Synthesis Cu(I)–CN-based MOF with in-situ generated cyanogroup by cleavage of acetonitrile: Highly efficient for catalytic cyclization of propargylic alcohols with CO2

Article abstract of DOI:10.1016/j.mcat.2020.111190

Developing a highly effective process for synthesis a cyano-bridged compound to avoid toxic organic or inorganic cyanides is very significant method for alleviating cyanides pollution. Here, a CN-based MOF catalyst (Cu(I)–CN–BPY) was synthesized by using copper ions coupled with Na₄W₁₀O₃₂ in CH₃CN under solvothermal conditions. The cyano-groups are generated in situ from the cleavage of C(sp³)–C(sp) in CH₃CN. Because Cu(I) sites have ability to activate π-activate internal alkynes of carbon–carbon triple bonds for carboxylic cyclization reactions, which was applied in the cyclization of propargylic alcohols with CO₂ and exhibited high efficiency with >95 % yields. For seeking out the active sites of MOF structure in carboxylic cyclization, we also synthesized two MOFs of Cu(I)–Cl–BPY and Cu(I)–I–BPY, and investigated for this reaction.

Full text of DOI:10.1016/j.mcat.2020.111190

Molecular Catalysis 496 (2020) 111190
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Synthesis Cu(I)–CN-based MOF with in-situ generated cyanogroup by
cleavage of acetonitrile: Highly efficient for catalytic cyclization of
propargylic alcohols with CO2
Zhuolin Shi, Jiachen Jiao, Qiuxia Han*, Yang Xiao, Laikuan Huang, Mingxue Li
Henan Key Laboratory of Polyoxometalate Chemistry, Institute of Molecular and Crystal Engineering, School of Chemistry and Chemical Engineering, Henan University,
Kaifeng, Henan, 475004, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Developing a highly effective process for synthesis a cyano-bridged compound to avoid toxic organic or inorganic
cyanides is very significant method for alleviating cyanides pollution. Here, a CN-based MOF catalyst (Cu(I)–
CN–BPY) was synthesized by using copper ions coupled with Na4W10O32 in CH3CN under solvothermal condi-
tions. The cyano-groups are generated in situ from the cleavage of C(sp3)–C(sp) in CH3CN. Because Cu(I) sites
Metal–organic framework
C–C cleavage
Carboxylic cyclization
Heterogenous catalysis
have ability to activate π-activate internal alkynes of carbon–carbon triple bonds for carboxylic cyclization re-
actions, which was applied in the cyclization of propargylic alcohols with CO2 and exhibited high efficiency with
>95 % yields. For seeking out the active sites of MOF structure in carboxylic cyclization, we also synthesized two
MOFs of Cu(I)–Cl–BPY and Cu(I)–I–BPY, and investigated for this reaction.
1. Introduction
stopped and many reports confirmed that copper ions can be applied to
cleave C–CN bonds under hydrothermal reactions [23–27].
Cyanides are widely applied in common life such as metal cleaning,
electroplating, precious metal extraction, biological probes and phar-
maceuticals synthesis [1]. Among all kinds of cyanides, the
cyano-bridged coordination polymers have attracted great interest from
chemists and governments due to their potential properties in molecule
magnetism, luminescence, heterogeneous catalysis and so on [2–4]. In
the conventional approaches for preparation cyano-bridged coordina-
tion polymers, inorganic cyanides and HCN have generally been used as
the nitrile sources, however, which are high toxicity and threatens
humans and the environment [5–14]. Compared with HCN and metal
cyanides, it would be a safer, greener and more economical strategy by
direct utilizing acetonitrile as a cyanogroup source through in situ
formed cyano-bridged bonds to generate cyano-based metal–organic
frameworks (MOFs) [15–18]. However, acetonitrile serving as a cyanide
source is quite a challenge. Thermodynamically, the C–CN bond cleav-
age of acetonitrile needs a strong bond dissociation energy of 133 kcal
molꢀ 1 [19]. Generally, a number of metals have been mediated for
splitting the C–CN bond of acetonitrile, occur with the assist of d10 metal
complexes with a Pt, Zn or Ag centrals [20–22]. In spite of such diffi-
culties, copper as one of the most inexpensive metals comparing above
metals, the exploration of copper promoted C–CN cleavage was never
For minimizing the emissions of carbon dioxide as a renewable C1
resource, considerable approaches have been proposed to transform it to
value chemicals [28–31[32]]. Among these strategies to fix and catalysis
for CO2, the cycloaddition with CO2 into cyclic carbonates demonstrates
the advantages of high selectivity and low cost and has been widely
applied [33–36]. Recently, reports have shown that transition
metal-based or ionic liquids (ILs) systems, such as some Ag@ILs or
Cu@ILs have high activity for promoting the resultant
clic carbonates in carboxylic cyclization reaction [37–39]. In particular,
Cu(I) sites have ability to activate -activate internal alkynes of car-
α-alkylidene cy-
π
bon–carbon triple bonds for carboxylic cyclization reactions [40,41].
