Urea-based porous organic frameworks: Effective supports for catalysis in neat water

Article abstract of DOI:10.1002/chem.201304046

Two urea-based porous organic frameworks, UOF-1 and UOF-2, were synthesized through a urea-forming condensation of 1,3,5-benzenetriisocyanate with 1,4-diaminobenzene and benzidine, respectively. UOF-1 and UOF-2 possess good hydrophilic properties and high scavenging ability for palladium. Their palladium polymers, Pd^II/UOF-1 and Pd^II/UOF-2, exhibit high catalytic activity and selectivity for Suzuki-Miyaura cross-coupling reactions and selective reduction of nitroarenes in water. The catalytic reactions can be efficiently performed at room temperature. Palladium nanoparticles with narrow size distribution were formed after the catalytic reaction and were well dispersed in UOF-1 and UOF-2. XPS analysis confirmed the coordination of the urea oxygen atom with palladium. SEM and TEM images showed that the original network morphology of UOF-1 and UOF-2 was maintained after palladium loading and catalytic reactions. Feeling supportive? Two urea-based porous organic frameworks were prepared by a facile urea-forming condensation and can serve as effective supports in palladium-catalyzed Suzuki-Miyaura cross-coupling reactions and reduction of nitroarenes in water. The reactions can even be efficiently performed at room temperature. The original morphology of the urea-based frameworks was maintained after the catalytic reactions.

Full text of DOI:10.1002/chem.201304046

DOI: 10.1002/chem.201304046
Full Paper
&
Sustainable Catalysis
Urea-Based Porous Organic Frameworks: Effective Supports for
Catalysis in Neat Water
[
a]
Liuyi Li, Zhilin Chen, Hong Zhong, and Ruihu Wang*
Chem. Eur. J. 2014, 20, 3050 – 3060
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Abstract: Two urea-based porous organic frameworks, UOF-
and UOF-2, were synthesized through a urea-forming con-
efficiently performed at room temperature. Palladium nano-
particles with narrow size distribution were formed after the
catalytic reaction and were well dispersed in UOF-1 and
UOF-2. XPS analysis confirmed the coordination of the urea
oxygen atom with palladium. SEM and TEM images showed
that the original network morphology of UOF-1 and UOF-2
was maintained after palladium loading and catalytic reac-
tions.
1
densation of 1,3,5-benzenetriisocyanate with 1,4-diamino-
benzene and benzidine, respectively. UOF-1 and UOF-2 pos-
sess good hydrophilic properties and high scavenging ability
II
II
for palladium. Their palladium polymers, Pd /UOF-1 and Pd /
UOF-2, exhibit high catalytic activity and selectivity for
Suzuki–Miyaura cross-coupling reactions and selective reduc-
tion of nitroarenes in water. The catalytic reactions can be
Introduction
which is beneficial for the stabilization of metal ions and nano-
particles in POFs through coordination interactions with urea.
In addition, the surrounding phenyl rings in urea-based POFs
can provide hydrophobic channels and void spaces that allow
organic species to efficiently penetrate and be absorbed into
Porous organic frameworks (POFs) have received great interest
owing to their unique features, such as high chemical stability,
a large surface area, permanent porosity, and a modular syn-
[
1]
[15]
thetic procedure. With a growing need in emerging applica-
POFs, which results in a microenvironment with high con-
[
2]
tions, various POFs with tailored functionalities have been
constructed by deliberately selecting building blocks with de-
centration of organic substrates inside the frameworks. These
unique properties are expected to induce a positive effect on
organic transformations in water. However, applications of
urea-based POFs in heterogeneous catalysis in water have not
been reported to date.
[
3]
sirable functional groups. In this context, POFs containing
[
4]
[5]
metal-binding sites, such as bipyridine, triazine rings, metal-
[
6]
[7]
[8]
loporphyrins, imines, and salen, have been widely used as
catalytic supports in oxidation reactions, photocatalysis, CO2
In our recent studies, we prepared a series of water-soluble
palladium complexes by the attachment of hydrophilic groups,
such as carboxylic acid and ammonium, to hydrophobic li-
gands. These complexes have demonstrated good catalytic ac-
tivities in Suzuki–Miyaura cross-coupling reactions in water, but
the separation difficulty of palladium catalysts results in poor
[
9]
conversions, and Suzuki–Miyaura cross-coupling reactions.
But these reactions are usually performed either in organic or
[
10]
mixed organic/aqueous solvents.
Water is a cheap, safe, and nontoxic reaction medium and
has thus attracted considerable attention. Generally, the effi-
cient implementation of heterogeneous catalysis in water re-
quires good aqueous dispersion of catalytic supports. However,
the majority of POFs mainly consist of hydrophobic aromatic
frameworks, which prevents their effective dispersion in water
and results in poor contact between the substrate and the
active sites. A feasible solution for overcoming the limitations
is to increase the affinity of POFs with water by introducing hy-
[16]
recyclability of the catalytic systems. As a continuous effort
[17]
to develop highly efficient and recyclable catalytic protocols,
herein we report two urea-based POFs (UOF-1 and UOF-2),
which were prepared by a facile urea-forming condensation of
1,3,5-benzenetriisocyanate with 1,4-diaminobenzene and ben-
zidine, respectively. They were used as effective supports in
palladium-catalyzed Suzuki–Miyaura cross-coupling reactions
and reduction of nitroarenes in water, and the reactions can
even be efficiently performed at room temperature.
[11]
drophilic linkages into the hydrophobic frameworks.
Al-
though the use of hydrophobic POFs in storage and separation
[
2h,12]
applications has been widely investigated,
studies of POFs
that contain hydrophilic linkages and their applications in het-
erogeneous catalysis in water have rarely been explored.
The urea group is known for its affinity with water. A urea-
forming condensation reaction can be readily performed under
Results and Discussion
The synthetic route to UOF-1 and UOF-2 is shown in Scheme 1.
