An excellently stable heterovalent copper–organic framework based on Cu4I4 and Cu(COO)2N2 SBUs: The catalytic performance for CO2 cycloaddition reaction and Knoevenagel condensation reaction

Article abstract of DOI:10.1016/j.inoche.2020.107940

A novel 3D heterovalent Cu-based MOF, [Cu₂Cu₄I₄(ANA)₄(DMF)₄]_n (denoted as complex 1), was successfully designed and synthesized by the self-assembly of CuI and 5-aminonicotinic acid (HANA)

Full text of DOI:10.1016/j.inoche.2020.107940

Journal Pre-proofs
An excellently stable heterovalent copper–organic framework based on Cu4I4
and Cu(COO)2N2 SBUs: The catalytic performance for CO2 cycloaddition re-
action and Knoevenagel condensation reaction
Feng Jin
PII:
S1387-7003(20)30422-6
DOI:
https://doi.org/10.1016/j.inoche.2020.107940
INOCHE 107940
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Inorganic Chemistry Communications
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Accepted Date:
28 March 2020
9 April 2020
11 April 2020
Please cite this article as: F. Jin, An excellently stable heterovalent copper–organic framework based on Cu4I4
and Cu(COO)2N2 SBUs: The catalytic performance for CO2 cycloaddition reaction and Knoevenagel
condensation reaction, Inorganic Chemistry Communications (2020), doi: https://doi.org/10.1016/j.inoche.
2020.107940
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1
An excellently stable heterovalent copper–organic
framework based on Cu4I4 and Cu(COO)2N2 SBUs: The
catalytic performance for CO2 cycloaddition reaction and
Knoevenagel condensation reaction
Feng Jin,a*
a Experiment Management Center, Dezhou University, Dezhou 253023, Shandong.
Corresponding Author: Feng Jin
E-mail address: dzjfeng@163.com

2
Abstract
A novel 3D heterovalent Cu-based MOF, [Cu2Cu4I4(ANA)4(DMF)4]n (denoted as
complex 1), was successfully designed and synthesized by the self-assembly of CuI and
5-aminonicotinic acid (HANA) under the solvothermal synthesis. The prepared sample
has the fascinating structure and was analysed in detail by lots of testing methods. Due
to the excellent stability and various catalytic sites, this material can catalyse CO2
cycloaddition reaction and Knoevenagel condensation reaction; meanwhile, complex 1
has outstanding recyclability and keep catalytic performance at least four cycles.
Keywords: heterovalent copper-organic framework; heterogeneous catalyst; stability;
CO2 cycloaddition reaction; Knoevenagel condensation reaction.
Metal–organic frameworks (MOFs) are a rapidly developing inorganic-organic hybrid
crystals,[1-3] which have been attracted a lot of interest due to their plentiful structures
and potentially actual applications, such as gas sorption/separation,[4-6] immobilizing
enzyme,[7,8] luminescence sensor,[9-11], optical device,[12-14] and heterogeneous
catalysis[15,16]. Contrast to traditional materials, MOFs have various unique properties
and merits, especially structural diversity, designability, and modification.[17-19] From
the previous researches, MOFs can be well designed and synthesized by the
coordination assembly of various metal ions/clusters and organic ligands.[20-23]
Meanwhile, diverse factors from internal and external environments all can affect the
crystal textures, such as pH value, temperature, pressure, template, solvent, and
countervailing anion.[24-27] Also, organic linkers play a vital role in the preparation of
MOFs, especially linkers with multifarious coordination sites of N and O donors,
because they are able to provide a variety of self-assembly configurations with metal
nodes to generate fascinating structures with unique properties.[28-30] Recently, MOFs
based on metal halides (especially copper(I) halides) as secondary building unit (SBU)
have been attracted lots of considerable attentions due to their attractive structures and
functional properties.[31-33] CuI-based SBUs offer a series of geometrically diverse

