Synthetic, spectral, structural and catalytic activity of infinite 3-D and 2-D copper(ii) coordination polymers for substrate size-dependent catalysis for CO2 conversion

Article abstract of DOI:10.1039/c9dt01457h

Two copper(ii) coordination polymers, viz. [Cu₂(OAc)₄(μ₄-hmt)_0.5]_n (1) and [Cu{C₆H₄(COO^-)₂}₂]_n·2C₉H₁₄N₃ (2), have been synthesized solvothermally and characterized. The solid-state structure reveals that 1 is an infinite three-dimensional (3D) motif with fused hexagonal rings consisting of Cu(ii) and hmt in a μ₄-bridging mode, while 2 is an infinite two dimensional (2D) motif containing Pht^-2 in a μ₁-bridging mode. CP 1 has a two-fold interpenetrated diamondoid network composed of 4-connected sqc6 topology with the point symbol of {6⁶}, while 2 has a Shubnikov tetragonal plane network possessing a 4-connected node with an sql topology with a point symbol of {4⁴·.6²}-VS [4·4·4·4·?·?]. Both CPs 1 and 2 serve as efficient catalysts for CO₂-based chemical fixation. Moreover, 1 demonstrates one of the highest reported catalytic activity values (%yield) among Cu-based MOFs for the chemical fixation of CO₂ with epoxides. 1 shows high efficiency for CO₂ cycloaddition with small epoxides but its catalytic activity decreases sharply with the increase in the size of epoxide substrates. The catalytic results suggested that the copper(ii) motif-catalyzed CO₂ cycloaddition of small substrates had been carried out within the framework, while large substrates could not enter into the framework for catalytic reactions. The high efficiency and size-dependent selectivity toward small epoxides on catalytic CO₂ cycloaddition make 1 a promising heterogeneous catalyst for carbon fixation and it can be used as a recoverable stable heterogeneous catalyst without any loss of performance. The solvent-free synthesis of the cyclic carbonate from CO₂ and an epoxide was monitored by in situ FT-IR spectroscopy and an exposed Lewis-acid metal site catalysis mechanism was proposed.

Full text of DOI:10.1039/c9dt01457h

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Trivedi, A. K. Yadav, G. Singh, A. Kumar, G. Kumar, A. Husain and N. Rath, Dalton Trans., 2019, DOI:
10.1039/C9DT01457H.
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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
rsc.li/dalton

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DOI: 10.1039/C9DT01457H
Journal Name
ARTICLE
Synthetic, Spectral, Structural and Catalytic activity of infinite
3-D and 2-D Copper(II) Coordination Polymers for Substrate
Size-Dependent Catalysis for CO2 Conversion
Received 00th January 20xx,
Accepted 00th January 20xx
Bhaskaran,a Manoj Trivedi,a* Anita K. Yadav,b Gurmeet Singh,a* Abhinav Kumar,c Girijesh
Kumar,d Ahmad Husain,e Nigam P. Rathf*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Two copper(II) coordination polymers, viz. [Cu2(OAc)4(4-hmt)0.5]n (1), and [Cu{C6H4(COO-)2}2]n.2C9H14N3 (2) have been
synthesized solvothermally and characterized. The solid-state structure reveals that 1 is an infinite three-dimensional (3D)
motif with fused hexagonal rings consisting of Cu(II) and hmt in 4-bridging mode, while 2 is infinite two dimensional (2D)
motif containing Pht-2 in 1-bridging mode. CP 1 has a two-fold interpenetrated diamondoid network comprising of 4-
connected sqc6 topology with the point symbol of {66}, while 2 has Shubnikov tetragonal plane network possessing 4-
connected node with sql topology with point symbol of {4^4.6^2}-VS [4.4.4.4.*.*]. Both CPs 1 and 2 serve as efficient
catalyst for CO -based chemical fixation. Moreover, 1 demonstrate some of the highest reported catalytic activity value
2
(%yield) among Cu-based MOFs for the chemical fixation of CO2 with epoxides. 1 show high efficiency for CO2
cycloaddition with small epoxides but its catalytic activity decreases sharply with increase in the size of epoxide substrates.
The catalytic results suggested that the copper(II) motifs catalyzed CO2 cycloaddition of small substrates had been carried
out within the framework, while large subtrates could not enter into the framework for catalytic reactions. The high
efficiency and size-dependent selectivity toward small epoxides on catalytic CO2 cycloaddition make 1 a promising
heterogeneous catalyst for carbon fixation and can be used as recoverable stable heterogeneous catalysts without losing
performance. The solvent-free synthesis of the cyclic carbonate from CO2 and an epoxide was monitored by in situ FT-IR
spectroscopy
and
an
exposed
Lewis-acid
metal
sites
catalysis
mechanism
was
proposed.
