Synthesis of Ultrafine and Highly Dispersed Metal Nanoparticles Confined in a Thioether-Containing Covalent Organic Framework and Their Catalytic Applications

Article abstract of DOI:10.1021/jacs.7b07918

Covalent organic frameworks (COFs) with well-defined and customizable pore structures are promising templates for the synthesis of nanomaterials with controllable sizes and dispersity. Herein, a thioether-containing COF has been rationally designed and used for the confined growth of ultrafine metal nanoparticles (NPs). Pt or Pd nanoparticles (Pt NPs and Pd NPs) immobilized inside the cavity of the COF material have been successfully prepared at a high loading with a narrow size distribution (1.7 ± 0.2 nm). We found the crystallinity of the COF support and the presence of thioether groups inside the cavities are critical for the size-controlled synthesis of ultrafine NPs. The as-prepared COF-supported ultrafine Pt NPs and Pd NPs show excellent catalytic activity respectively in nitrophenol reduction and Suzuki-Miyaura coupling reaction under mild conditions and low catalyst loading. More importantly, they are highly stable and easily recycled and reused without loss of their catalytic activities. Such COF-supported size-controlled synthesis of nanoparticles will open a new frontier on design and preparation of metal NP@COF composite materials for various potential applications, such as catalysis and development of optical and electronic materials.

Full text of DOI:10.1021/jacs.7b07918

Subscriber access provided by READING UNIV
Article
Synthesis of Ultrafine and Highly Dispersed Metal
Nanoparticles Confined in a Thioether-Containing Covalent
Organic Framework and Their Catalytic Applications
Shuanglong Lu, Yiming Hu, Shun Wan, Ryan McCaffrey, Yinghua Jin, Hongwei Gu, and Wei Zhang
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 02 Nov 2017
Downloaded from http://pubs.acs.org on November 2, 2017
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Journal of the American Chemical Society is published by the American Chemical
Society. 1155 Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 9
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
Synthesis of Ultrafine and Highly Dispersed Metal Nanoparticles
Confined in a Thioether-Containing Covalent Organic Framework
and Their Catalytic Applications
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Shuanglong Lu,†, ‡ Yiming Hu,‡ Shun Wan,§ Ryan McCaffrey,‡ Yinghua Jin,‡ Hongwei Gu*,† Wei
Zhang*, ‡
†Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials
Science, Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123,
China.
‡Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA
§NCO Technologies LLC, Concord, NC 28027, USA
ABSTRACT: Covalent organic frameworks (COFs) with wellꢀdefined and customizable pore structures are promising templates
for the synthesis of nanomaterials with controllable sizes and dispersity. Herein, a thioetherꢀcontaining COF has been rationally
designed and used for the confined growth of ultrafine metal nanoparticles (NPs). Pt or Pd nanoparticles (Pt NPs and Pd NPs)
immobilized inside the cavity of the COF material have been successfully prepared at a high loading with a narrow size distribution
(1.7 ± 0.2 nm). We found the crystallinity of the COF support and the presence of thioether groups inside the cavities are critical for
the sizeꢀcontrolled synthesis of ultrafine NPs. The asꢀprepared COFꢀsupported ultrafine Pt NPs and Pd NPs show excellent catalytic
activity respectively in nitrophenol reduction and SuzukiꢀMiyaura coupling reaction under mild conditions and low catalyst loading.
More importantly, they are highly stable and easily recycled and reused without loss of their catalytic activities. Such COFꢀ
supported sizeꢀcontrolled synthesis of nanoparticles will open a new frontier on design and preparation of metal NP@COF
composite materials for various potential applications, such as catalysis and development of optical and electronic materials.
organic porous polymers with wellꢀdefined pore structures.17ꢀ21
They have shown interesting potential applications in gas
INTRODUCTION
Ultrafine and highly dispersed metal nanoparticles (NPs)
have attracted extensive research interests within the past few
years due to their unique optical, electronic and mechanical
properties.1ꢀ2 Provided high surface area and surfaceꢀtoꢀ
volume ratio, ultrafine noble metal nanoparticles (Pt, Pd, etc.)
are of particular interest for the applications in heterogeneous
catalysis. These nanoparticles have been wildly used in
various reactions, such as dehalogenation,3 hydrogenation,4ꢀ5
coupling reactions,6 showing excellent catalytic activity, high
atom utilization, and easy recyclability. It has been proved that
ultrafine NPs with smaller sizes generally exhibit remarkable
catalytic activities.7ꢀ8 However, ultrafine NPs easily aggregate
due to their high surface energy, with greater aggregation
tendency for smaller particles. As a result, they gradually lose
their catalytic activity over time, hampering reusability and
thus limiting their practical applications. In order to obtain
stable nanoparticles with constant high catalytic activity,
researchers developed various supporting substrates,9ꢀ10
including porous silica,11 carbon materials,12 graphene,13 metal
oxide,14 polymers,15 and metal organic frameworks (MOFs).16
However, it remains a challenge to obtain ultrafine NPs with
high stability and a narrow size distribution.
