Reticular Synthesis of tbo Topology Covalent Organic Frameworks

Article abstract of DOI:10.1021/jacs.0c06605

The metal-organic framework (MOF) HKUST-1 with a tbo topology serves as an archetypal tunable and isoreticular framework platform for targeting desired applications, but the design and synthesis of tbo-covalent organic frameworks (COFs) remains a formidable challenge. Here we demonstrate the successful use of reticular chemistry as an appropriate strategy for the design and deliberate construction of COFs with a tbo topology. The judicious selection of the perquisite planar building blocks, 4-connected square tetramine of porphyrin and 3-connected trigonal trialdehydes of triphenylamine, allows the condensation of two tbo-COFs, the first examples of COFs with a tbo topology. The resulting COFs both crystallize in the cubic Pm3ˉ space group and adopt a non-interpenetrated open framework, in which each tritopic linker connects to three square units forming a truncated Td-octahedron and occupies the alternating triangular faces of the truncated octahedron. Owing to the presence of two different types of photoredox-active moieties, the two COFs can be efficient heterogeneous photocatalysts for the oxidative hydroxylation of arylboronic acids and the reductive defluoroalkylation of trifluoromethyl aromatics with alkenes. The present finding will provide an impetus to examine the potential of tbo-COFs as a new platform for engineering multifunctional materials via expansion and functionalization of building blocks.

Full text of DOI:10.1021/jacs.0c06605

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Article
Reticular Synthesis of tbo Topology Covalent Organic Frameworks
Xing Kang, Xing Han, Chen Yuan, Cheng Cheng, Yan Liu, and Yong Cui
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c06605 • Publication Date (Web): 25 Aug 2020
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Reticular Synthesis of tbo Topology Covalent Organic Frameworks
Xing Kang, Xing Han, Chen Yuan, Cheng Cheng, Yan Liu,* and Yong Cui*
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ABSTRACT: The metal-organic framework (MOF) HKUST-1 with a tbo topology serves as an archetypal tunable and isoreticular
framework platform for targeting desired applications, but the design and synthesis of tbo-covalent organic frameworks (COFs) remains a
formidable challenge. Here we demonstrate the successful use of reticular chemistry as an appropriate strategy for the design and deliberate
construction of COFs with a tbo-topology. The judicious selection of the perquisite planar building blocks, 4-connected square tetramine of
porphyrin and 3-connected trigonal trialdehydes of triphenylamine, allows the condensation of two tbo-COFs, the first examples of COFs
3
with a tbo topology. The resulting COFs both crystallize in the cubic Pm space group and adopt a non-interpenetrated open framework, in
which each tritopic linker connects to three square units forming a truncated Td-octahedron and occupies the alternating triangular faces of
the truncated octahedron. Owing to the presence of two different types of photoredox-active moieties, the two COFs can be efficient
heterogeneous photocatalysts for the oxidative hydroxylation of arylboronic acids and the reductive defluoroalkylation of
trifluoromethylaromatics with alkenes. The present finding will provide an impetus to examine the potential of tbo-COFs as a new platform
for engineering multifunctional materials via expansion and functionalization of building blocks.
32 connected net has been underexplored and in fact, only the fcc
1 ■ INTRODUCTION
33 topology has recently been discovered in 3D COFs by us and Zhong
34 et al using square and trigonal building blocks.10 It should be noted
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Covalent organic frameworks (COFs) are constructed from organic
building blocks into 2D and 3D networks through covalent
bonds1,2and are an emerging class of porous materials featuring
high surface areas, tunable pore sizes, shapes and functionalities.3,4
From a structural perspective, this area is dominated currently by
2D COFs that typically have eclipsed stacking structures with
unidirectional channels.5 In contrast, 3D COFs are far less explored,
although they can characteristically possess high surface areas,
35 that the combination of planar 3- and 4-connected organic
36 monomers can also generate 2D COFs under specific conditions.11
37 To address this challenge, it is important to note that a tbo-based
38 structure requires a C4-symmetric dicopper paddelwheel-type node
39 with specific geometrical constraints that limit the choice of organic
40 linkers.9 In this work, we illustrated that the judicious selection of
41 the perquisite planar building blocks, 4-connected square amine and
42 3-connected trigonal aldehydes, allowed the imine condensation of
43 two tbo-frameworks, the first examples of COFs with a tbo
44 topology. The two isostructural COFs adopt a non-interpenetrated
45 porous framework, in which each tritopic linker connects to three
46 square units forming a perfect truncated Td-octahedron and
47 occupies the alternating triangular faces of the truncated octahedron.
