Phosphine-Based Covalent Organic Framework for the Controlled Synthesis of Broad-Scope Ultrafine Nanoparticles

Article abstract of DOI:10.1002/smll.201906005

In this work, a phosphine-based covalent organic framework (Phos-COF-1) is successfully synthesized and employed as a template for the confined growth of broad-scope nanoparticles (NPs). Ascribed to the ordered distribution of phosphine coordination sites in the well-defined pores, various stable and well-dispersed ultrafine metal NPs including Pd, Pt, Au, and bimetallic PdAuNPs with narrow size distributions are successfully prepared as determined by transmission electron microscopy, X-ray photoelectron spectroscopy, inductively coupled plasma, and powder X-ray diffraction analyses. It is also demonstrated that the as-prepared Phos-COF-1-supported ultrafine NPs exhibit excellent catalytic activities and recyclability toward the Suzuki–Miyaura coupling reaction, reduction of nitro-phenol and 1-bromo-4-nitrobenzene, and even tandem coupling and reduction of p-nitroiodobenzene. This work will open many new possibilities for preparing COF-supported ultrafine NPs with good dispersity and stability for a broad range of applications.

Full text of DOI:10.1002/smll.201906005

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Phosphine-Based Covalent Organic Framework for the
Controlled Synthesis of Broad-Scope Ultrafine Nanoparticles
Rao Tao, Xiran Shen, Yiming Hu, Kun Kang, Yaqian Zheng, Shichang Luo, Shiyu Yang,
Wenliang Li, Shuanglong Lu, Yinghua Jin, Li Qiu,* and Wei Zhang*
1. Introduction
In this work, a phosphine-based covalent organic framework (Phos-COF-1)
is successfully synthesized and employed as a template for the confined
growth of broad-scope nanoparticles (NPs). Ascribed to the ordered distri-
bution of phosphine coordination sites in the well-defined pores, various
stable and well-dispersed ultrafine metal NPs including Pd, Pt, Au, and
bimetallic PdAuNPs with narrow size distributions are successfully prepared
as determined by transmission electron microscopy, X-ray photoelectron
spectroscopy, inductively coupled plasma, and powder X-ray diffraction
analyses. It is also demonstrated that the as-prepared Phos-COF-1-sup-
ported ultrafine NPs exhibit excellent catalytic activities and recyclability
toward the Suzuki–Miyaura coupling reaction, reduction of nitro-phenol
and 1-bromo-4-nitrobenzene, and even tandem coupling and reduction of
p-nitroiodobenzene. This work will open many new possibilities for pre-
paring COF-supported ultrafine NPs with good dispersity and stability for a
broad range of applications.
Phosphine groups, particularly triph-
enylphosphine (PPh3), have been widely
used as indispensable ligands for orga-
nometallic catalysis where they have
improved the reactivity and stability of
numerous catalysts since 1980s. Promi-
nent examples of metal phosphine
complexes include Wilkinson’s catalyst
(Rh(PPh3)3Cl),[1] Grubbs’ catalyst, and tet-
rakis (triphenylphosphine) palladium(0).[2]
Various phosphine-containing polymers
have been developed to coordinate metal
ions[3] as well as metal nanoparticles,[4]
forming heterogeneous organometallic
catalytic systems. These amorphous sys-
tems show excellent stability and reus-
ability. Inspired by these previous reports,
we envisioned that the incorporation of
PPh3 moieties into covalent organic frame-
works (COFs) would provide ordered dis-
tribution of phosphine coordination sites in porous structures,
forming highly accessible and regularly distributed catalytic
sites upon binding with metal ions and metal nanoparticle.
COFs represent a new type of burgeoning organic porous
materials[5] with well-defined pore structures and crystallinity,
which have received rapidly increasing attention since its first
report in 2005.[6] Their customizable compositions and struc-
tures, diverse functionalities, and high porosity have offered
extensive potential applications[5e,7] in catalysis, gas storage/
separation, energy storage and optoelectronics. Among var-
ious applications, COFs are particularly suitable for catalysis
research,[7b-d,8] especially in the field where they can serve
as supporting substrates for noble metal ions and/or their
corresponding nanoparticles (NPs). They have shown their
superiority in catalysis resulting from the unique compositions
and structures: i) ordered structures with evenly distributed
NP binding sites in the frameworks and ii) well-defined pore
structures that could enable confined growth and thus con-
trollable preparation of NPs; iii) well-isolated pore channels
effectively prevent NPs from aggregation throughout the syn-
thesis and usage. Accordingly, size (particularly on nanoscale)-
controlled synthesis of ultrafine metal NPs could be achieved
through rational design of COF templates. Several examples of
COF-supported NPs have been reported so far, however, with
limited control over their particle sizes.[8a] Moreover, the metal
R. Tao, X. R. Shen, K. Kang, Y. Q. Zheng, S. C. Luo, Prof. L. Qiu
School of Materials Science and Engineering
Yunnan Key Laboratory for Micro/Nano Materials & Technology
Yunnan University
Kunming 650091, China
E-mail: qiuli@ynu.edu.cn
Y. M. Hu, Dr. Y. H. Jin, Prof. W. Zhang
Department of Chemistry
University of Colorado
Boulder, CO 80309, USA
E-mail: wei.zhang@colorado.edu
S. Y. Yang
Research & Development Center of Yunnan Tin Group (Holding) Co. Ltd
Kunming 650000, China
Prof. W. L. Li
Faculty of Chemistry
Northeast Normal University
Jilin 130024, China
Prof. S. L. Lu
Key Laboratory of Synthetic and Biological Colloids
Ministry of Education
School of Chemical and Material Engineering
Jiangnan University
Wuxi 214122, China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.201906005.
