FULL PAPERS
DOI: 10.1002/asia.201000218
Two Robust Porous Metal–Organic Frameworks Sustained by Distinct
Catenation: Selective Gas Sorption and Single-Crystal-to-Single-Crystal
Guest Exchange
Rui Yang,[a] Lei Li,[a] Ying Xiong,[a] Jian-Rong Li,[b] Hong-Cai Zhou,[b] and
Cheng-Yong Su*[a]
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Chem. Asian J. 2010, 5, 2358 – 2368
Abstract: Assembly of copper(I) halide
with a new tripodal ligand, benzene1,3,5-triyl triisonicotinate (BTTP4), afforded two porous metal–organic
frameworks, [Cu2I2ACHTUNGRE(BTTP4)]2 CH3CN
(1·2 CH3CN)
and
[CuBrACHTUNGRE(BTTP4)](CH3CN·CHCl3·H2O)
(2·solvents), which have been characterized by IR spectroscopy, thermogravimetry (TG), single-crystal, and
powder X-ray diffraction (PXRD)
methods. Compound 1 is a polycatenated 3D framework that consists of 2D
(6,3) networks through inclined catenation, whereas 2 is a doubly interpenetrated 3D framework possessing the
ThSi2-type (ths) (10,3)-b topology.
Both frameworks contain 1D channels
of effective sizes 9 12 and 10 10 2,
which amounts to 43 and 40 % space
volume accessible for solvent molecules, respectively. The TG and variable-temperature PXRD studies indicated that the frameworks can be completely evacuated while retaining the
permanent porosity, which was further
verified by measurement of the desolvated complex [Cu2I2ACHTUNGRE(BTTP4)] (1’).
The subsequent guest-exchange study
on the solvent-free framework revealed
that various solvent molecules can be
Keywords: adsorption · host–guest
systems · metal–organic frameworks · polymers · solvent effects
Introduction
Metal–organic frameworks (MOFs) that consist of metal
nodes and organic linkers as porous solids have received
much attention on account of their potential applications in
the fields of gas storage,[1] separation,[2] and catalysis.[3] One
of the goals of crystal engineering is to make it possible to
design and synthesize MOFs with predetermined topology
and properties, usually through judicious selection of multitopic organic ligands as the spacers and metal ions or clusters as the junctures.[4] To create porous frameworks that
contain larger channels, one apparent option is to prepare
longer ligands. Meanwhile, the introduction of functional
groups into organic linkers is often anticipated as a means
of realizing the desired physical functionality with the
porous frameworks. One problem often encountered in the
construction of porous frameworks with a long organic
ligand is the well-known phenomenon of interpenetration or
catenation.[5] However, studies on the interwoven structures
revealed that the catenation may be beneficial for permanent porosity, framework stability, specific pore volume, and
improved performance in gas storage.[6]
A framework with permanent porosity should be robust
enough against the removal of the guest molecules. In many
cases, the guest exchange undergoes a single-crystal-to-
[a] R. Yang, L. Li, Y. Xiong, Prof. Dr. C.-Y. Su
MOE Laboratory of Bioinorganic and Synthetic Chemistry
State Key Laboratory of Optoelectronic Materials and Technologies
School of Chemistry and Chemical Engineering
Sun Yat-Sen University, Guangzhou 510275 (China)
Fax: (+ 86) 20-8411-5178
E-mail: cesscy@mail.sysu.edu.cn
[b] Dr. J.-R. Li, Prof. Dr. H.-C. Zhou
Department of Chemistry, Texas A&M University
PO Box 30012, College Station, 77842-3012 (USA)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201000218.
Chem. Asian J. 2010, 5, 2358 – 2368
adsorbed through a single-crystal-tosingle-crystal manner, thus giving rise
to the guest-captured structures [Cu2I2
ACHTUNGRE(BTTP4)]C6H6 (1·benzene), [Cu2I2
ACHTUNGRE(BTTP4)]2 C7H8 (1·2 toluene), and
[Cu2I2ACHTUNGRE(BTTP4)]2 C8H10 (1·2 ethylbenzene). The gas-adsorption investigation
disclosed that two kinds of frameworks
exhibited comparable CO2 storage capacity (86–111 mL g 1 at 1 atm) but
nearly none for N2 and H2, thereby implying its separation ability of CO2
over N2 and H2. The vapor-adsorption
study revealed the preferential inclusion of aromatic guests over nonaromatic solvents by the empty framework, which is indicative of selectivity
toward benzene over cyclohexane.
single-crystal (SCSC) procedure, which can be elucidated by
single-crystal X-ray diffraction analyses. Therefore, the
guest-uptake and -release behaviors can be directly observed,[7] thereby providing useful information in understanding the interactions between guests and host frameworks.
