Probe metal binding mode of imine covalent organic frameworks: cycloiridation for (photo)catalytic hydrogen evolution from formate

Article abstract of DOI:10.1039/d1sc01692j

Metalation of covalent organic frameworks (COFs) is a critical strategy to functionalize COFs for advanced applications yet largely relies on the pre-installed specific metal docking sites in the network, such as porphyrin, salen, 2,2′-bipyridine,etc.We show in this study that the imine linkage of simple imine-based COFs, one of the most popular COFs, readily chelate transition metal (Ir in this work)viacyclometalation, which has not been explored before. The iridacycle decorated COF exhibited more than 10-fold efficiency enhancement in (photo)catalytic hydrogen evolution from aqueous formate solution than its molecular counterpart under mild conditions. This work will inspire more functional cyclometallated COFs to be explored beyond catalysis considering the large imine COF library and the rich metallacycle chemistry.

Full text of DOI:10.1039/d1sc01692j

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Probe metal binding mode of imine covalent
organic frameworks: cycloiridation for (photo)
Cite this: Chem. Sci., 2021, 12, 7930
catalytic hydrogen evolution from formate†
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
a
b
c
Jiyun Hu,
Hamed Mehrabi,
Yin-Shan Meng,
Maddison Taylor,a
Jin-Hui Zhan, Qigeng Yan,e Mourad Benamara,e Robert H. Coridan
d
a
*
a
*
and Hudson Beyzavi
Metalation of covalent organic frameworks (COFs) is a critical strategy to functionalize COFs for advanced
applications yet largely relies on the pre-installed specific metal docking sites in the network, such as
porphyrin, salen, 2,20-bipyridine, etc. We show in this study that the imine linkage of simple imine-based
COFs, one of the most popular COFs, readily chelate transition metal (Ir in this work) via cyclometalation,
which has not been explored before. The iridacycle decorated COF exhibited more than 10-fold
efficiency enhancement in (photo)catalytic hydrogen evolution from aqueous formate solution than its
molecular counterpart under mild conditions. This work will inspire more functional cyclometallated
COFs to be explored beyond catalysis considering the large imine COF library and the rich metallacycle
chemistry.
Received 1st April 2021
Accepted 17th April 2021
DOI: 10.1039/d1sc01692j
rsc.li/chemical-science
linkages,38 which have been well studied in coordination
chemistry.39–41 If the imine linkages can be effectively exploited
Introduction
for binding metals, a huge COF catalyst library would be
accessible considering the large number of imine COFs. While
the rst COF-based heterogeneous catalyst was constructed on
an imine COF, in which palladium(II) was chelated by two imine
moieties from adjacent COF layers in 2011 (Fig. 1A),42 not much
progress has been achieved on using imines in COFs as intrinsic
ligands for transition metals during the past decade. Only
recently a new binding mode of stabilizing palladium(II) with
imine and amine defect from adjacent layers of an imine COF
was identied (Fig. 1B).43 Thus, it is of great interest to explore
both the scope of metal species and binding modes of imine-
based COFs.
(Photo)catalytic hydrogen evolution reaction (HER) from
water splitting is a promising clean energy production tech-
nique. COFs have been proved to be potential photocatalysts in
promoting HER.44–49 The system is typically comprised of COF
photocatalyst, Pt nanoparticle cocatalyst, and a sacricial
The designable ordered porous structure of covalent organic
frameworks (COFs) has attracted broad research interest in the
past decades.1–4 Although COFs are completely composed of
light elements (typically H, B, C, N, O, Si), their advanced
applications, particularly catalysis,5–8 have stressed the role of
metal species anchored onto the network.8–10 To accommodate
the target metal, a building unit with a suitable metal binding
site is required to be incorporated into the framework, which
usually meets complicated organic synthesis. Nevertheless,
plenty of functional linkers have been successfully imple-
mented into COFs, such as 2,20-bipyridine,11–14 phenanthro-
line,15–17 porphyrin,18–22 phthalocyanine,23–25 salen,26–29 hydroxy
groups,30–32
b-ketoenamine,33
phosphine,34,35
dehy-
drobenzoannulene,36,37 among others.9,10 As one of the most
popular COFs featuring both high crystallinity and stability,
Schiff base COFs possess uniformly distributed imine
aDepartment of Chemistry and Biochemistry, University of Arkansas, Fayetteville,
Arkansas, 72701, USA. E-mail: beyzavi@uark.edu
bMaterial Science and Engineering Program, University of Arkansas, Fayetteville,
Arkansas, 72701, USA
cState Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian,
116024, China
dState Key Laboratory of Multiphase Complex System, Institute of Process Engineering,
Chinese Academy of Sciences, Beijing, 100190, China. E-mail: jhzhan@ipe.ac.cn
eInstitute for Nanoscience
Arkansas, 72701, USA
& Engineering, University of Arkansas, Fayetteville,
† Electronic supplementary information (ESI) available. See DOI:
Fig. 1 Metal binding modes of imine-based COFs.