Treatment of 1,3,5-benzenetricarbonyl trichloride with sodium
azide followed by a Curtius rearrangement of thermolysis gave
rise to 1,3,5-benzenetriisocyanate and a subsequent urea-form-
ing condensation of 1,3,5-benzenetriisocyanate with 1,4-diami-
nobenzene and benzidine resulted in formation of UOF-1 and
UOF-2, respectively. UOF-1 and UOF-2 are fluffy powders that
are insoluble in water and common organic solvents.
[13]
mild conditions without releasing byproduct molecules. The
relatively low cost of the starting materials and avoidance of
precious-metal-catalyzed coupling chemistry make the urea
group an appealing linkage for the construction of POFs. Urea
[14]
linkages can be used as binding sites for transition metals,
UOF-1 and UOF-2 were characterized by using IR spectrosco-
[
a] Dr. L. Li, Dr. Z. Chen, Dr. H. Zhong, Prof. Dr. R. Wang
Key Laboratory of Coal to Ethylene Glycol and Its Related Technology
State Key Laboratory of Structural Chemistry
Fujian Institute of Research on the Structure of Matter
Chinese Academy of Sciences, Fuzhou, Fujian 350002 (China)
E-mail: ruihu@fjirsm.ac.cn
13
py, solid-state C NMR spectroscopy, elemental analysis, ther-
mogravimetric analysis (TGA), XRD, SEM, and TEM. In IR spectra
of UOF-1 and UOF-2 (Figure S1 in the Supporting Information),
disappearance of the stretching band of isocyanate at
À1
2
267 cm and concomitant emergence of the stretching band
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304046.
À1
of C=O at 1665 cm indicated complete condensation be-
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high relative pressure in the N2
adsorption isotherm showed the
presence of large pores, which
was ascribed to interparticle po-
rosity. The appearance of hyste-
resis demonstrated deformation
[20]
and swelling of the networks.
The specific surface areas calcu-
lated from Brunauer–Emmett–
Teller (BET) models of UOF-1 and
2
À1
UOF-2 were 113 and 91 m g ,
respectively. This unexpectedly
low surface area probably results
from framework interpenetra-
tion. The hydrogen bonds be-
tween urea–urea are probably
responsible for interpenetration
during the framework formation.
The pore sizes of the two sam-
ples, calculated by appropriate
fitting of density functional
theory (DFT) model to the iso-
therm, were centered at 1.3 nm
(
Figure S6b in the Supporting In-
formation).
Given that urea moieties may
impart hydrophilic character to
the polymers, the dispersion of
Scheme 1. Synthesis of UOFs and the corresponding palladium polymers.
polymers in H O and hexane
2
13
tween triisocyanate and the diamines. Solid-state C NMR
spectra further confirmed the presence of a carbonyl carbon
atom of the urea moiety at d=155 ppm, whereas other peaks
at d=139, 126, 118, and 105 ppm were assigned to carbon
atoms of the aryl rings (Figure 1). The broad linewidths suggest
that the carbon atoms have a heterogeneous surrounding.
XRD further confirmed their amorphous structures (Figure S2
in the Supporting Information). Elemental analyses of
UOF-1 and UOF-2 showed that the experimental values of C
and N are slightly lower than the corresponding theoretical
values; this deviation mainly resulted from the presence of
trapped guest molecules, which is common in porous materi-
als. The presence of guest molecules was further supported by
TGA; initial weight losses of 4.6 and 3.1% up to 1008C were
observed for UOF-1 and UOF-2, respectively, and the frame-
works of UOF-1 and UOF-2 were stable up to 221 and 2388C,
respectively (Figure S3 in the Supporting Information). Their
morphology was examined by using SEM (Figure 2a, b and
Figure S4a and b in the Supporting Information) and TEM (Fig-
ure 2c, d and Figure S5 in the Supporting Information). Inter-
estingly, UOF-1 and UOF-2 exhibited 3D network morphology,
which is much different from the granular morphology seen in
was investigated to probe the hydrophilicity of the urea-con-
[11,21]
taining polymers.
As shown in Figure S7 in the Supporting
Information, UOF-1 and UOF-2 were well distributed in water,
and they were retained in the aqueous phase in a water/
hexane biphasic system owing to the affinity of urea moieties
to H O. The wettability was further investigated by examining
2
the water contact angle (CA) measurements (Figure 3). The CA
between water and UOF-1 is 608. However, when a droplet of
salad oil was placed on the surface of UOF-1, the oil was quick-
ly adsorbed and the CA became almost 08. The strong oleophi-
licity of the two polymers was ascribed to the presence of hy-
drophobic compositions of phenyl rings and the porous struc-
ture of UOF-1. These results demonstrated that UOF-1 and
UOF-2 are good supports for catalysis in water because most
substrates are hydrophobic in catalytic reactions.
UOF-1 was selected to test the scavenging ability for palladi-
um. As shown in Figure S8 in the Supporting Information,
when a suspension of UOF-1 was stirred in a solution of
[Pd(OAc) ] in dichloromethane or in aqueous PdCl with a palla-
2
2
dium/urea molar ratio of 1:10, the yellowish supernatant
became colorless, which suggests a good affinity of UOF-1 for
palladium.
[
18]
most of the reported POFs.
To investigate the coordination properties of palladium with
II
II
The porous nature of UOF-1 and UOF-2 was studied by
urea units, Pd /UOF-1 and Pd /UOF-2 were prepared through
using N adsorption measurements at 77 K. As shown in Fig-
a simple treatment of UOF-1 and UOF-2 with [Pd(OAc) ] in di-
2
2
II
II
ure S6a in the Supporting Information, the N physisorption
chloromethane (Scheme 1). Pd /UOF-1 and Pd /UOF-2 are
2
13
isotherms of UOF-1 and UOF-2 showed type IV isotherm pat-
yellow powders. In their solid-state C NMR spectra, the chem-
ical shifts of the phenyl rings and urea units are almost identi-
[
19]
terns according to the IUPAC classification. A sharp rise at
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1
3
II
Figure 1. C CP/MAS NMR spectra for: a) UOF-1 and Pd /UOF-1, and
Figure 2. SEM images for: a) UOF-1 and b) UOF-2; TEM images for: c) UOF-
II
b) UOF-2 and Pd /UOF-2; *=spinning side bands.
