A 3D Anionic Metal Covalent Organic Framework with soc Topology Built from an Octahedral TiIV Complex for Photocatalytic Reactions

Article abstract of DOI:10.1002/anie.202102665

The construction of three-dimensional (3D) covalent organic frameworks (COFs) remains challenging due to the limited types of organic building blocks. With octahedral Ti^IV complex as the building unit, this study reports on the first 3D anionic

Full text of DOI:10.1002/anie.202102665

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Accepted Article
Title: A 3D Anionic Metal Covalent Organic Framework with soc
Topology Built from Octahedral Ti(IV) Complex for Photocatalytic
Reactions
Authors: Hui-Shu Lu, Wang-Kang Han, Xiaodong Yan, Chuan-Jie
Chen, Tengfei Niu, and Zhi-Guo Gu
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A 3D Anionic Metal Covalent Organic Framework with soc
Topology Built from Octahedral Ti(IV) Complex for Photocatalytic
Reactions
Hui-Shu Lu,+ Wang-Kang Han,+ Xiaodong Yan, Chuan-Jie Chen, Tengfei Niu* and Zhi-Guo Gu*
[*]
Mr. H. Lu, Dr. W. Han, Prof. Dr. X. Yan, Ms. C. Chen, Prof. Dr. T. Niu, Prof. Dr. Z. Gu
The Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material Engineering
Jiangnan University
Wuxi 214122, P.R. China
E-mail: niutf@jiangnan.edu.cn; zhiguogu@jiangnan.edu.cn
These authors contributed equally to this work.
[+]
Supporting information for this article is given via a link at the end of the document.
Abstract: The construction of three-dimensional (3D) covalent
organic frameworks (COFs) remains challenging due to the limited
types of organic building blocks. With octahedral Ti(IV) complex as
the building unit, this study reports on the first 3D anionic titanium-
based COF (Ti-COF-1) with an edge-transitive (6, 4)-connected soc
topology. Ti-COF-1 exhibits high crystallinity, superior stability, and
large specific surface area (1000.4 m2 g−1). Moreover, Ti-COF-1 has
a broad absorption band in the UV spectrum with an optical energy
gap of 1.86 eV, and exhibits high photocatalytic activity toward
Meerwein addition reactions. This research demonstrates an
attractive strategy for the design of 3D functional COFs.
COFs: (1) high crystallinity: linked by coordinate bonds, the
metal coordination centers may improve the crystallization of
MCOFs; (2) high porosity: MCOFs not only inherit voids from
MOCs but also have newly formed interconnected channels,
which can enrich the porosity of COFs; (3) superior stability: by
combining dynamic covalent bonds and coordinate bonds,
MCOFs will be more stable; (4) tunable functionality: MCOFs will
show promising applications in different areas, especially for
catalysis, due to their abundant metal centers.
Covalent organic frameworks (COFs), a new class of
crystalline porous materials with highly ordered framework
structures, have attracted widespread attention for use in
adsorption, separation, catalysis, and optoelectronics.[1-3] Most of
the reported COFs possess two-dimensional (2D) structures
with interlayer noncovalent interactions to form one-dimensional
pore channels.[4] By contrast, three-dimensional (3D) COFs
usually have interconnected porosity that ensures high
accessible surface area and abundant active sites.[5] However,
the limited types of building blocks (mainly tetrahedral organic
building units) restrict the development of 3D COFs. On the
other hand, the introduction of metal ions into porous COFs to
form metal covalent organic frameworks (MCOFs) may give
clues for the construction of 3D COFs, as the metal coordination
has
a
good spatial orientation.[6] Recently, Yaghi group
synthesized 3D MCOFs (COF-505 and COF-500) via the use of
tetrahedral metal complex as 4-connected building unit.[7]
However, the development of new topologies for 3D COFs using
metal complexes beyond tetrahedral configurations remains a
substantial challenge.
This study develops a new strategy to construct 3D MCOFs
with a new topology via the use of novel metal-complex building
blocks. Metal-organic cages (MOCs) formed by metal ions and
bridging chelate ligands usually have highly symmetric cage
structures,[8] making them good building blocks for MCOFs. As
shown in Scheme. 1, cubic MOCs can be assembled from 3-
connected C3-symmetric octahedral metal complexes and 4-
connected planar units.[9] If the vertices of cubic MOCs are
decorated with linking groups, these MOCs can connect with
each other to form 3D MCOFs. Based on this “MOC-to-MCOF”
strategy, the as-synthesized 3D MCOFs can inherit the following
advantages of MOCs, metal organic frameworks (MOFs) and
Scheme 1. The schematic illustration of the “MOC-to-MCOF” strategy for the
construction of a 3D COF with soc topology.