storage and separation, heterogeneous catalysis, optoelectronic
materials and energy storage.22ꢀ26 However, only recently have
they been reported as promising hosts to support NPs.27ꢀ32
COFꢀbased supports potentially could have several unique
advantages: (1) COFs have preꢀdesignable wellꢀdefined pore
structures which can be used to confine nanoparticle growth
and thus control the size of NPs; (2) the pore channels in
COFs are wellꢀisolated, thus the aggregation of entrapped NPs
can be minimized; (3) the chemical structures of COFs can be
easily tailored and surface functional groups that can anchor
metal NPs can be easily installed; (4) many COFs are stable
and thus would be resistant to decomposition under various
reaction conditions. Banerjee and coworkers reported COFꢀ
supported Au NPs and Pd NPs hybrid materials, which show
high catalytic activity in nitrophenol reduction and
Sonogashiraꢀcoupling, respectively.30ꢀ31 However, relatively
larger size of NPs (5 ± 3 nm for Au NPs and 7 ± 3 nm for Pd
NPs) compared to the pore size of the COF support (~1.8 nm)
were obtained with broad size distribution, which indicates
surface deposition of NPs rather than encapsulation inside the
pores. The lack of control of the nanoparticle size is likely due
to weak interactions between COF support and metal NPs.33
Excellent size control (2.4 ± 0.5 nm) and even distribution of
Pd NPs in a COF have
In the past decade, an impressive number of covalent
organic frameworks (COFs) have been reported as crystalline
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 9
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 1. (a) Synthesis of Thio-COF and (b) Schematic representation of the synthesis of Thio-COF supported PtNPs@COF and
PdNPs@COF. Top and side views of the energyꢀminimized models of Thio-COF (yellow, S; blue, N; grey, C; red, O) are shown in b.
been achieved by the Fischer group through gasꢀphase
infiltration of volatile organometallic precursors inside the
cavity of the 3D COFꢀ102 followed by photodecomposition of
the precursor to form Pd NPs.27
120 °C over 2 h. The reaction was kept at this temperature for
3 days and cooled to room temperature over 12 h. The orange
precipitate was collected by vacuum filtration, washed with a
large amount of acetone and CH2Cl2, and dried under vacuum
to yield the thioetherꢀcontaining COF (Thio-COF, 38 mg,
91 %). Elemental analysis calcd (%) for (C15H14NOS)n: C,
70.28; H, 5.50; N, 5.46. Found: C, 69.97; H, 6.50; N, 5.11.
Synthesis of PtNPs@COF and PdNPs@COF. In a typical
procedure, a wellꢀdispersed suspension of asꢀprepared Thio-
COF (4 mg) in methanol (3 mL) was mixed with a solution of
metal precursor K2PtCl4 (5 mg, 1.2×10ꢀ2 mmol) or K2PdCl4 (4
mg, 1.2×10ꢀ2 mmol) in water (2 mL). The mixture was brought
to dryness under vacuum with stirring to deposit metal
precursors in Thio-COF support. The mixture was reꢀ
dispersed in a mixed solvent of methanol and water (5 mL,
MeOH/H2O, v/v = 3:2). A solution of NaBH4 in methanol
(0.25 M, 2 mL) was added dropwise and the mixture was
stirred for 2 days. The product was collected through
centrifugation, washed with ethanol and dichloromethane
three times, and dried under vacuum for further use.
General procedure for the reduction of 4-nitrophenol
catalyzed by PtNPs@COF. To a cuvette were added an
aqueous solution of 4ꢀnitrophenol (1.0 × 10ꢀ3 M, 0.3 mL),
water (1 mL) and an aqueous solution of NaBH4 (0.5 M, 1
mL). A suspension of PtNPs@COF in water (0.5 mg/mL, 25
ꢁL or 50 ꢁL) was introduced, and the reaction progress was
monitored with the UVꢀVis spectroscopy. The initial bright
yellow solution gradually turned to colorless as the reaction
progressed.
These literature precedents suggest that encapsulation and
seedꢀgrowth of NPs inside the cavities are critical to confine
nanoparticle growth and control the size. We envision that the
presence of strong anchoring groups inside the pores would
facilitate the binding of metal ions/nanoparticles and
formation of nucleation sites inside the cavities, thus enabling
confined growth of nanoparticles and control of their particle
size. Previously, we have reported a sizeꢀcontrolled synthesis
of narrowlyꢀdistributed ultrafine gold nanoparticles (average
size: 1.9 nm) using a shapeꢀpersistent organic cage bearing
thioether groups as a template.34 We have demonstrated that
the presence of thioether group inside the cavity is crucial for
the sizeꢀcontrolled synthesis of Au NPs, serving as nucleation
sites for the metal deposition and NPs growth. Herein, we
report the synthesis of a thioetherꢀcontaining COF and its
application in controlled growth of ultrafine noble metal
nanoparticles. The COFꢀsupported Pd NPs and Pt NPs
(PtNPs@COF and PdNPs@COF) with narrow size
distribution (1.7 ± 0.2 nm) have been successfully prepared
with high distribution density. The asꢀprepared PtNPs@COF
and PdNPs@COF show excellent catalytic activities in
reduction of 4ꢀnitrophenol and the SuzukiꢀMiyaura coupling
reaction, respectively. Advantages of these COFꢀNP hybrid
catalysts over organic molecular cageꢀencapsulated NPs
include their efficient exposure of active sites, high stability
over multiple catalytic cycles, and easy recyclability. We also
demonstrate the critical importance of crystallinity of the COF
support and thioether functional groups in controlling the
formation of NPs with narrowꢀsize distribution.
