Construction of an Asymmetric Porphyrinic Zirconium Metal-Organic Framework through Ionic Postchiral Modification

Article abstract of DOI:10.1021/acs.inorgchem.0c02811

Herein, one kind of neutral chiral zirconium metal-organic framework (Zr-MOF) was reported from the porphyrinic MOF (PMOF) family with a metallolinker (MnIII-porphyrin) as the achiral polytopic linker [free base tetrakis(4-carboxyphenyl)porphyrin] and chiral anions. Achiral Zr-MOF was chiralized through the exchange of primitive anions with new chiral organic anions (postsynthetic exchange). This chiral functional porphyrinic MOF (CPMOF) was characterized by several techniques such as powder X-ray diffraction, Fourier transform infrared spectroscopy, ultraviolet-visible spectroscopy, 1H NMR, energy-dispersive X-ray spectroscopy, scanning electron microscopy, and Brunauer-Emmett-Teller measurements. In the resulting structure, there are two active metal sites as Lewis acid centers (Zr and Mn) and chiral species as Br?nsted acid sites along with their cooperation as nucleophiles. This CPMOF shows considerable bimodal porosity with high surface area and stability. Additionally, its ability was investigated in asymmetric catalyses of prochiral substrates. Interactions between framework chiral species and prochiral substrates have large impacts on the catalytic ability and chirality induction. This chiral catalyst proceeded asymmetric epoxidation and CO2 fixation reactions at lower pressure with high enantioselectivity due to Lewis acids and chiral auxiliary nucleophiles without significant loss of activity up to the sixth step of consecutive cycles of reusability. Observations revealed that chiralization of Zr-MOF could happen by a succinct strategy that can be a convenient method to design chiral MOFs.

Full text of DOI:10.1021/acs.inorgchem.0c02811

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Article
Construction of an Asymmetric Porphyrinic Zirconium Metal−
Organic Framework through Ionic Postchiral Modification
Kayhaneh Berijani and Ali Morsali*
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ABSTRACT: Herein, one kind of neutral chiral zirconium metal−organic
framework (Zr-MOF) was reported from the porphyrinic MOF (PMOF)
family with a metallolinker (MnIII-porphyrin) as the achiral polytopic linker
[free base tetrakis(4-carboxyphenyl)porphyrin] and chiral anions. Achiral
Zr-MOF was chiralized through the exchange of primitive anions with new
chiral organic anions (postsynthetic exchange). This chiral functional
porphyrinic MOF (CPMOF) was characterized by several techniques such
as powder X-ray diffraction, Fourier transform infrared spectroscopy,
1
ultraviolet−visible spectroscopy, H NMR, energy-dispersive X-ray spec-
troscopy, scanning electron microscopy, and Brunauer−Emmett−Teller
measurements. In the resulting structure, there are two active metal sites as
Lewis acid centers (Zr and Mn) and chiral species as Brønsted acid sites
along with their cooperation as nucleophiles. This CPMOF shows
considerable bimodal porosity with high surface area and stability. Additionally, its ability was investigated in asymmetric catalyses
of prochiral substrates. Interactions between framework chiral species and prochiral substrates have large impacts on the catalytic
ability and chirality induction. This chiral catalyst proceeded asymmetric epoxidation and CO2 fixation reactions at lower pressure
with high enantioselectivity due to Lewis acids and chiral auxiliary nucleophiles without significant loss of activity up to the sixth step
of consecutive cycles of reusability. Observations revealed that chiralization of Zr-MOF could happen by a succinct strategy that can
be a convenient method to design chiral MOFs.
environmental considerations with less energy consumption
such as the coupling of CO2 and epoxide or hydrocarbon
oxidation to useful materials. Because of the importance of
energy and environmental topics along with green chemistry,
herein we report on our investigations based on green
chemistry principles with an asymmetric direction. Before a
further explanation is provided, it should be said that our work
is divided into several steps as follows: (1) Types of catalytic
reactions: hydrocarbon oxidation and CO2 fixation are the
most significant methods that are used to remove dangerous
compounds, but they are commonly carried out under harsh
conditions. (2) Choice of catalyst: among the catalysts, metal−
organic frameworks (MOFs) as porous platforms are
appropriate candidates because of their structural properties.
