Salen(Co(III)) imprisoned within pores of a metal-organic framework by post-synthetic modification and its asymmetric catalysis for CO2 fixation at room temperature

Article abstract of DOI:10.1039/c7cc06522a

Herein, a new preparation strategy of chiral metal-organic frameworks (CMOFs) has been demonstrated. By adsorption and then post-synthetically modified (PSM) procedures, chiral salen(Co(iii)) could be imprisoned within the cages of an MOF and remained in its free form. This is the first report on the successful application of CMOFs in heterogeneous asymmetric catalysis for coupling CO₂ with epoxides to obtain optically active cyclic carbonates at room temperature.

Full text of DOI:10.1039/c7cc06522a

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Salen(Co(III)) imprisoned within pores of metal-organic
framework by postsynthetic modification and its asymmetric
Received 00th January 20xx,
Accepted 00th January 20xx
catalysis for CO fixation under room temperature
2
DOI: 10.1039/x0xx00000x
Danping Chen, Ran Luo, Meiyan Li, Mengqi Wen, Yan Li, Chao Chen* and Ning Zhang*
www.rsc.org/
A new preparation strategy of chiral metal-orangic frameworks
CMOFs) has been demonstrated. By adsorption and then post-
(
synthetically modified (PSM) procedures, chiral Salen(Co(III)) can
be imprisoned within the cages of MOF and kept as its free form.
It is the first example of CMOFs to be successfully applied in
2
heterougeous asymmetric catalysis for coupling CO with epoxides
to optically active cyclic carbonates under the room temperature.
Scheme 1 Scheme description of imprisoning homogeneous chiral Salen(Co(III)) catalyst
within the cage of MOF by two steps.
The development of heterogeneous asymmetric catalysts has
attracted much attention in both academia and industry field,
particularly in the pharmaceutical industry in consideration of the
transformation, or have small cavities that the chiral functionality
could not be accessible to the reactants. Hence, in order to settle
this issue, in the third strategy, some typically robust, mesoporous
1
, 2
reusability and the convenience of products’ separation. Chiral
metal-organic frameworks (CMOFs), well-known porous materials
featured in chirality and infinite network structures built from
organic struts and inorganic nodes, are deemed to be one of the
most promising heterogeneous asymmetric catalysts on account of
their well-defined porosity, facile tunability, large surface area, and
1
3
MOFs, being post-synthetically modified (PSM) by chiral privileged
reagents which normally used in homogeneous asymmetric
1
4-16
catalysis, have been developed.
However, the chiral reagents
suffered from covalent or coordinated modification could not keep
their original/free form and might even decrease their catalytic
activities. Therefore, in the fourth strategy, the encapsulation of
homogeneous asymmetric catalysts within the porous matrix of
MOFs support has been attempted. Nevertheless, the leaching
problem of the chiral active regents frequently limits their
recyclability, and the only case exhibited better recyclability is the
3
, 4
mild synthetic condition.
Several well-established strategies have been employed to
construct CMOFs. In the first strategy, CMOFs are prepared from
5
-7
achiral components via self-resolution or chiral induction. As for
the chiral components are not needed or only needed small
amounts for chiral induction, such strategy can reduce the
synthesized cost. Unfortunately, most of these CMOFs are in fact
racemic because their bulk samples contain both enantiomorphs
1
7
so-called ‘bottle around the ship’ technique.
In search for efficient and recyclable asymmetric catalysts, we
herein demonstrated a new preparation strategy of CMOFs. As
illustrated in Scheme 1, the first step of preparation procedure
included the adsorption of classic homogeneous asymmetric
catalyst metal Salen complex (Jacobsen catalyst) into the nanocages
of MOFs. Then the free amino groups decorated in the framework
of the MOF were acylated by anhydride via PSM technique. After
such two steps, the metal Salen complex could be entrapped and
imprisoned within the cages of MOF since the acylamide groups in
the window could block its leaching as a ‘fence’. Compared with the
above CMOFs systems obtained by PSM or chiral ligand approaches,
no structural modification of the chiral active species was required
for as-prepared CMOFs and thereby the metal Salen complex could
be kept as its free form. Additionally, the small dimensional
(
opposite handedness), which restricts their application in
asymmetric catalysis. The second strategy is to directly use chiral
3, 8-12
ligands as linkers to self-assembly construct CMOFs.
