Robust Biological Hydrogen-Bonded Organic Framework with Post-Functionalized Rhenium(I) Sites for Efficient Heterogeneous Visible-Light-Driven CO2 Reduction

Article abstract of DOI:10.1002/anie.202016710

A robust 2,2′-bipyridine (bpy)-derived biological hydrogen-bonded framework (HOF-25) has been realized depending on guanine-quadruplex with the assistance of π-π interaction, which reacts with Re(CO)₅Cl to give a post-functionalized HOF-25-Re. X-ray absorption fine structure spectroscopic study on HOF-25-Re confirms the covalent attachment of Re(bpy)(CO)₃Cl segments to this HOF. Robust and recycled HOF-25-Re bearing photocatalytic Re(bpy)(CO)₃Cl centers displays good heterogeneous catalytic activity towards carbon dioxide photoreduction in the presence of [Ru(bpy)₃]Cl₂ and triisopropanolamine in CH₃CN under visible-light irradiation, with both high CO production rate of 1448 μmol g^?1 h^?1 and high selectivity of 93 %. Under the same conditions, the experimental turnover number of HOF-25-Re (50) is about 8 times as that of the homogeneous control Re(bpy)(CO)₃Cl. The sustainably regenerated HOF-25-Re via crystallization and post-modification processes shows recovered photocatalytic performance.

Full text of DOI:10.1002/anie.202016710

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 8983–8989
International Edition: doi.org/10.1002/anie.202016710
German Edition: doi.org/10.1002/ange.202016710
CO2 Reduction
Hot Paper
Robust Biological Hydrogen-Bonded Organic Framework with Post-
Functionalized Rhenium(I) Sites for Efficient Heterogeneous Visible-
Light-Driven CO2 Reduction
Baoqiu Yu, Lianjie Li, Shanshan Liu, Hailong Wang,* Heyuan Liu, Chenxiang Lin, Chao Liu,
Hui Wu, Wei Zhou, Xiyou Li, Tianyu Wang, Banglin Chen,* and Jianzhuang Jiang*
Abstract: A robust 2,2’-bipyridine (bpy)-derived biological
hydrogen-bonded framework (HOF-25) has been realized
depending on guanine-quadruplex with the assistance of p-p
interaction, which reacts with Re(CO)5Cl to give a post-
functionalized HOF-25-Re. X-ray absorption fine structure
spectroscopic study on HOF-25-Re confirms the covalent
attachment of Re(bpy)(CO)3Cl segments to this HOF. Robust
and recycled HOF-25-Re bearing photocatalytic Re(bpy)-
(CO)3Cl centers displays good heterogeneous catalytic activity
towards carbon dioxide photoreduction in the presence of
[Ru(bpy)3]Cl2 and triisopropanolamine in CH3CN under
visible-light irradiation, with both high CO production rate
of 1448 mmolgÀ1 hÀ1 and high selectivity of 93%. Under the
same conditions, the experimental turnover number of HOF-
25-Re (50) is about 8 times as that of the homogeneous control
Re(bpy)(CO)3Cl. The sustainably regenerated HOF-25-Re via
crystallization and post-modification processes shows recov-
ered photocatalytic performance.
and mitigating anthropogenic CO2 emission simultaneously.[1]
In the study of CO2 conversion applicability, various crystal-
line porous materials including zeolites,[2] metal-organic
frameworks (MOFs),[1a,3] and covalent organic frameworks
(COFs),[1b, 4] have been estimated to transform carbon dioxide
molecules using organic reaction, hydrogenation, photocata-
lytic reduction, and electrocatalytic reduction. Compared
with amorphous porous materials, the well-defined structures
of these crystalline porous species are helpful in correlating
the structure with CO2 conversion property from the atomic
and molecular perspective.
