A microporous metal-organic framework catalyst for solvent-free strecker reaction and CO2 fixation at ambient conditions

Article abstract of DOI:10.1021/acs.inorgchem.9b03051

The self-assembly of zinc(II) acetate tetrahydrate, a flexible tetrapyridyl ligand, tetrakis(3-pyridyloxymethylene)methane (3-tpom), a bent dicarboxylic acid, and 4,4′-(dimethylsilanediyl)bis- benzoic acid (H₂L) under solvothermal conditions ha

Full text of DOI:10.1021/acs.inorgchem.9b03051

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Article
A Microporous Metal−Organic Framework Catalyst for Solvent-free
Strecker Reaction and CO2 Fixation at Ambient Conditions
Vijay Gupta and Sanjay K. Mandal*
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ABSTRACT: The self-assembly of zinc(II) acetate tetrahydrate, a
flexible tetrapyridyl ligand, tetrakis(3-pyridyloxymethylene)methane
(3-tpom), a bent dicarboxylic acid, and 4,4′-(dimethylsilanediyl)bis-
benzoic acid (H2L) under solvothermal conditions has resulted in the
formation of a microporous zinc(II)−organic framework, {[Zn2(3-
tpom)(L)2]·2H2O}n (1). The framework exhibits very good thermal
stability as evident from the thermogravimetric analysis, which is
further supported by variable temperature powder X-ray diffraction
analysis. The microporous nature of the framework has been
established by the gas adsorption analysis. The framework exhibits
exceptionally selective carbon dioxide adsorption in contrast with
other gases having comparatively larger kinetic diameters (3.64 Å for
N2 and 3.8 Å for CH4) under ambient conditions (298 K and 1 bar
pressure). Further, the framework decorated with catalytically active unsaturated metal sites acts as a good catalyst toward the
cycloaddition reaction of CO2 with epoxides and the three-component Strecker reaction at ambient conditions and without the
requirement of any solvent. The heterogeneous nature along with good catalytic activity at ambient and solvent-free conditions
entitles 1 as an excellent catalyst for these organic transformations.
feedstock and the product cyclic carbonates are important
chemical intermediates for several fine chemicals and
pharmaceuticals.18,20−27 Therefore, this approach not only
provides a route to a valuable chemical commodity but also
helps to reduce the greenhouse emissions.28−30 Taking into
account the issues of catalyst reusability, rigorous separation,
and purification of the product in homogeneous catalytic
processes,17,22,31−36 many efforts have been made in order to
develop efficient heterogeneous catalytic systems for the
chemical fixation of CO2 with epoxides,37−46 but most of
them suffer from low catalytic activity and require harsh
reaction conditions such as high temperatures and elevated
pressures of CO2 to activate the reaction. As discussed above,
MOFs have been cited as potential contenders for heteroge-
neous catalysis. By virtue of their large surface area, high CO2
selectivity, and pore surface decorated with Lewis/Brønsted
acidic or basic sites along with the exceptional physicochemical
stability, some of them have been tested as successful
heterogeneous catalyst for the chemical fixation of CO2 with
INTRODUCTION
■
Metal−organic frameworks (MOFs) have emerged as very
provocative porous crystalline materials and have captured
significant scientific interest due to their extraordinary
structural tunability and functionality.1,2 The mixing of
different building blocks (metal and organic ligands) into the
MOF structures imparts interesting physical and chemical
properties to the framework. The tailor-made synthesis and
various synthetic routes enable the realization of MOF
structures showing exceptional physicochemical stability and
multifunctionality. The structural tenability of MOFs materials
makes them potential candidates for a diverse range of
applications,3−5 in particular, for heterogeneous catalysis.6−9
Catalytic activity into the MOF structures can be incorporated
either by engineering coordinatively unsaturated metal
sites10,11 or by introducing catalytically active functional
groups into the linker backbone.10−12