General procedure for the Suzuki-Miyaura coupling
reaction catalyzed by PdNPs@COF. In a typical procedure
of the coupling reaction, arylhalide (1 mmol), phenylboronic
acid (1.1 mmol), K2CO3 (1.5 mmol), and PdNPs@COF (0.1
mol%) were mixed in DMF/H2O (3 mL, 1:1 v:v) in a Schlenk
tube. The reaction tube was evaluated and flushed with N2 gas
EXPERIMENTAL SECTION
o
three times. The mixture was heated at 50 C for 3 h with
Synthesis of Thio-COF. To a glass tube (the outer diameter is
10 mm and the inner diameter is 8 mm) were added
trialdehyde 1 (11 mg, 0.05 mmol), thioether substituted
diamine 2 (31 mg, 0.075mmol), 6 M acetic acid (0.2 mL),
dioxane (1.8 mL) and mesitylene (0.2 mL). The tube was
flash frozen at 77 K in liquid nitrogen and evacuated to the
internal pressure of ~100 mtorr, and then the tube was sealed
under an open flame. The mixture was first warmed to room
temperature and then the temperature was slowly raised to
stirring. The product was extracted with dichloromethane and
1
the yield was analyzed by H NMR integration of the crude
product mixture.
RESULTS AND DISCUSSION
The thioetherꢀcontaining imineꢀlinked COF (Thio-COF) was
synthesized through the condensation of trialdehyde 1 and
ACS Paragon Plus Environment

Page 3 of 9
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 2. Characterizations of PtNPs@COF: (a) lowꢀmagnification TEM image, the inset is the size distribution profile of Pt NPs; (b)
highꢀmagnification TEM image; (c) highꢀresolution TEM (HRꢀTEM) image; (d) highꢀangle annular darkꢀfield scanning transmission
electron microscopy (HAADFꢀSTEM) image; (e) selected area electron diffraction (SAED) pattern; (f) energy dispersive Xꢀray
spectroscopy (EDS) mapping of composition element, Pt, S, C and N respectively.
thioether substituted diamine 2 (Figure 1a). Under the
solvothermal conditions (1,4ꢀdioxane and mesitylene, 6M
HOAc, 120 °C, 3 days), Thio-COF was obtained as yellow
powder with an isolated yield of 91%. The COF material is not
soluble in water and common organic solvents such as
dichloromethane, acetone, ethanol, and tetrahydrofuran.
Thio-COF was characterized by elemental analysis, Fourier
transform infrared (FTꢀIR) spectroscopy, solid state 13C NMR
spectroscopy, scanning electron microscopy (SEM), and
thermal gravimetric analysis (TGA). The FTꢀIR spectrum of
Thio-COF (Figure S1) shows the characteristic C=O stretch
of a βꢀketoꢀenamine at ~1621 cmꢀ1. The structure of Thio-
COF was also supported by the solidꢀstate 13C crossꢀ
polarization magic angle spinning (CPꢀMAS) NMR spectrum
(Figure S2), showing the resonance signals around 148 ppm
and 185 ppm, which are corresponding to the resonance
signals of enamine and ketone carbons, respectively.35ꢀ37 The
scanning electron microscopy (SEM) images show irregular
shaped particles of Thio-COF (Figure S3). The elemental
composition detected by Energy Dispersive Xꢀray
Spectroscopy (EDS) agrees well with the theoretical values
and the element analysis result (Figure S4). TGA indicates
Thio-COF has a good stability, showing weight loss of 13%
at 300 oC (Figure S5).
assigned to planes (100), (110) and (220), respectively. It
indicates Thio-COF has good crystallinity with a long range
ordered molecular structure. Computational simulation and
Pawley refinement was performed to obtain the probable
structure of Thio-COF using Materials Studio software
package. Simulated models were constructed in the hexagonal
system, with layers lying on the ab plane. After the
geometrical energy minimization, the optimized parameters
for the unit cell are a = b = 38.82 Å, c = 4.05 Å (residuals: Rwp
= 6.89%, Rp = 5.37%). Through the comparison of the
simulated PXRD patterns with the experimental profile, we
determined that Thio-COF is in fully eclipsed AA stacking
mode with the interlayer distance of 4.05 Å (Table S1). The
theoretical pore size was determined to be 2.4 nm. Brunauerꢀ
EmmettꢀTeller (BET) surface area of Thio-COF is 50 m2 gꢀ1
(Figure S7). The relatively low surface area of Thio-COF is
probably due to the thin layers or small domains of the COF
material, similar to case reported by Banerjee on the high
crystalline COF (TpPa(MC), DaTp-CONs) with low porosity
(BET surface area of 53ꢀ61 m2 gꢀ1).38ꢀ39 The sample showed
only a slight decrease of crystallinity after high temperature
activation at 120 oC and BET surface area measurement.
Since thioether groups have strong binding interactions with
metal ions, we then explored the possibility of using Thio-
COF as a template to guide the nucleation and growth of
metal NPs inside the pores. Thioetherꢀbased COFs have
recently been reported to be able to detect and remove of Hg
ions.40ꢀ42 It was thus expected that the thioether groups
The crystallinity of Thio-COF was evaluated by powder Xꢀ
ray diffraction (PXRD) measurement (Figure S6, black). A set
of intensive peaks at 2θ = 3.04o, 4.99o and 10.23o were
observed in the experimental PXRD pattern, which can be
3
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 9
pointing inside the cavity would efficiently bind metal ions 4f5/2, which can be assigned to Pt(II) species. The presence of
1
2
3
4
5
6
7
8
and serve as the nucleation sites for confined growth of metal
NPs in the wellꢀdefined pores of the COF support. We used
wetꢀchemistry approach to synthesize Thio-COF-supported Pt
NPs (PtNPs@COF) (Figure 1b). The metal precursor was
deposited in the COF material by mixing K2PtCl4 solution
with a suspension of Thio-COF and evaporating all the
volatiles under vacuum with stirring. The mixture was reꢀ
dispersed and reduced with NaBH4 in methanol to provide
PtNPs@COF, which was thoroughly washed and dried before
the characterization and further use.