Most of the catalysts containment copper ions are homogeneous cata-
lysts that are tough to recycle, the like copper salts or complexes catalyst
systems. However, the leaching of active species in heterogeneous
catalysis may be reduced catalytic efficiency with copper complexes.
Therefore, to improve catalytic activity and reduce the cost, the explo-
rations on efficient catalysts of heterogeneous catalysis without noble
metals for this conversion are necessary and still quite a challenge.
Among various heterogeneous porous materials, MOFs are appealing
catalysts for heterogeneous catalysis because of their distinctive spe-
cialty with periodic, open structures and controllable pores, modifiable
* Corresponding author.
E-mail address: qiuxia_han@163.com (Q. Han).
https://doi.org/10.1016/j.mcat.2020.111190
Received 26 February 2020; Received in revised form 2 August 2020; Accepted 13 August 2020
Available online 15 September 2020
2468-8231/© 2020 Elsevier B.V. All rights reserved.

Z. Shi et al.
Molecular Catalysis 496 (2020) 111190
Fig. 1. (a) The coordination environment of Cu(I)–CN–BPY.
(A: 1-x, 1-y, 2-z; Color code: Cu1, teal spheres; Cu2, sky blue
spheres; C, gray spheres; N, blue spheres; all hydrogen atoms
are omitted for clarity). (b) The 1D chain-like of Cu(I)–
CN–BPY. (c) The connection scheme of two kinds of Cu(I) with
–CN bonds in the 2D layers of frame. (d) The 3D network frame
of Cu(I)–CN–BPY. (For interpretation of the references to
colour in the figure legend, the reader is referred to the web
version of this article.)
catalytic sites [42–48]. Cu(I)-based MOFs have efficient ability to
2.39; N, 16.73; Cu, 37.93; Found: C, 42.98; H, 2.41; N, 16.71; Cu, 37.90.
catalyze cycloaddition of CO2 with terminal alkynes to produce
α
-alkylidene cyclic carbonates under mild conditions, for example,
2.2. Catalysis
Zhao’s group reported an effective MOF catalyst with [Cu4I4] clusters to
catalyze the cyclization of propargylic alcohols with CO2 under mild
conditions, and the turnover number (TON) could reach up to 14400
[49].
Carboxylic cyclization of propargylic alcohols: before the catalysis
experiments, compound 1, 2 and 3 as catalysts were solvent exchanged
by CH3CN for three days, and dried in a vacuum at 100 ℃ to remove the
solvent molecules. The typical experiments were studied heteroge-
Inspired by the excellent work, we optimized the synthesis of a Cu
(I)–CN-based MOF, [Cu2(CN)2(BPY)] (Cu(I)–CN–BPY, compound 1), by
incorporating simple CuCl2, 4,4’-bipyridine (BPY) organic bridged
ligand and Na4W10O32 in mixable solvent of H2O and CH3CN under a
solvothermal reaction. In the process, CNꢀ was in situ obtained from the
neously with 20 μL TEA, 84 mg (1 mmol, about 100 μL) 2-methyl-3-
butyn-2-ol in 1 mL CH3CN as solvent, 0.05 mmol of Cu(I)–BPYs (about
15, 25 and 35 mg for Cu(I)–CN–BPY, Cu(I)–Cl–BPY and Cu(I)–I–BPY) in
autoclave with 0.5 MPa CO2 at appropriate temperatures and pre-
determined time. The pressure of CO2 was staid stable throughout re-
action. After the reaction, remains were purified by column
chromatography, and then performed by 1H NMR analysis for yield.