II
II
1
and d) UOF-2; and SEM images for: e) Pd /UOF-1, f) Pd /UOF-2,
0
g) Pd /UOF-1, and h) Pd-reduction. Scale bars: 1 mm (a, b, e, g, h) and
00 nm (c, d, f).
1
cal to their respective precursors (Figure 1). Two new strong
signals at d=188 and 23 ppm are assigned to the characteris-
tic peaks of carbonyl and methyl of the incorporated
[
Pd(OAc) ], respectively. In comparison with the carbonyl peak
2
[
7a]
of free [Pd(OAc) ] at d=190 ppm, the chemical shift of the
2
carbonyl group in the incorporated [Pd(OAc) ] was shifted
2
toward high field by d=2 ppm, which suggests weak coordi-
nation of palladium with urea units. However, no characteristic
À1
bands for [Pd(OAc) ] were observed at 1600–1650 cm in the
2
II
II
IR spectra of Pd /UOF-1 and Pd /UOF-2 owing to overlap of
the C=O stretching band of the urea moieties. Interestingly,
SEM analyses showed that the incorporation of palladium have
no obvious effect on their morphology (Figure 2e, f and Fig-
ure S4c and d in the Supporting Information), suggesting high
Figure 3. Images for contact angle measurement: Left: photograph of
a salad oil droplet, and right: photograph of a water droplet on a sample of
UOF-1.
[
7a]
parison with that of free [Pd(OAc) ] (338.40 eV),
Pd3d5/2
2
II
II
II
II
physicochemical stability of Pd /UOF-1 and Pd /UOF-2.
peaks in Pd /UOF-1 and Pd /UOF-2 were negatively shifted by
about 0.26 and 0.17 eV, respectively, due to the interactions of
To further investigate possible interactions between palladi-
um and the urea units, XPS analyses were performed. As
shown in Figure 4a and b, the binding energy of Pd3d in
[Pd(OAc) ] with the frameworks. In comparison with UOF-1 and
2
II
II
UOF-2, the binding energy of O1s in Pd /UOF-1 and Pd /UOF-2
was shifted from 531.57 to 531.94 eV and from 531.99 to
532.98 eV, respectively, whereas the binding energy of N1s re-
5/2
II
II
Pd /UOF-1 and Pd /UOF-2 was 338.14 and 338.23 eV, respec-
tively, which are assigned to the +2 oxidation state. In com-
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II
0
II
0
II
0
II
Figure 4. Pd3d XPS spectra for a) Pd /UOF-1 and Pd /UOF-1, b) Pd /UOF-2 and Pd /UOF-2; O1s spectra for c) UOF-1, Pd /UOF-1, and Pd /UOF-1, d) UOF-2, Pd /
0
II
0
II
0
UOF-2, and Pd /UOF-2; N1s spectra for e) UOF-1, Pd /UOF-1, and Pd /UOF-1; f) UOF-2, Pd /UOF-2, and Pd /UOF-2.
mained at 399.55 eV (Figure 4c–f). The above results showed
II
Table 1. Suzuki–Miyaura cross-coupling reaction catalyzed by Pd /UOF-
1 and Pd /UOF-2.
II
[a]
that [Pd(OAc) ] was successfully immobilized in the frameworks
2
through the coordination interaction of palladium with oxygen
atoms of the urea moieties. Pd contents were determined by
II
II
using ICP analyses in Pd /UOF-1 and Pd /UOF-2 and found to
1
2
[b]
Entry
[Pd]
R
R
Base
Yield [%]
be 16.87 and 16.83 wt%, which corresponds to 1.59 and
II
À1
1
2
3
4
Pd /UOF-1
4-COCH3
H
H
H
H
H
H
H
H
H
H
H
H
4-F
K2CO3
NEt
3
KOH
100 (97)
100 (96)
97
63
76
98 (98)
95 (92)
100 (98)
100 (99)
99 (99)
1
.58 mmolg , respectively.
II
Pd /UOF-1
4-COCH
4-COCH
3
3
II
II
Catalytic performances of Pd /UOF-1 and Pd /UOF-2 were in-
II
Pd /UOF-1
II
itially evaluated in a Suzuki–Miyaura cross-coupling reaction in
water. When the reaction of 4-bromoacetophenone with phe-
nylboronic acid was performed with K CO as a base in the
Pd /UOF-1
4-COCH3
KOAc
[
c]
II
5
6
7
8
9
Pd /UOF-1
4-COCH
4-COCH
3
3
K
K
K
K
2
CO
2
CO
2
CO
2
CO
3
3
3
3
[
d]
II
Pd /UOF-1
2
3
II
Pd /UOF-1
4-NO
4-OH
2
II
presence of 0.5 mol% Pd /UOF-1, complete conversion of 4-
II
Pd /UOF-1
II
bromoacetophenone was observed at 608C in 3 h (Table 1,
Pd /UOF-1
4-CHO
4-CN
K2CO3
II
entry 1). The use of NEt and KOH as bases also gave rise to 4-
10
Pd /UOF-1
K
K
K
K
2
CO
2
CO
2
CO
2
CO
3
3
3
3
3
[
e]
II
1
1
Pd /UOF-1
4-COOH
4-OCH
4-COCH
4-COCH3
– (100)
65
acetyldiphenyl in almost quantitative GC yields (Table 1, en-
tries 2 and 3), whereas KOAc was slightly less efficient in the
cross-coupling reaction and gave the target product in 63%
GC yield under the same conditions (Table 1, entry 4). When
palladium loading was decreased to 0.05 mol%, 4-acetylbi-
phenyl was obtained in 76% GC yield (Table 1, entry 5). Inter-
estingly, the catalytic system was also efficient at 258C; when
the reaction of 4-bromoacetophenone with phenylboronic was
performed at 258C for 24 h, 4-acetylbiphenyl was formed in
a 98% isolated yield (Table 1, entry 6). This was challenging to
achieve even for the homogeneous catalytic systems in
II
1
1
2
Pd /UOF-1
3
II
3
Pd /UOF-1
3
100 (99)
98 (98)
100 (99)
99 (99)
100 (98)
22
97 (95)
100 (96)
95 (93)
– (96)
II
14
15
Pd /UOF-1
4-CH O
3
K2CO3
II
Pd /UOF-1
4-COCH
4-COCH
4-COCH
3
3
3
4-CH
3-CH
2-CH
3
3
3
K
K
K
K
2
CO
2
CO
2
CO
2
CO
3
3
3
3
II
16
17
18
Pd /UOF-1
II
Pd /UOF-1
II
Pd /UOF-1
2- CH
3
4-COCH
3
II
19
20
Pd /UOF-2
4-COCH3
4-OH
4-NO
2
–
–
H
H
H
H
K2CO3
II
Pd /UOF-2
K
K
K
K
2
CO
2
CO
2
CO
2
CO
3
3
3
3
II
21
22
23
Pd /UOF-2
[
f]
II
Pd /UOF-1
[f]
II
Pd /UOF-1
4-CH
3
O
– (95)
[
16]
[a] Unless otherwise stated, the reactions were carried out by using aryl
bromide (0.5 mmol), arylboronic acid (0.75 mmol), K CO (1.0 mmol), and
Pd] (0.5 mol%) in H O (2 mL) for 3 h. [b] GC yield, isolated yields are
given in parenthesis. [c] [Pd] (0.05 mol%) was used. [d] T=258C, t=24 h.