48 Importantly, we selected triphenylamine and porphyrin as two
49 different types of functional molecules, since they represent two
50 types of important hole-conducting molecules with unique
51 photophysical and redox properties.12 This led to that the as-
52 prepared 3D COFs can be used as efficient photocatalysts for the
53 oxidative hydroxylation of arylboronic acids13 and the reductive
54 defluoroalkylation of trifluoromethylaromatics14.
10 numerous open sites and fascinating confinement effects.6,7 With
11 few exceptions, 3D COF have only been reported for networks
12 based on the tetrahedral building blocks, which are linked into
13 extended space by linear and trigonal monomers.2,6 Obviously, the
14 synthesis of 3D COFs remains a big challenge, especially for those
15 with unreported topologies, due to the limited choice of building
16 blocks and the difficulty of crystallization. Here we demonstrate the
17 successful use of reticular chemistry as an appropriate strategy for
18 the design and deliberate construction of HKUST-like tbo-COFs
19 by the combination of square and trigonal building blocks.
20
Reticular chemistry is concerned with the linking of molecular
21 building units to build crystalline extended frameworks and has
22 developed into a powerful tool for the synthesis of MOFs with
23 desired structural features.8,9 For example, the various combination
24 of planar inorganic nodes and 3-connected organic linkers allows
25 the formation of a rich library of 3D MOFs with ffc, tbo, fjh and
26 pto topologies, as shown in Figure 1.8,9 In particular, one of the
27 most well-known MOF materials-HKUST-1 that has a tbo
28 topology can be interpreted and regarded as constructed from 4-
29 connected square dicopper paddlewheel-based nodes and 3-
30 connected triangular 1,3,5-benzene-tricarboxylate struts.9a By
31 contrast, the synthesis of 3D COFs based on this unique (3,4)-
55 ■ RESULTS AND DISCUSSION
56 Synthesis and Characterization. As shown in Figure 1, COF-1
57 was prepared through the [3 + 4] condensation reaction of triangular
58 4',4''',4'''''-nitrilotris[(1,1'-biphenyl)-4-carbaldehyde] (NBC) and
59 square 5,10,15,20-tetra(4-aminophenyl)porphyrin [H2(TAP)] in a
60 12:3:2 (v/v/v) mixture of o-DBC/n-butanol/6 M acetic acid at 120
61 oC for 5 days, which was collected by filtration and washed with
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(3,4)-ffc (known COFs)10
(3,4)-fjh (unknown COFs)
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(3,4)-pto (unknown COFs)
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Figure 1. (a) Schematic representation of the (3,4)-connected 3D network with four different possible topologies ffc, fjh, tbo and pto. (b) Construction of
two 3D COFs with the tbo topology by imine condensation of square and trigonal linkages.
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Figure 2. PXRD patterns of COF-1 (a) and COF-2 (b) after Pawley refinement. PXRD profiles of experimental pattern (black curve), Pawley refined (red
curve), and calculated (blue curve) patterns from the modeled structure with the tbo topology; their difference (green curve).
