Significant Gas Adsorption and Catalytic Performance by a Robust CuII-MOF Derived through Single-Crystal to Single-Crystal Transmetalation of a Thermally Less-Stable ZnII-MOF

Article abstract of DOI:10.1002/chem.201503163

By using a bent tetracarboxylic acid ligand that incorporates a pendent -NH₂ functional group, a 3D Zn^II-framework (1) based on Zn₂(CO₂)₄ secondary building units and Zn₁₂(CO₂)

Full text of DOI:10.1002/chem.201503163

DOI: 10.1002/chem.201503163
Full Paper
&
Porous Materials
Significant Gas Adsorption and Catalytic Performance by a Robust
CuII–MOF Derived through Single-Crystal to Single-Crystal
Transmetalation of a Thermally Less-Stable ZnII–MOF
Tapan K. Pal,[a] Dinesh De,[a] Subhadip Neogi,[b] Pradip Pachfule,[c] S. Senthilkumar,[b]
Qiang Xu,*[c] and Parimal K. Bharadwaj*[a]
Abstract: By using a bent tetracarboxylic acid ligand that in-
corporates a pendent -NH2 functional group, a 3D ZnII-
framework (1) based on Zn2(CO2)4 secondary building units
and Zn12(CO2)24 supramolecular building blocks has been
synthesized. Framework 1 is thermally less stable, which pre-
cludes its application as a porous framework for gas adsorp-
tion or catalytic studies. This framework undergoes single-
crystal to single-crystal transmetalation to give isostructural
1Cu. Unlike 1, the CuII analogue is very stable and can be ac-
tivated by removing metal-bound lattice solvent molecules
by heating to afford 1Cu’. The activated 1Cu’ exhibits excellent
H2 storage (2.29 wt%) at 77 K and a high 32.1 wt% CO2
uptake at 273 K. Additionally, it displays significant selectivity
for CO2 adsorption over N2 and H2 and can catalyse size-se-
lective Knoevenagel condensation reactions.
Introduction
is a helpful approach to maintaining framework integrity after
a metal-exchange reaction.[5] By using this technique, multi-
functional MOFs can be obtained that may or may not be ac-
cessible by the de novo method. Moreover, transmetalation
can be used to fabricate the interior and exterior of MOFs with
improved properties.[6] With the aforesaid annotations in mind,
and evoking our experiences on bent carboxylates,[7] a new an-
Metal–organic frameworks (MOFs) are crystalline porous mate-
rials, for which most of the studies are focused towards sys-
tematic variation of porosity through 1) creation of unsaturat-
ed metal centres (UMCs) and 2) channel (pore) functionaliza-
tion with organic groups to augment their potential applica-
tions.[1] Formation of a thermally robust framework that con-
tains metal-bound solvent molecules is an essential
prerequisite for the creation of UMCs, whereas the position of
functional groups in the linker is crucial in the sense that it
should remain free.[2] Frequently, however, difficulties arise due
to thermal/chemical instability of the as-synthesized frame-
works because they may collapse upon solvent removal by
heating. Accordingly, much effort has been devoted to struc-
tural aspects of MOFs with a view to gaining superior stabil-
ity.[3] Post-synthetic modification (PSM), involving single-crystal
to single-crystal (SC–SC) transformation,[4] is a fascinating tech-
nique. The presence of secondary building units (SBUs) and
supramolecular building blocks (SBBs), based on metal clusters,
gular
2’-amino-[1,1’:3’,1’’-terphenyl]-3,3’’,5,5’’-tetracarboxylic
acid ligand (H4L; Figure 1a) has been designed. In this ligand,
the -NH2 group is placed to sterically preclude binding a metal
that is coordinated to the carboxylates.
