Synthesis and Characterization of an Isoreticular Family of Calixarene-Capped Porous Coordination Cages

Article abstract of DOI:10.1021/acs.inorgchem.0c03554

Functionalization of permanently porous coordination cages has been used to tune phase, surface area, stability, and solubility in this promising class of adsorbents. For many cages, however, these properties are intricately tied together, and installation of functional groups, for example, to increase solubility often leads to a decrease in surface area. Calixarene-capped cages offer the advantage in that they are cluster-terminated cages whose solid-state packing, and thus surface area, is typically governed by the nature of the capping ligand rather than the bridging ligand. In this work we investigate the influence of ligand functionalization on two series of isoreticular Ni(II)- and Co(II)-based calixarene-capped cages. The two types of materials described are represented as octahedral and rectangular prismatic coordination cages and can be synthesized in a modular manner, allowing for the substitution of dicarboxylate bridging ligands and the introduction of functional groups in specific locations on the cage. We ultimately show that highly soluble cages can be obtained while still having access to high surface areas for many of the isolated phases.

Full text of DOI:10.1021/acs.inorgchem.0c03554

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Article
Synthesis and Characterization of an Isoreticular Family of
Calixarene-Capped Porous Coordination Cages
Michael R. Dworzak, Meaghan M. Deegan, Glenn P. A. Yap, and Eric D. Bloch*
Cite This: Inorg. Chem. 2021, 60, 5607−5616
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ABSTRACT: Functionalization of permanently porous coordina-
tion cages has been used to tune phase, surface area, stability, and
solubility in this promising class of adsorbents. For many cages,
however, these properties are intricately tied together, and
installation of functional groups, for example, to increase solubility
often leads to a decrease in surface area. Calixarene-capped cages
offer the advantage in that they are cluster-terminated cages whose
solid-state packing, and thus surface area, is typically governed by
the nature of the capping ligand rather than the bridging ligand. In
this work we investigate the influence of ligand functionalization on
two series of isoreticular Ni(II)- and Co(II)-based calixarene-
capped cages. The two types of materials described are represented
as octahedral and rectangular prismatic coordination cages and can
be synthesized in a modular manner, allowing for the substitution of dicarboxylate bridging ligands and the introduction of
functional groups in specific locations on the cage. We ultimately show that highly soluble cages can be obtained while still having
access to high surface areas for many of the isolated phases.
can determine whether molecular materials, two-dimensional
MOFs, or three-dimensional MOFs are isolated.22,25 In
addition to this, functional groups have a pronounced effect
on thermal stability, solubility, crystal packing, and porosity.26
We have found that for most paddlewheel-based cages
solubility and porosity are inexorably linked where the highest
surface area cages are insoluble and the most soluble cages
display limited porosity.21
In addition to ligand-terminated paddlewheel-based cages
(Figure 1), functional groups can similarly be tuned in cluster-
terminated cages, either on the bridging ligand or on the
cluster caps. Ligand functionalization, for example, has been
used to control the nuclearity and solid-state packing in
zirconocene-based cages where the ratio between length and
width of the terephthalate-based bridging ligand has great
influence over the shape of the product cage(s).27 A subset of
cluster-capped permanently porous coordination cages are also
amenable to functionalization routes where either the bridging
or capping ligand can be modified to tune material proper-
ties.28,29 The CIAC (Changchun Institute of Applied
INTRODUCTION
■
Permanently porous coordination cages have received renewed
interest over the past few years as a result of their status as high
surface area molecular materials. In this regard, they have been
studied for gas separation,1,2 gas storage,3,4 catalysis,5,6 host−
guest chemistry,7 and a number of additional applications. As a
result of their molecular nature, they offer significant benefits
as compared to typical three-dimensional porous solids,
including metal−organic frameworks, with their potential
solubility making them well suited for solution-phase assembly
routes.8,9 In addition to this, their modularity allows for the
straightforward incorporation of functional groups10−15 and
provides a level of compatibility toward postsynthetic
modification strategies that is not possible for extended
structures.16−18 Functionalization of permanently porous
coordination cages has been used as a means to tune
solubility,19 gas adsorption properties,20 and bulk density21
and has been targeted at bridging ligands,22 capping ligands,23
and coordinatively unsaturated transition metal cation sites.24
For some porous coordination cages, however, ligand
functionalization can interfere with materials synthesis and
result in the formation of extended solids or nonporous
molecular materials. This is particularly the case for
paddlewheel-based cuboctahedral or octahedral structures
with isophthalic acid or carbazoledicarboxylic acid, respec-
tively. Both 5-position functionalization of isophthalic acid and
9-position functionalization of carbazoledicarboxylate have
been widely reported, and the nature of the functional group
Received: December 4, 2020
Published: March 30, 2021
https://doi.org/10.1021/acs.inorgchem.0c03554
© 2021 American Chemical Society
Inorg. Chem. 2021, 60, 5607−5616
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H2O,43 or Cl−.44 The μ4 species can also impart charge on
what would otherwise be a neutral cage, which can have
important implications for their stability, surface area, and
solubility.40
When TC4A and SC4A are used as SBUs, the rigidity of the
capping node allows for reliable structural regularity based on
the binding site angle of the bridging ligand. Specifically, when
a tritopic bridging ligand is used (i.e., trimesic acid), octahedral
molecular cages form with six SC4A metal cluster caps and
eight bridging ligands sitting in the faces of the octahedral
structure. When ditopic ligands (i.e., terephthalic acid,
isophthalic acid, etc.) are used, the resulting cage shape can
vary. For example, linear ligands still allow for octahedral
geometry but with 12 linear bridging ligands on the edges of
the structure, but when ligands of other angles become
introduced, the resulting geometry becomes more complex in
an effort to orient cap/cluster and ligand in the least strained
orientation (Figure 3).