reduces the anthropogenic greenhouse gas emission but also
generates valuable chemical commodity to decrease our
dependence on petrochemicals. Taking into account of the
issues of the product purifications and catalyst recycling in
homogeneous catalytic processes, some heterogeneous catalysts
have been developed for the CO2 chemical conversion.3
Particularly, the coupling of CO2 with epoxides into cyclic
carbonates is a very efficient route for CO2 utilization.4 The
cyclic carbonates are a kind of chemical intermediate in the
production of plastics, organic solvents, and so on.5
Homogeneous and heterogeneous catalysts have been
developed for the cycloaddition of CO2 and epoxides. To date,
considerable homogeneous catalysts, including quaternary
ammonium and phosphonium salts,6 ionic liquids,7 alkalimetal
salts,8 Schiff bases,9 and metal-centered salen complex,10 have
been employed to promote transformation. Although they show
effective conversion of the coupling of CO2 with epoxides into
cyclic carbonates, the inherent shortcomings of separation of
the product and recycling of the catalyst limit the wide
application of the homogeneous catalysts. Consequently, in
order to overcome the defect of the homogeneous catalyst,
heterogeneous catalysts, such as metal oxides,11 functional
polymers,12 zeolites,13 and porous organic polymers (POPs)14
have been developed. However, most of them need high
temperature (>100°C) to activate the reaction, which increases
Introduction
Carbon dioxide (CO2) gas is the most important anthropogenic
gas that has been cited as the leading culprit for average
temperature increase globally and subsequent climate changes.1
The emission of CO2 from the power plants actually can be
considered as an abundant carbon source. Besides the physical
adsorption and permanent underground deposition of CO2, an
alternative and more efficient strategy for addressing
anthropogenic CO2 emission issues should be the one leading to
catalytic chemical conversion of CO2 into value-added
chemicals and materials, so that the emitted CO2 can be reused
in the carbon recycling on the earth.2,3 This approach not only
a. Department of Chemistry, University of Delhi, Delhi-110007, INDIA. Email:
manojtri@gmail.com; gurmeet123@gmail.com.
b. Department of Chemistry, Rajdhani College, University of Delhi, Delhi-110015,
INDIA
c. Department of Chemistry, University of Lucknow, Lucknow-206007, INDIA
d. Department of Chemistry and Center of Advanced Studies in Chemistry, Panjab
University, Chandigarh-160014, INDIA
e. Department of Chemistry, DAV University Jalandhar, Jalandhar-144012, INDIA
f. Department of Chemistry & Biochemistry and Centre for Nanoscience, University
of Missouri-St. Louis, One University Boulevard, St. Louis, MO 63121-4499, USA.
Email: rathn@umsl.edu
Electronic Supplementary Information (ESI) available: Experimental details,
additional structural figures, crystallographic refinement details, PXRD, IR spectra.
CCDC 1429033, 1879768 for (1), and (2). For ESI and crystallographic data in CIF
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the cost of the reaction process. There are several requirements (2) and their catalysis for CO2 cycloaddition with VeipewoxAritdicelesOantlin1e
DOI: 10.1039/C9DT01457H
for an efficient heterogeneous catalyst for the cycloaddition of atm and room temperature.
CO2 and epoxide: first, high surface area and CO2 adsorption
capacity; second, enough Lewis/Brønsted acidic or basic sites
Experimental section
to active the epoxide/CO2; third, high stability and long
durability upon exposure to the reaction conditions. Owing to
their potential applications in various fields ranging from
Materials and Physical Measurements
molecular magnetism, adsorption science and gas storage to All the synthetic manipulations were performed under nitrogen
photoluminescence and catalysis, the design and synthesis of atmosphere. The solvents were dried and distilled before use
coordination polymers (CPs) have acquired an extensive following the standard procedures.61 Copper cyanide, copper
growth in recent years.15 Amongst the variety of organic thiocyanate, copper acetate hydrate, ammonia solution,
ligands, polycarboxylic acids and different N-donors hexamethylenetetramine, glacial acetic acid, o-phthalic acid, 1-
derivatives have been broadly explored as building blocks and (2-Pyridyl)piperazine and epoxides were purchased from Sigma
linkers for the construction of coordination networks.15,16 Much Aldrich Chemicals Pvt. Ltd. All the chemicals were used
research has also been focused on the development of new without further purification. Elemental analysis was performed
organic molecules with tunable properties for their subsequent using a Carlo Erba Model EA-1108 elemental analyzer and C,
use in crystal engineering.4 Hexamethylenetetramine (hmt; H and N are within ±0.4% of calculated values. IR (KBr)
C6H12N4), also known as hexamine or urotropine, can be spectrum was recorded using a Perkin-Elmer and Bruker FT-IR
considered as one such simple heterocyclic compound with a spectrophotometer. Electronic and emission spectra of 1 and 2
cage like structure which because of its low cost, commercial were obtained on a Perkin Elmer Lambda-35 and a Horiba
availability and high solubility in water and polar organic Jobin Yvon Fluorolog 3 spectrofluorometer, respectively.
solvents, has found a broad variety of applications, ranging Thermogravimetric analysis was carried out in nitrogen using a
from the production of phenolic resins and solid fuel tablets to TA DSC Q 200 instrument with a heating rate of 10°C min-1.