cleavage of the C–CN bond of acetonitrile by the synergistic effect Cu2+
4ꢀ
ions and W10
O
under hydrothermal reaction. Despite the structure of
32
Cu(I)–CN–BPY has been synthesised by Zhou and co-workers in 2011,
we improved the synthesis process of green cyanide source by replace-
ment K4Fe(CN)6⋅3H2O with acetonitrile [50].
3. Results and discussion
For seeking out the active sites of MOF structure in carboxylic
cyclization of propargylic alcohols, we also constructed two MOFs with
Cu-X (X = Cl, I) cluster, [Cu2Cl2(BPY)2] (Cu(I)–Cl–BPY, compound 2)
and [Cu2I2(BPY)2] (Cu(I)–I–BPY, compound 3), both of them have high
density of Cu(I) cores in their structure for this reaction and easily
synthesized by simple methods with low-cost materials [51]. Besides,
above three unique three-dimensional frameworks were synthesized by
4, 4’-bipyridine as the linker, thus all MOFs counld catalyze cyclization
of propargylic alcohols in heterogeneous manner. For convenience, we
called collectively of above three MOFs as Cu(I)–BPYs.
3.1. Structural description
Solvothermal reaction of Cu(NO3)2⋅3H2O, [(n-C4H9)N]4[W10O32
]
and 4, 4’-bipyridine gave compound 1 in a yield of 52.5 %. As we
known, polyoxometalates (POMs) are class of discrete anionic metal
oxide clusters, which commonly used as oxidation catalysts and pho-
4ꢀ
tocatalysts [52–54]. Especially, decatungstate anion W
O
10 32
has been
especially considered for its very important photocatalytic properties
[55,56]. In this synthesis process of synthesizing the Cu(I)–CN–BPY, we
4ꢀ
found the W
O
has an assistant function in prompting the cleavage of
2. Experimental
10 32
4ꢀ
C–C bonds of acetonitrile in the process. When in absence of W
(I)–CN–BPY cannot be obtained.
O
, Cu
10 32
2.1. Synthesis of Cu(I)–CN–BPY
Cu(I)–CN–BPY crystallizes in the monoclinic system with P2(1)/n
space group. The asymmetric Cu(I)–CN–BPY unit consists of two crys-
tallographically distinct copper ions, one BPY ligand and two –CN
bonds. The crystallographic data structure refinement for compounds 1,
2 and 3 are listed in Table S1. The relevant bond lengths and bond angles
are listed in Tables S2, S3 and S4. As shown in Fig. 1a, Cu1 ion is three-
coordinated in a slightly plane triangle environment, and the Cu1 center
is coordinated by one N(2) atom from the benzene ring of the BPY
ligand, two nitrogen atoms, N(3) and N(4), from the –CN bonds. Cu(2)
ion adopts tetrahedral coordination geometry with four nitrogen atoms,
one of the N(3) atoms from –CN bond is bound to the other Cu(1) center,
Cu(NO3)2⋅3H2O (60 mg, 0.25 mmol), [(n-C4H9)4N][W10O32] (50 mg,
0.019 mmol) and 4, 4’-bipyridine (24 mg, 0.15 mmol) were added in a
100 mL beaker and dissolved by 6.0 mL mixed solution (distilled water:
acetonitrile = 2: 1). The resulting was stirred over 10 h. Then adjusted
the above turbid solution to pH = 2.3 by the solution of 1 mol⋅Lꢀ 1 dilute
HCl. Then the mixture was moved into a 25 mL Teflon-lined autoclave,
and heated at 130 ℃. After the three days, yellow rhombic single
crystals were obtained with a yield of 52.49 %, based Cu element.