Figure 1. Cuboctahedral (left) and octahedral (center) paddlewheel-
based cages represent ligand-terminated structures and are amenable
to a wide range of surface functionalization. Cyclopentadienyl-capped
zirconium cages (right) can be functionalized on the linear ligands
that typically comprise these structures.
Chemistry) and MOSC (metal−organic supercontainer)
families of porous cages are relevant examples in this regard
and use calixarenes, a tunable class of cyclic oligomers, to form
capped metal clusters as nodes in cage formation.23,30−34
Calixarene-type molecules have been isolated with up to 90
phenol units in their structures35 and are commonly
encountered as 4-unit oligomers as caps in porous cages.
These four phenol units can then be bridged by either CH2, S,
or SO2 units, resulting in calix[4]arenes (C4A),36 thiacalix[4]-
arenes (TC4A),37 or sulfonylcalix[4]arenes (SC4A),38,39
respectively. These most often contain tert-butyl groups on
their upper rim with the phenol hydroxyl groups on the lower
rim. Among these three types of calixarenes, SC4As (Figure 2)
Figure 3. Structures of octahedral calixarene-capped cages based on
terephthalate (top) and isophthalate (bottom). In these structures,
the 2- and 5-position of the ligand groups, respectively, are potentially
functionalizable. These structures have previously been termed type II
or type III MOSCs.
Four subclasses of sulfonylcalix[4]arene-capped cages have
been previously reported that were based on tritopic, linear
ditopic, 120° bent ditopic, and 109.5° bent ditopic ligands with
a variety of transition metals.29,45,46 Functionalization of these
cages includes altering the metal in the tetranuclear cluster,
adding or removing alkyl groups from the upper rim of the
SC4A cap, and adding functional groups to the bridging ligand
at a given position.29,45 The synthesis of these functionalized
isoreticular cages has been limited to a nickel cage made with
5-sulfoisophthalic acid monolithium salt (Ni(5-SO3)) and two
terephthalate-based nickel structures containing 2-amino-
terephthalic acid (Ni-(2-NH2)) and the other with 2-
bromoterephthalic acid (Ni-(2-Br)).44,45
Figure 2. Calixarene-based cages typically adopt octahedral structures
where bridging ligands lie at either the face (top left) or edge (top
right) of the octahedron. Isophthalate-based cages are related
structures and adopt a truncated octahedral geometry.
offer milder synthesis conditions due to the oxygen atoms of
the −SO2 groups. The use of oxygen as a coordination site
allows for the formation of a more ideal octahedral geometry
between the metal cation and coordination atoms as opposed
to a more strained geometry seen when using TC4A.40 As
such, they are compatible with tetranuclear clusters that have
been isolated for Mg, Fe, Co, Ni, and Zn with a variety of di-
and tricarboxylate ligands.41 The tetranuclear clusters in these
cages contain a μ4 atom at their center which has been
reported in the past as a variety of species including O, OH−,42
It is important to note that, in contrast to paddlewheel- or
zirconium-based cages, the solid-state packing of these types of
cages has been largely governed by the nature of the functional
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collected via vacuum filtration. The product was dried at 120 °C for
24 h prior to use.
groups on the calixarene caps rather than ligands on the
carboxylate ligands that comprise the structures. This offers a
unique opportunity to potentially tune the solubility and thus
processability of these cages without altering the structures of
the materials in the solid state. Here, we present the design,
synthesis, and characterization of eight novel calixarene-capped
porous cages, six of which have been characterized by single-
crystal X-ray diffraction.
Synthesis of [(Ni4OHsc4A)6(Btc)8]6− (Ni-Btc).29 To a 20 mL
scintillation vial, Ni(NO3)2·6H2O (145 mg, 0.5 mmol), 1,3,5-
benzenetricarboxylic acid (69.3 mg, 0.33 mmol), and SC4A (85 mg,
0.1 mmol) were added and solvated in DMF (10 mL). The vial was
heated to 100 °C in a dry bath for 24 h before cooling slowly to room
temperature. Partial precipitation of the product as a green crystalline
solid was observed upon cooling, with additional crystal growth
observed after standing for an additional 24 h at room temperature.
The material was activated for gas adsorption measurements at 110
°C.
EXPERIMENTAL SECTION
■
General Considerations. All handling of materials and syntheses
were performed in air unless otherwise specified. All solvents, metal
salts, and organic ligands were obtained from commercial sources and
used without further purification. Me-H2-bdc,47 OProp-H2bdc,22 and
OHept-H2bdc48 were prepared via previously reported synthetic
protocols.
Synthesis of [(Co4OHsc4A)6(Btc)8]6− (Co-Btc).29 To a 20 mL
scintillation vial, CoCl2·6H2O (119 mg, 0.5 mmol), 1,3,5-benzene-
tricarboxylic acid (69.3 mg, 0.33 mmol), and SC4A (85 mg, 0.1
mmol) were added and solvated in DMF (10 mL). The vial was
heated to 100 °C in a dry bath for 24 h before cooling slowly to room
temperature. Partial precipitation of the product as a green crystalline
solid was observed upon cooling, with additional crystal growth
observed after standing for an additional 24 h at room temperature.
The material was activated for gas adsorption measurements at 110
°C.
Physical Methods. Thermogravimetric analyses (TGA) were
performed with a TA Q5000 SA under an N2 flow. Samples were
loaded onto a tared aluminum pan and heated from room
temperature to 600 °C at a rate of 2 °C/min. Powder X-ray
diffraction patterns were collected by using a Bruker D8 X-ray
diffractometer with a LynxEye position-sensitive detector operating
Synthesis of [(Ni4OHsc4A)6(Bdc)12]6− (Ni-p-Bdc).45 To six 20
mL scintillation vials, Ni(NO3)2·6H2O (145 mg, 0.5 mmol), 1,4-
benzenedicarboxylic acid (54.8 mg, 0.33 mmol), and SC4A (85 mg,
0.1 mmol) were added and solvated in DMF (10 mL). The vials were
heated to 100 °C in a dry bath for 24 h before cooling slowly to room
temperature. Vapor diffusion of Et2O into the resultant mixtures
yielded the product as a green polycrystalline solid. X-ray quality
single crystals were obtained upon vapor diffusion with EtOAc rather
than Et2O. The material was activated for gas adsorption measure-
ments at 300 °C.
