Inclusion complexes of organic salts with β-cyclodextrin as organocatalysts for CO2 cycloaddition with epoxides

Article abstract of DOI:10.1039/c7ra00416h

The inclusion complexes between β-cyclodextrin (β-CD) and the organic bases 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU)-based phenolates have been prepared by a simple method and also characterized by FT-IR, ¹H NMR, ¹⁹F NMR, TGA etc. Among these inclusion complexes, DBU-based 2,3,4,5,6-pentafluorophenolate as a guest compound bound by β-CD ([DBUH][PFPhO]/β-CD) has been employed as an easily-separable organocatalyst for the cycloaddition of CO₂ into cyclic carbonate and exhibited the best catalytic performance. High conversion of epoxides and excellent selectivity to carbonates could be achieved at 130 °C and under the 3.0 MPa CO₂ without additional organic solvents or additives. Additionally, the organocatalyst [DBUH][PFPhO]/β-CD exhibited the better recycability in consecutive catalytic recycles, as compared with that of corresponding DBU-based phenolates. The β-CD played a crucial role in immobilizing catalytically active species and thus improving the recyclability of the present organocatalysts. The detailed characterization indicated that phenolate anions had been bound inside the cavity of β-CD, while the [DBUH]⁺ cation was located outside of β-CD. More interestingly, it was observed that phenolate anions could dissociate from the β-CD cavity under the reaction temperature, but the inclusion compound could form on cooling the reaction mixture after reaction, which was extremely attractive for separation and recycling of the supramolecular organocatalysts. Finally, on the basis of the characterization above, a reaction mechanism for the present organocatalysts has been proposed.

Full text of DOI:10.1039/c7ra00416h

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Inclusion complexes of organic salts with b-
cyclodextrin as organocatalysts for CO2
Cite this: RSC Adv., 2017, 7, 14721
cycloaddition with epoxides†
Kun Li, Xiaohui Wu, Qingwen Gu, Xiuge Zhao, Mingming Yuan, Wenbao Ma, Wenxiu Ni
*
and Zhenshan Hou
The inclusion complexes between b-cyclodextrin (b-CD) and the organic bases 1,8-diazabicyclo-[5.4.0]
undec-7-ene (DBU)-based phenolates have been prepared by a simple method and also characterized
by FT-IR, 1H NMR, 19F NMR, TGA etc. Among these inclusion complexes, DBU-based 2,3,4,5,6-
pentafluorophenolate as a guest compound bound by b-CD ([DBUH][PFPhO]/b-CD) has been employed
as an easily-separable organocatalyst for the cycloaddition of CO2 into cyclic carbonate and exhibited
the best catalytic performance. High conversion of epoxides and excellent selectivity to carbonates
could be achieved at 130 ꢀC and under the 3.0 MPa CO2 without additional organic solvents or additives.
Additionally, the organocatalyst [DBUH][PFPhO]/b-CD exhibited the better recycability in consecutive
catalytic recycles, as compared with that of corresponding DBU-based phenolates. The b-CD played
a crucial role in immobilizing catalytically active species and thus improving the recyclability of the
present organocatalysts. The detailed characterization indicated that phenolate anions had been bound
inside the cavity of b-CD, while the [DBUH]+ cation was located outside of b-CD. More interestingly, it
was observed that phenolate anions could dissociate from the b-CD cavity under the reaction
temperature, but the inclusion compound could form on cooling the reaction mixture after reaction,
which was extremely attractive for separation and recycling of the supramolecular organocatalysts.
Finally, on the basis of the characterization above, a reaction mechanism for the present organocatalysts
has been proposed.
Received 11th January 2017
Accepted 1st March 2017
DOI: 10.1039/c7ra00416h
rsc.li/rsc-advances
A broad variety of homogeneous catalysts for the synthesis of
cyclic carbonates have been developed and reported so far, such
as quaternary ammonium and phosphonium salts,4 bromine,5
Introduction
Carbon dioxide (CO2) is a greenhouse gas that induces global
warming and climate change. Meanwhile, it always can be
regarded as an easily available and renewable carbon resource,
which is mainly due to its advantages of being abundant, cheap,
non-toxic, and nonammable in CO2 capture and conversion.1
Transformation of CO2 into highly valuable chemicals is of
great importance and has attracted increasing attention. One of
the most attractive synthetic goals starting from carbon dioxide
is the chemical xation of CO2 onto epoxide to afford the ve-
membered cyclic carbonates,2 such products are among the
most important feedstocks which have been extensively used as
aprotic polar solvents, electrolyte components in lithium
batteries, precursors for polycarbonates, and intermediates in
organic synthesis.3
ionic liquid,6 triphenylphosphine-phenol,7 metalloporphyrins8
and polyoxometalate.9 However, these catalysts generally suffer
from low to moderate catalytic activity, selectivity and poor
recyclability. Besides, a co-solvent/additive and harsh reaction
conditions such as high pressure and/or high temperature are
required in some cases. Meanwhile, the inherent corrosion,
toxicity and environmental problems associated with metallic
cations and halide anions are also concerned from the view-
point of green chemistry. Thus, the development of a highly
efficient, metal and halide ions-free environment-friendly
catalytic system for the cycloaddition of carbon dioxide with
epoxide are highly desirable.10 Meanwhile, the recovery of
homogeneous catalyst from products could also be a non-
ignorable defect which may result in the difficulty for further
product purication and catalyst reuses.11
Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis,
School of Chemistry & Molecular Engineering, East China University of Science and
Technology, Shanghai 200237, People's Republic of China. E-mail: houzhenshan@
ecust.edu.cn
By contrast, heterogeneous catalysts, due to its convenience
of separation and recovery, have received more attention and
extensive application than homogeneous analogues in the
catalytic synthesis of cyclic carbonates. From this point, a series
of inorganic oxides and organic polymers supported catalysts
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra00416h
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have been developed. For examples, these supports include b-CD played a crucial role in immobilizing catalytically active
mesoporous silica,12 polymers,13 graphene oxide,14 carbon species and thus improving the recyclability of the supramo-
nanotubes,15 chitosan (CS)16 etc. The synergistic effect of lecular organocatalysts. Furthermore, the reaction mechanism
hydroxyl groups of solid materials on promoting the reaction over the present supramolecular catalysts has been proposed on
has also been unveiled.
the basis of the detailed characterization.