They are constructed through strong bonds between organic
and inorganic sections that can be various models, and they
can act in different fields.1−4 The tunability in the components
INTRODUCTION
■
Nowadays, in industrialized societies, the preservation of the
environment and reduction of energy utilization are very
important issues because, unfortunately, there are various
sources that produce pollutants that are harmful not only for
the environment but also for human health, particularly
undesirable and unwanted chemical compounds in different
forms such as liquid, solid, and gas [for instance, hydrocarbons
(aromatic and aliphatic) and carbon dioxide (CO2)]. There-
fore, chemical transformations should help to convert these
environmental threats through chemical, photochemical,
biological, and electrochemical processes. It should be said
that these transformations are in the direction of green
chemistry ideology: elimination of pollution or hazardous
substances and reduction of energy consumption. Each day
new materials as catalysts are generated and used in chemical
conversion. The relationship between catalysis and the catalyst
structure is an essential factor. In this regard, there is one fact
that the most prevalent transformations are related to the
conversion of achiral hazardous and toxic substances to achiral
favorable products, for example, the transformation of CO2 to
CO, HCO2H, and CH4 or methanol (MeOH): the conversion
of benzene to phenol and water splitting are the most
complicated. However, there are many asymmetric chemical
transformations as appealing methods under economical and
Received: September 21, 2020
https://dx.doi.org/10.1021/acs.inorgchem.0c02811
© XXXX American Chemical Society
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Scheme 1. General Concept of the Acidic Centers Used with the Considered Chiral Products
of MOFs has led to recent developments in the synthesis of
these porous materials with novel designs.5 The vast range of
existing MOFs are electrically neutral with the usual building
blocks, intriguing properties, and different uses like separation
and sensing, photo- and electrocatalysis, gas sorption, and
optical activity.6−11 Ionic MOFs (IMOFs) are the other kinds
of MOFs that can be classified into two types: anionic or
cationic. They are similar to neutral MOFs, and they have
several uses as ion-exchange materials with different
applications, in particular, separation, adsorption, and catalysis
(gas or ion separation/adsorption and oxidative processes),
dye removal, drug delivery, and proton transfer.12−14 It is
surprising that the modulators used in the synthesis of MOFs,
specifically acidic types, can also produce defects that can act as
active centers. However, chirality, its relationship with MOFs,
and why it is significant are important questions for
researchers. Chirality is one of the important and essential
features that can exist in MOFs as either neutral or ionic. This
phenomenon is specifically revealed in diverse stereoselective
processes, especially in biochemistry to generate enantioen-
riched molecules, so it is very valuable.15 Chiral MOFs
(CMOFs) are the other kinds of modified MOFs whose
building blocks can be synthesized by chiral components,
functionalized by chiral agents, or constructed intrinsically
because of asymmetric space groups in the network. The
investigations demonstrate that using the pure chiral modifier
in design through a postsynthetic method is an effective
procedure without complexity in the creation of CMOFs
rather than others.16 Until now, utilizing chiral molecules has
been explored for suitable ability in chirality induction in
MOFs such as chiral postsynthetic modification of either a
metal node or an organic linker.17 Recently, these kinds of
chiral materials have attracted attention as hot topics because
they have all of the applications of simple MOFs with an
asymmetric direction, especially asymmetric catalysis.18,19
Among MOFs, zirconium-based MOFs (Zr-MOFs) to a
considerable extent were synthesized with specific topologies
via different organic linkers and connectivities with various
applications20−23 because they have chemical, mechanical, and
thermal stabilities with higher surface area and porosity than
the customary MOFs. It should not be forgotten that
zirconium is widely abundant at low cost.24 Most of the
organic linkers used in Zr-MOFs are bidentate carboxylic acids,
but they have been newly modified with multitopic linkers
such as hexatopic and tetratopic linkers, specifically by Zhou et
al. In this paper, we selected a topology, among them PCN-224
[6-c(she)], with a porphyrin−tetratopic linker as the main
platform to synthesize porphyrinic MOF (PMOF) because this
framework has easily passed every test.25,26 However, the
various chiral materials were extensively synthesized to
different cases by porphyrinic ligands, and few chiral Zr-
MOF have also been reported, but for the first time, we
reported a chiral porphyrinic Zr-MOF (CPMOF) with
removal of coordinated anions by tartrate anions.27−30 This
PMOF backbone based on Zr6 clusters (with defects such as
free centers from anions or weak coordination of anions to
metal sites), large channels (higher than 15 Å), and accessible
pores was chiralized via coordinated anion exchange. The
results showed that it can be a unique and promising chiral
porous platform, which has not been reported in the Zr-MOF
family. The porphyrinic metallolinkers in PMOFs can be active
as catalytic centers in the various transformations to optical-
related applications.31,32 Inspired by the chemical stability and
structural features of the afforded CPMOF and its extra-
ordinary chirality, we studied its catalytic capability in
enantioselective reactions. In this MOF, manganese tetrakis-
(4-carboxyphenyl)porphyrin [Mn(TCPP)] units are used as 4-
connected linkers, so they can simplify the substrate influence
in pores during the catalytic process.33 Besides, notably the
resulting structure consists of chiral tartrates that provide the
high chirality induction and acidic feature. We suggest that
chiral anions interact with the cationic framework because of
the exchange of chloride anions in the structure [Cl sources:
ZrCl4 and (Cl)Mn-porphyrin] to retain the charge neutrality of
the framework. Decoration of PMOF with specific chiral
molecules is necessary for improvement of the design. Our
selected chiral compound is a tartrate anion that bears two
effective neighboring asymmetric centers with two OH
functional groups (Brønsted acid sites). Plenty of them in
the resulting chiral architecture improve the chirality induction.
In this work, we presented the construction of a chiral PCN-
224 through postchiral modification, which is effective in
synthesizing chiral functional MOFs. This ideal CMOF has
advantages such as (i) having Zr clusters and Mn active sites,
(ii) the majority of the defects of the framework being formed
by modulators, and (iii) chiral species that can act as acidic
centers (Scheme 1). Considering the present CPMOF, the
relationship of structure chirality and defect activity is clearly
seen. Additionally, chiral ligands can show a nucleophilic role
in the considered catalytic system (CO2 fixation into
epoxides). So, the results demonstrated that designing with
this degree of precision can be valuable to the development of
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Hot Filtration. This general test was studied for both of the
catalytic reactions: enantioselective epoxidation and cycloaddition of
epoxide with CO2. The chiral Zr-MOF catalyst was removed after
reactions after 2 and 3 h, respectively. Then, the solution was stirred
under optimal reaction conditions without a catalyst.
CMOF chemistry. This CPMOF with these performances and
without any complication was used in two catalytic reactions in
an asymmetric direction: epoxidation of olefins and CO2
cycloaddition into epoxides, which are still important and
problematic, because the obtained products, epoxides and
cyclic carbonates, can be used as proper intermediates or
precursors in a wide range of syntheses.32 Because of these
reactions in the organic syntheses, oxygen and carbon trapping
in the considered substrates can happen, so they are
transformed into useful compounds. (3) Catalytic reaction
conditions: ambient temperature and pressure (according to
the kind of catalytic reaction) and determination of a real-time
reaction to prevent hazardous byproduct formation lead us to
the obtained results in the next sections.
