Water-medium organic reactions catalyzed by active and reusable Pd/Y Heterobimetal-organic framework

Article abstract of DOI:10.1021/cs400094x

A Pd/Y heterobimetallic MOF (denoted as Pd/Y-MOF) catalyst is synthesized by coordination of Pd(II) and Y(III) with 2,2′-bipyridine-5,5′- dicarboxylate acid (bpydc) under microwave irradiation condition and is characterized by XRD Rietveld refinement, FTIR, Raman, TG-DTA, and XPS. It is shown that the 3D extended framework is constructed by linking Pd(bpydc)Cl ₂ building blocks via Y(III) coordinating to carboxylic groups. Pd/Y-MOF exhibits higher catalytic activity than Pd(bpydc)Cl₂ in Suzuki-Miyaura coupling reaction and Sonogashira reaction owing to the highly dispersed Pd(II) sites in the layered structure of Pd/Y-MOF and the cooperative effect between Pd(II) and Y(III). The heterogeneity studies provide mechanistic evidence that the reaction proceeds on the surface of Pd(II) ions in the crystal framework. Thus, Pd/Y-MOF exhibits impressive size selectivity toward substrates. With the small-sized reactants, it displays comparable activities with Pd(OAc)₂ homogeneous catalyst. However, extremely poor activity in Suzuki-Miyaura coupling reaction with bulk substrates 1-iodonaphthalene and 4-(tert-butyl) iodobenzene is observed due to the inhibition of diffusion into the micropore channels. In addition, Pd/Y-MOF can be easily recycled and reused owing to the high stability of the framework formed by coordination of Y(III) with carboxylic group. The incorporation of Pd(II) into the crystal framework of Pd/Y-MOF prohibits the leaching of Pd(II) active species.

Full text of DOI:10.1021/cs400094x

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Article
Water-Medium Organic Reactions Catalyzed by Active
and Reusable Pd/Y Heterobimetal-Organic Framework
Jinping Huang, Wei Wang, and Hexing Li
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/cs400094x • Publication Date (Web): 29 May 2013
Downloaded from http://pubs.acs.org on May 31, 2013
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Water-Medium Organic Reactions Catalyzed by Active and
Reusable Pd/Y Heterobimetal-Organic Framework
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a
b,
a,
Jinping Huang, Wei Wang * and Hexing Li *
a
The Education Ministry Key Lab of Resource Chemistry, Shanghai Key Laboratory of Rare
Earth Functional Materials, Shanghai Normal University, Shanghai 200234, China
b
Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, NM
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KEYWORDS: Pd/Y heterobimetallic MOF (Pd/Y-MOF); Heterogeneous catalyst; Water-
medium clean organic reactions; Suzuki-Miyaura coupling reaction; Sonogashira reaction.
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ABSTRACT: A Pd/Y heterobimetallic MOF (denoted as Pd/Y-MOF) catalyst is synthesized by
coordination of Pd (II) and Y(III) with 2, 2’-bipyridine-5, 5’-dicarboxylate acid (bpydc) under
microwave irradiation condition and is characterized by XRD Rietveld refinement, FTIR, Raman,
TG-DTA and XPS. It is shown that the 3D extended framework is constructed by linking
Pd(bpydc)Cl building blocks via Y(III) coordinating to carboxylic groups. Pd/Y-MOF exhibits
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higher catalytic activity than Pd(bpydc)Cl in Suzuki-Miyaura coupling reaction and Sonogashira
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reaction owing to the highly dispersed Pd (II) sites in the layered structure of Pd/Y-MOF and the
cooperative effect between Pd (II) and Y(III). The heterogeneity studies provide mechanistic
evidence that the reaction proceeds on the surface of Pd (II) ions in the crystal framework. Thus,
Pd/Y-MOF exhibits impressive size selectivity towards substrates. With the small-sized reactants,
it displays comparable activities with Pd(OAc) homogeneous catalyst. However, extremely poor
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activity in Suzuki-Miyaura coupling reaction with bulk substrates 1-iodonaphthalene and 4-(tert-
butyl) iodobenzene is observed due to the inhibition of diffusion into the micropore channels. In
addition, Pd/Y-MOF can be easily recycled and reused owing to the high stability of the
framework formed by coordination of Y(III) with carboxylic group. The incorporation of Pd (II)
into the crystal framework of Pd/Y-MOF prohibits the leaching of Pd (II) active species.
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INTRODUCTION
Metal-organic frameworks (MOFs) represent a new class of functional materials formed by
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metal ions or clusters as nodes and polyfunctional organic ligands as linkers. The controllable
topology and geometry of framework and the tunable pore functionality render them highly
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attractive to various applications in catalysis, gas storage, chemical separations, sensors,
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drug delivery, and light harvesting etc. With the ability to assemble well-defined molecular
building blocks into microporous frameworks, MOFs are particularly suitable for immobilizing a
metal complex of catalytic competence in uniform dispersion and open channel, which may
display high efficiency and the advantage of easy catalyst separation from the reaction
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system.