e] K CO (1.5 mmol) was used. [f] 5-Bromo-2-hydroxybenzoic acid was
used as a substrate, with K CO (2.0 mmol) was used.
water.
2
3
To explore scope and generality of the catalytic system in
water, a series of aryl bromides and aryl boronic acids with dif-
ferent steric and electronic characters were tested. The effect
of varying aryl bromides was firstly investigated by using phe-
[
2
[
2
3
2
3
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nylboronic acid as a substrate (Table 1, entries 7–12). The aryl
bromides containing electron-withdrawing groups, such as
-
NO , -OH, -CHO, -CN, and -COOH, gave rise to desirable prod-
2
ucts in excellent isolated yields (Table 1, entries 7–11), whereas
the use of electron-rich 4-bromoanisole provided 4-methoxybi-
phenyl in 65% GC yield under the same conditions (Table 1,
entry 12). The cross-coupling reactions of 4-bromoacetophe-
none with electron-deficient and electron-rich aryl boronic
acids were also carried out, the corresponding biaryl products
were obtained in excellent isolated yields (Table 1, entries 13–
17). Note that the reactions of 4-bromoacetophenone with
ortho-, meta-, and para-methyl-substituted phenylboronic acid
produced the target products in quantitative yields (Table 1,
entries 14–16), which suggests the electronic and steric nature
of aryl boronic acids have no obvious effect on the cross-cou-
pling reactions. It should be mentioned that a quantitative
yield was achieved in the coupling reaction of 4-bromoaceto-
phenone with 2-methylphenylboronic acid (Table 1, entry 17),
whereas the reaction of 2-bromotoluene with 4-acetophenyl-
boronic acid only provided the same product in 22% GC yield
II
0
(Table 1, entry 18). It should be mentioned that the use of Pd /
Figure 5. TEM images and palladium size distributions for: a,b) Pd /UOF-1,
and c,d) Pd /UOF-2.
0
UOF-2 also gave the target products in excellent isolated yields
II
similar to those from Pd /UOF-1 (Table 1entries 19–21).
It is known that palladium nanoparticles (NPs) are usually in-
[22]
0
volved in palladium-catalyzed cross-coupling reactions.
To
399.55 eV (Figure 4e), whereas the O1s peak in Pd /UOF-1 and
0
discern the catalytic active species in Suzuki–Miyaura cross-
coupling reactions, after the reaction between 4-bromoaceto-
phenone and phenylboronic acid was conducted in the pres-
Pd /UOF-2 was shifted from 531.57 to 532.99 eV and from
531.99 to 533.31 eV, respectively, due to coordination of the
palladium NPs with oxygen atoms of the urea moieties. It
should be mentioned that the O1s peak of the acetate group
II
II
ence of Pd /UOF-1 and Pd /UOF-2 in water, the resultant mix-
ture was extracted with diethyl ether and then centrifuged.
The residues were successively washed with water and ethanol
to afford gray powders, which were ultrasonically dispersed in
anhydrous ethanol for TEM analysis. The palladium NPs were
well dispersed in the frameworks of UOF-1 and UOF-2, they
0
0
at 535.50 eV disappeared in Pd /UOF-1 and Pd /UOF-2, com-
II
II
pared with that in Pd /UOF-1 and Pd /UOF-2.
To assess whether the catalytic system functions in a hetero-
geneous pathway, mercury drop test and poison experiments
[24]
0
were carried out. When one drop of Hg was added to the
reaction mixture before the reaction of 4-bromoacetophenone
0
0
were denoted as Pd /UOF-1 and Pd /UOF-2, respectively
II
II
(
Figure 5). Their average sizes were (4.9Æ0.94) and (4.8Æ
and phenylboronic acid was performed, Pd /UOF-1 and Pd /
UOF-2 provided 4-acetylbiphenyl in 83 and 70% GC yields, re-
spectively (Table 2, entry 1), which was lower than that in the
absence of mercury under the same conditions (Table 1, en-
1
.1) nm, respectively, which are comparable with those formed
[
7a,23]
after the catalytic reaction.
SEM analyses of the 3D net-
work morphology of UOF-1 and UOF-2 showed no obvious
change after the Suzuki–Miyaura cross-coupling reaction (Fig-
ure 2g and Figure S4e in the Supporting Information).
tries 1 and 19). The use of substoichiometric amount of CS as
2
a poisoning agent in catalysis has been reported to be strong
[25]
To get an insight into palladium NPs in the urea-based
evidence for the formation of NPs. No catalytic activity was
detected when the reaction of 4-bromoacetophenone and
phenylboronic acid was performed in the presence of
0.5 equivalent of CS2 relative to palladium (Table 2, entry 2).
Poly(vinylpyridine) (PVPy) has been shown to be an excellent
scavenger for soluble palladium species due to its strong bind-
ing ability for palladium and insolubility in reaction media.
