Synthesis, crystal structure and gas uptake properties of a urea-functionalized rht-type metal-organic framework

Article abstract of DOI:10.1016/j.poly.2015.06.001

A hexatopic C₃-symmetrical 5,5′,5″-(((benzene-1,3,5-triyltris(azanediyl))tris(carbonyl))tris-(azanediyl))triisophthalic acid (H₆-1) linker containing tri-urea groups has been designed and synthesized through the reaction between 1,3,

Full text of DOI:10.1016/j.poly.2015.06.001

Polyhedron 99 (2015) 1–6
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, crystal structure and gas uptake properties
of a urea-functionalized rht-type metal–organic framework
⇑
Qiu-Yan Li, Ying Quan, Wei Wei, Jian Li, Han Lu, Rui Ni, Xiao-Jun Wang
Jiangsu Key Laboratory of Green Synthetic Chemistry for Functional Materials, School of Chemistry and Chemical Engineering, Jiangsu Normal University, Xuzhou 221116, PR China
a r t i c l e i n f o
a b s t r a c t
A hexatopic C3-symmetrical 5,50,500-(((benzene-1,3,5-triyltris(azanediyl))tris(carbonyl))tris-(azanediyl))-
triisophthalic acid (H6-1) linker containing tri-urea groups has been designed and synthesized through
the reaction between 1,3,5-benzenetriisocyanate and aniline, which was employed to construct a new
copper(II) metal–organic framework (MOF Cu-TUH) under the solvothermal reaction with Cu(NO3)2 in
N,N-dimethylformamide. As expected, single crystal X-ray diffraction study reveals that the MOF
Cu-TUH is composed of 3D packing of nanosized cuboctahedral, truncated tetrahedral, and truncated
octahedral cages with a (3,24)-connected rht-topology. Furthermore, the preliminary gas uptake investi-
gations of the activated MOF were conducted to evaluate its application in the area of gas storage and
purifications.
Article history:
Received 6 March 2015
Accepted 3 June 2015
Available online 21 June 2015
Keywords:
C3-Hexacarboxylate linker
Metal–organic framework
Crystal structure
Rht-topology
Ó 2015 Elsevier Ltd. All rights reserved.
Gas uptake
1. Introduction
isoreticular MOFs has recently emerged and gained extensive
attentions [16–30], where the cuboctahedral metal–organic poly-
Metal–organic frameworks (MOFs), also known as porous coor-
dination polymers (PCPs), are constructed by the self-assembly of
metal or metal-oxo nodes and multitopic bridging organic linkers
via coordination bonds of moderate strength typically under
solvothermal conditions [1–2]. Due to their chemical diversity,
permanent porosity, high surface area as well as adjustable pore
size and shape [1–5], MOFs have received extensive research and
industrial interest over the past decades and shown their great
promise in a wide variety of applications [6–11], including gas
storage and separation, heterogeneous catalysis, conductivity,
sensing, light harvesting, and biomedical area. A large number of
MOF materials with tremendous structures and specific applica-
tions have been constructed through the judicious combination
of metal centers and organic linkers. Moreover, various functional
groups can be readily incorporated into MOFs via either ligand
design or post-synthetic modifications [12–14], which affords a
unique advantage over other traditional porous materials, such
as zeolites.
hedra (MOP) with a chemical formula [M(II)2(BDC)2(H2O)2]12 (M
typically is Cu2+, BDC = 1,3-benzenedicarboxylate) is cross-linked
by a rigid triangular central core through the 24 vertices from
5-position of each BDC, resulting in a (3,24)-connected framework.
Moreover, such topology serves as an ideal platform for construc-
tions of novel MOFs, through combining such well-defined 24-con-
nected MOP with a suitable ligand that consists of three coplanar
BDC moieties having overall C3-symmetry [16–30]. In the context,
a series of dendritic hexacarboxylate organic linkers composed of
three coplanar isophthalates have been synthesized and employed
to construct such versatile rht-MOFs.
To enhance theperformanceand/or endowmore delicateproper-
ties of rht-MOFs, some polar groups such as acylamide and triazole
have been used to cross-link the BDC units except for alkyl [26–29].
Urea also is a very common polar group encountered in organic
chemistry, which recently has been incorporated into MOFs as
hydrogen-bond-donating (HBD) catalytic sites [31–33]. However,
to our knowledge, the urea has not been used to synthesize the hex-
acarboxylate organic ligand. In this work, we utilized the urea group
to cross-link three BDC units generating a new hexacarboxylate
linker, i.e., 5,50,500-(((benzene -1,3,5-triyltris(azanediyl))tris(car-
bonyl))tris (azanediyl))triisophthalic acid (H6-1, Scheme 1).
Subsequently, a urea-functionalized rht-MOF (Cu-TUH) was suc-
cessfully prepared. Subsequently, the gas sorption of the resulting
MOF was preliminarily investigated.
Because of the inherent modularity of MOFs, the rational design
and preparation of these materials with predetermined ordered
structures and topologies can be realized by the strategy of reticu-
lar chemistry [15]. Among the topologies, a fascinating rht-type of
⇑
Corresponding author.
E-mail address: xjwang@jsnu.edu.cn (X.-J. Wang).
http://dx.doi.org/10.1016/j.poly.2015.06.001
0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.

