Au-NHC@porous organic polymers: Synthetic control and its catalytic application in alkyne hydration reactions

Article abstract of DOI:10.1021/cs400983y

The synthetic control and functions of porous organic polymers (POPs) with N-heterocyclic carbene gold(I) (Au-NHC@POPs) are described in this article. A series of Au-NHC@POPs with tunable physical properties such as surface area and pore size distribution were first synthesized via Sonogashira chemistry by differing monomer strut lengths and concentration during polymerization; a controllable transition from nonporous to microporous and the coexistence of micro- and mesoporous structures in the framework were realized by varying the monomer concentration. To explain this phenomenon, we put forward a model assumption of a branch-branch cross effect. Additionally, Au-NHC@POPs1 was found to have superior catalytic activity in alkyne hydration reactions, and the catalyst could be used six times with a slight loss of activity.

Full text of DOI:10.1021/cs400983y

Research Article
pubs.acs.org/acscatalysis
Au-NHC@Porous Organic Polymers: Synthetic Control and Its
Catalytic Application in Alkyne Hydration Reactions
Wenlong Wang,†,‡ Anmin Zheng,§ Peiqing Zhao,† Chungu Xia,† and Fuwei Li*,†,∥
†State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of
Sciences, Lanzhou, Gansu 730000, P. R. China
‡University of the Chinese Academy of Sciences, Beijing, P. R. China
§State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Center for Magnetic Resonance, Wuhan
Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China
∥Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China
S
* Supporting Information
ABSTRACT: The synthetic control and functions of porous organic polymers
(POPs) with N-heterocyclic carbene gold(I) (Au-NHC@POPs) are described in this
article. A series of Au-NHC@POPs with tunable physical properties such as surface
area and pore size distribution were first synthesized via Sonogashira chemistry by
differing monomer strut lengths and concentration during polymerization; a
controllable transition from nonporous to microporous and the coexistence of
micro- and mesoporous structures in the framework were realized by varying the
monomer concentration. To explain this phenomenon, we put forward a model
assumption of a branch−branch cross effect. Additionally, Au-NHC@POPs1 was
found to have superior catalytic activity in alkyne hydration reactions, and the catalyst
could be used six times with a slight loss of activity.
KEYWORDS: N-heterocyclic carbene, porous organic polymers, synthetic control, alkyne hydration, cross-coupling
Catalytic POPs could be divided into two major classes:
POPs that are incorporated uniformly and covalently built-in
homogeneous catalysts as building blocks and POPs that can be
modified by postsynthesis26−31 (generally, catalysts are loaded
via noncovalent interactions). The first class has more flexibility
in building block design, higher thermal and chemical stability
due to the diversity of the organometallic catalysts, and all the
linkage of covalent bonds. For example, Lin et al. developed the
synthesis of bpy-ligated, Ir- and Ru-incorporating porous cross-
linked polymers (PCPs) for efficient photocatalysis using
Co2(CO)8-catalyzed alkyne trimerization.32 Cooper’s group
elaborated the synthesis of bpy-ligated complex-functionalized
conjugated microporous polymers (CMPs) via Sonogashira
chemistry as an efficient heterogeneous catalyst for reductive
amination.33 Jiang and co-workers reported the direct synthesis
and catalytic application of CMP-type iron porphyrin networks
via Suzuki−Miyaura coupling chemistry.34 For other catalytic
POPs, see refs 35−39.
1. INTRODUCTION
Homogeneous catalysts, especially organometallic catalysts,
play an extremely important role in the field of modern
catalysis. However, most precious organometallic catalysts have
not been widely used because of their complicated synthesis,
intricate recycling problem, and thus the metal contamination
of the products.1,2
So far, the main method of recycling organometallic
complexes is to heterogenize the homogeneous catalysts, for
instance, the classical anchoring method, in which the catalyst is
anchored to various kinds of supports, such as silica-based
materials3−7 and magnetic nanoparticles.8−10 However, the
catalytic moieties are inhomogeneously distributed and pendent
into the pore volume, which may cause pore blocking and
eventually deactivate the catalytic system.11 Recently, porous
organic materials, including porous organic polymers
(POPs),12−20 metal-organic frameworks (MOFs),21,22 and
covalent organic frameworks (COFs)23−25 have emerged as a
versatile platform for the deployment of catalysts because of its
porous nature and high surface area. Nonetheless, it should be
noted that MOFs and COFs are unstable in many circum-
stances such as acid, base, and moisture because of the whole
MOFs’ connection of coordination bonds and the presence of
boronate ester or imine groups in COFs, which severely limit
their applications in catalysis. In contrast, POPs are advanta-
geous compared to MOFs and COFs in terms of hydrothermal
stability and chemical robustness.