10.1039/d1sc01692j
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electron donor (e.g. ascorbic acid, triethanolamine) in an organometallic unit. FT-IR analysis revealed a new C]N
aqueous solution. Formic acid serves as an alternative high H2 vibration band at 1597 cmꢁ1, corresponding to the metal
storage density reservoir (4.3 wt%).50,51 The decomposition of coordinated imines, in addition to the free ones at 1623 cmꢁ1
HCOOH under basic conditions could release H2 with high for Py-1P–Ir COF and (Fig. 2B). The shiing of C]N vibration to
purity. However, some iridium-based homogeneous HCOOH lower wavenumber upon coordination to Ir was also observed
dehydrogenation catalysts displayed low stability and were for N-benzylideneaniline (L1) when forming the model irida-
prone to deactivate via nanoparticle formation.52,53 Immobili- cycle complex (L1–Ir, 1626 to 1582 cmꢁ1, Fig. S1†).62 The coor-
zation of the catalyst on proper solid support is expected to dination of imine N to iridium was further proved by XPS
extend the catalyst lifetime.54 Taking all of these considerations analysis (Fig. S2† and 2C). The N 1s XPS spectrum of Py-1P–Ir
together, in this work, we show that cyclometalation at the COF showed two subpeaks with binding energies of 399.0 and
imine site leads to an iridacycle functionalized imine COF for 400.0 eV, which were assigned to the free and coordinated imine
the rst time (Fig. 1C), which exhibits fascinating performance nitrogen, respectively (Fig. 2C). The increase of N 1s binding
in (photo)catalytic HER from aqueous formate solution.
energy upon coordination is consistent with that of the model
complex L1–Ir (399.2 to 399.8 eV, Fig. S3†). Solid-state cross-
polarization/magic angle spinning (CP/MAS) 13C NMR spec-
troscopy unambiguously conrmed the formation of the irida-
cycle in Py-1P–Ir COF. Three new signals at 173.7, 89.0, and
7.6 ppm appeared in the CP/MAS 13C NMR spectrum of Py-1P–Ir
COF (Fig. 2D). The broad peak at 173.7 ppm is assigned to the
iridium bonded carbon and the imine carbon of the iridacycle,
while the peaks at 89.0 and 7.6 ppm originate from the aromatic
and methyl carbons of Cp* ring, respectively, which matches
well with the 13C NMR spectrum of the model complex L1–Ir
(Fig. S4†). Meanwhile, the relative intensity of the free imine
carbon peak at 157.0 ppm of Py-1P–Ir COF decreased accord-
ingly compared to that of Py-1P COF. N2 adsorption experiment
was carried out to investigate the porosity of Py-1P–Ir COF
(Fig. 2E). The Brunauer–Emmett–Teller (BET) surface area of Py-
1P–Ir COF is 972 m2 gꢁ1, which is lower than 1960 m2 gꢁ1 of Py-
1P COF (Fig. S5 and S6†). The pore size is decreased to 1.57 nm
for Py-1P–Ir COF, compared to 2.19 nm for Py-1P COF, which is
expected for the pore wall metalation (Fig. 2F). The iridium
loading was determined to be 11.8 wt% by inductively coupled
plasma mass spectrometry (ICP-MS), corresponding to ca. 20%
imine metalation. Scanning electron microscope analysis
revealed that Py-1P–Ir COF was composed of aggregates of
nanometer-sized particles, similar to that of Py-1P COF
(Fig. S7†). Energy dispersive X-ray (EDX) analysis revealed the
homogeneous distributions of Ir and Cl elements over the
framework, and the Ir/Cl ratio is close to 1/1, as expected for the
Results and discussion
Synthesis and characterization
Metallacycles of Schiff base ligands have been well explored for
transition metals in organometallic chemistry.55–57 Neverthe-
less, introducing such type of complexes into COFs is unprec-
edented. To explore this new binding mode, a pyrene-based
imine COF (Py-1P) constructed from 1,3,6,8-tetrakis(4-
aminophenyl)pyrene and terephthalaldehyde was chosen as
a model system because of its high crystallinity58 and facile
synthesis.59 Iridium became the metal choice because of the
various catalytic applications of iridacycles.60,61 Metalation of Py-
1P COF was carried out by reuxing with [Cp*IrCl2]2 (Cp*,
pentamethylcyclopentadienyl) in methanol in the presence of
NaOAc for 24 hours (Scheme 1, see ESI† for details).
