Rational Design of Covalent Cobaloxime-Covalent Organic Framework Hybrids for Enhanced Photocatalytic Hydrogen Evolution

Article abstract of DOI:10.1021/jacs.0c02155

Covalent organic frameworks (COFs) display a unique combination of chemical tunability, structural diversity, high porosity, nanoscale regularity, and thermal stability. Recent efforts are directed at using such frameworks as tunable scaffolds for chemica

Full text of DOI:10.1021/jacs.0c02155

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Article
Rational Design of Covalent Cobaloxime-COF Hybrids
for Enhanced Photocatalytic Hydrogen Evolution
Kerstin Gottschling, Gökcen Savasci, Hugo A. Vignolo-González, Sandra Schmidt, Philipp
Mauker, Tanmay Banerjee, Petra Rovó, Christian Ochsenfeld, and Bettina V. Lotsch
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c02155 • Publication Date (Web): 20 Jun 2020
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Rational Design of Covalent Cobaloxime-COF Hybrids for Enhanced
Photocatalytic Hydrogen Evolution
Kerstin Gottschling,†,‡,¶,§ G¨okcen Savasci,†,‡,¶,§ Hugo Vignolo-Gonz´alez,† Sandra Schmidt,‡
Philipp Mauker,‡,§ Tanmay Banerjee,† Petra Rov´o,‡,§ Christian Ochsenfeld,‡,†,¶,§ and Bettina V.
Lotsch∗,†,‡,¶,§
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†Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany
‡Department of Chemistry, University of Munich (LMU), Butenandtstr. 5-13, 81377 Munich, Germany
¶Cluster of Excellence e-conversion, Lichtenbergstrasse 4a, 85748 Garching, Germany
§Center for Nanoscience (CeNS), Schellingstr. 4, 80799 Munich, Germany
Received June 19, 2020; E-mail: b.lotsch@fkf.mpg.de
hydrogen evolution reaction and have been used in hetero-
geneous systems with MOFs20,21 and carbon nitrides,22,23
Abstract:
as well as physisorbed to COFs.14 A major drawback of
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Covalent organic frameworks (COFs) display a unique com-
molecular proton reduction catalysts physisorbed to photo-
sensitizers is their photodeactivation over time24–26 and rate
limitations due to diffusion-controlled mechanisms. While
previous attempts14 used molecular cobaloxime catalysts in
solution, in this work we report photocatalytic hydrogen evo-
lution with molecular cobaloxime catalysts covalently teth-
ered to the COF backbone, yielding unprecedented insights
into the nature of the active site and the COF-co-catalyst
interface. By comparison with equivalent unbound, i.e.
physisorbed systems we show how the modification of the
hydrazone-based COF-42 and attachment of functionalized
chloro(pyridine)cobaloxime lead to more efficient hydrogen
evolution in a water/acetonitrile mixture under visible light
illumination in the presence of a sacrificial electron donor.
The structural composition of the photoreaction is verified
by computational and experimental methods including ad-
vanced high-resolution solid-state NMR techniques. These
results combine the advantages of fully heterogeneous sys-
tems with the tunability of molecular co-catalysts and lead
the way towards true single-site COF-based photocatalytic
systems with a high level of interfacial control.
bination of chemical tunability, structural diversity, high poros-
ity, nanoscale regularity, and thermal stability. Recent efforts
are directed at using such frameworks as tunable scaffolds for
chemical reactions. In particular, COFs have emerged as vi-
able platforms for mimicking natural photosynthesis. However,
there is an indisputable need for efficient, stable, and econom-
ical alternatives for the traditional platinum-based co-catalysts
for light-driven hydrogen evolution. Here, we present azide-
functionalized chloro(pyridine)cobaloxime hydrogen-evolution
co-catalysts immobilized on a hydrazone-based COF-42 back-
bone which shows improved and prolonged photocatalytic ac-
tivity with respect to equivalent physisorbed systems. Ad-
vanced solid-state NMR and quantum chemical methods allow
us to elucidate details of the improved photoreactivity and the
structural composition of the involved active site. We found
that a genuine interaction between the COF backbone and the
cobaloxime facilitates re-coordination of the co-catalyst during
the photoreaction, thereby improving the reactivity and hinder-
ing degradation of the catalyst. The excellent stability and pro-
longed reactivity make the herein reported cobaloxime-tethered
COF materials promising hydrogen evolution catalysts for fu-
ture solar fuel technologies.
Results and discussion
In previous studies, COF-4227 has been shown to be ac-
tive in photocatalytic hydrogen evolution reactions with con-
ventional hydrogen evolution co-catalysts such as platinum
nanoparticles or molecular chloro(pyridine)cobaloxime.14 At
the same time, this COF is a well-known and versatile plat-
form that is chemically robust due to its hydrazone-linked
structure.28,29 In this study, we used COF-42 as a platform
for covalent post-synthetic modification with cobaloxime
complexes. The synthesis of COF-42 by solvothermal acid-
catalyzed condensation of 1,3,5-triformylbenzene (TFB) and
2,5-diethoxyterephthalohydrazide (DETH) followed pub-
lished protocols.27 In order to provide functional sites
for the covalent attachment of the co-catalyst, 10 mol%
of DETH was replaced by the propargyl-containing 2,5-
bis(prop-2-yn-1-yloxy)terephthalohydrazide (DPTH) to ob-
tain the propargyl-modified pCOF10. The COFs were char-
acterized by FT-IR spectroscopy, sorption analysis, powder
X-ray diffraction (PXRD), magic-angle-spinning solid-state
NMR (ssNMR), and quantum-chemical calculations.
