Iron Coordination to Hollow Microporous Metal-Free Disalphen Networks: Heterogeneous Iron Catalysts for CO2 Fixation to Cyclic Carbonates

Article abstract of DOI:10.1002/chem.201904344

This work shows that a hollow and microporous metal-free N,N′-phenylenebis(salicylideneimine) (salphen) network (H-MSN) can be engineered by Sonogashira coupling of [tetraiodo{di(Zn-salphen)}] building blocks with 1,4-diethynylbenzene in the presence of silica templates and by successive Zn and silica etching. Iron(III) ions could be incorporated into the H-MSN to form hollow and microporous Fe–disalphen networks (H-MFeSN) with enhanced microporosity and surface area. The H-MFeSN showed efficient catalytic performance and recyclability in the CO₂ conversion to cyclic carbonates.

Full text of DOI:10.1002/chem.201904344

A Journal of
Accepted Article
Title: Iron Coordination to Hollow Microporous Metal-Free Salen
Networks: Heterogeneous Iron Catalysts for CO2 Fixation to
Cyclic Carbonates
Authors: Seung Uk Son, Kyoungil Cho, Sang Moon Lee, Hae Jin Kim,
Yoon-Joo Ko, and Eun Joo Kang
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To be cited as: Chem. Eur. J. 10.1002/chem.201904344
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Iron Coordination to Hollow Microporous Metal-Free Salen Networks:
Heterogeneous Iron Catalysts for CO2 Fixation to Cyclic Carbonates
Kyoungil Cho,[a] Sang Moon Lee,[b] Hae Jin Kim,[b] Yoon-Joo Ko,[c] Eun Joo Kang,*[d] and Seung Uk
Son*[a]
Abstract: This work shows that hollow and microporous metal-free
salen network (H-MSN) can be engineered by Sonogashira coupling
of tetraiodo-di(Zn-salen) building blocks with 1,4-diethynylbenzene in
the presence of silica templates and by successive Zn and silica
etching. Iron (III) ions could be incorporated into the H-MSN to form
hollow and microporous Fe-salen network (H-MFeSN) with enhanced
microporosity and surface area. The H-MFeSN showed efficient
catalytic performance and recyclability in the CO2 conversion to cyclic
carbonates.
Recently, various microporous organic network (MON) materials
have been prepared by the coupling of building blocks, showing
chemical stability and high surface areas.[9] Thus, the MON can
be a good platform of catalytic systems. There have been reports
on the development of catalysts based on the MON.[10] However,
iron-based MON catalysts have been relatively less explored.[11]
Our research group has studied the morphological engineering
of MON.[12] For example, using various hard templates, we have
engineered hollow MON materials.[13] The hollow morphologies
showed benefits such as shortened diffusion pathways of
substrates, enhancing the catalytic function of MON materials.[14]
If hollow MON material bearing metal-free coordination sites is
prepared, it can be a versatile platform to develop heterogeneous
transition metal catalysts. In this work, we report the synthesis of
hollow and microporous metal-free salen network (H-MSN) and
the complexation of irons to form hollow and microporous Fe-
salen catalyst (H-MFeSN), and its catalytic performance in the
CO2 conversion to cyclic carbonates.
Iron is the fourth most abundant element in the Earth’s crust.[1] In
the past, there were catalytic applications of iron such as the
Haber process.[2] During the last several decades, various
transition metals and ligands have been developed and applied to
catalytic reactions. Recently, based on the accumulated
knowledge in the transition metal catalysis, there has been
renewed interest in iron catalysis.[3]
Chemical fixation of CO2 is an important research subject.[4] For
example, cyclic carbonates have been prepared by the reaction
of CO2 with epoxides.[5] The cyclic carbonates have been used for
the synthesis of polymers and the electrolytes of batteries.[5]
Although numerous catalytic systems[5] including iron catalysts[6]
have been developed for this conversion, heterogeneous iron
catalysts are relatively rare.[7] Recently, we has reported efficient
and homogeneous iron catalysts for CO2 conversion to cyclic
carbonates.[8] However, the system suffered from the mixing of
catalysts with the resultant cyclic carbonates. Thus, more studies
on the efficient heterogeneous iron catalysts are required.
