From Zn(II) to Cu(II) framework via single-crystal to single-crystal metathesis with superior gas uptake and heterogeneous catalytic properties

Article abstract of DOI:10.1016/j.ica.2018.07.031

In this work, a Zn(II) framework, {[Zn₂(L)₄(H₂O)₄]·(7DMF)(7H₂O)}n (1_Zn), has been synthesized using a bent tetracarboxylic acid ligand (H₄L). The structure of 1_Zn contains [Zn₂(COO)₄] paddle-wheel secondary building units with axial sites occupied by water molecules. Interestingly, the isostructural Cu(II) framework {[Cu₂(L)₄(H₂O)₄]·(7DMF)(7H₂O)}_n (1_Cu) has been obtained via single-crystal to single-crystal metathesis reaction. The activated 1_Cu (denoted as a1_Cu; a stands for activated) has open coordination sites. This species showed enhanced CO₂ adsorption and heterogeneous catalytic properties for the Hantzsch coupling reaction involving condensation of an aldehyde with ethyl acetoacetate and ammonium acetate to produce 1,4-dihydropyridines, and three-component coupling of amines, aldehydes and alkynes to generate propargylic amines with high efficiency.

Full text of DOI:10.1016/j.ica.2018.07.031

Accepted Manuscript
Research paper
From Zn(II) to Cu(II) Framework via Single-Crystal to Single-Crystal Meta-
thesis with Superior Gas Uptake and Heterogeneous Catalytic Properties
Mayank Gupta, Dinesh De, Kapil Tomar, Parimal K. Bharadwaj
PII:
S0020-1693(18)30887-9
https://doi.org/10.1016/j.ica.2018.07.031
ICA 18377
DOI:
Reference:
Inorganica Chimica Acta
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Accepted Date:
7 June 2018
21 July 2018
21 July 2018
Please cite this article as: M. Gupta, D. De, K. Tomar, P.K. Bharadwaj, From Zn(II) to Cu(II) Framework via Single-
Crystal to Single-Crystal Metathesis with Superior Gas Uptake and Heterogeneous Catalytic Properties, Inorganica
Chimica Acta (2018), doi: https://doi.org/10.1016/j.ica.2018.07.031
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From Zn(II) to Cu(II) Framework via Single-Crystal to Single-Crystal
Metathesis with Superior Gas Uptake and Heterogeneous Catalytic Properties
Mayank Gupta, Dinesh De, Kapil Tomar, and Parimal K. Bharadwaj*
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India
ABSTRACT: In this work, a Zn(II) framework, {[Zn2(L)4(H2O)4]·(7DMF)(7H2O)}n (1Zn), has
been synthesized using a bent tetracarboxylic acid ligand (H4L). The structure of 1Zn contains
[Zn2(COO)4] paddle-wheel secondary building units with axial sites occupied by water
molecules. Interestingly, the isostructural Cu(II) framework {[Cu2(L)4(H2O)4]·(7DMF)(7H2O)}n
(1Cu) has been obtained via single-crystal to single-crystal metathesis reaction. The activated 1Cu
(denoted as a1Cu; a stands for activated) has open coordination sites. This species showed
enhanced CO2 adsorption and heterogeneous catalytic properties for the Hantzsch coupling
reaction involving condensation of an aldehyde with ethyl acetoacetate and ammonium acetate to
produce 1,4-dihydropyridines, and three-component coupling of amines, aldehydes and alkynes
to generate propargylic amines with high efficiency.
Keywords: Transmetalation; single crystal to single crystal; Gas Sorption; Catalysis.
INTRODUCTION
The design and synthesis of porous metal-organic frameworks (MOFs) has attracted remarkable
attention because of their potential application in gas adsorption, catalysis, and so on.1 For
potential applications, particularly in gas adsorption and catalysis, MOFs should have high
stability2 that is still a challenging problem. It is well known that MOFs built with multi-metal
clusters as nodes, known as the secondary building units (SBUs), impart rigidity to a framework
besides stabilizing the MOF.3 Recently, the metal ions exchange (also known as metathesis)4 or