The successful transformation of the starting materials
to pCOF10 was proven by the lack of residual aldehyde
Introduction
Identifying competitive alternatives to fossil-fuel-based
energy constitutes one of the main research goals of this
decade. Nature-inspired processes, like artificial photosyn-
thesis, guide the way to a green and sustainable solution.1–3
Covalent organic frameworks (COFs) have been emerging
as new materials in this context.4,5 COFs consist of light-
elements only and their bottom-up synthesis enables high
versatility and tunability on a molecular level while bene-
fiting from high stability and crystallinity due to covalent
bonding in plane and via π-π stacking out of plane.6–9 Most
reports of COFs as photosensitizers for light-driven hydro-
gen evolution use platinum as a co-catalyst;10–12 hydrogen
evolution rates up to 16.3 mmol h−1 g−1 have been reported
in this context.13 Recent studies showed that the precious
metal platinum can be replaced by earth-abundant molecu-
lar co-catalysts, namely chloro(pyridine)cobaloxime and re-
lated complexes.14–16 These co-catalysts are well-known and
well-defined while offering high tunability, which facilitate
their incorporation into photoactive organic and inorganic
systems.17–19 Cobaloximes feature low overpotential for the
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Figure 1. (A) Synthesis of pCOF10 by solvothermal condensation of triformyl benzene (TFB) and
a 9:1 mixture of 2,5-
diethoxyterephthalohydrazide (DETH) and 2,5-bis(prop-2-yn-1-yloxy)terephthalohydrazide (DPTH). (B) Eclipsed stacking model for
pCOF10. C, N and O atoms are represented in grey, blue and red. H atoms are omitted, the second and third layer are represented in
1
orange and yellow for clarity. (C) Solid state 1D 13C{ H} CP-MAS NMR spectrum of pCOF10 acquired at 11.7 T, 12 kHz MAS, 298 K,
and using cross-polarization times of 5 ms. Spinning side bands are marked with asterisks. Calculated shifts are marked with yellow
bars. The narrow signals labelled with crosses at 164 ppm, 37 ppm, and 32 ppm correspond to residual dimethylformamide. (D) Argon
adsorption isotherm of pCOF10
. Inlet: Pore size distribution from NLDFT calculations with cylindrical pores in equilibrium mode.
Resulting main pore size is 2.3 nm. (E) PXRD pattern of pCOF10 (open, green circles), Pawley refined profile (blue line) and calculated
XRD pattern for the idealized AA stacking (black line).
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stretching vibration in its FT-IR spectrum. Characteristic
hybrid samples are labeled as follows with the respective
numbering according to Figure 2: [1a]-COF for clicked lig-
ands; [Co-1a]-COF for COF-cobaloxime hybrid samples.
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C=O vibrations and signals originating from the hydrazone
bonds overlap at 1680 cm−1 (see Figure S13). New vibra-
tions emerged at 2250 cm−1 which could be assigned to the
propargyl groups confirming the successful incorporation of
DPTH building blocks into the COF backbone. This was
1
further supported by a 1D 13C{ H} ssNMR spectrum where
13C signals at 79 and 58 ppm can be assigned to the propar-
gyl functional group (Figure 1C). These shifts match the
corresponding chemical shift of the liquid state NMR of the
DPTH linker (see Supporting Information for experimental
details) and are also confirmed by quantum-chemical calcu-
lations (see Table S3).
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PXRD analysis confirmed the crystalline structure of
pCOF10. The PXRD pattern shows a strong reflection at
3.3◦ 2θ followed by smaller ones at 5.9, 7.0, 9.1 and a very
broad one at 26◦ 2θ. The experimental powder pattern was
compared to a simulated one (see Figure 1E) and the diffrac-
tion peaks assigned as the 100, 101, 200, 201 and 001 reflec-
tions, respectively. The peaks are broadened due to small
domain sizes in the COF particles, especially in the z direc-
tion, where the interlayer interactions are defined by π-π-
stacking only. Different possible orientations for the propar-
gyl functionality as well as slightly shifted AA’ stacking
modes lead to very similar powder patterns; due to broaden-
ing of the reflections in the experimental data, the different
orientations can not be distinguished. One of these possible
structural models is shown in Figure 1B, featuring an AA
Figure 2. (A) Structure of the azide-functionalized ligands 1a,
1b, 2 and (B) the azide-functionalized complexes Co-1a, Co-
1b, and Co-2. (C) Exemplary postsynthetic COF modification
towards [Co-1b]-COF. Synthesis conditions can be found in the
Supporting Information.