Figure 1 shows the synthetic schemes of H-MSN and H-MFeSN.
[a]
Mr. K. Cho, Prof. S. U. Son
Department of chemistry
Sungkyunkwan University
Suwon 16419, Korea
E-mail: sson@skku.edu
[b]
[c]
Dr. S. M. Lee, Dr. H. J. Kim
Korea Basic Science Institute
Daejeon 34133, Korea
Dr. Y. -J. Ko
Laboratory of Nuclear Magnetic Resonance,
National Center for Inter-University Research Facilities (NCIRF)
Seoul National University
Seoul 08826, Korea
[b]
Prof. E. J. Kang
Department of Applied Chemistry
Kyung Hee University
Yongin 17104, Korea
E-mail: ejkang24@khu.ac.kr
Supporting information for this article is given via a link at the end of
the document.
Figure 1. Synthesis of H-MSN and hollow H-MFeSN.
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First, we designed tetraiodo-di(Zn-salen) building block (denoted
as A in Figure 1), which is a new compound and characterizable
by nuclear magnetic resonance (NMR) and high resolution mass
(HR-MS) spectroscopy. When we used tetraiodo-di(metal-free
salen) building block for the synthesis of H-MSN, the Pd and Cu
catalysts were trapped in the salen ligands, resulting in the failure
of catalytic networking for MON. When we used tetraiodo-di(Fe-
salen) building block for the synthesis of H-MFeSN, the Fe ions
were etched during the silica etching process. In addition, the
chemical structure of tetraiodo-di(Fe-salen) building block could
not be characterized by NMR studies due to the paramagnetic
nature. Thus, we conducted the Sonogashira coupling of
tetraiodo-di(Zn-salen) building block with 1,4-diethynylbenzene in
the presence of silica spheres (an average diameter of 255 nm).
After silica etching with HF solution, H-MSN was obtained. The
reaction of H-MSN with FeCl3 in methanol resulted in H-MFeSN.
The morphologies of H-MSN and H-MFeSN were investigated
by scanning (SEM) and transmission electron microscopy (TEM).
(Figure 2a-f)
The SEM and TEM images of H-MSN showed its hollow
morphology with a diameter of 352 nm and a shell thickness of 48
nm. (Figure 2a-c) After Fe complexation to H-MSN, the resultant
H-MFeSN showed no change from the original hollow morphology.
(Figure 2d-f) The chemical surroundings of elements were
investigated by X-ray photoelectron spectroscopy (XPS). While
the N 1s orbital peak of H-MSN that appeared at 399.2 eV shifted
slightly to 399.1 eV in the XPS spectrum of H-MFeSN, the O1s
peak of H-MSN at 532.6 eV shifted significantly to 531.3 eV (H-
MFeSN) through Fe complexation, matching well with the
conventional observations of metal-phenolate species in the
literature.[15] (Figure 2g) The Zn 2p orbital peaks were not
observed in the range of 1010~1060 eV in the XPS spectra of H-
MSN and H-MFeSN, supporting that Zn ions were completely
etched. The Fe 2p1/2 and Fe 2p3/2 orbital peaks of H-FeMSN were
observed at 724.8 and 711.2 eV, respectively, matching well with
those of the Fe(III)-salen complexes reported in the literature.[16]
In the elemental analysis of H-MSN by energy dispersive X-ray
spectroscopy (EDS), Zn was not detected. (Figure 2h) In contrast,
the EDS analysis of H-MFeSN indicated that the Fe was
successfully incorporated into H-MSN.