Due to the
facile tenability of organic ligands, one can impart desirable chiral
functionality on them to achieve high activity and selectivity for
asymmetric catalysis. Indeed, most of CMOFs presented
asymmetric catalytic ability are prepared via this strategy. Yet, in
such case, many synthesized CMOFs have low chemical stability in
which the frameworks could not be sustained in the catalytic
reactants
could
freely
pass
through
the
window
and easily access the chiral catalytic center. The as-synthesized
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four cages of IRMOF-3 contained a Salen(Co(III)) complex. Given the
high activity of the Salen(Co(III)) catalyst, this small loading might be
preferred to minimize pore blocking and diffusion limitations which
1
7
lower the catalytic activity. The conversion of ca. 80% for the PSM
1
by the acetic anhydride was confirmed by H NMR spectroscopy
upon digestion of the sample in DCl/D
2
O/dimethyl sulfoxide
6
DMSO)-d (Fig. S5 for the detail). As expected, ICP analysis
2
3
(
suggested that the sample without the PSM treatment by the acetic
anhydride would leach the most of Salen(Co(III)) complexes after
3
the washing and soaking by CHCl solution. This in further indicated
that the amide could narrow the windows of the IRMOF-3 and the
Salen(Co(III)) was confined in the cages. Satisfactorily, the samples
of (R, R)-Salen(Co(III))@IRMOF-3-AM remained highly crystalline,
stable, and microporous after adsorption and PSM procedures, as
evidenced by powder XRD (PXRD, Fig. 1), thermal gravimetric
analysis (Fig. S6), and gas sorption analysis (Fig. S7). The Brunauer-
Emmett-Teller (BET) surface area of (R, R)-Salen(Co(III))@IRMOF-3-
Fig. 1 PXRD patterns of IRMOF-3 (a), fresh (R, R)-Salen(Co(III))@IRMOF-3-AM (b), and
used (R, R)-Salen(Co(III))@IRMOF-3-AM after three cycles for catalysis (c).
CMOFs were applied in the asymmetric catalysis for coupling CO
2
2
AM was determined to be ~475 m /g (based on two independent
with epoxides under the room temperature, and exhibited
expectable asymmetric catalytic activities and good recyclability.
IRMOF-3 was selected as the parent MOF. As shown in Fig. S1(a),
experiments), which was lower than that of the parent IRMOF-3.
This was not unexpected based on the introduction of bulky
Salen(Co(III)) and amide substituent into the pores.
4
its structure is made of Zn O tetranuclear clusters and connected by
The optical activity of the as-synthesized MOFs was confirmed by
solid-state circular dichroism (CD) spectroscopy with integrating
sphere. As shown in Fig. 2, there is no dichroism signal in IRMOF-3
since it does not have chiral nature. However, the observation
Cotton effect can be observed in the curve of (R, R)-
Salen(Co(III))@IRMOF-3-AM, which is consistent with the signal of
the powder sample of Salen(Co(III)) complex. It demonstrated that
9-21 the Salen(Co(III)) was intact during adsorption and the PSM
procedures and kept as its free form. In addition, (S, S)-
Salen(Co(III))@IRMOF-3-AM, the counterpart of (R, R)-
the benzene 1,4-dicarboxylic linker with a functional amino group,
to generate a cubic framework with the square channels. On
account of its rigid cubic framework, highly porous feature, and the
presence of amino groups, IRMOF-3 is a desirable material for the
penetration of the metal Salen complexes and postsynthetic
1
8
modification. Chiral Salen(Co(III)) complex was used as the
catalytic active center considering that it had been demonstrated to
1
be a highly efficient homogeneous enantioselective catalyst.
Recently, Lu et. al. used tBu-Salen(Co(III)) / quaternary ammonium
halide catalyst systems to catalyze the coupling reaction of CO
2
and
22
Salen(Co(III))@IRMOF-3-AM, was also synthesized. Those CD
spectra were nearly mirror images of each other, which indicated
the enantiomeric nature of the two materials.
racemic epoxides under the ambient temperature. The propylene
oxide (PO) conversion of 52.4% and propylene carbonate (PC) ee%
value of 42.9% are widely acceptable in view of the low chemical
2
reactivity of CO . Considering that the molecular size of tBu-
Salen(Co(III)) was larger than the window dimension of IRMOF-3,
the (R, R)-Salen(Co(III)) without tbu- group version was chosen
herein to be absorbed into the pore of IRMOF-3 (as shown in Fig.
S2(a)). Then, the acetic anhydride was added into the reaction
solution to produce the acetamide for confining the Salen(Co(III)) in
the cages. Finally, the products were rinsed by fresh CHCl
times and soaked in CHCl for 3 days with fresh CHCl added every
4 h, for which extensively rinsed the Salen(Co(III)) adhered the
outside surface of the solid and the unreacted alkyl anhydride.
