A Versatile CuII Metal-Organic Framework Exhibiting High Gas Storage Capacity with Selectivity for CO2: Conversion of CO2 to Cyclic Carbonate and Other Catalytic Abilities

Article abstract of DOI:10.1002/chem.201504747

A linear tetracarboxylic acid ligand, H₄L, with a pendent amine moiety solvothermally forms two isostructural metal-organic frameworks (MOFs) L_M (M=Zn^II, Cu^II). Framework L_Cu can also be obtained from

Full text of DOI:10.1002/chem.201504747

DOI: 10.1002/chem.201504747
Full Paper
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Metal–Organic Frameworks |Hot Paper|
A Versatile CuII Metal–Organic Framework Exhibiting High Gas
Storage Capacity with Selectivity for CO2: Conversion of CO2 to
Cyclic Carbonate and Other Catalytic Abilities
Dinesh De,[a] Tapan K. Pal,[a] Subhadip Neogi,[b] S. Senthilkumar,[b] Debasree Das,[c]
Sayam Sen Gupta,[c] and Parimal K. Bharadwaj*[a]
Abstract: A linear tetracarboxylic acid ligand, H4L, with
a pendent amine moiety solvothermally forms two isostruc-
tural metal–organic frameworks (MOFs) LM (M=ZnII, CuII).
Framework LCu can also be obtained from LZn by post-
synthetic metathesis without losing crystallinity. Compared
with LZn, the LCu framework exhibits high thermal stability
and allows removal of guest solvent and metal-bound water
molecules to afford the highly porous, LCu’. At 77 K, LCu’ ab-
sorbs 2.57 wt% of H2 at 1 bar, which increases significantly
to 4.67 wt% at 36 bar. The framework absorbs substantially
high amounts of methane (238.38 cm3 gÀ1, 17.03 wt%) at
303 K and 60 bar. The CH4 absorption at 303 K gives a total
volumetric capacity of 166 cm3 (STP)cmÀ3 at 35 bar
(223.25 cm3 gÀ1, 15.95 wt%). Interestingly, the NH2 groups in
the linker, which decorate the channel surface, allow a
remarkable 39.0 wt% of CO2 to be absorbed at 1 bar and
273 K, which comes within the dominion of the most
famous MOFs for CO2 absorption. Also, LCu’ shows pro-
nounced selectivity for CO2 absorption over CH4, N2, and H2
at 273 K. The absorbed CO2 can be converted to value-
added cyclic carbonates under relatively mild reaction condi-
tions (20 bar, 1208C). Finally, LCu’ is found to be an excellent
heterogeneous catalyst in regioselective 1,3-dipolar cycload-
dition reactions (“click” reactions) and provides an efficient,
economic route for the one-pot synthesis of structurally
divergent propargylamines through three-component
coupling of alkynes, amines, and aldehydes.
Introduction
mobile applications and those related to the ever-increasing
demand for energy.[8] On the other hand, the rising levels of
CO2 emissions pose a major problem for the global ecosys-
tem[9] and PMOFs could become an alternative to replace the
currently employed industrial method of chemical sorption of
CO2.[10] In addition, CO2 is an abundant C1 resource[11] and its
chemical conversion to value-added products is both environ-
mentally and practically attractive.[12] In this respect, formation
of cyclic carbonates through coupling of CO2 with epoxides is
a useful reaction. Cyclic carbonates formed thus have wide ap-
plications in the pharmaceutical and fine chemical industries.[13]
Although the formation of cyclic carbonate by homogeneous
catalysis has been industrialized,[14] the process requires high
temperature and pressure and has additional difficulties relat-
ed to product separation. The development of a heterogeneous
catalyst for synthesizing cyclic carbonate under mild conditions
remains a challenge.[15] Thus, the sequestration of CO2 with in
situ conversion should prove to be an attractive strategy for
efficiently and profitably reducing CO2 emission.