1
with a Cu Kα1 X-ray generator (λ = 1.54 Å). H and 13C NMR
spectra were measured by using a Bruker 400 MHz spectrometer, with
data processed by using MestReNova NMR software. Infrared (IR)
spectra were collected by using a Bruker ALPHA II ATR-IR
spectrometer with OPUS data processing software.
Gas Adsorption Measurements. Synthesized materials were
solvent exchanged with EtOH over the course of 3 days, replacing the
solvent once per day. Materials were obtained as free-flowing powders
by decanting the solvent and then evacuating the samples for 30 min
under dynamic vacuum at room temperature. Samples for gas
adsorption were activated under a positive nitrogen flow, with the
optimal activation temperature for each material determined by
increasing the activation temperature by 25 °C and recording single-
point N2/CO2 uptakes. Low-pressure gas adsorption measurements
up to 1.1 bar were recorded on a Micromeritics 3Flex gas adsorption
analyzer. N2 and CO2 surface areas were measured at 77 and 195 K,
respectively. Prior to measurements, samples were considered
activated when their outgas rate under static vacuum was ≤2 μbar/
min. All measurements were performed with ultrahigh-purity gases.
High-pressure isotherms were collected on a Sievert apparatus (PCT-
Pro-2000 from Hy-Energy Scientific Instruments) using ultrahigh-
purity gases. The total adsorption was calculated by using NIST
thermochemical properties of CH4 and the pore volume of each
material, as determined via 77 K N2 adsorption experiments.
Synthesis of p-tert-Butylsulfonylcalix[4]arene (TC4A). To a 1
L round-bottom flask, p-tert-butylphenol (64.5 g, 0.43 mol), elemental
sulfur (27.5 g, 0.86 mol), NaOH (8.86 g, 0.215 mol), and
tetraethylene gylcol dimethyl ether (19 mL) were added and stirred
under a flow of nitrogen. The mixture was heated to 230 °C over 4 h
and held at this temperature overnight. The mixture was allowed to
cool to room temperature, yielding a dark-brown solid to which
toluene (35 mL) and 4 M H2SO4 (78 mL) were added. The flask was
then sonicated for 30 min, and the solubilized material was transferred
to a separatory funnel. The organic phase was collected, and MeOH
(400 mL) was added, precipitating the product as a light-brown
powder that could be isolated by vacuum filtration. This process was
repeated until all of the initially generated solid was dissolved. The
collected product was dried at 120 °C for 24 h prior to use.
Synthesis of [(Co4OHsc4A)6(Bdc)12]6− (Co-p-Bdc).45 To six 20
mL scintillation vials, Co(NO3)2·6H2O (145 mg, 0.5 mmol), 1,4-
benzenedicarboxylic acid (54.8 mg, 0.33 mmol), and SC4A (85 mg,
0.1 mmol) were added and solvated in DMF (10 mL). The vials were
heated to 100 °C in a dry bath for 24 h before cooling slowly to room
temperature. Vapor diffusion of Et2O into the resultant mixtures
yielded the product as a red polycrystalline solid. X-ray quality single
crystals were obtained upon vapor diffusion with EtOAc rather than
Et2O. The material was activated for gas adsorption measurements at
275 °C.
Synthesis of [(Ni4OHsc4A)4(Bdc)8]4− (Ni-m-Bdc).46 To six 20
mL scintillation vials, Co(NO3)2·6H2O (145 mg, 0.5 mmol), 1,3-
benzenedicarboxylic acid (54.8 mg, 0.33 mmol), and SC4A (85 mg,
0.1 mmol) were added and solvated in DMF (10 mL). The vials were
heated to 100 °C in a dry bath for 24 h before cooling slowly to room
temperature. Vapor diffusion of Et2O into the resultant mixtures
yielded the product as a green polycrystalline solid. X-ray quality
single crystals were obtained upon vapor diffusion with EtOAc rather
than Et2O.The material was activated for gas adsorption measure-
ments at 250 °C.
Synthesis of [(Co4OHsc4A)4(Bdc)8]4− (Co-m-Bdc).46 To six 20
mL scintillation vials, Co(NO3)2·6H2O (145 mg, 0.5 mmol), 1,3-
benzenedicarboxylic acid (54.8 mg, 0.33 mmol), and SC4A (85 mg,
0.1 mmol) were added and solvated in DMF (10 mL). The vials were
heated to 100 °C in a dry bath for 24 h before cooling slowly to room
temperature. Vapor diffusion of EtOAc into the resultant mixtures
yielded the product as a red crystalline solid. The material was
activated for gas adsorption measurements at 150 °C.
Synthesis of [(Ni4OHsc4A)4(SO3−Li+-Bdc)8]4− (Ni-(5-Sulfo)).46
To a 20 mL scintillation vial, Ni(NO3)2·6H2O (145 mg, 0.5 mmol),
5-sulfo-1,3-benzenedicarboxylic acid monolithium salt (55.5 mg, 0.22
mmol), and SC4A (85 mg, 0.1 mmol) were added and solvated in
DMF (10 mL). The vials were heated to 100 °C in a dry bath for 24 h
before cooling slowly to room temperature. Partial precipitation of the
product as a green crystalline solid was observed upon cooling, with
additional crystal growth observed after standing for an additional 24
h at room temperature. The material was activated for gas adsorption
measurements at 325 °C.
