Copper(i) iodide cluster-based lanthanide organic frameworks: Synthesis and application as efficient catalysts for carboxylative cyclization of propargyl alcohols with CO2 under mild conditions

Article abstract of DOI:10.1039/c9dt01859j

Two metal-organic frameworks (MOFs), namely, [Dy₂Cu₄I₄(NA)₆(DMF)₂]_n (1) and [Gd₂Cu₂I₂(IN)₆(DMF)₄]·5DMF (2) (HNA = nicotinic acid, HIN = isonicotinic acid), constructed based on lanthanide ions and copper iodide clusters ([Cu₄I₄] and [Cu₂I₂]) were successfully synthesized and characterized. Compound 1 has a three-dimensional framework and compound 2 displays a two-dimensional plane with sql topology, respectively. Both of them exhibit high thermostability and solvent stabilities. Additionally, catalytic explorations reveal that 1 displays higher catalytic activity than 2 for the carboxylic cyclization of propargyl alcohols. More importantly, 1 also exhibits excellent catalytic performance in the carboxylation reactions of CO₂ and terminal propargylic alcohols with various substituents. To the best of our knowledge, this is the first example of non-noble metal based MOF catalysts for the carboxylative cyclization of propargyl alcohols with CO₂ under atmospheric pressure and at room temperature, which provides a highly promising approach for MOFs in the catalytic conversion of CO₂ to valuable chemicals.

Full text of DOI:10.1039/c9dt01859j

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Copper(I) iodide cluster-based lanthanide organic
frameworks: synthesis and application as efficient catalysts
for carboxylative cyclization of propargyl alcohols with CO2
under mild conditions
Zhilei Wu,ab Xingwang Lan, ab* Yaxin Zhang,a Meng Li,a Guoyi Baia
a
Key Laboratory of Chemical Biology of Hebei Province, Key Laboratory of Analytical Science
and Technology of Hebei Province, College of Chemistry and Environmental Science, Hebei
University, Baoding, 071002, P. R. China.
b
Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical
Engineering and Technology Collaborative Innovation Center of Chemical Science and
Engineering, Tianjin University, Tianjin, 300072, P. R. China.
Abstract: Two metal-organic frameworks (MOFs), namely, [Dy2Cu4I4(NA)6(DMF)2]n
(1) and [Gd2Cu2I2(IN)6(DMF)4]·5DMF (2) (HNA = nicotinic acid, HIN = isonicotinic
acid), constructed based on lanthanide ions and copper iodide clusters ([Cu4I4] and
[Cu2I2]) were successfully synthesized and characterized. Compound 1 has a three-
dimensional framework and compound 2 displays two-dimensional plane with sql
topology, respectively. Both of them exhibit high thermostability and solvent stabilities.
Additionally, catalytic explorations reveal that 1 displays higher catalytic activity than
2 for the carboxylic cyclization of propargyl alcohols. More importantly, 1 also exhibits
excellent catalytic performance in the carboxylation reactions of CO2 and terminal
propargylic alcohols with various substituents. To the best of our knowledge, this is the
first example of non-noble metal based MOFs catalysts for the carboxylative
cyclization of propargyl alcohols with CO2 under atmospheric pressure and room
temperature, which provides a highly promising approach for MOFs in the catalytic
conversion of CO2 to valuable chemicals.
Introduction
Increasing greenhouse effect associated with carbon dioxide (CO2) emission has
become an global issue.1-4 Capturing CO2 and converting it into high valuable
*Corresponding author. E-mail: hxlxw@sina.cn.
*Corresponding author. E-mail: baiguoyi@hotmail.com.

