Versatile Assembly of Metal-Coordinated Calix[4]resorcinarene Cavitands and Cages through Ancillary Linker Tuning

Article abstract of DOI:10.1021/jacs.7b03169

We propose a design strategy for assembly of metal-coordinated calix[4]resorcinarene cavitands and cages by tuning of the ancillary linkers. Assembly of newly functionalized cavitand with angular isophthalic acid analogs affords three intriguing metal-coordinated cavitands with deep cavities, 1a-1c. Further, by mediating appropriate spacers between two isophthalic acids, two bowl-shaped cavitands are successfully joined together to produce three elegant coordination cages with tunable sizes and shapes, 2a-2c. The cavitand and cage crystals possess considerable amount of accessible porosities, as clearly established by gas adsorption measurements. Remarkably, 1a-1c also exhibit high structural flexibilities, reversibly transforming between the open-pore and the narrow-pore structures, upon removal and sorption of guest molecules, as evidenced by diffraction and gas adsorption measurements. By combining experimental studies with density functional theory (DFT) calculations, we thoroughly elucidated the mechanism of the structural transformations in response to external stimuli in this new class of flexible porous solids.

Full text of DOI:10.1021/jacs.7b03169

Article
pubs.acs.org/JACS
Versatile Assembly of Metal-Coordinated Calix[4]resorcinarene
Cavitands and Cages through Ancillary Linker Tuning
Wen-Yuan Pei,†,⊥ Guohai Xu,‡,§,∥,⊥ Jin Yang,*,† Hui Wu,§ Banglin Chen,∥ Wei Zhou,*,§
and Jian-Fang Ma*,†
†Key Lab for Polyoxometalate Science, Department of Chemistry, Northeast Normal University, Changchun 130024, China
‡Key Laboratory of Jiangxi University for Functional Materials Chemistry, School of Chemistry and Chemical Engineering, Gannan
Normal University, Ganzhou, Jiangxi 341000, China
§NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-6102, United
States
∥Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249-0698, United States
S
* Supporting Information
ABSTRACT: We propose a design strategy for assembly of
metal-coordinated calix[4]resorcinarene cavitands and cages by
tuning of the ancillary linkers. Assembly of newly function-
alized cavitand with angular isophthalic acid analogs affords
three intriguing metal-coordinated cavitands with deep cavities,
1a−1c. Further, by mediating appropriate spacers between two
isophthalic acids, two bowl-shaped cavitands are successfully
joined together to produce three elegant coordination cages
with tunable sizes and shapes, 2a−2c. The cavitand and cage
crystals possess considerable amount of accessible porosities, as
clearly established by gas adsorption measurements. Remark-
ably, 1a−1c also exhibit high structural flexibilities, reversibly transforming between the open-pore and the narrow-pore
structures, upon removal and sorption of guest molecules, as evidenced by diffraction and gas adsorption measurements. By
combining experimental studies with density functional theory (DFT) calculations, we thoroughly elucidated the mechanism of
the structural transformations in response to external stimuli in this new class of flexible porous solids.
Importantly, the functionalized calix[4]resorcinarene cavitands
with bowl-shaped structures are also ideal scaffolds for
coordination cage assembly. Some typical dimeric calix[4]-
resorcinarene cages have been constructed through metal-
mediated assembly, in which two calix[4]resorcinarene-based
cavitands are directly joined together by metal ions.7b,12 To our
knowledge, however, coordination cage of two metal-
coordinated calix[4]resorcinarenes being held together by
ancillary poly(carboxylic acid) linkers remains almost unknown,
probably owing to the synthetic challenge.6−8
Here, we put forth a design strategy for assembly of metal-
coordinated calix[4]resorcinarene cavitands and tunable cages
through tuning of the ancillary linkers. On the basis of this idea,
two new bowl-shaped calix[4]resorcinarene cavitands (Me-
TPC4R and Pen-TPC4R) were designed by introducing four
chelating 2-(4H-pyrazol-3-yl)pyridine moieties at the upper rim
(Schemes 1 and S1 of the Supporting Information, SI).
