Alkyne Activation by a Porous Silver Coordination Polymer for Heterogeneous Catalysis of Carbon Dioxide Cycloaddition

Article abstract of DOI:10.1021/acscatal.6b03404

The widely studied porous coordination polymers, possessing large pores to adsorb waste carbon dioxide gas and further transform it into valuable chemical products, have been attracting research interest, both industrially and academically. The active silver(I) ions endow the specific alkynophilicity to activate C≡C bonds of alkyne-containing molecules via π activation. Incorporating catalytic Ag metal sites into the porous frameworks represents a promising approach to construct heterogeneous catalysts that cyclize propargylic alcohols with CO₂, which is highly desirable for the environmentally benign conversion of carbon dioxide to fine chemicals. We report the preparation of porous coordination polymers (PCPs) with active silver sites and efficient silver-silver bond formation by carefully modifying the coordination geometries of the silver sites. The decentralized silver(I) chains in the porous frameworks enable the efficient conversion of CO₂ and derivatives of acetylene to α-alkylidene cyclic carbonates in a heterogeneous manner. X-ray structure analysis reveals two kinds of substrate molecules positioned within the pores of the framework, which correspond to trapping and activated modes through the multiple interactions with the functional Ag chains. The example of tandem conversion of simple alkynes and carbon dioxide to α-alkylidene cyclic carbonates is also presented. The well-positioned catalytic silver(I) sites and the crystalline properties of the frameworks facilitated the structural analyses of the intermediates of each catalytic step, providing knowledge of the synergistic nature of the σ and π activation of C≡C bonds. The successful catalysis of azide-alkyne cycloaddition and synthesis of propargylic alcohols via terminal alkynes could also give another indicator for the activation properties of Ag sites.

Full text of DOI:10.1021/acscatal.6b03404

Research Article
pubs.acs.org/acscatalysis
Alkyne Activation by a Porous Silver Coordination Polymer for
Heterogeneous Catalysis of Carbon Dioxide Cycloaddition
Zhen Zhou,† Cheng He,† Lu Yang,† Yefei Wang,† Tao Liu,† and Chunying Duan*,†,‡
†State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, People’s Republic of China
‡Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300071, People’s Republic of China
S
* Supporting Information
ABSTRACT: The widely studied porous coordination
polymers, possessing large pores to adsorb waste carbon
dioxide gas and further transform it into valuable chemical
products, have been attracting research interest, both
industrially and academically. The active silver(I) ions endow
the specific alkynophilicity to activate CC bonds of alkyne-
containing molecules via π activation. Incorporating catalytic
Ag metal sites into the porous frameworks represents a
promising approach to construct heterogeneous catalysts that
cyclize propargylic alcohols with CO2, which is highly desirable
for the environmentally benign conversion of carbon dioxide
to fine chemicals. We report the preparation of porous
coordination polymers (PCPs) with active silver sites and
efficient silver−silver bond formation by carefully modifying the coordination geometries of the silver sites. The decentralized
silver(I) chains in the porous frameworks enable the efficient conversion of CO2 and derivatives of acetylene to α-alkylidene
cyclic carbonates in a heterogeneous manner. X-ray structure analysis reveals two kinds of substrate molecules positioned within
the pores of the framework, which correspond to trapping and activated modes through the multiple interactions with the
functional Ag chains. The example of tandem conversion of simple alkynes and carbon dioxide to α-alkylidene cyclic carbonates
is also presented. The well-positioned catalytic silver(I) sites and the crystalline properties of the frameworks facilitated the
structural analyses of the intermediates of each catalytic step, providing knowledge of the synergistic nature of the σ and π
activation of CC bonds. The successful catalysis of azide−alkyne cycloaddition and synthesis of propargylic alcohols via
terminal alkynes could also give another indicator for the activation properties of Ag sites.
KEYWORDS: silver coordination polymer, carbon dioxide, cyclic carbonates, multiple activation, heterogeneous catalyst
on homogeneous catalysis using relatively large amounts of Ag-
based catalysts. However, the low catalytic activity and the
leaching of active species make the alkyne activation by silver
complexes in the heterogeneous state challenging, and few
examples of heterogeneous catalysis for this conversion have
been reported.20,21
As one type of typical crystalline porous solid that resembles
zeolites, porous coordination polymers (PCPs) are able to
utilize a minimum amount of metal ions and organic ligands to
build maximum surface areas with predictable, controllable,
tailorable, and modifiable pores and cavities.22−24 Due to these
intrinsic structural attributes, porous coordination polymers
have emerged as promising candidates for catalysis because they
combine the benefits of heterogeneous catalysis, such as easy
postreaction separation and catalyst reusability, with those of
homogeneous catalysis, such as high selectivity and controll-
ability.25−29 PCP-based catalysis has been recently used for
INTRODUCTION
■
The use and transformation of abundant gaseous waste, such as
converting carbon dioxide as a C1 source into valuable chemical
products using cheap and easily obtained industrial chemicals,
fulfills the requirements of green chemistry and atom
economy.1−4 The nucleophilic addition of terminal alkynes to
various unsaturated electrophiles, which has been extensively
studied, enables the introduction of the alkyne functionality.5−7
The resultant propargylic alcohols directly react with CO2 for
conversion into cyclic carbonates with potential biological and
commercial value.8−10 In general, to ensure this conversion,
both catalytic processes require heavy-transition-metal catalysts
as well as a subsequent separation and purification of products
from the reagents, catalysts, and associated materials.11−13
A
procedure enabling multiple conversions of alkynes via a chain
of discrete catalytic steps and a single workup stage is highly
desirable for the environmentally benign conversion of carbon
dioxide to fine chemicals.14−16 Recently, reports have shown
that silver complexes have the potential to σ-activate terminal
alkynes and π-activate internal alkynes of CC bonds for
discrete reactions.17−19 Most of the studies have concentrated
Received: November 30, 2016
Revised: February 10, 2017
© XXXX American Chemical Society
2248
DOI: 10.1021/acscatal.6b03404
ACS Catal. 2017, 7, 2248−2256

ACS Catalysis
Research Article
Scheme 1. Representation of the Porous Silver Coordination Polymer for the Carboxylic Cyclization of a Phenyl Propargylic
Alcohol with CO2 to α-Alkylidene Cyclic Carbonates
CO2 fixation and transformations, such as cycloaddition of
epoxides, photochemical reduction, and carboxylation.30−33
Our group has also been dedicated to constructing several PCP
materials for the catalysis of CO2 cycloaddition to cyclic
carbonates in an asymmetric autotandem conversion or even
using raw power plant flue gas as a CO2 source in the past two
years.34−37 These developments have inspired the design and
preparation of previously poorly established silver-based PCPs
for the potential σ and π activation of CC bonds and tandem
transformation of alkyne derivatives to cyclic carbonates
directly with CO2.38−40
From a catalytic perspective, π activation by Ag ions is
different, since the silver-based catalytic properties are different
from those of the typical Lewis acid catalyst.41,42 The
fundamental mechanism of silver catalysts promoting π
interaction with a triple carbon−carbon bond is electrophilic
activation, which requires more space for the direct
interactions. However, the construction of three-dimensional
PCPs usually relies on the formation of a pattern of multiply
connected nodes, and this peculiar structural characteristic
remains an ongoing challenge for developing silver-based
catalysts in PCP systems due to the unfavorable ability of the
catalytic center Ag ions to connect to CC bonds via π
activation.