the organic linkers exchange5 have been reported. The metal ion exchange in single crystal to
single crystal (SC-SC) fashion is a powerful technique for obtaining new materials endowed with
enhanced stability and property. This process offers an alternative and effective route for
accessing materials when the conventional direct synthesis fails.6 As an example, MOFs with
[Zn2(COO)4] paddle-wheel SBUs often collapse when the axially coordinated solvent molecules
are removed exhibiting inferior applications.7 However, transmetalation of [Zn2(COO)4] SBU of
the pristine MOF with a metal ion such as Cu2+ (higher in the Irving-Williams series) without
losing crystallinity is a fascinating technique that afford more stable frameworks with superior
applications.5 Although the exchange of metal ions are simple, convenient and well reported, but
most of the reports deals with incomplete transmetalation.8 Recently, several groups including
our group have reported complete exchange of Zn2+ ions by the Cu2+ ions in a [Zn2(COO)4] SBU
containing MOF.7
Scheme 1. Synthesis of MOF 1Zn.
Herein,
we
report
the
solvothermal
synthesis
of
a
Zn(II)-MOF,
{[Zn2(L)4(H2O)4]·(7DMF)(7H2O)}n (1Zn) (Scheme 1), with paddle-wheel SBUs which was
found to be of low thermal stability. This compound readily underwent complete SC-SC
transmetalation at room temperature (RT) to afford the isostructural 1Cu, that was thermally

robust and could be activated to produce guest free framework a1Cu. This activated compound
exhibited significant capacity of adsorbing CO2 gas besides showing heterogeneous catalytic
activity.
EXPERIMENTAL SECTION
Materials and measurements. Reagent grade chemicals p-Anisidine (99%), 1-bromo-3,5-
dimethylbenzene (97%), trimethyl borate (99%), Zn(NO3)2∙6H2O (98%) and Cu(NO3)2∙3H2O
(99%) were obtained from Sigma-Aldrich and used as received. All the solvents and Bromine
were procured from S. D. Fine Chemicals (India) that were freshly purified prior to use.
Physical Measurements
The following Spectroscopic data were collected. IR spectra (KBr disk, 400−4000 cm−1)
were recorded on a Perkin-Elmer model 1320 spectrometer. Powder X-ray diffraction (PXRD)
patterns were recorded with a Bruker D8 Advance diffractometer equipped with nickel-filtered
Cu Kα (1.5418 Å) radiation. The tube voltage and current were 40 kV and 40 mA, respectively.
Microanalyses of all the compounds were carried out by using a Perkin-Elmer Series II,
elemental analyzer model 2400. Electron paramagnetic resonance (EPR) spectra were obtained
on a Bruker EMX EPR spectrometer. ICPMS measurements were carried out using Thermo-
iCAP Q. Thermogravimetric analyses (TGA) (heating rate of 5 °C/min under nitrogen
1
atmosphere) were performed with a Mettler Toledo Star System. H NMR and 13C NMR spectra
were recorded either on a JEOL ECX 500 FT (500, 125 MHz respectively) or on a JEOL ECS
400 FT (400, 100 MHz respectively) instrument in CDCl3 or in DMSO-d6 with Me4Si as the

