A molecular Pd(ii) complex incorporated into a MOF as a highly active single-site heterogeneous catalyst for C-Cl bond activation

Article abstract of DOI:10.1039/c4gc00314d

The organic palladium complex Pd(H₂bpydc)Cl₂ (H ₂bpydc = 2,2′-bipyridine-5,5′-dicarboxylic acid) was immobilized on a porous metal-organic framework UiO-67 (Zr₆O ₄(OH)₄(bpdc)₆, bpdc = para- biphenyldicarboxylate) using a direct incorporation strategy. The use of a large amount of the H₂bpdc ligand (90 mol% of the mixed ligands) that can't chelate the Pd complex allowed the formation of isolated Pd single active sites uniformly distributed in the MOF network. Pd(ii) doped UiO-67 is isostructural to the parent UiO-67 framework, with a high surface area and pore volume of ca. 2000 m² g^-1 and 0.79 cm³ g ^-1, respectively. The material was highly efficient in the catalytic conversion of aryl chlorides, showing remarkably higher activity than the homogeneous Pd counterparts. High yields were achieved in Heck and Suzuki-Miyaura coupling reactions of chloroarenes bearing a wide range of substituents. Moreover, the catalyst was recoverable and reusable, giving essentially identical activity after at least 5 cycles. The combination of the advantages of both homogeneous molecular Pd catalysts and solid MOF structures in this system may bring new opportunity in the development of highly active heterogeneous palladium catalysts for a variety of Pd-catalyzed transformations. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00314d

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A molecular Pd(II) complex incorporated into a
MOF as a highly active single-site heterogeneous
catalyst for C–Cl bond activation†
Cite this: DOI: 10.1039/c4gc00314d
Liyu Chen,a Sylvie Rangan,b Jing Li,*a,c Huanfeng Jianga and Yingwei Li*a
The organic palladium complex Pd(H2bpydc)Cl2 (H2bpydc = 2,2’-bipyridine-5,5’-dicarboxylic acid) was
immobilized on a porous metal–organic framework UiO-67 (Zr6O4(OH)4(bpdc)6, bpdc = para-biphenyldi-
carboxylate) using a direct incorporation strategy. The use of a large amount of the H2bpdc ligand
(90 mol% of the mixed ligands) that can’t chelate the Pd complex allowed the formation of isolated Pd
single active sites uniformly distributed in the MOF network. Pd(II) doped UiO-67 is isostructural to the
parent UiO-67 framework, with a high surface area and pore volume of ca. 2000 m2 g−1 and 0.79 cm3
g−1, respectively. The material was highly efficient in the catalytic conversion of aryl chlorides, showing
remarkably higher activity than the homogeneous Pd counterparts. High yields were achieved in Heck
and Suzuki–Miyaura coupling reactions of chloroarenes bearing a wide range of substituents. Moreover,
the catalyst was recoverable and reusable, giving essentially identical activity after at least 5 cycles. The
combination of the advantages of both homogeneous molecular Pd catalysts and solid MOF structures in
this system may bring new opportunity in the development of highly active heterogeneous palladium
catalysts for a variety of Pd-catalyzed transformations.
Received 21st February 2014,
Accepted 30th May 2014
DOI: 10.1039/c4gc00314d
www.rsc.org/greenchem
similar to its homogeneous counterparts, but also the advan-
tages of facile separation, possibility of recycling, and potential
Introduction
Homogeneous catalytic systems based on palladium play a application to continuous flow processes.4 So far, inorganic
very important role in organometallic catalysis, serving as a materials, organic polymers, and membranes have been used
strategic tool for organic transformation and total synthesis.1 as supports to anchor organometallic Pd catalysts.5 However,
Because of the limited availability and high price of Pd, from among these systems obtained via classical immobilization
the sustainablility point of view it is of great importance to methods, very few achieve high density as well as uniform dis-
develop Pd catalysts with full utilization of each Pd active site, tribution of isolated active single-sites. For example, the graft-
as well as excellent recyclability and negligible metal leaching ing of molecular Pd catalysts onto organic polymers often
in order to reduce the cost of the catalyst and pollution to the results in random anchoring of the ligands or active units on the
environment.2 Nevertheless, such a task remains a great chal- support, and consequently, mutual deactivation between sites that
lenge to date. Preparation of single-site heterogeneous cata- are too close to each other, and significantly reduced efficiency
lysts represents
a
suitable solution because single-site compared to their homogenous counterparts.6,7 Therefore, it is
heterogeneous catalysts can offer the selectivity and specificity essential to develop a heterogeneous Pd(II) catalytic system with a
of homogeneous single-site systems combined with easy cata- high Pd density as well as isolated single active sites.
