Unveiling the Local Structure of Palladium Loaded into Imine-Linked Layered Covalent Organic Frameworks for Cross-Coupling Catalysis

Article abstract of DOI:10.1002/anie.202004197

Layered covalent organic frameworks (2D-COFs), composed of reversible imine linkages and accessible pores, offer versatility for chemical modifications towards the development of catalytic materials. Nitrogen-enriched COFs are good candidates for binding Pd species. Understanding the local structure of reacting Pd sites bonded to the COF pores is key to rationalize interactions between active sites and porous surfaces. By combining advanced synchrotron characterization methods with periodic computational DFT modeling, the precise atomic structure of catalytic Pd sites attached to local defects is resolved within an archetypical imine-linked 2D-COF. This material was synthesized using an in situ method as a gel, under which imine hydrolysis and metalation reactions are coupled. Local defects formed in situ within imine-linked 2D-COF materials are highly reactive towards Pd metalation, resulting in active materials for Suzuki–Miyaura cross-coupling reactions.

Full text of DOI:10.1002/anie.202004197

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Unveiling the Local Structure of Palladium Loaded into Imine-
Linked Layered Covalent Organic Frameworks for Cross-
Coupling Catalysis
Authors: Ignacio Romero-Muñiz, Andreas Mavrandonakis, Pablo
Albacete, Alicia Vega, Valérie Briois, Félix Zamora, and Ana
Eva Platero-Prats
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10.1002/anie.202004197
Angewandte Chemie International Edition
RESEARCH ARTICLE
Unveiling the Local Structure of Palladium Loaded into Imine-
Linked Layered Covalent Organic Frameworks for Cross-
Coupling Catalysis
Ignacio Romero-Muñiz,[a] Andreas Mavrandonakis,[b] Pablo Albacete,[a] Alicia Vega,[a] Valérie Briois,[c]
Félix Zamora,*[a,d,e] Ana E. Platero-Prats*[a,d]
[a]
I. Romero-Muñiz, P. Albacete, A. Vega, Prof. F. Zamora, Dr. A. E. Platero-Prats
Departamento de Química Inorgánica, Facultad de Ciencias
Universidad Autónoma de Madrid
Campus de Cantoblanco, 28049 Madrid (Spain)
E-mail: felix.zamora@uam.es, ana.platero@uam.es
Dr. A. Mavrandonakis
[b]
Electrochemical Processes Unit
IMDEA Energy
Avenida Ramón de la Sagra 3, 28935 Móstoles, Madrid (Spain)
Dr. V. Briois
Synchrotron SOLEIL
L'Orme des Merisiers, Saint Aubin, BP48, 91192 Gif-sur-Yvette Cedex (France)
Prof. F. Zamora, Dr. A. E. Platero-Prats
Condensed Matter Physics Center (IFIMAC)
Universidad Autónoma de Madrid
[c]
[d]
Campus de Cantoblanco, 28049 Madrid (Spain)
Prof. F. Zamora
[e]
Instituto de Investigación Avanzada en Ciencias Químicas de la UAM
Universidad Autónoma de Madrid
Campus de Cantoblanco, 28049 Madrid (Spain)
Supporting information for this article is given via a link at the end of the document.
Abstract: Layered covalent-organic frameworks (2D-COFs),
composed of reversible imine-linkages and accessible pores, offer a
versatile platform for chemical modifications towards the development
of catalytic materials. Notably, nitrogen-enriched COF materials have
been proved as good candidates for binding palladium species.
Understanding the local structure of reacting palladium sites bonded
to the COF pores is key to rationalize interactions between active sites
and porous surfaces. By combining advanced synchrotron
characterization tools, such as Pair Distribution Function analyses
and X-ray Absorption Spectroscopy, with periodic computational
Density Functional Theory modeling, we resolve the precise atomic
structure of catalytic palladium sites attached to local-defects within
an archetypical imine-linked 2D-COF. This material has been
synthesized using an in-situ method as a gel, under which imine-
hydrolysis and metalation reactions are coupled. We demonstrate that
in-situ formed local defects within imine-linked 2D-COF materials are
highly reactive towards palladium metalation, resulting in active
materials for Suzuki-Miyaura cross-coupling reactions.
extremely light and flexible materials. COF synthesis is
remarkably dynamic, evolving from highly disordered polymers at
initial reaction times to porous crystalline frameworks at the end
of the synthesis. The reversible nature of the COF linkages allows
the starting disordered structures to reorganize into crystalline
stable forms. Is indeed the intrinsic flexibility of these materials
maybe one of their most unique yet underexplored structural
features.