internal standard. The ESI-Mass data were obtained in a WATERS-Q-Tof Premier Mass
Spectrometer.
X-Ray Structural Studies
The crystal data for 1Zn has been collected on a Bruker SMART CCD diffractometer (Mo Kα
radiation, λ = 0.71073 Å). The program SMART9 was used for collecting frames of data,
indexing reflections, and determining lattice parameters, SAINT6 for integration of the intensity
of reflections and scaling, SADABS10 for absorption correction, and SHELXTL11 for space group
and structure determination and least-squares refinements on F2. The crystal structure were
solved and refined by full-matrix least-squares methods against F2 by using the program
SHELXL-201412 using Olex-2 software.13 All the non-hydrogen atoms were refined with
anisotropic displacement parameters. Hydrogen positions were fixed at calculated positions and
refined isotropically. The solvent molecules in 1Zn and 1Cu are highly disordered. Therefore, the
PLATON-SQUEEZE14 program was used to remove those peaks. The solvent molecules were
calculated on the basis of combined study of TGA, elemental analysis, and removed electron
counts which are included in the molecular formula. The crystal and refinement data are
collected in Table S1, while selected bond distances and angles are given in Table S2 and S3.
Synthesis of ligand H4L. Synthesis of the ligand, 2'-amino-5'-methoxy-[1,1':3',1''-
terphenyl]-3,3'',5,5''-tetracarboxylic acid (H4L) was achieved in a multi-step procedure as
illustrated in Scheme 2.

Scheme 2. Synthetic procedure for the ligand, H4L
Synthesis of 2,6-dibromo-4-methoxybenzenamine (B). Compound 2,6-dibromo-4-
methoxylaniline was synthesized according to slightly modified procedure reported in
literature.15 A solution of p-anisidine (5 g, 40.63 mmol) in methanol:DCM (50:40 mL) was
vigorously stirred during the addition of bromine (4.9 mL, 95.48 mmol) drop wise over a period
of 30 min in an ice-bath and then left for overnight stirring. Then the mixture was transferred to a
500 mL beaker and left for evaporation in the hood. When the volume was reduced to ~ 40 mL,
white needle-shaped crystals appeared that was filtered off and washed with small amount of
cold methanol and dried in air. The reaction afforded 22.5 g (~75% yield) of the title compound.
Synthesis of tetraethyl 2'-amino-5'-methoxy-[1,1':3',1''-terphenyl]-3,3'',5,5''-
tetracarboxylate (D). A solution of 3,5-bis(ethoxycarbonyl)phenylboronic acid (8.4 g, 39.84

mmol), and 2,6-dibromo-4-methoxybenzenamine (4.0 g, 18.12 mmol) in 1,4-dioxane (190 mL)
was mixed with a solution of sodium carbonate (5.76 g, 54.33 mmol) and palladium
triphenylphosphine (656 mg) in water (10 mL). The mixture was allowed to stir at 90°C
overnight under nitrogen atmosphere. The mixture was allowed to cool to RT and then
evaporated to dryness. The desired compound was extracted with ethyl acetate in water. The
organic layer after drying over anhydrous sodium sulfate was evaporated under reduced pressure.
This compound was recrystallized from DCM:MeCN (1:9) as a pale yellow block shaped
1
crystals in ~85% yield, (8.2 g). H NMR (500 MHz, DMSO-d6): δ = 8.46 (s, 2H), 8.24 (s, 4H),
6.77 (s, 2H), 4.38 (q, 8H), 3.73 (s, 3H), 1.35 (t, 12H) ppm; 13C (125 MHz, DMSO-d6): δ =
164.87, 151.35, 140.48, 135.52, 134.13, 131.01, 128.09, 126.46, 116.00, 61.27, 55.57, 14.11
ppm. ESI-MS: (m/z): 564.22 (100%) [M+H]+ (Figs. S1–S3).
Synthesis of 3′-amino-1,1′:4′,1″-terphenyl-3,3″,5,5″-tetracarboxylic acid (H4L). To a
solution of the ester D (2.00 g, 3.65 mmol) in methanol (100 mL) and water (50 mL), KOH (1.46
g, 26.07 mmol) was added in hot condition. The mixture was refluxed overnight. After removal
of most of the solvent, water was added to fully dissolve the precipitate and acidified with
concentrated HCl up to pH ~3 in an ice-bath. The yellow precipitate formed was collected by
filtration, washed with ice cold water and dried in vacuo to obtain the ligand, H4L with an yield
1
of 1.46 g (90%). H NMR (500 MHz, DMSO-d6): δ = 8.41 (s, 2H), 8.10 (s, 4H), 6.88 (s, 2H),
3.70 (s, 3H) ppm; 13C NMR (125 MHz, DMSO-d6): δ = 166.38, 140.12, 133.74, 132.17, 131.63,
131.25, 130.64, 128.42, 125.81, 19.63; ESI-MS: (m/z): 450.08 (100%) [M-H]+. FTIR (KBr
pellets) (cm–1): 3441.79 (br), 2926.23 (m), 1707.80 (s), 1631.86 (w), 1598.69 (m), 1472.27 (m),
1427.76 (m), 1267.22 (w), 1211.03 (s), 1107.61 (w), 1044.89 (m), 920.85 (w), 862.87 (w),

