Quasi-homogeneous catalytic conversion of CO2into quinazolinones inside a metal-organic framework microreactor

Article abstract of DOI:10.1039/d1gc01677f

Management of CO2 has been attracting great attention in this century. Reaction of CO2 with 2-haloanilines and isocyanides is an attractive way for both converting CO2 and producing quinazolinones, which are key intermediates for the synthesis of various biologically active products. However, the heterogeneous and relatively inert nature of CO2 with 2-haloaniline and isocyanide reactants limits the types of suitable catalysts. Herein, we use metal-organic frameworks (MOFs) as a microreactor , in which Pd(PPh3)2Cl2 is well-dispersed as a single-molecular catalyst, and the reactants react in the molecular level through a quasi-homogeneous way to convert CO2 into quinazolinones under mild conditions with both promising yields over homogeneous catalysts and good recyclability as a heterogeneous reaction. The MOF-assisted single-molecular catalysis strategy should contribute to CO2 conversion, production of quinazolinone-type bioactive intermediates, and the epochal development of homo-and-heterogeneous catalysis. This journal is

Full text of DOI:10.1039/d1gc01677f

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Quasi-homogeneous catalytic conversion of CO2
into quinazolinones inside a metal–organic
framework microreactor†
Cite this: Green Chem., 2021, 23,
5456
Received 12th May 2021,
Accepted 26th June 2021
Zhenzhen Zhou,a Jian-Gong Ma, *a Jianbo Gaob and Peng Cheng
*
a,c
DOI: 10.1039/d1gc01677f
rsc.li/greenchem
Management of CO2 has been attracting great attention in this for such synthetic processes are extremely limited, and no
century. Reaction of CO2 with 2-haloanilines and isocyanides is an heterogeneous catalyst has been reported yet.
attractive way for both converting CO2 and producing quinazoli-
Metal–organic frameworks (MOFs), composed of organic
nones, which are key intermediates for the synthesis of various linkers with metal ions or clusters,4–6 have exhibited wide
biologically active products. However, the heterogeneous and rela- potential applications in the management of CO2 in recent
tively inert nature of CO2 with 2-haloaniline and isocyanide reac- years for their super adsorption capacity towards CO2 due to
tants limits the types of suitable catalysts. Herein, we use metal– their porous structure and ultrahigh surface area.7–25 Herein,
organic frameworks (MOFs) as a “microreactor”, in which Pd we make use of MOFs’ well-defined pores as a “microreactor”
(PPh3)2Cl2 is well-dispersed as a single-molecular catalyst, and the to provide an individual confined environment. For the first
reactants react in the molecular level through a “quasi-homo- time, the organometallic Pd(PPh3)2Cl2 is loaded and well-dis-
geneous” way to convert CO2 into quinazolinones under mild con- persed as a “single-molecular catalyst” in the pores of the MOF
ditions with both promising yields over homogeneous catalysts MIL-101(Cr), and the reactants 2-haloanilines and isocyanides
and good recyclability as a heterogeneous reaction. The MOF- are transported into the MOF pores by a solvent in the form of
assisted single-molecular catalysis strategy should contribute to molecules, and CO2 molecules are efficiently adsorbed into the
CO2 conversion, production of quinazolinone-type bioactive inter- pores by the MOF. As a result, the MOF acts as a “microreac-
mediates, and the epochal development of “homo-and-hetero- tor”, in which the reactants are arranged and react at the mole-
geneous” catalysis.
cular level catalyzed by the single-molecular Pd(PPh3)2Cl2 (s-Pd
(PPh3)2Cl2) through a “quasi-homogeneous” way. This MOF
reactor-based single-molecular approach enables the conver-
sion of CO2 into quinazolinones under 1 atm at a low tempera-
ture (50 °C), which exhibits promising activity and efficiency
over homogeneous catalysts as well as good reusability like
heterogeneous catalysts.
Pd(PPh3)2Cl2 was selected as the active center for the reaction
of CO2 with 2-haloanilines and isocyanides, because of its
specific activity towards C–X bond activation, C–C coupling and
C–N bond formation as well as the moderate coordination
ability and adjustable size of its phosphine group.26 MIL-101
(Cr) was used as a MOF microreactor due to its high stability,
highly porous structure and cage-type pores with a suitable size
(2.2–2.7 nm cavity diameter with 0.7 and 1.0 nm windows),27
which allowed the entry of the PPh3 ligand (0.9 nm) yet
restricted the escape of the formed complex (1.2 nm).28 The Pd
(PPh3)2Cl2 molecule was in situ assembled inside MIL-101’s
cage by the “ship in a bottle” method, as shown in Scheme 1.