Porous metal–organic frameworks (PMOFs) have continued to
attract intense research interest because of their potential ap-
plications in gas storage,[1] separation,[2] heterogeneous cataly-
sis,[3] and so on.[4] The application of MOFs is virtually dictated
by its structural features. The careful selection of the linkers,
and modification of pore size/shape and surface area can im-
prove the gas absorption capacity significantly.[5] Alternatively,
coordinatively unsaturated metal centers (UMCs), generated by
removing metal-bound solvent molecules, can enhance the
gas absorption ability.[6] In addition, UMCs can be utilized as
Lewis acids for catalytic reactions.[7] The supramolecular stor-
age of H2 and CH4 gases in PMOFs is of great significance for
[a] D. De, T. K. Pal, Prof. P. K. Bharadwaj
Department of Chemistry, Indian Institute of Technology Kanpur
Kanpur 208016, Uttar Pradesh (India)
E-mail: pkb@iitk.ac.in
[b] Dr. S. Neogi, S. Senthilkumar
Herein, we report a new linear NH2-functionalized ligand,
2’-amino-1,1’:4’,1’’-terphenyl-3,3’’,5,5’’-tetracarboxylic acid (H4L;
Figure 1a). The NH2 group attached to the middle benzene
ring of H4L precludes any metal coordination and can act as
a potential guest interaction site. Under solvothermal condi-
tions, the linker forms two isostructural, non-interpenetrated
PMOFs LM (M=ZnII, CuII), containing paddle-wheel secondary
Inorganic Materials and Catalysis Division
Central Salt and Marine Chemicals Research Institute (CSIR)
Bhavnagar 364002, Gujarat (India)
[c] D. Das, Dr. S. S. Gupta
CReST Chemical Engineering Division
CSIR-National Chemical Laboratory, Pune 411008 (India)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201504747.
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bonding units (SBUs). The accessibility of the ZnII ions from the
large channels facilitates post-synthetic metathesis reactions in
LZn, allowing us to derive the framework LCu by a single-crystal
to single-crystal transmetalation. A number of post-synthetic
metathesis reactions have been reported in recent years
leading to systems endowed with desirable properties.[16]
ces lie in the range 1.979(3)–2.024(2) Š, which are within
normal statistical error.[17] The assembly of the ligand and
paddle-wheel SBUs results in a NbO type or 64·82 topology.[18]
These paddle-wheel SBUs are extended in three dimensions
without any interpenetration, forming a robust structure with
large cages (Figure 1c) of dimension 13.78 Š (distance refers to
atom-to-atom connection throughout) occupied by DMF,
water, and ethanol as guests. Two different channels can be
seen when viewed along the crystallographic c axis (Fig-
ure 1d). Metal-bound water molecules as well as the amine
groups are directed towards the center of the channels. The
majority of the guest molecules are quite disordered. The sol-
vent composition (4DMF+3H2O+1EtOH) was established
from thermogravimetric weight-loss analysis (TGA), IR spectra,
and elemental analysis (see the Supporting Information). The
values are in good agreement with the PLATON-calculated sol-
vent-accessible void volume, which is found to be 62.2% of
the unit-cell volume. The structure of LCu is found to be iso-
structural with that of LZn. The Cu···Cu distance in the paddle-
wheel unit is found to be 2.625(8), which compares well with
the distances found in frameworks with similar structures.[18] All
CuÀO bond lengths are comparable to those found in the liter-
ature for CuII-containing complexes.[19] The total solvent-acces-
sible void volume of 62.2% in LCu is to the same as that of LZn.
Easy accessibility of the paddle-wheel ZnII dimer unit in LZn
(Figure S1 in the Supporting Information) encouraged us to ex-
plore the possibility of transmetalation with CuII ions. When
the as-synthesized crystals of LZn are soaked in a DMF solution
of Cu(NO3)2·3H2O at room temperature, all the ZnII ions are
completely replaced by CuII ions within 2 days without losing
crystallinity and the original shape and size are maintained
(Figure S2 in the Supporting Information). The lattice parame-
ters for this sample (LCu) are found to be same as those for the
as-synthesized LZn within statistical errors (Table S2 in the Sup-
porting Information). In addition, the PXRD pattern of the
metal-exchanged product is comparable to that of the mother
framework (Figure S3 in the Supporting Information). For the
metal-exchanged product, the X-ray fluorescence (XRF) spec-
troscopic studies demonstrate that no significant amount of
ZnII ion is left. The kinetics of metal exchange were monitored
by energy-dispersive X-ray spectroscopy (EDS), which showed
that nearly 50% of the ZnII ions are replaced by CuII within 5 h,
90% are exchanged within 20 h, and complete replacement re-
quires 40 h (Figure 2, and Figures S4 and S5 in the Supporting
Information).
Figure 1. a) Diagram of the linker H4L. b) Coordination environment around
the M2+ ions in LM. c) A view showing the NH2 group decorated cages.
d) View along the c axis, showing two types of pores.
We show that LCu can be activated by removing all solvent
molecules to form a highly porous structure (LCu’), which exhib-
its high-capacity gas absorption, and conversion of absorbed
CO2 into a value-added product. Its versatility as a hetero-
geneous catalyst is further probed with the 1,3-dipolar cyclo-
addition of azides to alkynes (“click” reactions) and three-
component coupling of alkynes, amines, and aldehydes
(A3-coupling).