We have been working on the crystal engineering of coordination cages and frameworks by using tripodal organic ligands.[8] For this paper, we have prepared a new large tripodal ligand, benzene-1,3,5-triyl triisonicotinate (BTTP4), as
shown in Scheme 1. Selection of this ligand in the assembly
Scheme 1. Molecular structure of the BTTP4 ligand.
of porous MOFs is as a result of the following considerations: a) the tritopic ligands that contain three pyridyl
donors have been verified to be useful for the construction
of porous frameworks,[3f, 9] b) the ligand as a whole is rigid
with three pyridyl donors that span about 1.3 nm regardless
of different conformations, and c) ester groups have been in-
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troduced to serve as functional organic sites for special
host–guest interaction performance, for example, mixed-gas
separation.[2a, 3f, 10] Reaction of this ligand with copper(I)
halide afforded two catenated MOFs, namely, [Cu2I2
ACHTUNGRE(BTTP4)]2 CH3CN
(1·2 CH3CN)
and
[CuBrACHTUNGRE(BTTP4)](CH3CN·CHCl3·H2O) (2·solvents). Two MOFs
show selective adsorption for CO2 over N2 or H2 at low temperature and atmospheric pressure, and the guest-exchange
study confirms framework robustness of 1, which retains the
single-crystal nature upon removal of the guest molecules
and adsorption of different aromatic hydrocarbons such as
benzene, toluene, or ethylbenzene.
Results and Discussion
Syntheses and Crystal Structures
The ligand BTTP4 was obtained in moderate yield from a
replacement reaction of freshly prepared 4-picolinoyl chloride and 1,3,5-trihydroxybenzene. The 1H NMR spectrum of
the product conforms well to the molecular structure illustrated in Scheme 1. By slow diffusion of a solution of CuI
(or CuBr) in CH3CN into the solution of CHCl3 and BTTP4
ligand, complex 1·2 CH3CN (or 2·solvents) was obtained as
crystals. The structure of complex 1·CH3CN has been unambiguously identified by single-crystal X-ray analysis, and the
purity of the bulk sample was confirmed by powder X-ray
diffraction (PXRD) measurements. For complex 2·solvents,
the single-crystal X-ray analysis was not able to conclusively
identify the solvents owing to severe disorder; therefore, elemental and thermogravimetric (TG) analyses were carried
out to determine the solvent content. The phase purity has
also been checked by PXRD. The desolvated complex 1’
was obtained by drying the crystals of 1·CH3CN under high
vacuum at room temperature, and the guest-inclusion complexes were obtained by immersing desolvated 1’ directly
into hot benzene, toluene, and ethylbenzene, thereby affording 1·benzene, 1·2 toluene, and 1·2 ethylbenzene, respectively.
The structural analysis of 1·2 CH3CN revealed a neutral
three-dimensional (3D) polycatenated framework consisting
of two sets of inclined two-dimensional (2D) networks of
(6,3) topology.[5b] The crystal has a chiral space group C2,
and the asymmetric unit contains one ligand, one Cu2I2 cluster, and two CH3CN molecules. It is surprising that two Cu +
ions in the Cu2I2 cluster have different coordination environments, as seen from Figure 1. Two I anions bridge two Cu +
cations to form a Cu2I2 cluster, in which one Cu + is in a
CuI2N2 tetrahedral geometry with two N atoms coming from
two different BTTP4 ligands, and the other Cu + is in a
CuI2N trigonal geometry that connects the third BTTP4
ligand. So the Cu2I2 cluster acts as a 3-connected node
rather than a 4-connected node; it joins three different
BTTP4 ligands to give rise to a 2D honeycomb (6,3) sheet.
To the best of our knowledge, such a coordination mode of
a Cu2I2 cluster is rare, and there is only one example available in the literature.[11] The 2D (6,3) network displays hex-
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Figure 1. Crystal structures of complexes 1 (left) and 2 (right). Top: molecular structures showing the coordination geometry of the Cu + ions
and the coordination mode of the BTTP4 ligand; middle: network topology and catenation; bottom: crystal packing showing 1D channels in the
b direction (guest molecules are omitted for clarity).