Incorporation of metal complex into a rigid lattice can effectively inhibit the leaching
of active sites and also strengthen the thermal stability of the heterogeneous catalyst, leading to
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the enhanced catalyst durability.
Nevertheless, to date the catalysis applications are mostly
focused on the MOFs as supports for metal nanoparticles or metal complexes through post-
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synthesis modification.
by Cr (III), Zn (II), or Ln (III) ions as Lewis acid.
Pd (II), Rh (II) and Ru(II)-complexes catalysts are widely used in organic reaction.
we have reported a Pd (II)-complex catalyst immobilized onto mesoporous polymer support,
The direct use of MOFs catalyst is limited to MOFs only constructed
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Recently,
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which can be easily recycled and repetitively used in various water-medium organic reactions.
However, it displays relatively poor activity due to the low dispersion of Pd (II) active sites and
the enhanced diffusion limit resulting from the pore blockage by metal-complex terminally
bonded to the pore surface. It is expected that the MOFs with Pd (II) coordinated into the
framework may exhibit improved activity. However, to our knowledge, no such Pd-MOF has
been used in catalysis so far possibly due to the poor stability. Recently, Bordiga et al have
reported the synthesis of a well defined heterobimetallic MOFs by coordinating Pt (II) with two
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Cl and two N atoms in 2, 2'-bipyridine-5, 5'-dicarboxylate (bpydc) to form Pt(bpydc)Cl building
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blocks, followed by linking together through Ln (III) ions bonding at carboxylic groups.
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noted that such heterobimetallic MOF displays strong stability, which may offer a promising way
to design new Pd (II), Rh (II) and Ru (II) complex catalysts in MOFs structure. The Ln (III) ions
serve as cornerstones for the MOFs framework while the noble metallic ions act as active sites
for catalysis. Herein, we report for the first time the synthesis of a Pd/Y heterobimetallic organic
framework (denoted as Pd/Y-MOF). Notably, it exhibits high activity in water-medium Suzuki-
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Miyaura coupling reaction and Sonogashira reaction. More interestingly, it also displays size-
selectivity to reactant molecules due to the diffusion limit in microporoes. The catalyst can be
recycled easily and used repetitively, showing great potential in practical application.
EXPERIMENTAL SECTION
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. Catalyst Synthesis. Pd/Y-MOF is synthesized under microwave irradiation conditions. In a
typical synthesis, K PdCl (0.098 g, 0.30 mmol), 2, 2’-bipyridine-5, 5’-dicarboxylic acid (0.073 g,
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0.30 mmol) and Y(NO ) ·6H O (0.50 g, 1.3 mmol) are mixed in a Teflon-lined vessel filled with
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0 mL of H O and stirred for 10 min at ambient temperature in air. Then, the vessel is closed in a
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polyether-ether-ketone (PEK) autoclave and heated rapidly to 100 ºC within 1 min by microwave.
The reaction system is kept at this temperature for 9 h. The resulting light yellow powder is
filtered and washed thoroughly with distilled water, followed by drying in air at 80 ºC. The
composition of the as-received Pd/Y-MOF is determined as C H Y Cl N O Pd based on both
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the ICP spectrum and elemental analysis. Experiment: C (28.3%), H (2.75), and N (5.53%);
calculation: C (26.5%), H (2.47%) and N (5.14%).
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. Characterization. Fourier transform infrared (FT-IR) spectra are collected on a Nicolet Magna
550 spectrometer. Raman spectroscopic measurement is performed by using a confocal
microprobe Raman system (LabRam II, Dilor, France). Powder X-ray diffraction pattern (XRD)
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is recorded on a Bragg-Brentano diffractometer (Rigaku D/Max-2000) with CuKa radiation
(λ=1.5418 Å) of graphite curve monochromator. The structure refinement is conducted by the
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GSAS suite program (the detailed procedure in the Supporting Information). Calculation of the
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accessible void space to the solvent molecules is performed by PLATON program using the as-
refined crystallographic information file (CIF). In situ temperature-dependent XRD is collected
in steps of 50 ºC on a Buhler furnace. The sample is smeared on a platinum sample plate,
allowing customized temperature programming from 50 to 900 ºC with a heating ramp of 10
ºC/min in air atmosphere. Thermogravimetric analysis and differential thermal analysis
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TG/DTA) are conducted on a DT-60 with a heating ramp of 10 ºC/min under oxygen
atmosphere, respectively. The surface electronic state is analyzed by X-ray photoelectron
spectroscopy (XPS, Perkin-Elmer PHI 5000C ESCA). All the binding energy values are
calibrated by using C = 284.6 eV as a reference. The content of Pd, Y and C, H, N are
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determined by inductively coupled plasma optical emission spectra (ICP, Varian VISTA-MPX)
and elemental analysis (Elementar Vario EL), respectively.