To prove the “MOC-to-MCOF” strategy, a new C3-symmetric
octahedral Ti(IV) complex Na2Ti(2,3-DHTA)3 (2,3-DHTA= 2,3-
dihydroxyterephthalaldehyde) was designed as a 6-connected
building unit, and an anionic 3D titanium-based COF (Ti-COF-1)
with soc topology was then successfully constructed for the first
time. In recent years, metal-catechol anionic MOCs have been
widely used for catalysis.[10] Ti-COF-1 presents high charge
1
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mobility, and is studied as promising catalyst for Meerwein
addition reactions.
EHOMO - Eg. It is worth noting that the low Eg of Ti-COF-1 can
enhance visible light harvesting and improve its photocatalytic
activity. Moreover, Ti-COF-1 was found to exhibit high thermal
stability with a 60% retention of its initial mass at 800 °C as
demonstrated by the thermogravimetric analysis (TGA) (Figure
S12). After submersion in NaOH (6 M) solutions, HCl (6 M)
Na2Ti(2,3-DHTA)3 was synthesized by titanium oxide
acetylacetonate and 2,3-DHTA in acetonitrile solution (Figure
1a; see SI). The precipitation of dark red crystals was achieved
via the slow diffusion of isopropyl ether. X-ray single-crystal
structure determination revealed that Ti(IV) center coordinated
with six oxygen atoms from the three 2,3-DHTA ligands, forming
an octahedral TiO6 coordination geometry, with an average Ti-O
bond length of 1.966 Å (Figure 1b and Table S4).[11] The six
aldehyde groups of Na2Ti(2,3-DHTA)3 were found to be oriented
in six different directions with C3 symmetry, suggesting that
solutions, or different organic solvents for
3 d, all the
characteristic FT-IR signals of Ti-COF-1 remained, confirming its
good chemical stability (Figure S13).
2-
Ti(2,3-DHTA)3 could be used as an octahedral building unit to
construct 3D MCOFs.
Ti-COF-1 was synthesized via an imine condensation
reaction between Na2Ti(2,3-DHTA)3 and 4, 4', 4'', 4'''-(pyrene-1,
3, 6, 8-tetrayl)tetraaniline (PyTTA) with 6 M acetic acid as a
catalyst in a mixture of o-dichlorobenzene, mesitylene and
acetonitrile at 120 °C for 7 d (Figure 1c; see SI). The FT-IR
spectrum of Ti-COF-1 (Figure 2a) revealed significantly
weakened signals from C=O vibration (1660 cm-1) and the
appearance of a C=N bond (1610 cm-1). The solid-state 13C
cross-polarization
magic-angle-spinning
(CP/MAS)
NMR
spectrum of Ti-COF-1 was then collected. A resonance signal at
158 ppm from the carbon in the C=N group was observed
(Figure 2b). The ultraviolet/visible diffuse reflectance
spectroscopy (UV/vis DRS) revealed a broad absorption band
with the edge at 700 nm for Ti-COF-1 (Figure 2c). The optical
energy gap (Eg) of Ti-COF-1 was estimated to be 1.86 eV by the
Tauc plot (Figure S11).[12] The highest occupied molecular
orbital (HOMO) energy level of Ti-COF-1 was measured to be
1.92 eV via VB X-ray photoelectron spectroscopy (XPS). The
lowest unoccupied molecular orbital (LUMO) energy level was
Figure 1. (a) Synthesis of Na2Ti(2,3-DHTA)3. (b) X-ray single crystal structure
of Na2Ti(2,3-DHTA)3. All H atoms, Na+ and solvent molecules have been
omitted for clarity (C: grey; O: red; Ti: yellow). (c) Synthesis of Ti-COF-1.
calculated to be 0.06 eV according to the equation ELUMO
=
Figure 2. (a) The FT-IR spectra of PyTTA, Na2Ti(2,3-DHTA)3 and Ti-COF-1. (b) The 13C CP/MAS NMR spectrum of Ti-COF-1. (c) The UV/vis DRS of Ti-COF-1.
(d) The experimental PXRD pattern (red), Pawley refined pattern (black), and calculated pattern (blue) of Ti-COF-1, and their differences (green). (e) The N2
adsorption/desorption isotherms and (f) pore distribution of Ti-COF-1.