lyst separation from reactants and products.3 The immobili-
Metal–organic frameworks (MOFs) have emerged as an
zation of single-site Pd(II) will not only lead to high activities important class of functional materials owing to their high
surface area, porosity, and chemical tunability.8 In recent
years, the employment of MOFs as heterogeneous catalysts has
aKey Laboratory of Fuel Cell Technology of Guangdong Province, School of Chemistry
attracted tremendous interest, especially in the use of MOFs as
and Chemical Engineering, South China University of Technology, Guangzhou 510640,
scaffolds for the encapsulation of metal (e.g., Pd, Au, Ru, and
China. E-mail: liyw@scut.edu.cn
Pt) nanoparticles.9 However, little attention has been paid to
the immobilization of metal ions on MOFs as single-site cata-
lysts.10 With open and uniform channel structures, MOFs are
particularly suitable for immobilizing well-defined molecular
catalysts with a high degree of metal dispersion. Recent
reports have illustrated some examples where palladium ions
bDepartment of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854,
USA
cDepartment of Chemistry and Chemical Biology, Rutgers University, Piscataway,
NJ 08854, USA. E-mail: jingli@rutgers.edu
†Electronic supplementary information (ESI) available: Experimental details,
additional reaction results, and spectra data for the products. See DOI:
10.1039/c4gc00314d
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are anchored on MOFs through direct or post-synthesis incor- dicarboxylic acid) ligands (90 mol% of the mixed ligands) that
poration. Post-synthesis modification has been applied to can’t chelate the Pd complex is essential in reducing the Pd
generate Pd(II)@MOFs,11 but the metal salts used as precur- usage as well as in ensuring the separation and dispersion of
sors tend to aggregate on the surface, leading to serious pore the metalloligand H2L1 and the single Pd cations in the indi-
blocking and a random distribution of Pd active sites on the vidual cages of the MOF architecture, and thus preventing the
MOF.6 While direct incorporation of organometallic catalysts Pd catalysts from mutual deactivation between sites that are
into the framework of MOFs allows completely isolated and too close together. It can be anticipated that such a rational
uniform catalytic centers to form, it has not been extensively design will lead to the highest single-site catalyst activity com-
investigated.7 A literature survey shows that there are only few pared with the other reported Pd(II)@MOFs because it allows
examples reporting the synthesis of Pd(II)@MOFs as hetero- maximum exposure of the active sites to reactants. The pre-
geneous catalysts using the direct incorporation method.12,13 pared palladium catalyst shows high activity in the Heck and
These Pd(II)@MOF catalysts were mostly utilized for carbon– Suzuki–Miyaura coupling reactions of a variety of aryl chlor-
carbon (C–C) coupling reactions. While this reaction represents ides. Moreover, the catalyst was recoverable and reusable,
one of the most useful transformations in organic synthesis, giving essentially identical activity after at least 5 cycles. Metal
the reported studies were shown to be efficient only for the acti- leaching was largely inhibited because of the strong inter-
vation of aryl bromides and iodides in the couplings.13 From a action between Pd(II) and the MOF network. This protocol
practical point of view, the use of aryl chlorides is desirable coincides with the concept of green chemistry as it could
because they are arguably the most useful class of aryl halide reduce the cost of catalysis and additional purification of the
substrate, due to their lower cost and the wider diversity of avail- final product as well as the potential heavy-metal pollution to
able compounds. However, the activation of aryl chlorides is the environment. To the best of our knowledge, this work rep-
much more difficult than aryl bromides or iodides. The ineffi- resents the first example of the incorporation of a palladium
ciency of these Pd(II) containing MOFs in the activation of aryl complex into MOFs for the efficient activation of aryl chlorides
chlorides could be attributed to their low specific surface area for carbon–carbon coupling reactions.