On the basis of this information, we report the incorporation
of the tetrakis(4-carboxyphenyl)ethylene (H4TCPE) moiety as
a four-point connected node to combine alkynophilic silver
ions in the porous and coordination frameworks. We envision
that the decentralized Ag chains in this structure exhibit
excellent potential to activate the π position of CC bonds,
and thus Ag-TCPE represents efficient catalytic capability in a
heterogeneous manner for the carboxylative cyclization of
propargylic alcohols with carbon dioxide (Scheme 1). The
successful catalysis of azide−alkyne cycloaddition also gives
another indicator of the activation properties of Ag sites.
Additionally, starting from nucleophilic addition of terminal
alkynes followed by cycloaddition of propargylic alcohols,
without further preparation, the tandem catalytic performance
of a silver-based PCP is presented. The intrinsic crystalline
properties of the frameworks facilitated deeply structural
analyses of the intermediates of each active and catalytic step,
providing powerful knowledge of π activation of CC bonds
by Ag sites.
EXPERIMENTAL SECTION
■
Materials and Instrumentation. All chemicals were of
reagent-grade quality, were obtained from commercial sources,
and were used without further purification. All starting reagents
were obtained from Acros or TCI and used as received. Carbon
dioxide (99.995%) was purchased from the Dalian Institute of
Special Gases and used as received. The elemental analyses of C
and H were performed on a Vario EL III elemental analyzer.
Inductively coupled plasma (ICP) analysis was performed on a
Jarrel-Ash J-A1100 spectrometer. The powder XRD diffracto-
grams were obtained on a Riguku D/MAX-2400 X-ray
diffractometer with a Cu sealed tube (λ = 1.54178 Å). IR
spectra were recorded as KBr pellets on a NEXUS instrument.
Thermogravimetric analyses (TGA) were carried out at a ramp
rate of 10 °C min−1 under a nitrogen flow with an SDTQ600
instrument. Solution UV−vis spectra were performed on a TU-
1900 spectrophotometer. Confocal laser scanning microscopy
(CLSM) micrographs were collected by an Olympus Fluoview
FV1000 instrument with λex 488 nm. Raman spectroscopy (Lab
Raman HR Evolution) measurements were performed using a
solid-state 683 nm laser. A laser power of 1−2.5% was used to
avoid degradation of the sample by the laser beam during the
Raman measurements. The morphology of the crystals was
observed using a scanning electron microscope (SEM,
QUANTA 450, FEI, US) at an accelerating voltage of 20.00
kV and a working distance of 10 mm. All samples were coated
with gold before SEM examination. CO2 sorption isotherms
were measured using an Autosorb-IQ-C analyzer of Quantach-
rome. 1H spectra were recorded on a Varian INOVA 400 MHz
spectrometer. Peak frequencies were referenced versus an
1
internal standard (TMS) for H NMR.
Preparation of Samples. All reagents were used as
purchased without further purification. Tetrakis(4-
carboxyphenyl)ethylene (H4TCPE) was prepared according
to the methods in the literature43 and characterized by H
1
NMR.
Synthesis of Ag-TCPE. A mixture of H4TCPE (30 mg, 0.06
mmol) and AgNO3 (134 mg, 0.2 mmol) was dissolved in 4 mL
of a mixed solvent of dimethylformamide (DMF) and DMA in
a screw-capped vial. The resulting mixture was kept in an oven
at 100 °C for 3 days. After the autoclave was cooled to room
temperature, dark red blocklike single crystals were separated,
washed with water, and air-dried. Yield: 10% (based on the
2249
DOI: 10.1021/acscatal.6b03404
ACS Catal. 2017, 7, 2248−2256

ACS Catalysis
Research Article
Figure 1. (a) Representation of Ag chains linked by each μ2-O bridge of the H4TCPE ligand. (b) Connected mode of Ag chains. (c) Structure of Ag-
TCPE showing the three-dimensional network. (d) Schematic representation of the Ag-TCPE network as tiling. Color code: Ag, violet; O, red; C,
green. H atoms are omitted for clarity.
crystal dried under vacuum). Anal. Calcd for C15H12O6Ag2: C,
35.74; H, 2.40; Ag, 42.80. Found: C, 35.90; H, 2.23; Ag, 43.24.
IR (KBr): 3436 (br, s), 1593 (s), 1530 (w), 1387 (m), 1196
(vs), 1066 (m), 794 (s) cm−1.
Ag-TCPE@1a. Ag-TCPE was first soaked in a methanol
solution (72 h) to remove guest molecules and then fully dried,
removing the alcohol molecules, in a vacuum oven (100 °C, 2
h). Crystals of Ag-TCPE@1a were obtained by soaking the
treated Ag-TCPE crystals in a (10 mmol) propargyl alcohol
dichloromethane solution for 48 h.