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chemicals is considered as a green strategy to cope with this situation. CO2 can serve
as low-cost, non-toxic, renewable, and inexhaustible C1 building block.5-7 To date,
various valuable chemicals derived from chemical fixation of CO2 have been obtained,
many of which are important products in chemical industry, such as carboxylic acids,8,9
organic carbonates,10 formic acid,11 and alcohols.12 The cyclization of propargylic
alcohols with CO2 for synthesizing asymmetrical organic carbonates attracted much
attention because the resultant products possess potential commercial value in the field
of biomedical industry and pharmaceutical chemistry.13-17 However, due to the high
thermodynamic stability and kinetic inertness of CO2, this catalytic process always
required very costly and harsh conditions such as: 1) highly active catalysts based on
noble metals (e.g., Ag, Pd, Ru), 2) large CO2 pressure and high temperature, 3) long
reaction time. It is thus highly desired to develop the mild catalytic system to furnish
the carboxylative cyclization of propargyl alcohol with CO2.
Metal–organic frameworks (MOFs) as porous crystalline materials are promising
candidates for the conversion of CO2,18 because they have large surface area, well-
defined pores, tunable and tailorable structure, abundant and small catalytic sites.19,20
More recently, many MOFs-based catalysts are employed for CO2 conversion,
including the cycloaddition of epoxides21 or aziridines,22 carboxylation of terminal
alkynes.23 Yet, there are very limited reports on MOFs-related catalysts for the CO2
conversion with propargylic alcohols. Duan’s group24 reported a porous Ag(I)-MOF
catalyzing this reaction under 5 bar CO2, 50 oC. By using MOF as host to encapsulate
catalytic species Ag(I), Fei and coworkers25 applied Ag(I)@MOF in this catalytic
systems and achieved satisfactory result at atmospheric pressure of CO2 and under room
temperature. Zhao et al.26 employed a noble-metal free heterometal–organic framework
based on In ions and [Cu4I4] clusters, which displayed high activity for this reaction (5
o
bar CO2, 50 C). These excellent works have demonstrated the incomparable
advantages of MOFs, but they are all associated with either noble metals or harsh
operation condition. Hence, exploring an inexpensive catalyst enabling the cyclization
of propargylic alcohols with CO2 at atmospheric pressure and room temperature is
highly desirable and challenging for the conversion of CO2. To the best of our

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knowledge, this kind of noble metal-free MOFs catalysts has not been reported at
present.
In this contribution, two compounds [Dy2Cu4I4(NA)6(DMF)2]n (1) and
[Gd2Cu2I2(IN)6(DMF)4]·5DMF (2) (HNA = nicotinic acid, HIN = isonicotinic acid)
were successfully synthesized and structurally characterized. They were both composed
of multinuclear lanthanide ions and copper iodide clusters ([Cu4I4] for 1 and [Cu2I2] for
2) and display excellent solvent stability. Interestingly, both of them exhibit highly
effective catalytic activity in the cycloaddition reaction of propargylic alcohols at room
temperature with atmospheric pressure of CO2.
Experimental Section
Materials and Methods
All the chemicals and solvents used for the synthesis were purchased commercially
without further purification. N2 sorption isotherms measurement was performed on a
Quantachrome IQ2 automatic volumetric instrument and the purity of N2 was 99.999%.
The elemental analysis data of C, H and N were collected on a Perkin-Elmer elemental
analyzer. Thermogravimetric analyses (TGA) experiments were implemented on a
NETZSCH TG 209 instrument under nitrogen atmosphere with a heating rate of
10°C/min. Powder X-ray diffraction data were collected on an Ultima IV X-ray
diffractometer using Cu-Kα radiation. The X-ray photoelectron spectroscopy (XPS)
were measured on Kratos Axis Ultra DLD multi-technique. Inductively coupled plasma
1
13
(ICP) tests were measured on ICP-9000(N+M). H NMR and C NMR spectra were
recorded at 600 MHz and 150 MHz on a Bruker spectrometer in CDCl3 using TMS as
the internal standard at room temperature. GC-MS spectra were performed on a Thermo
Fisher Trace 1300 +ISQ LT instrument.
Preparation of 1 and 2
Compound 1: A mixture of HNA (2 mmol), Dy(NO3)3·6H2O (0.5 mmol) and CuI
(0.5 mmol) were dissolved in 10 mL DMF. The obtained mixture was transferred into
a Teflon lined stainless steel vessel and heated at 120 oC for 24 h, and then cooled down
o
to room temperature at a rate of 5 C/h. Colorless cubic crystals were collected by
filtration and the yield of 1 was 46% (based on Dy(NO3)3·6H2O). Elemental analysis