Assembly of the cavitands with variable isophthalic acid analogs
and metal ions affords a series of intriguing isostructural metal-
coordinated cavitands with highly deep cavities, [Zn4(Me-
INTRODUCTION
■
Coordination-driven self-assembly based on metal−ligand
bonds has evolved as an efficient way to achieve discrete
metal−organic molecular architectures.1−5 In this facet, metal-
coordinated cavitands and cages with well-defined internal
voids have attracted a great deal of attention because of their
appealing structures and potential applications in gas sorption,
molecular recognition, and catalysis.6 In general, rational
assembly of highly directional ligands and appropriate metal
ions can produce elaborate metal-coordinated cavitands and
cages with various sizes, shapes, and cavities.7 In this context,
calixarenes, as a special family of macrocyclic host compounds,
are excellent ligand candidates,8 which have been extensively
utilized in supramolecular chemistry.9
As an important subset of calix[4]arenes, calix[4]-
resorcinarenes, featuring a bowl-shaped aromatic cavity, have
been recognized to be a greatly versatile class of molecules.10
The upper rims of the calix[4]resorcinarenes can be well
modified with various functional groups making them intriguing
cavitands.11 The utilization of the decorated cavitands in
combination with metal ions has resulted in a range of metal-
coordinated cavitands to date.12 Nevertheless, metal-coordi-
nated cavitands with highly deep cavities are exceedingly rare.
Received: March 31, 2017
© XXXX American Chemical Society
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was found that the angular isophthalic acid analogs support the
formation of the metal-coordinated cavitands during the
assembly of 1a−1c. When we varied the substituted groups
of the isophthalic acid analogs, isostructural metal-coordinated
cavitands were obtained. For the tetracarboxylic acid analogs,
appropriate length, geometry, and flexibility of the spacer
between two isophthalic acids are important factors that
influence the cage assembly.
Scheme 1. Structures of Me-TPC4R and Pen-TPC4R
Cavitands Used in This Work (Me-TPC4R: R = Methyl; Pen-
TPC4R: R = Pentyl)
Crystal Structures of 1a−1c. Single-crystal XRD reveals
that 1a−1c exhibit isoreticular structures with variable
isophthalic acid analogs (Scheme 2; Figures 1 and 2), and
crystallize in the same tetragonal space group P4/n (Table S1).
Hence, the structure of 1a will be described here in detail as a
representative.
Scheme 2. Angular Isophthalic Acid Analogs Used in This
Work
TPC4R)(L1)4]·6DMF·2H2O (1a), [Zn4(Me-TPC4R)(L2)4]·
3DMF·H2O (1b) and [Zn4(Me-TPC4R)(L3)4]·5DMF (1c)
(H2L1 = 5-aminoisophthalic acid, H2L2 = 5-hydroxyisophthalic
acid, H2L3 = 5-(pyridin-4-yl)isophthalic acid and DMF = N,N′-
dimethylformamide). Inspired and motivated by the successful
preparation of the metal-coordinated cavitands, we envision
that tetracarboxylic acid analogs with appropriate spacers
between two isophthalic acids may link two cavitands into
tunable cages in the presence of metal ions. To our great
delight, a family of nanoscale calix[4]resorcinarene-based cages
with tunable sizes, shapes, and cavities, [Zn8(Me-
TPC4R)2 (L4 )4 ]·5DMF·20H2 O (2a), [Zn8 (Pen-
TPC4R)2(L5)4]·7DMF (2b), and [Zn8(Pen-TPC4R)2(L6)4]·
6DMF (2c), were successfully prepared by using this strategy
(H4L4 = 5,5′-methylene-bis(oxy)diisophthalic acid, H4L5 = 5,5′-
(1,3-phenylenebis(methylene))bis(oxy)diisophthalic acid and
H4L6 = 5,5′-(1,4-phenylenebis(methylene))bis(oxy)-
diisophthalic acid). 2a−2c represent a new class of tunable
cages where two metal-coordinated calix[4]resorcinarene
cavitands are joined together by systematically tuning ancillary
poly(carboxylic acid)s.
