Pd(II)-NHDC-Functionalized UiO-67 Type MOF for Catalyzing Heck Cross-Coupling and Intermolecular Benzyne-Benzyne-Alkene Insertion Reactions

Article abstract of DOI:10.1021/acs.inorgchem.7b03271

A novel palladium N-heterocyclic bis-carbene dicarboxylate ligand (Pd-NHDC-H₂L) was successfully synthesized. In addition, an Pd-NHDC-containing UiO-67 type MOF (UiO-67-Pd-NHDC) was prepared on the basis of a size-matched ligand mixture of biphenyl-4,4′-dicarboxylic acid/Pd-NHDC-H₂L (9/1) and ZrCl₄ under solvothermal conditions. The obtained UiO-67-Pd-NHC MOF can be a highly heterogeneous catalyst to promote Heck cross-coupling and intermolecular benzyne-benzyne-alkene insertion reactions.

Full text of DOI:10.1021/acs.inorgchem.7b03271

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Pd(II)-NHDC-Functionalized UiO-67 Type MOF for Catalyzing Heck
Cross-Coupling and Intermolecular Benzyne−Benzyne−Alkene
Insertion Reactions
Yong-Liang Wei, Yue Li, Yun-Qi Chen, Ying Dong,* Jia-Jia Yao, Xin-Yue Han, and Yu-Bin Dong*
College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for
Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Shandong
Normal University, Jinan 250014, People’s Republic of China
S
* Supporting Information
ABSTRACT: A novel palladium N-heterocyclic bis-carbene dicarboxylate ligand
(Pd-NHDC-H2L) was successfully synthesized. In addition, an Pd-NHDC-
containing UiO-67 type MOF (UiO-67-Pd-NHDC) was prepared on the basis
of a size-matched ligand mixture of biphenyl-4,4′-dicarboxylic acid/Pd-NHDC-
H2L (9/1) and ZrCl4 under solvothermal conditions. The obtained UiO-67-
Pd-NHC MOF can be a highly heterogeneous catalyst to promote Heck cross-
coupling and intermolecular benzyne−benzyne−alkene insertion reactions.
UiO-67 type MOF (UiO-67-Pd-NHDC) which was generated
from the presynthesized palladium N-heterocyclic bis-carbene
dicarboxylate ligand Pd-NHDC-H2L, size-matched biphenyl-
4,4′-dicarboxylic acid, and ZrCl4 under solvothermal conditions.
The obtained UiO-67-Pd-NHDC NMOF can be a highly active
heterogeneous catalyst to promote Heck cross-coupling and
benzyne−benzyne−alkene insertion reactions.
INTRODUCTION
■
N-heterocyclic carbene (NHC) transition-metal complexes, espe-
cially Pd-NHC, have been widely used in catalyzing many types of
chemical reactions, including C−C coupling reactions, metathesis,
carbon−heteroatom cross-coupling, and atom-economical trans-
formations.1 However, the recycling and reuse of M-NHC cata-
lysts remains a demanding challenge, since the complexes are
typically of single use in a catalytic reaction.1
Heterogeneous catalysis was one of the earliest proposed and
demonstrated applications for metal−organic framework (MOF)
materials because MOFs themselves are inherent heterogeneous
catalysts due to their organic−inorganic hybrid composition and
polymeric nature.2 On the other hand, MOFs are still attractive
platforms for immobilizing catalytic species such as an M-NHC
moiety via a postsynthetic and in situ approach to generate new
heterogeneous catalytic systems.3 In comparison to various other
types of support-based M-NHC-loaded heterogeneous catalytic
systems, M-NHC-loaded MOF-based catalytic materials have
been much less explored.4
In principle, catalytic moieties, such as M-NHC, can be embed-
ded in MOF frameworks via a ligand premodification approach.5
That is, the MOFs can be directly prepared by the combination
of M-NHC-functionalized organic spacers and metal ions. In this
way, the catalytically active NHC-metal moiety could be logically
incorporated into MOFs. The embedding active catalytic spe-
cies in MOFs such as this would effectively avoid active catalytic
species leaching and aggregation during the catalytic process.5
In this contribution we report, as the first of its kind, a pal-
ladium N-heterocyclic bis-carbene (Pd-NHDC) moiety doped
EXPERIMENTAL SECTION
■
Materials and Instrumentation. All chemicals and solvents were
at least of analytical grade and were employed as received without fur-
ther purification. The elemental analysis was conducted on a PerkinElmer
1
Model 2400 analyzer. Solution-phase H NMR spectra were obtained
on an AM-400 spectrometer. Chemical shifts are reported in units
relative to TMS. Infrared spectra were obtained in the 400−4000 cm−1
range using a Bruker ALPHA FT-IR spectrometer. Powder X-ray diffrac-
tion (PXRD) measurements were performed at 293 K on a D8
ADVANCE diffractometer (Cu Kα, λ = 1.5406 Å). XPS spectra were
obtained from a PHI Versaprobe II instrument. Thermogravimetric
analyses were carried out on a TA Instrument Q5 simultaneous TGA
apparatus under flowing nitrogen at a heating rate of 10 °C/min. The
scanning electron microscopy (SEM) micrographs were recorded on a
Gemini Zeiss Supra TM scanning electron microscope equipped with
an energy-dispersive X-ray detector (EDS). High-resolution ESI data
were performed using a Bruker Daltonics Inc. instrument. GC analysis
was performed on a 7890B gas chromatograph (Agilent Technologies,
CA, USA) equipped with a capillary column (DB-1701, 30 m × 0.25 mm)
Received: December 29, 2017
© XXXX American Chemical Society
A
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using a flame ionization detector (FID). ICP measurement was per-
formed on an IRIS Intrepid (II) XSP and NU AttoM apparatus.
