Chiral Cadmium(II) Metal-Organic Framework from an Achiral Ligand by Spontaneous Resolution: An Efficient Heterogeneous Catalyst for the Strecker Reaction of Ketones

Article abstract of DOI:10.1021/acs.inorgchem.7b01915

A thermally stable cadmium-based chiral metal-organic framework (MOF), {[Cd₂(L)(H₂O)(DMF)]·3DMF·2H₂O}_n (1; DMF = N,N-dimethylformamide), has been synthesized from an achiral ligand by spontaneous resolution. The MOF features 1D open channels with a large density of active metal sites and has a 3,6-c binodal net with a rare sit 3,6-conn topology. The metal-bound water and DMF solvents could be easily removed along with the guest molecules in the lattice upon activation to afford the desolvated framework 1′. It exhibits microporous nature, as confirmed by the gas-sorption measurements with carbon dioxide uptake of 43.2 cm³ g^-1 at 273 K. The open metal sites in the framework make it an outstanding heterogeneous catalyst in the Strecker reaction for the synthesis of α-aminonitriles in a solvent-free state at room temperature with excellent conversion yields.

Full text of DOI:10.1021/acs.inorgchem.7b01915

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Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
Chiral Cadmium(II) Metal−Organic Framework from an Achiral
Ligand by Spontaneous Resolution: An Efficient Heterogeneous
Catalyst for the Strecker Reaction of Ketones
Ashish Verma, Kapil Tomar, and Parimal K. Bharadwaj*
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India
S
* Supporting Information
works.15 This happens because of the natural ability of V-shaped
ABSTRACT: A thermally stable cadmium-based chiral
metal−organic framework (MOF), {[Cd2(L)(H2O)-
(DMF)]·3DMF·2H2O}n (1; DMF = N,N-dimethylforma-
mide), has been synthesized from an achiral ligand by
spontaneous resolution. The MOF features 1D open
channels with a large density of active metal sites and has a
3,6-c binodal net with a rare sit 3,6-conn topology. The
metal-bound water and DMF solvents could be easily
removed along with the guest molecules in the lattice upon
activation to afford the desolvated framework 1′. It exhibits
microporous nature, as confirmed by the gas-sorption
measurements with carbon dioxide uptake of 43.2 cm3 g−1
at 273 K. The open metal sites in the framework make it
an outstanding heterogeneous catalyst in the Strecker
reaction for the synthesis of α-aminonitriles in a solvent-
free state at room temperature with excellent conversion
yields.
moieties to adopt a twisted geometry in the resulting networks.
Similarly, more or less a neglected ligand class is comprised of
bis(pyrazole)-16 and methylenebis(pyrazole)17-based systems,
where the chirality has also been observed, which is ascribed to
the restricted rotation of the two C−C-connected pyrazole rings
and the V shape of the molecule, respectively.
Herein, we have synthesized a novel V-shaped ligand, L,
comprised of a methylenebis(pyrazole) moiety. The introduc-
tion of L into coordination frameworks can be of special interest
in terms of chiral coordination engineering. From a mechanistic
viewpoint, for the ligand L, upon coordination with a metal ion,
the rotation freedom of N−CH2−N bonds of the bis(pyrazole)
moiety is restrained and the ligand can be locked in an
atropisomerically chiral conformation. Also, the strong and
directional bonding interactions could transmit the original
chirality from one metal center to another, which can lead to
formation of the chiral networks.
In this work, the synthesis of bis[4-(3,5-dicarboxyphenyl)-1H-
3,5-dimethylpyrazolyl]methane (Scheme1) was achieved and a
he fabrication of novel metal−organic frameworks (MOFs)
T
is of current interest in the field of supramolecular
chemistry, which originates from their potential applications as
functional materials1−4 as well as their intriguing molecular
topologies.5 In particular, the synthesis of chiral MOFs is of
immense interest in chemistry such as in enantioselective
catalysis6 and chiral separation7 as well as materials science
such as nonlinear optics8 and magnetism.9
Scheme 1. Schematic Diagram of the V-Shaped Ligand L and
Synthetic Procedure for 1
To date, the majority of chiral MOFs and coordination
polymers are prepared from enantiopure ligands, which make
them homochiral in nature.10 In contrast, the use of achiral
ligands to produce chiral MOFs under spontaneous resolution
can be more attractive.11 This is due to the easier synthesis of
achiral ligands in comparison to the laborious preparation of
chiral molecules. There are relatively few examples of the use of
achiral ligands in the self-assembly of chiral MOFs.12 So far, our
understanding of the fabrication of chiral MOFs structured from
achiral ligands is very limited and also unpredictable.13
Therefore, the development and investigation of new organic
struts and their use in the preparation of chiral MOFs are
necessary to gain in-depth knowledge of chiral induction in
solids.