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anhydrous DMF and acetone, and dried at 60 ºC under vacuum33 (e.g., tbo, pto, ffc, fjh, etc.) for COF-1/2 were constructed
overnight to afford COF-1 as black purple crystalline powders in34 according to Reticular Chemistry Structure Resource (RCSR).15
~80% yield. COF-2 was synthesized under similar conditions with35 After considering these possible interpenetrated and non-
the
exception
of
utilizing
5,10,15,20-tetrakis(4-36 interpenetrated nets with different space groups, the detailed
aminophenyl)porphinato]copper(II) [Cu(TAP)], giving crimson37 simulation (Figures S5-S23) clearly suggested that both COFs 1 and
crystalline solids in ~75% yield. These two COFs are insoluble in38 2 are proposed to adopt a non-interpenetrated tbo topology with the
water and common organic solvents such as THF, MeOH, DMSO
and DMF. Scanning electron microscopy (SEM) images showed
that both COFs display uniform aggregated spherical particles
3
39 cubic Pm space group (Figure 2). The 4-connected square amine
40 and 3-connected trigonal aldehyde building units are connected
41 through imine bonds to form an extended 3D framework of the tbo
42 topology (Figure 1). It is noteworthy that COF-1/2 are the first
43 examples of COFs with a tbo topology, isostructural to the well-
44 known HKUST-1 constructed from 4-connected square dicopper
45 paddlewheel nodes and 3-connected 1,3,5-benzene-tricarboxylate
46 struts. A full profile Pawley refinement was performed to obtain the
47 unit cell parameters (a = b = c = 95.3 Å, α = β = γ = 90°, Rp =
48 1.48%, Rwp = 2.22% for COF-1; a = b = c = 95.2 Å, α = β = γ =
49 90°, Rp = 1.44%, Rwp = 2.14 % for COF-2), which led to
50 satisfactorily low residual values and acceptable profile differences.
10 ranging in size from 0.8 to 1.6 μm in diameter (Figure S4 of the
11 Supporting Information, SI). The FT-IR spectra of the COFs
12 showed disappearance of C=O stretching signals and appearance of
13 strong C=N stretching vibration bands at 1615 and 1605 cm−1,
14 respectively, indicating the consumption of the starting materials
15 and successful polymerization (Figure S1). The 13C CP-MAS NMR
16 spectra further confirmed the formation of the imine linkages in the
17 COFs. The characteristic resonance peaks of imine carbons were
18 observed at 147 ppm for COF-1 and 145 ppm for COF-2. The
19 residual aldehyde signals did not appear (Figure S2).
51
The tbo-COFs consist of three types of polyhedral cages. In
52 COF-1, the smallest truncated tetrahedron is formed from four
53 NBC monomers and 12 amine groups from six H2(TAP) units with
54 a diameter of 14 Å (Figure 3a). The second cage is a truncated cube
55 and is formed from eight NBC and 24 amine groups from twelve
56 H2(TAP) units (Figure 3b). The largest cage can be described as a
57 truncated cuboctahedron and is formed by twelve H2(TAP)
58 monomers and 48 aldehyde groups from twenty-four NBC units
59 (Figure 3c). The truncated cube has a diameter of 20 Å, and the
60 truncated cuboctahedron has a diameter of 33 Å. The three different
61 polyhedra are closely packed together by sharing faces to result in
62 the tbo-topology COF; truncated tetrahedron and truncated cube
63 share a trigonal face, truncated tetrahedron and truncated
20 Crystal Structure. The exact periodical structures of the two COFs
21 were determined by powder X-ray diffraction (PXRD)
22 measurements in conjunction with structural simulations (Figure 2).
23 COF-1 exhibits an intense peak at 1.65° and relatively weak peaks
24 at 3.40°, 3.95°, 4.73°,7.11°, 7.91o, 9.20o, 9.89o, 17.1o, 19.9o, 23.1o,
25 corresponding to the (200), (222), (331), (422), (371), (237), (211),
1
26 (2 1), (541), (312) and (041) facets, respectively (Figure 2a).
27 Similar PXRD pattern was observed for COF-2, featuring peaks at
28 1.63° (200), 3.39° (222), 3.95° (331), 4.72° (422), 7.11° (371),
29 7.91o (237), 9.20o (211), 9.89o (2 1), 17.1 (541), 19.9 (312) and
30 23.1o (041) (Figure 2b). The structure models were built by using
31 the Materials Studio software package. Considering the geometry
32 of the precursors and the connection patterns, a few possible nets
o
o
1
64 cuboctahedron
a
hexagon, truncated cube and truncated
65 cuboctahedron an octagon
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Figure 3. (a-c) Three types of polyhedral cages in COF-1: truncated tetrahedron (red), cube (green), and cuboctahedron (blue). (d) Representative section of
the crystal structure of COF-1 showing the polyhedral cages. (e) The nanochannel of COF-1 viewed along the [100] direction. (f) Representation of tbo net,
based on face-sharing truncated cuboctahedron. (g) The structure of COF-1 with tbo topology.