The supramolecular chemistry played a signicant role in
many elds, while the supramolecular catalysis has recently
emerged and been well developed.17 The main strategy is to
assemble building blocks by weak interactions to elaborate new
Results and discussion
Characterizations of catalysts
catalytic systems, which could avoid the modication of the First, the different kinds of salts were synthesized by simply
catalyst backbone by means of multistep synthesis as usually neutralizing phenol derivatives with DBU in tetrahydrofuran
required to improve the catalytic performance of covalent cata- (THF). The b-CD was chosen as host molecules because it is
lysts. Especially, b-cyclodextrin (b-CD) has been used as a host highly thermally stable and owns abundant hydroxyl groups.
molecule to construct the supramolecular catalysts with the aid Through the hydrogen bond interaction, DBU-based phenolates
of the interactions of the host–guest type. b-CD are cyclic oligo- were thus included by b-CD in aqueous solution (Scheme 2).
saccharides connected by a-1,4 linkages (Scheme 1), frequently
In the rst step, a series of DBU-based phenolate salts were
characterized as a doughnut-shaped truncated cone.18 The rstly characterized by FT-IR spectra. As shown in Fig. 1b–f,
primary hydroxy groups (OHs) of glucose units are located on the aer the neutralization reaction, the characteristic peaks at
narrower rim of the cone, and the secondary OHs are on the wide 3600 cmꢁ1 of hydroxyl groups on phenol and other phenol
one. The b-CD's external surface is hydrophilic to make them be derivatives disappeared, and the new peaks at 2935 cmꢁ1 can be
soluble in water, while their hydrophobic cavity can host a variety assigned to the stretching vibration of N+–H.22 The peaks at
of guest organo-molecules leading to the formation of guest/b-CD 1646 cmꢁ1 and 2854 cmꢁ1 could be attributed to C]N+ and
inclusion complexes. Actually, inclusion complexes between b- C–H vibration on protonated DBU, respectively. The peak at
CD and various guest molecules have been employed as efficient 1499 cmꢁ1, 1593 cmꢁ1 could be assigned to C]C vibration on
catalysts and studied extensively.19
benzene ring.22–24 1H NMR spectra of DBU-based phenolates
On the other hand, organic Lewis base like 1,8-diazabicyclo- have been depicted in Fig. S1,† the signal of hydroxyl hydrogen
[5.4.0]-undec-7-ene (DBU), 7-methyl-1,5,7-triazabicyclo[4.4.0] in phenol moved to downeld aer the salts were formed (9–12
dec-5-ene (MTBD), 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) ppm). Additionally, the amidinium carbon in the DBU structure
and 1,4-diazabicyclo[2.2.2]octane (DABCO), etc., showed some moved to downeld from 161.8 ppm to 178.5 ppm (13C NMR
activity for chemical xation of CO2 into cyclic carbonates and spectra shown in Fig. S2†), as compared with that of free DBU (d
were regarded as an environmental-friendly catalysts due to the ¼ 161.5 ppm),25 revealing that DBU has been protonated aer
absence of halogen ions.20,21 However, these organic base- neutralization.23 All these results proved that the different kinds
derived materials normally acted as homogeneous catalysts, of DBU-based salts have been attained unambiguously.
resulting in difficult recovery of the catalysts. Taking into
Because all these DBU-based phenolates served as homoge-
account of the drawbacks of the homogeneous base catalysts, in neous catalysts in cycloaddition reaction of CO2 as shown from
our previous work, we have found that niobate salts of organic our experimental observation, we next have prepared the
base were highly efficient catalysts for CO2 cycloaddition, and inclusion complexes of DBU-based salts and b-CD with the aid
the catalyst was stable in the consecutive recycles.22 As far as we of the interactions of the host–guest type. The 1H NMR spectra
have known, the efficient synthesis of cyclic carbonate by using of all inclusion complexes was shown in Fig. S3.† Before the
the metal-free heterogeneous catalysts still remains a huge inclusion, the chemical shis of H-30 and H-50 protons of b-CD
challenge and is highly promising. Herewith, we attempted to were 3.60 and 3.56, respectively. Having included the DBU-
prepare inclusion complexes of DBU-based phenolates with b- based phenolates, the chemical shis of H-30 and H-50 moved
CD by a simple method and to use as heterogeneous organo- to 3.59 and 3.55. As shown in Scheme 1, the H-30 and H-50 were
catalysts for the cycloaddition of carbon dioxide with epoxide located inside the cavity of b-CD and formed the inner wall in it,
without additional solvents and additives. It was found that the when the guest molecules or the groups such as benzene rings
on guest molecules were included in the cavity of b-CD, the
shielding effect of these molecules could changed the chemical
shis of H-30 and H-50,26 but the H-20, H-40 and H-60 out of the
cavity still remained unchangeable. It was worth noticing that
the chemical shis of all protons on [DBUH]+ cation in all salts
did not undergo any obvious changes, while the resonance
signal from H6 protons in phenolates anions disappeared aer
inclusion (Fig. S1 vs. S3†). Moreover, the NMR signal from the
protons of OH-20 and OH-30 in b-CD moved to higher eld and
became boarder aer inclusion (Fig. S3a–S3f†), which might
result from the hydrogen bonding interaction between the OH-
Scheme 1 Chemical structures of b-CD.
14722 | RSC Adv., 2017, 7, 14721–14732
20 and OH-30 protons and [DBUH]+ cation. From these results
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Scheme 2 The synthetic route of b-CD inclusion complexes.
Fig. 1 The FT-IR spectra of (a) b-CD; (b) [DBUH][PhO]; (c) [DBUH][p-
FPhO]; (d) [DBUH][o-FPhO]; (e) [DBUH][p-NO2-PhO]; (f) [DBUH]
[PFPhO]; (g) [DBUH][PFPhO]/b-CD; (h) the recovered [DBUH][PFPhO]/
b-CD organocatalyst after recycling.