Results and Discussion
Synthesis and structure
The ligand H4L reacts with Zn(NO3)2·6H2O (1:6 molar ratio) sol-
vothermally to form a non-interpenetrated 3D MOF, LZn, which
However, this single-crystal–single-crystal (SC-SC) transmeta-
lation reaction is found to be irreversible as the CuII ions in LCu
cannot be replaced by ZnII upon keeping crystals of LCu in
a concentrated DMF solution of Zn(NO3)2·6H2O (1m) for as
long as a month. The ZnII ions of LZn could be replaced by
other transition-metal ions such as CoII or NiII only partially.
¯
crystallizes in the trigonal space group R3m (Table S1 in the
Supporting Information). The asymmetric unit consists of one-
quarter of the L4À ligand and one-eighth of the dinuclear
paddle-wheel SBU. Each metal ion in the paddle-wheel SBU
shows square-pyramidal coordination geometry (tꢀ0) with
equatorial coordination from four different bridging carboxy-
lates and an aqua ligand bound axially (Figure 1b). The aniline
moiety in the middle of the linker L4À has rotational freedom
and it exhibits four positional disorders. In the paddle-wheel
structure, the Zn···Zn separation is 3.002(9) Š. The ZnÀO distan-
Thermal stability
The TGA curves for LZn and LCu (Figures S6 and S7 in the Sup-
porting Information) show weight loss of 43.90% (calculated
43.88%) and 46.55% (calculated 46.52%), respectively, be-
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LZn’, does not show any significant absorption of N2 presuma-
bly owing to collapse of the framework during activation
(Figure S12 in the Supporting Information). In contrast, LCu’ ex-
hibits significant gas absorption properties. The N2 absorption
isotherm for LCu’ at 77 K is shown in Figure 3. It exhibits an
initial type I isotherm up to a relative pressure of 0.75, and
thereafter it is shifted to a type IV isotherm.[21]
Figure 2. Kinetic profile of the ZnII to CuII exchange process.
tween 30–3508C, corresponding to the loss of all solvent mole-
cules in the voids as well as coordinated water molecules.
Thereafter, a sharp loss of weight was observed in each case.
The TGA curves of both compounds do not show any distinct
plateau regions. However, when crystals of LZn and LCu are kept
in acetone for 7 days to remove DMF, EtOH, and H2O from the
voids, the resulting samples show distinct plateau regions (Fig-
ures S8 and S9 in the Supporting Information). The thermal
stability and phase purity of the compounds were also con-
firmed by variable-temperature powder X-ray diffraction meas-
urements (VT-PXRD; Figures S10 and S11 in the Supporting In-
formation). The experimental PXRD patterns for the as-synthe-
sized compounds are in good agreement with the correspond-
ing simulated patterns. Further, the VT-PXRD data indicate that
the overall framework integrity of LZn is retained only up to
a temperature of approximately 1008C, whereas the framework
LCu is stable up to at least 2008C. This behavior leads us to
postulate that ZnII ions, with a d10 electronic configuration, pro-
vide no ligand field stabilization energy and presumably the
vulnerability of the framework LZn originates from the removal
of the axial aqua ligand.[20] On the other hand, CuII in LCu has
a d9 electronic configuration that possesses ligand field stabili-
zation energy. Thus, despite removal of the axially coordinated
water ligands, the four-coordinated CuII center still has a
relatively stable structure.
Figure 3. N2 physisorption isotherms for LCu’ at 77 K.
The sorption of nitrogen reaches near-saturation at low rela-
tive pressure (P/P0 <0.02). After the initial steep rise, a plateau
region can be observed up to the relative pressure of 0.75, at
which point it rises further and attains saturation. A hysteresis
between the absorption and desorption is observed, which is
attributable to the trapping effect of the gas in the voids.[22]
The total gas uptake of LCu’ is found to be 636 cm3 gÀ1, corre-
sponding to a Brunauer–Emmett–Teller (BET) surface area of
1952 m2gÀ1 and a pore volume of 0.98 cm3 gÀ1.
Given the importance of developing protocols for reducing
the massive emission of anthropogenic CO2,[23] coupled with
the fact that the CO2 absorption capacity of a MOF is en-
hanced by accessible Lewis basic N sites, we next explored
CO2 absorption in LCu’ at 273 and 298 K up to a relative
pressure P/P0 =1 (Figure 4).