Among the various organic transformations, the chemical
exploitation of carbon dioxide (CO2) has been widely accepted
as a greener and sustainable route to the syntheses of fine
chemicals.13−16 However, the comparatively stable and inert
nature of CO2 limits the chemical fixation under mild
conditions. In this context, metal-catalyzed cycloaddition of
CO2 with high energy substrates such as epoxides or
azaridines, etc., to generate cyclic carbonates or carbamates
is a one of the potential methodologies for CO2 utilization17−19
as CO2 presents an inexpensive, nontoxic, and renewable C1
Received: October 17, 2019
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epoxides;47−57 unfortunately, these represent just a small
fraction of the entire MOF family. In view of these facts, we
intended to design a highly stable and robust MOF with
coordinatively unsaturated metal sites for efficient fixation of
CO2 to cyclic carbonates at ambient conditions. In addition,
among various Lewis-acid-catalyzed organic transformations,
the three-component Strecker reaction is an important C−N
bond forming reaction where the product α-aminonitriles are
adaptable starting materials for the synthesis of α-amino acid
and their derivatives.58
In the present work, we have synthesized a Zn(II)-based
metal−organic framework {[Zn2(3-tpom)(L)2]·2H2O}n (1)
using a bent dicarboxylic acid 4,4′-(dimethylsilanediyl)bis-
benzoic acid (H2L) and an heteroatom-functionalized semi-
rigid tetrapodal N-donor ligand, tetrakis(3-
pyridyloxymethylene)methane (3-tpom) (Scheme 1) with a
analysis was carried out under a flow of dinitrogen in the temperature
range of 30−500 °C (at a heating rate of 10 °C/min) on a Shimadzu
DTG-60H instrument. FTIR spectra of 1 and the ligands were
recorded in the 4000−400 cm−1 range on a PerkinElmer Spectrum I
spectrometer with samples prepared as KBr pellets. The recording of
powder X-ray diffraction data was accomplished by using parallel
beam geometry (2.5° primary and secondary solar slits, 0.5°
divergence slit with 10 mm height limit slit) on a Rigaku Ultima IV
diffractometer equipped with 3 KW sealed-tube Cu Kα X-ray
radiation (generator power settings: 40 kV and 40 mA) and a
DTex Ultra detector. Each sample was made into a fine powder using
a mortar and a pestle and was placed on a glass sample holder for the
room temperature measurement, while a copper sample holder was
used for the variable temperature measurement. Data were collected
over a 2-theta range of 5−50° (5−40° for VT-PXRD) with a scanning
speed of 2° per minute with 0.02° step. Gas sorption analysis was
carried out on a BELSORP MAX (BEL, Japan) volumetric adsorption
analyzer at different temperatures. Each gas of very high purity
(99.995%) was used for the adsorption measurements. Powdered
sample was activated at 393 K for 24 h and purged with ultrapure
nitrogen gas on cooling. The measurement temperatures for sorption
isotherms at different temperatures (263−298 K) were maintained
with a circulating water bath connected to a chiller.
Scheme 1. Chemical Structures of the Conformationally
Flexible 3-tpom Ligand and Bent Dicarboxylate H2L
Synthesis of {[Zn2(3-tpom)(L)2]·2H2O}n (1). 30 mg (0.1 mmol)
of H2L and 22 mg (0.05 mmol) of 3-tpom were placed in a Teflon
reactor along with 1 mL of DMF. A solution of 22 mg (0.1 mmol) of
zinc acetate dihydrate in 4 mL of an ethanol/water (1:3) mixture was
then added to this solution. The reactor was sealed in a stainless steel
vessel and heated in a programmable oven at 120 °C for 48 h followed
by slow cooling to room temperature at a rate of 3−4 °C/h. The
colorless block-shaped crystals thus obtained were collected via
filtration, washed with DMF and then with water, and air-dried. Yield:
42 mg (66%). Anal. calcd for C59H64N4O16Si2Zn2 (MW 1272.09):
calcd %C, 55.70; %H, 5.10; %N, 4.41. Found: %C, 56.02; %H, 5.46;
%N, 4.82. Selected FTIR peaks (KBr, cm−1): 3430 (br), 1624 (s),
1601 (m), 1576 (s), 1548 (m), 1482 (m), 1435 (s), 1390 (s), 1369
(s), 1283 (s), 1244 (s), 1190 (w), 1101 (s), 1056 (m), 1018 (m), 829
(m), 814 (s), 772 (s), 760 (s), 722 (m), 697 (m), 642 (w).