However, nearly complete conversion of 4-bromoacetophe-
none was observed when 150 equivalent of PVPy was added
to the catalytic system (Table 2, entry 3). The lack of effect of
PVPy on the catalytic system was mainly ascribed to its inabili-
ty to access active sites in pores of the frameworks. In contrast,
the use of 150 equivalent of pyridine or PPh3 as additives
almost completely quenched the cross-coupling reaction
(Table 2, entries 4 and 5). This is likely due to the fact that pyri-
0
0
frameworks, Pd /UOF-1 and Pd /UOF-2 were further investigat-
ed by using IR spectroscopy and XPS. The IR spectra showed
0
0
that the main characteristic peaks of Pd /UOF-1 and Pd /UOF-2
were similar to those of UOF-1 and UOF-2 (Figure S1 in the
Supporting Information). The XPS spectra indicated that the
Pd3d region was divided into two spin-orbital pairs (Fig-
ure 4a), which indicated the presence of two types of surface-
0
bound palladium species. For Pd /UOF-1, the binding energy
peaks at 335.55 (Pd3d5/2) and 340.80 eV (Pd3d3/2) were as-
0
signed to the Pd species, whereas the peaks at 337.65
II
(
Pd3d5/2) and 342.55 eV (Pd3d ) were attributed to the Pd
3/2
0
II
species. The ratio of surface Pd to Pd , as determined by ratio
of their relative peak areas, was 0.82. In comparison with XPS
spectra of UOF-1 and UOF-2, the binding energy peak of N1s
0
0
in Pd /UOF-1 and Pd /UOF-2 was not shifted and remains at
dine and PPh , as small molecules, could enter freely into the
3
Chem. Eur. J. 2014, 20, 3050 – 3060
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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II
II
Table 2. Summary of poisoning experiments for Pd /UOF-1 and Pd /UOF-
[
a]
2.
[
b]
[c]
Entry
Additive
Hg
Additive/Pd ratio
Conv. [%]
Conv. [%]
1
2
3
4
5
–
0.5
150
150
150
83
0
99
6
70
0
99
4
CS
2
PVPy
pyridine
PPh
3
5
0
II
II
Figure 6. Recyclability of Pd /UOF-1 and Pd /UOF-2 in the Suzuki–Miyaura
[
a] Reaction conditions: 4-bromoacetophenone (0.5 mmol), phenylboronic
cross-coupling reaction. Reaction conditions: 4-bromophenol (0.5 mmol),
II
II
acid (0.75 mmol), K
2
CO
3
(1.0 mmol), Pd /UOF-1 or Pd /UOF-2 (0.5 mol%)
II
II
phenylboronic acid (0.75 mmol), K
2 3
CO (1.5 mmol), and Pd /UOF-1 or Pd /
II
II
2
in H O (2.0 mL) at 608C for 3 h. [b] Pd /UOF-1. [c] Pd /UOF-2.
UOF-2 (0.5 mol%) in H O (2.0 mL) at 608C for 3 h.
2
pores of the frameworks to poison the palladium active spe-
cies. As a result, the catalytic system is heterogeneous. This ar-
gument was further confirmed by ICP analysis. After the reac-
tion of 4-bromoacetophenone and phenylboronic acid was
complete, ICP analyses showed that palladium leaching into
materials. Catalytic reduction of nitroarenes is one of the
simple and efficient methods for the production of aromatic
II
amines. The encouraging catalytic performances of Pd /UOF-
II
1 and Pd /UOF-2 in Suzuki–Miyaura cross-coupling reaction
prompted us to further explore their application in the selec-
tive reduction of nitroarenes. When the reduction reaction of
II
II
the organic phase for Pd /UOF-1 and Pd /UOF-2 was 6.260 and
.280 ppm, respectively, whereas palladium leaching to aque-
II
II
3
nitrobenzene was performed with 0.5 mol% Pd /UOF-1 or Pd /
UOF-2 as the catalytic precursor and three equivalent of NaBH4
as a reductive reagent in water at room temperature, aniline
was obtained in a full conversion in 1 h (Figure 7). After the re-
action was complete, the resultant mixture was extracted with
diethyl ether and the aqueous phase was centrifuged to afford
a gray solid, which was further washed with ethanol and water
ous phase was 0.084 and 0.080 ppm, respectively. Owing to
the negligible palladium leaching, the present catalytic system
may be appropriate for synthesis of pharmaceutical precursors
because the contamination of chemical intermediates by palla-
dium residues is of great concern for development of large-
scale syntheses, particularly in the pharmaceutical industry in
[
26]
II
which metal contaminants are strictly monitored. Biphenyl
derivatives of salicylic acid are known pharmaceutical inter-
and then used in the next run. Pd /UOF-1 maintained a catalytic
activity of 100% in four consecutive runs, but the selectivity of
aniline began to decrease from the third run; the selectivity of
aniline in the third run and the fourth runs is 97 and 86%, re-
[27]
mediates and possess high activity as tyrosinase inhibitors.
To our delight, the catalytic system is also efficient for the syn-
thesis of biphenyl derivatives of salicylic acid, and the reaction
of 5-bromo-2-hydroxybenzoic acid with phenylboronic acid
and 4-methoxyphenylboronic acid gave rise to the correspond-
ing products in 96 and 95% isolated yields, respectively
(Table 1, entries 22 and 23).
In addition to catalytic activity, recyclability is also crucial for
an outstanding heterogeneous catalyst. The catalytic recyclabil-
II
II
ity of Pd /UOF-1 and Pd /UOF-2 was evaluated by examining
the reaction between 4-bromophenol and phenylboronic acid
in the presence of 0.5 mol% palladium and K CO at 608C for
2
3
3
h. After every cycle, the mixture was extracted with diethyl
ether and the conversion of 4-bromophenol was determined
by GC. The remaining water phase was centrifuged to afford
a gray solid, which was further washed with ethanol and water
and then employed in the next run with fresh substrates and
II
II
base. Both Pd /UOF-1 and Pd /UOF-2 gave a complete conver-
sion of 4-bromophenol in the first and second run (Figure 6). A
slight loss in catalytic activity was observed in the fourth run
II
for Pd /UOF-1, for which the target product was obtained in
II
9
0% GC yield, whereas the reactivity of Pd /UOF-2 began to
decrease from the third run, so that 91 and 70% conversion of
-bromophenol was obtained in the third and fourth runs, re-
4
spectively.