CdII-ion-based chiral MOF, {[Cd2(L)(H2O)(DMF)]·3DMF·
2H2O}n (1; DMF = N,N-dimethylformamide; see the Support-
ing Information, SI), has been constructed. Remarkably, 1 has
been obtained by spontaneous resolution upon crystallization in
the absence of any other chiral source. Also, the MOF presents an
unprecedented 3,6-connected binodal sit 3,6-conn topology.
Because of the presence of potential Lewis acidic sites in 1, we
have explored the heterogeneous catalytic activity for the three-
component Strecker reaction in a solvent-free state, which is used
to prepare α-aminonitriles from a carbonyl, an amine, and a
cyano derivative. As reported earlier, ketones gave a very poor
yield in this reaction, for which In3+-based MOFs have been
effectively used as heterogeneous catalysts.18 A few other reports
Among the homochiral MOFs, chiral binaphthyl-based MOFs
are well investigated by the group of Lin.14 Because of the
restricted C−C bond rotation, a natural helicity is obtained in
these molecules. It has also been observed that V-shaped
exobidentate organic molecules can also induce helical net-
Received: July 27, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01915
Inorg. Chem. XXXX, XXX, XXX−XXX

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have also shown the Strecker reaction catalyzed by MOFs,19 but
d10 metal-ion-based MOFs have not been explored until now for
catalysis of this reaction.
symbol of {4.62}2{42.610.83}, which is assigned to a rare
topological type, sit 3,6-conn (topos&RCSR.ttd).
Upon analysis of the structure, the metal-ion centers are not
stereogenic and not responsible for the chirality induction; it is
more closely related to the V-shaped ligand. Here, metal
coordination of the chelating bis(pyrazole) moiety has locked the
ligand into one conformation. Therefore, rotation is restricted,
and with selective and directional coordination with the help of
the metal ion, it gives rise to a chiral network. To gain more
insight into the cause of the chirality in 1, we have carried out the
CD spectra of the ligand alone and the 1:1 Cd(ClO4)2/L
complex (see the SI) in DMF; it has been found that the free L
does not show any signal but the Cd(ClO4)2/L complex shows a
well-defined CD spectrum, asserting chirality generation in the
complex due to the restricted rotation (Figures S8 and S9). The
complex has also been characterized by electrospray ionization
mass spectrometry, which confirms 1:1 Cd(ClO4)2/L complex
formation (Figure S10). Moreover, we have carried out the CD
spectrum of the mother liquor after MOF filtration (Figure S35).
However, no CD activity has been shown probably because of
the fact that the solid MOF after crystallizing out is insoluble in
the solvent and any chance of the presence of other enantiomers
in the mother liquor is negligible.
The ligand L was synthesized in five steps starting from readily
available acetylacetone in 47.3% overall yield (Scheme S1). It has
been characterized by 1H and 13C NMR and mass spectrometry
(Figures S1−S5). The solvothermal reaction of Cd(NO3)2 with
L in DMF/water (H2O) for 3 days afforded colorless prismatic
crystals of 1 (see the SI). Single-crystal X-ray analysis reveals that
1 crystallizes in the chiral space group P1 and has a 3D open
structure constructed from Cd−L helical chains [Figure S11;
Flack parameter = 0.007(15)]. The solid-state circular dichroism
(CD) spectrum confirmed the chirality of the bulk material of 1,
which showed a positive Cotton effect (Figure S36). Single-
crystal X-ray structure analysis reveals that the asymmetric unit
contains two CdII ions, one ligand L4−, and one each of the
coordinated DMF and H2O molecules along with three DMF
and two H2O solvent molecules in the lattice. As shown in Figure
1a, the Cd1 center is in a distorted octahedral geometry by
TGA and variable-temperature powder X-ray diffraction (VT-
PXRD) measurements were carried out to examine the thermal
stability of the porous network. TGA of the as-synthesized,
methanol-exchanged, and activated samples of 1 was performed
(Figures S16 and S17). The thermal stability and phase purity of
1 was also confirmed by VT-PXRD (Figure S14). The
experimental PXRD patterns for the as-synthesized compound
were in good agreement with the simulated pattern of 1. Further,
the VT-PXRD data revealed that the overall framework integrity
of 1 is retained up to 210 °C.