The as-synthesized MOF, denoted as (R,
3
for three
3
3
2
R)-
Salen(Co(III))@IRMOF-3-AM, showed a color change from earthy
yellow of IRMOF-3 to dark brown (Fig. S3). Presumably, it indicated
that the (R, R)-Salen(Co(III)) had trapped into the cages of IRMOF-3,
evidenced by UV-Vis diffuse reflectance spectra (Fig. S4(a)). In
addition, the concentration of (R, R)-Salen(Co(III)) in the filtrate was
analyzed by UV-Vis spectroscopy. As shown in Fig. S4(b), the
intensity of the characteristic bands of the filtrate decreased Fig. 2 Solid-state CD spectra of IRMOF-3(black solid line), (R, R)-Salen(Co(III))@IRMOF-
3
-AM (blue solid line), (S, S)-Salen(Co(III))@IRMOF-3-AM (blue dash line), (R, R)-
sharply compared with that of the parent solution before the
adsorption, which indicated that most of (R, R)-Salen(Co(III)) were
trapped into the cage of IRMOF-3. Inductively coupled plasma (ICP)
analysis revealed the Zn/Co ratio of 32:1, suggesting that one out of
Salen(Co(III)) (red solid line), (S, S)-Salen(Co(III)) complex (red dash line), UMCM-1-NH
2
(
pink solid line) and tBu-(R, R)-Salen(Co(III))@UMCM-BM (green solid line).
2
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a
2
Table 1. Racemic epoxides / CO coupling results
diffusion difficulty of the reactants in the heterogeneous system.
Remarkably, ee% value was neither observed in the catalytic system
of the IRMOF-3 without (R, R)-Salen(Co(III)) active center, nor
presented in that of the (R, R)-Salen(Co(III))@IRMOF-3 without the
PSM treatment by acetyl anhydride (entry 6 and 7, Table 1). It
indicated that the majority of the adsorbed Salen(Co(III)) were
washed away form IRMOF-3 during the catalyst preparation as a
result of its large pore entrance size. In addition, IRMOF-3 gave
comparable PO conversion (ca. 20%) with (R, R)-
Salen(Co(III))@IRMOF-3. This transformation of epoxide to racemic
cyclic carbonates might be catalyzed either by the defects on the
surface and/or in the structure of the IRMOF-3, which also be
Conv. PC ee
Entry
Catalyst
Epoxides
(
%)
(%)
21
21
19
19
-18
0
1
2
3
4
5
6
7
8
9
(R,R)-Salen(Co)@IRMOF-3-AM
PO
PO
PO
PO
PO
PO
PO
PO
PO
25
26.5
26
26
24
20
18
31
32
8
b
(R,R)-Salen(Co)@IRMOF-3-AM(2nd)
(R,R)-Salen(Co)@IRMOF-3-AM(3rd)
(R,R)-Salen(Co)@IRMOF-3-AM(4th)
(S,S)-Salen(Co)@IRMOF-3-AM
IRMOF-3
2
8, 29
discussed in the literatures.
To evaluate the stability of the catalysts in such a liquid phase
reaction, the recyclability of (R, R)-Salen(Co(III))@IRMOF-3-AM has
been investigated. In the recycling experiments, after using in the
asymmetric cycloaddition reaction, the catalyst was washed with
PO three times and reused directly for the next run. PO conversions
and PC ee% for four repeated runs resulted in virtually constant
value (entry 2-4, Table 1.). The recovered catalyst was analyzed
with PXRD (Fig. 1), the result of which indicated that the catalyst
retained its structural identity. Moreover, the contents of Zn and Co
in the (R, R)-Salen(Co(III))@IRMOF-3-AM samples before and after
the reaction have been analyzed by ICP. The Zn/Co ratio of the
(R, R)-Salen(Co(III))@IRMOF-3
(R,R)-Salen(Co)
0
29
-31
<1
19
20
40
(S,S)-Salen(Co)
c
1
1
1
1
1
0
1
2
3
4
(R,R)-Salen(Co)@IRMOF-3-AM
(R,R)-Salen(Co)@IRMOF-3-AM
(R,R)-Salen(Co)@IRMOF-3-AM
tBu-(R, R)-Salen(Co(III))@UMCM-BM
tBu-(R, R)-Salen(Co(III))
SO
d
ECO
12
24
31
45
e
BO
PO
PO
f
41
a
Reaction was carried out in 50 mmol of neat epoxides, the catalysts (ca. 0.05
o
4 2
mmol equiv of Salen(Co)), 0.05 mmol of n-Bu NBr, 2.0 MPa CO at 25 C unless recovered sample was still 32:1, same with that of the fresh (R, R)-
otherwise noted. The quantitive analysis and configuration analysis of the
Salen(Co(III))@IRMOF-3-AM. This suggested that the catalyst had
maintained the catalytic active site after the catalytic reaction.
