Design and Construction of a Chiral Cd(II)-MOF from Achiral Precursors: Synthesis, Crystal Structure and Catalytic Activity toward C-C and C-N Bond Forming Reactions

Article abstract of DOI:10.1021/acs.inorgchem.8b03307

Using achiral components, a V-shaped dicarboxylic acid (H₂L) and a conformationally flexible bidentate linker (bpp), a thermally stable chiral metal organic framework {[Cd(bpp)(L)(H₂O)]·DMF}_n (1), where H₂L = 4,

Full text of DOI:10.1021/acs.inorgchem.8b03307

Article
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Design and Construction of a Chiral Cd(II)-MOF from Achiral
Precursors: Synthesis, Crystal Structure and Catalytic Activity toward
C−C and C−N Bond Forming Reactions
Vijay Gupta and Sanjay K. Mandal*
Department of Chemical Sciences, Indian Institute of Science Education and Research Mohali, Sector 81, Manauli PO, S.A.S. Nagar,
Mohali, Punjab 140306, India
S
* Supporting Information
ABSTRACT: Using achiral components, a V-shaped dicarboxylic acid
(H2L) and a conformationally flexible bidentate linker (bpp), a
thermally stable chiral metal organic framework {[Cd(bpp)(L)(H2O)]·
DMF}n (1), where H2L = 4,4′-(dimethylsilanediyl)bis-benzoic acid,
bpp = 1,3-bis(4-pyridyl)propane and DMF = N,N-dimethylformamide,
has been solvothermally synthesized and crystallographically charac-
terized. It consists of 1D helical chains linked at the cadmium centers
resulting in an overall 2D framework. Its microporous nature was
confirmed by gas-sorption measurements. Upon thermal activation of
1, where both guest DMF molecules present in the 1D open channels
and the coordinated H2O molecules are removed, its active metal site
shows Lewis acid character to be an excellent heterogeneous catalyst for
the C−C (Knoevenagel condensation reaction) and C−N (Strecker)
bond forming reactions.
opposite-handed helices often yields nonporous and achiral
structures.15 The other possibility involves individual homo-
chiral crystals that are formed via a process known as
spontaneous resolution.13 However, the bulk sample tends to
be an equal mixture of crystals with opposite handedness. So
far, our understanding of the bulk homochiral crystallization
from achiral precursors is an unpredictable16 but exceptionally
attractive process. Therefore, the design and development of
synthesis of chiral MOFs from achiral components is essential
to acquire the comprehensive expertise of inducing chirality
into the MOF architectures.
Owing to the chirality in coordination architectures, in most
cases chirality arises by the helical secondary structures.12b,17
The V-shaped exobidentate ligands are well-known to impart
helicity owing to their natural ability to endorse a twisted
conformation in the resulting framework.18 In addition, the
limited literature reports11b,19 reveal the uniqueness of
conformationally flexible ligands for the synthesis of chiral
frameworks adopting special geometries to induce chirality into
the overall framework. In view of these facts, we chose to use a
V-shaped dicarboxylic acid 4,4′-(dimethylsilanediyl)bis-ben-
zoic acid (H2L) along with a conformationally flexible 1,3-
bis(4-pyridyl)propane linker (bpp) (Scheme 1) for their
introduction into MOFs with a motive to engineer chirality
into the overall architecture. We envisioned that the natural
ability of V-shaped ligands to endorse a twisted conformation
INTRODUCTION
■
For some time special efforts have been directed to engineering
metal−organic frameworks (MOFs) not only because of their
potential applications as functional materials1−4 but also in
view of their assortment of captivating architectures and
topologies.5 In particular, there is currently a substantial
demand for sensible synthetic strategies to permit the
construction of chiral MOFs due to their intriguing potential
applications in enantioselective catalysis,6 chiral separation,7
nonlinear optical materials,8 and magnetic materials.9
Basically, there are two methods to generate chiral MOFs
with building blocks used for the construction of coordination