As for most of the reported active catalytic building blocks
described above, metal elements are coordinated with neutral
nitrogen ligands in a relatively unstable way. Recently, N-
heterocyclic carbenes (NHCs) have emerged as excellent
ligands with strong σ-electron donation ability, and their
Received: October 27, 2013
Revised: December 9, 2013
Published: December 11, 2013
© 2013 American Chemical Society
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coordination complexes with transition metals have found
outstanding applications in homogeneous catalysis.40−42 Never-
theless, most of them are synthesized through multiple steps
and can be used only one time in homogeneous catalysis.
Therefore, covalently built-in catalytic POPs based on NHC
complexes have great potential, and their development for
sustainable catalysis is highly desirable.
Scheme 2. Synthesis of Iodine-Functionalized [(IPr)AuCl]
Building Blocks
In addition, Cooper’s work reported that physical properties
such as micropore size and surface area of CMPs could be
finely tuned by varying the monomer strut length,43 and some
earlier works reported the use of different solvents to influence
porosity.44,45 It is well-known that large surface areas and a
suitable pore size distribution of the porous catalyst play a key
role in its catalytic activity. With our continuing studies of the
development of recyclable metal-NHC catalysts,46,47 herein, we
report the direct controllable synthesis of catalytic POPs (Au-
NHC@POPs) with tunable physical properties based on an N-
heterocyclic carbene gold(I) complex as building block via
Sonogashira coupling by varying the monomer strut length and
concentration during polymerization (Scheme 1).
of Yang et al.4 Iodine-functionalized [(IPr)AuCl] could be
prepared in 85% yield in 3-chloropyridine by the reaction of
NaAuCl4·2H2O with imidazolium salt and base (Na2CO3 or
K2CO3).48−50 Au-NHC coordination was established with the
absence of the 1H resonance NCHN proton of precursor 1. All
the alkyne monomers were synthesized according to reported
procedures.51,52
2.2. Synthesis of Au-NHC@POPs and NHC-salt@POPs.
The synthetic schemes of Au-NHC@POPs and NHC-salt@
POPs are shown in Scheme 1. To tune the physical properties
of Au-NHC@POPs, alkyne monomers with different lengths
were selected as linkers. Iodine-functionalized building block
[(IPr)AuCl] 2 (0.24 mmol) was polycondensed with alkyne
monomers 3 (0.12 mmol), 4 (0.16 mmol), and 5 (0.16 mmol)
under the condition of the same catalyst loading and solvent
amount (150 mL of toluene) via the Pd-catalyzed Sonogashira
coupling strategy to form Au-NHC@POPs. NHC-salt@POPs
were synthesized from [(IPr)HCF3SO3] 1 (0.24 mmol) and
monomer 3 (0.12 mmol) under the same conditions that were
used for Au-NHC@POPs (for detailed experiments, see the
Supporting Information).
Scheme 1. (a) Synthesis of Au-NHC@POPs Using the Pd-
Catalyzed Sonogashira Coupling Method and (b) Synthesis
of NHC-salt@POPs
3. RESULTS AND DISCUSSION
3.1. Characterization of the Material. N2 adsorption and
desorption isotherms of Au-NHC@POPs and NHC-salt@
POPs are shown in Figure 1. Compared with Au-NHC@
Figure 1. N2 adsorption−desorption isotherm measured at 77 K.