be used in organic synthesis, medicinal and materials The powder X-ray diffraction (PXRD) patterns were collected
chemistry.15u,17,18 As far as coordination chemistry is on a Bruker D8 Discover X-ray diffractometer (Cu-Kα 1.5405
concerned, hmt is a versatile ligand capable of adopting Å). X-Ray photoelectron spectroscopy(XPS) (PHI 5000 Versa
different coordination modes that span from the terminal Prob II, FEI Inc.) with Auger electron spectroscopy module
monodentate to bridging bi-, tri- and tetradentate modes.14u,17,18 was used for obtaining XPS spectra. The H and 13C NMR
1
In addition, due to the good H-bond accepting properties, hmt is spectra were recorded on a JEOL DELTA2 spectrometer at 400
very often trapped by metal- organic compounds forming MHz using TMS as an internal standard. The chemical shift
various molecular adducts and supramolecular structures.15u,19,20 values are recorded on the δ scale and the coupling constants (J)
The use of hmt as a simple and convenient linker with a are in Hz. GC-MS studies were done with the Shimadzu-2010
diamandoid-like geometry for the design of coordination instrument containing a DB-5/RtX-5MS-30Mt column of
polymers with other metals had been explored to a lesser 0.25mm internal diameter with an oven temperature range of
extent.15u However, due to the extensive development of crystal 90-180°C (5 min) at 4°C/min raised to 300°C at 4°C/min. Gas
engineering the interest in hmt had increased in recent years sorption isotherms were measured by using an ASAP 2020
thereby resulting in the synthesis of a variety of one-(1D), two- adsorption equipment.
(2D) and there-dimensional (3D) hmt-driven coordination Synthesis of [Cu2(OAc)4(4-hmt)0.5] (1). Copper cyanide
polymers bearing different metals.21-56 In addition, the aromatic (0.178 g, 2 mmol) was added slowly to a solution of CH3OH
carboxylic acid ligand, o-Phthalic acid has been widely (10 mL) and CH2Cl2 (10 mL) containing liquid NH3 (0.5 mL,
explored as linker in synthesising coordination polymers.57 The 30 mmol) and hexamethyleneamine (0.140 g, 1 mmol). The
immense popularity of o-phthalic acid as linker in coordination resulting solution was stirred at room temperature for 6 hours.
polymers is because of its ability to provide different modes of Slowly, a blue color precipitates appeared which was dissolved
coordination.58,59 In all the previously reported coordination by drop wise addition of glacial acetic acid. The resulting
polymers, at least one other ligand is found to be present solution was filtered and left for slow crystallization in room
together with o-phthalic acid. o-phthalic acid in the absence of temperature. Dark blue color block shaped crystals suitable for
other ligands resulted in monomeric complexes when allowed X-ray studies were obtained after four weeks. The same
to react directly with Cu(II) ion at room temperature.60 There compound was also isolated by the reaction of different copper
are very few report of coordination polymer based on o- salts such as copper(I) thiocyanate, copper(II) acetate hydrate,
phthalic acid alone without incorporating other ligands. and copper(II) nitrate hydrate under the similar condition
Although these studies were primarily focused on exploring mentioned above instead of copper cyanide. Yield: (0.980 g,
rich structural diversity and attractive molecular aesthetics of 50%). Anal. calc. for C11H18N2O8Cu2: C, 30.46; H, 4.15; N,
coordination polymers, the application of such compounds as 6.46. Found: C, 30.63; H, 4.45; N, 6.68. IR (cm-1, KBr):  =
functional materials still remain underdeveloped.26,29,30,37,38b 3306, 3204, 3168, 1624, 1552, 1435, 1260, 1021, 933, 722,
Herein, we describe the synthesis, spectral and structural 700, 649, 598, 459. UV/Vis: max ([dm3 mol-1 cm-1]) = 273
characterization of two copper coordination motif, namely (9090), 708 (20760).