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. Activity Test. Suzuki-Miyaura coupling reaction. As shown in Scheme 1, an aryl halide (0.50
mmol), methylphenylboronic acid (0.75 mmol), K CO (1.5 mmol), Pd/Y-MOF catalyst (0.018
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mmol) containing Pd (0.054 mmol) and 4.0 mL of H O are mixed and reacted at 80 ºC for a
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specified time under mild stirring. Then, the reaction system is extracted with ethyl acetate and
analyzed on a GC-17A gas chromatograph (SHIMADZU) equipped with a JWDB-5, 95%
dimethyl-5% diphenylpolysiloxane column and an FID detector. The column temperature is
programmed from 80 to 250 ºC at a speed of 10 ºC/min. N is used as carrier gas, and n-decane is
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used as an internal standard. The reaction conversion is calculated based on aryl halide since the
other reactant methylphenylboronic acid is in great excess. Besides the target product biphenyl,
no significant side product is observed via GC, showing the reaction is quite selective. The
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selectivity toward target product is further detected based on the ratio of the real biphenyl amount
determined by GC using commercial available biphenyl as a standard to the theoretical biphenyl
amount calculated from the conversion, which shows more than 99.5% selectivity by using
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chrolo-, bromo-, and iodobenzene as the reactant, implying that, as observed elsewhere,
Suzuki-Miyaura coupling reaction may proceed almost absolutely selective to the target product
in most cases, i.e., the yield is equal to the conversion.
X
B(OH)2
Pd/Y-MOF cat.
base, H2O
+
R
CH3
R
H3C
Scheme 1. Pd/Y-MOF catalyzed Suzuki-Miyaura reaction.
Sonogashira coupling reaction. As shown in Scheme 2, an aryl halide (0.50 mmol),
phenylacetylene (0.60 mmol), 1, 8-Diazabicyclo[5.4.0] undec-7-ene (DBU, 0.10 mL), n-decane
(
0.10 mL), Pd/Y-MOF catalyst (0.018 mmol) containing Pd (0.054 mmol) and 4.0 mL of H O are
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mixed and reacted at 90 C under air.
I
Pd/Y-MOF cat.
base, H2O
+
R
X
R
X
Scheme 2. Pd/Y-MOF catalyzed Sonogashira reaction.
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. Adsorption Test. The adsorption performance of Pd/Y-MOF catalyst is tested in the following
procedure: 2.0 mL of ethyl acetate containing a substrate (0.050 mmol) is mixed with the Pd/Y-
MOF catalyst containing Pd (0.054 mmol), followed by stirring at room temperature. The content
of the substrate in the solution at different time is analyzed by a gas chromatograph
(SHIMADZU GC-17A) as described above.
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. Recycling Test. To determine the durability and recyclability, the catalyst is allowed to
centrifuge after each run of reaction and the clear supernatant liquid is decanted slowly. The
catalyst is washed thoroughly with deionized water and ethanol, followed by vacuum drying at
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0 ºC overnight. Then, the catalyst is reused with fresh charge of reactant for subsequent recycle
under the identical reaction condition. The reproducibility of each activity test is checked by
repeating each result at least three times and is found to be within acceptable limit (± 5 %).
RESULTS AND DISCUSSION
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. Structural Characterization. The XRD pattern in Figure 1 displays a long-range crystalline
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structure in which all diffraction peaks coincide with the reported Pt/Y-MOF. No significant
diffraction peak of impurity or start material is observed, suggesting the formation of unique
phase of Pd/Y-MOF, isostructural Pt/Y-MOF in orthorhombic, Pbca (No. 61) space group.
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Therefore, the crystal structure refinement of Pd/Y-MOF is conducted by the GSAS suite using
atomic parameters of Pt/Y-MOF as an initial model (see Support Information for the refinement
details). The final fitting factors of Sig = 1.147, R = 5.626%, Rwnb = 5.280%, R = 3.943% and
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b
Rexp = 4.906% represent a satisfactory refinement result. The crystallographic parameters of
Pd/Y-MOF are listed in Table 1.
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Figure 1. Rietveld refinement of XRD patterns for Pd/Y-MOF. The observed counts and the
calculated patterns are indicated by blue dots and black line. The positions of Bragg reflections
are indicated by vertical bars in green and the difference between observed and calculated
patterns are shown by red line.
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Table 1. Structural and crystallographic parameters of Pd/Y-MOF.
Unit cell dimensions
Pd/Y-MOF formula
H C Cl N O Pd Y
40 36 6 6 23 3 2
a (Å)
b (Å)
18.0492(1)
20.0984(5)
Formula weight
Crystal system
Space group
1634.6
Orthorhombic
Pbca (No. 61)
c (Å)
Z
28.2613(1)
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Figure 2. The Pd and Y coordination environments.