Elemental analyses (EA) and ICP calcd (%) For Cu2C12H8N4: C, 42.95; H,
2

Z. Shi et al.
Molecular Catalysis 496 (2020) 111190
Fig. 2. (a) The 2D sheet of Cu(I)–CN–BPY. (b) The 3D network frame of Cu(I)–CN–BPY showed two kinds of pores. (c) Two kinds of channels interspersed by
cylindrical rods simulation in the 3D frame of Cu(I)–CN–BPY.
Fig. 3. (a) The coordination environment of Cu(I)–Cl–BPY. (A: 1-x, 0.5-y, z; Color code: Cu, sky blue spheres; C, gray spheres; N, blue spheres; all hydrogen atoms are
omitted for clarity). (b) The 2D sheet of Cu(I)–Cl–BPY. (c) The interpenetrated simulation of two 2D layers of frame. (d) The 3D network frame of Cu(I)–Cl–BPY. (For
interpretation of the references to colour in the figure legend, the reader is referred to the web version of this article.)
two of the N(4) atoms as nodes connect with Cu(2) ions and form
rhombus-like plane of a dimeric [Cu2(CN)2] secondary building unit
with the approximate cross sectional area of 2.49 × 3.66 Å, the
remaining one N(1) atom from the benzene ring of the BPY ligand. Each
[Cu2(CN)2] is connected by two Cu1 cations and two linear –CN bonds,
generating a 2D layer (Fig. 1c). Meanwhile, BPY ligands linked by
copper ions arranged alternately on the Cu–CN chain and like bridges
connect with [Cu2(CN)2] and Cu–CN chains forming a long channel,
which spiraled up and looks like a string of similar parallelogram shape
(Fig. 1b). The channels are then linked by N atoms to form a 3D network
frame, between the two benzene rings of BPY ligands with a consider-
able distance of 11.67 Å, which provided enough space for next catalysis
(Fig. 1d).
propargylic alcohols species, which making the cyclization reaction
more smoothly.
Solvothermal reaction of CuX (X = Cl, I) and 4, 4’-bipyridine gave Cu
(I)–Cl–BPY and Cu(I)–I–BPY in a yield of 81.42 % and 57.70 %,
respectively. Cu(I)–Cl–BPY crystallizes in the tetragonal system with I4
(1)/acd space group crystallize, and Cu(I)–I–BPY crystallizes in the
Orthorhombic system with Fdddspace group crystallize. Compound 2
and 3 are isostructural, therefore, Cu(I)–Cl–BPY is described as an
example below. An asymmetric unit of 2 consists of two crystallo-
graphically uniform copper atoms, two chlorine atoms, and four BPY
ligands. Copper atoms are connected to each other forming a copper
bridge bond, the Cu1 and Cu1A atoms are linked by a pair of chlorine
atoms form a dimeric [Cu2Cl2] SBU (Fig. 3a). Each [Cu2Cl2] SBU is
connected by four linear BPY groups, generating a 2D layer (Fig. 3b).
Compound 2 has two kinds of one-dimensional open channels with
approximate dimensions of 2.75 × 10.98 Å of A and 17.32 × 26.48 Å of
B. A high density of [Cu2Cl2] clusters are dispersed along the channels,
the specific structure of 2 improved capacity of absorbing CO2, which
vastly improve the selectivity in cyclization catalysis. All 2D sheets are
Cu(I)–CN–BPY has two types of pores which arranged on the cross
sections of dumbbell-shaped, the approximate size is 4.84 × 11.67 Å for
pore A and 3.55 × 15.45 for B (Fig. 2b, c). Both pores are architectures of
amphoteric channels, hydrophobic organic BPY ligands and –CN bonds
with alkaline sites could facilitate the capture of CO2. And the hydro-
philous copper ions can activate carbon–carbon triple bonds of
3

Z. Shi et al.
Molecular Catalysis 496 (2020) 111190
Fig. 4. SEM images of compounds 1, 2 and 3 display the morphology of Cu(I)–CN–BPY (a, b), Cu(I)–Cl–BPY (c, d) and Cu(I)–I–BPY (e, f).
vertical interpenetrated with each other and form to a 3D braided
structure could significantly enhance the stability of framework (Fig. 3c,
d). All the copper ions in compound 1, 2 and 3 are +1, calculated by the
BVS (bond valence sum) and consistent with results the X-ray structure
determination. TGA revealed all catalysts have high thermal stability
(Fig. S1).