2
Q.-Y. Li et al. / Polyhedron 99 (2015) 1–6
Scheme 1. Synthetic route for linker H6-1.
2.2.2. Synthesis of linker H6-1
2. Experimental
t
To a solution of the ester Bu6-1 (1.77 g, 1.6 mmol) in CH2Cl2
was added trifluoroacetic acid (TFA, 15 mL). The mixture was stir-
red at room temperature over 5 h, obtaining a precipitate. The sus-
pension was filtered, washed with CH2Cl2 to remove excess TFA,
and then dried under vacuum giving H6-1 (0.97 g, 1.3 mmol, yield:
81%) as light gray solid. 1H NMR (400 MHz, DMSO) d 13.23 (br, 6H),
8.98 (s, 3H), 8.90 (s, 3H), 8.31 (d, J = 1.2 Hz, 6H), 8.09 (s, 3H), 7.42 (s,
3H). 13C NMR (75 MHz, DMSO) d 166.71, 152.49, 140.55, 140.19,
131.83, 123.33, 122.81, 102.71. ESI-TOF-HRMS: m/z calcd for
2.1. Materials and methods
Unless specifically mentioned, all chemicals were purchased
from commercial source and used as received. Toluene and triethy-
lamine (TEA) were distilled over sodium/benzophenone ketyl and
CaH2 under a nitrogen atmosphere prior to use, respectively. Di-
tert-butyl 5-aminoisophthalate [31] and 1,3,5-triisocyanatoben-
zene [34] were prepared according to the literature procedures.
IR-spectra were recorded as KBr-pellet on a Perkin–Elmer 1760X
FT-IR spectrometer. NMR spectra were taken on Bruker AV400 at
room temperature. High-resolution mass spectral measurements
were carried out on a Waters Q-tof Premier MS. EI mass spectra
were obtained in a positive ion mode on a Waters GCT Premier.
Elemental analyses (C, H, and N) were obtained from EuroVector
Euro EA Elemental Analyzer. Thermogravimetric analysis (TGA)
was carried out on a TGA-Q500 thermoanalyzer with a heating rate
of 10 °C/min under nitrogen atmosphere. The powder X-ray diffrac-
tion (PXRD) measurements were taken on a Bruker D8 diffractome-
C
33H25N6O15: 745.1365. Found: 745.1378 [M+H]+.
2.2.3. Synthesis of MOF Cu-TUH
Compound H6-1 (20 mg, 0.027 mmol) and Cu(NO3)2ꢁ3H2O
(19 mg, 0.075 mmol) were dissolved in N,N-dimethylformamide
(DMF, 15 mL). Then, several drops of HNO3 were added into the
mixture. The solution was placed in a tightly capped 20 mL vial
and heated in an oven at 75 °C for 3 days. The green block crystals
were collected after cooling to room temperature. Then, these crys-
tals were washed by fresh DMF for several times (5 mg, yield: 20%).
Elemental analysis Calc. for Cu3(1)(H2O)3ꢁ5DMFꢁ10H2O: C, 37.71;
H, 5.21; N, 10.08. Found: C, 38.12; H, 5.41; N, 10.56 (%). Selected