DOI: 10.1002/smll.201906005
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Figure 1. a) Synthesis of Phos-COF-1 (conditions: 1,4-dioxane and mesitylene, aqueous acetic acid, 120 °C, 3 d); b) nitrogen adsorption and desorption
isotherms and pore size distribution of Phos-COF-1; c) PXRD patterns of Phos-COF-1: experimental (black), simulated PXRD patterns of eclipsed AA
(red) and staggered ABC (blue) stacking modes.
chelating sites have been limited to bipyridyl[9] and thioether
groups.[7b] To the best of our knowledge, phosphine coordina-
tion sites, which can bind a broad range of metal ions, have
never been explored in COFs.
Herein, we report the first example of a PPh3-based crys-
talline COF (Phos-COF-1) as a scaffold for broad-scope nano-
particle synthesis. Ultrafine (<2 nm) monometallic as well as
bimetallic nanoparticles, PdNPs, PtNPs, AuNPs, and PdAuNPs,
with narrow size distributions have been successfully obtained.
These nanoparticles exhibit high stability, excellent cata-
lytic activities and reusability toward nitrophenol reduction,
cross coupling, as well as tandem cross coupling-nitrophenol
reduction.
stretch at ≈1617 cm−1 supporting the formation of imine bonds.
The solid-state 13C cross polarization magic angle spinning
(CP-MAS) NMR spectrum (Figure S2, Supporting Information)
shows 13C resonance signal at 157 ppm, corresponding to the
carbon atom of the C N bond.[10] The peaks at 137 and 148 ppm

correspond to the resonance of the phenyl carbons attached
to the two imine bonds.[7a] The signals at 135, 128, 122, and
120 ppm can be assigned to the carbons of the phenyl groups
in PPh3 moieties.[3f] The nitrogen adsorption–desorption
isotherms of Phos-COF-1 were measured at 77 K to evaluate its
porosity. As shown in Figure 1b, Phos-COF-1 exhibits a type I
isotherm, showing a sharp increase at low relative pressure
(P/P0 < 0.01), which is characteristic of microporous materials.
The BET surface area is 818 m2 g−1, and the pore size calculated
by using QSDFT (Quenched Solid Density Functional Theory)
and cylindrical pore model shows a pore size distribution
mainly at 1.56 nm (Figure 1b, inset). The PXRD analysis with
Cu Kα radiation clearly shows that Phos-COF-1 is highly crystal-
line with a long-range structure order as depicted in Figure 1c.
An eclipsed AA stacking model, where the lone pairs of the
phosphines in the neighboring layers point toward the same
direction (Figure S3, Supporting Information), and a staggered
ABC stacking model, where the lone pairs of the phosphines
point in opposite ways (Figure S3, Supporting Information),
were investigated. The simulated models were constructed in
a hexagonal crystal system, with layers lying on the ab plane.
The total energy of each unit cell was calculated at the Perdew–
Burke–Ernzerhof functional and double numerical plus d-func-
tions basis set level after geometry and cell optimizations with
symmetry constraints using Dmol3 module. The optimized
eclipsed AA stacking model shows a unit cell of a = b = 26.2 Å,
c = 4.2 Å, α = β = 90°, γ = 120° in P1 symmetry. The
2. Results and Discussion
2.1. Synthesis and Characterization of Phos-COF-1
Phos-COF-1 was synthesized via cocondensation of PPh3-based
trialdehyde 1 and diamine 2 under the solvothermal conditions
(1,4-dioxane and mesitylene, aqueous acetic acid, 120 °C, 3 d),
in an isolated yield of 65% (Figure 1a). The obtained framework
is insoluble in water and common organic solvents such as
N, N-dimethylformamide, dichloromethane, acetone, ethanol,
dimethyl sulfoxide, chloroform and tetrahydrofuran.
