La-Metal-Organic Framework incorporating Fe3O4 nanoparticles, post-synthetically modified with Schiff base and Pd. A highly active, magnetically recoverable, recyclable catalyst for C–C cross-couplings at low Pd loadings

Article abstract of DOI:10.1016/j.jcat.2018.02.026

Unprecedented post-synthetic modification (PSM) applied on a new La-MOF containing Fe₃O₄ nanoparticles (NPs), via Schiff base build-up (with pyridine-2-carboxaldehyde) and PdCl₂ coordination, proved to be an advantageous strategy to produce a superior heterogeneous catalyst for Suzuki-Miyaura C–C bond formation. The Fe₃O₄@La-MOF-Schiff base-Pd ensemble (characterized by PXRD, EDS, TEM, HAADF-STEM, VSM, TGA) showed very active, quite stable, magnetically recoverable and reusable in coupling of bromo- or iodoaryls with arylboronic acids, affording practically quantitative yields of biaryls (>99%) and high TONs and TOFs. The catalyst has been recycled up to 12 times without significant loss of the catalytic activity. A synergistic cooperativity between Pd and the oxophilic La nodes, through the organic linker, is responsible for enhancing the catalyst activity and stability.

Full text of DOI:10.1016/j.jcat.2018.02.026

Journal of Catalysis 361 (2018) 116–125
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
La-Metal-Organic Framework incorporating Fe3O4 nanoparticles,
post-synthetically modified with Schiff base and Pd. A highly active,
magnetically recoverable, recyclable catalyst for CAC cross-couplings
at low Pd loadings
Gang Xiong a, Xiao-Ling Chen a, Li-Xin You a, Bao-Yi Ren a, Fu Ding a, Ileana Dragutan b, Valerian Dragutan b,
,
⇑
Ya-Guang Sun a,
⇑
a The Key Laboratory of Inorganic Molecule-Based Chemistry of Liaoning Province, Shenyang University of Chemical Technology, Shenyang 110142, China
b Institute of Organic Chemistry, Romanian Academy, Bucharest 060023, Romania
a r t i c l e i n f o
a b s t r a c t
Article history:
Unprecedented post-synthetic modification (PSM) applied on a new La-MOF containing Fe3O4 nanopar-
ticles (NPs), via Schiff base build-up (with pyridine-2-carboxaldehyde) and PdCl2 coordination, proved to
be an advantageous strategy to produce a superior heterogeneous catalyst for Suzuki-Miyaura CAC bond
formation. The Fe3O4@La-MOF-Schiff base-Pd ensemble (characterized by PXRD, EDS, TEM, HAADF-STEM,
VSM, TGA) showed very active, quite stable, magnetically recoverable and reusable in coupling of bromo-
or iodoaryls with arylboronic acids, affording practically quantitative yields of biaryls (>99%) and high
TONs and TOFs. The catalyst has been recycled up to 12 times without significant loss of the catalytic
activity. A synergistic cooperativity between Pd and the oxophilic La nodes, through the organic linker,
is responsible for enhancing the catalyst activity and stability.
Received 21 November 2017
Revised 22 February 2018
Accepted 26 February 2018
Keywords:
Metal-organic framework (MOF)
Fe3O4
Post-synthetic modification
Heterogeneous palladium catalyst
Magnetically recoverable
Suzuki-Miyaura cross-coupling
Ó 2018 Elsevier Inc. All rights reserved.
1. Introduction
MOFs, as target motifs, are frequently reacted with anhydrides
[11,12] and aldehydes [13–15] via PSM to create multidentate
As a new type of highly tailorable microporous materials,
metal–organic frameworks (MOFs) are at the core of a variety of
specific applications such as catalysis [1], gas storage [2], drug
delivery [3a], sensors [3b] or diversified multifunctional
materials [4]. In particular, MOFs have played a key role in hetero-
geneous catalysis as catalyst templates due to their high surface
area, adjustable pore size, robustness, controllable distribution of
the catalytically active sites, and increased activity and chemose-
lectivity [1,5]. Furthermore, post-synthetic modification (PSM) of
MOFs has emerged as an important tool for introducing a wide
range of functional groups into the complex MOF superstructures
extending thus their use in catalysis according to current needs
[6–8]. Presently, there are two PSM strategies for MOF functional-
ization. In the first, the newly introduced functions are connected
to backbones of the MOFs by covalent, coordination or covalent –
coordination modification [9,10]. For instance, amine groups on
coordination sites. The latter can further coordinatively bind metal
ions, catalytically active, such as Pd (II), in view of efficient applica-
tion in CAC cross couplings. The second method is to load nanopar-
ticles, such as Au, Pt, Ni and Cu, in the void spaces of MOFs by the
method of impregnation in metal salt solutions and reduction, then
use the resulting modified MOFs as catalysts in well-defined reac-
tions [16a]. However, a method implying the two above mentioned
strategies for tailoring La-MOFs, which might further widen the
perspectives of valorising these unique materials, has not been
previously [16b] reported.
Palladium-catalyzed cross-couplings, conducted in homoge-
neous and heterogeneous phase, have been extensively applied
in synthetic chemistry [17–21]. Among these, Suzuki-Miyaura
cross-coupling of aryl halides with arylboronic acids represents
an efficient way for forming carbon-carbon bonds, particularly
for synthesis of biaryls as building blocks of pharmaceutical prod-
ucts [17]. In spite of this progress, isolation of homogenous cata-
lysts from reaction mixtures is sometimes quite difficult and
metal leaching is problematic leading to highly toxic palladium
(II) residues [21a]. Although development of heterogeneous
⇑
Corresponding authors.
E-mail addresses: vdragutan@yahoo.com (V. Dragutan), sunyaguang@syuct.edu.
cn (Y.-G. Sun).
https://doi.org/10.1016/j.jcat.2018.02.026
0021-9517/Ó 2018 Elsevier Inc. All rights reserved.