[Cu2(OAc)4(4-hmt)0.5]n (1), [Cu{C6H4(COO-)2}2]n.2C9H14N3
2 | J. Name., 2012, 00, 1-3
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Synthesis of [Cu{C6H4(COO-)2}2]n.2C9H14N3 (2). Copper(II) stoichiometric ratio in a mixture of dichloromethane and
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acetate hydrate (0.182 g, 1 mmol) was added slowly to a methanol (1:1 V/V) containing hmt/1D-(O2I-:P10y.1r0id3y9/lC)p9iDpTe0r1a4z5i7nHe
solution of CH3OH (10 mL) and CH2Cl2 (10 mL) containing under solvothermal condition. All motifs are air stable solid,
liquid NH3 (0.5 mL, 30 mmol), 1-(2-Pyridyl)piperazine (0.163 insoluble in water and other common organic solvents but
g, 1 mmol). The resulting solution was stirred at room soluble in dimethyl sulfoxide and do not show any signs of
temperature for 4 hours. Slowly, a blue color precipitates decomposition in solution upon exposure to air. The hmt ligand
appeared which was dissolved by drop wise addition of o- adopts the less common tetradentate mode in 1. Compound 1
phthalic acid (0.166 g, 1 mmol) in 10 ml water. The resulting show bands at 929-1021 cm-1 and 1228-1260 cm-1
solution was filtered and left for slow crystallization in room corresponding to CN stretching of hmt moiety (See Fig. S1,
temperature. Dark blue color block shaped crystals suitable for ESI†). The carboxyl group of the acetate ligand bridges the
X-ray studies were obtained after one week. The same copper centers with both of their oxygen atoms. The νas CO2 in
compound was also isolated by the reaction of different copper 1 at 1552-1558 cm-1 and νs at 1418-1435 cm-1, is typical of the
salts such as copper(I) thiocyanate, copper(I) cyanide, and bridging carboxylate of the copper(II) carboxylate dimer.53b
copper(II) nitrate hydrate under the similar condition mentioned Compound 2 showed characteristic peaks due to the ligation of
above instead of copper acetate hydrate. Yield: (0.576 g, 80%). carboxylate. The peaks appearing in the region 1577-1608 cm-1
Anal. calc. for C34H36N6O8Cu: C, 56.66; H, 5.00; N, 11.66. can be attributed to νasym(COO-) vibrations while the peaks in
Found: C, 56.83; H, 4.88; N, 11.76. IR (cm-1, KBr):  = 2823, the range 1359-1400 cm-1 can be attributed to νsym(COO-)
2738, 2659, 2495, 1608, 1577, 1544, 1504, 1444, 1400, 1359, vibrations (See Fig. S2, ESI†). The acid-base properties of the
1307, 1273, 1219, 1172, 1082, 1031, 977, 941, 860, 835, 788, copper(II) coordination polymers 1 and 2 have been studied. In
744, 702, 648, 594, 538, 505, 447, 410. UV/Vis: max ([dm3 basic media, no structural change is observed in 1 and 2 when
mol-1 cm-1]) = 284 (8079), 690 (16081).
X-ray structure determination
A crystal of appropriate size was mounted on a glass fiber and
the intensity data for 1 were collected on an Oxford Xcalibur S
CCD area detector diffractometers using graphite
the solid is dispersed for 24 h in 1 M NaOH solution by the
PXRD. However, in acidic conditions, PXRD reveals a
structural change in 1 and 2 when the solid is dispersed for 24 h
in 1 M HCl solution. The single crystal X-ray refinement detail
for 1-2 are summarized in Table 1 (ESI†), and selected bond
lengths and angles and hydrogen bond parameters are presented
in Table 2 and Table 3 (ESI†), respectively. Fig. 1 presents the
overhead and perspective views of the crystal structures of 1
and 2. Compound 1 shows six-membered rings (See Fig. S3,
ESI†). This is a consequence of the hmt nodes adopting an
undistorted tetrahedral geometry and coordinating to four
spacers. Its ethyl analogue was published by Zaworotko et al. in
200153a and the phenyl analogue was reported by Ghosh et al.
in 2013.53b Compound 1 crystallizes in the tetragonal space
group P4(2)/nnm. The dinuclear copper carboxylate paddle
wheel spacers which exhibit square-pyramidal coordination to
monochromatized Mo-K radiation at 293(2) K. CrysAlisPro,
an Agilent Technologies software package62, was used for data
collection and data integration for 1. Intensity data sets for 2
was collected on a Bruker APEX II CCD area detector
diffractometer using graphite monochromatized Mo-K
radiation at 100(2) K. SAINT software packages were used for
data collection and data integration for 2. Structure solution and
refinement were carried out using the SHELXTL-PLUS
software package.63 The non-hydrogen atoms were refined with
anisotropy thermal parameters. All the hydrogen atoms were
treated using appropriate riding models. The computer
programme PLATON was used for analyzing the interaction
and stacking distances.63
four basal oxygens [Cu(1)-O(1)= 1.945(5) Å, Cu(1)-O(2)#2
=
1.965(5) Å, Cu(1)-O(2)#3 = 1.965(5) Å] and one apical nitrogen
[Cu(1)-N(1) = 2.343(6) Å] having CuCu separation of
2.6317(19) Å [Cu(1)-Cu(1)#2]. These distances are comparable
to other copper complexes having hmt as ligand.53 As expected,
the sheets of 1 are linear, and the packing of adjacent sheets
appears to be the consequence of their shape. The methyl
groups efficiently fill the hexagonal cavities, which have sides
of 1.17 nm and diagonals of 1.87 nm, and mitigate against the
inclusion of solvent molecules. The framework of 1 contains
hexagonal channels with dimension of 18.7 × 16.4 Å2 and made
up of six tetrahedrally coordinated hmt molecules (Fig. 2). The
3D framework of 1 can be simplified topologically by
considering the nodes corresponding to the centroid of organic
ligand as secondary building unit as a uninodal 4-connected net
and the linear bifunctional paddle wheel inorganic linkers (See
Fig. S4 to Fig. S6, ESI†).64 Topological analysis suggest that
compound 1 is a 3-D, two-fold diamondoid network comprising
of 4-connected sqc6 topology with the point symbol of {66}
(Fig. 3 & 4). The τ parameter (five-coordinated species)65 for
the coordination in 1 is 0 which indicated that the geometry is
General Experimental Procedure for 1/2-catalysed cycloaddition
of CO2 to cyclic carbonates
1 mol% of 1/2 (4.33 mg/4.89 mg/2.47 mg) and tetra-n-
tertbutylammonium bromide (TBAB, 0.3 mmol) were added
into the Schlenk reaction tube (10 mL). CO2 (balloon) and 20
mmol of epoxides were introduced into the reaction mixture
under stirring at 1 atm under solvent free environment at room
1
temperature for 18 h. The yields were calculated based on H
NMR analysis. The recovered catalyst was collected by
centrifuge, washed by fresh CH3Cl and dried in vacuum.