Supramolecular assemblies of these metal-coordinated
calix[4]resorcinarene cavitands and cages are fairly stable and
highly crystalline. After desolvation, they exhibit permanent gas-
accessible porosities. This was clearly established by gas
adsorption measurements. Remarkably, 1a−1c also exhibit
high structural flexibilities, reversibly transforming between the
open pore (op) and the activated narrow pore (np) form, upon
removal and reintroduction of guest molecules. This behavior is
similar to those found in flexible metal−organic framework
(MOFs) or soft porous crystals (SPCs), featuring reversible
structural transformations triggered by external stimuli.13,14
Therefore, 1a−1c represent a nice new addition to the family of
solid porous stimuli-responsive materials.4a,15 Importantly, by
combining experimental studies with density functional theory
(DFT) calculations, we were also able to thoroughly elucidate
the mechanism of the structural transformations in response to
external stimuli in this new class of flexible porous solids.
Figure 1. Side view of the structure of the metal-coordinated cavitand
[Zn4(Me-TPC4R)(L1)4] in 1a. Zn: pink; C: green; N: blue; O: red.
The large yellow sphere represents the inner cavity of the cavitand.
In 1a, each L1 linker bridges two neighboring Zn(II) ions
around the bowl rim to form a macrocyclic [Zn4(L1)4] unit, in
which each Zn(II) atom is further chelated by the 2-(4H-
pyrazol-3-yl)pyridine group at the upper rim, thus leading to an
appealing metal-coordinated cavitand (Figures 1 and 2). The
Zn(II)···Zn(II) distance across the calix[4]resorcinarene bowl
is approximately 13.3 Å (Figure S1a). It is of interest to note
that the cavity deepness of 1a is about 10.7 Å from the lower
rim to the upper rim (Figure S1b), and the diameter of the
bowl-shaped cavitand between the lower rim and the upper rim
varies from ca. 5.21 Å to ca. 16.20 Å. Adjacent cavitands are
packed against each other through π−π interactions between
the 2-(4H-pyrazol-3-yl)pyridine groups [the distances of
centroid-to-centroid and centroid-to-plane (based on the
pyridine groups) are 3.63 and 3.58 Å, respectively] to yield
fascinating supramolecular layers, in which the upper rims and
the lower rims of the bowls are alternately arranged.
Furthermore, adjacent supramolecular layers are stacked along
RESULTS AND DISCUSSION
■
Self-Assembly of 1a−1c and 2a−2c. All metal-coordi-
nated cavitands and cages were assembled by introducing
isophthalic acid or tetracarboxylic acid analogs into the
solvothermal reaction of Zn(NO3)2·6H2O and Me-TPC4R or
Pen-TPC4R systems. During the syntheses of 1a−1c and 2a−
2c, careful control of the reaction temperature and the solvent
ratio can favor the formation of large single crystals (see SI). It
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Figure 2. Structures of the metal-coordinated cavitands [Zn4(Me-TPC4R)(L1)4] in 1a, [Zn4(Me-TPC4R)(L2)4] in 1b, and [Zn4(Me-TPC4R)(L3)4]
in 1c.
the c axis, through van der Waals interaction, to furnish an
intriguing 3D porous architecture, as shown in Figure 3. The
total solvent-accessible volume is ca. 4279.6 Å3 (approximately
52.1% of the unit-cell volume), calculated by the PLATON
program.16
volume) is a little bigger than that of 1a. Since the pores of 1c
were partially occupied by the pyridyl groups of H2L3, the total
solvent-accessible volume for 1c is ca. 4212.0 Å3 (approx-
imately 48.3% of the unit-cell volume), which is the smallest
one of the three compounds. It needs to be pointed out that
although a range of metal-coordinated cavitands have been
reported thus far,7b 1a−1c, featuring the modular structures
and highly deep cavitand cavities, represent an unusual example
of metal-coordinated calix[4]resorcinarene cavitands involving
variable isophthalic acid analogs.
Crystal Structures of 2a−2c. Encouraged by the
successful assembly of the metal-coordinated cavitand
structures above, we conceived that two bowl-shaped calix[4]-
resorcinarene cavitands may be assembled into a coordination
cage by introducing tetracarboxylic acid analogs with
appropriate spacers between two isophthalic acids. A family
of coordination cages, 2a−2c, featuring tunable sizes, shapes,
and cavities, were successfully achieved by adjusting the spacers
of the tetracarboxylic acid linkers (Schemes 3 and S2).