agonal rings with a diameter of approximately 13 , measured by fitting a sphere from the centroid of the ring to the
van der Waals surface of its walls, which is large enough to
allow penetration of other sheets. Detailed structural analysis disclosed that every hexagonal ring is indeed interlocked
by another two parallel rings as shown in Figure 1. Therefore, there are two sets of 2D (6,3) sheets in the crystal lattice, one lying in the (11 1) plane and the other in the
( 111) plane. Two sets of networks interlace with each
other at a dihedral angle of 45.68 to cause a dimensional increase from 2D to 3D, thus representing a common polycatenation of 2D (6,3) networks in an inclined fashion.[12] The
whole framework is further stabilized by weak p–p stacking
interactions between the central benzene rings and pyridine
rings (centroid-to-centroid distances of 3.98 ) that belong
to adjacent (6,3) sheets. In spite of such twofold catenation,
1D hexagonal channels are formed along the b direction
(Figure 1), which have an effective aperture of 9 12 2
after considering the van der Waals radii. The CH3CN guest
molecules are accommodated inside the channels (depicted
in the Supporting Information), which accounts for 43 % solvent-accessible voids calculated by PLATON.[13]
When CuI was replaced by CuBr to react with TP4BE
under the same conditions, the complex 2·solvents was obtained as dark red block crystals. Single-crystal structural
analysis revealed a centrosymmetric space group P2/n with
an asymmetric unit that contained two ligands, two Cu + cations, two Br anions, and some free solvents that were badly
2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Robust Porous Metal–Organic Frameworks
disordered and could not be satisfactorily modeled. After
treating with the SQUEEZE procedure,[13] a twofold interpenetrated 3D framework of (10,3)-b topology was established, as shown in Figure 1. The solvent molecules were estimated by elemental and TG analyses, thus giving the overall formula [CuBrACHTUNGRE(BTTP4)](CH3CN·CHCl3·H2O), which is
in close agreement with the average electron counts of 170 e
per [CuBrACHTUNGRE(BTTP4)] squeezed from the voids. Different
from 1, the Cu + ion in 2 is in a CuBrN3 tetrahedral geometry to bond to three different BTTP4 ligands, and each
ligand also joins three Cu + ions. Thus, both Cu + ion and
BTTP4 ligand act as the 3-connected nodes to generate a ths
(10,3)-b 3D framework. The individual (10,3)-b framework
possesses large channels of the size 13 17 2 (atom-toatom separation) that run along the b axis. In the crystal lattice, two identical (10,3)-b frameworks interpenetrate each
other as seen in Figure 1 (also see the Supporting Information), thereby reducing the size of the effective channels to
10 10 2 (after subtraction of the van der Waals radii). The
potential solvent-accessible voids calculated by PLATON
amount to 40 %,[13] slightly smaller than that in 1.
Thermal Stability and Framework Robustness
Thermogravimetric analyses performed on the bulk samples
of as-prepared complexes 1·2 CH3CN and 2·solvents revealed that the two MOFs have similar thermal stability in
spite of their different structure topology (Figure 2). The
TG curves recorded under an N2 atmosphere indicated that
both crystals are apt to lose solvent guest molecules as soon
as they are isolated from the mother liquors. Compound
1·2 CH3CN lost all solvent molecules gradually from 30 to
110 8C. Afterward, a long plateau was observed up to 280 8C,
and then an abrupt weight loss appeared, which suggested
structure decomposition. Compound 2·solvents showed a
prolonged weight loss in the range of 30 to 150 8C that
amounted to 23.1 %, which may be attributable to the loss
of one CH3CN molecule, one CHCl3 molecule, and one
water molecule (calcd 23.4 %), quite consistent with the formula estimated from the results of elemental analysis. No
weight loss was observed up to 270 8C, and then rapid decomposition occurred.
To obtain desolvated samples at low temperature, we
treated the as-prepared samples under vacuum. It was found
that the solvent molecules could be completely evacuated
under various conditions, for example, heating at 60 8C for
6 h under vacuum, or even keeping as-prepared samples at
room temperature for 10 h under high vacuum. As seen in
Figure 2, the TG curves recorded for the desolvated samples
no longer show weight loss. A comparison of the PXRD patterns recorded for the as-prepared and desolvated samples
with the simulated ones from the single-crystal analyses
clearly denoted that the solid phase remained nearly intact
after desolvation (Figure 3) at low temperature.
To determine how the porous framework would be retained upon heating to decomposition, variable-temperature
powder X-ray diffraction (VT-PXRD) measurements were
Chem. Asian J. 2010, 5, 2358 – 2368
Figure 2. Thermogravimetric curves: a) as-prepared and desolvated samples of 1·2 CH3CN and 2·solvents, and b) desolvated sample of 1’ after exposure to EtOH or iPrOH vapors for 24 h and dipping in hot cyclohexane, benzene, toluene, or ethylbenzene for 10 h.
carried out. As shown in Figure 3, the diffraction profile of
1 was well retained until reaching 180 8C, but almost disappeared at 200 8C. Similarly, the diffraction peaks of 2
became blurred above 170 8C, and at 200 8C the phase
changed to an amorphous state. To further confirm the temperature of phase transition, differential scanning calorimetric (DSC) measurements were performed for 1·2 CH3CN.
The DSC curve in the Supporting Information exhibited an
evident phase change around 200 8C. These results disclose
that the frameworks in 1 and 2 are only stable up to 180 and
170 8C, respectively, which are both rather lower than the
decomposition temperatures observed from the TG curves.