766.14 (m), 674.04 (m), 656.91 (m). Anal. calcd. for C23H17NO9: C, 61.20; H, 3.80; N, 3.10%.
Found: C, 61.25; H, 3.78; N, 3.15% (Figs. S4–S7).
Synthesis of {[Zn2(L)4(H2O)4]·(7DMF)(7H2O)}n (1Zn). Zn(NO3)2∙6H2O (60 mg, 0.20
mmol), and H4L (20 mg, 0.044 mmol) were dissolved in 2 mL DMF and 1 mL H2O. The mixture
was placed in a Teflon-lined stainless steel autoclave and heated under autogenous pressure to 90
°C for 3 d and then allowed to cool to room temperature slowly. Block-shaped colorless crystals
of 1Zn were collected by filtration and washed with DMF and methanol. Finally the crystal was
dried in the air. Yield ∼55%. FTIR (KBr pellets) (cm–1): 3395.97 (broad), 1702.98 (m), 1618.21
(s), 1567.39(m), 1468.24(w), 1426.65 (m), 1363.42 (s), 1224.84 (w), 1208 (m), 1147.69 (w),
1042.29 (m), 920.37 (m), 856.77 (w), 778.43 (s), 717.06 (s). Anal. Calcd. For C51H80N10O26Zn2:
C, 44.39; H, 5.84; N, 10.15%. Found: C, 44.41; H, 5.82; N, 10.12% (Fig. S8).
Synthesis of {[Cu2(L)4(H2O)4]·(7DMF)(7H2O)}n (1Cu). The as-synthesized single
crystals of 1Zn were soaked in DMF for 2 days to remove any remaining reactants and unwanted
products that may be adhered. During the soaking process, the solvent was refreshed twice. After
that, the crystals were immersed in DMF solution of Cu(NO3)2·3H2O (0.05 M) at room
temperature for about 7 d. During this period, the solution was replaced with a fresh solution of
Cu(NO3)2·3H2O in DMF after each day. The Cu-exchanged crystals (1Cu) were washed
thoroughly with DMF and kept in DMF for 2 d to remove any excess metal salt from the pores of
the framework. FTIR (KBr pellets) (cm–1): 3388.99 (broad), 1620.14 (m), 1561.50 (s),
1468.17(w), 1426.47(m), 1363.63 (s), 1208.70 (m), 1147.87 (w), 1111.26 (w), 1042.99 (m),
920.37 (w), 856.59 (w), 777.33 (s), 718.67 (s). Anal. Calcd. For C51H82N10O26Cu2 (1Cu): C,
44.44; H, 6.0; N, 10.16 %. Found: C, 44.48; H, 6.12; N, 10.19% (Fig. S8).