Figure 1. (a) Coordination environment of cadmium(II). (b)
Coordination mode of the ligand L4−.
coordinating to two N atoms (N1 and N3) of the chelating
bis(pyrazolyl)methane moiety and four O atoms (O1, O2, O3,
and O5) of three carboxylate groups of ligand L4−. One of the
−COOH groups is in the chelating mode, while two others are in
monodentate and bidentate modes, respectively. The Cd2 center
also shows distorted octahedral geometry with ligation from four
O atoms (O4, O7, O8, and O9) of three carboxylate groups of
L4− and two O atoms of axially coordinated DMF and H2O
molecules, respectively. The two metal centers form a dinuclear
cluster (Cd···Cd distance, 3.818 Å) via two bridging COOH
groups of L4−. The angle between the two planes of the pyrazole
rings in L was 131.53° (Figure 1b). Also, the phenyl rings rotate
with angles of 42.42° and 55.80° with respect to the adjoining
pyrazole rings. The angle of the N2C22N4 moiety of bis(pyrazole)
is found to be 111.39°.
In the as-synthesized material, the coordinated DMF and H2O
as well as guest solvent molecules can readily be removed at high-
temperature and vacuum conditions to obtain the activated
compound 1′. It also exhibits open pores of ∼11 × 10 Å2, which
are readily accessible and present a surface lined with two open
CdII sites with chiral pores. Site I has a coordinated DMF solvent
molecule, while site II has a coordinated H2O molecule (Figure
S39). The porosity of 1 was examined on an activated sample by
the gas-sorption measurements. Activation was carried out by
placing the methanol-exchanged sample under vacuum at 120 °C
for 12 h. The gas-sorption measurements indicate that it can
adsorb appreciable amounts of carbon dioxide (CO2) but a
negligible amount of N2 at 77 K (Figure S33). The CO2
adsorption measurement at 273 K and 1 atm revealed a type I
isotherm with a saturated CO2 uptake of 43.2 cm3 g−1, which is
characteristic of a microsporous material corresponding to a
Brunauer−Emmett−Teller surface area of 382 m2 g−1 (Figure
S40). Furthermore, at 298 K, the maximum uptake of CO2 is 26
cm3 g−1 at 1 atm. The isosteric heat (Qst) of CO2 was also
calculated from the isotherms at 273 and 298 K. The initial Qst
value was found to be 32.69 kJ mol−1, which implies a high
binding affinity of the framework toward CO2 (Figure S34).
The microporous nature and coordinatively unsaturated open
metal sites, which are well positioned to interact with guest
molecules that enter the framework pores, suggested that 1′
should function as an active heterogeneous catalyst for Lewis acid
promoted reactions.
The isophthalate moiety of L4− and CdII dimer center form a
2D zigzag sheet structure perpendicular to the bc plane (Figure
S12). The sheets were further connected to each other with the
help of another ligand L4−, which is linked to the chelating
bis(pyrazolyl)methane moiety to form an overall 3D porous
structure (Figure S37). The network contains open channels
(with an opening of ∼11 × 10 Å2) with coordinated H2O and
DMF molecules that point toward the pore wall of the channels.
The cavities in the network are filled with DMF and H2O solvent
molecules. The total potential solvent-accessible void volume of
1 was 53.4% (611.9 Å3/1145.5 Å3) when coordinated solvent
molecules were also removed to form 1′.