2
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Figure 3. (a) The structure of Ti-COF-1 with soc topology. (b) Ti8 cubic cage decorated with 24 linking groups in Ti-COF-1. (c) Each of the four Ti8 cubic cages is
surrounded by vertically-crossed channels with a size of 16 Å. (d) The channels are interconnected to further form 3D gridding channels. (e) The “3  3  3” 3D
cavity view of Ti-COF-1 with soc topology. The embedded cavities and 3D channels are represented by yellow balls and blue polyhedrons, respectively. (f) The
model structure of Ti-COF-1. The 2D grids with different orientation are represented by different colors.
The powder X-ray diffraction (PXRD) pattern of Ti-COF-1
revealed diffraction peaks at 3.8°, 5.4°, 6.5°, and 7.7°, which
respectively correspond to the (011), (200), (112) and (202)
facets. The experimental PXRD pattern agrees well with the
theoretical simulation (Figure 2d). It is proposed that Ti-COF-1
adopts the I23 space group (a = b = c = 32.59 Å, α = β = γ = 90°,
Rwp = 7.17 % and Rp = 6.35 %). On the basis of the Ti(IV)
interlocked, forming the 3D framework (Figure S14). To the best
of our knowledge, Ti-COF-1 represents the first example of 3D
MCOFs with a soc topology.
The N2 adsorption/desorption isotherms of Ti-COF-1
exhibited a standard type-I curve with a rapid N2 uptake at
relatively low pressures (P/P0
<
0.1), which are the
characteristics of microporous frameworks.[13] Its Brunauer–
Emmett–Teller (BET) specific surface area was calculated to be
1000.4 m2 g−1 (Figure 2e). The pore size was calculated to be
about 1.55 nm by non-local density functional theory, which is
consistent with the theoretical pore diameter of 1.6 nm (Figure
2f).
2
octahedral complex Ti(2,3-DHTA)3 with six aldehyde grous
acting as the 6-connected nodes and the square organic ligand
PyTTA acting as the 4-connected building units, the topological
analysis reveals that Ti-COF-1 has an edge-transitive (6, 4)-
connected net with a soc topology (Figure 3a). Ti-COF-1
presents an anionic framework with Na+ as the counterions. As
shown in Figure 3b, Ti(IV) ions occupy the eight vertices, while
planar PyTTA units make up the six faces, forming a cubic
titanium-organic cage with a cavity diameter of 14 Å. The
vertices of each Ti8 cubic cage are decorated with 24 linking
groups. In Ti-COF-1, every four cages are spaced by two
perpendicular channels (Figure 3c), and each cage is
surrounded by connected channels (Figure 3d). As revealed by
Scanning electron microscopy (SEM) showed that Ti-COF-1
existed in the form of distorted cubes (Figure S15a), the size of
which is approximately 200 nm (Figure S15b). High-resolution
transmission electron microscopy (HRTEM) confirmed the cubic
morphology (Figure S15c). The high-resolution lattice fringes
revealed the high crystallinity of Ti-COF-1 (Figure S15d and
S15e). The symmetrically rectangularly alligned diffraction spots
as shown by the Fourier-filtered image (inset in Figure S15e)
indicate the highly ordered structure of Ti-COF-1. Moreover, the
dislocation diagram obtained by inverse Fourier transform
indicated quadrilateral gridding frameworks, which is consistent
with the soc structure (Figure S15f). Observation from the (200)
direction (Figure S15d-e) revealed distinct 1D channels with a
uniform diameter of 1.6 nm, which is consistent with the
theoretical pore diameter of 16 Å. The EDS mapping images
the 3D view (Figure 3e), channels of 16

16 Å2 are
interconnected vertically to form a 3D configuration. From
another point of view, Ti-COF-1 can be considered as an
interlocking 3D framework structure (Figure 3f). 2,3-DHTA can
be connected with planar PyTTA units to form 2D square grids,
and the coordination effect of Ti(IV) metals drive these 2D
square grids to be vertically intersected and mechanically
3
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revealled and confirmed the uniform distribution of the C, N, O
and Ti elements in Ti-COF-1 (Figure S15g).
withdrawing and electron-donating substituents including nitro,
ester and methoxyl groups, afforded the products with moderate
to good yields (Table 1, entries 1, 3, 4, 5 and 6). In addition, The
reaction cannot be carried out without light or a photocatalyst
(Table S1, entries 7 and 8), and low yield was obtained when
Na2Ti(2,3-DHTA)3 precursor was used instead of Ti-COF-1
(Table S1, entries 9 and 10). Compared to the other catalysts
(Table S2), Ti-COF-1 is an efficient heterogeneous catalyst for
photocatalytic Meerwein addition reactions. Moreover, Ti-COF-1
can be readily recycled, and its catalytic activity showed only
slight change after 5 runs (Figure S18a). Further, it was found
that the crystallinity of the recycled Ti-COF-1 was well
maintained (Figure S18b).