Fig. 2 19F NMR spectra of (a) [DBUH][PFPhO]; (b) [DBUH][PFPhO]/b-
CD. The concentration of [DBUH][PFPhO] in d6-DMSO was
0.054 mmol mLꢁ1
.
b-CD on [DBUH][PFPhO].19b What's more, from the 13C NMR
spectra (Fig. S4b and S4c†), aer inclusion the 13C NMR signals of
guest anion [PFPhO]ꢁ increased varying from 0.3 ppm to
0.8 ppm. Furthermore, the para carbon signals of phenolate
anions disappeared nearly (Fig. S4c†), while all protons in
[DBUH]+ did not show any obvious changes in chemical shis
aer inclusion (Fig. S4b and S4c†). This had been explained in
previous work that carbon atoms of guest molecules which are
deeper in the cavity seem to undergo shielding effects with b-
CD.26 Additionally, if the concentration of [DBUH][PFPhO]/b-CD
increased further, the 13C NMR signals did not showed much
change (Fig. S4d and S4e†), but the 19F NMR signals of guest
anion [PFPhO]ꢁ changed considerably (Fig. S5a and S5b†). All
these results also suggested that the b-CD had included the
[DBUH][PFPhO] successfully. Most likely, [DBUH]+ cation was
located outside of b-CD, while the hydrophobic phenol deriva-
tives as the guest molecules was bound inside the cavity of b-CD.
The thermal stability of the b-CD and the inclusion complex
catalysts [DBUH][PFPhO]/b-CD were investigated by TGA. As
shown in Fig. S6,† b-CD showed a slightly weight loss below
above, it suggested that the inclusion complexes between b-CD
and DBU-based phenolates have been formed.
Sequentially, taking inclusion complexes formed between
[DBUH][PFPhO] with b-CD as an example, FT-IR, 19F NMR and
13C NMR provided an additional evidence of inclusion
compound formation. As shown in Fig. 1a, b-CD exhibited the
characteristic peaks at 3373 cmꢁ1 (–OH), 2926 cmꢁ1 (C–H stretch)
(too closing to overlapped with the peaks of N+–H on protonated
DBU), 1160 cmꢁ1 (C–O–C stretch/O–H bend), 1250–1370 cmꢁ1
(C–H bend), 1029 cmꢁ1 (C–O stretch). Aer inclusion (Fig. 1f and
g), the catalyst still remains the characteristic peaks of b-CD,
which indicated that the skeleton structure of b-CD remained
untouched. The peaks at 2935 cmꢁ1 (N+–H stretch) and 1646
cmꢁ1 (C]N+ vibration) on DBU seems unchanged aer inclu-
sion. It should be noted that the peak at 1593 cmꢁ1 (C]C on
benzene ring) disappeared and the peak at 1499 cmꢁ1 (C]C on
benzene ring) became weaker aer inclusion. Meanwhile, 19F
NMR spectra (500 MHz) of [DBUH][PFPhO]/b-CD was shown in
Fig. 2. It was found that the chemical shis of uorine atoms on
the para, ortho and meta position changed from ꢁ192.67,
ꢁ172.08 and ꢁ171.31 to ꢁ192.27, ꢁ171.98 and ꢁ170.86 respec-
tively aer inclusion, which suggested the deshielding effects of
ꢀ
100 C, which could be attributed to the removal of the adsor-
bed H2O on the surface of b-CD. The b-CD started to decompose
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ꢀ
aer 300 C, which indicated its highly thermal stability. The considerably by using different DBU-based phenolates. Inter-
salt [DBUH][PFPhO] appeared an obviously weight loss at estingly, with the decrease of pKa value of the phenolate anions
150 ꢀC, aer inclusion with b-CD, the decomposition onset within catalysts (Table S1†), the catalytic activity was increased
temperature was around 200 ꢀC, indicating that the thermal (Fig. 3). It indicated that the stronger acidity of phenol deriva-
stability of [DBUH][PFPhO] was improved due to the formation tives really benets the reaction. Previous reports have sug-
of inclusion complexes. Scanning electron microscopy (SEM) gested that the parallel requirement of both Lewis base and
was also employed to observe the morphology of b-CD, [DBUH] Lewis acid in the xation of CO2.27 As a result, the increasing in
[PFPhO] and [DBUH][PFPhO]/b-CD, and their SEM images were acidity may promote the activation of CO2.28 From this stand-
shown in Fig. S7†. Compared with pure b-CD, the inclusion point, the salt [DBUH][PFPhO] formed with DBU and 2,3,4,5,6-
compounds have a lower dispersity (Fig. S7a and 7d vs. S7c and pentauorophenol owning the lowest pKa value of 5.50 showed
7f†). Modication of crystals can be added as a proof of the the highest catalytic activity under the present condition. On
formation of a solid inclusion complexes with hydrophobic the other hand, it was worth noticing that 2,3,4,5,6-penta-
cavity of b-CD.
uorophenol had CO2-philic properties, which presumably
facilitated CO2 conversion with increasing activity.29
Since the DBU-based phenolate catalyst was miscible with
cyclic carbonate aer reaction, leading to the decient recovery
of catalyst. In contrast, it was found that the inclusion
complexes as organocatalysts not only were separated easily
only by standing and ltration aer reaction but also even
afforded higher catalytic activity, in comparison with that of the
corresponding DBU-based phenolate catalysts (Fig. 3), which
was probably due to abundant hydroxyl on b-CD's surface.30
Moreover, it has been reported that the acidity of guest mole-
cules (here are phenol derivatives) bound to b-CD was consid-
erably higher than that of the corresponding free one,31 which
could result in stronger acidity aer inclusion in the present
situation and thus benets the reaction as well. Among all these
organocatalysts, [DBUH][PFPhO]/b-CD was chosen as a main
catalyst in the following investigation owing to its excellent
catalytic performance.
Catalyst performance
In the previous work, DBU has been proved as a well homoge-
neous catalyst for the cycloaddition reaction of CO2 with epox-
ides.20 In this work, for the sake of comparison, we synthesized
a serial of salts containing DBU and different kinds of phenol
derivatives, and their inclusion complexes of b-CD, which all
have been employed as our catalysts. The cycloaddition reaction
of propylene oxide (PO) and CO2 to propylene carbonate (PC)
was chosen as a model reaction to evaluate the catalytic
performance under the optimized reaction conditions. The
catalytic activities over these catalysts have been displayed in
Fig. 3. When only use the b-CD as catalyst, it can be seen that
only PO conversion of 29.8% was attained and 1,2-propanediol
was formed as a main product without any formation of cyclic
carbonates. When DBU and phenol were used as homogeneous
catalysts, the conversion of PO was 80% and 8.5%, respectively.
However, it was found that the catalytic activity was improved
Inuence of reaction parameters
All probable effects of reaction condition (temperature, CO2
pressure and reaction time and catalyst amount) were consid-
ered and investigated over the [DBUH][PFPhO]/b-CD catalyst.