Synthesis of Pd-NHDC-H2L. Synthesis of B. Compound A (1.49 g,
5 mmol), azodiisobutyronitrile (AIBN, 0.16 g, 0.5 mmol), and
N-bromosuccinimide (NBS, 1.78 g, 10 mmol) were stirred in CCl4
(50 mL) at 80 °C for 5 h (monitored by TLC). The product was puri-
fied by column chromatography (with 1% EtOAc/petroleum ether fol-
lowed by 5% EtOAc/petroleum ether as eluent) and recrystallized in
was sonicated for about 15 min and then placed in a preheated 100 °C
oven for 36 h. The solid was filtered and rinsed with DMF (10 mL)
overnight. After that, the as-synthesized sample was soaked in chlo-
roform for 2 days at room temperature. During this period, chloroform
was exchanged every 12 h. Finally, the sample was collected by centri-
fugation and dried under vacuum at room temperature to give the
activated samples. Light yellow crystalline solids of UiO-67-
Pd-NHDC were isolated in 49% yield. IR (KBr, cm−1): 3383 (s),
1657 (vs), 1594 (m), 1498 (w), 1416 (vs), 771 (w). Anal. Calcd for
[Zr6O4(OH)4C86.08H62.496Br0.416N0.832O24Pd0.208]: C, 46.42; H, 3.01;
Br, 1.49; N, 0.52. Found: C, 46.16; H, 2.94; Br, 1.60; N, 0.63. The
elemental analysis was very consistent with the ICP measurement
(Supporting Information).
General Catalytic Reaction Procedure for Heck Cross-
Coupling Reaction between Iodobenzene and Ethyl Acrylate.
A mixture of iodobenzene (0.5 mmol, 56 μL), ethyl acrylate (0.75 mmol,
82 μL), NEt3 (1.0 mmol, 140 μL), TBAB (0.5 mmol, 161 mg), and
UiO-67-Pd-NHDC (0.5 mol % Pd) in 2 mL of DMF was stirred at
120 °C for 1 h in an air atmosphere (monitored by TLC). After the
reaction was complete, the mixture was filtered, diluted with brine
(10 mL), and extracted with ethyl acetate (3 × 10 mL). The organic
phase was dried over anhydrous MgSO4 and concentrated under vacuum.
The residue was purified by column chromatography on silica gel using
hexane as eluent to give the product as a colorless oil (86.8 mg, 98%).
General Catalytic Reaction Procedure for Benzyne and Allyl
Bromide. 2-(Trimethylsilyl)phenyl triflate (1.6 mmol, 392 μL) was
added to a suspension of allyl bromide (0.40 mmol, 35.2 μL), UiO-67-
Pd-NHDC (5 mol % Pd), and CsF (3.2 mmol, 485 mg) in 4 mL of
CH3CN under an Ar atmosphere. The mixture was stirred at 80 °C for
6 h (monitored by TLC). After the reaction was complete, the mixture
was filtered and the filtrate was concentrated under reduced pressure.
The residue was purified by column chromatography on silica gel using
petroleum ether as eluent to give the product as white crystalline solids
(57.9 mg, 75%).
1
hexane to give compound B (1.39 g, 61%) as a white solid. H NMR
(400 MHz, CDCl3; δ (ppm)): 8.24 (s, 2H), 8.06 (d, J = 8.0 Hz, 2H),
7.36 (d, J = 8.0 Hz, 2H), 4.33 (d, J = 10.4 Hz, 2H), 4.17 (d, J =
10.4 Hz, 2H), 3.97 (s, 6H).13C NMR (100 MHz, CDCl3; δ (ppm)):
166.2, 143.0, 136.2, 132.0, 131.0, 130.1, 129.4, 52.5, 30.6. IR (KBr, cm−1):
2954 (w), 1724 (vs), 1436 (m), 1293 (s), 1217 (s), 993 (w), 768 (m),
617 (m). Anal. Calcd for C18H16Br2O4: C, 47.40; H, 3.54; Br, 35.04.