Crystallography. X-ray intensity data were measured on a
Bruker SMART APEX CCD-based diffractometer (Mo Kα
radiation, λ 0.71073 Å) using the SMART and SAINT
programs.44,45 The crystal structure was solved by direct
methods and further refined by full-matrix least-squares
refinements on F2 using the SHELXL-97 software, and an
absorption correction was performed using the SADABS
program.46 The remaining atoms were found from successive
full-matrix least-squares refinements on F2 and Fourier
syntheses. Non-H atoms were refined with anisotropic
displacement parameters. The hydrogen atoms within the
ligand backbones were fixed geometrically at calculated
distances and allowed to ride on the parent non-hydrogen
atoms. In the refinement of crystal data of Ag-TCPE@1a, to
help the stability of the refinement for the encapsulated related
substrate molecule, bond distances between several atoms were
fixed for the ideal geometrical shape. There are two and a half
solvent CH2Cl2 molecules within an asymmetric unit. C55, Cl5,
and Cl6 had 0.5 occupancy, and one Cl atom was disordered
into two parts (Cl0 and Cl2) with their SOF values refined as
0.35 and 0.65, respectively. Crystallographic data for Ag-TCPEs
are summarized in Table S1 in the Supporting Information.
Catalysis Details. Before the catalysis experiments, solvent-
exchange experiments were carried out to activate the catalysts
by removing the coordinated water molecules and solvent. The
crystal samples were immersed in methanol for 3 days and then
fully dried out at 100 °C under high vacuum for 2 h to produce
the activated samples.
Cyclization of Propargylic Alcohols with CO2. In a typical
reaction, the catalytic reactions were conducted in a 25 mL
autoclave reactor with propargylic alcohol (2 mmol) and Ph3P
(2.5 mol %) in 1 mL of CH3CN, purged with 0.5 MPa of CO2
under a constant pressure for 10 min to allow the system to
equilibrate. The vessel was set in an oil bath with frequent
stirring at a temperature of 50 °C for 36 h. At the end of the
reaction, the reactor was placed in an ice bath for 20 min and
then opened. The catalysts were separated by centrifugation,
and a small aliquot of the supernatant reaction mixture was
2250
DOI: 10.1021/acscatal.6b03404
ACS Catal. 2017, 7, 2248−2256

ACS Catalysis
Research Article
1
taken to be analyzed by H NMR to calculate the yields of the
reaction. The organic phase was collected and then purified by
flash column chromatography on silica gel (petroleum ether/
ethyl acetate) to isolate the desired products.
magnify effective contact regions of both sides, providing a
beneficial feature to capture and activate specific molecules.
The skeleton of the Ag-TCPE extends the chains to form 3D
frameworks with large quadrilateral channels of 15.4 × 15.4 Å2
along the c direction. The overall framework is depicted by a
Azide−Alkyne Cycloaddition. The catalytic reaction was
conducted in a 2 mL glass vial with a tetrafluoroethylene lid. To
a mixture of phenylacetylene (1 mmol) and benzyl azide (1
mmol, 1 equiv) in 0.1 mL of water was added the catalyst (1
mol % loading, based on each Ag site). The mixture was then
stirred at 50 °C for 24 h. The reaction mixture was diluted with
dichloromethane (10 mL). The catalysts were separated by
centrifugation. Subsequently, 10 mL of water was added, and
the organic layer was extracted with dichloromethane and dried
over anhydrous sodium sulfate. The organic phase was
concentrated under vacuum and purified through silica gel
column chromatography (ethyl acetate/hexanes) to afford the
pure product. All reported yields are isolated yields.
Tandem Conversion of Alkyne and CO2 to α-Methylene
Cyclic Carbonates. In a typical reaction, the catalytic reaction
was conducted in a 25 mL autoclave reactor, with the addition
of 2 mmol of phenylacetylene and acetone (1.5 equiv, 0.22 mL)
in the presence of the catalyst Ag-TCPE (1.0 mol %), t-BuOK
(5 mol %), Ph3P (5 mol %), and 1 mL of CH3CN, which were
directly placed in the reactor and then purged with 0.5 MPa
CO2 under constant pressure for 10 min to allow the system to
equilibrate. The vessel was set in an oil bath with frequent
stirring at a temperature of 40 °C for 72 h. At the end of the
reaction, the reactor was placed in an ice bath for 20 min and
then opened. The catalysts were separated by centrifugation,
and the reaction mixture was taken to be analyzed by 1H NMR
to calculate the yields of the reaction using durene as an
internal standard. The yields of the general reactions with
different substrates are indicated in Table S4 in the Supporting
Information.
Schlafli symbol of {64.82} {66}. In the absence of guests, the
̈
2
PLATON program gives a free void volume of 52.6% of the
crystal volume of Ag-TCPE, approximately 2930 Å3 per unit
cell. Dye-uptake studies were performed by soaking the crystals
in a methanol solution containing 2′,7′-dichlorofluorescein and
exhibit 9.8% uptake based on Ag-TCPE’s weight (Figure S8 in
the Supporting Information). Additionally, confocal laser
scanning microscopy of the guest-adsorbed crystals gave a
strong green fluorescence response (λex 488 nm) that can be
assigned to the emission of fluorescein (Figure S9 in the
Supporting Information). The uniform distribution of the dye
molecules throughout the crystals suggests that the dyes
penetrated deeply into the channels, rather than remaining on
the external surface.47 These results give potent evidence of
porous characteristics and the capacity of adsorbing and
accommodating substrate molecules in its pores. The
unsaturated metal sites in the MOF created by structural
defects would increase its interaction with CO2 molecules and
thus improve its CO2 uptake capacity and promote high
selectivity and efficient chemical transformations. The CO2
adsorption measurements at 273 and 298 K are reversible and
show q steady rise with an uptake of 56.4 cm3 g−1
(corresponding to 2.52 mmol g−1) and 42.2 cm3 g−1
(corresponding to 1.88 mmol g−1) for Ag-TCPE at 1 bar,
respectively (Figure S10 in the Supporting Information). This
suggests a sound interaction between the Ag-TCPE framework
and CO2 molecules. These features also render Ag-TCPE a
solid sorbent material for CO2 capture.