Firstly, the effect of the reaction temperature has been exam-
ined and the results were shown in Fig. 4a. With the increment
of temperature from 90 ꢀC to 130 ꢀC, the conversion of PO
increased signicantly from 20% to 96%. However, the PO
conversion leveled off under higher reaction temperature.
Consequently, it was suggested that 130 ꢀC was the appropriate
temperature for the cycloaddition reaction.
Then the impact of the CO2 pressure on cycloaddition reac-
tion of PO was studied and depicted in Fig. 4b. The conversion
of PO was increased in some extent with the reaction pressure
changing from 1.5–3.0 MPa. It could possibly be explained that
more CO2 can be dissolved in PO as increasing reaction pres-
sure,6e which facilitated the CO2 cyclic addition reaction.
However, when the reaction pressure continued to rise to
6.0 MPa, the conversion of PO has a slight decrease while the
selectivity has no obvious change. This might be related to
a dilution effect occurring that the excess CO2 reduced the
concentration of the catalyst in the vicinity or hinder the active
mass transport, which was also be observed in our group's
previous studies.22,32,33 The impact of reaction time under the
condition of 130 ꢀC and 3.0 MPa can also be obtained in Fig. 4c.
Fig. 3 The effect of different catalysts on the conversions of cyclo-
addition reaction. Reaction conditions: PO 0.7 mL (10 mmol), orga-
nocatalyst (1.5 mmol%), 130 ꢀC, 3 MPa. All catalysts showed over 98%
selectivities toward cyclic carbonates except that b-CD afford 1,2-
propanediol as a main by-product.
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Fig. 4 The effects of reaction temperature, pressure and time on conversion and selectivity of cycloaddition reaction. Reaction condition: PO
0.7 mL (10 mmol); organocatalyst 1.5 mmol%. (a) CO2 pressure 3 MPa; time 10 h; (b) temperature 130 ꢀC; time 10 h; (c) temperature 130 ꢀC; CO2
pressure 3 MPa.
The conversion of PO had a gradual increasing from 2 to 10 h the experimental data assuming pseudo-rst order reaction
and then remained almost unchanged with high selectivity all kinetics. The kinetic parameters were studied over tempe-
time. The substrate was almost completely converted into PC rature ranged from 110 to 140 ꢀC. The relationship of
within 10 h with high selectivity.
At last, we investigated the effect of the catalyst amount to different temperature was shown in Fig. S8.† Finally, as shown
the reaction and the results were shown in Table S2.† With the in Fig. 5, the activation energy (Ea) for the process was calcu-
ꢁln(1 ꢁ Con.) and PO remaining with reaction time at
increasing of the catalyst amount from 1 mmol% to 1.5 mmol%, lated using the Arrhenius equation, where Ea of [DBUH]
1
the conversion of PO had an obvious improvement. The [PFPhO]/b-CD was 43.1 kJ molꢁ1 while the Ea of [DBUH]
2
possible explanation is that increasing the catalytic activity [PFPhO] was 59.9 kJ molꢁ1. It can be seen that from this results
center indeed benets a lot to the reaction. However, the further that the inclusion of b-CD indeed owned lower Ea for the
increasing of catalyst amount leads to a sharp descent. Due to process and really beneted the cycloaddition reaction of CO2
the signicant molecular weight of b-CD, the excessive amount with epoxides. The activation energy under the present
of catalyst may be harmful to the mass transfer of the reactant conditions is consistent with that of the other many reports, in
and then decrease the conversion of PO. Thus, it was apparent which Ea was in the range of about 35–70 kJ molꢁ1
.
34
that the appropriate catalyst amount was 1.5 mmol%.
The substrate scope of the catalyst
Reaction kinetics
The results above demonstrated that the inclusion complexes of
Sequentially, the reaction kinetics of catalysts [DBUH] organic salt [DBUH][PFPhO] with b-CD was an highly efficient
[PFPhO]/b-CD and [DBUH][PFPhO] was investigated respec- organocatalyst for the cycloaddition of CO2 and PO without any
tively. The procedure for the studies of the kinetic parameters organic solvents and additives. Thus, a series of epoxides were
was similar to our group's previous work.33 PO (0.7 mL, 10 tested to synthesize the corresponding carbonates under opti-
mmol), catalysts (1.5 mmol%) was loaded into autoclave, CO2 mized conditions to check the scope of the substrates over the
was introduced to the reaction and kept at 3.0 MPa under the present catalyst and the results were shown in Table 1. It was
reaction temperature. The rate constant was determined from found that the organocatalyst was applicable to producing
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Fig. 5 Arrhenius plots for the CO2 cycloaddition to PC catalyzed by (a) [DBUH][PFPhO]/b-CD; (b) [DBUH][PFPhO].
cyclic carbonates from CO2 and various epoxides with high
conversions and selectivities (Table 1, entries 1–6). Compared
with other epoxides, 1,2-epoxyhexane and 1,2-epoxycyclohexane
needed a longer reaction time to produce corresponding cyclic
carbonates because of the higher steric hindrance (Table 1,
entries 5 and 6).
Catalyst stability
In the next step, the reusability of catalyst [DBUH][PFPhO]/b-CD
was examined under the optimal conditions. Aer the reaction,
the anhydrous ether was added to the reaction mixture. The
catalyst was recovered by a simple standing and ltration, fol-
lowed by washing with ethyl ether in 3 times and vacuum drying
at 60 ꢀC. Then the recovered catalyst can be reused for the next
recycle. The reusability of [DBUH][PFPhO]/b-CD catalyst was
shown in Fig. 6. It was found that the catalyst could be reusable
for at least up to four times with high product selectivity, while
the conversion of PO had a slight decrease in the consecutive
recycles. Subsequently, the FT-IR of the reused [DBUH][PFPhO]/
b-CD catalysts was displayed in Fig. 1h. Compared with the
fresh catalyst, aer recycling, the catalyst still remains the
characteristic peaks of b-CD, which indicated that the skeleton
Table 1 Different epoxides as substrates for the cycloaddition reac-
tion with carbon dioxide catalyzed by [DBUH][PFPhO]/b-CDa
Entry
Substrates
Products
Con. (%)
Sel. (%)
1
98.7
99.1
2
3
98.8
95.6
99.5
97.9
99.8
99.2
4
5b
94.8
81.7
99.1
6c
76.8d
a
Reaction conditions: PO 0.7 mL (10 mmol), organocatalyst (1.5
b
c
mmol%), 130 ꢀC, 3 MPa, 10 h. Reaction time: 30 h. Reaction time:
Fig. 6 Recyclability of [DBUH][PFPhO]/b-CD. Reaction conditions: PO
0.7 mL (10 mmol), organocatalyst (1.5 mmol%), 130 ꢀC, 3 MPa.