II
II
Figure 7. Reusability of Pd /UOF-1 and Pd /UOF-2 in catalytic reduction of ni-
Primary aromatic amines are widely used in the synthesis of
natural products, pharmaceutical intermediates, polymers, and
II
II
trobenzene. Reaction conditions: nitrobenzene (1 mmol), Pd /UOF-1 or Pd /
UOF-2 (0.5 mol%), H O (2.0 mL), and NaBH (3 mmol) at 258C for 1 h.
2
4
Chem. Eur. J. 2014, 20, 3050 – 3060
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
spectively. The conversion of nitrobenzene and the selectivity
of aniline in the fifth run were 80 and 70%, respectively. How-
was also achieved in 96% isolated yield through reduction of
4-nitroaniline (Table 3, entry 11). It should be mentioned that
reduction of 4-nitrobenzaldhyde gave rise to (4-aminophenyl)-
methanol in quantitative yield, in which CHO was concomitant-
ly reduced, no side products of polyimine or polyamine were
observed (Table 3, entry 12). Palladium catalysts are reported
to possess a high tendency toward dehalogenation, so much
effort was made to suppress side reactions through addition of
inhibitors. However, the reduction of 4-nitrochlorobenzene and
2-nitrochlorobenzene gave rise to the corresponding products
with 88 and 84% selectivity, respectively (Table 3, entries 13
and 14), in which the dechloro side-product aniline was ob-
tained in 12 and 18% yield, respectively. As anticipated, when
the reduction of nitrobenzene was performed in the absence
II
ever, when Pd /UOF-2 was used four times, no obvious loss in
catalytic activity and selectivity was observed, but the conver-
sion of nitrobenzene and the selectivity of aniline in the fifth
run were decreased to 90 and 78%, respectively.
Different nitro compounds were further used in the reduc-
II
tion of nitroarenes with Pd /UOF-2. When the reaction was per-
formed in the presence of three equivalents of NaBH in water
4
at room temperature for 1 h, a variety of nitro compounds
could be reduced to the corresponding amino compounds in
high yield and selectivity (Table 3). Various functional groups,
such as ÀOH, ÀNH , ÀOCH , ÀCOOH, and ÀCl, were well tolerat-
2
3
ed. Of these groups, electron-donating and -withdrawing
groups have no obvious effect on the activity and selectivity of
the reaction (Table 3, entries 1–7). In addition, ortho-, meta-,
and para-methyl-substituted nitrobenzenes were selectively re-
duced into the corresponding products in quantitative yield
II
of Pd /UOF-2 or with UOF-2 as a catalytic precursor, no aniline
was detected under the same conditions (Table 3, entries 15
and 16), which suggests that the palladium catalytic species is
necessary for the catalytic system.
(
Table 3, entries 5–7), which suggests that electronic character
After reduction of nitrobenzene in the presence of NaBH
4
II
and steric hindrance has no obvious effect on the catalytic
system. Interestingly, the catalytic system is also effective for
selective reduction of heterocyclic nitro compounds. If 4-nitro-
indole and 5-nitroindole were used as substrates, the corre-
sponding products were obtained in 94 and 100% isolated
yields, respectively (Table 3, entries 8 and 9). Notably, 1,3-dini-
trobenzene can be completely reduced to 1,3-benzendiamine
in quantitative isolated yield under the same conditions
and Pd /UOF-2 was complete, the resultant gray powder was
denoted as Pd-reduction, and was analyzed by using SEM and
TEM. SEM images showed that the morphology of Pd-reduc-
II
tion was identical to UOF-2 and Pd /UOF-2 (Figure 2h). TEM
images showed that the palladium NPs in Pd-reduction were
well dispersed and nearly spherical, and their average size was
0
(4.2Æ0.8) nm, which was slightly smaller than those in Pd /
UOF-2 (Figure 8a and b). For comparison, palladium NPs were
II
(Table 3, entry 10). Selective synthesis of 1,4-diaminobenzene
also directly prepared by reduction of Pd /UOF-2 with NaBH
4
[
a]
Table 3. Reduction of various nitroarenes.
Entry
1
Reactant
Product
Yield
Selectivity
[%]
Entry
10
Reactant
Product
Yield
[%]
Selectivity
[%]
[
b]
[b]
[%]
100 (98)
100
100 (100)
100
2
3
4
100 (98)
– (96)
100
96
11
12
13
100 (96)
100 (100)
100
100
100
88
100 (100)
100
5
100 (92)
100
14
100
84
[
d]
e]
6
7
100 (92)
100 (93)
100
100
15
16
0
0
0
0
[
[
c]
c]
[f]
8
9
– (94)
94
17
100
100
[
– (100)
100
II
[
a] Unless otherwise stated, the reaction conditions were as follows: substrate (1 mmol), Pd /UOF-2 (0.5 mol%), and NaBH
4
(3 mmol) in H
2
O (2 mL) for 1 h
II
at RT. [b] GC yield, isolated yields are given in parenthesis. [c] THF (0.1 mL) was added. [d] In the absence of Pd /UOF-2. [e] In the presence of UOF-2
1.6 mg). [f] In the presence of Pd-NaBH (0.5 mol%).
(
4
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Experimental Section
General information
All chemicals were commercially obtained and used without fur-
1
13
ther purification. H and C NMR spectra were recorded by using
a Bruker AVANCE III NMR spectrometer at 400 and 100 MHz, re-
spectively, with tetramethylsilane (TMS) as an internal standard.