More convincing evidence of porosity robustness that sustained the empty framework against guest removal was obtained by measuring the single-crystal structure of the desolvated complex 1’. After drying the single crystal of
1·2 CH3CN under high vacuum at room temperature for
10 h, reflection data was collected again. The refinement of
the structure reveals that the crystal lattice and coordination
framework remain almost unchanged (Table 1 and the Supporting Information), although the guest CH3CN molecules
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Table 1. Crystallographic data for 1·CH3CN, 2, and 1’.
1·2 CH3CN
empirical
formula
formula weight
space group
crystal system
a []
b []
c []
b [8]
V [3]
Z
1calcd [g cm 3]
T [K]
m [mm 1]
GOF
Rint
R1 (I > 2s(I))
wR2 (all data)
2
1’
C28H21Cu2I2N5O6 C48H30Br2Cu2N6O12 C24H15Cu2I2N3O6
904.38
C2
monoclinic
31.933(4)
7.8383(8)
18.115(2)
116.158(2)
4069.7(8)
4
1.476
173(2)
2.599
1.052
0.0173
0.0496
0.1438
1169.68
P2/n
monoclinic
30.267(3)
7.7321(8)
32.107(3)
115.235(2)
6796.8(12)
4
1.143
173(2)
1.850
1.047
0.0428
0.0759
0.1996
822.27
C2
monoclinic
32.093(6)
8.0032(9)
17.658(3)
113.49(2)
4159.5(11)
4
1.313
293(2)
13.160
1.035
0.0577
0.0664
0.1688
Figure 3. PXRD patterns of as-prepared and desolvated samples of 1 and
2 in comparison with the simulations from the single-crystal data (top),
and VT-PXRD patterns of as-prepared samples of 1 (middle) and 2
(bottom).
inside the 1D channels of 1·2 CH3CN have been completely
removed, justified by observation of no significant electron
residual (0.877 e 3) on the final differential electron-density map.
Gas Sorption and Selectivity
Figure 4. Gas adsorption/desorption isotherms for degassed samples of 1
(upper) and 2 (lower).
Both the porous frameworks of 1 and 2 were subject to gas
sorption with N2, H2, and CO2 at different temperatures
(Figure 4 and the Supporting Information). Before isotherm
measurements, the as-prepared sample of 1·2 CH3CN was
dried under high vacuum for 10 h to remove solvent molecules. For complex 2·solvents, the freshly prepared sample
was soaked in methanol to exchange the less volatile H2O
solvent, which was then followed by evacuation under a dynamic vacuum at 70 8C for 10 h. It is surprising that, although both 1 and 2 retain the stable frameworks and large
effective pore size notably bigger than the kinetic diameters
of N2 (3.64 ), H2 (2.89 ), and CO2 (3.3 ),[14] the frameworks display selective sorption of CO2 over N2 and H2. As
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Robust Porous Metal–Organic Frameworks
depicted in Figure 4, the sorption isotherms of N2 and H2
the ligands,[19] which can lead to an electric field to induce
the dipole in CO2. On the other hand, the large quadrupole
for 1 and 2 at 77 K revealed only surface adsorption, thus indicating that neither N2 nor H2 molecules can diffuse into
moment of the CO2 molecule (3.3–3.4 10 26 e.s.u.)[10c] can
the channels. At relatively high pressure, slight loading of
interact with the electric-field gradient, thus further contriN2 occurred, which may arise from capillary condensation.[15]
buting to the potential energy of adsorption. Therefore, the
adsorption selectivity of CO2 over N2 and H2 by the porous
On the contrary, the adsorption isotherms of CO2 measured
at 195 K for 1 and 2 exhibited satisfactory capacity for CO2
frameworks of 1 and 2 may be more attributable to the
host–guest interactions, thus reminding us of the fact that
storage of type-I profiles. As seen from Figure 4, framework
there should be some donor–acceptor affinity between the
1 has a CO2 adsorption capacity of 111 mL g 1
CO2 molecules and the Lewis acidic Cu + cations.[17e]
(5.0 mmol g 1) at 1 atm, which amounted to 4.1 CO2 molecules per formula unit. The Langmuir and BET surface
Since the isosteric heat of adsorption (Qst) is a sensitive
areas estimated from the CO2 sorption isotherm are 649 and
probe for adsorption nonuniformity (surface heterogeneity)
496 m2 g 1, respectively. The CO2 sorption of 2 revealed a
and hence for surface structure,[20] the adsorption isotherms
1
1
CO2 adsorption capacity of 86 mL g
of CO2 at different temperatures (273 and 283 K) were col(3.8 mmol g ) at
1 atm, which amounted to approximately 2.4 CO2 molecules
lected for 1 and used for the calculation of Qst from the
per metal ion at a pressure of 780 mmHg. The Langmuir
Clausius–Clapeyron equation (see Figure 5 and the Supportand BET surface areas of 2 are
calculated to be 417 and
260 m2 g 1, respectively.