45 h. d The main by-product was corresponding diol.
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structure of b-CD remained unchangeable. The peak at 1646 catalyst remain unchangeable, compared with its initial spectra
cmꢁ1 (C]N+ vibration) and the peak at 1499 cmꢁ1 (C]C on in 25 C (Fig. 7d). The change of chemical shis in 50 C and
benzene ring) were still visible in the spite of the intensity was 70 ꢀC may caused by the overowing of guest [DBUH][PFPhO]
decreased, which may caused by slightly leaching of [DBUH] from the b-CD's cavity. Aer cooled down the solution back to
[PFPhO] from the cavity of b-CD. Additionally, the new band at 25 ꢀC, the [DBUH][PFPhO] moved into cavity and kept its initial
1792 cmꢁ1 were assigned to the C]O stretching of the residual state. This revealed that organic salt [DBUH][PFPhO] might
ꢀ
ꢀ
propylene carbonate over catalyst surface.22,35
dissociate from cavity of b-CD under the reaction temperature,
but inclusion compound formed as cooling the reaction mixture
aer reaction, which was extremely attractive for separation and
recycling of the supramolecular organocatalysts.
Reaction mechanism
As we have mentioned above, the [DBUH][PFPhO] and b-CD can
form the inclusion complexes by means of the host–guest inter-
action. In order to further investigate the chemical state of
[DBUH][PFPhO] bound in the inclusion complexes during the
reaction, the 19F NMR spectra (500 MHz) of the [DBUH][PFPhO]/
b-CD had been determined at elevated temperature by simulating
reaction condition. The organocatalyst was dissolved in d6-
DMSO, and then the 19F NMR spectra was recorded in 25 ꢀC,
50 ꢀC and 70 ꢀC in turn. Aer that, the solution was cooled down
to 25 ꢀC and recorded the spectra of the catalyst. The results were
shown in Fig. 7. It was found that at 25 ꢀC, the chemical shis of
uorine atom on para, ortho and meta position was ꢁ192.27,
ꢁ171.98 and ꢁ170.86, respectively (Fig. 7a). While the tempera-
ture was raised to 50 ꢀC and 70 ꢀC, the chemical shis of all
uorine atoms moved slightly to the downeld, where was
ꢁ192.94, ꢁ172.21, ꢁ171.17 and ꢁ192.98, ꢁ172.24 and ꢁ171.17,
respectively (Fig. 7b and c). Increasing the concentration [DBUH]
[PFPhO]/b-CD in d6-DMSO, the chemical shis of all uorine
atoms moved to the downeld more obviously (Fig. S5a vs. S5c†).
When the solution cooled down back to 25 ꢀC, the spectra of
In order to further gain a deeper insight into the reaction
mechanism, DRIFTS of in situ PO adsorption was performed
over the [DBUH][PFPhO]/b-CD in 298 K. As shown in Fig. 8a, the
peaks at 1423, 1409 and 1399 cmꢁ1 can be assigned to C–H
bending vibration of gas-phase PO molecules.33 Aer PO was
introduced and purging with Ar, it was observed that the peaks
of gas-phase PO molecules disappeared, and the new peaks at
1458 and 1452 cmꢁ1 were formed (Fig. 8b and c). But these
peaks were disappeared with keeping purging with Ar more
longer time (Fig. 8d), indicating weak interaction of PO mole-
cules with organocatalyst at room temperature. The new peaks
could be assigned to C–O bending vibration of a probable
intermediate species forming between PO and [DBUH][PFPhO]
by hydrogen bond (species 2 in Scheme 3). This result suggested
that the [DBUH][PFPhO] acted a crucial role in activating PO
molecules. Next, the DRIFT of in situ CO2 adsorption over the
[DBUH][PFPhO]/b-CD was performed and the results have been
listed in Fig. 9. The gas CO2 exhibited its anti symmetric
stretching vibration peaks at 2367 and 2344 cmꢁ1 (Fig. 9a and
b), respectively.36 When the CO2 was introduced on [DBUH]
[PFPhO]/b-CD, the peak intensity was increased. But with
purging with Ar, no absorption peaks can be detected (Fig. 9h),
which suggested that CO2 could only physically absorbed on the
[DBUH][PFPhO]/b-CD and showed weak absorption ability on
catalyst at room temperature. Meanwhile, it should be noticed
Fig. 7 The 19F NMR spectra (500 MHz) of the [DBUH][PFPhO]/b-CD Fig. 8 DRIFT spectra of gas-phase PO (a) and in situ PO adsorbed on
(0.054 mmol mLꢁ1 in d6-DMSO) at different temperature. (a) 25 ꢀC; (b) [DBUH][PFPhO]/b-CD samples followed purging with the flow of Ar
50 ꢀC; (c) 70 ꢀC; (d) cooled down back to 25 ꢀC.
(50 mL minꢁ1), (b) 3 min; (c) 6 min; (d) 9 min.
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Scheme 3 The proposed reaction mechanism for cycloaddition of epoxide and CO2 catalyzed by [DBUH][PFPhO]/b-CD catalyst.
that when we introduced the PO and CO2 on [DBUH][PFPhO]/b- it can be deduced that the existence of the adsorbed PO
CD simultaneously, only CO2 peaks could be observed and the can promote the CO2 absorption on [DBUH][PFPhO]/b-CD
peak from PO adsorption was too weak to be observable. organocatalyst.