The maximum CO2 uptake capacity at 273 K is found to be
198.50 cm3 gÀ1 (39.0 wt%), which compares well with some of
the MOFs known for CO2 absorbing ability (see Table S3 in the
Supporting Information). The uptake value at 298 K is
98.12 cm3 gÀ1 (19.3 wt%). Although numerous MOFs with sig-
nificant CO2 uptake have been reported, only limited examples
of PMOFs exhibiting over 30.0 wt% CO2 uptake at 273 K and
1 bar are known (Table S3 in the Supporting Information).
The values obtained for LCu’ are substantial and can be at-
tributed to favorable interactions between CO2 and the host
framework. The isosteric heat of CO2 absorption (Qst, Figure 5)
evaluated from the isotherms at 273 and 298 K affords a value
of 27.1 kJmolÀ1, which is quite significant.[24]
Gas absorption studies
The highly porous character of LCu together with its robust
nature and available surface-functionalized voids, satisfy the
essential pre-requisites for gas sorption measurements. Prior to
the measurements, the as-synthesized sample was kept in ace-
tone for one week at room temperature for solvent exchange.
During this process, the acetone was refreshed twice a day.
The acetone-exchanged sample was then heated to 1308C
under ultra-high vacuum for 10 h to generate the fully de-sol-
vated sample, LCu’. The Zn-MOF that was similarly activated,
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Figure 4. CO2 physisorption isotherms for LCu’ at 273 and 298 K.
Figure 6. H2 physisorption isotherms for LCu’ at 77 K in: a) cm3 gÀ1, and
b) wt%.
Figure 5. Isosteric heat of CO2 absorption (Qst) for LCu’.
522.95 cm3 gÀ1 (4.67 wt%) at 36 bar pressure, which is quite
significant.
The ability of LCu’ for absorption of H2 gas at 77 K up to a rel-
ative pressure P/P0 =1.0 has also been studied and the results
are shown in Figure 6. LCu’ shows a typical type I behavior with
rapid increase of H2 gas uptake up to a relative pressure P/P0 =
0.2. The gas uptake reaches about 2.57wt% or about
288.28 cm3 gÀ1 at P/P0 =1. The absorption and desorption
follow the same path without hysteresis.
Next, the CH4 sorption studies of LCu’ at both low and high
pressures were carried out (Figure 7), and type I behavior with-
out any hysteresis is exhibited. At 1 bar pressure, the frame-
work shows uptake capacities of 36.97 cm3 gÀ1 (2.64 wt%) and
23.52 cm3 gÀ1 (1.68 wt%) at 273 and 298 K, respectively. The
framework absorbs substantially high amounts of methane
(238.38 cm3 gÀ1, 17.03 wt%) at 303 K and 60 bar (Figure 7b).
This uptake value is much higher than or comparable to some
well-known MOFs such as MIL-53(Al) and MIL-53(Cr) (8.8 wt%),
HKUST-1 (10.2 wt%), UMCM-1 (11.4 wt%), Ni-MOF-74
(11.9 wt%), CPO-27-Mg (13.7 wt%), and PCN-11 (14.0 wt%)
measured at different high pressures.[25] At 35 bar and 303 K,
the total volumetric methane uptake is found to be
166 cm3 (STP) cmÀ3, which is higher than or comparable to
some well-known MOFs (Table S5 in the Supporting
Information).
Noticeably, the hydrogen uptake value is higher than those
of MOF-5 (2.0wt%), MOF-505 (2.47wt%), Cu-BTT (2.42wt%),
Fe-BTT (2.3wt%), PCN-68 (1.87wt%), PCN-66 (1.79wt%), SNU-4
(2.07wt%), MOF-74(Mg) (2.2wt%), and many among the NOTT
series (Table S4 in the Supporting Information). As the H2
uptake capacity at 1 bar is still far from saturation, a larger H2
sorption capacity may be expected at higher pressures. As re-
vealed in Figure S13 (in the Supporting Information), the
uptake capacity of LCu’ indeed reaches
a
maximum of
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Figure 8. CO2, CH4, H2, and N2 absorption isotherm for LCu’ at 273 K.
The values for CO2 selectivity over N2, H2, and CH4 are found
to be 35, 207, and 8, respectively. Clearly, LCu’ exhibits better
CO2/N2 selectivity compared with other reported MOFs,[27]
suggesting its potential applications in the removal of CO2
from flue gases.