Heterogeneous catalysis
Catalyst 1Cu was activated by heating at 120 ºC under high vacuum for a period of 8 h in
order to remove all traces of lattice and coordinated solvent. Before activation crystals were
soaked in methanol for four days, which was changed two times per day with fresh methanol.
The sample thus produced (a1Cu) was used for Hantzsch condensation and three-component
coupling reaction involving amines, aldehydes and alkynes. Hantzsch condensation of aldehyde,
ethyl acetoacetate with NH4OAc without any salts or additives to afford 1,4-dihydropyridines,
with excellent to good yields. Dihydropyridines have emerged as one of the most important
classes of drugs for cardiovascular diseases and have more biological and medicinal interest.16
Furthermore, a1Cu shows three-component coupling of amines, aldehydes and alkynes to afford
propargylamines which are versatile intermediates in organic synthesis, also being important
structural elements of natural products and therapeutic drug molecules.17
General procedure for the synthesis of Hanstzsch esters (1,4-Dihydropyridines).
Reactions were carried out in a Schlenk tube with stirring at room temperature using 1ml dry
DCM as a solvent. Aldehyde (2 mmol), Ethylacetoacetate (4.1 mmol), NH4OAc (2 mmol) and
catalyst a1Cu (5 mol %) were added. Upon completion, the reaction mixture and catalyst were
separated by filtration to obtain crude compound. This crude mixture was extracted with DCM
and further purified with small column of silica gel using hexane/ethyl acetate as the eluent.
General procedure for the synthesis of propargylamines by one-pot A3-coupling
reactions. Synthesis and work-up procedure is same as above used in Hantzsch synthesis. Here
the reactions were carried out at 70 ºC using aldehyde (1 mmol), alkyne (1 mmol) and amine (1
mmol).

RESULTS AND DISCUSSION
A solvothermal reaction of the organic linker, H4L, and Zn(NO3)2 in a mixed solvent of N,N′-
dimethyl formamide (DMF) and H2O at 90 °C for 3 days afforded colorless block shaped
crystals of 1Zn. The structure was determined using single-crystal X-ray diffraction studies. On
the basis of single-crystal X-ray diffraction analysis, thermogravimetric analysis (TGA, Fig. S9),
and elemental analysis, 1Zn could be formulated as {[Zn2(L)4(H2O)4]·(7DMF)(7H2O)}n. The
phase purity of the bulk material was further confirmed using powder X-ray diffraction (PXRD,
Fig. S10). Single-crystal X-ray diffraction analysis revealed that 1Zn crystallized in the P63/mmc
space group with assymteric unit comprised of a quarter of the ligand, L4-, a Zn(II) ion and a
coordinated water molecule each with half occupancy. As expected, the framework nodes in 1Zn
were composed of paddlewheel dinuclear [Zn2(COO)4] secondary building units (SBUs; Fig. 1a)
(The Zn···Zn bond distance was 2.982(3) Å) which were bridged by the carboxylate groups of
L4-. The Zn···O bond distances fall in the range, 1.962(3) to 2.014(4) Å) to form a three
dimensional framework. As depicted in Fig. 1b two types of spherical cage were found in the
framework 1Zn: one larger cage (green color) of about 10 Å in diameter and the other smaller
cage (yellow color) of about 8 Å in diameter. These cages were further connected to each other
via small triangular windows of dimensions ~5 Å. The bigger cage was in close proximity with
the methoxy group while the smaller cages were decorated with the –NH2 groups.

(a)
(b)
Fig. 1. (a) Coordination environment around Zn2+ ions in 1Zn, (b) spherical cages in 1Zn.
The empty space in the framework was filled with disordered solvent molecules whose
composition was established from thermogravimetric weight-loss, IR spectra and
elemental analysis. The values were in good agreement with PLATON11 calculated
solvent-accessible void volume which was found to be 60.4% (4357.2/7208.9 Å3) of the
total cell volume.
1Zn was found to be stable up to 60 °C as confirmed by thermogravimetric analysis (Fig.
S9) and VTPXRD (Fig. S10). As Zn(II), a d10 ion without any ligand field stablization energy,
upon removal of axially bound water molecule, its coordination number changed from 5 to 4 and
the coordination geometry changed from square pyramidal to tetrahedral and collapsing of the
system. In case of Cu(II) SBUs, the departure of the axially bound water changed the
coordination geometry to a stable square planar. This encouraged us to explore post-synthetic
transmetalation in 1Zn. Upon dipping single crystals of 1Zn in a Cu(NO3)2·3H2O (0.05 M)