(BET, less than 100 m2 g−1) after Pd modification or the small
distance between the loaded Pd cations (Pd⋯Pd = 3–4 Å, too
short to attain single-site catalyst activity).14
Herein, we report the coupling reactions of aryl chlorides
over a palladium(II) complex, H2L1 (Pd(H2bpydc)Cl2, H2bpydc =
Experimental section
Materials preparation
2,2′-bipyridine-5,5′-dicarboxylic acid), which was built in a
Synthesis of (2,2′-bipyridine)-5,5′-dicarboxylic acid dimethyl
robust MOF using a direct incorporation method (Scheme 1). ester (Me2bpydc). An appropriate amount of (2,2′-bipyridine)-
We have chosen the UiO-67 type MOF for the incorporation of 5,5′-dicarboxylic acid (500 mg, 2 mmol) in methanol (30 mL)
the Pd complex because the UiO family of MOFs based on was added to concentrated sulfuric acid (2 mL), which had
Zr6O4(OH)4(CO2)12 secondary building units (SBUs) and dicar- been placed in an ice bath. After refluxing overnight, the solu-
boxylate bridging ligands possesses high specific surface areas tion was poured into water (40 mL), forming a white slurry.
with unprecedented stability.15 In contrast to the previous The pH of the slurry was adjusted to 7 with powdered Na2CO3.
reports on the synthesis of Pd(II)@MOFs by direct incorpor- The product was then extracted with chloroform, dried over
ation methods, in this work we have successfully developed a magnesium sulfate and evaporated to dryness. 1H NMR
novel mixed ligand strategy to prepare the MOF host in order (400 MHz, CDCl3) δ = 9.24 (s, 2H), 8.53 (d, J = 8.0 Hz, 2H), 8.40
to obtain isolated Pd single active sites in the MOF network in (d, J = 8.4 Hz, 2H), 3.93 (s, 3H).
a uniform way for highly efficient catalysis reactions. The use
Synthesis of [(5,5′-dimethoxylcarboxyl-2,2′-bipyridine-)palla-
of a large number of the H2L2 (H2bpdc, bpdc = para-biphenyl- dium(II)]dichloride (Me2L1). A mixture of PdCl2(CH3CN)2
(130 mg, 0.5 mmol) and dimethyl (2,2′-bipyridine)-5,5′-
dibenzoate (Me2bpydc) (136 mg, 0.5 mmol) in 8 mL of CH3CN
was stirred at 65 °C for 24 hours. The resulting yellow solid
was collected by filtration and was washed with CH3CN, and
then was heated at 100 °C for 12 h under vacuum.
Synthesis of [(5,5′-dicarboxy-2,2′-bipyridine-)palladium(II)]-
dichloride (H2L1). A solution of [(5,5′-dimethoxylcarboxyl-2,2′-
bipyridine-)palladium(II)]dichloride
(Me2L1)
(120
mg,
0.27 mmol) in tetrahydrofuran (THF) (10 mL), ethanol
(10 mL), and 3 M aqueous NaOH (10 mL) was stirred at 70 °C
for 5 hours. The solution was cooled to room temperature and
acidified to pH 1. A yellow solid was obtained, and washed
with H2O and methanol to afford the product. 1H NMR
(400 MHz, D2O) δ = 8.79 (s, 2H), 8.61 (d, J = 8.4 Hz, 2H), 8.37
(d, J = 8.4 Hz, 2H).
Scheme 1 Schematic representation of the synthesis of Pd(II) doped
UiO-67. The structure model (right) of Pd(II) doped UiO-67 shows the
incorporation of the L1 ligand into the framework. Colour coding: zirco-
nium, purple; carbon, grey; oxygen, red; nitrogen, blue; palladium,
yellow; chlorine, green. All H atoms are omitted for clarity.