(a)
(d)
(g)
(b)
(c)
According to the analysis of N2 adsorption-desorption isotherm
curves, surprisingly, the H-MFeSN showed a higher surface area
of 469 m2/g and a greater micropore volume of 0.13 cm3/g than
H-MSN which showed a surface area of 290 m2/g and a micropore
volume of 0.06 cm3/g. (Figure. 3a-b)
1 µm
200 nm
100 nm
(a)
(b)
(e)
(f)
1 µm
200 nm
100 nm
Zn 2p
O1s
O1s
H-MSN
N1s
N1s
Fe 2p
(c)
Fe 2p
Zn 2p
H-MFeSN
(h)
C
O
O
N
N
Fe
Fe
Zn
Zn
H-MSN
Figure 3. (a) N2 adsorption-desorption isotherm curves at 77K and (b) pore size
distribution diagrams (based on the DFT method) of H-MSN and H-MFeSN. (c)
The suggested structural flexibility of H-MSN.
H-MFeSN C
We suggest the reason as follows. In the conventional synthesis
of MON by Sonogashira coupling of building blocks, the building
block should be sufficiently rigid to form microporous networks. If
the building blocks have rotational flexibility, the microporosity of
Figure 2. SEM images of H-MSN (a-b) and H-MFeSN (d-e). TEM images of H-
MSN (c) and H-MFeSN (f). (g) XPS spectra and (h) EDS elemental mapping
images of H-MSN and H-MFeSN.
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MON can be reduced through the stacking of chemical moieties
in the MON. While Fe-salen moieties in the H-MFeSN can form
relatively rigid geometry, the metal-free salen in the H-MSN can
have structural flexibility between imine bonds and aromatic
substituents, which can be a reason for the reduced microporosity
and surface area of H-MSN. (Figure 3c) It is also noteworthy that
the MON materials prepared by Sonogashira coupling are rich in
connection defects, which can facilitate the structural flexibility.
We suggest that the multiple adsorption processes in the
adsorption isotherm of H-MSN and the different desorption
process in the desorption isotherm with big hysteresis may
originate from structural flexibility. In contrast, through Fe
complexation to salens, the structure of H-MFeSN became more
rigid and its microporoosity could be enhanced.
In the IR spectrum, the tetraiodo-metal-free salen building block
showed four main vibration peaks at 1600 (C=N), 1427, 1269 (C-
O), and 1164 cm-1. (indicated by asterisks in Figure 4b) In
comparison, the tetraiodo-Zn salen building block showed three
main vibration peaks at 1594 (C=N), 1513, and 1158 cm-1
(indicated by triangles in Figure 4b), due to the shift of C-O
vibration through metal coordination.[18] As expected, the IR
spectrum of H-MSN showed four main vibration peaks at 1600
(C=N), 1435, 1270 (C-O), and 1165 cm-1, resembling that of the
tetraiodo-metal-free salen compound. The IR spectrum of H-
MFeSN showed three main vibration peaks at 1594 (C=N), 1524,
and 1168 cm-1, resembling that of the tetraiodo-Zn-salen building
block. According to combustion elemental analysis, the N
contents in the H-MSN and H-FeMSN were analyzed to be 3.37
and 2.79wt%, respectively, corresponding to 1.20 and 0.996
mmol salens/g. Powder X-ray diffraction studies revealed that the
H-MSN and H-MFeSN are amorphous, which is a conventional
feature of MON materials prepared by the Sonogashira coupling
in the literature.[19] (Figure S1 in the SI) Thermogravimetric
analysis (TGA) showed that the H-MSN and H-MFeSN are stable
up to ~230 and ~220 oC, respectively. (Figure 4c)
The chemical structures of H-MSN and H-MFeSN were further
characterized by solid state 13C NMR and infrared (IR) absorption
studies. (Figure 4a-b)
(a)
Iron-salen complexes have shown promising performance as a
Lewis acid catalyst in various organic transformations.[20]
Considering the microporosity and the thermal stability of H-
MFeSN, we studied its catalytic performance in the CO2
conversion to cyclic carbonates. Table 1 and Figure 5 summarize
the results.