Among the expanding chemical variety of COF materials,
imine-linked frameworks have been one of the leading players
during the last years.[12]-[15] COF materials constructed from
condensation of amine and aldehyde linkers often result in
chemically stable materials.[16] However, the presence of water
can potentially reverse the imine formation, affording massive
hydrolysis and collapse of the framework. An elegant strategy to
avoid this undesired process has been reported by Banerjee and
co-workers, based on replacing hydrolysable imine bonding by
robust -ketoenamine linkages, taking advantage of the
irreversible imine-enamine tautomerism.[17],[18] Simply by
replacing a target trialdehyde building unit by its -substituted
hydroxyl analogue, a related -ketoenamine-linked COF material
with enhanced stability can be obtained. Recently, Dichtel and co-
workers have reported a versatile synthetic strategy to prepare -
ketoenamine COF materials starting from their imine-analogues
through ligand exchange methods.[19] Interestingly, this subtle
chemical modification of the COF architecture (i.e. replacing imine
by -ketoenamine linkages) might result in structural variations of
the framework, thereby playing a key role in the ultimate
properties of the material.[9]
Introduction
Covalent organic frameworks (COF) have been placed firmly
in chemistry as unique porous architectures[1,2] for their use in
applications related with water treatment,[3], [4] sensing,[5] energy
storage,[6]-[8] optoelectronics[9] and catalysis, including photo-[10]
and electro-activated[11] reactions.
A big family of these
architectures are composed of designable organic building units
linked through reversible covalent bonding, making COFs
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Nitrogen-enriched COF materials are ideal candidates for binding
catalytic metals, from single sites to few-atom clusters and bigger
[20]-[23]
nanoparticles
Palladium-catalyzed carbon-carbon cross-
couplings are unique tools in organic synthesis, in particular the
widely used Suzuki-Miyaura reaction due to the low toxicity of the
coupling partners and the mild conditions required.[24] One of the
main challenges linked to this type of catalysis is, however, the
recyclability of the palladium sites. One strategy is the use of
structurally-dynamic layered 2D-COF materials, where flexibility
arises from the tunable arrangement of purely organic layers. As
a proof of concept, Wang et al. in 2011 reported a pioneer work
in which the synthesis of a 2D imine-COF loaded with Pd(II) pre-
catalytic complexes that convert into Pd(0) nanoparticles during
the Suzuki-Miyaura reactions is described.[25] The direct
incorporation of homogenous palladium nanoparticles into
chemically-modified 2D COF materials (i.e. bearing ketoenamine
or triazine groups) also resulted in good catalytic materials for
one-pot Heck Sonogashira couplings[26] and multifold Heck
reactions.[27] Also, palladium(II) complexes incorporated into an
imine-linked triazine 2D-COF have been proved to catalyze
Hiyama couplings.[28] The use of a 3D imine-linked COF material
containing palladium acetate as heterogeneous catalyst showing
excellent conversion rates in cross-coupling reactions under flow
conditions was reported.[29] However, understanding the local
nature of palladium species loaded into COF frameworks under
conditions relevant for cross-coupling catalysis still remains under
debate.
Figure 1. Traditional metalation on a crystalline imine-linked COF material (left)
and in-situ method developed in this work, for which defect healing and
palladium binding are coupled (right).
Results and Discussion
The honeycomb imine-linked 2D-framework TAPB-BTCA
COF was prepared by condensation of 1,3,5-tris-(4-
In the present work, we combine experimental probes of local
and longrange atomic structure with computational modeling to
provide a more realistic picture of the catalytic palladium species
loaded into an archetypical defective imine-linked 2D COF
material. In particular, we report a room temperature one-pot
methodology to bind palladium complexes into local defects within
imine-linked 2D-COF platforms (Figure 1), affording active
catalytic materials for Suzuki-Miyaura cross-coupling reactions.