Results and discussion
Synthesis
Copper(II) coordination polymers, viz. [Cu2(OAc)4(4-hmt)0.5]n
(1), and [Cu{C6H4(COO-)2}2]n.2C9H14N3 (2) were obtained in
good yield by the reaction of MX (M=Cu: X= CN-, SCN-)/MX2
-
(M=Cu: X= CH3COO-, NO3 ) with liquid NH3 in
a
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perfectly square pyramidal. Compound 2 shows that phthalate carboxylate and the Cu(II) ion were found to be equal which is
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DOI: 10.1039/C9DT01457H
ion acts as a bridging ligand in the formation of infinite 2-D 52.89◦. The other two O-Cu(II)-O angles were of value
coordination polymer. Each Cu(II) ion is coordinated with four 127.2(2)◦. Interestingly the axial Cu(II)-O bonds were not
phthalate ligands completing an octahedral structure with six O perfectly perpendicular to the plane of four equatorial O atoms
atoms and two 1-(2-Pyridyl)piperazium ions in the lattice. Out from two chelating carboxylates. Each of the axial Cu(II)-O
of the four phthalate two coordinates to the Cu(II) through both bonds were found to be tilted slightly towards one of the two
O atoms of one carboxylate as chelate. These two chelating equatorial chelating carboxylate pairs. The O-Cu(II)-O angles
carboxylates are found to be in trans position to each other. involving one axial O atom and the O atoms from one pair of
Each of the other carboxylates (of these two phthalates) binds chelating carboxylate are 93.02◦ and 96.22◦ while these angles
to another Cu(II) through only one O atom while the other O with the other pair of carboxylate are 83.78◦ and 86.98◦. The
dangles as keto group.
distance between the two Cu(II) bound O atoms belonging to
the same carboxylate is 2.364 Å while the distance between the
two O atoms (on the same side of the coordination core)
belonging to two different carboxylates is 4.251 Å. Topological
analysis suggests that compound 2 is a Shubnikov tetragonal
plane network comprising of a 4-connected sql topology with
point symbol of {44.62}-VS [4.4.4.4.*.*] (Fig. 5).64a
Fig. 3. Two-fold interpenetrated diamondoid network comprising of 4-connected sqc6
topology in 1.
Fig. 1. Overhead and perspective views of 3D and 2D network (H and 1-(2-
Pyridyl)piperazium atoms omitted for clarity) seen in the crystal structure of
[Cu2(OAc)4(4-hmt)0.5] (1; a), and [Cu{C6H4(COO-)2}2]n.2C9H14N3 (2; b).
Fig. 4. Diamondoid 4-connected uninodal network in 1.
Fig. 2. Hexagonal channels in 1 made of six tetrahedrally coordinated hmt molecule.
The other two trans coordination sites of the Cu(II) are fulfilled
by carboxylate O atoms of phthalate of which the remaining
carboxylates chelate to two Cu(II). In this way, a two
dimensional network is generated where each phthalate acts as
a bridge between two Cu(II), and coordinating to one Cu(II) by
the monodentate carboxylate oxygen and to another Cu(II) by
chelating carboxylate. In compound 2, the two trans O-Cu(II)-O
angles created each by the two O atoms of a chelating
Fig. 5. Topology of 2 showing shubnikov tetragonal plane network comprising of 4-
connected node with sql topology.
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XPS behaviour.
about the decomposition and/or volatility of the coVmiepwoAurtinclde.OTnlhinee
TGA results (Fig. S9, ESI†) indicate thaDtOthI:e10f.1r0a3m9e/Cw9oDrTk01o4f571H,
and 2 are stable up to ca. 150°C, 170°C and then starts to
disrupt and decompose abruptly. The first major weight loss
(63.42%) occurred in the range of 150-280°C in 1 and then the
second minor weight loss (5.22%) is in between 281-404°C
leading to 31.36%, thermally stable final product which
corresponds to the calculated value 33.02% for Cu2O. On the
other hand, the weight loss was observed from 171°C to 505°C.
The first weight loss is a well-defined step with initiation
temperature 180°C due to loss of water molecules while the
second and the third weight losses are combined ones with
initiation temperatures ca. 258°C and 390°C, respectively
leading to 32.26% product which corresponds to the calculated
value 32.11% for CuO.