(b)
(c)
Table 1. Catalytic performance of H-MFeSN in the CO2 conversion with
propylene oxide (PO) to cyclic carbonates.[a]
Entry
Mol%
T
/ oC
Time
/ h
Yield
/%[b]
TON
TOF
/ h-1
1
0.0250
0.0250
0.0250
0.0250
0.0250
0.0500
0.0500
0.0500
0.0500
0.0500
0.0500
0.0500
0.0500
100
100
100
100
100
100
100
100
100
100
40
3
26.5
45.1
57.5
67.8
78.9
50.8
82.1
94.1
97.6
100
1060
1800
2300
2710
3160
1020
1640
1880
1950
2000
516
353
301
256
226
132
677
547
314
217
166
43
2
6
3
9
4
12
24
1.5
3
5
Figure 4. (a) Solid state 13C NMR spectra of H-MSN and H-MFeSN. (b) IR
spectra of metal-free salen building block, H-MSN, Zn-salen building block, and
H-MFeSN. (c) TGA curves of H-MSN and H-MFeSN.
6
7
8
6
The solid state 13C NMR spectrum of H-MSN showed 13C peaks
at 28.1, 33.6, 80, and 114.4~145.7 ppm, corresponding to methyl
groups, tertiary aliphatic carbons, alkynes, and aromatic groups,
respectively. (Figure 4a) The 13C peaks of the imine carbon and
phenolic carbon (aromatic carbon neighboring to OH group) of H-
MSN were observed at 159.6 ppm. It is noteworthy that the 13C
peaks of Zn-O-C in the Zn-salens were known to appear at ~170
ppm.[17] No 13C peak was detected at ~170 ppm in the 13C NMR
spectrum of H-MSN, supporting the metal-free salen moieties in
H-MSN. When we applied solid state 13C NMR study to H-MFeSN,
we could not get a meaningful spectrum, due to the paramagnetic
nature of Fe3+ ions. (Figure 4a)
9
9
10
11
12
13
12
12
12
12
25.8
52.8
76.4
60
1060
1530
88
80
127
[a] Reaction conditions: CO2 (20 bar), H-MFeSN (10.8 mg for 0.0250 mol%,
21.5 mg for 0.0500 mol%), tetrabutylammonium bromide (TBABr, 13.8 mg
for 0.100 mol%, 27.6 mg for 0.200 mol%), PO (42.9 mmol), neat. [b]
Conversion yield based on 1H NMR studies.
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Through literature survey, we figured out that most reactions of
CO2 conversion to cyclic carbonates by iron catalysts were
conducted at 100 oC under 20 bar CO2.[6-8] In this regard, we
scanned the experimental conditions of catalytic reactions at 100
oC under 20 bar CO2. (Entries 1-10 in Table 1) When we used
0.0250 mol% H-MFeSN and 0.100 mol% tetrabutylammonium
bromide (TBABr) with propylene oxide (PO) at 100 oC, the yields
of cyclic carbonate gradually increased to 67.8 and 78.9% after
12 and 24 h, respectively, corresponding to TONs of 2710 (TOF:
226 h-1) and 3160 (TOF: 132 h-1). (Entries 1-5 in Table 1). When
we increased the amount of H-MFeSN to 0.0500 mol% H-MFeSN
with 0.200 mol% TBABr and PO at 100 oC, the yield of cyclic
carbonate reached 82.1 and 94.1% after 3 and 6 h, respectively,
corresponding to TON of 1640 (TOF: 547 h-1) and 1880 (TOF: 314
h-1). (Entries 6-8 in Table 1). After 9 and 12 h, the yields further
increased to 97.6 and 100%, respectively, corresponding to TONs
of 1950 (TOF: 217 h-1) and 2000 (TOF: 166 h-1). (Entries 9-10 in
Table 1).