POPs2 (SBET = 350 m2/g) and Au-NHC@POPs3 (SBET = 258
m2/g), Au-NHC@POPs1 with monomer 3 as a linker has the
maximal surface area (SBET = 506 m2/g). This is mainly due to
the larger monomer with its four alkyne groups stretching to
four sides in a tetrahedral way that is different from that of
monomers 4 and 5 that stretched in a plane triangular way. In
addition, the surface area of NHC-salt@POPs (SBET = 249 m2/
g) is much smaller than that of Au-NHC@POPs1; this is
2. EXPERIMENTAL SECTION
2.1. Synthesis of N-Heterocyclic Carbene Gold(I)
Building Block and Alkyne Monomers. The synthetic
route of N-heterocyclic carbene gold(I) building blocks (2) is
shown in Scheme 2. Iodine-functionalized [IPr]·HSO3CF3 (1)
was synthesized according to the previously reported procedure
−
possibly because of the pore blocking of the CF3SO3 anion.
Nitrogen sorption of Au-NHC@POPs1 displayed a combina-
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tion of type I and type II sorption isotherm curves along with a
large adsorption at low pressure (P/P0 < 0.1), which is
suggestive of the coexistence of micro- and mesopores in the
framework (for relative pore size distribution curves, see Figure
S2 of the Supporting Information).
undetected levels, suggesting the bond formation of Sonoga-
shira coupling. The high degree of spectral similarity of the four
POPs also demonstrates the structural consistency of these
materials.
To further study the valence-state information of the Au-
NHC monomer in Au-NHC@POPs, characterization of the X-
ray photoelectron spectra (XPS) was employed to investigate
the variations of binding energy (BE) of Au between monomer
2 and Au-NHC@POPs. As shown in Figure 4, all the Au 4f7/2
Solid-state 13C NMR spectra (Figure 2) were measured to
confirm the structures of Au-NHC@POPs and NHC-salt@
Figure 4. XPS comparison of NHC-Au(I)Cl monomer 2, Au-NHC@
POPs1, Au-NHC@POPs2, and Au-NHC@POPs3.
and 4f5/2 peaks correspond well at 85.2 and 88.7 eV,
respectively, which strongly confirmed the well-retained state
of Au-NHC monomers 2 in Au-NHC@POPs. The Au-NHC
monomers were successfully doped into these POPs.
Figure 2. Solid-state NMR spectra of Au-NHC@POPs and NHC-
salt@POPs.
The composition of these POPs was determined by
combustion elemental analysis (EA for C, H, and N) and
atomic absorption spectroscopy (AAS for Au). All experimental
values (wt %; C 60.36, H 4.18, N 2.97, Au 18.45 for Au-NHC@
POPs1; C 60.12, H 4.09, N 3.01, Au 18.51 for Au-NHC@
POPs2; C 55.63, H 4.11, N 3.26, Au 21.42 for Au-NHC@
POPs3; and C 66.87, H 5.14, N 3.09 for NHC-salt@POPs) are
lower in comparison to the values calculated for an ideal
structure assuming a 100% cross-linking degree (wt %; C 64.82,
H 5.89, N 3.15, Au 22.15 for Au-NHC@POPs1; C 64.64, H
5.54, N 3.21, Au 22.55 for Au-NHC@POPs2; C 58.29, H 5.59,
N 3.88, Au 27.31 for Au-NHC@POPs3; and C 72.93, H 6.62,
N 3.47). This difference might be attributed to the residual
functional groups (I). Interestingly, the N/Au molar ratios were
>2. We found it because of the small part of insoluble catalysts
in nitromurlatic acid during the normal AAS sample
preparation. If the samples were treated with a high
temperature of 600 °C in the muffle furnace, then the samples
could be dissolved completely in the nitromurlatic acid.
Surprisingly, the reprepared AAS sample provided a higher
Au content (for details, see the Supporting Information). In
addition, thermogravimetric (TG) analysis showed all these
materials have thermal stability up to 300 °C (Figure S4 of the
Supporting Information).
POPs. The peaks at ∼22 and ∼29 ppm correspond to the
carbon atom of the -CH3 group and the tertiary carbon atom of
the -CH(CH3)2 group, respectively. The signals at ∼39 and
∼45 ppm are ascribed to the secondary carbon (-CH2) and the
quaternary carbon atom connected with the phenyl group (Ar-
C) of adamantane. The peak at ∼90 ppm is ascribed to sp-
hybridized -Car-CC-Car- sites. The peaks between 120 and
160 ppm are confirmed to be aromatic groups.