Phos-COF-1 was characterized by Fourier transform infrared
(FT-IR) spectroscopy, solid state 13C NMR spectroscopy, gas
adsorption analysis, powder X-ray diffraction (PXRD) anal-
ysis, scanning electron microscopy (SEM) and thermal gravi-
metric analysis (TGA). The FT-IR spectrum of Phos-COF-1

(Figure S1, Supporting Information) displays strong C N
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optimized staggered ABC stacking model shows a unit cell of
a = b = 29.6 Å, c = 4.2 Å, α = β = 90°, γ = 120° in P1 symmetry.
The experimental PXRD patterns of Phos-COF-1 show a set
of peaks at 2θ = 3.91, 7.05, 7.77, 11.77, 15.69, and 19.58 corre-
sponding to plane 100, 110, 200, 300, 400, and 500, respectively,
of the eclipsed AA stacking model. The observation of the full
set of 100, 200, 300, 400, and 500 planes indicates its ordered
structure in the vertical direction. The remaining experimental
peaks located at 2θ = 5.86, 11.81 match well with 110 and 220
planes of the staggered ABC stacking model. The presence of
both the eclipsed AA stacking and staggered ABC stacking crys-
tals in Phos-COF-1 is presumably due to the nonplanar frame-
work structure originated from the trigonal pyramidal geometry
of the PPh3 moieties. The SEM image shows that Phos-COF-1
is composed of regular agglomerated spheres of different
sizes (Figure S4, Supporting Information). TGA indicates that
Phos-COF-1 has an excellent thermal stability, showing a weight
loss of less than 5% at 400°C with the onset decomposition
temperature of 487 °C (Figure S5, Supporting Information).
high binding affinity toward various metal ions.[11] Therefore,
we next explored the possibility of using Phos-COF-1 as a tem-
plate to guide the growth of various metal NPs inside the pores.
We anticipate orderly distributed PPh3 functional groups in
the porous lattice could act as nucleation sites for the growth
of metal NPs. Moreover, the growth of nanoparticles inside the
pores of Phos-COF-1 would prevent their agglomeration during
preparation and usage. A two-phase liquid–liquid approach
developed by Brust et al.[12] was used to prepare Phos-COF-1
supported metal NPs including Pd NPs, Pt NPs, Au NPs and
PdAu NPs.
We first explored the formation of the well-known Pd NPs
using Phos-COF-1 as a solid support. In a typical procedure,
Phos-COF-1 was suspended in a methanol solution of K2PdCl4
to deposit Pd ions and form nucleation sites through metal
coordination interactions. Subsequent reduction of the Pd (II)
ions with NaBH4 in methanol provided PdNPs@Phos-COF-1
complex. The as-prepared PdNPs@Phos-COF-1 complex was
characterized by SEM-EDS (energy dispersive spectrum),
transmission electron microscope (TEM), X-ray photoelec-
tron spectroscopy (XPS), inductively coupled plasma (ICP)
and gas adsorption analysis. SEM-EDS (Figure S12, Sup-
porting Information) shows uniform distribution of various
elements including C, N, P, O, and Pd, while the TEM image
of PdNPs@Phos-COF-1 (Figure 2a, Figure S6a, Supporting
Information) show well-dispersed ultrafine Pd NPs with little
agglomeration. Further analysis showed the PdNPs have an
average size of 1.62 nm in diameter with a relatively narrow
2.2. Preparation and Characterization of PdNPs@Phos-COF-1
and Their Catalytic Activity
2.2.1. Preparation and Characterization of PdNPs@Phos-COF-1
Triphenyl phosphine is considered as one of the most com-
monly used ligands in organometallic catalysts owing to its
Figure 2. The TEM images and particle size distribution graphs of metal-NPs in Phos-COF-1: a) PdNPs@Phos-COF-1; b) PtNPs@Phos-COF-1;
c) AuNPs@Phos-COF-1; d) PdAuNPs@Phos-COF-1.
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Table 1. Catalytic performance of PdNPs@Phos-COF-1 in the Suzuki–
Miyaura coupling reaction.
size distribution, which is comparable to the pore size of
Phos-COF-1. Furthermore, doubling the dosage of Pd pre-
cursor led to the formation of larger size Pd NPs with a broader
size distribution (2.74 0.68 nm) according to TEM analysis
(PdNPs-2@Phos-COF-1, Figure S7, Supporting Information).
This result suggests that the loading amount of metal pre-
cursor is critical in control of the particle size. Excess loading
over the saturation point would promote overgrowth of NPs on
the surface of the COF with random size. Such extremely small
size with a narrow distribution (1.62 0.37 nm) of Pd NPs has
rarely been reported in the previous literatures.[7b,13]
Entrya)
1
Aryl halide
Time [h]
2
Yield [%]b)
>99
2
2
99
The oxidation state and the interaction of Pd with phosphine
were evaluated by XPS measurement (Figure S8a3,a4, Sup-
porting Information). The deconvolution of the doublet at Pd
3d region shows two sets of doublets, each of which contains a
lower energy (Pd 3d5/2) band and a higher energy (3d3/2) band.