When the reaction temperatures decreased to 80, 60, and 40 oC,
the yields of cyclic carbonates after 12 h were 76.4, 52.8, and
25.8%, respectively, corresponding to TONs of 1528 (TOF: 127
h-1), 1056 (TOF: 88 h-1), and 516 (TOF:43 h-1). (Entries 11-13 in
Table 1). When we used 0.200 mol% TBABr, 20 bar CO2, and PO
at 100oC in the absence of H-MFeSN, the yields of cyclic
carbonates were 33.1 and 36.3% after 9 and 12 h, respectively,
supporting the catalytic action of H-MFeSN. (Figure S2 in the SI)
Next, we scanned the range of epoxide substrates and Figure
5a summarizes the results. As alkyl substituents of epoxides
became longer, the reaction times taken to obtain >80% yields
became longer. When we used PO, 2-butyl-oxirane, and 2-hexyl-
o
oxirane, the yields of cyclic carbonates after 9 h at 100 C were
97.6, 63.3, and 48.1%, respectively. After 24 h, the yields of cyclic
carbonates obtained from 2-butyl-oxirane, and 2-hexyl-oxiran
reached 91.4 and 80.0%, respectively. The epoxides with ether
substituents also resulted in good conversions to cyclic
carbonates. When we used 2-methoxymethyl-oxirane and 2-
allyloxymethyl-oxirane, the yields of cyclic carbonates after 9 h at
100 oC were 70.9 and 64.0%, respectively. After 24 h, the yields
of cyclic carbonates obtained from 2-methoxymethyl-oxirane and
2-allyloxymethyl-oxirane reached 100 and 95.1%, respectively.
The epoxides with aromatic substituents showed good
conversions to cyclic carbonates. When we used 2-phenyl-
oxirane (styrene oxide, SO), 2-(4-chlorophenyl)-oxirane, and 2-
phenoxymethyl-oxirane, the yields of corresponding cyclic
carbonates obtained after 9 h at 100 oC were 78.0, 89.2, and
89.4%, respectively. After 24 h, the cyclic carbonate of SO
showed a yield of 98.9% (an isolated yield of 94.6%). In contrast,
cyclohexene oxide and cyclopentene oxide were relatively poor
substrates, yielding 60.0 and 44.8% of cyclic carbonates after 24
h at 100 oC.
(a)
Considering the thermal stability of H-MFeSN (Figure 4c), we
tested the recyclability of H-MFeSN. The H-MFeSN could be
recovered by centrifugation and reused after simple washing. As
shown in Figure 5b, the H-MFeSN maintained the catalytic
performance in the five successive reactions of CO2 with PO,
yielding 100, 98.8, 97.8, 99.8, and 100% of cyclic carbonates.
According to the SEM and IR studies, the H-MFeSN recovered
after five successive reactions showed the complete retention of
the original hollow morphology and chemical structure. (Figure 5c
and S3 in the SI)
(c)
(b)
Conversion
IsolatedYield
Before
In our recent report, homogenous iron catalysts showed TONs
of 184~380 with TOFs of 15~32 h-1 for the SO conversion to cyclic
carbonates (yields of 92~95%) at 100 oC.[8] Also, it is noteworthy
that the recent heterogeneous iron catalyst showed a TON of 57.2
with a TOF of 5.72 h-1 for the SO conversion to cyclic carbonate
(a yield of 92%) at 100 oC.[7] In comparison, the H-MFeSN showed
a TON of 1980 with a TOF of 82.4 h-1 for the SO conversion to
100 nm
After
o
cyclic carbonate (a yield of 98.9%) at 100 C. According to the
100 nm
recent review paper[21] and our literature survey,[6] homogeneous
iron catalysts showed TONs of 80~3480 with TOFs of 40~2733 h-
1 for the PO conversion to cyclic carbonates (yields ≥ 74%) at 100
oC. (Table S1 in the SI) In comparison, the H-MFeSN,
heterogeneous catalyst showed TONs of 1640~3160 with TOFs
of 132~547 h-1 for the PO conversion to cyclic carbonates (yields
Figure 5. (a) The yields of cyclic carbonates (Reaction conditions: 20 bar CO2,
0.0500 mol% H-MFeSN, 0.200 mol% TBABr, 42.9 mmol epoxides, 100 oC, neat.
Blue numbers: conversion yields, red numbers: isolated yields by column
chromatography). (b) Recyclability of 0.0500 mol% H-MFeSN (Reaction
conditions: 100 oC, 42.9 mmol PO, 20 bar CO2, 0.200 mol% TBABr, 12 h, blue:
conversion yields, red: isolated yields of products.) (c) SEM images of H-MFeSN
before and after five successive catalytic reactions.
o
≥ 78.9%) at 100 C. In this regard, the H-MFeSN is a promising
heterogeneous iron catalyst for the CO2 conversion to cyclic
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Experimental Section
carbonates and its excellent performance is attributable to the
hollow morphology and microporosity.