To verify the success of the tethering of the cobaloxime
and the unperturbed structural integrity of the covalently
modified hybrid COF-cobaloxime systems we performed the
same systematic experimental analysis as for the intact
˚
stacking mode with an interlayer distance of 3.5 A, which
is typical for structurally similar COFs.10,30,31 Note that
in the underlying structural model, one out of six DETH
linkers per pore was replaced by DPTH which results in a
functionalization degree of 16.6% instead of the statistically
pCOF10
. PXRD shows that the crystallinity of the COF
is preserved and the stacking mode does not change with
respect to pCOF10 (Figure S6). Sorption analysis shows the
expected reduction of the surface area according to Table
S1. Pore size distributions for the clicked samples were cal-
culated from Ar sorption isotherms as shown in Figure S5.
In all samples, the 2.3 nm pore size, as found in pCOF10, is
preserved with lower pore volume fraction while additional
smaller pores up to 1.9 nm occur as seen from optimized
pore models (see Figure S19). FT-IR spectra display all ex-
pected vibrations of the COF including propargyl vibrations
at ca. 3300 and 2300 cm−1. These vibrations are still visible
in ligand-tethered samples which hints to partial transfor-
mation. New triazole peaks are hidden in the region around
3100 cm−1 due to low intensity. The success of the click
reaction was further confirmed by the reduced intensity of
the propargyl signals relative to the other signals in the 1D
distributed 10% in the experimentally prepared pCOF10
Pawley refinement of the structure in the idealized AA
.
stacking mode suggests P2/m symmetry. For the modeled
˚
structure, the resulting cell parameters are a = 51.09 A,
b = 3.50 A, c = 29.48 A , α = γ = 90.00 and β = 89.94◦.
Sorption analysis revealed a mesoporous structure with pore
size of 2.3 nm and a Brunauer-Emmett-Teller (BET) sur-
◦
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face area of 1839 m2 −1, which matches the theoretically
g
expected values of the structural model well (see Figure 1D).
For the covalent attachment of the cobaloxime catalyst
to pCOF10, a postsynthetic click-chemistry approach was
chosen. The copper(I)-catalyzed Huisgen-type cycloaddi-
tion of azines and alkynes is known to be broadly appli-
cable with high yields and a high tolerance for functional
groups.32–36 Therefore, the pyridine which acts as axial
cobaloxime ligand was functionalized with an azide group
to yield the para-functionalized pyridine 1a, which forms
the azide-functionalized complex [Co-1a] and likewise, the
meta-functionalized analogues 1b and [Co-1b] were synthe-
sized, as depicted in Figure 2. Additionally, the equatorially-
functionalized chelating ligand 2 was synthesized as de-
scribed in the Supporting Information. It forms the azide-
functionalized catalyst [Co-2] by metal complexation as
before. Two strategies were tested for the attachment of
1
13C{ H} CP ssNMR spectrum upon addition of the azide
compounds. We did not observe any additional signals aris-
ing from the clicked compound, which is probably due to
signal superposition, especially in the aromatic region, and
due to lower signal intensity caused by a low functionaliza-
tion degree. UV-Vis diffuse reflectance spectra show two
additional broad absorption bands at 500 and 600 nm for
the cobaloxime containing samples (Figure S15). These
bands are due to the electronic transitions of the azide-
functionalized cobaloximes. Depending on the reaction con-
ditions (see supporting information for more details), the
cobaloxime loading can be adjusted within limits. For all
samples, the total cobaloxime amount was determined by
ICP analysis, and for [Co-1a]-COF it was additionally con-
firmed by fast-MAS 1H-detected NMR spectra. The values
range from 0.47 to 2.4 wt% for route II, while route I resulted
in higher cobaloxime amounts between 1.2 and 8.5 wt%. The
the cobaloxime complex to pCOF10
: i) metal complexa-
tion of azide-functionalized ligands with subsequent COF-
modification by click-reaction with the azide-functionalized
complexes, termed route I; ii) COF-modification by click-
reaction with azide-functionalized ligands with subsequent
complexation, termed route II (see Supporting Information
for experimental details). The resulting COF-cobaloxime
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highest cobaloxime content was found for [Co-1a]-COF as
directly observe four (1a-COF) and five ([Co-1a]-COF)
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can be seen in Table S2. The resulting functionalization de-
grees ranging from 2.0 to 15% are also listed there. Scanning
electron microscopy shows a flower-like morphology for all
samples. Elemental mapping showed a uniform distribution
of carbon, nitrogen, oxygen, and cobalt in the samples as
can be seen in the Supporting Information.
distinct proton resonances which correspond to the amide
proton (10.9 ppm), aromatic protons overlapping with the
olefin proton (7.2 ppm), methylene protons (3.9 ppm), and
methyl protons (1.7 ppm). For [Co-1a]-COF, we also ob-
serve a well-separated, downfield-shifted low-intensity peak
that belongs to the strongly hydrogen-bonded oxime proton
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(19.1 ppm). Note that all H signals are broader in the spec-
ssNMR analysis of the COF-cobaloxime hybrid sys-
tems
trum of [Co-1a]-COF relative to 1a-COF which indicates
that the cobaloxime functionalization process disrupted the
overall COF crystallinity to some extent. Cobaloxime con-
tains Co(III), which is, unlike Co(II), diamagnetic, therefore
the observed line broadening of [Co-1a]-COF cannot be a
consequence of paramagnetic relaxation enhancement; also,
residual CoCl2 salt is washed out during the sample prepara-
tion process. It is more likely that the post-synthetic modifi-
cation reduced the crystalline domain size and increased the
sample’s heterogeneity leading to a wider range of chemical
shifts for each sites.