RESULTS AND DISCUSSION
■
According to our goal, we synthesized an neutral asymmetric
PMOF as a chiral platform with a tetratopic ligand [manganese
(chloride) tetrakis(4-carboxyphenyl)porphyrin: Mn(Cl)-
TCPP]. The subsequent reaction in PCN-224(Mn(Cl)) was
the stripping of Cl− by replacement of tart− by adding a silver
salt of tartrate. Tartrate ligands are also able to interact with Cl
of Mn-porphyrin(s) and or most probably with unsaturated Zr
cluster(s); therefore, chiral PCN-224-Mn(tart) can easily be
formed postsynthetically. This catalyst could act as a chiral Zr-
MOF, whereas each of the MnIII-porphyrin, Zr cluster, and
general framework was shielded by anionic chiral auxiliary
ligands. For the first time, this work has been reported to
synthesize this kind of chiral porphyrinic Zr-MOF as a
significant perspective for preparing multipurpose chiral
porous materials. In the preparation of this CMOF, the
limitations were not observed, unlike other chiralization routes.
Chirality induction to the framework can happen through a
chiral transcription method (from tartrate ions to MOF).
Keeping in mind that chirality and asymmetric reactions have
important roles in many scientific fields, especially biosystems
to produce optically pure compounds, we designed an
unprecedented CMOF for asymmetric catalysis. Our system
can be known as a heterobimetal material and an achiral
electrophile−chiral nucleophile catalyst with unique features in
both asymmetric epoxidation and CO2 fixation. Two metallic
centers, Mn and Zr can act as Lewis acids/electrophiles and a
tartrate counterion as the nucleophile can act as chiral
functions and Brønsted acid, too.
EXPERIMENTAL SECTION
■
Syntheses. Tetrakis(4-carboxyphenyl)porphyrin (H2TCPP).
H2TCPP was prepared according to the procedure described in the
research paper.34 Briefly, a mixture of the following compounds was
stirred (4 h/120 °C): pyrrole (1.16 mmol), 4-carboxybenzaldehyde
(1.16 mmol), and 50 mL of propionic acid. The precipitated crystals
were separated with MeOH and by using of ice bath and washed with
the essential solvents (MeOH/hot H2O). Then, a purple solid was
purified through column chromatography with silica gel as the
stationary phase and chloroform with a little acetone as the mobile
phase.
Mn(TCPP)Cl. The metalated free-base porphyrin was prepared
under the explained conditions:35 0.082 mmol of TCPP, 0.82 mmol
of MnCl2, and 20 mL of N,N-dimethylformamide (DMF) were
refluxed for over 1 h. After removal of DMF, precipitated Mn(TCPP)
Cl was dissolved and reprecipitated by using NaOH (0.1 M) and HCl
(1 M), respectively. After recrystallization of the Mn-porphyrin, it was
dried and characterized.
PCN-224-MnIII(tart). PCN-224, PCN-224(Mn), and silver tartrate
were synthesized based on a previous report.26,27 Briefly, PCN-224
was synthesized in a 5 mL Pyrex vial without any Mn-porphyrin by
using ZrCl4, free-base porphyrin, and benzoic acid (30, 10, and 400
mg, respectively) in DMF (2 mL; 24 h/120 °C). Then, the obtained
crystals were filtered and investigated. In the synthesis of PCN-
224(Mn), in addition to the Zr source and benzoic acid, Mn(TCPP)
was also employed (10, 250, and 10 mg, respectively). After ultrasonic
dissolution of the mentioned mixture in a 5 mL Pyrex vial, it was
placed in an oven with the specific conditions (2 mL of DMF, 24 h/
120 °C). The synthesis of PCN-224-Mn(tart) was performed by
adding silver tartrate to a suspension of the heterogeneous mixture
[PCN-224-Mn(Cl)] in MeOH. Then, it was stirred for over 8 h under
ambient conditions. For removal of the impurity, washing of the
obtained PCN-224-Mn(tart) was done with DMF (3 × 2 mL) and
MeOH (5 × 2 mL) several times. Finally, after solvent exchange with
MeOH, it was dried at 80 °C.
Structural Characterization. PCN-224 as a porphyrinic
Zr-MOF was prepared by using the previously reported
procedure, and it was extensively investigated and charactrized
by different methods.26 Fourier transform infrared (FT-IR)
was utilized as a primitive strategy for investigation (Figure
S1). The usual characteristics in FT-IR of PCN-224-Mn(tart)
have been demonstrated: CC, around 1500 to 1620 cm−1;
C−Hdeformation, from 720 to 1150 cm−1; COOsy and COOasy,
1290−1714 cm−1; C−H, around 2850 and 2920 cm−1. The
intense peak at 1714 cm−1 is due to the chiral tartrate anions.28
The broad and weak peaks at about 3500 and 3300 cm−1 are
related to OH [PCN-224 and PCN-224-Mn(tart)] and
NHpyrrole (porphyrin base in PCN-224) bonds, respectively.
UV−vis is another analysis that is used for porphyrinic
compounds. So, we investigated the absorption spectra of
PCN-224 and PCN-224-Mn(tart) for comparison (Figure S2).
As is known, TCPP and Mn(TCPP) have two kinds of peaks
in UV−vis in two different regions, Soret and Q bands. In
Mn(TCPP), four Q bands of TCPP are decreased to two Q
bands; this conversion is due to an increase of the symmetry.
PCN-224 with a TCPP linker exhibits 1-Soret/4-Q bands:
429/518−551−591−647 nm. The number of peaks in PCN-
224 is similar to that in the free-base ligand with a sensible shift
toward high wavelength (L → M charge transfer due to
porphyrinic ligands to Zr clusters).36−39 PCN-224-Mn(tart)
demonstrates 1-Soret/2-Q bands: 479/576−612 nm, which
agree with Mn(TCPP) and confirm the insertion of Mn-
porphyrin into the PCN framework.39 To further study the
Catalyses. Epoxidation of Olefins. The catalytic epoxidation of
olefins was carried out with the described conditions: a ratio of 2.5
mmol of cocatalyst and model substrate [isobutyraldehyde (IBA)/
styrene], 0.03 g of catalyst, 1 bar of O2, and 5 mL of solvent
[acetonitrile (CH3CN)]. Then, this reaction mixture was stirred, and
the reaction proceeded for 8 h at 60 °C.