Synthesis of p-tert-Butylsulfonylcalix[4]arene (SC4A). In a 1
L round-bottom flask, TC4A (7 g, 46.7 mmol) and sodium perborate
tetrahydrate (14 g, 91.0 mmol) were added to a mixture of
chloroform (210 mL) and acetic acid (350 mL). The resultant
solution was heated with stirring at 50 °C for 18 h. After the solution
cooled to room temperature, H2O (300 mL) was added, and the
solution was transferred to a separatory funnel. The organic layer was
collected, and the solvent was removed via rotary evaporation before
an excess of Et2O was added; the precipitated white solid was
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Synthesis of [(Co4OHsc4A)4(SO3−Li+-Bdc)8]4− (Co-(5-Sulfo)).
To a 20 mL scintillation vial, Co(NO3)2·6H2O (145 mg, 0.5 mmol),
5-sulfo-1,3-benzenedicarboxylic acid monolithium salt (55.5 mg, 0.22
mmol), and SC4A (85 mg, 0.1 mmol) were added and solvated in
DMF (10 mL). The vials were heated to 100 °C in a dry bath for 24 h
before cooling slowly to room temperature. Partial precipitation of the
product as a pink crystalline solid was observed upon cooling, with
additional crystal growth observed after standing for an additional 24
h at room temperature. The material was activated for gas adsorption
measurements at 325 °C.
Monolith Formation. To three glass cells with internal diameters
of 5 mm and an internal height of 66 mm, 6, 12.5, and 25 mg of Ni-
(2-Me) were dosed. Each sample was solvated in 1 mL of benzene
and frozen in liquid nitrogen. Quartz wool was then introduced at the
top of each glass cell on top of each frozen sample before being
transferred to gas adsorption measurement tubes, submerged in liquid
nitrogen. The samples were subjected to reduced pressure, and once
the vacuum pressure reached below 0.01 mbar, the liquid nitrogen
bath was removed and replaced with an ice bath which remained for
18 h or until the vacuum pressure returned to <0.01 mbar.
Single-Crystal X-ray Diffraction. X-ray structural analysis was
performed for Ni-(2-Methyl), Ni-(5-Methyl), Ni-(dobdc), Ni-(2-Br),
Ni-(5-OProp), and Co-(5-OHept). Crystal data and refinement
details are shown in Table 1. Crystals were mounted by using viscous
Synthesis of [(Ni4OHsc4A)6(methyl-Bdc)12]6− (Ni-(2-Methyl)).
To six 20 mL scintillation vials, Ni(NO3)2·6H2O (145 mg, 0.5 mmol),
2-methyl-1,4-benzenedicarboxylic acid (59.5 mg, 0.33 mmol), and
SC4A (85 mg, 0.1 mmol) were added and solvated in DMF (10 mL).
The vials were heated to 100 °C in a dry bath for 24 h before cooling
slowly to room temperature. Vapor diffusion of EtOAc into the
resultant mixtures yielded the product as a green crystalline solid. The
material was activated for gas adsorption measurements at 250 °C.
Synthesis of [(Ni4OHsc4A)4(methyl-Bdc)8]4− (Ni-(5-Methyl)).
To six 20 mL scintillation vials, Ni(NO3)2·6H2O (145 mg, 0.5 mmol),
5-methyl-1,3-benzenedicarboxylic acid (59.5 mg, 0.33 mmol), and
SC4A (85 mg, 0.1 mmol) were added and solvated in DMF (10 mL).
The vials were heated to 100 °C in a dry bath for 24 h before cooling
slowly to room temperature. Vapor diffusion of Et2O into the
resultant mixtures yielded the product as a green polycrystalline solid.
X-ray quality single crystals were obtained upon vapor diffusion with
EtOAc rather than Et2O. The material was activated for gas
adsorption measurements at 225 °C.
Table 1. Cage Solubility
cage
Ni-(btc)
Co-(btc)
a
b
c
d
e
f
g
h
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
i
s
s
s
s
s
s
i
i
i
i
i
i
s
s
s
i
i
i
s
s
p
s
s
s
i
i
i
i
s
i
i
i
i
i
i
i
i
i
i
p
s
i
i
s
i
i
i
i
i
i
i
i
i
i
s
i
i
i
Ni-(p-bdc)
Co-(p-bdc)
Ni-(m-bdc)
Co-(m-bdc)
Ni-(5-Sulfo)
Co-(5-Sulfo)
Ni-(2-Me)
Ni-(5-Me)
Ni-(2-Br)
p
i
s
s
i
i
i
i
i
s
s
s
s
s
s
s
s
s
s
i
p
s
s
p
s
s
i
p
s
s
s
s
i
Synthesis of [(Ni4OHsc4A)6(Br-Bdc)12]6− (Ni-(2-Br)). To six 20
mL scintillation vials, Ni(NO3)2·6H2O (119 mg, 0.5 mmol), 2-
bromo-1,4-benzenedicarboxylic acid (80.9 mg, 0.33 mmol), and
SC4A (85 mg, 0.1 mmol) were added and solvated in DMF (10 mL).
The vials were heated to 100 °C in a dry bath for 24 h before cooling
slowly to room temperature. Vapor diffusion of Et2O into the
resultant mixtures yielded the product as a green polycrystalline solid.
X-ray quality single crystals were obtained upon vapor diffusion with
EtOAc rather than Et2O. The material was activated for gas
adsorption measurements at 150 °C.
Ni-(5-Br)
Ni-(dobdc)
Ni-(5-OPr)
i
s
s
s
i
p
Co-(5-OHept)
s
a
b
c
d
e
DMF. Chloroform. Methylene chloride. Acetone. THF.
f
g
h
Benzene. Toluene. Ethyl acetate.
Synthesis of [(Ni4OHsc4A)4(Br-Bdc)8]4− (Ni-(5-Br)). To six 20
mL scintillation vials, Ni(NO3)2·6H2O (119 mg, 0.5 mmol), 5-
bromo-1,3-benzenedicarboxylic acid (80.9 mg, 0.33 mmol), and
SC4A (85 mg, 0.1 mmol) were added and solvated in DMF (10 mL).