Cycloaddition of CO2 to styrene oxide
In view of the excellent CO2 uptake capacity of LCu’, its hetero-
geneous catalytic activity for the cycloaddition of CO2 with
styrene oxide (SO) was explored. Many synthetic chemicals[28]
and cyclic organic carbonates can be synthesized from CO2,
compounds that can be used for a wide range of applications
in industry.[29] Recently, thermally stable porous materials have
proven to be efficient heterogeneous catalysts for the coupling
of CO2 with epoxides.[30] To emulate this reaction with LCu’, the
coupling of CO2 with styrene oxide under similar pressure and
temperature (20 bar and 1208C) was carried out. As seen from
Table 1, LCu’ can serve as a good recoverable catalyst for the
solvent-free synthesis of cyclic carbonates in the presence of
co-catalytic amounts of tetrabutylammonium bromide (TBAB).
The maximum conversion is found to be approximately 50%.
The material can be reused without showing any significant
drop in its catalytic performance. The influence of CO2 pressure
on the conversion was also monitored by increasing the pres-
sure from 20 bar to 50 bar. However, a decrease in conversion
from 50 to 13% was observed upon increasing the pressure
(Table 1 and Figure S23 in the Supporting Information). This
probably occurs because of the fact that with the increased
pressure, CO2 reaches its critical pressure where it undergoes
liquefaction.[31] Both 1H NMR spectroscopy and GC results
(Figures S20–S25 in the Supporting Information) show no
noticeable amount of side products in this reaction.
Figure 7. a) Low-pressure CH4 physisorption isotherms for LCu’ at 273 and
298 K. b) High-pressure CH4 physisorption isotherms for LCu’ at 303 K.
The studies of CO2 capture and separation at low pressures
have practical implications, such as removing CO2 from flue
gases, landfill gases, natural gas, and is considered to be an im-
portant approach for reducing greenhouse-gas emissions and
clean energy generation. The substantial uptake of CO2 as well
as the stronger interactions between the LCu’ framework and
CO2 molecules highlights the potential improvement of ab-
sorptive selectivity for CO2 over other gases. To estimate the
effect of linker functionalization on gas selectivity, the absorp-
tion isotherms for CO2, CH4, H2, and N2 were measured at
273 K and 1 bar with the ideal absorbed solution theory (IAST),
based on single-component isotherms.[26] As shown in
Figure 8, N2 and H2 do not diffuse into the pores of LCu’, where-
as CH4 enters only slightly. We presume that the high quadru-
pole moment and polarizability of CO2 (13.4”10À4 Cm2 and
26.3”10À25 cm3, respectively) combined with the presence of
NH2 functionalities, induce better interactions with the
channels.
Regioselective “click” reaction
Meldal and Sharpless separately discovered the highly regiose-
lective Huisgen 1,3-dipolar cycloaddition of azides to alkynes
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after 3 h (~84% conversion) to get the same result with the fil-
trate. Thus, no catalytically active species leached into the
liquid phase and the conversion was only possible with the
solid catalyst (LCu’). Like Corma’s results with CuII-MOFs, we pre-
sume the participation of CuII rather than CuI in binding to the
acetylenic compound and its consequent activation leads to
the product. This is possibly due to the availability of an empty
coordination site on the metal in LCu’. With substrates such as
4-methylphenylacetylene, 4-fluorophenylacetylene, 1-ethynyl-
3,5-bis(trifluoromethyl) benzene, the yield was above 90%
(Table 2). However, when bulkier trimethylsilylacetylene was
used as the substrate, yield of the product was reduced to
<10%, suggesting the acetylenic compound was not accom-
modated as per the transition-state-geometry requirements for
driving the reaction to the product.
Table 1. Catalytic performance of LCu’ in the cycloaddition reaction of
CO2 to styrene oxide (SO).[a]
Entry
SO:LCu
t [h]
T [8C]
P [bar]
Conv. [%][b]
TON[c]
1
2
3
667
667
667
6
12
12
120
120
120
20
20
50
43
50
13
286
333
86.7
[a] LCu, after activation, was transferred at room temperature into the re-
actor in a glovebox. TBAB and styrene oxide were added in the autoclave
inside the glovebox. [b] Conversion was evaluated from the 1H NMR
spectra by integration of SO versus cyclic carbonate peaks (see the
Supporting Information). [c] Turnover number.