Bond formation was further confirmed by the FT-IR
comparison of monomer 2, monomer 3, Au-NHC@POPs1,
Au-NHC@POPs2, Au-NHC@POPs3, and NHC-salt@POPs
(Figure 3). In the spectra of Au-NHC@POPs and NHC-salt@
POPs, the C−H stretch and Ar−I stretch are diminished to
3.2. Synthesis and Characterization of Au-NHC@
POPs1 in Different Amounts of Solvent Systems. In
addition, X-ray powder diffraction analysis of Au-NHC@
POPs1 revealed no diffraction, implying that these POPs are
amorphous (Figure S3 of the Supporting Information). As we
know, the condensation reaction of COFs occurs in a reversible
and dynamic fashion.24,53 The chemical bonds of the forming
polymers have to close and open to yield the thermodynami-
Figure 3. FT-IR comparison of monomer 2, monomer 3, Au-NHC@
POPs1, Au-NHC@POPs2, Au-NHC@POPs3, and NHC-salt@POPs.
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cally stable yet crystalline structures. However, the condensa-
tion process of POPs takes place in an irreversible way, which is
totally different from that of COFs. Therefore, the idea of
kinetic control in the synthesis of POPs naturally came to mind.
In this work, the kinetically synthetic control of POPs was
realized by varying the monomer concentration. Monomer 2
(0.24 mmol) and monomer 3 (0.12 mmol) were poly-
condensed under the same catalyst loading conditions but
with different amounts of solvent (50, 80, 100, 120, and 150
mL of toluene) (for detailed experiments, see the Supporting
Information). To our delight, a controllable transition from
nonporous to microporous and the coexistence of micro- and
mesoporous structures in the framework were found (for N2
adsorption and desorption isotherms, see Figure 5). Corre-
the reaction rate was too fast (for time-dependent experimental
results, see Figure S1 of the Supporting Information) and
caused complete pore blocking by the branch−branch cross
effect (Figure S5 of the Supporting Information).
Nevertheless, with the gradually reduced concentration, the
reaction rate decreased and the branch−branch cross effect
weakened gradually. When the pore blocking reaches a certain
degree (in a 100 mL solvent system), microporous POPs with
the highest surface area (SBET = 798 m2/g) were prepared.
Finally, POPs (SBET = 506 m2/g) with the coexistence of micro-
and mesoporous structures in the framework were synthesized
in a 150 mL solvent system when the branch−branch cross
effect decreased to a much lower degree.
It is worth noticing that the Au content and other element
contents in each sample synthesized in different amounts of
solvent were very similar (wt %; C 60.36, H 4.18, N 2.97, Au
18.45 for Au-NHC@POPs1 synthesized in 150 mL; C 60.38, H
4.17, N 2.96, Au 18.42 for Au-NHC@POPs1 synthesized in
120 mL; C 60.35, H 4.18, N 2.95, Au 18.43 for Au-NHC@
POPs1 synthesized in 100 mL; C 60.37, H 4.15, N 2.97, Au
18.48 for Au-NHC@POPs1 synthesized in 80 mL; and C
60.34, H 4.17, N 2.97, Au 18.49 for Au-NHC@POPs1
synthesized in 50 mL).
3.3. Alkyne Hydration Reactions Catalyzed by Au-
NHC@POPs. Alkyne hydration reactions have gained prime
interest because of the wide availability of alkyne substrates and
the fundamental importance of the carbonyl motif in organic
synthesis.54Recently, Nolan and co-workers reported their
(NHC)Au/AgSbF6 catalytic system that is the most efficient
homogeneous system ever reported for an alkyne hydration
reaction.55 In this account, we also wish to investigate our
newly synthesized Au-NHC@POPs as a sort of efficient,
recoverable, and reusable heterogeneous catalyst in alkyne
hydration reactions.
Figure 5. N2 adsorption−desorption isotherm of Au-NHC@POPs1
prepared different amounts of solvent systems measured at 77 K and
pore size distribution calculated by NLDFT of Au-NHC@POPs1
prepared in 150 and 100 mL of toluene.
To begin, we explored this possibility of using phenyl-
acetylene as the substrate in a heterogeneous system.