frameworks. One is the likelihood of incorporating chiral
centers in either metal complexes or ligands, while the second
involves achiral ligands with no chiral auxiliaries for acquiring
either a chiral framework or enantiomers by spontaneous
resolution.10 Until now, most of the chiral MOFs and
coordination polymers are synthesized by the first method
using enantiopure ligands, which induces the homochirality in
the resulting framework. On the other hand, the second
method is more difficult,11 but attractive12 for the chiral MOFs
using achiral components, which can be easily prepared
compared to chiral components. The chirality can be conveyed
from achiral precursors through crystallization even without
the use of enantiopure building blocks.13 In the literature, very
few examples of self-assembled chiral MOFs from achiral
precursors have been reported.14 The chirality in these
frameworks arises because of the spatial organization (e.g.,
helix) of achiral precursors, but the interpenetration of
Received: November 28, 2018
© XXXX American Chemical Society
A
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heated to 100 °C for 48 h. Colorless rod-shaped crystals of 1 were
filtered out without cooling the mixture. Upon repeated washings with
DMF followed by water, these crystals were air-dried. Yield: 145 mg
(69%), based on the metal salt. Anal. Calcd for C32H37N3O6SiCd
(MW 700.13): Calc. %C, 54.89; %H, 5.33; %N, 6.00 and Found: %C,
55.05; %H, 6.10; %N, 5.88. Selected FTIR peaks (KBr, cm−1): 1670
(s), 1579 (s), 1530 (s), 1400 (s), 1307 (w), 1250 (m), 1225 (w),
1100 (s), 1017 (m), 812 (s), 767 (s), 724 (m), 659 (w), 509 (m).
Typical Experimental Procedure for the Knoevenagel
Reaction. In a typical reaction, a mixture of an aromatic aldehyde
(0.10 mmol), an active methylene compound (0.12 mmol), the
activated 1 (2 mg, 3 mol %) and 1 mL methanol was taken in a
capped glass vial. The reaction mixture was allowed to stir at room
temperature (25−30 °C) for 60 min. After that, the solid catalyst was
separated by centrifugation and washed with chloroform to recover
the catalyst. The supernatant was evaporated by using a rotary
evaporator and the crude product thus obtained was redissolved in
Scheme 1. Chemical Structure and Schematic
Representation of the Bent Dicarboxylic Acid H2L and
Conformationally Flexible Ligand bpp
and the rotation freedom of Py−CH2−CH2−CH2−Py bonds
of the bis(4-pyridyl)propane may act cooperatively to lock the
overall framework in a chiral conformation.
1
CDCl3 and analyzed by H NMR spectroscopy.
Herein, we report the synthesis of a Cd(II)-based helically
chiral MOF, {[Cd(bpp)(L)(H2O)]·DMF}n (1) (where, DMF
= N,N-dimethylformamide), using achiral organic ligands and
without the need of any chiral auxiliaries such as enantiopure
solvents, additives, catalysts, or a template. The polymer chain
in 1 consists of Cd(II) ion and L2− arranged along the twofold
screw axis in the form of a helix. These 1D helical chains are
further linked at the cadmium centers forming a 2D
framework. Further, the existence of exposed Lewis acidic
metal sites in 1 has been demonstrated by its heterogeneous
catalytic activity toward the carbon−carbon (Knoevenagel
condensation reaction) and carbon−nitrogen (Strecker) bond
forming reactions.
Typical Experimental Procedure for the Strecker Reaction.
In a typical reaction, a mixture of aldehyde/ketone (0.10 mmol),
aniline (0.10 mmol), and TMSCN (0.12 mmol) was taken in a
Schlenk tube along with 2 mg (3 mol %) of the activated 1. The
reaction mixture was allowed to stir at room temperature (25−30 °C)
for 6 h under nitrogen atmosphere in a solvent free condition. After
that, 1 mL chloroform was added and the solid catalyst was separated
by centrifugation. The supernatant was evaporated by using rotary
evaporator and the crude product thus obtained was redissolved in
1
CDCl3 and analyzed by H NMR spectroscopy.
Recycling of 1. After the catalytic reaction, the solid catalyst was
collected by centrifugation, washed with chloroform, and dried for 5 h
under vacuum for its reuse in the next experiment.