13
Solid-state C CP/MAS NMR was performed by using a Bruker SB
Avance III 500 MHz spectrometer equipped with a 4 mm double-
resonance MAS probe, a sample spinning rate of 7.0 kHz, a contact
time of 2 ms and pulse delay of 5 s. IR spectra were recorded for
KBr pellets by using a Perkin–Elmer Instrument. Thermogravimetric
analysis (TGA) was carried out by using a NETZSCH STA 449C in-
strument and heating samples from 30 to 7008C in a dynamic ni-
À1
trogen atmosphere at a heating rate of 108Cmin . Powder X-ray
diffraction (XRD) patterns were recorded in the range of 2q=5–408
by using a desktop X-ray diffractometer (RIGAKU-Miniflex II) with
CuKa radiation (l=1.5406 ꢁ). Nitrogen adsorption and desorption
isotherms were measured at 77 K by using a Micromeritics
ASAP 2020 system. The samples were degassed at 1008C for 5 h
before the measurements. Surface areas were calculated from the
adsorption data by using Brunauer–Emmett–Teller (BET) and Lang-
muir methods. The pore size distribution curves were obtained
from the adsorption branches by using a nonlocal density func-
tional theory (NLDFT) method. Field-emission scanning electron mi-
croscopy (SEM) was performed by using a JEOL JSM-7500F operat-
ed at an accelerating voltage of 3.0 kV. Transmission electron mi-
croscope (TEM) images were obtained by using a JEOL JEM-2010
instrument operated at 200 kV. X-ray photoelectron spectroscopy
Figure 8. TEM images and Pd size distributions for: a,b) Pd-reduction, and
c,d) Pd-NaBH
4
.
in H O and were denoted as Pd-NaBH . TEM analysis showed
2
4
that Pd-NaBH was nearly spherical with an average size of
4
(
4.6Æ0.6) nm (Figure 8c and d), which was slightly larger than
(XPS) measurements were performed by using a Thermo ESCA-
the size of Pd-reduction. The catalytic performance of Pd-
LAB 250 spectrometer with non-monochromatic AlKa X-rays as the
excitation source and with C1s (284.6 eV) as the reference line. In-
ductively coupled plasma spectroscopy (ICP) was measured by
using a Jobin Yvon Ultima2 instrument. Gas chromatography (GC)
was performed by using a Shimadzu GC-2014 equipped with a ca-
pillary column (RTX-5, 30 mꢂ0.25 mm) equipped with a flame ioni-
zation detector. Elemental analyses were performed by using an El-
ementar Vario MICRO Elemental analyzer.
NaBH was also investigated in the reduction of nitrobenzene.
4
Nitrobenzene was completely reduced to aniline by NaBH4
neat water at room temperature in 1 h (Table 3, entry 17).
Conclusion
Two urea-based porous organic frameworks have been suc-
cessfully prepared by facile urea-forming condensation. They
can serve as effective supports for palladium-catalyzed Suzuki–
Miyaura cross-coupling reaction and selective reduction of ni-
troarenes in neat water, excellent catalytic activity and selectiv-
ity with negligible palladium leaching were observed. Interest-
ingly, the catalytic reactions can be efficiently performed at
room temperature. The superior performances were mainly at-
tributed to good dispersion of urea-based frameworks in water
and a microenvironment with high concentration of organic
substrates inside the frameworks due to the presence of hy-
drophilic urea linkage and hydrophobic void. Palladium nano-
particles with narrow size distribution were formed after the
catalytic reaction and they are well stabilized by the synergistic
effects of the coordination of the urea moiety with palladium
and the confinement effect of frameworks. This study not only
provides a new approach for the construction of functionalized
POFs, but also opens a new avenue to developing heterogene-
ous catalytic systems in water.
Synthesis of 1,3,5-benzenetriisocyanate
1,3,5-Benzenetriisocyanate was prepared according to a modified
[28]
literature method. A solution of 1,3,5-benzenetricarbonyl trichlor-
ide (1.0 g, 3.8 mmol) in dry THF (10 mL) was added dropwise to an
aqueous solution (10 mL) of sodium azide (1.4 g, 22 mmol) at 08C.
The reaction mixture was stirred at 08C for 2 h to give 1,3,5-benze-
netricarbonyl triazide. Toluene (20 mL) and saturated aqueous
NaHCO (20 mL) were successively added. After phase separation,
3
the aqueous layer was extracted with toluene (3ꢂ10 mL). The
combined organic layers were successively washed with saturated
aqueous solutions of NaHCO3 and NaCl, and dried over MgSO4.
Subsequently, the toluene layer was gradually heated to 1008C
and stirred till gas evolution stopped. The resultant mixture was
concentrated under reduced pressure to give 1,3,5-benzenetriiso-
1
cyanate as a pale red solid (0.64 g, 85%). H NMR (400 MHz, CDCl ):
3
13
d=6.70 ppm (s, 3H); C NMR (100 MHz, CDCl ): d=135.7, 125.5,
3
118.6 ppm; FT-IR (KBr): n˜ =3091(w), 2261 (vs), 1675 (w), 1614 (s),
1
8
556 (s), 1484 (w), 1320 (w), 1263 (m), 1209 (m), 1100 (m), 898 (m),
53 (m), 665 (m), 562 (m), 452 cm (vw).
À1
Caution! 1,3,5-Benzenetricarbonyl triazide is explosive in the solid
state under heating. Therefore, it should preferentially be kept in
solution and be handled at 1 g scale maximum.
Chem. Eur. J. 2014, 20, 3050 – 3060
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Synthesis of UOF-1
nol (87 mg, 0.5 mmol), phenylboronic acid (92 mg, 0.75 mmol), and
K CO (138 mg, 1.0 mmol) in water (2.0 mL) at 608C for 3 h, the
2
3
A solution of 1,4-diaminobenzene (0.45 g, 4.2 mmol) in dry THF
mixture was cooled in ice water and was extracted with ethyl ace-
tate (3ꢂ5 mL). The combined organic layer was analyzed by using
GC. The gray powder in the aqueous phase was separated by cen-
trifugation, washed thoroughly with water, and then used for the
next run.