[a] T. K. Pal, D. De, Prof. P. K. Bharadwaj
Department of Chemistry, Indian Institute of Technology Kanpur
Uttar Pradesh, Kanpur 208016 (India)
E-mail: pkb@iitk.ac.in
[b] Dr. S. Neogi, S. Senthilkumar
Inorganic Materials and Catalysis Division, CSIR-CSMCRI
Bhavnagar, G. B. Marg, Gujarat 364002 (India)
[c] P. Pachfule, Prof. Q. Xu
National Institute of Advanced Industrial Science and Technology
Ikeda, Osaka, 563-8577 (Japan)
E-mail: q.xu@aist.go.jp
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201503163.
Figure 1. a) Ligand H4L; b) view of 3D architecture of framework; c) second-
ary building unit (SBU); and d) supramolecular building block (SBB).
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(Figure S9 in the Supporting Information). The metal replace-
ment was initially verified by the EPR spectrum (Figure S10 in
the Supporting Information) of the greenish-blue crystals. The
X-ray fluorescence spectroscopy (XFS) demonstrates that no
significant amount of Zn2+ ion is left in the metal-exchanged
framework. Also, the PXRD pattern of the metal-exchanged
product does not reveal the formation of any new phase (Fig-
ure S11 in the Supporting Information). These observations
clearly indicate a metal-ion exchange in a SC–SC manner. For-
tunately, framework 1 maintains single crystallinity, which
allows single-crystal X-ray data collection. The X-ray structural
investigation (Table S1 in the Supporting Information) indicates
that the transmetalated product has a similar structure (Fig-
ure S12 in the Supporting Information) to that of 1, with the
formula {[Cu2(L)(H2O)2]·(5DMF)·(H2O)} (1Cu). However, the M···M
distance in the paddlewheel SBU decreases in 1Cu (Table S2 in
the Supporting Information). Although ligand H4L bears close
resemblance to the previously reported diisophthalate
ligand,[10] the structure of 1 is novel due to inward positioning
of the -NH2 group in H4L, and exhibits 4,4-c net with different
ssa network topology (Figure S13 in the Supporting Informa-
tion). MOF 1Cu could be synthesized solvothermally by treating
Cu(NO3)2·3H2O with H4L, although in poor yields (<10%).
The kinetics of the Zn2+-to-Cu2+ ion exchange, monitored
by energy-dispersive X-ray spectroscopy (EDX), indicate a 50%
replacement of Zn2+ ions by Cu2+ within 2 d, whereas a 95%
exchange took place in 4 d. Complete exchange is achieved in
6 d (Figure S14 in the Supporting Information). However, the
reverse transmetalation does not occur even in a concentrated
solution of Zn(NO3)2·6H2O (0.5m) in DMF with a long exposure
time, even under warm conditions, which points to the greater
thermodynamic stability of 1Cu. Also, ZnII ions of 1 could not
be replaced by other transition-metal ions, such as Co2+ and
Ni2+. The variable-temperature PXRD experiment of 1Cu shows
that, in contrast to 1, it is highly stable and maintains frame-
work integrity up to at least 2008C (Figure S12 in the Support-
ing Information).
Results and Discussion
Under solvothermal conditions, H4L reacts with Zn(NO3)2·6H2O
(1:2 molar ratio) to form a non-interpenetrated 3D MOF,
{[Zn2(L)(H2O)2]·(5DMF)·(4H2O)} (1; L=L4À). It crystallizes in the
hexagonal space group P63/mmc (Table S1 in the Supporting
Information), in which the asymmetric unit contains one fourth
of the ligand, one ZnII ion and a water molecule (half occu-
pancy). The structure contains paddlewheel Zn2(COO)4 clusters
as SBUs (Figure 1c), in which every Zn2+ ion adopts distorted
square-pyramidal coordination. Four equatorial positions of
Zn2+ are occupied by carboxylate oxygen atoms from four dif-
ferent L units and the apical position is coordinated to one
water molecule. The bulk phase purity is confirmed by compar-
ing the experimental and simulated powder X-ray diffraction
(PXRD) patterns (Figure S7 in the Supporting Information). As
illustrated in Figure 1b, three isophthalate moieties of ligand L
link three SBUs to form a triangular window. Two such win-
dows are further linked by three
L units to produce
a Zn12(CO2)24 polyhedra (Figure 1d) with six vertices, five faces
and nine edges. The core cavity is accessible through two tri-
angular and three square faces (Figure 1d). The internal spher-
ical cavity of this polyhedron has a diameter of approximately
5.680(1) Š (distance refers to atom-to-atom connection
throughout), ignoring coordinated solvents, whereas the size
of the triangular aperture is approximately 5.08 Š. As expected,
the -NH2 groups remain free and are pointed towards the
cavity of Zn12(CO2)24 polyhedron. These polyhedral Zn12(CO2)24
units can be regarded as SBBs. A close look at the packing dia-
gram of 1 reveals that six SBBs are interconnected along the
crystallographic c axis to afford the overall 3D framework (Fig-
ure 1b).