As another kind of crystalline porous materials, hydrogen-
bonded organic frameworks (HOFs), which are constructed
by the self-assembly of organic or metal-organic building
blocks through hydrogen-bonding, have been conceptually
proposed in early 1990s,[5] but only recently been paid
increasingly attentions because of their uniqueness in term
of their easy solution processability and simple regeneration
through simple recrystallization as well as their versatile
applications including gas storage,[6] small molecule separa-
tion,[7] chiral separation,[8] sensing,[9] heterogeneous cataly-
sis,[10] and proton conduction.[11] In comparison with the
coordination and covalent bond in MOFs and COFs, the
instability and low-directionality of hydrogen-bonding nature
in HOFs are still the stumbling block to the establishment of
permanent porosity and thus the development of diverse
functions for this kind of materials. Combination of direc-
tional hydrogen-bonded synthons and suitable organic skel-
etons, which prefer to form strong p-p interactions, has been
demonstrated to effectively consolidate the frameworks of
HOFs.[12] Thus far, this strategy however has not yet been
widely spread due to the uncontrollable p-p interactions of
organic skeleton. It is therefore necessary to establish new
synthetic strategy for preparing robust functional HOFs and
even for realizing their post-functionalization towards a much
wider variety of applications.
Introduction
Carbon dioxide fixation and reduction is a promising
approach to address the increasing energy crisis and global
warming by outputting sustainable carbon fuels/chemicals
[*] B. Yu, L. Li, Prof. H. Wang, C. Lin, C. Liu, Prof. T. Wang, Prof. J. Jiang
Beijing Advanced Innovation Center for Materials Genome Engi-
neering, Beijing Key Laboratory for Science and Application of
Functional Molecular and Crystalline Materials, Department of
Chemistry and Chemical Engineering, School of Chemistry and
Biological Engineering, University of Science and Technology Beijing
Beijing 100083 (China)
E-mail: hlwang@ustb.edu.cn
jianzhuang@ustb.edu.cn
Prof. B. Chen
Department of Chemistry, University of Texas at San Antonio
San Antonio, TX 78249-0698 (USA)
E-mail: banglin.chen@utsa.edu
Herein, a biological HOF (HOF-25) has been designed
and afforded by hydrogen-bonded guanine-quadruplex (G-
quadruplex) directing the self-assembly of a 2,2’-bipyridine
(bpy)-derived building block (L) (Scheme 1 and Figure 1a).
This robust HOF is stabilized synergistically by the hydrogen-
bonded G-quadruplexes and their interlayered p-p interac-
tions, showing the permanent porosity with a Brunauer-
Emmett-Teller (BET) surface area of 173 m2 gÀ1 acquired
from the 196 K CO2 adsorption fitting. In addition, this HOF
can tolerate the soaking in aqueous solution with pH value
from 7.0 to 11.0 as well as common organic solvents as
S. Liu, Dr. H. Liu, Prof. X. Li
School of Materials Science and Engineering, China University of
Petroleum (East China)
Qingdao 266580 (China)
Dr. H. Wu, Dr. W. Zhou
Center for Neutron Research, National Institute of Standards and
Technology
Gaithersburg, MD 20899-6102 (USA)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202016710.
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Scheme 1. Synthesis of Boc6G2bpy and Building Block (L) for the Assembly of HOF-25 and HOF-25-Re.
covalent decoration of Re(bpy)(CO)3Cl units to the hydro-
gen-bonded reticular framework based on the extended X-ray
absorption fine structure spectroscopic (EXAFS) and PXRD
studies. As a result, by adding [Ru(bpy)3]Cl2·6H2O photo-
sensitizer in CH3CN containing triisopropanolamine (TIPA),
stable and recyclable HOF-25-Re heterogeneous catalyst
promotes the visible light photoreduction of carbon dioxide
with a high CO production rate of 1448 mmolgÀ1 hÀ1 and
constantly high CO selectivity of 93% for three cycles. The
photocatalytic performance of overused HOF-25-Re could be
recovered via a two-step treatment including crystallization
and post-modification. Interestingly, the turnover number
(TON) of heterogeneous HOF-25-Re (50) is obviously
superior to that of the homogeneous control Re(bpy)(CO)3Cl
(6) under the same photocatalytic conditions following the
electron transfer from photosensitizer to photocatalyst. These
results highlight the opportunity of rigid HOFs in heteroge-
nizing molecular catalyst for diverse transformations.