solution of DMF at room temperature, crystals gradually changed from colourless to greenish
blue within 7 d. Optical microscopic observation revealed that the pristine crystals retain their
original morphology during the exchange process (Fig. S11). Primarily EPR spectrum was taken
to confirm the exchange of Zn2+ with a Cu2+ ion (Fig. S12) of the greenish blue crystals. Also,
the PXRD pattern before and after the metal exchange remained unaltered (Fig. S13). The
exchange of Zn2+ with a Cu2+ ion and the kinetics (Fig. 2) were also monitored by energy
dispersive X-ray (EDX) (Fig. S14). It showed 70% replacement of Zn2+ by Cu2+ within 2 d,
while 95% exchange took place in 6 d. A complete exchange was achieved in 7 d. Inductively
coupled plasma mass spectrometry (ICP-MS) measurements showed 99.66% Cu(II) ions and
0.34 % Zn(II) ions present (see SI). The replacement of the Zn(II) ions in 1Zn could be achieved
only partially with metal ions like Co2+ and Ni2+. However, the Cu2+ ions in 1Cu could not be
replaced by other metal ions like Zn2+, Co2+ and Ni2+. Hence, the metal exchange process is
irreversible here. Fortunately, the single crystallinity of 1Cu was maintained after an exchange
period of 7 d, allowing single-crystal X-ray data collection. The X-ray structural study indicated
(Table S1) that 1Zn and the transmetalated product (1Cu) are iso-structural. It should be
mentioned here that all attempts for direct synthesis of 1Cu remained unsuccessful.
Fig. 2. Kinetic profile of the Zn(II) to Cu(II) exchange process.

The compound 1Cu was found to be stable beyond 180 °C, as confirmed by its
thermogravimetric analysis (Fig. S9) and VTPXRD (Fig. S15) measurements. As mentioned
o
earlier, the framework started break down in case of 1Zn at 60 C while 1Cu maintained its
integrity beyond 180 °C. Also, in case of 1Cu the crystallinity was preserved after the removal of
coordinating water molecule (Fig. S16). The reasonably high thermal stability, 3D porous
structure and presence of axially vacated paddle-wheel SBU prompted us to test the gas
adsorption measurements.
Gas adsorption studies.
Before gas sorption analysis, the as-synthesized compounds were solvent-exchanged with
dry methanol and then heated at 120°C under high vacuum for 12 h to yield fully activated
frameworks a1Zn and a1Cu. The permanent porosity of a1Zn and a1Cu were unambiguously
established by nitrogen adsorption experiments at 77 K. As shown in Fig. 3a, a1Zn showed
merely an uptake of 62.1 cc/g while a1Cu exhibited a reversible type I sorption behavior
revealing its microporous nature with a maximum N2 uptake of 494 cc g-1 corresponding to the
BET surface area of 1512 m2g-1.
Presence of the free amine groups prompted us to carry out CO2 adsorption
measurements at 273 and 298 K, and the results are illustrated in Fig. 3b. At 273 K and 1 bar
pressure, an adsorbed amount of 174 cc g-1 (34.2 wt %) while at 298 K an uptake of 116 cc g-1
(22.8 wt %) were recorded reached for a1Cu. On the other hand, for a1Zn, the maximum uptake at
273 was 23.3 cc g-1 (4.6 wt %) whereas at 298 K it dropped to merely 13.6 cc g-1 (2.7 wt %).