3
structural motifs, including the dimeric rhomboid [Cu2I2], the tetrameric cubane-like
[Cu4I4], the hexagonal-prism-shaped [Cu6I6], the zigzag [CuxIy]n chain, etc.[34-37] CuI
can be implemented as the metal source to construct MOFs, which is not only attributed
to the formation of various CuxIy clusters but also the easy conversation of Cu2+ ions
from Cu+ ions in air by the oxidation process in suit.[38] Cu2+ ions are easy to coordinate
with −COO− to form Cu(CO2)n SBUs;[39] meanwhile, Cu+ and I− ions can be linked with
N donors to generate much more stable [CuxIy]n SBUs.[40-44] It is significantly important
to implement suitable building linkers to synthesize heterovalent copper–organic
frameworks with fascinating textures and excellent properties. Hence, organic linkers
containing pyridine and –COOH are good candidates to assemble with CuI to construct
such MOFs.
According to the above discussions, we chose a 5-aminonicotinic acid (HANA) as
the organic linker, which has pyridine, carboxylic acid, and amino functional group.
CuI is the metal source to coordinate with HANA to prepare MOFs. Herein, a novel
three-dimensional (3D) heterovalent Cu–organic framework based on Cu4I4 and
Cu(COO)2N2 SBUs, formula as [Cu2Cu4I4(ANA)4(DMF)4]n (complex 1), was
successfully prepared under the solvothermal approach. The as-synthesized 1 possesses
a fascinating structure, which was analysed in detail by various testing methods. By
virtue of the excellent stability and lots of catalytic sites, complex 1 is a heterogeneous
catalyst for the CO2 cycloaddition reaction and Knoevenagel condensation reaction. In
addition, complex 1 owns the remarkable recyclability and retains the catalytic
performance at least four cycles.
From the single crystal X-ray diffraction analysis, it evidently shows that complex 1
belongs to the I41/a space group of the tetragonal system. As shown in Fig. 1a, the
asymmetrical unit of complex 1 has a half Cu atom, another Cu atom, one I atom, one
ANA– bridging organic linker, and a terminated DMF molecular. The ANA– bridging
linker has only one coordination fashion as 3-1: 1: 1 to connect with three Cu atoms
by pyridine, carboxylic acid, and amino functional groups (Fig. 1b). Notably,
Cu(COO)2N2 SBU can be found in complex 1 and the Cu2+ ion exists in this SBU,

4
which is attributed to the oxidation of Cu+ in air (Fig. 1c). On the other hand, the Cu+
in Cu4I4 SBU can coordinate with four N donors of pyridines from four ANA– bridging
linkers (Fig. 1d). The structure clearly illustrates that the heterovalent Cu–organic
framework is successfully prepared as expected. Finally, Cu(COO)2N2 and Cu4I4 SBUs
can be further connected with each other by these ANA– linkers to construct a novel
3D framework (Fig. 1e).
Fig. 1 (a) The asymmetrical unit; (b) the coordination mode of organic linker; (c, d) the coordination
environments of CuO3N2 and Cu4I4; and (e) viewing of the 3D structure in complex 1 (all H atoms
are removed for clarity).
As displayed in Fig. 2a, the thermogravimetric (TG) data of experimental complex
1 obviously can keep a platform and no any weight loss before ~250 °C, but a
significant weight loss can be found after increasing the heating temperature due to
losing the coordinated DMF molecules. Finally, the main skeleton collapses after the
temperature increases to ~520 °C. From the powder X-ray diffraction (PXRD) patterns
of fresh as-synthesized sample, these characteristic peaks are consistent well with those
of simulated patterns to prove the pure phase and uniformity (Fig. 2b). To further
estimate the stability, the as-synthesized samples were immersed in air for one month,
in water for one week at room temperature, after heating at 250 °C for one week, and
in pH aqueous solutions of pH = 3 and 13 at room temperature for one day, respectively.
Also, the corresponding PXRD patterns clearly show that the skeleton can be kept very

