Ambient Chemical Fixation of CO2 Using a Robust Ag27 Cluster-Based Two-Dimensional Metal–Organic Framework

Article abstract of DOI:10.1002/anie.202007122

Unprecedented double S^2? templated Ag₂₇ clusters have been stabilized by 5,10,15,20-tetra(4-pyridyl)porphyrin (TPyP-H₂) ligands to afford a robust 2D metal–organic framework (Ag27-MOF). This silver cluster-assembled materi

Full text of DOI:10.1002/anie.202007122

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Accepted Article
Title: Ambient Chemical Fixation of CO2 Using a Robust Ag27 Cluster-
Based Two-Dimensional Metal-Organic Framework
Authors: Meihua Zhao, Shan Huang, Qiang Fu, Weifeng Li, Rui Guo,
Qingxia Yao, Fenglong Wang, Ping Cui, Chen-Ho Tung, and
Di Sun
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10.1002/anie.202007122
Angewandte Chemie International Edition
RESEARCH ARTICLE
Ambient Chemical Fixation of CO2 Using a Robust Ag27 Cluster-
Based Two-Dimensional Metal-Organic Framework
Meihua Zhao, Shan Huang, Qiang Fu, Weifeng Li, Rui Guo, Qingxia Yao, Fenglong Wang, Ping Cui,*
Chen-Ho Tung, and Di Sun*
Abstract: Coinage metals (Cu, Ag, Au)-based catalysts have been
widely used in organic synthesis chemistry, however, robust and
heterogeneous silver-based catalysts are hard to access due to their
high photosensitivity and chemical instability. Herein, the
Among them, chemical fixation of CO2 into value-added
chemicals involving a series of atom-economic reactions,
such as hydrogenation, cycloaddition and carboxylation et al,
represents one of the most promising approaches.[2] Due to
the thermodynamic stability of CO2, exploration of highly
efficient catalysts has become the core topic for the scientific
community. Compared with the homogenous counterparts,
heterogeneous catalysts are much more appealing and
favoured in current industry mainly because of their
advantages in easy separation, recovery and recycling. In
−
unprecedented double S2 templated Ag27 clusters have been
stabilized by 5,10,15,20-tetra(4-pyridyl)porphyrin (TPyP-H2)
ligands to afford a robust 2D metal-organic framework (Ag27-MOF).
This silver cluster-assembled material serves as a highly efficient
heterogeneous catalyst for the cyclization of both terminal and
internal propargylamines with CO2 under atmospheric pressure.
The density functional theory (DFT) calculations illustrate that
the high catalytic activity and broad substrate scope are
attributable to the saddle-shaped metallic node in Ag27-MOF, which
the terms of CO2 chemical fixation,
a
series of
heterogeneous catalytic systems have been developed,
including supported metal nanoparticles,[3] immobilized
organometallic species[4] and metal-organic framework
(MOF)-based materials[5] et al. However, their catalytic
performances in CO2 chemical fixation are still not
satisfactory regarding to the recyclability, turnover number,
stability and so on.
features an accessible platform with high-density silver atoms as
π-
Lewis acid sites for activating triple bonds. As a result, different
C≡C
sterically hindered alkyne substrates can be effectively activated
through π-interactions with these cationic silver centers. This
study not only provides a new protocol to assembly robust silver
cluster based 2D MOF materials but also paves a new way for
the rational design of high-efficiency silver-based heterogeneous
catalysts.
MOFs-based catalysts for CO2 fixation have attracted
significant attention due to their adjustable pore size,
surfaces and functionalized metal nodes.[6, 7] Compared with
extensively investigated 3D MOF-based catalyst, 2D MOFs
have been rarely employed in catalytic organic
transformations,[8] probably because of their chemical
instability during the catalytic processes. Indeed, to the best
of our knowledge, only two 2D MOFs for catalytic CO2
fixation conducted under high-pressure/temperature
conditions have been described thus far.[9] And there have
been no reports of any chemical fixation of CO2 enabled by
2D MOFs under ambient conditions so far. Therefore, design
of a robust 2D MOF catalyst with desirable distribution of
active sites throughout the framework and delicate tail of
porosity remains a big challenge at present.