The vials were heated to 100 °C in a dry bath for 24 h before cooling
slowly to room temperature. Vapor diffusion of EtOAc into the
resultant mixtures yielded the product as a green crystalline solid. The
material was activated for gas adsorption measurements at 225 °C.
Synthesis of [(Ni4OHsc4A)6(Dobdc)12]6− (Ni-(Dobdc)). To six
20 mL scintillation vials, NiCl2·6H2O (119 mg, 0.5 mmol), 2,5-
dihydroxy-1,4-benzenedicarboxylic acid (65.4 mg, 0.33 mmol), and
SC4A (85 mg, 0.1 mmol) were added and solvated in DMF (10 mL).
The vials were heated to 100 °C in a dry bath for 24 h before cooling
slowly to room temperature. Vapor diffusion of CHCl3 into the
resultant mixtures yielded the product as a green crystalline solid. The
material was activated for gas adsorption measurements at 250 °C.
Synthesis of [(Ni4OHsc4A)4(OProp-Bdc)8]4− (Ni-(5-OProp)).
To six 20 mL scintillation vials Ni(NO3)2·6H2O (145 mg, 0.5 mmol),
5-propoxy-1,3-benzenedicarboxylic acid (74.0 mg, 0.33 mmol), and
SC4A (85 mg, 0.1 mmol) were added and solvated in DMF (10 mL).
The vials were heated to 100 °C in a dry bath for 24 h before cooling
slowly to room temperature. Vapor diffusion of MeOH into the
resultant mixtures yielded the product as a green crystalline solid. The
material was activated for gas adsorption measurements at 275 °C.
Synthesis of [(Co4OHsc4A)4(OHept-Bdc)8]4− (Co-(5-OHept)).
To six 20 mL scintillation vials Co(NO3)2·6H2O (145 mg, 0.5 mmol),
5-heptoxyoxyisophthalic acid (92.4 mg, 0.33 mmol), and SC4A (85
mg, 0.1 mmol) were added and solvated in DMF (10 mL). The vials
were heated to 100 °C in a dry bath for 24 h before cooling slowly to
room temperature. Vapor diffusion of MeOH into the resultant
mixtures yielded the product as a pink crystalline solid. The material
was activated for gas adsorption measurements at 225 °C.
oil onto a plastic mesh and cooled to the data collection temperature.
Data were collected on a Bruker-AXS APEX II DUO CCD
diffractometer with Cu Kα radiation (λ = 1.54178 Å) focused with
Goebel mirrors. Unit cell parameters were obtained from 48 data
frames, 0.5° ω, from different sections of the Ewald sphere. The unit-
cell dimensions, equivalent reflections, and systematic absences in the
diffraction data are uniquely consistent with R-3 and R3 for Ni-(2-
Methyl), Ni-(dobdc), and Ni-(2-Br) and with C2/c and Cc for Ni-(5-
OProp). No symmetry higher than triclinic was observed for Ni-(5-
Methyl) and Co-(5-OHept). Refinement in the centrosymmetric
space group options, R-3, C2/c, and P-1, yielded chemically
reasonable and computationally stable results of refinement. The
data were treated with multiscan absorption corrections.49 Structures
were solved by using intrinsic phasing methods50 and refined with
full-matrix, least-squares procedures on F2.
Residual electron density, solvent molecules, and atoms that cannot
be assigned a reasonable model were treated as diffused electron
density by using the Olex251 implementation of Squeeze. In Co-(5-
OHept), the ligand alkyl chain was confirmed by digestion followed
by NMR spectroscopy, and thus the electron density that was
deducted from the diffused alkyl chain ends could be assigned. A
dimethylammonium ion was located H-bonded to two DMF solvent
molecules in Co-(5-OHept); thus, accessible voids that were too
small to fit DMF were assigned to contain dimethylammonium.
Although the assignments of void contents were based on the best
spatial and electron count fit of the predominant reaction solvents, we
cannot discount the possibility of alternative void contents that might
consist of a combination of water, minor solvents, thermal
decomposition products of amide solvents, or solvated metal ions.
In initial solutions of Ni-(dobdc) there appeared to be a heavy atom
containing group that was severely disordered and subsequently
removed from the model; hence, its formula weight could be
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underestimated. The structures are located on special positions: on 3-
fold axes for Ni-(2-Methyl) and Ni-(2-Br); on 6-fold improper axis for
Ni-(dobdc); on a 2-fold for Ni-(5-OProp); and on inversion for Ni-
(5-Methyl) and Co-(5-OHept).
The symmetry unique 2,5-dihydroxyl phthalate ligands in Ni-
(dobdc) were found disordered in two positions with independently
refined site occupancy ratios of 52/48 and 52/48. The bromoph-
thalate ligands in Ni-(2-Br) were found to be each disordered, in
multiple positions, were treated as rigid groups based on an ordered
bromophthalate ligand in a previously reported structure,52 and were
each restrained to have unitary occupancy per ligand.