X-ray synchrotron Pair Distribution Function (PDF) analyses and
computational characterization indicate that palladium sites are
thermodynamically stabilized when binding local defects
containing amine terminations within the COF structures. Detailed
analyses combining PDF, Extended X-ray Absorption Fine
Structure (EXAFS) spectroscopy and Density Functional Theory
(DFT) modeling are applied to study in detail the real structural
nature of the reacting palladium species within an archetypical
defective imine-linked 2D-COF, under conditions relevant for
palladium metalation and Suzuki-Miyaura cross-coupling
catalysis.
aminophenyl)benzene
(TAPB)
and
1,3,5
benzene-
tricarbaldehyde (BTCA) at rt 30 °C, according to synthetic
methods reported previously by our group (see S2).[30] The
corresponding -ketoenamine analogue TAPB-TFP COF was
prepared by replacing BTCA with 2,4,6-trihydroxybenzene-1,3,5-
tricarbaldehyde (TFP) during the synthesis (see S2) for details).
The formation of the COFs was confirmed by powder X-ray
diffraction (PXRD) and attenuated total reflection Fourier-
transform infrared (ATR-FTIR) spectroscopy (Figure 2).
Interestingly, although crystalline materials are obtained after 72
h, the appearance of a disordered COF-like product at short
reaction times (2 h) was identified, suggesting the significant
presence of local defects within the frameworks.
The gel nature of these 2D COF systems before solvent
exchange, inspired us to explore chemical modifications such as
metalations directly as gels, conditions under which diffusion and
subsequent metal binding to the coordinating groups is favored.[31]
The incorporation of metal sites into COF structures, being an
excellent approach to achieve selectivity in heterogeneous
catalysis, is commonly carried out post-synthetically on the dried
pristine material. Hence, two different one-pot methods to load
palladium sites into nitrogen-donor groups within the COFs were
studied: i) In-situ metalation, where the palladium precursor was
directly added to the reacting disordered COF polymer (i.e. 2 h)
and let them further react until the material becomes crystalline
(i.e. 72 h). ii) Gel metalation, where the palladium precursor is
added to the crystalline COF as a gel and let them react further
(i.e. 24 h). We hypothesize that palladium metalation achieved
through the in-situ method would allow the incorporation of
palladium into local defects, affording a very interesting defective-
system for exploring catalysis.
2
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10.1002/anie.202004197
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RESEARCH ARTICLE
value of 888 m2g-1 determined for the pristine material (Figure 4a
and Table S8.1). Pore size distribution analyses performed using
a NLDFT model, indicated a main pore diameter of 15.2 Å for both
systems (Figure 4a), in agreement with the framework structure.
Additional minor contributions at 12.7 Å and 11.3 Å corresponding
to the presence of partially eclipsed layers were also observed,
being the population of those less abundant for the palladium-
loaded material.
Figure 3. Synchrotron PXRD data collected on the imine TAPB-BTCA and the
ketoenamine TAPB-TFP materials after in-situ palladium metalation in
comparison with the pristine systems.
Figure 2. (a) FT-IR and (b) synchrotron PXRD data collected for TAPB-BTCA
and TAPB-TFP 2D-COF materials.
Interestingly, surface area slightly increases after palladium
metalation for the ketoenamine BTCA-TFP material. BET
values of 664 m2g-1 and 550 m2g-1 were obtained for the in-situ
metalated and pristine systems, respectively (Figure 4b and Table
S8.1). Similar surface areas are obtained for the materials
metalated as gels (Table S8.1). Additionally, pore size distribution
of the in-situ metalated BTCA-TFP is in good agreement with the
values expected from crystallographic data (Figure 4b). These
results further corroborated that the palladium centers would favor
the occurrence of structural rearrangements coupled to the
simultaneous crystallization and tautomeric formation of the
ketoenamine form of BTCA-TFP.
Chemical composition analyses performed on the COF
materials after metalation showed that palladium modification is
more efficient following the in-situ metalation method, resulting in
14.9 and 11.8 wt % Pd loadings for imine and -ketoenamine
systems, respectively (see S4). While targeting same palladium
loadings, the gel-metalation afforded materials with systematically
lower content (ca. 10 wt%) of palladium than those synthesized
using the in-situ metalation procedure. These results
demonstrated that coupling palladium metalation with the
dynamic hydrolysis of the imine-linkages, as it occurs during the
in-situ method, would enhance the efficiency of the metalation
step during the synthesis.