Single-crystal XRD reveals that 2a crystallizes in the
orthorhombic system with space group Pccn, 2b in the
tetragonal system with space group I4/m, and 2c in the
triclinic system with space group P-1 (Table S2). The structure
Scheme 3. Tetracarboxylic Acid Analogs Used in This Work
Figure 3. Crystal structure of 1a. The 3D porous supramolecular
architecture is stabilized by π−π interactions and van der Waals
interactions. The large yellow spheres represent the inner cavities of
the cavitands, and the green cylinders represent the intersupermole-
cule 1-D pores along the crystallography c axis.
By varying the isophthalic acid analogs, isostructural 1b and
1c were also achieved (Figures 2, S2, and S3), demonstrating
the generality of this assembly strategy. Noticeably, 1c has a
much higher cavity deepness of ca. 12.5 Å (Figure S4) than
those of 1a and 1b. In 1c, the pyridyl groups are free and point
toward the interior of the channel, which partially occupied the
packing pores (Figure S3). The total solvent-accessible volumes
for 1b (ca. 4351.6 Å3, approximately 53.2% of the unit-cell
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Figure 4. Structures of [Zn4(Me-TPC4R)(L4)4] cage in 2a, [Zn8(Pen-TPC4R)2(L5)4] cage in 2b and [Zn8(Pen-TPC4R)2(L6)4] cage in 2c. The
methyl groups in 2a and the pentyl groups in 2b and 2c are omitted for clarity.
of the cage consists of two metal-coordinated calix[4]-
resorcinarene cavitands and four semiflexible tetracarboxylic
acid linkers, featured as a fascinating nanometer-sized
coordination cage. As shown in Figure 4, each of the four 2-
(4H-pyrazol-3-yl)pyridine groups at the upper rim chelates one
Zn(II) ion to produce a metal-coordinated [Zn4-TPC4R]8+
cavitand unit. Further, two [Zn4-TPC4R]8+ units are joined
together by four semiflexible tetracarboxylic acids to give a
remarkable [Zn8(TPC4R)2(Ln)4] nanocage (n = 4−6). As
expected, the modular cavitand units [Zn4(TPC4R)(Ln)4]4+ (n
= 1−3) in 1a−1c were well-maintained in cages 2a−2c, further
demonstrating the versatile applicability of our assembly
strategy.
It should be noted that 2a and 2b exhibit spindle-shaped
cavities, while 2c has a ellipsoid-shaped one, as illustrated in
Figure 4. This is ascribed to the different structural flexibility
and geometry of the tetracarboxylic acid linkers. The
longitudinal diameters of the cages are tuned by the variable
tetracarboxylic acid spacers, which well correspond to the
increasing length of the tetracarboxylic acids in the order of
H4L4 < H4L5 < H4L6. The longitudinal dimensions of the
internal cavity are approximately 2.3, 2.8, and 3.0 nm for 2a−
2c, respectively, identified as the distance between the two
opposite centroids of the two cavitand lower rims (Figure S5).
These are longer than most of the cages directly joined together
by two metal-coordinated calix[4]resorcinarenes reported in
the literature.12
The cages are stacked together through van der Waals
interactions, to form 3D crystals. As illustrated in Figures 5 and
S6, in addition to the large internal spherical cavities, there are
also obvious porosities presented between the cages of 2a. The
pore spaces are presumably occupied by disordered solvent
molecules. PLATON calculations indicate the presence of
approximately 43.7, 62.5, and 43.1% void space in 2a−2c,
respectively (Figures S7 and S8).16
Figure 5. Crystal structure of 2a. The large yellow spindle-shaped
spheres represent the inner cavities of the cages, and the green cylinder
represents the intersupermolecule 1-D pores along the crystallography
c axis.
High Structural Flexibility of 1a−1c. After describing the
crystal structures of the as-synthesized metal-coordinated
cavitands and cages, we next discuss the corresponding
desolvated phases. Guest molecules in the as-synthesized
crystals can be removed by acetone-exchange and activation
under vacuum at elevated temperature (see SI). Upon
desolvation, the PXRD patterns of 1a−1c changed (Figures
S9−S12), suggesting a structural transformation. Interestingly,
upon resolvation (immersing the activated samples in DMF),
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the PXRD patterns can almost be restored (Figures S9−S11),
implying that the process is reversible.
These observations prompted us to investigate the structural
flexibility in the metal-coordinated cavitand crystals in detail.17
Here, we focus on 1a as the representative in our discussion,
since the three compounds are isostructural and are expected to
behave similarly.