3
4
5
2
2
2
90
98
>99
The set of doublet at 335.9 eV (Pd 3d5/2) and 341.0 eV (Pd 3d3/2
)
correspond to Pd (0) species, supporting the successful reduc-
tion of Pd (II) to Pd (0).[14] Another set of doublet at 337.5 eV
(Pd 3d5/2) and 342.5 eV (Pd 3d3/2) can be assigned to Pd(II),
which are likely from PdO that has been commonly observed
in PdNPs exposed to air.[15] The spectrum of P 2p region shows
one singlet at 132.8 eV, which is higher than that of Phos-
COF-1 (131.7 eV) and the reported PPh3-based polymers,[3d]
indicating the strong electron donation of the PPh3 ligands to
Pd. These results reveal that the Pd is successfully decorated
on Phos-COF-1 through strong interactions with the phosphine
ligands. The ICP analysis showed that the Pd content in the
hybrid material is 2.84 wt%, which correlates well with the Pd
amount that was applied (3.14 wt%). Based on the Pd loading,
we could estimate PdNPs occupy <0.01% of the pore volume
available in Phos-COF-1. However, the BET surface area of
PdNPs@Phos-COF-1 was decreased drastically in comparison
with the original Phos-COF-1 (818 m2 g−1) based on nitrogen
adsorption–desorption isotherms (Figure S9, Supporting Infor-
mation). Other metal NPs@Phos-COF-1, which are described in
the following studies, were also almost nonporous. It is highly
likely that PdNPs mainly grow in the pores located on the sur-
face, blocking the entries and making the inside pores no longer
accessible. Therefore, the ratio of accessible pores on the surface
is likely more critical than the BET surface area when COFs are
used as templates for the confined growth of NPs.
6
7
2
3
89
90
a)Conditions: aryl halide (1.0 mmol), phenyl boronic acid (1.1 mmol), K2CO3
(1.5 mmol), PdNPs@Phos-COF-1 (0.35 mol% Pd loading), DMF/H2O
(1.5 mL/1.5 mL), 50 °C; b)The yields were estimated based on the proton peak integra-
tion in the 1H NMR spectra of the crude reaction mixture.
the same reaction conditions. Compared to the commercially
available Pd sources such as Pd(PPh3)2Cl2, Pd2(dba)3 and
Pd/C, PdNPs@Phos-COF-1 consistently provided higher yields
and higher reaction rates (Table 2). The reaction catalyzed by
PdNPs@Phos-COF-1 gave the highest turnover frequency
(TOF) number of 1648 h−1, surpassing many commonly used
homogeneous and heterogeneous Pd catalysts. In addition, to
confirm the critical role of Phos-COF-1 in the size-controlled
synthesis of PdNPs, PdNPs were also prepared without or
with other templates, such as tetraphenyl-COF (tetraphenyl
group instead of PPh3[16]) and PPh3-based amorphous polymer
(synthesis shown in the Supporting Information), which
Table 2. Catalytic performance of different Pd catalysts in the Suzuki–
Miyaura coupling reaction.
2.2.2. Catalytic Activity of PdNPs@Phos-COF-1
Entrya)
1
Catalyst
Pd [mol%]
0.35
Yield [%]b)
TOF [h-1]
1648
After confirming the formation of ultrafine PdNPs, we explored
the application of PdNPs@Phos-COF-1 in heterogeneous
catalysis. The catalytic activity of PdNPs@Phos-COF-1 was
examined in one of the representative Pd-catalyzed reactions,
Suzuki–Miyaura coupling, which has been widely applied for
PdNPs@
>99
Phos-COF-1
2
3
4
5
6
Pd(PPh3)2Cl2
Pd2(dba)3
Pd/C
0.35
0.35
0.35
0.35
0.35
61
94
94
72
21
356
1098
625
942
155

the facile formation of C C bonds. As shown in Table 1, excel-
lent catalytic activity of PdNPs@Phos-COF-1 was observed
with only 0.35 mol% Pd loading under mild conditions (50 °C,
2 h) for various substrates, including aryl iodides (90–99%
yield, entries 1–3) and aryl bromides (90–99%, entries 4–7).
Furthermore, the applied amount of the catalyst could be fur-
ther decreased to 0.1 mol% (catalyzing the reaction of p-bromo-
toluene) while keeping high efficiency (95%) under otherwise
K2PdCl4
PdNPs without
support
a)Conditions: aryl halide (1.0 mmol), phenyl boronic acid (1.1 mmol), K2CO3
(1.5 mmol), DMF/H2O (1.5 mL/1.5 mL), 50 °C; b)The yields were estimated based
on the proton peak integration in the 1H NMR spectra of the crude reaction mixtures.