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calcd (%) for C21H19Cu2DyI2N4O7 (1): C, 25.66; H, 1.95; N, 5.70; Found: C, 25.84; H,
2.07; N, 5.80.
Compound 2: HIN (2 mmol), Gd(NO3)3·6H2O (0.5 mmol), CuI (0.5 mmol) and 10
mL DMF were sealed in a Teflon lined stainless steel vessel and heated at 125 oC for
o
24 h, and then cooled down to room temperature at a rate of 5 C/h. Yellow needle-
shaped crystals were collected by filtration and the yield of 2 was 42% (based on
Gd(NO3)3·6H2O). Elemental analysis calcd (%) for C63H87Cu2Gd2I2N15O21 (2): C,
36.28; H, 4.20; N, 10. 07; Found: C, 36.45; H, 4.07; N, 10.13.
Catalytic reactions
The typical procedure following that: To a solution of propargylic alcohol (0.6 mmol)
in CH3CN (3 mL) was successively added catalyst (without catalyst for entry 1, 54 mg
of 1 for entry 2, 30 mg of 1 for entry 3, 12 mg of 1 for entry 4, 62 mg of 2 for entry 5,
30 mg of MIL-101 for entry 8, 30 mg of UiO-66 for entry 9, 30 mg of Gd-MOF for
entry 10, 30 mg of Dy-MOF for entry 11, Table 1) and DBU (1.0 equiv.) in the reaction
flask (10 mL) equipped with a CO2 (99.99%) balloon, and then it was vacuumed and
purged with CO2 three times to exclude air in the reaction system. Afterwards, the
o
reaction mixture was stirred at 25 C for 5 h. Upon completion, the yield was
determined by GC analysis. The crude products were purified by column
chromatography on silica gel (petroleum ether/ethyl acetate as an eluent) to obtain the
pure products. Following the same procedure, the reaction was conducted over 30 mg
of 1 without DBU in entry 6; In entry 7 the reaction was conducted over 30 mg of 1 in
air without CO2 balloon.
Crystallographic studies
Single-crystal X-ray diffraction data of compounds 1 and 2 were measured on a
Rigaku-RAPID diffractometer with graphitemonochromatic Mo Kα (λ = 0.71073 Å)
radiation using the ω–φ scan technique. The structures were solved by direct methods
and refined by full-matrix least squares on F2 with SHELXS-97 and SHELXL-97
programs. Anisotropic refinement was used for all non-hydrogen atoms, and all the
hydrogen atoms were placed geometrically and refined using a riding model. Because
the NA- anions and coordinated DMF molecules in 1 are disordered, their hydrogen

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atoms cannot be generated and included in the molecular formula directly. The structure
refinement data parameters of 1 and 2 are summarized in Table S1. The CCDC numbers
for compounds 1 and 2 are 1883655 and 1883656, respectively.
Crystal Structure of 1 and 2.
Crystallographic analyses reveal that 1 crystallizes in the tetragonal system with a I-
42d space group. In 1, four Cu(I) ions are connected together by four µ3-I- bridges to
form a tetra-nuclear Cu-cluster [Cu4I4] (Fig. 1b), in which one crystallographically
independent Cu(I) ion is located in a four coordinated environment with [NI3] set,
exhibiting a tetrahedron coordination geometry (Fig. 3). The Dy(III) ion is seven-
coordinated with a mono-capped octahedron geometry (Fig. 3), consisting of six
carboxylic oxygen atoms of different NA- and one O atom from unordered DMF
molecule, respectively. Adjacent Dy(III) ions are bridged by three η1:η1:μ2 carboxylates
to generate a 1D chain with neighboring Dy···Dy distance 4.783 Å (Fig. 1c). All the
Dy-O lengths are in the range of 2.277(7)-2.400(2) Å. The 1D chains are further
connected with [Cu4I4] clusters by NA- anions to give a 3D framework (Fig. 1a). The
total potential pore volume is about 21.9% calculated by the PLATON program.
However, the N2 absorption capacity measured at 77 K is very low (Fig. S1), so the
surface area of 1 are not experimentally determined.
Fig. 1 (a) Perspective view of the three dimensional structure of 1. (b) The structure of the cluster
[Cu4I4]. (c) The Dy(III)-based one dimensional chain bridged by carboxylates.
For compound 2, it crystallizes in the P-1 space group of triclinic system, with one
crystallographically independent Gd(III), Cu(I), three IN-, two I- and two coordinated
DMF molecules in the asymmetric unit. As showed in Figure 2, two different clusters