G. Xiong et al. / Journal of Catalysis 361 (2018) 116–125
117
catalytic systems made it possible to isolate and recycle the
recovered catalysts, some Pd leaching and cumbersome processes
such as filtration and clogging still remain current issues that
affect the reuse of conventional heterogeneous catalysts,
especially when solid inorganic bases are employed. To meet
these challenges, catalysts supported on magnetic nanoparticles
(MNPs) offered a convenient use circumventing separation by
centrifugation or tedious filtration, the catalyst being instead
recovered simply by applying an external magnet [22]. Thus, Zhang
2.1.2. Synthesis of magnetic Fe3O4 nanoparticles
FeCl3ꢀ6H2O (2.7 g) and FeSO4ꢀ7H2O (2.7 g) were added to dis-
tilled water (60 mL) and stirred until all the components were dis-
solved. A brown precipitate was generated by adding the right
amount of NH3ꢀH2O (28%), under mechanical stirring. After adjust-
ing the pH to 10, the solution was heated at 80 °C for 0.5 h. The
Fe3O4 was collected and washed 3 times with distilled water and
3 times with ethanol. After drying, the obtained black Fe3O4 pow-
der (1.5987 g) was dispersed into ethanol at a concentration of 7
mg/mL.
and coworkers [23a] synthesized
a palladium-2,2-bipyridine
complex supported on Fe3O4 nanoparticles and valorised it in
Suzuki-Miyaura couplings. This catalyst could be easily separated
magnetically. More recently, Astruc and coworkers [23b]
took advantage of magnetically recoverable ‘‘click” PEGylated
2.1.3. Synthesis of microcrystalline powder La-MOF
A microcrystalline powder of La-MOF was obtained by mixing
in water stoichiometric amounts of lanthanide chloride and dis-
odium 2-amino-benzene-1,4-dicarboxylate. Precipitation immedi-
ately occurred and the pale-yellow precipitate was filtered and
dried in air.
c-Fe2O3–Pd nanoparticle catalysts in the Suzuki–Miyaura,
Sonogashira, and Heck reactions, demonstrating intervention of
strong positive dendritic effects on catalyst loading, catalytic
activity, and recyclability. Also, superparamagnetic polymer
composite microspheres (Fe3O4/polymer), used to support
palladium nanoparticles, provided high reactivity in Suzuki
coupling reactions, while the catalyst could be easily recovered
and reused for five times [23c]. Further notable applications of
Pd on magnetic supports as catalysts in CAC bond formation
have also been reported [23d,23e].
2.1.4. Synthesis of single crystal La-MOF
NaOH (0.1 M) was added dropwise to 10 mL water solution of
H2abdc (0.018 g, 0.10 mmol) adjusting the pH value to 5–6, and
the resulting solution was introduced into a test tube. A new solu-
tion of La(NO3)3ꢀ6H2O (0.044 g, 0.10 mmol) in 2-propanol (8 mL)
was allowed to slowly diffuse into the above solution of H2abdc
through a buffering layer of 2-propanol and water (1:1). Colorless
cubic crystals suitable for X-ray crystal analyses were obtained
after four days (yield 20%), which are stable in air and insoluble
in water and common organic solvents
Prompted by these developments and continuing our research
program on Pd-catalyzed cross-coupling reactions [18b,19b,19c],
we have designed
a
highly efficient Fe3O4 nanoparticle-
supporting catalyst, synthesized via a PSM strategy operated on a
La-MOF [La(abdc)(Habdc)ꢀ9H2O]n (abdc = 2-amino-benzene-1,4-di
carboxylate), herein reported for the first time (Scheme 1). The
significant robustness and fine porosity of this La-MOF, provided
by the oxophilic La nodes and dicarboxylic organic linker,
coupled with adequate stability to solvents and chemical
reagents, were good reasons for choosing it as substrate for PSM
[24–26]. Besides, our synthesis protocol is very simple so that this
new catalyst can be made available on a large scale.
2. Experimental
2.1. Catalyst preparation
All starting materials and solvents were obtained and used
without further purification from commercial suppliers.
2.1.1. Synthesis of disodium 2-amino-benzene-1,4-dicarboxylate
Two equivalents of sodium hydroxide were added to a suspen-
sion of 2-amino-benzene-1,4-dicarboxylic acid (H2abdc) in dis-
tilled water and the mixture stirred until all components were
fully dissolved. After adjusting the pH of the solution to 6 and heat-
ing it carefully in air to dryness, a pale yellow powder of disodium
2-aminoterephthalate was collected.
Fig. 1. The 3D structure of La-MOF highlighting the 4-nodal topology of the La-MOF
subunit.
Scheme 1. Synthesis route of Fe3O4@La-MOF-Schiff base-Pd.

118
G. Xiong et al. / Journal of Catalysis 361 (2018) 116–125
2.1.5. Synthesis of the Fe3O4@La-MOF
washed with distilled water and ethanol for 3 times each. Then
A
suspension of nanosized Fe3O4 (7 mg/mL, 50 mL; see
the brown precipitate was isolated by using an external magnet
and dried in air to give the brown Fe3O4@La-MOF-Schiff base
(2.5825 g).
Section 1.2) was added to a solution of LaCl3ꢀ6H2O (2.1201 g) in
distilled water (20 mL), under mechanical stirring. After a few min-
utes, a solution of disodium 2-amino-benzene-1,4-dicarboxylate
(2.7014 g) in distilled water (20 mL) was added under mechanical
stirring; immediately a brown precipitate was generated. The
brown precipitate was collected and washed 3 times each with dis-
tilled water and ethanol. Then the brown precipitate was isolated
by using a magnet and dried to give Fe3O4@La-MOF (3.1039 g).
2.1.7. Synthesis of Fe3O4@La-MOF-Schiff base-Pd
A
mixture of Fe3O4@La-MOF-Schiff base (1.9636 g), PdCl2
(0.2215 g) and ethanol (100 mL) was mechanically stirred for 2
days, at room temperature. The precipitate was collected and
washed with distilled water and ethanol for 3 times, respectively.
Then the brown precipitate was isolated by using a magnet and
dried to give the brown Fe3O4@La-MOF-Schiff base-Pd (1.8921 g).