Introduction
The continuously increasing levels of CO2 in the
atmosphere has caused serious greenhouse effect and
global warming. Hence, the development of advanced CO2
capture and transformation technologies is of utmost
importance and has attracted considerable attention.[1]
[*]
M. Zhao, S. Huang, Prof. Q. Fu, R. Guo, Prof. P. Cui, Prof. C.-H.
Tung, Prof. D. Sun
School of Chemistry and Chemical Engineering, Key Lab of Colloid
and Interface Chemistry of Ministry of Education, State Key
Laboratory of Crystal Materials,Shandong University, Jinan 250100,
P. R. China.
E-mail: cuiping@sdu.edu.cn, dsun@sdu.edu.cn.
Prof. W. Li
School of Physics, Shandong University, Jinan 250100, P. R. China
Prof. Q. Yao, Prof. D. Sun
School of Chemistry and Chemical Engineering, and Shandong
Provincial Key Laboratory of Chemical Energy Storage and Novel
Cell Technology, Liaocheng University, Liaocheng 252000, PR China
Prof. F. Wang
Recently, a series of Ag-based MOFs have been
assembled and used as heterogeneous catalysts (Table S1).
Silver(I) ion is famous as soft π-Lewis acid in organic
catalytic reactions[10] and its versatile coordination
geometries[11] provided better compatibility for a wide range
of substrates when forming silver−π intermediates[12]. We
envisaged that the incorporation of high-density silver
centers into 2D framework as π-Lewis acid catalytic sites
would not only enhance the stability of the 2D MOF, but also
facilitate the activation of organic alkyne substrates,
especially for these bulky ones, and thus significantly
enhances their catalytic performance. Recently, the silver
clusters have been extensively studied due to their intriguing
aesthetic structures, catalysis, and luminescence
properties.[13] In 2017, Zang group demonstrated that
connecting silver clusters with organic linkers to form
periodic frameworks would greatly improve the chemical
stability of extremely unstable silver clusters, and they
School of Materials Science and Engineering, Shandong University,
Jinan 250061, P. R. China
Prof. P. Cui
Key Laboratory of Advanced Energy Materials Chemistry (Ministry
of Education), Nankai University, Tianjin 300071, China
and
College of Chemistry, Chemical Engineering and Materials Science
Shandong Provincial Key Laboratory of Clean Production of Fine
Chemicals, Shandong Normal University, 250014, P.R. China.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/anie.201xxxxxx.
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10.1002/anie.202007122
Angewandte Chemie International Edition
RESEARCH ARTICLE
disclosed the first silver cluster-assembled 3D MOF.[14]
Subsequently, Duan et al. constructed two TNS-Ag8 and
TOS-Ag4 3D MOFs and demonstrated their catalytic
cycloaddition of terminal propargylamines with CO2.[15] Most
recently, Zang and co-workers revealed Ag12TPyP shows
excellent photocatalytic activity by the efficient and selective
oxidation of a mustard simulant.[16] Inspired by these prior
research and our background on silver cluster chemistry,[14-
fashion is observed (Ag–S: 2.460(3)-2.852(3) Å), which
templates the formation of the silver cluster skeleton.
Additionally, CF3COO− auxiliary ligands function as chelating
bidentate and bridging bidentate ligands. Note that there
exist argentophilic interactions to consolidate the saddle-
shaped cluster, because the observed Ag⋯Ag distances of
2.883(5)-3.3678(15) Å are shorter than the sum of the van
der Waals radii of silver (3.44 Å).[20] Due to the multitopic
configuration of the TPyP-H2 ligand, it is bonded to four
saddle-shaped clusters through four pyridyl groups, forming
a square node (Figure 1c). Ag27-MOF adopts the 4-
connected uninodal net with {44ꞏ62}-sql net topology if both
saddle-shaped cluster and TPyP-H2 ligand are treated as 4-
connectors (Figure 1d and 1e). The 2D layers are further
stacked together to an extended 3D supramolecular
18]
we devote ourselves to installing silver clusters into 2D
MOFs and investigating their catalytic performance utilizing
the unique bonding preference between Ag and alkynyl.