As illustrated in Figure 2, each of the three crystallographically non-equivalent Pd atoms
constructs a square planar coordination mode connecting two N-atoms in a bpydc molecule and
two Cl-atoms in the remaining cis-positions with the average bond distances of dPd-N = 2.02(1) Å
and d(Pd-Cl) = 2.30(2), respectively. The N-Pd-N angle in Pd/Y-MOF severely contracts from
normal 90º to around 80.3º. Correspondingly, the Cl-Pd-N angle expands from normal 90º to
about 95.6º, indicating a typical mode of cis-chelating bipyridine. Raman spectra (Figure S2)
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display two peaks for Pd-Cl stretching at 350 and 309 cm indicative of the cis-geometry for
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[PdLCl ] complex.
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Table 2. The selected bonding distances and angles of Pd- and Y-coordination atoms in Pd/Y-
MOF.
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Bonding distance (Å)
Bonding angle (degree)
Pd1-N1
Pd1-N2
Pd1-Cl1
Pd1-Cl2
Pd1-Cl3
Pd2-N3
Pd2-N4
Pd2-Cl3
Pd2-Cl4
Pd3-N5
Pd3-N6
Pd3-Cl5
Pd3-Cl6
2.0171(1)
2.0111(1)
2.3005(1)
2.3061(2)
2.3005(1)
2.0314(1)
2.0148(1)
2.2964(2)
2.3066(1)
2.0238(1)
2.0194(1)
2.3096(1)
2.3014(1)
Y1-O1
Y1-O3
2.2429(2)
2.4907(1)
2.4347(1)
2.3365(1)
2.3044(1)
2.3382(1)
2.395(1)
N1- Pd1-N2
N1- Pd1-Cl1
N1- Pd1-Cl2
N2- Pd1-Cl1
N2- Pd1-Cl2
Cl1- Pd1-Cl2
Cl3- Pd2-Cl4
N3- Pd2-Cl3
N3- Pd2-Cl4
N4-Pd2-N3
N4- Pd2-Cl3
N4- Pd2-Cl4
N5- Pd3-Cl5
N5- Pd3-Cl6
N6- Pd3-N5
N6- Pd3-Cl5
N6- Pd3-Cl6
Cl5-Pd3-Cl6
80.675(4)
95.536(4)
176.238(5)
175.079(6)
95.574(4)
88.225(4)
88.562(4)
95.921(4)
174.836(6)
80.056(4)
175.498(5)
95.370(4)
174.879(5)
174.879(5)
80.140(4)
95.792(4)
175.461(6)
88.383(4)
Y1-O4
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Y1-O7
Y1-O11
Y1-OW1
Y1-OW2
Y1-OW3
Y2-O2
2.393(1)
2.2176(1)
2.3314(1)
2.4424(1)
2.4694(1)
2.2565(2)
2.4391(1)
2.376(1)
Y2-O5
Y2-O9
Y2-O10
Y2-O12
Y2-OW4
Y2-OW5
Y2-OW6
2.3622(1)
Two crystallographically independent Y atoms with similar coordination environment form
into Y dimers with d(Y1-Y2) = 5.439(3) Å. Each Y atom coordinates with eight O-atoms, five of
which are from the carboxyl in four different bpydc in (bpydc)PdCl primary units with d(Y-O)
2
around 2.24-2.49 Å. Except one bridged chelating bidentate coordination, the other carboxylic
groups are coordinated to Y ion in monodentate mode. The crystallographically non-equivalent Y
unit also contains three coordinated water molecules with d(Y-OW) around 2.33-2.44 Å. These
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Y dimers are located separately with distance around 11 Å in [010] direction, creating cages
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containing residual free water molecules. Calculation by PLATON program
gives 16.8%
accessible void space to the solvent molecules. This value is low comparetive to most MOFs
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materials. However, S. Bordigaet al have demostrated that the internal void of the Pt/Y-organic
framework is accessible to water, methanol, ethanol, and acetonitrile. The main bond distances
and angles are listed in Table 2.
Figure 3. The crystal structure of (a) Pd/Y-MOF view along [100] and (b) the units of
Pd(bpydc)Cl building blocks packed in a zigzag along [100].
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The crystal structure of Pd/Y-MOF is constructed by sheets stacking along [010] direction with
the space between layers around 3.3 Å (Figure 3). These sheets are formed by packing the
primary unit of Pd(bpydc)Cl in a zigzag along [100] direction in which each sheet rotates by 47º.
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The Pd (II) ions are uniformly distributed along [100] direction with the distance between two
neighboring Pd atoms around 9.6051(7) Å. Every two Y dimers connects three successive sheets
of Pd(bpydc)Cl through coordination of Y ions with O atoms of carboxylic groups in bpydc and
2
water molecules, leading to the formation of well-defined MOF framework.
Figure 4 shows the TG-DTA curves and in situ XRD patterns of Pd/Y-MOF in air atmosphere.
