3D cage Cofs: A dynamic three-dimensional covalent organic framework with high-connectivity organic cage nodes

Article abstract of DOI:10.1021/jacs.0c07732

Three-dimensional (3D) covalent organic frameworks (COFs) are rare because there is a limited choice of organic building blocks that offer multiple reactive sites in a polyhedral geometry. Here, we synthesized an organic cage molecule (Cage-6NH₂) that was used as a triangular prism node to yield the first cage-based 3D COF, 3D-CageCOF-1. This COF adopts an unreported 2-fold interpenetrated acs topology and exhibits reversible dynamic behavior, switching between a small-pore (sp) structure and a large-pore (lp) structure. It also shows high CO₂ uptake and captures water at low humidity (<40%). This demonstrates the potential for expanding the structural complexity of 3D COFs by using organic cages as the building units.

Full text of DOI:10.1021/jacs.0c07732

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Article
3D Cage COFs: A Dynamic Three-Dimensional Covalent Organic
Framework with High-Connectivity Organic Cage Nodes
QIANG ZHU, Xue Wang, Rob Clowes, Peng Cui, Linjiang Chen, Marc A. Little, and Andrew I. Cooper
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c07732 • Publication Date (Web): 06 Sep 2020
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3D Cage COFs: A Dynamic Three-Dimensional Covalent Organic
Framework with High-Connectivity Organic Cage Nodes
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Qiang Zhu,† Xue Wang,†‡ Rob Clowes,† Peng Cui,† Linjiang Chen,*†‡ Marc A. Little,*† Andrew I.
Cooper*†‡
†Department of Chemistry and Materials Innovation Factory, University of Liverpool, Liverpool, L7 3NY, UK.
‡Leverhulme Research Centre for Functional Materials Design, University of Liverpool, Liverpool, L7 3NY, UK.
Supporting Information Placeholder
ABSTRACT: Three-dimensional (3D) covalent organic frameworks (COFs) are rare because there is a limited choice of organic
building blocks that offer multiple reactive sites in a polyhedral geometry. Here, we synthesized an organic cage molecule
(Cage-6-NH2), which was used as a triangular prism node to yield the first cage-based 3D COF, 3D-CageCOF-1. This COF
adopts an unreported twofold interpenetrated acs topology and exhibits reversible dynamic behavior, switching between a
small-pore (sp) structure and a large-pore (lp) structure. It also shows high CO2 uptake and captures water at low humidity
(<40%). This demonstrates the potential for expanding the structural complexity of 3D COFs by using organic cages as the
building units.
topologies for COFs, and it is a central challenge for
expanding the functional scope of porous 3D frameworks.
INTRODUCTION
Covalent organic frameworks (COFs) have emerged as a
As with other porous 3D solids, such as zeolites,19 MOFs,6,
versatile class of crystalline porous solids that can be
20
and porous organic polymers,21 the interconnected pore
rationally designed and fine-tuned by synthesis to enhance
channels in 3D COFs may provide opportunities for superior
their functionality.1-4 The framework topology of a COF is
predetermined by the connectivity and geometry of the
building blocks and as for metal-organic frameworks
function,22, 23 such as enhanced molecular diffusivity. New
functions may also be possible: for example, inserting
rotatable dipolar linkers into certain framework topologies
could afford COFs with a net response to electric fields.24
Moreover, many porous 3D framework structures have
been shown to exhibit structural flexibility in response to
adsorption, as exemplified by breathing MOFs25 and
swellable porous polymers.26 Such complementary and
cooperative phenomena in porous solids can be beneficial in
applications such as controlled drug delivery, gas capture,
and size-selective catalysis.27-29 A key step to unlocking new
applications for COF materials is to increase the chemical
and structural diversity – for example, by accessing new
framework topologies using polyhedral organic nodes.
6
(MOFs),5, COF synthesis is often tolerant to changes in
linker size and chemical functionality. As such, different COF
topologies (or nets) can be purposefully targeted by
judicious choice of the building blocks.
To date, the majority of reported COFs are two-
dimensional (2D) layered structures with 1D pore channels,
where strong noncovalent interactions modulate the
interlayer packings.7 By contrast, COFs with 3D nets require
more than one non-planar building unit and these have
proven to be less synthetically accessible.4, 8 Consequently,
only eight distinct 3D nets in the Reticular Chemistry
Structure Resource (RCSR)9 have been reported for COFs:
dia,10 lon,11 bor,2 ctn,2 pts,12 rra,13 srs14 and ffc.15 The first
six of these were constructed using tetrahedral building
Organic cage molecules can be viewed as polyhedrons.30
Moreover, the vertices of a cage polyhedron may be
functionalized to link up with another building block for
framework structures31 or to expand its connectivity (e.g.,
12-arm octahedral cages32). Cages are therefore exciting
candidates for constructing 3D COFs with highly-connected
topologies. However, this strategy remains largely
unexplored because it is challenging to design and
synthesize organic cage molecules that are compatible with
reticular chemistry, that are shape-persistent, and that are
stable under COF synthetic conditions, not least because
organic cage molecules are often also assembled using
dynamic covalent chemistry.33 The first organic-cage-based
blocks with
4
points of extension, such as
tetraphenylmethane, tetraphenylsilane, adamantane, and
their derivatives.4 This is in stark contrast to endeavors that
have realized topologically more complex MOF nets,16
where metal clusters with up to 24 points of extension have
been reported.17, 18 However, organic molecules rarely have
a high number (>4) of reactive sites that are spatially
arranged to allow for extended inter-connection. This is a
major reason for the difficulty in broadening the range of
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COF was only recently reported, and this was a 2D net.34 The
cage molecule was used as a triangular node to connect with
a linear linker, yielding the prototypical 2D hcb net. To the
best of our knowledge, the use of organic cage molecules
with higher connectivity (more than 3 connecting sites) to
generate 3D COFs has not yet been reported.