Furthermore, we can easily found that the peak intensity
From what has been discussed above, we may draw the
became much stronger (Fig. 9d and e vs. g). From these results, conclusion that 2,3,4,5,6-pentauorophenol had CO2-philic
properties, and it may capture the CO2 and forms a critical
intermediate species during the reaction. Thus we attempted
to characterize this probable critical intermediate species
formed with [PFPhO]ꢁ anion and CO2. A stainless autoclave
was charged with [DBUH][PFPhO] (336 mg, 1.0 mmol) and
CO2 (1.0 MPa), and the mixture was stirred at a constant
temperature (60 ꢀC) for 24 h. Carefully release of CO2 gave
a white powder. The product was characterized by 13C NMR
spectra, a new signal at 159.2 ppm produced aer the
absorption of CO2 (Fig. S9b†), which can be assigned to
carbonate carbon.37 Based on the previous reports, and
observed product, a plausible intermediate species formed
with [PFPhO]ꢁ anion and CO2 can be proposed (Scheme S1†),
which exhibits the interaction between the electronegative
oxygen on [PFPhO]ꢁ and CO2 to form the intermediate
[PFPhO-CO2]ꢁ in CO2 cycloaddition reaction.38 Another new
signal at 129.4 ppm could be assigned to the gas CO2 which
was dissolved in DMSO.39 The solvent of CO2 adduction with
[DBUH][PFPhO] was vacuumed and the 13C NMR spectra was
Fig. 9 DRIFT spectra of (a) baseline; (b) gas-phase CO2; (c) gas-phase
CO2 on [DBUH][PFPhO]/b-CD; (d) in situ CO2 adsorbed on [DBUH]
[PFPhO]/b-CD samples with purging CO2 for 5 min; (e) for 15 min; (f)
gas-phase CO2 and PO on [DBUH][PFPhO]/b-CD; (g) in situ CO2 and
PO co-adsorbed on [DBUH][PFPhO]/b-CD samples with purging CO2;
(h) all samples purging with Ar (50 mL minꢁ1) for 5 min.
recorded again. As shown in Fig. S9c,† it was found that the
peaks at 159.2 and 129.4 ppm disappeared completely, which
indicated that the peaks at 129.4 ppm and 159.2 ppm were
highly relevant to CO2 adduction.
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Based on the results we have obtained, we proposed were also used without further purication. CO2 was supplied
a reasonable mechanism for this chemical xation reaction of by Shanghai Shangnong Gas Factory with a purity of >99.95%.
CO2 (Scheme 3). As the temperature increasing, the guest All kinds of epoxides were analytic grade and were used directly
compound [DBUH][PFPhO] dissociated from the cavity of b-CD without further purication. All solvents (analytic grade) were
under the reaction temperature, and could immobilize on the dried by using the methods that have been previously reported.
surface through the hydrogen bond. The proton in DBU-based
salts was coordinated with the oxygen of the epoxide through
a hydrogen bond, resulting in the activation of an epox-
ide,2c,4h,12b,28 and this is followed by CO2 insertion to form the
intermediate species [PFPhO-CO2]ꢁ bounding with the [DBUH]+
and PO through the hydrogen bond. Simultaneously, the
nucleophilic attack of [PFPhO-CO2]ꢁ on the less sterically
hindered b-carbon atom of the epoxide furnished the ring-
opened intermediate species, and then the intermediate
species made a subsequent intramolecular ring-closure, cyclic
carbonate was formed. With the temperature cooled down aer
reaction, the guest compound [DBUH][PFPhO] could be back to
the cavity of b-CD and the catalyst can be regenerated. It should
be noticed that the hydroxyl groups of the glucopyranose
monomers (such as cellulose and b-CD etc.) act as hydrogen
bond donor and thus activate the ring-opening reaction of
epoxides through the hydrogen bond on the oxygen atom of the
epoxide.20,30,40 In the present catalyst system, we could regard b-
CD as an additional activator for the ring-opening of epoxide
and thus promoted the cycloaddition of carbon dioxide with
epoxide.
Characterizations
1H NMR and 13C NMR spectra were recorded on a Bruker
AVANCE 400 MHz instrument (400 MHz 1H NMR), and 19F NMR
spectra were recorded on a Bruker AVANCE 500 MHz instru-
ment (500 MHz 19F NMR). The concentration of samples in
deuterated solvent was 0.054 mmol mLꢁ1, unless indicated
otherwise. The FT-IR spectra were recorded at RT on a Nicolet
Magna 550 FT-IR spectrometer. The thermal stability of cata-
lysts was determined by Thermogravimetry analysis (TGA)
method (heating rate: 10 ꢀC minꢁ1; N2 ow: 100 mL minꢁ1
)
using Perkin Elmer Pyris Diamond Analyzer. The scanning
electron microscopy (SEM) images were performed on JSM
electron microscopes (JEOL JSM-6360LV, Japan). In situ diffuse
reectance infrared Fourier transform spectroscopy (DRIFTS)
experiments were conducted on a Nicolet 6700 FTIR tted with
a liquid nitrogen cooled mercury-cadmium-telluride detector
(MCT). The DRIFTS cell (Harrick, HVC-DRP) tted with CaF2
windows were used as the reaction chamber that allowed
samples to be heated to 150 ꢀC. All the spectra were within the
range of 4000–1200 cmꢁ1 at the resolution of 4 cmꢁ1 and 64
scans. Prior to the adsorption experiments, the samples were
pretreated in Ar at 150 ꢀC for 4 h. Then the samples were cooled
down to 25 ꢀC in order to remove the contaminants and the
PO/CO2-adsorbed FT-IR spectra of the samples (40 mg) were
Conclusions
In summary, we have developed a series of DBU-based pheno-
lates and their inclusion complexes with b-CD to act as the
supramolecular organocatalysts in chemical xation of CO2 into
cyclic carbonates. High epoxides conversion and excellent
cyclocarbonates selectivity can be achieved under the optimal
reaction conditions. Among these catalysts, the organocatalyst
ꢀ
recorded at 25 C.
Preparation of DBU-based phenolates
Preparation of [DBUH][PFPhO]. The DBU-based 2,3,4,5,6-
[DBUH][PFPhO]/b-CD afforded the best catalytic performance. pentauorophenolate ([DBUH][PFPhO]) was prepared by
Furthermore, it can be easily separated aer reaction and neutralizing method. 1,8-Diazabicyclo-[5.4.0]undec-7-ene (DBU)
reused with high activity and selectivity. Based on all charac- (5 mmol, 0.75 mL) and 2,3,4,5,6-pentauorophenol (5 mmol,
terization, we proposed a reasonable mechanism for this reac- 0.92 g) were solved in 15 mL dried THF and stirred at 40 ꢀC for
tion, where b-CD played a crucial role in immobilizing 4 h in 100 mL Schlenk ask equipped with a magnetic stirrer
catalytically active species and thus improving the recyclability under N2 atmosphere. Aer the mixture was cooled to room
of the supramolecular catalyst. This easily-prepared, metal and temperature under nitrogen atmosphere, the solvent in the
halogen ion-free, inexpensive and recyclable catalytic system resulting mixture was evaporated under vacuum and then dried
ꢀ
has potential application for catalytically synthesizing cyclic under the vacuum at 60 C for 2 h to obtain the white powder
carbonates from CO2 and epoxides.