The valence of copper in 1 and 2 have been proved with the aid
of X-ray photoelectron spectroscopy (See Fig. S7 to Fig. S8,
ESI†). As shown in Fig. S10 to Fig. S11, the wide scan spectra
of 1 and 2 indicates that it consists of the elements Cu, N, O,
and C. The wide scan spectra of Cu2p having two absorption
band at 933.12 to 933.70 eV and 953.20 to 953.35 eV which
corresponds to the bonding energy of Cu2p3/2 and Cu2p1/2,
respectively. Along with this, there are two more absorption
bands corresponding to the strong satellites of Cu2p3/2 and
Cu2p1/2 indicate the presence of copper in +2 oxidation state.
Electronic and Emission Spectroscopy.
The UV-vis absorption spectra of 1 and 2 were measured in the
DMSO solution (Fig. 6). The absorption maxima from 273 to
284 nm of 1 and 2 in the ultraviolet region are assigned to
intraligand transitions. CPs 1 and 2 also exhibit broad bands in N2 Absorption.
the visible region with absorption maxima at 690 nm to 708 nm
The N2 adsorption/desorption isotherms of 1-2 were measured at 77
which are attributed to d-d transitions of copper(II) systems
(Fig. 6).64 Compounds 1 and 2 were non-emissive on excitation
at 690 nm to 708 nm. However, on excitation at 273 nm to 284
nm, 1 exhibited two broad emission peaks centered at 364 nm
K to investigate the apparent surface areas and pore volumes of the
materials. Compound 1 gave rise to reversible typical type-I
adsorption isotherm with a BET surface area up to 176 m2 g-1,
Langmuir surface area 234 m2 g-1, and a total micropore volume of
0.0810 cm-3 g as evaluated by Dubinin-Astakhov (DA) method
which indicated microporosity (Fig. S10, ESI†). The compound 2
was nonporous in nature.
Kinetic study by TGA for CO2 adsorption.
The CO2 mitigation studies were performed by using a TGA
(Fig. S11, ESI†). The compound 1 (5-10 mg) was placed in to
an alumina pan and adjusted operating temperature. 1 was
heated to 200°C to activate or remove moisture and kept
isothermally condition in N2 atmosphere for 30 min. It was then
brought to 50°C and kept again isothermally in CO2 atmosphere
for 1h. Compound 1 indicates 1.52% CO2 capturing in terms of
weight gain. It has been seen from BET that 1 showed high
surface area (>100 m2/gm).
Catalytic performances for the conversion of epoxides into cyclic
carbonates with CO2.
Fig. 6. Electronic spectra of 1 and 2 in DMSO.
The Cu(II) coordination polymers (1-2) with the incorporation
of copper metal sites and nitrogen-rich hexamethylene
tetraamine/1-(2-Pyridyl)piperazium units within the framework
inspired us to investigate its heterogeneous catalytic activity for
the cycloaddition reaction of CO2 with epoxides. Among CO2
chemical conversion reactions, catalyzed CO2 cycloaddition
with epoxides has been intensively investigated recently due to
their wide range of applications in the production of carbonates,
as an important synthetic intermediates in pharmaceutical and
electrochemical industries.68
Fig. 7. Emission spectra of 1-2 in DMSO on excitation at 273 nm, and 284 nm,
respectively.
O
and 755 nm. However, 2 showed one broad band at 368 nm
(Fig. 7). Luminescence response for these compounds is due to
ligand-to-metal charge transfer.57,58,66,67
O
O
O
1 mol% of 1 or 2
CO2
+
TBAB (0.3 mmol)
18h, RT
R
Thermogravimetric analysis.
R
Scheme 1. Catalytic cycloaddition of CO2 with epoxides to produce cyclic carbonates.
Thermogravimetric analyses (TGA) of 1-2 were performed
under nitrogen with heating rate 10˚C min-1 to gain information
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Table 1. 1/2 catalysed cycloaddition of epoxides with CO2.a
Entry
Substrate
Products
Yields (%)b/TONc
1
2
O
O
O
1.
95/1900
75/1500
O
O
O
O
O
2.
85/1700
60/1200
O
O
O
3.
4.
5.
87/1740
90/1800
96/1920
68/1360
65/1300
80/1600
O
O
Cl
Cl
Br
O
O
O
O
O
O
Br
O
HO
HO
O
NRd
NRd
O
6.
7.
6/120
6/120
O
O
H3
C
O
O
H3
C
O
O
O
O
8.
9.
6/120
5/100
NRd
O
O
NRd
O
O
O
b
aReaction conditions: epoxide (20 mmol), catalyst (1 mol%), and TBAB (0.3 mmol) under carbon dioxide (1 atm) at RT for 18 h. The yields were
determined by 1H NMR analysis.; cTON = (moles of product)/(moles of the catalyst).; dNR= No reaction.
Therefore, catalytic performance of compounds 1-2 in the per compound 1-2. A small decrease in yield was observed
cycloaddition of CO2 with epoxides to produce various when the hydroxyl was substituted with chloride or bromide
carbonates was explored (scheme 1). HKUST-1 was taken as a substituent. In the absence of co-catalyst TBAB, the reaction
catalyst in these heterogenous reactions as control experiments. catalysed by compound 1 or 2 yielded 10-15% after 18h.