Single Crystal X-ray Data Collection and Refinement. A
suitable single crystal of 1 was chosen using an optical microscope and
mounted in a nylon loop attached to a goniometer head. It was cooled
to 100 K with a programmed flow of nitrogen gas. Based on the
diffraction photographs and unit cell determination, its suitability was
judged for collecting data on a Kappa APEX II diffractometer
equipped with a sealed tube X-ray source with graphite mono-
chromated Mo Kα radiation (λ = 0.71073 Å) operating at 50 kV and
30 mA (with the crystal-to-detector distance fixed at 50 mm) through
the program APEX2,61 as described earlier from our laboratory.59
Data were integrated by the program SAINT61 to obtain values of F2
and σ(F2) for each reflection and to apply further correction for
Lorentz and polarization effects and absorption (SADABS).61 The
structure solution was accomplished by direct method using SHELXS
program of SHELXTL package and refined by full-matrix least-
squares methods with SHELXL-201462 within the Olex2 crystallo-
graphic software suite.63 The chosen space group based on systematic
absences was confirmed by the successful refinement of the structure.
For the completion of structure refinement with convergence, several
full-matrix least-squares/difference Fourier cycles were performed.
The treatment of disordered guest solvent molecules in 1 was handled
by the solvent mask option in Olex2 software,63 and the potential
solvent accessible area or void space was calculated using the
PLATON software.64 Final crystallographic parameters and basic
information pertaining to data collection and structure refinement are
summarized in Table 1. Selected bond lengths (Å) and bond angles
(degree) are listed in Table S1. All figures were drawn using Mercury
ver. 3.065 and Diamond ver. 3.2.66
motive to engineer a porous 3D framework with Lewis-acidic
unsaturated metal sites and basic oxygen functionalities for
selective CO2 capture and its fixation to cyclic carbonates. The
single-crystal X-ray diffraction (SCXRD) and thermogravi-
metric (TG) analysis reveal that 1 consists of a stable 3D
structure. The powder X-ray diffraction (PXRD) analysis
further corroborates thermal and chemical stability of 1 toward
different organic solvents. Gas adsorption analysis exhibits the
microporous nature of 1 and shows that 1 takes up a moderate
amount of CO2 at 1 bar pressure and exhibits very good
selectivity over other gases (N2 and CH4). Further, as a result
of the coordinatively unsaturated metal centers in the
framework, 1 shows excellent heterogeneous catalytic activity
for the chemical fixation of CO2 with epoxides at relatively
milder conditions. In addition, 1 is also found to catalyze the
three-component Strecker reaction involving carbonyl com-
pounds, aromatic amine, and trimethylsilyl cyanide in a
solvent-free condition and at ambient conditions to afford α-
aminonitriles in good yields.
EXPERIMENTAL SECTION
■
Materials and Physical Measurements. All reactions of n-BuLi
were carried out in oven-dried glassware under an inert nitrogen
atmosphere by using standard Schlenk techniques. Solvents were
predried with KOH followed by distillation over benzophenone−Na,
before their use in the reactions involving n-BuLi. The ligands H2L
and 3-tpom were synthesized using procedures in the literature.59,60
All other chemicals and reagents used for synthesis were purchased
from commercial vendors and were used as received.