3.2.2. IR spectra and PXRD pattern
The IR spectra of compounds 1, 2 and 3 are collected and shown in
Fig. 5a. Three compounds have similar characteristic bands at 1600 and
1411 cm–1, which assigned to the C–N and C–H bonds stretching vi-
brations of BPY groups, respectively. In addition, differing from com-
pounds 2 and 3, Cu(I)–CN–BPY has characteristic resonances of –CN
bond in the ranges of 2123 and 2097 cm–1. These characteristic vibra-
tion resonances confirm the existence of –CN bond, which forming in
situ at the solution of acetonitrile. The PXRD patterns of compounds 1, 2
and 3 are measured to verify purity of synthesized catalysts, shown in
Fig. 5(b–d). The bottom lines represent simulated pattern, which
calculated pattern based on the single-crystals prepared for catalysis.
The top lines represent experimental pattern, which collected after
synthesized catalysts. Its showing that almost strong peaks were well
matched between the simulated and experimental.
3.2. Characterizations of catalysts
3.2.1. Morphology characterization
Fig. 4 shows the morphology of the compounds 1, 2 and 3. It can be
seen that the samples of Cu(I)–CN–BPY are the prism-like solid blocks
with well-defined diamond shapes with smooth surfaces (Fig. 4a, b).
Compared to compound 1, the Cu(I)–Cl–BPY has
a cube-like
morphology, and almost ten times volume of compound 1 (Fig. 4c),
but some of samples were weathered with uneven surfaces (Fig. 4d). The
samples of Cu(I)–Cl–BPY are regularly hexagonal prisms shaped blocks,
but these samples were more weathered than compound 2.
3.2.3. BET and CO2-adsorption analysis
The N2 adsorption–desorption isotherms (Fig. 6a) of Cu(I)–I–BPY
4

Z. Shi et al.
Molecular Catalysis 496 (2020) 111190
Fig. 5. (a) The IR spectra of compounds 1, 2 and 3; correspond the black line to Cu(I)–CN–BPY, blue line to Cu(I)–Cl–BPY and red line to Cu(I)–I–BPY, respectively.
(b) The PXRD patterns of Cu(I)–CN–BPY. (c) PXRD patterns of Cu(I)–Cl–BPY. (d) PXRD patterns of Cu(I)–I–BPY. (For interpretation of the references to colour in the
figure legend, the reader is referred to the web version of this article.)
Fig. 6. (a) N2 adsorption–desorption isotherms and BJH pore distribution of Cu(I)–I–BPY. (b) CO2 adsorption isotherms of Cu(I)–BPYs at 273 K, the black line to Cu
(I)–Cl–BPY, blue line to Cu(I)–I–BPY and red line to Cu(I)–CN–BPY, respectively. (For interpretation of the references to colour in the figure legend, the reader is
referred to the web version of this article.)
displayed that the BET surface area was determined to be 18.93 m2 gꢀ 1
and BJH pore distribution determined the average pore diameter of Cu
(I)–I–BPY was 4.87 nm. The above results revealed that Cu(I)–I–BPY was
mesoporous structure with the BJH pore volume of 0.023 cm3 gꢀ 1. To
demonstrate the potential of three catalysts in CO2 gas absorption, we
measured the adsorption isotherm of CO2 at 278 K with a de-solvated
samples of Cu(I)–BPYs (Fig. 6b). The uptake value of CO2 was 7.3478
m2 gꢀ 1 for Cu(I)–CN–BPY, 1.2404 m2 gꢀ 1 and 1.1827 m2/g for Cu(I)–
I–BPY and Cu(I)–Cl–BPY, respectively. Cu(I)–I–BPY and Cu(I)–Cl–BPY
have similar and lower uptake values of CO2, compared with them, the
CO2 adsorption capacity of Cu(I)–CN–BPY is about 6 times higher. The
main reason for the higher gas uptake is presumably the presence of –CN
bonds in channels of Cu(I)–CN–BPY, which increased the containing of
nitrogen-atoms in framework and thus enhanced CO2 adsorption
5