The reactions were carried out using the epoxide (20 mmol) and However, in the absence of 1 or 2, the same reaction catalysed
carbon dioxide in the presence of a co-catalyst, tetrabutyl by the co-catalyst TBAB alone gave only 8% yield so the
ammonium bromide (TBAB, 0.3 mmol) and catalyst loading combination of 1 or 2 and TBAB gave the best result. The
[1-2, 1 mol%] at room temperature and 1 atm pressure for 18 h. comparison of the yields clearly indicates that the 1 shows
Yields of the obtained cyclic carbonates produced from CO2 greater performance than HKUST-1 but almost similar to
with different epoxides catalyzed by 1-2 and HKUST-1 has MOFs68e,68f,70 for catalytic CO2 cycloaddition with epoxides at
been determined under the identical conditions. As shown in the same conditions. The high catalytic activity of 1 can be
Table 1 and Fig.7, it has been observed that 1 showed high ascribed to the increase in CO2 affinity via the introduction of
efficiency in the CO2 cycloaddition as compared to compound the nitrogen-rich hexamethylenetetramine groups into the
2 especially with small-sized epoxides (Table 1; Entry 1 to 5) framework. To probe the size-selectivity in the cycloaddition
which is due to presence of higher number of available reaction of CO2 with epoxides, larger epoxide substrates such
undercoordinated metal ions in the paddlewheel units in 1.69 as 1,2-epoxypentane, 1,2-epoxyhexane, 1,2-epoxyoctane, and
The reaction yields of cyclic carbonates are 75-95% for 2- 1,2-epoxydodecane were employed. The product yields for 1
methyloxirane, corresponding to TON of 1500-1900; 60-85% showed sharp decreases and afforded only 5-6% (entries 6-9 in
for 2-ethyloxirane, corresponding to TON of 1200-1700; 68- Table 1) catalytic product and the corresponding TON values
87% for 2-(chloromethyl)oxirane, corresponding to TON of were 100-120. However, 2 showed no reaction. This suggests
1360-1740;
65-90%
for
2-(bromomethyl)oxirane, that large substrates are unable to access the catalytic sites
corresponding to TON of 1300-1800; 80-96% for 2- decorated inside the framework of 1 or 2. These observations
(hydroxymethyl)oxirane corresponding to TON of 1600-1920 also indicate that the former reactions with small substrates
6 | J. Name., 2012, 00, 1-3
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(entries 1-5 in Table 1) were indeed occurred within the carbonate capable of cyclization to form a cyclic carbonate with
View Article Online
DOI: 10.1039/C9DT01457H
framework of 1/2, and therefore verifying the size selectivity. the regeneration of the catalyst.
The remarkably high efficiency and the size selectivity to small
epoxides for catalytic CO2 cycloaddition confirm that 1 is a
suitable heterogeneous catalyst for carbon fixation.
A
comparison of catalytic efficiency of the 1 and 2 with other Cu-
MOFs catalysts, already described in literature for the
preparation of carbonates, revealed advantage of 1 in term of
better yield with shorter reaction time at room temperature
(Table 2). In the backdrop of these findings, 1 is found to be a
very excellent catalyst for efficient synthesis of small
carbonates. To examine the recyclability, taking the CO2
cycloaddition with glycidol as an example, the catalyst 1/2 were
successively reused in 5 runs without losing performance in the
catalytic efficiency (Fig. S12, ESI†). The stability of 1/2 was
also proven by the PXRD measurements, showing that the
PXRD patterns of the recycled 1/2 are in good agreement with
calculated pattern from its
Fig. 8. In situ FT-IR spectra of the synthesis of 2-(hydroxymethyl)oxirane catalyzed by
1 at room temperature under 1 atm CO2 pressure.
Fig.7. Yields of various cyclic carbonates prepared from the cycloaddition reaction of
CO2 with epoxides catalyzed by compounds 1(Red), 2(Green) and HKUST-1.
crystal data. The PXRD results indicate that the framework
retained its structural integrity very well after the catalytic
reactions (Fig. S13-Fig. S14, ESI†). To shed light on the
mechanism of the heterogeneous catalytic CO2 fixation, in situ
FT-IR measurements were performed to monitor the reaction
process of 1 at room temperature and 1 atm CO2 pressure (Fig.
8). The absorption intensities of the carbonyl group
characteristic peaks ((C=O)), centered at 1740 cm-1, rapidly
increased with the reaction time, indicating the formation of 2-
(hydroxymethyl)oxirane. The typical asymmetric vibrations
(2340 cm-1) of CO2 was observed after the addition of glycidol
during the reaction and disappeared at the end of the reaction.
Based on the structural features and catalytic performances as
well as the in situ FT-IR results, a tentative mechanism for the
1/2-catalyzed cycloaddition of epoxide and CO2 into cyclic
carbonates is proposed,70 and is shown in Fig. 9. The coupling
reaction is initiated by the ring-opening of the epoxide with the
help of Bu4NBr to afford a Cu-bound alkoxide in an SN2-type
reaction. The subsequent addition of CO2 to the ring-opened
epoxide was preferred by the presence of Bu4NBr, which
stabilized the polarized intermediate resulting in a metal
Fig. 9. The proposed mechanism for the cycloaddition reaction of epoxide and CO2 to
form cyclic carbonates catalyzed by 1 in the presence of TBAB.