Our catalytic experiments were initially performed on the
carboxylic cyclization of phenyl propargylic alcohols as a model
for reaction with carbon dioxide. In a typical experiment,
reactions were conducted in an autoclave reactor with
propargylic alcohol (2 mmol) and CO2 purged to 0.5 MPa in
1 mL of CH3CN at 50 °C. 1H NMR analysis revealed that Ag-
TCPE (0.5 mol % loading, based on each Ag site) could act as
an effective heterogeneous catalyst for this catalysis reaction in
the presence of 2.5 mol % triphenylphosphine (Ph3P) and
afforded nearly complete conversion within 36 h. No
rearrangement side product, such as α,β-unsaturated ketone,
was observed after the reaction. In the control experiments, no
detectable conversion occurred in the absence of catalysts, and
Ph3P itself was ineffective under the given reaction conditions.
However, Ph3P also plays a significant role in the catalytic
system, as there is no carboxylative product without adding it
(Table 1, entry 6). Notably, other silver(I) salts, such as
AgNO3, AgOAc, and AgSO3CF3 displayed negative activity
under identical conditions (Table 1, entries 1−3). The simple
mixture of AgNO3 and H4TCPE could also not afford the
detected conversion under the same reaction conditions (Table
1, entry 4). When other additives were used to replace Ph3P,
such as TBABr (tetra-n-butylammonium bromide), TBUP (tri-
n-butylphosphine), and DBU (1,8-diazabicyclo[5.4.0]undec-7-
ene), all of them gave low yields under identical conditions
(Table 1, entries 7−9). Internal propargylic alcohols with
various substituted R1 groups were subject to carboxylative
cyclization (Table 2). No significant conversion was observed
under identical reaction conditions, and all reactions gave
excellent yields. Additionally, 2-methyl-3-butyn-2-ol (2i) and 3-
RESULTS AND DISCUSSION
■
The synthesis of Ag-TCPE began with AgNO3 and H4TCPE in
a solvothermal reaction at 100 °C in a DMF/DMA mixed
solvent. Single-crystal X-ray structural analysis revealed that Ag-
TCPE crystallized in the C2c space group. The asymmetric unit
of this structure contains two independent, four-coordinate
Ag(I) ions, bound in a tetrahedral fashion with identical
coordination modes (neglecting the Ag···Ag interactions). Ag1
and Ag2 are bridged by two μ2-O atoms from the carboxylic
groups of two H4TCPE ligands and anchored by another
oxygen atom of the H4TCPE ligand and one lattice water
molecule, which can be considered as a potential catalytic metal
site after guest-molecule exchange, which would remove the
lattice water and solvent molecules (Figure 1). For each
H4TCPE ligand, one oxygen atom of the carboxylic groups acts
as a μ2-O bridge, whereas the other oxygen acts only as a μ-O
bridge to link each layer of the Ag chains. Notably, the Ag···Ag
distances between the silver pair bridged by one carboxylic
group of 2.85 Å (shorter than 3.44 Å, the Ag···Ag van der Waals
radius) indicate an argentophilic interaction. Considering the
weak Ag···Ag interaction between monatomic-bridged silver
pairs, the silver atoms are cross-linked to form one-dimensional
chains (Figure 1b). The close bonding of the Ag atoms
facilitates long-range charge and electron transfer and endows
the strong activating capacity. These functional Ag chains,
distributed on the tube walls, rather than the vertexes of the
framework, reduce the steric hindrance and at the mean time
2251
DOI: 10.1021/acscatal.6b03404
ACS Catal. 2017, 7, 2248−2256

ACS Catalysis
Research Article
Ag-TCPE by filtration after 12 h shut down the reaction, and
no additional conversions were observed for another 24 h
under the same reaction conditions. This observation suggested
that Ag-TCPE is a true heterogeneous catalyst in the catalytic
system and is stable enough to ensure that no leaching of metal
ions occurs during the catalysis process.48 For the heteroge-
neous catalysts, recyclability is an essential feature for them to
be considered for practical use in industrial applications. The
residual Ag-TCPE from the reaction isolated by simple
filtration was used for successive runs with freshly added
Ph3P. The yields of cyclic carbonates were not significantly
affected in additional cycles (Figure 2b), although the loss of
small amounts of the catalyst is unavoidable. The XRD patterns
and IR spectroscopy (Figures S5 and S7 in the Supporting
Information) of the reused crystals did not exhibit any
significant changes. Dye-uptake studies revealed that the dye-
uptake ability of the recovered catalyst (8.9%) was nearly
identical with that of the original catalyst, confirming the
chemical stability of fresh Ag-TCPE and that recovered after
each cycle.