9
While powder diffraction analysis provides long-range spa-
tial information such as approximate interlayer separations,
ssNMR provides us with short-range interatomic proximi-
ties, and hints about the position of the cobaloxime inside
the pore. To this end, we performed an in-depth structural
analysis of the clicked samples 1a-COF and [Co-1a]-COF
using 1H-detected, fast-MAS ssNMR at νrot = 55.55 kHz
at 700 MHz 1H Larmor frequency (16.4 T). The samples
based on [Co-1a] were chosen due to higher molecular sym-
metry compared to [Co-1b]. Both 1a-COF and [Co-1a]-
COF were studied by 1D and 2D 1H and 13C solid-state
NMR techniques. All 2D measurements were 1H-detected,
which significantly improved the sensitivity of the natural
abundance 13C measurements. In addition to the sensitivity
gain, we could exploit the 1H chemical shifts as well as the
1H - 1H correlations as sources of structural information.
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The good 1H resolution of the fast-MAS 1H spectrum
prompted us to measure 2D homonuclear correlation exper-
iments to gain deeper insight into the intramolecular inter-
action between the COF backbone and the cobaloxime co-
1
catalyst. We probed the relative H-1H distances using a 2D
double quantum-single quantum (DQ-SQ) correlation exper-
iments employing the R-symmetry-based R14−4 2 homonu-
clear recoupling sequence.37 The R14−4 2 is a γ-encoded sym-
metry sequence which suppresses all heteronuclear dipole-
dipole couplings and chemical shift terms in the first-order
Hamiltonian. We used a R = π0 element as the basic R-
symmetry block with a nutation frequency of 97.22 kHz
(3.5 × νrot). The homonuclear 2D 1H-1H DQ-SQ recou-
pling experiment relies on the generation of double quan-
tum coherences via homonuclear dipole-dipole coupling to
obtain through-space information of nearby protons. Due to
the double-quantum filter, the spectrum exhibits cross-peaks
only between protons that share direct dipolar interactions
with each other and thus no relayed magnetization transfer
occurs. For protonated organic solid materials, such as the
COFs of this study, the observation of a DQ peak is indica-
38,39
˚
tive of a proton-proton proximity that is ≤ 3.5 A.
The
relative signal intensities could well approximate interatomic
distances.39
Figure 3C and D show the 1H-1H DQ-SQ correlation
spectra of 1a-COF (yellow) and [Co-1a]-COF (blue). The
spectra reveal double-quantum correlations between both
distinct and identical environments, appearing at the off-
diagonal and diagonal positions, respectively. Diagonal
peaks are expected for the signals of the methyl and methy-
lene group, as well as between the resonances of the chemi-
cally equivalent aromatic sites. However, the weak diagonal
peak for the NH protons corresponds to an NH-NH autopeak
which is indicative of the dipolar interaction between COF
layers; the separation of NH protons within one layer is <
Figure 3. Solid-state NMR comparison of the 1H spectra of [1a]-
COF (yellow) and [Co-1a]-COF (blue) measured at 700 MHz 1H
Larmor frequency at νrot = 55.55 kHz. (A) Schematic structure
of the subsection of [Co-1a]-COF with proton labeling. (B) 1D
1H spectra of [1a]-COF (yellow) and [Co-1a]-COF (blue). Dis-
tinct 1H resonances are given in ppm and labelled with the cor-
responding atom labels as displayed in (A). (C) and (D) 1H-1H
DQ-SQ correlation spectra of [1a]-COF (yellow) and [Co-1a]-
COF (blue). Horizontal dashed lines indicate the 1H-1H con-
nectivities, and vertical solid lines reflect the individual 1H SQ
resonances. Assignments are given next to the dashed lines. In
(D) the assignment for only the two new connectivities are shown.
The skyline projection of both dimensions are also shown.