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Synthesis of Pd(II) doped UiO-67. ZrCl4 (70.0 mg, 0.3 mmol), GC-MS analysis (Shimadzu GCMS-QP5050A equipped with a
H2L1 (12.6 mg, 0.03 mmol) and H2L2 (H2bpdc, para-biphenyl- 0.25 mm × 30 m DB-WAX capillary column). The product was
dicarboxylic acid) (65.4 mg, 0.27 mmol) were dispersed in purified by silica gel chromatography. The product identifi-
DMF (10 mL) and sealed in a 25 mL tube at 100 °C for cation was conducted by GC-MS analysis and 1H NMR and 13
C
36 hours. The produced powders were isolated by centrifu- NMR measurement on a Bruker Advance 600 spectrometer or a
gation and dried at ambient temperature. Subsequently, the Bruker DRX-400 spectrometer using CDCl3 as a solvent and
as-synthesized sample was soaked in chloroform for three TMS as an internal standard.
18 hours periods at room temperature to remove DMF and
The recyclability tests
ligand precursors, then filtered off and dried under vacuum at
room temperature for 24 h. Finally, the sample was heated
under vacuum at 150 °C overnight to remove the solvents.
The reactions were performed under the same reaction con-
ditions as described above, except using the recovered catalyst.
Each time, the catalyst was isolated from the reaction solution
at the end of the reaction, washed with DMF, and then heated
at 150 °C under vacuum.
Catalyst characterization
The BET surface area measurements were performed with N2
adsorption/desorption isotherms at 77 K on a Micromeritics
ASAP 2020M instrument. Before the analysis, the samples were
evacuated at 150 °C for 12 h.
Powder X-ray diffraction patterns of the samples were
recorded on a Rigaku diffractometer (D/MAX-IIIA, 3 kW) using
Cu Kα radiation (40 kV, 30 mA, λ = 0.1543 nm). The size and
morphology of the Pd(II) doped UiO-67 were investigated by
Results and discussion
The [PdCl2(bpy)]-derived dicarboxylic acid H2L1 was syn-
thesized by treating PdCl2(CH3CN)2 with dimethyl (2,2′-bipyri-
dine)-5,5′-dicarboxylate, followed by base-catalyzed hydrolysis.
A mixture of H2L1 and H2L2 with a molar ratio of 1 : 9 was then
using
a transmission electron microscope (TEM, JEOL,
used to prepare
Pd(II)
doped UiO-67 with the
JEM-2010HR) with EDX analysis (Oxford INCA EDS) operated
at 300 kV. X-ray photoelectron spectroscopy (XPS) measure-
ments were performed on a Kratos Axis Ultra DLD system with
a base pressure of 10−9 Torr. The palladium content of the
samples were determined quantitatively by AAS on a HITACHI
Z-2300 instrument.
Zr6O4(OH)4(L1)x(L2)6−x formula, which allowed the incorpor-
ation of the H2L1 ligand into the UiO-67 framework with
biphenyldicarboxylate (bpdc) as the bridging ligand. Powder
X-ray diffraction (XRD) patterns of the Pd(II) doped UiO-67
were identical to those of the parent UiO-67 (Fig. 1), indicating
that Pd(II)–UiO-67 is isostructural to the parent UiO-67. In
order to confirm that L1 was really embedded into the network
of the MOF as proposed in Scheme 1, we have performed an
elemental analysis on the Pd doped UiO-67 sample which indi-
cated the N content was ca. 0.31 wt%. Based on the elemental
analysis results, the molar ratio of L1/(L1 + L2) was calculated
to be ca. 4.0%, giving a theoretical Pd content of ca. 1.16 wt%
with the assumption that all 2,2-bipyridine moieties were
bonded with PdCl2. This value was very close to the actual Pd
quantity (1.1 wt%) in the Pd doped UiO-67 as measured by
AAS. From the structure model depicted in Scheme 1 (and
Catalytic experiments
Heck coupling reaction. Typically, aryl halide (0.5 mmol),
olefin (0.75 mmol), K2CO3 (1.5 mmol), TBAB (0.3 mmol), hexa-
methylbenzene (0.3 mmol, as an internal standard for GC ana-
lysis), and palladium catalyst (0.46 mol%) were added to 5 mL
DMF. The reaction mixture was stirred at the desired tempera-
ture under a N2 atmosphere. The solution was filtered and
washed with brine and diethyl ether. The organic phase was
subsequently extracted with diethyl ether (3 × 20 mL), dried
over MgSO4, and concentrated in vacuo. The crude product was
quantified by GC-MS analysis (Shimadzu GCMS-QP5050A
equipped with a 0.25 mm × 30 m DB-WAX capillary column).