RESULTS AND DISCUSSION
Synthesis and Structural Characterization. Compound
1 was synthesized under solvothermal conditions by heating a
■
EXPERIMENTAL SECTION
■
Materials and Physical Measurements. All reactions carried
out with n-BuLi required oven-dried glasswares and an atmosphere of
nitrogen. Both diethyl ether and tetrahydrofuran (THF) were dried
and purified by standard protocols (predried with KOH and distilled
over benzophenone-Na) prior to use. All chemicals and other solvents
used for synthesis were obtained from commercial sources and were
used as received, without further purification. H2L was synthesized by
Scheme 2. Synthesis of 1
following the literature procedure20a and characterized by 1H and 13
C
NMR spectra recorded at room temperature on a 400 MHz Bruker
ARX400 NMR spectrometer.
mixture of Cd(NO3)2·4H2O, bpp, and H2L along with DMF/
H2O (2/1) mixture at 100 °C for 48 h (Scheme 2). Colorless
rod-shaped crystals of 1 suitable for single crystal X-ray
diffraction analysis were isolated by filtration in 69% yield.
Based on the single crystal X-ray diffraction analysis (Table
S1, Supporting Information), 1 crystallizes in the chiral space
group P21212 (No. 18) and has a 2D structure derived from
Cd−bpp−L2− helical chains [Flack parameter = 0.001(7)]. As
shown in Figure 1a, the asymmetric unit consists of one Cd(II)
ion, one bpp, one L2−, and one coordinated H2O molecule
along with one lattice DMF molecule. An ORTEP view of the
asymmetric unit with an atom numbering scheme is also
shown in Figure S4 in SI. The Cd center has a distorted
pentagonal bipyramidal geometry with an N2O5 coordination
environment due to binding of four oxygen atoms (O1, O2,
O3, O4) from carboxylates of two different L2− in a chelated
fashion and one oxygen (O5) from coordinating water at the
equatorial positions (Table S2, Supporting Information). The
two apical positions are occupied by two nitrogen atoms (N1
and N2) from two different bis-pyridyl ligands. The chelated
mode of carboxylate binding with the metal center is evident
by the difference (Δν = 179 cm−1) in the asymmetric (ν =
1579 cm−1) and symmetric (ν = 1400 cm−1) carbonyl
stretching of carboxylates in the FTIR spectrum of 1 (Figure
S3, Supporting Information). A strong peak at 1670 cm−1
FTIR spectra of samples as KBr pellets were recorded in the 4000−
400 cm−1 range on a PerkinElmer Spectrum I spectrometer. TG
(thermogravimetric) analysis was carried out in the temperature range
of 25−500 °C (at a heating rate of 10 °C/min) under a dinitrogen
atmosphere using an aluminum pan as a sample holder on a Shimadzu
DTG-60H instrument. Elemental analysis (C, H, and N) was carried
out using Leco TruSpec Micro CHNS analyzer. For the single crystal
X-ray diffraction, see details in the Supporting Information. Power X-
ray diffraction (PXRD) measurements were carried out on a Rigaku
Ultima IV diffractometer having settings and attachments as reported
earlier20b over a 2θ range of 5−50° with a scanning speed of 2° per
minute using a 0.02° step. Solid state CD spectrum was recorded on
Chirascan CD instrument. Gas sorption experiments were performed
using a BELSORP MAX (BEL JAPAN) volumetric adsorption
analyzer at different temperatures. Ultrapure (99.995%) N2, He, and
CO2 gases were used for the adsorption measurements. Each sample
was pretreated prior to data collection by grounding it to a powder
and heating at 373 K under very high vacuum for 24 h followed by
purging with nitrogen gas on cooling. For sorption isotherms at 273
and 298 K, the set temperature was maintained with a circulating
water bath connected to a chiller.