Therefore, it proved that (R, R)-Salen(Co(III))@IRMOF-3 could serve
as an good recoverable catalyst for the heterogeneous asymmetric
reaction products were conducted on Agilent 7890 GC with Agilent J&W Cyclosil
b
B Chiral capillary column (30m×0.25mm id ×0.25µm film). The recovered
c
d
e
f
catalyst. SO: styrene oxide. ECO: epichlorohydrin. BO: 1,2-epoxybutane. The
ee of 41% in our reaction system, but ee pf 42.9% reported in ref. [22].
catalysis from racemic epoxides and CO to optically active cyclic
2
carbonates. The performance of (R, R)-Salen(Co(III))@IRMOF-3 to
The chiral property of (R, R)-Salen(Co(III))@IRMOF-3-AM has asymmetrically catalyze CO reacting with other epoxides were also
2
been prompting us to explore its utilization as a heterogeneous studied under the room temperature. As summarized in entry 10-12,
asymmetric catalyst. Chemical fixation of CO is of great interest in Table 1, it showed reasonable asymmetric activities of ca. 20% ee
2
connection with the development of truly environmentally benign for the coupling reaction between CO and epichlorohydrin (ECO)
2
2
4
process and C1 chemistry.
One of the most promising (or epoxybutane (BO)). However, only 8% conversion and less 1% ee
methodologies in this area is the synthesis of cyclic carbonates via were observed for styrene oxide (SO). We supposed that the bulky
coupling of CO and epoxides. Direct synthesis of optically active size and steric hindrance of the reactant and product declined the
2
cyclic carbonates from the racemic epoxides is especially to be rate of diffusion over the cages of the catalyst, finally resulting in
expected for its value as precursors for pharmaceutical and the low asymmetric catalytic activities.
2
2, 25, 26
biomedical intermediates.
dozens of examples for the synthesis of the racemic cyclic CMOFs, the other MOF with a different topology, UMCM-1-NH2
carbonates that catalyzed by MOFs under the crucible condition (UMCM
University of Michigan Crystalline Material) was
To the best of our knowledge,
In order to further demonstrates this preparation strategy of
=
2
7
have been documented. As the synthetic condition of mild employed for the second parent MOF. Since UMCM-1-NH2
temperature is essential for avoiding the racemization of products, possesses a larger cage than IRMOF-3 (ca. 3.2 nm pore dimension of
the direct synthesis of optically active cyclic carbonates catalyzed by UMCM-1-NH , as shown in Fig. S1(b) and S2), it can facilely
2
MOFs is scarce. In view of the high activity of chiral Salen(Co(III)), it accommodate the tBu-(R, R)-Salen(Co(III)) with bigger molecular
is strongly believed that (R, R)-Salen(Co(III))@IRMOF-3-AM could size. Due to its more efficient enantioselective activity than that of
active CO and further asymmetrically catalyze such reaction under formal (R, R)-Salen(Co(III)) without tBu group, tBu-(R, R)-
2
the room temperature. As listed in table 1, (R, R)- Salen(Co(III)) imprisoned in the MOF might exhibit higher
Salen(Co(III))@IRMOF-3-AM gave an acceptable PO conversion of enantioselective activity than that of (R, R)-Salen(Co(III))@IRMOF-3-
2
5% and PC ee of 21%. The counterpart (S, S)- AM. The final product, denoted as tBu-(R, R)-Salen(Co(III))@UMCM-
Salen(Co(III))@IRMOF-3-AM showed the corresponding PO BM, was successfully prepared via two steps of the adsorption and
conversion of 24% and PC –ee of 18%. It can be manifested that the PSM, the procedures of which were similar with the preparation of
catalytic activity of those heterogenous catalysts was slightly lower (R, R)-Salen(Co(III))@IRMOF-3-AM. It was noteworthy that the PSM
than that of the homogeneous counterpart (R, R)-Salen(Co(III)) reagent was n-Butyric anhydride rather than acetic anhydride
(
31% conversion and 29% ee, entry 8 and 9, Table 1), owing to the because the cage window of UMCM-1-NH was so large that the
2
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acetamide group generated from PSM could not block the leaching 14.
W. T. Zhu, C. He, P. Y. Wu, X. Wu and C. Y. Duan, Dalton
Trans., 2012, 41, 3072-3077.
of the tBu-(R, R)-Salen(Co(III)). In consideration of this, the n-Butyric
1
5.