The solvent molecules present in the cavity are highly dis-
ordered and cannot be located unambiguously. However, the
exact solvent composition was established from a combination
of thermogravimetric weight loss and elemental analysis,
which are in good agreement with the solvent-accessible
volume calculated by using PLATON (4648 Š3, 65.13%).[8] The
PXRD patterns reveal that the framework turns amorphous at
508C (Figure S8 in the Supporting Information). Even solvent
exchanges by using different solvents like MeOH, EtOH, Me2CO
or CH2Cl2, followed by mild activation, lead to complete break-
down of the framework. This behaviour leads us to postulate
that Zn2+ ions with d10 electron configuration provide no
ligand-field stabilization energy and presumably the vulner-
ability of framework 1 originates from the removal of the axial
aqua ligand.[9] The unstable nature of 1 upon de-solvation pre-
vents us from obtaining a porous framework for gas-storage
applications.
The presence of porous channels and the high thermal sta-
bility of 1Cu led us to probe its gas sorption measurements. Ac-
tivation of the sample was accomplished by dipping crystals of
1Cu in EtOH for 7 d, followed by heating at 1508C under high
vacuum to produce the guest-free framework (1Cu’). During ac-
tivation, the colour of the crystals changed from greenish blue
to deep blue. However, the crystallinity of 1Cu’ was not suffi-
cient for single-crystal X-ray data collection. The activated
framework was first subjected to N2 gas sorption measure-
ments at 77 K up to a relative pressure (P/P0) of 1.0. It shows
a type I reversible isotherm (Figure 2a), which indicates the
microporous nature of the framework. The calculated Bruna-
uer–Emmett–Teller (BET) surface area was 1410 m2 gÀ1, with
a pore volume of 0.97 cm3 gÀ1 and a pore size in the range of
3.01 to 7.2 Š (Figure S15 in the Supporting Information).
The presence of paddlewheel SBUs and the accessibility of
polyhedral SBBs from the large channels provide an opportun-
ity to explore metal-ion exchange reactions through a post-
synthetic exchange (PSE) approach. When dipped in a solution
of Cu(NO3)2·3H2O (0.05m) in DMF at room temperature, the
colourless crystals of 1 gradually turns greenish blue in 6 d.
Optical microscopic observation shows that the crystals main-
tain their original shapes and sizes throughout the PSE process
Further interesting results were obtained from the H2 and
CH4 adsorption studies. Importantly, both these gases repre-
sent a potential replacement for depleted energy sources, for
which the main goal is to develop on-board gas-storage sys-
tems.[11] The H2 sorption isotherm of 1Cu’ at 77 K showed type I
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Figure 2. a) N2 adsorption isotherm of 1Cu at 77 K and b) H2 adsorption iso-
therm of 1Cu at 77 K.
behaviour (Figure 2b) with a storage capacity of 261 cm3 gÀ1
(2.29 wt%) at 1 bar (Figure 2b and Figure S16 in the Support-
ing Information). The adsorption and desorption isotherm
curves completely overlap with each other without any hys-
teresis. Noticeably, the hydrogen uptake shown by 1Cu’ is com-
parable and even better than many reported MOFs (Table S3 in
the Supporting Information). At 13 bar pressure, the H2 uptake
capacity of 1Cu’ reached a maximum of 364 cm3 gÀ1 (3.26 wt%;
Figure S17 in the Supporting Information). Such good storage
performance may be attributed to high electrostatic inter-
actions between H2 molecules and exposed CuII sites in the
structure.