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metal NPs with sizes less than 2 nm (Figure 2b,c). The sta-
tistic histograms of these PtNPs and AuNPs revealed that these
NPs have an average size of 2.06 and 1.78 nm, respectively,
with a narrow size distribution. These observations suggest
that Phos-COF-1 is an excellent supporting substrate for con-
trolled growth of a broad scope of nanoparticles. The Pt and
Au contents in the composite materials are determined to be
3.2 and 12.5 wt%, respectively, by the ICP analysis, which are
close to the amounts we applied (4.5 wt% for Pt and 16.2 wt%
for Au). The uniform composition and distribution of both Pt
and Au elements in COF were also characterized by SEM-EDS
(Figure S16c,f, Supporting Information), which agree well with
the TEM and ICP analysis, respectively. Similar to PdNPs, the
XPS revealed the presence of both metallic and oxidation state
of Pt and Au and their interactions with P (Figure S8b1,b3,c1,c3,
Supporting Information). Careful examination of the XPS
spectra (Figure S8b3, Supporting Information) showed that
the Pt (II) species are the reoxidized metal oxide (PtO).[17] The
P 2p binding energy of PtNPs@Phos-COF-1 was found to
be 132.4 eV and higher than that of Phos-COF-1 (131.7 eV),
supporting the electron donation of the triphenylphosphine
ligands to Pt. The coexistence of Au(0) and Au(I) was also
observed in AuNPs@Phos-COF-1(Figure S8c3, Supporting
Information). The peaks centered at 84.5 and 88.2 eV could
be assigned to 4f7/2 and 4f5/2 bands of Au(0) species, respec-
tively. The higher binding energy Au 4f7/2 peak at 85.4 eV and
4f5/2 peak at 89.0 eV were assigned to Au(I).[18] Meanwhile, the
P 2p binding energy of AuNPs@Phos-COF-1 was found to be
132.2 eV (Figure S8c1, Supporting Information), higher than
that of Phos-COF-1 (131.7 eV), again indicating the electron
donation of the triphenylphosphine ligands to Au and their
binding, similar to the case of PdNPs or PtNPs@Phos-COF-1.
Next, the catalytic performance of both ultrafine PtNPs
and AuNPs in heterogeneous catalysis was explored. For
PtNPs@Phos-COF-1, the reduction of 4-nitrophenol in the
presence of NaBH4 was employed, which is a representative
model reaction to verify the activity of noble metal catalysts and
can be easily monitored by UV–vis spectroscopy. As a control
experiment, we first conducted the 4-nitrophenol reduction
in the presence of Phos-COF-1 without metal NPs and moni-
tored the reaction by UV–vis spectroscopy. The absorption peak
at 400 nm, a characteristic peak of 4-nitrophenol and NaBH4
mixture, remains almost unchanged after 15 min, which indi-
cates the COF alone does not have any considerable catalytic
activity. By contrast, when PtNPs@Phos-COF-1 catalyst (0.05
eq, based on 4-nitrophenol) was introduced into the reac-
tion system, the absorption peak of 4-nitrophenol at 400 nm
decreased rapidly and almost disappeared within 35 min, and
a new peak at ≈300 nm corresponding to the reduction product
4-aminophenol appeared (Figure S13a, Supporting Informa-
tion), indicating the reduction was triggered immediately
upon the addition of PtNPs@Phos-COF-1. When the dosage
of PtNPs@Phos-COF-1 catalyst was doubled (0.1 eq. Pt), the
full conversion of 4-nitrophenol to 4-aminophenol was reached
within 7 min with the complete disappearance of 4-nitrophenol
peak at 400 nm (Figure 4). The catalytic efficiency of PtNPs@
Figure 3. The recyclability of PdNPs@Phos-COF-1 in the Suzuki–Miyaura
coupling reaction.
were studied as control experiments. As shown in Figure S10
(Supporting Information), significant agglomerations could be
observed from TEM images (other metal NPs such as Pt, Au
and bimetallic PdAu showed similar effect, as discussed later).
The catalytic performance of PdNPs without support was also
investigated, which only provided the product in 21% yield
(Entry 6, Table 2).