The successful formation of the iridacycle functionalized
COF Py-1P–Ir was conrmed by FT-IR spectroscopy, X-ray
photoelectron spectroscopy (XPS), and solid-state NMR char-
acterization, and the retention of the framework crystallinity
and porosity was examined by powder X-ray diffraction (PXRD)
and N2 adsorption–desorption isotherm analysis, respectively.
Py-1P–Ir COF showed strong diffraction peaks at 2q ¼ 3.74, 5.36,
7.56, 11.40, 23.52ꢀ, which are slightly shied to low 2q direction
(0.04 to 0.18ꢀ difference) compared to the diffraction pattern of
the parent Py-1P COF (Fig. 2A). This might result from the
crystal cell expansion aer introducing the bulky Ir
Scheme 1 Cyclometalation of Py-1P COF with iridium.
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Fig. 2 Comparison of the (A) PXRD patterns, (B) FT-IR spectra, (C) N 1s XPS spectra, (D) CP/MAS 13C NMR spectra, (E) N2 adsorption–desorption
isotherms, and (F) pore size distributions of Py-1P and Py-1P–Ir COFs. Pore size distribution was analysed using the nonlocal density functional
theory equilibrium model. The NMR spinning sidebands in (D) are marked with asterisks (*).
proposed structure (Fig. S8†). Besides, the formation of the found to favor the reaction that 48.9 and 213.8 mmol H2 were
iridacycle was also successfully performed on an azine linked afforded at pH ¼ 5.92, 3.72 respectively, while further lowering
COF (see ESI† for details), demonstrating the generality of the the pH to 1.94 did not lead to higher H2 production (200.2 mmol,
cyclometalation modication of imine-based COFs.
Fig. 3C). The increased H2 production came with compromised
H2 purity to ca. 70% under acidic conditions. The concentration
of HCOONa on the HER showed a volcano-type effect (Fig. 3D).
The H2 yield increased to 163.2 mmol from the reaction of 2 M
HCOONa but started to fall back to 137.5 mmol for 5 M HCOONa
and 92.6 mmol for 5 M HCOONa respectively thereaer. The H2
purities were all higher than 90% from the tested four HCOONa
concentrations. The decreased reaction efficiency might be due
to the decreased concentration of water, which is the proton
source for H2 production. Py-1P–Ir COF exhibited excellent
stability under all the tested conditions. All the recovered COF
samples preserved the crystallinity as shown by PXRD analysis
(Fig. S20–S22†). Interestingly, FT-IR and XPS indicate that the
imine linkage was partially reduced (Fig. S20–S23†). The imine
bond reduction is likely mediated by an iridium hydride inter-
mediate via an outer-sphere process.64–66 The involvement of Ir
in the imine reduction is supported by the fact that no imine
reduction was observed in Py-1P COF during HER catalysis
(Fig. S24†) and the catalytic reduction of N-benzylideneaniline
as an exogenous substrate in the presence of Py-1P–Ir COF was
observed (Fig. S25†). ICP-MS showed that the Ir concentrations
in the reaction ltrates were all below 60 ppb (Table S2†),
demonstrating the heterogeneous catalysis nature of the reac-
tion. The high stability of Py-1P–Ir COF allowed it to be recycled
for at least another four runs (Fig. 4). Interestingly, an initial
performance improvement was observed in the second cycle.