(
25 mL) was added dropwise to a solution of 1,3,5-benzenetriiso-
cyanate (0.56 g, 2.8 mmol) in toluene (25 mL) at 808C. After stirring
at 808C for 3 d, the precipitate was filtered and successively
washed with methanol (2ꢂ20 mL), THF (2ꢂ20 mL), and diethyl
ether (2ꢂ20 mL). The residue was dried in vacuo at 808C for 12 h
to give the target product as a brown powder (0.83 g, 83%). FT-IR
General procedures for reduction of nitroarenes
(
1
KBr): n˜ =3371 (s), 2257 (w), 1663 (s), 1611 (s), 1545 (s), 1514 (vs),
445 (m), 1306 (m), 1206 (s), 830 (w), 683 (vw), 512 cm (w); ele-
À1
NaBH4 (114 mg, 3 mmol) was added in one portion to a stirred
II
mental analysis calcd (%) for (C H N O) : C 59.50, H 4.16, N 23.13;
6
5
2
n
aqueous solution (2 mL) of nitroarenes (1.0 mmol) and Pd /UOF-
II
found: C 56.95, H 5.03, N 21.39.
1
or Pd /UOF-2 (0.5 mol%) at RT. After 1 h, the resultant mixture
was extracted with diethyl ether (3ꢂ5 mL). The combined organic
layer was analyzed by using GC, and concentrated under reduced
pressure to give the target products. The identity of the products
was confirmed by comparison with GC retention time of commer-
cial materials and literature NMR spectroscopic data.
Synthesis of UOF-2
Following the same procedures as synthesis of UOF-1, UOF-2 was
obtained as a brown powder (32% yield). IR (KBr): n˜ =3290 (s),
3
1
6
105 (w), 2949 (w), 1677 (m), 1609 (s), 1524 (s), 1500 (vs), 1439 (m),
315 (m), 1280 (m), 1204 (s), 1177 (s), 1004 (vw), 821 (m), 731 (w),
80 (w), 516 cm (w); elemental analysis calcd (%) for (C H N O) :
II
II
À1
Reuse of Pd /UOF-1 and Pd /UOF-2 in the reduction of nitro-
arenes
9
7
2
n
C 67.91, H 4.43, N 17.60; found: C 62.30, H 5.13, N 16.89.
After the reduction of nitrobenzene was complete, the resultant
mixture was extracted with diethyl ether (3ꢂ5 mL). The combined
organic layer was determined by using GC analysis. The gray
powder in the aqueous phase was separated by centrifugation,
washed with water thoroughly, and then used for the next run.
II
Synthesis of Pd /UOF-1
UOF-1 (0.20 g) was added to a solution of palladium acetate
0.17 g, 0.76 mmol) in dichloromethane (100 mL). The mixture was
stirred at RT for 48 h. The resultant solid was isolated by filtration
(
and washed with dichloromethane using a Soxhlet extraction for
0
0
II
Synthesis of Pd /UOF-1 and Pd /UOF-2
2
4 h, and then dried under vacuum at 808C for 12 h to give Pd /
II
UOF-1 as a gray powder (0.29 g, 80%). The Pd content in Pd /UOF-
After the cross-coupling reaction between 4-bromoacetophenone
1
1
1
5
was 16.87 wt% as determined by ICP. FT-IR (KBr): n˜ =3386 (vs),
658 (m), 1610 (s), 1560 (s), 1512 (vs), 1427 (s), 1404 (s), 1385 (s),
332 (m), 1307 (m), 1206 (s), 1050 (w), 1016 (w), 832 (w), 690 (w),
II
II
and phenylboronic acid in the presence of Pd /UOF-1 or Pd /UOF-2
was complete, the resultant mixture was extracted with diethyl
ether (3ꢂ5 mL). The residual suspension was centrifuged and
washed thoroughly with ethanol to afford Pd /UOF-1 or Pd /UOF-2
as gray powders.
À1
26 cm (w).
0
0
II
Synthesis of Pd /UOF-2
II
II
Synthesis of Pd-reduction
Following the same procedures as the synthesis of Pd /UOF-1, Pd /
UOF-2 was obtained as a gray powder (79% yield). The Pd content
After the reduction of nitrobenzene was finished, the resultant mix-
ture was extracted with diethyl ether (3ꢂ5 mL). The residues were
centrifuged, washed thoroughly with water and ethanol to afford
Pd-reduction as a gray powder.
II
in Pd /UOF-2 was 16.83 wt% as determined by ICP. FT-IR (KBr): n˜ =
396 (vs), 1650 (s), 1610 (s), 1499 (vs), 1429 (s), 1384 (s), 1320 (s),
205 (s), 1178 (s), 1049 (w), 821 (w), 713 (w), 689 (w), 552 (vw),
3
1
5
À1
26 cm (vw).
Synthesis of Pd-NaBH4
General procedures for the Suzuki–Miyaura cross-coupling
reaction
An aqueous solution of NaBH (0.1m, 114 mg, 3.0 mmol) was rapid-
ly added to a stirred suspension of Pd /UOF-2 (3.1 mg) in water
(2 mL). After 5 min, the resultant suspension was centrifuged and
washed thoroughly with ethanol to afford Pd-NaBH
powder.
4
II
II
II
Pd /UOF-1 or Pd /UOF-2 (0.5 mol%) was added to a mixture of aryl
bromide (0.5 mmol), arylboronic acid (0.75 mmol), and base
as a gray
4
(
1.0 mmol) in water (2.0 mL). After the mixture was stirred in a pre-
heated oil bath (608C) for the appropriate time, the resultant mix-
ture was cooled in ice water and the product was extracted with
ethyl acetate (3ꢂ5 mL). The combined organic layer was dried and
concentrated under reduced pressure. The crude products were
further purified by flash column chromatography on silica gel to
afford the desired products. The identity of the products was con-
firmed by comparison with literature spectroscopic data.
Acknowledgements
The authors acknowledge the 973 Program (2011CBA00502,
2010CB933501), the National Natural Science Foundation of
China (21273239) and the “One Hundred Talent Project” from
the Chinese Academy of Sciences for financial support.
II
II
Reuse of Pd /UOF-1 and Pd /UOF-2 in the Suzuki–Miyaura
cross-coupling reaction
Keywords: heterogeneous catalysis
· palladium · porous
After the Suzuki–Miyaura cross-coupling reaction was performed in
the presence of Pd /UOF-1 or Pd /UOF-2 (0.5 mol%), 4-bromophe-
organic frameworks · urea · water chemistry
II
II
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