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Figure 4. (a) CFM images obtained from COFs after incubation with dyes A-C. (b) Different dyes uptake released from COFs by UV-vis spectroscopy. (c)
The structures of dyes A-C.
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(Figures 3d-3f). The overall framework has interconnected 1D open
channels of 38.4 × 40.3 Å2 (Figure 3e). COF-2 has a similar cage
based network containing open channels in diameter of 38.2 × 40.1
Å2 (Figure S23c). The (3,4)-connected 3D COFs without the
employment of 3D monomers have been reported, in which case
the ffc topology was generated with tetragonal and triangular
monomers10. COFs 1 and 2 composed of triangular and square
building blocks represent the first examples of 3D COFs adopting
the tbo topology. The development of 3D COFs with new topology
19 4.169 wt %, corresponding to the 3:4 molar ratio of Cu(TAP)/NBC.
20 The structural model was also confirmed by elemental analysis
21 results of COF-1/2, which matched well with the expected
22 elemental ratios calculated for the (3,4)-connected frameworks.
23
Dye-uptake experiment in combination with confocal
24 fluorescence microscopy (CFM)16 were employed to evaluate the
25 dimensions of the guest-accessible pores. The activated COFs were
26 immersed in a THF solution of dyes with different sizes for 24 h.
27 The solution was decanted and the COFs were washed for several
28 times to remove the dye molecules adsorbed on the surfaces of the
29 COFs. CFM results showed that dye A (1.51 × 1.40 nm2) and dye
30 B (3.41 × 2.93 nm2) were distributed uniformly inside the COFs,
31 and the dye molecules and COFs after spectral separation can be
32 assigned to red and green fluorescence, respectively (Figure 4). In
33 the case of dye C with molecular size of 3.62 × 4.32 nm2, both
34 COFs did not display any major uptake, probably because the bulky
35 dye cannot access the pores in the frameworks due to its large size.
36 The amount
10 is of critical importance for enriching the structural diversity and
11 expanding the applications.
12
To quantitatively determine the ratios of the two monomers,
13 COF-1 was digested and analyzed by 1H NMR. Resonances for the
14 two linkers’ unique protons were observed in the expected regions
15 (Figure S30), and the ratio of H2(TAP) to NBC in COF-1 was
16 calculated to be ~ 3:4 by integrating the resonance peak intensities,
17 which is consistent with the structural model. Inductively coupled
18 plasma (ICP) analysis revealed that the Cu content in COF-2 is
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COFs 1 and 2 showed excellent thermal stability, and
(a)
56 thermogravimetric analysis (TGA) revealed that the frameworks
57 are thermally stable up to 360 oC (Figure S3). The chemical stability
58 of the two COFs was examined by PXRD on samples exposed to
59 various conditions (Figure S24). After immersed in common
60 organic solvents including DMSO, MeOH and THF for 48 h,
61 PXRD showed almost no changes in the diffraction patterns. Both
62 COFs retained their original structure in boiling water, but gave a
63 slightly decreased crystallinity. In the case of much harsher
64 conditions, COFs 1 and 2 maintained stability after being soaked in
65 aqueous NaOH solution (3M), however, they lost their crystallinity
66 and became amorphous after exposure to aqueous HCl solution (1
67 M) for 48 h.
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(b)
69 Photocatalysis. The as-synthesized COFs based on porphyrin and
70 triphenylamine backbones exhibit intriguing photophysical and
71 redox properties. Diffuse reflectance UV/Vis spectra of the COFs
72 are shown in Figure 5. It can be seen that both COFs 1 and 2 absorb
73 light in the UV and visible regions, with absorption edges at about
74 720 nm, which is blue-shifted by 230 nm in comparison to that of
75 the porphyrin monomers, due to the conjugation in the extended
76 frameworks. On the base of the Kubelka-Munk formula, the band
77 gaps of COFs were calculated to be ~1.7 eV, indicating that they
78 may be utilized as candidates for photocatalysts in visible-
79 lightdriven reactions. As a result, we tested the photocatalytic
80 activity of the two 3D COFs in the hydroxylation of arylboronic
81 acids and defluoroalkylation of trifluoromethylaromatics.