Synchrotron powder X-ray diffraction (PXRD) data collected
on the palladium-metalated materials showed that crystallinity is
retained for both COF chemistries studied. For the imine-linked
TAPB-BTCA materials, diffraction data demonstrates that
longrange order is not altered after palladium incorporation
within the structure. Interestingly, the palladium-loaded
ketoenamine TAPB-TFP system prepared using the in-situ
method showed improved crystallinity after metalation compared
to the pristine material (Figure 3). This observation could indicate
that palladium(II) centers may play a defect-healing role during
the COF crystallization due to their Lewis acidity, favoring the
stabilization of the ketoenamine over the enol-imine form (i.e.
favoring the palladium binding to amine groups). On the contrary,
this phenomenon is not critical during the metalation of the imine-
linked TAPB-BTCA due to the use of acetic acid as catalytic
solvent (see S2). Pawley analyses of the diffraction data indicated
that cell parameters and symmetry remained unaltered after
metalation with palladium for all the materials (see S3).
Nitrogen adsorption-desorption isotherms collected on the
activated imine TAPB-BTCA after palladium metalation indicated
that porosity is retained. For the imine-linked COF material
metalated using the in-situ method, a decreased BET surface
area value of 494 m2g-1 was determined, in comparison with a
Figure 4. Nitrogen adsorption-desorption isotherms and pore size distribution
plots for (a) for TAPB-BTCA (imine) and (b) TAPB-TFP (ketoenamine)
materials after in-situ palladium metalation in comparison with the pristine
systems.
3
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To assess the accessibility of the added palladium centers
within these two archetypical 2D-COF systems, the catalytic
activity of the metalated COF materials was explored for a Suzuki-
Miyaura cross-coupling as a model reaction. The coupling of p-
bromonitrobenzene and phenyl boronic acid using THF as a
solvent and base was tested under batch conditions at 80 °C (see
S12). Under these catalytic conditions, yields of ca. 70% were
determined after 2 h reaction time for both TAPB-BTCA and
TAPB-TFP systems metalated using the in-situ method. On the
contrary, lower yields were obtained for both materials metalated
using the gel-method (see S12.1). Additionally, lower yields and
higher leaching values were determined for the -ketoenamine
materials compared to their imine analogues. This suggests a
further stabilization of the palladium sites within defective-imine
COF systems, in contrast to the defect-free highly stable -
ketoenamine counterparts.
Being the oxidative addition the most critical step for
understanding the reactivity of the palladium centers loaded into
the imine-linked TAPB-BTCA in Suzuki-Miyaura couplings, a
wider substrate scope was carried out. To assess mechanistic
aspects related to this reaction step, different aryl halides
containing different stereo-electronic functionalities including
electron withdrawing (Table 1, entries 1e and 1f) and donating
(Table 1, entries 1e, 1g and 1h) groups, were selected. In all
cases, oxidative addition of the aryl halide to the palladium
centers is achieved yielding the desired cross-coupling product
(Table 1). The results evidence that the catalytic activity of the
palladium sites within the TAPB-BTCA depends on the electronic
properties of the aryl halides, reaching higher yields with
electrophilic substrates containing electron withdrawing
functionalities in para-positions.
in-situ metalated TAPB-BTCA and TAPB-TFP materials, the
presence of small palladium NPs of ca. 8 and 11 nm average size,
respectively, was exclusively identified after catalysis, in
agreement with the Suzuki-Miyaura mechanism (see S5). TEM
analyses showed that the post-catalysis palladium NPs are
homogeneously distributed within these materials. This result
evidenced an optimal metalation of the COF structure with
palladium following the in-situ metalation method (see S2). On the
contrary, a significant palladium segregation on the external COF
surface is determined after catalysis for the gel-metalated COF
materials (see S5). Diffraction data collected on the post-catalysis
materials did not show the presence of bulk palladium-black,
further supporting the nano-sized nature of the reduced palladium
species formed during catalysis.
To probe the local chemical environment of the palladium
sites within the 2D-COF systems, Pd K-edge Extended X-ray
Absorption Fine Structure (EXAFS) experiments were performed.