ter using Cu K
a
radiation (k = 1.5418 Å) at room temperature.
2.2. Preparations
Table 1
t
Crystal data and structure refinement for Cu-TUH.
2.2.1. Synthesis of compound Bu-1
To an aqueous solution (10 ml) of NaN3 (1.22 g, 19 mmol) was
added dropwise a tetrahydrofuran (THF, 5 ml) solution of 1,3,5-
benzenetricarbonyl trichloride (0.50 g, 1.9 mmol) while cooling
with an ice bath. The reaction suspension was stirred for another
2 h and then poured into water (50 ml), extracted with toluene
(50 ml ꢀ 2). The combined organic layer was washed by saturated
NaHCO3 solution and brine, dried over anhydrous Na2SO4 and con-
centrated by evaporating until a volume of around 40 ml was
reached. The obtained toluene solution of 1,3,5-benzenetricarbonyl
triazide was gradually heated to 120 °C and stirred for 2 h. The 1,3,5-
triisocyanatobenzene was formed in the solution, which was then
cooled to 80 °C and used as such without characterizations.
To the above obtained toluene solution of 1,3,4-triisocyanato-
benzene was added a dry THF (20 ml) solution of di-tert-butyl
5-aminoisophthalate (1.67 g, 5.7 mmol) at 80 °C. The reaction
mixture was stirred overnight and then evaporated in vacuo to
dryness. The crude product was purified by silica gel flash column
Empirical formula
Formula weight
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
C33H24Cu3N6O18
983.21
1.54184
tetragonal
I4/m
31.0865(6)
31.0865(6)
44.596(3)
90
90
90
43096(3)
16
b (Å)
c (Å)
a
(°)
b (°)
(°)
c
Volume (Å3)
Z
Dcalc (mg mꢂ3
)
0.604
0.962
7872
Absorption coefficient (mmꢂ1
F(000)
)
Crystal size (mm)
Index ranges
0.13 ꢀ 0.16 ꢀ 0.18
ꢂ34 6 h 6 12, ꢂ34 6 k 6 33,
ꢂ42 6 l 6 48
35442
14267 (0.136)
multi-scan
0.882 and 0.841
t
Reflections collected
chromatography (CH2Cl2/CH3OH, 100/2) to give Bu6-1 as a white
Independent reflections (Rint
Absorption correction
Maximum and minimum
transmission
)
solid (1.77 g, 1.6 mmol, yield: 84%). 1H NMR (400 MHz, DMSO) d
9.02 (s, 3H), 8.86 (s, 3H), 8.24 (d, J = 1.5 Hz, 6H), 8.01 (t,
J = 1.5 Hz, 3H), 7.37 (s, 3H), 1.57 (s, 54H). 13C NMR (101 MHz,
DMSO) d 164.12, 152.31, 140.45, 140.07, 132.17, 122.64, 122.26,
Refinement method
full-matrix least-squares on F2
0.879
R1 = 0.0860, wR2 = 0.2139
Goodness-of-fit (GOF) on F2
102.38, 81.30, 27.70. ESI-TOF-HRMS: m/z calcd for C57H73N6O15
:
Final R indices [I > 2
r
(I)]
1081.5115. Found: 1081.5134 [M+H]+.