Pt(II) species is likely due to the reoxidation of Pt(0) to Pt(II)
by exposure to air.5,43ꢀ44 The binding energy of Pt 4f7/2 peak of
the Pt(0) species in the PtNPs@COF has a higher value
compared to that of pure Pt NPs, while the binding energy of S
2p in PtNPs@COF has a negative shift compared to that of S
in Thio-COF (Figure S13). These shifts are likely caused by
metalꢀligand interactions (e.g. the charge transfer from Pt
nanoparticles to S atoms).45 We found the molar ratio of Pt/S
(1.38) determined by the XPS is much lower than the ratio
(5.13) determined by the TEMꢀEDS (Figure S14), which
suggests the majority of Pt NPs are encapsulated inside the
COF pores and only limited amount of Pt NPs are deposited
on the surface.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The typical transmission electron microscope (TEM) images
of PtNPs@COF (Figure 2a, b) reveal that the ultraꢀsmall Pt
NPs were uniformly distributed in the COF material. The
statistic histogram for more than 100 randomly chosen
particles in the Figure 2a shows that the ultrafine Pt NPs have
an average size of 1.70 nm in diameter with a narrow size
distribution (inset in Figure 2a). We found the reaction
conditions have significant effect on the resulting nanoparticle
size. When the reduction was conducted in water rather than
To probe the importance of structural order of the COF
support for the sizeꢀcontrolled growth of NPs, we used
amorphous thioetherꢀcontaining polymer with much higher
BET surface (588 m2gꢀ1), which was obtained from the same
monomers 1 and 2, as the supporting substrate for comparison
(Figure S9a, b). We found rather random distribution of Pt
NPs with low deposition density, indicating ordered COF
structure is preferred to obtain uniform and wellꢀdispersed Pt
NPs. As a control experiment, we also prepared Pt NPs in the
absence of a COF support. As expected, severe aggregation of
Pt NPs was observed (Figure S9c). Interestingly, when
analogous crystalline thioetherꢀfree COF was applied as a
supporting substrate, domains of evenly distributed Pt NPs as
well as highly aggregated Pt NPs were observed (Figure S9d).
This suggests, in the absence of metal stabilization sites in the
thioetherꢀfree COF, there is lack of control on the deposition
sites of NPs, thus substantial Pt NPs tend to grow on the
surface of COF materials and form NPs of uncontrollable
random size. Only limited amount of NPs are statistically
seeded in the interior of the COF support and grow in the
confined space with size control and protection from
agglomerization. It is therefore evident that both the structural
order of supporting substrate and thioether sites are
indispensable to obtain uniform and wellꢀdispersed Pt NPs.
The asꢀprepared PtNPs@COF is readily dispersed in water
and many organic solvents. Since wellꢀdispersed ultrafine NPs
generally exhibit high catalytic activities, next we explored the
application of PtNPs@COF in heterogeneous catalysis. The
catalytic activity of PtNPs@COF was examined by the
reduction of 4ꢀnitrophenol in the presence of NaBH4. This
reaction can be easily monitored by UVꢀVis spectroscopy and
has been widely used as a model reaction to evaluate the
activity of noble metal catalysts. This reaction also has a
practical significance since 4ꢀnitrophenol is identified as a
priority pollutant, while the product 4ꢀaminophenol is an
important precursor for medical industry. As a control
experiment, we first conducted the 4ꢀnitrophenol reduction in
the presence of Thio-COF itself (0.17 mg/mL, 50 ꢁL), and
monitored the reaction by the UVꢀVis spectra. The absorption
peak at 400 nm, a characteristic peak of 4ꢀnitrophenol and
NaBH4 mixture, remains almost unchanged after 10 min
(Figure S15a), which indicates the COF alone does not have
any considerable catalytic activity. By contrast, when the
PtNPs@COF catalyst
o
methanol, or the reaction temperature was raised to 50 C,
much larger size of NPs with broader size distribution were
formed (Figure S8b, c). The formation of COFꢀsupported Pt
NPs was further confirmed by the highꢀangle annular darkꢀ
field scanning transmission electron microscopy (HAADFꢀ
STEM) image (Figure 2d), which clearly shows the presence
of wellꢀdispersed Pt NPs embedded in the COF material
without any sign of agglomerization. We did not observe any
unꢀsupported Pt NPs, suggesting Thio-COF is a favorable
support for the growth of NPs. The high resolution TEM
image (HRꢀTEM) of the PtNPs@COF (Figure 2c) shows that
those Pt NPs located on the surface has interꢀplanar spacing of
0.23 nm corresponding to the plane of a typical Pt faceꢀ
centered cubic (fcc) structure. We found some Pt NPs don’t
show clear lattice fringes, which is likely caused by their
encapsulation inside the COF cavities. The corresponding
selected area electron diffraction (SAED) pattern in Figure 2e
shows the polycrystalline nature of Pt NPs and characteristic
diffraction rings at (111), (200), (220) and (311), which is
consistent with the PXRD pattern of Pt NPs supported by
Thio-COF (Figure S10).
The inductively coupled plasma (ICP) analysis reveals that
the experimental Pt content in the hybrid material is 34.36
wt%, which correlates well with the theoretical Pt amount we
applied (36.91 wt%). Such a high loading of ultrafine metal
NPs without aggregation has rarely been observed in previous
literature reports.15 This result also supports the excellent
confinement ability of Thio-COF material. The composition
of PtNPs@COF was further characterized by energy
dispersive spectrum (EDS) elemental mapping. As shown in
Figure 2f, the component elements Pt, S, C and N are all
evenly distributed in the hybrid material, which again supports
the excellent dispersity of Pt NPs in the Thio-COF material.