The product was purified by silica gel chromatography. The
product identification was conducted by GC-MS analysis and
1H NMR and 13C NMR measurement on a Bruker Advance 600
spectrometer or a Bruker DRX-400 spectrometer using CDCl3
as a solvent and TMS as an internal standard.
Suzuki–Miyaura coupling reaction. Typically, aryl halide
(0.5 mmol), phenylboronic acid (0.75 mmol), KOH (1.5 mmol),
hexamethylbenzene (0.3 mmol, as an internal standard for GC
analysis), and palladium catalyst (0.46 mol%) were added to a
DMF–EtOH mixture (5 mL DMF and 0.25 mL EtOH). The reac-
tion mixture was stirred at the desired temperature under a N2
atmosphere. The solution was filtered and washed with brine
and diethyl ether. The organic phase was subsequently
extracted with diethyl ether (3 × 20 mL), dried over MgSO4, and
Fig. 1 Powder XRD patterns of UiO-67 (a), and Pd(II) doped UiO-67
concentrated in vacuo. The crude product was quantified by samples recorded before (b) and after (c) the catalytic reactions.
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Fig. S1†), the molar ratio of L1/(L1 + L2) of the displayed struc- doping reactions, and to determine their coordination environ-
ture was calculated to be ca. 4.17 mol%, which is in good ment within the framework, we performed X-ray photoelectron
agreement with the actual value measured in the MOF spectroscopy (XPS) analysis on PdCl2(CH3CN)2, Me2L1
material. These results are consistent with the structure model (dimethyl esters of L1), and Pd(II)–UiO-67 samples. Fig. 4(a)
of the prepared Pd doped UiO-67 material shown in Scheme 1. shows that both Me2L1 and Pd(II) doped UiO-67 samples
The surface area and pore volume of the Pd-MOF were exhibit a Pd 3d5/2 band at around 338 eV, which is character-
determined from N2 adsorption/desorption isotherms at 77 K istic of divalent Pd. For comparison, the XPS spectrum of pris-
(Fig. 2), and were calculated to be ca. 2000 m2 g−1 and tine PdCl2(CH3CN)2 was also measured which confirmed the
0.79 cm3 g−1, respectively. The mean pore size obtained from Pd(II) environment. However, the Pd 3d lines of PdCl2(CH3CN)2
the Horvath–Kawazoe model was ca. 7.0 Å. The high porosity appear shifted to a higher binding energy compared to the Pd
of the framework allows close contact between the reactants 3d line of the other two compounds. This can be interpreted
and the Pd catalytic centers, and facilitates substrate diffusion as a similar Pd environment in both the MeL1 and Pd(II)
for efficient catalysis.
doped UiO-67 compounds, where the neighboring N is not
To evaluate the level of possible aggregation of the Pd involved in a CuN bond, but rather, placed in a C–N–C deloca-
cations during the MOF synthesis, TEM experiments were lized environment. This is confirmed by the N 1s spectra
carried out on the Pd(II) doped UiO-67 material. The results shown in Fig. 4(b), which indicate a larger binding energy for
shown in Fig. 3 suggest that there were no distinctly congre- the N 1s peak of the reference PdCl2(CH3CN)2 with respect to
gated Pd particles in the MOF. To confirm that the Pd ions those of the other two species. The absence of multiple fea-
were coordinated to 2,2′-bipyridine (bpy) moieties prior to the tures in the N 1s spectrum of the Pd(II) doped UiO-67 seems to
further confirm the presence of only one environment for N,
namely each Pd is bonded to the uncoordinated 2,2′-bipyridine
units. Additionally, the EDX analysis shows that the atomic
ratio of Pd : Cl is ca. 1 : 2 (Fig. S2†), which confirms that all the
palladium remains in its dichloride form.