The first weight-loss of 4.9% related to the endothermic process around 150 ºC can be assigned
to the removal of adsorbed water or organic solvents. Correspondingly, no significant structure
transformation is observed in XRD pattern and Pd/Y-MOF well retained its original structure up
to 500 ºC. Around 500 ºC, a strong exothermic peak occurs, corresponding to the weight loss of
6
5% due to the combustion of bpydc ligand. This amount is in good accordance with the
calculated value of bpydc (67%). The XRD patterns also reveal the formation of Y O (PDF 41-
2
3
1
5
105) and metallic Pd (PDF 46-1043) phases after 500 ºC. Further increase temperature above
00 ºC, PdO (PDF 43-1024) appears.
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-
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Temperature ( C )
Figure 4. TG-DTA curves (a) and in situ variable-temperature XRD patterns (b) of Pd/Y-MOF.
The XPS spectra of Pd/Y-MOF, Pd(bpydc)Cl and Y(CH COO) are illustrated in Figure 5. All
2
3
3
the Pd species in Pd/Y-MOF are present in +2 oxidation state, corresponding to binding energy
3
4
(
BE) of 337.7 and 342.9 eV of Pd3d5/2 and Pd3d3/2 level, respectively.
Compared with
Pd(bpydc)Cl , the Pd (II) BE in Pd/Y-MOF shifts negatively by 0.2 eV, suggesting that the N
2
atom donates partial electrons to Pd (II) and a stronger coordination of Pd (II) with the N atoms
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on pyridine ring in Pd/Y-MOF than that in Pd(bpydc)Cl . A possible reason is the formation of
2
Y-O coordination bond in which the O of carboxyl group gives partial electrons to Y(III). The
decreasing electron density around O atoms result in the increasing of electron density around the
0
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N atom in bipyridine ring, leading to the N atom further donate electrons to Pd (II). This is
confirmed by the positive BE shift of O and the negative shift of N in Pd/Y-MOF comparing to
those in Pd(bpydc)Cl . All Y-O bonding lengths in Pd/Y-MOF are generally shorter than that in
2
Y(CH COO) (2.33 Å, CSD CIF CEBFUL01), implying the stronger Y-O interaction in Pd/Y-
3
3
MOF (Table 2). The negative BE shift of Cl can also be explained in the similar way. Meanwhile,
FT-IR spectra (Figure S1) shows that in comparison with the free bpydc, the Pd/Y-MOF has no
-1
significant absorption bands of ν(C=O) at 1649 and 1258 and the bands at 2510-3020 cm ,
which indicates a complete deprotonation of carboxyl acid molecules and also confirms the
3
5
coordination between Y(III) ion and bpydc ligand. The blue shift of the absorbance bands in
-1
Raman spectra at 1446 and 1329 cm characterized by bipyridine molecule further provides the
evidence of the coordination between Pd (II) ion and bpydc through the N atom in bipyridine
3
3
ring.
337.9
Pd 3d5/2
O1s
Pd3d5/2
C1s
Pd(BPYDC)Cl2
Pd/Y-MOF
Pd3d3/2
Pd3p1/2
Pd 3d3/2343.1
N1s
Cl2p3
Y3d5
Cl2s
Pd(bpydc)Cl2
Pd/Y-MOF
100 200
334
336
338
340
342
344
346
0
300
400
500
600
700
Bind Energy (eV)
Bind Energy (eV)
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Y3d5/2 158.2
O1s 531.3
Y3d3/2
160.4
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Y(CH COO)3
Pd(BPYDC)Cl2
Pd/Y-MOF
Pd/Y-MOF
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154
156
158
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164
166
168
526
528
530
532
534
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538
540
542
Bind Energy (eV)
Bind Energy (eV)
Cl2p3/2197.5
Cl2p3/2199.1
Pd/Y-MOF
N1s 399.9
Pd/Y-MOF
Pd(BPYDC)Cl2
Pd(BPYDC)Cl2
392
394
396
398
400
402
404
406
408
194
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198
200
202
204
206
Bind Energy (eV)
Bind Energy (eV)
Figure 5. XPS spectra of Pd/Y-MOF, Pd(bpydc)Cl and Y(CH COO) .
2
3
3
2
. Catalysts Performance. A series of aryl halides and 4-methylphenylboronic acid are employed
as reactants in Suzuki-Miyaura coupling reaction to evaluate the catalytic performance of Pd/Y-
MOF. As shown in Table 3, the catalytic activity decreases from iodobenzene to bromobenzene
to chlorobenzene due to the enhanced difficulty for activating the C-halide bond. Suzuki-Miyaura
coupling reaction between chlorobenzene and 4-methylphenylboronic acid displays very low
conversion (22%) even after the reaction for 12 h (4 h more than other reactions) in the presence
of tetrabutylammonium bromide (TBAB). The presence of CH - and CH O-groups on the
3
3
iodobenzene slightly suppresses the reaction activity. Similar result is also observed by using
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Pd(OAc) homogeneous catalyst, suggesting that the electron-donating substitutes may stabilize
2
the C-halides bond against activation, leading to the poor activity. On the other hand, the
presence of electron-withdrawing substitute group (-NO ) on the iodobenzene greatly promotes
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the reaction and the conversion reaches 96% even after reaction for 6 h. It is also found that
Pd/Y-MOF shows much higher activity than the Pd(bpydc)Cl solid catalyst, possibly owing to
2
the exposed Pd (II) sites in the layered structure constructed by linking the Pd(bpydc)Cl building
2
blocks through Y(III) coordinating with carboxylic groups (see Figure 3). Meanwhile, as above
mentioned, the coodination of Y(III) and carboxylic groups results in N further donating electron
to Pd (II) (Figure 5). The corresponding increase in electron density on Pd (II) may facilitate the
initial adsorption and insertion of aryl halides molecules to Pd active sites according to common
3
6-38
mechanism of Pd-catalyzed carbon-carbon coupling reaction.