Table 1. Control Experiments of Carboxylic Cyclization of
Propargylic Alcohols with CO2
a
b
entry
catalyst
AgNO3
additive
yield (%)
1
2
3
PPh3
PPh3
PPh3
PPh3
PPh3
AgOAc
AgSO3CF3
AgNO3/H4TCPE
Ni-TCPE2
Ag-TCPE
Ag-TCPE
Ag-TCPE
Ag-TCPE
c
4
5
6
d
7
DBU
8
TBABr
TBUP
13
32
d
9
a
Reaction conditions unless specified otherwise: propargylic alcohols
(2 mmol), catalyst (0.5 mol %), additive (2.5 mol %), and CH3CN (1
b
mL) under CO2 (0.5 MPa), 50 °C and 36 h. The yields were
1
One of the most outstanding features of the porous materials
is their ability to accommodate molecules within their pores;
the robust crystallinity of these networks facilitates the X-ray
analysis of the encapsulated products.49,50 The definitive
identification of the captured structure gives more information
on the intermediate process of the catalytic reactions and has
become an increasingly general method for catalytic studies. As
postulated in the literature, the well-positioned catalytic Ag(I)
sites and the crystalline properties of the frameworks facilitate
deep structural analyses of the intermediates of the catalytic
steps and provide knowledge of the activation nature of the
CC bonds.51,52 Fortunately, the quality of substrate 1a-
impregnated Ag-TCPE crystals was sufficient for X-ray
structure analysis (noted as Ag-TCPE@1a). While the cell
dimensions of the crystals are almost the same as those of the
original, the strong interactions between the substrates and the
frameworks lead to a change in symmetry from the space group
C2/c to the space group P21/n. Clearly, the asymmetric unit of
this structure contains four independent Ag(I) ions, each
bound in a tetrahedral fashion with identical coordination
modes. As shown in Figure 3, two different types of 1a
molecules were observed positioned close to the functional Ag
chains within the frameworks. One type of 1a molecule
exhibited a weak Ag···O coordinate bond (the Ag···O
separation of 2.81 Å). It is postulated that this type of
interaction has the potential to supply the driving force for the
substrate trapping in the cavity of catalyst Ag-TCPE. Several
cooperative Ag···C interactions were also found, which
corresponded to the aromatic rings stabilizing the encapsulated
mode. The distances between the Ag ions and the aromatic
atoms of 3.29−3.33 Å were well within the reported range,53,54
probably providing additional driving force to fix the substrate
close to the catalytic sites with the potential activated bonds
well-positioned near the active silver sites. Another type of 1a
molecule exhibited stronger Ag···C interactions relevant to the
aromatic rings, with the average separation compressed from
3.30 to 3.00 Å, but showed much weaker Ag···O interactions
relevant to the hydroxyl group of propargylic alcohol (Ag···O
3.46 and 2.81 Å). The weak but significant Ag···C interactions
corresponding to the CC bonds were also found in this type
of interaction mode with the shortest intermolecular separation
of 3.16 Å. If the first type of substrate−framework interactions
was related to the trapping of the substrates, the latter is
determined by H NMR analysis using durene as internal standard.
c
d
H4TCPE (0.05 mmol). Solvent free.
Table 2. Ag-TCPE-Catalyzed Cyclization of Propargylic
Alcohols with CO2
a
a
Reaction conditions unless specified otherwise: propargylic alcohols
(2 mmol), catalyst (0.5 mol % loading, based on each Ag site), Ph3P
(2.5 mol %), and CH3CN (1 mL) under CO2 (0.5 MPa), 50 °C, and
b
36 h. The reaction time was 20 h. The yields were determined by 1H
NMR analysis using durene as internal standard.
methyl-1-pentyn-3-ol (2j) as substrates have also been
investigated and give excellent yields (>99%) within 20 h of
the reaction under an identical catalytic system, which enlarges
the feasibility of the catalyst Ag-TCPE. Increasing the bulk of
substituted R1 or R2 groups (i.e., 2e,h) caused the yield to
decrease. The size selectivity of the substrates suggested that
the catalytic processes occurred within the pores of the
catalysts.
The catalytic efficiency of Ag-TCPE was compared with that
of Ni-TCPE2, a previously reported framework containing
identical amounts of catalyst sites for each ion and a similar
framework skeleton as a reference.34 The fact that no detectable
amounts of the carboxylic cyclization products were found
under identical reaction conditions indicates that the effect of
the metal ions on this type of reaction is specific to typical π
activation of Ag ions. As is shown in Figure 2a, the removal of
2252
DOI: 10.1021/acscatal.6b03404
ACS Catal. 2017, 7, 2248−2256

ACS Catalysis
Research Article
Figure 2. (a) Catalytic traces of cyclization performed under optimal conditions with the catalyst Ag-TCPE filtered after 12 h. (b) Recycling
experiments at each run.
activation of propargylic alcohol molecules (Figure 4). In
comparison with the Raman spectrum of the free Ag-TCPE,
the spectrum of Ag-TCPE@1a exhibits two new peaks at 990
and 2225 cm−1, which correspond to the characteristic peak of
CC stretching.55 The obvious red shifts from 997 and 2230
cm−1 of free propargylic alcohol suggest the adsorption and the
1
activation of 1a within the channels of Ag-TCPE. The H
NMR signal of the OH group shifted from δ 3.41 to 3.46 ppm,
providing further evidence for the activation process. Addition-
ally, there is a small emerging peak at 3.54 ppm next to 3.46
ppm, which is attributed to the hydrogen-bonding interaction
between Ag-TCPE and 1a, leading to the enhancement of O
nucleophilicity. These two signals of the OH group also match
well with the trapping and activated modes of 1a captured by
the X-ray structure analysis.
To further examine the π-activation behavior of our Ag-based
PCP catalyst in the catalytic reactions, azide−alkyne cyclo-
addition was performed. This reaction, as a representative
example of “click” reactions, has been extensively studied and
widely employed in organic synthesis, medicinal chemistry, and
polymer science.56,57 Nevertheless, only a few studies regarding
silver-based catalysts have been reported.58−60 In the presence
of Ag-TCPE with a mixture of benzyl azide (1.0 mmol) and
phenylacetylene (1.0 mmol) in 0.1 mL of H2O, the substrate
exhibited 90% conversion after 24 h. Other phenylacetylene
substrates exhibited excellent to moderate yields under identical
conditions (Table 3). As is known, the synthesized silver
Figure 3. Structure of propargylic alcohol impregnated crystals Ag-
phenyl acetylide intermediate with Ag ions fixed to the terminal
CC bond cannot be directly reacted with benzyl azide
because there is no further π activation of extra Ag catalytic
sites.61 These results fully demonstrate that the framework
structure and decentralized Ag centers in our catalyst are
beneficial for the π activation to promote the catalysis.