2.1.6. Synthesis of Fe3O4@La-MOF-Schiff base
A
mixture of Fe3O4@La-MOF (2.7133 g), pyridine-2-
2.2. General reaction conditions for the Suzuki-Miyaura cross-coupling
reactions
carboxaldehyde (8 mmol) and ethanol (100 mL) was mechanically
stirred for 2 days. The formed precipitate was collected and
The typical experimental procedure for the Suzuki-Miyaura
coupling reactions using the Fe3O4@La-MOF-Schiff base-Pd as cat-
alyst is as follows. A mixture of aryl halide (1 mmol), boronic acid
(1.2 mmol), base (2.0 mmol, the indicated species), solvent (6 mL,
as indicated) and catalyst (the given amount) were placed in a Sch-
lenk tube and stirred in air, at the appropriate temperature for the
certain time (as indicated in Tables 1–3, S3 and S4). After comple-
tion of the reaction, a magnet was used to separate the catalyst and
then the mixture was extracted three times with ethyl acetate. The
product yield was determined by gas chromatography (GC) analy-
sis using hexadecane as the internal standard.
2.3. Characterization
Powder X-ray diffraction (PXRD) patterns of the samples were
collected on an X-ray diffractometer (BRUKER D8 ADVANCE) with
Cu K radiation. Thermal Gravimetric experiments were carried out
on a SDT Q600 instrument with a heating rate of 10 °C/min. The
Fig. 2. PXRD patterns for different samples indicating the presence of F3O4 NP in the
catalyst.
Fig. 3. (a and b) TEM image of the Fe3O4 nanoparticles; (c) SEM of Fe3O4@La-MOF-Schiff base-Pd; (d) STEM of Fe3O4@La-MOF-Schiff base-Pd; (e-f) TEM image of Fe3O4@La-
MOF-Schiff base-Pd.

G. Xiong et al. / Journal of Catalysis 361 (2018) 116–125
119
magnetic measurements were carried out by VSM at room temper-
ature. N2 gas sorption experiments were carried out on a V-Sorb
2800 TP volumetric gas sorption instrument. GC analyses were per-
formed on an Agilent Technologies 7890A gas chromatograph. X-
ray photoelectron spectroscopy was determined on Thermo VG
ESCALAB250. ICP-AES was determined on Thermo IRIS Advantage.
SEM images were taken with a Zeiss Gemini Ultra-55 analytical
scanning electron microscope at an accelerating voltage of 5 keV.
Energy-dispersive X-ray spectroscopy (EDS) was taken on a Zeiss
Gemini Ultra-55 analytical scanning electron microscope with an
EDAX EDS detector with Genesis software. TEM images were taken
on a JEOL 2100F operated at 120 keV equipped with Gatan Tridiem
imaging spectrometer. High-angle annular dark-field (HAADF)
scanning TEM (STEM) investigations using high angle annular
dark-field STEM (HAADF-STEM) were acquired with a 1-nm probe
at 200 kV with a monochromated FEI Tecnai F20 scanning-trans
mission-electron-microscope (STEM). 1H NMR spectra were
recorded on a Bruker BioSpin GmbH AVANCE III 500 MHz spec-
trometer operating at 500 MHz.
A suitable crystal was selected and mounted on a SuperNova,
Single source at offset, Eos diffractometer with
a
graphite-
-scan
monochromatic MoK radiation (k = 0.71073 Å) using the
a
x
technique. The crystal was kept at 120 K during data collection.
Using Olex21, the structure was solved with the Superflip2 struc-
ture solution program using Charge Flipping and refined with the
XL3 refinement package using Least Squares minimisation. The
occupancy sum of N1, N2, N3 atoms is 1. Hydrogen atoms of free
water molecules have not been localized. Crystal and final struc-
ture refinements of single crystal La-MOF have been listed in
Table S1. CCDC: 1522121. The data can be obtained free of charge
from the Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
Fig. 4. HAADF-STEM image and elemental mappings of Fe3O4@La-MOF-Schiff base-Pd show the presence of N, Cl, Fe, La and Pd atoms in the catalyst.
Fig. 5. EDS spectrum of Fe3O4@La-MOF-Schiff base-Pd.

120
G. Xiong et al. / Journal of Catalysis 361 (2018) 116–125
decrease in the BET surface. A further slighter decrease in the
3. Results and discussion
BET surface (32 m2/g) occurs after PSM with the Schiff base,
assigned to the effect of functionalization. However, the remaining,
still high, surface area will be conducive to an efficient spread of
the tethered Pd catalytic sites.
The X-ray structure analysis on a single crystal reveals that La-
MOF crystallized in the cubic system, space group Ia-3 (Table S1). It
presents a pcu topology 3D framework composed of abdc linkers
and linear tri-nuclear La units, which is isostructural to the known
neodymium compound [Nd(abdc)(Habdc)ꢀ9H2O]n [27]. The layered
structure of the powder La-MOF (Figs. 1 and S1) was confirmed by
PXRD analysis, as compared with single-crystal structure of La-
MOF.
Fe3O4@La-MOF has been readily synthesized by mixing aqueous
solutions of LaCl3, disodium 2-aminobenzene-1,4-dicarboxylate
and Fe3O4 nanoparticles under gentle mechanical stirring. Fe3O4@-
La-MOF was post-synthetically modified first by reaction with
pyridine-2-carboxaldehyde to give the imine derivative Fe3O4@-
La-MOF-Schiff base, and then by coordination with PdCl2 to afford
the palladium-functionalized framework Fe3O4@La-MOF-Schiff
base-Pd.
The cube morphology of Fe3O4@La-MOF-Schiff base-Pd (Fig. 3c
and d), similar to that of La-MOF (Fig. S4), evidences the mainte-
nance of the crystalline state. As in Fig. 3a, the black dots in
Fig. 3e and f represent embedded Fe3O4 nanoparticles. From TEM
analysis (Fig. 3a and b) and SEM image (Fig. S4(a)) of the Fe3O4
nanoparticles, it was inferred that their size is about 10 nm. The
typical TEM image of Fe3O4@La-MOF-Schiff base-Pd, as well as
the corresponding elemental mappings for the synthesized catalyst
(shown in Fig. 4), testify for an even distribution of the Fe3O4
nanoparticles and of Pd ions in the composite, crucial for a superior
performance of the heterogeneous promoter.