Direct single-crystal XRD measurement on activated 1a
sample was difficult. Thus, we relied on PXRD to elucidate the
structure of the desolvated phase. The PXRD pattern was
indexed to space group P4/n, which is the same as that of the
as-synthesized crystal. Therefore, the tetragonal crystal
symmetry of 1a remains unchanged before and after solvent
removal. The a/b axis and cell volume V, however, significantly
contracts from 25.82 and 8165.1 Å3 to 21.39 and 5503.8 Å3,
respectively, indicating that the structure of 1a is indeed highly
flexible. The coefficients of the contraction for a/b and V are
17.2% and 32.6%, respectively.
To elucidate the mechanism of the structural flexibility of 1a,
we performed a detailed computational first-principles inves-
tigation, based on dispersion-corrected density functional
theory (DFT-D) (see SI for details). Variable-cell relaxation
on the crystal structure of 1a was performed to simulate the
lattice contraction process upon activation. Cell dimensions
were changed smoothly on the fly, starting from those of the as-
synthesized crystal, shifting toward those of the desolvated
crystal. An animation consisting of snapshots of the structural
relaxation shows clearly how the structure of 1a evolves upon
lattice contraction (see the movie in SI). Essentially, the
cavitand supermolecules undergo large degree of rotation and
distortion, leading to narrowed intersupermolecule voids, a
denser packing, and a large crystal lattice shrinkage
(particularly, within the a-b plane). The calculated pore volume
reduction as a function of the lattice constant a decrease was
quantitatively shown in Figure 6.
Figure 7. Rietveld refinement of the experimental powder X-ray
diffraction pattern of activated 1a. The calculated pattern (line) well
corresponds to the experimental data (circles) as evidenced by the
difference pattern (line below observed and calculated patterns).
Vertical bars show the calculated positions of Bragg peaks. Goodness
of fit data: Rwp = 0.0868, Rp = 0.0635.
Comparing the experimental structures of the as-synthesized
phase and the desolvated phase, one can clearly see how the
supermolecular packing dramatically changes upon lattice
contraction. As shown in Figure 8(a,b), 1a transforms from
the op type to the np type upon guest removal, with a
significant void fraction decrease from 52.1% to 12.7%, as
calculated by PLANTON.16 The intersupermolecule 1-D pore
is almost completely diminished in the np structure.
While the internal cavity of each individual metal-
coordinated cavitand of 1a is largely remained, careful structural
comparison shows that the subunits of the cavitand also
undergo substantial distortion upon desolvation. The 2-(4H-
pyrazol-3-yl)pyridine moiety was twisted around the −CH2−
group, in which the angle between the Cphenyl and Npyrazol atoms
decreases from 103.1° to 95.8°, as illustrated in Figure 8(c,d).
Additionally, the L1 linker rotated around the Zn(II)···Zn(II)
axis in response to solvent removal. The dihedral angle between
the Zn4 plane and the phenyl ring of the L1 linker greatly
decreases from 50.8o to 28.7o (Figure S13), resulting in an
increase of the N···N distance across the cavitand from 16.2
to18.3 Å.
Noticeably, these findings represent an exceedingly rare
example where the structural transformation mechanism for the
discrete metal−organic architectures is fully elucidated.4a,15b It
also shows that theoretical calculations combining diffraction
measurements is a feasible strategy to uncover the mechanism
of reversible structural transition in porous crystals.
Gas Adsorption Induced Pore Opening in 1a and 1b.
The reversible structural transformation of the metal-coordi-
nated cavitands upon desolvation/solvation prompted us to
investigate their gas sorption properties, and how the structures
respond to adsorption/desorption of gas molecules. Measure-
ments were carried out on both activated 1a and 1b (1c is
omitted due to its lower porosity). We first tested N2
adsorption measurement at 77 K, and found that the N2
diffusion kinetics is very slow, most likely due to the narrow
pore openings of the np phases. Therefore, we focused on CO2
adsorption.
Figure 6. Calculated pore volume reduction of 1a with decreasing
lattice constant a upon desolvation.