Phenylacetylene (1 mmol) and Au-NHC@POPs1 (5 mg,
equivalent to 5.62 μmol of Au) as a catalyst together with
AgSbF6 (5 mg) as cocatalyst were refluxed at 120 °C in a
sealing tube loaded with H2O (330 μL) and CH3OH (660 μL)
for 24 h. As a result, the corresponding acetophenone was
obtained in 86% yield. It is worth mentioning that in pure
water, no product was observed, which implies that methanol is
not innocent in this reaction; this is possibly due to the
hydrophobicity of these materials and the fact that the pore
structure is not wetted without the addition of methanol.56 For
comparison, we then investigated other POPs as catalysts, and
Table 1 summarizes the catalytic performances of different
catalysts. Besides the acetophenone, no other products were
identified, suggesting all these catalysts were exclusively
selective. NHC-salt@POPs were inactive, which implied that
the Au active sites were essential for the present reactions or
spondingly, the BET surface areas increased gradually from a
very low point (SBET = 16 m2/g) to a peak value (SBET = 798
m2/g) and then decreased to a medium value (SBET = 506 m2/
g) (Figure 6). This is possibly due to the pore blocking caused
by the high reaction rate at high concentrations. To explain the
concentration-controlled synthesis of Au-NHC@POPs, a
model assumption of the branch−branch cross effect was
proposed as shown in Figure S5 of the Supporting Information.
When the polycondesation occurs in a 50 mL toluene system,
−
−
there is an ion exchange between SbF6 and CF3SO3 .
However, when AgSbF6 was removed from the catalyst system,
no product was observed. Therefore, these results clearly
demonstrate that the alkyne hydration could not be realized
without the assistance of AgSbF6. Au-NHC@POPs1 (86, 85,
86, and 73% yield for POPs prepared in different amounts of
solvent), Au-NHC@POPs2 (83%), and Au-NHC@POPs3
(75%) all exhibited >70% yields of the desired aryl ketone
except for nonporous Au-NHC@POPs1 (<10%, synthesized in
50 mL of toluene). This is possibly due to the small size of
phenylacetylene [0.76 nm (see Figure S6 of the Supporting
Figure 6. Changes in BET surface areas (Au-NHC@POPs1) in
different amounts of solvent.
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the newly synthesized heterogeneous catalyst, which exhibited a
yield (88%) close to that of Au-NHC@POPs1.
Table 1. Catalytic Performances of Different Catalyst
Systems
a
To further elucidate the substrate size effect, a larger
molecule [7j (Table 2)] up to 2.11 nm (see Figure S6 of the
b
SBET
CH3OH
AgSbF6
(mg)
yield
Table 2. Au-NHC@POPs1-Catalyzed Alkyne Hydration
Reactions
sample
(m2/g)
(μL)
(%)
a
c
c
c
d
e
f
Au-NHC@POPs1
Au-NHC@POPs1
Au-NHC@POPs1
Au-NHC@POPs1
Au-NHC@POPs1
Au-NHC@POPs1
Au-NHC@POPs1
Au-NHC@POPs2
Au-NHC@POPs3
NHC-salt@POPs
iodo-NHC-Au(I)
506
506
506
336
798
583
16
660
660
0
5
0
5
5
5
5
5
5
5
5
5
86
0
trace
73
86
85
trace
83
75
0
660
660
660
660
660
660
660
660
g
350
258
249
−
88
a
b
d
Reaction conditions: phenylacetylene (1 mmol), H2O (330 μL).
c
Isolated yield. Au-NHC@POPs1 synthesized in 150 mL of toluene.
e
Au-NHC@POPs1 synthesized in 80 mL of toluene. Au-NHC@
f
POPs1 synthesized in 100 mL of toluene. Au-NHC@POPs1
synthesized in 120 mL of toluene. Au-NHC@POPs1 synthesized in
g
50 mL of toluene.
Information)] that can pass in and out freely in those
micropores of the Au-NHC@POPs; therefore, no obvious
catalytic difference was observed among these porous catalysts.
To improve our understanding of the catalytic efficiency of Au-
NHC@POPs1 synthesized in different amounts of solvent,
kinetic studies of these catalysts were performed; a plot of the
GC yield versus reaction time and the TOF value of the initial 2
h are shown in Figure 7, and we can conclude that Au-NHC@
a
Reaction conditions: 6 (1.0 mmol), Au-NHC@POPs1 (synthesized
[b]
in 150 mL of toluene), MeOH (660 μL), H2O (330 μL). Isolated
yield.
synthesized in 50, 80, and 120 mL of toluene. Au-NHC@POPs1
synthesized in 100 mL of toluene.