Synthesis of {[Cd(bpp)(L)(H2O)]·DMF}n (1). A mixture of H2L
(90 mg, 0.3 mmol), bpp (60 mg, 0.3 mmol), and cadmium nitrate
tetrahydrate (92.5 mg, 0.3 mmol) was dissolved in 10 mL DMF. To
this solution, 5 mL water was added to obtain a clear solution, which
was then transferred to a glass vial. The glass vial was sealed and
B
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Figure 1. X-ray crystallographic analysis of 1: (a) A view of the asymmetric unit and the coordination environment around the Cd (II) center. (b)
Perspective view of the 2D framework; lattice DMF molecules present in the 1D channels are shown as their space-fill model. (c, d) Representation
of the 21 screw axis along b-axis and the twofold rotation axis along c-axis. (e) Space fill view of the framework where different 1D helical chains
fused at the Cd(II) centers are shown in different colors.
Figure 2. Simplified node and linker representation for 1, indicating 4-connected uninodal net, sql topology.
corresponds to the carbonyl stretching frequency of the lattice
DMF molecule.
the framework shown in Figure 1e depicts the fusion of
different 1D helical chains at the Cd(II) centers. Clearly, 1 is a
noninterpenetrating MOF, and its microporous nature
established by gas adsorption studies supports this further
(vide infra). To simplify the rather intricate structure of 1, the
Cd(II) center can be considered as a 4-connected node and
the ligands L2− and bpp as bent linkers, respectively. Further
examination of the node and linker representation reveals that
the framework exhibits a 4-connected uninodal net, sql
topology, with Schlafli point symbol {44.62} (Figure 2) as
determined by the TOPOS23 program (Table S3, Supporting
Information).
The Cd(II) centers act as a link between the helical chains
giving rise to an overall 2D framework with 1D open channels
with dimensions of 10.9 × 12.1 Å2 (7.1 × 9.0 Å2 without van
der Waals radii),21 which are filled with lattice DMF molecules
(Figure 1b). The total potential solvent-accessible void volume
for 1 was estimated to be 17.8% (577 Å3 out of the unit cell
volume 3224 Å3) using the PLATON software.22 The
dicarboxylate ligand (L2−) adopting a helical chain con-
formation, based on its connectivity with the Cd(II) centers in
1, can be observed by considering the 21 screw axis along the b-
axis (Figure 1c). On the other hand, the twofold rotation axis
along the c-axis provides the overall connectivity of all
components (Figure 1d). Furthermore, a space fill view of
Further structural analysis of 1 reveals that the metal center
is not stereogenic and can not induce chirality; thus, the V-
shaped dicarboxylate and the conformationally flexible bpp
C
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Table 1. Substrate Scope for the Knoevenagel Condensation
Reaction Catalyzed by 1
Table 2. Substrate Scope for the Strecker Reaction
Catalyzed by 1
a
a
a
Reaction conditions: Aldehyde (0.1 mmol), active methylene
compound (0.15 mmol), and reaction time: 60 min. bAverage percent
a
Reaction conditions: Aldehyde/ketone (0.1 mmol), amine (0.1
mmol), TMSCN (0.12 mmol) and reaction time: 6 h. bAverage
percent yield for a set of triplicate runs, calculated by 1H NMR of the
1
yield for a set of triplicate runs, calculated by H NMR of the crude
products. cNumber of moles of product per mole of catalyst. dIsolated
e
yield. Blank reaction.
c
crude products. Number of moles of product per mole of catalyst.
dIsolated yield. Blank reaction.
e
adopt a helical arrangement when coordinated to the metal
ion, giving rise to a chiral network. The helical chirality of the
framework was further established with the help of solid state
circular dichroism (CD) measurement (Figure S6, Supporting
Information).
ent powder X-ray diffraction analysis (Figure S9, Supporting
Information).