The obvious conclusion is that the Fe3O4 nanoparticles are cor-
respondingly embedded in the interior of the La-MOF. The content
of Fe3O4 in Fe3O4@La-MOF has been determined to be ca. 11 wt%.
In the Fe3O4@La-MOF-Schiff base-Pd, the amount of Fe3O4 corre-
sponds to a value of 9.35 wt% (considering the total weight of the
catalyst, after PSM with Schiff base and Pd). The EDS spectrum
showed, as expected, the concomitant presence of Fe, La and Pd
species (Figs. 5 and S5).
Powder X-ray diffraction (PXRD) (Fig. 2) illustrates that Fe3O4
nanoparticles have been successfully embedded in the La-MOF
while the crystalline state of the backbones was kept during
post-synthetic modification. Noteworthy, the characteristic signal
of Fe3O4 nanoparticles was present in the PXRD even after recy-
cling the catalyst for twelve times.
The magnetization saturation value of 37.9 emuꢀgꢁ1, taken from
the magnetic hysteresis loop curve of Fe3O4@La-MOF-Schiff base-
Pd (Fig. S6), evidences that the catalyst can be easily separated
from the reaction mixture, just by an external magnet. The binding
energy (400 eV) of N atoms in the Fe3O4@La-MOF-Schiff base
(inferred from XPS measurements) is slightly higher as compared
to that in Fe3O4@La-MOF-Schiff base-Pd (Fig. S7). The rationale
would be that electrons are transferred from N atoms to Pd atoms,
thus providing further support for a suitable coordination of palla-
dium atoms to the N atoms resulting in a good stability of the Pd
TGA curves (Fig. S2) demonstrate a considerable thermal stabil-
ity of the post-synthetically modified MOF. The N2 adsorption-
desorption isotherms (Fig. S3) showed a decrease of the BET
surface areas paralleling the different stages of the catalyst
preparation. According to data in Table S2, there is about a
50 m2/g difference in BET surface areas between La-MOF and
Fe3O4@La-MOF indicating that Fe3O4 has been primarily encapsu-
lated into La-MOF, as also confirmed from the TEM spectrum
(Fig. 3a and b). The relatively low content and small size of Fe3O4
nanoparticles are responsible for encapsulation and the first
Fig. 6. HAADF-STEM images of the recovered Fe3O4@La-MOF-Schiff base-Pd.

G. Xiong et al. / Journal of Catalysis 361 (2018) 116–125
121
complex. From an ICP-AES measurement on Fe3O4@La-MOF-Schiff
base-Pd we could deduce a palladium content of 0.873 wt% (8.23
ꢂ 10ꢁ3 mol% Pd). The actual distribution of Fe, La and Pd within
the catalyst, before and after use, could be properly evaluated from
the HAADF-STEM (Figs. 6 and S8) and TEM images (Figs. 7 and S9).
From Fig. 7a and c, as well as from Fig. S8, it appears that the size of
recovered Fe3O4@La-MOF-Schiff base-Pd has not been changed.
Practically, the morphology of the catalyst was destroyed by the
prolonged mechanical stirring, but PXRD indicated that the cata-
lyst was not decomposed. As inferred from the XPS (Fig. S10), the
valence of Pd is +2.
Schiff base-Pd acted, due to the presence of palladium active spe-
cies, as an effective catalyst, at reasonable catalyst loadings
(Table 2), the influence played by the catalyst amount, reaction
temperature and time have also been minutely examined (Tables
2 and 3). We found that a high yield (95%) could be obtained
employing a very small amount of the catalyst (3 mg). By raising
the catalyst amount to 8 mg, the yield exceeded 99%, showcasing
this catalyst as an excellent promoter of Suzuki-Miyaura CAC bond
formation. The particular catalytic ability of Fe3O4@La-MOF-Schiff
base-Pd was also far superior to that of PdCl2. Using PdCl2 as the
catalyst and working under same reaction conditions, a lower yield
was obtained (56%), as compared with Fe3O4@La-MOF-Schiff base-
Pd (yield >99%) (Table S3). Obviously, in comparison with PdCl2,
the Fe3O4@La-MOF-Schiff base-Pd catalyst displays a higher cat-
alytic activity at a lower Pd loading, exhibiting at the same time
a substantial robustness, easy magnetic recoverability, 12 times
recyclability, and a minimum noble metal leaching.
Overall, these determinations indicate that the catalyst has
been successfully prepared and used in cross-coupling reactions
according to our pre-established protocol.
The catalytic activity of the Fe3O4@La-MOF-Schiff base-Pd was
investigated in the Suzuki-Miyaura cross-couplings of selected aryl
halides with arylboronic acids (Table 1). Rewardingly, yields of
over 99% in different biaryl products have been attained for both
aryl iodides and bromides, under mild conditions. As expected, aryl
chlorides gave lower yields, readily accounted for by the reduced
reactivity of aryl chlorides in the oxidative addition rate-
determining step of the catalytic cycle [28], as compared to their
bromide and iodide counterparts.
Products from each stage of the catalyst preparation, i.e. Fe3O4,
La-MOF, Fe3O4@La-MOF, Fe3O4@La-MOF-Schiff base and Fe3O4@La-
MOF-Schiff base-Pd, were in turn used as possible initiators in a
standard reaction, between bromobenzene and phenylboronic acid
(Table S3). Predictably, results showed that only Fe3O4@La-MOF-
Optimization of main reaction parameters was further carried
out using different bases and solvents (Table S4). Data evidenced
that the best combination allowing high yields (>99%) to be
attained was K2CO3 as the base and ethanol as the solvent
(Table S4, entry 2). On raising the reaction temperature the yield
sharply increased. At 80 °C and a reaction time of 0.5 h, the yield
was as high as 99% (Table 3). In conclusion, ethanol as the solvent,
potassium carbonate as the base, 8 mg of catalyst, reaction time
0.5 h, and a temperature of 80 °C provide best results.
With a reduced amount of catalyst (1 mg; 8.23 ꢂ 10ꢁ3 mmol%
Pd) and an excess of bromobenzene (2 mmol; Table 1, entry 12),
Fig. 7. (a and b) TEM image of Fe3O4@La-MOF-Schiff base-Pd before use; (c and d) TEM image of the recovered Fe3O4@La-MOF-Schiff base-Pd.