The stability and reusability of PdNPs@Phos-COF-1
(0.35 mol%) was then examined in the reaction between
4-iodoanisole and phenylboronic acid. After each run, the
catalyst was collected through centrifugation, washed with
H2O and MeOH, and dried at 65 °C under vacuum for the next
use. The catalyst was recycled five times in total, and nearly com-
plete retention of activity (≥90% yield) was observed during the five
consecutive runs (Figure 3), indicating the excellent reusability of
PdNPs@Phos-COF-1 catalyst. The TEM image of the recycled cata-
lyst after the last run shows that PdNPs are still well dispersed
(Figure S11, Supporting Information). The XPS of PdNPs@
Phos-COF-1 recovered after the fifth use showed the amount
of oxidized Pd (Figure S8a3,a4, Supporting Information) was
increased compared to that in the as-prepared complex. The
inevitable oxidation of the catalytically active sites during the
reaction might account for the slightly reduced catalytic activity.
2.3. General Applicability of Phos-COF-1 as a Solid Support
for NP Growth
2.3.1. Synthesis of Other Monometallic Nanoparticles and Their
Catalytic Activity
Given the success in the synthesis of PdNPs@Phos-COF-1
with excellent catalytic activity and stability, we next used
Phos-COF-1 as a template for the confined growth of PtNPs
and AuNPs to demonstrate the general applicability of our
approach. To our delight, the well-dispersed ultrafine PtNPs and
AuNPs supported by Phos-COF-1 were successfully obtained.
The typical TEM characterization data showed well-dispersed
Phos-COF-1 is among the highest reported, even higher than
[7b]
that of PtNPs supported by our previous Thio-COF
or chal-
cogels, which require 1.3- and 6700-fold higher loading of Pt,
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Figure 4. The time-dependent UV–vis spectra of 4-nitrophenol reduction
catalyzed by 0.1 eq. PtNPs@Phos-COF-1.
Figure 5. Catalytic performance of Au catalyst (0.2 eq) in the reduction
reaction.
respectively, to achieve a comparable rate, demonstrating the
superior catalytic performance of PtNPs@Phos-COF-1. As a
control experiment, unsupported PtNPs showed significant
agglomerations by TEM analysis (Figure S10b, Supporting
Information). The catalysis study showed only 80% conversion
of 4-nitrophenol in 4 h under otherwise the same conditions,
indicating the lower activity of unsupported PtNPs compared
to PtNPs@Phos-COF-1 (Figure S15, Supporting Information).
To test the reusability and stability of PtNPs@Phos-COF-1,
consecutive reduction experiments were carried out by reusing
the catalyst in situ without any reactivation process by adding
fresh starting materials into the same reaction vessel after each
run. As shown in Figure S14 (Supporting Information), after up
to eight consecutive runs, the activity of PtNPs@Phos-COF-1
was almost constant, completing the reaction within 7–10 min.
The TEM image of the catalyst reused eight times shows slight
increase of the particle size (2.28 vs 2.06 nm, Figure 2b and
Figure S14b, Supporting Information).
Similarly, the application of AuNPs@Phos-COF-1 in het-
erogeneous catalysis was also explored using the hydride
reduction of 1-bromo-4-nitrobenzene, which is widely used
as a model reaction to test the activity for Au catalysts. With
0.2 eq. Au loading, the amine product was produced in 99%
yield at 25˚C within 2 h (Figure 5). By contrast, AuNPs without
Phos-COF-1 support (TEM image shown in Figure S10c, Sup-
porting Information) gave 95% yield at 25˚C within 2 h in the
first run, 81% yield in the second round and only 37% yield
in the third usage. Furthermore, when HAuCl4 alone (0.2 eq,
based on 1-bromo-4-nitrobenzene) was used as the catalyst,
only 30% conversion was achieved under otherwise the same
conditions (Figure 5); These results clearly showed the high
activity of AuNP@Phos-COF-1 in catalyzing the reduction
of 1-bromo-4-nitrobenzene. The reusability and stability of
AuNPs@Phos-COF-1 was also examined employing the same
method for PtNPs@Phos-COF-1. The catalyst was reused five
times, and almost complete retention of activity (>99% yield)
was observed over all five consecutive runs (Figure S17, Sup-
porting Information), demonstrating the excellent reus-
ability of AuNPs@Phos-COF-1 catalyst. The TEM image of
the catalyst after the final run shows well-dispersed particles,
albeit with slightly increased size (Figure S11c, Supporting
Information). The XPS analysis of PtNPs@Phos-COF-1 and
AuNPs@Phos-COF-1 recycled from the last run were per-
formed as well. As shown in Figure S8b,c (Supporting Infor-
mation), both phosphorus and metal elements are oxidized to
some extent, which might be due to the accumulated oxidation
effect of air during the catalysis and workup process.