The room-temperature CH4 uptake study of 1Cu’ under both
low- (Figure S18 in the Supporting Information) and high-pres-
sure conditions also displayed type I adsorption behaviour
without any hysteresis. The framework adsorbs a substantially
high 10.30 wt% of methane (144.25 cmÀ3 gÀ1) at 44 bar (Fig-
ure 3a). This uptake value is higher or comparable to some
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%), PCN-11 (14.0 wt%) meas-
ured at different high pressures.[12]
Figure 3. a) CH4 adsorption isotherm of 1Cu’ up to 50 bar, b) CO2 adsorption
isotherm, c) Qst of CO2 and d) CO2, CH4, H2 and N2 adsorption isotherms of
1Cu at 273 K.
Given the importance of developing protocols to reduce the
massive emission of CO2, we next explored the CO2 gas stor-
age[13] potential of 1Cu’ at different temperatures and pressures
of up to 1 bar (Figure 3b). The isotherms at 273 and 298 K dis-
played type I adsorption behaviour without hysteresis, in
which the maximum CO2 uptake at 273 K was found to be
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164.65 cm3 gÀ1 (32.1 wt%), whereas the value at 298 K approxi-
mated to 104.39 cm3 gÀ1 (20.5 wt%; Figure S19 in the Support-
ing Information). Though numerous MOFs with significant CO2
uptake have been reported, only limited examples of porous
MOFs that exhibit over 30.0 wt% CO2 uptake at 273 K and
1 atm (Table S4 in the Supporting Information) are known.
The isosteric heat of CO2 adsorption (Qst), evaluated by using
the Clausius–Clapeyron equation[14] from the isotherms at 273
and 298 K, gives a value of 37.51 kJmolÀ1 (Figure 3c) at zero
coverage, which indicates a strong adsorbate–adsorbant inter-
action. This value is lower than known MOFs with -NH2 groups,
such as bio-MOF-11 (45 kJmolÀ1), CAU-1 (48 kJmolÀ1), NH2-MIL-
53(Al) and USO-1-Al-A (50 kJmolÀ1), but significantly higher
than that of CuTATB-60, PCN-6 (35 kJmolÀ1), HKUST-1
(35 kJmolÀ1), IRMOF-1, MOF-5 (34 kJmolÀ1), MIL-53(Cr)
from the active sites was checked in a control experiment, in
which removal of the catalyst after 25 min resulted in com-
plete shutdown of the reaction as monitored by using GC-MS
(Figure S28 in the Supporting Information).