[c]
[d]
1,4-Dioxane instead of MeOH.
Au-NHC@POPs1
[e]
Supporting Information) was synthesized to test these different
catalysts (Au-NHC@POPs1 that were prepared in 150, 120,
100, 80, and 50 mL of toluene). An interesting observation is
the fact that 7j affords a 64% yield catalyzed by Au-NHC@
POPs1, which was prepared in 150 mL of toluene, but only a
trace or little (12%) product was detected when the reaction
was catalyzed by Au-NHC@POPs1, which was synthesized in
50, 80, 120, and 100 mL of toluene. This is most likely because
of the 2.11 nm large molecule that cannot pass through
micropore networks freely. However, Au-NHC@POPs1
prepared in 150 mL of toluene exhibits the coexistence of
micropores and mesopores that have a capacity for large
molecules. All the experiments described above demonstrated
that the pore size effect severely impacts catalytic efficiency.
Subsequently, we examined the scope of this catalytic system.
As shown in Table 2, the reactivity of terminal alkynes is better
than that of internal alkynes. Substrates with electron-donating
or electron-withdrawing groups all afforded good yields.
A favorable feature of the catalytic system that caught our
attention was the precipitation of Au-NHC@POPs from the
reaction mixture. Thus, we investigated the possibility of
recycling Au-NHC@POPs. The catalyst was collected by
centrifugation and then reused for the next round of alkyne
hydration. Because of the high thermal stability and exceptional
chemical robustness, Au-NHC@POPs1 was reused five times
with only a slight loss of activity (Figure 8a). To better test the
stability of catalyst Au-NHC@POPs1, a recycling experiment
was conducted by increasing the level of the alkyne substrate to
3.0 mmol with 5 mg of Au-NHC@POPs1 to achieve an ∼30%
yield of acetophenone (Figure 8, b). Recycling the catalyst Au-
NHC@POPs1 over five runs did not lead to a significant
Figure 7. Plot of yield (GC yield) vs reaction time and the TOF
(moles per hour) of the initial 2 h.
POPs1 synthesized in 100 mL had the best catalytic efficiecy
(initial 2 h TOF, 19.6 mol/h) and its analogues synthesized in
80 mL had the worst catalytic result with a 12.5 mol/h of the
initial 2 h TOF. Therefore, the morphology (surface areas and
pore size distrbution) of the Au-NHC@POPs1 played an
important role and had an impact on the early state of the
reactions. In addition, the catalytic activity of the homogeneous
counterpart (iodo-NHC-Au) was tested as a comparison with
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details and characterizations of all compounds,
TGA, IR, and pore size distribution profile. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: fuweili@licp.cas.cn.
■
Notes
The authors declare no competing financial interest.
Figure 8. Recyclability test of Au-NHC@POPs1 in the model of
phenylacetylene hydration: (a) 1 mmol of alkyne substrate and (b) 3
mmol of alkyne substrate.
ACKNOWLEDGMENTS
■
This work was supported by the Chinese Academy of Sciences
and the National Natural Science Foundation of China
(21002106 and 21133011).
decline in acetophenone yields. Analysis of the aqueous
reaction solution after each cycle by ICP-AES showed Au
element leaching did not reach the detection limit of 1 mg/L.
Moreover, when the recycling experiment had been completed,
the filtrate was used solely as the reaction medium without
adding the heterogeneous Au-NHC@POPs1, and then the
substrates were added to the reaction system to continue the
reaction. The result showed that the GC yield of acetophenone
was almost constant, which ensured the heterogeneous nature
of the catalytic process.
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4. CONCLUSION
In summary, for the first time, we have successfully prepared a
series of catalytic POPs materials (Au-NHC@POPs) with the
Au-NHC complex as building-in feedstock; their surface areas
and pore size distribution can be tuned not only by the
monomer structure but also through concentration control, and
a model assumption of the branch−branch cross effect was
proposed to explain the synthetic control. These Au-NHC@
POPs have been found to be efficient heterogeneous catalysts
for alkyne hydration reactions with good substrate tolerance
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of catalytic efficiency. Control experiments with the larger
alkyne proved the pore sized effect could severely impact the
catalytic activity of a specific molecule. In addition, Au-NHC@
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326
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