To investigate the morphology effect of H-MFeSN, we prepared
nonhollow MFeSN as a control system without using silica
templates. (Figure 6a).
General information
1H and 13C NMR spectra were obtained by a 400 MHz and 500 MHz Varian
spectrometers. HR-MS spectra were obtained by a JEOL JMS 700. SEM,
TEM, and EDS-elemental mapping were obtained by a JSM6700F and a
JEOL 2100F. XPS spectra were obtained by a Thermo VG spectrometer.
N2 isotherm curves were obtained at 77K by a Micromeritics ASAP2020.
Pore size distributions were analyzed by the DFT method. Solid state 13
C
NMR spectra were obtained at CP/TOSS mode by a 500 MHz Bruker
ADVANCE II NMR spectrometer. IR spectra were obtained by a Bruker
VERTEX 70 FT-IR spectrometer. TGA curves were obtained by a Seiko
Exstar 7300. Elemental analysis was conducted by a CE EA1110 analyzer.
Powder X-ray diffraction studies were conducted by a Rigaku MAX-2200.
Synthesis of tetraiodo-di(Zn-salen) building block
(a)
3-(t-Butyl)-2-hydroxy-5-iodobenzaldehyde was prepared by the synthetic
procedures reported in the literature.[22] For the preparation of tetraiodo-
di(metal-free salen), 1,2,4,5-benzenetetraamine tetrahydrochloride (1.0 g,
3.5 mmol), 3-(t-butyl)-2-hydroxy-5-iodobenzaldehyde (4.7 g, 16 mmol),
and methanol (120 mL) were added to a flame-dried 250 mL Schlenk flask.
The reaction mixture was stirred at 90oC for 2 h. After being cooled to room
temperature, orange solid was separated by filtration, washed with cold
methanol, and dried under vacuum. Characterization data of tetraiodo-
di(metal-free salen): Yield: 92%, 1H NMR (500 MHz, CDCl3) δ = 13.56 (s,
4H), 8.66 (s, 4H), 7.62 (s, 4H), 7.60 (s, 4H), 7.16 (s, 2H), 1.41 (s, 36H)
ppm, 13C NMR (125 MHz, CDCl3) δ = 163.2, 160.6, 158.2, 139.6, 138.9,
121.1, 111.1, 80.0, 35.2, 29.2 ppm, HR-MS: Calc. [M+H]+, C50H55N4O4I4,
1283.0402, Obs. 1283.0398.
For the preparation of tetraiodo-di(Zn-salen) building block, tetraiodo-
di(metal-free salen) (1.0 g, 0.78 mmol) and chloroform (70 mL) were added
to a flame-dried 250 mL Schlenk flask. After anhydrous zinc acetate (0.32
g, 1.7 g) in ethanol (70 mL) was added, the reaction mixture was stirred at
80oC for 2 h and turned to transparent deep red color. After the reaction
mixture was stirred for 2 h at room temperature, solvent was evaporated.
Red solid was separated by recrystalization in chloroform.
Characterization data of tetraiodo-di(Zn-salen) building block: Yield: 80%,
1H NMR (500 MHz, DMSO-d6) δ = 9.12 (s, 4H), 8.36 (s, 2H), 7.70 (s, 4H),
7.35 (s, 4H), 1.46 (s, 36H) ppm, 13C NMR (125 MHz, DMSO-d6) δ = 171.8,
162.2, 145.7, 142.3, 139.0, 122.8, 73.1, 35.7, 29.7 ppm, HR-MS: Calc.
[M+H]+, C50H51N4O4I4Zn2, 1406.8672, Obs. 1406.8679.