(c)
82
Phenols, as important intermediates, are widely found in natural
83 products and pharmaceutical drug candidates.13 Given their great
84 value in many fields, continuing efforts have been devoted to the
85 development of efficient methods for the synthesis of phenols over
86 the past decades. Recently, the hydroxylation of arylboronic acid
87 was shown to be a promising strategy to produce phenols via
88 photocatalytic aerobic oxidation.13 Porphyrin and triphenylamine
89 can
90
91 Table 1. Photocatalytic Hydroxylation of Arylboronic Acids.a,b
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40 Figure 5. (a) Solid-state UV spectra of COFs. (b) Tauc plot for absorption
41 spectra obtained with Kubelka-Munk function and the linear fit for direct
42 band gaps. (c) Solid-state CV with a scan rate of 50 mV s−1.
43
44 of dye molecules adsorbed was measured using UV-vis
45 spectrophotometry. COFs 1 and 2 could adsorb 26.4 and 32.6 wt%
46 dye A and 14.1 and 11.4 wt% dye B, respectively, but exhibited
100
47 negligible inclusion of dye C, indicating an obvious size selectivity.
101
48 In all cases, the inclusion samples gave almost the same PXRD
102
49 patterns as the pristine sample, suggesting that the frameworks
103
50 remain intact in solution (Figure S27). Moreover, the maximum
104
51 pore diameter and opening of the two COFs revealed by dye uptake
52 experiment were in the range of 3.4-4.3 nm, indicating that the
53 COFs have the non-interpenetrated tbo structures rather than the
54 interpenetrated structures with around 0.8 nm apertures.
105 a Reaction conditions: 4 (0.5 mmol), i-Pr2NEt (1.5 mmol), COF-1 (5 mol% based on
106 4), CH3CN (2.4 mL), H2O (0.6 mL), air, 440 nm LED as the light source, 30 h for COF
107 1 and 36 h for COF 2. b The yields of products were determined by 1H NMR spectra
108 using 4-bromo-1,2-dimethoxybenzene as an internal standard.
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serve as small-molecule photosensitizers to initiate the activation of55 decrease the reaction rate to give rise to the corresponding products
oxygen for active oxygen species. Hence, the COFs could be56 in 81-93% yields for 36 h. It is presumably due to that COF-2 has
utilized as photocatalysts for the aerobic oxidative hydroxylation of57 the weaker ability than 1 to produce singlet oxygen under
arylboronic acids.
58 photoirradiation, as a result of porphyrins with paramagnetic
Initially, phenylboronic acid (4a) was selected as the test59 copper ions that are poor photosensitizers.7b,17 For comparison,
substrates in combination with COF-1 as the photocatalyst for the60 control experiments using building units, H2(TAP) and Cu(TAP)
transformation of phenylboronic acids to phenols (Table S18).61 as photocatalysts were conducted. All of them were capable of
After screening various reaction conditions including solvent62 catalyzing the hydroxylation of 4-nitrophenylboronic acid,
system, reaction time, bases and temperature (Table S18), we found63 affording the product yields that are comparable (99% for NBC) or
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10 that 4a was oxidized to phenol (5a) with 91% yield in the system64 lower [83% for H2(TAP) and 67% for Cu(TAP)] than those of
11 of CH3CN/H2O with i-Pr2NEt at rt for 30 h. Encouraged by this65 COFs (Scheme S1). However, prolonging reaction time led to the
12 result, a variety of phenylboronic acid derivatives with various66 bleaching of all three monomers.18 In addition, the observed yields
13 electronic properties were subjected to the photocatalytic reactions67 are comparable to those of reported photocatalytic reactions using
14 and gave the corresponding products in 87-99% yields. Notably,68 heterogeneous and homogeneous photosensitizers (Table S23).13
15 substituted phenylboronic acid derivatives with electron-69
COF-1 can be readily recycled and reused for 10 runs without
16 withdrawing groups gave higher yields than that of substrates with70 obvious loss of activity (99%, 99%, 99%, 97%, 98%, 96%, 97%,
17 electron-rich substituents. For instance, substrates with electron-71 97%, 96% and 95% for 1-10 runs, respectively, in the hydroxylation
18 withdrawing substituents (4c and 4d) gave the targeted products in72 of 4c (Figure 6a). The recycled COF powder retained its crystallinity,
19 higher yield,
73 as evidenced by the retention of the major diffraction peaks
74 compared with the pristine sample. The loss of the expected 222
75 and 231 peaks is probably due to a structural distortion occurred
76 and/or some COF degradation in the recycle reactions. Moreover,
77 the recycled sample can adsorb ~20.6 wt % dye A in EtOH (Figure
78 4b), close to the pristine sample, indicating that COF-1 maintained
79 open channels after ten catalytic cycles.