Table 1. Photocatalytic Meerwein addition reactions by Ti-COF-1[a]
Yield
Entry
1
1
2
Product
TON[c]
202
[%][b]
75
F
2
3
4
5
72
62
67
61
194
167
181
165
HN
O2N
O
6
49
132
Figure 4. Proposed mechanism for the photocatalytic Meerwein addition
reactions on Ti-COF-1.
[a] Reaction conditions: catalyst Ti-COF-1 (10 mg, 0.37 mol % based on unit
cell), the arenediazonium salt (1 equiv, 0.25 mmol), alkene (2 equiv), and
water (1 equiv) were dissolved with 1 mL of CH3CN in a 10 mL snap vial
equipped with magnetic stirring bar at room temperature under a 27 W white
LED irradiation for 24 h. [b] Isolated yield. [c] TON is defined as the total moles
of product formed (nprod) divided by total moles of Ti-COF-1 catalyst unit cell[14]
Based on the previous studies,[16] and our experimental
results,
a possible mechanism for Ti-COF-1 to catalyze
Meerwein addition reactions is proposed (Figure 4). The porous
structure and anionic skeleton make Ti-COF-1 an excellent
adsorbent for cationic arenediazonium salts, which is favorable
for photocatalytic reactions. The coordination between catechol
ligands and metal ions synergistically improve the
electrochemical and photocatalytic properties.[17] Upon
irradiation, Ti(IV) coordinated active centers can efficiently
enhance the charge mobility, thereby exciting Ti-COF-1 to
produce electrons and holes. The adsorbed arenediazonium salt
1 is reduced by the photo-excited electrons to form aryl radical 4.
The subsequent addition reaction of the aryl radical to styrene 2
delivers the radical intermediate 5, which is oxidized by the hole
of Ti-COF-1 to form a cation 6. Finally, the intermediate 6 is
attacked by CH3CN to produce another intermediate 7, followed
by its hydrolysis to produce the amide product 3.
(ncat). TON = nprod / ncat
.
It is thus concluded that Ti-COF-1 possesses high porosity,
high crystallinity, superior stability, and appropriate HOMO and
LUMO energy levels. These characteristics meet the basic
criteria of a heterogeneous photocatalyst. Meerwein addition
reaction is a valuable synthetic method for aryl radical chemistry
and has been used in industrial processes for the synthesis of
drugs and fine chemicals.[15] Therefore, the photocatalytic
Meerwein addition reaction was selected as a model reaction to
demonstrate the photocatalytic activity of Ti-COF-1 (see SI).
As can been seen in Table 1, the Ti-COF-1-catalyzed
Meerwein addition reactions were found to tolerate various
arenediazonium salts and alkenes to afford desired products in
49-75 % yields with 132-202 turnover number (TON). In detail,
styrene with halide functional groups, such as -F and -Cl at the
para position produced the product with good yields (Table 1,
entries 2 and 3). Arenediazonium salts bearing electron-
In conclusion, the “MOC-to-MCOF” strategy was proven to
be an efficient method that produces 3D anionic COFs with soc
topology using octahedral Ti(IV) complex as the building unit. Ti-
COF-1 contains interconnected channels for high specific
surface area, possesses fast charge mobility, and exhibits
4
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This research can be extended to enrich the structure and
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[11] CCDC 2056407 of Na2Ti(2,3-DHTA)3 contains the supplementary
crystallographic data for this paper. These data are available free of
charge from the Cambridge Crystallographic Data Centre.
[12] J. Tauc, Mater. Res. Bull. 1968, 3, 37-46.
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This work was supported by the National Natural Science
Foundation of China (22075108, 21771089, 21905116), the
Natural Science Foundation of Jiangsu Province (BK20190614),
and MOE & SAFEA for the 111 Project (B13025).
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Keywords: covalent organic framework • three-dimensional •
topology • Ti(IV) complex • photocatalytic reactions
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5
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Entry for the Table of Contents
Octahedral Ti(IV) complex was used to construct the first 3D anionic titanium-based COF (Ti-COF-1) with the edge transitive (6, 4)-
connected soc topology. Ti-COF-1 shows high crystallinity, superior stability, abundant porosity and high photocatalytic activity for
Meerwein addition reactions. This research will extend to enrich the structure and functionality of 3D frameworks.
6
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