(c)
(b)
H-MFeSN
Substrate: SO
H-MFeSN
Nonhollow
MFeSN
Nonhollow
MFeSN
Substrate: PO
100 nm
Figure 6. (a) Synthetic scheme and (b) TEM image of nonhollow MFeSN (For
SEM images of nonhollow MFeSN, refer to Figure S3 in the SI). (c) Catalytic
performance of H-MFeSN and nonhollow MFeSN in the CO2 conversion to
cyclic carbonates (Reaction conditions: 0.0500 mol% catalysts, 20 bar CO2,
0.200 mol% TBABr, 42.9 mmol epoxides, 100 oC, neat).
SEM and TEM analysis showed that the nonhollow MFeSN has
irregular morphologies with a broad size range of 200 nm ~ 1 µm.
(Figures 6b and S3 in the SI). The surface area of nonhollow
MFeSN was measured to be 450 m2/g (c.f. H-MFeSN: 469 m2/g).
The N content of nonhollow MFeSN was analyzed to be 3.56wt%
(1.28 mmol salens/g). While 0.0500 mol% H-MFeSN produced
cyclic carbonates from PO with 50.8 and 82.1% yields after 1.5
and 3 h at 100oC, respectively, the 0.0500 mol% nonhollow
MFeSN produced the cyclic carbonates with 26.4 and 51.4%
yields, respectively. (Figure 6c) In the cases of SO, while H-
MFeSN produced cyclic carbonates with 78.0 and 85.2% yields
after 9 and 12 h at 100oC, respectively, the nonhollow MFeSN
produced cyclic carbonates with 48.9 and 51.9% yields,
respectively. (Figure 6c) These results indicate that the hollow
morphology of H-MFeSN is beneficial in the catalytic performance
due to the reduced diffusion pathways of substrates.
In conclusion, the formation of MON using new tetraiodo-di(Zn-
salen) building blocks in the presence of silica templates and
successive silica etching resulted in the H-MSN bearing metal-
free salen moieties. The Fe(III) complexation resulted in the
microporous iron catalysts (H-MFeSN) which showed excellent
catalytic performance in the CO2 conversion to cyclic carbonates
with TONs of 2960~3800 (TOFs of 135~213 h-1) for PO (yields ≥
74%) and TON of 1980 (TOF of 165 h-1) for SO (a yield of 98.9%)
at 100 oC. We believe that various metals can be coordinated to
H-MSN to result in the new catalysts.
Synthesis of H-MSN, H-MFeSN, and nonhollow MFeSN
Silica templates with a diameter of 255 nm were prepared by the Stöber
method.[22,12b,14] For the synthesis of H-MSN, silica spheres (0.30 g),
(PPh3)2PdCl2 (8.4 mg, 12 µmol), CuI (2.3 mg, 12 µmol), triethylamine (20
mL), and THF (10 mL) were added to a flame-dried 50 mL Schlenk flask
and the reaction mixture was sonicated for 1 h at room temperature. After
tetraiodo-di(Zn-salen) building block (0.17 g, 0.12 mmol) and 1,4-
diethynylbenzene (30 mg, 0.24 mmol) were added, the reaction mixture
was stirred at 80 oC for 2 days. After being cooled to room temperature,
the solid was separated by centrifugation, washed with acetone (40 mL)
thrice, methanol (40 mL) thrice, and methylene chloride (40 mL) thrice, and
dried under vacuum. After the solid was added to a mixture of aqueous HF
solution (48~51%, 5 mL), methanol (10 mL), and water (15 mL) in a 50 mL
Falcon tube, the reaction mixture was stirred for 2 h at room temperature.
Caution: The HF solution is extremely toxic and should be handled with
specific gloves in a hood. The excess HF solution should be treated with
NaOH solution. Solid (H-MSN) was separated by centrifugation, washed
with a mixture of methanol (10 mL) and water (30 mL) four times, methanol
(40 mL) twice, and acetone (40 mL) twice, and dried under vacuum.
For the preparation of H-MFeSN, H-MSN (0.52 g), FeCl3 (0.19 g, 1.2
mmol), methanol (120 mL), and triethylamine (0.27 mL, 1.9 mmol) were
added to a flame-dried 250 mL Schlenk flask. The reaction mixture was
heated at 60oC for 10 h. After being cooled to room temperature, solid (H-
MFeSN) was separated by centrifugation, washed with acetone (40 mL)
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thrice, methanol (40 mL) thrice, methylene chloride (40 mL) thrice, and
dried under vacuum.