Figure 1. Structures of L (a), proposed G-quadruplex grid (b), and AA
packing mode of HOF-25 (c and d). e) PXRD pattern comparison
between activated HOF-25 (red), as-synthesized HOF-25 (cyan), and
the simulated one (grey). f) A proposed structure of HOF-25-Re for
carbon dioxide reduction. [Octyl chains on the guanine unit are not
shown in these structures. C: grey; N: cyan; O: red; H: white; Re:
yellow; Cl: green].
Results and Discussion
Design and Assembly of HOF-25 and HOF-25-Re
Biological G-quadruplex moiety serves as a rigid hydro-
gen-bonded scaffold for the design and synthesis of various
supramolecular assemblies due to its eight Hoogsteen hydro-
gen bonds among four guanine units.[13] In particular, the
planar G-quadruplex prefers to form p-p stacking, further
consolidating the hydrogen-bonded assemblies. As a result,
indicated by the powder X-ray diffraction (PXRD) analysis.
The post-modification of HOF-25 with Re(CO)5Cl gives
a Re-functionalized porous HOF (HOF-25-Re) with the
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the guanine-decorated building blocks, which are able to pin
molecular catalysts, seem to be good candidates in construct-
ing robust porous HOFs as a new heterogenized platform.
Towards detailing such a proof-of-concept, the sonogashira
coupling of Boc-protected guanine derivative with 5,5’-
diethynyl-2,2’-bipyridine provided Boc6G2bpy as the precur-
sor of L (Scheme 1 and Figures S1 and S2). The biological
HOF-25 was obtained by the deprotection reaction of
Boc6G2bpy in a mixed solution of TFA and CH2Cl2 followed
by the subsequent self-assembly of L after simultaneous
solvent evaporation&diffusion (Figures S3 and S4). Reaction
of bpy groups in HOF-25 with Re(CO)5Cl in a solution of
toluene and methanol gave HOF-25-Re with the aim to graft
Re(bpy)(CO)3Cl functional sites on this HOF.
Characterization of HOF-25
The HOF-25 structure was investigated using PXRD
(Figures 1a–e). As shown in Figure 1e, there exists an intense
diffraction peak at 2q ꢀ 3.478 and several relatively weak
peaks at 4.728, 6.938, 10.388, 12.758, 13.838, and 25.808 in the
experimental PXRD pattern of the HOF-25 sample. Based on
peak indexing, HOF-25 crystal belongs to the tetragonal
system. Similar to G-quadruplex organic frameworks assem-
bled from guanine-functionalized naphthalene diimide deriv-
atives,[13a,b] G-quadruplex directs L to form a two-dimensional
supramolecular grid with a four-connected sql net (Figures 1a
and b). As detailed in the Supporting Information, an AA-
stacking structural model with space group of P4/m is
established (Figures 1c and d). This is supported by the Le
Bail fitting on as-synthesized HOF-25 with good agreement
factors a = b = 25.516(6) Š and c = 3.448(2) Š, Rp = 0.0595,
Figure 2. a) SEM micrograph of HOF-25. b) HRTEM micrograph of
HOF-25. c) EXAFS spectroscopic data of HOF-25-Re and Re(bpy)-
(CO)3Cl complex in energy R-space at Re L3-edge (solid lines are fitting
lines). d) CO2 adsorption (solid) and desorption (hollow) curves of
HOF-25 (196 K: square, 273 K: triangle) and HOF-25-Re (196 K: circle,
273 K: pentagon), respectively.
solution with pH 7.0–11.0 and common organic solvents
(including acetonitrile, methanol, toluene, and acetone) in
Figures S7 and S8. The robust bpy-containing HOF-25 with
excellent tolerance to common solvents is the premise of post-
modification and provides a new candidate to graft the
molecular metal complex. In addition, the robustness of
HOF-25 was also evidenced by the matched PXRD patterns
between as- synthesized and activated samples obtained by
degassing dichloromethane-exchanged HOF-25 at 258C (Fig-
ure 1e).