Synchrotron powder X-ray diffraction (PXRD) shows that
MIL-101(Cr) is preserved intact during the assembly of the Pd
(PPh3)2Cl2 molecule inside the MOF cage (Fig. 1a), which is
Carbon dioxide has been attracting great attention in this
century both as a major greenhouse gas and as an abundant,
economical, nontoxic and renewable C1 source.1 Consequently,
conversion of CO2 into value-added chemical materials is of
dual significance. Recently, reaction of CO2 with 2-haloanilines
and isocyanides exhibited a promising way both for fixation of
CO2 and for green production of quinazolinones,2 which are
key intermediates for the synthesis of a series of biologically
active products and medicines.3 However, because the reac-
tants including CO2 (gas), 2-haloanilines (solid) and isocya-
nides (liquid) are mutually heterogeneous, suitable catalysts
aDepartment of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry
(MOE) and Renewable Energy Conversion and Storage Center (RECAST), College of
Chemistry, Nankai University, Tianjin 300071, P. R. China.
E-mail: mvbasten@nankai.edu.cn, pcheng@nankai.edu.cn
bCentre of Excellence for Advanced Materials, Dongguan 523808, P. R. China
cFrontiers Science Center for New Organic Matter, Nankai University, Tianjin
300071, P. R. China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1gc01677f
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Scheme 1 Quasi-homogeneous conversion of CO2 into quinazolinones over the single-molecularly dispersed catalyst in a MIL-101 microreactor.
further confirmed by the scanning electron microscopy (SEM) (0) is observed, and the peak for Cl appears at 198.4 eV and
images (Fig. S1†). Solid-state NMR was used to confirm the 199.9 eV – the same as that in the Pd(PPh3)2Cl2 compound,
presence and the coordination of PPh3 with Pd ions. As shown which unambiguously confirms the coordination environment
in Fig. 1b, the solid-state NMR spectra of the encapsulated s- of the Pd(II) center and the formation of Pd(PPh3)2Cl2 mole-
Pd(PPh3)2Cl2 show exactly the same 31P signals as those of the cules inside the MOF microreactor. Inductively Coupled
independent Pd(PPh3)2Cl2, indicating the successful loading Plasma (ICP) analysis shows the content of the Pd element
of Pd(PPh3)2Cl2 inside the pores of MIL-101(Cr), which is loaded in MIL-101(Cr) as 4.65%. The incorporation of Pd
further confirmed by comparing the FT-IR spectra of Pd (PPh3)2Cl2 molecules is also illustrated by transmission elec-
(PPh3)2Cl2, MIL-101(Cr), PPh3@MIL-101(Cr) and s-Pd tron microscopy (TEM) through the well dispersed Pd, P and
(PPh3)2Cl2@MIL-101, as shown in Fig. 1d.23 X-ray photo- Cl elements in elemental mapping and the Energy-Dispersive
electron
spectroscopy
(XPS)
(Fig.
3)
of
s-Pd Spectroscopy (EDS) data (Fig. 2). The specific surface area,
(PPh3)2Cl2@MIL-101 shows a pattern quite similar to that of pore size and pore volume of MIL-101(Cr) after loading the Pd
the Pd(PPh3)2Cl2 compound, indicating the successful assem- (PPh3)2Cl2 molecules was measured by N2 physisorption ana-
bly of the Pd(PPh3)2Cl2 molecules inside the MOF microreac- lysis at 77 K (Fig. 1c), exhibiting a typical completely reversible
tor.29 In the XPS spectra, the peaks at 335.2 eV and 340.4 eV isotherm as the original MIL-101(Cr). The Brunauer–Emmett–
are well-assigned to 3d5/2 and 3d3/2 of Pd(II), and no peak of Pd Teller (BET) surface area (SBET) of MIL-101(Cr) and MIL-101
Fig. 1 (a) Synchrotron PXRD of s-Pd(PPh3)2Cl2@MIL-101. (b) N2 adsorption/desorption isotherms of (1) MIL-101(Cr) and (2) s-Pd(PPh3)2Cl2@MIL-101
at 77 K. (c) Solid-state NMR of s-Pd(PPh3)2Cl2@MIL-101. (d) FT-IR spectra of MIL-101(Cr), PPh3, Pd(PPh3)2Cl2, PPh3@MIL-101 and s-Pd
(PPh3)2Cl2@MIL-101.