The oxidation state and the interaction of Pt with S were
evaluated by Xꢀray photoelectron spectroscopy (XPS)
measurement. The spectrum of Pt 4f region shows a doublet
containing a lower energy (Pt 4f7/2) band and a higher energy
(Pt 4f5/2) band (Figure S12). The deconvolution of the
spectrum revealed two pairs of doublets: (1) centered at 71.3
eV for Pt 4f7/2 and 74.8 eV for Pt 4f5/2, which can be assigned
to Pt(0); (2) centered at 72.2 eV for Pt 4f7/2 and 76.0 eV for Pt
4
ACS Paragon Plus Environment

Page 5 of 9
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 3. (a) Timeꢀdependent UVꢀVis spectra of the reduction of
4ꢀnitrophenol (0.3 ꢁmol) catalyzed by 50 ꢁL PtNPs@COF (0.50
mg mLꢀ1); (b) Typical optical images of reaction mixtures at 0
min and 8 min; (c) Timeꢀdependent UVꢀVis spectra of the
reduction of 4ꢀnitrophenol (0.3 ꢁmol) catalyzed by 50 ꢁL
unsupported Pt NPs (0.17 mg mLꢀ1); (d) The recyclability of the
PtNPs@COF catalyst.
Figure 4. Characterizations of PdNPs@COF: (a) lowꢀ
magnification TEM image; (B) the size distribution profile of Pd
NPs; (C) Highꢀresolution TEM image; (D) EDS mapping of
composition elements, Pd, S, C and N respectively.
(25 ꢁL, 0.5 mg mLꢀ1) was introduced into the mixture, the
reduction was initiated immediately, showing the rapid
decrease of the 4ꢀnitrophenol absorption peak at 400 nm and
the appearance of a new peak at ~300 nm corresponding to 4ꢀ
aminophenol (Figure S15b). When the dosage of
PtNPs@COF catalyst (50 ꢁL, 0.5 mg mLꢀ1) was doubled, the
full conversion of 4ꢀnitrophenol to 4ꢀ aminophenol was
quickly reached in 8 min with the complete disappearance of
4ꢀnitrophenol peak at 400 nm (Figure 3a). This was even
obvious to the naked eye, showing the complete discoloration
of the solution from bright yellow to colorless (Figure 3b).
Our catalyst loading (0.04 ꢁmol Pt per 0.3 ꢁmol 4ꢀnitrophenol)
is ~4500 fold less than the loading of the previously reported
ultrafine Pt nanoparticles (0.2 mmol Pt per 0.3 ꢁmol 4ꢀ
nitrophenol) supported by chalcogels that show a comparable
reaction rate.5 When the same amount of unsupported Pt NPs
without COF were used as the catalyst of the 4ꢀnitophenol
reduction (Figure 3c), much longer reaction time (~ 50 min)
was required to reach near complete conversion likely due to
their larger sizes and aggregated structures.
Stability and reusability represent important parameters to
evaluate a heterogeneous catalyst for practical applications. A
successive addition experiment was performed to study the
stability and reusability of PtNPs@COF. After completion of
the first run, a fresh starting material (4ꢀnitrophenol, 1.0 × 10ꢀ3
M, 0.3 mL) was directly added to the reaction mixture of the
first run and the reaction was monitored as before. The
procedure was repeated until the completion of sixth cycle. As
shown in Figure 3d, the conversation of the 4ꢀnitrophenol for
each reaction cycle was constantly near 90%, indicating the
catalyst is stable under the reaction conditions and does not
undergo major decomposition and lose the activity gradually.
After reaction, the catalyst can be easily recycled through
centrifugation or natural settling. The TEM image shows the
recycled PtNPs@COF retain their structural features, still
having wellꢀdispersed Pt NPs with little aggregation (Figure
S16).
The excellent stability of Pt NPs could be attributed to the
thioetherꢀcontaining COF support, which facilitates anchoring
and confinement of NPs within the pores, thus efficiently
stabilizing these NPs.
Given the success in the synthesis of PtNPs@COF with high
stability and catalytic activity, we proceeded to use Thioꢀ COF
to grow Pd NPs to demonstrate the general applicability of
such COFꢀtemplated NP synthesis strategy. Pdꢀbased catalysts
are widely used in many organic reactions. It is therefore of
great significance to develop active and stable Pd catalysts.
The PdNPs@COF was prepared following the similar
procedure described for the synthesis of PtNPs@COF using
NaBH4 as the reducing agent. The typical TEM image shows
that asꢀobtained Pd NPs immobilized in ThioꢀCOF generally
have highly fine dispersity and ultraꢀsmall size, except for
several particles of relatively large size (Figure 4a). The
statistic of these Pd NPs reveals that these NPs have an
average size of 1.78 nm with a narrow size distribution (Figure
4b). The Pd content in the composite material is determined to
be 26.30 wt% by the ICP analysis, which agrees well with the
amount of Pd we applied (24.59 wt%). These observations
suggest that ThioꢀCOF is an excellent support for Pd NPs,
uptaking nearly all the Pd species and facilitating the
confinement and growth of Pd NPs. Highꢀresolution TEM
image (Figure 4c) confirms that the Pd NPs are in crystalline
phase with a crystal plane spacing of 0.23 nm, which
corresponds to the (111) plane of Pd NPs. Similar to the case
of PtNPs@COF, the EDS elemental mapping reveals even
element distribution and excellent dispersity of Pd NPs within
ThioꢀCOF (Figure 4d). The oxidation state of Pd species and
the interaction between Pd and S were investigated by the XPS
analysis (Figure S17 and S18), which shows the presence of
both Pd(0) and Pd(II) and binding of Pd NPs with thioether
5
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 9
Table 1 Catalytic performance of PdNPs@COF in the
SuzukiꢀMiyaura Coulpling reaction.