Cycloaddition of Epoxide with CO2. This asymmetric catalytic
process was performed under the explained conditions in a free
solvent: a ratio of 0.08 mmol from cocatalyst and model substrate
(tetrabutylammonium bromide/styrene epoxide), 0.03 g of catalyst,
and 1 bar of CO2 for 15 h at 60 5 °C.
Reusability Test. To study the PCN-224-Mn(tart) reusability in
both of the catalytic reactions, we separated the catalyst from the
reaction mixture by centrifugation, and then we washed, dried, and
reused it in a fresh reaction with a new catalytic cycle under optimized
conditions. Next, the progress of the catalytic reactions was
investigated by chiral gas chromatography (GC).
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Figure 1. (a) PXRD pattern of simulated PCN-224 and experimental PCN-224 and PCN-224-Mn(tart) (from top to bottom). (b) SEM and (c)
BET of PCN-224-Mn(tart).
Scheme 2. Brief Description of the Synthesis and Catalysis Processes of PCN-224-Mn(tart)
chiral Zr-MOF structure, 1H NMR was also used (Figure S3).
With this analysis, determination of the tartrate amount in the
structure was done through the digested CMOF. The digestion
process was accomplished in a concentrated D2SO4 solution.
The H atoms of the tetratopic ligand appeared at around 8−9
ppm. The presence of peaks at 7−8 ppm is probably related to
benzaldehyde generated from digestion of Zr-MOF and/or
benzoic acid, which may not be removed sufficiently from the
structure after activation. Protons of tartrate were seen at a low
chemical shift (4−5 ppm) to single peak form. This confirms
the existence of tartrate anions in the framework
(Zr:TCPP:tart ∼3.2:1:0.7). Also, inductively coupled plasma
(ICP) was used to determine the ratio of Zr to Mn in the final
framework (Zr:Mn ∼ 2:1). After metalation of the PCN-224
porphyrin ligand with Mn and the addition of the tartrate
anion during exchange, powder X-ray diffraction (PXRD) of
the resulting compound was investigated. PXRD monitoring of
the final structure, PCN-224-Mn(tart), showed that PCN-224
still maintained a crystalline structure and was not damaged
during the preparation process (Figure 1a). As is known, PCN-
224-Mn(tart) is prepared from two primitive substrates, PCN-
224-Mn(Cl) and silver tartrate, by exiting AgCl during the
preparation process. One logical reason to address the absence
of AgCl in the structure is that the PXRD pattern of AgCl has
clear and strong peaks from 2θ = ∼30 to 50° [(111), (200),
and (220) planes], which are related to the Cl− anions and Ag+
cations. To verify this subject, we reported the PXRD pattern
of our structure until 2θ = 50° (S4) and the characteristic
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a
Table 1. Effect of Various Parameters on the Oxidation of Styrene with O2
b
c
no.
catalyst
CPMOF
CPMOF
CPMOF
CPMOF
CPMOF
CPMOF
CPMOF
conditions
conversion (%)
epoxide selectivity (%)
ee (%) (Conf.)
d
1
2
3
4
5
6
7
8
9
10
CH3CN/25 °C/IBA/8 h/O2
CH3CN/60 °C/IBA/4 h/O2
EtOAc/60 °C/IBA/4 h/O2
n-hexane/60 °C/IBA/4 h/O2
MeOH/60 °C/IBA/4 h/O2
H2O/60 °C/IBA/4 h/O2
CH3CN/60 °C/IBA/4 h/air
CH3CN/60 °C/4 h/O2
<20
100
40
21
0
0
30
0
65
89
67
42
58 (R)
96 (S)
81 (R)
75 (R)
79
93 (S)
no cocatalyst
no catalyst
homogeneous form
CH3CN/60 °C/IBA/4 h/O2
CH3CN/60 °C/IBA/2 h/O2
<10
100
35
75
a
b
Reaction conditions: catalyst, 0.03 g; styrene, 2 mmol; cocatalyst, 5 mmol; CH3CN, 5 mL. Conversions were calculated with GC/flame
ionization detection (FID) by using chlorobenzene as the usual internal standard. All catalytic experiments were repeated twice. Determined by
c
d
GC with a chiral SGE-CYDEX-B capillary column. 1 atm of O2.
a
Table 2. Asymmetric Epoxidation of Various Olefins by Using PCN-224-Mn(tart)
a
Reaction conditions: catalyst, 0.03 g; substrate, 2 mmol (except for trans-stilbene, 0.5 mmol); cocatalyst, 5 mmol; temperature, 60 °C; time, 8 h.
Conversions (Conv.), epoxide selectivity, and ee were calculated with GC/FID (chiral column) by using chlorobenzene as the usual internal
standard. All catalytic experiments were repeated twice.