These compounds consistently deposit as multiple, high mosaicity,
weakly diffracting crystals, and the results herein represent the best of
several trials. To compensate for the low-resolution data, various
constraints and restraints were applied. To further conserve a
reasonable data/parameter ratio, all atoms in Co-(5-OHept) and
the solvated cation complexes in Ni-(2-Methyl) and Ni-(5-OProp)
were refined with isotropic displacement parameters. All other non-
hydrogen atoms were refined with anisotropic displacement
parameters. H atoms in cocrystallized methanol and water molecules
and those on the hydroxyl groups in Ni-(dobdc) could be neither
located nor calculated and were ignored. All other H atoms were
treated as idealized contributions with geometrically calculated
positions and with Uiso equal to (1.2−1.5)Ueq of the attached carbon
atom. Atomic scattering factors are contained in the SHELXTL
program library. The structures have been deposited at the Cambridge
Structural Database under the following CCDC deposition numbers:
2016862−2016867.
upon cooling of the reaction mixtures. To obtain crystalline
material for the balance of the ligands investigated here, vapor
diffusion of an antisolvent into the reaction mixture was
required. Large green and red crystals of nickel- and cobalt-
based MOSCs, respectively, form upon diffusion of Et2O into
solutions in 20 mL scintillation vials over the course of 3−5
days. These reactions typically produced large amounts of
highly crystalline solids with overall yields ranging from 20% to
70% based on the calixarene cap. Diffraction-quality single
crystals were obtained via diffusion of EtOAc into DMF
solutions in 4 mL scintillation vials over the course of ∼1 week.
Notably, for the truncated octahedral (isophthalic acid-based)
cages, increasingly polar antisolvents were required as the bulk
and/or chain length of appended functional groups were
increased.
X-ray diffraction data collected for these materials confirmed
their isostructural nature and further showed that solid-state
packing of these structures was generally not perturbed by
ligand functionalization. For both terephthalate- and iso-
phthalate-based cages, each molecule generally crystallized in
the same space group as the unfunctionalized parent structures,
with control over solid-state packing dominated by the
interactions between the calixarene caps rather than
interactions between the incorporated bridging ligand
functionality. The previously reported nickel−terephthalic
acid cage and all novel 1,4-bdc derivatives reported here
crystallize in the rhombohedral space group R-3. For these
octahedral cages, the incorporated functional groups sit in the
pore windows of the cage with no apparent intercage
interactions between the ligand-appended groups, and all
structures show minimal deviations in their unit cell
parameters (all axes 1.5 Å). A similar trend is observed for
the isophthalate-based cages, most of which crystallize in the
triclinic space group P-1. For these materials, the structures
were packed with all of calixarene ligands oriented toward one
another in the same plane and incorporated functional groups
protruding toward the space between sheets of cages. This
perturbs the separation between sheets of cages and slightly
alters the way cages pack in each sheet. This results in some
expansion of the unit cell parameters for the truncated
octahedra upon incorporation of functional groups. For one
of the synthesized materials, Ni-(5-Methyl), the incorporation
of a functional group resulted in the crystallization of this
material in the monoclinic space group C2/c. This was not
generally observed upon incorporation of larger functional
groups, as the packing for the Co-(5-Heptoxy) structure was
analogous to other isophthalate-based calixarene cages. In this
case, two-dimensional layers of cages are more widely spaced
as a result of the ligand functional groups. Work further
exploring the generality of these trends are ongoing in our
group.
RESULTS AND DISCUSSION
■
The syntheses of calixarene-based coordination cages typically
rely on the use of high-temperature solvothermal or hydro-
thermal conditions where a one-pot reaction of metal salt,
bridging ligand, and calixarene cap affords the targeted cage. It
has generally been shown that relatively more forcing
conditions are required to afford thiacalixarene-based cages,
likely a result of the strained geometry at the metal cation sites
that comprise the materials. By comparison, sulfonylcalix[4]-
arene capped cages are isolable under more facile conditions.
In this work, synthesis of three novel octahedral and five novel
truncated octahedral structures was achieved via the reaction of
5-sulfoisophthalic acid monolithium salt (5-Sulfo bdc), 2-
methylterephthalic acid (2-Methyl bdc), 5-methylisophthalic
acid (5-Methyl bdc), 2-bromoterephthalic acid (2-Bromo
bdc), 5-bromoisophthalic acid (5-Bromo bdc), 2,5-dihydroxy-
terephthalic acid (dobdc), 5-propylisophthalic acid (5-Propoxy
bdc), and 5-heptoxylisophthalic acid (5-Heptoxy bdc) with the
appropriate metal salts under solvothermal conditions
affording Co-(5-Sulfo), Ni-(2-Methyl), Ni-(5-Methyl), Ni-(2-
Br), Ni-(5-Br), Ni-(dobdc), Ni-(5-Propoxy), and Co-(5-
Heptoxy), respectively. All of the materials based on
functionalized isophthalic acid ligands afford truncated
octahedral structures while the terephthalate-based cages
adopt octahedral structures.
The ability to maintain consistent solid-state packing across
an isoreticular series of porous coordination cages is somewhat
unusual as control over cage packing is often dominated by
interactions between incorporated functional groups. This has
been demonstrated for a number of extensively studied
coordination cages including isophthalate and carbazole
dicarboxylate-linked paddlewheel systems as well as Cp-capped
Zr cages.26 Significant changes to solid-state packing upon
functional group variation can influence the solubility for these
systems and limit rational approaches to enhancing their
solubility. Thus, calixarene-capped cages offer a somewhat
unique opportunity to vary material solubility simply through
For the preparation of functionalized cages, we screened the
reaction of the aforementioned ligands with hydrated cobalt-
(II) and nickel(II) nitrate and chloride salts in the presence of
capping ligand. Ultimately, the cages we report here are readily
obtained by reacting the appropriate metal salts and their
corresponding ligand in the presence of SC4A at 100 °C in
DMF for 24 h. Although moderate color changes were
observed over the course of reactions, and in contrast to the
synthesis of most MOFs and many coordination cages, no
solid product phase was observed throughout the solvothermal
reactions. However, M-(5-sulfo) (M = Co, Ni) crystals formed
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level of solubility in polar solvents, likely a result of their
charged nature. The parent cage structures, M-(btc), M-(p-
bdc), and M-(m-bdc) (M = Co2+, Ni2+), are moderately
soluble in DMF, chloroform, and methylene chloride. The
truncated structures are additionally soluble in acetone and
THF. With eight newly synthesized materials in hand, we
sought to survey the solubility of these cages as a function of
structure type and functional group. It is notable that the
solubility of as-synthesized cages was generally low and was
somewhat irreproducible from batch to batch. Solvent
exchange and activation of cages prior to solubility screening
gave straightforward results, and cages typically displayed
higher solubilities after such treatment. The results of these
tests are listed in Table 1. The solubility of octahedral cages
increases with the addition of functional groups on the
bridging ligands of the cage. For instance, while the parent
nickel cage is soluble in DMF, chloroform, methylene chloride,
and acetone, the same octahedral cage bridged by dobdc4−
ligands exhibits solubility in at least three additional solvents
including tetrahydrofuran, benzene, and toluene. Compara-
tively, truncated octahedral cages exhibit slightly different
solubility characteristics than octahedral cages when function-
alized. This is due to the location of the functional group
residing closer to the outermost periphery of the cage, lending
more influence toward solubility in comparison to tereph-
thalate-based cages. The parent cobalt and nickel m-bdc cages
have less solubility when compared to the hydrocarbon and
alkoxy-functionalized analogues reported here. The addition of
a simple methyl group on the isophthalate ligand leads to a
highly soluble cage that can also be dissolved in benzene,
toluene, and ethyl acetate. As seen in Table 1, increasing the
size of the appended functional group generally increases the
solubility of the cage. Conversely, Ni-(5-Sulfo) and Co-(5-
Sulfo) are insoluble in all nonpolar and most polar solvents we
employed with the exception of DMF, in which the cages
remained highly soluble. This is likely a result of the higher
charge of this cage (12−) as compared to cages based on
ligands with neutral functional groups (4−).