Pd K-edge EXAFS data collected on representative systems
evidenced the occurrence of significant differences in the
palladium coordination environment during catalysis, depending
on the COF chemistry. For both imine and -ketoenamine COF
materials, two contributions centered at ~1.5 Å and ~1.9 Å in the
EXAFS spectra (without phase correction) are observed, linked to
PdN and PdCl bonds, respectively. Unexpectedly, the signal
corresponding to PdCl bonds disappears after catalysis, with the
concomitance appearance of a contribution centered at ~2.1 Å
which is attributed to PdBr bonds (Figure 5). Importantly, the
sharp contribution at ~1.5 Å which is attributed to PdN bonds
remains unaltered after catalysis, demonstrating the actual
binding of the palladium species to the COF framework (Figure 5).
These results demonstrated that labile chloride ligands are
completely exchanged by bromide groups formed during the
catalytic cycle in the reaction media, resulting in the main reacting
palladium(II) species.
Table 1. Suzuki-Miyaura cross-coupling scope using in-situ palladium-
metalated imine TAPB-BTCA as a catalyst.
Inspection of Pd K-edge X-ray Absorption Near Edge
Structure (XANES) data indicated a subtle shift of the absorption
edge determined at 24352 eV to ca. 2 eV lower energy values for
all the materials, in agreement with the partial reduction of
palladium(II) into palladium(0) during catalysis. EXAFS data
obtained on the post-catalysis showed the presence of an
expected contribution at ~2.45 Å (without phase correction),
which is associated with the formation of PdPd bonds after
catalysis (Figure 5). This result is in agreement with the formation
of Pd(0) NPs during catalysis, as further corroborated by TEM
(see S5). Importantly, the relative intensity of the PdPd bonds
contribution with respect to that of the PdBr bonds is significantly
higher for the imine TAPB-BTCA materials compared to the -
ketoenamine counterparts. This evidence agrees with the better
catalytic activity obtained for the metalated imine COFs in
comparison with their -ketoenamine analogues. In addition, for
the imine TAPB-BTCA metalated through the gel method, the
relative intensity of the PdPd contributions is clearly larger than
the contribution linked to PdBr bonds, suggesting a larger
formation of Pd(0) NPs compared to the in-situ metalation (Figure
5a). On the contrary, for the less active material (i.e. -
ketoenamine TAPB-TFP metalated as gel), no significant
contributions associated to PdPd bonds were identified (Figure
5b). These results demonstrated that the chemical coordination
environment of the palladium sites loaded within 2D-COF
materials is a structural fingerprint to understand their ultimate
catalytic activity.
To evaluate the stability of the palladium sites incorporated
within these 2D-COF materials towards aggregation during the
reduction-elimination step, under which palladium(II) gets
reduced to palladium(0),[32] further characterization was
performed. Transmission electron microscopy (TEM) analyses
were carried out on the materials before and after catalysis, to
identify the formation of palladium nanoparticles (NPs). For both
4
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Figure 5. The k3-weighted χ(r) Pd Kedge EXAFS spectra of metalated imine TAPB-BTCA COF (a) and β-ketoenamine TAPB-TFP COF (b) systems prepared
following either the in-situ or the gel methods, before and after catalysis
To better understand the local structural nature of the
palladium species loaded through the in-situ metalation method,
the energetics of defective and pristine imine TAPB-BTCA
materials were first explored computationally. We hypothesized
that under these synthetic conditions, in which the imine-bonding
is highly reversible, the presence of local defects resulting from
the hydrolysis of imine linkages to form amine and aldehyde
groups might be significant. Structural models showing different
relative orientation of the amine and aldehyde groups were
computed. Interestingly, periodic calculations demonstrated that
imine hydrolysis to give local defects in which the amine and the
aldehyde are pointing out towards different directions (trans-
defect conformers) are energetically favored by -5.4 kcal·mol-1
(Figure 6 and S11.1). On the contrary, imine hydrolysis to form
local defects where amine and aldehydes are pointing towards
the same direction (cis-defect conformers) are less favored by 3.4
kcal·mol-1, with a formation energy of -2.0 kcal mol-1 (Figure 6 and
S11.1). Interestingly, calculations showed that the energetics of
imine hydrolysis within these 2D-COF materials depends on the
relative location of the resulting amine and aldehydes groups in
the COF pores  more stable defective-COF structures are those
for which both aldehyde and amine groups are pointing out
towards different directions (trans-conformers, Figure 6). While
having related chemistries and similar average structures, these
computational studies also demonstrated that defective-TAPB-
BTCA structures are stabilized through an energetically more
favorable interlayer stacking compared to the idealized defect-
free systems (Tables S11.3 and S11.4).