Besides a little lower CO2
uptake than 1, compound 2 also
displayed more significant hysteresis between adsorption and
desorption curves. Although
such a hysteresis loop has often
been associated with a guest-induced framework rearrangement,[14, 16] it is unlikely that the Figure 5. Preferential CO2 location obtained from GCMC calculations with the van der Waals surface of the
framework of 2 can undergo CO2 molecule highlighted in transparency (left) and the isosteric heat of adsorption for CO2 in 1 (right).
any structural transformation
during gas adsorption, because
the VT-PXRD measurements indicated that the framework
ing Information).[21] It is evident that the Qst values are not
remained unchanged up to 170 8C, and the desolvated
constant as the degree of CO2 loading is varied. At low covframework kept the same PXRD profile as that of the
erage, the Qst value is high, up to about 70 kJ mol 1, which is
framework containing the solvents. From the structural analcomparable to that of amine-functionalized MOFs
yses we know that the frameworks in 1 and 2 show the fol(90 kJ mol 1),[21a] thus indicating a strong adsorbate–adsorblowing comparative features: a) compound 1 contains the
ent interaction. However, upon further loading of CO2, the
Cu2I2 cluster as a building unit with the I anion bridging
isosteric heat of adsorption decreased rapidly to about
15 kJ mol 1 and remained almost unchanged throughout the
two Cu + cations, whereas 2 contains the CuBr monomer as
a building unit with the Br anion being monodentate;
adsorption process. This Qst value is close to the vaporizab) the two frameworks show distinct catenation modes, but
tion enthalpy of CO2 (10.3 kJ mol 1 at 273 K) with a margin
c) the two frameworks possess similar 1D channels of comof error of 10 %, thereby suggesting fast deactivation of the
parable pore sizes and solvent-accessible voids. These differframework surface energy. Probably owing to such a low afences in metal–halide building units and framework catenafinity of gas to the host framework at high pressure, the
tion may be related to the slight difference in sorption/detotal uptake of CO2 in 1 is quite a lot lower than the theosorption behaviors of 1 and 2, thereby indicating that the
retical amount estimated from the crystal structural analysis.
different structural and topological features of 1 and 2 do
To further elucidate the adsorption mechanism, grand canhave some influence on their gas-uptake properties.
onical Monte Carlo (GCMC) simulations were employed to
The gas-sorption selectivity in MOFs is usually related to
understand the interactions between CO2 molecules and the
the size-exclusion effect.[17] In the present two MOFs, this
empty frameworks of 1 and 2. Calculations were carried out
with a model that considered the Coulomb and Lennardmechanism may not represent the only one, since the effecJones 6–12 potentials of the CO2 molecule and framework
tive pore sizes are much bigger than the kinetic diameters of
the studied N2, H2, and CO2 gases. It has been proven that
atoms, which were modeled with the universal force field
(UFF) embedded in the MS modeling 4.4 package (see the
the CO2 molecule can be induced in a polar environment to
Supporting Information).[22] As depicted in Figure 5 and the
give a dipole, thereby forming dipole-induced dipole interac[10c, 17e, 18]
+
tions.
Supporting Information, the preferred CO2-bonding sites
The frameworks of 1 and 2 contain Cu cations, halide anions, and p electrons and ester groups from
are around Cu + ions, thus resulting in Cu···O distances in
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the range of 2.72–2.85 . Such short distances clearly disclose a weak coordination interaction between the CO2 molecule and Cu + site, which is indicative of a chemisorption
interaction.[21a] The calculated adsorption enthalpies for 1
and 2 are 66.0 and 41.2 kJ mol 1, respectively, which are in
agreement with the results obtained from the adsorption isotherms.
To determine the adsorption selectivity for CO2 over N2,
the adsorption isotherm of N2 at 195 K was also collected
for 1 and compared with those of CO2. It was found that N2
uptake at 195 K is still negligible. The initial slopes of adsorption isotherms were calculated, and the ratios of these
slopes were used to estimate the CO2/N2 adsorption selectivity (see the Supporting Information).[21b] From these data, a
high CO2/N2 selectivity of 147:1 was obtained at 195 K.
However, the CO2/N2 selectivity is dramatically decreased
along with an increase in the temperature to 5:1 at 283 K.
This arises from the relatively low CO2 sorption at elevated
temperatures, thereby indicating that the interactions between CO2 and the host framework were significantly diminished at high temperatures.