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To probe whether the photocatalysis takes places within the
81 pores or on the surface of the COFs, more control experiments
82 were performed. When dye B and dye B@COF-1 were used as a
83 photocatalyst to catalyze hydroxylation of 4f, the product was
84 produced in 15% and 61% yield, respectively (Scheme S3),
85 indicating the catalysis can occur outside of the COF
. Also, we
86 studied the ability of the COF to encapsulate the substrate. The
87 result showed that the activated COFs can uptake 36.1 wt% 4f
88
(Table S17), which can be converted into the product in 83%
35
89 yield under irradiation, suggesting the catalytic reaction may
90 occur within the COF. It is thus likely that the photocatalytic
91 reaction can occur both inside and outside the COF. However,
92 attempts to prove the size selectivity of the substrate catalyzed
93 COFs by using sterically substrates have failed so far (Scheme S2).
(b)
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94
Importantly, when the reaction was performed in N2 instead of
95 air, the product was obtained in quite poor yield, highlighting the
96 important role of oxygen in the catalytic reaction. Previous studies
97 suggest that molecular O2 is capable of converting to active oxygen
98 species through energy/electron transfer from the excited
99 photosensitizer.13a To prove the active oxygen species generated by
100 COF-1, electron spin resonance (ESR) measurements were
101 conducted in which 2,2,6,6-tetramethylpiperidine (TEMP) and 5,5-
102 dimethyl-1-pyrroline-N-oxide (DMPO) were used as singlet
47
48 Figure 6. (a) Recycle results for the COF-1 catalyzed hydroxylation of 4-
49 nitrophenylboronic acid. (c) PXRD patterns of the recycled COF-1 after
50 ten catalytic cycles.
•-
103 oxygen 1O2 and superoxide radical anion O2 scavengers,
104 respectively. As shown in Figure 7, ESR spin-trapping experiments
105 suggest that O2 was converted into 1O2 and O2•- via single electron
106 transfer from the excited state of COFs during catalysis, consistent
107 with reported results13. Moreover, when TEMP was utilized as an
108 1O2 scavenger for the hydroxylation of 4-nitrophenylboronic acid, the
51
52 while 4-(tert-butyl)phenylboronic acid (4b) offered the desired
53 product with 90% yield under standard conditions. Compared to
54 COF-1, COF-2 was devoted to catalyzing the reaction and could
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yield of the product was 35% (Figure S33a), whereas when DMPO55 bistrifluoromethylbenzene
to
give
the
corresponding
was used as an O2•− scavenger, the yield was 23% (Figure S33b).56 defluoroalkylation products. For instance, when 1,3-
The quenching experiments thus indicated that O2 played a critical57 bistrifluoromethylbenzene 6 and 7b participated in this reaction, the
role in the reaction. To examine where the oxygen of the product58 product 8b was obtained in 80% yield. From the reactions of 6 with
comes from, an 18O-labeling experiment was conducted using59 7c, the product 8c was obtained in 81% yield. Moreover, when 7d
H218O to replace H2O. High-resolution mass (HRMS) analysis60 and 7f were subjected to
61
indicated that the oxygen atom in the product didn’t come from
water, but the molecular oxygen (Figure S34) 13c,13d. Besides, the
D-labeling experiment using D2O instead of H2O showed that the
62 Table 2. Defluoroalkylation of Trifluoromethylaromatics with Alkenes.a,b
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10 hydrogen of -OH in the product stemmed from water (Figure S35).