α = β ≈ 91°, γ = 120°). This result indicated that palladium
metalation on the TAPB-BTCA material involves a structural
rearrangement, resulting in a more efficient packing of layers.
While in both conformations the palladium chemical environment
is almost identical, these two models mainly differ in their -
stacking and metalation energies. After detailed analysis of the
individual bonding interactions for the most stable TAPB-BTCA
form (triclinic cell) (see Tables S11.3 and S11.4), its
corresponding metalation and - stacking energies were
estimated to be -23.2 and -49.3 kcal·mol-1, respectively.
Interestingly, for the defectfree non-metalated COF, the -
energy gain is estimated to be -48.7 kcal·mol-1. These results
demonstrated that, after palladium metalation, a more efficient -
 packing of the COF layers is identified, as a result of strong
interactions with the -[PdCl2] moieties located within adjacent
COF layers.
In a next step, the energetics of the palladium metalation was
computationally characterized in detail. Periodic structural models
of different palladium-metalated imine-linked TAPB-BTCA
systems were computed to explore the binding nature of
palladium to different sites for both defectfree and
defectivematerials. For the defectfree TAPB-BTCA material,
DFT calculations showed that palladium(II) only binds to imine
2
groups (푁푠푝 ) from adjacent COF layers, resulting in square-
planar geometries. This is the energetically favored metalation
mechanism with a computed energy gain of -11.6 kcal·mol-1 (see
Table S11.3) (triclinic, α ≈ 76°, β ≈ 100°, γ ≈ 118°). A less favorable
conformation with energy gain of -5.1 kcal·mol-1 was also
obtained, retaining the native cell symmetry (pseudo-hexagonal,
Figure 6. Reaction energies calculated for different defective-COF conformers
containing either cis- or trans-defects.
5
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Interestingly, a much richer structural variety was observed for
the palladium metalation on the defective TAPB-BTCA system.
The Density Functional Theory (DFT) calculations suggested that
palladium binding is more energetically favored to simultaneously
To further probe the local structure of the palladium sites
beyond the first coordination shell, synchrotron Pair Distribution
Function (PDF) analyses were performed. The PDF technique
provides local structural insights to a longer r-range than EXAFS,
resulting in a unique technique to assess the structural
characterization of chemically-modified COF systems with
compromised periodicity. Differential analysis of the PDF data
highlights new and modified atom–atom distances following
palladium metalation; by subtracting the PDF of the bare COF to
that of the metalated material, new atom–atom distances
associated with the added-palladium species are isolated. The
differential PDFs (d-PDFs) obtained by subtraction of the PDF
collected on bare TAPB-BTCA from that of the corresponding
metalated materials before catalysis showed contributions at ~1.9
Å and ~2.3 Å linked to PdN and PdCl distances (Figure 8a),
respectively, attributed to the coordination environment of the
palladium(II) sites. Interestingly, for the material prepared using
the in-situ method, the d-PDF shows additional sharp
contributions up to ~5 Å corresponding to up three shells, in
agreement with the periodic DFT model. These results evidenced
that the in-situ metalation results in well-defined palladium
species attached to the defective-COF compared to the gel-
method.
3
2
amine(푁푠푝 ) and imine(푁푠푝 ) groups from adjacent layers
than to exclusively imine groups, as observed for the defectfree
analogue. The difference in relative energy between these two
coordination scenarios is low with values of ~3.0 kcal·mol-1, as
anticipated due to the similar chemistries, suggesting the
plausible co-existence of both coordination environments.
Additionally, these calculations demonstrated that palladium
binding to the carbonyl terminations within the local defects is not
energetically favored (Table 2). Importantly, palladium-metalation
on the amine-imine defect-site is more energetically favored than
on the imine-imine sites within defective-COF materials. For low
concentration of defects (Figure 7 and Table S11.2), palladium
binding to either amine-imine or imine-imine sites has similar
energies, suggesting that both coordination environments might
coexist. However, with increasing number of defects within the
COF, the amine-imine site is energetically preferred. This finding
suggests that both the formation of local defects (imine hydrolysis)
and palladium binding (metalation) to the in-situ generated amine
groups might concur. This remarkable result from calculations
demonstrated that the defect-sites within TAPB-BTCA are more
reactive towards palladium metalation than the imine sites,
resulting in different palladium environments than expected
together with a more efficient metal loading (Table S4.1).