The 1H and 13C NMR spectra of H2L and 3-tpom were recorded at
25 °C on a Bruker ARX-400 spectrometer in DMSO-d6 and CDCl3
solutions, respectively. The chemical shifts are reported relative to the
residual solvent signals. The elemental analysis (C, H, N) was carried
out using Leco TruSpec Micro CHNS analyzer. Thermogravimetric
General Procedure for the Chemical Fixation of CO2 with
Epoxides Catalyzed by 1. In a typical reaction, 150 μL (2 mmol) of
epichlorohydrin was taken along with 0.5 mol % of the desolvated 1
and TBAB cocatalyst, and the mixture was allowed to react with CO2
B
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Table 1. Crystallographic Data and Structure Refinement
Scheme 2. Synthesis of 1
Parameters for 1
empirical formula
formula weight
temperature/K
crystal system
space group
a/Å
b/Å
c/Å
α/°
β/°
γ/°
volume/Å3
Z
ρcalc (g/cm3)
μ/mm−1
F(000)
crystal size/mm3
radiation
2θ range for data collection/° 3.048 to 50.072
index ranges
reflections collected
independent reflections
data/restraints/parameters
goodness-of-fit on F2
final R indexes [I ≥ 2σ(I)]
final R indexes [all data]
largest diff. peak/hole/e Å−3
Flack parameter
C57H52N4O12Si2Zn2
1171.94
100.15
monoclinic
Cc
8.3003(5)
26.7153(18)
26.3110(17)
90
90.138(2)
90
5834.3(6)
4
1.334
0.925
asymmetric unit. An ORTEP view of the asymmetric unit with
an atom labeling scheme is shown in Figure S1. Each Zn(II)
center adopts a N2O2-type tetrahedral coordination geometry
with four coordination sites being occupied by two oxygen
atoms from the carboxylate groups of two different L2− and
two nitrogen atoms from two different 3-tpom ligands. The
asymmetric unit is further expanded by full span of L2− and 3-
tpom to generate the overall 3D framework (Figure 1). Further
simplification of the framework into the node and linker
representation reveal that the framework exhibits a 4,4,4-
connected 3-nodal network topology (Figure S2), obtained by
the TOPOS program.67 The framework consists of open pores
functionalized with four basic oxygen atoms of the 3-tpom
ligand, and the coordinatively unsaturated Lewis-acidic metal
sites imparts dual functionality to the framework. The total
solvent accessible area or void space for 1 was estimated to be
13.1% (764/5834 Å3 per unit cell volume) by the PLATON
software.64 The presence of Lewis acidic metal sites and
heteroatom functionality in 1 may act cooperatively for
selective CO2 capture and its chemical fixation.
The phase purity of 1 in bulk was checked by PXRD
analysis. As shown in Figure S3, the PXRD pattern of the as-
synthesized, as well as the activated sample of 1, closely
matches with that of the simulated PXRD pattern (obtained
from the SCXRD data), indicating phase purity and retention
of crystallinity upon thermal activation of the framework. TGA
and in situ variable temperature PXRD measurements were
carried out in order to check the thermal stability of 1. The
TGA profile of 1 shows an initial weight loss of ≈3.5% in the
temperature range of 50−150 °C (Figure S4), which can be
attributed to the loss of lattice solvent molecules (ca. 2.98%).
The framework exhibits excellent stability up to 325 °C as no
weight loss can be seen up to this temperature, after which a
sudden weight loss indicates the framework decomposition.
The TGA profile of the desolvated/activated sample of 1 does
not show appreciable weight loss up to 325 °C, which further
supports that the initial weight loss in the as-synthesized
sample was due to the lattice solvent molecules. The in situ
variable temperature PXRD experiment further shows that 1 is
stable up to 300 °C (Figure 2a), and this data corroborates
well with the TG experiments. In addition, the stability of 1
toward common organic solvents was also tested by PXRD
diffraction analysis. The as-synthesized sample of 1 was
immersed in different solvents for two days. After separation
and drying of the sample, the PXRD data of 1 was recorded at
room temperature. As shown in Figure 2b, 1 exhibits very good
chemical stability toward these solvents in addition to its
excellent thermal stability.