2.3.2. Synthesis of Bimetallic Nanoparticles and Their Catalytic Activity
Encouraged by the successful synthesis of the three ultrafine
monometallic NPs, we then examined the synthesis of mixed
noble metal nanoparticles to further explore its general appli-
cability. As a proof of concept, the template synthesis and
catalytic performance of bimetallic Pd/AuNPs (denoted as
PdAuNPs@Phos-COF-1) were studied. We followed the sim-
ilar procedure to prepare PdAuNPs@Phos-COF-1 using 1:1
molar ratio of Pd and Au salts in MeOH. The TEM analysis
shows that ultrafine PdAuNPs with narrow size distribu-
tion were obtained as shown in Figure 2d and Figure S6d
(Supporting Information). The statistic histogram of these
PdAuNPs reveals that they have an average size of 1.03 nm
with a narrow size distribution (1.03
0.07 nm). When
no COF was employed, PdAuNPs showed large clusters
(Figure S10d, Supporting Information), further indicating the
size-controlled effect of COF support. The exact structure of
alloy or core-shell of the bimetallic PdAu nanoparticles was
hard to distinguish clearly even using the atomic resolution
aberration-corrected TEM. Nevertheless, the composition of
PdAuNPs@Phos-COF-1 was characterized by EDS elemental
mapping. As shown in Figure S18 (Supporting Information),
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highly stable and active heterogeneous metal catalysts using
PPh3-based COFs, which could be generally applicable for the
preparation of a broad scope of nanoparticles targeting various
applications. The synthesis of other PPh3-based COFs and their
related applications are currently ongoing in our group.
4. Experimental Section
Synthesis of Phos-COF-1: Trialdehyde 1 (104 mg, 0.30 mmol), diamine
2 (48 mg, 0.45 mmol), 6 m acetic acid (0.6 mL), dioxane (0.5 mL), and
mesitylene (1.5 mL) were added to a glass tube (the outer diameter was
10 mm and the inner diameter was 8 mm). The tube was degassed three
times through freeze–pump–thaw technique, and then evacuated to an
internal pressure of 150 mTorr and flame-sealed. The mixture was first
warmed to room temperature and kept in an oven at 120 °C for 3 d. An
orange solid was formed, which was collected by filtration, washed with
N,N-dimethylformamide (DMF) and tetrahydrofuran, and dried under
vacuum to yield the PPh3-based COF (Phos-COF-1, 90 mg, 65%).
Synthesis of Phos-COF-1 Supported PdNPs, PtNPs, and AuNPs:
Phos-COF-1 (100 mg) and K2PdCl4 (10 mg, 0.03 mmol) or K2PtCl4
(10 mg, 0.024 mmol) or HAuCl4 (33 mg, 0.097 mmol) were added in
MeOH (10 mL). The mixture was stirred for 24 h at room temperature,
and then a solution of NaBH4 in MeOH (2 m, 2 mL) was added
dropwise. The mixture was kept stirring for 2 h. The resulting solid
was isolated through centrifugation, washed with MeOH and CH2Cl2
three times, and then dried at 65 °C under vacuum overnight to yield
Phos-COF-1 supported PdNPs, PtNPs and AuNPs as a brown-colored
powder (100 mg, 96% yield), khaki-colored powder (91 mg, 87% yield),
and red-brown powder (109 mg, 92% yield), respectively. The Pd, Pt,
and Au contents were 2.8%, 3.2%, and 12.5% as determined by ICP,
respectively.
Synthesis of Phos-COF-1-Based PdAuNPs: Phos-COF-1 (30 mg),
K2PdCl4 (2.5 mg, 7.65 µmol), and HAuCl4 (2.6 mg, 7.65 µmol) were
added in MeOH (6.0 mL). The mixture was stirred for 24 h at room
temperature. A solution of NaBH4 in MeOH (2 m, 1.0 mL) was added
dropwise and the mixture was kept stirring for 2 h. The resulting solid
was isolated through centrifugation, washed with MeOH and CH2Cl2
each three times, and then dried at 65 °C under vacuum for 12 h to yield
PdAuNPs@Phos-COF-1 as a black-brown powder (28 mg, 86% yield).
General Procedure for the Suzuki–Miyaura Coupling Reaction
Catalyzed by PdNPs@Phos-COF-1: In a typical procedure, aryl halide
(0.5 mmol), phenylboronic acid (0.55 mmol), K2CO3 (0.75 mmol), and
PdNPs@Phos-COF-1 (0.35 mol %) were added in DMF/H2O (1.5 mL,
1/1 v/v) in a Schlenk tube. The reaction mixture was stirred for 2 h at
50 °C at nitrogen atmosphere, and then the crude product was extracted
with EtOAc and the yield was determined based on the integration
of the proton peaks in the 1H NMR spectrum of the crude product
mixture. After each run, the PdNPs@Phos-COF-1 was collected through
centrifugation, washed with MeOH three times, and dried at 65 °C
under vacuum for further use.
Scheme 1. The one-pot two-step tandem cross-coupling and hydride
reduction under the catalysis of PdAuNPs@Phos-COF-1.
the component elements C, N, O, Pd, and Au are all evenly
distributed in the hybrid material, which supports the excel-
lent dispersity of NPs in the Phos-COF-1 material.