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solvent, respectively. After the reaction, we carried out hot fil-
tration (Fig. S9†) and analyzed the Pd content of the filtrate by
an ICP test, which suggested that the leaching of Pd is almost
negligible (0.3 μg ml−1, ca. 0.02% of the catalyst).
With the optimized conditions in hand, we explored the
substrate scope of various 2-iodoanilines 1a–i (Table S2†). The
reactions of 2-iodoanilines bearing the electron-donating
groups such as methyl and methoxyl with isocyanide 2a and
CO2 afforded the desired products 3c and 3d in more than
95% yields, and halogen-substituted 2-iodoanilines were used
to produce 3e–i in ca. 90% yields, which confirmed the broad
application scope of s-Pd(PPh3)2Cl2 with the MOF reactor.
To illustrate the outstanding activity of the single-molecular
s-Pd(PPh3)2Cl2 in the MOF reactor, we compared the activities
of other known or relative catalysts under the optimized con-
ditions. The results are summarized in Table 2. Without a cata-
lyst, there is no product detected (Table 2, entry 2). When we
used the MOF MIL-101 (Cr) alone, the reaction hardly took
place (Table 2, entry 3), indicating the Pd complex as the cata-
lytic active center. When using Pd(PPh3)2Cl2 without the MOF
reactor, the yield of the quinazolinone was only 41.5%
(Table 2, entry 4), although Pd(PPh3)2Cl2 is a homogeneous
catalyst, which unambiguously confirms the significance of
the MOF as a microreactor and the single-molecular dis-
persion of s-Pd(PPh3)2Cl2. The other relative Pd compounds
including PdCl2, Pd(PPh3)4 and Pd(OAc)2 gave very poor yield
of ca. 40%. The reported catalysts for this reaction were quite
limited,2,3 and they were homogeneous catalytic systems and
required 10 times more Pd content, a higher temperature
(80 °C) and even 10 bar (9.9 atm) CO2 pressure.
Fig. 2 (a) TEM and EDS of s-Pd(PPh3)2Cl2@MIL-101. (b) HAADF-STEM
images of (c) C, (d) Cr, (e) Pd, (f) Cl and (g) P.
(Cr) with Pd(PPh3)2Cl2 are given as 2613 m2 g−1 and 1987 m2
g
−1 with pore volumes (Vp) 1.273 cm−3 g−1 and 1.038 cm−3 g−1
,
respectively, demonstrating that the cavities of MIL-101(Cr) are
occupied by the highly dispersed s-Pd(PPh3)2Cl2.
The MOF-loaded single-molecular catalyst s-Pd(PPh3)2Cl2
was then applied to process the reaction of CO2 with 2-haloani-
lines and isocyanides to produce quinazolinones. The reaction
was carried out under ambient pressure in MeCN. As the temp-
erature increased from 25 °C, the yield of the quinazolinone
product increased from 62.2 to 91.9% at 50 °C in 18 h cata-
lyzed by only 1 mol% s-Pd(PPh3)2Cl2, and then decreased
when the temperature increased further (Table 1, entries 1–6).
When 2 mol% catalyst was used, the yield decreased to 85.2%
(Table 1, entry 6), probably because more catalyst blocked the
MOF pores and affected the entrance of the reactant molecules
and the exit of the product. 18 h was proven to be the opti-
mized time for this reaction (Table 1, entries 8 and 9), and
DBU and MeCN were confirmed as the optimized base and
From the industrial perspective, the recyclability of a cata-
lyst is an important criterion for heterogeneous catalysis. The
catalyst recycling experiment was performed on the same
model reaction of introducing CO2 into a multicomponent
reaction of isocyanides. Upon completion of the reaction, the
Table 2 Screening of the compared reaction conditionsa
Table 1 Screening of the optimal reaction conditionsa
Entry
Cat. (mol%)
Base
Solvent
Temp. (°C)
Yield (%) Entry Cat. (mol%)
Temp. (°C) Yield (%)
1
2
3
4
5
6
1
1
1
1
1
1
2
1
1
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
25
40
50
60
70
80
50
50
50
62.2
60.3
91.9
89.4
85.0
90.5
85.2
85.4
82.9
1
2
3
4
5
6
7
8
s-Pd(PPh3)2Cl2@MIL-101 (1)
—
MIL-101(Cr) (1)
Pd(PPh3)2Cl2 (1)
PdCl2 (1)
Pd(PPh3)4 (1)
Pd(OAC)2 (1)
Pd(PPh3)2Cl2 (1) + MIL-101(Cr) (1) 50
50
50
50
50
50
50
50
91.9
0
Trace
41.5
44.9
40.5
39.5
42
7
8b
9c
9
PdCl2 (10) + PPh3 (20)
Pd(OAc)2 (10) + BuPAd2 (20)
80
80
92–97 (ref. 3)
91 (ref. 2)
10b
a Reaction conditions: 1a (0.60 mmol), 2a (0.90 mmol), catalyst s-Pd
(PPh3)2Cl2@MIL-101 (1% mol based on Pd), base (0.1 equiv.), solvent a Reaction conditions: 1a (0.60 mmol), 2a (0.90 mmol), catalyst, DBU
(4 ml), Schlenk tube, CO2 balloon, 50 °C, 18 h. b The reaction was base (0.1 equiv.), MeCN solvent (4 ml), Schlenk tube, CO2 balloon,
carried out for 12 h. c The reaction was carried out for 10 h.