Coordinated water and the guest molecules in as-synthesized
1 can be removed by heating under vacuum to obtain its
desolvated/activated form (Figure S10, Supporting Informa-
tion). As indicated earlier, the activated compound with readily
available open channels provides a surface with unsaturated
Cd(II) sites with chiral pore environment. Porosity in the
activated compound was examined by the gas-sorption
measurements. Prior to the adsorption measurements, the
methanol exchanged sample (∼100 mg) was activated by
placing the sample under ultravacuum at 100 °C for 12 h. The
low temperature gas sorption measurements reveal that 1
adsorbs an appreciable amount of CO2 (at 195 K) but much
lower amount of N2 (at 77 K) (Figure S11, Supporting
Information). The Brunauer−Emmett−Teller (BET) and the
Langmuir surface areas were estimated to be 61 m2 g−1 and
109 m2 g−1, respectively, based on the CO2 adsorption
measurement at 195 K (Figure S12 and S13, Supporting
Powder X-ray diffraction data of the as-synthesized, solvent
exchanged, and desolvated sample of 1 were recorded at room
temperature to confirm whether the single crystal structure
represents the bulk material or not (Figure S7, Supporting
Information). It was found that the experimental powder
patterns were in good agreement with the simulated powder
pattern (obtained from the single crystal data) of 1, which
confirmed the phase purity as well as retention of structural
integrity upon thermal desolvation/activation. In order to test
the thermal stability and structural variation as a function of
temperature of 1, thermogravimetric analysis (TGA) was
carried out for the single phase polycrystalline samples of as-
synthesized, solvent exchanged, as well as desolvated 1, under a
dinitrogen atmosphere (Figure S8, Supporting Information).
TGA data reveal that the compound is stable up to 200 °C,
which has been further corroborated by temperature-depend-
D
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Figure 3. Plausible mechanisms for the Knoevenagel condensation and the Strecker reactions catalyzed by 1.
Information). In order to determine the quantitative binding
strength of CO2 molecules with the framework, the isosteric
adsorption enthalpy (Qst) was calculated based on the
Clausius−Clapeyron equation, using the adsorption isotherms
collected at different temperatures (Figure S14 and S15,
Supporting Information). The Qst value at zero loading was
found to be ∼25 kJ mol−1, which indicates good binding
affinity of CO2 with the framework.
Catalysis Studies. Due to the microporous nature and
coordinatively unsaturated metal site in 1 (Figure S10,
Supporting Information), it was tested for its active role as a
heterogeneous catalyst for Lewis acid promoted reactions. The
Knoevenagel condensation and Strecker reactions are well-
known Lewis acid catalyzed C−C and C−N bond forming
reactions used for the synthesis of benzylidene malononitriles
and α-aminonitriles, respectively. As the Knoevenagel reaction
deals with aldehydes and compounds having active methylene
groups, it has wide applications for the preparation of several
fine chemicals and pharmaceuticals. Additionally, the product
benzylidene malononitrile is well-known for its biological
activity.24 On the other hand, α-aminonitriles obtained from
the three-component Strecker reaction are versatile building
blocks for the synthesis of α-amino acids and their
derivatives.25
To check the feasibility of 1 toward the Knoevenagel
condensation reaction, the reaction between benzaldehyde and
malononitrile was chosen as a test reaction. Both components
were placed in a capped glass vial along with a catalytic amount
(3 mol %) of 1. The reaction was performed at room
temperature using methanol as reaction medium. The progress
of the reaction was monitored by TLC and the final conversion
was calculated by 1H NMR spectroscopy (Figure S16,
Supporting Information). As a control experiment, the reaction
was repeated without any catalyst under the same conditions
(Entry 15, Table 1; Figure S17, Supporting Information). As
expected, only 17% yield was achieved. Inspired by the good
catalytic activity, the versatility of 1 was further tested by the
reaction of different types of substituted aromatic aldehydes
and active methylene compounds. It resulted in the
corresponding α,β-unsaturated cyano/ester derivatives in
good-to-excellent yields. A summary of the results is compiled
in Table 1.
There are reports where MOFs have been utilized for
Strecker reaction, but the scope of d10 metal ion-based MOFs
is very limited for this reaction.14a Therefore, benzaldehyde,
trimethylsilyl cyanide (TMSCN) and aniline as the test
substrates were added to 3 mol % of 1 placed in a Schlenk
tube. Without adding a solvent the reaction was performed at
room temperature under N2 atmosphere, while its progress was
1
followed by TLC. The final conversion was calculated by H
NMR spectroscopy (Figure S18, Supporting Information). As
a control experiment, the reaction was repeated without any
catalyst under the same conditions (Entry 14, Table 2; Figure
S19, Supporting Information), where only 27% yield was
obtained. The catalytic activity of 1 was further tested for
different substrates, especially for the more reluctant ketone
derivatives (Table 2).