Z. Shi et al.
Molecular Catalysis 496 (2020) 111190
reaching to highest yield.
Table 1
Cyclization of propargylic alcohols in Cu(I)–BPYs/TEA/CO2 system.a
The effect of various amounts of catalyst was also explored. After
adding 0.05 mmol of catalyst, the substrate was obviously activated and
the yield rose to 96 %. However, the increased reaction rate was grad-
ually turned slow with the amount of catalyst increasing. With time
increasing, the reaction yield reached to ~96 % at 24 h and declined
gradually despite of continued extension of reaction time. Hence, the
optimized reaction conditions of the carboxylic cyclization was found
that this reaction was carried out in an autoclave reactor with 2-methyl-
3-butyn-2-ol (0.5 mmol, 84 mg), 0.5 MPa CO2 in 1 mL of CH3CN and
stirring about 24 h at 50 ◦C. Control experiments indicated that the in-
dividual catalysts and raw materials of MOFs were hardly catalyze this
reaction under the same condition, which proved that assembly of the
metal centers with organic ligands into one framework and combining
with TEA can greatly improve catalytic efficiency (Table S5). The
existing TEA provided a yield of 33%, which proved TEA and catalysts
are also indispensable for the reaction. Finally, several typical prop-
argylic alcohols were further examined under identical reaction condi-
tions, and the yields of the target products are summarized in Table 1.
According to the model experiment, reactions were proceed with
various propargylic alcohols (1 mmol), catalyzed by pure crystals of Cu
(I)–BPYs (0.05 mmol), where a, b, and c represented by Cu(I)–CN–BPY,
Cu(I)–Cl–BPY, Cu(I)–I–BPY, respectively, and 0.5 MPa CO2 in 1 mL of
CH3CN at 50 ◦C. Notably, Cu(I)–BPYs showed the excellent catalytic
activity substrates of substituted R1 groups, providing a high yield from
93% to 96% of corresponding cyclic carbonates after 24 h reaction
(Entries 1, 2).
Entry
Substrate
Product
Yield
TONc
TOF/
h-1d
(%)b
1a, 96
1b, 95
19.2
19.0
0.80
0.79
1
2
1c, 94
18.8
0.78
2a, 94
2b, 93
18.8
18.6
0.78
0.77
2c, 96
19.2
0.80
3a, 59
3b, 56
11.8
11.2
0.49
0.47
3
3c, 53
10.6
0.44
4a,
<0.2
<0.01
<1
4
a
4b,
<0.2
<0.2
<0.01
<0.01
<1
4c, <1
Reaction conditions: 0.05 mmol catalysts, Cu(I)–CN–BPY(a), Cu(I)–Cl–BPY
(b), Cu(I)–I–BPY(c); 1 mmol substrates of propargylic alcohols, 1 mL CH3CN, 0.5
MPa CO2, 50 ◦C, 24 h.
b
The yields were determined by 1H NMR.
c
Turnover number was calculated by the ratio of moles of product/mol of
catalyst.
d
Turnover frequency was calculated by the ratio of TON/hour of catalytic
time.