Vapor Sorption and SCSC Guest Exchange
Adsorption selectivity of MOFs is also believed to be of
high importance in industry applications such as gas purification and liquid separation,[2a, 23] so the vapor-sorption properties of 1 were tested for hydrocarbon molecules such as
cyclohexane, benzene, toluene, and ethylbenzene, and compared with those of polar EtOH and iPrOH solvents. The
desolvated sample of 1’ was exposed in the EtOH or iPrOH
vapors for 24 h, or heated to reflux in cyclohexane, benzene,
toluene, or ethylbenzene for 10 h, respectively. Figure 2
shows the TG curves of the solvent guest-inclusion samples,
and the Supporting Information shows the PXRD patterns
of the samples before and after solvated guest exchange. It
is evident that the porous framework is robust against solvent desorption and adsorption, and displays little structural
change as seen from their PXRD patterns. In general, the
empty framework possesses good solvent-storage capacity;
however, the preference for aromatic guest molecules over
the nonaromatic cyclohexane guest molecule is clear. As
seen from the TG curves in Figure 2, the framework adsorbed cyclohexane molecules by less than 10 % (weight
basis), whereas it adsorbed benzene, toluene, or ethylbenzene molecules by more than 15 %. For comparison, TG
curves displayed about 10 % inclusion of EtOH and iPrOH
molecules. These observations imply that the structural character of the guest molecules plays some role in the solventloading processes.
More informative adsorption properties of the degassed
framework for solvent guest molecules were studied by
measuring the solvent vapor-adsorption isotherms, as depicted in Figure 6. In the low P/P0 region, cyclohexane showed
little uptake until a capillary condensation occurred at high
pressure of about P/P0 = 0.9. By contrast, benzene had a typical type-I isotherm that showed fast adsorption at low pres-
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Figure 6. Vapor adsorption/desorption isotherms for degassed samples of
1.
sure and gave a total sorption amount of 20.3 %, which corresponded to 2.7 benzene molecules per Cu2I2 unit. In comparison, adsorption of EtOH with an amount of 13.4 % was
observed, which also corresponds to 2.8 EtOH molecules
per Cu2I2 unit. Considering the molecular shape and size of
benzene and EtOH, more efficient adsorption of aromatic
guests by the empty framework is clear. In addition, the solvent-adsorption isotherms exhibited remarkable hysteresis
loops, thus indicating that desorption of solvent molecules
may have lagged owing to the formation of host–guest interactions. Taking into account the very similar boiling points
of benzene and cyclohexane (80.1 vs 80.7 8C), which causes
difficulty in industry separation, selective adsorption of benzene over cyclohexane by the present framework is of potential interest in their separation application.
Fortunately, the detailed structural information on solvent
guest exchange was able to be observed through a singlecrystal-to-single-crystal (SCSC) processes. As discussed
above, the desolvated complex 1’ can retain the single-crystal nature after removal of the guest CH3CN molecules.
Upon heating guest-free 1’ in aromatic solvents to load benzene, toluene, or ethylbenzene guests, its single crystallinity
was still retained well. The single-crystal X-ray analyses of
the guest-inclusion complexes disclosed that the coordination frameworks remained unchanged after adsorbing benzene, toluene, or ethylbenzene guest molecules. The cell
volume, network catenation, and pore size remained nearly
constant regardless of guest-molecule exchange (Tables 1
and 2). However, the interactions between the hosts and
guests, and even among the guest molecules, were different.
As shown in Figure 7, the CH3CN molecules in
1·2 CH3CN are aligned in different orientations to adhere to
the channel walls, thereby forming various weak C H···Oester
and C H···N intermolecular interactions between the guest
and host molecules. The guest CH3CN molecules themselves
are apparently not closely packed (see the Supporting Infor-
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Robust Porous Metal–Organic Frameworks
Table 2. Crystallographic data for 1·benzene, 1·2 toluene, and 1·2 ethylbenzene.
empirical
formula
formula weight
space group
crystal system
a []
b []
c []
b [8]
V [3]
Z
1calcd [g cm 3]
T [K]
m [mm 1]
GOF
Rint
R1 (I > 2s(I))
wR2 (all data)
1·benzene
1·2 toluene
1·2 ethylbenzene
C60H42Cu4I4N6O12
C38H31Cu2I2N3O6
C40H34Cu2I2N3O6
1800.76
P2
monoclinic
17.8991(10)
7.8787(5)
29.4265(18)
94.742(2)
4135.6(4)
2
1.446
173(2)
2.557
1.042
0.0424
0.0684
0.2043
1006.54
C2
monoclinic
30.375(6)
7.5302(15)
18.652(4)
115.721(3)
3843.5(13)
4
1.739
173(2)
2.761
1.037
0.0310
0.0470
0.1104
1033.58
C2
monoclinic
31.751
7.749
18.199
115.97
4025.6
4
1.705
173(2)
2.639
1.048
0.0372
0.0555
0.1459
mation). Two CH3CN molecules per formula unit are encapsulated inside the channels.