Q.-Y. Li et al. / Polyhedron 99 (2015) 1–6
3
IR (KBr) (cmꢂ1): 1657 (s), 1555 (m), 1377 (s), 1213 (m), 1107 (m),
784 (m), 732 (m).
Cu Ka radiation (k = 1.5418 Å) on an Agilent Technologies Super
Nova diffractometer. The structure was solved by the direct
methods and refined by full-matrix least-squares analyses on F2
(SHELXTL-97 [35]). In the structure, the solvent molecules are highly
2.3. Gas sorption
disordered and cannot be modeled, thus the SQUEEZE routine [36]
of PLATON was applied to remove the contributions of the solvent
molecules to the scattering. Two restraints (SIMU and ISOR) were
employed in the refinement. Crystallographic data for MOF
Cu-TUH is summarized in Table 1.
Low-pressure gas sorption measurements were performed by
using Quantachrome Instruments Autosorb-iQ (Boynton Beach,
Florida USA) with the extra-high pure gases. The as-synthesized
MOF Cu-TUH crystals were immersed in CH2Cl2 for 24 h, during
which time CH2Cl2 was replaced three times. The samples were
then moved into a sample cell and dried under vacuum at 60 °C
and 120 °C by using the ‘‘outgasser’’ function for 12 h and 6 h before
the measurements, respectively. The BrunauerꢂEmmettꢂTeller
(BET) surface area and total pore volume were calculated from
the N2 sorption isotherms at 77 K by using the Quantachrome
ASiQwin 2.01 software package. The isosteric heat of adsorption
(Qst) for H2 and CO2, defined as
3. Results and discussion
3.1. Synthesis and characterizations
The dendritic hexacarboxylate linker H6-1 was synthesized as
shown in Scheme 1. The reaction of di-tert-butyl 5-aminoisophtha-
late and 1,3,5-triisocyanatobenzene in the mixture of THF and
ꢀ
ꢁ
t
@ ln p
@T
toluene gave the precursor tert-butyl ester Bu6-1, which was fol-
Qst ¼ RT2
lowed by hydrolysis in the mixture of trifluoroacetic acid and
CH2Cl2 to give the urea functionalized C3-symmetric hexatopic
ligand H6-1 (synthetic details see Section 2). Here, tert-butyl group
was used to increase the solubility of the compounds.
qðClausius—Clapeyron equationÞ
was determined by using the H2 and CO2 absorption isotherms at 77
and 87 K as well as 273 and 298 K, respectively (ASiQwin 2.01).
Then, the solvothermal reaction of H6-1 and Cu(NO3)2 in N,N-
dimethylformamide (DMF) in the presence of a small amount of
HNO3 at 75 °C for 3 days afforded block green crystals of MOF
Cu-TUH. And the phase purity of the bulk sample was confirmed
from powder X-ray diffraction (PXRD) measurement. The as-syn-
thesized MOF crystals are insoluble in water and common organic
solvents such as acetone, methanol, ethanol, chloroform, DMF, and
2.4. X-Ray crystallography
The single crystal of MOF Cu-TUH for X-ray analysis was picked
up under a microscope from a freshly synthesized solution and
then mounted immediately. The diffraction data were collected
at room temperature using micro-focus graphite-monochromated
Fig. 1. Crystal structure of MOF Cu-TUH. (a) Organic linker 1 connects with Cu2 paddle-wheel units. (b) One linker 1 with three cuboctahedral cages around it, and its
simplified schematic view (c). (d) Schematic representation of (3,24)-connected network. The blue spheres refer to 24-connected nodes and the orange spheres refer to three-
connected nodes. Hydrogens and disordered solvent molecules are omitted for clarity. C, gray; O, red; N, blue; Cu, blue-green. (Colour online.)