Catalytic HER from formate
Iridium complexes have demonstrated their potential to be
homogeneous catalysts for hydrogen production from
formate.50–53,63 The homogeneous distribution of the single-site
iridacycle over the Py-1P–Ir COF with high surface area
prompted us to test its capability in promoting formate
decomposition to release H2. To our delight, Py-1P–Ir COF
produced 96.3 mmol H2 with a purity of 94% from 10 mL of
1.0 M HCOONa solution at 65 ꢀC in 6 hours (6.35 mmol catalyst
based on Ir, Fig. 3A). In contrast, L1–Ir gave only 7.9 mmol H2
with 70% purity under otherwise identical conditions. Py-1P
COF was catalytically inactive, showing even lower H2 produc-
tion (0.8 mmol, 42%) than the control reaction (2.3 mmol, 65%).
These results highlight the importance of the unique structure
of Py-1P–Ir COF. Then, we set to investigate the inuence of
reaction parameters on the hydrogen production efficiency
catalyzed by Py-1P–Ir COF, including temperature, pH, and
formate concentration.
As shown in Fig. 3B, Py-1P–Ir COF displayed higher efficiency
at higher temperature. An amount of 250.0 mmol H2 was ob-
tained at 85 ꢀC, which is ca. 2.6 times of that obtained at 65 ꢀC.
The high reactivity of Py-1P–Ir COF was manifested that 16.5
ꢀ
mmol H2 was formed even at 25 C. In all the tested tempera-
tures, the H2 purity was no less than 90%. Lowering the pH was
7932 | Chem. Sci., 2021, 12, 7930–7936
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Fig. 3 Catalytic hydrogen evolution from aqueous formate solution. The influence of catalyst (A), temperature (B), pH (C), and formate
concentration (D) on the reaction outcome (H2 quantity and purity are presented in orange column and grey line/square symbol form,
respectively. Purity refers to the H2 composition in the generated H2/CO2 mixture). Standard reaction condition: the reactions were carried out
by heating a 10 mL HCOONa solution (1.0 M, pH ¼ 7.52) containing 6.35 mmol catalyst (based on Ir) at 65 ꢀC for 6 h. The pH effect was studied at
room temperature. The concentration effect was studied at 65 ꢀC and the pHs of HCOONa solutions were not adjusted. See ESI† for details.
Pyrene-based COFs have exhibited excellent photophysical
properties which benet photocatalysis.67–70 Diffuse reectance
UV-vis absorption spectrum of Py-1P–Ir COF displayed prom-
inent absorption in 200–500 nm and a long tail to near-IR range
(500–800 nm, Fig. 5). The incorporation of iridium complex into
Py-1P COF did not change the absorption in 200–500 nm band
but strongly improved the absorption ability in the 500–800 nm
region. Encouraged by the excellent light absorption ability
across the whole UV-visible region of Py-1P–Ir COF, we further
Fig. 4 Recyclability of Py-1P–Ir COF. The reactions were carried out
in 1 M HCOONa at 85 ꢀC for 6 hours.
This improved reactivity of Py-1P–Ir COF is attributed to the
presence of more reactive Ir species in recovered material, in
which the Cl ligand has been replaced (see mechanism
discussion below). This is supported by the fact that no Cl
element was detected by EDX in Py-1P–Ir COF aer one HER
cycle (Fig. S26†). The average H2 production rate during the ve
cycles was calculated to be 4626 mmol gꢁ1 hꢁ1, corresponding to
a TOF of 7.3 hꢁ1. The recovered Py-1P–Ir COF aer ve cycles
was still crystalline (Fig. S28†). No metal nanoparticle formation
was observed by XPS (Fig. S29†) and TEM (Fig. S30†) analysis.
Fig. 5 Diffuse reflectance UV-vis spectra of Py-1P and Py-1P–Ir COF.