5
well after treating with these conditions. The result illustrates complex 1 has the
excellent physicochemical stability.
Fig. 2 (a) The TG data of as-synthesized complex 1; and (b) the PXRD profile of simulated and
experiment complex 1 after treating with various conditions.
Benefit from the excellent stability and various metal sites as Lewis acid sites,
complex 1 may be used as a potential heterogeneous catalyst for chemical fixation of
CO2 and epoxides to cyclic carbonates.[45-50] The prepared crystals were dried in air for
1 day before processing catalytic reactions. Epoxides (20 mmol), n-Bu4NBr (0.3 mmol),
and complex 1 (100 mg) were putted in a high-pressure reaction with 1 MPa CO2, which
was heated at 80 °C for 8 hours. The product of cyclic carbonate can be calculated by
the GC measurement with n-dodecane as the additive internal standard. Catalytic results
are all listed in Table 1. As seen in entries 1 and 2, the individual component for
epoxypropane and CO2 only show the low yields of complex 1 (~11%) or n-Bu4NBr
(~19%), respectively. However, a higher catalytic yield of >99% can be detected by
using complex 1 and n-Bu4NBr in the reaction system (entry 3). The results show that
this CO2 cycloaddition reaction almost can’t be efficiently catalysed by itself and can
fully react in the presence of complex 1 and n-Bu4NBr because of their synergistically
catalytic effect of metal centres as catalytic sites in complex 1 and n-Bu4NBr as co-
catalysis. To investigate the catalytic performance for different epoxides, complex 1
and n-Bu4NBr were both used to catalyse these reactions at the same reaction condition.

6
The yield of epichlorohydrin and CO2 can react completely (entry 4), but the other
catalytic yields obviously decrease to ~93%, ~87%, and ~72% for 1,2-epoxy-3-
allyloxypropane, benzyl phenylglycidyl ether, and tert-butyl glycidyl ether,
respectively (entries 5, 6 and 7), which may be attributed to the increasing molecule
sizes and different electronic effects of functional groups.[51-53] To estimate the
recyclability of complex 1, the cyclic sample can well keep the catalytic ability for
chemical transformation of epoxypropane and CO2 at least four cycles (entry 8 and Fig.
3a). The PXRD pattern of complex 1 after reusing four times is consistent well with
that of pristine complex 1 to prove the excellent stability of complex 1 in the successive
catalytic experiments (Fig. 3b).
Table 1. The catalytic yields of CO2 and various epoxides.
O
O
CO2
O
O
1 MPa
R
R
Entry
1
Catalyst
Epoxide
Product
Yield
%
O
O
O
O
O
O
complex 1
11
O
O
O
O
O
O
2
3
n-Bu4NBr
19
complex 1 + n-
>99
Bu4NBr

7
O
4
5
6
complex 1 + n-
>99
93
O
Cl
Bu4NBr
O
O
Cl
O
complex 1 + n-
O
O
Bu4NBr
O
O
O
O
O
complex 1 + n-
87
O
Bu4NBr
O
O
O
O
7
8
complex 1 + n-
72
O
Bu4NBr
O
O
O
O
O
O
Reused complex
>99
1 + n-Bu4NBr
O
O
Fig. 3 Reusability (a) and PXRD pattern (b) of complex 1.
According to the previous reported MOFs for catalyzing this reaction,[54-58] the