Table 3. Suzuki-Miyaura coupling reactions between aryl halides with 4-methylphenylboronic
a
acid.
X
B(OH)2
Pd/Y-MOF cat.
+
R
CH3
base, H2O
R
H C
3
Reaction
time (h)
Catalyst
Aryl halide
Product
Yields (%)
96
Pd/Y-MOF
Pd/Y-MOF
Pd/Y-MOF
I
8
CH3
CH3
CH3
Br
Cl
8
59
22
12
H C
CH3
3
Pd/Y-MOF
H C
3
I
8
86
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Pd/Y-MOF
Pd/Y-MOF
Pd/Y-MOF
H CO
I
I
8
6
8
8
H3CO
CH3
88
96
5.5
46
3
O N
2
CH3
O N
2
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I
CH3
I
Ph
Pd(bpydc)Cl2
Pd(OAc)2
I
I
8
8
CH3
65
96
CH3
H C
CH3
3
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
H C
3
I
8
8
8
92
79
95
I
CH3
I
Ph
[
a] Reaction conditions: aryl halide (0.50 mmol), 4-methylphenylboronic acid (0.75 mmol),
K CO (1.5 mmol), H O (4.0 mL), and a catalyst containing Pd (0.054 mmol) at 80 ºC. [b]
2
3
2
TBAB (0.30 mmol) is added.
Pd/Y-MOF shows the same catalytic activity as the Pd(OAc) homogeneous catalyst in Suzuki-
2
Miyaura coupling reaction between 4-methylphenylboronic acid and iodobenzene, obviously
owing to the extremely high dispersed Pd (II) active sites and the low diffusion limit. The activity
difference between Pd/Y-MOF and Pd(OAc) presents in bulky aryl halide substrate, which can
2
be attributed to the enhanced diffusion limit of reactant molecule in the micropore channel. It is
noted that using iodobenzene and 1-iodonaphthalene as reactant, Pd/Y-MOF catalyst affords the
discrepant conversion up to 50%, which is comparative to that of NaX zeolite-supported Pd (II)
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catalyst, taking into account that the micropore size of the Pd/Y-MOF framework is only about
3
.3 Å (see Figure 3). When 4-(tert-butyl) iodobenzene bearing a very large t-bu group is used in
the reaction, the Pd/Y-MOF exhibits extremely low activity due to both the inhibition of reactant
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molecules into micropore channels and electron effect. In contrast, the Pd(OAc) homogeneous
2
catalyst still shows relatively high activity for both small and bulk substrates owing to the
absence of diffusion limit.
40
30
20
1
4
-iodonaphthalene
-(tert-butyl)iodobenzene
iodobenzene
10
0
0
10
20
30
40
50
60
70
80
90
Time (min)
Figure 6. Adsorption test of Pd/Y-MOF catalyst for iodobenzene, 1-iodonaphthalene and 4-(tert-
butyl) iodobenzene.
We have attempted to evaluate the microporosity and surface area of the Pd/Y-MOF material by
the classical N adsorption at 77 K. The obtained adsorption capacity is as low as Pt/Y-MOF
2
25
reported by S.Bordiga, which is ascribed to nonpolar N can only perform weak intramolecular
2
interaction with the lattice and can not create the swelling of the pores that the molecular
penetrate inside the MOF needed. As a consequence, the surface area measured via N adsorption
2
only probes the external surface. To understand the great difference in Suzuki-Miyaura coupling
reaction efficiencies with iodobenzene, 1-iodonaphthalene and 4-(tert-butyl) iodobenzene as
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substrates, their adsorption performances on the Pd/Y-MOF catalyst are examined, respectively.
As shown in Figure 6, the Pd/Y-MOF catalyst displays more rapid adsorption to iodobenzene
than to 1-iodonaphthalene and 4-(tert-butyl) iodobenzene, obviously owing to the lower diffusion
limit. Meanwhile, Pd/Y-MOF catalyst exhibits much higher adsorption capacity for iodobenzene
than for 1-iodonaphthalene and 4-(tert-butyl) iodobenzene, because the large molecule size of
absorbates needs more space of the micrpores of Pd/Y-MOF catalyst and also blocks the pore
entrance, leading to the rapid stop of the adsorption for 1-iodonaphthalene and 4-(tert-butyl)
iodobenzene. These results clearly demonstrate that the diffusion of the reactant molecule in the
micropores of the Pd/Y-MOF catalyst plays a crucial role in determining the catalytic activity and
selectivity. In other words, the reactants and products in these reactions have to be sufficiently
small so that the reaction can take place inside the micropores of Pd/Y-MOF (diameter ≈ 3.3 Å).