The results obtained for both the Knoevenagel and Strecker
reactions reveal that the substrates with an electron with
drawing group gave the maximum conversion while a drop in
the product conversion was obtained for the substrates with
electron donating substituents. This supports the fact that an
electron withdrawing group increases the rate of a nucleophilic
attack at the carbonyl or imine groups compared to an electron
donating group. The stronger the electron-withdrawing ability
of the substituent, the easier the activation of carbonyls or
imines for nucleophilic attack is at the carbon center, and
consequently faster the reaction is. In all cases, products were
1
isolated and characterized by H and 13C NMR spectroscopy
to confirm their identity and purity (Figures S20−S73,
Supporting Information).
E
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To determine the recyclability of the catalyst, it was
separated via filtration, washed with chloroform and methanol,
and dried under vacuum at 100 °C for 5 h to regenerate the
active catalyst. In both cases, no loss of crystallinity or phase
purity of the catalyst was observed even after three cycles of
reactions, as evident from the PXRD patterns of the recovered
catalyst (Figure S74, Supporting Information). In addition, the
catalyst can be reused up to three cycles in both cases with a
slight loss (∼8−10%) in its catalytic activity (Figure S75,
Supporting Information). In order to find any catalyst leaching
into the product stream, the catalyst was separated from the
reaction mixture by centrifugation after a certain time and the
supernatant was subjected to the same reaction conditions. No
significant conversion was obtained (Figure S76, Supporting
Information) indicating the heterogeneous nature of 1 and its
active role as a catalyst in both the reactions.
The efficiency of 1, when compared to the catalytic activity
of various MOFs that have been used as heterogeneous
catalysts in the Knovenagel and Strecker reactions, is found to
be comparable with the reported examples.14a,26 Based on the
studies described above, plausible mechanisms are presented in
Figure 3. These are in accordance with reported proposals.26,27
The open Lewis-acidic metal center interacts with the
electrophilic carbonyl or imine group to impart polarization
of these groups, which leads to the enhancement of
electrophilic character of the carbon atom of carbonyl or
imine moiety. In addition, the Lewis basic O atom of the
coordinated carboxylate group is known to impart basic
character to the framework and can activate the corresponding
nucleophile; thus, it acts as the driving force of the reaction.
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: sanjaymandal@iisermohali.ac.in.
■
ORCID
Sanjay K. Mandal: 0000-0002-5045-6343
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge IISER Mohali, India for financial
support (to S.K.M.) and SRF from UGC New Delhi, India (to
V.G.). The use of the X-ray facility at IISER Mohali is
gratefully acknowledged.
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CONCLUSIONS
■
In summary, we have reported the synthesis and structural
characterization of a chiral Cd(II) MOF based on a relatively
simple V-shaped dicarboxylic acid and conformationally
flexible bis-pyridyl ligand, without the need of any chiral
auxiliary. The chirality induction has occurred due to the
helical arrangement of the polymeric chain around a twofold
screw-axis. The overall 2D framework represents the 4-
connected uninodal net, sql topology. The microporous nature
and the presence of open metal sites in the activated
framework has been utilized for its catalytic activity toward
Lewis acid catalyzed C−C and C−N bond forming
Knoevenagel condensation and Strecker reactions, respectively.
The structural integrity of the catalyst during the reaction was
retained and thus provided its heterogeneous nature.
Furthermore, this MOF can be easily separated and reused
for three catalytic cycles without significant loss of its activity
in both cases. Its utilization in many other organic trans-
formations is the future plan in our laboratory.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b03307.
■
S
Experimental details, characterization procedures, crys-
tallographic data with refinement details, selected bond
lengths and angles, TGA and PXRD patterns (PDF)
Accession Codes
CCDC 1879636 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
F
DOI: 10.1021/acs.inorgchem.8b03307
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