The recovered catalyst can be used at least five times with-
out any loss of activity. The PXRD pattern of the recovered
catalyst after five cycles was found to be unchanged (Fig-
ure S29 in the Supporting Information), which demonstrates
the framework integrity. In presence of the catalyst, the IR
spectrum of benzaldehyde experiences a 20 cmÀ1 redshift in
the C=O stretching frequency (Figure S30 in the Supporting In-
formation), presumably due to the interaction of carbonyl
group with the metal centre.[17] When a bulky aldehyde was
chosen as the substrate, the reaction did not proceed (Table 1),
which indicates that sterically encumbered aldehydes are not
able to fit inside the cavity of the framework. Finally, we found
the -NH2 groups remain inert towards any post-synthetic cova-
lent modification. A close look at the structure of 1Cu reveals
that the pendent -NH2 groups are positioned inward to steric-
ally preclude any post-synthetic covalent modification (Fig-
ure S31 in the Supporting information).[18]
(32 kJmolÀ1),
NOTT-140
(25 kJmolÀ1)
and
UMCM-1
(12 kJmolÀ1).[15] We also evaluated the CO2 adsorption select-
ivity of 1Cu’ over CH4, H2 or N2 with the ideal adsorbed solution
theory (IAST) based on single-component isotherms[16] at 273 K
(Figure 3d). As shown in Figure 3d, N2 and H2 gases do not dif-
fuse into the pores of 1Cu’, whereas CH4 gas enters only slight-
ly. We presume that the high quadrupole moment and polariz-
ability of CO2 (13.4”10À40 Cm2 and 26.3”10À25 cm3, respective-
ly) induce better interaction with the channels and interior
wall of the framework, composed of NH2 functionalities. The
CO2 selectivities over N2, H2 and CH4 were found to be 280,
288 and 6.95, respectively. Clearly, 1Cu’ exhibits better CO2/N2
selectivity compared with other reported MOFs,[13a] which sug-
gests that it may have potential applications for selective CO2
capture from flue gas.
Conclusion
By using angular tetracarboxylic acid H4L, which incorporates
pendent -NH2 groups, 3D ZnII-framework 1 has been synthe-
sized. The framework is thermally less stable and easily decom-
poses upon de-solvation. Post-synthetic transmetalation of the
ZnII ions by CuII ions, through a SC–SC transformation gener-
ates iso-structural framework 1Cu. Framework 1Cu shows high
thermal stability in the absence of lattice and metal-bound
water molecules. The activated framework, 1Cu’, exhibits
a large BET surface area with appreciable uptake of H2, CO2
and CH4 gases under both low- and high-pressure conditions.
Moreover, framework 1Cu’ selectively adsorbs an appreciable
amount of CO2 gas from a mixture of H2, N2 and CH4 at 273 K,
which provides some benefits towards the significant chal-
lenge of gas separation under ambient conditions. Additionally,
the presence of vacant coordination sites at the CuII centres
allows 1Cu’ to act as a heterogeneous catalyst for size-selective
Knoevenagel condensation reactions.
The open Cu2+ coordination sites and presence of -NH2
groups in 1Cu’ subsequently prompted us to explore its catalyt-
ic ability for Knoevenagel condensation reactions. In spite of
its simplicity, the challenges of Knoevenagel condensation in-
volve 1) base- or acid-catalysed condensation, 2) association of
high temperature or microwave irradiation, and 3) self-conden-
sation-/oligomerization-mediated byproducts. As a demonstra-
tion, aromatic aldehydes were chosen as substrates due to
their possible enclathration inside the cavity of the MOF. The
formation of the desired product in each case was confirmed
1
by using H and 13C NMR spectroscopy (Figures S20–27 in the
Supporting Information), and progress of the reaction was
monitored by using TLC. Table 1 summarizes the results with
diverse substrates. The absence of any leaching of metal ions
Experimental Section
Materials and method
Reagent-grade 2,6-dibromoaniline, 1-bromo-3,5-dimethylbenzene,
trimethyl borate, Zn(NO3)2·6H2O and Cu(NO3)2·3H2O were pur-
chased from Sigma–Aldrich and used as received. All solvents, such
as N,N’-dimethylformamide (DMF), hexane and ethyl acetate were
procured from S. D. Fine Chemicals, India, and their purification
and drying was accomplished according to standard protocols.
Table 1. Knoevenagel reactions catalysed by 1Cu’.
