Dynamic Transformation between Covalent Organic Frameworks and Discrete Organic Cages

and Discrete Organic Cages

Communication

Dynamic Transformation between Covalent Organic Frameworks

⊥

Zhen Shan, Xiaowei Wu, Bingqing Xu, You-lee Hong, Miaomiao Wu, Yuxiang Wang,

Yusuke Nishiyama, Junwu Zhu, Satoshi Horike, Susumu Kitagawa, and Gen Zhang *

Cite This: https://dx.doi.org/10.1021/jacs.0c11073

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sı Supporting Information

ABSTRACT: We propose a dynamic covalent chemistry (DCC)-induced linker exchange strategy for the structural transformation

between covalent organic frameworks (COFs) and cages for the ﬁrst time. Studies have shown that the COF-to-cage and cage-to-

COF transformations were realized by using borate bonds and imine bonds, respectively, as linkages. Self-sorting experiments

suggested that borate cages and imine COFs are thermodynamic minimum compounds. This research builds a bridge between

discrete and polymeric organic scaﬀolds and broadens the knowledge of chemistry and materials for porous materials science.

ynamic covalent chemistry (DCC) involves a series of

Covalent organic cages are constructed by connecting

organic building blocks via reversible covalent bonds into

zero-dimensional (0D) discrete architectures with tunable,

reversible chemical reactions controlled by thermody-

namic equilibrium. In light of the principle of DCC, small

organic building blocks are covalently assembled into complex

molecular architectures such as discrete assemblies (macro-

designable, and functionalizable nanospace. Since organic

cages and COFs follow the principle of DCC and share many

common precursor building blocks such as amines, aldehydes,

cycles, cages, molecular knots, rotaxanes, catenanes, etc.)

−9

and inﬁnite polymeric scaﬀolds

(amorphous and crystalline

and boronic acid molecules for their synthesis, we assume

that the dynamic transformation between these two crystalline

materials could occur via reversible bond exchanges under

thermodynamic control. In this contribution, we chose

boronate ester-bonded and imine-bonded COFs and cages as

framework materials and discrete architectures, respectively,

and systematically studied the dynamic transformation

between these two distinct crystalline porous materials via

the linker exchange process.

polymers). The nature of DCC enables the dynamic

transformation of discrete assemblies under homogeneous

phase equilibrium-controlled conditions. However, the dynam-

ic transformation of inﬁnite polymers in heterogeneous phases

remains a signiﬁcant challenge.

Covalent organic frameworks (COFs) are an emerging class

of crystalline porous materials constructed by covalent linking

of organic building blocks into two-dimensional (2D) sheets

To investigate the dynamic transformation between COFs

and cages, we initially synthesized boronate ester-linked COFs

built from catechol-functionalized tribenzotriquinacene

or three-dimensional (3D) frameworks. Thanks to their

tunable and designable structures and properties, COFs have

attracted extensive research attention in the ﬁelds of gas

(TBTQ), a core-twisted nonplanar polyphenolic compound,

and p-phenylene diboronic acid (DP), denoted as COF-

NUST-1 (Scheme 1). In the FT-IR spectra, strong stretching

vibration bands attributed to the formation of a boronate ester

separation and storage, sensing, catalysis, organic

electronics, energy conversion and storage, and environ-

mental and biological applications. Although the formation

of COFs follows the DCC principle and many dynamic

covalent bonds have been used in their construction, there are

few studies on their transformations compared with their

analogues, metal−organic frameworks (MOFs).

recently, Zhao et al. described the ﬁrst examples of COF-

−1

ring of B−O linkages were observed at ∼1324 cm , and the

almost disappearance of characteristic O−H stretching bands

−

(∼3284 cm ) indicated the complete condensation of two

monomers (Figure S2 ), which was also conﬁrmed by 1D

Very

H, B single-pulse NMR spectra and 2D H- C heteronuclear

to-COF transformations through linker exchange. Horike and

correlation solid-state NMR spectrum (Figures S4, S5, and

S7 ). Thermogravimetric analysis (TGA) revealed that COF-

co-workers achieved the transformation of single COFs to

hierarchical/composite COFs via postsynthetic modiﬁcation.

Amorphous organic polymers can also be transformed into

NUST-1 exhibited no weight loss under N upon heating to

COFs, as disclosed by Zeng and co-workers. Nevertheless, to

date these studies have mainly been focused on the

transformation between framework materials with similar

structures and properties or the conversion of one discrete

architecture into another. However, the transformation

between crystalline framework materials and discrete structures

Received: October 20, 2020

has never been investigated.

XXXX American Chemical Society

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Communication

Scheme 1. Synthesis of COF-NUST-1 and Cage-NUST-1 and the Structural Transformation

Figure 1. (a) PXRD pattern and Le Bail ﬁtting for COF-NUST-1. (b) Top and (c) side views of COF-NUST-1. (d) H NMR spectra of Cage-

NUST-1 (500 MHz, CDCl for Cage-NUST-1, THF-d for TBTQ and TP). (e, f) Structure of discrete Cage-NUST-1 showing the cage cavity.

00 °C (Figure S29). Scanning electron microscopy (SEM)

wavy lattice propensity (Figure 1b,c). The experimental

PXRD pattern showed good agreement with the simulated

pattern, which was conﬁrmed by low residual values and

images and optical pictures showed that COF-NUST-1

possessed a uniform cubic morphology (see Figures 2b and

S31 ), and TEM images showed a layered structure (Figure 2c).

The crystalline nature and structure of COF-NUST-1 was

elucidated by PXRD analysis (Cu Kα) (Figure 1). The PXRD

pattern for COF-NUST-1 (Figure 1a, black curve) exhibited

main diﬀraction peaks at 3.5°, 5.9°, and 6.8°, which were

assigned to the (100), (001), and (110) facets, respectively,

along with other minor peaks at 8 −20 °. Structural modeling

and Le Bail reﬁnement ﬁtting (Figure S9 ) were conducted to

build the crystal structure for COF-NUST-1, and it was

predicted that the most probable structure is a 2D net with the

eclipsed AA stacking, which is mainly contributed by the

structural self-complementarity of the twisted nodes and the

acceptable proﬁle diﬀerences (R_w_p= 5.6% and R = 3.3%).

The porosity of COF-NUST-1 was measured by nitrogen

sorption analysis at 77 K. The adsorption curves exhibited the

type-IV isotherm (Figure 2e), a characteristic of mesoporous

materials. Application of the Brunauer−Emmett−Teller (BET)

−1

model resulted in a surface area of 410 m g for COF-NUST-

1. A QSDFT calculation provided a narrow pore size of

about 24 Å for COF-NUST-1, in good agreement with the

simulated value from the eclipsed AA stacking model (Figure

S8 ).

Cage-NUST-1 was prepared from TBTQ and o-phenylene

diboronic acid (TP) with monitoring by H NMR spectros-

Journal of the American Chemical Society

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Figure 2. (a) Transformation from COF-NUST-1 to Cage-NUST-1 with the obvious color change. (b) Optical image of COF-NUST-1. (c) TEM

image of COF-NUST-1. (d) Optical image of Cage-NUST-1. (e) N sorption curves at 77 K for COF-NUST-1 and Cage-NUST-1. (f) Time-

dependent H NMR spectra for the transformation from COF-NUST-1 to Cage-NUST-1 (500 MHz, CDCl ).

copy (Figure 1d). The disappearance of characteristic signals

of −OH in TBTQ and the chemical shift of protons in the

benzene ring of TP were observed, which proved the

To study the possible transformation between COFs and

cages, COF-NUST-1 was put into a ﬂask containing THF and

molecular sieves (MS), a typical condition that was used in

COF/Cage-NUST-1 growth, followed by the addition of TP

(equivalent mole ratio; Figure S17). It should be noted that

COF-NUST-1 is stable and cannot be dissolved in THF,

remaining as a precipitate in the bottom while the upper THF

solution is clear (Figure 2a). Interestingly, the THF solution

quickly became turbid in 30 min, and as time went on, the

turbidity gradually turned clear and the color switched from

light blue to bluish-violet within 48 h (Figure 2a). Time-

formation of Cage-NUST-1. H NMR spectra showed only

one set of signals for the individual protons, indicating the high

symmetry of the structure. SEM images revealed that Cage-

NUST-1 showed a rhombic morphology (Figure S31 ). The

porosity of Cage-NUST-1 was much lower than that of COF-

NUST-1 (Figure 2e), consistent with the results from reported

work and demonstrating that inﬁnite framework materials

possess the natural merit regarding porosity and capacity

compared with discrete cage compounds. Currently the

obtained crystals were too small (Figure 2d), so we were not

able to determine the structure by X-ray diﬀraction. It is worth

dependent H NMR spectra were recorded to further

understand the transformation process, and they revealed

that with the passing of time, the characteristic peaks of TP at

6.6−7.1 ppm (H , H , H , H ) shifted gradually to 7.2−7.9

noting that Cage-NUST-1 is isostructural to a reported cage.

Atmospheric-pressure chemical ionization mass spectrometry

showed a peak at m/z = 1518.32556 as the dominant signal,

which correlated exactly to the molecular mass of Cage-NUST-

ppm and the peak for TBTQ (H , 7.38 ppm) also shifted to

7.24 ppm, accompanied by the emergence of a characteristic

peak of DP (H , 6.8 ppm) (Figure 2f). This phenomenon

(Figures 1e,f and S13).

suggested that the transformation was a step-by-step process of

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Communication

exchange with the signiﬁcant color changes (Figure 2a). Cage-

NUST-1 was obtained by removing the solvent and puriﬁed by

a recrystallization procedure. The FT-IR and H NMR results

for Cage-NUST-1 validated the successful COF-to-cage

transformation (Figure S16).

The experiment aiming to realize the conversion from Cage-

NUST-1 to COF-NUST-1 was also performed using a similar

method (Figure S18). Unfortunately, we were not able to

obtain the COF through this eﬀort. The addition of DP into

the THF solution of Cage-NUST-1 yielded no precipitate (no

COF) at room temperature, and a small amount of solid was

collected as an amorphous polymer upon heating of the system

Cage-1 has a needlelike shape (Figure S30). COF-LZU-1 was

prepared from TFB and p-phenylenediamine (PD) through

solvothermal reactions according to a reported procedure.

First, the transformation experiments from COF-LZU-1 to

Cage 1 were implemented following a similar strategy as used

in the boronate ester-linked system. COF-LZU-1 was

combined with solvent (ethyl acetate, 1,4-dioxane, etc.) and

ED, and with or without acetic acid as a catalyst at room

temperature, no Cage -1 was aﬀorded, as characterized by

PXRD analysis (Figure S25 ). Since the imine (>CN−) bond

is stronger than that of −B−O− (615 vs 515 kJ/mol), we

surmised that it might not be possible to manipulate these

transformations at room temperature, and thus, control of the

temperature was involved in this respect.

(Figure S19 ). We supposed that the feasibility of the change

from COF-NUST-1 to Cage-NUST-1 may be due to the

thermodynamic self-sorting issue. To prove this point, we

combined TBTQ, TP, and DP together in THF and MS for 72

h. Only Cage-NUST-1 was obtained in this self-sorting process

We then further carried out the transformation from Cage-1

to COF-LZU-1 by solvothermal reactions. An ampule

containing Cage-1 was combined with 1,4-dioxane and PD

in the presence of acetic acid as a catalyst (Figure 3a). After 3

days at 120 °C, a yellow solid was harvested at the bottom,

which was veriﬁed to be COF-LZU-1 as ascertained by PXRD

analysis together with other techniques (Figures 3b, S21, and

S22 ).

Time-dependent PXRD patterns were characterized to study

the kinetic transformation (Figure 3b). A yellow precipitate

was aﬀorded as an amorphous polymer within the ﬁrst period

of 12 h. The diﬀraction peak of the (100) facet at 4.8°

appeared in 24 h, indicating that ordered frameworks emerged.

After 72 h, highly crystalline COF-LZU-1 with (100), (110),

and (001) diﬀraction peaks at 4.8°, 8.1°, and 26.0°,

respectively, was formed. SEM images revealed that Cage-1

possessed a spherical morphology while COF-LZU-1 showed a

Figure S20 ), which demonstrated that DCC exchange of

precursor building blocks was the main driving force for the

COF-to-cage transformation, and the hindrance in going from

cages to COFs is mainly determined by thermodynamic and

kinetic control. To test the generality of this transformation,

we synthesized another borate-linked COF, COF-NUST-2

(Figure S3 ). The transformation from COF-NUST-2 to Cage-

NUST-1 was also successfully realized (Figure S17b ).

Encouraged by above achievements, we sought to explore

whether other types of linkages in the COF/cage family can

undergo such dynamic transformation. The use of imine

linkages has been the most prevalent strategy to construct

3,28

COFs/organic cages since 2009,

so we turned to two

12b

reported imine-linked COF/cage systems, COF-LZU-1 and

Cage-1 for further studies. Cage-1 was synthesized from

triformylbenzene (TFB) and ethylenediamine (ED) as

coralloid shape (Figure S31). The N

adsorption curve

exhibited the microporous characteristic of COF-LZU-1. The

28a

−1

described previously. The H NMR spectrum and PXRD

BET surface area was 823 m g , which is almost the same as

that for the COF synthesized directly (Figure S22c), and the

porosity was signiﬁcantly improved in going from Cage-1 to

COF-LZU-1 (Figure 3c).

peak of Cage-1 were consistent with those of reported work

(Figures 3 b and S15 ). Optical micromorphology revealed that

In addition, other imine-linked 2D COFs can also be

obtained by this method. For a proof-of-concept study,

conducting solvothermal reactions among Cage-1, 1,4-dioxane,

and 1,3,5-tris(4-aminophenyl)benzene in the presence of acetic

acid led to the successful construction of COF-TFPA (Figures

S23 and S24).

A self-sorting experiment was also performed. TFB, ED, and

PD were dissolved into ethyl acetate in a molar ratio of 2:3:3 at

room temperature, and a yellow solid precipitated with or

without the presence of acetic acid. The precipitate could be

partially dissolved in chloroform and was detected as Cage-1

by H NMR spectroscopy (Figure S26), and the remaining

undissolved material was con ﬁ rmed to be amorphous polymer

as checked by PXRD (Figure S27). When a temperature of 120

C was imposed on this system, COF-LZU-1 was obtained in

the presence of acetic acid as a catalyst (Figure S28). The self-

sorting results indicated that the DCC process plays a crucial

role in determining the ﬁnal products via diﬀerent routes

(

temperature, catalyst, etc.).

In summary, we have developed a DCC-induced linker

exchange strategy for the structural transformation between

inﬁnite COFs and discrete cages. The transformation mainly

depends on the reversible covalent bonds linking COFs and

cages. The studies showed that when boronate esters were

used as linkages, the COF-to-cage transformation could be

achieved, and when imine bonding was applied, the cage-to-

Figure 3. (a) Synthesis of Cage-1 and COF-LZU-1 and the structural

transformation. (b) Time-dependent PXRD patterns for the trans-

formation from Cage-1 to COF-LZU-1. (c) Nitrogen sorption curves

for COF-LZU-1 and Cage-1.

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

https://pubs.acs.org/10.1021/jacs.0c11073

Communication

COF conversion was accomplished. The transformation

process was determined by time-dependent NMR spectrosco-

py and PXRD analysis. Self-sorting experiments indicated that

the boronate ester COFs and imine cages are the

thermodynamic minimum compounds that controlled the

transformation. This work paves a new way to study the

crystalline nature of organic materials. Further studies

involving insightful experiments/mechanisms that show how

to control the transformations in a more elaborate way to

prepare more crystalline materials (MOFs, COFs, cycles, cages,

etc.) are greatly needed and are proceeding in our lab.

Technology, Nanjing, Jiangsu 210094, China; orcid.org/

0000-0002-7518-9683

Satoshi Horike − Institute for Integrated Cell-Material

Sciences, Institute for Advanced Study, Kyoto University,

Kyoto 606-8501, Japan; orcid.org/0000-0001-8530-

6364

Susumu Kitagawa − Institute for Integrated Cell-Material

Sciences, Institute for Advanced Study, Kyoto University,

Kyoto 606-8501, Japan; orcid.org/0000-0001-6956-

9543

Complete contact information is available at:

ASSOCIATED CONTENT

sı Supporting Information

■

Author Contributions

Z.S. and X.W. contributed equally.

Notes

⊥

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/jacs.0c11073.

The authors declare no competing ﬁnancial interest.

Experimental procedures and characterization data

Technology, Nanjing, Jiangsu 210094, China; orcid.org/

PDF)

ACKNOWLEDGMENTS

■

This work was ﬁnancially supported by startup funding from

Nanjing University of Science and Technology (AE89991/194,

AE89991/258, AE89991/259, AD41913, and AD41960) and

the Natural Science Foundation of Jiangsu Province

BK20200472 and BK20200476). G.Z. acknowledges the

support of the Thousand Young Talent Plan.

AUTHOR INFORMATION

■

Corresponding Author

Gen Zhang − Key Laboratory for Soft Chemistry and

Functional Materials of Ministry of Education, School of

Chemical Engineering, Nanjing University of Science and

(

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