Synthesis of propargylamines by one-pot A3-coupling
reactions
(commonly known as the “click” reaction) catalyzed by CuI spe-
cies to afford 1,4-disubstituted 1,2,3-triazoles.[32] This reaction
can be carried out either thermally or in the presence of
a copper catalyst.[33] Later, Corma and co-workers studied the
catalytic activity of various CuII-containing MOFs in
In a further demonstration of the heterogeneous catalytic
activity of LCu’, a one-pot three-component coupling reaction
the same reactions to afford 1,2,3-triazole deriva-
Table 2. Results obtained for the 1,3-dipolar cycloaddition reaction catalyzed by LCu’.[a]
tives.[34] Compound LCu’ is found to act as a good het-
erogeneous catalyst (Table 2) for the click reaction in
a regioselective manner. In a typical experiment,
5 wt% of LCu’ was allowed to react with benzyl azide
(1.0 mmol) and a derivative of acetylene (1.0 mmol)
under a dinitrogen blanket. Aliquots were taken
during the course of the reaction and analyzed by
GC using hexadecane as the external standard. Each
reaction was confirmed to be exclusively regioselec-
tive to the 1,4-substituted triazole by means of
1H NMR spectroscopic analysis of the isolated prod-
uct (Supporting Information).
Yield c
Entry
1
Solvent
CH2Cl2
Catalyst
[%][b]
LCu
LCu
LCu
’
’
’
96
2
3
CH2Cl2
CH2Cl2
94
93
Once the reaction was complete, the mixture was
filtered to separate the catalyst. The filtrate was
evaporated to dryness to obtain the product as
a white solid. The catalyst from this reaction was
washed several times with CH2Cl2 and then heated at
1308C under vacuum for 6 h to regenerate the active
catalyst. The catalyst was reused at least four times
without losing any significant activity (Figure S26 in
the Supporting Information). The overall structure of
the catalyst remained intact as revealed by the unal-
tered X-ray powder pattern (Figure S27 in the Sup-
porting Information). Under similar conditions, the
activated compound LZn’ does not catalyze any of
the reactions. Also, with Cu(NO3)2·3H2O the click reac-
tion does not proceed beyond 10% even after 24 h.
The possibility of leaching of metal ions into the solu-
tion and catalyzing the click reaction was probed by
filtering the whole suspension after 1 h (~45% con-
version) to remove the LCu’ catalyst. The filtrate did
not afford further conversion (Figure S28 in the Sup-
porting Information). This procedure was repeated
with another batch and the suspension was filtered
4
CH2Cl2
LCu
’
’
90
5
6
CH2Cl2
CH2Cl2
LCu
10
no catalyst
<5
7
8
9
neat
no catalyst
15
CH2Cl2
CH2Cl2
Cu(NO3)2·3H2O
<5
<5
LZn
’
[a] Reaction conditions: benzyl azide, b (1.0 mmol), acetylene, a (1.0 mmol), in CH2Cl2
(1 mL), cat. LCu’ (5 wt%), 508C for 4 h. [b] Yield of the isolated product.
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involving an aldehyde, an alkyne, and an amine (also known as
A3-coupling) was probed. The product in this coupling reaction
is a derivative of propargylamine, which is quite useful in or-
ganic synthesis especially in therapeutic drug molecules as
well as being an important structural element in natural prod-
ucts.[35] Naturally, various catalysts for A3-coupling reactions
have been reported.[36] In our method, 5 wt% of LCu’ was
stirred with an aldehyde (1.0 mmol), an alkyne (1.0 mmol), and
an amine (1.0 mmol) in CH2Cl2 solvent at 458C for 12 h under
a nitrogen atmosphere. The reaction of phenylacetylene with
para-formaldehyde and piperidine afforded the product in
95% yield (Table 3, entry 1). Aromatic aldehydes with different
functional groups undergo the A3-coupling reaction smoothly
and afford approximately 90% yield of the product (Table 3).
Noticeably, aromatic aldehydes with electron-withdrawing
groups afford higher yields compared with those with
electron-donating groups (Table 3, entries 3–5).
and then heated to 1308C under vacuum for 6 h to regenerate
the active catalyst. No significant loss of crystallinity and cata-
lytic activity were observed even after four runs (Figures S34
and S35 in the Supporting Information). Also, no significant
leaching of CuII was detected in the solution. Nevertheless, the
contribution of homogeneous catalyst by eventually leached
species after hot filtration was studied and the results
(Figure S36 in the Supporting Information) show negligible
activity in the filtrate.