Table 2. Formation energy (ΔEform) linked to the palladium metalation on the
imine TAPB-BTCA materials.
Palladium
binding
Relative
Energy
(kcal mol-1)
Relative
Energy[a]
(kcal mol-1)
Relative
Energy[b]
(kcal mol-1)
3
amine푁푠푝
)
0
-9.3
-6.3
-3.9
-0.9
2
imine푁푠푝
)
2
imine푁푠푝 
+3.0
+13.2
+29.7
2
imine푁푠푝 
Figure 8. Differential PDFs, d-PDFs, obtained by subtraction of the PDF
obtained for pristine TAPB-BTCA COF, showing the contributions attributed to
the palladium species before and after catalysis for the different metalation
methods used.
2
imine푁푠푝 
+3.9
+20.4
+9.3
+21.2
CC
2
carbonyl-푂푠푝 
After catalysis under conditions relevant for Suzuki-Miyaura
Coupling, new d-PDF peaks appeared at ~2.7 Å and at longer
distances corresponding to PdPd correlations within metallic
Pd(0) NPs with a face centered cubic (fcc) structure. Refinements
using a structural model of fcc Pd(0) against the experimental d-
PDF data provided an estimated average nanocrystal size,
assuming a spherical particle model (Figure 9). Refinements were
performed in the range of ~2.7-30 Å, beyond the contributions
associated with the coordination sphere of the palladium(II) sites
(i.e. a peak at ~2.42 Å is observed linked to PdIIBr distances
formed after catalysis, in agreement with EXAFS data). Average
Pd(0) crystal dimensions of ~3-4 nm were estimated for the
metalated TAPB-BTCA materials. These results are in
agreement with the particle size distribution determined by TEM
(~4-15 nm), proving that the palladium nanoparticles are
composed of about ~1-3 Pd(0) nano-crystals. These results
demonstrated the nano-size nature of the Pd(0) crystals formed
during the catalytic cycle, being stabilized either on the TAPB-
BTCA pores (in-situ) or at the external surface (gel) (see ESI,
Section S5).
NC
[a] Starting from defectfree COF. [b] Starting from defective COF.
Figure 7. Metalation energy (kcal·mol-1) versus the defect concentration in
COF. The %defect is defined as the number of defect sites per total number of
imine groups in the cell. The metalation energies are calculated from: COF +
PdCl2(CH3CN)2  COF-PdCl2 + 2CH3CN.
6
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10.1002/anie.202004197
Angewandte Chemie International Edition
RESEARCH ARTICLE
Conclusion
We have reported the palladation of an imine-linked layered COF
(TAPB-BTCA) and its β-ketoenamine analogue (TAPB-TFP)
using a single-step methodology (in-situ method), under which
metal binding and COF crystallization occur simultaneously.
Advanced synchrotron characterization combining Pd K-edge
EXAFS and PDF analyses corroborated the incorporation of well
defined -Pd(II)Cl2 moieties covalently attached to the COF pores.
Interestingly, computational studies showed that local defect-sites
within TAPB-BTCA, obtained as a result of imine-linkage
hydrolysis, are more reactive towards palladation than the ideal
pristine sites, resulting in different palladium environments than
originally envisaged. We also demonstrated that metalation
results in
a more energetically favored three-dimensional
arrangement, giving a more efficient packing of the COF layers
without disrupting the average porous structure. After Suzuki-
Miyaura coupling catalysis, a detailed analyses combining
EXAFS and PDF together with periodic DFT modeling
corroborated that the palladium reactive species within these
COFs are palladium(0) nanocrystals of 3 nm-size. Interestingly,
Pd(II)-Br2 moieties attached to the COF framework with a well-
defined local structure were also identified, which may play a key
role in the catalytic cycle. Our results suggest that the
incorporation of local defects within imine-linked COF materials
could be an interesting strategy to stabilize bonded palladium
sites for ultimate catalytic applications.
Figure 9. Fit of fcc Pd(0) NPs to the d-PDF obtained by subtracting the total
PDF of in-situ metalated TAPB-BTCA from that obtained after catalysis.