122
G. Xiong et al. / Journal of Catalysis 361 (2018) 116–125
Table 1
Suzuki-Miyaura cross-coupling reactions of various aryl halides with arylboronic acids using Fe3O4@La-MOF-Schiff base-Pd as catalyst.a
a
Reaction conditions: aryl halide (1.0 mmol), arylboronic acid (1.2 mmol), K2CO3 (2.0 mmol), ethanol (6 mL), catalyst (8 mg,
0.066 mmol% Pd), temperature (80 °C), time (0.5 h). bReaction conditions: catalyst (10 mg), temperature (80 °C), time (10 h).
-
c
Reaction conditions: bromobenzene (2.0 mmol), phenylboronic (2.4 mmol), K2CO3 (4.0 mmol), ethanol (12 mL), catalyst (1 mg, 8.23
ꢂ 10ꢁ3 mmol% Pd), temperature (80 °C), time (0.5 h).

G. Xiong et al. / Journal of Catalysis 361 (2018) 116–125
123
Table 2
Optimization of the catalyst amount in Suzuki-Miyaura coupling reaction promoted
by Fe3O4@La-MOF-Schiff base-Pd.
Entry
Catalyst (mg)
Yield (%)
1
2
3
4
5
6
3
5
8
10
15
20
95
98
>99
>99
>99
>99
Reaction conditions: bromobenzene (1.0 mmol), phenylboronic acid (1.2 mmol),
K2CO3 (2.0 mmol), ethanol (6 mL), catalyst Fe3O4@La-MOF-Schiff-Pd, temperature
80 °C, time 0.5 h.
Table 3
Evaluation of main reaction parameters (temperature and time) for the Suzuki-
Miyaura coupling reaction with the Fe3O4@La-MOF-Schiff base-Pd catalyst.
Fig. 8. Recycling of Fe3O4@La-MOF-Schiff base-Pd in Suzuki-Miyaura cross-cou-
pling (under optimal reaction conditions: Table 1, entry 1).
Entry
T (°C)
Time (h)
Yield (%)
1
2
3
4
5
40
60
80
80
80
1/2
1/2
1/6
1/3
1/2
52
89
93
95
>99
Reaction conditions: bromobenzene (1.0 mmol), phenylboronic acid (1.2 mmol),
K2CO3 (2.0 mmol), ethanol (6 mL), catalyst (8 mg).
the 88% yield was obtained, corresponding to a high turnover num-
ber (TON) of 21,443 and turnover frequency (TOF) of 42,886 hꢁ1
.
Both values are by far superior to other heterogeneous based-
MOFs Pd catalysts (e.g. TON of 15,800 and TOF of 1317 for the MoII
cluster coordination polymer/PdIIPd00.6; TOFs of: 396 for Pd/MIL-53
(Al)-NH2, 2190.5 for Pd/UiO-66-NH2, 2037 for IRMOF-3-PI-Pd, 1235
for UiO-67-3-PI-Pd) [29]. More importantly, our TOF ranks second
among highest values communicated for crystalline MOF-Pd cata-
lysts in the Suzuki-Miyaura cross-coupling of bromobenzene and is
very close to the best so far reported (43,890, for Pd/AZC) [30].
A heterogeneity test was carried out as well. First, the reaction
between bromobenzene and phenylboronic acid was run under
above optimal conditions giving >99% yield. The supernatant was
then added to the regular mixture of p-bromotoluene and phenyl-
boronic and let to react without further addition of catalyst. As
expected, no product was detected in this reaction. Consequently,
the reaction proceeds solely in heterogeneous mode, with no
leaching of Pd ions in the supernatant.
Recyclability of the catalyst was tested in the reaction of
iodobenzene and phenylboronic acid, in consecutive runs, separat-
ing the catalyst with an external magnet after each run (Fig. 8). It
was found that the Fe3O4@La-MOF-Schiff base-Pd could be reused
twelve times without a significant decrease in the yield. The ele-
mental mappings and TEM images of the recovered catalyst show
that palladium ions were still evenly distributed in the composite
(Figs. 6 and 7). The valence +2 of Pd was maintained (from XPS),
indicating that no Pd(0) nanoparticles have been generated during
the catalytic process (Fig. S10). According to PXRD tests (Fig. 2) the
crystalline phase and catalyst structure were fully preserved in the
used catalyst. Therefore, Fe3O4@La-MOF-Schiff base-Pd proved to
be a quite robust, high-performance heterogeneous catalyst for
activating aromatic CAX (X = Br, I) bonds in Suzuki-Miyaura
cross-coupling reactions.
Scheme 2. Proposed interaction of La, organic linker, solvent and base within the
Pd-catalytic cycle.
effect, La nodes could as well accelerate the reductive elimination
step, favouring generation of Pd(0) species able to resume a new
catalytic cycle. This kinetic behaviour has been also evidenced in
previous reports on La-MOF-Pd catalyzed cross-couplings.
[18b,19b] Scheme 2 gives insight into the role of La and organic
linker within the Pd catalytic cycle, essentially distinct from that
of the substrate, base and solvent [21b–21d].
Along with the kinetic effect, a stronger N-Pd bond that
increases the stability of the Pd complex, may substantially dimin-
ish Pd leaching in the overall catalytic process. Furthermore, La
ions foster a tridimensional MOF architecture with channels and
pores favouring decoration with Fe3O4 NP. Post-synthetic modifi-
cation via a Schiff-base to coordinatively bind Pd ions in a uniform
and stable arrangement results in an innovative, highly active
catalyst.