Topologically, each dinuclear metal cluster in 1 connects to six
adjacent clusters through half of the ligand L4−, which is acting as
a 3-connected node, affording a 3,6-connected framework
(Figure S38). The whole network can thus be viewed as a
binodal 3,6-c net with stoichiometry (3-c)2(6-c). It has a point
After reviewing the literature, we found that the Strecker
reaction, especially for ketones, has not been investigated by d10
metal-ion-based MOFs as catalysts. Therefore, catalytic activity
B
DOI: 10.1021/acs.inorgchem.7b01915
Inorg. Chem. XXXX, XXX, XXX−XXX

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of 1′ was undertaken for the three-component Strecker reaction
in a solvent-free state. This reaction provides a most direct and
viable method for the synthesis of α-aminonitriles, which are
versatile building blocks for the synthesis of α-amino acids and
their derivatives.20
The heterogeneous nature of 1′ was evaluated by performing the
leaching test (Figure S31). After the beginning of reaction for 1 h,
the solid 1′ was separated from the reaction mixture and the
mother liquor was transferred to an empty Schlenk flask. The
reaction progress was not observed after 4 h which proves that
the catalyst 1′ is necessary to perform the reaction.
To check the feasibility of 1′ for the three component Strecker
reaction, we began with the simplest aldehyde, benzaldehyde and
trimethylsilyl cyanide (TMSCN), and aniline as substrates. For
the reaction, catalytic amount of 1′ was placed in a Schlenk tube
followed by addition of three reactants. The reaction was
performed in a solvent free state at room temperature under N2
atmosphere. The increment of the reaction was monitored by
TLC analysis. Formation of the desired products, α-amino
To better understand the relationship between the structure
and reactivity, we proposed a mechanism for the reaction
catalyzed by 1 (Figure S32). The MOF 1 possesses an open-
framework structure having Lewis acidic sites that can activate
the imine intermediate (electrophile) and therefore lead to
inherently high surface reactivity per unit area. Besides this, 1′
possesses an accessible O atom of one monodentate coordinated
−COOH group, which imparts basic character (Lewis basicity)
to the framework and can help in driving the reaction by
activating the TMSCN (nucleophile).
1
nitriles, was confirmed by the H NMR spectra (Figures S19−
S29). The results of the reaction are summarized in Table 1. To
In summary, we report herein one chiral CdII MOF with a V-
shaped bis(pyrazole)-based achiral ligand by spontaneous
resolution. The MOF represents a rare sit 3,6-conn topology.
The activated framework is thermally stable and contains open
Lewis acid sites. The microporous nature of 1 was proven by the
gas-sorption experiment. The framework is able to catalyze the
three-component Strecker reaction in solvent-free conditions at
room temperature. To the best of our knowledge, it is the first
CdII-based MOF that can catalyze the Strecker reaction of
ketones in high conversion yields.
Table 1. Results Obtained for the Strecker Reaction Catalyzed
by 1′
a
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01915.
Experimental details, characterization procedures, crystal-
lographic data with refinement details, selected bond
lengths and angles, TGA, and PXRD patterns (PDF)
Accession Codes
CCDC 1562129 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
a
Reaction conditions: ketone (0.1 mmol), amine (0.1 mmol),
TMSCN (0.12 mmol), and catalyst 1′ (10 wt %) at 25 °C for 4 h.
b
Isolated yields after silica gel chromatography.
AUTHOR INFORMATION
Corresponding Author
*E-mail: pkb@iitk.ac.in.
■
our delight, benzaldehyde showed very good conversion yields
(99%) which further inspired us to check the more reluctant
substrate, different ketones, for the reaction. The results of the
catalytic activity of 1′ (Table 1) reveal that both electron-
withdrawing as well as electron donating groups attached to
ketone molecules gave good yields. We have also checked the
enantiomeric excess (ee) of the products but unfortunately, no ee
was observed. The probable reason can be the bulky phenyl
groups in the two substrates which possibly are not able to
penetrate inside the channels of the MOF and as a result cannot
induce asymmetric catalysis. The catalyst was separated by
filtration, washed with chloroform and methanol, followed by
drying under vacuum at 100 °C for 5 h to regenerate the active
catalyst 1′. The integrity of the framework was maintained even
after four cycles of the reaction, as confirmed by the PXRD of the
recovered catalyst (Figure S18). Also, it can be reused up to three
cycles with slight loss (11%) in its catalytic activity (Figure S30).
ORCID
Parimal K. Bharadwaj: 0000-0003-3347-8791
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from the MNRE,
New Delhi, India (to P.K.B.), and the SRF from IIT Kanpur,
India (to A.V.), and financial support in the form of a postdoc
fellowship from SERB (DST) (YSS/2015/001088) to K.T.
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DOI: 10.1021/acs.inorgchem.7b01915
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DOI: 10.1021/acs.inorgchem.7b01915
Inorg. Chem. XXXX, XXX, XXX−XXX