Features in the residual d-PDF (green) reflect the presence of a well-defined
local structure with a main signal attributed to Pd(II)-Br bonds.
To truly understand the structural nature of the molecular
palladium(II) bromide species formed during Suzuki-Miyaura
Coupling reactions, combined d-PDF and theoretical DFT studies
were further carried out. The formation of bromide anions during
catalysis might occur during the oxidative addition step, resulting
in the formation of postulated organanopalladium(II) species with
characteristic PdBr bonds. Interestingly, the PDF data collected
after catalysis on in-situ metalated TAPB-BTCA demonstrated
the presence of both palladium(0) nano-crystals of ~3 nm size and
well-defined palladium(II)Br2 sites. These palladium(II) species
have a well-defined local structure with a main peak centered at
~2.42 Å attributed to Pd(II)Br bonds. To explore the energetics
of these new formed Pd(II)-Br2 sites within the COF pores
compared the native dichloride species seen before catalysis,
periodic DFT calculations were performed. The residual d-PDF
obtained from subtracting the contribution of fcc Pd(0) NPs to the
d-PDF of in-situ metalated TAPB-BTCA after catalysis (Figure
10), highlighted the atomic contributions linked to the newformed
species containing PdIIBr bonds. Simulated PDF data using the
periodic DFT structural model (Figure 10b) matched well with the
contributions experimentally observed, demonstrating the single-
site local structure of the palladium(II)-Br2 reacting sites attached
to the COF.
Acknowledgements
We thank the Spanish Ministry of Science, Innovation and
Universities (RTI2018-096138-A-I00 and RTI2018-101049-B-I00)
and the Comunidad de Madrid for funding through the TALENTO
grants (2017-T1/IND5148 and 2017-T1/AMB-5264). A. E. P. -P.
and F. Z. acknowledge financial support from the Spanish Ministry
of Science and Innovation, through the “María de Maeztu”
Programme for Units of Excellence in R&D (CEX2018-000805-M).
The authors acknowledge the computing facilities of CSUC for
providing resources that contributed to the research results
reported within this paper. This work was supported by a public
grant overseen by the French National Research Agency (ANR)
as part of the “Investissements d’Avenir” program (reference:
ANR10-EQPX45). We acknowledge SOLEIL ROCK synchrotron
radiation beamline (proposal 20180480). The authors would like
to thank Diamond Light Source for rapid-access beamtime and to
Dr. Dean S. Keeble to perform X-ray total scattering experiments
(proposal CY22812). Part of this research were carried out at
P02.1 beamline at DESY (proposals I-20170717 EC and I-
20190208 EC), a member of the Helmholtz Association (HGF).
We would like to thank Dr. Michael Wharmby for assistance
during the experiment at P02.1. The research leading to this result
has been supported by the project CALIPSOplus under the Grant
Agreement 730872 from the EU Framework Programme for
Research and Innovation HORIZON 2020". I. R. -M.
acknowledges a FPI-UAM 2019 fellowship from Universidad
Autónoma de Madrid. P.A. acknowledges the financial support
from “Ayudas para Contratos Predoctorales para la Formación de
Doctores” Program of MINECO (Grant BES-2017-070838).
Acknowledgements Text.
Figure 10. d-PDF profiles for in-situ metalated TAPB-BTCA COF before and
after catalysis, in comparison with the data simulated from the periodic DFT
models.
7
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10.1002/anie.202004197
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RESEARCH ARTICLE
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Keywords: Layered Covalent-Organic Frameworks
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10.1002/anie.202004197
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RESEARCH ARTICLE
Entry for the Table of Contents
Insert graphic for Table of Contents here.
“Zoom-ing” into catalytic defects within layered covalent-organic frameworks. By combining advanced synchrotron
characterization tools and modeling, we provide a more realistic picture of the structure of catalytic palladium loaded into imine-linked
layered covalent-organic frameworks. The occurrence of local defects plays a key role in both chemical modifications through metalation
and catalytic activity in Suzuki-Miyaura cross-couplings.
Institute and/or researcher Twitter usernames: @anaplateroprats, @fzamora0, @amavr78,
@ignacio_romerom, @PabloAlbaceteC, @UAM_Madrid, @EcpuEnergy
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