(Fig. 1). NMR spectra of the model compound (SI, Figs. S6)
again indicated that the model compound had D3h symmetry
in solution, and the triangular prism-shape of the cage was
confirmed by single crystal X-ray structure (Fig. 1, SI, Table
S5 for refinement details). In the single crystal structure of
the model compound, the pendant amine groups were
distorted away from a perfect trigonal prismatic geometry,
although their geometry is likely to have been affected by
the close packing of the pendant aromatic groups in the
crystal structure (SI, Fig. S42).
Here, we synthesized a shape-persistent organic cage with
six pendant amine groups positioned in a trigonal prismatic
arrangement (Cage-6-NH2, Fig. 1). By reacting Cage-6-NH2
with the linear 2,5-dihydroxyterephthalaldehyde (DHTPA),
a 3D COF, 3D-CageCOF-1, adopting the acs topology, was
synthesized. In the extended structure of 3D-CageCOF-1,
there are two interpenetrated acs nets. 3D-CageCOF-1 was
found to undergo reversible transformations between a
large-pore (lp) and small-pore (sp) structure in response to
guest loading of dimethylformamide (DMF). Also, this
hydrophilic 3D-CageCOF-1 exhibited a high water uptake
(22 wt.%) at low relative humidity (40%) and a high carbon
dioxide uptake at 1 bar (204 mg/g, 273 K).
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The trigonal prismatic geometry of Cage-6-NH2, along
with its shape persistence in the solid state, makes it a
promising candidate for constructing several 3D topologies,
such as stp, tpt, sit, acs, and nia.40 Here we used the
combination of Cage-6-NH2 with a linear linker to target
COFs with acs nets according to reticular chemistry.35
DHTPA was chosen as the linear linker on the basis that
intramolecular hydrogen bonds between the hydroxyl
groups of DHTPA and the imine bonds in the COF product
might be beneficial for directing the supramolecular
assembly of the building blocks during synthesis and for
stabilizing the resulting COF structure.41 Atomistic COF
models were constructed using Cage-6-NH2 and DHTPA as
building blocks and the acs net as the blueprint (Fig. 2).42
Models of both the non-interpenetrated framework
structure and its twofold and threefold interpenetrated
forms were built (SI, Fig. S26). In contrast with tetrahedral-
based building units, which tend to form COFs with high
degrees of interpenetrations,11, 43 the bulky core of Cage-6-
NH2 enables the maximum interpenetration to be limited to
threefold, thus simplifying the task of solving the structure
by simulation, assuming that an acs net is formed. The
underlying structure of 3D-CageCOF-1 was analyzed by
powder X-ray diffraction (PXRD) combined with the
structural models, produced using the principles of reticular
chemistry.
CHO
O
NH2
HO
H2N
H2N
O
O
O
NH2
NH2
O
Dioxane, HOAc, MgSO4
O
NH2
Cage-6-NH2
Model compound
Figure 1. Synthesis of model compound and its single crystal
structure. Atom colors: C, grey; N, blue; O, red. H atoms are
omitted for clarity.
RESULTS AND DISCUSSION
The discovery of triangular prism-shaped building blocks
was important for increasing the topological diversity of
MOFs.35-38 However, triangular prisms have not been used
in COF synthesis because of the limited availability of
suitable building blocks. To address this, we synthesized the
triangular prism-shaped Cage-6-NH2 in two steps from a
previously reported hexa-nitro precursor that was reported
to have D3h symmetry (SI, Schemes S1-2).39 Importantly, the
cage structure of Cage-6-NH2 does not contain imine or
boronate ester bonds, which means that Cage-6-NH2 is, in
principle, compatible with a wide range of synthetic
conditions for COF formation. In addition, unlike planar
molecular building blocks that usually lead to layered COFs,
Cage-6-NH2 is decorated with 6 amine groups in a 3D
trigonal prismatic arrangement, ready to be extended in
three dimensions.
Despite screening several hydrothermal reaction
conditions for the reaction between Cage-6-NH2 and
DHTPA (SI, Figs. S7-8), we initially only isolated poorly
crystalline samples that had weak powder X-ray diffraction
(PXRD) patterns with a single peak at 5.3° (Cu-Kα, see SI,
Figs. S7-8). The nitrogen sorption derived Brunauer–
Emmett–Teller (BET) surface areas for these poorly
crystalline samples at 77 K were also low (ca. 230 m2 g-1, SI,
Fig. S12-13). We believe that the poor crystallinity of the
product was due to the poor solubilities of Cage-6-NH2 and
the oligomeric by-products in the reaction solvent, which
hindered reversibility and network self-healing during the
COF synthesis. Inspired by dynamic covalent chemistry
derived approaches for COF synthesis, aniline was
employed as a modulator.11 After optimizing the synthetic
conditions by including 7.5 molar equivalents of aniline per
Cage-6-NH2 as a modulator, and using 6M HOAc as the
catalyst, we produced a COF, 3D-CageCOF-1, with much
improved crystallinity (SI, Figs. S9-11).
The 3D symmetry of the amine groups in Cage-6-NH2 was
1
initially investigated in a solution using H and 13C NMR
spectroscopy (SI, Figs. S1-4), which indicated that the cage
had D3h symmetry like the previously reported hexa-nitro
precursor.39 To investigate this further, we synthesized a
model compound by reacting Cage-6-NH2 with 2-
hydroxybenzaldehyde, which has a phenol group that is
capable of forming hydrogen bonds with the cage molecule
2
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Figure 2. (a) Scheme for the synthesis of 3D-CageCOF-1 from Cage-6-NH2 and DHTPA, which can be topologically represented as a
triangular prism and a linear strut, respectively. Atom colors: C, white; N, blue; O, red. H atoms are omitted for clarity. (b, c) Two
views of an acs crystal net, where the purple nodes represent the cage-based building blocks, and (d, e) two comparable views of
the twofold interpenetrated acs-c net (c: catenated), with the cage nodes belonging to the different nets colored in purple or green.
The crystallinity of activated 3D-CageCOF-1 was
characterized by PXRD. By treating the reflections at 2.82°
and 5.65° (λ = 0.825015 Å) as the [100] and [200]
CageCOF-1 were 1040 and 1336 m2 g-1, respectively (SI,
Figs. S15-16). The measured BET surface area equates to
54% of the theoretical calculated nitrogen-accessible
surface area for the sp, twofold model shown in Fig. 3b.
The pore diameters derived by fitting to the nitrogen
isotherm are 8.8 and 5.6ꢀÅ, in a close agreement with the
pore diameters of the proposed model (8.5 and 5.1 Å; SI,
Fig. S17, Table S3 for comparison with other models).
Furthermore, the simulated nitrogen adsorption isotherm
uptake at 1 bar for the sp, twofold structure is also in very
good agreement with the experimental counterpart (SI,
Fig. S31).
reflections, respectively, in
a
trigonal/hexagonal
symmetry structure, we were able to determine that the
likely a=b cell edges of the activated 3D-CageCOF-1
structure were ~19.4 Å (SI, Fig. S27), in agreement with
the symmetry and cell dimensions that were calculated for
3
the activated model with P symmetry. Unfortunately, it
was not possible to unambiguously determine the c unit
cell edge from our PXRD data, and hence to produce a
meaningful fit, despite performing this measurement
using synchrotron radiation (SI, Fig. S27). However, by
using our structural models we were able to propose a
twofold interpenetrated acs-c net as the most likely
structure for 3D-CageCOF-1. The non-interpenetrated
structure optimized to a=b cell length of 24.0 , much
larger than the experimental values. Catenation of two
such COF networks led to a significant reduction in the cell
volume, owing to the strong non-covalent interactions
between the two networks, thus resulting in a contracted
structure referred to as a small-pore (sp) model in Fig. 3
and hereafter. The threefold interpenetration also
resulted in a contracted cell, similar in size to the twofold
cell. A comparison of these simulated models with the
experiment indicated that 3D-CageCOF-1 adopted an sp,
twofold interpenetrated structure (Fig. 3a; SI, Fig. S29).
Scanning electronic microscopy (SEM) indicated that
activated 3D-CageCOF-1 has a flake-like morphology (SI,
Fig. S32). Solid-state NMR cross-polarization magic angle
spinning (CP/MAS) confirmed the formation of imine
bonds at 164 ppm (SI, Fig. S35). Thermogravimetric
analysis (TGA) (SI, Fig. S36a) and differential scanning
calorimetry (DSC) (SI, Fig. S36b) indicate that the COF
starts to decompose at 400 °C.
Dynamic behaviors are common in 3D network
materials, and this is particularly well studied and
45
documented for MOFs.44,
Previous studies have also
revealed that 3D COF materials can reversibly respond to
stimuli, giving rise to applications in sensing46 and
47, 48
adsorption.27,
Initially, to investigate the chemical
stability of 3D-CageCOF-1, we immersed powdered
samples in a series of common organic solvents (MeOH,
CH3CN, THF, DMF) and water for 24 hours, washed the
samples by Soxhlet with acetone,
Nitrogen sorption analysis at 77 K revealed that 3D-
CageCOF-1 is microporous and has a type-II sorption
isotherm (SI, Fig. S14) with a pore volume of 0.50 cm3 g-1.
The BET and Langmuir surface areas calculated for 3D-
3
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