Entry
R
t [h]
Yield [%]
1
2
3
4
5
6
Ph
4-FPh
4-MePh
4-OMePh
1-naphthyl
anthryl
2
2
3
2
5
5
90
95
90
95
6
Physical measurements
Infrared (IR) spectra were performed (KBr disk, 400–4000 cmÀ1) by
using a Perkin–Elmer model 1320 spectrometer. Thermogravimetric
analyses (TGA) were acquired by using a Mettler Toledo Star
0
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system (heating rate 58CminÀ1). Powder X-ray diffraction spectra
(CuKa radiation, scan rate 38minÀ1, 293 K) were obtained by using
a Bruker D8 Advance Series 2 powder X-ray diffractometer. Micro-
analyses of all the compounds were carried out by using a Perkin–
Elmer Series II elemental analyser model 2400. 1H and 13C NMR
spectra (500, 400 and 125 MHz) were recorded in CDCl3 or
[D6]DMSO by using a JEOL ECX 500 FT instrument with Me4Si as
the internal standard. The following abbreviations are used to de-
scribe peak patterns: s=singlet, d=doublet, t=triplet, dd=doub-
let of doublets, br=broad and m=multiplet. The percentage of
metal ions after transmetalation reactions was measured by using
X-ray fluorescence (XRF) spectroscopy by using a Rigaku WD-XRF
(X-ray generation 4 kW, 60 kV, 150 mA) system and energy-dis-
persive X-ray spectroscopic data (EDX) were recorded by using
a JSM-6010A JEOL Tungsten-Electron Microscope (W-SEM). ESI
mass spectra were recorded by using a WATERS Q-TOF Premier
mass spectrometer. Melting points were recorded by using an elec-
trical melting point apparatus from PERFIT India and are uncorrect-
ed. Low-pressure gas adsorption measurements were performed
by using an automatic volumetric BELSORP-MINI-II adsorption
equipment and a static volumetric system (Micromeritics ASAP
2020). The high-pressure gas adsorption measurements were car-
ried out by using an automated high-pressure gas adsorption
system BELSORP-HP, (BEL, Japan). Prior to BET adsorption measure-
ments, as-synthesized compounds were immersed in ethanol for
5 d at RT to replace lattice guest molecules. The solvent-exchanged
frameworks were then heated to 1508C for 8 h under vacuum to
produce guest-free compounds. The PXRD pattern of the activated
samples matched that of the parent compound.
Synthesis and characterization of ligand H4L
Synthesis of tetra-acid ligand H4L was achieved in the following
steps, as shown in Scheme 1. Compound B was prepared accord-
ing to a reported procedure.[25]
Tetraethyl-2’-amino-[1,1’:3’,1’’-terphenyl]-3,3’’,5,5’’-tetra
carb-
oxylate (C): A solution of 2,6-dibromoaniline (1 g, 0.004 mol) and
(3,5-bis(ethoxycarbonyl)phenyl)boronic acid (2.33 g, 0.009 mol) in
DMF (15 mL) was mixed with a solution of Na2CO3 (1.69 g,
Scheme 1.
0.016 mol) and palladium acetate (10 mg) in water (10 mL). The
mixture was heated at 808C overnight. After cooling to RT, the re-
action mixture was extracted with ethyl acetate three times and
washed with water. The combine organic phase was dried over an-
hydrous Na2SO4. Evaporation of the solvent afforded the crude
product as a pale yellow solid. This was purified by column chro-
matography (silica gel; ethyl acetate/hexane 1:9) to obtain C as
a pale yellow solid (700 mg, 33% yield). M.p.: 1508C; 1H NMR
(500 MHz, CDCl3, 258C, Si(CH3)4): d=1.42 (t, 12H), 3.70 (s, 2H) 4.42
(q, 8H), 6.92 (s, 1H), 7.15 (d, 2H), 8.36 (d, 4H), 8.68 ppm (d, 2H);
13C NMR (125 MHz, CDCl3, 258C, Si(CH3)4): d=14.43, 29.78, 61.61,
118.67, 126.28, 129.73, 130.62, 131.81, 134.59, 140.21, 165.75 ppm;
elemental analysis calcd (%) for C30H31NO8: C 67.53, H 5.86, N 2.63;
found: C 67.35, H 5.75, N 2.50.