˚
˚
7 A, while the layer-to-layer distance is 3.5 A according to
powder crystal analysis. The two spectra look almost identi-
1
cal, the only considerable difference being the H cross-peaks
of the oxime 1H at 19.1 ppm with resonances at 8.7 and 3.4
ppm. In order to assign these two peaks, and thus uncover
the position of cobaloxime inside the pore, we performed
a detailed quantum-chemical study (vide infra). Based on
these studies we conclude the resonances at 8.7 and 3.4 ppm
to belong to the pyridine aromatic proton (H13), as well as
to a downfield shifted methyl proton of a neighboring ethoxy
Figure 3B compares the 1D 1H spectra of 1a-COF (yel-
low) and [Co-1a]-COF (blue). The high structural order
of these two-dimensional crystalline polymers is reflected in
the good resolution of the 1H signals; 1H line widths vary
between 800 and 1300 Hz for 1a-COF and between 1000
and 2000 Hz for [Co-1a]-COF. In the 1H spectra, we could
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group with which the cobaloxime is in close contact.
were used to elucidate the relative mobility of certain sites in
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the COF samples. In the 13C spectra recorded with d1 = 1 s
those signals are more intense that have considerably shorter
13C longitudinal relaxation time constants (T1 < 1 s), since
the signal recovery is proportional to 1−exp(−d1/T1). Such
short T1 is indicative of motions occurring on the inverse of
the Larmor frequency (few nanoseconds). The longitudinal
relaxation constant depends not only on the amplitude of
ns time-scale motion but also on the number of directly at-
tached protons: the more protons are directly bound to a
carbon the faster it relaxes via heteronuclear dipolar relax-
ation. This is reflected in the relative change of signal in-
tensities among the aromatic carbons. Besides, the methyl
resonance relaxes rapidly due to the free rotation around
the C-C axis in the ethyl group. The methyl resonance line
shape in the DP spectrum of [Co-1a]-COF is markedly dis-
torted presenting a shoulder at lower resonances. This signal
could be assigned to the methyl carbons of the cobaloxime
ligand. Otherwise, the signals of the covalently tethered lig-
and does not show any obvious sign of increased fast time-
scale flexibility, neither for [1a]-COF nor for [Co-1a]-COF.
In the spectrum of [Co-1a]-COF recorded with d1 = 1 s
the intensified resonances at 128 ppm indicate rather flexi-
ble aromatic sites, but due to strong overlaps in this region
we could not identify if this signal belongs to the ligand or
to some residual impurities which tend to show up stronger
in T1-weighted experiments. Selective 13C or 15N labeling at
specified positions at the ligand would help us to quantify
the amplitude and time scale of the ligand motion.
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The apparent lack of high-amplitude fast time-scale dy-
namics of the two COF frameworks were further validated
by comparing 1H-detected 2D CP-based 1H-13C correlation
spectra with INEPT-based 2D HSQC spectra (Fig. 4D, E).
High-amplitude ns time-scale motion results in inherent de-
coupling and thus leads to increased coherent lifetimes in
INEPT-based experiments and to decreased transfer efficien-
cies in CP-based experiments. In the HSQC spectrum of
both [1a]-COF and [Co-1a]-COF we observe only a single
methyl peak indicating that the COF backbone is generally
rigid on the ns time scale.
Figure 4. (A) Schematic structure of the subsection of [Co-1a]-
COF with carbon labeling. (B), (C) and (D) Comparison of the
natural abundance 13C one-dimensional solid-state NMR spec-
tra of [1a]-COF (blue shades) and [Co-1a]-COF (orange shades)
measured at 700 MHz 1H Larmor frequency at νrot = 55.55 kHz.
Direct polarization spectra recorded with d1 = 1 s (B) or with
long d1 = 25 s (C) are compared with CP MAS spectra (D). For
the CP MAS experiment, the carrier was centered at 130 ppm and
the CP was optimized to transfer magnetization to the aromatic
region. The CP contact time was 500 µs. Signals with short lon-
gitudinal relaxation times are enhanced in the 13C direct MAS
spectrum measured with 1 s recycle delay. The assignment of
the 13C resonances was obtained from 2D 1H-13C, and 1H-1H
correlation experiments, and from the quantum chemical calcula-
tions. The signals marked with crosses correspond to impurities,
e.g. to residual solvent signals.(E) and (F) 1H-detected 2D 1H-
13C correlation spectra of [1a]-COF (E) and [Co-1a]-COF (F)
recorded with 500 µs (red and green), or with 2250 µs (orange
and blue) CP contact times. The CP-based spectra are overlaid
with INEPT-based HSQC spectra which display only one methyl
cross-peak displayed with blue (E) and magenta (F) colors. For
each cross peak, the 1H and 13C assignments are displayed with
red and green colors, respectively. Signals marked with an as-
terisk are measurement artifacts and they do not appear in 1D
Computational studies
In order to provide a structural model for the position
and the orientation of the covalently tethered cobaloxime
co-catalyst inside the pore, we conducted a detailed in sil-
ico structural investigation of [1a]-COF and [Co-1a]-COF.
Atom positions and lattices of the periodic COF structure
of [1a]-COF were optimized on RI-PBE-D3/def2-TZVP40–43
level of theory using an acceleration scheme based on the
resolution of the identity (RI) technique and the continu-
ous fast multipole method (CFMM)44–46 implemented47,48
in Turbomole version V7.1.49 The obtained structure for the
[1a]-COF was then used to prepare parameters for molec-
ular dynamics simulations using antechamber.50 Force field
minimizations and subsequent dynamics were performed us-
ing the NAMD program package51,52 using GAFF param-
eters53 afterwards. NMR chemical shifts were then cal-
culated on B97-2/pcSseg-154,55 level of theory using the
FermiONs++56,57 program package, using cut models of
obtained structures to compare with experimental chemical
shifts and assign the resonances.