However, comparing to 2-methyl-3-butyn-2-ol, when increasing the
bulk of substituted R2 groups to phenyl propargylic alcohols caused the
yield decrease about 40 % (Entry 3). This result indicated that substrates
with benzene groups can not be adsorbed into the channels of Cu(I)–
BPYs, in turn, it suggested that substrates with smaller size were enter
into the pores of catalysts during the catalytic processes. In addition,
only trace of products (<1 %) were detected, when employed the sec-
ondary propargylic alcohols with benzene groups (Entry 4). From
another perspective, observe individual entries carefully, although
changed the substituent R1, R2 in the skeleton of propargylic alcohols,
the catalytic properties of the three kinds of catalysts are almost iden-
tical. All three catalysts have the common characteristic of high density
of Cu(I) sties fasten on channels, thus, the catalytic activity in carboxylic
cyclization of propargylic alcohols should be ascribed to Cu(I) sties.
As expected, the all above results revealed that three kinds of cata-
lysts all have effective heterogeneous catalysis for the cyclization with
propargylic alcohols of substituted R1 groups without benzene groups in
the presence of TEA and afforded nearly complete conversion with a
capacity.
3.3. Carboxylic cyclization of propargylic alcohols in Cu(I)–BPYs/TEA/
CO2 system
For the carboxylic cyclization of 2-methyl-3-butyn-2-ol as a model to
optimize reaction system under various reaction conditions are pre-
sented in Fig. S2. Initially, the influence of temperature carboxylic
cyclization of 2-methyl-3-butyn-2-ol was examined; reaction conditions:
triethylamine (TEA, 20
μL), Cu(I)–CN–BPY (0.05 mmol, 15 mg); 2-
methyl-3-butyn-2-ol (1 mmol, 84 mg), 24 h, 0.5 MPa CO2, and CH3CN
(1 mL). We found that the yield was increased with temperature in the
low temperature range of 30 ◦C to 50 ◦C, but it had a decrease with
further elevated the temperature to 60 ◦C, which possibly due to a slight
azeotrope of TEA and CH3CN during the temperature rise and partial
TEA overflow from solution. Therefore, the suitable reaction tempera-
ture is 50 ◦C, and the CO2 pressure was investigated at this temperature,
when the CO2 pressure was lifted from atmospheric pressure to 0.5 MPa,
Fig. 7. (a) Recyclability tests of compounds 1, 2 and 3; correspond the blue bar to Cu(I)–CN–BPY, orange bar to Cu(I)–Cl–BPY and yellow bar to Cu(I)–I–BPY,
respectively. (b) The PXRD patterns of Cu(I)–CN–BPY after recycled three times. (For interpretation of the references to colour in the figure legend, the reader is
referred to the web version of this article.)
6

Z. Shi et al.
Molecular Catalysis 496 (2020) 111190
Scheme 1. Possible mechanism of carboxylic cyclization the catalytic process.
highest yield up to 96 % within 24 h. Tertiary propargylic alcohols with
a phenyl group at the acetylenic terminus have lower reactivity, but it
suggests that carboxylic cyclization indeed occurred in the channels of
the MOFs, not on the external surface. It is noticeable, however, that this
reaction did not proceed when using secondary terminal propargylic
alcohol as substrate.
alcohols is more favorable to the electrophilic attack of the C atom in
CO2, at the same time, terminal alkyne was activated by Cu(I) sites and
–
–
forming the Cu(I)… C C bonds, then the propargylic carbonate inter-
–
mediate (B) was resulted. Finally, the oxygen atom attacks the carbon
atom of terminal alkyne which has been activated by Cu(I) sites, and the
intramolecular ring was closed with the C–O bond formation and
generated propargylic carbonate intermediate (C). Simultaneously,
carboxylic cyclization products are obtained process of proto-
demetallation and the regeneration of catalyst [39].
3.4. Heterogeneity and recyclability of Cu(I)–BPYs catalysts
To examine heterogeneity of the typical carboxylic cyclization sys-
tem with Cu(I)–CN–BPY catalyst in model catalysis system, after react-
ing 12 h, the catalyst was removed from carboxylic cyclization system.