peaks of AgCl were not seen in it. So, PXRD can confirm that
AgCl, probable Cl− anions, and coordinating Cl(s) to Mn
centers do not exist in CPMOF. Besides, the obtained pattern
from energy-dispersive spectroscopy (EDS) analysis of PCN-
224-Mn(tart) confirmed that the displacement of Cl with
tartrate happened successfully and also confirmed the PXRD
data. So, the Cl anions and coordinated Cl(s) to Mn(s) that
appear at less than 1 and 3 keV were not seen in the pattern of
the final structure (Figure S4).40−42 Of course, we used a
simple experimental procedure (titration) based on Vogel’s
textbook of quantitative chemical analysis to determine
chlorides of digested PCN-224-Mn(tart), in which no
detectable amount of Cl was obtained. Scanning electron
microscopy (SEM) of PCN-224-Mn(tart) was another reason
for porphyrin Zr-MOF stability (Figure 1b). Eventually, PCN-
224 has a high BET surface area in comparison to the other
porphyrinic MOFs. The results demonstrated that PCN-224
after metalation and ion exchange still maintains its high
surface area (1970 m2/g) and porosity (pore size approx-
imately 9 and 17 Å). This reduction can probably be assigned
to the existence of tartrate anions that entered into the pores
during the ion-exchange process (Figure 1c). Also, we
provided circular dichroism (CD) of the dispersed structure
to evaluate the optical activity. However, in this work, only one
kind of enantiomer, as the chiral source, has been employed
with a certain configuration (L-tartrate) for transmitting
chirality over PCN-224. As a result, the investigation of the
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Figure 2. (a and b) Chiral GC chromatogram and integration data of the styrene oxidation. (c) Aerobic oxidation reaction of styrene with O2/IBA.
1
(d) H NMR spectrum in dimethyl sulfoxide of the oxidation of styrene as the substrate scope.
CD spectrum was not complex and similar to the CD of chiral
species used (negative signal; Figure S7).
catalyst (conversion <10%) and cocatalyst (reaction stopped)
indicated that the reaction was not carried out in each of them.
Given that the solvent is a key agent in the proceeding
reaction, therefore we studied the effect of several solvents with
different polarities. In most of the epoxidation reactions,
CH3CN is chosen as the most suitable solvent based on its low
coordinating ability. Our results also showed that, among the
solvents examined, CH3CN can provide the proper environ-
ment for activation of the catalyst. In the optimization of the
catalytic parameters, IBA was selected to reduce molecular O2
for production of the radical oxidizing agent. The results
showed that employment of a cocatalyst was necessary (entry
8). One of the optimized reactions was investigation of the
PCN-224 activity without a chiral additive. As expected, unlike
chiral PCN-224(Mn) [CPCN-224(Mn) or PCN-224-Mn-
(tart)], achiral PCN-224(Mn) could not show any chirality
induction and enantiomeric excess (ee) that were logical.
After the essential experiments, the optimal conditions were
selected: temperature of 60 °C, 1 bar of O2 (generation of the
high epoxide selectivity),43,44 catalyst of 0.03 g, cocatalyst of 5
mmol, and CH3CN of 5 mL. Under these conditions, the
substrate scope (styrene) was catalyzed, producing two
stereoisomers of styrene epoxide (main product) and two
inevitable different kinds of aldehydes with low percent,
benzaldehyde and 2-phenylacetaldehyde (byproducts) (Table
2, entry 1; details in Figure 2). On the basis of the model
substrate size (calculated by Chem3D; Figure S5a) and pore
size of MOF, the reaction conversion was completed with 89%
epoxide selectivity and a unique level of ee [96% (S)]. These
results are evidences to confirm the essential role of the
catalyst.
Catalytic Activity. In enantioselective catalysis, the pore
size, number of the active catalytic sites, and kinds of chiral
ligands in the catalyst structure are important factors.
Considering the PCN-224-Mn(tart) properties as a chiral
porous solid, we used it to catalyze asymmetric catalytic
reactions due to Mn centers at the porphyrinic rings, Zr6
cluster defects, and neighboring hydroxyl groups in the chiral
ligand as acidic catalytic sources. In Scheme 2, the synthesis
and catalysis processes of PCN-224-Mn(tart) are briefly
shown. Two significant benefits of our study are having (i)
two distinct metallic Lewis acid centers and (ii) accessible
plentiful chiral counteranions and their cooperation in the
catalysis through strong hydrogen bonding as a Brønsted acid.
It is notable that catalyst separation using O2 as a friendly and
healthy oxidant, mild conditions in CO2 fixation, and the
synergistic effect of Brønsted/Lewis acid are positive points of
our system.
Enantioselective CC Epoxidation Reaction. The pre-
vious experiments showed and emphasized that the oxygen-
ation of olefins can happen over this chiral heterogeneous
catalyst because of Mn metallic centers within a ring of
porphyrinic linkers.27−30 For asymmetric epoxidation catalysis,
we investigated the different factors that affected the PCN-224-
Mn(tart) performance such as the oxidant, temperature,
solvent, time, and cocatalyst. According to the experiments
that have been presented in the following, we selected O2 as a
green oxidant source, a temperature of 60 °C, the solvent
CH3CN, a time of 8 h, and the cocatalyst IBA. As shown in
Table 1, the oxygenation of styrene (as an electron-rich olefin)
was performed at room temperature, and the absence of
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Figure 3. Recyclability of the catalyst (substrate/styrene): conversion, epoxide selectivity, and ee. (Top) Number of recycles: 0, fresh step. (Bottom
left) XRD and SEM of the catalyst used in the last cycle. (Bottom right) Results of the hot filtration test.