Having established the π-activation behavior of the Ag-
TCPE-catalyzed carboxylative cyclization protocol, we inves-
tigated the possibility of tandem conversion of alkyne and
carbon dioxide to α-alkylidene cyclic carbonates in related
reactions. Acetone and phenylacetylene were selected as the
first step to examine the conversion of propargylic alcohol
(reaction 1, Figure 5, top scheme). When phenylacetylene and
acetone were mixed in the presence of the catalyst Ag-TCPE
and t-BuOK (potassium tert-butoxide) was added at room
temperature under solvent-free conditions, the desired product
was isolated with considerable yields. Other terminal alkynes
with various alkyl substituents on the phenyl moiety at the para
TCPE@1a and enlarged views showing the positions of the substrate
and multiple-type interactions between the Ag ions and substrates.
possibly related to the π activation of the CC bonds. These
morphological differences can be attributed to the dynamic
interaction process from molecular trapping to the substrate
activation stage. From a mechanistic viewpoint, the dynamic
reversible weak Ag···O coordinate bonds first form to trap the
molecules of 1a within the pores of the catalyst, fix the
molecules close to the catalytic sites via the weak Ag···C
(aromatic ring) interactions, position the CC bonds close to
the silver chains, and finally form Ag···C (CC bond)
interactions, thus leading to a decrease in the electron density
in the triple bonds and the direct activation of them. This
unique decentralized structural distribution of Ag chains
endows the catalyst Ag-TCPE with the specific alkynophilicity.
1
Raman and H NMR measurements of the crystals Ag-
TCPE@1a were also carried out to verify the existence and
2253
DOI: 10.1021/acscatal.6b03404
ACS Catal. 2017, 7, 2248−2256

ACS Catalysis
Research Article
Figure 4. (a) Raman spectra of Ag-TCPE impregnated with propargyl alcohol (Ag-TCPE@1a, blue) and free Ag-TCPE (black). The inset gives the
1
Raman spectrum of free propargyl alcohol (1a). (b) Photographs and H NMR spectra of substrate 1a (top) and the treated crystals Ag-TCPE
impregnated with propargyl alcohol (bottom) in CD3CN-d3.
carbonates can be attributed to the dual-activation patterns of
the Ag-based catalysts. The fast adsorption of phenylacetylene
molecules is believed to facilitate the proximity between Ag
ions and the alkyne deprotonation by the additive base.
Subsequent nucleophilic addition of the activated terminal
alkyne to the carbonyl group generates the oxygen-centered
anion of the propargylic alcohol.64 Consequently, the terminal
alkyne is occupied via the protodemetalation process of σ-
coordinated Ag ions. However, the short distance facilitates
further π activation of the internal alkynes by Ag ions. The next
conversion occurs in the presence of CO2 and added Ph3P,
which plays a key role in the promotion of CO2 fixation and
activation. The nucleophilic attack on the silver propargylic
alcohol carbonate intermediate, which was generated by π
activation of the CC bonds, gives the corresponding
carboxylative cyclization product via a protodemetalation
process.
Table 3. Substrate Scope of Ag-TCPE-Catalyzed Azide−
a
Alkyne Cycloaddition
a
Reaction conditions: alkyne (1.0 mmol), benzyl azide (1.0 mmol),
catalyst (1 mol % loading, based on each Ag site) and H2O (0.1 mL) at
50 °C and 24 h. The yields were calculated using the isolated product.
CONCLUSION
■
position also produced the corresponding propargylic alcohols
in good to excellent yields (Figure 5a). To verify the potential
interactions between Ag-TCPE and phenylacetylene, the
phenylacetylene-uptake test was performed. After the catalyst
Ag-TCPE was added to water with 20 μL of phenylacetylene
sitting on the top, the organic layer gradually faded and
disappeared after 1 h, showing a fast uptake of phenylacetylene
molecules (Figure S13 in the Supporting Information). This
result reveals that the porous Ag-TCPE is capable of quickly
adsorbing the substrate phenylacetylene into its pores.
According to the above catalysis results for starting with
phenylacetylene and propargylic alcohol as substrates,
respectively, our protocol worked well for both internal and
terminal carbon−carbon triple bonds. A three-component
catalytic system for tandem reactions, consisting of phenyl-
acetylene, acetone, and CO2, has been established (reaction 3,
Figure 5, top scheme). The tandem reactions occurred
smoothly and gave the targeted carboxylative cyclization
products with ultimate 93% conversion and 73% selectivity
for Ag-TCPE (100[carbonate]/([carbonate] + [propargylic
alcohol]) %). Control experiments showed that the presence of
CH3CN as solvent could effectively avoid the reaction of CO2
with alkyne forming the unwanted propiolic acid in this tandem
reaction system.62,63 From the view of the plausible mechanism,
the smooth conversion of phenylacetylene into cyclic
In conclusion, a porous Ag(I)-based coordination polymer was
synthesized by incorporating the tetraphenylethylene moiety as
the backbone. The unique decentralized silver(I) chains in the
frameworks endow the specific alkynophilicity to activate CC
bonds, enabling the efficient conversion of CO2 and derivatives
of acetylene to α-alkylidene cyclic carbonates in a heteroge-
neous manner. The substrate propargylic alcohol could enter
the pores of catalyst Ag-TCPE and have multiple interactions
with Ag sites. The trapping and activated modes of propargylic
alcohol molecules have been captured within one single-crystal
structure. π activation and σ activation of the substrates enabled
successful tandem conversion of the phenylacetylene and
carbon dioxide to α-alkylidene cyclic carbonates in a single-
system catalysis reaction for the first time. The well-positioned
catalytic silver(I) sites and the crystalline properties of the
frameworks facilitated the structural analyses of the activation
intermediates and verified the existence of π activation of CC
bonds, which were further examined by the reaction of azide−
alkyne cycloaddition. This work provides a highly promising
approach for achieving continuous conversion by constructing a
dual-activation catalytic center in a single system, in which the
porous materials act as a microscopic chemical reactor for the
interaction and fast transport of the substrate molecule in its
cavities.
2254
DOI: 10.1021/acscatal.6b03404
ACS Catal. 2017, 7, 2248−2256

ACS Catalysis
Research Article
Figure 5. (top) Scheme of the tandem catalytic conditions. (a) Yields of the reaction of various phenylacetylene and acetone catalyzed by Ag-TCPE.
(b) Conversion and selectivity of the tandem carboxylative cyclization catalyzed by Ag-TCPE.