Similar results are also observed in the Sonogashira coupling reaction. As shown in Table 4,
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23
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44
45
46
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49
50
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Pd/Y-MOF catalyst also shows much higher activity than that of the Pd(bpydc)Cl owing to the
2
cooperative effect between Y(III) and Pd (II) in Pd/Y-MOF network. Meanwhile, a similar trend
is observed for the decreased activity in sequence of -H, -CH O, -CH and -C(CH ) on
3
3
3 3
iodobenzene, which further confirms that enhanced diffusion limit of the reactant with bulk
molecule and the electronic effect. The presence of electron-withdrawing group (-NO ) on
2
iodobenzene promoted the reaction activity due to the activation of C-I bond. It is interesting to
find that CH O-substituted group on iodobenzene and phenylacetylene plays a contrary role in
3
the Sonogashira coupling reaction catalyzed by Pd/Y-MOF. The presence of CH O-group on the
3
phenylacetylene promotes rather than hamperes reaction possibly owing to the activation of the
CC triple bond by an electron-donating group.
Table 4. Sonogashira reactions of aryl iodide with arylacetylene.
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I
Pd/Y-MOF cat.
+
R
X
base, H O
2
R
X
Yields
%)
Recycling
number
Reaction
time (h)
Catalyst
R
X
Product
(
0
1
2
3
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0
1
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1
2
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4
5
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9
0
Pd/Y-MOF
Pd/Y-MOF
Pd/Y-MOF
Pd/Y-MOF
Pd/Y-MOF
1
2
3
1
1
H
H
H
H
H
H
H
6
6
6
6
6
92
90
87
82
84
H
H C
CH3
3
H3CO
CH O
3
O N
Pd/Y-MOF
1
NO2
H
4
2
99
Pd/Y-MOF
Pd/Y-MOF
Pd/Y-MOF
1
1
1
1
H
H
CH O
6
6
6
6
OCH3
F
98
79
7.5
61
3
F
H
H
t-bu
H
Pd(bpydc)Cl2
Reaction conditions: iodobenzene or substituted iodobenzene (0.50 mmol), phenylacetylene or
substituted phenylacetylene (0.60 mmol), 1, 8-Diazabicyclo[5.4.0]undec-7-ene (DBU, 0.10 mL),
H O (4.0 mL), n-decane (0.10 mL), and a catalyst containing Pd ( 0.054 mmol) at 90 ºC.
2
3
. Heterogeneity Test. As shown in Figure 7, Pd/Y-MOF catalyst can be used repetitively in
Suzuki-Miyaura coupling reaction between iodobenzene and 4-methylphenylboronic acid for
more than 5 times without significant decrease in activity. Similarly, it also shows good
durability in the Sonogashira coupling reaction between iodobenzene and phenylacetylene. The
XRD patterns (Figure S3) reveal that the Pd/Y-MOF catalyst retains its original crystalline
structure well after being used repetitively for 3 times. The change of the relative intensities of
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some diffraction peaks may result from the micropores blockage from the solvent, showing
excellent hydrothermal stability of the MOF framework. The stabilizing effect of the Y(III)
coordinated with carboxyl of bpydc is essential to reserve the chemical microenvironment of the
Pd (II) active sites. Meanwhile, the ICP analysis demonstrates that the leaching of Pd (II) species
from the Pd/Y-MOF catalyst is neglected during the successive reactions because the Pd (II) ions
are incorporated into the MOF framework via coordination. The slight decrease in the activity in
each subsequent reaction can be attributed to the loss of trace Pd/Y-MOF catalyst during its
separation and washing process from the reaction system.
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100
80
60
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1
2
3
4
5
6
Recycling number
Figure 7. Durability test of Pd/Y-MOF catalyst in Suzuki-Miyaura coupling of iodobenzene and
-methylphenylboronic acid. Reaction conditions are given in Table 3.
4
The nature of the active Pd species (homogeneous vs heterogeneous) in C–C coupling reaction
2
8,40,41
has been a matter of much debate.