The palladium-catalyzed Heck coupling reaction is well
known as a powerful and versatile method to synthesize ary-
lated alkenes in fine chemical synthesis. Our initial studies
focused on the Heck reaction of aryl chlorides with olefins to
evaluate the catalytic activity of the Pd(II) doped
UiO-67 material. First, we chose 4-chloroacetophenone as sub-
strate to screen the reaction parameters. The reactions were
carried out with Pd(II) doped UiO-67 as a catalyst and DMF as a
solvent under a N2 atmosphere, and different bases were used
(Table 1, entries 1–4). It was found that potassium carbonate
worked the best compared with KOH, Cs2CO3 and K3PO4,
while the yield of the coupling product was still very low. An
◆
◊
Fig. 2 Nitrogen adsorption ( )/desorption ( ) isotherms at 77 K (top)
and micropore size distribution (bottom) of 1.1 wt% Pd(II) doped UiO-67.
Fig. 4 XPS spectra of Pd(II) doped UiO-67, Me2L1, and PdCl2(CH3CN)2
referenced to hydrocarbon C 1s. (a) Pd 3d; (b) N 1s; and (c) survey
photoemission spectrum of Pd(II) doped UiO-67. The inset in (c) indi-
cates the Pd 3d and Zr 3p peak positions.
Fig. 3 TEM images of Pd(II) doped UiO-67 (a and b).
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tion (entry 13). The poor yield of the coupling product demon-
strated that Pd was immobilized on the bpy moieties in the
Pd(II) doped UiO-67. It is well known that Pd(II) ions are easily
aggregated under such conditions, losing their catalytic activity
gradually. However, in the case of Pd(II) doped UiO-67, the 2,2′-
bipyridine moieties in the MOF could provide a strong coordi-
Table 1 Heck coupling of 4-chloroacetophenone with Pd(II) doped
UiO-67a
T
Time Yieldb nation environment for the immobilization of palladium,
Entry Base
Solvent
(°C) Additive (h)
(%)
thus preventing the agglomeration of Pd species during the
reaction.
1
2
3
4
5
6
KOH
K2CO3
Cs2CO3 DMF
DMF
DMF
100
100
100
100
120
100
—
—
—
—
20
20
20
20
20
20
24
24
24
24
24
24
24
24
24
5
15
11
7
18
95
8
Different aryl chlorides were employed in the reaction
(Table 2) to investigate the range of aryl chlorides that can be
tolerated in the Heck coupling reaction. Aryl chlorides of
various electronic characters reacted smoothly with styrene
(Table 2, entries 1–7). In general, electron-deficient aryl chlor-
ides exhibit higher activity than electron-rich ones. Heteroaryl
chlorides, e.g. 2-chloropyridine, also underwent coupling
smoothly and furnished the desired products in excellent
yields (entry 7).
K3PO4
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
DMF
DMF
DMF
—
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
7
Ethylene glycol 100
DMF + EtOH
H2O
DMF
DMF
DMF
DMF
DMF
DMF
8c
9
100
100
100
100
100
100
100
100
50
5
10
11d
12e
13f
14g
15h
99
—
49
34
—
—
We further investigated the coupling of different alkenes
with 4-chloroacetophenone. As shown in Table 2, styrene
derivatives reacted nicely with 4-chlorobenzaldehyde. For
example, 4-methylstyrene and 4-fluorostyrene afforded the
corresponding coupling products in 96% and 94% yield,
respectively (Table 2, entries 8 and 9). Also, the reactions of
methyl- or ethyl-acrylate with 4-chloroacetophenone both gave
the desired products in excellent yields (entries 10 and 11). In
particular, alkyl-substituted terminal olefins (e.g. octene),
which usually have low reactivity in the documented Heck
reactions, were found to be coupled in good yields (entry 12).