4. Conclusions
To account for the superior behaviour of our catalyst we assume
that La ions, due to their electropositive propensity, hard Lewis
acid character and their key role in building a rigid 3D configura-
tion by multiple coordination modes, enhance the stability of the
Pd complex. The latter process occurs via a synergistic cooperation
of the oxophilic La ions and the aromatic organic linker in transfer-
ring charge density to the N-Pd bond (Fig. S7). By this electronic
This catalyst demonstrated important beneficial attributes: (a)
accessibility from largely available reagents, by a straightforward
synthetic protocol which leaves only innocuous residues; (b)
robustness of the La-MOF template due to a rigid configuration
ensured by the oxophilic lanthanide nodes and the nucleophilic
organic linker; (c) superior catalytic activity as evidenced by the
quantitative yields and high TONs and TOFs attained at low

124
G. Xiong et al. / Journal of Catalysis 361 (2018) 116–125
(c) Z. Hu, B. Deibert, J. Li, Luminescent metal-organic frameworks for chemical
sensing and explosive detection, Chem. Soc. Rev. 43 (2014) 5815;
(d) J. Liu, L. Chen, H. Cui, J. Zhang, L. Zhang, C.-Y. Su, Applications of metal–
organic frameworks in heterogeneous supramolecular catalysis, Chem. Soc.
Rev. 43 (2014) 6011;
(e) P. Ramaswamy, N.E. Wong, G.K.H. Shimizu, MOFs as proton conductors–
challenges and opportunities, Chem. Soc. Rev. 43 (2014) 5913;
(f) V. Stavila, A.A. Talin, M.D. Allendorf, MOF-based electronic and opto-
electronic devices, Chem. Soc. Rev. 43 (2014) 5994.
catalyst loadings, under mild reaction conditions; (d) easy recover-
ability using external permanent magnets; and, quite importantly: (e)
proven recyclability for at least 12 runs without significant loss of
the catalytic activity. These merits recommending the Fe3O4@
La-MOF-Schiff base-Pd promoter for user-friendly, economical
CAC cross-couplings are poised to contribute to the development
of high-performance heterogeneous palladium catalytic systems.
[5] (a) A. Dhakshinamoorthy, A.M. Asiri, H. Garcia, Metal-organic frameworks as
heterogeneous catalysts in liquid phase reactions: why are they so
exceptional?, Chimica Oggi 33 (2015) 40;
Acknowledgements
(b) K Meyer, M. Ranocchiari, J.A. van Bokhoven, Metal organic frameworks for
photo-catalytic water splitting, Energy Environ. Sci. 8 (2015) 1923;
(c) A. Dhakshinamoorthy, H. Garcia, Metal-organic frameworks as solid
catalysts for the synthesis of nitrogen-containing heterocycles, Chem. Soc.
Rev. 43 (2014) 5750;
This work was supported by the Natural Science Foundation of
China (21671139 and 21501122), the Distinguished Professor
Project of Liaoning Province (2013204) and Doctoral Scientific
Research Foundation of Liaoning Province (201601193). Natural
Science Foundation of Liaoning Province (20170540713) and
Liaoning Province Department of Education Fund (LFD2017001).
F.D. acknowledges the Chinese Scholarship Council ([2012]5031)
for financial support. I.D. and V.D. gratefully acknowledge support
from the Romanian Academy, Institute of Organic Chemistry,
Bucharest, Romania.
(d) T. Zhang, W. Lin, Metal-organic frameworks for artificial photosynthesis
and photocatalysis, Chem Soc. Rev. 43 (2014) 5982.
[6] C. Chen, C.A. Allen, S.M. Cohen, Tandem postsynthetic modification of metal-
organic frameworks using an inverse-electron-demand Diels-Alder reaction,
Inorg. Chem. 50 (2011) 10534.
[7] J. Canivet, S. Aguado, Y. Schuurman, D. Farrusseng, MOF-supported selective
ethylene dimerization single-site catalysts through one-pot postsynthetic
modification, J. Am. Chem. Soc. 135 (2013) 4195.
[8] B. Li, Y.M. Zhang, D.X. Ma, L. Li, G.G. Li, G.D. Li, Z. Shi, S.H. Feng, A strategy
toward constructing
a
bifunctionalized MOF catalyst: post-synthetic
modification of MOFs on organic ligands and coordinatively unsaturated
metal sites, Chem. Commun. 48 (2012) 6151.
[9] S.M. Cohen, Postsynthetic methods for the functionalization of metal-organic
frameworks, Chem. Rev. 112 (2012) 970.
Appendix A. Supplementary data
[10] K.K. Tanabe, S.M. Cohen, Postsynthetic modification of metal–organic
frameworks-a progress report, Chem. Soc. Rev. 40 (2011) 498.
[11] S.J. Garibay, S.M. Cohen, Isoreticular synthesis and modification of frameworks
with the UiO-66 topology, Chem. Commun. 46 (2010) 7700.
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.jcat.2018.02.026.
[12] K. Peikert, F. Hoffmann, M. l Froba, Amino substituted Cu3(btc)2: a new metal-
organic framework with a versatile functionality, Chem. Commun. 48 (2012)
11196.
[13] J.W. Brown, N.N. Jarenwattananon, T. Otto, J.L. Wang, S. Glöggler, L.S. Bouchard,
Heterogeneous Heck coupling in multivariate metal–organic frameworks for
enhanced selectivity, Catal. Commun. 65 (2015) 105.
References
[1] (a) Y.-B. Huang, J. Liang, X.-S. Wang, R. Cao, Multifunctional metal-organic
framework catalysts: synergistic catalysis and tandem reactions, Chem. Soc.
Rev. 46 (2017) 126;
(b) J. Chen, K. Shen, Y. Li, Greening the processes in MOFs synthesis and MOFs-
involved sustainable catalysis, ChemSusChem 10 (2017) 3165;
(c) L. Oar-Arteta, T. Wezendonk, X. Sun, F. Kapteijn, J. Gascon, Metal organic
frameworks as precursors for the manufacture of advanced catalytic materials,
Mater. Chem. Front. 1 (2017) 1709;
(d) A. Dhakshinamoorthy, A.M. Asiri, H. Garcia, Mixed-metal or mixed-linker
metal organic frameworks as heterogeneous catalysts, Catal. Sci. Technol. 6
(2016) 5238;
(e) W.H. Dong, L. Zhang, C.H. Wang, C. Feng, N.Z. Shang, S.T. Gao, C. Wang,
Palladium nanoparticles embedded in metal–organic framework derived
porous carbon: synthesis and application for efficient Suzuki-Miyaura
coupling reactions, RSC Adv. 6 (2016) 37118;
(f) Y.-B. Huang, M. Shen, X. Wang, P. Huang, R. Chen, Z.-J. Lin, R. Cao, Water-
medium C-H activation over a hydrophobic perfluoroalkane-decorated metal-
organic framework platform, J. Catal. 333 (2016) 1;
[14] Z.G. Sun, G. Li, H.O. Liu, L.P. Liu, Salen-Co(II) complex incorporated into amino-
functionalized MIL-101(Cr) through postsynthetic modification as
a
cooperative catalyst for cyclohexane selective oxidation, Appl. Catal. A: Gen.