Rwp = 0.0782, (Figure S5). The simulated PXRD pattern
based on the present structural model is overall consistent
with the experimental data. The seven diffraction peaks
mentioned above correspond to (100), (110), (200), (300),
(320), (400), and (001) reflections, respectively (Figure 1e).
The strong (100) diffraction and moderately strong (110)
diffraction indicate a long-range order within the ab plane.
The distance of adjacent G-quadruplex centers in the square
grid is 2.54 nm. Grid-like hydrogen-bonded layers are further
stacked into a three-dimensional supramolecular structure via
interlayered p-p interactions with the layer-layer separation
of ꢀ 3.45 Š (Figures 1c and d). Note that the octyl chains on
the guanosine units are relatively “soft” and most likely
disordered. Therefore, they are not included in the structural
model. This is partly the reason for the difference in relative
intensities between the experimental and simulated PXRD
patterns. Scanning electron microscopy (SEM) photo shows
the nanoscale morphology of HOF-25 (Figure 2a). The
crystalline nature of HOF-25 is verified by high resolution
transmission electron microscope (HRTEM) images with
clear lattice fringes, indicating the d-spacing of 0.4 nm (Fig-
ure 2b).
Characterization of HOF-25-Re
After the post-functionalization of HOF-25 with Re-
(CO)5Cl, the hydrogen-bonded reticular structure is main-
tained by HOF-25-Re referring to the similar PXRD pattern
with the parent sample (Figures 1 f and S9). However, a slight
expansion of framework is observed according to the shift of
(100) diffraction due to the introduction of Re(CO)5Cl. The
nanoscale morphology and crystalline nature of this HOF
material were determined by the SEM and HRTEM charac-
terizations (Figure S10a and 10b). An ICP-OES analysis
indicates 1.1 wt% Re content in HOF-25-Re, corresponding
to the transformation of about one-sixteenth bpy units in
HOF to Re(bpy)(CO)3Cl. X-ray photoelectron spectra (XPS)
of HOF-25-Re together with Re(bpy)(CO)3Cl benchmark
have been collected (Figure S11), revealing the existence of
C, N, O, Re, and Cl for both samples. The monovalent
rhenium in HOF-25-Re is indicated by the two distinct Re 4f5/
HOF-25 shows good thermostablity with the decomposed
temperature higher than 4008C (Figure S6). Powder XRD
studies also reveal that the structure periodicity of pristine
HOF-25 was well preserved after soaking in the aqueous
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and Re 4f7/2 peaks appearing at the binding energy of 44.2
(Figure S16). After the covalent graft of Re(bpy)(CO)3Cl
units in HOF-25, the UV-vis diffuse reflectance spectrum
becomes much broader due possibly to the metal-to-ligand
charge transfer (MLCT) and extended delocalization upon
metalation involved,[15] (Figure S16). The Tauc plot discloses
an optical band gap of 2.25 eV (Figure S17). According to the
Mott-Schottky curves collected at frequencies of 500, 1000,
and 1500 Hz shown in Figure S18, the flat potential is
identified as À1.28 V vs. Ag/AgCl using Kubelka-Munk
(KM) method. The conduction band (LUMO) and valence
band (HOMO) level for HOF-25-Re therefore were calcu-
lated as À1.08 and 1.17 eV (vs. NHE), respectively (Fig-
ure S19).