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Table 1 Photocatalytic HER from aqueous formate solutiona
Entry
Catalyst
H2 (mmol)
Purity (%)
1
2
—
L1–Ir
0.7
2.5
84.5
31.6
0.6
10
82
93
92
68
71
58
3
Py-1P–Ir
Py-1P–Ir
Py-1P
Py-1P/L1–Ir
—
4b
5
6c
7d
19.5
2.0
a
Reaction conditions: 10 mL formate solution (1.0 M) containing 6.35
b
mmol catalyst was irradiated with 460 nm LED light for 6 h. Without
light at 45 ꢀC (the photothermal effect raised the reaction temperature
c
to ca. 42 ꢀC). Physical mixture of Py-1P COF and L1–Ir with the same
Ir and total mass loading as Py-1P–Ir COF. d Reaction ltrate of entry 3.
tested its photocatalytic HER performance from formate
decomposition. The reactions were carried out in 1 M formate
solution in the presence of 6.35 mmol catalyst (based on Ir)
under 460 nm light irradiation. As shown in Table 1, only
0.7 mmol H2 was detected in the absence of catalyst (entry 1). Py-
1P–Ir COF catalyzed the generation of 84.5 mmol H2 in high
purity (93%) in 6 hours (entry 3), which is ca. 34 times higher
than the molecular counterpart L1–Ir did (entry 2). The corre-
Fig. 6 (A) Proposed mechanism for Py-1P–Ir COF catalyzed HER from
formate; (B) calculated Gibbs free energy profile.
sponding H2 generation rate of 1358 mmol gꢁ1 hꢁ1 (TOF 2.2 hꢁ1
)
is comparable to the typical COF photocatalyst/Pt cocatalyst/
sacricial electron donor system.71 Py-1P COF failed to exert
any catalytic effect showing the critical role of iridium for the
reaction (entry 5, 0.6 mmol H2). Under the dark condition, Py-
1P–Ir COF produced 31.6 mmol H2 in 92% purity, proving it to be
a photocatalytic process (entry 4). A physical mixture of Py-1P
COF and L1–Ir displayed lower H2 production (19.5 mmol) and
poorer selectivity (71% purity), highlighting the importance of
covalent hybridization of Ir catalytic centers within the COF
(entry 6).72 The recovered Py-1P–Ir COF almost retained its
crystalline structure as evidenced by PXRD analysis (Fig. S31†).
Similarly, reduction of the imine bond was observed (Fig. S31†).
ICP-MS analysis of the Py-1P–Ir COF catalyzed reaction solution
revealed a low Ir concentration of 0.99 ppm, corresponding to
0.8% of the total Ir in Py-1P–Ir COF. Besides, the reaction ltrate
only produced 2.0 mmol H2 under the photocatalysis conditions
(entry 7), demonstrating the heterogeneous catalytic process by
Py-1P–Ir COF.
Based on literature reports,52,53 a proposed mechanism is
shown in Fig. 6A. The catalytic cycle is initiated by Cl ligand
replacement of IM-1 with formate to give HCOOꢁ coordinated
intermediate IM-2. CO2 extrusion from IM-2 affords the key
iridium hydride species IM-3. Then two pathways account for
hydrogen production depending on the reaction conditions
(pH). Under neutral or basic conditions, IM-3 reacts with one
molecular H2O to generate H2 and OHꢁ bonded Ir intermediate
IM-4, which could be reconverted to IM-1 or IM-2 via ligand
exchange to close the catalytic cycle. While under acidic
conditions, IM-3 reacts with HCOOH to release H2 and regen-
erate IM-2. The involvement of iridium hydride intermediate
was supported by NMR studies on L1–Ir in solution. Mixing L1–
Ir and excess HCOONa in CD3CN/D2O (v/v, 4/1) generated an
iridium hydride complex showing
a hydride signal at
ꢁ12.45 ppm in the 1H NMR spectrum (Fig. S32†).52,73 To further
support the proposed mechanism, we carried out DFT calcula-
tions. The computed Gibbs free energy prole of the prosed
reaction pathway is shown in Fig. 6B. IM-2 is slightly lower
(ꢁ1.56 kcal molꢁ1) in energy than IM-1, suggesting the rst step
ligand exchange is feasible. The transformation of IM-2 to IM-3
via CO2 extrusion is thermodynamically favored (DG
¼
1.88 kcal molꢁ1) with a small energy barrier of 21.85 kcal molꢁ1
(TS1). This is consistent with our experimental ndings that the
iridium hydride formed immediately at room temperature. The
reaction of IM-3 with H2O to give IM-4 and H2 is an energy
demanding process (DG ¼ 18.07 kcal molꢁ1) with an energy
barrier of 50.81 kcal molꢁ1 (TS2). In contrast, the reaction of
IM-3 with HCOOH to give IM-2 and H2 is energetically less
demanding (DG ¼ 2.11 kcal molꢁ1) and the activation energy is
also signicantly reduced to 28.68 kcal molꢁ1 (TS20). These
results match well with the observed higher reactivity of the
catalyst in lower pH conditions.