It is interesting that, although the TG measurement confirmed that the empty framework of 1’ can take up two benzene, toluene, or ethylene molecules per formula unit, a
single crystal that contained only half the amount of benzene guest, 1·benzene, which may be considered to be the
intermediate guest-adsorption state, was obtained. The absence of half of the guest molecules resulted in a symmetry
difference in the crystal lattice; therefore, the spacer group
of 1·benzene changed from C2 to P2, but the cell volume remained the same as 1’. The benzene molecules in 1·benzene
stand adjacent to the channel walls and take on an orientation that is perfectly parallel to the pyridyl ring of the
ligand, thereby forming weak offset face-to-face p–p interactions between the host and guest molecules, but separating the guest molecules completely (see the Supporting Information). On the contrary, in the fully guest-loaded complexes 1·2 toluene and 1·2 ethylbenzene, the aromatic guest
molecules seem to be inclined to pack closely (Figure 7 and
the Supporting Information). The arene molecules still lie
adjacent to the channel walls but no longer take an orientation parallel to the pyridyl rings. Instead, head-to-face C
H···p interactions between the host and guest molecules are
formed.
The above results suggest that the solvent adsorption may
be subject to combined influences from the host–guest and
guest–guest interactions. At the beginning, the host–guest
interactions may play a key role in initiating the guest
uptake. However, the formation of adequate guest–guest interactions may serve as the successive step in determining if
more guest molecules can be adsorbed. The aromatic guest
molecules can form closer packing inside the channels than
the nonaromatic guest molecules, so they are preferred by
the empty porous frameworks. This may account for the
reason why the empty frameworks show more efficient ad-
Chem. Asian J. 2010, 5, 2358 – 2368
Figure 7. Solvent guest molecule inclusion in the framework channels of
1 and their packing fashions. From top to bottom: 1·2 CH3CN, 1·benzene,
1·2 toluene, and 1·2 ethylbenzene.
sorption of benzene, toluene, and ethylbenzene over EtOH
and iPrOH, and selectivity toward benzene over cyclohexane. In this context, the porous frameworks of 1 and 2 possess versatile functional sites such as Cu + cations, halide
anions, p electrons, and ester groups, which facilitate formation of various supramolecular interactions between host
and guest molecules[24] such as Cu···O weak coordination
(for CO2), hydrogen bonds (for EtOH or iPrOH), and p–p
stacking (for aromatic molecules), thereby leading to adsorption selectivity such as CO2 over N2 and benzene over
cyclohexane.
2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
2365
C.-Y. Su et al.
FULL PAPERS
Conclusion
Two robust porous MOFs have been obtained from a new
tripodal ligand BTTP4 and CuI or CuBr; they contain distinct network catenations but similar 1D channels. Two
MOFs show selective gas adsorption for CO2 over N2/H2 at
low temperatures and atmospheric pressure. The framework
of 1’ is robust enough to maintain the single crystallinity despite removal of the solvents and the inclusion of guest molecules, including different aromatic hydrocarbons such as
benzene, toluene, and ethylbenzene. The solvent vapor isotherms and guest-exchange studies indicate that the empty
framework of 1’ prefers aromatic guests to the nonaromatic
solvents, as it shows adsorption selectivity toward benzene
over cyclohexane. The adsorption selectivity of the gases
and solvents is believed to owe itself to host–guest and even
guest–guest interactions, not just to the size selectivity as is
usually observed.
Experimental Section
All starting materials and solvents were obtained from commercial sources and used without further purification. Infrared spectra were measured
with a Nicolet/Nexus-670 FTIR spectrometer with Nujol in the 4000–
400 cm 1 region. 1H NMR spectra were recorded with a Varian Mercury
Plus 300 MHz spectrometer. The X-ray powder diffraction patterns were
measured with a Bruker D8 Advance diffractometer at 40 kV and 40 mA
with a Cu target tube and a graphite monochromator. For VT-PXRD
measurements, the diffraction patterns at different temperatures were recorded after the sample had stayed at the respective temperature for
30 min in air. Thermogravimetric analyses were performed under an N2
atmosphere at a heating rate of 10 8C min 1 with a NETZSCH TG 209
system. The CHN elemental analyses were performed with a Perkin–
Elmer 240 elemental analyzer. The sorption isotherms of compound 1
for N2 (77 K, 195 K), H2 (77 K), and CO2 (195 K) gas was measured with
an automatic volumetric sorption apparatus (BELSORP-max, Bel
Japan), whereas those of compound 1 for CO2 (273 K, 283 K) and compound 2 for N2, H2 (77 K), and CO2 (195 K) were performed with a Micromeritics ASAP 2020.
Benzene-1,3,5-triyl Triisonicotinate (BTTP4)
The BTTP4 ligand was prepared in an alternative method reported previously.[25] The stirred mixture of 4-picolinic acid (3.0 g, 24 mmol) and
freshly distilled thionyl chloride (10 mL) was heated to reflux for 4 h. An
excess amount of thionyl chloride was removed by distillation under reduced pressure, and the resulting crystalline powder of 4-picolinoyl chloride was used as prepared. A solution of 1,3,5-trihydroxybenzene (0.93 g,
7.5 mmol) and distilled triethylamine (8 mL) in dry THF (30 mL) was
added dropwise to a solution of the above-prepared 4-picolinoyl chloride
in THF (20 mL) in an ice bath. After the addition finished, the reaction
was maintained at room temperature for 10 h. The reaction mixture was
poured into water and the crude product of BTTP4 was collected by filtration. This crude product was dissolved in chloroform and filtered, and
the pure product was obtained from the filtrate after evaporation. Yield:
1.8 g, 55 %. 1H NMR (300 MHz, CDCl3, 25 8C): d = 8.87–8.89 (d, J =
6.0 Hz, 2 H), 7.99–8.01 (d, J = 6.0 Hz, 2 H), 7.47 ppm (s, 1 H).
ACHTUNGRE[Cu2I2ACHTUNGRE(BTTP4)]2 CH3CN (1·2 CH3CN)
CuI (6 mg, 0.03 mmol) was dissolved in acetonitrile (3 mL) and then
carefully layered onto a solution of BTTP4 (9 mg, 0.02 mmol) in chloroform (4 mL). After 1 w, orange block crystals suitable for single-crystal
X-ray analysis were formed at the interface of the two solutions. Yield:
www.chemasianj.org
ACHTUNGRE[CuBrACHTUNGRE(BTTP4)](CH3CN·CHCl3·H2O) (2·solvents)
Complex 2·solvents was synthesized in a similar way except that the CuI
was replaced by CuBr. After about 5 d, dark red crystals suitable for
single-crystal X-ray diffraction analysis were formed. Yield: 5 mg, 33 %
(based on the ligand). IR (KBr): ñ = 3432 (m), 1751 (s), 1607 (m), 1456
(w), 1412 (m), 1256 (s), 1133 (s), 1081 (m), 1059 (m), 754 (w), 695 cm 1
(w); elemental analysis calcd (%) for C27H21BrCl3CuN4O7: C 42.48,
H 2.77, N 7.34; found: C 41.79, H 2.79, N 7.08.
ACHTUNGRE[Cu2I2ACHTUNGRE(BTTP4)] (1’)
The single crystals of 1·2 CH3CN were dried under high vacuum for 10 h
at room temperature, thus giving rise to the solvent-free complex with an
empty framework.
ACHTUNGRE[Cu2I2ACHTUNGRE(BTTP4)]C6H6, [Cu2I2ACHTUNGRE(BTTP4)]2 C7H8, and
[Cu2I2ACHTUNGRE(BTTP4)]2 C8H10 (1·benzene, 1·2 toluene, and 1·2 ethylbenzene)
The crystal of the desolvated complex 1’ was dipped in the corresponding
aromatic solvents and heated at 80 8C for 10 h, thereby leading to formation of the guest-captured complexes 1·benzene, 1·toluene, and 1·ethylbenzene.
Crystal Structure Determination
Physical Methods
2366
8 mg, 45 % (based on the ligand). IR (KBr): ñ = 3436 (m), 1750 (s), 1608
(m), 1456 (w), 1417 (m), 1255 (s), 1136 (s), 1086 (w), 1060 (m), 756 (w),
691 cm 1 (w).
The diffraction data were collected with a Bruker SMART Apex CCD
system with graphite-monochromated MoKa radiation (l = 0.71073 ) for
complexes 1·2 CH3CN, 2, 1·benzene, 1·2 toluene, and 1·2 ethylbenzene at
173 K, and with an Oxford Gemini S Ultra diffractometer equipped with
CuKa radiation (l = 1.54178 ) for complex 1’ at 293 K by using f and w
scans.[26] Empirical absorption correction was applied with the SADABS
program[27] or the spherical harmonics implemented in the SCALE3 ABSPACK scaling algorithm.[28] Structural solution and refinement against
F2 were carried out with the SHELXL programs.[29] Anisotropical thermal factors were assigned to most of the non-hydrogen atoms except
those of the benzene molecules in 1·benzene, which were modeled by restraint FIX 66. The positions of the hydrogen atoms were generated geometrically, assigned isotropic thermal parameters, and allowed to ride on
their respective parent atoms before the final cycle of least-squares refinement. The MeCN molecules in 1·2 CH3CN were refined by fixing the
bond lengths reasonably. The guest molecules in 2 display severe disorder
so that the SQUEEZE program[13] was used to remove all the solvents,
thereby resulting in the empty framework. The crystallographic data for
1·2 CH3CN, 2, and 1’ are summarized in Table 1 and those for 1·benzene,
1·2 toluene, and 1·2 ethylbenzene are listed in Table 2.
CCDC 767072 (1·2 CH3CN), 767073 (1·benzene), 767074 (1·2 toluene),
767075 (1·2 ethylbenzene), 767076 (1’), and 767077 (2) contain the supplementary crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre at
www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
This work was supported by the 973 Program of China (2007CB815302)
and the NSFC (project nos. 20821001, 20773167, 20731005, and
U0934003).
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Received: March 22, 2010
Revised: May 4, 2010
Published online: August 16, 2010
2368
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2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2010, 5, 2358 – 2368