1
13C-detected 13C{ H} CP spectra.
Next, we assessed the relative flexibility of the two com-
pounds using 1D 13C NMR spectroscopy. Three different 1D
13C MAS spectra of [1a]-COF and [Co-1a]-COF are given
in Figure 4A and B, respectively. These spectra include
Using this data, we prepared 200 in silico 1H-1H DQ-SQ
1
13C{ H} cross-polarization (CP) MAS, and T1-weighted,
1
and H-13C 2D correlation spectra (see Supporting Informa-
direct-polarization (DP) 13C spectra recorded with short
(1 s) and long (25 s) recycle delay times. These latter spectra
tion for details) and used them to identify features that are
also present in the experimentally obtained ssNMR spectra.
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Figure 5. Direct comparison of quantum-chemically obtained 1H-13C (A, D, G, J) and 1H-1H DQ-SQ (B, E, H, K) 2D ssNMR spectra
with corresponding structural models of [Co-1a]-COF on the right (C, F, I, L). For a better comparison, the same NMR chemical shift
region is displayed as in the experimentally obtained spectra (Fig. 3C, D and Fig. 4D, E). In the 1H-13C 2D spectra blue and green
colors represent 1H-13C atom pairs that are within 6 and 2 A, respectively. In the 1H-1H DQ-SQ spectra, the orange color highlights
˚
the oxime proton cross-peaks. In C, F, I, and L the Co, Cl, O, N, and H atoms are displayed with pink, lime, red, blue, and white colors,
respectively.
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Such features include the number of cross-peaks, especially
cross-peaks of the oxime proton, their relative intensity ra-
translates into one [Co-1a] for every seven layers.
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tios, and their peak positions. The most distinctive factor
in the simulated 1H-1H DQ-SQ spectra is the presence of
oxime (H15) cross-peaks with resonances at around 8.7 and
3.4 ppm, which was used to categorize the simulated spec-
tra. These distinct chemical shifts suggest that the oxime
proton is interacting with an aromatic proton (at 8.7 ppm),
and with either an upfield shifted methylene proton or with
a downfield shifted methyl proton (at 3.4 ppm). There are
four different aromatic protons in [Co-1a]-COF: H1, H4,
H12, and H13, out of which only H4 and H13 can get closer
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than 3.5 A to H15.
To decide which resonance lead to the 3.4 ppm cross-peak
with H15, we analyzed the shielding effects of the glyoxime
group on the nearby ethoxy methyl and methylene protons.
The approach of the glyoxime oxygen towards the ethoxy
group induces a deshielding effect, consequently, both the
methyl and the methylene protons resonate at higher fre-
quencies (see Fig. S16 and SI text for more details), this
rules out the possibility that the cross-peak at 3.4 ppm would
stem from an upfield shifted methylene proton and leaves
only a downfield shifted methyl proton as a possible interac-
tion partner. Besides, we excluded the possibility that the
oxime proton shows a trivial intra-ligand cross-peak with the
glyoxime methyl protons, since (i) the distance between the
Figure 6. Front and side views of the MD simulated structural
model of [Co-1a]-COF showing a possible arrangement of the
co-catalyst. The linker and the cobaloxime group is depicted by
spheres and their carbon atoms are displayed with orange color.
Co, Cl, O, N, and H atoms are displayed with pink, lime, red,
blue, and white colors, C atoms of the backbone is light blue.
Photocatalytic activity
To probe whether there is a possible benefit of covalent
co-catalyst immobilization over simple physisorption,14,15
the COF-cobaloxime hybrid samples were tested for pho-
tocatalytic activity. In a typical photocatalysis experiment,
5 mg of COF hybrid were suspended in 10 mL of acetoni-
trile and water in a ratio of 4:1 at pH 8 containing 100 µL
triethanolamine (TEOA) as sacrificial donor. A housed Xe
lamp was used to illuminate the suspension interface with
a nominal beam spectral distribution similar to AM1.5G.
The beam intensity before experiments was then adjusted
to 100 mW cm−2. See ESI for more details. Photocatalytic
Hydrogen Evolution Reaction (HER) rates were quantified
in a continuous flow reactor as previously reported15 (Figure
7A). As a reference system, we compared the hybrid systems
to samples where [Co-1a] or [Co-1b] were added to the sus-
pension and physisorbed to COF-42 with a BET surface area
of 2336 m2 g−1 during photocatalysis. The maximum pho-
tonic efficiencies after in situ photoactivation of the samples
ranging from 2 to 8 wt% cobaloxime catalyst according to
ICP results can be found in Figure 7A. In the physisorbed
samples, an increase of the photonic efficiency was found
when increasing the catalyst amount from 2 to 4 wt% with
a maximum efficiency of 0.06% for [Co-1a] and 0.07% for
[Co-1b] at 4.0 wt%, while the efficiency is fairly constant
at higher percentages (0.06% to 0.08% at 5.0 and 8.0 wt%)
for [Co-1b]. This behaviour is expected for the system as
in the low-loading region, the photocatalytic activity scales
linearly with the co-catalyst amount while it reaches a max-
imum in the higher-loading region where the availability of
the co-catalyst is not limiting anymore.