M. Banerjee, S. Das, M. Yoon, H. J. Choi, M. H. Hyun, S. M.
Park, G. Seo and K. Kim, J. Am. Chem. Soc., 2009, 131,
anhydride was chosen to generate butyramide group, for the fact
that its long branch could well confine the tBu-(R, R)-Salen(Co(III))
7
524-7525.
molecular into the cage of UMCM-1-NH
for the PSM by the n-Butyric anhydride was confirmed by H NMR
spectroscopy upon the digestion of the sample in DCl/D O/dimethyl
sulfoxide (DMSO)-d (Fig. S8 for the detail).The product (R, R)-
2
. The conversion of >99%
1
16.
N. Candu, M. Tudorache, M. Florea, E. Ilyes, F. Vasiliu, I.
Mercioniu, S. M. Coman, I. Haiduc, M. Andruh and V. I.
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Commun., 2013, 49, 8021-8023.
2
6
1
7.
Salen(Co(III))@IRMOF-3-AM were further convinced by the various
characterizations such as Solid-state CD spectra (Figure 2 and Fig.
S9), PXRD (Fig. S10), TG (Fig. S11), ICP analysis, and dinitrogen 18.
M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter, M.
O'Keeffe and O. M. Yaghi, Science, 2002, 295, 469-472.
E. N. Jacobsen, Acc. Chem. Res., 2000, 33, 421-431.
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adsorption (Fig. S12), and then used as the asymmetric catalyst for
1
2
9.
0.
2
coupling CO with PO under the room temperature. As listed in
Entry 13, Table 1, it exhibited higher asymmetric activity than that
of (R, R)-Salen(Co(III))@IRMOF-3-AM. Its PC ee of 40% was even
comparable with its homogeneous counterpart (PC ee of 41%, Entry
4
043.
P. Miao, G. Li, G. Zhang and H. Lu, J. Energy Chem., 2014,
3, 507-512.
2
2
1.
2.
2
14, Table 1).
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Bai, H. Wang and R. Zhang, J. Am. Chem. Soc., 2004, 126,
In conclusion, the chiral metal Salen can be imprisoned in the
cage of MOFs and kept as its free form by two steps of the
adsorption and PSM procedures. The as-prepared catalyst exhibits
3
732-3733.
2
3.
C. Chen, C. A. Allen and S. M. Cohen, Inorg. Chem., 2011,
50, 10534-10536.
2
heterogeneous asymmetric activities for CO fixation under the
room temperature and could be easily recycled without significant 24.
T. Sakakura, J.-C. Choi and H. Yasuda, Chem. Rev., 2007,
107, 2365-2387.
loss of catalytic performance. This strategy could be generally
2
2
2
2
2
5.
6.
7.
8.
9.
X.-B. Lu and D. J. Darensbourg, Chem. Soc. Rev., 2012, 41,
applicable for CMOFs design and heterogenization of homogeneous
catalyst.
1
462-1484.
X.-B. Lu, W.-M. Ren and G.-P. Wu, Acc. Chem. Res., 2012,
5, 1721-1735.
This research was financed by the National Natural Science
Foundations of China (No. 21261017, 21561020 and 21661020) and
Jiangxi Province (No. 20153BCB23021, 20133ACB20001, and
4
M. H. Beyzavi, C. J. Stephenson, Y. Liu, O. Karagiaridi, J. T.
Hupp and O. K. Farha, Front. Energy Res., 2015, 2.
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Han, Green Chem., 2009, 11, 1031-1036.
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20132BAB213004). Danping Chen and Ran Luo contributed equally
to this work.
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Graphical and textual abstract
Salen(Co(III)) imprisoned within pores of metal-organic
framework by postsynthetic modification and its asymmetric
catalysis for CO fixation under room temperature
2
Danping Chen, Ran Luo, Meiyan Li, Mengqi Wen, Yan Li, Chao Chen*
and Ning Zhang*
Department of Chemistry, Nanchang University, Nanchang, 330031, P. R. China.
Fax: +86 791 83969514; Tel: +86 791 83779427;
E-mail: chaochen@ncu.edu.cn (C. Chen). nzhang.ncu@163.com (N. Zhang)
By two steps of adsorption and then post-synthetically modified (PSM) procedures,
chiral Salen(Co(III)) can be imprisoned within the cages of MOF and kept as its free
form. The as-prepared catalyst exhibits heterogeneous asymmetric activities for
coupling CO with epoxides to produce optically active cyclic carbonates under the
2
room temperature