The catalytic efficiency of Pd doped UiO-67 is superior to
that of the less porous catalysts with a similar building unit.
For example, the reaction of chlorobenzene and styrene in the
presence of Pd(II) doped UiO-67 produced the trans-1,2-diphe-
nylethylene product in 81% yield (Table 2, entry 5), while the
catalyst (Ln2[Pd(bpydc)Cl2]2[Pd(Hbpydc)Cl2]2, Ln = Sm, Eu, Gd,
or Tb) reported in the literature gave essentially no coupling
product (<5% yield) under similar conditions.13a The low
activity of the latter which has a similar building unit to that
prepared in this study may be attributed to the framework
structure, which has low porosity and small distances between
Pd⋯Pd (only 3.3–3.70 Å) that would restrain their catalytic
activity significantly.
The Pd(II) doped UiO-67 could be easily recovered by cen-
trifugation after removing the supernatant and washing with
DMF several times. The recovered catalyst was subsequently
used in successive runs without any loss of catalytic efficiency
(Table 2, entry 13). PXRD patterns for the used catalyst suggest
that the structural integrity of the MOF material was mostly
maintained after the catalytic reactions (Fig. 1). XPS measure-
ments of the recovered solid confirmed that the palladium
remained in its divalent form after the reaction (Fig. S3†). The
heterogeneous nature of Pd(II) doped UiO-67 was investigated
by a hot filtration experiment. The reaction in the presence of
Pd(II) doped UiO-67 was allowed to proceed only to approxi-
mately 28% conversion. At this point, the solution was quickly
filtrated and the solid was removed. The filtration was per-
a Reaction
conditions:
4-chloroacetophenone
(0.5
mmol),
phenylethylene (0.75 mmol), base (1.5 mmol), solvent (5 mL), Pd(II)
doped UiO-67 (0.46 mol% Pd), TBAB (0.3 mmol), under N2. b Yields
were determined by GC-MS analysis. c DMF (5 mL) and EtOH
(0.25 mL). d Parent UiO-67 as the catalyst. e Me2L1 as the catalyst.
f PdCl2(CH3CN)2 as the catalyst (0.46 mol% Pd). g PdCl2(CH3CN)2 as
the catalyst (46 ppb Pd). h PdCl2 as the catalyst (46 ppb Pd).
increase of temperature from 100 to 120 °C didn’t promote the
reaction significantly (entry 5). The addition of tetrabutyl-
ammonium bromide (TBAB) enhanced the yield remarkably
(entry 6), presumably due to the stabilization effect of TBAB
for the palladium species.16 The influence of solvent on the
catalytic performance of the present reaction system was inves-
tigated with the optimized base. It was reported that alcohol
can act as a hydrogen bond donor, and thus promote the dis-
sociation of halides and facilitate the reaction.17 However, in
the present work we found that the use of ethylene glycol,
ethanol mixed with DMF, or H2O as solvent were all ineffective
for this transformation (entries 7–9). A maximum of 99% con-
version of 4-chloroacetophenone (1a) was achieved with >99%
selectivity to 4-aceto-1,1′-biphenyl (3a) in 24 hours (entry 10).