466 (2013) 98.
[15] C.J. Doonan, W. Morris, H. Furukawa, O.M. Yaghi, Isoreticular metalation of
metal–organic frameworks, J. Am. Chem. Soc. 131 (2009) 9492.
[16] (a) C. Rösler, R.A. Fischer, Metal-organic frameworks as hosts for
nanoparticles, CrystEngComm 17 (2015) 199;
(b) J. Zhao, X. He, Y. Zhang, J. Zhu, X. Shen, D. Zhu, Highly water stable
lanthanide metal-organic frameworks constructed from 2,20-Disulfonyl-4,40-
biphenyldicarboxylic Acid: Syntheses, Structures, and Properties, Cryst.
Growth Des. 17 (2017) 5524.
[17] (a) J.G. de Vries, Palladium-Catalysed Coupling Reactions, in: M. Beller, H.-.U.
Blaser (Eds.), Organometallics as Catalysts in the Fine Chemical Industry,
Springer-Verlag, Berlin, Heidelberg, Germany, 2012, pp. 1–34;
(b) C. Torborg, M. Beller, Recent applications of palladium-catalyzed coupling
reactions in the pharmaceutical, agrochemical, and fine chemical industries,
Adv. Synth. Catal. 351 (2009) 3027;
(g) Y. Zhang, J.Y. Ying, Main-chain organic frameworks with advanced catalytic
functionalities, ACS Catal. 5 (2015) 2681.
[2] (a) I. Schwedler, S. Henke, M.T. Wharmby, S.R. Bajpe, A.K. Cheetham, R.A.
Fischer, Mixed-linker solid solutions of functionalized pillared-layer MOFs–
adjusting structural flexibility, gas sorption, and thermal responsiveness,
Dalton Trans. 45 (2016) 4230;
(c) A. Chatterjee, T.R. Ward, Recent advances in the palladium catalyzed
Suzuki-Miyaura cross-coupling reaction in water, Catal. Lett. 146 (2016) 820.
[18] (a) P. Gautam, B.M. Bhanage, J. Org. Chem. 80 (2015) 7810;
(b) L. You, W. Zhu, S. Wang, G. Xiong, F. Ding, B. Ren, I. Dragutan, V. Dragutan,
Y.G. Sun, High catalytic activity in aqueous Heck and Suzuki-Miyaura
reactions catalyzed by novel Pd/Ln coordination polymers based on 2,20-
(b) G. Chang, B. Li, H. Wang, T. Hu, Z. Bao, B. Chen, Control of interpenetration
in a microporous metal-organic framework for significantly enhanced C2H2/
CO2 separation at room temperature, Chem. Commun. 52 (2016) 3494;
(c) T.R.C. Van Assche, G.V. Baron, J.F.M. Denayer, Molecular separations with
breathing metal–organic frameworks: modelling packed bed adsorbers, Dalton
Trans. 45 (2016) 4416;
bipyridine-4,40-dicarboxylic acid as
(2016) 47;
a heteroleptic ligand, Polyhedron 115
(c) R.D.J. Froese, Ch. Lombardi, M. Pompeo, R.P. Rucker, M.G. Organ, Designing
Pd–N-heterocyclic carbene complexes for high reactivity and selectivity for
cross-coupling applications, Acc. Chem. Res. 50 (2017) 2244.
[19] (a) S.L. Huang, A.Q. Jia, G.X. Jin, Pd(diimine)Cl2 embedded heterometallic
compounds with porous structures as efficient heterogeneous catalysts, Chem.
Commun. 49 (2013) 2403;
´
(d) Z. Chen, K. Adil, Ł.J. Weselinski, Y. Belmabkhout, M. Eddaoudi,
A
supermolecular building layer approach for gas separation and storage
applications: the eea and rtl MOF platforms for CO2 capture and
hydrocarbon separation, J. Mater. Chem. A 3 (2015) 6276;
(e) Y. He, W. Zhou, G. Qian, B. Chen, Chem. Soc. Rev. 43 (2014) 5657;
(f) E. Barea, C. Montoro, J.A.R. Navarro, Toxic gas removal-metal-organic
frameworks for the capture and degradation of toxic gases and vapours, Chem.
Soc. Rev. 43 (2014) 5419.
(b) L. You, W. Zong, G. Xiong, F. Ding, S. Wang, B. Ren, I. Dragutan, V. Dragutan,
Y.-G. Sun, Cooperative effects of lanthanides when associated with palladium
in novel, 3D Pd/Ln coordination polymers. Sustainable applications as water-
stable, heterogeneous catalysts in carbon–carbon cross-coupling reactions,
Appl. Catal. A Gen. 511 (2016) 1;
(c) L.-X. You, H.-J. Liu, L.-X. Cui, F. Ding, G. Xiong, S.-J. Wang, B.-Y. Ren, I.
Dragutan, V. Dragutan, Y.-G. Sun, The synergistic effect of cobalt on a Pd/Co
catalyzed Suzuki-Miyaura cross-coupling in water, Dalton Trans. 45 (2016)
18455.
[3] (a) F. Ke, Y.P. Yuan, L.G. Qiu, Y.H. Shen, A.J. Xie, J.F. Zhu, X.Y. Tian, L.D. Zhang,
Facile fabrication of magnetic metal–organic framework nanocomposites for
potential targeted drug delivery, J. Mater. Chem. 21 (2011) 3843;
(b) J.W. Cao, Y.F. Gao, Y.Q. Wang, C.F. Du, Z.L. Liu, A microporous metal-organic
open framework containing uncoordinated carbonyl groups as postsynthetic
modification sites for cation exchange and Tb3+ sensing, Chem. Commun. 49
(2013) 6897.