Table 2 Reusability of PdNPs@COF in the SuzukiꢀMiyaura
Coulpling reaction.a
1
2
3
4
5
6
7
Yield (%) b
>99.0
Entry
1st batch
Time (h)
3
Yield (%) b
>99.0
Entry
1a
Aryl halides
Time (h)
3
I
8
9
2nd batch
3rd batch
4th batch
5th batch
3
3
3
3
98.0
98.0
99.0
98.0
2a
3a
4a
5a
3
3
3
3
>99.0
82.9
85.7
96.8
O
I
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
O
Br
Br
a
Reaction condition: arylhalide (1 mmol), phenylboronic acid
(1.1 mmol), K2CO3 (1.5 mmol), PdNPs@COF (0.5 mol%),
DMF/H2O (1.5 mL/1.5 mL), 50 ˚C. b The yield is based on the 1H
NMR integration.
Br
6a
7c
8d
3
3
3
>99.0
30.6
OHC
Br
I
reduced yields, 30.6% and 74.6%, were observed when the
unsupported Pd NPs and Pd(PPh3)2Cl2 were used as the
catalysts, respectively (entry 7ꢀ8). No conversion was
observed in the presence of Thio-COF alone, indicating the
COF material itself doesn’t have any catalytic activity towards
the SuzukiꢀMiyaura coupling reaction. The catalytic activity of
PdNPs@COF under such mild conditions (50 ˚C, 3 h) and
low loading (0.1 mol% Pd) surpasses many commonly used
homogeneous Pd catalysts, e.g. Pd(PPh3)2Cl2, as well as
previously reported Pdꢀbased heterogeneous catalysts,
including Pd(OAc)2 incorporated COF materials24 and the
Pd(II)ꢀcontaining MOF materials.50
74.6
I
a
Reaction condition: arylhalide (1 mmol), phenylboronic acid
(1.1 mmol), K2CO3 (1.5 mmol), PdNPs@COF (0.1 mol % Pd
loading), DMF/H2O (1.5 mL/1.5 mL), 50 C. b The yield is based
o
c
d
on the NMR integration. Pd NPs (0.1 mol %). Pd(PPh3)2Cl2
(0.1 mol %). The TOF numbers of PdNPs@COF, commercial
Pd/C, and Pd(PPh3)2Cl2 are 30444 hꢀ1, 185 hꢀ1, and 1048 hꢀ1,
respectively, under the same reaction condition, with 0.1 mol%
catalyst loading.
PdNPs@COF also exhibits excellent stability and
recyclability under the applied reaction conditions. The
catalysts were conveniently recycled through centrifugation
and washing with water and dichloromethane. The stability
and reusability of PdNPs@COF were evaluated using 0.5
mol% catalyst in the reaction between 1ꢀiodoꢀ4ꢀ
methylbenzene and phenylboronic acid. After the first run, the
catalyst was recycled and directly reused for the next run
without any other reactivation process. The catalyst was
recycled total five times, and almost complete retention of
activity (≥98% yield) was observed over all five consecutive
runs (Table 2), indicating the ultraꢀstable property of
PdNPs@COF catalyst. The recycled catalyst after the 5th run
was analyzed by the TEM (Figure S20). No obvious change in
the size of Pd NPs and aggregation between NPs were
observed. The excellent catalytic activity and stability of both
PtNPs@COF and PdNPs@COF can be attributed to the
following two factors: (1) the ultrafine, wellꢀdispersed nature
of these NPs is anticipated to significantly enhance their
accessible active surface area, thus both Pd and Pt NPs can
provide abundant catalytic sites; 2) wellꢀdefined cavities of
Thio-COF materials can facilitate easy mass transfer. They
can also provide protecting shell to maintain the good stability
of those NPs.
groups. Careful examination of the XPS spectrum showed that
the Pd(II) species are reꢀoxidized metal species (PdO), which
have been commonly observed in nanoparticle that are
exposed to air.44, 46ꢀ48
The catalytic activity of PdNPs@COF was examined in one
of the representative Pdꢀcatalyzed reactions, the Suzukiꢀ
Miyaura coupling reaction. The reactions between different
aryhalides and phenylboronic acid were performed under
similar reaction conditions as previously reported for polymerꢀ
supported Pd catalysts.49
A
mixture of arylhalide,
phenylboronic acid, K2CO3, and PdNPs@COF in DMF/H2O
was heated with stirring, and the crude product was analyzed
by 1H NMR spectroscopy (Table 1). Excellent catalytic
activity of PdNPs@COF was observed with only 0.1 mol% Pd
loading under mild conditions (50 ˚C, 3 h) for various
substrates, including aryl iodides (>99.0% yield, entry 1 and 2)
and less active aryl bromides (82.9%ꢀ99.0%, entry 3ꢀ6).
For comparison purpose, we also tested Pd NPs synthesized
without the COF support (the morphology of unsupported Pd
NPs is shown in Figure S19b) and the commonly used Pd
catalyst, Pd(PPh3)2Cl2, as a catalyst in the coupling of 1ꢀiodoꢀ
4ꢀmethylbenzene and phenylboronic acid under otherwise
identical conditions. In great contrast to the coupling reaction
(>99% yield) under the catalysis of PdNPs@COF,
significantly
6
ACS Paragon Plus Environment

Page 7 of 9
Journal of the American Chemical Society
(3)
(4)
(5)
(6)
(7)
(8)
Shiraishi, Y.; Takeda, Y.; Sugano, Y.; Ichikawa, S.; Tanaka, S.;
Hirai, T. Chem. Commun. 2011, 47, 7863.