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For the preparation of nonhollow MFeSN (Also, refer to Figure S3 in the
SI), tetraiodo-di(Fe-salen) building block was prepared as follows. In a
flame-dried 250 mL Schlenk flask, tetraiodo-di(metal free salen) (1.0 g,
0.78 mmol) and chloroform (70 mL) were added. After FeCl3 (0.28 g, 1.7
mmol) in ethanol (70 mL) and triethylamine (0.48 mL) were added, the
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di(Fe-salen) building block: Yield: 75% HR-MS: Calc. [M]+,
C50H50N4O4I4Cl2Fe2, 1459.8087, Obs. 1459.8075. NMR spectra could not
be obtained due to the paramagnetic nature. For the preparation of
nonhollow MFeSN, (PPh3)2PdCl2 (8.4 mg, 12 µmol), CuI (2.3 mg, 12 µmol),
triethylamine (20 mL), and THF (10 mL) were added to a flame-dried 50
mL Schlenk flask and the reaction mixture was sonicated for 1 h at room
temperature. After tetraiodo-di(Fe-salen) building block (0.18 g, 0.12
mmol) and 1,4-diethynylbenzene (30 mg, 0.24 mmol) were added, the
reaction mixture was stirred at 80 oC for 2 days. After being cooled to room
temperature, the solid was separated by centrifugation, washed with
acetone (40 mL) thrice, methanol (40 mL) thrice, and methylene chloride
(40 mL) thrice, and dried under vacuum.
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Procedures for catalytic reactions
After H-MFeSN (0.996 mmol salens/g, 10.8 mg for 0.0250 mol%, 21.5 mg
for 0.0500 mol%), tetrabutylammonium bromide (TBABr, 13.8 mg for
0.0250 mol% H-MFeSN, 27.6 mg for 0.0550 mol% H-MFeSN), and
epoxide (42.9 mmol) were added to an autoclave, CO2 (20 bar) was
charged at room temperature. The autoclave was heated at the given
temperature for the given reaction time. After being cooled to room
temperature, excess CO2 was discharged. The reaction mixture was
transferred to a 50 mL Falcon tube using methylene chloride. The catalyst
was recovered by centrifugation. After solvent being evaporated, cyclic
carbonates were isolated by column chromatograph and characterized by
1H and 13C NMR studies.[6-8] (Figure S4 in the SI) For the recyclability tests,
HMFeSN (21.5 mg, 0.0500 mol%), TBABr (27.6 mg), and PO (3.00 mL,
42.9 mmol) were added to an autoclave. After CO2 (20 bar) being charged,
the autoclave was heated at 100 oC for 12 h. After being cooled to room
temperature, excess CO2 was discharged. The mixture was transferred to
a 50 mL Falcon tube using methylene chloride. The catalyst was recovered
by centrifugation, washed with methylene chloride, acetone, and methanol,
dried under vacuum, and used for the next run.
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Acknowledgements
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This work was supported by “Next Generation Carbon Upcycling
Project” (Project No. 2017M1A2A2042517) through the National
Research Foundation (NRF) funded by the Ministry of Science
and ICT, Republic of Korea.
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Conflict of interest
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The authors declare no conflict of interest.
Keywords: microporous organic network • iron • catalyst •
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Entry for the Table of Contents
COMMUNICATION
Iron (III) was complexed to metal-
free salen network to form hollow
and microporous Fe-salen network
(H-MFeSN). The H-MFeSN showed
excellent performance as a
heterogeneous catalytic system in
the CO2 conversion to cyclic
carbonates.
Kyoungil Cho, Sang Moon Lee, Hae Jin
Kim, Yoon-Joo Ko, Eun Joo Kang*, and
Seung Uk Son*
Page No. – Page No.
Iron Coordination to Hollow
Microporous Metal-Free Salen
Networks: Heterogeneous Iron
Catalysts for CO2 Fixation to Cyclic
Carbonates
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