[4] (a) D. Zhao, Y. Cui, Y. Yang, G. Qian, CrystEngComm 18 (2016) 3746;
(b) Y.-N. Wang, P. Zhang, J.-H. Yu, J.-Q. Xu, 4-(4-Carboxyphenoxy) phthalate-
based coordination polymers and their application in sensing nitrobenzene,
Dalton Trans. 44 (2015) 1655;
[20] (a) V. Ramakrishna, N. Dastagiri Reddy, Synthesis of zwitterionic palladium
complexes and their application as catalysts in cross-coupling reactions of
aryl, heteroaryl and benzyl bromides with organoboron reagents in neat
water, Dalton Trans. 46 (2017) 8598;

G. Xiong et al. / Journal of Catalysis 361 (2018) 116–125
125
(b) L.F. Liu, Y.H. Zhang, Y.G. Wang, Phosphine-free palladium acetate catalyzed
Suzuki reaction in water, J. Org. Chem. 70 (2005) 6122.
characterization and application as catalysts for the Suzuki reactions, Appl.
Catal. A Gen. 475 (2014) 249;
[21] (a) Á. Molnár, A. Papp, Catalyst recycling-a survey of recent progress and
current status, Coord. Chem. Rev. 349 (2017) 1;
(d) M.O. Sydnes, Use of nanoparticles as catalysts in organic synthesis–cross-
coupling reactions, Curr. Org. Chem. 18 (2014) 312;
(b) Q.B. Liang, P. Xing, Z.G. Huang, J.J. Dong, K.B. Sharpless, X.X. Li, B. Jiang,
Palladium-catalyzed, ligand-free Suzuki reaction in water using aryl
fluorosulfates, Org. Lett. 17 (2015) 1942;
(c) C.F.R.A.C. Lima, A.S.M.C. Rodrigues, V.L.M. Silva, A.M.S. Silva, L.M.N.B.F.
Santos, Role of the base and control of selectivity in the Suzuki-Miyaura cross-
coupling reaction, ChemCatChem 6 (2014) 1291;
(e) M.O. Sydnes, The Use of Palladium on magnetic support as catalyst for
Suzuki-Miyaura cross-coupling reactions, Catalysts 7 (2017) 35.
[24] Y. Luo, G. Calvez, S. Freslon, C. Daiguebonne, T. Roisnel, O. Guillou, A family of
lanthanide-containing molecular open frameworks with high porosity: [Ln
(abdc)(Habdc), nH2O]1 with Ln = La–Eu and 8 ꢃ n ꢃ 11, Inorg. Chim. Acta 368
(2011) 170.
(d) Ch. Amatore, G. Le Duc, A. Jutand, Mechanism of palladium-catalyzed
suzuki-miyaura reactions: multiple and antagonistic roles of anionic ‘‘Bases”
and their countercations, Chem Eur. J. 19 (2013) 10082;
[25] J.H. Yang, D.F. Wang, W.D. Liu, X. Zhang, F.L. Bian, W. Yu, Palladium supported
on a magnetic microgel: an efficient and recyclable catalyst for Suzuki and
Heck reactions in water, Green Chem. 15 (2013) 3429.
(e) Ch. Amatore, A. Jutand, G. Le Duc, Mechanistic origin of antagonist effects
of usual anionic bases (OHꢁ, CO32ꢁ) as modulated by their countercations
(Na+, Cs+, K+) in palladium-catalyzed Suzuki-Miyaura reactions, Chem. Eur. J.
18 (2012) 6616.
[26] J.P. Huang, W. Wang, H.X. Li, Water-medium organic reactions catalyzed by
active and reusable Pd/Y heterobimetal-organic framework, ACS Catal.
(2013) 1526.
3
[27] X.Y. Chen, B. Zhao, W. Shi, J. Xia, P. Cheng, D.Z. Liao, S.P. Yan, Z.H. Jiang,
[22] S. Sobhani, Z.M. Falatooni, S. Asadi, M. Honarmand, Palladium-schiff base
complex immobilized covalently on magnetic nanoparticles as an efficient and
recyclable catalyst for heck and Suzuki cross-coupling reactions, Catal Lett.
146 (2016) 255.
Microporous metal-organic frameworks built on
connecting node, Chem. Mater. 17 (2005) 2866.
a Ln3 cluster as a six-
[28] T. Ichikawa, M. Netsu, M. Mizuno, T. Mizusaki, Y. Takagi, Y. Sawama, Y.
Monguchi, H. Sajiki, Development of unique heterogeneous palladium
a
[23] (a) Y.Q. Zhang, X.W. Wei, R. Yu, Fe3O4 nanoparticles-supported palladium-
bipyridine complex: effective catalyst for Suzuki coupling reaction, Catal. Lett.
135 (2010) 256;
catalyst for Suzuki-Miyaura reaction using (hetero) aryl chlorides and
chemoselective hydrogenation, Adv. Synth. Catal. 13 (2017) 2269.
[29] R. Sun, B. Liu, B.G. Li, S. Jie, Palladium(II)@zirconium-based mixed-linker
metal-organic frameworks as highly efficient and recyclable catalysts for
Suzuki and heck cross-coupling reactions, ChemCatChem 8 (2016) 3261.
[30] H. Choudhary, S. Nishimura, K. Ebitani, Tailored design of palladium species
grafted on an amino functionalized organozinc coordination polymer as a
highly pertinent heterogeneous catalyst, J. Mater. Chem. A 2 (2014) 18687.
(b) D. Wang, C. Deraedt, L. Salmon, C. Labrugere, L. Etienne, J. Ruiz, D. Astruc,
Efficient and magnetically recoverable ‘‘Click” PEGylated
c-Fe2O3–Pd
nanoparticle catalysts for Suzuki-Miyaura, Sonogashira, and heck reactions
with positive dendritic effects, Chem. Eur. J. 21 (2015) 1508;
(c) D. Yuan, H. Zhang, Nanosized palladium supported on diethylenetriamine
modified superparamagnetic polymer composite microspheres: synthesis,