11 Based on the above results, a plausible reaction mechanism for the
12 hydroxylation is proposed (Figure S36). First, the excited COF*
13 was generated under irradiation and quenched by the single electron
14 transfer to O2 for the formation of O2•−. Second, the phenylboronic
15 acid underwent oxidization, disproportionation and hydrolysis to
16 give the desired product. Final, COF-1 was regenerated in the
17 presence of electron donors, where the photoredox cycle is closed.
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78 a Reaction conditions: 6 (0.5 mmol), alkene (1.5 mmol), COF-1/2 (5 mol %), CySH
79 (10 mol %), HCOONa (1.5 mmol), H2O/DMSO (1/20 v/v, 5.0 mL), 440 nm LED as
80 the light source, N2, 60 oC, 30 h. b Isolated yields.
81
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82 the transformation, the expected products 8d and 8f were isolated
83 in 75% and 83% yields, respectively. Again, the alkene with
84 electron-withdrawing group 7e can also proceed smoothly in this
85 reaction, but the yield of the expected product 8e was lower.
86 Besides, COF-2 can also act as a photosensitizer for the
87 defluoroalkylation, affording the products in 65-81% yields (Table
88 2). Also, several tests showed that COF-1 was a heterogeneous and
89 recyclable catalyst in promoting the defluoroalkylation reaction
90 (Table S21 and Figure S37).
27
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31
32 Figure 7. ESR spectra of the COF 1 particles suspended in pure CH3CN
33 (a/c) and a CH3CN solution of the substrate 4a (b/d) in the presence of
34 TEMP/DMPO under irradiation for 2 mins.
35
36
The trifluoromethylaromatic (Ar-CF3) motif is widely dedicated
91
Again, control experiments showed that NBC gave a
37 to pharmaceuticals and photoelectric material design.19 Although
38 diverse methodologies have been developed for the cleavage of the
39 C-F bonds in Ar-CF3 substrates,20 the functionalization of Ar-CF3
40 groups remains a big challenge in organic synthesis.14 Recently, Jui
41 et al reported homogeneous photocatalysts for C-F
42 functionalization of trifluoromethylarenes with alkenes via Ar-CF3
43 activation, where single electron reduction could be generally
92 comparable yield (83%) to the two COFs and both H2(TAP) and
93 Cu(TAP) gave lower yields (61% and 55%, respectively) in
94 catalyzing the defluoroalkylation of trifluoromethylaromatics with
95 3-buten-1-ol (Scheme S1). The defluoroalkylation reactions
96 catalyzed by COF-1 gave comparable yields to the photocatalytic
97 reaction promoted by N-phenylphenothiazine (PTH), where two
98 distinct catalysts operate in concert to accomplish the transfer of
99 electrons and hydrogen atoms (Figure S39), although the
100 heterogeneous reactions required longer time, due to slow mass
101 diffusion in the porous media.14b Solid-state electrochemical
102 measurements displayed a redox potential at 0.65 V (vs SCE), due
103 to the redox potential of the COF-1+/COF-1 couple (Figure 5c). The
104 redox potential of the excited-state COF-1+/ COF-1* couple was
44 utilized in the formation and reaction of α,α-difluorobenzylic
45 radicals.
14b
The defluoroalkylation of trifluoromethylaromatics
46 with alkenes was selected as a test reaction catalyzed by COF-based
47 heterogeneous photocatalysts under irradiation with LED. The 1,3-
48 bistrifluoromethylbenzene and unactivated alkene structures were
49 initially selected as the model reaction for the systematic
50 assessment. The factors that influence this transformation were
51 thoroughly evaluated via control experiments, and the key results
52 are listed in Table S20.