Theoretical investigation not only revealed the mechanism of
the structural flexibility of 1a, but also provided us a reasonable
starting structural model to refine the PXRD data of the
activated 1a. Rietveld refinement of the PXRD data of activated
1a was carried out. A reasonable goodness of fit was achieved
(see Figure 7), and an experimental crystal structure was
obtained.
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Figure 8. Packing structures of 1a before (a) and after (b) desolvation. The structure transformation upon desolvation and resolvation is reversible.
Parts (c) and (d) show in more details the structural change of individual metal-coordinated cavitand unit.
As illustrated in Figure 9, CO2 adsorption in activated 1a and
1b exhibits interesting two steps, with hysteresis in desorption.
The presence of distinct adsorption steps in activated 1a and 1b
suggests that CO2 molecules are able to induce a structural
transformation between the contracted and the expanded
lattice. In the first step, the CO2 molecules get adsorbed into
the activated structure, and reach the first plateau in the
adsorption isotherm upon pore saturation. On the basis of the
CO2-196 K data of 1a, the first plateau (CO2 uptake ∼80 cc/g)
corresponds to a pore volume of ∼0.15 cc/g, close to the
calculated geometric pore volume of ∼0.12 cc/g of activated 1a.
This suggests that the first step of CO2 adsorption does not
involve significant lattice expansion (or pore opening), as
expected. With CO2 pressure increased to a critical point (often
called “gate pressure”), at ∼0.6 bar/196 K for 1a, the dense
intersupermolecule packing of the cavitands gets pushed open
to form an op structure. The CO2 adsorption proceeds into the
second step on the isotherm curve. The CO2 adsorption of 1b
at 196k is similar to that of 1a, and the first plateau corresponds
to CO2 uptake of ca. 95 cc/g (Figure 9b). There is no
significant lattice expansion (or pore opening). When the CO2
pressure increased to 0.33 bar (gate pressure), 1b was pushed
from the dense intersupermolecule packing structure to an op
structure. The CO2 adsorption increases sharply on the
isotherm curve. To certain extend, this gas adsorption induced
pore-opening process is analogous to the sample resolvation
process. Of course, the gas adsorption induced op structure
would not be as “open” as the fully solvated structure, since the
gas-cavitand interaction is much weaker than the solvent-
cavitand interaction. Indeed, the saturated CO2 uptake at 196 K
in 1a is ∼174 cc/g, corresponding to a pore volume of ∼0.33
cc/g, significantly lower than the theoretical “maximal” pore
volume of ∼0.56 cc/g of the as-synthesized 1a.
To further evaluate the CO2 binding nature, we performed
DFT-D calculations on activated 1a. We found that the initial
CO2 adsorption mainly takes place inside the cavity of the
metal-coordinated cavitand. Two strong adsorption sites are
identified, as schematically shown in Figures 10 and S14. Site I
is located near the upper rim of the cavitand, while site II is at
the center of the lower rim of the cavitand. The calculated static
CO2 binding energies are 31.6 and 27.7 kJ/mol, for the two
sites, respectively. These are consistent with the experimental
isosteric heat of adsorption Qst data (initial Qst ≈ 34.5 kJ/mol,
see Figure S15). Full occupation of the two sites inside the
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where majority of the adsorption takes place in the channel
pore.
For comparison purpose, CH4 adsorption in 1a/1b was also
investigated. Interestingly, from the isotherm data at 298 and
273 K, up to 65 bar, we do not observe a 2-step behavior
equivalent to that of the CO2 adsorption (Figure S16). The
CH4 uptakes of 1a and 1b are modest, ∼41 and 61 cm3(STP)/
g at 298 K and 65 bar, respectively. These are mainly from CH4
adsorption taking place inside the metal-coordinated cavitand,
similar to the first-step of CO2 adsorption. Clearly, the
interaction between CH4 and the cavitand is too weak to
induce a pore opening under the measurement condition, as
evidenced by the relatively low Qst of CH4 adsorption (Figure
S17) compared to that of CO2.
Permanent Porosity and Gas Adsorption in 2a. To
evaluate the potential of metal-coordinated cage crystals for
sorption related applications, we performed detailed gas
adsorption measurements, using 2a as a representative. Unlike
the three highly flexible metal-coordinated cavitand crystals, the
structure of 2a is relatively rigid, as shown by the PXRD
patterns before and after desolvation (Figure S18).