(3) Goeppert, A.; Czaun, M.; Jones, J. P.; Prakash, G. K. S.; Olah, G.
A. Chem. Soc. Rev. 2014, 43, 7995−8048.
(4) Hod, I.; Sampson, M. D.; Deria, P.; Kubiak, C. P.; Farha, O. K.;
Hupp, J. T. ACS Catal. 2015, 5, 6302−6309.
(5) Kumar, R. K.; Bi, X. H. Chem. Commun. 2016, 52, 853−868.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
■
S
ACS Publications website at DOI: 10.1021/acscatal.6b03404.
(6) Li, C. J. Acc. Chem. Res. 2010, 43, 581−590.
(7) Zhao, Y. Y.; Qiu, J. K.; Tian, L.; Li, Z. Y.; Fan, M. H.; Wang, J. J.
ACS Sustainable Chem. Eng. 2016, 4, 5553−5560.
(8) Kayaki, Y.; Yamamoto, M.; Ikariya, T. Angew. Chem., Int. Ed.
2009, 48, 4194−4197.
Experimental details, crystal data, additional catalytic
information, and NMR spectra for the products and
substrates (PDF)
Crystal data for Ag-TCPE (CIF)
Crystal data for Ag-TCPE@1a (CIF)
(9) Yoshida, S.; Fukui, K.; Kikuchi, S.; Yamada, T. J. Am. Chem. Soc.
2010, 132, 4072−4073.
(10) Yoshida, M.; Mizuguchi, T.; Shishido, K. Chem. - Eur. J. 2012,
18, 15578−15581.
AUTHOR INFORMATION
Corresponding Author
*E-mail for C.D.: cyduan@dlut.edu.cn.
ORCID
■
(11) Song, Q. W.; Yu, B.; Li, X. D.; Ma, R.; Diao, Z. F.; Li, R. G.; Li,
W.; He, L. N. Green Chem. 2014, 16, 1633−1638.
(12) Kikuchi, S.; Sekine, K.; Ishida, T.; Yamada, T. Angew. Chem., Int.
Ed. 2012, 51, 6989−6992.
Chunying Duan: 0000-0003-1638-6633
(13) Qin, C.; Feng, P.; Ou, Y.; Shen, T.; Wang, T.; Jiao, N. Angew.
Chem., Int. Ed. 2013, 52, 7850−7854.
Notes
The authors declare no competing financial interest.
(14) Fang, G. C.; Bi, X. H. Chem. Soc. Rev. 2015, 44, 8124−8173.
(15) Manjolinho, F.; Arndt, M.; Gooßen, K.; Gooßen, L. J. ACS
Catal. 2012, 2, 2014−2021.
ACKNOWLEDGMENTS
■
(16) Yoo, W. J.; Li, C. J. Adv. Synth. Catal. 2008, 350, 1503−1506.
(17) Halbes-Letinois, U.; Weibel, J. M.; Pale, P. Chem. Soc. Rev. 2007,
36, 759−769.
This work was supported by the National Nature Science
Foundation of China (Nos. 21531001 and 21421005) and the
Program for Changjiang Scholars and Innovative Research
Team in University (IRT1213).
(18) Karmakar, R.; Lee, D. Chem. Soc. Rev. 2016, 45, 4459−4470.
(19) Kim, S. H.; Kim, K. H.; Hong, S. H. Angew. Chem., Int. Ed. 2014,
53, 771−774.
REFERENCES
(20) Cui, M.; Qian, Q. L.; He, Z. H.; Ma, J.; Kang, X. C.; Hu, J. Y.;
Liu, Z. M.; Han, B. X. Chem. - Eur. J. 2015, 21, 15924−15928.
(21) Yang, Z. Z.; Zhao, Y. F.; Zhang, H. Y.; Yu, B.; Ma, Z. S.; Ji, G. P.;
Liu, Z. M. Chem. Commun. 2014, 50, 13910−13913.
■
(1) Aresta, M.; Dibenedetto, A.; Angelini, A. Chem. Rev. 2014, 114,
1709−1742.
(2) Markewitz, P.; Kuckshinrichs, W.; Leitner, W.; Linssen, J.; Zapp,
P.; Bongartz, R.; Schreiber, A.; Muller, T. E. Energy Environ. Sci. 2012,
5, 7281−7305.
(22) Furukawa, H.; Muller, U.; Yaghi, O. M. Angew. Chem., Int. Ed.
̈
2015, 54, 3417−3430.
2255
DOI: 10.1021/acscatal.6b03404
ACS Catal. 2017, 7, 2248−2256

ACS Catalysis
Research Article
(23) Zhang, J. P.; Zhang, Y. B.; Lin, J. B.; Chen, X. M. Chem. Rev.
2012, 112, 1001−1033.
(55) Yamakoshi, H.; Dodo, K.; Okada, M.; Ando, J.; Palonpon, A.;
Fujita, K.; Kawata, S.; Sodeoka, M. J. Am. Chem. Soc. 2011, 133, 6102−
6105.
(56) Thirumurugan, P.; Matosiuk, D.; Jozwiak, K. Chem. Rev. 2013,
113, 4905−4979.
(24) Sun, L. B.; Liu, X. Q.; Zhou, H. C. Chem. Soc. Rev. 2015, 44,
5092−5147.
(25) Yoon, M.; Srirambalaji, R.; Kim, K. Chem. Rev. 2012, 112,
1196−1231.
(57) Tiwari, V. K.; Mishra, B. B.; Mishra, K. B.; Mishra, N.; Singh, A.
S.; Chen, X. Chem. Rev. 2016, 116, 3086−3240.
(58) Connell, T. U.; Schieber, C.; Silvestri, I. P.; White, J. M.;
Williams, S. J.; Donnelly, P. S. Inorg. Chem. 2014, 53, 6503−6511.
(59) Wang, C. L.; Ikhlef, D.; Kahlal, S.; Saillard, J. Y.; Astruc, D.
Coord. Chem. Rev. 2016, 316, 1−20.
(26) Liu, Y.; Xuan, W. M.; Cui, Y. Adv. Mater. 2010, 22, 4112−4135.