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[Gd2] and [Cu2I2] are contained in 2. In the [Gd2] cluster, two eight-coordinated Gd(III)
ions are centrosymmetric and bridged by carboxylates of four different IN- in the
η1:η1:μ2 mode with Gd···Gd distance of 2.377 Å (Fig. 2a). The coordination spheres of
Gd1 and Gd1a are both completed by eight oxygen atoms: six carboxylic oxygen atoms
from five IN- anions and two oxygen atoms from two coordinated DMF molecules,
displaying distorted bicapped trigonal prism coordination geometry (Fig. 4). The Gd-O
distances are in the range of 2.304(3)-2.475(3) Å, which are similar to the reported Gd-
O bond lengths.27 Two different coordination modes of ligands IN- anions can be
observed in 2 (Scheme. 1). For the [Cu2I2] cluster, two I- anion as μ2-bridges bind two
Cu(I) atoms to form a [Cu2I2] unit, and two Cu(I) atoms are further linked by four IN-
through Cu-N bond (Fig. 2b). Both of two Cu(I) ions are four coordinated in a
tetrahedron geometry with the Cu···Cu distance of 2.594 Å (Fig. 4). Each [Gd2]/[Cu2I2]
cluster is attached to adjacent four [Cu2I2]/[Gd2] clusters through four IN- (Fig. 2c), thus
each cluster can be regarded as a 4-connected node and the whole structure can be
simplified as a 2D plane with sql topology (Fig. 2d).
Fig. 2 (a) The structure of the cluster [Gd2]. (b) The structure of the cluster [Cu2I2]. (c) The two
dimensional structure of 2. (d) The sql topological net of 2.
Fig. 3 The coordinated environments and coordination polyhedra for Cu(I) and Dy(III) in 1.

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Fig. 4 The coordinated environments and coordination polyhedra for Cu(I) and Gd(III) in 2.
O
O
O
O
Ln
Ln
Ln
Scheme 1 Coordination modes of the ligands observed in compounds 1 and 2.
Comparing 1 and 2, both of them contain [CunIn] clusters and lanthanide ions.
However, the lanthanide ion Dy(III) in 1 is seven-coordinated, distinctly different from
the eight-coordinated Gd(III) in 2, which is rarely seen in other lanthanide-based
compounds and may provide Lewis-acidity for catalysis. The oxidation states of Cu(I)
ions in them are further confirmed by X-ray photoelectron spectroscopy (XPS), the
typical peaks of 2p3/2 and 2p1/2 for Cu(I) ion can be clearly observed in the XPS
spectra of compounds 1 (932.08 and 951.85 eV) and 2 (931.92 and 951.91 eV),
respectively (Fig. 5).
Fig. 5 The XPS spectra of Cu(Ι) in compound 1(a) and 2(b).
Thermo-gravimetric analyses
In order to investigate the thermal stability of compounds 1 and 2, thermal
gravimetric analyses (TGA) of them were recorded under a N2 atmosphere in the

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o
temperature range of 50-800 C. For 1, almost no obvious weight loss was observed
o
until 255 C, suggesting that there were no guest molecules in the lattice of 1, as
confirmed by the single crystal X-ray diffraction. Subsequently, a weight loss of 7.14 %
between 255-400 oC can be seen, corresponding to the coordinated DMF molecules
(calcd: 7.43%). Due to the strong hydrogen-bonding interaction and the steric hindrance,
the coordinated DMF molecules are difficult to lose in the low temperature range. Then
the whole framework started to decompose above 400 oC (Fig. S2). For 2, from 50 oC
to 405 oC, about 17.27 % weight loss can be observed, which corresponds to the loss of
free DMF molecules (calcd: 17.51%). Then the framework begin to decompose sharply
o
above 405 C (Fig. S3). In general, the two compounds 1 and 2 have high thermal
stability.
Chemical stability
Fig. 6 The simulated and experimental PXRD patterns of compound 1 (a) and 2 (b) after being
immersed in different solvents for 12 hours.
Structural resistance to solvents is important for the application of MOFs, then the
powder X-ray diffraction (PXRD) patterns of 1 and 2 immersed in common solvents,
such as H2O, methanol, ethanol, acetonitrile, DMF, DMA, DMSO and 1,4-dioxane for
12 h are investigated. The simulated PXRD of 1 and 2 obtained from the crystal data
are well consistent with the experimental ones, suggesting that these treatments have
no effect on the structures of 1 and 2, both of them have excellent and extensive solvent
stability (Fig. 6).
Cyclization of propargyl alcohols with CO2
Considering that their unique structures, [CunIn] clusters are located at the center of