After determination of the optimized conditions, our study
began with various olefins, aromatic and aliphatic, like trans-
stilbene, 4-methylstyrene, α-methylstyrene, 1-phenyl-1-cyclo-
hexene, 1-decene, and 1-octene (Table 2). (E)-Stilbene (trans-
stilbene) was chosen for asymmetric epoxidation. The CC
oxidation of trans-stilbene was performed for formation of the
(R,R)-trans-stilbene epoxide isomer without any byproducts
(benzaldehyde, diol, or both of them) and configuration
change (entry 2).27 So, the obtained results were very
significant. The derivatives of styrene, 4-methyl, and α-
methylstyrene were also tested as important industrial
compounds. PCN-224-Mn(tart) could promote enantioselec-
tive epoxidation of these styrene derivatives with different
conversions and ee values (entries 3 and 4). The catalytic
results, under identical conditions, demonstrated that the
presence of steric hindrance around or close to the double
bond can be responsible for stereo- and enantioselectivity of
the generated products. 1-Phenyl-1-cyclohexene45 as a bulky
substrate was another olefin whose oxidation was investigated
with 75% conversion, 100% epoxide selectivity, and 88% ee
(entry 5). Two aliphatic olefins were also oxidized for 8 h with
the presented findings (entries 6 and 7). The moderate
conversions of 1-decene (70%) and 1-octene (79%) mean that
these linear terminal alkenes are less prone to oxidation
compared to the other aromatic olefins.46,47
The size of the starting substrate, steric effect, or steric
hindrance around of the active sites and catalyst structure−
substrate interaction degree can affect the results. Regarding
ee, it is interesting that, for oxidation of the considered olefins,
the ee values are high (84−100%). Mn(tart)-TCPP was also
tested as a homogeneous catalyst. The reaction was performed
under optimized conditions with complete conversion, 74%
epoxide selectivity, and 59% ee for 2 h. The ee in the
heterogeneous catalytic system shows the accessible volume of
chiral species in the framework. These charged species can
interact with substrates inside the framework, and so this
process can be effective in the product selectivity. However,
tartrate anions via the outer surface of the framework confer
chirality to the catalytic products with specific interactions.48,49
Mechanism. The probable mechanism of the oxidation of
olefins by MnIII/O2/aldehyde was previously presented. The
formation of an acyl radical, an acylperoxy radical, peroxy acid,
and finally MnV-oxo is proposed [MnIII-tart is converted to
tart-MnVO (tart as a trans-axial ligand binds to the Mn
center, which is plausible)].27 Although an understanding of
the mechanism needs further studies and investigations, an
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a
Table 3. Effect of Various Parameters on CO2 Cycloaddition to Styrene Epoxide
b
b
c
no.
catalyst
conditions
conversion (%)
carbonate selectivity (%)
ee (%) (Conf.)
1
2
3
4
5
6
CPMOF
25 °C/Bu4NBr/15 h/CO2
60 5 °C/15 h/CO2
60 5 °C/IBA/15 h/O2
60 5°C/Bu4NBr/15 h/CO2
60 5 °C/Bu4NBr/15 h/CO2
60 5 °C/Bu4NBr/15 h/CO2
31
15
<10
96
81
86
45
75
42
100
83
84
54 (R)
85 (S)
no cocatalyst
no catalyst
CPMOF
PCN-224 (without Mn and tart)
PCN-224-Mn (without tart)
94 (S)
a
b
Reaction conditions: catalyst, 0.03 g; substrate, 2 mmol; cocatalyst, 2.5 mmol; 1 bar of CO2; time, 15 h. Conversions (Conv.), cyclic carbonate
selectivity, and ee were determined with GC/FID (chiral column) by using chlorobenzene as the usual internal standard. All catalytic experiments
were repeated two times.
c
Figure 4. (a and b) Chiral GC chromatogram and integration data of CO2 fixation to styrene epoxide. (c) Reaction of CO2 cycloaddition of styrene
1
epoxide for the production of cyclic styrene carbonate. (d) H NMR spectrum in dimethyl sulfoxide of CO2 fixation of styrene epoxide as the
substrate scope.
important issue remains, that is, knowledge of how chirality
induction happens and produces a superior configuration is
mandatory. High enantioselectivity to epoxides is related to
several points: (1) the framework pore size, (2) close vicinity
of the one preferred face of the olefin (pro-S or -R face) to the
active Mn center to produce a suitable configuration with high
stability, and (3) the degree of noncovalent interactions of the
preferred face of the prochiral substrate (H atom of the olefinic
double bond) with chiral centers (interaction of the substrate
with the chiral surface and pore). On the basis of the catalytic
results, the S enantiomer is the major product in this catalytic
system. This observation shows that PCN-224-Mn(tart) has
the ability to discriminate between two kinds of chiral
configurations, and in the prochiral alkenes examined, one
face, R or S, tends to epoxidize. So, we did not see a racemic
mixture in the catalytic results.
Reusability Test. Also, the reusability of the catalyst was
investigated. The catalyst recovered in each run was washed
(CH3CN), dried, and employed in a fresh catalytic reaction to
the sixth step. 100% conversion, 89% epoxide selectivity, and
96% ee were decreased to 92% conversion, 87% epoxide
selectivity, and 94% ee. It is interesting that chiral PCN-224
was not destroyed and was stable after successive catalytic
reactions (SEM and PXRD in Figure 3 and BET in Figure S8).
The minor decrease in the conversion of the sixth cycle rather
than a fresh step depends on the negligible leaching of Mn
from the framework. Therefore, the Zr-based framework can
give stability to the porphyrin and prevent leaching/
aggregation of Mn(TCPP). All of recycle solutions at the
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a
Table 4. CO2 Cycloaddition to Various Olefin Epoxides
a
Reaction conditions: catalyst, 0.03 g; substrate, 2 mmol; cocatalyst, 2.5 mmol; 1 bar of CO2; temperature, 60 5 °C; time, 15 h. Conversions
(Conv.), cyclic carbonate selectivity, and ee were determined with GC/FID (chiral column) by using chlorobenzene as the usual internal standard.
All catalytic experiments were repeated two times.
end of the reaction were investigated with UV−vis, and it was
confirmed that no Mn-porphyrin has been leached (Figure
S6).
primitive investigations, the selected conditions were as
follows: temperature 60 5 °C; 1 bar of CO2; Bu4NBr as
the cocatalyst. The main investigated catalytic reactions are
denoted as (a) PCN-224 (without Mn), (b) PCN-224-Mn
(without tart), and (c) PCN-224-Mn(tart) (details in Table
3).
Finally, hot filtration was also investigated as a main strategy
to show the catalyst effect, which is commonly performed with
a heterogeneous catalyst. In the epoxidation reaction of the
model substrate, after 1 h, the catalyst was removed by
centrifugation. Then, the remaining solution was stirred under
optimized conditions, and it was monitored with GC every 1 h.