+
Found: C, 47.74; H, 3.50; Br, 35.46. ESI-MS: calcd for C18H17Br2O4
[M + H]+ m/z 456.9473, found m/z 456.9468.
Synthesis of C. N-Methylimidazole (NMI, 0.34 mL, 4.2 mmol) was
added dropwise to a CH3CN (15 mL) solution of B (0.91 g, 2 mmol).
The mixture was stirred at 80 °C for an additional 4 h. After removal
of the solvent under vacuum, the solid was washed with Et2O (3 × 10 mL)
and dried in vacuo to give compound C (1.19 g, 96%) as a white powder.
1H NMR (400 MHz, DMSO-d6; δ (ppm)): 8.95 (s, 2H), 8.04 (s, 2H),
7.99 (d, J = 8.0 Hz, 2H), 7.72 (s, 2H), 7.52 (s, 2H), 7.23 (d, J =
8.0 Hz, 2H), 5.44 (d, J = 16.0 Hz, 2H), 5.25 (d, J = 16.0 Hz, 2H), 3.90
(s, 6H), 3.80 (s, 6H). 13C NMR (100 MHz, DMSO-d6; δ (ppm)):
166.0, 142.9, 137.0, 133.2, 131.1, 130.6 (2C), 130.1, 124.2, 122.9, 53.0,
50.5, 36.3. IR (KBr, cm−1): 3447 (s), 3101 (m), 2057 (w), 1710 (vs),
1574 (w), 1439 (w), 1303 (s), 1160 (m), 972 (w), 760 (m), 616 (w).
Anal. Calcd for C26H28Br2N4O4: C, 50.34; H, 4.55; Br, 25.76; N, 9.03.
Found: C, 50.60; H, 4.59; Br, 25.40; N, 9.38. ESI-MS: calcd for
2+
C26H28N4O4 [M/2]+ m/z 230.1084, found m/z 230.1056.
Synthesis of Pd-NHDC-H2L. A mixture of C (0.62 g, 1 mmol) and
Pd(OAc)2 (0.22 g, 1 mmol) in DMSO (1 mL) was stirred at 60 °C for
6 h and at 130 °C for 4 h. Then the mixture was added dropwise to a
mixture of Et2O and CH2Cl2 (4/1 v/v) under vigorous stirring. The
resulting precipitate was filtered, washed with Et2O (5 mL), and dried
at room temperature to give Pd-NHDC-Me2L as slightly yellow crys-
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Pd-NHDC-H2L and
UiO-67-Pd-NHDC. As indicated in Scheme 1, the Pd-NHDC
decorated ligand Pd-NHDC-H2L was prepared on the basis of
Pd(OAc)2 and imidazolium double-attached biphenyl dimethyl
dicarboxylate in 69% yield. In comparison to intermediate C,
the absence of the signal for the imidazolium methine protons
(at 8.96 ppm) in the 1H NMR spectra of Pd-NHDC-Me2L and
Pd-NHDC-H2L indicated that the corresponding carbene spe-
cies was successfully generated from C.1f,g In addition, the disap-
pearance of carboxylmethyl protons (at 3.90 ppm) and the appear-
ance of carboxyl protons (at 13.10 ppm) in the 1H NMR spectrum
of Pd-NHDC-H2L after hydrolysis unambiguously affirmed the
formation of the dicarboxylate Pd-NHDC ligand (Figure 1).1f,g
The ESI-MS of Pd-NHDC-Me2L and Pd-NHDC-H2L showed
peaks at m/z 724.9329 and 694.8792 due to mononuclear Pd bis-
carbene [C26H27N4Br2O4Pd + H+] and [C24H21N4Br2O4Pd + H+]
species. In addition, a single downfield signal in the 13C NMR spec-
trum of Pd-NHDC-Me2L and Pd-NHDC-H2L at 170.9 ppm
also confirmed the formation of the corresponding carbene
carbons (Supporting Information).1f,g
1
talline solids (0.5 g, 69%). H NMR (400 Hz, DMSO-d6): δ (ppm)
7.91 (d, J = 8.0 Hz, 2H), 7.78 (s, 2H), 7.74 (s, 2H), 7.40 (d, J =
8.0 Hz, 2H), 7.30 (s, 2H), 5.77 (d, J = 16.0 Hz, 2H), 5.25 (d, J =
16.0 Hz, 2H), 3.84 (s, 6H), 3.75 (s, 6H). 13C NMR (100 MHz,
DMSO-d6): δ (ppm) 170.9, 166.3, 142.8, 137.6, 131.7, 130.5, 129.7,
127.6, 123.9, 122.5, 52.6, 50.2, 37.3. IR (KBr, cm−1): 3420 (s), 1717
(vs), 1436 (s), 1297 (s), 1114 (m), 768 (w). Anal. Calcd for
C26H26Br2N4O4Pd: C, 43.09; H, 3.62; Br, 22.05; N, 7.73. Found: C,
43.22; H, 3.94; Br, 21.57; N, 7.37. ESI-MS: calcd for
C26H27N4Br2O4Pd+ [M + H]+ m/z 724.9431, found 724.9329.