Then the remaining mixture was keep reacting for another 12 h. Finally,
it is proved that the conversion has hardly no addition after removing
catalysts. The recyclability of catalyst was performed with the model
catalysis and expressed that Cu(I)–CN–BPY can be reused at least three
times. Although a little of catalyst was inevitably lost, the yields of cyclic
carbonates had almost no influence when added reaction cycles
(Fig. 7a), the loss of small amounts of the catalyst is unavoidable. ICP
analysis of the filtrate after reaction revealed that almost negligible
detection of metal ions at the end of reaction. The patterns of Cu(I)–BPYs
after three reaction cycles showed in Fig. 7b and Fig. S3. The PXRD
patterns of the initial and recovered of catalysts were good similar,
which indicated that Cu(I)–BPYs were keeping the original structure
after several times of reaction. These results above suggested that all
three catalysts have high stability and capacity to heterogeneous catal-
ysis process.
4. Conclusion
We presented a green process to synthesize cyano-bridged MOF by
cleavage of the C–C bond of acetonitrile using transition metal ions
4ꢀ
coupled with decatungstate anion W
O
10 32
under hydrothermal reac-
tion. Cu(I)–CN–BPY was investigated as catalyst for carboxylic cycliza-
tion reactions with CO2. For seeking out the active sites of MOF structure
in carboxylic cyclization of propargylic alcohols, we also synthesized
two MOFs with Cu-X (X
= Cl, I) cluster, [Cu2Cl2(BPY)2] and
[Cu2I2(BPY)2], and investigated for this reaction. Cu-nodes in three
MOFs serve as efficient active catalytic centers for cyclization of various
tertiary propargylic alcohols with CO2 under mild conditions, and the
highest yield reached to 96 %. Three MOFs all exhibit significant sta-
bility and proved as excellent heterogeneous catalysts. Together with
the conventional approaches for preparation cyano-bridged coordina-
tion polymers, this approach may lead to the generation of MOFs con-
taining cyano-bridged ligands by using less toxic acetonitrile solvent.
CRediT author statement
3.5. Mechanism of the carboxylic cyclization of propargylic alcohols
process
Zhuolin Shi: Synthesis, Methodology, Software, Investigation,
Writing - Original Draft. Jiachen Jiao: Experiments on CV and Electro-
chemical Impedance Spectroscopy, Formal Analysis. Qiuxia Han: Re-
sources, Writing - Review & Editing, Supervision, Data Curation. Yang
Xiao: Catalytic Experiments and Results Analysis. Laikuang Huang: Ex-
periments on IR and PXRD. Mingxue Li: Review & Editing.
Based on the above results and literature reports, Cu(I)–Cl–BPY is
described as an example, a possible catalytic mechanism is proposed
(Scheme 1). Initially, Cu(I) sites of [Cu2Cl2] clusters can activate the
hydroxy of propargylic alcohols to form the intermediate state (A) with
TEA. Subsequently, after synergistically activated by Cu(I) sites and
TEA, the activated oxygen atom in hydroxyl group of propargylic
7

Z. Shi et al.
Molecular Catalysis 496 (2020) 111190
of cyanide-bridged ReI diimine multichromophores, Inorg. Chem. 58 (2019)
1988–2000, https://doi.org/10.1021/acs.inorgchem.8b02974.
Declaration of Competing Interest
[17] Y. Yamada, K. Oyama, R. Gates, S. Fukuzumi, High catalytic activity of
heteropolynuclear cyanide complexes containing cobalt and platinum ions: visible-
light driven water oxidation, Angew. Chem. Int. Ed. 54 (2015) 5613–5617, https://
doi.org/10.1002/anie.201501116.
The authors report no declarations of interest.
Acknowledgements
[18] Y. Gao, R. Broersen, W. Hageman, N. Yan, M.C.M. Hazeleger, G. Rothenberg,
S. Tanase, High proton conductivity in cyanide-bridged metal-organic frameworks:
understanding the role of water, J. Mater. Chem. A 3 (2015) 22347, https://doi.
org/10.1039/C5TA05280G.
This work was supported by the National Key R&D Program of China
(grant no. 2018YFB0605802), the Key Scientific Research Project of
Henan Higher Education Institutions (grant no. 20ZX006), Henan Uni-
versity First-class Discipline Cultivation Project (grant no.
2019YLZDJL10).
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