The photophysical behaviors of HOF-25-Re and Re-
(bpy)(CO)3Cl as heterogeneous and homogeneous phase,
respectively, in CH3CN were collected using nanosecond
transient absorption (TA) spectra upon the excitation at
420 nm (Figures 3a–c). For the TA spectra of Re(bpy)-
(CO)3Cl, the broad photoinduced absorption bands appear
at 420–550 and 600–1000 nm due to the MLCT of rhenium
complexes.[16] The photo-induced bleach band is not observed
due to the ground-state absorption of Re(bpy)(CO)3Cl
beyond the present testing range (Figure 3a). With respect
to the TA spectra of HOF-25-Re in Figure 3b, the photo-
induced bleach bands observed before 500 nm are close to the
ground-state absorption peak, being assigned to its ground
state bleach. An excited state absorption from 600 to 1000 nm
for HOF-25-Re is similar to that of Re(bpy)(CO)3Cl due to
the formation of the excited MLCT state. The normalized
single-wavelength dynamics curves of HOF-25-Re and Re-
(bpy)(CO)3Cl probed at 750 nm reveal that the excited state
of HOF-25-Re exhibits longer lifetime (t = 102.8 ns) than
(t = 31.2 ns) that of Re(bpy)(CO)3Cl (Figure 3c), suggesting
the effective role of Re-functionalized HOF in heterogeneous
phase in inhibiting charge recombination. These data indicate
2
and 41.9 eV (Figure S12), suggesting the graft of Re-func-
tional sites. Despite the similar XPS peak profiles, a shift from
the binding energies at 43.8&41.5 eV for Re(bpy)(CO)3Cl to
44.2&41.9 eV for HOF-25-Re is observed due possibly to the
introduction of acetylene groups to the former compound,
which influences the coordination field strength. In addition,
the characteristic v(CO) bands of Re(bpy)(CO)3Cl appear
between 2050 and 1850 cmÀ1 in the IR spectrum of HOF-25-
Re (Figure S13).[14] These results suggest the inclusion of
Re(bpy)(CO)3Cl catalytic active units in HOF-25-Re. The
possible generation of rhenium nanoparticles for HOF-25-Re
under the post-synthetic conditions is precluded by the
homogeneous Re atomic distribution in element energy
dispersive spectroscopy (EDS) mapping (Figure S10c). Fur-
thermore, X-ray absorption near-edge structure (XANES)
spectrum of HOF-25-Re at Re L3-edge is similar to that of
Re(bpy)(CO)3Cl (Figure S14), revealing the monovalent
nature of Re involved in this HOF. Fitting of extended X-
ray adsorption fine structure (EXAFS) spectroscopic data of
HOF-25-Re at Re L3-edge was performed on an octahedral
coordination geometry of ReI atom (Figure 2c), consisting of
one chloride anion, three carbonyl C atoms, and two bpy N
atoms. The Re-Cl, Re-C, and Re-N distances are fitted as 2.38,
2.10, and 2.01 Š, respectively (Table S1). These distances are
consistent with the Re(bpy)(CO)3Cl benchmark (Table S1),
confirming the covalent graft of Re(bpy)(CO)3Cl segments
on HOF-25.
Permanent Porosity Study
Similar to many porous organic crystals reported pre-
viously,[7b,d,h,8] no nitrogen sorption for activated HOF-25 and
HOF-25-Re was determined at 77 K due possibly to the octyl-
induced porosity dynamics, which blocks the gas diffusion.
The permanent porosities of these two HOFs were evaluated
by carbon dioxide sorption at 196 K. The BET surface area of
HOF-25 is estimated as 173 m2 gÀ1. Permanently porous
HOF-25 absorbs the CO2 uptake of 8.3 and 35.5 cm3 gÀ1
under 1 bar at 298 and 273 K, respectively (Figures 2d and
S15). After post-modification with Re(CO)3Cl segments, the
BET surface area of HOF-25-Re is slightly decreased to
149 m2 gÀ1 due to the partly porosity collapse (Figure 2d).