Conclusions
In summary, we identied a new metal binding mode of imine
COFs where cyclometalation with iridium readily occurred. The
iridium functionalized COF exhibited much enhanced catalytic
efficiency in HER from an aqueous formate solution than the
parent COF and the molecular counterpart in both thermal- and
photo-triggered reactions. The easily available imine-based
COFs, as well as various transition metals capable of forming
7934 | Chem. Sci., 2021, 12, 7930–7936
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metallacycles will largely advance surface organometallic 14 W. Zhong, R. Sa, L. Li, Y. He, L. Li, J. Bi, Z. Zhuang, Y. Yu and
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Author contributions
J. Hu and H. Beyzavi conceived the work. J. Hu performed all
experimental studies and data analysis. H. Mehrabi and R. H. 16 Y. Liu, Y. Ma, J. Yang, C. S. Diercks, N. Tamura, F. Jin and
Coridan performed GC analysis. Y.-S. Meng performed TGA and O. M. Yaghi, J. Am. Chem. Soc., 2018, 140, 16015–16019.
N2-sorption tests. M. Taylor was involved in linkers synthesis. J.- 17 H.-S. Xu, Y. Luo, X. Li, P. Z. See, Z. Chen, T. Ma, L. Liang,
H. Zhan performed the DFT calculations. Q. Yan and M.
Benamara performed XPS and TEM studies. J. Hu and H. Bey-
zavi wrote the manuscript with valuable inputs from all the
K. Leng, I. Abdelwahab, L. Wang, R. Li, X. Shi, Y. Zhou,
X. F. Lu, X. Zhao, C. Liu, J. Sun and K. P. Loh, Nat.
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authors. All authors reviewed and agreed with the content of the 18 S. Lin, C. S. Diercks, Y.-B. Zhang, N. Kornienko,
paper. H. Beyzavi led the project.
E. M. Nichols, Y. Zhao, A. R. Paris, D. Kim, P. Yang,
O. M. Yaghi and C. J. Chang, Science, 2015, 349, 1208–1213.
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Conflicts of interest
There are no conicts to declare.
20 H. Wang, H. Ding, X. Meng and C. Wang, Chin. Chem. Lett.,
2016, 27, 1376–1382.
21 G. Lin, H. Ding, R. Chen, Z. Peng, B. Wang and C. Wang, J.
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Acknowledgements
H. B. gratefully acknowledges the nancial support through the
startup funds from the University of Arkansas. J.-H. Zhan is 22 Y. Meng, Y. Luo, J. L. Shi, H. Ding, X. Lang, W. Chen,
grateful for the nancial support from the National Natural
Science Foundation of China (No. 21875255). Work by R. H. C
A. Zheng, J. Sun and C. Wang, Angew. Chem., Int. Ed., 2020,
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was supported by the U.S. Department of Energy (D.O.E.), Office 23 X. Ding, J. Guo, X. Feng, Y. Honsho, J. Guo, S. Seki,
of Science, Office of Basic Energy Sciences under Award Number
DE-SC-0020301. We thank Dr Barry J. Shaulis (University of
Arkansas, Department of Geosciences) for the ICP tests.
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