2424.0
0.15 × 0.15 × 0.1
Mo Kα (λ = 0.71073)
−9 ≤ h ≤ 9, −31 ≤ k ≤ 31, −31 ≤ l ≤ 31
36720
9764 [Rint = 0.0461, Rsigma = 0.0621]
9764/2/694
1.076
a
b
R1 = 0.0407, wR2 = 0.0973
R1 = 0.0502, wR2 = 0.1158
0.56/−0.36
0.033(15)
a
b
R1 = Σ∥F0| − |Fc∥/Σ|F0|. wR2 = [Σw(F0 − Fc2)2/Σw(F0 )2]1/2
,
2
2
where w = 1/[σ2(F0 ) + (aP)2 + bP], P = (F0 + 2Fc2)/3.
2
2
that was provided with a balloon filled with CO2. The reaction was
carried out at 40 °C in a solvent-free conditions. After the reaction
completion, the product was extracted with chloroform and the solid
catalyst was separated by filtration. The solvent was evaporated, and
1
the product yield was determined by H NMR spectroscopy.
General Procedure for the Three-Component Strecker
Reaction Catalyzed by 1. In a typical reaction, a mixture of
acetophenone (1 mmol), aniline (1 mmol), and TMSCN (1.25
mmol) was taken along with 1 mol % of the desolvated 1. The
reaction mixture was allowed to stir at room temperature (25−30 °C)
under an inert atmosphere of nitrogen in solvent-free conditions. The
product was extracted with chloroform and the solid catalyst was
separated by filtration. The product was obtained by solvent
evaporation, and the yield was determined by 1H NMR spectroscopy.
Recyclability of 1. The recyclability experiment for both of the
reactions was carried out under the similar reaction conditions. The
catalyst was retrieved by filtration, washed with chloroform, and
heated under vacuum for 5 h to regenerate the active catalyst, before
it was used for the next catalytic cycle.
RESULTS AND DISCUSSION
■
The reaction of Zn(OAc)2·2H2O with H2L and 3-tpom under
solvothermal conditions afforded colorless block-shaped
crystals of 1 in 66% yield (Scheme 2). Its molecular formula
was established as {[Zn2(3-tpom)(L)2]·2H2O}n by a combi-
nation of elemental analysis, TG, and SCXRD analyses.
Single-crystal X-ray diffraction analysis concedes that 1
crystallizes in a monoclinic crystal system, with the Cc space
group (no. 9), with lattice parameters a = 8.3003(5) Å, b =
26.7153(18) Å, and c = 26.3110(17) Å (Table 1). There are
two zinc atoms, two L2−, and one 3-tpom ligand in the
Gas adsorption studies were carried out in order to establish
the permanent porosity in 1. The PXRD and TG analysis
corroborates the retention of framework integrity and loss of
lattice solvent molecules upon thermal activation process
(Figures S3 and S4). The CO2 adsorption isotherm at 195 K
clearly demonstrate its microporous nature, and interestingly
enough, 1 takes up a much smaller amount of N2 (77 K)
compared to CO2 (195 K) (Figure S5).
The Brunauer−Emmett−Teller (BET) surface area and the
Langmuir surface area were estimated to be 135 and 180 m2
C
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Figure 1. Single-crystal structure of 1 with open channels (center) and enlarged views of the Lewis-basic and coordinatively unsaturated Lewis-
acidic metal sites in the framework.
Figure 2. (a) In situ variable-temperature powder XRD patterns of as-synthesized 1 and (b) powder XRD patterns of as-synthesized 1 after soaking
in different solvents (for 2 days) at room temperature, indicating the thermal robustness and stability of 1 toward these solvents.