Herein, we report our comprehensive studies on an
unprecedented
silver
cluster-based
2D
MOF,
{[Ag27S2(tBuS)14(CF3COO)8(TPyP-H2)](CF3COO)}n
(Ag27-
MOF), which was obtained by slowly diffusing a reaction
mother solution of AgStBu and CF3COOAg into 5,10,15,20-
tetra(4-pyridyl)porphyrin (TPyP-H2) solution. The catalytic
study illustrated that Ag27-MOF readily catalyses the CO2-
cycloadditions of both terminal and internal N-
propargylamines effectively under ambient conditions,
representing the first example of metal cluster-based 2D
MOFs as heterogeneous catalysts for CO2 fixation.
framework by C–H⋯F and C–H⋯
π interactions[21] along the
b axis (Figure S1b). Of note, tBuS− protected silver clusters
have been widely studied (Table S3), such as
[Ag262S100(tBuS)62(dppb)6],
[Ag344S124(tBuS)96],
[Ag320(tBuS)60S130(dppp)12] and {[Ag62S13(tBuS)32](BF4)4}.[19,
22] However, such double S2– templated saddle-shaped Ag27
cluster has never been reported and the formation of it
should be mainly dictated by the TPyP-H2 ligand.
Results and Discussion
The structure of Ag27-MOF was determined by single-
crystal X-ray crystallography (Table S2). Crystallizing in the
orthorhombic space group Cmcm, Ag27-MOF presents a
cationic 2D coordination structure based on the saddle-
shaped 27-nuclear silver(I) clusters linked by the multitopic
porphyrin-based ligands (Figures 1 and S1). The overall
charge balance was completed by a lattice CF3COO− serving
as a counterion, which could not be crystallographically
located due to its highly disordered nature. The chemical
formula of Ag27-MOF was further confirmed by elemental
analysis and thermogravimetric analysis (TGA, Figure S2).
The saddle-shaped Ag27 cluster with a C2v group symmetry
is protected by fourteen tBuS− and eight CF3COO− ligands,
as well as templated by two enclosed S2− ions (Figure 1a).
Such saddle configuration leads to seven silver atoms to be
exposed (Figure 1b), which could serve as potential active
sites for organic substrates activation. In this structure, the
asymmetric unit contains only one-fourth of the saddle-
shaped cluster, showing eight types of silver atoms (Ag1-
Ag8). Among them, one Ag1, two Ag2, and four Ag5 atoms
are on the belly of the saddle, and the remaining twenty silver
atoms are classified into five groups and distribute on the
back of the saddle. Ag1 atom on the crystallographic 4-fold
axis shows a two-coordinated geometry completed by two
S2− ions with an angle of 173.26(16)° and a Ag–S bond length
of 2.460(3) Å. Due to the steric bulkiness of TPyP-H2 ligand,
only four Ag6 atoms at the corner are coordinated by the
pyridyl nitrogen atoms, which reduces the steric hindrance
and provides a beneficial feature to magnify effective
interaction regions between silver atoms and guest
molecules. All tBuS− ligands show two kinds of bonding
fashions (μ3 and μ4) to connect silver atoms with Ag–S bond
lengths in the range of 2.396(3)-2.732(3) Å. As for in situ
generated S2− ion from the tBuS− ligand,[19, 20] μ6 bonding
Figure 1. (a) Crystallographic structure of the 27-nuclear silver cluster,
giving the coordination modes between the silver cluster and the
ligand. Hydrogen atoms are omitted for clarity. (b) Space-filling and
ball-and-stick representations of silver cluster, showing saddle-shape.