The solubility of coordination cages is not only important
for practical applications, where it can be leveraged for material
synthesis, purification, characterization, and utilization, it is an
important consideration in achieving high surface areas. In
most extended network solids, samples are thoroughly washed
with a high boiling synthesis solvent, typically an amide such as
DMF or DMA. After this process, they are exchanged with a
more volatile solvent to facilitate activation at more modest
temperatures. This procedure is particularly important for
MOFs with metal-bound solvent molecules as these are much
more difficult to remove than pore-bound solvent. Because of
the saturated coordination of the metal centers in each
tetranuclear cluster of these compounds, there are no exposed
metal sites for the coordination of solvent molecules. Given
their solubility in a range of solvents, particularly DMF, we
were able to forego exchanges with highly polar solvents. We
were somewhat limited, however, in which volatile solvents
could be used. Ultimately, room temperature washes with
EtOH were sufficient for removing DMF and unreacted metal,
ligand, or calixarene cap from the materials reported here. Fully
exchanged samples, as determined by the absence of a C−O
carbonyl stretch in the IR spectra of exchanged samples
(Figures S6−S24), were dried under dynamic vacuum at room
temperature before being transferred to gas adsorption tubes
for screening of optimal activation temperatures. For these, we
Figure 4. Solid-state packing of octahedral (a) and truncated
octahedral (b, c) cages. In both structures, cage−cage interactions
are governed by the tert-butyl groups on the cluster rather than
functional groups on the dicarboxylate ligands in a specific cage.
functional group incorporation while maintaining the same
solid-state packing and having a minimal effect on surface area.
As compared to carbazole dicarboxylate- or isophthalic acid-
based cages, calixarene-capped cages do display a moderate
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utilized flowing N2 rather than dynamic vacuum as it more
similarly represents the experimental parameters used in TGA
experiments (Figures S25−S32).
of these materials to the functionalized analogues reported
here, there is a slight decrease in uptake upon functional group
incorporation. The greatest decrease in gas adsorption in a
terephthalic acid cage is seen with Ni-(2-Br), which decreases
to 5.75 and 5.18 mmol/g for N2 and CO2, respectively, as
compared to 8.01 and 7.27 mmol/g. The decrease in
gravimetric uptake and surface area is expected given the
significantly increased molar mass of the 2-Br-bdc2− as
compared to p-bdc2−. The truncated cage with the greatest
decrease as compared to its unfunctionalized analogue is Co-
(5-Heptoxy), which displays saturation uptakes of just of 0.33
and 5.10 mmol/g for N2 and CO2, respectively. The trends in
gas uptake and surface area correlate with the positioning of
the functional group for each cage type. With the addition of a
functional group to an octahedral cage, the organic moiety sits
at the edge of the pore window. This reduces the availability
for gas to access the interior surface of the cage while the extra-
cage surface area is unchanged. On the other hand, upon
examination of truncated cages, the functional group must be
oriented away from the internal void of the cage; it does not
take up any space in the internal pore, although it can restrict
access to the pore which can inhibit uptake of gas. With the
addition of increasing sterically hindering groups this reduction
in pore window diameter only becomes more drastic, which
can explain the reduction in N2 adsorption in Ni-(5-Prop) and
Co-(5-Hept). Co-(5-OHept), for example, is minimally porous
to N2 while it has a CO2 accessible surface area on par with the
other cages.
As is often the case for MOFs, when comparing the optimal
activation temperatures of these materials with their respective
TGA measurements, there is no clear correlation between the
two. The TGA measurements are similar across all materials
tested where there is a slight decrease in weight percent due to
loss of solvent below 100 °C followed by a long plateau in
weight percent and apparent decomposition at ∼400 °C.
Regardless, the plateau between solvent loss and decom-
position shows no notable loss of weight that would give
insight into activation temperature. As a result, sample
activation screens were necessary to determine the optimum
activation temperature of each material. Temperature screens
were performed by activating samples at 25 °C intervals under
dynamic nitrogen flow from room temperature until gas uptake
decreased drastically (loss of ∼0.5−1.0 mmol/g). We generally
found that optimal activation, as judged by N2 or CO2
adsorption at 77 or 195 K, respectively, was achieved upon
heating samples to approximately 200−250 °C (Figures S33−
S45). Based on the evidence from the TGA measurements, all
solvent was evacuated from the system by 200 °C, which
suggests any change in uptake of N2 or CO2 from 200 to 400
°C is due to an alteration in the crystal packing of the system
and/or thermal decomposition.