In principle, mixed metal nanoparticles can catalyze more
than one type of reactions in one pot. We thus investigated
the possibility of conducting tandem cross-coupling and nitro
reduction using PdAuNPs@Phos-COF-1 as the catalyst. We
examined tandem Pd-catalyzed coupling and Au-catalyzed
reduction using 1-bromo-4-nitrobenzene as the substrate to
form 4-aminobiphenyl in one-pot fashion (Scheme 1). With
only 0.5 mol% of the catalyst loading, we observed an excellent
yield (estimated 90%) of the product from Suzuki–Miyaura cou-
pling reaction under mild conditions within 2 h. Then NaBH4
was directly added to the resulting reaction system to reduce
the nitro group. After stirring at the room temperature for 2 h,
the target 4-aminobiphenyl was obtained in 60% isolated yield
in two steps. Similar to the previous three types of metal nan-
oparticles, the changes in the valence of the elements before
and after catalysis were also characterized by XPS (Figure S8,
Supporting Information): oxidation state ratio decreased drasti-
cally for P, while increased for Au even with a new higher oxi-
dation state Au(III), which corresponds to the binding energy
of Au 4f7/2 peak at 89.4 eV and 4f5/2 peak at 92 eV.[19] This
“abnormal” phenomenon compared with AuNPs@Phos-COF-1
might suggest there is a redox reaction between P and Au in
the presence of Pd catalyst, albeit further investigations still
needed.
3. Conclusion
In summary, we have successfully synthesized a PPh3-based
COF (Phos-COF-1) for the first time and applied it to the
size-controlled synthesis of broad-scope mono- and bimetallic
nanoparticles. Highly stable and well-dispersed ultrafine Pd, Pt,
Au, and PdAuNPs with narrow size distributions were success-
fully obtained in the presence of Phos-COF-1, consisting of the
orderly distributed triphenylphosphine moieties in the porous
framework. The as-prepared Phos-COF-1-supported ultrafine
NPs show excellent catalytic activities toward Suzuki–Miyaura
coupling reaction, reduction of nitrophenol and 1-bromo-
4-nitrobenzene, and even tandem coupling and reduction of
p-nitroiodobenzene. More importantly, these metal NP catalysts
are highly stable and easily recycled without loss of catalytic
activity. Our work provides a novel strategy for preparation of
General Procedure for the Reduction of 4-Nitrophenol Catalyzed by
PtNPs@Phos-COF-1: To a Schlenk tube were added an aqueous solution
of 4-nitrophenol (1.5 × 10−2 m, 2.0 mL), water (1.0 mL), and an aqueous
solution of NaBH4 (20 m, 2.0 mL). A suspension of PtNPs@Phos-COF-1
in water (1.38 or 2.76 µmol, 30 mg mL−1, 0.28 or 0.56 mL) was
introduced, and the reaction progress was monitored with the UV–
vis spectroscopy. The initial bright yellow solution gradually turned to
colorless as the reaction progressed.
Procedure for the Reduction of 1-Bromo-4-Nitrobenzene Catalyzed by
AuNPs@Phos-COF-1: 1-Bromo-4-nitrobenzene (3.0 mg, 14.9 µmol), an
aqueous solution of NaBH4 (10 m, 1.5 mL), and AuNPs@Phos-COF-1
(1.0 mg, 0.64 µmol Au) were added in a Schlenk tube. The reaction
mixture was stirred for 2 h at 25 °C at nitrogen atmosphere. The crude
product was extracted with CH2Cl2. The yield was determined based
1
on the integration of the proton peaks in the H NMR spectrum of the
crude product mixture.
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Procedure for the Coupling and Reduction of p-Nitroiodobenzene
Catalyzed by PdAuNPs@COF: To each of two Schlenk tubes were
added p-nitroiodobenzene (7.5 mg, 0.03 mmol), phenylboronic acid
(4.0 mg, 0.033 mmol), K2CO3 (62 mg, 0.045 mmol), and PdAuNPs@
COF (1.4 mg, 0.36 µmol) and EtOH/H2O (0.34 mL, 1/1 v/v). The
reaction mixture was stirred for 2 h at 50 °C at nitrogen atmosphere. The
crude product of one tube was extracted with EtOAc and the yield was
determined based on the integration of the proton peaks in the 1H NMR
spectrum of the crude product mixture. To the other tube was added an
aqueous solution of NaBH4 (10 m, 5.0 mL), and the reaction mixture
was stirred for 2 h at 25 °C at nitrogen atmosphere. The crude product
was then extracted with CH2Cl2 and the yield was determined based
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The authors declare no conflict of interest.
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Keywords
catalysis, controlled synthesis, covalent organic frameworks, metal
nanoparticles, phosphine
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