18 h. b 10 bar CO2, Cs2CO3 base, 1,4-dioxane solvent and 7–16 h.
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Fig. 3 (a) The XPS full spectrum of s-Pd(PPh3)2Cl2@MIL-101. (b) The XPS full spectrum of pure Pd(PPh3)2Cl2. (c) After encapsulation of Pd spectrum.
(d) Pd spectrum. (e) After encapsulation of Cl spectrum. (f) After encapsulation of Cl spectrum.
catalyst was separated from the reaction mixture by filtration, ual environment. The catalyst Pd(PPh3)2Cl2 is loaded and well-
washed properly with dry ethanol and dried in a vacuum desic- dispersed as a single molecule in the pores of the MOF
cator. The recycled catalyst was then used for the same reac- MIL-101(Cr), and the reactants 2-haloanilines and isocyanides
tion up to five consecutive cycles, which was fully characterized are transported into the MOF pores by a solvent in the form of
afterwards. ICP tests of the fresh and reused catalyst after five molecules, and the CO2 molecules are efficiently adsorbed into
cycles confirmed the good recyclability of the composite cata- the pores by the MOF. It is the first time that phosphine com-
lyst (Table S1†) and suggested that the molar ratios of Pd : P pounds have been supported on MOFs as heterogeneous cata-
for both the fresh and recycled catalyst matched the molecular lysts for chemical fixation of CO2 to synthesize quinazolinones.
formula of Pd(PPh3)2Cl2 quite well, indicating the perfect This method is efficient and recyclable. MOFs are currently
reserve of the active center. The crystallinity of the support applied in more and more catalysis processes; thus, our mole-
remained unaltered in the recovered catalyst, as shown in the cular method may find vast use for efficiently enhancing the
TEM (Fig. S2†) images, and the dispersion of the Pd, P and Cl activities of MOF catalysts and remarkably saving the time and
elements was well maintained, as shown in the EDS mapping cost not only in CO2 conversion but also in other synthetic pro-
(Fig. S3†) throughout the chemical transformation. According cedures. Besides, the synthesis of new catalysts opens up a
to the XPS analysis (Fig. S6†), the valence state of Pd in catalyst new direction for the synthesis of supported catalysts. It is of
showed no change after five times experiment, and the solid- great significance for the catalytic application of MOFs.
state NMR spectra of the recovered s-Pd(PPh3)2Cl2@MIL-101
showed exactly the same 31P signals as those of the indepen-
dent Pd(PPh3)2Cl2 and fresh s-Pd(PPh3)2Cl2@MIL-101, which
proved the maintenance of s-Pd(PPh3)2Cl2 in the MOF cavity in
Conflicts of interest
There are no conflicts to declare.
the molecular form. The FT-IR spectra of the recovered catalyst
(Fig. S5†) confirmed the intact structural properties of the
recovered catalyst as well. Overall, all these characterization
methods clearly show the heterogeneous nature of the
MIL-101(Cr) supported single-molecular catalyst.
Acknowledgements
This work was supported by the National Key R&D Program of
China (2016YFB0600902), the NSFC (21771112 and 21875120)
and the Fundamental Research Funds for the Central
Universities, Nankai University (63181206 and 63213060). Dr J.
Gao acknowledged the support from the Program for
Conclusions
In summary, we make use of the unique porous metal–organic Guangdong Introducing Innovative and Entrepreneurial
frameworks (MOFs) as a “microreactor” to provide an individ- Teams (2016ZT06G025). We acknowledge Dr Qinfen Gu of the
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