Conclusions
We have synthesized highly porous MOFs with ZnII and CuII
under solvothermal conditions. The ZnII-MOF can be complete-
ly converted to the corresponding CuII-MOF in a SC-SC
transformation. Compared with the ZnII framework, the CuII
analogue is robust and can be activated by removing
metal-bound as well as lattice solvent molecules by heating to
afford LCu’. Framework LCu’ exhibits significant H2 and CH4 gas
absorption abilities. It absorbs appreciable amounts of CO2 gas
selectively from a mixture of H2, N2, and CH4 at 273 K owing to
the presence of pendant amine moiety in the framework. Fur-
Only very small amounts of propargylamine are produced in
the absence of the catalyst (Table 3, entry 9). The activity of
LCu’ is comparable with that of homogeneous copper salts (en-
tries 6 and 7). Once the reaction was over, the catalyst could
be recovered by filtration, washed with CH2Cl2 and acetone
Table 3. LCu’-catalyzed three-component coupling reactions of aldehydes, terminal alkynes, and amines.[a]
Entry
1
Catalyst
R1R2NH
R3COH
R4CꢁCH
Yield [%][b]
95
LCu
’
’
(HCOH)n
2
3
LCu
93
92
LCu
LCu
LCu
’
’
’
4
5
89
87
6
7
8
9
CuI
(HCOH)n
(HCOH)n
(HCOH)n
(HCOH)n
89
Cu(acac)2
85
LZn
’
<5
<4
no catalyst
[a] Reaction conditions: aldehyde (1.0 mmol), amine (1.0 mmol), alkyne (1.0 mmol), LCu’ (5 wt%), CH2Cl2 (1.0 mL), sealed tube, 458C, 12 h, under nitrogen.
[b] Yields of isolated products after flash chromatography.
Chem. Eur. J. 2016, 22, 3387 – 3396
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fully dissolve the precipitate. The aqueous solution was acidified
with concentrated HCl to pHꢀ2. The light yellow precipitate
formed was collected by filtration, washed with water and Et2O,
and dried in vacuum to obtain H4L (yield: 1.53 g, 97%). M.p.:
>3008C; 1H NMR (400 MHz,[D6]DMSO, 258C, Si(CH3)4): d=8.43–
8.38 (m, 2H), 8.36 (d, J=1.4 Hz, 2H), 8.20 (d, J=1.4 Hz, 2H), 7.22–
7.12 (m, 2H), 7.01 ppm (dd, J=1.7, 7.9 Hz, 1H); 13C NMR (100 MHz,
[D6]DMSO, 258C, Si(CH3)4): d=113.6, 115.2, 123.9, 128.5, 128.8,
130.9, 131.1, 131.8, 132.1, 133.5, 138.7, 140.0, 141.0, 146.2, 166.4,
166.6, 166.7 ppm; ESI-MS: (m/z): 420 (100%) [MÀH]; elemental
analysis calcd (%) for C22H15NO8: C 62.71, H 3.59, N 3.32; found: C
62.82, H 3.68, N 3.24.
thermore, the absorbed CO2 can be converted to cyclic carbon-
ate under relatively mild reaction conditions. The presence of
coordinatively unsaturated metal sites and large voids make
this compound an excellent heterogeneous catalyst for “click
reactions” as well as in the one-pot synthesis of A3-coupling
reactions, which shows versatility of the material.
Experimental Section
Materials and measurements
The metal salts and other reagent-grade chemicals were procured
from Sigma–Aldrich and used as received. All the solvents were
from S. D. Fine Chemicals, India. These solvents were purified by
following standard conventional methods prior to use.
Synthesis of {[Zn2(L)(H2O)2]·(4DMF)(3H2O)(EtOH)}n (LZn)
A mixture containing H4L (20 mg, 0.05 mmol) and Zn(NO3)2·6H2O
(85 mg, 0.28 mmol) in DMF (2 mL) and EtOH (1 mL) was sealed in
a Teflon-lined stainless steel autoclave and heated under autoge-
nous pressure to 908C for 48 h. Cooling to room temperature at
the rate of 108ChÀ1, afforded compound LZn as colorless block-
shaped crystals in 45% yield. FTIR (KBr pellets): 3434 (broad), 2931
(m), 1660 (s), 1585 (s), 1360 (s), 1101 (s), 780 cmÀ1 (s); elemental
analysis calcd (%) for C36H55N5O18Zn2: C 44.27, H 5.68, N 7.17;
found: C 44.41, H 5.79, N 7.11.
All spectroscopic and crystallographic studies are provided in the
Supporting Information.