(Fig. S1, S4b,† and 2a). 1H NMR (400 MHz, d6-DMSO): d 1.63 (m,
6H), 1.91 (m, 2H), 2.70 (d, J ¼ 10.4 Hz, 2H), 3.24 (t, J ¼ 5.7 Hz,
2H), 3.47 (t, J ¼ 5.8 Hz, 2H), 3.54 (m, 2H), 11.15 (s, 1H); 13C NMR
(100 MHz, d6-DMSO): d 19.0, 23.5, 26.0, 28.3, 31.2, 37.7, 47.8,
53.1, 124.2, 126.4, 137.1, 139.5, 141.8, 146.0, 165.2; 19F NMR
Experimental
Chemical materials
b-Cyclodextrins (b-CDs) was purchased from Sinopharm (500 MHz, d6-DMSO): d ꢁ192.67, ꢁ172.08, ꢁ171.31.
Chemical Reagent Co., Ltd., Shanghai. Organic bases 1,8- Preparation of [DBUH][PhO]. The DBU-based phenolate
diazabicyclo-[5.4.0]undec-7-ene (DBU) was also purchased from ([DBUH][PhO]) was prepared with the similar method but
Sinopharm Chemical Reagent Co., Ltd., and directly used replacing the 2,3,4,5,6-pentauorophenol with phenol (5 mmol,
without any further purication. Phenol and its other kinds of 0.47 g) to obtain the pale yellow liquid (Fig. S1 and S2†). 1H
substituted derivatives such as 2,3,4,5,6-pentauorophenol, 4- NMR (400 MHz, d6-DMSO): d 1.56 (m, 6H), 1.71 (m, 2H), 2.38
uorophenol, 2-uorophenol and 4-nitrophenol were all (m, 2H), 3.13 (t, J ¼ 5.3 Hz, 2H), 3.24 (m, 4H), 6.59 (t, J ¼ 7.2 Hz,
purchased from Shanghai Macklin Biochemical Co., Ltd., and 1H), 6.67 (d, J ¼ 8.1 Hz, 2H), 7.06 (t, J ¼ 7.6 Hz, 2H), 9.19 (s, 1H);
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13C NMR (100 MHz, d6-DMSO): d 21.4, 25.1, 27.5, 28.8, 34.5, 3.65 (dd, 14H, H-60), 4.82 (d, J ¼ 3.4 Hz, 7H, H-10), 5.1–5.5 (m,
41.6, 47.5, 52.1, 116.0, 116.2, 129.0, 160.6, 161.8.
14H), 6.66 (m, 2H), 7.10 (t, 2H).
Preparation of [DBUH][p-FPhO]. The DBU-based 4-uo-
[DBUH][p-FPhO]/b-CD. 1H NMR (400 MHz, d6-DMSO): d 1.57
rophenolate ([DBUH][p-FPhO]) was prepared with the similar (m, 6H), 1.73 (p, J ¼ 5.8 Hz, 2H), 2.38 (d, J ¼ 9.0 Hz, 2H), 3.12 (t, J
method but replacing the 2,3,4,5,6-pentauorophenol with 4- ¼ 5.5 Hz, 2H), 3.24–3.26 (m, 11H), 3.32 (m, 7H, H-40), 3.55 (m,
uorophenol (5 mmol, 0.56 g) to obtain the pale yellow liquid 7H, H-50), 3.59 (m, 7H, H-30), 3.65 (dd, 14H, H-60), 4.82 (d, J ¼
(Fig. S1 and S2†). 1H NMR (400 MHz, D2O): d 1.54 (m, 6H), 1.83 (p, 3.4 Hz, 7H, H-10), 5.1–5.5 (m, 14H), 6.63 (m, 2H), 6.90 (m, 2H).
J ¼ 5.8 Hz, 2H), 2.44 (d, J ¼ 9.0 Hz, 2H), 3.14 (t, J ¼ 5.5 Hz, 2H),
[DBUH][o-FPhO]/b-CD. 1H NMR (400 MHz, d6-DMSO): d 1.57
3.35 (m, 4H), 6.41 (dd, J ¼ 7.5, 5.6 Hz, 2H), 6.74 (t, J ¼ 8.9 Hz, 2H), (m, 6H), 1.82 (p, J ¼ 5.9 Hz, 2H), 2.41 (d, J ¼ 9.7 Hz, 2H), 3.18 (t, J
10.73 (s, 1H); 13C NMR (100 MHz, d6-DMSO): d 21.2, 25.1, 27.4, ¼ 5.5 Hz, 2H), 3.24–3.26 (m, 11H), 3.32 (m, 7H, H-40), 3.55 (m,
28.8, 34.2, 41.2, 47.6, 52.2, 115.1, 116.5, 152.8, 155.1, 157.7, 162.2. 7H, H-50), 3.59 (m, 7H, H-30), 3.65 (dd, 14H, H-60), 4.82 (d, J ¼
Preparation of [DBUH][o-FPhO]. The DBU-based 2-uo- 3.4 Hz, 7H, H-10), 5.1–5.5 (m, 14H), 6.41 (m, 1H), 6.64 (dd, J ¼
rophenolate ([DBUH][o-FPhO]) was prepared with the similar 12.9, 4.7 Hz, 1H), 6.74 (m, 1H), 6.86 (m, 1H).
method but replacing the 2,3,4,5,6-pentauorophenol with 2-
[DBUH][p-NO2-PhO]/b-CD. 1H NMR (400 MHz, d6-DMSO):
uorophenol (5 mmol, 0.56 g) to obtain the deep yellow liquid d 1.64 (m, 6H), 1.91 (m, 2H), 2.64 (d, J ¼ 8.0 Hz, 2H), 3.24 (s,
(Fig. S1 and S2†). 1H NMR (400 MHz, d6-DMSO): d 1.53 (m, 6H), 2H), 3.30 (m, 7H, H-20), 3.47 (s, 2H), 3.32 (m, 7H, H-40), 3.55 (m,
1.77 (p, J ¼ 5.9 Hz, 2H), 2.51 (d, J ¼ 9.7 Hz, 2H), 3.16 (t, J ¼ 5.5 Hz, 7H, H-50), 3.59 (m, 7H, H-30), 3.65 (dd, 14H, H-60), 4.47 (t, J ¼
2H), 3.32 (m, 4H), 6.31 (m, 1H), 6.73 (m, 2H), 6.86 (m, 1H), 11.98 5.6 Hz, 7H, OH-60), 4.83 (d, J ¼ 3.4 Hz, 7H, H-10), 5.5–6.0 (m,
(s, 1H); 13C NMR (100 MHz, d6-DMSO): d 20.3, 24.4, 26.9, 28.6, 14H), 6.03 (d, J ¼ 8.5 Hz, 2H), 7.76 (d, J ¼ 8.5 Hz, 2H).