(c) Coordination mode of the TPyP-H2 ligand, acting as a 4-connected
linker. (d) and (e) 2D structure along b and a directions, respectively,
and the sql topology for Ag27-MOF
. The polyhedron (purple)
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10.1002/anie.202007122
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RESEARCH ARTICLE
represents the saddle-shaped silver cluster unit, and square node
(sky-blue) represents the TPyP-H2 ligand.
silver nodes in the 2D framework, which would activate the
C≡C bonds through favorable Ag⋯
π interactions.[23]
Benzylprop-2-ynylamine 1a was first chosen as a model
compound and its reaction with CO2 was conducted under
atmospheric pressure of CO2 at room temperature with 1
mol% (per mole of Ag atoms) loading of Ag27-MOF and
1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU,10 mol%) as a
catalyst and a base, respectively. Various solvents were
firstly screened and it was found that the corresponding
product 5-methylene-3-(phenylmethyl)-2-oxazolidinone
2a was obtained with the best yield in 6 h when MeCN was
employed as a solvent, whereas the other solvents slowed
down the catalytic process remarkably and gave relatively
lower yields from 30% to 80% (Table 1, entries 1-6). In
addition, without any catalysts, no product was detected
(Table 1, entry 7). The nature of the catalytic reaction by
Ag27-MOF was then studied by catalyst filtration test
(Figure S5), which indicated that the cycloaddition
reaction completely stopped after removal of solid catalyst
Ag27-MOF at 3 h. In addition, ICP-MS analysis illustrated
a negligible amount of silver (1.2%) in the filtrate from the
reaction mixture, thus the leaching of Ag species can be
ruled out, confirming the genuine heterogeneous property
of catalyst Ag27-MOF. Moreover, the catalyst could be
recovered and reused for at least four reaction runs, with
the yield decreasing by only 7% at the fourth run (Figure
S6). PXRD, Raman, X-ray photoelectron spectroscopy
(XPS), and Fourier transform infrared (FTIR)
spectroscopic analyses of the recovered catalyst show
that the main skeleton is retained (Figures S7-S10).
Decreased diffraction intensities and broadened weaken
reflections were observed in the PXRD pattern, indicating
the small portion of long-range ordering crystalline state of
Ag27-MOF losses with the recycling. However, catalytic
sites in the micro-stucture remain unchanged and do not
fall out from the framework, thus concluding the post-
catalysis structure is stable. The similar PXRD change has
been already observed for many other recyclable MOF-
based catalysts.[15, 24] For comparison, the catalytic activity
of a serious of inorganic silver salts for this model reaction
has also been studied. Control experiments indicated that
the homogenous AgStBu and CF3COOAg provided low
yields under similar conditions (Table 1, entries 8 and 9).
Furthermore, the employment of CF3SO3Ag, AgNO3,
AgBF4 and AgOAc led to fair yields from 46% to 64%
(Table 1, entries 10-13). Moreover, using the mixed TPyP-
H2, AgStBu and CF3COOAg as the catalytic system only
afforded a 48% yield of 2a (Table 1, entry 14), illustrating
the crucial role of the 2D framework of Ag27-MOF in CO2
fixation. The most efficient heterogeneous catalysts for
cyclization of propargylamines with CO2 reported are
Powder X-ray diffraction (PXRD) analyses revealed that
both samples freshly synthesized and exposed under
ambient conditions (around four months) show good
phase purity and high stability, respectively (Figure S3).
