frꢃmont, I. M. Pujik, E. C. Ecarnot,
D. Amoroso, A. Bell, S. P. Nolan, Adv. Synth. Catal. 2007, 349,
380–2384; c) C. Boos, D. M. Bowles, C. Cai, A. Casimiro-Garcia,
2
2
008, 14, 10547; b) N. Ahlsten, B. Martꢁn-Matute, Chem. Commun.
011, 47, 8331; c) I. Cumpstey, S. Agrawal, K. E. Borbas, B. Martꢁn-
2
Matute, Chem. Commun. 2011, 47, 7827–7829; d) A. Bartoszewicz,
R. Marcos, S. Sahoo, A. K. Inge, X. Zou, B. Martꢁn-Matute, Chem.
Eur. J. 2012, 18, 14510–14519; e) N. Ahlsten, A. Bermejo Gꢀmez, B.
Martꢁn-Matute, Angew. Chem. 2013, 125, 6393–6396; Angew. Chem.
Int. Ed. 2013, 52, 6273–6276.
X. Chen, C. A. Hulford, S. M. Jennings, E. J. Kiser, D. W. Piotrow-
ski, M. Sammons, R. A. Wade, Tetrahedron Lett. 2011, 52, 7025–
7
029.
[
4] For a special issues on metal–organic frameworks, see: Acc. Chem.
Res. 2005, 38, 215; J. Solid State Chem. 2005, 178, 2409; Chem. Soc.
Rev. 2009, 38, 1201; Chem. Rev. 2012, 112, 673.
[
27] For a brief comparison of the performances of several recently re-
ported heterogeneous nanopalladium catalysts, see the Supporting
Information, Table S1, and: a) O. Verho, A. Nagendiran, E. V. John-
ston, C.-w. Tai, J.-E. Bꢈckvall, ChemCatChem 2013, 5, 612–618;
b) N. Shang, C. Feng, H. Zhang, S. Gao, R. Tang, C. Wang, Z. Wang,
Catal. Commun. 2013, 40, 111–115; c) K. Karami, M. Ghasemi,
N. H. Naeini, Catal. Commun. 2013, 38, 10–15; d) L. Isfahani, I.
Mohammadpoor-Baltork, V. Mirkhani, A. R. Khosropour, M. Mog-
hadam, S. Tangestaninejad, R. Kia, Adv. Synth. Catal. 2013, 355,
[
5] a) For reviews, see: H.-L. Jiang, Q. Xu, Chem. Commun. 2011, 47,
3
351–3370; b) M. C. Das, S. Xiang, Z. Zhang, B. Chen, Angew.
Chem. 2011, 123, 10696–10707; Angew. Chem. Int. Ed. 2011, 50,
1
8
0510–10520. J.-R. Li, J. Sculley, H.-C. Zhou, Chem. Rev. 2012, 112,
69–932.
[
[
6] For a review, see: P. Horcajada, R. Gref, T. Baati, P. K. Allan, G.
Maurin, P. Couvreur, G. Fꢃrey, R. E. Morris, C. Serre, Chem. Rev.
2
012, 112, 1232–1268.
9
57–972.
28] a) W. Lu, B. Wang, K. Wang, X. Wang, J. G. Hou, Langmuir 2003,
9, 5887–5891; b) F. Wang, C. Li, L.-D. Sun, C.-H. Xu, J. Wang, J. C.
7] For a review, see: F. A. Almeida Paz, J. Klinowski, S. M. F. Vilela,
J. P. C. Tomꢃ, J. A. S. Cavaleiro, J. Rocha, Chem. Soc. Rev. 2012, 41,
[
1
1
088–1110.
8] For a review, see: K. K. Tanabe, S. M. Cohen, Chem. Soc. Rev. 2011,
0, 498–519.
Yu, C.-H. Yan, Angew. Chem. 2012, 124, 4956–4960; Angew. Chem.
Int. Ed. 2012, 51, 4872–4876.
[
[
4
[
[
29] T. Egami, S. J. L. Billinge, Underneath the Bragg Peaks: Structural
Analysis of Complex Materials (Ed: R. W. Cahn), Pergamon,
Oxford, 2003.
30] a) S. J. L. Billinge, J. Solid State Chem. 2008, 181, 1695–1703; b) K.
Page, T. Proffen, H. Terrones, M. Terrones, L. Lee, Y. Yang, S.
Stemmer, R. Seshadri, A. K. Cheetham, Chem. Phys. Lett. 2004,
393, 385–388.
9] G. Fꢃrey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour, S.
Surblꢃ, I. Margiolaki, Science 2005, 309, 2040–2042.
[
10] a) J. Ehrenmann, S. K. Henninger, C. Janiak, Eur. J. Inorg. Chem.
011, 471–474; b) A. Khutia, H. U. Rammelberg, T. Schmidt, S.
2
Henninger, C. Janiak, Chem. Mater. 2013, 25, 790–798.