N2 adsorption isotherm at 77 K indicates that 2a has a
permanent porosity with a fully reversible type-I behavior
(Figure S19). The maximum N2 uptake is ∼320 cm3·g−1,
corresponding to a total pore volume of 0.499 cm3·g−1. The
experimental BET surface area is ∼1158 m2·g−1 (Langmuir
surface area ∼1302 m2·g−1). The pore size distribution derived
from the N2 adsorption data shows that the pore sizes are
mainly in the range of ∼6.5−14 Å (Figure S20a), consistent
with the geometric pore size analysis based on the crystal
structure (Figure S20b).
We next examined the high-pressure CO2 and CH4
adsorption capacities of 2a. The isotherm data are shown in
Figure 11. The CO2 adsorption of 2a can reach a maximum
uptake of 257 cm3(STP)/g at 40 bar and 298 K. CH4 uptakes at
298 K are ∼123 cm3(STP)/g and 157 cm3(STP)/g, at 35 and
65 bar, respectively, showing the potential of 2a as a promising
supramolecular porous material for CH4 storage.18 It is
instructive to compare the adsorption energetics of CO2/CH4
in 2a with those in 1a/1b. According to the experimental data
(Figure S21), the initial Qst values of CO2/CH4 in 2a are
approximately the same as those found in 1a/1b. This indicates
that the initial strong adsorption in the metal-coordinated cage
crystal takes place at locations similar to those found in the
metal-coordinated cavitand crystals, i.e., sites located inside the
cavitand.
Figure 9. CO2 total adsorption isotherms for 1a (a) and 1b (b),
measured at various temperatures.
CONCLUSIONS
■
In summary, we have demonstrated a design strategy for
assembly of metal-coordinated cavitands and tunable cages by
precise tuning of ancillary linkers. This strategy opens up a
versatile route to construct novel metal-coordinated cavitands
and cages. Remarkably, 1a−1c represent a new class of flexible
discrete metal−organic architectures, featuring a reversible
structural transition between the op and np phases in response
to guest molecules. Importantly, theoretical calculations
combined with diffraction measurements were successfully
employed to model the pore contraction/opening process, and
to elucidate the structural transformation mechanism. Our
metal-coordinated cage crystals (2a as a prototype) exhibit
permanent porosities and relatively high gas uptakes, and thus
may find promising applications for gas storage. Work is
currently under way in our lab to identify further potential
Figure 10. DFT-D-calculated CO2 adsorption sites inside the cavitand
of activated 1a. C and O of CO2 are shown in black and yellow,
respectively, for clarity.
cavitand corresponds to 10 CO2 per unit cell, i.e., an uptake of
∼51 cm3(STP)/g. This largely accounts for the first step of the
CO2 adsorption in activated 1a. In contrast, adsorption outside
the cavitand at low pressure is of much smaller amount due to
the narrow intersupermolecule pore space. At high pressure, a
widening of the intercavitand channel pore is triggered, leading
to the second step of CO2 uptake in the adsorption isotherm,
G
DOI: 10.1021/jacs.7b03169
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
AUTHOR INFORMATION
■
Corresponding Authors
*yangj808@nenu.edu.cn
*wzhou@nist.gov
*majf247@nenu.edu.cn
ORCID
Hui Wu: 0000-0003-0296-5204
Banglin Chen: 0000-0001-8707-8115
Wei Zhou: 0000-0002-5461-3617
Jian-Fang Ma: 0000-0002-4059-8348
Author Contributions
⊥These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (Grant No. 21471029 and 21301026),
and the state scholarship fund (Grant 201408360111) from
China Scholarship Council (CSC).
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.7b03169.
DFT calculations, synthetic procedures, characterization
details, gas adsorption isotherms, figures related to
crystals, Tables and other data (PDF)
Crystallographic data for 1a (CIF)
Crystallographic data for activated 1a (CIF)
Crystallographic data for activated 1a with CO2 (CIF)
Crystallographic data for 1b (CIF)
Crystallographic data for 1c (CIF)
Crystallographic data for 2a (CIF)
Crystallographic data for 2b (CIF)
Crystallographic data for 2c (CIF)
Movie evolving lattice contraction of 1a (AVI)
H
DOI: 10.1021/jacs.7b03169
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Journal of the American Chemical Society
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