(27) Slater, A. G.; Cooper, A. I. Science 2015, 348, aaa8075.
(28) Liu, J. W.; Chen, L. F.; Cui, H.; Zhang, J. Y.; Zhang, L.; Su, C. Y.
Chem. Soc. Rev. 2014, 43, 6011−6061.
(29) Manna, K.; Ji, P.; Greene, F. X.; Lin, W. J. Am. Chem. Soc. 2016,
138, 7488−7491.
(60) McNulty, J.; Keskar, K. Eur. J. Org. Chem. 2012, 2012, 5462−
5470.
́
(30) Guillerm, V.; Weselinski, Ł. J.; Belmabkhout, Y.; Cairns, A. J.;
(61) McNulty, J.; Keskar, K.; Vemula, R. Chem. - Eur. J. 2011, 17,
14727−14730.
D’Elia, V.; Wojtas, Ł.; Adil, K.; Eddaoudi, M. Nat. Chem. 2014, 6,
673−680.
(62) Riduan, S. N.; Zhang, Y. G.; Ying, J. Y. Angew. Chem., Int. Ed.
2009, 48, 3322−3325.
(31) Beyzavi, M. H.; Klet, R. C.; Tussupbayev, S.; Borycz, J.;
Vermeulen, N. A.; Cramer, C. J.; Stoddart, J. F.; Hupp, J. T.; Farha, O.
K. J. Am. Chem. Soc. 2014, 136, 15861−15864.
(32) Kajiwara, T.; Fujii, M.; Tsujimoto, M.; Kobayashi, K.; Higuchi,
M.; Tanaka, K.; Kitagawa, S. Angew. Chem., Int. Ed. 2016, 55, 2697−
2700.
(63) Yu, D. Y.; Zhang, Y. G. Green Chem. 2011, 13, 1275−1279.
(64) Schmidt, E. Y.; Cherimichkina, N. A.; Bidusenko, I. A.; Protzuk,
N. I.; Trofimov, B. A. Eur. J. Org. Chem. 2014, 2014, 4663−4670.
(33) Gao, W. Y.; Wu, H. F.; Leng, K. Y.; Sun, Y. Y.; Ma, S. Q. Angew.
Chem., Int. Ed. 2016, 55, 5472−5476.
(34) Zhou, Z.; He, C.; Xiu, J. H.; Yang, L.; Duan, C. Y. J. Am. Chem.
Soc. 2015, 137, 15066−15069.
(35) Zhou, Z.; Yang, L.; Wang, Y. F.; He, C.; Liu, T.; Duan, C. Y.
RSC Adv. 2016, 6, 108010−108016.
(36) Han, Q. X.; Qi, B.; Ren, W. M.; He, C.; Niu, J. Y.; Duan, C. Y.
Nat. Commun. 2015, 6, 10007.
(37) Guo, X. Y.; Zhou, Z.; Chen, C.; Bai, J. F.; He, C.; Duan, C. Y.
ACS Appl. Mater. Interfaces 2016, 8, 31746−31756.
(38) Liu, X. H.; Ma, J. G.; Niu, Z.; Yang, G. M.; Cheng, P. Angew.
Chem., Int. Ed. 2015, 54, 988−991.
(39) Yang, Z. Z.; Yu, B.; Zhang, H. Y.; Zhao, Y. Y.; Chen, Y.; Ma, Z.
S.; Ji, G. P.; Gao, X.; Han, B. X.; Liu, Z. M. ACS Catal. 2016, 6, 1268−
1273.
́
(40) García-Alvarez, J.; Díez, J.; Vidal, C.; Vicent, C. Inorg. Chem.
2013, 52, 6533−6542.
(41) Weibel, J. M.; Blanc, A.; Pale, P. Chem. Rev. 2008, 108, 3149−
3173.
(42) Furstner, A.; Davies, P. W. Angew. Chem., Int. Ed. 2007, 46,
̈
3410−3449.
̆
(43) Shustova, N. B.; McCarthy, B. D.; Dinca, M. J. Am. Chem. Soc.
2011, 133, 20126−20129.
(44) SMART Data collection software (version 5.629); Bruker AXS
Inc., Madison, WI, 2003.
(45) SAINT, Data reduction software (version 6.45); Bruker AXS Inc.,
Madison, WI, 2003.
(46) (a) Sheldrick, G. M. SHELXS-97, Program for Crystal Structure
Solution; Gottingen University, Gottingen, Germany, 1997. (b) Shel-
̈
̈
drick, G. M. SHELXL-97, Program for Crystal Structure Refinement;
Gottingen University, Gottingen, Germany, 1997.
̈
̈
(47) Cui, Y. J.; Song, T.; Yu, J. C.; Yang, Y.; Wang, Z. Y.; Qian, G. D.
Adv. Funct. Mater. 2015, 25, 4796−4802.
(48) Kayaki, Y.; Yamamoto, M.; Ikariya, T. J. Org. Chem. 2007, 72,
647−649.
(49) Inokuma, Y.; Yoshioka, S.; Ariyoshi, J.; Arai, T.; Hitora, Y.;
Takada, K.; Matsunaga, S.; Rissanen, K.; Fujita, M. Nature 2013, 495,
461−466.
(50) Lee, S.; Kapustin, E. A.; Yaghi, O. M. Science 2016, 353, 808−
811.
(51) Jing, X.; He, C.; Dong, D. P.; Yang, L. L.; Duan, C. Y. Angew.
Chem., Int. Ed. 2012, 51, 10127−10131.
(52) Brown, C. J.; Toste, F. D.; Bergman, R. G.; Raymond, K. N.
Chem. Rev. 2015, 115, 3012−3035.
(53) Zang, S. Q.; Mak, T. C. W. Inorg. Chem. 2008, 47, 7094−7105.
(54) Mak, T. C. W.; Zhao, X. L.; Wang, Q. M.; Guo, G. C. Coord.
Chem. Rev. 2007, 251, 2311−2333.
2256
DOI: 10.1021/acscatal.6b03404
ACS Catal. 2017, 7, 2248−2256