The reaction progress was not acceptable and satisfactory
(Figure 3). So, the presence of a catalyst that helps the catalytic
reaction proceed is essential. These results showed that this
CMOF with these achievements is known as a robust and
stable CMOF catalyst from the PCN family.
Enantioselective CO2 Cycloaddition to Epoxide Reaction.
Recently, Zr-MOFs are candidates for significant applications
because of their high stability.50,51 One essential reason for
their selection is the presence of random defects, which are
very important in these structures for different goals.51,52
Modulators, missing linker and cluster, can produce defects
that lead to vacancies and open Zr centers as active Lewis acids
with low connectivity. These accessible Lewis acid centers,
particularly defects, can induce catalytic ability into the
framework for use in various catalytic reactions such as CO2
chemical fixation. Usually, this reaction is carried out with low
yields under rough conditions (high temperature, pressure, and
time). Thus, the design of powerful and stable catalysts is of
great interest.53 Given that PCN-224 had been previously
extensively identified in detail, we found that our CMOF as a
chiral acid catalyst can be wonderfully used in the asymmetric
CO2 cycloaddition to epoxide.54 This reaction was also carried
out with styrene epoxide as the scope substrate (Figure S5b)
that could enter the framework with large pores. However, the
external surface of MOFs as defects can effect the development
of the catalytic reaction. We started our goal in the chiral
environment with styrene epoxide as a racemic substrate and
PCN-224-Mn(tart), which replete with Zr/Mn centers as
Lewis acids and tartrate species as Brønsted acids. After
To demonstrate the auxiliary effect of the tartrate anions as
nucleophiles, in addition to chirality induction and acidity, we
investigated PCN-224-Mn without tart. In the first and second
cases, the conversion of styrene epoxide to styrene carbonate
was carried out (81% and 86%, respectively). The results of the
metalated and nonmetalated PCN-224 showed that Zr nodes
are more effective than Mn as active sites. The low
performance of Mn(TCPP) can be assigned to the reported
findings that (i) the interaction of CO2 molecule with
Mn(TCPP) and (ii) the distance of CO2 with Mn(TCPP)
are weaker and longer than those of the CO2-Zr cluster.55
Thus, the obtained data agree with the structural properties.
When a chiral ligand was used in the framework, it was found
that tartrate anions could be the active catalytic centers and
they could increase the activity of PCN-224 due to a
synergistic effect with the other acidic centers. The low
conversion of PCN-224-Mn without tart rather than the PCN-
224-Mn(tart) catalyst showed that the present chiral anions
could accelerate the rate of reaction through the production of
ring-opened alkoxide. Notably, the excessive amount of tart
anions can play an important and key role in the reaction
development. In the third case, PCN-224-Mn(tart), the chiral
auxiliary ligand (tartrate), can act as both a nucleophile and a
Brønsted acid because of its nature (Conv. 96%; details in
Figure 4). A remarkable point is that additional product such
as diol was not seen during the reaction. This kind of product
(diol) is usually generated at temperatures above 90 °C in this
kind of catalytic process, while our catalytic system has mild
conditions (free solvent and 60
5 °C).56 Meanwhile, the
high selectivity of styrene carbonate can be related to its lack of
thermal degradation.57
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Figure 5. Recyclability of the catalyst (substrate/styrene epoxide): conversion, cyclic carbonate selectivity, and ee. (Top) Number of recycles: 0,
fresh step. (Bottom left) XRD and SEM of the catalyst used in the last cycle. (Bottom right) Results of the hot filtration test.
The production of cyclic carbonates as fine chemicals with
other racemic epoxides was also examined under the current
conditions. Two racemic aromatic epoxides and three aliphatic
epoxides were our selection (Table 4, entries 1−5). Styrene
epoxide and its derivative showed conversions of 96% and
87%, respectively. The catalyst exhibited more activity in CO2
fixation to styrene epoxide than its derivative, which can be
ascribed to the molecular size and steric hindrance. In
propylene epoxide catalysis, high conversion was observed
rather than those of its two derivatives with conversions of
99%, 91%, and 78%, respectively (entries 3−5). The obtained
results are evidence to show the superiority of PCN-224-
Mn(tart). However, the results revealed that PCN-224-
Mn(tart) has a capability to generate high ee values (90−98%).
As expected, tartrate groups are more around the clusters
and linkers, so they can transfer chirality to the environment
and directly activate epoxide or probably CO2.58−61 Of course,
free carboxylic acid groups of TCPP can also act as Brønsted
acids in promoting the reaction.62 Three catalyzed substrates
produced S-cyclic carbonates. It can be inferred that the chiral
component direction can affect the main product optical
activity. Certainly, the size of the substrate and MOF pores can
affect the conversion and cyclic carbonate selectivity. However,
it should be noticed that the selected substrates are in racemic
form (50:50) and PCN-224-Mn(tart) has the potential to
discriminate between S and R enantiomers of epoxide. So, in
addition to the major enantiomer of the chiral cyclic carbonate
produced, a chiral epoxide (R, S, and or both of them) that has
not been used as a reactant in the CO2 fixation catalytic
reaction is also seen. Several MOFs have been reported in
which their cation and anion sites could act in CO2 fixation
into propylene oxide but under harsh conditions like a pressure
of 20 bar and a temperature of 120 or 140 °C.63−66
The best results of asymmetric epoxidation and CO2 fixation
are listed in Table S1. It is noticed that asymmetric CO2
cycloaddition into epoxide reactions have been done less,
especially by using chiral heterogeneous catalysts.