Figure 3. Binary mixture adsorption isotherms and selectivities for (a) CO2/N2 (15/85) and (b) CO2/CH4 (50/50) mixtures at 298 K and 1 bar
pressure, obtained from IAST.
g−1, respectively (Figure S6), based on the CO2 adsorption
data at 195 K. The total pore volume was calculated to be
0.0608 cm3 g−1. Single-component gas adsorption measure-
ments were carried out for N2, CH4, and CO2 at 298 K and 1
bar pressure. As shown in Figure S7, 1 takes up a relatively
higher amount of CO2, which can be ascribed due to the
unsaturated metal sites and plausible quadrupole−quadrupole
interactions among CO2 molecules coupled with exposed
oxygen functionalities into the pores channels for enhanced
Lewis base−Lewis acid interactions.68,69 In contrast, 1 exhibits
D
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a
Table 2. Optimization of the Reaction Conditions for the Cycloaddition of CO2 with Epichlorohydrin Catalyzed by 1
b
d
e
entry
1 (mol %)
0.5
TBAB (mol %)
T (°C)
t (h)
yield (%)
TON
TOF (min−1
)
1
2
3
4
5
6
7
8
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
−
40
40
40
40
40
40
40
40
60
80
40
40
40
5
10
15
20
24
20
12
12
10
5
34
65
85
91
>99
96
97
>99
97
>99
10
15
68
130
170
182
198
96
0.22
0.21
0.18
0.15
0.013
0.08
0.26
0.07
0.32
0.66
0.0013
−
0.5
0.5
0.5
0.5
1
2
3
0.5
0.5
0.5
48.5
33
9
194
10
11
12
13
198
20
−
24
24
24
−
0.5
0.5
c
Zn(OAc)2
18
−
−
a
Reaction conditions: epichlorohydrin (2 mmol) was reacted with CO2 under one atmospheric pressure maintained with a balloon filled with CO2
b
gas, in the presence of 1 and cocatalyst tetra(tert-butyl) ammonium bromide (TBAB) in a solvent-free condition. Determined by 1H NMR of the
crude products. 1 mol % of zinc acetate dihydrate was used instead of 1. Number of moles of product per mole of catalyst. TON per minute.
c
d
e
much less uptake capacities for N2 and CH4 at the same
temperature and under the same pressure. The quantitative
binding strength of the CO2 molecules with the framework was
estimated by the Clausius−Clapeyron equation and a well-
known virial equation from the fits of their adsorption
isotherms at different temperatures (Figures S8 and S9). The
trend of isosteric adsorption enthalpy (Qst) with respect to
surface coverage is presented in Figure S10. The Qst value at
zero coverage (32.65 kJ mol−1) is comparable with the values
reported for some-well-known MOF materials (Table S2).
Although 1 represents very low CO2 uptake compared to well-
known MOF materials (Table S3), the selectivity of 1 for CO2
is comparatively higher than that of most of the MOFs. The
gas mixture adsorption selectivity was also evaluated using
ideal adsorption solution theory (IAST) developed by Myers
and Prausnitz for binary mixtures.70 The CO2−N2 and CO2−
CH4 selectivity at a general feed composition for flue-gas
(CO2/N2 = 15/85) and landfill gas (CO2/CH4 = 50/50) up to
1 bar pressure and at 298 K are presented in Figure 3. The
CO2/N2 and CO2/CH4 selectivity values of 87.6 and 15.7,
respectively, indicate the very good selectivity and separation
ability of 1 for CO2. Further the values are higher than the
selectivity values for well-known MOF and ZIF materials
(Table S5).