32.8, 47.7, 52.6, 112.8, 120.0, 119.4, 124.2, 152.5, 154.7, 163.4.
[DBUH][PFPhO]/b-CD. 1H NMR (400 MHz, d6-DMSO): d 1.64
Preparation of [DBUH][p-NO2-PhO]. The DBU-based 4- (m, 6H), 1.91 (m, 2H), 2.64 (d, J ¼ 10.2 Hz, 2H), 3.24 (t, J ¼
nitrophenolate ([DBUH][p-NO2-PhO]) was prepared with the 5.7 Hz, 2H), 3.47 (t, 2H), 3.53–3.55 (m, 9H), 3.59 (m, 7H, H-30),
similar method but replacing the 2,3,4,5,6-pentauorophenol 3.65 (m, 14H, H-60), 4.48 (s, 7H, OH-60), 4.83 (d, J ¼ 3.4 Hz, 7H,
with 4-nitrophenol (5 mmol, 0.70 g) to obtain the light yellow H-10), 5.73 (s, 7H, OH-30), 5.79 (d, J ¼ 6.4 Hz, 7H, OH-20); 13C
powder (Fig. S1 and S2†). 1H NMR (400 MHz, d6-DMSO): d 1.63 NMR (100 MHz, d6-DMSO): d 19.4, 23.9, 26.4, 28.7, 32.0, 38.1,
(m, 6H), 1.91 (m, 2H), 2.65 (d, J ¼ 8.0 Hz, 2H), 3.25 (s, 2H), 3.47 48.3, 53.8, 60.4, 72.5, 72.9, 73.6, 82.0, 102.4, 137.6, 139.9, 142.2,
(s, 4H), 6.06 (d, J ¼ 8.5 Hz, 2H), 7.78 (d, J ¼ 8.5 Hz, 2H), 10.10 (s, 146.1, 165.8; 19F NMR (500 MHz, d6-DMSO): d ꢁ192.27,
1H); 13C NMR (100 MHz, d6-DMSO): d 18.9, 23.4, 25.9, 28.2, ꢁ171.98, ꢁ170.86 (Fig. 2b).
31.6, 37.6, 47.8, 53.3, 118.7, 127.2, 129.6, 165.3, 178.5.
Purication of b-cyclodextrins. The commercial b-CD needs
to be re-crystallization before it was used as the support. The
method as followed have been reported previously: b-CD (10 g)
was added in 50 mL deionized water and heated until the b-CD
uate the catalytic performance. The catalyst (0.15 mmol) and PO
was completely solved. And then remove the impurities instantly
by ltration, the ltrate was stirred vigorously in ice bath to get the
Reaction of cycloaddition from CO2 and peroxides
The cycloaddition reaction of propylene oxide (PO) and CO2 to
propylene carbonate (PC) was used as a model reaction to eval-
(10 mmol) were added into the glass bush, and then put the bush
into the 50 mL stainless autoclave and then sealed it. The loading
of supramolecular organocatalyst was based on that of DBU-
based phenolates for the reaction. The reactor was purged with
hydrate b-CD. The above operation was repeated three times and
dried the hydrate b-CD at 80 ꢀC under the reduced pressure. The
puried sample was stored in ask under the N2 atmosphere
(Fig. S3a and S4a†). 1H NMR (400 MHz, d6-DMSO): d 3.30 (m, 7H,
CO2 for 3 times to remove the air into the reactor, and nally
charged with CO2 to the desired pressure. Aer the reaction, the
H-20), 3.35 (m, 7H, H-40), 3.56 (m, 7H, H-50), 3.60 (m, J ¼ 5.4 Hz,
reactor was cooled in ice-water bath immediately, and the CO2
was slowly vented while anhydrous ether was as absorption liquid
to absorb the PO and products entrained by CO2. Then, the
anhydrous ether in the trap was added to the reactor and washed
7H, H-30), 3.65 (dd, J ¼ 10.1, 7.5 Hz, 14H, H-60), 4.47 (t, J ¼ 5.6 Hz,
7H, OH-60), 4.83 (d, J ¼ 3.4 Hz, 7H, H-10), 5.68 (d, J ¼ 2.3 Hz, 7H,
OH-30), 5.74 (d, J ¼ 6.9 Hz, 7H, OH-20); 13C NMR (100 MHz, d6-
DMSO): d 59.9, 72.0, 72.4, 73.0, 81.5, 101.9.
the catalyst to extract products and substrate. The catalyst was
separated simply by standing and ltration, and the ltrate was
Preparation of inclusion complexes
subjected to GC-MS and GC analysis. The catalyst was washed
with anhydrous ether 3 times and dried in a vacuum at 60 ꢀC to
be used directly in next reaction.
The inclusion complexes were prepared by mixing the DBU-
based phenolates (1 mmol) and b-CD (1 mmol, 1.135 g) in
aqueous solution at 60 ꢀC for 6 h under the N2 atmosphere.
Aer the solution was cooled to room temperature, the water
was removed by rotary evaporation to get the solid powder.
Acknowledgements
Then drying the powder at 70 ꢀC for 2 h under the vacuum The authors are grateful for support from the National Natural
afforded the inclusion complexes as white powder (Fig. S3†).
Science Foundation of China (21373082) and innovation
[DBUH][PhO]/b-CD. 1H NMR (400 MHz, d6-DMSO): d 1.56 Program of Shanghai Municipal Education Commission
(m, 6H), 1.72 (m, 2H), 2.40 (m, 2H), 3.13 (t, 2H), 3.24–3.26 (m, (15ZZ031), and the Fundamental Research Funds for the
11H), 3.32 (m, 7H, H-40), 3.55 (m, 7H, H-50), 3.59 (m, 7H, H-30), Central Universities.
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