4
Q.-Y. Li et al. / Polyhedron 99 (2015) 1–6
DMSO. The thermal behaviors of MOF Cu-TUH were studied by
thermogravimetric analysis (TGA) under N2 from 20 to 700 °C
(Fig. S2). The first weight loss of 39% from 20 to 220 °C agrees with
the loss of lattice solvents and coordination water, while the sec-
ond one with 25% from 220 to 400 °C suggests the decompositions
of the MOF.
Fig. 2. Three types of cages and their overall 3D packing in the framework: a cuboctahedron (blue), a truncated tetrahedron (red) and a truncated octahedron (green). Spheres
indicate the void space within the three types of cavities. (Colour online.)

Q.-Y. Li et al. / Polyhedron 99 (2015) 1–6
5
3.2. Crystal structure of Cu-TUH
[a = b = 31.0865(6) Å, c = 44.596(3) Å, V = 43096(3) Å3], having the
framework formula of [Cu3(1)(H2O)3]. The framework node was
composed of two Cu(II) paddlewheel units coordinated by one car-
boxylate from each of four linkers and two axial water molecules
(Fig. 1a). As expected, the 3-connected organic linker 1 was con-
nected with three 24-connected supermolecular building blocks
(SBBs), as shown in Fig. 1b and c. The SBB consists of 12 dicopper
paddlewheels joined by 24 1,3-BDC linkers. Thus, the MOF
Cu-TUH exhibited the same non-interpenetrated (3,24)-connected
network as the other isoreticular rht-MOFs (Fig. 1d) [16–30].
The overall 3D framework of MOF Cu-TUH can be viewed as the
packing of three types of polyhedron: a cubotahedron (cub-Oh), a
truncated tetrahedron (T-Td), and a truncated octahedron (T-Oh)
(Fig. 2) , with the ratio of 1:2:1, respectively. The diameters of
the colored spheres, which represent the void inside the three
types of polyhedra, are estimated 12 Å for the cub-Oh (blue), 15 Å
for the T-Td (red), and 23 Å for the T-Oh (green) after the removal
of coordinated H2O molecules from the dicopper paddle-wheel.
Single-crystal X-ray diffraction revealed that MOF Cu-TUH crys-
tallized in the tetragonal space group I4/m with unit cell
3.3. Gas sorption properties
The total solvent-accessible volume for desolvated MOF Cu-
TUH after removal of guest solvates and coordinated water mole-
cules is estimated to be 76.8% by using the PLATON/VOID routine
[36], and the calculated density of the desolvated framework is
0.573 g cmꢂ3. The freshly as-synthesized MOF Cu-TUH crystals
Fig. 3. Nitrogen sorption isotherm for activated MOF Cu-TUH at 77 K
(STP = standard temperature and pressure; filled symbols, absorption; open sym-
bols, desorption).
Fig. 4. Powder XRD patterns of MOF Cu-TUH.
Fig. 5. Carbon dioxide and nitrogen adsorption isotherms (left) and hydrogen adsorption isotherms (right) for activated MOF Cu-TUH.

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Q.-Y. Li et al. / Polyhedron 99 (2015) 1–6
were exchanged with CH2Cl2 for several times at room tempera-
ture, and degassed under a dynamic vacuum, as described in our
previous procedure [4,28]. Then, the activated MOF Cu-TUH was
subjected to nitrogen sorption at 77 K (Fig. 3). However, out of
our expectation, it was found that the activated MOF showed a
quite low nitrogen adsorption capacity with the uptake only
40 cm3 gꢂ1 at 1 atm. The Brunauer–Emmett–Teller (BET) surface
area was estimated to be 68 m2 gꢂ1. The PXRD pattern of the acti-
vated sample exhibited a very weak intensity peak (Fig. 4), indicat-
ing the instability and collapse of the framework during this
solvent-exchange activation method. That may be attributed the
fact that the ligand H6-1 in MOF Cu-TUH is more flexible than
other hexacarboxylate linkers in rht-MOFs [16–30].
Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail:
deposit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/10.
1016/j.poly.2015.06.001.
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The CO2 adsorption isotherm was measured at 273 K, as shown
in Fig. 5, which shows an uptake of 37 cm3 gꢂ1 (7.3 wt %) CO2
at 1 atm, despite its low BET surface area. For comparison, the
N2 uptake of MOF Cu-TUH under the same conditions was
1.5 cm3 gꢂ1. Based on adsorption isotherms at different tempera-
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estimated to be moderately high (30–21 kJ molꢂ1) (Fig. S3). Next,
we further investigated the hydrogen uptake of MOF Cu-TUH in
low pressure range, as shown in Fig. 5. The desolvated framework
exhibited a H2 uptake capacity as high as 26 cm3 gꢂ1 at 77 K and
1 atm. And the Qst for H2 calculated from the isotherms at 77 and
87 K by using Clausius–Clapeyron equation was in the range of
5.6–1.3 kJ molꢂ1
.
4. Conclusion
In summary, we have successfully designed and synthesized a
C3-symmetric urea cross-linked hexa-carboxylate ligand, which
has been employed to construct a (3,24)-connected rht-MOF
Cu-TUH. Although it has a very large void in the framework via
the crystal structure analysis, however, the MOF exhibited a low
surface area and gas uptake capacities by using the solvent-
exchanged activation, which may be attributed the high flexibility
of the ligand. More delicate activation strategy should be con-
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groups in the framework, the MOF Cu-TUH acting as hydrogen-
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Acknowledgements
We are grateful for financial support from National Natural
Science Foundation of China (21302072), Natural Science
Foundation of Jiangsu Province (BK20130226), Natural Science
Foundation
of
Jiangsu
Higher
Education
Committee
(13KJB150014), the Scientific Research Foundation for the
Returned Overseas Chinese Scholars, State Education Ministry of
China, and the Priority Academic Program Development of Jiangsu
Higher Education Institutions (PAPD).
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Appendix A. Supplementary data
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CCDC 1000853 contain the supplementary crystallographic data
for MOF Cu-TUH. These data can be obtained free of charge via
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Cambridge Crystallographic Data Centre, 12 Union Road,
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