In the hybrid samples, an activity maximum rather than
a constant behavior is found for each hybrid type. For the
para-functionalized [Co-1a], the highest photonic efficiency
was found at 4.1 wt%, while for the meta-functionalized
[Co-1b] the maximum was found at 3.2 wt%. As be-
fore, a linear increase of the photonic efficiency in the low-
loading regime was observed. However, further increase in
cobaloxime loading resulted in lower activity in the immo-
bilized samples. We attribute this to a predominant pore
clogging effect of the active sites with increasing function-
alization. In general, the highest photonic efficiency was
achieved with [Co-1a]-COF with 0.14% followed by [Co-
1b]-COF with 0.11% Compared to the physisorbed sam-
ples with the corresponding cobaloxime content, the activ-
˚
H15 and H16 protons are > 3.5 A, and (ii) the calculated
chemical shift are below 2.9 ppm.
Out of the 200 simulated 1H-1H DQ-SQ spectra 27 (13)
contained two (three) oxime cross-peaks, among which 22
spectra have these peaks in the expected ppm range. By
considering the relative peak intensity ratios between the
oxime cross-peaks only 15 spectra have a more intense
aromatic-oxime than a methyl-oxime cross peak. Two of
such spectra, together with the simulated 1H-13C spectra
and corresponding structures, are displayed in Figure 5A-F.
As counter-examples, Fig. 5G-H and J-K display the spec-
tra of such structures (Fig. 5I and L) where three equally
intense peaks (Fig. 5H) or no oxime proton cross peak (Fig.
5K) appear in the simulated DQ-SQ spectra. The possibil-
ity that in reality, in a fraction of the [Co-1a]-COF pores
the cobaloxime does not interact with the pore wall cannot
be ruled out, nonetheless, our current data suggests that
when it does, it gets in close contact with the nearby ethoxy
group. It is also likely that this genuine interaction stabi-
lizes the complex and restricts the cocatalyst’s degradation
during the photocatalytic cycles. Note that at this stage,
both the ssNMR measurements and the in silico calcula-
tions were performed in a solvent-free environment. Future
ssNMR measurements with added acetonitrile/water mix-
ture accompanied with simulations in explicit solvent could
reveal if the cobaloxime stays attached to the pore wall or
wether it gains more flexibility and drifts towards the pore
center.
To inspect the spacial arrangement inside the pore, we
modelled [Co-1a]-COF including one tethered co-catalyst
based on the MD simulated structures (Fig. 6). The dis-
played ligand has the same orientation as in Fig. 5C. From
the side and front views it is apparent that the ligand spreads
over multiple layers and occupies a substantial portion of the
pore. Due to spacial confinements our model suggest that
no more than three [Co-1a] over three layers can fit into the
backbone, i.e. the maximum number of [Co-1a] per layer is
one. In our case, we have 13 mol% functionalization which
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Figure 7. (A) Comparison of photonic efficiencies for hybrid samples and COF-42 with physisorbed [Co-1a] and [Co-1b]. (B) Com-
parison of the hydrogen evolution rate of [Co-1b]-COF containing 3.2 wt% [Co-1b] and COF-42 with 4.0 wt% physisorbed [Co-1b]
and coarse-grained model fits of both systems. (C) Projection of the hydrogen evolution of [Co-1b]-COF containing 3.2 wt% [Co-1b]
and COF-42 with 4.0 wt% physisorbed [Co-1b] based on the coarse-grained models.
ity doubles for both systems. Additionally, to emphasize
the role of the complex environment of the cobaloxime over
the pure presence of Co(II) we performed a measurement
where we added CoCl2 to a suspension of pCOF10 and tri-
ethanolamine in the photocatalysis medium as well as exper-
iments where one of the components (COF, TEOA) was ex-
cluded (see Supporting Information). None of the reference
samples showed hydrogen evolution after several hours of ir-
radiation. For the hybrid samples, the close contact between
the cobaloxime and the COF pore wall—revealed by repre-
sentative solid state NMR and computational studies (vide
supra) with [Co-1a]-COF—might facilitate charge transfer
to the cobaloxime catalyst from the COF pore wall as also
observed from photoluminescence measurements (see Fig-
ure S11 and Figure S12) [Co-2]-COF shows a significantly
lower activity in CH3CN/H2O which is a known effect for
cobaloximes that lack equatorial protons. The protonation
of the oxime oxygen, which is necessary for the catalytic pro-
cess, is hindered in those cases.58,59 The catalytic activity
could not be improved by lowering the pH to 4. In this case,
different acids (ascorbic acid, acetic acid, and citric acid)
were tested which simultaneously served as sacrificial elec-
tron donors instead of the amine base TEOA. Even though
the stability of [Co-2]-COF is predicted to be higher than
for the other tested cobaloximes, the complex proved not to
be appropriate in our case. We compared the best perform-
ing [Co-1b]-COF sample (containing 3.2 wt% cobaloxime)
to COF-42 physisorbed with [Co-1b]. A sample with the
same amount of physisorbed cobaloxime was qualitatively
active in photocatalytic hydrogen evolution, but for precise
quantification we increased the catalyst amount to 4.0 wt%.