To make sure whether the Pd (II) active sites
incorporated into the Pd/Y-MOF framework or the homogeneous Pd (II) species leached from the
solid catalyst during reaction are truely catalyst, a series of tests including hot filtration, solid-
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phase poisoning test
and three-phase test.
are carried out (The detailed procedure see
Support Information). The present Pd/Y-MOF catalyst seems to catalyze the coupling reactions
in a truly heterogeneous manner. This conclusion is based on the following observations. 1)
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During the reaction, we remove the Pd/Y-MOF by microfiltration and allowed the reaction to
continue. The conversion stops increasing immediately (see Figure 8 and Figure S4). If a trace
amount of soluble Pd species are the active species, the catalytic conversion should increase
further, even after the removal of Pd/Y-MOF. 2) The coupling reaction is tested in the presence
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of poly(4-vinylpyridine), which is well known as a solid poison to trap homogeneous Pd
species in the solution phase through chelation. A comparison of conversion in C-C coupling
reactions clearly demonstrates that the catalytic efficacy of Pd/Y-MOF is not affected when the
PVPy is added to the reaction mixture (see Figure 8 and Figure S4), suggesting the reaction is
only catalyzed by the solid Pd/Y-MOF. 3) The three-phase test is considered a definitive test for
answering the homogeneity/ heterogeneity question. If leaching of active Pd species occurs, the
supported reactant will undergo the same chemical reaction as the reactant present in the liquid
medium. However, in the current case, the only product obtained in this process is derived from
the reactant (iodobenzene) in the liquid phase. The supported reactant, 4-iodoacetophenone,
which is imersed in the reaction solution but not directly contact with the solid catalyst remains
unreacted, demonstrating that both the Suzuki-Miyaura and the Sonogashira coupling reaction
are heterogeneous. 4) Microporous Pd/Y-MOF catalyst exhibits much higher activity for small-
sized substrates than the bulky molecular species. If the reactions are catalyzed by dissolved Pd
species, such size-selectivity will be very difficult to explain.
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Reaction time (h)
Figure 8. Plot of iodobenzene conversion of Suzuki-Miyaura coupling reactions, reaction
conditions are given in Table 3. (a) Pd/Y-MOF activity without any poison; (b) adding PVPy as a
poison at start of reaction; (c) removal of Pd/Y-MOF catalyst after 2 h reaction.
XPS analysis in Figure 9 shows almost identical patterns of Pd (II) in Pd/Y-MOF before and after
the Suzuki coupling reaction. No peaks corresponding to Pd (0) in the metallic state is detected.
The slight negative shifts and broadening of the Pd BE is ascribed to the oxidative addition of
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Figure 9. XPS spectra of Pd/Y-MOF before and after Suzuki-Miyaura coupling reaction
There have been numerous works indicating that the coupling reactions occurred on the surface
32,39,49,50
of the heterogeneous catalysts.
In cases where Pd (II) is used, in situ reduction to Pd (0)
is known to occur during the reaction and the Pd (0) is catalytically active species. Based on the
above heterogeneity studies, a plausible mechanism is proposed that the Suzuki-Miyaura and
Sonogashira coupling reaction proceed in heterogeneous phase. The Pd (II) of Pd/Y-MOF in the
coupling reaction seems to undergo a reversible process, in which Pd (II) is reduced to Pd (0) and
immediately oxidized back to the initial Pd (II) state in situ when the reaction is carried out under
the air atmosphere. The coordination of Pd-N and the stable Pd/Y-MOF frameworks prevent the
formation of Pd (0) nanoparticles and the agglomeration of Pd (0) species into large Pd
nanoparticles or Pd black.
CONCLUSIONS
We have synthesized a novel heterobimetallic Pd/Y-MOF catalyst through a facile microwave
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irradiation method. The 3D extended framework is constructed by Y(III) coordinating to
carboxylic groups on the Pd(bpydc)Cl building blocks. For the small-sized reactant molecules,
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the catalytic activity of Pd/Y-MOF is comparable to Pd(OAc) homogeneous catalyst owing to
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the high dispersion and accessibility of Pd (II) active sites in the layered structure and the
cooperative effect of Y(III) coordination. Besides the high activity and size selectivity, Pd/Y-
MOF can be easily recycled and reused owing to the excellent stability of MOF framework
derived from the strong interaction of Y(III) and carboxyl. The coordination of Pd (II) and N
atoms on bpydc prevented the leaching of Pd (II) active species under reaction conditions. Hot
filtration test, solid-phase poisoning test, three-phase test and XPS analysis demonstrate that the
catalytic nature of Pd/Y-MOF is heterogeneous. These studies indicate that the strategy can be
served as a general method for the development of MOFs-based heterogeneous catalysts by
fabrication of homogeneous counterparts into MOFs frameworks for practical catalytic
applications.
ASSOCIATED CONTENT
Supporting Information
The detailed Rietveld structure refinement, Raman and FT-IR spectra of the Pd/Y-MOF, the
reaction process of Suzuki-Miyaura and Sonogashira, and the XRD patterns of the Pd/Y-MOF
catalyst after being used repetitively in Sonogashira coupling reaction. This information is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
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Email: Hexing-Li@shnu.edu.cn; wwang@unm.edu
ACKNOWLEDGMENTS
This work is supported by the National Natural Science Foundation of China (20825724) and
Shanghai Government (10dj1400100).
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