105 calculated as -2.22 V on the basis of a free energy change (E0-0
)
106 between the ground state and the vibrationally excited state of 2.87
107 eV. The excited COF can initiate a photoinduced single electron
108 transfer to 1,3-bistrifluoromethylbenzene (E1/2 = -2.07 V) to for
109 radical anion 9, as well as the oxidized state COF-1+. The transfer
53
As illustrated in Table 2, this light-driven system effectively
54 couples a wide range of unactivated alkene structures with the 1,3-
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of electrons was confirmed by the Stern-Volmer quenching
49 Author
experiment where the fluorescence intensity of COF-1 decreased
with increasing concentrations of 6 in solution, but the other
reactants or reagents cannot quench COF-1* (Figure S38). Radical
anion 9 would facilitate the C-F bond cleavage to expel fluoride,
generating electrophilic radical 10. Intermolecular radical addition
to 7a would give nucleophilic radical 11, which would be polarity
matched for hydrogen atom transfer (HAT) from cyclohexanethiol
(CySH). This step can concurrently deliver the targeted product 8a
50 Xing Kang - School of Chemistry and Chemical Engineering,
51
52
53
Frontiers Science Center for Transformative Molecules and
State Key Laboratory of Metal Matrix Composites, Shanghai
Jiao Tong University, Shanghai 200240, China
54 Xing Han - School of Chemistry and Chemical Engineering,
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Frontiers Science Center for Transformative Molecules and
State Key Laboratory of Metal Matrix Composites, Shanghai
Jiao Tong University, Shanghai 200240, China
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10 and the thiyl species CyS•. The yields of the product 8a and by-
11 product 12 were related to the loadings of thiol (Table S22),
12 suggesting that the defluoroalkylation was accompanied with HAT.
13 It is likely that this catalytic process was depended on the formate
14 and thiol additives (Table S20, entries 14 and 16). Regeneration of
15 the catalysts by sodium formate (through HAT and SET) would
16 close both catalytic cycles, liberating the byproducts CO2 and NaF.
58 Chen Yuan - School of Chemistry and Chemical Engineering,
59
60
61
Frontiers Science Center for Transformative Molecules and
State Key Laboratory of Metal Matrix Composites, Shanghai
Jiao Tong University, Shanghai 200240, China
62 Cheng Cheng - School of Chemistry and Chemical Engineering,
63
64
65
Frontiers Science Center for Transformative Molecules and
State Key Laboratory of Metal Matrix Composites, Shanghai
Jiao Tong University, Shanghai 200240, China
17 ■ CONCLUSIONS
66 Author Contributions
18 In conclusion, we have demonstrated for the first time that
19 implementation of reticular chemistry using a judiciously designed
20 rigid square and trigonal building blocks allowed the construction
67 All authors have contributed to the design and implementation of
68 the studies reported here and have given approval to the final
69 version of the manuscript.
21 of two HKUST-1-like tbo-COFs. They crystallize in the cubic Pm
3
22
space group and adopt a non-interpenetrated open framework, in
70 Notes
23 which each tritopic linker connects to three square units forming a
24 truncated T -octahedron and occupies the alternating triangular
71 The authors declare no competing financial interest.
d
25 faces of the truncated octahedron. Both COFs bearing photoredox
26 triphenylamine and porphyrin moieties were efficient, recycle and
27 heterogeneous photocatalysts for the hydroxylation of arylboronic
28 acids to phenols and defluoroalkylation of trifluoromethylaromatics
29 with alkenes. The successful synthesis of the first two tbo-COFs
30 paves the way to expand this approach to other building blocks with
31 the aim to tune the pore system and functionality.
72 ■ ACKNOWLEDGMENT
73 This work was financially supported by the National Science
74 Foundation of China (Grant Nos. 21620102001, 91856204,
75 91956124, and 21875136), the National Key Basic Research
76 Program of China (2016YFA0203400), Key Project of Basic
77 Research of Shanghai (17JC1403100 and 18JC1413200), Shanghai
78 Rising-Star Program (19QA1404300), Postdoctoral Innovative
79 Talent Support Program (BX20190195) and the China postdoctoral
80 science foundation (2019M661483).
32 Supporting Information
33 Experimental procedures and characterization data. This material is
34 available free of charge via the Internet at http://pubs.acs.org.
35 Experimental procedures, characterization data and structural simulation
36 information (PDF).
81 ■ REFERENCES
37 ■ AUTHOR INFORMATION
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41
42
43
Frontiers Science Center for Transformative Molecules and
State Key Laboratory of Metal Matrix Composites, Shanghai
Jiao Tong University, Shanghai 200240, China; Email:
yongcui@sjtu.edu.cn
44 Yan Liu - School of Chemistry and Chemical Engineering,
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48
Frontiers Science Center for Transformative Molecules and
State Key Laboratory of Metal Matrix Composites, Shanghai
Jiao Tong University, Shanghai 200240, China; Email:
liuy@sjtu.edu.cn
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