Table 2. Surface Areas of Cages
The tunable solubility of these cages can be leveraged for
mechanical shaping and/or monolith formation. We have
previously focused on the densification of cage materials via
solvent processing. However, because of the increased recent
interest in mechanical shaping of MOFs, application of these
methods into cage systems is worth exploring. While methods
for manual shaping of MOFs, including pressing,53 spray
drying,54 and granulation,55 have been reported, methods
applied to porous cages have been limited. We initiated a
proof-of-concept study to show how functionalized calixarene-
capped cages can be used to create mechanically shaped
porous materials while still retaining the gravimetric uptake
and surface area of unaltered counterparts.
BET SA (m2/
g) (N2)
Lang SA
BET SA (m2/ Lang SA (m2/
cage
(m2/g) (N2)
g) (CO2)
g) (CO2)
Ni-(btc)
Ni-(p-
bdc)
Co-(p-
bdc)
Ni-(m-
bdc)
Co-(5-
Sulfo)
Ni-(2-
Methyl)
Ni-(5-
Methyl)
23029
52345
42345
80346
238
476
607
649
262
291
291
406
516
536
437
442
Ni-(2-Methyl), Ni-(5-Methyl), and Ni-(5-OProp), are
highly soluble in benzene which allows them to be isolated
as activated materials upon sublimation of solvent from frozen
samples. In this approach, the three cages largely retained their
shape immediately following this benzene freeze-drying (Bz
FD) procedure (Figure S140). Nitrogen adsorption isotherms
collected at 77 K confirmed that the cages retained gravimetric
surface area as compared to their counterparts activated under
more typical conditions with Ni-(2-Methyl)Bz FD, Ni-(5-
Methyl)Bz FD, and Ni-(5-OProp)Bz FD exhibiting BET
surface areas of 418, 425, and 421 m2/g, respectively (Figures
S108−S111). This supports the idea that manipulation of these
cages in the solid state has minimal influence over their ability
to adsorb gas making these materials good candidates for
mechanical shaping in the future. To further investigate
volume tuning of these materials, we focused on Ni-(2-Methyl)
as it maintains a robust structure upon freeze-drying, and it can
potentially be used to afford product phases with tunable
densities. Here, simply tuning the concentration of starting
solutions can be leveraged to adjust the density of shaped
product phases. Three solutions of constant volume and
varying cage concentrations were prepared, frozen, and the
Ni-(2-Br)
Ni-(5-Br)
Ni-
(dobdc)
Ni-(5-
OProp)
Co-(5-
OHept)
410
596
592
551
749
849
286
391
379
479
552
690
403
22
808
25
314
252
460
507
A clear advantage of calixarene-capped cages is their solid-
state packing is largely governed by the nature of the capping
ligand rather than the bridging di- or tricarboxylate ligands that
connect their tetranuclear clusters. As a result, while it is
expected that dramatic ligand functionalization will have a
negative effect on material surface area (Figures S92−S107),
dramatic tuning of solubility can be achieved. Surface areas of
previously reported cages generally support this hypothesis as
the gas adsorption characteristics of Ni-(p-bdc), Ni-(btc), and
Ni-(5-Sulfo) have been measured by Wang and co-workers and
show N2 uptakes (at 77 K and 1.0 bar) of 8.01, 8.96, and 5.13
mmol/g, respectively, and CO2 (at 195 K and 1.0 bar) uptakes
of 7.27, 7.70, and 5.69 mmol/g, respectively. Upon comparison
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solvent was sublimed to afford porous product where the
density and mechanical stability are highly tunable (Figure
S141).
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c03554
Funding
CONCLUSIONS
This material is based upon work supported by the U.S.
Department of Energy’s Office of Energy Efficiency and
Renewable Energy under the Hydrogen and Fuel Cell
Technologies and Vehicle Technologies Offices under Award
DE-EE0008813.
■
We have described a series of calixarene-capped porous
coordination cages that can be synthesized in a modular
fashion using a broad range of functionalized terephthalic and
isophthalic bridging ligands. The terephthalic acid-based cages
Notes
̅
presented in this work generally crystallize in the R3 space
The authors declare no competing financial interest.
group regardless of the functional group present on the
bridging ligand as cage−cage interactions are instead governed
by the nature of the calixarene cap. Similarly, the packing of
isophthalic acid-based cages in the solid state is largely based
on cap−cap interactions rather than the functional groups on
the 5-position of the isophthalic acid ligands they are based on.
The solubility of both types of cages increases significantly with
the addition of functional groups to their bridging ligands,
exhibiting subtle decreases in their surface areas. It is expected
that utilization of extended linear and bent linkers with
appropriate functional groups will lead to a large class of highly
porous cages that importantly still exhibit the hallmark
solubility of porous coordination cages. In addition, the
improved solubility of these materials compared to other cage
complexes offers potential benefits for a variety of applications,
most notably mechanical shaping for future applications in gas
adsorption that have virtually been exclusive to MOFs. These
observations motivate our ongoing interest in expanding the
chemistry of molecular metal−organic materials to comple-
ment studies of MOFs.
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■
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AUTHOR INFORMATION
Corresponding Author
Eric D. Bloch − Department of Chemistry and Biochemistry,
University of Delaware, Newark, Delaware 19716, United
States; orcid.org/0000-0003-4507-6247; Email: edb@
udel.edu
■
Authors
Michael R. Dworzak − Department of Chemistry and
Biochemistry, University of Delaware, Newark, Delaware
19716, United States
Meaghan M. Deegan − Department of Chemistry and
Biochemistry, University of Delaware, Newark, Delaware
19716, United States
Glenn P. A. Yap − Department of Chemistry and
Biochemistry, University of Delaware, Newark, Delaware
19716, United States
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