A mixture of Pd-NHDC-Me2L (0.44 g, 0.6 mmol) and LiOH·H2O
(0.10 g, 2.4 mmol) in THF (3.0 mL) and H2O (3.0 mL) was stirred at
room temperature for 3 h. After the removal of THF, the resulting
aqueous solution was acidified with diluted aqueous HBr solution
(pH 2−3). The resulting yellow precipitate was washed with H2O
(3 × 5 mL) and MeOH (2 × 5 mL) and dried at room temperature to
provide Pd-NHDC-H2L as yellow crystalline solids (0.36 g, 87%).
1H NMR (400 MHz, DMSO-d6; δ (ppm)): 13.11 (s, 2H), 7.88 (d, J =
4.0 Hz, 2H), 7.75 (d, J = 12.0 Hz, 4H), 7.36 (d, J = 8.0 Hz, 2H), 7.30
(s, 2H), 5.77 (d, J = 15.2 Hz, 2H), 5.22 (d, J = 15.2 Hz, 2H), 3.75
(s, 6H). 13C NMR (100 MHz, DMSO-d6; δ (ppm)): 170.8, 167.4,
142.6, 137.4, 131.4, 130.9, 130.7, 127.7, 123.9, 122.3, 50.3, 37.6. IR
(KBr, cm−1): 3446 (s), 3131 (s), 1715 (vs), 1608 (m), 1246 (s), 1116
(w), 771 (w). Anal. Calcd for C24H22Br2N4O4Pd: C, 41.38; H, 3.18;
Br, 22.94; N, 8.02. Found: C, 41.45; H, 3.51; Br, 22.90; N, 7.89. Pd wt
% was determined using ICP: calculated 15.27%, measured 14.98%.
ESI-MS: calcd for C24H21N4Br2O4Pd+ [M − H]+ m/z 694.9022, found
m/z 694.8792.
UiO-67-Pd-NHDC MOF was prepared by the combination of
Pd-NHDC-H2L/H2L′ (1/9) and ZrCl4 under solvothermal con-
ditions as light yellow crystalline solids in 49% yield (Scheme 1).
The IR spectrum of UiO-67-Pd-NHDC showed that the stretch-
ing vibration at 1715 cm−1 for the free characteristic carboxylate
of Pd-NHDC-H2L disappeared after the hydrothermal reac-
tion, implying that the Pd bis-carbene linkers were coordinated
to the zirconium clusters in UiO-67-Pd-NHDC. Figure 2a
shows that the PXRD pattern of UiO-67-Pd-NHDC is good
agreement with that of pristine UiO-67 MOF,6 indicating that
Synthesis and Characterization of UiO-67-Pd-NHDC. ZrCl4
(70.0 mg, 0.3 mmol), Pd-NHDC-H2L (20.8 mg, 0.03 mmol),
biphenyl-4,4′-dicarboxylic acid (65.4 mg 0.27 mmol), and 30 equiv
of acetic acid (515 μL) were dispersed in DMF (10 mL). The mixture
B
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a
Scheme 1. Synthesis of Pd-NHDC-H2L Ligand and UiO-67-Pd-NHDC MOF
a
A photograph of a sample of UiO-67-Pd-NHDC is shown as an insert.