Porous architecture of HOF-25-Re is helpful for the absorp-
tion and diffusion of CO2 molecule in solution.
Photophysical Investigation of HOF-25-Re
Re(bpy)(CO)3Cl is one of the most famous homogeneous
photocatalyst to drive CO2 reduction, but the dimerization-
driven deactivation limits its practical use.[3b] As a result, it is
still of significance to enhance their durability and to create
high performance photocatalytic derivatives. Before the
photocatalysis, the UV-vis diffuse reflectance spectrum of
HOF-25-Re was collected with HOF-25 as reference to
explore its light absorption scope. For HOF-25, there is
a broad absorption with two peaks at 280 and 419 nm
Figure 3. Transient adsorption spectra of Re(bpy)(CO)3Cl (a) and
HOF-25-Re (b) with 420 nm excitation. c) Normalized kinetic traces of
the 750 nm band for Re(bpy)(CO)3Cl (triangle) and HOF-25-Re (circle).
d) Steady state fluorescence spectra of [Ru(bpy)3]Cl2 upon the addition
of HOF-25-Re (lex =420 nm).
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the much more reaction opportunities between the excited
states of HOF catalyst and CO2, being favorable for CO2
reduction.
Heterogeneous Photocatalysis of HOF-25-Re
Among various photocatalysts for CO2 reduction, molec-
ular metal complexes have shown prominent superiority in
structure tailorability, mechanistic investigation, product
selectivity, and price.[17] Incorporation of molecular catalysts
with porous materials such as MOFs[1a,3a–c] and COFs[4]
effectively heterogenize the homogeneous catalysts for the
easy product isolation and catalyst recycle, prolonging their
service lives. In this case, HOF-25-Re seems to be a new
prototype to elucidate the possibility of heterogenizing
molecular catalysts by HOFs for photocatalytic CO2 reduc-
tion. As a consequence, the CO2 photoreduction performance
of 5-minute sonication-treated porous HOF-25-Re (5.0 mg)
was tested in the presence of TIPA sacrificial reagent in
CH3CN with the assistance of bpy (Figure S20). Upon visible-
light irradiation (300 W Xe lamp, l ꢁ 420 nm) and under the
CO2 pressure at 1.0 bar for 2.0 hours, gas chromatography
(GC) analysis detects a binary product containing CO and H2.
The CO evolution amount of 56 mmolgÀ1. This value is far
lower than that (4869 mmolgÀ1) for Re(bpy)(CO)3Cl homo-
geneous catalyst (5.0 mg) under the same reaction conditions
due to the weaker light absorption capability of HOF-25-Re
solid than molecular catalyst in solution. In addition, the
strong electron back-donation from Re ion to carbonyl ligand
of HOF-25-Re, as the indication of the much higher peak
intensity of Re L3-edge for this HOF catalyst than Re(bpy)-
(CO)3Cl benchmark (Figure S14), may be adverse to the
excitation under the present photocatalytic condition. Pro-
longing the reaction time to 12.0 hours, the generated CO
amount upon HOF-25-Re is slowly increased to 271 mmolgÀ1.
In contrast, the evolved CO amount with Re(bpy)(CO)3Cl is
constant from 2.0 to 12.0 hours due possibly to the dimeriza-
tion-driven deactivation. During the photocatalysis proceed-
ing in 12.0 hours, TON (4.8) of HOF-25-Re finally surpasses
that of homogeneous control (2.2), because the covalent
attachment of Re(bpy)(CO)3Cl on porous HOF precludes the
dimerization-driven deactivation of molecular photocatalysts.