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frameworks, we anticipate that the MOFs could deliver good catalytic ability, the
carboxylative cyclization of propargyl alcohol with CO2 was thus investigated over
compound 1 under mild conditions. The cyclization of 2-methyl-3-butyn-2-ol and CO2
was first chosen as model reaction to determine the optimal conditions, as illustrated in
Table 1. The control experiments suggested that no α-alkylidene cyclic carbonate
product was generated without catalyst (entry 1), and the Dy2O3 and Gd2O3, as the
building components of 1 and 2, could hardly drive the transformation of this reaction
(Table S2). Interestingly, the product could be obtained in a 95% yield with suitable
amount of 1 at room temperature and atmospheric pressure (entry 3). Under the same
reaction condition, the compound 2 containing [Cu2I2] cluster was also examined and
gave a yield as high as 84% (entry 5). Further control experiments indicated that no or
trace product was detected in the absence of the CO2 (entry 6) and 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU) (entry 7) or using other bases (Et3N, K2CO3,
and Cs2CO3) (Table S2). Other typical MOFs such as MIL-101 and UiO-66 showed no
catalytic activity under the same conditions. In addition, the other two isostructural
frameworks based on Gd(III) and Dy(III) (named Gd-MOF and Dy-MOF) were
synthesized according to the reported literature (confirmed by PXRD patterns, Fig. S4)
and applied in this reaction,28 they also have no catalytic activity. All these results
demonstrated that the copper iodide clusters in the structure act as the crucial catalytic
sites for this transformation. Compared with the reported the catalyst DMAM-PS-CuI,13
compounds 1 and 2 in this catalytic system display extremely prominent advantages
(room temperature, atmospheric CO2 pressure and short reaction time 5 h). Interestingly,
to verify the catalytic efficiency of 1 and 2, the pseudo-first-order reaction kinetics is
assumed to fit the kinetic traces. It was found that the reaction rate constant k1 (3.12
min-1) for compound 1 is higher than k2 (2.61 min-1) for compound 2 (Fig. S5),
suggesting that 1 features higher catalytic activity than that of 2 in this reaction system,
which can be attributed to the following aspects: 1) The 3D porous framework of 1
(Fig. 1a) could provide higher accessibility of metal sites than the 2D plane 2 with sql
topology; 2) The uncommon seven-coordinated Dy(III) with Lewis-acidity in 3D
framework may exert a synergistic catalytic effect. By contrast of the window size of 1

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and the substrate size (Fig. S6 and S7), it is likely that the catalytic reaction occurs
inside the MOF porosity. Moreover, recyclable performance is important for
heterogeneous catalyst. Hence the recyclable performance of 1 was also discussed, 1
can be separated from the reaction mixture by centrifugation. Inductively coupled
plasma (ICP) analysis of the reaction mixture filtrate reveals that no significant metal
ions leaching were observed. However, in the recycling process, decline of the catalytic
activity for 1 was observed: the yield of product decreases from 95% (1st run) to 68%
(4 th run). The mechanism was further investigated. The oxidation states of Cu ions in
the reused 1 are measured by X-ray photoelectron spectroscopy (XPS). As shown in
Fig. S8, the XPS spectra shows the characteristic peaks of 2p3/2 and 2p1/2 at 932.4 and
952.1 eV, suggesting the presence of Cu(I) in the reused sample, and the remaining
peaks at 940.1 and 961.8 eV indicate that Cu(II) ions also exist. These results suggest
that the Cu(I) ions in 1 as catalytic active site were partially oxidized to Cu(II) in the
reaction process, which results in the decease of catalytic activity.
Table 1. Carboxylative cyclization of propargyl alcohol with CO2.a
O
catalyst, DBU
O
CO2
+
O
HO
CH3CN, rt.
(balloon)
Entry
Catalyst
mol% of Cu
Yieldb(%)
none
0
1
2
Compound 1
Compound 1
Compound 1
Compound 2
Compound 1
Compound 1
MIL-101
18
10
4
96
95
80
81
˂1
˂1
˂1
2
3
4
5
10
10
10
6c
7d
8
9
UiO-66
10
11
Gd-MOF
3
Dy-MOF
2
aReaction conditions: 2-methyl-3-butyn-2-ol (0.6 mmol), catalyst (30 mg of entry 8-11), DBU (1.0
equiv.), CH3CN (3 mL), CO2 (balloon), 5 h, room temperature. b Determined by GC. c Without DBU.
d In air without CO2 balloon.