The porosity of UiO-67-Pd-NHDC was demonstrated by a
surface area analysis (Figure 2e). The N2 sorption measurement
(77 K) indicated that the porosity of the MOF is maintained
after Pd-NHDC doping. The lack of hysteresis demonstrated
that the adsorption is reversible. The Brunauer−Emmett−Teller
(BET) surface area of UiO-67-Pd-NHDC was found to be
1540 m2 g−1, which is smaller than that of UiO-67 (2113 m2 g−1).6b
This surface area decrease is clearly caused by the doped
Pd-NHDC species. The pore size, which was calculated by non-
local density functional theory (NLDET), shows narrow pore
width centered at 8.751 Å (Figure 2f), which is slightly less than
that for its parent UiO-67 (10.97 Å).6b
Heck Coupling Reaction Catalyzed by UiO-67-Pd-
NHDC. As we know, the palladium-catalyzed Heck coupling
reaction has evolved as one of the most important C−C form-
ing reactions in organic chemistry, in which Pd-NHC com-
plexes have been reported as effective types of catalysts in var-
ious C−C coupling reactions.8 For evaluation of the catalytic
activity of UiO-67-Pd-NHDC for Heck cross-coupling reac-
tions, the reaction between iodobenzene (1a) and ethyl acrylate
(2a) was chosen as the model reaction. As shown in Table 1,
the organic base NEt3 was found to be superior over K2CO3
(Table 1, entries 1 and 2). The yield of the coupling product of
3a from iodobenzene (1a) and ethyl acrylate (2a) was effec-
tively improved by the addition of tetra-n-butylammonium
bromide (TBAB) (Table 1, entries 3−6). In addition, the reac-
tions carried out in the presence of 1 or 0.5 mol % amount of
Pd species did not significantly affect the coupling yield within
1 h (yields 99 vs 98%, Table 1, entries 3 and 4). On the other
hand, the higher temperature was found to be beneficial to the
reaction. For example, when the reaction was conducted at 100 °C,
a high 93% yield was obtained but took a longer time (4 h)
(Table 1, entry 5). Notably, a 99% yield was obtained when the
reaction was performed in the presence of the metalloligand
Pd-NHDC-Me2L under the same reaction conditions, indicat-
ing that the Pd bis-carbene moiety in the MOF is responsible
for the catalytic activity but acts in a homogeneous way (Table 1,
entry 6). This Pd-NHDC-catalyzed process was further con-
firmed by an additional control experiment. As shown in Table 1
(entry 7), when the reaction was carried out in the presence of
UiO-67, no desired coupling product was detected under the
given reaction conditions.
1
Figure 1. H NMR (400 Hz, DMSO-d6) spectra of intermediate C,
Pd-NHDC-Me2L, and Pd-NHDC-H2L.
the crystallinity and structural integrity of the UiO-67 fram-
ework are well retained after the Pd-NHDC moiety doping. The
doped Pd amount (1.0 wt %) was determined by inductively cou-
pled plasma (ICP) analysis, and the Zr/Pd ratio in UiO-67-Pd-
NHDC is 1/0.0347 (Supporting Information). Therefore, the
corresponding formula of UiO-67-Pd-NHDC can be given as
Zr6O4(OH)4(Pd-NHDC-L)0.2L′5.8. The oxidation state of the Pd
species was characterized by X-ray photoelectron spectroscopy
(XPS). As shown in Figure 2b, the binding energies for Pd 3d5/2
and 3d3/2 are 338 and 343 eV, respectively, indicating that the
palladium oxidation state in UiO-67-Pd-NHDC is bivalent.7 Scan-
ning electron microscopy (SEM) measurement indicated that the
particle size of UiO-67-Pd-NHDC is around 900 nm (Figure 2c),
which was further confirmed by a dynamic light scattering (DLC)
measurement (Figure 2d). The energy-dispersive X-ray
spectrum (EDS) showed that the dispersion of Pd species in
the MOF matrix is uniform (Supporting Information).
C
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Figure 2. (a) PXRD patterns of UiO-67 and UiO-67-Pd-NHDC. (b) XPS spectrum of UiO-67-Pd-NHDC. (c) SEM image of UiO-67-Pd-NHDC.
(d) DLS measurement. (e) Nitrogen sorption isotherm of UiO-67-Pd-NHDC. (f) Density functional theory pore distribution for UiO-67-Pd-
NHDC determined from its nitrogen adsorption isotherm at 77 K.
a
Table 1. Model Heck Coupling Reaction of Iodobenzene (1a) with Ethyl Acrylate (2a)
b
entry
cat. (amt (mol % Pd))
base
t (h)
T (°C)
additive
yield (%)
1
2
3
4
5
6
7
UiO-67-Pd-NHDC (1)
UiO-67-Pd-NHDC (1)
UiO-67-Pd-NHDC (1)
UiO-67-Pd-NHDC (0.5)
UiO-67-Pd-NHDC (0.5)
Pd-NHDC-Me2L (0.5)
UiO-67 (1)
K2CO3
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
1
1
1
1
4
1
4
120
120
120
120
100
120
120
18
91
99
98
93
99
0
TBAB
TBAB
TBAB
TBAB
a
Reaction conditions: iodobenzene (0.5 mmol), ethyl acrylate (0.75 mmol), base (1.0 mmol), TBAB (0.5 mmol), DMF (2 mL), in an air
b
atmosphere. Yields were determined by GC analysis.