Mechanism. The mechanism of this reaction and chirality
induction happen based on the interaction between the active
acidic center (Zr; on the basis of the defect hypothesis in the
Zr cluster), Mn as the acid-assisted site, and auxiliary OH
groups of tartate with O atom of the epoxide ring (pro-S or -R
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face).67 Then, the present cocatalyst as a nucleophile (Br)
attacks the activated epoxide ring, and then ring opening
happens. It is interesting that chiral tartrate anions can help
with ring opening of epoxide.68 So, the reaction without a
nucleophile and with a counteranion can proceed, although
with low conversion. After attachment of CO2 to the negatively
opened ring, cyclic carbonate is produced. Another significant
point is that, during the reaction development (for 15 h),
byproducts are not observed.69 With development of the
reaction, increasing ee is seen, which could be related to
interaction of the chiral centers and substrate pro-R/S face.
Reusability Test. The conversion, cyclic carbonate
enantioselectivity, and ee results of the PCN-224-Mn(tart)
catalyst for six steps were investigated (Figure 5). A small
reduction in these findings was observed that was not related
to the catalyst ability but was due to the lost mass of the
catalyst during the recyclability test because, in per cycle, the
catalyst is separated from the reaction mixture, washed, and
dried. Again, the catalyst is employed in the next run. Findings
did not show the noticeable changes in SEM and PXRD
(Figure 5) and BET (Figure S8). Therefore, it can be said that
this chiral catalyst maintained its physical and chemical
properties. ICP analysis was also utilized to study recycle
solutions, and Zr leaching was not identified. A hot filtration
method was also investigated in this catalytic process; its
diagram is shown in Figure 5 (plot of the conversion versus
time). After removing catalyst from the solution mixture (after
3 h) and returning the temperature of the catalytic solution to
the optimized temperature, the solution was stirred under
optimal experimental conditions. Every 3 h, the progress of the
catalytic reaction was studied using GC. The findings gained in
this process were previously confirmed in the olefin
epoxidation section and reflect the catalyst importance.
One-Pot Asymmetric Epoxidation/CO2 Cycloaddition
Catalysis Reactions. Finally, it should be said that, for the
production of chiral cyclic carbonate, we investigated these
one-pot catalysis reactions via tandem reaction (asymmetric
epoxidation/CO2 cycloaddition) with styrene as the first
substrate [reaction pathway: (1) starting substrate, prochiral
styrene; (2) generated chiral epoxide, semiintermediate (as the
second substrate); (3) chiral cyclic carbonate, main product].
The obtained results were suitable in conversion (80%),
selectivity of carbonate (89%), and ee [93% (S)], and they
were close to the separated reaction results.
neutral chiral Zr-MOF, PCN-224-Mn(tart), that can be
considered to be a combinatorial multipurpose catalyst: achiral
electrophile−chiral nucleophile−chiral functional groups. Our
suggestion is the synthesis of a CMOF with unique chemical
stability through postchiral modification. In this method, chiral
ionic exchange happens in the outer and inner regions of the
framework. The ease of PCN-224-Mn(tart) preparation is the
first positive point. PCN-224-Mn(tart) having two different
metallic active sites (Zr/Mn: Lewis acid), abundant chiral
functional groups, and centers (Brønsted acid: to activate the
epoxide ring) can be selected as a unique chiral porous catalyst.
To ascertain that a CMOF has a pivotal role with
heterogeneous nature in the environmental catalysis with less
energy utilization, a series of olefins and epoxides with the
various sizes were used in the enantioselective oxidation and
CO2 fixation. The results show that PCN-224-Mn(tart) can
catalyze the catalytic reactions at low energy and ambient
pressure and temperature. Meanwhile, high selectivity to an
enantiomer in the asymmetric processes demonstrates that
high energy has not been employed for enantioseparation. In
fact, in the catalytic and reusability tests, PCN-224-Mn(tart)
exhibited excellent efficiency with ee to 100%. In total, the
presence of the accessible catalytic sites such as Mn centers, Zr
clusters, chiral functional groups, and hydrogen bonds is useful
in the catalyst structure. These reasons are the second positive
points of our catalyst. In the end, our comprehensive study not
only shows a simple strategy for the design of a new renewable
chiral Zr-based MOF as a chiral heterogeneous catalyst that
agrees well with green chemistry but also demonstrates a
unique vision to develop new chiral structures as asymmetric
platforms based on the MOF structure.
̅
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02811.
Details of materials and instrumentation, FT-IR and
1
UV−vis spectra, H NMR, EDS, PXRD, Chem3D data,
CD analysis, Barrett−Joyner−Halenda pore-size distri-
butions, and N2 adsorption isotherms (PDF)
AUTHOR INFORMATION
■
However, observations proposed that, in the asymmetric
tandem, from the second step, the reaction is performed
easily.70 Because the starting substrate of the second stage is
enantioriched epoxide (R or S), that CO2 addition leads to
cyclic carbonate asymmetrically with high ee, which it is
expected. On the other hand, in this manner, enantiodiscrimi-
nation of styrene epoxide by the catalyst is not necessary. So,
we maneuvered over asymmetric epoxidation and CO2 fixation,
separately. Of course, there is no difference in the kind of the
chiral styrene cyclic carbonate created because PCN-224-
Mn(tart) gave the S enantiomer in nontandem fixation, and, in
contrast, in a tandem reaction, the S enantiomer was also
generated.
Corresponding Author
Ali Morsali − Department of Chemistry, Faculty of Sciences,
Tarbiat Modares University, Tehran, Iran; orcid.org/
0000-0002-1828-7287; Email: morsali_a@modares.ac.ir
Author
Kayhaneh Berijani − Department of Chemistry, Faculty of
Sciences, Tarbiat Modares University, Tehran, Iran
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c02811
Notes
The authors declare no competing financial interest.
CONCLUSION
■
In the present research topic, the foregoing findings show that,
because of engineering of the synthesis process and
manipulation of the MOF framework by chiral anions, a
CMOF can be synthesized. For the first time, we report a
ACKNOWLEDGMENTS
■
The authors are grateful to Tarbiat Modares University for
supporting this research.
K
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