Even though it contained 20% less catalyst, the hybrid sam-
ple was 47% more active than the physisorbed one (163 vs.
simplified model. Based on the coarse-grained fits, we pro-
jected the total amount of hydrogen evolved by the samples
at full depletion (see Figure 7C). After 780 h, the projection
of the physisorbed sample reaches 35 µmol hydrogen evolved
while the value is 59 µmol for the hybrid sample, which is
a gain of 69%. Comparing the estimated turnover number
(TON) of both systems, the deviation gets even more obvi-
ous. While the TON after 780 h is simulated to be 81 for the
physisorbed sample, it increases by 110% to a value of 170
in the hybrid sample. We attribute this activity enhance-
ment to the local confinement in the COF-hybrid samples
as supported by MD simulations.
Cobaloximes are known to slowly decompose under pho-
tocatalytic conditions. The labile axial pyridine ligand de-
coordinates in the catalytic cycle due to a square-planar
Co(II) transition state. The catalyst in solution can then
possibly be reduced which limits its stability. Due to the
confinement between ligand and catalyst in the COF pores,
the re-coordination might be enhanced, hence counteract-
ing degradation, which leads to reactivation of the catalyst.
Additionally, charge transfer is favoured in the case of spa-
tial proximity of the co-catalyst and the pore wall. Both
effects result in higher overall activity as well as longevity.
Interestingly, the activation period for the hybrid samples is
significantly longer than for the physisorbed ones. This may
be attributed to the time-delayed accessibility of the cata-
lyst in the pores. Both limitations could be addressed via a
method that was recently published by Thomas et al.,60,61
where silica spheres were used to create an inverse-opal ar-
chitecture in the COF material. The so created macropores
could serve as channels for reagents and products. Also, im-
mobilization of the co-catalyst in a COF with larger pores
might have a similar effect.
111 µmol h−1 −1), see Figure 7A. Additionally, the long-
g
term stability increased significantly. After 20 h, the ph-
ysisorbed sample shows 52% of its initial activity, while the
hybrid sample maintains 80% of its initial activity. To get
an estimate of the longevity of the systems, we fitted the hy-
drogen evolution rates of both samples with a coarse-grained
model (Figure 7C) that was established in an earlier study
on photocatalysis with COFs and a Nickel-based oligomer as
co-catalyst.15 The model resulted in very precise fitting for
the physisorbed catalyst because of similarities to the origi-
nal Nickel-based system from where the coarse-grain fitting
model was obtained, while the hybrid sample showed a more
complex behaviour that is not perfectly mapped with this
Conclusion
In summary, we have developed a platform derived from
COF-42 as a support for the immobilization of cobaloxime
catalysts. The post-synthetic modification of propargyl-
functionalized COF-42 enabled the covalent tethering of
three different cobaloximes to form COF-cobaloxime hybrid
systems. This tethering significantly enhanced the photocat-
alytic activity of the system by more than 100% compared
to the physisorbates with the corresponding cobaloxime
amount. The high crystallinity of our materials allowed
for an in-depth solid-state 2D NMR characterization using
fast MAS and proton detection. In the 1D 1H spectrum of
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[Co-1a]-COF we could clearly identify the resonance corre-
sponding to the oxime proton based on its highly downfield
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shifted resonance. The 2D H-1H Dq-Sq experiment showed
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Associated Content
Supporting Information
Experimental procedures, COF synthesis, details of
molecular dynamic simulations and quantum chemical cal-
culations and additional measurements.
Author Information
Corresponding Authors
Bettina V. Lotsch (b.lotsch@fkf.mpg.de)
Notes
The authors declare no competing financial interests.
Acknowledgement Financial support is gratefully ac-
knowledged from the Max Planck Society, an ERC Start-
ing Grant (project COF Leaf, grant number 639233), the
Deutsche Forschungsgemeinschaft (DFG) via the SFB 1333
(project A03), the Cluster of Excellence e-conversion, and
the Center for Nanoscience. PR acknowledges the Deutsche
Forschungsgemeinschaft (DFG, German Research Founda-
tion) SFB 1309 – 325871075, project A3 and Fonds der
Chemischen Industrie. We thank Prof. T. Bein and Prof.
W. Schnick (University of Munich, LMU) for granting access
to the XRD facility and V. Duppel and M.-L. Schreiber for
the assistance with material analysis.
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Eisenberg, R. Making Hydrogen from Water Using a Homoge-
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