[
11] a) Y. K. Hwang, D.-Y. Hong, J.-S. Chang, S. H. Jhung, Y.-K. Seo, J.
Kim, A. Vimont, M. Daturi, C. Serre, G. Fꢃrey, Angew. Chem. 2008,
1
20, 4212–4216; Angew. Chem. Int. Ed. 2008, 47, 4144–4148; b) A.
Henschel, K. Gedrich, R. Kraehnert, S. Kaskel, Chem. Commun.
008, 44, 4192–4194; c) N. Maksimchuk, K. A. Kovalenko, V. P.
[31] C. Amatore, A. Jutand, G. Le Duc, Chem. Eur. J. 2012, 18, 6616–
6625.
2
[32] a) E. P. Urriolabeitia, in Oxidative Addition and Transmetallation–
Palladacycles: Synthesis Characterization and Applications (Eds: J.
Dupont, M. Pfeffer), Wiley-VCH, Weinheim, 2008, pp. 35–67;
b) K. C. Lam, T. B. Marder, Z. Lin, Organometallics 2007, 26, 758–
760.
[33] A. Scrivanti, V. Beghetto, M. Bertoldini, U. Matteoli, Eur. J. Org.
Chem. 2012, 264–268.
[34] R. A. Sheldon, M. Wallau, I. W. C. E. Arends, U. Schuchardt, Acc.
Chem. Res. 1998, 31, 485–493.
[35] For critical reviews on the nature of the active species with hetero-
geneous Pd catalysts, see: a) N. T. S. Phan, M. V. D. Sluys, C. W.
Jones, Adv. Synth. Catal. 2006, 348, 609–679; b) M. Pagliaro, V. Pan-
darus, R. Ciriminna, F. Bꢃland, P. D. Carꢉ, ChemCatChem 2012, 4,
432–445.
Fedin, O. Kholdeeva, Chem. Commun. 2012, 48, 6812–6814; d) L.
Bromberg, Y. Diao, H. Wu, S. A. Speakman, T. A. Hatton, Chem.
Mater. 2012, 24, 1664–1675.
[
[
[
12] F. Carson, S. Agrawal, M. Gustafsson, A. Bartoszewicz, F. Moraga,
X. Zou, B. Martꢁn-Matute, Chem. Eur. J. 2012, 18, 15337–15344.
13] A. Dhakshinamoorthy, M. Alvaro, H. Chevreau, P. Horcajada, T.
Devic, C. Serre, H. Garcia, Catal. Sci. Technol. 2012, 2, 324–330.
14] a) M. Gustafsson, A. Bartoszewicz, B. Martꢁn-Matute, J. Sun, J.
Grins, T. Zhao, Z. Li, G. Zhu, X. Zou, Chem. Mater. 2010, 22, 3316–
3
322; for a related heterogeneous catalyst, see: b) T. Yang, A. Bar-
toszewicz, J. Ju, J. Sun, Z. Liu, X. Zou, Y. Wang, G. Li, F. Liao, B.
Martꢁn-Matute, J. Lin, Angew. Chem. 2011, 123, 12763–12766;
Angew. Chem. Int. Ed. 2011, 50, 12555–12558.
17492
www.chemeurj.org
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 17483 – 17493

Pd Loading on MIL-101Cr-NH for Suzuki–Miyaura Reactions
2
FULL PAPER
[
[
[
36] S. Surblꢃ, C. Serre, C. Mellot-Draznieks, F. Millangea, G. Fꢃrey,
[41] A. P. Hammersley, S. O. Svensson, M. Hanfland, A. N. Fitch, D.
Hausermann, J. High Pressure Res. 1996, 14, 235–248.
[42] X. Qiu, J. W. Thompson, S. J. L. Billinge, J. Appl. Crystallogr. 2004,
37, 678–678.
[43] C. L. Farrow, P. Juhas, J. W. Liu, D. Bryndin, E. S. Bo zˇ in, J. Bloch, T.
Proffen, S. J. L. Billinge, J. Phys. Condens. Matter 2007, 19, 335219.
[44] Y. Huang, Z. Lin, R. Cao, Chem. Eur. J. 2011, 17, 12706–12712.
Chem. Commun. 2006, 284–286.
37] C. Messaoudi, T. Boudier, C. O. Sanchez Sorzano, S. Marco, BMC
Biochem. 2007, 8, 288.
38] C. O. Sorzano, C. Messaoudi, M. Eibauer, J. R. Bilbao-Castro, R.
Hegerl, S. Nickell, S. Marco, J. M. Carazo, BMC Biochem. 2009, 10,
1
24.
39] G. T. Herman, A. Lent, S. W. Rowland, J. Theor. Biol. 1973, 42, 1–
2.
[
[
3
40] W. S. Rasband, ImageJ, US National Institute of Health, Bethesda,
Received: July 5, 2013
MD, http://imagej.nih.gov/ij/, 1997–2012.
Published online: November 21, 2013
Chem. Eur. J. 2013, 19, 17483 – 17493
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
17493