The preferential adsorption and strong interaction of CO2
with the framework was further established by grand canonical
Monte Carlo (GCMC) simulation using a material studio
program71 (see Supporting Information, Figure S11). The
presence of high density Lewis-acidic unsaturatively coordi-
nated metal sites and highly selective CO2 affinity prompted us
to study the potential of 1 as an active heterogeneous catalyst
for the cycloaddition reaction of CO2 with epoxides to form
value-added cyclic carbonates at ambient conditions (30−40
°C and under 1 atm pressure of CO2). For a standard reaction,
epichlorohydrin was chosen with 1 as a catalyst (0.5 mol %) in
conjunction with tetra(tert-butyl)ammoniumbromide (TBAB)
as a cocatalyst (0.5 mol %). As shown in Table 2, 1 exhibits
very good catalytic activity for the cycloaddition reaction of
epichlorohydrin with CO2 at 40 °C with a yield of 99% over 24
h. The reaction condition was optimized for different reaction
parameters, such as temperature, reaction time, and amount of
the catalyst. With an increase in the amount of catalyst (3 mol
%) and reaction temperature (80 °C), the reaction completes
in only 12 and 5 h, respectively (entries 8 and 10, Table 2),
whereas a very low product yield in the absence of either 1 or
cocatalyst TBAB (entries 11 and 12, Table 2) indicates the
synergistic role of 1 and TBAB during the catalytic process.
In view of the good catalytic activity compared to other
MOF materials (Table S6) and green chemistry principles, we
further explored the catalytic activity of this system for the
solvent-free chemical fixation of CO2 with different epoxides at
ambient conditions (40 °C and 1 atm pressure of CO2) using
0.5 mol % of 1 and TBAB. The results are summarized in
Table 3. A very good catalytic activity was observed for the
cycloaddition reaction of propylene oxide, butylene oxide, as
well as epoxyhexane with CO2, resulting in the corresponding
cyclic carbonates in 99%, 94%, and 91% yield in each case
(entries 1, 2, and 4, Table 3). Further, the epoxides substituted
with long alkyl chains (allyl glycidyl ether and butyl glycidyl
ether) and phenyl ring (styrene oxide) also gives the cyclic
carbonates in good yields (entries 5−7, Table 3).
However, further increase in the size of the epoxide substrate
has resulted in a decrease in the product yield. In case of large-
sized phenyl glycidyl ether only 55% product yield was
obtained under the identical reaction conditions (entry 8,
Table 3). This can be due to steric hindrance of the bulkier
substrates to the active metal sites on the surface of 1.
In addition to the catalytic activity of 1 toward chemical
fixation of CO2 with epoxides, it was further tested for its
activity toward Lewis-acid-catalyzed Strecker reaction. The
three-component Strecker reaction between an aldehyde/
ketone, an amine, and cyano compounds is an important C−N
bond forming reaction which is a direct and viable method for
the synthesis of α-aminonitriles. However, there are reports
where MOF materials have been used as a heterogeneous
catalyst for this reaction, but these represent only a few
E
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Table 3. Substrate Scope for the Cycloaddition Reaction of
CO2 with Different Epoxides Catalyzed by 1
Table 4. Substrate Scope for the Three-Component Strecker
Reaction of Different Ketone Derivatives Catalyzed by 1
a
a
a
Reaction conditions: 2 mmol of epoxide was reacted with CO2 under
one atmospheric pressure maintained with a balloon filled with CO2
gas in the presence of 0.5 mol % of 1 and 0.5 mol % of cocatalyst
tetra(tert-butyl) ammonium bromide (TBAB) in a solvent-free
b
condition. Average yield for a set of triplicate experiments
c
1
determined by H NMR of the crude products. Number of moles
d
of product per mole of catalyst. TON per minute.
examples of MOFs derived from d10 metal ions. Also, the
ketone derivatives being less reactive than aldehydes are known
to exhibit very less yield for this reaction.72−75 Therefore, for a
standard reaction, acetophenone was reacted with aniline and
trimethylsilyl cyanide using 1 as the catalyst in a solvent-free
condition. The progress of the reaction was monitored by
1
a
TLC, and the final conversion was calculated by H NMR
Reaction conditions: ketone (0.2 mmol), amine (0.2 mmol),
b
TMSCN (0.25 mmol) and time (5 h). Average percent yield for a
spectroscopy. For a control experiment, a blank reaction
carried out in the absence of 1 gave a much lower product
yield, indicating the requirement of 1 for this reaction.