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The excellent catalytic activity of 1 in the cyclization of 2-methyl-3-butyn-2-ol with
CO2 encouraged us to further explore its generality. Based on the optimized reaction
conditions, several typical terminal propargylic alcohols with alkyl substituents were
examined. The yield determined by GC and isolated yield of the target products are
shown in Table 2. It should be noted that all the substrates could afford the
corresponding α-alkylidene cyclic carbonates smoothly with up to 98% yield and >99%
selectivity under mild conditions. All these results suggested that the 1 is an efficient
catalyst for the carboxylative cyclization of propargyl alcohol with CO2 at room
temperature and atmospheric pressure, which opens a new pathway to the CO2 chemical
conversion.
Table 2. Substrate scope for the cyclization of propargyl alcohols with CO2 over the
compound 1.a
O
OH
R1
R2
1
Compound (30 mg)
O
O
+
CO2
DBU, CH3CN, r.t., 5 h
R2
R1
2a-g
(balloon)
1a-g
O
O
O
O
O
O
O
O
O
O
O
O
a
b
a
b
a
b
a
b
2d
96% , 92%
2b
2a
98% , 95%
95% , 92%
2c
94% , 89%
O
O
O
O
O
O
O
O
O
a
b
b
a
b
2g
98% , 91%
2e
98%, 95%
2f
97% , 94%
aReaction conditions: propargylic alcohols (0.6 mmol), compound 1 (30 mg), DBU (1.0 equiv.),
CH3CN (3 mL), CO2 (balloon), 5 h, room temperature. Yield determined by GC. bIsolated yield.
According to these experimental results and previous reports,13,26,29,30 a plausible
mechanism for the carboxylation cyclization of propargyl alcohols with CO2 was
proposed, as illustrated in Fig. 3. Firstly, the cluster [Cu4I4] could coordinate with the
terminal alkynyl, which would react with the amphoteric salt generated from DBU and

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28
CO2 with the formation of the carbonate intermediate A. Afterwards, the carbonate
intermediate A underwent the intramolecular ring-closing step through O-nucleophilic
group attacking the alkynyl with the assistance of the cluster [Cu4I4] to form the
intermediate B. The corresponding cyclic carbonate products were finally generated
with the release of the cluster [Cu4I4].
Fig. 7. A plausible mechanism for the carboxylation cyclization of propargyl alcohols with CO2.
Conclusion
In summary, two metal-organic frameworks 1 and 2 were successfully fabricated
based on multinuclear lanthanide ions and copper iodide clusters ([Cu4I4] for 1 and
[Cu2I2] for 2). They both display excellent thermostability and solvent stabilities. The
[CunIn] clusters in the frameworks 1 and 2 acted not only as the building blocks but
active catalytic sites. The well-positioned [CunIn] clusters and unique structural
characteristics of the frameworks, especially the compound 1, featured highly catalytic
activity for the carboxylation reactions of CO2 and terminal propargylic alcohols with
various substituents at room temperature and atmospheric pressure. More importantly,
this work further indicate a highly promising approach with MOFs for the catalytic
conversion of CO2 to valuable chemicals.
Conflicts of interest
There are no conflicts to declare.

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Acknowledgements
The authors thank Professor G. Ma for his kind help. This work was supported by
the National Natural Science Foundation of China (21701039 and 21676068), Hebei
Natural Science Foundation (B2017201055, B2018201118), Hebei Higher Colleges
Science and Technology Research Program (QN2018049), and the 63rd China
Postdoctoral Science Foundation (2018M631745).
Notes and references
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546-550; (b) J. Wei, Q. Ge, R. Yao, Z. Wen, C. Fang, L. Guo, H. Xu, J. Sun, Nat.
Commun., 2017, 8, 15174-15181.
2 (a) C. A. Trickett, A. Helal, B. A. Al-Maythalony, Z. H. Yamani, K. E. Cordova and
O. M. Yaghi, Nat. Rev. Mater. 2017, 2, 17045; (b) P. Gao, S. Li, X. Bu, S. Dang, Z.
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