Moreover, its chemical stability in common solvents was
also testified. After soaking of Ag27-MOF in different
solvents (such as acetonitrile, N,N-dimethylformamide,
ethanol, methanol, dimethyl sulfoxide, chloroform, 1,4-
dioxane, and H2O) for at least 36 h at room temperature,
their peak positions of PXRD correspond well with the
theoretical pattern, indicating the preservation of
crystalline structure. Consistently, its stability in the
representative H2O and CHCl3 has also been confirmed
by the mass balance and inductively coupled plasma-
mass spectrometry (ICP-MS) studies (Table S4). TGA
study of Ag27-MOF was carried out to assess the thermal
behavior, in which a sharp weight loss from 100 °C was
observed, indicating decomposition of the material. Ag27-
MOF was tested for CO2 adsorption experiments at 273
and 300 K (Figure S4). The amount of CO2 uptakes is 14.5
(0.65) and 10.3 (0.46) cm3 g–1 (mmol g–1) at 273 and 300
K, corresponding to about 3.8 and 2.7 CO2 molecules per
formula unit, respectively. The excellent chemical stability
and significant affinity toward CO2 prompted us to assess
the potential application in serving as a heterogeneous
catalyst for CO2 fixation.
Table 1. Cycloaddition of benzylprop-2-ynylamine with CO2 using
various solvents and catalysts.a
TNS-Ag8 and TOS-Ag4
,
which promote CO2
incorporation into benzylprop-2-ynylamine in high yields
but require a much longer reaction time (Table 1, entries
15, 16). Additionally, the turnover number (TON) study for
this model reaction catalysed by Ag27-MOF revealed the
value is up to 1333, which is even higher than that for
presently the most efficient heterogeneous catalyst TOS-
Ag4 (930).[15]
We examined the catalytic activity of Ag27-MOF in the
CO2 cycloaddition of propargylamines given the exposed
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10.1002/anie.202007122
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RESEARCH ARTICLE
3D MOFs of TNS-Ag8 and TOS-Ag4 are not able to
promote the cyclization between CO2 and internal
propargylamines,[15] which is ascribed to their 3D
frameworks restricting the effective interactions between
bulky substrates and catalytic sites in the pores, thus
confirming the special advantage of accessible catalytic
platform of 2D MOF for broader substrate scope. Thus, it
is safe to conclude that the 2D MOF Ag27-MOF is among
the best catalysts for carboxylative cyclization of
propargylamines with CO2 compared with the known
homogenous and heterogeneous catalytic systems (Table
S5).
Table 2. Cycloaddition reactions of propargylamines with CO2.a
Figure 2. Density functional theory based calculations: the isosurface
plot of the differential charge density of 1c-Ag27-MOF (a) and 1i-
Ag27-MOF (b), density of state curves of Ag27-MOF and terminal
propargylamine 1c (c) and internal propargylamine 1i (d). The Ag, S,
C, N, H, and F atoms are shown in purple, yellow, gray, dark green,
light pink, and blue, respectively in (a) and (b). The electron
accumulation region is azure and value is at ±0.0007 Å-3.
To gain a deep understanding of how Ag27-MOF activates
propargylamines, the theoretical study was conducted using
the density function theory (DFT) calculations implemented
in the Vienna ab initio simulation package (VASP) (details of
the calculations can be found in the SI). The Ag2 atom was
chosen as a model catalytic site considering its spatial
convenience (Figure S12). Our results clearly indicate that,
benefitted from the opened-surface of the 2D structure, the
terminal 1c and internal 1i substrates can both easily bind to
Next, we turned our attention to examine the scope of
the catalytic reaction with various propargylamine
substrates (Table 2). Under the optimized reaction
conditions, terminal propargylamines bearing N-alkyl
substitutes afforded corresponding cyclized products in
high yields (2b 99%, 2c 97%, 2d 99%). In addition,
the exposed Ag atoms through π-interactions of C≡C bonds.
oxazolidinones 2e-2g could also be smoothly generated
Interestingly, Ag27-MOF can form more intimate binding with
internal 1i with an Ead value of −0.88 eV. Comparatively, for
1c adduct with Ag27-MOF, the corresponding Ead value of
−0.51 eV is relatively smaller. Consistently, the respective
in 6-8 hours from the propargylamine substrates with
substituted benzyl groups (2e 94%, 2f 96%, 2g 94%).