8
assumptive reaction can be deduced as follows: Firstly, this reaction is caused by the
interaction between the O atom in epoxide ring and the metal site in complex 1.
Subsequently, the epoxide ring will be attacked by Br– anion from n-Bu4NBr to open
the ring. At the same time, the O anion can quickly react with CO2 as C1 source to
synthesize an ionic alkycarbonate intermediate. Finally, the targeted cyclic carbonates
will be obtained from alkycarbonate anions by the cyclization process. Nevertheless,
the accurate mechanism is still far beyond our understanding nowadays.
Due to various N atoms in complex 1, the resultant sample may be used as a Lewis
basic catalysis for Knoevenagel condensation reaction. [59-70] After the as-synthesized
samples were dried in air before the reaction, complex 1 (100 mg) was added in a
toluene solution (5 mL) containing substrates with different functional groups (1.0
mmol) and malononitrile (1.05 mol). The above reaction was heated at 100 °C in an oil
bath for 10 hours under slowly stirring. The catalytic result was tested and calculated
by the GC approach. The catalytic yield of 2-benzylidenemalononitrile can reach up as
high as >99% after 10 hours (entry 1 of Table 2); however, this reaction has very low
catalytic performance (~9%) without complex 1. The catalytic result obviously
illustrates that complex 1 can efficiently catalyze this catalytic reaction (entry 2).
Different substrates with various functional groups were explored and investigated to
further evaluate the relationship between the structures of substrates and the
corresponding catalytic performances. The –F and –NO2 groups have the electron-
withdrawing effect, which both have high catalytic yields as high as >99% (entries 3
and 4 of Table 2). When the functional groups are substituted by other functional groups,
the catalytic performance has totally different catalytic yield. As seen in entries 5 and
6, the yields obviously reduce to 90% of only one electron donor –OMe group (entry
5) and 83% (entry 6) of two –OMe groups, respectively. Based on the catalytic yields,
it shows that the electron effect of functional groups on substrates will directly affect
the catalytic performance. Meanwhile, the reusability of complex 1 is also an important
factor to estimate this heterogeneous catalyst. As seen in entry 7 and Fig. 4b, complex
1 can catalyze the Knoevenagel condensation by using benzaldehyde as the substrate
model, which can keep the catalytic yield at least four times. The result shows that

9
complex 1 has the greatly potential application for the Knoevenagel condensation with
excellent reusability.
Table 2. Catalytic yields of Knoevenagel condensations by using complex 1.
CN
O
+
NC
CN
R
R
H
CN
Entry
1 [a]
Substrate
Product
Yield %
>99
CN
CN
O
O
CN
CN
2 [b]
3 [a]
4 [a]
5 [a]
6 [a]
9
>99
>99
90
CN
O
CN
F
F
CN
O
CN
O N
2
O N
2
CN
O
CN
O
O
83
CN
O
CN
O
O
O
O
7 [c]
>99
CN
O
CN
[a] Reaction conditions: substrates with various functional groups (1.0 mmol), malononitrile (1.05 mol), and catalyst (100 mg)
were added in toluene (5 mL) at 100 °C for 10 hours. [b] The same reaction conditions as in [a], but without the catalyst. [c] The
recyclability test after four cycles.

10
Fig. 4 (a) The reusability and (b) the PXRD pattern of complex 1 for this catalytic reaction.
In conclusion, a 3D excellently stable heterovalent copper–organic framework based
on Cu4I4 and Cu(COO)2N2 SBUs is successfully designed and synthesized by the
solvothermal synthesis. The prepared sample can be used as a bifunctional
heterogeneous catalyst for chemical fixation of CO2 and epoxide, and Knoevenagel
condensation reaction. Especially, the excellent stability of complex 1 can be used to
continuously catalyse both reactions at least four cycles.
Appendix A. Supplementary data
The experiments are listed in the supporting information. The CCDC reference number
is 1992434.
Acknowledgements
This research did not receive any specific funding.
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Graphical Abstract-Pictogram
This work exhibits an excellently stable heterovalent copper–organic framework
based on Cu4I4 and Cu(COO)2N2 SBUs, which can catalyse the CO2 cycloaddition
reaction and Knoevenagel condensation reaction with preferable recyclability.

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Highlights
1. A 3D heterovalent Cu–organic framework based on Cu4I4 and Cu(COO)2N2 SBUs
was successfully designed and synthesized.
2. The resultant sample can catalyse the CO2 cycloaddition reaction and Knoevenagel
condensation reaction.
3. Complex 1 has the remarkable stability and recyclability at least four cycles.

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Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests:

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Feng Jin: Supervision and writing this article.