X-ray structural studies
Single-crystal X-ray data of all the complexes were collected at
100 K by using a Bruker SMART APEX CCD diffractometer with
graphite monochromated MoKa radiation (l=0.71073 Š). The linear
absorption coefficients, scattering factors for the atoms and the
anomalous dispersion corrections were taken from the Internation-
al Tables for X-ray Crystallography.[19] The data integration and re-
duction were carried out by using SAINT[20] software. For each set
of data, empirical absorption correction was applied to the collect-
ed reflections with SADABS[21] and the space group was deter-
mined by using XPREP[22] The structure was solved by using direct
methods in SHELXTL-97[23] and refined on F2 by using full-matrix
least-squares in the SHELXL-97[24] program package. The unit cell
contains a large number of disordered solvent molecules that
could not be modelled as discrete atomic sites. Therefore, we em-
ployed PLATON/SQUEEZE[8] to calculate the diffraction contribution
of the solvent molecules and thus produce a set of solvent-free dif-
fraction intensities. In all compounds, the non-hydrogen atoms
were refined anisotropically. The hydrogen atoms attached to
carbon atoms were positioned geometrically and treated as riding
atoms by using the SHELXL default parameters. The hydrogen
atoms of the coordinated water molecule in both the structures
could not be located in the difference Fourier maps. CCDC
1414679 (1) and 1414680 (1Cu) contain the supplementary crystallo-
graphic data for this paper. These data are provided free of charge
by The Cambridge Crystallographic Data Centre. Lattice parameters
of the compounds, data collection and refinement parameters are
summarized in Table S1 (Supporting Information) and selected
bond lengths and bond angles are given in Table S5 (Supporting
Information).
2’-Amino-[1,1’:3’,1’’-terphenyl]-3,3’’,5,5’’-tetracarboxylic
acid
(H4L): The ester, tetraethyl 2’-amino-[1,1’:3’,1’’-terphenyl]-3,3’’,5,5’’-
tetracarboxylate (C; 1 g), was hydrolysed by refluxing with KOH
(5 n, 50 mL) for 72 h. The solution was filtered, the filtrate cooled
in an ice-bath and then acidified carefully with concentrated HCl
to obtain a pale yellow precipitate. After standing overnight in the
freezer, the solid was collected by filtration, washed many times
with water and finally dried at 808C to obtain H4L as a yellow
powder (yield: 700 mg, 88%). M.p. >3008C; 1H NMR (500 MHz,
CDCl3, 258C, Si(CH3)4): d=6.80 (t, 1H), 7.08 (d, 2H), 8.17 (d, 4H),
8.42 ppm (d, 2H); 13C NMR (125 MHz, CDCl3, 258C, Si(CH3)4): d=
122.88, 130.85, 133.99, 135.61, 137.24, 139.26, 145.52, 147.11,
171.89 ppm; elemental analysis calcd (%) for C22H15NO8: C 62.71, H
3.59, N 3.32; found: C 62.50, H 3.68, N 3.20.
Synthesis of {[Zn2(L)(H2O)2]·(5DMF)·(4H2O)} (1)
A mixture of H4L (20 mg, 0.05 mmol) and Zn(NO3)2·6H2O (28 mg,
0.1 mmol) in DMF (2 mL) and ethanol (1 mL) were heated at 908C
under autogenous pressure in a Teflon-lined stainless steel auto-
clave for 72 h, followed by cooling to RT at a rate of 108ChÀ1. Col-
ourless block-shaped crystals of 1 were isolated (yield: 50%). FT-IR
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Acknowledgements
We gratefully acknowledge financial support from the Depart-
ment of Science and Technology, New Delhi, India (J.C. Bose
National Fellowship to P.K.B.). T.K.P and D.D. thank the CSIR for
a SRF fellowship. P.P. and Q.X. are thankful to AIST for financial
support.
Keywords: carbon dioxide
·
hydrogen
·
metal–organic
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Received: August 11, 2015
Published online on November 20, 2015
Chem. Eur. J. 2015, 21, 19064 – 19070
www.chemeurj.org
19070
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim