Linkage Engineering by Harnessing Supramolecular Interactions to Fabricate 2D Hydrazone-Linked Covalent Organic Framework Platforms toward Advanced Catalysis

Platforms toward Advanced Catalysis

Article

Linkage Engineering by Harnessing Supramolecular Interactions to

Fabricate 2D Hydrazone-Linked Covalent Organic Framework

Cheng Qian, Weiqiang Zhou, Jingsi Qiao, Dongdong Wang, Xing Li, Wei Liang Teo, Xiangyan Shi,

Hongwei Wu, Jun Di, Hou Wang, Guofeng Liu, Long Gu, Jiawei Liu, Lili Feng, Yuchuan Liu,

Su Ying Quek, Kian Ping Loh, and Yanli Zhao*

Cite This: https://dx.doi.org/10.1021/jacs.0c08436

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sı Supporting Information

ABSTRACT: Covalent organic frameworks (COFs) are an

emerging class of crystalline porous polymers with tailor-made

structures and functionalities. To facilitate their utilization for

advanced applications, it is crucial to develop a systematic

approach to control the properties of COFs, including the

crystallinity, stability, and functionalities. However, such an

integrated design is challenging to achieve. Herein, we report

supramolecular strategy-based linkage engineering to fabricate a

versatile 2D hydrazone-linked COF platform for the coordination

of diﬀerent transition metal ions. Intra- and intermolecular

hydrogen bonding as well as electrostatic interactions in the

antiparallel stacking mode were ﬁrst utilized to obtain two

isoreticular COFs, namely COF−DB and COF−DT. On account of suitable nitrogen sites in COF−DB, the further metalation

of COF−DB was accomplished upon the complexation with seven divalent transition metal ions M(II) (M = Mn, Co, Ni, Cu, Zn,

Pd, and Cd) under mild conditions. The resultant M/COF−DB exhibited extended π-conjugation, improved crystallinity, enhanced

stability, and additional functionalities as compared to the parent COF−DB. Furthermore, the dynamic nature of the coordination

bonding in M/COF−DB allows for the easy replacement of metal ions through a postsynthetic exchange. In particular, the

coordination mode in Pd/COF−DB endows it with excellent catalytic activity and cyclic stability as a heterogeneous catalyst for the

Suzuki−Miyaura cross-coupling reaction, outperforming its amorphous counterparts and Pd/COF−DT. This strategy provides an

opportunity for the construction of 2D COFs with designable functions and opens an avenue to create COFs as multifunctional

systems.

INTRODUCTION

atomic scale. In particular, the linkage chemistry is crucial for

the design of 2D COFs because highly ordered structures often

rely on the formation of reversible linkages to connect organic

■

Covalent organic frameworks (COFs) are a class of crystalline

porous polymers constructed by reticulating organic subunits

into extended one-dimensional (1D), two-dimensional (2D),

34−36

linkers through thermodynamic equilibria.

Such a feature

−4

allows for the formation of crystalline COFs through dynamic

error correction. However, the same feature also impairs the

stability of COFs toward solvents and chemicals. For example,

imine-linked COFs have both good reversibility and functional

compatibility under solvothermal conditions but are suscep-

tible to decomposition or exfoliation under acidic or alkaline

and three-dimensional (3D) structures via covalent bonds.

Beneﬁting from the seminal work of Yaghi et al. in 2005,

numerous COFs have been developed and have shown great

potential in a wide variety of ﬁelds, including gas capture and

−9

10−12

13−16

24−27

separation,

energy storage,

optoelectronics,

sens-

and

7−19

20−23

ing,

catalysis,

biomedicine.

environmental remediation,

37,38

8−30

aqueous conditions.

Beyond the role of bridging the

Among all COFs, the synthesis of 2D COFs

is governed by both the lateral extension of the organic

monolayer via a covalent bond and the vertical stacking of

conjugated multilayers through noncovalent forces, endowing

them with periodic columnar π-arrays and open 1D

Received: August 6, 2020

1−33

channels.

The selection of appropriate linkers and

linkages and their stacking modes provides eﬃcient ways to

tailor the structures and functionalities of 2D COFs at the

XXXX American Chemical Society

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Scheme 1. Diﬀerent Coordination Modes between Metal-Binding Units and Metal Species in 2D COFs

Transition metals are coordinated with metal-binding units (such as nitrogen atoms) distributed at (a) linkages (ref 20), (b) linkers or linkages

ref 58), (c) linkers (ref 59), and (d) linkers and linkages (this work). Noncovalent interactions are presented in the local region of the frameworks

(

for better visuality.

linkers, the types and conformation of linkages also have a

nitrogen atom) in 2D COFs could aﬀect the properties of the

resulting materials. Three coordination modes between the

metal-binding units and the metal species have been well

established using 2D COFs as scaﬀolds thus far (Scheme 1a−

pronounced inﬂuence on the properties of COFs, such as their

40−42

19,43,44

magnetic, electronic,

and optical properties.

Signiﬁcant eﬀorts have been devoted to strengthening the

stability of the frameworks through the covalent modiﬁcation

20,58,59

c).

However, these coordination modes often introduce

of the linkages. Some 2D COFs with linkages such as amide,

catalytically active sites into COFs at the expense of decreased

crystallinity and porosity. This dilemma can be circumvented

by introducing the fourth coordination mode where metal

species are coordinated with metal-binding units distributed at

the linker and linkage to form a ﬁve-membered ring

conformation within the linkages (Schemes 1d and 2). Such

a distinct coordination mode restricts the bond rotation of the

linkage to allow for extended π-electron conjugation in 2D

COFs. This arrangement would also enhance the interlayer

interactions and introduce additional functionalities to the

resultant frameworks.

5,46

45,47

thiazole,

oxazole,

amine, cyclic carbamate and

51,52

thiocarbamate, thieno[3,2-c]pyridine, quinoline,

tetra-

hydroquinoline, and α-aminonitrile have been obtained

with enhanced chemical stability and improved π-electron

conjugation. However, the resultant COFs often suﬀer from

decreased porosity, an irreversible transformation, and limited

functionalities. These unsatisfactory aspects suggest that the

covalent modiﬁcation of linkages is not an optimal strategy for

improving the crystallinity, stability, and functionality of 2D

COFs.

The utilization of noncovalent or supramolecular inter-

actions for the linkage engineering provides an alternative

general approach for the synthetic control of 2D COFs.

Diﬀerent from covalent bonding, supramolecular interactions

feature reversible bonding behavior upon external stimuli.

Recent explorations have revealed that the introduction of

appropriate noncovalent interactions, such as intra- or

Herein, we developed a linkage engineering strategy based

on the supramolecular principle to fabricate a versatile 2D

hydrazone-linked COF platform for the coordination of seven

transition metal ions. The resultant materials feature a

combination of extended π-conjugation, improved crystallinity,

enhanced stability, and additional functionalities. This out-

come was realized by restricting the bond rotation of linkages

in a stepwise fashion by taking advantage of multiple

supramolecular interactions. We ﬁrst fabricated two 2D

hydrazone-linked COFs (i.e., COF−DB and COF−DT) with

antiparallel stacking in which the linkages were partially

restricted by intramolecular and intermolecular hydrogen

bonding as well as electrostatic interactions. Beneﬁting from

its unique coordination mode, the porosity and stability of

COF−DB were further improved through metal coordination,

which simultaneously endowed the ensuing metalated COF−

DB (denoted as M/COF−DB) with additional functionalities.

In addition, metal ions conﬁned within the open channels of

31,54

intermolecular hydrogen bonding,

molecular dipole mo-

5,56

ments,

hydrophobic interactions, and donor−acceptor

(

D−A) interactions, are capable of tuning the porosity,

stability and stacking behavior of 2D COFs. To the best of our

knowledge, however, one of the most widely known supra-

molecular interactions, i.e., metal coordination, was barely

utilized to improve the porosity and stability of 2D COFs.

Although numerous examples have demonstrated that 2D

COFs can serve as host materials to chelate metallic guests,

very little attention has been paid to how diﬀerent

coordination modes of the metal-binding units (such as the

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Scheme 2. Syntheses and Structures of 2D Hydrazone-Linked (COF−DB and COF−DT) and Metalated COFs

M/COF−DB can be readily replaced via a postsynthetic metal

exchange. In particular, Pd/COF−DB showed enhanced

activity and recycling stability for the Suzuki−Miyaura cross-

coupling reaction in comparison with its amorphous counter-

parts and Pd/COF−DT. This approach not only lays a solid

foundation for the development of 2D COFs toward advanced

applications but also provides an inspiration for the design of

functionalized COFs through supramolecular interactions.

tions but diﬀerent positions of the nitrogen atoms, i.e., 5,5′,5″-

(benzene-1,3,5-triyl)tripicolinaldehyde (BTTPA) and 4,4′,4″-

(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde (TATTA), were

used as triangular tritopic linkers. Theoretically, we would be

able to prepare four kinds of 2D hydrazone-linked COFs

through the permutation of these linkers. To our delight, we

successfully synthesized two isoreticular COFs, namely COF−

DB and COF−DT, via an acid-catalyzed solvothermal

polycondensation reaction of DMTHA with BTTPA and

TATTA, respectively (see the Supporting Information for

synthetic details). Attempts to yield the other two COFs only

resulted in the formation of amorphous polymers, suggesting

the important role of intramolecular hydrogen bonding in

facilitating the crystallization of COFs. Various synthetic

conditions were extensively screened to optimize the

RESULTS AND DISCUSSION

■

Synthesis and Characterization. As shown in Scheme 2,

two types of hydrazides with and without methoxy groups, i.e.,

,5-dimethoxyterephthalohydrazide (DMTHA) and tereph-

thalohydrazide (THA), were selected as linear ditopic linkers.

Two kinds of aldehydes sharing identical chemical composi-

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Figure 1. (a) FT-IR spectra of COF−DB, corresponding monomers, and the model compound DB. (b) FT-IR spectra of COF−DT,

corresponding monomers, and the model compound DT. C CP/MAS spectra of (c) COF−DB and (d) COF−DT. Asterisks denote spinning

sidebands. The assignments of the C chemical shifts are indicated in the chemical structures. The signals at 21 ppm originated from the solvent.

e) 2D SAXS/WAXS patterns of COF−DB. (f) HR-TEM image of COF−DB. The scale bar is 2 nm.

(

crystallinity of COF−DB and COF−DT (see the Supporting

Information for all the conditions screened). Eventually, highly

crystalline COF−DB was obtained in a mixture of dioxane and

mesitylene (1:1 v/v) in the presence of acetic acid (aq., 6 M)

at 120 °C for 72 h. Likewise, COF−DT was synthesized in a

mixture of dioxane and mesitylene (1:1 v/v) in the presence of

acetic acid (aq., 3 M) at 170 °C for 72 h. The COF−DB and

COF−DT products were collected as grayish yellow and pale-

yellow powders, respectively, with isolated yields of 84.4% and

XPS spectra ﬁ rmly supported the formation of hydrazone

bonds (Figures S1 and S2).

The atomic scale construction of the two COFs was further

conﬁrmed by solid-state cross-polarization with magic angle

spinning (CP/MAS) C NMR spectroscopy. The character-

istic resonance signals attributed to the chemical shift of the

CN bonds were observed at 146 and 150 ppm for COF−DB

and COF−DT respectively, which provided overwhelming

evidence for the formation of hydrazone linkages. The

resonance signals of other carbon atoms, including the methyl

and phenyl groups, were in accordance with their chemical

structures (Figure 1c and d). Thermogravimetric analysis

(TGA) revealed that the low-temperature onsets for the ﬁrst

derivatives of the traces were at 305 and 351 °C for COF−DB

and COF−DT, respectively (Figure S3), indicating they both

had a high thermal stability. Field-emission scanning electron

microscopy (FE-SEM) and transmission electron microscopy

(TEM) veriﬁed the phase purity of COF−DB and COF−DT,

which revealed highly uniform coral-like and ﬁber-like

morphologies, respectively (Figures S4 and S5).

6.8%. Both are insoluble in common organic solvents.

The as-prepared COFs were ﬁrst assessed by Fourier-

transform infrared (FT-IR) spectroscopy. In the FT-IR spectra,

the stretching vibration bands at 1655 and 1658 cm⁻assigned

to the newly formed CN bonds were clearly observed for

COF−DB and COF−DT, respectively. They were identical to

their respective model compounds, suggesting the successful

formation of hydrazone moieties from the polycondensation of

the amino groups of DMTHA with the aldehyde groups of

BTTPA or TATTA. The FT-IR spectra also showed that the

stretching vibration bands of the amino groups from DMTHA

−

−1

(

ν -NH , 3298 cm ; ν -NH , 3204 cm ) and the aldehyde

Crystalline structures of the two COFs were also elucidated

by powder X-ray diﬀraction (PXRD) analysis with Cu Kα

radiation. As shown in Figure 2, a strong diﬀraction peak

corresponding to the (100) facet was observed at 2.33° in the

experimental PXRD proﬁle of COF−DB, which indicated that

it possessed a long-range ordered structure. In addition to the

(100) facet, diﬀraction peaks at 4.09, 4.78, 6.34, 8.12, 14.13,

16.38, and 25.80° were attributed to the (110), (200), (210),

(220), (600), (411), and (001) facets, respectively. The

−

groups from BTTPA (HC = O, 1706 cm ) or TATTA (HC =

−

O, 1704 cm ) were dramatically attenuated after poly-

condensation, indicating a considerably high degree of

polymerization (Figure 1a and b). The compositions of

COF−DB and COF−DT were then investigated by X-ray

photoelectron spectroscopy (XPS). Three intense peaks

corresponding to C 1s, O 1s, and N 1s signals were observed

in the XPS patterns of the two COFs, and their high resolution

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Figure 2. Experimental and simulated PXRD patterns and atomic structures for COF−DB (left section) and COF−DT (right section), which are

shown as follows: experimental, black; Pawley reﬁned, red; and diﬀerence, gray. Simulated structures of (a) antiparallel, (b) eclipsed, and (c)

staggered stacking. (d and e) DFT-optimized crystal structures of antiparallel stacked COFs. (d) Unit cells of COFs viewed from the direction

perpendicular to and across the xy-plane (the lattice box was omitted). (e) Side view illustrating intralayer and interlayer hydrogen bonding.

successful synthesis of COF−DB was also corroborated by

small and wide-angle X-ray scattering (SAXS/WAXS) experi-

ments (Figures 1e and S6 ), suggesting the highly crystalline

nature of the structure. The high-resolution TEM image

revealed that COF−DB had a 2D layered structure with an

interlayer distance of approximately 3.3 Å (Figure 1f), which

was consistent with the π−π stacking distance obtained from

the PXRD data. For COF−DT, the most intense diﬀraction

peak assigned to the (100) facet appeared at 2.29°, while other

diﬀraction peaks at 4.03, 4.70, 6.20, 8.19, 10.31, and 26.42°

were assigned to the (110), (200), (210), (310), (410), and

(001) facets, respectively.

To determine the exact structures of the two COFs, three

proposed structures based on antiparallel, eclipsed, and

staggered stacking were simulated by density functional theory

(DFT) and universal force ﬁeld calculations. The staggered

structures could be ruled out by comparing the experimental

and simulated PXRD patterns from 10 to 15°, whereas the

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Figure 3. (a) PXRD patterns and digital photographs of COF−DB and M/COF−DB. The raw data for the PXRD patterns of COF−DB and M/

COF−DB samples are provided in Figures S76 −S83 . (b) Close comparison of the PXRD patterns of COF−DB and M/COF−DB from 5 to 30°,

suggesting that the π−π stacking distance of M/COF−DB was smaller than that of COF−DB. (c) Nitrogen adsorption (solid) and desorption

(

open) isotherms measured at 77 K. The colors represent the same materials as those shown in panels a−c.

simulated PXRD patterns of the antiparallel and eclipsed

structures were in line with the experimental ones. Recent

studies have suggested that intramolecular and intermolecular

hydrogen bonding could promote the crystallization of

hydrazone-linked COFs, and the dipole moments of C-

strongly suggest that both COF−DB and COF−DT adopt

antiparallel stacking facilitated by intralayer and interlayer

hydrogen bonding as well as electrostatic interactions. The

experimental PXRD patterns for COF−DB and COF−DT

agree well with the predicted ones generated from the

antiparallel stacking model belonging to the P6CC space

group. The unit cell parameters of two COFs were optimized

by Pawley reﬁnement to give good results as follows: a = b =

−

(

δ )O(δ ) and O(δ )−R(δ ) could induce adjacent layers

56,60

to preferably adopt antiparallel stacking.

Thus, we then

compared DFT-optimized COF−DB and COF−DT structures

in both antiparallel and eclipsed stacking. The total energy of

the antiparallel-stacked structures was 3.62 and 3.06 eV per

unit cell (containing two layers) more stable than that of

eclipsed stacked ones for COF−DB and COF−DT,

respectively. The π−π interlayer distances simulated by DFT

and universal force ﬁeld calculations for the antiparallel-stacked

structures were 3.5−3.6 Å, whereas those for the eclipsed-

stacked structures were ∼3.8 Å, also indicating that the

antiparallel-stacked structures agreed better with the exper-

imental PXRD results (Table S1). Besides reducing the

electrostatic repulsion between atomic species with similar

partial charges, the antiparallel stacking also favors the

formation of interlayer hydrogen bonding. Speciﬁcally, Figures

S7 and S8 show antiparallel stacking results in a redistribution

of the charge with the O atom in the CO group gaining

more electrons, thus strengthening the interlayer hydrogen

bonding marked by “b” and “c” in Figure 2e. The existence of

intralayer N−H···O (2.070/2.063 Å) and interlayer N−H···O

41.95 Å, c = 7.23 Å, α = β = 90.00° and γ = 120°, with R_w_p

2.99% and R = 2.33% for COF−DB; a = b = 43.12 Å, c = 7.28

Å, α = β = 90.00° and γ = 120°, with R = 2.96% and R =

2.48% for COF−DT. As revealed by the diﬀerence plots

(Figure 2, gray curves), the reﬁned results are consistent with

the predicted unit cell parameters.

The permanent porosity of COF−DB and COF−DT was

assessed by N adsorption−desorption experiments at 77 K.

COF−DB showed sharp uptake below P/P = 0.01 with a step

between P/P = 0.01−0.35, which was in good agreement with

the type IV sorption model for mesoporous materials. COF−

DT exhibited a similar nitrogen adsorption isotherm as well

(Figure S9 ). The Brunauer−Emmett−Teller (BET) speciﬁc

surface areas were calculated using the adsorption data within

−1

the 0.15 < P/P < 0.30 range to give 632 and 747 m g for

COF−DB and COF−DT, respectively (Figures S10 and S11 ).

A nonlocal density functional theory (NLDFT) model was

applied to the isotherms of COF−DB and COF−DT to

estimate their pore size distributions and total pore volumes. A

narrow pore size distribution with an average pore width of 31

Å was observed for both COFs (Figure S9 ), which was in good

agreement with the theoretical pore sizes of the hexagonal

(

2.343/2.429 Å) as well as C−H···O (2.920/3.040 Å)

hydrogen bonding in the antiparallel-stacked structures for

COF−DB/COF-DT restricts the molecular bond rotation,

thus facilitating the crystallinity of these COFs. These results

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pores (ca. 35 Å). The total pore volumes of COF−DB and

identical to those of the parent COF−DB, with a type IV

sorption proﬁle. The pore sizes of M/COF−DB were

approximately 31 Å as well (Figure S18). Notably, the BET

surface areas of M/COF−DB were higher than that of the

−1

COF−DT were estimated to be 0.49 and 0.20 cm g ,

respectively, at P/P = 0.99.

Stability of COF−DB upon Exposure to Various

Solvents. To assess the chemical stability of COF−DB, the

as-prepared COF−DB sample was soaked in various solvents

under ambient conditions for 48 h, including tetrahydrofuran

−1

parent COF −DB (632 m g ), ranging from 728 to 1047 m

−

(Figures S19 −S25 ). The signiﬁcant improvement in the

speciﬁc surface area and negligible change in the pore size

distribution for M/COF−DB suggested that the introduction

of the metal species did not block the mesoporous channels,

while improving the regularity of the resultant frameworks.

The inﬂuence of metal coordination on the stability of the

frameworks was also investigated. The M/COF−DB samples

(

THF), CHCl , toluene, CH OH, 1,4-dioxane, dimethylfor-

3 3

mamide (DMF), and H O. As indicated in the PXRD proﬁles,

the samples dispersed in THF, CHCl , toluene, CH OH, and

,4-dioxane could preserve the crystalline structures, while

those exposed to DMF and H O showed a loss of the bulk

crystallinity (Figure S12 ). The FT-IR spectra of samples

immersed in various solvents exhibited characteristic peaks

corresponding to the hydrazone moieties, which were almost

identical to those of the parent COF−DB (Figure S13). These

experimental results demonstrated that hydrazone-linked

COF−DB was chemically stable in most solvents, and the

apparent loss of crystallinity should be attributed to the

exfoliation of multilayers rather than the hydrolysis of covalent

hydrazone linkages in COF−DB. A similar phenomenon was

were examined by PXRD again after incubating in H O or

DMF under ambient conditions for 48 h. The PXRD patterns

of six M/COF−DB samples except Cd/COF−DB were well

preserved, suggesting that the layered stacking structures of the

M/COF−DB were well retained without the exfoliation upon

exposure to H O and DMF (Figures S26 and S27). TGA

curves also revealed that these M/COF−DB samples had a

better thermostability than the parent COF−DB (Figure S28).

To highlight the unique advantages of metal coordination in

M/COF−DB, Pd/COF−DT and amorphous Pd/COF−DB

were selected and synthesized as representative models for

comparison (Figure S29). The SEM and TEM images revealed

also demonstrated by Dichtel et al. The easy exfoliation of

hydrazone-linked COF−DB in H O or DMF should be

ascribed to the partial bond rotation of linkages in the 2D

extended network, leading to relatively weak interlayer

interactions. We envisioned that introducing the coordination

interactions with metal ions could further restrict the bond

rotation of linkages and lock the 2D sheets in a more planar

conformation. This arrangement would not only increase the

degree of π-conjugation in the 2D networks but also enhance

the stacking interactions over the 2D sheets.

that, after the coordination with Pd(OAc) , the morphologies

became more aggregated for both samples (Figures S4 and

S5 ). Amorphous Pd/COF−DB showed an increased BET

−1

surface area (192 m g ) and optimized pore size distribution

−1

as compared with amorphous COF−DB (48 m g ), whereas

−1

the BET surface area of Pd/COF−DT (342 m g ) was found

−1

to be lower than that of COF−DT (747 m g ) with the

preservation of the pore size distribution (Figures S30 −S34 ).

The thermal stability of Pd/COF−DT showed a negligible

change after the metalation of COF−DT (Figure S35 ). The

above results suggested that diﬀerent coordination modes did

exert signiﬁcant an inﬂuence on the porosity and stability of

the resultant frameworks.

Improved Porosity and Stability Promoted by Metal

Coordination. The structural design of COF−DB allows it to

act as a versatile platform for the chelation of a metallic species.

Toward this goal, we carried out the metalation of COF−DB

with diﬀerent metal ions, which could be easily realized by

immersing COF−DB in saturated solutions containing M

salts (M = Mn, Co, Ni, Cu, Zn, Pd, and Cd). A distinct color

change was observed for the resultant M/COF-DB samples.

The unchanged PXRD patterns of M/COF−DB proved that

the ordered structure of COF−DB was well preserved after the

metalation (Figure 3a). The interactions between COF−DB

and various metal ions were further investigated by FT-IR

spectroscopy. In comparison with the FT-IR spectrum of

parent COF−DB for the CN stretching vibration (1655

Inﬂuence on the Properties of Diﬀerent 2D COFs

Exerted by Metal Coordination. To get deeper insight into

the improved porosity and stability promoted by the metal

coordination, two model compounds, DB and DT (see the

Supporting Information for their chemical structures and

synthetic details), were ﬁrst synthesized to investigate their

diﬀerent coordination modes in Pd(II)/DB and Pd(II)/DT.

H NMR spectroscopy was carried out in DMSO-d . Upon the

−

cm ), an obvious blue shift was observed in the spectra of M/

addition of Pd(OAc) , a new set of resonance peaks was clearly

−

COF−DB, ranging from 1657 to 1663 cm , suggesting the

coordination of the metal species at the chelation site (Figure

S14 ). The SEM images showed that M/COF−DB possessed

similar morphologies to the parent COF−DB (Figure S15 ).

Notably, the even distribution of metal ions in the samples was

conﬁrmed by the scanning electron microscope energy-

dispersive X-ray spectra (SEM-EDS, Figure S16). The well-

dispersed metal species (black dots) could be clearly observed

in the TEM images as well (Figure S17 ). The metal contents of

the M/COF−DB samples were determined by inductively

coupled plasma atomic emission spectroscopy (ICP-AES)

analysis (Table S2 ). These data provided compelling evidence

for the strong coordination of metal ions with the metal-

binding units in COF−DB.

observed besides free DB, suggesting the occurrence of strong

interactions between DB and Pd(II) in forming a Pd(II)/DB

complex. In contrast, the addition of Pd(II) into a solution of

DT produced a negligible change in the chemical shift,

implying that the interactions between DT and Pd(II) were

very weak (Figures S36 and S37 ). To obtain more information

on the structural change caused by the metal coordination, the

photophysical properties of DB and DT before and after

coordinating with Pd(OAc) were also recorded by UV/vis

spectroscopy. Both DB and DT exhibited three absorption

bands at 309, 320, and 348 nm, suggesting that DB and DT

possessed a similar degree of π-electron delocalization before

the coordination with the metal species. Upon adding

Pd(OAc)₂to the solutions containing the two model

compounds separately, no noticeable shift was observed in

the UV/vis spectrum of Pd(II)/DT as compared to that of free

DT, whereas the maximum absorption band of Pd(II)/DB

The porosity of M/COF−DB was assessed by nitrogen

adsorption−desorption experiments. As shown in Figure 3c,

the adsorption isotherms of M/COF−DB were almost

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exhibited a large red shift to 500 nm from 348 nm in free DB.

The magnitude of the bathochromic shift is approximately 150

nm, which could be ascribed to the formation of a greater

most of the Mn(II) or Co(II) was readily replaced by Pd(II)

ions under mild conditions (Figures S41 and S42). Thus, these

results suggested that COF−DB might serve as a promising

and versatile platform to obtain diﬀerent metalated COFs

through a postsynthetic metal exchange.

degree of π-conjugation via the coordination with Pd(OAc) . A

remarkable visible color change from colorless to yellow was

also observed (Figure S38). Such a phenomenon could be

attributed to diﬀerent coordination modes between the organic

ligands and metal ions. The main feature of DB lies in its

distinct coordination mode through the pyridine and

hydrazone nitrogen atoms to serve as a bidentate ligand to

form a ﬁve-membered ring conformation upon complexation

with a metal species. This complexation mode can eﬀectively

increase the degree of π-conjugation of the system. Compound

DT, on the other hand, only depends on the nitrogen atoms of

the hydrazone linkages to form a sandwich-like structure with

metal ions, which had hardly any inﬂuence on the degree of π-

electron delocalization.

Heterogeneous Catalysis for the Suzuki−Miyaura

Cross-Coupling Reaction. The development of recyclable

heterogeneous catalytic systems is the impetus for many

industrial applications. In terms of their practical use,

heterogeneous catalysts outperform their homogeneous

counterparts due to their easy separation from reaction

mixtures and eﬃcient performance in continuous ﬂow

processes. The metalated COFs may serve as promising

systems for heterogeneous catalytic reactions. Given its good

crystallinity, stability, and functionality, Pd/COF−DB was

selected as a heterogeneous catalyst to investigate the Suzuki−

Miyaura cross-coupling reaction, which was well-known as a

useful method to form C−C bonds. As presented in Table 1,

A similar phenomenon was also observed for π-conjugated

COFs, and these interactions between metal-binding units and

metal ions were further ampliﬁed to exert an important

inﬂuence on the properties of the resulting materials. The

lowest-energy absorption band of Pd/COF−DB red-shifted to

a larger extent as compared to that of Pd/COF−DT, resulting

in a distinct color change from yellow to red (Figures 3a and

S39). The diﬀerence could be ascribed to the diﬀerent

coordination modes between Pd/COF−DB and Pd/COF−

DT. For M/COF−DB, such as Pd/COF−DB, the metal

species coordinated with the metal-binding units (nitrogen

atoms) distributed at linkers and linkages could suppress the

bond rotation of the linkages and lock the hexagonal 2D sheets

in a more planar conformation. This arrangement could be

veriﬁed by comparing the PXRD patterns of COF−DB and

M/COF−DB. The presence of the diﬀraction peak assigned to

the (001) facet revealed that the M/COF−DB samples had an

ordered arrangement of multilayers, which corresponded to the

π−π stacking distance between adjacent layers. The π−π

stacking distance in all M/COF−DBs, ranging from 3.43 to

Table 1. Catalytic Activity Tests of Pd/COF−DB in the

Suzuki−Miyaura Cross-Coupling Reaction

entry

time (h)

yield (%)

CH₃

CH O

NO₂

CH₃

CH O

NO₂

CHO

0.5

Reaction conditions (unless otherwise noted) are as follows: a

solution of aryl halide (1.0 mmol), phenylboronic acid (183 mg, 1.5

mmol), K CO (276 mg, 2 mmol), and Pd/COF−DB (5 mg) in 5

.35 Å, was smaller than that of the parent COF−DB (3.45 Å)

mL of EtOH/H O (1:1 v/v) was stirred at 80 °C for 30 min. Yields

were determined by gas chromatography. No catalyst was added.

(Figure 3b). Therefore, M/COF−DBs should have more

compact structures as a result of the synergistic supramolecular

interactions that increased the stacking interactions between

adjacent 2D layers in M/COF−DB, leading to extended π-

conjugation, improved crystallinity, and enhanced stability. By

contrast, the π−π stacking distance of M/COF−DTs, ranging

from 3.37 to 3.38 Å, was slightly larger than that of the parent

COF−DT (3.37 Å) (Figure S39 ). This result suggests that the

introduction of the routine coordination mode, where metal

species coordinate with metal-binding units (nitrogen atoms)

distributed at the linkages, could only weaken the interlayer

interactions to some extent. The loosely stacked layers may

account for the obvious decrease of the crystallinity and

stability of Pd/COF−DT.

Postsynthetic Metal Exchange. The dynamic nature of

coordination bonding inspired us to explore the possibility of

metal exchange in M/COF−DB. M/COF−DB (M = Mn and

Co) samples were selected and immersed in saturated

solutions containing Pd(II) at ambient conditions for 12 h.

The successful transformation of M/COF−DB (M = Mn and

Co) into M′/COF−DB (M′ = Pd) was demonstrated by the

dramatic change in the color of the samples and a negligible

diﬀerence in the PXRD patterns (Figure S40). SEM-EDS

analysis also revealed the homogeneous distribution of Pd(II)

ions in the exchanged Pd/COF−DB sample, indicating that

both electron-donating (entry 3) and electron-withdrawing

(entry 4) aryl iodide substrates gave rise to the expected biaryl

products in excellent yields (95−99%) within 30 min (entries

1−4). Similarly, high yields (95−99%) could also be obtained

for the less active aryl bromides (entries 5−9), suggesting the

superb eﬃciency of Pd/COF−DB for the Suzuki−Miyaura

reaction.

The recyclability and durability of Pd/COF−DB was further

assessed by subjecting it to four consecutive catalytic cycles

using bromobenzene and phenylboronic acid as reactants. It

was found that the catalytic activity of Pd/COF−DB could be

well maintained at a high level. However, Pd/COF−DT and

amorphous Pd/COF−DB showed a signiﬁcant loss of catalytic

activity after four cycles of testing, although their fresh catalysts

exhibited good catalytic activities (Figure S43). The well-

preserved ordered structure of the recycled Pd/COF−DB was

also veriﬁed by PXRD and SEM analysis, showing an

insigniﬁcant decrease in the diﬀraction intensity and an almost

unchanged morphology. In contrast, a signiﬁcant decrease in

the intensity of the PXRD pattern was observed for both Pd/

COF−DT and amorphous Pd/COF−DB, suggesting their

lower stability compared to Pd/COF−DB (Figure S44). The

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

https://pubs.acs.org/doi/10.1021/jacs.0c08436.

Article

low stability of Pd/COF−DT and amorphous Pd/COF−DB

was further supported by the aggregated morphologies from

SEM images (Figure S45 ). We then conducted a leaching

experiment to demonstrate that the reaction was indeed

catalyzed under heterogeneous conditions. Pd/COF−DB was

reﬂuxed under the same reaction conditions in the absence of

reactants. ICP-AES analysis of the ﬁltrate showed that less than

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

■

Experimental procedures, characterization data, supple-

mental ﬁgures, and computational details (PDF)

.0 ppm of Pd content was detected, indicating that the

coordination of Pd(OAc) with COF−DB is strong enough

AUTHOR INFORMATION

and the catalytic reaction proceeds through a heterogeneous

process. Based on the experimental results and previous

■

Corresponding Author

3,64

reports,

a plausible reaction mechanism for the formation

Yanli Zhao − Division of Chemistry and Biological Chemistry,

School of Physical and Mathematical Sciences and School of

Materials Science and Engineering, Nanyang Technological

University, 637371, Singapore; orcid.org/0000-0002-9231-

of biaryls catalyzed by Pd/COF−DB is outlined in Figure S46.

The superior catalytic activity and stability of Pd/COF−DB

could be attributed to the unique coordination mode after the

introduction of the metal species. Pd/COF−DB preserves its

periodic columnar arrays and 1D open mesopore channel well,

providing easy access to the active sites as well as the fast

diﬀusion of the reactants and products. In addition, the ICP-

AES analysis showed that the Pd content in Pd/COF−DB was

360 ; Email: zhaoyanli@ntu.edu.sg

Authors

Cheng Qian − Division of Chemistry and Biological Chemistry,

School of Physical and Mathematical Sciences, Nanyang

Technological University, 637371, Singapore; orcid.org/

0000-0002-1392-6975

.1× higher than that of Pd/COF−DT (Table S3), indicating

higher Pd loading for the former. Therefore, the coordination

mode in Pd/COF−DB not only serves as a noncovalent force

to stabilize the ordered structure but also facilitates the binding

of more metal ions on the COF scaﬀold for better catalytic

eﬃciency. The crystallinity, stability, and functionality can be

simultaneously integrated into Pd/COF−DB through the

combination of synergistic supramolecular interactions.

Weiqiang Zhou − Division of Chemistry and Biological

Chemistry, School of Physical and Mathematical Sciences,

Nanyang Technological University, 637371, Singapore

Jingsi Qiao − Centre for Advanced 2D Materials, National

University of Singapore, 117546, Singapore

Dongdong Wang − Division of Chemistry and Biological

Chemistry, School of Physical and Mathematical Sciences,

Nanyang Technological University, 637371, Singapore;

orcid.org/0000-0002-6278-0706

CONCLUSION

■

We have successfully developed a supramolecular strategy to

conduct linkage engineering to fabricate a versatile 2D

hydrazone-linked COF platform, demonstrating that diﬀerent

coordination modes have a signiﬁcant inﬂuence on the

properties of the resultant frameworks. Experimental and

computational studies reveal that inter- and intramolecular

hydrogen bonding as well as electrostatic interactions in

dipole-induced antiparallel stacking work together to partially

restrict the bond rotation of linkages within 2D COFs, laying

the foundation for the successful synthesis of COF−DB and

COF−DT. The bond rotation of the linkages in COF−DB can

be further restricted through the metal coordination, leading to

a more rigid and stable linkage conformation. This

conformation greatly improves the planarity of the 2D sheets

and strengthens the interlayer interactions. Furthermore,

COF−DB can serve as a versatile platform to chelate seven

diﬀerent divalent transition metal ions under mild conditions.

The resultant M/COF−DB samples exhibit extended π-

conjugation, improved crystallinity, enhanced stability, and

additional functionalities relative to the parent COF−DB. In

addition, metal ions anchored on the ordered channel walls of

M/COF−DB can be readily replaced via a postsynthetic metal

exchange. On account of the synergistic supramolecular

interactions, Pd/COF−DB shows excellent catalytic activity

and stability for the Suzuki−Miyaura cross-coupling reaction as

compared to its amorphous counterparts and Pd/COF−DT.

This work provides inspirations to design 2D COF-based

platforms for diverse applications. We believe that the

supramolecular design strategy described herein would

complement the existing covalent modiﬁcation approach and

promote the further development of 2D COFs for more

advanced applications.

Xing Li − Department of Chemistry, National University of

Singapore, 117543, Singapore

Wei Liang Teo − Division of Chemistry and Biological

Chemistry, School of Physical and Mathematical Sciences,

Nanyang Technological University, 637371, Singapore

Xiangyan Shi − Division of Chemistry and Biological Chemistry,

School of Physical and Mathematical Sciences, Nanyang

Technological University, 637371, Singapore; orcid.org/

000-0002-3784-5889

Hongwei Wu − Division of Chemistry and Biological Chemistry,

School of Physical and Mathematical Sciences, Nanyang

Technological University, 637371, Singapore

Jun Di − School of Materials Science and Engineering, Nanyang

Technological University, 639798, Singapore; orcid.org/

000-0002-6232-6466

Hou Wang − Division of Chemistry and Biological Chemistry,

School of Physical and Mathematical Sciences, Nanyang

Technological University, 637371, Singapore

Guofeng Liu − Division of Chemistry and Biological Chemistry,

School of Physical and Mathematical Sciences, Nanyang

Technological University, 637371, Singapore

Long Gu − Division of Chemistry and Biological Chemistry,

School of Physical and Mathematical Sciences, Nanyang

Technological University, 637371, Singapore

Jiawei Liu − Division of Chemistry and Biological Chemistry,

School of Physical and Mathematical Sciences, Nanyang

Technological University, 637371, Singapore; orcid.org/

0000-0003-4011-3950

Lili Feng − Division of Chemistry and Biological Chemistry,

School of Physical and Mathematical Sciences, Nanyang

Technological University, 637371, Singapore; orcid.org/

0000-0001-9459-6594

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Y.; Li, Z.; Chen, J.; Lu, Z. Tunable Redox Chemistry and Stability of

Article

Yuchuan Liu − Division of Chemistry and Biological Chemistry,

School of Physical and Mathematical Sciences, Nanyang

Technological University, 637371, Singapore

Su Ying Quek − Centre for Advanced 2D Materials and

Department of Physics, National University of Singapore,

(12) Gu, S.; Wu, S.; Cao, L.; Li, M.; Qin, N.; Zhu, J.; Wang, Z.; Li,

Radical Intermediates in 2D Covalent Organic Frameworks for High

Performance Sodium Ion Batteries. J. Am. Chem. Soc. 2019, 141,

(

623−9628.

13) Wan, S.; Gandara, F.; Asano, A.; Furukawa, H.; Saeki, A.; Dey,

17546, Singapore; orcid.org/0000-0003-4223-2953

S. K.; Liao, L.; Ambrogio, M. W.; Botros, Y. Y.; Duan, X.; Seki, S.;

Stoddart, J. F.; Yaghi, O. M. Covalent Organic Frameworks with High

Charge Carrier Mobility. Chem. Mater. 2011, 23, 4094−4097.

Kian Ping Loh − Centre for Advanced 2D Materials and

Department of Chemistry, National University of Singapore,

17546, Singapore; orcid.org/0000-0002-1491-743X

(14) Calik, M.; Auras, F.; Salonen, L. M.; Bader, K.; Grill, I.;

Handloser, M.; Medina, D. D.; Dogru, M.; Lobermann, F.; Trauner,

Complete contact information is available at:

https://pubs.acs.org/10.1021/jacs.0c08436

D.; Hartschuh, A.; Bein, T. Extraction of Photogenerated Electrons

and Holes from a Covalent Organic Framework Integrated

Heterojunction. J. Am. Chem. Soc. 2014, 136, 17802−17807.

(15) Jakowetz, A. C.; Hinrichsen, T. F.; Ascherl, L.; Sick, T.; Calik,

M.; Auras, F.; Medina, D. D.; Friend, R. H.; Rao, A.; Bein, T. Excited-

State Dynamics in Fully Conjugated 2D Covalent Organic Frame-

works. J. Am. Chem. Soc. 2019, 141, 11565−11571.

Author Contributions

These authors contributed equally to this work.

Notes

The authors declare no competing ﬁnancial interest.

(16) Yu, F.; Liu, W.; Li, B.; Tian, D.; Zuo, J.-L.; Zhang, Q.

Photostimulus-Responsive Large-Area Two-Dimensional Covalent

ACKNOWLEDGMENTS

■

Organic Framework Films. Angew. Chem., Int. Ed. 2019, 58, 16101−

This research is supported by the Singapore National Research

Foundation Investigatorship (No. NRF-NRFI2018-03). K.P.L.,

S.Y.Q., and J.Q. acknowledge support from the Singapore

National Research Foundation (NRF), Prime Minister’s

Oﬃce, under its Medium-Sized Centre Program and Grant

NRF-CRP16-2015-02. Computations were supported by the

National Supercomputing Centre in Singapore, as well as the

Centre for Advanced 2D Materials, funded under the NRF

Medium-Sized Centre Programs. We thank Dr. Tian-Guang

Zhan for helpful discussions.

(

6104.

17) Ding, S.-Y.; Dong, M.; Wang, Y.-W.; Chen, Y.-T.; Wang, H.-Z.;

Su, C.-Y.; Wang, W. Thioether-Based Fluorescent Covalent Organic

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