The turnover number (TON, defined as the molar ratio of
converted substrate to catalyst loading) and turnover frequency
(TOF, defined as the molar ratio of converted substrate to
catalyst loading per unit of time) for the model reaction under
the optimized conditions (Pd 0.5 mol %, 1 h, 120 °C, NEt3,
TBAB, DMF) are 196 and 196 h−1, respectively. In order to
demonstrate this coupling reaction in a heterogeneous catalytic
way, a hot leaching test was performed. As shown in Figure 3,
no further reaction occurred in the absence of UiO-67-Pd-NHDC
after ignition of the reaction at 20 min, indicating that Pd-NHDC
doped UiO-67 promoted the Heck cross-coupling reaction herein
in a heterogeneous way. The recyclability of UiO-67-Pd-NHDC
as a heterogeneous catalyst was also examined (Figure 3). After
each run, the solid catalyst was easily recollected by centrifugation,
washed with DMF (3 × 2 mL) and CHCl3 (3 × 2 mL), and dried
at 100 °C for 1 h and then was reused for the next catalytic run
under the same reaction conditions. The isolated yield was still up
to 96% even after five catalytic cycles. No valence or content
change of Pd species was detected on the basis of XPS and ICP
analysis (Supporting Information). The PXRD patterns demon-
strated that the structural integrity and crystalline nature were well
maintained after catalysis (Figure 3). On the other hand, no Pd
species leaching occurred after five catalytic cycles (ICP: Zr/Pd
ratio of 1/0.0341), which clearly resulted from the covalent double
N-heterocyclic carbene binding mode.
In addition, the model reaction between iodobenzene (1a)
and ethyl acrylate (2a) was also tested in the presence of UiO-67-
Pd-NHDC catalysts with different Pd loading amounts (UiO-67-
Pd-NHDC, containing 3.3 and 5.7 wt % Pd), which were syn-
thesized at different Pd-NHDC-H2L/H2L′ molar ratios (3/7 and
1/1) under the same solvothermal conditions (Supporting
Information). Notably, the coupling product of 3a can also be
generated in the same yield (98%) under the optimized conditions,
but extended reaction times were needed (1.5 h, Supporting
Information). The results demonstrated that the reaction substrates
might have more difficulty in accessing the Pd-NHDC catalytic
centers which are located in the MOF matrix. Therefore, the
catalysis based on UiO-67-Pd-NHDC herein should be an
D
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Figure 3. (left) Reaction time examination (black line) and leaching test (red line) for Heck cross-coupling reaction. (middle) Catalytic cycles. After
each run, the catalyst was isolated by centrifugation, washed with DMF (3 × 2 mL) and CHCl3 (3 × 2 mL), and dried at 100 °C for 1 h and then was
reused for the next catalytic run. (right) PXRD patterns of fresh UiO-67-Pd-NHDC and the catalyst after catalytic cycles.
internal and external surface synergistic catalytic process, and the
Pd-NHDC species should reside both inside and outside the MOF
particles. Taking into account the large-sized Pd-NHDC-H2L
linker and 12-connected Zr cluster node, the Pd-NHDC-H2L
linker located inside the MOF matrix might bind the Zr cluster
nodes via a reduced carboxyl−Zr6 connecting mode to generate a
defected UiO-67 type framework, which has been well dem-
onstrated by the reported results.9
Table 2. Heck Coupling Reactions of Various Aryl Halides
and Olefins Catalyzed by UiO-67-PdNHDC
a
b
entry
R1
X
R2
t (h) product yield (%)
To our knowledge, UiO-67-Pd-NHDC is the first Pd bis-
carbene embedded MOF material. In comparison to the Pd mono-
carbene moiety containing MOF ZnL0.42L0.58Cl2Pd0.58(H2O)1.16]·
9H2O reported by Wu et al.,3a UiO-67-Pd-NHDC exhibited a
comparable catalytic activity for the ArX (X = I, Br) and olefin
coupling reactions. Therefore, it could be a new valuable family
member for Pd-carbene MOFs as heterogeneous Heck cross-
coupling catalysts.
1
H
I
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Me
Ph
1
3a
3b
3c
3d
3e
3f
98
99
99
96
96
98
97
95
99
89
98
98
56
2
4-COOCH3
4-CN
I
1
3
I
1
4
4-OCH3
3-CH3
H
I
1
5
I
1
6
I
1
7
H
I
10
10
10
10
8
3g
3h
3i
8
4-OCH3
4-CN
I
Ph
With optimized reaction conditions in hand, the scope of the
reaction was explored (Table 2). It is noteworthy that the cat-
alytic system is tolerant to a wide range of functional groups
such as −CO2Me, −CN, and −OMe. As shown in Table 2, aryl
iodides with both electron-donating and electron-withdrawing
groups afforded cross-coupling products with excellent isolated
yields (95−99% for 3a−i, Table 2, entries 1−9). On the other
hand, acryl ester and styrene substrates exhibited different acti-
vities for the reactions. The maximum yields of the cross-coupling
between aryl iodides and acyl esters (Table 2, entries 1−6)
were obtained in only 1 h. However, longer time (10 h) was
needed for the aryl iodide and styrene coupling to reach full
conversion (Table 2, entries 7−9). On the other hand, the larger
4-phenyliodobenzene was also used as a substrate to perform this
Heck coupling reaction under the same conditions (Table 2,
entry 10). The coupling product was isolated in 89% yield for 3j,
suggesting that the catalytic activity of UiO-67-Pd-NHDC
might be generated from the collaboration of internal and out-
side surface catalytic processes. In comparison to aryl iodides,
the aryl bromides are less reactive, and their cross-coupling with
acryl esters was complete in 8 h, but still with excellent isolated
98% yield (3k,l) under the present reaction conditions (Table 2,
entries 10 and 11).