These results clearly implied that after transmetalation, the framework showed much more
improved adsorption capacity.7, 18
(a)
(b)
Fig. 3. Physisorption isotherms: (a) N2 at 77 K, and (b) CO2 at 273 and 298 K.
Catalytic study
(i) Hantzsch coupling. The efficacy of a1Cu as a catalyst in Hantzsch coupling reaction was
studied for the synthesis of 1,4-dihydropyridines. For this, benzaldehyde, ammonium acetate and
ethyl acetylacetonate were chosen as the substrates in the model reaction under different catalyst
loading at room temperature. The reaction was carried out in the presence of 2, 3, 4 and 5 wt %
of the catalyst under the same conditions, that showed improved yield with increasing the
catalyst loading. Further, increasing the catalyst loading (10 wt %) did not improve the yield
significantly. It should be pointed out that in the absence of catalyst, the reaction was very
sluggish and even after a prolonged reaction time (24 h), substantial amounts of starting

materials remained unreacted (Table 1, entry 12) with only ~13 % yield. It should be emphasized
that in this waste-free catalytic system, no byproduct was observed in the reaction.
The basic nature of the amine-decorated MOF played an important role in the synthesis
of 1,4-dihydropyridines with excellent yields (90%; Table 1, entry 1) in comparison to a similar
non-amine isostructural MOF (PCN-306) (83%) (Table 1, entry 2). The catalytic activity of a1Cu
is better than or comparable to other reported MOF, such as Er-MOF (92 % yield) reported by
Hajiashraf et al.19, TMU-33 (91.2 % yield) reported by Morsali et al.20, IRMOF-3 (89% yield)
reported by Morsali et al.21 When the model reaction was run using a1Zn, the product was
obtained in 24% yield in 12 h (Table 1, entry 13). In order to examine the scope of this process,
and to demonstrate the diversity of a1Cu, the optimized conditions were applied to a series of
substrates as collected in Table 1.
Table 1. Synthesis of 1,4-Dihydropyridines a
Entry Catalyst Aldehyde
Product
Yieldb
(%)
90
83
94
1
2
3
a1Cu
PCN-306
a1Cu

92
92
4
5
a1Cu
a1Cu
90
93
6
7
a1Cu
a1Cu
91
81
8
9
a1Cu
a1Cu
82
78
10
11
a1Cu
a1Cu
No
13
12
catalyst

24
13
a1Zn
aReaction conditions: Aldehyde (1 mmol), NH4OAc (1 mmol) and ethyl acetoacetate (4.1 equiv)
and cat. (5 wt %), r.t. for 4h. bIsolated yields based on aldehyde.
The possibility of recycling and reusing the catalyst was examined. After completion of
the reaction, the mixture was treated in CH2Cl2 and the catalyst was removed by filtration. The
o
catalyst was washed with CH2Cl2, dried and heated at 120 C under vacuum for 5 h to reuse in
the next run. The catalyst could be recycled at least four times without measurable loss of its
activity (Fig. S18). Formation of the desired product, 1,4-dihydropyridines, was confirmed by
1
the H NMR and 13C NMR data (Figs. S19−S38). Further investigation of the heterogeneous
nature of the catalyst as well as stability of the framework was carried out using a filtration test.
After 1 h of the reaction, the reaction mixture was filtered and the catalyst was removed. Then,
the supernatant was left stirring at the same reaction conditions. Within 3 h of further reaction
time, no distinguishable changes were recognized in the reaction conversion (Fig. S39). Also, the
PXRD pattern of 1Cu before and after the reaction indicates that the framework remained intact
after the catalytic reaction (Fig. S16). A tentative mechanism for the Hantzsch coupling
reactions20 was proposed and is given in the SI (Scheme S1).
(ii) A3-coupling reactions. The development of heterogeneous catalysts for preparing
propargylamines by the A3 coupling reaction remains an active research area.22 As is known,
propargylamines are versatile intermediates in organic synthesis, also being important structural
elements of natural products and therapeutic drug molecules.23 In this work, the A3-coupling
reaction was chosen to evaluate the activity of the a1Cu. The reaction afforded 100% selectivity
to the product of propargylamine. In an effort, the coupling reaction of benzaldehyde (1 mmol),
phenylacetylene (1.1 mmol), and pyrrolidine (1.1 mmol) was carried out using 5 mol % of a1Cu

at 70 °C for 12 h. The reaction produced 93% isolated yield (Table 2, entry 1). Subsequently,
various aldehydes, alkynes, and amines were similarly coupled as summarized in Table 2.
However, the reaction was found to be highly affected by the nature of the aldehyde. Aryl
aldehyde having electron donating group decreased the reactivity of the reaction and gave lower
yields (Table 2, entry 2). Aryl aldehydes having electron withdrawing group on the other hand,
displayed higher reactivity (Table 2, entry 3). In case of piperidine, we got slightly higher yield
(96%) (Table 2, entry 9) than that of pyrrolidine (93%). A variety of aromatic alkynes were
coupled with benzaldehyde and pyrrolidine in the presence of the a1Cu catalyst. In each case the
reaction afforded the corresponding propargylamines in excellent yields (Table 2). Formation of
1
13
the desired product, propargylamines, was confirmed by the H NMR and C NMR data (Figs.
S40−S57). The catalytic activity of a1Cu is better than or comparable to other reported MOF,
such as Cu(II)-MOF (89 % yield) reported by Kostakis et al.24, Cu(I)-MOF (89 % yield) reported
by Zhao et al.25, copper nanoparticles supported on MOF-5 (96% yield) reported by Wang et
al.26, Cu(I)-modified zeolites, CuI-USY (95% yield) reported by Sommer et al.27
After 3 h of reaction, the catalyst was separated by hot filtration and the filtrate was
allowed to react under identical reaction conditions for another 9 h. As a result, no further
conversion was detected after the catalyst removal (Fig. S58). As shown in Fig. S59 in the SI,
1Cu could be recycled for at least four runs without a significant loss of performance. PXRD data
of the recycled catalyst showed that its crystalline structure was retained (Fig. S17). A tentative
mechanism for the A3-coupling reactions was proposed based on previous report28 and is given
in the SI (Scheme S2)
Table 2. Synthesis of propargylamines by A3-coupling reactionsa

Entry Aldehyde
Alkene
Product
Yieldb
(%)
1
93
2
81
3
4
86
81
5
6
78
79
7
8
9
86
84
96

aReaction conditions: Aldehyde (1 mmol), acetylene (1.1 mmol), pyrrolidine (1.1 mmol), and
cat. (5 mol %), 70 °C for 12 h. bIsolated yields based on aldehyde.
Conclusions
We have successfully synthesized a thermally robust and porous Cu-framework 1Cu from
the isostructural less stable Zn-framework 1Zn using a post-synthetic metathesis approach. Unlike
the activated a1Zn, a1Cu exhibited significant N2 and CO2 adsorption properties. Compound a1Cu,
having both high populations of Lewis acid and basic sites, showed excellent catalytic
performance in the Hantzsch coupling reaction as well as three-component coupling of amines,
aldehydes and alkynes to generate propargylic amines with high efficiency.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the at DOI:
X-ray crystallographic data of 1Zn (CCDC 1838277) and 1Cu (CCDC 1838278) (CIF)
Materials and methods, X-ray structural studies, ESI-MS spectra, IR spectra, TGA, PXRD,
NMR spectra, and additional Fig.s (pdf).
AUTHOR INFORMATION
Corresponding Author
*E-mail: pkb@iitk.ac.in
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS

We gratefully acknowledge the financial support received from the DST and MNRE, New Delhi,
India (to P.K.B.), and SRF from the University Grants Commission (UGC), New Delhi, India to
M.G., D.D. thanks MNRE for a Research Associate fellowship, YSF to K.T. from DST (SERB)
(YSS/2015/001088/CS), New Delhi, India.
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