Synthesis of ligand H4L
Synthesis of the ligand 2’-amino-1,1’:4’,1’’-terphenyl-3,3’’,5,5’’-tetra-
carboxylic acid (H4L) was achieved in two steps (Scheme 1) by
slight modification of a literature procedure.[37]
Synthesis of 2’-amino-1,1’:4’,1’’-terphenyl-3,3’’,5,5’’-tetracarboxy-
Synthesis of {[Cu2(L)(H2O)2]·(5DMF)(4H2O)}n (LCu)
late:
A solution of 3,5-bis(ethoxycarbonyl)phenylboronic acid
Cu(NO3)2·3H2O (35 mg, 0.15 mmol) and H4L (20 mg, 0.05 mmol)
were dissolved in a mixture of DMF (2 mL), H2O (1 mL) and conc.
HCl (0.1 mL). The mixture was placed in a Teflon-lined stainless
steel autoclave and heated under autogenous pressure to 908C for
48 h and then allowed to cool to room temperature at a rate of
108ChÀ1. Blue-colored block-shaped crystals of LCu were collected
by filtration in ꢀ42% yield. FTIR (KBr pellets): 3422 (broad), 2929
(m), 1661 (s), 1590 (s), 1371 (s), 1103 (s), 776 (s), 730 cmÀ1 (s); ele-
mental analysis calcd (%) for C37H58N6O19Cu2: C 43.65, H 5.74, N
8.26; found: C 43.78, H 5.83, N 8.37.
(3.18 g, 11.95 mmol), and 2,5-dibromoaniline (1.00 g, 3.98 mmol) in
DMF (40 mL) was combined with a solution of sodium carbonate
(1.69 g, 15.94 mmol) and palladium acetate (30 mg) in water
(60 mL). The mixture was stirred at 608C overnight. After cooling
to room temperature, water (150 mL) was added to the reaction
mixture. The aqueous phase was then extracted three times with
ethyl acetate. The combined organic phases were dried with anhy-
drous sodium sulfate and evaporated to dryness to obtain the
crude product as a white solid. It was purified by column chroma-
tography using silica gel (200 mesh). Elution with 30% ethyl ace-
tate in n-hexane gave 2’-amino-1,1’:4’,1’’-terphenyl-3,3’’,5,5’’-tetra-
carboxylate as a white crystalline powder (yield: 2.02 g, 95% based
on 2,5-dibromoaniline). 1H NMR (500 MHz, CDCl3, 258C, Si(CH3)4):
d=1.43 (dt, J=9.01, 7.10 Hz, 12H), 3.88 (br s, 2H), 4.44 (quin, J=
7.18 Hz, 8H), 7.08 (d, J=1.83 Hz, 1H), 7.15 (dd, J=7.94, 1.83 Hz,
1H), 7.24 (s, 1H), 8.37 (d, J=1.22 Hz, 2H), 8.47 (d, J=1.22 Hz, 2H),
8.67 ppm (dd, J=14.05, 1.83 Hz, 2H); 13C NMR (125 MHz, CDCl3,
258C, Si(CH3)4): d=14.3, 61.5, 114.4, 117.7, 125.4, 129.4, 129.5,
131.2, 131.4, 131.7, 132.1, 134.2, 139.6, 140.2, 141.4, 144.0, 165.6,
165.8 ppm.
Synthesis of LCu by transmetalation
The as-synthesized single crystals of LZn were soaked in DMF for
2 days to remove any remaining reactants and unwanted products
present on the surface of the crystals. In this process, the solvent
was refreshed once. After the washing, the crystals of LZn were im-
mersed in a DMF solution of Cu(NO3)2·3H2O (0.1m) at 298 K for
about 30 h. During this period, the solution was replaced with
a fresh solution of Cu(NO3)2·3H2O in DMF three times. After decant-
ing the solution, the CuII-exchanged crystals were washed thor-
oughly with DMF and kept in DMF for 2 days to remove any
excess metal salt from the pores of the frameworks.
Synthesis of 2’-amino-1,1’:4’,1’’-terphenyl-3,3’’,5,5’’-tetracarbox-
ylic acid, (H4L): Tetraester (2.00 g, 3.75 mmol) was suspended in
methanol (180 mL) and water (20 mL). After adding KOH (1.47 g,
26.25 mmol), the reaction mixture was heated at reflux overnight.
After removal of most of the solvent in vacuo, water was added to
Acknowledgements
We gratefully acknowledge financial support from the Depart-
ment of Science and Technology, New Delhi, India (to P.K.B.)
and SRF from the Council of Scientific and Industrial Research,
New Delhi, India to D.D. and T.K.P.
Keywords: click reactions · CO2 absorption · cyclic carbonate ·
heterogeneous catalysis · metal–organic frameworks
Scheme 1. Synthetic route to H4L.
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Received: November 25, 2015
Published online on February 2, 2016
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