9
I
Ph
10
11
12
13
14
15
16
17
18
4-Ph
I
Ph
3j
4-NO2
4-COCH3
H
Br
Br
Br
Br
Cl
Cl
Cl
Cl
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
3k
3l
8
12
12
24
24
24
24
3m
3n
3o
3p
3q
3r
4-OCH3
H
49
c
<5
4-OCH3
4-COCH3
4-NO2
c
28
93
a
Reaction conditions: aryl halide (0.5 mmol), olefin (0.75 mmol),
NEt3 (1.0 mmol), catalyst (0.5 mol % Pd), TBAB (0.5 mmol), DMF
(2 mL), in an air atmosphere. Isolated yield. Determined by GC.
The characterization data for the products are provided in the
Supporting Information.
b
c
(Table 2, entry 15, 3o) or with an electron-donating group
(Table 2, entry 16, 3p) are much lower than those of substrates
with electron-withdrawing groups (Table 2, entries 17 and
18, 3q,r).
Benzyne−Benzyne−Alkene Insertion Reaction Cata-
lyzed by UiO-67-Pd-NHDC. Our previous study indicated
that Pd-NHC MOF can promote benzyne-intermediated three-
component cyclotrimerization.5c Herein, we wondered if UiO-
67-Pd-NHDC is able to catalyze the benzyne−benzyne−alkene
insertion reactions between benzyne and allyl halides. For this,
the reaction of allyl bromide (5a) with the benzyne precursor 4
(2-(trimethylsilyl)phenyl trifluoromethanesulfonate) was chos-
en as the model reaction to optimize the reaction conditions.
To optimize the conditions of the benzyne and allyl bromide
inserting reaction, exhaustive experimentation was conducted.
The coupling of aryl bromides containing functional groups
such as acetyl and nitro groups with ethyl acrylate gave the
corresponding products in very high yield (>98%, Table 2,
entries 11 and 12). The substrates without a substituted group
or with electron-donating groups, however, provided the cou-
pling products 3m,n in lower yields (Table 2, entries 13 and 14).
For aryl chlorides, the same phenomenon was observed: that
is, the coupling yields for the substrates without substituent
E
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As shown in Table 3, the optimized reaction conditions were
settled as follows: allyl bromide (5a, 1.0 equiv) and benzyne
Table 4. UiO-67-Pd-NHDC-Catalyzed Reaction of
2-(Trimethylsilyl)phenyl Triflate with Allyl Halides
a
a
Table 3. Model Benzyne−Benzyne−Alkene Insertion Reaction
amt of cat.
yield
b
entry
(mol % Pd)
T (°C)
solvent
CH3CN
base t (h) (%)
1
2
3
4
5
5
3
5
5
5
80
80
80
60
80
CsF
CsF
KF
6
6
75
50
21
53
64
CH3CN
CH3CN
CH3CN
6
CsF
CsF
12
6
CH3CN/
toluene (3/1)
6
UiO-67
80
CH3CN
CsF
6
a
Reaction conditions: 2-(trimethylsilyl)phenyl triflate (4, 1.6 mmol),
allyl bromide (5a, 0.40 mmol), base (3.2 mmol), solvent (4 mL), Ar
b
atmosphere. Isolated yield.
precursor (4, 4.0 equiv), UiO-67-Pd-NHDC (Pd, 5 mol %),
and CsF (8.0 equiv) in CH3CN at 80 °C for 6 h under an Ar
atmosphere. The reaction gave 6a in 75% yield under the given
conditions (Table 3, entry 1). When the reaction was per-
formed in CH3CN with a lower catalyst loading, 3 instead of
5 mol % Pd, the insertion product was obtained in 50% yield
(Table 3, entry 2). In addition, use of another source of fluoride
such as KF also gave a lower insertion yield of 21% (Table 3,
entry 3). When the reaction temperature was decreased (Table 3,
entry 4), the yield of the product was proportionately reduced
(yield 53%). In addition, an aromatic solvent such as toluene
was not propitious to the reaction, and a slightly lower yield of
63% was observed (Table 3, entry 5). It is noteworthy that no
desired benzyne−benzyne−alkene insertion product was detected
in 6 h when the reaction was performed in the presence of
UiO-67 under the given reaction conditions, indicating that the
Pd-NHDC moiety in the MOF is essential for the formation of
phenanthrene (6a).