Navarro, R.; Pawelec, B.; Trejo, J.; Mariscal, R.; Fierro, J. J.
Catal. 2000, 189, 184.
Liu, J.; He, K.; Wu, W.; Song, T. B.; Kanatzidis, M. G. J. Am.
Chem. Soc. 2017, 139, 2900.
Wan, Y.; Wang, H.; Zhao, Q.; Klingstedt, M.; Terasaki, O.; Zhao,
D. J. Am. Chem. Soc. 2009, 131, 4541.
Tsunoyama, H.; Sakurai, H.; Negishi, Y.; Tsukuda, T. J. Am.
Chem. Soc. 2005, 127, 9374.
Lei, Y.; Mehmood, F.; Lee, S.; Greeley, J.; Lee, B.; Seifert, S.;
Winans, R. E.; Elam, J. W.; Meyer, R. J.; Redfern, P. C. Science
2010, 328, 224.
White, R. J.; Luque, R.; Budarin, V. L.; Clark, J. H.; Macquarrie,
D. J. Chem. Soc. Rev. 2009, 38, 481.
CONCLUSIONS
1
2
3
4
5
6
7
8
In conclusion, we have developed a novel COFꢀtemplated
strategy for the sizeꢀcontrolled synthesis of stable and highly
dispersed unltrafine metal NPs. The thioetherꢀcontaining COF
was rationally designed and synthesized for the confined
growth of metal NPs. With the aid of the evenly distributed
thioether groups in the ordered framework structure, ultrafine
Pt or Pd nanoparticles (1.7 ± 0.2 nm) with a narrow size
distribution were successfully obtained. The COFꢀsupported
ultrafine nanoparticles show excellent catalytic activities (low
catalyst loading, mild reaction conditions, and high yielding)
towards the reduction of nitrophenol and SuzukiꢀMiyaura
coupling reaction. More importantly, these metal NP catalysts
are highly stable and easily recycled and reused without loss
of catalytic activity. The structure characterization of ultrafine
metal NPs suggests that they likely reside in the pores of the
COF structure, with only limited deposition on the surface.
The crystalline COF structure with thioether groups in the
wellꢀdefined cavity is critical for the successful synthesis of
NPs with narrow size distribution, providing favorable
accommodation sites for nanoparticles through metalꢀsulfur
binding interactions, confined space for sizeꢀcontrolled growth
of NPs, and also the protecting shell preventing the
aggregation of NPs. This work will open a new frontier on
design and preparation of highly stable metal NP@COF
composite materials with wellꢀdispersed NPs of narrow size
distribution. Further diversifying various COF structures and
different nanomaterials would provide great opportunities to
prepare functional composite nanomaterials for different
potential applications.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(9)
(10)
Chen, L.; Luque, R.; Li, Y. Chem. Soc. Rev. 2017, 46, 4614.
(11) Shang, L.; Bian, T.; Zhang, B.; Zhang, D.; Wu, L. Z.; Tung, C. H.;
Yin, Y.; Zhang, T. Angew. Chem., Int. Ed. 2014, 53, 250.
Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.;
Ryoo, R. Nature 2001, 412, 169.
Nethravathi, C.; Anumol, E. A.; Rajamathi, M.; Ravishankar, N.
Nanoscale 2011, 3, 569.
Wang, W. N.; An, W. J.; Ramalingam, B.; Mukherjee, S.;
Niedzwiedzki, D. M.; Gangopadhyay, S.; Biswas, P. J. Am. Chem.
Soc. 2012, 134, 11276.
Huang, Y. B.; Wang, Q.; Liang, J.; Wang, X.; Cao, R. J. Am.
Chem. Soc. 2016, 138, 10104.
Yang, Q.; Xu, Q.; Jiang, H. L. Chem. Soc. Rev. 2017, 46, 4774.
Cote, A. P.; Benin, A. I.; Ockwig, N. W.; O'Keeffe, M.; Matzger,
A. J.; Yaghi, O. M. Science 2005, 310, 1166.
Feng, X.; Ding, X.; Jiang, D. Chem. Soc. Rev. 2012, 41, 6010.
Ding, S. Y.; Wang, W. Chem. Soc. Rev. 2013, 42, 548.
Colson, J. W.; Dichtel, W. R. Nat. Chem. 2013, 5, 453.
Jin, Y.; Hu, Y.; Zhang, W. Nat. Rev. Chem. 2017, 1, s41570.
Furukawa, H.; Yaghi, O. M. J. Am. Chem. Soc. 2009, 131, 8875.
Colson, J. C., J. W.; Woll, A. R.; Mukherjee, A.; Levendorf, M.
P.; Spitler, E. L.; Shields, V. B.; Spencer, M. G.; Park, J.; Dichtel,
W. R. Science 2011, 332, 228.
Ding, S.ꢀY.; Gao, J.; Wang, Q.; Zhang, Y.; Song, W.ꢀG.; Su, C.ꢀ
Y.; Wang, W. J. Am. Chem. Soc. 2011, 133, 19816.
Xu, H.; Tao, S. S.; Jiang, D. L. Nat. Mater. 2016, 15, 722.
Du, Y.; Yang, H.; Whiteley, J. M.; Wan, S.; Jin, Y.; Lee, S.ꢀH.;
Zhang, W. Angew. Chem., Int. Ed. 2016, 55, 1737.
Kalidindi, S. B.; Oh, H.; Hirscher, M.; Esken, D.; Wiktor, C.;
Turner, S.; Van Tendeloo, G.; Fischer, R. A. Chem. Eur. J. 2012,
18, 10848.