With the addition of [Ru(bpy)3]Cl2·6H2O into the above
photocatalytic system and the irradiation of 2.0 hours, the CO
and H2 evolution rates are significantly increased to 1448 and
108 mmolgÀ1 hÀ1, respectively (Figure 4a). In particular for
CO evolution rate, it was enhanced 51 times in comparison
with that (28 mmolgÀ1 hÀ1) in the absence of photosensitizer
due to the introduction of electron transfer from [Ru-
(bpy)3]Cl2 to HOF-25-Re.[15,18] In the control experiment by
reducing the amount of HOF-25-Re (2.0 mg), the highly
selective photocatalytic performance, in term of the CO
evolution rates of 3030 mmolgÀ1 hÀ1, is much superior to that
of photocatalyst (5.0 mg) (Table 1). Such a rapid CO evolu-
tion rate and high product selectivity of 93% obviously
surpass those excellent photocatalysts such as Ni-TpBpy,[4a]
MAF-X27-OH,[19] and MOF-Co[20] under the help of [Ru-
(bpy)3]Cl2·6H2O photosensitizer (Table 1). The TON (50.4)
Figure 4. a) Time-dependent CO and H2 evolution for the visible-light
irradiated photocatalytic system with HOF-25-Re and CO2 at 1.0 bar in
presence of [Ru(bpy)3]Cl2·6H2O and TIPA in CH3CN. b) The TON
comparison of the CO2 photoreduction between HOF-25-Re and Re-
(bpy)(CO)3Cl in heterogeneous and homogeneous phase, respectively.
c) Stability test of HOF-25-Re photocatalyst with 2.0 h each run.
d) Mass spectrum of 13CO produced from the photocatalytic reduction
of 13CO2.
Table 1: Comparison of Reported Catalysts for Photocatalytic CO2
Reduction to CO with the Help of [Ru(bpy)3]Cl2 Photosensitizer under
Visible Light Irradiation in 1.0 atm CO2.
Catalyst
Rate of CO
Selectivity
of CO [%]
Ref.
[molgÀ1 hÀ1
]
HOF-25-Re[a]
HOF-25-Re[b]
Ni-TpBpy
MAF-X27-OH
MOF-Co
1448
3030
966
225.8
1140
3334
2003
1400
483
93
92
96
97
47
83
77
86
93
78
this work
this work
[4a]
[19]
[20]
[21]
[22]
[23]
[24]
BIF-29
Co3O4 platelets
Re-Bpy-sp2c-COF
PI-COF-TT
COF-367-Co NSs
10162
[25]
[a] HOF-25-Re (5.0 mg). [b] HOF-25-Re (2.0 mg).
based on Re atoms is about eight times than that (6.3) of
homogeneous control Re(bpy)(CO)3Cl at the same reaction
condition (Figure 4b). In contrast, there is only a little
amount of CO detected in the blank, non-photosensitizer,
non-sacrificial reagent, non-irradiation, HOF-25, and N2
control experiments (Figure S21), suggesting the Re(bpy)-
(CO)3Cl units in HOF-25-Re serve as catalytically active sites
for the present photocatalysis of CO2 reduction. The high CO/
H2 selectivity of HOF-25-Re (93%) is the same as that of the
homogeneous molecular counterpart Re(bpy)(CO)3Cl. In
particular, after the photo-driven conversion of CO2 to CO,
about 3.4% Re content in HOF-25-Re was determined to be
leached into the filtrate on the basis of the ICP data,
suggesting the moderate stability of Re(bpy)(CO)3Cl unit in
this HOF material. The high CO evolution rate above
1350 mmolgÀ1 hÀ1 and 93% product selectivity for heteroge-
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neous HOF-25-Re photocatalyst are able to be lasted for Conclusion
three cycles (Figure 4c). Photocatalytic CO2-to-CO conver-
sion upon HOF-25-Re for 16.0 hours (8 cycles) deactivated
this catalyst, giving low CO generation rate of
In summary, a robust porous biological HOF with the bpy
dock group to precisely load molecular catalyst has been
designed and prepared on the basis of rigid hydrogen-bonded
and p-p stacking metal-free G-quadruplex moieties. With the
help of sensitizer, the covalent graft of molecular photo-
catalyst on this porous HOF promotes efficient and selective
CO2-to-CO conversion in heterogeneous system. In partic-
ular, stable and recyclable HOFs were suggested as a prom-
inent platform in lastingly heterogenizing molecular catalysts.
These results not only illustrate a new strategy to construct
robust HOFs but also give a clear insight into porous HOF to
heterogenize molecular catalysts for carbon dioxide photo-
reduction. The present investigation opens a new avenue for
engineering functional HOFs by synergistic molecular design
and post-modification.
a
784 mmolgÀ1 hÀ1 and 68% selectivity. Self-assembly of L in
used HOF-25-Re enables the regeneration of HOF-25, and
further post-functionalization with Re centers can recover the
catalyst performance with the CO evolution rate of
1671 mmolgÀ1 hÀ1 and 92% selectivity (Figures S22 and S23),
indicating the sustainable advantage of HOF platform to
position molecular catalysts.
Photocatalysis Mechanism
During the photocatalysis, an electron transfer mecha-
nism from excited [Ru(bpy)3]Cl2·6H2O photosensitizer to
HOF-25-Re is suggested by their matched energy levels
(Figure S19).[18b, 20] This point was further probed by the
luminescent quenching experiment by adding the HOF-25-Re
to a CH3CN solution of [Ru(bpy)3]Cl2·6H2O and TIPA.[21]
The emission at 605 nm is assigned to the metal ligand charge
transfer (MLCT) excited state of [Ru(bpy)3]Cl2 (Figure 3d),
and the intensity is continuously reduced following the
increased amount of HOF-25-Re, implying the occurrence
of electron transfer from the excited state of photosensitizer
to generate a reduced state of HOF-25-Re.[18b,26] In addition,
the fitting of luminescent decay profile gives a lifetime
(161.3 ns) for photosensitizer (Figure S24). The addition of
HOF-25-Re to [Ru(bpy)3]Cl2 solution reduces the lifetime of
photosensitizer to 41.7 ns. Such shortened luminescent life-
time also suggests that photo-induced electrons transfer
occurs between [Ru(bpy)3]Cl2 and HOF-25-Re to trigger
the photocatalysis. Actually, the electron transfer from Ru
photosensitizer to Re photocatalyst in the present case is well
supported by the excellent photocatalytic systems containing
[Ru(bpy)3]+ and Re(bpy)(CO)3Cl components revealed pre-
viously.[15,18a]
Acknowledgements
Financial support from the National Natural Science Foun-
dation of China (Nos. 21631003, 21805005, and 22011540002),
the Fundamental Research Funds for the Central Universities
(No. FRF-BD-20-14A and FRF-IDRY-19-028), University of
Science and Technology Beijing is gratefully acknowledged.
We are also very grateful for the help and discussion from the
groups of Profs. Xianran Xing, Jiemin Liu, and Ichiro Hisaki.
Conflict of interest
The authors declare no conflict of interest.
Keywords: carbon dioxide reduction ·heterogeneous catalysis ·
hydrogen-bonded organic frameworks ·post-modification ·
visible-light photocatalysis
1
After the photocatalysis, H NMR spectroscopic analysis
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reaction liquid and the building block of HOF-25 (Fig-
ure S25), further indicating the high CO evolution selectivity
associated with the heterogeneous HOF catalyst. In addition,
powder XRD study reveals the porous periodic structure for
the used HOF-25-Re (Figure S26). The photocatalytic per-
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observed at m/z = 29 (Figures 4d and S27), confirming the
present CO2-to-CO conversion mechanism.
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Manuscript received: December 16, 2020
Revised manuscript received: January 22, 2021
Accepted manuscript online: January 25, 2021
Int. Ed. 2016, 55, 10667 –10671; Angew. Chem. 2016, 128, 10825 – Version of record online: March 9, 2021
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