Conclusions
In summary, we have synthesized two copper (II) coordination
polymers,
viz.
[Cu2(OAc)4(4-hmt)0.5]n
(1),
and
[Cu{C6H4(COO-)2}2]n.2C9H14N3 (2) and subsequently their
utilization as a heterogeneous catalyst for the cycloaddition of
carbon dioxide to epoxides. Significantly, the catalyst 1
possesses excellent recyclability, and it doesn’t showed decline
in activity during the first four cycles of reaction. The
constructed material incorporating both exposed metal sites and
nitrogen-rich hmt/1-(2-Pyridyl)piperazium groups make it
promising heterogeneous catalyst for CO2 chemical conversion
with small substrates, which have been confirmed by
remarkably high efficiency and size selectivity for catalytic
CO2 cycloaddition with epoxides. Research along this line is
going on in our laboratory.
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Table 2. Comparative catalytic performance of the 1 and 2 with others previously reported Cu-MOFs catalysts for cycloaddition of epoxides with CO2.
Entry
Cu-MOFs
Co-catalyst
Reaction conditions
T[K]/P[atm]/Time[h]
273/1 atm./48h
% yield
Ref.
1.
2.
3.
4.
5.
[Cu3(BTC)2] or HKUST-1
[Cu2(BPTC)(H2O)2]or MOF-505
[CuL1] or BIT-C
n-Bu4NBr
n-Bu4NBr
n-Bu4NBr
-
49
68g
68g
70j
273/1 atm./48h
48
333/1 atm./6h
95-99
10-73
42-95
[Cu(HIP)2(BPY)]
393/12 atm./6h
70g
68g
Cu2(Cu‐TACTMB)(H2O)3(NO3)2
MMCF‐2
or
or
n‐Bu4NBr
273/1 atm/48h
6.
Cu6(Cu‐TDPBPP)(HCO2)4(H2O)6
MMPF‐9
n‐Bu4NBr
273/1 atm/48h
30-87
70p
7.
8.
Cu4MTTP
n‐Bu4NBr
n‐Bu4NBr
n‐Bu4NBr
DBU
273/1 atm/48h
373/9.86 atm/3-12h
273/1 atm/48h
353/1atm/12h
83-96
>99
70e
70m
70n
[Cu7(H1L)2(TPT)3(H2O)6]
[ Cu2(C20H12N2O2)(COO)4]n
[Cu-ABF@ASMNPs]
9.
88-96
89-92
29-96
10.
11.
[Cu24(BDPO)12(H2O)12]·30DMF·14H2O
or (JUC1000)
TBABr
298/1 atm/48
70k
12.
13.
14.
15.
16.
[Cu2L(H2O)2]4H2O2DMF
[Cu2 (BDC)2 (DABCO)]
FJI-H14
n‐Bu4NBr
-
373/9.86 atm./2-6h
373/8 atm./12h
353/0.15 atm/24h
298/1 bar/8-24h
298/1 atm/36h
64->99
13
70l
70f
70r
70s
70t
n‐Bu4NBr
n‐Bu4NBr
n‐Bu4NBr
27-95
30-95
50
{[Cu6(L)3(H2O)6].(14DMF)(9H2O)}n
[Cu5(TPTC)3(BPDC-NH2)0.5(H2O)5] (1-
NH2)
17.
[Cu5(TPTC)3(BPDC-Urea)0.5(H2O)5] (1-
Urea)
n‐Bu4NBr
298/1 atm/36h
19-98
70t
18.
19.
[Cu2(OAc)4(4-hmt)0.5
[Cu{C6H4(COO-)2}2]n.2C9H14N3
]
n‐Bu4NBr
n‐Bu4NBr
273/1 atm./18h
273/1 atm./18h
87-96
60-80
Present work
Present work
n
Acknowledgements
Lin, C.S. Diercks, Y.-B. Zhang, N. Kornienko, E.M. Nichols,
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This project was financially supported by the Department of
Science and Technology, New Delhi, India (Grant No.
EMR/2016/006624). Special thanks are due to Professor P.J.
Sadler, University of Warwick, UK and Professor Josef Michl,
University of Colorado, USA for their kind encouragement. We
acknowledge funding from the National Science Foundation
(CHE0420497) for the purchase of the APEX II diffractometer.
4
5
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DOI: 10.1039/C9DT01457H
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DOI: 10.1039/C9DT01457H
Graphical Abstract: Synopsis and Pictogram
Copper(II) coordination polymers [Cu2(OAc)4(4-hmt)0.5]n (1), and [Cu{C6H4(COO-
)2}2]n.2C9H14N3 (2) have been synthesized and characterized. 1 showed high efficiency for
CO2 cycloaddition with small epoxides but its catalytic activity decreases sharply with
increase in the size of epoxide substrates.