Parent UiO-67 showed no signs of conversion, confirming
that the catalytic activity came from the Pd doped ligand (entry
11). For comparison, the reactivity of Me2L1, i.e. the esterified
L1 ligand of the MOF, in the Heck reaction was also examined
under identical conditions. As shown in Table 1, the yield of
the desired coupling product was very low when using Me2L1
as the catalyst (Table 1, entry 12). Therefore, we believe that
the incorporation of the metalloligand H2L1 into the porous
frameworks could stabilize the Pd catalytic center by efficiently
isolating the sites in a manner similar to the peptide architec-
ture of enzymes in biological systems. To further confirm that
the Pd atoms were successfully grafted on the bpy moieties of
the MOF rather than simply adsorbed on the solid surface,
PdCl2(CH3CN)2 was employed as a catalyst for the same reac-
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Table 2 Heck coupling reactions of various aryl chlorides with olefins over Pd(II) doped UiO-67a
Entry
1
Aryl chloride
Olefin
Product
Yieldb (%)
96
2
3
4
5
6
7c
97
95
96
81
95
90
8
96
94
97
9
10
11
90
85
95
12
13d
a Reaction conditions: aryl chloride (0.5 mmol), phenylethylene (0.75 mmol), K2CO3 (1.5 mmol), DMF (5 mL), Pd(II) doped UiO-67 (0.46 mol%
Pd), TBAB (0.3 mmol), 100 °C, 24 h, under N2. b Isolated yield based on 1. c 0.7 mol% Pd, 120 °C. d The fifth cycle.
formed at the reaction temperature in order to avoid re-depo- the reaction is predominantly heterogeneous. The Pd content
sition of dissolved palladium upon cooling. Only a trace of pal- of the reused catalyst was almost the same as the fresh one, as
ladium (ca. 46 ppb) could be detected in the hot filtrate. The determined by AAS. These studies indicated that the loss of
isolated solution was allowed to further react for 20 h, palladium active sites was negligible, which could account for
however, no further reactivity was observed under similar reac- the preservation of the catalytic activity of this catalytic system.
tion conditions (Fig. S4†). Moreover, a very small amount of
Encouraged by these interesting results, we further exam-
dissolved palladium (less than 0.1% of the total palladium) ined the catalytic activity of the Pd(II) doped UiO-67 for the
was detected in the solution at the end of the reaction. In Suzuki–Miyaura coupling reaction of aryl chlorides with phe-
order to assess the possible contribution of the leached Pd to nylboronic acid. The reaction parameters were first screened
the catalytic activity of Pd(II) doped UiO-67, we performed the using 4-nitro-1-chlorobenzene as a substrate (Table S1†). The
reaction using the same amount of dissolved palladium (ca. 46 best result was obtained when a DMF–EtOH mixture was used
ppb) under the same reaction conditions. No appreciable con- as a solvent and K2CO3 was used as a base, which afforded the
versions of 4-chloroacetophenone were observed (Table 1, desired product in 97% isolated yield (Table 3, entry 2). Under
entries 14 and 15). Although it cannot be excluded that such a the optimized conditions, different aryl chlorides reacted
low amount of palladium in solution may be active for the smoothly with phenylboronic acid to give the corresponding
reaction, it could be concluded that this contribution to the cross-coupling products in good to excellent yields (Table 3,
overall activity for the Pd(II) doped UiO-67 is very limited and entries 1–10). In general, aryl chlorides substituted with elec-
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with full utilization of each Pd active site, as well as excellent
recyclability and negligible metal leaching could coincide with
the concepts of green chemistry. This success in the prepa-
ration of single-site heterogeneous Pd catalysts by simply
immobilizing organometallic palladium onto MOFs might
bring new opportunities in the development of highly active
heterogeneous palladium catalysts for C–C couplings or other
Pd-catalyzed transformations.
Table 3 Suzuki–Miyaura coupling reactions of various aryl chlorides
over Pd(II) doped UiO-67a
Entry
1
Aryl chloride
Product
Yieldb (%)
95
2
97
96
94
88
90
85
80
Acknowledgements
3
4
This work was supported by NSF of China (20936001, 21322606,
and 21073065), Doctoral Fund of Ministry of Education of
China (20120172110012), Guangdong Natural Science Foun-
dation (S2011020002397 and 10351064101000000), and the Fun-
damental Research Funds for the Central Universities
(2013ZG0001). Prof. Jing Li is a Cheung Kong Scholar of South
China University of Technology.
5c
6
7c
8c
9c
90
88
Notes and references
10c
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4-CF3) exhibited higher activity than electron-rich ones (entries
1–7). Notably, even 2-chloroanisole (1i), which is both deacti-
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