1
set of triplicate runs, determined by H NMR of the crude products.
TON per minute. Number of moles of product per mole of catalyst.
c
d
e
Blank reaction.
The catalytic activity was further studied for a various ketone
derivative, ranging from substituted acetophenone, aliphatic,
and cycloaliphatic ketones to heteroaromatic ketone deriva-
tives. The results are summarized in Table 4. In all cases a
good product yield was obtained in each case except for 4-
methoxyacetophenone and 3-methyl acetophenone (entries 4
and 5, Table 4), which indicate the very good catalytic activity
of 1. The lower product yields in the cases of 4-
methoxyacetophenone and 3-methyl acetophenone are due
to the electron-donating effect of the substituents, which
decreases the nucleophilicity at the carbonyl carbon and
therefore decreases the reactivity and the product yields. In
order to confirm their identity and purity, the products in each
1
of the cases were isolated and characterized by H and 13C
NMR spectroscopy (Figures S15−S56).
As an important prerequisite of reusability and stability for
heterogeneous catalytic systems, the recycling experiments
were carried out for both reactions. The cycloaddition reaction
of epichlorohydrin and three-component reaction of aceto-
phenone, aniline, and trimethylsilyl cyanide were conducted
using 1 as a catalyst under the optimized reaction conditions.
After completion of the reaction, the catalyst was separated by
centrifugation, washed with DCM, dried under vacuum at 100
°C for 5 h to regenerate the active catalyst, and reused for the
F
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
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Article
next catalytic experiment. The PXRD of the recovered catalyst
in both cases exhibited no loss of crystallinity or phase purity
of the catalyst even after three cycles of the reactions (Figure
S57). Also, a product yield of 96% and 90% obtained in the
third cycle for the cycloaddition reaction and Strecker reaction,
respectively, indicate very little drop in the catalytic activity of
1 for both reactions (Figure S58). Further, to confirm the
heterogeneous nature of 1, the catalytic reactions were stopped
after a certain amount of time, and the catalyst was separated
from the reaction mixture by centrifugation. The reaction was
then continued under the same reaction conditions, during
which no significant increase in the product yield was obtained
(Figures S59−S60), indicating the heterogeneous nature of the
catalyst and its active role as a catalyst in both of the reactions.
The very good catalytic activity of 1, for both reactions, can
be attributed to the presence of the high density of Lewis
acidic coordinatively unsaturated metal sites and pore walls
functionalized with Lewis-basic oxygen atoms of the ligand.
This Lewis acid−base pair is known to act synergistically not
only to enhance the selectivity and uptake capacity of CO2 but
also to enhance the catalytic activity of the system.76,77
On the basis of the present studies and the reported
proposals,47−57,72−75 plausible mechanisms are presented in
Figures S61 and S62 for both the reactions. The Lewis-acidic
metal center interacts with the oxygen atom of epoxide or
carbonyl group (electrophile), leading to the activation of the
epoxy ring or the carbonyl group. In addition, the Lewis-basic
O atoms help in driving the reaction by polarization of the
CO2 and its interaction to the epoxy ring or activating the
corresponding nucleophile to generate the cyclic carbonates
and α-aminonitriles in good yields.
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Sanjay K. Mandal − Department of Chemical Sciences, Indian
Institute of Science Education and Research Mohali, Mohali,
Punjab 140306, India; orcid.org/0000-0002-5045-6343;
Email: sanjaymandal@iisermohali.ac.in
Author
Vijay Gupta − Department of Chemical Sciences, Indian Institute
of Science Education and Research Mohali, Mohali, Punjab
140306, India; orcid.org/0000-0002-1260-8836
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.9b03051
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from IISER
Mohali, India (to S.K.M.) and the 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
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In summary, we have synthesized a microporous 3D MOF,
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b03051.
■
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