After that, we examined the substrate scope of this reaction.
As shown in Table 4, in comparison to allyl bromide, allyl
iodide showed greater reaction activity and the corresponding
yield of 6a increased to 80% (Table 4, entry 2). Allyl chloride,
however, was less active and gave 6a in 51% yield (Table 4,
entry 3). The reaction of crotyl bromide and crotyl chloride
with 4 under the same conditions gave 73/27 and 76/24 mix-
tures of 6b and 7b in 78 and 53% combined yields (Table 4,
entries 4 and 5), respectively. Reactions of both cinnamyl bro-
mide and cinnamyl chloride (5f,g) afforded an approximately
7/3 mixture of 6c and 7c in 66 and 58% total yields (Table 4,
entries 6 and 7). The reaction of phenyl bromide (5h) gave 6d
and 7d in 47% yield with a larger isomer ratio of 93/7 (Table 4,
entry 8). In addition, the more sterically hindered substrate 5i
was also used to perform the reaction; the product yield was
65% with a 6e to 7e ratio of 61/39, further indicating that UiO-
67-Pd-NHDC possesses both internal and external surface
catalytic ability.
a
Reaction conditions: 2-(trimethylsilyl)phenyl triflate 4 (1.6 mmol,
392 μL), allyl halide (0.40 mmol), catalyst (5 mol % Pd), CsF
(3.2 mmol, 485 mg), 4 mL of CH3CN, Ar atmosphere, 80 °C, 6 h.
b
1
Isolated yield based on 5, product ratio determined by the H NMR
spectrum. The characterization data for the products are provided in
the Supporting Information. The commercial (E)-1-bromo-2-butene
contains ca. 14% of 3-bromo-1-butene.
c
Herein, UiO-67-Pd-NHDC can be reused at least five times
(66% isolated yield for the fifth catalytic cycle) for the model
reaction (Table 4, entry 1) of benzyne precursor (4) with allyl bro-
mide (5a), and its heterogeneous nature was well demonstrated
by a hot leaching experiment (Figure 4). The PXRD patterns
(Figure 4) and XPS and ICP analysis indicated that the structure of
UiO-67-Pd-NHDC and Pd species valence and amount were well
maintained after the catalytic cycles (Supporting Information).
Although the UiO-67-Pd-NHDC-catalyzed intermolecular
benzyne−benzyne−alkene insertion mechanism is not clear,
it is believed to be the same as those of the reported molecular
Pd(II) complexes.9 That is, the in situ generated benzyne
would insert into the π-allylpalladium halide complex ii which
was generated from allyl halide i and Pd(0) to provide the
arylpalladium intermediate iii. In the case of substituted allylic
halides, two regioisomers would be generated at this stage, which
could be the reason for the formation of the minor regioisomers
during the phenanthrene generation. After that, the second
benzyne would insert into arylpalladium intermediate iii, which
would afford the cyclized intermediate iv after the carbopalla-
dation. The final product of phenanthrene vii could be obtained
after β-hydride elimination of v and the following isomerization
step of vi (Scheme 2).
CONCLUSION
The benzyne−benzyne−alkene insertion has been tradition-
ally catalyzed by eco-unfriendly poisonous molecular Pd-phosphine
complexes in a homogeneous way.10 To our knowledge, this is the
first time the benzyne−benzyne−alkene insertion has been realized
via a Pd-NHC doped MOF catalyst in a heterogeneous way.
■
In summary, we report the new palladium N-heterocyclic
dicarbene moiety doped UiO-67 type NMOF UiO-67-Pd-
NHDC via combination of the presynthesized Pd-NHDC-H2L,
the size-matched biphenyl-4,4′-dicarboxylic acid, and ZrCl4
F
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Inorganic Chemistry
Article
Figure 4. (left) Catalytic cycles. After each run, the catalyst was isolated by centrifugation, washed with DMF (3 × 2 mL) and CHCl3 (3 × 2 mL),
and dried at 100 °C for 1 h and then was reused for the next catalytic run. (middle) Reaction time examination (black line) and leaching test
(red line) for model benzyne−benzyne−alkene insertion. (right) PXRD patterns of fresh UiO-67-Pd-NHDC and the catalyst after catalytic cycles.
Scheme 2. Plausible Mechanism for the Intermolecular
Benzyne−Benzyne−Alkene Insertion Reaction
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.inorgchem.7b03271.
■
S
Additional characterization data for UiO-67-Pd-NHDC,
its catalytic performance, and characterization data for
catalytic products (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail for Y.D.: dongyinggreat@163.com.
*E-mail for Y.-B.D.: yubindong@sdnu.edu.cn.
■
ORCID
Yu-Bin Dong: 0000-0002-9698-8863
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the NSFC (Grant
Nos. 21671122 and 21475078) and the Taishan Scholar’s
Construction Project.
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