Stegbauer, L.; Schwinghammer, K.; Lotsch, B. V. Chem. Sci.
2014, 5, 2789.
Kamiya, K.; Kamai, R.; Hashimoto, K.; Nakanishi, S. Nat.
Commun. 2014, 5, 5040.
Pachfule, P.; Panda, M. K.; Kandambeth, S.; Shivaprasad, S. M.;
Díaz, D. D.; Banerjee, R. J. Mater. Chem. A 2014, 2, 7944.
Pachfule, P.; Kandambeth, S.; Diaz Diaz, D.; Banerjee, R. Chem.
Commun. 2014, 50, 3169.
Kamai, R.; Kamiya, K.; Hashimoto, K.; Nakanishi, S. Angew.
Chem., Int. Ed. 2016, 55, 13184.
(12)
(13)
(14)
(15)
(16)
(17)
(18)
(19)
(20)
(21)
(22)
(23)
(24)
ASSOCIATED CONTENT
(25)
(26)
Supporting Information. Detailed experimental procedures, FTꢀ
IR spectra, solidꢀstate 13C CPꢀMAS NMR spectrum, Gas
adsorption data, TGA graph, SEM and TEM images, XPS
analysis, control experiments and structural simulation. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(27)
(28)
(29)
(30)
(31)
(32)
(33)
(34)
(35)
Corresponding Author
*wei.zhang@colorado.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
ChanꢀThaw, C. E.; Villa, A.; Katekomol, P.; Su, D.; Thomas, A.;
Prati, L. Nano Lett. 2010, 10, 537.
McCaffrey, R.; Long, H.; Jin, Y.; Sanders, A.; Park, W.; Zhang,
W. J. Am. Chem. Soc. 2014, 136, 1782.
Shinde, D. B.; Aiyappa, H. B.; Bhadra, M.; Biswal, B. P.; Wadge,
P.; Kandambeth, S.; Garai, B.; Kundu, T.; Kurungot, S.; Banerjee,
R. J. Mater. Chem. A 2016, 4, 2682.
Karak, S.; Kandambeth, S.; Biswal, B. P.; Sasmal, H. S.; Kumar,
S.; Pachfule, P.; Banerjee, R. J. Am. Chem. Soc. 2017, 139, 1856.
Zhu, Y.; Zhang, W. Chem Sci 2014, 5, 4957.
W. Z. acknowledges the support from Army Research Office
(w911NFꢀ12ꢀ1ꢀ0581) and K. C. Wong Education Foundation.
H. G. thanks financial support from the National Natural
Science Foundation of China (No. 21373006), the Priority
Academic Program Development of Jiangsu Higher Education
Institutions (PAPD). S. L. thanks the China Scholarship
Council for scholarship.
(36)
(37)
(38) Biswal, B. P.; Chandra, S.; Kandambeth, S.; Lukose, B.; Heine, T.;
Banerjee, R. J. Am. Chem. Soc. 2013, 135, 5328.
REFERENCES
(39)
(40)
Khayum, M. A.; Kandambeth, S.; Mitra, S.; Nair, S. B.; Das, A.;
Nagane, S. S.; Mukherjee, R.; Banerjee, R. Angew. Chem., Int. Ed.
2016, 55, 15604.
Ding, S. Y.; Dong, M.; Wang, Y. W.; Chen, Y. T.; Wang, H. Z.;
Su, C. Y.; Wang, W. J. Am. Chem. Soc. 2016, 138, 3031.
(1)
(2)
Eustis, S.; ElꢀSayed, M. A. Chem. Soc. Rev. 2006, 35, 209.
Yin, H.; Tang, H.; Wang, D.; Gao, Y.; Tang, Z. Acs Nano 2012, 6,
8288.
7
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 9
(41)
Huang, N.; Zhai, L.; Xu, H.; Jiang, D. J. Am. Chem. Soc. 2017,
139, 2428.
1
2
3
(42) Sun, Q.; Aguila, B.; Perman, J.; Earl, L. D.; Abney, C. W.; Cheng,
Y.; Wei, H.; Nguyen, N.; Wojtas, L.; Ma, S. J. Am. Chem. Soc.
2017, 139, 2786.
4
5
6
7
8
9
(43)
Zhong, H.; Liu, C.; Wang, Y.; Wang, R.; Hong, M. Chem. Sci.
2016, 7, 2188.
(44)
(45)
Li, L.; Zhou, C.; Zhao, H.; Wang, R. Nano Res. 2015, 8, 709.
Lee, H. I.; Joo, S. H.; Kim, J. H.; You, D. J.; Kim, J. M.; Park, J.ꢀ
N.; Chang, H.; Pak, C. J. Mater. Chem. 2009, 19, 5934.
Lee, A. F.; Hackett, S. F. J.; Hargreaves, J. S. J.; Wilson, K.
Green Chem. 2006, 8, 549.
Li, L.; Zhao, H.; Wang, J.; Wang, R. Acs Nano 2014, 8, 5352.
Wang, A.ꢀL.; Xu, H.; Feng, J.ꢀX.; Ding, L.ꢀX.; Tong, Y.ꢀX.; Li,
G.ꢀR. J. Am. Chem. Soc. 2013, 135, 10703.
(46)
(47)
(48)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(49)
(50)
Zhang, P.; Weng, Z.; Guo, J.; Wang, C. Chem. Mater. 2011, 23,
5243.
i Xamena, F. X. L.; Abad, A.; Corma, A.; Garcia, H. J. Catal.
2007, 250, 294.
8
ACS Paragon Plus Environment

Page 9 of 9
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
TOC
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
9
ACS Paragon Plus Environment