Gratifyingly, the internal propargylamine substrates
examined in this study also reacted well with CO2 and
C≡C bond distances increase by 0.0045 and 0.0051 Å for 1c
were readily transformed to products 2h
quantitatively (Table 2, entries 8-12). The geometry of exo
olefin of 2h 2l was confirmed to be the Z isomer by NOE
experiments, which indicates the activation of C C bonds
-2l almost
and 1i after adsorption, indicating no significant differences
of the activation for terminal and internal propargylamines by
Ag27-MOF. We further access the electron redistributions
caused by the Ag27-MOF binding (Figure 2a and 2b). For
both terminal 1c and internal 1i substrates, the electron
-
≡
of substrates by the catalytic silver sites is at the anti-
position with respect of the carbonate nucleophile in the
ring-closing step (Figure S11). It is salient to mention that
accumulation regions spread on C atoms of the C
≡C bonds
along the Ag−C directions with π* symmetry, whereas the
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10.1002/anie.202007122
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RESEARCH ARTICLE
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electron depletion regions feature σ symmetry and spread
along the C−C directions and also on Ag atoms. The
interactions and electron transfer between Ag27-MOF and
propargylamines can be also demonstrated by the density of
state (DOS) analysis (Figure 2c and 2d). The Ag−C bonds
are formed by the Ag-d orbital in complexation with the s and
p orbitals of 1c and 1i, resulting in the downshifted of
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active metal sites in the 2D framework enable it to exhibit an
excellent catalytic performance in catalytic transformation of
propargylamines through effective substrate-catalytic site π-
interactions.
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Conclusion
In summary, a 2D MOF Ag27-MOF has been constructed
by connecting the novel double S2− templated Ag27 clusters
with porphyrin ligands. The saddle-shaped cluster node and
existence of exposed catalytic sites facilitate Ag27-MOF to
be the most effective heterogeneous catalyst for
carboxylative cyclization of propargylamines with CO2. More
importantly, both terminal and internal propargylamines with
different sterically hindered substituents readily cyclize with
CO2 under ambient conditions. These preliminary results
indicate that the underestimated cluster-based 2D MOFs
would be attractive candidates as heterogeneous catalysts.
Development of more novel cluster-based 2D MOFs and
investigation of their catalytic performance in CO2 fixation are
currently in progress in our laboratory.
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Acknowledgements
We are grateful to the Natural Science Foundation of China
(21701188, 91961105, 21822107), the Natural Science
Foundation
of
Shandong
Province
(ZR2017MB005
ZR2019ZD45, and JQ201803), the Key R&D Program of
Shandong Province (2019GSF108158), Shandong Provincial Key
Laboratory of Clean Production of Fine Chemicals
(2019FCCEKL06) and the Fundamental Research Funds of
Shandong University (2018JC046), Project for Scientific
Research Innovation Team of Young Scholar in Colleges and
Universities of Shandong Province (2019KJC028).
Keywords: 2D metal-organic frameworks • heterogeneous
catalysis • silver cluster-assembled materials • CO2 fixation
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Entry for the Table of Contents
RESEARCH ARTICLE
M. Zhao, S. Huang, Q. Fu, W. Li, R.
Guo, Q. Yao, F. Wang, P. Cui,* Chen-
Ho Tung, D. Sun*
Page No. – Page No.
Ambient Chemical Fixation of CO2
Using a Robust Ag27 Cluster-Based
Two-Dimensional Metal-Organic
Framework
An Ag27 cluster-assembled 2D MOF has been constructed and employed as the
catalyst for carboxylative cyclization of propargylamines with CO2. Both the high TON
number and broad substrate scope confirm Ag27-MOF to be an excellent
heterogeneous catalyst, making a breakthrough in the use of 2D MOFs as high-
performance catalysts.
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